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Investigating mechanisms of myelin sheath length regulation and plasticity
Investigating mechanisms of myelin sheath length regulation and plasticity
Myelination of axons is important for proper functioning of the nervous system and breakdown of myelin can cause severe disabilities. By regulating nerve conduction, myelination is also critical for learning and memory processes. Myelination greatly influences conduction properties and remodeling of myelin has been proposed as a potential mechanism to adjust and modulated nervous system function. However, to date it is still unclear if existing myelin is able to remodel and therefore participate in brain plasticity. I used existing zebrafish transgenic lines and generated new constructs to visualize myelinated axons in the CNS and to perform in vivo life imaging. Furthermore, I developed a single cell ablation method with high spatial and temporal precision to selectively demyelinate axon stretches and assess remyelination and remodeling dynamics. Using these tools, I was able to describe the growth dynamics of single myelin sheaths and show that they are independent of neighboring sheaths and time of initiation. Myelin sheath growth can be divided into three different growth phases, an oligodendrocyte intrinsic, highly uniformly growth phase that lasts for about 8 hours, followed by a second variable growth phase, likely regulated by axon intrinsic mechanisms, in which sheath length differences are established. The last growth phase compensates for body growth, and is highly predictable by the length increase of the animal. By demyelination of short axon stretches I was able to show that myelin segments are able to deviate from their otherwise very stereotypic growth dynamics. Ablation of a myelin sheath resulted in reinitiation of fast sheath growth in the neighboring sheath to remyelinate the gap. A new sheath was formed in the gap and grew which often led to a pushing back of the neighboring sheaths that had invaded the demyelinated territory. Thereby, often re-establishing the pre-ablation pattern, indicating a homeostatic regulation of myelin sheath length along an axon. Similarly, partially myelinated axons regularly restored their pre-ablation pattern after demyelination. Together, these results indicated axonal control of myelin sheath length and node of Ranvier positioning to guide the restoration of pre-ablation patterns. Furthermore, I observed a high number of asymmetrically grown sheath that could not be explained by physical barriers like neighboring sheaths or axon collaterals, indicating the existence of a molecular growth barrier on the axon. To collect further evidence, I investigated the dynamics of the nodal marker Neurofascin and found that it forms clusters along unmyelinated axons which are predictive for node of Ranvier positions. In order to test if the formation of clusters and the positioning of nodes is axonal activity dependent I established an optogenetic setup for long-term stimulation of freely swimming fish. By manipulating axonal activity by optogenetics I was able to induce myelin sheath remodeling supporting the hypothesis of axonal regulation of node or Ranvier positioning, however, similar effects were observed in control animals. Together, I was able to describe the dynamics of myelin sheath growth and could show that existing myelin segments can remodel and are therefore able to participate in brain plasticity. Additionally, I collected evidence that node or Ranvier positioning and therefore also myelin sheath length are regulated by axonal mechanisms.
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Auer, Franziska
2019
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Auer, Franziska (2019): Investigating mechanisms of myelin sheath length regulation and plasticity. Dissertation, LMU München: Graduate School of Systemic Neurosciences (GSN)
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Abstract

Myelination of axons is important for proper functioning of the nervous system and breakdown of myelin can cause severe disabilities. By regulating nerve conduction, myelination is also critical for learning and memory processes. Myelination greatly influences conduction properties and remodeling of myelin has been proposed as a potential mechanism to adjust and modulated nervous system function. However, to date it is still unclear if existing myelin is able to remodel and therefore participate in brain plasticity. I used existing zebrafish transgenic lines and generated new constructs to visualize myelinated axons in the CNS and to perform in vivo life imaging. Furthermore, I developed a single cell ablation method with high spatial and temporal precision to selectively demyelinate axon stretches and assess remyelination and remodeling dynamics. Using these tools, I was able to describe the growth dynamics of single myelin sheaths and show that they are independent of neighboring sheaths and time of initiation. Myelin sheath growth can be divided into three different growth phases, an oligodendrocyte intrinsic, highly uniformly growth phase that lasts for about 8 hours, followed by a second variable growth phase, likely regulated by axon intrinsic mechanisms, in which sheath length differences are established. The last growth phase compensates for body growth, and is highly predictable by the length increase of the animal. By demyelination of short axon stretches I was able to show that myelin segments are able to deviate from their otherwise very stereotypic growth dynamics. Ablation of a myelin sheath resulted in reinitiation of fast sheath growth in the neighboring sheath to remyelinate the gap. A new sheath was formed in the gap and grew which often led to a pushing back of the neighboring sheaths that had invaded the demyelinated territory. Thereby, often re-establishing the pre-ablation pattern, indicating a homeostatic regulation of myelin sheath length along an axon. Similarly, partially myelinated axons regularly restored their pre-ablation pattern after demyelination. Together, these results indicated axonal control of myelin sheath length and node of Ranvier positioning to guide the restoration of pre-ablation patterns. Furthermore, I observed a high number of asymmetrically grown sheath that could not be explained by physical barriers like neighboring sheaths or axon collaterals, indicating the existence of a molecular growth barrier on the axon. To collect further evidence, I investigated the dynamics of the nodal marker Neurofascin and found that it forms clusters along unmyelinated axons which are predictive for node of Ranvier positions. In order to test if the formation of clusters and the positioning of nodes is axonal activity dependent I established an optogenetic setup for long-term stimulation of freely swimming fish. By manipulating axonal activity by optogenetics I was able to induce myelin sheath remodeling supporting the hypothesis of axonal regulation of node or Ranvier positioning, however, similar effects were observed in control animals. Together, I was able to describe the dynamics of myelin sheath growth and could show that existing myelin segments can remodel and are therefore able to participate in brain plasticity. Additionally, I collected evidence that node or Ranvier positioning and therefore also myelin sheath length are regulated by axonal mechanisms.