The dual role of astrocytes in myelination.
PhD thesis, University of Glasgow.
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Astrocytes are the most abundant cell within the central nervous system (CNS) and yet despite this, the true extent of their role in health and disease has not been fully elucidated. In the undamaged CNS, they are termed quiescent, where they maintain homeostasis. However, after injury or disease astrocytes become reactive where they are described as a physical and molecular barrier to regeneration. Emerging literature has suggested the existence of an additional phenotype of astrocyte, termed the activated astrocyte. These astrocytes are thought to enhance regeneration by creating a more growth-permissive environment for repair. In addition, it has also been reported that astrocytes may play a role in regulating myelination; however, it is unclear how the phenotype of the astrocytes may affect this process. Therefore, this thesis will focus on the variable phenotypic state of astrocytes and subsequently how this relates to their ability to support myelination.
Using an in vitro myelinating culture, where dissociated spinal cord cells were plated on a monolayer of astrocytes, myelination can be followed over time. Since it is hypothesised that astrocytes can affect myelination we used two protocols known to affect the reactive status of the astrocyte, i) activate the astrocytes by treating with ciliary neurotrophic factor (CNTF) or ii) induce a quiescent astrocyte state by plating them on Tenascin C (TnC).
It is hypothesised that CNTF changes the activation state of the astrocyte therefore making it more supportive to myelination. The addition of the astrocyte derived factor ciliary neurotrophic factor (CNTF) was shown to enhance myelination. My results demonstrate that CNTF addition does not lead to an increase in oligodendrocyte or microglia cell numbers or an increase in the diameter of the neurites, thus suggesting that this CNTF-induced increase in myelination is mediated via the astrocyte.
Conversely, culturing astrocytes on the extracellular matrix molecule Tenascin-C (TnC), a method to make the astrocytes quiescent (Holley et al., 2005), resulted in a reduction in myelination. Astrocytes cultured on TnC were shown have decreased expression of nestin, which is typically a marker for reactivity. A microarray gene study comparing gene expression of the various astrocyte phenotypes identified CXCL10 to be upregulated in astrocytes on TnC. Furthermore, the addition of CXCL10 into the myelination cultures resulted in a decrease in myelination. Conversely, the addition of anti-CXCL10 to myelinating cultures on quiescent astrocytes increased myelination.
Taken together, these data indicate that the astrocyte phenotype has considerable influence on myelination; where activated astrocytes support myelination whilst quiescent astrocytes do not. The identification of factors which may modify astrocyte phenotypes could lead to potential therapeutic strategies for CNS pathologies.
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