An autoantibody epitope comprising residues R660, Y661, and Y665 in the ADAMTS13 spacer domain identifies a binding site for the A2 domain of VWF

ADAMTS-13 conformation influences autoimmune recognition in immune thrombotic thrombocytopenic purpura

BACKGROUND

Patients with immune-mediated thrombotic thrombocytopenic purpura (iTTP) have anti-ADAMTS-13 immunoglobulin G (IgG) autoantibodies that enhance ADAMTS-13 clearance and/or inhibit its function. ADAMTS-13 normally circulates in a closed conformation, which is manifested by the interaction of the CUB domains with the central spacer domain. Disruption of the spacer-CUB interaction opens ADAMTS-13, which augments its proteolytic function but may also expose cryptic autoimmune epitopes that promote further autoantibody recognition.

OBJECTIVES

To explore differences in autoantibody binding to ADAMTS-13 in its closed or open conformations in patients with iTTP and to correlate these differences with disease-related parameters.

METHODS

We developed a novel assay to measure autoantibodies binding to closed and open ADAMTS-13. Autoantibody titer and IgG subclass binding to open or closed ADAMTS-13 were measured in 70 iTTP first presentation samples and correlated with clinical data, remission, and relapse.

RESULTS

In 70 patients with iTTP, the mean autoantibody titer against open ADAMTS-13 was, on average, approximately 2-fold greater than that against closed ADAMTS-13, suggesting that ADAMTS-13 opening increases epitope exposure and immune complex formation. Autoantibody titer against closed/open ADAMTS-13 and IgG subclass did not correlate with ADAMTS-13 antigen at presentation. Two patients with iTTP and persistent autoantibodies lost specificity for closed ADAMTS-13 in remission. Recognition of closed/open ADAMTS-13 and autoantibody IgG subclass between the first and second iTTP episodes were very similar.

CONCLUSION

ADAMTS-13 autoantibody binding is highly influenced by ADAMTS-13 conformation. Although this does not appear to modify the pathogenicity of autoantibodies, the autoantibody signature at relapse suggests that relapse represents re-emergence of the original autoimmune response rather than de novo presentation.

Open ADAMTS-13 conformation index predicts earlier relapse in immune-mediated thrombotic thrombocytopenic purpura

BACKGROUND

ADAMTS-13 adopts an open conformation in patients with immune-mediated thrombotic thrombocytopenic purpura (iTTP) in acute phase while being closed in healthy donors. We reported that a substantial number of patients with iTTP in remission with restored ADAMTS-13 activity (>50%) still had an open ADAMTS-13 conformation, although a closed conformation is expected given the extent of remission.

OBJECTIVES

To investigate whether open ADAMTS-13, represented by a conformation index >0.5, is associated with a risk of earlier ADAMTS-13 and/or clinical relapse.

METHODS

We collected follow-up data (ADAMTS-13 parameters, ADAMTS-13 and clinical relapse, and treatment) from 81 patients with iTTP in remission with ADAMTS-13 activity >50%.

RESULTS

During follow-up, 19 ADAMTS-13 and 10 clinical relapses were reported (median follow-up period, 20 months). First, open or closed ADAMTS-13 conformation was dichotomized based on the 0.5 conformation index cutoff. Open ADAMTS-13 (conformation index, >0.5) was not identified as a risk factor for ADAMTS-13 and clinical relapse (log-rank test and Cox regression model). In contrast, by identifying the optimal conformation index cutoff for relapse prediction, using classification and regression tree analysis, a conformation index >0.645 and >0.835 was shown to be a risk factor for ADAMTS-13 relapse (hazard ratio, 3.3; 95% CI, 1.3-8.3; P = .01) and clinical relapse (hazard ratio, 4.4; 95% CI, 1.3-15.3; P = .02), respectively.

CONCLUSION

Patients with open ADAMTS-13 with a conformation index >0.645 and >0.835 have a >3- and >4-fold higher risk of earlier ADAMTS-13 and clinical relapse, respectively. Hence, ADAMTS-13 conformation index could be used to complement ADAMTS-13 activity monitoring to timely notice ADAMTS-13 relapse and prevent clinical relapse.

Impact of N-glycan mediated shielding of ADAMTS-13 on the binding of pathogenic antibodies in immune thrombotic thrombocytopenic purpura

BACKGROUND

Thrombotic thrombocytopenic purpura (TTP) is a rare thrombotic disorder, with 1.5 to 6.0 cases per million per year. The majority of patients with TTP develop inhibitory autoantibodies that predominantly target the spacer domain of ADAMTS-13. ADAMTS-13 is responsible for cleaving von Willebrand factor (VWF) multimers, thereby regulating platelet adhesion at sites of high-vascular shear stress. Inhibition and/or clearance of ADAMTS-13 by pathogenic autoantibodies results in accumulation of VWF multimers that promotes the formation of platelet-rich microthrombi. Previously, we have shown that insertion of a single N-glycan (NGLY) in the spacer domain prevents the binding of antispacer domain antibodies.

OBJECTIVES

To explore whether NGLY mediated shielding of the ADAMTS-13 spacer domain effectively prevents binding of pathogenic antispacer autoantibodies in patients with immune-mediated TTP (iTTP).

METHODS

We screened 5 NGLY-ADAMTS-13 variants (NGLY3, NGLY7, NGLY8, NGLY3+7, and NGLY3+8) for binding of autoantibodies and for their activity in the presence and absence of 50 samples derived from patients with iTTP.

RESULTS

NGLY variants showed greatly reduced antibody binding, down to 27% of wild-type (wt) ADAMTS-13 binding. Moreover, NGLY variants of ADAMTS-13 remained more active in FRETS-VWF73 assay in the presence of the plasma samples from these 50 patients with acute phase iTTP when compared with wtADAMTS-13. On average, wtADAMTS-13 activity was reduced to 37% of regular levels in the presence of plasma, while NGLY3 and NGLY3+7 remained 69% and 81% active, respectively.

CONCLUSION

These results reinforce our previous findings that NGLYs shield ADAMTS-13 from antibody binding and hence restore ADAMTS-13 activity in the presence of autoantibodies.

Mechanisms of inhibition of human monoclonal antibodies in immune thrombotic thrombocytopenic purpura

Antibody binding to a plasma metalloprotease, a disintegrin and metalloproteinase with thrombospondin type 1 repeats 13 (ADAMTS13), is necessary for the development of immune thrombotic thrombocytopenic purpura (iTTP). Inhibition of ADAMTS13-mediated von Willebrand factor (VWF) cleavage by such antibodies clearly plays a role in the pathophysiology of the disease, although the mechanisms by which they inhibit ADAMTS13 enzymatic function are not fully understood. At least some immunoglobulin G-type antibodies appear to affect the conformational accessibility of ADAMTS13 domains involved in both substrate recognition and inhibitory antibody binding. We used single-chain fragments of the variable region previously identified via phage display from patients with iTTP to explore the mechanisms of action of inhibitory human monoclonal antibodies. Using recombinant full-length ADAMTS13, truncated ADAMTS13 variants, and native ADAMTS13 in normal human plasma, we found that, regardless of the conditions tested, all 3 inhibitory monoclonal antibodies tested affected enzyme turnover rate much more than substrate recognition of VWF. Hydrogen-to-deuterium exchange plus mass spectrometry experiments with each of these inhibitory antibodies demonstrated that residues in the active site of the catalytic domain of ADAMTS13 are differentially exposed to solvent in the presence and absence of monoclonal antibody binding. These results support the hypothesis that inhibition of ADAMTS13 in iTTP may not necessarily occur because the antibodies directly prevent VWF binding, but instead because of allosteric effects that impair VWF cleavage, likely by affecting the conformation of the catalytic center in the protease domain of ADAMTS13. Our findings provide novel insight into the mechanism of autoantibody-mediated inhibition of ADAMTS13 and pathogenesis of iTTP.

The C-terminal domains of ADAMTS1 contain exosites involved in its proteoglycanase activity

A disintegrin-like and metalloproteinase with thrombospondin type 1 motifs (ADAMTS1) is a protease involved in fertilization, cancer, cardiovascular development, and thoracic aneurysms. Proteoglycans such as versican and aggrecan have been identified as ADAMTS1 substrates, and Adamts1 ablation in mice typically results in versican accumulation; however, previous qualitative studies have suggested that ADAMTS1 proteoglycanase activity is weaker than that of other family members such as ADAMTS4 and ADAMTS5. Here, we investigated the functional determinants of ADAMTS1 proteoglycanase activity. We found that ADAMTS1 versicanase activity is approximately 1000-fold lower than ADAMTS5 and 50-fold lower than ADAMTS4 with a kinetic constant (kcat/Km) of 3.6 × 103 M-1 s-1 against full-length versican. Studies on domain-deletion variants identified the spacer and cysteine-rich domains as major determinants of ADAMTS1 versicanase activity. Additionally, we confirmed that these C-terminal domains are involved in the proteolysis of aggrecan as well as biglycan, a small leucine-rich proteoglycan. Glutamine scanning mutagenesis of exposed positively charged residues on the spacer domain loops and loop substitution with ADAMTS4 identified clusters of substrate-binding residues (exosites) in β3-β4 (R756Q/R759Q/R762Q), β9-β10 (residues 828-835), and β6-β7 (K795Q) loops. This study provides a mechanistic foundation for understanding the interactions between ADAMTS1 and its proteoglycan substrates and paves the way for development of selective exosite modulators of ADAMTS1 proteoglycanase activity.

