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Background ADAMTS13 circulates in a closed conformation, only achieving full proteolytic activity against von Willebrand Factor (VWF) following a substrate‐induced conformational change. A gain of function (GoF) ADAMTS13 variant (R568K/F592Y/R660K/Y661F/Y665F) is conformationally pre‐activated. Objectives To establish how the hyperactivity of GoF ADAMTS13 is manifest in experimental models mimicking the occlusive arterial thrombi present in acute ischaemic stroke. Methods The ability of GoF ADAMTS13 to dissolve VWF‐platelet agglutinates was examined using an assay of ristocetin‐induced platelet agglutination and in parallel flow models of arterial thrombosis. A murine model of focal ischaemia was used to assess the thrombolytic potential of GoF ADAMTS13. Results WT ADAMTS13 required conformational activation to attain full activity against VWF‐mediated platelet capture under flow. In this assay GoF ADAMTS13 had an EC50 value >5‐fold lower than wild type (WT) (0.73±0.21 nM and 3.81±0.97 nM, respectively). The proteolytic activity of GoF ADAMTS13 against pre‐formed platelet agglutinates under flow was enhanced >4‐fold compared to WT (EC50 values of 2.5±1.1 nM and 10.2±5.6 nM, respectively). In a murine stroke model GoF ADAMTS13 restored cerebral blood flow at a lower dose than WT ADAMTS13 and partially retained the ability to recanalise vessels when administration was delayed by 1 hour. Conclusion The limited proteolytic activity of WT ADAMTS13 in in vitro models of arterial thrombosis suggests an in vivo requirement for conformational activation. The enhanced activity of the GoF ADAMTS13 variant translates to a more pronounced protective effect in experimental stroke.

Summary 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.

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.