Exponential size distribution of von Willebrand factor

The interplay between adsorption and aggregation of von Willebrand factor chains in shear flows

Von Willebrand factor (VWF) is a giant extracellular glycoprotein that carries out a key adhesive function during primary hemostasis. Upon vascular injury and triggered by the shear of flowing blood, VWF establishes specific interactions with several molecular partners in order to anchor platelets to collagen on the exposed subendothelial surface. VWF also interacts with itself to form aggregates that, adsorbed on the surface, provide more anchor sites for the platelets. However, the interplay between elongation and subsequent exposure of cryptic binding sites, self-association, and adsorption on the surface remained unclear for VWF. In particular, the role of shear flow in these three processes is not well understood. In this study, we address these questions by using Brownian dynamics simulations at a coarse-grained level of resolution. We considered a system consisting of multiple VWF-like self-interacting chains that also interact with a surface under a shear flow. By a systematic analysis, we reveal that chain-chain and chain-surface interactions coexist nontrivially to modulate the spontaneous adsorption of VWF and the posterior immobilization of secondary tethered chains. Accordingly, these interactions tune VWF's extension and its propensity to form shear-assisted functional adsorbed aggregates. Our data highlight the collective behavior VWF self-interacting chains have when bound to the surface, distinct from that of isolated or flowing chains. Furthermore, we show that the extension and the exposure to solvent have a similar dependence on shear flow, at a VWF-monomer level of resolution. Overall, our results highlight the complex interplay that exists between adsorption, cohesion, and shear forces and their relevance for the adhesive hemostatic function of VWF.

von Willebrand factor unfolding mediates platelet deposition in a model of high-shear thrombosis

Thrombosis under high-shear conditions is mediated by the mechanosensitive blood glycoprotein von Willebrand factor (vWF). vWF unfolds in response to strong flow gradients and facilitates rapid recruitment of platelets in flowing blood. While the thrombogenic effect of vWF is well recognized, its conformational response in complex flows has largely been omitted from numerical models of thrombosis. We recently presented a continuum model for the unfolding of vWF, where we represented vWF transport and its flow-induced conformational change using convection-diffusion-reaction equations. Here, we incorporate the vWF component into our multi-constituent model of thrombosis, where the local concentration of stretched vWF amplifies the deposition rate of free-flowing platelets and reduces the shear cleaning of deposited platelets. We validate the model using three benchmarks: in vitro model of atherothrombosis, a stagnation point flow, and the PFA-100, a clinical blood test commonly used for screening for von Willebrand disease (vWD). The simulations reproduced the key aspects of vWF-mediated thrombosis observed in these experiments, such as the thrombus location, thrombus growth dynamics, and the effect of blocking platelet-vWF interactions. The PFA-100 simulations closely matched the reported occlusion times for normal blood and several hemostatic deficiencies, namely, thrombocytopenia, vWD type 1, and vWD type 3. Overall, this multi-constituent model of thrombosis enables macro-scale 3D simulations of thrombus formation in complex geometries over a wide range of shear rates and accounts for qualitative and quantitative hemostatic deficiencies in patient blood.

SIPA in 10 milliseconds: VWF tentacles agglomerate and capture platelets under high shear

Shear-induced platelet aggregation (SIPA) occurs under elevated shear rates (10 000 s-1) found in stenotic coronary and carotid arteries. The pathologically high shear environment can lead to occlusive thrombosis by SIPA from the interaction of nonactivated platelets and von Willebrand factor (VWF) via glycoprotein Ib-A1 binding. This process under high shear rates is difficult to visualize experimentally with concurrent molecular- and cellular-resolutions. To understand this fast bonding, we employ a validated multiscale in silico model incorporating measured molecular kinetics and a thrombosis-on-a-chip device to delineate the flow-mediated biophysics of VWF and platelets assembly into mural microthrombi. We show that SIPA begins with VWF elongation, followed by agglomeration of platelets in the flow by soluble VWF entanglement before mural capture of the agglomerate by immobilized VWF. The entire SIPA process occurs on the order of 10 milliseconds with the agglomerate traveling a lag distance of a few hundred microns before capture, matching in vitro results. Increasing soluble VWF concentration by ∼20 times in silico leads to a ∼2 to 3 times increase in SIPA rates, matching the increase in occlusion rates found in vitro. The morphology of mural aggregates is primarily controlled by VWF molecular weight (length), where normal-length VWF leads to cluster or elongated aggregates and ultra-long VWF leads to loose aggregates seen by others' experiments. Finally, we present phase diagrams of SIPA, which provides biomechanistic rationales for a variety of thrombotic and hemostatic events in terms of platelet agglomeration and capture.

