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La Polarité Cellulaire Planaire (PCP) est une voie de signalisation originellement identifiée chez les invertébrés pour son rôle dans l’établissement d’une asymétrie cellulaire perpendiculaire à l’axe apico‐basal. Elle définit une polarité dans le plan d’un épithélium et coordonne cette polarité dans tout l'épithélium. L'activation de la voie PCP conduit à une réorganisation ducyto squelette en passant par une modulation des zones d'adhésion, régulant ainsi la forme et les mouvements des cellules. La voie de signalisation de la PCP est conservée tout au long de l'évolution jusqu'au mammifères, et contrôle la morphogénèse de divers tissus dont les tissus épithéliaux et mésenchymateux, ainsi que pour les tissues cardiaques, osseux, pulmonaire ou encore rénaux, mais aussi le système nerveux pour n'en citer que quelques‐uns.Afin d'identifier le rôle de vangl2, un des gènes centraux de la PCP, dans la mise en place de la circuiterie hippocampale, nous avons créé un modèle murin où vangl2 est supprimé de façon conditionnelle (cKO) dans le télencéphale à des stades précoces de l’embryogénèse. J’ai d'abord montré que Vangl2 est enrichi dans les neurones immatures de la zone sous granulaire du DG, ainsi que dans l’arborisation des neurites (axones et dendrites) des cellules granulaires (CG) du gyrus denté (DG) de l’hippocampe. Ainsi, Vangl2 est enrichi dans le stratum lucidum (sl), une région dense en contacts synaptiques entre le DG et le CA3. Dans cette région a lieu une synapse très particulière entre l'axone des CG, la fibre moussue (Mf) qui forme des boutons géants (MfB) et les excroissances épineuse (TE) issues de la partie proximale des dendrites apicaux. L'analyse structurale et ultra structurale de ces épines démontre que l'élargissement et la complexification de la synapse MfB/TE est bloquée dans nos mutants, alors que les zones actives (PSD) des épines sont présentes, mais réorganisées. De façon intéressante,dans une zone plus distale des dendrites des neurones du CA3 (sl), les épines sont, elles, plus grosses, suggérant un remodelage complexe du réseau en l'absence de vangl2. Enfin, j’ai pu montrer que ces défauts morphologiques étaient corrélés à des problèmes de mémoire complexe (mémoire déclarative) qui dépendent de l’hippocampe mais aussi du cortex. Cette étude montre pour la première fois l’importance du signal PCP dans maturation in vivo d’un circuit hippocampique spécifique ainsi que ces conséquences cognitives. D'autres résultats in vitro montrent que la suppression de vangl2 augmente la vitesse de déplacement des cônes de croissance sur des substrats de N‐cadhérine. J’ai utilisé la microscopie en super résolution spt‐PALM‐TIRF pour montrer que cette augmentation de croissance est inversement proportionnelle à la vitesse du flux rétrograde d’actine. Des expériences de FRAP permettent de suggérer que les molécules de N‐cadhérine engagées dans des interactions hémophiliques (adhésion) est plus importante dans les mutants vangl2 Je propose que Vangl2 contrôle le recyclage et la stabilité des protéines N‐cadhérine dans les sites d’adhésion afin de réguler localement les dynamiques d’actine et par conséquent la croissance neuronale. / Planar Cell Polarity (PCP) is a signaling pathway originally known for its role in the establishment of cellular asymmetry perpendicular to the apico‐basal axis, in the plane of an epithelium. PCPsignaling has been shown to be crucial for many tissue patterning, including epithelial and mesenchymal tissue, but also cardiac, lung, bone, or kidney tissues, to cite a few. PCP signaling controls the regulation of cellular movement via the control of adhesion turnover and cytoskeleton reorganization. Vangl2 is one of the most upstream core PCP proteins that has been implicated in the recent years in various neuronal mechanisms, such as axonal guidance, dendrite morphogenesis or synaptogenesis. However, most of these studies rely on acute downregulation of the gene in vitro or in the use of a mouse presenting a spontaneous mutation of this gene, called Loop‐tail (Vangl2Lp) which causes the death of the embryo at birth. Moreover, the Vangl2Lp form of this protein has been described has a dominant‐negative form, making it difficult to untangle the molecular mechanism leading to the many phenotypes (included neuronal ones) reported inhomozygotes Looptail mice. To bypass this problem we created a conditional knockout (cKO) mouse in which vangl2 is deleted in the telencephalon during early embryogenesis. First, I analyzed the profile of expression of the protein during the first 3 weeks after birth, and I show