Crystal Structure of a Fibroblast Growth Factor Homologous Factor (FHF) Defines a Conserved Surface on FHFs for Binding and Modulation of Voltage-gated Sodium Channels

Abstract

Voltage-gated sodium channels (Na v ) 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 Na v ␣ 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 Na v s 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 Na v s.Voltage-gated sodium channels (Na v ) 3 produce sodium currents that underlie the initiation and propagation of action potentials in nerve and muscle cells. These channels are heteromeric membrane proteins composed of an ␣ subunit, which is sufficient for channel gating, and one or more auxiliary ␤ subunits, which tune voltage dependence and kinetics of channel gating (for review, see Ref.

Roles of intracellular fibroblast growth factors in neural development and functions

“…Alternative splicing at the N-terminus of intracellular FGFs does not contribute to their interaction with the C-terminal domain of Na v channels. FGF13A induces a greater shift in the voltage-dependence of inactivation when compared with FGF13B [10, 17,39]. This suggests that the alternatively spliced N-terminus of FGF13A is more effective in occluding the channel pore.…”

“…Therefore, the delayed neuronal migration and increased collateral branching of axons could disrupt the formation of the neural circuits and networks required for generating cognitive functions. At the moment, it remains unclear how synaptogenesis, action potential firing and synaptic transmission might be affected by loss of FGF13 function in adulthood, but some effects are expected because FGF13 may regulate neuronal excitability through modulating Na v channels [10, 17,32]. Therefore, FGF13-deficient mice may be a useful animal model for studying XLMR-related mechanisms.…”

“…Overexpression of FGF14-1a/b alters Na v channel currents, and shifts the voltage dependence of Na v channel activation and inactivation [17].…”

“…It would be interesting to study the correlation between the regulation of Na v channel activity and the signaling modules of intracellular FGFs in neurons. Analysis of the crystal structure of FGF13A reveals a conserved surface required for binding to the C-terminal domain of the pore-forming () subunits of Na v channels and for modulating the inactivation of Na v channels by partially occluding the inner face of the pore [17]. The 152-residue-long homologous region of intracellular FGFs is sufficient for channel binding, and the 18-residue-long, C-terminal extension of the homologous region plays an essential role in channel binding [17].…”

“…Analysis of the crystal structure of FGF13A reveals a conserved surface required for binding to the C-terminal domain of the pore-forming () subunits of Na v channels and for modulating the inactivation of Na v channels by partially occluding the inner face of the pore [17]. The 152-residue-long homologous region of intracellular FGFs is sufficient for channel binding, and the 18-residue-long, C-terminal extension of the homologous region plays an essential role in channel binding [17]. Alternative splicing at the N-terminus of intracellular FGFs does not contribute to their interaction with the C-terminal domain of Na v channels.…”