Size regulation of von Willebrand factor-mediated platelet thrombi by ADAMTS13 in flowing blood

Platelet-Neutrophil Association in NETs-Rich Areas in the Retrieved AIS Patient Thrombi

Histological structure of thrombi is a strong determinant of the outcome of vascular recanalization therapy, the only treatment option for acute ischemic stroke (AIS) patients. A total of 21 AIS patients from this study after undergoing non-enhanced CT scan and multimodal MRI were treated with mechanical stent-based and manual aspiration thrombectomy, and thromboembolic retrieved from a cerebral artery. Complementary histopathological and imaging analyses were performed to understand their composition with a specific focus on fibrin, von Willebrand factor, and neutrophil extracellular traps (NETs). Though distinct RBC-rich and platelet-rich areas were found, AIS patient thrombi were overwhelmingly platelet-rich, with 90% of thrombi containing <40% total RBC-rich contents (1.5 to 37%). Structurally, RBC-rich areas were simple, consisting of tightly packed RBCs in thin fibrin meshwork with sparsely populated nucleated cells and lacked any substantial von Willebrand factor (VWF). Platelet-rich areas were structurally more complex with thick fibrin meshwork associated with VWF. Plenty of leukocytes populated the platelet-rich areas, particularly in the periphery and border areas between platelet-rich and RBC-rich areas. Platelet-rich areas showed abundant activated neutrophils (myeloperoxidase+ and neutrophil-elastase+) containing citrullinated histone-decorated DNA. Citrullinated histone-decorated DNA also accumulated extracellularly, pointing to NETosis by the activated neutrophils. Notably, NETs-containing areas showed strong reactivity to VWF, platelets, and high-mobility group box 1 (HMGB1), signifying a close interplay between these components.

von Willebrand factor self-association is regulated by the shear-dependent unfolding of the A2 domain

von Willebrand factor (VWF) self-association results in the homotypic binding of VWF upon exposure to fluid shear. The molecular mechanism of this process is not established. In this study, we demonstrate that the shear-dependent unfolding of the VWF A2 domain in the multimeric protein is a major regulator of protein self-association. This mechanism controls self-association on the platelet glycoprotein Ibα receptor, on collagen substrates, and during thrombus growth ex vivo. In support of this, A2-domain mutations that prevent domain unfolding due to disulfide bridging of N- and C-terminal residues ("Lock-VWF") reduce self-association and platelet activation under various experimental conditions. In contrast, reducing assay calcium concentrations, and 2 mutations that destabilize VWF-A2 conformation by preventing coordination with calcium (D1498A and R1597W VWD type 2A mutation), enhance self-association. Studies using a panel of recombinant proteins that lack the A1 domain ("ΔA1 proteins") suggest that besides pure homotypic A2 interactions, VWF-A2 may also engage other protein domains to control self-association. Addition of purified high-density lipoprotein and apolipoprotein-A1 partially blocked VWF self-association. Overall, similar conditions facilitate VWF self-association and ADAMTS13-mediated proteolysis, with low calcium and A2 disease mutations enhancing both processes, and locking-A2 blocking them simultaneously. Thus, VWF appears to have evolved 2 balancing molecular functions in a single A2 functional domain to dynamically regulate protein size in circulation: ADAMTS13-mediated proteolysis and VWF self-association. Modulating self-association rates by targeting VWF-A2 may provide novel methods to regulate the rates of thrombosis and hemostasis.

Biomechanical thrombosis: the dark side of force and dawn of mechano-medicine

Arterial thrombosis is in part contributed by excessive platelet aggregation, which can lead to blood clotting and subsequent heart attack and stroke. Platelets are sensitive to the haemodynamic environment. Rapid haemodynamcis and disturbed blood flow, which occur in vessels with growing thrombi and atherosclerotic plaques or is caused by medical device implantation and intervention, promotes platelet aggregation and thrombus formation. In such situations, conventional antiplatelet drugs often have suboptimal efficacy and a serious side effect of excessive bleeding. Investigating the mechanisms of platelet biomechanical activation provides insights distinct from the classic views of agonist-stimulated platelet thrombus formation. In this work, we review the recent discoveries underlying haemodynamic force-reinforced platelet binding and mechanosensing primarily mediated by three platelet receptors: glycoprotein Ib (GPIb), glycoprotein IIb/IIIa (GPIIb/IIIa) and glycoprotein VI (GPVI), and their implications for development of antithrombotic 'mechano-medicine' .

Shear stress and platelet-induced tensile forces regulate ADAMTS13-localization within the platelet thrombus

BACKGROUND

The multimeric glycoprotein von Willebrand factor (VWF) mediates platelet adhesion and aggregation at the site of vessel injury. The adhesive activity of VWF is influenced by its multimer length which is regulated by the metalloprotease ADAMTS13. The ability of ADAMTS13 to regulate platelet thrombus growth in a shear-dependent manner has been described, however, the mechanistic basis of this action has not been well characterized.

METHODS

We developed an mCherry-tagged murine ADAMTS13 protein and utilized an ex vivo flow chamber system to visualize the localization of ADAMTS13 within the platelet thrombus under different conditions of shear. Using this system, we also assessed the influence of platelet-mediated tensile force on ADAMTS13 localization within the thrombus using gain-of-function GPIb binding and loss-of-function GPIIbIIIa binding mutants in VWF/ADAMTS13 DKO mice.

RESULTS

ADAMTS13 was visualized on the growing platelet thrombus under very high shear using ADAMTS13-mcherry. ADAMTS13-mCherry localized particularly at the top portion of the thrombus and reduced thrombus size as it grew to occlusion. At the pathological high shear of 7500 s-1, platelet-mediated tensile force, involving GPIb but not GPIIbIIIa receptors, influenced localization of ADAMTS13 to the thrombus under conditions of shear.

CONCLUSIONS

Tensile force applied on VWF produced by shear stress and platelet GPIb binding has a crucial role in ADAMTS13 activity at the site of thrombus formation. These results suggest that ADAMTS13 activity at the site of platelet thrombus formation is regulated by a shear stress and platelet-dependent feedback mechanism to prevent vessel occlusion and pathological thrombosis.

Leu33Pro (PlA) polymorphism of integrin beta3 modulates platelet Src pY418 and focal adhesion kinase pY397 phosphorylation in response to abnormally high shear stress

OBJECTIVES

Shear stress can activate platelet integrin-mediated signaling that leads to shear-induced platelet aggregation (SIPA) and eventually contribute to acute myocardial infarction. The major platelet integrin αIIbβ3 is polymorphic at residue 33 [Leu33Pro (PlA) polymorphism]. The Pro33 isoform has been shown to have a prothrombotic phenotype. In this work, we studied the impact of Leu33/Pro33 polymorphism on the shear-induced integrin-mediated Src and FAK activation in platelets.

