Central role of the P2Y12 receptor in platelet activation

Effect of cocoa (Theobroma cacao L.) on platelet function testing profiles in patients with coronary artery disease: ECLAIR pilot study

INTRODUCTION

This prospective pharmacodynamic nutraceutical study assessed the effect of a 1-week trial of 30 g/day of 65% cocoa (dark chocolate) (Theobroma cacao L.) consumption intervention on platelet reactivity.

METHODS

Patients with stable coronary artery disease (CAD) (n=20) who were on maintenance dual antiplatelet therapy of aspirin (ASA) 81 mg/day and clopidogrel 75 mg/day were recruited. Platelet function was evaluated with the VerifyNow P2Y₁₂ reaction unit (PRU) and aspirin reaction unit (ARU) assays (Werfen, Bedford, Massachusetts, USA) and assessed prior to initiation of and after a 1-week trial of 30 g/day of 65% cocoa consumption intervention. Results were compared with a paired t-test.

RESULTS

Cocoa augmented the inhibitory effect of clopidogrel, demonstrated by a reduction of 11.9% (95% CI 5.7% to 18.0%, p value 0.001), significantly decreasing the PRU by 26.85 (95% CI 12.22 to 41.48, p value 0.001). The inhibitory effect of ASA was not impacted by cocoa, reflected by a non-significant reduction in ARU of 17.65 (95% CI 21.00 to 56.3, p value 0.351). No patients experienced any serious adverse events.

CONCLUSIONS

Cocoa augmented the inhibitory effect of clopidogrel but not ASA. This nutraceutical study could be potentially informative and applicable for patients with stable CAD. Further long-term studies are required to confirm these exploratory findings.

TRIAL REGISTRATION NUMBER

NCT04554901.

[Functional Analysis of Apoptosis Signal-regulating Kinase Family in a Murine Model of Tumor Metastasis]

Stress-responsive signaling pathways convert cellular stresses into various physiological responses, such as cell proliferation, apoptosis, and inflammation. Signal pathway dysfunction thus induces abnormal cellular behaviors that may lead to tumorigenesis and tumor progression, including metastasis. Tumor metastasis is the spread of tumor cells from primary lesions to other distant tissues/organs. Several types of murine model which mimic the progression of human cancer have been established for preclinical studies to understand the biology of cancer. Mitogen-activated protein kinase (MAPK) cascades are one of the stress-responsive signaling pathways and are intricately involved in both tumor promotion and suppression. Here, we present the diverse roles of apoptosis signal-regulating kinase (ASK) family molecules in tumor formation and progression. ASK family is a member of MAPK kinase kinase (MAP3K) family in the c-Jun N-terminal kinase (JNK) and p38 MAPK pathways and comprises three family members, ASK1, ASK2, and ASK3. Accumulating evidence indicates that ASK1 controls tumorigenesis through the regulation of innate immunity and apoptosis. ASK2 also regulates tumorigenesis via apoptosis. Furthermore, analysis of the experimental lung metastasis model in mice suggests that host ASK1 deficiency attenuates tumor lung metastasis. In this symposium review, we discuss the potential roles of ASK family in the context of tumor metastasis.

The Effects of Andrographis paniculata on Platelet Activity in Healthy Thai Volunteers

Background. Andrographis paniculata (AP) has been used in Thai traditional medicine to treat various infections, including the common cold and fever. Its bioactive compound, andrographolide, has shown antiplatelet activities in an in vitro study model. Since clinical studies of the effects of AP on the human platelet function have never been reported, we investigated its effect on platelet activity in ten healthy volunteers. Methods. Two grams of AP was taken 3 times within one day. The blood was withdrawn by venipuncture before and 2 and 24 hours after the AP administration to analyze the effects of AP on platelet aggregation, the expression of enzyme cyclooxygenase (COX) mRNA and protein, and TXB₂, including P-selectin. Result. Even though there was no significant change in the studied parameters, this study exhibited patient-to-patient variability in platelet function. It was found that ADP-induced platelet aggregation tended to decrease after AP administration, while epinephrine-induced platelet aggregation in females tended to be higher than that in males for the entire study period. Moreover, COX-1 mRNA levels tended to decrease while P-selectin levels tended to rise after AP administration. Conclusion. These controversial results are possibly due to the multifactorial mechanisms of platelet aggregation as well as the multichemical composition of AP. Further study, probably at the molecular level, is needed to unveil the underlying mechanisms of action of AP.

