Understanding the pathogenesis of glaucoma, the second leading cause of blindness, is an important goal of vision research. Many studies have identified oxidative damage to the trabecular meshwork (TM) cells, leading to decreased outflow facility and increased intraocular pressure. Although the evidence linking oxidative damage to glaucoma is strong, the causes of oxidative damage in glaucoma are not known. In the proposed studies, we will test the hypothesis that oxidative damage to the TM is caused by excessive exposure to molecular oxygen and/or its metabolites. Our hypothesis is based on measurements of oxygen partial pressure (pO2), made in the human eye during surgery using a thin, flexible, fiberoptic probe. We identified large, stable oxygen gradients in the anterior chamber and tight regulation of pO2 in the anterior chamber angle, close to the TM. We also noted strong correlations between pO2 and important risk factors for glaucoma, including central corneal thickness, African-American heritage, and history of vitrectomy surgery. We believe that our studies are the first to link a physiologic variable, pO2 in the anterior chamber (AC), to these risks of glaucoma development. Our first specific aim will test whether the increased risk of glaucoma after vitrectomy is due to increased pO2 in the AC, causing damage to the TM. We will collaborate with colleagues at the University of Wisconsin-Madison for a longitudinal study in older Rhesus macaques that will sequentially undergo vitrectomy surgery and cataract extraction. We will map pO2 levels and measure aqueous humor antioxidants and outflow facility at each stage. At the conclusion of the study, we will determine the extent of TM cell loss and quantify the structural and oxidative damage to TM cells and their extracellular matrix. In this manner, we may correlate pO2 with oxidative damage to the TM.
Our second aim i s to determine whether the increased pO2 in African- Americans correlates with biochemical changes in the aqueous humor. We will expand our pilot study of metabolites in the aqueous humor, which suggested racial differences in mitochondrial metabolism. We will also analyze aqueous humor for its total antioxidant potential and concentration of known antioxidants, like ascorbate;studies that will determine whether there is a correlation between pO2 and the oxidant-antioxidant balance in the anterior chamber. We hypothesize that higher pO2 will be associated with depletion of anti- oxidants. Our third specific aim is to determine whether a non-invasive measure of corneal oxygen metabolism predicts intraocular pO2. We hypothesize that lower corneal oxygen consumption will correlate with increased pO2 in the AC. If correct, this test may provide an important means to determine those at risk of developing glaucoma. It will also offer a method to determine whether differences in intraocular oxygen are inherited and to identify the genes involved. Our novel approach will provide important information about the pathophysiology of open angle glaucoma by identifying factors responsible for the loss of aqueous outflow facility. The knowledge gained in these studies may lead to new therapies for and strategies to prevent this disease.
Numerous studies provide support for the theory that oxidative stress, the imbalance between damaging free radical molecules and the body's antioxidant defense mechanisms, plays a role in glaucoma by damaging the trabecular meshwork cells in the drain of the eye and also by potentially damaging the cells of the optic nerve. We hypothesize that the source of this damage is caused by excessive exposure of the trabecular meshwork cells to oxygen and/or its metabolites. We will evaluate the effects of elevated levels of oxygen, which we have correlated to several previously identified glaucoma risk factors, on the function and cellular changes of the trabecular meshwork in an animal model. We will also study the fluids of the eye for biochemical changes associated with oxidative stress and measure the cornea's use of oxygen and avoid invasive measurements with a contact lens. Relevance: Glaucoma, the second leading cause of blindness worldwide and frequent cause of visual impairment in the U.S., is an important public health issue. By extending our understanding of the causes of this condition, this research proposal will also expand potential pathways of therapy and prevention.
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