Thrombin binding to GPIbalpha induces platelet aggregation and fibrin clot retraction supported by resting alphaIIbbeta3 interaction with polymerized fibrin

We have investigated the mechanisms leading to platelet aggregation following thrombin interaction with the glycoprotein (GP) Ib-IX-V complex. We show that platelets desensitized for the two thrombin receptors, PAR-1 and PAR-4, are still able to aggregate in response to thrombin and that this aggregation can be inhibited by a monoclonal antibody (VM16d) that blocks thrombin binding to GPIbalpha, or by pretreatment of platelets with Mocarhagin, a protease that specifically cleaves GPIbalpha. The thrombin/GPIbalpha-initiated signaling cascade induces platelet shape change through activation of the Rho kinase p160ROCK, independent of calcium mobilization, transient MEK-1 phosphorylation as well as the cleavage of talin through a calcium-independent mechanism. This signaling cascade does not induce the exposure of high affinity alphaIIbbeta3 integrin receptors, nor does it lead to micro -calpain cleavage of filamin or the integrin cytoplasmic tail. In contrast, we provide evidence that binding of thrombin to GPIbalpha induces fibrin binding to resting alphaIIbbeta3 leading to fibrin-dependent platelet aggregation and clot retraction, that can be selectively inhibited by alphaIIbbeta3 antagonists such as RGDS, the dodecapeptide or lamifiban, as well as by the fibrin polymerization inhibitor GPRP-amide.     

Inhibition of platelet adhesion to fibrin(ogen) in flowing whole blood by Arg-Gly-Asp and fibrinogen gamma-chain carboxy terminal peptides.

We have employed synthetic peptides with sequences corresponding to the integrin receptor-recognition regions of fibrinogen as inhibitors of platelet aggregation and adhesion to fibrinogen- and fibrin-coated surfaces in flowing whole blood, using a rectangular perfusion chamber at wall shear rates of 300 s-1 and 1,300 s-1. D-RGDW caused substantial inhibition of platelet aggregation and adhesion to fibrinogen and fibrin at both shear rates, although it was least effective at blocking platelet adhesion to fibrin at 300 s-1. RGDS was a weaker inhibitor, and produced a biphasic dose-response curve; SDRG was inactive. HHLGGAKQAGDV partially inhibited platelet aggregation and adhesion to fibrin(ogen) at both shear rates. These results support the identification of an RGD-specific receptor, most likely the platelet integrin glycoprotein IIb:IIIa, as the primary receptor responsible for platelet:fibrin(ogen) adhesive interactions under flow conditions, and indicate that platelet adhesion to surface bound fibrin(ogen) is stabilized by multivalent receptor-ligand contacts.     

Fibronectin and coagulation factor XIII increases blood platelet adhesion to fibrin

Fibronectin is covalently linked to fibrin during clot formation by coagulation factor XIII and has been suggested as a possible mediator of platelet adhesion to collagen. Fixed human platelets were found to adhere to fibrin. This adhesion was significantly increased when fibronectin was incorporated into the fibrin network, and was supported by factor XIII.     

Beta3 tyrosine phosphorylation in alphaIIbbeta3 (platelet membrane GP IIb-IIIa) outside-in integrin signaling

