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

Jedi-1/MEGF12-mediated phagocytosis controls the pro-neurogenic properties of microglia in the ventricular-subventricular zone

Microglia are the primary phagocytes in the central nervous system and are responsible for clearing dead cells generated during development or disease. The phagocytic process shapes the phenotype of the microglia, which affects the local environment. A unique population of microglia reside in the ventricular-subventricular zone (V-SVZ) of neonatal mice, but how they influence this neurogenic niche is not well-understood. Here, we demonstrate that phagocytosis creates a pro-neurogenic microglial phenotype in the V-SVZ and that these microglia phagocytose apoptotic cells via the engulfment receptor Jedi-1. Deletion of Jedi-1 decreases apoptotic cell clearance, triggering the development of a neuroinflammatory phenotype, reminiscent of neurodegenerative and-age-associated microglia, that reduces neural precursor proliferation via elevated interleukin (IL)-1β signaling; inhibition of IL-1 receptor rescues precursor proliferation in vivo. Together, these results reveal a critical role for Jedi-1 in connecting microglial phagocytic activity to a phenotype that promotes neurogenesis in the developing V-SVZ.

Microfluidic assay of antiplatelet agents for inhibition of shear-induced platelet adhesion and activation

We have developed a microfluidic system to perfuse whole blood through a flow channel with an upstream stenotic region and a downstream protein capture region. This flow-based system was used to assay how effectively antiplatelet agents suppress shear-induced platelet adhesion and activation downstream of the stenotic region. Microcontact printing was used to covalently attach one of three platelet binding proteins [fibrinogen, collagen, or von Willebrand factor (vWf)] to the surface of the downstream capture region. Whole blood with an antiplatelet agent was transiently exposed to an upstream high wall shear rate (either 4860 s-1 or 11 560 s-1), and subsequently flowed over the downstream capture region where the platelet adhesion was measured. Several antiplatelet agents (acetylsalicylic acid, tirofiban, eptifibatide, anti-vWf, and anti-GPIbα) were evaluated for their efficacy in attenuating downstream adhesion. Following antibody blocking of vWf or GPIbα, downstream platelet activation was also assessed in perfused blood by flow cytometry using two activation markers (active GPIIb/IIIa and P-selectin). Acetylsalicylic acid demonstrated its inability to diminish shear-induced platelet adhesion to all three binding proteins. GPIIb/IIIa inhibitors (tirofiban and eptifibatide) significantly reduced platelet adhesion to fibrinogen. Antibody blocking of vWf or GPIbα effectively diminished platelet adhesion to all three capture proteins as well as platelet activation in perfused blood, indicating an essential role of vWf-GPIbα interaction in mediating shear-induced platelet aggregation.

Low intensity pulsed ultrasound (LIPUS) maintains osteogenic potency by the increased expression and stability of Nanog through spleen tyrosine kinase (Syk) activation

Mesenchymal stem cells (MSCs) are a powerful tool for cell-based, clinical therapies like bone regeneration. Therapeutic use of cell transplantation requires many cells, however, the expansion process needed to produce large quantities of cells reduces the differentiation potential of MSCs. Here, we examined the protective effects of low intensity pulsed ultrasound (LIPUS) on the maintenance of osteogenic potency. Primary osteoblastic cells were serially passaged between 2 and 12 times with daily LIPUS treatment. We found that LIPUS stimulation maintains osteogenic differentiation capacity in serially passaged cells, as characterized by improved matrix mineralization and Osteocalcin mRNA expression. Decreased expression of Nanog, Sox2, and Msx2, and increased expression of Pparg2 from serial passaging was recovered in LIPUS-stimulated cells. We found that LIPUS stimulation not only increased but also sustained expression of Nanog in primary osteoblasts and ST2 cells, a mouse mesenchymal stromal cell line. Nanog overexpression in serially passaged cells mimicked the recuperative effects of LIPUS on osteogenic potency, highlighting the important role of Nanog in LIPUS stimulation. Additionally, we found that spleen tyrosine kinase (Syk) is an important signaling molecule to induce Nanog expression in LIPUS-stimulated cells. Syk activation was regulated by both Rho-associated kinase 1 (ROCK1) and extracellular ATP in a paracrine manner. Interestingly, the LIPUS-induced increase in Nanog mRNA expression was regulated by ATP-P2X4-Syk Y323 activation, while the improvement of Nanog protein stability was controlled by the ROCK1-Syk Y525/526 pathway. Taken together, these results indicate that LIPUS stimulation recovers and maintains the osteogenic potency of serially passaged cells through a Syk-Nanog axis.

Signaling via P2Y12 may be critical for early stabilization of platelet aggregates

P2Y(12) receptor antagonism inhibits platelet aggregation by preventing adenosine diphosphate (ADP)-mediated amplification of activation pathways downstream of primary agonists, such as thrombin and collagen. However, the role of ADP signaling in maintaining aggregate stability and the effects of P2Y(12) antagonists on preestablished aggregates in vitro and arterial thrombus in vivo are not well understood. This study evaluated the impact of P2Y(12) signaling on platelet aggregate stability and early thrombotic occlusion using a reversible P2Y(12) antagonist, ticagrelor. There were 2 study objectives: (1) to determine if there was a time-dependent factor on the capacity of a P2Y(12) antagonist to affect human platelet aggregate stability in vitro using light transmission aggregometry and (2) to evaluate the extent of arterial thrombus reversal in a preclinical model upon administration of ticagrelor in vivo. Platelet aggregates were exposed to ticagrelor after ADP or collagen activation, monitored for stability by aggregometry, and visualized by microscopy. Freshly formed ADP- and collagen-induced platelet aggregates were more rapidly dispersed by a P2Y(12) antagonist than drug carrier control at clinically relevant concentrations (P < 0.05). However, stable aggregates were not noticeably affected. A murine arterial thrombosis model was used to evaluate thrombus stability in an in vivo mouse model. Thrombotic occlusion was induced by FeCl(3), followed by a bolus intravenous administration of ticagrelor or vehicle control. Doppler blood flow was monitored before injury and 30 minutes after bolus administration. Arteries were retrieved for inspection for residual thrombus. Early arterial thrombotic occlusion in vivo was partially reversed by ticagrelor administration. Blood flow through the injured artery increased, and thrombus size within the artery decreased (P < 0.05, n = 3). In conclusion, P2Y(12) antagonism disrupts the stability of newly formed platelet aggregates, promoting disaggregation, and reverses thrombotic vascular occlusion. Thus, in addition to activating platelets, signaling via P2Y(12) seems to be required for stabilizing platelet thrombi.

