Identification of Strain-specific Variants of Mouse Adamts13 Gene Encoding von Willebrand Factor-cleaving Protease

Two novel mutations in ADAMTS13 in a Chinese boy with congenital thrombocytopenic purpura: a case report

BACKGROUND

Mutations in the ADAMTS13 gene family have been reported to cause congenital thrombotic thrombocytopenic purpura (cTTP), a rare disease characterized by thrombocytopenia and hemolytic anemia. Nearly 150 causative mutations in ADAMTS13 have been identified; however, only a few of them were detected in Chinese patients.

CASE PRESENTATION

A 5-year-old Chinese boy presented with history of thrombocytopenic purpura, hemolytic anemia, and renal injury since the neonatal period. Gene analysis revealed two novel mutations in ADAMTS13: a missense mutation 332G > A (p:Gly111Glu) in exon4 and a nonsense mutation 3121C > T (p:Gln1041stop) in exon 24. Genetic analysis of his parents confirmed the heterozygous nature of the mutations.

CONCLUSION

We report two novel mutations in ADAMTS13 (332G > A, 3121C > T) in a Chinese boy. These two mutations may lead to early onset of cTTP and severe symptoms.

Domain-specific mechanical modulation of VWF-ADAMTS13 interaction

Hemodynamic forces activate the Von Willebrand factor (VWF) and facilitate its cleavage by a disintegrin and metalloprotease with thrombospondin motifs-13 (ADAMTS13), reducing the adhesive activity of VWF. Biochemical assays have mapped the binding sites on both molecules. However, these assays require incubation of two molecules for a period beyond the time allowed in flowing blood. We used a single-molecule technique to examine these rapid, transient, and mechanically modulated molecular interactions in short times under forces to mimic what happens in circulation. Wild-type ADAMTS13 and two truncation variants that either lacked the C-terminal thrombospondin motif-7 to the CUB domain (MP-TSP6) or contained only the two CUB domains (CUB) were characterized for interactions with coiled VWF, flow-elongated VWF, and a VWF A1A2A3 tridomain. These interactions exhibited distinctive patterns of calcium dependency, binding affinity, and force-regulated lifetime. The results suggest that 1) ADAMTS13 binds coiled VWF primarily through CUB in a calcium-dependent manner via a site(s) outside A1A2A3, 2) ADAMTS13 binds flow-extended VWF predominantly through MP-TSP6 via a site(s) different from the one(s) at A1A2A3; and 3) ADAMTS13 binds A1A2A3 through MP-TSP6 in a Ca²⁺-dependent manner to autoinhibit another Ca²⁺-independent binding site on CUB. These data reveal that multiple sites on both molecules are involved in mechanically modulated VWF–ADAMTS13 interaction.

Mouse germ line mutations due to retrotransposon insertions

Transposable element (TE) insertions are responsible for a significant fraction of spontaneous germ line mutations reported in inbred mouse strains. This major contribution of TEs to the mutational landscape in mouse contrasts with the situation in human, where their relative contribution as germ line insertional mutagens is much lower. In this focussed review, we provide comprehensive lists of TE-induced mouse mutations, discuss the different TE types involved in these insertional mutations and elaborate on particularly interesting cases. We also discuss differences and similarities between the mutational role of TEs in mice and humans.

Plasminogen Tochigi mice exhibit phenotypes similar to wild-type mice under experimental thrombotic conditions

Plasminogen (Plg) is a precursor of plasmin that degrades fibrin. A race-specific A620T mutation in Plg, also known as Plg-Tochigi, originally identified in a patient with recurrent venous thromboembolism, causes dysplasminogenemia with reduced plasmin activity. The Plg-A620T mutation is present in 3–4% of individuals in East Asian populations, and as many as 50,000 Japanese are estimated to be homozygous for the mutant 620T allele. In the present study, to understand the changes of thrombotic phenotypes in individuals with the mutant 620T allele, we generated knock-in mice carrying the homozygous Plg-A622T mutation (Plg^T/T), an equivalent to the A620T mutation in human Plg. Plg^T/T mice grew normally but showed severely reduced plasmin activity activated by urokinase, equivalent to ~8% of that in wild-type mice. In vitro fibrin clot lysis in plasma was significantly slower in Plg^T/T mice than in wild-type mice. However, all experimental models of electrolytic deep vein thrombosis, tissue factor-induced pulmonary embolism, transient focal brain ischaemic stroke, or skin-wound healing showed largely similar phenotypes between Plg^T/T mice and wild-type mice. Protein S-K196E mutation (Pros1^E/E) is a race-specific genetic risk factor for venous thromboembolism. Coexistence in mice of Plg^T/T and Pros1^E/E did not affect pulmonary embolism symptoms, compared with those in Pros1^E/E mice. Hence, the present study showed that the Plg-A622T mutation, which confers ~8% plasmin activity, does not increase the risk of thrombotic diseases in mice under experimental thrombotic conditions and does not modify the thrombotic phenotype observed in Pros1^E/E mice. Plg^T/T mice can be used to investigate the potential pathophysiological impact of the Plg-A620T mutation.

Generation of Anti-Murine ADAMTS13 Antibodies and Their Application in a Mouse Model for Acquired Thrombotic Thrombocytopenic Purpura

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening thrombotic microangiopathy linked to a deficiency in the metalloprotease ADAMTS13. In the current study, a novel mouse model for acquired TTP was generated to facilitate development and validation of new therapies for this disease. Therefore, a large panel (n = 19) of novel anti-mouse ADAMTS13 (mADAMTS13) monoclonal antibodies (mAbs) of mouse origin was generated. Inhibitory anti-mADAMTS13 mAbs were identified using the FRETS-VWF73 assay. Four mAbs strongly inhibited mADAMTS13 activity in vitro (∼68–90% inhibition). Injecting a combination of 2 inhibitory mAbs (13B4 and 14H7, 1.25 mg/kg each) in Adamts13^+/+ mice resulted in full inhibition of plasma ADAMTS13 activity (96 ± 4% inhibition, day 1 post injection), leading to the appearance of ultra-large von Willebrand factor (UL-VWF) multimers. Interestingly, the inhibitory anti-mADAMTS13 mAbs 13B4 and 14H7 were ideally suited to induce long-term ADAMTS13 deficiency in Adamts13^+/+ mice. A single bolus injection resulted in full ex vivo inhibition for more than 7 days. As expected, the mice with the acquired ADAMTS13 deficiency did not spontaneously develop TTP, despite the accumulation of UL-VWF multimers. In line with the Adamts13^-/- mice, TTP-like symptoms could only be induced when an additional trigger (rVWF) was administered. On the other hand, the availability of our panel of anti-mADAMTS13 mAbs allowed us to further develop a sensitive ELISA to detect ADAMTS13 in mouse plasma. In conclusion, a novel acquired TTP mouse model was generated through the development of inhibitory anti-mADAMTS13 mAbs. Consequently, this model provides new opportunities for the development and validation of novel treatments for patients with TTP. In addition, these newly developed inhibitory anti-mADAMTS13 mAbs are of great value to specifically study the role of ADAMTS13 in mouse models of thrombo-inflammatory disease.

