The platelet P2 receptors as molecular targets for old and new antiplatelet drugs

Characterization of P2Y receptor subtypes functionally expressed on neonatal rat cardiac myofibroblasts

BACKGROUND AND PURPOSE

Little is known about P2Y receptors in cardiac fibroblasts, which represent the predominant cell type in the heart and differentiate into myofibroblasts under certain conditions. Therefore, we have characterized the phenotype of the cells and the different P2Y receptors at the expression and functional levels in neonatal rat non-cardiomyocytes.

EXPERIMENTAL APPROACH

Non-cardiomyocyte phenotype was determined by confocal microscopy by using discoidin domain receptor 2, alpha-actin and desmin antibodies. P2Y receptor expression was investigated by reverse transcription-polymerase chain reaction and immunocytochemistry, and receptor function by cAMP and inositol phosphate (IP) accumulation induced by adenine or uracil nucleotides in the presence or absence of selective antagonists of P2Y(1) (MRS 2179, 2-deoxy-N(6)-methyl adenosine 3',5'-diphosphate diammonium salt), P2Y(6) (MRS 2578) and P2Y(11) (NF 157, 8,8'-[carbonylbis[imino-3,1-phenylenecarbonylimino(4-fluoro-3,1-phenylene)carbonylimino]]bis-1,3,5-naphthalene trisulphonic acid hexasodium salt) receptors. G(i/o) and G(q/11) pathways were evaluated by using Pertussis toxin and YM-254890 respectively.

KEY RESULTS

The cells (>95%) were alpha-actin and discoidin domain receptor 2-positive and desmin-negative. P2Y(1), P2Y(2), P2Y(4), P2Y(6) were detected by reverse transcription-polymerase chain reaction and immunocytochemistry, and P2Y(11)-like receptors at protein level. All di- or tri-phosphate nucleotides stimulated IP production in an YM-254890-sensitive manner. AMP, ADPbetaS, ATP and ATPgammaS increased cAMP accumulation, whereas UDP and UTP inhibited cAMP response, which was abolished by Pertussis toxin. MRS 2179 and NF 157 inhibited ADPbetaS-induced IP production. MRS 2578 blocked UDP- and UTP-mediated IP responses.

CONCLUSION AND IMPLICATIONS

P2Y(1)-, P2Y(2)-, P2Y(4)-, P2Y(6)-, P2Y(11)-like receptors were co-expressed and induced function through G(q/11) protein coupling in myofibroblasts. Furthermore, P2Y(2) and P2Y(4) receptor subtypes were also coupled to G(i/o). The G(s) response to adenine nucleotides suggests a possible expression of a new P2Y receptor subtype.

Acute arterial thrombosis in the absence of inflammation: the stress-related anti-inflammatory hormone ACTH participates in platelet-mediated thrombosis

OBJECTIVE

Despite the reciprocal relationship that exists between inflammation and thrombosis, we asked whether thrombosis can develop without inflammation, and whether stress-related hormones (ACTH and cortisol) influence platelet-mediated thrombosis.

METHODS

We investigated the role of ACTH and cortisol in platelet aggregation, as well as on the circulating levels of IL-6 in pigs subjected to different treatments. In control animals, deep vessel wall injury (DVWI) was induced in the right common carotid artery, while in the animals under study DVWI was induced 60 min after ACTH administration (subgroup 1) or not at all (subgroup 2). In an ex vivo study we evaluated whether ACTH or cortisol modulates platelet aggregation. Indeed, we assessed whether blocking the P2Y platelet receptors inhibits the effect of ACTH on platelet aggregation. Finally, we assessed whether ACTH mobilizes intracellular calcium and modulates intracellular cAMP in platelets ex vivo.

RESULTS

We found that the suppression of inflammation following ACTH administration was accompanied by acute arterial thrombosis in the zone of injury in vivo. Furthermore, ACTH but not cortisol amplifies the platelet aggregation induced ex vivo by agonists. Platelets do not express ACTH receptors which may explain why ACTH does not reduce intracellular levels of cAMP in platelets. Nevertheless, supraphysiological concentrations of ACTH increase calcium mobilization in platelets.

CONCLUSION

These results indicate for the first time that ACTH may fulfil an important role in acute arterial thrombosis by increasing the platelet aggregation induced by agonists, probably via a G(q)-coupled pathway.

