Arginine-glycine-aspartic acid- and fibrinogen gamma-chain carboxyterminal peptides inhibit platelet adherence to arterial subendothelium at high wall shear rates. An effect dissociable from interference with adhesive protein binding

Cadherin 6 has a functional role in platelet aggregation and thrombus formation

OBJECTIVE

Thrombosis occurs at sites of vascular injury when platelets adhere to subendothelial matrix proteins and to each other. Platelets express many surface receptor proteins, the function of several of these remains poorly characterized. Cadherin 6 is expressed on the platelet surface and contains an arginine-glycine-aspartic acid motif, suggesting that it might have a supportive role in thrombus formation. The aim of this study was to characterize the role of cadherin 6 in platelet function.

METHODS AND RESULTS

Platelet aggregation was inhibited by both antibodies and exogenous soluble cadherin 6. Platelet adhesion to immobilized cadherin 6 was inhibited by arginine-glycine-aspartic acid-serine tetrapeptides. Antibodies to α(IIb)β(3) inhibited platelet adhesion to cadherin 6. Because platelet aggregation occurs in fibrinogen and von Willebrand factor double-deficient mice, we investigated whether cadherin 6 is an alternative ligand for the integrin α(IIb)β(3). Platelet aggregation in fibrinogen and von Willebrand factor double-deficient mice was significantly inhibited by an antibody to cadherin 6. In flow-based assays, inhibition of cadherin 6 caused a marked reduction in thrombus formation in both human and mouse blood.

CONCLUSIONS

This study demonstrates the role of cadherin 6 as a novel ligand for α(IIb)β(3) and highlights its function in thrombus formation.

Surface structural conformations of fibrinogen polypeptides for improved biocompatibility

This work reports on how incorporation of silica nanocages into poly(urethane) copolymers (PU) affects conformational orientations of adsorbed fibrinogen and how different surfaces subsequently influenced HeLa cell attachment and proliferation. Incorporation of 2 wt% silica nanocages into poly(urethane) (PU4) substantially altered the surface topography of the films and some 50% of the surface was covered with the nanocages due to their preferential exposure. AFM studies revealed the deposition of a dense protein network on the soft polymeric domains of PU4 and much reduced fibrinogen adsorption on the hard nanocage domains. As on the bare SiO(2) control surface, fibrinogen molecules adsorbed on top of the hard nanocages mainly took the dominant trinodular structures in monomeric and dimeric forms. In addition, net positively charged long alpha chains were prone to being hidden beneath the D domains whilst gamma chains predominantly remained exposed. Dynamic interfacial adsorption as probed by spectroscopic ellipsometry revealed fast changes in interfacial conformation induced by electrostatic interactions between different segments of fibrinogen and the surface, consistent with the AFM imaging. On the PU surfaces without nanocage incorporation (PUA), however, adsorbed fibrinogen molecules formed beads-like chain networks, consistent with the structure featured on the soft PU4 domains, showing very different effects of surface chemical nature. Monoclonal antibodies specific to the alpha and gamma chains showed reduced alpha but increased gamma chain binding at the silicon oxide control and PU4 surfaces, whilst on the PUA, C18 and amine surfaces (organic surface controls) the opposite binding trend was detected with alpha chain binding dominant, showing different fibrinogen conformations. Cell attachment studies revealed differences in cell attachment and proliferation, consistent with the different polypeptide conformations on the two types of surfaces, showing a strong preference to the extent of exposure of gamma chains.

Staphylococcus aureus adhesion via Spa, ClfA, and SdrCDE to immobilized platelets demonstrates shear-dependent behavior

OBJECTIVE

The objective of this study is to delineate the molecular mechanisms responsible for Staphylococcus aureus-platelet adhesion as a function of physiologically relevant wall shear stresses.

METHODS AND RESULTS

A parallel plate flow chamber was used to quantify adhesion of wild-type, Spa-, ClfA- and SdrCDE- strains to immobilized platelet layers. In the absence of plasma, adhesion increases with increasing wall shear rate from 100 to 5000 seconds(-1). The presence of plasma significantly enhances adhesion at all shear levels. Addition of exogenous fibrinogen yields adhesion levels similar to plasma in the lower shear regimes, but has a diminishing effect on potentiating adhesion at higher shear rates. Alternatively, as shear rate increases von Willebrand factor (VWF) plays an increasingly significant role in mediating binding.

