Tropomyosin Tpm3.1 is Required to Maintain the Structure and Function of the Axon Initial Segment

Amr Abouelezz,Edna C Hardeman,Casper C Hoogenraad,Mikael Segerstråle,Holly Stefen,Peter W Gunning,David Micinski,Tomi Taira,Pirta Hotulainen,Rimante Minkeviciene,Thomas Fath

doi:10.2139/ssrn.3448139

Amr Abouelezz, Edna C Hardeman + Show 9 more

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https://doi.org/10.2139/ssrn.3448139

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Abstract

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.

Highlights

The proximal ends of axons in the vertebrate nervous system contain the axon initial segment (AIS)
We demonstrate that tropomyosin isoform Tpm3.1 co-localizes with actin patches and that the inhibition of Tpm3.1 led to a reduction in the density of actin patches
Tpm3.1 inhibition led to reduced accumulation of AIS structural and functional proteins, disruption in sorting somatodendritic and axonal proteins, and a reduction in firing frequency

Summary

Introduction

The proximal ends of axons in the vertebrate nervous system contain the axon initial segment (AIS). The AIS is a remarkably stable structure comprising a specialized membrane and protein complex. Central to this complex is ankyrin G (Kordeli et al, 1995; Rasband, 2010), which acts as an adaptor that recruits other AIS proteins (Jenkins and Bennett, 2001); ankyrin G recruits and binds to bIV-spectrin (Yang et al, 2007), neurofascin-186 (NF-186) (Ango et al, 2004), as well as sodium (Zhou et al, 1998) and KCNQ2/3 channels (Pan et al, 2006). The interaction of ankyrin G with microtubules (Freal et al, 2016; Kuijpers et al, 2016; Leterrier et al, 2011) and the binding of bIV-spectrin to actin filaments (Jenkins and Bennett, 2001; Leterrier et al, 2015) link the AIS complex to the cytoskeleton

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