Structure-function relationship of the platelet glycoprotein VI (GPVI) receptor: does it matter if it is a dimer or monomer?

GPVI is a critical signaling receptor responsible for collagen-induced platelet activation and a promising anti-thrombotic target in conditions such as coronary artery thrombosis, ischemic stroke, and atherothrombosis. This is due to the ability to block GPVI while having minimal effects on hemostasis, making it a more attractive target over current dual-antiplatelet therapy (DAPT) with acetyl salicylic acid and P2Y12 inhibitors where bleeding can be a problem. Our current understanding of how the structure of GPVI relates to function is inadequate and recent studies contradict each other. In this article, we summarize the structure-function relationships underlying the activation of GPVI by its major ligands, including collagen, fibrin(ogen), snake venom toxins and charged exogenous ligands such as diesel exhaust particles. We argue that contrary to popular belief dimerization of GPVI is not required for binding to collagen but serves to facilitate binding through increased avidity, and that GPVI is expressed as a mixture of monomers and dimers on resting platelets, with binding of multivalent ligands inducing higher order clustering.

Differential Inhibition of Human Atherosclerotic Plaque-Induced Platelet Activation by Dimeric GPVI-Fc and Anti-GPVI Antibodies: Functional and Imaging Studies

Background

Glycoprotein VI (GPVI) is the essential platelet collagen receptor in atherothrombosis, but its inhibition causes only a mild bleeding tendency. Thus, targeting this receptor has selective antithrombotic potential.

Objectives

This study sought to compare compounds interfering with platelet GPVI–atherosclerotic plaque interaction to improve current antiatherothrombotic therapy.

Methods

Human atherosclerotic plaque–induced platelet aggregation was measured in anticoagulated blood under static and arterial flow conditions (550/s, 1,100/s, and 1,500/s). Inhibition by dimeric GPVI fragment crystallizable region of IgG (Fc) masking GPVI binding sites on collagen was compared with that of 3 anti-GPVI antibodies: BLO8-1, a human domain antibody; 5C4, a fragment antigen-binding (Fab fragment) of monoclonal rat immunoglobulin G; and m-Fab-F, a human recombinant sFab against GPVI dimers.

Results

GPVI-Fc reduced plaque-triggered platelet aggregation in static blood by 51%, BLO8-1 by 88%, and 5C4 by 93%. Under arterial flow conditions, BLO8-1 and 5C4 almost completely inhibited platelet aggregation while preserving platelet adhesion on plaque. Inhibition by GPVI-Fc, even at high concentrations, was less marked but increased with shear rate. Advanced optical imaging revealed rapid persistent GPVI-Fc binding to collagen under low and high shear flow, upstream and downstream of plaque fragments. At low shear particularly, platelets adhered in plaque flow niches to GPVI-Fc–free segments of collagen fibers and recruited other platelets onto aggregates via ADP and TxA2 release.

Conclusions

Anti-GPVI antibodies inhibit atherosclerotic plaque-induced platelet aggregation under static and flow conditions more effectively than GPVI-Fc. However, potent platelet inhibition by GPVI-Fc at a higher shear rate (1,500/s) suggests localized antithrombotic efficacy at denuded or fissured stenotic high-risk lesions without systemic bleeding. The compound-specific differences have relevance for clinical trials targeting GPVI-collagen interaction combined with established antiplatelet therapies in patients with spontaneous plaque rupture or intervention-associated plaque injury.

Local delivery of soluble platelet collagen receptor glycoprotein VI inhibits thrombus formation in vivo

Platelet-mediated thrombus formation at the site of vascular injury is a major trigger for thrombo-ischemic complications after coronary interventions. The platelet collagen receptor glycoprotein VI (GPVI) plays a critical role in the initiation of arterial thrombus formation. Endothelial denudation of the right carotid artery in rabbits was induced through balloon injury. Subsequently, local delivery of soluble, dimeric fusion protein of GPVI (GPVI-Fc) (n = 7) or control Fc (n = 7) at the site of vascular injury was performed with a modified double-balloon drug-delivery catheter. Thrombus area within the injured carotid artery was quantified using a computer-assisted image analysis and was used as index of thrombus formation. The extent of thrombus formation was significantly reduced in GPVI-Fc- compared with control Fc-treated carotid arteries (relative thrombus area, GPVI-Fc vs. Fc: 9.3 +/- 4.2 vs. 2.3 +/- 1.7, p < 0.001). Local delivery of soluble GPVI resulted in reduced thrombus formation after catheter-induced vascular injury. These data suggest a selective pharmacological modulation of GPVI-collagen interactions to be important for controlling onset and progression of pathological arterial thrombosis, predominantly or even exclusively at sites of injured carotid arteries in the absence of systemic platelet therapy.

Relative antithrombotic effect of soluble GPVI dimer compared with anti-GPVI antibodies in mice

Glycoprotein VI (GPVI) is an essential platelet collagen receptor; therefore, the inhibition of GPVI-collagen interactions may be an attractive antithrombotic strategy. We have previously shown that targeting of GPVI with antibodies leads to the depletion of the receptor and to long-term antithrombotic protection in mice. An alternative agent to interfere with GPVI-collagen interactions might be soluble GPVI acting as a competitive inhibitor, thereby averting undesired effects on platelets. To test this, we expressed soluble dimeric human GPVI, comprising the extracellular domain of the receptor fused to the human immunoglobulin Fc domain (GPVI-Fc), and compared its antithrombotic potential with that of anti-GPVI antibodies in mice. In contrast to a recent report, we found by intravital fluorescence microscopy and ultrasonic flow measurements that GPVI-Fc had no effect on platelet adhesion and thrombus formation at the injured arterial wall, whereas anti-GPVI antibodies profoundly inhibited these processes. Similar results were obtained with a fusion protein comprising the extracellular domain of mouse GPVI and human IgG-Fc. This indicates that direct targeting of GPVI provides significantly stronger protection against arterial thrombosis than soluble GPVI dimer.

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.

A critical role for phospholipase Cgamma2 in alphaIIbbeta3-mediated platelet spreading

The interaction of fibrinogen with the integrin alphaIIbbeta3 plays a crucial role in platelet adhesion and platelet activation leading to the generation of intracellular signals that nucleate the reorganization of the cytoskeleton. Presently, we have only a limited understanding of the signaling cascades and effector proteins through which changes in the cytoskeletal architecture are mediated. The present study identifies phospholipase Cgamma2 (PLCgamma2) as an important target of the Src-dependent signaling cascade regulated by alphaIIbbeta3. Real time phasecontrast microscopy is used to show that formation of filopodia and lamellapodia in murine platelets on a fibrinogen surface is dramatically inhibited in the absence of PLCgamma2. Significantly, the formation of these structures is mediated by Ca2+ elevation and activation of protein kinase C, both directly regulated by PLC activity. With the involvement of Syk, SLP-76, and Btk, alphaIIbbeta3-induced PLCgamma2 activation partly overlaps with the pathway used by the collagen receptor glycoprotein VI. Important differences, however, exist between the two signaling cascades in that activation of PLCgamma2 by alphaIIbbeta3 is unaltered in murine platelets, which lack the FcR gamma-chain or the adaptor LAT, but is abolished in the presence of cytochalasin D. Therefore, PLCgamma2 plays not only a crucial role in activation of alphaIIbbeta3 by collagen receptors but also in alphaIIbbeta3-mediated responses.

Murine GPVI stimulates weak integrin activation in PLCgamma2-/- platelets: involvement of PLCgamma1 and PI3-kinase

Collagen stimulates platelet activation through a tyrosine kinase-based pathway downstream of the glycoprotein VI (GPVI)-Fc receptor (FcR) gamma-chain complex. Genetic ablation of FcR gamma-chain results in a complete inhibition of aggregation to collagen. In contrast, a steady increase in light transmission is induced by collagen in phospholipase Cgamma2-deficient (PLCgamma2-/-) platelets in a Born aggregometer, indicating a weak level of activation. This increase is inhibited partially in the presence of an alpha2beta1-blocking antibody or an alphaIIbbeta3 antagonist and completely by a combination of the 2 inhibitors. It is also abolished by the Src kinase inhibitor PP1 and reduced in the presence of the phosphatidylinositol (PI) 3-kinase inhibitor wortmannin. The GPVI-specific agonists convulxin and collagen-related peptide (CRP) also stimulate weak aggregation in PLCgamma2-/- platelets, which is inhibited by wortmannin and PP1. Collagen and CRP stimulate tyrosine phosphorylation of PLCgamma1 at its regulatory site, Tyr 783, in murine but not in human platelets through a Src kinase-dependent pathway. Adhesion of PLCgamma2-/- platelets to a collagen monolayer is severely reduced at a shear rate of 800 s-1, relative to controls, whereas it is abolished in FcR gamma-chain-/- platelets. These results provide strong evidence that engagement of GPVI stimulates limited integrin activation in PLCgamma2-/- platelets via PLCgamma1 and PI3-kinase.