A genetically-engineered von Willebrand disease type 2B mouse model displays defects in hemostasis and inflammation

von Willebrand disease (VWD)-type 2B is characterized by gain-of-function mutations in the von Willebrand factor (VWF) A1-domain, leading to increased affinity for its platelet-receptor, glycoprotein Ibα. We engineered the first knock-in (KI) murine model for VWD-type 2B by introducing the p.V1316M mutation in murine VWF. Homozygous KI-mice replicated human VWD-type 2B with macrothrombocytopenia (platelet counts reduced by 55%, platelet volume increased by 44%), circulating platelet-aggregates and a severe bleeding tendency. Also, vessel occlusion was deficient in the FeCl3-induced thrombosis model. Platelet aggregation induced by thrombin or collagen was defective for KI-mice at all doses. KI-mice manifested a loss of high molecular weight multimers and increased multimer degradation. In a model of VWF-string formation, the number of platelets/string and string-lifetime were surprisingly enhanced in KI-mice, suggesting that proteolysis of VWF/p.V1316M is differentially regulated in the circulation versus the endothelial surface. Furthermore, we observed increased leukocyte recruitment during an inflammatory response induced by the reverse passive Arthus reaction. This points to an active role of VWF/p.V1316M in the exfiltration of leukocytes under inflammatory conditions. In conclusion, our genetically-engineered VWD-type 2B mice represent an original model to study the consequences of spontaneous VWF-platelet interactions and the physiopathology of this human disease.

Prediction of Molecular Interaction between Platelet Glycoprotein Ibα and von Willebrand Factor using Molecular Dynamics Simulations

AIM

The molecular mechanism of the unique interaction between platelet membrane glycoprotein Ibα (GPIbα) and von Willebrand Factor (VWF), necessary for platelet adhesion under high shear stress, is yet to be clarified.

METHODS

The molecular dynamics simulation using NAMD (Nanoscale Molecular Dynamics) package with the CHARMM 22 (Chemistry at Harvard Macromolecular Mechanics) force field were used to predict dynamic structural changes occurring in the binding site of A1 domain of VWF and N terminus domain of GPIbα under water soluble condition.

RESULTS

The mean distance between the mass center of A1 domain of VWF and GPIbα in the stable form was predicted as 27.3 Å. The potential of mean force between the A1 domain of VWF and GPIbα were calculated in conditions of various distances of the mass center between them. All the calculated values were fitted to the Morse potential energy function curve. The maximum adhesive force between A1 domain of VWF and GPIbα was predicted as 62.3 pN by differentiating the potential of mean force with respect to the molecular distance.

CONCLUSIONS

The molecular dynamics simulation is useful for predicting the dynamic structure changes of protein bonds involved in platelet adhesion and for predicting the adhesive forces generated between their interactions.

Probing ADAMTS13 substrate specificity using phage display

Von Willebrand factor (VWF) is a large, multimeric protein that regulates hemostasis by tethering platelets to the subendothelial matrix at sites of vascular damage. The procoagulant activity of plasma VWF correlates with the length of VWF multimers, which is proteolytically controlled by the metalloprotease ADAMTS13. To probe ADAMTS13 substrate specificity, we created phage display libraries containing randomly mutated residues of a minimal ADAMTS13 substrate fragment of VWF, termed VWF73. The libraries were screened for phage particles displaying VWF73 mutant peptides that were resistant to proteolysis by ADAMTS13. These peptides exhibited the greatest mutation frequency near the ADAMTS13 scissile residues. Kinetic assays using mutant and wild-type substrates demonstrated excellent agreement between rates of cleavage for mutant phage particles and the corresponding mutant peptides. Cleavage resistance of selected mutations was tested in vivo using hydrodynamic injection of corresponding full-length expression plasmids into VWF-deficient mice. These studies confirmed the resistance to cleavage resulting from select amino acid substitutions and uncovered evidence of alternate cleavage sites and recognition by other proteases in the circulation of ADAMTS13 deficient mice. Taken together, these studies demonstrate the key role of specific amino acids residues including P3-P2’ and P11’, for substrate specificity and emphasize the importance in flowing blood of other ADAMTS13–VWF exosite interactions outside of VWF73.

Nxf1 natural variant E610G is a semi-dominant suppressor of IAP-induced RNA processing defects

Endogenous retroviruses and retrotransposons contribute functional genetic variation in animal genomes. In mice, Intracisternal A Particles (IAPs) are a frequent source of both new mutations and polymorphism across laboratory strains. Intronic IAPs can induce alternative RNA processing choices, including alternative splicing. We previously showed IAP I∆1 subfamily insertional mutations are suppressed by a wild-derived allele of the major mRNA export factor, Nxf1. Here we show that a wider diversity of IAP insertions present in the mouse reference sequence induce insertion-dependent alternative processing that is suppressed by Nxf1CAST alleles. These insertions typically show more modest gene expression changes than de novo mutations, suggesting selection or attenuation. Genome-wide splicing-sensitive microarrays and gene-focused assays confirm specificity of Nxf1 genetic modifier activity for IAP insertion alleles. Strikingly, CRISPR/Cas9-mediated genome editing demonstrates that a single amino acid substitution in Nxf1, E610G, is sufficient to recreate a quantitative genetic modifier in a co-isogenic background.

Allosteric activation of ADAMTS13 by von Willebrand factor

The metalloprotease ADAMTS13 cleaves von Willebrand factor (VWF) within endovascular platelet aggregates, and ADAMTS13 deficiency causes fatal microvascular thrombosis. The proximal metalloprotease (M), disintegrin-like (D), thrombospondin-1 (T), Cys-rich (C), and spacer (S) domains of ADAMTS13 recognize a cryptic site in VWF that is exposed by tensile force. Another seven T and two complement C1r/C1s, sea urchin epidermal growth factor, and bone morphogenetic protein (CUB) domains of uncertain function are C-terminal to the MDTCS domains. We find that the distal T8-CUB2 domains markedly inhibit substrate cleavage, and binding of VWF or monoclonal antibodies to distal ADAMTS13 domains relieves this autoinhibition. Small angle X-ray scattering data indicate that distal T-CUB domains interact with proximal MDTCS domains. Thus, ADAMTS13 is regulated by substrate-induced allosteric activation, which may optimize VWF cleavage under fluid shear stress in vivo. Distal domains of other ADAMTS proteases may have similar allosteric properties.

Von Willebrand Factor Abnormalities Studied in the Mouse Model: What We Learned about VWF Functions

Up until recently, von Willebrand Factor (VWF) structure-function relationships have only been studied through in vitro approaches. A powerful technique known as hydrodynamic gene transfer, which allows transient expression of a transgene by mouse hepatocytes, has led to an important shift in VWF research. Indeed this approach has now enabled us to transiently express a number of VWF mutants in VWF-deficient mice in order to test the relative importance of specific residues in different aspects of VWF biology and functions in an in vivo setting. As a result, mice reproducing various types of von Willebrand disease have been generated, models that will be useful to test new therapies. This approach also allowed a more precise identification of the importance of VWF interaction with subendothelial collagens and with platelets receptors in hemostasis and thrombosis. The recent advances gathered from these studies as well as the pros and cons of the technique will be reviewed here.

ADAMTS13 deficiency exacerbates VWF-dependent acute myocardial ischemia/reperfusion injury in mice

Epidemiologic studies suggest that elevated VWF levels and reduced ADAMTS13 activity in the plasma are risk factors for myocardial infarction. However, it remains unknown whether the ADAMTS13-VWF axis plays a causal role in the pathophysiology of myocardial infarction. In the present study, we tested the hypothesis that ADAMTS13 reduces VWF-mediated acute myocardial ischemia/reperfusion (I/R) injury in mice. Infarct size, neutrophil infiltration, and myocyte apoptosis in the left ventricular area were quantified after 30 minutes of ischemia and 23.5 hours of reperfusion injury. Adamts13(-/-) mice exhibited significantly larger infarcts concordant with increased neutrophil infiltration and myocyte apoptosis compared with wild-type (WT) mice. In contrast, Vwf(-/-) mice exhibited significantly reduced infarct size, neutrophil infiltration, and myocyte apoptosis compared with WT mice, suggesting a detrimental role for VWF in myocardial I/R injury. Treating WT or Adamts13(-/-) mice with neutralizing Abs to VWF significantly reduced infarct size compared with control Ig-treated mice. Finally, myocardial I/R injury in Adamts13(-/-)/Vwf(-/-) mice was similar to that in Vwf(-/-) mice, suggesting that the exacerbated myocardial I/R injury observed in the setting of ADAMTS13 deficiency is VWF dependent. These findings reveal that ADAMTS13 and VWF are causally involved in myocardial I/R injury.

