Force-induced cleavage of single VWFA1A2A3 tridomains by ADAMTS-13

Exploring ligands that target von Willebrand factor selectively under oxidizing conditions through docking and molecular dynamics simulations

The blood protein von Willebrand factor (VWF) is a large multimeric protein that, when activated, binds to blood platelets, tethering them to the site of vascular injury and initiating blood coagulation. This process is critical for the normal hemostatic response, but especially under inflammatory conditions, it is thought to be a major player in pathological thrombus formation. For this reason, VWF has been the target for the development of anti-thrombotic therapeutics. However, it is challenging to prevent pathological thrombus formation while still allowing normal physiological blood coagulation, as currently available anti-thrombotic therapeutics are known to cause unwanted bleeding, in particular intracranial hemorrhage. This work explores the possibility of inhibiting VWF selectively under the inflammatory conditions present during pathological thrombus formation. In particular, the A2 domain of VWF is known to inhibit the neighboring A1 domain from binding to the platelet surface receptor GpIbα, and this auto-inhibitory mechanism has been shown to be removed by oxidizing agents released during inflammation. Hence, finding drug molecules that bind at the interface between A1 and A2 only under oxidizing conditions could restore such an auto-inhibitory mechanism. Here, by using a combination of computational docking, molecular dynamics simulations, and free energy perturbation calculations, a ligand from the ZINC15 database was identified that binds at the A1A2 interface, with the interaction being stronger under oxidizing conditions. The results provide a framework for the discovery of drug molecules that bind to a protein selectively in the presence of inflammatory conditions.

Exploring ligands that target von Willebrand factor selectively under oxidizing conditions through docking and molecular dynamics simulations

The blood protein von Willebrand factor (VWF) is a large multimeric protein that, when activated, binds to blood platelets tethering them to the site of vascular injury initiating blood coagulation. This process is critical for the normal haemostatic response, but especially under inflammatory conditions it is thought to be a major player in pathological thrombus formation. For this reason, VWF has been the target for the development of anti-thrombotic therapeutics. However, it is challenging to prevent pathological thrombus formation while still allowing normal physiological blood coagulation as currently available anti-thrombotic therapeutics are known to cause unwanted bleeding in particular intracranial haemorrhage. This work explores the possibility of inhibiting VWF selectively under the inflammatory conditions present during pathological thrombus formation. In particular, the A2 domain of VWF is known to inhibit the neighboring A1 domain from binding to the platelet surface receptor GpIbα and this auto-inhibitory mechanism has been shown to be removed by oxidizing agents released during inflammation. Hence, finding drug molecules that bind at the interface between A1 and A2 only under oxidizing conditions could restore such auto-inhibitory mechanism. Here, by using a combination of computational docking, molecular dynamics simulations and free energy perturbation calculations, a ligand from the ZINC15 database was identified that binds at the A1A2 interface with the interaction being stronger under oxidizing conditions. The results provide a framework for the discovery of drug molecules that bind to a protein selectively in inflammatory conditions.

Force-induced biphasic regulation of VWF cleavage by ADAMTS13

It is crucial for hemostasis that platelets are rapidly recruited to the site of vascular injury by the adhesive ligand von Willebrand factor (VWF) multimers. The metalloproteinase ADAMTS13 regulates this hemostatic activity by proteolytically reducing the size of VWF and its proteolytic kinetics has been investigated by biochemical and single-molecule biophysical methods. However, how ADAMTS13 cleaves VWF in flowing blood remains poorly defined. To investigate the force-induced VWF cleavage, VWF A1A2A3 tridomains were immobilized and subjected to hydrodynamic forces in the presence of ADAMTS13. We demonstrated that the cleavage of VWF A1A2A3 by ADAMTS13 exhibited biphasic kinetics governed by shear stress, but not shear rate. By fitting data to the single-molecule Michaelis-Menten equation, the proteolytic constant kcat of ADAMTS13 had two distinct states. The mean proteolytic constant of the fast state (kcat-fast) was 0.005 ± 0.001 s-1, which is >10-fold faster than the slow state (kcat-slow = 0.0005 ± 0.0001 s-1). Furthermore, proteolytic constants of both states were regulated by shear stress in a biphasic manner, independent of the solution viscosity, indicating that the proteolytic activity of ADAMTS13 was regulated by hydrodynamic force. The findings provide new insights into the mechanism underlying ADAMTS13 cleaving VWF under flowing blood.

Force-Regulated Calcium Signaling of Lymphoid Cell RPMI 8226 Mediated by Integrin α4β7/MAdCAM-1 in Flow

MAdCAM-1 binds to integrin α₄β₇, which mediates the rolling and arrest of circulating lymphocytes upon the vascular endothelia during lymphocytic homing. The calcium response by adhered lymphocytes is a critical event for lymphocyte activation and subsequent arrest and migration under flow. However, whether the interaction of integrin α₄β₇ /MAdCAM-1 can effectively trigger the calcium response of lymphocytes remains unclear, as well as whether the fluid force affects the calcium response. In this study, we explore the mechanical regulation of integrin α₄β₇-induced calcium signaling under flow. Flou-4 AM was used to examine the calcium response under real-time fluorescence microscopy when cells were firmly adhered to a parallel plate flow chamber. The interaction between integrin α₄β₇ and MAdCAM-1 was found to effectively trigger calcium signaling in firmly adhered RPMI 8226 cells. Meanwhile, increasing fluid shear stress accelerated the cytosolic calcium response and enhanced signaling intensity. Additionally, the calcium signaling of RPMI 8226 activated by integrin α₄β₇ originated from extracellular calcium influx instead of cytoplasmic calcium release, and the signaling transduction of integrin α₄β₇ was involved in Kindlin-3. These findings shed new light on the mechano-chemical mechanism of calcium signaling in RPMI 8226 cells induced by integrin α₄β_7.

MD Simulation Reveals Regulation of Mechanical Force and Extracellular Domain 2 on Binding of DNAM-1 to CD155

Two extracellular domains of the adhesive receptor DNAM-1 are involved in various cellular biological processes through binding to ligand CD155, usually under a mechano-microenvironment. The first extracellular domain (D1) plays a key role in recognition, but the function of the second extracellular domain (D2) and effects of force on the interaction of DNAM-1 with CD155 remain unclear. We herein studied the interaction of DNAM-1 with CD155 by performing steered molecular dynamics (MD) simulations, and observed the roles of tensile force and D2 on the affinity of DNAM-1 to CD155. The results showed that D2 improved DNAM-1 affinity to CD155; the DNAM-1/CD155 complex had a high mechanical strength and a better mechanical stability for its conformational conservation either at pulling with constant velocity or under constant tensile force (≤100 pN); the catch-slip bond transition governed CD155 dissociation from DNAM-1; and, together with the newly assigned key residues in the binding site, force-induced conformation changes should be responsible for the mechanical regulation of DNAM-1's affinity to CD155. This work provided a novel insight in understanding the mechanical regulation mechanism and D2 function in the interaction of DNAM-1 with CD155, as well as their molecular basis, relevant transmembrane signaling, and cellular immune responses under a mechano-microenvironment.

