Axon-Targeting Motifs: Mechanisms and Applications of Enhancing Axonal Localisation of Transmembrane Proteins

Abstract

Neuronal polarity established in developing neurons ensures proper function in the mature nervous system. As functionally distinct cellular compartments, axons and dendrites often require different subsets of proteins to maintain synaptic transmission and overall order. Although neurons in the mature CNS do not regenerate throughout life, their interactions with their extracellular environment are dynamic. The axon remains an overall protected area of the neuron where only certain proteins have access throughout the lifespan of the cell. This is in comparison to the somatodendritic compartment, where although it too has a specialised subset of proteins required for its maintenance, many proteins destined for the axonal compartment must first be trafficked through the former. Recent research has shown that axonal proteins contain specific axon-targeting motifs that permit access to the axonal compartment as well as downstream targeting to the axonal membrane. These motifs target proteins to the axonal compartment by a variety of mechanisms including: promoting segregation into axon-targeted secretory vesicles, increasing interaction with axonal kinesins and enhancing somatodendritic endocytosis. In this review, we will discuss axon-targeting motifs within the context of established neuron trafficking mechanisms. We will also include examples of how these motifs have been applied to target proteins to the axonal compartment to improve both tools for the study of axon biology, and for use as potential therapeutics for axonopathies.

Keywords: axon transport; axon-targeting motif; neuronal polarity; protein trafficking; secretory pathway; somatodendritic; transcytosis; transmembrane protein.

Figure 1

Organisation of the pre-axonal exclusion zone (PAEZ) and axon initial segment (AIS) of mature central nervous system neurons. The PAEZ is located within the soma just proximal to the AIS. In the axon, the majority of microtubules are oriented with the plus-end pointing away from the soma. Plus-end directed kinesins drive anterograde transport and minus-end directed dyneins drive retrograde axonal transport of transmembrane proteins. The AIS is found in the axon hillock and is marked by a unique molecular architecture consisting of periodic rings of f-actin and a submembrane undercoat composed of ankyrin-G and βIV-spectrin. Furthermore, the AIS membrane is densely packed with transmembrane proteins such as voltage-gated ion channels and cell adhesion molecules.

Figure 2

Axon-targeting motifs influence diverse steps in the secretory and transcytotic trafficking pathways of transmembrane proteins to the axonal compartment. (A) A dicysteine palmitoylation motif found in Paralemmin and GAP-43 promotes association with lipid rafts in the trans-Golgi network, sorting transmembrane proteins into axon-targeted secretory vesicles. (B) Fused myosin VI binding domains of optineurin and DAB2 promote endocytosis of transmembrane proteins from the somatodendritic membrane for transcytotic delivery to the axon. (C) The C-terminus of APP drives association with light chains of kinesin-1, promoting axonal entry and anterograde trafficking. (D) Ankyrin-G (AnkG) binding domains found in Nav1.6 and Kv3.1 promote localisation to the axon initial segment potentially via association with AnkG on kinesin-3 driven vesicles or by anchoring transmembrane proteins in place following lateral diffusion into the AIS.