Claire Meissirel

Montse Soler- Lopez

Marc Dhenain

Keywords: 4 aminopyridine ABP, Actin-binding protein AC, Adenylate cyclase AChE, Acetylcholinesterase

First and foremost, I would like to

Un grand merci aussi aux amis « pas labos », Kelly, Tony, Laura, Tam, Pex, Tazo, Tris (et encore d'autres que je ne peux citer par soucis de place)! Merci d'avoir été là, et juste merci d'être vous! Don't change a thing either ;) Diolch yn fawr Mammy, Daddy! Thank you for always believing in me, for your continuous support through thick and thin, for your patience especially in the last few months (I know I've been a real pain in the back side!) and for everything that you do (I could make a list of things to thank you for as long as this thesis) you're the best! And mustn't forget the bros! Alex, Liam and Jonathan! What a great gang! U ma homies, u da best! Mes derniers remerciements vont à toi Pup. Toi qui es à mes côtés depuis le tout début. On en aura fait du chemin! On a grandi ensemble et je te remercie du fond du coeur pour tout ce que tu es, tout ce que tu m'as apporté et tout ce que tu as dû endurer! Si j'en suis là aujourd'hui, autant sur le plan professionnel que personnel, c'est grâce à toi! Love you long time <3

To my mams,

Résumé

La maladie d'Alzheimer (MA) est définie comme une maladie neurodégénérative où des altérations synaptiques mènent à la perte neuronale parallèlement à des défauts de mémoire et d'apprentissage. Il est établi que les dysfonctions synaptiques observées dans la MA sont initiées par les formes oligomériques du peptide β-amyloïde (Aβ), un dérivé protéolytique de l'Amyloïd Precursor Protein (APP). Cependant, le chemin qu'empreinte Aβ, selon son origine intra-ou extracellulaire, afin d'induire ces effets délétères et la façon dont ses effets sont maintenus et se propagent dans le cerveau restent encore à définir.

Dans cette étude, nous avons utilisé plusieurs formes mutées de l'APP qui conduisent à des peptides Aβ avec des signatures moléculaires uniques, tel que : la mutation Swedish (K670M/N671L) (APP swe ) qui augmentent la sécrétion (extracellulaire) d'Aβ; la mutation Osaka (E693Δ) (APP osa ) qui cause une accumulation intraneuronale (intracellulaire) d'Aβ; ainsi que la mutation Icelandic (A673T) (APP ice ) qui a été établi comme diminuant la production d'Aβ et protégeant contre la MA. Ces formes mutées d'APP ont été surexprimées dans des cultures de neurones corticaux murins et on permit : i) d'étudier la morphologie et fonction des épines dendritiques, l'élément post-synaptique, par microscopie confocale; ii) de tenter de mieux comprendre comment la pathologie se développe et se propage dans le cerveau et iii) d'identifier un nouveau partenaire d'intéraction avec l'Aβ faisant la lumière sur un possible rôle physiologique de ce peptide dans les neurones.

Nous montrons qu'une accumulation pathologique d'Aβ, due à la surexpression d'APP wt , APP swe et APP osa mais pas APP ice , induit une diminution significative de la densité synaptique particulièrement celle des épines les plus fonctionnelles, dites « mushroom ». Ses épines mushroom restantes présentent également une augmentation significative de leur volume et il semblerait que l'Aβ intracellulaire soit suffisant pour induire ses effets. Ses épines mushroom élargies présentent également une plasticité structurale altérée puisqu'elles n'ont pas augmenté d'avantage de volume à la suite d'une activation synaptique. Il semblerait que ceci soit la résultante d'un défaut de la dynamique activité-dépendante du cytosquelette d'actine dans les épines. Ces altérations de la morphologie, structure et plasticité synaptique serait dû à une intéraction, nouvellement identifiée, de l'Aβ avec l'actine et pourrait faire lumière sur un possible rôle physiologique de l'Aβ dans la plasticité synaptique activité-dépendante. De plus, nous montrons que le clivage amyloïde de l'APP est aussi activité-dépendant et que la séquence du peptide Aβ généré est aussi importante, dans l'induction de la synaptotoxicité, que sa concentration. En effet, car nous montrons que des concentrations pathologiques du peptide Aβ ice n'engendrent pas de perte ou de gonflement des épines mushroom. Enfin, nous mettons en lumière que l'Aβ sécrété dans le milieu extracellulaire affecte, non seulement le neurone sécrétant lui-même, mais aussi la densité synaptique des neurones sains avoisinant (qui ne surexpriment pas d'APP) d'une manière APP-dépendante, rappelant un mécanisme de propagation du type prion. L'ensemble de ces données démontrent que le clivage protéolytique de l'APP et la production d'Aβ qui en découle est un processus finement accordé, impliqué dans le remodelage de l'actine dans la plasticité synaptique activité-dépendante et ouvre de nouvelles voies pour le développement de stratégies thérapeutiques contre la MA.

I. Alzheimer's disease

A brief bit of history A.

On the 25 th of November 1901 at Frankfurt Hospital, German medical doctor Aloïs Alzheimer (Figure 1, left panel) receives a new patient. This 51 year-old woman named Auguste Dieter (Figure 1, right panel) has a set of marked cognitive disorders. Dr Alzheimer will note a reduction of memory and the comprehension of language, a very pronounced aphasia, an unpredictable behaviour, paranoia and auditory hallucinations [START_REF] Maurer | Auguste D and Alzheimer's disease[END_REF]. He observed, for example, that Mrs. Dieter was incapable of remembering the colour or shape of an object presented to her a few minutes before. She also had an Amnestic writing disorder and her spontaneous speech was full of paraphrasic derailments.

In 1906, Dr. Alzheimer will present this particular cognitive pathology that will take on his name in his article "Über einen eigenartigen schweren Erkrankungsprozeß der Hirnrinde" or "About a peculiar serious disease process of the cerebral cortex". The post-mortem autopsy of Auguste D, in 1911, will allow the description of two particular histological markers, present in her brain, characteristics of her pathology: neurofibrillary tangles and senile plaques.

A worldwide issue for public health B.

With a world population growing older, developed, as well as developing, countries are facing new major social and economic challenges which are neurodegenerative diseases. The emergence of these pathologies is characteristic of aging societies. Between the years 2000 and 2050, the number of people of over the age of 60 will rise from 65 million to 2 billion. Such a rapid increase will require important economic, social and medical measures in certain countries. This rise in the proportion of older people is due to a "demographic transition", corresponding to a decline in mortality and fecundity.

One of the principal consequences of this worldwide aging is the rise of pathologies such as dementia.

Dementia C.

Dementia is a chronic and evolving syndrome, where a person's cognitive capacities are more strongly affected than someone aging normally. These cognitive functions such as memory, learning, reasoning, orientation and attention decline progressively and irreversibly. This will elicit sensory, motor and behavioural impairments, affecting an elderly person's autonomy, creating a worldwide issue for public health; as this entails major medical and social costs. Several types of dementia exist, some can be classified and differ depending on what causes them.

The rise of Alzheimer's disease (AD) sparked a strong interest in the scientific and medical community. Unknown to the general public four decades ago, AD is actually at the origin of the majority of dementias encountered in elderly people, roughly 60 to 70 %. This neurodegenerative pathology usually starts with marked memory deficits followed by a progressive decline of all the other cognitive functions like language, judgement and mood, ultimately leading to the death of the patient (the symptoms will be further described in Part I.E.1).

Statistics of Alzheimer's disease D.

Since the early 80s, the scientific and medical community has been focusing on the identification of the symptoms, causes and risk factors of AD as well as potential therapeutic strategies. Although the knowledge on this disease is considerably expanding, the biological alterations leading to AD are still unclear. Amongst all the different types of dementia, 6 out of 10 cases are due to AD. Linked to aging, only 2 % of people under the age of 65 develop the disease and this is usually due to hereditary forms of the pathology which will be discussed further. However, AD affects more than 4 % of people over the age of 65, and 15 % of people over 80.

According to World Alzheimer Report, someone in the world develops dementia every 3 seconds.

There were an estimated 46.8 million people worldwide living with dementia in 2015 and this number is believed to be close to 50 million people in 2017. These numbers will almost double every 20 years, reaching 75 million in 2030 and 131.5 million in 2050. Approximately 70 % of these cases will be attributed to AD.

According to "France Alzheimer & maladies apparentées", in France, more than 850 000 people are affected by AD or a related disease. It is estimated that 1 in 4 people aged over 65 will develop AD by 2020, making this disease a true pandemic.

Clinical aspects of Alzheimer's disease E.

Symptoms 1.

 Memory disorders

One of the most obvious symptoms of AD is memory loss. Though forgetting someone's name or not remembering where you put your keys is quite natural, the development of AD is characterised by the appearance of memory deficits. These deficits range from light anterograde amnesia (forgetting recent facts, such as forgetting an appointment or forgetting where an everyday object is in the house) to severe retrograde amnesia (forgetting older facts, such as historical events or the name of a family member…). The person is going to become more and more dependent on their entourage and is going to have to ask several times the same information without being able to retain it [START_REF] Jahn | Memory loss in Alzheimer's disease[END_REF][START_REF] Sultzer | Neurobiology of delusions, memory, and insight in Alzheimer's disease[END_REF].

 Temporal-spatial disorientation

Patients can get lost in familiar environments and lose notion of time. They can also have difficulties to understand something if it doesn't happen immediately or forget where they are or how they got there.

 Mood disorders

AD patients are prone to mood swings and personality disorders. They can be delusional, get confused, anxious, depressed or even aggressive towards their close environment for no apparent reason [START_REF] Mograbi | On the relation among mood, apathy, and anosognosia in Alzheimer's disease[END_REF].

 Apathy, indifference AD patients who suffer from apathy turn in on themselves; lose their interest and their motivation even for activities and hobbies they usually enjoy. They can appear indifferent and depressed, expressing very little or no emotions towards something whether it be good or bad [START_REF] Mograbi | On the relation among mood, apathy, and anosognosia in Alzheimer's disease[END_REF][START_REF] Nobis | Apathy in Alzheimer's disease[END_REF].

 Aphasia, language disorders People affected by AD can have difficulties in participating or following a conversation. They can have a hard time finding their words or sometimes just stop in the middle of a sentence without knowing how to finish it [START_REF] Kirshner | Primary progressive aphasia and Alzheimer's disease: brief history, recent evidence[END_REF][START_REF] Whitwell | Working memory and language network dysfunctions in logopenic aphasia: a task-free fMRI comparison with Alzheimer's dementia[END_REF].

 Agnosia, recognition impairments AD sufferers can have difficulties in recognising objects or people of their close environment without presenting any sensory impairment. Agnosia is often at the root of many behaviour disorders where patients have ill-adapted attitudes towards certain objects which they are no longer capable of recognising [START_REF] Davis | Finger agnosia and cognitive deficits in patients with Alzheimer's disease[END_REF].

 Apraxia, gesture impairments Apraxia, which is difficult to perceive in early stages of AD, is characterised by the patient's growing difficulties to accomplish gestures which require motor coordination. Over time the AD patient can forget acquired movements, lose dexterity and eventually lose the ability to accomplish elaborate tasks such as hand writing. At advanced stages of the disease, the patient can even forget how to execute the most simple of tasks such as brushing their teeth or chewing their food [START_REF] Lesourd | Apraxia and Alzheimer's disease: review and perspectives[END_REF].

 Progression of the disease

Though the appearance of symptoms is gradual, this can greatly vary from one individual to another. In general, AD patients progressively lose their autonomy thus becoming more and more reliant on their entourage. Since the evolution of the pathology can take several decades, establishing a general timeline of the progression of the disease which fits all patients is difficult. However, the medical community have distinguished three phases over the course of which the pathology develops [START_REF] Sperling | The evolution of preclinical Alzheimer's disease: Implications for prevention trials[END_REF].

Firstly, there is the asymptomatic phase which can last more than a decade during which the patient undergoes anatomical as well as biological modifications, such as loss of neuronal density or variations in cerebrospinal fluid (CSF) composition. At this stage, these modifications do not induce any symptoms, more than likely due to compensatory mechanisms. From a clinical stand point, at this stage, the patient could only be distinguished from a healthy individual via elaborate neuropsychological tests.

Then comes the pre-dementia symptomatic phase which lasts 3 to 5 years, called MCI, for Mild Cognitive Impairments, during which the patient can still accomplish everyday tasks although certain cognitive impairments appear but without loss of autonomy. During this phase, the patient suffers from light executive function and memory deficits but not yet dementia [START_REF] Eshkoor | Mild cognitive impairment and its management in older people[END_REF][START_REF] Popp | Cerebrospinal fluid cortisol and clinical disease progression in MCI and dementia of Alzheimer's type[END_REF] (Figure 2).

Finally comes the symptomatic phase of dementia, the most severe stage of AD where the MCIs have progressed into full blown cognitive deficits with the emergence of behaviour disorders [START_REF] Aisen | Clinical Core of the Alzheimer's Disease Neuroimaging Initiative: progress and plans[END_REF][START_REF] Tan | Biomarkers for preclinical Alzheimer's disease[END_REF].

Diagnostic 2.

In order to identify whether a person is affected by AD or not, doctors firstly need to identify the presence of some form of dementia. For that, they use neurophysiological and behavioural tests. The most common test is the MMSE (for Mini Mental Status Evaluation) which globally evaluates cognitive functions [START_REF] Derouesne | Mini-Mental State Examination:a useful method for the evaluation of the cognitive status of patients by the clinician. Consensual French version[END_REF].

Since dementia is not always due to AD, doctors must then investigate for specific signs of the pathology. They usually resort to magnetic resonance imagery (MRI) [START_REF] Colliot | Magnetic resonance imaging for diagnosis of early Alzheimer's disease[END_REF] which allows the following of the evolution of cerebral atrophy. They can also use Positon Emission Tomography with Fluorodeoxyglucose (PET-FDG) (La [START_REF] Joie | Hippocampal subfield volumetry in mild cognitive impairment, Alzheimer's disease and semantic dementia[END_REF] in order to obtain functional imagery and bring to light a potential hypo-metabolism of certain areas of the brain. Single Photon Emission Computed Tomography (SPECT) can also be used to detect cerebral hypoperfusion in the temporal areas which is a characteristic lesion of AD [START_REF] Valotassiou | Alzheimer's disease: spect and pet tracers for beta-amyloid imaging[END_REF]. Lumbar puncture can also be performed, as well as amyloid PET scans, in order to verify for the presence of Amyloid Beta 42 peptide (Aβ 42 ) and Tau protein (both are histopathological biomarkers associated with AD which will be described in Part I.G) in the cerebrospinal fluid (CSF) and brain, and confirm the diagnosis for AD [START_REF] Olsson | CSF and blood biomarkers for the diagnosis of Alzheimer's disease: a systematic review and meta-analysis[END_REF][START_REF] Palmqvist | Detailed comparison of amyloid PET and CSF biomarkers for identifying early Alzheimer disease[END_REF]. Aβ accumulation starts early on in the development of the pathology and reaches a plateau when clinical symptoms occur. Biomarkers of synaptic dysfunction appear after and are strongly correlated with the severity of clinical symptoms. Tau protein accumulates later in the CSF and is correlated with Neurofibrillary Tangles (NFTs) and neuronal death (Figure 2).

Finally, other exams are performed in order to discard other causes of dementia (vitamin deficiency, hormone imbalance, infection, stroke…). Whilst these sets of exams enable doctors to assess quite specifically whether a patient is affected by AD, to this day there still isn't a formal diagnosis other than post-mortem brain autopsy. Recently, several promising lesser-invasive biomarkers have been brought to light as potential predictors of AD and brain amyloidosis. Cofilin 2 has been shown to be upregulated in the serum of AD animal models, AD patients and patients with MCI compared to controls [START_REF] Sun | Cofilin 2 in Serum as a Novel Biomarker for Alzheimer's Disease in Han Chinese[END_REF]. Also, high-precision plasma Aβ 42/40 in combination with age and APOE ε4 status (a genetic risk factor for AD, which will be further described in Part I.E.2.a) has been shown to be a very accurate predictor for AD and could be used in prevention trials to screen for individuals likely to be amyloid PETpositive and at risk for AD dementia [START_REF] Schindler | High-precision plasma β-amyloid 42/40 predicts current and future brain amyloidosis[END_REF].

Different forms of AD & risk factors F.

For several decades, AD was classed into two possible clinic cases depending on the age of the onset of the pathology. If a person younger than 65 years of age was diagnosed with AD, it was considered a "presenile dementia", where as if a similar diagnosis was given to a person over 65 it was considered an "Alzheimer-type senile dementia" [START_REF] Roth | The relationship between quantitative measures of dementia and of degenerative changes in the cerebral grey matter of elderly subjects[END_REF][START_REF] Roth | Correlation between Scores for Dementia and Counts of 'Senile Plaques' in Cerebral Grey Matter of Elderly Subjects[END_REF][START_REF] Tomlinson | Observations on the brains of demented old people[END_REF]. To this day, there isn't any formal evidence showing that AD is different depending on the age of onset. More recently, neuro-imaging, epidemiology and neuropathology research have highlighted the fact that AD is a multifactorial disease. On one hand, certain genetic factors are responsible for "familial forms of AD" (FADs) and generally result in an early onset of the pathology, some of these genetic factors will be further discussed below. On the other hand, other factors such as environmental factors seem responsible for "sporadic" AD, much more prevalent and with a later onset. Nevertheless, age still seems to be a factor in the development of AD and may reflect the effect of accumulating different risk factors throughout life.

Familial Alzheimer's disease (FAD) 1.

Familial Alzheimer's disease (FAD) or early-onset familial Alzheimer's disease (EOFAD) is an uncommon form of Alzheimer's disease that usually strikes earlier in life, usually between 40 and 50 years of age and is inherited in an autosomal dominant fashion [START_REF] Bertram | The genetic epidemiology of neurodegenerative disease[END_REF]. Familial AD requires the patient to have at least one first-degree relative with a history of AD. Nonfamilial cases of AD are referred to as "sporadic" AD, and encompass the majority of AD cases where genetic risk factors are minor or unclear. The genetic mutations which induce FADs are all localised on the genes coding for proteins involved in the production of Aβ: the Amyloid Precursor Protein (APP), Presinilin 1 (PS1) and Presinilin 2 (PS2) which are situated on chromosomes 21, 14 and 1 respectively.

A mutation on one of these genes will affect the production, the metabolism, the sequence and/or stability of the Aβ peptides found in AD brains (see Part II.F.).

a) Presinilins PS1 and PS2

Presinilins 1 and 2 are part of the γ-secretase complex, essential to Aβ production. Mutations on these genes will alter the formation process of the Aβ peptide in favour of an increased synthesis and aggregation.

PS1 gene is located on chromosome 14q24.3, and it is a vital component of the γ-secretase complex, which cleaves APP into Aβ fragments. To date, 215 pathogenic mutations have been identified in PS1 and account for up to 50% of EOFAD, with early age of onset. Mutant γ-secretase increases Aβ42 level while it decreases Aβ40 level, leading to an increase in the Aβ 42/40 ratio. In addition to their role in γ-secretase activity, PS1 mutations may compromise neuronal function, affecting GSK-3β activity and kinesin-I-based motility, thus leading to neurodegeneration [START_REF] Pigino | Alzheimer's presenilin 1 mutations impair kinesin-based axonal transport[END_REF].

PS2 gene is located on chromosome 1q31-q42, and it is very similar in structure and function to PS1. PS2 mutations are very rare, and to date only 13 pathogenic PS2 mutations have been detected in 29 families (Cruts et al., 2012a). PS2 is a main component of the γ-secretase complex [START_REF] Wakabayashi | Presenilins: members of the gamma-secretase quartets, but part-time soloists too[END_REF] and mutation of this protein alters the γ-secretase activity and results in an increase of the Aβ 42/40 ratio in a similar manner to the PS1 mutation [START_REF] Steiner | Uncovering gamma-secretase[END_REF][START_REF] Tanzi | Twenty years of the Alzheimer's disease amyloid hypothesis: a genetic perspective[END_REF]. People bearing PS2 mutations have a later onset of the pathology. It has been shown that β-secretase activity is enhanced by PS2 mutation, through reactive oxygen speciesdependent activation of extracellular signal-regulated kinase [START_REF] Park | Mutant presenilin 2 increases β-secretase activity through reactive oxygen species-dependent activation of extracellular signal-regulated kinase[END_REF]. Although, PS2 shows close homology to PS1, less amyloid peptide is produced by PS2.

b) APP

Mutations in the gene coding for APP were first brought to light by the observation that people with Down syndrome (trisomy 21) presented similar neuropathological features as people with AD.

This raised the question of the possible existence of genes located on chromosome 21 involved in AD pathogenesis. For the first time in 1991, a region containing the APP gene was identified and enabled the identification of a mutation implicated in an autosomal dominant form of AD [START_REF] Goate | Segregation of a missense mutation in the amyloid precursor protein gene with familial Alzheimer's disease[END_REF].

Currently, out of the 30 mutations observed on the APP gene, 25 of them are pathogenic. Indeed, most of the mutations on the APP gene are situated near or around the secretases' cleavage sites (α, β and γ). These types of mutations tune APP cleavage and, as a result, modulate not only Aβ peptide production but can also modulate Aβ's three-dimensional folding and aggregation capacities [START_REF] Streltsov | Crystal structure of the amyloid-β p3 fragment provides a model for oligomer formation in Alzheimer's disease[END_REF]. These APP mutations will be described in more detail in Part II.F.

While early-onset familial AD is estimated to account for only 3.5% of total Alzheimer's disease [START_REF] Harvey | The prevalence and causes of dementia in people under the age of 65 years[END_REF], it has presented a very useful model in studying various aspects of the disease.

Currently, the early-onset familial AD gene mutations guide the vast majority of animal model-based therapeutic discovery and development for AD.

Sporadic Alzheimer's disease 2.

Sporadic or Late-Onset Alzheimer's disease (LOAD) is considered multifactorial and is genetically far more complex than EOFAD with the possible involvement of multiple genes and environmental factors. Most cases of LOAD are sporadic with no family history of the disease.

a) Apolipoprotein E

Allelic variations for the gene coding for Apolipoprotein E (APOE, the major cholesterol carrier in the brain), on chromosome 19, represents the main genetic risk factor for LOAD [START_REF] Strittmatter | Apolipoprotein E: high-avidity binding to beta-amyloid and increased frequency of type 4 allele in late-onset familial Alzheimer disease[END_REF]. The APOE gene has three allelic variations: APOξ2 allele (5-10%), APOξ3 allele (70-75%) and

APOξ4 allele (15-20%). It has been shown that the APOξ4 allele increases the risk for AD by 20% [START_REF] Corder | Protective effect of apolipoprotein E type 2 allele for late onset Alzheimer disease[END_REF] and is associated with both early-and late-onset AD [START_REF] Borgaonkar | Linkage of late-onset Alzheimer's disease with apolipoprotein E type 4 on chromosome 19[END_REF].

The presence of one ξ4 allele is enough to increase three-fold the risk for AD whereas the presence of both copies increases the risk 12-fold [START_REF] Michaelson | APOE ε4: the most prevalent yet understudied risk factor for Alzheimer's disease[END_REF]. The APOE gene codes for a protein essential to lipid metabolism but also for hepatic clearance of Aβ peptides [START_REF] Kline | Apolipoprotein E, amyloid-ß clearance and therapeutic opportunities in Alzheimer's disease[END_REF]. Notably, a study has shown that patients with AD because of APOξ4 tend to have less Aβ in the Cerebrospinal Fluid (CSF) than healthy people, but this Aβ was found mostly in oligomeric form, suggesting a link between APOξ4 and oligomeric forms of Aβ [START_REF] Tai | Levels of soluble apolipoprotein E/amyloid-β (Aβ) complex are reduced and oligomeric Aβ increased with APOE4 and Alzheimer disease in a transgenic mouse model and human samples[END_REF]. Other studies have highlighted that the APOξ2 allelic variant may have a neuroprotective effect compared to its homolog APOξ4 [START_REF] Corder | Protective effect of apolipoprotein E type 2 allele for late onset Alzheimer disease[END_REF].

b) Other genetic factors

Before the era of large-scale Genome-Wide Association Study (GWAS), ε4 allele of the APOE gene was the only well-established risk factor for the pathogenesis of LOAD, but with technological advances, researchers have identified a number of regions of interest in the genome that may increase a person's risk for LOAD to varying degrees. It was striking to note that most of the genes identified by GWAS that could be linked with the Aβ cascade or tau pathology roughly cluster within three pathways: Lipid metabolism, Inflammatory response and Endocytosis [START_REF] Giri | Genes associated with Alzheimer's disease: an overview and current status[END_REF]) (Figure 3). These 29 or so genes and their role are briefly described in the table below (Table 2).

Gene Function

APOE

c) Environmental factors

Despite the fact that sporadic or late-onset AD is the most frequent form of the pathology; its origin is still to be understood. From a clinical stand point the elements which spark LOAD are various and heterogeneous. Nevertheless, several environmental risk factors have been identified and associated to disease development. One of the predominant risk factors is lifestyle. Indeed, an improper diet rich in saturated fats, salt and refined sugars may lead to obesity, hypertension and type II diabetes. These metabolic disorders raise the level of inflammation in the body and increase the incidence of AD. Alcohol and tobacco consumption are also aggravating factors as they are known to accelerate cellular aging. Other factors such as environmental pollution due to neurotoxic metals (lead, mercury, aluminium, cadmium and arsenic) and/or pesticides may alter Aβ production and Tau phosphorylation thus increasing the incidence of AD also [START_REF] Chin-Chan | Environmental pollutants as risk factors for neurodegenerative disorders: Alzheimer and Parkinson diseases[END_REF].

Furthermore, social context is also a determining factor in the development of AD. The level of education, physical activity and post-menopause treatments are amongst the parameters susceptible in modulating the risk of developing sporadic forms of the pathology [START_REF] Dosunmu | Environmental and dietary risk factors in Alzheimer's disease[END_REF][START_REF] Barnes | The projected effect of risk factor reduction on Alzheimer's disease prevalence[END_REF].

Nonetheless, several neuroprotective environmental factors susceptible to slow down or even decrease the incidence of AD have been brought to light. Performing regular physical activity, consuming foods rich in omega-3 and foods rich in antioxidants (fatty fish, nuts, seeds, fruit, vegetables, green tea…), the use of non-steroid anti-inflammatories and having a cognitively stimulating environment are some of them.

Histopathological aspects of Alzheimer's disease G.

Macroscopic lesions 1.

On a macroscopic level, AD is characterised by a cerebral atrophy mainly localised in the entorhinal cortex, hippocampus, amygdala, and in the temporal and frontal gyri. This translates as a weight reduction of the temporal, parietal and frontal lobes. Often a dilation of the cerebral ventricles can also be observed [START_REF] Perl | Neuropathology of Alzheimer's Disease[END_REF] (Figure 4).

Microscopic lesions 2.

Microscopic lesions that appear in the brain are not homogenous. The topography and the kinetics of the apparition of the lesions are crucial to identify the pathology. Two types of lesions can be distinguished, on one hand the ones associated with protein aggregation and on the other hand ones associated with synaptic and neuronal loss [START_REF] Duyckaerts | Classification and basic pathology of Alzheimer disease[END_REF].

On a tissue level, two main histopathological lesions, hallmarks of AD, can be identified: intracellular neurofibrillary tangles (NFTs) composed of hyperphosporylated Tau protein, and extracellular senile plaques composed of aggregated Aβ peptides that form insoluble antiparallel βsheets called fibrils (Figure 5).

It has been shown that these two hallmarks evolve differently in the brain during AD progression [START_REF] Braak | Demonstration of amyloid deposits and neurofibrillary changes in whole brain sections[END_REF]. A model of AD development, in 6 different stages, has been established (Figure 6).

NFTs first accumulate in the entorhinal cortex and hippocampus (Figure 6, stages I and II). During these so called "silent" stages, Mild Cognitive Impairments (MCIs) appear but are not determining signs of AD as these cognitive symptoms may also point towards potential other forms of dementia.

Then, NFTs progress to the limbic (Figure 6, stages III and IV) and cortical (Figure 6, stages V and VI) areas of the brain. At this stage, Fully Developed Alzheimer is attained and cognitive deficits are significant. As for amyloid deposits, which constitute senile plaques, they are initially found in the cortical areas (Figure 6, stage A). They then progress to the associated cortical areas such as the hippocampus (Figure 6, stage B) and finally to the rest of the cortex (Figure 6, stage C) including the sensori-motor cortex.

a) Intracellular Tau and neurofibrillary tangles

These lesions were first described by Alois Alzheimer in the early 20 th Century, during the study of his patient's brain (Alzheimer et al., 1907, translated into English in 1995). However, it wasn't until the 1960s that electron microscopy studies revealed the exact composition of these lesions; namely:

neurofilaments, microtubule associated proteins (MAP2), vimentin and tropomyosin proteins, elements of the proteasome, proteoglycans, inflammatory molecules, amyloïdogenic proteins (APP, presinilin, APE) and elements of the cytoskeleton such as Tau protein [START_REF] Smith | Quantitative solubilization and analysis of insoluble paired helical filaments from Alzheimer disease[END_REF].

Tau (Tubulin Associated Unit) protein is an organiser and stabiliser of microtubules (MTs). These MTs are, in a sense, the scaffold of cells as they shape and structure them. Tau phosphorylation allows the regulation of its binding to MTs. Unphosphorylated Tau has a higher affinity for MTs and so phosphorylated Tau remains detached from the MTs. The MT stabilisation and assembly role of Tau highlights its importance in axonal growth and cytoskeletal structure stability in the mature neuron. Interestingly, Tau is not the only MT-associated protein. In vivo studies in mice show that knocking out Tau does not significantly alter the stabilisation of the MTs. This suggests early developmental compensation mechanisms by other MAPs, mainly MAP1A which is found to be increased (D [START_REF] Ke | Lessons from Tau-Deficient Mice[END_REF]. This compensation doesn't seem to occur in adult brains [START_REF] Ke | Lessons from Tau-Deficient Mice[END_REF], which might explain why Tau pathology causes neuronal MT instability. In immature neurons, Tau is mainly present in axonal and somatodendritic compartments. In mature neurons, however, Tau is mainly found in the axon [START_REF] Burack | Site-specific regulation of Alzheimer-like tau phosphorylation in living neurons[END_REF] where they allow transport of cargo from the cell body to the axon terminals [START_REF] Avila | Tau Structures[END_REF] by interacting with transport proteins like dynein and kinesin [START_REF] Dixit | Differential regulation of dynein and kinesin motor proteins by tau[END_REF]. In addition, it has also been shown that the N-terminal projection of Tau has the ability to bind to the Annexin A2 protein at the level of the axonal plasma membrane, highlighting the ability of Tau to interact with cell membranes [START_REF] Sotiropoulos | Atypical, non-standard functions of the microtubule associated Tau protein[END_REF]. Through this membrane interaction, Tau is believed to play a role in various cell signalling pathways by interacting with transmembrane receptors [START_REF] Guo | Roles of tau protein in health and disease[END_REF].

Interestingly, Tau has also been found in the nucleus, where it is believed to bind to DNA and plays a role of protection against stressful stimuli such as hypothermia or hyperthermia which can cause DNA damage [START_REF] Guo | Roles of tau protein in health and disease[END_REF]. Additionally, Tau has been described to play a role in the regulation of genetic and epigenetic expression by binding and changing the conformation of DNA and its binding to histones. Furthermore, other roles have been found for Tau, including a role in the insulin signalling pathways [START_REF] Jolivalt | Defective insulin signaling pathway and increased glycogen synthase kinase-3 activity in the brain of diabetic mice: parallels with Alzheimer's disease and correction by insulin[END_REF]) (Figure 7).

Although Tau is mainly found in axons, it is also present in somatodendritic compartments and post-synaptic densities [START_REF] Wang | Tau in physiology and pathology[END_REF]. In dendrites and dendritic spines, Tau has an important role in synaptic plasticity. Tau acts as a scaffold at the level of the synapses, both in preand post-synaptic compartments. Tau is known to play a role in synaptic activity, as it interacts with

Fyn tyrosine kinase and binds to PSD-95, which regulates the function of post-synaptic receptors, such as NMDA receptors (NMDAr) [START_REF] Ittner | Dendritic function of tau mediates amyloid-beta toxicity in Alzheimer's disease mouse models[END_REF][START_REF] Arendt | Tau and tauopathies[END_REF]) (Figure 7).

In pathological conditions, it is precisely at the level of the postsynaptic density that Tau has been proposed to exert its deleterious effects. In the case of AD, studies using live microscopy and confocal imaging have demonstrated that Aβ causes the mislocalisation and abnormal phosphorylation of Tau at the postsynaptic level, abnormally increasing its abundance in this compartment [START_REF] Frandemiche | Activitydependent tau protein translocation to excitatory synapse is disrupted by exposure to amyloid-beta oligomers[END_REF][START_REF] Amar | The amyloid-β oligomer Aβ*56 induces specific alterations in neuronal signaling that lead to tau phosphorylation and aggregation[END_REF]. This may increase the interaction of Tau with Fyn tyrosine kinase and PSD-95, increasing the synaptic activity through NMDAr, which might lead to excitotoxicity. Furthermore, a study showed that Tau is required to cause Long-term Depression (LTD) in the hippocampus [START_REF] Kimura | Microtubule-associated protein tau is essential for long-term depression in the hippocampus[END_REF]. This could mean Tau plays a role in downregulating AMPA receptors (AMPAr), possibly leading to synapse pruning (Figure 7).

NFTs appear when Tau is abnormally phosphorylated and starts forming aggregates within the neuron. Studies have also reported that Tau cleavage by caspases generates peptides that are more prone to form aggregates [START_REF] Gamblin | Caspase cleavage of tau: linking amyloid and neurofibrillary tangles in Alzheimer's disease[END_REF]. These peptides aggregate with the non-cleaved Tau around MTs, where Tau gets hyperphosphorylated. This causes Tau to unbind from the MTs and cause their destabilisation whilst simultaneously aggregating into NFTs and accumulating in the cytosol [START_REF] Rissman | Caspase-cleavage of tau is an early event in Alzheimer disease tangle pathology[END_REF].

It is now widely accepted that Tau protein is the major component of NFTs. However, NFTs aren't a determining factor for AD since they are also found in frontotemporal dementia and other neurodegenerative pathologies known as tauopathies [START_REF] Delacourte | Tau pathology: a marker of neurodegenerative disorders[END_REF].

b) Aβ peptide and extracellular senile plaques

Although described for the first time by Alois Alzheimer in the early 20 th Century (Alzheimer et al., 1907, translated into English in 1995) after brain autopsy of his patient, the principal constituent of the extracellular senile plaques wasn't established until 1984 by Dr. Glenner's team. These plaques showed to be aggregates of a peptide named Amyloid Beta 42 peptide or Aβ [START_REF] Glenner | Alzheimer's disease: initial report of the purification and characterization of a novel cerebrovascular amyloid protein[END_REF].

Aβ is a physiological peptide, produced throughout the course of life. Senile plaques, however, are characteristic lesions of AD where Aβ peptides aggregate in an aberrant manner. Amyloid deposits are absent in young individuals and gradually appear with aging. In fact, it is only when these plaques grow considerably in number and size that they induce neuroinflammation, which is characterised by the activation of brain resident immune cells.

Although Aβ is the major constituent of senile plaques, there are other elements that participate in the making of these plaques. Indeed, during the formation of these plaques Aβ binds with ApoJ (or clusterin) [START_REF] Martin-Rehrmann | Association of apolipoprotein J-positive beta-amyloid plaques with dystrophic neurites in Alzheimer's disease brain[END_REF], ApoE, cholesterol [START_REF] Lesser | Cholesterol and LDL relate to neuritic plaques and to APOE4 presence but not to neurofibrillary tangles[END_REF], with cathepsin D, with components of the extracellular matrix such as thrombospondin, ICAM1 (Inter-Cellular Adhesion Molecule 1) and heparan sulfate proteoglycans, but also Ca 2+ , iron [START_REF] Everett | Nanoscale synchrotron X-ray speciation of iron and calcium compounds in amyloid plaque cores from Alzheimer's disease subjects[END_REF] and other metals (copper, zinc…) [START_REF] Ha | Metal ions differentially influence the aggregation and deposition of Alzheimer's beta-amyloid on a solid template[END_REF].

Furthermore, these amyloid deposits can aggregate into different forms in the brain [START_REF] Duyckaerts | Classification and basic pathology of Alzheimer disease[END_REF][START_REF] Levine | COMMENTARY: Molecular polymorphism of Aβ in Alzheimer's disease[END_REF].

The first correspond to "diffuse" forms where the deposits are relatively large, up to several hundred micrometres in diameter. These have little immunoreactivity and are difficult to identify by immunohistochemistry. Congo Red and Thioflavine S (two dyes which allow staining of aggregates)

do not allow the visualisation of these diffuse plaques as they are mainly composed of Aβ40.

However, they can be revealed with antibodies against Aβ [START_REF] Güntert | High sensitivity analysis of amyloid-beta peptide composition in amyloid deposits from human and PS2APP mouse brain[END_REF]. These types of plaques can be found in large numbers and sizes in individuals who allegedly have no cognitive deficits, which raises doubts as to their toxicity in the brain. In fact, it has been proposed that these forms of plaques are an early form of senile plaques in certain brain areas [START_REF] Duyckaerts | Classification and basic pathology of Alzheimer disease[END_REF] LeVine and Walker, 2010) (Figure 8).

The second form of plaques are described as "focal" which are spherical deposits much smaller in size but also much more dense. As they contain a lot more Aβ42 molecules, these focal deposits seem to form the core of senile plaques. They are surrounded by processes coming from nearby neurons and astrocytes [START_REF] Duyckaerts | Classification and basic pathology of Alzheimer disease[END_REF][START_REF] Levine | COMMENTARY: Molecular polymorphism of Aβ in Alzheimer's disease[END_REF] (Figure 8).

Other amyloid deposits, described as "stellar deposits" have also been identified. Smaller and likely associated to astrocytes, these deposits are less studied than the previous aforementioned forms although often observed [START_REF] Duyckaerts | Classification and basic pathology of Alzheimer disease[END_REF] (Figure 8).

In some cases, amyloid β can also accumulate in the walls of blood vessels, particularly in the ones present in the core of senile plaques, which induces CAA (Cerebral Amyloid Angiopathy) also known as congophilic angiopathy [START_REF] Yamada | Cerebral amyloid angiopathy[END_REF]).

It's the form and properties of the amyloid peptides which induce the formation of fibrils and eventually plaques on the outside of neurons. However, some Aβ can be found intracellularly [START_REF] Bayer | Intracellular accumulation of amyloid-Beta -a predictor for synaptic dysfunction and neuron loss in Alzheimer's disease[END_REF][START_REF] Tomiyama | A mouse model of amyloid beta oligomers: their contribution to synaptic alteration, abnormal tau phosphorylation, glial activation, and neuronal loss in vivo[END_REF][START_REF] Thal | Protein aggregation in Alzheimer's disease: Aβ and τ and their potential roles in the pathogenesis of AD[END_REF]. Quite intriguingly, this intracellular accumulation of Aβ42 correlates much more with neuronal cell death than the formation of extracellular senile plaques [START_REF] Christensen | Transient intraneuronal A beta rather than extracellular plaque pathology correlates with neuron loss in the frontal cortex of APP/PS1KI mice[END_REF] and the smaller oligomers seem more toxic than fibrillary Aβ deposits.

c) Links between Aβ and Tau

The hypothesis of the amyloid cascade originated from the observation that mutations which cause AD are always associated with alterations of the Amyloid Precursor Protein (APP) metabolism, leading to Aβ42 overproduction particularly. On the contrary, Tau mutations do not lead to AD.

Subsequently, it has been postulated that it may be the accumulation of neurotoxic amyloid peptides which drives Tau protein modifications leading to MT destabilisation, formation of NFTs, and eventually neuronal loss and memory deficits [START_REF] Hardy | Alzheimer's disease: the amyloid cascade hypothesis[END_REF]. Indeed, a study showed that Aβ42 fibrils induce NFT formation [START_REF] Götz | Formation of neurofibrillary tangles in P301l tau transgenic mice induced by Abeta 42 fibrils[END_REF]. It has also been proposed that Aβ can induce Tau cleavage by caspases, generating Tau fragments prone to aggregate and form NFTs [START_REF] Gamblin | Caspase cleavage of tau: linking amyloid and neurofibrillary tangles in Alzheimer's disease[END_REF].

Nevertheless, NFTs have been observed in brains in absence of amyloid deposits [START_REF] Braak | Frequency of stages of Alzheimer-related lesions in different age categories[END_REF] which suggests that both lesions occur independently and in parallel of one another, and that NFT formation might even precede amyloid plaque formation [START_REF] Duyckaerts | Classification and basic pathology of Alzheimer disease[END_REF].

Moreover, amongst the different transgenic mouse models of AD used in research, the ones that bare mutations only on APP and PS do not present any NFTs. On the other hand, the triple transgenic 3xTg-AD mouse model, developed by LaFerla's team, which bares the Swedish double mutation on APP (APPswe), a mutation on PS1 (PS1 M146V) and a Tau mutation (MAPT P301L) develop NFTs.

However, NFTs appear after amyloid plaque formation in this 3xTg-AD model [START_REF] Oddo | Triple-transgenic model of Alzheimer's disease with plaques and tangles: intracellular Abeta and synaptic dysfunction[END_REF]Billings et al., 2005). The possibility of a synergistic interaction between both Aβ and Tau pathways in the physiopathology of AD, driving synaptic dysfunction and neuronal degeneration, is widely accepted [START_REF] Small | Linking Abeta and tau in late-onset Alzheimer's disease: a dual pathway hypothesis[END_REF]) (Figure 9).

Propagation of the pathology throughout the brain H.

Extracellular Aβ deposits and intracellular Tau aggregates are both required to make the definite neuropathological diagnosis of AD. However, the interplay between these two proteins and the way they spread throughout the brain is still to be clearly determined. It is known that Tau pathology in the cell body of neurons precedes Aβ plaque formation and the two histopathological markers evolve and spread spatiotemporally differently in the brain. Indeed, Tau seems to spread outwards, from the centre to the cortex where as Aβ deposits do the opposite, spreading from the cortex to the centre [START_REF] Braak | Demonstration of amyloid deposits and neurofibrillary changes in whole brain sections[END_REF]. As NFTs appear first, this was long used as an argument to say that Tau was the initiating factor of AD.

Propagation of Tau 1.

From the observations made during brain autopsies of patients with tauopathies, that NFTs and other Tau aggregates progress in a stereotypical neuroanatomical pattern of spreading, came the hypothesis that Tau might spread from neuron to neuron. To support this hypothesis, several studies show that Tau is present in the extracellular compartment, that Tau can be released from donor cells and uptaken by recipient cells [START_REF] Goedert | The propagation of prion-like protein inclusions in neurodegenerative diseases[END_REF][START_REF] Mudher | What is the evidence that tau pathology spreads through prion-like propagation?[END_REF].

