Molecular Regulation of Neuronal Voltage-Gated Sodium Channels by Auxiliary SubunitsPublic
Voltage-gated Na channels are expressed in all neurons, and are responsible for the upstroke of the action potential. They are part of a complex of proteins that includes pore-forming α subunits and auxiliary subunits that modify their trafficking, gating, and function. The modulation of Na channels by auxiliary subunits is an important determinant of how neurons fire action potentials, as subunits regulate factors such as Na channel availability and resurgent Na current. Resurgent Na current results from intracellular open channel block of Na channels by an unidentified modulatory protein. Here, we have identified several candidate blocking proteins that may be responsible for producing resurgent current, and investigated the ways in which they modulate Na channel function in multiple cell types. First, we investigated the roles of subunits, NaVβ4 and FGF14-1a, in modulating Na channels in Purkinje cells using NaVβ4 and FGF14 knockout mice. We found that a knockout of NaVβ4 did not result in any observable change in Na current in Purkinje cells, while a knockout of FGF14 resulted in a decrease in resurgent current amplitude and a faster transient current decay. We also found that a subset of Na channels more likely to carry resurgent current was more susceptible to block by low levels of tetrodotoxin (TTX), and that this same subset of channels was more likely to be modulated by FGF14, suggesting an important role for FGF14 in modulating resurgent current. Next, we explored further questions about Na channel modulation and resurgent current in various cell types. We found that a knockout of NaVβ4 and FGF14 had similar effects on Na current in cerebellar granule cells and medium spiny neurons of the striatum as in Purkinje cells, though not in CA3 hippocampal pyramidal cells. By manipulating the driving force on Na ions, we found that resurgent current is indeed the result of channel block and unblock, and blocking peptides of different length revealed that the blocker likely shares a similar amino acid motif with NaVβ4. We also found that histamine may play a role in modulating resurgent current in Purkinje cells. Finally, we investigated the effects of co-expression of NaVβ4, FGF14-1a and 14-3-3η on Na channels in HEK-293 cells. We found that while expression of none of the candidate blocking proteins produced resurgent current in HEK-293 cells, all three modulated Na channel function, and that co-expression of one auxiliary subunit may alter the function of another subunit. The results of this work emphasize the complexity of Na channel modulation in neurons, and the importance of modulatory proteins in influencing Na channel function and resurgent current.
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