Tropomyosin Tpm3.1 is required to maintain the structure and function of the axon initial segment

The axon initial segment (AIS) is the site of action potential initiation and serves as a vesicular filter and diffusion barrier that help maintain neuronal polarity. Recent studies have revealed details about a specialized structural complex in the AIS. While an intact actin cytoskeleton is required for AIS formation, pharmacological disruption of actin polymerization compromises the AIS vesicle filter but does not affect overall AIS structure. In this study, we found that the tropomyosin isoform Tpm3.1 decorates a population of relatively stable actin filaments in the AIS. Inhibiting Tpm3.1 in cultured hippocampal neurons led to the loss of AIS structure, the AIS vesicle filter, the clustering of sodium ion channels, and reduced firing frequency. We propose that Tpm3.1-decorated actin filaments form a stable actin filament network under the AIS membrane which provides a scaffold for membrane organization and AIS proteins.