The epithelial Na+ channel, ENaC, forms the pathway for Na+ absorption in the kidney collecting duct and other epithelia (1, 2). Specific ENaC mutations lead to excessive channel expression at the cell surface, resulting in a genetic form of hypertension (Liddle's syndrome). Aldosterone and other steroid hormones regulate Na+ absorption by altering the expression of ENaC at the cell surface. Thus, understanding the mechanisms that control ENaC surface expression wifi provide critical new insights into the molecular mechanisms of Na+ homeostasis, and the pathogenesis of hypertension. Previous work found that Nedd4 and Nedd4-2 reduce ENaC surface expression by targeting the channel for degradation. A defect in this pathway is responsible for Liddle's syndrome. Conversely, serum and glucocorticoid-regulated kinase (SGK), a down-stream mediator of aldosterone, increases ENaC surface expression. Our preliminary studies suggest the novel hypothesis that these two pathways are not independent, but that they converge in a common pathway to regulate Na+ absorption. The goal of this project is to test the hypothesis that SGK modulates ENaC surface expression in part through Nedd4-2 binding and phosphorylation, and to understand the mechanisms involved. In the first specific aim, we will test the hypothesis that SGK phosphorylates Nedd4-2. We will identify the specific residues that are phosphorylated, and will test whether phoshorylation alters Nedd4-2 function.
In specific aim two, we will test the hypothesis that SGK binds to Nedd4-2. We will identify the sequences that mediate this interaction, and test the functional role of binding. Specific forms of hypertension are caused by defects in ENaC regulation by Nedd4 family members (Liddle's syndrome) and defects in mineralocorticoid and glucocorticoid signaling signalling (e.g. glucocorticoid-remediable aldosteronism). Thus, our work will provide a new understanding of these forms of hypertension. In addition, by elucidating the pathways and proteins responsible for blood pressure control, this work may provide important new insights into the molecular causes of essential hypertension.
