Differential regulation of cardiac sodium channels by intracellular fibroblast growth factors

Voltage-gated sodium (Na_V) channels are responsible for the initiation and propagation of action potentials. In the heart, the predominant Na_V1.5 α subunit is composed of four homologous repeats (I–IV) and forms a macromolecular complex with multiple accessory proteins, including intracellular fibroblast growth factors (iFGF). In spite of high homology, each of the iFGFs, iFGF11–iFGF14, as well as the individual iFGF splice variants, differentially regulates Na_V channel gating, and the mechanisms underlying these differential effects remain elusive. Much of the work exploring iFGF regulation of Na_V1.5 has been performed in mouse and rat ventricular myocytes in which iFGF13VY is the predominant iFGF expressed, whereas investigation into Na_V1.5 regulation by the human heart-dominant iFGF12B is lacking. In this study, we used a mouse model with cardiac-specific Fgf13 deletion to study the consequences of iFGF13VY and iFGF12B expression. We observed distinct effects on the voltage-dependences of activation and inactivation of the sodium currents (I_Na), as well as on the kinetics of peak I_Na decay. Results in native myocytes were recapitulated with human Na_V1.5 heterologously expressed in Xenopus oocytes, and additional experiments using voltage-clamp fluorometry (VCF) revealed iFGF-specific effects on the activation of the Na_V1.5 voltage sensor domain in repeat IV (VSD-IV). iFGF chimeras further unveiled roles for all three iFGF domains (i.e., the N-terminus, core, and C-terminus) on the regulation of VSD-IV, and a slower time domain of inactivation. We present here a novel mechanism of iFGF regulation that is specific to individual iFGF isoforms and that leads to distinct functional effects on Na_V channel/current kinetics.