The role of focal adhesion kinase in the regulation of cellular mechanical properties

The regulation of mechanical properties is necessary for cell invasion into connective tissue or intra- and extravasation through the endothelium of blood or lymph vessels. Cell invasion is important for the regulation of many healthy processes such as immune response reactions and wound healing. In addition, cell invasion plays a role in disease-related processes such as tumor metastasis and autoimmune responses. Until now the role of focal adhesion kinase (FAK) in regulating mechanical properties of cells and its impact on cell invasion efficiency is still not well known. Thus, this review focuses on mechanical properties regulated by FAK in comparison to the mechano-regulating protein vinculin. Moreover, it points out the connection between cancer cell invasion and metastasis and FAK by showing that FAK regulates cellular mechanical properties required for cellular motility. Furthermore, it sheds light on the indirect interaction of FAK with vinculin by binding to paxillin, which then impairs the binding of paxillin to vinculin. In addition, this review emphasizes whether FAK fulfills regulatory functions similar to vinculin. In particular, it discusses the differences and the similarities between FAK and vinculin in regulating the biomechanical properties of cells. Finally, this paper highlights that both focal adhesion proteins, vinculin and FAK, synergize their functions to regulate the mechanical properties of cells such as stiffness and contractile forces. Subsequently, these mechanical properties determine cellular invasiveness into tissues and provide a source sink for future drug developments to inhibit excessive cell invasion and hence, metastases formation.

PlaCor PRT measurement of shear-activated platelet aggregate formation in stable patients treated with single and dual antiplatelet therapy

Shear forces play a key role in thrombus formation and shear-based tests may better reflect physiological conditions in vivo compared with agonist-based tests. We evaluated the PlaCor PRT®, a novel platelet reactivity test based on shear-induced platelet aggregation, in patients with stable coronary artery disease (CAD) treated with single (SAPT) and dual antiplatelet therapy (DAPT). We examined 100 patients with multiple risk factors for CAD and/or documented stable CAD: 38 treated with SAPT, aspirin 100 mg qd, 62 treated with DAPT, aspirin 100 mg + clopidogrel 75 mg qd, compared with age- and sex-matched healthy volunteers without antiplatelet therapy (HV, n = 35). Measures of shear-induced platelet aggregation were performed with the PlaCor PRT®. In 25 patients in SAPT, the PlaCor test was also performed before and after a 12-hour-loading dose of clopidogrel 600 mg. The mean ± SD PRT time (seconds) in HV was 78 ± 13 and was significantly lower compared with SAPT (118 ± 16, p = 0.030) and to DAPT patients (242 ± 11, p < 0.0001). A statistically significant difference was also reported between SAPT and DAPT patients (p < 0.0001). After a loading dose of clopidogrel, the PRT time of SAPT patients increased significantly from 112 ± 20 to 254 ± 17, p < 0.0001. 2.7 and 26% of patients were considered as "poor responders" to single and dual antiplatelet therapy, respectively. This study shows that in patients with multiple risk factors for CAD and/or documented stable CAD, SAPT and DAPT play an important role in reducing platelet aggregation mediated by shear forces as evaluated with the novel PlaCor PRT®. Further studies will be required to confirm and assess the extent of these findings in patients with acute coronary syndromes.

Pathologic shear triggers shedding of vascular receptors: a novel mechanism for down-regulation of platelet glycoprotein VI in stenosed coronary vessels

Ligand-induced ectodomain shedding of glycoprotein VI (GPVI) is a metalloproteinase-dependent event. We examined whether shear force, in the absence of GPVI ligand, was sufficient to induce shedding of GPVI. Human-citrated platelet-rich plasma or washed platelets were subjected to increasing shear rates in a cone-plate viscometer, and levels of intact and cleaved GPVI were examined by Western blot and ELISA. Pathophysiologic shear rates (3000-10 000 seconds(-1)) induced platelet aggregation and metalloproteinase-dependent appearance of soluble GPVI ectodomain, and GPVI platelet remnant. Shedding of GPVI continued after transient exposure to shear. Blockade of α(IIb)β(3), GPIbα, or intracellular signaling inhibited shear-induced platelet aggregation but minimally affected shear-induced shedding of GPVI. Shear-induced GPVI shedding also occurred in platelet-rich plasma or washed platelets isolated from a von Willebrand disease type 3 patient with no detectable VWF, implying that shear-induced activation of platelet metalloproteinases can occur in the absence of GPVI and GPIbα ligands. Significantly elevated levels of sGPVI were observed in 10 patients with stable angina pectoris, with well-defined single vessel coronary artery disease and mean intracoronary shear estimates at 2935 seconds(-1) (peak shear, 19 224 seconds(-1)). Loss of GPVI in platelets exposed to shear has potential implications for the stability of a forming thrombus at arterial shear rates.

Bleeding manifestations of congenital and drug-induced defects of the platelet P2Y12 receptor for adenosine diphosphate

P2Y12, one of the two platelet receptors for adenosine diphosphate (ADP), plays a central role in platelet function. Defects of P2Y12 should be suspected when ADP, even at high concentrations (≥10 µM), is unable to induce full, irreversible platelet aggregation. Patients with congenital P2Y12 defects display a mild-to-moderate bleeding diathesis of variable severity, characterised by mucocutaneous bleeding and excessive post-surgical and post-traumatic blood loss. Drugs that inhibit P2Y12 are potent antithrombotic drugs, attesting the central role played by P2Y12 in platelet thrombus formation. Clopidogrel, the most widely used drug that inhibits P2Y12, is effective both in monotherapy and in combination with acetylsalicylic acid (ASA). Its most important drawback is the inability to inhibit adequately P2Y12-dependent platelet function in about 1/3 of patients, at the recommended therapeutic doses. The incidence of bleeding events is similar in ASA-treated and clopidogrel-treated patients; however, the combination of ASA and clopidogrel causes more bleeding than each drug in monotherapy. Compared to clopidogrel, new drugs inhibiting P2Y12, such as prasugrel and ticagrelor, decrease the risk of cardiovascular events and increase the risk of bleeding complications, because they adequately inhibit P2Y12-dependent platelet function in the vast majority of treated patients.

The platelet P2Y₁₂ receptor for adenosine diphosphate: congenital and drug-induced defects

P2Y₁₂, the G(i)-coupled platelet receptor for adenosine diphosphate (ADP), plays a central role in platelet function. Patients with congenital P2Y₁₂ defects display a mild to moderate bleeding diathesis, characterized by mucocutaneous bleedings and excessive post-surgical and post-traumatic blood loss. Defects of P2Y₁₂ should be suspected when ADP, even at high concentrations (≥ 10 μM), is unable to induce full, irreversible platelet aggregation. Tests that evaluate the degree of inhibition of adenylyl cyclase by ADP should be used to confirm the diagnosis. Drugs that inhibit P2Y₁₂ are potent antithrombotic drugs, attesting the central role played by P2Y₁₂ in platelet thrombus formation. Clopidogrel, the most widely used drug that inhibits P2Y₁₂, is effective both in monotherapy and in combination with acetylsalicylic acid. The most important drawback of clopidogrel is its inability to inhibit adequately P2Y₁₂-dependent platelet function in approximately one-third of patients who are therefore not protected from major cardiovascular events. New drugs, such as prasugrel and ticagrelor, which effectively inhibit P2Y₁₂ in the majority of patients, proved to be more efficacious than clopdidogrel in preventing major cardiovascular events. Although they increase the incidence of major bleedings, the net clinical benefit is in favor of the new P2Y₁₂ inhibitors.

