Molecular basis for ADP-induced platelet activation. II. The P2Y1 receptor mediates ADP-induced intracellular calcium mobilization and shape change in platelets

Platelet flow cytometry

Flow cytometry is a powerful and versatile tool which can be used to provide substantial phenotypic data on platelets by yielding quantitative information of their physical and antigenic properties. This includes surface expression of functional receptors, bound ligands, expression of granule components, interaction of platelets with other platelets via aggregation, or interaction with other blood components, such as leukocytes or the plasma coagulation system. Quantitative assessment of these parameters may facilitate the diagnosis of inherited or acquired platelet disorders, assist in the diagnosis of diseases associated with platelet activation, or assist in the monitoring of safety and efficacy of antiplatelet therapy.

Pharmacodynamic interplay of the P2Y(1), P2Y(12), and TxA(2) pathways in platelets: the potential of triple antiplatelet therapy with P2Y(1) receptor antagonism

INTRODUCTION

Previous work suggests that the extent of platelet inhibition by P2Y(1) receptor antagonism may be underappreciated, particularly in the context of dual antiplatelet therapy with aspirin and clopidogrel.

MATERIALS AND METHODS

Using P2Y(1), P2Y(12), and TxA(2) receptor antagonists individually and in combination, we assessed the incremental changes from baseline platelet reactivity in blood collected from healthy volunteers.

RESULTS

The P2Y(1) receptor antagonist further inhibited platelet activation and aggregation in several assay conditions ex vivo when combined with P2Y(12) and/or TxA(2) receptor blockers. Collagen and TRAP-induced platelet aggregation measured by light transmittance aggregometry was inhibited to a greater extent with the triple combination relative to each of the antagonists alone. The triple combination of P2Y(1), P2Y(12), and TxA(2) receptor antagonists also significantly shifted adenosine diphosphate (ADP)-stimulated platelet glycoprotein IIb/IIIa receptor and P-selectin expression compared to individual or dual antagonists.

CONCLUSIONS

These results substantiate that additional platelet inhibition occurs with the triple combination of P2Y(1), P2Y(12), and TxA(2) receptor antagonists and support further testing of P2Y(1) receptor antagonists as an option for alternative, synergistic, or triple antiplatelet therapy.

The P2Y(12) antagonists, 2MeSAMP and cangrelor, inhibit platelet activation through P2Y(12)/G(i)-dependent mechanism

Background

ADP is an important physiological agonist that induces integrin activation and platelet aggregation through its receptors P2Y₁ (Gα_q-coupled) and P2Y₁₂ (Gα_i-coupled). P2Y₁₂ plays a critical role in platelet activation and thrombosis. Adenosine-based P2Y₁₂ antagonists, 2-methylthioadenosine 5′-monophosphate triethylammonium salt hydrate (2MeSAMP) and Cangrelor (AR-C69931MX) have been widely used to demonstrate the role of P2Y₁₂ in platelet function. Cangrelor is being evaluated in clinical trials of thrombotic diseases. However, a recent study reported that both 2MeSAMP and Cangrelor raise intra-platelet cAMP levels and inhibit platelet aggregation through a P2Y₁₂-independent mechanism.

Methodology/Principal Findings

The present work, using P2Y₁₂ deficient mice, sought to clarify previous conflicting reports and to elucidate the mechanisms by which 2MeSAMP and Cangrelor inhibit platelet activation and thrombosis. 2MeSAMP and Cangrelor inhibited aggregation and ATP release of wild-type but not P2Y₁₂ deficient platelets. 2MeSAMP and Cangrelor neither raised intracellular cAMP concentrations nor induced phosphorylation of vasodilator-stimulated phosphoprotein (VASP) in washed human or mouse platelets. Furthermore, unlike the activators (PGI₂ and forskolin) of the cAMP pathway, 2MeSAMP and Cangrelor failed to inhibit Ca²⁺ mobilization, Akt phosphorylation, and Rap1b activation in P2Y₁₂ deficient platelets. Importantly, while injection of Cangrelor inhibited thrombus formation in a FeCl₃-induced thrombosis model in wild-type mice, it failed to affect thrombus formation in P2Y₁₂ deficient mice.

Conclusions

These data together demonstrate that 2MeSAMP and Cangrelor inhibit platelet function through the P2Y₁₂-dependent mechanism both in vitro and in vivo.

Platelet function in health and disease: from molecular mechanisms, redox considerations to novel therapeutic opportunities

Increased oxidative stress appears to be of fundamental importance in the pathogenesis and development of several disease processes. Indeed, it is well known that reactive oxygen species (ROS) exert critical regulatory functions within the vascular wall, and it is, therefore, plausible that platelets represent a relevant target for their action. Platelet activation cascade (including receptor-mediated tethering to the endothelium, rolling, firm adhesion, aggregation, and thrombus formation) is tightly regulated. In addition to already well-defined platelet regulatory factors, ROS may participate in the regulation of platelet activation. It is already established that enhanced ROS release from the vascular wall can indirectly affect platelet activity by scavenging nitric oxide (NO), thereby decreasing the antiplatelet properties of endothelium. On the other hand, recent data suggest that platelets themselves generate ROS, which may evoke pro-thrombotic responses, triggering many biological processes participating in atherosclerosis initiation, progression, and complication. That oxidative stress may alter platelet function is conceivable when considering that antioxidants play a role in the prevention of cardiovascular disease, although the precise mechanism accounting for changes attributable to antioxidants in atherosclerosis remains unknown. It is possible that the effects of antioxidants may be a consequence of their enhancing or promoting the antiplatelet effects of NO derived from both endothelial cells and platelets. This review focuses on current knowledge regarding ROS-dependent regulation of platelet function in health and disease, and summarizes in vitro and in vivo evidence for their physiological and potential therapeutic relevance.

