Le développement embryonnaire est un processus hautement contrôlé où différentes voies de signalisation se coordonnent pour la construction d'un organisme. L'une des principales voies de signalisation impliquées dans ce processus est la voie canonique Wnt. La longue quête pour comprendre la cascade de signalisation Wnt/β-catenine a révélé que la réponse transcriptionelle induite par le signal Wnt/β-catenine est dépendante du contexte, ou compétence, cellulaire. Peu de choses sont connues sur les évènements moléculaires qui influencent cette compétence cellulaire. Dans les embryons de X. laevis Wnt/β-catenine est le signal inducteur pour l'Organisateur de Spemann. On ne sait pas ce qui limite l'activité Wnt dans ce territoire et par voie de conséquence la taille de l'Organisateur. Les résultats présentés dans ce manuscrit de thèse montrent que le facteur de transcription Barhl2 affecte le développement de l'organisateur de Spemann. Nous démontrons que Barhl2 inhibe l'activité Wnt via son interaction avec le corépresseur Groucho et le facteur de transcription Tcf, et mobilise l'activité de Hdac1 qui agit sur la structure chromatinienne. En utilisant des expériences in vitro et in vivo sur des cellules en culture et des embryons de Xénope nous démontrons que la régulation de Barhl2 sur les activités Groucho-Tcf est maintenue pendant l'embryogenèse et joue un rôle dans le confinement des progéniteurs neuraux dans le cerveau. Ensemble, nos résultats fournissent un mécanisme nouveau et important agissant sur le contrôle de l'activité transcriptionelle Wnt et la compétence des cellules à répondre à ce signal. / Embryonic development is a highly controlled process where different signaling pathways participate into the elaboration of an organism. One of the main signaling pathways is the Wnt canonical pathway. The long-lasting search to understand Wnt/β-catenin transduction cascade revealed that the net transcriptional read out of Wnt/β-catenin signaling is highly dependent on the cellular context. In X. laevis embryos Wnt/β-catenin signaling is the informative signal for the Spemann Organizer induction. However little is known on what limits Wnt activity in this territory and consequently the size of the Spemann Organizer. The results presented in this manuscript provide evidence that the evolutionarily conserved transcription factor Barhl2 limits the development of the Spemann organizer. In this territory Barhl2 inhibits Wnt activity via its interaction with the co-repressor Groucho and the transcription factor Tcf. It participates to the recruitment of the chromatin remodeling enzyme, Hdac1 that represses the expression of Spemann organizer genes. Using a Xenopus tropicalis Tcf reporter line we demonstrate that Barhl2 inhibitory effect on Groucho-Tcf activities is maintained during embryogenesis and plays a role in the confinement of neural progenitors in the brain. Together, our results provide a new and important mechanism for the control of Wnt transcriptional activity.

Diencephalic development requires the transcription factors Pax6 and Barhl2 in order to proceed correctly. Both genes are necessary for the normal development of the organizer region known as the zona limitans intrathalamica (ZLI). The ZLI goes on to pattern the diencephalon via its secretion of the morphogen Shh, which inhibits the expression of Pax6. These findings suggest that interactions between Pax6, Barhl2 and Shh may be involved in the control of diencephalic development. This project aims to characterise these interactions and investigate their roles. The expression domains of Pax6 and Barhl2 were mapped during the early development of the mouse diencephalon. Qualitative approaches were employed to confirm the high complementarity of their expression domains and obtain evidence of a mutually repressive relationship existing between the two genes. The findings from a quantitative analysis suggested that this inhibition is incomplete within the thalamus. Investigations using the Pax6-null mutant mouse confirmed that in the absence of Pax6 the thalamic Barhl2 expression domain expands beyond the ventricular zone, the site of thalamic neurogenesis. The influence of Shh signalling on the expression of Pax6 and Barhl2 was investigated via a gain-of-function approach utilising in utero electroporation to activate the Shh pathway. This led to a downregulation of both Pax6 and Barhl2 within the thalamus. In Shh loss-of-function experiments drug treatment with the Shh antagonist vismodegib led to an upregulation of Barhl2 and the loss of the GABAergic pTh-R in the Pax6-null mutant thalamus, but not in the wild type thalamus, suggesting that Pax6 and Shh may be required to inhibit Barhl2 in order for GABAergic neurogenesis to proceed. Barhl2 expression was detected in the Shh-null mutant mouse confirming that, in contrast with their homologues in Drosophila, Shh may be expressed downstream of Barhl2. Together these findings have been used to develop a novel model of thalamic development in which Barhl2 induces ZLI development, inhibition of Barhl2 by Pax6 restricts its expansion, and secretion of Shh by the ZLI then goes on to inhibit Pax6 and Barhl2 in the pTh-R while mutual repression between Pax6 and Barhl2 modulates neurogenesis in the more caudal regions of the thalamic neuroepithelium.