GPVI inhibition by glenzocimab synergistically inhibits atherosclerotic plaque-induced platelet activation when combined with conventional dual antiplatelet therapy

Introduction

Aspirin and a potent platelet P2Y12 inhibitor, such as prasugrel or ticagrelor, are not always sufficient to prevent thrombus formation in patients with ST-elevation MI (STEMI), leading to “slow flow” or “no reflow” effects after stenting. GPIIb/IIIa inhibitors, such as eptifibatide, may help in this setting, but are not used routinely due to their bleeding risk. GPVI has critical roles in thrombosis and a minimal role in haemostasis. Here we tested whether depletion of GPVI has effects on thrombus formation after MI in an animal model and investigated the effects of a novel platelet GPVI inhibitor, glenzocimab (a Fab fragment of a monoclonal antibody), on platelet activation and thrombus formation when combined with aspirin and ticagrelor.

Methods

We used intravital microscopy in a murine model of ST-elevation myocardial infarction and ischaemia-reperfusion injury to investigate microvascular thrombosis. We investigated the antithrombotic effects of adding glenzocimab (previously known as ACT017) to blood from healthy donors and 20 patients with ACS treated with aspirin and ticagrelor. We compared the effect of glenzocimab with the GPIIb/IIIa inhibitor eptifibatide ex-vivo. We stimulated platelets with collagen and atherosclerotic plaque material that was sourced from patients undergoing carotid endarterectomy. We investigated effects on platelet aggregation, spreading, signalling, adhesion, thrombin generation, thrombus formation and clot stability ex vivo.

Results

Genetic depletion of GPVI in an animal model of myocardial infarction reduced microvascular thrombosis. Ex vivo, aspirin and ticagrelor partially inhibited atherosclerotic plaque-induced platelet aggregation (assessed by multiple electrode aggregometry) by 48% compared to control (34±3 vs. 65±4 U; P<0.001; Figure 1). Atherosclerotic plaque-induced platelet aggregation, adhesion, secretion and activation were critically dependent on platelet GPVI activation and were potently inhibited by glenzocimab. Glenzocimab alone reduced atherosclerotic plaque-induced platelet aggregation by 75% compared to control (16±4 vs. 65±4 U; P<0.001; Figure 1) and by over 95% when combined with aspirin and ticagrelor (3±1 vs 65±4 U; P<0.001; Figure 1). Furthermore, glenzocimab provided multiple synergistic antithrombotic effects when added to the blood of aspirin and ticagrelor-treated patients with ACS ex vivo. Glenzocimab and the GPIIb/IIIa inhibitor, eptifibatide, had many similar antithrombotic effects but glenzocimab had less effect on mechanisms of general haemostasis compared to eptifibatide, as assessed by ROTEM (Figure 2).

Conclusions

The addition of glenzocimab to aspirin and ticagrelor provides synergistic inhibition of multiple critical mechanisms of atherothrombosis. Glenzocimab and the GPIIb/IIIa inhibitor, eptifibatide, share many similar antithrombotic effects, although glenzocimab has less impact on mechanisms involved in haemostasis compared to eptifibatide.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Academy of Medical Sciences UK Clinical Lecturer Starter GrantRoyal Embassy of Saudi Arabia

Collagen or Collagen-related Peptide Cause [Ca2+]i Elevation and Increased Tyrosine Phosphorylation in Human Megakaryocytes

Since megakaryocytes are the cellular precursors of platelets we have investigated whether they share responses to platelet agonists, in particular collagen. Although previous studies have reported responses to thrombin in non-human megakaryocytes, through studies of single cell calcium responses and protein tyrosine-phosphorylation we demonstrate for the first time that both isolated human megakaryocytes and CD41/61-positive megakaryocytes derived in culture from CD34+ cells share responses to the platelet agonists collagen, collagen-related peptide and thrombin. The responses to either collagen or CRP were seen only in the most mature megakaryocytes and not in mega-karyocyte-like cell lines, suggesting that the response to collagen is a characteristic developed late during megakaryocyte differentiation. These primary cells offer the opportunity to use many molecular and cellular techniques to study and manipulate signalling events in response to platelet receptor agonists, which cannot be performed in the small, anucleate platelet itself.

Comparison of multiple electrode aggregometry with lumi-aggregometry for the diagnosis of patients with mild bleeding disorders

Essentials There is a clinical need for new technologies to measure platelet function in whole blood. Mild bleeding disorders were evaluated using multiple electrode aggregometry (MEA). MEA is insensitive at detecting patients with mild platelet function and secretion defects. More studies are required to investigate MEA in patients with a defined set of platelet disorders.

SUMMARY

Background Multiple electrode aggregometry (MEA) measures changes in electrical impedance caused by platelet aggregation in whole blood. This approach is faster, more convenient and offers the advantage over light transmission aggregometry (LTA) of assessing platelet function in whole blood and reducing preanalytical errors associated with preparation of platelet-rich plasma (PRP). Several studies indicate the utility of this method in assessing platelet inhibition in individuals taking antiplatelet agents (e.g. aspirin and clopidogrel). Objective Our current study sought to evaluate the ability of MEA in diagnosing patients with mild bleeding disorders by comparison with light transmission lumi-aggregometry (lumi-LTA). Methods Forty healthy subjects and 109 patients with a clinical diagnosis of a mild bleeding disorder were recruited into the UK Genotyping and Phenotyping of Platelets study (GAPP, ISRCTN 77951167). MEA was performed on whole blood using one or two concentrations of ADP, PAR-1 peptide, arachidonic acid and collagen. Lumi-LTA was performed in PRP using several concentrations of ADP, adrenaline, arachidonic acid, collagen, PAR-1 peptide and ristocetin. Results Of 109 patients tested, 54 (49%) patients gave abnormal responses by lumi-LTA to one or more agonists. In contrast, only 16 (15%) patients were shown to have abnormal responses to one or more agonists by MEA. Conclusions In this study we showed that MEA is less sensitive in identifying patients with abnormal platelet function relative to lumi-LTA.

Clustering of glycoprotein VI (GPVI) dimers upon adhesion to collagen as a mechanism to regulate GPVI signaling in platelets

Essentials Dimeric high-affinity collagen receptor glycoprotein VI (GPVI) is present on resting platelets. Spatio-temporal organization of platelet GPVI-dimers was evaluated using advanced microscopy. Upon platelet adhesion to collagenous substrates, GPVI-dimers coalesce to form clusters. Clustering of GPVI-dimers may increase avidity and facilitate platelet activation SUMMARY: Background Platelet glycoprotein VI (GPVI) binding to subendothelial collagen exposed upon blood vessel injury initiates thrombus formation. Dimeric GPVI has high affinity for collagen, and occurs constitutively on resting platelets. Objective To identify higher-order oligomerization (clustering) of pre-existing GPVI dimers upon interaction with collagen as a mechanism to initiate GPVI-mediated signaling. Methods GPVI was located by use of fluorophore-conjugated GPVI dimer-specific Fab (antigen-binding fragment). The tested substrates include Horm collagen I fibers, soluble collagen III, GPVI-specific collagen peptides, and fibrinogen. GPVI dimer clusters on the platelet surface interacting with these substrates were visualized with complementary imaging techniques: total internal reflection fluorescence microscopy to monitor real-time interactions, and direct stochastic optical reconstruction microscopy (dSTORM), providing relative quantification of GPVI cluster size and density. Confocal microscopy was used to locate GPVI dimer clusters, glycoprotein Ib, integrin α2 β1 , and phosphotyrosine. Results Upon platelet adhesion to all collagenous substrates, GPVI dimers coalesced to form clusters; notably clusters formed along the fibers of Horm collagen. dSTORM revealed that GPVI density within clusters depended on the substrate, collagen III being the most effective. Clusters on fibrinogen-adhered platelets were much smaller and more numerous; whether these are pre-existing oligomers of GPVI dimers or fibrinogen-induced is not clear. Some GPVI dimer clusters colocalized with areas of phosphotyrosine, indicative of signaling activity. Integrin α2 β1 was localized to collagen fibers close to GPVI dimer clusters. GPVI clustering depends on a dynamic actin cytoskeleton. Conclusions Platelet adhesion to collagen induces GPVI dimer clustering. GPVI clustering increases both avidity for collagen and the proximity of GPVI-associated signaling molecules, which may be crucial for the initiation and persistence of signaling.