Use of a mouse model to elucidate the phenotypic effects of the von Willebrand factor cleavage mutants, Y1605A/M1606A and R1597W

BACKGROUND

von Willebrand Factor (VWF) is tightly regulated by the metalloproteinase ADAMTS13, which cleaves VWF to reduce VWF multimer size and binding affinity for collagen and platelets.

OBJECTIVE

This study examines two VWF mutations, R1597W (enhanced cleavage) and Y1605A-M1606A (decreased cleavage), to determine their impact on VWF, in addition to ADAMTS13-mediated cleavage.

METHODS

In vitro mouse ADAMTS13 digestions were performed on recombinant proteins. VWF knockout mice received hydrodynamic injections of mouse Vwf cDNA, following which VWF antigen, multimer profile and VWF propeptide levels were determined. A ferric chloride injury model of thrombosis was also evaluated.

RESULTS

In vitro ADAMTS13 digestion of full-length mouse VWF required > 97-fold higher ADAMTS13 levels for Y1605A/M1606A, and 68% lower ADAMTS13 levels for R1597W compared with wild type. In vivo, R1597W had reduced VWF:Ag and both mutations exhibited increased VWF propeptide/VWF:Ag ratios. R1597W multimers show a lower molecular weight profile compared with wild type and Y1605A/M1606A mice. When co-injected with Adamts13 cDNA, Y1605A/M1606A multimers were larger compared with wild type, and R1597W showed only a single multimer band and decreased clearance via VWFpp/VWF:Ag ratio. R1597W was associated with reduced thrombus formation but normal platelet accumulation in a ferric chloride injury model while Y1605A/M1606A had a loss of occlusive thrombi but increased platelet accumulation compared with wild type.

CONCLUSIONS

This study demonstrates that mutations that alter ADAMTS13 cleavage also can affect VWF clearance, VWF antigen level, multimer structure and thrombotic potential in the VWF knockout hydrodynamic injection model.

Mouse endogenous retroviruses can trigger premature transcriptional termination at a distance

Endogenous retrotransposons have caused extensive genomic variation within mammalian species, but the functional implications of such mobilization are mostly unknown. We mapped thousands of endogenous retrovirus (ERV) germline integrants in highly divergent, previously unsequenced mouse lineages, facilitating a comparison of gene expression in the presence or absence of local insertions. Polymorphic ERVs occur relatively infrequently in gene introns and are particularly depleted from genes involved in embryogenesis or that are highly expressed in embryonic stem cells. Their genomic distribution implies ongoing negative selection due to deleterious effects on gene expression and function. A polymorphic, intronic ERV at Slc15a2 triggers up to 49-fold increases in premature transcriptional termination and up to 39-fold reductions in full-length transcripts in adult mouse tissues, thereby disrupting protein expression and functional activity. Prematurely truncated transcripts also occur at Polr1a, Spon1, and up to ∼5% of other genes when intronic ERV polymorphisms are present. Analysis of expression quantitative trait loci (eQTLs) in recombinant BxD mouse strains demonstrated very strong genetic associations between the polymorphic ERV in cis and disrupted transcript levels. Premature polyadenylation is triggered at genomic distances up to >12.5 kb upstream of the ERV, both in cis and between alleles. The parent of origin of the ERV is associated with variable expression of nonterminated transcripts and differential DNA methylation at its 5'-long terminal repeat. This study defines an unexpectedly strong functional impact of ERVs in disrupting gene transcription at a distance and demonstrates that ongoing retrotransposition can contribute significantly to natural phenotypic diversity.

The CXCR4/CXCR7/SDF-1 pathway contributes to the pathogenesis of Shiga toxin-associated hemolytic uremic syndrome in humans and mice

Hemolytic uremic syndrome (HUS) is a potentially life-threatening condition. It often occurs after gastrointestinal infection with E. coli O157:H7, which produces Shiga toxins (Stx) that cause hemolytic anemia, thrombocytopenia, and renal injury. Stx-mediated changes in endothelial phenotype have been linked to the pathogenesis of HUS. Here we report our studies investigating Stx-induced changes in gene expression and their contribution to the pathogenesis of HUS. Stx function by inactivating host ribosomes but can also alter gene expression at concentrations that minimally affect global protein synthesis. Gene expression profiling of human microvascular endothelium treated with Stx implicated a role for activation of CXCR4 and CXCR7 by their shared cognate chemokine ligand (stromal cell-derived factor-1 [SDF-1]) in Stx-mediated pathophysiology. The changes in gene expression required a catalytically active Stx A subunit and were mediated by enhanced transcription and mRNA stability. Stx also enhanced the association of CXCR4, CXCR7, and SDF1 mRNAs with ribosomes. In a mouse model of Stx-mediated pathology, we noted changes in plasma and tissue content of CXCR4, CXCR7, and SDF-1 after Stx exposure. Furthermore, inhibition of the CXCR4/SDF-1 interaction decreased endothelial activation and organ injury and improved animal survival. Finally, in children infected with E. coli O157:H7, plasma SDF-1 levels were elevated in individuals who progressed to HUS. Collectively, these data implicate the CXCR4/CXCR7/SDF-1 pathway in Stx-mediated pathogenesis and suggest novel therapeutic strategies for prevention and/or treatment of complications associated with E. coli O157:H7 infection.

ADAMTS13 reduces vascular inflammation and the development of early atherosclerosis in mice

ADAMTS13, a metalloprotease, plays a pivotal role in preventing spontaneous microvascular thrombosis by cleaving hyperactive ultra large von Willebrand factor multimers into smaller, less active multimers. Reduced ADAMTS13 activity in plasma has been described in many diseases associated with systemic inflammation. It remains uncertain, however, whether ADAMTS13 contributes to disease pathogenesis or rather simply serves as an inflammation-associated marker. We hypothesized that, by decreasing vascular inflammation, ADAMTS13 reduces the development of early atherosclerotic plaques. Using intravital fluorescence microscopy, we observed excessive leukocyte adhesion and accelerated atherosclerotic plaque formation at the carotid sinus of Adamts13(-/-)/ApoE(-/-) mice compared with ApoE(-/-) mice fed a high-fat Western diet. At 4 months of age, there was a significant increase in atherosclerosis in the aorta and aortic sinus of Adamts13(-/-)/ApoE(-/-) mice compared with ApoE(-/-) mice. Interestingly, we detected a 2-fold increase in macrophage recruitment to the atherosclerotic plaque of the Adamts13(-/-)/ApoE(-/-) mice compared with ApoE(-/-) mice, suggesting that the atherosclerotic lesions in these mice were not only larger but also more inflammatory. These findings reveal a new functional role for the antithrombotic enzyme ADAMTS13 in reducing excessive vascular inflammation and plaque formation during early atherosclerosis.

