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

Two-way Dispatched function in Sonic hedgehog shedding and transfer to high-density lipoproteins

The Sonic hedgehog (Shh) signaling pathway controls embryonic development and tissue homeostasis after birth. This requires regulated solubilization of dual-lipidated, firmly plasma membrane-associated Shh precursors from producing cells. Although it is firmly established that the resistance-nodulation-division transporter Dispatched (Disp) drives this process, it is less clear how lipidated Shh solubilization from the plasma membrane is achieved. We have previously shown that Disp promotes proteolytic solubilization of Shh from its lipidated terminal peptide anchors. This process, termed shedding, converts tightly membrane-associated hydrophobic Shh precursors into delipidated soluble proteins. We show here that Disp-mediated Shh shedding is modulated by a serum factor that we identify as high-density lipoprotein (HDL). In addition to serving as a soluble sink for free membrane cholesterol, HDLs also accept the cholesterol-modified Shh peptide from Disp. The cholesteroylated Shh peptide is necessary and sufficient for Disp-mediated transfer because artificially cholesteroylated mCherry associates with HDL in a Disp-dependent manner, whereas an N-palmitoylated Shh variant lacking C-cholesterol does not. Disp-mediated Shh transfer to HDL is completed by proteolytic processing of the palmitoylated N-terminal membrane anchor. In contrast to dual-processed soluble Shh with moderate bioactivity, HDL-associated N-processed Shh is highly bioactive. We propose that the purpose of generating different soluble forms of Shh from the dual-lipidated precursor is to tune cellular responses in a tissue-type and time-specific manner.

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.

CUB domains are not required for OVCH2 function in sperm maturation in the mouse epididymis

BACKGROUND

Ovochymase 2 (Ovch2) is an epididymis-specific gene that is required for male fertility. While a multitude of reproductive tract-specific genes required for male fertility have been identified, OVCH2 is thus far the first protein required for male fertility that contains Complement C1r/C1s, Uegf, Bmp1 (CUB) domains located in tandem in the C-terminus of the protein. Identifying the functional significance of this unique domain has implications in better understanding fertility and infertility and as a potential contraceptive target.

OBJECTIVE

The goals of these studies were to understand the influence and requirement of OVCH2 CUB domains in the localization and functional requirement of OVCH2 in sperm maturation and function.

MATERIALS AND METHODS

To this end, we performed in vivo localization analysis of OVCH2 and reproductive phenotype analysis of mice containing C-terminal FLAG tag on OVCH2, with either the entire protein intact, or CUB2 or both CUB1 and CUB2 genetically ablated. All mice were generated through the CRISPR/Cas9 gene editing approach.

RESULTS

We found that OVCH2 is specifically expressed in the proximal caput epididymidis, and the absence of CUB2 did not affect this localization pattern. Although the absence of both CUB domains significantly reduced sperm motility and progressive motility, this effect was not manifested in a reduction in fertility over a 6-month period mating trial, which showed no significant differences between control and CUB deletant mice. Further, the absence of one or both CUB domains did not affect reproductive organ structure or sperm morphology.

CONCLUSIONS

Our studies demonstrate that the CUB domains are not required for fertility in male mice, at least under the normal animal housing conditions our mice were tested in, and suggest that the enzymatic activity of the OVCH2 protease, in the absence of its CUB domains, is sufficient for normal sperm processing in the epididymis. Although our findings do not preclude the possibility that OVCH2 CUB domains are required under a yet-identified stress condition, our findings demonstrate that the most likely region for deleterious mutations in men with idiopathic infertility and the most vulnerable site for inhibition of OVCH2 protein function is in its protease domain, and not its CUB domains. Our findings have implications in the genetic screening of infertile men and the development of a novel non-hormonal male contraceptive by honing in on the more critical region of a functionally required protein.

