Single channel properties of P2X2 purinoceptors

Atomic Force Microscopy Imaging Demonstrates that P2X2 Receptors Are Trimers but That P2X6 Receptor Subunits Do Not Oligomerize

P2X receptors are cation-selective channels activated by extracellular ATP. The architecture of these receptors is still not completely clear. Here we have addressed this issue by both chemical cross-linking and direct imaging of individual receptors by atomic force microscopy (AFM). Cross-linking of the P2X(2) receptor produced higher order adducts, consistent with the presence of trimers. The mean molecular volume of the receptor determined by AFM (409 nm(3)) also points to a trimeric structure. P2X(2) receptors bearing His(6) epitope tags were incubated with anti-His(6) antibodies, and the resultant complexes were imaged by AFM. For receptors with two bound antibodies, the mean angle between the antibodies was 123 degrees , again indicating that the receptor is a trimer. In contrast, cross-linking of the P2X(6) receptor did not produce higher order adducts, and the mean molecular volume of the receptor was 145 nm(3). We conclude that P2X(2) receptors are trimers, whereas the P2X(6) receptor subunits do not form stable oligomers.

Factors affecting habituation of PC12 cells to ATP

Extracellular ATP triggers catecholamine secretion from PC12 cells by activating ionotropic purine receptors. Repeated stimulation by ATP leads to habituation of the secretory response. In this paper, we use amperometric detection to monitor the habituation of PC12 cells to multiple stimulations of ATP or its agonist. Cells habituate to 30 microm ATP slower than they do to 300 or 600 microm ATP. Modifying external Mg2+ affects the response of cells to 30 microm ATP, but does not affect habituation, suggesting that habituation does not necessarily correspond to either stimulus intensity or cellular response. Mg2+ affects the initial response of PC12 cells to 2MeSATP in a manner similar to ATP. Increasing external [Mg2+] to 3.0 mm, however, eliminates habituation to 2MeSATP. This habituation can be partially restored by costimulation with 100 microm UTP. Background application of UTP increases habituation to both ATP and 2MeSATP. This suggests that ATP-sensitive metabotropic (P2Y) receptors play a role in the habituation process. Finally, although Ca2+ influx through voltage-operated calcium channels does not appear to contribute to secretion during ATP stimulation, blocking these channels with nicardipine increases habituation. This suggests a role for voltage-operated calcium channels in the habituation process.

Gain and loss of channel function by alanine substitutions in the transmembrane segments of the rat ATP-gated P2X2 receptor

ATP opens ionotropic P2X channels through a process that is poorly understood. We made an array of mutant rat P2X2 channels containing unique alanine substitutions in the transmembrane segments with the goal of identifying possible secondary structure and mapping gating domains in the pore. Alteration of channel function was measured as a change in ATP potency, 2'-3'-O-(4-benzoylbenzoyl)ATP (BzATP) efficacy, and deactivation kinetics. Four mutants (V45A, Y47A, V51A, and D349A) failed to respond to ATP. Seven (H33A, Q37A, I40A, L41A, Y43A, F44A, and I50A) of 22 mutations in the first transmembrane segment (TM1) produced channels with altered potencies, and two mutants (Y43A and F44A) were active in the absence of agonist. The pattern of hits was consistent with a helical secondary structure. In contrast, nine (I328A, P329A, N333A, L338A, T339A, S340A, G342A, G344A, and S345A) of 24 mutations in the second transmembrane segment (TM2) resulted in a change in potency, but no regular pattern of impact was apparent. Many of the same mutations that altered ATP potency also changed the relative efficacy of the partial agonist BzATP. Together, these data suggest that both TM1 and TM2 participate in the conformational changes that occur during receptor activation and help to define domains involved in conformational switching within or near the pore.

Trimeric Architecture of Homomeric P2X2 and Heteromeric P2X1+2 Receptor Subtypes

Of the three major classes of ligand-gated ion channels, nicotinic receptors and ionotropic glutamate receptors are known to be organized as pentamers and tetramers, respectively. The architecture of the third class, P2X receptors, is under debate, although evidence for a trimeric assembly is accumulating. Here we provide biochemical evidence that in addition to the rapidly desensitising P2X1 and P2X3 receptors, the slowly desensitising subtypes P2X2, P2X4, and P2X5 are trimers of identical subunits. Similar (heteromeric) P2X subunits also formed trimers, as shown for co-expressed P2X1 and P2X2 subunits, which assembled efficiently to a P2X1+2 receptor that was exported to the plasma membrane. In contrast, P2X6 subunits, which are incapable of forming functional homomeric channels in Xenopus oocytes, were retained in the ER as apparent tetramers and high molecular mass aggregates. Altogether, we conclude from these data that a trimeric architecture is the structural hallmark of functional homomeric and heteromeric P2X receptors.

Extracellular ATP increases cation fluxes in human erythrocytes by activation of the P2X7 receptor

Canine erythrocytes are known to undergo a reversible increase in cation permeability when incubated with extracellular ATP. We have examined the expression and function of P2X receptors on human erythrocytes using confocal microscopy and a panel of anti-P2X(1-7) antibodies and have measured monovalent cation fluxes in the presence of various nucleotide agonists. Human erythrocytes expressed P2X7 receptors on all cells examined from eight of eight subjects, as well as P2X2 at a far lower staining intensity in six of eight subjects. ATP stimulated the efflux of 86Rb+ (K+) from human erythrocytes in a dose-dependent fashion with an EC50 of approximately 95 microM. Other nucleotides also induced an efflux of 86Rb+ from erythrocytes with an order of agonist potency of 2'- and 3'-O(4-benzoylbenzoyl) ATP (BzATP) > ATP > 2-methylthio-ATP (2MeSATP) > adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS), whereas ADP or UTP had no effect. ATP-induced efflux of 86Rb+ from erythrocytes was inhibited by extracellular Na+ and oxidized ATP, as well as by KN-62, an antagonist specific for the human P2X7 receptor. When erythrocytes were incubated in isotonic KCl medium, the addition of ATP stimulated an 86Rb+ influx approximately equal in magnitude to ATP-stimulated 86Rb+ efflux from the same cells. BzATP also stimulated the influx of 22Na+ into erythrocytes incubated in isotonic NaCl medium. Both ATP-induced efflux and influx of 86Rb+ and 22Na+ were impaired in erythrocytes from subjects who had inherited loss-of-function polymorphisms in the P2X7 receptor. These results suggest that the reversible permeabilization of erythrocytes by extracellular ATP is mediated by the P2X7 receptor.

