WNK bodies are multikinase complexes that facilitate NCC activation during hypokalemia

Abstract

Kidney‐Specific With‐No‐Lysine (KS‐WNK1) is a distal convoluted tubule (DCT) scaffold protein that serves as a docking site for the WNK‐SPAK/OSR1 kinase signaling pathway. During dietary K+ deprivation, KS‐WNK1 condenses with the active WNK‐SPAK/OSR1 cascade into large spherical membraneless structures in the DCT, termed WNK bodies. We propose that the assembly of these multikinase complexes promotes K+ conservation through the activation of the WNK‐SPAK/OSR1 pathway and its downstream target, the sodium‐chloride cotransporter (NCC). Mice lacking KS‐WNK1 are unable to form WNK bodies and have diminished activation of WNK‐SPAK/OSR1 pathway and NCC, resulting in a Gitelman‐like phenotype. A critical question is whether the blunted NCC activation in KS‐WNK1 KO mice is a direct consequence of the loss of WNK bodies, or a secondary effect from the generalized lack of KS‐WNK1. Here we address the issue by using CRISPR‐Cas technology to generate a mouse expressing a full‐length mutant version of KS‐WNK1 that does not form WNK bodies. The mutation was directed at a conserved N‐terminal hydrophobic motif (VFVIV ‐> QQQQQ) within exon 4A, and thus referred to as the “5Q” mouse. We confirmed in HEK293 cells that this mutation to KS‐WNK1 abolished WNK body formation. Next, we determiend the function of WNK bodies during 10 days of dietary K+ deprivation in the 5Q mouse versus littermates, assessing changes in WNK‐SPAK/OSR1 & NCC activation by immunostaining and western blot, and on electrolyte handling. We found that K+ restricted 5Q mice were unable to form spherical WNK bodies, and instead formed irregularly shaped aggregates around the nucleus and basolateral membrane. These malformed aggregates were enriched in phosphorylated SPAK/OSR1, which was nearly absent from the apical membrane. Consistent with intracellular sequestration of pSPAK/pOSR1, the abundance of pSPAK/pOSR1 was increased in immunoblots of kidney homogenates by 25% (males; p = 0.0007) & 30% (females; p = 0.0012), but phosphorylated NCC abundance (a surrogate for NCC activation) was decreased by 25% (males; p = 0.06) & 70% (females; p = 0.0005). This reduction in phospho‐NCC in the 5Q mutants resulted in a Gitelman‐like phenotype that was more pronounced in females. Female 5Q mice had a trend towards decreased plasma [K+] (0.4 meq/L reduction; p = 0.1), and significantly decreased [Cl‐] (4 meq/L reduction; p = 0.04) and increased [HCO3‐] (4 meq/L increase; p = 0.05). In conclusion, the KS‐WNK1 5Q mutation causes aberrant WNK body formation, resulting in mislocalization of the WNK‐SPAK/OSR1 pathway in K+ deprived mice. While the 5Q mice can activate SPAK, SPAK is unable to leave the dysfunctional aggregates to activate NCC at the apical membrane. Similar to KS‐WNK1 KO mice, this results in a Gitelman‐like phenotype that predominates in females. These findings demonstrate that KS‐WNK1 assembles WNK bodies to serve as functional protein scaffolds for multikinase complexes to enhance NCC activation during hypokalemia.