REVIEWING THE DYSREGULATION OF ADAMTS13 AND VWF IN SEPSIS

ABSTRACT

Sepsis is defined as a life-threatening organ dysfunction caused by excessive host response to infection, and represents the most common cause of in-hospital deaths. Sepsis accounts for 30% of all critically ill patients in the intensive care unit (ICU), and has a global mortality rate of 20%. Activation of blood coagulation during sepsis and septic shock can lead to disseminated intravascular coagulation, which is characterized by microvascular thrombosis. Von Willebrand factor (VWF) and ADAMTS13 are two important regulators of blood coagulation that may be important links between sepsis and mortality in the ICU. Herein we review our current understanding of VWF and ADAMTS13 in sepsis and other critical illnesses and discuss their contribution to disease pathophysiology, their use as markers of severe illness, and potential targets for new therapeutic development.

Management and follow-up of pregnancy-onset thrombotic thrombocytopenic purpura: the French experience

ABSTRACT

Pregnancy-onset thrombotic thrombocytopenic purpura (TTP) is a rare and life-threatening disease of which diagnosis and management requires experienced multidisciplinary teams. The mechanisms responsible for a deficiency in the disintegrin and metalloprotease with thrombospondin type 1 repeats, member 13 (ADAMTS13) leading to pregnancy-onset TTP may be congenital or acquired, and studying ADAMTS13 conformation could be of interest. The differential diagnosis between TTP and other pregnancy-associated thrombotic microangiopathies (TMA) is often challenging. Our retrospective multicenter study highlights the significance and the challenges associated with pregnancy-onset TTP and childbirth in terms of diagnosis, obstetric management, and follow-up aspects. Among 1174 pregnancy-onset TMA enrolled in the French Registry for TMA from 2000 to 2020, we identified 108 pregnancy-onset TTP: 52 immune-mediated TTP (iTTP, 48.1%), 27 acquired TTP of unidentified mechanism (uTTP, 25%), and 29 congenital TTP (cTTP, 26.9%). Data show that maternal outcome is good (survival rate: 95%) and fetal outcome is linked to the gestational age at the onset of the disease (survival rate: 75.5%). Three distinct entities with different natural histories emerged: pregnancy-onset iTTP appears similar to idiopathic iTTP, with an open ADAMTS13 conformation, and is marked by a relapse risk independent of subsequent pregnancies; pregnancy-onset uTTP appears to have a different pathophysiology with an unexpected open ADAMTS13 conformation and a very low relapse risk independent of subsequent pregnancies; finally, pregnancy-onset cTTP is characterized by the necessity of pregnancy as a systematic and specific trigger and a need for prophylactic plasmatherapy for subsequent pregnancies. This trial was registered at www.clinicaltrials.gov as #NCT00426686, and at the Health Authority and the French Ministry of Health (P051064/PHRC AOM05012).

Hemophagocytic lymphohistiocytosis is associated with deficiency and closed conformation of ADAMTS-13

Background

A disintegrin and metalloprotease with thrombospondin type 1 repeats, member 13 (ADAMTS-13) is the specific von Willebrand factor-cleaving protease and circulates in a closed and latent conformation due to a spacer/CUB1 domain interaction. ADAMTS-13 is allosterically activated after binding of its substrate or antibodies, inducing an open conformation. Recently, we suggested a potential role of plasmin (fibrinolysin) in hemostasis disorders reported in most patients with hemophagocytic lymphohistiocytosis (HLH), a rare and life-threatening condition related to a severe systemic inflammatory state. Most patients with HLH had a partial ADAMTS-13 deficiency, and plasmin could induce a truncation of the C-terminal part of ADAMTS-13 and thus an open conformation.

Objectives

To understand the effect of plasmin on ADAMTS-13, our study aimed to investigate ADAMTS-13 conformation in patients with HLH.

Methods

Forty-five critically ill patients with HLH were prospectively enrolled between April 2015 and December 2018. ADAMTS-13 activity was measured by fluorescent resonance energy transfer-VWF73 assay, ADAMTS-13 antigen, and conformation with our homemade 3H9-enzyme-linked immunosorbent assay and 1C4-enzyme-linked immunosorbent assay.