Akt and mitogen-activated protein kinase enhance C-type lectin-like receptor 2-mediated platelet activation by inhibition of glycogen synthase kinase 3α/β

BACKGROUND

The C-type lectin-like receptor 2 (CLEC-2) and the collagen receptor glycoprotein (GP)VI activate platelets through Src and Syk tyrosine kinases, and phospholipase Cγ2. The initial events in the two signaling cascades, however, are distinct, and there are quantitative differences in the roles of proteins downstream of Syk activation. The activation of Akt and mitogen-activated protein kinases (MAPKs) has been shown to enhance platelet activation by GPVI, but their role in CLEC-2 signaling is not known.

OBJECTIVES

We sought to investigate the role of the Akt and MAPK pathways in platelet activation by CLEC-2.

RESULTS

The CLEC-2 agonist rhodocytin stimulated phosphorylation of Akt and p38 and extracellular signal-related kinase (ERK) MAPKs, but with a delay relative to Syk. Phosphorylation of these proteins was markedly inhibited in the combined presence of apyrase and indomethacin, consistent with the reported feedback action of ADP and thromboxane A2 in CLEC-2 signaling. Phosphorylation of Akt and phosphorylation of ERK were blocked by the phosphoinositide 3-kinase (PI3K) inhibitor wortmannin and the protein kinase C (PKC) inhibitor Ro31-8220, respectively, whereas Syk phosphorylation was not altered. On the other hand, both inhibitors reduced phosphorylation of the Akt substrate glycogen synthase kinase 3α/β (GSK3α/β). Phosphorylation of GSK3α/β was also blocked by the Akt inhibitor MK2206, and reduced at late, but not early, times by the MEK inhibitor PD0325901. MK2206 and PD0325901 inhibited aggregation and secretion in response to a low concentration of rhodocytin, which was restored by GSK3α/β inhibitors.

CONCLUSIONS

These results demonstrate that CLEC-2 regulates Akt and MAPK downstream of PI3K and PKC, leading to phosphorylation and inhibition of GSK3α/β, and enhanced platelet aggregation and secretion.

An atypical IgM class platelet cold agglutinin induces GPVI-dependent aggregation of human platelets

Platelet cold agglutinins (PCA) cause pseudothrombocytopenia, spurious thrombocytopenia due to ex vivo platelet clumping, complicating clinical diagnosis, but mechanisms and consequences of PCA are not well defined. Here, we characterised an atypical immunoglobulin (Ig)M PCA in a 37-year-old woman with lifelong bleeding and chronic moderate thrombocytopenia, that induces activation and aggregation of autologous or allogeneic platelets via interaction with platelet glycoprotein (GP)VI. Patient temperature-dependent pseudothrombocytopenia was EDTA-independent, but was prevented by integrin αIIbβ3 blockade. Unstimulated patient platelets revealed elevated levels of bound IgM, increased expression of activation markers (P-selectin and CD63), low GPVI levels and abnormally high thromboxane (TX)A2 production. Patient serum induced temperature- and αIIbβ3-dependent decrease of platelet count in allogeneic donor citrated platelet-rich plasma (PRP), but not in PRP from Glanzmann's thrombasthenia or afibrinogenaemia patients. In allogeneic platelets, patient plasma induced shape change, P-selectin and CD63 expression, (14)C-serotonin release, and TXA2 production. Activation was not inhibited by aspirin, cangrelor or blocking anti-Fc receptor (FcγRIIA) antibody, but was abrogated by inhibitors of Src and Syk, and by a soluble GPVI-Fc fusion protein. GPVI-deficient platelets were not activated by patient plasma. These data provide the first evidence for an IgM PCA causing platelet activation/aggregation via GPVI. The PCA activity persisted over a five-year follow-up period, supporting a causative role in patient chronic thrombocytopenia and bleeding.

Use of next-generation sequencing and candidate gene analysis to identify underlying defects in patients with inherited platelet function disorders

BACKGROUND

Inherited platelet function disorders (PFDs) are heterogeneous, and identification of the underlying genetic defects is difficult when based solely on phenotypic and clinical features of the patient.

OBJECTIVE

To analyze 329 genes regulating platelet function, number, and size in order to identify candidate gene defects in patients with PFDs.

PATIENTS/METHODS

Targeted analysis of candidate PFD genes was undertaken after next-generation sequencing of exomic DNA from 18 unrelated index cases with PFDs who were recruited into the UK Genotyping and Phenotyping of Platelets (GAPP) study and diagnosed with platelet abnormalities affecting either Gi signaling (n = 12) or secretion (n = 6). The potential pathogenicity of candidate gene defects was assessed using computational predictive algorithms.

RESULTS

Analysis of the 329 candidate PFD genes identified 63 candidate defects, affecting 40 genes, among index cases with Gi signaling abnormalities, while 53 defects, within 49 genes, were identified among patients with secretion abnormalities. Homozygous gene defects were more commonly associated with secretion abnormalities. Functional annotation analysis identified distinct gene clusters in the two patient subgroups. Thirteen genes with significant annotation enrichment for 'intracellular signaling' harbored 16 of the candidate gene defects identified in nine index cases with Gi signaling abnormalities. Four gene clusters, representing 14 genes, with significantly associated gene ontology annotations were identified among the cases with secretion abnormalities, the most significant association being with 'establishment of protein localization.'

CONCLUSION

Our findings demonstrate the genetic complexity of PFDs and highlight plausible candidate genes for targeted analysis in patients with platelet secretion and Gi signaling abnormalities.

Evaluation of a whole blood remote platelet function test for the diagnosis of mild bleeding disorders

BACKGROUND

Mild platelet function disorders (PFDs) are complex and difficult to diagnose. The current gold standard test, light transmission aggregometry (LTA), including lumi-aggregometry, is time and labour intensive and blood samples must be processed within a limited time after venepuncture. Furthermore, many subjects with suspected PFDs do not show a platelet abnormality on LTA.

OBJECTIVE

To assess the diagnostic potential of an easy-to-use remote platelet function test (RPFT) as a diagnostic pre-test for suspected PFDs.

METHODS

A remote platelet function test was compared with lumi-aggregometry in participants recruited to the Genotyping and Phenotyping of Platelets Study (GAPP, ISRCTN 77951167). For the RPFT, whole blood was stimulated with platelet agonists, stabilized with PAMFix and returned to the central laboratory for analysis of P-selectin and CD63 by flow cytometry.

RESULTS

For the 61 study participants (42 index cases and 19 relatives) there was a good agreement between lumi-aggregometry and the RPFT, with diagnosis being concordant in 84% of cases (κ = 0.668, P < 0.0001). According to both tests, 29 participants were identified to have a deficiency in platelet function and 22 participants appeared normal. There were four participants where lumi-aggregometry revealed a defect but the RPFT did not, and six participants where the RPFT detected an abnormal platelet response that was not identified by lumi-aggregometry.

CONCLUSION

This study suggests that the RPFT could be an easy-to-use pre-test to select which participants with bleeding disorders would benefit from extensive platelet phenotyping. Further development and evaluation of the test are warranted in a wider population of patients with excessive bleeding and could provide informative screening tests for PFDs.

A novel mutation in the P2Y12 receptor and a function-reducing polymorphism in protease-activated receptor 1 in a patient with chronic bleeding

BACKGROUND

The study of patients with bleeding problems is a powerful approach in determining the function and regulation of important proteins in human platelets. We have identified a patient with a chronic bleeding disorder expressing a homozygous P2RY(12) mutation, predicting an arginine to cysteine (R122C) substitution in the G-protein-coupled P2Y(12) receptor. This mutation is found within the DRY motif, which is a highly conserved region in G-protein-coupled receptors (GPCRs) that is speculated to play a critical role in regulating receptor conformational states.

OBJECTIVES

To determine the functional consequences of the R122C substitution for P2Y(12) function.

PATIENT/METHODS

We performed a detailed phenotypic analysis of an index case and affected family members. An analysis of the variant R122C P2Y(12) stably expressed in cells was also performed.

RESULTS

ADP-stimulated platelet aggregation was reduced as a result of a significant impairment of P2Y(12) activity in the patient and family members. Cell surface R122C P2Y(12) expression was reduced both in cell lines and in platelets; in cell lines, this was as a consequence of agonist-independent internalization followed by subsequent receptor trafficking to lysosomes. Strikingly, members of this family also showed reduced thrombin-induced platelet activation, owing to an intronic polymorphism in the F2R gene, which encodes protease-activated receptor 1 (PAR-1), that has been shown to be associated with reduced PAR-1 receptor activity.