METHODS

Platelets of both genotypes were placed on immobilized fibrinogen or heat activated BSA and were exposed to physiological (500/s) or abnormally high (5000/s) shear rates for 2-10 min. Platelets after exposure to shear were analysed for Src pY418 and FAK pY397 activities.

RESULTS

Whereas physiological shear stress does not affect platelet signaling, abnormally high-shear stress considerably elevates Src and FAK phosphorylation in both Pro33 and Leu33 platelets. Both under static and flow conditions, Pro33 platelets exhibited a significantly higher Src and FAK activities than Leu33 platelets. Interestingly, even in the absence of the αIIbβ3-fibrinogen interaction, we could detect a shear-induced integrin-mediated signaling of Src and FAK in platelets. In parallel experiments in which platelets were pretreated with abciximab, an integrin αIIbβ3 antagonist, activation of both kinases by shear was inhibited.

CONCLUSION

Taken together, our data indicates an important role of αIIbβ3 and shows that Leu33Pro polymorphism modulates the integrin-mediated Src and FAK signaling in platelets in response to shear stress.

Highly elevated plasma level of von Willebrand factor accelerates the formation of platelet thrombus under high shear stress in plasma with deficient ADAMTS13 activity

Upshaw-Schulman syndrome (USS) is a thrombo-hemorrhagic disease caused by congenital deficiency of ADAMTS13 due to ADAMTS13 gene mutations. USS is characterized by repeated episodes of thrombocytopenia and microangiopathic hemolytic anemia that respond dramatically to infusions of fresh frozen plasma. There are two phenotypic expressions of USS: one is the early-onset type and the other, the late-onset type, is asymptomatic during childhood with the first bout of thrombotic thrombocytopenic purpura (TTP) developing after adolescence or during adulthood. We found that gravida with the latter phenotype developed thrombocytopenia and hemolytic anemia during the second or third trimesters, often followed by thrombotic microangiopathies (TMAs). These phenomena suggest that elevated plasma von Willebrand Factor (VWF) might be crucial because plasma levels of VWF antigen usually increase by 200-500% during this period of gestation. Here, we performed platelet function assays using a mixture of anti-coagulated blood from normal volunteers, human VWF, anti-ADAMTS13 monoclonal antibody A10, and purified plasma-derived ADAMTS13 to investigate the effect of plasma VWF levels on platelet thrombus formation in the context of deficient ADAMTS13. In vitro studies showed that mural thrombus formation and platelet aggregation under high shear stress were markedly augmented by increasing the amounts of exogenously added VWF when ADAMTS13 activity was deficient, as may be the case in the in vivo circulation of gravida with USS. These results suggest that highly elevated plasma VWF might accelerate platelet thrombus formation not only in the circulation but also on the surface of vascular endothelial cells in the setting of ADAMTS13 deficiency in USS.

ADAMTS13-mediated thrombolysis of t-PA-resistant occlusions in ischemic stroke in mice

Rapid vascular recanalization forms the basis for successful treatment of cerebral ischemia. Currently, tissue plasminogen activator (t-PA) is the only approved thrombolytic drug for ischemic stroke. However, t-PA does not always result in efficient thrombus dissolution and subsequent blood vessel recanalization. To better understand thrombus composition, we analyzed thrombi retrieved from ischemic stroke patients and found a distinct presence of von Willebrand factor (VWF) in various samples. Thrombi contained on average 20.3% ± 10.1% VWF, and this was inversely correlated with thrombus red blood cell content. We hypothesized that ADAMTS13 can exert a thrombolytic effect in VWF-containing thrombi in the setting of stroke. To test this, we generated occlusive VWF-rich thrombi in the middle cerebral artery (MCA) of mice. Infusion of t-PA did not dissolve these MCA occlusions. Interestingly, administration of ADAMTS13 5 minutes after occlusion dose-dependently dissolved these t-PA-resistant thrombi resulting in fast restoration of MCA patency and consequently reduced cerebral infarct sizes (P < .005). Delayed ADAMTS13 administration 60 minutes after occlusion was still effective but to a lesser extent (P < .05). These data show for the first time a potent thrombolytic activity of ADAMTS13 in the setting of stroke, which might become useful in treatment of acute ischemic stroke.

Pathological von Willebrand factor fibers resist tissue plasminogen activator and ADAMTS13 while promoting the contact pathway and shear-induced platelet activation

BACKGROUND

Under severe stenotic conditions, von Willebrand factor (VWF) multimerizes into large insoluble fibers at pathological shear rates.

OBJECTIVE

Evaluate the mechanics and biology of VWF fibers without the confounding effects of endothelium or collagen.

METHODS

Within a micropost-impingement microfluidic device, > 100-μm long VWF fibers multimerized on the post within 10 min using EDTA-treated platelet-free plasma (PFP) perfused at wall shear rates > 5000 s(-1) .

RESULTS

von Willebrand factor fiber thickness increased to > 10 μm as a result of increasing the shear rate to 10,000 s(-1) . In a stress-strain test, fibrous VWF had an elastic modulus of ~50 MPa. The insoluble VWF fibers were non-amyloid because they rapidly dissolved in trypsin, plasmin or 2% SDS, but were resistant to 50 nm ADAMTS13 or 100 nm tissue plasminogen activator in plasma. Following fiber formation, perfusion of low corn trypsin inhibitor (CTI)-treated (4 μg mL(-1) ), recalcified citrated plasma at 1500 s(-1) caused fibrin formation on the VWF fibers, a result not observed with purified type 1 collagen or a naked micropost. During VWF fiber formation, contact pathway factors accumulated on VWF because the use of EDTA/D-Phe-Pro-Arg chloromethylketone (PPACK)/apixaban/high CTI-treated PFP during VWF fiber formation prevented the subsequent fibrin production from low-CTI, recalcified citrated PFP. VWF fibers displayed FXIIa-immunostaining. When PPACK-inhibited whole blood was perfused over VWF fibers, platelets rolled and arrested on the surface of VWF, but only displayed P-selectin if prevailing shear rates were pathological. Platelet arrest on VWF fibers was blocked with αIIb β3 antagonist GR144053.

CONCLUSIONS

We report VWF fiber-contact pathway crosstalk and mechanisms of thrombolytic resistance in hemodynamic settings of myocardial infarction.