Results

ADAMTS-13 activity ranged from <10 to 65 IU/dL, and 41 of the 45 patients had a quantitative deficiency in ADAMTS-13 (activity <50 IU/dL). Twenty patients had a severe ADAMTS-13 deficiency (activity <20 IU/dL). ADAMTS-13 conformation was folded in all patients under normal conditions. Surprisingly, the switch of ADAMTS-13 conformation expected with the monoclonal antibody 17G2 (anti-CUB1) was disturbed in 6 patients (activity <20 IU/dL).

Conclusion

Our study reported that ADAMTS-13 conformation is closed in HLH and provides an indirect proof that plasmin is not able to massively degrade ADAMTS-13. Further studies on glycosylation and citrullination profiles of ADAMTS-13 are needed to understand their role in HLH.

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.

Molecular interplay of ADAMTS13-MDTCS and von willebrand Factor-A2: deepened insights from extensive atomistic simulations

Thrombotic thrombocytopenic purpura (TTP) is a rare and life-threatening disease. One hallmark is severe ADAMTS13 deficiency, causing ultra-large von Willebrand factor (VWF) multimers to accumulate, leading to microthrombi and lastly to microangiopathic hemolytic anemia and severe thrombocytopenia. Despite great success in recent decades, the molecular picture of the interaction between VWF and ADAMTS13 remains vague. Here, we utilized modern replica-exchange molecular dynamics simulations with the TIGER2h method to sample a vast configurational space of the isolated ADAMTS13-MDTCS domains and the exposure to its substrate and activating cofactor - the unraveled VWF-A2 domain. The sampling of binding sites and conformations was guided and filtered in agreement with available experimental evidence. We provide comprehensive information on exosites for each domain and direct pairs of interacting amino acids, for the first time. The major binding cluster for the active site of the MP domain contrasts the previous mapping of VWF-A2 residues and reciprocal binding pockets. Two major binding modes are revealed and provide access to conformational changes of an extended gatekeeper tetrad upon overcoming local latency during substrate binding and to a dedicated recruitment mechanism. Our work adds the first molecular interaction model that places previous experimental results in perspective to better understand disease-related mutations towards improved therapies. Numerous empirical targets are proposed to verify the given binding modes, to refine the overall picture of MP binding pockets, the role of Dis binding in MP activation and the passage of the Cys-rich domain through VWF-A2, thus deepening the understanding of a highly dynamic interplay.Communicated by Ramaswamy H. Sarma.

Metalloprotease domain latency protects ADAMTS13 against broad-spectrum inhibitors of metalloproteases while maintaining activity toward VWF

BACKGROUND

ADAMTS13 is a circulating metalloprotease that cleaves von Willebrand factor (VWF) in a shear-dependent manner. ADAMTS13 is secreted as an active protease but has a long half-life, suggesting that it is resistant to circulating protease inhibitors. These zymogen-like properties indicate that ADAMTS13 exists as a latent protease that is activated by its substrate.

OBJECTIVES

To investigate the mechanism of ADAMTS13 latency and resistance to metalloprotease inhibitors.

METHODS

Probe the active site of ADAMTS13 and variants using alpha-2 macroglobulin (A2M), tissue inhibitors of metalloproteases (TIMPs), and Marimastat.

RESULTS

ADAMTS13 and C-terminal deletion mutants are not inhibited by A2M, TIMPs, or Marimastat, but cleave FRETS-VWF73, suggesting that the metalloprotease domain is latent in the absence of substrate. Within the metalloprotease domain, mutating the gatekeeper triad (R193, D217, D252) or substituting the calcium-binding (R180-R193) or the variable (G236-S263) loops with corresponding features from ADAMTS5 did not sensitize MDTCS to inhibition. However, substituting the calcium-binding loop and an extended variable loop (G236-S263) corresponding to the S1-S1' pockets with those from ADAMTS5, resulted in MDTCS-GVC5 inhibition by Marimastat, but not by A2M or TIMP3. Substituting the MD domains of ADAMTS5 into full-length ADAMTS13 resulted in a 50-fold reduction in activity compared with the substitution into MDTCS. However, both chimeras were susceptible to inhibition, suggesting that the closed conformation does not contribute to the latency of the metalloprotease domain.

