Synaptic proteins expressed by oligodendrocytes mediate CNS myelination
Oligodendrocytes ensheath neuronal axons with myelin, a proteolipid-rich membrane that increases conduction velocity and provides trophic support. Our lab and others have provided evidence that vesicular release from neurons promotes myelin sheath growth. Complementarily, transcriptomic and proteomic approaches have revealed that oligodendrocytes express many proteins that allow dendrites to sense and respond to vesicular release at synapses. Do axon-myelin contacts use similar communication mechanisms as nascent synapses to form myelin sheaths on axons? To test this, we used fusion proteins to track synaptic vesicle localization and membrane fusion within spinal cord axons of zebrafish larvae during developmental myelination. Additionally, we used a CRISPR/Cas9-mediated GAL4 enhancer trap and genetically-encoded intrabody to detect expression and localization of PSD-95, a component of dendritic postsynaptic complexes, within oligodendrocytes. We found that synaptic vesicles accumulate at ensheathment sites over time and are exocytosed with variable patterning underneath myelin sheaths. Accordingly, we also found that most, but not all sheaths localized PSD-95 with patterning similar to exocytosis site location within the axon. By querying published transcriptome databases, we found that oligodendrocytes express numerous transsynaptic adhesion molecules that function across synapses to promote synapse formation and maturation. Disruption of candidate PDZ-binding transsynaptic adhesion proteins in oligodendrocytes revealed that these proteins have variable effects on sheath length and number. We focused on one candidate, Cadm1b (SynCAM1), and demonstrated that it localized to myelin sheaths where both its PDZ binding and extracellular adhesion to axons are required for myelin sheath growth. Our work reveals shared mechanisms of synaptic and myelin plasticity and provides new targets for mechanistic unraveling of activity-regulated myelination.