Role of Bruch Membrane Heparan Sulfate in Drusenogenesis in Age-Related Macular Degeneration

ABSTRACT Age-related macular degeneration (AMD) is the leading cause of blindness in developed countries. This devastating disease affects 196 million individuals and is predicted to increase to 288 million by 2050. Most of these patients suffer from the early and intermediate “dry” AMD and are currently without treatment options due to an incomplete mechanistic understanding of this complex disease. In early and intermediate AMD, large diffuse lipoprotein-rich deposits, named drusen, are deposited in the macula, and impair vision. Although drusen are a hallmark feature of AMD, the mechanism of drusenogenesis is unresolved. Drusen form between the retinal pigmented epithelium basal lamina (RPE BLam) and the inner collagenous layer (ICL) of Bruch’s membrane (BrM). RPE BLam and BrM are composed of glycosaminoglycans (GAG), including heparan sulfate (HS). Modification of GAG side chain residues creates binding sites for numerous growth factors, enzymes, and lipoproteins. This structural variation makes GAGs one of the most complex macromolecules found in nature and has been shown to be altered with aging and disease, including as an initiating event in subendothelial deposition of lipoproteins in atherosclerosis. Our understanding of GAGs in general, their role in BrM health, and how they change with aging and diseases, such as AMD, has lagged far behind other macromolecules despite their central role in biology. Our preliminary data show that HS is increased in AMD BrM compared to age-matched controls. In addition, AMD macula is rich in highly sulfated HS disaccharides (N-, 2-O and 6-O sulfation). Notably, Apolipoprotein E (APOE), a critical component of both lipoproteins and drusen, has binding sites to N-, 2-O and 6-O sulfated HS. In this proposal, we propose to test the hypothesis that an age-related increase in HS sulfation (N-, 2-O and 6-O) induces lipoprotein retention in BrM in early AMD. To address this hypothesis and elucidate the pharmacologic potential of this pathway, we propose the following aims: Aim 1: Determine what AMD risk factors are associated with BrM GAG composition. Aim 2: Determine if BrM HS structure or content promote lipoprotein binding in AMD. Aim 3: Determine if HS sulfation regulates lipoprotein retention in vivo in mice. If successful, the proposed experiments will establish alterations in HS sulfation as an initiating event in drusenogenesis and identify therapeutic targets for HS and lipoprotein binding for AMD. The mentored training program is designed to acquire new skills to support a career as a physician-scientist with expertise in glycobiology and lipoprotein biology. Towards this goal, a team of mentors have been assembled at UC San Diego, with expertise in heparan sulfate and lipoprotein metabolism.