Stabilizing role of platelet P2Y(12) receptors in shear-dependent thrombus formation on ruptured plaques

BACKGROUND

In most models of experimental thrombosis, healthy blood vessels are damaged. This results in the formation of a platelet thrombus that is stabilized by ADP signaling via P2Y(12) receptors. However, such models do not predict involvement of P2Y(12) in the clinically relevant situation of thrombosis upon rupture of atherosclerotic plaques. We investigated the role of P2Y(12) in thrombus formation on (collagen-containing) atherosclerotic plaques in vitro and in vivo, by using a novel mouse model of atherothrombosis.

METHODOLOGY

Plaques in the carotid arteries from Apoe(-/-) mice were acutely ruptured by ultrasound treatment, and the thrombotic process was monitored via intravital fluorescence microscopy. Thrombus formation in vitro was assessed in mouse and human blood perfused over collagen or plaque material under variable conditions of shear rate and coagulation. Effects of two reversible P2Y(12) blockers, ticagrelor (AZD6140) and cangrelor (AR-C69931MX), were investigated.

PRINCIPAL FINDINGS

Acute plaque rupture by ultrasound treatment provoked rapid formation of non-occlusive thrombi, which were smaller in size and unstable in the presence of P2Y(12) blockers. In vitro, when mouse or human blood was perfused over collagen or atherosclerotic plaque material, blockage or deficiency of P2Y(12) reduced the thrombi and increased embolization events. These P2Y(12) effects were present at shear rates >500 s(-1), and they persisted in the presence of coagulation. P2Y(12)-dependent thrombus stabilization was accompanied by increased fibrin(ogen) binding.

CONCLUSIONS/SIGNIFICANCE

Platelet P2Y(12) receptors play a crucial role in the stabilization of thrombi formed on atherosclerotic plaques. This P2Y(12) function is restricted to high shear flow conditions, and is preserved in the presence of coagulation.

Platelets undergo phosphorylation of Syk at Y525/526 and Y352 in response to pathophysiological shear stress

Atherosclerotic plaques can lead to partial vascular occlusions that produce abnormally high levels of arterial wall shear stress. Such pathophysiological shear stress can promote shear-induced platelet aggregation (SIPA), which has been linked to acute myocardial infarction, unstable angina, and stroke. This study investigated the role of the tyrosine kinase Syk in shear-induced human platelet signaling. The extent of Syk tyrosine phosphorylation induced by pathophysiological levels of shear stress (100 dyn/cm(2)) was significantly greater than that resulting from physiological shear stress (10 dyn/cm(2)). With the use of phospho-Syk specific antibodies, these data are the first to show that key regulatory sites of Syk at tyrosines 525/526 (Y525/526) and tyrosine 352 (Y352) were phosphorylated in response to pathophysiological shear stress. Increased phosphorylation at both sites was attenuated by pharmacological inhibition of Syk using two different Syk inhibitors, piceatannol and 3-(1-methyl-1H-indol-3-yl-methylene)-2-oxo-2,3-dihydro-1H-indole-5-sulfonamide (OXSI-2), and by inhibition of upstream Src-family kinases (SFKs). Shear-induced response at the Syk 525/526 site was ADP dependent but not contingent on glycoprotein (GP) IIb-IIIa ligation or the generation of thromboxane (Tx) A(2). Pretreatment with Syk inhibitors not only reduced SIPA and Syk phosphorylation in isolated platelets, but also diminished, up to 50%, the platelet-mediated thrombus formation when whole blood was perfused over type-III collagen. In summary, this study demonstrated that Syk is a key molecule in both SIPA and thrombus formation under flow. Pharmacological regulation of Syk may prove efficacious in treating occlusive vascular disease.

Future innovations in anti-platelet therapies

Platelets have long been recognized to be of central importance in haemostasis, but their participation in pathological conditions such as thrombosis, atherosclerosis and inflammation is now also well established. The platelet has therefore become a key target in therapies to combat cardiovascular disease. Anti-platelet therapies are used widely, but current approaches lack efficacy in a proportion of patients, and are associated with side effects including problem bleeding. In the last decade, substantial progress has been made in understanding the regulation of platelet function, including the characterization of new ligands, platelet-specific receptors and cell signalling pathways. It is anticipated this progress will impact positively on the future innovations towards more effective and safer anti-platelet agents. In this review, the mechanisms of platelet regulation and current anti-platelet therapies are introduced, and strong, and some more speculative, potential candidate target molecules for future anti-platelet drug development are discussed.

Protease-activated receptor-induced Akt activation--regulation and possible function

BACKGROUND

Thrombin induces the activation of the platelet serine/threonine kinase Akt. Akt activation is dependent on its phosphorylation at Thr308 and Ser473. The mechanism by which thrombin induces Akt phosphorylation is controversial, as is the role of Akt in platelet function.

OBJECTIVES

To investigate how protease-activated receptors (PARs) stimulate Akt and the role that Akt plays in human platelet function.

METHODS

Platelets were stimulated through PAR1 or PAR4. Specific inhibitors were used to evaluate, by Western blotting, signaling pathways regulating Akt phosphorylation, and the role of activated Akt was evaluated by aggregometry and flow cytometry.

RESULTS

Phospholipase C (PLC) controls Akt phosphorylation elicited by PARs. Stimulation of PAR1 or PAR4 resulted in rapid Akt phosphorylation, independently of secreted ADP and phosphatidylinositol-3-kinase (PI3K) activation. Akt phosphorylation approximately 60 s after PAR1 stimulation became entirely dependent on the purinergic receptor P2Y(12) and the activation of PI3K. In contrast, PAR4 partially sustained Akt phosphorylation independently of P2Y(12) and PI3K for up to 300 s. Pharmacologic inhibition of Akt reduced P-selectin expression and fibrinogen binding in platelets stimulated through PAR1, and delayed platelet aggregation in response to submaximal PAR1 or PAR4 stimulation, although aggregation at 300 s was unaffected.

CONCLUSIONS

Platelet PAR stimulation causes rapid Akt phosphorylation downstream of PLC, whereas with continuous stimulation, ADP and PI3K are required for maintaining Akt phosphorylation. Activated Akt regulates platelet function by modulating secretion and alpha(IIb)beta(3) activation.