CONCLUSIONS

Addition of plasma proteins potentiates S aureus-platelet interactions at all shear rates examined. Whereas fibrinogen plays a significant role in all shear regimes, VWF mediation becomes increasingly important as wall shear rate increases. Fibrinogen binding is dependent on bacterial adhesins ClfA and SdrCDE whereas Spa is the dominant receptor for VWF.

Alcohol and polyphenolic grape extract inhibit platelet adhesion in flowing blood

BACKGROUND

Moderate and prolonged alcohol consumption has been associated with decreased cardiovascular morbidity and mortality. Inhibition of platelet function in suspension attributes to these effects. Whether alcohol, red wine, or polyphenolic grape extracts (PGE) inhibit platelet adhesion is not known. We investigated platelet adhesion to fibrinogen and collagen in whole blood under standardised flow.

MATERIALS AND METHODS

Before perfusion was started, citrated whole blood from 95 volunteers was preincubated for five min with different alcohol concentrations, unfractioned red wine and PGE. Then, blood was perfused in a single-passage flow chamber over coverslips coated with human fibrinogen or collagen type III at shear rates of 300 s(-1) and 1600 s(-1).

RESULTS

Alcohol inhibited platelet adhesion to human fibrinogen at high shear rate (concentrations > or = 0.15 per thousand) and low shear rate (only at a concentration of 4.8 per thousand), whereas red wine (concentrations > or = 0.15 per thousand) inhibited platelet adhesion to human fibrinogen at both shear rates. In contrast, PGE (concentrations > or = 0.0225 g L(-1)) inhibited platelet adhesion to human fibrinogen only at low shear rate. None of these incubations affected adhesion to collagen.

CONCLUSIONS

Alcohol, red wine and PGE inhibit adhesion to fibrinogen but not to collagen. This inhibition might contribute to the cardioprotective effects of moderate alcohol consumption.

Simple Collagen-Like Peptides Support Platelet Adhesion Under Static But Not Under Flow Conditions: Interaction Via α2β1 and von Willebrand Factor With Specific Sequences in Native Collagen Is a Requirement to Resist Shear Forces

The aim of this study was to define the need for specific collagen sequences and the role of their conformation in platelet adhesion to collagen under both static and flow conditions. We recently reported that simple triple-helical collagen-related peptides (CRPs), GCP*(GPP*)10GCP*G and GKP*(GPP*)10GKP*G (single-letter amino acid code, P* = hydroxyproline; Morton et al,Biochem J 306:337, 1995) were potent stimulators of platelet activation and were able to support the adhesion of gel-filtered platelets examined under static conditions. The present study investigated whether these same peptides were able to support platelet adhesion under more physiologic conditions by examining static adhesion with platelet-rich plasma (PRP) and adhesion under flow conditions. In the static adhesion assay, we observed 20% surface coverage with platelet aggregates. In marked contrast, there was a total lack of adhesion under flow conditions examined at shear rates of 50 and 300 s−1. Thus, the interaction of platelets with the CRPs is a low-affinity interaction unable on its own to withstand shear forces. However, the addition of CRPs to whole blood, in the presence of 200 μmol/L D-arginyl-glycyl-L-aspartyl-L-tryptophan (dRGDW) to prevent platelet aggregation, caused an inhibition of about 50% of platelet adhesion to collagens I and III under flow. These results suggest that the collagen triple helix per se, as defined by these simple collagen sequences, plays an important contributory role in the overall process of adhesion to collagen under flow. The monoclonal antibody (MoAb) 176D7, directed against the α2 subunit of the integrin α2β1, was found to inhibit static platelet adhesion to monomeric but not fibrillar collagens I and III. However, under flow conditions, anti-α2 MoAbs (176D7 anf 6F1) inhibited adhesion to both monomeric and fibrillar collagens, indicating that α2β1 is essential for adhesion to collagen under flow, independent of collagen conformation, whether monomeric or polymeric. To obtain further insight into the nature of the different adhesive properties of CRPs and native collagen, we investigated the relative importance of von Willebrand factor (vWF) and the integrin α2β1 in platelet adhesion to collagen types I and III, using the same shear rate (300 s−1) as used when testing CRPs under flow conditions. Our results, together with recent data of others, support a two-step mechanism of platelet interaction with collagen under flow conditions. The first step involves adhesion via both the indirect interaction of platelet glycoprotein (GP) Ib with collagen mediated by vWF binding to specific vWF-recognition sites in collagen and the direct interaction between platelet α2β1 and specific α2β1-recognition sites in collagen. This suffices to hold platelets at the collagen surface. The second step occurs via another collagen receptor (thought to be GPVI) that binds to simple collagen sequences, required essentially to delineate the collagen triple helix. Recognition of the triple helix leads to strengthening of attachment and platelet activation.

