GRK4 and D3R regulation of NHE3 and NCC expression

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lXhe long-term objective is to determine the interaction among the five dopamine receptors and other G protein-coupled receptors (GPCRs) in the regulation of renal electrolyte transport and blood pressure. The D3 dopamine receptor (D3R), by itself, or via its interaction, with other dopamine receptors (e.g., DIR) and other GPCRs (e.g., angiotensin type 1 receptor), regulates renal sodium transport and blood pressure. Deletion of the D3R gene (D3R-/-) results in saltsensitive hypertension that is associated with increased renal expression of sodium/hydrogen exchanger type 3 (NHE3[SLC9A3]), sodium chloride exchanger (NCC[SLC12A3]) and alpha subunit of the epithelial sodium channel (ENaC [SCNNl A ]). Preliminary data show that the D3R ubiquitinates NHE3 and that the ubiquitination of NHE3 is due to D3R-mediated inhibition of USP48, an ubiquitinase. The overall hypothesis of project 3 is that the hypertension in D3R-/- mice is caused by increased activity and expression of NHE3 and NCC; their increased expression is caused by decreased degradation due to deubiquitination by USP48. Specific aim 1 will test the hypothesis that impaired D3R function, because of decreased expression (D3-/-) or because of constitutive desensitization by human GRK4 gamma variants (e.g., GRK4 gammpl42V), results in increased renal expression of NHE3 and NCC when NaCI intake is normal and increased renal expression of NCC and ENaC when NaCI intake is increased. The impaired ability of D3-/-mice to excrete sodium contributes to the development of hypertension. Specific aim 2 will test the hypothesis that D3R inhibits USP4S activity, preventing the deubiquitination of NHE3 and NCC; this preserves their ubiquitination, resulting in increased degradation and therefore, decreased expression. Decreased expression of D3R, or impairment of D3R function by human GRK4gamma 142V, increases USP48 expression and activity, promotes the deubiquitination and prevents the degradation of NHE3 and NCC, thus, increasing their expression levels. Hypertension is a complex polygenic disease. However, based on our findings, GRK4 regulation of a limited number of GPCRs, and the downstream regulation of genes/proteins by GPCRs, e.g., D3R, makes a single gene, GRK4, a key contributor in the pathogenesis of essential hypertension.