Biomedicine. Contact--how platelets touch von Willebrand factor

TMT-based proteomics analysis of growth advantage of triploid Apostichopus japonicus

Polyploid breeding can produce new species with a faster growth rate, higher disease resistance, and higher survival rate, and has achieved significant economic benefits. This study investigated the protein differences in the body wall of triploid Apostichopus japonicus and diploid A. japonicus using isotope-labeled relative and absolute quantitative Tandem Mass Tag technology. A total of 21,096 independent peptides and 4621 proteins were identified. Among them, there were 723 proteins with significant expression differences, including 413 up-regulated proteins and 310 down-regulated proteins. The differentially expressed proteins (DEPs) were enriched in 4519 Gene Ontology enrichment pathways and 320 Kyoto Encyclopedia of Genes and Genomes enrichment pathways. Twenty-two key DEPs related to important functions such as growth and immunity of triploid A. japonicus were screened from the results, among which 20 were up-regulated, such as cathepsin L2 cysteine protease and fibrinogen-like protein A. Arylsulfatase A and zonadhesin were down-regulated. The up-regulated proteins were mainly involved in oxidative stress response, innate immune response, and collagen synthesis in triploid A. japonicus, and the down-regulated proteins were mainly associated with the sterility of triploid A. japonicus. In addition, the transcriptome and proteome were analyzed jointly to support proteome data. In this study, the differences in protein composition between triploid and diploid A. japonicus were analyzed for the first time, and the results revealed the underlying reasons for the growth advantage of triploid A. japonicus.

Sequence features, structure, ligand interaction, and diseases in small leucine rich repeat proteoglycans

Small leucine rich repeat proteoglycans (SLRPs) are a group of active components of the extracellular matrix in all tissues. SLRPs bind to collagens and regulate collagen fibril growth and fibril organization. SLRPs also interact with various cytokines and extracellular compounds, which lead to various biological functions such cell adhesion and signaling, proliferation, and differentiation. Mutations in SLRP genes are associated with human diseases. Now crystal structures of five SLRPs are available. We describe some features of amino acid sequence and structures of SLRPs. We also review ligand interactions and then discuss the interaction surfaces. Furthermore, we map mutations associated with human diseases and discuss possible effects on structures by the mutations.

Device-Induced Hemostatic Disorders in Mechanically Assisted Circulation

Mechanically assisted circulation (MAC) sustains the blood circulation in the body of a patients undergoing cardiac surgery with cardiopulmonary bypass (CPB) or on ventricular assistance with a ventricular assist device (VAD) or on extracorporeal membrane oxygenation (ECMO) with a pump-oxygenator system. While MAC provides short-term (days to weeks) support and long-term (months to years) for the heart and/or lungs, the blood is inevitably exposed to non-physiological shear stress (NPSS) due to mechanical pumping action and in contact with artificial surfaces. NPSS is well known to cause blood damage and functional alterations of blood cells. In this review, we discussed shear-induced platelet adhesion, platelet aggregation, platelet receptor shedding, and platelet apoptosis, shear-induced acquired von Willebrand syndrome (AVWS), shear-induced hemolysis and microparticle formation during MAC. These alterations are associated with perioperative bleeding and thrombotic events, morbidity and mortality, and quality of life in MCS patients. Understanding the mechanism of shear-induce hemostatic disorders will help us develop low-shear-stress devices and select more effective treatments for better clinical outcomes.

Association of placenta-derived extracellular vesicles with pre-eclampsia and associated hypercoagulability: a clinical observational study

OBJECTIVE

Pre-eclampsia (PE) is a pregnancy-associated condition initiated by placental factors. We have demonstrated that placental extracellular vesicles (pcEVs) cause hypertension and proteinuria in pregnant and non-pregnant mice.

STUDY DESIGN

An observational study with both case-control and longitudinal designs.

SETTING

A single centre at the Department of Obstetrics and Gynaecology, Tianjin Medical University.

POPULATION

We collected blood samples and clinical information from 54 PE patients, 33 normally pregnant women at 30-36 gestational weeks and on postpartum days 1 and 4 for the cross-sectional study, and at 22-31, 32-35 and 36-40 weeks for the longitudinal study. Non-pregnant women were also recruited.

METHODS

Blood samples were analysed using flow cytometry, coagulation tests and ELISA.

MAIN OUTCOME MEASURES

The primary outcome was plasma pcEV and other extracellular vesicles (EVs), and their expressions of anionic phospholipids and von Willebrand factor (VWF). Secondary variables included coagulation, ADAMTS-13 and the anionic phospholipid-binding proteins.

RESULTS

Plasma pcEVs progressively increased from pregnant women during non-menstrual period (NW) to PE patients (interquartile range [IQR] for NW: 206/microlitre [116-255], normal pregnancy [NP]: 1108/microlitre [789-1969] and PE: 8487/microlitre [4991-16 752]) and predicted PE. EVs from endothelial cells, platelets and erythrocytes accounted for <10% of pcEVs. VWF became hyper-adhesive in PE patients and contributed to the pregnancy-associated hypercoagulability.

CONCLUSION

Placental, platelet- and endothelial cell-derived EVs were significantly elevated in PE patients, but only pcEVs predicted PE. These EVs played a causal role in the pregnancy-induced hypercoagulability.

TWEETABLE ABSTRACT

Placenta-derived extracellular vesicles predict pre-eclampsia and the associated hypercoagulability.

