Elementary mechanisms of calmodulin regulation of Na V 1.5 producing divergent arrhythmogenic phenotypes

In cardiomyocytes, Na 1.5 channels mediate initiation and fast propagation of action potentials. The Ca -binding protein calmodulin (CaM) serves as a de facto subunit of Na 1.5. Genetic studies and atomic structures suggest that this interaction is pathophysiologically critical, as human mutations within the Na 1.5 carboxy-terminus that disrupt CaM binding are linked to distinct forms of life-threatening arrhythmias, including long QT syndrome 3, a "gain-of-function" defect, and Brugada syndrome, a "loss-of-function" phenotype. Yet, how a common disruption in CaM binding engenders divergent effects on Na 1.5 gating is not fully understood, though vital for elucidating arrhythmogenic mechanisms and for developing new therapies. Here, using extensive single-channel analysis, we find that the disruption of Ca -free CaM preassociation with Na 1.5 exerts two disparate effects: 1) a decrease in the peak open probability and 2) an increase in persistent Na openings. Mechanistically, these effects arise from a CaM-dependent switch in the Na inactivation mechanism. Specifically, CaM-bound channels preferentially inactivate from the open state, while those devoid of CaM exhibit enhanced closed-state inactivation. Further enriching this scheme, for certain mutant Na 1.5, local Ca fluctuations elicit a rapid recruitment of CaM that reverses the increase in persistent Na current, a factor that may promote beat-to-beat variability in late Na current. In all, these findings identify the elementary mechanism of CaM regulation of Na 1.5 and, in so doing, unravel a noncanonical role for CaM in tuning ion channel gating. Furthermore, our results furnish an in-depth molecular framework for understanding complex arrhythmogenic phenotypes of Na 1.5 channelopathies.