Platelet-derived VWF-cleaving metalloprotease ADAMTS-13

ADAMTS13 bound to endothelial cells exhibits enhanced cleavage of von Willebrand factor

ADAMTS13 is a plasma metalloprotease that cleaves ultralarge von Willebrand factor multimers to generate less thrombogenic fragments. Although this cleavage can occur at the surface of endothelial cells, it is currently unknown whether this process involves binding of the ADAMTS13 to the endothelial cell plasma membrane. Using different assay systems, we present evidence that ADAMTS13 binds to endothelial cells in a specific, reversible, and time-dependent manner with a K(d) of 58 nm. This binding requires the COOH-terminal thrombospondin type 1 repeats of the protease. Binding is inhibited in the presence of heparin and by trypsin treatment of the cells. ADAMTS13 that was prebound to endothelial cells exhibited increased proteolysis of VWF as compared with ADAMTS13 present only in solution. These data support the notion that cleavage of VWF occurs mainly at the endothelial cell surface.

Characterization of conformation-sensitive antibodies to ADAMTS13, the von Willebrand cleavage protease

Background

The zinc metalloprotease ADAMTS13 is a multidomain protein that cleaves von Willebrand Factor (VWF) and is implicated in Thrombotic Thrombocytopenic Purpura (TTP) pathogenesis. Understanding the mechanism of this protein is an important goal. Conformation sensitive antibodies have been used to monitor protein conformation and to decipher the molecular mechanism of proteins as well as to distinguish functional and non-functional mutants.

Methodology/Principal Findings

We have characterized several antibodies against ADAMTS13, both monoclonal and polyclonal. We have used flow cytometry to estimate the binding of these antibodies to ADAMTS13 and demonstrate that antibodies raised against the TSP and disintegrin domains detect conformation changes in the ADAMTS13. Thus for example, increased binding of these antibodies was detected in the presence of the substrate (VWF), mainly at 37°C and not at 4°C. These antibodies could also detect differences between wild-type ADAMTS13 and the catalytically deficient mutant (P475S). The flow cytometry approach also allows us to estimate the reactivity of the antibody as well as its apparent affinity.

Conclusions/Significance

Our results suggest that these antibodies may serve as useful reagents to distinguish functional and non-functional ADAMTS13 and analyze conformational transitions to understand the catalytic mechanism.

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.

ADAMTS13 binds to CD36: a potential mechanism for platelet and endothelial localization of ADAMTS13

BACKGROUND

ADAMTS13 cleaves ultralarge von Willebrand factor (VWF) and plays a significant role in vascular biology and thrombotic thrombocytopenic purpura. CD36, a transmembrane protein present on endothelial cells and platelets (PLTs), binds to thrombospondin via three thrombospondin type 1 repeats. ADAMTS13 contains eight thrombospondin type 1 repeats.

STUDY DESIGN AND METHODS

An enzyme-linked immunoassay was used to explore the binding of recombinant human CD36 (rHuCD36) to recombinant human ADAMTS13 (rHuADAMTS13). A competition assay between rHuADAMTS13 and recombinant human (rHu)-thrombospondin-2 for binding to CD36 was then performed. Subsequently, binding of rHuADAMTS13 to PLT membrane fragments expressing CD36 (PLT glycoprotein IV) and glycoprotein Ib/IX was assessed. To examine the functional significance of an ADAMTS13-CD36 interaction, ADAMTS13 activity measured by a fluorescence resonance energy transfer assay was investigated in the presence of either rHuCD36 or concentrated PLTs.

RESULTS

rHuCD36 bound to rHuADAMTS13 in a dose-dependent fashion. rHu-thrombospondin-2 competed with ADAMTS13 for CD36 occupancy, but even high concentrations of rHu-thrombospondin-2 failed to completely block binding of rHuADAMTS13 to rHuCD36. rHuADAMTS13 bound to PLT membrane fragments expressing CD36 (PLT glycoprotein IV) in preference to PLT membrane fragments expressing glycoprotein Ib/IX. ADAMTS13 activity was not inhibited by the presence of either rHuCD36 or concentrated PLTs.

CONCLUSION

rHuADAMTS13 binds to both rHuCD36 and PLT membrane CD36 in vitro. The binding of CD36 to rHuADAMTS13 with retention of its enzymatic activity is consistent with a proposed role for CD36 in localizing ADAMTS13 on the endothelial cell surface where it regulates the cleavage of VWF.