The Extraordinary Role of Extracellular RNA in Arteriogenesis, the Growth of Collateral Arteries

Arteriogenesis is an intricate process in which increased shear stress in pre-existing arteriolar collaterals induces blood vessel expansion, mediated via endothelial cell activation, leukocyte recruitment and subsequent endothelial and smooth muscle cell proliferation. Extracellular RNA (eRNA), released from stressed cells or damaged tissue under pathological conditions, has recently been discovered to be liberated from endothelial cells in response to increased shear stress and to promote collateral growth. Until now, eRNA has been shown to enhance coagulation and inflammation by inducing cytokine release, leukocyte recruitment, and endothelial permeability, the latter being mediated by vascular endothelial growth factor (VEGF) signaling. In the context of arteriogenesis, however, eRNA has emerged as a transmitter of shear stress into endothelial activation, mediating the sterile inflammatory process essential for collateral remodeling, whereby the stimulatory effects of eRNA on the VEGF signaling axis seem to be pivotal. In addition, eRNA might influence subsequent steps of the arteriogenesis cascade as well. This article provides a comprehensive overview of the beneficial effects of eRNA during arteriogenesis, laying the foundation for further exploration of the connection between the damaging and non-damaging effects of eRNA in the context of cardiovascular occlusive diseases and of sterile inflammation.

Platelet-type von Willebrand disease: Local disorder of the platelet GPIbα β-switch drives high-affinity binding to von Willebrand factor

BACKGROUND

Mutations in the β-switch of GPIbα cause gain-of-function in the platelet-type von Willebrand disease. Structures of free and A1-bound GPIbα suggest that the β-switch undergoes a conformational change from a coil to a β-hairpin.

OBJECTIVES

Platelet-type von Willebrand disease (VWD) mutations have been proposed to stabilize the β-switch by shifting the equilibrium in favor of the β-hairpin, a hypothesis predicated on the assumption that the complex crystal structure between A1 and GPIbα is the high-affinity state.

METHODS

Hydrogen-deuterium exchange mass spectrometry is employed to test this hypothesis using G233V, M239V, G233V/M239V, W230L, and D235Y disease variants of GPIbα. If true, the expectation is a decrease in hydrogen-deuterium exchange within the β-switch as a result of newly formed hydrogen bonds between the β-strands of the β-hairpin.

RESULTS

Hydrogen-exchange is enhanced, indicating that the β-switch favors the disordered loop conformation. Hydrogen-exchange is corroborated by differential scanning calorimetry, which confirms that these mutations destabilize GPIbα by allowing the β-switch to dissociate from the leucine-rich-repeat (LRR) domain. The stability of GPIbα and its A1 binding affinity, determined by surface plasmon resonance, are correlated to the extent of hydrogen exchange in the β-switch.

CONCLUSION

These studies demonstrate that GPIbα with a disordered loop is binding-competent and support a mechanism in which local disorder in the β-switch exposes the LRR-domain of GPIbα enabling high-affinity interactions with the A1 domain.

Imbalance of von Willebrand factor and its cleaving protease ADAMTS13 during systemic inflammation superimposed on advanced cirrhosis

BACKGROUND & AIMS

Systemic inflammation in advanced cirrhosis represents a spectrum ranging from subclinical pathological bacterial translocation and immune activation to overt bacterial infection and sepsis. We hypothesized that systemic inflammation in cirrhosis is accompanied by a failure of ADAMTS13 to control the prothrombotic function of von Willebrand factor (VWF), which is increased in portal hypertension and hepatic fibrosis.

METHODS

Patients with Child A cirrhosis (n = 25), Child B/C cirrhosis without clinical features of systemic inflammation (n = 31), and Child B/C cirrhosis with overt bacterial infections or systemic inflammatory response syndrome (n = 24) were analysed for ADAMTS13 and associated parameters and were followed to determine transplant-free survival.

RESULTS

Plasma concentration and activity of ADAMTS13 were decreased in patients with systemic inflammation. Furthermore, ADAMTS13 inversely correlated with the extent of bacterial translocation and the severity of acute-phase reaction. As a function of reduced ADAMTS13 activity and increased VWF antigen, plasma from patients with superimposed inflammation strongly aggregated the platelet receptor glycoprotein Ib in presence of ristocetin. VWF:RCo correlated with higher concentrations of leucocytes and lipopolysaccharide-binding protein, organ dysfunction, augmented turnover of cross-linked intravascular fibrin, and the occurrence of acute kidney injury during follow-up. VWF:RCo of 390% or more predicted transplant-free survival in univariate analysis [HR = 8.24 (3.30-20.54)] and after adjustment for MELD [HR = 3.58 (1.30-9.88)]. However, adverse outcome was not associated with the accumulation of high-molecular weight VWF multimers.

CONCLUSIONS

Systemic inflammation complicating advanced cirrhosis is accompanied by reduced activity of ADAMTS13 promoting a prothrombotic function of VWF, which can be employed to predict clinical outcome.

