ALLISON, W.T.*; DANN, S.G.; HAWRYSHYN, C.W.; Univ. of Victoria, BC, Canada: Thyroid hormone control of opsin expression, cone death and regeneration during natural development

Background: Ultraviolet-sensitive (UV-) cones disappear from the retina of salmonid fishes during the metamorphosis that prepares them for marine life. Thyroid hormone mediates this metamorphosis during natural development. At sexual maturity, by the time fish return to freshwater, a population of putative UV-cones has been reported to reappear into the retina. Results: Immunohistochemistry or in situ hybridization labeling of photoreceptors, as well as electroretinograms, suggest that thyroid hormone application mimics natural development, including the loss and reappearance of UV-cones. Caspase inhibitors block the disappearance of UV-cones, indicating that they disappear via programmed cell death. Removal of thyroid hormone treatment results in the reappearance of UV-cones. BrdU incorporation indicates that proliferating stem cells are the source of these reappearing UV-cones. Upstream sequencing of opsin genes reveals transcription factor binding sites that implicate thyroid hormone in the regulation of apoptosis. Deiodinase gene expression shows that thyroid hormone levels are regulated within the retina. Conclusion: We support the hypothesis that, during natural development of salmonid fishes, UV-cones disappear via apoptosis and regenerate from proliferating stem cells. This provides a powerful model of cone photoreceptor death in which individually identifiable neurons can be induced to degenerate, analogous to the Manduca model of motor neuron programmed cell death. This also allows an examination of signals and gene expression inducing regeneration of cones from proliferating stem cells. Importantly, such studies can be conducted in vivo, without confounds of retinal damage, which is normally utilized to study regenerative events.