Renal Tubular Ubiquitin-Protein Ligase NEDD4-2 Is Required for Renal Adaptation during Long-Term Potassium Depletion.

Abstract

Adaptation of the organism to potassium (K⁺) deficiency requires precise coordination among organs involved in K⁺ homeostasis, including muscle, liver, and kidney. How the latter performs functional and molecular changes to ensure K⁺ retention is not well understood. Here, we investigated the role of ubiquitin-protein ligase NEDD4-2, which negatively regulates the epithelial sodium channel (ENaC), Na⁺/Cl^- cotransporter (NCC), and with no-lysine-kinase 1 (WNK1). After dietary K⁺ restriction for 2 weeks, compared with control littermates, inducible renal tubular NEDD4-2 knockout (<i>Nedd4L^Pax8/LC1</i> ) mice exhibited severe hypokalemia and urinary K⁺ wasting. Notably, expression of the ROMK K⁺ channel did not change in the distal convoluted tubule and decreased slightly in the cortical/medullary collecting duct, whereas BK channel abundance increased in principal cells of the connecting tubule/collecting ducts. However, K⁺ restriction also enhanced ENaC expression in <i>Nedd4L^Pax8/LC1</i> mice, and treatment with the ENaC inhibitor, benzamil, reversed excessive K⁺ wasting. Moreover, K⁺ restriction increased WNK1 and WNK4 expression and enhanced SPAK-mediated NCC phosphorylation in <i>Nedd4L^Pax8/LC1</i> mice, with no change in total NCC. We propose a mechanism in which NEDD4-2 deficiency exacerbates hypokalemia during dietary K⁺ restriction primarily through direct upregulation of ENaC, whereas increased BK channel expression has a less significant role. These changes outweigh the compensatory antikaliuretic effects of diminished ROMK expression, increased NCC phosphorylation, and enhanced WNK pathway activity in the distal convoluted tubule. Thus, NEDD4-2 has a crucial role in K⁺ conservation through direct and indirect effects on ENaC, distal nephron K⁺ channels, and WNK signaling.