The interphotoreceptor matrix (IPM) is the thin layer of extracellular material occupying the subretinal space in normal vertebrate eyes. Because it is located between the neural retina and the pigment epithelium (RPE), the IPM is in a strategic position to perform several important functions for the retina. The IPM could be involved in retinal nutrition, retinold transport, enzymatic digestion or phagocytosis shed disc material, and/or retinal adhesion. The long-term objective of the proposed research is to identify and investigate biochemical components of this matrix that may be required to maintain the health of the retina, and to examine how they function. Our studies currently focus on IPM proteins. The scope of this proposal is limited to four specific aims: (1) Correlate structure with function for interphotoreceptor retinol-binding protein (IRBP), and examine its mechanism of action at target tissues. Found only in the IPM, IRBP may have a role in transport of retinoids for the visual cycle. Methods include HPLC and fluorescence (for analysis of IRBP's ligands and their exchange with other proteins); physicochemical techniques (for examining IRBP's unusual size and shape, and as a probe for possible functional domains); biochemical binding studies of radiolabelled IRBP to RPE (and retina) membranes; and autoradiography of these tissues to search for endocytosis of IRBP and vitamin A. (2) Analyze the IPM, by in situ histochemistry, for hydrolytic enzymes (of RPE lysosomal orgin), which may begin the process of digesting shed disc packets prior to phagocytosis. (3) Identify which cell types from adjacent tissues are responsible for biosynthesis of IPM proteins; this study involves culturing pure cell populations and analyzing their secreted products. We shall investigate also the fate of IPM proteins, and the impact of certain retinal diseases (such as rodent degenerations) upon IPM-protein composition and metabolism. (4) Screen the IPM for factors enhancing the functioning of RPE and retina.
This aim i ncludes examining IPM influence upon (a) initiation of phagocytosis (by measuring engulfment of ROS by RPE cells); (b) retinal adhesion (by investigating flow of ions and water through the RPE); and (c) retinal nutrition (by analyzing concentration-gradients of nutrients and waste products in the IPM). It is anticipated that the proposed investigation of the IPM will increase our knowledge of how the health of the retina is maintained, and provide clues about faulty functioning in disease states.
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