Identification of amino acid residues essential for von Willebrand factor binding to platelet glycoprotein Ib. Charged-to-alanine scanning mutagenesis of the A1 domain of human von Willebrand factor

How Electricity Prevents Us from Bleeding to Death

Rapid tissue repair is also needed in the event of damage to blood vessels. Most of the essential steps that prevent us from bleeding to death involve the functions of Von Willebrand factor (VWF) and many of these are dependent on electrical forces.

VWF-Gly2752Ser, a novel non-cysteine substitution variant in the CK domain, exhibits severe secretory impairment by hampering C-terminal dimer formation

BACKGROUND

Von Willebrand factor (VWF) is a multimeric glycoprotein that plays important roles in hemostasis and thrombosis. C-terminal interchain-disulfide bonds in the cystine knot (CK) domain are essential for VWF dimerization. Previous studies have reported that missense variants of cysteine in the CK domain disrupt the intrachain-disulfide bond and cause type 3 von Willebrand disease (VWD). However, type 3 VWD-associated noncysteine substitution variants in the CK domain have not been reported.

OBJECTIVE

To investigate the molecular mechanism of a novel non-cysteine variant in the CK domain, VWF c.8254 G>A (p.Gly2752Ser), which was identified in a patient with type 3 VWD as homozygous.

METHODS

Genetic analysis was performed by whole exome sequencing and Sanger sequencing. VWF multimer analysis was performed using SDS-agarose electrophoresis. VWF production and subcellular localization were analyzed using ex vivo endothelial colony forming cells (ECFCs) and an in vitro recombinant VWF (rVWF) expression system.

RESULTS

The patient was homozygous for VWF-Gly2752Ser. Plasma VWF enzyme-linked immunosorbent assay showed that the VWF antigen level of the patient was 1.2% compared with healthy subjects. A tiny amount of VWF was identified in the patient's ECFC. Multimer analysis revealed that the circulating VWF-Gly2752Ser presented only low molecular weight multimers. Subcellular localization analysis of VWF-Gly2752Ser-transfected cell lines showed that rVWF-Gly2752Ser was severely impaired in its ER-to-Golgi trafficking.

CONCLUSION

VWF-Gly2752Ser causes severe secretory impairment because of its dimerization failure. This is the first report of a VWF variant with a noncysteine substitution in the CK domain that causes type 3 VWD.

Ancestrally Reconstructed von Willebrand Factor Reveals Evidence for Trench Warfare Coevolution between Opossums and Pit Vipers

Opossums in the tribe Didelphini are resistant to pit viper venoms and are hypothesized to be coevolving with venomous snakes. Specifically, a protein involved in blood clotting (von Willebrand factor [vWF] which is targeted by snake venom C-type lectins [CTLs]) has been found to undergo rapid adaptive evolution in Didelphini. Several unique amino acid changes in vWF could explain their resistance; however, experimental evidence that these changes disrupt binding to venom CTLs was lacking. Furthermore, without explicit testing of ancestral phenotypes to reveal the mode of evolution, the assertion that this system represents an example of coevolution rather than noncoevolutionary adaptation remains unsupported. Using expressed vWF proteins and purified venom CTLs, we quantified binding affinity for vWF proteins from all resistant taxa, their venom-sensitive relatives, and their ancestors. We show that CTL-resistant vWF is present in opossums outside clade Didelphini and likely across a wider swath of opossums (family Didelphidae) than previously thought. Ancestral reconstruction and in vitro testing of vWF phenotypes in a clade of rapidly evolving opossums reveal a pattern consistent with trench warfare coevolution between opossums and their venomous snake prey.

Platelet-inspired nanomedicine in hemostasis thrombosis and thromboinflammation

Platelets are anucleate cell-fragments derived predominantly from megakaryocytes in the bone marrow and released in the blood circulation, with a normal count of 150 000-40 000 per μl and a lifespan of approximately 10 days in humans. A primary role of platelets is to aid in vascular injury site-specific clot formation to stanch bleeding, termed hemostasis. Platelets render hemostasis by a complex concert of mechanisms involving platelet adhesion, activation and aggregation, coagulation amplification, and clot retraction. Additionally, platelet secretome can influence coagulation kinetics and clot morphology. Therefore, platelet defects and dysfunctions result in bleeding complications. Current treatment for such complications involve prophylactic or emergency transfusion of platelets. However, platelet transfusion logistics constantly suffer from limited donor availability, challenges in portability and storage, high bacterial contamination risks, and very short shelf life (~5 days). To address these issues, an exciting area of research is focusing on the development of microparticle- and nanoparticle-based platelet surrogate technologies that can mimic various hemostatic mechanisms of platelets. On the other hand, aberrant occurrence of the platelet mechanisms lead to the pathological manifestation of thrombosis and thromboinflammation. The treatments for this are focused on inhibiting the mechanisms or resolving the formed clots. Here, platelet-inspired technologies can provide unique platforms for disease-targeted drug delivery to achieve high therapeutic efficacy while avoiding systemic side-effects. This review will provide brief mechanistic insight into the role of platelets in hemostasis, thrombosis and thromboinflammation, and present the current state-of-art in the design of platelet-inspired nanomedicine for applications in these areas.

Prediction of binding characteristics between von Willebrand factor and platelet glycoprotein Ibα with various mutations by molecular dynamic simulation

INTRODUCTION

Binding of platelet glycoprotein (GP)Ibα with von-Willebrand factor (VWF) exclusively mediates the initial platelet adhesion to injured vessel wall. To understand the mechanism of biomedical functions, we calculated the dynamic fluctuating three-dimensional (3D) structures and dissociation energy for GPIbα with various single amino-acid substitution at G233, which location is known to cause significant changes in platelet adhesive characteristics.

MATERIAL AND METHODS

Molecular dynamics (MD) simulation was utilized to calculate 3D structures and Potential of Mean Force (PMF) for wild-type VWF bound with wild-type, G233A (equal function), G233V (gain of function), and G233D (loss of function) GPIbα. Simulation was done on water-soluble condition with time-step of 2 × 10^-15 s using NAnoscale Molecular Dynamics (NAMD) with Chemistry at HARvard Molecular Mechanics (CHARMM) force field. Initial structure for each mutant was obtained by inducing single amino-acid substitution to the stable water-soluble binding structure of wild-type.

RESULTS

The most stable structures of wild-type VWF bound to GPIbα in wild-type or any mutant did not differ. However, bond dissociation energy defined as difference of PMF between most stable structure and the structure at 65 Å mass center distances in G233D was 4.32 kcal/mol (19.5%) lower than that of wild-type. Approximately, 2.07 kcal/mol energy was required to dissociate VWF from GPIbα with G233V at mass center distance from 48 to 52 Å, which may explain the apparent "gain of function" in G233V.

CONCLUSION

The mechanism of substantially different biochemical characteristics of GPIbα with mutations in G233 location was predicted from physical movement of atoms constructing these proteins.

Long-ranged Protein-glycan Interactions Stabilize von Willebrand Factor A2 Domain from Mechanical Unfolding

von Willebrand Factor (vWF) is a large multimeric protein that binds to platelets and collagen in blood clotting. vWF A2 domain hosts a proteolytic site for ADAMTS13 (A Disintegrin and Metalloprotease with a ThromboSpondin type 1 motif, member 13) to regulate the size of vWF multimers. This regulation process is highly sensitive to force conditions and protein-glycan interactions as the process occurs in flowing blood. There are two sites on A2 domain (N1515 and N1574) bearing various N-linked glycan structures. In this study, we used molecular dynamics (MD) simulation to study the force-induced unfolding of A2 domain with and without a single N-linked glycan type on each site. The sequential pullout of β-strands was used to represent a characteristic unfolding sequence of A2. This unfolding sequence varied due to protein-glycan interactions. The force-extension and total energy-extension profiles also show differences in magnitude but similar characteristic shapes between the systems with and without glycans. Systems with N-linked glycans encountered higher energy barriers for full unfolding and even for unfolding up to the point of ADAMTS13 cleavage site exposure. Interestingly, there is not much difference observed for A2 domain structure itself with and without glycans from standard MD simulations, suggesting roles of N-glycans in A2 unfolding through long-ranged protein-glycan interactions.

