The von willebrand factor A3 domain does not contain a metal ion-dependent adhesion site motif

How unique structural adaptations support and coordinate the complex function of von Willebrand factor

ABSTRACT

von Willebrand factor (VWF) is a multimeric protein consisting of covalently linked monomers, which share an identical domain architecture. Although involved in processes such as inflammation, angiogenesis, and cancer metastasis, VWF is mostly known for its role in hemostasis, by acting as a chaperone protein for coagulation factor VIII (FVIII) and by contributing to the recruitment of platelets during thrombus formation. To serve its role in hemostasis, VWF needs to bind a variety of ligands, including FVIII, platelet-receptor glycoprotein Ib-α, VWF-cleaving protease ADAMTS13, subendothelial collagen, and integrin α-IIb/β-3. Importantly, interactions are differently regulated for each of these ligands. How are these binding events accomplished and coordinated? The basic structures of the domains that constitute the VWF protein are found in hundreds of other proteins of prokaryotic and eukaryotic organisms. However, the determination of the 3-dimensional structures of these domains within the VWF context and especially in complex with its ligands reveals that exclusive, VWF-specific structural adaptations have been incorporated in its domains. They provide an explanation of how VWF binds its ligands in a synchronized and timely fashion. In this review, we have focused on the domains that interact with the main ligands of VWF and discuss how elucidating the 3-dimensional structures of these domains has contributed to our understanding of how VWF function is controlled. We further detail how mutations in these domains that are associated with von Willebrand disease modulate the interaction between VWF and its ligands.

Computational identification of antibody-binding epitopes from mimotope datasets

Introduction: A fundamental challenge in computational vaccinology is that most B-cell epitopes are conformational and therefore hard to predict from sequence alone. Another significant challenge is that a great deal of the amino acid sequence of a viral surface protein might not in fact be antigenic. Thus, identifying the regions of a protein that are most promising for vaccine design based on the degree of surface exposure may not lead to a clinically relevant immune response. Methods: Linear peptides selected by phage display experiments that have high affinity to the monoclonal antibody of interest ("mimotopes") usually have similar physicochemical properties to the antigen epitope corresponding to that antibody. The sequences of these linear peptides can be used to find possible epitopes on the surface of the antigen structure or a homology model of the antigen in the absence of an antigen-antibody complex structure. Results and Discussion: Herein we describe two novel methods for mapping mimotopes to epitopes. The first is a novel algorithm named MimoTree that allows for gaps in the mimotopes and epitopes on the antigen. More specifically, a mimotope may have a gap that does not match to the epitope to allow it to adopt a conformation relevant for binding to an antibody, and residues may similarly be discontinuous in conformational epitopes. MimoTree is a fully automated epitope detection algorithm suitable for the identification of conformational as well as linear epitopes. The second is an ensemble approach, which combines the prediction results from MimoTree and two existing methods.

Removal of the vicinal disulfide enhances the platelet-capturing function of von Willebrand factor

A redox autoinhibitory mechanism has previously been proposed, in which the reduced state of the vicinal disulfide bond in the von Willebrand factor (VWF) A2 domain allows A2 to bind to A1 and inhibit platelet adhesion to the A1 domain. The VWF A1A2A3 tridomain was expressed with and without the vicinal disulfide in A2 (C1669S/C1670S) via the atomic replacement of sulfur for oxygen to test the relevance of the vicinal disulfide to the physiological platelet function of VWF under shear flow. A comparative study of the shear-dependent platelet translocation dynamics on these tridomain variants reveals that the reduction of the vicinal disulfide moderately increases the platelet-capturing function of A1, an observation counter to the proposed hypothesis. Surface plasmon resonance spectroscopy confirms that C1669S/C1670S slightly increases the affinity of A1A2A3 binding to glycoprotein Ibα (GPIbα). Differential scanning calorimetry and hydrogen-deuterium exchange mass spectrometry demonstrate that reduction of the vicinal disulfide destabilizes the A2 domain, which consequently disrupts interactions between the A1, A2, and A3 domains and enhances the conformational dynamics of A1-domain secondary structures known to regulate the strength of platelet adhesion to VWF. This study clarifies that the reduced state of the A2 vicinal disulfide is not inhibitory but rather slightly activating.