FAK association with multiple signal proteins mediates pressure-induced colon cancer cell adhesion via a Src-dependent PI3K/Akt pathway

Cancer cell adhesion is traditionally viewed as random, occurring if the cell's receptors match the substrate. Cancer cells are subjected to pressure and shear during growth against a constraining stroma, surgical manipulation, and passage through the venous and lymphatic system. Cells shed into a cavity such as the abdomen postoperatively also experience increased pressure from postoperative edema. Increased extracellular pressure stimulates integrin-mediated cancer cell adhesion via FAK and Src. PI 3-kinase (PI3K) inhibitors (LY294002 or wortmannin), Akt inhibitors, or Akt1 siRNA blocked adhesion stimulated by 15 mmHg pressure in SW620 or primary human malignant colonocytes. Pressure activated PI3K, tyrosine-phosphorylated and membrane-translocated the p85 subunit, and phosphorylated Akt. PI3K inhibitor (LY294002) prevented pressure-stimulated Akt Ser473 and FAK Tyr397, but not FAK576 or Src416 phosphorylation. PP2 inhibited PI3K activity and Akt phosphorylation. FAK siRNA did not affect pressure-induced PI3K activation but blocked Akt phosphorylation. Pressure also stimulated FAK or FAKY397F mutant translocation to the membrane. Akt inhibitor IV blocked pressure-induced Akt and FAK translocation. Pressure activated Src- and PI3K-dependently induced p85 interaction with FAK, and FAK with beta1 integrin. These results delineate a novel force-activated inside-out Src/PI3K/FAK/Akt pathway by which cancer cells regulate their own adhesion. These signals may be potential targets for inhibition of metastatic adhesion.

Pathological shear stress directly regulates platelet αIIbβ3signaling

Integrin mechanotransduction is a ubiquitous biological process. Mechanical forces are transduced transmembranously by an integrin's ligand-bound extracellular domain through its β-subunit's cytoplasmic domain connected to the cytoskeleton. This often culminates in the activation of tyrosine kinases directing cell responses. The delicate balance between hemostasis and thrombosis requires exquisitely fine-tuned integrin function, and balance is maintained in vivo despite that the major platelet integrin αIIbβ3is continuously subjected to frictional or shearing forces generated by laminar blood flow. To test the hypothesis that platelet function is regulated by the direct effects of mechanical forces on αIIbβ3, we examined αIIbβ3/cytoskeletal interactions in human platelets exposed to shear stress in a cone-plate viscometer. We observed that α-actinin, myosin heavy chain, and Syk coimmunoprecipitate with αIIbβ3in resting platelets and that 120 dyn/cm2shear stress leads to their disassociation from αIIbβ3. Shear-induced disassociation of α-actinin and myosin heavy chain from the β3tail is unaffected by blocking von Willebrand factor (VWF) binding to glycoprotein (Gp) Ib-IX-V but abolished by blocking VWF binding to αIIbβ3. Syk's disassociation from β3is inhibited when VWF binding to either GpIb-IX-V or αIIbβ3is blocked. Shear stress-induced phosphorylation of SLP-76 and its association with tyrosine-phosphorylated adhesion and degranulation-promoting adapter protein are inhibited by blocking ligand binding to αIIbβ3but not by blocking ligand binding to GpIb-IX-V. Chinese hamster ovary cells expressing αIIbβ3with β3truncated of its cytoskeletal binding domains demonstrate diminished shear-dependent adhesion and cohesion. These results support the hypothesis that shear stress directly modulates αIIbβ3function and suggest that shear-induced αIIbβ3-mediated signaling contributes to the regulation of platelet aggregation by directing the release of constraining cytoskeletal elements from the β3-tail.

Nucleotide receptor signaling in platelets

Upon injury to a vessel wall the exposure of subendothelial collagen results in the activation of platelets. Platelet activation culminates in shape change, aggregation, release of granule contents and generation of lipid mediators. These secreted and generated mediators trigger a positive feedback mechanism potentiating the platelet activation induced by physiological agonists such as collagen and thrombin. Adenine nucleotides, adenosine diphosphate (ADP) and adenosine triphosphate (ATP), released from damaged cells and that are secreted from platelet-dense granules, contribute to the positive feedback mechanism by acting through nucleotide receptors on the platelet surface. ADP acts through two G protein-coupled receptors, the Gq-coupled P2Y1 receptor, and the Gi-coupled P2Y12 receptor. ATP, on the other hand, acts through the ligand-gated channel P2X1. Stimulation of platelets by ADP leads to shape change, aggregation and thromboxane A2 generation. ADP-induced dense granule release depends on generated thromboxane A2. Furthermore, costimulation of both P2Y1 and P2Y12 receptors is required for ADP-induced platelet aggregation. ATP stimulation of P2X1 is involved in platelet shape change and helps to amplify platelet responses mediated by agonists such as collagen. Activation of each of these nucleotide receptors results in unique signal transduction pathways that are important in the regulation of thrombosis and hemostasis.

Targeting shear stress-induced platelet activation: is lesion-specific antiplatelet therapy a realistic clinical goal?

Platelets are mediators of physiologic hemostasis and pathologic thrombosis. They operate within distinctive vascular and rheologic microenvironments, and their participation in hemostasis or thrombosis is directed by distinct variables operating within the microenvironment. Thrombosis is not simply too much hemostasis: there is good evidence that triggering mechanisms of platelet aggregation under low shear stress conditions are different from those operating under high shear stress conditions. Such differences are hypothesized to exist in vivo and to separate mechanisms of microvascular hemostasis from mechanisms of arterial thrombosis, such as those involved in myocardial, cerebral and peripheral vascular ischemia and infarction. This separation forms the conceptual basis for the hypothesis that lesion-specific antithrombotic agents might some day be invented that inhibit arterial thrombosis without causing bleeding that arises from impaired hemostasis. The focus of much of the work in this field has been platelet aggregation initiated by shear-dependent von Willebrand factor binding to the platelet glycoprotein Ib-IX-V complex. It is hypothesized that by elucidating molecular mechanisms of platelet activation operating under pathologically elevated shear stresses, targets of lesion-specific therapies will one day be identified for use in clinical syndromes of arterial thrombosis.

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.

The Retention Index Test Homburg (RTH-II): Response to clopidogrel and comparison with aggregometry and CD62P-expression

The Retention Index Test Homburg (RTH-II) is quoted to detect effects of shear stress on platelets, which involve ADP receptor signaling. RTH-II might be a tool for monitoring antiplatelet therapy for compounds that interfere with ADP induced platelet activation and secretion. In a series of investigations, we used an ADP (2 microM) triggered RTH-II in parallel with light-transmittance aggregometry and flow cytometry in subjects before and after clopidogrel. A loading dose of 225 mg clopidogrel leads to a significant reduction (p < 0.01) in the ADP-stimulated retention index (RI) from 69 +/- 15 to 48 +/- 21%, in the aggregation response to 5 microM ADP (from 50 +/- 20 to 29 +/- 21%) and the expression of CD62P (from 64 +/- 11 to 41 +/- 17%). Correlation analysis showed that the RI corresponds significantly to CD62P-expression (p < 0.01) but not to aggregation. We also found a strong correlation (p < 0.01) between the ADP-stimulated RI and the expression of CD62P after stimulation with 2 microM ADP, whereas no correlation was seen for RI vs. binding of PAC-1 or aggregation. Platelets not retained in the filter had lower CD62P expression than measured in the sample before the filter passage (54 vs. 35%). A direct interaction of CD62P with platelet ligands might lead to enhanced retention in RTH and explain the correlation of RI with CD62P expression. The RTH-II might be a simple and easy to handle platelet function assay for monitoring effects on P2Y(12)-inhibitors on platelet degranulation, perhaps in addition to aggregometry.