Simple Collagen-Like Peptides Support Platelet Adhesion Under Static But Not Under Flow Conditions: Interaction Via α2β1 and von Willebrand Factor With Specific Sequences in Native Collagen Is a Requirement to Resist Shear Forces

The aim of this study was to define the need for specific collagen sequences and the role of their conformation in platelet adhesion to collagen under both static and flow conditions. We recently reported that simple triple-helical collagen-related peptides (CRPs), GCP*(GPP*)10GCP*G and GKP*(GPP*)10GKP*G (single-letter amino acid code, P* = hydroxyproline; Morton et al,Biochem J 306:337, 1995) were potent stimulators of platelet activation and were able to support the adhesion of gel-filtered platelets examined under static conditions. The present study investigated whether these same peptides were able to support platelet adhesion under more physiologic conditions by examining static adhesion with platelet-rich plasma (PRP) and adhesion under flow conditions. In the static adhesion assay, we observed 20% surface coverage with platelet aggregates. In marked contrast, there was a total lack of adhesion under flow conditions examined at shear rates of 50 and 300 s−1. Thus, the interaction of platelets with the CRPs is a low-affinity interaction unable on its own to withstand shear forces. However, the addition of CRPs to whole blood, in the presence of 200 μmol/L D-arginyl-glycyl-L-aspartyl-L-tryptophan (dRGDW) to prevent platelet aggregation, caused an inhibition of about 50% of platelet adhesion to collagens I and III under flow. These results suggest that the collagen triple helix per se, as defined by these simple collagen sequences, plays an important contributory role in the overall process of adhesion to collagen under flow. The monoclonal antibody (MoAb) 176D7, directed against the α2 subunit of the integrin α2β1, was found to inhibit static platelet adhesion to monomeric but not fibrillar collagens I and III. However, under flow conditions, anti-α2 MoAbs (176D7 anf 6F1) inhibited adhesion to both monomeric and fibrillar collagens, indicating that α2β1 is essential for adhesion to collagen under flow, independent of collagen conformation, whether monomeric or polymeric. To obtain further insight into the nature of the different adhesive properties of CRPs and native collagen, we investigated the relative importance of von Willebrand factor (vWF) and the integrin α2β1 in platelet adhesion to collagen types I and III, using the same shear rate (300 s−1) as used when testing CRPs under flow conditions. Our results, together with recent data of others, support a two-step mechanism of platelet interaction with collagen under flow conditions. The first step involves adhesion via both the indirect interaction of platelet glycoprotein (GP) Ib with collagen mediated by vWF binding to specific vWF-recognition sites in collagen and the direct interaction between platelet α2β1 and specific α2β1-recognition sites in collagen. This suffices to hold platelets at the collagen surface. The second step occurs via another collagen receptor (thought to be GPVI) that binds to simple collagen sequences, required essentially to delineate the collagen triple helix. Recognition of the triple helix leads to strengthening of attachment and platelet activation.

