Marie Deck

Gerben Van Hameren
email: gerben.van-hameren@inserm.fr

Nathalie Bernard-Marissal

Jérôme Devaux

Graham Campbell

Jade Berthelot

Alise Lattard

Jean-Jacques Médard

Roman Chrast

Nicolas Tricaud
email: nicolas.tricaud@inserm.fr

Flavin Mononucleotide FRET Fluorescence Resonance Energy Transfer FtMt Mitochondrial Ferritin GLUT1 Glucose Transporter 1 GPx Glutathione Peroxidase H 2 O 2 Hydrogen Peroxide HNE 4-Hydroxynonenal HS Heparan sulfate IL-2 Interleukin-2 IP3R Inositol 1,4,5-trisphosphate Receptor ISCs Iron-Sulfur Clusters IT Intrathecal ITR Inverted Terminal Repeats JXP Juxtaparanodes KGDH α-Ketoglutarate Dehydrogenase

Keywords: axonal activity, demyelination, MFN2, mitochondria, ROS

LPS

Prologue

The human nervous system is the biological system that transmits signals between the brain and the rest of the body. It can be subdivided in two parts: the central nervous system (CNS), consisting of the brain and the spinal cord, and the peripheral nervous system (PNS). The PNS connects the brain and spinal cord with the organs and the limbs of the periphery. The neurons of the PNS, which cell bodies are located in the spinal cord and in dorsal root ganglia, cover a large topographical area of the body ranging from the ganglia, located just outside the CNS, to nerve endings in the farthest extremities of the body. To send signals from the CNS to the periphery, the PNS neurons depolarize and then conduct action potentials through their axons, which can be extremely long. Neuronal axons in the PNS have close functional interactions with the surrounding Schwann Cells (SCs), which support long-term axonal survival and peripheral nerve function through the myelin sheath. On the opposite, defects in SCs can cause axonal degeneration. SCs can be present in the PNS both in a myelinating and nonmyelinating state. Myelinating SCs produce myelin, which is folded around the axon and is necessary for high nerve conduction velocity along axons. The axon is not myelinated in the space between two SCs and these unmyelinated areas, the nodes of Ranvier, are rich in ion channels, allowing action potentials to be regenerated.

When the PNS becomes damaged or does not function in a healthy way, this may lead to the development of peripheral neuropathies, such as hereditary Charcot-Marie-Tooth (CMT) diseases. In many patients that suffer from peripheral neuropathies, the interaction between neuronal axons and SCs is impaired, which often manifests as impaired motor or sensory functions. Several peripheral neuropathies indicate that mitochondrial physiology is critical for the maintenance of the PNS. Dysfunctional mitochondria are also involved in CNS diseases (such as Parkinson's disease, Multiple Sclerosis and Alzheimer's disease).

Mitochondria are organelles that are structured as two lipidic layers of discrete membranes and compartments in which proteins reside. These organelles are present in all human cells (except red blood cells), including neurons, and fulfill several functions that are essential for the cell to function in a healthy way, such as production of adenosine triphosphate (ATP) (the most-used energy form within cells), production of reactive oxygen species (ROS), and calcium storage. Mitochondrial function relies on a multitude of factors, including the structure of the mitochondrion, the composition of its membranes and the metabolites that are present in the cytosol. The vast majority of proteins (~99%) that define the mitochondrial structure and function are derived from transcription in the nucleus, translated in the cytosol and translocated into the mitochondria. Only a small proportion of proteins are derived from its own genetic material, the mitochondrial DNA (mtDNA). Most of the mtDNA genes code for proteins that are involved in the production of ATP and ROS.

ATP is produced via three metabolic pathways that use glucose or other metabolites: glycolysis in the cytosol, the tricarboxylic acid (TCA) cycle and the electron transport chain (ETC) in mitochondria. In short, glycolysis transforms glucose towards pyruvate, which can enter mitochondria and is used for the production of NADH and FADH 2 via a chain of enzymatic reactions. NADH and FADH 2 are used in the ETC as a source of electrons. These electrons are transferred from complex to complex within the mitochondrial inner membrane, while protons are exported in the intermembrane space to create a proton gradient towards the mitochondrial matrix. These protons re-enter the inner matrix of mitochondria via ATP synthase and this drives the ATP synthase to produce ATP from ADP and free phosphate.

The electrons that pass the ETC during the production of ATP may leak back into the inner matrix, and there it will reduce free oxygen to form ROS. ROS are mostly known for their role in DNA damage, protein oxidation and the oxidation of lipids. These are damaging processes, so an excess of ROS are considered toxic. However, ROS are not only damaging. They can regulate a whole range of cellular responses and therefore their role as a signaling molecule is more and more acknowledged. For example, ROS play important roles in immune responses, axonal regeneration and neurogenesis. Since mitochondria are central players in cellular function and viability, their involvement in many diseases, including peripheral neuropathies, is not surprising. Luckily, cells are equipped with competent antioxidant systems that can detoxify the cell by reducing the different types of ROS. Superoxide Dismutase (SOD), which is highly expressed in mitochondria, is an enzyme that reduces superoxide, which is a highly reactive type of ROS, to hydrogen peroxide (H 2 O 2 ). Hydrogen peroxide is less reactive, hence it is less damaging, but it can also diffuse longer distances and oxidize cellular components more distally. Glutathione Peroxidase and catalase are the antioxidant enzymes that can reduce hydrogen peroxide further to water and dioxygen.

The shape of mitochondria is correlated with its function. Mitochondria are able to merge with other mitochondria, a process which is called mitochondrial fusion, resulting in larger mitochondria. This fusion process is correlated with a higher production of ATP. The opposite process, mitochondrial fission is also possible, leading to the presence of multiple smaller mitochondria. Besides changes in mitochondrial shape, also changes in mitochondrial position are important in defining mitochondrial function. They can bind to the cellular cytoskeleton and move towards regions where the need for energy is high so that they can replenish that area with ATP. Continuous depolarization and repolarization requires ATP and subsequently, the production of ATP in the node of Ranvier is crucial for repeated regeneration of action potentials in this area. Nonetheless, physiology and function of nodal mitochondria are still controversial. Besides, the method to recruit metabolites for mitochondria to produce ATP is not established. In the CNS, lactate may be delivered by astrocytes and oligodendrocytes, in a system that was called the lactate shuttle. However, such a transport has never been demonstrated in the PNS and the mechanism of lactate production by Schwann cells is unknown, since most cells use pyruvate for oxidative phosphorylation instead of lactate production.

In this PhD thesis, I will investigate what the role of Schwann cells is in mitochondrial metabolism in the axon and how Schwann cells can produce such amount of lactate to transport to the axon. Also how lactate shortage will influence neuronal function is studied. Then I will explore how a mutation in the mitofusin2 (MFN2) gene, a model for CMT2A disease, can cause neuronal deficits. For this, we looked how the endoplasmic reticulum (ER) and mitochondria are affected by this mutation. Finally, I focus on mitochondrial physiology and find out how mitochondrial production of ATP and ROS is influenced by a range of healthy and neuropathic stimuli, such as action potential firing, spatial localization and demyelination.

During this project I used in vivo techniques, because mitochondrial physiology changes very fast when the system is altered. These techniques involve fluorescent probes delivered by viral vectors, multiphoton microscopy, live imaging. Coherent anti-Stokes Raman Scattering (CARS) imaging is used to visualize myelin, which allows for detection of internodes and nodes of Ranvier and for the assessment of the myelin sheath status. These techniques are used for experiments in vivo using either wildtype or transgenic mice.

By presenting my original data, I will show that cytosolic lactate and glucose, or mitochondrial ATP and H 2 O 2 levels can be measured in vivo using viral probes. When we looked for the mechanism how glial cells can produce enough lactate to feed the neurons they nurture, we found that the Warburg effect, which has previously been described mostly in cancer cells, can exist in the nervous system as well. The Warburg effect is a shift in metabolism from oxidative phosphorylation to aerobic glycolysis. Indeed, we showed that deleting Pyruvate kinase M2 (PKM2), the enzyme required for aerobic glycolysis, specifically in myelinating SCs in mice, resulted in correct myelin maintenance, but an alteration of the functions and the maintenance of myelinated axons of peripheral nerves. The lack of Warburg effect in glial cells becomes apparent only when neurons fire action potentials and thus require energy. In the PKM2 mutant mice, cytosolic lactate decreases following periods of induced action potential firing, which shows that lactate is used by mitochondria without replenishment of cytosolic lactate via the lactate shuttle. However, whereas mitochondria in healthy neurons upregulate ATP production upon nerve stimulation, mitochondria in lactate deprived neurons do not.

Secondly, by examining the effect of the MFN2 R94Q mutation on neurons, I found that this mutation causes locomotor defects and neurite degeneration in motoneurons specifically. This phenotype correlates with increased ER stress and a decreased contact area between the ER and mitochondria, abnormal mitochondria morphology and an impaired transport of mitochondria through axons.

Then to investigate further the effects of nerve activity on axonal mitochondria physiology, I show that besides ATP, H 2 O 2 levels increase as well when nerves are firing action potentials and energy need is higher. However, the dynamics of ATP is not the same as H 2 O 2 . Also, mitochondria are intrinsically more metabolically active in nodes of Ranvier than in internodes. Then, I will show how mitochondrial physiology changes in a couple of neuropathology models: the MFN2 R94Q mutation to model CMT2A and a demyelination model. In both pathologies, a decoupling between ATP production and H 2 O 2 production was observed.

Finally, I will provide a protocol on how to introduce transgenes into sciatic nerve cells. This protocol provides a list of the necessary equipment as well as a description of the surgical techniques to isolate the sciatic nerve in adult mice and mouse pups. In the next step, I will show how to inject viral particles into the nerve via micropulses and the procedure of closing the wound.

In conclusion, I will discuss these results in a larger context and create model on axon metabolism and axonal mitochondria physiology in normal and diseased conditions. I will also propose some perspectives for neuropathies and how mitochondria could be a therapeutic target.

All non-referenced figures are personally-made figures using Servier medical art, adapted with Microsoft Powerpoint and Adobe Photoshop 7.0.

Part A Introduction Chapter 1) Physiology and functions of mitochondria

The cells that build up the human body are filled with cytosol in which multiple cell components called organelles fulfill their specific role to ensure healthy cell function. The nucleus contains most of the cellular DNA and the Golgi apparatus and ribosomes fulfill their roles in protein synthesis. In this first chapter, I will introduce the origin, morphology and functions of mitochondria. These organelles are present in almost all human cells, where they play a role in several cellular processes. Mitochondria are often described as "the powerhouse of the cell", because of their function as energy producer. For this, mitochondria contain specialized groups of proteins that produce ATP, which is the most used energy source of human cells. How these proteins produce energy is also discussed in this chapter.

A mitochondrion

Mitochondrial biogenesis

According to the endosymbiont hypothesis, which is largely based on the genome that is contained within mitochondria, these organelles are of bacterial ancestry 1 . More specifically, the origin of mitochondria is the establishment of a phylum α-Proteobacteria as a symbiont into a host cell. Two main scenarios are hypothesized on how a phylum α-Proteobacteria has entered originally into a host cell. First, in the archezoan scenario, this host cell was a very primitive eukaryote, termed archezoan 2 . A second hypothesis, the symbiogenesis scenario, describes a single endosymbiotic event that involves the uptake of an α-Proteobacterium by a cell, resulting by the generation of mitochondria and as a consequence, the development of a nucleus and compartmentalization of this eukaryotic cell. An example of the symbiogenesis hypothesis is the hydrogen hypothesis 3 . The hydrogen hypothesis states that the origin of mitochondria is the result of symbiotic association between a strictly autotrophic archaebacterium, which is dependent on hydrogen, and an eubacterium that produces hydrogen as a byproduct of respiration through anaerobic heterotrophic metabolism. The host, the archaebacterim, depends on the hydrogen production of the symbiont, the eubacterium, which on its turn depends on the host's energy supply. A key point of symbiogenesis scenarios, including the hydrogen hypothesis, is that the complexity of the eukaryotic cell and its defining features emerged after the mitochondrial symbiosis, rather than before.

After this original genesis of a symbiont, these organelles have evolved, but its bacterial origin still has critical effects on mitochondrial function in humans. For example mitochondrial biogenesis results from the proliferation and growth of pre-existing mitochondria as the cell cannot produce mitochondria de novo. This is the reason why all our mitochondria are issued from the few mitochondria that were in our mother oocyte. Next to this autoreplication, mitochondrial biogenesis in human cells depends on fusion and fission. Mitochondria also contain their own DNA, called mitochondrial DNA (mtDNA), which code for 13 mitochondrial proteins 4 . Nonetheless, biogenesis of new mitochondria also involves the synthesis and import of 1000-1500 proteins that are encoded by the DNA in the nucleus 5 . A multitude of stimuli influences mitochondrial biogenesis, such as exercise, caloric restriction, low temperature, oxidative stress, cell division and renewal and differentiation. Because so many factors play a role, there is not only a large variety in numbers of mitochondria, but also in mitochondrial size and mass.

Lipids and proteins in the mitochondrial membranes

Mitochondria contain two bilipid layers that separate the compartments of the organelle 6 . The outer mitochondrial membrane separates the intermembrane space with the cytosol of the cell. The inner mitochondrial membrane separates the intermembrane space with the inner matrix of the mitochondrion (Fig. 1). The shape of the inner membrane involves many folds and notches, which are called cristae. The existence of two bilipid layers allows for the export of ions and other molecules from the inner matrix to the intermembrane space, which allow the formation of gradients, which is important for proper function of this organelle. During high respiratory activity, mitochondria have a small, condensed matrix volume, and the inner membrane is pulled away from the outer membrane, resulting in enlarged cristae, whereas when mitochondria are less active, they have a relatively large matrix volume with the inner membrane closely located to the outer membrane 7 . This shows that mitochondrial membranes actually have a really dynamic structure. The inner mitochondrial membrane is enriched in proteins and contains only about 20% of lipids, which is in contrast with the lipid-rich outer mitochondrial membrane 8 . Cardiolipin synthase, an enzyme located in the inner mitochondrial membrane, is involved in lipid biosynthesis by catalyzing the conversion of phosphatidylglycerol to cardiolipin. Cardiolipin is an essential lipid in the mitochondrial membranes, because it regulates mitochondrial membrane permeability and protein import. Cardiolipin is also involved in mitochondrial fusion and fission, which is no surprise considering that the mitochondrial membranes have to fuse or separate during these two processes. Cardiolipin reacts with multiple mitochondrial membranes and stabilizes their confirmation. Next to cardiolipin, ceramide is another lipid that is strongly involved in mitochondrial function. In contrast with cardiolipin, which is mostly present in the inner mitochondrial membrane, ceramide is present mostly in the outer mitochondrial membrane. It enables the formation of protein-permeable channels through witch pro-apoptotic proteins can travel from the mitochondria into the cytosol. Therefore cardiolipin and ceramide are known regulators of mitochondrial shape and permeability.

Fusion and fission

A part of mitochondrial biogenesis is the mechanism of mitochondrial fusion and fission (Fig. 2) 9 . Mitochondrial fusion and fission is a central mechanism that not only defines the mitochondrial number, but also the mitochondrial shape and size 10 . Even in other mitochondrial processes, including transport along axons and interactions with other organelles, such as the endoplasmic reticulum (ER), fusion and fission play a role. Mitochondrial dynamics involve the repartition of lipids, proteins or mtDNA within mitochondria and thereby these processes regulate the transport speed of these molecules through the cell 11 . Mitochondria adapt their shape continually through fusion, fission and motility in response to cellular stimuli, such as changes in energy demand 12 . Mitochondrial dynamics are essential to adapt mitochondria to different cellular functions, including maintaining cell viability or oppositely apoptosis. The fusion of mitochondria to form large mitochondria also increases the mitochondrial membrane potential and consequently increases the production of ATP 13 . In contrast, smaller mitochondria have a lower ATP production. Fission occurs when mitochondria become damaged, contain too high concentrations of deleterious components, such as mutated mtDNA 14 , or endure too much cellular stress. This way the damaged or unviable parts of the mitochondria can be separated from the healthy parts of the mitochondria. The disposed mitochondria parts can be degraded in lysosomes through autophagy while the viable parts can continue to function. Reversely, mitochondrial fusion allows cells to repair defective mitochondria with healthy ones in order to regenerate the mitochondrial DNA (see paragraph 1.4). Three dynamin-like GTPases are the central players in mitochondrial fusion. Mitofusin 1 and Mitofusin 2 (MFN1 and 2) are located in the outer membranes of mitochondria, while Optic Atrophy-1 (OPA1) is located in the inner membrane 10 . MFN1 and MFN2 interact with MFN proteins at the membranes of adjacent mitochondria. This interaction occurs via the formation of homo-and hetero-oligomers. Then, the outer membranes of the two mitochondria fuse. MFN1 interacts also with OPA1 for the fusion of the inner membranes of multiple mitochondria and the formation of cristae. Other proteins play a role in mitochondrial fission, which is the opposite process. Mitochondrial fission factor (MFF) and mitochondrial fission protein 1 (FIS1) are located at the outer membrane of mitochondria and act as ligands of dynalin-related protein 1 (DRP1), a cytoplasmic GTPase.

Mitochondrial DNA

A particular feature of mitochondria is the presence of mitochondrial DNA. mtDNA is circular shaped and consists of 16.5 kb. mtDNA contains 37 genes that code for 13 subunits of the ETC. In addition, mtDNA codes for 22 transfer ribonucleic acids (tRNAs) and 2 ribosomal RNAs (rRNAs) that are required for the translation of respiratory subunit messenger RNAs (mRNAs) within the mitochondrial matrix 15 . mtDNA is more susceptible to oxidative damage and as a consequence, mutations occur at a higher rate in mtDNA than in nuclear DNA. This high susceptibility is due to the high levels of ROS being generated by oxidative phosphorylation, lack of protective histones and a limited repair capacity for mtDNA damage. Intracellular ROS concentrations are correlated with changes in mtDNA copy number and the expression of mitochondrial respiratory genes. Increased oxidative stess during aging is as well associated with increased mtDNA copy number. Therefore, mitochondria require alternative protection methods to prevent accumulation of damaged mtDNA. One of the key protective measures is fusion and fission. Indeed, upon mitochondrial fusion, the mtDNA of each pre-existing mitochondrion is thought to recombine with each other 16 . This recombination of mtDNA allows healthy mtDNA to regenerate damaged mtDNA and thereby mitochondrial viability 17 .

Although mtDNA contains a few genes that are important for mitochondrial function, most genes that code for proteins required for mitochondrial function are located in nuclear genome and these proteins are transported into mitochondria. Nuclear respiratory factors 1 and 2 (NRF1 and NRF2) are transcriptional regulators that act on nuclear DNA. There, it regulates transcription of subunits required for oxidative phosphorylation within mitochondria and genes that are involved in maintaining mtDNA copy number via mtDNA replication. NRF1 is a homodimer and binds to DNA to function as a positive regulator of gene transcription 14 . Many respiratory genes are regulated through NRF1 and NRF2, including complex I of the ETC, complex II, complex III and ATP synthase. In addition, NRF1 and 2 regulate proteins involved in mtDNA transcription and replication. Also, NFR1 and 2 are involved in regulating proteins that play important roles in the import of mitochondrial proteins into the organelle. Another important group of proteins that act as regulators of mitochondrial biogenesis is the PPARγ coactivator-1 (PGC1) family (PGC1a, PGC1b and PRC). This group of proteins acts mainly as coordinators of a multitude of transcription factors that are regulating the mitochondrial biogenesis process. Therefore, PGC1 proteins work in an indirect manner and they bind to other transcription factors, such as NRF1, NRF2 and PPARs to regulate their activity. This way, those transcription factors have an increased activity and can stimulate mitochondrial biogenesis.

The activity of transcription factors, including transcription factors that are involved in mitochondrial biogenesis, are influenced by ROS 14 . Human cells respond to defective respiratory function by increasing the expression of nuclear and mitochondrial genes. The upregulation of these genes is managed via an H 2 O 2 -dependent signaling pathway 12 . Treatment of mouse embryonic cells with H 2 O 2 increases PGC1α and PGCβ mRNA levels 13 . PGC1α and PGC1β regulate gene expression of antioxidant genes such as catalase and superoxide dismutase. This shows that an interaction exists between mitochondrial biogenesis, ROS production and ROS defenses.

Mitochondrial functions in the cell

The main function of mitochondria is the production of ATP. The different components of the ETC have already been discussed shortly, but this system is actually the last phase in the production of ATP from metabolites. Glucose is the main energy source that is used by cells to produce ATP. Glucose is oxidized to carbon dioxide and water via glycolysis, the Kreb's cycle (also named the tricarboxylic acid (TCA) cycle), and the ETC. The latter two are referred to as oxidative phosphorylation. These are three synchronized pathways that involve a large range of enzymes and metabolic intermediates and result in the production of energy in the form of ATP 18 . The first pathway, glycolysis, is occurring in the cytosol and not in the mitochondria, but it is an important system in controlling mitochondrial metabolism.

Glycolysis

Glycolysis is the catabolism of glucose into pyruvate (Fig. 3) 19 . The formation of pyruvate from glucose also produces four ATP molecules, although two ATP molecules are used as well during this process, resulting in a net production of two ATP molecules. The produced pyruvate is being converted to lactate in anaerobic conditions, allowing the regeneration of nicotinamide adenine dinucleotide (NAD + ). Without NAD + , glycolysis cannot continue further and glyceraldehyde3-phosphate is formed 18 .

The function of glucose metabolism is to produce ATP and to provide carbon for reactions involving local activities 20 . In order for these pathways to be maintained, enough glucose needs to be present. Besides, there is a tight regulation of glycolysis via the presence of certain enzymes: hexokinase, phosphofructokinase and pyruvate kinase (PKM) 21 . Pyruvate kinase catalyzes the last step of glycolysis, converting phosphoenolpyruvate to pyruvate 22 . Several isoforms of pyruvate kinase exist, but the main two isoforms are PKM1 and PKM2. PKM2 is preferentially expressed by most types of cancer cells and it hypothesized that PKM2 promotes an alternative catalytic reaction that does not produce ATP, in contrast to the catalytic activity of PKM1 23 . PKM2 is a known target of ROS for H 2 O 2 , diamide and hypoxia have all been shown to inactivate PKM2. In contrast, the reductant DTT restores PKM2 activity. In addition, the isoform PKM1 is not inhibited by oxidation. More specifically, Cys358 of PKM2 appeared to be especially susceptible for oxidation and Cys358 oxidation decreases the level of PKM2. As a response to oxidative stress, the mutated Cys358 induces PKM2 dissociation thus preventing its enzymatic activity. PKM2 inhibition results in the increase of NADPH production, which prevents the accumulation of intracellular ROS. In multiple pathological conditions that are characterized by mitochondrial dysfunction, mitochondrial ROS production is increased. In these same diseases, glycolysis is also enhanced while oxidative phosphorylation is impaired. This suggests that ROS is involved in glycolysis (dys)regulation 24 .

The Citric Acid Cycle

Under aerobic conditions, the pyruvate produced by glycolysis is used for oxidative decarboxylation, resulting in the production of acetyl-CoA (Fig. 4) 25 . In this reaction, pyruvate dehydrogenase is the driving enzyme. Acetyl-CoA binds to oxaloacetate and citrate synthase drives its conversion into citrate. This enzymatic reaction is the first step of the TCA cycle and oxidative phosphorylation. During the TCA cycle, three NADH molecules, and one 1,5-dihydro-flavin adenine dinucleotide (FADH 2 ) molecule are formed. These compounds serve as electron transporter into the complexes of the ETC. The electrons transferred to oxygen to form ROS by the ETC are actually originating from the NADH and FADH 2 that are formed during TCA cycle 18 .

The TCA cycle consists of eight conversion steps 25 : The first step, the condensation step, has been mentioned above and involves the conversion of Acetyl-CoA to form citrate and is catalyzed by citrate synthase. Then, citrate is converted to isocitrate by aconitase in a two-step conversion process. The third step involves the conversion of isocitrate to alpha-ketoglutarate by isocitrate dehydrogenase. This step is also the first step where NADH is produced as a cofactor, which is later used to drive the ETC. Step 4 is catalyzed by α-ketoglutarate dehydrogenase (KGDH). This enzyme generates succinyl-coA from α-ketoglutarate. Then, succinyl-coA is converted to succinate by succinic thiokinase, a reaction during which GDP is converted to GTP. The sixth step involves the succinate conversion into fumaric acid by dehydrogenase. During this conversion, FADH 2 is formed as well from FAD. FADH 2 is a second molecule that drives the ETC, next to NADH. In step 7, fumarate hydratase catalyses the hydration of the double C=C double bond of fumaric acid to form malate. Finally, malate is dehydrogenated to produce oxaloacetate by malate dehydrogenase. This is a second step where NADH is formed from NAD. The formed oxaloacetate reacts with acetyl-coA and forms citrate again, which is the first step of another round of the TCA cycle. The NADH and FADH2 molecules that are formed during this cycle are used by complexes of the ETC. Therefore, both glycolysis and the TCA cycle are important pathways in the production of NADH and FADH 2 that provide electron to the ETC.

Not only is the TCA cycle important for providing substrates for the ETC, it appears that intermediates of the TCA cycle can modulate the concentration of mitochondrial ROS 26 . The central intermediate in this TCA-cycle mediated ROS homeostasis is α-ketoglutarate and the enzyme KGDH (Fig. 4). Indeed α-ketoglutarate plays a role in ROS scavenging while reducing NADH production and decreasing the activity of KGDH allows ketoglutarate increase and drop ROS levels 26 . KGDH possesses a sensor function of oxidative stress as its oxidation leads to inactivation of its enzymatic activity. Actually, a reduced KGDH activity is reported in many disorders. For example, expression of mitochondrial KGDH is reduced in demyelinated axons 27 . Supposedly, the decreased KGDH activity is an adaptive mechanism in an effort to detoxify the cell of ROS. Nevertheless, the cell's ATP production is also impaired, so in healthy conditions the production of energy is always a compromise with the production of toxic ROS.

Metabolism is affected by oxidative stress, since excessive oxidation can cause inactivation of enzymes that are involved in glycolysis, TCA cycle and the ETC, which decreases cell viability 28 . However, metabolism has evolved to adapt to conditions of high oxidative stress. These adaptive mechanisms are often independent from transcription, but instead rely on thiol-based switches of enzymes, which change enzymatic activity fast. As a consequence, metabolic changes can occur relatively fast as well. Indeed, metabolic changes have been detected within minutes of oxidative stress.

Warburg metabolism

Most cells use both glycolysis and the citric acid cycle for the production of ATP. Nonetheless, when cells are in hypoxic conditions, not all pyruvate that is produced via glycolysis is being used by mitochondria 29 . Instead, a portion of the pyruvate is enzymatically transformed into lactate. In this condition, cells rely predominantly on glycolysis for the production of ATP. The hypoxia-induced transition of pyruvate production towards lactate is regulated by lactate dehydrogenase (LDH), which is the enzyme that converts pyruvate to lactate 30 .

In the 1920s, the phenomenon of cells depending on glycolysis as the main source of ATP in aerobic conditions has first been described by Otto Warburg 31 . This aerobic glycolysis, also called Warburg metabolism, has first been demonstrated in several tumor types, including colorectal cancer, breast, lung and glioblastoma 32 . Several theories exist why switching to Warburg metabolism is beneficial for tumor cells 33 . Warburg metabolism is potentially an adaptation mechanism to match the increased biosynthetic needs that are the result of uncontrolled proliferation 34 . Increased glucose consumption by the tumor cell and hence increased aerobic glycolysis is then a carbon source for the generation of more nucleotides, lipids and proteins. A second advantage of Warburg metabolism for tumor cells is the acidification of the microenvironment due to elevated glucose consumption and increased lactate secretion 35 . This decreased pH level in the tumor's microenvironment increases the invasiveness. Overall, the potential to switch to Warburg metabolism is an important feature of a cell and it is essential to understand how cell metabolism is regulated both in physiological conditions and in cancer.

An example of a known regulator of Warburg metabolism is pyruvate kinase. Tumor cells have been shown to express exclusively PKM2 36 . Moreover, it has been demonstrated that a switch in splice isoform from PKM1 towards PKM2 is required for the shift in cellular metabolism to Warburg metabolism 37 . However, the exact mechanism how PKM2 regulates the transition towards Warburg metabolism has yet to be established.

Electron Transport Chain complexes

The inner mitochondrial membrane contains a set of proteins that cross this bilipidic layer. An important group of proteins located in the inner membrane includes the complexes of the ETC 38 . The role of the ETC in oxidative phosphorylation is to transfer electrons from electron donors to electron acceptors and the simultaneous transport of protons across the inner membrane in order to load the mitochondrial matrix with protons. These protons are then used as the driving force to produce ATP by ATP synthase. The ETC consists of 5 large protein complexes. These complexes are numbered as complex I (NADH dehydrogenase (ubiquinone), 45 protein subunits), complex II (succinate dehydrogenase, 4 protein subunits), complex III (ubiquinol-cytochrome C reductase, 10 protein subunits), complex IV (cytochrome C oxidase, 19 protein subunits), and complex V (ATP synthase, 19 protein subunits). It has been shown that these complexes are integrated in the inner mitochondrial membrane freely, but there is increasing evidence that the complexes can form respiratory chain supercomplexes 39 . Supercomplexes of two or even three respiratory chain complexes have been proposed, such as a complex III -complex IV supercomplex. Similar interactions include Complex I-III supercomplexes and further assembled interactions such as Complex I-III-IV 40 . The rate of supercomplex formation is different between different cell types, even between different cell types in the same tissue. It has been shown that neurons and astrocytes in the brain organize their mitochondrial respiratory chains differently, with a different proportion of respiratory chain complexes present freely or part of supercomplexes 41 . In astrocytes, complex I is assembled less frequently into supercomplexes resulting in more free complex I. In contrast, neuronal mitochondria contain less free complex I, since it is more frequently assembled into supercomplexes. An abundance of free complex I is caused by a low amount of complex III, which limits the amount of complex I that can be incorporated into supercomplexes. ATP synthase is a multisubunit complex that functions in the last step of the ETC. The structure of ATP-synthase resembles a mushroom and the globular protein complex that forms the "head" of the mushroom contains its catalytic sites. This subunit is called the F1 domain 42 . A large difference in proton concentration across the inner membrane of mitochondria creates a membrane potential, also called the proton-motive force. ATP synthase uses this force to drive the conversion of ADP and phosphate into ATP 43 .

The complexes of the ETC fulfill the main function of mitochondria: the synthesis of ATP. Besides, the activity of these complexes results in the production of ROS. The production of ROS is a central aspect of this dissertation hence their production by the ETC complexes will be discussed in detail in the next chapter.

Iron storage

Iron is used as a catalytic co-factor by proteins throughout the cell in various critical processes. It also acts as an epigenetic regulator as some promoters contain iron response elements 44 . A main source of cellular iron are mitochondria 45 . Synthesis of iron-sulfur clusters (ISCs) occurs within the matrix of mitochondria. Fe 2+ is donated by frataxin to a scaffold protein. Sulfur, derived from cysteine via Nfs1/ISD11 complex-mediated desulfuration, also binds to the same scaffold protein. The resulting scaffold protein with Fe 2+ and sulfur is the completed ISC. Then, the ISC is exported from mitochondria to the cytosol, although the exact export mechanism is still poorly understood. Possibly, the ATP binding cassette transporter Abcb7 enables the export of ISCs or ISC intermediates. Since ISCs are not stable, chaperones are required to export ISCs into the cytosol and protect them from degradation. However, ISCs have functions within mitochondria as well, for example in ETC complexes.

A particular mechanism in which ISCs are important is the production of heme, which is a compound required for the catalysis of several enzymatic activities such as a "storage" for electrons during electron transfer or redox chemistry. In addition, heme can store O 2 and O 2 oxidizing equivalents in the iron side chains for further selective oxidation of substrates 46 . In the first step of heme synthesis, aminolevulinic acid is produced from glycine and succinyl-CoA in the mitochondrial matrix 47 . ALAS2, an enzyme catalyzing this reaction, contains an iron response element and therefore the concentration of ALAS2 protein increases when iron is abundant. Aminolevulinic acid is transported into the cytosol, where it is metabolized into coproporphyrinogen III. Coproporphyrinogen III is imported back into the mitochondrial intermembrane space and converted into protoporphyrinogen IX, which is then transported into the mitochondrial inner matrix by the ATP binding cassette transporter Abcb6. In the inner matrix, protoporphyrin IX is then formed and protoporphyrin IX binds to Fe 2+ , a reaction catalyzed by ferrochelatase. Next to the iron response elements of ALAS2, iron can influence heme synthesis via ISCs, because ferrochelatase needs ISCs for its stability. When ISC synthesis is impaired, this will lead to unstable ferrochelatase, which will result in incomplete heme synthesis 47 .

Since mitochondria produce ROS via oxidative phosphorylation and import Fe 2+ to be used in ISC and heme synthesis, this creates a particular situation regarding ROS homeostasis. Indeed the imported Fe 2+ stimulates Fenton reactions (see Chapter 2), which amplify the production of ROS. To avoid this, mitochondria can either immediately use Fe 2+ in synthetic pathways or store it in a storage protein complex consisting of oligomerized mitochondrial ferritin (FtMt) 45 . FtMt forms ferroxidase centers that oxidize Fe 2+ to Fe 3+ , which is less redox active. Importantly, the expression of the FtMt gene does not contain an iron response element, making it independent from iron concentrations. Overexpression of FtMt lowers ROS production and enhances cell viability, showing that FtMt has antioxidant features via inhibition of the Fenton reaction.

Calcium uptake

Calcium is essential for the respiratory function of mitochondria 48 . Indeed, Ca 2+ has a stimulating effect on oxidative phosphorylation as it activates pyruvate dehydrogenase, isocitrate dehydrogenase, and KDGH 49 , as well as stimulation of the ATP synthase 50 , α-glycerophosphate dehydrogenase 51 , and the adenine nucleotide translocase (ANT) 52 . Mitochondria also participate to the regulation of intracellular calcium levels. Intracellular calcium concentrations can rise rapidly and mitochondria participate to the removal of Ca 2+ from the cytosol. Alternatively, cytoplasmic Ca 2+ can enter the sarcoplasmic reticulum or ER via its Ca 2+ -ATPase (SERCA) and the sarcolemmal Na + -Ca 2+ exchanger (NCX) 53 . Cytoplasmic Ca 2+ enters mitochondria passively via voltage-dependent anion channels (VDACs) and the mitochondrial Ca 2+ uniporter (MCU). However, most of the mitochondrial calcium actually originates from the ER, which contains a much higher concentration of calcium. Calcium can be exchanged between mitochondria and the ER via the connection between these two organelles: mitochondrial associated membranes (MAM). The MAM is a domain of the ER that forms close contacts with mitochondria, where lipids are transferred from the ER to mitochondria next to the exchange of calcium 54 . The contacts between ER and mitochondria are dynamic and ER tubules can form contact areas ranging from 10% of the mitochondrial surface to a complete enveloping of the mitochondrion 55 . Under physiological conditions, Ca 2+ influx into mitochondria is relatively low and stable. When cytoplasmic Ca 2+ becomes too high though, the flux of Ca 2+ into mitochondria increases. When this influx of Ca 2+ becomes too high, the membrane potential of mitochondria is dissipated, resulting in the inhibition of further Ca 2+ influx. In neurons, once intra-axonal homeostatic capability is exceeded, mitochondria are confronted with high Ca 2+ levels, which triggers the mitochondrial influx of Ca 2+ 56 . The mitochondrial NCX transporter (NCLX) enables the reverse transport of Ca 2+ from the mitochondrial matrix into the cytosol. NCLX is a cotransporter and exports Ca 2+ while importing sodium. Next to NCLX, the permeability transition (PT) pore is a route for Ca 2+ to exit mitochondria, when the pore is open. The PT pore is an assembly of preexisting proteins within the mitochondrial inner and outer membrane. It has Ca 2+ binding sites on the matrix side, which could regulate PT pore opening 57 .

