Effect of retinoic acid on tibial nerve regeneration after anastomosis in rats: histological and functional analyses

MiR-463-3p inhibits tibial nerve regeneration via post-transcriptional suppression of SPRR1A

Background: miRNAs have been involved in neural development, degeneration, and regeneration. MiR-463-3p is expressed in reproductive and nervous systems. In this study, the role of miR-463-3p in tibial nerve injury and regeneration was explored. Materials and methods: A model of tibial nerve injury was established with the crush method, and the levels of miR-463-3p were detected at days 0, 3, 7, 12, 18 and 24 post-injury. Then, primary tibial nerve cells were isolated from newborn mice, and miR-463-3p was respectively overexpressed and knocked down in cultured cells. Behaviors of tibial nerve cells were detected. Furthermore, bioinformatics technology was used to investigate the underlying mechanism. Results: The expression miR-463-3p was robustly increased in the injured tibial nerve in vivo and in tibial nerve cells treated with oxygen-glucose deprivation. The data on gain- and loss-of-function demonstrated that miR-463-3p negatively regulated including neurite length, percentage of cells with neurites, and cell branching in tibial nerve cells. Small proline-rich repeat protein 1 A (SPRR1A), an identified nerve regeneration associated genes, was identified as a target gene of miR-463-3p. Conclusion: Inhibition of miR-463-3p could increase SPRR1A expression in the tibial nerve tissue and improve regeneration of the tibial nerve post-injury in vivo.

A small molecule screen identifies in vivo modulators of peripheral nerve regeneration in zebrafish

Adult vertebrates have retained the ability to regenerate peripheral nerves after injury, although regeneration is frequently incomplete, often leading to functional impairments. Small molecule screens using whole organisms have high potential to identify biologically relevant targets, yet currently available assays for in vivo peripheral nerve regeneration are either very laborious and/or require complex technology. Here we take advantage of the optical transparency of larval zebrafish to develop a simple and fast pectoral fin removal assay that measures peripheral nerve regeneration in vivo. Twenty-four hours after fin amputation we observe robust and stereotyped nerve regrowth at the fin base. Similar to laser mediated nerve transection, nerve regrowth after fin amputation requires Schwann cells and FGF signaling, confirming that the fin amputation assay identifies pathways relevant for peripheral nerve regeneration. From a library of small molecules with known targets, we identified 21 compounds that impair peripheral nerve regeneration. Several of these compounds target known regulators of nerve regeneration, further validating the fin removal assay. Twelve of the identified compounds affect targets not previously known to control peripheral nerve regeneration. Using a laser-mediated nerve transection assay we tested ten of those compounds and confirmed six of these compounds to impair peripheral nerve regeneration: an EGFR inhibitor, a glucocorticoid, prostaglandin D2, a retinoic acid agonist, an inhibitor of calcium channels and a topoisomerase I inhibitor. Thus, we established a technically simple assay to rapidly identify valuable entry points into pathways critical for vertebrate peripheral nerve regeneration.

Retinoic acid regulates Schwann cell migration via NEDD9 induction by transcriptional and post-translational mechanisms

Schwann cell migration is essential during the regenerative response to nerve injury, however, the factors that regulate this phenomenon are not yet clear. Here we describe that retinoic acid (RA), whose production and signaling activity are greatly enhanced during nerve regeneration, increases Schwann cell migration. This is accompanied by the up-regulation of NEDD9, a member of the CAS family of scaffold proteins previously implicated in migratory and invasive behavior in gliomas, melanomas and the neural crest cells from which Schwann cells derive. This RA-induced NEDD9 accumulation is due to augmented mRNA levels, as well as an increase of NEDD9 protein stability. Although all NEDD9 phospho-isoforms present in Schwann cells are induced by the retinoid, the hormone also changes its phosphorylation status, thus altering the ratio between the different isoforms. Silencing NEDD9 in Schwann cells had no effect on basal migratory ability, but completely abrogated RA-induced enhanced migration. Collectively, our results indicate that RA could be a major regulator of Schwann cell migration after nerve injury, thus offering a new insight into peripheral nerve repair.

RAR/RXR and PPAR/RXR Signaling in Spinal Cord Injury

The retinoid acid receptors (RAR) and peroxisome proliferator-activated receptors (PPAR) have been implicated in the regulation of inflammatory reactions. Both receptor families contain ligand-activated transcription factors which form heterodimers with retinoid X receptors (RXR). We review data that imply RAR/RXR and PPAR/RXR pathways in physiological reactions after spinal cord injury. Experiments show how RAR signaling may improve axonal regeneration and modulate reactions of glia cells. While anti-inflammatory properties of PPAR are well documented in the periphery, their possible roles in the central nervous system have only recently become evident. Due to its anti-inflammatory function this transcription factor family promises to be a useful target after spinal cord or brain lesions.

