PROJECT SUMMARY / ABSTRACT Retinitis pigmentosa (RP) is the most common hereditary cause of blindness, affecting over 1.5 million people worldwide. RP has an extraordinarily variable etiology, with over 5,000 distinct mutations in more than 300 genes implicated in its pathogenesis, which suggests that the most productive therapeutic interventions would employ gene-independent strategies to target the cellular pathology shared across many mutations and patient groups. Our lab has demonstrated that one such common pathology is the impairment of cellular proteostasis. This condition arises from the insufficient capacity of the ubiquitin-proteasome system to process large amounts of misfolded or mistargeted mutant proteins. Reducing this stress by enhancing ubiquitin- independent protein degradation causes a major delay in the degeneration of photoreceptors bearing the P23H rhodopsin mutation, a commonly encountered cause of human RP. We propose to expand the mechanistic investigation of proteostatic imbalance in mouse models of photoreceptor degeneration and plan to further explore practical approaches for restoration of proteostatic equilibrium. Our overarching goal is to identify optimal therapeutic strategies and drug targets that could be used to ameliorate these devastating blinding conditions. We will investigate whether enhancement of proteasomal activity achieved by directly targeting the 20S proteasome core can alleviate retinal degeneration in mouse RP models. Progression of photoreceptor degeneration in mouse models of RP can be considerably delayed by overexpressing the 11S regulatory cap, which facilitates the accessibility of protein substrates to the proteolytic sites located inside the 20S core. We propose to address whether a similar therapeutic effect could be achieved through direct manipulation of the 20S core by two complementary approaches – genetic and pharmacologic. The therapeutic effects of these strategies will be assessed in two mouse models of RP of dissimilar etiology – rhodopsin P23H mutation and the BBS4 knockout – and will include retinal histological analysis, electroretinography, and optomotor responses. The results of these experiments will demonstrate whether direct activation of 20S proteasomes can serve as a therapeutic strategy to treat inherited blindness. These data will encourage further investigations of proteasome activity enhancers and ultimately advancement to clinical trials for RP. Furthermore, promising results in RP models will encourage the evaluation of these drugs in models of other retinal degenerations and potentially neurodegenerative diseases, all of which exhibit proteostatic stress as a pathophysiological feature.