Exposure of Von Willebrand Factor Cleavage Site in A1A2A3-Fragment under Extreme Hydrodynamic Shear

Von Willebrand Factor (vWf) is a giant multimeric extracellular blood plasma involved in hemostasis. In this work we present multi-scale simulations of its three-domains fragment A1A2A3. These three domains are essential for the functional regulation of vWf. Namely the A2 domain hosts the site where the protease ADAMTS13 cleavages the multimeric vWf allowing for its length control that prevents thrombotic conditions. The exposure of the cleavage site follows the elongation/unfolding of the domain that is caused by an increased shear stress in blood. By deploying Lattice Boltzmann molecular dynamics simulations based on the OPEP coarse-grained model for proteins, we investigated at molecular level the unfolding of the A2 domain under the action of a perturbing shear flow. We described the structural steps of this unfolding that mainly concerns the β-strand structures of the domain, and we compared the process occurring under shear with that produced by the action of a directional pulling force, a typical condition of single molecule experiments. We observe, that under the action of shear flow, the competition among the elongational and rotational components of the fluid field leads to a complex behaviour of the domain, where elongated structures can be followed by partially collapsed melted globule structures with a very different degree of exposure of the cleavage site. Our simulations pose the base for the development of a multi-scale in-silico description of vWf dynamics and functionality in physiological conditions, including high resolution details for molecular relevant events, e.g., the binding to platelets and collagen during coagulation or thrombosis.

A Continuum Model for the Unfolding of von Willebrand Factor

von Willebrand Factor is a mechano-sensitive protein circulating in blood that mediates platelet adhesion to subendothelial collagen and platelet aggregation at high shear rates. Its hemostatic function and thrombogenic effect, as well as susceptibility to enzymatic cleavage, are regulated by a conformational change from a collapsed globular state to a stretched state. Therefore, it is essential to account for the conformation of the vWF multimers when modeling vWF-mediated thrombosis or vWF degradation. We introduce a continuum model of vWF unfolding that is developed within the framework of our multi-constituent model of platelet-mediated thrombosis. The model considers two interconvertible vWF species corresponding to the collapsed and stretched conformational states. vWF unfolding takes place via two regimes: tumbling in simple shear and strong unfolding in flows with dominant extensional component. These two regimes were demonstrated in a Couette flow between parallel plates and an extensional flow in a cross-slot geometry. The vWF unfolding model was then verified in several microfluidic systems designed for inducing high-shear vWF-mediated thrombosis and screening for von Willebrand Disease. The model predicted high concentration of stretched vWF in key regions where occlusive thrombosis was observed experimentally. Strong unfolding caused by the extensional flow was limited to the center axis or middle plane of the channels, whereas vWF unfolding near the channel walls relied upon the shear tumbling mechanism. The continuum model of vWF unfolding presented in this work can be employed in numerical simulations of vWF-mediated thrombosis or vWF degradation in complex geometries. However, extending the model to 3-D arbitrary flows and turbulent flows will pose considerable challenges.

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, s_w, 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 s_w values and z-average diffusion coefficients, D_z. Molecular weights estimated using these values in the Svedberg equation ranged from 1.7 to 4.1 MDa. Frictional ratios calculated from D_z 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 s_w, D_z and molecular weight, s=k′Mas and D=k″MaD, yielded estimates of 0.51 and –0.49 for a_s and a_D, respectively, consistent with a random coil, in contrast to the a_s 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.

DNA binds to a specific site of the adhesive blood-protein von Willebrand factor guided by electrostatic interactions

Neutrophils release their intracellular content, DNA included, into the bloodstream to form neutrophil extracellular traps (NETs) that confine and kill circulating pathogens. The mechanosensitive adhesive blood protein, von Willebrand Factor (vWF), interacts with the extracellular DNA of NETs to potentially immobilize them during inflammatory and coagulatory conditions. Here, we elucidate the previously unknown molecular mechanism governing the DNA–vWF interaction by integrating atomistic, coarse-grained, and Brownian dynamics simulations, with thermophoresis, gel electrophoresis, fluorescence correlation spectroscopy (FCS), and microfluidic experiments. We demonstrate that, independently of its nucleotide sequence, double-stranded DNA binds to a specific helix of the vWF A1 domain, via three arginines. This interaction is attenuated by increasing the ionic strength. Our FCS and microfluidic measurements also highlight the key role shear-stress has in enabling this interaction. Our simulations attribute the previously-observed platelet-recruitment reduction and heparin-size modulation, upon establishment of DNA–vWF interactions, to indirect steric hindrance and partial overlap of the binding sites, respectively. Overall, we suggest electrostatics—guiding DNA to a specific protein binding site—as the main driving force defining DNA–vWF recognition. The molecular picture of a key shear-mediated DNA–protein interaction is provided here and it constitutes the basis for understanding NETs-mediated immune and hemostatic responses.