Coronary artery stents: Part I. Evolution of percutaneous coronary intervention

The subspecialty of interventional cardiology has made significant progress in the management of coronary artery disease over the past three decades with the development of percutaneous coronary transluminal angioplasty, atherectomy, and bare-metal and drug-eluting stents (DES). Bare-metal stents (BMS) maintain vessel lumen diameter by acting as a scaffold and prevent collapse incurred by angioplasty. However, these devices cause neointimal hyperplasia leading to in-stent restenosis and requiring reintervention in more than 20% of patients by 6 mo. DES (sirolimus and paclitaxel) prevent restenosis by inhibiting neointimal hyperplasia. However, DESs also delay endothelialization, causing the stents to remain thrombogenic for an extended, yet unknown, period of time. Late stent thrombosis is associated with a 45% mortality rate. Premature discontinuation of antiplatelet therapy, particularly clopidogrel, is the strongest predictor of stent thrombosis. Sixty percent of patients receive stents for off-label (unapproved) indications, which also increases the frequency of stent thrombosis. Clopidogrel and aspirin are the cornerstone of therapy in the prevention of stent thrombosis in both BMS and DES. Recommendations pertaining to the optimal duration of dual-antiplatelet therapy have been debated. Both the Food and Drug Administration and the American Heart Association/American College of Cardiologists, in association with other major societies, have made recommendations to extend the duration of dual-antiplatelet therapy in patients with DES to 1 yr. The 6-wk duration of dual-antiplatelet therapy in patients with BMS remains unchanged. All patients with coronary stents must remain on life-long aspirin monotherapy. Since the introduction of percutaneous transluminal coronary angioplasty for the treatment of coronary atherosclerosis, the practice of percutaneous coronary intervention has undergone a dramatic transformation from simple balloon dilation catheters to sophisticated mechanical endoprostheses. These advancements have impacted the practice of perioperative medicine. In this series of two articles, in Part I we will review the evolution of percutaneous coronary intervention and discuss the issues associated with percutaneous transluminal coronary angioplasty and coronary stenting; in Part II we will discuss perioperative issues and management strategies of coronary stents during noncardiac surgery.

Reduced Atherosclerotic Lesions in P2Y 1 /Apolipoprotein E Double-Knockout Mice

Background— The P2Y 1 receptor plays a key role in arterial thrombosis and is widely expressed in many cell types involved in atherosclerosis. The aim of this study was to evaluate its potential involvement in the development of atherosclerotic lesions.

Methods and Results— Apolipoprotein E–deficient ( ApoE −/− ) and P2Y 1 −/− / ApoE −/− mice were maintained on regular chow for 17 or 30 weeks before analysis of atherosclerotic lesions. At 17 weeks, lesions in the aortic sinus and entire aorta were smaller in P2Y 1 −/− / ApoE −/− compared with those in ApoE −/− animals. At 30 weeks, the aortic sinus lesions in P2Y 1 −/− / ApoE −/− mice were still diminished in size and displayed reduced inflammation, reflected by decreased macrophage infiltration and diminished VCAM-1 immunostaining, compared with those in ApoE −/− mice. They also had a lower smooth muscle cell content. Unexpectedly, bone marrow transplantation showed that the absence of the P2Y 1 receptor in blood cells only led to no significant modification of the lesion compared with control ApoE −/− reconstituted animals. Conversely, the absence of the P2Y 1 receptor except in blood cells resulted in a reduction in lesion size similar to that in control P2Y 1 −/− / ApoE −/− reconstituted mice, pointing to a role of non–hematopoietic-derived P2Y 1 receptors, most likely the endothelial or smooth muscle cell P2Y 1 receptors. In addition, although this was not statistically significant, plasma cholesterol levels were consistently decreased in P2Y 1 −/− animals, suggesting that a modification of lipid metabolism could be responsible for the observed phenotype.

Conclusion— The P2Y 1 receptor contributes to atherosclerosis, primarily through its role in non–hematopoietic-derived cells.

Triggers, targets and treatments for thrombosis

Thrombosis--localized clotting of the blood--can occur in the arterial or the venous circulation and has a major medical impact. Acute arterial thrombosis is the proximal cause of most cases of myocardial infarction (heart attack) and of about 80% of strokes, collectively the most common cause of death in the developed world. Venous thromboembolism is the third leading cause of cardiovascular-associated death. The pathogenic changes that occur in the blood vessel wall and in the blood itself resulting in thrombosis are not fully understood. Understanding these processes is crucial for developing safer and more effective antithrombotic drugs.

