The adult mammalian brain harbours at least two germinal - or neurogenic - niches in which new neurons are born throughout life. These neurogenic niches comprise the subependymal zone which lines the ventricular system, and the subgranular zone in the hippocampal dentate gyrus. Post-natal hippocampal neurogenesis was in fact first identified experimentally in the 1960s. However perhaps due partly to aforementioned institutionalised belief and partly to a lack of accessible experimental tools, the phenomenon of hippocampal neurogenesis was widely recognised by the scientific community only shortly before the millennium. Consequent study has established that adult hippocampal neurogenesis has been conserved through millions of years of evolution in nearly every mammalian species studied to date. Importantly, post-mortem studies and radioisotope carbon dating techniques suggest that it also occurs in humans. A great deal of this research has focused on understanding the inner workings of the cells that undergo the transformation to become new adult-born neurons. By contrast, relatively little is known about the potential regulatory role of the surrounding extracellular microenvironment. This might be useful to know in light of much evidence that the extracellular matrix is a key regulator of developmental neurogenesis. This thesis describes my study of whether extracellular matrix regulates hippocampal neurogenesis.

In the 1980s, it was widely accepted that new neurons are continuously generated in the dentate gyrus of the mammalian hippocampus. Since its acceptance, researchers have employed various techniques and behavioral paradigms to study the proliferation, differentiation, and functional role of adult-born neurons. This literature thesis aims to discuss how optogenetics is able to overcome the limitations of past techniques and provide the field with new insights into the functional role of neurogenesis. We will review the current knowledge on both adult hippocampal neurogenesis and optogenetics, present representative studies using optogenetics to investigate neurogenesis and discuss potential limitations and concerns involved in using optogenetics.

Hippocampal neurogenesis continues throughout life in mammals. These adult-generated dentate granule cells (DGCs) are generally believed to contribute to hippocampal memory processing and are generated at varying rates in response to neuronal network activity. Deep brain stimulation (DBS) allows clinicians to influence brain activity for therapeutic purposes and raises the possibility of targeted modulation of adult hippocampal neurogenesis. It has recently been shown that DBS may ameliorate cognitive decline associated with Alzheimer’s disease (AD), and while underlying mechanisms are unknown, one possibility is activity-dependent regulation of hippocampal neurogenesis. To this end, whether or not adult-generated DGCs can assume functional roles of developmentally-generated neurons, and stimulation-induced enhanced neurogenesis can benefit memory function in the normal and diseased brain, warrant study. First, we examined separate cohorts of developmentally- and adult-generated DGCs in intact mice and demonstrated similar rates of activation during hippocampus-dependent spatial memory processing, suggesting functional equivalence. Second, we examined the neurogenic and cognitive effects of targeted entorhinal cortex (EC) stimulation in mice using parameters analogous to clinical high frequency DBS. Stimulation increased the generation of DGCs. Moreover, stimulation-induced neurons were functionally recruited by hippocampal spatial memory processing in a cell age-dependent fashion that is consistent with DGC maturation. Importantly, stimulation facilitated spatial memory in the same maturation-dependent manner, and not when stimulation-induced promotion of adult neurogenesis was blocked, suggesting a causal relationship. Finally, we are in the process of testing whether similar stimulation facilitates spatial memory in a transgenic (Tg) disease model of AD that exhibits amyloid neuropathology and cognitive impairment. Preliminary results suggest stimulation promotes neurogenesis and rescues impaired spatial memory in Tg animals. When considered in the context of promising clinical results, this body of work suggests stimulation-induced neurogenesis could provide a novel therapeutic modality in settings where functional hippocampal regenerative therapy is desirable.

