Platelet Receptors for Collagen

Inhibition of platelet aggregation and the release of P-selectin from platelets by cilostazol

To evaluate the in vitro effects of cilostazol, a phosphodiesterase III inhibitor, on platelet responses, we measured platelet aggregation and the levels of soluble P-selectin, a glycoprotein present on the alpha-granule membrane in resting platelets, and cAMP. Platelet-rich plasma and washed platelets from healthy human volunteers were treated with cilostazol (5, 25 and 50 microM). Platelet-rich plasma was stimulated by ADP (1 and 5 microM) or collagen (5 microg/ml). Washed platelets were stimulated by thrombin (4 U/ml) in the presence or absence of 1 microM forskolin. In vehicle-treated samples, soluble P-selectin levels in response to 1 microM ADP-induced primary aggregation were similar to those of circulating levels of healthy volunteers but the levels in response to 5 microM ADP-induced secondary aggregation and collagen-induced aggregation increased markedly compared to those in response to primary aggregation. This result suggests that P-selectin is released from platelets according to the extent of platelet aggregation. Cilostazol inhibited platelet aggregation as well as P-selectin release in a concentration-dependent manner. Cilostazol inhibited completely thrombin-induced aggregation in the presence of 1 microM forskolin, when cAMP levels were two-fold higher than those in the absence of forskolin. Cilostazol, which increases intracellular cAMP in platelets, may be useful in the treatment of arterial occlusive diseases.

Integrin-independent tyrosine phosphorylation of p125(fak) in human platelets stimulated by collagen

Collagen fibers or a glycoprotein VI-specific collagen-related peptide (CRP-XL) stimulated tyrosine phosphorylation of the focal adhesion kinase, p125(fak) (FAK), in human platelets. An integrin alpha(2)beta(1)-specific triple-helical peptide ligand, containing the sequence GFOGER (single-letter nomenclature, O = Hyp) was without effect. Antibodies to the alpha(2) and beta(1) integrin subunits did not inhibit platelet FAK tyrosine phosphorylation caused by either collagen fibers or CRP-XL. Tyrosine phosphorylation of FAK caused by CRP-XL or thrombin, but not that caused by collagen fibers, was partially inhibited by GR144053F, an antagonist of integrin alpha(IIb)beta(3). The intracellular Ca(2+) chelator, BAPTA, and the protein kinase C inhibitor, Ro31-8220, were each highly effective inhibitors of the FAK tyrosine phosphorylation caused by collagen or CRP-XL. These data suggest that, in human platelets, 1) occupation or clustering of the integrin alpha(2)beta(1) is neither sufficient nor necessary for activation of FAK, 2) the fibrinogen receptor alpha(IIb)beta(3) is not required for activation of FAK by collagen fibers, and 3) both intracellular Ca(2+) and protein kinase C activity are essential intermediaries of FAK activation.

Rhodocytin induces platelet aggregation by interacting with glycoprotein Ia/IIa (GPIa/IIa, Integrin alpha 2beta 1). Involvement of GPIa/IIa-associated src and protein tyrosine phosphorylation

Although glycoprotein Ia/IIa (GPIa/IIa, integrin alpha(2)beta(1)) has established its role as a collagen receptor, it remains unclear whether GPIa/IIa mediates activation signals. In this study, we show that rhodocytin, purified from the Calloselasma rhodostoma venom, induces platelet aggregation, which can be blocked by anti-GPIa monoclonal antibodies. Studies with rhodocytin-coupled beads and liposomes loaded with recombinant GPIa/IIa demonstrated that rhodocytin directly binds to GPIa/IIa independently of divalent cations. In vitro kinase assays and Western blotting of GPIa immunoprecipitates revealed that Src and Lyn constitutively associate with GPIa/IIa and that Src activity increases transiently after rhodocytin stimulation. Src specifically associates with p130 Crk-associated substrate (Cas) in a manner dependent upon Cas phosphorylation, suggesting that Src is responsible for Cas tyrosine phosphorylation. While all these phenomena occur early after rhodocytin stimulation in a cAMP-resistant manner, tyrosine phosphorylation of Syk and phospholipase Cgamma2, intracellular Ca(2+) mobilization, and platelet aggregation occur later in a cAMP-sensitive manner. Cytochalasin D, which interferes with actin polymerization and blocks receptor clustering, inhibits all the rhodocytin-mediated signals we examined in this study. We suggest that rhodocytin, by clustering GPIa/IIa, activates GPIa/IIa-associated Src, which then mediates downstream activation signals.

