Rachel Thomas

TO WALK AS WE LISTEN TO THE CITY 1

Rachel Thomas CRESSON CNRS UMR 1563 EAG, 60 avenue de Constantine BP 2636 -38036 Grenoble Cédex 2 Email: rachel.thomas@grenoble.archi.fr Cities are built around the sense of sight, for those who can see. Since the XIXth century, and still today, this obviousness comes down through many work of literature and urban sociology about the mobility of the pedestrians in town. From Siegfried Kracauer to Georg Simmel, from Walter Benjamin to Lincoln A. Ryave and James N. Schenken, to walk in the city would almost exclusively request the visual attention of the pedestrian. But in all our towns and cities there are men and women, deprived of the sense of sight, going about their daily lives. How do people with impaired sight operate? What can we learn from them about the role of the senses in our cities and the process of getting about?

The aim of this article is not to focus on sight impediments. Experimental and cognitive psychology has been working on the subject for a long time. Rather we want to address the following question: how do the perceptible features of urban space (sound, sight, touch, hot and cold, etc.) contribute to the way we orientate ourself and we walk in the city ? Work on the mobility of the sight impaired has provided some answers to this question. On the one hand it reveals the perceptive skills (sound, visual, tactile, kinesthetic…) of pedestrians, notably the "affordances" [START_REF] Gibson | The ecological approach to visual perception[END_REF] of the perceptible urban environment. From this point of view movement around a town is initially considered as a process of adjustment between the skills of the pedestrian and the sensory features of the surround space. It is then defines as a process of anchoring of the pedestrian in urban public space : walk, in all its forms (to walk, run, stroll, cross…), indeed engages the pedestrian with the perceptible urban environment. In Return, this « anchoring » of the pedestrian takes part of the construction of the place, of the ambiance and of the city. The objective of this paper is to argue this position and notably to point out a relation of codetermination between the perceptible urban environment and the walk. So, the present paper is divided into three main parts : a presentation of an empirical method for gathering data on pedestrians' non-visual perception; a presentation of tactics for deciphering urban morphology; and lastly a consideration of the tactics adopted by the sight impaired to avoid obstacles. To conclude we shall consider the issue of the intersensory nature of the walk in the city and the merits of senseoriented thinking in architecture.

Gathering pedestrians' non-visual perception in cities in situ

The primacy granted to the visual experience in our contemporary societies makes us forget that the daily experience of the city engages the whole of our senses and the perceptible features of urban space. If the acoustic component of the urban environment causes a pollution which it is appropriate to understand, it must be studied like a component of the landscape of our cities and like an element determining of the walk of the pedestrian. The remark is true for the whole of the perceptible features of urban space. It arises methodological problems. How are we to gain access to the various sensory features of the urban space and the ways they are perceived in situ? How are we to observe and describe the process of adjustment between pedestrians' tactics and the perceptible environment?

Our method is based on an adaptation of the "commented route" method (Thibaud, 1999). Its aim is to "obtain reports on perception in movement, combining all perceptible features. With the commented route method we ask passers-by, regardless of whether they are regular users of the place or not, to travel a particular route in an urban environment and describe, as accurately as possible, what they perceive and feel as they proceed. Their comments, which are recorded then explored briefly in semi-directive interviews, are analysed to identify phenomena belonging to a shared experience. It is the redundancy and recurrence of similar descriptions made by different observers that demonstrates a community of perception for a given site" (Thibaud, 1999). In view of the specific target group with which we are working we made several adjustments to the survey protocol:

-Sight-impaired pedestrians need to memorise places and various points on which they take their bearings on routes used daily. The places along routes taken by the blind and sight impaired were necessarily selected from among other familiar routes, -Their perception of the various spaces and the perceptible environment, much as their motive capacity, varies from one person to the next depending on their disability. Our analysis of the results of the survey makes allowance for individual disabilities, notably partly or completely impaired vision2 . Lastly we proposed four route protocols to the sight impaired taking part in our study: one person following a customary route; two people following a route familiar to one but not to the other; taking pedestrians on an unknown route selected by the person conducting the survey; and guiding the investigator, temporarily sight impaired, along a route taken daily. Each person was asked to provide a detailed description of the spatial elements they perceived and any actions undertaken along the route. The aim of the four protocols was to increase pedestrians' ability to express their experience in words, in particular to describe the various perceptible elements of the environment enabling them to progress, find their bearings, decipher the urban morphology, avoid obstacles and cross roads. We identified a total of 15 routes in Grenoble, from day, in estival period. On each occasion participants were provided with a dictaphone to record their descriptions and comments. The investigator or a third party accompanied them and, if necessary, prompted more descriptive input. At the end of each walk we carried out a 30-minute semi-directive interview at a place along the route. The aim of this discussion was to gain a clearer picture of the various skills involved in walking the route, the obstacles and help afforded by the space and the manner in which each place was perceived.

Tactics used by the blind or sight impaired to decipher urban morphology

After analysis of the resulting corpus of data we can suggest several clearly established findings regarding the way in which people with sight disabilities decipher the urban morphology. In practical terms the form and topography of places are registered on the basis of the sensory data they provide for listening, seeing, feeling and sometimes touching. More specifically, if the route taken by pedestrians with no disabilities conforms to a logic of representation [START_REF] Lynch | L'image de la cité[END_REF], the route taken by partly or completely sight impaired pedestrians conforms to a logic of effectuation (or accomplishment). It is based on the body and senses reading the urban space. Let us take an example: how do sight-impaired pedestrians distinguish between a large urban highway, with several lanes of traffic, and a busy street in the centre open to motor vehicles. The spontaneous response is to suppose that they have particularly good hearing enabling them to distinguish clearly between the noise of road traffic and the conversation of passers-by. There is certainly no doubt about the auditory capacities of the sight impaired but they also know how to configure space. More specifically pedestrians select perceptible data from the environment mobilising their senses then organize them before acting. The work of configuration is done as part of the spatio-temporal dynamic of movement. It reflects the ability of sight impaired pedestrians to overcome the obstacles related to their disability by using sensory data from the environment.

Perception of the urban highway, and its recognition as such, is not just a question of hearing the noise of the vehicles. This perception is certainly of an auditory nature in the sight impaired, but it is discriminatory too. The urban highway is primarily characterised by a dilatation effect 3 : sounds (beat of steps and repeated tapping of the stick on the ground) produced by the sight impaired as they enter the site are reflected indirectly by the walls of buildings. Pedestrians realise that they carry further and that they can be clearly identified. Picking up the dilatation effect provides the sight impaired with an indication of the propagation area across which they are moving. It gives them an idea of the built-up dimensions of the site, enabling them to distinguish between urban highways and busy city streets. Furthermore the dilatation effect facilitates the sight impaired's work of sound marking and helps to keep them on a straight course. The dilatation effect combines with a wave effect. Once they are immersed in a place the sight impaired perceive road traffic as following a curve of intensity, with a similar shape to a wave and its ebb (crescendo, peak, break in the rapid or gradual sound, decrescendo). These sound cycles, interspersed with intervals of several seconds, correspond to vehicles stopping and starting at traffic lights. Most pedestrians treat this sound as an additional nuisance, but for the sight impaired hearing a wave effect not enables them to work out what is going on in a particular place but also to find their way through it. Those with partly impaired sight proceed differently. Given their cognitive and auditory abilities they pick up the wave effect but can also decipher the form and dimensions of the surrounding space thanks to their visual memory.

Recalling previous experience provides guidance on how to read a particular place.

Identifying a busy city street, not part of a pedestrian precinct, is a further indication of the hearing skills of the sight impaired. For the purposes of the survey most of the streets selected were located in the old town centre of Grenoble. They display various specific physical characteristics: symmetry and laterality of façades, narrowness and length of streets. On entering the site, the laterality and symmetry of the streets are perceived by picking up a reverberation effect. This effect takes the form of sound propagation of varying length once the source has stopped. As a result, in addition to the sound itself pedestrians hear the sound bouncing back from surfaces making up the surrounding space. The dimensions of the place are identified through the dull tonality of reverberated sound. Busy streets result in a narrowing effect. This sound effect is translated in a feeling of coming closer to the edge of a given space as pedestrians listen to the echo of the sound they themselves are making. The narrowing effect is the opposite of the dilatation effect.

