Identification of the platelet ADP receptor targeted by antithrombotic drugs

Extracellular nucleotides induce vasodilatation in human arteries via prostaglandins, nitric oxide and endothelium-derived hyperpolarising factor

1. The present study was aimed at examining P2 receptor-mediated vasodilatation in human vessels. The isometric tension was recorded in isolated segments of the human left internal mammary artery branches precontracted with 1 microM noradrenaline. 2. Endothelial denudation abolished the dilator responses. 3. The selective P2Y(1) agonist, 2-MeSADP, induced a potent vasodilatation (pEC(50)=6.9+/-0.1). The P2Y(1) antagonist of 10 microM, MRS 2216, shifted the 2-MeSADP concentration-response curve 1.1 log units to the right. The combined P2Y(1) and P2X agonist, 2-MeSATP, stimulated a dilatation with a potency similar to that of 2-MeSADP. Furthermore, MRS 2216 had a similar antagonistic effect on both 2-MeSATP and 2-MeSADP indicating that P2X receptors do not mediate vasodilatation. 4. Both the P2Y(2/4) agonist, UTPgammaS and the P2Y(6) agonist, UDPbetaS, stimulated potent dilatations (pEC(50)=7.8+/-0.4 for UTPgammaS and 8.4+/-0.2 for UDPbetaS). 5. The 2-MeSADP-induced nitric oxide (NO)-mediated dilatation was studied in the presence of 10 micro M indomethacin, 50 nM charybdotoxin and 1 microM apamin. The involvement of the endothelium-derived hyperpolarising factor (EDHF) was investigated in the presence of 0.1 mM L-NOARG and indomethacin. The involvement of prostaglandins was investigated in the presence of L-NOARG, charybdotoxin and apamin. Both NO, EDHF and prostaglandins mediated 2-MeSADP dilatation with similar efficacy (E(max)=25+/-5% for NO, 25+/-6% for EDHF and 27+/-5% for prostaglandins). 6. In conclusion, extracellular nucleotides induce endothelium-derived vasodilatation in human vessels by stimulating P2Y(1), P2Y(2/4) and P2Y(6) receptors, while P2X receptors are not involved. Endothelial P2Y receptors mediate dilatation by release of EDHF, NO and prostaglandins.

P2Y receptors modulate ion channel function through interactions involving the C-terminal domain

Nucleotide stimulation of G(q)-coupled P2Y receptors expressed in Xenopus laevis oocytes produces the activation of an endogenous voltage-gated ion channel, previously identified as the transient inward (T(in)) channel. Expression of human P2Y(1), human P2Y(2), rat P2Y(6), human P2Y(11), or skate P2Y receptors in oocytes resulted in modulation of the voltage dependence and inactivation gating of the channel. Expression of the human P2Y(4) receptor, rat M(1)-muscarinic receptor, and human B(1)-bradykinin receptor did not alter the properties of the T(in) channel. Replacement of the C-terminal domain of the human B(1)-bradykinin receptor with the C-terminal domains of either the human P2Y(1) or human P2Y(2) receptor resulted in voltage dependence and inactivation-gating properties, respectively, of the T(in) channel that were similar to those elicited by the respective native P2Y receptor. Systematic truncation of the C-terminal region of the human P2Y(1) receptor identified a short region responsible for modulation of the T(in) channel. This region contains a conserved sequence motif found in all P2Y receptors that modulates the voltage dependence of the T(in) channel. Synthetic 20-mer peptides from the C-terminal domains of human P2Y(1) and P2Y(2) receptors produced a shift in the voltage dependence and slowed inactivation gating, respectively, after injection into oocytes expressing human B(1)-bradykinin or truncated human P2Y(1) receptors. These results indicate that certain P2Y receptors are capable of modulating the voltage sensitivity and inactivation gating of an endogenous oocyte ion channel through interactions involving the C-terminal region of the receptor. Such modulation of ion channel function could also exist in native mammalian cells that express P2Y receptors.

Metabolic control of excessive extracellular nucleotide accumulation by CD39/ecto-nucleotidase-1: implications for ischemic vascular diseases

Platelets are responsible for maintaining vascular integrity. In thrombocytopenic states, vascular permeability and fragility increase, presumably due to the absence of this platelet function. Chemical or physical injury to a blood vessel induces platelet activation and platelet recruitment. This is beneficial for the arrest of bleeding (hemostasis), but when an atherosclerotic plaque is ulcerated or fissured, it becomes an agonist for vascular occlusion (thrombosis). Experiments in the late 1980s cumulatively indicated that endothelial cell CD39-an ecto-ADPase-reduced platelet reactivity to most agonists, even in the absence of prostacyclin or nitric oxide. As discussed herein, CD39 rapidly and preferentially metabolizes ATP and ADP released from activated platelets to AMP, thereby drastically reducing or even abolishing platelet aggregation and recruitment. Since ADP is the final common agonist for platelet recruitment and thrombus formation, this finding highlights the significance of CD39. A recombinant, soluble form of human CD39, solCD39, has enzymatic and biological properties identical to the full-length form of the molecule and strongly inhibits human platelet aggregation induced by ADP, collagen, arachidonate, or TRAP (thrombin receptor agonist peptide). In sympathetic nerve endings isolated from guinea pig hearts, where neuronal ATP enhances norepinephrine exocytosis, solCD39 markedly attenuated norepinephrine release. This suggests that NTPDase (nucleoside triphosphate diphosphohydrolase) could exert a cardioprotective action by reducing ATP-mediated norepinephrine release, thereby offering a novel therapeutic approach to myocardial ischemia and its consequences. In a murine model of stroke, driven by excessive platelet recruitment, solCD39 reduced the sequelae of stroke, without an increase in intracerebral hemorrhage. CD39 null mice, generated by deletion of apyrase-conserved regions 2 to 4, exhibited a decrease in postischemic perfusion and an increase in cerebral infarct volume when compared with controls. "Reconstitution" of CD39 null mice with solCD39 reversed these changes. We hypothesize that solCD39 has potential as a novel therapeutic agent for thrombotic diatheses.

Involvement of P2Y receptors in the differentiation of haematopoietic cells

The effects of extracellular nucleotides are mediated by multiple P2X ionotropic receptors and G protein-coupled P2Y receptors. These receptors are ubiquitous, but few physiological roles have been firmly identified. In this review article, we present a survey of the functional expression of P2Y receptors in the different haematopoietic lineages by analyzing the selectivity of these cells for the various adenine and uracil nucleotides as well as the second messenger signaling pathways involved. The pharmacological profiles of metabotropic nucleotide receptors are different among myeloid, megakaryoid, erythroid, and lymphoid cells and change during differentiation. A role of P2Y receptors in the differentiation and maturation of blood cells has been proposed: In particular the P2Y(11)receptor seems to be involved in the granulocytic differentiation of promyelocytes and in the maturation of monocyte-derived dendritic cells. It is suggested that the role of P2Y receptors in the maturation of blood cells may be more important than believed so far.

Effects of inhibition of P2Y(1) and P2Y(12) on whole blood clotting, coagulum elasticity and fibrinolysis resistance studied with free oscillation rheometry

INTRODUCTION

In vivo, initial platelet activation is likely caused by platelet contacts with collagen in the subendothelium or from the small amounts of thrombin formed by the tissue factor/factor VIIa complex. Our aim was to study the coagulative role of ADP released by the platelets after activation with strong stimuli such as collagen and/or thrombin, and the relative importance of the platelet ADP receptors P2Y(1) and P2Y(12).

MATERIALS AND METHODS

We used 10 Hz free oscillation rheometry to measure clotting time, clot elasticity and fibrinolysis resistance of non-anticoagulated whole blood. The platelets were activated with a collagen-related peptide (CRP), with the PAR1 thrombin receptor activating peptide TRAP-6 or by thrombin, the latter generated by small amounts of thromboplastin. To inhibit the platelet ADP receptors, we used the P2Y(1) antagonist MRS2179 and the P2Y(12) antagonist AR-C69931MX.