Properties of native P2X receptors in large multipolar neurons dissociated from rat hypothalamic arcuate nucleus

ATP, the ligand of P2X receptors, is a candidate of neurotransmitter or co-transmitter in the peripheral and the central nervous systems. Anatomical studies have revealed the wide distribution of P2X receptors in the brain. So far, P2X-mediated small synaptic responses have been recorded in some brain regions. To determine the physiological significance of postsynaptic ATP receptors in the brain, we have investigated the P2X responses in rat dissociated hypothalamic arcuate neurons by using the patch-clamp technique. ATP evoked inward currents in a concentration-dependent manner (EC(50)=42 microM) at a holding potential of -70 mV. The current-voltage relationship showed a marked inward rectification starting around -10 mV. Although neither 300 microM alphabeta-methylene-ATP nor 300 microM betagamma-methylene-ATP induced any currents, 100 microM ATPgammaS and 100 microM 2-methylthio-ATP evoked inward currents of which amplitude was about 60% of the control currents evoked by 100 microM ATP. PPADS, one of P2 receptor antagonists, inhibited the ATP-evoked currents in a time- and a concentration-dependent manners (IC(50)=19 microM at 2 min). Permeant Ca(2+) inhibited the ATP-evoked currents in the range of millimolars (IC(50)=7 mM); however, Cd(2+) (1-300 microM), a broad cation channel blocker, facilitated the currents with slow off-response. Zn(2+) in the range of 1-100 microM facilitated the currents whereas Zn(2+) at the concentrations over 100 microM inhibited the currents. These observations suggest that functional P2X receptors are expressed in the hypothalamic arcuate nucleus. The most likely subunit combinations of the P2X receptors are P2X(2)-homomultimer and P2X(2)/P2X(6)-heteromultimer.

Density-dependent changes of the pore properties of the P2X2 receptor channel

Ligand-gated ion channels underlie and play important roles in synaptic transmission, and it is generally accepted that the ion channel pores have a rigid structure that enables strict regulation of ion permeation. One exception is the P2X ATP-gated channel. After application of ATP, the ion selectivity of the P2X2 channel time-dependently changes, i.e. permeability to large cations gradually increases, and there is significant cell-to-cell variation in the intensity of inward rectification. Here we show P2X2 channel properties are correlated with the expression level: increasing P2X2 expression level in oocytes increases permeability to large cations, decreases inward rectification and increases ligand sensitivity. We also observed that the inward rectification changed in a dose-dependent manner, i.e. when low concentration of ATP was applied to an oocyte with a high expression level, the intensity of inward rectification of the evoked current was weak. Taken together, these results show that the pore properties of P2X2 channel are not static but change dynamically depending on the open channel density. Furthermore, we identified by mutagenesis study that Ile328 located at the outer mouth of the pore is critical for the density-dependent changes of P2X2. Our findings suggest synaptic transmission can be modulated by the local density-dependent changes of channel properties caused, for example, by the presence of clustering molecules.

[3H]A-317491, a Novel High-Affinity Non-Nucleotide Antagonist That Specifically Labels Human P2X2/3and P2X3Receptors

A-317491 is a potent and selective antagonist of P2X3 and P2X(2/3) receptors. In the present studies, the ability of [3H]A-317491 to label recombinant human P2X(2/3) and P2X(3) receptors was characterized. Using membranes prepared from 1321N1 cells expressing P2X(2/3) receptors, [3H]A-317491 specifically labeled high-affinity (Kd = 0.9 nM) recognition sites. High-affinity [3H]A-317491 binding was not detected in membrane preparations from native 1321N1 cells or cells expressing homomeric P2X1, P2X2, or P2X3 receptors. Specific [3H]A-317491 P2X3 receptors could only be reliably detected following treatment of intact P2X3 receptor-expressing cells with apyrase (1 U/ml) both before and during membrane preparation. Under these conditions, [3H]A-317491 also labeled high-affinity (Kd = 9 nM) binding sites. Lower affinity binding components (Kd values of 87-790 nM) were detected in both assays using higher ligand concentrations that likely represent nonfunctional recognition sites. [3H]A-317491 binding to both P2X(2/3) and P2X3 receptors was reversible, and ligand kinetic studies provided similar estimates of the high-affinity binding constants. Potent P2X3 receptor agonists 2-methylthio-ATP, 2,3-O-(4-benzoylbenzoyl)-ATP, and alpha,beta-methylene adenosine triphosphate also potently inhibited specific [3H]A-317491 binding to both P2X(2/3) and P2X3 receptors. The pharmacological profile for P2X receptor antagonists to inhibit [3H]A-317491 binding to P2X(2/3) and P2X3 receptors was highly correlated (r = 0.98, P < 0.05), and a similar rank order of potency was observed for blockade of P2X(2/3) receptor-mediated calcium influx. These data demonstrate that [3H]A-317491 is the first useful radioligand for the specific labeling of P2X3-containing channels.

Agonist-dependence of recovery from desensitization of P2X(3) receptors provides a novel and sensitive approach for their rapid up or downregulation

1. Fast-desensitizing P2X(3) receptors of nociceptive dorsol root ganglion (DRG) neurons are thought to mediate pain sensation. Since P2X(3) receptor efficiency is powerfully modulated by desensitization, its underlying properties were studied with patch-clamp recording. 2. On rat cultured DRG neurons, 2 s application of ATP (EC(50)=1.52 microm), ADP (EC(50)=1.1 microm) or alpha,beta-meATP (EC(50)=1.78 microm) produced similar inward currents that fully desensitized, at the same rate, back to baseline. Recovery from desensitization was much slower after ATP and ADP than after alpha,beta-meATP and, in all cases, it had sigmoidal time course. 3. By alternating the application of ATP and alpha,beta-meATP, we observed complete cross-desensitization indicating that these agonists activated the same receptors. This notion was confirmed by the similar antagonism induced by 2', 3'-O-(2,4,6,trinitrophenyl)-adenosine triphosphate (TNP-ATP). 4. Recovery from desensitization elicited by ATP was unexpectedly shaped by transient application of alpha,beta-methylene-adenosine triphosphate (alpha,beta-meATP), and vice versa. Thus, short-lasting, full desensitization produced by alpha,beta-meATP protected receptors from long-lasting desensitization induced by subsequent ATP applications. ATP and ADP had similar properties of recovery from desensitization. 5. Low nm concentrations of alpha,beta-meATP (unable to evoke membrane currents) could speed up recovery from ATP-induced desensitization, while low nm concentrations of ATP enhanced it. Ambient ATP levels were found to be in the pm range (52+/-3 pm). 6. The phenomenon of cross-desensitization and protection was reproduced by rP2X(3) receptors expressed by rat osteoblastic cell 17/2.8 or human embryonic kidney cell 293 cells, indicating P2X(3) receptor specificity. 7. It is suggested that transient application of an agonist that generates rapid recovery from desensitization, is a novel, powerful tool to modulate P2X(3) receptor responsiveness to the natural agonist ATP.

Subunit arrangement in P2X receptors

ATP-gated ionotropic receptors (P2X receptors) are distributed widely in the nervous system. For example, a hetero-oligomeric receptor containing both P2X2 and P2X3 subunits is involved in primary afferent sensation. Each subunit has two membrane-spanning domains. We have used disulfide bond formation between engineered cysteines to demonstrate close proximity between the outer ends of the first transmembrane domain of one subunit and the second transmembrane domain of another. After expression in HEK 293 cells of such modified P2X2 or P2X4 subunits, the disulfide bond formation is evident because an ATP-evoked channel opening requires previous reduction with dithiothreitol. In the hetero-oligomeric P2X2/3 receptor the coexpression of doubly substituted subunits with wild-type partners allows us to deduce that the hetero-oligomeric channel contains adjacent P2X3 subunits but does not contain adjacent P2X2 subunits. The results suggest a "head-to-tail" subunit arrangement in the quaternary structure of P2X receptors and show that a trimeric P2X2/3 receptor would have the composition P2X2(P2X3)2.

