ATP-induced P2X Receptor-Dependent Large Pore Formation: How Much Do We Know?

P2X receptors are a group of homo/hetero-trimeric membrane protein complexes with an integral ion channel that opens upon extracellular adenosine triphosphate (ATP) binding (North, 2002; Khakh and North, 2006). There are seven P2X subunits (P2X1-P2X7), all having a membrane topology of cytosolic N-and C-termini, and two transmembrane segments (TM1 and TM2) connected by a large extracellular domain (Figure 1A; Jiang L.-H. et al., 2013). During application of ATP for a few seconds, P2X receptors function as classical ligand-gated ion channels selectively permeable to small physiological cations such as Ca 2+ , Na + , and K + , with the exception of the human P2X5 receptor which exhibits significant Cl − permeability (Bo et al., 2003). Site-directed mutagenesis and functional studies of mammalian P2X receptors, in addition to the determination of the crystal structures of zebrafish P2X4 receptors in the apo, closed state and ATP-bound, open state, have defined the structural basis for ATP binding, ion permeation and channel gating (Kawate et al. Three ATP-binding pockets are located at the subunit interfaces (Figure 1B), each consisting of highly conserved residues from two adjacent subunits. Occupation of these sites by ATP or its synthetic analog agonists induces conformational changes of the extracellular domain which open the ion-permeating pathway formed by three TM2s (Figures 1C,D). The narrowest part of the ion-permeating pathway or the physical gate is provided by A347 and L351 in the crystal structures of zebrafish P2X4 receptor (Hattori and Gouaux, 2012) or the corresponding residues S342 and L346 in the structural models of rat and human P2X7 receptors (Figure 1D) (Bradley et al., 2011; Browne et al., 2013; Jiang L.-H. et al., 2013). It is well known that extended application of ATP to activate the P2X receptors for tens of seconds or minutes induces a remarkable increase in membrane permeability to large molecules of up to 900 Daltons, a phenotype often referred to as formation of large pores. This was originally documented in immune cells about three decades ago; ATP permeablized cell membranes to nucleotides (Cockcroft and Gomperts, 1979) and the cationic fluorescent dye ethidium in mast cells (Gomperts, 1983), or the anionic organic dyes lucifer yellow and carboxyfluoresceine in mast cells and macrophages (Bennett et al., 1981; Steinberg and Silverstein, 1987). These immune cells express the formerly named P2Z receptor, which is now known as P2X7 receptor, and heterologous expression of P2X7 receptors conferred ATP-induced large pore formation …