CNS myelin heterogeneity in species, single myelin sheaths and in Multiple Sclerosis
by Vasiliki Ilya Gargareta
Date of Examination:2024-06-17
Date of issue:2025-06-16
Advisor:Dr. Hauke Werner
Referee:Prof. Dr. Wolfram-Hubertus Zimmermann
Referee:Dr. Olaf Jahn
Referee:Prof. Dr. Christine Stadelmann-Nessler
Persistent Address:
http://dx.doi.org/10.53846/goediss-11334
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Description:Doctoral Cumulative Thesis
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Abstract
English
In the Central Nervous System (CNS), oligodendrocytes (OLGs) produce myelin which is a specialized plasma membrane that supports axons. Similarly, Schwann Cells (SCs) fulfill this role in the Peripheral Nervous System (PNS). Myelin facilitates fast, saltatory signal propagation important for cognitive and motor functions. Impairments in OLGs or myelin can cause demyelinating disorders like Multiple Sclerosis (MS) while dysfunctional SCs are associated with injuries or demyelinating neuropathies. In both aspects, myelin and many relating disorders are commonly investigated in mice as experimental models for humans. However, it was unknown whether their myelin protein composition is comparable. In Chapter 1 we investigated the conservation and divergence of the protein composition within the CNS myelin across humans and mice. Using quantitative proteomics, we analyzed purified myelin from the subcortical Normal Appearing White Matter (NAWM) of non-diseased humans. We identified that the relative abundance of PLP, MBP and CNP correlates well with that in WT C57Bl/6N mice. Yet, other proteins show exclusive expression in myelin of either species, as exemplified by PMP2 in humans or TSPAN2 in mice, an important finding for translating knowledge from mouse models to humans. Nevertheless, what is the role of PMP2 in human CNS myelin and what is its functional relevance? In Chapter 2, we investigate PMP2 expression in human CNS. PMP2 localizes in compact myelin and is a fatty acid binding protein with unknown functions. Through several experimental methods we observed mosaic expression of PMP2 with PMP2-immunopositive sheaths wrapping around large caliber axons. Immunoblot analysis also revealed evolutionary expression of PMP2 both in the CNS myelin of non-human primates and humans. Aiming to analyze the functional relevance of PMP2 in CNS myelin we generated transgenic mice where human PMP2 is expressed in mouse OLGs (PMP2OE). We find successful expression of PMP2 in purified CNS myelin while ultrastructural analysis of PMP2OE optic nerves showed no pathology. PMP2OE mice are thus suited to serve as a model for biological, functional and immunological experiments. However, it wasn’t known if the myelin protein composition is changed in the NAWM of MS patients and thus in Chapter 3, we unravel changes in the myelin proteome of subcortical NAWM from MS patients compared to non-diseased controls. While the relative abundance of high abundant CNS myelin proteins like PLP1, MBP and CNP was similar, other proteins like GSN and CFL1 showed increased abundance while PMP2 displayed decreased abundance in the NAWM myelin of MS patients. Information about the myelin protein composition can further aid in future translational researchers. Finally, in the PNS, SCs and myelin can be influenced by neuropathies and injuries. In order to investigate how injury might affect a model of large caliber axons we performed in Chapter 4, mild nerve compression injury in the sciatic nerve of mice lacking the PNS myelin protein CMTM6 from SCs, a model of normal myelination but increased axonal calibers. We observed decreased axonal calibers, increased pathological features and immune cell infiltration in CMTM6 cko mice compared to controls. Our findings support the idea that larger peripheral axons are especially susceptible to mechanical injury. In summary, this thesis provides insights into myelin protein composition diversity across mammalian species, identifies molecular sheath to sheath heterogeneity in the human CNS, reveals the myelin protein composition of NAWM in MS, and finally provides evidence that the deletion of CMTM6 in SCs makes large axonal diameters more susceptible to injury.
Keywords: Oligodendrocytes; Schwann Cells; Myelin; Multiple Sclerosis; Neuroscience