Frontiers in pathophysiology and management of thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a fatal disease in which platelet-rich microthrombi cause end-organ ischemia and damage. TTP is caused by markedly reduced ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) activity. Hereditary or congenital TTP (cTTP) is caused by ADAMTS13 gene mutations. In acquired or immune TTP (iTTP), ADAMTS13 activity is reduced by anti-ADAMTS13 autoantibodies. TTP is characterized by thrombocytopenia, hemolytic anemia, fever, renal dysfunction, and neuropsychiatric symptoms. Therapeutic plasma exchange (TPE) and immunosuppressive therapy are the mainstays of treatment. As untreated TTP has a high mortality rate, immediate initiation of TPE is recommended when TTP is suspected. Conventionally, corticosteroids have been used for immunosuppressive therapy. Current drug therapies include rituximab, an anti-CD20 antibody that is effective in newly diagnosed cases and refractory cases, as well as for relapse prevention, and caplacizumab, an anti- von Willebrand factor (VWF) nanobody that inhibits the binding of platelets to VWF and prevents microthrombi formation. Recombinant human ADAMTS13 is a promising treatment for cTTP. Although these therapeutic advances have improved the outcomes of TTP, early diagnosis and prompt initiation of appropriate therapy are necessary to achieve these outcomes.

TTP: From empiricism for an enigmatic disease to targeted molecular therapies

The 100th anniversary of the first description of Thrombotic Thrombocytopenic Purpura (TTP) as a disease by Dr. Eli Moschcowitz approaches. For many decades, TTP remained mostly a mysterious fatal condition, where diagnosis was often post-mortem. Initially a pentad of symptoms was identified, a pattern that later revealed to be fallible. Sporadic observations led to empiric interventions that allowed for the first impactful breakthrough in TTP treatment, almost 70 years after its first description: the introduction of plasma exchange and infusions as treatments. The main body of knowledge within the field was gathered in the latest three decades: patient registries were set and proved crucial for advancements; the general mechanisms of disease have been described; the diagnosis was refined; new treatments and biomarkers with improvements on prognosis and management were introduced. Further changes and improvements are expected in the upcoming decades. In this review, we provide a brief historic overview of TTP, as an illustrative example of the success of translational medicine enabling to rapidly shift from a management largely based on empiricism to targeted therapies and personalized medicine, for the benefit of patients. Current management options and present and future perspectives in this still evolving field are summarized.

Residues R1075, D1090, R1095, and C1130 Are Critical in ADAMTS13 TSP8-Spacer Interaction Predicted by Molecular Dynamics Simulation

ADAMTS13 (A Disintegrin and Metalloprotease with Thrombospondin type 1 repeats, member 13) cleaves von Willebrand Factor (VWF) multimers to limit the prothrombotic function of VWF. The deficiency of ADAMTS13 causes a lethal thrombotic microvascular disease, thrombotic thrombocytopenic purpura (TTP). ADAMTS13 circulates in a “closed” conformation with the distal domain associating the Spacer domain to avoid off-target proteolysis or recognition by auto-antibodies. However, the interactions of the distal TSP8 domain and the Spacer domain remain elusive. Here, we constructed the TSP8-Spacer complex by a combination of homology modelling and flexible docking. Molecular dynamics simulation was applied to map the binding sites on the TSP8 or Spacer domain. The results predicted that R1075, D1090, R1095, and C1130 on the TSP8 domain were key residues that interacted with the Spacer domain. R1075 and R1095 bound exosite-4 tightly, D1090 formed multiple hydrogen bonds and salt bridges with exosite-3, and C1130 interacted with both exosite-3 and exosite-4. Specific mutations of exosite-3 (R568K/F592Y/R660K/Y661F/Y665F) or the four key residues (R1075A/D1090A/R1095A/C1130A) impaired the binding of the TSP8 domain to the Spacer domain. These results shed new light on the understanding of the auto-inhibition of ADAMTS13.

Emerging Concepts in Immune Thrombotic Thrombocytopenic Purpura

Immune thrombotic thrombocytopenic purpura (iTTP) is an autoimmune disorder of which the etiology is not fully understood. Autoantibodies targeting ADAMTS13 in iTTP patients have extensively been studied, the immunological mechanisms leading to the breach of tolerance remain to be uncovered. This review addresses the current knowledge on genetic factors associated with the development of iTTP and the interplay between the patient’s immune system and environmental factors in the induction of autoimmunity against ADAMTS13. HLA-DRB1*11 has been identified as a risk factor for iTTP in the Caucasian population. Interestingly, HLA-DRB1*08:03 was recently identified as a risk factor in the Japanese population. Combined in vitro and in silico MHC class II peptide presentation approaches suggest that an ADAMTS13-derived peptide may bind to both HLA-DRB1*11 and HLA-DRB1*08:03 through different anchor-residues. It is apparent that iTTP is associated with the presence of infectious microorganisms, viruses being the most widely associated with development of iTTP. Infections may potentially lead to loss of tolerance resulting in the shift from immune homeostasis to autoimmunity. In the model we propose in this review, infections disrupt the epithelial barriers in the gut or lung, promoting exposure of antigen presenting cells in the mucosa-associated lymphoid tissue to the microorganisms. This may result in breach of tolerance through the presentation of microorganism-derived peptides that are homologous to ADAMTS13 on risk alleles for iTTP.

Anti-ADAMTS13 autoantibody profiling in patients with immune-mediated thrombotic thrombocytopenic purpura

Anti-A Disintegrin and Metalloproteinase with a ThromboSpondin type 1 motif, member 13 (ADAMTS13) autoantibodies cause a severe ADAMTS13 deficiency in immune-mediated thrombotic thrombocytopenic purpura (iTTP). ADAMTS13 consists of a metalloprotease (M), a disintegrin-like (D) domain, 8 thrombospondin type 1 repeats (T1-T8), a cysteine-rich (C), a spacer (S), and 2 CUB domains (CUB1-2). We recently developed a high-throughput epitope mapping assay based on small, nonoverlapping ADAMTS13 fragments (M, DT, CS, T2-T5, T6-T8, CUB1-2). With this assay, we performed a comprehensive epitope mapping using 131 acute-phase samples and for the first time a large group of remission samples (n = 50). Next, samples were stratified according to their immunoprofiles, a field that is largely unexplored in iTTP. Three dominant immunoprofiles were found in acute-phase samples: profile 1: only anti-CS autoantibodies (26.7%); profile 2: both anti-CS and anti-CUB1-2 autoantibodies (12.2%); and profile 3: anti-DT, anti-CS, anti-T2-T5, anti-T6-T8, and anti-CUB1-2 autoantibodies (8.4%). Interestingly, profile 1 was the only dominant immunoprofile in remission samples (52.0%). Clinical data were available for a relatively small number of patients with acute iTTP (>68), and no correlation was found between immunoprofiles and disease severity. Nevertheless, profile 1 was linked with younger and anti-T2-T5 autoantibodies with older age and the absence of anti-CUB1-2 autoantibodies with cerebral involvement. In conclusion, identifying acute phase and remission immunoprofiles in iTTP revealed that anti-CS autoantibodies seem to persist or reappear during remission providing further support for the clinical development of a targeted anti-CS autoantibody therapy. A large cohort study with acute iTTP samples will validate possible links between immunoprofiles or anti-domain autoantibodies and clinical data.

Characterization of the interactions of ADAMTS13 CUB1 domain to WT- and GOF-Spacer domain by molecular dynamics simulation

Metalloprotease ADAMTS13 specifically cleaves VWF (von Willebrand Factor) to prevent excessive platelet aggregation and thrombus formation at the sites of vascular injury. To avoid non-specific cleavage, ADAMTS13 has the auto-inhibition effect in which the Spacer domain in N-terminal interacts with the CUB1 domain in C-terminal, resulting in decreased proteolytic activity. Previous studies reported that exosite-3 in the Spacer domain was a key binding site in the Spacer-CUB1 interaction. When exosite-3 was mutated (R660K/F592Y/R568K/Y661F/Y665F, GOF), the auto-inhibition of ADAMTS13 was disrupted and the enzymatic activity was markedly increased. However, the characteristics of the Spacer-CUB1 interaction is not fully understood. Here, we constructed the model of Spacer-CUB1 complex by homologous modeling and molecular docking to characterize the Spacer-CUB1 binding and predict key amino acid residues via molecular dynamics simulation. Our data showed that G607-S610 was a non-reported potential binding site in the Spacer domain; GOF mutation attenuated the formation of hydrogen bond between exosite-3 and the CUB1 domain; Residues E1231, R1251, L1258, D1259 and T1261 in the CUB1 domain might play an important role in the Spacer-CUB1 interaction. Our study advances the understanding of the structural basis of the auto-inhibition of ADAMTS13 and provides information about the key residues in the binding interface.

