Mechanisms of retinal ganglion cell differentiation in the vertebrate retina.

The central nervous system gives rise to an extraordinary number of different cell types. Understanding the process of cell fate specification whereby these cells are generated is a fundamental challenge for developmental biologists. I n particular, this dissertation is focused on deciphering the molecular mechanisms that lead to the generation of retinal ganglion cells and understanding how the basic helix-loop-helix (bHLH) transcription factor Xath5 influences this process. Towards this aim, we took two approaches. First, we investigated the mechanisms by which Xath5 influences retinal ganglion cell genesis. Specifically, the retinal ganglion cell specific POUhomeodomain transcription factor XBrn3d was identified, and it was shown that Xath5 can directly induce the expression of this factor. This suggests that Xath5 is able to bias cells to adopt the retinal ganglion cell fate in part by coupling to retinal ganglion cell specific factors. Second, we sought to identify the regulatory mechanisms that determine the spatial and temporal pattern of Xath5 expression in vivo. Us ing transgenic Xenopus laevis embryos to perform in vivo promoter analysis, two promoters were identified in the 5'genomic region of the Xath5a locus. Within the proximal promoter region, two highly conserved E-box binding sites were identified, which, in the context of this proximal promoter, are necessary for retinal expression of a transgene. This indicates that Xath5 expression is likely regulated in part by auto- or cross-regulation with other bHLH factors. Collectively, these results enhance our understanding of retinal ganglion cell genesis by placing Xath5 in a molecular context and contributing to our knowledge of the mechanisms by which Xath5 influences the retinal ganglion cell fate.