The p.R1819_C1948delinsS mutation makes von Willebrand factor ADAMTS13-resistant and reduces its collagen-binding capacity

This report concerns abnormal ADAMTS13 (a disintegrin and metalloprotease with a thrombospondin type 1 motif, member 13) and collagen interactions coinciding with the p.R1819_C1948delinsS von Willebrand factor (VWF) mutation associated with the deletion of the C-terminus of the A3 domain (amino acids 1819-1947) in a patient with a history of bleeding. The von Willebrand disease (VWD) phenotype of the patient featured low plasma and platelet VWF, multimers with smears extending over the highest normal oligomers in plasma, but not platelets, and an impaired collagen-binding capacity. In vitro full-length p.R1819_C1948delinsS VWF expression showed impaired VWF release, increased cellular content with normally-multimerized VWF and impaired collagen binding. The recombinant p.R1819_C1948delinsS VWF fragment, extending from domains A2 to B3 (p.R1819_C1948delinsS A2-B3 VWF), was completely resistant to proteolysis by ADAMTS13 in the presence of 1·5 mol/l urea, unlike its normal counterpart. The defect stems from impaired ADAMTS13 binding to p.R1819_C1948delinsS A2-B3, analysed under static conditions. Partial deletion of the C-terminus of the A3 domain thus makes VWF resistant to ADAMTS13, interfering with ADAMTS13 binding to VWF, and impairing the collagen-binding capacity of VWF. The p.R1819_C1948delinsS mutation has both haemorrhagic features (defective collagen binding, reduced VWF levels) and prothrombotic (ADAMTS13 resistance) features, and the latter probably mitigate the patient's bleeding symptoms.

Interaction of von Willebrand factor with platelets and the vessel wall

The initiation of thrombus formation at sites of vascular injury to secure haemostasis after tissue trauma requires the interaction of surface-exposed von Willebrand factor (VWF) with its primary platelet receptor, the glycoprotein (GP) Ib-IX-V complex. As an insoluble component of the extracellular matrix (ECM) of endothelial cells, VWF can directly initiate platelet adhesion. Circulating plasma VWF en-hances matrix VWF activity by binding to structures that become exposed to flowing blood, notably collagen type I and III in deeper layers of the vessel along with microfibrillar collagen type VI in the subendothelium. Moreover, plasma VWF is required to support platelet-to-platelet adhesion - i. e. aggregation - which promotes thrombus growth and consolidation. For these reasons, understanding how plasma VWF interaction with platelet receptors is regulated, particularly any distinctive features of GPIb binding to soluble as opposed to immobilized VWF, is of paramount importance in vascular biology. This brief review will highlight knowledge acquired and key problems that remain to be solved to elucidate fully the role of VWF in normal haemostasis and pathological thrombosis.

A novel flow-based assay reveals discrepancies in ADAMTS-13 inhibitor assessment as compared with a conventional clinical static assay

BACKGROUND

Several static Bethesda-type assays are routinely used to determine ADAMTS-13-neutralizing autoantibodies in acquired thrombotic thrombocytopenic purpura (TTP), but the inhibitory activity of these antibodies has not been thoroughly evaluated under the more physiologic condition of flow.

OBJECTIVES

We investigated whether ADAMTS-13 inhibitor assessment with the FRETS-VWF73 assay is predictive for evaluation under flow.

METHODS

Anti-ADAMTS-13 autoantibodies were purified from patients with acquired TTP by chromatography involving an ADAMTS-13 affinity matrix and/or protein G. ADAMTS-13 activity was measured with the FRETS-VWF73 assay and a novel flow assay determining the ADAMTS-13-mediated decrease in platelet aggregate surface coverage, caused by perfusion of a suspension containing platelets, erythrocytes and von Willebrand factor (VWF) over a surface coated with extracellular matrix components. The neutralizing activities of ADAMTS-13 inhibitors were compared under static conditions and under flow by use of the two assays.

RESULTS

The suitability of the flow-based ADAMTS-13 activity assay for quantification of ADAMTS-13 inhibitors could be demonstrated by reversibility of the ADAMTS-13-dependent decrease in surface coverage upon addition of goat ADAMTS-13 antiserum. Testing the neutralizing activity of purified autoantibodies from six patients in the flow assay according to their FRETS-VWF73-based inhibitor titers gave rise to vastly different inhibitory effects, indicating a discrepancy in inhibitor assessment between static and flow conditions.

CONCLUSIONS

Anti-ADAMTS-13 autoantibodies may show inhibitory properties in vivo that are not consistent with the ADAMTS-13 inhibitor levels determined in routine static assays, possibly because certain epitopes are selectively exposed under shear. Consequently, the course of disease and treatment efficacy may vary among TTP patients, despite common inhibitor titers.

von Willebrand factor, Jedi knight of the bloodstream

When blood vessels are cut, the forces in the bloodstream increase and change character. The dark side of these forces causes hemorrhage and death. However, von Willebrand factor (VWF), with help from our circulatory system and platelets, harnesses the same forces to form a hemostatic plug. Force and VWF function are so closely intertwined that, like members of the Jedi Order in the movie Star Wars who learn to use "the Force" to do good, VWF may be considered the Jedi knight of the bloodstream. The long length of VWF enables responsiveness to flow. The shape of VWF is predicted to alter from irregularly coiled to extended thread-like in the transition from shear to elongational flow at sites of hemostasis and thrombosis. Elongational force propagated through the length of VWF in its thread-like shape exposes its monomers for multimeric binding to platelets and subendothelium and likely also increases affinity of the A1 domain for platelets. Specialized domains concatenate and compact VWF during biosynthesis. A2 domain unfolding by hydrodynamic force enables postsecretion regulation of VWF length. Mutations in VWF in von Willebrand disease contribute to and are illuminated by VWF biology. I attempt to integrate classic studies on the physiology of hemostatic plug formation into modern molecular understanding, and point out what remains to be learned.

C2362F mutation gives rise to an ADAMTS13-resistant von Willebrand factor

von Willebrand factor (VWF) multimers result from proteolysis by the metalloprotease ADAMTS13. Since C2362F-VWF features abnormally large multimers with their triplet oligomer structure replaced by a diffuse smear, we explored the susceptibility of C2362F-VWF to ADAMTS13. VWF-enriched blood samples, obtained by cryoethanol precipitation of plasma from a patient with von Willebrand disease (VWD) homozygous for the C2362F mutation and a normal subject, were submitted to cleavage by recombinant ADAMTS13 under static conditions in the presence of urea. C2362F-VWF proved completely ADAMTS13-resistant in vitro. At any concentration of recombinant ADAMTS13 (from 0.1 µM to 1 µM), there was no evidence of the abnormally large VWF multimers of C2362F-VWF disappearing, nor any increased representation of triplet multimer bands, unlike the situation seen in normal VWF. This is due partly to a defective ADAMTS13 binding to C2362F-VWF under static conditions, as seen in both the patient's and recombinant mutated VWF proteins. These findings were associated with a significantly shorter than normal survival of C2362F-VWF after DDAVP, demonstrating that proteolysis and VWF survival may be independent phenomena. Our findings clearly demonstrate that the loss of cysteine 2362 makes VWF resistant to proteolysis by ADAMTS13, at least partly due to an impaired ADAMTS13 binding to VWF. This suggests that the B2 domain of VWF is involved in modulating ADAMTS13 binding to VWF and the consequent proteolytic process. The C2362F-VWF mutation also enables a new abnormality to be identified in the VWF-ADAMTS13 relationship, i.e. an ADAMTS13-resistant VWF.