Conformational plasticity of ADAMTS13 in hemostasis and autoimmunity

A disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13) is a multidomain metalloprotease for which until now only a single substrate has been identified. ADAMTS13 cleaves the polymeric force-sensor von Willebrand factor (VWF) that unfolds under shear stress and recruits platelets to sites of vascular injury. Shear force–dependent cleavage at a single Tyr–Met peptide bond in the unfolded VWF A2 domain serves to reduce the size of VWF polymers in circulation. In patients with immune-mediated thrombotic thrombocytopenic purpura (iTTP), a rare life-threatening disease, ADAMTS13 is targeted by autoantibodies that inhibit its activity or promote its clearance. In the absence of ADAMTS13, VWF polymers are not adequately processed, resulting in spontaneous adhesion of blood platelets, which presents as severe, life-threatening microvascular thrombosis. In healthy individuals, ADAMTS13–VWF interactions are guided by controlled conversion of ADAMTS13 from a closed, inactive to an open, active conformation through a series of interdomain contacts that are now beginning to be defined. Recently, it has been shown that ADAMTS13 adopts an open conformation in the acute phase and during subclinical disease in iTTP patients, making open ADAMTS13 a novel biomarker for iTTP. In this review, we summarize our current knowledge on ADAMTS13 conformation and speculate on potential triggers inducing conformational changes of ADAMTS13 and how these relate to the pathogenesis of iTTP.

A human monoclonal antibody against the distal carboxyl terminus of ADAMTS-13 modulates its susceptibility to an inhibitor in thrombotic thrombocytopenic purpura

BACKGROUND

Immune thrombotic thrombocytopenic purpura (iTTP) is a potentially fatal thrombotic microangiopathy, resulting from a severe deficiency of plasma ADAMTS-13 (A Disintegrin And Metalloproteinase with ThromboSpondin type 1 motif, member 13) activity. IgG-type autoantibodies are primarily responsible for the inhibition of plasma ADAMTS-13 activity. However, the mechanism underlying autoantibody-mediated inhibition is not fully understood.

OBJECTIVE

The purpose of the present study is to determine the role of IgG autoantibodies against various carboxyl-terminal domains of ADAMTS-13 in regulating ADAMTS-13 activity and its inhibition.

METHOD

Various human monoclonal antibodies isolated by phage display, recombinant protein expression and purification, and biochemical analyses were employed for the study.

RESULTS

Our results demonstrate for the first time that a human monoclonal antibody fragment, the single chain fragment of the variable region (scFv) isolated from a patient with acute iTTP that binds the distal carboxyl-terminus of ADAMTS-13, is able to activate ADAMTS-13 and increase the proteolytic cleavage of a FRETS-VWF73 substrate; moreover, binding of such a human monoclonal antibody against the carboxyl-terminus of ADAMTS-13 to plasma ADAMTS-13 appears to modulate inhibition by another human monoclonal antibody (i.e., scFv4-20), also isolated from an iTTP patient, that targets the spacer domain of ADAMTS-13. These results provide new insights into our understanding of the pathogenesis of iTTP.

Alternative splicing of the metalloprotease ADAMTS17 spacer regulates secretion and modulates autoproteolytic activity

ADAMTS proteases mediate biosynthesis and breakdown of secreted extracellular matrix (ECM) molecules in numerous physiological and disease processes. In addition to their catalytic domains, ADAMTS proteases contain ancillary domains, which mediate substrate recognition and ECM binding and confer distinctive properties and roles to individual ADAMTS proteases. Although alternative splicing can greatly expand the structural and functional diversity of ADAMTS proteases, it has been infrequently reported and functional consequences have been rarely investigated. Here, we characterize the structural and functional impact of alternative splicing of ADAMTS17, mutations in which cause Weill-Marchesani syndrome 4. Two novel ADAMTS17 splice variants, ADAMTS17A and ADAMTS17B, were investigated by structural modeling, mass spectrometry, and biochemical approaches. Our results identify a novel disulfide-bridged insertion in the ADAMTS17A spacer that originates from inclusion of a novel exon. This insertion results in differential autoproteolysis of ADAMTS17, and thus, predicts altered proteolytic activity against other substrates. The second variant, ADAMTS17B, results from an in-frame exon deletion and prevents ADAMTS17B secretion. Thus, alternative splicing of the ADAMTS spacer significantly regulates the physiologically relevant proteolytic activity of ADAMTS17, either by altering proteolytic specificity (ADAMTS17A) or by altering cellular localization (ADAMTS17B).

N-glycan-mediated shielding of ADAMTS13 prevents binding of pathogenic autoantibodies in immune-mediated TTP

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is an autoimmune disorder caused by the development of autoantibodies targeting different domains of ADAMTS13. Profiling studies have shown that residues R568, F592, R660, Y661, and Y665 within exosite-3 of the spacer domain provide an immunodominant region of ADAMTS13 for pathogenic autoantibodies that develop in patients with iTTP. Modification of these 5 core residues with the goal of reducing autoantibody binding revealed a significant tradeoff between autoantibody resistance and proteolytic activity. Here, we employed structural bioinformatics to identify a larger epitope landscape on the ADAMTS13 spacer domain. Models of spacer-antibody complexes predicted that residues R568, L591, F592, K608, M609, R636, L637, R639, R660, Y661, Y665, and L668 contribute to an expanded epitope within the spacer domain. Based on bioinformatics-guided predictions, we designed a panel of N-glycan insertions in this expanded epitope to reduce the binding of spacer domain autoantibodies. One N-glycan variant (NGLY3-ADAMTS13, containing a K608N substitution) showed strongly reduced reactivity with TTP patient sera (28%) as compared with WT-ADAMTS13 (100%). Insertion of an N-glycan at amino acid position 608 did not interfere with processing of von Willebrand factor, positioning the resulting NGLY3-ADAMTS13 variant as a potential novel therapeutic option for treatment of iTTP.

Evaluation of Immuno-Rolling Circle Amplification for Multiplex Detection and Profiling of Antigen-Specific Antibody Isotypes

Antibody characterization is essential for understanding the immune system and development of diagnostics and therapeutics. Current technologies are mainly focusing on the detection of antigen-specific immunoglobulin G (IgG) using bulk singleplex measurements, which lack information on other isotypes and specificity of individual antibodies. Digital immunoassays based on nucleic acid amplification have demonstrated superior performance by allowing the detection of single molecules in a multiplex and sensitive manner. In this study, we demonstrate for the first time an immuno-rolling circle amplification (immuno-RCA) assay for the multiplex detection of three antigen-specific antibody isotypes (IgG, IgA, and IgM) and its integration with microengraving. To validate this approach, we used the autoimmune disease immune-mediated thrombotic thrombocytopenic purpura (iTTP) as the model disease with anti-ADAMTS13 autoantibodies as the diagnostic target molecules. To identify the anti-ADAMTS13 autoantibody isotypes, we designed a pool of three unique antibody-oligonucleotide conjugates for identification and subsequent amplification and visualization via RCA. To validate this approach, we first confirmed an assay specificity of >88% and a low limit of detection of 0.3 ng/mL in the spiked buffer. Subsequently, we performed a dilution series of an iTTP plasma sample for the multiplex detection of the three isotypes with higher sensitivity compared to an enzyme-linked immunosorbent assay. Finally, we demonstrated single-cell analysis of human B cells and hybridoma cells for the detection of secreted antibodies using microengraving and achieved a detection of 23.3 pg/mL secreted antibodies per hour. This approach could help to improve the understanding of antibody isotype distributions and their roles in various diseases.

Crystal structure of ADAMTS13 CUB domains reveals their role in global latency

ADAMTS13 is a plasma metalloprotease that is essential for the regulation of von Willebrand factor (VWF) function, mediator of platelet recruitment to sites of blood vessel damage. ADAMTS13 function is dynamically regulated by structural changes induced by VWF binding that convert it from a latent to active conformation. ADAMTS13 global latency is manifest by the interaction of its C-terminal CUB1-2 domains with its central Spacer domain. We resolved the crystal structure of the ADAMTS13 CUB1-2 domains revealing a previously unreported configuration for the tandem CUB domains. Docking simulations between the CUB1-2 domains with the Spacer domain in combination with enzyme kinetic functional characterization of ADAMTS13 CUB domain mutants enabled the mapping of the CUB1-2 domain site that binds the Spacer domain. Together, these data reveal the molecular basis of the ADAMTS13 Spacer-CUB interaction and the control of ADAMTS13 global latency.

An Integrative Structural Biology Analysis of Von Willebrand Factor Binding and Processing by ADAMTS-13 in Solution

Von Willebrand Factor (vWF), a 300-kDa plasma protein key to homeostasis, is cleaved at a single site by multi-domain metallopeptidase ADAMTS-13. vWF is the only known substrate of this peptidase, which circulates in a latent form and becomes allosterically activated by substrate binding. Herein, we characterised the complex formed by a competent peptidase construct (AD13-MDTCS) comprising metallopeptidase (M), disintegrin-like (D), thrombospondin (T), cysteine-rich (C), and spacer (S) domains, with a 73-residue functionally relevant vWF-peptide, using nine complementary techniques. Pull-down assays, gel electrophoresis, and surface plasmon resonance revealed tight binding with sub-micromolar affinity. Cross-linking mass spectrometry with four reagents showed that, within the peptidase, domain D approaches M, C, and S. S is positioned close to M and C, and the peptide contacts all domains. Hydrogen/deuterium exchange mass spectrometry revealed strong and weak protection for C/D and M/S, respectively. Structural analysis by multi-angle laser light scattering and small-angle X-ray scattering in solution revealed that the enzyme adopted highly flexible unbound, latent structures and peptide-bound, active structures that differed from the AD13-MDTCS crystal structure. Moreover, the peptide behaved like a self-avoiding random chain. We integrated the results with computational approaches, derived an ensemble of structures that collectively satisfied all experimental restraints, and discussed the functional implications. The interaction conforms to a 'fuzzy complex' that follows a 'dynamic zipper' mechanism involving numerous reversible, weak but additive interactions that result in strong binding and cleavage. Our findings contribute to illuminating the biochemistry of the vWF:ADAMTS-13 axis.

