The effect of WNK4 on the Na⁺-Cl^- cotransporter is modulated by intracellular chloride

It is widely recognized that the phenotype of familial hyperkalemic hypertension is mainly a consequence of increased activity of the renal Na⁺-Cl^- cotransporter (NCC) because of altered regulation by with no-lysinekinase 1 (WNK1) or WNK4. The effect ofWNK4 on NCC, however, has been controversial because both inhibition and activation have been reported. It has been recently shown that the long isoform of WNK1 (L-WNK1) is a chloride-sensitive kinase activated by a low Clconcentration. Therefore, we hypothesized that WNK4 effects on NCC could be modulated by intracellular chloride concentration ([Cl-]_i), and we tested this hypothesis in oocytes injected with NCC cRNA with or without WNK4 cRNA. At baseline in oocytes, [Cl-]_i was near 50 mM, autophosphorylation of WNK4 was undetectable, andNCC activity was either decreased or unaffected byWNK4. A reduction of [Cl-]_i, either by low chloride hypotonic stress or coinjection of oocytes with the solute carrier family 26 (anion exchanger)-member 9 (SLC26A9) cRNA, promoted WNK4 autophosphorylation and increased NCC-dependent Na⁺ transport in a WNK4-dependent manner. Substitution of the leucine with phenylalanine at residue 322 ofWNK4, homologous to the chloride-binding pocket in L-WNK1, convertedWNK4 into a constitutively autophosphorylated kinase that activated NCC, even without chloride depletion. Elimination of the catalytic activity (D321A or D321K-K186D) or the autophosphorylation site (S335A) in mutant WNK4-L322F abrogated the positive effect on NCC. These observations suggest that WNK4 can exert differential effects on NCC, depending on the intracellular chloride concentration.