| Microglia are the only resident immune cells in the CNS, and they are major producers of inflammatory molecules. The overarching hypothesis guiding this thesis is that microglia are activated by pathologic paradigms of hypoxia, to induce the expression of pro-inflammatory/neurotoxic molecules and contribute to neuropathology. Initial studies demonstrated that a single sustained hypoxia/reoxygenation (HRO) event was sufficient to induce microglial inflammatory gene expression, and that this inflammatory response was modulated by extracellular nucleotides through activation of P2X purinergic receptors. We next investigated the effects of multiple, brief HRO events (intermittent hypoxia) on microglial inflammatory activities. Intermittent hypoxia (IH), is a hallmark feature of sleep disordered breathing, and it causes significant neurological deficits. Oxidative stress and inflammatory pathways play a central role in IH-induced neuropathology; however, the cellular source(s) and mechanisms underlying IH-induced inflammation are poorly understood. This thesis explores the role of microglia in IH-induced neuroinflammation, and identifies key pathways modulating these responses. We found that IH up-regulated microglial pro-inflammatory genes/proteins in rodent models of IH, effects that were CNS region-specific and had a time course correlating with neuron death. Microglial inflammatory responses to IH were mediated, in part, through activation of the toll-like receptor 4 (TLR4) pathway, as IH-mediated inflammation was attenuated in primary microglia derived from TLR4-deficient mice. In our final studies, we investigated the ability of chronic IH exposure to induce epigenetic changes known to regulate microglial inflammatory responses. Using our newly developed flow cytometry method, we found JMJD3 and JMJD5 to be enriched in microglia compared to other CNS cell types, and microglial JMJD3 expression was increased by IH ( in vitro and in vivo). IH-induced inflammation in cultured microglia was attenuated in the presence of the JMJD3 inhibitor, GSK-J4, suggesting that JMJD3 is an important mediator of IH-induced pro-inflammatory gene transcription in microglia. Overall, this research provides fundamental information regarding the biological responses of microglia following exposure to IH, and has identified 3 novel mechanistic pathways whereby IH-induces microglial inflammation, laying the foundation for future studies into the therapeutic control of microglial phenotypes in disorders in which hypoxia is a component. |