The platelet integrin alphaIIbbeta3 not only binds fibrinogen and von Willebrand factor to mediate platelet aggregation and adhesion, it also serves as a signaling receptor. Platelet agonists such as ADP, thrombin and collagen induce "inside-out" signaling which activates the receptor function of alphaIIbbeta3 for soluble fibrinogen. Subsequent platelet aggregation leads to "outside-in" signaling, inducing platelet aggregate stabilization and triggering a variety of functions important to platelet physiology. This review focuses on the role of beta3 tyrosine phosphorylation in alphaIIbbeta3 outside-in signaling. Tyrosine phosphorylation of beta3 in platelets is a dynamic process which is initiated upon platelet aggregation and also by adhesion of platelets to immobilized fibrinogen. Tyrosine phosphorylation occurs on the beta3 integrin cytoplasmic tyrosine (ICY) domain, a conserved motif found in the beta subunits of several integrins. Beta3 ICY domain tyrosine phosphorylation induces the recruitment of two proteins to the cytoplasmic domains of alphaIIbbeta3: the cytoskeletal protein myosin, important to clot retraction; and the signaling adapter protein Shc, important to platelet stimulation, The critical role of beta3 tyrosine phosphorylation to platelet function was established by the diYF mouse, a novel strain which expresses an alphaIIbbeta3 in which the two beta3 ICY domain tyrosines have been mutated to phenylalanine. These mice are selectively impaired in outside-in alphaIIbbeta3 signaling, with defective aggregation and clot-retraction responses in vitro, and an in vivo bleeding defect which is characterized by a pronounced tendency to rebleed. Taken together, the data suggest that the beta3 tyrosine phosphorylation signaling mechanism is important to alphaIIbbeta3 function and might be applicable to a wide variety of integrin-mediated events.     

Regulation of tissue factor-induced coagulation and platelet aggregation in flowing whole blood

Photochemically induced thrombosis (a thrombin-dependent process) was measured in rats treated with moderate doses of anticoagulants, but which appeared to be unchanged. We considered the possibility that platelet-inhibiting agents, which also indirectly inhibit coagulation, would act as more potent antithrombotic agents. Inhibitors used as such were prostaglandin E1 (PGE1), which elevates cyclic AMP levels, and the P2Y12 ADP-receptor antagonist, AR-C69931MX. Effects of these agents were investigated in an ex vivo model system, in which whole blood under coagulant conditions was perfused over fibrinogen at defined wall shear rate. Perfusion of blood (rat or human) in the presence of tissue factor resulted in deposition of activated platelets and subsequent aggregate formation, along with exposure of procoagulant phosphatidylserine (PS) on the platelet surface and formation of fibrin fibers. In the presence of PGE1 aggregation was completely inhibited, but platelet adhesion and PS exposure were only party reduced, while fibrin formation was hardly affected. Treatment with AR-C69931MX caused similar, but less complete effects. These results indicate that in tissue factor-triggered blood under conditions of flow: (i) the platelet procoagulant response is independent of aggregate formation; (ii) the platelet-inhibiting effect of PGE1 and AR-C69931MX is sufficient to suppress aggregation, but not platelet adhesion and coagulation. These platelet inhibitors thus maintain their aggregation-inhibiting effect at sites of thrombin formation.     

Fibrinogen adsorption,platelet adhesion and thrombin generation at heparinized surfaces exposed to flowing blood

Thrombus formation at an artificial surface in contact with blood is a complex process that encompasses accretion of platelets from flowing blood and fibrin deposition. Platelet adhesion and fibrin formation are intimately intertwined reactions that are triggered by different sets of surface adsorbed plasma proteins. To dissect the contribution of protein adsorption and platelet adhesion to thrombin formation, a coherent study was performed with non-coated (NC) and heparin-coated (HC) surfaces. Thrombin production in whole blood, platelet adhesion and protein adsorption were studied using an amidolytic thrombin assay, a dynamic platelet adhesion assay and ellipsometry, respectively. Thrombin generation in flowing whole blood exposed to HC surfaces was greatly diminished when compared with NC surfaces. However, separate platelet adhesion and protein adsorption studies with anticoagulated whole blood revealed that platelets do not adhere because fibrinogen is not available in the protein layer that was deposited during the perfusion. These findings indicate that the in vitro thrombogenicity of a material cannot be predicted from platelet adhesion and protein adsorption data when these measurements are performed with anti-coagulated blood or platelet rich plasma. Preincubation of NC and HC surfaces with fibrinogen or 2000-fold diluted plasma resulted in similar amounts of surface-bound fibrinogen and mediated massive platelet adhesion from flowing whole blood. These results indicate that a) platelet adhesion correlates with the availability of surface-bound fibrinogen and b) NC and HC surfaces are indistinguishable with respect to protein (fibrinogen) adsorption and platelet adhesion. It is apparent that the heparinized surface used in our studies exerts its anti-thrombogenic properties by neutralizing locally formed thrombin and not by reducing fibrinogen-dependent platelet adhesion.     

