Preferential Targeting of Na(v)1.6 Voltage-Gated Na(+) Channels to the Axon Initial Segment during Development

During axonal maturation, voltage-gated sodium (Na(v)) channels accumulate at the axon initial segment (AIS) at high concentrations. This localization is necessary for the efficient initiation of action potentials. The mechanisms underlying channel trafficking to the AIS during axonal development have remained elusive due to a lack of Na(v) reagents suitable for high resolution imaging of channels located specifically on the cell surface. Using an optical pulse-chase approach in combination with a novel Na(v)1.6 construct containing an extracellular biotinylation domain we demonstrate that Na(v)1.6 channels are preferentially inserted into the AIS membrane during neuronal development via direct vesicular trafficking. Single-molecule tracking illustrates that axonal channels are immediately immobilized following delivery, while channels delivered to the soma are often mobile. Neither a Na(v)1.6 channel lacking the ankyrin-binding motif nor a chimeric K(v)2.1 channel containing the Na(v) ankyrinG-binding domain show preferential AIS insertion. Together these data support a model where ankyrinG-binding is required for preferential Na(v)1.6 insertion into the AIS plasma membrane. In contrast, ankyrinG-binding alone does not confer the preferential delivery of proteins to the AIS.