Exploiting the kinetic interplay between GPIbα-VWF binding interfaces to regulate hemostasis and thrombosis

Platelet-von Willebrand factor (VWF) interactions must be tightly regulated in order to promote effective hemostasis and prevent occlusive thrombus formation. However, it is unclear what role the inherent properties of the bond formed between the platelet receptor glycoprotein Ibα and the A1 domain of VWF play in these processes. Using VWF-A1 knock-in mice with mutations that enhance (I1309V) or disrupt (R1326H) platelet receptor glycoprotein Ibα binding, we now demonstrate that the kinetic interplay between two distinct contact surfaces influences the site and extent to which platelets bind VWF. Incorporation of R1326H mutation into the major site shortened bond lifetime, yielding defects in hemostasis and thrombosis comparable to VWF-deficient animals. Similarly, disrupting this region of contact with an allosteric inhibitor impaired human platelet accrual in damaged arterioles. In contrast, the I1309V mutation near the minor site prolonged bond lifetime, which was essential for the development of a type 2B-like VWD phenotype. However, combining the R1326H and I1309V mutations normalized both bond kinetics and the hemostatic and thrombotic properties of VWF. These findings broaden our understanding of mechanisms governing platelet-VWF interactions in health and disease, and underscore the importance of combined biophysical and genetic approaches in identifying potential therapeutic avenues for treating bleeding and thrombotic disorders.

Targeting von Willebrand factor as a novel anti-platelet therapy; application of ARC1779, an Anti-vWF aptamer, against thrombotic risk

Excessive activation of platelets is a causative factor for thrombotic diseases such as acute coronary syndrome or stroke, and various anti-platelet drugs were developed. Aspirin and clopidogrel have been used as gold standards for anti-platelet therapies, however, their clinical limitations including bleeding problem have increased the demand driving development of novel anti-platelet drugs with new targets. Among several activating pathways leading to platelet aggregation, the interaction between von Willebrand factor (vWF) and glycoprotein Ib, which mainly occurs under high shear stress in arterioles, is recently suggested to be a new promising target. The anti-thrombotic efficacy of anti-vWF agents, such as ARC1779, has been proved in several preclinical and clinical studies. Here, we will discuss the potential benefits of targeting vWF as a novel antiplatelet therapy, providing an insight into the role of vWF in increased thrombotic risk.

Platelets in pulmonary vascular physiology and pathology

Almost a trillion platelets pass through the pulmonary circulation every minute, yet little is known about how they support pulmonary physiology or contribute to the pathogenesis of lung diseases. When considering this conundrum, three questions jump out: Does platelet production in the lungs occur? Why does severe thrombocytopenia—which undercuts the principal physiological role of platelets to effect hemostasis—not lead to pulmonary hemorrhage? Why does atherothrombosis—which platelets initiate, maintain, and trigger is other critically important arterial beds—not develop in the pulmonary artery? The purpose of this review is to explore these and derivative questions by providing data within a conceptual framework that begins to organize a subject that is largely unassembled.

Novel antiplatelet activity of protocatechuic acid through the inhibition of high shear stress-induced platelet aggregation

Bleeding is the most common and serious adverse effect of currently available antiplatelet drugs. Many efforts are being made to develop novel antithrombotic agents without bleeding risks. Shear stress-induced platelet aggregation (SIPA), which occurs under abnormally high shear stress, plays a crucial role in the development of arterial thrombotic diseases. Here, we demonstrate that protocatechuic acid (PCA), a bioactive phytochemical from Lonicera (honeysuckle) flowers, selectively and potently inhibits high shear (>10,000 s(-1))-induced platelet aggregation. In isolated human platelets, PCA decreased SIPA and attenuated accompanying platelet activation, including intracellular calcium mobilization, granule secretion, and adhesion receptor expression. The anti-SIPA effect of PCA was mediated through blockade of von Willebrand factor binding to activated glycoprotein Ib, a primary and initial event for the accomplishment of SIPA. Conspicuously, PCA did not inhibit platelet aggregation induced by other endogenous agonists like collagen, thrombin, or ADP that are important in both pathological thrombosis and normal hemostasis. Antithrombotic effects of PCA were confirmed in vivo in a rat arterial thrombosis model, where PCA significantly delayed the arterial occlusion induced by FeCl(3). Of particular note, PCA did not increase bleeding times in a rat tail transection model, whereas conventional antiplatelet drugs, aspirin, and clopidogrel substantially prolonged it. Collectively, these results suggest that PCA may be a novel antiplatelet agent that can prevent thrombosis without increasing bleeding risks.

Mapping paratope on antithrombotic antibody 6B4 to epitope on platelet glycoprotein Ibalpha via molecular dynamic simulations

Binding of platelet receptor glycoprotein Ibα (GPIbα) to the A1 domain of von Willebrand factor (vWF) is a critical step in both physiologic hemostasis and pathologic thrombosis, for initiating platelet adhesion to subendothelium of blood vessels at sites of vascular injury. Gain-of-function mutations in GPIbα contribute to an abnormally high-affinity binding of platelets to vWF and can lead to thrombosis, an accurate complication causing heart attack and stroke. Of various antithrombotic monoclonal antibodies (mAbs) targeting human GPIbα, 6B4 is a potent one to inhibit the interaction between GPIbα and vWF-A1 under static and flow conditions. Mapping paratope to epitope with mutagenesis experiments, a traditional route in researches of these antithrombotic mAbs, is usually expensive and time-consuming. Here, we suggested a novel computational procedure, which combines with homology modeling, rigid body docking, free and steered molecular dynamics (MD) simulations, to identify key paratope residues on 6B4 and their partners on GPIbα, with hypothesis that the stable hydrogen bonds and salt bridges are the important linkers between paratope and epitope residues. Based on a best constructed model of 6B4 bound with GPIbα, the survival ratios and rupture times of all detected hydrogen bonds and salt bridges in binding site were examined via free and steered MD simulations and regarded as indices of thermal and mechanical stabilizations of the bonds, respectively. Five principal paratope residues with their partners were predicted with their high survival ratios and/or long rupture times of involved hydrogen bonds, or with their hydrogen bond stabilization indices ranked in top 5. Exciting, the present results were in good agreement with previous mutagenesis experiment data, meaning a wide application prospect of our novel computational procedure on researches of molecular of basis of ligand-receptor interactions, various antithrombotic mAbs and other antibodies as well as theoretically design of biomolecular drugs.

