Transport Mechanism and Renal Function of a Newly Recognized Na+/H+ Exchanger

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Project Summary Hypertension is a major health problem affecting more than 1 in 4 adults in the United States and is an independent risk factor for heart and kidney failure. Approximately 95% of cases are idiopathic and classified as essential hypertension. Increased Na+-Li+ countertransport (SLC) is a well-characterized inheritable trait and known marker for essential hypertension and diabetic nephropathy. SLC represents an alternative mode of Na+/H+ exchange and correlates with sodium transport in the renal tubule. Recently, our laboratory identified NHA2, a novel sodium proton (Na+/H+) antiporter that mediates SLC, and is expressed in the distal nephron of the kidney. NHA2 is a member of a phylogenetically distinct and uncharacterized branch of the superfamily of metazoan cation proton transporters that includes the well-known NHE family of transporters. In Aim 1, we will generate and test model structures of NHA2 based on similarity with bacterial and archaeal orthologs of known structures in outward and inward facing conformations. The predictive power of the structural models, combined with functional screening of transport phenotypes in yeast, will be tested against a database of human variants and patient electronic medical records in an innovative PheWAS approach to link genotypes to disease phenotypes. Our preliminary observations implicate NHA2 in renal cyst formation and point to a potential role in nephrogenic diabetes insipidus, a major complication in patients prescribed lithium for bipolar and other neurological disorders. Therefore, in Aim 2 we will use renal epithelial cell models and 3-dimensional cysts to determine the role of NHA2 in salt and pH homeostasis. Modern Western diet is high in sodium and is known to be associated with hypertension. In preliminary experiments, we have observed elevation of NHA2 transcript and protein in mice fed a high salt diet. Experiments in Aim 3 will investigate the salt induction of NHA2 in the kidney and the role of NHA2 in salt and pH homeostasis by extrapolation to animal models. We will directly test whether NHA2 is responsible for SLC activity in red cells from mouse and human. This proposal will provide the first mechanistic and functional insights on a novel human Na+ transporter and its relevance to hypertension and human disease.