After traumatic brain injury oligodendrocytes regain a plastic phenotype and can become astrocytes

After acute brain injuries various response cascades are evoked that direct the formation of the glial scar. Here, we report that acute lesions associated with a disruption of the blood-brain barrier trigger a re-programming within the oligodendrocyte lineage. In PLP-DsRed1/GFAP-EGFP and PLP-EGFPmem/GFAP-mRFP1 transgenic mice with cortical injuries, we transiently found PLP transgene-labelled cells with activated GFAP promoter activity adjacent to the lesion site. We termed them AO cells, based on their concomitant activity of astro- and oligodendroglial genes. By fate mapping using PLP- and GFAP-split Cre complementation and NG2-CreERT2 mice we observed that major portions of AO cells surprisingly differentiated into astrocytes. Using repeated long-term in vivo two-photon laser-scanning microscopy (2P-LSM) we followed oligodendrocytes after injury. We observed their conversion into astrocytes via the AO cell stage with silencing of the PLP promoter and simultaneous activation of the GFAP promoter. In addition, we provide evidence that this oligodendrocyte-to-astrocyte conversion depends on local cues. At the lesion site higher expression levels of various glial differentiation factors were detected. And indeed, local injection of IL-6 promoted the formation of AO cells. In summary, our findings highlight the plastic potential of oligodendrocytes in acute brain trauma. An altered environmental milieu affects gene expression programs of mature oligodendrocytes and induces a plastic differentiation stage with astrogliogenic potential via transitional AO cells. ### Competing Interest Statement The authors have declared no competing interest. * ### Abbreviations BMP4 : bone morphogenetic protein 4 BrdU : 5-bromo-2’-deoxyuridine CNTF : ciliary neurotrophic factor dbi : days before injury dpi : days post injury GLAST : glutamate/aspartate transporter IL-6 : interleukin 6 LIF : leukemia inhibitory factor MCAO : middle cerebral artery occlusion AO cell : astro-and oligodendroglial -precursor cell OPC : oligodendrocyte precursor cell R26-tdT : Rosa26ActinflSTOPfltdTomato SWI : stab wound injury tam : tamoxifen tdT : tdTomato wbi : weeks before injury wpi : weeks post injury 2P-LSM : two-photon laser-scanning microscopy