ADP and AMP Induce Interleukin-1β Release from Microglial Cells through Activation of ATP-Primed P2X7 Receptor Channels

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Abstract
P2X7 is a subtype of ATP-gated channels that is highly expressed in astrocytes, microglia, and other immune cells. Activation of P2X7 purinoceptors by ATP or 3′-O-(4-benzoyl)-benzoyl ATP (BzATP) induces the formation of cytolytic pores and provokes release of interleukin-1β from immune cells. We investigated the actions of other endogenous nucleotides on recombinant and microglial P2X7 receptors using electrophysiology, fluorescence imaging, and interleukin-1β release measurement. We found that initial application of ADP or AMP toXenopus oocytes expressing P2X7 receptors was ineffective. However, when ADP and AMP, but not UTP or adenosine, were applied after a brief exposure to ATP or BzATP, they activated P2X7 receptors in a dose-dependent manner. Moreover, responses to ADP and AMP were also elicited after exposure to low concentrations of ATP and were recorded several minutes after removal of ATP from the extracellular medium. Whole-cell recordings from mouse microglial cells showed that significant responses to ADP and AMP were elicited only after ATP application. YO-PRO-1 dye uptake imaging revealed that, unlike ATP, prolonged application of ADP or AMP did not cause an opening of large cytolytic pores in mouse microglial cells. Finally, ADP and AMP stimulated the release of interleukin-1β from ATP-primed mouse and human microglial cells. We conclude that selective sensitization of P2X7 receptors to ADP and AMP requires priming with ATP. This novel property of P2X7 leads to activation by ATP metabolites and proinflammatory cytokine release from microglia without cytotoxicity.