Synaptic proteins expressed by oligodendrocytes mediate CNS myelination

Abstract

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.Communication in the central nervous system (CNS) depends on billions of connections. Synapses, the connections between neurons, are plastic structures that grow and change with experience-evoked neuronal activity. Additionally, connections form between neurons and oligodendrocytes, the myelinating cell type of the CNS. Myelin sheaths also are plastic structures, mutable in length, number, and thickness 1-3 . Some of this plasticity may be triggered by experience, because motor learning paradigms increase myelination 4,5 and social and sensory deprivation paradigms reduce myelination of relevant brain regions 2,6,7 . What accounts for myelin sheath plasticity? One possibility is that neuronal activity tunes ensheathment, similar to activitydependent plasticity at synapses 8 . Consistent with this possibility, Demerens and colleagues first demonstrated more than 20 years ago that inhibiting action potential propagation with tetrodotoxin (TTX) reduced myelination of cultured neurons 9 . Activity-mediated communication from axons to myelin sheaths could ensure that active neuron...

The RNA Binding Protein FMRP Promotes Myelin Sheath Growth

“…FMRP binds dozens of transcripts encoding synaptic proteins [44,52], many of which are also expressed in the oligodendrocyte lineage [30]. Interestingly, our lab has shown that disruption of synaptic protein function in oligodendrocytes can lead to excess myelin sheaths of diminished length [35], which is comparable to FMR1-I304N overexpression (Fig. 8).…”

“…Larvae homozygous for the loss-of-function fmr1 hu2787 allele do not display any obvious physical defects, and grow into viable, fertile adults [34]. To examine individual oligodendrocytes in the developing spinal cord, we mosaically expressed an oligodendrocyte-specific membrane tethered reporter, mbpa:EGFP-CAAX, which distinctly labels myelin sheaths [35]. We injected mbpa:EGFP-CAAX into 1-cell wild-type and maternalzygotic fmr1 hu278 (mzfmr1 -/-) embryos, in which any potential maternal contribution of FMRP is absent.…”