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Cell biology of myelin wrapping, plasticity and turnover

Periodic Reporting for period 3 - Myelination (Cell biology of myelin wrapping, plasticity and turnover)

Reporting period: 2017-07-01 to 2018-12-31

Our main goal is to understand how myelin forms. To realize this goal we study the mechanisms of myelin formation during the development of mice and zebrafish. We analyze the factors that determine whether and to what extent an axon will be myelinated or not, the forces that drive myelin around the axon, the structural basis of myelin plasticity and the mechanisms of myelin turnover in the adult. To realize these aims we combine genetics, biochemistry, proteomics and imaging. We hope that our project would not only explain how myelin is generated during brain development, but also how myelin reforms in demyelinating diseases.
Myelin is synthesized as a stable multilamellar membrane, but the mechanisms of membrane degradation under normal aging are unknown. We found that myelin pieces were gradually released from aging myelin sheaths, which are subsequently cleared by microglia. Myelin fragmentation increased with age and led to the formation of insoluble lysosomal inclusions in microglia. Thus, age-related myelin fragmentation is substantial, leading to lysosomal storage and contributing to microglial senescence and immune dysfunction in aging.

We made the surprising finding that the self-limiting inflammatory response, which is necessary for remyelination to occur, is maladaptive in the CNS of aged mice. We found that cholesterol-rich myelin debris overwhelms the efflux capacity of phagocytes resulting in a phase transition of free cholesterol into crystals, which induces lysosomal rupture and inflammasome stimulation.

Within the compacted myelin sheath there is a system of uncompacted, cytoplasmic-rich channels. This cytosolic space connects the mature oligodendroglial soma with the innermost membrane of the myelin sheath facing the axon. The aim was to determine the biogenesis and the function of the cytoplasmic channels. We used high-pressure freezing to improve tissue preservation of CNS white matter tracts and to elucidate myelin structure close to its native state. Using this technique, we identified CNP as an essential protein in setting up and maintaining normal cytoplasmic regions within myelin sheaths. At the molecular level, we find that CNP acts together with F-actin to antagonize the membrane adhesive forces exerted by polymerizing MBP molecules. One model of how CNP could exert such a function is by forming pillars anchored to the membrane by the actin cytoskeleton in the cytoplasmic space of the myelin sheath.

Since the last reporting period, we have made further progress in identifying the mechanisms in myelin growth. One major goal was to identify adhesion systems that mediate axon-glia interactions. Adhesion molecules play a key role in target recognition and the acquisition of positional identity in the central nervous system. While cell-cell adhesions are widely used to generate specific connections in neuronal development, the association of myelin to axons provides a particular challenge to the function of adhesive systems. Myelination is a dynamic process in which the oligodendroglial process, while connecting to a target axon, continuously moves around the axon and thereby constantly forms and breaks adhesive contacts with the axonal surface. In this project, we combined data from different experimental systems that have led to a new model of how adhesion molecules operate in myelin membrane growth and targeting. We suggest that two adhesive systems work together to coordinate the wrapping of the leading edge at the inner tongue around the axon, underneath the previously deposited membrane. Our model implies that the lateral cytoplasmic-rich membranous pockets of each myelin layer are kept in close contact with the axonal surface by the function of the paranodal adhesion molecules. While these cytoplasmic-rich lateral edges move towards the future node where they align and position as paranodal loops, they provide a shelter for the movement of the inner tongue. We believe that this is necessary as the leading edge of the growing myelin sheath needs to move with low adhesiveness multiple times around the axon. Thus, one function of the axoglial paranodal adhesion molecules could be to protect the promiscuous leading edge from targeting surfaces unselectively. We propose that myelin is targeted to and expands around the axon in progressive stages: during the interaction of oligodendroglial processes with appropriate axons, Mag binds to gangliosides on the axonal surface and, by restricting the periaxonal space, displaces the axoglial paranodal adhesion molecules to the lateral edges of the developing sheath. While the paranodal adhesion molecules remain tightly associated to each other, the interactions of Mag with axonal gangliosides are constantly renewed. In the absence of only the paranodal adhesion molecules, the interaction of Mag with the axons appears to be sufficient to guide the movement of the inner tongue around the axon. Likewise, if only Mag is deleted, the specific targeting of the inner tongue to the axon is functioning with only very few axons being surrounded by two or more myelin sheaths. However, the combined deficiency of the paranodal adhesion molecules and Mag, results in severe disturbances by disrupting both mechanisms that control myelin targeting and growth. Now, the leading edge has not only lost its fixation to the axon, but also its boundaries that keep it clamped to the interior of the myelin sheath. As a result, the leading edge starts to move out of the myelin sheath, across or beneath another sheath, and even on top of nodes In addition, as the lateral edges of the extending myelin layers fall back, the leading edge moves in some cases on top of previously formed layers resulting in the formation of double myelinated axons. The paper containing this data has been submitted for publication.
This project aims at improving our understanding of myelin formation in normal development. This knowledge will be important to design strategies of how to improve remyelination in diseases like multiple sclerosis.