Memory in repetitive protein–protein interaction series

Interactions between proteins coordinate biological processes in an organism and may impact its responses to changing environments and diseases through feedback systems. Feedback systems function by using changes in the past to influence behaviors in the future, which we refer to here as memory. Here, we summarized several observations made, ideas conceptualized, and mathematical models developed for quantitatively analyzing memory effects in repetitive protein–protein interactions (PPIs). Specifically, we consider how proteins on the cell or in isolation retain information about prior interactions to impact current interactions. The micropipette, biomembrane force probe, and atomic force microscopic techniques were used to repeatedly assay PPIs. The resulting time series were analyzed by a previous and two new models to extract three memory indices of short (seconds), intermediate (minutes), and long (hours) timescales. We found that interactions of cell membrane, but not soluble, T cell receptor (TCR) with peptide-major histocompatibility complex (pMHC) exhibits short-term memory that impacts on-rate, but not off-rate of the binding kinetics. Peptide dissociation from MHC resulted in intermediate- and long-term memories in TCR–pMHC interactions. However, we observed no changes in kinetic parameters by repetitive measurements on living cells over intermediate timescales using stable pMHCs. Parameters quantifying memory effects in PPIs could provide additional information regarding biological mechanisms. The methods developed herein also provide tools for future research.

Moderate Constraint Facilitates Association and Force-Dependent Dissociation of HA-CD44 Complex

Binding of cell surface glycoprotein CD44 to hyaluronic acid (HA) is a key event for mediating cell adhesion, motility, metastasis, inflammatory responses and tumor development, but the regulation mechanism and its molecular basis under diverse mechanical constraints remain unclear. We herein investigated interaction of CD44 HABD (HA binding site domain) to HA through free and steered molecular dynamics (MD) simulations as well as atomic force microscope (AFM) measurement using different constraints on HA. The middle, two ends or both of the constrained HA chains were fixed for MD simulations, while one and two biotin-avidin linkage or physical absorption were used to immobilize HA on substrates for AFM experiments, to model HA chains with low, moderate and high HA flexibilities, respectively. We found that binding of CD44 to moderate fixed HA was possessed of a better thermo-stability, a lower mechanical strength and a higher dissociation probability, while higher adhesive frequency, smaller rupture force and shorter lifetime were assigned to CD44 on the two biotin-immobilized HA rather than one biotin-immobilized or physically absorbed HA on substrates, suggesting a moderate HA flexibility requirement in favor of association and force-induced dissociation of CD44-HA complex. Tensile-induced convex conformation of HA chain was responsible for reduction of complex mechano-stability and did inversely a shrunken CD44 HABD under stretching; transition from catch bond to slip bond governed CD44-HA interaction. This study uncovered the regulation mechanism and its molecular basis for CD44-HA affinity under diverse mechano-microenvironments and provided a new insight into CD44-HA interaction-mediated cell inflammatory responses and tumor development.

Bitiscetin-3, a Novel C-Type Lectin-like Protein Cloned from the Venom Gland of the Viper Bitis arietans, Induces Platelet Agglutination and Inhibits Binding of Von Willebrand Factor to Collagen

Bitiscetin-1 (aka bitiscetin) and bitiscetin-2 are C-type lectin-like proteins purified from the venom of Bitis arietans (puff adder). They bind to von Willebrand factor (VWF) and—at least bitiscetin-1—induce platelet agglutination via enhancement of VWF binding to platelet glycoprotein Ib (GPIb). Bitiscetin-1 and -2 bind the VWF A1 and A3 domains, respectively. The A3 domain includes the major site of VWF for binding collagen, explaining why bitiscetin-2 blocks VWF-to-collagen binding. In the present study, sequences for a novel bitiscetin protein—bitiscetin-3—were identified in cDNA constructed from the B. arietans venom gland. The deduced amino acid sequences of bitiscetin-3 subunits α and β share 79 and 80% identity with those of bitiscetin-1, respectively. Expression vectors for bitiscetin-3α and -3β were co-transfected to 293T cells, producing the heterodimer protein recombinant bitiscetin-3 (rBit-3). Functionally, purified rBit-3 (1) induced platelet agglutination involving VWF and GPIb, (2) did not compete with bitiscetin-1 for binding to VWF, (3) blocked VWF-to-collagen binding, and (4) lost its platelet agglutination inducing ability in the presence of an anti-VWF monoclonal antibody that blocked VWF-to-collagen binding. These combined results suggest that bitiscetin-3 binds to the A3 domain, as does bitiscetin-2. Except for a small N-terminal fragment of a single subunit—which differs from that of both bitiscetin-3 subunits—the sequences of bitiscetin-2 have never been determined. Therefore, by identifying and analyzing bitiscetin-3, the present study is the first to present the full-length α- and β-subunit sequences and recombinant expression of a bitiscetin-family toxin that blocks the binding of VWF to collagen.

Force-enhanced biophysical connectivity of platelet β3 integrin signaling through Talin is predicted by steered molecular dynamics simulations

Platelet β3-integrin signaling through Talin is crucial in platelet transmembrane signaling, activation, adhesion, spreading and aggregation, and remains unclear in mechano-microenvironments. In order to examine Talin-β3 integrin biophysical connectivity, a series of “ramp-clamp” steered molecular dynamics (SMD) simulations were performed on complex of F3 domain of Talin and cytoplasmic tail of β3 integrin to imitate different force-loads in platelet. Pull-induced allostery of the hydrophobic pocket in F3 domain might markedly enhance complex rupture-force (> 150pN) and slow down breakage of the complex; the complex should mechano-stable for its conformational conservation under loads (≤ 80pN); increasing force below 60pN would decrease the complex dissociation probability, and force-induced extension of β5 strand on Talin and binding site residues, ASP⁷⁴⁰ and ALA⁷⁴² as well as Asn⁷⁴⁴, on β3-integrin were responsible for the force-enhanced linkage of the Talin-β3 integrin. Force might enhance biophysical connectivity of β3-integrin signaling through Talin by a catch bond mechanism, which be mediated by the force-induced allostery of complex at clamped stage. This work provides a novel insight into the force-regulated transmembrane β3-integrin signaling and its molecular basis for platelet activation, and exhibited a potential power of the present computer strategy in predicting mechanical regulation on ligand-receptor interaction under loads.

Single-molecule imaging of von Willebrand factor reveals tension-dependent self-association

von Willebrand factor (VWF) is an ultralong concatemeric protein important in hemostasis and thrombosis. VWF molecules can associate with other VWF molecules, but little is known about the mechanism. Hydrodynamic drag exerts tensile force on surface-tethered VWF that extends it and is maximal at the tether point and declines linearly to 0 at the downstream free end. Using single-molecule fluorescence microscopy, we directly visualized the kinetics of binding of free VWF in flow to surface-tethered single VWF molecules. We showed that self-association requires elongation of tethered VWF and that association increases with tension in tethered VWF, reaches half maximum at a characteristic tension of ∼10 pN, and plateaus above ∼25 pN. Association is reversible and hence noncovalent; a sharp decrease in shear flow results in rapid dissociation of bound VWF. Tethered primary VWF molecules can recruit more than their own mass of secondary VWF molecules from the flow stream. Kinetics show that instead of accelerating, the rate of accumulation decreases with time, revealing an inherently self-limiting self-association mechanism. We propose that this may occur because multiple tether points between secondary and primary VWF result in lower tension on the secondary VWF, which shields more highly tensioned primary VWF from further association. Glycoprotein Ibα (GPIbα) binding and VWF self-association occur in the same region of high tension in tethered VWF concatemers; however, the half-maximal tension required for activation of GPIbα is higher, suggesting differences in molecular mechanisms. These results have important implications for the mechanism of platelet plug formation in hemostasis and thrombosis.

