Genetic ablation of the fibroblast growth factor (Fgf) 14 gene in mice or a missense mutation in Fgf14 in humans causes ataxia and cognitive deficits. These phenotypes suggest that the neuronally expressed Fgf14 gene is essential for regulating normal neuronal activity. Here, we demonstrate that FGF14 interacts directly with multiple voltage‐gated Na⁺ (Nav) channel α subunits heterologously expressed in non‐neuronal cells or natively expressed in a murine neuroblastoma cell line. Functional studies reveal that these interactions result in the potent inhibition of Nav channel currents (I_Na) and in changes in the voltage dependence of channel activation and inactivation. Deletion of the unique amino terminus of the splice variant of Fgf14, Fgf14‐1b, or expression of the splice variant Fgf14‐1a modifies the modulatory effects on I_Na, suggesting an important role for the amino terminus domain of FGF14 in the regulation of Na_v channels. To investigate the function of FGF14 in neurones, we directly expressed Fgf14 in freshly isolated primary rat hippocampal neurones. In these cells, the addition of FGF14‐1a–GFP or FGF14‐1b–GFP increased I_Na density and shifted the voltage dependence of channel activation and inactivation. In fully differentiated neurones, FGF14‐1a–GFP or FGF14‐1b–GFP preferentially colocalized with endogenous Nav channels at the axonal initial segment, a critical region for action potential generation. Together, these findings implicate FGF14 as a unique modulator of Nav channel activity in the CNS and provide a possible mechanism to explain the neurological phenotypes observed in mice and humans with mutations in Fgf14.

Fibroblast growth factor 14 (FGF14) belongs to the intracellular FGF homologous factor subfamily of FGF proteins (iFGFs) that are not secreted and do not activate tyrosine kinase receptors. The iFGFs, however, have been shown to interact with the pore-forming (α) subunits of voltage-gated Na⁺(Na_v) channels. The neurological phenotypes seen inFgf14^−/−mice and the identification of anFGF14missense mutation (FGF14^F145S) in a Dutch family presenting with cognitive impairment and spinocerebellar ataxia suggest links between FGF14 and neuronal functioning. Here, we demonstrate that the expression of FGF14^F145Sreduces Na_vα subunit expression at the axon initial segment, attenuates Na_vchannel currents, and reduces the excitability of hippocampal neurons. In addition, and in contrast with wild-type FGF14, FGF14^F145Sdoes not interact directly with Na_vchannel α subunits. Rather, FGF14^F145Sassociates with wild-type FGF14 and disrupts the interaction between wild-type FGF14 and Na_vα subunits, suggesting that the mutant FGF14^F145Sprotein acts as a dominant negative, interfering with the interaction between wild-type FGF14 and Na_vchannel α subunits and altering neuronal excitability.

Voltage-gated sodium channels (Nav) produce sodium currents that underlie the initiation and propagation of action potentials in nerve and muscle cells. Fibroblast growth factor homologous factors (FHFs) bind to the intracellular C-terminal region of the Nav alpha subunit to modulate fast inactivation of the channel. In this study we solved the crystal structure of a 149-residue-long fragment of human FHF2A which unveils the structural features of the homology core domain of all 10 human FHF isoforms. Through analysis of crystal packing contacts and site-directed mutagenesis experiments we identified a conserved surface on the FHF core domain that mediates channel binding in vitro and in vivo. Mutations at this channel binding surface impaired the ability of FHFs to co-localize with Navs at the axon initial segment of hippocampal neurons. The mutations also disabled FHF modulation of voltage-dependent fast inactivation of sodium channels in neuronal cells. Based on our data, we propose that FHFs constitute auxiliary subunits for Navs.

Long-term depression (LTD) is a well-known form of synaptic plasticity of principal neurons in the mammalian brain. Whether such changes occur in interneurons is still controversial. CA3 hippocampal interneurons expressing Ca ²⁺ -permeable AMPA receptors exhibited LTD after tetanic stimulation of CA3 excitatory inputs. LTD was independent of NMDA receptors and required both Ca ²⁺ influx through postsynaptic AMPA receptors and activation of presynaptic mGluR7-like receptors. These results point to the capability of interneurons to undergo plastic changes of synaptic strength through joint activation of pre- and postsynaptic glutamate receptors.