von Willebrand disease: clinical and laboratory lessons learned from the large von Willebrand disease studies

During the past 25 years, our knowledge concerning the pathogenesis, diagnostic strategies, and treatment of von Willebrand disease (VWD) has increased significantly. Following the immunological differentiation of factor VIII (FVIII) and von Willebrand factor (VWF) in the 1970s and the cloning of the FVIII and VWF genes in the mid-1980s, substantial progress has been made in our understanding of this, the most common inherited bleeding disorder. We now recognize that VWD represents a range of genetic diseases all with the clinical endpoint of increased mucocutaneous bleeding. The molecular pathology of Type 2 and 3 VWD is now comprehensively documented and involves rare sequence variants at the VWF locus. In contrast, the genetic causation of Type 1 disease remains incompletely defined and in many cases appears to involve genetic determinants in addition to or instead of VWF. The diagnostic triad of a personal history of excessive mucocutaneous bleeding, laboratory tests for VWF that are consistent with VWD, and a family history of the condition remain the keystone to VWD identification. In the laboratory, measurement of VWF antigen and function continue to be the most important diagnostic studies, and while our understanding of the molecular genetic pathology of VWD has advanced considerably in the past decade, genetic testing as a component of diagnosis is limited to certain distinct subtypes of the disorder. Treatment of VWD has been relatively unchanged for the past decade and continues to involve either stimulation of the release of intrinsic VWF with desmopressin or the infusion of VWF concentrates.

Elevated plasma von Willebrand factor and propeptide levels in Malawian children with malaria

Background

In spite of the significant mortality associated with Plasmodium falciparum infection, the mechanisms underlying severe disease remain poorly understood. We have previously shown evidence of endothelial activation in Ghanaian children with malaria, indicated by elevated plasma levels of both von Willebrand factor (VWF) and its propeptide. In the current prospective study of children in Malawi with retinopathy confirmed cerebral malaria, we compared these markers with uncomplicated malaria, non malarial febrile illness and controls.

Methods and Findings

Children with cerebral malaria, mild malaria and controls without malaria were recruited into the study. All comatose patients were examined by direct and indirect ophthalmoscopy. Plasma VWF and propeptide levels were measured by ELISA. Median VWF and propeptide levels were significantly higher in patients with uncomplicated malaria than in children with non-malarial febrile illness of comparable severity, in whom levels were higher than in non-febrile controls. Median concentrations of both markers were higher in cerebral malaria than in uncomplicated malaria, and were similar in patients with and without retinopathy. Levels of both VWF and propeptide fell significantly 48 hours after commencing therapy and were normal one month later.

Conclusions

In children with malaria plasma VWF and propeptide levels are markedly elevated in both cerebral and mild paediatric malaria, with levels matching disease severity, and these normalize upon recovery. High levels of both markers also occur in retinopathy-negative ‘cerebral malaria’ cases, many of whom are thought to be suffering from diseases other than malaria, indicating that further studies of these markers will be required to determine their sensitivity and specificity.

Severe Plasmodium falciparum malaria is associated with circulating ultra-large von Willebrand multimers and ADAMTS13 inhibition

Plasmodium falciparum infection results in adhesion of infected erythrocytes to blood vessel endothelium, and acute endothelial cell activation, together with sequestration of platelets and leucocytes. We have previously shown that patients with severe infection or fulminant cerebral malaria have significantly increased circulatory levels of the adhesive glycoprotein von Willebrand factor (VWF) and its propeptide, both of which are indices of endothelial cell activation. In this prospective study of patients from Ghana with severe (n = 20) and cerebral (n = 13) P. falciparum malaria, we demonstrate that increased plasma VWF antigen (VWF:Ag) level is associated with disproportionately increased VWF function. VWF collagen binding (VWF:CB) was significantly increased in patients with cerebral malaria and severe malaria (medians 7.6 and 7.0 IU/ml versus 1.9 IU/ml; p<0.005). This increased VWF:CB correlated with the presence of abnormal ultra-large VWF multimers in patient rather than control plasmas. Concomitant with the increase in VWF:Ag and VWF:CB was a significant persistent reduction in the activity of the VWF-specific cleaving protease ADAMTS13 (approximately 55% of normal; p<0.005). Mixing studies were performed using P. falciparum patient plasma and normal pooled plasma, in the presence or absence of exogenous recombinant ADAMTS13. These studies demonstrated that in malarial plasma, ADAMTS13 function was persistently inhibited in a time-dependent manner. Furthermore, this inhibitory effect was not associated with the presence of known inhibitors of ADAMTS13 enzymatic function (interleukin-6, free haemoglobin, factor VIII or thrombospondin-1). These novel findings suggest that severe P. falciparum infection is associated with acute endothelial cell activation, abnormal circulating ULVWF multimers, and a significant reduction in plasma ADAMTS13 function which is mediated at least in part by an unidentified inhibitor.

