von Willebrand factor cleaved from endothelial cells by ADAMTS13 remains ultralarge in size

Backbone resonance assignments of the A2 domain of mouse von Willebrand factor

von Willebrand factor (vWF) is an adhesive plasma protein that is important for platelet adhesion in normal hemostasis in response to vascular injury. Although large vWF multimers are released from storage granules of platelets and (sub-)endothelial cells in response to hemostatic stimuli, for normal physiological function, vWF multimers are required to be cleaved into smaller multimeric forms. The plasma metalloproteinase ADAMTS13 specifically cleaves the peptide bond located in the middle of the A2 domain of vWF (vWF-A2), but the cleavage site is buried inside the structure of vWF and is difficult to access in the absence of elevated flow shear stress. On the other hand, in the presence of high vascular shear stress, the structure of vWF-A2 is supposed to be unfolded, thereby becoming accessible for proteolysis by ADAMTS13. However, the atomic-level mechanism underlying shear-induced structural changes of vWF-A2 remains unclear and to date no solution NMR information is available. In this study, we present the backbone 1H, 13C, and 15N resonance assignments of mouse vWF-A2; side chain assignments of 13Cβ are also provided. Secondary structure propensity analysis based on the assigned chemical shifts showed that mouse vWF-A2 forms similar secondary structures in solution to the previously determined crystal structure of human vWF-A2. The obtained NMR assignment data will contribute to an atomic-level characterization of shear-induced unfolding of vWF-A2 in solution.

Plasma ADAMTS13 activity and von Willebrand factor antigen and activity in patients with subarachnoid haemorrhage

Increased von Willebrand factor (VWF) and reduced ADAMTS13 activity are associated with arterial thrombosis. This may also be the culprit mechanism implicated in delayed cerebral ischaemia after aneurysmal subarachnoid haemorrhage (SAH). It was our objective to determine plasma VWF and ADAMTS13 in patients with SAH and healthy subjects; and to explore the levels of those markers and outcome after SAH. Forty consecutive patients were enrolled between September 2007 and April 2014 in a pilot study. Plasma samples were collected from SAH patients on post-bleed day (PBD) 0, 1, 3, 5, 7 and 10 and healthy controls. VWF antigen (VWFAg) and VWF activity (VWFAc) were determined by enzyme-linked immunoassay and collagen binding assay, respectively. ADAMTS13 activity was determined by the cleavage of a fluorescent substrate. Univariate descriptive statistics and cluster analyses were performed based on outcomes in the group with SAH only. Mean age of SAH patients was 52.4 years (26-84 years) and 30 (75 %) were women. 12/40 (30 %) had a high Hunt and Hess grade (IV-V) and 25 (62.5 %) were treated with coil embolisation. Plasma VWFAg and VWFAc were significantly higher in SAH patients than those in healthy subjects on each PBD (p<0.0001). Concurrently, plasma ADAMTS13 activity in SAH patients was significantly lower than that in healthy subjects (p<0.0001). Among those with SAH, cluster analysis demonstrated that patients with higher VWFAg and VWFAc and/or lower ADAMTS13 activity might be at risk of increased mortality. In conclusion, the relative deficiency of plasma ADAMTS13 activity in SAH patients may associate with worse outcome.

ADAMTS13 and von Willebrand factor in thrombotic thrombocytopenic purpura

Pathogenesis of thrombotic thrombocytopenic purpura (TTP) was a mystery for over half a century until the discovery of ADAMTS13. ADAMTS13 is primarily synthesized in the liver, and its main function is to cleave von Willebrand factor (VWF) anchored on the endothelial surface, in circulation, and at the sites of vascular injury. Deficiency of plasma ADAMTS13 activity (<10%) resulting from mutations of the ADAMTS13 gene or autoantibodies against ADAMTS13 causes hereditary or acquired (idiopathic) TTP. ADAMTS13 activity is usually normal or modestly reduced (>20%) in other forms of thrombotic microangiopathy secondary to hematopoietic progenitor cell transplantation, infection, and disseminated malignancy or in hemolytic uremic syndrome. Plasma infusion or exchange remains the initial treatment of choice to date, but novel therapeutics such as recombinant ADAMTS13 and gene therapy are under development. Moreover, ADAMTS13 deficiency has been shown to be a risk factor for the development of myocardial infarction, stroke, cerebral malaria, and preeclampsia.