CONCLUSIONS

Our study is the first to demonstrate a patient with deficits in two stimulatory GPCR pathways that regulate platelet activity, further indicating that bleeding disorders constitute a complex trait.

CLEC-2-dependent activation of mouse platelets is weakly inhibited by cAMP but not by cGMP

BACKGROUND

The activation of platelet CLEC-2 by podoplanin on lymphatic endothelial cells (LECs) has a critical role in prevention of mixing of lymphatic and blood vasculatures during embryonic development. Paradoxically, LECs release cAMP and cGMP-elevating agents, prostacyclin (PGI2 ) and nitric oxide (NO), respectively, which are powerful inhibitors of platelet activation. This raises the question of how podoplanin is able to activate CLEC-2 in the presence of the inhibitory cyclic nucleotides.

OBJECTIVES

We investigated the influence of cyclic nucleotides on CLEC-2 signaling in platelets.

METHODS

We used rhodocytin, CLEC-2 monoclonal antibody, LECs and recombinant podoplanin as CLEC-2 agonists on mouse platelets. The effects of the cyclic nucleotide-elevating agents PGI2 , forskolin and the NO-donor GSNO were assessed with light transmission aggregometry, flow cytometry, protein phosphorylation and fluorescent imaging of platelets on LECs.

RESULTS

We show that platelet aggregation induced by CLEC-2 agonists is resistant to GSNO but inhibited by PGI2 . The effect of PGI2 is mediated through decreased phosphorylation of CLEC-2, Syk and PLCγ2. In contrast, adhesion and spreading of platelets on recombinant podoplanin, CLEC-2 antibody and LECs is not affected by PGI2 and GSNO. Consistent with this, CLEC-2 activation of Rac, which is required for platelet spreading, is not altered in the presence of PGI2 .

CONCLUSIONS

The present results demonstrate that platelet adhesion and activation on CLEC-2 ligands or LECs is maintained in the presence of PGI2 and NO.

Utility of the ISTH bleeding assessment tool in predicting platelet defects in participants with suspected inherited platelet function disorders

BACKGROUND

The ISTH bleeding assessment tool (ISTH-BAT) was developed to record bleeding symptoms and to aid diagnosis in patients with a possible bleeding disorder.

OBJECTIVES

To investigate the utility of the ISTH-BAT in predicting functional defects in platelet activation in participants with suspected inherited platelet function disorders.

PATIENTS/METHODS

Participants with clinical evidence of excessive bleeding and suspected inherited platelet function disorders and healthy volunteers were recruited to the Genotyping and Phenotyping of Platelets study (GAPP; ISRCTN 77951167). The ISTH-BAT questionnaire was applied by a trained investigator prior to lumiaggregometry.

RESULTS

One hundred participants were included (79 with suspected inherited platelet function disorders, and 21 healthy volunteers). The ISTH-BAT score in participants with suspected inherited platelet function disorders (median 12; interquartile range [IQR] 8-16) was significantly higher than in healthy volunteers (median 0; IQR 0-0). There was no difference between participants with suspected inherited platelet function disorders with a platelet defect detected by lumiaggregometry (median 11; IQR 8-16) and those with normal platelet function (median 12; IQR 8-14) (P > 0.05). The ISTH-BAT score was not associated with a demonstrable platelet defect on platelet function testing (area under the receiver operating characteristic curve = 0.501 [95% confidence interval 0.372-0.630, P = 0.98] and odds ratio 1.01 [95% confidence interval 0.93-1.09, P = 0.91]).

CONCLUSIONS

The ISTH-BAT is a powerful tool for documenting lifelong bleeding history. However, the score obtained is not predictive of the presence of a platelet defect on lumiaggregometry in patients with suspected inherited platelet function disorders.

Genotyping and phenotyping of platelet function disorders

The majority of patients with platelet function disorders (PFDs) have normal platelet counts and mild day-to-day bleeding symptoms, but are at risk of major hemorrhage at times of trauma, surgery, or childbirth. This group is challenging to investigate, because the assays are often time-intensive and labour-intensive, and interpretation is difficult, especially in patients with mild disorders. In addition, interuser variability in performance of the assays, including the currently accepted gold standard, light transmission aggregometry, makes the results difficult to compare between laboratories. Furthermore, a similar pattern of mucocutaneous bleeding is seen in disorders in other components of the hemostatic pathway, including type 1 von Willebrand disease (VWD). We have undertaken an extensive investigation of patients with clinically diagnosed excessive bleeding, using a genotyping and platelet phenotyping approach based on lumi-aggregometry, and other specialist tests of platelet function, in combination with Sanger and next-generation sequencing (NGS). We found a functional defect in ~ 60% of patients, the majority being associated with feedback pathways of platelet activation. Function-disrupting mutations were identified in known and novel genes, and coinheritance with other genetic disorders of hemostasis, including type 1 VWD, was shown. A significant number of mutations are heterozygous and unlikely to cause extensive bleeding in isolation, consistent with incomplete penetrance of inheritance of bleeding disorders and a multifactorial etiology for excessive bleeding in many patients. Mucocutaneous bleeding is a complex trait, and this has important implications for NGS in the assessment of a PFD.

Glycoprotein Ibα and FcγRIIa play key roles in platelet activation by the colonizing bacterium, Streptococcus oralis

BACKGROUND

Infective endocarditis (IE) is characterized by thrombus formation on a cardiac valve. The oral bacterium, Streptococcus oralis, is recognized for its ability to colonize damaged heart valves and is frequently isolated from patients with IE. Platelet interaction with S. oralis leads to the development of a thrombotic vegetation on heart valves, which results in valvular incompetence and congestive heart failure.

OBJECTIVE

To investigate the mechanism through which platelets become activated upon binding S. oralis.

PATIENTS AND METHODS

Platelet interactions with immobilized bacteria under shear conditions were assessed using a parallel flow chamber. S. oralis-inducible platelet reactivity was determined using light transmission aggregometry. Dense granule secretion was measured by luminometry using a luciferin/luciferase assay.

RESULTS

Using shear rates that mimic physiological conditions, we demonstrated that S. oralis was able to support platelet adhesion under venous (50-200 s(-1) ) and arterial shear conditions (800 s(-1) ). Platelets rolled along immobilized S. oralis through an interaction with GPIbα. Following rolling, platelet microaggregate formation was observed on immobilized S. oralis. Aggregate formation was dependent on S. oralis binding IgG, which cross-links to platelet FcγRIIa. This interaction led to phosphorylation of the ITAM domain on FcγRIIa, resulting in dense granule secretion, amplification through the ADP receptor and activation of RAP1, culminating in platelet microaggregate formation.

CONCLUSIONS

These results suggest a model of interaction between S. oralis and platelets that leads to the formation of a stable septic vegetation on damaged heart valves.

Recommendations for the Standardization of Light Transmission Aggregometry: A Consensus of the Working Party from the Platelet Physiology Subcommittee of SSC/ISTH

Light transmission aggregometry (LTA) is the most common method used to assess platelet function. However, there is no universal standard for its performance. The Platelet Physiology Subcommittee of the Scientific and Standardization Committee (SSC) of the International Society on Thrombosis and Haemostasis formed a working party of experts with the aim of producing a series of consensus recommendations for standardizing LTA. Due to a lack of investigations that directly compared different methodologies to perform LTA studies, there were insufficient data to develop evidence-based guidelines. Therefore, the RAND method was used, which obtains a formal consensus among experts about the appropriateness of health care interventions, particularly when scientific evidence is absent, scarce and/or heterogeneous. Using this approach, each expert scored as "appropriate", "uncertain" or "inappropriate" a series of statements about the practice of LTA, which included pre-analytical variables, blood collection, blood processing, methodological details, choice of agonists and the evaluation and reporting of results. After presentation and public discussion at SSC meetings, the assessments were further refined to produce final consensus recommendations. Before delivering the recommendations, a formal literature review was performed using a series of defined search terms about LTA. Of the 1830 potentially relevant studies identified, only 14 publications were considered to be actually relevant for review. Based upon the additional information, 6 consensus statements were slightly modified. The final statements were presented and discussed at the SSC Meeting in Cairo (2010) and formed the basis of a consensus document, which is the subject of the present report. This article is protected by copyright. All rights reserved.