Pathological shear stress directly regulates platelet αIIbβ3signaling

Integrin mechanotransduction is a ubiquitous biological process. Mechanical forces are transduced transmembranously by an integrin's ligand-bound extracellular domain through its β-subunit's cytoplasmic domain connected to the cytoskeleton. This often culminates in the activation of tyrosine kinases directing cell responses. The delicate balance between hemostasis and thrombosis requires exquisitely fine-tuned integrin function, and balance is maintained in vivo despite that the major platelet integrin αIIbβ3is continuously subjected to frictional or shearing forces generated by laminar blood flow. To test the hypothesis that platelet function is regulated by the direct effects of mechanical forces on αIIbβ3, we examined αIIbβ3/cytoskeletal interactions in human platelets exposed to shear stress in a cone-plate viscometer. We observed that α-actinin, myosin heavy chain, and Syk coimmunoprecipitate with αIIbβ3in resting platelets and that 120 dyn/cm2shear stress leads to their disassociation from αIIbβ3. Shear-induced disassociation of α-actinin and myosin heavy chain from the β3tail is unaffected by blocking von Willebrand factor (VWF) binding to glycoprotein (Gp) Ib-IX-V but abolished by blocking VWF binding to αIIbβ3. Syk's disassociation from β3is inhibited when VWF binding to either GpIb-IX-V or αIIbβ3is blocked. Shear stress-induced phosphorylation of SLP-76 and its association with tyrosine-phosphorylated adhesion and degranulation-promoting adapter protein are inhibited by blocking ligand binding to αIIbβ3but not by blocking ligand binding to GpIb-IX-V. Chinese hamster ovary cells expressing αIIbβ3with β3truncated of its cytoskeletal binding domains demonstrate diminished shear-dependent adhesion and cohesion. These results support the hypothesis that shear stress directly modulates αIIbβ3function and suggest that shear-induced αIIbβ3-mediated signaling contributes to the regulation of platelet aggregation by directing the release of constraining cytoskeletal elements from the β3-tail.

von Willebrand factor and factor VIII are independently required to form stable occlusive thrombi in injured veins

von Willebrand factor (VWF) protects factor VIII (FVIII) from proteolysis and mediates the initial contact of platelets with the injured vessel wall, thus playing an important role in hemostasis and thrombosis. VWF is crucial for the formation of occlusive thrombi at arterial shear rates. However, with only a few conflicting studies published, the role of VWF in venous thrombosis is still unclear. Using gene-targeted mice, we show that in ferric chloride-injured veins platelet adhesion to subendothelium is decreased and thrombus growth is impaired in VWF(-/-) mice when compared with wild type (WT). We also observed increased embolization in the VWF(-/-) mice, which was due to lower FVIII levels in these mice as recombinant factor VIII (r-FVIII) restored thrombus stability. Despite normalization of blood clotting time and thrombus stability after r-FVIII infusion, the VWF(-/-) venules did not occlude. Transgenic platelets lacking the VWF receptor GPIbalpha extracellular domain showed decreased adhesion to injured veins. But, after a delay, all the injured venules occluded in these transgenic mice. Thus, VWF likely uses other adhesion receptors besides GPIbalpha in thrombus growth under venous shear conditions. Our studies document crucial roles for VWF and FVIII in experimental thrombosis under venous flow conditions in vivo.

Targeting shear stress-induced platelet activation: is lesion-specific antiplatelet therapy a realistic clinical goal?

Platelets are mediators of physiologic hemostasis and pathologic thrombosis. They operate within distinctive vascular and rheologic microenvironments, and their participation in hemostasis or thrombosis is directed by distinct variables operating within the microenvironment. Thrombosis is not simply too much hemostasis: there is good evidence that triggering mechanisms of platelet aggregation under low shear stress conditions are different from those operating under high shear stress conditions. Such differences are hypothesized to exist in vivo and to separate mechanisms of microvascular hemostasis from mechanisms of arterial thrombosis, such as those involved in myocardial, cerebral and peripheral vascular ischemia and infarction. This separation forms the conceptual basis for the hypothesis that lesion-specific antithrombotic agents might some day be invented that inhibit arterial thrombosis without causing bleeding that arises from impaired hemostasis. The focus of much of the work in this field has been platelet aggregation initiated by shear-dependent von Willebrand factor binding to the platelet glycoprotein Ib-IX-V complex. It is hypothesized that by elucidating molecular mechanisms of platelet activation operating under pathologically elevated shear stresses, targets of lesion-specific therapies will one day be identified for use in clinical syndromes of arterial thrombosis.

Leucine-rich repeats 2-4 (Leu60-Glu128) of platelet glycoprotein Ibalpha regulate shear-dependent cell adhesion to von Willebrand factor

Glycoprotein (GP) Ib-IX-V binds von Willebrand factor (VWF), initiating thrombosis at high shear stress. The VWF-A1 domain binds the N-terminal domain of GPIbalpha (His1-Glu282); this region contains seven leucine-rich repeats (LRR) plus N- and C-terminal flanking sequences and an anionic sequence containing three sulfated tyrosines. Our previous analysis of canine/human and human/canine chimeras of GPIbalpha expressed on Chinese hamster ovary (CHO) cells demonstrated that LRR2-4 (Leu60-Glu128) were crucial for GPIbalpha-dependent adhesion to VWF. Paradoxically, co-crystal structures of the GPIbalpha N-terminal domain and GPIbalpha-binding VWF-A1 under static conditions revealed that the LRR2-4 sequence made minimal contact with VWF-A1. To resolve the specific functional role of LRR2-4, we compared wild-type human GPIbalpha with human GPIbalpha containing a homology domain swap of canine for human sequence within Leu60-Glu128 and a reverse swap (canine GPIbalpha with human Leu60-Glu128) for the ability to support adhesion to VWF under flow. Binding of conformation-specific anti-GPIbalpha antibodies and VWF binding in the presence of botrocetin (which does not discriminate between species) confirmed equivalent expression of wild-type and mutant receptors in a functional form competent to bind ligand. Compared with CHO cells expressing wild-type GPIbalpha, cells expressing GPIbalpha, where human Leu60-Glu128 sequence was replaced by canine sequence, supported adhesion to VWF at low shear rates but became increasingly ineffective as shear increased from 50 to 2000 s(-1). Together, these data demonstrate that LRR2-4, encompassing a pronounced negative charge patch on human GPIbalpha, is essential for GPIbalpha.VWF-dependent adhesion as hydrodynamic shear increases.

