Structure of von Willebrand factor-cleaving protease (ADAMTS13), a metalloprotease involved in thrombotic thrombocytopenic purpura

Effects of naturally occurring mutations in CUB-1 domain on synthesis, stability, and activity of ADAMTS-13

INTRODUCTION

Upon stimulation, endothelial cells release von Willebrand factor (VWF) in the unusually large (UL) and hyperactive forms that are rapidly cleaved by ADAMTS-13. Mutations in the ADAMTS13 gene result in ULVWF-mediated thrombosis found in patients with familial thrombotic thrombocytopenia purpura (TTP). ADAMTS-13 fits in the consensus of the ADAMTS family metalloproteases, but also contains two unique C- terminal CUB domains. Studying mutations in CUB domains could provide insights into the functional role of these domains.

METHODS

Three naturally occurring mutations (C1213Y, W1245del and K1256FS) in the CUB-1 domain found in patients with TTP were expressed in Hela cells. The secretion, stability and VWF-cleaving activity of the mutants under static and flow conditions were examined.

RESULTS

The mutations impaired secretion of ADAMTS-13 to apical surface, but not to extracellular matrix of transfected Hela cells. C1213Y and K1256FS also accelerated, whereas W1245del delayed, extracellular degradation of the mutants. The mutations also resulted in a moderate decrease in cleaving plasma VWF under static conditions. However, the mutated ADAMTS-13 bound to VWF substrate similarly as the wild-type metalloprotease and remained active in cleaving (UL)VWF under flow conditions.

CONCLUSIONS

The CUB-1 domain is critical for ADAMTS-13 secretion and stability upon secretion. ADAMTS-13 deficiency found in TTP patients could be resulted from reduced ADAMTS-13 secretion and, in the case of C1213Y and K1256FS accelerated degradation. W1245del is highly resistant to degradation and active in cleaving VWF.

Production, crystallization and preliminary crystallographic analysis of an exosite-containing fragment of human von Willebrand factor-cleaving proteinase ADAMTS13

ADAMTS13 is a reprolysin-type metalloproteinase belonging to the ADAMTS (a disintegrin and metalloproteinase with thrombospondin type 1 motif) family. It specifically cleaves plasma von Willebrand factor (VWF) and regulates platelet adhesion and aggregation. ADAMTS13 is a multi-domain enzyme. In addition to the N-terminal metalloproteinase domain, the ancillary domains, including a disintegrin-like domain, a thrombospondin-1 type 1 repeat, a Cys-rich domain and a spacer domain, are required for VWF recognition and cleavage. In the present study, a fragment of the ADAMTS13 ancillary domains (ADAMTS13-DTCS; residues 287-685) was expressed using CHO Lec cells, purified and crystallized. Diffraction data sets were collected using the SPring-8 beamline. Two ADAMTS13-DTCS crystals with distinct unit-cell parameters generated data sets to 2.6 and 2.8 A resolution, respectively.

A team player: the disintegrin domain of ADAMTS13

In this issue of Blood, de Groot and colleagues identify novel interactions between the disintegrin domain of ADAMTS13 and the A2 domain of VWF. This provides another piece in the puzzle of how this domain of ADAMTS13 works in concert with other domains to bind and process VWF.

Essential role of the disintegrin-like domain in ADAMTS13 function

ADAMTS13 is a highly specific multidomain plasma metalloprotease that regulates the multimeric size and function of von Willebrand factor (VWF) through cleavage at a single site in the VWF A2 domain. The precise role that the ADAMTS13 disintegrin-like domain plays in its function remains uncertain. Truncated ADAMTS13 variants suggested the importance of the disintegrin-like domain for both enzyme activity and specificity. Targeted mutagenesis of nonconserved regions (among ADAMTS family members) in the disintegrin-like domain identified 3 of 8 ADAMTS13 mutants (R349A, L350G, V352G) with reduced proteolytic activity. Kinetic analyses revealed a 5- to 20-fold reduction in catalytic efficiency of VWF115 (VWF residues 1554-1668) proteolysis by these mutants. These residues form a predicted exposed exosite on the surface of the disintegrin-like domain that lies approximately 26 A from the active site. Kinetic analysis of VWF115 carrying the D1614A mutation suggested that Arg349 in the ADAMTS13 disintegrin-like domain interacts directly with Asp1614 in VWF A2. We hypothesize that this interaction assists in positioning the scissile bond within the active site of ADAMTS13 and therefore plays a major role in determining cleavage parameters (K(m) and k(cat)), as opposed to binding affinity (K(d)) of ADAMTS13 for VWF, the latter being primarily determined by the spacer domain.

Molecular basis of ADAMTS13 dysfunction in thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a thrombotic microangiopathic disorder characterized by thrombocytopenia, hemolytic anemia, neurological and renal manifestations, and fever. It is associated with dysfunctional von Willebrand factor (VWF) proteolysis and the occurrence of VWF- and platelet-rich thrombi in the microcirculation of multiple organs, including the kidneys. Von Willebrand factor is a large glycoprotein that circulates in plasma as a series of multimers, and it plays a major role in primary hemostasis by inducing the formation of platelet plugs at sites of vascular injury and high-shear stress. Its activity is dependent on the sizes of the multimers, with ultra-large (UL) VWF multimers being biologically very potent. The ULVWF multimers are rapidly degraded upon their secretion from endothelial cells in normal individuals but not in the circulation of TTP patients, causing the formation of disseminated thrombi in the latter. The defective breakdown of VWF is attributed to a severely deficient activity of the VWF-cleaving protease ADAMTS13, a plasma metalloprotease synthesized in the liver, kidneys, and endothelium. This protease rapidly degrades VWF-platelet strings under flow by proteolytic cleavage of the VWF subunit, thereby regulating the size of the platelet thrombus. Congenital TTP occurs due to ADAMTS13 mutations, with the usual debut occurring during the first years of life, while acquired TTP is associated with auto-antibodies against ADAMTS13.

Plasma ADAMTS13 activity parallels the APACHE II score, reflecting an early prognostic indicator for patients with severe acute pancreatitis

OBJECTIVE

Severe acute pancreatitis (SAP) frequently progresses to pancreatitis-associated multiorgan failure (MOF) with high mortality. Decreased plasma ADAMTS13 activity (ADAMTS13:AC) results in the accumulation of unusually large von Willebrand factor multimers (UL-VWFM) and the formation of platelet thrombi, ultimately leading to MOF. The purpose of the study was to investigate the potential role of ADAMTS13:AC in the severity of SAP.

MATERIAL AND METHODS

Plasma ADAMTS13:AC and its related parameters were sequentially determined in 13 SAP patients. ADAMTS13:AC was determined by the chromogenic act-ELISA.

