A C-terminal lysine that controls human P2X4 receptor desensitization

Phosphoinositides regulate P2X4 ATP-gated channels through direct interactions

P2X receptors are ATP-gated nonselective cation channels highly permeable to calcium that contribute to nociception and inflammatory responses. The P2X(4) subtype, upregulated in activated microglia, is thought to play a critical role in the development of tactile allodynia following peripheral nerve injury. Posttranslational regulation of P2X(4) function is crucial to the cellular mechanisms of neuropathic pain, however it remains poorly understood. Here, we show that the phosphoinositides PI(4,5)P(2) (PIP(2)) and PI(3,4,5)P(3) (PIP(3)), products of phosphorylation by wortmannin-sensitive phosphatidylinositol 4-kinases and phosphatidylinositol 3-kinases, can modulate the function of native and recombinant P2X(4) receptor channels. In BV-2 microglial cells, depleting the intracellular levels of PIP(2) and PIP(3) with wortmannin significantly decreased P2X(4) current amplitude and P2X(4)-mediated calcium entry measured in patch clamp recordings and ratiometric ion imaging, respectively. Wortmannin-induced depletion of phosphoinositides in Xenopus oocytes decreased the current amplitude of P2X(4) responses by converting ATP into a partial agonist. It also decreased their recovery from desensitization and affected their kinetics. Injection of phosphoinositides in wortmannin-treated oocytes reversed these effects and application of PIP(2) on excised inside-out macropatches rescued P2X(4) currents from rundown. Moreover, we report the direct interaction of phospholipids with the proximal C-terminal domain of P2X(4) subunit (Cys(360)-Val(375)) using an in vitro binding assay. These results demonstrate novel regulatory roles of the major signaling phosphoinositides PIP(2) and PIP(3) on P2X(4) function through direct channel-lipid interactions.

Molecular shape, architecture, and size of P2X4 receptors determined using fluorescence resonance energy transfer and electron microscopy

P2X receptors are ATP-gated nonselective cation channels with important physiological roles. However, their structures are poorly understood. Here, we analyzed the architecture of P2X receptors using fluorescence resonance energy transfer (FRET) microscopy and direct structure determination using electron microscopy. FRET efficiency measurements indicated that the distance between the C-terminal tails of P2X₄ receptors was 5.6 nm. Single particle analysis of purified P2X₄ receptors was used to determine the three-dimensional structure at a resolution of 21Å_; the orientation of the particle with respect to the membrane was assigned by labeling the intracellular C termini with 1.8-nm gold particles and the carbohydrate-rich ectodomain with lectin. We found that human P2X₄ is a globular torpedo-like molecule with an approximate volume of 270 nm³ and a compact propeller-shaped ectodomain. In this structure, the distance between the centers of the gold particles was 6.1 nm, which closely matches FRET data. Thus, our data provide the first views of the architecture, shape, and size of single P2X receptors, furthering our understanding of this important family of ligand-gated ion channels.

Functional characterization of P2Y1 versus P2X receptors in RBA-2 astrocytes: elucidate the roles of ATP release and protein kinase C

A physiological concentration of extracellular ATP stimulated biphasic Ca(2+) signal, and the Ca(2+) transient was decreased and the Ca(2+) sustain was eliminated immediately after removal of ATP and Ca(2+) in RBA-2 astrocytes. Reintroduction of Ca(2+) induced Ca(2+) sustain. Stimulation of P2Y(1) receptors with 2-methylthioadenosine 5'-diphosphate (2MeSADP) also induced a biphasic Ca(2+) signaling and the Ca(2+) sustains were eliminated using Ca(2+)-free buffer. The 2MeSADP-mediated biphasic Ca(2+) signals were inhibited by phospholipase C (PLC) inhibitor U73122, and completely blocked by P2Y(1) selective antagonist MRS2179 and protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) whereas enhanced by PKC inhibitors GF109203X and Go6979. Inhibition of capacitative Ca(2+) entry (CCE) decreased the Ca(2+)-induced Ca(2+) entry; nevertheless, ATP further enhanced the Ca(2+)-induced Ca(2+) entry in the intracellular Ca(2+) store-emptied and CCE-inhibited cells indicating that ATP stimulated Ca(2+) entry via CCE and ionotropic P2X receptors. Furthermore, the 2MeSADP-induced Ca(2+) sustain was eliminated by apyrase but potentiated by P2X(4) allosteric effector ivermectin (IVM). The agonist ADPbetaS stimulated a lesser P2Y(1)-mediated Ca(2+) signal and caused a two-fold increase in ATP release but that were not affected by IVM whereas inhibited by PMA, PLC inhibitor ET-18-OCH(3) and phospholipase D (PLD) inhibitor D609, and enhanced by removal of intra- or extracellular Ca(2+). Taken together, the P2Y(1)-mediated Ca(2+) sustain was at least in part via P2X receptors activated by the P2Y(1)-induced ATP release, and PKC played a pivotal role in desensitization of P2Y(1) receptors in RBA-2 astrocytes.

