Emerging roles for P2X1 receptors in platelet activation

A Platelet Reactivity ExpreSsion Score derived from patients with peripheral artery disease predicts cardiovascular risk

Platelets are key mediators of atherothrombosis, yet, limited tools exist to identify individuals with a hyperreactive platelet phenotype. In this study, we investigate the association of platelet hyperreactivity and cardiovascular events, and introduce a tool, the Platelet Reactivity ExpreSsion Score (PRESS), which integrates platelet aggregation responses and RNA sequencing. Among patients with peripheral artery disease (PAD), those with a hyperreactive platelet response (>60% aggregation) to 0.4 µM epinephrine had a higher incidence of the 30 day primary cardiovascular endpoint (37.2% vs. 15.3% in those without hyperreactivity, adjusted HR 2.76, 95% CI 1.5-5.1, p = 0.002). PRESS performs well in identifying a hyperreactive phenotype in patients with PAD (AUC [cross-validation] 0.81, 95% CI 0.68 -0.94, n = 84) and in an independent cohort of healthy participants (AUC [validation] 0.77, 95% CI 0.75 -0.79, n = 35). Following multivariable adjustment, PAD individuals with a PRESS score above the median are at higher risk for a future cardiovascular event (adjusted HR 1.90, CI 1.07-3.36; p = 0.027, n = 129, NCT02106429). This study derives and validates the ability of PRESS to discriminate platelet hyperreactivity and identify those at increased cardiovascular risk. Future studies in a larger independent cohort are warranted for further validation. The development of a platelet reactivity expression score opens the possibility for a personalized approach to antithrombotic therapy for cardiovascular risk reduction.

Mechanisms Underlying Dichotomous Procoagulant COAT Platelet Generation-A Conceptual Review Summarizing Current Knowledge

Procoagulant platelets are a subtype of activated platelets that sustains thrombin generation in order to consolidate the clot and stop bleeding. This aspect of platelet activation is gaining more and more recognition and interest. In fact, next to aggregating platelets, procoagulant platelets are key regulators of thrombus formation. Imbalance of both subpopulations can lead to undesired thrombotic or bleeding events. COAT platelets derive from a common pro-aggregatory phenotype in cells capable of accumulating enough cytosolic calcium to trigger specific pathways that mediate the loss of their aggregating properties and the development of new adhesive and procoagulant characteristics. Complex cascades of signaling events are involved and this may explain why an inter-individual variability exists in procoagulant potential. Nowadays, we know the key agonists and mediators underlying the generation of a procoagulant platelet response. However, we still lack insight into the actual mechanisms controlling this dichotomous pattern (i.e., procoagulant versus aggregating phenotype). In this review, we describe the phenotypic characteristics of procoagulant COAT platelets, we detail the current knowledge on the mechanisms of the procoagulant response, and discuss possible drivers of this dichotomous diversification, in particular addressing the impact of the platelet environment during in vivo thrombus formation.

Thrombo-Inflammation: A Focus on NTPDase1/CD39

There is increasing evidence for a link between inflammation and thrombosis. Following tissue injury, vascular endothelium becomes activated, losing its antithrombotic properties whereas inflammatory mediators build up a prothrombotic environment. Platelets are the first elements to be activated following endothelial damage; they participate in physiological haemostasis, but also in inflammatory and thrombotic events occurring in an injured tissue. While physiological haemostasis develops rapidly to prevent excessive blood loss in the endothelium activated by inflammation, hypoxia or by altered blood flow, thrombosis develops slowly. Activated platelets release the content of their granules, including ATP and ADP released from their dense granules. Ectonucleoside triphosphate diphosphohydrolase-1 (NTPDase1)/CD39 dephosphorylates ATP to ADP and to AMP, which in turn, is hydrolysed to adenosine by ecto-5'-nucleotidase (CD73). NTPDase1/CD39 has emerged has an important molecule in the vasculature and on platelet surfaces; it limits thrombotic events and contributes to maintain the antithrombotic properties of endothelium. The aim of the present review is to provide an overview of platelets as cellular elements interfacing haemostasis and inflammation, with a particular focus on the emerging role of NTPDase1/CD39 in controlling both processes.

Calcium Signalling through Ligand-Gated Ion Channels such as P2X1 Receptors in the Platelet and other Non-Excitable Cells

Ligand-gated ion channels on the cell surface are directly activated by the binding of an agonist to their extracellular domain and often referred to as ionotropic receptors. P2X receptors are ligand-gated non-selective cation channels with significant permeability to Ca(2+) whose principal physiological agonist is ATP. This chapter focuses on the mechanisms by which P2X1 receptors, a ubiquitously expressed member of the family of ATP-gated channels, can contribute to cellular responses in non-excitable cells. Much of the detailed information on the contribution of P2X1 to Ca(2+) signalling and downstream functional events has been derived from the platelet. The underlying primary P2X1-generated signalling event in non-excitable cells is principally due to Ca(2+) influx, although Na(+) entry will also occur along with membrane depolarization. P2X1 receptor stimulation can lead to additional Ca(2+) mobilization via a range of routes such as amplification of G-protein-coupled receptor-dependent Ca(2+) responses. This chapter also considers the mechanism by which cells generate extracellular ATP for autocrine or paracrine activation of P2X1 receptors. For example cytosolic ATP efflux can result from opening of pannexin anion-permeable channels or following damage to the cell membrane. Alternatively, ATP stored in specialised secretory vesicles can undergo quantal release via the process of exocytosis. Examples of physiological or pathophysiological roles of P2X1-dependent signalling in non-excitable cells are also discussed, such as thrombosis and immune responses.

Blood cells: an historical account of the roles of purinergic signalling

The involvement of purinergic signalling in the physiology of erythrocytes, platelets and leukocytes was recognised early. The release of ATP and the expression of purinoceptors and ectonucleotidases on erythrocytes in health and disease are reviewed. The release of ATP and ADP from platelets and the expression and roles of P1, P2Y(1), P2Y(12) and P2X1 receptors on platelets are described. P2Y(1) and P2X(1) receptors mediate changes in platelet shape, while P2Y(12) receptors mediate platelet aggregation. The changes in the role of purinergic signalling in a variety of disease conditions are considered. The successful use of P2Y(12) receptor antagonists, such as clopidogrel and ticagrelor, for the treatment of thrombosis, myocardial infarction and stroke is discussed.