RESULTS

Both antagonists significantly retarded the clotting induced by CRP. The effects were most pronounced with AR-C69931MX. For TRAP-6, the same trend was seen, but the retardation was only significant with AR-C69931MX. Clotting induced by small amounts of thromboplastin was not affected by any ADP-receptor antagonist. Addition of both antagonists did not change the results as compared to samples with AR-C69931MX alone. Nor did the antagonists, one at a time or in concert, effect fibrinolysis or the elastic properties of the clot.

CONCLUSION

We conclude that ADP-receptor inhibition prolongs the clotting time for whole blood activated by CRP, but that it does not affect the properties of the subsequently formed coagulum.

Inhibitory effects of glycoprotein IIb/IIIa antagonists and aspirin on the release of soluble CD40 ligand during platelet stimulation

BACKGROUND

Glycoprotein (GP) IIb/IIIa antagonists inhibit platelet aggregation, an activity attributed to the clinical benefits of these drugs in settings that involve acute coronary thrombosis. However, platelet activation and subsequent aggregation are now known to cause the release of a soluble form of CD40 ligand (sCD40L), a prothrombotic and proinflammatory protein with GP IIb/IIIa binding activity and an established role in atherosclerotic lesion progression. The present study was designed to determine what effect GP IIb/IIIa antagonists have on the release of sCD40L.

METHODS AND RESULTS

Doses of eptifibatide, abciximab, and tirofiban that inhibited platelet aggregation by at least 80% also inhibited sCD40L release in vitro (by 85%, 57%, and 80%, respectively). When platelets were stimulated with a thrombin receptor agonist, inhibition by GP IIb/IIIa antagonists occurred without affecting the release of betaTG, an alpha-granule protein. Unexpectedly, concentrations of the 3 antagonists that blocked aggregation by only 20% to 50% potentiated the release of sCD40L (by 19% to 26%). Platelets from aspirin-treated individuals were partially protected from sCD40L release, but only when the agonist was collagen, an affect augmented by the addition of GP IIb/IIIa antagonists.

CONCLUSIONS

These studies suggest that the mechanisms responsible for the clinical benefits of GP IIb/IIIa antagonists (at doses that optimally inhibit aggregation) and of aspirin may not be limited to the inhibition of thrombosis through their blockade of platelet aggregation but may also involve the inhibition of inflammation and thrombosis through their blockade of sCD40L release. These studies also provide a mechanism by which suboptimal doses of GP IIb/IIIa antagonists may be proinflammatory.

Platelet receptors and their role in diseases

The absence or deficiency of specific platelet glycoprotein receptors has a well-defined role in causing several rare bleeding disorders such as Bernard-Soulier syndrome or Glanzmann's thrombasthenia. Several new rare disorders caused by defects in other receptors or their signalling pathways have recently been described. Platelet receptors are also often targets for antibodies in pathological conditions. The roles of platelet receptors or their polymorphism variants in diseases such as cardiovascular disorders have started to be intensively investigated over the last 5 years. Many of these findings still remain controversial. Recent evidence points to a fundamental role for platelets and their receptors in the origins of atherosclerosis. Studies on the role of platelet receptors in diseases such as asthma, diabetes and HIV are still at an early stage.

In vivo blockade of platelet ADP receptor P2Y12 reduces embolus and thrombus formation but not thrombus stability

OBJECTIVE

ADP is a key platelet agonist in thromboembolism. One of the receptors involved in ADP-induced platelet activation is the P2Y12 receptor, which is a target for antithrombotic drugs.

METHODS AND RESULTS

Here, we present first evidence for a differential role of this receptor in thrombus and embolus formation in vivo. Anesthetized rabbits were treated with the selective P2Y12 antagonists AR-C69931 MX (3 microg x kg x min(-1) IV) or clopidogrel (25 mg/kg orally). Efficacy of these treatments was monitored by aggregation and thrombin generation measurements in blood samples ex vivo. Mesenteric arterioles were mechanically injured; thrombus growth and subsequent embolus formation were visualized by real-time intravital microscopy. AR-C69931 MX and clopidogrel significantly (P<0.05) reduced the total duration of embolization (by 52% and 36%, respectively), and fewer and smaller emboli were produced. The size of the initial thrombus was significantly reduced (P<0.005), but its stability was unaffected: plug formation was still effective.

CONCLUSIONS

These findings demonstrate that ADP and its P2Y12 receptor are involved in thrombus growth and especially in the formation of emboli on the downstream side of the initial thrombus. This may explain the beneficial effects of P2Y12 receptor antagonists in secondary prevention of ischemic events in patients with arterial thrombosis.

Purinergic P2Y12 receptor blockade inhibits shear-induced platelet phosphatidylinositol 3-kinase activation

Pathologically elevated shear stress triggers aspirin-insensitive platelet thrombosis. Signaling mechanisms involved in shear-induced platelet thrombosis are not well understood. To investigate these, we examined the hypothesis that functionally important platelet phosphatidylinositol 3-kinase (PI3-K) activity is stimulated by an in vitro shear stress of 120 dynes/cm(2) (shear rate of 6,000 sec(-1)). Phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) production was examined in washed human platelets subjected to pathological shear stress in a cone-plate viscometer. PIP(3) production peaks 30 s after shear begins and is initiated by von Willebrand factor (VWF) binding to the glycoprotein (Gp) Ib-IX-V complex. Inhibiting PI3-K with wortmannin or 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) results in the inhibition of shear-induced platelet aggregation. In resting platelets, class IA PI3-K associates with the tyrosine kinase Syk. Within 30 s of beginning shear, PI3-K-associated Syk becomes tyrosine phosphorylated. Inhibiting Syk activation with piceatannol results in the inhibition of PIP(3) production and aggregation. Selective blockade of the P2Y(12) receptor results in the inhibition of Syk phosphorylation, PIP(3) production, and aggregation. These results indicate that shear-induced VWF binding to platelet GpIb-IX-V stimulates functionally important PI3-K activity. PI3-K activation is signaled by rapid feedback amplification that involves P2Y(12) receptor-mediated activation of Syk.

Involvement of a purinergic pathway in the sympathetic regulation of motility in rat ileum

We investigated extrinsic neuronal regulation of intestinal motility. The mesenteric nerve stimulation (MNS; duration 0.5 ms, 10 Hz for 30 s) evoked relaxation in the longitudinal muscle direction of the isolated rat ileum. The MNS-induced relaxation was abolished by guanethidine (2 microM) or propranolol (10 microM), but was not affected by prazosin (10 microM), rauwolscine (10 microM), hexamethonium (100 microM) or capsaicin (1 microM). Exposure to a high concentration (100 microM) of ATP (ATP-desensitization) or ADP (ADP-desensitization) reduced the MNS-induced relaxation to 44.7% or 32.5% of the control (P<0.01), respectively. P2 purinoceptor antagonists [suramin (100 microM) and reactive blue-2 (RB-2, 50 microM)] or small conductance Ca(2+)-activated K(+) channel blocker, apamin (0.5 microM), significantly decreased the relaxation to 54.4% and 25.6% or 19.4% of the control (P<0.01), respectively, whereas selective P2Y(1) purinoceptor antagonist MRS2179 (10 microM) failed to affect the relaxation. Furthermore, exogenous ATP (1 microM) or ADP (1 microM) elicited relaxation in the rat ileum, which was almost abolished by reactive blue-2 (50 microM, 9.1% of control remained, P<0.05). In contrast, relaxation induced by noradrenalin (10 microM) was not antagonized by ATP-desensitization, apamin (0.5 microM) or reactive blue-2 (50 microM). From the present results, we conclude that noradrenergic sympathetic nerves might regulate intestinal motility mediated through a purinergic inhibitory neuronal pathway in the rat small intestine.

Molecular bases of defective signal transduction in the platelet P2Y12 receptor of a patient with congenital bleeding

We have identified structural attributes required for signal transduction through a seven-transmembrane-domain receptor. Platelets from a patient (AC) with a congenital bleeding disorder had normal shape change but reduced and reversible aggregation in response to 4 microM ADP, similar to normal platelets with blocked P2Y(12) receptor. The response to 20 microM ADP, albeit still decreased, was more pronounced and was reduced by a P2Y(12) antagonist, indicating some residual receptor function. ADP failed to lower the adenylyl cyclase activity stimulated by prostaglandin E(1) in the patient's platelets, even though the number and affinity of 2-methylthioadenosine 5'-[(33)P]diphosphate-binding sites was normal. Analysis of the patient's P2Y(12) gene revealed a G-to-A transition in one allele, changing the codon for Arg-256 in the sixth transmembrane domain to Gln, and a C-to-T transition in the other allele, changing the codon for Arg-265 in the third extracellular loop to Trp. Neither mutation interfered with receptor surface expression but both altered function, since ADP inhibited the forskolin-induced increase of cAMP markedly less in cells transfected with either mutant P2Y(12) as compared with wild-type receptor. These studies delineate a region of P2Y(12) required for normal function after ADP binding.