On each occasion the sight impaired pedestrian "acts" on their perception of spatial morphology by becoming a producer of sounds (beat of steps and repeated tapping of the stick on the ground, whistling…) and by taking advantage of the sound qualities of a given place. Pedestrians without disabilities behave in the same way but have greater difficulty expressing their experience in words (Thomas, 2005). The reason for this silence is probably to be found in Western culture and once again the primacy it accords to the sense of sight. It is nevertheless fair to say that any movement depends on a process that gives shape to the perceptible public urban space. This configuration activity proceeds in three times : selection of perceptible qualities necessary to orientate and to walk, combination of these qualities to give them an operational character, finally incorporation of these perceptible qualities in the movement. This last phase of the activity of configuration is observed from a certain number of "behavioural indices" (Thomas, 2005) as modification of the trajectories, variations speeds of walk or adjustment of the body language. From this point of view, this configuration activity involves at one and the same time the physical and sensory qualities of a place, a pedestrian's steps, their body and their perceptive attention.

Tactics used by the blind or sight impaired to avoid obstacles

The tactics for avoiding obstacles are based on the same rationale. On the other hand they vary depending on the disability and bring into play several sensory features. Three types of obstacle need to be avoided by people with impaired sight when walking through a town : small immobile obstacles (bollards, signposts, flower tubs, etc.), bulky immobile obstacles (bus shelters, telephone boxes, public bench, etc.) and moving obstacles (cyclists, pedestrians, roller-skaters, etc.).

Small immobile obstacles represent a danger and a common source of accidents for people with sight disabilities. The reason is that this type of obstacle can only be detected with a stick, appearing suddenly as they move forward. Avoiding such obstacles affects the linearity of the course taken, the continuity of movement and sometimes the body integrity of the person. To overcome this problem the sight impaired memorize their daily route and the location of street furniture. This brings into a play a cognitive skill. Contrary, the presence of bulky immobile obstacles are more of a bother than a real obstacle for such pedestrians. Their detection requires intersensory skills, i. e. which imply two differents perceptible feature at least :

-For the sight impaired the key skill is auditory-tactile: the perception, at a distance, of this type of street furniture is based on sound. The object's mass changes the acoustics of the environment, creating a filtering effect. The obstacles constitutes a screen to the sound signal sent by sight impaired pedestrians when they tap the ground with their stick. When it is deflected by the object, the spectral envelope of the sound is changed. When the sight impaired pick up this sound it tells them about the physical characteristics of the obstacle. This auditory skill goes hand in hand with a tactile skill. The sight impaired are particularly sensitive to heat and air movements. They can thus detect air flows caused by their own movement and reflected back by objects in the vicinity. They refer to a sense of contact around their forehead, caused by variations in the pressure on their skin as they approach bulky obstacles. Psychologists refer to this skill as "facial perception" or "obstacle sense". -The partly sight impaired rely more on a visual-tactile skill. Poor sight does not alter the perception of contrasting colours and brightness. The partly sight impaired can consequently detect bulky obstacles because of the contrast in colour that it causes in their line of sight. Having identified it, they can find their way round it with their stick. Lastly, detecting moving obstacles brings into play the auditory skills of the sight impaired. It is the sudden change in the tonality of the sounds they hear or the arrival of a new sound that alerts them to a potential danger. At the time of the investigation, blind people of birth anticipated the trajectory of the urban buses which skirted the way that we borrow by differentiating de auditu noise of the buses'engines rolling to the gasoline of those rolling to gas. Once made distinction, it was easy for them to choose the ways which was not borrowed by the bus and so to walk without danger. The tactics of the pedestrian, who anticipates de auditu the speed and the trajectory of a vehicle before crossing, corresponds to the same logic : it's the variation of the level and of the tonality of the sounds which alerts him nearest arrival of a moving obstacle and wich authorizes (or not) him to cross.

Contrary to a generally accepted idea, none of these skills come into play in isolation. Each one combines with others to make it easier for pedestrians accurately to register their surroundings and get around town. Such skills are based on the perceptible environment of the public urban space. This perceptible environment is neither neutral, homogenous, nor single-sensed from the point of view of the pedestrian. It has an "effective motor" in the sense that it presents "holds" -for sight, hearing, touch and air displacement -to walk. It is their combination between skills of the pedestrian and the perceptible feature of urban space makes it possible to pass the disabling situations of mobility. But, because for each sense corresponds to a particular form of behaviour and way of apprehending space, these "holds" differently models the action and the perception of the pedestrian.

Conclusion

In the introduction of this article, we affirmed to have recourse to the example of the mobility of the sight impaired to choose how the perceptible feature of urban space contribute to the way we walk. What can we say now ? With which general information can we conclude ?

The first precisely relates to the sensory nature of walk in urban public space. From our point ok view, walk is a synesthesic activity which requests senses of the pedestrian and perceptible feature of urban public space. Through the example of mobility of the sight impaired, we show in what walk requests as much the capacity of listening of the pedestrian, its sensibility to heat or to the movements of air as the sound, tactile or thermal environment. Precisely, walk relies on a process of codetermination between the action of the pedestrian and the perceptible "hods" of the sensory environment. More than a simple translation of people between a point A and a point B or a combination of physical and biological process, walk generates a "carnal reports" between the pedestrian and the city.

The second contribution of our work more directly relates to the field of the design and architecture. Making allowance for the city's practical perceptible dimension changes the way we think about urban design and calls into question the primacy given to the sense of sight. It also upsets the manner in which the topic of accessibility is addressed in design. At present, to make our towns and cities accessible we flatten the ground and eliminate obstacles. This approach to design negates pedestrians' perceptive skills and the perceptible features of urban space. It sometimes produces routes that favour some people but hinder others. Focussing debate and design on the perceptible dimension of the urban experience would undoubtedly enable us to address disabilities in new ways and re-appraise the role played by the environment in the creation and the appropriation of the city (Thomas, 2005).

Blindness (cecity) is defined as the condition of any person whose central sight with their best eye, after correction, is less than or equal to 1/20 of normal sight. Impaired vision (amblyopia) corresponds to a value of 4/10 for the best eye.

The notion of a "sound effect" is a theoretical tool developed by JF.Augoyard at Cresson in 1995. Its purpose is to characterise the many ways in which citizens reorganise everyday physical sound data, depending on the context and local situation. More specifically it defines "interaction between the physical sound environment, the sound environment of a socio-cultural community and each individual's inner soundscape"(Augoyard and Torgue, 1995).


Pascale Barberger-Gateau
email: pascale.barberger-gateau@isped.u-bordeaux2.fr

Cilia Samieri

M Lanie Plourde

Epid Miologie

Biostatistique

Pascale Barberger

Dietary omega 3 polyunsaturated fatty acids and Alzheimer s disease: ' interaction with apolipoprotein E genotype

Keywords: Alzheimer Disease, genetics, metabolism, Animals, Apolipoproteins E, genetics, Diet, Fatty Acids, Omega-3, genetics, metabolism, Genetic Predisposition to Disease, Genotype, Humans Alzheimer s disease ', apolipoprotein E, fatty acids, omega 3, nutrition

Epidemiological studies suggest a protective role of omega-3 poly-unsaturated fatty acids (n-3 PUFA) against Alzheimer s disease ' (AD). However, most intervention studies of supplementation with n-3 PUFA have yielded disappointing results. One reason for such discordant results may result from inadequate targeting of individuals who might benefit from the supplementation, in particular because of their genetic susceptibility to AD. The 4 allele of the apolipoprotein E gene (ApoE) is a genetic risk factor for late-onset ε AD. ApoE plays a key role in the transport of cholesterol and other lipids involved in brain composition and functioning. The action of n-3 PUFA on the aging brain might therefore differ according to ApoE polymorphism. The aim of this review is to examine the interaction between dietary fatty acids and ApoE genotype on the risk for AD. Carriers of the 4 allele tend to be the most responsive to changes in dietary fat and cholesterol. Conversely, several epidemiological ε studies suggest a protective effect of long-chain n-3 PUFA on cognitive decline only in those who do not carry the 4 but with ε inconsistent results. An intervention study showed that only non-carriers had increased concentrations of long-chain n-3 PUFA in response to supplementation. The mechanisms underlying this gene-by-diet interaction on AD risk may involve impaired fatty acids and cholesterol transport, altered metabolism of n-3 PUFA, glucose or ketones, or modification of other risk factors of AD in 4 ε carriers. Further research is needed to explain the differential effect of n-3 PUFA on AD according to ApoE genotype.