New highly active antiplatelet agents with dual specificity for platelet P2Y1 and P2Y12 adenosine diphosphate receptors

Currently approved platelet adenosine diphosphate (ADP) receptor antagonists target only the platelet P2Y12 receptor. Moreover, especially in patients with acute coronary syndromes, there is a strong need for rapidly acting and reversible antiplatelet agents in order to minimize the risk of thrombotic events and bleeding complications. In this study, a series of new P(1),P(4)-di(adenosine-5') tetraphosphate (Ap4A) derivatives with modifications in the base and in the tetraphosphate chain were synthesized and evaluated with respect to their effects on platelet aggregation and function of the platelet P2Y1, P2Y12, and P2X1 receptors. The resulting structure-activity relationships were used to design Ap4A analogs which inhibit human platelet aggregation by simultaneously antagonizing both P2Y1 and P2Y12 platelet receptors. Unlike Ap4A, the analogs do not activate platelet P2X1 receptors. Furthermore, the new compounds exhibit fast onset and offset of action and are significantly more stable than Ap4A to degradation in plasma, thus presenting a new promising class of antiplatelet agents.

Low angle light scattering analysis: a novel quantitative method for functional characterization of human and murine platelet receptors

BACKGROUND

Determinations of platelet receptor functions are indispensable diagnostic indicators of cardiovascular and hemostatic diseases including hereditary and acquired receptor defects and receptor responses to drugs. However, presently available techniques for assessing platelet function have some disadvantages, such as low sensitivity and the requirement of large sample sizes and unphysiologically high agonist concentrations. Our goal was to develop and initially characterize a new technique designed to quantitatively analyze platelet receptor activation and platelet function on the basis of measuring changes in low angle light scattering.

METHODS

We developed a novel technique based on low angle light scattering registering changes in light scattering at a range of different angles in platelet suspensions during activation.

RESULTS

The method proved to be highly sensitive for simultaneous real time detection of changes in size and shape of platelets during activation. Unlike commonly-used methods, the light scattering method could detect platelet shape change and aggregation in response to nanomolar concentrations of extracellular nucleotides. Furthermore, our results demonstrate that the advantages of the light scattering method make it a choice method for platelet receptor monitoring and for investigation of both murine and human platelets in disease models.

CONCLUSIONS

Our data demonstrate the suitability and superiority of this new low angle light scattering method for comprehensive analyses of platelet receptors and functions. This highly sensitive, quantitative, and online detection of essential physiological, pathophysiological and pharmacological-response properties of human and mouse platelets is a significant improvement over conventional techniques.

Platelet biology: the role of shear

Platelets are anucleated fragments produced by megakaryocytes that circulate in the blood. Platelets are involved in the initial cellular response to damaged endothelium and migrate to this area to prevent excessive bleeding. What is becoming more acknowledged over the last few decades is that blood flow (hemodynamics) plays a critical role in platelet function. The purpose of this review is to summarize the current understanding of platelet biology with particular focus on the role of hemodynamics. The emerging concept of shear microgradients, which are challenging the traditional model of platelet function, will also be introduced in the review.

ATP antagonizes thrombin-induced signal transduction through 12(S)-HETE and cAMP

In this study we have investigated the role of extracellular ATP on thrombin induced-platelet aggregation (TIPA) in washed human platelets. ATP inhibited TIPA in a dose-dependent manner and this inhibition was abolished by apyrase but not by adenosine deaminase (ADA) and it was reversed by extracellular magnesium. Antagonists of P2Y1 and P2Y12 receptors had no effect on this inhibition suggesting that a P2X receptor controlled ATP-mediated TIPA inhibition. ATP also blocked inositol phosphates (IP1, IP2, IP3) generation and [Ca(2+)]i mobilization induced by thrombin. Thrombin reduced cAMP levels which were restored in the presence of ATP. SQ-22536, an adenylate cyclase (AC) inhibitor, partially reduced the inhibition exerted by ATP on TIPA. 12-lipoxygenase (12-LO) inhibitors, nordihidroguaretic acid (NDGA) and 15(S)-hydroxy-5,8,11,13-eicosatetraenoic acid (15(S)-HETE), strongly prevented ATP-mediated TIPA inhibition. Additionally, ATP inhibited the increase of 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid (12(S)-HETE) induced by thrombin. Pretreatment with both SQ-22536 and NDGA almost completely abolished ATP-mediated TIPA inhibition. Our results describe for the first time that ATP implicates both AC and 12-LO pathways in the inhibition of human platelets aggregation in response to agonists.

Harnessing the platelet signaling network to produce an optimal hemostatic response

Once released into the circulation by megakaryocytes, circulating platelets can undergo rapid activation at sites of vascular injury and resist unwarranted activation, which can lead to heart attacks and strokes. Historically, the signaling mechanisms underlying the regulation of platelet activation have been approached as a collection of individual pathways unique to agonist. This review takes a different approach, casting platelet activation as the product of a signaling network, in which activating and restraining mechanisms interact in a flexible network that regulates platelet adhesiveness, cohesion between platelets, granule secretion, and the formation of a stable hemostatic thrombus.

Modified diadenosine tetraphosphates with dual specificity for P2Y1 and P2Y12 are potent antagonists of ADP-induced platelet activation

BACKGROUND

Diadenosine 5',5'''-P(1),P(4)-tetraphosphate (Ap(4)A), a natural compound stored in platelet dense granules, inhibits ADP-induced platelet aggregation. Ap(4)A inhibits the platelet ADP receptors P2Y(1) and P2Y(12), is a partial agonist of P2Y(12), and is a full agonist of the platelet ATP-gated ion channel P2X1. Modification of the Ap(4)A tetraphosphate backbone enhances inhibition of ADP-induced platelet aggregation. However, the effects of these Ap(4)A analogs on human platelet P2Y(1), P2Y(12) and P2X1 are unclear.

OBJECTIVE

To determine the agonist and antagonist activities of diadenosine tetraphosphate analogs towards P2Y(1), P2Y(12), and P2X1.

METHODS

We synthesized the following Ap(4)A analogs: P(1),P(4)-dithiotetraphosphate; P(2),P(3)-chloromethylenetetraphosphate; P(1)-thio-P(2),P(3)-chloromethylenetetraphosphate; and P(1),P(4)-dithio-P(2),P(3)-chloromethylenetetraphosphate. We then measured the effects of these analogs on: (i) ADP-induced platelet aggregation; (ii) P2Y(1)-mediated changes in cytosolic Ca(2+); (iii) P2Y(12)-mediated changes in vasodilator-stimulated phosphoprotein phosphorylation; and (iv) P2X1-mediated entry of extracellular Ca(2+).

