Dependence of axon initial segment formation on Na+ channel expression

Spinal motor neurons were isolated from embryonic rats, and grown in culture. By 2 days in vitro, the axon initial segment was characterized by colocalization and clustering of Na+ channels and ankyrinG. By 5 days, NrCAM, and neurofascin could also be detected at most initial segments. We sought to determine, as one important aim, whether Na+ channels themselves played an essential role in establishing this specialized axonal region. Small hairpin RNAs (shRNAs) were used to target multiple subtypes of Na+ channels for reduced expression by RNA interference. Transfection resulted in substantial knockdown of these channels within the cell body and also as clusters at initial segments. Furthermore, Na+ currents originating at the initial segment, and recorded under patch clamp, were strongly reduced by shRNA. Control shRNA against a nonmammalian protein was without effect. Most interestingly, targeting Na+ channels also blocked clustering of ankyrinG, NrCAM, and neurofascin at the initial segment, although these proteins were seen in the soma. Thus, both Na+ channels and ankyrinG are required for formation of this essential axonal domain. Knockdown of Na+ channels was somewhat less effective when introduced after the initial segments had formed. Disruption of actin polymerization by cytochalasin D resulted in multiple initial segments, each with clusters of both Na+ channels and ankyrinG. The results indicate that initial segment formation occurs as Na+ channels are transported into the nascent axon membrane, diffuse distally, and link to the cytoskeleton by ankyrinG. Subsequently, other components are added, and stability is increased. A computational model closely reproduced the experimental results.