Identification of a promising drug candidate for the treatment of type 2 diabetes based on a P2Y(1) receptor agonist

The activation by extracellular nucleotides of pancreatic P2Y receptors, particularly, the P2Y(1)R subtype, increases insulin secretion. Therefore, we developed analogues of the P2Y(1)R receptor agonist 2-MeS-ADP, as potential antidiabetic drugs. Analogue 3A was found to be a potent P2Y(1)R agonist (EC(50) = 0.038 μM vs 0.0025 μM for 2-MeS-ADP) showing no activity at P2Y(2/4/6)Rs. Analogue 3A was stable at pH 1.4 (t(1/2) = 7.3 h) and resistant to hydrolysis vs 2-MeS-ADP by alkaline phosphatase (t(1/2) = 6 vs 4.5 h), human e-NPP1 (4% vs 16% hydrolysis after 20 min), and human blood serum (30% vs 50% hydrolysis after 24 h). Intravenous administration of 3A in naive rats decreased blood glucose from 155 mg/dL to normal values, ca. 87 mg/dL, unlike glibenclamide, leading to subnormal values (i.e., 63 mg/dL). Similar observations were made for streptozotocin (STZ)-treated and db(+)/db(-) mouse models. Furthermore, 3A inhibits platelet aggregation in vitro and elongates bleeding time in mice (iv administration of 30 mg of 3A/kg), increasing bleeding time to 16 vs 9 min for Prasugrel. Oral administration of 30 mg/kg 3A to rats increased tail bleeding volume, similar to aspirin. These findings suggest that 3A may be an effective treatment for type 2 diabetes by reducing both blood glucose levels and platelet aggregation.

P2Y(12) receptors in platelets and other hematopoietic and non-hematopoietic cells

The P2Y(12) receptor is a Gi-coupled ADP receptor first described in blood platelets where it plays a central role in the complex processes of activation and aggregation. Platelet granules store important amounts of ADP which are released upon stimulation by interaction of platelets with the damaged vessel wall. Therefore, the P2Y(12) receptor is a key player in primary hemostasis and in arterial thrombosis and is an established target of antithrombotic drugs like the thienopyridine compounds ticlopidine, clopidogrel, and prasugrel or the direct, reversible antagonists ticagrelor and cangrelor. Beyond the platelet physiology and pharmacology, recent studies have revealed the expression of the P2Y(12) receptor in other hematopoietic cells including leukocyte subtypes and microglia in the central nervous system as well as in vascular smooth muscle cells. These studies indicate putative roles of the P2Y(12) receptor in inflammatory states and diseases of the brain, lung, and blood vessels. The selective role of P2Y(12) among other P2 receptors as well as the possible impact of P2Y(12) targeting drugs in these processes remain to be evaluated.

Evolving role of platelet function testing in coronary artery interventions

The substantial reduction in ischemic events provided by the dual antiplatelet regimen with aspirin and clopidogrel is well documented in patients with acute coronary syndrome and patients undergoing percutaneous coronary intervention. Recently the variable response to the antiplatelet agents has received considerable attention after several “boxed warnings” on clopidogrel. This led to intense controversy on pharmacokinetic, pharmacodynamic, and pharmacogenomic issues of antiplatelet drugs, especially clopidogrel. Research use of platelet function testing has been successfully validated in identifying new antiplatelet drugs like prasugrel and ticagrelor. These platelet function assays are no longer regarded just as a laboratory phenomenon but rather as tools that have been shown to predict mortality in several clinical trials. It is believed that suboptimal response to an antiplatelet regimen (pharmacodynamic effect) may be associated with cardiovascular, cerebrovascular, and peripheral arterial events. There has been intense controversy about this variable response of antiplatelet drugs and the role of platelet function testing to guide antiplatelet therapy. While the importance of routine platelet function testing may be uncertain, it may be useful in high-risk patients such as those with diabetes mellitus, diffuse three vessels coronary artery disease, left main stenosis, diffuse atherosclerotic disease, and those with chronic renal failure undergoing percutaneous coronary intervention. It could also be useful in patients with suspected pharmacodynamic interaction with other drugs to assure the adequacy of platelet inhibition. While we wait for definitive trials, a predictive prognostic algorithm is necessary to individualize antiplatelet therapy with P2Y12 inhibitors based on platelet function assays and genetic testing.

Arterial thrombosis--insidious, unpredictable and deadly

The formation of blood clots--thrombosis--at sites of atherosclerotic plaque rupture is a major clinical problem despite ongoing improvements in antithrombotic therapy. Progress in identifying the pathogenic mechanisms regulating arterial thrombosis has led to the development of newer therapeutics, and there is general anticipation that these treatments will have greater efficacy and improved safety. However, major advances in this field require the identification of specific risk factors for arterial thrombosis in affected individuals and a rethink of the 'one size fits all' approach to antithrombotic therapy.

Clopidogrel, a P2Y12 receptor antagonist, potentiates the inflammatory response in a rat model of peptidoglycan polysaccharide-induced arthritis

The P2Y12 receptor plays a crucial role in the regulation of platelet activation by several agonists, which is irreversibly antagonized by the active metabolite of clopidogrel, a widely used anti-thrombotic drug. In this study, we investigated whether reduction of platelet reactivity leads to reduced inflammatory responses using a rat model of erosive arthritis. We evaluated the effect of clopidogrel on inflammation in Lewis rats in a peptidoglycan polysaccharide (PG-PS)-induced arthritis model with four groups of rats: 1) untreated, 2) clopidogrel-treated, 3) PG-PS-induced, and 4) PG-PS-induced and clopidogrel-treated. There were significant differences between the PG-PS+clopidogrel group when compared to the PG-PS group including: increased joint diameter and clinical manifestations of inflammation, elevated plasma levels of pro-inflammatory cytokines (IL-1 beta, interferon (IFN) gamma, and IL-6), an elevated neutrophil blood count and an increased circulating platelet count. Plasma levels of IL-10 were significantly lower in the PG-PS+clopidogrel group compared to the PG-PS group. Plasma levels of platelet factor 4 (PF4) were elevated in both the PG-PS and the PG-PS+clopidogrel groups, however PF4 levels showed no difference upon clopidogrel treatment, suggesting that the pro- inflammatory effect of clopidogrel may be due to its action on cells other than platelets. Histology indicated an increase in leukocyte infiltration at the inflammatory area of the joint, increased pannus formation, blood vessel proliferation, subsynovial fibrosis and cartilage erosion upon treatment with clopidogrel in PG-PS-induced arthritis animals. In summary, animals treated with clopidogrel showed a pro-inflammatory effect in the PG-PS-induced arthritis animal model, which might not be mediated by platelets. Elucidation of the mechanism of clopidogrel-induced cell responses is important to understand the role of the P2Y12 receptor in inflammation.

