Cellular and molecular mechanisms of myelinated axon formation and regeneration in vivo.
Final Report Abstract
The majority central nervous system (CNS) axons are eventually ensheathed with myelin, a fatty structure formed by oligodendrocytes as they iteratively wrap their cell membranes around the axon. This ensheathment critically supports axon function by restricting nerve impulse propagation to short unmyelinated gaps between individual myelin sheaths (the nodes of Ranvier), and by the local provision of metabolites to the axon. Axonal myelination is not as stereotyped as it was long believed. Instead, axons can display different extents of myelination with highly variable numbers and position of myelin along the axon. Furthermore, it has become clear that myelination is a dynamic process that can occur almost lifelong and which may adaptively change in response to experience and neuronal activity – with implications for interneuronal communication and learning. Damage to myelin, as it can occur in diseases and injuries, impairs axon function and ultimately leads to its degeneration when damaged myelin is not repaired in a regenerative process called remyelination. The mechanisms that orchestrate how an axon gets myelinated and repaired along its length in the healthy and damaged central nervous system are not well understood. This proposal aimed to investigate the mechanisms underlying the formation of myelinated axons with nodes of Ranvier in different positions, the dynamics of nodes and myelin following CNS damage and oligodendrocyte death, and the mechanisms by which myelin of correct number and length can regenerate. To address these objectives, we used in vivo live cell microscopy, genetic and cellular manipulations in zebrafish as model organism, which offers unique opportunities to study these processes at unprecedented resolution. Over the course of this project, we have generated a suite of new transgenic reagents and over 20 new transgenic lines allowing us to study for the first time how individual axons with nodes of Ranvier in different positions get myelinated axon their length in real time. We could show that individual myelin sheaths only grow dynamically for a short period of time of no more than about three days after their respective initiation. After this time, myelin sheath length changes are restricted to compensate for animal body growth. As a consequence of this cellular behaviour, axon-myelination patterns remain largely stable once they have been established. However, despite this stereotyped myelin growth seen in the healthy CNS, we were able to show that myelin sheaths are in principle able to dynamically remodel in length. To show this, we had developed novel assays of experimental demyelination using targeted ablation of individual oligodendrocytes. Our work revealed that myelin exhibited remodelling that was characterised by length changes of remaining myelin sheaths as well as the formation of new sheaths. To our surprise, remodelling dynamics frequently led to a reestablishment of myelin sheath positioning similarly to how it was before demyelination. Therefore, our work revealed the existence of an apparent homeostatic control of axon-myelin patterns by the controlled maintenance and remodeling myelin sheath length. How might the formation, maintenance, and remodelling of axon-myelin patterns be regulated? Our work revealed the presence of multiple mechanisms that likely act synergistically. One mechanism was based on contact-inhibition by myelin sheath that appose each other to form a node of Ranvier in the remaining gap. In addition, we were able to reveal axonal mechanisms that can restrict myelin growth and as a consequence pre-determine node of Ranvier positioning. Lastly, we were able to identify axon Neurofascin as a crucial molecular player in this process. Neurofascin clusters in distinct positions along axons prior to myelination and thus prefigures node position. Moreover, CRISPR/Cas9 deficient animals exhibit aberrant internodal distances, indicating that premyelinating Neurofascin clusters may act as a stop signal for growing myelin sheaths. Together, the work of this project provided first-time insights in the dynamics and control of how myelinated axons are formed, maintained and repaired by cell-intrinsic and -extrinsic parameters.
Publications
- Insights into mechanisms of central nervous system myelination using zebrafish. GLIA 2016, 64:333-49
Czopka T
(See online at https://doi.org/10.1002/glia.22897) - Neue Ansätze zur Analyse von Axon-Oligodendrozyten Kommunikation in vivo. Neuroforum 2017, 23:231-38
Czopka T, Auer F
(See online at https://doi.org/10.1515/nf-2017-0010) - Evidence for myelin sheath remodelling in the central nervous system revealed by in vivo imaging. Current Biology 2018, 28:549-559.e3
Auer F, Vagionitis S, Czopka T
(See online at https://doi.org/10.1016/j.cub.2018.01.017) - Visualisation and time-lapse microscopy of myelinating glia in vivo in zebrafish. Methods in Molecular Biology 2018, 1791:25-35
Vagionitis S, Czopka T
(See online at https://doi.org/10.1007/978-1-4939-7862-5_3) - Functionally distinct subgroups of oligodendrocyte precursor cells integrate nervous system activity and execute myelin formation. Nature Neuroscience 2020, 23:363-374
Marisca R, Hoche T, Agirre E, Hoodless LJ, Barkey W, Auer F, Castelo-Branco G, Czopka T
(See online at https://doi.org/10.1038/s41593-019-0581-2)