Subunit arrangement in P2X receptors

ATP-gated ionotropic receptors (P2X receptors) are distributed widely in the nervous system. For example, a hetero-oligomeric receptor containing both P2X2 and P2X3 subunits is involved in primary afferent sensation. Each subunit has two membrane-spanning domains. We have used disulfide bond formation between engineered cysteines to demonstrate close proximity between the outer ends of the first transmembrane domain of one subunit and the second transmembrane domain of another. After expression in HEK 293 cells of such modified P2X2 or P2X4 subunits, the disulfide bond formation is evident because an ATP-evoked channel opening requires previous reduction with dithiothreitol. In the hetero-oligomeric P2X2/3 receptor the coexpression of doubly substituted subunits with wild-type partners allows us to deduce that the hetero-oligomeric channel contains adjacent P2X3 subunits but does not contain adjacent P2X2 subunits. The results suggest a "head-to-tail" subunit arrangement in the quaternary structure of P2X receptors and show that a trimeric P2X2/3 receptor would have the composition P2X2(P2X3)2.

Chicken DT40 cells stably transfected with the rat P2X7 receptor ion channel: a system suitable for the study of purine receptor-mediated cell death

We have generated and characterised a clone of chicken DT40 lymphocytes stably transfected with the rat P2X(7) receptor (rP2X(7)). Successful transfection was confirmed by Western blotting. Under voltage clamp, P2X(7)-expressing cells responded to ATP and dibenzoyl-ATP (Bz-ATP) (a more potent P2X(7) receptor agonist) with a rapidly activating and sustained inward current. The EC(50) values for these agonists were 305 and 15 microM, respectively. Bz-ATP evoked Ca(2+) and Mn(2+) influx into transfected cells as determined by Fura-2 spectrofluorimetry. Responses to Bz-ATP were inhibited by pre-treatment of cells with oxidised ATP. Treatment of cells with Bz-ATP for up to 24hr produced time- and concentration-dependent cell death. This was associated with an increase in caspase-3-like activity, exposure of phosphatidylserine on the outside of cell membrane and DNA cleavage, indicating death by apoptosis. Pre-treatment with Z-VAD-fmk, a pan-caspase inhibitor, reduced the DNA fragmentation and phosphatidylserine externalisation, but did not affect overall rates of cell death at 24hr, implicating caspase-independent mechanisms. The properties of rP2X(7) receptors expressed in DT40 cells are similar to those described for other expression systems. Because DT40 cells lack functionally detectable endogenous P2 receptors and are highly amenable to genetic manipulation, stably transfected DT40 cells provide a novel and potentially useful model system in which to investigate the intracellular signal transduction pathways associated with P2X(7) receptor stimulation, in particular those involved in induction of cell death.

Intracellular calcium measurements as a method in studies on activity of purinergic P2X receptor channels

Extracellular nucleotide-activated purinergic receptors (P2XRs) are a family of cation-permeable channels that conduct small cations, including Ca2+, leading to the depolarization of cells and subsequent stimulation of voltage-gated Ca2+ influx in excitable cells. Here, we studied the spatiotemporal characteristics of intracellular Ca2+ signaling and its dependence on current signaling in excitable mouse immortalized gonadotropin-releasing hormone-secreting cells (GT1) and nonexcitable human embryonic kidney cells (HEK-293) cells expressing wild-type and chimeric P2XRs. In both cell types, P2XR generated depolarizing currents during the sustained ATP stimulation, which desensitized in order (from rapidly desensitizing to nondesensitizing): P2X3R > P2X2b + X4R > P2X2bR > P2X2a + X4R > P2X4R > P2X2aR > P2X7R. HEK-293 cells were not suitable for studies on P2XR-mediated Ca2+ influx because of the coactivation of endogenously expressed Ca2+-mobilizing purinergic P2Y receptors. However, when expressed in GT1 cells, all wild-type and chimeric P2XRs responded to agonist binding with global Ca2+ signals, which desensitized in the same order as current signals but in a significantly slower manner. The global distribution of Ca2+ signals was present independently of the rate of current desensitization. The temporal characteristics of Ca2+ signals were not affected by voltage-gated Ca2+ influx and removal of extracellular sodium. Ca2+ signals reflected well the receptor-specific EC50 values for ATP and the extracellular Zn2+ and pH sensitivities of P2XRs. These results indicate that intracellular Ca2+ measurements are useful for characterizing the pharmacological properties and messenger functions of P2XRs, as well as the kinetics of channel activity, when the host cells do not express other members of purinergic receptors.

Contributions of P2X3 homomeric and heteromeric channels to acute and chronic pain

ATP acts as a fast neurotransmitter by activating a family of ligand-gated ion channels, the P2X receptors. Functional homomeric (P2X(3)) and heteromeric (P2X(2/3)) receptors are highly localised on primary sensory afferent neurons that transmit nociceptive sensory information. Activation of these P2X(3)containing channels may provide a specific mechanism whereby ATP, released via synaptic transmission or by cellular injury, elicits pain. The physiological relevance of the pro-nociceptive actions of ATP is supported by data demonstrating that the exogenous peripheral or spinal administration of ATP and other P2X receptor agonists elicits nociceptive behaviour and increases sensitivity to noxious stimuli in both humans and laboratory animals. The nociceptive effects of P2X receptor agonists are also enhanced in the presence of inflammatory mediators. Both permanent (P2X(3) gene knockout) and transient (P2X(3) antisense) receptor gene disruption studies in laboratory rodents have provided hypoalgesic phenotypes, further supporting a role for P2X(3) subunits in contributing to the expression of pain. More recently, the acute systemic administration of a highly selective non-nucleotide P2X(3) antagonist, A317491, has been shown to fully block specific types of chronic inflammatory and neuropathic pain in animal models in the absence of cardiovascular and CNS side effects associated with other analgesic compounds. Therefore, both genetic and pharmacological approaches have provided converging evidence that activation of P2X(3)-containing channels is an important mediator of persistent nociceptive signalling. The available data also indicate potential discrete roles for homomeric P2X(3) and heteromeric P2X(2/3) receptor activation in acute and chronic pain.

