Regulation of CFTR Bicarbonate Channel Activity by WNK1: Implications for Pancreatitis and CFTR-Related Disorders

Aberrant epithelial bicarbonate (HCO₃^-) secretion caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene is associated with several diseases including cystic fibrosis and pancreatitis. Dynamically regulated ion channel activity and anion selectivity of CFTR by kinases sensitive to intracellular chloride concentration ([Cl^-]_i) play an important role in epithelial HCO₃^- secretion. However, the molecular mechanisms of how [Cl^-]_i-dependent mechanisms regulate CFTR are unknown.

We examined the mechanisms of the CFTR HCO₃^- channel regulation by [Cl^-]_i-sensitive kinases using an integrated electrophysiological, molecular, and computational approach including whole-cell, outside-out, and inside-out patch clamp recordings and molecular dissection of WNK1 and CFTR proteins. In addition, we analyzed the effects of pancreatitis-causing CFTR mutations on the WNK1-mediated regulation of CFTR.

Among the WNK1, SPAK, and OSR1 kinases that constitute a [Cl^-]_i-sensitive kinase cascade, the expression of WNK1 alone was sufficient to increase the CFTR bicarbonate permeability (P_HCO3/P_Cl) and conductance (G_HCO3) in patch clamp recordings. Molecular dissection of the WNK1 domains revealed that the WNK1 kinase domain is responsible for CFTR P_HCO3/P_Cl regulation by direct association with CFTR, while the surrounding N-terminal regions mediate the [Cl^-]_i-sensitivity of WNK1. Furthermore, the pancreatitis-causing R74Q and R75Q mutations in the elbow helix 1 of CFTR hampered WNK1-CFTR physical associations and reduced WNK1-mediated CFTR P_HCO3/P_Cl regulation.

The CFTR HCO₃^- channel activity is regulated by [Cl^-]_i and a WNK1-dependent mechanism. Our results provide new insights into the regulation of the ion selectivity of CFTR and the pathogenesis of CFTR-related disorders.