Role of mast cells and pro-inflammatory mediators on the intestinal secretion induced by cholera toxin

Recent data suggest that diarrhea caused by Vibrio cholerae involves a pro-inflammatory mediators release, such as cytokines, prostaglandin and nitric oxide. The aim of this study was to investigate the role of mast cells and their mediators in the intestinal secretion induced by cholera toxin. We examined the dose responses, time course and role of mast cells and pro-inflammatory mediators in cholera toxin intestinal secretory response, in vivo. Cholera toxin caused a dose-dependent secretion, in ligated small intestine loops, at 18 h. Rats treated with 48/80 compound or ketotifen had a significant decrease in the intestinal secretory response. Cholera toxin secretion was significantly reduced by an unspecific histamine/serotonin receptor antagonist, histamine receptor antagonist, phospholipase A2 and cyclooxygenase inhibitors, platelet-activating factor (PAF) receptor antagonists and TNF-alpha synthesis blockers. On the other hand, pretreatment with a specific serotonin receptor antagonist and lipoxygenase inhibitors failed to block this effect. Analysis of the intestinal fluid from rats injected with cholera toxin, revealed that cholera toxin induces the release of IL-1beta and TNF-alpha into fluid. The data suggest that, at least in part, mast cells are involved in cholera toxin-induced secretion, as well as point to the importance of histamine, prostaglandins, PAF, IL-1beta and TNF-alpha in this process.

PKD: a new protein kinase C-dependent pathway in platelets

Protein kinase D (PKD, also known as PKCmu) is closely related to the protein kinase C superfamily but is differentially regulated and has a distinct catalytic domain that shares homology with Ca(2+)-dependent protein kinases. PKD is highly expressed in hematopoietic cells and undergoes rapid and sustained activation upon stimulation of immune receptors. PKD is regulated through phosphorylation by protein kinase C (PKC). In the present study, we show that PKD is expressed in human platelets and that it is rapidly activated by receptors coupled to heterotrimeric G-proteins or tyrosine kinases. Activation of PKD is mediated downstream of PKC. Strong agonists such as convulxin, which acts on GPVI, and thrombin cause sustained activation of PKC and PKD, whereas the thromboxane mimetic U46619 gives rise to transient activation of PKC and PKD. Activation of PKD by submaximal concentrations of phospholipase C-coupled receptor agonists is potentiated by G(i)-coupled receptors (eg, adenosine diphosphate and epinephrine). This study shows that PKD is rapidly activated by a wide variety of platelet agonists through a PKC-dependent pathway. Activation of PKD enables phosphorylation of a distinct set of substrates to those targeted by PKC in platelets.

Platelet activating factor-induced apoptosis is inhibited by ectopic expression of the platelet activating factor G-protein coupled receptor

The pro-inflammatory lipid mediator platelet activating factor (PAF: 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) accumulates in ischemia, epilepsy, and human immunodeficiency virus-1-associated dementia and is implicated in neuronal loss. The present study was undertaken to establish a role for its G-protein coupled receptor in regulating neurotoxicity. PC12 cells do not express PAF receptor mRNA as demonstrated by northern analysis and RT-PCR. In the absence of the G-protein coupled receptor, PAF (0.1-1 micro m) triggered chromatin condensation, DNA strand breaks, oligonucleosomal fragmentation, and nuclear disintegration characteristic of apoptosis. Lyso-PAF (0.001-1 micro m), the immediate metabolite of PAF, did not elicit apoptotic death. Concentrations of PAF or lyso-PAF that exceeded critical micelle concentration had physicochemical effects on plasma membrane resulting in necrosis. Apoptosis but not necrosis was inhibited by the PAF antagonist BN52021 (1-100 micro m) but not CV3988 (0.2-20 micro m). Ectopic PAF receptor expression protected PC12 transfectants from ligand-induced apoptosis. PAF receptor-mediated protection was inhibited by CV3988 (1 micro m). These data provide empirical evidence that: (i) PAF can initiate apoptosis independently of its G-protein coupled receptor; (ii) PAF signaling initiated by its G-protein coupled receptor is cytoprotective to PC12 cells; (iii) the pro- and anti-apoptotic effects of PAF on PC12 cells can be pharmacologically distinguished using two different PAF antagonists.

[The role of platelet activating factor (PAF) in physiology and pathology of the central nervous system]

This review describes the role of platelet activating factor (PAF) in the central nervous system injury. Cerebral ischaemia, traumatic injury of central nervous system, metabolic, toxic and degenerative neuropathy, and also the increase in Ca2+ concentration in the cell, are strong stimulators of PAF synthesis and its release from cell membranes. Neurons, glial and microglial cells, monocyte cell populations, macrophages and endothelial cells of blood vessels are the targets of platelet activating factor. The release of PAF leads to ischaemia of nervous tissue, acute traumatic or nontraumatic injuries, degenerative and metabolic nervous system disorders in adults. The use of PAF receptor antagonists prevents partially cell injury in central nervous system and leukocyte adhesion to endothelial cells.

Differential role of glycolipid-enriched membrane domains in glycoprotein VI- and integrin-mediated phospholipase Cgamma2 regulation in platelets

The platelet collagen receptor glycoprotein VI (GPVI) and the fibrinogen receptor integrin alphaIIbbeta3 trigger intracellular signalling cascades involving the tyrosine kinase Syk, the adapter SLP-76 and phospholipase Cgamma2 (PLCgamma2). Similar pathways are activated downstream of immune receptors in lymphocytes, where they have been localized in part to glycolipid-enriched membrane domains (GEMs). Here we provide several lines of evidence that GPVI-mediated tyrosine phosphorylation of PLCgamma2 in platelets is dependent on GEM-organized signalling and utilizes the GEM resident adapter protein LAT (linker for activation of T cells). In sharp contrast, although fibrinogen binding to platelets stimulates alphaIIbbeta3-dependent activation of Syk and tyrosine phosphorylation of SLP-76 and PLCgamma2, it does not utilize GEMs to promote these responses or to support platelet aggregation. These results establish that GPVI and alphaIIbbeta3 trigger distinct patterns of receptor signalling in platelets, leading to tyrosine phosphorylation of PLCgamma2, and they highlight the role of GEMs in compartmentalizing signalling reactions involved in haemostasis.

Glycoprotein VI-mediated platelet fibrinogen receptor activation occurs through calcium-sensitive and PKC-sensitive pathways without a requirement for secreted ADP

Collagen activates platelets by transducing signals through glycoprotein VI (GPVI). It is not clear whether collagen can directly activate fibrinogen receptors on the adherent platelets without a role for positive feedback agonists. We investigated the contribution of secondary G protein signaling to the mechanism of GPVI-stimulated platelet aggregation using the GPVI-selective agonists, convulxin and collagen-related peptide (CRP) as well as collagen. Adenosine diphosphate (ADP) scavengers or ADP receptor antagonists shifted the concentration-response curve slightly to the right at low concentrations of convulxin, whereas platelet aggregation at higher concentrations of convulxin was unaffected by these agents. ADP receptor antagonists shifted the concentration-response curve of collagen- or CRP-induced platelet aggregation to the right at all the concentrations. Protein kinase C inhibitor, Ro 31-8220, or a calcium chelator 5,5'-dimethyl-BAPTA shifted the concentration-response curve of convulxin-induced platelet aggregation to the right. In addition, pretreatment with both Ro 31-8220 and dimethyl-BAPTA resulted in total inhibition of convulxin-mediated aggregation. Blockade of either the calcium- or protein kinase C-regulated pathway leads to inhibition of fibrinogen receptor activation on platelets adherent to collagen, but inhibition of both pathways leads to abolished fibrinogen receptor activation. We conclude that collagen-induced activation of fibrinogen receptor on adherent platelets through GPVI signaling occurs without any significant role for secreted ADP or thromboxane A(2). Furthermore, protein kinase C- and calcium-regulated pathways independently contribute to GPVI-mediated platelet aggregation.