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.

Haemolysis induced by α-toxin from Staphylococcus aureus requires P2X receptor activation

Recently, it was documented that α-haemolysin (HlyA) from Escherichia coli uses erythrocyte P2 receptors cause lysis. This finding was surprising as it appeared firmly established that HlyA-dependent pore formation per se is sufficient for full cell lysis. We discovered that HlyA induced a sequential process of shrinkage and swelling and that the final haemolysis is completely prevented by blockers of P2X receptors and pannexin channels. This finding has potential clinical relevance as it may offer specific pharmacological interference to ameliorate haemolysis inflicted by pore-forming bacterial toxins. In this context, it is essential to know whether this is specific to HlyA-induced cell damage or if other bacterial pore-forming toxins involve purinergic signals to orchestrate haemolysis. Here, we investigate if the haemolysis produced by α-toxin from Staphylococcus aureus involves P2 receptor activation. We observed that α-toxin-induced haemolysis is completely blocked by the unselective P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid. Moreover, several selective blockers of P2X(1) and P2X(7) ionotropic receptors abolished haemolysis in murine and equine erythrocytes. Inhibitors of pannexin channels partially reduced the α-toxin induced lysis. Thus, we conclude that α-toxin, similar to HlyA from E. coli produces cell damage by specific activation of a purinergic signalling cascade. These data indicate that pore-forming toxins in general require purinergic signalling to elicit their toxicity.

The purinergic G protein-coupled receptor 6 inhibits effector T cell activation in allergic pulmonary inflammation

We show that the P2Y(6) receptor, a purinergic G protein-coupled receptor with a high affinity for the nucleotide uridine diphosphate, is an important endogenous inhibitor of T cell function in allergic pulmonary inflammation. Mice conditionally deficient in P2Y(6) receptors [p2ry6 (flox/flox);cre/+ mice] exhibited severe airway and tissue pathology relative to P2Y(6)-sufficient [p2ry6 (flox/flox)] littermates (+/+ mice) when treated intranasally with an extract of the dust mite Dermatophagoides farinae (Df). P2Y(6) receptors were inducibly expressed by lung, lymph node, and splenic CD4(+) and CD8(+) T cells of Df-treated +/+ mice. Df-restimulated P2Y(6)-deficient lymph node cells produced higher levels of Th1 and Th2 cytokines, and polyclonally stimulated P2Y(6)-deficient CD4(+) T cells proliferated faster than comparably stimulated P2Y(6)-sufficient cells. The absence of P2Y(6) receptors on CD4(+) cells, but not APCs, was sufficient to amplify cytokine generation. Thus, P2Y(6) receptors protect the lung against exuberant allergen-induced pulmonary inflammation by inhibiting the activation of effector T cells.

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.

P2Y12 receptor in platelet activation

ADP plays an important role in hemostasis and thrombosis. The P2Y12 receptor, activated by ADP, plays a central role in platelet activation and thrombus formation. Thus, the P2Y12 receptor has been an effective target for antithrombotic drugs.

Docking-based virtual screening of potential human P2Y12 receptor antagonists

Platelet plays essential roles in hemostasis and its dysregulation can lead to arterial thrombosis. P2Y12 is an important platelet membrane adenosine diphosphate receptor, and its antagonists have been widely developed as anti-coagulation agents. The current P2Y12 inhibitors available in clinical practice have not fully achieved satisfactory anti-thrombotic effects, leaving room for further improvement. To identify new chemical compounds as potential anti-coagulation inhibitors, we constructed a three-dimensional structure model of human P2Y12 by homology modeling based on the recently reported G-protein coupled receptor Meleagris gallopavo β1 adrenergic receptor. Virtual screening of the modeled P2Y12 against three subsets of small molecules from the ZINC database, namely lead-like, fragment-like, and drug-like, identified a number of compounds that might have high binding affinity to P2Y12. Detailed analyses of the top three compounds from each subset with the highest scores indicated that all of these compounds beard a hydrophobic bulk supplemented with a few polar atoms which bound at the ligand binding site via largely hydrophobic interactions with the receptor. This study not only provides a structure model of P2Y12 for rational design of anti-platelet inhibitors, but also identifies some potential chemicals for further development.

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.

Ser352 and Ser354 in the carboxyl terminus of the human P2Y(1) receptor are required for agonist-promoted phosphorylation and internalization in MDCK cells

BACKGROUND AND PURPOSE

The P2Y(1) receptor promotes chloride secretion in epithelial cells, a process critical for regulation of extracellular ion and fluid levels. Here we have examined the role of phosphorylation in agonist-induced internalization of P2Y(1) receptors.

EXPERIMENTAL APPROACH

A high-affinity radiolabelled antagonist, MRS2500, was used to quantify cell surface-binding sites of P2Y(1) receptors in Madin-Darby canine kidney (MDCK) epithelial cells, following exposure to agonists. The regions in the carboxyl terminus involved in both agonist-induced internalization of the receptor and its phosphorylation were identified by mutational analysis.

KEY RESULTS

Endogenous and stably expressed recombinant P2Y(1) receptors rapidly internalized with similar time courses in response to agonist in MDCK cells, ensuring that the levels of recombinant receptor achieved by retroviral infection did not adversely affect function of the internalization machinery. Four protein kinase C inhibitors of varying specificity did not affect internalization of recombinant receptors. Agonist-promoted internalization of a series of truncated P2Y(1) receptors identified a region between residues 349 and 359 in the carboxyl terminus as critical for regulation. Two amino acids within this region, Ser352 and Ser354, were shown to be both necessary and sufficient for agonist-promoted receptor phosphorylation and internalization.