Variable responsiveness to clopidogrel and aspirin among patients with acute coronary syndrome as assessed by platelet function tests

Unresponsiveness to clopidogrel or aspirin has been reported in patients with acute coronary syndrome (ACS). Platelet aggregometry (PA) and the Impact-R [Cone and Plate(let) Analyzer (CPA) technology, measuring whole blood platelet adhesion under flow conditions] were compared in detecting laboratory unresponsiveness to clopidogrel and aspirin among ACS patients. Platelet-rich plasma (PRP) samples were evaluated in 404 patients by PA using adenosine diphosphate (ADP) and arachidonic acid (AA) and whole blood samples by the Impact-R ADP- and AA-response tests. The first cohort (n=114) was assayed by PA on days 1 and 4 of the onset of ACS. A patient with relative decrease of </=10% in ADP-induced maximal platelet aggregation after clopidogrel treatment was defined as laboratory non-responding (NR) patient to clopidogrel. This relative value correlated well with an absolute value of ADP-induced aggregation >/=70%. A patient with an absolute value of AA-induced maximal aggregation >/=60% was defined as laboratory NR patient to aspirin. The second cohort (n=290) was tested on day 4 by both systems and results analyzed by receiver operating characteristic curve. The following cut-off values of the Impact-R surface coverage were obtained: </=2.8% and </=3.4% for clopidogrel and aspirin NR patients, respectively. The incidence of NR patients to clopidogrel and aspirin, according to the two methods was 27% and 22%, respectively. Impact-R compared to PA in detecting clopidogrel and aspirin NR patients revealed: 79% and 82% agreement, 71% and 73% sensitivity, 83% and 86% specificity, respectively. In conclusion, the Impact-R and PA results demonstrated high degree of similarity.

P2 receptors in cardiovascular regulation and disease

The role of ATP as an extracellular signalling molecule is now well established and evidence is accumulating that ATP and other nucleotides (ADP, UTP and UDP) play important roles in cardiovascular physiology and pathophysiology, acting via P2X (ion channel) and P2Y (G protein-coupled) receptors. In this article we consider the dual role of ATP in regulation of vascular tone, released as a cotransmitter from sympathetic nerves or released in the vascular lumen in response to changes in blood flow and hypoxia. Further, purinergic long-term trophic and inflammatory signalling is described in cell proliferation, differentiation, migration and death in angiogenesis, vascular remodelling, restenosis and atherosclerosis. The effects on haemostasis and cardiac regulation is reviewed. The involvement of ATP in vascular diseases such as thrombosis, hypertension and diabetes will also be discussed, as well as various heart conditions. The purinergic system may be of similar importance as the sympathetic and renin-angiotensin-aldosterone systems in cardiovascular regulation and pathophysiology. The extracellular nucleotides and their cardiovascular P2 receptors are now entering the phase of clinical development.

Identification of a unique co-operative phosphoinositide 3-kinase signaling mechanism regulating integrin alpha IIb beta 3 adhesive function in platelets

Phosphoinositide (PI) 3-kinases play an important role in regulating the adhesive function of a variety of cell types through affinity modulation of integrins. Two type I PI 3-kinase isoforms (p110 beta and p110 gamma) have been implicated in G(i)-dependent integrin alpha(IIb)beta(3) regulation in platelets, however, the mechanisms by which they coordinate their signaling function remains unknown. By employing isoform-selective PI 3-kinase inhibitors and knock-out mouse models we have identified a unique mechanism of PI 3-kinase signaling co-operativity in platelets. We demonstrate that p110 beta is primarily responsible for G(i)-dependent phosphatidylinositol 3,4-bisphosphate (PI(3,4)P(2)) production in ADP-stimulated platelets and is linked to the activation of Rap1b and AKT. In contrast, defective integrin alpha(IIb)beta(3) activation in p110 gamma(-/-) platelets was not associated with alterations in the levels of PI(3,4)P(2) or active Rap1b/AKT. Analysis of the effects of active site pharmacological inhibitors confirmed that p110 gamma principally regulated integrin alpha(IIb)beta(3) activation through a non-catalytic signaling mechanism. Inhibition of the kinase function of PI 3-kinases, combined with deletion of p110 gamma, led to a major reduction in integrin alpha(IIb)beta(3) activation, resulting in a profound defect in platelet aggregation, hemostatic plug formation, and arterial thrombosis. These studies demonstrate a kinase-independent signaling function for p110 gamma in platelets. Moreover, they demonstrate that the combined catalytic and non-catalytic signaling function of p110 beta and p110 gamma is critical for P2Y(12)/G(i)-dependent integrin alpha(IIb)beta(3) regulation. These findings have potentially important implications for the rationale design of novel antiplatelet therapies targeting PI 3-kinase signaling pathways.