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.

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.

Role of metalloproteinases in platelet function

Platelets contain and release matrix metalloproteinases (MMPs), their inhibitors (TIMPs) and disintegrin metalloproteinases (ADAMs) including MMP-1, MMP-2, MMP-3, MMP-9, MT1-MMP (MMP-14), ADAM-10, ADAM-17, ADAMTS-13, TIMP-1, TIMP-2 and TIMP-4. These proteins exert several effects regulating platelet functions such as agonist-stimulated platelet adhesion and aggregation, tumour cell-induced platelet aggregation and platelet-leukocyte aggregation. In this review, mechanisms of MMPs, TIMPs and ADAMs on platelets are discussed.

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.

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: 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.

An IAP retrotransposon in the mouse ADAMTS13 gene creates ADAMTS13 variant proteins that are less effective in cleaving von Willebrand factor multimers

Severe deficiency of ADAMTS13, a von Willebrand factor (VWF)-cleaving metalloprotease, causes thrombotic thrombocytopenic purpura. When analyzed with VWF multimers, but not with an abbreviated VWF peptide (VWF73) as the substrate, the plasma ADAMTS13 activity levels of mouse strains segregated into a high and a low group that differed by approximately 10 fold. Low ADAMTS13 activity was detected in mice containing 2 alleles of intracisternal A-type particle (IAP) retrotransposon sequence in the ADAMTS13 gene. Molecular cloning of mouse ADAMTS13 identified 2 truncated variants (IAP-a and IAP-b) in the low-activity mice. Both of the IAP variants lacked the 2 carboxyl terminus thrombospondin type 1 repeat (TSR) and CUB domains of full-length ADAMTS13. The IAP-b variant also had splicing abnormalities affecting the spacer domain sequence and had miniscule enzymatic activity. Compared with full-length ADAMTS13, the IAP-a variant was approximately one ninth as active in cleaving VWF multimers but was only slightly less active in cleaving VWF73 peptide. Recombinant human ADAMTS13 was also less effective in cleaving VWF multimers than VWF73 when the C-terminal TSR sequence was deleted. In summary, the carboxyl terminus TSR sequence is important for cleaving VWF multimers. Assay results should be interpreted with caution when peptide substrates are used for analysis of variant ADAMTS13 proteins.

ADAMTS13 assays and ADAMTS13-deficient mice

PURPOSE OF REVIEW

Thrombotic thrombocytopenic purpura can be induced by acquired or congenital deficiency of the plasma von Willebrand factor-cleaving protease, ADAMTS13. Measurement of ADAMTS13 activity is important for the diagnosis and treatment of microangiopathies including thrombotic thrombocytopenic purpura. Phenotypic analysis of mice lacking the Adamts13 gene is valuable for understanding the pathogenesis of microangiopathies.

RECENT FINDINGS

The minimum substrate for ADAMTS13 activity was identified as 73 amino acid residues in the A2 domain of von Willebrand factor, called VWF73. Several new assays have been developed using this sequence. The VWF73-based assays are rapid, quantitative, and easy to handle, and are well correlated with the measures from previous assays. Mice lacking the Adamts13 gene were produced. The mice were viable and fertile. They showed a prothrombotic state but no symptoms of spontaneous thrombocytopenia, hemolytic anemia, or microvascular thrombosis were observed.

SUMMARY

VWF73-based ADAMTS13 assays will significantly facilitate the accurate diagnosis of microangiopathies and contribute to the improved clinical treatment of these diseases. Accumulated clinical information on patients with ADAMTS13 deficiency and mice lacking the Adamts13 gene indicates that additional environmental or genetic susceptibility factors are required to trigger thrombotic thrombocytopenic purpura.

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.

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.

