GPR17-expressing oligodendrocyte precursor cells differentially react to damage in experimental autoimmune encephalomyelitis and cuprizone-induced demyelination

Introduction In the central nervous system, oligodendrocytes provide support to axons thanks to the production of a myelin sheath. During their maturation oligodendroglial precursors (OPCs) follow a very precise differentiation program, finely orchestrated by transcription factors, epigenetic factors and microRNAs, a class of small non-coding RNAs involved in post-transcriptional regulation. Any alterations in this program can potentially contribute to dysregulated myelination, impaired remyelination and neurodegenerative conditions, as it happens in multiple sclerosis. Methods Eight-week-old age male C57BL/6 mice were fed with 0.2% cuprizone supplemented diet ad libitum for 5 weeks to induce de-myelination and were then switched to normal diet for further 3 weeks to allow spontaneous re- myelination. Stereotaxic injection of the detergent lysolecithin was performed in the subcortical white matter of C57BL/6 adult mice to produces a focal demyelinating injury. Lentiviral infection was performed in the same site 9 days later, then animals were sacrificed at 21 days post-lesion. Results Recently, we identified miR-125a-3p as a new actor of oligodendroglial maturation, that could also be involved in the pathological consequences of multiple sclerosis, showing that its over-expression impairs, whereas its silencing promotes, oligodendrocyte maturation (Lecca et al., Sci Rep, 2016). To shed light on the mechanism underlying this effect, we performed a microarray analysis on OPCs after miR- 125a-3p over-expression. This analysis suggested that miR-125a-3p is indeed involved in the regulation of biological processes important for OPC maturation, such as cell-cell interaction and morphological differentiation. Interestingly, we also found that miR-125a-3p levels were up-regulated in vivo in presence of de-myelinating conditions. To evaluate whether miR-125a-3p modulation may influence the progression of remyelination in vivo, we overexpressed the miR-125a-3p by lentiviral approach in a focal lysolecithin-mediated demyelinating lesion in the subcortical white matter of adult mice. Interestingly, also in this case, we found that miRNA-overexpressing OPCs persisted in an immature (i.e. PDGRα+/NG2+) state. Relevant to the human disease, we found that miR- 125a-3p levels are altered in the cerebrospinal fluid of multiple sclerosis patients in the active phase (relapsing), suggesting that it could be a potential biomarker of pathology. Conclusion The identification of new pathogenetic mechanisms regulated by miRNAs provides new means for treatments of diseases. Based on these results, we hypothesize that antago-miRNA for miR-125a-3p may help to promote oligodendrocyte maturation in demyelinating conditions. Sponsored by Fondazione Cariplo, grant n° 2014-1207 to DL.