RESULTS

Ap(4)A analogs with modifications in the phosphate backbone inhibited both P2Y(1) and P2Y(12), and showed no agonist activity towards these receptors. The dithio modification increased inhibition of P2Y(1), P2Y(12), and platelet aggregation, whereas the chloromethylene modification increased inhibition of P2Y(12) and platelet aggregation, but decreased P2Y(1) inhibition. Combining the dithio and chloromethylene modifications increased P2Y(1) and P2Y(12) inhibition. As compared with Ap(4)A, each modification decreased agonist activity towards P2X1, and the dual modification completely eliminated P2X1 agonist activity.

CONCLUSIONS

As compared with Ap(4)A, tetraphosphate backbone analogs of Ap(4)A have diminished activity towards P2X1 but inhibit both P2Y(1) and P2Y(12) and, with greater potency, inhibit ADP-induced platelet aggregation. Thus, diadenosine tetraphosphate analogs with dual receptor selectivity may have potential as antiplatelet drugs.

Molecular basis of selective antagonism of the P2X1 receptor for ATP by NF449 and suramin: contribution of basic amino acids in the cysteine-rich loop

BACKGROUND AND PURPOSE

The cysteine-rich head region, which is adjacent to the proposed ATP-binding pocket in the extracellular ligand-binding loop of P2X receptors for ATP, is absent in the antagonist-insensitive Dictyostelium receptors. In this study we have determined the contribution of the head region to the antagonist action of NF449 and suramin at the human P2X1 receptor.

EXPERIMENTAL APPROACH

Chimeras and point mutations in the cysteine-rich head region were made between human P2X1 and P2X2 receptors. Mutant receptors were expressed in Xenopus oocytes and P2X receptor currents characterized using two-electrode voltage clamp.

KEY RESULTS

The chimera replacing the region between the third and fourth conserved cysteine residues of the P2X1 receptor with the corresponding part of P2X2 reduced NF449 sensitivity a thousand fold from an IC₅₀ of ∼1 nM at the P2X1 receptor to that of the P2X2 receptor (IC₅₀∼1 µM). A similar decrease in sensitivity resulted from mutation of four positively charged P2X1 receptor residues in this region that are absent from the P2X2 receptor. These chimeras and mutations were also involved in determining sensitivity to the antagonist suramin. Reciprocal chimeras and mutations in the P2X2 receptor produced modest increases in antagonist sensitivity.

CONCLUSIONS AND IMPLICATIONS

These data indicate that a cluster of positively charged residues at the base of the cysteine-rich head region can account for the highly selective antagonism of the P2X1 receptor by the suramin derivative NF449.

P2 receptors for extracellular nucleotides in the central nervous system: role of P2X7 and P2Y₂ receptor interactions in neuroinflammation

Extracellular nucleotides induce cellular responses in the central nervous system (CNS) through the activation of ionotropic P2X and metabotropic P2Y nucleotide receptors. Activation of these receptors regulates a wide range of physiological and pathological processes. In this review, we present an overview of the current literature regarding P2X and P2Y receptors in the CNS with a focus on the contribution of P2X7 and P2Y(2) receptor-mediated responses to neuroinflammatory and neuroprotective mechanisms.

P2 receptors and platelet function

Following vessel wall injury, platelets adhere to the exposed subendothelium, become activated and release mediators such as TXA(2) and nucleotides stored at very high concentration in the so-called dense granules. Released nucleotides and other soluble agents act in a positive feedback mechanism to cause further platelet activation and amplify platelet responses induced by agents such as thrombin or collagen. Adenine nucleotides act on platelets through three distinct P2 receptors: two are G protein-coupled ADP receptors, namely the P2Y(1) and P2Y(12) receptor subtypes, while the P2X(1) receptor ligand-gated cation channel is activated by ATP. The P2Y(1) receptor initiates platelet aggregation but is not sufficient for a full platelet aggregation in response to ADP, while the P2Y(12) receptor is responsible for completion of the aggregation to ADP. The latter receptor, the molecular target of the antithrombotic drugs clopidogrel, prasugrel and ticagrelor, is responsible for most of the potentiating effects of ADP when platelets are stimulated by agents such as thrombin, collagen or immune complexes. The P2X(1) receptor is involved in platelet shape change and in activation by collagen under shear conditions. Each of these receptors is coupled to specific signal transduction pathways in response to ADP or ATP and is differentially involved in all the sequential events involved in platelet function and haemostasis. As such, they represent potential targets for antithrombotic drugs.

The P2X1 receptor and platelet function

Extracellular nucleotides are ubiquitous signalling molecules, acting via the P2 class of surface receptors. Platelets express three P2 receptor subtypes, ADP-dependent P2Y1 and P2Y12 G-protein-coupled receptors and the ATP-gated P2X1 non-selective cation channel. Platelet P2X1 receptors can generate significant increases in intracellular Ca(2+), leading to shape change, movement of secretory granules and low levels of α(IIb)β(3) integrin activation. P2X1 can also synergise with several other receptors to amplify signalling and functional events in the platelet. In particular, activation of P2X1 receptors by ATP released from dense granules amplifies the aggregation responses to low levels of the major agonists, collagen and thrombin. In vivo studies using transgenic murine models show that P2X1 receptors amplify localised thrombosis following damage of small arteries and arterioles and also contribute to thromboembolism induced by intravenous co-injection of collagen and adrenaline. In vitro, under flow conditions, P2X1 receptors contribute more to aggregate formation on collagen-coated surfaces as the shear rate is increased, which may explain their greater contribution to localised thrombosis in arterioles compared to venules within in vivo models. Since shear increases substantially near sites of stenosis, anti-P2X1 therapy represents a potential means of reducing thrombotic events at atherosclerotic plaques.