Light chain of factor VIII is sufficient for accelerating cleavage of von Willebrand factor by ADAMTS13 metalloprotease

We previously demonstrated that coagulation factor VIII (FVIII) accelerates proteolytic cleavage of von Willebrand factor (VWF) by A disintegrin and metalloprotease with thrombospondin type 1 repeats (ADAMTS13) under fluid shear stress. In this study, the structural elements of FVIII required for the rate-enhancing effect and the biological relevance of this cofactor activity are determined using a murine model. An isolated light chain of human FVIII (hFVIII-LC) increases proteolytic cleavage of VWF by ADAMTS13 under shear in a concentration-dependent manner. The maximal rate-enhancing effect of hFVIII-LC is ∼8-fold, which is comparable with human full-length FVIII and B-domain deleted FVIII (hFVIII-BDD). The heavy chain (hFVIII-HC) and the light chain lacking the acidic (a3) region (hFVIII-LCΔa3) have no effect in accelerating VWF proteolysis by ADAMTS13 under the same conditions. Although recombinant hFVIII-HC and hFVIII-LCΔa3 do not detectably bind immobilized VWF, recombinant hFVIII-LC binds VWF with high affinity (K(D), ∼15 nM). Moreover, ultra-large VWF multimers accumulate in the plasma of fVIII(-/-) mice after hydrodynamic challenge but not in those reconstituted with either hFVIII-BDD or hFVIII-LC. These results suggest that the light chain of FVIII, which is not biologically active for clot formation, is sufficient for accelerating proteolytic cleavage of VWF by ADAMTS13 under fluid shear stress and (patho) physiological conditions. Our findings provide novel insight into the molecular mechanism of how FVIII regulates VWF homeostasis.

Essential domains of a disintegrin and metalloprotease with thrombospondin type 1 repeats-13 metalloprotease required for modulation of arterial thrombosis

OBJECTIVE

A disintegrin and metalloprotease with thrombospondin type 1 repeats-13 (ADAMTS13) inhibits platelet aggregation and arterial thrombosis by cleavage of von Willebrand factor. However, the structural components of ADAMTS13 required for inhibition of arterial thrombosis are not fully defined.

METHODS AND RESULTS

Using recombinant proteins and a murine model, we demonstrated that an ADAMTS13 variant truncated after either the eighth thrombospondin type 1 repeat or the spacer domain inhibits ferric chloride-induced arterial thrombosis in ADAMTS13(-/-) mice with efficacy similar to that of full-length ADAMTS13. The results obtained from monitoring thrombus formation in carotid and mesenteric arteries were highly concordant. Further analyses by site-directed mutagenesis and human monoclonal antibody inhibition assay revealed that the Cys-rich and spacer domains of ADAMTS13, particularly the amino acid residues between Arg559 and Glu664 in the spacer domain, may be critical for modulation of arterial thrombosis in vivo. Finally, the thrombosis-modulating function of ADAMTS13 and variants/mutants was highly correlated with the von Willebrand factor-cleavage activity under fluid shear stress.

CONCLUSIONS

Our results suggest that the amino terminus of ADAMTS13, specifically the variable region of the spacer domain, is crucial for modulation of arterial thromboses under (patho)physiological conditions. These findings shed more light on the structure-function relationship of ADAMTS13 in vivo and may be applicable for rational design of protein- or gene-based therapy of arterial thromboses.

Atherosclerosis and thrombosis: insights from large animal models

Atherosclerosis and its thrombotic complications are responsible for remarkably high numbers of deaths. The combination of in vitro, ex vivo, and in vivo experimental approaches has largely contributed to a better understanding of the mechanisms underlying the atherothrombotic process. Indeed, different animal models have been implemented in atherosclerosis and thrombosis research in order to provide new insights into the mechanisms that have already been outlined in isolated cells and protein studies. Yet, although no model completely mimics the human pathology, large animal models have demonstrated better suitability for translation to humans. Indeed, direct translation from mice to humans should be taken with caution because of the well-reported species-related differences. This paper provides an overview of the available atherothrombotic-like animal models, with a particular focus on large animal models of thrombosis and atherosclerosis, and examines their applicability for translational research purposes as well as highlights species-related differences with humans.

Escherichia coli-derived von Willebrand factor-A2 domain fluorescence/Förster resonance energy transfer proteins that quantify ADAMTS13 activity

The cleavage of the A2 domain of von Willebrand factor (VWF) by the metalloprotease ADAMTS13 regulates VWF size and platelet thrombosis rates. Reduction or inhibition of this enzyme activity leads to thrombotic thrombocytopenic purpura (TTP). We generated a set of novel molecules called VWF-A2 FRET (fluorescence/Förster resonance energy transfer) proteins, where variants of yellow fluorescent protein (Venus) and cyan fluorescent protein (Cerulean) flank either the entire VWF-A2 domain (175 amino acids) or truncated fragments (141, 113, and 77 amino acids) of this domain. These proteins were expressed in Escherichia coli in soluble form, and they exhibited FRET properties. Results show that the introduction of Venus/Cerulean itself did not alter the ability of VWF-A2 to undergo ADAMTS13-mediated cleavage. The smallest FRET protein, XS-VWF, detected plasma ADAMTS13 activity down to 10% of normal levels. Tests of acquired and inherited TTP could be completed within 30 min. VWF-A2 conformation changed progressively, and not abruptly, on increasing urea concentrations. Although proteins with 77 and 113 VWF-A2 residues were cleaved in the absence of denaturant, 4M urea was required for the efficient cleavage of larger constructs. Overall, VWF-A2 FRET proteins can be applied both for the rapid diagnosis of plasma ADAMTS13 activity and as a tool to study VWF-A2 conformation dynamics.

Mutation and ADAMTS13-dependent modulation of disease severity in a mouse model for von Willebrand disease type 2B

Von Willebrand disease (VWD)–type 2B originates from a gain-of-function mutation in von Willebrand factor (VWF), resulting in enhanced platelet binding. Clinical manifestations include increased bleeding tendency, loss of large multimers, thrombocytopenia, and circulating platelet aggregates. We developed a mouse model to study phenotypic consequences of VWD-type 2B mutations in murine VWF: mVWF/R1306Q and mVWF/V1316M. Both mutations allow normal multimerization but are associated with enhanced ristocetin-induced platelet aggregation, typical for VWD-type 2B. In vivo expression resulted in thrombocytopenia and circulating aggregates, both of which were more pronounced for mVWF/V1316M. Furthermore, both mutants did not support correction of bleeding time or arterial vessel occlusion in a thrombosis model. They further displayed a 2- to 3-fold reduced half-life and induced a 3- to 6-fold increase in number of giant platelets compared with wild-type VWF. Loss of large multimers was observed in 50% of the mice. The role of ADAMTS13 was investigated by expressing both mutants in VWF/ADAMTS13 double-deficient mice. ADAMTS13 deficiency resulted in more and larger circulating platelet aggregates for both mutants, whereas the full multimer range remained present in all mice. Thus, we established a mouse model for VWD-type 2B and found that phenotype depends on mutation and ADAMTS13.