Current status of high on-treatment platelet reactivity in patients with coronary or peripheral arterial disease: Mechanisms, evaluation and clinical implications

Antiplatelet therapy with aspirin or clopidogrel or both is the standard care for patients with proven coronary or peripheral arterial disease, especially those undergoing endovascular revascularization procedures. However, despite the administration of the antiplatelet regiments, some patients still experience recurrent cardiovascular ischemic events. So far, it is well documented by several studies that in vitro response of platelets may be extremely variable. Poor antiplatelet effect of clopidogrel or high on-treatment platelet reactivity (HTPR) is under investigation by numerous recent studies. This review article focuses on methods used for the ex vivo evaluation of HTPR, as well as on the possible underlying mechanisms and the clinical consequences of this entity. Alternative therapeutic options and future directions are also addressed.

MicroRNAs and cardiovascular diseases

Coronary artery diseases (CAD) and heart failure have high mortality rate in the world, although much progress has been made in this field in last two decades. There is still a clinical need for a novel diagnostic approach and a therapeutic strategy to decrease the incidence of CAD. MicroRNAs (miRNAs) are highly conserved noncoding small RNA molecules that regulate a large fraction of the genome by binding to complementary messenger RNA sequences, resulting in posttranscriptional gene silencing. Recent studies have shown that specific miRNAs are involved in whole stage of atherosclerosis, from endothelium dysfunction to plaque rupture. These findings suggest that miRNAs are potential biomarkers in early diagnosis and therapeutic targets in CAD. In the present review, we highlight the role of miRNAs in every stage of atherosclerosis, and discuss the prospects of miRNAs in the near future.

Pistacia chinensis Methanolic Extract Attenuated MAPK and Akt Phosphorylations in ADP Stimulated Rat Platelets In Vitro

Pistacia chinensis (Chinese pistache) is a widely grown plant in southern China where the galls extract is a common practice in folk medicine. However, extracts from this plant have never been attempted for their cardiovascular protective effects in experimental setting. Here therefore we aimed to investigate the antiplatelet activity of Pistacia chinensis methanolic extract (PCME) in ADP stimulated rat platelets in vitro. PCME (2.5-20 μg/mL) inhibited ADP-induced platelet aggregation. While PCME diminished [Ca(2+)]i, ATP, and TXA2 release in ADP-activated platelets, it enhanced cAMP production in resting platelets. Likewise, PCME inhibited fibrinogen binding to αIIbβ3 and downregulated JNK, ERK, and Akt phosphorylations. Thus, PCME contains potential antiplatelet compounds that could be deployed for their therapeutic values in cardiovascular pathology.

Aspirin: pharmacology and clinical applications

Antiplatelet therapy has been documented to reduce risks of cardiovascular disease after acute myocardial infarction, coronary artery bypass graft, and in chronic atrial fibrillation patients, amongst other risk factors. Conventional management of thrombosis-based disorders includes the use of heparin, oral anticoagulants, and the preferred antiplatelet agent aspirin. Interestingly, aspirin was not intended to be used as an antiplatelet agent; rather, after being repurposed, it has become one of the most widely prescribed antithrombotic drugs. To this end, there have been several milestones in the development of antiplatelet agents in the last few decades, such as adenosine diphosphate receptor inhibitors, phosphodiesterase inhibitors, and GPIIb/IIIa inhibitors. However, given some of the limitations of these therapies, aspirin continues to play a major role in the management of thrombotic and cardiovascular disorders and is expected to do so for years to come.

