Mechanism of Gating and Isoform-Specific Inhibition in Renal CLC Chloride Channels

Hyponatremia is a prevalent disorder marked by excess water retention and substantial morbidity, motivating interest in the CLC-Ka chloride channel as a therapeutic target. Selectively inhibiting CLC-Ka without affecting the closely related CLC-Kb is essential for preventing serious side effects. However, developing isoform-selective inhibitors has been challenging because most small molecules do not distinguish between CLC-Ka and CLC-Kb, and the basis for selectivity in the few known exceptions remains unclear. The small molecule BIM1 preferentially inhibits CLC-Ka over CLC-Kb, providing an opportunity to dissect isoform-specific pharmacology. To investigate this mechanism, we determined cryo-EM structures of BIM1 and BIM15, a related nonselective analog, bound to a CLC-K variant engineered to match the human CLC-Ka binding pocket. Structural and computational analyses reveal that inhibition and isoform selectivity are anchored by interactions with a conserved lysine, with surrounding binding-site residues subtly tuning the local electrostatic environment to promote or disfavor these contacts. These analyses further identify a dynamic extracellular loop that intermittently occludes the access pathway, indicating its role as a gate for ions and inhibitors. BIM15 engages this gating loop more extensively than BIM1, suggesting that differential loop engagement contributes to inhibitor selectivity. To probe how gating reshapes this region, we solved the structure in the presence of Ca²⁺, which favors channel opening, and found the gating loop ordered and withdrawn from the pathway. Together, these findings elucidate how CLC-K channels gate and how subtle binding-site differences and loop dynamics shape isoform-specific drug binding, providing a foundation for designing next-generation CLC-Ka inhibitors.