The secretion of Tau has been described to occur in multiple ways. Interestingly, the vast majority (90%) of secreted Tau is released as a free protein, whereas only a small fraction is bound to microvesicles such as exosomes or ectosomes [START_REF] Katsinelos | Unconventional Secretion Mediates the Trans-cellular Spreading of Tau[END_REF]. The mechanisms of this secretion are still not fully understood, though it has been reported that Tau may exit the cell through diffusion or through the formation of nanotubes between cells [START_REF] Tardivel | Tunneling nanotube (TNT)-mediated neuron-to neuron transfer of pathological Tau protein assemblies[END_REF]. It has also been reported that phosphorylation of Tau increases Tau secretion through exosomes [START_REF] Katsinelos | Unconventional Secretion Mediates the Trans-cellular Spreading of Tau[END_REF]. This makes sense in the light of the fact that phosphorylated Tau is more available for secretion as unphosphorylated Tau would rather tend to bind to MTs.

The uptake mechanism of Tau by recipient cells is not fully described yet but several studies seem to indicate that i) small oligomeric species of Tau, not monomeric, are spontaneously uptaken by the recipient cells in vitro ii) this would occur through simple diffusion or endocytosis [START_REF] Wu | Small misfolded Tau species are internalized via bulk endocytosis and anterogradely and retrogradely transported in neurons[END_REF][START_REF] Wu | Neuronal activity enhances tau propagation and tau pathology in vivo[END_REF]) (Figure 10).

Interestingly, Tau fibrils bind to transmembrane protein APP and increase its internalisation as well as increasing intracellular aggregation of Tau [START_REF] Takahashi | Extracellular association of APP and tau fibrils induces intracellular aggregate formation of tau[END_REF].

Most convincingly, several studies have showed that intracerebral injection of misfolded Tau (whether it be from brain extracts from transgenic mice with Tau P301S mutation or purified Tau fibrils from P301S brain extracts or preformed recombinant Tau fibrils) into the brain of transgenic mice expressing human wild-type Tau, not only induced Tau pathology but also the spread of this pathology along a path that is neuroanatomically connected [START_REF] Ahmed | A novel in vivo model of tau propagation with rapid and progressive neurofibrillary tangle pathology: the pattern of spread is determined by connectivity, not proximity[END_REF][START_REF] De Calignon | Propagation of tau pathology in a model of early Alzheimer's disease[END_REF][START_REF] Iba | Synthetic tau fibrils mediate transmission of neurofibrillary tangles in a transgenic mouse model of Alzheimer's-like tauopathy[END_REF][START_REF] Dujardin | Neuron-to-neuron wild-type Tau protein transfer through a trans-synaptic mechanism: relevance to sporadic tauopathies[END_REF][START_REF] Mudher | What is the evidence that tau pathology spreads through prion-like propagation?[END_REF]. [START_REF] Mudher | What is the evidence that tau pathology spreads through prion-like propagation?[END_REF]. Tau proteins can be transferred from donor cells (green) to recipient cells (orange) using different routes. Pathway indicated by blue arrows: tau proteins are released in the medium by extracellular vesicles like exosomes and ectosomes. Violet pathway: Around 90% of tau in the extracellular space is found as free protein, passive diffusion facilitated by a membraneous transporter/receptor (?2) or active exocytosis (?3) might contribute to this process. Uptake of free tau species by recipient cells, including APPmediated endocytosis has been reported. Whether free or aggregated tau is taken up by other mechanisms such as diffusion (?4) or non-receptor mediated endocytosis/macropinocytosis (?5) has not been resolved. Nor is it known how membranebound tau can escape from vesicles and enter the cytoplasm of recipient cells (?6). Red pathway: Tau was shown to be present inside nanotubes connecting cells in vitro and to allow its interneuronal transfer. This mechanism could potentially participate in prion-like propagation of tau pathology but whether it is a mode of transcellular transfer of seeding-competent tau species in vivo needs to be investigated.

Additionally, Tau propagation is closely linked to neuronal activity [START_REF] Wu | Neuronal activity enhances tau propagation and tau pathology in vivo[END_REF]. Indeed, an increased neuronal activity leads to an increase in the release and spread of Tau. The fact that Tau spreads along neuroanatomically connected tracts and that the spreading is neuronal activitydependent strongly suggests that the propagation of Tau occurs in a transsynaptic fashion [START_REF] Guo | Roles of tau protein in health and disease[END_REF]. Different conformers of Tau with a functional diversity have been identified and described as "strains" of Tau. Those strains of aggregated Tau seem to be stably transferred from neuron to neuron [START_REF] Sanders | Distinct tau prion strains propagate in cells and mice and define different tauopathies[END_REF]. This means that an aggregated strain of Tau is able to leave a neuron, enter the recipient neuron and seed aggregation inside the new neuron in manner that templates its conformation [START_REF] Kaufman | Tau Prion Strains Dictate Patterns of Cell Pathology, Progression Rate, and Regional Vulnerability In Vivo[END_REF].

Taken together, all these data point towards Tau as being a "prion-like" protein, as it has been proposed by Stanley Prusiner.

Propagation of Aβ 2.

Although NFTs appear before Aβ deposits in the pathogenesis of AD, FADs are only provoked by mutations of genes implicated in APP cleavage; no mutations of Tau are able to induce or reproduce the symptoms of AD. Moreover, in the past twenty years, an increasing number of studies have highlighted the toxicity of the soluble oligomeric forms of Aβ that form long before the plaques, and NFTs, bringing oligomers of Aβ at the forefront of AD development (Walsh and Selkoe, 2007).

Intensive neuropathological examination of AD brains revealed that Aβ deposits appear to follow distinct pathways, progressively spreading through interconnected brain regions, rather than emerging randomly in the brain over time [START_REF] Braak | Demonstration of amyloid deposits and neurofibrillary changes in whole brain sections[END_REF][START_REF] Eisele | Propagation of Aß pathology: hypotheses, discoveries, and yet unresolved questions from experimental and human brain studies[END_REF]. This gradual spread in the brain highlights an underlying, molecular mechanism evocative of Aβ acquiring a self-propagating conformation upon misfolding and aggregation (Condello and Stöehr, 2018).

The first hints that pointed towards Aβ as being a "prion-like" protein dates back from the 50s to the mid-80s. During that time, a number of children with conditions such as short stature were treated with growth hormone that had been isolated from cadaveric human pituitary glands. It was later discovered that some of these batches of hormones were contaminated with PrP prions, resulting in several children developing iatrogenic Creutzfeldt-Jacob disease (CJD) [START_REF] Rudge | Iatrogenic CJD due to pituitary-derived growth hormone with genetically determined incubation times of up to 40 years[END_REF]. It has been recently discovered that some of these cases, who unfortunately died of CJD at ages ranging from 36 to 51, had significant amounts of Aβ in the form of Aβ plaques or Aβ deposits [START_REF] Jaunmuktane | Evidence for human transmission of amyloid-β pathology and cerebral amyloid angiopathy[END_REF]. These findings raise the possibility that some batches of growth hormones were not only contaminated by PrP prions, but also with Aβ seeds that originated from pituitary glands collected from patients who died with AD [START_REF] Walker | The Prion-Like Properties of Amyloid-β Assemblies: Implications for Alzheimer's Disease[END_REF].

Though the most convincing evidence for Aβ self-propagation came from in vivo studies where Aβ aggregates derived from AD patients or aged transgenic mice were injected into young transgenic mice expressing human Aβ (TgAPP23) [START_REF] Meyer-Luehmann | Exogenous induction of cerebral betaamyloidogenesis is governed by agent and host[END_REF]. The injection of Aβ-containing brain homogenates was sufficient to induce potent deposition of endogenous, host expressed Aβ.

Notably, the removal of aggregated Aβ by chemical denaturation or immunodepletion abrogated the seeding potential of these preparations, which designates Aβ directly as being the self-propagating entity. These experiments suggest a mechanism where Aβ aggregates form spontaneously in focal areas of the aging brain and start to self-propagate from neuron to neuron, whether it is via interconnected neural circuits or by physical proximity of the neurons [START_REF] Wei | Amyloid beta from axons and dendrites reduces local spine number and plasticity[END_REF].

More recently, further evidence for iatrogenic induction of Aβ suspected in patients exposed to pituitary-derived hormones, dural grafts or surgical instruments contaminated with Aβ has been brought to light. For the first time, long-term memory and learning impairments have been demonstrated in mouse lemurs (Microcebus murinus), a non-transgenic non-human primate model by inoculation with Alzheimer brain homogenates. The brains of these animals, which underwent intracerebral injections with Alzheimer human brain extracts, displayed parenchymal and vascular depositions of Aβ, as well tau lesions, close to the inoculation site [START_REF] Gary | Encephalopathy induced by Alzheimer brain inoculation in a non-human primate[END_REF]. Furthermore, progressive cerebral atrophy and neuronal loss was also observed in the hippocampus and entorhinal cortex of these animals, which are interconnected brain structures. The results of this study further support the prion-like nature of Aβ and its self-propagation from neuron to neuron.

Another direct proof that Aβ oligomers are sufficient for self-propagation came from transmission studies using highly purified brain-derived Aβ or synthetic Aβ fibrils formed in vitro [START_REF] Stöhr | Purified and synthetic Alzheimer's amyloid beta (Aβ) prions[END_REF]. The study highlighted that synthetic Aβ fibrils alone are sufficient for propagation in mice demonstrating quite evidently that Aβ shares the same self-propagating features as PrP prions.

Moreover, Aβ shares additional similarities with PrP prions such as the rich β-sheet structure of Aβ fibrils as well as an increased stability towards chemical denaturation and protease digestion [START_REF] Fritschi | Aβ seeds resist inactivation by formaldehyde[END_REF][START_REF] Watts | Serial propagation of distinct strains of Aβ prions from Alzheimer's disease patients[END_REF]. Under harsh denaturing conditions, Aβ loses its self-propagating activity, highlighting that distinct structural motifs are required for templating and propagation [START_REF] Meyer-Luehmann | Exogenous induction of cerebral betaamyloidogenesis is governed by agent and host[END_REF], bringing forward the notion of Aβ strains.

These Aβ strains and their impact on AD pathogenesis will be further described in Part II.F.2.

APP-dependent propagation 3.

Another possible mechanism for disease propagation involves the Amyloid Precursor Protein (APP), the transmembrane protein from which Aβ is cleaved, and a specific subset of 100nm diameter extracellular vesicles called exosomes. These exosomes, secreted by all mammalian cells, enable the transfer of material from one cell to another, allowing intricate intercellular communication. It has recently been shown in neuronal cells overexpressing APP harbouring the Swedish mutation (see Part II. F. 1) that a specific subset of exosomes originating from these neuronal cells is highly enriched with APP and its cleavage fragments, especially CTF-α and CTF-η (the proteolytic cleavage of APP will be further described in the next chapter). These exosomes specifically target and are endocytosed by other neurons where these CTFs and APP can undergo further processing. These results bring to light a new mechanism by which AD pathology may spread from neuron to neuron [START_REF] Laulagnier | Amyloid precursor protein products concentrate in a subset of exosomes specifically endocytosed by neurons[END_REF].

II. Amyloid Precursor Protein processing and Amyloidogenesis

The Amyloid-β peptide (Aβ) is a cleavage product of a large transmembrane protein, namely the Amyloid Precursor Protein (APP), of which its role, function, trafficking and processing will be described below.

APP -Background A.

APP is a 130 kDa type-I transmembrane glycoprotein which is composed of a large extracellular Nterminal domain and a short cytoplasmic C-terminal tail [START_REF] Müller | Not just amyloid: physiological functions of the amyloid precursor protein family[END_REF]. The APP gene is located on the long arm of chromosome 21 and contains 18 exons that are alternatively spliced. It is part of a protein family which includes APP-like protein 1 (APLP1) and APLP2, many of which are conserved across several species including invertebrate C. elegans and D. melanogasters [START_REF] Shariati | Redundancy and divergence in the amyloid precursor protein family[END_REF]. Prokaryotes, plants, and yeasts do not appear to possess members of the APP family of proteins. The evolutionary appearance of the APP family of proteins therefore seems to coincide with the evolution of the earliest nervous systems with functioning synapses [START_REF] Shariati | Redundancy and divergence in the amyloid precursor protein family[END_REF]) but also coincides with other events such as the appearance of lipoprotein receptors [START_REF] Dieckmann | Lipoprotein receptors--an evolutionarily ancient multifunctional receptor family[END_REF]. Interestingly, APLP1 is found only in mammals and lacks two exons found in both APP and APLP2, suggesting that it diverged during evolution from the latter gene [START_REF] Zheng | Biology and pathophysiology of the amyloid precursor protein[END_REF]. Amongst these three genes, only APP contains the motif required for Aβ formation. The APP splice variants range from 365 to 770 amino acids with three major Aβ-containing isoforms: APP770, APP751 and APP695, with the number indicating amino acid length [START_REF] Zheng | Biology and pathophysiology of the amyloid precursor protein[END_REF]. APP770 and APP751 isoforms are expressed in most tissues. APP770 is the full length isoform which hasn't undergone any splicing. This isoform has a Kunitz domain, inhibitor of serine proteases, and an OX-2 domain, an antigen found on the surface of neurons and certain immune cells. These domains are found on exons 7 and 8 respectively. APP751 isoform undergoes splicing of exon 8 and so loses the OX-2 domain; whereas APP695, which is predominantly expressed in neurons, undergoes splicing of exons 7 and 8, losing both Kunitz and OX-2 domains [START_REF] Coburger | Analysis of the overall structure of the multi-domain amyloid precursor protein (APP)[END_REF].

The extracellular domain of APP contains E1 and E2 domains. The E1 domain is reported to function as the major interaction interface for dimerization of cellular APP/APLPs [START_REF] Soba | Homo-and heterodimerization of APP family members promotes intercellular adhesion[END_REF].

Although some studies suggest E2 domain has the same trans-dimerization capacity [START_REF] Wang | The X-ray structure of an antiparallel dimer of the human amyloid precursor protein E2 domain[END_REF], biochemical assays have failed to confirm this phenomenon [START_REF] Soba | Homo-and heterodimerization of APP family members promotes intercellular adhesion[END_REF].

Besides its implication in different pathologies such as AD or Chronic Traumatic Encephalopathy (CTE), APP is also involved in numerous physiological processes, although its actual function remains unclear. These processes include synapse formation and repair [START_REF] Priller | Synapse formation and function is modulated by the amyloid precursor protein[END_REF], anterograde transport to synapses [START_REF] Satpute-Krishnan | A peptide zipcode sufficient for anterograde transport within amyloid precursor protein[END_REF], transport of iron [START_REF] Duce | Iron-export ferroxidase activity of βamyloid precursor protein is inhibited by zinc in Alzheimer's disease[END_REF], cell adhesion, via an RHDS motif similar to the adhesion region of integrins which can be found along with APP at the surface of axons, as well as neurite outgrowth via its interaction with integrin β1 [START_REF] Young-Pearse | Secreted APP regulates the function of full-length APP in neurite outgrowth through interaction with integrin beta1[END_REF]. APP also has binding domains to bind to different metals such as copper and zinc, and to bind to the extracellular matrix (heparin, collagen and laminin) [START_REF] Thinakaran | Amyloid precursor protein trafficking, processing, and function[END_REF]. In all, a trophic role as well as a cell adhesion role for APP has perhaps been the most consistently and arguably the best-established functions for the molecule.

As its name states, APP is a precursor, of Aβ but also of other peptides. These peptides are produced via cleavage of APP by several enzymes called secretases. Amongst these secretases there are: I) the α-secretases, part of the ADAM (A Desintegrin And Metalloprotease) enzyme family [START_REF] Vincent | α-Secretase in Alzheimer's disease and beyond: mechanistic, regulation and function in the shedding of membrane proteins[END_REF], II) the β-secretases, also known as BACE1 (Beta-site Cleaving Enzyme 1) [START_REF] Chami | BACE1 is at the crossroad of a toxic vicious cycle involving cellular stress and βamyloid production in Alzheimer's disease[END_REF], and III) the γ-secretases, composed of 4 proteins, Aph1 (Anterior pharynx defective 1 homolog), Pen2 (Presenilin enhancer 2 homolog), Nicastrin, and the Presinilins (PS1 and PS2) which form the catalytic domain of the γ-secretase complex [START_REF] Wolfe | Toward the structure of presenilin/γ-secretase and presenilin homologs[END_REF][START_REF] Gertsik | Complex regulation of γ-secretase: from obligatory to modulatory subunits[END_REF].

Depending on which secretase and in what order they will cleave APP, two main cleavage pathways can be distinguished: an amyloïdogenic pathway, which yields Aβ peptides, and a non-amyloïdogenic pathway.

APP processing and trafficking B.

The amyloïdogenic pathway, β-and γ-secretases 1.

The amyloïdogenic pathway is the pathway that generates Aβ peptides by consecutive cleavage by β-secretase and γ-secretase. The first step in Aβ generation is cleavage of APP at the β-cleavage site by β-secretase. The putative β-secretase BACE1 is a membrane-bound aspartyl protease with a characteristic type I transmembrane domain near the C-terminus [START_REF] Vassar | Beta-secretase cleavage of Alzheimer's amyloid precursor protein by the transmembrane aspartic protease BACE[END_REF] and is thought to be the rate-limiting factor in Aβ generation from APP. β-secretase is mainly found in Golgi membranes and endosomes where the environment is acidic, providing an optimal environment for BACE1 activity [START_REF] Rajendran | Alzheimer's disease beta-amyloid peptides are released in association with exosomes[END_REF]. Following β-cleavage, two fragments are generated: the large N-terminal sAPP-β fragment and the C-terminal fragment C99, also called β-CTF. sAPP-β has been given a role in axonal pruning and neuronal death since a study showed that this soluble fragment may be a death receptor-6 ligand which activates caspase 6 [START_REF] Nikolaev | APP binds DR6 to trigger axon pruning and neuron death via distinct caspases[END_REF]. However, the question about sAPP-β remains since some studies demonstrate that sAPP-β also stimulates neurite outgrowth [START_REF] Chasseigneaux | Functions of Aβ, sAPPα and sAPPβ : similarities and differences[END_REF].

After β-cleavage, the C-terminal fragment β-CTF is then cleaved, at the transmembrane level, by the γ-secretase complex which completes the amyloïdogenic pathway by rendering an Aβ peptide ranging from 38 to 43 aa depending on the location of γ-cleavage, and a final C-terminal fragment of APP called AICD (APP IntraCellular Domain) (Figure 12).

The γ-secretase complex is not specific to APP and has other substrates vital for the organism such as Notch [START_REF] De Strooper | A presenilin-1-dependent gamma-secretase-like protease mediates release of Notch intracellular domain[END_REF]. However, several modulators of γ-secretase activity have been identified to decrease Aβ production without interfering with cleavage of the other substrates [START_REF] Kounnas | Modulation of gamma-secretase reduces betaamyloid deposition in a transgenic mouse model of Alzheimer's disease[END_REF], showing that Aβ production can be modulated without major side effects. This final C-terminal fragment, which is common to both APP cleavage pathways, may act as a transcription factor as it translocates to the nucleus to regulate certain genes such as: p53 (a tumour suppressor), GSK-3β (a kinase implicated in Tau phosphorylation) but also APP and BACE (Pardossi-Piquard and Checler, 2012). The non-amyloïdogenic pathway, α-and γ-secretases 2.

This pathway is characterised by the cleavage of APP by α-secretase, the α-cleaving site is within the Aβ sequence of APP, precluding Aβ generation. Following α-secretase cleavage, two fragments are generated: the N-terminal fragment sAPP-α and the C-terminal fragment named C83 or α-CTF.

This cleavage occurs at the plasma membrane. Studies report that the soluble sAPP-α fragment has neurotrophic properties and improves neuron survival in vitro and protects from excitotoxicity (van der Kant and Goldstein, 2015) despite having only 17 aa difference with sAPP-β. The α-CTF fragment then undergoes γ-cleavage and generates two new fragments: P3, which contains the aa 17 to 43 of the Aβ sequence, and the AICD fragment (Figure 13).

Other cleavage pathways of APP 3.

This binary vision of APP cleavage has recently evolved since the recent discovery of new cleavage pathways of APP. One particular pathway is the η-secretase (eta-secretase) pathway whereby APP is first cleaved by MT1-MMP and MT5-MMP (Membrane-Type Matrix Metalloproteinases) which have the η-secretase activity [START_REF] Ahmad | Cleavage of amyloid-beta precursor protein (APP) by membrane-type matrix metalloproteinases[END_REF][START_REF] Baranger | MT5-MMP Promotes Alzheimer's Pathogenesis in the Frontal Cortex of 5xFAD Mice and APP Trafficking in vitro[END_REF][START_REF] Baranger | MT5-MMP is a new pro-amyloidogenic proteinase that promotes amyloid pathology and cognitive decline in a transgenic mouse model of Alzheimer's disease[END_REF] releasing a N-terminal fragment sAPP-η and a C-terminal fragment CTF-η. CTF-η is then cleaved by γ-secretase, releasing an amyloid-η (Aη) peptide and AICD. This new Aη peptide has been found in brains of AD mouse models as well as in human AD brains [START_REF] Willem | η-Secretase processing of APP inhibits neuronal activity in the hippocampus[END_REF]. This discovery was made in a study with mice treated with a BACE1 inhibitor where the inhibition effectively decreased Aβ production but led to an accumulation of Aη which induced significant neuronal impairments. In particular, Long-Term Potentiation (LTP), the electrophysiological correlate of memory and learning, was decreased in the hippocampi of these Aη burdened mouse brains. These findings bring to light the possible negative outcome of using BACE1 inhibition as a therapeutic strategy to reduce Aβ load in AD brains in an attempt to alleviate memory deficits; and also strengthen the notion of the multitude of Aβ strains and their consequences, as well as the complexity of sporadic AD pathogenesis.

Other minor alternative cleavage pathways have also been discovered such as the δ-secretase pathway [START_REF] Zhang | Delta-secretase cleaves amyloid precursor protein and regulates the pathogenesis in Alzheimer's disease[END_REF] and cleavage by meprin [START_REF] Jefferson | Metalloprotease meprin beta generates nontoxic N-terminal amyloid precursor protein fragments in vivo[END_REF] which won't be described in this manuscript.

Intracellular trafficking of APP 4.

 Biosynthesis and progression through the secretory pathway

In non-polarised mammalian cells, APP follows the canonical protein maturation pathway (Figure 14). It is synthesised in the Endoplasmic Reticulum (ER) and then transported through the Golgi Apparatus to the trans-Golgi Network (TGN) and eventually to the Plasma Membrane (PM) [START_REF] Haass | Trafficking and proteolytic processing of APP[END_REF]. During its transit to the PM, newly synthesised APP undergoes post-translational modifications such as N-and O-linked glycosylation, ectodomain and cytoplasmic phosphorylation, as well as tyrosine sulphation. Based on APP overexpression studies in cultured cells, only around 10% of newly synthesised APP reaches the PM, whereas the majority of APP at steady-state is localised in the Golgi apparatus and the TGN. APP which eventually reaches the PM is internalised within minutes of arrival to the cell surface because of the "YENPTY" internalisation motif near the Cterminus of APP (residues 682-687 of APP695 isoform) [START_REF] Lai | Characterization of sorting signals in the beta-amyloid precursor protein cytoplasmic domain[END_REF][START_REF] Marquez-Sterling | Trafficking of cell-surface beta-amyloid precursor protein: evidence that a sorting intermediate participates in synaptic vesicle recycling[END_REF].

After endocytosis, APP is sorted to endosomes, a portion of endocytosed molecules is recycled to the cell surface and some are also sorted to lysosomes for degradation [START_REF] Haass | Targeting of cell-surface beta-amyloid precursor protein to lysosomes: alternative processing into amyloid-bearing fragments[END_REF].

 APP trafficking in neurons

Neurons are highly polarised into soma, axons, and dendrites, all of which perform different functions and therefore are equipped with distinct sets of proteins and lipids that finely regulate protein trafficking. Disturbances in this system could affect APP processing and have been linked to AD pathogenesis [START_REF] Morfini | Axonal transport defects in neurodegenerative diseases[END_REF]. The transport from the ER to Golgi and TGN is thought to be similar in non-polarised mammalian cells and in the neuronal soma. However, after leaving the TGN in neurons, APP is transported to axons and dendrites in post-Golgi transport vesicles [START_REF] Kins | Subcellular Trafficking of the Amyloid Precursor Protein Gene Family and Its Pathogenic Role in Alzheimer's Disease[END_REF]. APP delivery to the axons makes use of the fast axonal transport system, with kinesin-1 as the microtubule motor protein [START_REF] Kins | Subcellular Trafficking of the Amyloid Precursor Protein Gene Family and Its Pathogenic Role in Alzheimer's Disease[END_REF]. APP vesicles continuously move unidirectionally, with an average speed of 4.5 µm/s, reaching maximal speeds up to 10 µm/s. This is among the fastest transport velocities measured in cultured neurons. Significant retrograde transport with slightly slower kinetics was also observed [START_REF] Kaether | Axonal Membrane Proteins Are Transported in Distinct Carriers: A Two-Color Video Microscopy Study in Cultured Hippocampal Neurons[END_REF]. Little is known about the fate of the axonal transport carrier vesicles. Where do they fuse with axonal plasma PM? Where do the retrograde carriers go? A small fraction of the axonal APP has been suggested to undergo transcytotic transport to dendrites [START_REF] Simons | Intracellular routing of human amyloid protein precursor: Axonal delivery followed by transport to the dendrites[END_REF] but the significance and kinetics of this process need to be determined. Likewise, a detailed study of dendritic transport kinetics of APP is lacking. Furthermore, the sorting signals mediating axonal and/or dendritic transport of APP remain elusive as it has been reported that APP is transported into axons and dendrites without apparent sorting signals [START_REF] Back | β-amyloid precursor protein can be transported independent of any sorting signal to the axonal and dendritic compartment[END_REF]. At this point, the only certainty is that APP trafficking in neurons is fundamentally different [START_REF] Haass | Trafficking and proteolytic processing of APP[END_REF]. Newly synthesised APP molecules (black bars) mature through the constitutive secretory pathway (1). Once APP reaches the plasma membrane, it is rapidly internalised [START_REF] Cipriani | Alzheimer and his disease: a brief history[END_REF], and sorted through the endocytic and recycling compartments back to the cell surface [START_REF] Tiwari | Alzheimer's disease: pathogenesis, diagnostics, and therapeutics[END_REF] or degraded in the lysosome.

from that of other cells, and more work has to be done to fully understand polarized APP sorting in neurons.

Aβ production and clearance C.

Aβ production sites 1.

Although the amyloïdogenic and non-amyloïdogenic pathways of APP are well described, the actual localisation of APP and the α-, β-and γ-secretase complexes are still to be clearly identified as much as APP and Aβ trafficking in neurons. Notably, numerous studies show that Aβ accumulates intracellularly however the precise production site of the peptide is still under debate. Two hypotheses have been proposed for Aβ production (LaFerla et al., 2007;[START_REF] Aguzzi | Protein aggregation diseases: pathogenicity and therapeutic perspectives[END_REF]:  Either APP is endocytosed then cleaved, leading to an intracellular Aβ production possibly followed by Aβ exocytosis (Figure 15, left side).

 Or β-and γ-cleavage occurs directly at the neuron's plasma membrane and the release of Aβ into the extracellular space is immediate (Figure 15, right side). In this case, it is still to be determined whether Aβ or at least a fraction of the secreted Aβ is then uptaken by the surrounding cells to form intracellular pools of Aβ. This is why it is important to understand where the APP fragments and the β-/ γ-secretases are located in order to determine exactly where Aβ is produced during APP trafficking. Indeed, Aβ production may potentially occur where APP and β-: γ-secretase complexes are (for review : LaFerla et al., 2007;van der Kant and Goldstein, 2015).

 Non-amyloïdogenic pathway

On one hand, the APP found at the plasma membrane is mainly cleaved by α-secretase, releasing the sAPP-α fragment into the extracellular space and leaving the C83 fragment (or α-CTF) within the plasma membrane (Figure 16, 2) before being internalised and addressed to early endosomes (Figure 16, 3). Since α-cleavage occurs within the Aβ sequence of APP, it precludes Aβ production and therefore constitutes the non-amyloïdogenic pathway.

 Amyloïdogenic pathway

On the other hand, the APP that still hasn't been processed (Figure 16, 1) is endocytosed then internalised in early endosomes (Figure 16, 3). Early endosomes have optimal pH for β-secretase activity. When APP is addressed to endocytosis sites via its sequestration within lipid rafts, this precludes α-cleavage and therefore promotes the amyloïdogenic pathway. o β-secretase cleavage:

Although BACE1 and APP can be found in separate intracellular vesicle pools they, however, can be found in the same somato-dendritic compartments in conditions of neuronal activity [START_REF] Das | Activity-induced convergence of APP and BACE-1 in acidic microdomains via an endocytosis-dependent pathway[END_REF]. BACE1 and APP can also be internalized separately from the plasma membrane and interact with one another if they are in the same early endosome [START_REF] Sannerud | ADP ribosylation factor 6 (ARF6) controls amyloid precursor protein (APP) processing by mediating the endosomal sorting of BACE1[END_REF]. Notably, while APP is internalised via a clathrin-dependent endocytosis, BACE1 however is internalised via an ARF6dependent (ADP Ribosylation Factor 6) endocytosis [START_REF] Sannerud | ADP ribosylation factor 6 (ARF6) controls amyloid precursor protein (APP) processing by mediating the endosomal sorting of BACE1[END_REF]. As the pH at the plasma membrane (pH ≈ 7.5) is not optimal for β-secretase activity, BACE1 cleaves APP only once it is internalised in the most acidic environment of the endosomal system (optimal pH ≈ 4. 5-5.5).

Together these data suggest that APP cleavage by BACE1 takes places within early endosomes to generate sAPP-β and C99 (β-CTF) within the endosomal lumen. The sAPP-β fragment is then sorted to recycling endosomes to be readdressed to the plasma membrane (Figure 16, 4) or it can be sorted to the lysosome for degradation (Figure 16,8). C99 from β-cleavage and C83 from α-cleavage are retained within the membranes of early endosomes (Figure 16, 3) before being sorted to late endosomes (Figure 16,5). Once they are in the late endosomes, these fragments are either addressed to the Trans-Golgi network (Figure 16,6) and then exocytosed (Figure 16, 7) or sorted to lysosomes for degradation (Figure 16,8). (d). Full-length APP can be endocytosed from the PM (1) or cleaved by the α-secretase [START_REF] Cipriani | Alzheimer and his disease: a brief history[END_REF] to release sAPPα in the extracellular environment as well as generating C83 that is also endocytosed. In the early endosomes (EE), full-length APP is cut by the β-secretase generating C99 and sAPPβ [START_REF] Tiwari | Alzheimer's disease: pathogenesis, diagnostics, and therapeutics[END_REF]. Full-length APP and soluble APP fragments generated in the endosome might be recycled to the PM via recycling endosomes (RE) [START_REF] Greenfield | Endoplasmic reticulum and trans-Golgi network generate distinct populations of Alzheimer beta-amyloid peptides[END_REF] or can be trafficked toward late endosomes (LEs) [START_REF] Laferla | Intracellular amyloid-beta in Alzheimer's disease[END_REF]. In the LEs, these fragments can subsequently be sorted toward the TGN [START_REF] Walsh | A beta oligomers -a decade of discovery[END_REF], exocytosed [START_REF] Shankar | Natural oligomers of the Alzheimer amyloid-beta protein induce reversible synapse loss by modulating an NMDA-type glutamate receptor-dependent signaling pathway[END_REF], or further trafficked to the lysosome for degradation [START_REF] Shankar | Amyloid-beta protein dimers isolated directly from Alzheimer's brains impair synaptic plasticity and memory[END_REF]. Cutting of C99 and C83 by the γ-secretase might occur in the TGN, the LE, or in both compartments, therefore possibly releasing Aβ and P3 either via the secretory pathway or via exocytosis. Cutting by the γ-secretase also releases the AICD that can translocate to the nucleus [START_REF] Walsh | Naturally secreted oligomers of amyloid beta protein potently inhibit hippocampal long-term potentiation in vivo[END_REF]. o γ-secretase cleavage:

According to the literature, the exact location of γ-cleavage is still currently under debate and it suggests that C99 (β-CTF) and C83 (α-CTF) cleavage by the γ-secretase complex occurs within the TGN and/or in late endosomes. Following this cleavage, Aβ or P3 is released via the secretory pathway or exocytosis depending on the initial β-or α-cleavage respectively. The third released fragment is AICD and may act as a transcription factor (Figure 16,9). These peptides can be found in senile plaques and are more inclined to undergo oligomerisation and fibrillogenesis [START_REF] Kim | Abeta40 inhibits amyloid deposition in vivo[END_REF].

In the light of these data, there is no consensus concerning the production site of Aβ. It can be produced in endosomes, in the TGN where APP undergoes strong glycosylation or both structures. It is possible that the location of β-CTF γ-cleavage, in late endosomes or in the TGN, might influence where the γ-secretase complex cleaves β-CTF and influence the size of the generated Aβ peptide; indeed, one location might favour Aβ 42 production over Aβ 40 . Furthermore, cleavage within the TGN could lead the generated Aβ peptide towards the constitutive secretory pool and therefore release Aβ into the extracellular space, facilitating amyloid plaque formation. Cleavage in late endosomes could release Aβ in the lumen of the organelle from which it could either be degraded later in the endosome or exocytosed to be secreted. It is also important to keep in mind that mutations on APP itself might also influence these parameters.

Aβ peptide degradation and clearance 2.

Aβ peptides can be degraded by neuropeptidases, by truncation of their N-terminus in the cerebral interstitial environment (extracellular space) or cleared via specific receptors which allow them to cross the blood-brain barrier. Several Aβ degrading neuropeptidases have been identified [START_REF] Miners | Aβ-degrading enzymes: potential for treatment of Alzheimer disease[END_REF] including neprilysin (NEP), insulin-degrading enzyme, and endothelin-converting enzyme. However, the neuropeptidase recognised as being the most efficient in degrading Aβ is NEP, a zinc-dependent metalloprotease enzyme [START_REF] Iwata | Identification of the major Abeta1-42-degrading catabolic pathway in brain parenchyma: suppression leads to biochemical and pathological deposition[END_REF]. It has been shown, for example, that Aβ 42 degradation in conditions of synaptic activity occurs mainly via NEP but this clearing process declines over time as it has been reported that NEP declines with age [START_REF] Tampellini | Synaptic activity reduces intraneuronal Abeta, promotes APP transport to synapses, and protects against Abeta-related synaptic alterations[END_REF].

Despite having identified the enzymes involved in Aβ degradation and clearance their regulation is still to be clearly determined. It has been reported that NEP could be regulated by AICD generated from γ-cleavage of APP [START_REF] Grimm | Neprilysin and Aβ Clearance: Impact of the APP Intracellular Domain in NEP Regulation and Implications in Alzheimer's Disease[END_REF]. In this light, γ-secretase could indirectly participate in the regulation of clearing mechanisms of Aβ peptides by NEP.

Taken together, this set of Aβ degrading enzymes constitute potential therapeutic targets capable of modulating endogenous Aβ concentration.

Toxic and physiologic roles of Aβ D.

Aβ peptides are naturally produced by neuronal metabolism and can be detected in the plasma or CSF of non-AD patients [START_REF] Haass | Targeting of cell-surface beta-amyloid precursor protein to lysosomes: alternative processing into amyloid-bearing fragments[END_REF][START_REF] Walsh | The oligomerization of amyloid betaprotein begins intracellularly in cells derived from human brain[END_REF]. In these conditions, the physiological presence of Aβ peptides does not induce any neurodegenerative processes. Studies have shown that in physiological conditions a "normal" production of Aβ occurs but the proportion of the different forms of Aβ differ from the proportions found in pathological conditions. Particularly, the Aβ 40 /Aβ 42 ratio is modified in pathological conditions. Some studies suggest that the Aβ 40 /Aβ 42 ratio is a determining factor of toxicity, fibrillogenesis and pathological distribution of Aβ and highlight the fact that Aβ 40 and Aβ 42 have very different roles. Aβ 42 promotes accumulation and deposition of amyloid plaques whereas Aβ 40 does not [START_REF] Kim | Abeta40 inhibits amyloid deposition in vivo[END_REF][START_REF] Jan | The ratio of monomeric to aggregated forms of Abeta40 and Abeta42 is an important determinant of amyloid-beta aggregation, fibrillogenesis, and toxicity[END_REF].

In pathologic conditions, the Aβ 40 /Aβ 42 ratio is reduced. A number of studies have shown that during AD, Aβ 42 is increased due to a hyper-activation of the amyloïdogenic pathway and/or due to mutations affecting the actors of Aβ 42 synthesis in both animal models and humans [START_REF] Li | Moderate reduction of gamma-secretase attenuates amyloid burden and limits mechanism-based liabilities[END_REF] and/or due to defects in Aβ clearance mechanisms. Other studies have shown that Aβ load in the CSF varies according to synaptic activity [START_REF] Cirrito | Synaptic activity regulates interstitial fluid amyloid-beta levels in vivo[END_REF] and circadian rhythms [START_REF] Musiek | Sleep, circadian rhythms, and the pathogenesis of Alzheimer disease[END_REF].

Notably, inhibition of synaptic activity decreases the amount of Aβ secreted into the extracellular space whereas increased synaptic activity or disrupted circadian rhythm provokes an increase of Aβ secretion into the extracellular space.

It has been proposed that, in physiologic conditions, Aβ selectively depresses excitatory synaptic transmission via NMDAr and inversely, synaptic activity modulates β-cleavage. This activitydependent modulation of endogenous Aβ production, at the synaptic level, may normally participate in a negative feedback loop that could keep neuronal hyperactivity in check [START_REF] Kamenetz | APP processing and synaptic function[END_REF].

In this light, it is possible that synaptic depression from excessive Aβ could contribute to cognitive decline during early AD by disrupting this feedback system and disease progression (Figure 17). This points towards APP as being the gun targeting the synapse, Aβ is the bullet and synaptic activity is the trigger.

The different forms of Aβ E.

The Aβ peptide can be found under different conformations: alpha-helices, intermediate soluble forms such as monomers and oligomers, but also insoluble beta-sheet rich forms such as protofibrils and fibrils which are at the origin of amyloid plaques (Figure 18).

Aβ peptides have a tendency to aggregate. This is why, in most cases, the majority of Aβ in AD brains is aggregated in the form of fibrils which themselves tend to form amyloid plaques. Several

Senile plaques Figure 18: Aβ assembly states (adapted from La Ferla et al., 2007). Aβ can exist in multiple assembly states: monomers, oligomers, protofibrils and fibrils. It is the ability of this peptide to fibrils and other intermediate states that impart the unique pathophysiological characteristics of AD pathology. Fibril formation is a complex, nucleation-dependent process. The mechanism driving this process, particularly in the elderly brain,

is not yet understood, but it appears to be closely related to protein misfolding. In its monomeric state, Aβ does not appear to be neurotoxic. In contrast, oligomeric and protofibrillar species impede synaptic plasticity processes.

studies have investigated the aggregation properties of Aβ peptides. The Aβ 42 peptide is the most hydrophobic compared to its shorter counterparts. Its solubility is therefore lower and its aggregating capacity higher (approximately 70 times more rapid than Aβ 40 ). The two extra residues in the C-terminal of Aβ 42 give it a more structured organisation, compared to Aβ 40 , responsible for its toxicity and more potent aggregating properties [START_REF] Sgourakis | The Alzheimer's peptides Abeta40 and 42 adopt distinct conformations in water: a combined MD / NMR study[END_REF]. The reason why these extra two residues on Aβ 42 , compared to Aβ 40 , change its toxic and aggregating properties is still to be discovered. However, the capacity of Aβ to aggregate and form organised fibrillary structures not only depends on the nature of Aβ (40/42), the pH, temperature, and concentration of the peptide but mostly depends on the hydrophobic interactions of the peptides. These hydrophobic interactions enable them to go from an α-helix monomeric form to a "random coil" monomeric structure, whereby the monomeric units adopt random conformations, to eventually a β-sheet structure [START_REF] Zagorski | NMR studies of amyloid beta-peptides: proton assignments, secondary structure, and mechanism of an alpha-helix----beta-sheet conversion for a homologous, 28-residue, Nterminal fragment[END_REF][START_REF] Ahmed | Structural conversion of neurotoxic amyloid-beta(1-42) oligomers to fibrils[END_REF]. NMR spectroscopy based studies have shown that Aβ oligomers do not possess this β-sheet conformation, characteristic of fibrils, but are rather composed of loosely aggregated strands with a turn conformation, placing Phe19 in contact with Leu34 [START_REF] Ahmed | Structural conversion of neurotoxic amyloid-beta(1-42) oligomers to fibrils[END_REF] (Figure 19,A). It is the association of several β-sheet units that polymerise in a parallel manner that lead to the formation of protofibrils which themselves will associate into insoluble "coiled" fibrils (Figure 19, B) that will eventually aggregate into extracellular amyloid plaques.

Figure 19: Illustration of the molecular model of in turn conformation (adapted from Ahmed et al., 2010). (A) Oligomeric Aβ 42 has a structural conformation whereby Phe19 and Leu34 are in contact. (B) Stacked model of fibrillar Aβ 42 that contact via Gln15 and Gly37 interaction between the N-and C-termini of each β-strands.

It has therefore been shown that soluble monomers aggregate to form oligomers and that these soluble oligomeric forms of Aβ range in size from 4 to 100 kDa. It is these forms that are described as being the most toxic forms and responsible for AD cognitive deficits [START_REF] Lambert | Diffusible, nonfibrillar ligands derived from Abeta1-42 are potent central nervous system neurotoxins[END_REF][START_REF] Haass | Soluble protein oligomers in neurodegeneration: lessons from the Alzheimer's amyloid beta-peptide[END_REF][START_REF] Tomiyama | A mouse model of amyloid beta oligomers: their contribution to synaptic alteration, abnormal tau phosphorylation, glial activation, and neuronal loss in vivo[END_REF]. Molecular biology studies on mouse primary neuronal cultures have shown the enhanced toxicity of Aβ oligomers compared to protofibrils and fibrils [START_REF] Ahmed | Structural conversion of neurotoxic amyloid-beta(1-42) oligomers to fibrils[END_REF]. Several types of globular oligomers have been identified ranging from dimers, trimers, tetramers up 24-mers, with a higher molecular weight. These globular oligomers are coined ADDLs for "Aβ-derived diffusible ligands". So far, a detailed description of the toxicity of wild-type oligomeric Aβ has been given. However, the complexity of AD pathophysiology goes beyond the wild-type form of the peptide. Indeed, although most AD cases are sporadic, FADs are usually far more aggressive forms of the pathology and are very informative in understanding AD pathogenesis. The different pathogenic mutations that have been identified on APP either lead to increased wild-type Aβ production or generate mutated toxic forms of Aβ. These mutated Aβs can each adopt different conformations, have different aggregating properties, can be located differently inside and outside the cell, and generally behave differently. Studying these mutated forms of the peptides and their consequences, through transgenic animal models and human cases, have helped tremendously in understanding AD in the past four decades. These studies have brought to light the notion of Aβ strains, that each strain may behave and evolve in its own way and that AD patients might even have a combination of several strains within the brain. This has added a level of difficulty in understanding and finding therapeutic strategies for sporadic and familial AD. Nevertheless, using these mutated forms of Aβ and studying their consequences on biological processes might give an insight into the common denominator which gives rise to the cognitive impairments observed in AD. Some of these most remarkable mutations on APP, with unique signatures, will be described in the next section.

