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The role of Pax6 and Wls in rhombic lip compartmentation and cerebellar development Yeung, Joanna


The transcription factor Pax6 is prominently expressed in the glutamatergic neurons during cerebellar development. When mutated, the Pax6-null Sey cerebellar granule cells (GC) are disorganized and foliation is disrupted. To elucidate the gene network regulated by Pax6 in cerebellar development, the Sey mutant transcriptome was analyzed and differentially expressed genes were identified. In this thesis, I examine some of these genes and their role in cerebellar development, as well as the relationship of these molecules with Pax6. Wntless (Wls) is up-regulated in the Sey cerebellum. Wls expression is restricted to the interior face of the rhombic lip (iRL) in normal cerebellar development. Whereas in the Sey cerebellum, Wls-expressing cells expand into the EGL, indicating that Pax6 normally suppresses Wls expression. Furthermore, examination of Wls and other rhombic lip (RL) markers in the wildtype embryos demonstrates that the RL is dynamically demarcated into four molecularly distinct compartments. Conversely, Tbr1 and Tbr2 are down-regulated in the Sey cerebellum. These are cell markers of cerebellar nuclei (CN) neurons and unipolar brush cells (UBCs), respectively. The absence of CN neurons and UBCs in the Sey mutant are revealed using standard immunohistochemistry and Nissl staining. Cell death analysis demonstrates that there is enhanced cell death in Sey mutant CN neuron progenitors, GCs and UBCs. Cell proliferation analysis also shows a reduction in the Sey RL progenitor pool during the genesis of UBCs and GCs. To elucidate the requirement of Wls in cerebellar development, I examine the conditional Wls knockout (Wls-cKO) in which Wls is ablated in the RL during mid-gestation. Ectopic Pax6-expressing cells are found in the Wls-cKO RL indicating that Wls normally suppresses Pax6. The Wls-cKO cerebellum displays a smaller vermis and foliation defects. Granule cells are disorganized and UBCs are missing from the mutant cerebellum. The lack of Wls also affected cells of the VZ-lineage such as Purkinje cells and interneurons. This thesis provides novel insights into the molecular network underpinning cerebellar development, in particular the requirement of Pax6 and Wls. This work also reveals the spatial compartments in the developing RL that are defined by Pax6, Wls and other molecular markers.

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