Conformation Of Von Willebrand Factor Research Articles

The Variable Heavy Chain Antibody Syn-VWFA1 Inhibits the Interaction of Platelets to Von Willebrand Factor in Solution but Not to Immobilized Von Willebrand Factor Thereby Inhibiting 3-Dimensional but Not 2-Dimensional Thrombus Formation

Neutralizing the interaction between GP1bα and von Willebrand Factor (VWF) is an attractive strategy for treatment and prevention of platelet-VWF agglutinate formation in von Willebrand disease type 2B and thrombotic thrombocytopenic purpura. The aim of this study was to characterize a llama-derived variable heavy chain antibody (VHH) targeting the A1 domain of active VWF (Syn-VWFA1), in terms of its interference with platelet VWF interaction. Syn-VWFA1 (1.5 µM) abolished platelet binding to ristocetin-activated VWF (native and Haemate® P VWF) and R1306W VWF (VWF type 2B) in a flow-cytometric platelet function assay (Figure 1). Moreover, Syn-VWFA1 dose-dependently inhibited ristocetin-activated VWF and R1306W VWF platelet agglutination, but not collagen- and ADP-induced platelet aggregation. Under flow conditions, Syn-VWFA1 (1.8 µM) did not affect adhesion of platelets to collagen at high shear. In the shear- and collagen-dependent PFA-200, Syn-VWFA1 prolonged the closure time, but only at high concentrations (>3 µM), while GPIba binding to immobilized VWF remained unaffected by Syn-VWFA1. Syn-VWFA1 increased the cleavage of VWF multimers by ADAMTS13. Furthermore, by applying deuterium exchange, we found a small region in the β1 to α1 loop (1288-1293) in the A1 domain of VWF, which is likely to constitute the nanobody binding site (Figure 2) and binding of Syn-VWFA1 to the A1 domain results in a conformational change exposing regions in the α2 and α3 helices. In conclusion, Syn-VWFA1 dose-dependently blocks platelet-binding to activated VWF in solution, but not to (collagen-) immobilized active VWF. This implies that immobilization of VWF to collagen affects VWF conformation in such a way (1) that access of Syn-VWFA1 to its epitope in the A1 domain is hindered or (2) that conformational changes following Syn-VWFA1 binding are blocked, while allowing platelet binding. This implies that Syn-VWFA1 has potential treatment opportunities for patients with increased circulating active VWF, to prevent 3-dimensional thrombus formation (pathological platelet-VWF aggregate formation) but not 2-dimensional thrombus formation (leaving physiological hemostasis at sites of vascular injury intact).

Conformation of von Willebrand factor in shear flow revealed with stroboscopic single-molecule imaging

Abstractvon Willebrand factor (VWF) is a multimeric blood protein that acts as a mechanical probe, responding to changes in flow to initiate platelet plug formation. Previously, our laboratory tests had shown that using single-molecule imaging that shear stress can extend surface-tethered VWF, but paradoxically, we found that the required shear stress was higher than reported for free-in-flow VWF, an observation inconsistent with basic physical principles. To resolve this inconsistency critical to VWF’s molecular mechanism, we measured free-VWF extension in shear flow using pulsed laser stroboscopic imaging of single molecules. Here, laser pulses of different durations are used to capture multiple images of the same molecule within each frame, enabling accurate length measurements in the presence of motion blur. At high shear stresses, we observed a mean shift in VWF extension of <200 nm, much shorter than the multiple-micron extensions previously reported with no evidence for the predicted sharp globule-stretch conformational transition. Modeling VWF with a Brownian dynamics simulation, our results were consistent with VWF behaving as an uncollapsed polymer rather than the theorized compact ball. The muted response of free VWF to high shear rates implies that the tension experienced by free VWF in physiological shear flow is lower than indicated by previous reports and that tethering to platelets or the vessel wall is required to mechanically activate VWF adhesive function for primary hemostasis.