Tachykinin receptor types: Classification and membrane signalling mechanisms

The use of selective agonists in both functional and binding studies has provided unequivocal evidence for the existence of three types of tachykinin receptor (NK(1), NK(2) and NK(3)); there is also preliminary evidence for the existence of further subtypes. These results have been confirmed by the development of selective antagonists and by the identification and cloning of three distinct cDNA sequences. All three receptors belong to the superfamily of G protein coupled receptors and are linked to the phosphoinositide transmembrane-signalling pathway. The purpose of this article is to review recent developments in the pharmacology of each receptor with emphasis on the NK(3) type. In particular, the need to use selective agonists and antagonists to identify each receptor type is stressed.

Distinct and overlapping functional roles of Src family kinases in mouse platelets

BACKGROUND AND OBJECTIVES

Src family kinases (SFKs) play a critical role in initiating and propagating signals in platelets. The aims of this study were to quantitate SFK members present in platelets and to analyze their contribution to platelet regulation using glycoprotein VI (GPVI) and intregrin αIIbβ3, and in vivo.

METHODS AND RESULTS

Mouse platelets express four SFKs, Fgr, Fyn, Lyn and Src, with Lyn expressed at a considerably higher level than the others. Using mutant mouse models, we demonstrate that platelet activation by collagen-related peptide (CRP) is delayed and then potentiated in the absence of Lyn, but only marginally reduced in the absence of Fyn or Fgr, and unaltered in the absence of Src. Compound deletions of Lyn/Src or Fyn/Lyn, but not of Fyn/Src or Fgr/Lyn, exhibit a greater delay in activation relative to Lyn-deficient platelets. Fibrinogen-adherent platelets show reduced spreading in the absence of Src, potentiation in the absence of Lyn, but no change in the absence of Fyn or Fgr. In mice double-deficient in Lyn/Src or Fgr/Lyn, the inhibitory role of Lyn on spreading on fibrinogen is lost. Lyn is the major SFK-mediating platelet aggregation on collagen at arterial shear and its absence leads to a reduction in thrombus size in a laser injury model.

CONCLUSION

These results demonstrate that SFKs share individual and overlapping roles in regulating platelet activation, with Lyn having a dual role in regulating GPVI signaling and an inhibitory role downstream of αIIbβ3, which requires prior signaling through Src.

A role for adhesion and degranulation-promoting adapter protein in collagen-induced platelet activation mediated via integrin α(2) β(1)

BACKGROUND

Collagen-induced platelet activation is a key step in the development of arterial thrombosis via its interaction with the receptors glycoprotein (GP)VI and integrin α(2) β(1) . Adhesion and degranulation-promoting adapter protein (ADAP) regulates α(IIb) β(3) in platelets and α(L) β(2) in T cells, and is phosphorylated in GPVI-deficient platelets activated by collagen.

OBJECTIVES

To determine whether ADAP plays a role in collagen-induced platelet activation and in the regulation and function of α(2) β(1).

METHODS

Using ADAP(-/-) mice and synthetic collagen peptides, we investigated the role of ADAP in platelet aggregation, adhesion, spreading, thromboxane synthesis, and tyrosine phosphorylation.

RESULTS AND CONCLUSIONS

Platelet aggregation and phosphorylation of phospholipase Cγ2 induced by collagen were attenuated in ADAP(-/-) platelets. However, aggregation and signaling induced by collagen-related peptide (CRP), a GPVI-selective agonist, were largely unaffected. Platelet adhesion to CRP was also unaffected by ADAP deficiency. Adhesion to the α(2) β(1) -selective ligand GFOGER and to a peptide (III-04), which supports adhesion that is dependent on both GPVI and α(2) β(1), was reduced in ADAP(-/-) platelets. An impedance-based label-free detection technique, which measures adhesion and spreading of platelets, indicated that, in the absence of ADAP, spreading on GFOGER was also reduced. This was confirmed with non-fluorescent differential-interference contrast microscopy, which revealed reduced filpodia formation in ADAP(-/-) platelets adherent to GFOGER. This indicates that ADAP plays a role in mediating platelet activation via the collagen-binding integrin α(2) β(1). In addition, we found that ADAP(-/-) mice, which are mildly thrombocytopenic, have enlarged spleens as compared with wild-type animals. This may reflect increased removal of platelets from the circulation.

Platelets and the innate immune system: mechanisms of bacterial-induced platelet activation

It has become clear that platelets are not simply cell fragments that plug the leak in a damaged blood vessel; they are, in fact, also key components in the innate immune system, which is supported by the presence of Toll-like receptors (TLRs) on platelets. As the cells that respond first to a site of injury, they are well placed to direct the immune response to deal with any resulting exposure to pathogens. The response is triggered by bacteria binding to platelets, which usually triggers platelet activation and the secretion of antimicrobial peptides. The main platelet receptors that mediate these interactions are glycoprotein (GP)IIb-IIIa, GPIbα, FcγRIIa, complement receptors, and TLRs. This process may involve direct interactions between bacterial proteins and the receptors, or can be mediated by plasma proteins such as fibrinogen, von Willebrand factor, complement, and IgG. Here, we review the variety of interactions between platelets and bacteria, and look at the potential for inhibiting these interactions in diseases such as infective endocarditis and sepsis.

Src family kinases are essential for primary aggregation by G(i) -coupled receptors

INTRODUCTION AND BACKGROUND

Adrenaline stimulates biphasic aggregation in plasma through the G(i) -coupled α(2A) -adrenoreceptor. In the present study, we demonstrate that both primary and secondary wave aggregation induced by adrenaline in plasma is blocked by two structurally distinct inhibitors of Src family kinases, dasatinib and PD0173952.

METHODS AND RESULTS

In contrast, primary aggregation is partially inhibited or unaffected in the presence of inhibitors of cyclo-oxygenase, phosphoinositide (PI) 3-kinases, and P2Y(1) and P2Y(12) ADP receptors, although secondary aggregation is abolished. The ability of adrenaline to inhibit adenylyl cyclase and to synergize with platelet agonists in mediating platelet activation in plasma is retained in the presence of Src family kinase inhibition. Moreover, adrenaline does not activate Src family kinases, as determined by western blotting of their regulatory tyrosines, suggesting that constitutive signaling from Src family kinases may underlie their role in activation. Adrenaline is widely used in clinical laboratories for investigation of patients with suspected bleeding disorders. In a group of 90 unrelated patients with a clinically diagnosed platelet bleeding disorder, we identified four who did not exhibit primary wave aggregation in response to adrenaline, although the catecholamine potentiated the response to other agonists, and five who failed to undergo secondary wave aggregation. In contrast, adrenaline stimulated biphasic aggregation in 60 controls. All of the patients with a defective response to adrenaline had impaired ADP-induced platelet activation.

CONCLUSIONS

The present results indicate a previously unappreciated role for Src family kinases in mediating G(i) signaling in plasma, and demonstrate heterogeneity in response to adrenaline in patients with a clinically diagnosed platelet disorder.

GPVI and CLEC-2 in hemostasis and vascular integrity

SUMMARY

The glycoprotein VI (GPVI)-FcR gamma-chain complex initiates powerful activation of platelets by the subendothelial matrix proteins collagen and laminin through an immunoreceptor tyrosine-based activation motif (ITAM)-regulated signaling pathway. ITAMs are characterized by two YxxL sequences separated by 6-12 amino acids and are found associated with several classes of immunoglobulin (Ig) and C-type lectin receptors in hematopoietic cells, including Fc receptors. Cross-linking of the Ig GPVI leads to phosphorylation of two conserved tyrosines in the FcR gamma-chain ITAM by Src family tyrosine kinases, followed by binding and activation of the tandem SH2 domain-containing Syk tyrosine kinase and stimulation of a downstream signaling cascade that culminates in activation of phospholipase Cgamma2 (PLCgamma2). In contrast, the C-type lectin receptor CLEC-2 mediates powerful platelet activation through Src and Syk kinases, but regulates Syk through a novel dimerization mechanism via a single YxxL motif known as a hemITAM. CLEC-2 is a receptor for podoplanin, which is expressed at high levels in several tissues, including type 1 lung alveolar cells, lymphatic endothelial cells, kidney podocytes and some tumors, but is absent from vascular endothelial cells and platelets. In this article, we compare the mechanism of platelet activation by GPVI and CLEC-2 and consider their functional roles in hemostasis and other vascular processes, including maintenance of vascular integrity, angiogenesis and lymphogenesis.

