PROJECT SUMMARY Although purine nucleotide homeostasis is critical to photoreceptor health, basic aspects of purine metabo- lism and its regulation in photoreceptors are unknown. Here, we analyze the critical role of inosine monophos- phate dehydrogenase 1 (IMDPH1) in retinal function and disease. IMPDH1 catalyzes and regulates a critical step in metabolic flow through the parallel de novo adenine and guanine nucleotide biosynthesis pathways. At least a dozen dominant missense mutations in IMPDH1 cause photoreceptor degeneration. We will leverage the recent advances we made with initial support from a two-year R21 award. We combined in vitro biochemical analyses of the human and zebrafish retinal IMPDH1 variants with in situ characterization of mutant forms of IMPDH1 in zebrafish, a well-established model for investigating the causes of photoreceptor degeneration. We showed that the structural and biochemical properties of human retinal variants are conserved in zebrafish. Im- portantly, our preliminary data also show that a transgenic zebrafish line expressing mutant IMPDH1 recapitu- lates the retinal degeneration seen in human disease. We propose to exploit our extensive collection of reagents and animal lines to define the structure, function, and metabolic role of IMPDH1 in healthy and dis- eased photoreceptors. We use a multidisciplinary approach that combines characterization of IMPDH1 structure and function using cryo-electron microscopy and biochemical dissection with cell biological and metabolic strat- egies in intact retinas.