In silico design of fusion keratinocyte growth factor containing collagen-binding domain for tissue engineering application

Keratinocyte growth factor (KGF) is a potential therapeutic factor in wound healing. However, its applications have been restricted due to its low stability, short half-life, and limited target specificity. We aimed to immobilize KGF on collagen-based biomaterials for long-lasting and targeted therapy by designing fusion forms of KGF with collagen-binding domains (CBD) from natural origins. Twelve fusion proteins were designed consisting of KGF and CBDs with different lengths and amino acid compositions. Three-dimensional (3D) structures of the fusions were predicted by homology modeling. Physiochemical properties and secondary structure of the fusions were evaluated by bioinformatics tools. Moreover, the effect of the CBDs on the 3D structure and dynamic behavior of the fusions was investigated by molecular dynamics (MD) simulation. The binding affinity of the fusions to collagen, KGF receptor, and heparin was assessed using docking tools. Our results demonstrated that fusions with small CBDs like CBD of mammalian collagenase and decapeptide CBD of von Willebrand factor (VWF) were more stable and properly folded than those with larger CBDs. On the other hand, the insertion of bulky CBDs, including Fibronectin CBD and CBD of Clostridium histolyticum collagenase, into KGF resulted in stronger binding to collagen. Therefore, very small or large CBDs are inappropriate for constructing KGF fusions because they suffer from low collagen affinity or poor stability. By comparing the results of MD simulation and docking, this study proposed that CBDs belonging to Vibrio mimicus metalloprotease and A3 domain of VWF would be good candidates to produce stable fusions with proper affinities toward collagen and KGF receptors. Moreover, the secondary structure analysis showed that the overall structure of KGF and CBDs was better preserved when CBDs were inserted at the C-terminal of KGF. This computational information about novel KGF fusions may help find the best constructs for experimental studies.

Subunit Flexibility of Multimeric von Willebrand Factor/Factor VIII Complexes

Von Willebrand factor (VWF) is a plasma glycoprotein that participates in platelet adhesion and aggregation and serves as a carrier for blood coagulation factor VIII (fVIII). Plasma VWF consists of a population of multimers that range in molecular weight from ∼ 0.55 MDa to greater than 10 MDa. The VWF multimer consists of a variable number of concatenated disulfide-linked ∼275 kDa subunits. We fractionated plasma-derived human VWF/fVIII complexes by size-exclusion chromatography at a pH of 7.4 and subjected them to analysis by sodium dodecyl sulfate agarose gel electrophoresis, sedimentation velocity analytical ultracentrifugation (SV AUC), dynamic light scattering (DLS), and multi-angle light scattering (MALS). Weight-average molecular weights, M _w, were independently measured by MALS and by application of the Svedberg equation to SV AUC and DLS measurements. Estimates of the Mark-Houwink-Kuhn-Sakurada exponents , α_s, and α_D describing the functional relationship between the z-average radius of gyration, , weight-average sedimentation coefficient, s _w, z-average diffusion coefficient, D _z , and M _w were consistent with a random coil conformation of the VWF multimer. Ratios of to the z-average hydrodynamic radius, , estimated by DLS, were calculated across an M _w range from 2 to 5 MDa. When compared to values calculated for a semi-flexible, wormlike chain, these ratios were consistent with a contour length over 1000-fold greater than the persistence length. These results indicate a high degree of flexibility between domains of the VWF subunit.