New Antiplatelet Strategies in Atherothrombosis and Their Indications

Antiplatelet agents (APA) are used to reduce the risk of major cardiovascular events in various settings. When used for secondary prevention, antiplatelet monotherapy is associated with a relative risk reduction of such ischemic events of 25% compared to a placebo. New strategies are based on dual APA therapy. Aspirin-clopidogrel combination therapy is effective in situations of acute vessel injury such as myocardial infarction, coronary stenting and, possibly, peripheral stenting. GPIIb/IIIa inhibitors and loading doses of clopidogrel also have a place in these acute settings. In contrast, the aspirin-clopidogrel combination has proven disappointing in stable patients with cardiovascular disease, with no beneficial effect and, often, more bleeding events. Combination therapy with aspirin and extended-release dipyridamole may be more beneficial than very low doses of aspirin in ischemic stroke, but its use is limited by adverse effects. Overall, aspirin remains the first-line monotherapy of choice for patients with atherothrombosis, while clopidogrel is a valuable alternative. New antiplatelet strategies are in the pipeline, and clinically relevant laboratory tests of APA response may soon help to tailor treatment.

Primary and secondary agonists can use P2X(1) receptors as a major pathway to increase intracellular Ca(2+) in the human platelet

In the platelet, it is well established that many G-protein- and tyrosine kinase-coupled receptors stimulate phospholipase-C-dependent Ca(2+) mobilization; however, the extent to which secondary activation of adenosine 5'-triphosphate (ATP)-gated P2X(1) receptors contributes to intracellular Ca(2+) responses remains unclear. We now show that selective inhibition of P2X(1) receptors substantially reduces the [Ca(2+)](i) increase evoked by several important agonists in human platelets; for collagen, thromboxane A(2), thrombin, and adenosine 5'-diphoshate (ADP) the maximal effect was a reduction to 18%, 34%, 52%, and 69% of control, respectively. The direct contribution of P2X(1) to the secondary Ca(2+) response was far greater than that of either P2Y receptors activated by co-released ADP, or via synergistic P2X(1):P2Y interactions. The relative contribution of P2X(1) to the peak Ca(2+) increase varied with the strength of the initial stimulus, being greater at low compared to high levels of stimulation for both glycoprotein VI and PAR-1, whereas P2X(1) contributed equally at both low and high levels of stimulation of thromboxane A(2) receptors. In contrast, only strong stimulation of P2Y receptors resulted in significant P2X(1) receptor activation. ATP release was detected by soluble luciferin:luciferase in response to all agonists that stimulated secondary P2X(1) receptor activation. However, P2X(1) receptors were stimulated earlier and to a greater extent than predicted from the average ATP release, which can be accounted for by a predominantly autocrine mechanism of activation. Given the central role of [Ca(2+)](i) increases in platelet activation, these studies indicate that ATP should be considered alongside ADP and thromboxane A(2) as a significant secondary platelet agonist.

Platelet-neutrophil-interactions: Linking hemostasis and inflammation

Platelets are essential for primary hemostasis, but they also play an important pro-inflammatory role. Platelets normally circulate in a quiescent state. Upon activation, platelets can secrete and present various molecules, change their shape as well as the expression pattern of adhesion molecules. These changes are associated with the adhesion of platelets to leukocytes and the vessel wall. The interaction of platelets with neutrophils promotes the recruitment of neutrophils into inflammatory tissue and thus participates in host defense. This interaction of neutrophils with platelets is mainly mediated through P-selectin and beta(2) and beta(3) integrins (CD11b/CD18, CD41/CD61). Platelets can also interact with endothelial cells and monocytes. Adherent platelets promote the 'secondary capture' of neutrophils and other leukocytes. In addition, platelets secrete neutrophil and endothelial activators inducing production of inflammatory cytokines. Thus, platelets are important amplifiers of acute inflammation.