Identification and characterization of a novel member of olfactomedin-related protein family, hGC-1, expressed during myeloid lineage development

We have cloned a novel hematopoietic granulocyte colony-stimulating factor (G-CSF)-induced olfactomedin-related glycoprotein, termed hGC-1 (human G-CSF-stimulated clone-1). mRNA differential display was used in conjunction with a modified two-phase liquid culture system. Cultures were enriched for early precursors of erythroid, myeloid, and megakaryocytic lineages, which were isolated after induction with erythropoietin, G-CSF, and thrombopoietin, respectively. RNA from the enriched cells was subjected to differential display analysis to identify lineage-specific expressed genes. One clone specifically induced by G-CSF, hGC-1, was characterized. The 2861 bp cDNA clone of hGC-1 contained an open reading frame of 1530 nucleotides, translating into a protein of 510 amino acids with a signal peptide and six N-linked glycosylation motifs. The protein sequence of hGC-1 showed it to be a glycoprotein of the olfactomedin family, which includes olfactomedin, TIGR, Noelin-2 and latrophilin-1. Olfactomedin-like genes show characteristic tissue-restricted patterns of expression; the specific tissues expressing these genes differ among the family members. hGC-1 was strongly expressed in the prostate, small intestine, and colon, moderately expressed in the bone marrow and stomach, and not detectable in other tissues. In vitro translation and ex vivo expression showed hGC-1 to be an N-linked glycoprotein. The hGC-1 gene locus mapped to chromosome 13q14.3. Together, our findings indicate that hGC-1 is primarily expressed as an extracellular olfactomedin-related glycoprotein during normal myeloid-specific lineage differentiation, suggesting the possibility of a matrix-related function for hGC-1 in differentiation.

Variable protection of beta 3-integrin--deficient mice from thrombosis initiated by different mechanisms

Platelet integrin alpha IIb beta 3 (GPIIb/IIIa) plays a central role in the initiation of arterial thrombosis, but its contribution to disseminated microvascular thrombosis is less well defined. Therefore, wild-type mice (beta 3(+/+)), beta 3-integrin-deficient mice (beta 3(-/-)), and wild-type mice treated with a hamster monoclonal antibody (1B5) that blocks murine alpha IIb beta 3 function were tested in models of large-vessel and microvascular thrombosis. In the large-vessel model, ferric chloride was used to injure the carotid artery, and the time to thrombosis was measured. In beta 3(+/+) mice, the median time to occlusion was 6.7 minutes, whereas occlusion did not occur in any of the beta 3(-/-) mice tested (P <.001). Fab and F(ab')(2) fragments of 1B5 increased the median time to occlusion. To initiate systemic intravascular thrombosis, prothrombotic agents were administered intravenously, and platelet thrombus formation was monitored by the decrease in circulating platelet count. Three minutes after the injection of adenosine diphosphate (ADP), collagen + epinephrine, or tissue factor, the platelet counts in beta 3(+/+) mice decreased by 289, 424, and 429 x 10(3)/microL, respectively. beta 3(-/-) mice and wild-type mice pretreated with 1B5 Fab (1 mg/kg, IP) were nearly completely protected from the effects of ADP. In contrast, beta 3(-/-) mice were only partially protected from the effects of collagen + epinephrine and minimally protected from the effects of tissue factor. In all cases, less fibrin became deposited in the lungs of beta 3(-/-) mice than in wild-type mice. These results suggest that though alpha IIb beta 3 plays a dominant role in large-vessel thrombosis, it plays a variable role in systemic intravascular thrombosis. (Blood. 2001;98:1055-1062)

Antibody against platelet membrane glycoprotein VI in a patient with systemic lupus erythematosus

Platelet-collagen interaction is important in primary hemostasis and collagen receptors on the platelet surface include membrane glycoprotein (GP) Ia/IIa and VI. Platelets from a 47-year-old woman with systemic lupus erythematosus (SLE) and a mild bleeding symptom showed a defective collagen-induced aggregation and an impaired adhesion to collagen surface. The patient's platelets had a markedly decreased content of GPVI. The patient had an antibody against GPVI in serum and the patient's plasma induced aggregation and release reaction of normal platelets. These findings indicate that GPVI is an important receptor for collagen on the platelet surface, and that anti-GPVI antibody activates the platelets, resulting in aggregation. This is the first documented case of SLE who acquired a platelet-aggregating anti-GPVI antibody.

Platelet adhesion enhances the glycoprotein VI-dependent procoagulant response: Involvement of p38 MAP kinase and calpain

In the final stages of activation, platelets express coagulation-promoting activity by 2 simultaneous processes: exposure of aminophospholipids, eg, phosphatidylserine (PS), at the platelet surface, and formation of membrane blebs, which may be shed as microvesicles. Contact with collagen triggers both processes via platelet glycoprotein VI (GPVI). Here, we studied the capacity of 2 GPVI ligands, collagen-related peptide (CRP) and the snake venom protein convulxin (CVX), to elicit the procoagulant platelet response. In platelets in suspension, either ligand induced full aggregation and high Ca(2+) signals but little microvesiculation or PS exposure. However, most of the platelets adhering to immobilized CRP or CVX had exposed PS and formed membrane blebs after a prolonged increase in cytosolic [Ca(2+)](i). Platelets adhering to fibrinogen responded similarly but only when exposed to soluble CRP or CVX. By scanning electron microscopic analysis, the bleb-forming platelets were detected as either round, spongelike structures with associated microparticles or as arrays of vesicular cell fragments. The phosphorylation of p38 mitogen-activated protein kinase (MAPK) elicited by CRP and CVX was enhanced in fibrinogen-adherent platelets compared with that in platelets in suspension. The p38 inhibitor SB203580 and the calpain protease inhibitor calpeptin reduced only the procoagulant bleb formation, having no effect on PS exposure. Inhibition of p38 also downregulated calpain activity. We conclude that the procoagulant response evoked by GPVI stimulation is potentiated by platelet adhesion. The sequential activation of p38 MAPK and calpain appears to regulate procoagulant membrane blebbing but not PS exposure.

Platelet endothelial cell adhesion molecule-1 serves as an inhibitory receptor that modulates platelet responses to collagen

Platelet responses to collagen are mediated by the combined actions of the integrin alpha2beta1, which serves as a major collagen-binding receptor, and the GPVI/FcRgamma-chain complex, which transmits collagen-specific activation signals into the cell interior through the action of an immunoreceptor tyrosine-based activation motif within the cytoplasmic domain of the FcRgamma-chain. Despite much progress in identifying components of the signaling pathway responsible for collagen-induced platelet activation, virtually nothing is known about the regulatory elements that modulate this important hemostatic event. PECAM-1, a recently recognized member of the inhibitory receptor family, contains a functional immunoreceptor tyrosine-based inhibitory motif within its cytoplasmic domain that, when tyrosine phosphorylated, recruits and activates the protein-tyrosine phosphatase, SHP-2. To test the hypothesis that PECAM-1 functions to regulate GPVI/FcRgamma-chain-mediated platelet activation, the responses of wild-type versus PECAM-1-deficient murine platelets to GPVI-specific agonists were compared. Four distinct GPVI/FcRgamma-chain-dependent responses were found to be significantly exaggerated in platelets derived from PECAM-1-deficient mice, including Mg++-independent adhesion to immobilized fibrillar collagen, collagen-induced platelet aggregation, platelet aggregation induced by the GPVI-specific agonist collagen-related peptide, and GPVI/FcRgamma-chain-induced dense granule secretion. Together, these data provide compelling evidence that PECAM-1 modulates platelet responses to collagen, and they implicate this novel member of the inhibitory receptor family in the regulation of primary hemostasis.

Defective platelet aggregation in polycythaemia vera is not caused by impaired calcium signaling, phospholipase D activation or decreased amounts of focal adhesion proteins

We have previously demonstrated that platelets in polycythaemia vera (PV) exhibit decreased aggregation after stimulation with platelet activating factor (PAF) and reduced expression of GPIIIa on both resting and stimulated platelets. In the present study, we investigated if these results were related to changes in the mobilization of intracellular calcium, activation of phospholipase D (PLD) or amounts of GPIIIa and the intracellular tyrosine kinases Fak, Syk, Grb2, Shc and rhoA. Intracellular calcium levels were not different in resting platelets from 14 PV patients and 15 healthy controls (median 43 nmol/L, range 10-114, vs. 36 nmol/L, range 10-119). After stimulation with PAF (1 micromol/L) an equal increase was seen (125 nmol/L for PV platelets, range 67-257, vs. 113 nmol/L for controls, range 60-250). Also formation of phosphatidyl ethanol (PEt) was similar after exposure to 0.5 U/ml thrombin (0.28% PEt of total phospholipid, range 0.16-1.10, vs. 0.24 for controls, range 0.11-2.3) and 1 micromol/L PMA (0.25, range 0.16-0.32, vs. 0.14, range 0.09-0.6). In contrast to the reduced amount of GPIIIa on the surface of PV platelets, immunoblotting on whole cell lysates showed no reduction in PV patients compared to controls, indicating the possibility of an impaired incorporation of GPIIIa to the cell membrane. Levels of Fak, Syk, Shc, Grb2 and rhoA appeared equal in patients and controls. Similar intracellular proteins were tyrosine phosphorylated after stimulation with thrombin, PAF and PMA. In summary, defective platelet aggregation after stimulation with PAF is caused by neither defective mobilization of intracellular calcium nor, in contrast to the situation in PV granulocytes, an impaired activation of PLD. Moreover, no apparent differences in the intracellular amounts of Fak, Syk Shc, Grb2 and rhoA could be detected between PV and control platelets.