Immunolocalization of P2Y1 and TPalpha receptors in platelets showed a major pool associated with the membranes of alpha -granules and the open canalicular system

P2Y(1) and thromboxane-prostanoid-alpha (TPalpha) receptors on platelets belong to the G-protein-coupled 7-transmembrane domain family. They transmit signals for shape change, mobilization of calcium, and platelet aggregation. Immunogold labeling with a monoclonal antibody (MoAb) to the amino-terminal domain of P2Y(1) and a polyclonal antibody to the C-terminal domain of TPalpha revealed that while present at the platelet surface, both receptors were abundantly represented inside the platelet. Specifically, receptors were found in membranes of alpha-granules and elements of the open-canalicular system. A similar organization was found in mature megakaryocytes. Activation of platelets by adenosine diphosphate (ADP) and the thromboxane A(2) (TXA(2)) analog, I-BOP [1S-(1 alpha,2 beta(5Z),3 alpha-(1E,3S)4 alpha)-7-(3-(3- hydroxy-4-(p-iodophenoxy)-1-butenyl)-7-oxabicyclo(2.2.1)hept-2-yl)-5-heptenoic acid], increased the labeling of both P2Y(1) and TPalpha at the surface and in intracellular pools, suggesting that activation resulted in greater antibody accessibility to the receptor. A return to a platelet discoid shape and to basal values of labeling accompanied receptor desensitization. Platelets lacking the P2Y(12) ADP receptor normally expressed P2Y(1) and TPalpha, both before and after activation. Studies with the anti-ligand-induced binding site (anti-LIBS) MoAb, AP-6, confirmed that stored fibrinogen associated with internal pools of alpha(IIb)beta(3) at the start of secretion in a microenvironment containing agonist receptors. Pharmacologic antagonism of ADP or TXA(2) receptors in antithrombotic therapy may need to take into account blockade of internal receptor pools.

Identification of P2Y12-dependent and -independent mechanisms of glycoprotein VI-mediated Rap1 activation in platelets

Glycoprotein (GP) VI is a critical platelet collagen receptor, yet the steps involved in GPVI-mediated platelet activation remain incompletely understood. Because activation of Rap1, an abundant small guanosine triphosphatase (GTPase) in platelets, contributes to integrin alpha(IIb)beta(3) activation, we asked whether and how GPVI signaling activates Rap1 in platelets. Here we show that platelet Rap1 is robustly activated upon addition of convulxin, a GPVI-specific agonist. Using a reconstituted system in RBL-2H3 cells, we found that GPVI-mediated Rap1 activation is dependent on FcRgamma but independent of another platelet collagen receptor, alpha(2)beta(1). Interestingly, GPVI-mediated Rap1 activation in human platelets is largely dependent on adenosine diphosphate (ADP) signaling through the P2Y(12) and not the P2Y(1) receptor. However, experiments with specific ADP receptor antagonists and platelets from knockout mice deficient in P2Y(1) or the P2Y(12)-associated G-protein, Galphai(2), indicate that human and murine platelets also have a significant P2Y(12)-independent component of GPVI-mediated Rap1 activation. The P2Y(12)-independent component is dependent on phosphatidylinositol 3-kinase and is augmented by epinephrine-mediated signaling. P2Y(12)-dependent and -independent components are also observed in GPVI-mediated platelet aggregation, further supporting a role for Rap1 in aggregation. These results define mechanisms of GPVI-mediated platelet activation and implicate Rap1 as a key signaling protein in GPVI-induced platelet signaling.

Purinergic modulation of neuronal activity in the mesolimbic dopaminergic system in vivo

ATP and its metabolite adenosine activate membrane receptors, termed P2 and P1, respectively. In the present study, the modulation of the mesolimbic neuronal circuit by ATPergic and adenosinergic mechanisms was investigated by microdialysis in the nucleus accumbens (NAc) and by telemetrically recorded EEG from both the NAc and the ventral tegmental area (VTA) of freely moving rats. The basal extracellular dopamine concentration was enhanced after accumbal perfusion with the ATP analog 2-methylthio ATP (2-MeSATP; 100 microM); by contrast, adenosine (100 microM) caused a reduction of extracellular dopamine. When given alone, the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 20 microM) decreased the concentration of dopamine, whereas the P1 receptor antagonist 8-(p-sulfophenyl)theophylline (8-SPT; 100 microM) increased it. In the same animals, P2 receptor stimulation by 2-MeSATP caused neuronal activation, indicated by an elevation of the absolute power in the EEG of the NAc mainly by enhancement of the relative power in the alpha band (8-13 Hz) of the EEG spectrum. By contrast, adenosine led to a depression of the absolute power in the VTA accompanied by an elevation of the delta-band power (0.4-6 Hz) in the NAc corresponding to a slowing of neuronal activity. When given alone, PPADS reduced the absolute EEG power in the NAc accompanied by a decrease in the high-frequency power, but had no effects on the VTA. 8-SPT on its own enhanced the total power in both the NAc and the VTA, reflected by an enhancement in the slow and the high-frequency bands. Whereas the 8-SPT-evoked changes of EEG pattern as well as of dopamine concentration in the NAc were abolished by the co-application of PPADS, the 8-SPT-induced EEG changes in the VTA persisted under these conditions. In conclusion, the accumbal neuronal output, reflected by both dopamine release and neuronal electrical activity, is modulated in a functionally antagonistic manner by P2 and P1 receptor stimulation. It is suggested that an inhibitory GABAergic feedback projection to the VTA is stimulated by adenosine, either directly or indirectly via glutamate release.

P2Y6 receptor mediates colonic NaCl secretion via differential activation of cAMP-mediated transport

Extracellular nucleotides are important regulators of epithelial ion transport. Here we investigated nucleotide-mediated effects on colonic NaCl secretion and the signal transduction mechanisms involved. Basolateral UDP induced a sustained activation of Cl(-) secretion, which was completely inhibited by 293B, a specific inhibitor of cAMP-stimulated basolateral KCNQ1/KCNE3 K(+) channels. We therefore speculated that a basolateral P2Y(6) receptor could increase cAMP. Indeed UDP elevated cAMP in isolated crypts. We identified an epithelial P2Y(6) receptor using crypt [Ca(2+)](i) measurements, RT-PCR, and immunohistochemistry. To investigate whether the rat P2Y(6)elevates cAMP, we coexpressed the P2Y(1) or P2Y(6) receptor together with the cAMP-regulated cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel in Xenopus oocytes. A two-electrode voltage clamp was used to monitor nucleotide-induced Cl(-) currents. In oocytes expressing the P2Y(1) receptor, ATP transiently activated the endogenous Ca(2+)-activated Cl(-) current, but not CFTR. In contrast, in oocytes expressing the P2Y(6)receptor, UDP transiently activated the Ca(2+)-activated Cl(-) current and subsequently CFTR. CFTR Cl(-) currents were identified by their halide conductance sequence. In summary we find a basolateral P2Y(6) receptor in colonic epithelial cells stimulating sustained NaCl secretion by way of a synergistic increase of [Ca(2+)](i) and cAMP. In support of these data P2Y(6) receptor stimulation differentially activates CFTR in Xenopus oocytes.

The role of platelet receptors and adhesion molecules in coronary artery disease

Platelets play a significant role in coronary artery disease through interactions with each other and with other cell types. These interactions are mediated by certain receptors on the surface of platelets and other cells which can lead to intra-coronary thrombus formation and occlusion that may result in acute coronary syndromes. The important roles of the currently available anti-platelet therapies have been well established in many clinical outcome trials in cardiovascular patients. An understanding of these different interactions provides the clinician with a background that supports the clinical importance of currently available anti-platelet therapies. Moreover, knowledge of the mechanisms of cellular crosstalk will lead to important advances in the development of better antithrombotic therapies.