Inhibition of the platelet P2Y12 receptor for adenosine diphosphate potentiates the antiplatelet effect of prostacyclin

BACKGROUND

Activation of two receptors for adenosine diphosphate (ADP), P2Y(1) and P2Y(12), is necessary for ADP-induced platelet aggregation (PA). It is generally believed that the antithrombotic effects of drugs inhibiting P2Y(12), such as clopidogrel, are uniquely mediated by inhibition of P2Y(12)-dependent PA. However, as P2Y(12) is negatively coupled to adenylyl cyclase (AC), its inhibition may also exert antithrombotic effects through the potentiation of prostacyclin (PGI(2)), which inhibit PA by stimulating AC.

OBJECTIVES

To test whether inhibition of P2Y(12) potentiates the antiplatelet effects of PGI(2).

METHODS

We measured the effects of PGI(2) (0.01-10 microm) on PA of washed human platelets induced by thrombin (0.5 U mL(-1)) in the presence or absence of ARC69931MX (anti-P2Y(12)) or MRS2500 (anti-P2Y(1)).

RESULTS

PGI(2) inhibited PA in the presence of anti-P2Y(12), but not in the presence of anti-P2Y(1) or in the absence of inhibitors. In contrast, dibutyryl-cyclicAMP inhibited PA both in the presence and absence of anti-P2Y(1) or anti-P2Y(12). PGI(2) increased platelet cyclicAMP levels only in the absence of thrombin or in the presence of thrombin plus anti-P2Y(12).

CONCLUSIONS

PGI(2) did not inhibit PA induced by thrombin, because its effect on AC was prevented by released ADP interacting with P2Y(12). Anti-P2Y(12) drugs, by rescuing AC activity, potentiate the antiplatelet effect of PGI(2), which may contribute to their antithrombotic effect.

Effect of physiologic shear stresses and calcium on agonist-induced platelet aggregation, secretion, and thromboxane A2 formation

INTRODUCTION

Platelets in vivo react under low-shear venous-flow as well as high-shear arterial-flow conditions. Because most studies were carried out at low shear stresses, platelet granule secretion at high shear has not been examined thoroughly. We investigated the secretion of all three types of platelet granules and thromboxane A(2) formation at high shear after stimulation with ADP or thrombin.

MATERIALS AND METHODS

Washed human platelets were reacted rapidly (<5 s) in a quenched-flow system simulating high-shear arterial-flow conditions (30 dyn/cm(2)). For comparison, we employed a low-shear stirring system (1-5 dyn/cm(2)). Serotonin release and membrane exposure of P-selectin (alpha), CD63 (dense), and CD107a (lysosomes) were used to assess granule secretion. Aggregation was evaluated by resistive-particle counting of the remaining platelet singlets.

RESULTS AND CONCLUSIONS

ADP and thrombin induced similar strong levels of aggregation ( approximately 70%) at high shear by 5 s. Thrombin also caused release of about 40% of all alpha and dense granules within 5 s. However, by 5 s at high shear, ADP failed to induce significant granule secretion or thromboxane A(2) formation (<5%, p>0.05). By 10 min at low shear, ADP caused secretion and thromboxane A(2) formation only at non-physiological, micromolar extracellular Ca(2+) concentrations. These results emphasize the ability of thrombin to initiate multiple aspects of platelet function within seconds, while ADP was only able to induce rapid aggregation.

Brief review: Coronary drug-eluting stents and anesthesia

PURPOSE

Anesthesiologists managing patients with drug-eluting stents (DES) face the challenge of balancing the risks of bleeding vs perioperative stent thrombosis (ST). This article reviews DES and the influence of antiplatelet medications related to their use. A perioperative management algorithm is suggested. Novel P2Y12 antagonists currently under investigation, including cangrelor and prasugrel are considered, as well as their potential role in modification of perioperative cardiovascular risks and management of patients with DES.

SOURCE

A PubMed search of the relevant literature over the period 1985-2005 was undertaken using the terms "drug-eluting stent", "coronary artery stent", "bare metal stent", "antiplatelet medication", "aspirin", "clopidogrel."