Roles of ectodomain and transmembrane regions in ethanol and agonist action in purinergic P2X2 and P2X3 receptors

The present work investigated sites of ethanol action in ATP-gated P2X receptors (P2XRs) using chimeric strategies that exploited the differences in ethanol response between P2X2R (inhibition) and P2X3R (potentiation). We tested ethanol (10-200mM) effects on ATP- and alpha,beta-methylene-ATP (alpha,beta-meATP)-induced currents in wildtype P2X2, P2X3 and chimeric P2X2/P2X3Rs expressed in Xenopus oocytes using two-electrode voltage-clamp (-70mV). Exchanging ectodomain regions of P2X2 and P2X3Rs reversed wildtype ethanol responses. Substituting back portions of the P2X2R ectodomain at TM interfaces in chimeras that contained the P2X3R ectodomain restored wildtype P2X2R-like ethanol response. Point mutations that replaced non-conserved ectodomain residues at TM interfaces of P2X3Rs with homologous P2X2R residues identified positions that reversed the direction (304) or changed the magnitude (53, 55 and 313) of ethanol response. Homologous substitutions in P2X2Rs did not significantly alter wildtype P2X2R-like ethanol responses. These findings suggest that ectodomain segments at TM interfaces play key roles in determining qualitative and quantitative responses to ethanol of P2X2 and P2X3Rs. Studies that substituted TM regions of P2X3R with respective P2X2R TMs indicate that the TM1, but not the TM2, region plays a role in determining the magnitude of ethanol response. Studies with ATP and alpha,beta-meATP support prior indications that TM regions are important in agonist desensitization and suggest that both ectodomain and TM regions play roles in determining agonist potency and selectivity. Overall, these findings are the first to identify potential targets for ethanol in P2X2 and P2X3Rs and should provide insight into the sites of ethanol action in other P2XRs.

Cysteine substitution mutagenesis and the effects of methanethiosulfonate reagents at P2X2 and P2X4 receptors support a core common mode of ATP action at P2X receptors

The agonist binding site of ATP-gated P2X receptors is distinct from other ATP-binding proteins. Mutagenesis on P2X(1) receptors of conserved residues in mammalian P2X receptors has established the paradigm that three lysine residues, as well as FT and NFR motifs, play an important role in mediating ATP action. In this study we have determined whether cysteine substitution mutations of equivalent residues in P2X(2) and P2X(4) receptors have similar effects and if these mutant receptors can be regulated by charged methanethiosulfonate (MTS) compounds. All the mutants (except the P2X(2) K69C and K71C that were expressed, but non-functional) showed a significant decrease in ATP potency, with >300-fold decreases for mutants of the conserved asparagine, arginine, and lysine residues close to the end of the extracellular loop. MTS reagents had no effect at the phenylalanine of the FT motif, in contrast, cysteine mutation of the threonine was sensitive to MTS reagents and suggested a role of this residue in ATP action. The lysine-substituted receptors were sensitive to the charge of the MTS reagent consistent with the importance of positive charge at this position for coordination of the negatively charged phosphate of ATP. At the NFR motif the asparagine and arginine residues were sensitive to MTS reagents, whereas the phenylalanine was either unaffected or showed only a small decrease. These results support a common site of ATP action at P2X receptors and suggest that non-conserved residues also play a regulatory role in agonist action.

