von Willebrand factor (VWF) is a large adhesive glycoprotein with established functions in hemostasis. It serves as a carrier for factor VIII and acts as a vascular damage sensor by attracting platelets to sites of vessel injury. VWF size is important for this latter function, with larger multimers being more hemostatically active. Functional imbalance in multimer size can variously cause microvascular thrombosis or bleeding. The regulation of VWF multimeric size and platelet-tethering function is carried out by ADAMTS13, a plasma metalloprotease that is constitutively active. Unusually, protease activity of ADAMTS13 is controlled not by natural inhibitors but by conformational changes in its substrate, which are induced when VWF is subject to elevated rheologic shear forces. This transforms VWF from a globular to an elongated protein. This conformational transformation unfolds the VWF A2 domain and reveals cryptic exosites as well as the scissile bond. To enable VWF proteolysis, ADAMTS13 makes multiple interactions that bring the protease to the substrate and position it to engage with the cleavage site as this becomes exposed by shear. This article reviews recent literature on the interaction between these 2 multidomain proteins and provides a summary model to explain proteolytic regulation of VWF by ADAMTS13. Many of the coordinated processes involved in hemostasis are driven by proteolytic reactions in which proteases cleave specific substrate bonds. Bond cleavage can lead to activation, propagation, or inactivation of a biochemical process. Control of proteolytic reactions is complex and, in its broadest sense, embraces localization of the protease and substrate, recruitment of cofactors for the purpose of acceleration of cleavage and then, direct or indirect inhibition of the protease to terminate its action. The plethora of proteolytic reactions in hemostasis provides examples of both general and process-specific mechanisms of control, which are all needed to ensure effective coordination. The proteolytic regulation of von Willebrand factor (VWF) function by its cleaving protease, ADAMTS13, falls into the latter category (ie, process-specific control), as it has certain unique features that set it apart from other hemostatic reactions. VWF is a large adhesive glycoprotein, necessary for initial platelet tethering and subsequent platelet adhesion. 1 It is a multiadhesive protein that can interact with cell surface, extracellular matrix, and plasma protein ligands through specific domain binding sites. [2][3][4][5][6] VWF is synthesized as a multimeric protein, which is central to its physiologic role. VWF function as a vessel wall damage sensor and initiator of primary hemostasis is highly dependent on its multimeric size. 7 The larger VWF multimers in plasma are the most hemostatically reactive not only because they contain more ligand binding sites, but also because they are more conformationally responsive to vascular shear forces. 1 In circulation, a first level of functional control of VWF is provided...

The multimeric size and the function of circulating von Willebrand factor are modulated via its proteolytic cleavage by the plasma metalloproteinase, AD-AMTS13. It is unclear how ADAMTS13 activity is regulated within the vascular system. In the absence of a regulatory mechanism, ADAMTS13 activity might compromise platelet adhesion at sites of vascular injury. We hypothesized that at sites of vascular injury, ADAMTS13 activity could be regulated locally by coagulation proteinases. Initiation of coagulation in human plasma resulted in the disappearance of added full-length recombinant ADAMTS13. This loss was inhibited by hirudin. Using purified proteins, we showed that ADAMTS13 is proteolyzed at several cleavage sites by thrombin in a time-and concentration-dependent manner. Furthermore, this proteolysis ablated ADAMTS13 activity against purified von Willebrand factor. Preincubation of thrombin with soluble thrombomodulin, but not heparin, inhibited the proteolysis of AD-AMTS13, suggesting the involvement of IntroductionVon Willebrand factor (VWF) is a large (2050 amino acid/ϳ250 kDa) multidomain glycoprotein whose functions are critical for normal hemostasis. 1 VWF has 2 principal roles that influence the hemostatic process: (1) it acts as a carrier protein for coagulation factor VIII 2 and (2) it mediates rapid adhesion of platelets to sites of vascular perturbation. The latter is one of the first hemostatic events following endothelial disruption and occurs through the specific binding of VWF to exposed subendothelial matrix proteins (principally collagen). 3 Once immobilized, VWF affinity for the glycoprotein (GP) Ib-IX-V receptor complex on the surface of circulating platelets is significantly enhanced. 4 This results in the tethering of platelets at sites of vascular damage and in the formation of a primary platelet plug. Tethered platelets are subsequently activated and expose phosphatidylserine-rich surfaces that are critical for efficient thrombin generation to occur. 5 VWF is constitutively secreted into the blood by endothelial cells as multimers of varying size that differ predominantly in the number of component VWF units. A significant proportion is also stored within Weibel-Palade bodies, predominantly as "ultra-large" multimers (UL-VWF) 6 that may exceed 2 ϫ 10 4 kDa. 1 This pool is released on demand in response to endothelial cell activation. 7 The properties of circulating VWF are, in part, dependent on its molecular size. Larger VWF multimers not only bind circulating platelets more readily than smaller forms, but also undergo marked conformational changes in response to the rheologic forces exerted by the circulating blood. 8 Under normal flowing conditions, VWF multimers circulate in a globular form. However, when VWF is exposed to increased shear forces, these molecules unravel into a "stringlike" conformation. This increases the number of exposed platelet/matrix binding sites and thus enhances the platelet tethering potential of the VWF molecule.Thrombotic thrombocytopenic purpura (TTP) is...

To define the role of the plasminogen activators (PAs) tissue PA (t-PA) and urokinase PA (u-PA) in vascular wound healing, neointima formation and reendothelialization were evaluated after electric or mechanical arterial injury in mice with a single or combined deficiency of t-PA (t-PA-/-) and/or u-PA (u-PA-/-). In both models, neointima formation and neointimal cell accumulation were reduced in u-PA-/- and in t-PA-/-/u-PA-/- arteries but not in t-PA-/- arteries. The electric injury model was used to characterize the underlying cellular mechanisms. Topographic analysis of vascular wound healing in electrically injured wild-type and t-PA-/- arteries revealed a similar degree of migration of smooth muscle cells from the noninjured borders into the necrotic center. In contrast, in u-PA-/- and t-PA-/-/u-PA-/- arteries, smooth muscle cells accumulated at the uninjured borders but failed to migrate into the necrotic center. Cultured u-PA-/- but not t-PA-/- smooth muscle cells also failed to migrate in vitro after scrape wounding. Proliferation of smooth muscle cells was not affected by PA deficiency. Reendothelialization after electric injury was similar in all genotypes. In situ analysis revealed markedly elevated u-PA zymographic activity, mRNA, and immunoreactivity in smooth muscle cells, endothelial cells, and leukocytes within 1 week after injury, eg, when cells migrated into the wound. Thus, u-PA plays a significant role in vascular wound healing and arterial neointima formation after injury, most likely by affecting cellular migration.