Heparin-binding domain of fibrin mediates its binding to endothelial cells

Spreading of human umbilical vein endothelial cells (ECs) on fibrin requires thrombin cleavage of fibrinopeptide B (FPB) and subsequent exposure of the new beta 15-42 N-terminus. To further understand the interactions between ECs and fibrin beta 15-42 sequences, binding of fibrin(ogen) to EC monolayers was measured with polyclonal anti-fibrinogen (FBG) in parallel with monoclonal anti-FBG (18C6, beta 1-21; J88B, gamma 63-78) and anti-fibrin (T2G1, beta 15-21) antibodies in an indirect enzyme-linked immunosorbent assay. To accomplish this, large, soluble fragments of fibrin were prepared by cyanogen bromide (CNBr) cleavage (fibrin-CNBr); CNBr-cleaved FBG (FBG-CNBr) served as the control ligand. N-terminal fibrin-CNBr bound to EC monolayers and cells in suspension in a dose-dependent and saturable manner. By contrast, FBG-CNBr bound only 50% as well to EC monolayers, with no significant binding of intact FBG, C-terminal FBG plasmic fragment D, or N-terminal plasmic fragment E, which lacks beta 1-53. ECs bound the peptide beta 15-42-bovine serum albumin (BSA) conjugate but neither a scrambled beta 15-42 peptide conjugate nor conjugates of beta 24-42, beta 18-27, or beta 18-31. Binding of fibrin-CNBr was inhibited 54% by the beta 15-42-BSA conjugate and 17% by the B beta 1-42-BSA conjugate but not by free beta 15-42 peptide or RGDS-cell binding peptide. Binding of fibrin-CNBr was inhibited > 95% by heparin in a concentration-dependent manner; the same concentrations of heparin inhibited binding of beta 15-42-BSA by > 75% but not the dose-dependent binding of fibronection to ECs. These data suggest that in their native conformation, FBG B beta 15-42 sequences are unavailable for binding to ECs and that thrombin-induced exposure of beta 15-42 is required for binding by a heparin-dependent, RGD-independent mechanism at the new N-terminus of fibrin.

Platelet adhesion at high shear rates: the roles of von Willebrand factor/GPIb and the beta 1 integrin alpha 2 beta 1

We have previously described a monomeric rvWf fragment, Leu504-Lys728 that contains one disulfide bond linking Cys509-Cys695. This fragment, VCL, has previously been shown to inhibit vWf-ristocetin, asialo-vWf, and botrocetin-induced vWf binding and aggregation of platelets. VCL inhibited 50% of vWf binding to heparin, but it did not inhibit vWf binding to type I collagen. At a high shear force (2600-1 sec), VCL inhibited platelet adhesion to the subendothelial surface of human umbilical arteries. The maximum inhibition of platelet adhesion was 83 +/- 4% at a VCL concentration of 7.6 mumol/L. Various monoclonal anti-Very Late Activation antigens (VLA) antibodies were added to the VCL and tested for their ability to enhance the inhibition of platelet adhesion at high shear forces. Of all of the VLA antibodies tested, only the anti-VLA-2 antibody (176D7) inhibited platelet aggregation in the absence of VCL and enhanced the inhibition of platelet adhesion in the presence of VCL. The VLA-2 antibody and VCL together inhibited 96 +/- 4% of platelet adhesion at high shear forces.

Whole-blood platelet aggregation predicts in vitro and in vivo primary hemostatic function in the elderly

Increased platelet aggregation is associated with higher coronary artery disease mortality. Enhanced platelet aggregation in platelet-rich plasma has also been described in the elderly. To define age-related changes in primary hemostasis, we studied 37 elderly and 31 young blood donors. There were no significant age-related differences in whole-blood platelet aggregation, platelet adherence and thrombus formation on human umbilical artery segments, or bleeding time. Plasma fibrinogen was significantly higher in elderly men and women, whereas activated factor VII was elevated only in elderly women. Collagen-induced platelet aggregation was significantly correlated with platelet adherence to the subendothelium in elderly (r = .488, P = .002) but not in young donors. Accordingly, collagen-induced platelet aggregation showed a significant inverse correlation with bleeding time only in the elderly (r = -.401, P = .014). Arachidonic acid-induced platelet aggregation was significantly associated with platelet adherence to the subendothelium (r = .658, P = .003) and bleeding time (r = -.540, P = .021) only in elderly men. In young donors, ADP-induced platelet aggregation was significantly correlated with platelet adherence to the thrombogenic adventitial surface (r = .395, P = .031); in the elderly this association only approached significance (r = .315, P = .058). Whole-blood platelet aggregation in response to collagen and arachidonic acid may be more useful in predicting primary hemostatic function in the elderly than in the young. Furthermore, in the elderly, the correlation between platelet aggregation in whole blood and platelet-arterial wall interactions in vitro and in vivo may contribute to the ability of this test to predict coronary risk.