ADP induces partial platelet aggregation without shape change and potentiates collagen-induced aggregation in the absence of Gαq

Platelets from Gαq knockout mice are unable to aggregate in response to physiological agonists like adenosine 5′-diphosphate (ADP), thromboxane A2, thrombin, or collagen, although shape change still occurs in response to all of these agonists except ADP. ADP-induced platelet aggregation results from simultaneous activation of the purinergic P2Y1receptor coupled to calcium mobilization and shape change and of a distinct P2 receptor, P2cyc, coupled through Gi to adenylyl cyclase inhibition, which is responsible for completion and amplification of the response. P2cyc could be the molecular target of the antithrombotic drug clopidogrel and the adenosine triphosphate (ATP) analogs AR-C69931MX, AR-C67085, and AR-C66096. The aim of the present study was to determine whether externally added ADP could still act through the Gi pathway in Gαq-deficient mouse platelets and thereby amplify the residual responses to agonists such as thrombin or collagen. It was found that (1) ADP and adrenaline still inhibited cyclic AMP accumulation in Gαq-deficient platelets; (2) both agonists restored collagen- but not thrombin-induced aggregation in these platelets; (3) the effects of ADP were selectively inhibited in vitro by the ATP analog AR-C69931MX and ex vivo by clopidogrel and hence were apparently mediated by the P2cyc receptor; and (4) high concentrations of ADP (100 μmol/L) induced aggregation without shape change in Gαq-deficient platelets through activation of P2cyc. Since adrenaline was not able to induce platelet aggregation even at high concentrations, we conclude that the effects of ADP mediated by P2cyc are not restricted to the inhibition of adenylyl cyclase through Gi2.

ADP induces partial platelet aggregation without shape change and potentiates collagen-induced aggregation in the absence of Gαq

Platelets from Gαq knockout mice are unable to aggregate in response to physiological agonists like adenosine 5′-diphosphate (ADP), thromboxane A2, thrombin, or collagen, although shape change still occurs in response to all of these agonists except ADP. ADP-induced platelet aggregation results from simultaneous activation of the purinergic P2Y1receptor coupled to calcium mobilization and shape change and of a distinct P2 receptor, P2cyc, coupled through Gi to adenylyl cyclase inhibition, which is responsible for completion and amplification of the response. P2cyc could be the molecular target of the antithrombotic drug clopidogrel and the adenosine triphosphate (ATP) analogs AR-C69931MX, AR-C67085, and AR-C66096. The aim of the present study was to determine whether externally added ADP could still act through the Gi pathway in Gαq-deficient mouse platelets and thereby amplify the residual responses to agonists such as thrombin or collagen. It was found that (1) ADP and adrenaline still inhibited cyclic AMP accumulation in Gαq-deficient platelets; (2) both agonists restored collagen- but not thrombin-induced aggregation in these platelets; (3) the effects of ADP were selectively inhibited in vitro by the ATP analog AR-C69931MX and ex vivo by clopidogrel and hence were apparently mediated by the P2cyc receptor; and (4) high concentrations of ADP (100 μmol/L) induced aggregation without shape change in Gαq-deficient platelets through activation of P2cyc. Since adrenaline was not able to induce platelet aggregation even at high concentrations, we conclude that the effects of ADP mediated by P2cyc are not restricted to the inhibition of adenylyl cyclase through Gi2.