Modulation of ATP-induced currents by zinc in acutely isolated hypothalamic neurons of the rat

1. Whole-cell patch-clamp and fast perfusion were used to study the effects of zinc on adenosine 5'-triphosphate (ATP)-induced responses of histaminergic neurons. 2. At 10-30 micro M ATP, Zn(2+) had biphasic effects on ATP responses. Zn(2+) at 3-100 micro M increased the ATP-induced currents, but inhibited them at higher concentrations. 3. At 300 micro M ATP, Zn(2+) predominantly but incompletely inhibited the currents. 4. At 5 and 50 micro M, Zn(2+) shifted to the left the concentration-response curve for ATP-induced currents, without changing the maximal response. At 1 mM, Zn(2+) inhibited ATP-induced currents in a noncompetitive way, reducing the maximal response by 58%. .Zn(2+) increased the decay time of ATP-evoked currents nine fold with an EC(50) of 63 micro M. Upon removal of high concentrations of Zn(2+), there was a rapid increase of the current followed by a slow decline towards the response amplitude seen with ATP alone. The appearance of a tail current is consistent with a Zn(2+)-induced increase of ATP affinity and an inhibition of its efficacy. 6. Thus, Zn(2+) acts as a bidirectional modulator of ATP receptor channels in tuberomamillary neurons, which possess functional P2X(2) receptors. The data are consistent with the existence of two distinct modulatory sites on the P2X receptor, which can be occupied by Zn(2+). 7. Our data suggest that zinc-induced potentiation of ATP-mediated currents is caused by the slowing of ATP dissociation from the receptor, while inhibition of ATP-induced currents is related to the suppression of ATP receptor gating.

Bimodal action of protons on ATP currents of rat PC12 cells

The mode of action of extracellular protons on ATP-gated P2X2 receptors remains controversial as either enhancement or depression of ATP-mediated currents has been reported. By investigating, at different pH, the electrophysiological effect of ATP on P2X2 receptors and complementing it with receptor modelling, the present study suggests a unified mechanism for both potentiation and inactivation of ATP receptors by protons. Our experiments on patch-clamped PC12 cells showed that, on the same cell, mild acidification potentiated currents induced by low ATP concentrations (<0.1 mM) and attenuated responses to high ATP concentrations (>1 mM) with emergence of current fading and rebound. To clarify the nature of the ATP/H+ interaction, we used the Ding and Sachs's "loop" receptor model which best describes the behavior of such receptors with two open states linked via one inactivated state. No effects by protons could be ascribed to H+-mediated open channel block. However, by assuming that protons facilitated binding of ATP to resting as well as open receptors, the model could closely replicate H+-induced potentiation of currents evoked by low ATP doses plus fading and rebound induced by high ATP doses. The latter phenomenon was due to receptor transition to the inactive state. The present data suggest that the high concentration of protons released with ATP (and catecholamines) from secretory vesicles may allow a dual action of H+ on P2X2 receptors. This condition might also occur on P2X2 receptors of central neurons exposed to low pH during ischemia.

Pharmacological and biophysical properties of the human P2X5 receptor

We constructed a full-length human P2X5 purinoceptor cDNA by incorporating a sequence corresponding to exon 10, which is missing in cDNAs cloned previously from human tissues. We studied the functional properties by patch-clamp recording and fluorescence imaging after expression in human embryonic kidney 293 cells. ATP (1-100 microM; half-maximal current at 4 microM) elicited inward currents at -60 mV; these persisted during brief (2 s) applications but declined during longer applications. The peak current was dependent on the holding potential and showed little rectification; however, both the desensitization during the application and the decline in the current when ATP was washed out were slower at +30 mV than at -60 mV. 2',3'-O-(4-Benzoyl)-benzoyl-ATP and alphabeta-methylene-ATP mimicked the action of ATP (half-maximal concentrations 6 and 161 microM, respectively). The currents were inhibited by suramin, pyridoxal-5-phosphate-6-azo-2',4'-disulfonic acid and Brilliant Blue G, with half-maximal inhibition at 3, 0.2, and 0.5 microM, respectively; 2',3'-O-(2',4',6'-trinitrophenol)-ATP (1 microM) was ineffective. Removing divalent cations did not significantly alter ATP concentration-response curves. Reversal potential measurements showed that the human P2X5 receptor was permeable to calcium (PCa/PNa = 1.5) and N-methyl-d-glucamine (NMDG) (PNMDG/PNa = 0.4); it was also permeable to chloride (PCl/PNa = 0.5) but not gluconate (Pgluc/PNa = 0.01) ions. The permeability to NMDG developed as quickly as the channel opened, in contrast to the P2X7 receptor where the NMDG permeability develops over several seconds. Cells expressing human P2X5 receptors also rapidly accumulated the propidium dye YO-PRO-1 in response to ATP.

Expression and function of P2X purinoceptors in rat histaminergic neurons

(1) The pharmacology of ATP responses and the expression pattern of seven known subunits of the P2X receptor were investigated in individual histaminergic neurons of the tuberomamillary nucleus (TM). (2) ATP (3-1000 micro M) evoked fast non-desensitizing inward currents in TM neurons. 2-methylthioATP (2MeSATP) displayed the same efficacy but a lower potency, EC(50)s 84 micro M versus 48 micro M, when compared with ATP. Adenosine-diphosphate (ADP), uridine-triphosphate (UTP) and alpha beta methylene-ATP (alphabeta-meATP) were inactive. (3) ATP-mediated whole cell currents were potentiated by acidification of the recording solution (pH 7.5 and 6.6 were compared). (4) Single-cell RT-PCR (scRT-PCR) analysis revealed that the P2X(2) receptor is expressed in all PCR-positive neurons. Each of the P2X(1), P2X(3), P2X(4), P2X(5) and P2X(6) mRNAs were detected in less than 35% of the cells. (5) Suramin antagonized ATP responses with an IC(50) of 4.2 micro M and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 1 micro M) reduced ATP responses to 43% of control, when antagonists were pre-applied 90s before the agonist. Cibacron blue (3 micro M) given together with ATP potentiated control responses by 67%, but inhibited it to 10% after pre-application. (6) 2',3'-O-(2,4,6-Trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) antagonized ATP responses with an IC(50) of 7 micro M. (7) Pharmacological properties of ATP responses together with scRT-PCR data suggest that P2X(2) is the major purinoceptor on the soma of TM neurons, however the presence of heteromeric P2X(2/5) receptors in some neurons cannot be excluded.

P2 receptor expression profiles in human vascular smooth muscle and endothelial cells

P2 receptors mediate the actions of the extracellular nucleotides ATP, ADP, UTP, and UDP, regulating several physiologic responses including cardiac function, vascular tone, smooth muscle cell (SMC) proliferation, platelet aggregation, and the release of endothelial factors. P2 receptor characterization has been hampered by the lack of selective antagonists. The aim of the current study was to investigate the mRNA and protein expression of P2X and P2Y receptors in human SMC and in endothelial cells (EC). Smooth muscle cells were obtained from human mammary artery and EC from human umbilical vein. Using real-time PCR, the authors established quantitative mRNA assays. Protein expression was studied using Western blotting with recently developed antibodies. The P2X1 receptor was highly specific for human SMC, while the P2X4 was the highest expressed receptor in EC. The P2Y2 receptor was present in both SMC and EC. UTP-mediated effects in these cells are likely to be mediated by P2Y2 and not P2Y4 receptors since the latter had considerably lower expression. The P2Y6 receptor was expressed in both SMC and EC. The P2Y1 and surprisingly the P2Y11 receptors were the most abundantly expressed P2Y receptors in the endothelium. Overall, Western blotting confirmed the mRNA findings in most aspects, and most interestingly, indicated oligomerization of the P2Y1 receptor that may be important for its function. In conclusion, P2X1, P2Y2, and P2Y6 are the most expressed P2 receptors in SMC and are thus probably mediating the contractile and mitogenic actions of extracellular nucleotides. The P2X4, P2Y11, P2Y1, and P2Y2 are the most expressed P2 receptors in EC, and are most likely mediating release of nitric oxide, endothelium-dependent hyperpolarizing factor (EDHF), and t-PA induced by extracellular nucleotides. These findings will help to direct future cardiovascular drug development against the large P2 receptor family.