CONCLUSIONS AND IMPLICATIONS

Our results firmly establish Ser352 and Ser354 in the carboxyl terminus of P2Y(1) receptors as critical residues for agonist-induced receptor internalization in MDCK cells. As the mechanism mediating this regulation requires phosphorylation of these key residues, the relevant receptor-regulated protein kinase can now be identified.

Extracellular Ca(2+) modulates ADP-evoked aggregation through altered agonist degradation: implications for conditions used to study P2Y receptor activation

ADP is considered a weak platelet agonist due to the limited aggregation responses it induces in vitro at physiological concentrations of extracellular Ca²⁺ [(Ca²⁺)_o]. Lowering [Ca²⁺]_o paradoxically enhances ADP-evoked aggregation, an effect that has been attributed to enhanced thromboxane A₂ production. This study examined the role of ectonucleotidases in the [Ca²⁺]_o-dependence of platelet activation. Reducing [Ca²⁺]_o from millimolar to micromolar levels converted ADP (10 μmol/l)-evoked platelet aggregation from a transient to a sustained response in both platelet-rich plasma and washed suspensions. Blocking thromboxane A₂ production with aspirin had no effect on this [Ca²⁺]_o-dependence. Prevention of ADP degradation abolished the differences between low and physiological [Ca²⁺]_o resulting in a robust and sustained aggregation in both conditions. Measurements of extracellular ADP revealed reduced degradation in both plasma and apyrase-containing saline at micromolar compared to millimolar [Ca²⁺]_o. As reported previously, thromboxane A₂ generation was enhanced at low [Ca²⁺]_o, however this was independent of ectonucleotidase activity_. P2Y receptor antagonists cangrelor and MRS2179 demonstrated the necessity of P2Y₁₂ receptors for sustained ADP-evoked aggregation, with a minor role for P2Y₁. In conclusion, Ca²⁺-dependent ectonucleotidase activity is a major factor determining the extent of platelet aggregation to ADP and must be controlled for in studies of P2Y receptor activation.

Dimerization of G protein-coupled purinergic receptors: increasing the diversity of purinergic receptor signal responses and receptor functions

It is well accepted that G protein-coupled receptors (GPCRs) arrange into dimers or higher-order oligomers that may modify various functions of GPCRs. GPCR-type purinergic receptors (i.e. adenosine and P2Y receptors) tend to form heterodimers with GPCRs not only of the different families but also of the same purinergic receptor families, leading to alterations in functional properties. In the present review, we focus on current knowledge of the formation of heterodimers between metabotropic purinergic receptors that activate novel functions in response to extracellular nucleosides/nucleotides, revealing that the dimerization seems to be employed for 'fine-tuning' of purinergic signaling. Thus, the relationship between adenosine and adenosine triphosphate is likely to be more and more intimate than simply being a metabolite of the other.

Antiplatelet combinations for prevention of atherothrombotic events

Antiplatelet therapy plays a fundamental role in reducing atherothrombotic events by several pathways. The present work reviews available evidence on antiplatelet therapy both for primary prevention and in the presence of established peripheral, cerebral, or cardiac ischemic disease. Due to the importance of adherence to therapy to achieve optimal effects, special attention is given to the use of fixed-dose oral formulations in the clinical subset of patients in whom double antiplatelet therapy has proven indications.

Identification of P2Y12 single-nucleotide polymorphisms and their influences on the variation in ADP-induced platelet aggregation

INTRODUCTION

Although P2Y12 has a significant role in normal hemostasis and thrombosis, no genetic study has been described about the association between P2Y12 variants and the extent of ADP-induced platelet activation in the Korean population.

MATERIALS AND METHODS

The expression levels of two reference sequences of P2Y12 mRNA transcripts (variants 1 and 2) were examined in the whole blood before direct DNA sequencing. The subjects were screened for single-nucleotide polymorphisms (SNPs) in P2Y12 by direct DNA sequencing (n=50). Frequencies of P2Y12 single nucleotide polymorphisms (SNPs), linkage disequilibrium blocks, haplotype structures, and haplotype-tagging SNPs were determined. The effects of genetic variation in the P2Y12 gene on the extent of ADP-induced platelet aggregation were studied in healthy Korean men (n=40).

RESULTS

Variant 2 (NM 176876.1) was the predominantly expressed form in all subjects, but variant 1 was also weakly expressed in all cases (n=10). A total of 20 SNPs were identified: 2 in exons, 5 in introns, and 8 and 5 in the 5'-untranslated regions of the known P2Y12 RNA variants 1 and 2, respectively. Genetic analysis of the P2Y12 SNPs and haplotypes revealed a statistically significant association between P2Y12 haplotype, denoted H3, and an increase in the ADP-induced platelet aggregation response relative to that for the reference haplotype H1 (P=0.01).

CONCLUSIONS

Application of these findings to the development of a multivariate model might be useful in explaining the variable outcome of antiplatelet drug therapy in Asian populations.

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.

Molecular pharmacology, physiology, and structure of the P2Y receptors

The P2Y receptors are a widely expressed group of eight nucleotide-activated G protein-coupled receptors (GPCRs). The P2Y(1)(ADP), P2Y(2)(ATP/UTP), P2Y(4)(UTP), P2Y(6)(UDP), and P2Y(11)(ATP) receptors activate G(q) and therefore robustly promote inositol lipid signaling responses. The P2Y(12)(ADP), P2Y(13)(ADP), and P2Y(14)(UDP/UDP-glucose) receptors activate G(i) leading to inhibition of adenylyl cyclase and to Gβγ-mediated activation of a range of effector proteins including phosphoinositide 3-kinase-γ, inward rectifying K(+) (GIRK) channels, phospholipase C-β2 and -β3, and G protein-receptor kinases 2 and 3. A broad range of physiological responses occur downstream of activation of these receptors ranging from Cl(-) secretion by epithelia to aggregation of platelets to neurotransmission. Useful structural models of the P2Y receptors have evolved from extensive genetic analyses coupled with molecular modeling based on three-dimensional structures obtained for rhodopsin and several other GPCRs. Selective ligands have been synthesized for most of the P2Y receptors with the most prominent successes attained with highly selective agonist and antagonist molecules for the ADP-activated P2Y(1) and P2Y(12) receptors. The widely prescribed drug, clopidogrel, which results in irreversible blockade of the platelet P2Y(12) receptor, is the most important therapeutic agent that targets a P2Y receptor.

