Detrimental and protective action of microglial extracellular vesicles on myelin lesions: astrocyte involvement in remyelination failure

Marta Lombardi,Marta Fumagalli,Paola Giussani,Paola Viani,Elisabetta Bonfanti,Linda Chaabane,Claudia Verderio,Maria P Abbracchio,Antonio Uccelli,Alice Gualerzi,Nicole Kerlero De Rosbo,Annalisa Buffo,Martina Gabrielli,Cecilia Garlanda,Federica Scaroni,Roberta Parolisi

doi:10.1007/s00401-019-02049-1

Abstract

Microglia are highly plastic immune cells which exist in a continuum of activation states. By shaping the function of oligodendrocyte precursor cells (OPCs), the brain cells which differentiate to myelin-forming cells, microglia participate in both myelin injury and remyelination during multiple sclerosis. However, the mode(s) of action of microglia in supporting or inhibiting myelin repair is still largely unclear. Here, we analysed the effects of extracellular vesicles (EVs) produced in vitro by either pro-inflammatory or pro-regenerative microglia on OPCs at demyelinated lesions caused by lysolecithin injection in the mouse corpus callosum. Immunolabelling for myelin proteins and electron microscopy showed that EVs released by pro-inflammatory microglia blocked remyelination, whereas EVs produced by microglia co-cultured with immunosuppressive mesenchymal stem cells promoted OPC recruitment and myelin repair. The molecular mechanisms responsible for the harmful and beneficial EV actions were dissected in primary OPC cultures. By exposing OPCs, cultured either alone or with astrocytes, to inflammatory EVs, we observed a blockade of OPC maturation only in the presence of astrocytes, implicating these cells in remyelination failure. Biochemical fractionation revealed that astrocytes may be converted into harmful cells by the inflammatory EV cargo, as indicated by immunohistochemical and qPCR analyses, whereas surface lipid components of EVs promote OPC migration and/or differentiation, linking EV lipids to myelin repair. Although the mechanisms through which the lipid species enhance OPC maturation still remain to be fully defined, we provide the first demonstration that vesicular sphingosine 1 phosphate stimulates OPC migration, the first fundamental step in myelin repair. From this study, microglial EVs emerge as multimodal and multitarget signalling mediators able to influence both OPCs and astrocytes around myelin lesions, which may be exploited to develop novel approaches for myelin repair not only in multiple sclerosis, but also in neurological and neuropsychiatric diseases characterized by demyelination.

Highlights

Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system characterized by secondary decline in myelin repair, loss of neurons and progressive disability [3, 28]
Our study reveals a previously unrecognised role of extracellular vesicles (EVs) produced by microglia in the control of remyelination, the spontaneous process by which oligodendrocyte precursor cells (OPCs) differentiate into myelin-forming cells, restore myelin sheaths to protect axons from degeneration and allow fast signal transmission [24]
It gives a first identification of the distinct molecular components of EVs involved in astrocyte detrimental transition as well as in promotion of OPC migration and/ or differentiation, unveiling new targets for modulation of myelin repair

Summary

Introduction

Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system characterized by secondary decline in myelin repair, loss of neurons and progressive disability [3, 28]. During disease chronicization up to progressive MS, remyelination is gradually impaired, primarily due to the inability of OPCs to differentiate into myelinating cells, and resulting from inefficient OPC activation and recruitment to myelin lesions [60]. More than 80% of MSspecific genes identified at actively demyelinating lesions are related to microglia activation and T cell-mediated inflammation [66]. The negative impact of microglia on oligodendrocytes may occur through astrocytes, recognised players in MS immunopathology [12, 21], which are transformed to harmful A1 cells by inflammatory microglial mediators released from microglia (IL1α, TNF and C1q) [55]. In the absence of pro-regenerative microglia, OPC differentiation becomes less efficient, eventually resulting in impaired myelin repair [64, 87], whereas a switch of microglia to an anti-inflammatory phenotype promotes remyelination and ameliorates the clinical signs of experimental autoimmune encephalomyelitis (EAE) [100], an animal model of MS. Despite the demonstration of both detrimental and beneficial effects of microglia on myelin lesion, the mode(s) of action of these cells in aiding or inhibiting remyelination is still largely unclear

Methods

Results

Conclusion