Prospective study of hemostatic alterations in children with acute lymphoblastic leukemia

In a group of newly diagnosed acute lymphocytic leukemia (ALL) children we evaluated a number of hemostatic and inflammatory markers at diagnosis and at different time points during chemotherapy for the remission induction to identify alterations in the plasma levels of prothrombotic markers before and during the course of chemotherapy. The following plasma markers were evaluated: thrombin-antithrombin complex (TAT), D-Dimer, plasminogen activator inhibitor 1 (PAI-1), antithrombin, fibrinogen, von Willebrand factor (VWF) antigen and high molecular weight VWF (HMW-VWF) multimers, P-selectin, tumor necrosis factor alpha (TNF-alpha), and interleukin 6 (IL-6). Plasma samples were collected at the following time points: at T0 (baseline) and T1 (+24 days of therapy), T2 (+36 days therapy), and T3 (+64 days therapy). The results show that, at diagnosis, ALL children presented with laboratory signs of increased thrombin generation and fibrin formation (i.e. high TAT and D-dimer levels), fibrinolysis inhibition (i.e. high PAI-1 level), endothelial activation (i.e., high HMW-VWF and soluble P-selectin levels) and inflammation (i.e. high TNF-alpha and IL-6 levels). After starting induction therapy, the thrombin generation markers and inflammatory cytokines significantly decreased. To the opposite, PAI-1 and P-selectin significantly increased, suggesting an insult by chemotherapy on the vascular endothelium. These effects were more evident during steroid administration. Symptomatic venous thromboembolism (VTE) episodes developed in two cases during induction therapy, which did not allow the evaluation of the predictive value for VTE of laboratory markers.

Pathophysiology of thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a disorder with characteristic von Willebrand factor (VWF)-rich microthrombi affecting the arterioles and capillaries of multiple organs. The disorder frequently leads to early death unless the patients are treated with plasma exchange or infusion. Studies in the last decade have provided ample evidence to support that TTP is caused by deficiency of a plasma metalloprotease, ADAMTS13. When exposed to high shear stress in the microcirculation, VWF and platelets are prone to form aggregates. This propensity of VWF and platelet to form microvascular thrombosis is mitigated by ADAMTS13, which cleaves VWF before it is activated by shear stress to cause platelet aggregation in the circulation. Deficiency of ADAMTS13, due to autoimmune inhibitors in patients with acquired TTP and mutations of the ADAMTS13 gene in hereditary cases, leads to VWF-platelet aggregation and microvascular thrombosis of TTP. In this review, we discuss the current knowledge on the pathogenesis, diagnosis and management of TTP, address the ongoing controversies, and indicate the directions of future investigations.

Fluid shear induces conformation change in human blood protein von Willebrand factor in solution

Many of the physiological functions of von Willebrand Factor (VWF), including its binding interaction with blood platelets, are regulated by the magnitude of applied fluid/hydrodynamic stress. We applied two complementary strategies to study the effect of fluid forces on the solution structure of VWF. First, small-angle neutron scattering was used to measure protein conformation changes in response to laminar shear rates (G) up to 3000/s. Here, purified VWF was sheared in a quartz Couette cell and protein conformation was measured in real time over length scales from 2-140 nm. Second, changes in VWF structure up to 9600/s were quantified by measuring the binding of a fluorescent probe 1,1'-bis(anilino)-4-,4'-bis(naphthalene)-8,8'-disulfonate (bis-ANS) to hydrophobic pockets exposed in the sheared protein. Small angle neutron scattering studies, coupled with quantitative modeling, showed that VWF undergoes structural changes at G < 3000/s. These changes were most prominent at length scales <10 nm (scattering vector (q) range >0.6/nm). A mathematical model attributes these changes to the rearrangement of domain level features within the globular section of the protein. Studies with bis-ANS demonstrated marked increase in bis-ANS binding at G > 2300/s. Together, the data suggest that local rearrangements at the domain level may precede changes at larger-length scales that accompany exposure of protein hydrophobic pockets. Changes in VWF conformation reported here likely regulate protein function in response to fluid shear.

Platelet adhesion under flow

Platelet-adhesive mechanisms play a well-defined role in hemostasis and thrombosis, but evidence continues to emerge for a relevant contribution to other pathophysiological processes, including inflammation, immune-mediated responses to microbial and viral pathogens, and cancer metastasis. Hemostasis and thrombosis are related aspects of the response to vascular injury, but the former protects from bleeding after trauma, while the latter is a disease mechanism. In either situation, adhesive interactions mediated by specific membrane receptors support the initial attachment of single platelets to cellular and extracellular matrix constituents of the vessel wall and tissues. In the subsequent steps of thrombus growth and stabilization, adhesive interactions mediate platelet-to-platelet cohesion (i.e., aggregation) and anchoring to the fibrin clot. A key functional aspect of platelets is their ability to circulate in a quiescent state surveying the integrity of the inner vascular surface, coupled to a prompt reaction wherever alterations are detected. In many respects, therefore, platelet adhesion to vascular wall structures, to one another, or to other blood cells are facets of the same fundamental biological process. The adaptation of platelet-adhesive functions to the effects of blood flow is the main focus of this review.

Mechanisms of microvascular thrombosis in thrombotic thrombocytopenic purpura

Recent studies have demonstrated that thrombotic thrombocytopenic purpura (TTP), a serious thrombotic disorder affecting the arterioles and capillaries of multiple organs, is caused by a profound deficiency in the von Willebrand factor cleaving metalloprotease, ADAMTS13. ADAMTS13, a 190-kD plasma protease originating primarily in hepatic stellate cells, prevents microvascular thrombosis by cleaving von Willebrand factor when the substrate is conformationally unfolded by high levels of shear stress in the circulation. Deficiency of ADAMTS13, due to genetic mutations or inhibitory autoantibodies, leads to accumulation of superactive forms of vWF, resulting in vWF-platelet aggregation and microvascular thrombosis. Analysis of ADAMTS13 has led to the recognition of subclinical TTP and atypical TTP presenting with thrombocytopenia or acute focal neurological deficits without concurrent microangiopathic hemolysis. Infusion of plasma replenishes the missing ADAMTS13 and ameliorates the complications of hereditary TTP. The patients are at risk of both acute and chronic renal failure if they receive inadequate plasma therapy. The more frequent, autoimmune type of TTP requires plasma exchange therapy and perhaps immunomodulatory measures. Current studies focus on the factors affecting the phenotypic severity of TTP and newer approaches to improving the therapies for the patients.