Vwf K1362A resulted in failure of protein synthesis in mice

Von Willebrand factor (VWF) is synthesized in megakaryocytes and endothelial cells (ECs) and has two main roles: to carry and protect coagulation factor VIII (FVIII) from degradation by forming VWF-FVIII complex; and to mediate platelet adhesion and aggregation at sites of vascular injury. Previous research using the HEK293 cell line revealed that the VWF K1362 mutation interacted directly with platelet glycoprotein Ib (GPIb). Vwf K1362A knock-in (KI) mice were therefore generated to verify the in vivo function of residue 1362 in binding to platelet GPIb. The Cre-loxP system was employed to introduce the Vwf K1362A mutation systemically in mice. In blood coagulation analysis, the VWF antigen (VWF:Ag) of Lys1362Ala KI homozygous (homo) mice was below the sensitivity of detection by enzyme-linked immunosorbent assay. FVIII activities (FVIII:C) were 47.9 ± 0.3 and 3.3 ± 0.3% (K1362A heterozygous (hetero) and K1362A KI homo mice, respectively) compared to wild-type mice. Immunohistochemical staining analysis revealed that VWF protein did not exist in ECs of K1362A KI homo mice. These results indicated that VWF protein synthesis of K1362A was impaired after transcription in mice. K1362 seems to represent a very important position not only for VWF function, but also for VWF synthesis in mice.

New Concepts and Mechanisms of Platelet Activation Signaling

Upon blood vessel injury, platelets are exposed to adhesive proteins in the vascular wall and soluble agonists, which initiate platelet activation, leading to formation of hemostatic thrombi. Pathological activation of platelets can induce occlusive thrombosis, resulting in ischemic events such as heart attack and stroke, which are leading causes of death globally. Platelet activation requires intracellular signal transduction initiated by platelet receptors for adhesion proteins and soluble agonists. Whereas many platelet activation signaling pathways have been established for many years, significant recent progress reveals much more complex and sophisticated signaling and amplification networks. With the discovery of new receptor signaling pathways and regulatory networks, some of the long-standing concepts of platelet signaling have been challenged. This review provides an overview of the new developments and concepts in platelet activation signaling.

A biophysical view on von Willebrand factor activation

The process of hemostatic plug formation at sites of vascular injury crucially relies on the large multimeric plasma glycoprotein von Willebrand factor (VWF) and its ability to recruit platelets to the damaged vessel wall via interaction of its A1 domain with platelet GPIbα. Under normal blood flow conditions, VWF multimers exhibit a very low binding affinity for platelets. Only when subjected to increased hydrodynamic forces, which primarily occur in connection with vascular injury, VWF can efficiently bind to platelets. This force-regulation of VWF's hemostatic activity is not only highly intriguing from a biophysical perspective, but also of eminent physiological importance. On the one hand, it prevents undesired activity of VWF in intact vessels that could lead to thromboembolic complications and on the other hand, it enables efficient VWF-mediated platelet aggregation exactly where needed. Here, we review recent studies that mainly employed biophysical approaches in order to elucidate the molecular mechanisms underlying the complex mechano-regulation of the VWF-GPIbα interaction. Their results led to two main hypotheses: first, intramolecular shielding of the A1 domain is lifted upon force-induced elongation of VWF; second, force-induced conformational changes of A1 convert it from a low-affinity to a high-affinity state. We critically discuss these hypotheses and aim at bridging the gap between the large-scale behavior of VWF as a linear polymer in hydrodynamic flow and the detailed properties of the A1-GPIbα bond at the single-molecule level.

Adhesive Forces between A1 Domain of von Willebrand Factor and N-terminus Domain of Glycoprotein Ibα Measured by Atomic Force Microscopy

AIM

von Willebrand factor (VWF) plays an important role in the regulation of hemostasis and thrombosis formation, particularly under a high shear rate. However, the adhesive force due to the molecular interaction between VWF and glycoprotein Ibα (GPIbα) has not been fully explored. Thus, we employed atomic force microscopy to directly measure the adhesive force between VWF and GPIbα.

METHODS

We measured the adhesive force between VWF and GPIbα at the molecular level using an atomic force microscope (AFM). An AFM cantilever was coated with recombinant N-terminus VWF binding site of GPIbα, whereas a cover glass was coated with native VWF.

RESULTS

The adhesive force at the molecular level was measured using an AFM. In the presence of 1 μg/mL VWF, the adhesion force was nearly 200 pN. As per the Gaussian fit analysis, the adhesive force of a single bond could have been 54 or 107 pN.

CONCLUSION

Our consideration with the Gaussian fit analysis proposed that the adhesive force of a single bond could be 54 pN, which is very close to that obtained by optical tweezers (50 pN).

Storage and secretion of naturally occurring von Willebrand factor A domain variants

Von Willebrand disease (VWD) is a bleeding disorder characterized by reduced plasma von Willebrand factor (VWF) levels or functionally abnormal VWF. Low VWF plasma levels in VWD patients are the result of mutations in the VWF gene that lead to decreased synthesis, impaired secretion, increased clearance or a combination thereof. However, expression studies of variants located in the A domains of VWF are limited. We therefore characterized the biosynthesis of VWF mutations, located in the VWF A1-A3 domains, that were found in families diagnosed with VWD. Human Embryonic Kidney 293 (HEK293) cells were transiently transfected with plasmids encoding full-length wild-type VWF or mutant VWF. Six mutations in the A1-A3 domains were expressed. We found that all mutants, except one, showed impaired formation of elongated pseudo-Weibel-Palade bodies (WPB). In addition, two mutations also showed reduced numbers of pseudo-WPB, even in the heterozygous state, and increased endoplasmic reticulum retention, which is in accordance with the impaired regulated secretion seen in patients. Regulated secretion upon stimulation of transfected cells reproduced the in vivo situation, indicating that HEK293 cells expressing VWF variants found in patients with VWD can be used to properly assess defects in regulated secretion.

Platelet adhesion involves a novel interaction between vimentin and von Willebrand factor under high shear stress

The interaction between platelet receptor glycoprotein Ibα and the A1 domain of von Willebrand factor (VWF) mediates tethering/translocation of platelets to sites of vascular injury. Unexpectedly, we observed platelets translocating over A1A2A3 domains protein slower than on A1 domain at high shear stress. This observation suggests an additional interaction between A domains and an adhesive receptor. We investigated vimentin because we have data showing the interaction of vimentin with the A2 domain of VWF. Moreover, vimentin is expressed on the platelet surface. This novel interaction was analyzed by using purified VWF, recombinant proteins, anti-vimentin antibodies, parallel flow chamber adhesion assays, flow cytometry, and vimentin-deficient murine platelets. The active form of VWF bound to vimentin, and the purified A2 domain blocked that binding. The interaction of a gain-of-function A1A2A3 mutant with platelet was reduced using anti-vimentin antibody. Platelet adhesion to wild-type (WT) A1A2A3 protein, collagen, and fibrin(ogen) was inhibited (32-75%) by anti-vimentin antibody under high shear stress. Compared with WT mice, platelets from vimentin-deficient mice had a reduced flow-dependent adhesion to both collagen and purified murine VWF. Last, the vimentin knockout mice had a prolonged tail bleeding time. The results describe that platelet vimentin engages VWF during platelet adhesion under high shear stress.

