mRNAs and Protein Synthetic Machinery Localize into Regenerating Spinal Cord Axons When They Are Provided a Substrate That Supports Growth

Abstract

Although intra-axonal protein synthesis is well recognized in cultured neurons and during development in vivo, there have been few reports of mRNA localization and/or intra-axonal translation in mature CNS axons. Indeed, previous work indicated that mature CNS axons contain much lower quantities of translational machinery than PNS axons, leading to the conclusion that the capacity for intraaxonal protein synthesis is linked to the intrinsic capacity of a neuron for regeneration, with mature CNS neurons showing much less growth after injury than PNS neurons. However, when regeneration by CNS axons is facilitated, it is not known whether the intra-axonal content of translational machinery changes or whether mRNAs localize into these axons. Here, we have used a peripheral nerve segment grafted into the transected spinal cord of adult rats as a supportive environment for regeneration by ascending spinal axons. By quantitative fluorescent in situ hybridization combined with immunofluorescence to unambiguously distinguish intra-axonal mRNAs, we show that regenerating spinal cord axons contain ␤-actin, GAP-43, Neuritin, Reg3a, Hamp, and Importin ␤1 mRNAs. These axons also contain 5S rRNA, phosphorylated S6 ribosomal protein, eIF2␣ translation factor, and 4EBP1 translation factor inhibitory protein. Different levels of these mRNAs in CNS axons from regenerating PNS axons may relate to differences in the growth capacity of these neurons, although the presence of mRNA transport and likely local translation in both CNS and PNS neurons suggests an active role in the regenerative process.

Axonal ribosomes and mRNAs associate with fragile X granules in adult rodent and human brains

“…However, it seems likely that axonal translation is a general phenomenon that is regulated by different mechanisms in other neurons. Indeed, axonal translation might be induced in some neurons only in response to specific physiological or pathological cues (4,61). In support of this idea, retinal ganglion cell axons harbour ribosome-associated mRNAs in the adult nervous system yet do not contain FXGs (5,14).…”

Protein synthetic machinery and mRNA in regenerating tips of spinal cord axons in lamprey

“…Although mRNAs have been found in axons of some specific brain regions, e.g., oxytocin and vasopressin mRNAs in the hypothalamo‐hypophyseal tract (Mohr et al, ), and odorant receptor mRNAs in olfactory axon terminals in the olfactory bulb (Ressler et al, ), evidence for mRNA in axons of mature CNS neurons in vivo has been difficult to obtain, and has required indirect or signal‐augmenting procedures. When peripheral nerve segments were grafted into the transected spinal cords of adult rats, invading spinal axons contained 5S rRNA, phosphorylated S6, and multiple translation‐related factors (Kalinski et al, ). The authors did not look for these translational elements in nongrafted injured spinal cord axons, but the levels of translational machinery were comparable to those of injured sciatic nerve, i.e., much greater than those found by others in the axons of optic nerve (Verma et al, ).…”

A predominantly glial origin of axonal ribosomes after nerve injury

“…Images were post‐processed with ImageJ (http://imagej.nih.gov/ij/) to generate orthogonal and maximum projections. To extract the colocalization signals of the ribosomal signals (RPL26 or RFP) with the axonal marker (Tubb3, CGRP, IB4, NF200, or ChAT) the RG2B plug‐in for ImageJ was used as previously described (Kalinski et al, ). The extracted colocalization signals were displayed as heat maps (ImageJ plug‐in, http://www.samuelpean.com/heatmap-histogram/).…”