Physiological role of plasma and its components and the clinical implications of different methods of apheresis: A narrative review

Nowadays, therapeutic plasmapheresis (TP) is accepted as part of the treatment for specific groups of diseases. The availability of different methods, including double filtration and adsorption, increases selectivity for the removal of substances. However, the use of these techniques requires a thorough understanding of the characteristics and components of plasma. By considering pivotal papers from several databases, the aim of this narrative review is to describe the characteristics of plasma related to apheresis techniques. We have tried to cover the clinical implications including physiology, estimation of plasma volume, viscosity, and a description of its components including the size, volume of distribution, and half-lives of the different substances to be removed or maintained depending on the clinical situation and applied apheresis technique. Applying this knowledge will help us to choose the right method and dosage and improve the efficacy of the procedure by preventing or addressing any complications.

Occlusive thrombosis in arteries

Thrombus formation in major arteries is life threatening. In this review article, we discuss how an arterial thrombus can form under pathologically high shear stresses, with bonding rates estimated to be the fastest Kon values in biochemistry. During occlusive thrombosis in arteries, the growth rate of the thrombus explodes to capture a billion platelets in about 10 min. Close to 100% of all platelets passing the thrombus are captured by long von Willebrand factor (vWF) strands that quickly form tethered nets. The nets grow in patches where shear stress is high, and the local concentration of vWF is elevated due to α-granule release by previously captured platelets. This rapidly formed thrombus has few red blood cells and so has a white appearance and is much stronger and more porous than clots formed through coagulation. Understanding and modeling the biophysics of this event can predict totally new approaches to prevent and treat heart attacks and strokes.

A mechano-reactive coarse-grained model of the blood-clotting agent von Willebrand factor

The von Willebrand Factor (vWF) is a large blood glycoprotein that aids in hemostasis. Within each vWF monomer, the A2 domain hosts a cleavage site for enzyme ADAMTS13, which regulates the size of vWF multimers. This cleavage site can only be exposed when an A2 domain unfolds, and the unfolding reaction energy landscape is highly sensitive to the force conditions on the domain. Based on previous optical tweezer experimental results, we advance here a new activated A2 monomer model (AA2MM) for coarse-grained modeling of vWF that accurately represents the force-based probabilistic change between the unfolded/refolded states. A system of springs is employed to mimic the complex mechanical response of vWF monomers subject to pulling forces. AA2MM was validated by comparing monomer scale simulation results to data from prior pulling experiments on vWF monomer fragments. The model was further validated by comparing multimer scale Brownian dynamics simulation results to experiments using microfluidic chamber microscopy to visualize tethered vWF proteins subject to flow. The A2 domain unfolding reaction was studied in bulk flow simulations (pure shear and elongation flow), giving evidence that elongational flow drives the vWF size regulation process in blood. The mechanoreactive, coarse-grained AA2MM accurately describes the complex mechanical coupling between human blood flow conditions and vWF protein reactivity.

Prediction of Sub-Monomer A2 Domain Dynamics of the von Willebrand Factor by Machine Learning Algorithm and Coarse-Grained Molecular Dynamics Simulation

We develop a machine learning tool useful for predicting the instantaneous dynamical state of sub-monomer features within long linear polymer chains, as well as extracting the dominant macromolecular motions associated with sub-monomer behaviors of interest. We employ the tool to better understand and predict sub-monomer A2 domain unfolding dynamics occurring amidst the dominant large-scale macromolecular motions of the biopolymer von Willebrand Factor (vWF) immersed in flow. Results of coarse-grained Molecular Dynamics (MD) simulations of non-grafted vWF multimers subject to a shearing flow were used as input variables to a Random Forest Algorithm (RFA). Twenty unique features characterizing macromolecular conformation information of vWF multimers were used for training the RFA. The corresponding responses classify instantaneous A2 domain state as either folded or unfolded, and were directly taken from coarse-grained MD simulations. Three separate RFAs were trained using feature/response data of varying resolution, which provided deep insights into the highly correlated macromolecular dynamics occurring in concert with A2 domain unfolding events. The algorithm is used to analyze results of simulation, but has been developed for use with experimental data as well.

