FGF12A Regulates Nav1.5 via CaM-regulated and CaM-independent Mechanisms

Opening of the cardiac voltage-gated Na+ channel (Nav1.5) is responsible for robust depolarization of the cardiac action potential, while inactivation, which rapidly follows, allows for repolarization. Regulation of both the voltage- and time-dependent kinetics of Nav1.5 inactivation can alter the ability of the heart to initiate and sustain a re-entrant arrhythmia. The C-terminal domain (CTD) of Nav1.5 has been shown to modulate fast inactivation of the channel, and multiple auxiliary proteins bind to the CTD, including calmodulin (CaM) and intracellular fibroblast growth factor 12A (FGF12A). Recently, a non-canonical CaM-binding site was also discovered on the N-terminal of A-splice variants of iFGFs. We performed cut-open Vaseline gap (COVG) voltage-clamp to test whether FGF12A with and without CaM regulates Nav1.5 gating. In WT Nav1.5 channels, FGF12A with and without CaM present had a minimal effect on the voltage dependence of both activation and inactivation. Conversely, when CaM is absent on the Nav1.5 CTD (IQ/AA), a dramatic shift in steady-state inactivation (SSI) occurred, regardless of whether CaM was present on FGF12A. These two distinct mechanisms are operative in Nav1.5 LQT3 mutations where FGF12A requires CaM to shift in the voltage-dependence of inactivation, but not to inhibit the persistent late current. We conclude that there are two distinct mechanisms by which FGF12A modulates the Nav1.5 channel: CaM-regulated alteration of the voltage dependence of inactivation and CaM-independent inhibition of persistent late current.