The N-terminal flanking region of the A1 domain regulates the force-dependent binding of von Willebrand factor to platelet glycoprotein Ibα

Binding of platelet glycoprotein Ibα (GPIbα) to von Willebrand factor (VWF) initiates platelet adhesion to disrupted vascular surface under arterial blood flow. Flow exerts forces on the platelet that are transmitted to VWF-GPIbα bonds, which regulate their dissociation. Mutations in VWF and/or GPIbα may alter the mechanical regulation of platelet adhesion to cause hemostatic defects as found in patients with von Willebrand disease (VWD). Using a biomembrane force probe, we observed biphasic force-decelerated (catch) and force-accelerated (slip) dissociation of GPIbα from VWF. The VWF A1 domain that contains the N-terminal flanking sequence Gln(1238)-Glu(1260) (1238-A1) formed triphasic slip-catch-slip bonds with GPIbα. By comparison, using a short form of A1 that deletes this sequence (1261-A1) abolished the catch bond, destabilizing its binding to GPIbα at high forces. Importantly, shear-dependent platelet rolling velocities on these VWF ligands in a flow chamber system mirrored the force-dependent single-bond lifetimes. Adding the Gln(1238)-Glu(1260) peptide, which interacted with GPIbα and 1261-A1 but not 1238-A1, to whole blood decreased platelet attachment under shear stress. Soluble Gln(1238)-Glu(1260) reduced the lifetimes of GPIbα bonds with VWF and 1238-A1 but rescued the catch bond of GPIbα with 1261-A1. A type 2B VWD 1238-A1 mutation eliminated the catch bond by prolonging lifetimes at low forces, a type 2M VWD 1238-A1 mutation shifted the respective slip-catch and catch-slip transition points to higher forces, whereas a platelet type VWD GPIbα mutation enhanced the bond lifetime in the entire force regime. These data reveal the structural determinants of VWF activation by hemodynamic force of the circulation.

An optimized fluorogenic ADAMTS13 assay with increased sensitivity for the investigation of patients with thrombotic thrombocytopenic purpura

BACKGROUND

Most ADAMTS13 assays use non-physiological conditions (low ionic strength, low pH, barium chloride), are subject to interference from plasma proteins, hemoglobin and bilirubin, and have limited sensitivity, especially for inhibitors.

OBJECTIVES

We addressed these constraints by designing a substrate that can be used in undiluted plasma.

METHODS

A polypeptide was expressed in E. coli that corresponds to von Willebrand factor Gln(1599) -Arg(1668) , with mutations N1610C and K1617R and an N-terminal Gly. Substrate FRETS-rVWF71 was prepared by modifying Cys(1610) with DyLight 633 (abs 638 nm, em 658 nm) and the N-terminus with IRDye QC-1 (abs 500-800 nm). Assays were performed at pH 7.4 in 150 mm NaCl, 10 mm CaCl2 .

RESULTS

Serum and plasma anticoagulated with citrate or heparin had equivalent ADAMTS13 activity with FRETS-rVWF71. Neither bilirubin (≤ 20 mg dL(-1) ) nor hemoglobin (≤ 20 g L(-1) ) interfered with product detection. Assays with FRETS-rVWF71 and FRETS-VWF73 gave similar results (R(2 ) = 0.95) for plasma from 80 subjects with thrombotic microangiopathy, 22 subjects with other causes of thrombocytopenia, and 20 healthy controls. The limit of detection with FRETS-rVWF71 for ADAMTS13 activity was ≤ 0.3%. Inhibitor assays with FRETS-rVWF71 gave titers ~2.5-fold higher than with FRETS-VWF73 and clearly distinguished patients with and without inhibitors.

CONCLUSIONS

FRETS-rVWF71 is suitable for ADAMTS13 assays in minimally diluted plasma or serum without interference from proteins, bilirubin or free hemoglobin in plasma. Optimized detection of ADAMTS13 inhibitors will facilitate the monitoring of antibody responses during the treatment of thrombotic thrombocytopenic purpura.

A mechanism for localized dynamics-driven affinity regulation of the binding of von Willebrand factor to platelet glycoprotein Ibα

Binding of the A1 domain of von Willebrand factor (vWF) to glycoprotein Ibα (GPIbα) results in platelet adhesion, activation, and aggregation that initiates primary hemostasis. Both the elevated shear stress and the mutations associated with type 2B von Willebrand disease enhance the interaction between A1 and GPIbα. Through molecular dynamics simulations for wild-type vWF-A1 and its eight gain of function mutants (R543Q, I546V, ΔSS, etc.), we found that the gain of function mutations destabilize the N-terminal arm, increase a clock pendulum-like movement of the α2-helix, and turn a closed A1 conformation into a partially open one favoring binding to GPIbα. The residue Arg(578) at the α2-helix behaves as a pivot in the destabilization of the N-terminal arm and a consequent dynamic change of the α2-helix. These results suggest a localized dynamics-driven affinity regulation mechanism for vWF-GPIbα interaction. Allosteric drugs controlling this intrinsic protein dynamics may be effective in blocking the GPIb-vWF interaction.

An HMM-based algorithm for evaluating rates of receptor-ligand binding kinetics from thermal fluctuation data

MOTIVATION

Abrupt reduction/resumption of thermal fluctuations of a force probe has been used to identify association/dissociation events of protein-ligand bonds. We show that off-rate of molecular dissociation can be estimated by the analysis of the bond lifetime, while the on-rate of molecular association can be estimated by the analysis of the waiting time between two neighboring bond events. However, the analysis relies heavily on subjective judgments and is time-consuming. To automate the process of mapping out bond events from thermal fluctuation data, we develop a hidden Markov model (HMM)-based method.

RESULTS

The HMM method represents the bond state by a hidden variable with two values: bound and unbound. The bond association/dissociation is visualized and pinpointed. We apply the method to analyze a key receptor-ligand interaction in the early stage of hemostasis and thrombosis: the von Willebrand factor (VWF) binding to platelet glycoprotein Ibα (GPIbα). The numbers of bond lifetime and waiting time events estimated by the HMM are much more than those estimated by a descriptive statistical method from the same set of raw data. The kinetic parameters estimated by the HMM are in excellent agreement with those by a descriptive statistical analysis, but have much smaller errors for both wild-type and two mutant VWF-A1 domains. Thus, the computerized analysis allows us to speed up the analysis and improve the quality of estimates of receptor-ligand binding kinetics.

Characterisation of mutations and molecular studies of type 2 von Willebrand disease

Type 2 von Willebrand disease (VWD) is characterised by qualitative defects in von Willebrand factor (VWF). Exon 28 of the VWF gene is known to be a hot spot for type 2 VWD mutations. The goal of this study was to characterise the mutations in VWF exon 28 and understand the molecular basis of phenotypes through in vitro and in silico studies. Mutation screening was performed in 56 type 2 VWD patients through direct sequencing. Expression vectors for five mutations were transiently expressed in 293-EBNA cells to understand the mutations pathology. Furthermore, in silico structure analysis was performed for 13 missense mutations. A total of 16 including eight novel mutations were detected in 23 (41%) patients. Of these, 15 were missense (including seven V1439M, A1464P, M1495L, I1509V, R1527Q, N1635I and A1647D novel ones) and one was a novel gene conversion. Expression studies and characterisation of recombinant VWF suggested the loss of VWF function for mutants P1266Q, V1439M and N1635I and gain of function for mutant R1308C. No apparent defect was seen in mutant N1231S. In silico structure analysis suggested the probable gain or loss of hydrogen/van der Waals interactions in 10 mutant proteins. In conclusion, type 2A mutations and gene conversion were found to be a common cause of type 2 VWD. Expression studies suggest the mutations N1635I for type 2A(II), P1266Q and V1439M for type 2M, R1308C for type 2B VWD and N1231S as a non-causative variant. Moreover, in silico studies of the mutants show the probable cause of respective phenotypes.