The In Vitro Replication Cycle of Achromobacter xylosoxidans and Identification of Virulence Genes Associated with Cytotoxicity in Macrophages

Achromobacter xylosoxidans is an opportunistic pathogen implicated in a wide variety of human infections including the ability to colonize the lungs of cystic fibrosis (CF) patients. The role of A. xylosoxidans in human pathology remains controversial due to the lack of optimized in vitro and in vivo model systems to identify and test bacterial gene products that promote a pathological response. We have previously identified macrophages as a target host cell for A. xylosoxidans-induced cytotoxicity. By optimizing our macrophage infection model, we determined that A. xylosoxidans enters macrophages and can reside within a membrane bound vacuole for extended periods of time. Intracellular replication appears limited with cellular lysis preceding an enhanced, mainly extracellular replication cycle. Using our optimized in vitro model system along with transposon mutagenesis, we identified 163 genes that contribute to macrophage cytotoxicity. From this list, we characterized a giant RTX adhesin encoded downstream of a type one secretion system (T1SS) that mediates bacterial binding and entry into host macrophages, an important first step toward cellular toxicity and inflammation. The RTX adhesin is encoded by other human isolates and is recognized by antibodies present in serum isolated from CF patients colonized by A. xylosoxidans, indicating this virulence factor is produced and deployed in vivo. This study represents the first characterization of A. xylosoxidans replication during infection and identifies a variety of genes that may be linked to virulence and human pathology. IMPORTANCE Patients affected by CF develop chronic bacterial infections characterized by inflammatory exacerbations and tissue damage. Advancements in sequencing technologies have broadened the list of opportunistic pathogens colonizing the CF lung. A. xylosoxidans is increasingly recognized as an opportunistic pathogen in CF, yet our understanding of the bacterium as a contributor to human disease is limited. Genomic studies have identified potential virulence determinants in A. xylosoxidans isolates, but few have been mechanistically studied. Using our optimized in vitro cell model, we identified and characterized a bacterial adhesin that mediates binding and uptake by host macrophages leading to cytotoxicity. A subset of serum samples from CF patients contains antibodies that recognize the RTX adhesion, suggesting, for the first time, that this virulence determinant is produced in vivo. This work furthers our understanding of A. xylosoxidans virulence factors at a mechanistic level.

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.

Von Willebrand factor: A key glycoprotein involved in thrombo-inflammatory complications of COVID-19

COVID-19 is an ongoing public health emergency that has affected millions of people worldwide and is still a threat to many more. One of the pathophysiological features of COVID-19 is associated with the activation of vascular endothelial cells (ECs) leading to the disruption of vascular integrity, coagulation and inflammation. An interlink mechanism between coagulation and inflammatory pathways has been reported in COVID-19. Multiple components are involved in these pathological pathways. Out of all, Von Willebrand Factor (VWF) is one of the primary components of coagulation pathway and also a mediator of vascular inflammation that plays an important role in thrombo-inflammation that further leads to acute respiratory distress syndrome (ARDS). The thrombo-inflammatory co-morbidities such as hyper-coagulation, thrombosis, ARDS etc. have become the major cause of mortality in the patients of COVID-19 admitted to the ICU. Thus, VWF can be explored as a potential target to manage COVID-19 associated co-morbidities. Supporting this hypothesis, there are literature reports which disclose previous attempts to target VWF for the management of thrombo-inflammation in other pathological conditions. The current report summarizes emerging insights into the pathophysiology, mechanism(s), diagnosis, management and foundations for research on this less explored clinically relevant glycoprotein as coagulation biomarker in COVID-19.

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

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

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

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

Investigating the clearance of VWF A-domains using site-directed PEGylation and novel N-linked glycosylation

BACKGROUND

Previous studies have demonstrated that the A1A2A3 domains of von Willebrand factor (VWF) play a key role in regulating macrophage-mediated clearance in vivo. In particular, the A1-domain has been shown to modulate interaction with macrophage low-density lipoprotein receptor-related protein-1 (LRP1) clearance receptor. Furthermore, N-linked glycans within the A2-domain have been shown to protect VWF against premature LRP1-mediated clearance. Importantly, however, the specific regions within A1A2A3 that enable macrophage binding have not been defined.

OBJECTIVE AND METHODS

To address this, we utilized site-directed PEGylation and introduced novel targeted N-linked glycosylation within A1A2A3-VWF and subsequently examined VWF clearance.