Effects of cyclooxygenase inhibitors on ex vivo cysteinyl-leukotriene production by whole human blood allowed to clot spontaneously. Comparison to stimulated blood.

In ex vivo experiments we have investigated the effects of single oral administrations of aspirin (100 or 1000 mg) or ibuprofen (200 or 1200 mg) on the formation of immunoreactive thromboxane (TX) B2 and cysteinyl-leukotrienes (LT) by whole human blood in vitro. Whole human blood was allowed to clot spontaneously and in addition, in the presence of the chemotactic tripeptide f-Met-Leu-Phe (FMLP, 1 microM), or the non-physiological ionophore A 23187 (10 microM). Oral pretreatment with aspirin or ibuprofen led to dose-dependent inhibition of the TXB2 production in the presence of any of the stimuli used. After drug pretreatment the cysteinyl-LT production was not significantly enhanced in blood stimulated with FMLP or allowed to clot spontaneously as compared to values prior to oral drug administration. However, pronounced inhibition of the cyclooxygenase pathway was accompanied by an enhanced cysteinyl-LT formation when ionophore A 23187 was used as a stimulus. This could also be shown in experiments where the cyclooxygenase inhibitors were added in vitro. The higher oral dose of aspirin reduced the ex vivo cysteinyl-LT production in spontaneously clotting blood. By reverse phase HPLC immunoreactive cysteinyl-LT were characterized as a mixture of LTD4 and LTE4 as well as small amounts of LTC4. The data show that in the presence of therapeutic cyclooxygenase inhibitors no enhanced formation of cysteinyl-LT occurs in human blood in vitro after physiological stimuli such as contact activation of the clotting cascade or FMLP.     

GPIIb-IIIa antagonist-induced reduction in platelet surface factor V/Va binding and phosphatidylserine expression in whole blood

In addition to inhibition of platelet aggregation, GPIIb-IIIa antagonists may reduce thrombotic events via other mechanisms. In a novel whole blood flow cytometric system, we investigated the effects of GPIIb-IIIa antagonists, in the presence or absence of thrombin inhibitors, on platelet surface-bound factor V/Va and platelet surface phospholipids. Diluted venous blood was incubated with either buffer or a GPIIb-IIIa antagonist (abciximab, tirofiban, or eptifibatide). Some samples were pre-incubated with clinically relevant concentrations of unfractionated heparin (UFH), a low molecular weight heparin, a direct thrombin inhibitor, or buffer only. Platelets were then activated and labeled with mAb V237 (factor V/Va-specific) or annexin V (binds phosphatidylserine), fixed, and analyzed by flow cytometry. In the absence of thrombin inhibitors, GPIIb-IIIa antagonists (especially abciximab) significantly reduced agonist-induced platelet procoagulant activity, as determined by reduced binding of V237 and annexin V. At high pharmacologic concentrations, unfractionated heparin and enoxaparin, but not hirudin, further reduced factor V/Va binding to the surface of activated platelets in the presence of GPIIb-IIa antagonists. Agonist-induced platelet procoagulant activity was reduced in a patient with Glanzmann's thrombasthenia. We conclude that GPIIb-IIIa antagonists reduce platelet procoagulant activity in whole blood and heparin and enoxaparin augment this reduction. Fibrinogen binding to GPIIb-IIIa is important in the generation of platelet procoagulant activity.     