Filamin A Binding Stabilizes Nascent Glycoprotein Ibα Trafficking and Thereby Enhances Its Surface Expression

The glycoprotein (Gp) Ib-IX-V complex is essential for platelet-mediated hemostasis and thrombosis. The cytoplasmic domain of its largest polypeptide subunit GpIbalpha possesses a binding region for filamin A, which links GpIb-IX-V to the platelet cytoskeleton. There is evidence that filamin A binding to GpIbalpha directs the surface expression of GpIb-IX. To investigate the mechanism of this effect, we examined GpIbalpha biosynthesis in Chinese hamster ovary (CHO) cells stably co-expressing wild-type or mutant GpIbalpha with GpIbbeta, GpIX with and without filamin A. We observed that surface GpIbalpha expression is enhanced in CHO cells co-expressing human filamin A. In comparison with cells expressing only GpIbalpha, GpIbbeta, and GpIX (CHO-GpIbalpha/betaIX), lysates from CHO-GpIbalpha/betaIX + filamin A-expressing cells showed greater amounts of immature, incompletely O-glycosylated and fully mature GpIbalpha, but lesser amounts of the approximately 15-kDa C-terminal peptide released when the extracellular domain of GpIbalpha is cleaved by proteases. When filamin A binding is eliminated by truncation of GpIbalpha at C-terminal residue 557 or by a deletion between amino acids 560-570, the decreased synthesis of mature GpIbalpha is accompanied by decreased immature GpIbalpha and by an increased immunodetectable C-terminal peptide. The synthesis of mature GpIbalpha in CHO-GpIbalpha/betaIX cells is eliminated by brefeldin A (which inhibits transport out of the endoplasmic reticulum (ER)) and restored by lactacystin (which inhibits proteasomal degradation). These results suggest that GpIbalpha binds to filamin A within the ER and that filamin A binding directs post-ER trafficking of GpIbalpha to the cell surface.

Signalling through the platelet glycoprotein Ib-V–IX complex

The glycoprotein Ib-V-IX is one of the major adhesive receptors expressed on the surface of circulating platelets. It is composed of four different polypeptides-GPIbalpha, GPIbbeta, GPIX, and GPV-and represents a multifunctional receptor able to interact with a number of ligands, including the adhesive protein von Willebrand factor, the coagulation factors thrombin, factors XI and XII, and the membrane glycoproteins P-selectin and Mac-1. Interaction of GPIb-V-IX with the subendothelial von Willebrand factor is essential for primary haemostasis, as it initiates platelet adhesion to the subendothelial matrix at the sites of vascular injury even under high flow conditions. Upon interaction with von Willebrand factor, GPIb-V-IX initiates transmembrane signalling events for platelet activation, which eventually result in integrin alpha(IIb)beta(3) stimulation and platelet aggregation. The investigation of the biochemical mechanisms for platelet activation by GPIb-V-IX has attracted increasing attention during the last years. This review will describe and discuss recent findings that have provided new insights into the events underlying GPIb-V-IX transmembrane signalling. In particular, it will summarise basic concepts on the structure of this receptor, extracellular ligands, and intracellular interactors potentially involved in transmembrane signalling. The recently suggested role of membrane Fc receptors in GPIb-V-IX-initiated platelet activation will also be discussed, along with the involvement of lipid metabolising enzymes, tyrosine kinases, and the cytoskeleton in the crosstalk between GPIb-V-IX and integrin alpha(IIb)beta(3).

Proteolytic inactivation of ADAMTS13 by thrombin and plasmin

The multimeric size and the function of circulating von Willebrand factor are modulated via its proteolytic cleavage by the plasma metalloproteinase, ADAMTS13. It is unclear how ADAMTS13 activity is regulated within the vascular system. In the absence of a regulatory mechanism, ADAMTS13 activity might compromise platelet adhesion at sites of vascular injury. We hypothesized that at sites of vascular injury, ADAMTS13 activity could be regulated locally by coagulation proteinases. Initiation of coagulation in human plasma resulted in the disappearance of added full-length recombinant ADAMTS13. This loss was inhibited by hirudin. Using purified proteins, we showed that ADAMTS13 is proteolyzed at several cleavage sites by thrombin in a time- and concentration-dependent manner. Furthermore, this proteolysis ablated ADAMTS13 activity against purified von Willebrand factor. Preincubation of thrombin with soluble thrombomodulin, but not heparin, inhibited the proteolysis of ADAMTS13, suggesting the involvement of thrombin exosite I (and not exosite II) in ADAMTS13 recognition. Plasmin also cleaved ADAMTS13 into similar fragments, resulting in the loss of ADAMTS13 activity. This study demonstrates the susceptibility of ADAMTS13 to proteolytic inactivation and suggests possible roles for thrombin and plasmin at sites of vascular injury.

