| Voltage-gated sodium channels are composed of alpha and beta subunits. They are localized at high density at nodes of Ranvier in myelinated axons, allowing for rapid and efficient saltatory conduction of action potentials. The goal of this research was to define the molecular composition of sodium channel complexes at nodes of Ranvier. We investigated beta1 interactions with neurofascin-186, neurofascin-155, N&barbelow;euron-glia r&barbelow;elated c&barbelow;ell a&barbelow;dhesion m&barbelow;olecule (NrCAM), contactin, beta2, and beta3. We report that beta1 interacts with contactin, NrCAM, neurofascin-155, beta2, and neurofascin-186, but not beta3. Cells expressing Nav1.2, beta1, and contactin have 4-fold higher peak sodium currents and correspondingly higher cell surface 3H-saxitoxin binding than cells expressing Nav1.2 alone. Neurofascin-186 and beta2 also increase Nav1.2 cell surface expression in the presence of beta1 but not to the same extent as contactin. NrCAM, Neurofascin-155, and beta3, however, do not increase Nav1.2 cell surface expression. Coimmunoprecipitation experiments show that contactin interacts specifically with beta1 and not beta2 subunits.;Sodium channels and contactin co-localize at CNS nodes. In beta1 (-/-) mice, despite co-localization at CNS nodes, contactin no longer interacts with the sodium channel complex. In the PNS, contactin is primarily paranodal, but is transiently co-localized with sodium channels at nodes during development and remyelination. Studies with chimeras in which the Ig-domain of beta1 was exchanged with that of beta2 show that the Ig-domain of beta1 interacts with contactin. Nav1.2 associates with ankyrinG in transfected cells and this interaction is enhanced in the presence of beta1 but not beta1 Y181 E, a mutant that does not interact with ankyrinG. beta1Y181E does not modulate Nav1.2 function despite association with Na v1.2 and contactin. beta1Y181E increases Nav1.2 cell surface expression, but not as efficiently as wild-type beta1.;Taken together, our results make significant contributions to understanding the molecular composition of the sodium channel complex at nodes of Ranvier. Our results suggest the novel idea that beta1 stabilizes sodium channel clusters at nodes for long periods of time and functions in axo-glial communication through association with neurofascin-155 in paranodal septate-like junctions. Thus, beta1 subunits are multi-functional molecules that play key roles in electrical excitability in neurons. |