Determination of platelet factor 3 in whole blood by a chromogenic peptide substrate assay

A newly developed chromogenic peptide substrate assay for the determination of platelet factor 3 (PF 3) in plasma was modified for application to whole blood. The PF 3 activity values found in whole blood from healthy volunteers were essentially the same as those of the corresponding platelet-rich plasma samples. The PF 3 values obtained in whole blood taken in the anti-coagulant EDTA/citrate/PGE 1/ /theophylline were stable for three hours or more after drawing the blood, which indicated that no platelet release had taken place. When citrate was used as an anticoagulant a marked increase in PF 3 activity started 10 to 20 minutes after drawing the blood. Release of β-thromboglobulin (β-tg) was found to occur simultaneously with the release of PF 3 under these conditions. This means that normal transfusion blood contains a large amount of PF 3 activity as well as other platelet release products. Infusion of such material into patients subjected to major surgery or trauma might increase the risk of thrombosis or disseminated intravascular coagulation. Generation of PF 3 activity in platelet-rich plasma upon addition of collagen and during blood clotting has been studied.     

Sequential adhesion of platelets and leukocytes from flowing whole blood onto a collagen-coated surface: requirement for a GpVI-binding site in collagen

The adhesion of leukocytes to immobilised platelets may contribute to inflammatory and thrombotic responses in damaged tissue. To investigate the conditions under which platelets and leukocytes might be deposited together in vessels, we perfused fluorescently-labelled whole blood through glass capillaries coated with various collagen preparations. Video-microscopic observations of the surface showed that platelets formed numerous, individual, rolling and stationary attachments to surfaces coated with acid-soluble, monomeric collagen. However, leukocyte interactions with the deposited platelets were rare. If the blood was washed out, the adherent platelets became more activated, and many rolling adherent leukocytes were observed if a second bolus of blood was perfused over them. This suggested that platelet activation had initially been inadequate to support leukocyte capture. Next, fibrillar collagen was adsorbed to the capillaries to present an ordered array of peptide motifs to platelet receptor glycoprotein (Gp)VI and transduce an activating signal. In this case, platelets were deposited in discrete, stable aggregates and the bound platelets captured many flowing leukocytes. Alternatively, acid-soluble collagen was seeded with collagen-related peptide (CRP) known to contain a GpVI-binding motif. Again, platelet adhesion became stable, and numerous flowing leukocytes were captured. Addition of antibody against GpVI or against P-selectin greatly reduced leukocyte adhesion to the platelets. Thus, in whole blood, platelets binding to exposed collagen need to be activated through GpVI in order to expose sufficient P-selectin to allow efficient capture of flowing leukocytes to take place.     

3H-imipramine binding to freshly prepared platelet membranes in depression

3H-Imipramine binding was measured in freshly prepared platelet membranes from 47 drug-free major depressives and 46 healthy controls. Where possible, platelet binding in depressed subjects was repeated following treatment. A significant negative correlation was found between Bmax and assay protein concentration and Bmax values were corrected for this effect. Adjusted Bmax was significantly lower (by 14%) in female depressed patients than in female control subjects, and the difference was of similar magnitude premenopausally and postmenopausally. No such difference was found in males. Kd did not differ significantly between depressed and control subjects. Multiple regression analysis confirmed significant effects on Bmax of presence of depressive illness, age (positive correlation), and season (higher in summer). Within the depressed sample, Bmax was significantly lower in those subjects with obsessional features. Endogenicity (Research Diagnostic Criteria or Newcastle), dexamethasone suppression test result, drug-free interval, family history of depression, depressive psychosis, suicidal ideation, and past history of suicide attempts were not significantly related to Bmax. Paired comparisons revealed no significant effect on Bmax of 6 weeks' treatment with imipramine, maprotiline, or BRL 14342 or of a course of electroconvulsive therapy.     

