The finely tuned regulation of neuronal firing relies on the integrity of ion channel macromolecular complexes. Minimal disturbances of these tightly regulated networks can lead to persistent maladaptive plasticity of brain circuitry. The intracellular fibroblast growth factor 14 (FGF14) belongs to the nexus of proteins interacting with voltage-gated Na+ (Nav) channels at the axonal initial segment. Through isoform-specific interactions with the intracellular C-terminal tail of neuronal Nav channels (Nav1.1, Nav1.2, Nav1.6), FGF14 controls channel gating, axonal targeting and phosphorylation in neurons effecting excitability. FGF14 has been also involved in synaptic transmission, plasticity and neurogenesis in the cortico-mesolimbic circuit with cognitive and affective behavioral outcomes. In translational studies, interest in FGF14 continues to rise with a growing list of associative links to diseases of the cognitive and affective domains such as neurodegeneration, depression, anxiety, addictive behaviors and recently schizophrenia, suggesting its role as a converging node in the etiology of complex brain disorders. Yet, a full understanding of FGF14 function in neurons is far from being complete and likely to involve other functions unrelated to the direct regulation of Nav channels. The goal of this Mini Review article is to provide a summary of studies on the emerging role of FGF14 in complex brain disorders.

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Novel proteoforms with single amino acid variations represent proteins that often have altered biological functions but are less explored in the human proteome. We have developed an approach, searching high quality shotgun proteomic data against an extended protein database, to identify expressed mutant proteoforms in glioma stem cell (GSC) lines. The systematic search of MS/MS spectra using PEAKS 7.0 as the search engine has recognized 17 chromosome 19 proteins in GSCs with altered amino acid sequences. The results were further verified by manual spectral examination, validating 19 proteoforms. One of the novel findings, a mutant form of branched-chain aminotransferase 2 (p.Thr186Arg), was verified at the transcript level and by targeted proteomics in several glioma stem cell lines. The structure of this proteoform was examined by molecular modeling in order to estimate conformational changes due to mutation that might lead to functional modifications potentially linked to glioma. Based on our initial findings, we believe that our approach presented could contribute to construct a more complete map of the human functional proteome.

The voltage‐gated sodium (Nav) channel complex is comprised of pore‐forming α subunits (Nav1.1–1.9) and accessory regulatory proteins such as the intracellular fibroblast growth factor 14 (FGF14). The cytosolic Nav1.6 C‐terminal tail binds directly to FGF14 and this interaction modifies Nav1.6‐mediated currents with effects on intrinsic excitability in the brain. Previous studies have identified the FGF14 V160 residue within the FGF14 core domain as a hotspot for the FGF14:Nav1.6 complex formation. Here, we used three short amino acid peptides around FGF14 V160 to probe for the FGF14 interaction with the Nav1.6 C‐terminal tail and to evaluate the activity of the peptide on Nav1.6‐mediated currents. In silico docking predicts FLPK to bind to FGF14 V160 with the expectation of interfering with the FGF14:Nav1.6 complex formation, a phenotype that was confirmed by the split‐luciferase assay (LCA) and surface plasmon resonance (SPR), respectively. Whole‐cell patch‐clamp electrophysiology studies demonstrate that FLPK is able to prevent previously reported FGF14‐dependent phenotypes of Nav1.6 currents, but that its activity requires the FGF14 N‐terminal tail, a domain that has been shown to contribute to Nav1.6 inactivation independently from the FGF14 core domain. In medium spiny neurons in the nucleus accumbens, where both FGF14 and Nav1.6 are abundantly expressed, FLPK significantly increased firing frequency by a mechanism consistent with the ability of the tetrapeptide to interfere with Nav1.6 inactivation and potentiate persistent Na + currents. Taken together, these results indicate that FLPK might serve as a probe for characterizing molecular determinants of neuronal excitability and a peptide scaffold to develop allosteric modulators of Nav channels.

Multiple voltage-gated Na+ (Nav) channelopathies can be ascribed to subtle changes in the Nav macromolecular complex. Fibroblast growth factor 14 (FGF14) is a functionally relevant component of the Nav1.6 channel complex, a causative link to spinocerebellar ataxia 27 (SCA27) and an emerging risk factor for neuropsychiatric disorders. Yet, how this protein:channel complex is regulated in the cell is still poorly understood. To search for key cellular pathways upstream of the FGF14:Nav1.6 complex, we have developed, miniaturized and optimized an in-cell assay in 384-well plates by stably reconstituting the FGF14:Nav1.6 complex using the split-luciferase complementation assay. We then conducted a high-throughput screening (HTS) of 267 FDA-approved compounds targeting known mediators of cellular signaling. Of the 65 hits initially detected, 24 were excluded based on counter-screening and cellular toxicity. Based on target analysis, potency and dose-response relationships, 5 compounds were subsequently repurchased for validation and confirmed as hits. Among those, the tyrosine kinase inhibitor lestaurtinib was highest ranked, exhibiting submicromolar inhibition of FGF14:Nav1.6 assembly. While providing evidence for a robust in-cell HTS platform that can be adapted to search for any channelopathy-associated regulatory proteins, these results lay the potential groundwork for repurposing cancer drugs for neuropsychopharmacology.