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.

Physical forces regulating hemostasis and thrombosis: Vessels, cells, and molecules in illustrated review

This illustrated review focuses on the physical forces that regulate hemostasis and thrombosis. These phenomena span from the vessel to the cellular to the molecular scales. Blood is a complex fluid with a viscosity that varies with how fast it flows and the size of the vessel through which it flows. Blood flow imposes forces on the vessel wall and blood cells that dictates the kinetics, structure, and stability of thrombi. The mechanical properties of blood cells create a segmented flowing fluid whereby red blood cells concentrate in the vessel core and platelets marginate to the near-wall region. At the vessel wall, shear stresses are highest, which requires a repertoire of receptors with different bond kinetics to roll, tether, adhere, and activate on inflamed endothelium and extracellular matrices. As a thrombus grows and then contracts, forces regulate platelet aggregation as well as von Willebrand factor function and fibrin mechanics. Forces can also originate from platelets as they respond to the external forces and sense the stiffness of their local environment.

MD Simulations on a Well-Built Docking Model Reveal Fine Mechanical Stability and Force-Dependent Dissociation of Mac-1/GPIbα Complex

Interaction of leukocyte integrin macrophage-1 antigen (Mac-1) to platelet glycoprotein Ibα (GPIbα) is critical for platelet-leukocyte crosstalk in hemostasis and inflammatory responses to vessel injuries under hemodynamic environments. The mechano-regulation and its molecular basis for binding of Mac-1 to GPIbα remain unclear, mainly coming from the lack of crystal structure of the Mac-1/GPIbα complex. We herein built a Mac-1/GPIbα complex model through a novel computer strategy, which included a flexible molecular docking and system equilibrium followed by a "force-ramp + snapback" molecular dynamics (MD) simulation. With this model, a series of "ramp-clamp" steered molecular dynamics (SMD) simulations were performed to examine the GPIbα-Mac-1 interaction under various loads. The results demonstrated that the complex was mechano-stable for both the high rupture force (>250 pN) at a pulling velocity of 3 Å/ns and the conformational conservation under various constant tensile forces (≤75 pN); a catch-slip bond transition was predicted through the dissociation probability, examined with single molecular AFM measurements, reflected by the interaction energy and the interface H-bond number, and related to the force-induced allostery of the complex; besides the mutation-identified residues D222 and R218, the residues were also dominant in the binding of Mac-1 to GPIbα. This study recommended a valid computer strategy for building a likely wild-type docking model of a complex, provided a novel insight into the mechanical regulation mechanism and its molecular basis for the interaction of Mac-1 with GPIbα, and would be helpful for understanding the platelet-leukocyte interaction in hemostasis and inflammatory responses under mechano-microenvironments.

Predicting pathological von Willebrand factor unraveling in elongational flow

The globular-to-unraveled conformation transition of von Willebrand factor (vWF), a large polymeric glycoprotein in human blood plasma, is a crucial step in the process of clotting at sites of vascular injury. However, unraveling of vWF multimers in uninjured vasculature can lead to pathology (i.e., thrombus formation or degradation of vWF proteins by enzyme ADAMTS13, making them nonfunctional). To identify blood flow conditions that might induce pathological unraveling of vWF multimers, here we have computed the globular-to-unraveled transition rate of vWF multimers subjected to varying strain rate elongational flow by employing an enhanced sampling technique, the weighted ensemble method. Weighted ensemble sampling was employed instead of standard brute-force simulations because pathological blood flow conditions can induce undesired vWF unraveling on timescales potentially inaccessible to standard simulation methods. Results here indicate that brief but periodic exposure of vWF to the elongational flow of strain rate greater than or equal to 2500 s^-1 represents a source of possible pathology caused by the undesired unraveling of vWF multimers.

Device-Induced Hemostatic Disorders in Mechanically Assisted Circulation

Mechanically assisted circulation (MAC) sustains the blood circulation in the body of a patients undergoing cardiac surgery with cardiopulmonary bypass (CPB) or on ventricular assistance with a ventricular assist device (VAD) or on extracorporeal membrane oxygenation (ECMO) with a pump-oxygenator system. While MAC provides short-term (days to weeks) support and long-term (months to years) for the heart and/or lungs, the blood is inevitably exposed to non-physiological shear stress (NPSS) due to mechanical pumping action and in contact with artificial surfaces. NPSS is well known to cause blood damage and functional alterations of blood cells. In this review, we discussed shear-induced platelet adhesion, platelet aggregation, platelet receptor shedding, and platelet apoptosis, shear-induced acquired von Willebrand syndrome (AVWS), shear-induced hemolysis and microparticle formation during MAC. These alterations are associated with perioperative bleeding and thrombotic events, morbidity and mortality, and quality of life in MCS patients. Understanding the mechanism of shear-induce hemostatic disorders will help us develop low-shear-stress devices and select more effective treatments for better clinical outcomes.

Mechanical Forces Impacting Cleavage of Von Willebrand Factor in Laminar and Turbulent Blood Flow

Von Willebrand factor (VWF) is a large multimeric hemostatic protein. VWF is critical in arresting platelets in regions of high shear stress found in blood circulation. Excessive cleavage of VWF that leads to reduced VWF multimer size in plasma can cause acquired von Willebrand syndrome, which is a bleeding disorder found in some heart valve diseases and in patients receiving mechanical circulatory support. It has been proposed that hemodynamics (blood flow) found in these environments ultimately leads to VWF cleavage. In the context of experiments reported in the literature, scission theory, developed for polymers, is applied here to provide insight into flow that can produce strong extensional forces on VWF that leads to domain unfolding and exposure of a cryptic site for cleavage through a metalloproteinase. Based on theoretical tensile forces, laminar flow only enables VWF cleavage when shear rate is large enough (>2800 s−1) or when VWF is exposed to constant shear stress for nonphysiological exposure times (>20 min). Predicted forces increase in turbulence, increasing the chance for VWF cleavage. These findings can be used when designing blood-contacting medical devices by providing hemodynamic limits to these devices that can otherwise lead to acquired von Willebrand syndrome.

