A role for P2X7in microglial proliferation

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Abstract
Microglia, glial cells with an immunocompetent role in the CNS, react to stimuli from the surrounding environment with alterations of their phenotypic response. Amongst other activating signals, the endotoxin lipopolysaccharide (LPS) is widely used as a tool to mimic bacterial infection in the CNS. LPS‐activated microglia undergo dramatic changes in cell morphology/activity; in particular, they stop proliferating and differentiate from resting to effector cells. Activated microglia also show modifications of purinoreceptor signalling with a significant decrease in P2X7expression. In this study, we demonstrate that the down‐regulation of the P2X7receptor in activated microglia may play an important role in the antiproliferative effect of LPS. Indeed, chronic blockade of the P2X7receptor by antagonists (oxidized ATP, KN62 and Brilliant Blue G), or treatment with the ATP‐hydrolase apyrase, severely decreases microglial proliferation, down‐regulation of P2X7receptor expression by small RNA interference (siRNA) decreases cell proliferation, and the proliferation of P2X7‐deficient N9 clones and primary microglia, in which P2X7expression is down‐regulated by siRNA, is unaffected by either LPS or P2X7antagonists. Furthermore, flow cytometric analysis indicates that exposure to oxidized ATP or treatment with LPS reversibly decreases cell cycle progression, without increasing the percentage of apoptotic cells. Overall, our data show that the P2X7receptor plays an important role in controlling microglial proliferation by supporting cell cycle progression.

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