The uptake and release of calcium by mitochondria is important for neuronal function, because many cellular processes respond to the concentration of calcium within the cytosol. The processing of calcium in the cytoplasm is performed by sensor proteins that can bind to Ca 2+ , which results in a reversible conformation change 58 . For example, the release of neurotransmitters by healthy neurons is dependent on calcium levels in the cell. Upon depolarization, voltage-gated calcium channels open and calcium enters at the axonal terminal. There, Ca 2+ promotes fusion of synaptic vesicles with the presynaptic plasma membrane via acting on proteins of the SNARE superfamily. The fusion of synaptic vesicles and the plasma membrane results in a release of the vesicles' content, for example neurotransmitters, into the synaptic cleft.

Apoptosis

Finally mitochondria are also essential to trigger programmed cell death 56 . Release of cytochrome C from mitochondria regulates the assembly of caspase molecules and these caspases induce the cell degradation process. The exact export route of cytochrome C is not known, but its release is regulated by multiple proteins, such as proteins of the Bcl-2 family and Bax family 59 . Bcl-2 proteins block the release of cytochrome C and hence are antiapoptotic. On the other hand, insertion of Bax proteins into the mitochondrial membrane stimulates cytochrome C release and promotes apoptosis.

Another regulator of cytochrome C release is opening of the PT pore. However, cytochrome C does not exit through the PT pore, and the way how the PT pore stimulates cytochrome C release is still unknown. The PT pore is triggered by high concentrations of Ca 2+ within the mitochondrial matrix and several other stimuli, including ROS and the depletion of ATP 48 . PT pore opening is accompanied by a burst of ROS production. This spike of ROS opens more PT pores, so an autoamplification process is induced. This burst of ROS may be the cause of cytochrome C release via the oxidation of cardiolipin. In addition to the release of mitochondrial Ca 2+ into the cytosol through PT pores, it has been shown that cytochrome C binds to ER inositol 1,4,5-trisphosphate receptor (IP3R). This binding triggers the release of Ca 2+ from the ER, so the concentration of Ca 2+ into the cytosol increases even further. Overload of Ca 2+ is the initial cellular trigger of the apoptotic cascade. The close proximity between the ER and mitochondria enables this cross-talk via cytochrome C.

Conclusion

So Mitochondria are highly dynamic organelles, since they are able to fuse with other mitochondria (mitochondrial fusion), split into two or more smaller mitochondria (fission), and migrate through cells by following the cytoskeleton. The shape and location of a specific mitochondrion define its functionality. In textbooks mitochondria are often referred to as "the powerhouse of the cell". Even though mitochondria indeed fulfill an important role in energy production, since oxidative phosphorylation produces much more ATP than cytosolic glycolysis, limiting the function of mitochondria to just being a battery is too reductive. After all, mitochondria fulfill an important role in iron storage, calcium homeostasis and regulation of apoptosis as well. In addition, mitochondria produce reactive oxygen species, which will be the topic of chapter 2.

Chapter 2) Reactive oxygen species

In chapter 1, the production of ATP by the electron transport chain has been presented. This process of proton transport coincides with the transport of electrons from complex to complex. Most of this electron transport results in the formation of water, but part of these electrons leak back into the mitochondrial matrix, where it oxidizes oxygen. Therefore, the electron transport chain produces reactive oxygen species (ROS) next to ATP. ROS can be present in different forms and these different forms have distinct features, although all of them can cause damage to the cell.

However, what is the exact mechanism how the transport of electrons gives rise to ROS? Besides, is ROS only produced by mitochondria or do other ROS sources exist? Furthermore, how do ROS interact with other molecules to cause damage?

In this chapter, we will introduce the different types of ROS and how they are produced. Then, the damaging effects of ROS will be described and how antioxidants can prevent this damage. Finally, the role of ROS as signaling molecules will be discussed.

The different types of ROS

ROS are chemically reactive molecules derived from the reduction of molecular oxygen. In human cells, ROS are generated as a byproduct of oxidative phosphorylation in mitochondria. The first step of ROS production is the reduction of molecular oxygen (O 2 ) to anion superoxide (O 2

•-

).

H 2 O 2 is a non-radical oxidant present in virtually all aerobic organisms. H 2 O 2 is formed from O 2

•-by Mn-containing SOD that catalyzes the following reaction:

2O 2 • -+ 2H + → H 2 O 2 + O 2 H 2 O 2 is considerably less reactive than O 2 •-60 .
Therefore, H 2 O 2 can diffuse from the inner matrix of mitochondria through the mitochondrial membranes into the cell's cytoplasm. H 2 O 2 can even diffuse in such a far radius that it is able to interact with the cell's nucleus. H 2 O 2 is scavenged to water by glutathione peroxidase-1 (GPx) to limit its harmful effects 61 .

The generation of the highly reactive hydroxyl radical (OH•) is catalyzed by iron via the Fenton reaction. OH• is an extremely reactive type of ROS 62 . It immediately attracts electrons from nearby molecules, turning them into a radical, which will result in a cascade of ROS. The classical Fenton mechanism predicts that H 2 O 2 is reduced at the iron centers of the electron transport chain, which generates free OH•.

Fe 2+ + H 2 O 2 à Fe 3+ + OH -+ OH•
The Fe 2+ can be regenerated though the interaction of Fe 3+ with O 2

•- . Fe 3+ + O 2 •-à Fe 2+ + O 2
The combination of these two reactions is called the Haber-Weiss reaction. As most other types of ROS, formation of HO• occurs mainly within mitochondria 63 . The reason for this is that the precursors of OH•, i.e. O 2

•-and H 2 O 2 , are being produced within mitochondria as well. The catalysts for OH• formation, low molecular weight iron, is also abundant there.

Next to the classical enzymatic conversion of O 2

•-towards H 2 O 2 , O 2

•-can react with nitric oxide (NO•), which is produced enzymatically by nitric oxide synthase in the cytosol and can diffuse freely into the mitochondria 64 to form peroxynitrite (ONOO -) 65;66 . ONOO -can be generated in the cytosol as well, but the primary location of ONOO -formation is mitochondria. An alternative route for ONOO -formation is the reaction of nitroxyl anion with O 2 , where the nitroxyl anion formation in mitochondria is the result of NO• reduction by mitochondrial electron donors such as ubiquinol or cytochrome C 67 . There, it can react with its targets, which results in oxidation, nitration and nitrosation of mitochondrial components. Since mitochondria contain many targets for ONOO -oxidation or nitrisatiation, such as metalloproteins, thiolgroups, and CO 2 , intramitochondrially formed ONOO -has a shorter half-life than its counterpart formed in the cytosol. Like oxidation by O 2

•-or H 2 O 2 , oxidation by ONOO -significantly affect mitochondrial homeostasis and physiology. ONOO -reacts with complexes of the ETC as well, but these reactions are not the same as reactions between the ETC and NO•. Whereas NO• favors the inhibition of cytochrome C oxidase, ONOO -mainly reacts to complexes I, II and ATP synthase 68 . ONOO -reacting with complex I leads to inactivation, depending at least partially on thiol oxidation and the formation of a snitrosothiol-derivative of the complex. Complex II inhibition is most likely dependent on the thiol oxidation of the succinate dehydrogenase that is present in the dicarboxylate binding site. Moreover, ONOO -inactivates and nitrates ATP synthase, potentially by targeting the critical tyrosine of ATP synthase, which would account for the decrease in activity. ONOO - may also diffuse over the mitochondrial membranes into the cytosol, since its mean diffusion distance has been shown to be 3-4 µm, which is larger than the mean mitochondrial length being 3 µm in length and 0.6 µm in diameter. However, ONOO -undergoes reactions during this diffusion. Protonisation of ONOO -results in ONOOH, which can be scavenged by GPx.

Mitochondrial ROS production

The production of ROS starts in the mitochondrial electron transport chain via a leak of electrons, mainly from complex I and complex III (Fig. 5), that reduce O 2 to O 2

•-

. Complex I pumps protons from the inner matrix to the intermembrane space. This export of protons creates a proton gradient, which is used for ATP formation by ATP synthase. Complex I is the entry point into the respiratory chain for electrons from NADH generated during the TCA cycle 38 . It contains a Flavin mononucleotide (FMN) cofactor that accepts electrons from NADH and passes them through to a Coenzyme Q (CoQ) reduction site. In between, the electrons pass a chain of seven iron-sulfur clusters (ISCs). These ISCs are located in the hydrophilic arm of Complex I and are shielded from O 2 . This means that O 2 enters complex I at the FMN and Coenzyme Q sites and that is where electrons can reduce this O 2 . The isolated complex I produces O 2 • -when the FMN site is fully reduced. Therefore the proportion of the FMN that is fully reduced is an important factor for ROS production. On its turn the proportion of reduced FMN is dependent on the ration of NADH/NAD + . When complex I is impaired or inhibited, electrons flow back into FMN and this will produce an increased level of O 2 • -production. In addition, when NADH accumulates due to extremely low energy demand, this will lead to an increased O 2 • -production. However, only small amounts of O 2 • - are produced in normally respiring mitochondria, because the TCA cycle is not much active and therefore NADH/NAD + ratio is relatively low. A second mechanism of O 2 • -formation in complex I exists, named reverse electron transport (RET). Instead of an increased NADH/NAD + ratio or a fully reduced FMN site, RET occurs when CoQ is reduced 69 . With a sufficient proton motive force, electrons will then flow back into complex I and will reduce NAD + to NADH at the FMN site. When the electron supply to the CoQ pool originates from succinate, α-glycerophosphate or fatty acid oxidation, instead of pyruvate oxidation in the TCA cycle, a lot of ROS is created through this RET process. This particular respiration occurs in macrophages in response to bacterial infections and in cardiac and neuronal cells after ischemic periods 70 .

Mitochondrial complex II is also called succinate dehydrogenase. It plays a role both in the ETC and in the TCA cycle, where it catalyzes the conversion of succinate to fumarate (see Chapter 1). Complex II consists of four subunits, including one subunit that contains a chain of three ISCs 71 . Apart from complex I, complex II is the second entry point for electrons. Succinate oxidation generates FADH, which gives rise to two electrons that are transferred from the flavin site of complex II, through the ISCs towards CoQ, which is then reduced to ubiquinol. The contribution of complex II to ROS production in healthy conditions is controversial 72 , although increasing evidence is provided for its role in pathologic conditions, such as cancer 73 .

Like complex I, complex III pumps protons over the mitochondrial membrane (Fig. 5). Complex III works via two reduction steps involving ubiquinonic structures 60 . First, two electrons are transferred from CoQ ubiquinone towards two cytochrome C molecules 38 .

Then, two electrons are used to reduce quinone to quinol. During those processes, electrons leak back into the inner matrix, thereby participating in the formation of O 2

•-(Fig. 5) 74 . In contrast with complex I, complex III has just a single ISC. Another difference with complex I is that O 2

•-produced by complex III is not only leaking back into the inner matrix, but also into the intermembrane space 75 . However, the maximal amount of ROS produced by complex III is negligible when compared to the amount of ROS that can be produced by complex I through RET 38 .This means that when complex I is producing high amounts of O 2 • -, such as, then the extra ROS produced by complex III becomes irrelevant.

Complex IV, also called cytochrome C oxidase, catalyzes the final step of the ETC. This complex contains 13 subunits, of which three catalytic subunits, and four metal centers: Cu A , Cu B , Fe a and Fe a3 76 . Complex IV accepts electrons from cytochrome C and transfers them to the Fe a metal center. From there, these electrons are transferred to the bimetallic center Fe a3 /Cu B , where molecular oxygen is converted to water. Next to this electron transfer, complex IV also contributes to the generation of the mitochondrial proton gradient. During each reduction of O 2 to water, eight protons are taken up from the mitochondrial matrix, of which four are being used for water formation 77 . The other four protons are being pumped across the inner mitochondrial membrane. Mitochondrial ROS production by the ETC is dependent on a multitude of stimuli. One of those stimuli is the concentration of a 2+ within the cell. Indeed, cellular Ca 2+ drives myocyte enhancer factor 2 (MEF2) expression through mitochondrial activation of caspase-3 60 , which on its turn increases the expression of NADH dehydrogenase, an essential complex I component. In addition, MEF2 also decreases the expression of antioxidant enzymes such as SOD and hydrogen peroxidase. The conformation of the ETC is another factor determining the amount of ROS that is produced in mitochondria. Indeed, when complex I is present freely, which is a situation that is more common in astrocytes than in neurons, ROS production is higher than when it is assembled into a supercomplex 41 . NADH-derived electrons are inefficiently transferred to ubiquinone by the free complex I, because the distance is larger. In addition, the low proportion of complex III in astrocyte mitochondria inhibits the transfer of electrons to complex IV and deactivates complex I, because the ubiquinone pool is reduced.

NADPH oxidase

Although mitochondria are the main producers of ROS, this organelle is not the only origin of cellular ROS. NADPH oxidase is a plasma membrane bound enzymatic complex that can produce superoxide through the oxidation of NADPH 78 . While mitochondrial ROS is normally correlated with the production of ATP, ROS production is the main function of NADPH oxidase. In normal conditions, this enzyme is hardly active, but it is activated in particular conditions that require ROS production, such as the immune response mediated by neutrophils. Once the enzyme is activated, NADPH is oxidized to NADPH + and an electron is transferred to oxygen. This way, superoxide is produced to kill bacteria and fungi that are ingested inside the phagosomes. The activity of NADPH oxidase is strictly regulated and many intercellular signaling pathways are involved, but the strongest factor is the presence of Ca 2+ in the cytosol, indicating that NADPH oxidase could also play a role in apoptosis and neurodegeneration 60 .

NADPH oxidase activity and mitochondria can interact with each other [START_REF] Dikalov | Cross talk between mitochondria and NADPH oxidases[END_REF] . For example, upon binding of Angiotensin II (AngII) on the AngII type 1 receptor, NADPH oxidase is activated through protein kinase C (PKC). Activated NADPH oxidase then produces ROS, which, among other effects, open mitochondrial K ATP channels. The opening of these mitochondrial channels increases potassium influx into the mitochondria intermembrane space, swelling it and induces the production of mitochondrial ROS [START_REF] Andrukhiv | Opening mitoKATP increases superoxide generation from complex I of the electron transport chain[END_REF] . The interaction between NADPH oxidase and mitochondrial ROS can also work the other way around, making the pathway even more complex. Diazoxide, a mitoK ATP channel activator, triggers the production of mitochondrial ROS [START_REF] Andrukhiv | Opening mitoKATP increases superoxide generation from complex I of the electron transport chain[END_REF] , but ROS production by NADPH oxidase is also increased. As diazoxide does not directly affect NADPH oxidase activity, this suggests that production of mitochondrial ROS can influence NADPH oxidase activity. Inhibition of SOD2, which results in increased mitochondrial ROS levels, also increases NADPH oxidase activity and potentiate the effect of AngII [START_REF] Dikalova | Therapeutic Targeting of Mitochondrial Superoxide in Hypertension[END_REF] . In addition, increasing SOD2 levels decreased mitochondrial ROS production, which prevented AngII induced activity of NADPH oxidase. These results confirm that changes in mitochondrial ROS production affect the production of ROS by NADPH oxidase. Altogether, the activation of NADPH oxidases may increase the production of mitochondrial ROS and vice versa. This crosstalk between mitochondrial ROS and NADPH oxidases play an important role in the homeostasis of redox cell signaling. When ROS levels rise to excessive amounts due to overstimulation by AngII, high glucose, fat, or hypoxia for example, then a vicious cycle is triggered between mitochondrial ROS and NADPH oxidase resulting in more ROS production, which may contribute to the development of pathological conditions. Indeed, it has been shown that in some neurodegenerative diseases, such as Alzheimer's disease, Huntington's disease or Parkinson's disease, NADPH oxidase activity is increased [START_REF] Park | NADPH-oxidase-derived reactive oxygen species mediate the cerebrovascular dysfunction induced by the amyloid beta peptide[END_REF] in addition to the production of mitochondrial ROS.

How ROS cause damage to DNA, proteins and lipids

ROS can damage cells in several of different ways. It can oxidize DNA inside the nucleus, but also proteins and lipids throughout the cell. DNA damage will trigger a general shutdown of RNA synthesis [START_REF] Williamson | UV Irradiation Induces a Non-coding RNA that Functionally Opposes the Protein Encoded by the Same Gene[END_REF] , although the expression of some genes may actually be increased [START_REF] Fleming | Oxidative DNA damage is epigenetic by regulating gene transcription via base excision repair[END_REF] .

DNA oxidation

The main components of DNA are the nucleobases that are held together by deoxyribose and phosphate groups. The four types of nucleobases, thymine, adenine, cytosine and guanine can be oxidized by ROS [START_REF] Cadet | DNA Base Damage by Reactive Oxygen Species, Oxidizing Agents, and UV Radiation[END_REF] .

On thymidine, ROS oxidation adds a 5,6-pyrimidine covalent bond and substracts an H-atom from a methyl group. ROS preferably target C5 and to a lesser extent C6, which gives rise to the C6-yl and C5-yl radicals. These radical sites of thymidine react with O 2 to give rise to hydroperoxyl radicals [START_REF] Wagner | Thymidine Hydroperoxides: Structural Assignment, Conformational Features, and Thermal Decomposition in Water[END_REF] . The second pathway of ROS mediated thymine oxidation involves H-aton abstraction from the methyl group. This results in the 5-(uracilyl) methyl radical. Then, the 5-(uracilyl) methyl radical is converted into peroxyl radical after the reaction with O 2 and further to hydroperoxide radical. This radical is then reduced to 5hydroxymethyluracil and 5-formyluracil, which can also react with guanine and adenine bases to produce intrastrand or interstrand cross-links between the methyl group of thymine and the C8 position of guanine or adenine. ROS oxidation of cytosine and 5-methylcytosine also involves the formation of peroxyl radicals and hydroperoxides. Cytosine hydroperoxides rapidly decompose into intermediate compounds, such as uracil hydroperoxides and cyclic endoperoxide. These intermediates can form labile products such as cytosine glycol. The resulting products are analogues of uracil, which leads to altered transcription. Guanine oxidation by ROS results in two main degradation products: 8-oxo-7,8dihydroguanine and 2,6-diamino-4-hydroxy-5-formamidopyrimidine [START_REF] Cadet | DNA Base Damage by Reactive Oxygen Species, Oxidizing Agents, and UV Radiation[END_REF] , which are then further degraded. Another type of guanine oxidation product is oxazolone [START_REF] Cadet | Oxidatively generated damage to the guanine moiety of DNA: mechanistic aspects and formation in cells[END_REF] . The formation of oxazolone leads to base pair transversions and it creates stop points for DNA polymerase β, thereby blocking DNA synthesis [START_REF] Duarte | In vitro DNA synthesis opposite oxazolone and repair of this DNA damage using modified oligonucleotides[END_REF] . The oxidation of adenine is similar to guanine oxidation, leading to 8-oxo-7,8-dihydroadenine and 4,6-diamino-5-formamidopyrimidine. The formation of both these oxidation products is initiated by addition of •OH to C8 and then either a one-electron or a reduction step [START_REF] Cadet | Oxidatively generated base damage to cellular DNA[END_REF] . Interestingly, the yield of the adenine oxidation products is approximately a ten-fold lower than guanine oxidation products. Possibly, the reason behind this disproportion is that the majority of initial oxidation reactions occur at C4 of adenine [START_REF] Vieira | Pattern of OH radical reaction with adenine and its nucleosides and nucleotides: Characterization of two types of isomericOHadduct and their unimolecular transformation reactions[END_REF] , leading to an adduct radical that undergoes rapid dehydration to Ade-N6-yl-radicals. However, these Ade-N6-yl-radicals induce reactions that involves electron of H-atom abstraction with normal adenine as a result. Thus, the adenine can be regenerated and this might explain the low yield of adenine oxidation products after DNA alteration by ROS. Besides, the strong oxidizing properties of Ade-N6-yl radicals may enable electron transfers in DNA thereby transferring damage from adenine to guanine, which is an alternative explanation for the low adenine oxidation yield.

These modifications to the DNA bases and oxidation of the deoxyribose backbone of DNA can lead to different types of DNA damage (Fig. 6), including, DNA single-strand breaks, DNA double-strand breaks [START_REF] Woodbine | Endogenously induced DNA double strand breaks arise in heterochromatic DNA regions and require ataxia telangiectasia mutated and Artemis for their repair[END_REF] , bulky adducts, abasic sites, interstrand and intrastrand crosslinking, insertions and deletions [START_REF] Castillo | Galantamine protects against beta amyloid peptide-induced DNA damage in a model for Alzheimer's disease[END_REF] . This DNA damage ultimately results in halted or false gene transcription and subsequently malformed proteins.

Protein oxidation

Next to DNA oxidation, ROS can also directly oxidize proteins forming protein oxidation products [START_REF] Cecarini | Protein oxidation and cellular homeostasis: Emphasis on metabolism[END_REF] . There are many different ways that ROS can induce protein oxidation, but metal catalyzed oxidation and amino acid oxidation are the main ones. Because several proteins interact with metal ions for their functions, the presence of H 2 O 2 together with these metal ions, such as Fe 3+ and Cu 2+ ions, or Fe 2+ and Cu + , attack amino acid residues close to the metal binding site [START_REF] Stadtman | Free radical-mediated oxidation of free amino acids and amino acid residues in proteins[END_REF] inactivating the proteins.

Concerning amino acids oxidation, two different pathways, the diamide pathway and the αamidation pathway, can induce cleavage of peptide bonds inside proteins. For example OH• reacts with proteins to form alkyl radicals 93;94 . Alkyl radicals can crosslink with each other to form protein aggregates or react with O 2 to generate alkylperoxide radicals. After reaction with Fe 2+ or HO 2

•

, alkylperoxide can form alkoxyl radicals, which are capable of reacting with almost any organic substance [START_REF] Stadtman | Protein Oxidation[END_REF] and therefore can attack the adjacent amino acids as well. The cleavage of the peptide bond can also be obtained more specifically by the reaction of OH• with the glutamyl, prolyl and aspartyl residues of the protein chain.

Several other amino acids can be specifically modified via side chain reactions with ROS. Amino acids that include aromatic side chain groups and those containing sulfydryl groups are most susceptible to react with ROS. Examples of aromatic amino acids are phenylalanine, tryptophan, histidine, and tyrosine. Upon oxidation, these amino acids can form mono-and dihydroxy derivatives [START_REF] Cecarini | Protein oxidation and cellular homeostasis: Emphasis on metabolism[END_REF] . Hydroxyl derivative nitrotyrosine can interact with other tyrosine radicals within the same molecule or between other molecules and thereby inactivate the protein and form harmful protein aggregates. In addition, the oxidation of methionine and cysteine sulfhydryl residues form sulfoxide, sulfenic acids and disulfide bridges [START_REF] Stadtman | Protein Oxidation in Aging andAge-Related Diseases[END_REF] . However, oxidation of cysteine and methionine residues is also reversible via the reduction of cysteine by thioredoxin and glutaredoxin and methionine by the methionine sulfoxide reductase system.

However, the main product of protein oxidation, whatever the type of protein oxidation, is the carbonyl residue. Carbonyl groups can be formed via secondary reactions with 4hydroxynonenal (HNE) or with reducing sugars or their oxidation product [START_REF] Castro | 4-Hydroxynonenal (HNE) modified proteins in metabolic diseases[END_REF] . Once formed, carbonyl groups can react with α-amino groups of lysine residues. These reactions lead to the formation of intra-or inter-molecular crosslinks and the formation of protein aggregates. A main problem with protein aggregates is that they are unable to be degraded via normal protein degradation mechanisms, leading to further accumulation of oxidized proteins and cellular dysfunction.

Lipid peroxidation

In addition to DNA and proteins damage, lipids are a third cellular component that can be heavily damaged by ROS [START_REF] Ayala | Lipid Peroxidation: Production, Metabolism, and Signaling Mechanisms of Malondialdehyde and 4-Hydroxy-2-Nonenal[END_REF] . In general, lipids are damaged via lipid peroxidation, which involves the abstraction of a hydrogen from the carbon chain replacing it with an oxygen. This results in lipid peroxyl radicals and hydroperoxides. Glycolipids, phospholipids and cholesterol are the main targets of peroxidation. The process of lipid peroxidation includes three steps: initiation, propagation and termination. The first step involves a ROS-induced abstraction of allylic hydrogen, resulting in the carbon-centered lipid radical. Then, in the propagation phase, lipid radicals react with oxygen to form a lipid peroxy radical. The peroxy radical is able to induce the abstraction of hydrogen from another lipid molecule to form a new lipid radical. This creates a chain reaction of lipid radical formation and lipid peroxy radical generation. Another product formed during the propagation phase is lipid hydroperoxyde. During termination, antioxidants donate a hydrogen atom to the lipid peroxy radical to form nonradical products. Once lipid peroxidation is initiated, a propagation of chain reactions will take place until termination products are produced.

Lipid peroxidation can result in a variety of oxidation products. The main primary products are lipid hydroperoxides (LOOH). Many different aldehydes can also be formed as secondary oxidation products, including malondialdehyde (MDA), propanal, hexanal and HNE. MDA has been shown to be the most mutagenic type of lipid peroxidation product [START_REF] Niedernhofer | a Product of Lipid Peroxidation, Is Mutagenic in Human Cells[END_REF] , whereas HNE is the most toxic 100 . This high toxicity is the result of the rapid reactions between HNE and thiol and amino groups. Besides, reactive aldehydes can act both as signaling molecules and as cytotoxic products. HNE acts as a signaling molecule for cell proliferation and/or differentiation, cell survival, autophagy, senescence, apoptosis, and necrosis by regulating several transcription factors sensible to stress, such as NRF2, activating protein-1 (AP-1), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-!B), and peroxisomeproliferator-activated receptors (PPAR) 101 .

ROS vs antioxidants

In many diseases, including neuropathies, ROS play a central role. However, this does not necessarily mean that the production of ROS has increased. Instead, antioxidant defenses, especially antioxidant enzymes, can be impaired. Multiple antioxidants exist in cells and each one is regulated independently, which allows a large range of scavenging of ROS. The main antioxidant enzymes are GPx, catalase and SOD proteins.

MnSOD

There are three SOD proteins in cells: the cytoplasmic Cu/ZnSOD (SOD1), the mitochondrial MnSOD (SOD2), and the extracellular Cu/ZnSOD (SOD3). All three isoforms are scavengers of H 2 O 2 , but I will focus on SOD2 in particular, because SOD2 is the only isoform present in the mitochondrial matrix. SOD2 is a mitochondrial manganese (Mn) containing enzyme (MnSOD), which is composed of a 96 kDa homotetramer 102 . The active site of SOD2 contains Mn in order to catalyze the disproportionation of O 2

•-to oxygen and H 2 O 2 . In SOD1 and SOD3, Cu or Zn respectively fulfills this catalytic function. The Mn is produced in the cytoplasm and then transported into the mitochondria, where it can fulfill its function. SOD2 contains a structure with a N-terminal helical hairpin and a C-terminal alpha/beta domain.

Both the hairpin and C-terminal domains direct the ligand of SOD2 to its catalytic Mn site. These active sites are stabilized by two identical four-helix bundles that are symmetrically assembled from the N-terminal hairpin to form a tetrameric interface.

In some pathological conditions, SOD2 can insert iron instead of Mn. Recently it has been shown that FeSOD2 actually does have enzymatic activity, but not as the traditional antioxidant that SOD is known for (Fig. 7) 103 . Bound to Fe, SOD forms an alternative isoform with peroxidase activity and the switch from Mn to iron binding allows FeSOD to promote oxidative stress at the expense of H 2 O 2 .

This shows that the binding of either Mn or Fe at its catalytic site is important for the regulation of SOD2 activity. However, the main regulation of SOD activity works via the expression of SOD genes. Indeed a multitude of transcription factors can regulate the expression of SOD. These transcription factors include NF-κB, AP-1, AP-2 and Sp1, as well as CCAAT-Enhancer-Binding Proteins (C/EBP) 104 . NF-!B is a redox-sensitive transcriptional factor that is considered a regulator of genes that acts as an "immediate responder" to harmful cellular stimuli. All three SOD genes have been shown to be regulated by NF-κB. In contrast with SOD1, which expression is not strongly regulated, SOD2 transcription can rapidly be modulated in response to oxidative stress. The NF-κB site is located within the second intron of the SOD2 gene, which implies a positive effect of NF-κB binding to SOD2 transcription 105 . Nonetheless, p50, which is another member of the NF-κB family, also regulates SOD2 expression but negatively. Sp1 is a transcription factor that contains a zinc-finger domain that binds directly to the DNA. It stimulates gene transcription through one of its glutamine-rich domains. DNA domains that are rich in GC base pairs, such as the promotor regions of all 3 SOD genes, are targets for Sp1 binding. Multiple Sp1 binding sites are required to induce SOD2 expression 106 . The 5'flanking region of the SOD2 gene contains a unique DNA looping structure allowing synergistically activation of transcription. This DNA looping structure is formed by the interactions between local and distant Sp1, indicating its influence on SOD2 expression. The unique single-strand structure of the SOD2 promoter modulates the interaction of transcription factors, such as NF-κB and Sp1, with the SOD2 promoter. AP-2 is a family of transcription factors that can bind directly to the cis-element of its target gene. In addition, it can crosstalk with other transcription factors in order to change its target gene expression. AP-2 is an interesting transcription factor, since it can increase SOD1 transcription whereas it suppresses SOD2 by inhibiting Sp1-dependent transcription 107 . Methylation of the AP-2 site in the SOD2 promoter reduces AP-2 binding and subsequently SOD2 transcription is increased.

FOXO3a is a transcription factor that can bind to the 5'-flanking region of SOD2, and this way stimulates its expression. During ageing, Akt activity becomes increased, which is thought to be responsible for FOXO3a phosphorylation and hence its inactivation. This mechanism might be the cause of the age-related downregulation of MnSOD. Based on the overlapping binding sites with NF-κB, a dual NF-κB/FOXO regulation has been suggested 108 . Regulation of SOD2 expression and activity is essential in cellular function. For example, increased levels of SOD2 reduces the mitochondrial superoxide level, protects the mitochondrial morphology and functions, and provides resistance against oxidative cytotoxicity 109 . In contrast, complete ablation of SOD2 causes neonatal lethality in mice. These mice have inactivated ISCs in oxidative phosphorylation and TCA enzymes 110 , which results in dilated cardiomyopathy and neurodegeneration 102 , which soon cause the mice's death. In contrast, heterozygous mutant SOD2 animals have a partial oxidative phosphorylation defect involving a reduced respiratory control ratio. In these mice, the predisposition for opening of the PT pore is increased. The opening of the PT pore creates a channel through the inner and outer mitochondrial membranes that allows free diffusion of molecules of 1500 kDa or smaller. This results in the efflux of protons and the loss of the membrane potential. This leads in the swelling of the mitochondria and the initiation of apoptosis 110 . A dysregulation of SOD expression has been involved in several diseases and in particular in cancer. Oxidative stress is elevated when cancer is at an early stage and the amount of antioxidant enzymes is low. In this condition, the mutation of tumor suppression genes and activation of oncogenes is facilitated. Even though MnSOD initially has a tumor suppressor effect via mediation of ROS levels and the modulation of redox-related transcription factors, its role shifts during cancer progression 111 . Therefore, MnSOD expression is higher in aggressive cancer forms compared to benign cancers, when the disease is at a later stage. Next to alterations in gene transcription factors, polymorphisms in the SOD2 gene are the cause of affected SOD2 expression in cancer cells. Mutations in the SOD2 promoter region have been detected in various cancers, showing how this can be an underlying cause of the decreased MnSOD expression at the initial phase of the disease. These mutations create an extra DNA binding site for inhibitory transcription factor AP-2 and impair the interaction between DNA and proteins that are required for transcription initiation.

Glutathione Peroxidase

Human cells contain four major GPx isoforms 112 . The classical GPx (GPx1) is located in cytosol, nucleus and mitochondria of liver, lung and kidney cells, whereas GPx2 is located in the cytosol and nucleus (so not within mitochondria) of cells in the gastrointestinal tract. Plasma GPx (GPx3) is a secreted protein found in the cytosol in of cells located in many tissues including kidney, lung, epididymus, vas deferens, placenta, seminal vesicle, heart and muscle. Lastly, phospholipid GPx (GPx4) is present in the nucleus, cytosol and mitochondria and bound to membranes. GPx4 is broadly distributed in different tissues. GPx1, 2 and 3 act as homotetramers, whereas GPx4 is a monomer.

GPx catalyzes reactions that reduces H 2 O 2 to water, and hydroperoxide (ROOH) is reduced to and the less reactive ROH alcoholic counterpart 110 :

H 2 O 2 + 2GSH → 2H 2 O + GSSG ROOH + 2GSH → ROH + GSSG + H 2 O
Although H 2 O 2 is not the most reactive kind of ROS, it is involved in damaging Fenton reactions and is a direct precursor to the more reactive •OH. Therefore, mice lacking GP X were thought to exhibit impaired mitochondrial function in tissues that express high levels of mitochondrial GPx1. However, initial studies in GPx1 mutant mice did not show any effect of the mutation indicating that loss of GPx1 is not sufficient to cause toxic oxidative stress. However, when challenged by paraquat or H 2 O 2 , the mutant mice appeared to be more susceptible to oxidative damage. Likely there is a compensatory mechanism for the GPx1 deficiency in place as well. Logically those compensatory mechanisms involve other GPx isoforms, such as Gpx4, or catalase.

It has been shown that mitochondrial ROS production is correlated to cellular protein disulfide content. In addition, it has been shown that cells overexpressing GPx also contain less protein with disulfide bonds. Moreover, GPx overexpression results in a reduction of the mitochondrial membrane potential, indicating that GPx acts on the mitochondrial electron flux over the mitochondrial membranes 113 . These are two strong indicators that GPx overexpression can alter the ROS production by mitochondria, which has a functional effect on proteins in the cytoplasm. It also suggests that other oxidation reactions, such as oxidation of lipids, can be diminished by GPx overexpression. Interestingly, another effect of the overexpression of GPx is the decrease in ATP levels. Therefore, an excess of GPx is thought to cause a reduction in proton-motive force, which results not only in less disulfide bonds in proteins, but also a decreased cellular ATP content. Since GPx is mainly an antioxidant, this pathway is driven by a reduction in ROS.