RESULTS

Within 1 or 2 days after admission, ADAMTS13:AC was lower in SAP patients (mean 28%) than in healthy controls (99%), and gradually recovered in the 11 survivors but further decreased in the 2 non-survivors. Patients with higher sepsis-related organ failure assessment (SOFA) scores showed lower ADAMTS13:AC than those without these scores. The inhibitor against ADAMTS13 was undetectable. On day 1, von Willebrand factor antigen (VWF:Ag) was higher (402%, p<0.001) in SAP patients than in controls (100%). VWF:Ag gradually decreased in the survivors, except in the 3 patients needing a necrosectomy, but remained high in the non-survivors. ADAMTS13:AC was inversely correlated with the APACHE II score (r=-0.750, p<0.005), and increased plasma concentrations of interleukin 6 (IL-6) and IL-8 at admission. UL-VWFM-positive patients had lower ADAMTS13:AC and decreased serum calcium concentrations, but higher VWF:Ag and IL-8 concentrations than UL-VWFM-negative patients.

CONCLUSIONS

Plasma ADAMTS13:AC was closely related to the APACHE II score. This intimate relationship may serve as an early prognostic indicator for SAP patients. The imbalance between decreased ADAMTS13:AC and increased UL-VWFM could contribute to SAP pathogenesis through enhanced thrombogenesis.

A functional calcium-binding site in the metalloprotease domain of ADAMTS13

ADAMTS13 regulates the multimeric size of von Willebrand factor (VWF). Its function is highly dependent upon Ca(2+) ions. Using the initial rates of substrate (VWF115, VWF residues 1554-1668) proteolysis by ADAMTS13 preincubated with varying Ca(2+) concentrations, a high-affinity functional ADAMTS13 Ca(2+)-binding site was suggested with K(D(app)) of 80 muM (+/- 15 muM) corroborating a previously reported study. When Glu83 or Asp173 (residues involved in a predicted Ca(2+)-binding site in the ADAMTS13 metalloprotease domain) were mutated to alanine, Ca(2+) dependence of proteolysis of the substrate was unaffected. Consequently, we sought and identified a candidate Ca(2+)-binding site in proximity to the ADAMTS13 active site, potentially comprising Glu184, Asp187, and Glu212. Mutagenesis of these residues within this site to alanine dramatically attenuated the K(D(app)) for Ca(2+) of ADAMTS13, and for D187A and E212A also reduced the V(max) to approximately 25% of normal. Kinetic analysis of the Asp187 mutant in the presence of excess Ca(2+) revealed an approximately 13-fold reduction in specificity constant, k(cat)/K(m), contributed by changes in both K(m) and k(cat). These results were corroborated using plasma-purified VWF as a substrate. Together, our results demonstrate that a major influence of Ca(2+) upon ADAMTS13 function is mediated through binding to a high-affinity site adjacent to its active site cleft.

Correction of murine ADAMTS13 deficiency by hematopoietic progenitor cell-mediated gene therapy

ADAMTS13, a metalloprotease primarily synthesized in liver and endothelial cells, cleaves von Willebrand factor (VWF) at the central A2 domain, thereby reducing the sizes of circulating VWF multimers. Genetic or acquired deficiency of plasma ADAMTS13 activity leads to a potentially fatal syndrome, thrombotic thrombocytopenic purpura (TTP). To date, plasma infusion or exchange is the only proven effective therapy for TTP. In search for a better therapy, an autologous transplantation of hematopoietic progenitor cells transduced ex vivo with a self-inactivating lentiviral vector encoding a full-length murine Adamts13 and an enhanced green fluorescent protein (GFP) reporter gene was performed in Adamts13(-/-) mice after irradiation. All recipient mice showed detectable ADAMTS13 antigen and proteolytic activity in plasma despite only low levels of bone marrow chimerism. The levels of plasma ADAMTS13 were sufficient to eliminate the ultralarge VWF multimers and offered systemic protection against ferric chloride-induced arterial thrombosis. The data suggest that hematopoietic progenitor cells can be genetically modified ex vivo and transplanted in an autologous model to provide adequate levels of functional ADAMTS13 metalloprotease. This success may provide the basis for development of a novel therapeutic strategy to cure hereditary TTP in humans.

Correction of ADAMTS13 deficiency by in utero gene transfer of lentiviral vector encoding ADAMTS13 genes

Deficiency of A Disintegrin And Metalloprotease with ThromboSpondin (ADAMTS13) results in thrombotic thrombocytopenic purpura (TTP). Plasma infusion or exchange is the only effective treatment to date. We show in this study that an administration of a self-inactivating lentiviral vector encoding human full-length ADAMTS13 and a variant truncated after the spacer domain (MDTCS) in mice by in utero injection at embryonic days 8 and 14 resulted in detectable plasma proteolytic activity (approximately 5-70%), which persisted for the length of the study (up to 24 weeks). Intravascular injection via a vitelline vein at E14 was associated with significantly lower rate of fetal loss than intra-amniotic injection, suggesting that the administration of vector at E14 may be a preferred gestational age for vector delivery. The mice expressing ADAMTS13 and MDTCS exhibited reduced sizes of von Willebrand factor (vWF) compared to the Adamts13(-/-) mice expressing enhanced green fluorescent protein (eGFP). Moreover, the mice expressing both ADAMTS13 and MDTCS showed a significant prolongation of ferric chloride-induced carotid arterial occlusion time as compared to the Adamts13(-/-) expressing eGFP. The data demonstrate the successful correction of the prothrombotic phenotypes in Adamts13(-/-) mice by a single in utero injection of lentiviral vectors encoding human ADAMTS13 genes, providing the basis for developing a gene therapy for hereditary TTP in humans.

Thrombotic thrombocytopenic purpura related to severe ADAMTS13 deficiency in children

Thrombotic thrombocytopenic purpura (TTP) related to a severely deficient activity of the von Willebrand factor cleaving protease, ADAMTS (A Disintegrin And Metalloprotease with ThromboSpondin type 1 repeats) 13, is a life-threatening event, the onset of which may occur as early as childhood. TTP is either inherited (Upshaw-Schulman syndrome) via ADAMTS13 gene mutations (neonatal onset) or acquired via anti-ADAMTS13 autoantibodies (childhood onset). TTP is due to platelet- and von-Willebrand-factor-rich thrombi of the microvasculature, inducing mechanical hemolytic anemia, consumption thrombocytopenia, and multivisceral ischemia. Clinical course consists of relapsing acute events triggered mostly by infections, associated icterus and hyperbilirubinemia, severe hemolytic anemia with schistocytosis and a negative Coombs test, severe thrombocytopenia, and sometimes symptoms related to visceral ischemia (renal failure, central nervous system vascular events, other organ failure). The recently available ADAMTS13 laboratory investigation combining measurement of ADAMTS13 activity in plasma, search for an ADAMTS13 circulating inhibitor, and anti-ADAMTS13 IgG and ADAMTS13 gene sequencing is a crucial addition to TTP diagnosis. Plasma exchanges are first-line treatment of acquired TTP, combined with steroids and immunosuppressive drugs. Curative treatment of acute events in Upshaw-Schulman syndrome relies on plasma infusions (provider of active ADAMTS13). Guidelines for preventive treatment of relapses are not clearly established but should associate plasmatherapy and caution to triggers of relapses. Therapeutic perspectives are focused on the development of concentrated plasma-derived ADAMTS13 or recombinant ADAMTS13.

