Voltage-Gated Na⁺Channel β1 Subunit-Mediated Neurite Outgrowth Requires Fyn Kinase and Contributes to Postnatal CNS DevelopmentIn Vivo

Abstract

Voltage-gated Naϩ channel ␤1 subunits are multifunctional, participating in channel modulation and cell adhesion in vitro. We previously demonstrated that ␤1 promotes neurite outgrowth of cultured cerebellar granule neurons (CGNs) via homophilic adhesion. Both lipid raft-associated kinases and nonraft fibroblast growth factor (FGF) receptors are implicated in cell adhesion molecule-mediated neurite extension. In the present study, we reveal that ␤1-mediated neurite outgrowth is abrogated in Fyn and contactin (Cntn) null CGNs. ␤1 protein levels are unchanged in Fyn null brains, whereas levels are significantly reduced in Cntn null brain lysates. FGF or EGF (epidermal growth factor) receptor kinase inhibitors have no effect on ␤1-mediated neurite extension. These results suggest that ␤1-mediated neurite outgrowth occurs through a lipid raft signaling mechanism that requires the presence of both fyn kinase and contactin. In vivo, Scn1b null mice show defective CGN axon extension and fasciculation indicating that ␤1 plays a role in cerebellar microorganization. In addition, we find that axonal pathfinding and fasciculation are abnormal in corticospinal tracts of Scn1b null mice consistent with the suggestion that ␤1 may have widespread effects on postnatal neuronal development. These data are the first to demonstrate a cell-adhesive role for ␤1 in vivo. We conclude that voltage-gated Na ϩ channel ␤1 subunits signal via multiple pathways on multiple timescales and play important roles in the postnatal development of the CNS.

Functional reciprocity between Na + channel Na v 1.6 and β1 subunits in the coordinated regulation of excitability and neurite outgrowth

“…The neurite length distribution was increased by FGF in both WT and Scn8a null CGNs (Fig. 1H), demonstrating that, similar to Scn1b neurons (22), Scn8a null neurons are not deficient in downstream mechanisms of neurite outgrowth that converge between lipid raft-mediated and non-raft-mediated pathways (26). β1 protein expression was unchanged in the cerebella of Scn8a null mice, suggesting that the lack of response of Scn8a null CGNs to the β1 monolayer was not due to a reduction in β1 in the CGNs (Fig.…”

“…TTX had no effect on fibroblast growth factor (FGF; 20 ng/mL)-mediated neurite outgrowth, which is β1-independent (P < 0.001; Fig. 1 C and D) (22). Furthermore, FGF had no additive effect on the neurite outgrowth of CGNs plated on β1-expressing monolayers, suggesting that FGFmediated and β1-mediated neurite outgrowth mechanisms ultimately activate a common signaling pathway (P = 0.57; n = 300).…”

“…As a member of the Ig superfamily of cell adhesion molecules (CAMs), β1 mediates cellular aggregation, cytoskeletal recruitment, and extracellular matrix interactions in vitro (11). β1 also promotes neurite outgrowth in cerebellar granule neurons (CGNs) via a lipid raft mechanism involving trans homophilic cell adhesion, fyn kinase, and contactin (22,23). Axonal path finding and fasciculation are disrupted in Scn1b null mice and zebrafish scn1bb morphants (22,24).…”

“…β1 also promotes neurite outgrowth in cerebellar granule neurons (CGNs) via a lipid raft mechanism involving trans homophilic cell adhesion, fyn kinase, and contactin (22,23). Axonal path finding and fasciculation are disrupted in Scn1b null mice and zebrafish scn1bb morphants (22,24). Thus, β1 functions as a CAM to regulate neuronal migration, axon guidance, and fasciculation during CNS development.…”

Contactin-1 regulates myelination and nodal/paranodal domain organization in the central nervous system

“…Third, Contactin directly associates with sodium channel-β1 subunits to increase channel surface expression and current density in vitro (16,54,55). Contactin deficiency could thus limit the number of sodium channels on axon surfaces, and thereby decrease neuronal activity, which is crucial for myelination and oligodendrocyte differentiation in vitro and in vivo (56)(57)(58)(59).…”