Activation of the inflammasome, a complex that processes interleukin-1β (IL-1β), occurs in response to activation of the adenosine triphosphate receptor P2X7 on macrophages. Activation of P2X7 also triggers the opening of a nonselective, large pore in the plasma membrane. Pelegrin and Surprenant found that pannexin-1 (panx1), a protein that produces hemichannel currents when ectopically expressed in oocytes, was present in monocytes, macrophages, astrocytes, and various cell lines. Panx1, when expressed in transfected HEK cells, coimmunoprecipitated with the P2X7 receptor and colocalized with the P2X7 receptors in ATP-stimulated cells. Using either an siRNA to knock down panx1 or a peptide inhibitor to block the pore function of panx1, the authors showed that dye uptake (a measure of pore opening) in response to P2X7 required panx1. When overexpressed in cells that lack P2X7 receptors, panx1 caused constitutive dye uptake, and when overexpressed in cells that contain P2X7 receptors, panx1 increased the rate of dye uptake in response to ATP but did not cause constitutive dye uptake. Inhibition of panx1 function in macrophages or a monocyte cell line prevented P2X7-mediated stimulation of caspase 1 activation and IL-1β release. However, inhibition of panx1 function did not prevent intracellular K+ depletion, which has been proposed as an activator of the inflammasome. The authors propose that opening of the pannexin1 channel in response to P2X7 activation may allow extracellular ATP to enter the cell and the increased intracellular ATP concentration may contribute to the activation of caspase 1.
P. Pelegrin, A. Surprenant, Pannexin-1 mediates large pore formation and interleukin-1β release by the ATP-gated P2X7 receptor. EMBO J. 25, 5071-5082 (2006). [PubMed]