New treatment options for immune-mediated hematological disorders

In recent years, there has been a tremendous increase in the number of clinical studies with monoclonal antibodies and small molecules in the treatment of hematological malignancies. Clinical observations have shown that some of these molecules may also aid in the treatment of immune-mediated hematological disorders. Moreover, immunotherapy has become an important treatment cornerstone in other, non-hematological, auto-immune diseases. This paper reviews the current state of the use of these new molecules in the treatment of the most frequently encountered immune-mediated hematological disorders: auto-immune hemolytic anemia (AIHA), idiopathic thrombocytopenic purpura (ITP), and thrombotic thrombocytopenic purpura (TTP).

Changes in plasma von Willebrand factor and ADAMTS13 levels associated with left atrial remodeling in atrial fibrillation

INTRODUCTION

Previous studies have shown raised plasma von Willebrand factor (VWF) levels in patients with atrial fibrillation (AF). However, little is known about changes of VWF associated with VWF-cleaving protease (ADAMTS13) in AF. The aim of this study was to examine the relationship between changes in plasma VWF and ADAMTS13 levels, and left atrial remodeling in AF patients.

MATERIALS AND METHODS

We measured plasma VWF and ADAMTS13 antigen levels in 70 paroxysmal AF (PAF) patients, 56 chronic AF (CAF) patients, and 55 control subjects.

RESULTS

Plasma VWF levels (mU/ml) were significantly higher in CAF and PAF patients compared with the controls (2103 +/- 743, 1930 +/- 676, 1532 +/- 555, respectively, P < 0.0001 in CAF vs. controls, P = 0.001 in PAF vs. control), while ADAMTS13 levels (mU/ml) were significantly lower in CAF and PAF patients compared with the controls (795 +/- 169, 860 +/- 221, 932 +/- 173, respectively, P = 0.0002 in CAF vs. controls, P = 0.04 in PAF vs. control). The VWF/ADAMTS13 ratio was significantly higher in patients with CAF than PAF or controls (2.81 +/- 1.30, 2.34 +/- 0.92, 1.73 +/- 0.83, respectively; P = 0.01 in CAF vs. PAF, P < 0.0001 in CAF vs. controls). There was a significant correlation between the VWF/ADAMTS13 ratio and left atrial diameter (positive correlation; r = 0.275, P = 0.0002) and left atrial appendage flow velocity (negative correlation; r = -0.345, P = 0.0018).

CONCLUSIONS

These findings suggest that the imbalance between plasma VWF and ADAMTS13 levels caused by left atrial remodeling might be closely associated with intra-atrial thrombus formation in AF patients.

Inhibition of ADAMTS-7 and ADAMTS-12 degradation of cartilage oligomeric matrix protein by alpha-2-macroglobulin

OBJECTIVE

As we previously reported, ADAMTS-7 and ADAMTS-12, two members of ADAMTS (a disintegrin and metalloprotease with thrombospondin motifs) family, degrade cartilage oligomeric matrix protein (COMP) in vitro and are significantly induced in the cartilage and synovium of arthritic patients [Liu CJ, Kong W, Ilalov K, Yu S, Xu K, Prazak L, et al. ADAMTS-7: a metalloproteinase that directly binds to and degrades cartilage oligomeric matrix protein. FASEB J 2006;20(7):988-90; Liu CJ, Kong W, Xu K, Luan Y, Ilalov K, Sehgal B, et al. ADAMTS-12 associates with and degrades cartilage oligomeric matrix protein. J Biol Chem 2006;281(23):15800-8]. The purpose of this study was to determine (1) whether cleavage activity of ADAMTS-7 and ADAMTS-12 of COMP are associated with COMP degradation in osteoarthritis (OA); (2) whether alpha-2-macroglobulin (a(2)M) is a novel substrate for ADAMTS-7 and ADAMTS-12; and (3) whether a(2)M inhibits ADAMTS-7 or ADAMTS-12 cleavage of COMP.

METHODS

An in vitro digestion assay was used to examine the degradation of COMP by ADAMTS-7 and ADAMTS-12 in the cartilage of OA patients; in cartilage explants incubated with tumor necrosis factor-alpha (TNF-alpha) or interleukin-1-beta (IL-1beta) with or without blocking antibodies; and in human chondrocytes treated with specific small interfering RNA (siRNA) to knockdown ADAMTS-7 or/and ADAMTS-12. Digestion of a(2)M by ADAMTS-7 and ADAMTS-12 in vitro and the inhibition of ADAMTS-7 or ADAMTS-12-mediated digestion of COMP by a(2)M were also analyzed.

RESULTS

The molecular mass of the COMP fragments produced by either ADAMTS-7 or ADAMTS-12 were similar to those observed in OA patients. Specific blocking antibodies against ADAMTS-7 and ADAMTS-12 dramatically inhibited TNF-alpha- or IL-1beta-induced COMP degradation in the cultured cartilage explants. The suppression of ADAMTS-7 or ADAMTS-12 expression by siRNA silencing in the human chondrocytes also prevented TNF-alpha- or IL-1beta-induced COMP degradation. Both ADAMTS-7 and ADAMTS-12 were able to cleave a(2)M, giving rise to 180- and 105-kDa cleavage products, respectively. Furthermore, a(2)M inhibited both ADAMTS-7- and ADAMTS-12-mediated COMP degradation in a concentration (or dose)-dependent manner.

CONCLUSION

Our observations demonstrate the importance of COMP degradation by ADAMTS-7 and ADAMTS-12 in vivo. Furthermore, a(2)M is a novel substrate for ADAMTS-7 and ADAMTS-12. More significantly, a(2)M represents the first endogenous inhibitor of ADAMTS-7 and ADAMTS-12.

The Role Of von Willebrand Factor In Hemorrhagic And Thrombotic Disorders

von Willebrand factor (VWF) is a multimeric plasma protein that mediates platelet adhesion as well as platelet aggregation at sites of vascular injury and acts as a carrier of factor VIII. Although acquired or inherited VWF deficiency is associated with a bleeding tendency, there is increasing evidence that VWF has a pivotal role in thrombogenesis. In fact, while the presence in the plasma of unusually large VWF multimers, due to a congenital or acquired deficiency of a VWF-cleaving metalloprotease, has been implicated in the pathogenesis of thrombotic thrombocytopenic purpura, high plasma levels of VWF have been associated with an increased risk of both arterial and venous thrombosis. The role of VWF in normal and pathological hemostasis is discussed in this review, and important advances in the pathophysiology, diagnosis, and treatment of VWF-associated disorders are also described.