Evidence for non-independent gating of P2X2 receptors expressed in Xenopus oocytes

BACKGROUND

P2X2 receptor is an ATP-activated ion channel which is widely expressed in the nervous system, and mediates synaptic transmission.

RESULTS

We recorded currents of P2X2 receptors expressed in Xenopus oocytes from outside-out patches and have found that currents recorded from patches containing a single or multiple P2X2 channels differ in a manner suggesting positive cooperativity. First, the currents from multichannel patches exhibit simultaneous transitions more frequently than predicted from the activity of independent channels. Second, the mean open lifetime at the current level of a single channel in a multichannel burst is about six times longer than the open time of currents from single channel patches, a trend opposite to what is expected of independent channels. These results indicate that the channels have positive cooperativity and that the longer opening is due to a slower closing rate. Third, from kinetic analysis the likelihood of the cooperative model is significantly larger than that of the independent model. Fourth, the open channel noise of currents from patches containing multiple channels is less than half that from a single channel, which is consistent with the channel properties being different when they are active in groups.

CONCLUSION

Taken together, our results suggest that P2X2 receptors are non-independent, but interact with positive cooperativity.

Molecular physiology of P2X receptors

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40-50% identical in amino acid sequence. Each subunit has two transmembrane domains, separated by an extracellular domain (approximately 280 amino acids). Channels form as multimers of several subunits. Homomeric P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels have been most fully characterized following heterologous expression. Some agonists (e.g., alphabeta-methylene ATP) and antagonists [e.g., 2',3'-O-(2,4,6-trinitrophenyl)-ATP] are strongly selective for receptors containing P2X1 and P2X3 subunits. All P2X receptors are permeable to small monovalent cations; some have significant calcium or anion permeability. In many cells, activation of homomeric P2X7 receptors induces a permeability increase to larger organic cations including some fluorescent dyes and also signals to the cytoskeleton; these changes probably involve additional interacting proteins. P2X receptors are abundantly distributed, and functional responses are seen in neurons, glia, epithelia, endothelia, bone, muscle, and hemopoietic tissues. The molecular composition of native receptors is becoming understood, and some cells express more than one type of P2X receptor. On smooth muscles, P2X receptors respond to ATP released from sympathetic motor nerves (e.g., in ejaculation). On sensory nerves, they are involved in the initiation of afferent signals in several viscera (e.g., bladder, intestine) and play a key role in sensing tissue-damaging and inflammatory stimuli. Paracrine roles for ATP signaling through P2X receptors are likely in neurohypophysis, ducted glands, airway epithelia, kidney, bone, and hemopoietic tissues. In the last case, P2X7 receptor activation stimulates cytokine release by engaging intracellular signaling pathways.

Control of P2X(2) channel permeability by the cytosolic domain

ATP-gated P2X channels are the simplest of the three families of transmitter-gated ion channels. Some P2X channels display a time- and activation-dependent change in permeability as they undergo the transition from the relatively Na(+)-selective I(1) state to the I(2) state, which is also permeable to organic cations. We report that the previously reported permeability change of rat P2X(2) (rP2X(2)) channels does not occur at mouse P2X(2) (mP2X(2)) channels expressed in oocytes. Domain swaps, species chimeras, and point mutations were employed to determine that two specific amino acid residues in the cytosolic tail domain govern this difference in behavior between the two orthologous channels. The change in pore diameter was characterized using reversal potential measurements and excluded field theory for several organic ions; both rP2X(2) and mP2X(2) channels have a pore diameter of approximately 11 A in the I(1) state, but the transition to the I(2) state increases the rP2X(2) diameter by at least 3 A. The I(1) to I(2) transition occurs with a rate constant of approximately 0.5 s(-1). The data focus attention on specific residues of P2X(2) channel cytoplasmic domains as determinants of permeation in a state-specific manner.

The role of histidine residues in modulation of the rat P2X(2) purinoceptor by zinc and pH

P2X(2) receptor currents are potentiated by acidic pH and zinc. To identify residues necessary for proton and zinc modulation, alanines were singly substituted for each of the nine histidines in the extracellular domain of the rat P2X(2) receptor. Wild-type and mutant receptors were expressed in Xenopus oocytes and analysed with two-electrode voltage clamp. All mutations caused less than a 2-fold change in the EC(50) of the ATP concentration-response relation. Decreasing the extracellular pH from 7.5 to 6.5 potentiated the responses to 10 microM ATP of wild-type P2X(2) and eight mutant receptors more than 4-fold, but the response of the mutant receptor H319A was potentiated only 1.4-fold. The H319A mutation greatly attenuated the maximal potentiation that could be produced by a drop in pH, shifted the pK(a) (-log of dissociation constant) of the potentiation to a more basic pH as compared with P2X(2) and revealed a substantial pH-dependent decrease in the maximum response with a pK(a) near 6.0. Substituting a lysine for H319 reduced the EC(50) for ATP 40-fold. Zinc (20 microM) potentiated the responses to 10 microM ATP of wild-type P2X(2) and seven histidine mutants by approximately 8-fold but had virtually no effect on the responses of two mutants, H120A and H213A. Neither H120A nor H213A removed the voltage-independent inhibition caused by high concentrations of zinc. The observation that different mutations selectively eliminated pH or zinc potentiation implies that there are two independent sites of action, even though the mechanisms of pH and zinc potentiation appear similar.

Heteromultimeric P2X(1/2) receptors show a novel sensitivity to extracellular pH

Rat P2X(1) and P2X(2) subunits were coexpressed in defolliculated Xenopus oocytes and the resultant P2X receptors studied under voltage-clamp conditions. Extracellular ATP elicited biphasic inward currents, involving an initial rapidly inactivating (P2X(1)-like) component and a later slowly inactivating (P2X(2)-like) component. The maximum amplitude of P2X(1)-like ATP responses was increased in some cells by lowering extracellular pH (from 7.5 to 6.5), whereas P2X(2)-like responses and those of homomeric rP2X(1) and rP2X(2) receptors were not changed by this treatment. Concentration-response (C/R) curves for ATP for pH-enhanced P2X(1)-like responses were biphasic, and clearly distinct from monophasic ATP C/R curves for homomeric rP2X(1) and rP2X(2) receptors. Under acidic (pH 5.5 and 6.5) and alkaline (pH 8.5) conditions, ATP C/R curves for P2X(1)-like responses showed increases in agonist potency and efficacy, compared with data at pH 7.5, but the same was not true of homomeric rP2X(1) and rP2X(2) receptors. ATP C/R curves for P2X(2)-like responses overlay C/R curves for homomeric rP2X(2) receptors, and determinations of agonist potency and efficacy were identical for P2X(2)-like and P2X(2) responses at all pH levels tested. Our results show that P2X(1)-like responses possessed the kinetics of homomeric P2X(1) receptors but an acid sensitivity different from homomeric P2X(1) and P2X(2) receptors. In contrast, the P2X(2)-like responses exactly matched the profile expected of homomeric P2X(2) receptors. Thus, coexpression of P2X(1) and P2X(2) subunits yielded a mixed population of homomeric and heteromeric P2X receptors, with a subpopulation of novel pH-sensitive P2X receptors showing identifiably unique properties that indicated the formation of heteromeric P2X(1/2) ion channels.