N-terminal flanking region of A1 domain in von Willebrand factor stabilizes structure of A1A2A3 complex and modulates platelet activation under shear stress

von Willebrand factor (vWF) mediates platelet adhesion and thrombus formation via its interaction with the platelet receptor glycoprotein (GP)Ibα. We have analyzed two A1A2A3 tri-domain proteins to demonstrate that the amino acid sequence, Gln(1238)-Glu(1260), in the N-terminal flanking region of the A1 domain, together with the association between the A domains, modulates vWF-GPIbα binding and platelet activation under shear stress. Using circular dichroism spectroscopy and differential scanning calorimetry, we have described that sequence Gln(1238)-Glu(1260) stabilizes the structural conformation of the A1A2A3 tri-domain complex. The structural stabilization imparted by this particular region inhibits the binding capacity of the tri-domain protein for GPIbα. Deletion of this region causes a conformational change in the A1 domain that increases binding to GPIbα. Only the truncated protein was capable of effectively blocking ristocetin-induced platelet agglutination. To determine the capacity of activating platelets via the interaction with GPIbα, whole blood was incubated with the N-terminal region truncated or intact tri-A domain protein prior to perfusion over a fibrin(ogen)-coated surface. At a high shear rate of 1,500 s(-1), platelets from blood containing the truncated protein rapidly bound, covering >90% of the fibrin(ogen) surface area, whereas the intact tri-A domain protein induced platelets to bind <10%. The results obtained in this study ascertain the relevant role of the structural association between the N-terminal flanking region of the A1 domain (amino acids Gln(1238)-Glu(1260)) and the A1A2A3 domain complex in preventing vWF to bind spontaneously to GPIbα in solution under high shear forces.

A pH-regulated dimeric bouquet in the structure of von Willebrand factor

At the acidic pH of the trans-Golgi and Weibel-Palade bodies (WPBs), but not at the alkaline pH of secretion, the C-terminal ∼1350 residues of von Willebrand factor (VWF) zip up into an elongated, dimeric bouquet. Six small domains visualized here for the first time between the D4 and cystine-knot domains form a stem. The A2, A3, and D4 domains form a raceme with three pairs of opposed, large, flower-like domains. N-terminal VWF domains mediate helical tubule formation in WPBs and template N-terminal disulphide linkage between VWF dimers, to form ultralong VWF concatamers. The dimensions we measure in VWF at pH 6.2 and 7.4, and the distance between tubules in nascent WPB, suggest that dimeric bouquets are essential for correct VWF dimer incorporation into growing tubules and to prevent crosslinking between neighbouring tubules. Further insights into the structure of the domains and flexible segments in VWF provide an overall view of VWF structure important for understanding both the biogenesis of ultralong concatamers at acidic pH and flow-regulated changes in concatamer conformation in plasma at alkaline pH that trigger hemostasis.

Phylogenetic and functional analysis of histidine residues essential for pH-dependent multimerization of von Willebrand factor

von Willebrand factor (VWF) is a multimeric plasma protein that mediates platelet adhesion to sites of vascular injury. The hemostatic function of VWF depends upon the formation of disulfide-linked multimers, which requires the VWF propeptide (D1D2 domains) and adjacent D'D3 domains. VWF multimer assembly occurs in the trans-Golgi at pH ~ 6.2 but not at pH 7.4, which suggests that protonation of one or more His residues (pK(a) ~6.0) mediates the pH dependence of multimerization. Alignment of 30 vertebrate VWF sequences identified 13 highly conserved His residues in the D1D2D'D3 domains, and His-to-Ala mutagenesis identified His³⁹⁵ and His⁴⁶⁰ in the D2 domain as critical for VWF multimerization. Replacement of His³⁹⁵ with Lys or Arg prevented multimer assembly, suggesting that reversible protonation of this His residue is essential. In contrast, replacement of His⁴⁶⁰ with Lys or Arg preserved normal multimer assembly, whereas Leu, Met, and Gln did not, indicating that the function of His⁴⁶⁰ depends primarily upon the presence of a positive charge. These results suggest that pH sensing by evolutionarily conserved His residues facilitates the assembly and packaging of VWF multimers upon arrival in the trans-Golgi.

GPIbα-vWF rolling under shear stress shows differences between type 2B and 2M von Willebrand disease

Both type 2B and type 2M von Willebrand disease result in bleeding disorders; however, whereas type 2B has increased binding affinity between platelet glycoprotein Ibα and von Willebrand factor (vWF), type 2M has decreased binding affinity between these two molecules. We used R687E type 2B and G561S type 2M vWF-A1 mutations to study binding between flowing platelets and insolubilized vWF mutants. We measured rolling velocities, mean stop times, and mean go times at 37°C using high-speed video microscopy. The rolling velocities for wt-wt interactions first decrease, reach a minimum, and then increase with increasing shear stress, indicating a catch-slip transition. By changing the viscosity, we were able to quantify the effects of force versus shear rate for rolling velocities and mean stop times. Platelet interactions with loss-of-function vWF-A1 retain the catch-slip bond transition seen in wt-wt interactions, but at a higher shear stress compared with the wt-wt transition. The mean stop time for all vWF-A1 molecules reveals catch-slip transitions at different shear stresses (gain-of-function vWF-A1 < wt vWF-A1< loss-of-function vWF-A1). The shift in the catch-slip transition may indicate changes in how the different mutants become conformationally active, indicating different mechanisms leading to similar bleeding characteristics.

Targeting von Willebrand factor and platelet glycoprotein Ib receptor

Atherothrombotic events, such as acute coronary syndrome or stroke, are the result of platelet activation. Von Willebrand factor (vWF), a multimeric glycoprotein, plays a key role in aggregation of platelets, especially under high-shear conditions. Acting as bridging element or ligand between damaged endothelial sites and the glycoprotein Ib (GPIb) receptor on platelets, vWF is responsible for platelet adhesion and aggregation. This vWF activation and further platelet aggregation mainly occurs under high shear stress present in small arterioles or during deficiency of the vWF-cleaving protease ADAMTS13. There are several substances targeting vWF itself or its binding receptor GPIb on platelets. Two antibodies are directed against vWF: AJW200, an IgG4 humanized monoclonal antibody, and 82D6A3, a monoclonal antibody of the collagen-binding A-3 domain of vWF. ALX-0081 and ALX-0681 are bivalent humanized nanobodies targeting the GPIb binding site of vWF. Aptamers are oligonucleotides with drug-like properties that share some of the attributes of monoclonal antibodies. ARC1779 is a second-generation, nuclease-resistant aptamer, binding to the activated vWF A1 domain and ARC15105 is a chemically advanced follower with an assumed higher affinity to vWF. Antibodies targeting GPIbα are h6B4-Fab, a murine monoclonal antibody; GPG-290, a recombinant, chimeric protein containing the amino-terminal 290 amino acids of GPIbα linked to human IgG1 Fc; and the monoclonal antibody SZ2. There are a number of promising preclinical results and development of some agents (AJW 200, ARC1779 and ALX-0081) has already reached Phase II trials.

Flow-induced beta-hairpin folding of the glycoprotein Ibalpha beta-switch

Flow-induced shear has been identified as a regulatory driving force in blood clotting. Shear induces beta-hairpin folding of the glycoprotein Ibalpha beta-switch which increases affinity for binding to the von Willebrand factor, a key step in blood clot formation and wound healing. Through 2.1-micros molecular dynamics simulations, we investigate the kinetics of flow-induced beta-hairpin folding. Simulations sampling different flow velocities reveal that under flow, beta-hairpin folding is initiated by hydrophobic collapse, followed by interstrand hydrogen-bond formation and turn formation. Adaptive biasing force simulations are employed to determine the free energy required for extending the unfolded beta-switch from a loop to an elongated state. Lattice and freely jointed chain models illustrate how the folding rate depends on the entropic and enthalpic energy, the latter controlled by flow. The results reveal that the free energy landscape of the beta-switch has two stable conformations imprinted on it, namely, loop and hairpin--with flow inducing a transition between the two.