A high-throughput biotin-avidin-ELISA for studying expression of platelet membrane glycoproteins and its clinical application

Platelet membrane glycoproteins (GPs) are critical for normal platelet adhesion, activation and aggregation. To define the abnormalities in surface GP expression on circulating platelets and provide a better biomarker of bleeding and thrombotic disorders, we have developed a accurate, time-saving and high-throughput biotin-avidin enzyme-linked immunosorbent assay (BA-ELISA) with the monoclonal antibodies (mAbs), 7E3 against the complex of GPIIb and GPIIIa (GPIIb/IIIa), SZ-51 against P-selectin, and SZ-2 against GPIb, respectively. The levels of P-selectin and GPIIb/IIIa were measured in patients with acute myocardial infarction (AMI), intracerebral hemorrhage (ICH), or diabetes mellitus (DM)) and healthy subjects. Inhibition of GP expression was evaluated with SZ-21, an inhibitory mAb against GPIIIa and aspirin, respectively. The sensitivity of BA-ELISA is high enough to detect platelet count as low as 3.13 x 10(9)/L in platelet-rich plasma (PRP). Both the inter-assay and intra-assay coefficient variation are less than 10%. Adenosine diphosphate (ADP)-induced or non-ADP-induced expression of P-selectin and GPIIb/IIIa was significantly higher in AMI, ICH or DM than that in controls (P < 0.01 for each). Either SZ-21 or aspirin can inhibit the ADP-induced expression of P-selectin and GPIIb/IIIa. Importantly, a high correlation was detected between BA-ELISA and flow cytometry methods. These observations indicate that BA-ELISA is a sensitive and high-throughput assay for evaluating platelet GP expression. The newly developed BA-ELISA can be popularized in community hospitals, because it does not require sophisticated equipments and reagents. This method is suitable for screening inhibitors of platelet activation and has a potential in use for diagnostic purposes.

Preinterventional clopidogrel response variability for coil embolization of intracranial aneurysms: clinical implications

BACKGROUND AND PURPOSE

Thromboembolism is one of the most serious complications in coil embolization for intracranial aneurysms, and antiplatelet premedication may reduce this complication. However, interindividual variation exists in the efficacy of CPG. This study sought to elucidate the clinical implications of preinterventional CPG response variability in patients who undergo coil embolization for intracranial aneurysms.

MATERIALS AND METHODS

CPG premedication was given to 186 consecutive patients with 209 aneurysms who underwent elective coil embolization, and the response to the premedication was measured by a point-of-care antiplatelet function test (VerifyNow assay). Patients were stratified into 4 quartiles according the test results, and their correlation with the occurrence of periprocedural complications was analyzed. The contribution of a variety of variables to the high PRU was also tested.

RESULTS

In this cohort, rates of thromboembolic events and all adverse events were 7.5% and 9.1%, respectively. The quartiles of the P2Y12 reaction unit of the ADP channel (PRU) showed a significant tendency toward thromboembolic events (P = .013) and all procedure-related adverse events (P = .009), while those of the BASE and percentage inhibition did not. Thromboembolic events occurred in 17.0% and procedure-related adverse events, in 21.3% of the patients in the fourth quartile. Female sex was the only significant factor related to the fourth quartile of PRU in the multiple logistic regression analysis (P = .014).

CONCLUSIONS

Procedure-related thromboembolic events occurred more frequently in patients in the upper quartile with higher PRU, especially in the fourth quartile. Further study including a large number of patients is expected to confirm this finding.

Interaction of new, very potent non-nucleotide antagonists with Arg256 of the human platelet P2Y12 receptor

The P2Y(12) receptor plays a crucial role in platelet aggregation. In the present study, we analyzed the properties of non-nucleotide antagonists at the recombinant human P2Y(12) receptor and searched for amino acids involved in the molecular interaction. Receptor function was assessed by measuring the cAMP response element (CRE)-directed luciferase expression in Chinese hamster ovary cells. The cellular cAMP production was accelerated by forskolin; 2-methylthio-ADP was used to activate the wild-type P2Y(12) receptor or mutant constructs. 2-Methylthio-ADP inhibited the CRE-dependent luciferase expression with an IC(50) value of approximately 1 nM. The anthraquinone derivative reactive blue 2 used at increasing concentrations shifted the concentration-response curve of 2-methylthio-ADP to the right in a manner compatible with competitive antagonism (pA(2) value, 7.4). Its analog, 1-amino-4-[4-phenylamino-3-sulfophenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (PSB-0739), showed a markedly higher antagonistic potency with a pA(2) value of 9.8. In cells expressing the R256A-mutant receptor, the potencies of both reactive blue 2 (apparent pK(B), 5.9) and PSB-0739 (apparent pK(B), 9.1) were decreased. The same was true for the pure reactive blue 2 meta- and para-isomers and for the ortho-isomer cibacron blue 3GA. In contrast, the analog, 1-amino-4-[4-anilino-phenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate, lacking a sulfonic acid residue at ring D (PSB-0826), showed similar pK(B) values at wild-type (8.4) and R256A-mutant receptors (8.3). In summary, the results demonstrate that PSB-0739 is the most potent competitive non-nucleotide antagonist at the human P2Y(12) receptor described so far. The results also indicate that the sulfonic acid residue at ring D is involved in the interaction of antagonists derived from reactive blue 2 with the residue Arg256 of the human P2Y(12) receptor.

