Fibroblast Growth Factor Homologous Factor 2B: Association with Na_v1.6 and Selective Colocalization at Nodes of Ranvier of Dorsal Root Axons

Abstract

Voltage-gated sodium channels interact with cytosolic proteins that regulate channel trafficking and/or modulate the biophysical properties of the channels. Na v 1.6 is heavily expressed at the nodes of Ranvier along adult CNS and PNS axons and along unmyelinated fibers in the PNS. In an initial yeast two-hybrid screen using the C terminus of Na v 1.6 as a bait, we identified FHF2B, a member of the FGF homologous factor (FHF) subfamily, as an interacting partner of Na v 1.6. Members of the FHF subfamily share ϳ70% sequence identity, and individual members demonstrate a cell-and tissue-specific expression pattern. FHF2 is abundantly expressed in the hippocampus and DRG neurons and colocalizes with Na v 1.6 at mature nodes of Ranvier in myelinated sensory fibers in the dorsal root of the sciatic nerve. However, retinal ganglion cells and spinal ventral horn motor neurons show very low levels of FHF2 expression, and their axons exhibit no nodal FHF2 staining within the optic nerve and ventral root, respectively. Thus, FHF2 is selectively localized at nodes of dorsal root sensory but not ventral root motor axons. The coexpression of FHF2B and Na v 1.6 in the DRG-derived cell line ND7/23 significantly increases the peak current amplitude and causes a 4 mV depolarizing shift of voltage-dependent inactivation of the channel. The preferential expression of FHF2B in sensory neurons may provide a basis for physiological differences in sodium currents that have been reported at the nodes of Ranvier in sensory versus motor axons.

Polarized localization of voltage-gated Na + channels is regulated by concerted FGF13 and FGF14 action

“…2) and our observations that FGF13 can affect other ion channels, such as voltage-gated Ca 2+ channels in cardiac myocytes (34). Another possibility is that the FGF13 within the AIS is in transition to developing nodes of Ranvier, because FGF13 has been found concentrated in nodes of Ranvier in dorsal root ganglion neurons (43). Nevertheless, the different roles for FGF13 and FGF14 in hippocampal neurons are consistent with the importance of cellular context in determining the actions of any specific FHF (44), a phenomenon that is also highlighted by the opposing effects of FGF13 in hippocampal neurons compared with the heart.…”

Fibroblast growth factor homologous factors: Evolution, structure, and function

“…This effect was not observed in similar studies on Nav1.6. By contrast, ectopic expression FHF2b increased the maximal activatable whole cell Nav1.6 sodium currents by 2-fold (Wittmack et al, 2004). This may be attributable to either increased surface channel density or to more efficient activation of available surface channels.…”

“…Since then, it has been shown that FHFs can bind to Nav1.5, which is expressed in cardiac myocytes (Liu et al, 2003), and to Nav1.6, the principal channel mediating initiation and propagation of action potentials in most mature neurons of the central and peripheral nervous systems (Shtraizent and Goldfarb, 2005;Wittmack et al, 2004). Native complexes of FHFs with sodium channels have been detected by coimmunoprecipitation from extracts of brain or dorsal root ganglia (M. Goldfarb, unpublished data).…”

“…Native complexes of FHFs with sodium channels have been detected by coimmunoprecipitation from extracts of brain or dorsal root ganglia (M. Goldfarb, unpublished data). FHF2 colocalizes with Nav1.6 at conduction nodes of Ranvier in myelinated sensory axons (Wittmack et al, 2004). Although an early report suggested that only specific sodium channels associate with FHFs, there is now no evidence showing such restriction.…”

Another piece to the intracellular FGF/Na + channel puzzle

“…Although this diversity has clearly complicated the field, FGFs' role as Na v channel interacting proteins has been solidified by the discovery of a highly conserved Na v interaction site within the FGF core domain that interacts with specific amino acids within the Na v C termini (2). FGF14 and Na v channels are enriched at the axon initial segment (AIS) of several neuronal types, whereas FGF13 colocalizes with Na v 1.6 at nodes of Ranvier of dorsal roots of primary afferents (5). Of note is that nodal localization was seen using a pan FGF13 antibody, but an antibody that is specific to FGF13S (also known as FGF13A or FHF2A) did not immunolabel nodes of Ranvier, showing important isoform and splice variant differences (6).…”