Cloning, characterization, and functional studies of human and mouse glycoprotein VI: a platelet-specific collagen receptor from the immunoglobulin superfamily

Injuries to the vessel wall and subsequent exposure of collagen from the subendothelial matrix result in thrombus formation. In physiological conditions, the platelet plug limits blood loss. However, in pathologic conditions, such as rupture of atherosclerotic plaques, platelet-collagen interactions are associated with cardiovascular and cerebral vascular diseases. Platelet glycoprotein VI (GPVI) plays a crucial role in collagen-induced activation and aggregation of platelets, and people who are deficient in GPVI suffer from bleeding disorders. Based on the fact that GPVI is coupled to the Fc receptor (FcR)-gamma chain and thus should share homology with the FcR chains, the genes encoding human and mouse GPVI were identified. They belong to the immunoglobulin (Ig) superfamily and share 64% homology at the protein level. Functional evidence demonstrating the identity of the recombinant protein with GPVI was shown by binding to its natural ligand collagen; binding to convulxin (Cvx), a GPVI-specific ligand from snake venom; binding of anti-GPVI IgG isolated from a patient; and association to the FcR-gamma chain. The study also demonstrated that the soluble protein blocks Cvx and collagen-induced platelet aggregation and that GPVI expression is restricted to megakaryocytes and platelets. Finally, human GPVI was mapped to chromosome 19, long arm, region 1, band 3 (19q13), in the same region as multiple members of the Ig superfamily. This work offers the opportunity to explore the involvement of GPVI in thrombotic disease, to develop alternative antithrombotic compounds, and to characterize the mechanism involved in GPVI genetic deficiencies. (Blood. 2000;96:1798-1807)

Ultrastructural analysis of megakaryocytes in GPV knockout mice

Lesions in the genes for GPIb alpha, GPIb beta or GPIX result in a bleeding diathesis, the Bernard-Soulier syndrome (BSS), which associates a platelet adhesion defect with thrombocytopenia, giant platelets and abnormal megakaryocytes (MK). The role of GPV, also absent in BSS, was recently addressed by gene targeting in mice. While a negative modulator function for GPV on thrombin-induced platelet responses was found in one model, the absence of GP V had no effect on GPIb-IX expression or platelet adhesion. Our study extends previous results and reports that electron microscopy of bone marrow from the GPV knockout mice revealed a normal MK ultrastructure and development of the demarcation membrane system (DMS). There was a usual presence of MK fragments in the bone marrow vascular sinus. Immunogold labelling of MK from the knockout mice showed a normal distribution of GPIb-IX in the DMS and on the cell surface. The distribution of fibrinogen, vWF and P-selectin was unchanged with, interestingly, P-selectin also localised within the DMS in both situations. Thus GPV is not crucial to MK development and platelet production, consistent with the fact that no mutation in the GPV gene has as yet been described in BSS.

Chronic urticaria serum induces histamine release, leukotriene production, and basophil CD63 surface expression--inhibitory effects ofanti-inflammatory drugs

BACKGROUND

A role of potential histamine-releasing autoantibodies against the high-affinity IgE receptor on the surface of basophils and mast cells is discussed in the pathogenesis of chronic urticaria. This so-called autoimmune urticaria may be diagnosed by a positive intracutaneous autologous serum skin test, which is found in about 30% of patients with chronic urticaria.

OBJECTIVE

Our purpose was, first, to compare the effect of complement-inactivated sera of 20 patients with chronic urticaria and positive autologous serum skin tests, 20 patients with chronic urticaria and negative skin tests, and 20 control subjects without chronic urticaria (10 atopic and 10 nonatopic subjects) and, second, to analyze the effect of anti-inflammatory drugs on the serum activity.

METHODS

The following assay systems were used: release of histamine in whole blood samples, surface expression of the activation marker CD63 on basophils, and sulfidoleukotriene de novo production in leukocyte suspensions. Whole blood, basophils, and leukocyte suspensions were obtained from a nonatopic and an atopic donor.

RESULTS

Sera of patients with autologous serum skin test positive chronic urticaria resulted not only in significantly increased histamine release compared with skin test-negative chronic urticaria sera but also in a significant higher induction of basophil CD63 surface expression and sulfidoleukotriene de novo production. However, serum activity was neither characteristic for chronic urticaria nor for chronic urticaria with a positive autologous serum skin test. Preincubation with dapsone, chloroquine, and lidocaine dose dependently resulted in a significant reduction of all histamine release, CD63 expression, and sulfidoleukotriene production. In addition, mizolastine was able to inhibit serum-induced sulfidoleukotriene production.

CONCLUSION

Further studies investigating the in vivo effect of these drugs will have to clarify their role in the management of the subset of patients with chronic urticaria demonstrating serum-induced inflammatory effects.

Thrombin facilitates primary platelet adhesion onto vascular surfaces in the absence of plasma adhesive proteins: studies under flow conditions

BACKGROUND AND OBJECTIVE

The effect of local and circulating thrombin on platelet adhesion onto vascular surfaces was explored in the absence of plasma adhesive proteins using flow conditions.

DESIGN AND METHODS

To study the local effects of thrombin, denuded rabbit aorta segments were incubated with thrombin concentrations of 0.001, 0.01 and 0.1 U/mL. To evaluate the effects of circulating thrombin, the same concentrations were added to perfusates consisting of washed platelets and washed red blood cells suspended in a human albumin solution (5%). In some experiments, purified von Willebrand's factor (vWF) (Haemate-P) was added to the perfusates (0. 8 U/mL of vWF, final concentration). A humanized chimeric antibody to the GPIIb-IIIa complex (Reopro) was used to determine the role of this glycoprotein on platelet adhesion under the conditions described. The effect of blocking GPIb was also assessed. Perfusions were carried out at 800 s(-1) for 10 min. The interaction of platelet with the vessel surface was morphometrically evaluated and expressed as percentage of surface coverage (%SC). Changes in the surface expression of the major platelet antigens were also analyzed by flow cytometry.

RESULTS

Incubation of subendothelial surfaces with thrombin enhanced platelet deposition with respect to control levels (increases in SC of 64%, 79% and 86% with 0.001, 0.01 and 0.1 U/mL of thrombin, respectively). Low concentrations of thrombin (0.001 and 0.01 U/mL) incorporated in the perfusates resulted in a similar pro-adhesive effect (increases in SC of 64% and 71%, respectively) while the highest concentration (0.1 U/mL) failed to produce a pro-adhesive effect due to the augmented formation of platelet aggregates with subsequent thrombocytopenia (15+/-1 vs. 160+/-5x10(9) plt/L in the perfusates). Similar results were obtained when VWF was present in the perfusate. Reduction of platelet deposition by blockade of GPIIb-IIIa (to 5.3+/-0.7%) was partially restored by thrombin. Blockade of GPIb prevented platelets from adhering even when thrombin was present (%SC of 2.0+/-0.8%). No significant changes in the distribution of platelet membrane glycoproteins during perfusion experiments were detected.

INTERPRETATION AND CONCLUSIONS

Our results suggest that thrombin facilitates primary platelet adhesion onto vascular surfaces even in the absence of plasma adhesive proteins. This effect seems to be mainly dependent on the GPIb/vWF axis.

Thrombopoietin combined with early-acting growth factors effectively expands human hematopoietic progenitor cells in vitro

Thrombopoietin (TPO) is established as a powerful stimulant of megakaryocyte differentiation and platelet production both in vivo and in vitro. In preparation for future transplantation of ex vivo expanded CD34+ hematopoietic progenitor cells (HPCs), we have examined the in vitro effect of TPO on cultures of HPC when combined with other early-acting hematopoietic growth factors (GFs) in an attempt to decrease post-transplant thrombocytopenia and accelerate engraftment. By adding TPO to all possible combinations of GM-CSF, IL-3, and c-kit ligand (CKL) in a suspension culture system, we found a significant increase in both relative and absolute numbers of cells in cultures containing TPO of the megakaryocytic lineage and CD34+ cells after 14 days of culture. The most efficient GF combinations for expansion of cell populations of the megakaryocytic lineage and HPCs were TPO, GM-CSF, and CKL, which increased the number of cells of the megakaryocytic lineage 78 fold and the number of CD34+ cells 1.8 fold. The number of CD34+ cells decreased in the cultures containing GM-CSF and CKL with no TPO present, and the number of cells of the megakaryocytic lineage was increased merely 27 fold. Based on our findings, we suggest adding cells from HPCs expanded in cultures containing TPO, GM-CSF, and CKL to unexpanded stem cells for stem cell transplantation.