Characterization of the UDP-glucose receptor (re-named here the P2Y14 receptor) adds diversity to the P2Y receptor family

The cloning of a human G-protein-coupled receptor (GPCR) that specifically responds to UDP-glucose and related sugar-nucleotides has been reported recently. This receptor has important structural similarities to known members of the P2Y receptor family but also shows a distinctly different pharmacological response profile. Here, the IUPHAR Subcommittee for P2Y receptor nomenclature and classification review the current knowledge of this receptor and present their reasons for including this receptor in the P2Y receptor family as the P2Y(14) receptor.

Blockade of the platelet P2Y12 receptor by AR-C69931MX sustains coronary artery recanalization and improves the myocardial tissue perfusion in a canine thrombosis model

OBJECTIVE

Reperfusion therapy for myocardial infarction is limited by a significant reocclusion rate and less optimal myocardial tissue perfusion due to excessive platelet accumulation and recruitment at the sites of vascular injury. We assessed the influence of a selective P2Y(12)-receptor antagonist (AR-C69931MX), in conjunction with thrombolytic therapy, on the prevention of platelet aggregation and thrombus formation.

METHODS AND RESULTS

A canine coronary electrolytic injury thrombosis model was used. Tissue-type plasminogen activator (t-PA; 1 mg/kg in phase I, 0.5 mg/kg in phase II in the AR-C69931MX group, and 1 mg/kg in the placebo group in phase I and II) was administered 30 minutes after thrombus formation; either saline or AR-C69931MX (4 micro g x kg(-1) x min(-1)) was given to all animals intravenously 10 minutes before t-PA administration for a total of 2 hours. All animals received heparin (80 U/kg) as an intravenous bolus followed by a continuous infusion of 17 U x kg(-1) x h(-1). Myocardial tissue perfusion was evaluated by use of the colored microsphere technique and real-time myocardial contrast echocardiography. The incidences of reocclusion and cyclic flow variation were significantly decreased in the AR-C69931MX group (P<0.05). Myocardial tissue flow with AR-C69931MX treatment improved significantly at 20 and 120 minutes after reflow, whereas tissue flow with placebo remained at a level similar to that during occlusion (P<0.05).

CONCLUSIONS

The adjunctive administration of AR-C69931MX blocked ADP-mediated platelet aggregation and recruitment and prevented platelet-mediated thrombosis, resulting in prolongation of reperfusion time and a decrease in reocclusion and cyclic flow variations. Importantly, myocardial tissue perfusion was significantly improved in the P2Y(12) antagonist group.

Effect of clopidogrel administration to healthy volunteers on platelet phosphorylation events triggered by ADP

The action of clopidogrel on platelet receptors was analysed using platelets obtained from 11 healthy volunteers given 75 mg of clopidogrel daily for 8 d. Samples of blood were taken before treatment and after 8 d of medication. Determination of 2-methylthioadenosine diphosphate trisodium (2MesADP)-induced platelet aggregation, serine/threonine and tyrosine phosphorylations were performed in the absence or presence of the P2Y1-receptor-specific antagonist: adenosine 3'-phosphate 5'-phosphate (A3P5P) or the strong inhibitor of GPIIb/IIIa activation: SR121566.

MAJOR CONCLUSIONS

1). Serine and threonine phosphorylations of the myosin light chain (P20) and pleckstrin (P47) do not behave similarly, although they are both recognized as the result of phospholipase C pathway stimulation triggered by the P2Y1 receptor. P47 is strongly affected by the A3P5P, and this appears to be highly dependent on P2Y12. However, P20 phosphorylation occurs in the presence of A3P5P, suggesting that the P2Y12 receptor signal contributes to P20 phosphorylation mediated by a calcium-independent pathway. The results suggest that P2Y1 and P2Y12 receptors interact to modulate the phosphorylation of P20 and P47. 2). The inside-out signalling dependent on both P2Y12 and P2Y1 is necessary for GPIIb/IIIa activation. 3). Clopidogrel and SR121566 inhibited the increase in tyrosine phosphorylation induced by 2MesADP and concomitantly inhibited platelet aggregation, indicating that most of the phosphorylations are GPIIb/IIIa dependent. However, neither clopidogrel nor SR121566 inhibited the first wave of 80 kDa substrate (cortactin) which is involved in the reorganization of the cytoskeleton necessary for shape change and which appeared to be essentially P2Y1 dependent.

A novel inhibitor of Rho-associated protein kinase, Y-27632, ameliorates hepatic ischemia and reperfusion injury in rats

BACKGROUND

A Rho-ROCK signal system induces vascular contraction and neutrophil migration, both of which are characteristic features found with ischemia and reperfusion injury of the liver. We tested our hypothesis that a novel ROCK I inhibitor, Y-27632, attenuates hepatic ischemia and reperfusion injury.

METHODS

Rats underwent 70% partial hepatic ischemia for 120 minutes and subsequent reperfusion. Y-27632 of 10mg/kg was given orally 1 hour before ischemia, while distilled water was given to the control animals. One week animal survival, systemic hemodynamics, hepatic tissue blood flow, liver function tests, plasma endothelin-1, serum hyaluronic acid levels, myeloperoxidase activity and malondialdehyde level in liver tissue, membrane attack complex-1 and intracellular adhesion molecule-1 staining, and histological architecture were analyzed.

RESULTS

Y-27632 prolonged 1-week animal survival from 25% of untreated animals to 75% accompanied with significant amelioration of hepatic tissue blood flow, liver function tests and histological architecture without any adverse effects on systemic hemodynamics. In addition, plasma endothelin-1 and serum hyaluronic acid levels decreased markedly compared to the control, concomitant with remarkable suppression of membrane attack complex-1 stain positive neutrophils infiltration, myeloperoxidase activity and malondialdehyde level.

CONCLUSION

Present study suggests that activation of a Rho-ROCK signal system is associated with ischemia and reperfusion injury of the liver, and that Y-27632 may be an attractive agent for application in major liver resection using temporary inflow occlusion and hepatic preservation.

Attenuation of the P2Y receptor-mediated control of neuronal Ca2+ channels in PC12 cells by antithrombotic drugs

1. In PC12 cells, adenine nucleotides inhibit voltage-activated Ca(2+) currents and adenylyl cyclase activity, and the latter effect was reported to involve P2Y(12) receptors. To investigate whether these two effects are mediated by one P2Y receptor subtype, we used the antithrombotic agents 2-methylthio-AMP (2-MeSAMP) and N(6)-(2-methyl-thioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene-ATP (AR-C69931MX). 2. ADP reduced A(2A) receptor-dependent cyclic AMP synthesis with half maximal effects at 0.1-0.17 micro M. In the presence of 30 micro M 2-MeSAMP or 100 nM AR-C69931MX, concentration response curves were shifted to the right by factors of 39 and 30, indicative of pA(2) values of 6.1 and 8.5, respectively. 3. The inhibition of Ca(2+) currents by ADP was attenuated by 10-1000 nM AR-C69931MX and by 3-300 micro M 2-MeSAMP. ADP reinhibited Ca(2+) currents after removal of 2-MeSAMP within less than 15 s, but required 2 min to do so after removal of AR-C69931MX. 4. ADP inhibited Ca(2+) currents with half maximal effects at 5-20 micro M. AR-C69931MX (10-100 nM) displaced concentration response curves to the right, and the resulting Schild plot showed a slope of 1.09 and an estimated pK(B) value of 8.7. Similarly, 10-100 micro M 2-MeSAMP also caused rightward shifts resulting in a Schild plot with a slope of 0.95 and an estimated pK(B) of 5.4. 5. The inhibition of Ca(2+) currents by 2-methylthio-ADP and ADPbetaS was also antagonized by AR-C69931MX, which (at 30 nM) caused a rightward shift of the concentration response curve for ADPbetaS by a factor of 3.8, indicative of a pA(2) value of 8.1. 6. These results show that antithrombotic drugs antagonize the inhibition of neuronal Ca(2+) channels by adenine nucleotides, which suggests that this effect is mediated by P2Y(12) receptors.