The different mutations of APP F.

Over the last couple of decades, numerous mutations on APP have been found in FADs. These mutations usually lead to aggressive forms of AD with an early onset and can be filed roughly into two categories: i) mutations that affect Aβ production; ii) mutations that affect Aβ conformation.

APP mutations affecting Aβ production 1.

To date, about 50 pathogenic mutations of APP have been reported ("AD&FTD Mutation Database," http://www.molgen.ua.ac.be/admutations), most of which affect the proteolysis of APP in such a way that Aβ 42 levels are changed relative to other Aβ isoforms (Suzuki et al., 1994;[START_REF] Scheuner | Secreted amyloid beta-protein similar to that in the senile plaques of Alzheimer's disease is increased in vivo by the presenilin 1 and 2 and APP mutations linked to familial Alzheimer's disease[END_REF][START_REF] Kwok | Novel Leu723Pro amyloid precursor protein mutation increases amyloid beta42(43) peptide levels and induces apoptosis[END_REF]De Jonghe et al., 2001;Di Fede et al., 2009;Cruts et al., 2012). Most of the pathogenic mutations of APP (Figure 20) are autosomal dominant and occur near the β-secretase cleavage site (aa 670 to 682), near the γ-secretase cleavage site (aa 713 to 724) [START_REF] Ringman | Genetic heterogeneity in Alzheimer disease and implications for treatment strategies[END_REF], or within the Aβ sequence of APP (the latter will be described in Part II.F.2).

a) Mutations affecting β-cleavage

 Swedish (K670M/N671L)

The Swedish (Swe) double mutation K670M/N671L, located just before the β-cleavage site, is probably the most renown and used mutation in AD research. The most commonly used transgenic AD mouse models usually carry this mutation, such as Tg2576 mice, J20 mice (which also carry the V717F Indiana mutation on APP), APP/PS1-21 (which also carry a mutation on PS1) and 3xTg mice (which also carry mutations on PS1 and Tau). This mutation leads to a general 20% increase of Aβ 40

and Aβ 42 production and secretion without affecting Aβ 40 /Aβ 42 ratio (Citron et al., 1992;[START_REF] Cai | Release of excess amyloid beta protein from a mutant amyloid beta protein precursor[END_REF].

 Taiwanese (D678H)

The Taiwanese D678H mutation increases general Aβ 40 and Aβ 42 levels and also favours the amyloïdogenic pathway, decreasing Aβ 40 /Aβ 42 ratio. The mutant Aβ is also more susceptible to the formation of ion-induced Aβ oligomers and exhibits greater toxicity in vitro compared with wild-type Aβ42 [START_REF] Chen | Amyloid-beta (Aβ) D7H mutation increases oligomeric Aβ42 and alters properties of Aβ-zinc/copper assemblies[END_REF].

 Leuven (E682K)

The Leuven E682K mutation at the β' site in APP shifts BACE1 cleavage toward the β-site and causes a significant increase in total Aβ levels and also decreases Aβ 40 /Aβ 42 ratio [START_REF] Zhou | Amyloid precursor protein mutation E682K at the alternative β-secretase cleavage β'-site increases Aβ generation[END_REF].

b) Mutations affecting γ-cleavage

 Austrian (T714I)
This mutation is thought to affect APP processing by γ-secretase and alters the Aβ [START_REF] Cruts | Novel APP mutation V715A associated with presenile Alzheimer's disease in a German family[END_REF].

The French V715M mutation is unusual as it was found to reduce total Aβ production. However, this mutation caused significant reduction in Aβ 40 levels with no change in Aβ 42 levels, leading to a decreased Aβ 40 /Aβ 42 ratio [START_REF] Ancolio | Unusual phenotypic alteration of beta amyloid precursor protein (betaAPP) maturation by a new Val-715 --> Met betaAPP-770 mutation responsible for probable early-onset Alzheimer's disease[END_REF].

A study using NMR spectroscopy found that these two mutations at the same position, altered the structure and dynamics of the transmembrane domain in APP, making it more accessible to γsecretase for cleavage in position 42 of Aβ sequence, and consequently shifting production toward Aβ 42 [START_REF] Chen | Familial Alzheimer's mutations within APPTM increase Aβ42 production by enhancing accessibility of ε-cleavage site[END_REF].

 Iberian (I716F) and Florida (I716V)

Both of these mutations decrease Aβ 40 /Aβ 42 ratio by increasing γ-secretase specificity for cleavage in position 42 of Aβ sequence (De Jonghe et al., 2001;[START_REF] Eckman | A new pathogenic mutation in the APP gene (I716V) increases the relative proportion of A beta 42(43)[END_REF][START_REF] Guardia-Laguarta | Clinical, neuropathologic, and biochemical profile of the amyloid precursor protein I716F mutation[END_REF][START_REF] Herl | Mutations in amyloid precursor protein affect its interactions with presenilin/gamma-secretase[END_REF].

 Indiana (V717F), V717G, London (V717I), V717L

The aa corresponding to codon 717 is within the transmembrane region of APP, mutations at this position all alter relative levels of Aβ peptides leading to decreased Aβ 40 /Aβ 42 ratio, usually by increasing Aβ 42 levels and decreasing (or not changing) Aβ 40 levels (De Jonghe et al., 2001;[START_REF] Eckman | A new pathogenic mutation in the APP gene (I716V) increases the relative proportion of A beta 42(43)[END_REF][START_REF] Herl | Mutations in amyloid precursor protein affect its interactions with presenilin/gamma-secretase[END_REF]Suzuki et al., 1994;[START_REF] Tamaoka | APP717 missense mutation affects the ratio of amyloid beta protein species (A beta 1-42/43 and a beta 1-40) in familial Alzheimer's disease brain[END_REF].

The London V717I missense mutation, the first described and best characterised of all APP mutations actually affects both β-and γ-secretase cleavage sites, resulting in increased Aβ 42 levels. It is to note, the initial clinical case of a Chinese AD patient with the London mutation had a more aggressive form of AD than Western AD patients with the same mutation. This finding brings to light the fact that the phenotypes of AD patients with identical APP mutations are affected by ethnic differences, environment or possibly other unknown factors [START_REF] Zhang | Clinical characterization of an APP mutation (V717I) in five Han Chinese families with early-onset Alzheimer's disease[END_REF].

 Australian (L723P) and Belgian (K724N)

Both of these mutations also decreased Aβ 40 /Aβ 42 ratio by increasing Aβ 42 production and decreasing Aβ 40 production [START_REF] Kwok | Novel Leu723Pro amyloid precursor protein mutation increases amyloid beta42(43) peptide levels and induces apoptosis[END_REF][START_REF] Theuns | Alzheimer dementia caused by a novel mutation located in the APP Cterminal intracytosolic fragment[END_REF].

 Flemish (A692G) and A693V

Although these two mutations are actually located within the Aβ sequence, they render APP a better substrate for β-secretase cleavage consequently increasing overall Aβ production [START_REF] Farzan | BACE2, a beta -secretase homolog, cleaves at the beta site and within the amyloid-beta region of the amyloid-beta precursor protein[END_REF]Di Fede et al., 2009). The A673V mutation will be further discussed in Part II.F.3.

APP mutations affecting Aβ sequence 2.

Mutations found within the Aβ sequence of APP (aa 692 to 705) are more complex as they mostly result in a variety of mutated Aβ peptides with each mutant having their own degree of toxicity, aggregating properties, localisation inside/outside the cell and/or resistance to degradation.

a) The hotspot for Aβ mutations (aa 693 to 694 of APP)

Most mutations within the Aβ sequence of APP are located on amino acids 693 and 694 of APP.

These mutations are clustered around the central hydrophobic Aβ core near the α-secretase cleavage site, resulting in an array of polymorphic aggregates in a mutation-dependent manner [START_REF] Dai | The genes associated with early-onset Alzheimer's disease[END_REF].

 Osaka (E693Δ)

The Osaka E693Δ mutation is a deletion of a glutamate at position 693. This particular mutation was initially found in a Japanese pedigree where the patients had severe AD-like cognitive deficits but PET-scans using Pittsburgh compound-B showed they lacked the Aβ-plaque hallmark of the disease.

Although the secretion of total Aβ is markedly reduced by this mutation, the variant Aβ is more resistant to proteolytic degradation. Furthermore, the Aβ-osaka (Aβosa) peptide accumulates intracellularly and shows unique aggregation properties of enhanced oligomerisation but no fibrillisation conferring it with high synaptotoxicity as it has been shown to inhibit LTP more potently than wild-type Aβ [START_REF] Nishitsuji | The E693Delta mutation in amyloid precursor protein increases intracellular accumulation of amyloid beta oligomers and causes endoplasmic reticulum stress-induced apoptosis in cultured cells[END_REF][START_REF] Takuma | Amyloid-beta E22Delta variant induces synaptic alteration in mouse hippocampal slices[END_REF][START_REF] Tomiyama | A mouse model of amyloid beta oligomers: their contribution to synaptic alteration, abnormal tau phosphorylation, glial activation, and neuronal loss in vivo[END_REF][START_REF] Tomiyama | A new amyloid beta variant favoring oligomerization in Alzheimer's-type dementia[END_REF][START_REF] Umeda | Intracellular amyloid β oligomers impair organelle transport and induce dendritic spine loss in primary neurons[END_REF].

Moreover, studies using Aβ 40 with the Osaka deletion showed that the internalization and binding of this variant to PC12 neurons was enhanced 6-fold which would in part explain the high level of intraneuronal Aβ seen in this Japanese pedigree [START_REF] Poduslo | Alzheimer's disease amyloid β-protein mutations and deletions that define neuronal binding/internalization as early stage nonfibrillar/fibrillar aggregates and late stage fibrils[END_REF].

 Arctic (E693G), Italian (E639K) and Dutch (E693Q)

These three point missense mutations cause an overall decrease of Aβ 40 and Aβ 42 levels in plasma.

Additionally, low levels of Aβ 42 are detected in condition media from HEK293 cells transfected with APP E693G and APP E693K . However, fibrillisation studies demonstrate no difference in fibrillisation rate, but Arctic, Italian and Dutch Aβ form protofibrils at a much higher rate and in larger quantities than wild-type Aβ with higher stability [START_REF] Nilsberth | The "Arctic" APP mutation (E693G) causes Alzheimer's disease by enhanced Abeta protofibril formation[END_REF]Poduslo and Howell, 2015).

In addition, these mutations are found to confer resistance to neprilysin-catalysed proteolysis of Aβ [START_REF] Tsubuki | Dutch, Flemish, Italian, and Arctic mutations of APP and resistance of Abeta to physiologically relevant proteolytic degradation[END_REF].

 Iowa (D694N)

The Iowa D694N autosomal dominant mutation has been shown to affect Aβ peptide structure in vitro resulting in the formation of a turn rather than a bend motif [START_REF] Krone | Effects of familial Alzheimer's disease mutations on the folding nucleation of the amyloid betaprotein[END_REF]. Additional in vitro experiments have shown that the Iowa mutation promotes fibrillogenesis of Aβ and results in greater Aβ-induced toxicity [START_REF] Van Nostrand | Pathogenic effects of D23N Iowa mutant amyloid beta -protein[END_REF], 2002).

b) Other

 Tottori (D678N)

The D678N mutation alters an amino acid within the Aβ region of APP, specifically at position 7 (D7N). The mutation does not affect Aβ generation from APP [START_REF] Hori | The Tottori (D7N) and English (H6R) familial Alzheimer disease mutations accelerate Abeta fibril formation without increasing protofibril formation[END_REF], but does alter the assembly kinetics of the peptide. Mutant Aβ displays accelerated secondary structure transitions and an increased propensity to form relatively large oligomers. The oligomers are also more efficient nucleators of fibril formation, and are significantly more cytotoxic than wild-type peptides [START_REF] Ono | Effects of the English (H6R) and Tottori (D7N) familial Alzheimer disease mutations on amyloid beta-protein assembly and toxicity[END_REF].

Not all mutations on APP are toxic 3.

 Icelandic (A673T) the protective mutation This rare mutation was initially found in one Finnish subject who lived until age 104.8 years and showed little beta-amyloid pathology. Although dementia was noted at 104 years of age, this was attributed to likely hippocampal sclerosis [START_REF] Kero | Amyloid precursor protein (APP) A673T mutation in the elderly Finnish population[END_REF]. Because of the low amount of parenchymal plaque pathology at 104.8 years of age, [START_REF] Kero | Amyloid precursor protein (APP) A673T mutation in the elderly Finnish population[END_REF] suggested that this variant protects again amyloid pathology and AD.

It has been reported that some of the reasons why this mutation is allegedly protective against AD is because: i) Aβ ice has a lower rate of aggregation (Poduslo and Howell, 2015) ii) it has a lower rate of β-cleavage. Indeed, studies using APP ice overexpression in HEK293 cells, as well as in human iPSC lines that express APP ice at endogenous levels, showed an approximate 30 to 40% reduction in the formation of amyloïdogenic peptides [START_REF] Jonsson | A mutation in APP protects against Alzheimer's disease and age-related cognitive decline[END_REF][START_REF] Maloney | Molecular mechanisms of Alzheimer disease protection by the A673T allele of amyloid precursor protein[END_REF]. Although this reduction might seem quite significant, it is important to keep in mind that the amyloïdogenic pathway accounts for only 10% of total APP processing (Sinha and Lieberburg, 1999).

 A673V the mutation with two faces

A673V is a particular recessive mutation whereby the homozygous state is very amyloïdogenic and the heterozygous state is not. In the homozygous state this mutation not only leads to increased Aβ production but also enhances Aβ aggregation and toxicity. However, in the heterozygous state, when there is a mixture of mutated and wild-type Aβ peptides they both aggregate more slowly than either peptide alone [START_REF] Di Fede | Good gene, bad gene: new APP variant may be both[END_REF].

These Aβ peptides, with different aggregation states or fibrillary structures, imply multiple pathways of binding/internalisation for the different Aβ peptides. These findings bring to light how subtle changes in the primary structure of Aβ translate into dramatic changes in the clinical expression and distinct neuropathology. Therefore these different variants will provide greater insight into the mechanisms of the disease.

Therapeutic strategies G.

To this day, the only treatments that exist to treat AD are directed towards alleviating the symptoms more than treating the causes. Amongst these treatments, four of them are acetylcholinesterase inhibitors and the fifth is an NMDAr antagonist. Older studies had shown that cholinergic neurons were particularly damaged in AD [START_REF] Coyle | Alzheimer's disease: a disorder of cortical cholinergic innervation[END_REF] and the number of remaining cholinergic neurons correlated with AD patients' cognitive performances [START_REF] Baskin | Brain choline acetyltransferase and mental function in Alzheimer disease[END_REF]. Acetylcholinesterase inhibitors (AChE) block the enzymes responsible for acetylcholine degradation inducing an increase in acetylcholine concentration in the synaptic cleft, thus prolonging the action of the neurotransmitter on the post-synaptic element. This allows counterbalancing the decrease of synaptic influx of acetylcholine due to selective neurodegeneration of presynaptic neurons. However, these treatments are effective only when there is still synthesis and release of acetylcholine meaning they lose their efficacy when neuronal degeneration is too advanced [START_REF] Ellis | Cholinesterase inhibitors in the treatment of dementia[END_REF]. In parallel of AChE, a non-competitive NMDAr antagonist, memantine, is used for treating AD especially in more advanced and severe stages of the pathology. This molecule is able to block NMDAr during low but continuous release of glutamate, which occurs during neuronal death.

Although these treatments are currently used in AD therapy, their efficacy is only relative.

More recently, new therapeutic strategies have been developed to target AD's abnormal Aβ peptide accumulation in order to decrease/restore these levels back to physiological levels in the brain [START_REF] Giacobini | Alzheimer disease therapy--moving from amyloid-β to tau[END_REF]:

Decreasing Aβ production 1.
One of the therapeutic strategies is to target the proteolytic enzymes of APP processing and therefore targeting Aβ production (γ-and β-secretases).

a) Inhibition of γ-secretase

Several inhibitors of the γ-secretase complex have been tested, but none of them showed any significant therapeutic efficacy and their development has been aborted during clinical trials. Also, several problems were recurrently encountered, such as difficulties in making the molecule cross the blood-brain barrier. Amongst these γ-secretase inhibitors, two molecules that modulate γ-secretase activity without altering Notch's signalling pathway have been developed: Tarenflurbil and Avagacestat. Tarenflurbil was abandoned during phase 3 of the clinical trials due to lack of effect on patients that were currently at a mild stage of the pathology. Avagacestat was also abandoned because of cognitive deterioration of the patients. Another inhibitor of the γ-secretase, Semagacestat, 30 times stronger than Tarenflurbil but also interferes with Notch signalling, was also abandoned due to lack of selectivity but also because of cognitive deterioration of the patients.

b) Inhibition of β-secretase

Another very promising therapeutic strategy, which has been undergoing clinical trials in the last few years, is BACE inhibitors. Since BACE1 is the β-cleaving enzyme of APP and is thought to be the rate limiting factor in Aβ production, both academia and industry have invested substantial resources into developing chemical compounds to inhibit BACE1 function. Several BACE inhibitors are currently being tested in Phase 2 and/or 3 trials (Table 3). Atabecestat (JNJ-54861911) was also discontinued in 2018 because treatment group not only had worsened cognitive symptoms than placebo group; they also reported more depression, anxiety, and sleep problems than controls. Only two trials are still ongoing, Elenbecestat (E2609), which is coming to term in 2020, and Umibecestat (CNP520), which is terminating in 2023/2024 (http://www.alzforum.org/therapeutics).

One of the main issues with this therapeutic strategy is that BACE1 has only been recently discovered, and the research community are only starting to unravel its role outside APP processing.

It has been shown that BACE1 has many other substrates, indicating it may be involved in various other physiological functions (Zhu et al., 2018a). Notably, it has several roles at the synapse including functions in synaptic structure and synaptic plasticity (Table 4). Indeed, BACE1 can be found at the pre-and post-synaptic structures and recent studies of Bace1 -/- mice have been very informative in order to understand these roles (Table 5). These findings highlight the complexity of using BACE1 inhibition as a therapeutic strategy and give insight into why these BACE1 inhibitor trials have failed to improve cognitive deficits, if not worsen them. Although BACE1 inhibition yielded underwhelming results during clinical trials, at this point, the only proposed solution to minimise negative side effects due to BACE1's other functions within the brain is to only lower the activity of BACE1 by 50% (Bace1 +/-mice) as this suppresses Aβ generation by 40% without inducing synaptic deficits [START_REF] Devi | Effects of BACE1 haploinsufficiency on APP processing and Aβ concentrations in male and female 5XFAD Alzheimer mice at different disease stages[END_REF][START_REF] Sadleir | Aβ reduction in BACE1 heterozygous null 5XFAD mice is associated with transgenic APP level[END_REF] (Table 5).

Immunotherapies 2.

This second therapeutic strategy is based on the development of active or passive immunotherapies, against the Aβ peptide. These approaches aim at promoting the clearance of already existing Aβ peptides in extracellular amyloid plaques and/or to reduce its accumulation via cellular immunity.

Despite encouraging results from trials using non-specific immunologic approaches (immunoglobuline, IVIg, gammagard) on AD transgenic animals; these effects weren't reproduced in AD patients. A first active vaccine against the Aβ peptide (AN1792) was developed then discontinued due to harmful inflammatory reactions. Furthermore, post-mortem studies showed that there was no correlation between the amount of Aβ clearance in amyloid plaques in the brains of the patients and their synaptic integrity. Afterwards, a monoclonal antibody against soluble and insoluble (fibrillary) Aβ, Bapineuzumab, was developed and showed promising results on transgenic AD animals as it significantly reduced the accumulation of Aβ. However, clinical trials were discontinued due to lack of therapeutic efficacy. Another monoclonal antibody was developed, targeting an epitope in the central region of Aβ, Solanezumab. Despite Solanezumab having shown positive results in AD transgenic animals with an improvement of cognition, it failed to reproduce these effects in humans. Regardless of these failures, several new monoclonal antibodies and six new vaccines are currently under clinical trials and focus on patients in early stages of AD and even prodromal AD. However, the lack of biomarkers to detect early preclinical phases of AD and the complexity of the measures to track the cognitive evolution of patients with presymptomatic/early AD constitutes are real challenge to demonstrate the beneficial effects of these therapeutic approaches since these patients present no clinical symptoms of AD yet.

Although many doubts have been expressed regarding the validity of the amyloid hypothesis due to the lack of effect of these therapeutic strategies targeting Aβ, a new study show encouraging preliminary results and reinforce the hypothesis of the amyloid cascade [START_REF] Sevigny | The antibody aducanumab reduces Aβ plaques in Alzheimer's disease[END_REF]. This

Breaking news

At the time of finishing writing this manuscript, biotechnology firm Biogen and its partner Eisai announced plans to abandon the two clinical trials for Alzheimer's treatment using the drugs Elenbecestat and Aducanumab. The decision to end the studies came after a data safety monitoring board found that the benefits of administering these drugs "did not outweigh the risks".

Decreasing Aβ aggregation 3.

Aβ aggregation into fibrils, which is at the basis of amyloid plaque formation, has driven researchers to develop anti-aggregating compounds, which preclude the oligomerisation of Aβ peptides, or promote the clearance of these aggregates.

Several compounds, such as Tramiprosat or Scyllo-inositol, have been developed but without any successful results once tested on AD patients.

Increasing Aβ clearance 4.

The fourth therapeutic approach is based on the use of compounds which decrease Aβ levels in the plasma. Notably, Neprilysine, a zinc metallo-protease which degrades Aβ peptides in the brain, has been the subject of many studies in vitro and also using transgenic AD animals. Despite positive results in decreasing plasma Aβ levels in vitro, the concentration of Aβ in AD transgenic mouse models was not decreased. This suggests that this therapeutic approach will have limited efficacy.

Counteracting the toxic effects of Aβ 5.

Another major route in the development of therapeutic strategies is to counteract the deleterious effects induced by Aβ, especially the toxic effects inflicted on synapses and synaptic plasticity processes. Indeed, it is now well-established that Aβ induces a reduction in the number of synapses (i.e. a decrease in dendritic spine density) and a decrease of Long-term Potentiation, the experimental correlate of memory and learning processes (both will be further described in the next chapters)which leads to AD-related cognitive and memory defects. Many studies have investigated potential diffusible factors that could counteract these effects. One of which being Vascular Endothelial cell Growth Factor (VEGF), which is already known to be required for the action in antidepressant therapies. This molecule, and its receptor VEGFR2, are involved in increasing NMDAdependent postsynaptic responses [START_REF] De Rossi | A critical role for VEGF and VEGFR2 in NMDA receptor synaptic function and fear-related behavior[END_REF]. This molecule has been shown to be abundant in the CNS but decreased in the CSF of AD patients as well as transgenic AD animal models, due to an interaction of VEGF with Aβ, impeding VEGF function. It has been shown that VEGF treatment on AD transgenic mouse hippocampal neurons counteracts Aβ-induced synaptic dysfunction and restores long-term synaptic plasticity processes [START_REF] Martin | Impact du VEGF sur les altérations synaptiques dans la maladie d'Alzheimer[END_REF], opening a new route for therapeutic molecule development.

The main difficulty in the development of all these therapeutic strategies is the discrepancy between the clinical efficacy of these therapies on AD transgenic animals and the efficacy on humans. Together these data and studies strongly suggest an evolution of the amyloid hypothesis in the coming years; and that finding a one-size-fits-all therapeutic approach is proving to be very difficult. Thus, a more made-to-measure, personalised therapeutic strategy might be the way forward to treat this multi factorial pathology that is AD.

Aβ and Synaptotoxicity H.

Historically, senile plaques, made up of Aβ fibrils, were considered as the initiating factor and at the root cause of neurotoxic and neurodegenerative phenomena observed during AD [START_REF] Hardy | Alzheimer's disease: the amyloid cascade hypothesis[END_REF]. However, the cognitive deficits observed, and the significant loss of synapses, do not correlate with this hallmark of AD. Rather, it is now widely accepted that it is the soluble oligomers of Aβ that are responsible of the first synaptic and cognitive alterations that are at the origin of AD pathogenesis. Insoluble amyloid deposits that constitute the plaques are seen, these days, more as a reservoir of bioactive oligomers [START_REF] Haass | Soluble protein oligomers in neurodegeneration: lessons from the Alzheimer's amyloid beta-peptide[END_REF]. This is why, the recent evolution of the amyloid cascade hypothesis (Figure 21) brings to light that it is the oligomers of Aβ that are responsible for the first synaptic and cognitive deficits observed in AD. This hypothesis has become the dominating model of the development of this pathology [START_REF] Glenner | Alzheimer's disease: initial report of the purification and characterization of a novel cerebrovascular amyloid protein[END_REF][START_REF] Hardy | Alzheimer's disease: the amyloid cascade hypothesis[END_REF] albeit the linearity of this cascade is still controversial [START_REF] De Strooper | The Cellular Phase of Alzheimer's Disease[END_REF]. Studies have revealed that synapse loss is the best suited marker of observed cognitive deficits (Selkoe, 2002;[START_REF] Shankar | Natural oligomers of the Alzheimer amyloid-beta protein induce reversible synapse loss by modulating an NMDA-type glutamate receptor-dependent signaling pathway[END_REF]. Since, numerous studies have confirmed the synaptotoxic effects of the soluble oligomeric forms of Aβ (for review: Shankar and Walsh, 2009).

However, even though the Aβ peptide has been identified in the mid-80s as the main component of extracellular senile plaques, studies carried out on cultures of primary neurons treated with synthetic or human brain extracted Aβ peptides as well as studies using transgenic AD animal models have shown the existence of intracellular Aβ [START_REF] Gouras | Intraneuronal Abeta42 accumulation in human brain[END_REF][START_REF] Grundke-Iqbal | Amyloid protein and neurofibrillary tangles coexist in the same neuron in Alzheimer disease[END_REF]for review: LaFerla et al., 2007;[START_REF] Ripoli | Intracellular accumulation of amyloid-β (Aβ) protein plays a major role in Aβ-induced alterations of glutamatergic synaptic transmission and plasticity[END_REF][START_REF] Tampellini | Impaired β-amyloid secretion in Alzheimer's disease pathogenesis[END_REF] and particularly inside synapses [START_REF] Pickett | Non-Fibrillar Oligomeric Amyloid-β within Synapses[END_REF]. It seems the intracellular accumulation of Aβ oligomers is an upstream event in the formation of senile plaques in AD pathogenesis, and is responsible for the early dysfunctions in AD. Several studies show that the intraneuronal accumulation of Aβ peptides coincides with synaptic dysfunction along with the memory alterations dependent of the hippocampus, the cerebral structure implicated in spatial memory (Billings et al., 2005;LaFerla et al., 2007;[START_REF] Umeda | Intracellular amyloid β oligomers impair organelle transport and induce dendritic spine loss in primary neurons[END_REF]. 6). Notably, Aβ can interact with post-synaptic receptors such as cholinergic nicotinic receptors α7nAchR but also ionotropic glutamatergic NMDA (N-methyl-D-aspartic acid) receptors [START_REF] Dinamarca | Postsynaptic Receptors for Amyloid-β Oligomers as Mediators of Neuronal Damage in Alzheimer's Disease[END_REF]. It is important to note that these receptors interact with oligomeric and not fibrillary forms of Aβ [START_REF] Nimmrich | Amyloid beta oligomers (A beta(1-42) globulomer) suppress spontaneous synaptic activity by inhibition of P/Q-type calcium currents[END_REF] and that their function is altered upon this interaction which leads to synaptic defects. Currently, it is well accepted that the prime target of Aβ is the synapse [START_REF] Li | Environmental novelty activates β2-adrenergic signaling to prevent the impairment of hippocampal LTP by Aβ oligomers[END_REF][START_REF] Ripoli | Intracellular accumulation of amyloid-β (Aβ) protein plays a major role in Aβ-induced alterations of glutamatergic synaptic transmission and plasticity[END_REF]Shankar and Walsh, 2009), which subsequently induces the synaptic alterations and neuronal defects observed during the development of AD, and ultimately results in memory impairments.

III. The excitatory glutamatergic synapse

The chemical synapse A.

In the central nervous system, synaptic transmission between two neurons occurs through distinct contact points called synapses, which can be electrical or chemical. Electrical synapses are a minority in vertebrate CNS; communication occurs via ion exchanges and a direct passage of current through gap junctions. The majority of synapses found in the CNS are chemical. They are at the basis of neurotransmission and can be excitatory or inhibitory. These chemical synapses are constituted of pre-and postsynaptic elements separated by a gap called the synaptic cleft. Together these elements form the functional unit that transmits nervous messages from one neuron to another (Figure 22). In this part, we will only focus on the architecture and function of the excitatory glutamatergic synapse. Electron microscopy studies have enabled to identify excitatory glutamatergic synapses by the presence of: i) a presynaptic area encompassing numerous small vesicles which contain the neurotransmitter (glutamate) and ii) an electron-dense postsynaptic area known as the postsynaptic density (PSD) (Harris et al., 1992). The pre-and postsynaptic membranes are approximately 20 nm apart, this gap delimitates the synaptic cleft. This narrow space is spread across a relatively wide area and enables a rapid increase of glutamate concentration whilst limiting its diffusion outside the synaptic cleft [START_REF] Isaacson | The uptake inhibitor L-trans-PDC enhances responses to glutamate but fails to alter the kinetics of excitatory synaptic currents in the hippocampus[END_REF][START_REF] Savtchenko | The optimal height of the synaptic cleft[END_REF]. The structure of the synapses is maintained by adhesion proteins which link the pre-and postsynaptic elements together and organise the different components of the synapse. This architecture enables to minimise excitotoxicity phenomena and optimises the transmission of electrochemical signals.

Glutamatergic neurotransmission B.

There are several types of chemical synapses depending on the neurotransmitter that is released (Table 7). The majority of the excitatory neurotransmission in the CNS is carried out by glutamatergic excitatory synapses. These types of synapses are very abundant in the hippocampus and cortex. Glutamatergic neurotransmission plays an essential role in brain development and in the molecular mechanisms of memory and learning. This neurotransmitter activates two types of receptors, which are pharmacologically different and are classed depending on their transduction modalities, namely: metabotropic receptors and ionotropic receptors [START_REF] Dhami | Regulation of metabotropic glutamate receptor signaling, desensitization and endocytosis[END_REF][START_REF] Zhu | Structure and symmetry inform gating principles of ionotropic glutamate receptors[END_REF].

Neurotransmitter

Glutamate receptors and synaptic transmission C.

The presynaptic element contains the machinery which enables the release of glutamate by regulating synaptic glutamate-containing vesicle transport and fusion to the plasma membrane.

Neurotransmitter release from this presynaptic element is calcium-dependent and relies on both activation of voltage-gated channels [START_REF] Katz | A study of synaptic transmission in the absence of nerve impulses[END_REF] and intracellular calcium release from reservoirs present in the presynaptic element [START_REF] Emptage | Calcium stores in hippocampal synaptic boutons mediate short-term plasticity, store-operated Ca2+ entry, and spontaneous transmitter release[END_REF]. The glutamate released into the synaptic cleft binds and activates different types of receptors located on the pre-and postsynaptic elements. Two broad categories of receptors can be distinguished: i) metabotropic receptors coupled to G proteins (mGluRs) ii) ionotropic receptors coupled to ion channels which include AMPAr (α-amino3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor), NMDAr (N-methyl-D-aspartate receptor) and kainate receptors.

Metabotropic receptors 1.

Metabotropic receptors (mGluRs) are G-protein coupled receptors. There are eight sub-types of mGluRs subdivided into three families depending on their sequence homology and their coupling to secondary messengers (Figure 23) (for review: [START_REF] Nicoletti | Metabotropic glutamate receptors: from the workbench to the bedside[END_REF]. mGluR1 and mGluR5 belong to group I; mGluR2 and mGluR3 belong to group II; and mGluR4, mGluR6, mGluR7 and mGluR8 belong to group III (Table 8; mGluRs section).

Group I includes receptors positively coupled to phospholipase C (PLC) via Gq-type G-proteins.

PLC activation in turn induces the release of diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3). IP3 will induce intracellular release of Ca 2+ stored in the endoplasmic reticulum [START_REF] Endoh | Characterization of modulatory effects of postsynaptic metabotropic glutamate receptors on calcium currents in rat nucleus tractus solitarius[END_REF].

Metabotropic receptors from group II and III are negatively coupled to adenylate cyclase (AC) via inhibitory G-proteins which will reduce the activity of AC and decrease the amount of cyclic adenosine monophosphate (AMPc) in the cell (for review [START_REF] Conn | Pharmacology and functions of metabotropic glutamate receptors[END_REF]. AMPc is implicated in the activation of protein kinase A (PKA), phosphorylates kainate receptors, but also AMPAr and potentiates their activity.

The mGluRs regulate and participate in synaptic transmission via several effectors such as protein kinase C (PKC), inositol triphosphate receptors or membrane ion channels. Amongst these three groups of receptors, metabotropic receptors from group I are localised at the postsynaptic level, whereas receptors from group II are localised at the pre-and postsynaptic level, and receptors from group III are preferentially expressed at the presynaptic level where they modulate the release of neurotransmitters by acting as presynaptic autoreceptors (Table 8, mGluRs section). A number of studies have shown that metabotropic receptors from group I are involved in numerous psychiatric disorders and in synaptic plasticity (for review [START_REF] Bhattacharyya | Inside story of Group I Metabotropic Glutamate Receptors (mGluRs)[END_REF]. The variety of roles held by glutamate metabotropic receptors reflects in the diversity of their localisation around the glutamatergic synapse but also in their pharmacological characteristics as well as their sensitivity to agonists/antagonists (Table 9, mGluRs section). Metabotropic glutamate receptors have slow activation kinetics and are not involved in fast synaptic transmission. Indeed, their activation via glutamate does not result in the opening of a channel but rather in the activation of signalling cascades implicated in relatively slow modulations of synaptic transmission [START_REF] Niswender | Metabotropic glutamate receptors: physiology, pharmacology, and disease[END_REF].

Ionotropic receptors 2.

Ionotropic receptors (iGluRs) are channel receptors permeable to sodium (Na + ), calcium (Ca 2+ ) and potassium (K + ) ions (Table 8, iGluRs section).

In the CNS, there are three groups of ionotropic receptors. They are classed depending on their pharmacological characteristics, their selective sensitivity to agonists, their voltage sensitivity and their localisation (Table 9, iGluRs section). These receptors were named after the agonist that enabled to identify them: AMPA (α-amino3-hydroxy-5-methyl-4-isoxazolepropionic acid), NMDA (Nmethyl-D-aspartate) and kainate (KA). Each subunit of iGluRs is composed of four domains: an N-terminal domain, an agonist binding site (Glutamate or Glycine) formed by the spatial folding of two extracellular loops S1 and S2. These two loops are linked to the transmembrane domain (TM) composed of hydrophobic regions (TM I-IV) that defines a part of the pore of the tetrameric receptor. Lastly, there is the cytosolic C-terminal domain (Figure 24). This domain is the most variable between the different subunits of iGluRs and participates in the trafficking and anchoring of iGluRs to scaffold proteins at the plasma membrane.

The function of iGluRs is conditioned by their localisation in the synaptic complex, at the presynaptic level they modulate neurotransmitter release (kainate type receptors), and at the postsynaptic density (PSD) they participate in synaptic transmission (AMPA, NMDA and KA). Furthermore, iGluRs can also be extrasynaptic and be activated either by glutamate diffusion outside the synaptic cleft, this phenomenon is coined a "spillover", or by glutamate release for glial cells.

a) AMPA receptors

The AMPA receptor (AMPAr) is a complex formed of four subunits: GluA1, GluA2, GluA3 and GluA4 (Table 8, iGluRs section) produced by different genes [START_REF] Hollmann | Cloned glutamate receptors[END_REF][START_REF] Traynelis | Glutamate receptor ion channels: structure, regulation, and function[END_REF]. Therefore, it is a tetrameric glutamatergic receptor. The types of subunits which form the receptor dictate its electrophysiological properties. AMPAr forms an ion channel that is permeable to K + and Na + but is also permeable to Ca 2+ . The GluA2 subunit is responsible for blocking the passage of calcium through the pore (Table 9, iGluRs section) therefore AMPAr that possess a GluA2 subunit are permeable to calcium ions.

AMPA receptors are the main actors in fast synaptic transmission and are present in all glutamatergic synapses. These receptors are, therefore, rapidly activated. They trigger glutamatergic neurotransmission and have high motility within the membrane. They enable rapid conversion of glutamate release from presynaptic vesicle into electrical activity in the post synaptic neuron.

Therefore these receptors are mainly concentrated on the postsynaptic membrane of dendritic spines, within the PSD. However, their localisation at the subcellular level seems variable. Although they are present in large quantities at the synapse, they are also present outside the synapse where they diffuse freely in the plasma membrane. Once these extrasynaptic AMPAr reach the synapse, however, their motility is greatly reduced thus indicating that these extrasynaptic AMPAr might serve as a reservoir of postsynaptic receptors (for review: [START_REF] Groc | AMPA and NMDA glutamate receptor trafficking: multiple roads for reaching and leaving the synapse[END_REF].

b) NMDA receptors

NMDA receptors (NMDAr), contrary to AMPA receptors, need simultaneous fixation of glutamate (or NMDA) and a co-agonist, glycine or D-serine, on two sites located on two different subunits, in order to be activated [START_REF] Mothet | D-serine is an endogenous ligand for the glycine site of the N-methyl-D-aspartate receptor[END_REF]. Furthermore, this receptor forms an ion channel that is permeable to Na + , K + but also Ca 2+ , and is blocked by an extracellular Mg 2+ ion when the membrane is at resting potential. This membrane potential-dependent Mg 2+ makes this NMDAr a real sensor of simultaneous postsynaptic neuron depolarization and presynaptic glutamate release [START_REF] Yuste | Mechanisms of calcium influx into hippocampal spines: heterogeneity among spines, coincidence detection by NMDA receptors, and optical quantal analysis[END_REF]. Simultaneous activation of both pre-and postsynaptic elements is therefore required to activate these receptors, and the subsequent Ca2+ entry allows them to play an essential role in the establishment of certain synaptic plasticity processes.

NMDA receptors are also heterotetramers composed of two mandatory GluN1 subunits associated to GluN2 (A to D) or GluN3 (A and B). Each of these subunits has an extracellular Nterminal domain (NTD), an agonist binding domain (ABD), a transmembrane domain that forms the channel pore, and an intracellular C-terminal domain of which the length varies depending on the type of subunit [START_REF] Paoletti | NMDA receptor subunit diversity: impact on receptor properties, synaptic plasticity and disease[END_REF]. The ABD forms a central pocket that binds to: the co-agonist (glycine or D-serine) for GluN1 and GluN3; and glutamate for GluN2 (A -D) [START_REF] Paoletti | NMDA receptor subunit diversity: impact on receptor properties, synaptic plasticity and disease[END_REF][START_REF] Yao | Conformational analysis of NMDA receptor GluN1, GluN2, and GluN3 ligand-binding domains reveals subtype-specific characteristics[END_REF]. Each receptor subtype has specific biophysical, pharmacological and signalling pathway properties [START_REF] Paoletti | NMDA receptor subunit diversity: impact on receptor properties, synaptic plasticity and disease[END_REF].

The spatiotemporal expression of the different subunit mRNAs varies significantly, particularly GluN2 [START_REF] Monyer | Developmental and regional expression in the rat brain and functional properties of four NMDA receptors[END_REF]. During mouse embryonic development of the CNS, only receptors containing GluN2B and GluN2D are present. GluN2B is expressed throughout the brain whereas GluN2D is restricted to the diencephalon, mesencephalon and brain stem. Receptors with GluN2A subunits start progressively to be expressed after birth, contrary to GluN2D containing receptors that decrease rapidly [START_REF] Monyer | Developmental and regional expression in the rat brain and functional properties of four NMDA receptors[END_REF]. This same inverted tendency is observed for GluN2A and GluN2B, with a maximal expression of GluN2B during the second postnatal week, then a progressive decline to then be restricted to anterior brain structures. In parallel, GluN2A expression increases and spreads progressively throughout the cortex, hippocampus and cerebellum. Finally, GluN2C containing receptors are expressed after birth, mainly in the cerebellum and, to a lesser extent, in the olfactory bulb. Spatiotemporal expression modulation of these different subunits could play a crucial role in regulating neuronal circuit remodelling and adaptation to afferent sensory experiences [START_REF] Sanz-Clemente | Diversity in NMDA receptor composition: many regulators, many consequences[END_REF].

Although NMDA receptors are always expressed at the postsynaptic density (PSD), whether the synapse is active or not [START_REF] Takumi | Different modes of expression of AMPA and NMDA receptors in hippocampal synapses[END_REF], some NMDAr are also expressed extra-and presynaptically. Extrasynaptic NMDAr are organised in clusters which can be mobilized to the synapse by lateral diffusion during synaptic plasticity processes [START_REF] Papouin | Organization, control and function of extrasynaptic NMDA receptors[END_REF]. It is believed that extrasynaptic NMDAr are mainly composed of GluN2B subunits. Indeed, GluN2B

preferentially interacts with SAP102 rather than PSD95, contrary to GluN2A, which increases its diffusion capacity thus increasing the extrasynaptic pool of GluN2B containing receptors. At the presynaptic level, NMDAr can be found in both excitatory and inhibitory synapses [START_REF] Bouvier | Presynaptic NMDA receptors: Roles and rules[END_REF]. These receptors modulate neurotransmitter release but can also be implicated in presynaptic signalling. Notably, it has been suggested that presynaptic receptors could be a source of calcium influx involved during LTD processes [START_REF] Bidoret | Presynaptic NR2A-containing NMDA receptors implement a high-pass filter synaptic plasticity rule[END_REF].

Together these data highlight that NMDA receptor subunit composition influence synaptic transmission by modulating their motility, their synaptic or extrasynaptic localisation, and even their conductance [START_REF] Paoletti | NMDA receptor subunit diversity: impact on receptor properties, synaptic plasticity and disease[END_REF].

c) Kainate receptors

Kainate receptors are ubiquitously expressed in the CNS and can be found at both pre-and postsynaptic levels. Contrary to AMPA and NMDA receptors, kainate receptors are not much involved fast glutamatergic synaptic transmission. However, they play an important role in regulating neuronal network activity and plasticity [START_REF] Sihra | Kainate receptors: multiple roles in neuronal plasticity[END_REF]. They can also regulate postsynaptic membrane depolarisation as well as presynaptic neurotransmitter release [START_REF] Huettner | Kainate receptors and synaptic transmission[END_REF].

