Targeting Arrhythmogenic Late Sodium Current by FHF1 _A -Derivative FixR in Heart Failure Cardiomyocytes

BACKGROUND: Enhanced late Na + current (I Na,L ) in heart failure (HF) contributes to cardiomyocyte proarrhythmia. In addition to CaMKII (Ca 2+ /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. METHODS: 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 . RESULTS: 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 Ca 2+ 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 . CONCLUSIONS: 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.