Insights Into Immunothrombosis: The Interplay Among Neutrophil Extracellular Trap, von Willebrand Factor, and ADAMTS13

Both neutrophil extracellular traps (NETs) and von Willebrand factor (VWF) are essential for thrombosis and inflammation. During these processes, a complex series of events, including endothelial activation, NET formation, VWF secretion, and blood cell adhesion, aggregation and activation, occurs in an ordered manner in the vasculature. The adhesive activity of VWF multimers is regulated by a specific metalloprotease ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motifs, member 13). Increasing evidence indicates that the interaction between NETs and VWF contributes to arterial and venous thrombosis as well as inflammation. Furthermore, contents released from activated neutrophils or NETs induce the reduction of ADAMTS13 activity, which may occur in both thrombotic microangiopathies (TMAs) and acute ischemic stroke (AIS). Recently, NET is considered as a driver of endothelial damage and immunothrombosis in COVID-19. In addition, the levels of VWF and ADAMTS13 can predict the mortality of COVID-19. In this review, we summarize the biological characteristics and interactions of NETs, VWF, and ADAMTS13, and discuss their roles in TMAs, AIS, and COVID-19. Targeting the NET-VWF axis may be a novel therapeutic strategy for inflammation-associated TMAs, AIS, and COVID-19.

Diverse activities of von Willebrand factor in traumatic brain injury and associated coagulopathy

Traumatic brain injury (TBI) is a leading cause of death and disability. Patients with isolated TBI lose a limited amount of blood to primary injury, but they often develop secondary coagulopathy, resulting in delayed or recurrent intracranial and intracerebral hematoma. TBI-induced coagulopathy is closely associated with poor outcomes for these patients, including death. This secondary coagulopathy is consumptive in nature, involving not only brain-derived molecules, coagulation factors, and platelets, but also endothelial cells in a complex process now called blood failture. A key question is how a localized injury to the brain is rapidly disseminated to affect systemic hemostasis that is not directly affected the way it is in trauma to the body and limbs, especially with hemorrhagic shock. Increasing evidence suggests that the adhesive ligand von Willebrand factor (VWF), which is synthesized in and released from endothelial cells, plays a paradoxical role in both facilitating local hemostasis at the site of injury and also propagating TBI-induced endotheliopathy and coagulopathy systemically. This review discusses recent progress in understanding these diverse activities of VWF and the knowledge gaps in defining their roles in TBI and associated coagulopathy.

Biphasic Force-Regulated Phosphorylation Site Exposure and Unligation of ERM Bound with PSGL-1: A Novel Insight into PSGL-1 Signaling via Steered Molecular Dynamics Simulations

The PSGL-1-actin cytoskeleton linker proteins ezrin/radixin/moesin (ERM), an adaptor between P-selectin glycoprotein ligand-1 (PSGL-1) and spleen tyrosine kinase (Syk), is a key player in PSGL-1 signal, which mediates the adhesion and recruitment of leukocytes to the activated endothelial cells in flow. Binding of PSGL-1 to ERM initials intracellular signaling through inducing phosphorylation of Syk, but effects of tensile force on unligation and phosphorylation site exposure of ERM bound with PSGL-1 remains unclear. To answer this question, we performed a series of so-called “ramp-clamp” steered molecular dynamics (SMD) simulations on the radixin protein FERM domain of ERM bound with intracellular juxtamembrane PSGL-1 peptide. The results showed that, the rupture force of complex pulled with constant velocity was over 250 pN, which prevented the complex from breaking in front of pull-induced exposure of phosphorylation site on immunoreceptor tyrosine activation motif (ITAM)-like motif of ERM; the stretched complex structure under constant tensile forces <100 pN maintained on a stable quasi-equilibrium state, showing a high mechano-stabilization of the clamped complex; and, in consistent with the force-induced allostery at clamped stage, increasing tensile force (<50 pN) would decrease the complex dissociation probability but facilitate the phosphorylation site exposure, suggesting a force-enhanced biophysical connectivity of PSGL-1 signaling. These force-enhanced characters in both phosphorylation and unligation of ERM bound with PSGL-1 should be mediated by a catch-slip bond transition mechanism, in which four residue interactions on binding site were involved. This study might provide a novel insight into the transmembrane PSGL-1 signal, its biophysical connectivity and molecular structural basis for cellular immune responses in mechano-microenvironment, and showed a rational SMD-based computer strategy for predicting structure-function relation of protein under loads.

Prediction of Catch-Slip Bond Transition of Kindlin2/β3 Integrin via Steered Molecular Dynamics Simulation

Kindlin2 is believed to be crucial in integrin activation, which mediates the cell-extracellular matrix adhesion and signaling, but the mechanoregulation of the interaction between Kindlin2 and integrin remains unclear. Here, we performed the so-called "ramp-clamp" steered molecular dynamics simulation on the crystal structure of Kindlin2 bound with β3 integrin. The results showed that the complex had a better mechanical strength for its rupture force of about 200 pN under pulling with the velocity of 1 Å/ns, and was mechanostable for its conformational conservation under constant tensile force (≤60 pN). The catch-slip bond transition with a force threshold of 20 pN was demonstrated by the dissociation probability, the interaction energy, the interface H-bond number, and the force-induced allostery of the complex. This study might provide a novel insight into force-dependent Kindlin2/integrin-related signaling and its structural basis in cellular processes as well as a rational SMD-based computer strategy for predicting the structure-function relationship of the stretched complex.

METALLOPROTEASE ADAMTS-13

Introduction. The signifi cance of ADAMTS-13 extends beyond its key role in the pathogenesis of thrombotic thrombocytopenic purpura (TTP); there is evidence of a relationship between a decrease in the ADAMTS-13 activity and thrombotic events in acute myocardial infarction and ischemic stroke.

Aim. To generalise available information on the structure and function of the metalloprotease ADAMTS-13.

General findings. The biological function of ADAMTS-13 consists in the cleavage of ultra-large von Willebrand factor (vWF) multimers. The fact that its defi ciency causes the development of TTP provides a basis for understanding the function of vWF–cleaving protease. ADAMTS-13 has a domain structure. The functional roles of most ADAMTS-13 domains, as well as the key role of the ADAMTS-13-vWF interaction in the regulation of haemostasis, are defi ned. The conformational activation of ADAMTS-13 by vWF constitutes an important aspect of its function. After getting into the bloodstream, ultra-large vWF multimers quickly adopt a closed conformation, which becomes very resistant to ADAMTS-13 proteolysis in the absence of shear stress. Ultra-large plasma vWF multimers regain their sensitivity to ADAMTS-13 after being exposed to high fl uid shear stress, which unfolds the central vWF-A2 domain. The unfolding of a vWF molecule under shear stress conditions reveals previously hidden exosites in domain A2, which gradually increase the binding affi nity between ADAMTS-13 and vWF. The mechanism underlying the production of autoantibodies against ADAMTS-13 is unknown and requires further study. The masking of cryptic epitopes in the closed conformation of ADAMTS-13 prevents the formation of autoantibodies. Early antigen recognition of ADAMTS-13 occurs through surface-exposed epitopes in the C-terminal domains. More detailed information on the mechanisms underlying the interaction between ADAMTS-13 and the vWF can improve the understanding of mechanisms involved in the regulation of the coagulation system.

Conflict of interest: the authors declare no confl ict of interest.

Financial disclosure: the study had no sponsorship.

