PROJECT SUMMARY/ABSTRACT Myelination is essential for information transfer and circuit function in the brain. New oligodendrocytes are continuously generated throughout life, and recent studies showed that blocking the generation of new oligodendrocytes disrupts learning and memory consolidation. Mature oligodendrocytes differentiate from oligodendrocyte precursor cells (OPCs), a population of motile, proliferative precursors that tiles the CNS and persists throughout life. OPCs receive direct synapses from neurons, and it has been hypothesized that they integrate neuronal firing information to regulate their differentiation and myelination. Significant research has been aimed at understanding the intracellular signaling pathways that regulate the maturation of oligodendrocyte lineage cells (oligodendroglia), yet molecular mechanisms underlying neuronal activity- dependent myelination are unknown. Furthermore, little is known about the excitation-transcription coupling events that link extracellular neuronal stimulation with intracellular signaling dynamics and gene expression changes in oligodendroglia. Activity-dependent changes in intracellular calcium drive myriad processes in neurons related to synaptic plasticity and learning, yet the effects of calcium dynamics in OPCs have not been carefully studied. Here, I will test the hypothesis that changes in neuronal activity modulate OPC calcium release from intracellular stores to control their differentiation following learning. Aim 1 uses fast in vivo time lapse imaging combined with viral expression of neurotoxins and holographic optogenetic stimulation to test how manipulations in neuronal firing modulate OPC calcium signaling. Aim 2 combines in vitro molecular biochemistry with longitudinal imaging and behavioral assays to assess if activity-dependent calcium release from intracellular stores drives oligodendrogenesis. Results from these studies will provide essential insights into the neuron-oligodendroglia interactions that govern myelin plasticity and may uncover novel therapeutic targets for the treatment of demyelinating diseases.