The cellular and molecular pathways of sub-retinal pigment epithelial (sub-RPE) deposit formation, the hallmark of the early 'dry' form of age-related macular degeneration (AMD), and the leading cause of vision loss in the elderly, are not known. Epidemiology studies have identified age, n-6 polyunsaturated fatty acid (PUFA) dietary intake and oxidant injury as risks for AMD. Further, studies from several laboratories, including our own, suggest that pathways regulating lipid and cholesterol processing and secretion play a pivotal role in deposit formation. However, the link between dietary intake of PUFAs and signaling pathways that promote production of deposits formation is not known. Our goal therefore is to identify the pathogenic mechanisms by which lipids, including PUFAs, regulate deposit formation. Peroxisome proliferator activating receptors (PPARs) are nuclear receptors that act as lipid sensors. Of the three isoforms, PPAR/d mediates the regulatory effects of dietary fatty acids, including n-6 PUFAs, on gene expression and stimulates the proliferation of peroxisomes, organelles involved in fatty acid oxidation. PPAR transcriptional activity is coupled with upregulation of molecules associated with cholesterol and lipid efflux, altered extracellular matrix (ECM) synthesis, and mitochondrial dysfunction; these families of molecules are also associated with deposit formation. With this in mind, we asked a corollary question: does dietary lipid activation of the PPAR/d signaling pathway in RPE cells stimulate deposit formation? In our preliminary studies, we found that exposure of RPE cell cultures to native and oxidized derivatives of n-6 PUFAs resulted in profound cellular changes in the expression of molecules associated with and regulating deposit formation. The changes included (1) increased synthesis and secretion of ECM molecule collagen IV; (2) increased expression of cholesterol and lipid efflux regulatory genes ABCA1 and CD36; (3) accumulation of damaged mitochondria and reactive oxygen species; (4) activation of PPAR /d; and (5) upregulation of PPAR/d specific target genes regulating ECM molecules, and lipid secretion. Based on this preliminary data, we hypothesize that dietary n-6 fatty acids stimulate production of molecules found in deposits through activation of PPAR/d and increased proliferation of peroxisomes. Accumulation of deposits requires lipid and cholesterol secretion by the RPE and dysregulated synthesis of ECM molecules by the RPE, leads to trapping of lipids. We further propose that additional 'stressors' on the PPAR/d pathway (i.e., age, oxidants) lead to upregulation of these processes and further compromise RPE cell function, mediated by mitochondrial dysfunction. To test this hypothesis we will use a combination of cell culture assays with human RPE cells and mouse models of deposits to investigate the role of PPARs in deposit formation. We will also investigate if decreasing activity of PPAR/d can slow the progression of AMD in murine models of deposit formation.
Age-related macular degeneration (AMD) afects 30% of individuals over the age of 65 years and is the leading cause of vision loss in the Western World. Of the three clinical sub-types, the early 'dry' form of the disease effects over 85% of the patient population. Risk factors for developing the disease include advanced age, dietary intake of lipids and oxidant injury. The objective of this grant is to investigate signaling pathways used by dietary lipids that cause phenotypic pathology of early 'dry' AMD, specifically (1) production and accumulation of lipid and protein rich-extracellular deposits and (2) functional changes in lipid metabolic processing cellular organelles. We also propose to investigate the modulating effect of additional AMD risk factors, age and oxidant injury, on disease development. Currently there are no therapeutic options available for early 'dry' AMD. Understanding the mechanisms underlying deposit formation and identifying signaling pathways key to disease initiation and progression will open up new avenues for therapeutic strategies in early 'dry' AMD.
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