Molecular cloning, in vitro expression and functional characterization of canine ADAMTS13

A disintegrin and metalloproteinase with thrombospondin type 1 motifs, number 13 (ADAMTS13) is a plasma zinc metalloprotease also known as von Willebrand factor (VWF)-cleaving protease. Deficiency of ADAMTS13 activity is known to cause thrombotic thrombocytopenic purpura (TTP) in humans. We isolated the canine ADAMTS13 cDNA, which encodes 1502 amino acids, and expressed the recombinant protein to evaluate VWF-cleaving ability. Although the propeptide domain was longer and the TSP1 repeat domain was shorter than those in other species, the overall structures were similar to human and mouse ADAMTS13. Recombinant canine ADAMTS13 cleaved the 250-kDa VWF monomer into two fragments of 150 kDa and 120 kDa. Furthermore, high molecular weight VWF multimers were abolished based on the activity of ADAMTS13. These results could facilitate research into hemostatic disorders such as TTP in dogs.

Unraveling the scissile bond: how ADAMTS13 recognizes and cleaves von Willebrand factor

von Willebrand factor (VWF) is a large adhesive glycoprotein with established functions in hemostasis. It serves as a carrier for factor VIII and acts as a vascular damage sensor by attracting platelets to sites of vessel injury. VWF size is important for this latter function, with larger multimers being more hemostatically active. Functional imbalance in multimer size can variously cause microvascular thrombosis or bleeding. The regulation of VWF multimeric size and platelet-tethering function is carried out by ADAMTS13, a plasma metalloprotease that is constitutively active. Unusually, protease activity of ADAMTS13 is controlled not by natural inhibitors but by conformational changes in its substrate, which are induced when VWF is subject to elevated rheologic shear forces. This transforms VWF from a globular to an elongated protein. This conformational transformation unfolds the VWF A2 domain and reveals cryptic exosites as well as the scissile bond. To enable VWF proteolysis, ADAMTS13 makes multiple interactions that bring the protease to the substrate and position it to engage with the cleavage site as this becomes exposed by shear. This article reviews recent literature on the interaction between these 2 multidomain proteins and provides a summary model to explain proteolytic regulation of VWF by ADAMTS13.

Distributions of transposable elements reveal hazardous zones in mammalian introns

Comprising nearly half of the human and mouse genomes, transposable elements (TEs) are found within most genes. Although the vast majority of TEs in introns are fixed in the species and presumably exert no significant effects on the enclosing gene, some markedly perturb transcription and result in disease or a mutated phenotype. Factors determining the likelihood that an intronic TE will affect transcription are not clear. In this study, we examined intronic TE distributions in both human and mouse and found several factors that likely contribute to whether a particular TE can influence gene transcription. Specifically, we observed that TEs near exons are greatly underrepresented compared to random distributions, but the size of these “underrepresentation zones” differs between TE classes. Compared to elsewhere in introns, TEs within these zones are shorter on average and show stronger orientation biases. Moreover, TEs in extremely close proximity (<20 bp) to exons show a strong bias to be near splice-donor sites. Interestingly, disease-causing intronic TE insertions show the opposite distributional trends, and by examining expressed sequence tag (EST) databases, we found that the proportion of TEs contributing to chimeric TE-gene transcripts is significantly higher within their underrepresentation zones. In addition, an analysis of predicted splice sites within human long terminal repeat (LTR) elements showed a significantly lower total number and weaker strength for intronic LTRs near exons. Based on these factors, we selectively examined a list of polymorphic mouse LTR elements in introns and showed clear evidence of transcriptional disruption by LTR element insertions in the Trpc6 and Kcnh6 genes. Taken together, these studies lend insight into the potential selective forces that have shaped intronic TE distributions and enable identification of TEs most likely to exert transcriptional effects on genes.

Sequences derived from transposable elements (TEs) are major constituents of mammalian genomes and are found within introns of most genes. While nearly all TEs within introns appear harmless, some de novo intronic TE insertions do disrupt gene transcription and splicing and cause disease. It is unclear why some intronic TEs perturb gene transcription whereas most do not. Here, we examined intronic TE distributions in both human and mouse genes to gain insight into which TEs may be more likely to affect transcription. We found evidence that TEs near exons are likely subject to strong negative selection but the size of the region under selection or “underrepresentation zone” differs for different TE classes. Strikingly, all reported human disease-causing intronic TE insertions fall within these underrepresentation zones, and the proportion of TEs contributing to chimeric TE-gene transcripts is significantly higher when TEs are located in these zones. We also examined insertionally polymorphic mouse TEs located within underrepresentation zones and found evidence of transcriptional disruption in two genes. Given the growing appreciation for ongoing activity of TEs in human, our results should be of value in prioritizing insertionally polymorphic TEs for study of their potential contributions to gene expression differences and phenotypic variability.

Pathologic mechanisms of type 1 VWD mutations R1205H and Y1584C through in vitro and in vivo mouse models

Type 1 VWD is the mild to moderate reduction of VWF levels. This study examined the mechanisms underlying 2 common type 1 VWD mutations, the severe R1205H and more moderate Y1584C. In vitro biosynthesis was reduced for both mutations in human and mouse VWF, with the effect being more severe in R1205H. VWF knockout mice received hydrodynamic injections of mouse Vwf cDNA. Lower VWF antigen levels were demonstrated in both homozygous and heterozygous forms for both type 1 mutations from days 14-42. Recombinant protein infusions and hydrodynamic-expressed VWF propeptide to antigen ratios demonstrate that R1205H mouse VWF has an increased clearance rate, while Y1584C is normal. Recombinant ADAMTS13 digestions of Y1584C demonstrated enhanced cleavage of both human and mouse VWF115 substrates. Hydrodynamic-expressed VWF shows a loss of high molecular weight multimers for Y1584C compared with wild-type and R1205H. At normal physiologic levels of VWF, Y1584C showed reduced thrombus formation in a ferric chloride injury model while R1205H demonstrated similar thrombogenic activity to wild-type VWF. This study has elucidated several novel mechanisms for these mutations and highlights that the type 1 VWD phenotype can be recapitulated in the VWF knockout hydrodynamic injection model.

The distal carboxyterminal domains of murine ADAMTS13 influence proteolysis of platelet-decorated VWF strings in vivo

BACKGROUND

The multidomain metalloprotease ADAMTS13 regulates the size of von Willebrand factor (VWF) multimers upon their release from endothelial cells. How the different domains in ADAMTS13 control VWF proteolysis in vivo remains largely unidentified.

METHODS

Seven C-terminally truncated murine ADAMTS13 (mADAMTS13) mutants were constructed and characterized in vitro. Their ability to cleave VWF strings in vivo was studied in the ADAMTS13(-/-) mouse.

RESULTS

Murine MDTCS (devoid of T2-8 and CUB domains) retained full enzyme activity in vitro towards FRETS-VWF73 and the C-terminal T6-8 (del(T6-CUB)) and CUB domains (delCUB) are dispensable under these assay conditions. In addition, mADAMTS13 fragments without the spacer domain (MDT and M) had reduced catalytic efficiencies. Our results hence indicate that similar domains in murine and human ADAMTS13 are required for activity in vitro, supporting the use of mouse models to study ADAMTS13 function in vivo. Interestingly, using intravital microscopy we show that removal of the CUB domains abolishes proteolysis of platelet-decorated VWF strings in vivo. In addition, whereas MDTCS is fully active in vivo, partial (del(T6-CUB)) or complete (delCUB) addition of the T2-8 domains gradually attenuates its activity.

CONCLUSIONS

Our data demonstrate that the ADAMTS13 CUB and T2-8 domains influence proteolysis of platelet-decorated VWF strings in vivo.

