| The paired-class homeobox gene Crx is essential for vertebrate retinal photoreceptor differentiation and maintenance. Mutations in human CRX cause photoreceptor degeneration diseases, including retinitis pigmentosa, cone-rod dystrophy, and Leber congenital amaurosis. Targeted disruption of the Crx gene results in failure of formation of outer segments in photoreceptors and degeneration of photoreceptors in young mice. Mammalian Crx is also required for the pineal gland function.; In my thesis, I studied the function of the zebrafish crx gene in retinal development. I showed that crx expression emerged in a ventronasal patch in the retina around 24 hpf, before any retinal progenitors exit the cell cycle. The expression domain was then expanded in a wave into the dorsotemporal region of the retina. As differentiation of the retina progressed, crx became restricted to the outer layers of the retina. In adult zebrafish retina, crx is expressed in the outer nuclear layer by both rods and cones, and in the outer region of the inner nuclear layer, where bipolar cells reside. Antisense Morpholino oligonucleotides were then used to knock down the crx function. With Morpholinos, I demonstrated that crx is required for expression of opsin genes and that it positively regulates the expression of other photoreceptor-specific genes. My results also showed that crx has additional roles in retinal development in zebrafish, which have not been described previously in mammals. First, I found that zebrafish crx regulates transcription of homeobox genes that are specifically expressed in the INL. Second, crx promotes retinal determination during regionalization of the optic primordia. Third, crx promotes withdrawal of retinal progenitors from the cell cycle. These results suggest that crx not only is required for photoreceptor differentiation but also plays a role in early retinal development and is involved in multiple steps during retinal development.; My research also showed that a closely related gene, otx5, plays a role in regulating pineal and retinal gene expression, with stronger regulation in the pineal gland. According to a phylogenetic study, zebrafish otx5 and crx genes are co-orthologs. My results indicated that there is subfunctionalization otx5/crx gene family in zebrafish, with crx dominating in the retina, otx5 in the pineal. |
