Detrimental and Protective Action of Microglial Extracellular Vesicles on Myelin Lesions: Astrocyte Involvement in Remyelination Failure

Microglia (MG) are the immune cells of the brain. They have an enormous plasticity in response to brain injury, acquiring different activated phenotypes depending on the stimuli they receive. During multiple sclerosis, MG participate to myelin injury and repair by influencing the function of oligodendrocyte precursor cells (OPCs), the brain cells that differentiate into myelin-forming cells. However, the mode(s) of action of MG in promoting or inhibiting brain repair is still largely unknown. Here, we explored the action of Extracellular Vesicles (EVs) produced in vitro by either pro-inflammatory or pro-regenerative MG on OPCs at myelin lesion induced by lysolecithin injection in the mouse corpus callosum. Immunolabelling for myelin proteins and electron microscopy showed that EVs released by pro-inflammatory MG (i-EVs) impaired remyelination, whereas EVs produced by MG co-cultured with immunosuppressive mesenchymal stem cells (MSC-EVs) promoted OPC recruitment and differentiation at the lesion site. The molecular mechanisms underlying the detrimental/beneficial action of MG EVs were dissected using primary OPC cultures, a powerful system with easy access for EVs. i-EVs promoted OPC differentiation in monoculture, while they caused a clear block of OPC maturation when OPCs were co-cultured with astrocytes, implicating these cells in remyelination failure. Moreover, biochemical fractionation of EV components revealed that astrocyte may be converted in oligotoxic cells by i-EV cargo whereas the lipid components of EVs promoted OPC maturation and migration as indicated by immunohistochemical and qPCR analyses. While the lipid species that enhance OPC maturation remains undefined, we demonstrated a central role for EV-associated S1P in stimulating OPC migration, a fundamental process in remyelination. Collectively, these results uncover a role for microglial EVs as key players in myelin repair, given their ability to influence OPC and astrocyte functions at myelin lesion, and support the study of EV content and function for developing novel approaches for remyelination