that Vangl2 is specifically targeted to the arborization of granular cells (GC) of the dentate gyrus (DG) of the hippocampus, and excluded from cell bodies. Also, the protein was highly enriched in immature neurons of the subgranular zone of the DG, and in the stratum lucidum, a region of high‐density contacts between the GC and the CA3. In this region, a special type of synapse is formed: the Mossy Fiber Bouton (MfB) / Thorny Excrescence (TE) synapse. These synapses are bigger and more complex than conventional synapses. I then performed a structural and ultrastructural analysis of the DG/CA3 circuit in the Vangl2 cKO mice in order to understand the role of Vangl2 in the hippocampus maturation. For this, I used stereotaxic mice infection viruses, and Serial block face scanning electron microscopy (SBFsEM) with 3D reconstruction. Results show that in cKO mice, Mfs fasciculation is mildly impacted, and that the enlargement and complexification of the MfB/TE synapse is arrested, with TEs almost absent. I was able to link these morphological abnormalities to deficits in complex hippocampal‐dependent learning tasks. This work demonstrates for the first time the importance of PCP signaling for the in vivo maturation of a specific hippocampal circuit and its specific cognitive consequences. Next, I attempted to identify the functional consequences of vangl2 deletion on young hippocampal neuron maturation. My results confirm that Vangl2 is expressed in young hippocampal neurons and that the deletion of the gene affected neurite outgrowth on Ncadherin substrate. I used spt‐PALM‐TIRF super‐resolution microscopy to show that this increased neurite outgrowth was inversely proportional to a decrease in actin retrograde flowand to a decrease in the number of directed actin trajectories. These results strongly suggest that N‐cadherin adhesions are affected by Vangl2 deletion. FRAP experiments demonstratedthat in Vangl2 cKO neurons the recovery of N‐cadherin molecules engaged in homophilicbindings (adhesion) was decreased, suggesting that the turnover of N‐cadherin involved inadhesion is reduced. Altogether, I propose that Vangl2 controls the turnover/stability of Ncadherin proteins at adhesion sites to regulate local actin dynamics and consequently neuronal outgrowth

The mechanisms that order cellular packing geometry are critical for the functioning of many tissues, but are poorly understood. Here we investigate this problem in the developing wing of Drosophila. The surface of the wing is decorated by hexagonally packed hairs that are uniformly oriented towards the distal wing tip. They are constructed by a hexagonal array of wing epithelial cells. We find that wing epithelial cells are irregularly arranged throughout most of development but become hexagonally packed shortly before hair formation. During the process, individual cell junctions grow and shrink, resulting in local neighbor exchanges. These dynamic changes mediate hexagonal packing and require the efficient delivery of E-cadherin to remodeling junctions; a process that depends on both the large GTPase Dynamin and the function of Rab11 recycling endosomes. We suggest that E-cadherin is actively internalized and recycled as wing epithelial cells pack into a regular hexagonal array. Hexagonal packing furthermore depends on the activity of the Planar Cell Polarity proteins. The Planar Cell Polarity group of proteins coordinates complex and polarized cell behavior in many contexts. No common cell biological mechanism has yet been identified to explain their functions in different tissues. A genetic interaction between Dynamin and the Planar Cell Polarity mutants suggests that the planar cell polarity proteins may modulate Dynamin-dependent trafficking of E-cadherin to enable the dynamic remodeling of junctions. We furthermore show that the Planar Cell Polarity protein Flamingo can recruit the exocyst component Sec5. Sec5 vesicles also co-localizes with E-cadherin and Flamingo. Based on these observations we propose that during the hexagonal repacking of the wing epithelium these proteins polarize the trafficking of E-cadherin-containing exocyst vesicles to remodeling junctions. The work presented in this thesis shows that one of the basic cellular functions of planar cell polarity signaling may be the regulation of dynamic cell adhesion. In doing so, the planar cell polarity pathway mediates the acquisition of a regular packing geometry of Drosophila wing epithelial cells. We identify polarized exocyst-dependent membrane traffic as the first basic cellular mechanism that can explain the role of PCP proteins in different developmental systems.