Project Summary Many different genes and mutations have been associated with photoreceptor degeneration. This enormous genetic heterogeneity requires the development of treatment approaches targeting common mechanisms and pathways, which can effectively treat a condition regardless of the underlying genetic cause. Mutations in several genes involved in guanine nucleotide homeostasis lead to photoreceptor degeneration, likely due to the unique metabolic demands for cyclic GMP in photoreceptor signaling. Despite this critical importance, basic aspects of regulation of purine metabolism in photoreceptors have not been investigated rigorously. This proposal is focused on the critical role of IMP dehydrogenase 1 (IMDPH1), the enzyme that regulates flux through the parallel de novo adenine and guanine nucleotide biosynthesis pathways. Nine different missense mutations in IMPDH1 are associated with dominant forms of retinitis pigmentosa or Lebers congenital amaurosis, each with varying severity of disease phenotype. None of the mutations has a direct effect on the intrinsic biochemical activity of the enzyme, and the mutations do not have deleterious effects in tissues other than the retina. For many years progress has been stalled on the molecular mechanism of IMPDH1-induced retinopathies due to a lack of animal models to study the disease pathology, and the lack of a clear defect at the enzyme level. IMDPH forms micron- scale dynamic filaments both in vitro and in vivo in response to increased demand for guanine nucleotides. We have now discovered that the IMPDH1 retinal degeneration mutations have a direct effect on the filament form, either promoting constitutive assembly or preventing polymerization entirely, with concomitant defects in allosteric regulation of the enzyme activity. Here, we propose a collaborative and initial effort to define the structure, function, and metabolic role of IMPDH1 in healthy photoreceptors and in transgenic animals that express IMPDH1 mutant alleles associated with photoreceptor degeneration. Our multidisciplinary approach combines cryoEM structural analysis with imaging and metabolic measurements in intact retinas. Our work will provide novel insight into the unique role of IMPDH1 in photoreceptors, and lay the groundwork for analyzing more mutations and for future targeted therapies to prevent blindness due to cell death.