Proteomic analysis of integrin alphaIIbbeta3 outside-in signaling reveals Src-kinase-independent phosphorylation of Dok-1 and Dok-3 leading to SHIP-1 interactions

BACKGROUND AND OBJECTIVES

Outside-in integrin alphaIIbbeta3 signaling involves a series of tyrosine kinase reactions that culminate in platelet spreading on fibrinogen. The aim of this study was to identify novel tyrosine phosphorylated signaling proteins downstream of alphaIIbbeta3, and explore their role in platelet signaling.

METHODS AND RESULTS

Utilizing proteomics to search for novel platelet proteins that contribute to outside-in signaling by the integrin alphaIIbbeta3, we identified 27 proteins, 17 of which were not previously shown to be part of a tyrosine phosphorylation-based signaling complex downstream of alphaIIbbeta3. The proteins identified include the novel immunoreceptors G6f and G6b-B, and two members of the Dok family of adapters, Dok-1 and Dok-3, which underwent increased tyrosine phosphorylation following platelet spreading on fibrinogen. Dok-3 was also inducibly phosphorylated in response to the GPVI-specific agonist collagen-related peptide (CRP) and the PAR-1 and -4 agonist thrombin, independently of the integrin alphaIIbbeta3. Tyrosine phosphorylation of Dok-1 and Dok-3 was primarily Src kinase-independent downstream of the integrin, whereas it was Src kinase-dependent downstream of GPVI. Moreover, both proteins inducibly interacted with Grb-2 and SHIP-1 in fibrinogen-spread platelets.

CONCLUSIONS

This study provides new insights into the molecular mechanism regulating alphaIIbbeta3-mediated platelet spreading on fibrinogen. The novel platelet adapter Dok-3 and the structurally related Dok-1 are tyrosine phosphorylated in an Src kinase-independent manner downstream of alphaIIbbeta3 in human platelets, leading to an interaction with Grb2 and SHIP-1.

The novel Syk inhibitor R406 reveals mechanistic differences in the initiation of GPVI and CLEC-2 signaling in platelets

BACKGROUND

Syk is a key mediator of signaling pathways downstream of several platelet surface receptors including GPVI/FcRgamma collagen receptor, the C-type lectin receptor CLEC-2, and integrin alphaIIbbeta3. A recent study identified the novel small molecule R406 as a selective inhibitor of Syk.

OBJECTIVES

The present study evaluates the role of Syk in human platelets using the novel inhibitor R406.

METHODS

Agonist-induced GPVI and CLEC-2 signaling were assessed using aggregometry, immunoprecipitation and western blotting to determine the effects of R406 on platelet activation.

RESULTS

We demonstrate R406 to be a powerful inhibitor of Syk in human platelets. R406 abrogated shape change and aggregation induced by activation of GPVI and CLEC-2, and reduced platelet spreading on fibrinogen. The inhibitory effect of R406 was associated with inhibition of tyrosine phosphorylation of signaling proteins that lay downstream of Syk for all three receptors, including PLCgamma2. Strikingly, R406 markedly inhibited tyrosine phosphorylation of CLEC-2 and Syk downstream of CLEC-2 activation, whereas phosphorylation of Syk downstream of GPVI and integrin alphaIIbbeta3 was unaffected.

CONCLUSIONS

The inhibitory effect of R406 provides direct evidence of a role for Syk in GPVI, CLEC-2 and integrin alphaIIbbeta3 signaling in human platelets. Further, the results demonstrate a critical role for Syk in mediating tyrosine phosphorylation of CLEC-2, suggesting a novel model in which both Src and Syk kinases regulate tyrosine phosphorylation of the C-type lectin receptor leading to platelet activation.

Molecular basis of platelet activation by an alphaIIbbeta3-CHAMPS peptide

BACKGROUND

A novel method, known as computed helical anti-membrane protein (CHAMP), for the design of peptides that bind with high affinity and selectivity to transmembrane helices was recently described and illustrated using peptides that bind alphaIIb- and alphav-integrin subunits, which induce selective activation of integrins alphaIIbbeta3 and alphavbeta3, respectively.

OBJECTIVES

In the present study, we have investigated the ability of an alphaIIb-CHAMPS peptide (termed integrin-activatory-peptide or IAP) to stimulate protein tyrosine phosphorylation and aggregation in human and mouse platelets.

METHODS

The ability of IAP to stimulate platelet aggregation and dense granule secretion was measured in washed preparations of human and mouse platelets. Samples were taken for measurement of tyrosine phosphorylation.

RESULTS

IAP stimulates robust tyrosine phosphorylation of the tyrosine kinase Syk and the FcR gamma-chain, but only weak phosphorylation of PLCgamma2. Aggregation to low but not high concentrations of IAP is reduced in the presence of the Src kinase inhibitor, PP1, or by inhibitors of the two feedback agonists, ADP and thromboxane A(2) (TxA(2)) suggesting that activation is reinforced by Src kinase-driven release of ADP and TxA(2). Unexpectedly, aggregation by IAP is only partially inhibited in human and mouse platelets deficient in integrin alphaIIbbeta3. Further, IAP induces partial aggregation of formaldehyde-fixed platelets.

CONCLUSIONS

The present study demonstrates that the alphaIIb-CHAMPS peptide induces platelet activation through integrin alphaIIbbeta3-dependent and independent pathways with the former mediating tyrosine phosphorylation of FcR gamma-chain and Syk. The use of the alphaIIb-CHAMPS peptide to study integrin alphaIIbbeta3 function is compromised by non-integrin-mediated effects.

Differential roles for the adapters Gads and LAT in platelet activation by GPVI and CLEC-2

BACKGROUND

The adapter proteins SLP-76 and LAT have been shown to play critical roles in the activation of PLCgamma2 in platelets downstream of GPVI/FcRgamma and the C-type lectin receptor CLEC-2. SLP-76 is constitutively associated with the adapter Gads in platelets, which also binds to tyrosine phosphorylated LAT, thereby providing a potential pathway of regulation of SLP-76.

OBJECTIVE

In the present study, we have compared the role of Gads alongside that of LAT following activation of the major platelet glycoprotein receptors using mice deficient in the two adapter proteins.

RESULTS

Gads was found to be required for the efficient onset of aggregation and secretion in response to submaximal stimulation of GPVI and CLEC-2, but to be dispensable for activation following stronger stimulation of the two receptors. Gads was also dispensable for spreading induced through integrin alpha(IIb)beta(3) or the GPIb-IX-V complex. Further, Gads plays a negligible role in aggregate formation on collagen at an arteriolar rate of shear. In stark contrast, platelets deficient in the adapter LAT exhibit a marked decrease in aggregation and secretion following activation of GPVI and CLEC-2, and are unable to form stable aggregates on collagen at arteriolar shear.

CONCLUSIONS

The results demonstrate that Gads plays a key role in linking the adapter LAT to SLP-76 in response to weak activation of GPVI and CLEC-2 whereas LAT is required for full activation over a wider range of agonist concentrations. These results reveal the presence of a Gads-independent pathway of platelet activation downstream of LAT.

Identification of a novel, actin-rich structure, the actin nodule, in the early stages of platelet spreading

BACKGROUND

During platelet spreading, the actin cytoskeleton undergoes marked changes, forming filopodia, lamellipodia and stress fibres. In the present study, we report the identification of a novel actin-rich structure, termed an actin nodule, which appears prior to lamellipodia and stress fibre formation.

METHODS

Platelet spreading was monitored using human platelets and mouse GFP-actin platelets using real-time and end-point DIC, and fluorescent and electron microscopy (EM).

RESULTS

We identified a small, novel actin structure, the actin nodule, in the early stages of adhesion and spreading, which we hypothesize to be a precursor of lamellipodia and stress fibres. Nodule formation shows an inverse correlation to Rho kinase and myosin-II activity, is independent of PI3-kinase, but dependent on Src kinase activity. Actin nodules contain multiple proteins, including Arp2/3, Fyn, Rac, and beta1- and beta3- integrins, but not Src. EM analysis revealed that actin filaments extend in all directions from the nodules. Actin nodules are present on multiple matrices, including fibrinogen, laminin and VWF + botrocetin.

CONCLUSION

This work identifies a novel platelet actin structure, which we propose is a precursor to both lamellipodia and stress fibres and acts to drive platelet spreading.

Globular adiponectin induces platelet activation through the collagen receptor GPVI-Fc receptor gamma chain complex

BACKGROUND

The adipocyte-derived cytokine, adiponectin (Ad), exerts potent vascular effects, although the direct effects of Ad on blood platelets are unclear.

