WNK3 Bypasses the Tonicity Requirement for K-Cl Cotransporter Activation via a Phosphatase-Dependent Pathway

SLC12A cation/Cl⁻ cotransporters are mutated in human disease, are targets of diuretics, and are collectively involved in the regulation of cell volume, neuronal excitability, and blood pressure. This gene family has two major branches with different physiological functions and inverse regulation: K-CI cotransporters (KCC1-KCC4) mediate cellular CI⁻ efflux, are inhibited by phosphorylation, and are activated by dephosphorylation; Na-(K)-CI cotransporters (NCC and NKCC1/2) mediate cellular CI- influx and are activated by phosphorylation. A single kinase/phosphatase pathway is thought to coordinate the activities of these cotransporters in a given cell; however, the mechanisms involved are as yet unknown. We previously demonstrated that WNK3, a paralog of serine-threonine kinases mutated in hereditary hypertension, is coexpressed with several cation/CI⁻ cotransporters and regulates their activity. Here, we show that WNK3 completely prevents the cell swelling-induced activation of KCC1-KCC4 in Xenopus oocytes. In contrast, catalytically inactive WNK3 abolishes the cell shrinkage-induced inhibition of KCC1-KCC4, resulting in a