Cytoskeleton and Membrane Organization at Axon Branches

Abstract

Axon branching is a critical process ensuring a high degree of interconnectivity for neural network formation. As branching occurs at sites distant from the soma, it is necessary that axons have a local system to dynamically control and regulate axonal growth. This machinery depends on the orchestration of cellular functions such as cytoskeleton, subcellular transport, energy production, protein- and membrane synthesis that are adapted for branch formation. Compared to the axon shaft, branching sites show a distinct and dynamic arrangement of cytoskeleton components, endoplasmic reticulum and mitochondria. This review discusses the regulation of axon branching in the context of cytoskeleton and membrane remodeling.

Keywords: actin; axon branching; cytoskeleton; endoplasmic reticulum; membraneremodeling and neurons; microtubules; mitochondria.

FIGURE 1

Cellular processes during the maturation of axon branches. (A) Premature branch: Actin patches form along the axon and push the plasma membrane to initiate the formation of a filopodium. Mitochondria stall at branch sites and the translation machinery accumulates to locally synthetize new cytoskeletal proteins (Spillane et al., 2013; Armijo-Weingart and Gallo, 2017). (B) Mature branch: Dispersed actin filaments reorient and form actin bundles. Microtubules and ER can co-migrate into the growing branch to stabilize it and vesicles supply membrane components. Intracellular organelles such as mitochondria, endoplasmic reticulum, synaptic and dense-core vesicles redistribute and accumulate at the branch site to support the high demand for energy, proteins, lipid- and membrane components.

FIGURE 2

Remodeling of cytoskeleton and plasma membrane at axon branches. (A) Schematic representation of a mature, growing axon branch. Mitochondria increase in numbers through fission mediated by mitochondria fission factors or via constriction by the wrapping of endoplasmic reticulum. (B) Mechanisms to regulate membrane expansion (1) and retraction (2) at axon branches. Membrane expansion in response to attraction guidance cues is mediated by the fusion of synaptic vesicles with the plasma membrane or by exocytosis using SNARE proteins. The retraction of membranes is initiated by repulsion guidance cues and facilitated by endocytosis of membrane material. (C) Mechanisms for the reorganization of the cytoskeleton at axon branches. Bundled microtubule arrays are fragmented by microtubule-severing enzymes such as spastin and katanin to increase the local microtubule/tubulin pool available for polymerization. Tubulin-nucleation factors like MAP7 and SSNA1 promote and stabilize microtubule growth at axonal branches. Actin-microtubule crosslinking factors like drebrins and septins are suggested to promote the entry of microtubules into actin-rich filopodia. Note that processes at the primary growth cone and axon branch formation have similar cellular mechanisms for cytoskeletal rearrangements and responses to attractive or repulsive signaling through exocytosis and endocytosis (discussed in Dent et al., 2003; Winkle et al., 2016).