The Von Willebrand factor-ADAMTS-13 axis: a two-faced Janus in bleeding and thrombosis

Von Willebrand factor (VWF), a blood multimeric protein with a very high molecular weight, plays a crucial role in the primary hemostasis, the physiological process characterized by the adhesion of blood platelets to the injured vessel wall. Hydrodynamic forces are responsible for the VWF multimers conformational transitions from a globular to a stretched linear conformation. These characteristics render this protein a valuable object to be investigated by mechanochemistry, the biophysical chemistry branch that studies the effects of shear forces on protein conformation. This review will focus on the structural elements of the VWF molecule involved in the biochemical response to shear forces. The stretched VWF conformation favors the interaction with the platelet GpIb and at the same time with ADAMTS-13, the zinc-protease that cleaves VWF in the A2 domain, limiting its prothrombotic capacity. It is important to consider the level or the function of VWF or ADAMTS-13 always in relation each other, keeping in mind that in many thrombotic forms of microangiopathies the reduction of the ratio between the ADAMTS-13 activity and the VWF level (lower than 0.5) can be a valuable parameter to predict a real thrombotic risk. Hence, a significant increase in VWF level alone, even without any reduction of ADAMTS-13 concentration, would still be responsible for a significant reduction of the ADAMTS-13/VWF ratio, which ultimately could reflect or predict a prothrombotic risk. Future studies will have to validate the concept whether ADAMTS-13/VWF ratio could a valuable and reliable biomarker to predict or confirm the presence of thrombotic risk in several morbid conditions.

Osteoprotegerin modulates platelet adhesion to von Willebrand factor during release from endothelial cells

BackgroundPlatelet‐binding Von Willebrand Factor (VWF) strings assemble upon stimulated secretion from endothelial cells. ObjectivesTo investigate the efficiency of platelet binding to multi‐molecular VWF bundles secreted from endothelial cells and to investigate the role of osteoprotegerin, a protein located in Weibel‐Palade bodies that interacts with the VWF platelet binding domain. MethodsThe nanobody VWF/AU‐a11 that specifically binds to VWF in its active platelet‐binding conformation was used to investigate the conformation of VWF. ResultsUpon stimulated secretion from endothelial cells, VWF strings were only partially covered with platelets, while a VWD‐type 2B mutation or ristocetin enhanced platelet binding by 2–3‐fold. Osteoprotegrin, reduces platelet adhesion to VWF by 40% ± 18% in perfusion assays. siRNA‐mediated down‐regulation of endothelial osteoprotegerin expression resulted in a 1.8‐fold increase in platelet adhesion to VWF strings.Upon viral infection, there is a concordant rise in VWF and osteoprotegerin plasma levels. Unexpectedly, no such increase was observed in plasma of desmopressin‐treated hemophilia A‐patients. In a mouse model, osteoprotegerin expression was low in liver endothelial cells of vehicle‐treated mice, and concanavalin A‐treatment increased VWF and osteoprotegerin expression 4‐ and 40‐fold, respectively. This increase was translated in a 30‐fold increased osteoprotegerin/VWF ratio in plasma. ConclusionsRelease of VWF from endothelial cells opens the platelet‐binding site, irrespective of the presence of flow. However, not all available platelet‐binding sites are being occupied, suggesting some extent of regulation. Part of this regulation involves endothelial proteins that are co‐secreted with VWF, like osteoprotegerin. This regulatory mechanism may be of more relevance under inflammatory conditions.

Von Willebrand Factor A1 Domain Binds to Clusters II and IV of Low-Density Lipoprotein Receptor-Related Protein 1

