Résumé confidentiel / Résumé confidentiel

The purpose of the thesis is to explore the role of BMP signaling in developing vertebrate midbrain. BMP signaling plays important roles in various tissues and stages of neural development to regulate cell fate, proliferation, differentiation, morphogenesis and more. We observed that several major BMPs are expressed not only at the roof plate but also the floor plate of the midbrain. This has led us to ask the role of BMP signaling in dorsal and ventral midbrain patterning. Despite ventral experiments, we found that BMP signaling does not regulate ventral cell fate specification in the midbrain. Instead BMPs profoundly influence the shape and early morphogenesis of the midbrain neural plate as it closes into a neural tube. During neural tube closure, one of the early events occurring at the ventral midline is median hinge point (MHP) formation. Failure to form MHP leads to neural tube closure defects, the 2nd most common birth defects in humans. However, the molecular mechanisms underlying MHP formation are not well known. We found that the lowest BMP signaling occurs at the MHP during early neurulation and BMP blockade is necessary and sufficient for MHP formation. Interestingly, we also demonstrated that BMP blockade directs MHP formation by regulating the apicobasal polarity pathway and this regulation may be mediated by biochemical interactions between pSMAD5 and the apical protein, PAR3. Additionally, our time-lapse data suggest that BMP blockade slows cell cycle progression by increasing duration of G1 to S transition and S phase which leads cell nuclei stay at the basal location longer. This mimics basal nuclear migration seen at the MHP where low BMP signaling occurs. Thus, we conclude that BMP signaling regulates neural tube closure via the apicobasal polarity pathway and in a cell cycle dependent manner at the ventral midline. We observed that BMP signaling is necessary and sufficient for the dorsal cell fate specification in a context-dependent manner and ventral BMP signaling affects dorsal cell fates. Taken together, we propose the idea that BMP signaling has distinct roles in different contexts. BMPs regulate tissue morphogenesis in the ventral midbrain and dorsally cell fate specification. / text

La cytoarchitecture du néocortex repose sur la coordination spatiotemporelle des taux de production des neurones ‐via la régulation du cycle cellulaire de leurs progéniteurs‐ et de leur migration radiale vers la surface corticale. Chez le primate, les couches supragranulaires de l'aire 17 sont plus développées que celles de l'aire 18, conséquence d'une prolifération et d'une production neuronale accrues dans l'aire 17 à E77‐80. Des observations en vidéomicroscopie bi‐photonique, sur tranches organotypiques de cortex, révèlent que la migration radiale est plus rapide dans l'aire 17. Les variations aire‐spécifiques des taux de prolifération et de migration neuronale sont donc congruentes. L'étude des mécanismes moléculaires qui sous‐tendent la régulation coordonnée de la prolifération et de la migration est centrée sur le régulateur du cycle cellulaire p27kip1, qui via son domaine C‐terminal promeut la migration en inhibant la GTPase RhoA. Ce rôle pléiotrope de p27 a été exploré dans la migration nucléaire intercinétique (INM) qu'effectuent les précurseurs de la zone ventriculaire corticale, en synchronie avec les phases du cycle cellulaire. Des formes mutantes de p27 ou des shRNA ont été électroporés spécifiquement dans les neuroblastes d'embryons murins à E14‐15. Des observations en vidéomicroscopie bi‐photonique sur tranches organotypiques révèlent que le domaine C-terminal de p27 affecte l'INM, promeut les décisions différenciatives et la migration radiale. P27 se place donc au sein d'un réseau moléculaire contrôlant conjointement, et de façon aire-spécifique, les divisions successives des précurseurs corticaux, ainsi que la migration des neurones qui en sont issus / Cortical cytoarchitecture relies on the spatiotemporal coordination of neuronal production rate, precursors cell-cycle control and neuronal radial migration towards the cortical plate. In the primate, area 17 supragranular layers are more developed than in area 18, due to higher proliferation and neuronal production rates in area 17 between E77-80. Two-photon videomicroscopy observations on cortical organotypic slices revealed that radial migration is faster in area 17 than 18. This indicates that area-specific variations of proliferation and migration rates are congruent during corticogenesis. The study of molecular mechanisms underlying the coordinated regulation of proliferation and migration focused on the cell-cycle regulator p27kip1, which promotes migration, via inhibition of the Rhoa GTPase by its C-terminal domain. This p27 dual function could play a major role during the Interkinetic Nuclear Migration (INM) performed by cortical precursor cells from the ventricular zone, in synchrony with the cell-cycle phases. Mutant forms of p27 or shRNA were electroporated into neuroblasts of E14-15 mice embryos. Two-photon videomicroscopy observations on organotypic slices revealed that p27 affects INM, promotes differentiative divisions and neuronal radial migration, though its C-terminal domain. P27 is thus part of a molecular network which finely tunes, in an area-specific manner, the successive rounds of divisions of precursor, as well as the migratory behavior of the newborn neurons