Catalase

Catalase is an enzyme that is found in almost all aerobically respiring organisms. Catalase consists of four protein subunits, each containing a heme group and a molecule of NADPH, which prevents deactivation 114 . Catalase is present in many tissues, but its activity is highest in liver, erythrocytes, kidney and adipose tissue, whereas its activity is relatively low in heart and brain. Catalase is the first antioxidant enzyme that has been characterized to catalyze the two stages of converting H 2 O 2 to water and oxygen 115 :

catalase-Fe(III) + H 2 O 2 → compound I compound I + H 2 O 2 → catalase-Fe(III) + 2 H 2 O + O 2
The kenetics of catalase is particularly high and therefore it is almost impossible to saturate catalase in vivo. Catalase can prevent mitochondrial permeability transitions by the transition of H 2 O 2 to water and oxygen 116 . Catalase has also been shown to prevent a loss of mitochondrial depolarization, suggesting that the inner mitochondrial membrane permeabilisation is caused by mitochondrial H 2 O 2 .

Within cells, catalase is mainly located within peroxisomes and in the cytosol. In healthy neurons, this antioxidant can have major effects in cellular function, because it can efficiently regulate redox signaling by increasing or decreasing H 2 O 2 concentrations. For example, catalase has been shown to regulate the influx of Ca 2+ and Na + via transient receptor potential V (TRPV) channels 117 . Intracellular ROS are potent activators of these plasma membrane channels, meaning that ROS will open TRPV channels through which Ca 2+ and Na + will enter the neuron. In heart and liver cells, catalase has been detected in mitochondria as well, but in neuronal mitochondria only extremely low catalase concentrations are found, compared to its concentration in the cytosol 118 In addition, as catalase inhibitors did not have an effect on the scavenging activity of ROS in mitochondria, this indicates that catalase is functionally irrelevant for brain mitochondria 119 .

Changes in the catalase levels and polymorphisms in the gene coding for catalase are associated with many different diseases, such as hypertension, diabetes mellitus, insulin resistance, dyslipidaemia, asthma, bone metabolism or vitiligo 120 .

The role of ROS as a signaling molecule

When ROS are being produced in physiological levels, those molecules are used in signaling pathways. These pathways include a wide variety of biological mechanisms, including cell proliferation, differentiation, and maturation. Compared to other signaling mechanisms, redox signaling is much less specific, since it can react on many different targets. Whereas the traditional method of signaling works via a receptor-ligand binding process, ROS signaling works via the oxidation of redox-sensitive amino acid residues. These residues are often located at the active sites or catalytic domains of protein tyrosine phosphatases. Since serine/threonine phosphatases are susceptible to oxidative inactivation, ROS can modulate signal transduction by decreasing phosphatase activity.

The signaling role is particularly important for H 2 O 2 because it can diffuse over longer distances than the other types of ROS. For H 2 O 2 to act as a signaling molecule, it must first evade reduction by the antioxidants that are present in between the mitochondrion and the proteins that are sensitive to H 2 O 2 levels and then trigger a response of that peroxide-sensitive protein. Certain features of proteins make them susceptible to oxidation by H 2 O 2 . Because of the low pH of the cytosol, most cytosolic proteins are protonated and therefore protected from H 2 O 2 oxidation. The few cytosolic proteins that do contain a deprotonated cysteine residue are specific targets for H 2 O 2 and hence those proteins are suitable for being H 2 O 2 sensors. Indeed, ROS induces the reversible oxidation of thiol groups of key cysteine residues in many proteins, including transcriptional regulators, kinases, phosphatases, structural proteins, metabolic enzymes, and SUMO ligases. The cysteine oxidation in these proteins often leads to altered activities.

ROS changes protein activity and gene expression in cells

The response of human cells to increased levels of H 2 O 2 is an adaptation to the environment by stimulating the reduction of ROS 121;122 . This adaptation of the cell occurs via the control of single enzymatic activities or via the control of transcription (Fig. 8) 123 . Moreover, H 2 O 2 increases the transcription of antioxidant genes, which increases the concentration of oxidant reducing enzymes in the cell. Those antioxidants reduce the level of ROS and this negative feedback loop ensures the homeostatic balance of ROS within the cell. However, this negative feedback only occurs up to a threshold level of peroxide concentration. If the peroxide concentration rises above that threshold, despite the action of increased antioxidant production, different patterns of p53-regulated gene expression are initiated that stimulate the expression of pro-oxidants involved in apoptosis 124 . The negative feedback turns into a positive feedback that leads to apoptotic death of the cell.

Oxidation of the transcriptional activator OxyR is another remarkable example of H 2 O 2 dependent regulation of gene expression. Oxidation of OxyR does not influence DNA binding per se, since both oxidized and reduced OxyR are able to bind DNA. However, only the oxidized form of OxyR can activate transcription of antioxidant genes 125 . It has been shown that activation of OxyR by H 2 O 2 is dependent on the oxidation of cysteine 199, which results in a sulfenic acid derivative. The sulfenic acid derivative formation is sufficient to activate OxyR dependent transcription 126 , but the activation becomes more stable by the formation of a more stronger disulfide bond between cysteines 199 and 208. One of the target genes of OxyR is glutaredoxin1, which becomes increasingly expressed upon OxyR activation. Glutaredoxin1 can reduce OxyR and deactivate it, thereby creating a negative feedback loop 127 Transcription factors that contain Fe 2+ are also suitable candidates for being a H 2 O 2 sensor. H 2 O 2 reacts easily with Fe 2+ , which is functioning as a co-factor in several proteins. Indeed, the PerP transcriptional repressor reacts to H 2 O 2 in a mechanism that is thiol independent. Instead, PerR works via the metal-catalyzed oxidation of histidine residues. It contains a structural Zn 2+ binding site and a regulatory Fe 2+ binding site 128 . When PerR binds to Fe 2+ to its regulatory site, it connects to DNA and represses the expression of target genes. These target genes include antioxidant enzymes, such as catalase. H 2 O 2 activates genes that were previously repressed by PerR, because Fe 2+ coordinating histidine residues of PerR become oxidized. Therefore, PerR can no longer bind to Fe 2+ or to DNA 129 .The oxidation of the critical residues of PerP is on its turn catalyzed by Fe 2+ , showing that PerR activity is modulated by both H 2 O 2 and Fe. Whereas the oxidation of thiol groups is reversible, the oxidation of PerR histidine residues is not.

Next to regulation of ROS homeostasis, H 2 O 2 can also activate signaling pathways to stimulate cell proliferation, differentiation, migration, or apoptosis. In addition, it can regulate the conjugation of proteins to the SUMO ubiquitin-like modifier in a concentration-dependent manner. Active macrophages produce H 2 O 2 levels in a range that inhibits the conjugation of SUMO and hence reduces sumoylation. When H 2 O 2 levels become elevated, the deconjugation of SUMO is inhibited. This causes an increased sumoylation rate 130 . Sumoylation is an important regulator of localization, activity, and stability of many proteins. Therefore, H 2 O 2 can result in many biological responses via altered protein modification through sumoyliation.

The role of ROS in misfolded protein degeneration

The ubiquitin-proteasome system (UPS) is the main biological mechanism in the degradation of soluble proteins in cytosol and nucleus 131 . In the UPS, 26S proteasome is the major cellular proteolytic machinery. 26S proteasome degradation is not specific to nonfunctional proteins as it degrades folded and functional proteins as well in an ATP/Ub-dependent matter. Nonetheless, oxidative stress enhances the activity of 26S proteasome, in order to degrade nonfunctional proteins that are supposed to be present in abundance under conditions of high oxidative stress. The increased 26S proteasome activity should then protect the cells from these oxidatively damaged proteins. However, when ROS levels continue to be elevated, 26S proteasome disassembles and the UPS is deactivated. This gives rise to 20S proteasome, which is the core of 26S proteasome. Whereas 26S proteasome is ATP/Ub-dependent and relatively susceptible for oxidative stress itself, the 20S proteasome works in an ATP/Ubindependent way and it is more resistant against oxidation. Initially, the disassembly of 26S proteasome is still reversible when oxidative stress decreases, so the 26S proteasome can still reassemble. It has been observed that within several hours after reassemble of 26S proteasome, there is hyperactivation of the ubiquitinating system and proteolysis. If elevated oxidative stress is continued for a longer period of time, proteasomal activities are inhibited and new proteasomal components are being produced. This gives rise to newly-formed 20S and 26S. At one point, oxidative stress continues for too long or rises to a sublethal level, which impairs the proteolytic system and results in the accumulation of aggregates of damaged, abnormal proteins 132 . Besides its role in the degradation of damaged proteins, the proteasomal system also controls the degradation of proteins that act as redox switches, some of which are discussed earlier, such as the transcription factors hypoxia-induciblefactor-1 (HIF-1), NRF2 or NFκB. The UPS itself is subjected to fluctuations in oxidative stress, even when those levels do not exceed concentrations that trigger 26S proteasome disassembly. Ubiquitination enzymes are inactivated by disulfide bond formation, S-nitrosylation, and Sglutathionylation. The proteolytic activity of the proteasome requires β-subunits in the catalytic 20S core with intact Cys-residues. These Cys-residues are extremely sensitive for oxidation.

A second mechanism of protein degradation is autophagy. Autophagy is an intracellular process that degrades cytosolic components inside lysosomes. Mild oxidative stress, redoxmediated signaling or oxidative modifications of macromolecules stimulate autophagy and hence prevent the aggregation of non-functional and damaging proteins and affected organelles. An oxidative stress-induced form of microautophagy has also been described, that involves mitochondria-derived vesicles (MDVs). These MDVs are transported to the lysosomes and contain mainly oxidized proteins. Oxidative stress induces the generation of MDVs 133 . Basal ROS levels contribute to healthy rates of autophagy, which is necessary to maintain protein turnover and proteostasis. This way, protein degradation via autophagy is actually a pro-survival mechanism. When oxidative stress increases, autophagy can either act as pro-survival function or contribute to cell death and the equilibrium between the two is tightly regulated. However, when oxidative stress exceeds a certain threshold, lysosomes release ROS and proteases into the cytosol and crucial cellular components are being degraded, eventually resulting in autophagy cell death 134 .

ROS as a signaling molecule in the immune system

Substantial evidence has revealed that ROS are essential second messengers in innate and adaptive immune cells 135 . In the innate immune response, the toll-like receptor (TLR) ligand lipopolysaccharide (LPS) stimulates the production of ROS by NADPH oxidase and mitochondria. This increase in ROS levels activates inflammatory cytokines, is essential for immune response pathways induced by TLRs and optimizes the activity of macrophages. Furthermore, RIG-I-like receptors (RLRs) also signal through mitochondrial ROS and NLRP3, a component of the inflammasome, is activated by mitochondrial ROS. In adaptive immunity, ROS is an important activator of T-cells. ROS stimulates proliferation of T-cells and its production of the cytokine interleukin-2 (IL-2). As a response to initial T-cell activation by mitochondrial ROS, NADPH oxidase becomes involved to sustain T-cell activation. In addition to T-cell activation, ROS are also involved in B-cell activation. As discussed shortly in the role of H 2 O 2 promoting either gene expression of antioxidants or promoting apoptosis, the amount of ROS is important in determining the effect of H 2 O 2 as a signaling molecule in immune responses. A low amount of ROS production benefits immune responses by activation of T-cells and B-cells, and the immune response is more efficient without causing tissue damage 136 . However, when ROS levels rise higher, this can exacerbate the immune response via elevated levels of pro-inflammatory cytokines 137 . For example, NRG-deficient mice have higher levels of ROS and their immune response to pathogens is worse than wildtype mice, resulting in worsened pneumonia and sepsis. Thus, slight elevations in ROS levels may enhance immune system function, whereas high levels of ROS could promote a pathological response.

ROS as a signaling molecule in neurogenesis

Redox signaling plays an important role in the differentiation of various cell lineages from their respective precursors 138 . In addition, ROS levels play a role in clonal expansion of stem cells in the proliferative niches in the brain, such as the hippocampus and the subventricular zone (SVZ). Because ROS regulate the proliferation and differentiation of neuronal stem cells (NSCs), tight regulation of ROS production is required to maintain stemness properties of the NSCs. In contrast, increased levels of ROS in neuronal progenetor cells (NPCs) in p53 -/-mice induce a shift in the NPC differentiation balance towards the neuronal lineage 139 .

ROS as a signaling molecule for neuronal calcium

Ca 2+ is a central player in intracellular mechanisms and reactions and hundreds of proteins are involved in Ca 2+ signaling 56;140 . Therefore, the intracellular concentration of Ca 2+ is highly regulated. In human cells, there is close interaction between the levels of ROS, ATP and calcium and mitochondria are important regulating organelles that maintain the balance in this triad (Fig. 8) 56 . Stimuli that cause changes in cytoplasmic Ca 2+ , ATP or ROS trigger feedback loops that should restore the balance in time or reach a new equilibrium. If a new balance cannot be obtained, this will lead to a decline of ATP levels, an increase of ROS production and increased intracellular Ca 2+ concentration. ROS can act as a signaling molecule to regulate the intracellular Ca 2+ concentration by acting on a variety of channels through which calcium can enter the cytosol. Important Ca 2+ channels in neurons are the voltage-dependent Ca 2+ channels (VDCCs). There are several subtypes of VDCCs and the reaction to ROS differs between subtypes. H 2 O 2 accelerates the opening of P/Q-type of Ca 2+ channels, so H 2 O 2 stimulates the entry of Ca 2+ into the cell. In contrast, ROS decreases the influx of Calcium through L-type Ca 2+ channels in muscle cells, although no significant effect of H 2 O 2 on L-type Ca 2+ channels is reported in pancreatic β-cells.

Intracellular Ca 2+ release from the ER or sarcoplasmic reticulum (SR) is mediated by ryanodine receptors (RyR) and IP3R. RyR and IP3R channels contain many cysteine residues. As mentioned earlier, deprotonated cysteine residues are especially susceptible to oxidation by ROS. Changes in the redox state of RyR and IP3R would affect their activities. ROS open RyR channels and this way enhances Ca 2+ release from the ER. In neurons, activation of RyR subtype 3 by ROS might modify Ca 2+ -dependent long-term potentiation and long-term depression. O 2

•-enhances IP3-induced Ca 2+ release through IP3Rs from fractionated vascular smooth muscle SR, suggesting that, like RyR, ROS increase IP3R channel opening. Also ATP-dependent calcium channels, including the plasma membrane Ca 2+ -ATPases (PMCA) and SERCA, as well as Na + /Ca 2+ exchangers NCX, are sensitive to ROS regulation. ROS effectively inhibits Ca 2+ transport by SERCA in smooth muscle cells. H 2 O 2 and O 2

•-can completely uncouple the hydrolytic reaction of PMCA and inhibit the hydrolytic reaction of SERCA 141 . The effect of ROS on NCX is not fully understood yet, because several contradictory results have been reported. An interesting feature of calcium signaling is that it can be administered intracellularly in Ca 2+ sparks. Ca 2+ sparks play an important role in local control of Ca 2+ signaling in many types of cells. ROS also has a regulatory effect on the frequency that these Ca 2+ sparks are occurring 142 . Like the general influx and efflux of calcium that has been described before, the effect of ROS on Ca 2+ sparks vary depending on cell type.

ROS and wound healing and axonal regeneration

Although ROS are central players in neurodegeneration through pathways involving Bax, caspase, PARP and MAPKs, ROS also play an important role in tissue repair, including axonal regeneration. However, the production of this damage induced H 2 O 2 production seems not to occur in mitochondria, but by the H 2 O 2 producing enzyme Duox1, which is a type of NADPH oxidase and is present in epidermal cells. ROS signaling in regenerative tissue induces two parallel pathways. H 2 O 2 triggers apoptosis, while activating the JNK pathway 143 . Both events are necessary for proliferation of epidermal cells around the wound and subsequently required for tissue repair. In addition, the reaction of tissue damage involves the recruitment of immune cells into the wound in order for tissue repair to occur. The H 2 O 2 that is secreted by damaged tissue results in a gradient of H 2 O 2 , with a high concentration at the wound site and lower concentrations further away from the wound 144 . Because of the fast diffusion of H 2 O 2 , the damaged area can quickly be located by immune cells. Those immune cells follow the gradient of H 2 O 2 and migrate into the wound site. ROS is involved in the regeneration of nerve tissue in the same way that H 2 O 2 is triggering wound healing by signaling to epidermal cells and inducing repair pathways. It is shown that blocking H 2 O 2 production by Duox1 prevents axonal regeneration after tissue damage 145 . Interestingly, a gradient of H 2 O 2 that recruits leukocytes after tissue damage seems not to be required for promotion of axon regeneration 146 .

The fact that mitochondrial ROS production seems not to play a central role in tissue regeneration nor axonal regeneration does not mean that mitochondria are not involved in regenerative mechanisms altogether 147 . The energy consumption during axonal regeneration is high, because many energy dependent processes, including the resealing of the cut membrane, formation of a new growth cone and mobility, are happening. The nerve also requires energy to activate a growth program that involves the synthesis of new proteins, transport, and assembly to reorganize the new cytoskeleton of the regenerated axon. To accommodate the rise in energy demand after nerve injury, mitochondria are transported to the regeneration area of the axon. Both anterograde and retrograde transport of mitochondria in regenerating axons is increased within hours and is sustained for more than 21 days 148 .

Conclusion

Several forms of reactive oxygen species are generated in cells and different forms have different features. For example, superoxide anions are highly reactive and therefore are acting extremely fast with molecules in its direct environments. On the other hand, hydrogen peroxide is a type of ROS that is less reactive, but can diffuse over longer distances and can oxidize distal targets. ROS are often considered as a negative by-product of ATP synthesis by the electron transport chain in mitochondria. Indeed, mitochondria produce superoxide anions, which give rise to the other types of ROS, but mitochondria are not the only source of ROS. In general, ROS can be considered damaging for cells, because they oxidize DNA, proteins and lipids and these oxidation products are often non-functional or unstable. Fortunately, the cells are equipped with a competent antioxidant system that consists of a group of antioxidant enzymes, which reduce ROS to water. However, defining ROS as a by-product of ATP synthesis is too reductive. Low amounts of ROS can have physiological signaling functions that can regulate gene expression.

Chapter 3) Mitochondria in the peripheral nervous system

As described in the prologue, our lab focuses on the biology of peripheral nerves. Several peripheral neuropathies indicate that mitochondria, both in the Schwann cells and in the peripheral axons of the nerves are critical for the maintenance of the axon-glia physiology. Compared to the CNS, PNS axons can be much longer and large axons are myelinated by a single Schwann cell. In the CNS, oligodendrocytes myelinate several axons and astrocytes, which are not myelinating, also participate to the metabolic maintenance of the axon. So if the overall principle of the metabolic maintenance of the axon-glia unit may be the same, some particularities may be specific for the PNS Schwann cell-axon unit.

One of the roles of Schwann cells is to form the myelin sheath that covers large axons that are critical for peripheral nerves function. The formation of the myelin sheath induces a spatial heterogeneity along the myelinated axons as specialized regions such as nodes of Ranvier form between two successive myelin segments. While one important functional particularity of the axon is to allow the transit of mitochondria from the cell body to the synapse, the presence of several domains along myelinated axons appears to change the dynamic of mitochondria movements. So in this chapter, I will discuss the development, features and functioning of the PNS. I will describe the role of Schwann cells and their roles in the nervous system. One of its roles is to form the myelin sheath that covers large axons that are critical for peripheral nerve function. The formation of the myelin sheath induces a spatial heterogeneity along the myelinated axons as specialized region such as node of Ranvier form between two successive myelin segments. While one important functional particularity of the axon is to allow the transit of mitochondria from the cell body to the synapse, the presence of several domains along myelinated axons appears to change the dynamic of mitochondrial movements. Finally, I will discuss how mitochondria can migrate towards different areas within the neuron and how nodes of Ranvier change the physiology and the movement of these axonal mitochondria.

Peripheral nervous system development

The PNS consists of two main cell types: neurons and myelinating Schwann Cells (mSC). Those two cell types have close functional interactions. Nevertheless, the development of these two cell types is not the same.

The development of peripheral nerves starts with migration and differentiation of neural crest cells towards neurons. These developed neurons then form axons that have to find their way to their peripheral targets 149 . The terminal point of the developing axon, the growth cone, leads the axon along its route. Initially, the first developing axons grow in an axon-free environment and form a path for later developing axons. However, because the axonal targets may be far away from the soma, later developing axons can use these paths to grow more efficiently by creating intermediate targets on specialized cells called guidepost cells and this way form axon networks. The individual segments that connect to intermediate targets are not longer than a few hundred micrometers. Guided and influenced by a contact attraction factors (secreted semaphorins and netrins), chemoattractants (netrins), contact repulsion factors (Eph ligands, transmembrane samaphorins and extracellular matrix proteins), and chemorepulsion molecules (Ig CAMs, cadherins and extracellular matrix proteins), the growth cone eventually finds the correct target of its respective axon 150 . All these mechanisms act simultaneously and in a coordinated manner for a proper pathfinding 151 . The definitive peripheral nerve contains axons with a diameter of 0.1-1.5 mm within large bundles surrounded by two cellular layers. The outer layer is made of migrating and proliferating SCs. SC development starts, like neurons of the PNS, with migration and differentiation of neural crest cells 152 . Expression of Sox10, Neuregulin-1 (NRG1) 153 , Pax3 154 , fatty acid binding protein 7 (Fabp7) and P0 155 in neural crest cells drives the development towards SC lineage by stimulating differentiation of neural crest cells to Schwann Cell Progenitor (SCP) cells (Fig. 10) 156 . SCPs proliferate and migrate together with the developing axon 157 and during this process, the axon stimulates SCP survival. Once SCP migration stops, these cells differentiate into immature SCs, which is regulated by the expression of NRG1, Notch and ErbB2/3 158 . Immature SCs determine which axons become myelinated and which axons become non-myelinated Remak bundles via a process called radial sorting. In the last phase of SC development, immature SCs either differentiate into myelinating SCs or non-myelinating Remak SCs. These Remak SCs are associated with C fiber nociceptors, post-ganglionic sympathetic and parasympathetic fibers, and motor nerve terminals at the neuromuscular synapses. Naturally, these classes of neurons have axons that are not myelinated. The differentiation of immature SCs into myelinating SCs requires signaling of a large variation of signaling factors, such as Krox20, NRG1, and BDNF 156 .

Figure 10 Schwann cell development. Neural crest cells differentiate into Schwann cell precursor cells and then into immature Schwann cells. These Immature Schwann cells either differentiate into pro-myelin Schwann cells and further into myelinating Schwann cells or into non-myelinating Schwann cells (Remak cells). After nerve damage, both myelinating and non-myelinating Schwann cells can dedifferentiate into Immature Schwann cells.

Neurons have a signaling role for survival of the developing SC precursors, but SCs have a role in neuronal development as well. This is exceptionally apparent in the development of the neuromuscular junction (NMJ). During development, one NMJ is innervated by multiple axonal nerve endings coming from the same neuron. During early post-natal development, most of these axonal nerve endings retract and the remaining axonal endpoint gets strengthened 159 . SCs play a central role in deciding which axonal endpoint gets supported. The perisynaptic SCs can detect the activity of the associated nerve ending by detecting intracellular Ca 2+ levels in the perisynaptic SC. Additionally, these perisynaptic SCs can detect the release of neurotransmitters by the nerve ending as well 160;161 . Increased axonal activity leads to increased neurotransmitter release and increased intracellular Ca 2+ levels. These factors are determining factors of the perisynaptic SCs for which nerve ending to support during competition between the nerve endings during post-natal development 162 .

Myelination

The myelin sheath is a greatly extended plasma membrane of myelinating cells that wrap several times around axons (Fig. 11) 163 . It consists of 70% lipids in which proteins are embedded, with myelin basic protein (MBP) and proteolipid protein 20 (PLP20) being the most predominant. The myelin sheath is shaped as multiple layers (lamellae) of modified and compacted plasma membrane that are regularly breached by channels filled by cytoplasm (Schmidt Lantermann incisures). Cytoplasm containing membrane loops (paranodal loops) form the lateral extremities of the myelin sheath 164 . Longitudinal and transverse channels filled with cytoplasm (Cajal bands) are located at the outermost layer, where the myelin sheath adheres to the plasma membrane of the Schwann cell 165 .

Once an initial contact between mSC and axon is made, the myelin membrane of the mSC advances in a spiraling motion 166 . The membrane extends along the entire axonal segment and then moves underneath the growing sheath to form the future internode 167 . The inner tongue continues to advance underneath the earlier generated myelin layers, resulting in an increasing number of myelin layers (Fig. 11) 154 . Because the inner tongue generates the newest myelin membrane that leads to the radial growth of myelin, the newly synthesized membrane components need to be transported from the mSC soma through the developing myelin sheath to its innermost spiral. Therefore, the Schmidt-Lantermann incisures are made within the compact myelin, which create a helical path for transport 168;169 . For the lateral growth of the myelin layers, membrane components also have to be transported to the lateral sides of each myelin layer, the paranodal loops. At one point, many of the radial channels close, resulting in elevated transport of membrane components to the paranodal loops and hence, the internodal length increases. The final stage of myelin formation is the compaction of the myelin layers. This is achieved by the insertion of MBP into the membrane, which starts already when the myelin sheath is just a few layers thick. Although radial growth occurs in the innermost layer, compaction starts at the outermost layers.

The role of Schwann cells

One role of myelin sheath formation by Schwann cells is to ensure efficient conduction of action potentials via electrical insulation. In between two Schwann cells are small areas where the axon is not myelinated called the nodes of Ranvier, which enable the fast propagation of action potentials from node to node (salutatory conduction). Another role of Schwann cells is to transport molecules to the axon to provide (metabolic) support.

The node of Ranvier

Myelinated axons are divided into distinct areas with different physiology, protein setup and sometimes different function within the PNS. These areas include the paranodal axoglial junctions (PNJ), juxtaparanodes (JXP) and internodes (Fig. 12) 170;171 . At the nodes of Ranvier, microvilli emerge from the edges of the mSCs that myelinate the flanking internodes 172 . The nodes of Ranvier contain several sodium transporters that allow the generation of action potentials. Na + /K + ATPases, Na + /Ca 2+ exchangers and voltage-gated Na + channels are all clustered within the node of Ranvier 173 . The α-subunit of Na + channels form a pore and βsubunits anchor this pore to both extracellular and intracellular components. In addition to Na + channels, nodes of Ranvier are enriched in K + channels 174 and the cytoskeletal and scaffolding proteins Ankyrin G (AnkG) and βIV spectrin. AnkG and spectrin connect the Na + and K + channels to the cytoskeleton of the node of Ranvier 175 . Additionally, the role of these scaffold proteins is to link Na + and K + to the extrinsic interactions, including interactions with the mSCs and the extracellular matrix (Fig. 12).

The formation of the node of Ranvier involves both axonal stimuli and stimuli coming from the mSC. Cytoskeletal proteins and ion channels are recruited to the place where mSCs connect to the axon. AnkG is an important neuronal regulator in the formation of the node, because AnkG is required for Na + channel clustering at the axon initial segment (AIS) that structurally and functionally resemble nodes of Ranvier. Nodal Na + channels, K + channels and bIV spectrin all contain Ankyrin-binding motifs, indicating that AnkG plays a central role in the accumulation of all these nodal factors to the same location, which is the developing node of Ranvier. When the AnkG-binding motif of Na + channels becomes phosphorylated by CK2, a protein kinase that is enriched in the node, the affinity for AnkG becomes much higher 176 . In accordance, mutations in βIV spectrin cause disrupted nodes, leading to ataxia and premature death 177 . When Na + channels initially cluster at the nodes, this is followed by the formation of the paranodal junction and then the clustering of K + channels at the juxtaparanodal region 178 . The initial Na + clustering occurs at sites adjacent to the SC extremities 179 . When the internodal regions become larger, these extremities displace the clusters of Na + channels until two clusters meet and fuse, forming 1 cluster where a new node of Ranvier is formed.

Propagation of action potentials

The focus on the density of sodium and potassium channels in the node of Ranvier shows the importance of those channels in the function of the node of Ranvier. These ion channels are major players that allow fast propagation of action potentials from node to node. In the resting state, sodium channels are closed, whereas potassium channels are open, which creates a high concentration of potassium within the neuron, but a high concentration of sodium in the extracellular matrix. Overall, the cytosolic side of the membrane contains a negative potential. Regeneration of action potentials at the node of Ranvier follows the same four steps as initiation of the action potential at the neuronal soma. Initially, opening of voltage gated Na + channels causes depolarization of the axonal membrane. Voltage-gated sodium channels are responsible for the rising phase of the action potential in membranes of neurons. Depolarization of the membrane induces a conformational change that opens the pore 180 and Na + enters the axon, raising the membrane potential by the increased concentration of positive ions in the cytosol of the axon. Meanwhile, the potassium channels remain closed. When the depolarization reaches a certain threshold, the other sodium channels open and a fast influx of sodium increases the membrane to a point that the membrane potential even becomes positive. Then, the sodium channels close while the potassium channels reopen, allowing the fast efflux of potassium towards the extracellular space. During this phase, the membrane potential decreases and at a certain point falls even lower than the resting state potential. The membrane potential recovers from this hyperpolarization to the equilibrium of the resting state potential by the sodium potassium ATPase that transfers sodium and potassium over the cell membrane against its concentration gradient. Voltage-gated sodium channels are located within the node of Ranvier whereas voltage-gated potassium channels are located at the juxtaparanode (Fig. 11). Sodium ions are transported out of the neuron by Na + /K + ATPases. This ion-pump is located along the plasmalemma of PNS neurons and at the nodal, but not the paranodal or internodal, axolemma 173 . In the first step of the Na + /K + ATPase reaction, binding of ATP accelerates the conformational transition that releases of K + in the cytosol. Upon potassium release, the ATPase becomes phosphorylated, which requires Mg + or ATP-Fe 2+ as a co-factor and the binding of 3 Na + ions 181 . After phosphorylation, a reversion of the conformational transition occurs and the sodium ions are extruded from the cytosol.

Bioenergetics of the axon

Myelination enables fast propagation of action potentials along the axon, through the formation of the node of Ranvier and the electrical isolation that it provides along the internodes, forcing action potentials to jump from one node to the next. This is thought to save energy, because it limits the regeneration of action potentials, which is a highly energy demanding process 182;183 , to the nodes of Ranvier. However, myelin synthesis and maintenance also costs energy 184 . Nonetheless, the fast signal transduction that myelination of axons enables, has big advantages in neuronal function. Tight wrapping of myelin around the axon is limiting entry of energy-rich metabolites, such as glucose, into the axon. Only at nodes of Ranvier, where no myelin is covering the axonal membrane, direct entry of metabolites is possible 185 . However, there are other transport systems occurring that allows the presence of sufficient metabolites inside the cytosol of the axon. This additional metabolic support is required, because next to basic survival mechanisms of the cell, neurons also require a lot of metabolites to produce enough ATP for depolarization and repolarization, which is constantly happening in firing neurons. Even though energy demand, and thus the necessity of the presence of metabolites, is higher in the node of Ranvier than in internodes, it is debated that Na + /K + ATPases are present along the internode as well 186 , indicating that metabolites need to be present in internodes too.

Besides action potential propagation, other critical ATP-dependent processes exist in the axon. Indeed, the intracellular transport of organelles, proteins, lipids and RNA along the axon is dependent on ATP, because axonal transport along microtubule tracks is mediated by two classes of ATP-dependent motor proteins (i.e. kinesins and dynein) 187 . Some authors suggest that this axonal transport energy cost can reach even 25% of the total energy consumption of the neuron 188 . The transport of mitochondria by these motor proteins to highenergy demand regions such as nodes of Ranvier is discussed in paragraph 3.5 and 3.6.

The abundance of mitochondria at the synapse indicates that these organelles also have functional roles in synaptic transmission as well. When the action potential reaches the axonal terminal, the depolarization of the cell membrane triggers opening of the voltage gated calcium channels. The influx of calcium at the synapse induces the fusion of synaptic vesicles filled with neurotransmitters with the cell membrane and the neurotransmitters are released in the synaptic cleft. The buffering of the calcium that entered the cell is one of the functions of mitochondria at the synapse. However, the production of ATP in this region is another important task, because the steps of synaptic transmission require energy 189 . The remobilization of synaptic vesicles, which are immobile before depolarization, and exocytosis are both ATP-dependent processes 189;190 . Then, ATP is also hydrolyzed for endocytosis, which is the production of new vesicles from the cell membrane, and for filling these new vesicles with neurotransmitter 189;190 . In addition, the membrane at the synaptic terminal contains Na + /K + ATPase proteins, which use ATP like in the other parts of the axon. As synaptic terminals mitochondria can only come from the axon, this suggests that axonal transport of mitochondria is critical for the maintenance of the presynaptic terminals and therefore the maintenance of the axon itself.

Role of the Schwann cell and of the myelin sheath in the metabolic support of axons

Although glucose is the main energy metabolite for neurons, an alternative energy source exists in the form of lactate. Systemic administration of lactate maintains functional activity of neurons for several hours. In the CNS, the lactate that is required when the neuron is deprived of glucose was initially thought to be delivered only by astrocytes because they are also in contact with microvessels through their podocytes 191 . This transfer of lactate from astrocytes to neurons was called the lactate shuttle 192 (Fig. 13). When glutamatergic neurotransmission rates are changed, this lactate shuttle ensures that normal glucose utilization and energy consumption can continue. Glial cells have a higher glycolytic rate than neurons and some hypotheses even suggest that neurons depend completely on external lactate transfer and merely use glycolysis to maintain their antioxidant status and viability (via the production of NADPH for the reduction of glutathione). Since a myelin sheath limits free access of metabolites into the neurons, it has been suggested that next to astrocytes, also oligodendrocytes can be the source of lactate and play a role in the lactate shuttle. The most likely route for metabolites to be transferred from oligodendrocyte are small, extended spaces of non-compacted myelin that provides a connection between the glial cell's soma and the periaxonal space underneath the myelin. Neuronal or synaptic activity can influence the rate of the lactate shuttle and therefore is an important factor of mitochondrial function 194 . Synaptic activity induces these mechanisms via activating changes in gene expression, which results in long-lasting adaptation to this stimulus. Examples of such adaptive responses are activity-dependent dendritic remodeling and improving neuronal protection. The upregulated genes as a response of synaptic activity are called activity-regulated inhibitors of death (AID) genes. The exact working mechanisms on how these AID genes improve neuroprotection are not well understood. However, it seems that the functional changes induced by AID gene expression work via protection of mitochondria.

