INTERPHOTORECEPTOR RETINOID BINDING PROTEIN IN THE CONE VISUAL CYCLE

In the classical visiual cycle, retinoids are transferred between the retinal pigment epithelium (RPE) and the photoreceptor cells.  A novel cone specific cycle has recently been discovered, showing the alternate cycle of retinoids between Müller cells and cone photoreceptor cells.  The main retinoids of interest that are transferred in the novel cycle are all-trans retinol (atROL) and 11-cis-retinol (11cROL).  It has been shown that interphotoreceptor retinoid binding protein (IRBP), the most abundant protein in the interphotoreceptor matrix, can bind to these retinoids, which suggests its purpose as a transfer protein.  Thus, we hypothesized that IRBP contains the ability to protect 11cROL under high photopic conditions. Biochemical assays, were performed to test the retinoid protection abilities of IRBP.  Retinoids exposed to 2,000 lux at 10 min time intervals, in the presence and absence of IRBP were extracted and analyzed by high performance liquid chromatography.  After 30 minutes of light exposure, a significant reduction of 11cROL and atROL in the absence of IRBP was observed.  The addition of IRBP resulted in complete protection from light induced degradation during the 30 minute experimental period.  Our results showed that even after 30 min of high light exposure, IRBP maintains its protection of the retinoids from light induced degradation in situ.  This suggests that IRBP may act as a transport protein in the cone visual cycle, shuttling retinoids between the cellular components.  This project will help us close the gap on our long-term goal of obtaining a complete understanding of the novel cone visual cycle.