Sex differences in platelet adherence to subendothelium: relationship to platelet function tests and hematologic variables

Men have significantly more atherosclerotic disease than women. Platelet-mediated thrombosis plays a role in the initiation of myocardial infarction and stroke. Citrated whole blood from male and female donors was perfused through an annular system over everted human umbilical artery segments. Comparisons were made between platelet adherence and thrombus formation on subendothelium, platelet aggregation in citrated whole blood, hematologic variables, and the bleeding time. Platelet spreading and adherence were approximately 22% greater with male blood (P < 0.001), whereas thrombus formation on subendothelium and collagen- and arachidonic acid-induced platelet aggregation did not show sex-related differences. Platelet aggregation with adenosine diphosphate was greater in women, related to their lower hematocrit values. By contrast, in women hematocrit values showed a slight but significant positive correlation with platelet adherence on subendothelium. Fibrinogen was significantly correlated with collagen- and adenosine-diphosphate-induced platelet aggregation and with platelet adherence, spreading, and thrombus formation on subendothelium. The mean bleeding time was slightly longer in women than in men (P = 0.118). Platelet aggregation was not associated with the bleeding time except for collagen-induced platelet aggregation in males; the latter was significantly correlated with platelet adherence and spreading in both sexes, while arachidonic acid-induced platelet aggregation predicted platelet adherence and spreading in males. Male blood shows enhanced primary hemostatic activity; this may predispose men to atherosclerosis.

Purification and characterization of human platelet von Willebrand factor

Platelet von Willebrand factor (vWf) was purified from human platelet concentrates. The multimeric structure of the purified platelet vWf was similar to that observed in the initial platelet lysate, and, like the platelet lysate, the purified platelet vWf contained higher molecular weight multimers than plasma vWf. The apparent molecular weight of the reduced platelet vWf subunit was similar to the plasma vWf subunit. The N-terminal amino acid of the purified platelet and plasma vWf was blocked. In concentration dependent binding to botrocetin- or ristocetin-stimulated platelets, 125I-plasma vWf bound with a higher affinity than platelet. The ristocetin cofactor activity per mg of purified plasma vWf was 5-fold greater than the platelet vWf activity. Platelet and plasma vWf bound to collagen with similar affinities; however, platelet vWf bound to thrombin-stimulated platelets and to heparin with a higher affinity than plasma vWf. The differences in the binding affinity(s) of plasma and platelet vWf to platelet GPIb and GPIIb/IIIa and extracellular matrix proteins may reflect different roles for plasma and platelet vWf in the initial stages of haemostasis and thrombosis.

Platelet size distribution measurements as indicators of shear stress-induced platelet aggregation

The mechanisms underlying shear stress-induced platelet aggregation (SIPA) were investigated by measuring changes in the platelet size distributions resulting from the exposure of human platelet-rich plasma (PRP) to well-defined shear stresses in a modified viscometer. Exposure of PRP to a shear stress of 100 dyne/cm2 for 1 min at 37 degrees C resulted in the loss of single platelets, an overall shift in the distribution to larger particle sizes, and the generation of platelet fragments. Treatment of PRP prior to shearing with a monoclonal antibody directed against platelet glycoprotein (GP) IIb-IIIa (integrin alpha IIb beta 3) at a concentration that completely inhibited ADP-induced platelet aggregation also inhibited SIPA. Furthermore, incubation of PRP with a recombinant fragment of von Willebrand factor (vWF) that abolishes ristocetin-induced platelet agglutination significantly inhibited but did not eliminate SIPA. Pretreatment of PRP with the tetrapeptides RGDS or RGDV, which constitute the GP IIb-IIIa peptide recognition sequences on fibrinogen and vWF, almost completely blocked platelet aggregation at 100 dyne/cm2, whereas the negative control peptide RGES had no discernible effect. Finally, incubation of PRP with a monoclonal antibody directed against the platelet vitronectin receptor (integrin alpha v beta 3) did not affect SIPA. These results indicate that both GP IIb-IIIa and GP Ib, the latter through its interaction with vWF, are required for SIPA at 100 dyne/cm2; that the interaction of GP IIb-IIIa with its adhesive ligands under shear stress can be inhibited by RGD-containing peptides; and that the vitronectin receptor on platelets, which shares the same beta 3 subunit as GP IIb-IIIa, plays no role in SIPA.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue-type plasminogen activator and its inhibitor in rat aorta. Effect of endotoxin