A monoclonal antibody directed against human von Willebrand factor induces type 2B-like alterations

We have previously described a monoclonal antibody (mAb), 1C1E7, against von Willebrand factor (VWF), that increases ristocetin-induced platelet aggregation (RIPA) and induces a preferential binding of the high-molecular-weight multimers of VWF to platelet GPIb. Further investigations using a rotational viscometer at a shear rate of 4000 s(-1) could now demonstrate that shear-induced platelet aggregation (SIPA) is significantly increased with 1C1E7 and that this could be completely inhibited by the anti-GPIb mAb 6D1. In contrast, platelet adhesion to a collagen surface at a shear rate of 2600 s(-1), using a rectangular perfusion chamber, was significantly inhibited in the presence of 1C1E7. When citrated whole blood was incubated with 1C1E7, a spontaneous binding of VWF to the platelet GPIb could be demonstrated by flow cytometric analysis. Parallel to this, a decrease of the highest molecular weight multimers of VWF in the plasma was found. Platelets with bound VWF on their surface were able to form macroaggregates but were no longer able to adhere. These phenomena are very similar to the alterations described in von Willebrand's disease type 2B. The epitope of this mAb could be localized to the N-terminal part of the subunit; therefore a distant conformational change in the A1 domain of VWF is suggested.

von Willebrand factor binding to platelet glycoprotein Ib-IX-V stimulates the assembly of an alpha-actinin-based signaling complex

BACKGROUND

Pathological shear stress induces platelet aggregation that is dependent on von Willebrand factor (VWF) binding to glycoprotein (Gp)Ib-IX-V and phosphatidylinositol 3-kinase activation. We tested the hypothesis that pathological shear stress stimulates phosphatidylinositol 3,4,5-trisphosphate (PIP3) synthesis by directing the assembly of a molecular signaling complex that includes class IA phosphatidylinositol 3-kinase (PI 3-KIA).

METHODS

Platelets were subjected to 120 dynes cm-2 shear stress in a cone-plate viscometer. Resting and sheared platelets were lyzed, immunoprecipitations of PI 3-KIA performed, or lipids extracted for PIP3 measurements. alpha-Actinin was incubated with phosphatidylinositol 4,5-bisphosphate (PIP2), immunoprecipitated, and used as a substrate for in vitro PI 3-KIA activity.

RESULTS

Pathological shear stress induces biphasic PIP3 production. In resting platelets, PI 3-KIA associates with alpha-actinin and PIP2. After exposure to shear stress, alpha-actinin and PIP2 rapidly disassociate from PI 3-KIA. PI 3-KIA then gradually re-associates with PIP2 and alpha-actinin, and this complex becomes linked to GpIb alpha through the cytoskeleton. PIP3 production and the observed changes in the association between alpha-actinin, PIP2, and PI 3-KIA are inhibited when VWF binding to GpIb alpha is blocked. In a cell-free system, alpha-actinin binds PIP2 and when the alpha-actinin-PIP2 complex is added to platelet PI 3-KIA, PIP3 production is stimulated.

CONCLUSIONS

These results suggest that pathological shear-induced VWF binding to GpIb-IX-V stimulates PIP3 production through the assembly of an alpha-actinin-based complex that colocalizes PI 3-KIA with substrate PIP2.

Filamin A binding to the cytoplasmic tail of glycoprotein Ibalpha regulates von Willebrand factor-induced platelet activation

We examined the hypothesis that filamin A binding to the cytoplasmic tail of platelet glycoprotein Ibalpha (GpIbalpha) is regulated by pathologic shear stress and modulates von Willebrand factor (VWF)-induced platelet activation. To begin, we examined filamin binding to GpIbalpha in Chinese hamster ovary cells coexpressing mutant human GpIb-IX and wild-type human filamin A. We observed that many different deletions and truncations N-terminal to GpIbalpha's cytoplasmic domain residue 594 disrupted filamin A binding, but that binding was unaffected by 14 different point mutations in hydrophilic residues between amino acids 557 and 593. To try to narrow GpIbalpha's filamin A-binding domain, we next measured the effect of several cytoplasmic domain peptides on human filamin A binding to a GST-GpIbalpha cytoplasmic domain fusion protein. One peptide (residues 557-575; designated "A4 peptide") inhibited filamin A binding to the GST-GpIbalpha cytoplasmic domain fusion protein and competed with GpIbalpha for binding to filamin A. When the A4 peptide was delivered to intact human platelets using a carrier peptide, we observed the dose-dependent inhibition of VWF-induced platelet aggregation in response to both ristocetin and shear stress. The effect of the A4 peptide on shear-induced platelet aggregation was accompanied by the attenuation of shear-induced filamin A binding to GpIbalpha and diminished shear-dependent protein tyrosine phosphorylation. These results suggest that shear-dependent VWF-induced platelet activation affects filamin A binding to GpIb-IX-V, and that filamin A binding to the cytoplasmic tail of GpIbalpha regulates proaggregatory tyrosine kinase signaling.