New antithrombotic drugs

Thrombosis, both venous and arterial, is a major cause of morbidity and mortality worldwide. Consequently, there is an ongoing search for new antithrombotic drugs, particularly novel antiplatelet agents and anticoagulants. A better understanding of the biochemical pathways involved in platelet activation and coagulation and of the links between these systems and the impact of thrombosis on inflammation has led to the identification of new targets for antithrombotic drugs. This paper focuses on these new targets and new antiplatelet drugs and anticoagulants and describes the major advances in the continuing search for more potent antithrombotic drugs that have limited effects on hemostasis.

Impact of bivalirudin or unfractionated heparin on platelet aggregation in patients pretreated with 600 mg clopidogrel undergoing elective percutaneous coronary intervention

AIMS

The aim of this study was to assess the impact of bivalirudin or unfractionated heparin (UFH) on platelet aggregation in patients, pretreated with a 600 mg loading dose clopidogrel, undergoing elective percutaneous coronary intervention (PCI).

METHODS AND RESULTS

Patients (n = 100) were recruited consecutively in the setting of a double-blind, randomized trial. Bivalirudin or UFH was administered during PCI. Blood was drawn immediately before PCI, following administration of bivalirudin or UFH directly after PCI, and 24 h after PCI. Adenosine diphosphate (ADP)-induced platelet aggregation was assessed with light transmission aggregometry (LTA) and multiple electrode aggregometry (MEA). Before PCI, ADP-induced platelet aggregation was similar in UFH and bivalirudin patients (P = 0.99 for LTA; P = 0.28 for MEA). Administration of bivalirudin during PCI resulted in significant additional suppression of platelet aggregation (P = 0.012 for LTA; P = 0.008 for MEA). Administration of UFH did not have a significant influence on platelet aggregation (P = 0.42 for LTA; P = 0.78 for MEA). Platelet aggregation was again similar in the two groups 24 h after PCI (P > 0.05 for LTA and MEA).

CONCLUSION

Bivalirudin, given during PCI in patients pretreated with 600 mg of clopidogrel, is in contrast to UFH associated with further inhibition of platelet aggregation.

Raloxifene-induced acceleration of platelet aggregation

A 59-year-old postmenopausal woman diagnosed to have primary osteoporosis began to take 60 mg daily of oral raloxifene. The platelet aggregation induced by 1 microM adenosine diphosphate (ADP) and the alpha2-antiplasmin activity were accelerated significantly after 8 weeks from the beginning of raloxifene-treatment, and gradually deteriorated up to 24 weeks. ADP markedly caused the phosphorylation of Akt in the platelets obtained at 24 weeks. Although there were no subjective complaints at 24 weeks, the medication was stopped with her consent to avoid any adverse effects due to thrombus formation. The platelet hyper-aggregability and Akt phosphorylation induced by ADP disappeared at 4 weeks after the cessation of medication. These results strongly suggest that raloxifene caused the acceleration of platelet aggregation and subclinical thrombus formation through the Akt signal pathway in this case.

Genetic and pharmacologic evidence that Rac1 GTPase is involved in regulation of platelet secretion and aggregation

BACKGROUND

Rac1 GTPase, a member of the Ras-related Rho GTPase family, is the major Rac isoform present in platelets and has been shown to be involved in cell actin cytoskeleton reorganization and adhesion. Agonists that induce platelet secretion and aggregation also activate Rac1 GTPase, raising the possibility that Rac1 GTPase may be involved in regulation of platelet function.

