Ionic Mechanisms of Impulse Propagation Failure in the FHF2-Deficient Heart.

Park, David S; Shekhar, Akshay; Santucci Iii, John; Redel-Traub, Gabriel; Solinas, Sergio Mg; Mintz, Shana; Lin, Xianming; Chang, Ernest W; Narke, Deven; Xia, Yuhe; Goldfarb, Mitchell; Fishman, Glenn I

Rationale: Fibroblast growth factor homologous factors (FHFs) are key regulators of sodium channel inactivation. Mutations in these critical proteins have been implicated in human diseases including Brugada syndrome, idiopathic ventricular arrhythmias, and epileptic encephalopathy. The underlying ionic mechanisms by which reduced sodium channel availability in Fhf2 knockout mice predisposes to abnormal excitability at the tissue level are not well defined. Objective: Using animal models and theoretical multicellular linear strands, we examined how FHF2 orchestrates the interdependency of sodium, calcium, and gap junctional conductances to safeguard cardiac conduction. Methods and Results: Fhf2KO mice were challenged by reducing calcium conductance using verapamil or by reducing gap junctional conductance using carbenoxolone or by backcrossing into a connexin 43 heterozygous (Cx43+/-) background. All conditions produced conduction block in Fhf2KO mice, with Fhf2WT showing normal impulse propagation. To explore the ionic mechanisms of block in Fhf2KO hearts, multicellular linear strand models incorporating FHF2-deficient sodium channel inactivation properties were constructed and faithfully recapitulated conduction abnormalities seen in mutant hearts. The mechanisms of conduction block in mutant strands with reduced calcium conductance or gap junction uncoupling are very different. Enhanced sodium channel inactivation due to FHF2 deficiency shifts dependence onto calcium current to sustain electrotonic driving force, axial current flow, and action potential generation from cell-to-cell. In the setting of gap junction uncoupling, slower charging time from upstream cells conspires with accelerated sodium channel inactivation in mutant strands to prevent sufficient downstream cell charging for action potential propagation. Conclusions: FHF2-dependent effects on sodium channel inactivation ensure adequate sodium current reserve to safeguard against numerous threats to reliable cardiac impulse propagation.