Introduction

Prevention of age-related cognitive decline and dementia has become a public health challenge. Dementia results from a dynamic interaction between genetically determined, non-modifiable, pathological processes and environmental exposures that are potentially preventable or reversible . Nutrition, a major lifelong environmental factor, offers promising perspectives for the prevention of late-life [1 ] dementia. Long-chain poly-unsaturated fatty acids (PUFA) are major components of neuron membranes where they exert a functional and structural role . Broadly speaking, PUFA of the omega-3 (n-3) and omega-6 (n-6) series have opposite effects on inflammation,

[2 -3 ]
atherogenesis, thrombosis, and gene expression, and their respective proportions influence the physical properties of biological membranes where they are incorporated . A competition exists between the n-3 and n-6 families for the synthesis of long-chain PUFA from their [START_REF] Schmitz | The opposing effects of n-3 and n-6 fatty acids[END_REF] respective precursors alpha-linolenic and linoleic acids . Moreover, the biosynthesis of long-chain n-3 PUFA is very limited in humans [START_REF] Schmitz | The opposing effects of n-3 and n-6 fatty acids[END_REF] [

. These must therefore be obtained through diet. Fish and seafood are the main dietary source of long-chain n-3 PUFA under the form of 5 ] eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) . [START_REF] Bourre | Contributions (in 2005) of marine and fresh water products (finfish and shellfish, seafood, wild and farmed) to the French dietary intakes of vitamins D and B12, selenium, iodine and docosahexaenoic acid: impact on public health[END_REF] Several prospective epidemiological studies have suggested a protective role of fish consumption or dietary n-3 PUFA [7 -11 ] [9 ] against risk of dementia or Alzheimer s disease (AD). However, conflicting results exist as well . Moreover, most intervention ' [12 -15 ] studies based on supplementation with n-3 PUFA have failed to evidence any protective effect on AD or cognitive decline [16 -17 ] [ 18 -except in some patients with very mild cognitive impairment , . We examined the reasons for such inconsistencies in a 20 ] [16 21 -22 ] recent review

. A particular explanation for these discordant results may lie in ineffective targeting of individuals who might benefit [START_REF] Cunnane | Fish, docosahexaenoic acid and Alzheimer s disease[END_REF] from high doses of n-3 PUFA, in particular because of their genetic susceptibility to AD. The 4 allele of the apolipoprotein E gene ε (ApoE) is the main genetic risk factor for late-onset AD . The ApoE plays a key role in the transport of cholesterol and other lipids [START_REF] Blennow | Alzheimer s disease[END_REF] involved in brain composition and functioning . The action of n-3 PUFA on the aging brain might therefore differ according to ApoE [START_REF] Solfrizzi | Circulating biomarkers of cognitive decline and dementia[END_REF] polymorphism, suggesting a gene-environment interaction. In this review we examine the interaction between dietary n-3 PUFA and ApoE genotype on the risk for AD at the light of biological data and human studies.

Role of ApoE genotype in AD

Despite the recent discovery of susceptibility genes for AD thanks to Genome Wide Assessment of large population-based samples, the main genetic risk factor for sporadic late-onset AD remains by far the ApoE 4 allele (ApoE4) . The ApoE gene has three different ε [START_REF] Lambert | Genome-wide association study identifies variants at CLU and CR1 associated with Alzheimer s ' disease[END_REF] alleles: 2, 3 and 4, that give rise to six genotypes: 2/2, 2/3, 2/4, 3/3, 3/4, and 4/4. ApoE3 is the most common isoform . The

ε ε ε ε ε ε ε ε ε [27 ]
frequency of the ApoE4 decreases following a North-to-South gradient in normal middle-aged populations as well as in patients with late-onset AD . A meta-analysis showed that the relative increase in the frequency of ApoE4 in patients with AD compared with [START_REF] Panza | Decreased frequency of apolipoprotein E epsilon 4 allele from Northern to Southern Europe in Alzheimer s disease patients and centenarians[END_REF] controls is substantially less in African Americans (32.2 vs. 19.0 ) than Caucasians (36.7 vs. 13.6 ) . The ApoE4 increases the % % % % [START_REF] Farrer | Effects of age, sex, and ethnicity on the association between apolipoprotein E genotype and Alzheimer disease: a meta-analysis[END_REF] risk of AD by 15 times in Caucasian homozygotes . Although ApoE4 is the major genetic risk factor for AD, its mode of action [START_REF] Farrer | Effects of age, sex, and ethnicity on the association between apolipoprotein E genotype and Alzheimer disease: a meta-analysis[END_REF] remains partly unknown and several mechanisms have been proposed . Indeed, ApoE4 is the only molecule that has been [START_REF] Mahley | Apolipoprotein E4: a causative factor and therapeutic target in neuropathology, including Alzheimer s disease[END_REF] [ [START_REF] Cedazo-M Nguez Í A | Apolipoprotein E and Alzheimer s disease: molecular mechanisms and therapeutic opportunities[END_REF] associated with all the biochemical disturbances characteristic of the disease: beta-amyloid (A ) peptide deposition, tangle formation, β oxidative stress, inflammation, lipid homeostasis deregulation, synaptic plasticity loss and cholinergic dysfunction . ApoE4 may [START_REF] Cedazo-M Nguez Í A | Apolipoprotein E and Alzheimer s disease: molecular mechanisms and therapeutic opportunities[END_REF] increase A in senile plaques and impair its clearance . Recently, a study using voxel-based analysis of C labelled Pittsburgh β [START_REF] Mahley | Apolipoprotein E4: a causative factor and therapeutic target in neuropathology, including Alzheimer s disease[END_REF] 11 compound B (PIB)-positron emitting tomography (PET) and magnetic resonance imaging (MRI) showed higher levels of A plaque β deposition in AD patients with the ApoE4 allele compared to age-matched ApoE4-non carriers with similar levels of cognitive impairment and brain atrophy . The neuropathological changes associated with AD occur as soon as 30 years of age, that is decades before the [START_REF] Drzezga | Effect of APOE genotype on amyloid plaque load and gray matter volume in Alzheimer disease[END_REF] clinical onset of the disease, but in ApoE4 carriers the accumulation of A peptide in senile plaques is more pronounced at every age β except in the oldest old . There is therefore a large window of action for environmental risk factors that will modulate the clinical [START_REF] Kok | Apolipoprotein E-dependent accumulation of Alzheimer disease-related lesions begins in middle age[END_REF] expression of late-onset AD. In addition to their increased burden of A , ApoE4 carriers may be particularly sensitive to some β environmental factors, in particular dietary fat. We focus here on the antioxidant and immunomodulatory/anti-inflammatory properties of ApoE that are particularly relevant to lipid metabolism given the high susceptibility to peroxidation and the anti-inflammatory [START_REF] Jofre-Monseny | Impact of apoE genotype on oxidative stress, inflammation and disease risk[END_REF] properties of n-3 PUFA.

ApoE and oxidative stress

Increased oxidative stress is a key feature in AD . Markers of oxidative damage precede pathological changes in experimental [START_REF] Querfurth | Alzheimer s Disease[END_REF] models and are observed very early in the clinical course of the disease . A is a potent generator of free radicals (reactive oxygen [35 -36 ] β species, ROS, and reactive nitrogen species) . The production of free radical species leads to peroxidation of membrane lipids. In [START_REF] Querfurth | Alzheimer s Disease[END_REF] synaptosomes isolated from mice that expressed only one of the 2, 3 or 4 human ApoE alleles, treatment with A induced a ε ε ε β significantly higher increase in ROS and markers of lipid peroxidation in synaptosomes containing the 4 allele .

ε [START_REF] Lauderback | Apolipoprotein E modulates Alzheimer s A beta (1 42)-induced oxidative damage to ' [ ]synaptosomes in an allele-specific manner[END_REF] Oxidative stress results from an imbalance between production of free radicals and anti-oxidant defence mechanisms. These defence mechanisms involve enzymatic systems such as glutathion peroxidase (GSH-Px) and catalase (CAT) and non-enzymatic antioxidant nutrients including vitamin E, carotenoids and selenium. In vitro, the ApoE isoforms are associated with different antioxidant capacities, the highest being observed with the 2 and the lowest with the 4 isoforms . Plasma concentrations of vitamin E, a powerful ε ε [START_REF] Miyata | Apolipoprotein E allele-specific antioxidant activity and effects on cytotoxicity by oxidative insults and beta-amyloid peptides[END_REF] fat-soluble antioxidant, vary according to ApoE genotype, the highest concentrations being found in 2/2 individuals in one study . In ε [START_REF] Ortega | Influence of apolipoprotein E genotype on fat-soluble plasma antioxidants in Spanish children[END_REF] another study, no significant difference in vitamin E concentration was observed between ApoE4 carriers and non-carriers in older AD patients while in control subjects vitamin E level was significantly higher in ApoE4 carriers . However, this difference disappeared [START_REF] Mas | Functional vitamin E deficiency in ApoE4 patients with Alzheimer s disease[END_REF] after normalization according to triglycerides (TG) and cholesterol levels. For the authors, this increased plasma vitamin E level in ApoE4 controls could be associated with retention of vitamin E in plasma lipoproteins and a functional vitamin E deficiency in peripheral tissues [

. The protective effect of some dietary antioxidants may also depend on ApoE genotype. Vitamin E from food, but not other 40 ] antioxidants, was associated with a reduced risk of AD only among individuals without the ApoE4 allele . Conversely, higher serum [START_REF] Morris | Dietary intake of antioxidant nutrients and the risk of incident Alzheimer disease on a biracial community study[END_REF] beta-carotene was associated with lower risk of cognitive decline only in ApoE4 carriers among high-functioning older persons . In an [START_REF] Hu | Association between serum beta-carotene levels and decline of cognitive function in high-functioning older persons with or without apolipoprotein E 4 alleles: MacArthur studies of successful aging[END_REF] elderly Chinese cohort, carriers of the ApoE4 allele had significantly lower Selenium levels measured in nails .

