Diabetic retinopathy (DR) is the leading cause of vision loss in developed countries and affects 90% of type 1 diabetic patients (T1DM) with at least 15 years disease duration. Treatments such as laser photocoagulation and anti-VEGF agents are often able to treat proliferative diabetic retinopathy (PDR) and macular edema; however, clinical treatment for early stages of DR based on the mechanisms attributed to hyperglycemia have not been successful. Due to the epidemiological observation that the Joslin 50-Year Medalist Study cohort (n>900), a group with 50 or more years of T1DM, did not develop severe DR (prevalence 35%) despite limited early tools for glycemic control, we focused on finding an endogenous retinal protective factor. Using proteomic analysis of the retina and vitreous, retinoid binding protein-3 (RBP3) was found to be elevated in those Medalists with no-mild DR compared to those with PDR even in the presence of hyperglycemia. Preliminary data showed that this pattern of RBP3 elevation in the vitreous with no-mild DR compared to active or quiescent PDR is consistent in both T1D and T2D patients. Purified RBP3 inhibited the actions of hyperglycemia and VEGF on retinal endothelial cells and pericytes. Interestingly, purified RBP3 stimulated signaling cascades of p-AKT and p-ErK in pericytes and endothelial cells at nanomolar concentrations. Overexpression of RBP3 by subretinal injections of lentivirus prevented increases in capillary permeability and VEGF expression, neural retinal dysfunction measured by ERG and OCT, the formation of acellular capillaries and pericyte apoptosis in diabetic Lewis rats, all suggest the protective ability of the RBP3 protein. These studies suggest that RBP3 has functions other than transporting retinoids between photoreceptors and retinal pigmented epithelial cells. To confirm that RBP3 could have protective and novel actions in the retina against the adverse effects of hyperglycemia, we propose three specific aims: Specific Aim 1: To evaluate RBP3 levels in the vitreous and aqueous fluids from non-diabetic controls, people with type 1 and type 2 diabetes of various duration, age, severity of DR and glycemic control. Specific Aim 2: To determine whether elevating the level of RBP3 in the retina or vitreous can decrease retinal or vascular abnormalities induced by diabetes in rodents. Specific Aim 3: To characterize RBP3's signaling and actions in retinal pericyte and endothelial cells.