Pharmacochemistry of the platelet purinergic receptors

Platelets contain at least five purinergic G protein-coupled receptors, e.g., the pro-aggregatory P2Y(1) and P2Y(12) receptors, a P2Y(14) receptor (GPR105) of unknown function, and anti-aggregatory A(2A) and A(2B) adenosine receptor (ARs), in addition to the ligand-gated P2X1 ion channel. Probing the structure-activity relationships (SARs) of the P2X and P2Y receptors for extracellular nucleotides has resulted in numerous new agonist and antagonist ligands. Selective agents derived from known ligands and novel chemotypes can be used to help define the subtypes pharmacologically. Some of these agents have entered into clinical trials in spite of the challenges of drug development for these classes of receptors. The functional architecture of P2 receptors was extensively explored using mutagenesis and molecular modeling, which are useful tools in drug discovery. In general, novel drug delivery methods, prodrug approaches, allosteric modulation, and biased agonism would be desirable to overcome side effects that tend to occur even with receptor subtype-selective ligands. Detailed SAR analyses have been constructed for nucleotide and non-nucleotide ligands at the P2Y(1), P2Y(12), and P2Y(14) receptors. The thienopyridine antithrombotic drugs Clopidogrel and Prasugrel require enzymatic pre-activation in vivo and react irreversibly with the P2Y(12) receptor. There is much pharmaceutical development activity aimed at identifying reversible P2Y(12) receptor antagonists. The screening of chemically diverse compound libraries has identified novel chemotypes that act as competitive, non-nucleotide antagonists of the P2Y(1) receptor or the P2Y(12) receptor, and antithrombotic properties of the structurally optimized analogues were demonstrated. In silico screening at the A(2A) AR has identified antagonist molecules having novel chemotypes. Fluorescent and other reporter groups incorporated into ligands can enable new technology for receptor assays and imaging. The A(2A) agonist CGS21680 and the P2Y(1) receptor antagonist MRS2500 were derivatized for covalent attachment to polyamidoamine dendrimeric carriers of MW 20,000, and the resulting multivalent conjugates inhibited ADP-promoted platelet aggregation. In conclusion, a wide range of new pharmacological tools is available to control platelet function by interacting with cell surface purine receptors.

P2Y receptors in health and disease

The purine- and pyrimidine-sensitive P2Y receptors belong to the large group of G-protein-coupled receptors that are the target of approximately one-third of the pharmaceutical drugs used in the clinic today. It is therefore not unexpected that the P2Y receptors could be useful targets for drug development. This chapter will discuss P2Y receptor-based therapies currently used, in development and possible future developments. The platelet inhibitors blocking the ADP-receptor P2Y(12) reduce myocardial infarction, stroke, and mortality in patients with cardiovascular disease. Clopidogrel (Plavix) was for many years the second most selling drug in the world. The improved P2Y(12) inhibitors prasugrel, ticagrelor, and elinogrel are now entering the clinic with even more pronounced protective effects. The UTP-activated P2Y(2) receptor stimulates ciliary movement and secretion from epithelial cells. Cystic fibrosis is a monogenetic disease where reduced chloride ion secretion results in a severe lung disease and early death. No specific treatment has been available, but the P2Y(2) agonist Denufosol has been shown to improve lung function and is expected to be introduced as treatment for cystic fibrosis soon. In preclinical studies, there are indications that P2Y receptors can be important for diabetes, osteoporosis, cardiovascular, and atherosclerotic disease. In conclusion, P2Y receptors are important for the health of humans for many diseases, and we can expect even more beneficial drugs targeting P2Y receptors in the future.

P2X1 receptor mobility and trafficking; regulation by receptor insertion and activation

P2X1 receptors for ATP contribute to signalling in a variety of cell types and following stimulation undergo rapid desensitisation (within 1 s), and require ∼5 min to recover. In HEK293 cells P2X1 receptors C-terminally tagged with enhanced green fluorescent protein (P2X1-eGFP) were predominantly expressed at the cell surface. Following > 90% photo-bleaching of P2X1-eGFP within a 6 μm² circle at the cell surface fluorescence recovery after photo-bleaching (FRAP) was fit with a time constant of ∼60 s and recovered to ∼75% of pre-bleach levels. Following activation of the P2X1 receptor with α,β-methylene ATP the associated calcium influx doubled the FRAP recovery rate. The protein synthesis inhibitor cycloheximide had only a small effect on repeated FRAP and indicated a limited contribution of new P2X1 receptors to the FRAP. Inhibition of trafficking with brefeldin A reduced recovery and this effect could be reversed following receptor activation. In contrast, the dynamin inhibitor dynasore had no effect on FRAP under unstimulated conditions but reduced the level of recovery following agonist stimulation. In functional studies both brefeldin A and dynasore increased the recovery time from desensitisation. Taken together these studies demonstrate for the first time an important role of receptor recycling on P2X1 receptor responsiveness.

P2Y12 or P2Y1 inhibitors reduce platelet deposition in a microfluidic model of thrombosis while apyrase lacks efficacy under flow conditions

Determination of the patient-specific response to antiplatelet agents facilitates proper dosing for both acute and chronic prophylaxis. "Closed" systems (with or without flow) may fail to predict pharmacological potency in situations where platelets rapidly accumulate under flow conditions at a site of thrombosis ("Open" systems). Using an 8-channel microfluidic flow assay of human whole blood with corn trypsin inhibitor (+/- PPACK) perfused over focal zones of collagen, dose-response curves were measured for pharmacological agents at a wall shear rate of 210 s(-1). The P2Y(1) inhibitor MRS 2179 (IC(50) = 0.233 +/- 0.132 microM) and P2Y(12) inhibitor 2-MeSAMP (IC(50) = 2.558 +/- 0.799 microM) were potent blockers of secondary platelet accumulation under flow, while the P2X(1) inhibitor (NF 449) and apyrase failed to reduce platelet accumulation. MRS 2179 and 2-MeSAMP had undetectable effects on initial platelet adhesion to collagen. Numerical simulation of convective-diffusive transport and apyrase-mediated catalytic degradation of ADP indicated that ultra-high concentrations of apyrase ( approximately 2000 U mL(-1)) would be required to have the same effect under flow as much lower concentrations (1 U mL(-1)) currently used in closed systems (aggregometry or cone-and-plate viscometer). This is the first evaluation of IC(50) values for P2Y(12) and P2Y(1) antagonists under controlled flow conditions. Evaluation of antiplatelet agents in open flow systems demonstrates that inhibition of either ADP by apyrase or antagonism of P2X(1) signaling had no inhibitory effect on platelet accumulation. This technique provides a platform for rapidly investigating effects of antithrombotic therapies simultaneously in a model injury system.

Agonist and antagonist effects of diadenosine tetraphosphate, a platelet dense granule constituent, on platelet P2Y1, P2Y12 and P2X1 receptors

INTRODUCTION

Diadenosine 5',5'''-P(1),P(4)- tetraphosphate (Ap(4)A) is stored in platelet dense granules, but its effects on platelet function are not well understood.

METHODS AND RESULTS

We examined the effects of Ap(4)A on platelet purinergic receptors P2Y(1), P2Y(12) and P2X(1). Flow cytometry was used to measure the effects of Ap(4)A in the presence or absence of ADP on: a) P2Y(12)-mediated decrease in intraplatelet phosphorylated vasodilator stimulated phosphoprotein (VASP), b) P2Y(1)-mediated increase in platelet cytosolic Ca(2+), and c) P2X(1)-mediated intraplatelet entry of extracellular Ca(2+). ADP-stimulated platelet shape change (P2Y(1)-mediated) and aggregation (P2Y(1)- and P2Y(12)-mediated) were measured optically. Ap(4)A inhibited 3 microM ADP-induced: a) platelet aggregation (IC(50) 9.8+/-2.8 microM), b) P2Y(1)-mediated shape change, c) P2Y(1)-mediated increase in platelet cytosolic Ca(2+) (IC(50) 40.8+/-12.3 microM), and d) P2Y(12)-mediated decrease in VASP phosphorylation (IC(50)>250 microM). In the absence of added ADP, Ap(4)A had agonist effects on platelet P2X(1) and P2Y(12), but not P2Y(1), receptors.