Evidence that the purinergic receptor P2Y12 potentiates platelet shape change by a Rho kinase-dependent mechanism

ADP activates human platelets through two G-protein coupled receptors, P2Y1 and P2Y12, to induce a range of functional responses. Here we have addressed the role and mechanism of P2Y12 in modulating ADP-induced platelet shape change. Although the response depended upon activation of P2Y1, it was potentiated by P2Y12 as the P2Y12-selective antagonists AR-C69931MX and 2MeSAMP partially inhibited shape change in the later phase of the response. This was paralleled by inhibition of pseudopod formation, platelet spheration, actin polymerisation and myosin light chain phosphorylation. P2Y12 is known to couple to activation of PI3 kinase and inhibition of adenylate cyclase, but we showed that neither of these signalling events couples to regulation of shape change by this receptor. However, by assessment of phosphorylation of its major substrate myosin light chain phosphatase, we provide direct evidence for activation of Rho kinase by ADP, and that although P2Y1 is required for activation of Rho kinase, P2Y12 is able to potentiate its activity. We conclude that P2Y12 plays a potentiatory role in ADP-induced shape change through regulation of the Rho kinase pathway, potentiating both myosin phosphorylation and actin polymerisation, and this forms part of an important signalling pathway additional to its well-established Gi-coupled pathways.

Identification of endogenous surrogate ligands for human P2Y12 receptors by in silico and in vitro methods

Endogenous ligands acting on a human P2Y12 receptor, one of the G-protein coupled receptors, were searched by in silico screening against our own database, which contains more than 500 animal metabolites. The in silico screening using the docking software AutoDock resulted in selection of cysteinylleukotrienes (CysLTs) and 5-phosphoribosyl 1-pyrophosphate (PRPP), with high free energy changes, in addition to the known P2Y12 ligands such as 2MeSADP and ADP. These candidates were subjected to an in vitro Ca2+ assay using the CHO cells stably expressing P2Y12-G16alpha fusion proteins. We found that CysLTE4 and PRPP acted on the P2Y12 receptor as agonists with the EC50 values of 1.3 and 7.8 nM, respectively. Furthermore, we analyzed the phylogenetic relationship of the P2Y, P2Y-like, and CysLT receptors based on sequence alignment followed by evolutionary analyses. The analyses showed that the P2Y12, P2Y13, P2Y14, GPR87, CysLT-1, and CysLT-2 receptors formed a P2Y-related receptor subfamily with common sequence motifs in the transmembrane regions.

Pharmacological profiles of cloned mammalian P2Y-receptor subtypes

Membrane-bound P2-receptors mediate the actions of extracellular nucleotides in cell-to-cell signalling. P2X-receptors are ligand-gated ion channels, whereas P2Y-receptors belong to the superfamily of G-protein-coupled receptors (GPCRs). So far, the P2Y family is composed out of 8 human subtypes that have been cloned and functionally defined; species orthologues have been found in many vertebrates. P2Y1-, P2Y2-, P2Y4-, P2Y6-, and P2Y11-receptors all couple to stimulation of phospholipase C. The P2Y11-receptor mediates in addition a stimulation of adenylate cyclase. In contrast, activation of the P2Y12-, P2Y13-, and P2Y14-receptors causes an inhibition of adenylate cyclase activity. The expression of P2Y1-receptors is widespread. The receptor is involved in blood platelet aggregation, vasodilatation and neuromodulation. It is activated by ADP and ADP analogues including 2-methylthio-ADP (2-MeSADP). 2'-Deoxy-N6-methyladenosine-3',5'-bisphosphate (MRS2179) and 2-chloro-N6-methyl-(N)-methanocarba-2'-deoxyadenosine 3',5'-bisphosphate (MRS2279) are potent and selective antagonists. P2Y2 transcripts are abundantly distributed. One important example for its functional role is the control of chloride ion fluxes in airway epithelia. The P2Y2-receptor is activated by UTP and ATP and blocked by suramin. The P2Y2-agonist diquafosol is used for the treatment of the dry eye disease. P2Y4-receptors are expressed in the placenta and in epithelia. The human P2Y4-receptor has a strong preference for UTP as agonist, whereas the rat P2Y4-receptor is activated about equally by UTP and ATP. The P2Y4-receptor is not blocked by suramin. The P2Y6-receptor has a widespread distribution including heart, blood vessels, and brain. The receptor prefers UDP as agonist and is selectively blocked by 1,2-di-(4-isothiocyanatophenyl)ethane (MRS2567). The P2Y11-receptor may play a role in the differentiation of immunocytes. The human P2Y11-receptor is activated by ATP as naturally occurring agonist and it is blocked by suramin and reactive blue 2 (RB2). The P2Y12-receptor plays a crucial role in platelet aggregation as well as in inhibition of neuronal cells. It is activated by ADP and very potently by 2-methylthio-ADP. Nucleotide antagonists including N6-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene-ATP (=cangrelor; AR-C69931MX), the nucleoside analogue AZD6140, as well as active metabolites of the thienopyridine compounds clopidogrel and prasugrel block the receptor. These P2Y12-antagonists are used in pharmacotherapy to inhibit platelet aggregation. The P2Y13-receptor is expressed in immunocytes and neuronal cells and is again activated by ADP and 2-methylthio-ADP. The 2-chloro-5-nitro pyridoxal-phosphate analogue 6-(2'-chloro-5'-nitro-azophenyl)-pyridoxal-alpha5-phosphate (MRS2211) is a selective antagonist. mRNA encoding for the human P2Y14-receptor is found in many tissues. However, a physiological role of the receptor has not yet been established. UDP-glucose and related analogues act as agonists; antagonists are not known. Finally, UDP has been reported to act on receptors for cysteinyl leukotrienes as an additional agonist--indicating a dual agonist specificity of these receptors.

Botrocetin/VWF-induced signaling through GPIb-IX-V produces TxA2 in an alphaIIbbeta3- and aggregation-independent manner

Binding of von Willebrand factor (VWF) to the platelet membrane glycoprotein (GP) Ib-IX-V complex initiates a signaling cascade that causes alphaIIbbeta3 activation and platelet aggregation. Previous work demonstrated that botrocetin (bt)/VWF-mediated agglutination activates alphaIIbbeta3 and elicits adenosine triphosphate (ATP) secretion in a thromboxane A2 (TxA2)- and Ca2+-dependent manner. This agglutination-elicited TxA2 production occurs in the absence of ATP secretion. However, the signaling components and signaling network or pathway activated by GPIb-mediated agglutination to cause TxA2 production have not been identified. Therefore, the focus of this study was to elucidate at least part of the signal transduction network or pathway activated by GPIb-mediated agglutination to cause TxA2 production. The phosphatidylinositol 3-kinase (PI3K) selective inhibitor wortmannin, and mouse platelets deficient in Lyn, Src, Syk, Src homology 2 (SH2) domain-containing leukocyte protein 76 (SLP-76), phospholipase Cgamma2 (PLCgamma2), linker for activation of T cells (LAT), or Fc receptor gamma-chain (FcRgamma-chain) were used for these studies. LAT and FcRgamma-chain were found not to be required for agglutination-driven TxA2 production or activation of alphaIIbbeta3, but were required for granule secretion and aggregation. The results also clearly demonstrate that bt/VWF-mediated agglutination-induced TxA2 production is dependent on signaling apparently initiated by Lyn, enhanced by Src, and propagated through Syk, SLP-76, PI3K, PLCgamma2, and protein kinase C (PKC).