On the contribution of the first transmembrane domain to whole-cell current through an ATP-gated ionotropic P2X receptor

Scanning cysteine mutagenesis was used to identify potential pore-forming residues in and around the first transmembrane domains of ionotropic P2X(2) receptor subunits. Twenty-eight unique cysteine-substituted mutants (R28C-Y55C) were individually expressed in HEK293 cells by lipofection. Twenty-three of these were functional as assayed by application of ATP to transfected voltage-clamped cells. Individual mutants varied in their sensitivity to ATP; otherwise, currents through functional mutant receptors resembled those of the homomeric wild-type (WT) receptor. In five (H33C, R34C, I50C, K53C, and S54C) of 23 functional mutants, coapplication of 30 microm ATP and 500 nm Ag(+) irreversibly inhibited inward current evoked by subsequent applications of ATP alone. These inhibitions did not result in a lateral shift in the agonist concentration-response curve and are unlikely to involve a modification of the agonist binding site. Two (K53C and S54C) of the five residues modified by Ag(+) applied in the presence of ATP when the channels were gating were also modified by 1 mm (2-aminoethyl)methanethiosulfonate applied in the absence of ATP when the channels were closed. These data suggest that domains near either end of the first transmembrane domain influence ion conduction through the pore of the P2X(2) receptor.

Polar residues of the second transmembrane domain influence cation permeability of the ATP-gated P2X(2) receptor

P2X receptors are simple polypeptide channels that mediate fast purinergic depolarizations in both nerve and muscle. Although the depolarization results mainly from the influx of Na(+), these channels also conduct a significant Ca(2+) current that is large enough to evoke transmitter release from presynaptic neurons. We sought to determine the molecular basis of this Ca(2+) conductance by a mutational analysis of recombinant P2X(2) receptors. Wild type and 31 mutant P2X(2) receptors were expressed in HEK-293 cells and studied under voltage-clamp. We found that the relative Ca(2+) permeability measured from the reversal potentials of ATP-gated currents was unaffected by neutralizing fixed charge (Asp(315), Asp(349)) near the mouths of the channel pore. By contrast, mutations that changed the character or side chain volume of three polar residues (Thr(336), Thr(339), Ser(340)) within the pore led to significant changes in P(Ca)/P(Cs). The largest changes occurred when Thr(339) and Ser(340) were replaced with tyrosine; these mutations almost completely abolished Ca(2+) permeability, reduced P(Li)/P(Cs) by about one-half, and shifted the relative permeability sequence of Cs(+), Rb(+), K(+), and Na(+) to their relative mobility in water. Our results suggest that the permeability sequence of the P2X(2) receptor arises in part from interactions of permeating cations with the polar side chains of three amino acids located in a short stretch of the second transmembrane domain.

On the contribution of the first transmembrane domain to whole-cell current through an ATP-gated ionotropic P2X receptor

Scanning cysteine mutagenesis was used to identify potential pore-forming residues in and around the first transmembrane domains of ionotropic P2X(2) receptor subunits. Twenty-eight unique cysteine-substituted mutants (R28C-Y55C) were individually expressed in HEK293 cells by lipofection. Twenty-three of these were functional as assayed by application of ATP to transfected voltage-clamped cells. Individual mutants varied in their sensitivity to ATP; otherwise, currents through functional mutant receptors resembled those of the homomeric wild-type (WT) receptor. In five (H33C, R34C, I50C, K53C, and S54C) of 23 functional mutants, coapplication of 30 microm ATP and 500 nm Ag(+) irreversibly inhibited inward current evoked by subsequent applications of ATP alone. These inhibitions did not result in a lateral shift in the agonist concentration-response curve and are unlikely to involve a modification of the agonist binding site. Two (K53C and S54C) of the five residues modified by Ag(+) applied in the presence of ATP when the channels were gating were also modified by 1 mm (2-aminoethyl)methanethiosulfonate applied in the absence of ATP when the channels were closed. These data suggest that domains near either end of the first transmembrane domain influence ion conduction through the pore of the P2X(2) receptor.

Differential assembly of rat purinergic P2X7 receptor in immune cells of the brain and periphery

ATP-gated P2X(7) purinoceptors are found in most immune cells of the periphery and the brain where their activation leads to multiple downstream events such as cell permeabilization, apoptosis, and/or cytokine release. P2X(7) receptors do not form heteromeric receptors with any of the other six P2X subunits, and it is not known what type of homomeric assemblies the P2X(7) subunit makes. We constructed and purified an ectodomain protein of the rat P2X(7) receptor (amino acids 60-323) and used this to generate a monoclonal antibody (Ab) with which to probe P2X(7) receptors in central and peripheral immune cells. In HEK cells expressing rat P2X(7) receptors, the Ab increased the maximum current evoked by BzATP by 3-8-fold with a 5-fold leftward shift in EC(50) concentration. This Ab recognized only a non-denatured, multimeric form of the receptor on blue native-PAGE but did not recognize the denatured form on SDS-PAGE. A C-terminal polyclonal P2X(7) Ab recognized both monomeric subunits on SDS-PAGE and a multimeric complex on blue native-PAGE in this heterologous expression system. With Western blotting using these two Abs, native P2X(7) receptors in peritoneal macrophage and bone marrow cells are shown to exist as a strongly bound multimeric complex, whereas P2X(7) receptors in brain glia and/or astrocytes appear to form only as monomeric subunits.

Activation-dependent changes in receptor distribution and dendritic morphology in hippocampal neurons expressing P2X2-green fluorescent protein receptors

ATP-gated P2X(2) receptors are widely expressed in neurons, but the cellular effects of receptor activation are unclear. We engineered functional green fluorescent protein (GFP)-tagged P2X(2) receptors and expressed them in embryonic hippocampal neurons, and report an approach to determining functional and total receptor pool sizes in living cells. ATP application to dendrites caused receptor redistribution and the formation of varicose hot spots of higher P2X(2)-GFP receptor density. Redistribution in dendrites was accompanied by an activation-dependent enhancement of the ATP-evoked current. Substate-specific mutant T18A P2X(2)-GFP receptors showed no redistribution or activation-dependent enhancement of the ATP-evoked current. Thus fluorescent P2X(2)-GFP receptors function normally, can be quantified, and reveal the dynamics of P2X(2) receptor distribution on the seconds time scale.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Molecular physiology of P2X receptors and ATP signalling at synapses

ATP is found in every cell, where it is a major source of energy. But in the nervous system, ATP also has additional actions, which include its role in fast synaptic transmission and modulation. Here I discuss the 'fast' actions of ATP at synapses, the properties of the receptors that are activated by ATP and the physiology of ATP signalling, with emphasis on its role in pain processing.