Heyde Syndrome: Aortic Stenosis and Beyond

Heyde described aortic stenosis and gastrointestinal bleeding in the 1950s. Since then, a link with intestinal angiodysplasia and abnormalities of von Willebrand factor (VWF) has been noted. Loss of the highest-molecular-weight multimers of VWF and bleeding also have been described in subaortic stenosis in hypertrophic cardiomyopathy, in isolated mitral and aortic insufficiency, in endocarditis, in patients with prosthetic valve stenosis or regurgitation, and in patients with left ventricular assist devices (LVADs). Bleeding tends to recur with local treatment of angiodysplasias, whereas cardiac repair or removal of LVAD eliminates VWF dysfunction is curative of bleeding in the majority.

Shear-Induced Extensional Response Behaviors of Tethered von Willebrand Factor

We perform single-molecule flow experiments using confocal microscopy and a microfluidic device for shear rates up to 20,000 s^-1 and present results for the shear-induced unraveling and elongation of tethered von Willebrand factor (VWF) multimers. Further, we employ companion Brownian dynamics simulations to help explain details of our experimental observations using a parameterized coarse-grained model of VWF. We show that global conformational changes of tethered VWF can be accurately captured using a relatively simple mechanical model. Good agreement is found between experimental results and computational predictions for the threshold shear rate of extension, existence of nonhomogenous fluorescence distributions along unraveled multimer contours, and large variations in extensional response behaviors. Brownian dynamics simulations reveal the strong influence of varying chain length, tethering point location, and number of tethering locations on the underlying unraveling response. Through a complex molecule like VWF that naturally adopts a wide distribution of molecular size and has multiple binding sites within each molecule, this work demonstrates the power of tandem experiment and simulation for understanding flow-induced changes in biomechanical state and global conformation of macromolecules.

Electrostatic Steering Enables Flow-Activated Von Willebrand Factor to Bind Platelet Glycoprotein, Revealed by Single-Molecule Stretching and Imaging

Von Willebrand factor (VWF), a large multimeric blood protein, senses changes in shear stress during bleeding and responds by binding platelets to plug ruptures in the vessel wall. Molecular mechanisms underlying this dynamic process are difficult to uncover using standard approaches due to the challenge of applying mechanical forces while monitoring structure and activity. By combining single-molecule fluorescence imaging with high-pressure, rapidly switching microfluidics, we reveal the key role of electrostatic steering in accelerating the binding between flow-activated VWF and GPIbα, and in rapidly immobilizing platelets under flow. We measure the elongation and tension-dependent activation of individual VWF multimers under a range of ionic strengths and pH levels, and find that the association rate is enhanced by 4 orders of magnitude by electrostatic steering. Under supraphysiologic salt concentrations, strong electrostatic screening dramatically decreases platelet binding to VWF in flow, revealing the critical role of electrostatic attraction in VWF-platelet binding during bleeding.

Advancing multimer analysis of von Willebrand factor by single-molecule AFM imaging

The formation of hemostatic plugs at sites of vascular injury crucially involves the multimeric glycoprotein von Willebrand factor (VWF). VWF multimers are linear chains of N-terminally linked dimers. The latter are formed from monomers via formation of the C-terminal disulfide bonds Cys2771-Cys2773', Cys2773-Cys2771', and Cys2811-Cys2811'. Mutations in VWF that impair multimerization can lead to subtype 2A of the bleeding disorder von Willebrand Disease (VWD). Commonly, the multimer size distribution of VWF is assessed by electrophoretic multimer analysis. Here, we present atomic force microscopy (AFM) imaging as a method to determine the size distribution of VWF variants by direct visualization at the single-molecule level. We first validated our approach by investigating recombinant wildtype VWF and a previously studied mutant (p.Cys1099Tyr) that impairs N-terminal multimerization. We obtained excellent quantitative agreement with results from earlier studies and with electrophoretic multimer analysis. We then imaged specific mutants that are known to exhibit disturbed C-terminal dimerization. For the mutants p.Cys2771Arg and p.Cys2773Arg, we found the majority of monomers (87 ± 5% and 73 ± 4%, respectively) not to be C-terminally dimerized. While these results confirm that Cys2771 and Cys2773 are crucial for dimerization, they additionally provide quantitative information on the mutants' different abilities to form alternative C-terminal disulfides for residual dimerization. We further mutated Cys2811 to Ala and found that only 23 ± 3% of monomers are not C-terminally dimerized, indicating that Cys2811 is structurally less important for dimerization. Furthermore, for mutants p.Cys2771Arg, p.Cys2773Arg, and p.Cys2811Ala we found 'even-numbered' non-native multimers, i.e. multimers with monomers attached on both termini; a multimer species that cannot be distinguished from native multimers by conventional multimer analysis. Summarizing, we demonstrate that AFM imaging can provide unique insights into VWF processing defects at the single-molecule level that cannot be gained from established methods of multimer analysis.