RESULTS

Conjugation with a 40-kDa polyethylene glycol (PEG) moiety significantly extended the half-life of A1A2A3-VWF in VWF-/- mice in a site-specific manner. For example, PEGylation at specific sites within the A1-domain (S1286) and A3-domain (V1803, S1807) attenuated VWF clearance in vivo, compared to wild-type A1A2A3-VWF. Furthermore, PEGylation at these specific sites ablated binding to differentiated THP-1 macrophages and LRP1 cluster II and cluster IV in-vitro. Conversely, PEGylation at other positions (Q1353-A1-domain and M1545-A2-domain) had limited effects on VWF clearance or binding to LRP1.Novel N-linked glycan chains were introduced at N1803 and N1807 in the A3-domain. In contrast to PEGylation at these sites, no significant extension in half-life was observed with these N-glycan variants.

CONCLUSIONS

These novel data demonstrate that site specific PEGylation but not site specific N-glycosylation modifies LRP1-dependent uptake of the A1A2A3-VWF by macrophages. This suggests that PEGylation, within the A1- and A3-domains in particular, may be used to attenuate LRP1-mediated clearance of VWF.

Evidence for the Misfolding of the A1 Domain within Multimeric von Willebrand Factor in Type 2 von Willebrand Disease

Von Willebrand factor (VWF), an exceptionally large multimeric plasma glycoprotein, functions to initiate coagulation by agglutinating platelets in the blood stream to sites of vascular injury. This primary hemostatic function is perturbed in type 2 dysfunctional subtypes of von Willebrand disease (VWD) by mutations that alter the structure and function of the platelet GPIbα adhesive VWF A1 domains. The resulting amino acid substitutions cause local disorder and misfold the native structure of the isolated platelet GPIbα-adhesive A1 domain of VWF in both gain-of-function (type 2B) and loss-of-function (type 2M) phenotypes. These structural effects have not been explicitly observed in A1 domains of VWF multimers native to blood plasma. New mass spectrometry strategies are applied to resolve the structural effects of 2B and 2M mutations in VWF to verify the presence of A1 domain structural disorder in multimeric VWF harboring type 2 VWD mutations. Limited trypsinolysis mass spectrometry (LTMS) and hydrogen-deuterium exchange mass spectrometry (HXMS) are applied to wild-type and VWD variants of the single A1, A2, and A3 domains, an A1A2A3 tridomain fragment of VWF, plasmin-cleaved dimers of VWF, multimeric recombinant VWF, and normal VWF plasma concentrates. Comparatively, these methods show that mutations known to misfold the isolated A1 domain increase the rate of trypsinolysis and the extent of hydrogen-deuterium exchange in local secondary structures of A1 within multimeric VWF. VWD mutation effects are localized to the A1 domain without appreciably affecting the structure and dynamics of other VWF domains. The intrinsic dynamics of A1 observed in recombinant fragments of VWF are conserved in plasma-derived VWF. These studies reveal that structural disorder does occur in VWD variants of the A1 domain within multimeric VWF and provides strong support for VWF misfolding as a result of some, but not all, type 2 VWD variants.

Interaction of von Willebrand factor with platelets and the vessel wall

The initiation of thrombus formation at sites of vascular injury to secure haemostasis after tissue trauma requires the interaction of surface-exposed von Willebrand factor (VWF) with its primary platelet receptor, the glycoprotein (GP) Ib-IX-V complex. As an insoluble component of the extracellular matrix (ECM) of endothelial cells, VWF can directly initiate platelet adhesion. Circulating plasma VWF en-hances matrix VWF activity by binding to structures that become exposed to flowing blood, notably collagen type I and III in deeper layers of the vessel along with microfibrillar collagen type VI in the subendothelium. Moreover, plasma VWF is required to support platelet-to-platelet adhesion - i. e. aggregation - which promotes thrombus growth and consolidation. For these reasons, understanding how plasma VWF interaction with platelet receptors is regulated, particularly any distinctive features of GPIb binding to soluble as opposed to immobilized VWF, is of paramount importance in vascular biology. This brief review will highlight knowledge acquired and key problems that remain to be solved to elucidate fully the role of VWF in normal haemostasis and pathological thrombosis.