ADAMTS13 activity and inhibitor

Thrombotic thrombocytopenic purpura (TTP) is often associated with acquired or congenital deficiency of the von Willebrand factor-cleaving metalloprotease, ADMATS13 (Lammle B et al., J Thromb Haemost 2005;3:1663-1675; Schneppenheim et al., Blood 2003;101:1845-1850). Although undetectable levels of enzyme activity (<10%) are diagnostic of inherited or acquired TTP in the correct clinical setting (absence is specific), not all patients diagnosed with TTP have severe protease deficiency, and it is therefore not recommended as an initial test for diagnosis (Copelovitch and Kaplan, Pediatr Nephrol, in press). Many prospective and retrospective studies have demonstrated that patients with severe protease deficiency have a higher likelihood of relapse, making it helpful as an indicator of recurrence. The short-term prognostic usefulness of ADAMTS13 testing during acute TTP warrants further investigation because of limited prospective studies (Ferrari S et al., Blood 2007;109:2815-2822; Peyvandi et al., Haematologica 2008;93:232-239).

Von Willebrand factor, ADAMTS13, and thrombotic thrombocytopenic purpura

Discoveries during the past decade have revolutionized our understanding of idiopathic thrombotic thrombocytopenic purpura (TTP). Most cases in adults are caused by acquired autoantibodies that inhibit ADAMTS13, a metalloprotease that cleaves von Willebrand factor within nascent platelet-rich thrombi to prevent hemolysis, thrombocytopenia, and tissue infarction. Although approximately 80% of patients respond to plasma exchange, which removes autoantibody and replenishes ADAMTS13, one third to one half of survivors develop refractory or relapsing disease. Intensive immunosuppressive therapy with rituximab appears to be effective as salvage therapy, and ongoing clinical trials should determine whether adjuvant rituximab with plasma exchange also is beneficial at first diagnosis. A major unanswered question is whether plasma exchange is effective for the subset of patients with idiopathic TTP who do not have severe ADAMTS13 deficiency.

Frequency of Pro475Ser polymorphism of ADAMTS13 gene and its association with ADAMTS-13 activity in the Korean population

PURPOSE

The in vitro study suggested that proline to serine polymorphism in codon 475 (C1423T) of the A Disintegrin and Metalloprotease with ThromboSpondin type 1 repeats-13 (ADAMTS-13) gene is related to reduced activity of ADAMTS-13. In this study, the frequency of the Pro475Ser polymorphism in Koreans was studied and plasma ADAMTS-13 activity was measured to find out whether this polymorphism contributes to decreased ADAMTS-13 activity in Koreans.

PATIENTS AND METHODS

The frequency of the C1423T allele of the ADAMTS13 gene was studied along with measuring plasma ADAMTS-13 activity in 250 healthy Korean individuals.

RESULTS

The allele frequency of C1423T polymorphism was 4%, and the median activity of CT type was 107 (69-143)%, which was lower than in controls with the CC genotype [118 (48-197)%, (p=0.021)].

CONCLUSION

Therefore, the Pro475Ser polymorphism seems to be popular in the Korean population, and attenuates ADAMTS-13 plasma activity.

Decreased Activity of Plasma ADAMTS13 May Contribute to the Development of Liver Disturbance and Multiorgan Failure in Patients with Alcoholic Hepatitis

BACKGROUND

The pathogenesis of alcoholic hepatitis (AH) remains unclear and the prognosis of severe alcoholic hepatitis (SAH) is very poor. Deficiency of von Willebrand factor (VWF)-cleaving protease (VWF-CP/ADAMTS13) results in an increase of the plasma unusually large VWF multimer and leads to platelet clumping, which causes microcirculatory disturbance and finally multiorgan failure. The aim of this study was to explore the potential role of ADAMTS13 on the development of liver disturbance and multiorgan failure in AH.

METHODS

The activity of plasma ADAMTS13 and its clinical correlation were determined in 14 patients with AH, 4 with SAH (Maddrey score, mean 62), and 10 with alcoholic liver cirrhosis (LC).

RESULTS

The activity of the plasma ADAMTS13 significantly decreased in patients with AH (mean 59%, p < 0.001), SAH (17%, p < 0.001) and LC (76%, p < 0.02) as compared with the healthy subjects (102%, n = 60). The activity was markedly lower in SAH than in AH (p < 0.02) and LC (p < 0.02). In three nonsurvivors with SAH who had multiorgan failure, it was extremely low (4.5%, 5.0%, and 16.0%, respectively), but in a survivor with SAH it remained mild decrease (44%). In AH, the protease activity increased at the recovery stage (42% --> 75%, p < 0.05). In the univariate analysis, the activity correlated with 10 clinical variables including functional liver capacity, inflammation signs, renal function, and platelet count in patients with AH and SAH. Among these, multivariate analysis showed that serum total bilirubin and C-reactive protein independently correlated with the protease activity.

CONCLUSION

The activity of plasma ADAMTS13 markedly decreased in SAH in addition to AH. The activity was closely related to hyperbilirubinemia and inflammation signs, and was extremely low in nonsurvivors with SAH and multiorgan failure. The marked decrease of plasma ADAMTS13 may, in part, contribute to not only the progression of liver disturbance in AH, but also the development of multiorgan failure in SAH through microcirculatory disturbance.

Extensive contacts between ADAMTS13 exosites and von Willebrand factor domain A2 contribute to substrate specificity

The metalloprotease ADAMTS13 efficiently cleaves only the Tyr(1605)-Met(1606) bond in the central A2 domain of multimeric von Willebrand factor (VWF), even though VWF constitutes only 0.02% of plasma proteins. This remarkable specificity depends in part on binding of the noncatalytic ADAMTS13 spacer domain to the C-terminal alpha-helix of VWF domain A2. By kinetic analysis of recombinant ADAMTS13 constructs, we show that the first thrombospondin-1, Cys-rich, and spacer domains of ADAMTS13 interact with segments of VWF domain A2 between Gln(1624) and Arg(1668), and together these exosite interactions increase the rate of substrate cleavage by at least approximately 300-fold. Internal deletion of Gln(1624)-Arg(1641) minimally affected the rate of cleavage, indicating that ADAMTS13 does not require a specific distance between the scissile bond and auxiliary substrate binding sites. Smaller deletions of the P2-P9 or the P4'-P18' residues on either side of the Tyr(1605)-Met(1606) bond abolished cleavage, indicating that the metalloprotease domain interacts with additional residues flanking the cleavage site. Thus, specific recognition of VWF depends on cooperative, modular contacts between several ADAMTS13 domains and discrete segments of VWF domain A2.