OBJECTIVES

To rigorously define the role of Rac1 in platelet regulation.

METHODS

We have used a dual approach of gene targeting in mice and pharmacologic inhibition of Rac1 by NSC23766, a rationally designed specific small molecule inhibitor, to study the role of Rac1 in platelet function.

RESULTS

Platelets from mice as well as human platelets treated with NSC23766 exhibited a significant decrease in: (i) active Rac1 species and phosphorylation of the Rac effector, p21-activated kinase; (ii) expression of P-selectin and secretion of adenosine triphosphate induced by thrombin or U46619; and (iii) aggregation induced by adenosine 5'-diphosphate, collagen, thrombin and U46619, a stable analog of thromboxane A(2). NSC23766 did not alter the cAMP or cGMP levels in platelets. Consistent with the requirement of Rac1 for normal platelet function, the bleeding times in Rac1(-/-) mice or mice given NSC23766 were significantly prolonged.

CONCLUSIONS

Our data show that deficiency or inhibition of Rac1 GTPase blocks platelet secretion. The inhibition of secretion, at least in part, is responsible for diminished platelet aggregation and prolonged bleeding times observed in Rac1 knockout or Rac1 inhibitor-treated mice.

Antithrombotic and antiplatelet activities of Korean red ginseng extract

The antithrombotic and antiplatelet activities of Korean red ginseng extract (KRGE) were examined on rat carotid artery thrombosis in vivo and platelet aggregation in vitro and ex vivo. The KRGE significantly prevented rat carotid arterial thrombosis in vivo in a dose-dependent manner. Administration of the KRGE to rats significantly inhibited adenosine diphosphate (ADP)- and collagen-induced platelet aggregation ex vivo, although it failed to prolong coagulation times such as activated partial thromboplastin and prothrombin time indicating that the antithrombotic effect of the red ginseng may be due to its antiplatelet aggregation rather than anticoagulation effect. In line with the above observations, the red ginseng inhibited the U46619-, arachidonic acid-, collagen- and thrombin-induced rabbit platelet aggregations in vitro in a concentration-dependent manner, with IC(50) values of 390 +/- 15, 485 +/- 19, 387 +/- 11 and 335 +/- 15 microg/ml, respectively. Consistently, serotonin secretion was also inhibited by ginseng in the same pattern. These results suggest that the red ginseng has a potent antithrombotic effect in vivo, which may be due to the antiplatelet rather than the anticoagulation activity, and the red ginseng intake may be beneficial for individuals with high risks of thrombotic and cardiovascular diseases.

A multiple dose study of prasugrel (CS-747), a novel thienopyridine P2Y12 inhibitor, compared with clopidogrel in healthy humans

AIMS

This double-blind, placebo-controlled trial was designed to evaluate the pharmacodynamics, pharmacokinetics, safety, and tolerability of prasugrel (CS-747, LY640315), a novel thienopyridine P2Y(12) ADP receptor antagonist compared with clopidogrel, during multiple oral dosing in healthy subjects.

METHODS

Thirty subjects received placebo, prasugrel 5 mg, 10 mg, or 20 mg, or clopidogrel 75 mg orally, daily for 10 days. Platelet aggregation, bleeding time, and prasugrel metabolites were measured and adverse events were recorded.

RESULTS

Inhibition of ADP-induced platelet aggregation reached steady state by day 3 following prasugrel 10 and 20 mg compared with 5 days for clopidogrel 75 mg or prasugrel 5 mg. Compared with placebo, at 24 h after the last dose of study drug, inhibition of platelet aggregation using (20 microm) ADP was significantly higher in the prasugrel 10 mg group (58.2 +/- 4.9% vs. 9.2 +/- 4.0%, P < 0.001) with no difference in the clopidogrel group (15.7 +/- 6.8% vs. 9.2 +/- 4.0%, P = 0.78). With 5 microm ADP, inhibition of platelet aggregation with prasugrel 10 mg and clopidogrel 75 mg was significantly higher than with placebo (prasugrel 10 mg, 70.5 +/- 4.7%; clopidogrel 75 mg, 36.5 +/- 9.0%; vs. placebo, 11.3 +/- 5.1%; P < 0.0001 and P = 0.02). On day 10 at 4 h postdose, bleeding time was prolonged with prasugrel 10 mg (prasugrel 10 mg, 706 +/- 252 s vs. placebo, 221 +/- 38 s, P = 0.05) but not with clopidogrel (283 +/- 56 s, P = 0.98). There were no clinically significant bleeding events, serious adverse events, or discontinuations of the study drug.