Schwann cell demyelination and remyelination

The state of the myelin relies on a balance between positive and negative regulators that modulate mSCs between myelination and dedifferentiation/demyelination. Whereas positive regulators stimulate transitions from immature SC toward mature mSC, negative regulators promote the reverse transition towards immature SC at the expense of myelination. Examples of positive regulators that support myelination are Krox-20, Sox10, NF-κB and cyclic AMP (cAMP). Promyelin transcription factors in mSCs, such as Krox-20/Egr2 are activated by axonal signals. On the other side of the myelination balance are the negative regulators c-Jun, Notch, Sox2 and NT3. For example, an upregulation of c-Jun is observed in nuclei of SCs in demyelinating diseases 195 . The promyelination factors and negative regulators can also interact with each other to form a pathway or feedback loops. At physiological levels, the anti-myelination factor c-Jun inhibits myelination via the inhibition of pro-myelination regulators Krox-20 or cAMP 196 . In contrast, the transcription factor CHOP inhibits myelination by its interaction with c-Jun. When the nerve becomes damaged, this balance between positive and negative regulators is disturbed. mSCs and non-myelinating Remak cells that surround axons respond to nerve damage and axonal degeneration by rapidly activating c-Jun 197 , which induces the transdifferentiation of myelinating SCs and Remak cells into repair SCs distal to the nerve injury. Importantly, other factors such as Sox2 and Id2 are also upregulated after nerve damage, which suggest a role for these genes in the regeneration process as well and possibly in transdifferentiation of SCs. These repair SCs form regeneration tracks, also called bands of Bungner. The bands of Bungner guide regenerating axons along the nerve in order to reconnect the neuronal soma with its corresponding synapses. In addition to its function as a guide, repair SCs break down inhibitors of axonal regeneration. This works via myelin autophagy and by attracting macrophages to the injury site (Fig. 14) 198 . Moreover, repair SCs support survival of the regenerating neurons by secreting trophic factors. c-Jun is the driving force behind the transition into repair SCs by promoting expression of genes that fulfill all these support mechanisms. Inactivation of c-Jun in SCs results in a strong impairment of nerve regeneration after injury. Importantly, the shift towards a repair SC phenotype is reversible, since these Bungner cells associate with the axons they helped regenerating and then transform back to either a myelinating SCs or a Remak cell 199 .

Difference of mitochondria in internodes and the node of Ranvier

As mentioned before, mitochondria are able to adapt their shape and function in response to their environment. Indeed, a couple of significant differences between SC mitochondria and axonal mitochondria have been observed. It has been reported that condensed mitochondria (indicating small matrix volume and high respiratory activity) are abundant in the axoplasm, but not in SCs 200 . Nonetheless, also the location within the same cell changes mitochondrial morphology. Since the nodes have particular energy needs, the roles of mitochondria that are located in or near the node of Ranvier are of interest. Even though majority of mitochondria (>90%) is located in internodes, internodes also contain the largest volume of the axon. Therefore, the density of mitochondria is a more interesting factor in investigating local energy homeostasis and energy homeostasis in nodes of Ranvier in particular. Although the density of mitochondria is already high throughout the axons in PNS neurons, the mitochondrial density is shown to be even higher at the paranode-node-paranode (PNP) region. Mitochondria residing in the PNP are also smaller than internodal mitochondria. Furthermore, these mitochondria contained fewer cristae compared to mitochondria residing in internodes. The higher density in the nodal axoplasm has not been observed in the CNS. In contrast, mitochondria are absent in 39% of nodes of Ranvier in the cerebellum and even 66% of nodes in the optic nerve 201 . Axonal firing appeared to decrease mitochondrial motility of mitochondria within PNP regions, whereas the motility of internodal mitochondria is not influenced 201 . As a result, the density of mitochondria in the nodal axoplasm increases in response to axonal firing.

The high density of mitochondria at PNPs suggests a need for ATP production at these regions. This would suggest that mitochondria produce more ROS in nodes of Ranvier as well. Another indication for increased ROS production by mitochondria located in the node of Ranvier is the observation that nodes of Ranvier are the initial sites of mitochondrial damage induced by oxidative stress 202 . In nodes of Ranvier, oxidative stress initially induces depolarized mitochondria, resulting in changes in the morphology of mitochondrial membranes and disruption of cristae. Mitochondrial transport also becomes impaired as a result of these morphological disruptions. This shows that nodes of Ranvier are sensitive to oxidative stress, which is possibly the result of an already elevated ROS concentration due to a higher mitochondrial density. The mitochondrial changes induced by oxidative stress expanded from the nodes of Ranvier bidirectionally into internodes, showing that mitochondria are more sensitive to ROS the closer they are located to nodal areas.

Anterograde and retrograde mitochondrial movement

Mitochondria are dynamic organelles and their shape can be changed mainly via fusion and fission. Nonetheless, another important feature of neuronal mitochondria is their transport along the axon, using the cytoskeleton as guidance. Estimations on the percentage of motile mitochondria ranges between 10% and 30% and this transport is not continuous for mitochondria can frequently change direction, pause or stop completely 203 . These mobility patterns are possible because of the ability of mitochondria to connect to anterograde or retrograde motor proteins and to a docking and anchoring machinery. This connection between mitochondria and motor proteins usually involves receptors located at mitochondrial membranes (Fig. 15) 204 . Kinesin superfamily protein (KIF) is a family of proteins guiding mitochondrial transport by converting ATP energy into unidirectional motion along microtubules 205 . For example, KIF5 proteins play a key role in anterograde transport of neuronal mitochondria. Each KIF5 heavy chain contains an amino-terminal motor domain, whereas carboxy-terminal domain connects to kinesin light chains or directly to mitochondria or mitochondrial adaptor proteins. Mutations in the KIF5b gene result in clustering of mitochondria in the soma, due to impaired transport along the axon towards synapses. In addition to KIF5, two members of the kinesin-3 family, KIF1B-α and KLP6, modulate mitochondrial transport, since it has been shown that mutant forms of KIF1B-α and KLP6 decrease their migration velocity and the density of mitochondria within the axon 206 . The protein Milton is involved in mitochondrial transport in neurons by association with KIF5. Milton not only connects to KIF5, but also with mitochondria via the interaction with mitochondrial rho (Miro), which is a Rho family GTPase located in the mitochondrial outer membrane. Miro contains two Ca 2+ -binding sites and two GTPase domains and connects mitochondria to Milton and KIF5. As a result, the Miro-Milton complex connects mitochondria to its motor protein and thus this complex is required for anterograde transport of mitochondria. Mutations in genes coding for Milton or Miro result in impaired mitochondrial transport and depletion of mitochondria in synapses. Two Milton orthologues (TRAK1 and TRAK2) have been identified in mammals that interact with KIF5 207 . Interestingly, these Milton proteins not only transport mitochondria, but are also involved in transporting other organelles through neurons. Another protein that can connect mitochondria to KIF5 is called syntabulin, which binds directly to the cargo-binding site of KIF5 and attaches to the outer mitochondrial membrane via its C-terminal domain 208 . Fasciculation and elongation protein-ζ1 (FEZ1) is a protein responsible for axonal outgrowth.

FEZ1 is yet another KIF5 adaptor protein that is involved in axonal transport of mitochondria. Finally, RAN-binding protein 2 (RANBP2) can bind mitochondria to KIF5 to undergo anterograde transport. Inhibition of RANBP2 and KIF5B or KIF5C has been shown to result in mitochondrial clustering near the nucleus, although the role of RANBP2 in neurons has not been established. Opposed to anterograde transport, mitochondria can also move in a retrograde direction. Instead of kinesin proteins that regulate anterograde mitochondrial transport, it is cytoplasmic dynein that is the driving protein in retrograde transport. Cytoplasmic dynein consists of two dynein heavy chains and several dynein intermediate chains, dynein light intermediate chains and dynein light chains. Dynein mutations located in heavy chains impair retrograde mitochondrial movement regarding migration velocity and run length 209 . The interaction between mitochondria and dynein is much less described than mitochondria's interaction with kinesin. Supposedly, the dynein light chain protein TCTEX1 binds to VDAC1, which is located in mitochondrial outer membrane. Dynactin binds to both cytoplasmic dynein and to microtubules via its p150 component, which can regulate the interaction between dynein with its cargo. It has been shown that dynactin can bind to mitochondria as well in vitro. However, in vivo analysis showed that dynactin only has a coordinating function in bidirectional movement of organelles and is not required to link those organelles to dynein. Snaptin is another motor adaptor protein that links endocytic organelles to dynein in neurons. However, an involvement of snaptin in mitochondria-dynein connection has not been established, so a clear demonstration of a stable connection between this organelle with dynein is still lacking.

Nonetheless, dynein-mediated retrograde transport of mitochondria seems to require the action of Miro as well. Next to Miro's effect on anterograde transport through kinesin, loss of Miro gene impairs dynein-mediated mitochondrial transport. This shows that Miro has more roles than being a KIF5 adaptor protein, even though a Miro-dynein association has not been established either.

Mitochondria movement to high energy demand regions

The functional reason behind migration of mitochondria is that ATP has a limited diffusion capacity, which causes problems in long axons in particular. To avoid ATP shortages in the distal synapses, mitochondria migrate along axons. For the same reason, mitochondria are able to dock at locations where energy dependency is high and be released when they have to move somewhere else or to be regenerated for example.

Transport and spatial distribution of mitochondria over the axons is directly correlated with synaptic activity. The driving forces behind mitochondrial transport to high demand regions such as synapses and nodes of Ranvier in response to synaptic activity are ATP and Ca 2+ 210 . Mitochondrial velocity increases when they enter cellular regions with high ATP levels and decreases when local ATP concentrations are low. Due to the high use of energy in synapses and nodes of Ranvier, ATP is depleted in these regions, and hence mitochondria slow down. Similarly, mitochondrial movement decreases when local Ca 2+ concentrations increase. Local Ca 2+ increases are mostly the result of Ca 2+ influx through voltage-dependent Ca 2+ channels at presynaptic terminals or nodes of Ranvier. In other words, action potentials open these Ca 2+ channels, Ca 2+ enters the neuron and mitochondrial velocity decreases 211 .

The notion that ATP and Ca 2+ influence mitochondrial velocity suggests that these molecules are influencing motor proteins, motor adaptor proteins or their interaction with mitochondria. The paradigm on why mitochondrial binding to these classes of proteins involves multiple adaptor proteins is that neurons are able to regulate mitochondrial transport by different mechanisms in response to different physiological signals. Indeed, it has been shown that ATP/ADP and Ca 2+ can influence the interaction of mitochondria with KIF5 through these adaptor proteins, which shows that mitochondrial transport and distribution is regulated to fulfill mitochondrial functions at the location where it is most needed. In addition, Miro protein has been identified as a Ca 2+ sensor 212 . In neurons expressing Miro with mutations in its Ca 2+ sensing domains, mitochondria no longer slow down in areas with high Ca 2+ levels. These mutated Miro proteins cannot bind Ca 2+ , showing that Miro is an important candidate as the main regulator of mitochondrial movement to and recruitment at the nodes of Ranvier and activated synapses in response to increased action potential firing.

Next to variable mitochondrial speeds, mitochondria can become stationary and reversely, stationary ones can be remobilized and redistributed. Factors that can change mitochondrial velocity can similarly switch stationary mitochondria to mobile ones. These factors include changes in axonal physiology, but also changes in axonal activity. Vice versa, the state of mitochondria is an important factor in axonal and synaptic function. For example, mitochondrial anchoring is required for synaptic transmission and for axonal branching and maintenance. The requirement for mitochondrial docking provides a need for factors that regulate mitochondrial docking as well. Syntaphilin acts as a static anchor that immobilizes mitochondria. Synaptaphilin contains a carboxyl-terminal mitochondria-targeting domain and an axon-sorting sequence, which make it possible to be located in the mitochondrial outer membrane of mitochondria that are targeted to axons. Syntaphilin arrests mitochondrial movement by coupling mitochondria to microtubules. Deletion of syntaphilin in mice results in an increase of motile mitochondria but a reduction of mitochondria within axons. Conversely, over-expression of syntaphilin results in immobile mitochondria. Syntaphilin is also involved in activity-dependent immobilization of axonal mitochondria. In neurons that fire action potentials, the Miro-Ca 2+ pathway is activated, but this does not result in reduced mitochondrial velocity or mitochondrial docking in syntaphilin deficient mice. On the other hand, dendritic mitochondria are immobilized, showing syntaphilin's selectivity for axonal mitochondria. Syntaphilin specifically binds to the KIF5 C-terminal tail and this binding is facilitated via Miro-Ca 2+ . Binding of syntaphilin to KIF5 inhibits ATPase activity and prevents the assembly of the Miro1-TRAK2-KIF5 transport complex. Interestingly, neuronal activity stimulates the recruitment of syntaphilin into the mitochondrial membrane, suggesting that syntaphilin undergoes dynamic translocation in response to neuronal activity.

The central role that calcium appears to play in mitochondrial function and mitochondrial movement points to the ER with its MAM as a potential regulator of these processes. 213 However, the influence of the ER on such distally located mitochondria depends on the presence of ER all the way throughout the axon. Reticulon-4 is a protein that promotes ER tubulation, and subsequently influences ER-mitochondria interactions in the axon 214 . Other ER tubulation enzymes fulfil a similar function, including receptor expression enhancing protein 1 (REEP1), which is a known regulator of both ER network formation and mitochondria membrane dynamics. Likewise, the microtubule severing protein spastin plays a role in connecting ER tubules to the cytoskeleton and in the trafficking of mitochondria along axons. Hence, migration of mitochondria and tubulation of the ER go hand in hand, suggesting the role of the ER in mitochondrial movement to energy dependent areas.

Conclusion

Myelinating Schwann cells have several functions to support neuronal functions and axonal mitochondria. The myelin sheath has metabolic advantages and increases action potential conductivity, but it also creates spatial differences in the axon and the effects of these spatial differences on mitochondrial physiology have only partially been investigated. Also the lactate shuttle is a process that has not been fully understood yet. Since most cells use pyruvate for oxidative phosphorylation, instead of transforming it to lactate, the origin of lactate that is transported from Schwann cells to axons is not established. Also the effect of the lactate shuttle on neuronal function and mitochondrial function is unknown.

Furthermore, the movement of mitochondria along the axon to their target destination has been shown to be a complex mechanism. The connection between mitochondria and the cytoskeleton works via a complex of multiple proteins, but the regulation of this complex has to be investigated further.

Chapter 4) Mitochondria and ROS in neuropathies

The role of nervous system is to send signals from the brain to the organs and back. In the conduction of these signals, mitochondria play an important role. However, the nervous system can become damaged or may not function properly, which may result in the development of neuropathies. Multiple sorts of neuropathies exist, depending on the mechanism that causes the neuropathy and the cell type and region of the nervous system that is affected. These different sorts of neuropathies have different symptoms.

Several of the most important neuropathies both in the CNS and PNS involve mitochondria and dysfunctional mitochondrial functions and/or physiology. In this chapter I will discuss the role of mitochondria and ROS in amyotrophic lateral sclerosis, Alzheimer's disease, Parkinson's disease, multiple sclerosis and Charcot-Marie-Tooth diseases.

Amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neuronal disease that progressively affects motor functions 215 . The incidence rate of ALS is 2.7 per 100,000 people and patients suffer from muscle weakness, twitching, and cramping. Most cases are sporadic, although 5% of the cases showed a hereditary component 216 . While the exact pathological mechanisms remain unclear, the role of mitochondria and the ROS they produce have been demonstrated.

Most studies point towards ROS as a central player in ALS pathology. Mutations in the SOD1 gene have been detected in around 20% of familial ALS cases 217 . SOD1 mutations decrease the quenching capacity of H 2 O 2 , which can have several downstream effects. Indeed, numerous studies demonstrated a correlation between ALS and increased oxidative stress. Increased protein carbonyl levels, indicating elevated oxidative damage to proteins, have been detected in both the spinal cord and motor cortex of ALS patients 218 . In addition, increased 3nitrotyrosine levels have been measured within large ventral horn neurons 219 . Next to protein oxidation, lipid oxidation was detected in motor neurons, astrocytes and macrophages. Oxidative damage to DNA has been found to be elevated in the spinal cord of ALS patients, especially in the ventral horn. Not only neurons, but also other brain components can contain more oxidative damaged DNA, proteins or lipids, including the CSF 220 . Next to increased oxidative stress, other mitochondrial abnormalities, such as disrupted axonal transport, have been reported in ALS patients. The SOD1 mutations in ALS patients correlate with reduced astroglial glutamate transporter EAAT2, which leads to over-stimulation of glutamate receptors of neurons and excessive neuronal firing. In addition, this glutamate excitotoxicity causes excessive calcium influx, which results in neurodegeneration.

Interestingly, in transgenic mice with mutant SOD1, the most severely oxidized protein appeared to be SOD1 itself 221 . This finding suggests that mutant SOD1 is not only incapable of quenching oxygen radicals, but also inhibits the function of normal SOD1 expressed by the normal allele. Nevertheless, some SOD1 mutations involved in ALS have been shown to leave the reducing capacity of the protein intact, suggesting that alternative mechanisms are probably existing.

Alzheimer's disease

Alzheimer's disease (AD) is a dementia disorder with progressive deterioration of cognitive function and memory loss. In Europe, 11.08 per 1000 persons are diagnosed with Alzheimer every year 222 . The exact pathological pathway is not known due to the many genes that have been shown to be involved. Nonetheless, two major causes have been characterized being the formation of amyloid plaques and the formation of intracellular neurofibrillary tangles.

ROS have been linked to AD as well 223 . For example, DNA and RNA oxidation is increased in AD brains. Also increased protein carbonyl levels have been observed in AD frontal lobe, hippocampus and superior middle temporal gyrus. Besides, nitrotyrosine and dityrosine crosslinked proteins are elevated 8-fold and 3-fold respectively in hippocampal and neocortical regions of AD brain. Oxidation-induced crosslinking of proteins may result in the resistance of lesions against degradation, even though there is abundant ubiquitination of these proteins. Lipid peroxidation and the subsequent altered phospholipid composition have also been detected in AD brains. Interestingly, the antioxidant response by SOD is elevated in AD brains, which suggests that the normal compensatory mechanisms against ROS may be insufficient in AD patients.

The increased ROS levels and oxidative stress in AD brains is correlated with mitochondrial dysfunctions. For example, mutations in the MFN2 gene in AD patients result in a disbalance in mitochondrial fusion and fission and an abnormal distribution of mitochondria in the hippocampus 224 . Knockout of MFN2 in mice resulted in neurodegeneration in the hippocampus. This phenotype resembles many of the characteristics of AD. Another explanation of mitochondrial dysfunction in AD is a dysfunctional ER-mitochondria interaction. MAM are uniformly distributed throughout the hippocampal neurons and impaired MAM function results in neuronal degeneration. Interestingly, MAM function appeared to be upregulated in AD patients, leading to increased ER-mitochondrial interaction 225 . Similarly, neuronal cells exposed to amyloid β, an AD mouse model (APPSwe/Lon) and cortical tissue from AD patients all showed affected MAM. In mice, MAM-associated proteins are upregulated already at 2 months of age, which is before the presence of visible amyloid plaques 55 . In an early stage of AD, the increased expression of MAM proteins could indicate a stress response, where neurons try to maintain proper function. Although most MAM-associated proteins were shown to be upregulated, σ1R, a molecular chaperone located in the ER-mitochondria interface, was significantly decreased. In accordance, decreased σ1R was detected in hippocampus tissue from AD patients.

Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder that results in the loss of dopaminergic neurons of the substantia nigra that project to the putamen 226 . The incidence of Parkinson's disease is 17.2 per 1000 persons per year 227 and patients suffer mainly symptoms that impair motor functions, such as tremor at rest, rigidity, akinesia or bradykinesia and postural instability 228 .

A defective ETC has been an established marker of Parkinson's disease. In particular a defective complex I has been associated with the pathogenesis of PD. Blocking complex I by 1-methyl-4-phenyl-1,2,3,4-tetrahydropyridine or rotenone results in neurodegeneration selectively in the substantia nigra pars compacta, which are also the cells that are most affected by PD pathology. Moreover, decreased complex I activity was shown in brains of patients with sporadic PD 229 . A decreased complex I activity may lead to decreased ATP production and an increase in the production of ROS. Some studies reported only a 25-30% decrease in complex I activity in PD patients, which is not sufficient to cause ATP depletion 230 , although others have shown a decrease up to 60% 231 . Nonetheless, increased oxidative damage to proteins, lipids, and DNA has been observed in postmortem brain samples from PD patients 232 . However, this oxidative damage and increased levels of ROS may be derived from sources other than the neuronal mitochondria. For example, neighboring glial cells may be causing the observed damage to PD brains. In addition to possible decreased ATP and increased ROS production, calcium homeostasis is affected by PD pathology. It has been shown that PINK1, a regulator of calcium release via the mitochondrial Na + /Ca 2+ exchanger, is affected in PD. PINK1 ablation indeed results in impaired Ca 2+ efflux from mitochondria, accumulation of mitochondrial Ca 2+, increased production of mitochondrial ROS, decreased mitochondrial respiration and a lowered threshold for Ca 2+ -dependent opening of the mitochondrial permeability transition pore complex in dopaminergic neurons. This results in increased apoptosis 233 . Another protein that is connected to PD is parkin. Mutant parkin results in morphological and functional mitochondrial defects, including a decrease in the membrane potential (by 30%), complex I activity (by 45%), ATP production (by 58%) and an increase mitochondrial fragmentation 234 . This shows that mitochondrial morphology is also different in PD mitochondria.

Finally, a protein that plays an important role in Parkinson's disease is α-synuclein. In Parkinson's disease, aggregates of α-synuclein accumulate in Lewy bodies located in neurons and these aggregates are thought to contribute to neuronal loss. In healthy neurons, αsynuclein is associated with mitochondria as it regulates the balance between mitochondrial fission and fusion 235 . However, overexpression of α-synuclein increases the flux of calcium from the ER to mitochondria, and the number of ER-mitochondria contact sites increases 236 . These observations show that MAM are involved in mitochondrial physiology in Parkinson's disease, including calcium homeostasis 237 .

Multiple Sclerosis

Multiple Sclerosis (MS) is a chronic inflammatory disease of the CNS with a crude prevalence of 151.2 per 100,000 French inhabitants 238 . MS is characterized by the presence of demyelinated lesions in both white and gray matter and it progresses in episodes of exacerbated immune responses followed by remission periods 239 . These immune responses involve widespread microglial activation. Over time, MS patients develop neuronal and axonal degeneration with extensive cortical demyelination. MS pathology manifests by a variety of symptoms including fatigue, numbness and tingling muscle spasms, stiffness and weakness, motor problems, and organ failure.

The activated microglia and macrophages are thought to be the initial source of ROS that cause damage in MS lesions 240 . Iron mediated ROS has been shown to cause oxidative stress in MS lesions 241 , since iron is abundant in myelin 241;242 and during demyelination, this iron becomes accessible for ROS 242 to induce damage to lipids and proteins. Indeed, MS patients that were treated with iron chelator deferoxamine showed an improvement of their condition. It is thought that by chelating the iron deferoxamine prevents the production of hydroxyl radical from H 2 O 2 and O 2

•-

. Mitochondrial dysfunction and an increase of mitochondrial ROS contribute as well in progressive MS. Indeed, activity of mitochondrial complex IV is increased in active MS lesions even though mitochondrial mass is also increased in MS lesions 243 .

Cuprizone treatment is used in animal studies as a model for toxic demyelination, like in MS 244 . Cuprizone treatment and the resulting demyelination in the nervous system causes changes in mitochondria, such as increased complex IV activity 245 and increased ROS levels. Additionally, decreased mnSOD and CuZnSOD are measured 246 , indicating that the elevated ROS levels are possibly caused via both increase mitochondrial activity as antioxidant failure. Cuprizone also induced decreased levels of glutathione, the main electron donor of the antioxidant GPx, which leads to low GPx activity.

Cuprizone treatment eventually affects ER function. Also Rab32, a known regulator of the MAM, is highly expressed in active MS lesions of both human MS patients and mouse models 247 . The high expression of Rab32 leads to changes in mitochondrial physiology and the induction of apoptosis. Impaired ER function disrupts protein and lipid synthesis as well, including proteins and lipids that would build up the myelin sheath in a healthy condition. Therefore MAM function, including the exchange of Ca 2+ between ER and mitochondria, contributes to the further breakdown of the myelin sheath and controls neuronal degeneration in active MS lesions 247 During inflammatory events, the increase of ROS induces further mitochondrial damage and reduced ATP production, which inhibits axonal mitochondrial transport. Also transport of other axonal compounds is inhibited in MS, which affects axonal integrity 239 .

All these mechanisms indicate that the pathologic immune response in MS triggers a feedback loop that involves demyelination, mitochondrial dysfunction, elevated ROS production and ER failure (Fig. 16) and this mechanism progressively destroys the myelin sheath and eventually results in neuronal death.

Charcot-Marie-Tooth disease

CMT disease is a genetically and phenotypically heterogeneous group of disorders with a prevalence of CMT is 1 per 2500 people 248 . The first signs of CMT are found distally in the feet and progress proximally. Symptoms of CMT patients include a combination of peripheral nerve related motor and sensory deficits, high foot arches and curled toes. These symptoms present themselves as length-dependent paresis, muscle atrophy and areflexia. The chronic nature of CMT leads to the development of foot deformities, hammertoes and high-arched feet in approximately 70% of patients. As the disease progresses, the hands may also become deformed. Sensory system symptoms are common as well, which include loss of vibration and joint position sense and decreased pain and temperature sensation. Patients also report numbness and tingling in their feet and hands. Scoliosis occurs in approximately one-third of CMT cases.

CMT can be divided in demyelinating forms (CMT type 1) and axonal forms (CMT type 2) 249 . Interestingly, clinical symptoms are not different between the demyelinating or axonal forms. The age of onset of both CMT1 and CMT2 is in the first or second decade of life, but the severity of the disease can be highly variable with some patients suffering severe symptoms at an early age and others showing only mild features in their whole lifetime. For example, patients with CMT1A, which is the most common form of CMT, rarely will ever need the use of a wheelchair. On the other hand, CMT2A has greater severity with most patients requiring a wheelchair at a young age 250 . Details about the genetic bases of CMT diseases and the pathomechanisms can be found in Sziget & Lupski, 2009 239 .

Diagnostic tests may include electrophysiological tests and sural nerve biopsies. These electrophysiological tests are especially helpful to distinguish between demyelinating CMT type 1 and axonal CMT type 2, the former showing a NCV below 38 m/s and the latter being associated with normal NCV 250 , but reduced compound muscle action potential. Sometimes an intermediate CMT is classified when the patient cannot be classified confidently as either CMT1 or CMT2 251 . These intermediate CMT patients have features of both demyelination and axonopathy. Sural nerve biopsies from CMT1 patients reveal segmental demyelination and onion bulb formation. CMT2 patient sural nerves show axonal degeneration, but a healthy myelin sheath.

CMT Type 1

Although CMT1 and CMT2 cannot be distinguished based on the patients' symptoms, the cellular deficits that cause these symptoms differ between the two forms. This is also reflected by the detection of genetic factors, which are different between CMT1 and CMT2. Not surprising, the demyelinating forms of CMT show mutations in genes involved in myelin production or its regulation. For example, it has been shown that elevating cJun in SCs of a CMT1A mouse model prevents myelin loss of myelinated sensory axons 252 . This prevention is correlated with an improvement of behavioral symptoms in these mice. Sox2 has also been identified to be a physiological regulator of SC myelination in vivo that is implicated in myelination disorders in the PNS, such as CMT1 253 . Next to hereditary causes, acquired inflammatory etiologies, exposure to toxic compounds such as lead or chemotherapeutic agents or vitamin deficiencies as thiamine or B12, are factors that contribute to CMT disease development.

Deletion of serine 63 from P0 glycoprotein (P0S63del) causes CMT1B neuropathy in humans and a CMT1-like phenotype in mice. As a result of a misfolded P0S63del, SCs develop an increased unfolded protein response and increase the expression of CHOP. This transcription factor has previously been associated with apoptosis in ER-stress and apoptosis. However, ablation of CHOP in S63del mice rescues their motor deficit and partially prevents from demyelination, indicating that S63del causes the phenotype effects through CHOP 254 . Next to myelination problems, mitochondrial deficits have been correlated with c-Jun, CHOP and the unfolded protein response. Upon activation of c-Jun and CHOP-10, expression of genes that are involved in the unfolded protein response of specifically mitochondrial proteins (UPRmt) is increased. UPRmt involves mitochondrial chaperones and proteases that remove dysfunctional proteins from the mitochondrial matrix 255 .

CMT Type 2

The genetic causes of CMT2 are partially distinct from CMT1. The most common mutations observed in CMT2 are in genes encoding for MPZ, HSPB1 and MFN2 256 . The detection of MPZ mutations in CMT2 patients is remarkable, because MPZ is a protein component of compact myelin in the PNS, where it plays a crucial role in myelin formation and adhesion 257 . CMT2 patients with mutations in the MPZ gene have been shown to suffer from severe axonal neuropathy with Wallerian-like degeneration and clusters of regeneration. However, no evidence of demyelination was observed in these patients. The precise mechanism on how this myelin related gene causes the axonal CMT2 is not fully understood. HSPB1 belongs to a family of heat shock proteins and is involved in a multitude in cellular mechanisms. In peripheral nerves however, its most prominent role is the stimulation of neurofilament formation and dynamics of cytoskeletal proteins 258 . This finding links HSPB1 mutations with impaired transport of cargo, and possibly impaired mitochondrial transport, which has also been reported in CMT2. More than 60 mutations in the MFN2 gene have been identified in CMT2A patients 259 and almost all these mutations are single point mutations in the coding sequence that cause an amino acid substitution. MFN2 mutations mainly cause autosomal dominant CMT, though families with homozygous or compound heterozygous mutations have been identified 260 . Especially the amino acid residue at position 94 of the MFN2 gene appeared to be highly susceptible to be mutated in CMT2A patients. The most common substitutions reported at this position are the R94Q and R94W point mutations. Hence, models mimicking the R94Q mutation in vitro or in vivo have been useful to research the biological effects 261 . Interestingly, MFN2 mutant mice show an increased proportion of small and medium sized axons. Fusion impairments have been reported to cause neurite atrophy in purkinje cells 262 , so possibly MFN2 related fusion impairments also cause the shift towards smaller nerve fibers. The presence of smaller nerve fibers does not indicate that axons are lost. Indeed, CMT2A patients with MFN2 mutations, as well as MFN2 R94Q mice models, do not show any sign of axonal loss 263 .

One murine model of CMT2A, overexpressing MFN2 T105M shows an improper mitochondrial distribution and tight clusters of mitochondria within axons of motorneurons 264 . However, this result was thought to be the result of increased expression of this mutant MFN2 and the subsequent accumulation of this protein, rather than the mutation per se.

In other mouse models and in CMT2A patients, neither such extreme overexpression of MFN2, nor a depletion of mitochondria in axons has been observed. In contrast, a 30% increased number of mitochondria in axons have been detected in mutant MFN2 expressing neurons in the sciatic nerve. The increase in mitochondrial density in axons of MFN2 mutant mice can be caused by multiple reasons, including impaired retrograde transport of mitochondria and an increased number of fission events, while fusion is impaired. OPA1, the other protein involved in the mitochondrial fusion process, interestingly is not involved with CMT2A. Instead mutations in the OPA1 gene cause another neuropathy being dominant optic atrophy (DOA) 265 . These patients suffer from visual loss, central visual field deficits and problems with color vision. These symptoms develop due to atrophy of the optic disc and retinal ganglion cells 266 . Secondary symptoms that only occur in a minority of patients include neurosensory hearing loss, myopathy, peripheral neuropathy and multiple sclerosis-like illness. DOA patients also show heterogenous mitochondrial network and clumps in discrete areas of the cytoplasm 265 , so the abnormal distribution of mitochondria in these patients may cause the pathology. Furthermore, OPA1 mutant fibroblasts show a higher percentage of mitochondria containing balloon-like structures and complete fragmented mitochondria 267 . Since both MFN2 and OPA1 are involved in mitochondrial fusion, it is striking that mutations in the respective gene causes such different pathology. This suggests that next to mitochondrial fusion, OPA1 and MFN2 have other functions in the cell.

MFN2 is required for axonal mitochondrial transport 268 . In neurons derived from MFN2 knockout mice or neurons expressing mutant MFN2, migrating axonal mitochondria undergo pauses that take a longer time than axonal mitochondria in healthy neurons. In addition, anterograde and retrograde migration speed is decreased in mitochondria with defect MFN2. That result suggests that the attachment to microtubules becomes impaired when MFN2 is not present of mutated. Importantly, MFN2 defects do not affect the transport of other organelles, showing that the effect is specific to mitochondria. Knockdown of Miro2 results in identical mitochondrial transport deficits, suggesting that mitochondria in MFN2 mutants are no longer able to connect to Miro2 and hence the kinesin motors. Mutations in Opa1, the other protein required for mitochondrial fusion, results in impaired fusion, but not in decreased mitochondrial transport, indicating that mitochondrial fusion and mitochondrial transport are not inherently connected.

Changes in the physiology of mitochondria caused by MFN2 mutations have an effect on the functionality of these mitochondria 269 . Changes in cellular energy metabolism, calcium homeostasis and other cellular functions are possible. Indeed, mutations in the MFN2 gene result in a reduced mitochondrial respiration and a decrease in respiratory complex subunits. Importantly, a decrease in the mitochondrial membrane potential is observed. This is correlated with a reduction of the efficacy of oxidative phosphorylation 270 . In contrast, oxygen consumption is increased in MFN2 mutant cells in certain conditions and cell types, showing that the effect of mutations in the MFN2 gene can be highly variable between cell types. However, a stable depletion of the MFN2 gene induces adaptive processes that counteract disorganization of cell metabolism and function. The reduced oxidative phosphorylation efficiency is compensated by increased glycolysis 269 .

Differences in energy metabolism in MFN2 mutant mitochondria can result in downstream effects, including altered ROS production. Sebastian et al. reported that in muscle and liver tissue, MFN2 deficiency leads to increased H 2 O 2 concentrations, differences in ROS response and mitochondrial dysfunction 271 . This is possibly due to a dysregulated antioxidant response, because the mutation c.140A>G, p.His47Arg in SOD1 is connected to CMT2 like pathologies 272 . Also SOD1 -/-mice develop a peripheral neuropathy phenotype that affects mostly motor neurons, while sensory neuron function remain normal 273 . Nerve conduction velocity in these SOD1 -/-mice is impaired in mostly motor nerves, whereas the sensory sural nerve is unaffected. The effect of increased ROS production by MFN2 mutant mitochondria is an increase in JNK phosphorylation and IRS1 phosphorylation, which leads to insulin resistance. In addition, palmitate induced ROS production is reversed by upregulating the expression of MFN2. Also in the reverse experiment, knockdown of MFN2 by antisense MFN2, an increase in ROS production was detected. These results show that mitochondrial ROS production is increased when MFN2 function is impaired, but the underlying mechanism is not well understood.

In addition to increased ROS production caused by MFN2 mutations, changes in ATP production in MFN2 mutant mitochondria have been observed. However, this might not be the downstream effect of different energy metabolism. In mouse models for CMT2, several mutations in the gene coding for ATP synthase are observed 274 . A specific mutation that is found to be correlating with disease severity is m.9185T>C. Remarkably, ATP synthase activity is measured but in the reverse direction, meaning that ATP is hydrolyzed to produce ADP and inorganic phosphate. In addition, the m8993T>G mutation is associated with impaired assembly and stability of ATP synthase with reduced the activity of ATP synthase in mice. Same impairments in ATP synthase, either impaired assembly, impaired function, or both are detected in CMT2 patients that appeared to have the m.9185T>C mutation. Even though the direct link between ETC deficits and neuronal degeneration is not fully described yet, several hypotheses are proposed. Possibly, axonal degeneration is caused by a decreased axonal membrane potential due to Na + /K + -ATPase failure caused by a decrease in ATP production by MFN2 mutant mitochondria. On the other hand, even though a decrease in mitochondrial ATP production is detected in MFN2 mutant fibroblasts due to a less efficient oxidative phosphorylation, total cellular ATP production is found to be unchanged, because other ATP producing mechanisms, such as glycolysis, compensate for the lack of mitochondrial ATP 270 . However, these compensatory mechanisms require greater oxygen consumption, so ATP production may still become impaired due to MFN2 deficits in situations when the cell is located in a low oxygen environment or when the cell is very active such as firing axons.