PI 3-kinase p110beta: a new target for antithrombotic therapy

Platelet activation at sites of vascular injury is essential for the arrest of bleeding; however, excessive platelet accumulation at regions of atherosclerotic plaque rupture can result in the development of arterial thrombi, precipitating diseases such as acute myocardial infarction and ischemic stroke. Rheological disturbances (high shear stress) have an important role in promoting arterial thrombosis by enhancing the adhesive and signaling function of platelet integrin alpha(IIb)beta(3) (GPIIb-IIIa). In this study we have defined a key role for the Type Ia phosphoinositide 3-kinase (PI3K) p110beta isoform in regulating the formation and stability of integrin alpha(IIb)beta(3) adhesion bonds, necessary for shear activation of platelets. Isoform-selective PI3K p110beta inhibitors have been developed which prevent formation of stable integrin alpha(IIb)beta(3) adhesion contacts, leading to defective platelet thrombus formation. In vivo, these inhibitors eliminate occlusive thrombus formation but do not prolong bleeding time. These studies define PI3K p110beta as an important new target for antithrombotic therapy.

P2Y12 receptors play a significant role in the development of platelet microaggregation in patients with diabetes

Ninety-eight diabetic patients (type 2) were studied together with 24 healthy normotensive controls. Microaggregates (particle scale, <25 microm) of platelets were detected by a laser scattering system. Microaggregates in the control group showed a time-dependent reversible change; however, they existed continuously in 82 of 98 diabetic patients. When platelets of diabetics were stimulated by a shear stress alone without ADP, 74 also showed spontaneous and irreversible microaggregates even though they were not observed in all control subjects. In control subjects, microaggregates were inhibited by MRS2279 (a P2Y1 antagonist), but not AR-C69931MX (a P2Y12 antagonist). However, AR-C69931MX prevented irreversible microaggregates in diabetic patients. When either aspirin or ticlopidine was administered to diabetic patients with irreversible microaggregates, both drugs significantly decreased microaggregates induced by a low dose of ADP. Ticlopidine additionally reduced the microaggregates induced by shear stress alone. In conclusion, microaggregates of platelets via P2Y12 receptors could play a key role in the hypersensitivity of platelets in diabetic patients, and the measurement of microaggregation could be a useful marker to estimate of thrombogenesis. These findings present a possible new means for patients with diabetes to prevent ischemic events.

Signalling through the platelet glycoprotein Ib-V–IX complex

The glycoprotein Ib-V-IX is one of the major adhesive receptors expressed on the surface of circulating platelets. It is composed of four different polypeptides-GPIbalpha, GPIbbeta, GPIX, and GPV-and represents a multifunctional receptor able to interact with a number of ligands, including the adhesive protein von Willebrand factor, the coagulation factors thrombin, factors XI and XII, and the membrane glycoproteins P-selectin and Mac-1. Interaction of GPIb-V-IX with the subendothelial von Willebrand factor is essential for primary haemostasis, as it initiates platelet adhesion to the subendothelial matrix at the sites of vascular injury even under high flow conditions. Upon interaction with von Willebrand factor, GPIb-V-IX initiates transmembrane signalling events for platelet activation, which eventually result in integrin alpha(IIb)beta(3) stimulation and platelet aggregation. The investigation of the biochemical mechanisms for platelet activation by GPIb-V-IX has attracted increasing attention during the last years. This review will describe and discuss recent findings that have provided new insights into the events underlying GPIb-V-IX transmembrane signalling. In particular, it will summarise basic concepts on the structure of this receptor, extracellular ligands, and intracellular interactors potentially involved in transmembrane signalling. The recently suggested role of membrane Fc receptors in GPIb-V-IX-initiated platelet activation will also be discussed, along with the involvement of lipid metabolising enzymes, tyrosine kinases, and the cytoskeleton in the crosstalk between GPIb-V-IX and integrin alpha(IIb)beta(3).

Differential requirements for calcium and Src family kinases in platelet GPIIb/IIIa activation and thromboxane generation downstream of different G-protein pathways

G(12/13) or G(q) signaling pathways activate platelet GPIIb/IIIa when combined with G(i) signaling. We tested whether combined G(i) and G(z) pathways also cause GPIIb/IIIa activation and compared the signaling requirements of these events. Platelet aggregation occurred by combined stimulation of G(i) and G(z) pathways in human platelets and in P2Y1-deficient and G alpha(q)-deficient mouse platelets, confirming that the combination of G(i) and G(z) signaling causes platelet aggregation. When G(i) stimulation was combined with G(z) stimulation, there was a small mobilization of intracellular calcium. Chelation of intracellular calcium decreased the extent of this platelet aggregation, whereas it abolished the G(q) plus G(i)-mediated platelet aggregation. Costimulation of G(i) plus G(z) pathways also caused thromboxane generation that was dependent on outside-in signaling and was inhibited by PP2, a Src family tyrosine kinase inhibitor. Src family tyrosine kinase inhibitors also inhibited platelet aggregation and decreased the PAC-1 binding caused by costimulation of G(i) and G(z) signaling pathways in aspirin-treated platelets. However, Src family kinase inhibitors did not affect G(q) plus G(i)-mediated platelet aggregation. We conclude that the combination of G(i) plus G(z) pathways have different requirements than G(q) plus G(i) pathways for calcium and Src family kinases in GPIIb/IIIa activation and thromboxane production.

Review article: management of oesophageal adenocarcinoma -- control of acid, bile and inflammation in intervention strategies for Barrett's oesophagus