OBJECTIVE

The influence of globular Ad (gAd) on blood platelet function was investigated.

RESEARCH DESIGN AND METHODS

We measured platelet aggregation and tyrosine phosphorylation signaling events in human and mouse platelets. The ability of gAd to activate Glycoprotein VI (GPVI) activity was determined with a NFAT luciferase reporter assay.

RESULTS

gAd, but not full length Ad, induced rapid aggregation and granule secretion of human and mouse platelets through a pathway that is ablated under conditions of Src kinase inhibition, indicating a tyrosine kinase-dependent mechanism. Consistent with this, gAd stimulates rapid tyrosine phosphorylation of several proteins in human and mouse platelets. The pattern of increase in tyrosine phosphorylation was similar to that induced by collagen, with the tyrosine kinase Syk and PLCgamma2 being identified among the list of tyrosine phosphorylated proteins. As collagen activates platelet through the GPVI-Fc receptor gamma-chain (FcRgamma) complex, we used FcRgamma null platelets (which also lack GPVI) to explore the mechanism by which gAd stimulates platelets. Stimulation of tyrosine phosphorylation and platelet aggregation by gAd was abolished in FcRgamma null platelets and markedly reduced in the absence of PLCgamma2. Further, GPVI was confirmed as a collagen receptor for gAd by increased luciferase activity in Jurkat T-cells transfected with GPVI.

CONCLUSIONS

We identify gAd as a novel ligand for GPVI that stimulates tyrosine kinase-dependent platelet aggregation. Our data raise the possibility that gAd may promote unwanted platelet activation at sites of vascular injury.

Substance P receptors in the nervous system and possible receptor subtypes

The ability of various related peptides and substance P analogues to compete for the binding of 3H-labelled substance P to rat brain membranes corresponds with their known biological activities, providing a simple model for studies of peptide receptors in the central nervous system. In salivary gland and brain slices substance P and related peptides stimulate the rate of incorporation of phosphatidylinositol, offering an alternative biochemical model for substance P receptor studies. Two types of receptor may be responsible for the actions of substance P on peripheral tissues: the SP-P type, where all tachykinins are approximately equally active, and the SP-E type, where eledoisin and kassinin are more potent than the other tachykinins. Alkyl esters of substance P appear to act as selective SP-P agonists.

Collagen promotes sustained glycoprotein VI signaling in platelets and cell lines

BACKGROUND

Glycoprotein (GP)VI is the major signaling receptor for collagen on platelets and signals via the associated FcRgamma-chain, which has an immunoreceptor tyrosine-containing activation motif (ITAM).

OBJECTIVE

To determine why GPVI-FcRgamma signals poorly, or not at all, in response to collagen in hematopoietic cell lines, despite robust responses to the GPVI-reactive snake venom toxin convulxin.

METHODS AND RESULTS

Using a nuclear factor of activated T-cells (NFAT) transcriptional reporter assay, a sensitive readout for sustained ITAM signaling, we demonstrate collagen-induced GPVI-FcRgamma signaling in hematopoietic cell lines. This is accompanied by relatively weak but sustained protein tyrosine phosphorylation, in contrast to the stronger but transient response to convulxin. Sustained signaling by collagen is also observed in platelets and is necessary for the maintenance of spreading on collagen. Finally, in cell lines, the inhibitory collagen receptor leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1), which is not expressed on platelets but is present on most hematopoietic cells, inhibits GPVI responses to collagen but not convulxin.

CONCLUSION

The inability of previous studies to readily detect GPVI collagen signaling in cell lines is probably because of the weak but sustained nature of the signal and the presence of the inhibitory collagen receptor LAIR-1. In platelets, we propose that GPVI-FcRgamma has evolved to transmit sustained signals in order to maintain spreading over several hours, as well as facilitating rapid activation through release of feedback agonists and integrin activation. The establishment of a cell line NFAT assay will facilitate the molecular dissection of GPVI signaling and the identification of GPVI antagonists in drug discovery.

MyosinIIa contractility is required for maintenance of platelet structure during spreading on collagen and contributes to thrombus stability

BACKGROUND

MyosinIIs are adenosine triphosphate-driven molecular motors that form part of a cell's contractile machinery. They are activated by phosphorylation of their light chains, by either activation of myosin light chain (MLC) kinase or inhibition of MLC phosphatase via Rho kinase (ROCK). MyosinIIa phosphorylation underlies platelet rounding and stress fiber formation.

OBJECTIVE

To identify the functional significance of myosinIIa in platelet spreading and thrombus formation on collagen using inhibitors of ROCK (Y27632) and myosinII (blebbistatin).

RESULTS

Stress fiber formation on collagen is inhibited by both Y27632 and blebbistatin. A substantial proportion of spread platelets generate internal holes or splits on collagen, presumably because of a reduction in contractile strength. Platelet integrity, however, is maintained. In an in vitro model, thrombus embolization on collagen is increased in the presence of Y27632 and blebbistatin at intermediate shear, leading to a reduction in platelet aggregate growth. Moreover, Y27632 causes a marked reduction in thrombus formation in an in vivo laser-injury model.

CONCLUSIONS

MyosinIIa contractility is required for maintenance of platelet structure during spreading on collagen and contributes to thrombus stability.

Differential regulation of adapter proteins Dok2 and Dok1 in platelets, leading to an association of Dok2 with integrin alphaIIbbeta3

BACKGROUND

We previously demonstrated that Dok2 is rapidly phosphorylated on tyrosine residues in platelets in response to thrombin, the immunoreceptor tyrosine-based activation motif-coupled collagen receptor glycoprotein (GP) VI, and by integrin alphaIIbbeta3.

OBJECTIVES AND METHODS

In this study we further delineate the regulation of phosphorylation of Dok2 and compare this to the related adapter Dok1.

RESULTS

We demonstrate expression of Dok1 in platelets and the unexpected observation that the adapter protein undergoes tyrosine phosphorylation in response to thrombin but not to GPVI or integrin alphaIIbbeta3. Furthermore, Dok1 phosphorylation is transient, peaking at 30 s and returning to basal by 5 min, whereas Dok2 phosphorylation is delayed but sustained. Dok2 phosphorylation, but not that of Dok1, is inhibited by Src kinase inhibitors and by chelation of intracellular calcium. Further, phosphorylation of Dok2 by thrombin and integrin alphaIIbbeta3 in mouse platelets is independent of Syk and phospholipase Cgamma2. Additionally, Dok2 coimmunoprecipitates with integrin alphaIIbbeta3 downstream of Src kinases.

CONCLUSIONS

These results demonstrate differential modes of regulation of Dok1 and Dok2 in platelets. Further, they raise the interesting possibility that Dok2 plays an important role in integrin outside-in signaling through a physical and functional interaction with integrin alphaIIbbeta3.

von Willebrand factor mediates platelet spreading through glycoprotein Ib and alpha(IIb)beta3 in the presence of botrocetin and ristocetin, respectively

BACKGROUND

von Willebrand factor (VWF) plays a critical role in the process of hemostasis by mediating flow-dependent adhesion and spreading of platelets on exposed extracellular matrix proteins following vascular injury. To accomplish this, VWF binds to two distinct platelet receptors: glycoprotein (GP)Ib-IX-V and integrin alpha(IIb)beta3.

OBJECTIVE

To evaluate the ability of GPIb and alpha(IIb)beta3 to mediate platelet adhesion and lamellipodia formation on immobilized VWF in the presence of the biochemical modulators, ristocetin and botrocetin.

RESULTS

In the presence of botrocetin and inhibitors of adenosine diphosphate (ADP) and thromboxane A2 (TxA2), VWF is able to support formation of lamellipodia through a GPIb-dependent mechanism that is independent of alpha(IIb)beta3 and PI3-kinase. Lamellipodia formation under these conditions is incomplete. In marked contrast, in the presence of ristocetin, VWF stimulates formation of fully spread lamellipodia through a pathway that is dependent upon alpha(IIb)beta3 and PI3-kinase. Furthermore, alpha(IIb)beta3 also supports platelet spreading on VWF alone, but only in the absence of inhibitors of ADP and TxA2. The localization of filamentous actin and the Arp2/3 complex in platelets on VWF in the presence of botrocetin and ristocetin are distinct, yielding disparate lamellipodium kinetic signatures. Interestingly, botrocetin significantly enhances platelet adhesion to VWF under flow in whole blood in an alpha(IIb)beta3-independent manner, while ristocetin augments washed platelet adhesion and spreading to VWF under flow in an alpha(IIb)beta3-dependent manner.