Reduced thrombus cohesion in an ex vivo human model of arterial thrombosis induced by clopidogrel treatment: kinetics of the effect and influence of single and double loading-dose regimens

OBJECTIVE

To compare the effects of clopidogrel on ex vivo thrombogenesis with those on ADP-dependent platelet aggregation, and to compare single and double loading-dose regimens.

METHODS AND RESULTS

Step 1: Volunteers (n=12) received clopidogrel 75 mg/day for 8 days. ADP-induced platelet aggregation was measured in platelet-rich plasma (PRP). Thrombogenesis was measured in an ex vivo model. Clopidogrel produced rapid platelet inhibition, increasing up to day 5. Maximal intensity of platelet aggregation correlated with density of platelet thrombus, surface of collagen covered by platelets and thrombus cross-sectional surface (p<0.001). Step 2: On day 1, volunteers (n=60) randomly received clopidogrel 75 mg, a single 300-mg loading dose or two 300-mg loading doses separated by a 12-h interval. On day 2, all volunteers received clopidogrel 75 mg. Both loading dose regimens enhanced platelet inhibition at all time points (p<0.03 vs. clopidogrel 75 mg). After 3 h, the antiplatelet effect of a loading dose was substantial, and the mean decrease in dense thrombus surface was greater in the loading-dose groups than in the 75 mg group (p=0.041 for the single loading dose). Ex vivo, there were no significant differences between loading-dose groups.

CONCLUSIONS

Clopidogrel reduces arterial thrombus cohesion by an effect that correlates with inhibition of ADP-induced platelet aggregation. A single 300-mg loading dose provides a rapid onset of such an antithrombotic effect, which was more significant at 24 h with the double loading dose.

A receptor linked to a Gi-family G-protein functions in initiating oocyte maturation in starfish but not frogs

The stimulation of oocyte maturation by 1-methyladenine in starfish, and by a steroid in frogs, has been proposed to involve G-protein-coupled receptors. To examine whether activation of receptors linked to G(i) or G(z) was sufficient to cause oocyte maturation, we expressed mammalian G(i)- and G(z)-linked receptors in starfish and frog oocytes. Application of the corresponding agonists caused meiosis to resume in the starfish but not the frog oocytes. We confirmed that the receptors were effectively expressed in the frog oocytes by using a chimeric G-protein, G(qi), that converts input from G(i)- and G(z)-linked receptors to a G(q) output and results in a contraction of the oocyte's pigment. These results argue against G(i) or G(z) functioning to cause maturation in frog oocytes. Consistently, maturation-inducing steroids did not cause pigment contraction in frog oocytes expressing G(qi), and G(z) protein was not detectable in frog oocytes. For starfish oocytes, however, our results support the conclusion that G(i) functions in 1-methyladenine signaling and suggest the possibility of using frog oocyte pigment contraction as an assay to identify the 1-methyladenine receptor. To test this concept, we coexpressed G(qi) and a starfish adenosine receptor in frog oocytes and showed that applying adenosine caused pigment contraction.

Scientific and therapeutic advances in antiplatelet therapy

Over the past decade, the platelet has emerged as a pivotal entity in cardiovascular diseases. Indeed, the 'preeminence of the platelet' has spawned a variety of drugs that have been shown in large-scale randomized trials to improve patient outcomes in acute coronary syndromes and percutaneous revascularization procedures. Although the platelet was initially viewed only as a bystander in haemostasis, it is now evident that the platelet is in fact a key mediator of thrombosis as well as of inflammation. New insights at the cellular and genomic levels will probably generate novel drugs to inhibit platelet function more effectively and safely than previously possible.

Thrombopoietin Enhances Rapid Current Responses Mediated by P2X1 Receptors on Megakaryocytic Cells in Culture

The effect of thrombopoietin (TPO), a magakaryocytopoietic cytokine, on the functional maturation of megakaryocytes was studied by using cell culture and patch-clamp techniques focusing on purinergic 2X(1) (P2X(1))-receptors, which are expressed specifically on platelets and their progenitors. Meg-01 cells, one of the typical human megakaryocytic cell lines, were cultured and studied by using a whole-cell patch electrode. In control cells cultured in RPMI1640 medium, an application of adenosine nucleotide (ADP, 40 microM) evoked transient inward currents with amplitudes of 45 +/- 19 pA (at -43 mV). Based on kinetic, ionic, and pharmacological properties as well as on previously reported findings, these currents were thought to be mediated by P2X(1) receptors. When Meg-01 cells were cultured for 7-9 d in a medium to which the differentiation-inducing agent phorbol ester (PMA; 10 nM) or TPO (100 ng/ml) had been added, the responses of the cells to ADP increased to about 150% of the control with PMA and to about 200% of the control value with TPO. A combination of the two agents enhanced the response of the cells to ADP to about 570% of the control value. These results suggest that phorbol ester and TPO cause cellular differentiation of Meg-01 cells and enhance the level of expression of P2X(1)-receptors on cell membranes in a synergetic manner. The effect of TPO on the induction of P2X(1)-receptors on mouse megakaryocytes in culture was more obvious.

Lineage-specific overexpression of the P2Y1 receptor induces platelet hyper-reactivity in transgenic mice

In order to investigate the role of the platelet P2Y1 receptor in several aspects of platelet activation and thrombosis, transgenic (TG) mice overexpressing this receptor specifically in the megakaryocytic/platelet lineage were generated using the promoter of the tissue-specific platelet factor 4 gene. Studies of the saturation binding of [33P]2MeSADP in the presence or absence of the selective P2Y1 antagonist MRS2179 indicated that wild-type (WT) mouse platelets bore 150 +/- 31 P2Y1 receptors and TG platelets 276 +/- 34, representing an 84% increase in P2Y1 receptor density. This led to a well defined phenotype of platelet hyper-reactivity in vitro, as shown by increased aggregations in response to adenosine 5'-diphosphate (ADP) and low concentration of collagen in TG as compared with WT platelets. Moreover, overexpression of the P2Y1 receptor enabled ADP to induce granule secretion, unlike in WT platelets, which suggests that the level of P2Y1 expression is critical for this event. Our results further suggest that the weak responses of normal platelets to ADP are due to a limited number of P2Y1 receptors rather than to activation of a specific transduction pathway. TG mice displayed a shortened bleeding time and an increased sensitivity to in vivo platelet aggregation induced by infusion of a mixture of collagen and epinephrine. Overall, these findings emphasize the importance of the P2Y1 receptor in hemostasis and thrombosis and suggest that variable expression levels of this receptor on platelets might play a role in thrombotic states in human, which remains to be assessed.

Pharmacological identification of P2X1, P2X4 and P2X7 nucleotide receptors in the smooth muscles of human umbilical cord and chorionic blood vessels

To ascertain the role of extracellular adenosine 5'-triphosphate (ATP) receptors in human placenta circulation, we identified and pharmacologically characterized the P2X receptor population in its superficial vessels. Total RNA was extracted from segments of chorionic and umbilical arteries and veins of terminal placentae delivered by vaginal or Caesarian births. Polymerase chain reaction (PCR), followed by sequencing of the products, identified the presence of P2X 1, 4, 5, 6, and 7mRNAs in smooth muscle from chorionic and umbilical arteries and veins. Umbilical vessels proximal to the fetus expressed the same population of P2X subtypes, except for the P2X(5), but additionally expressed the P2X(2). Rings of chorionic vessels contracted upon addition of nucleotides and analogs with the following relative rank order of potencies in arteries and veins: alpha,beta-methyleneATP>beta,gamma-methyleneATP>PNP>ATP=diBzATP>2-MeSATP>ADP>AMP; in umbilical vessels alpha,beta-methyleneATP was at least 100-fold more potent than ATP. Nucleotide potency was less than that of PGF(2alpha) or endothelin-2, but had the same magnitude as serotonin. ATP-desensitized receptors evidenced cross desensitization to alpha,beta-methyleneATP, 2-MeSATP and diBzATP, effect not observed when desensitization was elicited by alpha,beta-methyleneATP, confirming the presence of various P2X receptor subtypes in the smooth muscles of these vessels. The vasocontractile efficacy of alpha,beta-methyleneATP was unaltered by endothelium removal, while that of ATP was significantly attenuated and those elicited by 2-MeSATP were blunted, indicating the presence of additional endothelial nucleotide receptors. These results suggest that P2X receptors participate in the humoral regulation of placental blood flow.

