Once high intraocular pressure is detected, one way that glaucoma can be prevented and treated is by reducing the rate of secretion of intraocular fluid in an effort to reduce net intraocular pressure. Current drug therapies have some shortcomings, however, and it would be preferable to have a target that has a more unique role in the control of intraocular fluid secretion. This pilot research grant application proposes to test the new hypothesis that phospholemann, a small membrane protein that is phosphorylated by multiple protein kinases, could be such a target. We show that in the anterior segment of the eye, phospholemann is expressed specifically in the non-pigmented layer of the ciliary epithelium and co-localizes with Na,K-A TPase, the active transport enzyme that provides the driving force for fluid secretion. In the similar secretory organ of the brain ventricles, the choroid plexus, phospholemann is associated in a complex with the Na,K-ATPase. Phospholemman is a homolog of two other proteins that have been shown to associate with and modulate Na,K-ATPase in the kidney. We will test the hypothesis that phospholemann binds to Na,K-ATPase in ciliary epithelium, and that it is phosphorylated by protein inases that are implicated in the regulation of intraocular fluid secretion. Two different strains of mutant mice will be used to evaluate phospholemann's role in glaucoma. In one strain, DBA/2J, progressive symptoms of glaucoma develop because of blockage of the outflow pathway, mimicking human glaucomas. We will assess compensatory and pathological changes in phospholemman and in the Na,K-ATPase itself. The other strain is a knockout of phospholemann. We will investigate the ciliary body, retina, and optic nerve for evidence of damage from a deficiency in the regulation of intraocular fluid secretion. The idea that phospholemann and its homologs may be regulators of Na,K-ATPase is quite new, and as an intermediate of signal transduction, phospholemman may integrate the activation of multiple signaling pathways. If successful the work promises to lead to a new paradigm for understanding the regulation and pharmacological control of intraocular fluid secretion.
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