CONCLUSIONS

These observations demonstrate that VWF is able to induce lamellipodia formation through distinct receptors, and has important consequences for investigation of the role of VWF-GPIb interactions in the context of platelet regulation.

Mapping the platelet proteome: a report of the ISTH Platelet Physiology Subcommittee

Proteomic technology has the potential to transform the way we analyze platelet biology, through the determination of platelet protein composition and its modification upon stimulation and with disease. We are a considerable way from achieving these goals, however, because of significant limitations in current methodology. It is therefore important to consider the extent to which these aims can be met and the way that proteomic data should be presented and used. These issues are discussed in the present paper by the Platelet Physiology Subcommittee of the ISTH Scientific Standardisation Committee (SSC). It is recommended that proteomic information be combined with data from other experimental approaches to establish a database on protein expression and function in platelets.

GPVI and integrin alphaIIb beta3 signaling in platelets

This review summarizes recent developments in our understanding of the molecular basis of platelet activation by two distinct types of surface receptor, the immunoglobulin GPVI, and the integrin alphaIIb beta3 (also known as GPIIbIIIa). These two classes of receptor signal through similar yet distinct tyrosine kinase-based signaling cascades leading to activation of phospholipase C gamma2. The significance of these signaling cascades in platelet adhesion and platelet aggregation at arterial rates of shear is discussed.

Evaluation of the role of platelet integrins in fibronectin-dependent spreading and adhesion

BACKGROUND

Recent studies have shown that platelet adhesion and subsequent aggregation can occur in vivo in the absence of the two principal platelets adhesive ligands, von Willebrand factor and fibrinogen. These results highlight a possible role for fibronectin in supporting thrombus formation.

OBJECTIVE AND METHODS

To evaluate the platelet integrins and subsequent activation pathways associated with fibronectin-dependent platelet adhesion utilizing both human and murine platelets.

RESULTS

Platelets can adhere to fibronectin via the integrin alpha(IIb)beta(3), leading to formation of lamellipodia. This is mediated through an interaction with the tenth type III domain in fibronectin. Spreading on fibronectin promotes alpha(IIb)beta(3)-mediated Ca(2+) mobilization and tyrosine phosphorylation of focal adhesion kinase and phospholipase C gamma2. In contrast, studies with blocking antibodies and mice demonstrate that alpha(5)beta(1) and alpha(v)beta(3) support adhesion and promote formation of filopodia but not lamellipodia or tyrosine phosphorylation of these proteins. Further, neither alpha(5)beta(1) nor alpha(v)beta(3) is able to induce formation of lamellipodia in the presence of platelets agonists, such as collagen-related-peptide (CRP).

CONCLUSIONS

These observations demonstrate that integrins alpha(5)beta(1) and alpha(v)beta(3) support platelet adhesion and the generation of filopodia but that, in contrast to the integrin alpha(IIb)beta(3), are unable to promote formation of lamellipodia.

The Endothelium and Platelets in Cardiovascular Disease: Potential Targets for Therapeutic Intervention

The interaction between platelets and endothelium has been implicated in many disease processes. It is now known that platelets may be involved in the initiation of atheroma, modulate various inflammatory responses, and contribute to endothelial dysfunction, in addition to their classical role in thombosis. Indeed, various drugs targeting platelets and endothelium have been shown to be of benefit in disease states. The last few years has seen an exponential increase in our knowledge of platelet and endothelial biology. As a result of this, many new drugs, for example the ADP-antagonists and the GpIIbIIIa antagonists, have been developed for use in the clinical setting. There is also renewed interest in the use of older drugs (for example, the statins) in modulating platelet-endothelial interactions. This review focuses on the recent advances in our understanding of the interaction(s) between platelets and endothelium and how this knowledge could be manipulated for therapeutic gain.

c-Cbl negatively regulates platelet activation by glycoprotein VI

BACKGROUND

The adapter protein c-Cbl has emerged as having a potential role in negative regulation of immune receptor signaling. The major platelet-signaling receptor for collagen, glycoprotein VI (GpVI), is associated with the Fc receptor (FcR) gamma-chain, and signals through a similar pathway to immune receptors. c-Cbl is tyrosine-phosphorylated in response to stimulation of GpVI, whereas phosphorylation of c-Cbl in thrombin-activated platelets is dependent on fibrinogen binding to the integrin GpIIb/IIIa.

OBJECTIVE

To investigate the role of c-Cbl in platelet signaling.

METHODS

Murine platelets lacking functional c-Cbl or Src family kinases were analyzed.

RESULTS

Phosphorylation of c-Cbl through GpVI is reduced in murine platelets deficient in the Src-family kinases Fyn and Lyn, demonstrating that they lie upstream of c-Cbl phosphorylation. Phosphorylation of several proteins of the GpVI-signaling pathway, including the FcR gamma-chain, Syk and phospholipase Cgamma2 (PLCgamma2), is increased in the absence of c-Cbl. In line with this, aggregation is potentiated in response to the GpVI-specific collagen-related peptide (CRP) after a slight delay. A delay in potentiation is also seen in response to stimulation by thrombin.

CONCLUSIONS

These observations demonstrate that c-Cbl negatively regulates platelet responses to GpVI agonists and to thrombin, with the latter effect possibly being mediated downstream of GpIIb/IIIa. c-Cbl may play a physiological role in helping to prevent unwanted platelet activation in vivo.

Activation of GPVI by collagen is regulated by alpha2beta1 and secondary mediators

We have compared the roles of adenosine diphosphate (ADP), thromboxanes and the integrin alpha(2)beta(1) in the activation of washed platelets by collagen in the presence of the alpha(IIb)beta(3) antagonist lotrafiban. The stimulation of protein tyrosine phosphorylation by a collagen suspension is markedly delayed in the presence of the above inhibitors but shows substantial recovery with time. In comparison, activation of phospholipase C (PLC), Ca(2+) elevation and dense granule secretion are more severely suppressed by the above inhibitors. alpha(2)beta(1) blockade has a slightly greater inhibitory effect on all of the above responses than a combination of ADP receptor antagonists and cyclooxygenase inhibitor. Platelets exposed to a collagen monolayer show robust elevation of Ca(2+) that is delayed in the presence of the above inhibitors and which is accompanied by alpha-granule secretion. These results demonstrate that secondary mediators and alpha(2)beta(1) modulate collagen-induced intracellular signaling but have negligible effect on GPVI signaling induced by the specific agonist convulxin. This work supports the postulate that the major role of alpha(2)beta(1) is to increase the avidity of collagen for the platelet surface and by doing so enhance activation of GPVI. Therefore we propose an important role of secondary mediators in collagen-induced signaling is the indirect regulation of GPVI signaling via activation of alpha(2)beta(1).

International Union of Pharmacology. XXXI. Recommendations for the nomenclature of multimeric G protein-coupled receptors

A receptor is defined by the International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR) as a protein, or a complex of proteins, which recognizes physiologically relevant ligands that can regulate the protein to mediate cellular events (Ruffolo et al., 2000). This definition does not include associated proteins, which are not required for agonist recognition and/or receptor assembly. Thus, G proteins are not included in the nomenclature of G protein-coupled receptors (GPCRs). Similarly, proteins which modify receptor disposition, such as proteins with a PDZ domain (Sheng and Sala, 2001), and which associate with the cytosolic portion of the receptor are not included. The question arises, however, as to the way to name multimeric receptors where subunits influence receptor assembly and agonist recognition. The essential issue is whether to name the individual proteins or the association of proteins? NC-IUPHAR recommends that, where possible, the functional receptor complex be given a different name from that of the subunits.

Signalling components underlying platelet aggregation to a Ca2+ ionophore and a phorbol ester

Although it is well established that G(q)- and G(i)-coupled receptors can combine to mediate platelet aggregation, the signalling events underlying the synergy are not fully characterised. This study has used the calcium ionophore, A23187, and phorbol ester, PMA, to investigate this question. We show that aggregation to submaximal but not maximal concentrations of ionophore is partially inhibited by antagonism of the P2Y(12) ADP receptor or PKC blockade. However, a full aggregation response can be restored under these conditions by addition of PMA or ADP. Studies using PI 3-kinase inhibitors demonstrate that this is the second messenger pathway that restores aggregation by the G(i)-coupled receptor in the presence of PKC blockade. However, under normal circumstances, PI 3-kinase activity is not a major requirement for aggregation to the ionophore. PMA stimulates a weak aggregation which takes several minutes to reach a maximum. Threshold concentrations of PMA and a G(i)-coupled receptor agonist when added alone show no effect on aggregation, but when combined induce aggregation responses. This study demonstrates that calcium and PKC interact synergistically with a G(i)-coupled receptor agonist to mediate aggregation, and also with each other. Activation of G(i) supports aggregation in part through the PI 3-kinase pathway. High concentration of ionophore on their own can induce aggregation independent of PKC and activation of G(i). Multiple signalling pathways mediate platelet aggregation and their relative importance depends on experimental conditions.