Retinoic acid in the development, regeneration and maintenance of the nervous system

Retinoic acid (RA) is involved in the induction of neural differentiation, motor axon outgrowth and neural patterning. Like other developmental molecules, RA continues to play a role after development has been completed. Elevated RA signalling in the adult triggers axon outgrowth and, consequently, nerve regeneration. RA is also involved in the maintenance of the differentiated state of adult neurons, and disruption of RA signalling in the adult leads to the degeneration of motor neurons (motor neuron disease), the development of Alzheimer's disease and, possibly, the development of Parkinson's disease. The data described here strongly suggest that RA could be used as a therapeutic molecule for the induction of axon regeneration and the treatment of neurodegeneration.

Effects of inflammatory cytokines IL-1beta, IL-6, and TNFalpha on the intracellular localization of retinoid receptors in Schwann cells

It was investigated whether retinoic acid (RA) and the proinflammatory cytokines IL-1beta, IL-6, and TNFalpha influence the intracellular distribution of retinoic acid receptors (RAR) and retinoid X receptors (RXR) in Schwann cells. This question arose because nuclear translocation of RARalpha, RXRalpha, and RXRbeta was observed after nerve injury, and because mutual interactions exist between the signal transduction pathways of RA and proinflammatory cytokines. Schwann cell primary cultures from the rat sciatic nerve were incubated with IL-1beta, IL-6, and TNFalpha, with all-trans RA and with a combination of IL-1beta and RA. After incubation periods ranging from 5 min to 5 h, the intracellular distributions of RARalpha, RARbeta, RXRalpha, and RXRbeta were analyzed. All three cytokines caused a shift of RARalpha from the cytosolic compartments into the cell nuclei. This was also observed with RA, and combining RA with IL-1beta produced an additive effect. IL-1beta and IL-6 also affected the distribution of RARbeta, although immunoreactivity of this receptor always remained stronger in the cytosol. No effect of the cytokines on RXRalpha or RXRbeta was observed, whereas RA treatment caused a stronger nuclear signal of both receptors. Effects on the subcellular localization of retinoid receptors may provide a link in a feedback loop between RA/RAR and cytokines.

Regulation of retinoic acid receptors alpha, beta and retinoid X receptor alpha after sciatic nerve injury

Cell culture experiments indicated that activation of the retinoic acid signaling system is involved in axonal regeneration. This hypothesis was tested with sciatic nerve injury in the rat. Since the effect of retinoic acid is mediated via retinoic acid receptors and retinoid X receptors, we investigated mRNA and protein expression of these receptors during injury-induced degeneration and regeneration. Seven days after crush injury, transcript concentrations of all retinoic acid receptors and of retinoid X receptor alpha were significantly higher than in non-lesioned nerves. Protein levels of retinoic acid receptor alpha, retinoic acid receptor beta and retinoid X receptor alpha were upregulated 4, 7 and 14 days after injury. In degenerating nerves a significant increase of retinoic acid receptor alpha was detected 7 and 14 days, and of retinoic acid receptor beta 14 and 21 days after complete transection. Immunohistochemical staining of retinoid receptors revealed their expression in Schwann cells and macrophages. In addition, we observed that retinoic acid receptor alpha and retinoid X receptor alpha appeared in the cell nuclei of macrophages during the lesion-induced inflammatory reaction, and that retinoid X receptor alpha-staining co-localized with some regenerating axons. Experiments with Schwann cell primary cultures revealed an effect of retinoic acid on the expression of the neuregulin receptor ErbB3, suggesting that one function of retinoic acid consists in the regulation of neuroglial interactions after peripheral nerve injury.

New therapeutic target for CNS injury? The role of retinoic acid signaling after nerve lesions

Experiments with sciatic nerve lesions and spinal cord contusion injury demonstrate that the retinoic acid (RA) signaling cascade is activated by these traumatic events. In both cases the RA-synthesizing enzyme is RALDH-2. In the PNS, lesions cause RA-induced gene transcription, intracellular translocation of retinoid receptors, and increased transcription of CRBP-I, CRABP-II, and retinoid receptors. The activation of RARbeta appears to be responsible for neurotrophic and neuritogenic effects of RA on dorsal root ganglia and embryonic spinal cord. While the physiological role of RA in the injured nervous system is still under investigation three domains of functions are suggested: (1) neuroprotection and support of axonal growth, (2) modulation of the inflammatory reaction by microglia/macrophages, and (3) regulation of glial differentiation. Few studies have been performed to support nerve regeneration with RA signals in vivo, but a large number of experiments with neuronal and glial cell cultures and spinal cord explants point to beneficial effects of RA, so that future therapeutic approaches will likely focus on the activation of RA signaling.