Hippocampal neurogenesis continues throughout life in mammals. These adult-generated dentate granule cells (DGCs) are generally believed to contribute to hippocampal memory processing and are generated at varying rates in response to neuronal network activity. Deep brain stimulation (DBS) allows clinicians to influence brain activity for therapeutic purposes and raises the possibility of targeted modulation of adult hippocampal neurogenesis. It has recently been shown that DBS may ameliorate cognitive decline associated with Alzheimer’s disease (AD), and while underlying mechanisms are unknown, one possibility is activity-dependent regulation of hippocampal neurogenesis. To this end, whether or not adult-generated DGCs can assume functional roles of developmentally-generated neurons, and stimulation-induced enhanced neurogenesis can benefit memory function in the normal and diseased brain, warrant study. First, we examined separate cohorts of developmentally- and adult-generated DGCs in intact mice and demonstrated similar rates of activation during hippocampus-dependent spatial memory processing, suggesting functional equivalence. Second, we examined the neurogenic and cognitive effects of targeted entorhinal cortex (EC) stimulation in mice using parameters analogous to clinical high frequency DBS. Stimulation increased the generation of DGCs. Moreover, stimulation-induced neurons were functionally recruited by hippocampal spatial memory processing in a cell age-dependent fashion that is consistent with DGC maturation. Importantly, stimulation facilitated spatial memory in the same maturation-dependent manner, and not when stimulation-induced promotion of adult neurogenesis was blocked, suggesting a causal relationship. Finally, we are in the process of testing whether similar stimulation facilitates spatial memory in a transgenic (Tg) disease model of AD that exhibits amyloid neuropathology and cognitive impairment. Preliminary results suggest stimulation promotes neurogenesis and rescues impaired spatial memory in Tg animals. When considered in the context of promising clinical results, this body of work suggests stimulation-induced neurogenesis could provide a novel therapeutic modality in settings where functional hippocampal regenerative therapy is desirable.

It was long believed within the scientific community that the adult brain was unable to generate new neurons. In the end of the 1990s the consensus changed and it is since believed that the adult brain can and does generate new neurons after birth, a process referred to as adult neurogenesis. Adult neurogenesis takes place in two places in the adult brain: the subventricular zone (SVZ) in close proximity to the olfactory bulb and the subgranular zone (SGZ) in the hippocampus. The level of adult hippocampal neurogenesis (AHN) can be upregulated and one part of the aim was to examine the effect of voluntary chronic aerobic exercise (VCAE) on AHN. It is clear that voluntary chronic aerobic exercise reliably increases AHN. Still, the function of these new brain cells is under debate. Spatial learning and memory are among the main abilities that have been focused on. The other part of the aim was to examine the effect of VCAE and AHN on spatial learning and memory. The reviewed literature suggests that both AHN and spatial learning and memory increase together from VCAE, although it does not show causation, that an increase of AHN from VCAE causally effects spatial learning and memory. More studies are needed to investigate if a causal relationship exists.

Les dernières années témoignent d'une remarquable accélération dans la compréhension des facteurs génétiques impliqués dans la déficience intellectuelle (DI) et de nombreux gènes responsables ont été identifiés. Néanmoins, les mécanismes cellulaires et moléculaires sous-jacents à la DI sont encore mal connus. Une hypothèse attractive est que les mutations à l’origine de DI affectent la neurogenèse hippocampique adulte (NGA), une forme de plasticité qui joue un rôle crucial dans la mémoire. L'objectif de ce projet est d’entreprendre une analyse comparative de la NGA chez trois modèles murins de pathologies d’origine génétique, menant à une DI sévère, impliquant des gènes localisés sur le chromosome X et participant à différentes voies de signalisation susceptibles de moduler la NGA : le syndrome de Coffin-Lowry (gène rsk2), la dystrophie musculaire de Duchenne (gène dmd) et une DI liée au gène pak3. Mes recherches actuelles montrent que ces trois modèles présentent des déficits cognitifs dépendants de l’hippocampe, dont des altérations de la fonction de séparation de patterns. Nous avons également mis en évidence des altérations de la NG adulte, avec, entre autres, des altérations du recrutement des jeunes neurones par l’apprentissage qui pourraient contribuer aux déficits cognitifs observés en particulier dans la fonction de séparation de patterns. Toutefois, selon les gènes en cause, les déficits ne sont pas observés dans les mêmes étapes de la NGA ni dans les mêmes situations comportementales. L’ensemble de ces résultats laisse donc suggérer que chacun des gènes étudiés pourrait jouer un rôle différent dans la NGA, mais qu'in fine des altérations de cette forme de plasticité contribuent, au moins en partie, aux déficits cognitifs associés à la DI dans les trois modèles. Ensemble, ces résultats apportent des informations supplémentaires qui seront directement pertinentes pour d’autres pathologies neuro-développementales conduisant à des déficits cognitifs liés à des altérations de la NG, et pourraient ouvrir de nouvelles pistes thérapeutiques. / Recent years have shown a remarkable acceleration in the understanding of genetic factors involved in intellectual disability (ID) and many genes responsible have been identified. However, the cellular and molecular underlying mechanisms are still poorly understood. An attractive hypothesis is that mutations causing ID may affect adult hippocampal neurogenesis (ANG), a form of plasticity that plays a crucial role in learning and memory. The objective of this project was to undertake a comparative analysis of adult hippocampal neurogenesis in three mouse models of genetic diseases involving genes located on the X chromosome and participating in different signalling pathways that may modulate ANG: the Coffin-Lowry syndrome (rsk2 gene), Duchenne muscular dystrophy (dmd gene) and ID due to mutation of the pak3 gene. My current research shows that these three models present hippocampal dependent cognitive deficits. Among these deficits, major deficits in spatial pattern separation function have been highlighted. We also showed specific alterations of basal ANG, together with alterations in the recruitment of young newborn neurons by learning that could contribute to the observed cognitive deficits, in particular in pattern separation function. However, depending on the genes involved, the deficits are not observed in the same steps of adult NG and in the same behavioural situations. In all, the results suggest that each of the genes plays a different role in ANG, but finally that alterations of this form of plasticity may contribute to the cognitive deficits associated with ID in the three models. Together, these results provide additional information that will be directly relevant to other neurodevelopmental disorders leading to cognitive deficits related to NG alterations, and could open new therapeutic tracks.

