Intracellular β-Nicotinamide Adenine Dinucleotide Inhibits the Skeletal Muscle ClC-1 Chloride Channel*

Background: Weakly voltage-dependent ClC-1 chloride channels regulate skeletal muscle excitability. Results: Intracellular NAD(H) altered the voltage dependence of ClC-1 gating, and specific mutations attenuated this effect. Conclusion: NAD(H) directly inhibits ClC-1 chloride channels by binding to intracellular domains. Significance: ClC-1 inhibition by NAD(H) may play an important role in muscle fatigue and the pathophysiology of myotonia congenita. ClC-1 is the dominant sarcolemmal chloride channel and plays an important role in regulating membrane excitability that is underscored by ClC-1 mutations in congenital myotonia. Here we show that the coenzyme β-nicotinamide adenine dinucleotide (NAD), an important metabolic regulator, robustly inhibits ClC-1 when included in the pipette solution in whole cell patch clamp experiments and when transiently applied to inside-out patches. The oxidized (NAD+) form of the coenzyme was more efficacious than the reduced (NADH) form, and inhibition by both was greatly enhanced by acidification. Molecular modeling, based on the structural coordinates of the homologous ClC-5 and CmClC proteins and in silico docking, suggest that NAD+ binds with the adenine base deep in a cleft formed by ClC-1 intracellular cystathionine β-synthase domains, and the nicotinamide base interacts with the membrane-embedded channel domain. Consistent with predictions from the models, mutation of residues in cystathionine β-synthase and channel domains either attenuated (G200R, T636A, H847A) or abrogated (L848A) the effect of NAD+. In addition, the myotonic mutations G200R and Y261C abolished potentiation of NAD+ inhibition at low pH. Our results identify a new biological role for NAD and suggest that the main physiological relevance may be the exquisite sensitivity to intracellular pH that NAD+ inhibition imparts to ClC-1 gating. These findings are consistent with the reduction of sarcolemmal chloride conductance that occurs upon acidification of skeletal muscle and suggest a previously unexplored mechanism in the pathophysiology of myotonia.