Age-impaired remyelination is associated with dysregulated microglial transitions.

Multiple sclerosis (MS) is a chronic, inflammatory condition characterized by demyelination and neurodegeneration. Regeneration of lost myelin, or remyelination, occurs in people with MS but is prone to failure and impaired with age. Remyelination is facilitated by microglia but our understanding of the microglial response during remyelination is incomplete. Here, we profiled the microglial response in the lysolecithin mouse model of remyelination using single-cell RNA sequencing. We found several distinct microglial states during the early stages of remyelination that coalesced into a resolved microglial state defined by myelin transcripts. This resolved state was also present in MS brains. As remyelination becomes inefficient with age, we profiled microglia from both young and middle-aged mice during remyelination and found a delay in several microglial states, in concordance with delayed remyelination. Overall, microglia synchronously initiate a multi-faceted response during efficient remyelination in young mice, which becomes dysregulated with age.