C-terminal peptide of thrombospondin-1 induces platelet aggregation through the Fc receptor gamma-chain-associated signaling pathway and by agglutination

A peptide from the C-terminal domain of thrombospondin-1 (Arg-Phe-Tyr-Val-Val-Met-Trp-Lys; known as 4N1-1) has been reported to induce platelet aggregation and to bind to the integrin-associated protein (IAP), which is also known as CD47. In this study, it was discovered that 4N1-1 or its derivative peptide, 4N1K, induces rapid phosphorylation of the Fc receptor (FcR) gamma chain, Syk, SLP-76, and phospholipase C gamma2 in human platelets. A specific inhibitor of Src family kinases, 4-amino-4-(4-methylphenyl)-7-(t-butyl) pyrazola[3,4-d]pyrimidine, prevented phosphorylation of these proteins, abolished platelet secretion, and reduced aggregation by approximately 50%. A similar inhibition of aggregation to 4N1-1 was obtained in the presence of Arg-Gly-Asp-Ser in mouse platelets deficient in FcR gamma chain or SLP-76 and in patients with type I Glanzmann thrombasthenia. These results show that 4N1-1 signals through a pathway similar to that used by the collagen receptor glycoprotein (GP) VI. The alphaIIbbeta3-independent aggregation induced by 4N1-1 was also observed in fixed platelets and platelets from patients with Bernard-Soulier syndrome, which are deficient in GPIbalpha. Surprisingly, the ability of 4N1-1 to stimulate aggregation and tyrosine phosphorylation was not altered in platelets pretreated with anti-IAP antibodies and in IAP-deficient mice. These results show that the C-terminal peptide of thrombospondin induces platelet aggregation through the FcR gamma-chain signaling pathway and through agglutination. The latter pathway is independent of signaling events and does not use GPIbalpha or alphaIIbbeta3. Neither of these pathways is mediated by IAP.

The transmembrane adapter LAT plays a central role in immune receptor signalling

The transmembrane adapter LAT (linker for activation of T cells) plays a central role in signalling by ITAM bearing receptors expressed on T cells, natural killer cells, mast cells and platelets. Receptor engagement leads to the phosphorylation of tyrosine residues present in the intracellular domain of LAT and formation of a multiprotein complex with other adapter molecules and enzymes including Grb2, Gads/SLP-76 and PLCgamma isoforms. These signalling events predominantly take place in glycolipid-enriched membrane domains. The constitutive presence of LAT in GEMs enables its function as the main scaffolding protein for the organization of GEM-localized signalling. The study of LAT-deficient mice and LAT-deficient cell lines further emphasizes the importance of LAT for these signalling cascades but also defines the existence of LAT-independent events downstream of the Syk-family kinase-ITAM complex.

Regulation of phospholipase C gamma isoforms in haematopoietic cells: why one, not the other?

Phospholipase C gamma (PLCgamma) isoforms are critical for the generation of calcium signals in haematopoietic systems in response to the stimulation of immune receptors. PLCgamma is unique amongst phospholipases in that it is tightly regulated by the action of a number of tyrosine kinases. It is itself directly phosphorylated on a number of tyrosines and contains several domains through which it can interact with other signalling proteins and lipid products such as phosphatidylinositol 3,4,5-trisphosphate. Through this network of interactions, PLCgamma is activated and recruited to its substrate, phosphatidylinositol 4,5-bisphosphate, at the membrane. Both isoforms of PLCgamma, PLCgamma1 and PLCgamma2, are present in haematopoietic cells. The signalling cascade involved in the regulation of these two isoforms varies between cells, though the systems are similar for both PLCgamma1 and PLCgamma2. We will compare these cascades for both PLCgamma1 and PLCgamma2 and discuss possible reasons as to why one form of PLCgamma and not the other is required for signalling in specific haematopoietic cells, including T lymphocytes, B lymphocytes, platelets, and mast cells.

Signalling events underlying platelet aggregation induced by the glycoprotein VI agonist convulxin

We have investigated the role of secretion and intracellular signalling events in aggregation induced by the glycoprotein (GP)VI-selective snake venom toxin convulxin and by collagen. We demonstrate that aggregation induced by threshold concentrations of convulxin undergoes synergy with ADP acting via the P2Y12 receptor whereas there is no synergy via the P2Y1 receptor or with thromboxanes. On the other hand, apyrase, the P2Y12 receptor antagonist, AR-C67085, and indomethacin only marginally inhibit aggregation induced by convulxin. In comparison, these inhibitors severely attenuate the response to collagen. In order to investigate whether the weak inhibitory action against convulxin is due to release of agonists other than ADP from dense granules, experiments were performed on murine platelets deficient in this organelle (pearl mice platelets). A slightly greater reduction in aggregation induced by convulxin was observed in pearl platelets than in the presence of inhibitors of ADP, but a maximal response was still attained. Importantly, inhibition of protein kinase C further reduced the response to convulxin in pearl platelets demonstrating a direct role for the kinase in aggregation. Chelation of intracellular Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N,N',N'-tetraacetic acid (acetoxymethyl)ester (BAPTA-AM) abolished aggregation induced by convulxin under all conditions. Activation of phospholipase C by convulxin was potentiated by ADP acting through the P2Y12 receptor. In conclusion, we show that Ca2+ and protein kinase C, but not release of the secondary agonists ADP and thromboxane A2, are required for full aggregation induced by convulxin, whereas the response induced by collagen shows a much greater dependence on secretion of secondary agonists.

A novel viper venom metalloproteinase, alborhagin, is an agonist at the platelet collagen receptor GPVI

The interaction of platelet membrane glycoprotein VI (GPVI) with collagen can initiate (patho)physiological thrombus formation. The viper venom C-type lectin family proteins convulxin and alboaggregin-A activate platelets by interacting with GPVI. In this study, we isolated from white-lipped tree viper (Trimeresurus albolabris) venom, alborhagin, which is functionally related to convulxin because it activates platelets but is structurally different and related to venom metalloproteinases. Alborhagin-induced platelet aggregation (EC50, <7.5 microg/ml) was inhibitable by an anti-alphaIIbbeta3 antibody, CRC64, and the Src family kinase inhibitor PP1, suggesting that alborhagin activates platelets, leading to alphaIIbbeta3-dependent aggregation. Additional evidence suggested that, like convulxin, alborhagin activated platelets by a mechanism involving GPVI. First, alborhagin- and convulxin-treated platelets showed a similar tyrosine phosphorylation pattern, including a similar level of phospholipase Cgamma2 phosphorylation. Second, alborhagin induced GPVI-dependent responses in GPVI-transfected K562 and Jurkat cells. Third, alborhagin-dependent aggregation of mouse platelets was inhibited by the anti-GPVI monoclonal antibody JAQ1. Alborhagin had minimal effect on convulxin binding to GPVI-expressing cells, indicating that these venom proteins may recognize distinct binding sites. Characterization of alborhagin as a GPVI agonist that is structurally distinct from convulxin demonstrates the versatility of snake venom toxins and provides a novel probe for GPVI-dependent platelet activation.

The role of ITAM- and ITIM-coupled receptors in platelet activation by collagen

The major activation-inducing collagen receptor glycoprotein VI (GPVI) has been cloned within the last two years. It is a member of the Ig superfamily of proteins and is constitutively associated with the ITAM-bearing Fc receptor gamma-chain (FcR gamma-chain). GPVI signals through a pathway that involves several of the proteins used by Fc, B- and T-lymphocyte receptors and which takes place in glycolipid-enriched membrane domains in the plasma membrane known as GEMs. Responses to GPVI are regulated by PECAM-1 (CD31) and possibly other ITIM-bearing receptors. Despite a pivotal role for GPVI, there are important differences between signalling events to collagen and GPVI-specific ligands. This may reflect a role for co-receptors in the response to collagen.