In salt-sensitive hypertension, an increase in NaCl intake raises vascular volume and therefore blood pressure. However, Cl- intake may be as much or more important than Na+ in the generation of salt-sensitive hypertension. Aldosterone increases renal NaCl absorption by increasing NaCl transporter insertion in the apical plasma membrane, which stimulates NaCl absorption, thereby raising blood pressure. This steroid hormone increases apical plasma membrane abundance of these renal Na+ and Cl- transporters, at least in part, through regulation of the ubiquitin ligase, NEDD4-2, expressed in the aldosterone-sensitive region of the nephron. Of the NEDD4-2-regulated NaCl transporters, the best studied are the thiazide-sensitive NaCl cotransporter, NCC, and the epithelial Na+ channel, ENaC. NEDD4-2 binds to ENaC, which ubiquitinates the transporter, resulting in its endocytosis and degradation. With aldosterone administration, Sgk1 is stimulated, thereby phosphorylating NEDD4-2, which prevents its association with ENaC. The hypertension observed in mice with NEDD4-2 gene ablation and in people with certain polymorphisms of NEDD4-L, the human homologue of rodent NEDD4-2, occurs, in part, from the increased renal Na+ and Cl- transporter abundance. In the cortical collecting duct (CCD) NEDD4-2 is expressed in both principal and intercalated cells. Within intercalated cell subtypes, NEDD4-2 protein expression is highest in type B cells, suggesting that Cl- transport in type B intercalated cells i modulated by NEDD4-2. While principal cell NEDD4-2 function has been well studied, little is known about NEDD4-2 in intercalated cells. As such, we generated intercalated cell NEDD4-2 null mice using Cre-lox technology. With intercalated cell NEDD4-2 gene ablation, we observed a substantial increase in both electroneutral Cl- absorption and HCO3- secretion in the CCD and increased mean arterial blood pressure. We hypothesize that NEDD4-2 associates with intercalated cell Cl- transporters such as pendrin and ClC-5, which leads to their ubiquitination, endocytosis and degradation. We hypothesize further that aldosterone increases Cl- absorption in the CCD, in part, by reducing the association of these transporters with NEDD4-2. This proposal will dissect the mechanism whereby intercalated cell NEDD4-2 alters Cl- transport by intercalated cells.
Aims of the proposal are to determine the following: 1) If aldosterone increases CCD Cl- absorption by stimulating Cl- /HCO3- and Cl-/H+ exchange in tandem, 2) if aldosterone acts through NEDD4-2 to increase Cl- absorption by intercalated cells of the CCD and 3) what intercalated cell Cl- transporters are targeted by NEDD4-2 in the CCD. To accomplish these objectives, we will examine the effect of aldosterone and NEDD4-2 on intercalated cell transporter abundance and function in mice both in vivo and in vitro, using quantitative real time PCR, immunohistochemistry and immunofluorescence, immunogold cytochemistry, immunoblots, electrophysiology and transport studies in renal tubules perfused in vitro.
Hypertension has grown to epidemic levels in the industrialized world and is a major risk factor for cardiovascular and renal disease. Blood pressure rises with increased salt (NaCl) intake due to increased NaCl uptake by the kidney. The purpose of this proposal is to determine how a minority cell type in kidney (the intercalated cell) regulates Cl- uptake by the kidney.
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