Identification of Photoreceptor Protein, Retinol Binding Protein 3 (RBP3), as a Protective Factor for Diabetic Retinopathy

Abstract

Purpose : The Joslin Medalist Study characterized 1023 people with type 1 diabetes duration of 50 years or more. Over 35% of these individuals exhibit only no-mild diabetic retinopathy (DR), independent of glycemic control, suggesting the presence of endogenous protective factors against DR. The goal of the study is to identify these protective factors with a high therapeutic potential to prevent or stop the progression of DR.

Methods : Proteomic analysis by mass spectrometry was performed to assess differences in retinal and vitreous protein profiles from Medalists with no-mild DR versus those with proliferative DR (PDR). ELISA assays were developed in addition to immunoblot analysis to confirm the expression of protective factors in Medalist ocular fluids and to correlate vitreous to serum levels. Retinol Binding Protein 3 (RBP3) capacity to prevent or ameliorate DR progression was tested in diabetic rodents.

Results : Proteomic analysis identified elevated concentrations of photoreceptor-secreted RBP3 in the retina and vitreous of Medalists with no-mild DR versus those with PDR (p=0.04 and p=0.005, respectively), independent of glycemic control. A novel ELISA RBP3 assay with high sensitivity (1x10^{-9 to -10} M) and specificity (no detection of immunoglobulin G, albumin, or RBP4) and immunoblot analysis confirmed initial mass spectrometry findings of an inverse correlation between vitreous RBP3 levels and DR severity (p<0.01) and vitreous vascular endothelial growth factor (VEGF) levels (p<0.05) in Medalists and other type 1 and 2 diabetic patients (n=54). Serum RBP3 concentrations were 1000-fold less than those in vitreous, but correlated with vitreous RBP3 concentration and DR severity (p<0.05, all). Intravitreous injection of RBP3 and its specific overexpression in rodents inhibited actions of VEGF and normalized diabetes-induced capillary permeability, abnormal neural retinal function, retinal thinning and acellular capillaries (p<0.05, all).

Conclusions : A neuroretinal protein, RBP3, was identified with the novel potential protective ability to neutralize toxic effects of hyperglycemia in the retina of patients with extreme duration of diabetes. RBP3 concentrations in vitreous and serum correlate with DR severity. These findings identify the first neuroretinal protein to act on retinal vasculature, suggesting numerous therapeutic and diagnostic possibilities.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.