The effect of shear stress on protein conformation: Physical forces operating on biochemical systems: The case of von Willebrand factor

Macromolecules and cells exposed to blood flow in the circulatory tree experience hydrodynamic forces that affect their structure and function. After introducing the general theory of the effects of shear forces on protein conformation, selected examples are presented in this review for biological macromolecules sensitive to shear stress. In particular, the biochemical effects of shear stress in controlling the von Willebrand Factor (VWF) conformation are extensively described. This protein, together with blood platelets, is the main actor of the early steps of primary haemostasis. Under the effect of shear forces >30 dyn/cm², VWF unfolding occurs and the protein exhibits an extended chain conformation oriented in the general direction of the shear stress field. The stretched VWF conformation favors also a process of self aggregation, responsible for the formation of a spider web network, particularly efficient in the trapping process of flowing platelets. Thus, the effect of shear stress on conformational changes in VWF shows a close structure-function relationship in VWF for platelet adhesion and thrombus formation in arterial circulation, where high shear stress is present. The investigation of biophysical effects of shear forces on VWF conformation contributes to unraveling the molecular interaction mechanisms involved in arterial thrombosis.

Accelerated clearance alone explains ultra-large multimers in von Willebrand disease Vicenza

BACKGROUND

von Willebrand disease (VWD) Vicenza is characterized by low plasma von Willebrand factor (VWF) levels, the presence of ultra-large (UL) VWF multimers and less prominent satellite bands on multimer gels, and the heterozygous amino acid substitution R1205H in the VWF gene. The pathogenesis of VWD Vicenza has been elusive. Accelerated clearance is implicated as a cause of low VWF level.

OBJECTIVES

We addressed the question, whether the presence of ultra-large multimers is a cause, or a result of accelerated VWF clearance, or whether it is an unrelated phenomenon.

PATIENTS/METHODS

We studied the detailed phenotype of three Hungarian patients with VWD Vicenza, expressed the mutant VWF-R1205H in 293T cells and developed a mathematical model to simulate VWF synthesis and catabolism.

RESULTS

We found that the half-life of VWF after DDAVP was approximately one-tenth of that after the administration of Haemate P, a source of exogenous wild-type (WT) VWF (0.81 + or - 0.2 vs. 7.25 + or - 2.38 h). An analysis of recombinant mutant VWF-R1205H showed that the biosynthesis and multimer structure of WT and mutant VWF were indistinguishable. A mathematical model of the complex interplay of VWF synthesis, clearance and cleavage showed that decreasing VWF half-life to one-tenth of normal reproduced all features of VWD Vicenza including low VWF level, ultra-large multimers and a decrease of satellite band intensity.

CONCLUSION

We conclude that accelerated clearance alone may explain all features of VWD Vicenza.

Identification of amino acid residues responsible for von Willebrand factor binding to sulfatide by charged-to-alanine-scanning mutagenesis

von Willebrand factor (VWF) performs its hemostatic functions through binding to various proteins. The A1 domain of VWF contains binding sites of not only physiologically important ligands, but also exogenous modulators that induce VWF-platelet aggregation. Sulfatides, 3-sulfated galactosyl ceramides, that are expressed on oligodendrocytes, renal tubular cells, certain tumor cells and platelets, have been suggested to interact with VWF under some pathological conditions. The binding of VWF to sulfatide requires the A1 domain, but its binding sites have not been precisely identified. Here, we report that alanine mutations at Arg1392, Arg1395, Arg1399 and Lys1423 led to decreased VWF–sulfatide binding. These sites have been reported to be the binding sites for platelet membrane glycoprotein (GP) Ib and/or snake venom botrocetin, and, interestingly, are identical to the monoclonal antibody (mAb) NMC4 epitope previously reported to inhibit the VWF-GPIb interaction. We observed that NMC4 also inhibited VWF interaction with sulfatides in a dose-dependent manner. Thus, we conclude that VWF binding sites of sulfatide overlap those of platelet GPIb and botrocetin.

Platelet glycoprotein Ibalpha forms catch bonds with human WT vWF but not with type 2B von Willebrand disease vWF

Arterial blood flow enhances glycoprotein Ibalpha (GPIbalpha) binding to vWF, which initiates platelet adhesion to injured vessels. Mutations in the vWF A1 domain that cause type 2B von Willebrand disease (vWD) reduce the flow requirement for adhesion. Here we show that increasing force on GPIbalpha/vWF bonds first prolonged ("catch") and then shortened ("slip") bond lifetimes. Two type 2B vWD A1 domain mutants, R1306Q and R1450E, converted catch bonds to slip bonds by prolonging bond lifetimes at low forces. Steered molecular dynamics simulations of GPIbalpha dissociating from the A1 domain suggested mechanisms for catch bonds and their conversion by the A1 domain mutations. Catch bonds caused platelets and GPIbalpha-coated microspheres to roll more slowly on WT vWF and WT A1 domains as flow increased from suboptimal levels, explaining flow-enhanced rolling. Longer bond lifetimes at low forces eliminated the flow requirement for rolling on R1306Q and R1450E mutant A1 domains. Flowing platelets agglutinated with microspheres bearing R1306Q or R1450E mutant A1 domains, but not WT A1 domains. Therefore, catch bonds may prevent vWF multimers from agglutinating platelets. A disintegrin and metalloproteinase with a thrombospondin type 1 motif-13 (ADAMTS-13) reduced platelet agglutination with microspheres bearing a tridomain A1A2A3 vWF fragment with the R1450E mutation in a shear-dependent manner. We conclude that in type 2B vWD, prolonged lifetimes of vWF bonds with GPIbalpha on circulating platelets may allow ADAMTS-13 to deplete large vWF multimers, causing bleeding.

N1421K mutation in the glycoprotein Ib binding domain impairs ristocetin- and botrocetin-mediated binding of von Willebrand factor to platelets

von Willebrand disease (VWD) is a common inheritable bleeding disorder caused by deficiency of von Willebrand Factor (VWF), which is involved in platelet adhesion and aggregation. We report a family consisting of three patients with VWD characterized by an apparently normal multimeric pattern, moderately decreased plasma factor VIII (FVIII) and VWF levels, and disproportionately low-plasma VWF:RCo levels. The patients were found to be heterozygous for the novel N1421K mutation, caused by a 4263C > G transversion in exon 28 of the VWF gene coding for the A1 domain. Botrocetin- and ristocetin-mediated binding of plasma VWF to GPIb were reduced in the patients. In vitro mutagenesis and expression in COS-7 cells confirmed the impairment of the mutant in botrocetin- and ristocetin-mediated VWF binding to GPIb. VWF collagen binding capacity was unaffected in plasma from the heterozygous individuals as well as in medium from transfected COS-7 cells. Our findings indicate that the N1421K substitution in the VWF affects the GPIb binding site or a recognition element by a conformational change of the A1 domain.

L1503R is a member of group I mutation and has dominant-negative effect on secretion of full-length VWF multimers: an analysis of two patients with type 2A von Willebrand disease

Type 2A von Willebrand disease (VWD) is characterized by decreased platelet-dependent function of von Willebrand factor (VWF); this in turn is associated with an absence of high-molecular-weight multimers. Sequence analysis of the VWF gene from two unrelated type 2A VWD patients showed an identical, novel, heterozygous T-->G transversion at nucleotide 4508, resulting in the substitution of L1503R in the VWF A2 domain. This substitution, which was not found in 60 unrelated normal individuals, was introduced into a full-length VWF cDNA and subsequently expressed in 293T cells. Only trace amount of the mutant VWF protein was secreted but most of the same was retained in 293T cells. Co-transfection experiment of both wild-type and mutant plasmids indicated the dominant-negative mechanism of disease development; as more of mutant DNA was transfected, VWF secretion was impaired in the media, whereas more of VWF was stored in the cell lysates. Molecular dynamic simulations of structural changes induced by L1503R indicated that the mean value of all-atom root-mean-squared-deviation was shifted from those with wild type or another mutation L1503Q that has been reported to be a group II mutation, which is susceptible to ADAMTS13 proteolysis. Protein instability of L1503R may be responsible for its intracellular retention and perhaps the larger VWF multimers, containing more mutant VWF subunits, are likely to be mal-processed and retained within the cell.