Plasminogen activator (PA) and PA inhibitor (PAI) antigen, activity, and mRNA were analyzed in the three layers of rat aorta, and the effect of endotoxin on PA and PAI was studied. All PA activity in aorta was identified as tissue-type PA (TPA) activity; no urokinase-type PA was detected. In the tunica adventitia TPA activity, TPA antigen, and TPA mRNA were detected, whereas in the tunica media TPA antigen and TPA mRNA, but no TPA activity, were found. PAI activity was detected in the tunica media, explaining the absence of TPA activity in this layer. Removal of the endothelial cells had no effect on TPA antigen and PAI activity in intima-media preparations. Also, similar amounts of PAI-1 mRNA were found in intima-media preparations, irrespective of the presence or absence of the intima. Immunohistochemical staining showed that TPA immunoreactivity was present in all three layers of the aorta, whereas PAI-1 immunoreactivity was found in medial smooth muscle cells but not in endothelial cells. After endotoxin treatment, TPA activity was decreased in extracts of the total aorta and of the adventitia, although TPA antigen and TPA mRNA were unchanged. PAI-1 mRNA was strongly increased in the tunica adventitia and in the tunica media, as was PAI activity in the tunica media. Thus, endotoxin decreased TPA activity by increasing the synthesis of PAI-1; TPA was unaffected. Our observations in rat aorta differ from observations in mouse aorta and in rat carotid artery, and they caution against extrapolation from one tissue (or species) to another.

Thrombin plays a key role in late platelet thrombus growth and/or stability. Effect of a specific thrombin inhibitor on thrombogenesis induced by aortic subendothelium exposed to flowing rabbit blood

Thrombin is involved in the pathogenesis of venous and arterial thrombosis. This study addressed the question of the relative importance of thrombin in the early and late phases of thrombogenesis. The effect of Ro 46-6240 (1.43 mg/kg bolus and 0.1 mg/kg per minute i.v.), a novel, selective thrombin inhibitor on thrombogenesis induced by rabbit aorta subendothelium, was measured ex vivo in a perfusion chamber model after a short (5-minute) and long (30-minute) exposure time to rabbit native blood. The role of the perfusion time was assessed at shear rates of 100/s, 650/s, and 2600/s. These shear rates mimic blood flow conditions found in veins, arteries, and small or stenosed arteries, respectively. Fibrin deposition and platelet thrombus formation on subendothelium were evaluated by microscopic morphometry. In the presence of Ro 46-6240, fibrin deposition was abolished at both perfusion times and at all shear rates. In the 5-minute experiments, thrombus height was reduced by Ro 46-6240 at shear rates of 100/s (85%) and 650/s (35%) but not at a shear rate of 2600/s, whereas thrombus area was not affected at any shear rate. In contrast, both thrombus height and thrombus area were reduced (60% to 90%) by Ro 46-6240 in the 30-minute perfusion groups at all wall shear rates. The antithrombotic effect of Ro 46-6240 after 30-minute perfusion was confirmed by the minimal increase in the pressure difference between the entrance and the exit of the perfusion chamber compared with the control groups.(ABSTRACT TRUNCATED AT 250 WORDS)

