Axon initial segment cytoskeleton comprises a multiprotein submembranous coat containing sparse actin filaments

Area of Science:

• Neuroscience
• Cell Biology
• Cytoskeleton Dynamics

Background:

• The axon initial segment (AIS) is crucial for neuronal function, regulating action potential initiation and polarity.
• The role of actin dynamics in AIS function, particularly in maintaining axon-dendritic polarity, is not fully understood.
• Existing models lack clarity on the precise structure and organization of actin filaments within the AIS.

Purpose of the Study:

• To elucidate the detailed architecture of the AIS cytoskeleton, focusing on actin filament organization.
• To investigate the structural contribution of actin to the AIS and its potential roles in cellular processes.
• To challenge and refine existing models of AIS structure and function.

Main Methods:

• Platinum replica electron microscopy (PREM) was employed to visualize the ultrastructure of the AIS cytoskeleton.
• Immunogold PREM was used to identify specific protein components within the AIS coat.
• Analysis was performed on both mature and developing hippocampal neurons.

Main Results:

• The AIS cytoskeleton features a dense coat of proteins, including ankyrin G, spectrin, neurofascin, and voltage-gated sodium channels, overlying microtubule bundles.
• Contrary to previous assumptions, highly organized actin arrays or dense meshworks were not observed.
• Two distinct populations of sparse actin filaments were identified: short, stable filaments and longer, dynamic filaments.

Conclusions:

• Stable actin filaments likely contribute to the structural integrity of the AIS diffusion barrier.
• Dynamic actin filaments may be involved in the remodeling of the AIS coat.
• These findings offer a revised understanding of actin's role in AIS structure and neuronal polarity.