CONCLUSIONS

Compared with clopidogrel 75 mg, prasugrel 10 mg and 20 mg daily for 10 days resulted in more rapid, more consistent, and higher levels of platelet inhibition.

Critical role of ADP interaction with P2Y12 receptor in the maintenance of alpha(IIb)beta3 activation: association with Rap1B activation

OBJECTIVE

Platelet integrin alpha(IIb)beta3 plays a crucial role in platelet aggregation, and the affinity of alpha(IIb)beta3 for fibrinogen is dynamically regulated. Employing modified ligand-binding assays, we analyzed the mechanism by which alpha(IIb)beta3 maintains its high-affinity state.

METHODS AND RESULTS

Washed platelets adjusted to 50 x 10(3) microL(-1) were stimulated with 0.2 U mL(-1) thrombin or 5 microm U46619 under static conditions. After the completion of alpha(IIb)beta3 activation and granule secretion, different kinds of antagonists were added to the activated platelets. The activated alpha(IIb)beta3 was then detected by fluorescein isothiocyanate (FITC)-labeled PAC1. The addition of 1 mum AR-C69931MX (a P2Y12 antagonist) or 1 mm A3P5P (a P2Y1 antagonist) disrupted the sustained alpha(IIb)beta3 activation by approximately 92% and approximately 38%, respectively, without inhibiting CD62P or CD63 expression. Dilution of the platelet preparation to 500 microL(-1) also disrupted the sustained alpha(IIb)beta3 activation, and the disruption by such dilution was abrogated by the addition of exogenous adenosine 5'-diphosphate (ADP) in a dose-dependent fashion. The amounts of ADP released from activated platelets determined by high-performance liquid chromatography were compatible with the amounts of exogenous ADP required for the restoration. We next examined the effects of antagonists on protein kinase C (PKC) and Rap1B activation induced by 0.2 U mL(-1) thrombin. Thrombin induced long-lasting PKC and Rap1B activation. AR-C69931MX markedly inhibited Rap1B activation without inhibiting PKC activation.

CONCLUSIONS

Our data indicate that the continuous interaction between released ADP and P2Y12 is critical for the maintenance of alpha(IIb)beta3 activation.

Identification of a potent inverse agonist at a constitutively active mutant of human P2Y12 receptor

Human platelets express two P2Y receptors: G(q)-coupled P2Y(1), and G(i)-coupled P2Y(12). Both P2Y(1) and P2Y(12) are ADP receptors on human platelets and are essential for ADP-induced platelet aggregation that plays pivotal roles in thrombosis and hemostasis. Numerous constitutively active G protein-coupled receptors have been described in natural or recombinant systems, but in the P2Y receptors, to date, no constitutive activity has been reported. In our effort to identify G protein coupling domains of the human platelet ADP receptor, we constructed a chimeric hemagglutinin-tagged human P2Y(12) receptor with its C terminus replaced by the corresponding part of human P2Y(1) receptor and stably expressed it in Chinese hamster ovary-K1 cells. It is interesting that the chimeric P2Y(12) mutant exhibited a high level of constitutive activity, as evidenced by decreased cAMP levels in the absence of agonists. The constitutive activation of the chimeric P2Y(12) mutant was dramatically inhibited by pertussis toxin, a G(i) inhibitor. The constitutively active P2Y(12) mutant retained normal responses to 2-methylthio-ADP, with an EC(50) of 0.15 +/- 0.04 nM. The constitutively active P2Y(12) mutant caused Akt phosphorylation that was abolished by the addition of pertussis toxin. Pharmacological evaluation of several P2Y(12) antagonists revealed (E)-N-[1-[7-(hexylamino)-5-(propylthio)-3H-1,2,3-triazolo-[4,5-d]-pyrimidin-3-yl]-1,5,6-trideoxy-beta-d-ribo-hept-5-enofuranuronoyl]-l-aspartic acid (AR-C78511) as a potent P2Y(12) inverse agonist and 5'-adenylic acid, N-[2-(methylthio)ethyl]-2-[(3,3,3-trifluoropropyl)thio]-, monoanhydride with (dichloromethylene)bis[phosphonic acid] (AR-C69931MX) as a neutral antagonist. In conclusion, this is the first report of a cell line stably expressing a constitutively active mutant of human platelet P2Y(12) receptor and the identification of potent inverse agonist.