Assembly of Weibel-Palade body-like tubules from N-terminal domains of von Willebrand factor

Endothelial cells assemble von Willebrand factor (VWF) multimers into ordered tubules within storage organelles called Weibel-Palade bodies, and tubular packing is necessary for the secretion of VWF filaments that can bind connective tissue and recruit platelets to sites of vascular injury. We now have recreated VWF tubule assembly in vitro, starting with only pure VWF propeptide (domains D1D2) and disulfide-linked dimers of adjacent N-terminal D'D3 domains. Assembly requires low pH and calcium ions and is reversed at neutral pH. Quick-freeze deep-etch electron microscopy and three-dimensional reconstruction of negatively stained images show that tubules contain a repeating unit of one D'D3 dimer and two propeptides arranged in a right-handed helix with 4.2 units per turn. The symmetry and location of interdomain contacts suggest that decreasing pH along the secretory pathway coordinates the disulfide-linked assembly of VWF multimers with their tubular packaging.

Cloning of the cDNA for murine von Willebrand factor and identification of orthologous genes reveals the extent of conservation among diverse species

Interaction of von Willebrand factor (VWF) with circulating platelets promotes hemostasis when a blood vessel is injured. The A1 domain of VWF is responsible for the initial interaction with platelets and is well conserved among species. Knowledge of the cDNA and genomic DNA sequences for human VWF allowed us to predict the cDNA sequence for murine VWF in silico and amplify its entire coding region by RT-PCR. The murine VWF cDNA has an open reading frame of 8,442 bp, encoding a protein of 2,813 amino acid residues with 83% identity to human pre-pro-VWF. The same strategy was used to predict in silico the cDNA sequence for the ortholog of VWF in a further six species. Many of these predictions diverged substantially from the putative Reference Sequences derived by ab initio methods. Our predicted sequences indicated that the VWF gene has a conserved structure of 52 exons in all seven mammalian species examined, as well as in the chicken. There is a minor structural variation in the pufferfish Takifugu rubripes insofar as the VWF gene in this species has 53 exons. Comparison of the translated amino acid sequences also revealed a high degree of conservation. In particular, the cysteine residues are conserved precisely throughout both the pro-peptide and the mature VWF sequence in all species, with a minor exception in the pufferfish VWF ortholog where two adjacent cysteine residues are omitted. The marked conservation of cysteine residues emphasizes the importance of the intricate pattern of disulfide bonds in governing the structure of pro-VWF and regulating the function of the mature VWF protein. It should also be emphasized that many of the conserved features of the VWF gene and protein were obscured when the comparison among species was based on the putative Reference Sequences instead of our predicted cDNA sequences.

Two Cys residues essential for von Willebrand factor multimer assembly in the Golgi

Von Willebrand factor (VWF) dimerizes through C-terminal CK domains, and VWF dimers assemble into multimers in the Golgi by forming intersubunit disulfide bonds between D3 domains. This unusual oxidoreductase reaction requires the VWF propeptide (domains D1D2), which acts as an endogenous pH-dependent chaperone. The cysteines involved in multimer assembly were characterized by using a VWF construct that encodes the N-terminal D1D2D'D3 domains. Modification with thiol-specific reagents demonstrated that secreted D'D3 monomer contained reduced Cys, whereas D'D3 dimer and propeptide did not. Reduced Cys in the D'D3 monomer were alkylated with N-ethylmaleimide and analyzed by mass spectrometry. All 52 Cys within the D'D3 region were observed, and only Cys(1099) and Cys(1142) were modified by N-ethylmaleimide. When introduced into the D1D2D'D3 construct, the mutation C1099A or C1142A markedly impaired the formation of D'D3 dimers, and the double mutation prevented dimerization. In full-length VWF, the mutations C1099A and C1099A/C1142A prevented multimer assembly; the mutation C1142A allowed the formation of almost exclusively dimers, with few tetramers and no multimers larger than hexamers. Therefore, Cys(1099) and Cys(1142) are essential for the oxidoreductase mechanism of VWF multimerization. Cys(1142) is reported to form a Cys(1142)-Cys(1142) intersubunit bond, suggesting that Cys(1099) also participates in a Cys(1099)-Cys(1099) disulfide bond between D3 domains. This arrangement of intersubunit disulfide bonds implies that the dimeric N-terminal D'D3 domains of VWF subunits align in a parallel orientation within VWF multimers.

[Platelet--vessel wall interactions]

The vascular endothelium is a thromboresistant surface, allowing the free flowing of blood cell elements. Platelets are predominantly involved in the rapid response to a vascular lesion, exposing the underlying thrombogenic subendothelium and leading to initiation of thrombus formation. Thrombus growth requires on the one hand, the recruitment of circulating platelets to the luminal side of the thrombus and on the other hand, the assembly of the proteins of the blood coagulation cascade on the platelet catalytic surface leading to thrombin formation. High shear forces are necessary for the dual role of von Willebrand factor (VWF) in the initiation of platelet thrombus formation and in its growth and stabilization. In hemodynamic conditions, platelet adhesion depends on the interaction between VWF and the platelet receptor glycoprotein Ib (GPIb). This interaction is the only one able to resist to the high shear rates that prevail in arterioles, the microcirculation or stenosed arteries. Thereafter, the interaction between VWF and the alphaIIbbeta3 integrin allows the definitive arrest of platelets and induces thrombus formation. Thus, high shear forces by themselves are able to induce platelet activation/aggregation, without added exogenous agonist. VWF is synthesised by endothelial cells as a series of multimers of different sizes. The multimers with the highest molecular weight, the so-called ultra-large multimers, are strongly thrombogenic by their increased ability to bind platelet GPIb and to induce the formation of circulating aggregates. These ultra-large multimers are normally cleaved by the ADAMTS13 metalloprotease into smaller multimers which are also less thrombogenic. The in vivo proteolysis of VWF by ADAMTS13 depends on the high shear rates, which increase the opening of multimers anchored to the endothelial cell layer and the exposure of the cleavage site of VWF by ADAMTS13. An ADAMTS13 deficiency thus likely would result in the accumulation of ultra-large multimers on the endothelial surface, which retains platelets on the activated endothelium and results in micro-thrombi formation, as seen in thrombotic thrombocytopenic purpura. Platelet-VWF interactions are also involved in inflammation. Activation of endothelial cells induces the release of VWF from the Weibel-Palade bodies as well as the surface expression of VWF and P-selectin. These molecules allow leukocyte and platelet rolling on endothelial cells, and expression of E-selectin, VCAM-1 and other adhesion molecules. Recently, it has been shown that activated platelets allow transient activation of intact, non stimulated endothelial cells, thus increasing the inflammation process. VWF and platelet P-selectin have been shown to be essential to this process. Thus, platelet--vessel wall interactions are involved in thrombosis and inflammation essentially via VWF.

