Summary
Voltage-gated Na+ (NaV) channels initiate neuronal action potentials. NaV channels are composed of a transmembrane domain responsible for voltage-dependent Na+ conduction and a cytosolic C-terminal domain (CTD) that regulates channel function through interactions with many auxiliary proteins, including fibroblast growth factor homologous factors (FHFs) and calmodulin (CaM). Most ion channel structural studies have focused on mechanisms of permeation and voltage-dependent gating but less is known about how intracellular domains modulate channel function. Here we report the crystal structure of the ternary complex of a human NaV CTD, an FHF, and Ca2+-free CaM at 2.2 Å. Combined with functional experiments based on structural insights, we present a platform for understanding the roles of these auxiliary proteins in NaV channel regulation and the molecular basis of mutations that lead to neuronal and cardiac diseases. Furthermore, we identify a critical interaction that contributes to the specificity of individual NaV CTD isoforms for distinctive FHFs.
Rationale
Fibroblast growth factor homologous factors (FHFs), a subfamily of fibroblast growth factors (FGFs) that are incapable of functioning as growth factors, are intracellular modulators of Na+ channels and have been linked to neurodegenerative diseases. Although certain FHFs have been found in embryonic heart, they have not been reported in adult heart, and they have not been shown to regulate endogenous cardiac Na+ channels nor participate in cardiac pathophysiology.
Objective
We tested whether FHFs regulate Na+ channels in murine heart.
Methods and Results
We demonstrated that isoforms of FGF13 are the predominant FHFs in adult mouse ventricular myocytes. FGF13 binds directly to, and co-localizes with the Na 1.5 Na+ V channel in the sarcolemma of adult mouse ventricular myocytes. Knockdown of FGF13 in adult mouse ventricular myocytes revealed a loss-of-function of NaV1.5: reduced Na+ current (INa) density, decreased Na+ channel availability, and slowed INa recovery from inactivation. Cell surface biotinylation experiments showed a ~45% reduction in NaV1.5 protein at the sarcolemma after FGF13 knockdown, whereas no changes in whole-cell NaV1.5 protein nor mRNA level were observed. Optical imaging in neonatal rat ventricular myocyte monolayers demonstrated slowed conduction velocity and a reduced maximum capture rate after FGF13 knockdown.
Conclusion
These findings show that FHFs are potent regulators of Na+ channels in adult ventricular myocytes and suggest that loss-of-function mutations in FHFs may underlie a similar set of cardiac arrhythmias and cardiomyopathies that result from NaV1.5 loss-of-function mutations.
Ca2+ regulates voltage-gated Na+ (NaV) channels and perturbed Ca2+ regulation of NaV function is associated with epilepsy syndromes, autism, and cardiac arrhythmias. Understanding the disease mechanisms, however, has been hindered by a lack of structural information and competing models for how Ca2+ affects NaV channel function. Here, we report the crystal structures of two ternary complexes of a human NaV cytosolic C-terminal domain (CTD), a fibroblast growth factor homologous factor, and Ca2+/calmodulin (Ca2+/CaM). These structures rule out direct binding of Ca2+ to the NaV CTD, and uncover new contacts between CaM and the NaV CTD. Probing these new contacts with biochemical and functional experiments allows us to propose a mechanism by which Ca2+ could regulate NaV channels. Further, our model provides hints towards understanding the molecular basis of the neurologic disorders and cardiac arrhythmias caused by NaV channel mutations.
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