Elucidating the Structural Constituents of Fibroblast Growth Factor 14 that Confer its Modulatory Effects on the Voltage-Gated Sodium Channel 1.6

Abstract

Disruption of protein:protein interactions (PPIs) between voltage-gated Na+ (Nav) channels and their auxiliary proteins is implicated in the etiology of channelopathies. Despite restoration of these perturbed PPIs serving as a novel therapeutic approach, such efforts to develop PPI-based pharmacotherapies are hindered by lack of insights into the structural constituents of regulatory proteins that confer their modulatory effects on Nav channel kinetics. Focusing on the PPI between Nav1.6 and its auxiliary protein fibroblast growth factor 14 (FGF14), we previously identified clusters of amino acids on the β12 sheet and β8-β9 loop of FGF14 that are at its PPI interface with the C-terminal domain (CTD) of Nav1.6 and are predicted to disproportionately contribute to the Gibbs energy of their protein:protein binding. To develop chemical probes, we derived peptidomimetics from the PLEV and EYYV motifs of the β12 sheet and β8-β9 loop, respectively. Crucially, a FGF14PLEV derivative caused a depolarizing shift in the V1/2 of Nav1.6 channel activation and steady-state inactivation, but did not affect long-term inactivation. Conversely, a FGF14EYYV derivate caused an increase in the long-term inactivation of Nav1.6 channels, but did not affect the V1/2 of activation or steady-state inactivation, suggesting that the β12 sheet and β8-β9 loop constituents of FGF14 regulate divergent kinetic properties of Nav1.6 channels. Based upon molecular docking of these two derivatives, it is hypothesized that their divergent regulation of Nav1.6 channel kinetics is conferred by differential interactions with the EF hand like (EFL) and IQ domains of the CTD. Exploration of this hypothesis will enable an increasingly refined understanding of Nav channel regulation and provide insights into developing therapeutics targeted against PPI interfaces between Nav channels and their auxiliary proteins.