No description available.

La maladie d'Alexander est une maladie neurodégénérative due à des mutations hétérozygotes du gène GFAP codant le principal filament intermédiaire des astrocytes matures. Nous avons étudié l'effet des mutations GFAP dans l'hippocampe d'un patient avec AxD infantile et de deux souris knockin, l'une portant une mutation dans le rod domain (p.R85C) et l'autre dans le tail domain (p.T409I). Chez le patient, nous décrivons pour la première fois: (i) des changements morphologiques sévères des cellules GFAP+ dans la zone subgranulaire du gyrus denté, qui ont perdu la plupart de leurs processus radiaux; (ii) une réactivité microgliale; (iii) et un déficit de la neurogénèse hippocampique postnatale. Nous avons trouvé des anomalies similaires dans les deux souris knockin, plus sévères chez les homozygotes. La comparaison de ces modèles a montré que ces anomalies prédominent chez les souris GFAPT409I, tandis que l’accumulation de GFAP est supérieure chez les souris GFAPR85C. La comparaison des deux modèles de souris a montré que les conséquences pathologiques dépendent la localisation de la mutation dans la GFAP. Ces résultats suggèrent qu'en plus du gain évident de fonction, d'autres dysfonctions astrocytaires dans peuvent être dues à une perte de fonction. De plus, nous avons traité les souris mutantes avec de la ceftriaxone, connu pour son effet neuroprotecteur, mais nous n'avons observé aucun effet significatif. Enfin, la mégalencéphalie étant fréquente chez les patients AxD, nous avons mesuré la quantité d'eau cérébrale chez les souris mutantes GFAP. Nous avons trouvé une augmentation significative de la teneur en eau chez les souris GFAPR85C/R85C âgées d'un an. Nous avons observé une localisation anormale de l'AQP4 dans les astrocytes des asouris mutées, pouvant participer au déséquilibre hydrique cérébral. / Alexander disease is a neurodegenerative disorder caused by heterozygous mutations of GFAP gene coding the major intermediate filament of mature astrocytes. We studied the effect of GFAP mutation in the hippocampus of infantile onset AxD patient and two novel knockin mouse models, one bearing a mutation located in the rod domain (p.R85C), and the other bearing a mutation located in the tail domain (p.T409I) of mouse Gfap. In the AxD patient, we describe for the first time: (i) obvious morphological changes of GFAP+ cells in the subgranular zone of the dentate gyrus, which have lost most of their radial processes; (ii) microglial reactivity; (iii) and deficit in postnatal hippocampal neurogenesis. We found similar abnormalities in the two knockin mouse lines, more obvious in homozygous mice. A comparison of these mouse models showed that pathological findings predominated in the GFAPT409I mice, whereas GFAP accumulated in larger amounts in the GFAPR85C mice. The comparison of the two mouse models showed that their pathological consequences depend on the location of the mutated residues in GFAP. These findings suggest that in addition to the evident gain of GFAP function, other astrocyte dysfunctions in this disease may be due to a loss of function of GFAP. In addition, we treated the mice mutants with ceftriaxone, which has been reported to have a neuroprotective effect, but we observe no significant effect. Finally, AxD patients have often megalencephaly, therefore we measured the brain water content in AxD mouse models. We found a significant increase in brain water content in the one year old GFAPR85C/R85C mice vs controls. We observed mislocalization of AQP4 in mutant mice astrocytes that can participated to water imbalance in brain.