CONCLUSION

The metalloprotease domain protects ADAMTS13 from inhibitors and exists in a latent state that is partially maintained by loops flanking the S1 and S1' specificity pockets.

A novel mechanism underlying allosteric regulation of ADAMTS-13 revealed by hydrogen-deuterium exchange plus mass spectrometry

Background

ADAMTS-13, a plasma metalloprotease, cleaves von Willebrand factor. ADAMTS-13 activity appears to be regulated through allosteric inhibition by its distal C-terminus.

Objectives

The objective of this study was to better understand how domain-domain interactions may affect ADAMTS-13 conformations and functions.

Methods

We performed deuterium-hydrogen exchange plus mass spectrometry to assess the number and rate of deuterium incorporation into various peptides of full-length ADAMTS-13 and its truncated variants.

Results

Under physiological conditions, a bimodal distribution of deuterium incorporation was detected in the peptides from metalloprotease (217-230 and 282-304), cysteine-rich (446-482), and CUB (for complement C1r/C1s, Uegf, Bmp1) domains (1185-1214, 1313-1330, 1341-1347, 1358-1378, and 1393-1407) of full-length recombinant ADAMTS-13, but not of truncated variants. These results suggest that the full-length ADAMTS-13 undergoes conformational changes. On removal of the middle and distal C-terminal domains, the number and rate of deuterium incorporation were increased in the peptides from cysteine-rich (445-467, 467-482, and 495-503) and spacer domains (621-642 and 655-654) but decreased in the peptides from metalloprotease (115-124, 217-230, and 274-281). Moreover, most peptides, except for 217-230 and 1357-1376, exhibited a pD-dependent deuterium incorporation in the full-length ADAMTS-13, but not in the truncated variant (eg, MDTCS or T5C). These results further suggest that the bimodal deuterium incorporation observed in the peptides from the full-length ADAMTS-13 is the result of potential impact from the middle to distal C-terminal domains. Surface plasmon resonance revealed the direct binding interactions between the distal and proximal domains of ADAMTS-13.

Conclusion

Our results provide novel insight on how intramolecular interactions may affect conformations of ADAMTS-13, thus regulating its proteolytic functions.

Mechanisms of ADAMTS13 regulation

Recombinant ADAMTS13 is currently undergoing clinical trials as a treatment for hereditary thrombotic thrombocytopenic purpura, a lethal microvascular condition resulting from ADAMTS13 deficiency. Preclinical studies have also demonstrated its efficacy in treating arterial thrombosis and inflammation without causing bleeding, suggesting that recombinant ADAMTS13 may have broad applicability as an antithrombotic agent. Despite this progress, we currently do not understand the mechanisms that regulate ADAMTS13 activity in vivo. ADAMTS13 evades canonical means of protease regulation because it is secreted as an active enzyme and has a long half-life in circulation, suggesting that it is not inhibited by natural protease inhibitors. Although shear can spatially and temporally activate von Willebrand factor to capture circulating platelets, it is also required for cleavage by ADAMTS13. Therefore, spatial and temporal regulation of ADAMTS13 activity may be required to stabilize von Willebrand factor-platelet strings at sites of vascular injury. This review outlines potential mechanisms that regulate ADAMTS13 in vivo including shear-dependency, local inactivation, and biochemical and structural regulation of substrate binding. Recently published structural data of ADAMTS13 is discussed, which may help to generate novel hypotheses for future research.

ADAMTS13 conformations and mechanism of inhibition in immune thrombotic thrombocytopenic purpura

ADAMTS13, a plasma metalloprotease that cleaves von Willebrand factor, is crucial for normal hemostasis. Acquired autoantibody-mediated deficiency of plasma ADAMTS13 results in a potentially fatal blood disorder, immune thrombotic thrombocytopenic purpura (iTTP). Plasma ADAMTS13 protease appears to exist in multiple conformations. Under physiological conditions, plasma ADAMTS13 exists predominantly in its "closed" conformation (or latent form), which may be activated by lowering pH, ligand binding, and binding of an antibody against the distal domains of ADAMTS13. In patients with iTTP, polyclonal antibodies target at various domains of ADAMTS13. However, nearly all inhibitory antibodies bind the spacer domain, whereas antibodies that bind the distal C-terminal domains may activate ADAMTS13 through removing its allosteric inhibition. Additionally, the anti-C-terminal antibodies may alter the potency of inhibitory antibodies towards ADAMTS13 activity. This review summarizes some of the most recent knowledge about the ADAMTS13 conformation and its mechanism of inhibition by its autoantibodies.