Furthermore, these receptors also take part in the modulation of presynaptic plasticity processes in the hippocampus as they facilitate long-term potentiation (LTP) at the synapses between mossy fibres and CA3 neurons [START_REF] Lauri | Synaptic activation of a presynaptic kainate receptor facilitates AMPA receptor-mediated synaptic transmission at hippocampal mossy fibre synapses[END_REF].

In order to ensure proper synaptic transmission at the synapse, AMPA and NMDA receptors along with a host of other components are all finely organised on the postsynaptic element: the dendritic spine, which will be described in the next section.

The Dendritic spine D.

Background 1.

Dendritic spines form the postsynaptic compartment which receives the nervous influx of excitatory glutamatergic synapses. These spines were initially discovered by Ramòn y Cajal in 1888.

They noticed the presence of these distinct membrane protrusions all along the dendritic arborisation of neurons (Figure 23). These protrusions were later coined "dendritic spines". It wasn't until the development of electron microscopy that more information was available about these structures. In 1959, George Gray showed, for the first time, the contact point upstream of the dendritic spine: the axon terminal or axonal bouton (Figure 24). Gray managed to distinguish two categories of synapses. The first was symmetrical synapses with two electron-dense zones: one on the presynaptic and one on the postsynaptic side; both of these zones were of same size [START_REF] Guillery | Observations of synaptic structures: origins of the neuron doctrine and its current status[END_REF]. These symmetrical synapses are now known to be characteristic of inhibitory synapses. The second category of synapses was asymmetrical synapses, with an electron-dense zone on the postsynaptic side only, and corresponds mainly to excitatory synapses. [START_REF] Guillery | Early electron microscopic observations of synaptic structures in the cerebral cortex: a view of the contributions made by George Gray (1924-1999)[END_REF]. Electron-microscopic image of an asymmetrical synapse. "a" is the presynaptic electrondense zone. "b" is the dense material in the synaptic cleft and "c" is the postsynaptic density. "pre": presynaptic; "den": dendrite.

Dendritic spine morphology 2.

Dendritic spines are small (typically 0.5 -2 µm in length) membranous protrusions that house the essential postsynaptic components, including the PSD, actin cytoskeleton, and a variety of "supporting" organelles (Figure 25). Spines occur at a density of 1 to 10 spines per µm of dendrite length on principal neurons [START_REF] Sorra | Overview on the structure, composition, function, development, and plasticity of hippocampal dendritic spines[END_REF]. Typical spines have a bulbous head (receiving a single presynaptic bouton) connected to the parent dendrite through a thin spine neck (Figure 25).

The surface of the spine head can be divided into three concentric circles. The centre, averaging at 500 to 1000 nm in diameter, is the core of the PSD housing the AMPArs and NMDArs (Figure 25, red zone), just opposite the presynaptic release site. Around the core is a 100-200 nm ring (Figure 25, blue zone) called the perisynaptic domain, where mGluRs are preferentially enriched. Beyond this perisynaptic domain is the extrasynaptic domain [START_REF] Scheefhals | Functional organization of postsynaptic glutamate receptors[END_REF]. This highly organised spatial segregation has important functional implications for synaptic efficacy, where the peri-and extrasynaptic domains act as reservoirs for glutamate receptors.

Because the neck hinders diffusion of molecules to and from the parent dendritic shaft, spines serve as microcompartments in which biochemical changes in one individual synapse can be isolated from other synapses on the same neuron [START_REF] Kennedy | Integration of biochemical signalling in spines[END_REF][START_REF] Macgillavry | The internal architecture of dendritic spines revealed by superresolution imaging: What did we learn so far?[END_REF].

The geometry of the spine neck determines, for example, calcium efflux into the dendritic shaft, thus modulating the degree of calcium elevation in the spine head following NMDAr activation. This spine neck diffusion barrier is said to be modulated by synaptic activity [START_REF] Tønnesen | Spine neck plasticity regulates compartmentalization of synapses[END_REF]. Dendritic spines are highly heterogeneous structures that show dynamic motility, especially during development and synaptic plasticity processes [START_REF] Tada | Molecular mechanisms of dendritic spine morphogenesis[END_REF]. Their number, size and shape undergo plastic changes correlated with long-term modifications of synaptic strength and interneuronal connectivity (Hayashi and Majewska, 2005). Spine shape has been broadly categorised as "mushroom", "thin" or "stubby" (which will be further described in Part III.C.2.a, b and c) (Figure 26). Though electron-microscopy studies tend to show more of a continuum between these categories, there is growing evidence that different spine shapes and sizes reflect different developmental stages and/or altered strength of synapses (Hayashi and Majewska, 2005;Yuste and Bonhoeffer, 2001). Sophisticated imaging experiments indicate that the volume of spine heads can increase with stimuli that strengthen synapses and can decrease with stimuli that weaken synapses (Hayashi and Majewska, 2005;[START_REF] Kasai | Structure-stability-function relationships of dendritic spines[END_REF]. The molecular mechanisms that coordinate synaptic strength with spine morphogenesis are still under investigation.

Spines with large heads are generally stable, express large numbers of AMPAr, and contribute to strong synaptic connections. In contrast, spines with small heads are more motile, less stable, and contribute to weak synaptic connections [START_REF] Holtmaat | Experience-dependent and cell-typespecific spine growth in the neocortex[END_REF]. In vivo timelapse studies show that spines turn over at various rates in the mouse brain; a large proportion of mushroom spines are persistent, with lifetimes up to several months [START_REF] Trachtenberg | Long-term in vivo imaging of experience-dependent synaptic plasticity in adult cortex[END_REF]. Nevertheless, it is currently believed that a subset of spines can undergo changes in shape or number in response to experience and other factors in the adult brain, thereby relating spine morphology to synaptic plasticity and long-term memory formation.

a) Thin spines

Thin spines represent the majority of dendritic spines in the adult hippocampus and cortex (Harris et al., 1992). These spines are particularly dynamic and show the most spontaneous morphology changes. They have distinct morphological characteristics such as a defined neck, which is much longer than the diameter of the neck, and a defined small head, that is barely wider than the spine neck (Figure 26) and faces an axonal bouton. The dynamics of these spines enable them to maintain an excellent structural flexibility. These changes in shape are associated to neurotransmission modifications on the presynaptic side and enable the spines to adapt their shape according to an increased or decreased stimulus. Therefore, because of their plastic nature, these thin spines are also called "learning spines" [START_REF] Bourne | Do thin spines learn to be mushroom spines that remember?[END_REF][START_REF] Harris | Dendritic spines: cellular specializations imparting both stability and flexibility to synaptic function[END_REF].

b) Stubby spines

Stubby spines are intermediate spines in the process of maturation. These spines are also considered as axon-dendritic contacts. They don't have a mature shape as they do not have a neck at the base of the dendrite (Figure 26). Their length is approximately the same as their head-width.

c) Mushroom spines

Mushroom spines represent approximately 25% of the spines present in the CNS [START_REF] Bourne | Do thin spines learn to be mushroom spines that remember?[END_REF]. These are characteristic of mature spines as they have a defined thin neck, a welldefined bulbous head with a diameter significantly superior to the neck diameter and this head is at least 0.6 µm [START_REF] Bourne | Do thin spines learn to be mushroom spines that remember?[END_REF][START_REF] Sorra | Overview on the structure, composition, function, development, and plasticity of hippocampal dendritic spines[END_REF]. Spine head volume ranges from 0.01 to 0.8 µm 3 [START_REF] Gipson | Structural and functional plasticity of dendritic spines -root or result of behavior?[END_REF].

These spines have the highest concentration of PSD components. Mushroom spines are the active/mature spines that ensure glutamatergic synaptic transmission.

How a thin spine matures into, eventually, a mushroom spine will be described in the following section.

Dendritic spine morphogenesis 3.

Dendritic spines appear early on during post-natal development, after neurite formation. To date, there is no consensus in the scientific community about the formation process spines or spinogenesis. There are currently three models of spine development (Figure 27).

The earliest model (studied on pyramidal neurons) proposes that spine formation is initiated by the axon terminal that makes contact with the dendrite directly. Then, the postsynaptic element develops from this contact point and forms a dendritic spine [START_REF] Miller | Maturation of rat visual cortex. II. A combined Golgi-electron microscope study of pyramidal neurons[END_REF].

The second model came from a study on Purkinje cells [START_REF] Sorra | Overview on the structure, composition, function, development, and plasticity of hippocampal dendritic spines[END_REF], where the proposed hypothesis is that the dendritic spine develops independently from the axon terminal. Dendritic spine would emerge and mature from the dendrite without axonal contact.

The last model, based on pyramidal neuron studies, proposes that filopodia (the most immature form of spine) are the precursors of spines. These filopodia emerge from the dendrite and make contact with a nearby axon. This contact initiates the maturation of both the axon terminal and the dendritic spine. More recent optogenetic and ultrastructure studies seem to support this last hypothesis [START_REF] Hotulainen | Defining mechanisms of actin polymerization and depolymerization during dendritic spine morphogenesis[END_REF][START_REF] Korobova | Molecular Architecture of Synaptic Actin Cytoskeleton in Hippocampal Neurons Reveals a Mechanism of Dendritic Spine Morphogenesis[END_REF]. The size, shape, motility, maturation and stability of dendritic spines depend largely on actin, the primary cytoskeleton within spines. A complex network of regulatory proteins control actin arrangement and spine morphogenesis.

Actin cytoskeleton: the scaffold of dendritic spines E.

Despite having a small volume, dendritic spines are actin-rich protrusions with thousands of proteins, among other molecular components involved indifferent functions [START_REF] Murakoshi | Postsynaptic signaling during plasticity of dendritic spines[END_REF]) (Figure 28). Both monomeric (G-actin) and filamentous (F-actin) actin are present in spines, below the PSD. Whereas spine heads mainly consist of a network of branched and F-actin, in the neck filaments they usually form long bundles lengthwise of the spine apparatus (Hotulainen and Hoogenraad, 2010).

In the last few years, some studies have proposed the existence of different actin pools within the dendritic spine. A dynamic pool is believed to be found below the spine surface, whereas another pool is thought to be more stable to support the overall structure of the spine. A third pool of stable F-actin has been described upon glutamate uncaging or LTP induction [START_REF] Honkura | The Subspine Organization of Actin Fibers Regulates the Structure and Plasticity of Dendritic Spines[END_REF], and its confinement to dendritic spines seems to require CAMKII activity. Ca 2+ /calmodulin-dependent protein kinase II (CaMKII) along with actin are important molecules involved in synapse structure and plasticity. Indeed, in addition to its signalling function, CaMKII plays a structural role via direct interaction with actin filaments, [START_REF] Okamoto | The roles of CaMKII and F-actin in the structural plasticity of dendritic spines: a potential molecular identity of a synaptic tag?[END_REF]) (Figure 28). Actin filaments in the spine head are very dynamic and show a high turnover, with a total protein replacement every 2 -3 minutes [START_REF] Honkura | The Subspine Organization of Actin Fibers Regulates the Structure and Plasticity of Dendritic Spines[END_REF]. Furthermore, previous studies show that the degree of actin polymerisation affects the various aspects of dendritic spine morphology [START_REF] Murakoshi | Postsynaptic signaling during plasticity of dendritic spines[END_REF].

One of the most relevant roles of actin cytoskeleton in mature spines is to modulate spine head structure and size in response to synaptic activity [START_REF] Star | Rapid turnover of actin in dendritic spines and its regulation by activity[END_REF]. Additionally, it contributes to overall structure of synapses, organising the PSD, anchoring and stabilising postsynaptic receptors, localising the translation machinery (Hotulainen and Hoogenraad, 2010) and is said to serve as a molecular sieve to counteract the free diffusion of unbound signalling molecules, possibly in order to favour their interaction with their substrates [START_REF] Macgillavry | The internal architecture of dendritic spines revealed by superresolution imaging: What did we learn so far?[END_REF].

Several actin-binding proteins (ABPs) as well as other actin-associated proteins are enriched in dendritic spines and cooperate to regulate actin-based cellular events (Figure 28). Some of them play a major role in actin nucleation like the Arp2/3 complex, which orchestrates de novo actin polymerisation, and its activators Cortactin, Abp1, N-WASP, WAVE-1 and Abl interactor 2 (Abi2). All of these proteins are essential to spine structure and morphology since the alteration of any of these proteins results in dendritic spine and synaptic structural impairments [START_REF] Bellot | The structure and function of actin cytoskeleton in mature glutamatergic dendritic spines[END_REF].

Other actin cytoskeleton-interacting proteins participate in F-actin severing (ADF/cofiline and gelsolin), actin bundling (calpolin, dystrophin) or actin polymerisation (profilin). Upon NMDAr activation, calcineurin/PP2B causes a dephosphorylation of cofilin through slingshot protein phosphatase 1 (SSH1) activity, and active cofilin is translocated into dendritic spines for spine remodelling [START_REF] Pontrello | Cofilin under control of βarrestin-2 in NMDA-dependent dendritic spine plasticity, long-term depression (LTD), and learning[END_REF]. Thus cofiline is essential in controlling the turnover of F-actin at synapses and a dysregulation F-actin may result in alterations in spine morphology and density, leading to impaired associative learning. Recently, in our research group, cofilin phosphorylation was found to be dysregulated in APP/PS1-21 mice as well as in cultured neurons exposed to Aβ oligomers leading to the formation of aberrant cofilin-actin rods, which impact spine morphological remodelling, block axonal trafficking and thus may contribute to deficits in synaptic plasticity [START_REF] Rush | Synaptotoxicity in Alzheimer's Disease Involved a Dysregulation of Actin Cytoskeleton Dynamics through Cofilin 1 Phosphorylation[END_REF].

Together, these data highlight the pivotal role of actin cytoskeleton in the formation and elimination, motility and stability, size and shape of dendritic spines; making the actin cytoskeleton a major actor of synaptic plasticity.

Synaptic plasticity F.

Synaptic plasticity defines the set of molecular mechanisms that take place within the synapse in order to modify its neurotransmission properties depending on its use. Although very few neurons renew themselves within the brain, synaptic connections, however, undergo many changes that can be short-term or longer-term. Indeed, the brain is capable of positively or negatively modulating the efficacy of synaptic connections in response to neuronal activity. This is an essential characteristic for the formation and function of neuronal networks involved in the molecular mechanisms of learning and memory.

The strength of neurotransmission can be increased or decreased for a few minutes, hours or even up to several months. This is called Long-Term Potentiation or Depression of synaptic function [START_REF] Barnes | Memory deficits associated with senescence: a neurophysiological and behavioral study in the rat[END_REF]. This model is considered as the reference for the study of the molecular and cellular mechanisms of learning and memory consolidation, and has enabled tremendous understanding of the molecular processes underlying cognitive function.

Long-term Potentiation (LTP) 1.

Long-term potentiation (LTP) is the experimental paradigm that reproduces one of the types of synaptic plasticity associated to the molecular events at the origin of memory consolidation processes. It can be induced by high-frequency electrical stimulation of the presynaptic neuron.

In 1973, the research group of Bliss and Lomo showed that high frequency stimulation of the perforant path fibres in the hippocampus induced a persistent increase of synaptic transmission efficacy in the dentate gyrus [START_REF] Bliss | Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path[END_REF]. This has also been evidenced in the connections between the Schaeffer collaterals and the dendritic spines of CA3 and CA1 hippocampal areas [START_REF] Alger | Long-term and short-term plasticity in the CA1, CA3, and dentate regions of the rat hippocampal slice[END_REF]) (Figure 29). LTP is not inherent to hippocampal excitatory synapses as it has also been demonstrated in synapses of many other brain regions such as the amygdala, the cortex and the cerebellum [START_REF] Maroun | Stress reverses plasticity in the pathway projecting from the ventromedial prefrontal cortex to the basolateral amygdala[END_REF][START_REF] Martin | Synaptic plasticity and memory: an evaluation of the hypothesis[END_REF][START_REF] Xin | Role of phosphorylation of ERK in induction and maintenance of LTP of the C-fiber evoked field potentials in spinal dorsal horn[END_REF].

In vivo electrical stimulation in animal models can induce an LTP that persists for several days [START_REF] Douglas | Long-term potentiation of the perforant path-granule cell synapse in the rat hippocampus[END_REF]. Pharmacological modulation of this LTP can affect the learning capacities of these animals, particularly with the use NMDAr antagonist AP-V [START_REF] Morris | Selective impairment of learning and blockade of long-term potentiation by an N-methyl-D-aspartate receptor antagonist, AP5[END_REF]. Indeed, LTP induction depends mostly on synaptic glutamate release and subsequent postsynaptic Ca 2+ entry through glutamate-activated NMDAr, this process in coined NMDA-dependent LTP.

However, LTP can also be induced via voltage-gated calcium channels or via mGluRs (mGluR1,5), this is called NMDA-independent LTP [START_REF] Lanté | Low-frequency stimulation induces a new form of LTP, metabotropic glutamate (mGlu5) receptor-and PKA-dependent, in the CA1 area of the rat hippocampus[END_REF][START_REF] Whitlock | Learning induces long-term potentiation in the hippocampus[END_REF]. 3) which in turn project in CA1 ( 4). This pathway enabled to demonstrate LTP, which has been observed in each of the 3 synaptic connections (2, 3 and 4) represented in this diagram.

During a brief low frequency synaptic transmission (Figure 30), the action potential coming from the axon terminal induces a Ca 2+ influx in the presynaptic compartment. This Ca 2+ influx enables synaptic glutamate-containing vesicle exocytosis and subsequent glutamate release into the synaptic cleft. This glutamate then binds to postsynaptic AMPAr and NMDAr. AMPAr opens and lets Na + ions enter the postsynaptic compartment inducing membrane depolarisation. This enables the expulsion of Mg 2+ blocking NMDAr and this, in turn, induces an ion influx after binding of glutamate and coagonist to NMDAr. This series of events can induce an EPSP (Excitatory postsynaptic potential) but will not induce the prolonged activation of NMDAr necessary for LTP induction. LTP has two phases: the induction phase and the consolidation phase. The induction phase can experimentally reproduced by a high-frequency stimulation of the presynaptic neuron like the ones used in LTP protocol (a short stimulation, less than 1 second, at 100 Hz, for example). This type of stimulation induces a sustained depolarisation of the plasma membrane via AMPAr activation following glutamate release into the synaptic cleft. This provokes Mg 2+ ion expulsion from NMDAr channel (Figure 30). This expulsion allows the passage of massive Ca2+ ion influx through NMDAr, dramatically rising postsynaptic Ca 2+ concentration. NMDAr are therefore activated once glutamate release from axon terminal, glutamate binding to receptors and postsynaptic membrane depolarisation occur concomitantly. NMDAr are therefore the main players in Ca2+ influx during NMDA-dependent LTP [START_REF] Collingridge | Excitatory amino acids in synaptic transmission in the Schaffer collateral-commissural pathway of the rat hippocampus[END_REF].

Intracellular Ca 2+ increase is central during the consolidation phase of LTP. It induces transduction pathway activation leading to potentiation of the postsynaptic compartment. LTP induction is associated with activation of transduction pathways involving protein kinases such as protein kinases PKA and PKC [START_REF] Roberson | Transient activation of cyclic AMP-dependent protein kinase during hippocampal long-term potentiation[END_REF], ERKs [START_REF] Bading | Stimulation of protein tyrosine phosphorylation by NMDA receptor activation[END_REF], PI3K [START_REF] Man | Activation of PI3-kinase is required for AMPA receptor insertion during LTP of mEPSCs in cultured hippocampal neurons[END_REF] and CAMKs [START_REF] Malenka | The impact of postsynaptic calcium on synaptic transmission--its role in long-term potentiation[END_REF]. All these protein kinases participate in the increase of synaptic strength and efficacy. Notably, protein neosynthesis occurs following phosphorylation by the kinases of transcription factors such as CREB [START_REF] Kandel | The molecular biology of memory storage: a dialogue between genes and synapses[END_REF]. Another example is the activation of CAMKII which induces the phosphorylation of GluA1 at serine 831 of non-synaptic AMPAr and subsequently increases their integration to the PSD; therefore increasing both the number of AMPAr on the postsynaptic membrane and their conductance (for review: [START_REF] Lisman | Mechanisms of CaMKII action in long-term potentiation[END_REF].

The expression level of NMDAr subunits also seems to play a role in the establishment of LTP.

Notably, GluN2B subunits give a slow inactivation rate to NMDAr, responsible of longer-lasting current and bestow a longer opening-time of NMDAr, enabling a longer Ca 2+ influx than GluN1/GluN2A. Moreover, NMDAr GluN1/GluN2B interact more with CAMKII implicated in AMPAr and ERF phosphorylation. Several studies support the hypothesis that GluN1/GluN2B-type NMDAr facilitate LTP induction [START_REF] Barria | NMDA receptor subunit composition controls synaptic plasticity by regulating binding to CaMKII[END_REF]. However, others have shown an implication of GluN2A in LTP induction [START_REF] Bellone | Rapid bidirectional switching of synaptic NMDA receptors[END_REF][START_REF] Matta | mGluR5 and NMDA receptors drive the experience-and activity-dependent NMDA receptor NR2B to NR2A subunit switch[END_REF]. Thus, the implication of these different subunits in LTP is still under debate.

Long-term Depression (LTD) 2.

It was in the 80s that Long-term Depression (LTD) was discovered. It is, in a way, the "opposite" mechanism of LTP. It is a type of synaptic plasticity present in the synapses of the cerebellum, the cerebral cortex, the hippocampus and the striatum.

LTD is induced by a low-frequency stimulation of the presynaptic neuron (1 -5 Hz) which provokes a weak to moderate Ca 2+ influx in the postsynaptic neuron following NMDAr activation [START_REF] Lüscher | NMDA receptor-dependent long-term potentiation and long-term depression (LTP/LTD)[END_REF]. NMDAr are responsible for calcic influx during LTP but also during LTD.

However, whilst LTP is induced by a strong depolarisation and a massive calcic influx in the dendritic spine, LTD is induced by a moderate depolarisation and a weak Ca 2+ influx which translates into phosphatase protein activation [START_REF] Mulkey | Involvement of a calcineurin/inhibitor-1 phosphatase cascade in hippocampal long-term depression[END_REF] such as calcineurin and PP1 (protein phosphatase 1). These phosphatases will induce endocytosis of AMPAr from the PSD leading to a reduction of the number of AMPAr at the PSD associated to a decrease in dendritic spine volume.

It has been suggested that this insertion/endocytosis of AMPAr during LTP and LTD occur at the perisynaptic level where the receptors reach the postsynaptic density via lateral diffusion [START_REF] Lüscher | NMDA receptor-dependent long-term potentiation and long-term depression (LTP/LTD)[END_REF].

Dendritic spine dynamics, the basis of synaptic plasticity G.

Actin dynamics in dendritic spines 1.

In dendritic spines, the effective remodelling of the actin cytoskeleton is essential for adequate morphological dynamics. In general, actin filaments in spines are exceptionally short and dynamic [START_REF] Frost | Single-molecule discrimination of discrete perisynaptic and distributed sites of actin filament assembly within dendritic spines[END_REF][START_REF] Hotulainen | Defining mechanisms of actin polymerization and depolymerization during dendritic spine morphogenesis[END_REF][START_REF] Korobova | Molecular Architecture of Synaptic Actin Cytoskeleton in Hippocampal Neurons Reveals a Mechanism of Dendritic Spine Morphogenesis[END_REF][START_REF] Koskinen | Myosin IIb controls actin dynamics underlying the dendritic spine maturation[END_REF][START_REF] Star | Rapid turnover of actin in dendritic spines and its regulation by activity[END_REF].

In terms of turnover rate, mature spines are proposed to have two distinct pools of actin filaments regulating spine shape and dynamics during maturation and activity [START_REF] Honkura | The Subspine Organization of Actin Fibers Regulates the Structure and Plasticity of Dendritic Spines[END_REF][START_REF] Star | Rapid turnover of actin in dendritic spines and its regulation by activity[END_REF]. The stable pool of F-actin occupies 5-20% of total spine F-actin. These stable pools of F-actin have a turnover rate of 17 minutes and are found in structures generally located at the base of the spine head, where they form the structural backbone of the spine [START_REF] Honkura | The Subspine Organization of Actin Fibers Regulates the Structure and Plasticity of Dendritic Spines[END_REF].

The size of the stable pool in dendritic spines increases during neuronal maturation [START_REF] Koskinen | Myosin IIb controls actin dynamics underlying the dendritic spine maturation[END_REF]. Conversely, the dynamic pool of F-actin, which represents 80-95% of total spine F-actin, has a turnover of 40 seconds, approximately 25 times faster than the stable pool. There is also a third pool of filamentous actin with an intermediate turnover rate. This pool appears upon LTP induction and is responsible for spine head growth. Accordingly, this pool was coined the "enlargement pool" [START_REF] Honkura | The Subspine Organization of Actin Fibers Regulates the Structure and Plasticity of Dendritic Spines[END_REF].

The interplay between the actin cytoskeleton and synaptic plasticity 2.

a)

The signalling pathways that regulate F-actin networks LTP and LTD correlate with the enlargement and shrinkage of dendritic spines, respectively (Cingolani and Goda, 2008;Matsuzaki et al., 2004;Zhou et al., 2004). Therefore, the organisation and dynamics of the F-actin network requires to be adjusted accordingly to the strength of synaptic transmission [START_REF] Saneyoshi | The Ca2+ and Rho GTPase signaling pathways underlying activity-dependent actin remodeling at dendritic spines[END_REF]. This readjustment is finely tuned by signalling pathways that regulate F-actin networks during synaptic plasticity. These pathways give rise to a sequence of events that lead to morphological remodelling of dendritic spines where the actin cytoskeleton sustains both spine structure and function (Figure 31).

b) F-actin reorganisation during synaptic plasticity

Early electron-microscopy studies already reported that LTP induction would increase dendritic spine head and PSD [START_REF] Buchs | Induction of long-term potentiation is associated with major ultrastructural changes of activated synapses[END_REF]. More recent studies using two-photon glutamate uncaging (Matsuzaki et al., 2004) and time-lapse STED microscopy [START_REF] Tønnesen | Spine neck plasticity regulates compartmentalization of synapses[END_REF] confirmed these findings and also showed that spine enlargement was accompanied by GluA1 recruitment and increased GluA1-mediated current as well as a widening and shortening of the spine neck.

These changes in spine head volume display two distinct phases:

a transient large volume increase which takes place in the first 5 minutes of LTP and where spine head volume may increase up to 200-400%.

-Followed by long-lasting smaller volume increase for small dendritic spines which takes place within 60 minutes after LTP and where spine volume increases up to 50-150%.

Spine neck changes, however, seem to display only a long-lasting change with an approximate 30% increase in spine neck width and 30% decrease in spine neck length [START_REF] Tønnesen | Spine neck plasticity regulates compartmentalization of synapses[END_REF].

These modifications were abrogated by pharmacological treatments that increased F-actin depolymerisation (latrunculin-A) confirming that F-actin supports transient and long-lasting changes of dendritic spine head.

Honkura et al showed in 2008, using two-photon glutamate uncaging, that single spine activation (which triggered spine enlargement) induced the formation of a transient enlargement F-actin pool distributed throughout the spine head, displaying a distinct turnover compared to the stable and dynamic F-actin pools (Figure 32). The localisation and dynamics of the enlargement pool often synchronised with spine membrane protrusions, as if F-actin polymerisation drove the enlargement.

However, increased volumes triggered by LTP last longer than the life-time of the enlargement pool, suggesting that the long-lasting spine enlargement might be sustained by transfer of F-actin from the enlargement into the stable pool. Similar studies, using an adaptation of the two-photon glutamate uncaging to induce LTD named LFU (Low-frequency uncaging), were performed to explore the effects of LTD on dendritic spine remodelling. Results showed an approximate 25% shrinkage of dendritic spines [START_REF] Oh | Synapse-specific and size-dependent mechanisms of spine structural plasticity accompanying synaptic weakening[END_REF].

Spine shrinkage following LTD was shown to be also F-actin dependent since inhibition of ADF/cofilin, calcineurin or Arp2/3 complex blocked spine shrinkage [START_REF] Hayama | GABA promotes the competitive selection of dendritic spines by controlling local Ca2+ signaling[END_REF][START_REF] Nakamura | PICK1 inhibition of the Arp2/3 complex controls dendritic spine size and synaptic plasticity[END_REF][START_REF] Wang | Independent expression of synaptic and morphological plasticity associated with long-term depression[END_REF]. Therefore, as for LTP, regulation of F-actin organisation and dynamics might be critical mechanisms for structural LTD.

Numerous studies have demonstrated that synaptic activity modifies spine morphology, structure and function. In physiological conditions, dendritic spines adapt to the incoming information.

However this adaptation is usually the front line target in neurodegenerative diseases, particularly in the case of Alzheimer's disease. Indeed, dendritic spine alterations and eventually loss induced by Aβ oligomers is one the first steps of AD pathogenesis and may be at the origin of the cognitive impairments in early AD.

IV. Aβ pathology and excitatory synapses

Extracellular amyloid plaques composed of Aβ peptides are among the principal pathological characteristics observed during AD development. Many other additional structural and functional alterations are also observed such as inflammatory responses and oxidative stress [START_REF] Mcgeer | Inflammation, anti-inflammatory agents and Alzheimer disease: the last 12 years[END_REF]. The combination of all these alterations gives rise to synaptic impairments and eventually loss.

Most studies point towards Aβ accumulation in the brain as the main culprit of these synaptic dysfunctions which are at the basis of disease progression.

Extracellular amyloid plaques were considered as one of the main causative agents of AD and particularly because of cortical and hippocampal amyloid deposits. However, it is now widely accepted that it is in fact the soluble oligomeric Aβ species that are at the origin of synaptic dysfunction and loss, and AD onset rather than extracellular Aβ plaques and NFTs [START_REF] Gong | Alzheimer's disease-affected brain: presence of oligomeric A beta ligands (ADDLs) suggests a molecular basis for reversible memory loss[END_REF][START_REF] Mclean | Soluble pool of Abeta amyloid as a determinant of severity of neurodegeneration in Alzheimer's disease[END_REF]. Studies using certain mutations of the amyloid peptide found in FADs support this hypothesis, especially the Osaka mutation which causes intraneuronal accumulation of Aβ and absence of amyloid plaques while still inducing severe cognitive impairments. This is why using FAD mutation-based forms of Aβ to investigate Aβ-induced effects on neurons are crucial to understand the role of Aβ, not only in AD pathogenesis but also to get an insight into the physiological role of Aβ in the brain.

The impact of Aβ on synaptic transmission B.

Alterations of synaptic activity and cognitive function 3.

At this point in time, the role of the Aβ peptide within synapses and the effects of synaptic activity on the Aβ peptide are some of the centre pieces of the unresolved puzzle in the comprehension of AD.

Notably, it has been shown that synaptic activity increases Aβ secretion in the extracellular space [START_REF] Cirrito | Synaptic activity regulates interstitial fluid amyloid-beta levels in vivo[END_REF][START_REF] Kamenetz | APP processing and synaptic function[END_REF]. It is this extracellular Aβ which then provokes LTP reduction [START_REF] Shankar | Amyloid-beta protein dimers isolated directly from Alzheimer's brains impair synaptic plasticity and memory[END_REF], LTD facilitation and alters the quantity of synaptic proteins [START_REF] Snyder | Regulation of NMDA receptor trafficking by amyloid-beta[END_REF] as well as the structure of the synapse in vitro and in vivo [START_REF] Almeida | Betaamyloid accumulation in APP mutant neurons reduces PSD-95 and GluR1 in synapses[END_REF][START_REF] Coleman | Synaptic slaughter in Alzheimer's disease[END_REF][START_REF] Hsieh | AMPAR removal underlies Abeta-induced synaptic depression and dendritic spine loss[END_REF]Selkoe, 2002). Therefore, it seems that chronic exposure to extracellular Aβ induces synaptic alterations which could lead to the accumulation of Aβ peptides at the origin of extracellular amyloid plaque formation. This suggests that synaptic activity could contribute to AD development and progression. Furthermore, it has also been shown that, after a traumatic brain injury, there is an extracellular accumulation of Aβ during the recovery phase of cognitive function [START_REF] Brody | Amyloid-beta dynamics correlate with neurological status in the injured human brain[END_REF].

Although synaptic activity and cognitive/neuronal activity aren't quite equivalent, studies have shown that cognitive activity stimulation have rather a protective effect against AD progression. Indeed, it seems that a higher education is associated to a reduction in the risk of developing AD [START_REF] Stern | Cognitive reserve in ageing and Alzheimer's disease[END_REF]. Moreover, studies based on APPswe/PS1ΔE9 transgenic AD mice showed that an enriched, cognitively stimulating environment slows down the formation of amyloid plaques and upregulates genes implicated in learning and memory processes [START_REF] Lazarov | Environmental enrichment reduces Abeta levels and amyloid deposition in transgenic mice[END_REF].

Alterations of the number and function of synaptic receptors 4.

Several studies have shown that oligomeric Aβ peptides exert their neurotoxic effects through alterations of glutamatergic neurotransmission. Although electrophysiological studies have brought to light that Aβ oligomers cause synaptic dysfunctions, these peptides also alter glutamatergic receptor function and quantity at the synapse. To support this, Aβ has been found to interact with numerous receptors found at the PSD, such as NMDAr [START_REF] De Felice | Abeta oligomers induce neuronal oxidative stress through an N-methyl-D-aspartate receptor-dependent mechanism that is blocked by the Alzheimer drug memantine[END_REF][START_REF] Decker | N-methyl-D-aspartate receptors are required for synaptic targeting of Alzheimer's toxic amyloid-β peptide oligomers[END_REF], AMPAr and mGluR5 [START_REF] Renner | Deleterious effects of amyloid beta oligomers acting as an extracellular scaffold for mGluR5[END_REF], neurotrophin p75 NTR [START_REF] Kraemer | A role for the p75 neurotrophin receptor in axonal degeneration and apoptosis induced by oxidative stress[END_REF], Prion protein (PrP c ) [START_REF] Laurén | Cellular prion protein mediates impairment of synaptic plasticity by amyloid-beta oligomers[END_REF], glutamate transporter [START_REF] Li | Soluble oligomers of amyloid Beta protein facilitate hippocampal long-term depression by disrupting neuronal glutamate uptake[END_REF], ephrinB2 receptor [START_REF] Cissé | Reversing EphB2 depletion rescues cognitive functions in Alzheimer model[END_REF] and ephrinA4 [START_REF] Fu | Blockade of EphA4 signaling ameliorates hippocampal synaptic dysfunctions in mouse models of Alzheimer's disease[END_REF][START_REF] Vargas | EphA4 activation of c-Abl mediates synaptic loss and LTP blockade caused by amyloid-β oligomers[END_REF] as well as acetylcholine α-7 nicotinic receptor (α7nAChRs) [START_REF] Wang | beta-Amyloid(1-42) binds to alpha7 nicotinic acetylcholine receptor with high affinity. Implications for Alzheimer's disease pathology[END_REF]. It has also been shown that Aβ interacts with other proteins of the PSD such as PSD-95 [START_REF] Pham | Progressive accumulation of amyloid-beta oligomers in Alzheimer's disease and in amyloid precursor protein transgenic mice is accompanied by selective alterations in synaptic scaffold proteins[END_REF]. In pathological conditions, all of these synaptic receptors and proteins constitute potential binding partners to oligomeric forms of Aβ and these interactions may be involved in synaptic dysfunctions and synaptic plasticity alterations (for review: [START_REF] Tu | Oligomeric Aβ-induced synaptic dysfunction in Alzheimer's disease[END_REF].

Alterations of synaptic plasticity 5.

It has been brought to light that synaptic plasticity processes of LTP and LTD are significantly altered by oligomeric forms of the Aβ peptide. Indeed, several studies have shown that LTP was altered/decreased, and LTD was in some cases facilitated, by wild-type oligomeric Aβ as well as mutated toxic forms of Aβ [START_REF] Li | Soluble Aβ oligomers inhibit long-term potentiation through a mechanism involving excessive activation of extrasynaptic NR2Bcontaining NMDA receptors[END_REF][START_REF] Li | Soluble oligomers of amyloid Beta protein facilitate hippocampal long-term depression by disrupting neuronal glutamate uptake[END_REF][START_REF] Selkoe | Soluble oligomers of the amyloid beta-protein impair synaptic plasticity and behavior[END_REF][START_REF] Shankar | Natural oligomers of the Alzheimer amyloid-beta protein induce reversible synapse loss by modulating an NMDA-type glutamate receptor-dependent signaling pathway[END_REF][START_REF] Tomiyama | A new amyloid beta variant favoring oligomerization in Alzheimer's-type dementia[END_REF][START_REF] Walsh | Naturally secreted oligomers of amyloid beta protein potently inhibit hippocampal long-term potentiation in vivo[END_REF]. Furthermore, it has also been shown that the initial spine enlargement phase of LTP is altered in APP overexpressing/Aβ overproducing neurons meaning that structural plasticity is also impaired [START_REF] Wei | Amyloid beta from axons and dendrites reduces local spine number and plasticity[END_REF].

Among the different hypotheses explaining the effects of Aβ oligomers on the mechanisms of synaptic plasticity, one of them proposes that LTP inhibition may be due to internalisation and endocytosis of NMDAr and AMPAr induced by Aβ oligomers. Application of Aβ oligomers on acute hippocampal slices provokes a decrease of AMPAr concentration at the surface of dendritic spines, and more precisely an endocytosis of GluA2 AMPAr, which leads to synaptic transmission depression [START_REF] Hsieh | AMPAR removal underlies Abeta-induced synaptic depression and dendritic spine loss[END_REF][START_REF] Zhao | Inhibition of calcineurin-mediated endocytosis and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors prevents amyloid beta oligomer-induced synaptic disruption[END_REF]. Other studies have shown that incubating neuronal cultures with Aβ oligomers increases NMDAr endocytosis via calcineurin and STEP (Striatal enriched phosphatase) activity [START_REF] Snyder | Regulation of NMDA receptor trafficking by amyloid-beta[END_REF]. Taken together, these studies show that Aβ oligomers induce AMPAr endocytosis, especially GluA1 and GluA2; leading to spine loss and is responsible for reduced NMDAr response [START_REF] Hsieh | AMPAR removal underlies Abeta-induced synaptic depression and dendritic spine loss[END_REF][START_REF] Lacor | Synaptic targeting by Alzheimer's-related amyloid beta oligomers[END_REF][START_REF] Miñano-Molina | Soluble oligomers of amyloid-β peptide disrupt membrane trafficking of αamino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor contributing to early synapse dysfunction[END_REF][START_REF] Shankar | Natural oligomers of the Alzheimer amyloid-beta protein induce reversible synapse loss by modulating an NMDA-type glutamate receptor-dependent signaling pathway[END_REF].

Another hypothesis proposes that the changes observed at the level of synaptic receptors could lead to modifications of intracellular signalling pathways which may be responsible for the effects on synaptic plasticity induced by Aβ oligomers (Figure 33). Notably, excessive activation of GluN2B-type NMDAr by Aβ oligomers could affect the function of certain enzymes, and calcium-dependent protein kinases and phosphatases such as calcineurin, which could eventually lead to synaptic dysfunctions and excitotoxicity due to alterations of certain signalling cascades and mitochondrial homeostasis. According to certain studies, Aβ oligomers alter different kinases and phosphatases such as, for example, GSK3β (Glycogen synthase kinase 3β), Cdk5 (cyclin-dependent kinase 5), members of the MAPK (mitogen-activated protein kinase) family like PAK (p21-activated kinase) or ERF (extracellular-signal-related kinase), AMPK (AMP-activated protein kinase), CaMKII [START_REF] Dolan | The role of tau kinases in Alzheimer's disease[END_REF][START_REF] Mairet-Coello | The CAMKK2-AMPK kinase pathway mediates the synaptotoxic effects of Aβ oligomers through Tau phosphorylation[END_REF][START_REF] Thornton | AMP-activated protein kinase (AMPK) is a tau kinase, activated in response to amyloid β-peptide exposure[END_REF][START_REF] Zhu | Activation of glycogen synthase kinase-3 inhibits long-term potentiation with synapse-associated impairments[END_REF] but also CREB (cAMP response element binding protein) transcription factor. On the other hand, studies based on models which overexpress APP showed that alterations of mitochondrial dynamics, transport and function are associated with synaptic dysfunction and loss [START_REF] Balietti | Early selective vulnerability of synapses and synaptic mitochondria in the hippocampal CA1 region of the Tg2576 mouse model of Alzheimer's disease[END_REF]. This suggests that Aβ may affect mitochondrial function thus leading to intracellular calcium concentration dysregulations.

However, a recent study showed that oligomeric Aβ toxicity could be more NMDAr-dependent rather than calcium flux variations alone, since synaptic alterations are attenuated when NMDAr are blocked by AP-V but not when ion flux are blocked by calcium chelator like BAPTA or when NMDA ion channels are blocked by MK-801 or memantine [START_REF] Birnbaum | Calcium flux-independent NMDA receptor activity is required for Aβ oligomer-induced synaptic loss[END_REF].

Other studies suggest that the effects of Aβ peptides observed on synaptic plasticity may essentially be due to excessive activation of extrasynaptic NMDAr. Activation of these extrasynaptic receptors is thought to alter calcic regulation processes and oxidative mechanisms due to hindered mitochondria. Furthermore, activation of extrasynaptic NMDAr may be responsible for a preferential shift towards the amyloïdogenic pathway [START_REF] Bordji | Activation of extrasynaptic, but not synaptic, NMDA receptors modifies amyloid precursor protein expression pattern and increases amyloid-ß production[END_REF] leading to increased Aβ production.

More recently, a few studies have showed that Aβ oligomers alter glutamate reuptake/recycling thus leading to glutamate leakage outside the synaptic cleft ("glutamate spillover" phenomenon) thus activating extrasynaptic NMDAr with subsequent LTP inhibition and activation of proapoptotic signalling cascades [START_REF] Li | Soluble Aβ oligomers inhibit long-term potentiation through a mechanism involving excessive activation of extrasynaptic NR2Bcontaining NMDA receptors[END_REF][START_REF] Varga | Amyloid-β1-42 Disrupts Synaptic Plasticity by Altering Glutamate Recycling at the Synapse[END_REF]. This extrasynaptic NMDAr recruitment could explain why Aβ oligomers facilitate LTD. Indeed, it has been shown that LTD induction by Aβ peptides induces the preferential activation of GluN2B-type NMDAr, a subpopulation of receptors that are more localised at the extrasynaptic level (Figure 33) [START_REF] Kervern | Selective impairment of some forms of synaptic plasticity by oligomeric amyloid-β peptide in the mouse hippocampus: implication of extrasynaptic NMDA receptors[END_REF].

