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Huntington's disease (HD) is a fatal neurodegenerative disorder characterized by progressive loss of striatal and cortical neurons. HD is caused by an abnormal polyglutamine (polyQ) expansion in Huntingtin protein (HTT). HTT controls vesicular trafficking along axons in neurons through interaction with components of the molecular motor machinery. Arginine methylation is one of the most abundant post-translational modifications (PTMs) and is catalyzed by protein arginine methyltransferases (PRMTs). Recent evidence supports a key role for arginine methylation in neurodegeneration and particularly in polyglutamine diseases. However, whether HTT is methylated at arginine residues has not been investigated yet and the role of arginine methylation in HD pathogenesis remains to be fully elucidated. In this thesis, I show that vesicle-associated HTT is methylated in vivo at two evolutionarily conserved arginine residues, namely R101 and R118. Methylation of HTT at R118 is catalyzed by Protein Arginine Methyltransferase 6 (PRMT6), which localizes on vesicles together with HTT, whereas further analyses are required to identify the enzyme(s) responsible for R101 methylation. Interestingly, loss of PRMT6-mediated R118 methylation reduces the association of HTT with vesicles, impairs anterograde axonal transport and exacerbates polyQ-expanded HTT toxicity. Conversely, PRMT6 overexpression improves the global efficiency of anterograde axonal transport and rescues cell death in neurons expressing polyQ-expanded HTT. These findings establish a crucial role of arginine methylation as a modulator of both normal HTT function and polyQ-expanded HTT toxicity and identify PRMT6 as a novel modifier of HD pathogenesis. Importantly, defects in HTT methylation may contribute to neurodegeneration in HD and promoting arginine methylation of HTT might represent a new therapeutic strategy for HD.

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De plus en plus de preuves montrent que le transport axonal rapide (FAT) joue un rôle crucial au cours des maladies neurodégénératives (NDs). La maladie de Huntington est une maladie neurodégénérative causée par une expansion anormale de polyglutamines dans la partie Nterminale de la protéine huntingtine (HTT) : une grande protéine d’échafaudage impliquée dans la régulation du transport. La présence de HTT mutante comme l’absence de la HTT induisent des défauts de transport chez les mammifères. Chez la Drosophile, la HTT mutante reproduit le phénotype observée chez les mammifères, cependant la fonction conservée de la HTT chez la Drosophile melanogaster (DmHTT) n’est pas encore clairement établie. Ici nous mettons en évidence que DmHTT s’associe aux vésicules, aux microtubules et intéragit avec la proteine dynéine. Dans les neurones corticaux de rat, DmHTT remplace partiellement la HTT de mammifère dans le transport axonal rapide, et les drosophiles invalidées pour la HTT montrent des défauts de transport axonal in vivo. Ces résultats suggèrent que la fonction de la HTT est conservée dans le modèle Drosophile.Le FAT est un processus qui requiert un apport constant d’énergie. Les mitochondries sont les principales sources de production d’ATP de la cellule. Cependant nous avons démontré que le FAT ne dépend non pas de cette source d’énergie là, contrairement à ce que l’on pensait, mais de l’ATP glycolytique produit par les vésicules. La dérégulation de GAPDH ou de PK, les deux enzymes glycolytiques productrices d’ATP, ralentit le transport vésiculaire. Néanmoins, l’invalidation de GAPDH n’affecte pas le transport mitochondrial. En outre, toutes les enzymes glycolytiques sont associées à des vésicules dynamiques et sont capables de produire leur propre ATP. Enfin nous montrons que l’ATP produit est suffisant pour assurer leur propre transport, prouvant l’autonomie énergétique des vésicules pour le transport. / Growing evidence support the idea that impairments in Fast Axonal Transport (FAT) play a crucial role in Neurodegenerative Diseases (NDs). Huntington’s Disease is neurodegenerative disorder caused by an abnormal polyglutamine expansion in the N-Terminal part of huntingtin (HTT), a large scaffold protein implicated in transport regulation. Both the presence of the mutated HTT as the loss of HTT leads to transport defects in mammals. In the fruit fly overexpression of the mutant HTT recapitulates the phenotype observed in mammals. However, it is still unclear whether HTT’s function is conserved in D. melanogaster. Here, we show that D. melanogaster HTT (DmHTT) associates with vesicles, microtubules, and interacts with dynein. In rat cortical neurons, DmHTT partially replaces mammalian HTT in fast axonal transport, and DmHTT KO flies show axonal transport defects in vivo. These results suggest that HTT function in transport is conserved in D. melanogaster.FAT is a process that requires a constant supply of energy. Mitochondria are the main producers of ATP in the cell. However, we have demonstrated that FAT does not depend on this source of energy, as previously thought, but it depends on glycolytic ATP produced on vesicles. Perturbing GAPDH or PK, the two ATP generating glycolytic enzymes, slows down vesicular transport. However, knocking down GAPDH does not affect mitochondrial transport. Furthermore, all of the glycolytic enzymes are associated with dynamic vesicles, and are capable of producing their own ATP. Finally, we show that this ATP production is sufficient to sustain their own transport, demonstrating the energetical autonomy of vesicles for transport.

