| Neuronal immunoglobulin superfamily cell adhesion molecules (IGSF CAMs) are important for cellular adhesion, cellular migration, myelination of central and peripheral axons, axonal pathfinding and neurite outgrowth. Members of the IGSF are characterized by at least one V- or C set-extracellular Ig like domain composed of two beta sheets with 7--9 beta-strands. Based on amino acid and structural homology, beta-subunits of voltage-gated sodium channels belong to the IGSF. We previously demonstrated that sodium channel beta1 and beta2 subunits participate in homophilic cellular adhesion in Drosophila Schneider's line 2 cell aggregation assays, as well as cellular migration in response to extracellular matrix molecules. Here we examined whether beta subunits play roles in neurite outgrowth and axonal pathfinding within the central nervous system (CNS). Using acutely dissociated cerebellar granule neurons grown on fibroblast monolayers, it was demonstrated that the sodium channel beta1 subunit is permissive for, and enhances, neurite outgrowth ∼1.5 fold over basal lengths, similar to other IGSF CAMs, whereas the beta2 subunit inhibits basal neurite outgrowth. Furthermore, we demonstrate a role for the non-receptor tyrosine kinase, fyn, in beta1 mediated neurite outgrowth. In vivo labeling experiments utilizing 5'bromodeoxyuridine (BrdU) and 1,1' dioctadecyl-3,3,3'3'-tetramethyl-indoncarbocyanine perchlorate (DiI) in Scn1b null mice demonstrate a functional role for beta1 in the cellular migration of cerebellar granule neurons as well axonal pathfinding in the corticospinal tract. These studies are the first functional demonstration of beta subunit-mediated cell adhesion in neurons. We conclude that sodium channel beta subunits are multifunctional molecules that play roles in channel modulation, regulation of channel cell surface expression, formation of sodium channel signaling complexes, and cell adhesion. |
