Mitochondrial dynamics and purine metabolism in zebrafish cone photoreceptors

Abstract

Photoreceptors, the light-detecting cells in the retina, are vital for vision. Despite being post-mitotic and having high energy demands, photoreceptors survive throughout a human’s lifespan. However, many different proteins, including inosine monophosphate dehydrogenase 1 (IMPDH1), can cause vision loss when mutated. In this thesis, I describe a strategy used by zebrafish photoreceptors to remain healthy throughout their lifespan. I also present new hypotheses to explain why cone photoreceptors degenerate when faced with a genetically-induced metabolic insult. Although photoreceptors are mostly glycolytic, they have abundant mitochondria that help with energy demand. We found that cone photoreceptor mitochondria are dynamic depending on the time of day and energy demands. Further, we found that at night, cone mitochondria extrude material out of cones. We expanded our studies to evaluate how larval zebrafish cone mitochondria handle stress. In stressed and unstressed conditions, cone mitochondria migrate away from the ellipsoid region. These unhealthy mitochondria are ejected from the cell, and Müller glia take them up for turnover. We also tested a metabolic stressor by expressing mutant forms of Impdh1a in cone photoreceptors. In humans, these mutations leadto photoreceptor degeneration. Some Impdh1a mutations are hyperactive in vitro so we designed the first Impdh1a mutant animal models to analyze enzyme activity in vivo. We found no evidence for Impdh1a hyperactivity in zebrafish cone photoreceptors and instead found other metabolic phenotypes that could contribute to disease. We also found that Impdh1a mutants form large mislocalized filaments that may disrupt important protein-protein interactions. These studies are important for understanding cone health in the context of daily energy demand changes, stress, and disease.