CONCLUSION

Ap(4)A, a constituent of platelet dense granules, is a) an antagonist of platelet P2Y(1) and P2Y(12) receptors, where it inhibits the effects of ADP, and b) an agonist of platelet P2X(1) and P2Y(12) receptors.

Abnormal expression of P2X family receptors in Chinese pediatric acute leukemias

Nucleotides are new players in intercellular communication network. P2X family receptors are ATP-gated plasma membrane ion channels with diverse biological functions. Their distribution patterns and significance in pediatric leukemias have not been established. Here we investigated the expression of P2X receptors in BMMC samples from Chinese pediatric acute leukemias. Real-time PCR and Western blot results showed that P2X1, P2X4, P2X5 and P2X7 receptors were simultaneously over expressed in leukemias compared with controls, whereas P2X2, P2X3 and P2X6 were absent or marginally expressed in both groups. It was worth noting that the co-expression feature of them, especially between P2X4 and P2X7, could be observed and the highest expression of P2X7 was detected in relapsed patients. Moreover, concomitant decrease of P2X4, P2X5 and P2X7 expressions was observed at CR stage in a follow-up study. Functional P2X7 was also verified. These results suggested that P2X1, P2X4, P2X5 and P2X7 were hematopoiesis-related P2X receptors, and their signaling, especially for P2X7, might play important roles in pediatric leukemias. P2X receptors might co-operatively contribute to the malignant phenotype in human pediatric leukemias.

P2X(1) receptor inhibition and soluble CD39 administration as novel approaches to widen the cardiovascular therapeutic window

Thrombus formation at sites of disrupted atherosclerotic plaques is a leading cause of death and disability worldwide. Although the platelet is now recognized to be a central regulator of thrombus formation, development of antiplatelet reagents that selectively target thrombosis over hemostasis represents a challenge. Existing prophylactic antiplatelet therapies are centered on the use of aspirin, an irreversible cyclooxygenase inhibitor, and a thienopyridine such as clopidogrel, which inactivates the adenosine diphosphate-stimulated P2Y(12) receptor. Although these compounds are widely used and have beneficial effects for patients, their antithrombotic benefit is complicated by an elevated bleeding risk and substantial or partial "resistance." Moreover, combination therapy with these two drugs increases the hemorrhagic risk even further. This review explores the possibility of inhibiting the platelet-surface ionotropic P2X(1) receptor and/or elevating CD39/NTPDase1 activity as new therapeutic approaches to reduce overall platelet reactivity and recruitment of surrounding platelets at prothrombotic locations. Because both proteins affect platelet activation at an early stage in the events leading to thrombosis but are less crucial in hemostasis, they provide new strategies to widen the cardiovascular therapeutic window without compromising safety.

Ectodomain lysines and suramin block of P2X1 receptors

P2X(1) receptors belong to a family of cation channels gated by extracellular ATP; they are found inter alia in smooth muscle, platelets, and immune cells. Suramin has been widely used as an antagonist at P2X receptors, and its analog 4,4',4'',4'''-[carbonylbis(imino-5,1,3-benzenetriylbis(carbonylimino))] tetrakis-benzene-1,3-disulfonic acid (NF449) is selective for the P2X(1) subtype. Human and mouse P2X(1) receptors were expressed in human embryonic kidney cells, and membrane currents evoked by ATP were recorded. ATP (10 nm to 100 microm) was applied only once to each cell, to avoid the profound desensitization exhibited by P2X(1) receptors. Suramin (10 microm) and NF449 (3-300 nM) effectively blocked the human receptor. Suramin had little effect on the mouse receptor. Suramin and NF449 are polysulfonates, with six and eight negative charges, respectively. We hypothesized that species differences might result from differences in positive residues presented by the large receptor ectodomain. Four lysines in the human sequence (Lys(111), Lys(127), Lys(138), and Lys(148)) were changed individually and together to their counterparts in the mouse sequence. The substitution K138E, either alone or together with K111Q, K127Q, and K148N, reduced the sensitivity to block by both suramin and NF449. Conversely, when lysine was introduced into the mouse receptor, the sensitivity to block by suramin and NF449 was much increased for E138K, but not for Q111K, Q127K, or N148K. The results explain the marked species difference in antagonist sensitivity and identify an ectodomain lysine residue that plays a key role in the binding of both suramin and NF449 to P2X(1) receptors.

Potential therapeutic targets for ATP-gated P2X receptor ion channels

P2X receptors make up a novel family of ligand-gated ion channels that are activated by binding of extracellular ATP. These receptors can form a number of homomeric and heteromeric ion channels, which are widely distributed throughout the human body. They are thought to play an important role in many cellular processes, including synaptic transmission and thrombocyte aggregation. These ion channels are also involved in the pathology of several disease states, including chronic inflammation and neuropathic pain, and thus are the potential targets for drug development. The recent discovery of potent and highly selective antagonists for P2X(7) receptors, through the use of high-throughput screening, has helped to further understand the P2X receptor pharmacology and provided new evidence that P2X(7) receptors play a specific role in chronic pain states. In this review, we discuss how the P2X family of ion channels has distinguished itself as a potential new drug target. We are optimistic that safe and effective candidate drugs will be suitable for progression into clinical development.

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.