Internal Tensile Force and A2 Domain Unfolding of von Willebrand Factor Multimers in Shear Flow

Using Brownian molecular dynamics simulations, we examine the internal dynamics and biomechanical response of von Willebrand factor (vWF) multimers subject to shear flow. The coarse grain multimer description employed here is based on a monomer model in which the A2 domain of vWF is explicitly represented by a nonlinear elastic spring whose mechanical response was fit to experimental force/extension data from vWF monomers. This permits examination of the dynamic behavior of hydrodynamic forces acting on A2 domains as a function of shear rate and multimer length, as well as position of an A2 domain along the multimer contour. Force/position data reveal that collapsed multimers exhibit a force distribution with two peaks, one near each end of the chain; unraveled multimers, however, show a single peak in A2 domain force near the center of multimers. Guided further by experimental data, significant excursions of force acting on a domain are associated with an increasing probability for A2 domain unfolding. Our results suggest that the threshold shear rate required to induce A2 domain unfolding is inversely proportional to multimer length. By examining data for the duration and location of significant force excursions, convincing evidence is advanced that unfolding of A2 domains, and therefore scission of vWF multimers by the size-regulating blood enzyme ADAMTS13, happen preferentially near the center of unraveled multimers.

Von Willebrand factor and the aortic valve: Concepts that are important in the transcatheter aortic valve replacement era

Since the approval of the first transcatheter aortic valve replacement (TAVR) device in 2011, this technology has undergone substantial enhancements and exponential growth. However, valve thrombosis and residual paravalvular leaks (PVL) are among the challenges that require further investigation. Recently, monitoring von Willebrand factor (vWF) multimers has emerged as a tool to help evaluate the severity of PVL after TAVR. Following TAVR, vWF large multimers recovery have been documented. The role of large vWF multimers recovery and their interactions with platelets, and the endothelium have not been entirely elucidated. In this review, we discuss vWF synthesis and its role in aortic stenosis. We further provide an overview of the studies that investigated changes affecting vWF multimers following TAVR and the role of HMW vWF multimers monitoring in the determination of PVL severity. We also offer potential future directions for what will be fertile ground for research in this field.

Gastrointestinal Bleeding in Native and Prosthetic Valve Disease

Gastrointestinal bleeding with severe aortic stenosis was originally described in the 1950s by Heyde, although for years, the association was debated. Further discovery of mechanisms and the ubiquity and severity of acquired von Willebrand syndrome in the left ventricular assist device therapy have removed any doubts. At this time, gastrointestinal bleeding from intestinal angiodysplasia in patients with turbulence-related proteolysis of the highest molecular weight multimers of von Willebrand factor is now known to occur in patients with aortic stenosis, and also subaortic obstruction and associated mitral insufficiency in hypertrophic cardiomyopathy, isolated mitral and aortic insufficiency, endocarditis, and in patients with prosthetic valve dysfunction, either from stenosis or insufficiency. The degree of loss of high molecular weight multimers correlates with lesion severity, and tests of von Willebrand factor function have been proposed as important biomarkers of the severity of valve dysfunction, including in-lab testing for paravalvular leak during transcatheter aortic valve replacement. Bleeding tends to recur after endoscopic or surgical therapy, but cardiac repair is curative in the great majority.

