Project Summary Circular RNAs (circRNAs) are a novel class of covalently closed RNA species derived from “back splicing” of pre-mRNAs. Mounting evidence suggests the essential roles of circRNAs in governing healthy brain development and their abnormalities in neurological and neuropsychiatric disorders. Many circRNAs are unique and highly abundant in the human brain, which are thought to underlie the sophisticated function of human brains and the fragility of various brain diseases. Mechanistically, circRNAs can function through sponging microRNAs or RNA-binding proteins, which broadly regulate numerous biological pathways. Our current knowledge of molecular mechanisms that regulate circRNA biogenesis in the human brain is still in its infancy. In particular, circRNA biology in human glial cells are poorly understood. Whether neurons and glia cells possess distinct circRNA landscapes and downstream interactomes remain entirely unknown. The biological functions of circRNAs in governing brain development and modulating lesion repair are vastly elusive. These prevailing knowledge gaps limit the current understanding of the complex etiology of many brain diseases. Our long-term goals are to elucidate the regulation and function of circRNAs in healthy and diseased brains, which may help to develop novel therapeutics against brain illnesses. In this application, we focus on circRNA biology in oligodendroglia (OL). OLs are responsible for myelination of the central nervous system and affected in numerous diseases, represented by multiple sclerosis and schizophrenia. Our preliminary data revealed that the RNA-binding protein QKI advances biogenesis of a human OL circRNA, which can promote differentiation of human and rodent OLs. We established state-of-the-art technical platforms to identify circRNA landscapes and interactomes in human OL and neurons. We hypothesize that human circRNAs play essential roles in controlling OL and myelin development, and QKI mediates developmental signals to enhance human OL circRNA biogenesis. In Aim 1, we will determine how QKI regulates OL circRNA biogenesis to advance OL differentiation. In Aim 2, we will determine developmental regulation of human OL circRNA landscapes, downstream pathways, and mechanisms of circRNA action in OLs from multiple platforms with integrated analyses. In Aim 3, we will explore whether human OL circRNA pathways can promote OL lineage development in human induced pluripotent stem cell (iPSC)-derived oligodendrocyte spheres (hOLS) or myelin lesion repair in a well-established mouse model. Findings from these studies will provide novel insights on fundamental rules governing human OL function and myelin repair.