The first but not the second thrombospondin type 1 repeat of ADAMTS5 functions as an angiogenesis inhibitor

Angiogenesis is critical for tumour growth and metastasis where factors that regulate this process are potential targets for development of anti-cancer drugs. In this study, we show that the first TSR domain of the extracellular matrix protease ADAMTS5, unlike the second TSR, has anti-angiogenic activities where it inhibits endothelial cell tube formation on Matrigel, reduces cell proliferation and attachment, while promoting cell apoptosis and migration, all in a dose-dependent manner. Furthermore, it influences the architecture of endothelial cells by disrupting actin stress fibres and reducing focal adhesions, likely via suppressing RhoA activation. TSR1 of ADAMTS5 is therefore a novel anti-angiogenic peptide and could serve as a prototype for future development into anti-cancer drugs.

Evidence that high von Willebrand factor and low ADAMTS-13 levels independently increase the risk of a non-fatal heart attack

BACKGROUND

A disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS-13) may influence von Willebrand factor (VWF) levels and consequently the risk of myocardial infarction (MI). Moreover, ADAMTS-13 influences hemostatic plug formation in mouse models. We therefore studied their associations in the Glasgow MI Study (GLAMIS).

METHODS AND RESULTS

We measured ADAMTS-13 and VWF antigen levels by ELISAs in stored plasma from a case-control study of 466 MI cases and 484 age- and sex-matched controls from the same north Glasgow population. There was no correlation between ADAMTS-13 and VWF levels in cases or controls. ADAMTS-13 levels correlated positively with serum cholesterol and triglycerides and body mass index, and negatively with high-density lipoprotein-cholesterol. VWF levels correlated with age, fibrinogen and C-reactive protein. In multivariable analyses including risk factors, VWF correlated positively with risk of MI, and ADAMTS-13 correlated negatively with risk of MI. These associations were independent of each other. The association of ADAMTS-13 with risk of MI was observed only in multivariable analysis.

CONCLUSIONS

VWF and ADAMTS-13 levels were not associated in this study, and showed associations with MI risk in opposite directions but of similar strength. The association of ADAMTS-13 with MI is influenced by lipid levels, and consequently requires further investigation.

Prevalence of the ADAMTS-13 missense mutation R1060W in late onset adult thrombotic thrombocytopenic purpura

BACKGROUND

Thrombotic thrombocytopenic purpura (TTP) is most commonly associated with deficiency or inhibition of von Willebrand factor-cleaving protease (ADAMTS-13) activity. ADAMTS-13 mutations and polymorphisms have been reported in childhood congenital TTP, but their significance in adult onset TTP remains unclear.

OBJECTIVES

We sought to identify common ADAMTS-13 mutations in adults with late onset TTP and to investigate whether they may predispose acute clinical episodes of the disorder in adulthood.

PATIENTS/METHODS/RESULTS

We detected a missense mutation (C3178T) in exon 24 of ADAMTS-13 in 6/53 (11.3%) adult onset TTP patients, but no normal controls (n = 100). Three of the patients had pregnancy-associated TTP; three had chronic relapsing acute idiopathic TTP. C3178T encodes an arginine to tryptophan (R1060W) substitution in the TSP1-7 domain of ADAMTS-13. In vitro expression of mutant and wild-type ADAMTS-13 demonstrated that R1060W caused severe intracellular retention of ADAMTS-13 (<5% secretion) without affecting its metalloprotease activity. One homozygous and five heterozygous patients were identified. No other causative mutations were discovered, yet all six patients had ADAMTS-13 activity levels <5% at presentation (normal: 66-126%). Antibodies/inhibitors to ADAMTS-13 were detected in three/five heterozygous patients, and all six patients had subnormal antigen levels. Six asymptomatic first-degree relatives, including those of two probands with antibodies, were also heterozygous for C3178T; all but one had subnormal ADAMTS-13 activity.

CONCLUSION

The high prevalence of R1060W ADAMTS-13 in adult onset TTP, together with its absence in childhood congenital TTP cases reported elsewhere, suggests it may be a factor in the development of late onset TTP.

Pathogenesis of thrombotic microangiopathies

Profound thrombocytopenia and microangiopathic hemolytic anemia characterize thrombotic microangiopathy, which includes two major disorders: thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS). TTP has at least three types: congenital or familial, idiopathic, and nonidiopathic. The congenital and idiopathic TTP syndromes are caused primarily by deficiency of ADAMTS13, owing to mutations in the ADAMTS13 gene or autoantibodies that inhibit ADAMTS13 activity. HUS is similar to TTP, but is associated with acute renal failure. Diarrhea-associated HUS accounts for more than 90% of cases and is usually caused by infection with Shiga-toxin-producing Escherichia coli (O157:H7). Diarrhea-negative HUS is associated with complement dysregulation in up to 50% of cases, caused by mutations in complement factor H, membrane cofactor protein, factor I or factor B, or by autoantibodies against factor H. The incomplete penetrance of mutations in either ADAMTS13 or complement regulatory genes suggests that precipitating events or triggers may be required to cause thrombotic microangiopathy in many patients.

ADAMTS13 related markers and von Willebrand factor in plasma from patients with thrombotic microangiopathy (TMA)

The ADAMTS13 (a disintegrin and metalloprotease with a thrombospondin type I domain 13) related markers were measured in the plasma of healthy volunteers and thrombotic microangiopathy (TMA) patients including thrombotic thrombocytopenic purpura (TTP) to examine their efficacy in the diagnosis of TTP. The plasma levels of the ADAMTS13 antigen and ADAMTS13-factor XI complex were significantly lower in TMA patients with a significant decreased ADAMTS13 activity (and these patients were considered to have TTP) than in the healthy volunteers. The plasma levels of ADAMTS13 antigens closely correlated with those of ADAMTS13-factor XI complex. Autoantibody for ADAMTS 13 was also positive in almost all TTP patients. In addition, the ratio of von Willebrand factor (VWF)/ADAMTS13 activity was significantly high in TTP suggesting that this ratio might be more useful for the differential diagnosis of TTP than the ADAMTS13 assay alone. These findings suggest that ADAMTS13 related markers are useful for the diagnosis and analysis of TTP.

Human endothelial cells synthesize and release ADAMTS-13

Hepatic stellate cells have been considered to be a primary source for human plasma ADAMTS-13, the von Willebrand factor (VWF)-cleaving metalloprotease. In this study, ADAMTS-13 antigen was detected by immunofluorescence in both venous (HUVECs) and arterial endothelial cells (HUAECs) using both polyclonal antibodies made against peptides found in various domains of human ADAMTS-13, as well as by a monoclonal antibody against the ADAMTS-13 metalloprotease domain. ADAMTS-13 antigen had an intra-cellular distribution in endothelial cells distinct from the Weibel-Palade body location of VWF, and was released from the cells during 48 h in culture. The mRNA for ADAMTS13 was detected in HUVECs and HUAECs using reverse transcription-polymerase chain reaction (RT-PCR), indicating that the enzyme is synthesized in these cells. The ADAMTS-13 protein was immunoprecipitated from HUVECs and had an approximate M(r) of 170 kDa, similar to the molecular mass of recombinant ADAMTS-13. The ADAMTS-13 in HUVEC and HUAEC lysates had enzymatic activity using both static and flow assays. We conclude that ADAMTS-13 is synthesized in human endothelial cells, and released constitutively. The vast number of endothelial cells in the body may be an important source of ADAMTS-13.