Immunogenic hotspots in the spacer domain of ADAMTS13 in immune-mediated thrombotic thrombocytopenic purpura

BACKGROUND

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is caused by anti-ADAMTS13 autoantibodies inducing a severe deficiency of ADAMTS13. Epitope mapping studies on samples obtained during acute iTTP episodes have shown that the iTTP immune response is polyclonal, with almost all patients having autoantibodies targeting the spacer domain of ADAMTS13.

OBJECTIVES

To identify the immunogenic hotspots in the spacer domain of ADAMTS13.

PATIENTS/METHODS

A library of 11 full-length ADAMTS13 spacer hybrids was created in which amino acid regions of the spacer domain of ADAMTS13 were exchanged by the corresponding region of the spacer domain of ADAMTS1. Next, the full-length ADAMTS13 spacer hybrids were used in enzyme-linked immunosorbent assay to epitope map anti-spacer autoantibodies in 138 samples from acute and remission iTTP patients.

RESULTS

Sixteen different anti-spacer autoantibody profiles were identified with a similar distribution in acute and remission patients. There was no association between the anti-spacer autoantibody profiles and disease severity. Almost all iTTP samples contained anti-spacer autoantibodies against the following three regions: amino acid residues 588-592, 602-610, and 657-666 (hybrids E, G, and M). Between 31% and 57% of the samples had anti-spacer autoantibodies against amino acid regions 572-579, 629-638, 667-676 (hybrids C, J, and N). In contrast, none of the samples had anti-spacer autoantibodies against amino acid regions 556-563, 564-571, 649-656, and 677-685 (hybrids A, B, L, and O).

CONCLUSION

We identified three hotspot regions (amino acid regions 588-592, 602-610, and 657-666) in the spacer domain of ADAMTS13 that are targeted by anti-spacer autoantibodies found in a large cohort of iTTP patients.

ADAMTS proteases in cardiovascular physiology and disease

The a disintegrin-like and metalloproteinase with thrombospondin motif (ADAMTS) family comprises 19 proteases that regulate the structure and function of extracellular proteins in the extracellular matrix and blood. The best characterized cardiovascular role is that of ADAMTS-13 in blood. Moderately low ADAMTS-13 levels increase the risk of ischeamic stroke and very low levels (less than 10%) can cause thrombotic thrombocytopenic purpura (TTP). Recombinant ADAMTS-13 is currently in clinical trials for treatment of TTP. Recently, new cardiovascular roles for ADAMTS proteases have been discovered. Several ADAMTS family members are important in the development of blood vessels and the heart, especially the valves. A number of studies have also investigated the potential role of ADAMTS-1, -4 and -5 in cardiovascular disease. They cleave proteoglycans such as versican, which represent major structural components of the arteries. ADAMTS-7 and -8 are attracting considerable interest owing to their implication in atherosclerosis and pulmonary arterial hypertension, respectively. Mutations in the ADAMTS19 gene cause progressive heart valve disease and missense variants in ADAMTS6 are associated with cardiac conduction. In this review, we discuss in detail the evidence for these and other cardiovascular roles of ADAMTS family members, their proteolytic substrates and the potential molecular mechanisms involved.

Modifying ADAMTS13 to modulate binding of pathogenic autoantibodies of patients with acquired thrombotic thrombocytopenic purpura

Antibodies that develop in patients with immune thrombotic thrombocytopenic purpura (iTTP) commonly target the spacer epitope R568/F592/R660/Y661/Y665 (RFRYY). In this study we present a detailed contribution of each residue in this epitope for autoantibody binding. Different panels of mutations were introduced here to create a large collection of full-length ADAMTS13 variants comprising conservative (Y←→F), semi-conservative (Y/F→L), non-conservative (Y/F→N) or alanine (Y/F/R→A) substitutions. Previously reported Gain-of-Function (GoF, KYKFF) and truncated 'MDTCS' variants were also included. Sera of 18 patients were screened against all variants. Conservative mutations of the aromatic residues did not reduce the binding of autoantibodies. Moderate resistance was achieved by replacing R568 and R660 by lysines or alanines. Semi-conservative mutations of aromatic residues show a moderate effectiveness in autoantibody resistance. Non-conservative asparagine or alanine mutations of aromatic residues are the most effective. In the mixtures of autoantibodies from the majority (89%) of patients screened, autoantibodies targeting the spacer RFRYY epitope have preponderance compared to other epitopes. Reductions in ADAMTS13 proteolytic activity were observed for all full-length mutant variants, in varying degrees. The greatest activity reductions were observed in the most autoantibody-resistant variants (15-35% residual activity in FRETS-VWF73). Among these, a triple-alanine mutant RARAA showed activity in a VWF multimer assay. This study shows that non-conservative and alanine modifications of residues within the exosite-3 spacer RFRYY epitope in full-length ADAMTS13 resist the binding of autoantibodies from iTTP patients, while retaining residual proteolytic activity. Our study provides a framework for the design of autoantibody-resistant ADAMTS13 variants for further therapeutic development.

Upshaw-Schulman syndrome-associated ADAMTS13 variants possess proteolytic activity at the surface of endothelial cells and in simulated circulation

ADAMTS13 regulates the hemostatic activity of von Willebrand factor (VWF). Determined by static assays, proteolytic activity <10IU/dL in patient plasma, in absence of ADAMTS13 autoantibodies, indicates Upshaw-Schulman syndrome (USS); the congenital form of Thrombotic Thrombocytopenic Purpura (TTP). We have recently functionally characterized sixteen USS-associated ADAMTS13 missense variants under static conditions. Here, we used two assays under shear flow conditions to analyze the activity of those seven mutants with sufficiently high residual secretion plus two newly identified variants. One assay determines cleavage of VWF strings bound to the surface of endothelial cells. The other, light transmission aggregometry-based assay, mimics degradation of VWF-platelet complexes, which are likely to be present in the circulation during TTP bouts. We found that 100 ng/ml of all variants were able to cleave about 80–90% of VWF strings even though 5 out of 9 exhibited activity ≤1% in the state-of-the-art static assay at the same concentration. These data indicate underestimation of ADAMTS13 activity by the used static assay. In simulated circulation, two variants, with missense mutations in the vicinity of the catalytic domain, exhibited only minor residual activity while all other variants were able to effectively break down VWF-platelet complexes. In both assays, significant proteolytic activity could be observed down to 100 ng/ml ADAMTS13. It is thus intriguing to postulate that most variants would have ample activity if secretion of 10% of normal plasma levels could be achieved.

ADAMTS7: Recombinant Protein Expression and Purification

ADAMTS7 is a secreted protease that is predominantly expressed in tissues of the cardiovascular system and tendon. Although recent evidence suggests that it may play a role in the etiology of coronary artery disease, its physiological function and substrates are unknown. The enzyme undergoes extensive posttranslational modifications, including chondroitin sulfate attachment, N and O-linked glycosylation, and a two-step activation process. For the benefit of scientists who study the function of ADAMTS7 and its role in disease, this chapter provides an introduction to the chemical and functional properties of the various ADAMTS7 domains, as well as a protocol for the recombinant expression and purification of ADAMTS7.

METALLOPROTEASE ADAMTS-13

Introduction. The signifi cance of ADAMTS-13 extends beyond its key role in the pathogenesis of thrombotic thrombocytopenic purpura (TTP); there is evidence of a relationship between a decrease in the ADAMTS-13 activity and thrombotic events in acute myocardial infarction and ischemic stroke.

Aim. To generalise available information on the structure and function of the metalloprotease ADAMTS-13.

General findings. The biological function of ADAMTS-13 consists in the cleavage of ultra-large von Willebrand factor (vWF) multimers. The fact that its defi ciency causes the development of TTP provides a basis for understanding the function of vWF–cleaving protease. ADAMTS-13 has a domain structure. The functional roles of most ADAMTS-13 domains, as well as the key role of the ADAMTS-13-vWF interaction in the regulation of haemostasis, are defi ned. The conformational activation of ADAMTS-13 by vWF constitutes an important aspect of its function. After getting into the bloodstream, ultra-large vWF multimers quickly adopt a closed conformation, which becomes very resistant to ADAMTS-13 proteolysis in the absence of shear stress. Ultra-large plasma vWF multimers regain their sensitivity to ADAMTS-13 after being exposed to high fl uid shear stress, which unfolds the central vWF-A2 domain. The unfolding of a vWF molecule under shear stress conditions reveals previously hidden exosites in domain A2, which gradually increase the binding affi nity between ADAMTS-13 and vWF. The mechanism underlying the production of autoantibodies against ADAMTS-13 is unknown and requires further study. The masking of cryptic epitopes in the closed conformation of ADAMTS-13 prevents the formation of autoantibodies. Early antigen recognition of ADAMTS-13 occurs through surface-exposed epitopes in the C-terminal domains. More detailed information on the mechanisms underlying the interaction between ADAMTS-13 and the vWF can improve the understanding of mechanisms involved in the regulation of the coagulation system.