[43 ]

ApoE genotype may also influence activity of enzymes involved in antioxidant defence mechanisms such as GSH-Px and CAT. In AD patients as well as in age-matched controls, the ApoE4 carriers had significantly lower activities of GSH-Px and CAT, and lower serum total antioxidant status compared to non ApoE4 carriers . The decreased antioxidant activity of ApoE4 could therefore contribute to its [START_REF] Kharrazi | Association between enzymatic and non-enzymatic antioxidant defense mechanism with apolipoprotein E genotypes in Alzheimer disease[END_REF] association with AD .

[33 ]

ApoE and inflammation

In AD, the accumulation of misfolded proteins leads to inflammatory damage . In addition to low-grade inflammation associated [START_REF] Querfurth | Alzheimer s Disease[END_REF] with brain aging, A deposition is accompanied by activation of microglia which will contribute to eliminate A . In turn, chronically β β activated microglia will release chemokines and pro-inflammatory cytokines . Acute systemic inflammation is also suspected to [START_REF] Querfurth | Alzheimer s Disease[END_REF] exacerbate chronic brain inflammation thereby increasing the rate of cognitive decline in AD patients . The ApoE4 allele is also [START_REF] Holmes | Systemic inflammation and disease progression in Alzheimer disease[END_REF] reported to directly promote inflammation which may contribute to its deleterious effect in AD . The activation of the amyloid cascade [START_REF] Finch | Systemic inflammation, infection, ApoE alleles, and Alzheimer disease: a position paper[END_REF] in ApoE transgenic mice results in the activation of microgliosis and astrogliosis in the hippocampus of ApoE4, but not in ApoE3 transgenic mice. This differential impact of ApoE genotype was limited to this brain area specifically affected in AD and was not observed in septal neurons . Paradoxically, this exacerbation of brain inflammation linked to ApoE4 is not observed at peripheral level where [START_REF] Belinson | ApoE4-dependent Abeta-mediated neurodegeneration is associated with inflammatory activation in the hippocampus but not the septum[END_REF] inverse associations are reported. Indeed, in a comparative analysis of three large population-based studies, lower levels of plasma C-Reactive protein (CRP), a biomarker for inflammation, were observed in ApoE4 carriers . Similarly, in a population of cognitively [START_REF] Chasman | Qualitative and quantitative effects of APOE genetic variation on plasma C-reactive protein, LDL-cholesterol, and apoE protein[END_REF] normal Italian elderly, ApoE4 carriers had a lower risk of high CRP than non-carriers . The same association was observed in [START_REF] Ravaglia | Apolipoprotein E e4 allele affects risk of hyperhomocysteinemia in the elderly[END_REF] nonagerians . However, there is an interaction between peripheral markers of inflammation and ApoE on cognitive performance. In a [START_REF] Rontu | Apolipoprotein E genotype is related to plasma levels of C-reactive protein and lipids and to longevity in nonagenarians[END_REF] cross-sectional study conducted in older community dwellers, participants with at least one 4 allele and CRP in the highest tertile had the ε greatest odds of impaired memory . This finding suggests, as already hypothesized by Jofre-Monseny et al.

, that the detrimental [51 ] [33 ] effect of ApoE might be partly counteracted by lower levels of CRP.

This large body of evidence suggests that ApoE4 is a key player in the pathogenesis of AD . A large part of this effect might be [START_REF] Cedazo-M Nguez Í A | Apolipoprotein E and Alzheimer s disease: molecular mechanisms and therapeutic opportunities[END_REF] explained by the major role of ApoE in lipid metabolism.

ApoE genotype, lipid metabolism and AD

Most dietary fats are ingested in the form of TG. They are efficiently absorbed in the small intestine towards facilitated diffusion and uptaken by enterocytes in the forms of fatty acids and monoglycerides ( ) . Once fatty acids have entered the cell, they are Figure 1 [START_REF] Mu | The digestion of dietary triacylglycerols[END_REF] re-esterified with glycerol back into TG and phospholipids (PL) for transport in the blood chylomicrons to the liver and other tissues. via All lipoproteins, with the exception of chylomicrons, are produced by the liver, and begin as molecules packed full with TG, with the exception of high density lipoprotein (HDL). Very low density lipoprotein (VLDL) is the most TG-rich lipoprotein, which through sequential lipolysis of TG by lipoprotein lipase leads to the formation of low density lipoprotein (LDL). Lipoproteins are composed of lipids and proteins named apolipoproteins which bind to specific receptors. ApoE is specific to LDL receptor which is at the root of cholesterol homeostasis but is also essential for normal catabolism of TG-rich lipoprotein constituent.

Throughout life and increasing with age, neurons must be remodelled and repair to maintain neurotransmission . Through its lipid [START_REF] Mahley | Apolipoprotein E4: a causative factor and therapeutic target in neuropathology, including Alzheimer s disease[END_REF] transport function, ApoE plays a key role in the transport and metabolism of cholesterol, TG and other lipids within many organs including the brain , . ApoE is also the main ligand for LDL receptors in the periphery, while in the central nervous system it is the main [25 53 ] ligand for the LDL receptor-related protein . In the brain ApoE plays an important role in the maintenance and repair of neurons, by [START_REF] Hooijmans | Fatty acids, lipid metabolism and Alzheimer pathology[END_REF] distributing lipids necessary for proliferation, synaptogenesis and myelinisation of axons [START_REF] Cedazo-M Nguez Í A | Apolipoprotein E and Alzheimer s disease: molecular mechanisms and therapeutic opportunities[END_REF] Allelic variations in ApoE impair the protein structure, function and metabolism . For instance, in ApoE4 carriers, substitution of [START_REF] Mahley | Apoliprotein E: far more than a lipid transport protein[END_REF] an amino acid changes the protein structure because of the formation of a salt bridge between an arginine at position 61 and a glutamic acid at position 255, causing the isoform to bind preferentially to VLDL .

[54 ]

ApoE and cholesterol

Other mechanisms linking ApoE and risk of AD possibly involve cholesterol homeostasis in the brain since ApoE is its principal carrier . In the brain, cholesterol has a crucial role in development of neuronal plasticity and function . All the cholesterol used in

[53 ] [55 ]
the brain is synthesized within the central nervous system since the blood-brain barrier restricts direct transport of cholesterol from peripheral circulation, in the absence of vascular injury .

[55 ]

Cholesterol forms complexes with ApoE for delivery to neurons and is an essential component of neuronal membranes where it is concentrated in lipid rafts . Rafts are membrane micro-environments for the assembly of -secretases and -secretases and processing [START_REF] Querfurth | Alzheimer s Disease[END_REF] β γ of amyloid precursor protein into A . ApoE isoforms influence the composition of membrane lipid rafts . ApoE4 is the least

β [34 ] [56 ]
effective of the three isoforms in promoting healthy lipid membrane turnover. ApoE4 also enhances A production and decreases its β clearance, leading to higher aggregation of A in the brain especially when there is an overabundance of esterified cholesterol that β decreases membrane lipid turnover .

[34 ]

Peripheral serum cholesterol levels are also influenced by ApoE genotype . In a large sample of healthy European men, the

[57 -59 ]
ApoE genotypes 2/2 and 3/2, 4/2, 3/3, 4/3, and 4/4 were associated with respectively increasing levels of serum total and LDL

ε ε ε ε ε ε cholesterol .
This relationship may however be disturbed in the presence of AD. In a case-control study, cholesterol levels were [START_REF] Giltay | Serum cholesterol, apolipoprotein E genotype and depressive symptoms in elderly European men: The FINE study[END_REF] significantly higher in ApoE4 individuals within the control group, whereas no significant relationship was found between ApoE4 and cholesterol in the AD cases .

