PROJECT SUMMARY Outer retinal degenerative diseases (RDDs) resulting in photoreceptor (PR) cell death are a leading cause of visual impairment worldwide, but options for rescuing or restoring vision in many of these patients are limited. Human pluripotent stem cell (hPSC)-derived PR transplantation is increasingly being studied as a therapeutic strategy for these patients, and neural regeneration within the retina has recently been identified as an area of strategic focus by the National Eye Institute (NEI). Several preclinical studies have shown some degree of visual restoration with bolus-delivered PR transplants in animal models, and clinical trials studying the safety and efficacy of bolus-delivered fetal-derived retinal precursors in patients with severe retinal degeneration are currently underway. Despite these recent successes, the field still faces several critical roadblocks before clinical PR replacement therapy can be realized for most RDD patients. Current strategies for bolus subretinal delivery of dissociated PRs fail to accurately reconstitute the complex organization of the outer retina, and they are often accompanied by disorganization, unpredictable dosing, and overall low cell counts immediately after injection due to reflux into the vitreous cavity. Further, it remains unclear whether visual responses commonly attributed to transplanted donor PRs are actually due to anatomic integration and functional synapse formation within the host degenerate retina. Indeed, the efficiency of synapse formation following PR transplantation, and the relationship between de novo synaptogenesis and measurements of visual function has not been tested to date. Here, we seek to use state-of-the-art biomaterials and PR scaffolds along with rigorous synaptic tracing methodologies to address these challenges in a rat model of severe photoreceptor degeneration. In Aim 1, we will use a novel micro-patterned, biodegradable scaffold for targeted hPSC-PR transplantation to assess the retention, survival, and maturation of bolus-delivered and scaffold-delivered PRs in vivo. In Aim 2, we will define the synaptic connectivity of hPSC-PRs in degenerate retinal explants and live host degenerate retinal tissue with an innovative monosynaptic retrograde tracing assay. The University of Wisconsin-Madison fosters the ideal scientific and intellectual environment for successful completion of these aims with strong, collaborative research communities spanning the fields of ophthalmology, biomedical engineering, and regenerative medicine. The research proposal and fellowship training plans detailed here seek to address current roadblocks within the field of PR replacement while also providing the necessary skillset to address the next generation of challenges facing the burgeoning field of retinal regeneration.