Introduction. Von Willebrand factor (VWF) is a large multimeric plasma glycoprotein involved in hemostasis and thrombosis. Genetic and in vivo studies have identified low-density lipoprotein receptor-related protein 1 (LRP) as the major clearance receptor of VWF. The ligand-binding moiety of LRP is represented by complement-type repeats grouped into four clusters, among which clusters II and IV are prominent in ligand binding. In in vitro studies under shear stress conditions, VWF acquires an unfolded conformation and interacts with LRP. More than 30 mutations of VWF have been reported to associate with its increased plasma clearance and result in type 2B von Willebrand disease. The majority of these mutations are located within the VWF A1 domain (A1), which is in agreement with previous reports that isolated A1 binds to LRP. However, the interactive site of LRP has not been identified.Objective. We aim to characterize the LRP binding site for VWF by investigating the interaction of individual LRP clusters II-IV to different A1 variants.Experimental Approach. An A1-containing large N-terminal homo-dimeric proteolytic fragment of VWF (SPIII) was produced upon its proteolytic digestion by V8-protease. Alternatively, we generated three variants of recombinant A1: (i) A1(residues 1261-1478), (ii) a VWD type 2B mutant, A1(V1316M) (residues 1261-1478) and (iii) an N-terminus truncated form, (residues 1269-1478). These proteins were tested for binding to immobilized LRP, purified from human placenta, and recombinant LRP clusters II and IV by surface plasmon resonance.Results. We did not find any interaction between the SPIII proteolytic fragment and LRP, suggesting that this fragment may still preserve a folded conformation of VWF that masked its LRP-binding site. In turn, all three forms of recombinant A1 were found to bind LRP and its clusters II and IV. Notably, A1(V1316M) and A1(1269-1478) had higher affinity for LRP and its clusters than A1. This result is consistent with the enhanced clearance of the VWF(V1316M) in vivo and better exposure of the A1 site in a truncated form of A1 to the platelet receptor GpIb-alpha reported previously.Conclusions. Our data demonstrate that LRP has at least two sites for binding to VWF, which are located within clusters II and IV. This finding is in accordance with previous observations that LRP clusters II and IV can serve as “functional duplicates” for binding selected ligands. DisclosuresNo relevant conflicts of interest to declare.

Conformation of the von Willebrand factor/factor VIII complex in quasi-static flow

Von Willebrand factor (VWF) is a plasma glycoprotein that circulates noncovalently bound to blood coagulation factor VIII (fVIII). VWF is a population of multimers composed of a variable number of ∼280 kDa monomers that is activated in shear flow to bind collagen and platelet glycoprotein Ibα. Electron microscopy, atomic force microscopy, small-angle neutron scattering, and theoretical studies have produced a model in which the conformation of VWF under static conditions is a compact, globular “ball-of-yarn,” implying strong, attractive forces between monomers. We performed sedimentation velocity (SV) analytical ultracentrifugation measurements on unfractionated VWF/fVIII complexes. There was a 20% per mg/ml decrease in the weight-average sedimentation coefficient, sw, in contrast to the ∼1% per mg/ml decrease observed for compact globular proteins. SV and dynamic light scattering measurements were performed on VWF/fVIII complexes fractionated by size-exclusion chromatography to obtain sw values and z-average diffusion coefficients, Dz. Molecular weights estimated using these values in the Svedberg equation ranged from 1.7 to 4.1 MDa. Frictional ratios calculated from Dz and molecular weights ranged from 2.9 to 3.4, in contrast to values of 1.1–1.3 observed for globular proteins. The Mark–Houwink–Kuhn–Sakurada scaling relationships between sw, Dz and molecular weight, and , yielded estimates of 0.51 and –0.49 for as and aD, respectively, consistent with a random coil, in contrast to the as value of 0.65 observed for globular proteins. These results indicate that interactions between monomers are weak or nonexistent and that activation of VWF is intramonomeric.

Targeting shear gradient activated von Willebrand factor by the novel single-chain antibody A1 reduces occlusive thrombus formation in vitro.

Intraluminal thrombus formation precipitates conditions such as acute myocardial infarction and disturbs local blood flow resulting in areas of rapidly changing blood flow velocities and steep gradients of blood shear rate. Shear rate gradients are known to be pro-thrombotic with an important role for the shear-sensitive plasma protein von Willebrand factor (VWF). Here, we developed a single-chain antibody (scFv) that targets a shear gradient specific conformation of VWF to specifically inhibit platelet adhesion at sites of shear rate gradients (SRG) but not in areas of constant shear. Microfluidic flow channels with stenotic segments were used to create SRG during blood perfusion. VWF-GPIbα interactions were increased at sites of SRG compared to constant shear rate of matched magnitude. The scFv-A1 specifically reduced VWF-GPIbα binding and thrombus formation at sites of SRG but did not block platelet deposition and aggregation under constant shear rate in upstream sections of the channels. Significantly, the scFv A1 attenuated platelet aggregation only in the later stages of thrombus formation. In the absence of shear, direct binding of scFv-A1 to VWF could not be detected and scFVA1 did not inhibit ristocetin induced platelet agglutination. We have exploited the pro-aggregatory effects of SRG on VWF dependent platelet aggregation and developed the shear gradient-sensitive scFv-A1 antibody that inhibits platelet aggregation exclusively at sites of SRG. The lack of VWF inhibition in non-stenosed vessel segments places scFV-A1 in an entirely new class of anti-platelet therapy for selective blockade of pathological thrombus formation while maintaining normal hemostasis.

