Wnt and Lithium promote Gliogenesis in the adult optic nerve
Student thesis: Doctoral Thesis
The central nervous system (CNS) is made up mainly of neurones, the signalling cells, and glia. The main types of glia are astrocytes and oligodendrocytes. Astrocytes are multifunctional cells with vital homeostatic roles, whereas oligodendrocytes are highly specialised to form axonal myelin sheaths, which are essential for rapid neuronal communication. Although glia are involved in every neurodegenerative disease, they have been less well studied as drug targets than neurones. Lithium has been used for decades as a mood stabilising agent and may have major therapeutic potential as a treatment in diverse neurodegenerative diseases. The mode of action of lithium is generally attributed to its potent inhibitory effect on glycogen synthase kinase 3 (GSK3), an enzyme that regulates the Wnt/-catenin pathway. The aim of this study was to examine the effects of lithium and Wnt signalling in glial cells of the adult CNS. First, I developed an organotypic culture model of the adult mouse optic nerve that maintains cellular, axon and myelin integrity in vitro. This model was used to examine the effects of lithium and Wnt on adult glial cells, using confocal microscopy, immunohistochemistry, microarray, western blot and qRT-PCR. A key finding was that lithium acts through the canonical Wnt/β -catenin pathway to stimulate gliogenesis in the adult optic nerve, acting via diverse cell-cycle and cytoskeletal remodelling mechanisms. Lithium profoundly increased oligodendrocytes by downregulating inhibitors of differentiation, such as Wnt, bone morphometric protein (BMP) and inhibitor of differentiation (ID)4. Furthermore, lithium also acted through non-GSK3 targets, including Cxcl1 and leukemia inhibitory factor (LIF), which promote oligodendrocyte differentiation. An important finding was that lithium, and to a lesser extent Wnt, induced the generation of a novel astrocyte morphology, with highly polarised morphology and axon growth promoting phenotype. Notably, I identified a number of novel glial genes in the optic nerve and provide evidence that lithium induces astrocyte polarisation through the inhibition lysyl oxidase and upregulation of the cell-cell adhesion molecule corneodesmosin. In summary, this PhD has demonstrated for the first time that Wnt regulates gliogenesis in the adult CNS and indicates that glial cells are a potential target of lithium in neurodegenerative diseases.
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