Calmodulin acts as a selective switch in FGF12A-mediated regulation of the cardiac sodium channel Nav1.5

The voltage-gated Na⁺ channel (Nav1.5) is responsible for the rapid depolarization of the cardiac action potential and is associated with the initiation of neuronal action potentials in the central nervous system. Nav1.5 channel gating is modulated by its C-terminal domain (CTD) and the multiple auxiliary proteins that are bound to it, including calmodulin (CaM) and intracellular fibroblast growth factors (iFGFs). A non-canonical CaM-binding site was recently discovered on the N-terminus of iFGF A-splice variants. To test whether FGF12A, CaM-bound or not, regulates Nav1.5 gating, we performed cut-open Vaseline gap (COVG) voltage-clamp on Nav1.5 heterologously expressed in Xenopus laevis oocytes. We observed that only CaM-bound FGF12A (wild-type FGF12A) shifted the voltage-dependence of steady-state inactivation (SSI) of Nav1.5. The effect of CaM-bound FGF12A was amplified when CaM was absent from the Nav1.5 CTD (IQ/AA variant) or when the channel carried LQT3 variants. CaM-bound FGF12A additionally shifted the activation of voltage-sensing domain IV (VSD-IV) within the Nav1.5 channel as assessed by voltage-clamp fluorometry (VCF). Conversely, the ability of FGF12A to reduce late sodium current, I_Na,L, did not depend on CaM binding. We conclude that FGF12A regulates Nav1.5 through two distinct mechanisms: CaM-bound FGF12A is required to shift the voltage-dependence of inactivation, but CaM binding is not necessary for the inhibition of pathogenic I_Na,L. These findings reveal that CaM acts as a selective switch within the FGF12A regulatory complex, enabling voltage-dependent inactivation shifts while late current suppression proceeds independently. This bifurcated architecture is likely conserved across A-type iFGF interactions with Nav channel alpha-subunits.