The key to understanding any system, including physiologic and pathologic systems, is to obtain a truly comprehensive view of the system. The purpose of this dissertation was to develop foundational analytical and modeling tools, which would enable such a comprehensive view to be obtained of any physiological or pathological system by combining experimental, clinical, and theoretical viewpoints. Specifically, we focus on the development of analytical and modeling techniques capable of predicting and prioritizing the mechanisms, emergent dynamics, and underlying principles necessary in order to obtain a comprehensive system understanding. Since physiologic systems are inherently complex systems, our approach was to translate the philosophy of complex systems into a set of applied and quantitative methods, which focused on the relationships within the system that result in the system's emergent properties and behavior. The result was a set of developed techniques, referred to as relational modeling and analysis that utilize relationships as either a placeholder or bridging structure from which unknown aspects of the system can be effectively explored. These techniques were subsequently tested via the construction and analysis of models of five very different systems: synaptic neurotransmitter spillover, secondary spinal cord injury, physiological and pathological axonal transport, and amyotrophic lateral sclerosis and to analyze neurophysiological data of in vivo cat spinal motoneurons. Our relationship-based methodologies provide an equivalent means by which the different perspectives can be compared, contrasted, and aggregated into a truly comprehensive viewpoint that can drive research forward.

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La méthylation de l’ADN est une modification majeure du génome des eucaryotes permettant de moduler l’expression génique et contrôler le développement des mammifères. La protéine Mecp2 (Methyl CpG binding protein 2), dont le gène est situé sur le chromosome X, appartient à la famille des protéines de liaison à l’ADN méthylé. Sur la base de sa structure et de ses interactions Mecp2 a été décrit comme un répresseur de l’expression des gènes. A l’heure actuelle, son implication en tant qu’activateur de la transcription et organisateur de la structure chromatinienne lui confère un rôle plus global dans la régulation de l’épigénome. Des mutations de MECP2 conduisent à des troubles neurologiques dont le principal est le syndrome de Rett (RTT). Cette pathologie dominante liée à l’X affecte principalement les jeunes filles (incidence: 1/15000 naissances). Même si les causes précises du phénotype RTT ne sont pas connues, le profil d’expression de Mecp2 est en lien avec la synaptogenèse, la maturation et la maintenance des réseaux neuronaux. A mon arrivée en thèse l’équipe qui m’a accueilli venait d’identifier des déficits neuronaux, affectant notamment les groupes catécholaminergiques bulbaires et périphériques, à l’origine de troubles respiratoires chez un modèle murin de cette pathologie. Mon travail de thèse a permis de caractériser l’évolution postnatale des déficits moteurs et physiologiques affectant la souris Mecp2-déficiente. L’étude de structures catécholaminergiques d’intérêt telles que la Substantia Nigra et le Locus Coeruleus a révélé que les neurones dopaminergiques et noradrénergiques centraux ont un métabolisme affecté. Le nombre de neurones immunomarqués apparait significativement réduit dans ces groupes ce qui résulterait d’une perte progressive du phénotype « catécholaminergique », en l’absence de mort cellulaire. Nos données suggèrent que ces atteintes constituent un corrélat neuropathologique aux troubles comportementaux observés chez les souris Mecp2-déficientes. Ainsi certains troubles moteurs ont pu être améliorés, à l’aide d’un agent pharmacologique pro-dopaminergique, la L-Dopa. En relation avec les déficits en Bdnf (Brain-derived neurotrophic factor) décrits chez les patientes et les souris Mecp2-déficientes, nous avons identifié qu’une modification du dosage de Mecp2 induit une dérégulation de gènes (Htt, Hap1) codant des protéines impliquées dans le transport intracellulaire des vésicules de Bdnf. Nos travaux nous permettent de postuler que chez la souris Mecp2-déficiente, une altération de la dynamique de transport des vésicules chargées en Bdnf pourrait exacerber le déficit d’expression de cette neurotrophine. Notre traitement des souris Mecp2-déficientes par la cystéamine, une molécule capable d’agir sur les contenus, la libération et la sécrétion du Bdnf permet d’augmenter la survie des animaux et de réduire leurs troubles moteurs. Nos résultats montrent que les déficiences en Mecp2 entrainent des déficits de transport axonal du Bdnf qui s’ajoutent aux déficits de production du Bdnf. Par ailleurs, avec l’utilisation d’agents pharmacologiques agissant sur ce transport, nous offrons de nouvelles perspectives thérapeutiques. / DNA methylation is the major modification of eukaryotic genomes and plays an essential role in mammalian development. The protein Mecp2 (Methyl CpG binding protein 2), encoded by a gene located on the X chromosome, belongs to the ‘Methyl Binding domain’ protein family. Based on its structure and its interactions Mecp2 has historically been described as a repressor of expression for many genes. Currently, its involvement as an activator of transcription and its role in chromatin architecture suggests that it could be a global regulator of the epigenome. Mutations in MECP2 lead to