Fibroblast Growth Factor Homologous Factors Modulate Cardiac Calcium Channels
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 Ca2+ channel, CaV1.2, and the ryanodine receptor, RyR2, in the dyad. Immunocytochemical analysis revealed overall T-tubule structure and localization RyR2 were unaffected by FGF13 knockdown in adult ventricular cardiomyocytes, but localization of CaV1.2 was affected. FGF13 knockdown decreased CaV1.2 current density, and reduced the amount of CaV1.2 at the surface due to aberrant localization of the channels. CaV1.2 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 CaV1.2, we demonstrated that FGF13 also regulated Ca2+-induced Ca2+ release, indicated by a smaller Ca2+ transient after FGF13 knockdown. Further, FGF13 knockdown caused a profound decrease in the cardiac action potential half width.
Conclusions: This study demonstrates that FHFs are not only potent modulators voltage-gated Na+ channels, but also affect Ca2+ channels and their function. We predict that FHF loss-of-function mutations would adversely affect currents through both Na+ and Ca2+ cannels, suggesting that FHFs may be arrhythmogenic loci, leading to arrhythmias through a novel, dual-ion channel mechanism.
- homologous factor
- calcium channel
- cardiac electrophysiology
- fibroblast growth factor
- Received February 19, 2013.
- Revision received June 25, 2013.
- Accepted June 26, 2013.