Mechanochemistry of von Willebrand factor.

Von Willebrand factor (VWF), a blood multimeric protein with a very high molecular weight, plays a crucial role in the primary haemostasis, the physiological process characterized by the adhesion of blood platelets to the injured vessel wall. Hydrodynamic forces are responsible for extensive conformational transitions in the VWF multimers that change their structure from a globular form to a stretched linear conformation. This feature makes this protein particularly prone to be investigated by mechanochemistry, the branch of the biophysical chemistry devoted to investigating the effects of shear forces on protein conformation. This review describes the structural elements of the VWF molecule involved in the biochemical response to shear forces. The stretched VWF conformation favors the interaction with the platelet GpIb and at the same time with ADAMTS-13, the zinc-protease that cleaves VWF in the A2 domain, limiting its prothrombotic capacity. The shear-induced conformational transitions favor also a process of self-aggregation, responsible for the formation of a spider-web like network, particularly efficient in the trapping process of flowing platelets. The investigation of the biophysical effects of shear forces on VWF conformation contributes to unraveling the molecular mechanisms of many types of thrombotic and haemorrhagic syndromes.

Analytical characterization and reference interval of an enzyme-linked immunosorbent assay for active von Willebrand factor.

BackgroundInteraction of von Willebrand factor (VWF) with platelets requires a conformational change that exposes an epitope within the VWF A1 domain, enabling platelet glycoprotein Ibα binding. Quantification of this ‘‘active” conformation of VWF has been shown to provide pathophysiological insight into conditions characterized by excessive VWF-platelet interaction.MethodsWe developed an immunosorbent assay based on a variable heavy chain antibody fragment against the VWF A1 domain as a capture antibody. Assay performance in terms of specificity (binding to active R1306W- and sheared VWF), precision, accuracy, linearity, limits of detection and stability were determined. Active VWF, VWF antigen, VWF ristocetin cofactor activity, VWF:GP1bM and VWF propeptide were measured in citrated plasma and platelet-VWF binding in whole blood from 120 healthy individuals to establish a reference interval for active VWF and to assess associations with other VWF parameters.ResultsIntra- and inter-assay CVs were between 2.4–7.2% and 4.1–9.4%, depending on the level. Mean recovery of spiked recombinant R1306W VWF was 103±3%. The assay was linear in the range of 90.1–424.5% and had a limit of quantification of 101%. The reference interval for active VWF was 91.6–154.8% of NPP. Significant, positive correlations between active VWF and all other VWF parameters were found, with the strongest correlation with VWF:GP1bM binding.ConclusionsWe developed and validated an immunosorbent assay for the accurate detection of active VWF levels in plasma. The assay fulfilled all analytical criteria in this study and a reference interval was established, allowing its use to quantify active VWF in pathological conditions for future research.

Von Willebrand Factor for Aortic Valve Intervention: From Bench to Real-Time Bedside Assessment.

VWF (von Willebrand factor ) is a circulating multimeric blood glycoprotein. VWF plays a major role in primary hemostasis by promoting the adhesion of platelets to subendothelial collagen at sites of vascular damage and thereby promoting platelet aggregation. VWF is synthesized in endothelial cells and megakaryocytes. The VWF units dimerize and are transported into the Golgi apparatus, where disulfide bonds are formed leading to formation of VWF multimers. This large subunit combination is required to support its hemostatic function. VWF has the unique features to be circulating in an inactive coiled form, hiding binding domains for platelet receptors and subendothelial collagen. At the site of vascular injury, VWF binds to the exposed collagen and unfolds. Once VWF is unfolded, the VWF A1 domain is exposed allowing the binding of platelets via GP Ib (glycoprotein IB) receptor. After platelet activation, GP IIb/IIIa (glycoprotein IIb/IIIa) receptor becomes able to bind VWF C1 domain. The VWF conformation and activity is intimately related to shear conditions and blood flow. At high shear rate (beyond 10–15 pN), it becomes unfolded exposing binding sites but also the cleavage site in VWF A2 domain for ADAMTS13 (adisintegrin-like and metalloprotease thrombospondin) protease that conducts to its proteolysis. Overall, these environmental changes generate the modification of the conformation of VWF …