Other studies have shown that Aβ peptides are also capable of creating pores in the lipid bilayer of membranes which could contribute to the aberrant calcium influx inside neurons [START_REF] Demuro | Calcium dysregulation and membrane disruption as a ubiquitous neurotoxic mechanism of soluble amyloid oligomers[END_REF][START_REF] Lin | Single-cell screening of cytosolic [Ca(2+)] reveals cell-selective action by the Alzheimer's Aβ peptide ion channel[END_REF]. Nevertheless, according to the literature, it seems unlikely that Aβ interaction with membrane receptors and the associated neurotoxic phenomena are the only events responsible for Aβ synaptotoxicity. Indeed, an increasing number of studies carried out on AD animal models as well as in vitro studies on neuronal cultures have highlighted a recurrent issue: the existence of intracellular Aβ inside neurons [START_REF] Grundke-Iqbal | Amyloid protein and neurofibrillary tangles coexist in the same neuron in Alzheimer disease[END_REF] and their potential role in AD pathogenesis (for review: [START_REF] Gouras | Critical role of intraneuronal Aβ in Alzheimer's disease: technical challenges in studying intracellular Aβ[END_REF].

All of these molecular and cellular mechanisms involved in the synaptotoxicity induced by Aβ oligomers constitute a large panel of perturbations affecting NMDAr-dependent signalling pathways, calcium homeostasis, proteins involved in various signalling cascades, and mitochondrial function. All of these Aβ-related defective mechanisms all converge towards a dysfunctional dendritic spine, and as we stated earlier, dendritic spine function and morphology are tightly linked together, meaning that morphological Aβ-related defects are also observed. These morphological alterations are a readout for the dendritic spine functional defect and are reflected through the state of the actin cytoskeleton in the spine.

Figure 33: Schematic diagram of the potential mechanisms of oligomeric Aβ-induced synaptic dysfunction (adapted from Tu et al., 2014). At pathological concentrations, Aβ oligomers may interact with multiple astrocytic, microglial, and neuronal synaptic proteins, including α7-AChRs and NMDARs, triggering a series of synaptotoxic events. These events include aberrant activation of NMDArs (especially NR2B-containing extrasynaptic NMDArs), elevated neuronal calcium influx, calcium-dependent activation of calcineurin/PP2B and its downstream signalling pathways, involving cofilin,GSK-3β, CREB, and MEF2. This results in aberrant redox reactions and severing/depolymerising F-actin, tau-hyperphosphorylation, endocytosis of AMPArs, and eventually leads to synaptic dysfunction and cognitive impairment.

The impact of Aβ on dendritic spine morphology C.

Alterations of the synapse 3.

As well as altering synaptic transmission, Aβ oligomers drastically impact the density and morphology of dendritic spines, the physical support of synapses [START_REF] Androuin | Evidence for altered dendritic spine compartmentalization in Alzheimer's disease and functional effects in a mouse model[END_REF]Dorostkar et al., 2015). This effect is expected since, as we stated earlier, spine morphology reflects the efficacy of synaptic transmission. Furthermore, these dendritic spine structural defects correlate much better with the cognitive impairments observed in AD, compared to all the other AD biomarkers, such as NFTs (for review: [START_REF] Masliah | Synaptic remodeling during aging and in Alzheimer's disease[END_REF]Selkoe, 2002). Numerous studies have established a strong link between functional alterations induced by Aβ oligomers on synaptic receptors and synaptic transmission, and the resulting morphological alterations of dendritic spines [START_REF] Androuin | Evidence for altered dendritic spine compartmentalization in Alzheimer's disease and functional effects in a mouse model[END_REF][START_REF] Frandemiche | Activitydependent tau protein translocation to excitatory synapse is disrupted by exposure to amyloid-beta oligomers[END_REF][START_REF] Miller | Tau phosphorylation and tau mislocalization mediate soluble Aβ oligomer-induced AMPA glutamate receptor signaling deficits[END_REF][START_REF] Shankar | Natural oligomers of the Alzheimer amyloid-beta protein induce reversible synapse loss by modulating an NMDA-type glutamate receptor-dependent signaling pathway[END_REF][START_REF] Talantova | Aβ induces astrocytic glutamate release, extrasynaptic NMDA receptor activation, and synaptic loss[END_REF][START_REF] Um | Alzheimer amyloid-β oligomer bound to postsynaptic prion protein activates Fyn to impair neurons[END_REF]. Aβ oligomers induce this synapse loss whether the oligomers are synthetic, secreted by mutated APP overexpressing cells or extracted from brains of AD patients, whereas monomers and fibrils seem relatively inert [START_REF] Lacor | Synaptic targeting by Alzheimer's-related amyloid beta oligomers[END_REF][START_REF] Lambert | Diffusible, nonfibrillar ligands derived from Abeta1-42 are potent central nervous system neurotoxins[END_REF].

In parallel of this spine number reduction there are also alterations of the expression of several proteins of the synaptic compartments such as proteins associated to presynaptic vesicular membranes like synaptophysine or Tau [START_REF] Arendt | Synaptic degeneration in Alzheimer's disease[END_REF][START_REF] Frandemiche | Activitydependent tau protein translocation to excitatory synapse is disrupted by exposure to amyloid-beta oligomers[END_REF].

Tau protein, which plays an essential role in the assembly and stability of microtubules, is essentially present in axons and more rarely in dendrites, where Tau interacts with Fyn kinase and regulates postsynaptic NMDAr function [START_REF] Ittner | Dendritic function of tau mediates amyloid-beta toxicity in Alzheimer's disease mouse models[END_REF]. Aβ oligomers induce rapid phosphorylation and translocation of Tau to dendritic spines, similar to what is observed following synaptic activation via GABA inhibition with Bic/4-AP protocol [START_REF] Frandemiche | Activitydependent tau protein translocation to excitatory synapse is disrupted by exposure to amyloid-beta oligomers[END_REF]. However, complementary experiments of protein interactions between Tau and actin suggest that this Aβinduced translocation of Tau is unstable as its interaction with actin in the spines is more labile than when the translocation is synaptic-activity-induced [START_REF] Frandemiche | Activitydependent tau protein translocation to excitatory synapse is disrupted by exposure to amyloid-beta oligomers[END_REF]. Furthermore, this spatial translocation of Tau is characterised by a different phosphorylation motif of the protein depending on whether it is in response to synaptic activity or in response to neuronal stimulation via Aβ oligomers. Indeed, synaptic activation induces phosphorylation of tyrosine 205 whereas in presence of Aβ oligomers this phosphorylation is reduced and the serine 404 is phosphorylated [START_REF] Frandemiche | Activitydependent tau protein translocation to excitatory synapse is disrupted by exposure to amyloid-beta oligomers[END_REF]. Miller and colleagues confirmed with cultured neurons from AD mouse models harbouring the Swedish mutation that Tau translocation in spines depends on its phosphorylation [START_REF] Miller | Tau phosphorylation and tau mislocalization mediate soluble Aβ oligomer-induced AMPA glutamate receptor signaling deficits[END_REF]. Therefore, these oligomers are responsible for functional alterations of Tau which modifies its interaction with the actin cytoskeleton and therefore modifies the architecture of the spine.

Alterations of the actin cytoskeleton 4.

Aβ oligomers induce important alterations of the dendritic spines which contribute to synapse loss [START_REF] Penzes | Impaired regulation of synaptic actin cytoskeleton in Alzheimer's disease[END_REF]. Studies have highlighted that dendritic spine morphology is tightly linked to its function. Actin filaments, composed of actin molecules and other actin-binding proteins, form the skeletal network inside dendritic spines which plays a crucial role in spine morphogenesis, maintenance and plasticity. Oligomeric forms of Aβ induce dendritic spine loss [START_REF] Shankar | Natural oligomers of the Alzheimer amyloid-beta protein induce reversible synapse loss by modulating an NMDA-type glutamate receptor-dependent signaling pathway[END_REF] which explains the reduction of spine density observed in AD patients' brains as well as in transgenic animal models [START_REF] Knobloch | Dendritic spine loss and synaptic alterations in Alzheimer's disease[END_REF].

The mechanisms at the origin of spine loss are still unclear. Nevertheless, one the principal hypothesis of oligomeric Aβ-induced synaptotoxicity suggests that spine collapse is NMDAdependent. Indeed, NMDA blockade is sufficient to induce spine loss similar to what is obtained in presence of Aβ oligomers. Spine loss due to the presence of Aβ oligomers can be abrogated by treatment with antibodies against Aβ or treatment with molecules that prevent Aβ aggregation, but also require a signalling cascade involving NMDAr, calcineurin and cofilin [START_REF] Shankar | Natural oligomers of the Alzheimer amyloid-beta protein induce reversible synapse loss by modulating an NMDA-type glutamate receptor-dependent signaling pathway[END_REF]. These findings suggest that Aβ oligomers may activate NMDA-dependent signalling pathways which promote LTD induction. Aβ oligomers may activate or inhibit NMDAr which induces the activation of a calcium-dependent kinase, calcineurin. This phosphatase may in turn dephosphorylate cofilin (ie: activate this filamentous actin depolymerising protein). Subsequently, cofilin dephosphorylation by calcineurin induces progressive depolymerisation of actin filaments which structure the spine [START_REF] Cheng | Amyloid beta-protein fragments 25-35 and 31-35 potentiate long-term depression in hippocampal CA1 region of rats in vivo[END_REF][START_REF] Liu | Truncation and activation of calcineurin A by calpain I in Alzheimer disease brain[END_REF][START_REF] Rush | Synaptotoxicity in Alzheimer's Disease Involved a Dysregulation of Actin Cytoskeleton Dynamics through Cofilin 1 Phosphorylation[END_REF]. These Aβ oligomers may, therefore, provoke dendritic spine loss by affecting NMDAr which may alter postsynaptic calcium influx and subsequent modifications of downstream signalling pathways ultimately leading to spine weakening, loss and eventually decrease in dendritic spine density [START_REF] Shankar | Natural oligomers of the Alzheimer amyloid-beta protein induce reversible synapse loss by modulating an NMDA-type glutamate receptor-dependent signaling pathway[END_REF].

All together, these studies carried out on the mechanistic action of Aβ seem to indicate that actin disruption is in part responsible for the observed effects, induced by Aβ, on spine structure, morphology and density. The effect of Aβ oligomers seems also to occur through NMDAr perturbations and cause downstream signalling pathway modifications. Therefore, there is no consensus on the action of these oligomers neither on dendritic spines or synaptic neurotransmission nor on the mechanism of action depending on the localisation of these Aβ oligomers, whether there are intracellular or extracellular.

Intracellular vs extracellular Aβ D.

Quite interestingly, so far, all the described effects of Aβ oligomers on spine function and morphology depend on the presence of Aβ oligomers in the extracellular space, whether they are exogenously added or secreted by overproducing cells. However, numerous studies have confirmed the presence of intraneuronal Aβ (Billings et al., 2005;[START_REF] Gouras | Critical role of intraneuronal Aβ in Alzheimer's disease: technical challenges in studying intracellular Aβ[END_REF][START_REF] Gouras | Intraneuronal beta-amyloid accumulation and synapse pathology in Alzheimer's disease[END_REF][START_REF] Gouras | Intraneuronal Abeta accumulation and origin of plaques in Alzheimer's disease[END_REF][START_REF] Gouras | Intraneuronal Abeta42 accumulation in human brain[END_REF][START_REF] Grundke-Iqbal | Amyloid protein and neurofibrillary tangles coexist in the same neuron in Alzheimer disease[END_REF]LaFerla et al., 2007;[START_REF] Tomiyama | A new amyloid beta variant favoring oligomerization in Alzheimer's-type dementia[END_REF].

Intraneuronal Aβ accumulation constitutes an essential and determining event in AD pathogenicity. More and more studies support the concept that intracellular Aβ oligomers play a critical role in the development of synaptic impairments at the origin of characteristic AD cognitive deficits (Billings et al., 2005;[START_REF] Gouras | Critical role of intraneuronal Aβ in Alzheimer's disease: technical challenges in studying intracellular Aβ[END_REF][START_REF] Gouras | Intraneuronal beta-amyloid accumulation and synapse pathology in Alzheimer's disease[END_REF][START_REF] Gouras | Intraneuronal Abeta accumulation and origin of plaques in Alzheimer's disease[END_REF][START_REF] Gouras | Intraneuronal Abeta42 accumulation in human brain[END_REF]LaFerla et al., 2007;[START_REF] Mohamed | Aβ internalization by neurons and glia[END_REF][START_REF] Mori | Intraneuronal Abeta42 accumulation in Down syndrome brain[END_REF][START_REF] Oddo | Triple-transgenic model of Alzheimer's disease with plaques and tangles: intracellular Abeta and synaptic dysfunction[END_REF][START_REF] Sheng | Lipopolysaccharide-inducedneuroinflammation increases intracellular accumulation of amyloid precursor protein and amyloid beta peptide in APPswe transgenic mice[END_REF]. This concept was initially difficultly accepted by the scientific community since the first studies demonstrating the presence of intracellular Aβ were carried out three decades ago [START_REF] Grundke-Iqbal | Amyloid protein and neurofibrillary tangles coexist in the same neuron in Alzheimer disease[END_REF] and the antibodies used at the time were not capable of differentiating Aβ from APP inside the neurons.

Intracellular Aβ accumulation: an early event in AD 3.

It is now more widely accepted that intracellular Aβ accumulation constitutes an early event in humans and animal models of AD. Studies have brought to light that intracellular Aβ accumulation precedes NFT and senile plaque apparition (D'Andrea et al., 2001;[START_REF] Gouras | Intraneuronal Abeta42 accumulation in human brain[END_REF]. Furthermore, LTP defects and cognitive impairments which appear in the triple transgenic AD mouse model correlate with the presence of intracellular Aβ, before the apparition of NFTs or senile plaques (Billings et al., 2005;[START_REF] Oddo | Triple-transgenic model of Alzheimer's disease with plaques and tangles: intracellular Abeta and synaptic dysfunction[END_REF].

Forms of intracellular Aβ oligomers 4.

Concerning the specific form of Aβ peptide that accumulates inside neurons, several studies have investigated, by immunocytochemistry, which oligomeric form of Aβ was mainly present inside the cells. Using antibodies against the C-terminus of Aβ 40 or Aβ 42 , these studies showed that it is mostly the oligomeric Aβ 42 peptide which accumulates intracellularly (D'Andrea et al., 2001;Echeverria and Cuello, 2002;[START_REF] Gouras | Intraneuronal Abeta42 accumulation in human brain[END_REF][START_REF] Näslund | Correlation between elevated levels of amyloid beta-peptide in the brain and cognitive decline[END_REF][START_REF] Ohyagi | Intraneuronal amyloid β42 enhanced by heating but counteracted by formic acid[END_REF]Tabira et al., 2002).

However, a recent study showed that not only oligomeric Aβ can be found inside neurons, but also monomeric, aggregated and fibrillary forms can also be found intracellularly [START_REF] Pickett | Non-Fibrillar Oligomeric Amyloid-β within Synapses[END_REF].

Intraneuronal localisation of Aβ and consequences of its accumulation 5.

A study carried out on an AD transgenic mouse model (APP/PS1) showed via high-resolution imaging techniques that Aβ accumulates in the synaptic cleft and inside synapses, at the presynaptic terminal but even more so at the postsynaptic terminals close to amyloid plaques [START_REF] Pickett | Non-Fibrillar Oligomeric Amyloid-β within Synapses[END_REF]. These results are in accordance with the general idea that both pre-and postsynaptic compartments are functionally affected by Aβ oligomers, and that potsynaptic terminals play a particularly important role in intracellular Aβ-induced synaptic loss. Studies carried out cultured neurons also demonstrate the existence of intracellular Aβ accumulation and associated synaptic dysfunctions [START_REF] Greenfield | Endoplasmic reticulum and trans-Golgi network generate distinct populations of Alzheimer beta-amyloid peptides[END_REF][START_REF] Skovronsky | Detection of a Novel Intraneuronal Pool of Insoluble Amyloid β Protein that Accumulates with Time in Culture[END_REF][START_REF] Takahashi | Oligomerization of Alzheimer's beta-amyloid within processes and synapses of cultured neurons and brain[END_REF]. Particularly, data show that oligomeric Aβ, Aβ 40 and Aβ 42 can accumulate in several intracellular compartments (such as the endoplasmic reticulum, Golgi complex, late endosomes, mitochondria and exocytosis vesicles) but also in the cytosol (in this case Aβ is rather found as "clusters") [START_REF] Zheng | Intracellular Localization of Amyloid-β Peptide in SH-SY5Y Neuroblastoma Cells[END_REF]. This intracellular accumulation of Aβ may be responsible for synaptic dysfunction [START_REF] Meyer-Luehmann | Exogenous induction of cerebral betaamyloidogenesis is governed by agent and host[END_REF][START_REF] Takahashi | Oligomerization of Alzheimer's beta-amyloid within processes and synapses of cultured neurons and brain[END_REF][START_REF] Takahashi | Intraneuronal Alzheimer abeta42 accumulates in multivesicular bodies and is associated with synaptic pathology[END_REF] leading to possible more severe cellular pathologies. Indeed, it is now known that Aβ accumulation inside neurons precedes degenerative events in all animal models presenting neuronal loss and synaptic dysfunction [START_REF] Wirths | Neuron loss in transgenic mouse models of Alzheimer's disease[END_REF]. Several studies have shown that intracellular Aβ accumulation coincided with electrophysiological and behavioural perturbations characteristic of AD animal models (Billings et al., 2005;[START_REF] Knobloch | Dendritic spine loss and synaptic alterations in Alzheimer's disease[END_REF]. Furthermore, electrophysiological studies based on primary cultured neurons and acute hippocampal brain slices of AD mouse models, harbouring an APP mutation, show that synaptic activity increases the production/release of extracellular Aβ but reduces intracellular Aβ [START_REF] Tampellini | Synaptic activity reduces intraneuronal Abeta, promotes APP transport to synapses, and protects against Abeta-related synaptic alterations[END_REF]. Also, Tampellini and collaborators show that despite intra-and extracellular Aβ toxicity, under synaptic activation PSD-95 protein expression in transgenic mice is restored to control levels. These data, therefore, suggest a protective role of synaptic activity against synaptic defects induced by Aβ. To conclude, the effect of synaptic activity on the intracellular pool of Aβ is a key element in understanding AD pathogenesis.

The relationship between intra-and extracellular Aβ E.

Whether a focus is made on the possible mechanisms explaining the origin of an intracellular and extracellular pool of Aβ or on the effects of these two pools at the neuronal and synaptic level, any studies converge towards the importance of the particularly evident interconnection between the intracellular and extracellular Aβ pools.

Origin of intracellular Aβ 3.

The intraneuronal Aβ pool may originate from an APP cleavage within the neurons on one hand, or from an internalisation of extracellular Aβ on the other. Although both of these mechanisms are distinct and regulated differently, understanding which of these mechanisms is most important or contributes the most to AD pathogenicity may highlight crucial information in order to identify new potential therapeutic targets for treating AD.

On one hand, several studies have described an in situ intraneuronal production of Aβ 1-42 [START_REF] Greenfield | Endoplasmic reticulum and trans-Golgi network generate distinct populations of Alzheimer beta-amyloid peptides[END_REF][START_REF] Nathalie | Processing of amyloid precursor protein and amyloid peptide neurotoxicity[END_REF]) and its intracellular accumulation, which have been reviewed by several [START_REF] Bayer | Intracellular accumulation of amyloid-Beta -a predictor for synaptic dysfunction and neuron loss in Alzheimer's disease[END_REF]LaFerla et al., 2007;[START_REF] Mohamed | Aβ internalization by neurons and glia[END_REF].

On the other hand, other studies converge towards a mechanism involving an internalisation of Aβ from the extracellular pool (D'Andrea et al., 2001;LaFerla et al., 2007;[START_REF] Ohyagi | Intraneuronal amyloid β42 enhanced by heating but counteracted by formic acid[END_REF].

Notably, it has been shown that cells treated with synthetic Aβ peptides exhibited an intracellular accumulation of Aβ 42 and this accumulation was prohibited when endocytosis was blocked [START_REF] Knauer | Intracellular accumulation and resistance to degradation of the Alzheimer amyloid A4/beta protein[END_REF]. However, another possible mechanism of Aβ internalisation has been suggested such as the passive diffusion of extracellular Aβ through the plasma membrane [START_REF] Li | Moderate reduction of gamma-secretase attenuates amyloid burden and limits mechanism-based liabilities[END_REF]. Other studies showed that the mechanism of Aβ internalisation could occur through Aβ binding to α7nACh nicotinic receptors followed by endocytosis of the complex resulting in inhibition of this receptor function and consequently leading to cognitive and memory defects [START_REF] Dziewczapolski | Deletion of the α7 Nicotinic Acetylcholine Receptor Gene Improves Cognitive Deficits and Synaptic Pathology in a Mouse Model of Alzheimer's Disease[END_REF][START_REF] Wang | beta-Amyloid(1-42) binds to alpha7 nicotinic acetylcholine receptor with high affinity. Implications for Alzheimer's disease pathology[END_REF]. It has also been suggested that Aβ may be internalised in the cells of CA1 region of the hippocampus but not the other hippocampal regions and this intracellular accumulation of Aβ may be responsible for a reduction of PSD-95 and GluR1 synaptic expression, resulting in synaptic activity perturbations [START_REF] Almeida | Betaamyloid accumulation in APP mutant neurons reduces PSD-95 and GluR1 in synapses[END_REF][START_REF] Bahr | Amyloid beta protein is internalized selectively by hippocampal field CA1 and causes neurons to accumulate amyloidogenic carboxyterminal fragments of the amyloid precursor protein[END_REF]. Others have brought to light the role of integrins and NMDAr in Aβ internalisation by showing a facilitation of Aβ internalisation with the use of integrin antagonists but an inhibition of internalisation with NMDA antagonists [START_REF] Bi | Uptake and pathogenic effects of amyloid beta peptide 1-42 are enhanced by integrin antagonists and blocked by NMDA receptor antagonists[END_REF][START_REF] De Felice | Abeta oligomers induce neuronal oxidative stress through an N-methyl-D-aspartate receptor-dependent mechanism that is blocked by the Alzheimer drug memantine[END_REF].

Together these data bring to light the dynamic relationship that seems to exist between these two pools of Aβ and the potential detrimental role of this relationship.

Functional relationship between the intra-and extracellular Aβ pools 4.

Aβ internalisation from the extracellular medium and intraneuronal Aβ production from APP cleavage seem to be linked by what we could call an "autocatalytic vicious circle", meaning the intracellular accumulation of Aβ 42 induces an overproduction of neosynthesised Aβ 42 inside neurons (for review: Mohamed and Posse de Chaves, 2011).

Accumulating evidence supports the hypothesis of this mechanism. Indeed, it has been shown that extracellular application of Aβ 42 on cultures of human embryonic kidney cells (HEK) increases the intracellular production of neosynthesised Aβ 42 [START_REF] Yang | Intracellular accumulation of insoluble, newly synthesized abetan-42 in amyloid precursor protein-transfected cells that have been treated with Abeta1-42[END_REF].

Another study carried out on hippocampal neurons has brought to light, through electrophysiological and confocal imaging methods, the major role of Aβ 42 intracellular accumulation in the alterations of synaptic glutamatergic transmission [START_REF] Ripoli | Intracellular accumulation of amyloid-β (Aβ) protein plays a major role in Aβ-induced alterations of glutamatergic synaptic transmission and plasticity[END_REF]. This study showed that Aβ 42 synaptotoxicity occurs independently from an interaction with membrane receptors and brings forward the hypothesis of an internalisation from the extracellular space and intracellular accumulation which play a significant role in synaptic perturbations. Notably, via a fluorescent marking of Aβ 42 perfused in the extracellular medium of neuronal cultures, it has been shown that this fluorescent marking was found inside the somatic and dendritic compartments of the cultured neurons within 20 minutes. When a variant of Aβ 42 (Aβ 42

MO

), oxidised on methionine 35 residue preventing it from crossing the plasma membrane [START_REF] Ripoli | Effects of different amyloid β-protein analogues on synaptic function[END_REF], was applied on the cultured neurons, it was still located in the extracellular space after 20 minutes and did not affect synaptic neurotransmission or plasticity. However, when this variant was directly applied inside the neurons, via the electrophysiological recording pipette, it induced synaptic depression and inhibited LTP in a similar fashion than intracellular Aβ 42 . This study suggests that the synaptotoxic effects of intracellular Aβ requires its internalisation and depends on its interaction with intracellular partners [START_REF] Ripoli | Intracellular accumulation of amyloid-β (Aβ) protein plays a major role in Aβ-induced alterations of glutamatergic synaptic transmission and plasticity[END_REF].

Regarding familial mutations of AD, a study recently evidenced an important intracellular increase of Aβ 42 /Aβ 40 ratio associated to a PS2 mutation. This study showed the existence of a specificity of action of the γ-secretase complex depending on its localisation in the cell and its PS1/PS2 subunit composition. The PS1/γ-secretase complex is ubiquitously distributed throughout the cell and notably at the plasma membrane. However, the PS2/γ-secretase complex is preferentially located in late endosomes/lysosomes where it will target its substrate in these compartments then generate an intracellular pool of Aβ peptides, especially of the Aβ 42 isoforms [START_REF] Sannerud | Restricted Location of PSEN2/γ-Secretase Determines Substrate Specificity and Generates an Intracellular Aβ Pool[END_REF].

Consequently, this data suggests that it is the restricted localisation of PS2 in late endosomes/lysosomes which contributes to the generation of the intracellular Aβ pool associated to AD development [START_REF] Bayer | Intracellular accumulation of amyloid-Beta -a predictor for synaptic dysfunction and neuron loss in Alzheimer's disease[END_REF][START_REF] Gouras | Intraneuronal beta-amyloid accumulation and synapse pathology in Alzheimer's disease[END_REF].

Furthermore, this discovery of the Osaka mutation (APP-ΔE693), a mutation found in a Japanese pedigree about a decade ago, reinforce the importance of the role of intracellular Aβ in the mechanisms of synaptotoxicity observed in AD [START_REF] Tomiyama | A new amyloid beta variant favoring oligomerization in Alzheimer's-type dementia[END_REF]. This mutation induces early disease onset (45±4 years of age). Animal models harbouring this mutation have been generated and enable the analysis of the role of intracellular Aβ. According to the studies carried out on these transgenic animals, the synaptic perturbations are mainly caused by intracellular soluble Aβ oligomers.

Together these data highlight the importance of the interplay between intra-and extracellular pools of Aβ in AD pathogenicity and especially bring forward the underrated intracellular Aβ pool as a key player in synaptotoxicity setup. When many deleterious consequences of extracellular Aβ on glutamatergic neurotransmission have already been identified; one more repercussion of this Aβ secretion outside the neuron still needs to be outlined and that is its effect on surrounding neurons.

Aβ secretion and spreading of the disease in the brain 5.

Many studies have focused on the effects and consequences of Aβ treatment/overproduction on neurons as a cell-autonomous mechanism. Indeed, in most cases, observations and data were acquired from what we would call "pathological neurons", the effects are observed on the cell that is overexpressing/overproducing Aβ. However, one very important part of the story of AD pathogenesis, especially in the cases of sporadic AD, still needs to be clarified; that is how the disease spreads throughout the brain. Some have investigated the effect of Aβ secretion of one neuron on a neighbouring "healthy neuron" (that does not overproduce Aβ). Indeed, it has been shown in organotypic hippocampal mouse brain slices that one "healthy neuron" surrounded by "infected neurons" (neurons overexpressing APPswe) has a significant decrease of NMDA and AMPA transmission compared to a "healthy neuron" surrounded by "non-infected neurons" [START_REF] Kamenetz | APP processing and synaptic function[END_REF]. Meaning that Aβ may act intercellularly, where an infected neuron induces synaptic depression onto a healthy nearby neuron and this seems to occur in a concentration-dependent manner of Aβ [START_REF] Kamenetz | APP processing and synaptic function[END_REF].

Other studies have shown that Aβ that is secreted from one neuron, whether it is dendritic or axonal Aβ, decreases spine density of nearby healthy neurons and alters the structural synaptic plasticity of the healthy neuron's remaining spines by hindering LTP-induced spine enlargement [START_REF] Wei | Amyloid beta from axons and dendrites reduces local spine number and plasticity[END_REF]. It has also been shown, as previously discussed, that extracellular Aβ may be internalised by a cell and cause an increase of intracellular neosynthesised Aβ (for review [START_REF] Mohamed | Aβ internalization by neurons and glia[END_REF][START_REF] Yang | Intracellular accumulation of insoluble, newly synthesized abetan-42 in amyloid precursor protein-transfected cells that have been treated with Abeta1-42[END_REF].

Interestingly, the evidence accumulated of the effects of "pathological neurons" on "healthy neurons" is very similar to what is observed on the pathological neuron itself. Together these studies bring to light a potential neuron-to-neuron spreading mechanism of the disease whereby: one Aβ overproducing neuron alters a healthy nearby neuron's spine density, structural plasticity, neurotransmission and induces an intracellular increase of neosynthesised Aβ within that healthy nearby neuron. The healthy nearby neuron subsequently becomes pathological and, in turn, may affect other healthy nearby neurons. This initial mechanism could be, in physiological conditions where Aβ levels are stable, a negative feedback loop to keep neuronal activity in check [START_REF] Kamenetz | APP processing and synaptic function[END_REF] but in conditions of excess Aβ this mechanism could become substantially problematic, ultimately leading to synaptic defects and cognitive impairments. This spreading of the disease is reminiscent of a "prion-like" mechanism.

Together these studies point at the hypothesis that the synaptotoxic phenomena induced by Aβ oligomers are due to a set of events involving i) the intra-and extracellular localisation of Aβ oligomers as well as ii) the concentration, sequence thus conformation, of these oligomers. It is, therefore, crucial to identify the events that are at the origin of Aβ 42 internalisation and/or overproduction, and the intracellular interacting partners responsible for the synaptotoxic effects observed in AD as well as the mechanisms of spreading of the pathology in the brain. More detailed studies are needed to identify the intracellular molecular targets in order to bring to light new therapeutic strategies which would prevent and/or alleviate the synaptic and cognitive alterations of AD.

V. The research project

In view of the bibliographical data aforementioned, several problematics subside, particularly in the case of the toxic species of Aβ since various studies show conflicting results depending on the species and/or concentration of Aβ oligomers used. Among the different subjects that still need to be addressed, the extra-and/or intracellular origin of Aβ and the pathway by which Aβ induces its synaptotoxic effects are some of the most controversial subjects to date. Many studies have highlighted spine loss as one of the major defects observed in consequence of excess and/or toxic Aβ, and most likely explain AD onset and the subsequent cognitive impairments. However, less focus has been made on the state of the remaining spines of Aβ-burdened neurons and the way this synaptotoxicity is sustained as it propagates throughout the brain. It is well accepted that Aβ disrupts the synaptic actin cytoskeleton organisation, ultimately leading to spine degeneration; however the mechanistic behind this phenomenon is still unclear. Also, most studies converge towards the hypothesis that propagation of the disease occurs via extracellular Aβ which can be internalised then re-exocytosed and re-internalised by other neurons. Some data suggests that extracellular Aβ causes an increase of neosynthesised intracellular Aβ which could affect neighbouring neurons in a "prionlike" mechanism by which Aβ oligomers induce Aβ overproduction which propagates from neuron to neuron.

Indeed, several steps within this "auto-catalytic vicious circle" of Aβ internalisation from the extracellular space and intraneuronal Aβ production need a more in-depth analysis. Shedding some light on the driving force of amyloïdogenic APP processing and the subsequent impact of its proteolytic derivatives on excitatory synapses, in pathological but also possibly in physiological conditions, therefore embodies a critical and essential step in order to improve our comprehension of the molecular mechanisms leading to synaptic dysfunction in AD.

Furthermore, accumulating evidence show that Aβ may take various conformations, and aggregation properties depending on its mutation, and therefore takes on a more or less toxic role depending on this sequence. Investigating the common denominator that is affected by all these different variants of Aβ, which all lead to synaptic defects and subsequent cognitive impairments, is crucial. Indeed, with the rising notion that one individual with AD may in fact have several strains of misfolded Aβ peptides within the brain and the recent failings of β-secretase inhibition and immunotherapies in clinical trials; it is critical, at this point, to shed some light on this potential common denominator in order to better understand AD pathogenesis and find new therapeutic targets.

In this thesis project, we investigated the synaptotoxic effects induced by intra-and/or extracellular Aβ and the possible mechanism behind disease propagation in the brain, using different variants of APP. The reasoning is that most studies on AD have used synthetic human Aβ oligomers at very variable concentrations, often leading to debatable results. In our approach we bypass these uncertainties by using different mutants of APP which give rise to Aβ peptides with unique molecular signatures. This strategy enables us to study the impact of intracellular and secreted (extracellular)

Aβ on the morphology and function of dendritic spines in conditions closer to the physiopathology of AD.

 The different mutants of APP used

In our study we focused on 3 mutants of Human neuronal APP 695 that generate different pools of Aβ but all induce a very aggressive form of AD and one mutant which, allegedly, protects against AD.

Firstly, there is APP 695 wild-type which itself induces synaptic alterations such as loss of synapses as well as long term plasticity defects when it is overexpressed. Secondly, we selected the well-studied Swedish mutation (K670N/M671L) which leads to an increased production of Aβ secreted into the extracellular space, favouring the formation amyloid plaques. The third pathologic variant we have chosen is the Osaka mutation (E693Δ). This particular mutation leads to an intracellular accumulation of Aβ and an absence of amyloid plaques in brains of patients as well as in animal models harbouring this mutation. Lastly, the final variant used carries the Icelandic mutation which, unlike the other mutants, produces an Aβ peptide which allegedly protects against AD and promotes better cognitive aging.

 Investigating Aβ-induced dendritic spine structural plasticity alterations

As the excitatory synapse is one of the frontline targets of Aβ peptides and dendritic spine morphology and function are tightly related, we looked into the impact of intra-and/or extracellular and non-toxic Aβ on spine morphology.

To do so, we studied spine density and spine volume. First this analysis was carried out in an AD transgenic mouse model harbouring the Swedish mutation in order to highlight the defects observed.

Then we assessed whether these defects were transposable to cultured cortical neurons overproducing the various Aβ mutants. This was carried out in both resting conditions as well and conditions of synaptic activity.

 A look into activity-dependent actin dynamics and the presence of Aβ Since the actin cytoskeleton frames the spine and reorganises itself following synaptic activation,

and it has been reported that this mechanism is impaired in AD, we analysed dendritic spine actin dynamics in cultured cortical neurons overexpressing the various mutants of APP, in resting conditions and following synaptic activation.

This allowed us to evaluate the impact of our different variants of Aβ on structural plasticity of the remaining spines, and provide a potential synaptotoxic target of intracellular Aβ: the actin cytoskeleton.

 Synaptic activity as a modulator of both spine plasticity and APP processing

As synaptic transmission shapes the dendritic spine depending on its activity and this mechanism is influenced by the presence of Aβ. We next assessed whether synaptic activity may also modulate APP processing and subsequent Aβ production in neurons overexpressing the various mutants of APP, using a β-secretase inhibitor. This would shed some light on the possible mechanistic behind the "auto-catalytic vicious circle" of Aβ.

 Exploring a possible direct functional link between Actin and Aβ

Since activity-dependent actin dynamics was disrupted in spines due to the presence of Aβ we questioned a possible interaction between these two partners. After identification of a potential actin-binding sequence on Aβ, a subsequent mutant of the peptide was generated which, allegedly, could no longer interact with actin.

Spine morphology and activity-dependent structural plasticity and APP processing was also investigated using this new variant of Aβ coined Aβ3M.

 Aβ synaptotoxicity: sequence vs concentration

It has been reported that the synaptotoxic effects of Aβ are due to excess Aβ production and conversely, that the neuroprotective effect of some Aβ mutants (Icelandic) are due to a decrease in Aβ production. Here we analysed the effects of equal "pathological" concentrations of our various toxic and non-toxic Aβs on dendritic spine morphology and plasticity to assess Aβ sequence vs Aβ concentration.

Introduction

Alzheimer's disease (AD) is defined as a neurodegenerative disorder where neuronal defects lead to neuronal loss and concurrent memory impairments. As first described in the 1900s, two histopathological signatures can be found in the brain, namely intracellular neurofibrillary tangles and extracellular senile plaques composed of β-amyloid (Aβ) peptides 2 .

Aβ is generated by the amyloïdogenic proteolytic cleavage of type I trans-membrane Amyloid

Precursor Protein (APP) by β-secretase (BACE) and γ-secretase sequentially 3 . Although APP mainly matures through the canonical secretory pathway, beginning in the endoplasmic reticulum (ER) to the Golgi apparatus, where it undergoes post-translational modifications before vesicular transport to the plasma membrane; it may also be directly processed in the ER/intermediate compartment. Aβ generated in the ER is essentially Aβ 42 and is not appointed for secretion 4 , suggesting the existence of several pools of Aβ produced by neurons: a secreted extracellular pool and another pool which accumulates intracellularly 5 . Furthermore, Aβ peptides have the capacity to aggregate into multiple forms of oligomers, and eventually into protofibrils and fibrils 5 . It is now well-established that Aβ toxicity and subsequent cognitive impairments are due to oligomeric forms of Aβ 6 . However, the pathway by which they induce their deleterious effects, whether it is initiated by intra-and/or extracellular Aβ pools, is still unclear. Indeed, several studies have shown that extracellular Aβ oligomers hinder learning and memory processes on many levels such as impairing long-term potentiation, decreasing glutamatergic synaptic transmission and altering synapse morphology 7-12 . In contrast, other studies have demonstrated that similar memory defects in various transgenic AD models precede, if not exclude, extracellular Aβ accumulation into plaques 13-15 . Intracellular Aβ accumulation and associated cognitive deficits have also been observed in human AD brains 16,17 suggesting that this could be a main feature in AD memory and learning impairments.

Dendritic spines are the highly heterogeneous postsynaptic compartments of neurons, varying in size and number, and their architecture depends mainly on their dynamic actin cytoskeleton 18 . Spine shape has been broadly categorised as "mushroom", "thin" or "stubby". Although electronmicroscopy studies tend to show more of a continuum between these categories, there is growing evidence that different spine morphologies reflect different developmental stages and/or altered strength of synapses whereby thin and stubby spines are the least -and mushroom spines are the most -mature and functional 19,20 . Dendritic spine morphology and function are therefore intimately linked 21 and this relationship has been shown to be dysfunctional in AD, ultimately leading to synapse and, eventually, neuronal loss 22,23 . It has been proposed that AD is in fact a synaptopathy 24 and studying dendritic spine morphological and functional alterations could shed some light on the underlying mechanisms of AD pathogenesis.

Despite familial AD (FAD) only representing less than 1% of all AD cases, these rare hereditary forms, mostly due to mutations on APP and γ-secretase's PS1/PS2 genes, provide crucial insight into the mechanistic of sporadic AD onset and progression 25 .

Thus, in this study, to gain more insight on Aβ synaptotoxicity, we used several mutated forms of APP which give rise to Aβ peptides with unique molecular signatures, such as the well-studied Swedish mutation (K670M/N671L) (APP swe ) which increases secreted (extracellular) Aβ 1 ; the Osaka mutation (E693Δ) (APP osa ) which causes intraneuronal (intracellular) accumulation of Aβ 17 ; and the Icelandic mutation (A673T) (APP ice ) which has been reported to decrease Aβ production and protect against AD 26 . These mutated forms of APP were overexpressed in cultured mouse cortical neurons in order to study spine morphology and function. Our results will allow a deeper understanding of the mechanisms leading to synaptotoxicity and disease propagation throughout the brain and bring to light that Aβ sequence is as important in inducing synaptotoxicity as Aβ concentration.

Materials and Methods

Transgenic animals

Primary cultures of cortical neurons

Mouse cortical neurons were cultured from 14-to Lyon,France) as described previously 27 . After extraction of the embryonic brains, the cerebral membranes were removed and the cortices were dissected, mechanically dissociated and cultured in Dulbecco's Modified Eagle's Medium supplemented with 5% horse serum, 5% foetal bovine serum and 1mM glutamine (all from Sigma, Lyon, France) on 24-well plates (Falcon, Corning, N.Y., USA; Becton Dickinson, Le Pont de Claix, France) for biochemical experiments.

Neurons were seeded on 35 mm glass-bottom dishes (MatTek, Ashland, MA, USA) at a final concentration of two cortical hemispheres per dish for confocal experiments. All plates, dishes, and coverslips were coated with 0.1 mg/mL poly-D-lysine and 0.02 mg/mL laminin (Sigma, Lyon, France).

Cultures were maintained at 37°C in a humidified atmosphere containing 5% CO2/95% air. days in vitro (DIV), cytosine arabinoside (AraC, 10 µM; Sigma, Lyon, France) was added to inhibit proliferation of non-neuronal cells in cultures used for biochemistry experiments; 98% of the cells were considered as neuronal. The day before the experiments, cells were washed in DMEM.

Treatments were performed on neuronal cultures at 14-15 DIV.

Lentivirus production

Lentiviruses were produced in HEK293T cells from the following plasmids: pLenti-APP wt -mCherry, pLenti-APP wt3M -mCherry, pLenti-APP swe -mCherry, pLenti-APP swe3M -mCherry, pLenti-APP osa -mCherry and pLenti-APP ice -mCherry. To generate infectious lentiviral particles, sequences of various human APP 695 mutants were cloned into pLenti-C-mCherry vector. pLenti-C mCherry vector was a gift from Dr Christophe Bosc (Grenoble Institute of Neuroscience) and was derived from pLenty-C-mGFP (Snapgene, Chicago, IL, USA). psPAX2 is a packaging plasmid encoding HIV-1 gag/pol sequences under the control of a SV40 promoter (Addgene plasmid # 12259). pCMV-VSV-G is an envelope-expressing plasmid encoding for VSV-G glycoprotein under the control of a CMV promoter (Addgene plasmid # 8454).

For the virus production, cells were transfected using Ca 2+ -phosphate in cell culture dishes (100 x 15 mm) with a given lentiviral plasmid and the two helper plasmids psPAX2 and pCMV-VSV-G. Six hours after transfection, the medium was changed to remove transfection reagent in the conditioned medium to which the virus is secreted. 48 h after transfection conditioned medium was spun at 250g

for 5 min at 4°C before being collected and filtered using a 0.45 μm sterile filter (Sarstedt, Nuembrecht, Germany). Then virus particles were pelleted by ultra-centrifugation for 2 h at 4 °C and 20,000 rpm in a Beckman SW32Ti swinging bucket rotor. Supernatant was discarded and virus was suspended in PBS (X100 concentrated according to the initial volume of supernatant) and aliquots of the viral solution were snap-frozen in liquid nitrogen before storage at -80°C until use. Lentivirus titration was performed by FACS analysis after viral transduction of HEK293T and was estimated around 2 10 8 UI/mL.

For viral transduction, lentiviral solutions were diluted (1:10) in complete culture medium and 50 µL of the diluted preparation were added to the culture medium of a 24-well plate (for human neuronal cultures). Cell culture supernatant or lysates were harvested 48 h to 72 h post-infection.