Conclusion

Mitochondria and ROS play an important role in the nervous system and when mitochondria malfunction or when the equilibrium between ROS and antioxidants becomes in disbalance, neuropathies can develop such as Amyotrophic Lateral Sclerosis, Multiple Sclerosis, Alzheimer's disease and Parkinson's disease. In the PNS, peripheral neuropathies such as Charcot-Marie Tooth diseases can develop due to mitochondrial dysfunction and an excess of ROS. Some diseases, such as ALS and CMT2, are the result of neuronal malfunctioning or neurodegeneration, whereas pathologic demyelination is the cause of MS and CMT1 for example. However, our understanding of the effect of pathological conditions on mitochondrial function is still lacking. Especially knowledge about changes of mitochondrial physiology in pathologic conditions in vivo is limited.

Chapter 5) In-vivo imaging of peripheral nerve mitochondria

The biological mechanisms in the PNS and the role that mitochondria play in these mechanisms appear to be complex. Importantly, when these mechanisms do not function correctly, this can lead to neuropathies. Therefore, there is a need to better understand the biological processes in the PNS and what goes wrong on a cellular and molecular level. Because mitochondrial function is greatly different in an in vitro setting or in dissected nerve tissue than in a living organism, it is important to observe what happens in healthy and diseased nerves in vivo. For this, advanced research tools are developed. During my thesis I contributed to develop such techniques to decipher the role of mitochondria and metabolic changes that occurs in healthy and diseases peripheral nerves.

In this chapter I will discuss several techniques that can be used for in vivo imaging. These techniques include viral vectors with fluorescent probes, multiphoton microscopy, FRET imaging and CARS imaging.

Expression of fluorescent markers in the developing mouse embryo

Fluorescent probes

To measure cellular processes, genetically-encoded probes can be used that emit fluorescent signals. These fluorescent probes can either be cytosolic or targeted to cellular compartments such as mitochondria. Manufactured probes include mito-AT1.03, Laconic, FLII12Pglu-δ6, mito-GCaMP2 and mito-roGFP2-Orp1. Relative ATP levels are measured using the YFP/CFP fluorescence ratio. AT1.03 appeared to have a sensitive range between 1 and 10 mM at 37 °C. It is sensitive to ATP selectively, since addition of up to 10mM GTP or ADP has been shown not to have an effect on the YFP/CFP fluorescent ratio, in contrast what is observed with the addition of ATP. Even deoxyadenosine triphosphate (dATP) does not change the fluorescent ratio of AT1.03. Within the pH-range of 7.1 to 8.5, pH variations does not influence the emission ratio of AT1.03. This indicates that small pH fluctuations around the normal cytoplasmic pH (which is near 7.3) or mitochondrial pH (7.8) will not affect the AT1.03 fluorescent ratio. Nonetheless, temperature changes appeared to have a stronger effect on the AT1.03 sensitivity. The dissociation constant elevates approximately five-fold by an increase of 10 °C. Next to its specificity and sensitivity, the kinetics of ATP binding and dissociation are important factors of AT1.03 functionality. The rate of ATP binding was established at 1.7 x 10 -2 mM -1 s -1 and the rate of ATP release at 9.8 x 10 -2 s -1 . This indicates that ATP can be monitored with rates up to 0.1 s -1 . AT1.03 targeted to cellular compartments, showed that differences in ATP concentration were present in different cellular structures. Surprisingly, AT1.03 located in the mitochondrial matrix has been shown to have significant lower YFP/CFP emission ratios than in the cytoplasm or in the nucleus. The nucleus also shows a lower AT1.03 emission ratio than the cytoplasm, but to a smaller extent, suggesting that ATP can pass the nuclear membrane almost freely. The mitochondrial target sequences should not impair AT1.03 sensitivity as the target sequence is cleaved after transport into mitochondria. Although that observation seems paradoxical, considering mitochondria being the main producer of ATP, possibly the nucleotide translocator pumps ATP from the mitochondrial matrix into the cytosol, thereby maintaining a relative high ADP/ATP ratio, which is preferable for ATP synthase to produce ATP from ADP and phosphate.

Laconic

Laconic is a FRET-based probe as well, with the FRET pair being mTFP and Venus, bound to the DNA-binding domain of LldR, which is a bacterial transcription regulator that also contains a lactate-binding/regulatory domain 278 . mTFP is located at the N-terminus of LldR flanked by linkers, and Venus located at the C-terminus. Opposite to AT1.03, FRET efficiency decreases upon binding of lactate, meaning that the mTFP/Venus fluorescent ratio decreases when lactate is bound. Laconic detection is sensitive for a large range: from at least 1 µM up to 10 mM. Also opposite to AT1.03, laconic shows some change in specificity in milieus of different pH. Although in vitro, some interference was observed from pyruvate and citrate, this should not have significant effects in other conditions. Pyruvate only showed interference at high concentration, whereas in a physiological condition, pyruvate concentration is 10-50 times lower than lactate, indicating that endogenous pyruvate should not interfere with lactate sensing under physiological conditions. Citrate increased FRET ratio at a concentration of 1 mM, although no effect was measured at lower concentrations. Like pyruvate, cytosolic citrate levels are below the concentration in which it affects Laconic. However, mitochondrial citrate is 10 times higher than in the cytosol, suggesting that mitochondrial citrate may affect mitochondrial lactate sensing by mitochondria-targeted Laconic. Extreme redox changes did not change the Laconic emission ratio.

FLII12Pglu-δ6

FLII12Pglu-δ6 is another FRET based probe, connecting eCFP and citrine to the glucose/galactose binding protein of Escherichia coli (MglB) 279 . Citrine was inserted into the backbone of MglB in order to reduce rotation of citrine and hence preventing limited resonance energy transfer. To limit this rotation even further, a number of amino acids of the linker domains was reduced. For FLII12Pglu-δ6, 6 amino acids were deleted (hence δ6). FLII12Pglu-δ6 is a probe that detects glucose and it has a high dynamic ratio in vitro and in vivo. FLII12Pglu-δ6 appeared to be sensitive to glucose levels from 0.05 mM to 9.6 mM, with a K 0.5 at 600 µM in vivo and a normalized Δratio of 0.74. In the cytosol of HepG2 cells in vitro, FLII12Pglu-700µδ6 showed saturation over 10 mM glucose. The kinetics of FLII12Pglu-700µδ6 showed a K 0.5 of 1.54±0.30 mM glucose, and a Vmax of 3.91 ± 0.22 Δratio/min. These values are in accordance with the GLUT1 affinity to D-glucose on plasma membrane.

RoGFP2-Orp1

Redox sensitive GFPs (roGFPs) are ratiometric probes that detect the redox status of its environment (Fig. 18). roGFPs harbor an engineered dithiol/disulfide switch on their surface, which determines their wavelength of excitation 280 . RoGFP2 can be converted into an H 2 O 2 specific probes by fusion with the microbial H 2 O 2 sensor Oxidant Receptor Peroxidase 1 (Orp1) 280;281 . Orp1 is known to respond to H 2 O 2 with high specificity. Because of this, the fusion protein roGFP2-Orp1 is also highly sensitive to H 2 O 2 , as it has shown not to react with other oxidants, such as GSSG, cysteine, hydroxyethyl disulfide or hydroascorbic acid, but a significant ratiometric change could be observed induced by H 2 O 2 . Upon detection of H 2 O 2 by Orp1, de resulting disulfide bridge on Orp1 is transferred to the roGFP subunit, which then can be detected as the conformational shift alters the excitation spectrum of roGFPs. The oxidized roGFP that contains disulfide bridges is detected with an excitation peak at 405 nm, whereas the excitation peak of the reduced roGFP (that contains -SH groups) is at 488 nm 282 . The formation of disulfide bridges on roGFP is reversible via reduction by cellular thioredoxin (Trx) or GRx/GSH 283 . RoGFP2-Orp1 appeared to be sensitive to H 2 O 2 levels in a range between 0.1-10 µM in vitro and 1-50 µM in living cells 281 .

GCaMP2

To measure cellular and mitochondrial calcium levels, a fluorescent indicator for Ca 2+ was developed: GCaMP2. These calcium sensors include a M13 fragment from myosin light chain kinase, a circularly permutated EGFP (cpEGFP) and calmodulin (CaM) located N to C terminally. When Ca 2+ binds to CaM, conformational changes due to the Ca 2+ -CaM-M13 interaction induce a subsequent conformational change in cpEGFP, so that the fluorescence intensity increases 284 . Indeed, GCaMP is sensitive to Ca 2+ levels with a Kd of approximately 235 nM 284 . The Kd of mitochondria-targeted GCaMP2 is slightly variable from one cell to another, since the Kd has been established at 136 nM in HéLA cells and 189 nM in cardiac myocytes 285 . The effect of pH on mito-GCaMP2 is shown to be minimal 285 .

Viral vectors

These fluorescent probes may be useful tools to detect physiological changes in cells, but they need to be expressed in living cells first. To detect fluorescent probes in vivo, genetically modified mice expressing DNA that codes for the fluorescent proteins are generally needed. Conventionally, DNA plasmids coding for fluorescent proteins were introduced into embryonic stem cells or into mouse embryos and then mice have to be bred. However, a faster and easier method is injecting viral vectors that contain DNA coding for fluorescent probes into mice postnatally. Efficient gene transfer and stable transgene expression are important features to bring DNA into cells and express this newly delivered DNA 286 .

Adeno-associated virus (AAV)-derived vectors are excellent vectors for gene delivery in vivo, because of their low toxicity and low immunogenicity, highly efficient gene transfer and longterm expression in several cell types, including neurons. AAVs are non-enveloped, helperdependent parvoviruses with a capsid architecture of "25 nm in diameter 287 . AAVs package "4.7 kb genome flanked by "145 bp inverted terminal repeats (ITRs) on the 5 and 3 ends. Wildtype AAV genome is a linear single stranded DNA consisting of two open reading frames (ORFs). This wildtype AAV requires coinfection by adenoviruses or herpes simplex viruses to transfect neurons successfully. It has been shown that wildtype AAV is able to integrate into the (human) host genome at a specific locus in chromosome 19. The integration of AAVs at this specific locus is mediated through its ITR and Rep68 or Rep78 proteins. However the rep gene is undesirable for AAV transducing vectors, since it strongly inhibits gene expression and cell growth 288 . Recombinant AAVs lack endogenous viral genes 289 , so it allows for transduction of a wide variety of cells at a high titer with a low risk insertional mutagenesis and immunogenicity. Twelve AAV vector serotypes with different transduction profiles have been constructed. For example, AAV5 is mainly used for transfection of sensory neurons, whereas AAV2, 6 and 9 is used for spinal motor neurons. Successful transduction by AAV vectors occurs in several steps 290 . To target primary sensory and motor neurons, two routes of delivery have successfully been used, depending on the target location. AAVs can be delivered via direct intraganglionic or intraspinal injection. Direct intraganglionic injection of an AAV vector results in expression of the AAV in sensory neurons with very little expression in other locations 291 . The second route is intrathecal (IT) delivery. IT delivery results in transduction of sensory and spinal cord neurons, but also in other non-neuronal cell types 292 . When the AAV vectors are injected, the virus has to enter the neurons. First, AAVs bind to the receptors located on the cell surface. Surface exposed regions on the AAV capsid mediate this initial interaction with the host cell surface (Huang et al., 2014). In addition, cell surface glycans have been identified as the preferred receptor to bind to by many AAV serotypes. AAV serotypes 1, 5, and 6 bind N-linked sialic acid (SA), whereas AAV4 is the only natural AAV isolate that binds O-linked SA moieties on mammalian cell surfaces 293 . AAV serotype 1,5 and 6 bind N-linked SA, whereas AAV4 binds O-linked SA moieties and AAV2, AAV3 and AAV6 bind heparan sulfate (HS) proteoglycans. AAV9 requires N-terminal galactose residues to perform successful gene transfer 294 . Then, the AAVs are taken up into the cell and translocated towards the nucleus. This translocation is followed by entry into the nucleus, uncoating of the capsid and release of the new genome. Finally, a second strand of DNA needs to be synthesized and gene transcription needs to occur for successful gene expression.

Multiphoton microscopy

Conventional fluorescent microscopy is based on a linear effect: One photon is absorbed by the fluorescent molecule and then emits a single fluorescent photon 295 . In conventional fluorescence, if the excitation power is increased by a factor of two, then twice the amount fluorescence is generated as well. Multiphoton microscopy is not based on this linear effect, but relies on nonlinear interactions between the excitation laser and the fluorescent molecule. Two-photon excitation is the most commonly used multiphoton non-linear imaging technique. Two-photon absorption is the interaction of two photons with the same molecule within a time interval less than 10 -18 seconds. With normal light illumination the probability of twophotons arriving within this time interval is extremely low. Therefore, fluxes in the range of 10 20 -10 30 photons per centimeter per second are necessary to generate a two-photon interaction. In microscopy, powerful pulsed lasers generate photon fluxes with such a high density. Actually, the average laser power is kept relatively low, but since the laser photons are condensed in small time packets, these pulses contain a power high enough to produce high density photon fluxes. To compress photons not only in time, but also in space, high numerical aperture objectives are used. The combination of concentration the photons in time and in space, increases the probability of a nonlinear excitation event. Because a two-photon excitation event requires two photons to interact with the same particle at the same time, the probability of an absorption event is not linearly dependent on the excitation intensity. Moreover, the excitation light is focused at a small area. These two factors sharply decrease the probability of absorption events outside the focal area. Therefore, no pinhole is required to reduce light originating from different parts of the sample, such as in a two-photon setup, which is in contrast to a conventional confocal setup. This property allows more of the excitation laser beam to reach deep into the sample as well.

Although two-photon excitation can be used for several kinds of optical transition, it is most often used to generate fluorescence. Fluorescence is the absorption and re-emission of light by a molecule. In a standard fluorescence process, photons of the right energy interact with a sample molecule and are absorbed. This absorption of the photon and the energy it contains cause the sample molecule to transition to an excited electronic state (Fig. 19) 295;296 . This excited state is unstable and within 10 -8 seconds or shorter, the molecule returns to its ground state by emitting a new photon 297 . During this process, energy is lost, so the emitted photon contains less energy than the photon that the molecule absorbed earlier. Since the wavelength of photons is inversely correlated to the amount of energy these photons contain, the wavelength of the emitted photon is therefore longer than the excitation photons. In twophoton microscopy, each photon of the excitation laser has just one-half of the energy that is necessary to excite the molecule. Therefore the wavelengths of these excitation photons is around two times longer than the wavelength required for conventional fluorescent microscopy. Molecules that are excited by two-photon excitation emit photons with the same features as after one-photon excitation, including the same wavelength. The consequence is a reverse Stokes shift, meaning that the emitted photons contain more energy than each of the two photons that were necessary for excitation. This means as well that wavelength of emitted photons have a shorter wavelength than the excitation photons. The biphoton setup therefore requires an appropriate filter to collect the fluorescence of interest. Two-photon imaging is the best option for imaging structures in vivo. It allows deeper imaging compared to confocal microscopy, which is a strong advantage when excision of the sample is not possible 298 . Two-photon microscopy also has less phototoxicity, because first the use of two photons with a shorter wavelength allows to use infrared light and second only a small part of the sample is illuminated. Although two-photon excitation allows imaging in deeper samples and is preferred for in vivo imaging, it also has a set of limitations. Aside for the higher costs, accelerated photobleaching can cause problems in two-photon imaging 299 . Higher photon density activates more photobleaching pathways, even though this photobleaching is condensed to the imaging area and does not affect the path between laser and focal point, which is the case in confocal imaging. The photobleaching in two photon microscopy is especially detrimental in thin slices of less than 10 µm. Besides, two-photon excitation can lead to interaction with chromophores that are naturally present in the sample.

CARS imaging

CARS is a third-order nonlinear optical process consisting of two stimulating scattering steps 296 . In the first step, a pump photon (ωp) excites molecules in the the sample and brings it to a higher energetic state, but this elevated energetic state is not high enough to emit a detectable emission photon. Instead a Stokes photon (ωs), coming from a second laser beam, causes a scattering of photon on the molecules' electrons and thereby decreasing the sample's energy.

Depending on the chosen wavelengths of pump and stoke photons, the sample molecules become in a vibrational sub-state, which is specific to the chemical properties of the molecules. Coherent generation of anti-Stokes photons by another pump photon illumination can only be efficient when the frequency difference between the pump and the Stokes photons is equal to the vibrational frequency of the sample. Then, the pump photon excites the substate sample again in a specific set of molecules. Whereas the first excitation resulted in an energy state that was not high enough for the sample to emit a photon, the second excitation elevates the energy state even further, because the sample was already in a vibrational substate. After the second excitation by the pump photon, the sample will emit a blue-shifted anti-Stokes photon (ωas). The anti-Stokes signal can be detected by the detector of a microscope (Fig. 20) 300

The effect of anesthesia

Using multiphoton microscopy for in vivo experiments requires the connection to an anesthesia supply system. A regular anesthetic used for rodent in vivo experiments is isoflurane, which is a general anesthetic and needs to be inhaled by the animal. Therefore a constant flow of isoflurane needs to be administered to the animal, while it is examined by multiphoton microscopy. However, in experiments that test mitochondrial function, the effect of isoflurane on mitochondria needs to be considered.

It has been shown that isoflurane induces ROS accumulation in HEK-293 cells 301 . However, the duration of treating mitochondria appeared to be an important factor in ROS accumulation. Treatment of HEK-293 cells with 2% isoflurane for 15 to 30 minutes does not induce ROS accumulation, but treatment for a longer period of time (60 to 90 minutes) increases ROS levels. Other studies have shown that isoflurane can induce a range of other changes to cells and mitochondria, such as an increase in cytosolic calcium concentration, opening of mitochondrial PT pores, decreasing mitochondrial membrane potential and release of cytochrome C. These pathways might actually describe the pathway in which isoflurane induces ROS accumulation and potential activation of caspase-3.

On the other hand, isoflurane has also been reported to activate cardioprotective signaling through ROS sensitive pathways, because only low concentrations of ROS are induced by isoflurane 302 . These low ROS concentrations activate preconditioning pathways such as PKC, Akt, and glycogen synthase kinase β pathways. These pathways can actually lead to a delay in opening of mitochondrial PT pores, which is in contrast with the results reported by Sun et al. 301 . Previous studies also demonstrated that ROS scavengers actually abolish isofluraneinduced reduction of myocardial infarct sizes. Isoflurane increases ROS generation with complex I-, but not complex II-linked substrates in submitochondrial particles. Besides, isoflurane only alters activity of complex I, but not of the other complexes in solubilized mitochondria. Interestingly, in the presence of complex I inhibitors, isoflurane actually enhances ROS production from complex I-linked substrates. In this situation, the inhibitory effect of isoflurane on complex I is enhanced, resulting in isoflurane induced ROS production. Even though isoflurane does not alter complex II activity, isoflurane induced ROS production by complex III has been observed as well. However, an increased ubiquinone/ubiquinol ratio enhances ROS generation at complex III. Electrons are transferred to oxygen from reduced downstream cytochrome b L to oxygen in a reverse reaction via oxidized ubiquinone. Indeed, exogenous ubiquinone enhances ROS generation at complex I. In addition, inhibition of complex I by isoflurane further increases the ubiquinone/ubiquinol ratio by decreasing complex I activity and forming reduced ubiquinol, thereby enhancing ROS generation at complex III. In a similar way, inhibition of complex II increases ubiquinol and also enhances ROS generation by complex III. This shows that inhibition of upstream complexes of the ETC, including inhibition of complex I by isoflurane, can increase ROS production by complex II through changes in the ubiquinone/ubiquinol ratio. However, isoflurane has not been shown to influence mitochondrial function in the peripheral axons in vivo. Besides, only long-term exposure of isoflurane to the direct environment of cells showed an effect of the anesthetic. In addition, I checked the impact of the isoflurane anesthesia on my imaging experiments changing the anesthesia protocol to ketamine/xylazine and found no effect. This indicates that isoflurane can be used in the experiments that are discussed in part B of this thesis.

Conclusion

To investigate cellular and mitochondrial function in vivo several techniques have been developed. For example, the use of viral vectors allows for the introduction of fluorescent markers in the nervous system. These viruses, such as AAVs, will infect neurons or Schwann cells and the fluorescent proteins will be expressed at their target location, which could be the cytosol or an organelle. These fluorescent markers can be detected by microscopes that can detect fluorescent light. Since fluorescent light of various colors exist, it allows for the detection of multiple cellular structures when labelled with fluorescent tags of a different color. More recent advancements of fluorescent microscopy involve fluorescent markers that shift color in different conditions. Examples of those types of fluorescent markers are redoxbased fluorescent probes and Fluorescence Resonance Energy Transfer (FRET) based fluorescent probes. Next to standard fluorescence, label-less imaging techniques can be used for in vivo microscopy, which create a detectable fluorescent signal based on the molecular properties of the specific samples. An example of a label-less imaging technique is Coherent Anti-Stokes Raman Scattering (CARS), which can be used to visualize the myelin sheath, based on the lipid-rich structure of myelin. The type of anesthesia is an important factor in in vivo studies, but isoflurane can safely be used without impacting the results.

Part B Results

Chapter 6) Cancer-like metabolism in myelinating glia protects axons

The previous chapters provided a biological and technical background for the results that will be explained in the following chapters. These chapters are either published or under review or in preparation for publication at the time of writing. Therefore, chapter 6 till 9 are presented as separate sections with their own (short) introductions, figure numbering and reference sections.

First, I will present my research on the metabolic support of Schwann cells to axons. In chapter 3, I discussed how astrocytes, oligodendrocytes and supposedly SC could provide lactate to the axon, which axonal mitochondria can use for oxidative phosphorylation. This transport of lactate is named the lactate shuttle. However for this lactate shuttle to occur, glial cells need to produce sufficient amount of lactate. In most cells, including neurons, mostly PKM1 is expressed while PKM2 is not expressed, leading to the production of pyruvate, which is used in the TCA cycle within mitochondria to produce ATP. However in a small number of cell types, such as cancer cells, a shift from oxidative phosphorylation towards aerobic glycolysis has been observed, which is correlated with a shift in the balance from PKM1 towards PKM2. The metabolic shift from oxidative phosphorylation to glycolysis as the main source of ATP in cancer cells has been referred to as the Warburg effect and this allows for the production of ATP via aerobic glycolysis. Therefore, the Warburg effect provides a condition in which the cancer cells can proliferate faster. A healthy use of the Warburg effect has recently been proposed in the nervous system. Similar to cancer cells, it has been shown that oligodendrocytes can rely completely on glycolysis instead of oxidative phosphorylation and still remain viable. In contrast, completely glycolytic neurons are not viable. Moreover astrocytes or oligodendrocytes that cannot provide lactate to neurons induce neuronal degeneration 303 , suggesting that the eventual Warburg effect in glial cells would be essential for the maintenance of the nervous systems. This suggests that the shift in metabolism towards glycolysis that occurs in oligodendrocytes is an important mechanism for neurons. The Warburg effect allows for the production of lactate that can be transferred towards neurons. However, a direct demonstration of the role of the Warburg effect in glial cells in order to support neuronal function and survival is lacking.

In this chapter, we will demonstrate the Warburg effect in the PNS. We also show that a defective Warburg effect, which is induced by a specific and conditional knockout of PKM2 in myelinating SC, results in impaired neuronal function and defective ATP synthesis by axonal mitochondria. For this we will use viral vectors that will induce expression of fluorescent probes that detect cytosolic glucose or lactate or mitochondrial ATP.

Introduction

Metabolism is an essential parameter of neurons' function and survival 1,2,3 . Glial cells play a critical role in this matter as they metabolize glucose into lactate and export it to the axon as a rapid fuel for mitochondria 4,5,6 . Recently mitochondrial respiration was shown to be dispensable for myelinating oligodendrocytes suggesting these cells uses aerobic glycolysis for ATP production. This glycolytic metabolism also named Warburg effect, almost exclusively used by cancer cells, has the particularity to produce lactate instead of pyruvate 7,8,9 . So, do myelinating cells use a cancer-like metabolism to support axonal physiology? Here we show that PKM2, the glycolytic enzyme required for Warburg effect 10 , is expressed in mature myelinating Schwann cells (mSC), oligodendrocytes and astrocytes. Genetic ablation of PKM2 in mature mSC results in lactate shortage in nerves and in active myelinated axons. Despite an increased glucose mobilization, stimulated axons of mutant mice failed to upregulate mitochondrial ATP production. While the myelin sheath was correctly maintained mutant mice had reduced motor performances. Axonal electrophysiological properties were maintained but axon terminals retracted and motor neurons showed an increased expression of neuronal stress marker caspase 3. Further challenging lactate production using dichloroacetate prevented mutant mice functional recovery after inducing peripheral neuropathy. Our results show that mSC provide lactate to axons to maintain their physiology and this requires an aerobic glycolysis metabolism to produce the critical lactate. As oligodendrocytes and astrocytes also express PKM2 in mature brain, it is likely that a similar unusual metabolism exists in the brain. Such as in cancer cells, this may dramatically change the response of the nervous system to many external stimuli and metabolic perturbations, suggesting that part of the neurodegenerative disorders' particularities results from this metabolism of glial cells 11,12,13 .

Results

Schwann cell metabolism shifts to aerobic glycolysis upon maturation

In order to investigate the role of Warburg effect in glial cells functions in vivo in mice, we focused on myelinating Schwann cells (mSC). Indeed, these glial cells are the main cellular partner of peripheral nerves axons, so perturbations of mSC Warburg effect is likely to directly affect them. In addition, live imaging of these axons is more amenable than in the central nervous system. As most cells express PKM1, we first investigated a potential shift from PKM1 to PKM2 expression during the maturation of the mSC, when glial cells are required to maintain axonal homeostasis 14 . We collected nerve samples that contain mostly glial cells mRNA and we investigated PKM mRNA expressions using quantitative RT-PCR. PKM1 mRNA was downregulated from day 2 to day 28 while PKM2 expression on the opposite increased at the same time (Fig. 1A). To go further we immunostained sciatic nerve cross-sections with PKM1 or PKM2 specific antibodies together with E-cadherin and 2H3 antibodies, as marker of mSC and axons respectively (Fig 1B). At day 4, E-cadherin and 2H3

were not yet well expressed but PKM1 and PKM2 were both expressed in the cytoplasm of mSC that surrounded axons (Fig. 1B a,b). A similar distribution was seen in mSC at day 15 with a more explicit localization of E-cadherin (Fig. 1B c,d). 2H3 colocalized with PKM2 in axons (Fig. 1B d). At day 30 PKM2 replaced PKM1 in the cytoplasm of mSC while the reverse occurred in axons (Fig. 1B e,f). This shift from PKM1 to PKM2 expression in mSC was confirmed in fully adult mice at 5 months (Fig. 1B g,h). So both at mRNA and protein levels PKM expression in glial cells shifted from PKM1 to PKM2 isoforms when the myelinaxon couple enters the maintenance stage, suggesting mature mSC enter the Warburg effect metabolic mode.

To investigate the role of this Warburg effect in mSC we then specifically and conditionally deleted PKM2 isoform in myelinating glia of mice. A mouse strain expressing floxed PKM2 intron alleles 15 was crossed with mice expressing the Cre recombinase under the inducible and myelinating cells-specific promoter PLP-ERt 16 . Injecting tamoxifen in Cre positive and floxed PKM2 positive mice (mutant) at 1 month postnatal resulted in a significant decrease of PKM 2 expression in sciatic and saphenous nerves as seen in immunoblotting (Fig. 1C), quantitative RT-PCR (Fig. 1D), and immunostaining (Fig. 1E j,l). We used cre-negative PKM2-floxed positive littermates injected with tamoxifen in the same conditions as control animals. Both immunostaining and RT-PCR indicated that PKM1 was re-expressed in PKM2deleted mSC (Fig. 1E i,k), suggesting that these recombined cells practiced mitochondrial respiration instead of Warburg effect. In order to check whether this reversal of the metabolism had an impact on the production of lactate by the mSC we measured free lactate in sciatic nerves of mutant and control mice. Mutant mice had significantly less lactate that control ones (Fig. 1F). So, the increased consumption of pyruvate by mitochondrial respiration in mSC of mutant mice resulted in a shortage of lactate production.

Neuronal physiology is affected by PKM2 deletion in Schwann Cells

According to the lactate shuttle theory this lack of lactate should also result in shortage of axonal lactate. We investigated this assumption expressing a lactate-detecting probe (Laconic, 17 ) in peripheral axons using an AAV transduction approach. An AAV9 expressing the probe was injected in the spinal cord of new born mutant and control mice that were then treated with tamoxifen at one month. We performed live-imaging of probe-labelled axons that cross the sciatic nerve of 7 to 9 weeks old anesthetized mice (Suppl. Fig. 1). In basal conditions no difference could be seen between control and mutant animals (Suppl. Fig. 2). However when axons were challenged with electrical stimulations axonal lactate significantly dropped in mutant mice while control mice axons were able to maintain their lactate homeostasis (Fig. 2A), showing that mSC lactate is required for lactate homeostasis in axons. Beside lactate, another way axons feed their mitochondria to produce ATP is glucose-derived pyruvate. Thus, we also investigated the glucose content in axons in the same conditions using a virus expressing a glucose-specific probe (Δglu6, 18 )(Suppl. Fig. 1B). Upon electrical stimulations glucose increased in axons of mutant mice while it remained stable in control mice (Fig. 2B). This indicated that mutant mice axons mobilized glucose upon stimulations while this was not necessary in control mice axons. Taken together we conclude that mutant mice axons, unable to maintain their lactate homeostasis, mobilized glucose instead in order to sustain the production of ATP. We next investigated the production of ATP by axonal mitochondria in vivo using a mitochondria-targeted fluorescent probe detecting ATP (ATeam, 19 ). When axons were electrically stimulated to challenge axonal metabolism mutant mice mitochondria failed to increase ATP production while this production increased in control mice (Fig. 2C). Thus, in absence of Warburg effect in mSC, lactate homeostasis in axons is impaired and, despite the mobilization of glucose, axonal mitochondria respiration is impaired.

FIGURE 2 Mutant mice axons display an altered metabolic response to electrical stimulations.

The ratio of fluorescent probes (R) was normalized over the average ratio at time points before the first stimulation (R0). Segments at the top of each Control mice graphs show stimulation periods. Lines show linear regressions of the values over time starting with the last value before the first stimulation. P-values indicate the statistical significance of the linear regression test: is there a relationship between x and y (if regression line is y=ax+b, the null hypothesis is b=0)? A-Axonal lactate increased after stimulations and remained stable on the long term in Control mice (n= 20 axons, 7 mice) while it decreased in Mutant mice (n= 7 axons, 4 mice) . B-Axonal glucose remained stable over time upon stimulations in Control mice while it increased in Mutant mice (n= 6 and 9 axons respectively, 3 mice). C-Mitochondrial ATP increased over time upon stimulations in Control mice axons while it remained stable in Mutant mice axons (n= 16 and 14 axons, respectively 5 mice each). Error bars represent SEM.

Unbalanced axonal metabolism affects motor performance

Does this unbalanced axonal metabolism affect peripheral nerves functions? To answer this question, we investigated the motor behavior of mutant and control mice using Rotarod and grip test. The longitudinal analysis of mutant mice revealed a significant deficit in both tests starting 4 (Rotarod) and 10 months (grip test) after birth (respectively 3 and 9 months after PKM2 deletion in mSC) (Fig. 3A,B). So Warburg effect suppression in mSC impairs peripheral nerves functions. We also measured the nerve conduction velocity of sciatic nerves and no deficit was found (Fig. 3C). As nerve conduction velocity reflects the myelination status of the nerves, this suggested that the myelin sheath was not affected by PKM2 deletion. These data indicated that impaired peripheral nerves functions did not result from a defective myelination but more likely from axonal dysfunctions. We next investigated the electrophysiological properties of sciatic nerve axons ex vivo in artificial CSF looking at fibers recruitment, refractory period and fatigability at high frequency stimulation trains. No significant difference could be detected between mutant and control mice nerves (Suppl. Fig. 2), suggesting that peripheral nerves defects observed in mutant mice do not result from an altered conduction of action potentials along axons. However, mutant mice showed an increased number of retracted neuromuscular junctions at 12 months of age (Fig. 3D,E) and their spinal cord motor neurons displayed a higher expression of caspase 3 (Fig. 3F, Suppl. Fig. 3), a marker of neuronal stress and axonal degeneration 20 , suggesting an alteration of the axonal physiology and maintenance. Several axonopathies indicate that mitochondrial trafficking is essential for the axonal maintenance. However, tracking axonal mitochondria in vivo revealed no significant decrease in their movements in mutant mice versus control. (Fig. 3G). Mitochondrial movement decreased after nerve stimulation (Fig. 3G), suggesting their docking along the axon. Indeed, a temporary docking of mitochondria in active nerves has been observed in previous studies in vitro 21;22;23;24 , which is thought to promote replenishing cytosolic ATP. Taken together these data indicated that Warburg effect in mSC is required for the maintenance of trafficking of mitochondria in myelinated axons. This defect is correlated with neuronal stress linked to axonal dysfunction and retraction of presynaptic terminals at neuromuscular junctions.

Effect of dichloroacetate on peripheral nerve function

To investigate this further, we treated mutant and control mice with dichloroacetate (DCA), a drug that inhibits pyruvate dehydrogenase kinase (PDK) and therefore promotes pyruvate dehydrogenase activity (PDH) (Fig. 4a). As this enzyme catalyzes the production of acetyl-CoA from pyruvate, DCA promotes the mitochondrial consumption of pyruvate and decreases the production of lactate 25 . Thus, DCA treatment has been considered for patients suffering congenital lactic acidosis 26 . This treatment however leads to a reversible peripheral neuropathy, which is mimicked in rodents 27 , suggesting DCA prevents lactate supply to peripheral axons. We analyzed DCA-treated mutant and control mice behavior using the Rotarod test and measured nerve conduction velocity. Over the three weeks of treatment Rotarod performance significantly decreased but independently of the genotype (Fig. 4b). When we stopped the treatment control mice recovered over 3 weeks on the Rotarod while mutant mice did not (Fig. 4B), showing that axonal function was definitely damaged in these mice. These data confirm that Warburg effect in mSC is required for the long-term maintenance of peripheral axons physiology.