Robust thrombolytic and anti-inflammatory action of a constitutively active ADAMTS13 variant in murine stroke models

Advances in our understanding of ADAMTS13 structure, and the conformation changes required for full activity, have rejuvenated the possibility of its use as a thrombolytic therapy. We have tested a novel Ala1144Val ADAMTS13 variant (constitutively active [ca] ADAMTS13) that exhibits constitutive activity, characterized using in vitro assays of ADAMTS13 activity, and greatly enhanced thrombolytic activity in 2 murine models of ischemic stroke, the distal FeCl3 middle cerebral artery occlusion (MCAo) model and transient middle cerebral artery occlusion (tMCAO) with systemic inflammation and ischemia/reperfusion injury. The primary measure of efficacy in both models was restoration of regional cerebral blood flow (rCBF) to the MCA territory, which was determined using laser speckle contrast imaging. The caADAMTS13 variant exhibited a constitutively active conformation and a fivefold enhanced activity against fluorescence resonance energy transfer substrate von Willebrand factor 73 (FRETS-VWF73) compared with wild-type (wt) ADAMTS13. Moreover, caADAMTS13 inhibited VWF-mediated platelet capture at subphysiological concentrations and enhanced t-PA/plasmin lysis of fibrin(ogen), neither of which were observed with wtADAMTS13. Significant restoration of rCBF and reduced lesion volume was observed in animals treated with caADAMTS13. When administered 1 hour after FeCl3 MCAo, the caADAMTS13 variant significantly reduced residual VWF and fibrin deposits in the MCA, platelet aggregate formation, and neutrophil recruitment. When administered 4 hours after reperfusion in the tMCAo model, the caADAMTS13 variant induced a significant dissolution of platelet aggregates and a reduction in the resulting tissue hypoperfusion. The caADAMTS13 variant represents a potentially viable therapeutic option for the treatment of acute ischemic stroke, among other thrombotic indications, due to its enhanced in vitro and in vivo activities that result from its constitutively active conformation.

An Optimized Purification Design for Extracting Active ADAMTS13 from Conditioned Media

ADAMTS13 is a hemostatic enzyme that breaks down pro-thrombotic ultra-large multimers of von Willebrand factor (VWF). The deficiency of ADAMTS13 increases VWF-mediated thrombogenic potential and may lead to thrombotic thrombocytopenic purpura (TTP). Recently, clinical studies have shown the development of acquired TTP after COVID-19 infection and a correlation between low ADAMTS13 plasma levels and increased mortality. As a result, investigating ADAMTS13 as a potential recombinant therapeutic is of broad interest in the field of hematology. ADAMTS13 is considered challenging to purify in its biologically active state. Current purification methods utilize immobilized metal ions, which can interfere with ADAMTS13 metalloprotease activity. For this reason, we optimized an alternative strategy to isolate milligram quantities of highly active recombinant ADAMTS13 (rADAMTS13) from conditioned media after exogenous expression in human cell line, HEK293. HEK293 cells stably expressing C-terminal V5-His-tagged ADAMTS13 were grown in two parallel systems, culture bottles and flasks, for identifying an optimal cultivation strategy. Subsequently, we employed anion exchange followed by anti-V5-tag affinity chromatography to purify rADAMTS13, and extracted rADAMTS13 of high specific activity while preserving its native post-translational modifications. In addition, this process has been optimized and scaled up to produce active rADAMTS13 at levels sufficient for laboratory-scale structural, enzymatic, and biochemical studies.

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.

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.