WNK4 regulates activity of the epithelial Na⁺ channel in vitro and in vivo
2007
Ring, Aaron M. | Cheng, Sam X. | Leng, Qiang | Kahle, Kristopher T. | Rinehart, Jesse | Lalioti, Maria D. | Volkman, Heather M. | Wilson, Frederick H. | Hebert, Steven C. | Lifton, Richard P.
Homeostasis of intravascular volume, Na⁺, Cl⁻, and K⁺ is interdependent and determined by the coordinated activities of structurally diverse mediators in the distal nephron and the distal colon. The behavior of these flux pathways is regulated by the renin-angiotensin-aldosterone system; however, the mechanisms that allow independent modulation of individual elements have been obscure. Previous work has shown that mutations in WNK4 cause pseudohypoaldosteronism type II (PHAII), a disease featuring hypertension with hyperkalemia, due to altered activity of specific Na-Cl cotransporters, K⁺ channels, and paracellular Cl⁻ flux mediators of the distal nephron. By coexpression studies in Xenopus oocytes, we now demonstrate that WNK4 also inhibits the epithelial Na⁺ channel (ENaC), the major mediator of aldosterone-sensitive Na⁺ (re)absorption, via a mechanism that is independent of WNK4's kinase activity. This inhibition requires intact C termini in ENaC β- and γ-subunits, which contain PY motifs used to target ENaC for clearance from the plasma membrane. Importantly, PHAII-causing mutations eliminate WNK4's inhibition of ENaC, thereby paralleling other effects of PHAII to increase sodium balance. The relevance of these findings in vivo was studied in mice harboring PHAII-mutant WNK4. The colonic epithelium of these mice demonstrates markedly increased amiloride-sensitive Na⁺ flux compared with wild-type littermates. These studies identify ENaC as a previously unrecognized downstream target of WNK4 and demonstrate a functional role for WNK4 in the regulation of colonic Na⁺ absorption. These findings support a key role for WNK4 in coordinating the activities of diverse flux pathways to achieve integrated fluid and electrolyte homeostasis.
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