The Role of Ascl1 in NG2 Cells in the Spinal Cord
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NG2 cells, one of the major glial cell populations within the central nervous system (CNS), are highly proliferative cells identified by the expression of the NG2 proteoglycan. Throughout life, NG2 cells can differentiate into oligodendrocytes to myelinate axon fibers, or they can be maintained in a proliferative or quiescent state as NG2 cells indefinitely. A recent study showed that deletion of tumor suppressor genes specifically within NG2 cells was sufficient to produce brain tumors in a mouse model, indicating that NG2 cells may serve as a potential cell of origin for gliomas. At present, however, understanding of how NG2 cells are regulated to proliferate or differentiate in the CNS remains incomplete. Interestingly, Ascl1, a proneural basic-helix-loop-helix (bHLH) transcription factor that is highly expressed in neural progenitor cells, is also expressed in NG2 cells. Although previous studies have shown that the loss of Ascl1 affects the initial specification and differentiation of NG2 cells, the specific role of Ascl1 in NG2 cells during embryonic and postnatal development remains unknown. In this study, we investigated the direct requirement of Ascl1 in NG2 cells during embryonic and postnatal development in the grey matter (GM) and white matter (WM) of the spinal cord. More specifically, we conditionally deleted Ascl1 specifically within NG2 cells (Ascl1-CKO) at E14.5 or at P30 using an NG2-CreERT2 mouse strain in which the tdTomato (tdTom) fluorescence reporter was also incorporated to allow direct visualization of the development of NG2-labeled (tdTom+) cells. We found that Ascl1-CKO at E14.5 resulted in a decrease in the number of tdTom+ cells in the GM, but an increased number of tdTom+ cells in the WM. In contrast, Ascl1-CKO at P30 resulted in a significant reduction in the number of tdTom+ cells in both the GM and WM. Quantification of the percentage of tdTOM-labeled cells that had differentiated to mature oligodendrocytes revealed that Ascl1-CKO at E14.5 does not affect NG2 cell differentiation, while Ascl1-CKO at P30 accelerated NG2 cell differentiation. Taken together, these findings indicate that Ascl1 is differentially required to regulate the number of NG2 cells in the GM and WM during embryonic development, whereas Ascl1 is essential for regulating both the differentiation and number of NG2 cells in the adult CNS.