The role of platelets in atherothrombosis

Platelets have evolved highly specialized adhesion mechanisms that enable cell-matrix and cell-cell interactions throughout the entire vasculature irrespective of the prevailing hemodynamic conditions. This unique property of platelets is critical for their ability to arrest bleeding and promote vessel repair. Platelet adhesion under conditions of high shear stress, as occurs in stenotic atherosclerotic arteries, is central to the development of arterial thrombosis; therefore, precise control of platelet adhesion must occur to maintain blood fluidity and to prevent thrombotic or hemorrhagic complications. Whereas the central role of platelets in hemostasis and thrombosis has long been recognized and well defined, there is now a major body of evidence supporting an important proinflammatory function for platelets that is linked to host defense and a variety of autoimmune and inflammatory diseases. In the context of the vasculature, experimental evidence indicates that the proinflammatory function of platelets can regulate various aspects of the atherosclerotic process, including its initiation and propagation. The mechanisms underlying the proatherogenic function of platelets are increasingly well defined and involve specific adhesive interactions between platelets and endothelial cells at atherosclerotic-prone sites, leading to the enhanced recruitment and activation of leukocytes. Through the release of chemokines, proinflammatory molecules, and other biological response modulators, the interaction among platelets, endothelial cells, and leukocytes establishes a localized inflammatory response that accelerates atherosclerosis. These inflammatory processes typically occur in regions of the vasculature experiencing low shear and perturbed blood flow, a permissive environment for leukocyte-platelet and leukocyte-endothelial interactions. Therefore, the concept has emerged that platelets are a central element of the atherothrombotic process and that future therapeutic strategies to combat this disease need to take into consideration both the prothrombotic and proinflammatory function of platelets.

Clopidogrel in acute coronary syndrome: implications of recent study findings

The platelet ADP receptor antagonist clopidogrel is recommended for the treatment of patients with acute coronary syndrome and/or percutaneous coronary intervention. Patients who received a coronary stent in particular should be protected by sufficient antiplatelet therapy to prevent stent thrombosis. Clopidogrel is a prodrug and has to undergo extensive metabolization before the active metabolite can irreversibly bind to platelets. This makes clopidogrel treatment susceptible to genetic and drug interactions. Recent study findings suggest that initial treatment with a higher dose of clopidogrel may be superior to the currently approved dose. It is not clear whether this approach will be sufficient to entirely overcome clopidogrel hyporesponsiveness, which worsens outcomes in up to one-third of patients. Newer antiplatelet agents are emerging but clopidogrel remains the best established treatment option, with more than 120,000 patients treated in randomized trials and 12 years of clinical postmarketing experience.

Platelet P2Y₁₂ receptor inhibition: an update on clinical drug development

Antiplatelet therapy is the cornerstone of treatment for patients with coronary artery disease. Since adenosine diphosphate (ADP) represents one of the most important mediators of thrombosis, the inhibition of the platelet ADP receptor, in particular the P2Y₁₂ subtype, plays a pivotal role in secondary prevention of recurrent atherothrombotic events in high-risk settings. Numerous clinical trials have shown the efficacy of clopidogrel, an inhibitor of the ADP P2Y₁₂ receptor, in patients presenting with an acute coronary syndrome and undergoing percutaneous coronary intervention. However, laboratory and clinical experience with clopidogrel have led to understanding some of the limitations of this drug, the most important of which is its broad range in interindividual response variability, resulting in the development of novel ADP P2Y₁₂ receptor-inhibiting strategies. This article provides an overview of ADP P2Y₁₂ receptor-inhibiting strategies, including high clopidogrel dosing regimens and novel agents under advanced clinical development.

Differential phosphorylation of myosin light chain (Thr)18 and (Ser)19 and functional implications in platelets

BACKGROUND

Myosin IIA is an essential platelet contractile protein that is regulated by phosphorylation of its regulatory light chain (MLC) on residues (Thr)18 and (Ser)19 via the myosin light chain kinase (MLCK).

OBJECTIVE

The present study was carried out to elucidate the mechanisms regulating MLC (Ser)19 and (Thr)18 phosphorylation and the functional consequence of each phosphorylation event in platelets.

RESULTS

Induction of 2MeSADP-induced shape change occurs within 5s along with robust phosphorylation of MLC (Ser)19 with minimal phosphorylation of MLC (Thr)18. Selective activation of G(12/13) produces both slow shape change and comparably slow MLC (Thr)18 and (Ser)19 phosphorylation. Stimulation with agonists that trigger ATP secretion caused rapid MLC (Ser)19 phosphorylation while MLC (Thr)18 phosphorylation was coincident with secretion. Platelets treated with p160(ROCK) inhibitor Y-27632 exhibited a partial inhibition in secretion and had a substantial inhibition in MLC (Thr)18 phosphorylation without effecting MLC (Ser)19 phosphorylation. These data suggest that phosphorylation of MLC (Ser)19 is downstream of Gq/Ca(2+) -dependent mechanisms and sufficient for shape change, whereas MLC (Thr)18 phosphorylation is substantially downstream of G(12/13) -regulated Rho kinase pathways and necessary, probably in concert with MLC (Ser)19 phosphorylation, for full contractile activity leading to dense granule secretion. Overall, we suggest that the amplitude of the platelet contractile response is differentially regulated by a least two different signaling pathways, which lead to different phosphorylation patterns of the myosin light chain, and this mechanism results in a graded response rather than a simple on/off switch.