Catching platelets from the bloodflow: the role of the conformation of von Willebrand factor

The mechanics of platelet initial adhesion due to interactions between GPIb receptor with von Willebrand factor (vWf) multimers is essential for thrombus growth and the regulation of this process. Multimeric structure of vWf is known to make adhesion sensitive to the hydrodynamic conditions, providing intensive platelet aggregation in bulk fluid for high shear rates. But it is still unclear how it affects the dynamics of platelet motion near vessel walls and efficiency of their adhesion to surfaces. Our goal is to resolve the principal issues in the mechanics of platelet initial attachmentviaGPIb-vWf bonds in near-wall flow conditions: when the platelet tends to roll or slide and how this dynamics depends on the size, conformation and adhesive properties of the vWf multimers. We employ a 3D computer model based on a combination of the Lattice Boltzmann method with mesoscopic particle dynamics for explicit simulation of vWf-mediated blood platelet adhesion in shear flow. Our results reveal the link between the mechanics of platelet initial adhesion and the physico-chemical properties of vWf multimers. This has implications in further theoretical investigation of thrombus growth dynamics, as well as the interpretation of in vitro experimental data.

Margination and stretching of von Willebrand factor in the blood stream enable adhesion

The protein von Willebrand factor (VWF) is essential in primary hemostasis, as it mediates platelet adhesion to vessel walls. VWF retains its compact (globule-like) shape in equilibrium due to internal molecular associations, but is able to stretch when a high enough shear stress is applied. Even though the shear-flow sensitivity of VWF conformation is well accepted, the behavior of VWF under realistic blood flow conditions remains poorly understood. We perform mesoscopic numerical simulations together with microfluidic experiments in order to characterize VWF behavior in blood flow for a wide range of flow-rate and hematocrit conditions. In particular, our results demonstrate that the compact shape of VWF is important for its migration (or margination) toward vessel walls and that VWF stretches primarily in a near-wall region in blood flow making its adhesion possible. Our results show that VWF is a highly optimized protein in terms of its size and internal associations which are necessary to achieve its vital function. A better understanding of the relevant mechanisms for VWF behavior in microcirculation provides a further step toward the elucidation of the role of mutations in various VWF-related diseases.

Single-Molecule Activation by Flow: Force-Induced Conformational Transitions Regulate the Ligand Binding Affinity of a Large, Polymeric Mechanosensor Protein

Mechanical force is a critical regulator for physiological and pathophysiological processes. von Willebrand factor (VWF), a large, polymeric blood glycoprotein, is proposed to elongate in response to hydrodynamic forces in hemorrhage and become activated to bind to the vessel wall and aggregate platelets to form a plug. To provide direct evidence for this hypothesis, we simultaneously visualized the effect of hydrodynamic force on VWF conformation and the binding of its ligand, the platelet membrane receptor GPIbα.Single VWF concatemers were surface-immobilized and subjected to shear flow in a microfluidic device with rapid air pressure-actuated changes in flow. Using dual-color VWF and GPIbα fluorescence, dynamics of VWF elongation and binding of GPIbα were recorded in real time with total internal reflection microscopy. Increase in shear flow induced a rapid conformational transition in VWF from a compact to an elongated form. In alternating stop-flow and start-flow cycles, dynamics were measured of VWF re-compaction and re-extension, which exhibited kinetic hysteresis during re-compaction. GPIbα rapidly bound to flow-stretched VWF and rapidly dissociated once VWF collapsed, suggesting that elongation of VWF exposed and/or increased the affinity of its binding sites for GPIbα. GPIbα binding affinity increased with increasing shear stress, and along the length of extended VWF molecules, GPIbα localization was enriched in the region of VWF subjected to higher forces, i.e. in upstream regions. The VWF concatemer and GPIbα dissociation constant was also measured under shear flow.The results provide direct evidence for the hypothesis that increased hydrodynamic shear induces a conformational elongation in VWF, exposing high-affinity, shear-dependent binding sites for platelet GPIbα. This dynamic, force-driven mechanism enables rapid and precise regulation of hemostasis, providing a paradigm for mechanosensory control of receptor-ligand binding affinity.