Changes of platelet cytoskeletal proteins following platelet aggregation induced by collagen and platelet membrane glycoprotein in Japanese black cattle with delta-storage pool deficiency

Five Japanese Black cattle showing the tendency of persistent hemorrhage were diagnosed as delta-storage pool deficiency because of lack of dense bodies in platelets. There was no significant difference in the platelet count, fibrinogen concentration, prothrombin time and activated partial thromboplastin time between the cases and normal control cattle. However, the maximum platelet aggregation rate and the values of myosin in the cytoskeletons during platelet aggregation induced by collagen were significantly lower in the cases compared with those in normal control cattle. The quantities of platelet membrane glycoprotein were in the range of 94-160 kDa and were not different between the cases and control cattle. However, a decrease of thrombospondin in alpha-granules in platelet cytoplasm were suspected in two of the 5 cases.

The platelet thrombin receptor and postoperative bleeding

BACKGROUND

We hypothesized that small amounts of thrombin desensitize the platelet thrombin receptor during cardiopulmonary bypass (CPB), resulting in postoperative platelet dysfunction and bleeding.

METHODS

Seventy-nine patients were entered into a study designed to measure changes in platelet thrombin receptor function during CPB and to correlate them to postoperative bleeding. In addition to measurements of clinical blood loss, platelet function tests of aggregation, activation, and cell-cell adhesion were used. The thrombin receptor agonist peptide (TRAP) was used to activate the platelets. Flow cytometry was used to measure various platelet surface markers and platelet-white cell interactions during CPB.

RESULTS

Compared with preoperative values, both aggregometry and flow cytometry measured a significant reduction of TRAP-induced activation immediately and up to 24 hours after CPB. The response of other activating agents returned to normal by 24 hours. Postoperatively, 8 of 79 patients required excessive blood transfusion (> or = 10 units of blood products) and had significantly decreased TRAP-induced aggregation response.

CONCLUSIONS

Our results show that (1) platelet activation, aggregation, and adhesion to leukocytes induced by TRAP are reduced after CPB, (2) decreased thrombin receptor responsiveness is associated with excessive postoperative blood loss, and (3) because the aggregation and activation responses are different for TRAP and thrombin, there may be a second thrombin receptor on platelets that is protected from damage during CPB. These results imply that prevention of the CPB-induced effects on the thrombin receptor will lessen postoperative morbidity associated with blood transfusion.

CD63 associates with tyrosine kinase activity and CD11/CD18, and transmits an activation signal in neutrophils

As a member of the tetraspan family, it has been hypothesized that CD63 may be associated with signal transduction; however, its role in leukocyte function is unknown. To examine the potential ability of CD63 to activate neutrophils, the effects of five CD63 mAbs, AHN-16, -16.1, -16.2, -16.3, and -16.5, were examined for their ability to alter neutrophil adhesion to HUVEC monolayers. These CD63 Abs increased neutrophil adhesion to resting and TNF-stimulated HUVEC monolayers. This increase in neutrophil adhesion caused by CD63 Abs was blocked by a CD18 Ab and was associated with up-regulation of CD11/CD18 and down-regulation of CD62L on the neutrophil surface. CD11/CD18 was also found to be associated with CD63. This increase in neutrophil adhesion required physiologic extracellular calcium concentrations at or near the time of CD63 Ab binding. The incubation of CD63 Abs with cells in the absence of calcium for 10 min before repletion of calcium resulted in no increase in neutrophil adhesion. Protein kinase activity was detected in neutrophils associated with CD63. Most of the protein kinase activity associated with these Ags was tyrosine kinase activity, with a lesser amount of threonine and serine kinase activities. Src family kinases Lyn and Hck accounted for much of the associated tyrosine kinase activity. The data suggest that CD63 Ab binding to the neutrophil surface triggers a transient activation signal that requires extracellular calcium and regulates the adhesive activity of CD11/CD18. Associated protein kinase activity may play a role in signal transduction by CD63 to regulate other cell functions.

A platelet activation-specific monoclonal antibody that recognizes a receptor-induced binding site on canine fibrinogen

An activation-specific monoclonal antibody (MoAb) termed "Canine Activated Platelet 1" (CAP1) has been developed and partially characterized. Flow cytometric studies of isolated canine platelets, using adenosine diphosphate (ADP) and platelet activating factor (PAF) as agonists, demonstrated that CAPI binding site number was proportional to agonist strength and agonist concentration. MoAb CAP1 binding was diminished by ethylenediamine-tetraacetic acid, suggesting that the antigen was either stabilized by calcium or antigen binding to the platelet surface was mediated by calcium. ADP-activated gel-filtered platelets also demonstrated reduced binding of MoAB CAP1 even in the presence of 1 mM CaCl2. Binding of MoAb CAP1 could be partially restored by activating gel-filtered platelets with PAF, suggesting that the antigen was either present within platelet granule membranes or was exposed after binding of released proteins(s) with a platelet receptor. A monoclonal antibody to human platelet glycoprotein IIIa (GPIIIa), which cross-reacts with canine platelet GPIIIa regardless of platelet activation status, did not inhibit binding of MoAb CAP1. MoAb CAP1 bound to isolated canine fibrinogen captured on polystyrene microtiter plates in the absence of platelet proteins. Immunoblots indicated that MoAb CAP1 recognizes nonreduced fibrinogen as well as a plasmin digest of isolated canine fibrinogen. Results of the present studies suggest that MoAb CAP1 recognizes a receptor-induced binding site on canine fibrinogen.

G-protein-coupled receptors in HL-60 human leukemia cells

1. HL-60 human leukemia cells are a widely employed model system for the analysis of signal transduction processes mediated via regulatory heterotrimeric guanine nucleotide-binding proteins (G-proteins). HL-60 promyelocytes are pluripotent and can be differentiated into neutrophilic or monocytic cells. 2. HL-60 cells express formyl peptide-, complement C5a-, leukotriene B4 (LTB4)- and platelet-activating factor receptors, receptors for purine and pyrimidine nucleotides, histamine H1- and H2-receptors, beta 2-adrenoceptors and prostaglandin receptors. 3. The major G-proteins in HL-60 cells are pertussis toxin (PTX)-sensitive Gi-proteins (Gi2 > Gi3). Gs-proteins and G-proteins of the Gq-family (e.g., G16) are expressed, too. 4. G-protein-regulated effector systems in HL-60 cells are adenylyl cyclase and phospholipase C-beta 2 (PLC-beta 2) and, possibly, phospholipase D (PLD), nonselective cation (NSC) channels and NADPH oxidase. 5. The expression of signal transduction pathways in HL-60 cells strongly depends on the differentiation state of cells. 6. Formyl peptides, via Gi-proteins, mediate activation of PLC, PLD, NSC channels, NADPH oxidase and azurophilic granule release and are referred to as full secretagogues. In dibutyryl cAMP (Bt2cAMP)-differentiated HL-60 cells, C5a and LTB4 are partial and incomplete secretagogues, respectively. There are substantial differences in the Gi-protein activations induced by formyl peptides, C5a and LTB4. 7. In HL-60 promyelocytes, purine and pyrimidine nucleotides mediate activation of PLC and NSC channels largely via PTX-insensitive G-proteins and induce functional differentiation. In Bt2cAMP-differentiated HL-60 cells, they additionally activate PLD, NADPH oxidase and granule release via PTX-sensitive and -insensitive pathways. ATP and UTP are partial secretagogues. Multiple types of receptors (i.e., P2Y- and P2U-receptors and pyrimidinocyeptors) may mediate the effects of nucleotides in HL-60 cells. 8. Bt2cAMP- and 1 alpha,25-dihydroxycholecalciferol-differentiated HL-60 cells express H1-receptors coupled to Gi-proteins and PTX-insensitive G-proteins. In the former cells, histamine mediates activation of PLC and NSC channels, and in the latter, activation of NSC channels. Histamine is an incomplete secretagogue in these cells. 9. HL-60 promyelocytes express H2-receptors coupled to adenylyl cyclase, PLC, and NSC channels. There are substantial differences in the agonist/antagonist profiles of H2-receptor-mediated cAMP formation and rises in cytosolic Ca2+ concentration, indicative of the involvement of different H2-receptor subtypes. H2-receptors mediate functional differentiation of HL-60 cells. 10. Certain cationic-amphiphilic histamine receptor ligands (i.e., 2-substituted histamines, lipophilic guanidines, and a histamine trifluoromethyl-toluidide derivative) show stimulatory effects in HL-60 cells that are attributable to receptor-independent activation of Gi-proteins.