Conclusion

Mutant mice phenotype was close to axonal peripheral neuropathy such as CMT2 diseases and ageing in the behavior and to ALS, Parkinson and Alzheimer diseases in the molecular aspects (altered mitochondrial physiology, altered axonal metabolic homeostasis, neuronal stress, axon terminals defects). Therefore, we propose that myelinating cells alter their metabolism in adult animals toward the cancer-like aerobic glycolysis in order to produce the lactate required for the long-term maintenance of axonal physiology. While such metabolic switch is pretty common in cancer cells, Warburg effect has been shown to occur in activated macrophages 28 , adult stem cells 29 and retinal cells 30 . This uncommon property appears to be an adaptation to particular physiological cell conditions 31 . However, this dramatically changes the response of the system to many external stimuli and perturbations, suggesting that part of the neurodegenerative diseases particularities results from this exceptional metabolism of glial cells.

Material and Methods: Animals

Schwann cell specific ablation of PKM2 in adult mice (Plp1-cre ERT ;PKM2 f/f ) was obtained by crossing Plp1-cre ERT (JAX: 005975) 16 mice with PKM2 f/f (JAX: 024048) 15 to generate Plp1-Cre ERT ;Pkm2 +/f animals, which were in turn backcrossed with PKM2 f/f mice. All transgenic lines were kept in C57/Bl6 background. Genotyping for all mutants was performed by PCR strategies using standard procedures and appropriate primers. Animals were kept under French and EU regulations, following recommendations of the local ethics committee.

Tamoxifen administration: The recombination of the floxed allele was induced by intraperitoneal injection of tamoxifen (Sigma, T-5648 ) dissolved in corn oil (Sigma, C-8267) once every 24 hours for a total of 5 consecutive days, in 1 month animals at 180 µg/ gram mouse weight.

DCA administration: dichloroacetate (DCA, Sigma) dissolve in water was administrated by gavaging six days per week during seven weeks, at 500 mg/ kg mouse weight.

Behavioural studies

Rotarod tests: Locomotor coordination was performed with a rotating rod apparatus (Bioseb). Mice from 2 to 12 month were placed on the rotating rod and challenged using the following step: on day 1, mice was familiarized to the behavioral room and learned to stay on the Rotarod at constant speed of 8 rpm for 2 min. At day 2, locomotion performance was assessed on the accelerating Rotarod with a rate of 4 to 40 rpm for 2 min. Latency to fall was recorded on 3 trials separating by at least 15 min pauses. Data are expressed as the means from the 3 trials for each animal, normalized according to animal weight, +-standard error of the mean (SEM).

Griptest: muscular strength for the 4 limbs was assessed with a grip test apparatus (Bioseb). Mice were held by the tail and allowed to grab the grid and then pulled backwards in a horizontal plane. The maximum force (measured in Newtons) applied to the grid was recorded. Data are expressed as the means from 3 trials for each animal, normalized according to animal weight, +-standard error of the mean (SEM).

Hotplate test: sensitivity was tested with a hotplate (Bioseb). The mouse was placed on the surface of the plate heated at 55°C and the timer was started. The timer is stop at the instant the animals was reacting to discomfort by lifting its paw from the plate and shaking or licks its paw or jumps off. A maximal cut-off time of 20 s was used to prevent tissue damage. Data are expressed as the means from 2 trials for each animal separated by at least 30 min pauses, +-standard error of the mean (SEM).

Motor nerve conduction velocity measurement: mice were anesthetized with 2% isoflurane and maintain at 37°c on a hotplate. Left and right sciatic nerves were successively stimulated at the sciatic notch (proximal stimulation) and the ankle (distal stimulation) via a pair of steel needle electrodes (AD Instruments, MLA1302) with supramaximal pulses (7V) of 0.05 milliseconds delivered using a PowerLab 26T (AD Instruments ML4856). The distance between the 2 sites of stimulation was measured alongside the skin surface with fully extended legs. The latencies of the CMAP were recorded with a second pair of electrodes inserted between the digits of the hind paw and measured from the stimulus artefact to the onset of the M-waves deflection. NCV was calculated by dividing the distance between sciatic notch and ankle sites of stimulation by the subtraction of the distal latency from the proximal latency.

Histological analysis

Immunohistochemistry

For immunohistochemistry on cryosection, nerves were dissected and fixed 2hours in PFA4% at 4°C. For spinal cord and DRG, mice were first perfused transcardially with PBS, and spinal cord and DRG were dissected and fixed in PFA4% overnight or 10min respectively. All tissues were then transferred to 30% sucrose overnight before embedding into Optimum Cutting Temperature (OCT, Tissue-Tek) medium. Cryostat sections of nerves or spinal cord (12 μm) were dried at room temperature (RT) for 15 min, washed in PBS, incubated 1h at RT in a blocking solution (0.25% Triton X-100 and 10% normal Goat serum in PBS) and incubated with primary antibodies in blocking solution overnight at 4°C. For immunochemistry on teased fibers, nerves were isolated from mice, fixed for 10 min in Zamboni's fixative, washed in PBS and subsequently teased on glass slides. Slides were then dried overnight at RT and immunostained as described above. Primary antibodies: PKM1: 1/2500 (NBP2-14833, Novus Biologicals); PKM2 1/500 (SAB4200095, Sigma Aldrich); 2H3:1/80 (DSHB); Cleaved caspase3 1/400 (Cell Signaling); SMI32 1/1000; E-Cadherin 1/300 (MABT26, Merck); SV2 1/100 (DSHB). Secondary donkey antibodies coupled to Alexa Fluor 488, Alexa Fluor 594, or Alexa Fluor 647 1/1000 (Invitrogen) and TO-PRO-3 iodide 1/1000 (Invitrogen, T3605). Imaged were acquired with a Zeiss confocal microscope LSM710.

For neuromuscular junctions immunostaining, soleus and gastrocnemius muscles were dissected and fixed 20 minutes in PFA4% and incubated in 25% sucrose solution at 4°C for 24 h. Tissues were embedded in OCT and stored at -80°C before processing. Neuromuscular Junctions (NMJ) occupancy was quantified on 25-µm and 16-µm thick longitudinal tibialis and soleus sections, respectively. Muscle sections were incubated overnight at 4°C in blocking solution (2% BSA, 10% NGS, 0.1% Triton and PBS) with mouse anti-SV2 (Developmental Hybridoma Bank, 1/50). Sections were next incubated with goat anti-mouse IgG1 Cy3-conjugated antibody (Jackson ImmunoResearch, 1/500) and Bungarotoxin-488 (Molecular Probes, 1/500), and mounted in Mowiol mounting medium. NMJ were imaged on Axioplan fluorescence microscope (Zeiss) with a 20x objective.

Image analysis was done on tissue sections of at least 3 mice. Images that did show at least one caspase3 positive cells were not included in the statistical analysis.

Electron microscopy

Sciatic nerves were isolated from mice and immediately fixed in 2.5% glutaraldehyde and 4% PFA for 2h at RT, and postfixed in 2.5% glutaraldehyde in PHEM buffer (1X, pH 7.4) overnight at 4°C. They were then rinsed in PHEM buffer and post-fixed in a 0.5% osmic acid for 2h at dark and room temperature. After two washs in PHEM buffer, the cells were dehydrated in a graded series of ethanol solutions (30-100%). The cells were embedded in EmBed 812 using an Automated Microwave Tissue Processor for Electronic Microscopy, Leica EM AMW. Thin sections (70 nm; Leica-Reichert Ultracut E) were collected at different levels of each block. These sections were counterstained with uranyl acetate 1.5% in 70% Ethanol and lead citrate and observed using a Tecnai F20 transmission electron microscope at 200KV in the CoMET MRI facilities, INM, Montpellier France. Semithin (1 µm) cross section were stained with toluidine blue (Sigma-Aldrich, 89640-5G) and observed with a Nanozoomer Hamamatsu. The g-ratio was determined using the ImageJ GRatioCalculator plug-in (CIF, UNIL).

Lactate assay

To measure lactate content in sciatic nerves of Control and Mutant animals of 4 months and 12 months, equal-length nerves of 2 cm were dissected and immediately homogenized with Lysing Matix D tubes (MP Biomedical) in the lactate assay buffer and proceed according the instruction of the lactate assay kit (MAK064, Sigma-Aldrich).The absorbance at 570 nm was measured with microplate reader (CLARIOstar, BMG Labtech)

Western Blot

Sciatic nerves were dissected from mice, after removal of the epineurium and perineurium, the nerves were frozen in liquid nitrogen and conserved at -80°C. Protein were extracted from whole sciatic nerves after homogenization by sonication in standard RIPA lysis buffer and the protein concentration was measured using a BCA protein assay kit (Pierce). 20 μg proteins were directly analyzed by western blot using standard procedures with 12% SDS-PAGE and transferred on PVDF membranes for immunoblotting. Primary antibodies: Rabbit anti-Pkm1 1/2500 (NBP2-14833, Novus Biologicals); Rabbit anti-Pkm2 1/2500 (SAB4200095, Sigma Aldrich); Mouse anti-α-β-Actin 1/10000 (C1.AC-15, #A1978, Sigma Aldrich). Secondary antibodies, Peroxidase Goat anti Rabbit or Mouse (H+L) (Jackson Immuno Research) were used at 1/10000 dilution.

Semi-quantitative rtPCR

Whole sciatic nerves from 2 and 10 days old mice (P2 and P10) or endoneuria from 1 (P28), 3 and 12 months-old mice were dissected at indicated time-points. Tissues were lysed using Trizol reagent and mechanical homogenization (TissueLyser II, Qiagen). Total RNA was extracted with the RNeasy lipid tissue kit (Qiagen), and RNA quality and concentration were verified by ND-1000 spectrophotometer (NanoDrop). cDNA was synthesized using 70-200ng of the RNA with the PrimeScript RT kit (Takara), following manufacturer's protocols. PKM1 and PKM2 splicing assays at different developmental time-points were conducted as previously described in Clower et al. 32 using the following primers PKM-F: 5'-ATGCTGGAGAGCATGATCAAGAAGCCACGC-3' and PKM-R: 5'-CAACATCCATGGCCAAGTT-3' with PCR annealing step at 60°C during 35 cycles and using HotStart Taq Polymerase (Qiagen). Digestions of cleaned 502bp-PCR products (with DNA Extraction kit, Qiagen) by NcoI (NEB), producing approx. 250 bp bands revealing PKM1 transcript, and PstI (NEB), producing approx. 280+220 bp bands revealing PKM2 transcript), were performed for 2h at 37°C. Digested PCR products were resolved on a 2% agarose gel, imaged by ChemiDoc XRS+ system and quantified using Image lab software version 3.0 (Biorad). The ratio between detected intensity of uncut and cut products was used to assess relative amount of PKM1 and PKM2.

Probes construction and AAV preparation

pAAV-Laconic and pAAV-ΔGlu6 were obtained by digesting Laconic/pcDNA3.1(-) (gift from Luis Felipe Barros; Addgene plasmid #44238) 17 and pcDNA3.1 FLII12Pglu-700uΔ6 (gift from Wolf Frommer; Addgene plasmid # 17866) 18 by BamH1/HindIII (NEB) and cloned into pAAV-MCS (Cell Biolabs, Inc.) under of a CMV promoter. Clones were validated by sequencing. pcDNA-mito-AT1.03 (from H. Imamura, Tokyo, Japan) 19 was digested with XhoI/HindIII (NEB), blunted and cloned into the CMV promoter controlled pAAV-MCS. Mitochondria-targeting tags were two tandem copies of CoxVIII. pAAV9 were produced at UPV, Universitat Autonoma de Barcelona or at INSERM U1089 University of Nantes, France.

In vivo injection in spinal cord

The pups between 1 and 3 days were covered in aluminum foil and completely surrounded in ice for 3-4 min, until a completely cryoanesthetizia. Cryoanesthetized neonates were injected using a thin glass needle filled with colored viral and hold by a micromanipulator (IM-3C, Narishige Japan Group) directly in the spinal cord. 1 µL of the viral solution was injected slowly with short pressure pulses using a microinjector (Pneumatic Picopump PV820, World Precision Instruments) coupled to a 3 MHz function pulse generator (GFG8215, Langlois). The injection site was then cleaned with betadine (Vetoquinol, cat. No. 3042413) and pups were warmed in hands. When fully awake, pups were put back to the mother and the littermates.

Imaging and stimulation of saphenous nerves in living mice

3-4 weeks after the viral injection, mice were anesthetized with a constant low (1.5 l/min) of oxygen+ 5% of isoflurane in an anesthesia induction box (World precision Instrument, Ref.EZ-B800) for 5 min. Thereafter the anesthesia was maintained with a mask delivering 2% isoflurane at 0.8 L/min. The eyes were protected with Ocry-gel (TVM, cat. No. 48026T613/3). The mouse was placed on the back in a silicone mold, the hind leg was shaved and the paws were immobilized using small pins. The incision area was disinfected with betadine (Vetoquinol, cat. No. 3042413). After incision of the skin of the thigh, connective tissue was carefully removed to expose the saphenous nerve. For electrophysiological experiments, the saphenous nerve was lifted up in the middle of the thigh and isolated electrically from the surrounding muscle tissue using a plastic trip. A pair of stimulating platinum electrodes (World precision Instrument, PTM23B05KT) held by micromanipulators (U-31CF, Narishige) was carefully placed under the saphenous nerve, at the ankle. The recording electrode (AD Instruments, MLA 1203) was placed at the groin. A reference needle electrode was inserted in the groin area and a ground electrode is placed in the tail. Once the electrodes were positioned the animal was transferred in a chamber at 37°C under the twophoton microscope (LSM 7 MP OPO, Zeiss), electrodes are connected to a Powerlab 26T (AD Instrument; ML4856). Supramaximal stimulus (between 4 and 12 mA, 150 µs) was delivered at a frequency of 10 Hz during 5 min. Time-lapse of the middle portion of the exposed nerve in the thigh were acquired every 5 min during 1 h with 20X objective lens (LD CApochromat, 421887-9970, Zeiss. For Laconic, ΔGlu6, and mito-ATeam alike, a 870 nm excitation wavelength was used to obtain both emission wavelength 475 nm for mTFP (Laconic) or CFP (ΔGlu6, mito-Ateam) and emission wavelength 527 nm for Venus (Laconic, mito-ATeam) or YFP (ΔGlu6).

Image analysis

We used ImageJ software to analyze the relative lactate, glucose or mitochondrial ATP levels in peripheral axons. The acquired images for each wavelength (em. 475 nm for mTFP or CFP and em. 527 nm for Venus or YFP) were aligned using the Template Matching plugin. We defined a Region of Interest (ROI) encompassing the cytosolic area of one axon or encompassing all labelled mitochondria of one axon and the mean fluorescent intensity in the ROI was measured on both images. These light intensities were then corrected for background light intensity determined as an area within the nerve where no fluorescent signal from the viral probe can be observed. The Citrine/CFP (ΔGlu6) or Venus/mTFP (Laconic) or Venus/CFP (mito-Ateam) ratio was then calculated from the 2 values for mean light intensity. The MTrackJ plugin in ImageJ was used for analysis of mitochondrial movement. For at least 5 images over a time span of 20 minutes, the location of the same mitochondrion was marked, the distance between marked locations was measured and migration velocity was calculated. n= 65 (Control before stimulation), 75 (Mutant before stimulation), 76 (Control after stimulation), 45 (Mutant after stimulation) mitochondria.

Ex vivo nerve electrophysiology

Sciatic nerves were dissected out and transferred into oxygenated artificial cerebrospinal fluid (ACSF) containing 126 mM NaCl, 3 mM KCl, 2 mM CaCl 2 , 2 mM MgSO 4 , 1.25 mM NaH 2 PO 4 , 26 mM NaHCO 3 , and 10 mM dextrose, pH 7.4-7.5. The nerves were desheated, and cut into 2 cm segments. Nerves were then placed in a three compartment recording chamber and perfused (1-2 ml/min) in 36°C ACSF equilibrated with 95% O 2 -5% CO 2 . The distal end was stimulated supramaximally (40 µs duration) through two electrodes isolated with Vaseline, and recordings were performed at the proximal end. Signals were amplified and digitized at 500 kHz. Measurements were made once the effects had reached a steady state. The delay and duration of compound action potentials (CAPs) were calculated at half the maximal amplitude and at the maximal amplitude. For recruitment analysis, the amplitude of CAPs was measured and plotted as a function of the stimulation intensity. For refractory period analysis, two stimuli were applied at different intervals, and the amplitude of the second CAP was measured and plotted as a function of the stimulus interval. To ensure that the amplitude of the second response was accurately assessed, the first response was subtracted from all the recordings. For train stimulation, nerves were stimulated for 200ms at frequencies ranging from 100 to 1 kHz. The amplitude of each evoked CAP was measured and plotted as a function of time. Conduction velocities were estimated from latencies.

Chapter 7) Altered MAM in CMT2A neuropathy

In the previous chapter, we showed that Schwann cells produce lactate via the Warburg effect and transport this lactate to the neuron where it is used in mitochondrial respiration. Mitochondrial physiology in neurons also depends on intraneuronal cues, for example the connection between mitochondria with the endoplasmic reticulum (ER). The ER is an organelle that contains a large amount of calcium, which can be transported into the mitochondrial matrix through pores in mitochondria associated membranes (MAM). Calcium regulates multiple functions within mitochondria, such as improving the activity of enzymes that are part of oxidative phosphorylation and changing conformation of proteins that form the interaction between mitochondria and the cytoskeleton.

The MAM is an area of the ER with a high density of calcium pores, but interestingly MFN2 protein, which is a protein involved in mitochondrial fusion, is also located in there. This suggests that MFN2 also has a role in calcium transport. MFN2 mutations can cause the peripheral neuropathy CMT2A, so possibly calcium transport towards mitochondria is impaired in patients suffering from this disease. However, the effect of MFN2 mutations on ER-mitochondria interaction is not known.

In this chapter, our results on ER-mitochondria interactions in a model for CMT2A neuropathy with MFN2 deficient mitochondria will be presented and discussed as well as downstream effects such as dendrite length, neuronal survival and mitochondrial morphology.

Chapter 8) Dynamics of ATP and ROS production in healthy and neuropathological peripheral nerves

ATP and ROS production are two main functions of axonal mitochondria within peripheral nerves. However, mitochondrial activity is not stable over time, because their environment is constantly changing due to constant depolarization and repolarization of the cell membrane, which changes the composition of the cytosol in respect to glucose, lactate, calcium and other molecules. In addition, mitochondria encounter spatial differences, such as differences between nodes of Ranvier and internodes, during their migration through the axon, which may change mitochondrial ATP and ROS production.

Besides these changes in healthy conditions, mitochondria also respond to pathological changes. In the previous chapter, I described how MFN2 deficient mitochondria show impaired ER-mitochondria interactions and altered morphology and decreased migration velocity through axons. However, changes in ATP and ROS production in the CMT2A pathology have not yet been established. Furthermore, in what way pathologic demyelination changes mitochondrial ATP and ROS production in vivo is not known.

In this chapter, I will present my results on mitochondrial function in healthy peripheral nerves, in MFN2 deficient peripheral nerves and in demyelinated axons in vivo.

ATP synthases

exchanges through the plasma membrane, first through voltage-gated ion channels to depolarize, then through the sodium-potassium ATPase to repolarize [2,3]. Therefore, the production of ATP in axons is crucial for repeated regeneration of APs. Both in the central nervous system (CNS) and in the peripheral nervous system (PNS), myelination significantly reduces the energy cost of AP propagation through the sequestration of the AP firing machinery at the node of Ranvier and axon initial segment [4].

Mitochondria appear to be the main source of cellular ATP and these organelles are abundant in CNS [5,6] and PNS [7] axons. Some reports indicated that mitochondria were more abundant in the node of Ranvier [8]. In addition, axonal AP activity and axo-glial junctions regulates the recruitment of mitochondria in the nodal area [9]. Therefore, a well-accepted idea is that axonal AP activity could locally stimulate ATP production in axonal mitochondria [10]. However, this nodal enrichment in mitochondria remains controversial [11] and the dynamic of local mitochondrial ATP production following AP is actually unknown.

As an intrinsic by-product of ATP production, mitochondria also produce reactive oxygen species (ROS) [12,13]. The mitochondrial electron transport chain (ETC) and NAPDH oxidases are the major sources of superoxide (O 2 •-), a highly reactive type of ROS, within cells [14]. O 2 Indeed ROS can directly oxidize and damage DNA, carbohydrates, proteins or lipids [17].

In several neurodegenerative diseases and axonopathies, such as Parkinson's disease [18], multiple sclerosis (MS) [19] and Charcot-Marie-Tooth diseases (CMT) [20], mitochondrial dysfunction and increased levels of ROS have been shown to be involved. Whether axonal mitochondria are involved is still unclear and investigating the production of ATP and ROS by these mitochondria in active or diseased axons in real time in vivo would constitute a first answer.

The importance of mitochondrial homeostasis for the normal function of myelinated axons [9,21] is highlighted by mutations in the mitofusin 2 (MFN2) gene. Indeed, MFN2 has several functions in mitochondria physiology, among which mitochondrial fusion [22] and transport along axons [23], and mutations in its gene result in axonal form of peripheral neuropathy (CMT2A) in humans and in mice [24,25]. In particular, MFN2 loss-of-function in axonal mitochondria decreases the expression of oxidative phosphorylation subunits [26], suggesting that MFN2 mutations also affect ATP and ROS production. However, this remains to be shown in vivo.

MS is a chronic inflammatory disease affecting myelinated tracts in the brain [27]. MS is characterized by demyelinated lesions resulting from the loss of the myelin sheath and the progressive form of MS shows successive events of myelin degeneration and regeneration that damage axons [27]. A common hypothesis is that demyelinated axons are more demanding in ATP in order to maintain their functions and their integrity [28]. Recent data showed that axonal mitochondria complex IV activity is increased in MS lesions and the total mitochondrial mass is increased [29], suggesting that axonal mitochondria are indeed producing more ATP. In addition, while activated microglia and macrophages are thought to be the initial source of ROS, axonal mitochondria dysfunction and an increase of mitochondrial ROS contribute to progressive MS [30]. Taken together this suggests that the myelin sheath plays a significant role in the maintenance of axonal mitochondria homeostasis and in particular in the production of ATP and ROS in mitochondria.

To answer these questions, we set up an in vivo approach using a H 2 O 2 -sensitive GFP (roGFP-Orp1) [31] and a fluorescent ATP sensor (ATeam) [32]

Results

Validation of the mito-roGFP-Orp1 and mito-ATeam probes

We used ATeam, which is a genetically-encoded fluorescence probe based on fluorescence resonance energy transfer (FRET), to detects ATP [32]. This probe consists of the cyan fluorescent protein (CFP) mseCFP, the yellow fluorescent protein (YFP) variant monomeric Venus, both linked at the ε-subunit of the Bacillus subtilis FoF1-ATP synthase. In the presence of ATP, the ε-subunit retracts to bring the two fluorescent proteins close to each other, thereby increasing FRET efficiency. The conformational change is reversible. Relative ATP levels are measured using the Venus/CFP fluorescence ratio.

Redox sensitive GFPs (roGFPs) harbor an engineered dithiol/disulfide switch on their surface, which determines their wavelength of excitation [33]. RoGFP2 has been converted into an H 2 O 2 specific probes by fusion with the microbial H 2 O 2 sensor Oxidant Receptor Peroxidase 1 (Orp1) [31]. The reaction is reversible via reduction by cellular thioredoxin (Trx) or GRx/GSH [34]. Relative suggesting that this H 2 O 2 production reflects the oxidative phosphorylation process.

ATP and H2O2 production are altered in CMT2A neuropathic mice

We recently showed that MFN2 R94Q mice, a model for CMT2A disease [25] where MFN2 is defective, displayed altered mitochondria motility and clustering in peripheral nerve axons [41]. We used this model to measure the level of ATP in axonal mitochondria of myelinated axons in neuropathic conditions. In non-stimulated conditions, control and mutant mice axonal mitochondrial ATP production were similar (Fig 4a). After the first stimulation, similar to control mice, mitochondrial ATP production increased in some axons whereas others did not respond, resulting in statistically non-significant variation (Fig 4b). However, after a second period of nerve stimulation, as opposed to controls, MFN2 R94Q axonal mitochondria failed to upregulate ATP production (Fig 4b-c). This indicates that dysfunctional MFN2 does not hinder the basal production of ATP by axonal mitochondria but impairs their ability to up-regulate their production in response to axonal activity.

Similar to ATP, we found no difference between controls and MFN2 R94Q mice in the basal H 2 O 2 levels (Fig 4d [42,43], these data indicate that oxidative phosphorylation is decoupled from ATP production in mitochondria of these neuropathic mice.

ATP and H2O2 production in nodes of Ranvier and internodes

The enrichment of mitochondria at the node of Ranvier and whether these nodal mitochondria are more metabolically active remains controversial. In order to detect potential spatial differences between nodal and internodal mitochondria, we used CARS microscopy. Gaps in CARS signal between two myelinated internodes are nodes of Ranvier (Fig 5a) [38,43,44].

Combining two-photon imaging of fluorescent probes with CARS imaging allowed us to analyze mitochondrial physiology in nodes of Ranvier versus internodes defined as areas distant of more than 5 µm from the node (Fig 5a , suggesting again a decoupling between oxidative phosphorylation and the production of ATP in mitochondria of these mutant mice.

ATP and H 2 O 2 production in mitochondria of demyelinated axons

The myelin sheath is critical to maintain the node of Ranvier on myelinated axons and is therefore likely to play a significant role in the homeostasis of axonal mitochondria. As demonstrated in Fig 5a, the myelination state of the axons in the mouse sciatic nerve can be assessed in vivo by CARS imaging. We therefore investigated how this demyelination affects axonal mitochondria physiology in vivo. To induce demyelination we used lysophosphatidylcholine (LPC) and we followed the production of mitochondrial ATP and H 2 O 2 in axons during demyelination and during the restoration of the myelin sheath (remyelination). LPC, which is a signaling molecule resulting from the hydrolysis of phophatidylcholine by phospholipase A2 [45] acts in mSC and oligodendrocytes as a demyelinating signal [46,47,48]. LPC was injected in the sciatic nerve of adult mice and demyelination occurred immediately to culminate one week after the injection with the formation of myelin ovoids and debris as seen using CARS (Fig

Discussion

Although it has been shown that PNS axonal mitochondria, like most mitochondria, produce both ATP and ROS, the regulations of their production are still not well understood in vivo and in situ. We used a combination of viral delivery of mitochondria-targeted fluorescent probes to active peripheral neurons and two-photon and CARS live imaging in mice to fill that gap. The mito-ATeam and mito-roGFP-Orp1 fluorescent probes had been previously validated in vitro [51] and in vivo [33] in other systems but we also validated them in vivo in our system. We did not attempt to measure absolute values of ATP or H2O2 with each probe fluorescence in vivo because firstly, this was difficult as probes are targeted to mitochondrial matrix and secondly, it was not necessary as we aimed to compare different genotypes and conditions in particular with time. So only relative values obtained in the same imaging conditions are shown here. We found that the range of detection was large enough to cover the changes occurring in axonal mitochondria in vivo. Moreover the probe sensitivity was sufficient to observe variations in real-time using a 5 minutes delay between measures. This delay was partly imposed by relative low speed of our imaging system (1 minute to create 10 scans of 200µm 2 over 40 μm depth for each wavelength), but also by the intrinsic limits of our set-up: electrical stimulation induced slight contractions of saphenous nerve surrounding muscles which made the pictures blurred during the stimulation. The probes alterations were also reversible fast enough to observe changes in both directions. Mito-ATeam was stable, but mito-roGFP-Orp1 showed a slight increase of its fluorescence ratio over time under our experimental settings. These results are not explained by photobleaching or degradation of the probe, since the total fluorescent signal does not decrease. This change in ratio of fluorescence may be caused by air oxidation [52]. However, buffered artificial cerebrospinal fluid as an incubation medium for the saphenous nerve did not prevent the probe oxidation.

Nonetheless, this slow oxidation of mito-roGFP-Orp1 had negligible effects on our experiments, since it was too slow to cause significant changes in our relatively short term imaging.

Axonal activity induced stereotypic changes in ATP and H 2 O 2 production by axonal mitochondria. Shortly after the stimulation, H 2 O 2 production increased and dropped immediately and then ATP increased for several minutes. This was stereotypic, because it occurred almost identically after both stimulations we did. After the first stimulation, not all axonal mitochondria produced more ATP, suggesting that either our stimulation protocol was not mobilizing enough axons or axonal mitochondria are deactivated, similar to some ion channels. This limitation was lifted by the second stimulation. On the attempts we did for further stimulations, sometimes the ATP level was maintained at high range and sometimes mitochondria did not respond anymore. The reason for these variations in the long term is unknown and we did not pursue this further. However, taken together, this suggests that ATP production in axonal mitochondria is positively correlated with the level of axonal activity and can eventually reach a plateau in our conditions. Due to technical reasons we could not measure H 2 O 2 levels in mitochondria after several stimulations. However, looking at the first two stimulations, we conclude that H 2 O 2 production in mitochondria is also positively correlated with axonal activity but not cumulative. It would be interesting to look further for the molecular mechanisms that allow mitochondria to detect axolemma depolarization or ion channels activity on a very short period of time (around one minute in our detection capacities). This fast kinetic suggests the role of ions, such as calcium ions, which are mobilized during AP firing at the node of Ranvier [53], or kinases. A mechanism involving calcium has been shown to anchor mitochondria at the node of Ranvier [54], suggesting that mitochondria are associated with the axolemma there. Consistently, we found nodal mitochondria to be more metabolically active than internodal ones. However, our data following stimulations were recorded on mitochondria independently of their location on axons. As the node is much smaller than the internodes (1 mm versus 600 mm in average in the mouse nerve) the probability we recorded nodal mitochondria is extremely small. So the molecular mechanism that mobilizes mitochondria after AP firing has to exist also in internodal mitochondria.

Looking at the known origin of ROS in mitochondria, our data suggest that the H 2 O 2 production we detected in axonal mitochondria reflects the loading of the mitochondrial matrix with protons before the production of ATP. However, since the H 2 O 2 production drops very quickly and, contrary to the ATP production, it is not cumulative, a more complex scheme has to be taken in consideration. Indeed, H 2 O 2 is formed by SOD enzyme from O are removing it quickly. In addition, these enzymes' activities change following fast posttranslational modifications such as phosphorylation [56] or environmental changes [57].

While this may explain why H 2 O 2 levels are not incremental following several stimulations, further investigation will be needed to clarify underlying mechanisms.

Several CNS and PNS neuropathies are linked to mitochondrial dysfunctions in neurons [18,19,20]. Our data show that the electrical activity of neurons directly regulates the physiology and the respiration of axonal mitochondria. As axonal mitochondria are the most abundant among neuronal mitochondria, this suggests that axonal mitochondria dysfunctions may severely impact neuronal physiology and survival. We evaluated this hypothesis using a couple of peripheral neuropathy mouse model, such as a model of CMT2A disease expressing mutant MFN2 R94Q [25]. While in vitro data had shown increased H 2 O 2 levels [58], but no change in ATP production [59] in neuronal mitochondria of these mice, in vivo we did not measure significant differences in these parameters between resting MFN2 R94Q mice and control mice. A first explanation for this discrepancy is that cultured cells are living in an environment that is significantly different from the in vivo conditions. This is especially true in terms of metabolism [60]. Another explanation is that mitochondria physiology is altered, but this can only be seen when mitochondria are challenged. Environmental changes in culture may constitute this challenge, while more physiologically relevant challenges have to be found in vivo. Our electrical stimulations were physiologically meaningful challenges for axons and, similar to control mice, we observed ATP levels increasing in several axons of mutant mice, but not all, after a first nerve stimulation. However, while control mice mitochondria increased their ATP production further following a second stimulation, no increase was observed in mutant mice mitochondria. We have shown that Mitochondria Associated Membranes (MAM), which mediate interactions between the endoplasmic reticulum and mitochondria [41], are impaired in these mutant mice. So we suggest that an impaired Ca 2+ uptake by mitochondria through MAM [61] could alter the ATP production as it has been shown that calcium is required for the TCA cycle [62]. However, mutant mice mitochondria were still able to produce a large amount of H 2 O 2 following both stimulations, suggesting the ETC and therefore the TCA cycle are working properly. Yet this H 2 O 2 production was completely abnormal: it was largely delayed and was sustained for over at least 10 minutes. As discussed before the levels of H 2 O 2 result from an equilibrium between several parameters, so it is difficult to explain this abnormal production. Nevertheless, a relationship between mitochondrial shape and ROS has been shown in vitro [63]. It has been suggested that smaller mitochondria result in intra-mitochondrial redistribution of cytochrome c and the pro-aging molecule p66shc that acts trough ROS upregulation [64,65]. Smaller fragmented mitochondria, such as MFN2 mutants [63], correlate with a malfunctioning antioxidant system and an increased production of ROS [58,64]. In any case, our data show that an altered MFN2 function lead to the dysfunction of axonal mitochondria only when axons are firing. This dysfunction is strongly deleterious as mitochondria don't produce more ATP but show a sustained production of deleterious ROS. This functional decoupling between ATP and H 2 O 2 production, occurring in the most abundant mitochondria of the neurons and in conditions of normal firing activity, is likely to be a significant cause of the neuronal dysfunction in this peripheral neuropathy.

Looking at the node of Ranvier of MFN2 mutant mice, we found that, similar to control mice, nodal mitochondria were more metabolically active than internodal ones. As MFN2 mutant mitochondria do not increase ATP production in active axons, this suggests that nodal mitochondria are intrinsically more active even in absence of firing. This is consistent with the fact that motile mitochondria slow down in the nodes of Ranvier [6] and that they have a different morphology [66]. Moreover, nodal mitochondria may participate to axonal degeneration since this process starts in nodes of Ranvier due to high ROS levels [67]. It would have been interesting to investigate changes occurring in ATP and H 2 O 2 production of nodal mitochondria after electrical stimulation but this was impossible due to the small size of the node and the change of focus that occurs following stimulations.

The myelin sheath that covers axons appears to be an important regulator of axonal metabolism and of mitochondria physiology. Indeed in MS, demyelination alters axonal mitochondria [19,29,68] and we show here that nodes of Ranvier devoid of myelin house more metabolically active mitochondria. To go further, we therefore investigated the impact of the loss of myelin on axonal mitochondria. LPC was the right tool because it induced demyelination without destroying axons [46,47,48]. LPC-induced demyelination resulted in a decrease of ATP levels in axonal mitochondria. This decrease was strongest after two weeks and was reversed by 3 weeks, showing that ATP production within neurons is impaired until complete remyelination. This decrease was unexpected, since AP propagation in demyelinated axons is more energy demanding: the nodal machinery and the Na + /K + ATPase are diffused all along the axolemma and all the energy-effective benefits of myelination are lost [69]. In demyelinated area of brains affected by MS, an upregulation of ETC complexes protein expression and more complexes I and IV activity [68] was observed. Moreover, demyelinated axons showed more mitochondria [29], which is consistent with a higher energy demand.