Space and Time Resolved Detection of Platelet Activation and von Willebrand Factor Conformational Changes in Deep Suspensions

Tracking cells and proteins' phenotypic changes in deep suspensions is critical for the direct imaging of blood-related phenomena in in vitro replica of cardiovascular systems and blood-handling devices. This paper introduces fluorescence imaging techniques for space and time resolved detection of platelet activation, von Willebrand factor (VWF) conformational changes, and VWF-platelet interaction in deep suspensions. Labeled VWF, platelets, and VWF-platelet strands are suspended in deep cuvettes, illuminated, and imaged with a high-sensitivity EM-CCD camera, allowing detection using an exposure time of 1 ms. In-house postprocessing algorithms identify and track the moving signals. Recombinant VWF-eGFP (rVWF-eGFP) and VWF labeled with an FITC-conjugated polyclonal antibody are employed. Anti-P-Selectin FITC-conjugated antibodies and the calcium-sensitive probe Indo-1 are used to detect activated platelets. A positive correlation between the mean number of platelets detected per image and the percentage of activated platelets determined through flow cytometry is obtained, validating the technique. An increase in the number of rVWF-eGFP signals upon exposure to shear stress demonstrates the technique's ability to detect breakup of self-aggregates. VWF globular and unfolded conformations and self-aggregation are also observed. The ability to track the size and shape of VWF-platelet strands in space and time provides means to detect pro- and antithrombotic processes.

Modeling the cleavage of von Willebrand factor by ADAMTS13 protease in shear flow

Von Willebrand factor (VWF) is a key protein in hemostasis as it mediates adhesion of blood platelets to a site of vascular injury. A proper distribution of VWF lengths is important for normal functioning of hemostatic processes, because a diminished number of long VWF chains may significantly limit blood clotting and lead to bleeding, while an abundant number of long VWFs may result in undesired thrombotic events. VWF size distribution is controlled by ADAMTS13 protease, which can cleave VWF chains beyond a critical shear rate when the chains are stretched enough such that cleavage sites become accessible. To better understand the cleavage process, we model VWF cleavage in shear flow using mesoscopic hydrodynamic simulations. Two cleavage models are proposed, a geometrical model based on the degree of local stretching of VWF, and a tension-force model based on instantaneous tension force within VWF bonds. Both models capture the susceptibility of VWF to cleavage at high shear rates; however, the geometrical model appears to be much more robust than the force model. Our simulations show that VWF susceptibility to cleavage in shear flow becomes a universal function of shear rate, independent of VWF length for long enough chains. Furthermore, VWF is cleaved with a higher probability close to its ends in comparison to cleaving in the middle, which results into longer circulation lifetimes of VWF multimers. Simulations of dynamic cleavage of VWF show an exponential distribution of chain lengths, consistently with available in vitro experiments. The proposed cleavage models can be used in realistic simulations of hemostatic processes in blood flow.

Mutation G1629E Increases von Willebrand Factor Cleavage via a Cooperative Destabilization Mechanism

The large multimeric glycoprotein von Willebrand Factor (VWF) plays a pivotal adhesive role during primary hemostasis. VWF is cleaved by the protease ADAMTS13 as a down-regulatory mechanism to prevent excessive VWF-mediated platelet aggregation. For each VWF monomer, the ADAMTS13 cleavage site is located deeply buried inside the VWF A2 domain. External forces in vivo or denaturants in vitro trigger the unfolding of this domain, thereby leaving the cleavage site solvent-exposed and ready for cleavage. Mutations in the VWF A2 domain, facilitating the cleavage process, cause a distinct form of von Willebrand disease (VWD), VWD type 2A. In particular, the VWD type 2A Gly1629Glu mutation drastically accelerates the proteolytic cleavage activity, even in the absence of forces or denaturants. However, the effect of this mutation has not yet been quantified, in terms of kinetics or thermodynamics, nor has the underlying molecular mechanism been revealed. In this study, we addressed these questions by using fluorescence correlation spectroscopy, molecular dynamics simulations, and free energy calculations. The measured enzyme kinetics revealed a 20-fold increase in the cleavage rate for the Gly1629Glu mutant compared with the wild-type VWF. Cleavage was found cooperative with a cooperativity coefficient n = 2.3, suggesting that the mutant VWF gives access to multiple cleavage sites of the VWF multimer at the same time. According to our simulations and free energy calculations, the Gly1629Glu mutation causes structural perturbation in the A2 domain and thereby destabilizes the domain by ∼10 kJ/mol, promoting its unfolding. Taken together, the enhanced proteolytic activity of Gly1629Glu can be readily explained by an increased availability of the ADAMTS13 cleavage site through A2-domain-fold thermodynamic destabilization. Our study puts forward the Gly1629Glu mutant as a very efficient enzyme substrate for ADAMTS13 activity assays.

