Correct myelin targeting by oligodendrocytes in vivo requires a balance between myelin production and available axonal space

Centre for Neuroregeneration, University of Edinburgh

The ensheathment of axons by myelin, made by oligodendrocytes (OLs) in the CNS, is indispensable for nervous system function, by speeding up action potential propagation [1] and by metabolically supporting axons [2]. How OLs target their myelinating processes to specific axons in a complex milieu that also includes axons not fated for myelination, dendrites, neuronal somas and other cells remains incompletely understood. Here we investigated whether myelin targeting was influenced by the overall balance between available permissive axonal space and myelin production. We first examined myelination in the developing anterior spinal cord of kinesin-binding protein zebrafish mutants, which have fewer target axons, but where OL number and development is normal [3]. Live imaging of OL morphology and of axonal myelination revealed that in mutants, where target axonal space is halved, individual OLs still make most of their sheaths, but converge on fewer axons, resulting in shorter myelin sheaths. OLs did not appear to myelinate incorrect axonal tracts, but abnormally extended large spherical membrane profiles in addition to myelin sheaths. Remarkably, a dual reporter of myelin and of spinal neurons revealed these profiles to be ensheathed neuronal somas, suggesting that normally unmyelinated somas can be ectopically wrapped if there is a surplus of OL processes. To independently test whether an imbalance between OL processes and axonal space could cause soma ensheathment, we then sought to manipulate OL number in a normal axonal environment. We found that wildtype zebrafish treated with compounds DLH001 and DLH002, which we recently identified as greatly increasing OL number [4], recapitulated the ensheathed somas phenotype. Conversely, wildtype fish treated with a Gli1 inhibitor, which reduced OL number, decreased the normally low number of ensheathed somas even further. To further test whether matching axonal space to myelin production prevents ectopic soma wrapping, we then reasoned that providing extra axonal surface would rescue the phenotype. We previously showed that injection of a notch1a morpholino generates surplus Mauthner neurons and their large-diameter axons, which are the first myelinated in the zebrafish spinal cord [5]. notch1a morpholino injections were indeed able to decrease the number of ensheathed somas to near wildtype levels, showing that soma ensheathment was due to an imbalance between axonal surface and myelin production. Collectively, our data reveal a novel regulatory mechanism that contributes to appropriate targeting of myelin to axons.

References

1. Sherman DL, Brophy PJ. Mechanisms of axon ensheathment and myelin growth. Nature reviews Neuroscience. 2005;6(9):683-90.
2. Saab AS, Tzvetanova ID, Nave KA. The role of myelin and oligodendrocytes in axonal energy metabolism. Current opinion in neurobiology. 2013;23(6):1065-72.
3. Almeida R, Lyons D. Oligodendrocyte Development in the Absence of Their Target Axons In Vivo. PloS one. 2016;11(10):e0164432.
4. Cole K, Lyons D, personal communication (2016).
5. Almeida RG, Czopka T, Ffrench-Constant C, Lyons DA. Individual axons regulate the myelinating potential of single oligodendrocytes in vivo. Development. 2011;138(20):4443-50.