PRINCIPAL FINDINGS

Delayed re-endothelialization may render both sirolimus-eluting and paclitaxel-eluting stents susceptible to thrombosis for a longer duration than bare metal stents. Stent thrombosis may be associated with resistance to antiplatelet medication. In patients with a DES, a preoperative cardiology consultation is essential. Elective surgery should be postponed if the duration between DES placement and noncardiac surgery is less than six months. For semi-emergent procedures, both aspirin and clopidogrel should be continued during surgery unless clearly contraindicated by the nature of the surgery. If the risk of bleeding is high, then modification of antiplatelet medications should be considered on a case-by-case basis.

CONCLUSION

A profound increase in the number of patients with DES requires anesthesiologists to be familiar with their associated antiplatelet medications, and strategies for risk modification of ST and possible hemorrhagic complications in the perioperative setting.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

Bacillus pasteurii urease shares with plant ureases the ability to induce aggregation of blood platelets

Ureases (EC 3.5.1.5) are highly homologous enzymes found in plants, bacteria and fungi. Canatoxin, an isoform Canavalia ensiformis urease, has several biological properties unrelated to its ureolytic activity, like platelet-aggregating and pro-inflammatory effects. Here, we describe that Bacillus pasteurii urease (BPU) also induces aggregation of rabbit platelets, similar to the canatoxin-induced effect (ED(50) 0.4 and 0.015 mg/mL, respectively). BPU induced-aggregation was blocked in platelets pretreated with dexamethasone and esculetin, a phospholipase A(2) and a lipoxygenase inhibitor, respectively, while platelets treated with indomethacin, a cyclooxygenase inhibitor, showed increased response to BPU. Methoxyverapamil (Ca(2+) channel blocker) and AMP (ADP antagonist) abrogated urease-induced aggregation, whereas the PAF-acether antagonist Web2170 had no effect. We concluded that platelet aggregation induced by BPU is mediated by lipoxygenase-derived eicosanoids and secretion of ADP from the platelets through a calcium-dependent mechanism. Potential relevance of these findings for bacterium-plant interactions and pathogenesis of bacterial infections are discussed.

Monitoring modulators of platelet aggregation in a microtiter plate assay

Platelets play a central role in maintaining biological hemostasis. Inappropriate platelet activation is responsible for thrombotic diseases such as myocardial infarction and stroke. Therefore, novel agents that can inhibit platelet activation are necessary. However, assays that monitor platelet aggregation are generally time-consuming and require high volumes of blood and specialized equipment. Therefore, a medium- to high-throughput assay that can monitor platelet aggregation would be considered useful. Such an assay should be sensitive, comparable to the "gold standard" assay of platelet aggregometry, and able to monitor multiple samples simultaneously but with low assay volumes. We have developed such a microtiter assay. It can assay an average of 60 independent treatments per 60 ml blood donation and demonstrates greater sensitivity than the current gold standard assay, namely platelet aggregation in stirring conditions in a platelet aggregometer. The microtiter plate (MTP) assay can detect known inhibitors of platelet function such as indomethacin, aspirin, and ReoPro. It is highly reproducible when using standard doses of agonists such as thrombin receptor-activating peptide (20 microM) and collagen (0.19 mg/ml). Finally, the MTP assay is rapid and sensitive and can detect unknown platelet-modulating agents from a library of compounds.

Inherited disorders of platelets: an update

PURPOSE OF REVIEW

To overview inherited syndromes that affect platelets and to discuss current data on the molecular origin and management of these rare diseases.

RECENT FINDINGS

An increasing number of genes responsible for inherited thrombocytopenias have been identified and these now extend to glycosylation defects. Although Glanzmann thrombasthenia remains the predominant disorder of platelet function, knowledge is increasing of pathologies concerning primary receptors for adhesion and signalling, the activation and secretory pathways, and even the development of procoagulant activity.

SUMMARY

These syndromes affect cell adhesion, cell activation, and cell-to-cell contact interactions fundamental in cell biology. Studies on the pathophysiology of alphaIIbbeta3 in platelets have helped unravel the molecular mechanisms of integrin function, and the information gained has resulted in improved antithrombotic therapy. The establishment of national registries and the use of state-of-the-art genomic and proteomic technologies will accelerate progress and help to define how mutations affecting a much larger range of proteins contribute alone or in combination to defining specific platelet phenotypes.