Quantification of recombinant and platelet P2Y(1) receptors utilizing a [(125)I]-labeled high-affinity antagonist 2-iodo-N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate ([(125)I]MRS2500)

The ADP-activated P2Y(1) receptor is broadly expressed and plays a crucial role in ADP-promoted platelet aggregation. We previously synthesized 2-iodo-N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate (MRS2500), as a selective, high-affinity, competitive antagonist of this receptor. Here we report utilization of a trimethylstannyl precursor molecule for the multi-step radiochemical synthesis of a [(125)I]-labeled form of MRS2500. [(125)I]MRS2500 bound selectively to Sf9 insect cell membranes expressing the human P2Y(1) receptor but did not specifically bind to membranes isolated from empty vector-infected cells. Binding of [(125)I]MRS2500 to P2Y(1) receptors was saturable with a Kd of 1.2nM. Known agonists and antagonists of the P2Y(1) receptor inhibited [(125)I]MRS2500 binding to P2Y(1) receptor-expressing membranes with potencies in agreement with those previously observed in functional assays of this receptor. A high-affinity binding site for [(125)I]MRS2500 also was observed on intact human platelets (Kd=0.61nM) and mouse platelets (Kd=1.20nM) that exhibited the pharmacological selectivity of the P2Y(1) receptor. The densities of sites observed were 151 sites/platelet and 229 sites/platelet in human and mouse platelets, respectively. In contrast, specific binding was not observed in platelets isolated from P2Y(1) receptor(-/-) mice. Taken together, these data illustrate the synthesis and characterization of a novel P2Y(1) receptor radioligand and its utility for examining P2Y(1) receptors natively expressed on human and mouse platelets.

Recent therapeutic approaches to platelet activation in coronary artery disease

We have examined the role of platelets and platelet activation, and related emerging therapeutic approaches, in acute coronary syndromes (ACS). The role of platelets in atherothrombosis and ACS is critical, since platelet activation is a key step in the manifestation of these syndromes. Adhesion of sub-endothelial collagen and von Willebrand Factor (vWF) to the glycoprotein (GP) platelet receptors (GPIaIIa and GPIb/IX/V respectively) stimulates platelet activation. During activation, platelets present pseudopodia, which ensures a tighter adhesion to the sub-endothelial matrix and, via GPIIbIIIa receptors, facilitates platelet aggregation and platelet binding to fibrinogen and vWF. Although all laboratory methods estimating platelet activation and antiplatelet therapy have specific limitations, the use of antiplatelet agents such as aspirin, clopidogrel, and GPIIbIIIa inhibitors remains essential in ACS prevention and treatment. Platelet-related genetic polymorphisms can modulate the response to these agents. Presently, antiplatelet intervention remains an important therapeutic modality, with novel antiplatelet therapies, such as prasugrel and ticagrelor under clinical investigation.

Role of phosphoinositide 3-kinase beta in platelet aggregation and thromboxane A2 generation mediated by Gi signalling pathways

PI3Ks (phosphoinositide 3-kinases) play a critical role in platelet functional responses. PI3Ks are activated upon P2Y12 receptor stimulation and generate pro-aggregatory signals. P2Y12 receptor has been shown to play a key role in the platelet aggregation and thromboxane A2 generation caused by co-stimulation with Gq or Gz, or super-stimulation of Gi pathways. In the present study, we evaluated the role of specific PI3K isoforms alpha, beta, gamma and delta in platelet aggregation, thromboxane A2 generation and ERK (extracellular-signal-regulated kinase) activation. Our results show that loss of the PI3K signal impaired the ability of ADP to induce platelet aggregation, ERK phosphorylation and thromboxane A2 generation. We also show that Gq plus Gi- or Gi plus Gz-mediated platelet aggregation, ERK phosphorylation and thromboxane A2 generation in human platelets was inhibited by TGX-221, a PI3Kbeta-selective inhibitor, but not by PIK75 (a PI3Kalpha inhibitor), AS252424 (a PI3Kgamma inhibitor) or IC87114 (a PI3Kdelta inhibitor). TGX-221 also showed a similar inhibitory effect on the Gi plus Gz-mediated platelet responses in platelets from P2Y1-/- mice. Finally, 2MeSADP (2-methyl-thio-ADP)-induced Akt phosphorylation was significantly inhibited in the presence of TGX-221, suggesting a critical role for PI3Kbeta in Gi-mediated signalling. Taken together, our results demonstrate that PI3Kbeta plays an important role in ADP-induced platelet aggregation. Moreover, PI3Kbeta mediates ADP-induced thromboxane A2 generation by regulating ERK phosphorylation.

Mutational analysis of residues important for ligand interaction with the human P2Y(12) receptor

The P2Y(12) receptor, a Gi protein-coupled receptor, plays a central role in platelet activation. In this study, we did a mutational analysis of residues possibly involved in the ligand interactions with the human P2Y(12) receptor. Mutant receptors were stably expressed in CHO-K1 cells with an HA-tag at the N-terminus. Expression of wild-type and mutant receptors was confirmed by detecting the HA-tag on the cell membrane. Residues in transmembrane helical domains (TMs) 3, 5, 6, and 7, which are homologous to residues important for P2Y(1) receptor activation and ligand recognition, were replaced by site-directed mutagenesis. ADP-induced inhibition of forskolin-stimulated cAMP levels in the presence or absence of antagonist AR-C69931MX were investigated for each of the mutant receptors. F104S and S288P significantly increased agonist-induced receptor function without affecting the antagonism by AR-C69931MX. Arg256 in TM6 and Arg 265 in extracellular loop 3 (EL3) are more important for antagonist recognition than effect on agonist-mediated receptor function. Compared to wild-type P2Y(12) receptor, mutations in Arg 256 or/and Arg 265 significantly increased the sensitivity to antagonist AR-C69931MX. Our study shows that the cytosolic side of TM3 and the exofacial side of TM5 are critical for P2Y(12) receptor function, which is different from P2Y(1). Arg 256 in TM6 and Arg265 in EL3 appear to play a role in antagonist recognition rather than effects on agonist-induced receptor function.