Pilus Assembly in Gram-Positive Bacteria

Pili of Gram-positive bacteria are unique structures on the bacterial surface, assembled from covalently linked polypeptide subunits. Pilus assembly proceeds by transpeptidation reactions catalyzed by sortases, followed by covalent anchoring of the filament in the peptidoglycan layer. Another distinctive property is the presence of intramolecular isopeptide bonds, conferring extraordinary chemical and mechanical stability to these elongated structures. Besides their function in cell adhesion and biofilm formation, this section discusses possible application of pilus constituents as vaccine components against Gram-positive pathogens.

von Willebrand factor, Jedi knight of the bloodstream

When blood vessels are cut, the forces in the bloodstream increase and change character. The dark side of these forces causes hemorrhage and death. However, von Willebrand factor (VWF), with help from our circulatory system and platelets, harnesses the same forces to form a hemostatic plug. Force and VWF function are so closely intertwined that, like members of the Jedi Order in the movie Star Wars who learn to use "the Force" to do good, VWF may be considered the Jedi knight of the bloodstream. The long length of VWF enables responsiveness to flow. The shape of VWF is predicted to alter from irregularly coiled to extended thread-like in the transition from shear to elongational flow at sites of hemostasis and thrombosis. Elongational force propagated through the length of VWF in its thread-like shape exposes its monomers for multimeric binding to platelets and subendothelium and likely also increases affinity of the A1 domain for platelets. Specialized domains concatenate and compact VWF during biosynthesis. A2 domain unfolding by hydrodynamic force enables postsecretion regulation of VWF length. Mutations in VWF in von Willebrand disease contribute to and are illuminated by VWF biology. I attempt to integrate classic studies on the physiology of hemostatic plug formation into modern molecular understanding, and point out what remains to be learned.

Evolution of integrin I domains

In humans, an ~200-residue "inserted" I domain, a von Willebrand factor A domain (vWFA), buds out from the β-propeller domain in 9 of 18 integrin α subunits. The vWFA domain is not unique to the α subunit as it is an integral part of all integrin β subunits and many other proteins. The βI domain has always been a component of integrins but the αI domain makes its appearance relatively late, in early chordates, since it is found in tunicates and later diverging species. The tunicate αI domains are distinct from the human collagen and leukocyte recognizing integrin α subunits, but fragments of integrins from agnathastomes suggest that the human-type αI domains arose in an ancestor of the very first vertebrate species. The rise of integrins with αI domains parallels the enormous changes in body plan and systemic development of the chordate line that began some 550 million or more years ago.

A structure of a collagen VI VWA domain displays N and C termini at opposite sides of the protein

Von Willebrand factor A (VWA) domains are versatile protein interaction domains with N and C termini in close proximity placing spatial constraints on overall protein structure. The 1.2 Å crystal structures of a collagen VI VWA domain and a disease-causing point mutant show C-terminal extensions that place the N and C termini at opposite ends. This allows a “beads-on-a-string” arrangement of multiple VWA domains as observed for ten N-terminal domains of the collagen VI α3 chain. The extension is linked to the core domain by a salt bridge and two hydrophobic patches. Comparison of the wild-type and a muscular dystrophy-associated mutant structure identifies a potential perturbation of a protein interaction interface and indeed, the secretion of mutant collagen VI tetramers is affected. Homology modeling is used to locate a number of disease-associated mutations and analyze their structural impact, which will allow mechanistic analysis of collagen-VI-associated muscular dystrophy phenotypes.

