SummaryTo investigate the role of microRNAs in regulating oligodendrocyte (OL) differentiation and myelination, we utilized transgenic mice in which microRNA processing was disrupted in OL precursor cells (OPCs) and OLs by targeted deletion of Dicer1. We found that inhibition of OPC-OL miRNA processing disrupts normal CNS myelination and that OPCs lacking mature miRNAs fail to differentiate normally in vitro. We identified three miRNAs (miR-219, miR-138, and miR-338) that are induced 10–100× during OL differentiation; the most strongly induced of these, miR-219, is necessary and sufficient to promote OL differentiation, and partially rescues OL differentiation defects caused by total miRNA loss. miR-219 directly represses the expression of PDGFRα, Sox6, FoxJ3, and ZFP238 proteins, all of which normally help to promote OPC proliferation. Together, these findings show that miR-219 plays a critical role in coupling differentiation to proliferation arrest in the OL lineage, enabling the rapid transition from proliferating OPCs to myelinating OLs.Highlights•Mature microRNAs are required for normal compact myelin development in CNS and PNS•miR-219, induced in OLs, is necessary and sufficient to promote OL differentiation•miR-219 represses inhibitors of OL differentiation PDGFRa, Sox6, FoxJ3, and ZFP238•miRNAs couple initiation of OL differentiation to inhibition of OPC proliferation

Dicer1 and miR-219 Are Required for Normal Oligodendrocyte Differentiation and Myelination