Unbalanced expression of ADAMTS13 and von Willebrand factor in mouse endotoxinemia

INTRODUCTION

Secondary ADAMTS13 deficiency may occur in septic patients. The expression of ADAMTS13 in mouse endotoxinemia was studied.

METHODS

The blood and mRNA expression levels of ADAMTS13 and von Willebrand factor were measured in lipopolysaccharide-injected mice.

RESULTS

The plasma ADAMTS13 activity in wild-type mice was significantly decreased at 2 h after lipopolysaccharide injection, and this decrease in ADAMTS13 activity preceded the decrease in ADAMTS13 mRNA expression in the liver and continued for 24 h. However, no decreases in the plasma ADAMTS13 activity after lipopolysaccharide injection were observed in mice pretreated with a neutrophil elastase inhibitor or in plasminogen-deficient mice, suggesting that the decrease in ADAMTS13 activity was processed efficiently by the coordinated actions of plasmin and neutrophil elastase. von Willebrand factor mRNA was abundantly expressed in the lung and moderately in the kidney, but showed relatively low expression in the liver without lipopolysaccharide injection. However, von Willebrand factor mRNA expression in the liver was significantly increased after lipopolysaccharide injection and this high expression level continued for 24 h after the injection. The von Willebrand factor and ADAMTS13 mRNA expression levels in these organs changed in the opposite manners following lipopolysaccharide administration. Furthermore, the blood von Willebrand factor level increased after lipopolysaccharide administration, in contrast to the decrease in the blood ADMTS13 level after lipopolysaccharide administration.

CONCLUSION

These data suggest that imbalance between the blood von Willebrand factor and ADAMTS13 levels may occur in endotoxinemia, and that this may partly contribute to the thrombotic state associated with endotoxinemia.

Platelet-VWF complexes are preferred substrates of ADAMTS13 under fluid shear stress

Endothelial cells secrete prothrombotic ultralarge von Willebrand factor (VWF) multimers, and the metalloprotease ADAMTS13 cleaves them into smaller, less dangerous multimers. This reaction is stimulated by tensile force applied to the VWF substrate, which may occur on cell surfaces or in the circulating blood. The cleavage of soluble VWF by ADAMTS13 was accelerated dramatically by a combination of platelets and fluid shear stress applied in a cone-plate viscometer. Platelet-dependent cleavage of VWF was blocked by an anti-GPIbalpha monoclonal antibody or by a recombinant soluble fragment of GPIbalpha that prevents platelet-VWF binding. Multimeric gel analysis showed that shear and platelet-dependent cleavage consumed large VWF multimers. Therefore, ADAMTS13 preferentially acts on platelet-VWF complexes under fluid shear stress. This reaction is likely to account for a majority of VWF proteolysis after secretion and to determine the steady-state size distribution of circulating VWF multimers in vivo.

Two mechanistic pathways for thienopyridine-associated thrombotic thrombocytopenic purpura: a report from the SERF-TTP Research Group and the RADAR Project

OBJECTIVES

We sought to describe clinical and laboratory findings for a large cohort of patients with thienopyridine-associated thrombotic thrombocytopenic purpura (TTP).

BACKGROUND

The thienopyridine derivatives, ticlopidine and clopidogrel, are the 2 most common drugs associated with TTP in databases maintained by the U.S. Food and Drug Administration (FDA).

METHODS

Clinical reports of TTP associated with clopidogrel and ticlopidine were identified from medical records, published case reports, and FDA case reports (n = 128). Duration of thienopyridine exposure, clinical and laboratory findings, and survival were recorded. ADAMTS13 activity (n = 39) and inhibitor (n = 30) were measured for a subset of individuals.

RESULTS

Compared with clopidogrel-associated TTP cases (n = 35), ticlopidine-associated TTP cases (n = 93) were more likely to have received more than 2 weeks of drug (90% vs. 26%), to be severely thrombocytopenic (84% vs. 60%), and to have normal renal function (72% vs. 45%) (p < 0.01 for each). Compared with TTP patients with ADAMTS13 activity >15% (n = 13), TTP patients with severely deficient ADAMTS13 activity (n = 26) were more likely to have received ticlopidine (92.3% vs. 46.2%, p < 0.003). Among patients who developed TTP >2 weeks after thienopyridine, therapeutic plasma exchange (TPE) increased likelihood of survival (84% vs. 38%, p < 0.05). Among patients who developed TTP within 2 weeks of starting thienopyridines, survival was 77% with TPE and 78% without.

CONCLUSIONS

Thrombotic thrombocytopenic purpura is a rare complication of thienopyridine treatment. This drug toxicity appears to occur by 2 different mechanistic pathways, characterized primarily by time of onset before versus after 2 weeks of thienopyridine administration. If TTP occurs after 2 weeks of ticlopidine or clopidogrel therapy, therapeutic plasma exchange must be promptly instituted to enhance likelihood of survival.

The cooperative activity between the carboxyl-terminal TSP1 repeats and the CUB domains of ADAMTS13 is crucial for recognition of von Willebrand factor under flow

ADAMTS13 cleaves von Willebrand factor (VWF) between Tyr(1605) and Met(1606) residues at the central A2 subunit. The amino-terminus of ADAMTS13 protease appears to be sufficient to bind and cleave VWF under static and denatured condition. However, the role of the carboxyl-terminus of ADAMTS13 in substrate recognition remains controversial. Present study demonstrates that ADAMTS13 cleaves VWF in a rotation speed- and protease concentration-dependent manner on a mini vortexer. Removal of the CUB domains (delCUB) or truncation after the spacer domain (MDTCS) significantly impairs its ability to cleave VWF under the same condition. ADAMTS13 and delCUB (but not MDTCS) bind VWF under flow with dissociation constants (K(D)) of about 50 nM and about 274 nM, respectively. The isolated CUB domains are neither sufficient to bind VWF detectably nor capable of inhibiting proteolytic cleavage of VWF by ADAMTS13 under flow. Addition of the TSP1 5-8 (T5-8CUB) or TSP1 2-8 repeats (T2-8CUB) to the CUB domains restores the binding affinity toward VWF and the inhibitory effect on cleavage of VWF by ADAMTS13 under flow. These data demonstrate directly and quantitatively that the cooperative activity between the middle carboxyl-terminal TSP1 repeats and the distal carboxyl-terminal CUB domains may be crucial for recognition and cleavage of VWF under flow.

