Molecular mechanisms of choroid fissure closure and ventral retina formation in the zebrafish eye
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During optic cup morphogenesis, the neuroectodermal layers of the optic vesicle (OV) invaginate ventrally, and fuse at the choroid fissure (CF) along the proximo-distal axis such that the retina and retinal pigment epithelium (RPE) are confined within the cup. Failure of CF closure results in colobomas, which are characterized by the persistence of a cleft or hole at the back of the eye. While CF closure is a critical aspect of ocular development, the molecular and cellular mechanisms underlying this process are poorly understood. My research examined CF closure and colobomas using zebrafish as a model system. In the first study, I determined that early cell fate changes within the eye field could cause colobomas using the zebrafish mutant blowout. Colobomas in blowout resulted from defects in optic stalk morphogenesis whereby the optic stalk extended into the retina and impeded the edges of the CF from meeting and fusing. Positional cloning of blowout identified a nonsense mutation in patched1, a negative regulator of the Hedgehog pathway. Up-regulation of Hedgehog pathway activity causes disruption in the patterning of the OV into proximal and distal territories, revealing that cell fate determination, mediated by Hedgehog signaling, is intimately involved in regulating CF closure. In the second study, I examined Bcl6 function and regulation during zebrafish eye development. bcl6 encodes a transcriptional repressor expressed in the ventral retina during zebrafish eye development. Loss of Bcl6 function leads to colobomas along with up-regulation of p53, a previously known Bcl6 target, and an increase in the number of apoptotic cells in the retina, demonstrating that Bcl6 plays a critical role in preventing apoptosis in the retina during early eye development. I also showed that Vax1 and Vax2 act upstream of bcl6 in the ventral retina. Furthermore, I identified functional interactions between Bcl6, Bcor and Hdac1 during eye development, demonstrating that Bcl6 functions along with Bcor and Hdac1 to mediate cell survival by regulating p53 expression. Together my studies expand the gene regulatory network involved in cell fate determination and cell survival during CF closure and ventral retina formation, and provide mechanistic insight into coloboma formation.