Conflict of interest: the authors declare no confl ict of interest.

Financial disclosure: the study had no sponsorship.

Synergistic effects of ADAMTS13 deficiency and complement activation in pathogenesis of thrombotic microangiopathy

Severe deficiency of plasma ADAMTS13 activity is the primary cause of thrombotic thrombocytopenic purpura (TTP) whereas overwhelming activation of complement via an alternative pathway results in atypical hemolytic uremic syndrome (aHUS), the prototypes of thrombotic microangiopathy (TMA). However, clinical and pathogenic distinctions between TTP and aHUS are often quite challenging. Clinical reports have suggested that complement activation may play a role in the development of TTP, which is caused by severe deficiency of plasma ADAMTS13 activity. However, the experimental evidence to support this hypothesis is still lacking. Here, we show that mice with either Adamts13 -/- or a heterozygous mutation of complement factor H (cfh) at amino acid residue of 1206 (ie, cfh W/R ) alone remain asymptomatic despite the presence of occasional microvascular thrombi in various organ tissues. However, mice carrying both Adamts13 -/- and cfh W/R exhibit thrombocytopenia, low haptoglobin, increased fragmentation of erythrocytes in peripheral blood smear, increased plasma levels of lactate dehydrogenase activity, blood urea nitrogen, and creatinine, as well as an increased mortality rate, consistent with the development of TMA. Moreover, mice with a homozygous mutation of cfh (ie, cfh R/R ) with or without Adamts13 -/- developed severe TMA. The mortality rate in mice with Adamts13 -/- cfh R/R was significantly higher than that in mice with cfh R/R alone. Histological and immunohistochemical analyses demonstrated the presence of disseminated platelet-rich thrombi in terminal arterioles and capillaries of major organ tissues in these mice that were either euthanized or died. Together, our results support a synergistic effect of severe ADAMTS13 deficiency and complement activation in pathogenesis of TMA in mice.

Crystal structure and substrate-induced activation of ADAMTS13

Platelet recruitment to sites of blood vessel damage is highly dependent upon von Willebrand factor (VWF). VWF platelet-tethering function is proteolytically regulated by the metalloprotease ADAMTS13. Proteolysis depends upon shear-induced conformational changes in VWF that reveal the A2 domain cleavage site. Multiple ADAMTS13 exosite interactions are involved in recognition of the unfolded A2 domain. Here we report through kinetic analyses that, in binding VWF, the ADAMTS13 cysteine-rich and spacer domain exosites bring enzyme and substrate into proximity. Thereafter, binding of the ADAMTS13 disintegrin-like domain exosite to VWF allosterically activates the adjacent metalloprotease domain to facilitate proteolysis. The crystal structure of the ADAMTS13 metalloprotease to spacer domains reveals that the metalloprotease domain exhibits a latent conformation in which the active-site cleft is occluded supporting the requirement for an allosteric change to enable accommodation of the substrate. Our data demonstrate that VWF functions as both the activating cofactor and substrate for ADAMTS13.

Exosites in Hypervariable Loops of ADAMTS Spacer Domains control Substrate Recognition and Proteolysis

ADAMTS (A Disintegrin-like and Metalloproteinase domain with Thrombospondin type 1 Motif)-1, -4 and -5 share the abilities to cleave large aggregating proteoglycans including versican and aggrecan. These activities are highly relevant to cardiovascular disease and osteoarthritis and during development. Here, using purified recombinant ADAMTS-1, -4 and -5, we quantify, compare, and define the molecular basis of their versicanase activity. A novel sandwich-ELISA detecting the major versican cleavage fragment was used to determine, for the first time, kinetic constants for versican proteolysis. ADAMTS-5 (k_cat/K_m 35 × 10⁵ M⁻¹ s⁻¹) is a more potent (~18-fold) versicanase than ADAMTS-4 (k_cat/K_m 1.86 × 10⁵ M⁻¹ sec⁻¹), whereas ADAMTS-1 versicanase activity is comparatively low. Deletion of the spacer domain reduced versicanase activity of ADAMTS-5 19-fold and that of ADAMTS-4 167-fold. Co-deletion of the ADAMTS-5 cysteine-rich domain further reduced versicanase activity to a total 153-fold reduction. Substitution of two hypervariable loops in the spacer domain of ADAMTS-5 (residues 739–744 and 837–844) and ADAMTS-4 (residues 717–724 and 788–795) with those of ADAMTS-13, which does not cleave proteoglycans, caused spacer-dependent reductions in versicanase activities. Our results demonstrate that these loops contain exosites critical for interaction with and processing of versican. The hypervariable loops of ADAMTS-5 are shown to be important also for its aggrecanase activity. Together with previous work on ADAMTS-13 our results suggest that the spacer domain hypervariable loops may exercise significant control of ADAMTS proteolytic activity as a general principle. Identification of specific exosites also provides targets for selective inhibitors.

Proteomic discovery of substrates of the cardiovascular protease ADAMTS7

The protease ADAMTS7 functions in the extracellular matrix (ECM) of the cardiovascular system. However, its physiological substrate specificity and mechanism of regulation remain to be explored. To address this, we conducted an unbiased substrate analysis using terminal amine isotopic labeling of substrates (TAILS). The analysis identified candidate substrates of ADAMTS7 in the human fibroblast secretome, including proteins with a wide range of functions, such as collagenous and noncollagenous extracellular matrix proteins, growth factors, proteases, and cell-surface receptors. It also suggested that autolysis occurs at Glu-729-Val-730 and Glu-732-Ala-733 in the ADAMTS7 Spacer domain, which was corroborated by N-terminal sequencing and Western blotting. Importantly, TAILS also identified proteolysis of the latent TGF-β-binding proteins 3 and 4 (LTBP3/4) at a Glu-Val and Glu-Ala site, respectively. Using purified enzyme and substrate, we confirmed ADAMTS7-catalyzed proteolysis of recombinant LTBP4. Moreover, we identified multiple additional scissile bonds in an N-terminal linker region of LTBP4 that connects fibulin-5/tropoelastin and fibrillin-1-binding regions, which have an important role in elastogenesis. ADAMTS7-mediated cleavage of LTBP4 was efficiently inhibited by the metalloprotease inhibitor TIMP-4, but not by TIMP-1 and less efficiently by TIMP-2 and TIMP-3. As TIMP-4 expression is prevalent in cardiovascular tissues, we propose that TIMP-4 represents the primary endogenous ADAMTS7 inhibitor. In summary, our findings reveal LTBP4 as an ADAMTS7 substrate, whose cleavage may potentially impact elastogenesis in the cardiovascular system. We also identify TIMP-4 as a likely physiological ADAMTS7 inhibitor.

Phylogenetic and functional analysis of ADAMTS13 identifies highly conserved domains essential for allosteric regulation

The metalloprotease ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats member 13) prevents microvascular thrombosis by cleaving von Willebrand factor (VWF) within platelet-rich thrombi, and cleavage depends on allosteric activation of ADAMTS13 by the substrate VWF. Human ADAMTS13 has a short propeptide, metalloprotease (M), disintegrin-like (D), thrombospondin-1 (T), Cys-rich (C), and spacer (S) domains (proximal domains), followed by 7 T and 2 CUB (complement components C1r and C1s, sea urchin protein Uegf, and bone morphogenetic protein-1) domains (distal domains). Distal domains inhibit the catalytic proximal domains; binding of distal T8-CUB domains to the VWF D4 domain relieves autoinhibition and promotes cleavage of the nearby VWF A2 domain. However, the role of specific ADAMTS13 distal domains in this allosteric mechanism is not established. Assays of plasma ADAMTS13 from 20 placental mammals, birds, and amphibians show that allosteric regulation is broadly conserved, and phylogenetic analysis of 264 vertebrates shows the long propeptide, T3, T4, T6, and T6a domains have been deleted several times in placental mammals, birds, and fish. Notably, pigeon ADAMTS13 has only 3 distal T domains but was activated normally by human VWF D4 and cleaved VWF multimers, preferentially under fluid shear stress. Human ADAMTS13 constructed to resemble pigeon ADAMTS13 retained normal allosteric regulation and shear-dependent cleavage of VWF. Thus, the T3-T6 domains of human ADAMTS13 are dispensable. Conversely, deletion of T7 or T8 abolished allosteric activation. For most species, some sequence changes in the VWF substrate can markedly increase the rate of cleavage, suggesting that ADAMTS13 and VWF have not evolved to be optimal enzyme-substrate pairs. These properties may reflect evolutionary pressure to balance the risk for VWF-dependent bleeding and thrombosis.