Oesophagitis is associated with Barrett's metaplasia in about 10% of individuals. The UK has one of the highest world-wide prevalences of Barrett's metaplasia, with 1% of adults having the condition, resulting in an incidence of oesophageal adenocarcinoma two to three times that seen in either Europe or North America. In addition, the conversion rate to cancer in individuals with Barrett's metaplasia in UK surveillance programmes is twice that observed in the USA (0.96% per year vs. 0.4% per year), lending further support to the notion that the UK is a high-risk region. The evidence base on what can be achieved with medical therapy to reduce the risk of dysplasia or the development of adenocarcinoma needs to be strengthened with data from randomized controlled trials, as existing data have many limitations. Patients with Barrett's metaplasia respond variably to proton pump inhibitor therapy (even high-dose therapy 'normalizes' acid reflux in only 85% of cases), and symptom control is a poor determinant of the adequacy of suppression of acid reflux. Gastro-oesophageal reflux is implicated in the pathogenesis of Barrett's metaplasia, and ex vivo and in vitro evidence suggests that its attenuation reverses proliferation and biological variables over days, and perhaps the metaplastic histology to a degree over years. The effect of proton pump inhibitor therapy on cancer risk in the long term is essentially unknown. Acid suppressant therapy or anti-reflux surgery on its own does not result in the complete regression of the metaplastic epithelium. Bile acids, present especially frequently in the refluxate of Barrett's oesophagus patients, are also likely to influence the development and persistence of metaplasia. Barrett's metaplasia is replaced by a squamous epithelium when acid reflux is well controlled and the epithelium is physically destroyed by ablation with argon plasma coagulation or photodynamic therapy. These modalities are invasive and are not likely to be useful in the routine management of patients with Barrett's oesophagus without dysplasia or cancer. Why metaplasia does not fully regress once external initiating stimuli are removed is a mystery. There is some evidence to implicate a variety of molecules, including cyclo-oxygenase-2, tumour necrosis factor-alpha, beta-catenin nuclear translocation and mitogen-activated protein kinase signalling, because they are expressed preferentially in metaplastic rather than normal or inflamed squamous oesophageal mucosa. The use of non-steroidal anti-inflammatory drugs, including aspirin, is associated with a decreased incidence of oesophageal adenocarcinoma. There is therefore a great need for randomized controlled trials to assess the outcomes of such chemopreventive therapy in patients with Barrett's metaplasia.

Reciprocal cross-talk between P2Y1 and P2Y12 receptors at the level of calcium signaling in human platelets

Adenosine diphosphate (ADP), an important platelet agonist, acts through 2 G-protein-coupled receptors (GPCRs), P2Y(1) and P2Y(12), which signal through Gq and Gi, respectively. There is increasing evidence for cross-talk between signaling pathways downstream of GPCRs and here we demonstrate cross-talk between these 2 ADP receptors in human platelets. We show that P2Y(12) contributes to platelet signaling by potentiating the P2Y(1)-induced calcium response. This potentiation is mediated by 2 mechanisms: inhibition of adenylate cyclase and activation of phosphatidylinositol 3 (PI 3)-kinase. Furthermore, the Src family kinase inhibitor PP1 selectively potentiates the contribution to the calcium response by P2Y(12), although inhibition of adenylate cyclase by P2Y(12) is unaffected. Using PP1 in combination with the inhibitor of PI 3-kinase LY294002, we show that Src negatively regulates the PI 3-kinase-mediated component of the P2Y(12) calcium response. Finally, we were able to show that Src kinase is activated through P2Y(1) but not P2Y(12). Taken together, we present evidence for a complex signaling interplay between P2Y(1) and P2Y(12), where P2Y(12) is able to positively regulate P2Y(1) action and P2Y(1) negatively regulates this action of P2Y(12). It is likely that this interplay between receptors plays an important role in maintaining the delicate balance between platelet activation and inhibition during normal hemostasis.

Luminal flow induces eNOS activation and translocation in the rat thick ascending limb. II. Role of PI3-kinase and Hsp90

Endothelial nitric oxide synthase (eNOS) regulates NaCl absorption by the thick ascending limb of the loop of Henle (THAL). We found that augmenting luminal flow induces eNOS activation and translocation to the apical membrane of THALs (Ortiz PA, Hong NJ, and Garvin JL. Am J Physiol Renal Physiol 287: F274-F280, 2004). In other cells, eNOS activation by shear stress is mediated by phosphatidylinositol 3-OH kinase (PI3)-kinase. We hypothesized that luminal flow induces eNOS activation via PI3-kinase. Pretreatment of THALs with wortmannin, a PI3-kinase inhibitor, significantly reduced flow-induced nitric oxide (NO) release by 75% (from 53.6 +/- 6 to 13.2 +/- 5.7 pA/mm). Increasing luminal flow from 0 to 20 nl/min induced eNOS translocation to the apical membrane, whereas in the presence of wortmannin eNOS translocation was prevented (basolateral = 32 +/- 2%, middle = 38 +/- 1%, apical = 30 +/- 1%, n = 5, not significant vs. no flow). We next studied which PI3-kinase product mediates eNOS translocation. Addition of PI(3,4,5)P(3) (5 microM) in the absence of flow increased NO levels (P < 0.05) and induced eNOS translocation to the apical membrane (from 40 +/- 4 to 60 +/- 2% of total eNOS, n = 6, P < 0.05). Incubation with PI(3,4)P(2) or PI(4,5)P(2) did not change eNOS localization. We next tested whether heat shock protein (Hsp)90 is involved in eNOS translocation. The Hsp90 inhibitor geldanamycin blocked flow-induced eNOS translocation to the apical membrane (n = 6). Flow also induced translocation of Hsp90 to the apical membrane (from 35 +/- 2 to 57 +/- 2%; P < 0.05) in a PI3-kinase-dependent manner. We conclude that luminal flow induces eNOS translocation and activation in the THAL via PI3-kinase and that Hsp90 is involved in eNOS translocation to the apical membrane.

ATP augments von Willebrand factor-dependent shear-induced platelet aggregation through Ca2+-calmodulin and myosin light chain kinase activation

Shear stress triggers von Willebrand factor (VWF) binding to platelet glycoprotein Ibalpha and subsequent integrin alpha(IIb)beta(3)-dependent platelet aggregation. Concomitantly, nucleotides are released from plateletdense granules, and ADP is known to contribute to shear-induced platelet aggregation (SIPA). We found that the impaired SIPA of platelets from a Hermansky-Pudlak patient lacking dense granules was restored by exogenous l-beta,gamma-methylene ATP, a stable P2X(1) agonist, as well as by ADP, confirming that in addition to ADP (via P2Y(1) and P2Y(12)), ATP (via P2X(1)) also contributes to SIPA. Likewise, SIPA of apyrase-treated platelets was restored upon P2X(1) activation with l-beta,gamma-methylene ATP, which promoted granule centralization within platelets and stimulated P-selectin expression, which is a marker of alpha-granule release. In addition, during SIPA, platelet degranulation required both extracellular Ca(2+) and VWF-glycoprotein Ibalpha interactions without involving alpha(IIb)beta(3). Neither platelet release nor SIPA was affected by protein kinase C inactivation, even though protein kinase C blockade inhibits platelet responses to collagen and thrombin in stirring conditions. In contrast, inhibiting myosin light chain (MLC) kinase with ML-7 reduced platelet release and SIPA by 30%. Accordingly, the potentiating effect of P2X(1) stimulation on the aggregation of apyrase-treated platelets coincided with intensified phosphorylation of MLC and was abrogated by ML-7. SIPA-induced MLC phosphorylation occurred exclusively through released nucleotides and selective antagonism of P2X(1) with MRS2159-reduced SIPA, ATP release, and potently inhibited MLC phosphorylation. We conclude that the P2X(1) ion channel induces MLC-mediated cytoskeletal rearrangements, thus contributing to SIPA and degranulation during VWF-triggered platelet activation.