Exosite interactions contribute to tension-induced cleavage of von Willebrand factor by the antithrombotic ADAMTS13 metalloprotease

Von Willebrand factor (VWF) is a multimeric protein that mediates platelet adhesion at sites of vascular injury, and ADAMTS13 (a disintegrin and metalloprotease with thrombospondin)is a multidomain metalloprotease that limits platelet adhesion by a feedback mechanism in which fluid shear stress induces proteolysis of VWF and prevents disseminated microvascular thrombosis. Cleavage of the Tyr(1605)-Met(1606) scissile bond in the VWF A2 domain depends on a Glu(1660)-Arg(1668) segment in the same domain and on the noncatalytic spacer domain of ADAMTS13, suggesting that extensive enzyme-substrate interactions facilitate substrate recognition. Based on mutagenesis and kinetic analysis, we find that the ADAMTS13 spacer domain binds to an exosite near the C terminus of the VWF A2 domain. Deleting the spacer domain from ADAMTS13 or deleting the exosite from the VWF substrate reduced the rate of cleavage approximately 20-fold. A cleavage product containing the exosite was a hyperbolic mixed-type inhibitor of ADAMTS13 proteolysis of either VWF multimers or model peptide substrates but only if the ADAMTS13 enzyme contained the spacer domain. The specificity of this unique mechanism depends on tension-induced unfolding of the VWF A2 domain, which exposes the scissile bond and exosite for interaction with complementary sites on ADAMTS13.

Conformational stability and domain unfolding of the Von Willebrand factor A domains

Von Willebrand factor (VWF), a multimeric multidomain glycoprotein secreted into the blood from vascular endothelial cells, initiates platelet adhesion at sites of vascular injury. This process requires the binding of platelet glycoprotein Ib-IX-V to the A1 domain of VWF monomeric subunits under fluid shear stress. The A2 domain of VWF monomers contains a proteolytic site specific for a circulating plasma VWF metalloprotease, A Disintegrin and Metalloprotease with Thrombospondin motifs, member #13 of the ADAMTS enzyme family (ADAMTS-13), that functions to reduce adhesiveness of newly released, unusually large (UL), hyperactive forms of VWF. Shear stress assists ADAMTS-13 proteolysis of ULVWF multimers allowing ADAMTS-13 cleavage of an exposed peptide bond in the A2 domain. Shear stress may induce conformational changes in VWF, and even unfold regions of VWF monomeric subunits. We used urea as a surrogate for shear to study denaturation of the individual VWF recombinant A domains, A1, A2, and A3, and the domain triplet, A1-A2-A3. Denaturation was evaluated as a function of the urea concentration, and the intrinsic thermodynamic stability of the domains against unfolding was determined. The A1 domain unfolded in a 3-state manner through a stable intermediate. Domains A2 and A3 unfolded in a 2-state manner from native to denatured. The A1-A2-A3 triple domain unfolded in a 6-state manner through four partially folded intermediates between the native and denatured states. Urea denaturation of A1-A2-A3 was characterized by two major unfolding transitions: the first corresponding to the simultaneous complete unfolding of A2 and partial unfolding of A1 to the intermediate state; and the second corresponding to the complete unfolding of A3 followed by gradual unfolding of the intermediate state of A1 at high urea concentration. The A2 domain containing the cleavage site for ADAMTS-13 was the least stable of the three domains and was the most susceptible to unfolding. The low stability of the A2 domain is likely to be important in regulating the exposure of the A2 domain cleavage site in response to shear stress, ULVWF platelet adherence, and the attachment of ADAMTS-13 to ULVWF.

Measurement of von Willebrand factor binding to a recombinant fragment of glycoprotein Ibalpha in an enzyme-linked immunosorbent assay-based method: performances in patients with type 2B von Willebrand disease

Type 2B von Willebrand disease (VWD) is characterised by an increased affinity of von Willebrand factor (VWF) for its platelet receptor glycoprotein Ib (GPIb). This feature is usually studied in vitro by a ristocetin-dependent VWF platelet-binding assay, which has some limitations as it requires [e.g. (radio)-labelled anti-VWF antibodies and normal formaldehyde-fixed platelets]. We, here, extended the applicability of an enzyme-linked immunosorbent assay-based method previously described for the measurement of ristocetin co-factor activity that used a recombinant fragment of GPIb (rfGPIb alpha) and horseradish peroxidase-labelled rabbit anti-human VWF antibodies for measuring the captured ristocetin-VWF complexes on the rfGPIb alpha. Thirty-one type 2B VWD patients from 15 families with eight different known mutations were studied. VWF in plasma from 28 of these patients bound better than normal VWF at 0.2 mg/ml ristocetin, with the ratio, optical density (OD) patient/OD normal pool plasma, higher than 1.8. For two of the three other patients with no enhanced response of plasma VWF, the platelet lysate VWF showed an enhanced binding capacity; for the last patient, the results in other members of the family are unequivocal. We conclude that, this new method for measurement of plasma or platelet VWF-binding capacity offers great advantages for correct type 2B VWD diagnosis.

Shielding of the A1 Domain by the D′D3 Domains of von Willebrand Factor Modulates Its Interaction with Platelet Glycoprotein Ib-IX-V

Soluble von Willebrand factor (VWF) has a low affinity for platelet glycoprotein (GP) Ibalpha and needs immobilization and/or high shear stress to enable binding of its A1 domain to the receptor. The previously described anti-VWF monoclonal antibody 1C1E7 enhances VWF/GPIbalpha binding and recognizes an epitope in the amino acids 764-1035 region in the N-terminal D'D3 domains. In this study we demonstrated that the D'D3 region negatively modulates the VWF/GPIb-IX-V interaction; (i) deletion of the D'D3 region in VWF augmented binding to GPIbalpha, suggesting an inhibitory role for this region, (ii) the isolated D'D3 region inhibited the GPIbalpha interaction of a VWF deletion mutant lacking this region, indicating that intramolecular interactions limit the accessibility of the A1 domain, (iii) using a panel of anti-VWF monoclonal antibodies, we next showed that the D'D3 region is in close proximity with the A1 domain in soluble VWF but not when VWF was immobilized; (iv) destroying the epitope of 1C1E7 resulted in a mutant VWF with an increased affinity for GPIbalpha. Our results support a model of domain translocation in VWF that allows interaction with GPIbalpha. The suggested shielding interaction of the A1 domain by the D'D3 region then becomes disrupted by VWF immobilization.

The interaction of von Willebrand factor-A1 domain with collagen: mutation G1324S (type 2M von Willebrand disease) impairs the conformational change in A1 domain induced by collagen

BACKGROUND

It is established that the A3 domain in von Willebrand factor (VWF) contains the major collagen-binding site. However, there are conflicting reports describing the capacity of the A1 domain to interact with collagen types I and III.

METHODS

In this study, we have used recombinant VWF-A1 polypeptides, as well as conformation-specific monoclonal antibodies (mAb), to analyze the A1-collagen interaction.

RESULTS

The A1 domain bound to collagen with K(d) approximately 8.0 nm and this binding was blocked by the mAb 6G1, which blocks the interaction between ristocetin and VWF. In addition, collagen-bound A1 protein was able to support flow-dependent adhesion of platelets, demonstrating that the binding sites for collagen and glycoprotein (GP)Ib are different. Analysis with two conformation-specific mAb demonstrated that the structure of the A1 domain changed as a result of the binding to collagen. In contrast, the antibodies failed to detect conformational change in the G1324S mutant (type 2M von Willebrand disease). Thus, direct binding to collagen induces a change in the structural conformation within the VWF-A1 domain, and the G1324S substitution prevents this conformational change.

CONCLUSION

This study has shown that the isolated A1 domain can simultaneously bind to collagen and platelet GPIb, supporting platelet adhesion under high-flow conditions. In addition, this study has used mAb to demonstrate that the binding of the isolated A1 domain or full-length VWF to collagen is accompanied by a conformational change in A1 domain.