P-selectin regulates platelet-activating factor synthesis and phagocytosis by monocytes

Adhesion molecules on endothelial cells or platelets may regulate localization and activation of leukocytes at sites of tissue injury, infection, or thrombosis. In these studies, we found that human peripheral blood monocytes adhered specifically to immobilized P-selectin (CD62P), Chinese hamster ovary cells transfected with a cDNA for P-selectin, or endothelial cells stimulated to express P-selectin on the cell surface. P-selectin did not directly stimulate synthesis of the lipid autoacoid platelet-activating factor (PAF); however, incubation on immobilized P-selectin primed monocytes for increased synthesis of PAF in response to opsonized zymosan particles. P-selectin did not stimulate increased surface expression of integrin CD11b/CD18 and did not enhance binding of iC3b-coated erythrocytes, a CD11b/CD18-mediated functional response. P-selectin increased PAF production by monocytes incubated with unopsonized zymosan particles that stimulate this response by interaction with the beta-glucan receptor. Further, phagocytosis of unopsonized zymosan particles, another response triggered by the beta-glucan receptor, was increased following the adherence of monocytes to P-selectin. These data suggested that P-selectin primed monocytes for increased PAF synthesis through regulation of the beta-glucan receptor or regulation of signal transduction mechanisms that are linked to the receptor. P-selectin expressed on endothelial cells or platelets may serve both to localize monocytes at sites of vascular inflammation or thrombosis and to prime the cells for subsequent responses that augment inflammation.

Leukocyte interactions with vascular endothelium. New insights into selectin-mediated attachment and rolling

Among the earliest signs of inflammation is the capture of leukocytes from the blood stream and their subsequent rolling along the endothelium of postcapillary venules. This commentary summarizes recent insight into the molecular basis of leukocyte rolling gained from gene-targeted mice, Ab blocking studies, and in vitro and in vivo reconstitution assays. These data reveal how the selectins individually and collectively contribute to the process of leukocyte capture and subsequent rolling on vascular endothelium.

Mechanisms of oxidized chylomicron-induced leukocyte-endothelial cell adhesion

The objectives of this study were to determine whether oxidatively modified chylomicrons (oxCM) can elicit leukocyte-endothelial cell adhesion in the mesenteric microcirculation and to define the mechanisms underlying the oxCM-induced adhesive interactions. Oxidation of chylomicrons (CM) with the peroxyl radical generator 2,2'-azobis(2-amidinopropane)hydrochloride was associated with the formation of thiobarbituric acid-reactive substances and lipid hydroperoxides. Leukocyte rolling, adherence, and emigration as well as erythrocyte velocity were monitored in rat mesenteric venules infused with either native CM or oxCM. oxCM, but not native CM, increased the numbers of rolling, adherent, and emigrated leukocytes. The oxCM-induced leukocyte adherence was significantly blunted by pretreating the animals with either superoxide dismutase, a platelet-activating factor (PAF) receptor antagonist, or monoclonal antibodies (MAb) directed against either CD11/CD18 or intracellular adhesion molecule 1. A MAb against P-selectin reduced oxCM-induced leukocyte rolling but not adherence. These findings suggest that the increased plasma oxCM levels associated with ingestion of oxidized lipids may promote leukocyte adhesion through a mechanism that involves the superoxide anion, PAF, and adhesion receptors on leukocytes and endothelial cells.

Regulation of P-selectin by tumor necrosis factor-alpha

The levels of P-selectin mRNA and polypeptide were analyzed in bovine capillary cells treated with or without the cytokine tumor necrosis factor-alpha. The 3 kb P-selectin mRNA was upregulated three- to five-fold in cytokine-stimulated cells. The increase in mRNA correlated with a dramatic but short-lived increase in P-selectin polypeptide as determined by metabolic-labeling and immunoadsorption. These data confirm earlier studies on mouse P-selectin expressed in a mouse endothelioma cell line and further indicate that P-selectin function can be regulated not only by rapid translocation to the cell surface but also by cytokine-stimulation of P-selectin biosynthesis.

Induction of superoxide anion production from monocytes an neutrophils by activated platelets through the P-selectin-sialyl Lewis X interaction

Activated platelets expressing P-selectin on their surface are known to adhere to monocytes and neutrophils. We examined the possibility that the leukocytes were activated by their adhesion to activated platelets and demonstrated that P-selectin-dependent platelet adhesion to neutrophils and monocytes induced production of extracellular superoxide anion (O2-) by these leukocytes. Leukocyte membrane glycoproteins containing Ser/Thr-linked carbohydrate chains were responsible for the signal reception leading to the leukocyte activation. Cytokines were shown to influence these processes. For example, treatments of neutrophils with interleukin-8 (IL-8) or granulocyte colony-stimulating factor (G-CSF) potentiated the P-selectin-induced O2- production. Furthermore, interleukin-1 (IL-1) and interferon-gamma (IFN-gamma) induced surface expression of P-selectin on platelets in the presence of a low concentration of thrombin and consequently enhanced their adhesion capacity to leukocytes. These results indicated that the adhesion of activated platelets to the leukocytes through the interaction between P-selectin and its carbohydrate ligand, sialyl Lewis X (LeX), was a crucial step for the activation of leukocyte function and supported the notion that activated platelets were actively involved in the inflammatory processes.

Two sites on P-selectin (the lectin and epidermal growth factor-like domains) are involved in the adhesion of monocytes to thrombin-activated endothelial cells

P-selectin, also known as GMP-140, PADGEM or CD62, is expressed on the surface of thrombin-activated platelets and endothelial cells (EC). It is a member of the selectin family of adhesion molecules that regulate leucocyte interactions with the blood vessel wall. In this study we have found that peptides derived from both the lectin (residues 19-34 and 51-61) and epidermal growth factor (EGF)-like (residues 127-139) domains inhibit the adhesion of peripheral blood mononuclear cells (PBMC), elutriated monocytes and a monocytic cell line (U937) to thrombin-activated EC. This inhibition occurred in a concentration-dependent manner and the peptide most active at the lowest concentrations was the one derived from the EGF-like motif (127-139). The scrambled forms of these peptides, identical in amino acid composition to the authentic peptides but with altered sequences, were not inhibitory. Thrombin-activated platelets supported adhesion of U937 cells and this adhesion was dramatically inhibited by the two peptides derived from the lectin-like domain (residues 19-34 and 51-61). All three peptides, when conjugated to BSA and coated on plastic plates, mediated U937 cell adhesion. This study shows, for the first time, that two sites on P-selectin, the lectin and EGF-like domains, are involved in the adhesion of monocytes to thrombin-activated EC.

Neutrophil proteinase cathepsin G is proteolytically active on the human platelet glycoprotein Ib-IX receptor: characterization of the cleavage sites within the glycoprotein Ib alpha subunit

The proteolytic activity of the neutrophil serine-proteinase cathepsin G (CG) on platelet adherence receptors, the glycoprotein (GP) Ib-IX complex and the integrin alpha IIb beta 3, has been investigated. In the range 50 to 200 nmol/l, CG is a potent platelet agonist which induces shape change, granule exocytosis and aggregation. Investigation of the proteolysis of the receptors' subunits during the course of platelet activation by CG was performed by immunoblot analysis of platelet proteins using a panel of specific antibodies. Exposure of platelets for 3 min at 37 degrees C to CG at a concentration that induces full cell activation resulted in an extensive cleavage of the N-terminal region of the extracellular domain of GPIb alpha, the largest (relative molecular mass, M(r), 143,000) of the three subunits constituting the GPIb-IX complex. In contrast, no detectable proteolytic modification of the two other subunits, GPIb beta and GPIX, was detected. Similarly, we observed that neither of the two subunits of the alpha IIb beta 3 receptor were proteolytically modified by CG. Cleavage of GPIb alpha by CG leaves a remnant of the polypeptide chain with M(r) approx. 106,000 in the plasma membrane, while releasing into the extracellular milieu the N-terminal domain with M(r) in the range 40,000 to 46,000. N-terminal sequencing of the CG-derived fragments of GPIb alpha indicated that the Leu275-Tyr276 peptide bond was the primary cleavage site for this proteinase. Proteolysis of GPIb alpha was already detectable at concentrations of CG as low as 25 nmol/l, while with 200 nmol/l the cleavage was detected as soon as 10 s after exposure of platelets to the proteinase. Comparison of the kinetics and concentration dependency for the proteolysis of GPIb alpha and for the activation of platelets by CG showed that cleavage of the GPIb-IX receptor is an early event that accompanies exocytosis and aggregation. Quantitative evaluation of the conversion of GPIb alpha into its membrane fragment indicated that, under optimal conditions, a maximum of approx. 50% of the total GPIb alpha can be affected by proteolysis. However, this proteolysis was > 90% complete when platelets were in the presence of the potent antagonist prostacyclin, suggesting that cellular redistribution of the GPIb-IX receptor may also occur during activation by CG. These results thus indicate that the very early phase of platelet activation by CG is accompanied by extensive modifications in the structure and expression of the GPIb-IX receptor, an effect that might be of functional significance for the interaction of platelets with the vessel wall.

