Mechanisms of Purinergic Receptor 2X Modulation of Microglial Gene Expression: Influence of Hypoxia and Re-Oxygenation

Following hypoxic brain injury, microglial cell activation and their release of pro- and anti-inflammatory mediators as well as neurotrophic factors contributes to the neuronal environment within the central nervous system (CNS). Among the proteins important for modulating the inflammatory function of microglia are the P2 purinergic receptors for which extracellular adenine nucleotides are ligands. Although P2X7 receptor activation is recognized to regulate processing and release of cytokines, little is known concerning its role in regulating the transcription of inflammatory genes. We demonstrate for the first time that both early growth response transcription factors (Egr) and P2X7 receptors via the MAPK (MEK and p38)-Egr pathways are necessary for BzATP-mediated modulation of iNOS, and stimulation of TNF-alpha and IL-6 gene expression.;Little is known concerning the modulatory effects of extracellular adenine nucleotides during hypoxia. We hypothesized that P2X7 receptor activation would decrease their pro-inflammatory activities in sustained hypoxia and in hypoxia followed by reoxygenation similar to under normoxia. Surprisingly, the effects of BzATP were reversed in cells exposed to hypoxia/re-oxygenation (HRO); demonstrating differential inflammatory gene regulation by P2X4/P2X7 receptors depending upon the history of oxygen exposure. Collectively, these data support the novel idea that the history of oxygen exposure alters purinergic receptor modulation of microglial inflammatory activity.;Microglial phenotypes can vary widely, even in a single pathological state and across CNS regions. Therefore, microglial contributions to inflammation can differ between early and late disease states. We hypothesized that intermittent hypoxia would exacerbate microglial inflammatory expression. However, acute treatment caused the greatest increase in inflammatory and anti-inflammatory/neurotrophic expression. We also illustrate the importance of studying gene expression from microglia directly as it is apparent that homogenates misrepresent the gene expression of microglia.;We propose that the ratio of P2X4/P2X7 influences the balance of opposing microglial activities via regulation of Egr expression; but whether the net effects are beneficial or detrimental depend on the history of tissue oxygen exposure. Understanding how to regulate this pathway is key for minimizing microglial's pro-inflammatory effects and to identify selective therapeutic targets that can minimize neurotoxic microglial effects while promoting their neurotrophic actions.