Micromolar Ca2+ concentrations are essential for Mg2+-dependent binding of collagen by the integrin alpha 2beta 1 in human platelets

Integrin receptor alpha(2)beta(1) requires micromolar Ca(2+) to bind to collagen and to the peptide GPC(GPP)(5)GFOGER(GPP)(5)GPC (denoted GFOGER-GPP, where O represents hydroxyproline), which contains the minimum recognition sequence for the collagen-binding alpha(2) I-domain (Knight, C. G., Morton, L. F., Peachey, A. R., Tuckwell, D. S., Farndale, R. W., and Barnes, M. J. (2000) J. Biol. Chem. 275, 35-40). Platelet adhesion to these ligands is completely dependent on alpha(2)beta(1) in the presence of 2 mm Mg(2+). However, we show here that this interaction was abolished in the presence of 25 microm EGTA. Adhesion of Glanzmann's thrombasthenic platelets, which lack the fibrinogen receptor alpha(IIb)beta(3), was also inhibited by micromolar EGTA. Mg(2+)-dependent adhesion of platelets was restored by the addition of 10 microm Ca(2+), but millimolar Ca(2+) was inhibitory. Binding of isolated alpha(2)beta(1) to GFOGER-GPP was 70% inhibited by 50 microm EGTA but, as with intact platelets, was fully restored by the addition of micromolar Ca(2+). 2 mm Ca(2+) did not inhibit binding of isolated alpha(2)beta(1) to collagen or to GFOGER-GPP. Binding of recombinant alpha(2) I-domain was not inhibited by EGTA, nor did millimolar Ca(2+) inhibit binding. Our data suggest that high affinity Ca(2+) binding to alpha(2)beta(1), outside the I-domain, is essential for adhesion to collagen. This is the first demonstration of a Ca(2+) requirement in alpha(2)beta(1) function.

Contraction of collagen matrices mediated by alpha2beta1A and alpha(v)beta3 integrins

The (beta)1-null fibroblastic cell line GD25 and its derivatives were studied to gain an understanding of the roles of (beta)1 and (beta)3 integrins in the initial (1-hour) contraction of collagen gels. Stable transfectants of GD25 cells expressing the (beta)1A splice variant of (beta)1 ((beta)1A-GD25) did not express (alpha)2(beta)1A and did not adhere to collagen. After transfection of (alpha)2 into (beta)1A-GD25 cells, the (alpha)2(beta)1A-GD25 transfectants contracted collagen gels in the presence of serum, whereas (beta)1A-GD25 cells did not. The GD25 parental cells, however, also contracted collagen gels. Collagen gel contraction by GD25 cells was blocked by antibodies to (alpha)v(beta)3 or a RGD-containing peptide, indicating that (alpha)v(beta)3 is the integrin responsible for mediation of contraction by GD25 cells. Collagen gel contraction by (alpha)2(beta)1A-GD25 cells was not inhibited by antibodies to (alpha)v(beta)3 or RGD-containing peptide, but was inhibited by anti-(alpha)2 antibody. Flow cytometry demonstrated negligible expression of (alpha)v(beta)3 by (beta)1A-GD25 and (alpha)2(beta)1A-GD25 cells when compared to GD25 cells. Platelet derived growth factor (PDGF) and sphingosine-1-phosphate (S1P) enabled gel contraction by (alpha)2(beta)1A-GD25 and GD25 cells, respectively, in the absence of serum. PDGF-stimulated contraction by (alpha)2(beta)1A-GD25 cells was attenuated in the presence of inhibitors of phosphatidylinositol-3-kinase whereas such inhibitors had no effect on S1P-stimulated contraction by GD25 cells. These experiments using the (beta)1-null GD25 cells and (beta)1A and (alpha)2(beta)1A transfectants demonstrate that (alpha)2(beta)1A and (alpha)v(beta)3 independently mediate collagen gel contraction and are regulated by different serum factors and signaling pathways.

Protein disulfide isomerase mediates integrin-dependent adhesion

Cell adhesion is mediated by the integrin adhesion receptors. Receptor-ligand interaction involves conformational changes in the receptor, but the underlying mechanism remains unclear. Our earlier work implied a role for sulfhydryls in integrin response to ligand binding in the intact blood platelet. We now show that non-penetrating blockers of free sulfhydryls inhibit beta(1) and beta(3) integrin-mediated platelet adhesion regardless of the affinity state of the integrin. Removal of the inhibitors prior to adhesion fully restores adhesion despite the irreversible nature of inhibitor-thiol interaction, indicating sulfhydryl exposure in response to adhesion. We further show that blocking protein disulfide isomerase (PDI) inhibits adhesion. These data indicate that: (a) ecto-sulfhydryls are necessary for integrin-mediated platelet adhesion; (b) disulfide exchange takes place during this process; (c) surface PDI is involved in integrin-mediated adhesion.

