Epigenetic perturbation unravels diverse origins, state conversions and de-differentiation of cortical astrocytes

The role of epigenetic modifiers in astrogenesis is less understood compared to neurogenesis. For example, the Disruptor of telomeric silencing 1 like, DOT1L, conferring H3K79 methylation safeguards neural progenitors from premature differentiation in various brain regions, however, its role in astrogenesis has not yet been reported. Here, we decipher its role during astrogenesis using an Emx1-cre driven DOT1L conditional knock-out during the development of mouse cerebral cortex from E12.5 till P0. We describe two astrocyte types at P0, distinguishable by expression of phagocytosis markers genes. DOT1L loss-of-function (LOF) increased the number of those astrocytes that express fewer phagocytosis marker genes, while it affects a larger number of differentially expressed genes in those astrocytes that strongly express homeostasis marker genes. Our findings establish DOT1L as a barrier between these astrocyte states, limiting differentiation of the astrocytes expressing phagocytosis marker genes. We also identify multiple developmental origins of astrocytes. In addition to apical progenitors (AP), the prevailing stem cell population considered as origin of cortical astrocytes, we identify neural stem cells (NSC) in the dorsal telencephalon differentiating towards astrocytes, and Nkx2.1-lineage derived astrocytes, originating from the ventral telencephalon. This finding refines the prevailing view that dorsal telencephalon APs are the only source of embryonic cortical astrocytes, and enter astrogliogenesis after neurogenesis. We show that the expression of Emx1 discriminates different astrocyte origins during cortical development, and we describe a new set of transcription factors (TFs) that mark the state conversion between the homeostasis or phagocytosis expression programs in astrocytes. DOT1L orchestrates different TF networks, of which Lhx2, Nfia, Sox6, and Creb5 drive astrogliogenesis, and Nfia, Nfib, and Tcf4 affect astrocyte state conversion. Importantly, our analysis also shows that NSCs, APs and astrocytes de-differentiate within this lineage trajectory, and lowering the DOT1L-mediated epigenetic barrier strongly favours appearance of astrocytes with limited phagocytosis properties and reactivation of progenitor programs.