Regulation of mRNA Translation in Axons

Axons can extend long distances from the neuronal cell body, and mRNA translation in axons is used to locally generate new proteins in these distal reaches of the neuron’s cytoplasm. Work over the past two decades has shown that axonal mRNA translation occurs in many different organisms and different neuronal systems. The field has progressed substantially over this time, moving from documenting mRNA translation in axons to understanding how axonal mRNA translation is regulated and what the protein products do for the neuron. Translational regulation in axons extends beyond merely controlling activity of the protein synthesis machinery. Transport of mRNAs into axons, stability of the mRNAs within the axons, and sequestration of mRNAs away from the translational machinery each contribute to determining what proteins are generated in axons, as well as when and where those proteins are generated within the axon. It is now known that thousands of different mRNAs can localize into axons. Based on unique responses to different axonal translation regulating stimuli and events, there clearly is specificity for when different mRNA populations are translated. How that specificity is driven is just now beginning to be understood, and studies emerging over the last five years point to multiple mechanisms for imparting specificity for regulation of axonal protein synthesis responses.

Expanding Axonal Transcriptome Brings New Functions for Axonally Synthesized Proteins in Health and Disease

Intra-axonal protein synthesis has been shown to play critical roles in both development and repair of axons. Axons provide long-range connectivity in the nervous system, and disruption of their function and/or structure is seen in several neurological diseases and disorders. Axonally synthesized proteins or losses in axonally synthesized proteins contribute to neurodegenerative diseases, neuropathic pain, viral transport, and survival of axons. Increasing sensitivity of RNA detection and quantitation coupled with methods to isolate axons to purity has shown that a surprisingly complex transcriptome exists in axons. This extends across different species, neuronal populations, and physiological conditions. These studies have helped define the repertoire of neuronal mRNAs that can localize into axons and imply previously unrecognized functions for local translation in neurons. Here, we review the current state of transcriptomics studies of isolated axons, contrast axonal mRNA profiles between different neuronal types and growth states, and discuss how mRNA transport into and translation within axons contribute to neurological disorders.

Axonal maintenance, glia, exosomes, and heat shock proteins

Of all cellular specializations, the axon is especially distinctive because it is a narrow cylinder of specialized cytoplasm called axoplasm with a length that may be orders of magnitude greater than the diameter of the cell body from which it originates. Thus, the volume of axoplasm can be much greater than the cytoplasm in the cell body. This fact raises a logistical problem with regard to axonal maintenance. Many of the components of axoplasm, such as soluble proteins and cytoskeleton, are slowly transported, taking weeks to months to travel the length of axons longer than a few millimeters after being synthesized in the cell body. Furthermore, this slow rate of supply suggests that the axon itself might not have the capacity to respond fast enough to compensate for damage to transported macromolecules. Such damage is likely in view of the mechanical fragility of an axon, especially those innervating the limbs, as rapid limb motion with high impact, like running, subjects the axons in the limbs to considerable mechanical force. Some researchers have suggested that local, intra-axonal protein synthesis is the answer to this problem. However, the translational state of axonal RNAs remains controversial. We suggest that glial cells, which envelop all axons, whether myelinated or not, are the local sources of replacement and repair macromolecules for long axons. The plausibility of this hypothesis is reinforced by reviewing several decades of work on glia-axon macromolecular transfer, together with recent investigations of exosomes and other extracellular vesicles, as vehicles for the transmission of membrane and cytoplasmic components from one cell to another.

Increases in Retrograde Injury Signaling Complex-Related Transcripts in Central Axons following Injury

Axons in the peripheral nervous system respond to injury by activating retrograde injury signaling (RIS) pathways, which promote local axonal protein synthesis (LPS) and neuronal regeneration. RIS is also initiated following injury of neurons in the central nervous system (CNS). However, regulation of the localization of axonal mRNA required for LPS is not well understood. We used a hippocampal explant system to probe the regulation of axonal levels of RIS-associated transcripts following axonal injury. Axonal levels of importinβ1 and RanBP1 were elevated biphasically at 1 and 24 hrs after axotomy. Transcript levels forβ-actin, a prototypic axonally synthesized protein, were similarly elevated. Our data suggest differential regulation of axonal transcripts. At 1 hr after injury, deployment of actinomycin revealed that RanBP1, but not importinβ1, requires de novo mRNA synthesis. At 24 hrs after injury, use of importazole revealed that the second wave of increased axonal mRNA levels required importinβ-mediated nuclear import. We also observed increased importinβ1 axonal protein levels at 1 and 6 hrs after injury. RanBP1 levels and vimentin levels fluctuated but were unchanged at 3 and 6 hrs after injury. This study revealed temporally complex regulation of axonal transcript levels, and it has implications for understanding neuronal response to injury in the CNS.