Association between diabetic retinopathy and genetic variations in α2β1 integrin, a platelet receptor for collagen

Platelets might be involved in the pathogenesis of diabetic microangiopathy. Wide interindividual variations in the density of a platelet collagen receptor (2β1 integrin or glycoprotein Ia/IIa) are reportedly associated with polymorphism(s) in the gene encoding the  subunit of the receptor, including a Bgl II polymorphism in intron 7. The aim of the present study was to determine the relationship between the Bgl II polymorphism and the susceptibility to diabetic microangiopathy. A case-control study comparing 227 patients with type II diabetes mellitus (119 with versus 108 without diabetic retinopathy) as well as 169 nondiabetic subjects demonstrated that genotypes with Bgl II (+) allele had a significant increase in the risk for retinopathy. The odds ratio for Bgl II (+/+) to Bgl II (−/−) was 3.41 (95% CI, 1.49-7.78, P = .0036) when analysis was confined to those with a disease duration of diabetes of 10 years or more. The present study suggests that the presence of a Bg II (+) allele is a genetic risk factor for diabetic retinopathy.

Defective platelet aggregation and increased resistance to thrombosis in purinergic P2Y(1) receptor-null mice

ADP is a key agonist in hemostasis and thrombosis. ADP-induced platelet activation involves the purinergic P2Y(1) receptor, which is responsible for shape change through intracellular calcium mobilization. This process also depends on an unidentified P2 receptor (P2cyc) that leads to adenylyl cyclase inhibition and promotes the completion and amplification of the platelet response. P2Y(1)-null mice were generated to define the role of the P2Y(1) receptor and to determine whether the unidentified P2cyc receptor is distinct from P2Y(1). These mice are viable with no apparent abnormalities affecting their development, survival, reproduction, or the morphology of their platelets, and the platelet count in these animals is identical to that of wild-type mice. However, platelets from P2Y(1)-deficient mice are unable to aggregate in response to usual concentrations of ADP and display impaired aggregation to other agonists, while high concentrations of ADP induce platelet aggregation without shape change. In addition, ADP-induced inhibition of adenylyl cyclase still occurs, demonstrating the existence of an ADP receptor distinct from P2Y(1). P2Y(1)-null mice have no spontaneous bleeding tendency but are resistant to thromboembolism induced by intravenous injection of ADP or collagen and adrenaline. Hence, the P2Y(1) receptor plays an essential role in thrombotic states and represents a potential target for antithrombotic drugs.

Studies of adhesion-dependent platelet activation: distinct roles for different participating receptors can be dissociated by proteolysis of collagen

The molecular differences between native-type collagen type I fibrils (NC) and their pepsinated monomers (PC) were used to uncover receptors involved in platelet-collagen interaction along the adhesion-activation axis. The platelet-depositing capacity of NC and PC under blood flow and their adhesive properties and respective morphologies, aggregation, procoagulant capacity, and tyrosine phosphorylation were compared under different cationic milieus, including or excluding the glycoprotein (GP) Ia/IIa. NC was consistently a more preferable and activating substrate than PC during flow (5 minutes) and in platelet aggregation. In PPACK-treated blood, both NC (3.3-fold) and PC (2.7-fold) increased platelet attachment on elevation of the shear rate from 500 to 1640 s(-1), whereas in citrated blood, adhesion and thrombus growth on PC were negligible under the high shear rate, unlike on NC (1.9-fold increase). The complete lack of platelet deposition on PC in citrated blood could be overcome by restoring physiological Mg(2+) concentration, and in contrast to NC, platelets interacting with PC were highly dependent on Mg(2+) during adhesion, aggregation, and procoagulant response. Monoclonal antibody (mAb 131.7) against GP IV inhibited platelet deposition to NC in citrated blood (2 minutes) by 49%, which was not further increased by coincubation with mAb against GP Ia (6F1). These results stress the importance of GP Ia/IIa in shear-resistant platelet deposition on collagen monomers. In native fibers, however, the preserved quaternary structure with telopeptides activates additional platelet receptors capable of substituting GP Ia/IIa and GP IV.