Force Sensing by the Vascular Protein Von Willebrand Factor is Tuned by a Strong Intermonomer Interaction

The large plasma glycoprotein von Willebrand factor (VWF) senses hydrodynamic forces in the blood stream and responds to elevated forces with abrupt elongation, thereby increasing its adhesiveness to platelets and collagen. Remarkably, forces on VWF are elevated at sites of vascular injury, where VWF's hemostatic potential is important to mediate platelet aggregation and to recruit platelets to the subendothelial layer. Adversely, elevated forces in stenosed vessels lead to an increased risk of VWF-mediated thrombosis. To dissect the remarkable force sensing ability of VWF, we have performed atomic force microscopy (AFM)-based single-molecule force measurements on dimers, the smallest repeating subunits of VWF multimers. We have identified a strong intermonomer interaction that involves the D4 domain and critically depends on the presence of divalent ions. Dissociation of this strong interaction occurred at forces above ∼50 pN and provided ∼80 nm of additional length to the elongation of dimers. Corroborated by the static conformation of VWF, visualized by AFM imaging, we estimate that under physiological conditions approximately half of VWF's constituent dimers are firmly closed via the strong intermonomer interaction. As firmly closed dimers drastically shorten VWF's effective length contributing to force sensing, they can be expected to markedly tune VWF's sensitivity to hydrodynamic flow in the blood and to thereby significantly affect VWF's function in hemostasis and thrombosis.

Force sensing by the vascular protein von Willebrand factor is tuned by a strong intermonomer interaction

The large plasma glycoprotein von Willebrand factor (VWF) senses hydrodynamic forces in the bloodstream and responds to elevated forces with abrupt elongation, thereby increasing its adhesiveness to platelets and collagen. Remarkably, forces on VWF are elevated at sites of vascular injury, where VWF's hemostatic potential is important to mediate platelet aggregation and to recruit platelets to the subendothelial layer. Adversely, elevated forces in stenosed vessels lead to an increased risk of VWF-mediated thrombosis. To dissect the remarkable force-sensing ability of VWF, we have performed atomic force microscopy (AFM)-based single-molecule force measurements on dimers, the smallest repeating subunits of VWF multimers. We have identified a strong intermonomer interaction that involves the D4 domain and critically depends on the presence of divalent ions, consistent with results from small-angle X-ray scattering (SAXS). Dissociation of this strong interaction occurred at forces above [Formula: see text]50 pN and provided [Formula: see text]80 nm of additional length to the elongation of dimers. Corroborated by the static conformation of VWF, visualized by AFM imaging, we estimate that in VWF multimers approximately one-half of the constituent dimers are firmly closed via the strong intermonomer interaction. As firmly closed dimers markedly shorten VWF's effective length contributing to force sensing, they can be expected to tune VWF's sensitivity to hydrodynamic flow in the blood and to thereby significantly affect VWF's function in hemostasis and thrombosis.

Shear-stress-induced conformational changes of von Willebrand factor in a water-glycerol mixture observed with single molecule microscopy.

The von Willebrand factor (VWF) is a human plasma protein that plays a key role in the initiation of the formation of thrombi under high shear stress in both normal and pathological situations. It is believed that VWF undergoes a conformational transition from a compacted, globular to an extended form at high shear stress. In this paper, we develop and employ an approach to visualize the large-scale conformation of VWF in a (pressure-driven) Poiseuille flow of water-glycerol buffers with wide-field single molecule fluorescence microscopy as a function of shear stress. Comparison of the imaging results for VWF with the results of a control with λ-phage double-stranded DNA shows that the detection of individual VWF multimers in flow is feasible. A small fraction of VWF multimers are observed as visibly extended along one axis up to lengths of 2.0 μm at high applied shear stresses. The size of this fraction of molecules seems to exhibit an apparent dependency on shear stress. We further demonstrate that the obtained results are independent of the charge of the fluorophore used to label VWF. The obtained results support the hypothesis of the conformational extension of VWF in shear flow.