Polymer particles that switch shape in response to a stimulus

Particle engineering for biomedical applications has unfolded the roles of attributes such as size, surface chemistry, and shape for modulating particle interactions with cells. Recently, dynamic manipulation of such key properties has gained attention in view of the need to precisely control particle interaction with cells. With increasing recognition of the pivotal role of particle shape in determining their biomedical applications, we report on polymeric particles that are able to switch their shape in real time in a stimulus-responsive manner. The shape-switching behavior was driven by a subtle balance between polymer viscosity and interfacial tension. The balance between the two forces was modulated by application of an external stimulus chosen from temperature, pH, or chemical additives. The dynamics of shape switch was precisely controlled over minutes to days under physiological conditions. Shape-switching particles exhibited unique interactions with cells. Elliptical disk-shaped particles that are not phagocytosed by macrophages were made to internalize through shape switch, demonstrating the ability of shape-switchable particles in modulating interaction with cells.

De novo protein synthesis in mature platelets: a consideration for transfusion medicine

Platelet function in thrombosis and haemostasis is reasonably well understood at the molecular level with respect to the proteins involved in cellular structure, signalling networks and platelet interaction with clotting factors and other cells. However, the natural history of these proteins has only recently garnered the attention of platelet researchers. De novo protein synthesis in platelets was discovered 40 years ago; however, it was generally dismissed as merely an interesting minor phenomenon until studies over the past few years renewed interest in this aspect of platelet proteins. It is now accepted that anucleate platelets not only have the potential to synthesize proteins, but this capacity seems to be required to fulfil their function. With translational control as the primary mode of regulation, platelets are able to express biologically relevant gene products in a timely and signal-dependent manner. Platelet protein synthesis during storage of platelet concentrates is a nascent area of research. Protein synthesis does occur, although not for all proteins found in the platelet protein profile. Furthermore, mRNA appears to be well preserved under standard storage conditions. Although its significance is not yet understood, the ability to replace proteins may form a type of cellular repair mechanism during storage. Disruption by inappropriate storage conditions or processes that block protein synthesis such as pathogen reduction technologies may have direct effects on the ability of platelets to synthesize proteins during storage.

A structural basis for the inhibition of collagen-stimulated platelet function by quercetin and structurally related flavonoids

BACKGROUND AND PURPOSE

Molecular mechanisms underlying the links between dietary intake of flavonoids and reduced cardiovascular disease risk are only partially understood. Key events in the pathogenesis of cardiovascular disease, particularly thrombosis, are inhibited by these polyphenolic compounds via mechanisms such as inhibition of platelet activation and associated signal transduction, attenuation of generation of reactive oxygen species, enhancement of nitric oxide production and binding to thromboxane A(2) receptors. In vivo, effects of flavonoids are mediated by their metabolites, but the effects and modes of action of these compounds are not well-characterized. A good understanding of flavonoid structure-activity relationships with regard to platelet function is also lacking.

EXPERIMENTAL APPROACH

Inhibitory potencies of structurally distinct flavonoids (quercetin, apigenin and catechin) and plasma metabolites (tamarixetin, quercetin-3'-sulphate and quercetin-3-glucuronide) for collagen-stimulated platelet aggregation and 5-hydroxytryptamine secretion were measured in human platelets. Tyrosine phosphorylation of total protein, Syk and PLCgamma2 (immunoprecipitation and Western blot analyses), and Fyn kinase activity were also measured in platelets. Internalization of flavonoids and metabolites in a megakaryocytic cell line (MEG-01 cells) was studied by fluorescence confocal microscopy.

KEY RESULTS

The inhibitory mechanisms of these compounds included blocking Fyn kinase activity and the tyrosine phosphorylation of Syk and PLCgamma2 following internalization. Principal functional groups attributed to potent inhibition were a planar, C-4 carbonyl substituted and C-3 hydroxylated C ring in addition to a B ring catechol moiety.

CONCLUSIONS AND IMPLICATIONS

The structure-activity relationship for flavonoids on platelet function presented here may be exploited to design selective inhibitors of cell signalling.

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.

Hereditary defect of the platelet ADP receptor(s)

Four patients with a previously unrecognized congenital disorder of platelet function have recently been described. Their platelets aggregate very poorly to exogenous ADP. The abnormality is likely due to a severe defect of the platelet ADP receptor that is coupled to adenylate cyclase, as suggested by the following findings: 1) ADP does not normally lower cAMP levels of PGE1-treated platelets; 2) platelet shape change induced by ADP is normal; 3) the binding of [radiolabelled]ADP to formalin-fixed platelets or of the ADP analogue [radiolabelled]2-MeS-ADP to fresh platelets is severely defective. Since all patients that have been described were born from consanguineous parents, the condition seems to be inherited as an autosomal recessive trait. Platelets of an obligate heterozygote have intermediate binding sites for 2-M eS-ADP, undergo a normal primary wave of aggregation induced by exogenous ADP, but do not normally secrete the content of their granules when stimulated by release-inducing agonists. Studies of normal platelets treated with acetylsalycilic acid revealed that ADP potentiates platelet secretion directly, and that the full complement of its platelet receptors appears to be necessary for this function.

