Muller glia and Notch signaling in zebrafish retinal development and regeneration

Zebrafish continuously add new rod photoreceptors to their retinas as a consequence of persistent body growth over their lifespan. Additionally, zebrafish have the ability to regenerate photoreceptors destroyed by injury. We currently do not know the identity of the cells generating new photoreceptors in zebrafish. Muller glia (a type of radial glial cell) proliferate in response to retinal injury, but the fate of their progeny has not been determined. In the first part of my dissertation I tested the hypothesis that Muller glia can function as stem cells in the zebrafish retina. To test this hypothesis I created transgenic zebrafish with GFP-labeled glia, enabling me to trace the lineage of proliferating Muller glia. In the growing, uninjured retina I identified GFP+ cells that differentiated into rod photoreceptors. I next exposed transgenic zebrafish to intense light to destroy their photoreceptors. In the lesioned retinas I observed GFP+ cells that differentiated into cone photoreceptors. These results demonstrate that Muller glia can generate photoreceptors in both the growing and regenerating zebrafish retina.; In the second part of my dissertation I performed experiments to investigate the role of Notch signaling in retinal development and regeneration. Notch signaling is involved in several modes of function in the vertebrate retina. I first used in situ hybridization to characterize Notch expression in the retina. Several Notch family members were expressed during retinal development. In the regenerating retina I observed an upregulation of Notch signaling within the lesion site, suggesting Notch may play a role in regulating regeneration. To further test the role of Notch signaling in development, I examined retinas from zebrafish embryos in which Notch signaling was inactivated by either the mindbomb (mib) mutation or by treatment with gamma-secretase inhibitors, both of which block Notch signal transduction. In both groups I observed an absence of Muller glia and retinal lamination, but no effect on neurogenesis. These results suggest Notch has an instructive role in retinal gliogenesis and Muller glia may facilitate proper retinal lamination.; In conclusion my results indicate a role for both Muller glia and the Notch signaling pathway in retinal development and regeneration.