[40 ]

Hence, we could anticipate that higher plasma cholesterol levels would correlate with higher A aggregation in the brain of ApoE4 β AD patients. However, in post-mortem studies of AD patients, higher total cholesterol and LDL levels in the blood did correlate with higher A 42-amyloid levels in the brain but independently of ApoE genotype . These correlations were not observed in healthy β [START_REF] Kuo | Elevated low-density lipoprotein in Alzheimer s disease correlates with brain A beta ' 1-42 levels[END_REF] controls. In epidemiological studies, there is an association between higher serum cholesterol levels in midlife and increased risk of AD [ but this association was independent of ApoE genotype . Conversely, a decrease in serum total cholesterol from midlife to 62 -63 ] [START_REF] Kivipelto | Apolipoprotein E epsilon 4 allele, elevated midlife total cholesterol level, and high midlife systolic blood pressure are independent risk factors for late-life Alzheimer disease[END_REF] late-life was associated with early stages of dementia, independently of ApoE genotype . Lower blood cholesterol in later life [64 -65 ] the controversy, several studies show a modification of the association between serum cholesterol level and cognitive performance by ApoE genotype. In three cross-sectional studies conducted in various populations, higher plasma total and LDL cholesterol were associated with increased AD risk in the non carriers of the ApoE4 allele, whereas cholesterol was not associated with increased AD risk in the carriers . Similarly, in a cross-sectional study conducted in very old (> 85 years) non-demented individuals, there was a [66 -68 ] significant positive correlation between higher total or LDL cholesterol and better memory performances in ApoE4 non-carriers . No [START_REF] West | Better memory functioning associated with higher total and low-density lipoprotein cholesterol levels in very elderly subjects without the apolipoprotein e4 allele[END_REF] correlation was observed in ApoE4 carriers or with HDL cholesterol. The Longitudinal Aging Study Amsterdam yielded controversial results since in separate analyses by ApoE status, there was a significant positive association between total cholesterol level and change in information processing speed only in the ApoE4 carriers, suggesting that a higher total cholesterol level at baseline was significantly associated with a slower rate of decline in these subjects . A common limitation to these studies is the lack of longitudinal data on the [START_REF] Van Den Kommer | Total cholesterol and oxysterols: Early markers for cognitive decline in elderly?[END_REF] interaction between ApoE genotype and change in serum cholesterol levels under treatment or dietary modification on the rate of cognitive decline or risk of dementia.

ApoE and fatty acids

Whereas cholesterol in the brain is produced and regulated endogenously , the fatty acid content of neuronal membranes can be [START_REF] Shobab | Cholesterol in Alzheimer s disease[END_REF] modulated by dietary intake and ApoE polymorphism in animals . ApoE4 is associated with the inhibition of lipid metabolism, [START_REF] Lane | Lipid homeostasis and apolipoprotein E in the development and progression of Alzheimer s disease[END_REF] inducing inefficient delivery of PUFA, in particular DHA, to neurons and altered lipid membrane homeostasis . DHA is the most [START_REF] Lane | Lipid homeostasis and apolipoprotein E in the development and progression of Alzheimer s disease[END_REF] abundant component of membrane phospholipids in the brain. ApoE is a constituent of the TG rich lipoproteins VLDL but there are inconsistencies in the literature regarding the essentiality of ApoE in hepatic VLDL synthesis and secretion . After the phospholipids, [START_REF] Minihane | ApoE genotype, cardiovascular risk and responsiveness to dietary fat manipulation[END_REF] TG are the next richest source of DHA in plasma . The most important role of ApoE in lipoprotein metabolism is to be a high-affinity [START_REF] Plourde | Unresolved issues in the link between docosahexaenoic acid and Alzheimer s disease[END_REF] ligand for receptor to the LDL-receptor family and to deliver cholesterol to neurons. Although the role of ApoE in brain cholesterol homeostasis is well documented, its potential role into fatty acid transport and delivery to the brain is less understood with the exception of mentioning that ApoE is important for synaptic plasticity . Hence, at this point, no direct connection between ApoE genotypes and [START_REF] Kim | The role of apolipoprotein E in Alzheimer s disease[END_REF] fatty acids delivered to the brain is yet possible to establish. This is supported by the data obtained by Fraser et al.

showing that the [START_REF] Fraser | Fatty acid composition of frontal, temporal and parietal neocortex in the normal human brain and in Alzheimer s disease[END_REF] relative proportions of fatty acids in frontal, temporal and parietal cortex in AD and control brains was not significantly different by ApoE genotype. However, that study measured the overall fatty acid composition of the cerebral cortex without regard to subcellular distribution, especially in relation to membrane lipid rafts, which is a limitation, particularly for DHA .

[74 ]

Modification by ApoE genotype of the effect of dietary n-3 PUFA on cognition

Epidemiological studies

Few epidemiological studies have examined the interaction between n-3 PUFA status and ApoE on cognitive decline or risk for dementia or AD. They are summarized in . table 1

Dietary studies

The Cardiovascular Health Cognition Study was the first to evidence an interaction with borderline significance between consumption of fatty fish and presence of the ApoE4 allele on the risk for AD . Fatty fish appeared to have little or no association with risk for AD [START_REF] Huang | Benefits of fatty fish on dementia risk are stronger for those without APOE epsilon4[END_REF] in those with the ApoE4 but was associated with significantly lower risk in those without the ApoE4 allele. Similar results were observed in the French Three-City (3C) study which reported a significant protective effect of fish consumption against all-cause dementia only in ApoE4 non-carriers . However, there was no statistical interaction between fish consumption and ApoE genotype on the risk for AD.

[10 ]

Moreover, the same study showed a deleterious effect of n-6 rich oils (sunflower or grape seed oil) when their consumption was not counterbalanced by consumption of n-3 rich foods (oils or fish) among ApoE4 non-carriers. This detrimental effect was not observed in ApoE4 carriers.

Conversely, among ApoE4 carriers moderate intake of PUFA (2 quartile) at midlife was associated with decreased risk of dementia, nd while moderate intake of saturated fats was associated with an increased risk in the Cardiovascular Risk Factors, Aging, and Incidence of Dementia (CAIDE) study . No association was found between intake of fat, whatever its type, and risk of dementia in ApoE4 [79 -80 ] non-carriers. However, that study did not discriminate between n-3 and n-6 PUFA and was therefore not included in . table 1

Biological studies

Most biological studies did not show any interaction between blood n-3 PUFA status and ApoE genotype on cognitive evolution in older persons. Indeed, among the five published or in press prospective epidemiological studies that examined whether ApoE4 may modify the relationship between long-chain n-3 PUFA blood status and dementia or cognitive decline, three , 14, 81 did not find any [START_REF] Beydoun | Plasma n-3 fatty acids and the risk of cognitive decline in older adults: the Atherosclerosis Risk in Communities Study[END_REF] ]

modification effect and only the first one reported data stratified by ApoE status ( ). In the Atherosclerosis Risk in [START_REF] Beydoun | Plasma n-3 fatty acids and the risk of cognitive decline in older adults: the Atherosclerosis Risk in Communities Study[END_REF] table 1 Communities study , an inverse association was observed between higher EPA DHA in plasma cholesterol esters and verbal fluency [START_REF] Beydoun | Plasma n-3 fatty acids and the risk of cognitive decline in older adults: the Atherosclerosis Risk in Communities Study[END_REF] + in ApoE4 non-carriers, but not in carriers despite a non-significant interaction term. The lack of significant protective association in ApoE4 carriers might therefore be attributed to a lack of power in this smaller subgroup. In the Canadian Study of Health and Aging , [START_REF] Kroger | Omega-3 fatty acids and risk of dementia: the Canadian Study of Health and Aging[END_REF] there was no significant modification of risk of dementia or AD associated with n-3 PUFA, DHA, or EPA concentrations, treated in quartiles or as continuous variables, by ApoE4 status in models adjusted for sex and education (data not shown by the authors and therefore not reproduced in ). However, this study found a paradoxical protective association with higher blood mercury Table 1 concentration, which can be considered as a proxy biomarker for fatty fish intake. The presence of both high n-3 PUFA and high mercury concentration was associated with a significantly reduced risk by 43 for dementia and by 46 for AD, independently of ApoE genotype

% % [ . 14 ]
Two studies found a statistically significant interaction ( ), with a protective effect in ApoE4 non-carriers only in one study table 1 [START_REF] Whalley | n-3 Fatty acid erythrocyte membrane content, APOE epsilon4, and cognitive variation: an observational follow-up study in late adulthood[END_REF] and conversely, a protective association only in ApoE4 carriers in the other study . In the first study, children aged 11 years old with [START_REF] Samieri | Omega-3 fatty acids and cognitive decline: modulation by ApoE epsilon4 allele and depression[END_REF] higher general intelligence had higher erythrocyte n-3 PUFA, but the correlation was significant only for those without the ApoE4 allele [

. The same population was retested at 64 years old and their general intelligence was even more highly positively correlated with 77 ] erythrocyte n-3 PUFA, but there was still no correlation in ApoE4carriers. The 3C study yielded inverse results since higher plasma EPA and DHA proportions were associated with slower decline in working memory assessed by Benton Visual Retention Test among ApoE4 carriers only

. Decline of performance in other cognitive domains ( global cognition, executive functioning and verbal fluency) was [START_REF] Samieri | Omega-3 fatty acids and cognitive decline: modulation by ApoE epsilon4 allele and depression[END_REF] i.e not related to plasma long-chain n-3 PUFA content, neither in the whole sample nor according to ApoE4 carrier status. Notably, in that study, baseline plasma EPA and DHA proportions did not differ according to ApoE genotype.

