FGF12A Counteracts Long Qt Syndrome-Linked Inactivation Deficiency

Abstract

Cardiac sodium channels (Nav1.5) are responsible for the initiation and propagation of the action potential. Multiple proteins interact and modulate its gating kinetics, including FGF12, the most dominant intracellular FGF in human heart. Two isoforms encoded by the FGF12 gene are FGF12a and FGF12b, differing in N-terminal sequences. We investigate the mechanism of FGF12 regulation of Nav1.5 and its interaction with disease-linked mutations. Nav1.5 comprises 4 homologous domains (DI-DIV). Within each domain, 6 transmembrane segments (S1-S6) constitute the voltage sensing domain (VSD; S1-S4) and the channel pore (S5-S6). By probing changes in fluorescence from a fluorophore tethered onto S4, voltage-clamp fluorometry (VCF) allows simultaneous monitoring of individual VSD movement and ion channel kinetics in the cut-open oocyte. Both FGF12 isoforms shifted the steady-state inactivation curve toward depolarizing potentials (FGF12a:+7.4mV, FGF12b:+7.2mV). VCF experiments identified a shift in DI- (FGF12a:+35.5mV, FGF12b:+33.7mV) and DIII-VSD activation (FGF12a:+43.6mV, FGF12b:+34.2mV). The deactivation of DIII-VSD was previously linked to the channel inactivation and the IFM motif within the DIII-DIV linker (Hsu et al, 2017). We hypothesized an IFM role in mediating FGF12 modulation of Nav1.5 kinetics and found that the FGF12 effect on DIII-VSD was removed by the inactivation-deficient IQM(F1486Q) mutant. Interestingly, FGF12a partially rescued the inactivation of IQM, increasing the percentage of inactivated channels at 40mV from 46% relative to the peak to 79%. Consistent results were also found with a long QT Syndrome-linked variant, deltaKPQ, where FGF12a nearly abolished the late Na+ current in this inactivation-deficient mutant. Both FGF12a and FGF12b interact with DIII-DIV linker to modulate NaV1.5 kinetics. FGF12a is able to restore inactivation impaired by disease-linked mutations. A small molecule mimicking the FGF12a effect could be a potent inhibitor of late Na+ current underlying various cardiac pathologies including arrhythmia and heart failure.