A splice variant of ADAMTS13 is expressed in human hepatic stellate cells and cancerous tissues

Although ADAMTS13, the von Willebrand factor (VWF)-cleaving protease, is expressed in a range of tissues, the physiological significance of tissue-specific ADAMTS13 alternative splicing isoforms have yet to be clarified. Screening a panel of human tissues and cell lines revealed a spliced ADAMTS13 transcript in hepatic stellate cells and a hepatoma cell line that retains the 25th intron. A nonsense codon within the intron truncates the protease, which gains 64 novel amino acids in lieu of both CUB domains. This isoform, while retaining VWF-cleaving capability, accumulates intracellularly and its biological inaccessibility may prevent its participation in regulating haemostasis and other physiologic functions.

Pathophysiology of thrombotic thrombocytopenic purpura

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

The function of ADAMTS13 in thrombogenesis in vivo: insights from mutant mice

Recently, two independent groups have established ADAMTS13-deficient mice using gene-targeting techniques. In humans, genetic or acquired deficiency in ADAMTS13 leads to a potentially fatal syndrome, thrombotic thrombocytopenic purpura (TTP). Surprisingly, ADAMTS13-deficient mice are viable with no apparent signs of TTP. However, these mouse models indicate that ADAMTS13 down-regulates platelet adhesion and aggregation in vivo, and ADAMTS13 deficiency can provide enhanced thrombus formation at the site of vascular lesions. In addition, ADAMTS13 by cleaving hyperactive ultra-large von Willebrand factor multimers not only down-regulates thrombosis but also inflammation. ADAMTS13-congenic mice that carry a truncated form of ADAMTS13 lacking the C-terminal domains have also been developed. Phenotypes of the congenic mice indicate the physiological significance of the C-terminal domains of ADAMTS13 in down-regulating thrombus growth. The studies mentioned here in different mouse models uncover the in vivo function of ADAMTS13 and strengthened the understanding of the mechanism of systemic disease TTP.

Species-dependent variability of ADAMTS13-mediated proteolysis of human recombinant von Willebrand factor

BACKGROUND

von Willebrand factor (VWF) is composed of a series of multimers, the sizes of which are regulated by the plasma metalloprotease ADAMTS13.

OBJECTIVE

Proteolysis of human recombinant VWF (rVWF) by ADAMTS13 present in the plasma of different species typically used as preclinical animal models was investigated to evaluate the efficacy and safety of rVWF.

METHODS

Degradation of rVWF was studied in vitro under moderate denaturing conditions and was monitored by multimer analysis, residual collagen binding, and immunoblot analysis. In vivo cleavage was determined by administration of rVWF to cynomolgus monkeys, rabbits and VWF-deficient mice and subsequent analysis of plasma samples by immunoblot. Plasma ADAMTS13 levels were determined with a synthetic human VWF peptide (FRETS-VWF73).

RESULTS

From the animals tested, only rabbit plasma was as efficient as human plasma in proteolysing rVWF in vitro. Mouse plasma virtually failed to cleave rVWF. Administration of human rVWF resulted in ADAMTS13-specific cleavage products in rabbits and, to a lesser extent, in cynomolgus monkeys at various doses of rVWF. Virtually no cleavage occurred in mice. ADAMTS13 activity levels in rabbit and monkey plasma were similar to those in human plasma and were not significantly altered upon infusion of rVWF up to very high doses, indicating that rVWF did not lead to an exhaustion of endogenous ADAMTS13 in both species.

CONCLUSIONS

The differences in susceptibility to cleavage of rVWF by different species need to be considered when interpreting the physiology of human rVWF from results of tests in animal models.

The first deletion mutation in the TSP1-6 repeat domain of ADAMTS13 in a family with inherited thrombotic thrombocytopenic purpura

The inherited deficiency of ADAMTS13 is usually associated with severe forms of thrombotic thrombocytopenic purpura. Among the mutations identified in the ADAMTS13 gene, none have been described on the TSP1-6 repeat domain. We investigated an Iranian family with a history of chronic recurrent thrombotic thrombocytopenic purpura, severe ADAMTS13 deficiency and a heterogeneous pattern of clinical symptoms among affected members. Genetic analysis revealed a homozygous deletion of nucleotides 2930-2935 (GTGCCC) in exon 23 of ADAMTS13, leading to the replacement of Cys977 by a Trp and the deletion of Ala978 and Arg979 in the TSP1-6 repeat domain. To explore the mechanism of ADAMTS13 deficiency, in vitro expression studies were performed. Western blotting, pulse-chase labeling and immunofluorescence studies demonstrated a secretion pathway defect of the mutant protein, with no intracellular accumulation. This finding is consistent with the severe ADAMTS13 deficiency but does not explain the heterogeneous clinical picture of the 3 siblings carrying the same mutation.

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

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

Modeling the neurovascular niche: murine strain differences mimic the range of responses to chronic hypoxia in the premature newborn

Preterm birth results in significant cognitive and motor disabilities, but recent evidence suggests that there is variable recovery over time. One possibility that may explain this variable recovery entails variable neurogenic responses in the subventricular zone (SVZ) following the period of chronic hypoxia experienced by these neonates. In this report, we have characterized the responses to chronic hypoxia of two mouse strains that represent a wide range of susceptibility to chronic hypoxia. We determined that C57BL/6 pups and neural progenitor cells (NPCs) derived from them exhibit a blunted response to hypoxic insult compared with CD-1 pups and NPCs. Specifically, C57BL/6 pups and NPCs exhibited blunted in vivo and in vitro proliferative and increased apoptotic responses to hypoxic insult. Additionally, C57BL/6 NPCs exhibited lower baseline levels and hypoxia-induced levels of selected transcription factors, growth factors, and receptors (including HIF-1alpha, PHD2, BDNF, VEGF, SDF-1, TrkB, Nrp-1, CXCR4, and NO) that determine, in part, the responsiveness to chronic hypoxic insult compared with CD-1 pups and NPCs, providing insight into this important and timely problem in perinatology.

Microarray-based global mapping of integration sites for the retrotransposon, intracisternal A-particle, in the mouse genome

Mammalian genomes contain numerous evolutionary harbored mobile elements, a part of which are still active and may cause genomic instability. Their movement and positional diversity occasionally result in phenotypic changes and variation by causing altered expression or disruption of neighboring host genes. Here, we describe a novel microarray-based method by which dispersed genomic locations of a type of retrotransposon in a mammalian genome can be identified. Using this method, we mapped the DNA elements for a mouse retrotransposon, intracisternal A-particle (IAP), within genomes of C3H/He and C57BL/6J inbred mouse strains; consequently we detected hundreds of probable IAP cDNA-integrated genomic regions, in which a considerable number of strain-specific putative insertions were included. In addition, by comparing genomic DNAs from radiation-induced myeloid leukemia cells and its reference normal tissue, we detected three genomic regions around which an IAP element was integrated. These results demonstrate the first successful genome-wide mapping of a retrotransposon type in a mammalian genome.

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.

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.

Insights into von Willebrand factor proteolysis: clinical implications

The proteolysis of von Willebrand factor (VWF) by the recently discovered metalloprotease ADAMTS13 (a disintegrin and metalloprotease with thrombospondin repeats), is a normal processing step in VWF biochemistry. Emerging data indicate that this step may be influenced by a variety of factors, some of which favour increased proteolysis and some of which compromise proteolysis. The former may predispose to bleeding, whilst the latter appears to be the underlying mechanism for thrombotic thrombocytopenic purpura (TTP). The new insights support the concept of "risk" in bleeding, particularly in the case of type 1 von Willebrand disease (VWD), in much the same way that risk is considered in venous thrombosis. This review presents relevant current knowledge of VWF proteolysis by ADAMTS13, and a novel model of how this may be implicated in type 1 VWD is proposed, based on events at the vessel wall at a time of haemostatic challenge.

