Regulation of Persistent Na Current by Interactions between β Subunits of Voltage-Gated Na Channels

The beta subunits of voltage-gated Na channels (Scnxb) regulate the gating of pore-forming alpha subunits, as well as their trafficking and localization. In heterologous expression systems, beta 1, beta 2, and beta 3 subunits influence inactivation and persistent current in different ways. To test how the beta 4 protein regulates Na channel gating, we transfected beta 4 into HEK (human embryonic kidney) cells stably expressing Na(V)1.1. Unlike a free peptide with a sequence from the beta 4 cytoplasmic domain, the full-length beta 4 protein did not block open channels. Instead, beta 4 expression favored open states by shifting activation curves negative, decreasing the slope of the inactivation curve, and increasing the percentage of noninactivating current. Consequently, persistent current tripled in amplitude. Expression of beta 1 or chimeric subunits including the beta 1 extracellular domain, however, favored inactivation. Coexpressing Na(V)1.1 and beta 4 with beta 1 produced tiny persistent currents, indicating that beta 1 overcomes the effects of beta 4 in heterotrimeric channels. In contrast, beta 1(C121W), which contains an extracellular epilepsy-associated mutation, did not counteract the destabilization of inactivation by beta 4 and also required unusually large depolarizations for channel opening. In cultured hippocampal neurons transfected with beta 4, persistent current was slightly but significantly increased. Moreover, in beta 4-expressing neurons from Scn1b and Scn1b/Scn2b null mice, entry into inactivated states was slowed. These data suggest that beta 1 and beta 4 have antagonistic roles, the former favoring inactivation, and the latter favoring activation. Because increased Na channel availability may facilitate action potential firing, these results suggest a mechanism for seizure susceptibility of both mice and humans with disrupted beta 1 subunits.