Regulation of large-conductance Ca²⁺-activated K⁺ channels by WNK4 kinase

Large-conductance, Ca²⁺-activated K⁺ channels, commonly referred to as BK channels, have a major role in flow-induced K⁺ secretion in the distal nephron. With-no-lysine kinase 4 (WNK4) is a serinethreonine kinase expressed in the distal nephron that inhibits ROMK activity and renal K⁺ secretion. WNK4 mutations have been described in individuals with familial hyperkalemic hypertension (FHHt), a Mendelian disorder characterized by low-renin hypertension and hyperkalemia. As BK channels also have an important role in renal K⁺ secretion, we examined whether they are regulated by WNK4 in a manner similar to ROMK. BK channel activity was inhibited in a rabbit intercalated cell line transfected with WNK4 or a WNK4 mutant found in individuals with FHHt. Coexpression of an epitope-tagged BK α-subunit with WNK4 or the WNK4 mutant in HEK293 cells reduced BK α-subunit plasma membrane and whole cell expression. A region within WNK4 encompassing the autoinhibitory domain and a coiled coil domain was required for WNK4 to inhibit BK α-subunit expression. The relative fraction of BK α-subunit that was ubiquitinated was significantly increased in cells expressing WNK4, compared with controls. Our results suggest that WNK4 inhibits BK channel activity, in part, by increasing channel degradation through an ubiquitin-dependent pathway. Based on these results, we propose that WNK4 provides a cellular mechanism for the coordinated regulation of two key secretory K⁺ channels in the distal nephron, ROMK and BK.