Interaction of the 268-282 region of glycoprotein Ibalpha with the heparin-binding site of thrombin inhibits the enzyme activation of factor VIII

Activation of factor VIII (FVIII) by thrombin plays a fundamental role in the amplification of the coagulation cascade and takes place through specific proteolytic cleavages at Arg(372), Arg(740) and Arg(1689). Full FVIII activation requires cleavage at Arg(372), a process involving the alpha-thrombin exosite-II; referred to as heparin-binding site (HBS). The present study was aimed at investigating the effect of glycoprotein Ibalpha (GpIbalpha; 1-282 fragment) binding to thrombin HBS on FVIII activation. Similar experiments were also performed using a synthetic peptide modelled on the 268-282 sequence of GpIbalpha, and sulphated successfully at all tyrosine residues present along its sequence, at positions 276, 278 and 279. Both GpIbalpha 1-282 and the sulphated GpIb 268-282 peptides induced a progressive decrease (up to 70%) in activated FVIII generation, assessed by coagulation and FXa-generation assays. Furthermore, SDS/PAGE and Western-blot experiments showed that the specific appearance of the 44 kDa A2 domain on cleavage of the FVIII Arg(372)-Ser(373) peptide bond was delayed significantly in the presence of either GpIbalpha 1-282 or GpIb 268-282 peptide. Moreover, the effect of the latter on thrombin-mediated hydrolysis of a peptide having the sequence 341-376 of FVIII was investigated using reverse-phase HPLC. The k (cat)/ K (m) values of the FVIII 341-376 peptide hydrolysis by thrombin decreased linearly as a function of the GpIbalpha 268-282 peptide concentration, according to a competitive inhibition effect. Taken together, these experiments suggest that the sulphated 268-282 region of GpIbalpha binds to thrombin HBS, and is responsible for the inhibition of the Arg(372)-Ser(373) bond cleavage and activation of FVIII.

Activation of pp125FAK by type 2B recombinant von Willebrand factor binding to platelet GPIb at a high shear rate occurs independently of alpha IIb beta 3 engagement

Shear-induced platelet aggregation (SIPA) involves the sequential interaction of von Willebrand factor (VWF) with both glycoprotein Ib (GPIb) and alphaIIbbeta3 receptors. Type 2B recombinant VWF (2B-rVWF), characterized by an increased affinity for GPIb, induces strong SIPA at a high shear rate (4000 s-1). Despite the increased affinity of 2B-rVWF for GPIb, patients with type 2B von Willebrand disease have a paradoxical bleeding disorder, which is not well understood. The purpose of this study was to determine if SIPA induced by 2B-rVWF was associated with alphaIIbbeta3-dependent platelet activation. To this end, we have addressed the influence of 2B-rVWF (Val553Met substitution) on SIPA-dependent variations of tyrosine protein phosphorylation (P-Tyr) and the effect of alphaIIbbeta3 blockers. At a high shear rate, 2B-rVWF induced a strong SIPA, as shown by a 92.7% +/- 0.4% disappearance of single platelets (DSP) after 4.5 minutes. In these conditions, increased P-Tyr of proteins migrating at positions 64 kd, 72 kd, and 125 kd were observed. The band at 125 kd was identified as pp125FAK using anti-phospho-FAK antibody. This effect, which required a high level of SIPA (> 70% DSP), was observed at 4000 s-1 but not at 200 s-1. Monoclonal antibodies (MoAbs) 6D1 (anti-GPIb) and 328 (anti-VWF A1 domain), completely abolished SIPA and p125FAK phosphorylation mediated by 2B-rVWF. In contrast, neither RGDS peptide nor MoAb 7E3, both known to block alphaIIbbeta3 engagement, had any effect on SIPA and pp125FAK. The size of aggregates formed at a high shear rate in the presence of 2B-rVWF was decreased by genistein, demonstrating the biologic relevance of pp125FAK. These findings provide a unique mechanism whereby the enhanced interaction of 2B-rVWF with GPIb, without engagement of alphaIIbbeta3, is sufficient to induce SIPA but does not lead to stable thrombus formation.

Scientific and therapeutic advances in antiplatelet therapy

Over the past decade, the platelet has emerged as a pivotal entity in cardiovascular diseases. Indeed, the 'preeminence of the platelet' has spawned a variety of drugs that have been shown in large-scale randomized trials to improve patient outcomes in acute coronary syndromes and percutaneous revascularization procedures. Although the platelet was initially viewed only as a bystander in haemostasis, it is now evident that the platelet is in fact a key mediator of thrombosis as well as of inflammation. New insights at the cellular and genomic levels will probably generate novel drugs to inhibit platelet function more effectively and safely than previously possible.

Pharmacogenomics in anticoagulant drug development

The emergence of pharmacogenomic-guided anticoagulant drug development has unraveled novel approaches in the management of patients and ensured individualized therapy to one and all. Gene expression profiling will be useful in the diagnoses of various diseases, in preclinical phases of drug development and in developing markers for adverse drug reactions and desired pharmacological effects. Hence, the adverse drug reactions can be avoided by withdrawing a particular drug. Through cheminformatics, decisions could be made in anticoagulant drug discovery, tailored to the individual needs of the patient at the right dosage and right time. As the human genome is now completely mapped, gene-based single nucleotide polymorphism will be valuable in the diagnosis of diseases. In this review, various polymorphism of coagulation factors will be discussed. Newer anticoagulant drugs could be withdrawn from drug discovery and development pipelines should they exhibit hepatic metabolism requiring CYP450 enzymes known to manifest single nucleotide polymorphism resulting in adverse drug reactions. Pharmacogenomics and cheminformatics should be incorporated in the current study designs of prospective clinical trials. Pharmacogenomic and pharmacogenetic data should be included in the Investigational New Drug (IND) applications, which would enable the FDA to better understand its true impact on pharmacoeconomics. Pharmacogenomics will eventually revolutionize anticoagulant drug development and future practice of medicine.