Multiple electrode aggregometry: a new device to measure platelet aggregation in whole blood

Several methods are used to analyse platelet function in whole blood. A new device to measure whole blood platelet aggregation has been developed, called multiple electrode platelet aggregometry (MEA). Our aim was to evaluate MEA in comparison with the single platelet counting (SPC) method for the measurement of platelet aggregation and platelet inhibition by aspirin or apyrase in diluted whole blood. Platelet aggregation induced by different concentrations of ADP, collagen and TRAP-6 and platelet inhibition by apyrase or aspirin were determined in citrateor hirudin-anticoagulated blood by MEA and SPC. MEA indicated that spontaneous platelet aggregation was lower, and stimulated platelet aggregation was higher in hirudin- than citrate-anticoagulated blood. In hirudin-anticoagulated, but not citrate-anticoagulated blood, spontaneous platelet aggregation measured by MEA was inhibited by apyrase. For MEA compared with SPC the dose response-curves of agonist-induced platelet aggregation in citrate- and hirudin-blood showed similar EC50 values for TRAP, and higher EC50 values for ADP (non-significant) and collagen (p < 0.05). MEA and the SPC method gave similar results concerning platelet-inhibition by apyrase and aspirin. MEA was more sensitive than SPC to the inhibitory effect of aspirin in collagen-induced aggregation. In conclusion, MEA is an easy, reproducible and sensitive method for measuring spontaneous and stimulated platelet aggregation, and evaluating antiplatelet drugs in diluted whole blood. The use of hirudin as an anticoagulant is preferable to the use of citrate. MEA is a promising technique for experimental and clinical applications.     

Variability in platelet responses to collagen--comparison between whole blood perfusions, traditional platelet function tests and PFA-100.

The purpose of this study was to determine if the results obtained in platelet function tests and whole blood perfusions are associated with those in platelet function analyser (PFA)-100. We used collagen type I monomers and fibrils to analyse the distinct roles of glycoprotein (GP) Ia/IIa and other collagen receptors in flowing blood under a high shear rate (1600/s) and in aggregation studies. Also, anticoagulation [citrate vs. D-phenylalanyl-1-prolyl-1 arginine chloromethyl ketone (PPACK)] was varied to enhance the functions of GP Ia/IIa, since it has been shown that the cation-poor environment of citrated blood impairs GP Ia/IIa-dependent platelet recruitment. Large interindividual variability (45-fold) was detected in deposition of platelets in whole blood perfusions over collagen monomers, whereas this variation was only fourfold in fibrils. In PFA, this variation was reduced to 2.5-fold. However, platelet deposition on monomers is associated with epinephrine-enhanced PFA (r=-.49, P<.03), whereas platelet deposition on fibrils is correlated with adenosine diphosphate (ADP)-enhanced PFA (r=-.47, P<.05), suggesting a distinct synergism between epinephrine and monomers (GP Ia/IIa) as well as ADP with fibrils (other collagen receptors). Donors with 807 C/C polymorphism of GP Ia (n=14) had longer lag phase in aggregation experiments compared with C/T (n=7) both by monomers and fibrils (P<.04), but these polymorphisms with their mild impact on GP Ia/IIa activity did not markedly differ in other tests. In conclusion, the results obtained in perfusion studies and PFA experiments correlated, but PFA fails to reveal the large-scale variability related to collagen-induced platelet responses.     

Acute myocardial infarction: measurement of arachidonate end-products in whole blood as an index of platelet cyclo-oxygenase activity in vivo

The endogenous arachidonic acid metabolism was investigated ex vivo, in separated serum from clotted whole blood, soon after the onset of acute myocardial infarction (3.3 +/- 0.7 hr). A group of eight consecutive male patients was selected, since no evidence was obtained of any associated disease known to increase platelet activity or any recent exposure to cyclo-oxygenase inhibitors. This group of patients compared to an age and sex matched control group showed a large decrease in the platelet cyclo-oxygenase end-products in whole blood: thromboxane B2 (TXB2), 12-hydroxy-5-cis, 8-cis, 10-trans-heptadecatrienoic acid (HHT) and 6-keto-PGF1 alpha (p less than .01). In addition, platelet lipoxygenase produced an increased amount of 12-hydroperoxy-5,8,10,14-eicosatetraenoic acid (12-HPETE) as measured by its reduced metabolite 12-HETE (p less than .05). Furthermore, the TXB2 plasma concentration was significantly elevated in patients (p less than .01), confirming the enhanced platelet reactivity during the early stages of acute myocardial infarction. These results point out that a decreased level of cyclo-oxygenase end-products and an increased level of lipoxygenase end-product in serum is consistent with a previous in vivo cyclo-oxygenase hyperactivity.     

