Regulation of the FGF14:Nav Interactome by a GSK-3 Centered Pathway

Psychiatric disorders are complex, multifactorial diseases characterized by disruption of neural circuits, accompanied by an imbalance of excitatory and inhibitory inputs leading to dysregulations in neuronal excitability. Recent research focused on uncovering novel causes of psychiatric disorders have highlighted the importance of the axonal initial segment (AIS), a highly specialized neuronal structure critical for spike initiation of the action potential. The AIS contains a high density of voltage-gated sodium channels, the fundamental component of the neuron driving the initiation and propagation of the action potential, which interact with accessory and regulatory proteins including ankyrin-G, β-IV-spectrin, neurofascin, and intracellular fibroblast growth factors (iFGFs, FGF11-14). Dysregulation of the sodium channel complex is associated with a panorama of psychiatric disorders in both animal models as well as human genome-wide association studies. However, the cellular signaling networks responsible for exerting the fine-grained control of this critical macromolecular complex are poorly understood. Protein kinases crucially regulate cellular pathways, governing intracellular protein signaling and genetic programs through residue-specific phosphorylation. Recent studies have shown that the assembly and trafficking of FGF14, an intracellular fibroblast growth factor critical for the maintenance of the Nav complex, in neurons is controlled via a GSK-3 dependent signaling pathway (Shavkunov et al., 2013). Additionally, inhibition of GSK-3 reduces the assembly of FGF14:Nav1.2 complexes, redistributes FGF14 and Nav channels from the axonal compartment to the somatodendritic compartment, and bidirectionally modulates Nav currents. Conducting a high-throughput screening of over 400 small molecule protein kinase inhibitors, we have identified a panel of kinases that we show through downstream studies to comprise a focused GSK-3 interactome regulating the Nav complex. By combining biochemical assays, high-content confocal imaging, mass spectrometry, and patch-clamp electrophysiology, we show that alteration of this focused GSK-3 interactome, comprised of Akt, Wee1, and CK2, disrupts Nav complex assembly, changes the phosphorylation status of the FGF14 N-terminus, alters the sub-cellular localization and expression of Nav complex proteins, and affects neuronal firing. These findings suggest that FGF14 may act as a GSK-3 sensitive switch to fine-tune neuronal excitability by altering protein:protein interactions at the level of the Nav complex.