PI3-kinase is essential for ADP-stimulated integrin alpha(IIb)beta3-mediated platelet calcium oscillation, implications for P2Y receptor pathways in integrin alpha(IIb)beta3-initiated signaling cross-talks

Phosphatidylinositol 3-kinase (PI3K) pathway is important for platelet activation. Recent studies showed that PI3K and oscillative calcium could cross talk to each other and positively regulate integrin alpha (IIb)beta3-mediated outside-in signaling. However, the mechanism of this feedback regulation remains to be further characterized. Here we found that treatments of both PI3K inhibitor wortmannin and P2Y1 inhibitor A3P5P could inhibit granular secretion in platelets. Additionally, when RGD-substrate adherent platelets were treated with the ADP scavenger apyrase to deplete the granular-released ADP, their attachments in engaging with substrates became looser and the frequency of calcium oscillation decreased. Since it is known that ADP stimulates the PI3K and calcium signal primarily through P2Y12 and P2Y1 receptors respectively, our data indicated that integrin alpha(IIb)beta3 downstream PI3K and calcium activation might be not completely coupled to integrin associated signaling complex, but in part through feedback stimulation by granular released ADP. Our data indicates the important roles of PI3K and granular released ADP in coordinating the feedback regulations in integrin alpha(IIb)beta3-mediated platelet activation.

Zebrafish: a genetic model for hemostasis and thrombosis

Here we review the zebrafish hemostatic system, its relevance to mammalian hemostasis, and its efficacy as a vertebrate genetic model to further the understanding of hemostasis and thrombosis.

P2Y2 receptor activation regulates the expression of acetylcholinesterase and acetylcholine receptor genes at vertebrate neuromuscular junctions

At the vertebrate neuromuscular junction (nmj), ATP is known to be coreleased with acetylcholine from the synaptic vesicles. We have previously shown that the P2Y1 receptor is localized at the nmj. Here, we extend the findings to show that another nucleotide receptor, P2Y2, is also localized there and with P2Y1 jointly mediates trophic responses to ATP. The P2Y2 receptor mRNA in rat muscle increased during development and peaked in adulthood. The P2Y2 receptor protein was shown to become restricted to the nmjs during embryonic development, in chick and in rat. In both rat and chick myotubes, P2Y1 and P2Y2 are expressed, increasing with differentiation, but P2Y4 is absent. The P2Y2 agonist UTP stimulated there inositol trisphosphate production and phosphorylation of extracellular signal-regulated kinases, in a dose-dependent manner. These UTP-induced responses were insensitive to the P2Y1-specific antagonist MRS 2179 (2'-deoxy-N6-methyl adenosine 3',5'-diphosphate diammonium salt). In differentiated myotubes, P2Y2 activation induced expression of acetylcholinesterase (AChE) protein (but not control alpha-tubulin). This was shown to arise from AChE promoter activation, mediated by activation of the transcription factor Elk-1. Two Elk-1-responsive elements, located in intron-1 of the AChE promoter, were found by mutation to act in this gene activation initiated at the P2Y2 receptor and also in that initiated at the P2Y1 receptor. Furthermore, the promoters of different acetylcholine receptor subunits were also stimulated by application of UTP to myotubes. These results indicate that ATP regulates postsynaptic gene expressions via a common pathway triggered by the activation of P2Y1 and P2Y2 receptors at the nmjs.