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-axonalmRNAs,weshow that regenerating spinal cord axons contain β-actin, GAP-43, Neuritin, Reg3a, Hamp, and Importin β1 mRNAs. These axons also contain 5S rRNA, phosphorylated S6 ribosomal protein, eIF2 α translation factor, and 4EBP1 translation factor inhibitory protein. Different levels of thesemRNAsinCNSaxons from regeneratingPNSaxonsmayrelate to differences in the growth capacity of these neurons, although the presence ofmRNAtransport and likely local translation in both CNS and PNS neurons suggests an active role in the regenerative process.