Molecular biology of ADAMTS13 and diagnostic utility of ADAMTS13 proteolytic activity and inhibitor assays

ADAMTS13, a reprolysin-like metalloprotease, limits platelet-rich thrombus formation in the small arteries by cleaving von Willebrand factor (vWF) at the Tyr1605-Met1606 peptide bond. Deficiency of plasma ADAMTS13 activity, due to either an inherited or an acquired etiology, may lead to a potentially lethal syndrome, thrombotic thrombocytopenic purpura (TTP). Molecular cloning and characterization of the ADAMTS13 gene have provided further insight into the structure-function relationships, biosynthesis, and regulation of the ADAMTS13 protease, in addition to understanding the pathogenesis of TTP and perhaps other thrombotic disorders. ADAMTS13 consists of a short propeptide, a typical reprolysin-like metalloprotease domain, followed by a disintegrin-like domain, first thrombospondin type 1 (TSP1) repeat, Cys-rich domain, and spacer domain. The carboxyl terminus of ADAMTS13 has seven more TSP1 repeats and two CUB domains. ADAMTS13 is synthesized mainly in hepatic stellate cells, but also in vascular endothelial cells. Recognition and cleavage of vWF require the proximal carboxyl terminal domains, but not the middle and distal carboxyl terminal domains. Cleavage of vWF appears to be modulated by shear force, binding to platelet or platelet glycoprotein-1balpha, heparin, inflammatory cytokine (interleukin-6), and chloride ion. At the site of thrombus formation, the ADAMTS13 may be inactivated by thrombin, plasmin, and factor Xa. Having a sensitive and specific assay for ADAMTS13 activity is not only critical to understand the basic biology of ADAMTS13 protease, but also to facilitate a more timely and accurate clinical diagnosis of TTP, and to initiate potentially life-saving plasma exchange therapy. Although many assays have been developed and tested for clinical applications, the fluorescent resonance energy transfer-vWF73 assay appears to be the simplest and most promising assay to date.

A fibrinogen deficiency accelerates the initiation of LDL cholesterol-driven atherosclerosis via thrombin generation and platelet activation in genetically predisposed mice

Mice with combined deficiencies of the low-density lipoprotein receptor (LDLR(-/-)) and the catalytic component of an apolipoprotein B-edisome complex (APOBEC1(-/-)) that converts apoB-100 to apoB-48 have been characterized, and this model of LDL cholesterol-driven atherosclerosis was applied to an investigation of the role of fibrinogen (Fg) in the genesis and progression of the plaque. LDLR(-/-)/APOBEC1(-/-)/FG(-/-) (L(-/-)/A(-/-)/FG(-/-)) triple-deficient mice presented more advanced plaque in their aortic trees and aortic sinuses at 24, 36, and 48 weeks of age compared to L(-/-)/A(-/-) mice, a feature that may result from enhanced platelet activation in these former mice. This is supported by the presence of hypercoagulability, increased CD61 and CD62P on resting platelets, and higher plasma soluble P-selectin in L(-/-)/A(-/-)/FG(-/-) mice as compared to L(-/-)/A(-/-), FG(-/-), or wild-type mice. The elevated higher molecular weight forms of von Willebrand factor (VWF) in L(-/-)/A(-/-)/FG(-/-) mice, revealed by increased VWF collagen binding activity, perhaps resulting from down-regulation of its cleaving metalloproteinase, ADAMTS13, further indicates enhanced platelet activation. Thus, the earlier arterial plaque deposition in L(-/-)/A(-/-)/FG(-/-) mice appears to contain a contribution from enhanced levels of thrombin and activated platelets, a synergistic consequence of an Fg deficiency combined with a high LDL cholesterol concentration.

Mechanisms of the interaction between twoADAMTS13 gene mutations leading to severe deficiency of enzymatic activity

The inherited deficiency of the von Willebrand factor-cleaving protease ADAMTS13 is associated with rare forms of thrombotic thrombocytopenic purpura (TTP). We investigated a woman with a family history of chronic recurrent TTP and undetectable plasma levels of ADAMTS13 activity. Genetic analysis revealed two missense mutations in the heterozygous state: p.Val88Met substitution in the metalloprotease domain and p.Gly1239Val substitution in the first CUB domain of ADAMTS13. To explore the mechanism of ADAMTS13 deficiency in this patient, the wild type (WT; ADAMT13(WT)) and each mutant construct (ADAMTS13(Val88Met), ADAMTS13(Gly1239Val)) were transiently expressed in HEK 293 and COS-7 cells. To recapitulate the compound heterozygous state of the patient, both mutant ADAMTS13 proteins were also expressed together. The p.Val88Met mutation led to a defect of secretion of the protease associated with a reduction of enzymatic activity, the p.Gly1239Val mutation led to a secretion defect causing intracellular accumulation of the protease. The mechanistic effects of the mutations were further explored by means of differential immunofluorescence, that demonstrated an homogeneous distribution of ADAMTS13(WT) in the Cis-Golgi and endoplasmic reticulum (ER) compartments, a reduction of ADAMTS13(Val88Met) in both compartments, while ADAMTS13(Gly1239Val) failed to reach the Cis-Golgi compartment and remained in the ER.

Analysis on the Molecular Species and Concentration of Circulating ADAMTS13 in Blood

ADAMTS13 is the metalloprotease responsible for the proteolytic degradation of von Willebrand factor (VWF). A severe deficiency of this VWF-cleaving protease activity causes thrombotic thrombocytopenic purpura. This protease, comprising 1,427 amino acid residues, is composed of multiple domains, i.e., a preproregion, a metalloprotease domain, a disintegrin-like domain, a thrombospondin type-1 motif (Tsp1), a cysteine-rich domain, a spacer domain, seven Tsp1 repeats, and two CUB domains. We prepared one polyclonal and seven monoclonal antibodies recognizing distinct epitopes spanning the entire ADAMTS13 molecule. Of these antibodies, two of the monoclonal ones, which recognize the disintegrin-like and cysteine-rich/spacer domains, respectively, abolished the hydrolytic activity of ADAMTS13 toward both a synthetic substrate, FRETS-VWF73, and the natural substrate, VWF. In addition, these antibodies blocked the binding of ADAMTS13 to VWF. These results revealed that the region between the disintegrin-like and cysteine-rich/spacer domains interacts with VWF. Employing these established polyclonal and monoclonal antibodies, we examined the molecular species of ADAMTS13 circulating in the blood by immunoprecipitation followed by Western blot analysis, and estimated the plasma concentration of ADAMTS13 by enzyme-linked immunosorbent assay. These studies indicated that the major fraction of ADAMTS13 in blood plasma consisted of the full-length form. The concentration of ADAMTS13 in normal plasma was approximately 0.5-1 microg/ml.

Complete deficiency in ADAMTS13 is prothrombotic, but it alone is not sufficient to cause thrombotic thrombocytopenic purpura

ADAMTS13 is a plasma metalloproteinase that regulates platelet adhesion and aggregation through cleavage of von Willebrand factor (VWF) multimers. In humans, genetic or acquired deficiency in ADAMTS13 causes thrombotic thrombocytopenic purpura (TTP), a condition characterized by thrombocytopenia and hemolytic anemia with microvascular platelet thrombi. In this study, we report characterization of mice bearing a targeted disruption of the Adamts13 gene. ADAMTS13-deficient mice were born in the expected mendelian distribution; homozygous mice were viable and fertile. Hematologic and histologic analyses failed to detect any evidence of thrombocytopenia, hemolytic anemia, or microvascular thrombosis. However, unusually large VWF multimers were observed in plasma of homozygotes. Thrombus formation on immobilized collagen under flow was significantly elevated in homozygotes in comparison with wild-type mice. Thrombocytopenia was more severely induced in homozygotes than in wild-type mice after intravenous injection of a mixture of collagen and epinephrine. Thus, a complete lack of ADAMTS13 in mice was a prothrombotic state, but it alone was not sufficient to cause TTP-like symptoms. The phenotypic differences of ADAMTS13 deficiencies between humans and mice may reflect differences in hemostatic system functioning in these species. Alternatively, factors in addition to ADAMTS13 deficiency may be necessary for development of TTP.