Dissecting the pathophysiology of immune thrombotic thrombocytopenic purpura: interplay between genes and environmental triggers

Although outstanding progress has been made in understanding the pathophysiology of thrombotic thrombocytopenic purpura (TTP), knowledge of the immunopathogenesis of the disease is only at an early stage. Anti-ADAMTS13 auto-antibodies were shown to block proteolysis of von Willebrand factor and/or induce ADAMTS13 clearance from the circulation. However, it still remains to identify which immune cells are involved in the production of anti-ADAMTS13 autoantibodies, and therefore account for the remarkable efficacy of the B-cell depleting agents in this disease. The mechanisms leading to the loss of tolerance of the immune system towards ADAMTS13 involve the predisposing genetic factors of the human leukocyte antigen class II locus DRB1*11 and DQB1*03 alleles as well as the protective allele DRB1*04, and modifying factors such as ethnicity, sex and obesity. Future studies have to identify why these identified genetic risk factors are also frequently to be found in the healthy population although the incidence of immune-mediated thrombotic thrombocytopenic purpura (iTTP) is extremely low. Moreover, the development of recombinant ADAMTS13 opens a new therapeutic era in the field. Interactions of recombinant ADAMTS13 with the immune system of iTTP patients will require intensive investigation, especially for its potential immunogenicity. Better understanding of iTTP immunopathogenesis should, therefore, provide a basis for the development of novel therapeutic approaches to restore immune tolerance towards ADAMTS13 and thereby better prevent refractoriness and relapses in patients with iTTP. In this review, we address these issues and the related challenges in this field.

Prediction of spacer-α6 complex: a novel insight into binding of ADAMTS13 with A2 domain of von Willebrand factor under forces

Force-regulated cleavage of A2 domain of von Willebrand factor (vWF) by ADAMTS13 is a key event in preventing thrombotic thrombocytopenic purpura (TTP). Recognition and cleavage depend on cooperative and modular contacts between several ADAMTS13 subdomains and discrete segments of vWF A2 domain. Spacer domain of ADAMTS13 contains an important exosite interacting with α6 helix of unfold A2 domain, but it remains unclear whether stretching of α6 regulates binding to spacer. To understand the molecular mechanism underlying the interactions between spacer and α6 under stretching, we successfully predicted spacer-α6 complex by a novel computer strategy combined the steered molecular dynamics (SMD) and flexible docking techniques. This strategy included three steps: (1) constant-velocity SMD simulation of α6; (2) zero-velocity SMD simulations of α6, and (3) flexible dockings of α6 to spacer. In our spacer-α6 complex model, 13 key residues, six in α6 and seven in spacer, were identified. Our data demonstrated a biphasic extension-regulated binding of α6 to spacer. The binding strength of the complex increased with α6 extension until it reaches its optimum of 0.25 nm, and then decreased as α6 extension further increased, meaning that spacer is in favor to binding with a partially extended α6, which may contribute to the optimal contact and proteolysis. Changes of interface area and intermolecular salt bridge may serve as the molecular basis for this characteristic. These findings provide a novel insight into mechano-chemical regulation on interaction between ADAMTS13 and vWF A2 domain under forces.

Monoclonal antibodies against metzincin targets

The metzincin clan of metalloproteinases includes the MMP, disintegrin and metalloproteinase (ADAM) and ADAM with thrombospondin motifs families, which cleave extracellular targets in a wide range of (patho)physiological processes. Antibodies constitute a powerful tool to modulate the activity of these enzymes for both therapeutic and research purposes. In this review, we give an overview of monoclonal antibodies (mAbs) that have been tested in preclinical disease models, human trials and important studies of metzincin structure and function. Initial attempts to develop therapeutic small molecule inhibitors against MMPs were hampered by structural similarities between metzincin active sites and, consequently, off-target effects. Therefore, more recently, mAbs have been developed that do not bind to the active site but bind to surface-exposed loops that are poorly conserved in closely related family members. Inhibition of protease activity by these mAbs occurs through a variety of mechanisms, including (i) barring access to the active site, (ii) disruption of exosite binding, and (iii) prevention of protease activation. These different modes of inhibition are discussed in the context of the antibodies' potency, selectivity and, importantly, the effects in models of disease and clinical trials. In addition, various innovative strategies that were used to generate anti-metzincin mAbs are discussed. LINKED ARTICLES: This article is part of a themed section on Translating the Matrix. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.1/issuetoc.

Pathophysiology of thrombotic thrombocytopenic purpura and hemolytic uremic syndrome

Thrombotic microangiopathies are rare disorders characterized by the concomitant occurrence of severe thrombocytopenia, microangiopathic hemolytic anemia, and a variable degree of ischemic end-organ damage. The latter particularly affects the brain, the heart, and the kidneys. The primary forms, thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS), although their clinical presentations often overlap, have distinctive pathophysiologies. TTP is the consequence of a severe ADAMTS-13 deficiency, either immune-mediated as a result of circulating autoantibodies, or caused by mutations in ADAMTS-13. HUS develops following an infection with Shiga-toxin producing bacteria, or as the result of excessive activation of the alternative pathway of the complement system because of mutations in genes encoding complement system proteins.

Mass spectrometry-assisted identification of ADAMTS13-derived peptides presented on HLA-DR and HLA-DQ

Formation of microthrombi is a hallmark of acquired thrombotic thrombocytopenic purpura. These microthrombi originate from insufficient processing of ultra large von Willebrand factor multimers by ADAMTS13 due to the development of anti-ADAMTS13 autoantibodies. Several studies have identified the major histocompatibility complex class II alleles HLA-DRB1*11, HLA-DQB1*03 and HLA-DQB1*02:02 as risk factors for acquired thrombotic thrombocytopenic purpura development. Previous research in our department indicated that ADAMTS13 CUB2 domain-derived peptides FINVAPHAR and LIRDTHSLR are presented on HLA-DRB1*11 and HLA-DRB1*03, respectively. Here, we describe the repertoire of ADAMTS13 peptides presented on HLA-DQ. In parallel, the repertoire of ADAMTS13-derived peptides presented on HLA-DR was monitored. Using HLA-DR- and HLA-DQ-specific antibodies, we purified HLA/peptide complexes from ADAMTS13-pulsed monocyte-derived dendritic cells. Using this approach, we identified ADAMTS13-derived peptides presented on HLA-DR for all 9 samples analyzed; ADAMTS13-derived peptides presented on HLA-DQ were identified in 4 out of 9 samples. We were able to confirm the presentation of the CUB2 domain-derived peptides FINVAPHAR and LIRDTHSLR on HLA-DR. In total, 12 different core-peptide sequences were identified on HLA-DR and 8 on HLA-DQ. For HLA-DR11, several potential new core-peptides were found; 4 novel core-peptides were exclusively identified on HLA-DQ. Furthermore, an in silico analysis was performed using the EpiMatrix and JanusMatrix tools to evaluate the eluted peptides, in the context of HLA-DR, for putative effector or regulatory T-cell responses at the population level. The results from this study provide a basis for the identification of immuno-dominant epitopes on ADAMTS13 involved in the onset of acquired thrombotic thrombocytopenic purpura.

Acquired thrombotic thrombocytopenic purpura

The von Willebrand factor (VWF)-cleaving metalloprotease, ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 motifs-13) is the only known target of the dysregulated immune response in acquired TTP. Autoantibodies to ADAMTS13 either neutralize its activity or accelerate its clearance, thereby causing a severe deficiency of ADAMTS13 in plasma. As a consequence, size regulation of VWF is impaired and the persistence of ultra-large VWF (ULVWF) multimers facilitates micro vascular platelet aggregation causing microangiopathic haemolytic anaemia and ischaemic organ damage. Autoimmune TTP although a rare disease with an annual incidence of 1.72 cases has a mortality rate of 20% even with adequate therapy.

We describe the mechanisms involved in ADAMTS13 autoimmunity with a focus on the role of B- and T-cells in the pathogenesis of this disorder. We discuss the potential translation of recent experimental findings into future therapeutic concepts for the treatment of acquired TTP.

An open conformation of ADAMTS-13 is a hallmark of acute acquired thrombotic thrombocytopenic purpura

Essentials Conformational changes in ADAMTS-13 are part of its mode-of-action. The murine anti-ADAMTS-13 antibody 1C4 discriminates between folded and open ADAMTS-13. ADAMTS-13 conformation is open in acute acquired thrombotic thrombocytopenic purpura (TTP). Our study forms an important basis to fully elucidate the pathophysiology of TTP.

SUMMARY

Background Acquired thrombotic thrombocytopenic purpura (aTTP) is an autoimmune disorder characterized by absent ADAMTS-13 activity and the presence of anti-ADAMTS-13 autoantibodies. Recently, it was shown that ADAMTS-13 adopts a folded or an open conformation. Objectives As conformational changes in self-antigens play a role in the pathophysiology of different autoimmune diseases, we hypothesized that the conformation of ADAMTS-13 changes during acute aTTP. Methods Antibodies recognizing cryptic epitopes in the spacer domain were generated. Next, the conformation of ADAMTS-13 in 40 healthy donors (HDs), 99 aTTP patients (63 in the acute phase versus 36 in remission), 12 hemolytic-uremic syndrome (HUS) patients and 63 sepsis patients was determined with ELISA. Results The antibody 1C4 recognizes a cryptic epitope in ADAMTS-13. Therefore, we were able to discriminate between a folded and an open ADAMTS-13 conformation. We showed that ADAMTS-13 in HDs does not bind to 1C4, indicating that ADAMTS-13 circulates in a folded conformation. Similar results were obtained for HUS and sepsis patients. In contrast, ADAMTS-13 of acute aTTP patients bound to 1C4 in 92% of the cases, whereas, in most cases, this binding was abolished during remission, showing that the conformation of ADAMTS-13 is open during an acute aTTP episode. Conclusions Our study shows that, besides absent ADAMTS-13 activity and the presence of anti-ADAMTS-13 autoantibodies, an open ADAMTS-13 conformation is also a hallmark of acute aTTP. Demonstrating this altered ADAMTS-13 conformation in acute aTTP will help to further unravel the pathophysiology of aTTP and lead to improved therapy and diagnosis.