P-selectin induces the expression of tissue factor on monocytes

P-selectin on activated platelets and stimulated endothelial cells mediates cell adhesion with monocytes and neutrophils. Since activated platelets induce tissue factor on mononuclear leukocytes, we examined the effect of P-selectin on the expression of tissue factor activity in monocytes. Purified P-selectin stimulated tissue factor expression on mononuclear leukocytes in a dose-dependent manner. Chinese hamster ovary (CHO) cells expressing P-selectin stimulated tissue factor procoagulant activity in purified monocytes, whereas untransfected CHO cells and CHO cells expressing E-selectin did not. Anti-P-selectin antibodies inhibited the effects of purified P-selectin and CHO cells expressing P-selectin on monocytes. Incubation of CHO cells expressing P-selectin with monocytes leads to the development of tissue factor mRNA in monocytes and to the expression of tissue factor antigen on the monocyte surface. These results indicate that P-selectin upregulates the expression of tissue factor on monocytes as well as mediates the binding of platelets and endothelial cells with monocytes and neutrophils. The binding of P-selectin to monocytes in the area of vascular injury may be a component of a mechanism that initiates thrombosis.

Dynamics of neutrophil rolling over stimulated endothelium in vitro

Prior to extravasation at sites of acute inflammation, neutrophils roll over activated endothelium. Neutrophil rolling is often characterized by the average rolling velocity. An additional dynamic feature of rolling that has been identified but not extensively studied is the fluctuation in the rolling velocity about the average. To analyze this characteristic further, we have measured the instantaneous velocity of bovine neutrophils interacting with lipopolysaccharide-stimulated bovine aortic endothelium at shear stresses of 1, 2, 3, and 4 dynes/cm2. The average velocities are quantitatively similar to those reported for human neutrophils rolling over reconstituted P-selectin at a surface density of 400 sites/microns 2. At all shear stresses tested, the population average variance in the instantaneous velocity is at least 2 orders of magnitude higher than the theoretical variance generated from experimental error, indicating that the neutrophils translate with a nonconstant velocity. Possible sources of the variance are discussed. These include "macroscopic" sources such as topological heterogeneity in the endothelium and microscopic sources, such as inherent stochastic formation and breakage of the receptor-ligand bonds that mediate the rolling. Regardless of the ultimate source of the variance, these results justify the use of mathematical models that incorporate stochastic processes to describe bond formation and breakage between the neutrophil and the endothelium and hence are able to generate variable velocity trajectories.

Adhesive proteins in platelet-endothelial interactions

Adhesion molecules mediate the interaction between endothelium and platelets as well as other blood cells and the endothelium. The structure and function of some of these molecules will be reviewed and discussed. The expression of these molecules is largely affected by disease states such as hypertension, diabetes, and cardiac failure. Determination of adhesion molecules expressed on the surface of endothelial cells and platelets by cytoflowmetry enables a new approach to estimate the activity state of these cells and might be helpful to identify patients with an increased thrombotic risk.

Protective effects of selectin chimeras in neutrophil-mediated lung injury

Recombinant selectin chimeric molecules featuring the joining of the extracellular domains of L-, P-, and E-selectin to the CH2 and CH3 domains of human IgG1 have been evaluated for their ability to protect against neutrophil-dependent lung injury in rats after systemic activation of C caused by vascular infusion of cobra venom factor (CVF) or lung injury that follows intrapulmonary deposition of IgG immune complexes. Previous studies using anti-selectin antibodies have suggested that the former model is P-selectin dependent, whereas the latter is E-selectin dependent. Requirements for L-selectin have not been identified because of lack of reagents. For the current studies employing the CVF model of lung injury, infusion of P-selectin-Ig chimera reduced injury (as assessed by changes in permeability and hemorrhage) in a dose-dependent manner, with parallel reductions in lung myeloperoxidase (MPO) content. Similar results were obtained with the L-selectin-Ig chimera, whereas the E-selectin-Ig chimera was not protective and failed to alter MPO content. In contrast, in the IgG immune complex model of lung injury, the L- and E-selectin-Ig chimeras both showed dose-related protective effects and reductions in MPO content, whereas the P-selectin-Ig chimera failed to protect against injury and did not alter MPO content in this model of lung injury. In all cases of blocking of injury, this was incomplete, suggesting multi-selectin engagement or inadequate amounts of selectin-Ig chimeras employed. These data indicate that neutrophil recruitment and attendant lung injury in the CVF model are L- and P-selectin dependent and E-selectin-independent, whereas in the IgG immune complex model, neutrophil recruitment and lung injury are L- and E-selectin-dependent but independent of P-selectin. Thus, differing selectin requirements for acute inflammatory lung injury have been identified.

Modulation of the activity and assessment of the receptor selectivity in a series of new RGD-containing peptides

We have investigated the structure-activity relationship of a series of new synthetic RGD analogs and their potential use as specific platelet aggregation inhibitors. Twelve short linear peptides showed high potency to inhibit aggregation in ADP-stimulated dog platelets. In order to assess the selectivity of these analogs towards platelet integrin GPIIb-IIIa, a new cell adhesion inhibition system was devised which was able to discriminate between the two closely related beta 3-integrins of the vasculature, GPIIb-IIIa (alpha IIb beta 3), present in platelets, and the vitronectin receptor (alpha v beta 3), expressed in endothelial cells and platelets. As reported for other peptides by Scarborough et al. (1993, J. Biol. Chem. 268, 1066), the analogs containing lysine instead of arginine in position 1 showed increased selectivity towards GPIIb-IIIa. One of them, in which the piperidine carboxylic group was attached to the N-terminus of KGDW, not only strongly inhibited platelet aggregation, but also selectively abolished cell adhesion mediated by GPIIb-IIIa without effect on the vitronectin receptor.

Thrombospondin and a 140 kd fragment promote adhesion and neurite outgrowth from embryonic central and peripheral neurons and from PC12 cells

The ability of thrombospondin (TSP), an extracellular matrix glycoprotein, and two proteolytic fragments to support adhesion and neurite outgrowth from embryonic dorsal root ganglia, spinal cord neurons, and PC12 cells was examined. Anti-TSP antibodies or a synthetic peptide (GRGDS) containing an RGD cell-binding region was also added to cells plated on TSP. TSP and its 140 kd fragment were more efficient than laminin controls in supporting adhesion. Neurites formed on laminin, on varying concentrations of TSP, and particularly the 140 kd fragment. The amino-terminal heparin-binding domain supported little adhesion and outgrowth. Both adhesion and process outgrowth on TSP were inhibited by addition of anti-TSP antibodies, but not GRGDS.

Thrombospondin-exposed human monocytes display augmented luminol-enhanced chemiluminescence upon receptor triggering

It was tested whether the exposure to blood platelet thrombospondin (TSP) influences the function of monocytes. TSP-treated monocytes displayed luminol-enhanced chemiluminescence (CL) upon triggering with polyclonal or monoclonal anti-TSP. This response was mediated by an Fc receptor, since F(ab')2 fragments were without effect. Evidence is provided that a CL signal was induced only when antibodies bound to TSP and Fc receptors of the same monocyte. TSP-treated monocytes exerted enhanced CL to aggregated IgG when compared with untreated or albumin-treated cells, suggesting that TSP up-regulated the cells' capacity to mediate Fc receptor-dependent generation of reactive oxygen. A similar enhancement was observed when TSP-treated cells were stimulated with anti-CD36, or with fMLP. Upon stimulation of TSP-pretreated cells with monoclonal anti-fibrinogen (Fg), a much stronger enhancement was noted, which was similar in magnitude to that induced by anti-TSP. The effect of anti-Fg cannot be explained by a trace contamination of TSP with Fg alone. In contrast to receptor-mediated CL, PMA-induced and zymosan-induced CL were influenced little by TSP pretreatment. IgG-mediated phagocytosis was not enhanced in TSP-treated cells. Thus, TSP selectively modulates certain monocyte functions which could be of physiological relevance.

Barbourin. A GPIIb-IIIa-specific integrin antagonist from the venom of Sistrurus m. barbouri

Sixty-two snake venoms were screened to identify those which specifically inhibit the adhesive protein binding function of the glycoprotein (GP) IIb-IIIa complex, the receptor-mediating platelet aggregation. Although 52 of these venoms inhibited GPIIb-IIIa, only one of these, from the southeastern pigmy rattlesnake, Sistrurus m. barbouri, was specific for GPIIb-IIIa versus other integrins. The peptide responsible for this activity, termed barbourin, was sequenced and found to be highly homologous to other peptides of the viper venom GPIIb-IIIa antagonist family but was the first member which did not contain the Arg-Gly-Asp (RGD) amino acid sequence, believed to be required for inhibition of receptor function. Instead, barbourin contains the sequence, Lys-Gly-Asp (KGD). The conservative Lys for Arg substitution appears to be the sole structural feature which imparts integrin specificity to barbourin, since venom peptide analogs with Lys substitutions were also specific for GPIIb-IIIa. Thus, barbourin represents a new structural model useful for designing potent and GPIIb-IIIa-specific compounds that may have therapeutic value as platelet aggregation inhibitors.