Platelet-delivered ADAMTS13 inhibits arterial thrombosis and prevents thrombotic thrombocytopenic purpura in murine models

ADAMTS13 metalloprotease cleaves von Willebrand factor (VWF), thereby inhibiting platelet aggregation and arterial thrombosis. An inability to cleave ultralarge VWF resulting from hereditary or acquired deficiency of plasma ADAMTS13 activity leads to a potentially fatal syndrome, thrombotic thrombocytopenic purpura (TTP). Plasma exchange is the most effective initial therapy for TTP to date. Here, we report characterization of transgenic mice expressing recombinant human ADAMTS13 (rADAMTS13) in platelets and its efficacy in inhibiting arterial thrombosis and preventing hereditary and acquired antibody-mediated TTP in murine models. Western blotting and fluorescent resonance energy transfer assay detect full-length rADAMTS13 protein and its proteolytic activity, respectively, in transgenic (Adamts13(-/-)Plt(A13)), but not in wild-type and Adamts13(-/-), platelets. The expressed rADAMTS13 is released on stimulation with thrombin and collagen, but less with 2MesADP. Platelet-delivered rADAMTS13 is able to inhibit arterial thrombosis after vascular injury and prevent the onset and progression of Shigatoxin-2 or recombinant murine VWF-induced TTP syndrome in mice despite a lack of plasma ADAMTS13 activity resulting from the ADAMTS13 gene deletion or the antibody-mediated inhibition of plasma ADAMTS13 activity. These findings provide a proof of concept that platelet-delivered ADAMTS13 may be explored as a novel treatment of arterial thrombotic disorders, including hereditary and acquired TTP, in the presence of anti-ADAMTS13 autoantibodies.

Fluid Mechanics of Blood Clot Formation

Intravascular blood clots form in an environment in which hydrodynamic forces dominate and in which fluid-mediated transport is the primary means of moving material. The clotting system has evolved to exploit fluid dynamic mechanisms and to overcome fluid dynamic challenges to ensure that clots that preserve vascular integrity can form over the wide range of flow conditions found in the circulation. Fluid-mediated interactions between the many large deformable red blood cells and the few small rigid platelets lead to high platelet concentrations near vessel walls where platelets contribute to clotting. Receptor-ligand pairs with diverse kinetic and mechanical characteristics work synergistically to arrest rapidly flowing cells on an injured vessel. Variations in hydrodynamic stresses switch on and off the function of key clotting polymers. Protein transport to, from, and within a developing clot determines whether and how fast it grows. We review ongoing experimental and modeling research to understand these and related phenomena.

Update on von Willebrand factor multimers: focus on high-molecular-weight multimers and their role in hemostasis

Normal hemostasis requires von Willebrand factor (VWF) to support platelet adhesion and aggregation at sites of vascular injury. VWF is a multimeric glycoprotein built from identical subunits that contain binding sites for both platelet glycoprotein receptors and collagen. The adhesive activity of VWF depends on the size of its multimers, which range from 500 to over 10 000 kDa. There is good evidence that the high-molecular-weight multimers (HMWM), which are 5000-10 000 kDa, are the most effective in supporting interaction with collagen and platelet receptors and in facilitating wound healing under conditions of shear stress. Thus, these HMWM of VWF are of particular clinical interest. The unusually large multimers of VWF are, under normal conditions, cleaved by the plasma metalloproteinase ADAMTS13 to smaller, less adhesive multimers. A reduction or lack of HMWM, owing to a multimerization defect of VWF or to an increased susceptibility of VWF for ADAMTS13, leads to a functionally impaired VWF and the particular type 2A of von Willebrand disease. This review considers the biology and function of VWF multimers with a particular focus on the characterization of HMWM - their production, storage, release, degradation, and role in normal physiology. Evidence from basic research and the study of clinical diseases and their management highlight a pivotal role for the HMWM of VWF in hemostasis.

