PROJECT SUMMARY The long-term goal of this project is to understand how oligodendrocytes, the myelinating glia cell type of the vertebrate central nervous system, are formed during development. In the ventral spinal cord and dorsal forebrain, neural progenitors first produce neurons followed by oligodendrocyte precursor cells (OPCs), which migrate and divide to populate the neural tube. Subsequently, many OPCs stop dividing and differentiate as myelinating oligodendrocytes. Whereas we know a considerable amount about the mechanisms that promote oligodendrocyte differentiation and myelination, we know very little about mechanisms that regulate the very earliest steps in specifying progenitors for oligodendrocyte fate. Based on our in vivo, live imaging-based fate mapping studies, we discovered that different spinal cord and forebrain progenitor cells produce neurons and OPCs. We have now used single cell RNA-seq analyses to identify a transcriptional state of these pre-OPC progenitors, and have identified candidate cell-intrinsic programs and cell-extrinsic pathways that are conserved between nervous system regions and across vertebrate species. Using zebrafish and mice as model systems, this project will identify the molecular mechanisms that guide formation of OPCs from neural progenitors. Specific Aim 1 will test a hypothesis that the pioneer transcription factor Ascl1 establishes the epigenetic and transcriptional state of pre-OPCs. Aim 2 will investigate the mechanisms by which Notch signaling selects a subset of neural progenitors for a pre-OPC fate. Specific Aim 3 will test a hypothesis that the transcription factor Gsx2 helps specify a pre-OPC state and/or regulates a pre-OPC to OPC transition. The results of this project have the potential for important new insights into developmental myelination, the causes of myelin disease and potential strategies to restore myelin following disease and injury.