Effect of MicroRNA-34c on Regenerative Axon Growth in Dorsal Root Ganglion Neurons of the Peripheral Nervous System

Abstract

The peripheral nervous system is unique in its ability to regenerate axons after injury. Our lab previously assessed the miRNA expression levels of the regenerating sciatic nerve which indicated miR-34c up-regulation. MicroRNAs are non-protein coding small RNA molecules, ~20 nt in length, that have been shown to control many basic cellular processes by post-transcriptional regulation. The purpose of this study was to identify the effect of miR-34c on regenerating axons of DRG neurons. 9 to 12 week CD-1 male mice used for this study were subjected to sciatic nerve crush lesion. Five days after surgery, animals were sacrificed, and DRGs were dissected and dissociated physically and enzymatically. DRG neurons were then grouped as follows: 1) non-transfected, 2) miR-34c mimic, 3) miR-34c inhibitor, 4) mimic negative control, and 5) inhibitor negative control. Cells were grown for 48 hours on glass cover slips, fixed, and incubated with primary beta-tubulin III antibody and with secondary antibody conjugated with a fluorophore. Cells were viewed on an inverted fluorescent microscope, and images were captured using a digital camera. Axon length and branches were measured using ImageJ. Inhibition of miR-34c showed a 27% increase in axon length for naive cultures (p<0.05), and a 23% increase in regenerating cultures (p<0.05), when compared to the inhibitor negative control. Also, a potential miR-34c target named Neuron Navigator 1 (NAV1) expression showed 43% increase after inhibition (p<0.05). Over-expression of miR-34c showed a 16% decrease in branching of regenerating cultures (p<0.05), as well as a 52% decrease in NAV1 expression. In conclusion, miR-34c inhibitor promoted axon growth in regenerating cultures resulting in longer axons and increased NAV1 expression. Over-expression of miR-34c produced a degenerative response in regenerating neurons resulting in decreased branching and decreased NAV1 expression.