Carboxyl terminus of ADAMTS13 directly inhibits platelet aggregation and ultra large von Willebrand factor string formation under flow in a free-thiol-dependent manner

OBJECTIVE

ADAMTS13 (A Disintegrin And Metalloprotease with Thrombospondin type 1 repeats, 13) cleaves von Willebrand factor (VWF), thereby inhibiting thrombus formation. Proteolytic cleavage relies on the amino-terminal (MDTCS) domains, but the role of the more distal carboxyl-terminal domains of ADAMTS13 is not fully understood. A previous study demonstrated the presence of multiple surface-exposed free sulfhydryls on ADAMTS13 that seemed to interact with those on VWF under shear. Here, we determined the physiological relevance of such an interaction in antithrombotic responses under flow.

APPROACH AND RESULTS

A microfluidic assay demonstrated that a carboxyl-terminal fragment of ADAMTS13, comprising either 2 to 8 thrombospondin type 1 (TSP1) repeats and CUB domains (T2C) or 5 to 8 Thrombospondin type 1 (TSP1) repeats and CUB domains (T5C), directly inhibited platelet adhesion/aggregation on a collagen surface under arterial shear. In addition, an intravital microscopic imaging analysis showed that the carboxyl-terminal fragment of ADAMTS13 (T2C or T5C) was capable of inhibiting the formation and elongation of platelet-decorated ultra large (UL) VWF strings and the adhesion of platelets/leukocytes on endothelium in mesenteric venules after oxidative injury. The inhibitory activity of T2C and T5C on platelet aggregation and ULVWF string formation were dependent on the presence of their surface free thiols; pretreatment of T2C and T5C or full-length ADAMTS13 with N-ethylmaleimide that reacts with free sulfhydryls abolished or significantly reduced its antithrombotic activity.

CONCLUSIONS

Our results demonstrate for the first time that the carboxyl terminus of ADAMTS13 has direct antithrombotic activity in a free-thiol-dependent manner. The free thiols in the carboxyl-terminal domains of ADAMTS13 may also contribute to the overall antithrombotic function of ADAMTS13 under pathophysiological conditions.

Structure-function and regulation of ADAMTS-13 protease

ADAMTS-13, a plasma reprolysin-like metalloprotease, cleaves von Willebrand factor (VWF). Severe deficiency of plasma ADAMTS-13 activity results in thrombotic thrombocytopenic purpura (TTP), while mild to moderate deficiencies of plasma ADAMTS-13 activity are emerging risk factors for developing myocardial and cerebral infarction, pre-eclampsia, and malignant malaria. Moreover, Adamts13(-/-) mice develop more severe inflammatory responses, leading to increased ischemia/perfusion injury and formation of atherosclerosis. Structure-function studies demonstrate that the N-terminal portion of ADAMTS-13 (MDTCS) is necessary and sufficient for proteolytic cleavage of VWF under various conditions and attenuation of arterial/venous thrombosis after oxidative injury. The more distal portion of ADAMTS-13 (TSP1 2-8 repeats and CUB domains) may function as a disulfide bond reductase to prevent an elongation of ultra-large VWF strings on activated endothelial cells and inhibit platelet adhesion/aggregation on collagen surface under flow. Remarkably, the proteolytic cleavage of VWF by ADAMTS-13 is accelerated by FVIII and platelets under fluid shear stress. A disruption of the interactions between FVIII (or platelet glycoprotein 1bα) and VWF dramatically impairs ADAMTS-13-dependent proteolysis of VWF in vitro and in vivo. These results suggest that FVIII and platelets may be physiological cofactors regulating VWF proteolysis. Finally, the structure-function and autoantibody mapping studies allow us to identify an ADAMTS-13 variant with increased specific activity but reduced inhibition by autoantibodies in patients with acquired TTP. Together, these findings provide novel insight into the mechanism of VWF proteolysis and tools for the therapy of acquired TTP and perhaps other arterial thrombotic disorders.

