Lesion-remote astrocytes govern microglia-mediated white matter repair.

Spared regions of the damaged central nervous system undergo dynamic remodelling and exhibit a remarkable potential for therapeutic exploitation(1). Lesion-remote astrocytes (LRAs), which interact with viable neurons and glia, undergo reactive transformations whose molecular and functional properties are poorly understood(2). Here, using multiple transcriptional profiling methods, we investigated LRAs from spared regions of mouse spinal cord following traumatic spinal cord injury. We show that LRAs acquire a spectrum of molecularly distinct, neuroanatomically restricted reactivity states that evolve after spinal cord injury. We identify transcriptionally unique reactive LRAs in degenerating white matter that direct the specification and function of local microglia that clear lipid-rich myelin debris to promote tissue repair. Fuelling this LRA functional adaptation is the secreted matricellular protein CCN1. Loss of astrocyte-derived CCN1 results in excessive, aberrant activation of local microglia, characterized by abnormal molecular specification, impaired debris processing reflected by the intracellular accumulation of myelin and axon debris, and dysregulated lipid metabolism with distinctive attenuation in lipid droplet accumulation. Mechanistically, we find that CCN1 binds microglial SDC4 to augment lipid storage, linking this signalling axis to a vital repair-associated lipid buffering response in debris-clearing microglia. Accordingly, microglial deficits resulting from astrocyte CCN1 depletion culminate in blunted clearance of white matter debris and impaired neurological recovery from spinal cord injury. Ccn1-expressing white matter astrocytes are induced by local myelin damage and are generated in diverse demyelinating disorders in mice and humans, pointing to their fundamental, evolutionarily conserved role in white matter repair. Our findings show that context-specific cues shape regionally distinct LRA reactivity states with functional adaptations that orchestrate multicellular processes underlying neural repair and influence disease outcome.