Identification and characterization of zebrafish thrombocytes

To analyse primary haemostasis in the zebrafish we have identified and characterized the zebrafish thrombocyte by morphologic, immunologic and functional approaches. Novel methods were developed for harvesting zebrafish blood with preservation of thrombocytes, and assaying whole blood adhesion/aggregation responses in microtitre plates. Light and electron microscopy of the thrombocyte illustrated morphological characteristics including the formation of aggregates, pseudopodia, and surface-connected vesicles analagous to the platelet canalicular system. Immunostaining with polyclonal antisera versus human platelet glycoproteins demonstrated the presence of glycoprotein Ib and IIb/IIIa-like complexes on the thrombocyte surface. Whole blood assays for adhesion/aggregation and ATP release showed ristocetin-induced adhesion without ATP release, and platelet agonist (collagen, arachidonic acid) induced aggregation with ATP release. Blood harvested from zebrafish treated with aspirin demonstrated inhibition of arachidonic acid induced aggregation and agonist induced ATP release, consistent with at least partial dependence on an intact cyclo oxygenase pathway. The combined morphologic immunologic and functional evidence suggest that the zebrafish thrombocyte is the haemostatic homologue of the mammalian platelet. Conservation of major haemostatic pathways involved in platelet function and coagulation suggests that the zebrafish is a relevant model for mammalian haemostasis and thrombosis.

Review article: platelet-collagen interactions: membrane receptors and intracellular signalling pathways

Platelet adhesion to and activation by exposed subendothelial collagen plays a critical role in normal haemostasis and pathological thrombosis. Recent advances in elucidating the mechanisms underlying platelet-collagen interaction support a 'two-site, two-step' model. Direct platelet binding to integrin alpha2beta1 mainly sustains adhesion and allows recognition of glycoprotein VI. The latter interaction is responsible for characteristic intracellular signalling events leading to p72Syk and PLCgamma2 activation. The present review describes the known collagen receptors on platelets and discusses the current understanding of signal transduction promoted by collagen.

Effects of ethanol on platelet responses associated with adhesion to collagen

Adhesion of platelets to collagen in damaged blood vessels or ruptured atherosclerotic plaques is important in hemostasis and arterial thrombosis. Adhesion to collagen results in secretion of granule contents and formation of thromboxane A2; thromboxane A2 and released ADP synergistically promote aggregation around platelets adherent to collagen. Ethanol inhibits collagen-induced platelet aggregation, secretion, arachidonate mobilization, and thromboxane A2 formation but does not inhibit platelet adhesion to de-endothelialized rabbit aortae. We investigated whether ethanol affects the initial signalling events and responses of platelets adherent to collagen, independent of the actions of secondary agonists. Suspensions of washed human platelets, labelled by incorporation of [3H]oleate into phospholipids, were used to measure platelet adhesion to collagen by a filtration method; studies were done in the presence of an ADP-removing system and blockers of receptors for thromboxane A2, platelet-activating factor, serotonin, and fibrinogen. Ethanol (87 mM) did not affect the rate or extent of platelet adhesion to collagen or secretion of [14C]serotonin from prelabelled platelets adherent to collagen, but ethanol did inhibit thromboxane A2 formation. Previous studies showed that ethanol does not affect platelet stimulation by arachidonate, leading to the suggestion that reduced mobilization of arachidonate, rather than inhibition of its conversion to thromboxane A2, is responsible for inhibition by ethanol of thromboxane A2 formation. Here, we show by a gel mobility shift assay and immunoblotting, that ethanol delays the collagen-induced increase in the phosphorylation of cytosolic phospholipase A2, the enzyme responsible for arachidonate mobilization. However, ethanol has no effect on collagen-induced tyrosine phosphorylation of phospholipase Cgamma2, determined by immunoprecipitation and immunoblotting. Thus, ethanol's effect on signal transduction in collagen-adherent platelets occurs distal to phosphorylation of phospholipase Cgamma2 but proximal to phosphorylation of cytosolic phospholipase A2.