Le déclin cognitif associé au vieillissement chez l’Homme, impacte fortement la vie quotidienne des personnes âgées. Si la pratique musicale apparait comme une activité de loisir prometteuse pour le maintien d’un bon fonctionnement cognitif au cours du vieillissement, les mécanismes neurobiologiques sous-jacents sont à l’heure actuelle, mal connus. L’objectif de ce travail a donc été d’étudier les effets d’une exposition tardive et chronique à la musique sur les performances comportementales et certains processus neurobiologiques au cours du vieillissement chez le rat Wistar. Si quelques études rapportent qu’une exposition à la musique améliore les performances d’apprentissage et de mémoire, associé à une augmentation de la neurogenèse hippocampique et du BDNF chez le Rongeur jeune adulte, il n’en existe aucune qui se soit intéressée à ces effets chez le Rongeur âgé. Des rats d’âge médian ont été répartis dans 2 groupes, l’un exposé à de la musique et l’autre à du bruit blanc, puis inclus dans une étude longitudinale, au cours de laquelle les performances comportementales ont été évaluées jusqu’à l’âge de 24 mois, suivi d’analyses biologiques. Ainsi, nous avons pu montrer qu’une exposition chronique à la musique démarrant à un âge médian, réduit le déclin cognitif associé au vieillissement. En revanche, la neurogenèse hippocampique et le BDNF n’apparaissent pas comme des mécanismes neurobiologiques potentiels impactés par la musique chez le rat âgé. / Cognitive decline associated to aging impacts daily life of elderly. While the music practice appears as promising leisure activity to prevent cognitive decline in elder, little is known about the neurobiological mechanisms involved. The aim of this work was to study the effects of music exposure on behavioral performances and some neurobiological processes across aging in rats. Indeed, improved behavioral performances together with an increased hippocampal neurogenesis and a higher BDNF expression were reported after music exposure in both young and adult animals. Yet, no study has so far investigated these effects in aged rats. After a fine appraisal of the cognitive state in middle-aged Wistar rats (15 months), they were divided in two groups, exposed either to classic music or to white noise. Thereafter, a longitudinal follow up of 9 months was performed. We observed for the first time that chronic music exposure alleviated age-related cognitive decline. However, contrary to what was observed in adult animals, we did not reported any differences in age-related changes of hippocampal neurogenesis and BDNF expression. These promising results of a beneficial effect of music exposure in the field of aging still lay open the question about the underlying mechanisms in the context of aging of the beneficial effect of music exposure.

Due to two different mutations in the gene that encodes the a1A subunit of voltage-activated CaV 2.1 calcium ion channels, the compound heterozygous tottering/leaner (tg/tgla) mouse exhibits numerous neurological deficits. Human disorders that arise from mutations in this voltage dependent calcium channel are familial hemiplegic migraine, episodic ataxia-2, and spinocerebellar ataxia 6. The tg/tgla mouse exhibits ataxia, movement disorders and memory impairment, suggesting that both the cerebellum and hippocampus are affected. To gain greater understanding of the many neurological abnormalities that are exhibited by the 90-120 day old tg/tgla mouse the following aspects were investigated: 1) the morphology of the cerebellum and hippocampus, 2) proliferation and death in cells of the hippocampal dentate gyrus and 3) changes in basic biochemical parameters in granule cells of the cerebellum and hippocampus. This study revealed no volume abnormalities within the hippocampus of the mutant mice, but a decrease in cell density with the pyramidal layer of CA3 and the hilus of the dentate gyrus. Cell size in the CA3 region was unaffected, but cell size in the hilus of the dentate gyrus did not exhibit the gender difference seen in the wild type mouse. The cerebellum showed a decrease in volume without any decrease in cerebellar cellular density. Cell proliferation and differentiation in the subgranular zone of the hippocampal dentate gyrus remained normal. This region also revealed a decrease in cell death in the tg/tgla mice. Basal intracellular calcium levels in granule cells show no difference within the hippocampus, but an increase in the tg/tgla male cerebellum compared to the wild type male cerebellum. There was no significant difference in granule cell mitochondrial membrane potential within the wild type and mutant animals in either the hippocampus or cerebellum. The rate of reactive oxygen species (ROS) production in granule cells revealed no variation within the hippocampus or cerebellum. The amount of ROS was decreased in cerebellar granule cells, but not granule cells of the hippocampus. Inducing ROS showed no alteration in production or amount of ROS produced in the hippocampus, but did show a ceiling in the amount of ROS produced, but not rate of production, in the cerebellum.