neurological disorders, among which Rett syndrome (RTT). This dominant X-linked pathology mainly affects girls (incidence: 1/15000 live births). Although the precise causes of the RTT phenotype are unknown, the pattern of Mecp2 expression is related to synaptogenesis, maturation and neuromaintenance. Before my integration in the ‘Human Neurogenetics’ team, this group identified neural deficits, affecting brainstem and peripheral catecholaminergic cell groups, causing respiratory disturbances in a mouse model of this disease. My thesis work enabled the characterization of the postnatal physiological and motor deficits affecting the Mecp2-deficient mice. The study of catecholaminergic structures of interest such as the substantia nigra pars compacta and the locus coeruleus has revealed that the central noradrenergic and dopaminergic neurons are affected in their metabolism. The number of immunolabelled neurons of these groups appears significantly reduced and would result in a gradual loss of the mature ‘catecholaminergic’ phenotype, in the absence of cell death. Our data suggest that these defects are a neuropathological correlate for behavioral disorders observed in Mecp2-deficient mice. Some motor deficits have been improved, with L-Dopa, a pro-dopaminergic drug. In relation with Bdnf (Brain-derived neurotrophic factor) reduction described in patients and Mecp2-deficient mice, we identified that a change in the dosage of Mecp2 deregulates genes (Htt, Hap1) encoding proteins involved in the intracellular transport of Bdnf. Our work allows to postulate that in the Mecp2-deficient neurons, an altered dynamics of Bdnf vesicles transport could exacerbate the deficit of expression of this neurotrophin. Our treatment of Mecp2-deficient mice with cysteamine, a molecule able to increase Bdnf contents and enhancing its release and secretion, increased the survival of the animals and reduced their motor defects. Our results show that the Mecp2-deficiencies lead to alteration in the axonal transport of Bdnf in addition to deficits in Bdnf production. In addition, by the use of pharmacological agents that affect this transport, we offer new therapeutic perspectives.

O exercício físico pode atenuar os efeitos do envelhecimento sobre o sistema nervoso central, por meio do aumento da expressão de neurotrofinas, tais como fator neurotrófico derivado do cérebro (BDNF), o qual promove a ramificação dendrítica e melhora da maquinaria sináptica, pela interação com seu receptor TrkB. Receptores TrkB são produzidos no corpo da célula e transportados aos terminais axonais, por meio de SLP1, CRMP2, Rab27B e Sortilina onde são ancorados para realizar seu papel fisiológico. Sabendo que a relação entre o tráfego de receptores de neurotrofinas e o treinamento físico ainda é pouco conhecida, o objetivo do presente trabalho é analisar os níveis do receptor TrkB, bem como de seus transportadores anterógrados e retrógrados, no sistema nervoso central de ratos idosos, modelos de neurodegeneração, expostos a diferentes protocolos de treinamento físico moderado. Os ratos do primeiro grupo experimental foram expostos a 1mg/kg/dia de Rotenona ou DMSO durante 4 semanas, depois, juntamente com a exposição à rotenona, realizaram treinamento físico moderado em esteira, 5 vezes por semana, durante 40 minutos; ou permaneceram em repouso. Os ratos do segundo grupo experimental realizaram 6 semanas de treinamento, sendo em seguida expostos à rotenona por 4 semanas, e subdividos em dois grupos, um que continuou o exercício e outro que ficou sedentário. Os resultados encontrados sugerem que o treinamento físico parece reverter ou prevenir de maneira geral os danos presentes na neurodegeneração considerando as proteínas do tráfego de BDNF e seu receptor, e ainda, que a magnitude e direção destas alterações está diretamente relacionada ao protocolo de treinamento físico, bem como, a região do sistema nervoso central analisada / Physical exercise can attenuate the effects of aging on the central nervous system by increasing the expression of neurotrophins such as brain-derived neurotrophic factor (BDNF), which promotes dendritic branching and enhances synaptic machinery, through interaction with its receptor TrkB. TrkB receptors are synthesized in the cell body and are transported to the axonal terminals, through SLP1, CRMP2, Sortilin and Rab27B, to where receptors are anchored to perform its physiological role. However, the aspects of the neurotrophin receptors traffic after physical training is still a matter of investigation. Thus, the present study aims to analyze the expression levels of TrkB receptor and their anterograde carriers in aged Lewis rats, model of neurodegeneration, and its relationship with moderate exercise training. Rats from the first experimental group were exposed to 1mg/kg/day of Rotenone (ROT) or DMSO for 4 weeks, and then subjected or not to moderate exercise running on treadmill, five days a week, 40 minutes a day, combined with the drug. Rats from the second experimental group were trained for 6 weeks, followed by exposure to rotenone during 4 weeks, rats were then subdivided into two groups, one that continued the exercise and the other became sedentary. Results suggest that exercise training appears to reverse or prevent the impairment related to neurodegeneration considering the proteins involved in BDNF signaling, and also that the magnitude and direction of these changes in directly related to the physical training protocol, as well as the area of the central nervous system analyzed