Fibroblast Growth Factor Homologous Factors Modulate Cardiac Calcium Channels

Abstract

Rationale : Fibroblast growth factor (FGF) homologous factors (FHFs; FGF11–14) are intracellular modulators of voltage-gated Na + channels, but their cellular distribution in cardiomyocytes indicated that they performed other functions. Objective : We aimed to uncover novel roles for FHFs in cardiomyocytes, starting with a proteomic approach to identify novel interacting proteins. Methods and Results : Affinity purification of FGF13 from rodent ventricular lysates followed by mass spectroscopy revealed an interaction with junctophilin-2, a protein that organizes the close apposition of the L-type Ca 2+ channel Ca V 1.2 and the ryanodine receptor 2 in the dyad. Immunocytochemical analysis revealed that overall T-tubule structure and localization of ryanodine receptor 2 were unaffected by FGF13 knockdown in adult ventricular cardiomyocytes but localization of Ca V 1.2 was affected. FGF13 knockdown decreased Ca V 1.2 current density and reduced the amount of Ca V 1.2 at the surface as a result of aberrant localization of the channels. Ca V 1.2 current density and channel localization were rescued by expression of an shRNA-insensitive FGF13, indicating a specific role for FGF13. Consistent with these newly discovered effects on Ca V 1.2, we demonstrated that FGF13 also regulated Ca 2+ -induced Ca 2+ release, indicated by a smaller Ca 2+ transient after FGF13 knockdown. Furthermore, FGF13 knockdown caused a profound decrease in the cardiac action potential half-width. Conclusions : This study demonstrates that FHFs not only are potent modulators of voltage-gated Na + channels but also affect Ca 2+ channels and their function. We predict that FHF loss-of-function mutations would adversely affect currents through both Na + and Ca 2+ channels, suggesting that FHFs may be arrhythmogenic loci, leading to arrhythmias through a novel, dual-ion channel mechanism.