Depletion of Microglia Increases Cortical Oligodendrocyte Density During Remyelination

Cortical demyelination is a critical contributor to progressive disease in multiple sclerosis (MS). The barriers to cortical remyelination following demyelination are not fully understood, and there are no remyelinating treatments for MS. We previously took advantage of the spatial and temporal resolution of longitudinal in vivo imaging to study cortical oligodendrocyte regeneration following cuprizone-induced demyelination and found that oligodendrocyte regeneration was impaired. In this study, we investigated whether cortical reactive microglia disrupt oligodendrocyte regeneration. To do so, we used a combination of in situ RNA and immunofluorescence labeling to characterize cortical microglia reactive states following cuprizone-mediated demyelination. We then depleted cortical microglia by administering a Csf1r inhibitor during the recovery period from cuprizone and quantified oligodendrocyte recovery. We found that following cortical demyelination, deep cortical microglia change morphology, downregulate homeostatic markers (P2RY12, TMEM119), and upregulate a marker (CD68) associated with activated macrophages. These reactive changes persisted through early recovery post-cuprizone but resolved by late recovery. Depleting cortical microglia post-cuprizone restored the baseline density of deep cortical ASPA+ oligodendrocytes at early and late recovery. There were also more deep cortical BCAS1+ differentiating oligodendrocytes at early recovery when microglia were depleted, suggesting that transient deep cortical reactive microglia impair oligodendrocyte differentiation following demyelinating injury. Together, we found that cortical microglia adopt spatially restricted reactive functions after demyelination and deep cortical reactive microglia transiently reduce differentiating oligodendrocytes. A potential therapeutic strategy for progressive MS could involve targeting transiently reactive microglia at the right time and place in cortical lesions to promote oligodendrocyte regeneration.