Intervention studies

One recent study evaluating the impact of fish oil on cognitive performance in the healthy elderly reported that after 26 weeks of supplementation with either 0.4 or 1.8 g/d of long-chain n-3 PUFA ApoE4 carriers had improved scores on tests of the cognitive domain of attention compared to the placebo contrarily to non-carriers . Plasma EPA and DHA concentrations were given but no information was [START_REF] Van De Rest | Effect of fish oil on cognitive performance in older subjects: A randomized, controlled trial[END_REF] provided as to whether they changed differentially in relation to ApoE4 genotype. progression of cognitive decline in ApoE4 non-carriers, but no effect in ApoE4 carriers. In the Memory Improvement with DHA Study (MIDAS) RCT, improved learning and episodic memory functions were observed in older adults with mild memory complaints receiving 900 mg algal DHA per day but the publication does not mention whether ApoE genotype was taken into account .

[22 ]

Post-hoc analyses of RCT in small subgroups must be considered with caution when the randomisation was not stratified according to ApoE genotype. Analyses according to ApoE genotype should be systematically planned in the protocol of trials of dietary supplementation with n-3 PUFA for the primary or secondary prevention of cognitive decline in AD.

Putative mechanisms underlying this interaction

Impaired fatty acids transport to the brain

The link between the way EPA and DHA are transported and cognition may involve delivery of fatty acids for membrane repair and signalling pathways within the brain. It is suggested that DHA-enriched LDL are taken up more efficiently by the brain than other lipoproteins , whereas others suggest that it is the unesterified form of DHA that most efficiently reaches the brain . One major limitation to the delivery of fatty acids to the human brain is to cross the blood brain barrier. Two general models have been proposed: the first approach suggests a passive diffusion of unesterified plasma fatty acids while the second suggests uptake of

[87 -89 ]
lipoproteins via a receptor-mediator process, likely the LDL-receptor , . Since ApoE is the main ligand for LDL-receptor protein in [86 88 ] the central nervous system and that the 4 allele seems to impact the efficiency of the binding of ApoE with the LDL-receptor, it is

[53 ] ε
hence possible that the delivery of EPA and DHA to the brain is altered in ApoE4 carriers. However, this hypothesis yet has to be investigated in mice expressing human ApoE4.

Modification by ApoE of the effect of dietary EPA and DHA on blood lipids

Another mechanism explaining the interaction between dietary PUFA and ApoE genotype on rate of cognitive decline and risk for AD may lie in a modification by ApoE of the effect of EPA and DHA intake on blood lipid profile.

Modification by ApoE of the effect of dietary EPA and DHA on plasma fatty acid profile

A gene-by-diet interaction has recently been reported for the incorporation of EPA and DHA into plasma lipids of ApoE4 carriers and non-carriers . The plasma fatty acid profiles of 8 carriers and 20 non-carriers of ApoE4 showed that ApoE4 carriers had significantly [START_REF] Plourde | Plasma n-3 fatty acid response to an n-3 fatty acid supplement is modulated by apoE 4 but not by the common PPAR-alpha L162V polymorphism in men[END_REF] EPA and DHA in plasma TG at baseline . However, after receiving a supplement of 1.9 g/d of EPA and 1.1g/d of DHA for 6 higher [START_REF] Plourde | Plasma n-3 fatty acid response to an n-3 fatty acid supplement is modulated by apoE 4 but not by the common PPAR-alpha L162V polymorphism in men[END_REF] weeks, there were significant gene-by-diet interactions in the incorporation of both EPA in free fatty acids and DHA in plasma TG such that in carriers of ApoE4, increases in the two fatty acids was lower compared to the non-carriers . The mechanism underlying the gene-by-diet interaction is possibly linked with the way EPA and DHA are transported by lipoproteins. After dietary intake, most dietary lipids are transported by chylomicrons but small amounts circulate in the blood stream as free fatty acids bound to albumin. Chylomicrons are TG-rich lipoproteins produced in intestinal cells after the intake of a meal, while lipoproteins are produced by hepatic cells. All lipoproteins, with the exception of HDL, begin as molecules rich in TG. VLDL is the most TG-rich lipoprotein, which through sequential lipolysis of TG by lipoprotein lipase leads to the formation of LDL. Although many studies report the incorporation of EPA and DHA into blood total lipids, phospholipids, TG, cholesteryl esters or red blood cells, there are only a few studies that report how EPA and DHA are transported in the blood and how much is incorporated into each type of lipoprotein. In studies of supplementation with EPA DHA, both + fatty acids were incorporated into VLDL-, LDL-and HDL-TG within 4h after fish-oil intake . Altogether, the limited number of [91 -93 ] available studies suggests that DHA is rapidly and preferentially incorporated into TG fraction of most lipoproteins after its intake, and that within a few hours its concentration tends to decrease. Hence, the metabolism of EPA and DHA is most of the time in transition and the lap of time of this transition may be altered by ApoE4 genotype. However, to our knowledge, no studies have evaluated the impact of ApoE4 genotype on the transport of EPA and DHA by lipoproteins.

Modification by ApoE of the effect of EPA and DHA on blood cholesterol

Some of the modifying effect of ApoE genotype on the relationship between dietary n-3 PUFA and risk of AD might be mediated through cholesterol. Indeed, blood cholesterol concentration is partly determined by dietary intake of n-3 and n-6 PUFA but also by [START_REF] Motoyama | Association of serum n-6 and n-3 polyunsaturated fatty acids with lipids in 3 populations of middle-aged men[END_REF] genetics, in particular ApoE polymorphism. A gene-nutrient interaction might therefore be expected.

Carriers of ApoE4 have been shown to have a greater lipid response to dietary changes . In 2003, a systematic review of the [START_REF] Anil | The impact of EPA and DHA on blood lipids and lipoprotein metabolism: influence of apoE genotype[END_REF] literature reported the effect of genetic variation, including ApoE polymorphism, on the lipid response to dietary intervention . The [START_REF] Masson | Genetic variation and the lipid response to dietary intervention: a systematic review[END_REF] authors reviewed intervention studies to modify dietary cholesterol, dietary fat or other compounds of the diet. Of the 46 interventions that modified the fat composition of the diet, 15 found significantly different total, LDL-and HDL-cholesterol responses, with carriers of the ApoE4 allele tending to be the most responsive to changes in dietary fat. Recently, others have reported a gene-by-diet interaction such that total cholesterol in ApoE4 carriers was increased following DHA supplementation, which is possibly due to an increase in LDL-cholesterol . In plasma VLDL fraction of ApoE4 carriers, intake of DHA resulted in a significant 32 reduction in LDL uptake [START_REF] Olano-Martin | Contribution of apolipoprotein E genotype and docosahexaenoic acid to the LDL-cholesterol response to fish oil[END_REF] 2 % relative to control supporting alterations in the transport and uptake of lipoproteins . However, this study did not investigate whether [START_REF] Olano-Martin | Contribution of apolipoprotein E genotype and docosahexaenoic acid to the LDL-cholesterol response to fish oil[END_REF] the incorporation of DHA into the different types of lipoproteins was different based on ApoE4 genotype, so whether the possible changes are at the root of the LDL uptake differences still needs further investigations.

Altogether, these data suggest that ApoE genotype interacts with dietary PUFA in the transport and uptake of lipoproteins, thereby changing cholesterol levels.

Modification by ApoE of the effect of EPA and DHA on blood TG

A major effect of dietary n 3 PUFA on plasma lipids is a reduction of the concentration of plasma TG , an effect that may be [START_REF] Nagao | Bioactive lipids in metabolic syndrome[END_REF] modified by ApoE genotype. A study in healthy elderly Brazilian women showed higher TG serum levels in 2 compared to 4 carriers ε ε [99 . However, in the presence of high spontaneous intake of total fat or a low relative intake of PUFA, ApoE4 carriers lost protection against ] hypertriglyceridemia .

[99 ]

Administration of fish oil in a RCT showed a double treatment by sex by ApoE genotype interaction . Indeed, the greatest

[100 ]
decreases in TG concentration with either 0.7 g EPA DHA/d or 1.8 g EPA DHA/d were observed in ApoE4 males.