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.

P2X(1) receptor-deficient mice establish the native P2X receptor and a P2Y6-like receptor in arteries

The contribution of P2 receptors to vasoconstriction of mouse mesenteric arteries was determined using wild-type (WT) and P2X(1) receptor-deficient (KO) animals. alpha,beta-methylene ATP (alpha,beta-meATP) and ATP evoked transient inward currents and constrictions of WT mesenteric arteries. In contrast, alpha,beta-meATP (100 microM) and ATP (100 microM) failed to evoke responses in KO arteries from a range of vascular beds. Nerve stimulation (100 pulses at 10 Hz) evoked constrictions of mesenteric arteries. For WT arteries, the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2'-5'-disulfonate (PPADS) (30 microM) reduced the amplitude of response by approximately 50%; the residual constriction was abolished by prazosin (0.1 microM). In KO mice, vasoconstriction induced by nerve stimulation was reduced in amplitude by approximately 50%, unaffected by PPADS, but was abolished by prazosin. ADP (1 mM) (a P2Y(1), P2Y(12), and P2Y(13) receptor agonist) was ineffective. Because ATP had no effect on mesenteric artery tone from KO mice, this rules out the contribution of P2Y(2) receptors. The P2Y(4) receptor agonist ITP also failed to contract mesenteric arteries. However, UTP and UDP evoked sustained contractions of mesenteric arteries with similar potency (EC(50) approximately 10 microM). Complementary studies using reverse-transcriptase polymerase chain reaction showed that mesenteric arteries express P2Y(1), P2Y(2), and P2Y(6) receptors. These results demonstrate that homomeric P2X(1) receptors underlie the artery smooth muscle P2X receptor phenotype and contribute approximately 50% to sympathetic neurogenic vasoconstriction and indicate the presence of a UTP- and UDP-sensitive P2Y(6)-like receptor, but not vasoconstrictor P2Y(2) or P2Y(4) receptors, on mouse mesenteric arteries.

Induction of COX-2 and reactive gliosis by P2Y receptors in rat cortical astrocytes is dependent on ERK1/2 but independent of calcium signalling

The present study has been aimed at characterizing the ATP/P2 receptor (and transductional pathways) responsible for the morphological changes induced in vitro by alphabetamethyleneATP on rat astrocytes obtained from cerebral cortex, a brain area highly involved in neurodegenerative diseases. Exposure of cells to this purine analogue resulted in elongation of cellular processes, an event reproducing in vitro a major hallmark of in vivo reactive gliosis. alphabetamethyleneATP-induced gliosis was prevented by the P2X/P2Y blocker pyridoxalphosphate-6-azophenyl-2'-4'-disulfonic acid, but not by the selective P2X antagonist 2',3'-O-(2,4,6-trinitrophenyl)-ATP, ruling out a role for ligand-gated P2X receptors. Conversely, the Gi/Go protein inactivator pertussis toxin completely prevented alphabetamethyleneATP-induced effects. No effects were induced by alphabetamethyleneATP on intracellular calcium concentrations. RT-PCR and western blot analysis showed that alphabetamethyleneATP-induced gliosis involves up-regulation of cyclooxygenase-2 (but not lipooxygenase). Also this effect was fully prevented by pyridoxalphosphate-6-azophenyl-2'-4'-disulfonic acid. Experiments with inhibitors of mitogen-activated protein kinases (MAPK) suggest that extracellular signal regulated protein kinases (ERK)1/2 mediate both cyclooxygenase-2 induction and the associated in vitro gliosis. These findings suggest that purine-induced gliosis involves the activation of a calcium-independent G-protein-coupled P2Y receptor linked to ERK1/2 and cyclooxygenase-2. Based on the involvement of cyclooxygenase-2 and inflammation in neurodegenerative diseases, these findings open up new avenues in the identification of novel biological targets for the pharmacological manipulation of neurodegeneration.

Protein kinase C- and calcium-regulated pathways independently synergize with Gi pathways in agonist-induced fibrinogen receptor activation

Platelet fibrinogen receptor activation is a critical step in platelet plug formation. The fibrinogen receptor (integrin alphaIIbbeta3) is activated by agonist-mediated G(q) stimulation and resultant phospholipase C activation. We investigated the role of downstream signalling events from phospholipase C, namely the activation of protein kinase C (PKC) and rise in intracellular calcium, in agonist-induced fibrinogen receptor activation using Ro 31-8220 (a PKC inhibitor) or dimethyl BAPTA [5,5'-dimethyl-bis-(o-aminophenoxy)ethane-N,N,N', N'-tetra-acetic acid], a high-affinity calcium chelator. All the experiments were performed with human platelets treated with aspirin, to avoid positive feedback from thromboxane A2. In the presence of Ro 31-8220, platelet aggregation caused by U46619 was completely inhibited while no effect or partial inhibition was seen with ADP and the thrombin-receptor-activating peptide SFLLRN, respectively. In the presence of intracellular dimethyl BAPTA, ADP- and U46619-induced aggregation and anti-alphaIIbbeta3 antibody PAC-1 binding were completely abolished. However, similar to the effects of Ro 31-8220, dimethyl BAPTA only partially inhibited SFLLRN-induced aggregation, and was accompanied by diminished dense-granule secretion. When either PKC activation or intracellular calcium release was abrogated, aggregation and fibrinogen receptor activation with U46619 or SFLLRN was partially restored by additional selective activation of the G(i) signalling pathway. In contrast, when both PKC activity and intracellular calcium increase were simultaneously inhibited, the complete inhibition of aggregation that occurred in response to either U46619 or SFLLRN could not be restored with concomitant G(i) signalling. We conclude that, while the PKC- and calcium-regulated signalling pathways are capable of inducing activating fibrinogen receptor independently and that each can synergize with G(i) signalling to cause irreversible fibrinogen receptor activation, both pathways act synergistically to effect irreversible fibrinogen receptor activation.

Immunoreactive localisation of P2Y1 receptors within the rat and human nodose ganglia and rat brainstem: comparison with [alpha 33P]deoxyadenosine 5'-triphosphate autoradiography

The present study employed standard peroxidase immunohistochemistry to map the distribution of P2Y(1) receptors in the rat brainstem and nodose ganglia and characterised the binding profile of [alpha(33)P]dATP. Binding of [alpha(33)P]dATP was fully displaceable by adenosine 5'-triphosphate (ATP), and was found on both human and rat nodose ganglia, and throughout the rat brainstem, including the nucleus tractus solitarius and ventrolateral medulla. [Alpha(33)P]dATP binding in the human nodose ganglia was significantly displaced by both 2-methylthio ATP and alpha,beta-methylene ATP, but not by uridine 5'-triphosphate, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, 8,8'-(carbonylbis(imino-4,1-phenylenecarbonylimino-4,1-phenylenecarbonylimino))bis(1,3,5-naphtalenetrisulfonic) acid (NF279) or N-ethylcarboxamidoadenosine. [Alpha(33)P]dATP binding in the rat nodose ganglia and brainstem was significantly displaced by only 2-methylthio ATP, suggesting that [alpha(33)P]dATP is binding to P2Y receptors in the rat. Binding of [alpha(33)P]dATP was also significantly displaced by alpha,beta-methylene adenosine 5'-diphosphate, suggesting a component of the binding is to endogenous ecto-5'-nucleotidase, however, almost all binding could be displaced by a combination of receptor agonists (2-methylthio ATP, uridine 5'-triphosphate and alpha,beta-methylene ATP), suggesting preferential binding to receptors. Immunoreactivity to P2Y(1) receptor (P2Y(1)-IR) exhibited similar distribution patterns to [alpha(33)P]dATP binding, with a clear topographic profile. Particularly dense P2Y(1)-IR labeling was evident in cells and fibres of the dorsal vagal complex. Immunolabeling was also present in the dorsal motor nucleus of the vagus and nucleus ambiguus, indicating the possibility of P2Y(1) receptors on vagal efferents. Unilateral vagal ligation was also performed to examine the transport of P2Y(1) receptor, using both immunohistochemistry and [alpha(33)P]dATP autoradiography. Accumulations of both P2Y(1)-IR and [alpha(33)P]dATP binding were apparent adjacent to both ligatures, suggesting bi-directional transport of P2Y(1) receptors along the rat vagus nerve. This current study represents the first description of P2Y(1) receptor distribution within the rodent brainstem and nodose ganglion and also characterises [alpha(33)P]dATP binding to P2Y receptors.

