Chronic changes in oligodendrocyte sub‐populations after middle cerebral artery occlusion in neonatal mice

Neonatal stroke is common and causes life‐long motor and cognitive sequelae. Because neonates with stroke are not diagnosed until days‐months after the injury, chronic targets for repair are needed. We evaluated oligodendrocyte maturity and myelination and assessed oligodendrocyte gene expression changes using single cell RNA sequencing (scRNA seq) at chronic timepoints in a mouse model of neonatal arterial ischemic stroke. Mice underwent 60 min of transient right middle cerebral artery occlusion (MCAO) on postnatal day 10 (p10) and received 5‐ethynyl‐2′‐deoxyuridine (EdU) on post‐MCAO days 3–7 to label dividing cells. Animals were sacrificed 14 and 28–30 days post‐MCAO for immunohistochemistry and electron microscopy. Oligodendrocytes were isolated from striatum 14 days post‐MCAO for scRNA seq and differential gene expression analysis. The density of Olig2+EdU+ cells was significantly increased in ipsilateral striatum 14 days post‐MCAO and the majority of oligodendrocytes were immature. Density of Olig2+EdU+ cells declined significantly between 14 and 28 days post‐MCAO without a concurrent increase in mature Olig2+EdU+ cells. By 28 days post‐MCAO there were significantly fewer myelinated axons in ipsilateral striatum. scRNA seq identified a cluster of “disease associated oligodendrocytes (DOLs)” specific to the ischemic striatum, with increased expression of MHC class I genes. Gene ontology analysis suggested decreased enrichment of pathways involved in myelin production in the reactive cluster. Oligodendrocytes proliferate 3–7 days post‐MCAO and persist at 14 days, but fail to mature by 28 days. MCAO induces a subset of oligodendrocytes with reactive phenotype, which may be a therapeutic target to promote white matter repair. Main Points Focal arterial ischemic stroke in full‐term equivalent neonatal mice alters proliferation and maturation of oligodendrocytes and myelination. Single cell transcriptomics reveal a unique subset of oligodendrocytes with an immune phenotype that persist 2 weeks after ischemia.