Dynamic Interplay of Calmodulin and Fibroblast Growth Factor Homologous Factors in Regulating NA Channels

Abstract

In recent years, the ubiquitous Ca2+-binding protein calmodulin (CaM), and the non-canonical intracellular fibroblast growth factor homologous factors (FHF) have emerged as two potent modulators of voltage-gated sodium channels with profound implications for the excitability of neuronal, cardiac, and skeletal muscle cells. Curiously, both regulatory molecules associate with the Na channel through a well-conserved channel carboxy-terminus with CaM preassociating to a canonical IQ motif and FHF interacting with the closely apposed preIQ - EF hand surface. Even so, the interaction of these auxiliary proteins is thought to evoke distinct and independent channel regulatory effects. CaM interaction with the Na channel confers a rapid Ca2+-dependent feedback regulation (Cell 157(7):1657) while FHF appear to alter various voltage-dependent properties of the canonical channel fast inactivation gate (Cell. Mol. Life Sci. 59:1067). Here, we demonstrate a novel role of FHF to allosterically suppress Ca2+/CaM-dependent regulation of skeletal muscle Na channels (NaV1.4). In the absence of FHF, the wild-type NaV1.4 channels exhibit rapid and robust Ca2+-dependent inhibition mediated by a resident CaM. However, co-expression of FHF1 entirely abolishes Ca2+-dependent inhibition of these channels. Similarly, co-expression of various isoforms of FHF (FHF2-4) with NaV1.4 results in a graded suppression of Ca2+/CaM regulation. These results argue that Ca2+-regulation of Na channels can be potently tuned by differential expression of various FHF isoforms, as are often associated with various pathophysiological conditions. Overall, the dynamic interplay of CaM and FHF identified here vastly enriches the modulatory landscape of Na channels and highlights the sophisticated mechanisms by which cytosolic proteins can tune cellular excitability.