Abstract 2700: Intracellular FGF13 Functions as an Endogenous Regulator of Myocardial Voltage-Gated Sodium (Nav) Channels
Background: Although structurally related to the classical Fibroblast Growth Factor family of peptides, the intracellular FGFs (iFGFs) are not secreted and do not activate tyrosine kinase receptors. Previous studies have shown that members of the iFGF subfamily interact with the pore forming (α) subunits of voltage-gated Na+ (Nav) channels, suggesting that iFGFs might function in the regulation of cardiac Nav channels.
Methods and Results: Quantitative RT-PCR analyses revealed robust expression of FGF13 and much lower, but detectable, levels of expression of the other iFGFs, FGF11 > FGF14 > FGF12, in adult mouse ventricles. Short hairpin interfering RNAs (shRNAs) targeting FGF13 were screened in heterologous cells expressing an EGFP-tagged FGF13, and validated shRNAs were introduced into isolated neonatal mouse ventricular myocytes, together with a construct encoding EGFP, using the AMAXA nucleofector system. Whole-cell voltage-clamp recordings from EGFP-positive, FGF13-targeted shRNA-expressing ventricular myocytes revealed that the (mean±SEM) peak Nav current density (120±37 pA/pF; n = 26) was significantly (P < 0.001) lower than in cells expressing EGFP alone (265±42 pA/pF; n = 19). In contrast, peak Nav current densities in cells transfected with a control, non-targeting shRNA (n = 21) or with a shRNA targeted against FGF11 (n = 11) were indistinguishable from control cells expressing EGFP alone. In addition to decreasing peak Nav current amplitudes, expression of the FGF13-targeted shRNA, resulted in a marked hyperpolarizing (−13±2 mV) shift in the voltage-dependence of Nav channel inactivation, an effect that will reduce Nav channel availability and further attenuate functional Nav current densities. In contrast, the kinetic properties of mouse ventricular Nav currents were unaffected by expression of the FGF13-targeted shRNA.
Conclusions: These results reveal a novel role for FGF13 in controlling the myocardial membrane excitability via regulation of the functional cell surface expression and properties of ventricular Nav channels and suggest that targeting FGF13-Nav channel interactions might represent a novel therapeutic strategy to augment myocardial Nav channel functioning.
This research has received full or partial funding support from the American Heart Association, Midwest Affiliate (Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, South Dakota & Wisconsin).