P2Y-receptors mediating an inhibition of the evoked entry of calcium through N-type calcium channels at neuronal processes

In the search for P2-receptors modulating the stimulation-evoked entry of calcium at processes of PC12 cells differentiated in the presence of nerve growth factor and neurotrophin-3, electrically evoked increases in free calcium were assessed by fura-2 microfluorimetry. Omission of calcium and addition of cadmium (100 microM) or the N-type calcium channel blocker omega-conotoxin GVIA (0.5 microM) abolished or markedly reduced the evoked responses. The P2Y-receptor agonists 2-methylthio adenosine 5'-diphosphate (2-methylthio-ADP), ADP, and adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS) inhibited the electrically evoked entry of calcium without any changes in basal calcium concentrations. 2-Methylthio-ADP was the most potent agonist. Adenosine, P(1),P(4)-di(adenosine-5')-tetraphosphate (Ap4A), UDP, and UTP (30 microM each) had no effect. The effect of ADPbetaS (30 microM) was abolished by the P2-antagonists reactive blue 2 (3 microM), suramin (100 microM), 2-methylthio-AMP (10 microM), p-chloromercuriphenyl sulfonic acid (1 microM), and AR-C 69931MX [N(6)-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene adenosine 5'-triphosphate] (300 nM). In contrast, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (10 microM), the selective P2Y1-receptor antagonist MRS 2179 (N(6)-methyl-2'-deoxyadenosine 3',5'-bisphosphate; 10 microM), as well as the adenosine A(1)-receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine; 100 nM), caused no change. Pretreatment with pertussis toxin abolished the effect of ADPbetaS. Reverse transcriptase-polymerase chain reaction revealed the presence of mRNA for P2Y12-receptors in nondifferentiated and differentiated PC12 cells. The results indicate that processes of differentiated PC12 cells possess P2Y12-receptors coupling to pertussis toxin-sensitive G-proteins and mediating an inhibition of the stimulation-evoked entry of calcium through omega-conotoxin GVIA-sensitive calcium channels. This suggests a role of P2Y12-receptors in neuromodulation in addition to their involvement in platelet aggregation.

Construction of hypothetical three-dimensional structure of P2Y1 receptor based on Fourier transform analysis

G protein-coupled receptors constitute a large family of homologous transmembrane proteins that represents one of the most important classes of confirmed drug targets. For novel drug discovery, the 3D structure of target protein is indispensable. To construct hypothetical 3D structures of G protein-coupled receptors, several prediction methods have been proposed. But none of the them has confirmed a correct ligand binding site. In this study we constructed the 3D structure of bovine rhodopsin using the prediction method proposed by Donnelly et al., with some modification. We found that our 3D model showed a good agreement with the reported retinal binding site. Using the similar method, we constructed the 3D structure of the P2Y1 receptor; one of the G protein-coupled receptors, and showed a binding site of an endogenous ligand, ADP, on the basis of the 3D model and in vitro experimental data. These results should be valuable for design of a specific antagonist for P2Y1 receptor.

Microglial activation by purines and pyrimidines

Microglial activation by purines and pyrimidines is reviewed, with emphasis on the actions of adenosine 5'-triphosphate (ATP) on chemotaxis or releases of plasminogen and cytokines from microglia. ATP activates microglia, causing morphological changes with membrane ruffling. Activated microglia exhibit chemotaxis to ATP. Microglia stimulated by a low concentration of ATP (approximately 30-50 microM) rapidly release plasminogen (within 5-10 min), which may protect neurons. Microglia stimulated by a higher concentration of ATP release tumor necrosis factor-alpha (TNF-alpha), 2-3 h after the stimulation and interleukin-6 (IL-6), 6 h after the stimulation. It is reported that TNF-alpha stimulation causes an increase in the expression of IL-6 receptor mRNA and expression in neuronal cells (März et al. 1996. Brain Res 706:71-79). After binding with gp130, the IL-6 receptor matures and can accept IL-6 molecules. It is speculated that neurons may require several hours to prepare for the full reception of IL-6, which induces a more efficient protective effect by IL-6 after stimulation with TNF-alpha. After neurons are ready to accept IL-6 fully, microglia release IL-6 to neurons. Stronger and longer stimulation by ATP may change the function of microglia and cause cell death. The conditions evoking the heavy stimulation would result from serious injury. Activated microglia act as scavenger cells that induce apoptosis in damaged neurons by releasing toxic factors, including NO, and removing dead cells, their remnants, or dangerous debris by phagocytosis. These actions lead to a suitable environment for tissue repair and neural regeneration. The fate of neurons may therefore be regulated in part by ATP through the activation of microglia.

Human Ntera-2/D1 neuronal progenitor cells endogenously express a functional P2Y1 receptor

We report here that human Ntera-2/D1 (NT-2) cells, an undifferentiated committed neuronal progenitor cell line, endogenously express a functional P2Y(1) receptor, while other P2Y subtypes, except perhaps P2Y(4), are not functionally expressed. Quantitative RT-PCR analysis showed that NT-2 cells abundantly express mRNA for P2Y(1) and P2Y(11) receptors, while P2Y(2) and P2Y(4) receptors were detected at considerably lower levels. Western blot analysis also demonstrated expression of P2Y(1) receptors and Galpha(q/11) subunits. Various nucleotides induced intracellular Ca(2+) mobilisation in NT-2 cells in a concentration-dependent manner with a rank order potency of 2-MeSADP > 2-MeSATP > ADP > ATP > UTP > ATPgammaS, a profile resembling that of human P2Y(1) receptors. Furthermore, P2Y(1) receptor-specific (A3P5P) and P2Y-selective (PPADS, suramin) antagonists inhibited adenine nucleotide-induced Ca(2+) responses in a concentration-dependent manner, consistent with expression of a P2Y(1) receptor. Moreover, of seven adenine nucleotides tested, only Bz-ATP and ATPgammaS elicited small increases in cAMP formation suggesting that few, if any, functional P2Y(11) receptors were expressed. P2Y(1) receptor-selective adenine nucleotides, including 2-MeSADP and ADP, also induced concentration-dependent phosphorylation and hence, activation of the extracellular-signal regulated protein kinases (ERK1/2). NT-2 cells, therefore, provide a useful neuronal-like cellular model for studying the precise signalling pathways and physiological responses mediated by a native P2Y(1) receptor.

Costimulation of Gi- and G12/G13-mediated signaling pathways induces integrin alpha IIbbeta 3 activation in platelets

Platelet activation is a complex process induced by a variety of stimuli, which act in concert to ensure the rapid formation of a platelet plug at places of vascular injury. We show here that fibrillar collagen, which initiates platelet activation at the damaged vessel wall, activates only a small fraction of platelets in suspension directly, whereas the majority of platelets becomes activated by mediators released from collagen-activated platelets. In Galpha(q)-deficient platelets that do not respond with activation of integrin alpha(IIb)beta(3) to a variety of mediators like thromboxane A2 (TXA2), thrombin, or ADP, collagen at high concentrations was able to induce aggregation, an effect that could be blocked by antagonists of the TXA2 or P2Y12 receptors. The activation of TXA2 or P2Y12 receptors alone, which in Galpha(q)-deficient platelets couple to G12/G13 and Gi, respectively, did not induce platelet integrin activation or aggregation. However, concomitant activation of both receptors resulted in irreversible integrin alpha(IIb)beta3-mediated aggregation of Galpha(q)-deficient platelets. Thus, the activation of G12/G13- and Gi-mediated signaling pathways is sufficient to induce integrin alpha(IIb)beta3 activation. Although G(q)-mediated signaling plays an important role in platelet activation, it is not strictly required for the activation of integrin alpha(IIb)beta3. This indicates that the efficient induction of platelet aggregation through G-protein-coupled receptors is an integrated response mediated by various converging G-protein-mediated signaling pathways involving G(q) and G(i) as well as G12/G13.