Thrombotic Thrombocytopenic Purpura in Humans and Mice

Thrombotic thrombocytopenic purpura (TTP) is a disorder of blood coagulation that presents classically with the pentad of fever, thrombocytopenia, microangiopathic hemolytic anemia, renal dysfunction and mental status changes. However, the clinical presentation can be quite variable making the diagnosis difficult in many cases. “Hyaline” microthrombi composed primarily of platelets and Von Willebrand Factor (VWF) are found in the small vessels of affected organs and represent the pathological hallmark of the disease. The accompanying tissue ischemia is thought to explain the clinical TTP signs and symptoms. Pathogenesis of TTP has been linked to dysfunction of ADAMTS13, a metalloprotease whose only known substrate is VWF. Interestingly, further investigation into the natural history of TTP has demonstrated that ADAMTS13 deficiency likely is necessary, but not sufficient for the development of this disease, suggesting that additional genetic and/or environmental factors are required for TTP pathogenesis. Recently, a mouse model of TTP was established that recapitulates many of the key clinical features of this disease, including the requirement for further genetic and environmental factors in addition to ADAMTS13 deficiency. Therefore, in addition to being useful for the direct study of disease pathophysiology in vivo , this mouse model may also play a key role in elucidating some of the important environmental and genetic contributors to disease pathogenesis. Here we will review TTP in humans, and then discuss recent information gained from the analysis of ADAMTS13-deficient mice.

Serial changes in von Willebrand factor-cleaving protease (ADAMTS13) and prognosis after acute myocardial infarction

Von Willebrand factor (VWF), a cofactor in platelet adhesion and aggregation, increases hemostasis and thrombosis. Recently, a metalloprotease that cleaves VWF multimers has been identified, namely ADAMTS13. The aim of this study was to investigate the relation between serial changes in plasma VWF and ADAMTS13 and the prognosis after acute myocardial infarction (AMI). We measured serial changes of plasma VWF and ADAMTS13 antigen levels in 92 patients with AMI and 40 control subjects. VWF levels were significantly higher in patients with AMI compared with controls (p <0.01) on admission, peaked 3 days after admission, and remained high for 14 days. In contrast, on admission, ADAMTS13 levels were significantly lower in patients with AMI compared with controls (p <0.0001), with minimum antigen levels reached after 3 days, and remained lower for 14 days. The ratio of VWF/ADAMTS13 antigen levels was higher in patients with AMI compared with controls throughout the time course. Cox hazards analysis revealed that the early increase of VWF and VWF/ADAMTS13 ratio levels and the early decrease of ADAMTS13 levels were significant predictors of future thrombotic events during the 1-year follow-up period. Kaplan-Meier analysis demonstrated that patients with major decreases of ADAMTS13 levels and high increases of VWF/ADAMTS13 levels had significantly greater probabilities for development of thrombotic events (p = 0.0104 and 0.0209, respectively). In conclusion, these findings suggest that monitoring the changes of VWF and ADAMTS13 antigen levels in the early phase might be valuable for predicting and preventing thrombosis during 1-year follow-up in patients with AMI.

Podocytes express ADAMTS13 in normal renal cortex and in patients with thrombotic thrombocytopenic purpura

Congenital thrombotic thrombocytopenic purpura (TTP) is associated with ADAMTS13 mutations. The major site of ADAMTS13 synthesis is the liver. Expression in other tissues, and in TTP, has not been shown. In this study, ADAMTS13 protein expression was investigated in normal kidney and in renal tissue from two TTP patients, with a compound heterozygous mutation (P353L and P457L) and a homozygous mutation (4143insA). Real-time polymerase chain reaction demonstrated ADAMTS13 mRNA in normal kidney. ADAMTS13 was detected in the glomeruli and tubuli of normal and TTP kidney using anti-ADAMTS13 antibodies. In the glomeruli, expression was localised to podocytes (as demonstrated by counterstaining with two podocyte markers) and endothelium. Similar distribution was detected in the TTP kidneys. Electron microscopy detected ADAMTS13 in podocytes, endothelium and glomerular basement membrane. Cultured human podocytes expressed ADAMTS13 mRNA and protein, and podocyte lysate exhibited von Willebrand factor-cleaving activity. Mutation expression studies of the P353L and P457L mutations showed partially impaired secretion and lower activity of the secreted mutants. Impaired secretion has previously been shown for the 4143insA mutation. Podocyte-derived ADAMTS13 may offer local protection in the high-shear microcirculation of the glomerulus. The mutations in the two TTP patients studied enabled protein expression in the podocytes but affected protease secretion.

A new look at Weibel-Palade body structure in endothelial cells using electron tomography

Multimers of von Willebrand Factor (vWF), a protein mediating blood clotting in response to vascular injury, are stored as tubular structures by endothelial cells in specific organelles, the Weibel-Palade Bodies (WPBs). To date very little is known about the 3D structure of WPBs in relation to the organization of the tubules. Therefore, we have initiated a thorough electron microscopic study in human umbilical vein endothelial cells (HUVECs) using electron tomography to gain further understanding of the ultrastructure of WPBs. We found that in addition to the well-documented cigar-shape, WPBs adopt irregular forms, which appeared to result from homotypic fusion. In transverse views of WPBs the tubular striations appear evenly spaced, which indicates a high level of organization that is likely to involve an underlying scaffold of structural proteins. Additionally, we found that the tubular striations twisted in an orderly fashion, suggesting that they are stored within the WPBs by a spring-loading mechanism. Altogether these data suggest that WPBs undergo a relatively complex maturation process involving homotypic fusion. Although the mechanism of assembly of vWF multimers into tubules is still unknown, the curled arrangement of the tubules within WPBs suggests a high degree of folding of the protein inside the organelle.

Thrombotic thrombocytopenic purpura: a thrombotic disorder caused by ADAMTS13 deficiency

A serious disorder with characteristic microvascular thrombosis involving the brain and other organs, thrombotic thrombocytopenic purpura (TTP) typically presents with thrombocytopenia, hemolysis with schistocytes on blood smears, and mental changes or seizures. It may progress rapidly to a fatal end if the patient is not treated immediately with plasma. Recent advances have shown that TTP is caused by deficiency of a circulating, von Willebrand factor cleaving metalloprotease, ADAMTS13. This new knowledge will provide clues to improve the diagnosis and management of this intriguing disease.

Diagnosis and treatment of transplantation-associated thrombotic microangiopathy: real progress or are we still waiting?

Transplantation-associated thrombotic microangiopathy (TA-TMA) is an infrequent but devastating syndrome that occurs in allogeneic hematopoietic stem cell transplant recipients, and is associated with a variety of transplantation-related factors, including conditioning regimens, immunosuppressive agents, GVHD and opportunistic infections. Progress in managing this condition has been hampered by lack of a consensus definition and poor understanding of the pathophysiology of the disorder. Two different groups recently have proposed consensus definitions, yet they fail to distinguish the primary syndrome from the secondary causes, such as a variety of infections, medication exposure or other conditions. Increasing evidence suggests that TA-TMA is a multifactorial disorder that is distinct from thrombotic thrombocytopenic purpura (TTP), and likely represents the final common pathway of a number of endothelial cell insults. TA-TMA responds poorly to conventional treatment for TTP, including plasma exchange, but newer agents, including daclizumab and defibrotide show promise. In addition, other agents known to modify endothelial responses to injury, including statins, prostacyclin analogues, endothelin-receptor antagonists and free radical scavengers, may lead to improved outcomes for patients affected by this disorder.