Effects of cathepsin G pretreatment of platelets on their subsequent responses to aggregating agents

Cathepsin G, a proteolytic enzyme from activated leukocytes, can interact with platelets during inflammation and thrombosis. Platelets that have been exposed to cathepsin G in thrombi may recirculate if they are freed during fibrinolysis. To determine whether some of the subsequent functions of such platelets would be impaired, we investigated the responses of cathepsin G-pretreated platelets to agonists that they would encounter in the circulation. Suspensions of washed human platelets were labeled with [14C]serotonin and resuspended in Tyrode-albumin solution (with 2 mM Ca2+ and apyrase). After 15 minute incubation with 400 nM cathepsin G at 37 degrees C, 52+/-3% of [14C]serotonin had been released, and glycoprotein Ib was degraded. The platelets were washed and resuspended in fresh medium to remove cathepsin G and released materials. Ristocetin-induced agglutination was abolished, indicating that the binding site for von Willebrand Factor on glycoprotein Ib had been removed. Aggregation and release of residual [14C]serotonin in response to 0.1-1.0 U/mL thrombin was blocked or greatly reduced by the cathepsin G pretreatment. This inhibition is probably largely due to cleavage by cathepsin G of some of the protease-activated receptors at the C-terminal side of Ser42 so that the tethered ligand is lost. Pretreatment with cathepsin G did not affect responses to ADP or a low concentration of platelet-activating factor in the presence of fibrinogen, indicating that receptors for these agonists were unaffected and that the function of the fibrinogen receptor, GPIIb/IIIa was unchanged. Responses to cathepsin G, the thrombin receptor-activating peptide SFLLRN, collagen, or the thromboxane A2 mimetic U46619 were partially inhibited, even in the presence of added fibrinogen. Platelet adhesion to a collagen-coated surface was 51+/-7% inhibited, which may indicate cleavage of a collagen receptor or receptors; this may partly account for strong inhibition of collagen-induced aggregation and release of granule contents; additionally, as shown by inhibition of responses to U46619, the function of the thromboxane A2 receptor may be compromised. Thus, although cathepsin G activates platelets, if they recirculate after interaction with it, their subsequent adhesion to damaged vessel walls, aggregation, and release of granule contents induced by thrombin and collagen will be diminished.

Collagens and atherosclerosis

Smooth muscle cells in the atherosclerotic lesions of diseased arteries produce new extracellular matrix, largely collagenous in nature, which is responsible in part for the occlusion of the vessel lumen by the atherosclerotic plaque. These smooth muscle cells express a different phenotype, responsive to growth factors, to that of the differentiated, nondividing contractile cell in the media. Specific collagens may be involved in the regulation of phenotype and in the migration of the cells to the site of lesion growth. Collagens may also be involved in the calcification of lesions, in the retention of low-density lipoprotein in the vessel wall and in smooth muscle cell survival. Glycation of collagen may promote atherogenesis. Effects as summarized in this short review, are not always, at first sight, consistent. The following points should be kept in mind, though, when considering the response of a cell to collagen. Any effect may be governed not just by the identity of the collagen type as such but by its state of polymerization: monomeric collagen, for instance, whether in solution or immobilized on plastic, may express different effects to the same collagen type when presented in its native polymerized state, e.g., as fibers. The precise identity of the cell and its location may be important: SMCs in secondary culture may not necessarily respond to any given collagen exactly as SMCs within the lesion or possess precisely the same properties, albeit both types are regarded as expressing the same (synthetic) phenotype. Effects may not necessarily be directly attributable to collagen, but to some other matrix constituent bound to collagen.