Shigatoxin triggers thrombotic thrombocytopenic purpura in genetically susceptible ADAMTS13-deficient mice

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening illness caused by deficiency of the vWF-cleaving protease ADAMTS13. Here we show that ADAMTS13-deficient mice are viable and exhibit normal survival, although vWF-mediated platelet-endothelial interactions are significantly prolonged. Introduction of the genetic background CASA/Rk (a mouse strain with elevated plasma vWF) resulted in the appearance of spontaneous thrombocytopenia in a subset of ADAMTS13-deficient mice and significantly decreased survival. Challenge of these mice with shigatoxin (derived from bacterial pathogens associated with the related human disease hemolytic uremic syndrome) resulted in a striking syndrome closely resembling human TTP. Surprisingly, no correlation was observed between plasma vWF level and severity of TTP, implying the existence of TTP-modifying genes distinct from vWF. These data suggest that microbe-derived toxins (or possibly other sources of endothelial injury), together with additional genetic susceptibility factors, are required to trigger TTP in the setting of ADAMTS13 deficiency.

Modulation of ADAMTS13 secretion and specific activity by a combination of common amino acid polymorphisms and a missense mutation

Sequence analysis of the ADAMTS13 locus of 2 patients with hereditary thrombotic thrombocytopenic purpura (TTP) revealed the homozygous presence of 4 single nucleotide polymorphisms (SNPs) (R7W, Q448E, P618A, A732V) and a rare missense mutation (R1336W). Analysis of the individual effect of any amino acid exchanges showed that several sequence variations can interact with each other, thereby altering the phenotype of ADAMTS13 deficiency. Introduction of polymorphisms R7W, Q448E, and A732V had no or only minor effects on ADAMTS13 secretion. In contrast, P618A, R1336W, and the A732V-P618A combination strongly reduced ADAMTS13-specific activity and antigen levels. Surprisingly, R7W and Q448E were positive modifiers of ADAMTS13 secretion in the context of P618A and A732V but neither could rescue the severely reduced specific activity conferred by P618A. However, in the context of R1336W, polymorphisms R7W and Q448E enhanced the detrimental effect of the missense mutation and led to undetectable enzyme activity. We show that dependent on the sequence context, the same polymorphisms might be either positive or negative modifiers of gene expression. Our results might therefore be widely relevant to understanding the influence of polymorphisms on the phenotypic expression of complex diseases.

Measurement of ADAMTS13 activity and inhibitors

PURPOSE OF REVIEW

Acquired or congenital deficiency in the plasma von Willebrand factor-cleaving protease ADAMTS13 causes life-threatening thrombotic thrombocytopenic purpura. This condition is characterized primarily by thrombocytopenia and microangiopathic hemolytic anemia, accompanied by variable degrees of neurologic dysfunction, renal failure, and fever. Measurement of ADAMTS13 activity is important in the diagnosis of microangiopathies such as thrombotic thrombocytopenic purpura. This review introduces both established and emerging assays for ADAMTS13 activity, focusing on their impact on clinical practice.

RECENT FINDINGS

Previously established assays are useful screening methods to identify suspected thrombotic thrombocytopenic purpura. Novel assays measuring ADAMTS13 activity using either recombinant peptides or synthetic substrates directly measure the activity quantitatively. These assays can also detect neutralizing autoantibodies in the plasma of patients with acquired ADAMTS13 deficiency. Although ADAMTS13 in control subjects exhibits a broad variation in activity, ranging from 30 to 200%, significant decreases in ADAMTS13 activity have been observed in several physiologic and pathologic conditions. A portion of thrombotic thrombocytopenic purpura patients, however, did not display severe ADAMTS13 deficiency, suggesting that as-yet-unidentified environmental or genetic factors may contribute to the etiology of thrombotic thrombocytopenic purpura.

SUMMARY

New assays measuring ADAMTS13 activity will contribute significantly to the accurate diagnosis of microangiopathies, ultimately leading to improved clinical treatment of these diseases. These assays may also help to clarify the role of ADAMTS13 activity in additional thrombotic disorders, including disseminated intravascular coagulation, stroke, and myocardial infarction.

ADAMTS13 turns 3

It has now been 3 years since the von Willebrand factor (VWF)–cleaving protease implicated in thrombocytopenic purpura (TTP) pathogenesis was identified as ADAMTS13 (adisintegrin-like and metalloprotease with thrombospondin type 1 motif 13). More than 50 ADAMTS13 mutations resulting in familial TTP have been reported. Considerable progress has also been realized toward understanding the role of ADAMTS13 in normal hemostasis, as well as the mechanisms by which ADAMTS13 deficiency contributes to TTP pathogenesis. Measurement of ADAMTS13 activity in TTP and other pathologic conditions also remains a focus of a substantial clinical research effort. Building on these studies, continued investigation of ADAMTS13 and VWF holds considerable promise for advancing the understanding of TTP pathogenesis and should lead to improved diagnosis and treatment for this important hematologic disease.

Interplay between ADAMTS13 and von Willebrand factor in inherited and acquired thrombotic microangiopathies

The presence of unusually large multimers of von Willebrand factor (VWF) is thought to be a major pathogenic factor for thrombotic thrombocytopenic purpura (TTP). ADAMTS13 is a protease that regulates the multimeric size and function of VWF by cleaving VWF. Hence, congenital or acquired deficiency of ADAMTS13 causes life-threatening illness of TTP. Mutations in the ADAMTS13 gene cause inherited TTP, and the development of autoantibodies that inhibit ADAMTS13 activity frequently are associated with acquired TTP. ADAMTS13 consists of 1,427 amino acid residues and is composed of multiple structural and functional domains, containing a signal peptide, a propeptide, a reprolysin-like metalloprotease domain, a disintegrin-like domain, a thrombospondin type-1 (Tsp1) motif, a cysteine-rich domain, a spacer domain, seven additional Tsp1 repeats, and two CUB domains. In particular, the cysteine-rich/spacer domains are essential for VWF cleavage and are the principal epitopes recognized by autoantibodies in patients with acquired TTP. Therefore, it is likely that these domains are involved in the recognition and binding of ADAMTS13 to VWF. ADAMTS13 circulates in the blood in an active state, and efficiently cleaves unfold form of VWF induced under shear stress caused by blood flow, preventing the accumulation of pathogenic unusually large VWF multimers (ULVWF). Thus, ADAMTS13 helps maintain vascular homeostasis by preventing the excess thrombus formation.

ADAMTS13 is expressed in hepatic stellate cells

ADAMTS13 is a circulating zinc metalloprotease that cleaves the hemostatic glycoprotein von Willebrand factor (VWF) in a shear-dependent manner. Deficiency in ADAMTS13, owing to genetic mutations or autoimmune inhibitors, causes thrombotic thrombocytopenic purpura (TPP). Northern blot analysis has shown that ADAMTS13 is expressed primarily in the liver. By using real-time RT-PCR, we confirmed that in mice the liver had the highest level of the ADAMTS13 transcript. To identify the liver cell-type-specific origin of ADAMTS13, we used in situ hybridization techniques to investigate the pattern of ADAMTS13 expression in the liver; analyzed the ADAMTS13 proteolytic activity in the culture media of fractionated liver cells; and confirmed ADAMTS13 expression with RT-PCR analysis and cloning of the mouse ADAMTS13 gene. The results revealed that ADAMTS13 was expressed primarily in cell fractions enriched in hepatic stellate cells. The mouse ADAMTS13 cloned from primary hepatic stellate cells was similar to its human counterpart in digesting VWF and was susceptible to suppression by EDTA or the IgG inhibitors of patients with TTP. Since hepatic stellate cells are believed to play a major role in the development of hepatic fibrosis and cirrhosis, the identification of the liver cell-type expressing ADAMTS13 will have important implications for understanding pathophysiological mechanisms regulating ADAMTS13 expression.