+ +

High TG are linked to cognitive decline and risk of dementia. An experimental study in obese mice showed that lowering TG can reverse their cognitive impairment . In the 3C study we showed an association between hypertriglyceridemia and increased risk of

[101 ]
all-cause and vascular dementia . However, this effect was independent of ApoE4 genotype.

[102 ]

N-3 PUFA could therefore contribute to lower the risk of cognitive impairment and dementia associated with high TG in ApoE4

carriers. However, the potential modifying effect of ApoE genotype on this relationship still lacks scientific support.

Modification by ApoE of the susceptibility to lipid peroxidation

As discussed in section 2.1, ApoE4 carriers have a lower activity of defence mechanisms against oxidative stress. One hypothesis to explain why ApoE4 carriers are not protected against risk of cognitive decline by fish or DHA intake is possibly linked to their altered LDL metabolism leading to higher susceptibility to peroxidation . Lipids are needed to repair neuronal membrane and ApoE4

[103 ]

genotype apparently fails to protect lipids from peroxidation. Therefore, the oxidised lipids needed to be removed from damaged neuronal membranes are possibly replaced by other oxidised lipids . Moreover, formation of lipid peroxidation products may contribute to tau

[103 ]
protein phosphorylation in AD . Since EPA and DHA are susceptible to oxidation because they are highly unsaturated, their transport

[104 ]
via LDL is thus possibly more vulnerable to oxidation in ApoE4 carriers. The residence time of LDL in ApoE4 carriers may also be longer since ApoE is the main ligand to the LDL-receptor in the periphery and that carrying the 4 allele lowers the binding of ApoE to its ε LDL-receptor , . The higher peroxidation of lipoproteins in ApoE4 carriers may hence contribute to their higher risk of AD. neuro-pathological and vascular changes in late-life dementia . N-3 PUFA may decrease the negative impact of these risk factors and

[1 ]
enhance compensatory repair mechanisms . In addition to its direct effects on lipid metabolism discussed above, ApoE genotype might [START_REF] Cunnane | Fish, docosahexaenoic acid and Alzheimer s disease[END_REF] also modulate the impact of dietary n-3 PUFA on inflammation, vascular risk factors and overall nutritional status. However, very few studies have analysed the interaction between EPA, DHA and ApoE genotype on these risk factors of dementia. The aim of this more speculative section is to raise new hypotheses that would deserve further research.

Inflammation

N-3 PUFA have potent anti-inflammatory properties . Indeed, EPA is the precursor of anti-inflammatory eicosanoids and DHA is [START_REF] Schmitz | The opposing effects of n-3 and n-6 fatty acids[END_REF] the precursor of neuroprotectin D1 (NPD1). NPD1 represses the activation of inflammatory signalling mediators such as prostaglandins synthesized from arachidonic acid (long-chain n-6 PUFA) by cyclooxygenase-2 .

[3 ]

As discussed in section 2.2, some studies suggest that inflammatory status might also be modulated by ApoE genotype. In the Cardiovascular Health Cognition Study, the reduction in risk for AD associated with use of non-steroidal anti-inflammatory drugs was greater among ApoE4 carriers . Hence, ApoE4 carriers might also have an enhanced response to the anti-inflammatory effects of n-3

[105 ]

PUFA. This hypothesis was investigated in a small subsample of the OmegAD RCT which evaluated the impact of a supplementation with 2320 mg EPA DHA/d on cognitive functioning in patients with mild to moderate AD , . However, plasma biomarkers of

+ [ 21
106 ] inflammation (interleukin-6, tumor necrosis factor-, CRP) did not differ between the intervention and placebo groups nor was there any α difference for inflammation markers in cerebrospinal fluid. There was no interaction with ApoE genotype indicating a similar lack of impact of n-3 PUFA supplementation on biomarkers of inflammation in ApoE4 carriers and non-carriers. However, this was a very small study (N 35) and the potential differential effect of n-3 PUFA on inflammation according to ApoE genotype still deserves further = research.

Glucose metabolism

During healthy aging, brain glucose uptake may decrease significantly in specific cortical regions , an effect that may be more

[107 ]
pronounced in the elderly with deteriorating cognitive function such as AD . Reduction of cerebral metabolic rates for glucose was

[108 ]
also found in asymptomatic ApoE4 carriers as young as 20 to 30 years old in the same brain regions as clinically affected AD patients [109 . These results are the earliest sign of brain hypometabolism abnormality yet found in living person at risk of AD and suggest that in ] ApoE4 carriers, hypometabolism precedes the onset of clinical AD and is not a consequence of the disease itself. Glucose is the main cerebral energy substrate and during fasting or low carbohydrate intake ketones are glucose s primary replacement fuel . Hence, in

' [110 ]
the elderly and in ApoE4 carriers, increasing blood ketones and potentially their uptake by the brain may be a good strategy to counterbalance their lower brain glucose uptake. The capacity to produce ketones seems similar in the elderly (>70 y old) compared to the young (between 18 25 y old) and is potentially not depending upon ApoE genotype . Higher blood ketones facilitated cognitive

- [111 ]
performance in ApoE4 non-carriers but not in the carriers . Similar results in a larger scale study with 152 subjects diagnosed with mild to moderate AD confirmed that higher ketone levels in ApoE4 carriers had no specific changes in the scores of cognitive tests .

[113 ]

Whether increasing blood ketones improves cognition by raising brain uptake of ketone bodies needs investigation.

Hyperhomocysteinemia

Elevated serum homocysteine is a cardiovascular risk factor which is also associated with increased risk of cognitive decline presence of the ApoE4 allele also modified the relationship between hyperhomocysteinemia and cognitive performance in one study: the inverse association between total homocysteinemia and cognitive scores was consistently higher in ApoE4 carriers .

[120 ]

There is an inverse association between plasma homocysteine and platelet phospholipid DHA concentration . Administration of [121 ] fish oil significantly decreased plasma homocysteine in rats and hyperlipemic men . Conversely, DHA concentration may be

[122 ] [123 ]
affected by homocysteine concentration . However, to our knowledge, no published study examined the impact of ApoE genotype on [124 ] the link between homocysteine and DHA in AD.

Obesity

There is increasing evidence that obesity at midlife is a risk factor for cognitive impairment and dementia later [125 -126 ] [127 -130 ] in life, although the latter association became non significant after further adjustment for vascular disorders and ApoE4 status in one study . Obesity contributes to maintain a chronic inflammatory status. In vitro, DHA inhibits the differentiation of pre-adipocytes to [130 ] adipocytes . In obese adults, there was an inverse correlation between total n-3 PUFA, EPA and body mass index (BMI) which was [131 ] not observed in normal weight individuals . Moreover, obese individuals had lower n-3 PUFA concentrations which might contribute [132 ] to their increased risk of cognitive decline.

However, the link between ApoE genotype, obesity, n-3 PUFA and dementia risk is not yet clearly established. Indeed, a recent review concluded that few studies have attempted to link human ApoE isoforms to obesity, and they have produced somewhat conflicting results [

. Obesity results from an imbalance between energy intake and actual needs. One study evidenced an interaction between high energy 133 ] intake and ApoE4 on the risk of dementia. Indeed, in the WHICAP higher calorie intake was associated with increased risk of AD only in those who had at least one ApoE4 allele . High total fat intake was also associated with increased risk for AD in the same individuals.

[134 ]

However, the modulating effect of n-3 PUFA on these relationships was not studied. Further research is therefore needed to understand how ApoE genotype could modulate the impact of n-3 PUFA on obesity and subsequent cognitive decline.

global nutritional status

In older adults there is a U-shaped association between BMI and dementia, with increased dementia risk in obese and underweight individuals . Poor nutritional status is associated with higher risk of dementia in older persons .

[ survival of individuals whose healthy diet may have contributed to reverse the negative selection pressure associated with ApoE4. The quality of the diet would be less essential for the survival of ApoE4 non-carriers. Significant weight loss and decrease in BMI were observed in older women with AD who were ApoE4 carriers but not in non-carriers . In cognitively normal control women, no

[139 ] significant weight loss was observed whatever the ApoE4 status.

In AD patients included in the OmegAD RCT, weight gain and appetite improved in the EPA DHA group independently of the ApoE + genotype . Not carrying the ApoE4 allele and higher DHA plasma levels during the treatment period were both positively associated [140 ] with weight gain. Unfortunately, the same study did not report the differential effects of n-3 PUFA supplementation on cognition according to ApoE genotype . Hence, DHA and/or EPA might also work through mechanisms that could contribute to improve overall [START_REF] Freund-Levi | Omega-3 Fatty Acid Treatment in 174 Patients With Mild to Moderate Alzheimer Disease: OmegAD Study: A Randomized Double-blind Trial[END_REF] nutritional status of AD patients. This effect might be modulated by ApoE4 genotype but the direction and magnitude of the interaction is not yet clearly established.