Weibel-Palade bodies: a window to von Willebrand disease

Weibel-Palade bodies (WPBs) are the storage organelles for von Willebrand factor (VWF) in endothelial cells. VWF forms multimers that assemble into tubular structures in WPBs. Upon demand, VWF is secreted into the blood circulation, where it unfolds into strings that capture platelets during the onset of primary hemostasis. Numerous mutations affecting VWF lead to the bleeding disorder von Willebrand disease. This review reports the recent findings on the effects of VWF mutations on the biosynthetic pathway of VWF and its storage in WPBs. These new findings have deepened our understanding of VWF synthesis, storage, secretion, and function.

Unwinding the von Willebrand factor strings puzzle

von Willebrand factor (VWF) is amongst others synthesized by endothelial cells and stored as ultra-large (UL) VWF multimers in Weibel-Palade bodies. Although UL-VWF is proteolysed by ADAMTS13 (a disintegrin-like and metalloprotease domain with thrombospondin type-1 motif, number 13) on secretion from endothelial cells, in vitro experiments in the absence of ADAMTS13 have demonstrated that a proportion of these UL-VWF multimers remain anchored to the activated endothelium. These multimers unravel, bind platelets, and wave in the direction of the flow. These so-called VWF "strings" have also been visualized in vivo, lining the lumen of activated mesenteric veins of Adamts13(-/-) mice. Various studies have demonstrated the extraordinary length of these VWF strings, the availability of their platelet binding and ADAMTS13 cleavage sites, and the possible nature of their endothelial attachment. VWF strings are also capable of tethering leukocytes and parasite-infected red blood cells. However, the majority of studies have been performed in the absence of ADAMTS13, a condition only experienced in thrombotic thrombocytopenic purpura. A normal functional role of VWF strings in healthy persons or in other disease pathologies remains unclear. In this review, we discuss some of the puzzling characteristics of VWF strings, and we debate whether the properties of VWF strings in the absence of ADAMTS13 might be relevant for understanding (patho)physiologic mechanisms.

Antigen and substrate withdrawal in the management of autoimmune thrombotic disorders

Prevailing approaches to manage autoimmune thrombotic disorders, such as heparin-induced thrombocytopenia, antiphospholipid syndrome and thrombotic thrombocytopenic purpura, include immunosuppression and systemic anticoagulation, though neither provides optimal outcome for many patients. A different approach is suggested by the concurrence of autoantibodies and their antigenic targets in the absence of clinical disease, such as platelet factor 4 in heparin-induced thrombocytopenia and β(2)-glycoprotein-I (β(2)GPI) in antiphospholipid syndrome. The presence of autoantibodies in the absence of disease suggests that conformational changes or other alterations in endogenous protein autoantigens are required for recognition by pathogenic autoantibodies. In thrombotic thrombocytopenic purpura, the clinical impact of ADAMTS13 deficiency caused by autoantibodies likely depends on the balance between residual antigen, that is, enzyme activity, and demand imposed by local genesis of ultralarge multimers of von Willebrand factor. A corollary of these concepts is that disrupting platelet factor 4 and β(2)GPI conformation (or ultralarge multimer of von Willebrand factor oligomerization or function) might provide a disease-targeted approach to prevent thrombosis without systemic anticoagulation or immunosuppression. Validation of this approach requires a deeper understanding of how seemingly normal host proteins become antigenic or undergo changes that increase antibody avidity, and how they can be altered to retain adaptive functions while shedding epitopes prone to elicit harmful autoimmunity.

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.