Cloning, expression and functional characterization of the full-length murine ADAMTS13

Functional deficiency or absence of the human von Willebrand factor (VWF)-cleaving protease (VWF-cp), recently termed ADAMTS13, has been shown to cause acquired and congenital thrombotic thrombocytopenic purpura (TTP), respectively. As a first step towards developing a small animal model of TTP, we have cloned the complete (non-truncated) murine Adamts13 gene from BALB/c mice liver poly A+ mRNA. Murine ADAMTS13 is a 1426-amino-acid protein with a high homology and similar structural organization to the human ortholog. Transient expression of the murine Adamts13 cDNA in HEK 293 cells yielded a protein with a molecular weight of approximately 180 kDa which degraded recombinant murine VWF (rVWF) in a dose-dependent manner. The cleavage products of murine rVWF had the expected size of 140 and 170 kDa. Murine ADAMTS13 was inhibited by EDTA and the plasma from a TTP patient.

FRETS-VWF73, a first fluorogenic substrate for ADAMTS13 assay

A plasma metalloprotease, ADAMTS13, cleaves von Willebrand factor (VWF) multimers and downregulates their activity in platelet aggregation. Functional ADAMTS13 deficiency leads to the accumulation of hyperactive large VWF multimers, inducing a life-threatening disease, thrombotic thrombocytopenic purpura (TTP). Although measuring ADAMTS13 activity is important in TTP diagnosis, existing methods require time and skill. Here, we report a fluorescence resonance energy transfer (FRET) assay for ADAMTS13 activity. We developed a synthetic 73-amino-acid peptide, FRETS-VWF73. Cleavage of this substrate between two modified residues relieves the fluorescence quenching in the intact peptide. Incubation of FRETS-VWF73 with normal human plasma quantitatively increased fluorescence over time, while ADAMTS13-deficient plasma had no effect. Quantitative analysis could be achieved within a 1-h period using a 96-well format in commercial plate readers with common filters. The FRETS-VWF73 assay will be useful for the characterization of thrombotic microangiopathies like TTP and may clarify the importance of ADAMTS13 activity as a predictive marker for various thrombotic diseases.

Proteolytic inactivation of ADAMTS13 by thrombin and plasmin

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

Epitope mapping of ADAMTS13 autoantibodies in acquired thrombotic thrombocytopenic purpura

Severe deficiency of the von Willebrand factor (VWF)-cleaving protease ADAMTS13 can lead to thrombotic thrombocytopenic purpura (TTP), a disease associated with the widespread formation of platelet-rich thrombi in many organs. Autoantibodies that inactivate ADAMTS13 are the most frequent cause of acquired TTP. Little is known about epitope specificity and reactivity of anti-ADAMTS13 antibodies. In this study, a series of ADAMTS13 domains were expressed in Escherichia coli, and the reactivity of purified recombinant fragments with anti-ADAMTS13 auto-antibodies from 25 patients with severe ADAMTS13 deficiency was evaluated in vitro. All TTP plasmas contained antibodies directed against the cysteine-rich spacer (cys-rich/spacer) domain of ADAMTS13. In the plasma of 3 patients, antibodies were detected that reacted exclusively with the cys-rich/spacer domain, underscoring the importance of this region for functional activity of ADAMTS13. In 64% of the plasmas, antibodies reacted with the 2 CUB domains, and in 56% they reacted with the isolated first thrombospondin type 1 (TSP-1) repeat and with the compound fragment consisting of the catalytic, the disintegrin-like, and the TSP1-1 domain. Less frequently, in 28% of the plasmas, antibodies reacted with the TSP1 repeats 2 to 8. Unexpectedly, antibodies reacted with the propeptide region in 20% of the plasmas. In conclusion, this study shows that even though anti-ADAMTS13 autoantibodies react with multiple domains of the protease, the cys-rich/spacer domain is consistently involved in antibody reactivity.

Recent advances in thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is characterized by microangiopathic hemolytic anemia and thrombocytopenia, accompanied by microvascular thrombosis that causes variable degrees of tissue ischemia and infarction. Intravascular coagulation is not a prominent feature of the disorder. Plasma exchange can induce remissions in approximately 80% of patients with idiopathic TTP, but patients have a much worse prognosis when thrombotic microangiopathy is associated with cancer, certain drugs, infections, or tissue transplantation. Recently, acquired autoimmune deficiency of a plasma metalloprotease named ADAMTS13 was shown to cause many cases of idiopathic TTP. This review describes our current understanding of how to use this knowledge clinically. In Section I, Dr. Joel Moake describes the presentation of thrombotic microangiopathy, emphasizing the pathophysiology of idiopathic TTP. Platelets adhere to ultra-large (or "unusually large") von Willebrand factor (ULVWF) multimers that are immobilized in exposed subendothelial connective tissue and secreted into the circulation in long "strings" from stimulated endothelial cells. ADAMTS13 cleaves ULVWF multimers within growing platelet aggregates under flowing conditions, and this normally limits platelet thrombus formation. If ADAMTS13 is absent, either congenitally or due to acquired autoantibodies, platelet-rich microvascular thrombosis proceeds unchecked and TTP ensues. Plasma exchange is effective therapy for idiopathic TTP, probably because it replenishes the deficient ADAMTS13 and removes some of the pathogenic autoantibodies and endothelial-stimulating cytokines. Some patients have a type of thrombotic microangiopathy after transplantation/chemotherapy but do not have severe ADAMTS13 deficiency. The pathogenesis of their disease must differ but remains poorly understood. In Section II, Dr. Toshiyuki Miyata describes recent advances in assay methods that should facilitate routine laboratory testing of ADAMTS13 for patients with thrombotic microangiopathy. ADAMTS13 cleaves a single Tyr-Met bond in domain A2 of the VWF subunit. ADAMTS13 assays based on the cleavage of plasma VWF multimers have been used extensively but require considerable time and expertise to perform. A recombinant substrate containing 73 amino acid residues of VWF domain A2 has been devised that allows short incubation times and rapid product detection by gel electrophoresis or immunoassay. These results should encourage the development of even simpler assays that can be performed in most clinical laboratories. In Section III, Dr. James George provides an update on the long-term prospective study of thrombotic microangiopathy in the Oklahoma TTP-HUS Registry. At presentation, the clinical distinction between idiopathic TTP, various forms of secondary thrombotic microangiopathy, and even Shiga toxin-associated hemolytic uremic syndrome (HUS) can be problematic because the symptoms and laboratory findings often overlap. Consequently, plasma exchange usually is administered to any patient with thrombotic microangiopathy if there is doubt about the cause. The role of ADAMTS13 testing in choosing therapy remains uncertain, but the results do appear to have prognostic significance. Severe ADAMTS13 deficiency is specific for idiopathic TTP and identifies a subgroup with a high likelihood of response to plasma exchange, and high-titer ADAMTS13 inhibitors correlate strongly with a high risk of relapsing disease. Patients with normal ADAMTS13 activity have a much worse prognosis, although many factors probably contribute to this difference. Longitudinal study of these patients will continue to clarify the relationship of ADAMTS13 deficiency to the clinical course of thrombotic microangiopathy.