Aβ measurements by ELISA assay

To assess the level of « total » Aβ secreted into the medium or in cell lysate, after 72 h infection of cortical neurons with lentivirus producing various APP mutants we performed an ELISA assay. For this assay, samples (200 µL of cell culture medium or lysates corresponding to 150 µg of proteins) and standards were incubated overnight at 4°C in a maxisorb 96-well plate (Therm Fisher, Waltham, MA, USA). The Aβ (1-42 aa) standards were prepared using synthetic Aβ ranging from 0.1 to 2 µg/mL. The plates were then aspirated and blocked with 3% bovine serum albumin (BSA) in PBS buffer for at least 1 h at 37°C. The samples and standards were added to the plates and incubated at RT for 1 h.

The 6E10 antibodies diluted to 0.2 µg/mL in blocking buffer was incubated in the wells for 1 h at RT.

The plates were washed three times with wash buffer (0.05% Tween 20 in PBS buffer containing 1% BSA). Horseradish peroxidase conjugated anti mouse antibodies (Jackson Laboratories, Cambridgeshire, UK), diluted 1:5000 in blocking buffer, was added to the wells for 1 h at 37°C. Then the plates were washed three times with wash buffer and once with PBS. The colorimetric substrate, Ultra TMB-ELISA (Thermo Fisher, Waltham, MA, USA), was added and allowed to react for 15 min, after which the enzymatic reaction was stopped with addition of 1 M H 2 SO 4 . Reaction product was quantified using a Molecular Devices Vmax spectrophotometer measuring the difference in absorbance at 405 nm and 650 nm. The low end sensitivity of this assay is 50 ng/mL (14 nM; data not shown).

Plasmids

cDNAs of WT human APP 695 and the Swedish mutant (APP swe ) were cloned into pmcherry-N1 vector (Snapgene, Chicago, IL, USA) using the BamHI and AgeI restriction sites. Then, using overlapping PCR and the Infusion Kit (Thermo Scientific, Waltham, MA, USA) we generated the various APP mutant-mCherry plasmids as described by manufacturer's instructions. These plasmids included: APP osa -mCherry corresponding to the deletion of E22 in the Aβ 1-42 sequence; APP ice -mCherry (A598T); APP wt3M -mCherry (G625A/L630A/G633A); APP osa3M -mCherry and APP swe3M -mCherry.

All constructions in pmCherry vector were verified by sequencing.

Neuronal transfection

Transfections were performed on cortical neuron cultures after 12 DIV with calcium phosphate precipitation. Growth medium (DMEM and sera) was removed and kept at 37°C until the last step of transfection. Cells were washed in DMEM and incubated for 30 min in DMKY buffer containing the following: 1 mM kynurenic acid, 0.9 mM NaOH, 0.5 mM HEPES, 10 mM MgCl 2 and phenol red 0.05%, pH 7.4. Then, 3 µg of the plasmids containing the APP 695 mutants APP x -mCherry (human neuronal Amyloid Precursor Protein) and Actin-GFP (for FRAP experiments) or LifeAct-GFP, a peptide which specifically binds filamentous actin 28 , were mixed with CaCl 2 (120 mM) in HBS containing the following: 25 mM HEPES, 140 mM NaCl, and 0.750 mM Na 2 HPO 4 , pH7.06) and left for 15 min to precipitate the DNA.

Two types of transfections were performed as follows:

Co-transfection: 3 μg of the plasmids containing the APP 695 (full-length human neuronal Amyloid

Precursor Protein) mutants fused to an mCherry tag (APP 695x -mCherry) and Actin-GFP (for FRAP experiments) or LifeActin-GFP, a peptide which specifically binds to filamentous actin, were mixed with 1 M CaCl 2 and HBS buffer (20 mM HEPES, 150 mM NaCl, 1.5 mM Na 2 HPO 4 , pH 7.4) and left for 15 min to precipitate the DNA. Plasmids were then applied to cells for 60 min.

Two-step transfection: 3 μg of APP 695x -mCherry plasmid (mixed with 1 M CaCl 2 and HBS buffer)

was first applied to cells for 40 min. Then, 3 μg of LifeActin-GFP plasmid (mixed with 1M CaCl 2 and HBS buffer) was added to the cells for 40 min.

Transfection medium was replaced with conditioned growth medium and cultures were returned to the incubator until use at DIV 14-15.

Confocal imaging

Transfected neurons were placed in HBBSS solution containing the following (in mM): 110 NaCl, 5

KCl, 2 CaCl 2 , 0.8 MgSO 4 , 1 NaH 2 PO 4 , 12 HEPES, 5 D-glucose, 25 NaHCO 3 , and 10 glycine (all from Sigma, Lyon France) 1.5-2 h before experiments. Neurons were visualised using a Nikon Ti C2

confocal microscope with a Nikon 60X water-immersion objective and NIS-Elements software (Nikon, Melville, NY, USA). Excitation of GFP and mCherry fluorophores was performed with an argon laser excited at 488nm (emission filtered at 504-541 nm) and at 543 nm (emission filtered at 585-610 nm)

respectively. Images were acquired as Z-stacks (tridimensional section) with 0.3 μM per step immediately before and 15 min after treatment. The acquired images were then deconvoluted using AutoQuantX3 software (Media Cybernetics, Abingdon, Oxon, UK).

Spine density and volume was assessed using NeuronStudio software (CNIC -Mount Sinai School of Medecine).

Fluorescence recovery after photobleaching experiments

Fluorescence recovery after photobleaching (FRAP) was performed on cultured neurons 48 h after transfection. Images were acquired with an inverted Nikon Eclipse Ti C2 confocal microscope with a Nikon 60x water objective with a 1.33 numerical aperture. Actin-GFP in the spine head was bleached at 405 nm and the fluorescence recovery was measured for 80 s (at 1 s/frame). Fluorescent signal analysis was performed with the Nikon software Nis-Elements.

Production of recombinant HIS-tagged Aβ proteins

To make the plasmids for the fusion protein Aβ(His) of various mutants of human β-amyloid 1-42 protein (Aβ wt , Aβ osa : ΔE22, Aβ ice : A2T, Aβ wt3M : G29A/L34A/G37A), the cDNA containing the sequence for these human Aβ 1-42 were obtained from synthetic oligonucleotides (Sigma, Lyon, France) (containing a Nde1 restriction site as forward primers and a PspXI restriction site as reverse primers) using overlapping PCR. PCR products were then cloned into a pet28a-vector (Novagen, Paris, France)

and subsequently constructed as various mutant HIS-Aβ 1-42 expressing plasmid (pet28a-AβHis pet28a-Aβ(His)osa: ΔE22, pet28a-Aβ(His)ice: A2T and pet28a-Aβ(His)wt3M: G29A/L34A/G37A). The resulting plasmids were verified by sequencing. Escherichia Coli BL21 (DE3) was transformed with the fusion protein plasmids and a single colony was chosen to grow in a 250 mL starter culture in Luria broth (LB medium) overnight at 37°C. The next day, 10 mL of culture was diluted in 1L LB culture medium. When the culture reached an OD 600nm of 0.8, isopropyl-beta-D-thiogalactopyranoside (IPTG)

was added to 1 mM for induction. The culture was grown for an additional 4 h and the cells harvested by centrifugation at 4000g for 20 min. The pellet was re-suspended in 10 mL ice-cold PBS and lysed by sonication at ice-cold temperature. The cell extract was then centrifuged at 20,000g for 15 min at 4°C. The pellet was re-suspended in 10 mL of 8 M urea in PBS and sonicated as previously described before centrifugation at 20,000g for 15 min at 4°C. The supernatant (5 mL) was diluted with 15 mL of binding buffer (PBS with 10 mM imidazole at pH 8.0). Before affinity purification using nickel-nitriloacetic acid (NTA) column purification, samples were filtered on 0.45 µm. The Ni-NTA column (3 mL of protino Ni-NTA Agarose from Macherey Nagel) was equilibrated with binding buffer prior to loading the sample on the column. Then the column was washed with the washing buffer (PBS with 30 mM imidazole at pH 8.0) with 5-10 column volumes. The protein was then eluted with the elution buffer (PBS with 500 mM imidazole at pH 7.4). The absorbance at 280 nm was used to monitor the elution but the concentration of the fusion proteins was estimated by comparing the intensity of the band of the protein on SDS-PAGE with that of known quantity of BSA (Sigma, Lyon, France). A final concentration of 100 µM was obtained and aliquots were stored at -80°C. Aliquots from all subsequent purification steps were analysed by SDS-PAGE (Laemmli, 1970), and the identity of Aβ 1-42 and mutants was verified by western blots using 4G8 monoclonal antibodies against Aβ sequence (4G8).

Aβ binding assay

This assay was adapted from 29 . Briefly, cortical neuron cultures were lysed by Dounce homogenisation in PBS and then centrifuged at 100,000 × g for 1 h at 4°C. The pellet was filtered at 1 kHz using the Patchmaster Multi-channel data acquisition software (HEKA Elektronik Dr.

Schulze GmbH, Wiesenstrasse, Germany. The initial slope of the fEPSPs was measured to avoid population spike contamination. For LTP experiments, test stimuli (0.2 ms pulse width) were delivered once every 15 s and the stimulus intensity was set to give baseline fEPSP slopes that were 50% of maximal evoked slopes.

Slices that showed maximal fEPSP sizes < 1mV were rejected. LTP was induced by applying 2 trains of 100 stimuli at 100 Hz with an interval of 20 s.

Statistical analysis

Statistical analyses were performed with Graphpad 7.0 Software (La Jolla, CA, USA) using nonparametric test: Mann-Whitney or Wilcoxon Signed Rank Test or Kruskal-Wallis followed by Dunn's multiple comparison test or RM two-way ANOVA followed by Tukey's multiple comparison test.

Significance was set at 0.05. Results are expressed as the mean ± SEM from independent experiments.

Results

AD transgenic mouse model displays impaired dendritic spine density and mushroom spine volume in the hippocampus and cortex

We first looked at the morphology of dendritic spines of our transgenic AD mouse model, Taken together these data show a synaptic morphological alteration of these AD mouse brains at 3-months-old, shifting from a functional to a less functional set of spines with enlarged mushroom spines.

Primary cortical cell cultures transfected with pathogenic forms of APP have similar impaired spine density and mushroom spine volume

Given the results we found with the AD mouse brains, which highly express the human form of APP transgene bearing the Swedish mutation (KM670/671NL), we next wanted to assess whether similar results could be obtained by overexpressing mutated human neuronal forms of APP in primary cortical cell cultures. To do so, we selected various mutants of APP that are pro-AD: APP wt , APP swe , APP osa and one protective mutation against AD: APP ice which were fused to mCherry and cotransfected with LifeActin-GFP (LA-GFP), a small peptide which specifically binds to filamentous actin without disrupting actin stoichiometry (fig2.A Top and middle row), which enables the visualisation of the dendritic arbour and spines (fig2.A, Bottom row). Spine density analysis showed a marked decrease in total spine density of neurons overexpressing APP wt -, APP swe -and APP osa -mCh with 20.86±3.07% (***p=0.0002), 25.75±1.29% (****p<0.0001) and 24.49±0.71% (****p<0.0001) reduction respectively, compared to control neurons which only overexpress LA-GFP (fig2.B, comparing the whole bars of bar graph). However, overexpression of APP ice -mCh had no effect on total spine density (fig2.B). When we looked at spine subpopulation we also found a strong decrease of mushroom spine density when APP wt -, APP swe -or APP osa -mCh was overexpressed with 39.20±2.58% ( †p=0.0456), 63.80±1.17% ( † † †p=0.0002) and 67.62±1.34% ( † † † †p<0.0001) reduction respectively, compared to control (fig2.B, comparing the black segments of bar graph). As seen in the AD mouse brain (fig1.), there was also an increase in thin spine density for neurons overexpressing APP wt -, APP swe -and APP osa -mCh (fig2.B, § §p=0.0063; § §p=0.0084 and § § §p=0.0003 respectively, comparing they grey segments of bar graph). Once again APP ice -mCh had no effect on mushroom spine density and distribution (fig2.B). Moreover, we found that APP wt -, APP swe -and APP osa -mCh expressing neurons also had enlarged mushroom spines, with an increase of 121.81±6.43% (*p=0.025); 151.38±5.40% (**p=0.0012) and 141.75±12.14% (**p=0.0058) respectively, compared to control (fig2.C).

Together these data show that our study design replicates what is observed with AD transgenic mouse brains in vivo, namely an impaired spine density and enlarged mushroom spines.

Spine density and volume impairments observed in pathogenic APP-transfected neurons are driven by Aβ production

To decipher whether the effects observed in our APP-transfected neurons were due to an overproduction of Aβ and not just an effect induced by the mutated APPs alone, we treated the neurons which had the strongest impairments, namely APP swe -and APP osa -mCh transfected cells, with BACE1 Inhibitor IV [1µM] (βSecI) just after transfection until day of confocal imaging. This treatment shuts down the amyloid pathway of APP. We evaluated dendritic spine density and volume of the treated neurons (fig3.A). Our data show that inhibition of the amyloid pathway restored spine density close to control values (fig3.B), especially for mushroom spine density (fig3.B, comparing the black segments on bar graph). Furthermore, the volume of mushroom spines was also restored to control values (fig3.C). These results confirm that the observed effects are indeed Aβ-induced and that intracellular Aβ is sufficient, since restoration was also observed in APP osa -mCh transfected cells.

Processing of APP wt , APP swe , APP osa and APP ice in infected neurons generates different localisations of Aβ pools

Since our different mutants of APP are said to yield Aβ in different ways (sequence, quantity, localisation), we evaluated the amount of Aβ found in cell lysate (fig4.A) and extracellular medium (fig4.B) of neurons expressing APP wt , APP swe , APP osa or APP ice . For this, we infected cultured cortical neurons with the different APPs (APP x ) and performed ELISA assays to analyse the production of the different Aβs (Aβ x ) using 6E10 antibody. Our results showed that APP wt and APP ice produced Aβ wt and Aβ ice respectively and these peptides were detected in both cell lysate and extracellular medium.

APP osa yielded mostly intracellular Aβ osa . APP swe produced the most Aβ which could be found in both cell lysate and extracellular medium (fig4.A and B). Neurons overexpressed similar levels of APP as confirmed by western blot (fig4.C) that is processed into C-terminal fragments (CTF) and Aβ peptides.

These data show that overexpression of our different mutants of APP in neurons resulted in similar levels of APP with an increased production of Aβ and differences in the localisation of the peptide.

APP wt , APP ice produce Aβ peptides that are both intracellular and secreted; APP osa produces mostly intracellular Aβ peptides; and APP swe produced the most Aβ intracellularly and secreted.

Synaptic activity modulates APP processing

Several studies have shown that synaptic activity modulates APP processing and Aβ production 30- 32 . We wanted to see if we could observe this effect in our study model by assessing the red marking Interestingly, APP swe -mCh had a very marked 51.09± 2.99% decrease of red marking after BIC15 (fig5.G).

In order to confirm that these observed effects are indeed due to the processing of APP following synaptic activation, we carried out the same set of experiments looking with APP swe -and APP osa -mCh transfected neurons using βSecI pretreatment (fig5.E and F). Interestingly, we noticed an approximate 2-fold increased marking of APP swe and APP osa when βSecI was present, before BIC15

(data not shown). This seems to point towards an accumulation of APP, most likely in vesicular compartments (given the dot shaped appearance of the marking), when the amyloid pathway is inhibited (fig5.E and F, middle and right panels). After synaptic activity was induced by BIC15, there was no significant decrease of neither APP swe -nor APP osa -mCh marking (fig5.H, 1.96±2.12% and 3.75±1.03% decrease respectively), compared to without βSecI.

Together these data show that our study model allows visualisation of APP processing, mostly the amyloïdogenic pathway, and that it is indeed modulated by synaptic activity.

Spine morphology modulation mediated by synaptic activity is impaired in toxic Aβ overproducing neurons

One of the main characteristics of dendritic spines is their ability to reshape via different modulators such as synaptic activity 33-37 and it is reported that this plasticity is altered in AD, prior to neuronal and memory loss 38-40 . Synaptic activation is known to induce dendritic spine enlargement 34,41,42 . Here we wanted to evaluate this phenomenon in our APP x -mCh transfected neurons by measuring mushroom spine volume before and after synaptic activation by BIC15, using LA-GFP marking as readout (fig6.A). Consistently with literature 34,43 , we found a 117.64±2.18% (*p=0.0156) increase in mushroom spine volume in control neurons (expressing LA-GFP only) after BIC15 (fig6.B). Similarly, APP ice -mCh transfected neurons, overproducing Aβ ice (see fig3.), showed a 116.3±3.34% (*p=0.0156) increase in mushroom spine volume after BIC15 (fig6.B). However, neurons transfected with APP wt -or APP swe -mCh, which overproduce the wild-type form of Aβ (see fig3.) and display enlarged mushroom spines (see fig2.E), failed to increase the volume of these spines after BIC15 (fig6.B). APP osa -mCh (fig6.B) transfected neurons which overproduce intraneuronal Aβ osa (see fig3.) and displayed enlarged mushroom spines (see fig2.E) also failed to increase spine volume after BIC15, suggesting that Aβ overproduction leads to no spine volume variation. We carried out the same experiment using βSecI pretreatment. It is to note that this inhibition of the amyloid pathway not only inhibits the overproduction of Aβ coming from the APP x -mCh overexpression but also shuts down the endogenous amyloid processing of APP. Meaning there is hardly any Aβ in these cells on the day of imaging. Interestingly, the neurons pretreated with βSecI all failed to increase mushroom spine volume after BIC15, even the control and APP ice -mCh neurons (fig6.C) bringing to light the necessity of the presence of Aβ to induce activity-dependent remodelling of the dendritic spine.

Taken together, these results show that: on one hand, overproduction of toxic forms of Aβ (wt and osa) lead to enlarged mushroom spines which fail to respond to synaptic activity but not the overproduction Aβ ice ; and on the other hand, absence of Aβ also causes mushroom spines to not respond to synaptic activity, suggesting that Aβ is important for mushroom spine formation, structure and regulation. As dendritic spine remodelling depends on its actin cytoskeleton dynamics 21 and this remodelling seems hindered in an Aβ-dependent manner, we next questioned the possible link between actin and Aβ.

Activity-dependent actin dynamics in mushroom spines is altered in toxic Aβ overproducing neurons, but also in absence of Aβ

To further investigate the Aβ-dependent spine morphology and remodelling impairments observed previously, we next evaluated the actin dynamics within dendritic spines in resting conditions and in the context of synaptic activity. For this, we performed Fluorescence Recovery After Photobleaching (FRAP) experiments on individual mushroom spines of neurons co-expressing Actin-GFP (ActGFP) with either APP wt -, APP swe -, APP osa -or APP ice -mCh, in resting conditions and after BIC15 (fig7.A and B). Briefly, an ROI was placed on individual mushroom spines and ActGFP was photobleached (only spines with >80% loss of fluorescence were kept for analysis), fluorescence recovery was monitored for 2 min following photobleaching (fig7.A and B). By normalising the fluorescence to prebleached intensity of the spine of ActGFP we obtained fluorescence recovery curves expressed as % of prebleached fluorescence in spine (fig7.C and E). In control neurons only overexpressing ActGFP, and in resting conditions, we obtained 80% fluorescence recovery after 80 s before reaching a plateau (fig7.A, C and D), which is consistent with what is found in literature 44 . This was also the case for all the other conditions where the different APP x -mCh were overexpressed, including the condition with βSecI pretreatment (fig7.D). This result shows that in absence of synaptic activity, Aβ overproduction or even absence of Aβ does not affect actin dynamics in spines.

However Taken together these results demonstrate that: 1) Aβ is required for activity-dependent stabilisation in spines, 2) excess or toxic form of Aβ increases this activity-dependent stabilisation and

3) absence of Aβ prevents activity-dependent stabilisation. This brings to light a possible direct interaction between Aβ and actin.

Aβ wt and Aβ osa decrease in vitro depolymerisation of actin

Since Aβ seems to modulate actin cytoskeleton stabilisation in the spines, we next wanted to assess whether there is a possible interaction between actin and Aβ. To do so, we carried out an in vitro actin depolymerisation assay. In control conditions, in absence of Aβ, actin depolymerises, translated as a decrease of fluorescence (fig8.A). This Δfluorescence (from starting point to plateau) is of 58.4±0.94% (fig8.B). This Δfluorescence is significantly decreased when actin is in presence of As we hypothesised a possible direct interaction of Aβ with the actin cytoskeleton, we compared with Aβ sequence with other actin binding proteins (ABPs). This brought to light a sequence of 3 common amino-acids GXXXXXLXXG (fig9.A) between Aβ and ABPs involved in actin bundling (namely:

calponin, L-plastin, dystrophin and ABP280). We mutated these 3 amino-acids into 3 Alanine within the Aβ sequence and generated the 3M mutation in order to lose the potential interaction of Aβ with actin. This mutation was applied to APP wt -, APP swe -and APP osa -mCh, giving rise to APP wt3M -mCh, APP swe3M -mCh and APP osa3M -mCh. Preliminary in vitro bi-complementation assays with actin and Aβ wt3M showed an absence of fluorescence hence a loss of interaction between these 2 proteins, compared to actin with Aβ (data not shown).

We first analysed the spine morphology of cortical neurons transfected with APP wt3M -, APP swe3Mor APP osa3M -mCh and LA-GFP (fig9.B). Unlike APP wt -, APP swe -or APP osa -mCh overexpressing neurons, spine density (total as well as the different spine subtypes) was not affected when APP wt3M -, APP swe3M -or APP osa3M -mCh was overexpressed (fig9.C), neither was mushroom spine volume (fig9.D) compared to control neurons.

We next assessed spine morphology modulation by synaptic activity in APP wt3M , APP swe3M and APP osa3M expressing neurons (fig10.A). Unlike APP wt , APP swe or APP osa overexpressing neurons, where mushroom spines were enlarged and failed to increase in size after BIC15, APP wt3M , APP swe3M and APP osa3M overexpressing ones had mushroom spines with the same volume as control neurons (only overexpressing LA-GFP); and these spines were able to significantly increase their volume by 119.05±2.65% (* p=0.0156); 118.87±5.33% (*p=0.0313) and 116.69±3.73% (*p=0.0469) respectively after BIC15 (fig10.A and B).

These first results indicate that APP wt3M , APP swe3M and APP osa3M have no effect on spine morphology and do not disrupt synaptic plasticity, pointing towards a loss of synaptotoxicity compared to their non-3M counterparts.

As these APP x3M -mCh seemingly lost their toxicity, we next questioned whether this effect could be due to a modification of the processing of these APPs which would subsequently modify Aβ production. We first evaluated the red marking in APP wt3M -, APP swe3M -and APP osa3M -mCh expressing neurons before and after BIC15 (fig10.C, D and E) and compared them to their respective non-3M counterparts (fig10.F). Consistent with the results obtained previously (fig5.) BIC15 induced a decrease in red marking of APP wt3M -, APP swe3M -and APP osa3M -mCh (fig10.C, D and E, right panels). This decrease was of 26.80±3.92% for APP wt3M -mCh and of 21.19±2.41% for APP osa3M -mCh, similar to what was obtained with APP wt -mCh and APP osa -mCh (fig10.F). APP swe3M -mCh also displayed a similar strong 47.23±1.08% decrease in red marking as APP swe -mCh (fig10.F).

Taken together, these results show that with APP wt3M , APP swe3M and APP osa3M we are in the same context of Aβ overproduction as with APP wt , APP swe and APP osa ; only we do not have any signs of synaptotoxicity.

The 3M mutation abrogates the effect of toxic Aβ on actin in vitro and in cortical neuron dendritic spines Since the 3M mutation caused loss of synaptotoxicity when inserted in APP wt , APP swe and APP osa , we wanted to validate that this loss of toxicity could be due to a loss of interaction of Aβ with actin.

To do so, we carried out the in vitro actin depolymerisation assay (fig7.) using Aβ wt3M (fig11A. and B.)

As expected, Aβ wt3M, compared to Aβ wt , no longer decreased the depolymerisation rate of actin (fig11.B, p=0.5391), but rather behaved like Aβ ice (fig11.A). These results validate that the 3M mutation causes loss of interaction of Aβ with actin.

We next examined actin dynamics in the context of synaptic plasticity using FRAP on mushroom spines of neurons overexpressing either APP wt3M -, APP swe3M -or APP osa3M -mCh and ActGFP. Like previously seen in fig9., in control conditions (before BIC15), there was no difference in actin dynamics across the different mutants; they all had 80% FRAP (fig11.C, D and E). However, after Here also we noted Aβ's loss of toxicity when it bares the 3M mutation, since Aβ wt induced LTP impairment whilst Aβ wt3M did not ( †p=0.0128, Aβ wt vs Aβ wt3M ).

Neurons overproducing secreted toxic Aβ affect neighbouring neurons through APP, whereas neurons overproducing toxic intracellular Aβ or non-toxic Aβ do not

Several studies have shown that Aβ overproduction in one cell affects the neighbouring healthy cells and could explain the spreading of the pathology across the brain 11,45 . Here we examined the effects of an APP wt -, APP swe -, APP osa -or APP ice -mCh overexpressing neuron on a healthy nearby neuron. To do this, we first transfected either of the APP x -mCh in cultured cortical neurons then, 30 min later, we added LA-GFP. This allowed us to have one neuron expressing both LA-GFP and one of the APP x -mCh (APP neuron) next to another neuron which only expresses LA-GFP (healthy neuron) (fig14.A). We then examined the spine density of the dendrites of the healthy neuron depending on their distance from the APP neuron. Results showed that neurons overproducing secreted Aβ, namely APP wt -and APP swe -mCh overexpressing neurons, decrease the spine density of the nearby healthy neuron. Indeed, APP wt neuron significantly decreased spine density of healthy neuron from 0 (dendrites from both neurons are overlapping) to 20 µm (distance from APP neuron). APP swe neuron had an even stronger impact on healthy neuron by decreasing its spine density from a range of 0 to 40 µm. However, APP osa neuron had no effect on healthy neuron, neither did APP ice neuron. These results show that, in order for the pathology to propagate from cell to cell, Aβ has to be of a toxic sequence and secreted (fig14.B).

Furthermore, we wanted to assess the role of APP in this spreading of toxic effects. We carried out the same set of experiments using APP knock-out cultured cortical neurons. Here, one APPKO neuron is overexpressing both APP swe and LA-GFP and the neighbouring healthy APPKO neuron is only overexpressing LA-GFP. Interestingly, when the healthy neuron has no APP, the toxic effects induced by the APPKO(+ APP swe ) neuron do not occur, demonstrating the implication of APP in the spreading of the pathology (fig14.B). To verify that the absence of effect of Aβ osa and Aβ ice was not attributed to an impaired interaction with APP, we carried out an Aβ binding assay (fig14.C) where the different Aβ x were loaded on a nitrocellulose membrane and neuronal plasma membrane extracted proteins were added. The Aβ-APP interaction was revealed with 22C11. Our results show that all Aβ wt , Aβ osa , Aβ ice and synthetic Aβ (Aβ synth ) are capable of interacting with APP at the plasma membrane further supporting our hypothesis that amyloid pathology might spread from neuron to neuron via APP.

Discussion

In the last decade, Aβ peptides have been identified as the main conductor driving synaptotoxicity and have emerged as the pivot in the pathophysiological development of AD. Although it is now wellestablished that Aβ accumulation is the trigger of synaptic deficits, and has multiple effects on neurons 24 , the pathway by which the Aβ peptides exert and sustain their synaptotoxicity is still under debate. In this study, we get a deeper insight into the way Aβ initially impairs synapse morphology and function, and how APP processing sustains and propagates these effects in a synaptic activitydependent manner. Here we show that Aβ accumulation induces a decrease in spine density especially mushroom spines, accompanied by an increased volume of the remaining mushroom spines, and that intracellular Aβ is sufficient to induce these effects. These enlarged mushroom spines have impaired structural plasticity as they did not increase in volume following synaptic activation and this seems to be due to defective activity-dependent actin dynamics in the spines. This alteration of synaptic morphology, structure and plasticity seems to be due to a newly-identified interaction between actin and Aβ, hinting a possible physiological role for Aβ in activity-dependent synaptic plasticity. We also show that synaptic activity modulates amyloïdogenic APP processing, further exacerbating these synaptic defects. Furthermore, we show that Aβ sequence is as important as Aβ concentration in inducing synaptic alterations. Lastly, we bring to light that secreted Aβ, not only affects the Aβ-secreting neuron itself, but also affects nearby neurons in an APP-dependent manner, reminiscent of a prion-like mechanism.

One of the first issues we wanted to address was the pathway which Aβ follows to induce its synaptotoxic effects. Is it extracellular Aβ that causes synaptotoxicity? Or rather could it be intracellular Aβ and/or internalised Aβ from the extracellular space? In our study we show that intracellular Aβ is sufficient to induce synaptic alterations, since APP osa overexpression led to intraneuronal accumulation of Aβ, no Aβ secretion and spine density decrease along with abnormal spine head enlargement. Several have brought to light that intracellular Aβ accumulation precedes NFT and senile plaque apparition 16,46 . Our results support this hypothesis and point towards intracellular Aβ as the initiating factor of synaptic distress leading to the early cognitive impairments in AD. Our results also show that overexpression of APP swe and APP wt lead to an increase in intracellular Aβ, as well as an increase in secreted Aβ which could be uptaken by neighbouring neurons.

Dendritic spines adapt their shape thus function to match the incoming synaptic activity 21 . Longterm potentiation (LTP) leads to spine enlargement, whereas Long-term depression (LTD) leads to spine shrinkage 35,47,48 . In our study, we used BIC15 protocol which mimics a chemical LTP-type synaptic activation. We confirmed spine enlargement following synaptic activation. However in pathological neurons, where a form of toxic APP is overexpressed, mushroom spines are already enlarged and fail to undergo activity-dependent spine enlargement, indicating synaptic plasticity defects in these neurons, presumably due to intracellular Aβ. Interestingly, although an excess amount of intracellular Aβ leads to impaired synaptic plasticity, an absence of Aβ also seems adverse.

Indeed, upon βSecI treatment, which was carried out just after transfection and inhibition of β-secretase was maintained until the end of the experiments, effectively all amyloïdogenic processing was blocked in the cells. Although this inhibition prevented toxic Aβ-induced spine density decrease, our results show that absence of Aβ still leads to failed spine enlargement following synaptic activation and brings to light a possible physiological role for Aβ in activity-dependent spine stabilisation. Our FRAP data further confirmed this possible role as excess intracellular toxic Aβ led to actin overstabilisation in spines whereas absence of Aβ, as well as protective Aβ ice , led to no activitydependent stabilisation. Interestingly, these results were confirmed in cultured cortical neurons from APPKO mice (data not shown) where activity-dependent actin remodelling and subsequent spine enlargement did not occur. However, this model is not ideal to study the effects of absence of Aβ as APPKO neurons had overall decreased spine density, pointing out a possible implication of APP or its proteolytic derivatives in spine structure and maintenance. This decrease in spine density in APPKO neurons has already been reported by others and, according to these studies, this could be either due to the lack of sAPPα, the N-terminal fragment of APP generated in the non-amyloïdogenic pathway after cleavage by α-secretase 49 or due to the lack of interaction of cell-surface APP with proteins from the extracellular matrix 50 . More recently, it has also been evidenced that dendritic spines from APPKO neurons also have altered pre-and post-synaptic protein and receptor content, further supporting the role of APP in spine structure and function 51 .

The interplay between intracellular Aβ and actin which had been hinted by others 52,53 , led us to question a potential interaction between them. We identified a common 3 amino-acid actin-binding sequence between Aβ and other actin-binding proteins involved in actin bundling, and so we generated the 3M mutation within the Aβ sequence. After confirming that this mutation did not hinder APP processing or Aβ production we reported a loss of synaptotoxicity of Aβ (wt, osa) when it harboured the 3M mutation as well as a loss of activity-dependent actin overstabilisation in the spines. In 2003, Kamenetz and collaborators 45 proposed a physiological activity-dependent negative feed-back system, where Aβ production modulates synaptic transmission, and synaptic activity modulates Aβ production, to keep neural hyperactivity in check. In this light, excess Aβ would drive synaptic depression and eventually spine loss. Our findings support this hypothesis and add that: i) this mechanism might occur via excess Aβ preventing proper activity-dependent actin remodelling in the spines and ii) absence of Aβ also leads to improper activity-dependent actin-remodelling in the spines. This suggests that the pathway to synaptotoxicity depends not only on a balance of the quantity of Aβ produced but also on the sequence of Aβ and its ability to interact with the actin cytoskeleton in spines.

Several have reported that synaptic activity modulates APP processing 31,45 . Our results support these findings and bring to light that activity-dependent amyloïdogenic processing of APP seems predominant at the intracellular level. Indeed, our results show that the mCherry signal inside our cultured cortical neurons from transfected APP x -mCh constructs decreased following synaptic activity, especially APP swe , and this decrease was largely blocked by β-secretase inhibition.

Additionally, according to several studies 16,46,54-57 it is preferably the toxic Aβ 42 isoform that is produced intracellularly. Our findings shed some light on the possible purpose of activity-dependent processing of APP, where generated Aβ regulates activity-dependent actin remodelling and subsequent synaptic transmission. Excess Aβ or conversely, lack of Aβ, both ultimately lead to defective activity-dependent synaptic plasticity and transmission. This indicates that activitydependent amyloïdogenic processing of APP is likely to be a finely tuned equilibrium and misbalancing this mechanism would lead to the early cognitive impairments of AD. In the bigger picture, this provides possible answers about the recent failings of β-secretase inhibition as treatment for AD in clinical trials 58 . In addition to the fact that BACE1 has other substrates involved in various synaptic processes, blocking this enzyme and subsequent Aβ production would still lead to synaptic defects, ultimately worsening cognitive impairments.

It is now well established that Aβ toxicity arises from pathological/excessive amounts of Aβ.

Indeed, nowadays the most used AD transgenic animal models carry the FAD Swedish mutation on APP, which increases Aβ production 59 . Conversely, it has been proposed that less Aβ would be protective against AD. This has been suggested following the discovery of the protective Icelandic mutation which, allegedly, leads to a 30 to 40% reduction in Aβ generation 26 . Although this might seem quite significant, it is to note that the amyloïdogenic processing accounts for only 10% of total APP processing 60 . Furthermore, other studies based on FAD mutations such as the Arctic mutation, showed that toxicity may occur from an imbalance of Aβ 42 /Aβ 40 ratio when total Aβ is unchanged 61 , highlighting that Aβ sequence may be as important as Aβ concentration in exerting synaptic dysfunction. In our study we used equal "pathological" concentrations of Aβ wt , Aβ osa , Aβ ice and Aβ wt3M (100 nM) on cultured cortical neurons to assess spine morphology, as well as on acute mouse hippocampal slices for LTP electrophysiological experiments. As expected, we observed the toxic effects of Aβ wt and Aβ osa in both models. Interestingly, however, neither Aβ ice nor Aβ wt3M induced any alterations on spine morphology or LTP. This puts forward the importance of the sequence of Aβ as well as its concentration, and that the Icelandic mutation may in fact be protective against AD via its sequence along with its decreased concentration. Interestingly, although Aβ ice has no common mutated amino-acids with Aβ wt3M , throughout our study they both induced very similar effects regarding actin depolymerisation rate, spine morphology and activity-dependent spine actin stabilisation. This led us to question whether their sequence may induce similar three-dimensional folding of the peptide, preventing their interaction with actin. Poduslo and Howell highlighted the importance of Aβ sequence in Aβ aggregation/fibril formation in the development of neuronal degeneration 25 . Taken together, with our findings, we show that Aβ sequence may lead to different peptide behaviour in the cells: where it preferentially localises or how it aggregates/misfolds, and it is capital that this aspect is taken into account when developing new therapeutic strategies.

The last question we wanted to address regarding the way Aβ exerted and sustained its synaptotoxicity in early stages of AD was how the disease propagated throughout the brain, particularly relevant for sporadic AD cases. Our results showed that mutant APP overexpression, which leads to increased secreted Aβ in the extracellular space (wt and swe), not only did it more than likely influence the Aβ overproducing neuron itself, but also affected nearby "healthy" neurons which did not overexpress APP. This effect seemed to be proportional to the amount of secreted Aβ.

Indeed, APP swe overexpressing neurons, which secreted the most Aβ, decreased spine density of "healthy" neuron at an effective distance of 40 µm; whereas APP wt overexpressing neurons, which also secreted Aβ but less than with APP swe , decreased "healthy" neuron's spine density at an effective distance of 20 µm. It would seem the more Aβ is secreted into the extracellular space, the more the "healthy" neuron's spine density is affected. These results outline that the rate of Aβ production and secretion into the extracellular space, which can be in part modulated by the sequence of Aβ, might dictate the severity of sporadic AD pathology development. Interestingly, when we carried out the same experiments using APPKO cultured mouse cortical neurons, the "healthy" APPKO neuron was not affected by the nearby Aβ secreting neuron. This suggests that the effects observed on the "healthy" wild-type neuron are due to APP. It has been proposed that Aβ internalisation may occur via its binding to plasma membrane constituents 62 . Here we propose that Aβ internalisation from the extracellular space may occur via cell-surface APP, which in turn affects synaptic function in the recipient "healthy" neuron, reminiscent of a prion-like mechanism where the disease would progress from neuron to neuron.

To conclude, our study has brought to light a novel interaction between Aβ and the actin cytoskeleton revealing a potential physiological function for Aβ in activity-dependent spine stabilisation and a possible new route for therapeutic development. This function in structural plasticity is seemingly dependent of an equilibrium of the quantity and sequence of Aβ, and offbalancing this quantity and sequence would lead to synaptic dysfunction which propagates from one neuron to the neighbouring neuron if Aβ is secreted into the extracellular space. In the light of our findings and recent evidence showing that patients with sporadic AD may, in fact, have several strains of Aβ within the brain 63 , we show that unravelling AD development entails an extra level of complexity where Aβ quantities and sequence have to both be taken into account in order to find

Figures

VII. Discussion & Perspectives

In the last decade, Aβ peptides have been identified as the main conductor driving synaptotoxicity and have emerged as the pivot in the pathophysiological development of AD. Indeed, AD has been coined as a synaptopathy, meaning that synaptic defects are the initiating factor leading to memory and learning impairments. Although it is now well-established that Aβ accumulation is the trigger of these synaptic deficits, and has multiple effects on neurons (Selkoe, 2002), the pathway by which the Aβ peptides exert and sustain their synaptotoxicity is still under debate. In this study, we get a deeper insight into the way Aβ initially impairs synapse morphology and function, and how APP processing sustains and propagates these effects in a synaptic activity-dependent manner, using different mutants of APP with unique molecular signatures. Here, we show that Aβ accumulation induces a decrease in spine density especially mushroom spines, accompanied by an increased volume of the remaining mushroom spines, and that intracellular Aβ is sufficient to induce these effects. These enlarged mushroom spines have impaired structural plasticity as they did not increase in volume following synaptic activation and this seems to be due to defective activity-dependent actin dynamics in the spines. This alteration of synaptic morphology, structure and plasticity seems to be due to a newly-identified interaction between actin and Aβ, hinting a possible physiological role for Aβ in activity-dependent synaptic plasticity. We also show that synaptic activity modulates APP processing, ostensibly the amyloïdogenic pathway, which further exacerbates these synaptic defects when Aβ is already abounding. Furthermore, we show that Aβ sequence is as important as Aβ concentration in inducing deleterious effects, as a pathological concentration of a non-toxic mutant of Aβ (Aβ ice , Aβ wt3M ) does not induce synaptotoxicity. Lastly, we bring to light that secreted Aβ, not only affects the Aβ-secreting neuron itself, but also affects nearby neurons in an APP-dependent manner, reminiscent of a prion-like mechanism.

Intracellular Aβ: the instigator of the early cognitive alterations in A. AD?

Intracellular Aβ was initially difficultly accepted within the scientific community, yet more and more evidence have confirmed their presence and subsequent toxicity inside neurons [START_REF] Gouras | Critical role of intraneuronal Aβ in Alzheimer's disease: technical challenges in studying intracellular Aβ[END_REF][START_REF] Gouras | Intraneuronal beta-amyloid accumulation and synapse pathology in Alzheimer's disease[END_REF][START_REF] Gouras | Intraneuronal Abeta accumulation and origin of plaques in Alzheimer's disease[END_REF][START_REF] Gouras | Intraneuronal Abeta42 accumulation in human brain[END_REF][START_REF] Grundke-Iqbal | Amyloid protein and neurofibrillary tangles coexist in the same neuron in Alzheimer disease[END_REF]LaFerla et al., 2007;[START_REF] Tomiyama | A mouse model of amyloid beta oligomers: their contribution to synaptic alteration, abnormal tau phosphorylation, glial activation, and neuronal loss in vivo[END_REF].

Although many studies have identified the detrimental effects of Aβ through extracellular application or its presence in the extracellular space [START_REF] Selkoe | Soluble oligomers of the amyloid beta-protein impair synaptic plasticity and behavior[END_REF][START_REF] Shankar | Amyloid-beta protein dimers isolated directly from Alzheimer's brains impair synaptic plasticity and memory[END_REF][START_REF] Shankar | Natural oligomers of the Alzheimer amyloid-beta protein induce reversible synapse loss by modulating an NMDA-type glutamate receptor-dependent signaling pathway[END_REF][START_REF] Walsh | Naturally secreted oligomers of amyloid beta protein potently inhibit hippocampal long-term potentiation in vivo[END_REF], the question of the pathway which Aβ follows to induce these effects is still unclear. Is it extracellular Aβ that causes synaptotoxic effects? Or rather could it be intracellular Aβ and/or internalised Aβ from the extracellular space? In our study we show that intracellular Aβ is sufficient to induce synaptic alterations, since APP osa overexpression led to intraneuronal accumulation of Aβ, no Aβ secretion and spine density decrease along with abnormal spine head enlargement. Our ELISA assay results also show that overexpression of APP swe and APP wt led to an increase in intracellular Aβ, as well as an increase in secreted Aβ. Furthermore, unpublished data from our team show that LTP inhibition by extracellular Aβ application requires β-secretase cleavage of APP, entailing an intracellular generation of Aβ that disrupts synaptic transmission. Several have brought to light that intracellular Aβ accumulation precedes NFT and senile plaque apparition (D'Andrea et al., 2001;[START_REF] Gouras | Intraneuronal Abeta42 accumulation in human brain[END_REF] and correlates much better with AD cognitive decline. Together, our data support this hypothesis point out intracellular Aβ as the instigator of synaptic distress leading to the early cognitive impairments in AD.

Regulation of dendritic spine actin dynamics: a physiological role B. for Aβ?