New antithrombotic strategies for resistant thrombotic processes

Life-threatening thrombo-occlusive events producing heart attacks and strokes develop in patients at sites of atherosclerotic arterial stenoses when plaques rupture, a process resistant to both aspirin and heparin. Resistant thrombotic complications are also troublesome during therapeutic thrombolytic or mechanical interventions for symptomatic atherosclerotic vascular disease, including angioplasty, various types of atherectomies, endarterectomy, endovascular stent deployment, or implanted small caliber vascular grafts. In this review therapeutic strategies for more effective management of these resistant, platelet-dependent, occlusive thrombi are discussed, including: a) inhibition of platelet recruitment by anti-GPIIb/IIIa monoclonal antibodies, naturally occurring peptides containing RGD sequences, or synthetic competitive analogs; b) direct inactivation of thrombin bound to thrombus by natural or synthetic antithrombin peptides; c) interruption of thrombin's production by natural or synthetic antagonists of Factor Xa or extrinsic and intrinsic coagulation pathways; and d) elimination of thrombogenicity at sites of vascular injury by immediately restoring confluent endothelium or prior therapy with dietary n-3 fatty acids. However, antagonists of both GPIIb/IIIa- and thrombin-dependent platelet recruitment produce equivalent inhibition of thrombus formation and platelet hemostatic function. Interestingly, hemostasis is spared by therapies that inhibit thrombin's production. Recommendations for development strategies are related to the relative hemostatic risks and antithrombotic benefits.

Aggregate formation is more strongly inhibited at high shear rates by dRGDW, a synthetic RGD-containing peptide

The effect of D-Arg-Gly-Asp-Trp (dRGDW), a synthetic RGD-containing peptide, on platelet adhesion and aggregate formation on various purified adhesive proteins and the extracellular matrix of endothelial cells was investigated with anticoagulated blood recirculating through a parallel-plate perfusion chamber. Aggregate formation on the extracellular matrix of phorbol myristate acetate (PMA)-stimulated endothelial cells and on collagen type I was more strongly inhibited by dRGDW at higher shear rates than at a low shear rate. Platelet adhesion to the extracellular matrix of nonactivated and PMA-stimulated endothelial cells was inhibited by dRGDW, especially at high shear rates, probably as a consequence of the inhibition of platelet spreading. Inhibition by dRGDW of platelet adhesion to von Willebrand factor, fibronectin, and fibrinogen was almost complete, indicating that platelet adhesion to these substrates is mediated through RGD-directed receptors. Platelet adhesion to laminin was not inhibited by the peptide, whereas platelet adhesion to collagen was increased as a consequence of the inhibition of aggregate formation. Our results show that dRGDW is a strong inhibitor of platelet adhesion and aggregate formation, especially at high shear rates.

Interactions of thermally denatured fibrinogen on polyethylene with plasma proteins and platelets

During the investigation of fibrin deposition onto hydrophobic polymers in contact with human blood, a model was developed in which fibrinogen was denatured and irreversibly coated onto a polyethylene surface by heating to 70 degrees C for 10 min. The denatured fibrinogen-polyethylene surface is resistant to fluid wall shear rates of up to 550 s-1 and the fibrinogen does not desorb in the presence of plasma proteins. Compared to uncoated polyethylene, little albumin or fibrinogen adsorbs to heat-denatured fibrinogen. Thrombin binds to the denatured fibrinogen-coated polyethylene with low affinity and also acts on the surface-bound denatured fibrinogen and cleaves fibrinopeptide A (FPA) quantitatively. Washed, 51Cr-labeled platelets do not adhere to the thermally denatured fibrinogen at either low or high shear rates compared to surfaces coated with undenatured fibrinogen (p < 0.01). These observations support the role of the D domain of fibrinogen in platelet adhesion because this is the region that is denatured by heating. In contrast, the E domain of fibrinogen is not altered by heating to 70 degrees C and hence remains susceptible to thrombin and/or plasmin cleavage. The characteristics of this surface are such that it can be used to develop fibrin-coated surfaces for use in studies of thrombus formation on artificial surfaces.

A monomeric von Willebrand factor fragment, Leu-504--Lys-728, inhibits von Willebrand factor interaction with glycoprotein Ib-IX [corrected]

