Trace Element Dynamics in the Vertebrate Eye

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Abstract Iron is an essential element for normal physiological functions. However, excess it can cause extensive tissue damage and participates in numerous ocular pathologies including cataractogenesis and retinal degenerations such as age-related macular degeneration. The study of ocular iron metabolism has been a focus of this laboratory for many years. We have made recent novel observations about iron's physiological role. We found that iron regulates synthesis and secretion of the neurotransmitter glutamate by ocular tissues and neurons. This is of fundamental clinical relevance since iron and glutamate are both dysregulated in neurodegeneration. In high quantities, glutamate can be excitotoxic in the central nervous system as well as the retina. Additionally, in retinal pigmented epithelial cells (RPE) and lens epithelial cells (LEC) iron regulates the activity of the transcription factor, hypoxia-inducible factor, which in turn regulates the synthesis of dozens of proteins. Our preliminary data indicate that hypoxic conditions stimulate glutamate release, another critically important observation since hypoxic conditions occur in stroke and retinal ischemia. Furthermore, there are profound changes in the structure of the iron storage protein ferritin in lenses that occur with age, cataractogenesis and differentiation. We will continue to explore how these changes affect iron storage in ferritin and the protection against iron damage such storage provides. Unfortunately, little is known about how iron levels are regulated in the eye which is isolated from the systemic circulation by blood ocular barriers (BOB). The proposal's hypothesis is that intraocular tissues have unique and independent systems for regulating iron uptake into and efflux from the eye across the BOBs. Their polarized location and iron-regulated quantity within ocular tissues allows for proper control of intraocular iron levels. Hypoxia, hemorrhage and inflammation significantly impact iron uptake storage, utilization and efflux. The resulting dysregulation of iron metabolism plays a critical role in ocular pathology. We will use an innovative integrated approach to determine how the BOB's regulate iron levels in intraocular tissues. The two specific aims utilize normal and pathological human eyes as well as normal canine eyes and tissue cultures of cells which form the BOBs, e.g., RPE and CE. Additionally, the lens will be used to assess how iron handling strategies adapt for survival in a normally hypoxic environment. We will utilize a state-of-the-art live-cell imaging quantitative fluorescence microscope with total internal reflection fluorescence for quantifying events at the plasma membrane and allow for measurement of dynamic processes underlying these complex interactions in four dimensions (4D) in living cells. It is the goal of this proposal to determine how intraocular iron levels are controlled and the specific role(s) iron has in ocular pathology in order to provide a basis for development of therapeutic modalities needed for prevention and treatment of ocular disease.