Targeting protein: protein interaction sites for drug development against voltage-gated sodium channels

Voltage-gated sodium (Nav) channels are responsible for initiation and propagation of action potentials, which contribute to control of neuronal excitability. Malfunction of specific Nav channel isoforms is associated with a wide range of brain disorders including psychological, neurological and developmental disorders. Unfortunately, currently available drugs targeting Nav channels are directed against highly conserved domains of the α-subunit of all Nav channels, and as such they have severe side effects, including cardiac malfunction. Fortunately, the macromolecular complex of Nav channels is a source of less conserved protein-protein interaction (PPI) interfaces that represent a novel opportunity for designing isoform-specific chemical leads targeting Nav channels. The macromolecular complex of Nav channels is regulated by a number of accessory proteins. Very few proteins regulate the functional properties of Nav channels as potently as the intracellular fibroblast growth factor 14 (FGF14). FGF14 is a biologically relevant accessory protein of the neuronal Nav channel complex controlling gating, stability, and trafficking of native Nav channels. Through a monomeric interaction with the intracellular C-terminal tail of Nav channel α subunits, FGF14 binds and modulates the activity of Nav channels in an isoform-specific manner. By applying luciferase-based assays, patch clamp electrophysiology, and intrinsic fluorescence studies, we have identified the β9 loop at the interface of FGF14 as critical for binding to Nav1.6 channels. Based on this information, we have identified a short sequence on FGF14 and designed a peptidomimetic (ZL181) fragment as an effective probe for modulating Nav1.6 channels as measured by luciferase-based assay. This peptidomimetic was further evaluated with purified proteins, in silico docking, and whole-cell patch clamp electrophysiology in both in vitro and ex vivo systems. Overall, our data demonstrated that a novel peptidomimetic (ZL181) can modulate the functional properties of Nav1.6 channels and can suppress neuronal excitability in nucleus accumbens medium spiny neurons. The new knowledge gained from this study might be useful for the treatment of Nav1.6 channel-related brain disorders such as epilepsy, schizophrenia and cognitive disorders.