Nevertheless, H2O2 levels strongly increased in axonal mitochondria of fully demyelinated axons in absence of general oxidative stress in the tissue. This decoupling of mitochondrial oxidative phosphorylation and ATP production is in accordance with previous studies that reported a decreased ATP production, together with increased ROS levels in demyelinating diseases [70]. This suggests that actually, mitochondrial ETC complexes are more active and produce more ROS, but this does not lead to more ATP production. Such a mechanism has been shown in particular conditions where the goal of mitochondrial activity is to produce heat. Uncoupling proteins such as UCP2 permeabilize the inner mitochondrial membrane to dissipate the proton gradient [71]. This may not be the sole utility of uncoupling as this process has been shown to occur in several neurodegenerative diseases [72]. However, so far we failed to detect unusual UCP2 expression in demyelinated nerves. The high production of H 2 O 2 during demyelination is probably not without consequences as ROS can alter the function of the ATP synthase [73]. This may explain why ATP production remains weak in mitochondria of axons that are being remyelinated two weeks after LPC injection, despite that ROS production is back to normal. This suggests that during demyelination, axons may recruit more mitochondria [29], or use another metabolism, such as aerobic glycolysis, in order to cover their need in ATP.

While very little amount of data exists on the homeostasis of axonal mitochondria in demyelinated axons of the PNS, in the CNS, demyelination in progressive MS results in axonal mitochondria dysfunction and ROS increase [74]. The data we collected in PNS myelinated axons on ROS production by axonal mitochondria are consistent with this. This suggests that, despite the large difference that exists between PNS and CNS myelinating glia, their similar role in segregating axonal firing machinery at nodes of Ranvier is essential for mitochondria homeostasis in axons. Nevertheless, the electrical isolation of the axon and the formation of the node of Ranvier is not the only function of the myelin sheath as it also largely participates to the metabolic homeostasis of the axon through the lactate shuttle process [75,76]. This is particularly true for the CNS myelin [76]. In addition, MS disease is significantly different from peripheral nerve diseases, in particular because of the important role played by CNS glial cells astrocytes and microglia. So, it would be essential to confirm

Mouse strains

Either Swiss mice (Janvier, France) or MFN2 R94Q mice [25] were used in the performed

In vivo virus injection in spinal cord

A thin borosilicate glass capillary (Harvard Apparatus, Ref. 30-0016) was pulled with a Vertical Micropipette Puller (Sutter Instruments, P30-682) to form a glass needle. The glass needle was filled with viral solution (1 μl) and a 1 day old mouse is restrained in a position that exposes the lower back. The needle is injected through the skin using a micromanipulator and introduced into the spinal cord. The viral solution was injected over 2 minutes with short pressure pulses using a Picopump (World Precision Instrument) coupled to a pulse generator.

After injection, the injection site is cleaned with betadine solution (Vetoquinol, cat. no.

3042413) for disinfection and the pup is placed back with its mother and littermates.

Immunohistochemistry

One month after the viral injection, the sciatic nerve was dissected and fixed in Zamboni's fixative [77] for 10 min at room temperature. After fixation, the dissected sciatic nerves are washed in PBS and incubated in successive glycerol baths (15, 45, 60

Imaging of sciatic nerve in living mice

One month after the viral injection, the mice were anesthetized with a constant flow (1.5 l/min) of oxygen + 5% of isoflurane in an anesthesia box (World Precision Instruments, Ref.

EZ-B800) for 5 min. Thereafter the anesthesia was maintained with a mask delivering 2% isoflurane at 0.8 l/min. The eyes were protected by eye protection gel (Ocry-gel, TVM, cat.

no. 48026T613/3). Intraperitoneal injection of 0.1 mg/kg buprenorphine was used for presurgery analgesia. The mouse was placed in a silicone mold, lying on its belly, shaved on his hind paw and the paws immobilized using small pins. The incision area was disinfected with betadine solution (Vetoquinol, cat. no. 3042413). The skin was cut using scissors, fat tissue removed, the gluteus superficialis and biceps femoris muscles were separated to reveal a cavity crossed by the sciatic nerve, and the sciatic nerve was lifted up using a small spatula. A long plastic strip was placed underneath the sciatic nerve and this strip fixed using magnets.

Demyelination procedure

After imaging of the sciatic nerve in vivo for at least 30 minutes, 5 µl of 1 mg/ml lysophotphatidyl choline (LPC) was injected directly into the sciatic nerve using a Hamilton syringe (Ref. 80930) to induce demyelination. Injection of 5 µl PBS is used as a negative control. After injection, the nerve was placed back to its original location in the body. The skin of the incision was realigned together using a blunt scalpel and stapled along the wound with two clips (Fine Science Tools; 12020-00). The area around the wound was disinfected again with betadine solution. The anesthesia mask was removed and the mouse was monitored until it had woken up. Motor function of the injected and non-injected hindlimb was followed by lifting the animal by the tail. At 1 week after LPC injection, the non-injected hindlimb showed a normal postural reflex characterized by spreading of leg whereas the injected hindlimb showed abnormal reflexes such as tremors, clasping or retracting of the paw. This behavior has been described and linked to deficient myelination in the PNS in previous studies [78]. The severity of abnormal hindlimb function decreased at 2 weeks after LPC injection and no difference in leg function between the injected and non-injected hindlimb could be observed anymore at 3 weeks after LPC injection.

Saphenous nerve stimulation in living mice

After skin incision and removal of connecting tissue, the saphenous nerve is lifted up and isolated using a plastic strip (Fig 2a). To stimulate the saphenous nerve, two platinum kapton microscope imaging parameters were maintained over all different regions we imaged.

Images, acquired with ZEN software (Zeiss), were saved in .czi format.

Data and statistical analysis

We Upon injection of CellROX deep red into the sciatic nerve, fluorescence signal could be detected indicative of oxidative stress. No significant differences were observed between the time points of the demyelination process, nor nerves injected with PBS. In the contralateral nerves, which were not injected with LPC or PBS, a fluorescence signal was detected as well. Before CellROX injection, no fluorescence signal was observed. At the injection site, where tissue is locally damaged due to insertion of the syringe, a much stronger fluorescence signal is observed than at the region where mitochondrial H 2 O 2 was measured (Fig. 6C).

Chapter 9) In vivo introduction of viral vectors into mouse sciatic nerves

Next to the expression of fluorescent probes in the neurons to observe the axonal cytosol or axonal mitochondria, fluorescent probes can be expressed in myelinating Schwann cells as well.

In this chapter, I provide a protocol to inject viral vectors into the sciatic nerve of mice in order to infect Schwann cells. In this protocol, a distinction is made between sciatic nerve injection in adult mice and mouse pups. This protocol has been published in Methods of Molecular Biology -Myelin in 2018.

Expression of fluorescent probes in Schwann cell mitochondria

Chapter 10) Discussion

Combining the data of different experiments and projects, I propose an attempt to create a model of the axon metabolism and axonal mitochondria physiology in normal and in some disease conditions. In addition, I would like to illustrate how its metabolism and energy production changes in response to different stimuli.

Energy production in peripheral nerves

The production of energy in the form of ATP is essential for healthy axonal physiology. Indeed, axons have to regenerate and propagate action potentials all along their length and they act as a major trafficking highway for mitochondria and vesicles between the cell body and the presynaptic terminals. However, axons are very fine structures (up to 2-3µm) compared to a full cell (more than 20µm usually), isolated in middle of the nervous system, so finding and storing metabolic resources may be difficult. Indeed CNS axons are known to be unable to store glucose, such as glycogen. So where do those metabolites come from? Also, what happens to the neuron when metabolites are scarce? We have shown that myelinating Schwann cells shift from a PKM1 driven mitochondrial respiration metabolic state to a PKM2 aerobic glycolysis metabolic state when they enter a mature state, maintaining myelin and nurturing axons. When PKM2, the enzyme that favors lactate instead of pyruvate as glycolysis end product, is knocked down in Schwann cells, the Warburg effect is suppressed and pyruvate, which remains the sole glycolysis end product, is only used for oxidative phosphorylation. As a consequence, less lactate is produced by mSC. This leads to an abnormal metabolism in axons. Indeed, the cytosolic content of lactate decreases in axons upon successive stimulations of the nerve, indicating that the lactate that is used during the firing of action potentials is not replenished in absence of the Warburg effect in Schwann cells. Meanwhile, cytosolic glucose is recruited as an alternative energy source, but this is not sufficient to allow the upregulation of mitochondrial ATP production. As a result, oxidative phosphorylation is affected as we have shown by decreased ATP production in axonal mitochondria of PKM2 knockout mice.

What are the functional consequences of this glial lactate-starvation metabolism in the axon?

The abnormal axonal metabolism and decreased ATP production by axonal mitochondria result in the development of motor problems over time. These motor problems are not the result of impaired myelin function as shown by normal nerve conduction velocity, but rather due to impaired neuronal function. This result was verified using DCA to decrease the levels of lactate even further. We showed that addition of DCA results in a drop of motor performance in both PKM2 mutants and controls due to a global shortage of lactate, but controls were able to recover motor performance after halting DCA administration, whereas mutants could not. This result indicates that motor performance is indeed dependent on Schwann cell lactate production through Warburg effect.

The glial lactate-starvation axonal metabolism causes impaired motor performance, but how exactly? While we could not detect any significant change in the propagation of action potentials, We found that the nerve endings retracted in PKM2 knockout mice, indicating that the maintenance of nerve endings at the neuromuscular junction is impaired. In addition we show an increased percentage of motorneurons showing caspase 3 expression, indicating they experience stress. Finally, the transport of mitochondria along the axon was also affected as more mitochondria were found to be stuck in mutant mice axons.

This represents the first confirmation that lactate shuttle is also essential in the PNS to provide SC lactate to the myelinated axons. Indeed so far only the expression of MCT transport has been reported to support this theory in the PNS. In addition, this work provides the first demonstration of the existence of an aerobic glycolysis metabolism in a glial cell that supports axonal function. Indeed, while several indications exist that astrocytes and oligodendrocytes are glycolytic cells, suggesting they may use Warburg effect, we show here that PKM2, the enzyme required to produce Warburg effect, is expressed in mature mSC and is required there for the maintenance of functional axons in the PNS. This discovery has several implications for the understanding of the PNS biology and by extrapolation of the CNS. Indeed the Warburg effect may be a useful target for therapeutic strategies against PNS and CNS diseases, especially in neurodegenerative diseases in which cell fate is determined by prosurvival and apoptotic signals. Stimulating the Warburg effect and the subsequent delivery of lactate to the axon could be a strong pro-survival signal, which could prevent neurodegeneration.

The environment changes the metabolic rate of mitochondria

So we showed that ATP production by axonal mitochondria relies on the production and transport of lactate by Schwann cells. This indicates that axonal mitochondria activity is regulated by external and eventually internal stimuli. By extension we also reasoned that reactive oxygen species produced by active mitochondria as the other product of oxidative phosphorylation could also be influenced by these stimuli.

For a long time, mitochondria at the nodes of Ranvier were supposed to be more active based on structural differences observed between internodal and nodal mitochondria and on the important role of ATP in the maintenance of the action potential propagation process that occurs at the node. However, as no one had ever seen ATP production in these mitochondria, this remained merely an assumption. We looked at the production of ATP in axonal mitochondria and compared different spatial locations of these mitochondria within the axon of the same neuron. We found that the location of mitochondria changes the intrinsic ATP and ROS production. Mitochondria that are located in the node of Ranvier, an area where the axon is not myelinated, is rich in ion channels and where the electric signal of action potentials are reinitiated, produce more ATP and ROS than in mitochondria located in the internodes. This observation suggests that these mitochondria are intrinsically more active, possibly because the need for ATP is higher in this region. Why the energy need in nodes of Ranvier is higher than in internodes is not clear yet. Nodes of Ranvier are rich in voltage gated ion channels, but these channels do not require ATP. Na + /K + ATPase activity does require ATP, but whether Na + /K + ATPase is enriched in the node of Ranvier is under debate. Another hypothesis is that in nodes of Ranvier more ATP is used for calcium buffering by the ATP-utilizing calcium pump SERCA of the endoplasmic reticulum.

Then, what happens to ATP production by axonal mitochondria when the axon is active and conducts action potentials? We showed that after multiple successive periods of induced action potentials, mitochondria upregulate their ATP production. Likely, this upregulation of ATP production is an adaptation to an increased energy dependency of the axon during action potential firing. However, the upregulation of ATP is not limited to mitochondria located in the nodes of Ranvier, which shows that action potential firing has an effect on mitochondria in internodes as well.

In addition to increased ATP production, mitochondrial ROS production increases for a short period of time after each period of induced action potential firing. Naturally, these ROS are the byproducts of ATP production as it immediately precedes the increase in mitochondrial ATP. However, because ROS have signaling effects, these increase of ROS levels potentially results in downstream effects. For example, ROS levels have an influence on gene expression. However, the mitochondria in our model are located in the axon, thus far away from the nuclear DNA that is in the cell body. Therefore, the effect of ROS produced by axonal mitochondria on neuronal gene expression in neurons is nihil. The signaling roles of ROS more probably influence local proteins activity. The elevated ROS levels may as well have signaling effects on Schwann cells, because of the close physical interaction they have with the axons.

While mitochondrial activity increases, they also slow down. This docking of axonal mitochondria may be the result of ROS signaling, because the motor protein KIF5 is sensitive to ROS levels and therefore, elevated ROS levels can cause a disconnect of mitochondria from the microtubule. In parallel, neuronal firing causes an influx of calcium into the cytosol though voltage-dependent Ca 2+ channels. These calcium ions interact with Miro, hence causing the docking of mitochondria. Possibly, mitochondria halt their migration to replenish cytosolic ATP locally.

Our results show that indeed mitochondrial physiology is influenced by multiple stimuli such as spatial differences and nerve activity. However, the levels of ATP and ROS, the two main products of mitochondria, are influenced in an independent matter. Whereas ROS production shows spikes, the increase in ATP production happens gradually, showing that the dynamics between ATP and ROS levels are not the same. Since mitochondrial ROS production is the result of proton translocation by the electron transport chain, this implies that a formation of a stronger proton gradient precedes increased ATP production. Furthermore, the level of H 2 O 2 is not elevated any longer once ATP production is upregulated, probably because the antioxidant defenses that are present inside mitochondria are sufficient to adapt to elevated ROS levels and reduce those levels quickly. Together, these results show that the level of ROS cannot simply be extrapolated from the amount of ATP produced or vice versa. The model of mitochondrial physiology is more complex than often is assumed.

Axonal mitochondria in pathological conditions

So mitochondria can change their ATP production depending on their location or on nerve activity. Moreover, this increase of ATP production is without too many negative ramifications for we showed that ROS levels are increased for only a very short period of time. However, pathologic conditions change the healthy adaptation of mitochondria to physiological stimuli.

Deficient mitofusin 2, a protein involved in mitochondrial fusion, results in CMT2A or CMT6A disease, which characterized by motor and sensory deficits. We have shown that in mice the morphology of these fusion-deficient mitochondria changes: they are shorter in length and abnormally clustered in peripheral nerve axons. MFN2 deficient mitochondria also show decreased transport velocity. This may be the result of MFN2 no longer interacting with the Miro-Milton complex, the main protein complex that connects mitochondria to the cytoskeleton. Now, we have shown how this altered morphology correlates with pathologic physiology too. First, we have demonstrated that the contact area of MFN2 mutant mitochondria with the endoplasmic reticulum becomes impaired, which may result in decreased transport of calcium from the ER to mitochondria. Since calcium is a co-factor for oxidative phosphorylation, decreased calcium transport may cause impaired mitochondrial ATP production. Indeed, MFN2 mutant mitochondria fail to upregulate ATP production upon action potentials, even though basal ATP levels are the same. Interestingly, MFN2 mutations have critical consequences for ROS production too. The upregulation of ROS was delayed compared to littermates that did not express the MFN2 mutation, but elevated levels of ROS were maintained in mitochondria for a longer period of time, probably resulting in increased oxidative stress. In short, mitochondrial ATP levels were maintained while ROS levels soared. This shows that the MFN2 mutation causes a pathological decoupling of ATP and ROS production. These results also imply that mitochondrial shape correlates with mitochondrial physiology. Even though we show that MFN2 mutations affect the contact between mitochondria and ER, mitochondrial shape, transport and physiology in peripheral axons, these data do not prove a causal relationship between ER-mitochondria contact and mitochondrial physiology. Possibly, the observed changes are independent effects of the MFN2 mutation.

A second pathological condition was mimicked through the induction of demyelination. This mimics multiple sclerosis and CMT1 pathology that affects the myelin sheath around axons in the CNS and PNS respectively. In this condition, ATP decreased and ROS production increased. This decoupling of ATP production and ROS production suggests or that mitochondrial ETC complexes are more active, the ETC complexes produce more ROS or the antioxidant defense are less efficient.

In first hypothesis, as increased ROS production does not correlate with more ATP production, this suggests that protons reenter the mitochondrial matrix without driving ATP production by passing through ATP synthase. Such a mechanism has been shown in particular conditions where the goal of mitochondrial activity is to produce heat. Uncoupling proteins such as UCP2 permeabilize the inner mitochondrial membrane to dissipate the proton gradient, without the production of ATP molecules. In the second hypothesis is the rate of proton translocation normal or lower, but the percentage of electrons that leak back into the matrix and form ROS is higher, possibly via reverse electron transport. This reverse electron transport could be the result of altered metabolism, because the lactate shuttle becomes impaired due to the reduced connection between mSC and axon. Mitochondrial physiology would then resemble the mice with an impaired Warburg effect. Indeed both models showed a decreased ATP production by axonal mitochondria. Unfortunately we did not check the production of ROS in PKM2 knockout mice. In the third hypothesis, an increased ROS production is the result of a less efficient antioxidant defense. Indeed, myelin has antioxidant capacities, but an absence of this antioxidant defense should not affect the antioxidant capacity within mitochondria, which is where we measured the ROS levels. Nonetheless, decreased SOD levels have been measured in demyelinating disorders and SOD2 may even switch into a pro-oxidant in manganese deprived conditions.

Taken together our combined data show that the presence of a myelin sheath is an essential factor that determines the function of axonal physiology and mitochondria. Indeed, a healthy myelin sheath allows for neuronal maintenance and salutatory conduction, while it regulates mitochondrial activity.

Perspectives to neuropathies

In order to find a solution for diseases that involve an impaired function of the nervous system, a better understanding of the dynamics between ATP and ROS production is lacking.

In the model that I lay out in this thesis, we show that interaction between cell types is an important factor in neuronal function, and these interactions between cell types have often been overlooked or ignored.

Indeed, the role of the Warburg effect in the nervous system has never been directly investigated and its role in the development of neurodegenerative diseases is a research field that is practically untouched. In addition, the interaction between myelinating cells and axons has widespread downstream consequences, as observed with the change in mitochondrial physiology in axons and the subsequent behavioral defects.

Secondly, for future research it should be understood that the physiology of axonal and, by extrapolation, neuronal mitochondria is highly sensitive to their environment and since alterations in their ROS and ATP production have such widespread impact on neuronal function, mitochondria play without a doubt a central role in the development or progression of symptoms of many neuropathies of the PNS.

Furthermore, we showed that mitochondrial function is not as static as often is assumed. In many research projects, either ROS or ATP is measured, but detection of both ATP and ROS in the same experimental setup is seldom. Generally, ROS has been considered as a byproduct of ATP production and it is assumed that ROS levels always increase when ATP production is upregulated. We demonstrated that this common assumption is much too reductive. When these ATP and ROS productions become decoupled, this may have critical consequences for the development of neuropathies.

Taken together I strongly believe our experiments and results provide a next big step towards a better understanding of the peripheral nervous system in health and disease.

Conclusion

In the presented research, I provided a dynamic mechanism how the peripheral nerve axons function and how axons work together with neighboring cell types such as Schwann cells, with a focus on mitochondria.

A key element in axons is energy, because all the major axonal cellular processes require energy, especially the continuous depolarization and repolarization of the neuronal membrane and organelles trafficking. This energy is produced in the form of ATP and it is likely that energy cannot efficiently be stored in axons (under a glycogen form for example) due to the lack of space eventually. In addition, the scarcity of space probably result in the production of energy in active axons having negative consequences: the production of reactive oxygen species, which are eager to react with the molecules in their environment and cause damage.

As a consequence, complex pathways regulate ATP and ROS production in axons. First, the correlation between mitochondrial shape and mitochondrial function is revealed when mitochondrial shape is abnormal. I used a model in which mitochondria are unable to fuse due to mutations in MFN2 genes, resulting in an abnormal shape, because mitochondria are decreased in length. I have shown that these abnormally shaped mitochondria also differ in their production of ATP and ROS and in the function of MAMs, although the nerve needs to be challenged first.

The second factor influencing mitochondrial function is the intracellular environment. We show that upon nerve stimulation, mitochondria are able to upregulate ATP production. Meanwhile ROS production also increases but with a different dynamic: a spike in ROS production just before a gradual ATP increase. This suggests that the concentration of antioxidant enzymes is an important factor in the pathway of mitochondrial bioenergetics.

Next to the interplay between ATP production and ROS production, the interplay between mitochondria and other organelles appears to be another factor in mitochondrial physiology. Through MAM, calcium may enter mitochondria from the endoplasmic reticulum, which changes enzymatic activity in oxidative phosphorylation.

The presence of metabolites in the axonal cytoplasm involves a third type of interplay being the influence of other cell types. Especially mSCs appeared to have a major effect on mitochondrial physiology in the axon. Indeed, first they use the Warburg effect to produce more lactate, which is then transported into the neuron through MCT transporters via the lactate shuttle. Additionally, mSCs produce a myelin sheath, which presence strongly influences axonal mitochondrial function. Mitochondria in regions where the axon is not myelinated produce more ATP and more ROS. Moreover, pathological myelin loss results in decreased ATP production and elevated levels of ROS.

Factors that determine axonal function often involve counterparts that influence each other: ATP with ROS, axon with Schwann cell, endoplasmic reticulum with mitochondrion, oxidative phosphorylation with glycolysis, ATP synthase with uncoupling proteins, glucose with lactate, node of Ranvier with internode. These counterparts allow an energy-efficient function of the nerve and enable adaptation to changing environments to a certain extent.

In conclusion, mitochondria are dynamic organelles that are sensitive to their environment and respond accordingly by changing its physiology. This is essential when mitochondria need to adapt to changing environments, such as action potential firing. However, pathologic stimuli can result in mitochondria that work in disbalance. Unbalanced mitochondria physiology can have disastrous effects for the nerve, because ROS are often damaging for cell components and enough ATP production is essential for axonal function. These malfunctioning mitochondria may be central players in developing symptoms of neurodegenerative diseases, such as multiple sclerosis, CMT2A, Alzheimer disease or Parkinson's.

In this thesis, I elaborated a model for ATP and ROS production by axonal mitochondria in vivo. To sum up this model, the following statements reflect the original discoveries from my research:

-The Warburg effect in Schwann cells is essential for healthy axonal metabolism and ATP production in mitochondria (1). -Blocking the Warburg effect affects neuronal physiology characterized by decreased synaptic contact at the neuromuscular junction and increased cellular stress in motor neurons (2). -Blocking the Warburg effect causes motor problems caused by neuronal failure, but not Schwann cell or myelin sheath deficiencies (3). -Demyelination results in a pathologic decoupling of ATP and ROS production by axonal mitochondria (4). -Following induced action potentials, mitochondrial ATP and ROS production increase, but with separate dynamics (5). -Mitochondria located in the node of Ranvier intrinsically are more active than mitochondria located in internodes ( 6). -Mitofusin2-deficient neurons show mitochondria with a different morphology in vivo, together with an affected transport (7). -Mitofusin2-deficient motorneurons show shorter dendritic length (8). -Mitofusin2-deficient neurons show decreased contact area between endoplasmic reticulum and mitochondria (9). -Mitofusin2-deficient mitochondria do not produce altered levels of ATP or ROS in rest, but in active nerves, affected mitochondrial physiology is revealed (10).

In conclusion, mitochondrial physiology within myelinated axons is an energetic interplay involving a multitude of counterparts. Pour obtenir ces résultats, des techniques d'imagerie avancées ont été utilisées pour observer les nerfs périphériques de souris transgéniques. Ces transgènes sont introduits par injection des vecteurs viraux qui induisent l'expression de sondes fluorescentes dans les cellules neuronales. Ces sondes fluorescentes sont détectées par microscopie multiphotonique. En plus du modèle de production d'ATP et DRO dans les nerfs périphériques, je présente un protocole de marquage du nerf sciatique par des vecteurs viraux.

The nervous system consists of several cell types that interact with each other in order to conduct action potentials from the neuronal soma through axons to the synapse. In peripheral nerves, Schwann cells interact with neurons by wrapping around the axon and creating a myelin sheath. This myelin sheath allows for fast conduction of action potentials from node of Ranvier to node of Ranvier, which are small unmyelinated areas of the axon. In addition, Schwann cells transfer lactate to the neuron, which the axonal mitochondria use to produce energy in the form of ATP. This is necessary, because many cellular processes, such as the conduction of action potentials use ATP. The production of ATP involves three mechanisms: anaerobic glycolysis in the cytosol and the Kreb's cycle and electron transport chain within mitochondria. However, the production of ATP by mitochondria also results in the production of reactive oxygen species (ROS), which cause cell damage. ROS can be present in several different forms and these different forms have specific properties. For example, superoxide anions are highly reactive and subsequently react rapidly with the molecules in their environment. Hydrogen peroxide on the other hand is less reactive but hence can diffuse over longer distances and react with their targets more distally. Fortunately, the cell contains a competent antioxidant system, which can reduce ROS to water. When mitochondria malfunction or when the equilibrium between ROS and antioxidants becomes in disbalance, neuropathies can develop, such as amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), Alzheimer's disease or Parkinson's disease. In the PNS, peripheral neuropathies can develop such as Charcot-Marie-Tooth disease as a result from an excess of ROS.

In this thesis, I will provide an elaborate model for ATP and ROS production by axonal mitochondria in vivo. I will show how Schwann cells use the Warburg effect, the shift in metabolism from oxidative phosphorylation to anaerobic glycolysis, to produce lactate, which is then transported to the neuron for energy production. I also demonstrate that without the Warburg effect in Schwann cells neuronal metabolism would be impaired, leading to impaired ATP production, neuronal deficits and motor problems. Following action potential firing, not only ATP is produced by mitochondria, but also ROS, although with independent dynamics. In addition, I show that mitochondrial physiology is affected by several neuropathologies. In mitofusin2 deficient mice, a model for CMT2A, contact between the endoplasmic reticulum and mitochondria is impaired next to affected mitochondrial morphology and function. Also demyelination causes deficits in mitochondrial ATP and ROS production, showing a pathologic decoupling between ATP and ROS.

To obtain these results, advanced imaging techniques were used to image peripheral nerves of transgenic mice. These transgenes were introduced in mice via injection of viral vectors which induce expression of fluorescent probes in neuronal cells. These fluorescent probes were detected via multiphoton microscopy. Next to the model for ATP and ROS production in peripheral nerves, I provide a protocol for introducing viral vectors into mouse sciatic nerves.

Figure 1

 1 Figure 1 Structure of a mitochondrion. The inner matrix (pink) is separated from the intermembrane space (green) by a bilipid layer. A second bilipid layer separates the intermembrane space with the cytosol.

Figure 2 : 9 .

 29 Figure 2: The process of mitochondrial fusion and fission 9 . Multiple small mitochondria can fuse to form one larger mitochondrion. The opposite reaction is mitochondrial fission, in which the large mitochondrion can separate to form multiple smaller mitochondria. These small mitochondria are more susceptible for degradation via mitophagy.

Figure 3 :

 3 Figure 3: Schematic diagram of glycolysis

Figure 4 :

 4 Figure 4: A schematic overview of oxidative phosphorylation. Pyruvate, produced by glycolysis, is transformed by Acetyl-CoA. Then, in a series of enzymatic reactions that form the TCA cycle, NADH and FADH is formed. These 2 compounds drive the electron transport chain to pump protons across the mitochondrial inner membrane. The resulting proton gradient is used to produce ATP.

Figure 5 :

 5 Figure 5: Schematic overview of the electron transport chain that depicts the sites of superoxide production. Protons are being translocated by complex I, III, and IV. In this process, free electrons are produced as well, which are delivered to coenzyme Q (Q), and further towards cytochrome C (Cyt C). Some electrons leak back into the inner matrix and those electrons reduce electrons to form superoxide. Superoxide can be enzymatically degraded towards H 2 O 2 and further to H 2 O.

Figure 6

 6 Figure 6 Types of DNA damage that can be the result of oxidation by ROS, being the formation of 8-OHdG adducts, mismatching of DNA bases, breaks of either 1 strand or both strands, and crosslinking between the different strands or within the same strand.

Figure 7 :

 7 Figure 7: Incorporation of different cofactors in SOD results in opposite reactions. When Manganese is incorporated, SOD fulfills its known function as an antioxidant. However, SOD2 can function as a prooxidant in situations when manganese is deficient or iron is enriched in the environment. In these situations, SOD incorporates Fe.

Figure 8 :

 8 Figure 8: ROS can function as a signaling molecule and can change gene expression at the DNA level. When ROS is produced by mitochondria, NADPH oxidase or potential other sources, it stimulates the activity of transcription factors such as NF-!B. This will result in DNA binding of NF-!B and hence increased expression of certain genes, for example genes responsible for antioxidant functions or inflammatory responses.

Figure 9 :

 9 Figure 9: The ATP-ROS-calcium triad are important regulators in axonal degeneration. High cytosolic Ca 2+ concentrations are a well-established stimulus that triggers a positive feedback loop leading up to Wallerian degeneration. Calcium can enter the axon, but then can be buffered by mitochondria. Decreased ATP levels induce intracellular calcium levels. Meanwhile ROS stimulates the opening of PT pores, resulting in an efflux of calcium into the cytosol. This way, ATP and ROS either prevent or stimulate axonal degeneration via cytosolic calcium levels.

Figure 11

 11 Figure 11 Schematic representation of myelination 154 . a) Oligodendrocyte myelination involves the creation of an outfolding that connects to the axon. Then this connection increases contact area and a myelin sheath is created by wrapping around the axon multiple times. b) Schwann cells engulf the axon and then wrap around axons to create a myelin sheath.

Figure 12 :

 12 Figure 12: Structure of the node of Ranvier and its surrounding areas 171 . Most parts of the axon are surrounded by layers of myelin, although myelin is absent in small regions called the nodes of Ranvier. These areas are rich in iron channels, which enable salutatory conduction. Paranodes and juxtaparanodes are the regions flanking nodes of Ranvier. These regions contain the paranodal loops of myelin and K + channels at the axonal side.

Figure 13 :

 13 Figure 13: Schematic overview of the lactate shuttle from oligodendrocyte to neuron. Glucose, coming from the blood stream, is taken up by oligodendrocyte via GLUT1 transporters. Via glycolysis, this glucose is then transformed into lactate, which can be transported towards the axon through MCT1 and MCT2. In the axon, lactate is transformed back towards pyruvate, which can be used by axonal mitochondria to produce ATP via oxidative phosphorylation.

Figure 14 :

 14 Figure 14: Demyelination and remyelination 198 . In healthy condition, axons are surrounded by a myelin sheath (a). However, damage to the axon can result in a destruction of myelin and possibly the axon itself (b). As a response repair SCs and macrophages become active and remove the damaged myelin debris. Then, repair SCs guide the regenerating axon towards its target (c). Then, a new myelin sheath is formed (d).

Figure 15 :

 15 Figure 15: Transport of mitochondria along the cytoskeleton. Different motor proteins are involved in different transport direction. For anterograde transport, mitochondria recruit the Miro-milton complex and binds to KHC to connect to microtubules (a). In contrast, retrograde transport involves dynein proteins, but the proteins located within mitochondrial membranes that can connect to dynein are still not identified (b).

Figure 16 :

 16 Figure 16: Steps in the development of MS pathology that form a feedback loop. Immune cells produce ROS that induces demyelination. This demyelination causes mitochondrial dysfunction in oligodendrocytes and neurons, which elevates ROS levels. This mitochondrial ROS causes damage to the endoplasmic reticulum and mitochondrial associated membranes. The failure of the ER on its turn contributes to the demyelination and mitochondrial dysfunction. Demyelination also induces antioxidant failure which exacerbates ROS. Mitochondrial dysfunction affects ATP production.

Figure 17

 17 Figure 17 Schematic representation of AT1.03

5. 1 . 1

 11 AT1.03 AT1.03 is a genetically-encoded fluorescence probe based on fluorescence resonance energy transfer (FRET) that detects ATP (Fig. 17

  )275 . FRET is a distance dependence physical process by which energy is transferred from an excited fluorescent molecule, an energy donor, to another fluorophore, an energy acceptor 276 . The efficiency of FRET is dependent on the distance between the energy donor and acceptor fluorophores, making it a sensitive technique for investigating a variety of biological mechanisms that induce changes in molecular proximity. FRET microscopy captures fluorescent signals from the interaction of the labeled molecules277 . This interaction includes quenching of donor channel signals and increase of the acceptor channel signal. Several FRET microscopy techniques exist and are used to test various biological mechanisms such as organelle structure, conjugated antibodies, cytochemical identification, and oxidative metabolism.AT1.03 consists of the cyan fluorescent protein (CFP) mseCFP, the yellow fluorescent protein (YFP) variant monomeric Venus, both linked at the εsubunit of the Bacillus subtilis FoF1-ATP synthase. In presence of ATP, the ε-subunit retracts to bring the two fluorescent proteins close to each other, thereby increasing FRET efficiency. The conformational change is reversible.

Figure 18

 18 Figure 18 Schematic representation of roGFP-Orp1

Figure 19 :

 19 Figure 19: Energy-level diagrams of four different microscopy techniques: One-photon fluorescence microscopy, twophoton fluorescence microscopy, 3-color CARS microscopy and 2-color CARS microscopy). In One-photon microscopy, the sample molecule reaches a higher energetic state upon excitation. After loss of energy, a lower energy photon is reemitted. In two-photon microscopy, two low-energy photons are used to reach this same high energetic state of the sample molecule, although the same emission is obtained. This means that the excitation photons contain less energy than the emitted photon. CARS imaging requires two steps. First, a pump photon excites the sample, followed by a Stokes photon that induces electron scattering, which results in a vibrational sub-state. In step 2, the pump photon excites the sample again, now to an energetic state that is high enough to emit an anti-Stokes photon, which can be detected.

Figure 20 :

 20 Figure 20: The setup of a microscope equipped with lasers for two-color CARS imaging. The two laser beams, the pump beam and the Stokes beam, are directed through the objective to the sample. After passing through a filter, the created anti-Stokes emitted photons are detected by the microscope's detector.

FIGURE 1 PKM2

 1 FIGURE 1 PKM2 is efficiently deleted in mature mSC of Mutant mice. A-Quantitative RT-PCR shows decreased PKM1 and increased PKM2 expressions during mSC maturation (P: postnatal days). Normalized on P2 values. n=4 animals. B-Immunostaining on mouse sciatic nerve sections shows decreased PKM1 (left panels) and increased PKM2 (right panels) expressions in mSC (E-cadherin) during maturation. m: months. sc=Schwann cell; a=axon. C-Western blots show lower PKM2 and higher PKM1 amounts in Mutant versus Control mice sciatic nerves. Actin serves as loading control. KDa=Kilo Dalton. n=3 mice. D-Quantitative RT-PCR shows decreased PKM2 and increased PKM1 expressions in Mutant (Mut, n=7) versus Control (Cont, n=5) mouse sciatic nerves. E-Immunostainings, as described in B, show PKM2 lower in Mutant mSC (panel l) while PKM1 is higher (arrowheads and Insert panel k). F-Biochemical measure of lactate shows a reduced concentration in Mutant versus Control sciatic nerves (n=6 mice) and 12 months old). All scale bars = 10mm. Statistics show two-tailed Student t-test. Error bars represent SD in A and D, and SEM in F. AU=arbitrary unit.

