Targeting Arrhythmogenic Late Sodium Current by FHF1_A-Derivative FixR in Heart Failure Cardiomyocytes

Enhanced late Na⁺ current (I_Na,L) in heart failure (HF) contributes to cardiomyocyte proarrhythmia. In addition to CaMKII (Ca²⁺/calmodulin-dependent protein kinase II), FGF (fibroblast growth factor) homologous factors 1-4 (FHF1-4) also modulate I_Na,L, and targeting these pathways may provide benefits in HF.

Reverse-transcriptase quantitative polymerase chain reaction analysis of cardiac Na⁺ channel and FHF splice isoforms was performed in human arrhythmogenic HF with reduced ejection fraction (HFrEF) and in translational animal models of HFrEF and HF with preserved ejection fraction (HFpEF). We tested the effects of an engineered peptide, FixR (FHF inhibiting x region), on cardiomyocyte I_Na,L and electrophysiology in rabbit HFrEF and murine models of HFrEF and HFpEF. We also tested the in vivo electrophysiological effects of FixR via adenoviral delivery in transgenic mice with cardiomyocyte-specific overexpression of CaMKIIδ_C.

Expression of key FHF long isoforms (FHF1_A in humans, FHF2_S in rabbits, and FHF2_VY in mice) that have inhibitory effects on I_Na,L was reduced in human, rabbit, and murine failing hearts. I_Na,L was markedly enhanced in both HFrEF and HFpEF, and the FixR cell-penetrating peptide significantly reduced pathological I_Na,L in both forms of HF. FixR had no effect on transient peak Na⁺ current, L-type Ca²⁺ current, or major K⁺ currents in rabbit ventricular myocytes. FixR markedly attenuated proarrhythmogenic action potential changes (increased action potential duration, short-term variability, and alternans susceptibility) and delayed afterdepolarizations in both rabbit and murine HFrEF and HFpEF cardiomyocytes. These cellular antiarrhythmic effects were mimicked by the selective I_Na,L inhibitor GS967 and the CaMKII inhibitor AIP (autocamtide-2 inhibitory peptide). FixR also attenuated QT prolongation and in vivo arrhythmia susceptibility in transgenic mice with cardiomyocyte-specific overexpression of CaMKIIδ_C.

In HF, I_Na,L is increased, and FHF splice isoform expression is altered, allowing a novel mechanism to therapeutically target I_Na,L. FixR, an FHF1_A-derived peptide, is a potent and selective I_Na,L inhibitor and has antiarrhythmic properties in both HFrEF and HFpEF cardiomyocytes.