The type 2B p.R1306W natural mutation of von Willebrand factor dramatically enhances the multimer sensitivity to shear stress

Shear stress triggers conformational stretching of von Willebrand factor (VWF), which is responsible for its self-association and binding to the platelet receptor glycoprotein (GP)Ibα. This phenomenon supports primary hemostasis under flow. Type2B VWF natural mutants are considered to have increased affinity for platelet GPIbα. To assess the mechanism responsible for the enhanced interaction of the p.R1306W VWF mutant with the platelet receptor. The interaction of GPIbα with wild-type (WT) and p.R1306W VWF multimers and A1-A2-A3 constructs was investigated with surface plasmon resonance spectroscopy. Analysis of the static VWF conformation in solution was performed with dynamic light scattering spectroscopy. The shear stress-induced self-association of VWF multimers was investigated with atomic force microscopy (AFM) over a 0-60dyncm(-2) range. WT VWF did not interact with GPIbα under static conditions, whereas the mutant at ~2μgmL(-1) already bound to the receptor. By contrast, the WT and p.R1306W-A1-A2-A3 constructs showed comparable affinities for GPIbα (Kd ~20nm). The hydrodynamic diameter of resting R1306W VWF multimers was significantly greater than that of the wild type (210±60nm vs. 87±22nm). At shear forces of <14dyncm(-2) , the p.R1306W multimers rapidly changed conformation, entering a regime of self-aggregation, which, in contrast, was induced for WT VWF by shear forces of >30dyncm(-2) . Mechanical stretching AFM experiments showed that p.R1306W multimers needed less energy per length unit (~10pN) to be stretched than the WT protein. The increased affinity of p.R1306W VWF for GPIbα arises mostly from higher sensitivity to shear stress, which facilitates exposure of GPIbα binding sites.

Elucidating Hydrodynamic Force Regulation of Von Willebrand Factor in Hemostasis

Von Willebrand factor (VWF) is a multimeric plasma glycoprotein that plays a pivotal role in hemostasis - it serves as a ligand for platelet adhesion and aggregation forming a hemostatic plug. The hemostatic potential of VWF is regulated by hydrodynamic forces arising in the circulation. Hydrodynamic shear at sites of bleeding activate the binding of VWF to platelet receptor GpIb. Hydrodynamic forces in blood flow also attenuated thrombosis promoting ultralarge VWF by activating cleavage in the VWF A2 domain by the metalloprotease ADAMTS13. Here, we investigated how mechanical forces, which occur in the circulation, regulate the structural conformation and function of VWF at the molecular level using a combination single-molecule manipulation and computational modeling. We created various hydrodynamic flows to mimic a wide range of blood flow conditions, while directly visualizing the conformational dynamics of single VWF molecules. First, we developed a assay that uses a microfluidic device with a cross-slot geometry to study VWF under elongational flow and developed a custom “shear wheel” microscope to study VWF under shear flow. These techniques will enable us to understand how hydrodynamic forces in the bloodstream act on VWF to cause quaternary unfolding, and how this in turn regulates adhesive activity. Second, we developed a kinetic model of force-induced enzymatic cleavage of A2 by ADAMTS13. Our simulations, using parameters from single-molecule force experiments, predict the known healthy size distribution of VWF. We also found that calcium ions protects health-sized VWF from cleavage, and that A2 mutations related to the heritable bleeding disorder von Willebrand disease (VWD) shift hemostatic potential by destabilizing the domain structure under force. These newly developed experimental techniques and findings will lead to a better understanding of bleeding disorders and help progress the therapeutic development of VWD.

Vimentin Exposed On Platelets Serves As an Adhesive Receptor for Von Willebrand Factor