Anti-ADAMTS13 Antibodies and a Novel Heterozygous p.R1177Q Mutation in a Case of Pregnancy-Onset Immune-Mediated Thrombotic Thrombocytopenic Purpura

In this study, we investigated a case of pregnancy-onset thrombotic thrombocytopenic purpura (TTP). The patient had severely decreased ADAMTS13 ( ad isintegrin a nd m etalloprotease with t hrombo s pondin type 1 motif, member 13) activity levels during acute phase and the presence of inhibitory anti-ADAMTS13 autoantibodies was demonstrated, which led to the diagnosis of immune-mediated TTP. However, ADAMTS13 activity was only mildly restored during remission, although inhibitory anti-ADAMTS13 antibodies were no longer detected. We hypothesized that genetic abnormalities could account for this discrepancy between ADAMTS13 activity and antigen. Genetic analysis revealed the presence of two heterozygous substitutions on the same allele: a single nucleotide polymorphism (SNP) c.2699C > T (p.A900V), located in the beginning of the T5 domain, and a mutation c.3530G > A (p.R1177Q) located in the third linker region of ADAMTS13. In vitro testing of those substitutions by expression of recombinant proteins revealed a normal secretion but a reduced ADAMTS13 activity by the novel p.R1177Q mutation, which could partially explain the subnormal activity levels found during remission. Although changes in the linker region might induce conformational changes in ADAMTS13, the p.R1177Q mutation in the third linker region of ADAMTS13 did not expose a cryptic epitope in the metalloprotease domain. In conclusion, we report on an immune-mediated pregnancy-onset TTP patient who had inhibitory anti-ADAMTS13 autoantibodies during acute phase, but not during remission. Genetic analysis confirmed the diagnosis of immune-mediated TTP and revealed the novel p.R1177Q mutation which mildly impaired ADAMTS13 activity.

ADAMTS-13 and von Willebrand factor: a dynamic duo

von Willebrand factor (VWF) is a key player in hemostasis, acting as a carrier for factor VIII and capturing platelets at sites of vascular damage. To capture platelets, it must undergo conformational changes, both within its A1 domain and at the macromolecular level through A2 domain unfolding. Its size and this function are regulated by the metalloproteinase ADAMTS-13. Recently, it has been shown that ADAMTS-13 undergoes a conformational change upon interaction with VWF, and that this enhances its activity towards its substrate. This review summarizes recent work on these conformational transitions, describing how they are controlled. It points to their importance in hemostasis, bleeding disorders, and the developing field of therapeutic application of ADAMTS-13 as an antithrombotic agent in obstructive microvascular thrombosis and in cardiovascular disease.

The ADAMTS131239-1253 peptide is a dominant HLA-DR1-restricted CD4+ T-cell epitope

Acquired thrombotic thrombocytopenic purpura is a rare and severe disease characterized by auto-antibodies directed against “A Disintegrin And Metalloproteinase with Thrombospondin type 1 repeats, 13^th member" (ADAMTS13), a plasma protein involved in hemostasis. Involvement of CD4⁺ T cells in the pathogenesis of the disease is suggested by the IgG isotype of the antibodies. However, the nature of the CD4⁺ T-cell epitopes remains poorly characterized. Here, we determined the HLA-DR-restricted CD4⁺ T-cell epitopes of ADAMTS13. Candidate T-cell epitopes were predicted in silico and binding affinities were confirmed in competitive enzyme-linked immunosorbent assays. ADAMTS13-reactive CD4⁺ T-cell hybridomas were generated following immunization of HLA-DR1 transgenic mice (Sure-L1 strain) and used to screen the candidate epitopes. We identified the ADAMTS13^1239–1253 peptide as the single immunodominant HLA-DR1-restricted CD4⁺ T-cell epitope. This peptide is located in the CUB2 domain of ADAMTS13. It was processed by dendritic cells, stimulated CD4⁺ T cells from Sure-L1 mice and was recognized by CD4⁺ T cells from an HLA-DR1-positive patient with acute thrombotic thrombocytopenic purpura. Interestingly, the ADAMTS13^1239–1253 peptide demonstrated promiscuity towards HLA-DR11 and HLA-DR15. Our work paves the way towards the characterization of the ADAMTS13-specific CD4⁺ T-cell response in patients with thrombotic thrombocytopenic purpura using ADAMTS13^1239–1253-loaded HLA-DR tetramers.

ADAMTS-13 glycans and conformation-dependent activity

Essentials The impact of N-linked glycosylation on ADAMTS-13 function has not been fully explored. The activity of glycan modified ADAMTS-13 was investigated under static and shear stress conditions. Terminal sialic acid on the metalloprotease domain glycans are important for ADAMTS-13 activity. The CUB domain glycans modulate ADAMTS-13 activity.

SUMMARY

Background ADAMTS-13 activity can be regulated by its conformation, whereby interactions between the C-terminal CUB domains and the spacer domain maintain ADAMTS-13 in a closed conformation. ADAMTS-13 contains 10 N-linked glycans, with four sites present in theTSP2 through to CUB domains that may contribute to its conformation. Objectives/Methods We hypothesized that glycosylation contributes to ADAMTS-13 conformation and function. The proteolytic activity of glycan-modified ADAMTS-13 was assessed under static and shear stress conditions. Results Enzymatic removal of terminal silaic acid or entire N-linked glycan chains decreased activity against FRETS-VWF73 at pH 7.4 and against full-length von Willebrand factor (VWF) under shear stress. Using truncated ADAMTS-13, we demonstrated that this was attributable to loss of sialic acid from the glycans in the metalloprotease domain and an effect of N-linked glycosylation in the TSP2 through to CUB domains. Mutation of the N-linked glycan sites in the MDTCS domains reduced or abolished protein expression. However, the N707Q, N828Q, N1235Q and N1354Q (TSP2, TSP4, CUB1, and CUB2 domains, respectively) variants were expressed normally. Interestingly, the N707Q and N828Q variants showed reduced activity against FRETS-VWF73, but normal activity under flow conditions. In contrast, the N1235Q and N1354Q variants had enhanced activity against FRETS-VWF73 and VWF under shear stress. Immunoprecipitation experiments confirmed that loss of N-linked glycans in the CUB domains significantly reduced the interaction with the spacer domain and enhanced binding to the 6A6 anti-ADAMTS-13 antibody, which recognizes a cryptic epitope in the metalloprotease domain. Conclusions Together, these data demonstrate that the N-linked glycans of ADAMTS-13 play a crucial role in regulating ADAMTS-13 activity.

Mechanisms of Autoantibody-Induced Pathology

Autoantibodies are frequently observed in healthy individuals. In a minority of these individuals, they lead to manifestation of autoimmune diseases, such as rheumatoid arthritis or Graves' disease. Overall, more than 2.5% of the population is affected by autoantibody-driven autoimmune disease. Pathways leading to autoantibody-induced pathology greatly differ among different diseases, and autoantibodies directed against the same antigen, depending on the targeted epitope, can have diverse effects. To foster knowledge in autoantibody-induced pathology and to encourage development of urgently needed novel therapeutic strategies, we here categorized autoantibodies according to their effects. According to our algorithm, autoantibodies can be classified into the following categories: (1) mimic receptor stimulation, (2) blocking of neural transmission, (3) induction of altered signaling, triggering uncontrolled (4) microthrombosis, (5) cell lysis, (6) neutrophil activation, and (7) induction of inflammation. These mechanisms in relation to disease, as well as principles of autoantibody generation and detection, are reviewed herein.

Thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP; also known as Moschcowitz disease) is characterized by the concomitant occurrence of often severe thrombocytopenia, microangiopathic haemolytic anaemia and a variable degree of ischaemic organ damage, particularly affecting the brain, heart and kidneys. Acute TTP was almost universally fatal until the introduction of plasma therapy, which improved survival from <10% to 80-90%. However, patients who survive an acute episode are at high risk of relapse and of long-term morbidity. A timely diagnosis is vital but challenging, as TTP shares symptoms and clinical presentation with numerous conditions, including, for example, haemolytic uraemic syndrome and other thrombotic microangiopathies. The underlying pathophysiology is a severe deficiency of the activity of a disintegrin and metalloproteinase with thrombospondin motifs 13 (ADAMTS13), the protease that cleaves von Willebrand factor (vWF) multimeric strings. Ultra-large vWF strings remain uncleaved after endothelial cell secretion and anchorage, bind to platelets and form microthrombi, leading to the clinical manifestations of TTP. Congenital TTP (Upshaw-Schulman syndrome) is the result of homozygous or compound heterozygous mutations in ADAMTS13, whereas acquired TTP is an autoimmune disorder caused by circulating anti-ADAMTS13 autoantibodies, which inhibit the enzyme or increase its clearance. Consequently, immunosuppressive drugs, such as corticosteroids and often rituximab, supplement plasma exchange therapy in patients with acquired TTP.

