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The role of astrocytes for oligodendrocyte death and remyelination

Claudia Wrzos
Doctoral thesis
Date of Examination: 2012-03-06
Date of issue: 2013-01-30
Advisor: Stadelmann-Nessler, Christine Prof. Dr.
Referee: Stadelmann-Nessler, Christine Prof. Dr.
Referee: Müller, Michael Prof. Dr.

Persistent Address: http://hdl.handle.net/11858/00-1735-0000-000D-F1F4-E

 

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Abstract

English

Astrocytes are the most abundant cells in the central nervous system (CNS) with wide-ranging functions. The first part of my thesis deals with Neuromyelitis optica (NMO), an inflammatory demyelinating disease of the CNS. Aquaporin 4 (AQP4), a water channel expressed in high density at perivascular astrocytic endfeet, has recently been identified as a target of the humoral immune response in NMO. I developed two in vivo models to assess the extent, time course, and mechanisms of tissue damage and repair after astrocyte depletion. A patient derived, recombinant anti-AQP4 (rAb-53) antibody was i.v. transferred into immunized rats (experimental autoimmune encephalomyelitis (EAE)/NMO model) or focally injected into the rat cerebral cortex (focal NMO model). One hour after a single cerebral injection of complement-binding rAb-53, a selective depletion of astrocytes was found. 3hrs after antibody injection, in addition, a diminution of oligodendrocytes was observed. 24hrs after antibody injection the astrocyte depleted areas reached their full extent and revealed loss of oligodendrocytes and oligodendrocyte precursor cells (OPCs). After two weeks, astrocytic repopulation of the lesion was nearly complete. However, the prelesional density of oligodendrocytes was not yet achieved. To assess the role of excess glutamate and ATP on oligodendroglial cell death, N-methyl D-aspartate (NMDA) and P2X7 receptor antagonists were applied in vivo and in vitro. However, no clear-cut effect of NMDA and P2X7 receptor antagonists on oligodendroglial cell death was observed. In the systemic EAE/NMO model, administration of rAb-53 to preimmunized rats demonstrated huge perivascular astrocyte depleted areas in the spinal cord accompanied by significant loss of oligodendrocytes and OPCs after 30hrs. In addition, kidney pathology with detached cells of the inner medullary collecting ducts, where AQP4 is expressed, and infiltrating macrophages were observed. In summary, both models successfully mimicked human NMO lesions. The human rAb-53 was able to induce astrocyte depletion with a single cerebral injection or when administered to immunized rats. Activation of the complement system was found to be a prerequisite for astrocyte lysis. Furthermore, this work demonstrates that oligodendroglial cell death follows astrocyte depletion quite rapidly. However, oligodendroglial repair of astrocyte depleted lesions is slow compared to classical demyelinated lesions. In the second part of my thesis the role of fibroblast growth factor 9 (FGF9) on demyelination (DM) and remyelination (RM) was assessed. To provide overexpression of this growth factor in vivo, an adeno-associated virus 6 (AAV-6)-based vector containing the cDNA of FGF9 under the control of the GFAP promoter was constructed. This FGF9-AAV-6-based vector was intracerebrally injected in cuprizone-fed mice and rats with focal EAE to study a possible role of FGF9 on DM and RM. In both models, no clear effect of FGF9 on RM was found. However, in both models FGF9 led to widespread activation and proliferation of astrocytes and similarly, activation and proliferation of oligodendroglia. Thus, my results indicate that FGF9 has an important role for the homeostasis of glial cells, however, rather influences cell number and activation state than repair.
Keywords: Astrocytes
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