Competitive antagonism of recombinant P2X(2/3) receptors by 2', 3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP)

TNP-ATP has become widely recognized as a potent and selective P2X receptor antagonist, and is currently being used to discriminate between subtypes of P2X receptors in a variety of tissues. We have investigated the ability of TNP-ATP to inhibit alpha,beta-methylene ATP (alpha,beta-meATP)-evoked responses in 1321N1 human astrocytoma cells expressing recombinant rat or human P2X(2/3) receptors. Pharmacological responses were measured using electrophysiological and calcium imaging techniques. TNP-ATP was a potent inhibitor of P2X(2/3) receptors, blocking both rat and human receptors with IC(50) values of 3 to 6 nM. In competition studies, 10 to 1000 microM alpha,beta-meATP was able to overcome TNP-ATP inhibition. Schild analysis revealed that TNP-ATP was a competitive antagonist with pA(2) values of -8.7 and -8.2. Inhibition of P2X(2/3) receptors by TNP-ATP was rapid in onset, reversible, and did not display use dependence. Although the onset kinetics of inhibition were concentration-dependent, the TNP-ATP off-kinetics were concentration-independent and relatively slow. Full recovery from TNP-ATP inhibition did not occur until >/=5 s after removal of the antagonist. Because of the slow off-kinetics of TNP-ATP, full competition with alpha,beta-meATP for receptor occupancy could be seen only after both ligands had reached a steady-state condition. It is proposed that the slowly desensitizing P2X(2/3) receptor allowed this competitive interaction to be observed over time, whereas the rapid desensitization of other P2X receptors (P2X(3)) may mask the detection of competitive inhibition by TNP-ATP.

Identification of amino acid residues contributing to the ATP-binding site of a purinergic P2X receptor

P2X receptor subunits have intracellular N and C termini, two membrane-spanning domains, and an extracellular loop of about 280 amino acids. We expressed the rat P2X(2) receptor in human embryonic kidney cells, and used alanine-scanning mutagenesis on 30 residues with polar side chains conserved among the seven rat P2X receptor subunits. This identified a region proximal to the first transmembrane domain which contained 2 lysine residues that were critical for the action of ATP (Lys(69) and Lys(71)). We substituted cysteines in this region (Asp(57) to Asp(71)) and found that for S65C and I67C ATP-evoked currents were inhibited by methanethiosulfonates. At I67C, the inhibition by negatively charged ethylsulfonate and pentylsulfonate derivatives could be overcome by increasing the ATP concentration, consistent with a reduced affinity of ATP binding. The inhibitory action of the methanethiosulfonates was prevented by pre-exposure to ATP, suggesting occlusion of the binding site. Finally, introduction of negative charges into the receptor by mutagenesis at this position (I67E and I67D) also gave receptors in which the ATP concentration-response curve was right-shifted. The results suggest that residues close to Ile(67) contribute to the ATP-binding site.

Roles of individual N-glycans for ATP potency and expression of the rat P2X1 receptor

P2X(1) receptor subunits assemble in the ER of Xenopus oocytes to homotrimers that appear as ATP-gated cation channels at the cell surface. Here we address the extent to which N-glycosylation contributes to assembly, surface appearance, and ligand recognition of P2X(1) receptors. SDS-polyacrylamide gel electrophoresis (PAGE) analysis of glycan minus mutants carrying Gln instead of Asn at five individual NXT/S sequons reveals that Asn(284) remains unused because of a proline in the +4 position. The four other sites (Asn(153), Asn(184), Asn(210), and Asn(300)) carry N-glycans, but solely Asn(300) located only eight residues upstream of the predicted reentry loop of P2X(1) acquires complex-type carbohydrates. Like parent P2X(1), glycan minus mutants migrate as homotrimers when resolved by blue native PAGE. Recording of ATP-gated currents reveals that elimination of Asn(153) or Asn(210) diminishes or increases functional expression levels, respectively. In addition, elimination of Asn(210) causes a 3-fold reduction of the potency for ATP. If three or all four N-glycosylation sites are simultaneously eliminated, formation of P2X(1) receptors is severely impaired or abolished, respectively. We conclude that at least one N-glycan per subunit of either position is absolutely required for the formation of P2X(1) receptors and that individual N-glycans possess marked positional effects on expression levels (Asn(154), Asn(210)) and ATP potency (Asn(210)).

Single channel properties of P2X ATP receptors in outside-out patches from rat hippocampal granule cells

The single channel properties of P2X ATP receptors were investigated in outside-out patches from hippocampal granule cells in brain slices from 12-day-old rats. The results demonstrate that functional P2X ATP receptors are expressed in hippocampal granule cells and, combined with previously published information on the P2X subunits expressed in the hippocampus, suggest that the receptors may be heteromers of the P2X4 and P2X6 subunits or P2X1, P2X2, P2X4 and P2X6 subunits. Two distinct types of P2X channel openings were observed. A flickery P2X receptor channel was observed in three patches with a mean chord conductance of 32 +/- 6 pS, a mean open time of 1.0 +/- 0.3 ms and a mean burst length of 11 +/- 5 ms at a membrane potential of -60 mV. A large conductance P2X receptor was observed in 19 out of 98 patches with a mean conductance of 56 +/- 1. 8 pS, a linear current-voltage relationship between -80 and +60 mV with a reversal potential around 0 mV, a mean open time of 2.6 +/- 0. 2 ms and a mean burst length of 8.8 +/- 1.8 ms at -60 mV. At an ATP concentration of 1 mM, these channels exhibited a low steady-state open probability (Popen, 0.07 +/- 0.008; n = 15), little apparent desensitisation and were also activated by alpha,beta-methylene ATP (alpha,beta-meATP, 40 microM; Popen, 0.007 +/- 0.0002; conductance, 57 +/- 1.1 pS; n = 3). No decrease in the single channel conductance was observed on increasing the free extracellular calcium concentration from 0.3 to 0.85 mM. Channel closed time distributions were fitted with five exponential components with time constants (and relative areas) of 90 micros (20 %), 0.77 ms (32 %), 10 ms (15 %), 90 ms (18 %) and 403 ms (15 %) at 1 mM ATP. Of these, the first two components are suggested to represent gaps within single activations of the receptor based on the lack of agonist concentration dependence of these two shut time components between 1 microM and 1 mM ATP. Suramin (40 microM) significantly increased the single channel conductance (19 +/- 7 %; n = 5) and produced a small decrease in Popen (39 +/- 9 %; n = 5) by decreasing mean open time, burst length and total open time per burst. These actions of suramin are not consistent with simple competitive antagonism.