Thrombospondin binds to the surface of human osteosarcoma cells and mediates platelet-osteosarcoma cell interaction

We have previously shown that thrombospondin (TSP) is synthesized and secreted by human MG-63 osteosarcoma cells. In this study, the secretion and cell surface expression of TSP by two different human osteosarcoma cell lines (MG-63 and TE-85) as well as the involvement of TSP in the platelet-aggregating activity of these tumor cells were studied. Using a sandwich enzyme-linked immunosorbent assay, MG-63 cells secreted 3-fold as much TSP as TE-85 cells at 48 h (0.17 +/- 0.01 (SD) versus 0.06 +/- 0.006 micrograms/10(6) cells, P = 0.007). Binding of exogenous 125I-TSP to MG-63 and TE-85 cells in monolayer indicated that binding was time and concentration dependent, saturable, and inhibited by excess cold TSP. However, despite a similar affinity, MG-63 cells had 10-fold more TSP-binding sites than TE-85 cells (402,394 +/- 130,346 versus 36,748 +/- 7,708 TSP-binding sites/cell; P = 0.002). Similar binding differences of 125I-TSP were observed with both osteosarcoma cell lines in suspension. A fluorescence-activated cell-sorting analysis was used in conjunction with an anti-TSP polyclonal antibody, and binding of endogenous TSP to MG-63 and TE-85 cells in suspension was investigated. Addition of an anti-TSP antibody to MG-63 and TE-85 cells in suspension increased the mean fluorescence intensity 50-fold when compared to an irrelevant antibody. Moreover, the fluorescence intensity of MG-63 cells with an anti-TSP polyclonal antibody was increased by 40% when compared to TE-85 cells. Since TSP was expressed on the surface of osteosarcoma cells, the involvement of this glycoprotein in the platelet-aggregating activity of MG-63 and TE-85 cells was therefore investigated using an anti-TSP polyclonal antibody and two monoclonal antibodies (P10 and MA-II), the epitopes of which lie within the Mr 140,000 non-heparin-binding fragment and the Mr 25,000 heparin-binding fragment of TSP, respectively. Preincubation of MG-63 cells (1 x 10(6) cells/ml) with either an anti-TSP polyclonal antibody (100 micrograms/ml) or monoclonal antibody P10 (15 micrograms/ml) inhibited by 80% other platelet-aggregating activity of these tumor cells, while anti-TSP monoclonal antibody MA-II (15 micrograms/ml) had no effect. In sharp contrast, the anti-TSP polyclonal antibody (100 micrograms/ml) only exhibited a slight inhibitory effect on platelet aggregation induced by TE-85 cells when using a low concentration of tumor cells (0.6 x 10(6) cells/ml).(ABSTRACT TRUNCATED AT 400 WORDS)

Thrombospondin-induced adhesion of human platelets

Washed human unactivated platelets attached and spread on thrombospondin (TSP)-coated microtiter plates. Platelet adhesion was promoted by divalent cations Mn2+, Mg2+, and Ca2+ as compared to buffer having all divalent cations complexed with EDTA. TSP-dependent adhesion was inhibited by anti-TSP fab fragments, an anti-TSP monoclonal antibody, an RGD-containing peptide, complex-specific anti-glycoprotein (GP)IIb-IIIa monoclonal antibodies (A2A9 or AP-2) and anti-VLA-2 monoclonal antibodies (6F1 and Gi9), but not by rabbit preimmune fab fragments, mouse IgG, an anti-GPIIIa monoclonal antibody, or monoclonal antibodies against either the human vitronectin receptor, glycocalicin, or GPIV. At saturating concentrations, anti-GPIIb-IIIa inhibited adhesion by 40-60%. Glanzman's thrombasthenic platelets, which lack GPIIb-IIIa, adhered to TSP to the same extent as anti-GPIIb-IIIa-treated normal platelets or 40-60% as well as untreated normal platelets. Antibody 6F1 (5-10 micrograms/ml) inhibited platelet adhesion of both normal and thrombasthenic platelets by 84-100%. Both VLA-2 antibodies also inhibited collagen-induced platelet adhesion, but had no effect on fibronectin-induced adhesion of normal platelets. These data indicate that platelets specifically adhere to TSP and that this adhesion is mediated through GPIIb-IIIa and/or VLA-2.

Adhesion protein GMP140 inhibits superoxide anion release by human neutrophils

The respiratory burst of blood neutrophils has a critical role in the destruction of microorganisms and tissue damage in inflammation. Neutrophils adhere in a dose-dependent fashion to granule membrane protein 140 (GMP140), a member of the LEC-CAM (lectin/epidermal growth factor/complement-binding domain cell adhesion molecule) family of adhesion proteins when it is immobilized onto plastic surfaces. Adherence to GMP140 was associated with less superoxide anion generation than adherence to other surfaces, an effect that is especially remarkable after activation of neutrophils with tumor necrosis factor alpha, an agent that on other surfaces promotes adhesion and spreading. However, on GMP140 the cells fail to spread and instead remain rounded and refractile. Neutrophils adhering to GMP140 were also deficient in superoxide anion generation to formylmethionylleucylphenylalanine. Furthermore, fluid-phase GMP140 also inhibited the superoxide generation by neutrophils stimulated by tumor necrosis factor alpha. The effect of GMP140 was reversible by washing and was inhibited by anti-GMP140 Fab antibody. GMP140 appears to be a natural antiinflammatory molecule that may prevent the inappropriate activation of neutrophils in the circulation.

Human carcinoma cells express receptors for distinct domains of thrombospondin

Thrombospondin (TSP), an adhesive glycoprotein, is incorporated into the extracellular matrix, mediates cell attachment and spreading, chemotaxis, haptotaxis, and may participate in the directed movement of cells in metastasis. Evidence from several model systems suggests that these functions may be mediated by different domains within the TSP molecule. Radioligand binding assays on 11B squamous carcinoma cells with 125I-Radioligand binding assays on 11B squamous carcinoma cells with 125I-TSP demonstrated the presence of 1.2 x 10(6) sites/cell with an apparent Kd of 74 nM. Binding studies using TSP fragments demonstrated that both the NH2 terminal heparin-binding domain (HBD) and the COOH terminal fragment with a molecular weight of 140,000 (140K) retained the ability to bind 11B cells in a time-dependent, dose-dependent, saturable, and specific manner. The HBD bound to 11B cells with an apparent Kd of 1.2 microM at 1.4 x 10(6) sites/cell. Binding of 140K to cells demonstrated half-maximal binding at 36 nM and a Bmax of 1.9 x 10(5) sites/cell. The binding of 140K also showed a high degree of positive cooperativity with a Hill slope of +3.5, suggesting that binding one 140K molecule to cells leads to increased binding of additional 140K molecules. In addition, the HBD and 140K showed no cross-competition in binding assays. Therefore, it appears likely that these distinct TSP domains bind to separate sites on the cell surface. Neither vitronectin or the peptide RGDS were able to inhibit the binding of TSP or 140K to 11B cells. Based on these data, there appears to be more than one distinct receptor on 11B cells for TSP; one receptor class which mediates the binding of the HBD and a second receptor class which mediates the binding of the Mr 140,000 fragment.

Characterization of platelet aggregation induced by the human melanoma cell line HMV-I: roles of heparin, plasma adhesive proteins, and tumor cell membrane proteins

We investigated the in vitro mechanism of platelet aggregation induced by HMV-I human melanoma cells. HMV-I cells, in the absence of exogenous plasma proteins, induced platelet aggregation, followed by the release reaction. Heparin at an anticoagulant concentration had no effect on the aggregation. Calcium ion was essential for this tumor cell-platelet interaction and could not be replaced by magnesium. Among the adhesive proteins containing RGD sequences that have been reported to enhance experimental metastasis, fibrinogen and thrombospondin significantly enhanced the aggregation induced by HMV-I cells, fibronectin and von Willebrand factor inhibited it, and vitronectin had no effect. To identify the platelet-aggregating factor(s) of the tumor cells, we have developed a monoclonal antibody against HMV-I cells that can inhibit HMV-I cell-induced platelet aggregation. Immunoprecipitation analysis revealed that this antibody recognized an Mr 71,000 membrane protein. These results suggest that the association between the tumor cells and platelets is mediated by the Mr 71,000 membrane protein recognized by this monoclonal antibody.