Hypoxia Induces Trained Immunity-like Changes in Histone Modifications and Gene Expression in Microglia
Elizabeth A. Kiernan, Andrea C. Ewald, Jyoti J. Watters University of Wisconsin Madison, USA
Pathological hypoxia (Hx), such as the intermittent Hx experienced during sleep-disordered breathing, is associated with changes in peripheral macrophage function and multiple neurodegenerative diseases. In the periphery, pathogen exposure primes macrophage inflammation and elicits trained immunity, a process involving activation of Hx signaling pathways and histone modifications at glycolytic and inflammatory genes. Although Hx increases CNS inflammation and cognitive dysfunction, underlying molecular mechanisms whereby Hx alters the long-term functions of microglia, resident CNS macrophages, remain poorly understood. We tested the hypothesis that Hx would elicit immune memory changes in microglia similar to the effects of peripheral pathogen exposure. We exposed primary microglia to Hx (1% O2) for 24 hours and then returned them to normal culture conditions (normoxia; 21% O2) for 3 days. We then challenged them with lipopolysaccharide (LPS) and assessed inflammatory gene expression to ascertain if Hx could prime microglial inflammatory gene responses. Microglia exposed to Hx had enhanced inflammatory gene responses to LPS compared to normoxia-exposed microglia, even 3 days after Hx exposure. To determine if Hx elicits epigenetic changes similar to those involved in trained immunity, we performed ChIP-Seq for histone 3 lysine 4 trimethylation (H3K4me3) and found H3K4me3 enrichment at inflammatory and glycolytic genes involved in trained immunity. Lastly, using pharmacological manipulation, we tested if changes in gene expression and histone modifications were dependent on H3K4me3 methyltransferases, and/or reductions in metabolic cofactors such as α-ketoglutarate necessary for histone demethylase activity. We found that unlike trained immunity, Hx-induced microglial priming was independent of methyltransferase activity and could not be blocked upon α-ketoglutarate addition. We conclude that Hx can elicit memory-like changes in microglia similar to peripheral macrophage pathogen priming, but find that the pathways involved in Hx effects are unique. Elucidation of these underlying cellular mechanisms will identify novel targets to restore appropriate microglial signaling and gene responses in neural disorders associated with Hx.
This work was supported by NIH R01NS085226 and F31NS100229.
Credits: None available.
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