Differential sensitivity of human platelet P2X1 and P2Y1 receptors to disruption of lipid rafts

ATP-stimulated P2X1 and ADP-stimulated P2Y1 receptors play important roles in platelet activation. An increase in intracellular Ca2+ represents a key signalling event coupled to both of these receptors, mediated via direct gating of Ca2+-permeable channels in the case of P2X1 and phospholipase-C-dependent Ca2+ mobilisation for P2Y1. We show that disruption of cholesterol-rich membrane lipid rafts reduces P2X1 receptor-mediated calcium increases by approximately 80%, while P2Y1 receptor-dependent Ca2+ release is unaffected. In contrast to artery, vas deferens, bladder smooth muscle, and recombinant expression in cell lines, where P2X1 receptors show almost exclusive association with lipid rafts, only approximately 20% of platelet P2X1 receptors are co-expressed with the lipid raft marker flotillin-2. We conclude that lipid rafts play a significant role in the regulation of P2X1 but not P2Y1 receptors in human platelets and that a reserve of non-functional P2X1 receptors may exist.

Regulation of platelet functions by P2 receptors

The main role of blood platelets is to ensure primary hemostasis, which is the maintenance of vessel integrity and cessation of bleeding upon injury. While playing a major part in acute arterial thrombosis, platelets are also involved in inflammation, atherosclerosis, and angiogenesis. ADP and ATP play a crucial role in platelet activation, and their receptors are potential targets for antithrombotic drugs. The ATP-gated cation channel P2X(1) and the two G protein-coupled ADP receptors, P2Y(1) and P2Y(12), selectively contribute to platelet aggregation and formation of a thrombus. Owing to its central role in the growth and stabilization of a thrombus, the P2Y(12) receptor is an established target of antithrombotic drugs such as clopidogrel. Studies in P2Y(1) and P2X(1) knockout mice and selective P2Y(1) and P2X(1) antagonists have shown that these receptors are also attractive targets for new antithrombotic compounds. The potential role of platelet P(2) receptors in the involvement of platelets in inflammatory processes is also discussed.

(N)-methanocarba-2MeSADP (MRS2365) is a subtype-specific agonist that induces rapid desensitization of the P2Y1 receptor of human platelets

Adenosine diphosphate (ADP) initiates and maintains sustained aggregation of platelets through simultaneous activation of both the Gq-coupled P2Y1 receptor and the Gi-coupled P2Y12 receptor. We recently described the synthesis and P2Y1 receptor-specific agonist activity of (N)-methanocarba-2MeSADP (MRS2365). Consequences of selective activation of the P2Y1 receptor by MRS2365 have been further examined in human platelets. Whereas MRS2365 alone only induced shape change, addition of MRS2365 following epinephrine treatment, which activates the Gi/z-linked, alpha2A-adrenergic receptor, resulted in sustained aggregation that was indistinguishable from that observed with ADP. Conversely, the platelet shape change promoted by ADP in the presence of the GPIIb/IIIa antagonist eptifibatide was similar to that promoted by MRS2365. Preaddition of the high affinity P2Y1 receptor antagonist MRS2500 inhibited the effect of MRS2365, whereas addition of MRS2500 subsequent to MRS2365 reversed the MRS2365-induced shape change. Preactivation of the P2Y1 receptor with MRS2365 for 2 min resulted in marked loss of capacity of ADP to induce aggregation as evidenced by a greater than 20-fold rightward shift in the concentration effect curve of ADP. This inhibitory effect of P2Y1 receptor activation was dependent on the concentration of MRS2365 (EC50 = 34 nm). The inhibitory effect of preincubation with MRS2365 was circumvented by activation of the Gq-coupled 5-HT2A receptor suggesting that MRS2365 induces loss of the ADP response as a consequence of desensitization of the Gq-coupled P2Y1 receptor. The time course of MRS2365-induced loss of aggregation response to epinephrine was similar to that observed with ADP. These results further demonstrate the P2Y1 receptor selectivity of MRS2365 and illustrate the occurrence of agonist-induced desensitization of the P2Y1 receptor of human platelets studied in the absence of P2Y12 receptor activation .