Targeted deletion of ectonucleoside triphosphate diphosphohydrolase 1/CD39 leads to desensitization of pre- and postsynaptic purinergic P2 receptors

We previously reported that ATP coreleased with norepinephrine from cardiac sympathetic nerves activates presynaptic P2X purinoceptors (P2XR), thereby enhancing norepinephrine exocytosis. Blockade of ectonucleoside triphosphate diphosphohydrolase 1 (E-NTPDase1/CD39) potentiates norepinephrine exocytosis, whereas recombinant soluble CD39 (solCD39) in-hibits it. This suggested that CD39 gene (Entpd1) deletion would enhance purinergic and adrenergic signaling by preserving ATP and its norepinephrine-releasing activity. However, we found that the neurogenic contractile response of vasa deferentia from Entpd1-null (CD39(-/-)) mice was attenuated and accompanied by reduced activity of pre- and postsynaptic P2XR, whereas contractile responses to K(+) or norepinephrine remained intact. In addition, the magnitude of ATP and norepinephrine exocytosis from cardiac synaptosomes was decreased in CD39(-/-) mice. Inhibition of E-NTPDase1/CD39, or solCD39 administration, did not affect the attenuated contractile response of vasa deferentia from CD39(-/-) mice. Notably, Entpd1 deletion and pharmacological P2XR desensitization in control mice similarly attenuated vasa deferentia responses. Thus, excessive and prolonged ATP exposure resulting from CD39 deletion desensitizes pre- and postjunctional P2XR at the sympathetic neuromuscular junction. This diminishes purinergic activity directly and adrenergic activity indirectly. It remains to be determined whether this desensitization results from receptor internalization, changes in receptor conformation or phosphorylation. Shutdown of ATP signaling in CD39(-/-) mice may represent a defense mechanism for the prevention of purinergic overstimulation. Our findings emphasize the cardioprotective role of neuronal CD39: by reducing presynaptic facilitatory effects of neurotransmitter ATP, CD39 attenuates norepinephrine release and its dysfunctional consequences. Moreover, by virtue of its antithrombotic action CD39 can potentially prevent the transition from myocardial ischemia to infarction.

Extracellular ATP is involved in the induction of apoptosis in murine hematopoietic cells

Extracellular nucleotides have multiple biological actions in processes such as proliferation, differentiation, chemotaxis, and cytokine secretion through P2X receptors on the cell surface. To determine the biological activity of adenosine triphosphate (ATP) and the expression of P2 nucleotide receptors in murine bone marrow-derived hematopoietic cells and stem cells/progenitor cells, we investigated the effects of ATP in assays of cell proliferation and cell death in vitro. Our results demonstrated that several subtypes of P2X receptors were expressed on hematopoietic cells and that P2X7, in particular, was partially expressed in hematopoietic stem cells/progenitor cells. In addition, stimulation of hematopoietic cells with high concentrations of ATP caused severe inhibition of cell proliferation despite the presence of cytokine stimulation. We analyzed the apoptotic effects of stimulation with several different dosages of ATP and confirmed the enhanced apoptotic activity in hematopoietic cells and progenitor cells. Antagonists, against P2X receptors and ATP, suramin and oxidized ATP, inhibited the induction of cell death for murine hematopoietic cells. Our data suggest that extracellular nucleotides may provide a novel and powerful tool for regulating the cell fate of hematopoietic stem cells.

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.

Current developments in anti-platelet therapy

Platelets play a life-saving role in hemostasis and blood clotting at sites of vascular injury and consequently are similarly involved in the pathological counterpart, namely thrombosis. Thus, anti-platelet therapy has become a mainstay in treatment and/or prophylaxis of conditions like myocardial infarction, stroke and other cardiovascular diseases. Acetyl-salicylic acid (ASA, aspirin) is still the most widely used agent and considered as the prototype antiplatelet drug. Since platelet activation occurs via several pathways that are not influenced by ASA, several other compounds have been developed to add to its beneficial effect. Currently four main classes of antiplatelet agents are available for clinical use: acetyl-salicylic acid (ASA), phosphodiesterase (PDE) inhibitors, thienopyridines and the intravenous GPIIb/IIIa antagonists. This article gives a concise review of these four classes of anti-platelet agents, using ASA as the reference standard.

Contribution of P2X1 receptor intracellular basic residues to channel properties

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

Molecular and electrophysiological characterization of transient receptor potential ion channels in the primary murine megakaryocyte

The molecular identity of platelet Ca(2+) entry pathways is controversial. Furthermore, the extent to which Ca(2+)-permeable ion channels are functional in these tiny, anucleate cells is difficult to assess by direct electrophysiological measurements. Recent work has highlighted how the primary megakaryocyte represents a bona fide surrogate for studies of platelet signalling, including patch clamp recordings of ionic conductances. We have now screened for all known members of the transient receptor potential (TRP) family of non-selective cation channels in murine megakaryocytes following individual selection of these rare marrow cells using glass micropipettes. RT-PCR detected messages for TRPC6 and TRPC1, which have been reported in platelets and megakaryocytic cell lines, and TRPM1, TRPM2 and TRPM7, which to date have not been demonstrated in cells of megakaryocytic/platelet lineage. Electrophysiological recordings demonstrated the presence of functional TRPM7, a constitutively active cation channel sensitive to intracellular Mg(2+), and TRPM2, an ADP-ribose-dependent cation channel activated by oxidative stress. In addition, the electrophysiological and pharmacological properties of the non-selective cation channels stimulated by the physiological agonist ADP are consistent with a major role for TRPC6 in this G-protein-coupled receptor-dependent Ca(2+) influx pathway. This study defines for the first time the principal TRP channels within the primary megakaryocyte, which represent candidates for Ca(2+) influx pathways activated by a diverse range of stimuli in the platelet and megakaryocyte.

The impact of blood rheology on the molecular and cellular events underlying arterial thrombosis

There is an increasing appreciation of the importance of disturbed blood flow, especially turbulent flow, in the pathogenesis of vascular disease. However, the precise mechanism(s) by which rheological changes accelerate the atherothrombotic process remains incompletely understood. Atherosclerotic lesions typically develop in vascular regions exhibiting bifurcated or curved architectures. Such regions exhibit complex blood flow profiles with considerable divergence from uniform laminar flow. These altered flow behaviours can promote deposition of pro-atherogenic lipids and proteins to the vessel wall and modulate the adhesive function of endothelial, platelets and leukocytes. Once developed, atherosclerotic lesions can further exacerbate flow disturbances, establishing a potential hazardous cycle of accelerated atherogenesis. At the cellular level, alterations in fluid flow can lead to significant changes in signal transduction, leading to a variety of functional and morphological changes. In particular, disturbed rheology has a significant impact on the adhesion and activation mechanisms utilised by platelets and leukocytes with high shear, playing an important role in accelerating platelet activation and thrombus growth. This review focuses on the impact of blood rheology on the cellular and molecular events underlying thrombosis, with particular emphasis on the role of platelets in this process.

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.

Plasma ectonucleotidases prevent desensitization of purinergic receptors in stored platelets: importance for platelet activity during thrombus formation

BACKGROUND

Platelets (PLTs) contain purinergic receptors for ATP (P2X1) and ADP (P2Y1 and P2Y12) that rapidly desensitize upon stimulation with these nucleotides. In vivo, this is antagonized by ectonucleotidases on the surface of endothelial cells and white blood cells (WBCs). The receptor desensitization of ATP- and ADP-induced responses of PLTs stored in plasma without WBCs was investigated.

