Renal tubular SGK1 deficiency causes impaired K+ excretion via loss of regulation of NEDD4-2/WNK1 and ENaC.

The stimulation of postprandial K(+) clearance involves aldosterone-independent and -dependent mechanisms. In this context, serum- and glucocorticoid-induced kinase (SGK)1, a ubiquitously expressed kinase, is one of the primary aldosterone-induced proteins in the aldosterone-sensitive distal nephron. Germline inactivation of SGK1 suggests that this kinase is fundamental for K(+) excretion under conditions of K(+) load, but the specific role of renal SGK1 remains elusive. To avoid compensatory mechanisms that may occur during nephrogenesis, we used inducible, nephron-specific Sgk1(Pax8/LC1) mice to assess the role of renal tubular SGK1 in K(+) regulation. Under a standard diet, these animals exhibited normal K(+) handling. When challenged by a high-K(+) diet, they developed severe hyperkalemia accompanied by a defect in K(+) excretion. Molecular analysis revealed reduced neural precursor cell expressed developmentally downregulated protein (NEDD)4-2 phosphorylation and total expression. γ-Epithelial Na(+) channel (ENaC) expression and α/γENaC proteolytic processing were also decreased in mutant mice. Moreover, with no lysine kinase (WNK)1, which displayed in control mice punctuate staining in the distal convoluted tubule and diffuse distribution in the connecting tubule/cortical colleting duct, was diffused in the distal convoluted tubule and less expressed in the connecting tubule/collecting duct of Sgk(Pax8/LC1) mice. Moreover, Ste20-related proline/alanine-rich kinase phosphorylation, and Na(+)-Cl(-) cotransporter phosphorylation/apical localization were reduced in mutant mice. Consistent with the altered WNK1 expression, increased renal outer medullary K(+) channel apical localization was observed. In conclusion, our data suggest that renal tubular SGK1 is important in the regulation of K(+) excretion via the control of NEDD4-2, WNK1, and ENaC

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

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 (Nedd4L(Pax8/LC1) ) 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 Nedd4L(Pax8/LC1) 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 Nedd4L(Pax8/LC1) 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