Bleeding in critical care associated with left ventricular assist devices: pathophysiology, symptoms, and management

Chronic heart failure (HF) is a growing health problem, and it is associated with high morbidity and mortality. Left ventricular assist devices (LVADs) are nowadays an important treatment option for patients with end-stage HF not only as a bridging tool to heart transplantation but also, as a permanent therapy for end-stage HF (destination therapy). The use of LVAD is associated with a high risk for bleeding complications and thromboembolic events, including pump thrombosis and ischemic stroke. Bleeding is the most frequent complication, occurring in 30% to 60% of patients, both early and late after LVAD implantation. Although the design of LVADs has improved over time, bleeding complications are still the most common complication and occur very frequently. The introduction of an LVAD results in an altered hemostatic balance as a consequence of blood-pump interactions, changes in hemodynamics, acquired coagulation abnormalities, and the strict need for long-term anticoagulant treatment with oral anticoagulants and antiplatelet therapy. LVAD patients may experience an acquired coagulopathy, including platelet dysfunction and impaired von Willebrand factor activity, resulting in acquired von Willebrand syndrome. In this educational manuscript, the epidemiology, etiology, and pathophysiology of bleeding in patients with LVAD will be discussed. Because hematologist are frequently consulted in cases of bleeding problems in these individuals in a critical care setting, the observed type of bleeding complications and management strategies to treat bleeding are also reviewed.

Dynamic bonds and their roles in mechanosensing

Mechanical forces are ubiquitous in a cell's internal structure and external environment. Mechanosensing is the process that the cell employs to sense its mechanical environment. In receptor-mediated mechanosensing, cell surface receptors interact with immobilized ligands to provide a specific way to receive extracellular force signals to targeted force-transmitting, force-transducing and force-supporting structures inside the cell. Conversely, forces generated endogenously by the cell can be transmitted via cytoplasmic protein-protein interactions and regulate cell surface receptor activities in an 'inside-out' manner. Dynamic force spectroscopy analyzes these interactions on and inside cells to reveal various dynamic bonds. What is more, by integrating analysis of molecular interactions with that of cell signaling events involved in force-sensing and force-responding processes, one can investigate how dynamic bonds regulate the reception, transmission and transduction of mechanical signals.

Bleeding signs due to acquired von Willebrand syndrome at diagnosis of chronic myeloid leukaemia in children

A considerable proportion of patients with chronic myeloid leukaemia (CML) may present at diagnosis with high platelet counts. This may result in thrombosis or bleeding complications due to binding of von Willebrand factor (VWF) multimers to platelets. Paediatric CML is very rare and no systematic investigation on clinical complications of elevated platelets has been reported. Data on platelet count and associated haemostaseological complications were retrospectively analysed in a cohort of 156 children with CML. Fifty-one percent (81/156) patients presented with thrombocytosis (platelet count> 500 × 10⁹ /l), and were extreme (>1 000 × 10⁹ /l) in 23/156 (16%). There were no cases of thrombosis but mild bleeding signs were present in 12% (n = 9) children with thrombocytosis. Bleeding occurred without correlation to elevated platelet counts and was associated with reduced large VWF multimers, indicating a diagnosis of acquired von Willebrand syndrome (AVWS), which resolved after initiation of CML treatment. Patients with paediatric CML frequently exhibit high platelet counts not resulting in thrombosis. In patients with thrombocytosis mild bleeding signs due to a low percentage of large VWF multimers can be demonstrated. AVWS may be underdiagnosed in paediatric CML (Clinical-Trials.gov NCT00445822, 9 March 2007).

Domain-specific mechanical modulation of VWF-ADAMTS13 interaction

Hemodynamic forces activate the Von Willebrand factor (VWF) and facilitate its cleavage by a disintegrin and metalloprotease with thrombospondin motifs-13 (ADAMTS13), reducing the adhesive activity of VWF. Biochemical assays have mapped the binding sites on both molecules. However, these assays require incubation of two molecules for a period beyond the time allowed in flowing blood. We used a single-molecule technique to examine these rapid, transient, and mechanically modulated molecular interactions in short times under forces to mimic what happens in circulation. Wild-type ADAMTS13 and two truncation variants that either lacked the C-terminal thrombospondin motif-7 to the CUB domain (MP-TSP6) or contained only the two CUB domains (CUB) were characterized for interactions with coiled VWF, flow-elongated VWF, and a VWF A1A2A3 tridomain. These interactions exhibited distinctive patterns of calcium dependency, binding affinity, and force-regulated lifetime. The results suggest that 1) ADAMTS13 binds coiled VWF primarily through CUB in a calcium-dependent manner via a site(s) outside A1A2A3, 2) ADAMTS13 binds flow-extended VWF predominantly through MP-TSP6 via a site(s) different from the one(s) at A1A2A3; and 3) ADAMTS13 binds A1A2A3 through MP-TSP6 in a Ca²⁺-dependent manner to autoinhibit another Ca²⁺-independent binding site on CUB. These data reveal that multiple sites on both molecules are involved in mechanically modulated VWF–ADAMTS13 interaction.

AFM Imaging Reveals Multiple Conformational States of ADAMTS13

Background

ADAMTS13 (A disintegrin and metalloprotease with a thrombospondin type 1 motif 13) cleaves Von Willebrand factor (VWF) to regulate its size, thereby preventing aberrant platelet aggregation and thrombus. Deficiency of ADAMTS13 caused by either genetic mutations or by inhibitory autoantibodies against ADAMTS13 leads to thrombotic thrombocytopenic purpura (TTP). Recently, ADAMTS13 was reported to adopt a “closed” conformation with lower activity and an “open” one resulting from the engagements of VWF D4-CK domains or antibodies to the distal domains of ADAMTS13, or mutations in its spacer domain. These engagements or mutations increase ADAMTS13 activity by ~ 2.5-fold. However, it is less known whether the conformation of ADAMTS13 is dynamic or stable.

Results

Wild type ADAMTS13 (WT-ADAMTS13) and the gain-of-function variant (GOF-ADAMTS13) with five mutations (R568K / F592Y / R660K / Y661F / Y665F) in spacer domain were imaged by atomic force microscopy (AFM) at pH 6 and pH 7.5. The data revealed that at both pH 6 and pH 7.5, WT-ADAMTS13 adopted two distinct conformational states (state I and state II), while an additional state (state III) was observed in GOF-ADAMTS13. In the present study, we propose that state I is the “closed” conformation, state III is the “open” one, and state II is an intermediate one. Comparing to pH 7.5, the percentages of state II of WT-ADAMTS13 and state III of GOF-ADAMTS13 increased at pH 6, with the decrease in the state I for WT-ADAMTS13 and state I and state II for GOF-ADAMTS13, suggesting lower pH extended the conformation of ADAMTS13.

Conclusion

Both WT- and GOF-ADAMTS13 exist multiple conformational states and lower pH might alter the tertiary structure and/or disrupt the intra-domain interactions, increasing the flexibility of ADAMTS13 molecules.

Electronic supplementary material

The online version of this article (10.1186/s13036-018-0102-y) contains supplementary material, which is available to authorized users.