STUDY DESIGN AND METHODS

ATP- and ADP-induced PLT shape change (shear-induced) aggregation and Ca2+ signaling were measured in the presence or absence of plasma. Degradation of nucleotides in plasma was quantified by high-performance liquid chromatography.

RESULTS

Washed PLTs became refractory for ATP and ADP in shape change, aggregation, and Ca2+ responses during a 90-minute incubation at 37 degrees C. The PLT responses mediated by P2X1, P2Y1, and P2Y12 receptors gradually reduced or disappeared. When plasma was present, however, the PLTs persistently showed high responses to ATP and ADP. Heat treatment of plasma abolished this effect. Also under conditions of flow and high shear, PLTs in plasma kept high P2X1 activity, mediating aggregate formation. In isolated plasma, not containing WBCs, nucleotides were degraded in the order of ADP/UDP>ATP/UTP. Degradation of ATP was partly inhibited by blocking the ecto-NTPDase CD39, whereas degradation of both ATP and ADP was inhibited by blocking ectopyrophosphatase/phosphodiesterase activity. Part of the nucleotide-degrading activities appeared to be membrane-bound.

CONCLUSION

Ectonucleotidases in plasma preserve the functionality of P2X1 and P2Y receptors. Upon PLT storage, these plasma activities are essential to ensure adequate (shear-dependent) formation of aggregates and thrombi.

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.

Characterization of functional P2X(1) receptors in mouse megakaryocytes

INTRODUCTION

Although accumulating evidence within the past 5 years strongly supports the importance of platelet P2X(1) receptors in hemostasis and thrombosis, P2X(1) receptors of platelet and/or its progenitor cell, megakaryocyte, have not been fully characterized. The aim of this study was to electrophysiologically and pharmacologically characterize the functional P2X(1) receptors on mouse megakaryocytes.

MATERIALS AND METHODS

The currents in response to nucleotides were examined using the patch-clamp whole-cell recording.

RESULTS

The agonist profile of megakaryocyte P2X(1) receptors was ATP>alpha,beta-methylene ATP>beta,gamma-methylene ATP. The P2X(1) receptors exhibited substantial monovalent as well as divalent cation permeability and the ratios of Na(+) to Cs(+) and Ca(2+) to Cs(+) permeability were 1 and 2.5, respectively. P2X receptor antagonists except suramin significantly inhibited the P2X(1) responses with an IC(50) values of 0.4 microM for pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS), 0.3 microM for 2',3'-O-(2,4,6-trinitophenyl)-adenosine 5'-triphosphate (TNP-ATP), 20 microM for reactive blue 2 (RB2), or 160 microM for 8,8'-(carbonylbis(imino-3,1-phenylene carbonylimino)bis(1,3,5-naphthalenetrisulfonic acid) (NF023), respectively. Suramin had no significant effect on the P2X(1) responses. In rat megakaryocytes, suramin similarly had no significant effect on the P2X(1) responses, but abolished the P2Y receptor-mediated responses, indicating that the suramin was active under present experimental condition.

CONCLUSIONS

These results provide the basic properties of mouse megakaryocyte P2X(1) receptors and would be helpful to examine the P2 receptor signaling in platelets and megakaryocytes.

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.

P2X1 stimulation promotes thrombin receptor-mediated platelet aggregation

P2X1 receptors are ATP-gated channel demonstrated to be involved in multiple platelet responses, although in vitro analysis has been complicated by the effects of rapid desensitization. To further investigate potential roles of P2X1 receptors in platelet activation, the current study employed methods which maximally preserved P2X1 functionality. In preliminary in vivo studies, P2X1-deficiency reduced thrombus formation following the laser-induced, but not FeCl3-induced injury. Given the multiple potential mechanisms involved in thrombus formation in vivo, including tissue-factor/thrombin generation pathways, subsequent studies were designed to investigate the effects of P2X1 inhibition or stimulation on platelet activation in vitro; specifically, the interaction of P2X1 with thrombin receptor stimulation. Aggregation initiated by low/threshold levels of a protease-activated receptor (PAR)4 agonist was reduced in P2X1-deficient murine platelets, and inhibition of P2X1 in wild-type platelets similarly reduced PAR4-mediated aggregation. In human platelets, aggregation to low/threshold stimulation of PAR1 was inhibited with the P2X1 antagonist MRS2159. In addition, P2X1 stimulation primed human platelet responses, such that subsequent sub-threshold PAR1 responses were converted into significant aggregation. Selective ADP receptor inhibitors attenuated P2X1-mediated priming, suggesting that the synergy between P2X1 and sub-threshold PAR1 stimulation was in part because of enhanced granular release of ADP. Overall, the present study defines a novel interaction between platelet P2X1 and thrombin receptors, with P2X1 functioning to amplify aggregation responses at low levels of thrombin receptor stimulation.

Sp1/3 and NF-1 mediate basal transcription of the human P2X1 gene in megakaryoblastic MEG-01 cells

BACKGROUND

P2X1 receptors play an important role in platelet function as they can induce shape change, granule centralization and are also involved in thrombus formation. As platelets have no nuclei, the level of P2X1 expression depends on transcriptional regulation in megakaryocytes, the platelet precursor cell. Since nothing is known about the molecular mechanisms regulating megakaryocytic P2X1 expression, this study aimed to identify and functionally characterize the P2X1 core promoter utilized in the human megakaryoblastic cell line MEG-01.

RESULTS

In order to identify cis-acting elements involved in the transcriptional regulation of P2X1 expression, the ability of 4.7 kb P2X1 upstream sequence to drive luciferase reporter gene expression was tested. Low promoter activity was detected in proliferating MEG-01 cells. This activity increased 20-fold after phorbol-12-myristate-13-acetate (PMA) induced differentiation. A transcription start site was detected 365 bp upstream of the start codon by primer extension. Deletion analysis of reporter constructs indicated a core promoter located within the region -68 to +149 bp that contained two Sp1 sites (named Sp1a and Sp1b) and an NF-1 site. Individual mutations of Sp1b or NF-1 binding sites severely reduced promoter activity whereas triple mutation of Sp1a, Sp1b and NF-1 sites completely abolished promoter activity in both untreated and PMA treated cells. Sp1/3 and NF-1 proteins were shown to bind their respective sites by EMSA and interaction of Sp1/3, NF-1 and TFIIB with the endogenous P2X1 core promoter in MEG-01 cells was demonstrated by chromatin immunoprecipitation. Alignment of P2X1 genes from human, chimp, rat, mouse and dog revealed consensus Sp1a, Sp1b and NF-1 binding sites in equivalent positions thereby demonstrating evolutionary conservation of these functionally important sites.

