von Willebrand factor A1 domain: stuck in the middle

The well-being of any living organism requires a strict regulation of virtually every biological process to maintain its homeostatic balance. Such a regulation results from a delicate balance between activation pathways and pathways that antagonize these processes. The hemostatic system, designed to limit blood loss when the vessel wall is damaged, is not an exception in this regard; vascular injury provokes a cascade of events that ultimately produces a stabilized thrombus at the site of injury, while in parallel to these events a series of actions is initiated to prevent undesired occlusion of vessels elsewhere in the vasculature. One of the main players in hemostasis is von Willebrand factor (VWF), a multimeric protein that functions as a carrier protein for coagulation factor (F) VIII. In addition, VWF facilitates thrombus formation at sites of vascular injury by bridging platelets to the exposed subendothelial matrix, particular under conditions of high shear. Not unexpectedly, VWF–platelet interactions are controlled by various mechanisms, which are needed to prevent premature formation of platelet clumps that would occlude the vascular system. In this issue of the journal, Martin et al. [1] describe a novel pathway interfering with VWF–platelet interactions. Before giving further details of their work, we examine what we already know about the mechanisms that prevent premature VWFinduced platelet aggregation. Endothelial cells are the dominant source of circulating VWF. Mature VWF consists of repeating subunits with a distinct domain structure (D1-D2-D¢-D3-A1-A2-A3-B-C1-C2CK), which are assembled into multimers via covalent Cand N-terminal disulphide coupling. This process generates a heterologous pool of differentially sized multimers, which can be as large as 40-mers with a molecular weight of 10 million Dalton. When released from endothelial cells, VWF has the peculiar capacity to spontaneously bind platelets [2,3]. This may be beneficial under conditions of immediate need on damage to the endothelium and consequent blood loss. However, in some cases, the presence of circulating active, platelet-binding VWF is undesirable as it could provoke unwanted formation of platelet-rich thrombi that occlude the vasculature. This platelet-binding capacity of freshly released VWF originates from a subset of VWFmolecules that are qualified as being ultra or unusually large (UL) [4]. UL-VWF multimers form long platelet-binding strings (up to several millimeters) that are anchored at the cell surface following endothelial secretion [3]. While anchored and hence exposed to hydrodynamic forces, these VWF bundles are vulnerable substrates for the plasma protease ADAMTS-13 [3]. ADAMTS-13-mediated proteolysis results in smaller-sized multimers that have lost their ability to bind platelets spontaneously. In addition, it seems conceivable that proteolysis also induces molecular transformations that contribute to the encryption of the VWF platelet-binding site, which is located in its A1 domain. Indeed, in steady-state conditions, VWF and platelets coexist in the circulation without interacting, partially owing to the fact that the platelet-binding site is shielded by its adjacent D¢-D3 domains [5]. A misbalance in downregulating mechanisms may result in circulating active VWF, as is illustrated by the occurrence of thrombotic micro-angiopathy that is associated with ADAMTS-13 deficiency [6]. However, is our body only relying on the action of ADAMTS-13 to prevent undesired VWF-mediated platelet clumping? Probably not, because ADAMTS-13 deficiency alone is not always sufficient to provoke thrombotic complications, as is apparent from ADAMTS-13-deficient mice that (dependent on the mouse strain) express no phenotype and from ADAMTS-13-deficient patients who may remain symptom-free until adulthood [7,8]. Preliminary studies by one of us have revealed that plasma contains a potential inhibitor (i.e. b2-glycoprotein I), which interferes with Correspondence: Peter J. Lenting, Department of Clinical Chemistry and Haematology, Laboratory for Thrombosis and Haemostasis, G.03. 550, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands. Tel. : + 31 302507610; fax: + 31 302505418; e-mail: p.j.lenting@ umcutrecht.nl