The Histone Methyltransferase Ezh2 Regulates Cell Cycle and Oligodendrocyte Differentiation From NG2 Cells

Oligodendrocyte progenitor cells (OPCs, NG2 cells, polydendrocytes) are a population of glial cells in the central nervous system that continue to proliferate and differentiate into adulthood. The mechanisms underlying the decision to proliferate and self-renew or to generate myelinating oligodendrocytes are not completely understood. Histone methylation is a posttranslational modification of chromatin that has been shown to control cell fate by regulating transcription in various stem and progenitor cell types. The methylation of histone H3 lysine 27 (H3K27me3) is catalyzed by Ezh2, a component of the Polycomb Repressive Complex 2 that induces chromatin compaction, thereby repressing transcription of target genes. In this thesis we show that Ezh2 regulates distinct cell cycle dynamics in NG2 cells and further modulates their generation of oligodendrocytes. Thus, we identify a novel molecular mechanism which controls cell cycle in this resident population of brain progenitor cells with important implications for understanding development and disease. Using a transgenic mouse model to induce the deletion of Ezh2 specifically in NG2 cells, we examined the cell cycle dynamics and fate of NG2 cells in vivo. In combination with cell cycle phase-specific markers, we further identified at what stage NG2 cell division was disrupted with the loss of Ezh2. We also discovered a compensatory population of NG2 cells that maintained Ezh2 expression in the knockout, but also had altered cell cycle dynamics. In addition to cell cycle, we found that Ezh2 modulated the generation of oligodendrocytes, particularly during a precise temporal window after division. To understand the molecular changes associated with these findings, we performed single-cell RNA-sequencing and discovered several differentially expressed genes that provide novel insight into the role of Ezh2 in NG2 cells, as well as in cells exhibiting compensatory proliferation. The cell cycle length of NG2 cells has been shown to elongate with age, but these findings provide a molecular mechanism by which cell cycle time is regulated. It is critical for NG2 cells to properly expand during development to generate oligodendrocytes in a timely manner, and our data show that Ezh2 is important for cell cycle and oligodendrocyte differentiation