Apical sorting of ADAMTS13 in vascular endothelial cells and Madin-Darby canine kidney cells depends on the CUB domains and their association with lipid rafts

ADAMTS13 biosynthesis appeared to occur mainly in hepatic stellate cells, but detection of ADAMTS13 mRNA in many other tissues suggests that vascular endothelium may also produce ADAMTS13. We showed that ADAMTS13 mRNA and protein were detectable in human umbilical vein endothelial cells, aortic endothelial cells, and endothelium-derived cell line (ECV304). ADAMTS13 in cell lysate or serum-free conditioned medium cleaved von Willebrand factor (VWF) specifically. ADAMTS13 and VWF were localized to the distinct compartments of endothelial cells. Moreover, ADAMTS13 was preferentially sorted into apical domain of ECV304 and Madin-Darby canine kidney (MDCK) cells. Apical sorting of ADAMTS13 depended on the CUB domains and their association with lipid rafts. A mutation in the second CUB domain of ADAMTS13 (4143-4144insA), naturally occurring in patients with inherited thrombotic thrombocytopenic purpura, resulted in a significant reduction of ADAMTS13 secretion and a reversal of its polarity in MDCK cells. These data demonstrated that ADAMTS13 is synthesized and secreted from endothelial cells; the apically secreted ADAMTS13 from endothelial cells may contribute significantly to plasma ADAMTS13 proteases. The data also suggest a critical role of the CUB domains and a novel cargo-selective mechanism for apical sorting of a soluble ADAMTS protease in polarized cells.

Increased ADAMTS-13 proteolytic activity in rat hepatic stellate cells upon activation in vitro and in vivo

INTRODUCTION

ADAMTS-13 is a member of A Disintegrin And Metalloprotease with ThromboSpondin type 1 repeats (ADAMTS) family, primarily synthesized in hepatic stellate cells (HSCs), one of the major cell types transdifferentiating into myofibroblasts during liver fibrosis. However, the association between ADAMTS-13 expression and HSC activation or liver fibrosis is not known.

METHODS

In this study, we determined the ADAMTS-13 mRNA, protein, and activity in isolated primary HSCs upon activation on a plastic dish and in liver after administration of carbon tetrachloride (CCl(4)) in rats.

RESULTS

We showed that ADAMTS-13 antigen and proteolytic activity in the activated rat HSCs were dramatically increased, whereas ADAMTS-13 mRNA in these cells was only minimally altered. Similarly, the ADAMTS-13 antigen and proteolytic activity in rat liver after CCl(4) injury were also significantly increased, whereas the ADAMTS-13 mRNAs in these liver tissues were only slightly increased compared with normal. Surprisingly, despite the dramatic up-regulation of ADAMTS-13 protein synthesis in the activated HSCs after CCl(4) administration, the plasma levels of ADAMTS-13 protease in rats did not increase concordantly.

CONCLUSION

We conclude that the up-regulation of ADAMTS-13 protein expression in rat HSCs during activation in vitro and in vivo suggests the possibility of ADAMTS-13 proteolysis, an important part of function of the activated HSCs, perhaps through modulation of liver regeneration or formation of liver fibrosis after various injuries. The data also suggest the minimal contribution of the activated HSCs in regulation of plasma levels of ADAMTS-13 protease.

Hemolytic uremic syndrome-associated Shiga toxins promote endothelial-cell secretion and impair ADAMTS13 cleavage of unusually large von Willebrand factor multimers

Shiga toxin 1 (Stx-1) and Stx-2 produced by enterohemorrhagic Escherichia coli cause the diarrhea-associated hemolytic uremic syndrome (HUS). This type of HUS is characterized by obstruction of the glomeruli and renal microvasculature by platelet-fibrin thrombi, acute renal failure, thrombocytopenia, microvascular hemolytic anemia, and plasma levels of von Willebrand factor (VWF)-cleaving protease (ADAMTS13) activity that are within a broad normal range. We investigated the mechanism of initial platelet accumulation on Stx-stimulated endothelial cells. Stx-1 or Stx-2 (1-10 nM) stimulated the rapid secretion of unusually large (UL) VWF multimeric strings from human umbilical vein endothelial cells (HUVECs) or human glomerular microvascular endothelial cells (GMVECs). Perfused normal human platelets immediately adhered to the secreted ULVWF multimeric strings. Nanomolar concentrations (1-10 nM) of the Shiga toxins were as effective in inducing the formation of ULVWF-platelet strings as millimolar concentrations (0.1-20 mM) of histamine. The rate of ULVWF-platelet string cleavage by plasma or recombinant ADAMTS13 was delayed by 3 to 10 minutes (or longer) in the presence of 10 nM Stx-1 or Stx-2 compared with 20 mM histamine. Stx-induced formation of ULVWF strings, and impairment of ULVWF-platelet string cleavage by ADAMTS13, may promote initial platelet adhesion above glomerular endothelial cells. These processes may contribute to the evolution of glomerular occlusion by platelet and fibrin thrombi in diarrhea-associated HUS.