PROJECT SUMMARY Retinitis pigmentosa (RP) and age-related macular degeneration (AMD) are blinding diseases caused by the degeneration of rods and cones, leaving the rest of the visual system intact but unable to respond to light. A synthetic chemical photoswitch, named DENAQ, can restore visual responses in blind mouse models of RP. Previous studies showed that DENAQ imparts light-sensitivity on action potential firing in retinal ganglion cells (RGC), but how this occurs is unclear. The goal of this project is to elucidate the mechanism of DENAQ photosensitization, crucial for enabling discovery of improved drug candidates and for optimizing photo- stimulation strategies for vision restoration. The first aim is to understand why DENAQ selectively photosensitizes retinas from mice with dead rods and cones while having no effect on healthy retinas with intact rods and cones. We will test the hypothesis that degeneration leads to enhanced entry of DENAQ into RGCs and/or enhanced action on ion channels underlying spontaneous firing in RGCs. The second aim is to identify which RGCs are photosensitized by DENAQ. In the healthy retina, some RGCs fire at light onset, some at offset, and some at onset and offset. Studies will determine which are photosensitized by DENAQ, and whether local degeneration of rods and cones leads to spatially constrained RGC photosensitization, of particular relevance for AMD, a localized degenerative disease. Other studies will reveal whether DENAQ photosensitization applies to human RGCs in tissue samples obtained during surgical retinectomy. The third aim is to exploit our findings to optimize vision restoration. Information about the ion channels targeted by DENAQ will enable development of more specific photoswitches. Subcellular localization of these channels in RGCs will enable more spatially-precise photo-control. Finally imaging studies in vivo will reveal signals transmitted from the DENAQ-treated retina to the brain of blind mice, validating the functional integrity of the visual system and providing a platform for optimizing retinal stimulation patterns to best recapitulate normal visual responses.