Synaptic input and Ca²⁺ activity in zebrafish oligodendrocyte precursor cells contribute to myelin sheath formation

In the nervous system, only one type of neuron–glial synapse is known to exist: that between neurons and oligodendrocyte precursor cells (OPCs), yet their composition, assembly, downstream signaling and in vivo functions remain largely unclear. Here, we address these questions using in vivo microscopy in zebrafish spinal cord and identify postsynaptic molecules PSD-95 and gephyrin in OPCs. The puncta containing these molecules in OPCs increase during early development and decrease upon OPC differentiation. These puncta are highly dynamic and frequently assemble at 'hotspots'. Gephyrin hotspots and synapse-associated Ca²⁺ activity in OPCs predict where a subset of myelin sheaths forms in differentiated 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/dlg4b, gphnb 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.