Biophysical nanotools for single-molecule dynamics

The focus of the cell biology field is now shifting from characterizing cellular activities to organelle and molecular behaviors. This process accompanies the development of new biophysical visualization techniques that offer high spatial and temporal resolutions with ultra-sensitivity and low cell toxicity. They allow the biology research community to observe dynamic behaviors from scales of single molecules, organelles, cells to organoids, and even live animal tissues. In this review, we summarize these biophysical techniques into two major classes: the mechanical nanotools like dynamic force spectroscopy (DFS) and the optical nanotools like single-molecule and super-resolution microscopy. We also discuss their applications in elucidating molecular dynamics and functionally mapping of interactions between inter-cellular networks and intra-cellular components, which is key to understanding cellular processes such as adhesion, trafficking, inheritance, and division.

The D' domain of von Willebrand factor requires the presence of the D3 domain for optimal factor VIII binding

The D'-D3 fragment of von Willebrand factor (VWF) can be divided into TIL'-E'-VWD3-C8_3-TIL3-E3 subdomains of which TIL'-E'-VWD3 comprises the main factor VIII (FVIII)-binding region. Yet, von Willebrand disease (VWD) Type 2 Normandy (2N) mutations, associated with impaired FVIII interaction, have been identified in C8_3-TIL3-E3. We now assessed the role of the VWF (sub)domains for FVIII binding using isolated D', D3 and monomeric C-terminal subdomain truncation variants of D'-D3. Competitive binding assays and surface plasmon resonance analysis revealed that D' requires the presence of D3 for effective interaction with FVIII. The isolated D3 domain, however, did not show any FVIII binding. Results indicated that the E3 subdomain is dispensable for FVIII binding. Subsequent deletion of the other subdomains from D3 resulted in a progressive decrease in FVIII-binding affinity. Chemical footprinting mass spectrometry suggested increased conformational changes at the N-terminal side of D3 upon subsequent subdomain deletions at the C-terminal side of the D3. A D'-D3 variant with a VWD type 2N mutation in VWD3 (D879N) or C8_3 (C1060R) also revealed conformational changes in D3, which were proportional to a decrease in FVIII-binding affinity. A D'-D3 variant with a putative VWD type 2N mutation in the E3 subdomain (C1225G) showed, however, normal binding. This implies that the designation VWD type 2N is incorrect for this variant. Results together imply that a structurally intact D3 in D'-D3 is indispensable for effective interaction between D' and FVIII explaining why specific mutations in D3 can impair FVIII binding.

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.

The role of ADAMTS13 testing in the diagnosis and management of thrombotic microangiopathies and thrombosis

ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motif, 13) is a metalloprotease responsible for cleavage of ultra-large von Willebrand factor (VWF) multimers. Severely deficient activity of the protease can trigger an acute episode of thrombotic thrombocytopenic purpura (TTP). Our understanding of the pathophysiology of TTP has allowed us to grasp the important role of ADAMTS13 in other thrombotic microangiopathies (TMAs) and thrombotic disorders, such as ischemic stroke and coronary artery disease. Through its action on VWF, ADAMTS13 can have prothrombotic and proinflammatory properties, not only when its activity is severely deficient, but also when it is only moderately low. Here, we will discuss the biology of ADAMTS13 and the different assays developed to evaluate its function in the context of TTP, in the acute setting and during follow-up. We will also discuss the latest evidence regarding the role of ADAMTS13 in other TMAs, stroke, and cardiovascular disease. This information will be useful for clinicians not only when evaluating patients who present with microangiopathic hemolytic anemia and thrombocytopenia, but also when making clinical decisions regarding the follow-up of patients with TTP.

Quantification of Von Willebrand Factor Cleavage by adamts-13 in Patients Supported by Left Ventricular Assist Devices

Patients supported by left ventricular assist devices (LVADs) often present with the loss of large von Willebrand factor (VWF) multimers. This VWF deficiency is believed to contribute to the bleeding diathesis of patients on LVAD support and is caused by excessive VWF cleavage by the metalloprotease ADAMTS-13 under high shear stress. However, only a small percentage of patients who have suffered the loss of large VWF multimers bleed. The actual rates of VWF cleavage in these patients have not been reported, primarily because of the lack of reliable detection methods. We have developed and validated a selected reaction monitoring (SRM) mass spectrometry method to quantify VWF cleavage as the ratio of the ADAMTS-13-cleaved peptide MVTGNPASDEIK to the ILAGPAGDSNVVK peptide. The rate of VWF cleavage was found to be 1.26% ± 0.36% in normal plasma. It varied significantly in patient samples, ranging from 0.23% to 2.5% of total VWF antigen, even though all patients had the loss of large VWF multimers. Von Willebrand factor cleavage was greater in post-LVAD samples from patients in whom bleeding had developed, but was mostly reduced in patients in whom thrombosis had developed. This SRM method is reliable to quantify the rate of VWF cleavage in patients on LVAD support.

Prediction of spacer-α6 complex: a novel insight into binding of ADAMTS13 with A2 domain of von Willebrand factor under forces

Force-regulated cleavage of A2 domain of von Willebrand factor (vWF) by ADAMTS13 is a key event in preventing thrombotic thrombocytopenic purpura (TTP). Recognition and cleavage depend on cooperative and modular contacts between several ADAMTS13 subdomains and discrete segments of vWF A2 domain. Spacer domain of ADAMTS13 contains an important exosite interacting with α6 helix of unfold A2 domain, but it remains unclear whether stretching of α6 regulates binding to spacer. To understand the molecular mechanism underlying the interactions between spacer and α6 under stretching, we successfully predicted spacer-α6 complex by a novel computer strategy combined the steered molecular dynamics (SMD) and flexible docking techniques. This strategy included three steps: (1) constant-velocity SMD simulation of α6; (2) zero-velocity SMD simulations of α6, and (3) flexible dockings of α6 to spacer. In our spacer-α6 complex model, 13 key residues, six in α6 and seven in spacer, were identified. Our data demonstrated a biphasic extension-regulated binding of α6 to spacer. The binding strength of the complex increased with α6 extension until it reaches its optimum of 0.25 nm, and then decreased as α6 extension further increased, meaning that spacer is in favor to binding with a partially extended α6, which may contribute to the optimal contact and proteolysis. Changes of interface area and intermolecular salt bridge may serve as the molecular basis for this characteristic. These findings provide a novel insight into mechano-chemical regulation on interaction between ADAMTS13 and vWF A2 domain under forces.