Abstract 259Platelet adhesion, activation, and aggregation in the vasculature are necessary events in both life-saving hemostasis and pathological thrombosis. Thrombosis may occur in patients presenting with several clinical conditions including atherosclerosis, cardiovascular disease, and inflammation. Von Willebrand factor (VWF) is a multimeric plasma glycoprotein that plays a critical role in mediating platelet adhesion, activation, and aggregation on the exposed subendothelium in order to maintain hemostasis under arterial flow conditions. On the other hand, VWF permits the stabilization of platelets adherent to components of ruptured atherosclerotic plaques, leading to artery-occluding thrombus formation. The initial interaction of activated or hyperadhesive VWF with platelets occurs via the interaction between the A1 domain of VWF and the platelet receptor glycoprotein (GP)Ibα. This engagement is responsible for reducing the velocity of rapidly flowing platelets, allowing the rolling platelets to interact with the second binding site on VWF for the platelet receptor GPIIb/IIIa; a binding site that is located within the C domains of VWF. Therefore, the hyperadhesive property of VWF apparently relies on the synchronized interaction of the two platelet surface receptors, GPIbα and GPIIb/IIIa. Despite this concept, we and others have speculated that other binding site in VWF synergistically works with the A1 domain to quickly capture the extremely fast flowing platelets. We have obtained interesting results from studies using a monomeric A1A2A3 domain protein that lacks the binding site for GPIIb/IIIa. For example, the rolling velocity of platelets over an A1A2A3-coated surface was markedly lower than that seen with use of the single A1 domain. This observation suggests the possibility of an additional binding site in the A domains for platelets. Given the similar hyperadhesive features of the A1A2A3 protein and plasma VWF, we proposed to look for a potential receptor on platelets with a recognition site within the A domains of VWF. We suggested examining vimentin because, it was identified as a binding protein for the isolated A2 domain of VWF in our laboratory, and vimentin has been found on the surface of platelets. First, both full length VWF and recombinant A1A2A3 proteins efficiently bound to human vimentin only in the presence of the modulator ristocetin, indicating that vimentin preferably interact with the active conformation of VWF. In fact, a constitutively active A1A2A3 protein (containing a gain-of-function mutation in A1 domain) had a binding activity for vimentin higher than that of wild type (WT) A1A2A3 in the absence of ristocetin. Second, anti-vimentin monoclonal antibody blocked the interaction of that mutant A1A2A3 to activated washed platelets using flow cytometry. Third, we then examined the effect of anti-vimentin antibody on flow-dependent platelet adhesion to A1A2A3-coated surface at high shear stress. In comparison to whole blood incubated with irrelevant IgG molecule as a negative control, the anti-vimentin antibody blocked 75% platelet adhesion to the triple-A domain protein. Finally, whole blood from vimentin-deficient or WT mice was perfused over a surface coated with murine VWF at high shear rate. In comparison to platelets from WT mice, vimentin-deficient platelets had a significant reduced platelet adhesion to VWF (25% of WT). Similarly, vimentin-deficient platelets had a reduced platelet adhesion to collagen (20% of WT) under high flow conditions. This platelet-collagen interaction is initially mediated by VWF. These interesting results indicate that vimentin on platelets serves as a receptor for VWF, and this binding may participate in the initial interaction of circulating platelets with VWF under flow conditions. Disclosures:No relevant conflicts of interest to declare.

Light Chain of Factor VIII Is Sufficient for Accelerating Cleavage of von Willebrand Factor by ADAMTS13 Metalloprotease

We previously demonstrated that coagulation factor VIII (FVIII) accelerates proteolytic cleavage of von Willebrand factor (VWF) by A disintegrin and metalloprotease with thrombospondin type 1 repeats (ADAMTS13) under fluid shear stress. In this study, the structural elements of FVIII required for the rate-enhancing effect and the biological relevance of this cofactor activity are determined using a murine model. An isolated light chain of human FVIII (hFVIII-LC) increases proteolytic cleavage of VWF by ADAMTS13 under shear in a concentration-dependent manner. The maximal rate-enhancing effect of hFVIII-LC is ∼8-fold, which is comparable with human full-length FVIII and B-domain deleted FVIII (hFVIII-BDD). The heavy chain (hFVIII-HC) and the light chain lacking the acidic (a3) region (hFVIII-LCΔa3) have no effect in accelerating VWF proteolysis by ADAMTS13 under the same conditions. Although recombinant hFVIII-HC and hFVIII-LCΔa3 do not detectably bind immobilized VWF, recombinant hFVIII-LC binds VWF with high affinity (K(D), ∼15 nM). Moreover, ultra-large VWF multimers accumulate in the plasma of fVIII(-/-) mice after hydrodynamic challenge but not in those reconstituted with either hFVIII-BDD or hFVIII-LC. These results suggest that the light chain of FVIII, which is not biologically active for clot formation, is sufficient for accelerating proteolytic cleavage of VWF by ADAMTS13 under fluid shear stress and (patho) physiological conditions. Our findings provide novel insight into the molecular mechanism of how FVIII regulates VWF homeostasis.