Shear-Induced Unfolding and Enzymatic Cleavage of Full-Length VWF Multimers

Proteolysis of the multimeric blood coagulation protein von Willebrand Factor (VWF) by ADAMTS13 is crucial for prevention of microvascular thrombosis. ADAMTS13 cleaves VWF within the mechanosensitive A2 domain, which is believed to open under shear flow. In this study, we combine fluorescence correlation spectroscopy (FCS) and a microfluidic shear cell to monitor real-time kinetics of full-length VWF proteolysis as a function of shear stress. For comparison, we also measure the Michaelis-Menten kinetics of ADAMTS13 cleavage of wild-type VWF in the absence of shear but partially denaturing conditions. Under shear, ADAMTS13 activity on full-length VWF arises without denaturing agent as evidenced by FCS and gel-based multimer analysis. In agreement with Brownian hydrodynamics simulations, we find a sigmoidal increase of the enzymatic rate as a function of shear at a threshold shear rate γ˙1/2 = 5522/s. The same flow-rate dependence of ADAMTS13 activity we also observe in blood plasma, which is relevant to predict hemostatic dysfunction.

Biophysical approaches promote advances in the understanding of von Willebrand factor processing and function

The large multimeric plasma glycoprotein von Willebrand factor (VWF) is essential for primary hemostasis by recruiting platelets to sites of vascular injury. VWF multimers respond to elevated hydrodynamic forces by elongation, thereby increasing their adhesiveness to platelets. Thus, the activation of VWF is force-induced, as is its inactivation. Due to these attributes, VWF is a highly interesting system from a biophysical point of view, and is well suited for investigation using biophysical approaches. Here, we give an overview on recent studies that predominantly employed biophysical methods to gain novel insights into multiple aspects of VWF: Electron microscopy was used to shed light on the domain structure of VWF and the mechanism of VWF secretion. High-resolution stochastic optical reconstruction microscopy, atomic force microscopy (AFM), microscale thermophoresis and fluorescence correlation spectroscopy allowed identification of protein disulfide isomerase isoform A1 as the VWF dimerizing enzyme and, together with molecular dynamics simulations, postulation of the dimerization mechanism. Advanced mass spectrometry led to detailed identification of the glycan structures carried by VWF. Microfluidics was used to illustrate the interplay of force and VWF function. Results from optical tweezers measurements explained mechanisms of the force-dependent functions of VWF's domains A1 and A2 and, together with thermodynamic approaches, increased our understanding of mutation-induced dysfunctions of platelet-binding. AFM-based force measurements and AFM imaging enabled exploration of intermonomer interactions and their dependence on pH and divalent cations. These advances would not have been possible by the use of biochemical methods alone and show the benefit of interdisciplinary research approaches.

Von Willebrand factor processing

Von Willebrand factor (VWF) is a multimeric glycoprotein essential for primary haemostasis that is produced only in endothelial cells and megakaryocytes. Key to VWF's function in recruitment of platelets to the site of vascular injury is its multimeric structure. The individual steps of VWF multimer biosynthesis rely on distinct posttranslational modifications at specific pH conditions, which are realized by spatial separation of the involved processes to different cell organelles. Production of multimers starts with translocation and modification of the VWF prepropolypeptide in the endoplasmic reticulum to produce dimers primed for glycosylation. In the Golgi apparatus they are further processed to multimers that carry more than 300 complex glycan structures functionalized by sialylation, sulfation and blood group determinants. Of special importance is the sequential formation of disulfide bonds with different functions in structural support of VWF multimers, which are packaged, stored and further processed after secretion. Here, all these processes are being reviewed in detail including background information on the occurring biochemical reactions.

Internal tension in a collapsed polymer under shear flow and the connection to enzymatic cleavage of von Willebrand factor

By means of Brownian hydrodynamics simulations we show that the tension distribution along the contour of a single collapsed polymer in shear flow is inhomogeneous and above a threshold shear rate exhibits a double-peak structure when hydrodynamic interactions are taken into account. We argue that the tension maxima close to the termini of the polymer chain reflect the presence of polymeric protrusions. We establish the connection to shear-induced globule unfolding and determine the scaling behavior of the maximal tensile forces and the average protrusion length as a function of shear rate, globule size, and cohesive strength. A quasi-equilibrium theory is employed in order to describe the simulation results. Our results are used to explain experimental data for the shear-sensitive enzymatic degradation of von Willebrand factor.

Treatment of acquired von Willebrand syndrome in aortic stenosis with transcatheter aortic valve replacement

OBJECTIVES

This study sought to investigate the prevalence of abnormal von Willebrand multimers (AbM) in patients undergoing transcatheter aortic valve replacement (TAVR) and the impact of TAVR on the underlying factor variances.