Identification of amino acid residues essential for heparin binding by the A1 domain of human von Willebrand factor

Platelet adhesion is mediated by von Willebrand factor (VWF) that binds platelet glycoprotein Ib (GPIb). Previous observations suggested that heparin competitively inhibits the binding of VWF to GPIb and may down-regulate platelet adhesion. We performed charged-to-alanine scanning mutagenesis of domain A1 and studied dose-dependent binding to heparin-Sepharose beads. Mutations at Lys1362 and Arg1395, at which the GPIb binding was markedly decreased, showed 41% and 42% binding, respectively. Clustered mutations in the segments 1332KDRKR1336 and 1405KKKK1408, which have been proposed as heparin binding sequences, showed 72% and 52% binding, respectively. However, single alanine substitutions within these clusters showed normal binding. Our findings suggest that heparin may inhibit the binding of VWF to GPIb by interacting with GPIb binding and interpret why some hemorrhagic complications of heparin therapy are not predictable based on techniques for monitoring the conventional anticoagulant effects of heparin.

Structure and function of snake venom toxins interacting with human von Willebrand factor

Hemostatic plug formation is a complex event mediated by platelets, subendothelial matrices and von Willebrand factor (VWF) at the vascular injury. Snake venom proteins have an excellent potency to regulate the interaction between VWF and platelet membrane receptors in vitro. Two protein families, C-type lectin-like proteins and Zn(2+)-metalloproteinases, have been found to affect platelet-VWF interaction. Botrocetin and bitiscetin from viper venom are disulfide-linked heterodimers with C-type lectin-like motif, and modulate VWF to elicit platelet glycoprotein Ib (GPIb)-binding activity via the A1 domain of VWF leading to the platelet agglutination. The crystal structures of botrocetin and bitiscetin together with complex from the VWF A1 domain indicate the following: (1) a central concave domain formed by two subunits of botrocetin or bitiscetin provides the binding site for VWF, (2) these modulators directly bind to the A1 domain of VWF in close proximity to the GPIb binding site, (3) both modulators induce no significant conformational change on the GPIb-binding site of the A1 domain but could provide a supplemental platform fitting for GPIb. These results suggest that the modulating mechanisms of these venoms are different from those performed by either antibiotic ristocetin in vitro or extremely high shear stress in vivo. Other modulator toxins include kaouthiagin and jararhagin, chimeric proteins composed of metalloproteinase, disintegrin-like and Cys-rich domains. These toxins cleave VWF and reduce its platelet agglutinating or collagen-binding activity. Kaouthiagin from cobra venom specifically cleaves between Pro708 and Asp709 in the C-terminal VWF A1 domain resulting in the decrease of the multimer structure of VWF. Recently a plasma proteinase, which specifically cleaves VWF into a smaller multimer, has been elucidated to be a reprolysin-like metalloproteinase with thrombospondin motif family (ADAMTS). This endogenous metalloproteinase (ADAMTS-13) specifically cleaves between Tyr842 and Met843 in the A2 domain of VWF regulating its physiological hemostatic activity. These VWF-binding snake venom proteins are suitable probes for basic research on platelet plug formation mediated by VWF, for subsidiary diagnostic use for von Willebrand disease or platelet disorder, and might be potently applicable to the regulation of VWF in thrombosis and hemostasis. Structural information of these venom proteins together with recombinant technology might strongly promote the construction of a new antihemostatic drug in the near future.

O-linked glycosylation and functional incompatibility of porcine von Willebrand factor for human platelet GPIb receptors

BACKGROUND

Xenograft rejection is associated with vascular inflammation, thrombocytopenia and the accelerated consumption of coagulation factors. Primary biological incompatibilities of the xenograft in the regulation of clotting appear to amplify pathological processes associated with rejection. The functional incompatibility of porcine von Willebrand factor (vWF) expressed within the xenograft vasculature may heighten interactions with the primate platelet receptor GPIb, hence augmenting formation of platelet microthrombi and vascular injury. Here, we address the functional impact of O-linked glycosylation of the vWF A1 domain on primate platelet activation.

METHODS

Recombinant human or porcine vWF A1-domains were transiently over-expressed in COS-7 cells as FLAG-tagged fusion protein, linked to plasma membranes via GPI anchors. O-linked glycosylation was blocked by the addition of phenyl-alpha-GalNAc2 to cultures. Expressed vWF-A1 domains were characterized utilizing cytofluometric- and Western blot analyses.

RESULTS

Cytofluometric analysis confirmed equivalent levels of human and porcine vWF A1-domain expression irrespective of the levels of O-linked glycosylation. Differential glycosylation patterns of vWF-A1 under these conditions were confirmed by Western blot analyses. Native porcine vWF A1-domains had enhanced human platelet activation potential when compared with human recombinant vWF A1. However, the loss of O-linked glycosylation abolished differences in aggregatory responses between human and porcine vWF A1 domains.

CONCLUSIONS

Various degrees of O-linked glycosylation of vWF-A1-domains modulate levels of functional interaction with platelet receptor GPIb and consequent platelet aggregation responses in vitro. These data may have implications for outcomes of xenotransplantation. We speculate that alterations in glycosylation of vWF and other adhesion proteins associated with the targeting of the alpha1,3-Gal-epitope in mutant swine may have salutatory effects on the primate platelet activation observed in these xenografts.

Impact of O-linked glycosylation of the VWF-A1-domain flanking regions on platelet interaction

This study investigated the functional impact of O-linked glycosylation of von Willebrand Factor (VWF) A1 domains on the interaction with platelet receptors. Native or mutant VWF-A1-domains were transiently overexpressed on COS-7 cells as membrane glycosylphosphatidylinositol (GPI)-anchored FLAG-tagged fusion proteins. Cytofluometric analysis assured comparable levels of A1-domain expression among native and mutant homologues as well as for different culture conditions. Expressing native VWF-A1-domains under O-linked glycosylation blocking conditions increased the platelet aggregatory responses observed for fully glycosylated forms. Utilizing a neuronal network for prediction of O-linked glycosylation of mammalian proteins, threonine (T) and serine (S) residues located in the VWF-A1-loop flanking regions - not in the loop itself - were determined to be glycosylated n-terminal at amino acids T485, S490, T492 and T493 and c-terminal at T705. Simultaneous selective charge-to-alanine mutation of S490, T492 and T493 led to gain in aggregatory responses. When compared with native forms, equivalent alterations of T485 did not dictate functional differences. Any alanine-substitution for T705 revealed a substantial loss in aggregatory effects - possibly as a result of structural desintegration of the VWF-A1-binding site for glycoprotein (GP) Ib. These data suggest specific O-linked glycosylation of the amino-terminal VWF-A1-loop-flanking region to have a negative regulatory impact on the A1-domain affinity of non-activated human VWF for human platelet-GPIb.

A Covalent Oxidoreductase Intermediate in Propeptide-dependent von Willebrand Factor Multimerization

The assembly of von Willebrand factor multimers in the Golgi apparatus requires D1D2 domains of the von Willebrand factor propeptide, which may act as an oxidoreductase to promote disulfide bond formation or rearrangement between two D3 domains in the mature subunit. This mechanism predicts that the propeptide should form a transient intrachain disulfide bond with the D3 domain before multimerization. Such an intermediate was detected using truncated subunits that simplify the analysis of the multimerization process. When only the D1D2D'D3 region of von Willebrand factor was expressed in baby hamster kidney cells, the propeptide and D'D3 formed an intrachain disulfide-linked species in the endoplasmic reticulum that could be identified by two-dimensional gel electrophoresis after cleavage with thrombin or furin. This intermediate rearranged in the Golgi to form free propeptide and D'D3 dimers that were secreted. A similar intracellular disulfide-linked species was identified in cells expressing the propeptide and D'D3 as separate proteins and in cells expressing full-length von Willebrand factor. These results support a model in which the propeptide acts as an oxidoreductase to promote von Willebrand factor multimerization in the Golgi apparatus.