Synapses in oligodendrocyte precursor cells are dynamic and contribute to Ca2+ activity

In the nervous system, only one type of neuron-glial synapse is known to exist: that between neurons and oligodendrocyte precursor cells (OPCs). Neuron-OPC synapses are thought to bridge neuronal activity to OPCs. However, their composition, assembly, downstream signaling, and in vivo functions remain largely unclear. Here, we use zebrafish to address these questions and identify postsynaptic molecules PSD-95 and Gephyrin in OPCs. They increase during early development and decrease upon OPC differentiation. PSD-95 and Gephyrin in OPCs are highly dynamic and frequently assemble at "hotspots." Gephyrin hotspots and synapse-associated Ca2+ activity in OPCs predict where a subset of myelin sheaths form in oligodendrocytes. Further analyses reveal that spontaneous synaptic release is integral to OPC Ca2+ activity, while evoked synaptic release contributes only in early development. Finally, disruption of the synaptic genes dlg4a&b, gephyrinb, and nlgn3b impairs OPC differentiation and myelination. Together, we propose that neuron-OPC synapses are dynamically assembled and can predetermine myelination patterns through Ca2+ signaling.