PROJECT SUMMARY A diverse group of autosomal-dominant mutations that cause human blindness as a result of rod and cone degeneration includes multiple mutations in two genes, GUCY2D coding for retinal guanylyl cyclase 1 (RetGC1) and GUCA1A coding for guanylyl cyclase activating protein 1 (GCAP1). These mutations explicitly trigger photoreceptor death via a common mechanism – by abnormally elevating cyclic GMP production in the dark. Although the biochemical origin of the dominant GUCY2D and GUCA1A retinopathies became better understood over the past decade, potential approaches to their therapy, such as using gene editing or RNA interference to suppress particular alleles causing the disease, continue to present a major conceptual challenge. This proposal, conforming to the NEI mission to support research with respect to blinding eye diseases, visual disorders and mechanisms of visual function, explores the foundation of a new approach for the prospective therapy – targeting the common biochemical pathway underlying GUCY2D and GUCA1A dominant retinopathies. The proposal is built on the following main findings: (i) understanding that deregulation of RetGC1 activity is the key to the photoreceptor death caused by the gain-of-function mutations in GUCY2D and GUCA1A; (ii) development of mouse genetic models for studying the biochemical and physiological mechanisms of degeneration incited by the mutant RetGC1 and GCAP1; (iii) recent findings that the abnormal increase in cGMP production that leads to photoreceptor death can be effectively opposed by three different biochemical processes, such as acceleration of cGMP decay in the dark by recombinant phosphodiesterase 5 (PDE5r), stemming RetGC1 activation by GCAP1 using a newly designed protein inhibitor of guanylyl cyclase (PIGC), and enhancing protection of photoreceptors against the presence of deregulated RetGC1 using retinal degeneration-3 (RD3) protein. The research plan of this proposal pursues three Specific Aims, each addressing a new original concept designed to avert photoreceptor dystrophy caused by deregulation of RetGC1. Specific Aim 1 explores the biochemical and physiological mechanisms underlying rescue of photoreceptors in mouse models harboring the degenerative mutants of RetGC1 and GCAP1 using ectopic expression of PDE5r. Aim 2 explores mechanisms of their rescue using PIGC. Aim 3 explores the mechanisms supporting the survival of photoreceptors using RD3. We reason that achieving these Specific Aims will help in developing approaches to the future therapy of the dominant GUCY2D and GUCA1A retinopathies.