Acquired coagulopathy in patients with left ventricular assist devices

Chronic heart failure (HF) is a major emerging healthcare problem, associated with a high morbidity and mortality. Left ventricular assist devices (LVADs) have emerged as a successful treatment option for patients with end-stage HF. Despite its great benefit, the use of LVAD is associated with a high risk of complications. Bleeding, pump thrombosis and thromboembolic events are frequently observed complications, with bleeding complications occurring in over a third of the patients. Although the design of the third-generation LVAD has improved greatly, these hemostatic complications still occur. The introduction of an LVAD into the circulatory system results in an altered hematological balance as a consequence of blood-pump interactions, changes in hemodynamics, the rheology, and the concomitant need for anticoagulation while implanted with an LVAD. The majority, if not all, LVAD patients experience a form of platelet dysfunction and impaired von Willebrand factor activity, leading to acquired coagulopathy disorders. Different diagnostic tools and treatment strategies have been reported; however, they require validation in LVAD patients. The present review focuses on acquired coagulopathies, describing the incidence, impact and underlying mechanism of acquired coagulopathy disorders in patients supported by LVADs. In addition, we will discuss diagnostic and management strategies for these acquired coagulopathies.

ADAMTS-13 and von Willebrand factor: a dynamic duo

von Willebrand factor (VWF) is a key player in hemostasis, acting as a carrier for factor VIII and capturing platelets at sites of vascular damage. To capture platelets, it must undergo conformational changes, both within its A1 domain and at the macromolecular level through A2 domain unfolding. Its size and this function are regulated by the metalloproteinase ADAMTS-13. Recently, it has been shown that ADAMTS-13 undergoes a conformational change upon interaction with VWF, and that this enhances its activity towards its substrate. This review summarizes recent work on these conformational transitions, describing how they are controlled. It points to their importance in hemostasis, bleeding disorders, and the developing field of therapeutic application of ADAMTS-13 as an antithrombotic agent in obstructive microvascular thrombosis and in cardiovascular disease.

A multiplexed magnetic tweezer with precision particle tracking and bi-directional force control

Background

In the past two decades, methods have been developed to measure the mechanical properties of single biomolecules. One of these methods, Magnetic tweezers, is amenable to aquisition of data on many single molecules simultaneously, but to take full advantage of this "multiplexing" ability, it is necessary to simultaneously incorprorate many capabilities that ahve been only demonstrated separately.

Methods

Our custom built magnetic tweezer combines high multiplexing, precision bead tracking, and bi-directional force control into a flexible and stable platform for examining single molecule behavior. This was accomplished using electromagnets, which provide high temporal control of force while achieving force levels similar to permanent magnets via large paramagnetic beads.

Results

Here we describe the instrument and its ability to apply 2–260 pN of force on up to 120 beads simultaneously, with a maximum spatial precision of 12 nm using a variety of bead sizes and experimental techniques. We also demonstrate a novel method for increasing the precision of force estimations on heterogeneous paramagnetic beads using a combination of density separation and bi-directional force correlation which reduces the coefficient of variation of force from 27% to 6%. We then use the instrument to examine the force dependence of uncoiling and recoiling velocity of type 1 fimbriae from Eschericia coli (E. coli) bacteria, and see similar results to previous studies.

Conclusion

This platform provides a simple, effective, and flexible method for efficiently gathering single molecule force spectroscopy measurements.

Receptor-mediated cell mechanosensing

Mechanosensing depicts the ability of a cell to sense mechanical cues, which under some circumstances is mediated by the surface receptors. In this review, a four-step model is described for receptor-mediated mechanosensing. Platelet GPIb, T-cell receptor, and integrins are used as examples to illustrate the key concepts and players in this process.

Mechanosensing describes the ability of a cell to sense mechanical cues of its microenvironment, including not only all components of force, stress, and strain but also substrate rigidity, topology, and adhesiveness. This ability is crucial for the cell to respond to the surrounding mechanical cues and adapt to the changing environment. Examples of responses and adaptation include (de)activation, proliferation/apoptosis, and (de)differentiation. Receptor-mediated cell mechanosensing is a multistep process that is initiated by binding of cell surface receptors to their ligands on the extracellular matrix or the surface of adjacent cells. Mechanical cues are presented by the ligand and received by the receptor at the binding interface; but their transmission over space and time and their conversion into biochemical signals may involve other domains and additional molecules. In this review, a four-step model is described for the receptor-mediated cell mechanosensing process. Platelet glycoprotein Ib, T-cell receptor, and integrins are used as examples to illustrate the key concepts and players in this process.

Angiodysplasia in von Willebrand Disease: Understanding the Clinical and Basic Science

Severe and intractable gastrointestinal bleeding caused by angiodysplasia is a debilitating problem for up to 20% of patients with von Willebrand disease (VWD). Currently, the lack of an optimal treatment for this recurrent problem presents an ongoing challenge for many physicians in their management of affected patients. Over the past few years, studies have pointed to a regulatory role for the hemostatic protein, von Willebrand factor (VWF), in angiogenesis, providing a novel target for the modulation of vessel development. This article will review the clinical implications and molecular pathology of angiodysplasia in VWD.

Mutual A domain interactions in the force sensing protein von Willebrand factor

The von Willebrand factor (VWF) is a glycoprotein in the blood that plays a central role in hemostasis. Among other functions, VWF is responsible for platelet adhesion at sites of injury via its A1 domain. Its adjacent VWF domain A2 exposes a cleavage site under shear to degrade long VWF fibers in order to prevent thrombosis. Recently, it has been shown that VWF A1/A2 interactions inhibit the binding of platelets to VWF domain A1 in a force-dependent manner prior to A2 cleavage. However, whether and how this interaction also takes place in longer VWF fragments as well as the strength of this interaction in the light of typical elongation forces imposed by the shear flow of blood remained elusive. Here, we addressed these questions by using single molecule force spectroscopy (SMFS), Brownian dynamics (BD), and molecular dynamics (MD) simulations. Our SMFS measurements demonstrate that the A2 domain has the ability to bind not only to single A1 domains but also to VWF A1A2 fragments. SMFS experiments of a mutant [A2] domain, containing a disulfide bond which stabilizes the domain against unfolding, enhanced A1 binding. This observation suggests that the mutant adopts a more stable conformation for binding to A1. We found intermolecular A1/A2 interactions to be preferred over intramolecular A1/A2 interactions. Our data are also consistent with the existence of two cooperatively acting binding sites for A2 in the A1 domain. Our SMFS measurements revealed a slip-bond behavior for the A1/A2 interaction and their lifetimes were estimated for forces acting on VWF multimers at physiological shear rates using BD simulations. Complementary fitting of AFM rupture forces in the MD simulation range adequately reproduced the force response of the A1/A2 complex spanning a wide range of loading rates. In conclusion, we here characterized the auto-inhibitory mechanism of the intramolecular A1/A2 bond as a shear dependent safeguard of VWF, which prevents the interaction of VWF with platelets.

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

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.

BROWNIAN DYNAMICS SIMULATION OF CATCH TO SLIP TRANSITION OVER A MODEL ENERGY LANDSCAPE

We perform Brownian dynamics simulation (BDS) of catch to slip transition over a model energy landscape. Through our BDS we demonstrate that for forces below the critical force the bond rupture occurs mostly through the catch pathway while for forces above the critical force the bond rupture occurs mostly through the slip pathway. We also demonstrate that the shoulder in the bond rupture force distribution switches to peak as the loading rate increases progressively and the bond lifetime is maximized at the model dependent critical force. The force dependent bond lifetime obtained via transforming the bond rupture force distribution at a given loading rate is in excellent agreement with that obtained from our BDS at constant forces. An alternative to the current mechanism of catch to slip transition is presented and validated through BDS.