Adenosine triphosphate accelerates recovery from hypoxic/hypoglycemic perturbation of guinea pig hippocampal neurotransmission via a P(2) receptor

The present study was designed to assess the effects of adenosine triphosphate (ATP) on hippocampal neurotransmissions under the normal and hypoxic/hypoglycemic conditions. ATP reversely depressed population spikes (PSs), which were monitored in the dentate gyrus of guinea pig hippocampal slices, in a dose-dependent manner at concentrations ranged from 0.1 micro M to 1 mM. A similar depression was obtained with the P(2) receptor agonist, alpha,beta-methylene ATP (alpha,beta-MeATP), and the effect was inhibited by the P(2) receptor antagonists, suramin and PPADS. The inhibitory action of ATP or alpha,beta-MeATP was inhibited by the gamma-aminobutyric acid(A) (GABA(A)) receptor antagonist, bicuculline, but it was not affected by theophylline, a broad inhibitor of adenosine (P(1)) receptors, tetraethylammonium, a broad inhibitor of K(+) channels, or ecto-protein kinase inhibitors. ATP or alpha,beta-MeATP enhanced GABA release from guinea pig hippocampal slices, that was inhibited by deleting extracellular Ca(2+) or in the presence of tetrodotoxin, while ATP had no effect on GABA release from cultured rat hippocampal astrocytes or postsynaptic GABA-gated channel currents in cultured rat hippocampal neurons. Twenty-minutes deprivation of glucose and oxygen from extracellular solution abolished PSs, the amplitude recovering to about 30% of basal levels 50 min after returning to normal conditions. ATP or alpha,beta-MeATP accelerated the recovery after hypoxic/hypoglycemic insult (approximately 80% of basal levels). Adenosine diphosphate and adenosine monophosphate accelerated the recovery, but to a much lesser extent, and adenosine had no effect. The results of the present study thus suggest that ATP inhibits neuronal activity by enhancing neuronal GABA release via a P(2) receptor, perhaps a P2X receptor, thereby protecting against hypoxic/hypoglycemic perturbation of hippocampal neurotransmission.

Purines and pyrimidines are not involved in NANC relaxant responses in the rabbit vaginal wall

1. Non-adrenergic non-cholinergic (NANC) relaxant responses were elicited by electrical field stimulation (EFS) in rabbit vaginal wall strips after treatment with guanethidine and scopolamine and raising smooth muscle tone with phenylephrine. Under these conditions treatment with NOS inhibitors revealed a non-nitrergic NANC relaxant response. The possible role of purines and pyrimidines in these non-nitrergic NANC responses was investigated. 2. Exogenous application of ATP, ADP, adenosine, UTP, or UDP (all at 0.03-10 mM) induced concentration-dependent relaxant responses. 3. Responses to exogenous application of ATP were reduced by the general P2 antagonist cibacron blue (500 micro M), but not by suramin (100 micro M) and were unaffected by L-NAME (500 micro M), omega-conotoxin GVIA (omega-CTX, 500 nM) or tetrodotoxin (TTX, 1 micro M). 4. Responses to exogenous application of adenosine were reduced by the A(2A) antagonist ZM-241385 (30 micro M). 5. ATP- and ADP-induced responses were unaffected by the G-protein inhibitor pertussis toxin (100 ng ml(-1)), whilst ADP- but not ATP-induced responses were reduced by GDPbetaS (100 micro M), which stabilizes G-proteins in their inactive state. 6. EFS-induced non-nitrergic NANC relaxant responses were unaffected by suramin, cibacron blue, ZM-241385, pertussis toxin or GDPbetaS, but were completely inhibited by TTX. 7. Exogenous application of ATP (10 mM) and adenosine (10 mM) increased intracellular cyclic adenosine-3', 5'-monophosphate (cAMP). However, non-nitrergic NANC responses were not associated with increased cAMP. Neither non-nitrergic NANC responses nor responses to ATP or adenosine were associated with increased intracellular cyclic guanosine-3', 5'-monophosphate (cGMP) concentrations. 8. These results suggest that adenosine A(2A) receptors and P2 receptors are present in the rabbit vaginal wall, but that they are not involved in non-nitrergic NANC relaxant responses.

Specific diadenosine pentaphosphate receptor coupled to extracellular regulated kinases in cerebellar astrocytes

In this study, we show specific intracellular responses evoked by the stimulation of astrocytes with the P1,P5-di(adenosine-5')pentaphosphate, Ap5A. The stimulation of astrocytes with micromolar concentrations of the dinucleotide elicited rapid increases in intracellular calcium concentration ([Ca2+]i), showing an EC50 value of 15.27 +/- 0.61 micro m. Moreover, the stimulation of cells with nanomolar concentrations of Ap5A, unable to induce calcium responses, increased the phosphorylated forms of extracellular-signal regulated kinase 1/2 (ERK) with an EC50 value of 9.8 +/- 2.4 nm. The maximal activation was observed at 100 nm Ap5A, which was similar to that produced by epidermal growth factor (EGF) under the same experimental conditions. The present data reported here indicate that Ap5A mediated these effects by interacting with a specific receptor, not yet identified, which was different from the P2Y1 and P2Y2/P2Y4 receptors present in all individual astrocytes.

P2X(1)-mediated activation of extracellular signal-regulated kinase 2 contributes to platelet secretion and aggregation induced by collagen

Adenosine triphosphate (ATP) and its stable analog, alpha,beta-methylene ATP, activate the platelet P2X(1) ion channel, causing a rapid Ca(++) influx. Here, we show that, in washed apyrase-treated platelets, alpha,beta-methylene ATP elicits reversible extracellular signal-regulated kinase 2 (ERK2) phosphorylation through a Ca(++)- and protein kinase C-dependent pathway. In contrast, high-performance liquid chromatography-purified adenosine diphosphate (ADP) did not trigger ERK2 phosphorylation. alpha,beta-Methylene ATP also activated the ERK2 pathway in P2X(1)-transfected HEK293 cells but not in cells expressing mutated P2X(1)delL nonfunctional channels. Because ATP released from the dense granules during platelet activation contributes to platelet aggregation elicited by low doses of collagen, and because collagen causes ERK2 phosphorylation, we have investigated the role of P2X(1)-mediated ERK2 activation in these platelet responses. We found that the antagonism of P2X(1) with ADP or desensitization of this ion channel with alpha,beta-methylene ATP both resulted in impaired ERK2 phosphorylation, ATP secretion, and platelet aggregation induced by low concentrations of collagen (< or = 1 microg/mL) without affecting the minor early dense granule release. Selective MEK1/2 inhibition by U-0126 and Ca(++) chelation with EGTA (ethyleneglycoltetraacetic acid) behaved similarly, whereas the PKC inhibitor GF109203-X totally prevented collagen-induced secretion and ERK2 activation. In contrast, when elicited by high collagen concentrations (2 microg/mL), platelet aggregation and secretion no longer depended on P2X(1) or ERK2 activation, as shown by the lack of their inhibition by alpha,beta-methylene ATP or U-0126. We thus conclude that mild platelet stimulation with collagen rapidly releases ATP, which activates the P2X(1)-PKC-ERK2 pathway. This process enhances further degranulation of the collagen-primed granules allowing platelet aggregation to be completed.

Effects of a P2Y(12) receptor antagonist on the response of equine platelets to ADP. Comparison with human platelets

Horses show susceptibility to platelet-related disorders. Equine platelets differ from human platelets in some of their responses, so information available about human platelets must be validated in the horse. Aggregation of platelets by ADP involves both P2Y(1) and P2Y(12) receptors on the platelet surface. We have compared the effect of the P2Y(12) antagonist, AR-C67085, on equine and human platelets in vitro using turbidimetric aggregometry to measure the rate and final extent of aggregation. Aggregation profiles, concentration-response curves and pA(2) values show that the rate of aggregation of equine platelets is much more susceptible to inhibition by AR-C67085 than that of human platelets. This species difference may reflect differences in the relative numbers of P2Y(1) and P2Y(12) receptors, or in intracellular signalling pathways, but will need to be considered by equine clinicians before using P2Y(12) antagonists in the treatment of thrombotic conditions.