Extracellular ATP facilitates flow-induced vasodilatation in rat small mesenteric arteries

ATP can be released from endothelial cells, and this release is increased by intraluminal flow in blood vessels. In the present study, the effect of extracellular ATP (1 microM) on flow-induced vasodilatation was investigated in isolated and pressurized rat small mesenteric arteries. In the absence of extracellular ATP, only 46% of arteries developed dilatation in response to flow, and this response was both transient and unstable. In marked contrast, with ATP present, all vessels developed a prolonged and stable dilatation in response to flow. Even in the vessels that failed to respond to flow in the absence of ATP, dilatation could be stimulated once ATP was present. The ability of ATP to facilitate flow-induced vasodilatation was mimicked by UTP (1 microM), a P2Y agonist, or 3'-O-(4-benzoyl)benzoyl ATP (BzATP; 10 microM), an agonist for P2X1, P2X7, and P2Y11 purinoceptors. The involvement of P2X7 purinoceptors was further supported by the inhibitory effect of KN-62 (1 microM), a P2X7 antagonist, on the action of BzATP. P2X1 and P2X3 purinoceptors were not involved because their receptor agonist alpha,beta-methylene ATP had no effect. The facilitating effect of ATP on flow dilatation was also attenuated by the combined application of reactive blue 2 (100 microM), a P2Y antagonist, and suramin (100 microM), a nonselective P2X and P2Y antagonist. Furthermore, flow-induced dilatation obtained in the presence of ATP was reproducible. In contrast, in the additional presence of the ectonucleotidase inhibitor ARL-67156 (10 microM), although the first dilatation was normal, the responses to the second and later exposures to flow were greatly attenuated. The nonhydrolyzable ATP analogs adenosine-5'-(3-thiotriphosphate)trilithium salt (1 microM) and adenosine 5'-(beta,gamma-imido) triphosphate tetralithium salt hydrate (10 microM) had similar effects to those of ARL-67156. These data suggest that ATP acts through both P2X and P2Y purinoceptors to facilitate flow-induced vasodilatation and that ectonucleotidases prevent this effect by degrading ATP on the endothelial cell surface.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

von Willebrand factor binding to platelet glycoprotein Ib-IX-V stimulates the assembly of an alpha-actinin-based signaling complex

BACKGROUND

Pathological shear stress induces platelet aggregation that is dependent on von Willebrand factor (VWF) binding to glycoprotein (Gp)Ib-IX-V and phosphatidylinositol 3-kinase activation. We tested the hypothesis that pathological shear stress stimulates phosphatidylinositol 3,4,5-trisphosphate (PIP3) synthesis by directing the assembly of a molecular signaling complex that includes class IA phosphatidylinositol 3-kinase (PI 3-KIA).

METHODS

Platelets were subjected to 120 dynes cm-2 shear stress in a cone-plate viscometer. Resting and sheared platelets were lyzed, immunoprecipitations of PI 3-KIA performed, or lipids extracted for PIP3 measurements. alpha-Actinin was incubated with phosphatidylinositol 4,5-bisphosphate (PIP2), immunoprecipitated, and used as a substrate for in vitro PI 3-KIA activity.

RESULTS

Pathological shear stress induces biphasic PIP3 production. In resting platelets, PI 3-KIA associates with alpha-actinin and PIP2. After exposure to shear stress, alpha-actinin and PIP2 rapidly disassociate from PI 3-KIA. PI 3-KIA then gradually re-associates with PIP2 and alpha-actinin, and this complex becomes linked to GpIb alpha through the cytoskeleton. PIP3 production and the observed changes in the association between alpha-actinin, PIP2, and PI 3-KIA are inhibited when VWF binding to GpIb alpha is blocked. In a cell-free system, alpha-actinin binds PIP2 and when the alpha-actinin-PIP2 complex is added to platelet PI 3-KIA, PIP3 production is stimulated.

CONCLUSIONS

These results suggest that pathological shear-induced VWF binding to GpIb-IX-V stimulates PIP3 production through the assembly of an alpha-actinin-based complex that colocalizes PI 3-KIA with substrate PIP2.

Purification and functional reconstitution of the human P2Y12 receptor

The human P2Y12 receptor (P2Y12-R) is a member of the G protein coupled P2Y receptor family, which is intimately involved in platelet physiology. We describe here the purification and functional characterization of recombinant P2Y12-R after high-level expression from a baculovirus in Sf9 insect cells. Purified P2Y12-R, Gbeta1gamma2, and various Galpha-subunits were reconstituted in lipid vesicles, and steady-state GTPase activity was quantified. GTP hydrolysis in proteoliposomes formed with purified P2Y12-R and Galphai2beta1gamma2 was stimulated by addition of either 2-methylthio-ADP (2MeSADP) or RGS4 and was markedly enhanced by their combined presence. 2MeSADP was the most potent agonist (EC50 = 80 nM) examined, whereas ADP, the cognate agonist of the P2Y12-R, was 3 orders of magnitude less potent. ATP had no effect alone but inhibited the action of 2MeSADP; therefore, ATP is a relatively low-affinity antagonist of the P2Y12-R. The G protein selectivity of the P2Y12-R was examined by reconstitution with various G protein alpha-subunits in heterotrimeric form with Gbeta1gamma2. The most robust coupling of the P2Y12-R was to Galphai2, but effective coupling also occurred to Galphai1 and Galphai3. In contrast, little or no coupling occurred to Galphao or Galphaq. These results illustrate that the signaling properties of the P2Y12-R can be studied as a purified protein under conditions that circumvent the complications that occur in vivo because of nucleotide metabolism and interconversion as well as nucleotide release.

Filamin A binding to the cytoplasmic tail of glycoprotein Ibalpha regulates von Willebrand factor-induced platelet activation

We examined the hypothesis that filamin A binding to the cytoplasmic tail of platelet glycoprotein Ibalpha (GpIbalpha) is regulated by pathologic shear stress and modulates von Willebrand factor (VWF)-induced platelet activation. To begin, we examined filamin binding to GpIbalpha in Chinese hamster ovary cells coexpressing mutant human GpIb-IX and wild-type human filamin A. We observed that many different deletions and truncations N-terminal to GpIbalpha's cytoplasmic domain residue 594 disrupted filamin A binding, but that binding was unaffected by 14 different point mutations in hydrophilic residues between amino acids 557 and 593. To try to narrow GpIbalpha's filamin A-binding domain, we next measured the effect of several cytoplasmic domain peptides on human filamin A binding to a GST-GpIbalpha cytoplasmic domain fusion protein. One peptide (residues 557-575; designated "A4 peptide") inhibited filamin A binding to the GST-GpIbalpha cytoplasmic domain fusion protein and competed with GpIbalpha for binding to filamin A. When the A4 peptide was delivered to intact human platelets using a carrier peptide, we observed the dose-dependent inhibition of VWF-induced platelet aggregation in response to both ristocetin and shear stress. The effect of the A4 peptide on shear-induced platelet aggregation was accompanied by the attenuation of shear-induced filamin A binding to GpIbalpha and diminished shear-dependent protein tyrosine phosphorylation. These results suggest that shear-dependent VWF-induced platelet activation affects filamin A binding to GpIb-IX-V, and that filamin A binding to the cytoplasmic tail of GpIbalpha regulates proaggregatory tyrosine kinase signaling.