Role of fluid shear stress in regulating VWF structure, function and related blood disorders

Abstract

Von Willebrand factor (VWF) is the largest glycoprotein in blood. It plays a crucial role in primary hemostasis via its binding interaction with platelet and endothelial cell surface receptors, other blood proteins and extra-cellular matrix components. This protein is found as a series of repeat units that are disulfide bonded to form multimeric structures. Once in blood, the protein multimer distribution is dynamically regulated by fluid shear stress which has two opposing effects: it promotes the aggregation or self-association of multiple VWF units, and it simultaneously reduces multimer size by facilitating the force-dependent cleavage of the protein by various proteases, most notably ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type repeats, motif 1 type 13). In addition to these effects, fluid shear also controls the solution and substrate-immobilized structure of VWF, the nature of contact between blood platelets and substrates, and the biomechanics of the GpIbα-VWF bond. These features together regulate different physiological and pathological processes including normal hemostasis, arterial and venous thrombosis, von Willebrand disease, thrombotic thrombocytopenic purpura and acquired von Willebrand syndrome. This article discusses current knowledge of VWF structure-function relationships with emphasis on the effects of hydrodynamic shear, including rapid methods to estimate the nature and magnitude of these forces in selected conditions. It shows that observations made by many investigators using solution and substrate-based shearing devices can be reconciled upon considering the physical size of VWF and the applied mechanical force in these different geometries.

Keywords: ADAMTS-13; Hydrodynamic force; blood; flow chamber; platelet; rheology; thrombosis; von Willebrand factor.

Fig. 1.

Mature VWF. (A) Mature VWF is a multi-domain protein. It contains the A1, A2 and A3 domains that are flanked by two D-domain assemblies. These D-domains include the VWD, cysteine-8(C8)-fold, trypsin inhibitor-like (TIL) structure and E-modules. Following this, the protein contains six VW C-domains followed by the C-terminal cysteine knot (CTCk). Mature VWF includes 10 O-linked glycans depicted by open circle and 13 N-linked glycans depicted by filled circles. Thirteen percent of the N-glycans carry ABO(H) blood group determinants. VWF has a number of binding partners that are shown below the schematic. The A2-domain is also cleaved by the enzyme ADAMTS13. (B) Approximate representation of VWF based on existing structural knowledge. (C) Though the carbohydrates of VWF are usually represented by small lollipops, their physical dimension can be fairly large, in particular the N-linked glycans. (Colors are visible in the online version of the article; http://dx.doi.org/10.3233/BIR-15061.)

Fig. 2.

Role of shear stress in VWF related biology. Shear stress exerts force on multimeric VWF and causes structural changes in globular A1, A2 and A3 domains, allowing them to carry out their respective functions. Shear stress also regulates the binding of VWF to various plasma proteins and surface receptors on platelets, endothelial cells and even prokaryotic cells in circulation. In a number of different blood disorders, high shear stress either causes enhanced VWF proteolysis (bleeding disorders) or the formation of VWF-rich thrombi (thrombotic disorders). Finally, shear stress is important for VWF and platelet aggregation in physiology. Abbreviations: Shiga-toxin-hemolytic uremic syndrome (stx-HUS), thrombotic thrombocytopenic purpura (TTP), acquired von Willebrand syndrome (AvWS).

Fig. 3.

Similarity of geometry in biological particles of interest: (A)–(D) Biological particles of interest can be approximated as rigid dumbbells composed of two (un)equal spheres linked by a thin tether in many cases. (E)–(F) In other cases, these entities can be modeled as beads or disks attached to a wall by a string.

Fig. 4.

Domain level organization of ADAMTS-13 showing potential binding interactions with VWF. ADAMTS-13, like VWF, contains multiple subunits including a divalent-ion dependent metalloprotease domain (M) followed by a disintegrin-like (D), thrombospodin-1 repeat (TSP-1), cysteine-rich (C), spacer (S), seven more TSPs and two CUB domains. The protease has several VWF-binding exosites that bind both VWF-A2 and the D4-CK segment of VWF as indicated.