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The structure of a VWA domain (N5) of collagen VI at 1.2 Å is presented

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N and C termini of the domain are at opposite ends

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The structure with a myopathy-causing mutation shows altered interaction interface

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The impact of mutations in collagen VI VWA domains was analyzed

Urea-temperature phase diagrams capture the thermodynamics of denatured state expansion that accompany protein unfolding

We have analyzed the thermodynamic properties of the von Willebrand factor (VWF) A3 domain using urea-induced unfolding at variable temperature and thermal unfolding at variable urea concentrations to generate a phase diagram that quantitatively describes the equilibrium between native and denatured states. From this analysis, we were able to determine consistent thermodynamic parameters with various spectroscopic and calorimetric methods that define the urea-temperature parameter plane from cold denaturation to heat denaturation. Urea and thermal denaturation are experimentally reversible and independent of the thermal scan rate indicating that all transitions are at equilibrium and the van't Hoff and calorimetric enthalpies obtained from analysis of individual thermal transitions are equivalent demonstrating two-state character. Global analysis of the urea-temperature phase diagram results in a significantly higher enthalpy of unfolding than obtained from analysis of individual thermal transitions and significant cross correlations describing the urea dependence of ΔH0 and ΔCP0 that define a complex temperature dependence of the m-value. Circular dichroism (CD) spectroscopy illustrates a large increase in secondary structure content of the urea-denatured state as temperature increases and a loss of secondary structure in the thermally denatured state upon addition of urea. These structural changes in the denatured ensemble make up ∼40% of the total ellipticity change indicating a highly compact thermally denatured state. The difference between the thermodynamic parameters obtained from phase diagram analysis and those obtained from analysis of individual thermal transitions illustrates that phase diagrams capture both contributions to unfolding and denatured state expansion and by comparison are able to decipher these contributions.

Weibel-Palade bodies: a window to von Willebrand disease

Weibel-Palade bodies (WPBs) are the storage organelles for von Willebrand factor (VWF) in endothelial cells. VWF forms multimers that assemble into tubular structures in WPBs. Upon demand, VWF is secreted into the blood circulation, where it unfolds into strings that capture platelets during the onset of primary hemostasis. Numerous mutations affecting VWF lead to the bleeding disorder von Willebrand disease. This review reports the recent findings on the effects of VWF mutations on the biosynthetic pathway of VWF and its storage in WPBs. These new findings have deepened our understanding of VWF synthesis, storage, secretion, and function.

Implications for collagen I chain registry from the structure of the collagen von Willebrand factor A3 domain complex

Fibrillar collagens, the most abundant proteins in the vertebrate body, are involved in a plethora of biological interactions. Plasma protein von Willebrand factor (VWF) mediates adhesion of blood platelets to fibrillar collagen types I, II, and III, which is essential for normal haemostasis. High affinity VWF-binding sequences have been identified in the homotrimeric collagen types II and III, however, it is unclear how VWF recognizes the heterotrimeric collagen type I, the superstructure of which is unknown. Here we present the crystal structure of VWF domain A3 bound to a collagen type III-derived homotrimeric peptide. Our structure reveals that VWF-A3 interacts with all three collagen chains and binds through conformational selection to a sequence that is one triplet longer than was previously appreciated from platelet and VWF binding studies. The VWF-binding site overlaps those of SPARC (also known as osteonectin) and discodin domain receptor 2, but is more extended and shifted toward the collagen amino terminus. The observed collagen-binding mode of VWF-A3 provides direct structural constraints on collagen I chain registry. A VWF-binding site can be generated from the sequences RGQAGVMF, present in the two α1(I) chains, and RGEOGNIGF, in the unique α2(I) chain, provided that α2(I) is in the middle or trailing position. Combining these data with previous structural data on integrin binding to collagen yields strong support for the trailing position of the α2(I) chain, shedding light on the fundamental and long-standing question of the collagen I chain registry.