Mood

Older persons with depressive symptoms are at higher risk of cognitive decline and AD . Moreover, several studies have [141 -143 ] reported an association between the ApoE4 allele and depression among older adults . Studies examining the impact of long-chain efficiency may be explained in part by a differential effect of n-3 PUFA according to ApoE genotype which could mask a protective association in some individuals. In the 3C study, higher plasma EPA was associated with slower decline on Benton Visual Retention Test in ApoE4 carriers, or in subjects with high depressive symptoms at baseline . Both interactions were independent, i.e. the highest [START_REF] Samieri | Omega-3 fatty acids and cognitive decline: modulation by ApoE epsilon4 allele and depression[END_REF] protective effect was observed in depressive ApoE4 carriers. This finding suggests that the impact of n-3 PUFA on cognition in ApoE4 carriers is mediated by more than a decrease in depressive symptoms.

In the OmegAD RCT, no significant overall treatment effects on neuropsychiatric symptoms of AD patients were found .

[154 ]

However, significant positive treatment effects on the scores in the Neuro-Psychiatric Inventory agitation domain in ApoE4 carriers and on the Montgomery-Asberg Depression Rating Scale in ApoE4 non-carriers were found. Such inconsistent results emphasize the need for further research on the mechanisms underlying the effect of n-3 PUFA on mood disorders in AD patients according to ApoE genotype and their potential impact on subsequent cognitive decline.

General discussion and conclusion

As suggested in a recent review, the presence of the ApoE4 allele might alter the course of late-life cognitive decline through changes in levels of both AD pathology and vascular injury, as well as through modifications in compensatory repair mechanisms . AD likely

[1 ]
occurs when many contributing risk factors, including genetics, converge and are counterbalanced by few protective factors . Dietary

[155 ]

n-3 PUFA are now understood to modify some of these risk factors. The ApoE4 interaction with PUFA metabolism is a nutrient-gene interaction which must be taken into account when assessing the risk for AD and evaluating the impact of dietary intake or supplementation with n-3 PUFA. Statistical analyses should systematically search for interactions with, and not only adjust for, ApoE genotype. In case of a significant interaction, stratified analyses should be provided. protection and repair mechanisms. Indeed, ApoE4 carriers have higher serum total and LDL cholesterol, impaired brain metabolism, and more often depressive symptoms. Nevertheless, ApoE4 carriers often have a lower level of some risk factors related to cognitive decline or dementia such as inflammation, hyperhomocysteinemia, and overweight, with inconsistent results. When considering the impact of dietary supplementation with n-3 PUFA on these biomarkers, ApoE4 carriers tend to be more responsive but few data are available from epidemiological or intervention studies. Paradoxically, ApoE4 carriers are less sensitive to the protective effects of n-3 PUFA on cognition in most studies. Further research is needed to understand the patho-physiological mechanisms underlying the differential effect of dietary PUFA on AD risk or cognitive decline according to ApoE genotype in order to better target intervention studies and nutritional recommendations on potential beneficiaries.

Acknowledgements:

We thank S. CS was funded by the Carnot Institute LISA (Lipids for Industry, Safety and Health).

MP was supported by a fellowship given by the Fonds de la recherche en sant du Qu bec and her research is currently supported by Improving neuro-psychiatric symptoms X Improving depressive symptoms X X stronger effect similar effect in both groups = ? unknown or discordant results

[71 96 ]

 96 Modification by ApoE of the effect of dietary EPA and DHA on other risk factors of dementiaPresence of the ApoE4 might accelerate the rate of cognitive decline through changes in the level of both AD pathology and vascular injury . Inflammation and metabolic disorders belong to the large set of factors that contribute to the accumulation of[1 ]

  controlled trial ROS : Reactive oxygen species TG : Triglycerides VLDL : Very low density lipoprotein WHICAP : Washington Heights-Inwood Columbia Aging Project References: 1 . Fotuhi M , Hachinski V , Whitehouse PJ . Changing perspectives regarding late-life dementia . Nat Rev Neurol . 5 : 649 -58 2009 ; 2 . Rapoport SI . Arachidonic acid and the brain . J Nutr . 138 : 2515 -2520 2008 ;

Figure 1

 1 Figure 1 Metabolism of lipoproteins starting with absorption in the small intestine. LPL lipoprotein lipase, HL hepatic lipase, VLDL very low = = = density lipoproteins, IDL intermediate density lipoproteins, LDL low density lipoproteins = =

  Impact of n-3 PUFA on risk factors of dementia and AD according to ApoE genotype: summary of findings.Effect of dietary n-3 PUFA on:Stronger effect of n-3 PUFA in: ApoE4 carriers ApoE4 non-carriers Incorporation of EPA and DHA in blood TG X Blood total, LDL or HDL cholesterol X weight gain in AD patients = =

  Although the underlying mechanisms are not clear, nutritional status may be associated with ApoE genotype. Surprisingly, in the Cache County Study on Memory and Aging participants with a diet that provided a greater variety of recommended foods were more likely to have at least one copy of the 4 allele than were those with less varied diets . This finding may be explained by selective

	135 ]	[135 -137 ]
	ε	[138 ]

  The reasons why some effects of n-3 PUFA are observed in the ApoE4 carriers and others in non-carriers, as summarized in , are not yet fully understood. It has been suggested table 2 that ApoE4 carriers may be more vulnerable to environmental factors because of compromised brain reserve or poor brain

	[80 ]	[156 ]

  Cunnane, Research Center on Aging, University of Sherbrooke, Sherbrooke, Canada, for his careful revision of the manuscript and relevant comments. CF benefited from financial support of the ANR--The French National Research Agency under the , project COGINUT ANR-06-PNRA-005 . Programme National de Recherche en Alimentation et nutrition humaine »

	«	»

«

Agence Nationale de la Recherche » «

Table 1

 1 Association between n-3 PUFA and cognitive outcomes stratified by ApoE status in prospective epidemiological studies.

	Reference Exposure	Outcome	ApoE4 carriers	ApoE4 non-

carriers N Measure of association (95 CI) or value

	[ 75 ]	per week Fatty fish consumption: 2 to 4 servings	8-yr incidence of dementia	474 HR 0.91 (0.48 to 1.71) =	1570 HR 0.60 (0.40 to 0.89) =
	[ 10 ]	Fish consumption: at least once a week 4-yr incidence of dementia	1479 HR 1.28 (0.58 to 2.83) =	5944 HR 0.60 (0.41 to 0.89) =
	[ 76 ]	esters (for 1 SD) EPA DHA in plasma cholesteryl +	Cognitive decline on Word Fluency Test	650 OR 0.80 (0.45 to 1.63) =	1601 OR 0.61 (0.43 to 0.87) =
	[ 77 ].	Total n-3 PUFA in erythrocyte membranes (log)	Cognitive performance on a score derived by principal component analysis	38	r = NS	0.02	75	0.35 < 0.01 r = P
	[ 78 ]	EPA and DHA in total plasma fatty acids (for 1 SD)	Cognitive decline on Benton Visual Retention Test	235 DHA: 0.061, β= EPA: 0.077, β= P = P = .003 .01	984 DHA: 0.001, β= EPA: 0.001, β= P = P = .93 .93
					in subjects with low depressive symptoms; β=		in subjects with low depressive symptoms;
					0.172, <.001 P		0.096,	.04
					in subjects with high depressive symptoms.

% P N Measure of association (95 CI) or % P value

[START_REF] Kroger | Omega-3 fatty acids and risk of dementia: the Canadian Study of Health and Aging[END_REF]

[START_REF] Kroger | Omega-3 fatty acids and risk of dementia: the Canadian Study of Health and Aging[END_REF]

[START_REF] Kroger | Omega-3 fatty acids and risk of dementia: the Canadian Study of Health and Aging[END_REF]

[START_REF] Kroger | Omega-3 fatty acids and risk of dementia: the Canadian Study of Health and Aging[END_REF]

[START_REF] Kroger | Omega-3 fatty acids and risk of dementia: the Canadian Study of Health and Aging[END_REF]

[START_REF] Kroger | Omega-3 fatty acids and risk of dementia: the Canadian Study of Health and Aging[END_REF]

[START_REF] Kroger | Omega-3 fatty acids and risk of dementia: the Canadian Study of Health and Aging[END_REF]

[START_REF] Kroger | Omega-3 fatty acids and risk of dementia: the Canadian Study of Health and Aging[END_REF]