Overexpression of miR-338-5p in exosomes derived from mesenchymal stromal cells provides neuroprotective effects by the Cnr1/Rap1/Akt pathway after spinal cord injury in rats

•miR-338-5p-overexpressing exosomes provided neuroprotective effects after SCI in rats.•miR-338-5p-overexpressing exosomes alleviated H2O2-induced cell injury in PC12 cells.•miR-338-5p directly targeted the 3́-untranslated region of Cnr1.•miR-338-5p modulated the expression of active Rap1 by suppressing the target gene Cnr1.•Active Rap1 activated the PI3K/Akt pathway through upregulation of miR-338-5p expression. Growing evidence has shown that microRNAs (miRNAs) play crucial roles in the physiopathology of spinal cord injury (SCI). Recent studies have confirmed that miR-338-5p regulates myelination, suggesting a potential role in the treatment of SCI. However, the molecular mechanism of miR-338-5p on SCI is still unknown. Recently, exosomes have emerged as an ideal vector to deliver therapeutic molecules such as miRNAs. Here, we explored the effects of miR-338-5p-overexpressing exosomes derived from bone marrow-derived mesenchymal stromal cells (BMSCs) on SCI. In vivo, a model of contusion SCI in rats was established, and we observed that overexpression of miR-338-5p in exosomes profoundly increased the expression levels of neurofilament protein-M and growth-associated protein-43 and decreased those of myelin-associated glycoprotein and glial fibrillary acidic protein, which provided neuroprotective effects after acute SCI. In an in vitro study, we found that overexpression of miR-338-5p in exosomes repressed cell apoptosis following H2O2-induced oxidative stress injury in PC12 cells. Additionally, we confirmed that cannabinoid receptor 1 (Cnr1) was the target gene of miR-338-5p by dual-luciferase reporter assays and that Rap1 was the downstream gene by the KEGG pathway analysis. We found that miR-338-5p increased cAMP accumulation as a consequence of downregulated expression of the target gene Cnr1, and then, Rap1 was activated by cAMP. Eventually, the activation of the PI3K/Akt pathway attenuated cell apoptosis and promoted neuronal survival by cAMP-mediated Rap1 activation. In brief, these findings showed that exosomes overexpressing miR-338-5p were a promising treatment strategy for SCI.