CONCLUSION

This study has identified and characterized the P2X1 promoter utilized in MEG-01 cells and shown that binding of Sp1/3 and NF-1 to elements in the direct vicinity of the transcription start site is essential for basal transcription. Targeting the function of these transcription factors in megakaryocytes may therefore provide a basis for the future therapeutic manipulation of platelet P2X1 function.

The platelet P2 receptors in arterial thrombosis

ADP and ATP play a crucial role in hemostasis and thrombosis and their receptors are potential targets for antithrombotic drugs. The ATP-gated channel P2X1 and the two G protein-coupled P2Y1 and P2Y12 ADP receptors selectively contribute to platelet aggregation. Due to its central role in the formation and stabilization of a thrombus, the P2Y12 receptor is a well established target of antithrombotic drugs like clopidogrel which has proved efficacious in many clinical trials and experimental models of thrombosis. Competitive P2Y12 antagonists have also been shown to be effective in experimental thrombosis as well as in several clinical trials. Studies in P2Y1 and P2X1 knock-out mice and experimental thrombosis models using selective P2Y1 and P2X1 antagonists have shown that, depending on the conditions, these receptors could also be potential targets for new antithrombotic drugs. Since both P2X1 and P2Y1 receptor inhibition result in milder prolongation of the bleeding time as compared to P2Y12 inhibition, the idea is put forward that combination of P2 receptor antagonists could improve efficacy with diminished hemorrhagic risk. However, further studies are required to validate such a point of view.

The novel suramin analogue NF864 selectively blocks P2X1 receptors in human platelets with potency in the low nanomolar range

The role of ATP-stimulated P2X1 receptors in human platelets is still unclear. They may act alone or in synergy with other pathways, such as P2Y1 or P2Y12 receptors, to accelerate and enhance calcium mobilisation, shape change and aggregation. To date very few pharmacological means of selectively inhibiting platelet P2X1 receptors have been described, although recent work has shown that suramin is a useful lead compound for the development of high-affinity P2X1 antagonists. We therefore investigated the effects of a series of bivalent and tetravalent suramin analogues on alphabeta meATP (P2X1 receptors)-induced or ADP (P2Y1 receptors)-induced intracellular calcium increases and shape change, as well as on ADP-induced aggregation (P2Y1 & P2Y12 receptors) in human platelets. Changes in intracellular calcium were measured using standard fluorescence techniques, while shape change and aggregation were determined by turbidimetry. The novel tetravalent compound NF864 (8,8',8'',8'''-(carbonylbis(imino-5,1,3-benzenetriyl-bis(carbonylimino)))tetrakis-naphthalene-1,3,5-trisulfonic acid-dodecasodium salt) proved to be the most potent platelet P2X1 antagonist reported to date, blocking alphabeta meATP-induced Ca2+ increases and shape change in a concentration-dependent manner, with a pA2 of 8.17 and 8.49, respectively. The ability to inhibit the platelet P2X1 receptor displayed the following order : NF864 > NF449 > or = NF110 > NF023 = MK-HU1 = suramin. A different antagonistic profile was observed for ADP-induced Ca2+ increases, shape change and aggregation; however, overall four compounds showed sufficient ability to selectively inhibit P2X1 responses, with the order NF110 > NF449 > or = NF864 > or = MK-HU1. Therefore, these compounds should prove useful tools for investigating the functional significance of platelet P2X1 receptors in thrombosis and haemostasis, NF864 being the most promising compound.

Involvement of P2 receptors in the growth and survival of neurons in the CNS

Extracellular adenosine 5'-triphosphate (ATP) has been recognized as a ubiquitous, unstable signalling molecule, acting as a fast neurotransmitter and modulator of transmitter release and neuronal excitability. Recent findings have demonstrated that ATP is a growth factor participating in differentiation, cell proliferation, and survival, as well as a toxic agent that mediates cellular degeneration and death. Potential sources of extracellular purines in the nervous system include neurons, glia, endothelium, and blood. A complex family of ectoenzymes rapidly hydrolyzes or interconverts extracellular nucleotides, thereby either terminating their signalling action or producing an active metabolite of altered purinoceptor selectivity. Most effects are mediated through the 2 main subclasses of specific cell surface receptors, P2X and P2Y. Members of these P2X/Y receptor families are widely expressed in the central nervous system (CNS) and are involved in glia-glia and glia-neuron communications, whereby they play important physiological and pathophysiological roles in a variety of biological processes. After different kinds of "acute" CNS injury (e.g., ischemia, hypoxia, mechanical stress, axotomy), extracellular ATP can reach high concentrations, up to the millimolar range, flowing out from cells into the extracellular space, exocytotically, via transmembrane transport, or as a result of cell damage. In this review, P2 receptor activation as a cause or a consequence of neuronal cell activation or death and/or glial activation is described. The involvement of P2 receptors is also described under different "chronic" pathological conditions, such as pain, epilepsia, toxic influence of ethanol or amphetamine, retinal diseases, Alzheimer's disease (AD), and possibly, Parkinson's disease. The relationship between changes in P2 receptor expression and the specific response of different cell types to injury is extremely complex and can be related to detrimental and/or beneficial effects. The present review therefore considers ATP acting via P2 receptors as a potent regulator of normal physiological and pathological processes in the brain, with a focus on pathophysiological implications of P2 receptor functions.

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

Platelet activation by ADP and ATP plays a crucial role in haemostasis and thrombosis, and their so-called P2 receptors are potential targets for antithrombotic drugs. The ATP-gated channel P2X1 and the 2 G protein-coupled P2Y1 and P2Y12 ADP receptors selectively contribute to platelet aggregation. The P2Y1 receptor is responsible for ADP-induced shape change and weak and transient aggregation, while the P2Y12 receptor is responsible for the completion and amplification of the response to ADP and to all platelet agonists, including thromboxane A2 (TXA2), thrombin, and collagen. The P2X1 receptor is involved in platelet shape change and in activation by collagen under shear conditions. Due to its central role in the formation and stabilization of a thrombus, the P2Y12 receptor is a well-established target of antithrombotic drugs like ticlopidine or clopidogrel, which have proved efficacy in many clinical trials and experimental models of thrombosis. Competitive P2Y12 antagonists have also been shown to be effective in experimental thrombosis as well as in several clinical trials. Studies in P2Y1 and P2X1 knockout mice and experimental thrombosis models using selective P2Y1 and P2X1 antagonists have shown that, depending on the conditions, these receptors could also be potential targets for new antithrombotic drugs.