Evidence that the platelet integrin alphaIIb beta3 is regulated by the integrin-linked kinase,ILK, in a PI3-kinase dependent pathway

Platelet aggregation is mediated by the integrin alphaIIb beta3 which is activated by intracellular signals during platelet activation. We have attempted to determine if ILK ("Integrin-Linked Kinase") is involved in the regulation of alphaIIb beta3 function. ILK co-immunoprecipitated with beta3 in stimulated platelets. Using confocal microscopy, ILK was detected in the cytoplasm of resting platelets. ADP or PMA stimulation led to its translocation to the plasma membrane. In parallel, there was a transient increase in ILK kinase activity, association with and phosphorylation of beta3. Inhibition of PI3-kinase by two unrelated inhibitors (wortmannin and LY294002) prevented ILK-related functions. However, it did not prevent the conformational change in alphaIIb beta3 (shown by PAC-1 binding), although integrin affinity for fibrinogen was decreased as measured using FITC-fibrinogen. Furthermore, aggregate formation was reduced. Thus ILK transiently associates with and phosphorylates beta3 in a PI3-kinase dependent manner suggesting that it participates at an intermediate stage in a critical mechanism for assuring large stable aggregates.     

Effect of platelet encapsulated Iloprost on platelet aggregation and adhesion to collagen and injured blood vessels in vitro.

A novel approach to site-directed delivery of drugs in vivo using blood platelets as carrier vehicles is being investigated. In this context some initial studies are reported on the effect of platelet encapsulated anti-platelet drugs on platelet aggregation and adhesion to fibrillar collagen and injured arteries in vitro. The stable prostacyclin analogue Iloprost has been encapsulated within human and pig platelets by high voltage electroporation (Hughes and Crawford 1989 and 1990). After resealing the platelets, the packaged drug has a negligible effect upon platelet adhesion to a surface of fibrillar collagen or to damaged aorta (stripped to the tunica media to simulate deep injury). The rate of platelet recruitment to the collagen shows no dose dependency with respect to intracellular Iloprost concentrations. After high Iloprost loading, as few as 2% drug loaded platelets in a mixture with control (sham encapsulated) platelets, inhibit agonist-induced platelet aggregation > 50%. The prior deposition of a "lawn" of Iloprost-loaded platelets onto fibrillar collagen or damaged aorta has a substantial inhibitory effect (50-70%) upon the secondary recruitment of normal platelets compared with recruitment to a "lawn" of normal platelets. This inhibition of secondary recruitment occurs even in the presence of a platelet activator. If reduction of platelet recruitment to a vessel wall lesion results in a decrease in the local concentration of platelet granule-derived smooth muscle cell chemotactic and proliferative factors, this site-directed drug delivery may well have application for the prevention of restenosis following balloon angioplasty procedures.     

The kinetics of platelet and fibrin deposition on to damaged rabbit carotid arteries in vivo: involvement of platelets in the initial deposition of fibrin

Thrombus formation in the rabbit carotid artery has been studied kinetically in vivo using a minimally invasive technique utilising radioisotopes. Clamping of the carotid artery for 5 min resulted in the simultaneous accumulation of platelets and fibrin at the site of injury over the next 45 min. Under the electron microscope the response was seen to range from platelet monolayer adhesion to mural thrombus formation with fibrin deposition. In animals rendered thrombocytopenic, fibrin deposition was impaired during the first 15-20 minutes after injury. Basic coagulation times and fibrinogen concentration were within normal limits. In addition the injured vessels in these animals accumulated more radiolabelled albumin, but not erythrocytes, than injured vessels in control animals. The results may imply a role for platelets in the enhancement of fibrin deposition during the early part of the response to injury and in contributing to the maintenance of normal permeability following vessel injury.