A positive inotropic effect of ATP in the human cardiac atrium

We studied contractile effects in isolated electrically driven (1 Hz) atrial preparations from patients undergoing cardiac bypass surgery. ATP concentration dependently (10, 30, and 100 microM) and rapidly decreased force of contraction (negative inotropic effect, NIE) and thereafter more slowly increased force of contraction. The maximum positive inotropic effect (PIE) at 100 microM ATP amounted to 152% of the predrug value (n = 9) and was stable and could be washed out fast and completely. The PIE did not affect time parameters of contraction (time to peak tension and time of relaxation). Moreover, a similar NIE and PIE were noted with adenosine 5'-O-(2-thiotriphosphate) (100 microM). In contrast 2-methyl-thio-ATP did not exert a NIE but only a PIE. In a second set of experiments, preparations were first incubated for 30 min with purinoreceptor antagonists and, in their continuous presence, 100 microM ATP was applied. However, the PIE and NIE of ATP could neither be blocked with suramin (100 and 500 microM), pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (50 microM), nor reactive blue 2 (30, 100, and 500 microM), which are known blockers for subtypes of P(2) receptors, or 1,3-dipropyl-cyclopentvl-xanthine (1 and 10 microM), a subtype (A(1) adenosine) P(1) receptor blocker. Likewise, the inhibitor of phospholipase C (PLC) activity (U-73122) and the inhibitor of adenylate cyclase activity (SQ-022563) (10 microM each) failed to affect the NIE and the PIE of ATP. We tentatively suggest that the PIE of ATP might be mediated via P(2X4)-like receptors. In summary, we describe a novel biphasic effect of ATP on force contraction in the isolated human atrium. It is conceivable that ATP plays a physiological role in the human heart, for instance, after cardiac injury to sustain contractility.

Cysteine substitution mutants give structural insight and identify ATP binding and activation sites at P2X receptors

P2X receptors for extracellular ATP are a distinct family of ligand-gated cation channels involved in physiological processes ranging from synaptic transmission to muscle contraction. Common ATP binding motifs are absent from P2X receptors, and the extent of the agonist binding site is unclear. We used cysteine-scanning mutagenesis, radiolabeled 2-azido ATP binding, and methanethiosulfonate (MTS) compounds to identify amino acid residues involved in ATP binding and gating of the human P2X1 receptor. The pattern of MTSEA [(2-aminoethyl)methanethiosulfonate hydrobromide] biotinylation was also used to determine the accessibility of substituted cysteine residues and whether this changed on addition of ATP. Analysis of cysteine-substituted mutants of the last 44 amino acid residues (S286-I329) in the extracellular loop before the second transmembrane segment showed that N290, F291, R292, and K309 mutants had reduced ATP potency and 2-azido ATP binding. MTS reagents produced additional shifts in ATP potency at these residues, suggesting that they are directly involved in ATP binding; the effects were dependent on the charge of the MTS reagent at K309C; one explanation for this is that K309 interacts directly with the negatively charged phosphate of ATP. The remainder of the cysteine substitutions had little or no effect on ATP potency. However, at the mutants D316C, G321C, A323C, and I328C, MTS reagents did not change ATP potency but modified agonist-evoked responses, suggesting that this region may contribute to the gating of the channel.

Contribution of P2X1 receptor intracellular basic residues to channel properties

The intracellular amino and carboxy termini of P2X receptors have been shown to contribute to the regulation of ATP evoked currents. In this study we produced, and expressed in Xenopus oocytes, individual alanine point mutants of positively charged amino acids (eight lysine, seven arginine and one histidine) in the intracellular domains of the human P2X1 receptor. The majority of these mutations had no effect on the amplitude, time-course or rectification of ATP evoked currents. In contrast the mutant K367A was expressed at normal levels at the cell surface however ATP evoked currents were reduced by >99% and desensitised more rapidly demonstrating a role of K367 in channel regulation. This is similar to that previously described for T18A mutant channels. Co-expression of T18A and K367A mutant P2X1 receptors produced larger ATP evoked responses than either mutant alone and suggests that these amino and carboxy terminal regions interact to regulate channel function.