N-linked glycans within the A2 domain of von Willebrand factor modulate macrophage-mediated clearance

Enhanced von Willebrand factor (VWF) clearance is important in the etiology of von Willebrand disease. However, the molecular mechanisms underlying VWF clearance remain poorly understood. In this study, we investigated the role of VWF domains and specific glycan moieties in regulating in vivo clearance. Our findings demonstrate that the A1 domain of VWF contains a receptor-recognition site that plays a key role in regulating the interaction of VWF with macrophages. In A1-A2-A3 and full-length VWF, this macrophage-binding site is cryptic but becomes exposed following exposure to shear or ristocetin. Previous studies have demonstrated that the N-linked glycans within the A2 domain play an important role in modulating susceptibility to ADAMTS13 proteolysis. We further demonstrate that these glycans presented at N1515 and N1574 also play a critical role in protecting VWF against macrophage binding and clearance. Indeed, loss of the N-glycan at N1515 resulted in markedly enhanced VWF clearance that was significantly faster than that observed with any previously described VWF mutations. In addition, A1-A2-A3 fragments containing the N1515Q or N1574Q substitutions also demonstrated significantly enhanced clearance. Importantly, clodronate-induced macrophage depletion significantly attenuated the increased clearance observed with N1515Q and N1574Q in both full-length VWF and A1-A2-A3. Finally, we further demonstrate that loss of these N-linked glycans does not enhance clearance in VWF in the presence of a structurally constrained A2 domain. Collectively, these novel findings support the hypothesis that conformation of the VWF A domains plays a critical role in modulating macrophage-mediated clearance of VWF in vivo.

Cooperative unfolding of distinctive mechanoreceptor domains transduces force into signals

How cells sense their mechanical environment and transduce forces into biochemical signals is a crucial yet unresolved question in mechanobiology. Platelets use receptor glycoprotein Ib (GPIb), specifically its α subunit (GPIbα), to signal as they tether and translocate on von Willebrand factor (VWF) of injured arterial surfaces against blood flow. Force elicits catch bonds to slow VWF-GPIbα dissociation and unfolds the GPIbα leucine-rich repeat domain (LRRD) and juxtamembrane mechanosensitive domain (MSD). How these mechanical processes trigger biochemical signals remains unknown. Here we analyze these extracellular events and the resulting intracellular Ca(2+) on a single platelet in real time, revealing that LRRD unfolding intensifies Ca(2+) signal whereas MSD unfolding affects the type of Ca(2+) signal. Therefore, LRRD and MSD are analog and digital force transducers, respectively. The >30 nm macroglycopeptide separating the two domains transmits force on the VWF-GPIbα bond (whose lifetime is prolonged by LRRD unfolding) to the MSD to enhance its unfolding, resulting in unfolding cooperativity at an optimal force. These elements may provide design principles for a generic mechanosensory protein machine.

Acquired von Willebrand syndrome associated with left ventricular assist device

Left ventricular assist devices (LVAD) provide cardiac support for patients with end-stage heart disease as either bridge or destination therapy, and have significantly improved the survival of these patients. Whereas earlier models were designed to mimic the human heart by producing a pulsatile flow in parallel with the patient's heart, newer devices, which are smaller and more durable, provide continuous blood flow along an axial path using an internal rotor in the blood. However, device-related hemostatic complications remain common and have negatively affected patients' recovery and quality of life. In most patients, the von Willebrand factor (VWF) rapidly loses large multimers and binds poorly to platelets and subendothelial collagen upon LVAD implantation, leading to the term acquired von Willebrand syndrome (AVWS). These changes in VWF structure and adhesive activity recover quickly upon LVAD explantation and are not observed in patients with heart transplant. The VWF defects are believed to be caused by excessive cleavage of large VWF multimers by the metalloprotease ADAMTS-13 in an LVAD-driven circulation. However, evidence that this mechanism could be the primary cause for the loss of large VWF multimers and LVAD-associated bleeding remains circumstantial. This review discusses changes in VWF reactivity found in patients on LVAD support. It specifically focuses on impacts of LVAD-related mechanical stress on VWF structural stability and adhesive reactivity in exploring multiple causes of AVWS and LVAD-associated hemostatic complications.

Linker regions and flexibility around the metalloprotease domain account for conformational activation of ADAMTS-13

BACKGROUND

Recently, conformational activation of ADAMTS-13 was identified. This mechanism showed the evolution from a condensed conformation, in which the proximal MDTCS and distal T2-CUB2 domains are in close contact with each other, to an activated, open structure due to binding with von Willebrand factor (VWF).

OBJECTIVES

Identification of cryptic epitope/exosite exposure after conformational activation and of sites of flexibility in ADAMTS-13.

METHODS

The activating effect of 25 anti-T2-CUB2 antibodies was studied in the FRETS-VWF73 and the vortex assay. Cryptic epitope/exosite exposure was determined with ELISA and VWF binding assay. The molecular basis for flexibility was hypothesized through rapid automatic detection and alignment of repeats (RADAR) analysis, tested with ELISA using deletion variants and visualized using electron microscopy.

RESULTS

Eleven activating anti-ADAMTS-13 antibodies, directed against the T5-CUB2 domains, were identified in the FRETS-VWF73 assay. RADAR analysis identified three linker regions in the distal domains. Interestingly, identification of an antibody recognizing a cryptic epitope in the metalloprotease domain confirmed the contribution of these linker regions to conformational activation of the enzyme. The proof of flexibility around both the T2 and metalloprotease domains, as shown by by electron microscopy, further supported this contribution. In addition, cryptic epitope exposure was identified in the distal domains, because activating anti-T2-CUB2 antibodies increased the binding to folded VWF up to ~3-fold.

CONCLUSION

Conformational activation of ADAMTS-13 leads to cryptic epitope/exosite exposure in both proximal and distal domains, subsequently inducing increased activity. Furthermore, three linker regions in the distal domains are responsible for flexibility and enable the interaction between the proximal and the T8-CUB2 domains.

Shear-Dependent Interactions of von Willebrand Factor with Factor VIII and Protease ADAMTS 13 Demonstrated at a Single Molecule Level by Atomic Force Microscopy

Vital functions of mammals are only possible due to the behavior of blood to coagulate most efficiently in vessels with particularly high wall shear rates. This is caused by the functional changes of the von Willebrand Factor (VWF), which mediates coagulation of blood platelets (primary hemostasis) especially when it is stretched under shear stress. Our data show that shear stretching also affects other functions of VWF: Using a customized device to simulate shear conditions and to conserve the VWF molecules in their unstable, elongated conformation, we visualize at single molecule level by AFM that VWF is preferentially cleaved by the protease ADAMTS13 at higher shear rates. In contrast to this high shear-rate-selective behavior, VWF binds FVIII more effectively only below a critical shear rate of ∼30.000 s(-1), indicating that under harsh shear conditions FVIII is released from its carrier protein. This may be required to facilitate delivery of FVIII locally to promote secondary hemostasis.