Dendritic spines adapt their shape thus function to match the incoming synaptic activity (Hotulainen and Hoogenraad, 2010). Long-term potentiation (LTP) leads to spine enlargement, whereas Long-term depression (LTD) leads to spine shrinkage (Cingolani and Goda, 2008;Matsuzaki et al., 2004;Zhou et al., 2004). In our study, we used a Bic/4-AP (BIC15) protocol which mimics LTPtype synaptic activation. We confirmed spine enlargement following synaptic activation. However in pathological neurons, where a form of toxic Aβ is in excess, mushroom spines are already enlarged and fail to undergo activity-dependent spine enlargement, indicating synaptic plasticity defects in these neurons, presumably due to intracellular Aβ. To support these findings, an earlier study carried out by our team also showed aberrant actin-F stabilisation in spines due to application of synthetic oligomeric Aβ peptides [START_REF] Rush | Synaptotoxicity in Alzheimer's Disease Involved a Dysregulation of Actin Cytoskeleton Dynamics through Cofilin 1 Phosphorylation[END_REF]. Interestingly, although an excessive amount of intracellular Aβ leads to impaired synaptic plasticity, an absence of Aβ also seems adverse. Indeed, upon βSecI treatment, which was carried out just after transfection and inhibition of β-secretase was maintained until the end of the experiments, effectively all amyloïdogenic processing was blocked in the cells. Although this inhibition prevented toxic Aβ-induced spine density decrease, our results

show that absence of Aβ still leads to failed spine enlargement following synaptic activation and brings to light a possible physiological role for Aβ in activity-dependent spine stabilisation. Our FRAP data further confirmed this possible role as excess intracellular toxic Aβ lead to actin overstabilisation in spines whereas absence of Aβ, as well as protective Aβ ice , lead to no activity-dependent stabilisation.

It is noteworthy that these results were confirmed in cultured cortical neurons from APPKO mice (see Supplementary data) where activity-dependent actin remodelling (Supplementary Data, FRAP experiments, figures C. and D.) and subsequent spine enlargement (Supplementary Data, spine volume analysis before/after BIC15, figure B.) did not occur. However, this model is not ideal to study the effects of absence of Aβ as APPKO neurons had overall decreased spine density, including decreased mushroom spines (Supplementary Data, Spine density analysis, figure A.), pointing out a possible implication of APP or its proteolytic derivatives in spine formation, structure and maintenance. This decrease in spine density in APPKO neurons has already been reported by others with different explanations as to why this is [START_REF] Lee | Beta amyloidindependent role of amyloid precursor protein in generation and maintenance of dendritic spines[END_REF]Weyer et al., 2014). On one hand, according to Lee and collaborators this is likely due to a lacking interaction of cell surface APP with proteins from the extracellular matrix [START_REF] Lee | Beta amyloidindependent role of amyloid precursor protein in generation and maintenance of dendritic spines[END_REF]. On the other hand, more recent data from Weyer and coworkers suggested that this decreased spine density in APPKO neurons is due to the lack of sAPPα, the soluble N-terminal fragment of APP generated in the non-amyloïdogenic pathway after cleavage by α-secretase (Weyer et al., 2014b). Furthermore, a very recent study by Martinsson and collaborators also showed that dendritic spines from APPKO neurons have altered pre-and postsynaptic proteins and receptors [START_REF] Martinsson | APP depletion alters selective pre-and post-synaptic proteins[END_REF]. Moreover, these data also show that there is no compensation by APP-like proteins (APLPs), which doesn't contain the Aβ sequence. Together these studies support the hypothesis that APP and its cleavage products are important for spine formation and maintenance. Our data also support this hypothesis whilst highlighting that Aβ is more involved in activity-dependent actin remodelling during synaptic plasticity. This interplay between intracellular Aβ and actin which had been hinted by others [START_REF] Kommaddi | Aβ mediates F-actin disassembly in dendritic spines leading to cognitive deficits in Alzheimer's disease[END_REF]Maloney et al., 2005), led us to question a potential interaction between them. We identified a common 3 amino-acid actin-binding sequence between Aβ and other actin-binding proteins and lead us to generate the 3M mutation within the Aβ sequence. After confirming that this mutation did not hinder APP processing or Aβ production, we reported a loss of synaptotoxicity of Aβ (wt, osa) when it harboured the 3M mutation as well as a loss of activity-dependent actin overstabilisation in the spines. In 2003, Kamenetz and collaborators, proposed a physiological activity-dependent negative feed-back system, where Aβ production modulates synaptic transmission, and synaptic activity modulates Aβ production, to keep neural hyperactivity in control.

In this light, excess Aβ would drive synaptic depression and eventually spine loss. Our findings support this hypothesis and add that: i) this mechanism might occur via excess Aβ preventing proper activity-dependent actin remodelling in the spines and ii) absence of Aβ also leads to improper activity-dependent actin-remodelling in the spines. This suggests that the pathway to synaptotoxicity depends not only on a balance of the quantity of Aβ produced and where it is produced, but also on the sequence of Aβ and its ability to interact with the actin cytoskeleton in spines since Aβ osa and Aβ wt seems to interact with actin, whereas Aβ ice and Aβ wt3M do not.

Although our results may have identified a new role physiological role for Aβ it is important to highlight that there are some caveats that will need to be addressed. Indeed, a major question which is still under debate is the exact localisation of intraneuronal Aβ. Since our data show that Aβ is able to interact with the actin cytoskeleton this entails that some Aβ is localised in the cytoplasm. This is still debated within the scientific community although there are some data to support this. Indeed, some studies have showed that, at sub-lethal concentrations, Aβ42 can be translocated to the nucleus of neuroblastoma cells and act as a repressor of transcription of the genes LRP1 and KAI1 and also regulates the gene expression of growth factors such as insulin-like growth factor binding proteins 3 and 5 (IGFBP3/5) [START_REF] Barucker | Alzheimer amyloid peptide aβ42 regulates gene expression of transcription and growth factors[END_REF][START_REF] Barucker | Nuclear translocation uncovers the amyloid peptide Aβ42 as a regulator of gene transcription[END_REF]. Although these studies did not specifically look for intracytoplasmic Aβ peptides they did, however, detect the presence of Aβ in the nucleus implying that Aβ has to, at some point, be in the cytoplasm in order to travel to the nucleus. As it is known that Aβ is generated within endosomes with an acidic milieu, since BACE1 is active at pH 6, the question of how Aβ can get into the cytoplasm still remains to be answered. The presence of intracellular Aβ has been shown in human AD and Down syndrome brains as well as in numerous transgenic rodent models of AD [START_REF] Gouras | Intraneuronal beta-amyloid accumulation and synapse pathology in Alzheimer's disease[END_REF][START_REF] Kobro-Flatmoen | Reelin-immunoreactive neurons in entorhinal cortex layer II selectively express intracellular amyloid in early Alzheimer's disease[END_REF][START_REF] Welikovitch | Evidence of intraneuronal Aβ accumulation preceding tau pathology in the entorhinal cortex[END_REF]. Although we were able to detect intracellular Aβ within the neurons, our data did not allow us to identify the exact localisation of intracellular Aβ. However, a dynamic relationship exists between pools of intracellular and extracellular Aβ [START_REF] Oddo | A dynamic relationship between intracellular and extracellular pools of Abeta[END_REF]. Thus, we speculate that, in physiological conditions, cytoplasmic Aβ would originate from two possible pathways: i) a processing of APP inside the neurons and/or ii) via uptake of Aβ from the extracellular space.

The first involves the maturation pathway of APP. Newly biosynthesised APP travels through the TGN via the secretory pathway before reaching the plasma membrane. In this section of the pathway, APP is within TGN vesicles with its N-terminal domain on the outside and its C-terminal domain on the inside of the vesicle. Then, when APP reaches the plasma membrane it is rapidly reendocytosed. Now in these endosomes APP is orientated the opposite way with its N-terminal domain on the inside and its C-terminal domain on the outside. At both of these stages, APP can be cleaved by BACE-1, generating two pools of Aβ: one destined to remain inside the neuron (processing of newly biosynthesised APP) and one destined to be secreted (processing of endocytosed APP).

Accordingly, we speculate that β-cleaved newly biosynthesised APP generates Aβ into the cytosol whereas β-cleaved endocytosed APP generates Aβ which remains in the vesicles in order to be secreted.

The second pathway to intracytoplasmic Aβ involves secretion of Aβ into the extracellular space and re-internalisation into the cytoplasm either by simple diffusion of the peptide through the plasma membrane or via a partner that, in the light of our findings, could be APP.

It has been shown that intracellular Aβ accumulation in AD vulnerable neurons leads to more prominent endosomal damage [START_REF] Willén | Aβ accumulation causes MVB enlargement and is modelled by dominant negative VPS4A[END_REF][START_REF] Yang | Loss of endosomal/lysosomal membrane impermeability is an early event in amyloid Abeta1-42 pathogenesis[END_REF]. In this light, we further hypothesise that, in pathological conditions, these endosomal alterations lead to an Aβ leakage from the endosomes into the cytoplasm, further exacerbating the deleterious synaptotoxic effects. Due to the aggregating nature of the peptide and the multitude of cleavage products of APP of which their sequences overlap with one another; it makes it quite complex to target and track Aβ specifically. We have to bear in mind that this is a recurring issue and one of the main difficulties in the field of Aβ research. Nevertheless, our results confirm the presence of an intracellular pool of Aβ that is sufficient to induce synaptic alterations and bring to light that this pool may have a role in synaptic plasticity when in homeostatic concentrations.

In order to get a better understanding of the physiological role of Aβ in actin interaction and remodelling following synaptic activity, it would be of interest to generate a transgenic mouse harbouring the 3M mutation within its endogenous mouse APP gene. This transgenic APP wt3M model would allow: i) behavioural assessments ii) In vivo electrophysiological recordings as well as on acute hippocampal slices iii) ex vivo neuron architecture evaluation (spine density, spine volume, dendritic arbour length and branching). We speculate that this transgenic animal would be similar to the APPKO mouse (in the sense that they both lack Aβ to interact with actin) but without the synaptic defects linked to the absence of APP. The stream of data that would be provided by this model would give a greater insight into the neurotransmission state of neuronal networks and unravel this potential novel physiological role for Aβ.

Activity-dependent amyloïdogenic processing of APP: a finely C. tuned equilibrium?

As higher levels of education have been associated with decreased risks of developing AD, one could easily envisage that higher levels of synaptic activity would be neuroprotective. However, the role of synaptic activity in the context of AD is complex. AD brain dysfunction is initiated particularly in areas known to be more chronically active, known as the "default network". It has also been shown in AD mouse models that excessive brain activity from stress or seizures worsens brain βamyloidosis, while reduced brain activity or excessive sleep also damages synapses in the AD models but not wild-type models [START_REF] Mohajeri | Passive immunization against beta-amyloid peptide protects central nervous system (CNS) neurons from increased vulnerability associated with an Alzheimer's disease-causing mutation[END_REF]Tampellini et al., 2010). Moreover, our team has shown that APP/PS1 mice have hyperactive hippocampal neurons at 1 month of age but become hypoactive at 3 months (unpublished data). This raises the question of the role of synaptic activity in AD development. Several have reported that synaptic activity modulates APP processing [START_REF] Kamenetz | APP processing and synaptic function[END_REF]Tampellini and Gouras, 2010). Our results support these findings and bring to light that activity-dependent amyloïdogenic processing of APP seems predominant at the intracellular level. Indeed, our results show that the mCherry fluorescence inside our cultured cortical neurons from transfected APP x -mCh constructs decreased following synaptic activity, especially APP swe , and this decrease was largely blocked by β-secretase inhibition. Additionally, according to several studies (D'Andrea et al., 2001;Echeverria and Cuello, 2002;[START_REF] Gouras | Intraneuronal Abeta42 accumulation in human brain[END_REF][START_REF] Näslund | Correlation between elevated levels of amyloid beta-peptide in the brain and cognitive decline[END_REF][START_REF] Ohyagi | Intraneuronal amyloid β42 enhanced by heating but counteracted by formic acid[END_REF]Tabira et al., 2002) it is preferably the toxic Aβ 42 isoform that is produced intracellularly.

Our findings shed some light on the possible purpose of activity-dependent processing of APP, where generated Aβ regulates activity-dependent actin remodelling and subsequent synaptic transmission.

In this context, excess Aβ or conversely lack of Aβ, both ultimately lead to defective activitydependent synaptic plasticity and transmission. In the bigger picture, this raises concerns, and provides possible answers, about the recent failings of β-secretase inhibition as treatment for AD in clinical trials (Zhu et al., 2018a). Indeed, in addition to the fact that BACE1 has other substrates involved in various synaptic processes, long term inhibition of this enzyme and subsequent Aβ production would still lead to synaptic defects, ultimately worsening cognitive impairments. Taken all together, our data indicate that activity-dependent amyloïdogenic processing of APP is likely to be a finely tuned equilibrium and misbalancing this mechanism would lead to the early cognitive impairments of AD.

So what is the factor that tips the scale in favour of excess Aβ and AD development? One of the possibilities, and one of the major risk factors for AD development, is aging. Aging impacts all organs and biological pathways, and is a risk factor for numerous diseases. What is more, although everybody and every living organism ages, the aging process in each individual is essentially unique as it is tainted by events one may have had to endure and lifestyle choices one may have made throughout their life, such as the amount of physical activity, the amount of brain activity (higher education, stress…), sickness, diet and so one and so forth. Therefore, the molecular and cellular mechanisms critical for how aging impacts AD remain unclear. Nevertheless, these mechanisms of aging are crucial for AD pathogenesis, as it has been shown in many rodent models of AD and even APPKO animals. Indeed, in the case of AD rodents, although these animals are genetically programmed to develop the pathology, the symptoms do not appear until adulthood, at several months of age. Even in the case of APPKO mice, these animals do not present any particular deficits at a young age despite a reduction in spine density; however they do present defective LTP at older age, further demonstrating the importance of the process of aging in neurodegenerative pathology development. Common themes shared among various age-related neurodegenerative diseases such as Parkinson's disease and AD are aberrant protein aggregations and early synapse dysfunction. As we now know, synapses are early targets of damage in AD and reduced brain activity is a well-known outcome of the disease. The brain uses a large proportion of energy, with synaptic transmission being its main source of energy use [START_REF] Harris | Synaptic energy use and supply[END_REF], and energy metabolism in the aging brain is affected by numerous factors. Altered energy metabolism with aging can influence several mechanisms such as early protein accumulation/aggregation due to altered protein degradation/clearance, and protein aggregation/accumulation in turn can impact brain energy metabolism. In this light, it is comprehensible that the homeostatic state of Aβ and the dynamics of actin cytoskeleton could be shifted due to these age-related altered mechanisms, thus becoming a burden to vulnerable neurons overtime, ultimately causing synaptic dysfunction then loss. This intricate interplay between altered energy metabolism in the aging brain and protein sorting mechanisms requires further investigation and could open new routes for AD therapy and probably several other neurodegenerative diseases.

To get a deeper insight into this activity-dependent processing of APP, it would be interesting to look more closely at the localisation of this processing within neurons. If Aβ has a role in actin remodelling in spines, then one could envisage that APP would be cleaved by β-secretase at the spines. To test this hypothesis, it would be of interest to study the potential colocalisation of APP and β-secretase at the spines. This could be done either by i) co-transfection of APP and BACE1, each protein fused to fluorescent tag of different colours, or ii) by bi-complementation assays whereby APP and BACE1 are each fused to one half of a fluorescent tag and if the proteins interact, a fluorescence will emit [START_REF] Das | Visualization of APP and BACE-1 approximation in neurons: new insights into the amyloidogenic pathway[END_REF][START_REF] Das | Activity-induced convergence of APP and BACE-1 in acidic microdomains via an endocytosis-dependent pathway[END_REF]. Also, to go further into the regulation of amyloïdogenic processing of APP in neurons, we could investigate the two pathways (secretory/endocytic) of β-cleavage of APP using double fluorescently tagged APP construct (mcherry-APP swe -EYFP). In this construction, mcherry fluorescent protein was inserted into the ectodomain and YFP fused to the C-terminal domain of APP695 swe . While unprocessed APP appeared in yellow, the presence of red or green puncta revealed the predominant processing pathway involved. Red puncta correspond to the accumulation of the shredded ectodomain in endocytic vesicles and reveal a predominant endocytic pathway for APP processing. Whereas green puncta correspond to the accumulation of C-terminal fragments and reveal a processing of APP during the secretory pathway. Unpublished data from our team already show that in presence of extracellular Aβ, green puncta is mainly observed in neurons due to the C-terminal fragments sequestrated in vesicles whereas shredded soluble APP ectodomain fragments were released in the cytosol. These results were further confirmed by subcellular fractionation and Western-Blot analysis of APP fulllength and APP fragments in endosome-, lysosome-, endoplasmic reticulum-and cytosol-enriched fractions obtained from primary cortical neurons exposed to Aβ for 30 min. After Aβ application, APP localised in all fractions, but sAPP was also found in the cytosol. This result confirms the processing of APP during the secretory pathway. Remarkably, we also observed an increase of Aβ oligomers in the cytosol of Aβ-treated neurons compared to control, further confirming the presence of intracytoplasmic Aβ pool. It would now be of crucial importance to carry out the same set of experiments using BIC15 protocol, which would demonstrate and confirm activity-dependent amyloïdogenic processing of APP and intracytoplasmic Aβ generation. Nevertheless, this stream of data already point out an involvement of Aβ in APP processing, potentially revealing a vicious circle where amyloïdogenic processing of APP is modulated by both synaptic activity and Aβ, highlighting how this mechanism may get out of hand in pathological conditions, ultimately leading to excess Aβ production and subsequent AD cognitive impairments.

Aβ sequence over Aβ concentration? D.

Many studies have established that Aβ toxicity arises from pathological/excessive amounts of Aβ. Indeed, nowadays the most used AD transgenic animal models carry the FAD Swedish mutation on APP, which increases Aβ production (Citron et al., 1992). Conversely, it has been proposed that less Aβ would be protective against AD. This has been suggested following the discovery of the protective Icelandic mutation which, allegedly, leads to a 30 to 40% reduction in Aβ generation [START_REF] Maloney | Molecular mechanisms of Alzheimer disease protection by the A673T allele of amyloid precursor protein[END_REF]. Although this might seem quite significant, it is to note that the amyloïdogenic processing accounts for only 10% of total APP processing (Sinha and Lieberburg, 1999). Furthermore, other studies based on FAD mutations such as the Arctic mutation (E693G), showed that toxicity may occur from an imbalance of Aβ 42 /Aβ 40 ratio when total Aβ levels are unchanged [START_REF] Nilsberth | The "Arctic" APP mutation (E693G) causes Alzheimer's disease by enhanced Abeta protofibril formation[END_REF], highlighting that Aβ sequence may be as important as Aβ concentration in exerting synaptic dysfunction. In our study we used equal "pathological" concentrations of Aβ wt , Aβ osa , Aβ ice and Aβ 3M (100 nM) on cultured cortical neurons to assess spine morphology, as well as on acute mouse hippocampal slices for LTP electrophysiological experiments. As expected, we observed the toxic effects of Aβ wt and Aβ osa in both models. Interestingly, however, neither Aβ ice nor Aβ 3M induced any alterations on spine morphology or LTP. This puts forward the importance of the sequence of Aβ as well as its concentration since its sequence may lead to different behaviour of the peptides in the cells, whether it is where it localised or how it aggregates/misfolds. A study by Poduslo and Howell in 2015 showed the immediate transition to fibrils of the Osaka (E693Δ) Aβ mutant and the rapid transition from aggregates to fibrils of the Arctic (E693G), Dutch (E693Q) and Icelandic (A673T) mutants highlighting the importance of aggregates/fibrils in AD pathogenesis. This study also brings to light that the reproducibility of the footprints of the mutated Aβ peptides and their uniqueness for each mutation suggest an individuality that was not recognised up until these findings and which likely contribute to differences in the clinical and pathological features of AD. Further supporting these findings, it has recently been evidenced that patients with sporadic AD may, in fact, have several strains of Aβ within the brain, creating an extra level of complexity in elaborating therapeutic strategies as each strain implies an individuality that could potentially modulate the clinical and neuropathological expression of AD (Condello and Stöehr, 2018).

AD pathology propagation in the brain: a prion-like APP-E. dependent mechanism?

The last question we wanted to address regarding the way Aβ exerted and sustained its synaptotoxicity in early stages of AD was how the disease propagated throughout the brain. Indeed, this is especially relevant for sporadic AD cases. Our results showed that mutant APP overexpression, which leads to increased secreted Aβ in the extracellular space (wild-type and swedish), not only did it more than likely influence the Aβ overproducing neuron itself, but also affected nearby "healthy" neurons which did not overexpress APP. This effect seemed to be proportional to the amount of secreted Aβ, the more Aβ is secreted into the extracellular space, and the farthest the "healthy" neuron's spine density was affected. Interestingly, when we carried out the same experiments using APPKO cultured mouse cortical neurons, the "healthy" APPKO neuron was not affected by the nearby Aβ secreting neuron. This suggests that the effects observed on the "healthy" wild-type neuron are dependent of APP expression. It has been proposed that Aβ internalisation may occur via its binding to plasma membrane constituents [START_REF] Bharadwaj | Role of the cell membrane interface in modulating production and uptake of Alzheimer's beta amyloid protein[END_REF]. Indeed, Aβ entry into cells could potentially occur through ligand-receptor type interactions such as: direct or indirect interaction with integrins, receptor for advanced glycation end products (RAGE), and APP itself [START_REF] Verdier | Amyloid beta-peptide interactions with neuronal and glial cell plasma membrane: binding sites and implications for Alzheimer's disease[END_REF].

Other interactions with TrkA, p75NTR, some G-proteins, NMDA, and AMPA receptors [START_REF] Benilova | The toxic Aβ oligomer and Alzheimer's disease: an emperor in need of clothes[END_REF], and/or interactions via the prion protein [START_REF] Laurén | Cellular prion protein mediates impairment of synaptic plasticity by amyloid-beta oligomers[END_REF][START_REF] Resenberger | The cellular prion protein mediates neurotoxic signalling of β-sheet-rich conformers independent of prion replication[END_REF] have also been reported. Here we propose that Aβ internalisation from the extracellular space mainly occurs via APP on the plasma membrane, which in turn affects synaptic function in the recipient neuron. In the light of our findings and some of our unpublished data mentioned above, we can further speculate that the healthy recipient neuron also undergoes modulations of its processing of APP due to Aβ internalisation. This would further increase intracellular Aβ levels and subsequent synaptic deficits, reminiscent of a prion-like mechanism where the disease would progress from neuron to neuron that are either interconnected [START_REF] Gary | Encephalopathy induced by Alzheimer brain inoculation in a non-human primate[END_REF] or just simply in physical proximity [START_REF] Wei | Amyloid beta from axons and dendrites reduces local spine number and plasticity[END_REF].

To go further into this investigation, it would now be of great interest to study in further detail the state of the "healthy" nearby neuron that is "contaminated" by Aβ. Does this recipient neuron have an increased APP processing? Also, to give further support to the "prion-like" mechanism of disease propagation it would be worthy to take a look at the state of the remaining spines, of the "healthy" nearby neuron, that are in the vicinity of the pathological neuron. Are these spines affected in the same way as the pathological neuron? To do so, it would be relevant to carry out FRAP experiments on these remaining spines. Moreover, to further support the idea that APP is necessary for disease propagation from neuron to neuron it would be of interest to carry out the same experiment of neuron treatment with 24 h of pathological concentrations of oligomeric Aβ but with APPKO cultured cortical neurons and assess spine density, spine sub-population distribution and spine volume. These data would provide a deeper understanding of the mechanistic behind disease propagation from neuron to neuron. Taking all our results together, our study has brought to light a novel interaction between Aβ and the actin cytoskeleton revealing a potential physiological function for Aβ in activity-dependent spine stabilisation (Figure 34) and a possible new route for therapeutic development. This function in structural plasticity is seemingly dependent of an equilibrium of the quantity and sequence of Aβ, and off-balancing this quantity and sequence would lead to synaptic dysfunction which propagates from one neuron to the neighbouring neuron if Aβ is secreted into the extracellular space (Figure 35 and 36). In the light of our findings and recent evidence showing that patients with sporadic AD may, in fact, have several strains of Aβ within the brain (Condello and Stöehr, 2018), we show that unravelling AD development entails an extra level of complexity where Aβ quantities and sequence have to be taken into account in order to find new therapeutic strategies. Our findings support the idea that there is no one-size-fits-all for AD treatment and that it should rather be made-to-measure for each individual AD patient.

Take-home messages F.

In this section I sum up the main messages and ideas to take home from this thesis, outlining the effects of the balance of Aβ on activity-dependent actin remodelling in dendritic spine plasticity, when there is a sufficient amount (Physiologic condition, This Aβ, which could be interacting with the actin cytoskeleton, could in part explain the increased spine volume in resting conditions. (Right side) Upon synaptic activation, as APP is more abundant, amyloïdogenic processing of APP is even more increased, generating excessive amounts of intracellular Aβ peptides which bind to the actin cytoskeleton in spines; further decreasing the depolymerisation rate of, and ultimately overstabilising actin cytoskeleton, which results in no increase in spine volume. If the Aβ sequence allows the peptide to be secreted into the extracellular space, it can then "contaminate" neighbouring neurons via an APP-dependent mechanism. This stream of events could in part explain synaptic dysfunction and eventual spine loss, observed in AD.

Synaptotoxicity in Alzheimer's disease: Influence of APP processing on excitatory synapses

Alzheimer's disease (AD) is defined as a neurodegenerative disorder where synaptic defects lead to neuronal loss and concurrent memory impairments. It is now well-established that synaptic dysfunction in AD is initiated by oligomeric forms of the amyloid-β peptide (Aβ), a proteolytic derivative of Amyloid Precursor Protein (APP). However, the pathway by which Aβ induces its deleterious effects, whether it is due to intra-and/or extracellular Aβ pools, and how these effects are sustained and propagated throughout the brain, are still unclear.

In this study, we used several mutated forms of APP which give rise to Aβ peptides with unique molecular signatures, such as: the Swedish mutation (K670M/N671L) (APP swe ) which increases secreted (extracellular) Aβ; the Osaka mutation (E693Δ) (APP osa ) which causes intraneuronal (intracellular) accumulation of Aβ; and the Icelandic mutation (A673T) (APP ice ) which has been reported to decrease Aβ production and protect against AD. These mutated forms of APP were overexpressed in cultured mouse cortical neurons in order to: i) study the morphology and function of dendritic spines, the post-synaptic element of synapses, by confocal microscopy, ii) get a better insight into pathology development and propagation and iii) identify a novel interacting partner bringing to light the possible physiologic role of Aβ in neurons.

We report that pathological Aβ accumulation, due to APP wt , APP swe and APP osa overexpression but not APP ice overexpression induces a significant decrease in spine density especially mushroom spines, accompanied by a significantly increased volume of the remaining mushroom spines, and that intracellular Aβ is sufficient to induce these effects. These enlarged mushroom spines have impaired structural plasticity as they did not increase in volume following synaptic activation seemingly as a result of defective activity-dependent actin dynamics in the spines. This alteration of synaptic morphology, structure and plasticity seems to be due to a newly-identified interaction between actin and Aβ, hinting a possible physiological role for Aβ in activitydependent synaptic plasticity. We also show that synaptic activity modulates amyloïdogenic APP processing which, in pathological conditions, further exacerbates these synaptic defects. Furthermore, we show that Aβ sequence is as important as Aβ concentration in inducing synaptic alterations since pathological concentrations of Aβ harbouring the Icelandic mutation had no effect on spine density or volume. Lastly, we bring to light that secreted Aβ, not only affects the Aβ-secreting neuron itself, but also affects spine density of nearby neurons in an APP-dependent manner, reminiscent of a prion-like mechanism. Together these results demonstrate that APP processing is a finely tuned equilibrium involved in actin-remodelling during activitydependent synaptic plasticity and opens a new route for AD therapeutic strategies.

Figure 1 :

 1 Figure 1: Portraits of Aloïs Alzheimer (left) and Auguste Dieter (right).

Figure 2 :

 2 Figure 2: Timeline of the apparition of AD biomarkers and cognitive impairments at preclinical stages (adapted from Tan et al., 2014). The horizontal axis indicates clinical stages of AD: preclinical AD, Mild Cognitive Impairments (MCI), and dementia. The vertical axis indicates the relative values of each biomarker. Aβ is identified in cerebrospinal fluid (CSF) Aβ42ELISA assays or PET amyloid imaging. Synaptic dysfunction is evidenced by functional imagery (FDG-PET or MRI). The horizontal "cut-points" line represents the threshold for the identification of the different stages.

Figure 3 :

 3 Figure 3: Major pathways involved in AD and affected genes (adapted from[START_REF] Giri | Genes associated with Alzheimer's disease: an overview and current status[END_REF]

Figure 4 :

 4 Figure 4: Cross section of a normal brain versus AD brain (adapted from Crimins et al., 2013). AD patient brains are characterised by gross atrophy of the hippocampus (arrow in A) and cortical thinning (arrowhead in A).

Figure 5 :

 5 Figure 5: Hallmarks of AD (adapted from Nixon et al., 2007). AD brain stained with Bielschowsky silver revealing extracellular β-amyloid plaques (arrowheads) and intracellular neurofibrillary tangles (arrows).

Figure 6 :

 6 Figure 6: Evolution of the spatial distribution of NFTs and senile plaques during AD (adapted from Braak and Braak, 1991). The shades of grey show low (white) to high (black) density of NFTs (left panel) and senile plaques (right panel).

Figure 7 :

 7 Figure 7: Identified functions and localisations of Tau in neurons (adapted from Sotiropoulos et al., 2017). Tau protein is located in several compartments in the neuron. In axons, Tau stabilises MTs and regulates axonal transport. At the synapses, Tau interacts with membrane proteins and plays a role in neuronal activity as well as synapse plasticity. In the nucleus, Tau protects DNA and nuclear RNA, while regulating genetic and epigenetic expression.

Figure 8 :

 8 Figure 8: Macroscopic lesions and the different aspects of amyloid deposits (adapted from Duyckaerts et al., 2009). Left panel: Image of a normal brain versus an advanced stage of AD brain. Right panels: the different types of amyloid deposits identifiable by immunohistochemistry. (A) Diffuse amyloid peptide deposits. (B) Focal deposits (arrowheads) much denser, which form the core of amyloid plaques, surrounded by a halo of much less dense amyloid peptides. (C) Stellar deposits (arrows).

Figure 9 :

 9 Figure 9: Two Hypothesized Models Linking Core Features of Alzheimer's Disease (adapted from Small and Duff, 2008). The amyloid hypothesis assumes a serial model of causality, whereby abnormal elevations in Aβ drive tau hyperphosphorylation and other downstream manifestations of the disease. According to the dual pathway hypothesis, Aβ elevations and tau hyperphosphorylation can be linked by separate mechanisms driven by a common upstream molecular defect.

Figure 10 :

 10 Figure 10: Different pathways identified for Tau propagation (adapted from[START_REF] Mudher | What is the evidence that tau pathology spreads through prion-like propagation?[END_REF]. Tau proteins can be transferred from donor cells (green) to recipient cells (orange) using different routes. Pathway indicated by blue arrows: tau proteins are released in the medium by extracellular vesicles like exosomes and ectosomes. Violet pathway: Around 90% of tau in the extracellular space is found as free protein, passive diffusion facilitated by a membraneous transporter/receptor (?2) or active exocytosis (?3) might contribute to this process. Uptake of free tau species by recipient cells, including APPmediated endocytosis has been reported. Whether free or aggregated tau is taken up by other mechanisms such as diffusion (?4) or non-receptor mediated endocytosis/macropinocytosis (?5) has not been resolved. Nor is it known how membranebound tau can escape from vesicles and enter the cytoplasm of recipient cells (?6). Red pathway: Tau was shown to be present inside nanotubes connecting cells in vitro and to allow its interneuronal transfer. This mechanism could potentially participate in prion-like propagation of tau pathology but whether it is a mode of transcellular transfer of seeding-competent tau species in vivo needs to be investigated.

Figure 11 :

 11 Figure 11: Self-propagating conformational strains of Aβ in AD and their potential impact on diagnostic tools and treatments (adapted from Condello et al., 2017). (Left panel) Different Aβ strains may lead to different pathological phenotypes and kinetics that may differ between patients. Given the wide heterogeneity seen sporadic AD, a mixture of strains could be observed in patients ("hybrid") resulting in mixed/hybrid pathology. (Right panel) Differential strains of Aβ and potential hybrids make diagnosis and potential treatment even more complex.

Figure 12 :

 12 Figure 12: The amyloïdogenic pathway (adapted from O'Brien and Wong, 2011). APP cleavage by β-secretase generates sAPP-β and β-CTF fragments. Subsequent γ-cleavage by γ-secretase generates Aβ peptide and AICD.

Figure 13 :

 13 Figure 13: The non-amyloïdogenic pathway (adapted from O'Brien and Wong, 2011). APP cleavage by α-secretase generates sAPP-α and α-CTF fragments. Subsequent γ-cleavage by γ-secretase generates P3 peptide and AICD.

Figure 14 :

 14 Figure 14: Intracellular trafficking of APP (adapted from[START_REF] Haass | Trafficking and proteolytic processing of APP[END_REF]. Newly synthesised APP molecules (black bars) mature through the constitutive secretory pathway(1). Once APP reaches the plasma membrane, it is rapidly internalised (2), and sorted through the endocytic and recycling compartments back to the cell surface[START_REF] Tiwari | Alzheimer's disease: pathogenesis, diagnostics, and therapeutics[END_REF] or degraded in the lysosome.

Figure 15 :

 15 Figure 15: Possible Aβ production pathways (adapted from Aguzzi and O'Connor, 2010). APP is cleaved firstly by βsecretase, generating two fragments, sAPP-β (brown bars) and a membrane peptide C99 or β-CTF (grey bars). C99 is then cleaved by γ-secretase generating the Aβ peptide (orange squares). APP may be cleaved in two ways: either inside endosomes (left side) or directly at the plasma membrane (right side).

Figure 16 :

 16 Figure 16: APP Proteolytic Products and Intracellular Trafficking of APP (adapted from Van der Kant and Goldstein, 2015).APP is synthesized in the ER (a) and trafficked via the TGN (b) to the PM (c) or endosomes(d). Full-length APP can be endocytosed from the PM(1) or cleaved by the α-secretase[START_REF] Cipriani | Alzheimer and his disease: a brief history[END_REF] to release sAPPα in the extracellular environment as well as generating C83 that is also endocytosed. In the early endosomes (EE), full-length APP is cut by the β-secretase generating C99 and sAPPβ[START_REF] Tiwari | Alzheimer's disease: pathogenesis, diagnostics, and therapeutics[END_REF]. Full-length APP and soluble APP fragments generated in the endosome might be recycled to the PM via recycling endosomes (RE)[START_REF] Greenfield | Endoplasmic reticulum and trans-Golgi network generate distinct populations of Alzheimer beta-amyloid peptides[END_REF] or can be trafficked toward late endosomes (LEs)[START_REF] Laferla | Intracellular amyloid-beta in Alzheimer's disease[END_REF]. In the LEs, these fragments can subsequently be sorted toward the TGN[START_REF] Walsh | A beta oligomers -a decade of discovery[END_REF], exocytosed[START_REF] Shankar | Natural oligomers of the Alzheimer amyloid-beta protein induce reversible synapse loss by modulating an NMDA-type glutamate receptor-dependent signaling pathway[END_REF], or further trafficked to the lysosome for degradation[START_REF] Shankar | Amyloid-beta protein dimers isolated directly from Alzheimer's brains impair synaptic plasticity and memory[END_REF]. Cutting of C99 and C83 by the γ-secretase might occur in the TGN, the LE, or in both compartments, therefore possibly releasing Aβ and P3 either via the secretory pathway or via exocytosis. Cutting by the γ-secretase also releases the AICD that can translocate to the nucleus[START_REF] Walsh | Naturally secreted oligomers of amyloid beta protein potently inhibit hippocampal long-term potentiation in vivo[END_REF].

  The exact fragments generated by γ-cleavage vary in size. Studies have identified several Aβ peptides made up of 34 to 50 aa and approximately 90% of these Aβ peptides are Aβ 40 . Although Aβ 42/43 peptides represent the smallest fraction generated by γ-cleavage, these peptides are the ones that are the most aggregation-prone and induce most of the neurotoxic effects observed in AD.

Figure 17 :

 17 Figure 17: Negative feedback model indicating proposed interaction between neural activity and APP processing (adapted from Kamenetz et al., 2003). Neural activity regulates B-secretase activity on APP. Formation of AB depresses synaptic transmission. Synaptic depression decreases neural activity.

Figure 20 :

 20 Figure 20: Sequence and mutations of APP (adapted from Bemporad et al., 2019).The extracellular N-terminal domain (residues 18-671), the Aβ42 sequence (residues 672-713), and the intra-cellular C-terminal domain (residues 714-770) are shown in green, orange, and grey, respectively. The transmembrane domain encompasses residues 700-723. Mutations that have been shown to induce an enhancement of the Aggregation propensity are reported above. Mutations that alter the processing of the APP sequence are reported below.

  . a Numbers refer to the study codes in the ClinicalTrials.gov database. b Data from preclinical human studies or phase 1 studies. Although BACE inhibition effectively reduces Aβ levels with all of these different molecules, most of them failed to show any therapeutic benefits. Recently, clinical trial for Verubecestat (MK-8931), a BACE1 and 2 inhibitor, failed to rescue cognitive impairment in patients with prodromal AD. Several other trials have been discontinued before the end of trial period such as LY3202626, Lanabecestat (AZD3293) and Atabecestat (JNJ-54861911). LY3202626, although a potent BACE1 inhibitor, alone induced no cognitive improvement in AD patients. It was combined with an immunotherapy using injection of a monoclonal antibody against a pyroglutamate form of Aβ aggregated in plaques Donanemab, in another trial which also failed to show any improvement. Lanabecestat (AZD3293), a BACE1 and 2 inhibitor, effectively reduced Aβ levels in plasma and CSF of patients up to 70% but the trial was prematurely discontinued in 2018 due to lack of efficacy on cognitive symptoms.

  spine density[START_REF] Hu | BACE1 deficiency causes altered neuronal activity and neurodegeneration[END_REF][START_REF] Savonenko | Alteration of BACE1dependent NRG1/ErbB4 signaling and schizophrenia-like phenotypes in BACE1-null mice[END_REF]

  study is based on the development of a new human monoclonal antibody, coined Aducanumab, which selectively targets Aβ aggregates and more specifically soluble oligomers and insoluble fibrils. Preclinical tests carried out on transgenic AD animals show that an analogue of Aducanumab crosses the blood-brain barrier, binds to parenchymal Aβ in the brain and decreases soluble and insoluble Aβ in plaques in a time-and dose-dependent manner, which enabled the launch of clinical trials. These trials were performed on patients with mild-AD which were administered a placebo or 1, 3, 6 or 10 mg/kg doses of Aducanumab every month for 2 years. The results seem encouraging since several reports show that patients treated with Aducanumab present a significant reduction of amyloid plaques in a time-and dose-dependent manner. The development of this antibody is currently in phase 3 of clinical trials and should reinforce the importance of targeting the Aβ peptide in the treatment of AD.

Figure 21 :

 21 Figure 21: The amyloid cascade hypothesis (adapted from Selkoe and Hardy, 2016). Illustration of the sequence of major events leading to the development of AD.

Figure 22 :

 22 Figure 22: Illustration of a chemical synapse. The action potential arrives at the presynaptic axonal terminal (black arrow) and provokes the opening of voltage-gated calcium channels. Calcium entry induces neurotransmitter-containing vesicle fusion to the presynaptic plasma membrane and release of neurotransmitter into the synaptic cleft. Neurotransmitter (eg: Glutamate) then binds to postsynaptic receptors. This binding allows the opening of postsynaptic ion channels creating an influx of cations (Na+ , K + , Ca 2+ ) in the postsynaptic element which will induce a depolarisation of the membrane whereas an influx of anions (Cl -) will induce a hyperpolarisation. This ion influx enables the transmission of the nervous signal.

Figure 23 :

 23 Figure 23: Illustration of a metabotropic glutamate receptor (adapted from: Spooren et al., 2001). The neurotransmitter (glutamate) binding domain in the N-terminal region is followed by a heptahelix domain (the region with 7 transmembrane domains) and a cytosolic C-terminal domain (COOH).

Figure 24 :

 24 Figure 24: Schematic diagram of the general structure of ionotropic glutamate receptors (adapted from Bristol, n.d.). The N-terminal domain is extracellular whereas the shorter C-terminal domain is intracellular.

Figure 23 :

 23 Figure 23: Original illustration of Ramon y Cajal. Silver-stained pyramidal neuron (left). Close-up of a dendritic branch with dendritic spines (right).

Figure 24 :

 24 Figure 24: The difference in thickness of pre-and postsynaptic density is one of the first criteria to differentiate synapses (adapted from[START_REF] Guillery | Early electron microscopic observations of synaptic structures in the cerebral cortex: a view of the contributions made by George Gray (1924-1999)[END_REF]. Electron-microscopic image of an asymmetrical synapse. "a" is the presynaptic electrondense zone. "b" is the dense material in the synaptic cleft and "c" is the postsynaptic density. "pre": presynaptic; "den": dendrite.

Figure 25 :

 25 Figure 25: Schematic diagram of a mature mushroom-shaped spine, showing the PSD, the perisynaptic membrane and other organelles (adapted from Sheng and Hoogenraad, 2007). The endocytic zone (EZ) is located lateral of the PSD in extrasynaptic regions of the spine, where it may be associated with clathrine-coated vesicles (CCV) and recycling endosomes (RE). Smooth endoplasmic reticulum (SER), polyribosomes (PR) and mitochondria (M) are found mainly at the base of the spine neck but may extend into the spine. The abundant cytoskeleton (brown lines) is connected to the PSD and determines spine structure and motility. Other abbreviation: SA, spine apparatus.

Figure 26 :

 26 Figure 26: Schematic diagram of the different types of spines. Spine shape translates synaptic strength and functionality but also spine maturity. Thin spines are less mature and functional whereas mushroom spines are mature and functional. Stubby spines are at an intermediate stage.

Figure 27 :

 27 Figure 27: The different models of spinogenesis (adapted from: Yuste and Bonhoeffer, 2004). (a) Sotelo's model, 1990. This model proposes that spines from Purkinje cells develop independently from axonal fibres. (b) Miller & Peter's model, 1981. The authors propose a model where axon terminals stimulate spine formation. (c) Filopodia model, Vaughn, 1989. Vaughn proposes a "synaptotropic" hypothesis where filopodia associate to axons and form dendritic spines upon contact.

Figure 28 :

 28 Figure 28: Organisation of proteins and protein-protein interactions in the postsynaptic density (PSD) (adapted from Sheng and Hoogenraad, 2007). Schematic diagram of the network of proteins in the PSD, with edge of PSD depicted at right. Only major families and certain classes of PSD proteins are shown. Contacts between proteins indicate an established interaction between them. Domain structure is shown only for PSD-95 (PDZ domain, SH3 domain, GuK domain).Other scaffold proteins are coloured yellow; signalling enzymes, green; actin binding proteins, pink. CaMKII (calcium/calmodulindependent protein kinase II) is depicted as dodecamer. Unnamed proteins signify the many other PSD proteins that are not illustrated in this diagram. Abbreviations: AKAP150, A-kinase anchoring protein 150 kDa; CAM, cell adhesion molecule; Fyn, a Src family tyrosine kinase; GKAP, guanylate kinase-associated protein; H, Homer; IRSp53, insulin receptor substrate 53 kDa; KCh ,K + channel; mGluR, metabotropic glutamate receptor; nNOS, neuronal nitric oxide synthase; RTK,receptor tyrosine kinases (e.g., ErbB4, TrkB); SPAR, spine-associated RapGAP.

