ASCL1 and OLIG2 Expression Dynamics Control Glial Cell Fate and Regional Diversity in the Dorsal Forebrain

Gliogenesis is a multistep process that begins with the specification of glial progenitors (GPs) into migrating and proliferating precursor cells, which later differentiate into mature astrocytes and oligodendrocytes. How these developmental processes are coordinated to generate the diverse glial lineages in gray matter (GM) and white matter (WM) in the brain remains poorly understood. Here, we show that the basic-helix-loop-helix (bHLH) transcription factor ASCL1 serves as a direct mechanistic link between glial cell fate specification, migration, proliferation, and differentiation in the dorsal forebrain. Notably, ASCL1 is dynamically expressed in GPs, initiating in the ventricular zone (VZ), peaking in the intermediate zone (IZ), but is downregulated once GPs enter the cortical plate. Lineage tracing of ASCL1+ GPs demonstrates that they subsequently co-express OLIG2 to generate both astrocytes and oligodendrocytes in an "outside-in" pattern starting from the upper cortex inward to the corpus callosum, the opposite pattern of neurogenesis. Gain- and loss-of-function experiments further reveal that a sustained ASCL1 expression is essential for inducing sufficient levels of OLIG2 required to specify oligodendrocyte precursor cell (OPC) fate. Interestingly, a persistent ASCL1 expression also maintains OPCs into postnatal stages by promoting their self-renewal while suppressing their differentiation into postmitotic oligodendrocytes. Together, these findings establish ASCL1 as a key regulator of the spatiotemporal order of glial lineage diversity in cortical GM and callosal WM and implicate ASCL1 dysregulation as an underlying mechanism in the pathogenesis of gliomas.