The distal carboxyl-terminal domains of ADAMTS13 are required for regulation of in vivo thrombus formation

ADAMTS13 is a multidomain protease that limits platelet thrombogenesis through the cleavage of von Willebrand factor (VWF). We previously identified 2 types of mouse Adamts13 gene: the 129/Sv-strain Adamts13 gene encodes the long-form ADAMTS13 having the same domains as human ADAMTS13, whereas the C57BL/6-strain Adamts13 gene encodes the short-form ADAMTS13 lacking the distal C-terminal domains. To assess the physiologic significance of the distal C-terminal domains of ADAMTS13, we generated and analyzed 129/Sv-genetic background congenic mice (Adamts13(S/S)) that carry the short-form ADAMTS13. Similar to wild-type 129/Sv mice (Adamts13(L/L)), Adamts13(S/S) did not have ultralarge VWF multimers in plasma, in contrast to 129/Sv-genetic background ADAMTS13-deficient mice (Adamts13(-/-)). However, in vitro thrombogenesis under flow at a shear rate of 5000 s(-1) was accelerated in Adamts13(S/S) compared with Adamts13(L/L). Both in vivo thrombus formation in ferric chloride-injured arterioles and thrombocytopenia induced by collagen plus epinephrine challenge were more dramatic in Adamts13(S/S) than in Adamts13(L/L) but less than in Adamts13(-/-). These results suggested that the C-terminally truncated ADAMTS13 exhibited decreased activity in the cleavage of VWF under high shear rate. Role of the C-terminal domains may become increasingly important under prothrombotic conditions.

Function of von Willebrand factor in haemostasis and thrombosis

The physiological protection against bleeding is secured by platelet adhesion to the site of injury and sealing of the defect. The first step involves the arrest of platelets that have adhered to subendothelial structures, primarily collagen, at the site of injury. Under conditions of low shear rates, platelet adhesion to the damaged vessel wall is mediated by several proteins, including von Willebrand factor (VWF). However, under conditions of high shear, aggregation occurs only in the presence of soluble VWF. In solution, VWF becomes immobilized via its A3 domain on the fibrillar collagen of the vessel wall and acts as an intermediary between collagen and the platelet receptor glycoprotein Ibalpha (GPIbalpha), which is the only platelet receptor that does not require prior activation for bond formation. After GPIbalpha binds to the A1 domain of its main ligand VWF, further activation of the platelet via intracellular signalling occurs, allowing other receptors to engage VWF and collagen and thereby reinforcing permanent adhesion. On this first layer of adherent platelets, soluble VWF binds and uncoils, thereby attracting more platelets. Platelet interaction with immobilized and soluble VWF may also generate platelet-derived microparticles that exhibit pro-coagulant activity. Full growth of a multilayered platelet aggregate comprises binding of the platelet receptor integrin alphaIIbbeta3 to VWF and fibrinogen. In addition, the surface of the activated platelets accelerates the coagulation cascade, which, by its end product fibrin, stabilizes the growing platelet thrombus. This article summarizes the characteristics and role of VWF in the coagulation cascade.

Association Between Thrombotic Microangiopathy and Reduced ADAMTS13 Activity in Malignant Hypertension

The thrombotic microangiopathy observed in malignant hypertension is similar to that of thrombotic thrombocytopenic purpura, which is associated with a deficiency of ADAMTS13, a von Willebrand factor (VWF)–cleaving protease that cleaves large prothrombogenic multimers. We hypothesized that ADAMTS13 is deficient in malignant hypertension and that the severity of thrombotic microangiopathy is associated with decreased ADAMTS13 activity. We included 20 patients with malignant and 20 patients with severe hypertension, and 20 matched normotensive individuals served as control subjects. VWF, active VWF, and free hemoglobin were assessed to explore predictors of ADAMTS13 activity. Patients with malignant hypertension had lower ADAMTS13 activity (80%; interquartile range: 53% to 130%) compared with control subjects (99% interquartile range: 82% to 129%; P <0.01) but not compared with patients with severe hypertension ( P =0.14). ADAMTS13 activity negatively correlated with lactic dehydrogenase levels after logarithmic transformation ( r =−0.65; P <0.001) and was associated with platelet count ( r =0.34; P =0.04) and the presence of schistocytes ( r =−0.37; P =0.02). Apart from the association with thrombotic microangiopathy, ADAMTS13 was inversely associated with creatinine ( r =−0.42; P =0.008). Increasing levels of VWF were associated with a decrease in ADAMTS13 activity ( r =−0.34; P =0.03). There was no significant association between ADAMTS13 activity and other parameters, including blood pressure. In conclusion, ADAMTS13 is decreased in malignant hypertension and associated with the severity of thrombotic microangiopathy, likely because of the release of VWF after endothelium stimulation. A severe deficiency could not be demonstrated. More studies are needed to identify the role of ADAMTS13 in the thrombotic microangiopathy and ischemic complications of malignant hypertension.

Degradation of circulating von Willebrand factor and its regulator ADAMTS13 implicates secreted Bacillus anthracis metalloproteases in anthrax consumptive coagulopathy

Pathology data from the anthrax animal models show evidence of significant increases in vascular permeability coincident with hemostatic imbalances manifested by thrombocytopenia, transient leucopenia, and aggressive disseminated intravascular coagulation. In this study we hypothesized that anthrax infection modulates the activity of von Willebrand factor (VWF) and its endogenous regulator ADAMTS13, which play important roles in hemostasis and thrombosis, including interaction of endothelial cells with platelets. We previously demonstrated that purified anthrax neutral metalloproteases Npr599 and InhA are capable of cleaving a variety of host structural and regulatory proteins. Incubation of human plasma with these proteases at 37 degrees C in the presence of urea as a mild denaturant results in proteolysis of VWF. Also in these conditions, InhA directly cleaves plasma ADAMTS13 protein. Npr599 and InhA digest synthetic VWF substrate FRETS-VWF73. Amino acid sequencing of VWF fragments produced by InhA suggests that one of the cleavage sites of VWF is located at domain A2, the target domain of ADAMTS13. Proteolysis of VWF by InhA impairs its collagen binding activity (VWF:CBA) and ristocetin-induced platelet aggregation activity. In plasma from anthrax spore-challenged DBA/2 mice, VWF antigen levels increase up to 2-fold at day 3 post-infection with toxigenic Sterne 34F(2) strain, whereas VWF:CBA levels drop in a time-dependent manner, suggesting dysfunction of VWF instead of its quantitative deficiency. This conclusion is further supported by significant reduction in the amount of VWF circulating in blood in the ultra-large forms. In addition, Western blot analysis shows proteolytic depletion of ADAMTS13 from plasma of spore-challenged mice despite its increased expression in the liver. Our results suggest a new mechanism of anthrax coagulopathy affecting the levels and functional activities of both VWF and its natural regulator ADAMTS13. This mechanism may contribute to hemorrhage and thrombosis typical in anthrax.