Local elongation of endothelial cell-anchored von Willebrand factor strings precedes ADAMTS13 protein-mediated proteolysis

Platelet-decorated von Willebrand factor (VWF) strings anchored to the endothelial surface are rapidly cleaved by ADAMTS13. Individual VWF string characteristics such as number, location, and auxiliary features of the ADAMTS13 cleavage sites were explored here using imaging and computing software. By following changes in VWF string length, we demonstrated that VWF strings are cleaved multiple times, successively shortening string length in the function of time and generating fragments ranging in size from 5 to over 100 μm. These are larger than generally observed in normal plasma, indicating that further proteolysis takes place in circulation. Interestingly, in 89% of all cleavage events, VWF strings elongate precisely at the cleavage site before ADAMTS13 proteolysis. These local elongations are a general characteristic of VWF strings, independent of the presence of ADAMTS13. Furthermore, large elongations, ranging in size from 1.4 to 40 μm, occur at different sites in space and time. In conclusion, ADAMTS13-mediated proteolysis of VWF strings under flow is preceded by large elongations of the string at the cleavage site. These elongations may lead to the simultaneous exposure of many exosites, thereby facilitating ADAMTS13-mediated cleavage.

A shear-based assay for assessing plasma ADAMTS13 activity and inhibitors in patients with thrombotic thrombocytopenic purpura

BACKGROUND

Severe deficiency of plasma ADAMTS13 activity is a frequent finding in patients with hereditary and acquired thrombotic thrombocytopenic purpura (TTP). To date, plasma ADAMTS13 activity is determined by cleavage of either predenatured von Willebrand factor (VWF) or small peptides derived from the VWF-A2 domain. The physiologic relevance of the assay results is uncertain.

STUDY DESIGN AND METHODS

We sought to develop a novel shear-based assay to assess plasma ADAMTS13 activity and inhibitors. We also compared this assay with a fluorogenic peptide assay.

RESULTS

We found that an incubation of purified plasma VWF with 0.5 to 1.0 µL of citrated plasma under constant vortexing at 2500 rpm for 60 minutes in the presence of 5 mmol/L CaCl(2) and 1.7 µmol/L ZnCl(2) and low concentration of NaCl resulted in the maximal cleavage of VWF. The cleavage product could be separated by a 2.5% agarose gel and detected by Western blotting. The assay revealed that plasma and recombinant ADAMTS13 are highly sensitive to inhibition by zinc and chloride ions. Under the optimal conditions, the shear-based assay appeared to be more sensitive than the guanidine-denaturization assay for determining plasma ADAMTS13 activity.

CONCLUSIONS

Our fluid shear-based assay may be useful for investigating basic biologic function and regulation of ADAMTS13 metalloprotease. It may also be applicable for assessing plasma ADAMTS13 activity and inhibitors in TTP patients.

Factor VIII and platelets synergistically accelerate cleavage of von Willebrand factor by ADAMTS13 under fluid shear stress

Previous studies have demonstrated that factor VIII (FVIII) or platelets alone increase cleavage of von Willebrand factor (VWF) by ADAMTS13 under mechanically induced shear stresses. We show in this study that the combination of FVIII and platelets at the physiological concentrations is more effective than either one alone. In the absence of FVIII, lyophilized platelets increase the formation of cleavage product by 2-3-fold. However, in the presence of physiological concentration of FVIII (1 nm), the formation of VWF cleavage product increases dramatically as a function of increasing platelets with the maximal rate enhancement of approximately 8-fold. Conversely, in the presence of a physiological concentration of lyophilized platelets (150 x 10(3)/microl), the half-maximal concentration of FVIII required to accelerate VWF proteolysis by ADAMTS13 reduces by approximately 10-fold (to approximately 0.3 nm) compared with that in the absence of platelets ( approximately 3.0 nm). Further studies using the FVIII derivative that lacks an acidic region (a3), an antiplatelet glycoprotein 1balpha IgG, and a purified recombinant VWF-A1 domain or glycoprotein 1balpha-stripped platelets demonstrate that the synergistic rate-enhancing effect of FVIII and platelets depends on their specific binding interactions with VWF. Our findings suggest that FVIII and platelets are cofactors that regulate proteolysis of multimeric VWF by ADAMTS13 under physiological conditions.