Platelet-independent adhesion of calcium-loaded erythrocytes to von Willebrand factor

Adhesion of erythrocytes to endothelial cells lining the vascular wall can cause vaso-occlusive events that impair blood flow which in turn may result in ischemia and tissue damage. Adhesion of erythrocytes to vascular endothelial cells has been described in multiple hemolytic disorders, especially in sickle cell disease, but the adhesion of normal erythrocytes to endothelial cells has hardly been described. It was shown that calcium-loaded erythrocytes can adhere to endothelial cells. Because sickle erythrocyte adhesion to ECs can be enhanced by ultra-large von Willebrand factor multimers, we investigated whether calcium loading of erythrocytes could promote binding to endothelial cells via ultra-large von Willebrand factor multimers. We used (immunofluorescent) live-cell imaging of washed erythrocytes perfused over primary endothelial cells at venular flow rate. Using this approach, we show that calcium-loaded erythrocytes strongly adhere to histamine-stimulated primary human endothelial cells. This adhesion is mediated by ultra-large von Willebrand factor multimers. Von Willebrand factor knockdown or ADAMTS13 cleavage abolished the binding of erythrocytes to activated endothelial cells under flow. Platelet depletion did not interfere with erythrocyte binding to von Willebrand factor. Our results reveal platelet-independent adhesion of calcium-loaded erythrocytes to endothelium-derived von Willebrand factor. Erythrocyte adhesion to von Willebrand factor may be particularly relevant for venous thrombosis, which is characterized by the formation of erythrocyte-rich thrombi.

A model for the conformational activation of the structurally quiescent metalloprotease ADAMTS13 by von Willebrand factor

Blood loss is prevented by the multidomain glycoprotein von Willebrand factor (VWF), which binds exposed collagen at damaged vessels and captures platelets. VWF is regulated by the metalloprotease ADAMTS13, which in turn is conformationally activated by VWF. To delineate the structural requirements for VWF-mediated conformational activation of ADAMTS13, we performed binding and functional studies with a panel of truncated ADAMTS13 variants. We demonstrate that both the isolated CUB1 and CUB2 domains in ADAMTS13 bind to the spacer domain exosite of a truncated ADAMTS13 variant, MDTCS (K_D of 135 ± 1 0.1 nm and 86.9 ± 9.0 nm, respectively). However, only the CUB1 domain inhibited proteolytic activity of MDTCS. Moreover, ADAMTS13ΔCUB2, unlike ADAMTS13ΔCUB1-2, exhibited activity similar to wild-type ADAMTS13 and could be activated by VWF D4-CK. The CUB2 domain is, therefore, not essential for maintaining the inactive conformation of ADAMTS13. Both CUB domains could bind to the VWF D4-CK domain fragment (K_D of 53.7 ± 2.1 nm and 84.3 ± 2.0 nm, respectively). However, deletion of both CUB domains did not prevent VWF D4-CK binding, suggesting that competition for CUB-domain binding to the spacer domain is not the dominant mechanism behind the conformational activation. ADAMTS13ΔTSP8-CUB2 could no longer bind to VWF D4-CK, and deletion of TSP8 abrogated ADAMTS13 conformational activation. These findings support an ADAMTS13 activation model in which VWF D4-CK engages the TSP8-CUB2 domains, inducing the conformational change that disrupts the CUB1-spacer domain interaction and thereby activates ADAMTS13.

The class I scavenger receptor CD163 promotes internalization of ADAMTS13 by macrophages

Internalization of ADAMTS13 by macrophages may contribute to its clearance from the circulation. Here we investigated endocytic mechanisms that contribute to the uptake of ADAMTS13 by macrophages. Human monocyte-derived macrophages were used to monitor the uptake of fluorescently labeled recombinant ADAMTS13 by flow cytometry. Internalization of ADAMTS13 was blocked upon addition of the cell-permeable dynamin inhibitor dynasore. Partial blocking of ADAMTS13 uptake was observed by using mannan; however, uptake was not affected by an antibody that blocked binding to the macrophage mannose receptor CD206, which suggests that other endocytic receptors contribute to the internalization of ADAMTS13 by macrophages. A pull-down with ADAMTS13 and subsequent mass spectrometric analysis identified the class I scavenger receptor CD163 as a candidate receptor for ADAMTS13. Blocking experiments with monoclonal anti-CD163 antibody EDHu-1 resulted in decreased ADAMTS13 internalization by macrophages. Pronounced inhibition of ADAMTS13 uptake by EDHu-1 was observed in CD163 high-expressing macrophages. In agreement with these findings, CD163-expressing Chinese hamster ovary cells were capable of rapidly internalizing ADAMTS13. Surface plasmon resonance revealed binding of ADAMTS13 to scavenger receptor cysteine-rich domains 1-9 and 1-5 of CD163. Taken together, our data identify CD163 as a major endocytic receptor for ADAMTS13 on macrophages.

Conformational quiescence of ADAMTS-13 prevents proteolytic promiscuity

Essentials Recently, ADAMTS-13 has been shown to undergo substrate induced conformation activation. Conformational quiescence of ADAMTS-13 may serve to prevent off-target proteolysis in plasma. Conformationally active ADAMTS-13 variants are capable of proteolysing the Aα chain of fibrinogen. This should be considered as ADAMTS-13 variants are developed as potential therapeutic agents. Click to hear Dr Zheng's presentation on structure function and cofactor-dependent regulation of ADAMTS-13 SUMMARY: Background Recent work has revealed that ADAMTS-13 circulates in a 'closed' conformation, only fully interacting with von Willebrand factor (VWF) following a conformational change. We hypothesized that this conformational quiescence also maintains the substrate specificity of ADAMTS-13 and that the 'open' conformation of the protease might facilitate proteolytic promiscuity. Objectives To identify a novel substrate for a constitutively active gain of function (GoF) ADAMTS-13 variant (R568K/F592Y/R660K/Y661F/Y665F). Methods Fibrinogen proteolysis was characterized using SDS PAGE and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Fibrin formation was monitored by turbidity measurements and fibrin structure visualized by confocal microscopy. Results ADAMTS-13 exhibits proteolytic activity against the Aα chain of human fibrinogen, but this is only manifest on its conformational activation. Accordingly, the GoF ADAMTS-13 variant and truncated variants such as MDTCS exhibit this activity. The cleavage site has been determined by LC-MS/MS to be Aα chain Lys225-Met226. Proteolysis of fibrinogen by GoF ADAMTS-13 impairs fibrin formation in plasma-based assays, alters clot structure and increases clot permeability. Although GoF ADAMTS-13 does not appear to proteolyse preformed cross-linked fibrin, its proteolytic activity against fibrinogen increases the susceptibility of fibrin to tissue-type plasminogen activator (t-PA)-induced lysis by plasmin and increases the fibrin clearance rate more than 8-fold compared with wild-type (WT) ADAMTS-13 (EC50 values of 3.0 ± 1.7 nm and 25.2 ± 9.7 nm, respectively) in in vitro thrombosis models. Conclusion The 'closed' conformation of ADAMTS-13 restricts its specificity and protects against fibrinogenolysis. Induced substrate promiscuity will be important as ADAMTS-13 variants are developed as potential therapeutic agents against thrombotic thrombocytopenic purpura (TTP) and other cardiovascular diseases.

Identification of glycans on plasma-derived ADAMTS13

Patients suffering from acquired thrombotic thrombocytopenic purpura develop autoantibodies directed toward the plasma glycoprotein ADAMTS13. Here, we studied the glycan composition of plasma-derived ADAMTS13. Purified ADAMTS13 was reduced, alkylated, and processed into peptides with either trypsin or chymotrypsin. Glycopeptides were enriched using zwitterionic HILIC zip-tips and analyzed by tandem mass spectrometry employing higher-energy collision dissociation fragmentation. Upon detection of a diagnostic ion of a glycan fragment, electron transfer dissociation fragmentation was performed on the same precursor ion. The majority of N-linked glycans were of the complex type containing terminal sialic acids and fucose residues. A high mannose-containing glycan was attached to Asn614 in the spacer domain. Six O-linked glycans mostly terminating in sialic acid were found dispersed over ADAMTS13. Five O-linked glycans were attached to a Ser and one to Thr. All 6 O-linked glycans contained a terminal sialic acid. O-fucosylation is a common posttranslational modification of thrombospondin type 1 repeats. We identified 7 O-fucosylation sites in the thrombospondin (TSP) type 1 repeats. Unexpectedly, one additional O-fucosylation site was found in the disintegrin domain. This O-fucosylation site did not meet the proposed consensus sequence CSX(S/T)CG. C-mannosylation sites were identified in TSP1, linker TSP4-TSP5, and TSP8. Overall, our findings highlight the complexity of glycan modifications on ADAMTS13, which may have implications for its interaction with immune- or clearance receptors containing carbohydrate recognition domains.