Myelin pruning by microglia during development
by Ulrich Weikert
Date of Examination:2019-04-24
Date of issue:2019-04-26
Advisor:Prof. Dr. Mikael Simons
Referee:Prof. Dr. Mikael Simons
Referee:Prof. Dr. Thomas A. Bayer
Persistent Address:
http://dx.doi.org/10.53846/goediss-7415
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Abstract
English
Myelin is a lipid rich membrane produced by oligodendrocytes in the central nervous system. During development, oligodendrocyte precursor cells find their target axons and start to wrap their processes around axons, thereby creating a multilayered insulating sheath. Those sheaths are regularly distributed along the axon with small, intersecting gaps called Nodes of Ranvier. This insulation results in increase in signaling conduction velocity. Furthermore, recent publications suggest that myelin is a dynamic structure that can adapt to environmental changes. I investigated the epigenetic control of oligodendrocytes during motor skill learning, since it was suggested in literature, that myelin is necessary for complex learning. Mice were subjected to a complex running wheel and the corpus callosum, a region with high myelin density, were collected for batch isolation of tissue specific methylated DNA immunoprecipitation (Bits-meDIP), before and several days after complex learning. Thereby i could not identify changes of differentially methylated regions or genes. Myelin plasticity is also visible during development, where the formation of the myelin sheath is accompanied by myelin outfoldings, which are normally linked to disease pathology. Here, I investigated the ultrastructural changes of myelin by 2D and 3D electron microscopy during optic nerve development. Thus, I could see degenerated myelin debris additional to outfoldings in wild type optic nerves. Interestingly, degenerated myelin was not only seen in the extracellular space, but also still attached to otherwise normal looking myelin sheath. Additionally, microglia were found to be associated with myelin outfoldings and phagocytose degenerated myelin. This phenomenon has already been described for demyelinating disease models, however not yet for normal myelin development. Activation of microglia during that time seemed to be dependent on triggering receptor on myeloid cells 2 (TREM2) signaling, since TREM2 deficient animals showed less expression of the activation markers. Despite that RNA sequencing of white and grey matter microglia at 14 and 60 days after birth did not show an increase in genes usually expressed in disease-associated microglia (DAM) – a microglial subtype which arises in a Trem2-dependent manner. Pathway analysis showed that P14 microglia in cortex and corpus callosum expressed genes related to extracellular matrix interaction, cytokine signaling, focal adhesion and protein digestion, when compared to P60 microglia. Hence, redundant myelin during development is unlikely to be sufficient to trigger severe disease related gene expression.
Keywords: myelin; pruning; microglia; development; oligodendrocytes