Multiple domains of ADAMTS13 are targeted by autoantibodies against ADAMTS13 in patients with acquired idiopathic thrombotic thrombocytopenic purpura

BACKGROUND

Type G immunoglobulins against ADAMTS13 are the primary cause of acquired (idiopathic) thrombotic thrombocytopenic purpura. However, the domains of ADAMTS13 which the type G anti-ADAMT13 immunoglobulins target have not been investigated in a large cohort of patients with thrombotic thrombocytopenic purpura.

DESIGN AND METHODS

Sixty-seven patients with acquired idiopathic thrombotic thrombocytopenic purpura were prospectively collected from three major U.S. centers. An enzyme-linked immunosorbent assay determined plasma concentrations of anti-ADAMTS13 type G immunoglobulins, whereas immunoprecipitation plus western blotting determined the binding domains of these type G immunoglobulins.

RESULTS

Plasma anti-ADAMTS13 type G immunoglobulins from 67 patients all bound full-length ADAMTS13 and a variant truncated after the eighth TSP1 repeat (delCUB). Approximately 97% (65/67) of patients harbored type G immunoglobulins targeted against a variant truncated after the spacer domain (MDTCS). However, only 12% of patients' samples reacted with a variant lacking the Cys-rich and spacer domains (MDT). In addition, approximately 37%, 31%, and 46% of patients' type G immunoglobulins interacted with the ADAMTS13 fragment containing TSP1 2-8 repeats (T2-8), CUB domains, and TSP1 5-8 repeats plus CUB domains (T5-8CUB), respectively. The presence of type G immunoglobulins targeted against the T2-8 and/or CUB domains was inversely correlated with the patients' platelet counts on admission.

CONCLUSIONS

This multicenter study further demonstrated that the multiple domains of ADAMTS13, particularly the Cys-rich and spacer domains, are frequently targeted by anti-ADAMTS13 type G immunoglobulins in patients with acquired (idiopathic) thrombotic thrombocytopenic purpura. Our data shed more light on the pathogenesis of acquired thrombotic thrombocytopenic purpura and provide further rationales for adjunctive immunotherapy.

Amino acid residues Arg(659), Arg(660), and Tyr(661) in the spacer domain of ADAMTS13 are critical for cleavage of von Willebrand factor

Previous studies have shown that ADAMTS13 spacer domain is required for cleavage of von Willebrand factor (VWF). However, the exact amino acid residues within this domain critical for substrate recognition are not known. Epitope mapping of anti-ADAMTS13 immunoglobulin G from patients with thrombotic thrombocytopenic purpura and sequence alignment of the ADAMTS13 spacer domains of human, mouse, and zebrafish with these of human and murine ADAMTS1, a closely related member of ADAMTS family, have provided hints to investigate the role of the amino acid residues between Arg(659) and Glu(664) of the ADAMTS13 spacer domain in substrate recognition. A deletion of all these 6 amino acid residues (ie, Arg(659)-Glu(664)) from the ADAMTS13 spacer domain resulted in dramatically reduced proteolytic activity toward VWF73 peptides, guanidine-HCl denatured VWF, and native VWF under fluid shear stress, as well as ultralarge VWF on endothelial cells. Site-directed mutagenesis, kinetic analyses, and peptide inhibition assays have further identified a role for amino acid residues Arg(659), Arg(660), and Tyr(661) in proteolytic cleavage of various substrates under static and fluid shear stress conditions. These findings may provide novel insight into the structural-function relationship of ADAMTS13 and help us to understand pathogenesis of thrombotic thrombocytopenic purpura and other arterial thromboses associated with compromised VWF proteolysis.