Functional imaging of shear-dependent activity of ADAMTS13 in regulating mural thrombus growth under whole blood flow conditions

The metalloprotease ADAMTS13 is assumed to regulate the functional levels of von Willebrand factor (VWF) appropriate for normal hemostasis in vivo by reducing VWF multimer size, which directly represents the thrombogenic activity of this factor. Using an in vitro perfusion chamber system, we studied the mechanisms of ADAMTS13 action during platelet thrombus formation on a collagen surface under whole blood flow conditions. Inhibition studies with a function-blocking anti-ADAMTS13 antibody, combined with immunostaining of thrombi with an anti-VWF monoclonal antibody that specifically reflects the VWF-cleaving activity of ADAMTS13, provided visual evidence for a shear rate–dependent action of ADAMTS13 that limits thrombus growth directly at the site of the ongoing thrombus generation process. Our results identify an exquisitely specific regulatory mechanism that prevents arterial occlusion under high shear rate conditions during mural thrombogenesis.

The kidney in thrombotic thrombocytopenic purpura

The kidney is commonly affected in thrombotic thrombocytopenic purpura (TTP), a multi-system disorder with microvascular thrombosis of the capillaries and arterioles. Nevertheless, due to difference in its diagnostic criteria, the frequency and severity of renal dysfunction in TTP remains controversial. Recent studies indicate that the severe deficiency of a von Willebrand factor (VWF) cleaving protease, ADAMTS13, is the main cause of platelet thrombosis in TTP; it is now possible to define TTP at the molecular level. Among patients with acquired TTP due to inhibitory antibodies of ADAMTS13, renal dysfunction is usually mild; oliguria, fluid overload, hypertension, and need of dialysis support are infrequent. When any of these complications occur, one must re-examine the diagnosis of TTP and consider the possibility that the patient has another cause for these complications. In a patient with hereditary TTP, acute renal failure may ensue. However, the renal dysfunction is mostly reversible if the patients are promptly treated with plasma to replenish ADAMTS13. Patients with TTP, particularly of the hereditary type, may develop chronic renal failure. This complication may be a consequence of repeated insults by overt or subclinical microvascular thrombosis to the kidney, or it may have a separate cause. Therapy of hereditary TTP should aim not only to prevent acute exacerbations but also to minimize the risk of irreversible renal injury.

Molecular pathways used by platelets to initiate and accelerate atherogenesis

PURPOSE OF REVIEW

The response to injury model in the development of atherosclerosis is broadly accepted by the scientific audience. Platelets are generally not believed to be involved in the initiation of atherosclerosis. New data imply, however, that the response to injury model is too simple for a complete understanding of the inflammatory disease atherosclerosis. The involvement of platelets in the initiation of atherosclerotic lesion formation is critical in directing the atherosclerotic process into regeneration or ongoing vascular injury.

RECENT FINDINGS

Platelets internalize oxidized phospholipids and promote foam cell formation. Platelets also recruit circulating blood cells including progenitor cells to the vessel, that are able to differentiate into foam cells or endothelial cells depending on conditions. Platelets express various scavenger receptors that are able to regulate LDL-uptake. LDL-laden platelets are internalized by adherent progenitor cells that in turn differentiate into macrophages and foam cells.

SUMMARY

An expanding body of evidence continues to build on the role of platelets as initial actors in the development of atherosclerotic lesions. Platelets bind to leukocytes, endothelial cells, and circulating progenitor cells and initiate monocyte transformation into macrophages. Therefore platelets regulate the initiation, development and total extent of atherosclerotic lesions.

Adhesion mechanisms in platelet function

Platelet adhesion is an essential function in response to vascular injury and is generally viewed as the first step during which single platelets bind through specific membrane receptors to cellular and extracellular matrix constituents of the vessel wall and tissues. This response initiates thrombus formation that arrests hemorrhage and permits wound healing. Pathological conditions that cause vascular alterations and blood flow disturbances may turn this beneficial process into a disease mechanism that results in arterial occlusion, most frequently in atherosclerotic vessels of the heart and brain. Besides their relevant role in hemostasis and thrombosis, platelet adhesive properties are central to a variety of pathophysiological processes that extend from inflammation to immune-mediated host defense and pathogenic mechanisms as well as cancer metastasis. All of these activities depend on the ability of platelets to circulate in blood as sentinels of vascular integrity, adhere where alterations are detected, and signal the abnormality to other platelets and blood cells. In this respect, therefore, platelet adhesion to vascular wall structures, to one another (aggregation), or to other blood cells, represent different aspects of the same fundamental biological process. Detailed studies by many investigators over the past several years have been aimed to dissect the complexity of these functions, and the results obtained now permit an attempt to integrate all the available information into a picture that highlights the balanced diversity and synergy of distinct platelet adhesive interactions.

The presence of active von Willebrand factor under various pathological conditions

PURPOSE OF REVIEW

To highlight mechanisms that regulate the balance between latent and active von Willebrand factor (VWF), and describe pathological conditions leading to increased levels of active VWF.

RECENT FINDINGS

Levels of circulating active VWF are increased in von Willebrand disease type 2B, HELLP syndrome, malaria and antiphospholipid syndrome.

SUMMARY

Freshly secreted VWF consists of ultra-large multimers that interact spontaneously with platelets at the endothelial cell surface. Proteolysis of ultra-large VWF by a member of the disintegrin and metalloprotease with thrombospondin motif family (ADAMTS13) reduces both multimeric size and accessibility of platelet-adhesion sites. The resulting VWF molecules circulate as inactive multimers, which regain their platelet-adhesion capacity upon binding to the subendothelial matrix, in particular under conditions of high shear. Unfortunately, mechanisms responsible for suppression of circulating plasma levels of active VWF are hampered in a number of pathological conditions, leading to VWF-platelet aggregates associated with thrombotic complications or thrombocytopenia. A recently developed assay allowed us to monitor the presence of circulating active VWF and we found that several diseases are characterized by increased levels. Further analysis provided insight into mechanisms contributing to the presence of active VWF, which revealed that beta2-glycoprotein I may act as a natural regulator of VWF-platelet interactions.