FIGURE 3

 3 FIGURE 3 Mutant mice show motor deficit and display axonal defects. A-Rotarod latency to fall and B-grip strength were measured between 2 and 12 or 16 months postnatal respectively on Control (n=21) and Mutant (n=25) mice. Statistical tests show twoway non-repeated ANOVA Sidak's post-hoc tests. C-Nerve conduction velocity (NCV) measured in Control and Mutant mice (n=10) at 6, 10 and 12 months postnatal show no significant difference (ns, two-way non-repeated ANOVA statistical tests, Sidak's post-hoc tests). D-Soleus muscle neuromuscular junctions of 12 months old Mutant and Control mice were stained for presynaptic SV2 and postsynaptic acetylcholine receptor (a-bungarotoxin). Scale bar= 100mm. E-Innervated (complete overlap), partially innervated (partial overlap) and denervated (no overlap) junctions were counted on sections as shown in D for Mutant mice and Control mice. Two tailed Student t-test. n= 4 mice (12 months). F-Spinal cord sections of Mutant (Mut) and Control (Cont) mice were immunostained for Caspase 3, motor neurons markers ChAT and Neurofilament (see Figure S3). Caspase 3/Neurofilament-positive neurons are more abundant in Mutant (n=20 images, 3 animals) than in Control (n= 14, 2 animals). Two tailed Student t-test. G-Axonal mitochondria labelled with mito-Dsred2 were imaged in vivo before and after electrical stimulation. Upper panels show typical kymographs. Tracked mitochondria are shown with colour dots. Mutant mice mitochondria follow a straight pattern indicating they are immobile or slowly moving. Lower panel: Mitochondria speed was plotted according to the genotype before and after stimulation. Statistical analysis show one-way ANOVA Tukey's post-hoc test. n are provided in Material and Methods. Error bars represent SEM.

FIGURE 4 PKM2

 4 FIGURE 4 PKM2 deletion in mSC impairs motor performance recovery after DCA treatment. A-Metabolic pathways directly affected by the manipulations. LDH: lactate dehydrogenase; PDH: pyruvate dehydrogenase; PDK: PDH kinase. DCA inhibits PDK, which inhibits PDH. B-Rotarod latency to fall was measured in Control and Mutant mice during a 7 weeks daily treatment with DCA and then during a three weeks recovery period without treatment. Statistical analysis show two-way non-repeated ANOVA Sidak's post-hoc test from week 7 to week 10. n= 5 animals for each genotype. Error bars represent SEM.

  FIGURE S1 Lactate, glucose and mitochondrial ATP levels in resting axons of Mutant and Control saphenous nerves are not different. As described in Figure S3, Laconic (A), dGlu6 (B) and mito-Ateam (C) fluorescence ratios were measured in resting axons of the saphenous nerves of Mutant and Control mice. No significant differences could be detected in these conditions. P-values show two-tailed unpaired Student T-test. A: n=20 axons in 7 Mutant mice, 7 axons in 4 Control mice. B: n=9 axons in 3 Mutant mice, 15 axons in 6 Control mice. C: n=14 axons in 5 Mutant mice, 16 axons in 5 Control mice. Error bars represent SEM.

A

  Figure S2. Characterization of consequences of the overexpression of MFN2 WT or MFN2 R94Q in motor and sensory neurons (A) Motor and sensory neurons were infected with AAV6-hsyn-hMFN2WT or AAV6-hsyn-hMFN2 R94Q (10 TU per cell), 2 days after plating. Level of expression of MYC-tagged-MFN2 was evaluated at 8 dpi with anti-myc staining. (B) As a control for specificity of antibodies used for proximity ligation assay, the experiment was performed without VDAC1 antibody, using only the IP3R antibody. No signal was detected under this condition as compared to experiment performed in presence of both antibodies (IP3R + VDAC1). (C-D) Western blot on spinal cord extracts from 12 month-old WT (n=3) and 12 month-old CMT2A Tg (n=3) mice. (C) Proteins levels of two MAM proteins: VAPB and SIGMAR1 and GAPDH as a reference protein. (D) Levels of expression of VAPB and SIGMAR1 relative to GAPDH. Values are expressed as mean intensity ratio (protein of interest/GAPDH) ± sem (from n=3 animals). Statistical analysis: two-tailed unpaired student's t-test.

Figure S3 .

 S3 Figure S3. Characterization of parameters related to ER and mitochondria biology (A) Intracellular calcium measurements using Fura-Red ratiometric imaging (F490/F440) after KCl (25mM) exposure in MFN2WT (n=16) and MFN2R94Q cells (n=25). Half time recovery (sec) to basal values was calculated for each cell. Data are expressed as mean ± sem. Each dot represents one cell. Statistical analysis: two-tailed unpaired student's t-test. (B) Velocity histogram showing mitochondrial transport in sciatic nerve axons of 1-month-old WT (n=24 axons from 7 mice) and CMT2A Tg (n=12 axons from 6 mice) mice. Mitochondria are classified according to fast (>0.9 μm/min), medium (>0.6 and <0.9 μm/min), slow (>0.3 and <0.6 μm/min) and very slow (<0.3 μm/min) transport velocities. Statistical analysis: repeated-measure two-way ANOVA (C) Electron microscopy assessment of mitochondrial length in lumbar spinal cord motoneurons of 12 month-old WT (n=76 mitochondria from 5 mice) and CMT2A Tg mice (n=76 mitochondria from 5 mice). Statistical analysis: two-tailed unpaired student's t-test. *p<0.05.

  H 2 O 2 levels are measured using the ratio of emitted fluorescence lights when excited at 800nm (oxidized form) or 940nm (reduced form). All experiments were done in the same imaging conditions and analysis. As both probes are ratiometric, the measured values are independent of the number of mitochondria or of the absolute fluorescence intensity of the probe. Both probes were targeted to the mitochondrial matrix.We first validated mito-roGFP-Orp1 and mito-ATeam probes in vivo. Injection of AAV9mito-roGFP-Orp1 and AAV9-mito-ATeam in the spinal cord of mouse pups one day after birth (P1) resulted in expression of the fluorescent probes in multiple axons that are part of sciatic and saphenous nerves (Fig1a). Coherent Anti-stokes Raman Scattering (CARS) microscopy uses the non-linear interactions between light and molecules to generate light from lipids without labeling[35]. As myelin is enriched with lipids, CARS microscopy allows visualizing the myelin sheath around axons in vivo[36,37,38]. Using this technique, we observed that the fluorescent probe signal was encompassed by the myelin sheath, which shows the probes are expressed in myelinated axons (Fig1a). We then performed an immunostaining of teased fibers for the mitochondrial marker TOM20 and observed a partial colocalisation between fluorescent probes and mitochondria (Fig 1b), showing probes are expressed in axonal mitochondria, but not all of them. Consistently, mitochondria located in The fluorescent signal of mito-ATeam was recorded every 5 minutes for 20 minutes after stimulation (Fig 3a). At each time point the fluorescence ratio (R) was corrected for the mean fluorescence ratio of that same axon before nerve stimulation (R0). After a first stimulation, the fluorescence ratio of mito-ATeam increased in several axons, but not in all of them (44%), resulting in a slight non-significant average increase (Fig 3a). After the second stimulation, all axons responded significantly increasing mitochondrial ATP production (Fig 3a). In the same conditions, nerve stimulations significantly increased mitochondrial H 2 O 2 production just 1 minute after the stimulation period (Fig 3b). H 2 O 2 level then quickly went back and stabilized at pre-stimulation values (Fig 3b). These data show that axonal mitochondria very quickly adapt to the axonal activity up-regulating their production of ATP. Spikes in mitochondrial H 2 O 2 production were also observed just before the surge of ATP in mitochondria (Fig 3c),

  6a, week 1)[37,49]. As soon as two weeks after LPC injection, remyelination occurred and some thin myelin sheaths reappeared around axons (Fig 6a, week 2). Axons were completely remyelinated three weeks after injection of LPC (Fig 6a, week 3). No demyelination occurred in control mice injected with PBS (Fig 6a last panel). Axonal integrity is not directly affected by LPC (EV Figure 1) [50] and axonal mitochondria remained visible using either mito-ATeam or mito-roGFP-Orp1 probes during the entire process of demyelination and remyelination (Fig 6a inserts).We therefore measured the fluorescence ratio for these probes in non-stimulated axonal mitochondria during the process. One week after LPC injection, when axons are completely demyelinated, ATP production was slightly lower than before demyelination, but not significantly (Fig 6b), while H 2 O 2 levels peaked (Fig6c). ATP levels were significantly lower in axonal mitochondria at two weeks after LPC injection, while H 2 O 2 levels were back to control and pre-demyelination levels (Fig6b-c). Finally, three weeks after injection, when remyelination is completed, ATP levels were back to control and pre-demyelination levels as are the H 2 O 2 levels (Fig6b-c). No change either in ATP or H 2 O 2 levels were detected at one week after injection with PBS in control mice (Fig6b-c). Moreover, increase in ROS did not result from a general oxidative stress in the demyelinated nerve, since CellRox dye did not show any significant increase, except at the LPC injection site where we did not image our probe (EVFig 2). Taken together, these data indicate that the myelin sheath has a profound impact on axonal mitochondria physiology. Mitochondria manage to maintain their ATP production during myelin break-down, but they generate more H 2 O 2 . The process of remyelination leads to decreased production of ROS, while ATP production is temporally diminished before full remyelination. These data therefore indicate that demyelination induces the decoupling of ATP production from oxidative phosphorylation.

4 .

 4 the data we obtained in PNS myelinated axons in CNS myelinated axons in vivo, in order to definitely conclude on the alteration of ATP and ROS production by axonal mitochondria in demyelinated lesion of progressive MS. To conclude, combining several novel techniques such as two-photon and CARS non-linear live imaging coupled to electrical nerve stimulation and genetically-encoded fluorescent probes delivered by AAV vectors, we were able to observe the production of ATP and H 2 O 2 by axonal mitochondria in real time and in living and active axons of mouse peripheral nerves. We demonstrated that mitochondria are more metabolically active at the node of Ranvier and we characterized the role of AP firing in the dynamics of mitochondrial ATP and H 2 O 2 production. Moreover we show the deleterious alterations that occur in axonal mitochondria physiology of two mouse models of neuropathy, peripheral axonal CMT2A and demyelinating peripheral neuropathies. Together, our data thus provide new insight into the role of axonal mitochondria under physiological and pathological conditions. Materials and methods 4.1. Cloning pLPCX mito-roGFP-Orp1 (Addgene; #64992) was digested with XhoI/ClaI, blunted and cloned into pAAV-MCS (Cell Biolabs, Inc.) under the control of a CMV promoter. Clones were validated by sequencing. Likewise, pcDNA-mito-ATeam (from H. Imamura, Tokyo, Japan) was digested with XhoI/HindIII, blunted and cloned into the CMV promoter controlled pAAV-MCS. Mitochondria-targeting tags were CoxVIII for roGFP-Orp1 and two tandem copies of CoxVIII for mito-ATeam. AAV viral particles were produced at the viral vector production centre at the Centre de Biotecnologia Animal I Teràpia Gènica in Barcelona, Spain or the University of Nantes, France.

  experiments. Mice were kept in the animal facility of the Institute for Neurosciences of Montpellier in clear plastic boxes and subjected to standard light cycles. All animal experiments were conducted in accordance with the French Institutional and National Regulation CEEA-LR-11032.

  used ImageJ software to analyze the relative ATP or H 2 O 2 levels in mitochondria of peripheral axons. The acquired images for each wavelength were aligned using the Template Matching plugin. We defined a Region of Interest (ROI) encompassing all labeled mitochondria of the same axon and the mean fluorescent intensity on the ROI was measured on both images. These light intensities were then corrected for background light intensity determined as an area within the nerve where no fluorescent signal from the viral probe can be observed. Either the ATeam Venus/CFP ratio or the oxidized roGFP/reduced roGFP ratio was then calculated from the 2 values for mean light intensity. Statistical significance for the effect of time on probe stability was determined using linear regression. The effects of the positive controls and negative controls on probe validation were determined using Student two-tailed T-tests. The effect of nerve stimulation was determined using one-way ANOVA and Dunnett's post-hoc tests. In addition, the threshold for a responding axon was set at 20% change in probe fluorescence ratio. The differences between MFN2 mutants and wild-type mice were determined using either Student two-tailed T-tests or two-way ANOVA with Sidak post-hoc tests. Differences between internodes and node of Ranvier mitochondria were determined using paired two-tailed T-tests. The effect of demyelination was determined using paired two-tailed T-tests.

Figure 1 :

 1 Figure 1: In vivo validation of the fluorescent probes. (A) Several axons expressing mito-roGFP-Orp1 or mito-ATeam probe can be observed in teased fibers of mouse sciatic nerve 1 month after the virus injection. CARS imaging (inserts) shows that axons expressing mito-roGFP-Orp1 or mito-ATeam (green) are surrounded by a myelin sheath (red). Scale bars = 3 µm. (B) mito-roGFP-Orp1 expression (green) partially colocalizes with mitochondrial marker TOM20 (red) in the axon. Similarly, mito-ATeam partially colocalizes with TOM20 in axonal mitochondria. Scale bars = 3 µm. (C) A fluorescent signal of mito-roGFP-Orp1 is detected when the probe is reduced and when it is oxidized by H 2 O 2 . The ratio of mito-roGFP-Orp1 fluorescence measured in vivo (n=6 axons; 6 mice) shows a slight increase over time. Scale bar = 10 µm (D) Mito-roGFP-Orp1 fluorescence ratio decreases with DTT (p=0.006; n=3 axons, 3 mice) and increases after injection of diamide into the nerve (p=0.003; n=4 axons, 4 mice). (E) A fluorescent signal of the CFP subunit and Venus subunit of mito-ATeam is detected. Mito-ATeam fluorescence ratio measured in vivo shows no significant change over time (n=18 axons; 3 mice). Scale bar = 10 µm. (F) Mito-ATeam fluorescence ratio increases after injection of 0.4M (p=0.06; n=3 axons, 3 mice) and 1M (p=7.5E-4; n=6 axons, 4 mice) ATP into the sciatic nerve. All error bars show SEM. Statistical analysis shows Student two-tailed T-tests.

Figure 2 :

 2 Figure 2: Setup of the saphenous nerve imaging experiment.

Figure 3 :

 3 Figure 3: Effect of burst nerve stimulation on mitochondrial ATP and H 2 O 2 levels.

Figure 4 :

 4 Figure 4: ATP and H 2 O 2 levels of MFN2 R94Q mice. (A) Graph showing mito-ATeam fluorescence ratio in resting axonal mitochondria of wildtype or mutant MFN2 R94Q mice (Student two-tailed T-test p=0.809; n=18 axons, 7 mice per group). (B) Graph showing mito-ATeam fluorescence ratio normalized on pre-stimulation values (R/R0) following two successive nerve stimulation periods (black bars at the top). The first period of stimulation resulted in a slight, non-significant increase; the second stimulation period did not generate any change (n=14 axons in 5 mice). (C) Graphs showing mito-ATeam R/R0 in wild-type (Fig. 3A) and mutant MFN2 R94Q mice (B panel) were overlaid to show the relative dynamics of ATP levels in both genotypes after nerve stimulations (Two way

Figure 5 :

 5 Figure 5: ATP and H 2 O 2 levels are different along axons. (A) Two-photons imaging of CFP (blue) and Venus (yellow) fluorescence of mito-ATeam was combined with CARS imaging of the myelin sheath (red). CARS imaging gap shows a node of Ranvier (arrowhead). Using the combined images, mitochondria located in the node of Ranvier and mitochondria located in internodes can be identified (right panel). Scale bar = 10 µm (B) Graph showing mito-ATeam fluorescence ratio in axonal mitochondria located in nodes of Ranvier (N) or internodes (IN) of wild-type (WT) or MFN2 R94Q mice (MFN2 mutant). In wild-type mice, ATP levels are higher in mitochondria in nodes of Ranvier than in internodes (p=0.035; n=14 axons in 7 mice). In MFN2 R94Q mice, ATP levels in node of Ranvier mitochondria are equal to internode mitochondria (n=7 nodes in 3 mice). (C) Graph showing mito-roGFP-Orp1 fluorescence ratio in axonal mitochondria located in nodes of Ranvier or internodes of wild-type or mutant MFN2 R94Q mice. In both wild-type mice and mutant MFN2 R94Q mice, H 2 O 2 levels are higher in mitochondria in nodes of Ranvier than in internodes (WT: p=0.009; n=8 axons in 3 mice. MFN2 R94Q : p=0.019; n=9 axons in 3 mice). Error bars show SEM. Statistical tests are paired two-sided T-tests.

Figure 6 :

 6 Figure 6: Impact of demyelination on axonal mitochondria ATP and H 2 O 2 . (A) CARS imaging is used to visualize myelin. At week 0, before injection of LPC, most axons are myelinated. 1 week after LPC injection, axons are demyelinated and myelin debris and ovoids are observed. At week 2, thinly myelinated axons are observed in between the myelin debris (arrowheads) and at week 3, almost all axons are myelinated again, thus resembling a healthy nerve. PBS injection induces small conformational changes, but no formation of ovoids or debris. Inserts show that neuronal mitochondria remain visible throughout the whole process. (B) Graph showing mito-ATeam fluorescence ratio (R) in axonal mitochondria normalized on pre-demyelination values (R0) following demyelination. Following LPC injection, ATP levels are unchanged after 1 week (p=0.085; n=5 mice; 27 axons), decreased after 2 weeks (p=0.019; n=3 mice, 24 axons) and restored after 3 weeks (p=0.491; n=5 mice; 31 axons). PBS injection results in no significant change (p=0.799; n=3 mice; 23 axons). (C) Graph showing mito-roGFP-Orp1 fluorescence ratio (R) in axonal mitochondria normalized on pre-demyelination values (R0) following demyelination. Following LPC injection, H 2 O 2 levels are increased after 1 week (p=0.031; n=10 mice, 17 axons), and unchanged after 2 weeks (p=0.663; n=5 mice, 9 axons) and after 3 weeks (p=0.450; n=8 mice, 13 axons). PBS injection results in no significant change (p=0.121; n=3 mice, 5 axons).

  The products of glucose metabolism via glycolysis, pyruvate and lactate, can be released and taken up by monocarboxylate transporter proteins (MCTs). MCT1 is located in the myelin while MCT2 is located into the axonal membrane. Inhibition of MCT1 is not tolerated by neurons and results in axonal degeneration. This suggests that the transfer of lactate from oligodendrocytes to neurons is essential for neuronal viability. Although indications for the existence of the lactate shuttle exist in oligodendrocytes, a transfer of lactate between Schwann cells and neurons is unknown. The few indications of a lactate shuttle in the PNS are based on expression of MCT1 and MCT4 in Schwann cells, mainly in regions of noncompact myelin193 . Downregulation of MCT1 in mSC/neuron cocultures increases myelination and decreases neurofilament, but a direct transfer of lactate has not been demonstrated.

TABLE S Electrophysiological characteristics of Control and Mutant mice.

 S

		Control	Mutant
	Amplitude (mV)	2.5 ± 1.8	3.2 ± 2.5
	Duration (ms)	0.34 ± 0.06	0.37 ± 0.09
	CV V½ (m.s -1 )	52.1 ± 15.8	45.2 ± 16.1
	CV Vmax (m.s -1 )	33.8 ± 8.5	30.6 ± 9.4
	n	10 (5 animals)	10 (5 animals)

FIGURE S3 Mutant mouse motor neurons display neuronal stress marker Caspase 3. Spinal

cord cryosections of Mutant and Control mice were immunostained for neuronal stress marker Caspase 3 (red), neuronal marker Neurofilament (NF, green) and motor neurons marker ChAT (blue). While Neurofilament staining is not different, more motor neurons express Caspase 3 in Mutant mice than in Control mice. Scale bar= 50mm.

  •-is rapidly converted to H 2 O 2 by superoxide dismutase (SOD) and H 2 O 2 is then

	reduced to H 2 O and O 2 by mitochondrial glutathione peroxidase (GPx) and cytosolic enzymes
	[15]. H 2 O 2 can also react with iron or O 2	•-to form hydroxyl radicals (OH•), which is then
	reduced by molecular hydrogen to H 2 O [16]. If not reduced by antioxidants, all types of ROS
	are highly toxic for the cell and in particular for mitochondria, where they are produced.

  targeted to mitochondria to image and measure the dynamics of ATP and H 2 O 2 production in mitochondria of myelinated axons in myelinated axons of the PNS. Using this approach, we show that both ATP and H 2 O 2 levels are increased in mitochondria residing in nodes of Ranvier and that nerve stimulation sharply increases ATP and H 2 O 2 production by mitochondria in few minutes. While MFN2 mutations had no effect on basal levels of ATP and H 2 O 2 in axons, it prevented the increase of ATP production after nerve stimulation. At the opposite, an increased H 2 O 2 production was

	observed. Moreover, pathological demyelination reduced ATP production while enhancing
	mitochondrial ROS, showing that both neuropathic conditions decoupled the production of
	ATP from the production of ROS.

  2 •-, which directly results from the ETC. Moreover, H 2 O 2 is then changed in H 2 O and O 2 by several reducing enzymes, such as mitochondrial GPx, so actually the amount of H 2 O 2 we detected is the result of equilibrium between O 2 •-production and SOD and reducing enzymes' activities. As these enzymes are highly expressed in mitochondria [55], they probably do not constitute a limiting factor and the H 2 O 2 levels we detected still reflect axonal mitochondria metabolic activation. However, H 2 O 2 levels drop quickly, probably because reducing enzymes

  The next day, the samples were washed in PBS and incubated for 1h at room temperature with secondary donkey antibodies coupled to Alexa568 (1/1000, Invitrogen, Ref.

	A10042) or Alexa488 (1/1000, Invitrogen, Ref. A21202). Finally the samples were washed in
	PBS and mounted in Immu-mount (Thermo Scientific). Images were acquired at room
	temperature using a 20× or 40× objective, a Zeiss confocal microscope LSM710, and its
	associated software.
	, 66% in PBS) for 18 to
	24 h each before freezing at -20 °C. The nerves were cut in small pieces in 66% glycerol and
	the epineurium sheaths removed. Small bundles of fibers were teased in double-distilled water
	on Superfrost slides and dried for 3 hours at room temperature. Some nerves were frozen in
	O.C.T. Compound (Tissue-Tek, Ref. 4583) and longitudinal sections were cut using a cryostat
	(Leica Biosystems, CM3050). For immunostaining, the teased fibers or longitudinal sections
	were incubated for 1 h at room temperature in blocking solution (10% goat serum and 0.3%
	TritonX100 in PBS). Then, the samples were incubated with TOM20 (FL-145) primary rabbit

antibody (1/500, Santa Cruz, Ref. sc-11415), mouse anti-Myelin Basic Protein (1/500, Merck, Ref. NE1019) or rabbit NF-200 (1/500, Sigma, Ref. N4142) in blocking solution overnight at 4 °C.

  Montpellier as well for the countless good times at Stade Philippides, but also other (running) events around Montpellier and even as far as Edinburgh: Alexis Aufore, Mathieu Brydniak, Marion Béchard, Jean-Michel Blenet, Gabriël Carvalho, Remy Choquet, Odile Conrardy, Guilhem Debarros, Joao Ferreira, Tom Gaudy, Catherine Grimal, Jean-Pierre Grimal, Manon Grimal, Lorna Handley, Jeff Holley, Sophie Imus, Julien Jeanjean, Julie Jerome, Raphael Lamiral, Gabriël Lecat, Marie Loury, Atica Mamoudi, Jennifer Mercier, Sophie Noel, Franc Puyoo, Florie Seznec, Zaina Tazeroualti, Sylvain Téchené and le coach Jean-Luc.My stay in Montpellier would not have been possible without the people that taught me the necessary skills in preparation of becoming a PhD student. My stay Coleman lab at Sun City, Arizona was a very influential period in my life and for that I want to thank Dr. Paul Coleman, Diego Mastroeni, Elaine Delvaux, Jennifer Nolz, Amanda Lewis and Yuyan Tan. Also former supervisors at Maastricht University had played a crucial role, especially Daniël van den Hove, Jos Prickaerts and Guido Haenen. Special thanks to my dearest friends during the Research Master at Maastricht, Netherlands. Artemis Iatrou, Koen Meuwissen, Dila Suay and Britt van Hagen: the saying out of sight, out of mind definitely does not apply to you guys! Finally, I want to thank the homefront, my family. Wim van Hameren, Paul van Hameren, Henriëtte van Hameren and Wim van der Linden: Thank you for everything. Also thanks to the developers of Skype and Whatsapp for making regular contact with them possible.Le système nerveux est composé de plusieurs types cellulaires qui interagissent entre eux pour conduire le potentiel d'action du corps cellulaire, le long de l'axone jusqu'aux synapses. Au niveau du système nerveux périphérique (SNP), les cellules de Schwann interagissent avec les neurones en s'enroulant autour l'axone et en créant une gaine de myéline. Cette gaine de myéline permet une conduction saltatoire rapide du potentiel d'action d'un petit espace non myélinisé au suivant, appelé noeud de Ranvier. De plus, les cellules de Schwann transportent du lactate vers le neurone qui l'utilise pour produire de l'énergie sous forme d'ATP via notamment les mitochondries. Cette production est primordiale car de nombreux processus cellulaires ont besoin d'ATP pour fonctionner dont la propagation de l'influx nerveux. Trois mécanismes sont impliqués dans la production d'ATP : la glycolyse dans le cytosol, le cycle de Krebs et la chaîne de transport d'électrons dans les mitochondries. Cependant, la production mitochondriale d'ATP génère également des dérivés réactifs de l'oxygène (DRO) à l'origine du stress oxydatif. Les DRO existent sous différentes formes avec des caractéristiques propres pouvant toutes endommager la cellule. Par exemple, l'ion superoxyde est très réactif donc il réagit extrêmement rapidement avec les molécules environnantes. A l'opposé, le péroxyde d'hydrogène est un DRO peu réactif pouvant diffuser sur de grandes distances et oxyder des cibles plus distales. Heureusement, les cellules sont équipées d'un système antioxydant composé d'un groupe d'enzymes antioxydantes qui transforment les DRO en eau. Malheureusement en cas de dysfonctionnement mitochondrial ou déséquilibre entre les DRO et antioxydants, des neuropathies peuvent se développer, comme la Sclérose Latérale Amyotrophique (SLA), la Sclérose En Plaques (SEP), les maladies d'Alzheimer et Parkinson. Dans le SNP, des neuropathies périphériques peuvent se développer en raison d'excès de DRO comme la maladie Charcot-Marie-Tooth (CMT).Dans cette thèse, je propose un nouveau modèle détaillé de la production mitochondriale d'ATP et DRO in vivo. Je montre comment les cellules de Schwann utilisent l'effet Warburg, la transition métabolique depuis la phosphorylation oxydative vers la glycolyse aérobique, pour produire du lactate qui est ensuite transporté vers le neurone pour la production d'énergie. Je démontre également que sans l'effet Warburg des cellules de Schwann, le métabolisme neuronal est détérioré menant à la production diminuée d'ATP, des déficits neuronaux et des problèmes moteurs. Suite à l'activité nerveuse, la mitochondrie fabrique indépendemment de l'ATP mais aussi des DRO avec leurs dynamiques respectives. De plus, je montre que la physiologie mitochondriale est affectée par des les neuropathies. Chez les souris déficiente mitofusin2, un modèle pour CMT2A, le contact entre le réticulum endoplasmique et les mitochondries est diminué et s'accompagne d'un changement de la morphologie et fonction mitochondriale. D'autre part, la démyélinisation provoque un déficit de production d'ATP et de DRO, montrant une dissociation pathologique entre la production d'ATP et de DRO.

	Resumé en Français

Transport of cargo through axons

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgements

To every single person who has helped me during my time in Montpellier, helped preparing me in the years before arriving in France, I would like to say:

Abstract

Mitochondria are critical for the function and maintenance of myelinated axons notably through ATP production. A by-product of this activity is reactive oxygen species (ROS), which are highly deleterious for neurons. While ROS and metabolism are involved in several neurodegenerative diseases, it is still unclear how axonal activity or myelin modulates ATP and ROS production in axonal mitochondria. We imaged and quantified mitochondrial ATP and hydrogen peroxide (H 2 O 2 ) in resting or stimulated peripheral nerve myelinated axons in vivo, using genetically-encoded fluorescent probes, two-photon time-lapse and CARS imaging. ATP and H 2 O 2 productions are intrinsically higher in nodes of Ranvier even in resting conditions. Axonal firing increased both ATP and H 2 O 2 productions but with different dynamics. In neuropathic MFN2 R94Q mice, mimicking Charcot-Marie-Tooth 2A disease, defective mitochondria failed to upregulate ATP production following axonal activity.

However, H 2 O 2 production was dramatically sustained. Mimicking demyelinating peripheral neuropathy resulted in a reduced production of ATP while H 2 O 2 level soared. Taken together, our results suggest that ATP and ROS productions are decoupled under neuropathic conditions, which may compromise axonal function and integrity.

Introduction

While the nervous system, and the brain in particular, represents around 2% of the body mass, it consumes up to 20% of the glucose we mobilize every day [1]. This high energy expenditure in the nervous system is firstly due to the synaptic activity that requires high amounts of adenosine tri-phosphate (ATP) [1]. A second energy demanding process is the propagation of action potentials (APs) along axons. Indeed, conduction of APs involves ion Schwann cells surrounding axons were labeled with TOM20 antibody but not with fluorescent probes. Both probes could be detected in axons of the mouse saphenous and sciatic nerves in vivo.

Then the ratio of the fluorescent intensity of oxidized/reduced roGFP-Orp1 (R) was

The effect of nerve stimulation on ATP and H 2 O 2 production

While ATP is supposed to be critical for the firing of axons, ATP production in mitochondria of functionally active axons has never been observed in vivo. We used the real-time imaging approach previously described in combination with a setup for electrical nerve stimulation based on previous work [39] (Fig 2a-d). We used the saphenous nerve, which is mostly composed of sensory axons, hence its stimulation leads to only limited unwanted contraction of the paw's muscles [40]. After recording the probe fluorescence for at least 20 minutes without stimulation, APs were induced by an electrical burst stimulation protocol including 3 bursts of 50Hz for 30 seconds spaced by a 60 second recovery period (Fig 2e).

Two-photon image acquisition

All in vivo images were obtained with a two-photon microscope LSM 7 MP OPO (Zeiss, France) coupled to a dark microscope incubator (L S1 Dark, Zeiss) in which the temperature was maintained at 37 °C (Heating Unit XL S, Zeiss, France). Mitochondria images were acquired by time-lapse recording varying from one image every minute to one image every 5 minutes during 1 hour. Each image is a stack at Maximum Intensity (ZEN software, Zeiss) of 10 scans over 40 μm depth. For ATeam imaging, a single track at 850 nm excitation wavelength is used to obtain both the CFP (em. 475 nm) and Venus (em. 527 nm) image at the same time point. For roGFP imaging, the two images were acquired for each time point using alternating tracks at 940nm and 800nm. Change of track was set after each stack. For Coherent Anti-Stokes Raman Scattering (CARS) imaging, 2 synchronized laser lines at excitation wavelengths 836 nm and 1097 nm are used simultaneously thanks to the OPO system. Each scan was acquired with constant laser intensity (20% for 940 nm, 10% for 850 nm, 10% for 800nm, 15% for 836nm, 4% 1097nm) at a 512 × 512 pixel resolution and

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgements

In addition we thank the MRI imaging platform, supported by the French National Research In a healthy sciatic nerve (Before injection), axons (red) are surrounded by a myelin sheath (green). This myelin sheath is severely damaged following injection of LPC into the sciatic nerve (LPC + 1w), but no sign of axonal degeneration. Injection of PBS (PBS + 1w) does not affect axon or myelin sheath physiology. Scale = 10 µm.

THANK YOU!

First, I'd like to thank my promoter Dr. Nicolas Tricaud, who has helped me over the whole process of the project. The project that was initially designed looked very different than what is the end result here before you. These changes in direction were based on unexpected results that often arise in challenging projects like these. Thanks to the guidance of Dr. Tricaud, I was able to continue the project by trying to answer our research questions in a different way or pursue more important or more interesting hypotheses. Also he has shown me how to anticipate multiple results and how to respond accordingly. Finally, thanks to him I was able to improve my scientific writing enormously.

Also the other members of team 6, past or present, I would like to thank for their help with my work, especially Marie Deck and Graham Campbell, who have seen the development of this project closely and were able to give me countless technical or theoretical advice. I thank Benoit Gautier for his training in several experimental techniques and help with the presentation of my data. Thanks to Jade Berthelot for her help within the lab and for all her French lessons. I will never forget the difference between un moule and une moule! Hélène, with whom I have shared the PhD experience in parallel, Scarlette Abbou and Elisa Gonzalez are to be thanked for the help with several experiments as well. Furthermore, I would like Jerôme Devaux and Heather Richardson for their advice and for our interesting discussions. Also I would like thank Mallory Poncelet, Alise Lattard, Sophie Guelfie, Sergio Gonzalez, Aymeline Staigre and Jérémie El Bechir, and in particular Liz Tavares, for the comments during lab meetings and for the good vibes both in the lab and outside. However, research cannot be possible without the support from the platforms. I would like to thank the employees from the animal facility platform for taking good care of the mice, especially Laura Cerramon and Jerôme Sarniguet. Also, the help from Hassan Boukhaddaoui from the imaging platform was essential for the use of the multiphoton microscope.

My work was also highly influenced by the advice that I got from the weekly joint-meetings with the Jeanneteau lab from the IGF. Freddy Jeanneteau, Marga Arango-Lievano, Amelie Borie and Yann Dromard: thank you for this. Other guidance from external advisors showed to be key moments to push the projects to a next phase. For this, great thanks to Marie-Luce Vignais, Benedicte Dargent, Roman Chrast and Nathalie and Bernard-Marissal.

But also the (mental) support of the other people working at the INM was very much appreciated, especially the other researchers working on the first floor. Pascalle Bomont, Karolina Campbell, Zoe Geraghty, Léa Lescouzères, Aurora Scrivo, Simona Torriano, Mikhaela Diakitou, Gregor Dubois, Viki Kalatzis, Carla Sanjurjo-Sorriano, Nejla Erkilic and Dasha Mamaeva, thank you for the good atmosphere. Special thanks to Laura Dumas for her corrections in my French writing.

Next to people that supported me with my science, I also would like to thank a group of people that made sure I had a great time during my stay in Montpellier. First, Hannah Emlein from Labex EpiGenMed. Also Leentje den Ceuninck, Alexandra Miliu, Ksenia Semenovskaya and Annika Urbanek. I want to thank my team mates from M.A.R.C.