Research Highlights

CNS Myelin Turnover Mechanism Clarified: Astrocytes Eliminate Cellular Debris Resulting from Removal of Myelin Segments as the Optic Nerve Shortens During Frog Metamorphosis

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SBEM-based reconstruction shows a phagocyte (red) using lamellar processes (dark red) to internalize axonal material containing both myelin (blue) and axoplasm (yellow). (Scale bar: 5 μm.)

18-08-2015 La Jolla

In the central nervous system of nearly all vertebrates, oligodendrocytes provide support and insulation to axons by wrapping them in myelin sheaths that enable fast and efficient transfer of information across long distances. Axons with larger diameters have correspondingly thicker myelin sheaths provided by neighboring oligodendrocytes.

Too much or too little myelination leads to neuropathy. For example, the persistence of regions of excess myelin can lead to neurodegeneration, and debris myelin is known to provoke inflammation. So regulation of myelin on axons is essential to proper functioning of the vertebrate nervous system. However, the mechanisms involved in myelin segment plasticity, as this waxing and waning of myelin is sometimes called, have been poorly understood in part because of the difficulty of studying the process in mammals over a protracted period as the animals mature.

This maturation process in frogs, by contrast, is relatively short, making frogs a more useful subject of study. The project described here focused on a model organism, Xenopus laevis, the African Clawed Frog.

During metamorphosis, remodeling of the frog head takes place. The head becomes smaller and more triangular between the Nieuwkoop and Faber (NF) pre-metamorphic stage 58 and immediately post-metamorphic climax at stage 66, a period that takes place over approximately one week.

The number, length, and thickness of individual myelin segments vary with the species and region of the nervous system. In the absence of pathology, myelin segments are highly stable structures with plasticity limited to growth-related increases in myelin thickness. The proper thickness and length of these segments are likely established during the myelination process, which involves elongation, shortening, and removal of individual segments. Still, once established, some myelin segments need to be modified to accommodate axonal growth. Developmental increases in axon diameter are complemented by the addition of membrane wrap material to the myelin segments, thereby maintaining a near-linear relationship between axon diameter and myelin thickness.

During metamorphosis, the optic nerve (ON) in the frog head and its associated axons rapidly shorten in length, leading to abnormally folded myelin on the axons. These formations suggest that myelin might be used in the remodeling process that takes place during metamorphosis, thereby serving as a model system with which to study myelin plasticity.

In fact, measurements taken from dissected ONs confirmed previous studies, showing that the ON decreased in length by approximately half, whereas the cross-sectional area doubled in thickness. However, because the increase in thickness is transient (taking place only at the climax of metamorphosis, stage 62), the ON ultimately experiences a net loss of volume.

This study discovered that myelin segments also decrease in length in response to ON shortening. Electron microscopy-based analyses revealed that such shortening is accomplished by myelin-axon detachments and protrusions from intact myelin segments. Surprisingly, the research team found that astrocytes remove this waste material using phagocytosis, a process that eliminates cell debris to ward against infection and potential pathological effects. As opposed to axonal pathologies where the removal of degenerating myelin is usually associated with macrophages, in this case it is the astrocytes that selectively remove myelin. In fact, this study showed they remove approximately 25% of all myelin present. This mechanism likely contributes to other instances of myelin remodeling and prevents the persistence of potentially pathological myelin.

Scientific knowledge of the roles played by astrocytes has expanded following the discovery that they express phagocytic machinery. Prior to this study, a role for astrocytes in the clearance of myelin has been observed only in pathological contexts. This study provided evidence that astrocytes can also clear myelin in a developmentally regulated non-pathological manner. The continual removal of small amounts of dystrophic myelin from intact axons could provide a mechanism to prevent large-scale loss of myelin and the resulting fragmentation of axons that triggers a pathological immune response. Furthermore, this research team observed that the breakdown of myelin debris within astrocytes appeared to be very slow and suspects that at least some of this material is transferred to “professional phagocytes” near the pericapillary astrocytic endfoot processes or to professional circulating scavengers after crossing the vasculature.

In summary, this work demonstrates that, in a developing vertebrate ON, individual myelin segments can shorten in response to shortening in axon length, revealing a plasticity of myelin segments not previously known. Through volume EM-based quantification, the team demonstrated that axon myelin segments shorten through selective astrocytic phagocytosis of small intra-segmental myelin dystrophies. This mechanism may be relevant to maintenance of myelin segments or in instances where myelin segment length may need to be modified, such as to accommodate new segments replacing damaged ones during normal myelin turnover, aging, or regeneration.

This project is a collaboration among researchers at The Johns Hopkins University School of Medicine, the Hugo W. Moser Research Institute at Kennedy Krieger, Inc., and the National Center for Microscopy and Imaging Research.

Funding Source: This work was supported by Grant R01 EY019960 and a Catalyst for a Cure grant from the Glaucoma Research Foundation (to N.M.-A.). This work was also supported by National Center for Research Resources Grant 5P41RR004050, National Institute on Drug Abuse Human Brain Project Grant DA016602, and National Institute of General Medical Sciences Grants 5R01GM82949 and 5P41GM103412 (to M.H.E.).

Relevant Publication: Mills, E.A., Chung-ha, O.D., Bushong, E.A., Boassa, D., Kim, K.Y., Ellisman, M.H., and Marsh-Armstrong, N. (2015). Astrocytes phagocytose focal dystrophies from shortening myelin segments in the optic nerve of Xenopus laevis at metamorphosis. Proceedings of the National Academy of Sciences, 112(33), 10509-10514.