Mechanism of activation and functional role of protein kinase Ceta in human platelets

The novel class of protein kinase C (nPKC) isoform eta is expressed in platelets, but not much is known about its activation and function. In this study, we investigated the mechanism of activation and functional implications of nPKCeta using pharmacological and gene knock-out approaches. nPKCeta was phosphorylated (at Thr-512) in a time- and concentration-dependent manner by 2MeSADP. Pretreatment of platelets with MRS-2179, a P2Y1 receptor antagonist, or YM-254890, a G(q) blocker, abolished 2MeSADP-induced phosphorylation of nPKCeta. Similarly, ADP failed to activate nPKCeta in platelets isolated from P2Y1 and G(q) knock-out mice. However, pretreatment of platelets with P2Y12 receptor antagonist, AR-C69331MX did not interfere with ADP-induced nPKCeta phosphorylation. In addition, when platelets were activated with 2MeSADP under stirring conditions, although nPKCeta was phosphorylated within 30 s by ADP receptors, it was also dephosphorylated by activated integrin alpha(IIb)beta3 mediated outside-in signaling. Moreover, in the presence of SC-57101, a alpha(IIb)beta3 receptor antagonist, nPKCeta dephosphorylation was inhibited. Furthermore, in murine platelets lacking PP1cgamma, a catalytic subunit of serine/threonine phosphatase, alpha(IIb)beta3 failed to dephosphorylate nPKCeta. Thus, we conclude that ADP activates nPKCeta via P2Y1 receptor and is subsequently dephosphorylated by PP1gamma phosphatase activated by alpha(IIb)beta3 integrin. In addition, pretreatment of platelets with eta-RACK antagonistic peptides, a specific inhibitor of nPKCeta, inhibited ADP-induced thromboxane generation. However, these peptides had no affect on ADP-induced aggregation when thromboxane generation was blocked. In summary, nPKCeta positively regulates agonist-induced thromboxane generation with no effects on platelet aggregation.

RhoA downstream of G(q) and G(12/13) pathways regulates protease-activated receptor-mediated dense granule release in platelets

Platelet secretion is an important physiological event in hemostasis. The protease-activated receptors, PAR 1 and PAR 4, and the thromboxane receptor activate the G(12/13) pathways, in addition to the G(q) pathways. Here, we investigated the contribution of G(12/13) pathways to platelet dense granule release. 2MeSADP, which does not activate G(12/13) pathways, does not cause dense granule release in aspirin-treated platelets. However, supplementing 2MeSADP with YFLLRNP (60muM), as selective activator of G(12/13) pathways, resulted in dense granule release. Similarly, supplementing PLC activation with G(12/13) stimulation also leads to dense granule release. These results demonstrate that supplemental signaling from G(12/13) is required for G(q)-mediated dense granule release and that ADP fails to cause dense granule release because the platelet P2Y receptors, although activate PLC, do not activate G(12/13) pathways. When RhoA, downstream signaling molecule in G(12/13) pathways, is blocked, PAR-mediated dense granule release is inhibited. Furthermore, ADP activated RhoA downstream of G(q) and upstream of PLC. Finally, RhoA regulated PKCdelta T505 phosphorylation, suggesting that RhoA pathways contribute to platelet secretion through PKCdelta activation. We conclude that G(12/13) pathways, through RhoA, regulate dense granule release and fibrinogen receptor activation in platelets.

Hematopoietic lineage cell-specific protein-1 (HS1) regulates PAR-mediated ERK activation and thromboxane generation in platelets

Thrombin-induced platelet activation leads to tyrosine phosphorylation of hematopoietic lineage cell-specific protein-1 (HS1), a 75 kDa adapter protein expressed exclusively in cells of hematopoietic lineage. We have shown HS1 to be a functionally important signaling molecule downstream of PAR-4 and GPVI collagen receptor. We have thus begun to elucidate PAR signaling pathway of HS1 phosphorylation, and its functional implications. PAR-1 and PAR-4 activating peptides (SFLLRN and AYPGKF, respectively) induced HS1 phosphorylation in a Gq-dependent manner as shown by incubation with the Gq inhibitor, YM254890. Consistently, HS1 phosphorylation was abolished in platelets from Gq deficient mice upon AYPGKF stimulation. Treatment with ADP receptor antagonists did not affect HS1 phosphorylation. Pretreatment of platelets with Src kinase inhibitors abolished HS1 phosphorylation. Further Syk activation, as measured by tyrosine phosphorylation of Syk (residues 525/526), in response to PAR activation was abolished in the presence of Src inhibitors. HS1 null mice show inhibition of PAR-mediated thromboxane A2 generation compared to wild type littermates. Phosphorylation of Erk, a key signaling molecule in thromboxane generation, was also diminished in HS1 null mice platelets. Based on these findings, we conclude that tyrosine phosphorylation of HS1 occurs downstream of both PAR-1 and PAR-4. HS1 phosphorylation is a Gq mediated response regulated by Src kinases. Thus, HS1 may mediate PAR-induced thromboxane generation through regulation of Erk phosphorylation.

Dysregulation of Ca2+ movement in platelets from patients with acute ischaemic stroke

1. Platelets play a pivotal role during acute ischaemic stroke. An increase in cytosolic Ca(2+) concentrations ([Ca(2+)](i)) triggers intracellular signal transduction, leading to platelet aggregation and thrombosis. In the present study, we examined the differences between platelets from acute ischaemic stroke patients and at-risk controls in terms of the increase in platelet [Ca(2+)](i). 2. Thirty-one patients with acute ischaemic stroke and 27 at-risk controls were enrolled in the present study. Platelet [Ca(2+)](i) was measured using the fluorescent dye fura-2 after stimulation with 100 micromol/L arachidonic acid (AA), 10 micromol/L ADP, 1 micromol/L platelet-activation factor (PAF) and 0.1 U/mL thrombin. 3. Basal [Ca(2+)](i) was higher in the stroke group compared with at-risk controls, irrespective of the presence or absence of extracellular Ca(2+). In Ca(2+)-containing medium, both PAF and ADP, but not AA and thrombin, significantly increased platelet [Ca(2+)](i) in the stroke group compared with the at-risk controls. However, in Ca(2+)-free medium, only PAF significantly increased platelet [Ca(2+)](i) in the stroke group compared with the at-risk controls. Basal [Ca(2+)](i) and PAF-induced platelet [Ca(2+)](i) increases were still higher in the stroke group at the subacute stage than in the at-risk controls. 4. The results of the present study provide direct evidence that Ca(2+) signalling in platelets from acute ischaemic stroke patients was altered in response to particular stimuli. The dysregulation of Ca(2+) movement in platelets may persist up to the subacute stage of ischaemic stroke.