Abstract
In the adult nervous system axon caliber varies widely amongst different tracts. When considering a given axon, its diameter can further fluctuate in space and time, according to processes including the distribution of organelles and activity-dependent mechanisms. In addition, evidence is emerging supporting that in axons circumferential tension/contractility is present. Axonal diameter is generically regarded as being regulated by neurofilaments. When neurofilaments are absent or low, microtubule-dependent mechanisms can also contribute to the regulation of axon caliber. Despite this knowledge, the fine-tune mechanisms controlling diameter and circumferential tension throughout the lifetime of an axon, remain largely elusive. Recent data supports the role of the actin-spectrin-based membrane periodic skeleton and of non-muscle myosin II in the control of axon diameter. However, the cytoskeletal arrangement that underlies circumferential axonal contraction and expansion is still to be discovered. Here, we discuss in a critical viewpoint the existing knowledge on the regulation of axon diameter, with a specific focus on the possible role played by the axonal actin cytoskeleton.
Highlights
During development, after reaching their synaptic targets, axons increase their caliber several fold to achieve the large diameters needed for the rapid conduction of action potentials
In the initial model of the organization of the membrane periodic skeleton, actin rings were proposed to be made of short filaments (Xu et al, 2013), likely assisted by additional actinbinding proteins (Figure 1)
Further supporting the involvement of myosin II in longitudinal axonal contractility, disruption of actin filaments in embryos treated with cytochalasin D and latrunculin A (Spector et al, 1989), significantly impaired axonal contraction
Summary
After reaching their synaptic targets, axons increase their caliber several fold to achieve the large diameters needed for the rapid conduction of action potentials. The discovery of the membrane periodic skeleton opened new perspectives on how the actin cytoskeleton might support the neuronal architecture and function. In the initial model of the organization of the membrane periodic skeleton, actin rings were proposed to be made of short filaments (Xu et al, 2013), likely assisted by additional actinbinding proteins (Figure 1).
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