Activation of the GP IIb-IIIa complex induced by platelet adhesion to collagen is mediated by both alpha2beta1 integrin and GP VI

alpha2beta1 integrin, CD36, and GP VI have all been implicated in platelet-collagen adhesive interactions. We have investigated the role of these glycoproteins on activation of the GP IIb-IIIa complex induced by platelet adhesion to type I fibrillar and monomeric collagen under static conditions. In the presence of Mg2+, platelet adhesion to fibrillar collagen induced activation of the GP IIb-IIIa complex and complete spreading. Anti-alpha2beta1 integrin and anti-GP VI antibodies inhibited the activation of the GP IIb-IIIa complex by about 40 and 50%, respectively, at 60 min although minimal inhibitory effects on adhesion were seen. Platelet spreading was markedly reduced by anti-alpha2beta1 integrin antibody. The combination of anti-alpha2beta1 integrin with anti-GP VI antibody completely inhibited both platelet adhesion and activation of the GP IIb-IIIa complex. Anti-CD36 antibody had no significant effects on platelet adhesion, spreading, and the activation of the GP IIb-IIIa complex at 60 min. Aspirin and the thromboxane A2 receptor antagonist SQ29548 inhibited activation of the GP IIb-IIIa complex about 30% but had minimal inhibitory effect on adhesion. In the absence of Mg2+, there was significant activation of the GP IIb-IIIa complex but minimal spreading was observed. Anti-GP VI antibody completely inhibited adhesion whereas no effect was observed with anti-alpha2beta1 integrin antibody. Anti-CD36 antibody partially inhibited both adhesion and the activation of the GP IIb-IIIa complex. Platelet adhesion to monomeric collagen, which requires Mg2+ and is exclusively mediated by alpha2beta1 integrin, resulted in partial activation of the GPIIb-IIIa complex and spreading. No significant effects were observed by anti-CD36 and anti-GP VI antibodies. These results suggest that both alpha2beta1 integrin and GP VI are involved in inside-out signaling leading to activation of the GP IIb-IIIa complex after platelet adhesion to collagen and generation of thromboxane A2 may further enhance expression of activated GP IIb-IIIa complexes.

The number of platelet glycoprotein Ia molecules is associated with the genetically linked 807 C/T and HPA-5 polymorphisms

BACKGROUND

The neutral 807 C/T (Phe224) polymorphism (807 C/T polymorphism) of the glycoprotein (GP)Ia gene has been recently associated with the number of GPIa molecules on the platelet surface. The association of the number of GPIa molecules with other GPIa polymorphisms, such as HPA-5 (Glu/Lys505) (HPA-5 polymorphism), involved in alloimmune thrombocytopenias is unknown.

STUDY DESIGN AND METHODS

The association of the HPA-5 polymorphism with the number of GPIa molecules on the platelet surface in 159 white blood donors was investigated. The genetic linkage between the HPA-5 and the 807 C/T polymorphisms in 316 individuals was also determined.

RESULTS

Both the 807 C/T and HPA-5 polymorphisms correlate with the number of GPIa molecules on the platelet surface. The 807 T and HPA-5b alleles are associated with increased numbers of GPIa molecules on the platelet surface. Moreover, the HPA-5b allele is genetically linked to 15.8 percent of the 807 C alleles. Therefore, the number of GPIa molecules on the platelet surface is dependent on both GPIa polymorphisms as follows: 807 T/T, HPA-5 a/a > 807 C/T, HPA-5 a/b > 807 C/T, HPA-5 a/a > 807 C/C, HPA-5 a/b > 807 C/C, HPA-5 a/a.

CONCLUSION

Two GPIa polymorphisms (807 C/T and HPA-5) responsible for the variability in the numbers of GPIa/IIa molecules on the platelet surface in whites have been identified. Despite the genetic linkage between the two polymorphisms, their influence on the number of GPIa molecules on the platelet surface may occur through different mechanisms.

