The ataxia3 mutation in the N-terminal cytoplasmic domain of sodium channel Na v1.6 disrupts intracellular trafficking

Abstract

The ENU-induced neurological mutant ataxia3 was mapped to distal mouse chromosome 15. Sequencing of the positional candidate gene Scn8a encoding the sodium channel Na v1.6 identified a T>C transition in exon 1 resulting in the amino acid substitution p.S21P near the N terminus of the channel. The cytoplasmic N-terminal region is evolutionary conserved but its function has not been well characterized. ataxia3 homozygotes exhibit a severe disorder that includes ataxia, tremor, and juvenile lethality. Unlike Scn8a null mice, they retain partial hindlimb function. The mutant transcript is stable but protein abundance is reduced and the mutant channel is not detected in its usual site of concentration at nodes of Ranvier. In whole-cell patch-clamp studies of transfected ND7/23 cells that were maintained at 37°C, the mutant channel did not produce sodium current, and function was not restored by coexpression of /31 and /32 subunits. However, when transfected cells were maintained at 30°C, the mutant channel generated voltage-dependent inward sodium currents with an average peak current density comparable with wild type, demonstrating recovery of channel activity. Immunohistochemistry of primary cerebellar granule cells from ataxia3 mice demonstrated that the mutant protein is retained in the cw-Golgi. This trafficking defect can account for the low level of Na v1.6-S21P at nodes of Ranvier in vivo and at the surface of transfected cells. The data demonstrate that the cytoplasmic N-terminal domain of the sodium channel is required for anterograde transport from the Golgi complex to the plasma membrane. Copyright © 2009 Society for Neuroscience.