The Role of Pigment Epithelium-Derived Factor and Its Receptor in Retinal Cells

Abstract

The retina is the light-sensitive tissue in the back of the eye comprised of several cell types. The photoreceptor layer of the retina is composed of two cell types: rods, which are responsible for night and peripheral vision, and cones which are responsible for central detailed color vision. Immediately adjacent to the photoreceptor layer is a monolayer of hexagonal cells that form the retinal pigment epithelium (RPE). One of the most important functions of the RPE is the daily phagocytosis of photoreceptor outer segments (POS), where POS bind to the RPE cell surface and are internalized and degraded in a tightly regulated signaling pathway. The RPE also secretes trophic factors towards the RPE to maintain the health of the neural retina. One such factor is pigment epithelium-derived factor (PEDF), a 50 kDa secreted glycoprotein. PEDF is a member of the serine protease inhibitor superfamily based on sequence identity and homology with other serpins. PEDF lacks the ability to inhibit serine proteases, but possesses neurotrophic, anti- angiogenic, anti-tumorigenic, and anti-inflammatory activities. PEDF-receptor (PEDF-R) is a cell surface receptor for PEDF encoded by the gene PNPLA2. Upon binding to PEDF-R, PEDF stimulates the enzymatic lipase activity of PEDF-R, leading to cell survival. In the retina, PEDF-R is present in photoreceptor inner segments and the RPE. In this thesis, the cell specific roles of PEDF and PEDF-R in photoreceptors and the RPE are explored. First, the data show that single point alterations to the portion of PEDF which binds to PEDF-R lead to both enhanced or abrogated binding and subsequent retinoprotective activity in photoreceptors. Building upon our current understanding of PEDF and PEDF-R interactions in photoreceptors, we developed retinal cell-based high-throughput assays to identify retinoprotective compounds. Second, by examining the role of PEDF-R in RPE cells, we found that PEDF-R depletion led to a deficiency in the degradation of POS, altered expression of fatty acid metabolism related genes, and an accumulation of lipid droplets. The presented work provides insights into the activities of two retinoprotective proteins that can be exploited for the purpose of identifying therapeutics to treat retinal disorders.