Role of autocrine stimulation on the effects of cyclic AMP on protein and lipid phosphorylation in collagen-activated and thrombin-activated platelets

We compared several responses in thrombin-stimulated and collagen (type I)-stimulated platelets with and without forskolin and inhibitors of autocrine stimulation (IAS: an ADP-removing system of creatine phosphate/creatine phosphokinase, Arg-Gly-Asp-Ser peptide to prevent fibrinogen/fibronectin binding to GPIIb/IIIa, SQ 29.548 as a thromboxane A2 receptor antagonist, cyproheptadine as a serotonin receptor antagonist, BN 52021 as a platelet-activating factor receptor antagonist). The pattern of tyrosine-phosphorylated proteins, the phosphorylation of lipids in the polyphosphoinositide cycle and phosphorylation of pleckstrin (P47) were studied as markers for signal-transducing responses, exposure of CD62 (P-selectin) and CD63 (Glycoprotein 53), as well as secretion of ADP + ATP and beta-N-acetyl-glycosaminidase were studied as final activation responses. Clear differences between thrombin-stimulated and collagen-stimulated platelets were observed. First, practically all protein-tyrosine phosphorylation induced by thrombin was inhibited by IAS, while a partial inhibition was observed for collagen; the phosphorylation due to collagen alone was apparently stimulated by elevation of cAMP. Secondly, the other responses to thrombin were inhibited by increased levels of cAMP, independent of autocrine stimulation. In contrast, only the autocrine part of the collagen-induced responses was inhibited by elevation of cAMP. Thus, the inhibition by elevated cAMP seen in collagen-stimulated platelets seems to be due to removal of the G-protein-mediated activation from secreted autocrine stimulators either by IAS or forskolin. The remaining activity is a pure collagen effect which is not affected by elevated levels of cAMP.

Identification in collagen type I of an integrin alpha2 beta1-binding site containing an essential GER sequence

The collagen type I-derived fragment alpha1(I)CB3 is known to recognize the platelet collagen receptor integrin alpha2beta1 as effectively as the parent collagen, although it lacks platelet-aggregatory activity. We have synthesized the fragment as seven overlapping peptides that spontaneously assemble into triple helices. On the basis of their capacity to bind purified alpha2 beta1 and the recombinant alpha2 A-domain, and their ability to support alpha2 beta1-mediated cell adhesion, we identified two peptides, CB3(I)-5 and -6, which contain an alpha2 beta1 recognition site. Synthesis of the peptide CB3(I)-5/6, containing the overlap sequence between peptides 5 and 6, allowed us to locate the binding site within the 15-residue sequence, GFP*GERGVEGPP*GPA (where P* represents hydroxyproline), corresponding to residues 502-516 of the collagen type I alpha1 chain. The Glu and Arg residues in the GER triplet were found to be essential for recognition since substitution of either residue with Ala caused a loss of alpha2 A-domain binding. By contrast, substitution of the Glu in GVE did not reduce binding, but rather enhanced it slightly. We were unable to detect significant recognition of alpha2 beta1 by the peptide CB3(I)-2 containing the putative alpha2 beta1 recognition sequence DGEA. Peptides CB3(I)-1 to -6, together with peptide CB3(I)-5/6, exhibited good platelet-aggregatory activity, in some cases better than collagen. However, peptide CB3(I)-7 was inactive, suggesting the presence of an inhibitory element that might account for the lack of aggregatory activity of the parent alpha1(I)CB3 fragment.

Flow cytometric analysis of platelet activation by different collagen types present in the vessel wall

The interaction of platelets with collagens of the vessel wall is a critical event in primary haemostasis. Although numerous studies have examined the ability of various collagen types to support platelet adhesion, little is known concerning the relative ability of different collagens to elicit specific activation markers in platelets. In this report, flow cytometric analysis has been utilized to evaluate the ability of various native collagen types to elicit secondary activation events in human platelets. Collagen types I, III, V and VI induced alpha-granule secretion and up-regulation of cell surface glycoprotein (GP) IIb/IIIa. In contrast, collagen type IV did not elicit these responses in the concentration ranges examined. Dose-response curves for alpha-granule secretion induced by the various collagen types indicated that human type III and human type I collagens were less effective than human type V, human type VI and calf skin type I. In addition, the ability of these various collagens to activate GPIIb/IIIa to its ligand binding conformation was even more heterogenous with only human type VI and calf skin type I readily promoting this transition. These data demonstrate that flow cytometric analysis of collagen-induced platelet activation is feasible and that collagen-mediated alpha-granule secretion and membrane glycoprotein redistribution in human platelets are separate events from activation of GPIIb/IIIa.