Figure 29 :

 29 Figure 29: Schematic diagram of a rodent hippocampal slice showing the different regions, excitatory pathways and synaptic connections (adapted from Purves et al., 2001). The excitatory pathways are illustrated with a (+) sign. Influx coming from the entorhinal cortex follows the perforant pathway (1) then goes through the mossy fibres of the dentate gyrus (2) that are connected to the Schaeffer collaterals in CA3 (3) which in turn project in CA1 (4). This pathway enabled to demonstrate LTP, which has been observed in each of the 3 synaptic connections(2, 3 and 4) represented in this diagram.

Figure 30 :

 30 Figure 30: Schematic representation of the main synaptic mechanisms of LTP. During simple neurotransmission, NMDAr are blocked by an Mg 2+ ion, the synapse is in "resting condition" (left). During high-frequency synaptic activation, the presynaptic compartment releases glutamate that binds to AMPAr and NMDAr. This stimulation induces postsynaptic membrane depolarisation allowing Mg 2+ ion to be ejected from NMDAr ① and provokes massive Ca² + influx②, which is the major secondary messenger that activates several signalling pathways leading to actin reorganisation to increase spine volume ③ and recruitment of more AMPAr, PSD-95 and NMDAr to the spine ④. This sequence of events, as well as the subsequent structural and functional changes within the synapse, leads to increased synaptic strength via induction and consolidation of long-term potentiation.

Figure 31 :

 31 Figure 31: Actin remodelling signalling pathways (adapted from Chazeau and Giannone, 2016). Selected signalling cascades driving F-actin remodelling upon Ca 2+ influx via GluN receptors. CAMKII is a key triggering protein both involved in functional and structural LTP. CAMKII can activate small RhoGTPases (Rac1, Cdc42, RhoA) by phosphorylating GEFs (Tiam1, Karilin7) and GAPs (p250GAAP). This will control the spatiotemporal activation of several ABPs (Arp2/3, myosin II, cofilin). (Arrows) indicate positive regulation. (T-shaped bars) indicate negative regulation. (GluN) NMDAr. (CaM) calmodulin. (CaMK) calcium-calmodulin dependent protein kinase. (GEF) guanine nucleotide exchange factor. (GAP) GTPase activating protein. (βPIX) β PAK interacting exchange factor. (Tiam1) T cell lymphoma invasion and metastasis-inducing protein 1. (Rac1) Ras-related C3 botulinium toxin substrate 1. (Cdc42) cell division cycle 42. (RhoA) Ras homologous member A. (ROCK) Rho-associated, coiled-coil containing protein kinase. (PAK) p21-activated kinase. (LIMK) LIM-kinase. (N-WASP) neuronal Wiskott-Aldrich syndrome protein. (WAVE) Wasp-family verprolin homologous protein. (MLC) myosin light chain. (Arp2/3) actin-related protein 2/3.

Figure 32 :

 32 Figure 32: A model for F-actin reorganisation during LTP (adapted from Chazeau and Giannone, 2016). LTP induces a transient and long-lasting increase of the spine head size, a shortening and widening of spine neck and a concomitant ABPs and CaMKII spine recruitment. Activated CaMKII will dissociate from F-actin and phosphorylate multiple proteins leading to a fast F-actin reorganisation and a transient spine head enlargement. This transient reorganisation is characterised by the formation of an enlargement F-actin pool, an increase in the F-actin/G-actin ratio and increased concentration of cofilin andArp2/3. During the long-lasting spine head enlargement, most ABPs return to their basal concentration, suggesting the formation of a larger dynamic and stable F-actin pool. Those larger dendritic spines most likely provide a "tag" for the capture of newly synthesised synaptic proteins in order to sustain late LTP.

  Thy1-YFP-H (The Jackson Laboratory, Ellsworth, Maine, USA, B6 Cg-Tgn 2Jrs) mice were crossed with heterozygote APP/PS1-21 (APP/PS1) mice to generate C57BL/6Thy1-eYFP APP/PS1-21 mouse colony. These mice were used at 3 months of age for spine density and mushroom spine volume analysis in the hippocampus and cortex. All experiments involving animals were conducted in accordance with the policy of Institut des Neurosciences de Grenoble and French legislation, in compliance with the European Community Council Directive of November 24, 1986 (86/609/EEC). The research involving animals was authorised by the Direction Départementale de la protection des populations-Préfecture de l'Isère France and by the ethics committee of Institut des Neurosciences de Grenoble accredited by the French Ministry of Research.

  C57BL/6Thy1-eYFP APP/PS1-21. This mouse is an APP/PS1-21 with an added eYFP under the Thy1 neuronal promoter which allows us to visualise neurons by confocal microscopy without having to infect, transfect or label them. Images were acquired with 350 µm brain slices of 3month-old animals by confocal microscopy and isolated dendrites from the hippocampus were analysed (fig1.A). Our results show that APP/PS1 Thy1-eYFP mice have a significant 22.79±5.04% ( § § § §p<0.0001) decreased total spine density (fig1.B, comparing the whole bars) compared to their C57BL/6Thy1-eYFP (Wild-type Thy1-YFP) littermates. When we looked in more detail at the spine subpopulation (thin, stubby and mushroom spines) of these mice we found that there was a 138.64±13.65% ( † † † †p<0.0001) increase in thin spines (fig1.B, comparing the grey segments of the bar graphs). Yet, most remarkably, the major spine loss was due to a strong 44.50±4.35% (****p<0.0001) decrease in mushroom spine density (fig1.B, comparing the black segments of the bar graph) in the AD mouse hippocampus compared to wild-type Thy1-YFP littermates. We next measured the volume of the remaining mushroom spines in the AD mice and found a significant 114.64±5.51% (***p=0.0003) increase in volume of these spines compared to wild-type Thy1-YFP littermates (fig1.C). We carried out the same set of analyses for the cortices of these AD mice (fig1. D) and found a similar decrease of 27.43±5.18% ( § §p=0.0099) in total spine density (fig1.E, comparing the whole bars) as well as a strong decrease of 40.59±6.43% (***p=0.0002) of mushroom spine density (fig1.E, comparing the black segments of the bar graph) compared to their wild-type Thy1-YFP littermates. The volume of the remaining mushroom spines tended also to be increased compared to wild-type Thy1-YFP littermates (fig1.F) though non-significant (p=0.07871).

  induced by APP x -mCh expression in neurons before and after synaptic activity. Here we quantified and compared the red APP marking in the soma of APP wt -(fig5.A), APP swe -(fig5.B), APP osa -(fig5.C) and APP ice -mCh (fig5.D) in neurons cotransfected with LA-GFP, before synaptic activation (Before BIC15) versus after induction of synaptic activity (After BIC15: incubation with bicuculline methiodide [50 µM] and 4-aminopyridine [2,5 mM] for 15 min). Our results show that red APP wt , APP osa and APP ice marking area decreased by 23.80±1.16%; 20.46±3.37% and 23.07±3.96% respectively (fig5.G).

Figure 1 .

 1 Figure 1. APP/PS1-21 Thy1-YFP mice have a decreased spine density and increased mushroom spine volume, in the hippocampus and cortex, at 3 months old. (A) Representative confocal images of dendrites with spines from acute hippocampal (Hp) slices of (Left panel) wild-type Thy1-YFP and (Right panel) APP/PS1-21 Thy1-YFP mice at 3 mo (scale bar = 2 µm). (B) Bar graphs (mean ± SEM) show the distribution of dendritic spine subtypes (spine type and density per µm of dendrite) per condition. § § § §p<0.0001 when compared to total spine density (whole bar) of wild-type Thy1-YFP Hp; ****p<0.0001 when compared to mushroom spine density (black segment in bar) of wild-type Thy1-YFP Hp; † † † †p<0.0001 when compared to thin spine density (grey segment in bar) of wild-type Thy1-YFP Hp. (C) Bar graphs (mean ± SEM) show mushroom spine volume (in µm 3 ) per condition. ***p<0.001 when compared to wild-type Thy1-YFP Hp. (D) to (F) are the same as (A) to (C) but for cortical (Cx) slices. § §p<0.01 when compared to total spine density (whole bar) of wild-type Thy1-YFP Cx; ***p<0.001 when compared to mushroom spine density (black segment in bar) of wild-type Thy1-YFP Cx; †p<0.05 when compared to thin spine density (grey segment in bar) of wild-type Thy1-YFP Cx. Mann-Whitney test. N=3 animals per strain; n=19 to 23 neurons per condition.

Figure 2 .Figure 3 .

 23 Figure 2. Overexpression of APP wt , APP swe and APP osa in cortical cell cultures decrease spine density and increase mushroom spine volume whereas APP ice overexpression does not. (A, Top row) Representative confocal images of cultured cortical neurons overexpressing APP wt -mCh, APP swe -mCh, APP osa -mCh, APP ice -mCh and (A, middle row) LifeActin-GFP (LA-GFP) (scale bar = 10 µm). Small square inlay show representative dendritic spines (scale bar = 2 µm). (A, bottom row) Representative dendrite portions (scale bar = 5 µm). (B) Bar graphs (mean ± SEM) show the distribution of dendritic spine subtypes (spine type and density per µm of dendrite) per condition. ***p<0.001; ****p<0.0001 when compared to total spine density (whole bar) of control neurons (neurons only expressing LA-GFP); †p<0.05; † † †p<0.001; † † † †p<0.0001 when compared to mushroom spine density (black segment in bar) of control neurons; ‡p<0.05 when compared to stubby spine density (white segment in bar) of control neurons; § §p<0.01; § § §p<0.001 when compared to thin spine density (grey segment in bar) of control neurons. (C) Bar graphs (mean ± SEM) show mushroom spine volume (in µm

Figure 4 .

 4 Figure 4. Quantification of intracellular and extracellular Aβ x from neurons infected with APP wt , APP swe , APP osa or APP ice . (A) Bar graphs (mean ± SEM) show that cortical neurons infected with APP wt , APP swe , APP osa or APP ice produce intracellular Aβ x , especially APP swe and APP osa . (B) Bar graphs (mean ± SEM) show that cortical neurons infected with APP wt , APP swe , APP osa or APP ice produce extracellular Aβ x especially APP swe . (C) Westernblot of infected neuron lysate showing no difference in overall APP levels using Y188 antibody. N=4 different cortical neuron cultures.

Figure 5 .

 5 Figure 5. Synaptic activity decreases red marking in APP x -mCh, especially APP swe -mCh, expressing cortical neurons and is abrogated by βSecI pretreatment. (A, Top row) Representative confocal images of cortical neuron overexpressing APP wt -mCh and LA-GFP, before synaptic activation. (Top left panel) overlay of APP wt -mCh and LA-GFP. (Top middle panel) outline of neuron with APP wt -mCh marking. (Top right panel) cell body of neuron. (Bottom row) same neuron with decreased red marking after synaptic activation with BIC15 protocol (BIC15: incubation with bicuculline methiodide [50 µM] and 4-aminopyridine [2,5 mM] for 15 min). (B) Same as (A) with cortical neuron overexpressing APP swe -mCh and LA-GFP before and after BIC15. (C) Same as (A) with cortical neuron overexpressing APP osa -mCh and LA-GFP before and after BIC15. (D) Same as (A) with cortical neuron overexpressing APP ice -mCh and LA-GFP before and after BIC15. (E, Top row) Representative confocal images of cortical neuron pretreated with βsecI (1 µM post-transfection) and overexpressing APP swe -mCh + LA-GFP before BIC15. (Top left panel) overlay of APP swe -mCh and LA-GFP. (Top middle panel) outline of neuron with increased APP swe -mCh fluorescence. (Top right panel) cell body of neuron. (Bottom row) same neuron with no decreased red marking after BIC15 protocol. (F) Same as (E) with cortical neuron pretreated with βsecI (1 µM post-transfection) and overexpressing APP osa -mCh + LA-GFP before and after BIC15, showing no decrease in red marking. (G) Bar graphs (mean ± SEM) show a similar decrease of APP x -mCh red marking as APP wt -mCh after BIC15 protocol (expressed as % variation of APP x -mCh fluorescence normalised to APP x -mCh fluorescence before BIC15) except APP swe -mCh which displays a significant decrease. ****p<0.0001 compared to APP wt -mCh. Kruskal-Wallis followed by Dunn's test. (H) Bar graphs (mean ± SEM) show an inhibition of red marking decrease induced by BIC15 protocol when neurons are pretreated with βsecI. **p<0.01 APP swe vs APP swe +βsecI; *p<0.05 APP osa vs APP osa +βsecI. Mann-Whitney test. All scale bars = 10 µm. N=5 to 6 neurons per condition from at least 3 different cultures.

Figure 6 .

 6 Figure 6. Enlarged mushroom spines, induced by APP wt , APP swe or APP osa overexpression, lose their ability to increase their volume after synaptic activation protocol (BIC15). Mushroom spines analysed by confocal microscopy after neurons transfected with APP x -mCh and LA-GFP. (A, Top panels) Representative confocal images of a mushroom spine per condition before synaptic activation (scale bar = 1 µm). (Bottom panels) the same mushroom spines per condition after synaptic activation (BIC15: incubation with bicuculline methiodide 50 µM and 4-aminopyridine 2,5 mM for 15 min). (B) Bar graphs (mean ± SEM) show mushroom spine volume before (CT) and after BIC15 per condition. *p<0.05; n.s. p>0.05 when compared to mushroom spine volume before BIC15. (C) Bar graphs (mean ± SEM) show mushroom spine volume before and after BIC15 per condition when there is no Aβ present. n.s. p>0.05 when compared to mushroom spine volume before BIC15. Wilcoxon Signed Rank test. N=7 neurons per condition from at least 3 different cultures.

Figure 7 .

 7 Figure 7. Activity-dependent actin dynamics in mushroom spines is altered by APP wt , APP swe , APP osa but also APP ice and βsecI treatment. (A) Representative FRAP timelapse of a mushroom spine in resting conditions (no synaptic activation) showing 80% fluorescence recovery. (B) Representative FRAP timelapses of mushroom spines pretreated with BIC15. (Top row) representative FRAP timelapse of mushroom spine pretreated with BIC15 showing activity-dependent stabilisation of actin cytoskeleton in spine (Act-GFP only) with 50% fluorescence recovery. (Middle row) representative FRAP timelapse of mushroom spine pretreated with BIC15 when there is no activity-dependent stabilisation of actin cytoskeleton in spine (>60% fluorescence recovery).(Bottom row) representative FRAP timelapse of mushroom spine pretreated with BIC15 when there is activitydependent over-stabilisation of actin cytoskeleton in spine (<50% fluorescence recovery). (C) Representative FRAP curves (% FRAP normalised to prebleached intensity of the spine of Actin-GFP +/-APP x -mCh transfected neurons +/-βsecI) of Actin-GFP in resting conditions (before BIC15 protocol). (D) Bar graphs show the plateau values (mean ± SEM of the last 10 points for each FRAP curve) for each condition before BIC15 protocol. No statistical differences across the different conditions. (E) Representative FRAP curves of Actin-GFP (+/-APP x -mCh +/-βsecI) after BIC15 protocol showing either activity-dependent 50% stabilisation (Actin-GFP only) or no activity-dependent stabilisation (APP ice -mCh, APP swe +βsecI) or an over-stabilisation (APP wt -mCh, APP swe -mCh, APP osa -mCh) of actin cytoskeleton in spines. (F) Bar graphs (mean ± SEM) of plateau values for each condition after BIC15 protocol showing an activity-dependent stabilisation of actin cytoskeleton in spines of Actin-GFP only expressing neurons (50% fluorescence recovery); no activity-dependent stabilisation (>60% fluorescence recovery) of actin (for neurons expressing Actin-GFP and APP ice -mCh or APP swe -mCh+βsecI); and an activitydependent over-stabilisation (<50% fluorescence recovery) of actin in neurons expressing Actin-GFP and APP wt -mCh or APP swe -mCh or APP osa -mCh.*p<0.05 when compared to Act-GFP only BIC15; † † † †p<0.0001 APP swe BIC15 vs APP swe +βsecI BIC15. Kruskal-Wallis followed by Dunn's test. N=at least 25 spines per condition from at least 3 different cultures.

Figure 8 .

 8 Figure 8. Aβ wt and Aβ osa , but not Aβ ice significantly decreases depolymerisation rate of actin in vitro. (A) Depolymerisation curve of actin in presence of 100 nM of either Aβ wt , Aβ osa and Aβ ice . (B) Bar graph (mean ± SEM) shows a significant decrease of the depolymerisation rate of actin (expressed as Δfluorescence from beginning to mean plateau value) in presence of Aβ wt and Aβ osa but not Aβ ice . *p=0.0382 for CT vs Aβ wt ; n.s. p=0.9245 for CT vs Aβ ice ; *p=0.0276 for CT vs Aβ osa . Kruskal-Wallis followed by Dunn's test. N=3 independent experiments.

Figure 9 .Figure 10 .

 910 Figure 9. Overexpression of APP swe3M and APP osa3M and APP wt3M in cortical cell cultures did not induce a decrease in spine density or an increase in mushroom spine volume. (A) Comparison of Aβ wt with other actin binding proteins involved in actin bundling showing a common sequence and design of Aβ 3M mutation. (B, Top row) Representative confocal images of cultured cortical neurons overexpressing APP swe3M -mCh, APP osa3M -mCh, APP wt3M -mCh and (B, Middle row) LA-GFP (scale bar = 10 µm). (B, Bottom row) Representative dendrite portions (scale bar = 5 µm). (C) Bar graphs (mean ± SEM) show the distribution of dendritic spine subtypes (spine type and density per µm of dendrite) per condition. n.s. p>0.05 when compared to Ctrl (LA-GFP only) total spine density (whole bar); ***p<0.001 APP swe vs APP swe3M , APP osa vs APP osa3M and APP wt vs APP wt3M . (D) Bar graphs (mean ± SEM) show mushroom spine volume (in µm

Figure 13 .

 13 Figure 13. Long-term potentiation in acute mouse hippocampal slices is impaired by exposure to Aβ wt and Aβ osa but not by Aβ wt3M or Aβ ice . Long-term potentiation (LTP) in the hippocampal CA1 region was induced by delivering two 100 Hz protocols (2 x 100 Hz) with an interval of 20 sec to the Schaffer collateral/commissural pathway. Aβ x peptides were added to the ACSF bath (final concentration of Aβ x : 100 nM) 15 min prior to recording. 2 x 100 Hz was delivered after 15 min of stable baseline. Each point on the graph represents the mean ± SEM. **p=0.0017; ****p<0.0001 when comparing the last 10 time points of fEPSP Slope (% of baseline) to Control conditions (Ctrl). †p=0.0128 Aβ wt vs Aβ wt3M . Kruskal-Wallis followed by Dunn's test. N=at least 5 slices per condition.

Figure 14 .

 14 Figure 14. Overexpression of APP wt , APP swe in cortical cell cultures decrease spine density of neighbouring healthy wild-type neuron, in an APP-dependent manner, whereas APP ice and APP osa overexpression does not. (A) Representative confocal images of cultured cortical neurons where the neuron on the left is overexpressing either APP wt -mCh, APP swe -mCh or APP osa -mCh or APP ice -mCh and LA-GFP (APP neuron); and the neuron on the right is only overexpressing LA-GFP (healthy neuron) (scale bar = 10 µm). (B) Bar graphs (mean ± SEM) show spine density of healthy neuron depending on the distance from APP neuron. *p<0.05; **p<0.01 when compared to control condition (both neurons only overexpress LA-GFP) at equivalent distance. RM two-way ANOVA followed by Tukey's multiple comparison test. N=at least 3 neurons per condition from 3 different cultures. (C) Dotblot of Aβ synth , Aβ wt , Aβ osa or Aβ ice with added wild-type neuronal membrane extracted protein revealed with Ponceau red (to show total protein load before membrane extracted protein added) and 22C11 (to reveal APP from membrane extracted protein bound to Aβ x ) showing an interaction of the different Aβ x with APP at the neuronal plasma membrane.

Figure 34 )

 34 , an excessive amount (Pathologic condition: excess Aβ, Figure 35) or a lack/absence of the peptide ("Pathologic" condition: absence Aβ, Figure 36).

Figure 34 :

 34 Figure 34: Potential novel physiological role for Aβ in activity-dependent actin stabilisation in dendritic spines. (Left side)An excitatory glutamatergic synapse in resting conditions. (Right side) Upon synaptic activation, amyloïdogenic processing of APP is increased, generating intracellular Aβ peptides (that can be secreted and reinternalised depending on its sequence?) which bind to the actin cytoskeleton in spines; decreasing the depolymerisation rate of, and ultimately stabilising, actin cytoskeleton. This stream of events could contribute to the activity-dependent increase in volume of mushroom spines, revealing a physiological role for Aβ in synaptic plasticity.

Figure 35 :

 35 Figure 35: When the physiological role becomes pathological with excess Aβ and leads to synaptic dysfunction and eventually loss. (Left side) An excitatory glutamatergic synapse in resting conditions where there is excess intracellular Aβ.This Aβ, which could be interacting with the actin cytoskeleton, could in part explain the increased spine volume in resting conditions. (Right side) Upon synaptic activation, as APP is more abundant, amyloïdogenic processing of APP is even more increased, generating excessive amounts of intracellular Aβ peptides which bind to the actin cytoskeleton in spines; further decreasing the depolymerisation rate of, and ultimately overstabilising actin cytoskeleton, which results in no increase in spine volume. If the Aβ sequence allows the peptide to be secreted into the extracellular space, it can then "contaminate" neighbouring neurons via an APP-dependent mechanism. This stream of events could in part explain synaptic dysfunction and eventual spine loss, observed in AD.

Figure 36 :

 36 Figure 36: Absence of Aβ could also lead to synaptic dysfunction and eventually loss. (Left side) An excitatory glutamatergic synapse in resting conditions where there is no intracellular Aβ. (Right side) Upon synaptic activation, as amyloïdogenic processing of APP no longer occurs (either by absence of APP or by inhibition of β-secretase), no Aβ is generated thus there is no Aβ to potentially interact with the actin cytoskeleton in spine. Therefore there is no decrease of the depolymerisation rate of actin cytoskeleton, which results in an impaired activity-dependent stabilisation of actin in spines ultimately leading to no increase in spine volume. This stream of events could also lead to synaptic dysfunction and eventual spine loss.

Table 1 : Genes implicated in risk of early-onset Alzheimer's disease (adapted from Giri et al., 2016).

 1

	Gene Symbol	Location	Function	Pathway
	APP	21q21.3	Neuronal development, Synaptic formation and repair, β-Amyloid production	APP processing
	PS1	14q24.3	γ-Secretase activity, Intracellular signalling, β-Amyloid production	APP processing
	PS2	1q42.13	γ-Secretase activity, β-Amyloid production, Synaptic plasticity APP processing

Table 2 : Common and rare gene variants associated with Alzheimer's disease identified by GWAS (adapted from Giri et al., 2016).

 2

  40 /Aβ 42 ratio approximately 11-fold, simultaneously increasing Aβ 42 and decreasing Aβ 40 secretion (Kumar-Singh et al., 2000). The German V715A increases Aβ 42 and decreases Aβ 40 levels. It has been observed that Aβ 40 /Aβ 42

	 German (V715A) and French (V715M)

ratio is increased fourfold in HEK293 cells and primary neuronal cultures

(De Jonghe et al., 2001;

Table 3 : Ongoing BACE-Inhibitor Clinical trials (adapted from Zhu et al

 3

., 2018). NCT: National Clinical Trial

Table 4 : BACE1 substrates involved in synaptic plasticity (adapted from Zhu et al., 2018).

 4

Table 5 : Consequences of genetic deletion and pharmacological inhibition of BACE1 on synapses (adapted from Zhu et al., 2018).

 5

Table 6 : Examples of receptors and transporters that interact with Aβ

 6

	Aβ binding receptors/transporters		Effects	Reference
	NMDA receptors		 	↗ neurotoxic calcic influx Endocytosis of NMDAr	De Felice et al., 2007 Snyder et al., 2005
	AMPA receptors		Calcineurin-dependent endocytosis of AMPAr	Zhao et al., 2010
	GABA receptors		Inhibition of GABAr	Ulrich, D., 2015
	αν type Integrins		Inhibition of LTP (Long Term Potentiation)	Wang et al., 2008
	Prion protein receptors (PrP	c )	 	Binding at lipid rafts Activates Src Kinase Fyn	Um et al., 2013
				Receptor internalisation
	Nicotinic α7nACh receptors		Intracellular accumulation of Aβ	Wang et al., 2000
				Induces NMDAr endocytosis
				Binding to receptor or via its ligand ApoE
	Lipoprotein-related receptors (LRP1)		Induces LRP1 endocytosis	Fuentealba et al., 2010
				Intracellular accumulation of Aβ

Table 7 : Examples of neurotransmitters and their postsynaptic effects.

 7

Table 8 : Overview of the principal characteristics of ionotropic and metabotropic glutamate receptors.

 8

	Family						iGluRs				mGluRs
	Receptor		AMPA			Kainate		NMDA	Group I	Group II	Group III
	Structure of complex				Homo/hetero-oligomeric	Homo/hetero-oligomeric
										GluN1
	Subunits	GluA1 to 4	GluK1 to 5	GluN2 (A, B, C, D)	mGluR1 and 5	mGluR2 and 3	mGluR4, 6, 7, 8
										GluN3 (A, B)
	Permeability
	(iGluRs) or induced effects	Na	+ , K	+ , (Ca	2+ )	Na	+ , K	+ , (Ca	2+ )	Na Ca 2+ +++ + , K + ,	↑ PLC	↓ AC
	(mGluRs)
	Localisation	Postsynaptic	Pre-and postsynaptic	Postsynaptic	Postsynaptic	Presynaptic

Table 9 : Particularities that differentiate metabotropic and ionotropic glutamate receptors.

 9

	Family		iGluRs			mGluRs
	Receptor	AMPA	Kainate	NMDA	Group I	Group II	Group III
	Particularities	-GluA2 impermeable to Ca 2+ -Fast excitatory synaptic transmission	-2 binding sites glutamate release with different affinities -Involved in epilepsy -Modulates	-Channel blocked by Mg synaptic plasticity 2+ at resting potential -Learning, memory, induction of	Regulation of neuronal excitability via ion channel modulation	Involved in inhibition of glutamate and other neurotransmitter release
				+++
	Agonists	AMPA	Kainate	NMDA	(S)-3,5-DHPG	DCG-IV
	Antagonists	DNQX, NBQX, CNQX	GYKI 53655	AP-V	AIDA, MPEP	LY341495
	Voltage sensitivity	Voltage-dependent	Voltage-dependent	Voltage-dependent	G-Protein coupled receptor

  , when we looked at what happened after BIC15 results were very different. In control neurons, synaptic activation resulted in 49.04±2.63% FRAP (fig7.B top row, E and F) which means synaptic activity induces actin stabilisation in spines (50% S). In neurons overproducing toxic Aβ wt and Aβ osa , such as APP wt -mCh, APP swe -mCh and APP osa -mCh overexpressing neurons, synaptic activation

	resulted in 31.40±2.70% (*p=0.0362); 31.56±1.76% (*p=0.0380) and 30.78±2.22% (*p=0.0457) FRAP

respectively (fig7.B bottom row, E and F) which means there is an over stabilisation of actin in spines (Over S). Interestingly, in neurons overproducing Aβ ice or neurons that have no Aβ (APP swe + βSecI pretreatment), synaptic activity did not induce actin stabilisation in spines with 68.08±2.40% (*p=0.0115) and 80.30±1.82% (****p<0.0001) respectively (fig7.B middle row, E and F) which means there is no activity-dependent stabilisation of actin in spines (No S).

common sequence with other actin binding proteins and mutating this sequence within APP causes loss of generated Aβ synaptotoxicity without modifying APP processing

  Aβ wt and Aβ osa to 41.56±0.22% (*p=0.0382) and 41.64±0.71% (*p=0.0276) respectively, whereas in presence of Aβ ice there is no statistical difference (fig8.B,).These first results indicate that toxic Aβ wt and Aβ osa seem to interact with actin and hinder its depolymerisation rate, whereas non-toxic Aβ ice does not.

	Aβ has a

Exogenous application of toxic Aβ wt and Aβ osa on neurons causes mushroom spine loss and enlarged remaining ones but not Aβ wt3M nor Aβ ice

  So far, all the effects observed seemed to be mainly due to the intracellular pool of Aβ, since Aβ osa , which is mostly intracellular, induced the same toxic effects as excessive wild-type Aβ. We next wanted to test the effects of extracellular Aβ wt , Aβ osa , Aβ ice and Aβ wt3M at equal pathological concentrations on spine morphology. For this, we first imaged LA-GFP expressing cultured cortical neurons (fig12.A). Then we incubated these neurons for 24 h with 100 nM of either: Aβ wt , Aβ osa , Aβ ice Aβ wt and Aβ osa application respectively. These results show that, at equivalent concentrations, toxic Aβ wt and Aβ osa induce synaptotoxic effects whereas non-toxic Aβ ice and Aβ wt3M do not. This demonstrates that the 3M mutation induces Aβ loss of toxicity, since Aβ wt is toxic and Aβ wt3M is not (fig12.C, D and G, comparing Aβ wt vs Aβ wt3M : ##p=0.0034 for total spine density; ###p=0.0004 for mushroom spine density and ####p<0.0001 for mushroom spine volume), and Aβ ice seems to behave in the same way as Aβ wt3M .

	or Aβ wt3M , and then imaged them again (fig12.B). We then analysed spine density (fig12.C to F) and
	volume of mushroom spines (fig12.G). Our results showed that total spine density was affected by 24
	h of Aβ wt and Aβ osa with a 15.23±3.36% (****p<0.0001) and 28.67±3.12% (****p<0.0001) decrease
	respectively (fig12.A, B: 2 nd and 3 rd column, and C). This loss of spines was mainly due to a strong
	decrease in mushroom spine density, other spine subtypes were not significantly affected (fig12.D, E
	and F). Furthermore, the volume of the remaining mushroom spines was increased by 112.66±2.34%
	(****p<0.0001) and 114.14±1.84% (****p<0.0001) after Long-
	BIC15, synaptic activation resulted in no actin stabilisation with 69.39±2.98%; 67.39±1.31% and
	73.31±3.34% FRAP in spines for APP swe3M -, APP osa3M -and APP wt3M -mCh overexpressing neurons
	respectively (fig11.F and G), similar to what was obtained with Aβ ice overproducing neurons or in
	absence of Aβ with βSecI pretreatment (see fig8.E and F). This indicates that, when Aβ wt3M or Aβ osa3M
	is overproduced in neurons, activity-dependent actin stabilisation in spines no longer occurs.

term potentiation is impaired by exogenous application of Aβ wt and Aβ osa but not with Aβ ice or Aβ wt3M

  To go further into the investigation of Aβ's synaptotoxicity depending on its sequence, we next carried out functional tests by evaluating synaptic plasticity via electrophysiological recordings. To do so, we performed long-term potentiation experiments (LTP) on wild-type mouse acute hippocampal slices incubating in 100 nM of Aβ wt , Aβ osa , Aβ ice or Aβ wt3M peptides. We observed an impaired LTP for slices incubating in Aβ wt (**p=0.0017, compared to Ctrl) and even more so for slices incubating in Aβ osa (****p<0.0001, compared to Ctrl). Aβ ice and Aβ wt3M , however, had no effect on LTP (fig13.).

  (Bottom row) same neuron with no decreased red marking after BIC15 protocol. (F) Same as (E) with cortical neuron pretreated with βsecI (1 µM post-transfection) and overexpressing APP osa -mCh + LA-GFP before and after BIC15, showing no decrease in red marking. (G) Bar graphs (mean ± SEM) show a similar decrease of APP x -mCh red marking as APP wt -mCh after BIC15 protocol (expressed as % variation of APP x -mCh fluorescence normalised to APP x -mCh fluorescence before BIC15) except APP swe -mCh which displays a significant decrease. ****p<0.0001 compared to APP wt -mCh. Kruskal-Wallis followed by Dunn's test. (H) Bar graphs (mean ± SEM) show an inhibition of red marking decrease induced by BIC15 protocol when neurons are pretreated with βsecI. **p<0.01 APP swe vs APP swe +βsecI; *p<0.05 APP osa vs APP osa +βsecI. Mann-Whitney test. All scale bars = 10 µm. N=5 to 6 neurons per condition from at least 3 different cultures.

The 3M mutation abrogates the effect of toxic Aβ on actin in vitro and in cortical neuron dendritic spines. (A)

  of APP wt3M -mCh, APP swe3M -mCh and APP osa3M -mCh as their non-3M counterpart. n.s. p>0.05 APP wt vs APP wt3M ; APP swe vs APP swe3M ; APP osa vs APP osa3M . All scale bars = 10 µm. Mann-Whitney test. N=6 neurons per condition from at least 3 different cultures. Depolymerisation curve of actin in presence of 100nM of either Aβ wt , Aβ wt3M (and Aβ ice to show similar behaviour with Aβ wt3M ). (B) Bar graph (mean ± SEM) showing a significant decrease of the depolymerisation rate of actin (expressed as Δfluorescence from beginning to mean plateau value) in presence of Aβ wt but not Aβ wt3M . *p=0.0219 for CT vs Aβ wt . Kruskal-Wallis followed by Dunn's test. N=3 independent Bar graphs show the plateau values (mean ± SEM of the last 10 points for each FRAP curve) for each condition before BIC15 protocol. No statistical differences across the different conditions. (F) Representative FRAP curves of Actin-GFP (+/-APP x -mCh) after BIC15 protocol showing either activitydependent 50% stabilisation (Actin-GFP only) or no activity-dependent stabilisation (APP swe3M -mCh, APP osa3M -mCh, APP wt3M -mCh) or an over-stabilisation (APP wt -mCh, APP swe -mCh, APP osa -mCh) of actin cytoskeleton in spines. (G) Bar graphs (mean ± SEM) of plateau values for each condition after BIC15 protocol showing an activity-dependent stabilisation of actin cytoskeleton in spines of Actin-GFP only expressing neurons (50% fluorescence recovery); no activity-dependent stabilisation (>60% fluorescence recovery) of actin (for neurons expressing Actin-GFP and APP swe3M -mCh or APP osa3M -mCh, or APP wt3M -mCh); and an activity-dependent overstabilisation (<50% fluorescence recovery) of actin in neurons expressing Actin-GFP and APP wt -mCh or APP swe -mCh or APP osa -mCh.***p<0.001; ****p<0.0001 when compared to Act-GFP only BIC15; † † † †p<0.0001 APP swe BIC15 vs APP swe3M BIC15, APP osa BIC15 vs APP osa3M BIC15, APP wt BIC15 vs APP wt3M BIC15. Kruskal-Wallis followed by Dunn's test. N=at least 19 spines per condition from at least 3 different cultures.

	Figure 11. experiments. (C) Representative FRAP timelapse of a mushroom spine showing no actin stabilisation (80% fluorescence recovery). (D) Representative FRAP curves (% FRAP normalised to prebleached intensity of the spine of Actin-GFP +/-APP x -mCh transfected neurons) of Actin-GFP in control conditions (before BIC15 protocol). (E) Figure 12.

Exogenous application of Aβ wt and Aβ osa induces mushroom spine density reduction and mushroom spine enlargement but not Aβ ice nor Aβ wt3M . (A, Top row)

  Representative confocal images of cultured cortical neurons expressing LA-GFP before treatment with exogenous application of Aβ wt (scale bar = 10 µm). (Bottom row) Dendrite portions with mushroom spines (white arrows) before treatment with different Aβ (scale bar = 5 µm). (B, Top row) Representative confocal images of the same neurons after 24 h incubation with 100 nM of different Aβ (scale bar = 10 µm). (Bottom row) Dendrite portions with less mushroom spines (white arrows) after 24 h incubation with Aβ wt and Aβ osa but not with Aβ ice or Aβ wt3M (scale bar = 5 µm). (C) Quantification of total spine density (mean ± SEM) shows a reduction of total number of spines after treatment with Aβ wt and Aβ osa . ****p<0.0001 when compared to treatment with Veh; ##p<0.01 Aβ wt vs Aβ wt3M (D) Quantification of mushroom spine density (mean ± SEM) show a reduction of number of mushroom spines after treatment with Aβ wt and Aβ osa . ****p<0.0001 when compared to treatment with Veh; ###p<0.001 Aβ wt vs Aβ wt3M Quantification of (E) stubby spine and (F) thin spine density (mean ± SEM) show no changes after treatment with the different Aβ. (G) Quantification of mushroom spine volume (mean ± SEM) shows an increase after treatment with Aβ wt and Aβ osa but not with Aβ ice or Aβ wt3M . ****p<0.0001 when compared to treatment with Veh; ####p<0.0001 Aβ wt vs Aβ wt3M . RM two-way ANOVA followed by Tukey's multiple comparison test. N=6 neurons from at least 3 different cultures.

https://alzheimersnewstoday.com/aducanumab/ https://www.the-scientist.com/news-opinion/biogen--eisai-end-two-late-stage-trials-for-alzheimers-treatment-66431

) per condition. *p<0.05; **p<0.01 when compared to control neurons (expressing only LA-GFP). Kruskal-Wallis followed by Dunn's test. N=7 neurons per condition from at least 3 different cultures.

) per condition. n.s. p>0.05 when compared to Ctrl (LA-GFP only); ****p<0.0001 APP swe vs APP swe3M , APP osa vs APP osa3M and APP wt vs APP wt3M . Kruskal-Wallis followed by Dunn's test. N=7 neurons per condition from at least 3 different cultures.

Acknowledgments

Acknowledgments

This research was supported by the FEDER -CoInside program and the "Fondation pour la Recherche Médicale" FRM FDT201805005139.

α C-terminal Fragment (Non-Amyloïdogenic pathway) C99/β-CTF: β C-terminal Fragment (Amyloïdogenic pathway) CA1/3: Hippocampal Cornu Ammonis Region 1/

 An APP-dependent propagation of the pathology throughout the brain

To get an insight into how the disease propagates throughout the brain at AD onset, we used an innovative system whereby one neuron overexpresses one of the APP mutants and a nearby neuron does not. This technique enables to investigate the effect of a "pathological neuron" on a nearby "healthy" neuron. This experiment was carried out in both wild-type and APP knock-out (APPKO) mouse cultured cortical neurons in order to assess the importance of APP in disease progression from neuron to neuron.

VI. Results

Introduction

A.

The results of this thesis are presented in the attached document, currently the subject of a research article to be submitted for publication:

Research article B.

APP mutations unveil distinct roles for Aβ: while intracellular Aβ modulates synaptic plasticity, extracellular Aβ participates in Alzheimer's disease propagation R.L.Powell 1,2 ; M.Jacquier-Sarlin 1 ; S.Boisseau 1,2 ; E.Borel 1,2 ; F.Lanté 1 ; A. seemingly as a result of defective activity-dependent actin dynamics in the spines. These synaptic alterations seem to be due to a newly-identified interaction between actin and Aβ, hinting a possible physiological role for Aβ in activity-dependent synaptic plasticity. This synaptic activity modulates amyloïdogenic APP processing which would explain synaptic dysfunction in pathological conditions.

Furthermore, we show that Aβ sequence is as important as Aβ concentration in inducing synaptic alterations. Lastly, we bring to light that neurons secreting Aβ also affect nearby neurons in an APPdependent manner, reminiscent of a prion-like mechanism. Together these results demonstrate that APP processing is a finely tuned equilibrium involved in actin-remodelling during activity-dependent synaptic plasticity and opens a new route for AD therapeutic strategies.

resuspended in 1% Triton X-100 in PBS and centrifuged at 16,000 × g for 30 min. The supernatant was kept and used as detergent-soluble membrane proteins in farwestern experiments. For this purpose, various Aβ mutants (4 µg) or the vehicle control (PBS) was applied to nitro-cellulose membrane, and the membrane was blocked with 5% BSA for 2 h at room temperature. The membrane was then incubated with detergent-extracted membrane proteins for 12 h at 4°C, followed by incubation with the APP monoclonal antibody 22C11 and development by ECL.

Actin depolymerisation assay

Pyrene-labeled muscle actin (20%) was mixed with dark actin at 2 µM in general actin buffer ( 5mM Tris-HCl pH 8, 0.2 mM CaCl 2 ) following the manufacturer's instructions (Cytoskeleton, Inc., Denver, CO, USA). Into wells of a black assay 96 well plate, 2 µM of pyrene actin were mixed along with 1 mM dithiothreitol (DTT) and 0. Stimulating electrodes (bipolar microelectrodes) were placed in the stratum radiatum to stimulate the Schaffer collaterals pathway. Field EPSPs (fEPSPs) were recorded in the stratum radiatum using a recording glass pipette filled with ACSF and were amplified with an EPC 10 Amplifier Patchmaster Multi-channel (HEKA Elektronik Dr. Schulze GmbH, Wiesenstrasse, Germany). Recordings were new therapeutic strategies. Our findings support the idea that there is no one-size-fits-all for AD treatment and that it should rather be made-to-measure for each individual AD patient.

VIII. Supplementary data

Supplementary Figure: Cultured cortical neurons from APPKO mice are not the best model for studying the effects of the absence of Aβ, as absence of APP causes altered spine density. (A) Bar graph (mean ± SEM) showing the distribution of dendritic spine subtypes (spine type and density per µm of dendrite) in wild-type (wt) neurons only overexpressing LA-GFP (no APP), wt neurons overexpressing LA-GFP and pretreated with βsecretase inhibitor [1 µm] (LAGFP βSecI) and APPKO neurons overexpressing LA-GFP (APPko). *p<0.05 when comparing total spine density (whole bar) of APPKO neurons vs wt neurons (no APP) or vs wt neurons pretreated with βSecI (LAGFP βSecI); †p<0.05 when comparing mushroom spine density (black segment in bar) of APPKO neurons vs wt neurons (no APP); §p<0.05 when comparing mushroom spine density (black segment in bar) of APPKO neurons vs wt neurons pretreated with βSecI (LAGFP βSecI). Mann-Whitney test. N=at least 4 neurons per condition from at least 3 different cultures. (B), (C), and (D)

show that activity-dependent actin remodelling in spines of cultured cortical neurons from APPKO mice is similar to wt neurons pretreated with βSecI, demonstrating that Aβ is involved in this process, not APP. In preparation:

IX. List of publications

Amyloid beta oligomers: a novel actin binding complex that influence synapse structure and function in Alzheimer's disease Rebecca Powell 1,2 , Adrien Paumier 1,2 , Jose Martinez-Hernandez §1,2 , Travis J. Rush