BACKGROUND

An association between the acquired von Willebrand syndrome (aVWS) and valvular aortic stenosis (AS) has been established in the past and surgical aortic valve replacement (SAVR) shown to lead to factor recovery. Prevalence and course of AbM in patients treated with TAVR though has not yet been described comprehensively.

METHODS

Ninety-five consecutive patients underwent TAVR at our institution. Hemostaseologic testing was performed before and up to 1 week after TAVR. Transvalvular and right heart hemodynamics as well as bleeding episodes were recorded and analyzed with descriptive statistics.

RESULTS

Baseline prevalence of AbM was 42% with an average high-molecular-weight multimer (HMWM) count of 16.2 ± 3.3%. Pressure gradients correlated significantly with the extent of HMWM deficiency (r = -0.63 [p < 0.0001]). Following valve implantation, HMWM increased proportional to the drop in mean pressure gradient and normalized in most of the patients. However, residual aortic regurgitation/leakage led to inferior HMWM recovery but prosthesis-patient mismatch (PPM) was rare and left HMWM uninfluenced. We saw no association of transfusion with AbM and 1-year mortality was unaffected by AbM.

CONCLUSIONS

AbM in patients with AS undergoing TAVR is frequent. However, TAVR is capable of correcting AbM and therefore possibly aVWS in patients with AS. As opposed to SAVR, bleeding and transfusion requirement in TAVR patients was not associated with severe HMWM deficiency; PPM was rare and HMWM were uninfluenced by the procedure. Aortic regurgitation after TAVR adversely influenced HMWM recovery.

Single-molecule force measurements of the polymerizing dimeric subunit of von Willebrand factor

Von Willebrand factor (VWF) multimers are large adhesive proteins that are essential to the initiation of hemostatic plugs at sites of vascular injury. The binding of VWF multimers to platelets, as well as VWF proteolysis, is regulated by shear stresses that alter VWF multimeric conformation. We used single molecule manipulation with atomic force microscopy (AFM) to investigate the effect of high fluid shear stress on soluble dimeric and multimeric forms of VWF. VWF dimers are the smallest unit that polymerizes to construct large VWF multimers. The resistance to mechanical unfolding with or without exposure to shear stress was used to evaluate VWF conformational forms. Our data indicate that, unlike recombinant VWF multimers (RVWF), recombinant dimeric VWF (RDVWF) unfolding force is not altered by high shear stress (100dynes/cm^{2} for 3 min at 37^{∘}C). We conclude that under the shear conditions used (100dynes/cm^{2} for 3 min at 37^{∘}C), VWF dimers do not self-associate into a conformation analogous to that attained by sheared large VWF multimers.

Force-sensitive autoinhibition of the von Willebrand factor is mediated by interdomain interactions

Von Willebrand factor (VWF) plays a central role in hemostasis. Triggered by shear-stress, it adheres to platelets at sites of vascular injury. Inactivation of VWF has been associated to the shielding of its adhesion sites and proteolytic cleavage. However, the molecular nature of this shielding and its coupling to cleavage under shear-forces in flowing blood remain unknown. In this study, we describe, to our knowledge, a new force-sensory mechanism for VWF-platelet binding, which addresses these questions, based on a combination of molecular dynamics (MD) simulations, atomic force microscopy (AFM), and microfluidic experiments. Our MD simulations demonstrate that the VWF A2 domain targets a specific region at the VWF A1 domain, corresponding to the binding site of the platelet glycoprotein Ibα (GPIbα) receptor, thereby causing its blockage. This implies autoinhibition of the VWF for the binding of platelets mediated by the A1-A2 protein-protein interaction. During force-probe MD simulations, a stretching force dissociated the A1A2 complex, thereby unblocking the GPIbα binding site. Dissociation was found to be coupled to the unfolding of the A2 domain, with dissociation predominantly occurring before exposure of the cleavage site in A2, an observation that is supported by our AFM experiments. This suggests that the A2 domain prevents platelet binding in a force-dependent manner, ensuring that VWF initiates hemostasis before inactivation by proteolytic cleavage. Microfluidic experiments with an A2-deletion VWF mutant resulted in increased platelet binding, corroborating the key autoinhibitory role of the A2 domain within VWF multimers. Overall, autoinhibition of VWF mediated by force-dependent interdomain interactions offers the molecular basis for the shear-sensitive growth of VWF-platelet aggregates, and might be similarly involved in shear-induced VWF self-aggregation and other force-sensing functions in hemostasis.