Pharmacology of cloned P2X receptors

There are seven P2X receptor cDNAs currently known. Six homomeric (P2X1, P2X2, P2X3, P2X4, P2X5, P2X7) and three heteromeric (P2X2/P2X3, P2X4/P2X6, P2X1/P2X5) P2X receptor channels have been characterized in heterologous expression systems. Homomeric P2X1 and P2X3 receptors are readily distinguishable by their rapid desensitization, the agonist action of alpha beta methyleneATP, and the block by 2',3'-O-(2,4,6-trinitrophenyl)-ATP. P2X2 receptors are unique among homomeric forms in their potentiation by low pH. Homomeric P2X4 receptors are much less sensitive to antagonism by suramin and pyridoxal 5-phosphate-6-azo-2',4'-disulfonic acid. Homomeric P2X7 receptors are the only form in which 2',3'-O-(4-benzoylbenzoyl)-ATP is more potent than ATP. The heteromeric P2X2/P2X3 receptor resembles P2X2 in slow desensitization kinetics and potentiation by low pH and is similar to P2X3 with respect to agonism by alpha beta methyleneATP and block by 2',3'-O-(2,4,6-trinitrophenyl)-ATP. Other agonists, antagonists, and ions that can be used to differentiate among the receptors are discussed.

Coexpression of rat P2X2 and P2X6 subunits in Xenopus oocytes

Transcripts for P2X(2) and P2X(6) subunits are present in rat CNS and frequently colocalize in the same brainstem nuclei. When rat P2X(2) (rP2X(2)) and rat P2X(6) (rP2X(6)) receptors were expressed individually in Xenopus oocytes and studied under voltage-clamp conditions, only homomeric rP2X(2) receptors were fully functional and gave rise to large inward currents (2-3 microA) to extracellular ATP. Coexpression of rP2X(2) and rP2X(6) subunits in Xenopus oocytes resulted in a heteromeric rP2X(2/6) receptor, which showed a significantly different phenotype from the wild-type rP2X(2) receptor. Differences included reduction in agonist potencies and, in some cases (e.g., Ap(4)A), significant loss of agonist activity. ATP-evoked inward currents were biphasic at the heteromeric rP2X(2/6) receptor, particularly when Zn(2+) ions were present or extracellular pH was lowered. The pH range was narrower for H(+) enhancement of ATP responses at the heteromeric rP2X(2/6) receptor. Also, H(+) ions inhibited ATP responses at low pH levels (<pH 6.3). The pH-dependent blocking activity of suramin was changed at this heteromeric receptor, although the potentiating effect of Zn(2+) on ATP responses was unchanged. Thus, the rP2X(2/6) receptor is a functionally modified P2X(2)-like receptor with a distinct pattern of pH modulation of ATP activation and suramin blockade. Although homomeric P2X(6) receptors function poorly, the P2X(6) subunit can contribute to functional heteromeric P2X channels and may influence the phenotype of native P2X receptors in those cells in which it is expressed.

Coexpression of rat P2X2 and P2X6 subunits in Xenopus oocytes

Transcripts for P2X(2) and P2X(6) subunits are present in rat CNS and frequently colocalize in the same brainstem nuclei. When rat P2X(2) (rP2X(2)) and rat P2X(6) (rP2X(6)) receptors were expressed individually in Xenopus oocytes and studied under voltage-clamp conditions, only homomeric rP2X(2) receptors were fully functional and gave rise to large inward currents (2-3 microA) to extracellular ATP. Coexpression of rP2X(2) and rP2X(6) subunits in Xenopus oocytes resulted in a heteromeric rP2X(2/6) receptor, which showed a significantly different phenotype from the wild-type rP2X(2) receptor. Differences included reduction in agonist potencies and, in some cases (e.g., Ap(4)A), significant loss of agonist activity. ATP-evoked inward currents were biphasic at the heteromeric rP2X(2/6) receptor, particularly when Zn(2+) ions were present or extracellular pH was lowered. The pH range was narrower for H(+) enhancement of ATP responses at the heteromeric rP2X(2/6) receptor. Also, H(+) ions inhibited ATP responses at low pH levels (<pH 6.3). The pH-dependent blocking activity of suramin was changed at this heteromeric receptor, although the potentiating effect of Zn(2+) on ATP responses was unchanged. Thus, the rP2X(2/6) receptor is a functionally modified P2X(2)-like receptor with a distinct pattern of pH modulation of ATP activation and suramin blockade. Although homomeric P2X(6) receptors function poorly, the P2X(6) subunit can contribute to functional heteromeric P2X channels and may influence the phenotype of native P2X receptors in those cells in which it is expressed.

Block by extracellular Mg2+ of single human purinergic P2X4 receptor channels expressed in human embryonic kidney cells

Single channel properties of human P2X4 receptors expressed in human embryonic kidney cells have been investigated by outside-out mode patch clamp recordings. P2X4 channel activity was characterized by very fast kinetics. The current-voltage relationship was strongly non-linear at potentials <-100 mV. A slope conductance of approximately 9 pS was estimated at the approximately linear part of the current-voltage relation (>-100 mV). External Mg2+ reversibly decreased the amplitude of ATP-evoked single channel currents in a concentration-dependent manner but independent of the membrane potential. Additionally, extracellular Mg2+ shortened the mean open time whereas the mean closed time was not affected. Thus, Mg2+ ions are proposed to inhibit the function of human P2X4 receptors by means of an open-channel block with a Mg2+ binding site at the exterior surface of the pore.

Inactivation of P2X2 purinoceptors by divalent cations

1. P2X2 channels are activated by extracellular ATP. Despite being commonly described as non-desensitizing, P2X2 receptors do desensitize or inactivate. In the unspliced, 472 amino acid isoform of the P2X2 receptor, inactivation required membrane disruption and the presence of extracellular Ca2+. 2. The ability to inactivate whole-cell currents developed slowly after breaking in. In contrast, currents from excised patches exhibited rapid (approximately 100 ms) inactivation with a dependence on extracellular Ca2+, ATP and voltage. 3. The inactivation rate increased with the fourth power of [Ca2+] suggesting that the functional channel may be a tetramer. Ca2+ had both a higher affinity and a larger Hill coefficient for inactivation than Mg2+, Ba2+ or Mn2+. Trivalent cations at concentrations up to the solubility product of ATP had no effect. The change in apparent co-operativity with ionic species suggests the presence of experimentally unresolved ligand-insensitive kinetic steps. 4. Based on the weak voltage dependence of inactivation and the lack of effect of intracellular Ca2+ buffers, the Ca2+-binding sites are probably located near the extracellular surface of the membrane. 5. The recovery from inactivation was slow, with a time constant of approximately 7 min. 6. Ca2+-sensitive inactivation only appeared when the membrane was disrupted in some manner. Treatment with actin and microtubule reagents did not induce inactivation, suggesting that an intact cytoskeleton is not necessary. 7. Inactivation rates observed in different patch configurations suggest that the induction of Ca2+-dependent inactivation was due to the loss of a diffusible cofactor located in the membrane or the cytoplasm.