von Willebrand factor interaction with glycoprotein Ib alpha (GPIb alpha) plays a critical role in the initial phase of platelet adhesion at high shear rates, and it may also play a role in platelet thrombus formation in partially occluded arteries. Previous studies have indicated that two peptides, Cys-474--Pro-488 (peptide 153) and Ser-692--Pro-708 (peptide 154), inhibit von Willebrand factor--GPIb alpha interaction. We have expressed a recombinant fragment of von Willebrand factor, Leu-504--Lys-728 [corrected], with a single intrachain disulfide bond linking residues Cys-509--Cys-695 and examined its ability to inhibit von Willebrand factor--GPIb alpha interactions and platelet adhesion at high shear forces. This recombinant fragment, named VCL, inhibits ristocetin-induced, botrocetin-induced, and asialo-von Willebrand factor-induced platelet aggregation and binding to platelets at an IC50 = 0.011-0.260 microM, significantly lower than the IC50 of peptide 153 or 154, IC50 = 86-700 microM. Peptides 153 and 154 did not result in any inhibition of platelet adhesion (IC50 greater than 500 microM). In contrast, VCL inhibited 50% of platelet adhesion at 0.94 microM and at 7.6 microM inhibited greater than 80% of platelet adhesion to human umbilical artery subendothelium at high shear forces. VCL inhibited the contact and spreading of platelets and also caused a marked decrease in thrombus formation. These studies indicate that VCL may be an effective antithrombotic agent in preventing arterial thrombus formation in areas of high shear force.

Pulsed laser and thermal ablation of atherosclerotic plaque: morphometrically defined surface thrombogenicity in studies using an annular perfusion chamber

Although clinical trials using laser and thermal angioplasty devices have been underway, the effects of pulsed laser and thermal ablation of atherosclerotic plaque on surface thrombogenicity are poorly understood. This study examined the changes in platelet adherence and thrombus formation on freshly harvested atherosclerotic aorta segments from Watanabe-heritable hyperlipidemic rabbits after ablation by two pulsed laser sources (308-nm xenon chloride excimer and 2,940-nm erbium:yttrium-aluminum-garnet [YAG] lasers) and a prototype catalytic hot-tip catheter. Specimens were placed in a modified Baumgartner annular chamber and perfused with citrated whole human blood, followed by quantitative morphometric analysis to determine the percent surface coverage by adherent platelets and thrombi in the treated and contiguous control areas. Pulsed excimer laser ablation of plaque did not change platelet adherence or thrombus formation in the treated versus control zones. However, photothermal plaque ablation with a pulsed erbium:YAG laser resulted in a 67% reduction in platelet adherence, compared with levels in control areas (from 16.7 +/- 2.2% to 5.5 +/- 1.8%; p less than 0.005). Similarly, after plaque ablation using a catalytic thermal angioplasty device, there was a 74% reduction in platelet adherence (from 29.2 +/- 5.1% to 7.7 +/- 1.6%; p less than 0.005) and a virtual absence of platelet thrombi (from 8.6 +/- 2.3% to 0.03 +/- 0.03%; p less than 0.005). This reduced surface thrombogenicity after plaque ablation with either an erbium:YAG laser or a catalytic hot-tip catheter suggests that thermal modifications in the arterial surface ultrastructure or thermal denaturation of surface proteins, or both, may be responsible for reduced platelet adherence. These in vitro findings indicate that controlled thermal plaque ablation by catheter-based techniques may elicit endovascular responses that can reduce early thrombus formation during angioplasty procedures.

Vitronectin receptor has a role in bone resorption but does not mediate tight sealing zone attachment of osteoclasts to the bone surface

During bone resorption, osteoclasts form a tight attachment, the sealing zone, around resorption lacunae. Vitronectin receptor has previously been shown to be expressed in osteoclasts and it has been suggested that it mediates the tight attachment at the sealing zone. In this study we have shown that glycine-arginine-glycine-aspartic acid-serine pentapeptide inhibits bone resorption by isolated osteoclasts and drastically changes the morphology of the osteoclasts. When the vitronectin receptor was localized by immunofluorescence in rat and chicken osteoclasts cultured on bone slices, it was found to be distributed throughout the osteoclast cell membrane except in the sealing zone areas. Immunoperoxidase staining of rat bone sections at the light microscopical level also revealed intense staining of the cell membrane with occasional small unstained areas, probably corresponding to the sealing zones. Immunoelectron microscopy confirmed the results obtained by light microscopy showing specific labeling only at the ruffled borders and basolateral membranes (0.82 and 2.43 gold particles/microns of membrane, respectively), but not at the sealing zone areas (0.06 gold particles/microns of membrane). Both alpha v and beta 3 subunits of the vitronectin receptor were similarly localized. These results strongly suggest that, although the vitronectin receptor is important in the function of osteoclasts, it is not mediating the final sealing zone attachment of the osteoclasts to the mineralized bone surface.