Is pannexin the pore associated with the P2X7 receptor?

Abstract

The P2X7 receptor (P2X7R), an ATP-gated cation channel, is expressed predominantly in leukocytes. Activation of P2X7R has been implicated in the formation of a cytolytic pore (i.e., a large conductance channel) that allows the passage of molecules up to 900 Da in macrophages. At least two hypotheses have been presented to explain the conversion of a nonselective cation channel to a cytolytic pore. One hypothesis suggests that the pore is a separate molecular structure activated by P2X7R, and the second asserts that this is an intrinsic property of P2X7R (pore dilation). Based on connexin knockout and hemichannel antagonist studies, some groups have concluded that connexins and pannexins, the hemichannel-forming proteins in vertebrates, are fundamental components of the large conductance channel associated with P2X7R. Dye uptake and electrophysiology experiments were used to evaluate the efficacy and specificity of some hemichannel antagonists under conditions known to open the large conductance channel associated with P2X7R. Hemichannel antagonists and interference RNA (RNAi) targeting pannexin-1 did not affect P2X7R macroscopic currents [ATP, 1,570±189 pA; ATP+100 μM carbenoxolone (CBX), 1,498±100 pA; ATP+1 mM probenecid (Prob), 1,522±9 pA] or dye uptake in a FACS assay (ATP, 63±5%; ATP+100 μM CBX, 51.51±8.4%; ATP+1 mM Prob, 57.7±4.3%) in mouse macrophages. These findings strongly suggest that the high-permeability pore evident after prolonged P2X7R activation does not occur through connexin or pannexin hemichannels in murine macrophages. Another membrane protein may be involved in P2X7R pore formation.

Activation, Permeability, and Inhibition of Astrocytic and Neuronal Large Pore (Hemi)channels

“…Expression of P2X 7 allowed for agonist-induced permeation of three different fluorescent dyes (ethidium, Yo-Pro, and carboxyfluorescein). Although a few reports suggest inherent lack of dye permeability in P2X 7 (42)(43)(44), P2X 7 -mediated large molecule and fluorescent dye permeability (in the absence of Px1) have, in analogy with the present data, been documented in mammalian cell lines (40,76), thus arguing against the proposed requirement for Px1 to act as the large pore channel during P2X 7 -mediated dye uptake. FIGURE 6.…”

Nucleotide signalling during inflammation

“…Interestingly, P2X 7 R is capable of activating the NOD-like-receptor-mediated inflammasome assembly with pro-caspase 1 proteolytic activation and subsequent pro-IL-1 and pro-IL-18 cleavage and release of their biologically active forms 65 . The long C-terminal ‘tail’ of P2X 7 R allows it to undergo a conformational change resulting in the so called ‘permeability transition’; that is, P2X 7 R changes from a cationic channel to a wider pore, allowing transmembrane fluxes of small hydrophilic molecules (including ATP) with a molecular mass of approximately 900 (refs 69, 70). …”

The P2X7 Receptor Channel: Recent Developments and the Use of P2X7 Antagonists in Models of Disease

“…However, there is now growing evidence suggesting that pannexin-1 cannot be the major pore pathway used by P2X7. For example, no defect in ATP-mediated YO-PRO-1 2+ uptake was observed in macrophages from pannexin-1 knockout (KO) mice (Qu et al, 2011), siRNA knockdown of pannexin-1 in mouse macrophages failed to affect P2X7 responses (Alberto et al, 2013), and pharmacological blockade of pannexin-1 did not affect ATP-induced dye uptake in transfected HEK-293 cells or human monocytes (Bhaskaracharya et al, 2014).…”