O-fucosylation is required for ADAMTS13 secretion

ADAMTS13 is a plasma metalloproteinase that cleaves von Willebrand factor to smaller, less thrombogenic forms. Deficiency of ADAMTS13 activity in plasma leads to thrombotic thrombocytopenic purpura. ADAMTS13 contains eight thrombospondin type 1 repeats (TSR), seven of which contain a consensus sequence for the direct addition of fucose to the hydroxyl group of serine or threonine. Mass spectral analysis of tryptic peptides derived from human ADAMTS13 indicate that at least six of the TSRs are modified with an O-fucose disaccharide. Analysis of [(3)H]fucose metabolically incorporated into ADAMTS13 demonstrated that the disaccharide has the structure glucose-beta1,3-fucose. Mutation of the modified serine to alanine in TSR2, TSR5, TSR7, and TSR8 reduced the secretion of ADAMTS13. Mutation of more than one site dramatically reduced secretion regardless of the sites mutated. When the expression of protein O-fucosyltransferase 2 (POFUT2), the enzyme that transfers fucose to serines in TSRs, was reduced using siRNA, the secretion of ADAMTS13 decreased. A similar outcome was observed when ADAMTS13 was expressed in a cell line unable to synthesize the donor for fucose addition, GDP-fucose. Although overexpression of POFUT2 did not affect the secretion of wild-type ADAMTS13, it did increase the secretion of the ADAMTS13 TSR1,2 double mutant but not that of ADAMTS13 TSR1-8 mutant. Together these findings indicate that O-fucosylation is functionally significant for secretion of ADAMTS13.

Prophylactic fresh frozen plasma may prevent development of hepatic VOD after stem cell transplantation via ADAMTS13-mediated restoration of von Willebrand factor plasma levels

We initially conducted a multicenter, randomized trial (n=43), and subsequently a questionnaire study (n=209) of participating hospitals, to evaluate whether infused fresh frozen plasma (FFP) could prevent the occurrence of hepatic veno-occlusive disease (VOD) after stem cell transplantation (SCT). Forty-three patients were divided into two groups: 23 receiving FFP infusions and 20 not receiving it. VOD developed in three patients not receiving FFP. Plasma von Willebrand factor (VWF) antigen levels were lower at days 0, 7 and 28 after SCT in patients receiving FFP than in those not receiving it, whereas plasma ADAMTS13 activity (ADAMTS13:AC) did not differ between them. Plasma VWF multimer (VWFM) was demonstrated to be defective in the high approximately intermediate VWFM during the early post-SCT phase, but there was a significant increase in high VWFM just before VOD onset. This suggests that a relative enzyme-to-substrate (ADAMTS13/high-VWFM) imbalance is involved in the pathogenesis of VOD. To strengthen this hypothesis, the incidence of VOD was apparently lower in patients receiving FFP infusions than in those not receiving it (0/23 vs 3/20) in the randomized trial. Further, the results combined with the subsequent questionnaire study (0/36 vs 11/173) clearly showed the incidence to be statistically significant (0/59 vs 14/193, P=0.033).

FSH stimulates the expression of the ADAMTS-16 protease in mature human ovarian follicles

We report the characterization of full-length human ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs)-16, a novel member of the disintegrin and metalloproteinase with thrombospondin motifs (hence ADAMTS) family. ADAMTS-16 is highly expressed both in the kidney and in the ovary, where it is predominantly expressed in the parietal granulosa cells of pre-ovulatory follicles but only slightly expressed in cells of the cumulus oophorus. In fully differentiated luteinizing granulosa cells, follicle-stimulating hormone and forskolin induces expression of ADAMTS-16, suggesting that it is regulated via the cAMP pathway. Luteinizing hormone had a minor effect on the expression of ADAMTS-16. ADAMTS-16 is capable of cleaving alpha2-macroglobulin (MG), a common substrate for proteases, which is present at high concentrations in the follicular fluid of ovarian follicles. These studies provide the first evidence that ADAMTS-16 is an active protease and suggest a physiological role of ADAMTS-16 in ovarian follicles, at least during the pre-ovulatory phase.

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.

Further characterization of ADAMTS-13 inactivation by thrombin

BACKGROUND

The multimeric size and platelet-tethering function of von Willebrand factor (VWF) are modulated by the plasma metalloprotease, a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS-13). In vitro ADAMTS-13 is susceptible to proteolytic inactivation by thrombin.

OBJECTIVES

In this study, we aimed to characterize the inactivation of ADAMTS-13 by thrombin and to assess its physiological significance.

METHODS AND RESULTS

By N-terminal sequencing of cleavage products, and by mutagenesis, we identified the principal thrombin cleavage sites in ADAMTS-13 as R257 and R1176. Using a library of 76 thrombin mutants, we highlighted the functional importance of exosite I on thrombin in the proteolysis of ADAMTS-13. Proteolysis of ADAMTS-13 by thrombin caused an 8-fold reduction in its affinity for VWF that contributed to its loss of VWF-cleaving function. Intriguingly, thrombin-cleaved ADAMTS-13 both bound and proteolyzed a short recombinant VWF A2 domain substrate (VWF115) normally. Following activation of coagulation in normal plasma, endogenous ADAMTS-13, but not added ADAMTS-13, appeared resistant to coagulation-induced fragmentation. An estimation of the K(m) for ADAMTS-13 proteolysis by thrombin was appreciably higher than the physiological concentration of ADAMTS-13. This was corroborated by the comparatively low affinity of ADAMTS-13 for thrombin (K(D) 95 nM).

CONCLUSIONS

Together, our data suggest that ADAMTS-13 is protected from rapid proteolytic inactivation by thrombin in normal plasma. Whether this remains the case under pathological situations involving elevated/sustained generation of thrombin remains unclear.

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.

Hepatic stellate cell damage may lead to decreased plasma ADAMTS13 activity in rats

ADAMTS13 is gaining attention, because its deficiency causes thrombotic thrombocytopenic purpura. Although its regulatory mechanism is not fully understood, we wondered if hepatic stellate cells (HSCs) play a role, because ADAMTS13 mRNA is exclusively expressed in the liver and primarily in HSCs. Plasma ADAMTS13 activity was markedly reduced in dimethylnitrosamine-treated rats, where HSC apoptosis is an essential event, but not in carbon tetrachloride- or thioacetamide-treated rats without HSC apoptosis. Furthermore, plasma ADAMTS13 activity was also reduced in 70% hepatectomized rats, where HSC loss occurs. These results suggest that HSC may be involved in the regulation of plasma ADAMTS13 activity.