Structural and functional correlation of ADAMTS13

PURPOSE OF REVIEW

ADAMTS13 represents a landmark in a journey that began over 80 years ago with a single clinical case. Thrombotic thrombocytopenic purpura exemplifies how von Willebrand factor can be responsible for life-threatening thrombosis. This review summarizes recent progress on ADAMTS13, which prevents this deadly event.

RECENT FINDINGS

Recent advances are summarized in four main areas. First, the core ADAMTS13-binding site is contained in a short sequence in the A2 domain, but other domains affect this interaction. Mutations from thrombotic thrombocytopenic purpura and von Willebrand disease provide clues for the structural prerequisites and regulation of von Willebrand factor cleavage. Second, studies are unraveling the reasons why urea, BaCl2, and low ionic strength are required to cleave von Willebrand factor under static conditions. Third, studies on thrombotic thrombocytopenic purpura and ADAMTS13-knockout mice suggest that ADAMTS13 deficiency alone may not be sufficient to cause thrombotic thrombocytopenic purpura. Finally, ADAMTS13 could be an antithrombotic agent for thrombotic thrombocytopenic purpura and other thrombotic conditions.

SUMMARY

Study of ADAMTS13 has exploded since this metalloprotease was characterized. This knowledge reveals the nature of ADAMTS13's interaction with von Willebrand factor and the pathogenesis of clinical thrombotic thrombocytopenic purpura, especially in relation to ADAMTS13.

Shear-induced unfolding triggers adhesion of von Willebrand factor fibers

von Willebrand factor (VWF), a protein present in our circulatory system, is necessary to stop bleeding under high shear-stress conditions as found in small blood vessels. The results presented here help unravel how an increase in hydrodynamic shear stress activates VWF's adhesion potential, leading to the counterintuitive phenomena of enhanced adsorption rate under strong shear conditions. Using a microfluidic device, we were able to mimic a wide range of bloodflow conditions and directly visualize the conformational dynamics of this protein under shear flow. In particular, we find that VWF displays a reversible globule-stretch transition at a critical shear rate gamma(crit) in the absence of any adsorbing surface. Computer simulations reproduce this sharp transition and identify the large size of VWF's repeating units as one of the keys for this unique hydrodynamic activation. In the presence of an adsorbing collagen substrate, we find a large increase in the protein adsorption at the same critical shear rate, suggesting that the globule unfolding in bulk triggers the surface adsorption in the case of a collagen substrate, which provides a sufficient density of binding sites. Monitoring the adsorption process of multiple VWF fibers, we were able to follow the formation of an immobilized network that constitutes a "sticky" grid necessary for blood platelet adhesion under high shear flow. Because areas of high shear stress coincide with a higher chance for vessel wall damage by mechanical forces, we identified the shear-induced increase in the binding probability of VWF as an effective self-regulating repair mechanism of our microvascular system.

Formation of platelet strings and microthrombi in the presence of ADAMTS-13 inhibitor does not require P-selectin or beta3 integrin

BACKGROUND

Ultra-large von Willebrand factor (ULVWF) and the receptor P-selectin are released from endothelial Weibel-Palade bodies during injury or inflammation. VWF mediates platelet adhesion and P-selectin promotes leukocyte rolling. ADAMTS-13 limits the duration of platelet adhesion by cleaving the ULVWF. In the absence of ADAMTS-13, long VWF filaments decorated with platelets form. Recent in vitro studies suggested that P-selectin might anchor these platelet strings to endothelium, but whether the same mechanism exists in vivo remains to be elucidated.

METHODS

We address the role of P-selectin and beta(3) integrin in platelet string formation in vivo using intravital microscopy by infusing inhibitory ADAMTS-13 antibody in P-selectin-/- and beta(3)-deficient mice and activating the endothelium by injecting histamine.

RESULTS

We show that inhibition of ADAMTS-13 combined with endothelial activation leads to similar extents of platelet string formation in wild-type, P-selectin- and integrin beta(3)-deficient mice. Further, in venules the platelet strings can coalesce into VWF-platelet aggregates. This process utilizes neither the platelet beta(3) integrin nor P-selectin. We also show in vitro that platelets can act as a bridge between the VWF fibers and that VWF can self-associate even in areas devoid of platelets.

CONCLUSIONS

The formation or retention of the platelet strings does not require P-selectin or the endothelial VWF receptor alpha(v)beta(3). Furthermore, in the presence of low ADAMTS-13 activity, VWF-dependent and alpha(IIb)beta(3)-independent platelet clustering occurs in veins, as has been shown at high arterial shear rates. Our study further supports the importance of regulation of VWF multimer size upon secretion from Weibel-Palade bodies.

ADAMTS13 and microvascular thrombosis

Interaction between platelet and von Willebrand factor, a circulating adhesive glycoprotein, is essential for hemostasis under the high shear environments of arterioles and capillaries. If unregulated, this interaction may lead to unwarranted platelet thrombosis. ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 motif, number 13), a plasma zinc metalloprotease synthesized primarily in the stellate cells of the liver, cleaves shear stress-activated von Willebrand factor, thereby preventing the occurrence of von Willebrand factor-platelet interaction in the circulation. A profound deficiency of ADAMTS13, due to genetic mutations or autoimmune inhibition, results in intravascular von Willebrand factor platelet aggregation and widespread microvascular thrombosis characteristic of thrombotic thrombocytopenic purpura. Cloning of ADAMTS13 and structure-function analyses of the enzyme are leading to exciting advances in the diagnosis and therapy of this hitherto mysterious disease.

Activation-independent platelet adhesion and aggregation under elevated shear stress

Platelet aggregation, which contributes to bleeding arrest and also to thrombovascular disorders, is thought to initiate after signaling-induced activation. We found that this paradigm does not apply under blood flow conditions comparable to those existing in stenotic coronary arteries. Platelets interacting with immobilized von Willebrand factor (VWF) aggregate independently of activation when soluble VWF is present and the shear rate exceeds 10 000 s(-1) (shear stress = 400 dyn/cm(2)). Above this threshold, active A1 domains become exposed in soluble VWF multimers and can bind to glycoprotein Ibalpha, promoting additional platelet recruitment. Aggregates thus formed are unstable until the shear rate approaches 20 000 s(-1) (shear stress = 800 dyn/cm.(2)). Above this threshold, adherent platelets at the interface of surface-immobilized and membrane-bound VWF are stretched into elongated structures and become the core of aggregates that can persist on the surface for minutes. When isolated dimeric A1 domain is present instead of native VWF multimers, activation-independent platelet aggregation occurs without requiring shear stress above a threshold level, but aggregates never become firmly attached to the surface and progressively disaggregate as shear rate exceeds 6000 s(-1). Platelet and VWF modulation by hydrodynamic force is a mechanism for activation-independent aggregation that may support thrombotic arterial occlusion.