Shear stress-induced unfolding of VWF accelerates oxidation of key methionine residues in the A1A2A3 region

VWF is required for platelet adhesion to sites of vessel injury, a process vital for both hemostasis and thrombosis. Enhanced VWF secretion and oxidative stress are both hallmarks of inflammation. We recently showed that the neutrophil oxidant hypochlorous acid (HOCl) inhibits VWF proteolysis by ADAMTS13 by oxidizing VWF methionine 1606 (M1606) in the A2 domain. M1606 was readily oxidized in a substrate peptide, but required urea in multimeric plasma VWF. In the present study, we examined whether shear stress enhances VWF oxidation. With an HOCl-generating system containing myeloperoxidase (MPO) and H(2)O(2), we found that shear stress accelerated M1606 oxidation, with 56% becoming oxidized within 1 hour. Seven other methionine residues in the VWF A1A2A3 region (containing the sites for platelet and collagen binding and ADAMTS13 cleavage) were variably oxidized, one completely. Oxidized methionines accumulated preferentially in the largest VWF multimers. HOCl-oxidized VWF was hyperfunctional, agglutinating platelets at ristocetin concentrations that induced minimal agglutination using unoxidized VWF and binding more of the nanobody AU/VWFa-11, which detects a gain-of-function conformation of the A1 domain. These findings suggest that neutrophil oxidants will both render newly secreted VWF uncleavable and alter the largest plasma VWF forms such that they become hyperfunctional and resistant to proteolysis by ADAMTS13.

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.

Cloning and characterization of the osteoarthritis-associated gene DVWA

Osteoarthritis (OA) is one of the most prevalent skeletal diseases. Recently, we identified a novel gene on chromosome 3p24.3, named DVWA (double von Willebrand factor A domains), and its functional variants, which are associated with susceptibility to knee OA. Here we report the cloning and characterization of the DVWA gene. DVWA consisted of seven exons and had four alternative splicing variants, which encoded long (385 amino acid) and short (276 amino acid) proteins (L-DVWA and S-DVWA, respectively). S-DVWA was an N-terminal truncated form of L-DVWA and lacked a signal peptide and a part of a VWA domain. L-DVWA and S-DVWA transcripts were mainly expressed in articular cartilage. Immunoblot analysis using epitope-tagged proteins showed L-DVWA in the conditioned media and S-DVWA only in the cell, consistent with the in silico prediction. We also cloned the murine counterpart of DVWA, which was found to be identical to Col6a4, which has recently been reported. L-DVWA had 73% identity to the N-terminal sequence of the 2,309-amino acid Col6a4 protein. The mouse Dvwa/Col6a4 mRNA was present mainly in the small intestine in embryos and adults, but not in cartilage. The amino acid sequence of L-DVWA was conserved in higher species than chicken, but that of S-DVWA was unique in human. Knockdown of DVWA by siRNAs increased expression of chondrocyte matrix genes. Our study indicates that DVWA is evolutionally very unique, which, together with its specific expression in articular cartilage, suggests its specific role in human cartilage metabolism.

Architects at the bacterial surface - sortases and the assembly of pili with isopeptide bonds

The cell wall envelope of Gram-positive bacteria can be thought of as a surface organelle for the assembly of macromolecular structures that enable the unique lifestyle of each microorganism. Sortases - enzymes that cleave the sorting signals of secreted proteins to form isopeptide (amide) bonds between the secreted proteins and peptidoglycan or polypeptides - function as the principal architects of the bacterial surface. Acting alone or with other sortase enzymes, sortase construction leads to the anchoring of surface proteins at specific sites in the envelope or to the assembly of pili, which are fibrous structures formed from many protein subunits. The catalysis of intermolecular isopeptide bonds between pilin subunits is intertwined with the assembly of intramolecular isopeptide bonds within pilin subunits. Together, these isopeptide bonds endow these sortase products with adhesive properties and resistance to host proteases.

Protein stability in the presence of cosolutes

Protein scientists have long used cosolutes to study protein stability. While denaturants, such as urea, have been employed for a long time, the attention became focused more recently on protein stabilizers, including osmolytes. Here, we provide practical experimental instructions for the use of both stabilizing and denaturing osmolytes with proteins, as well as data evaluation strategies. We focus on protein stability in the presence of cosolutes and their mixtures at constant and variable temperature.

von Willebrand factor to the rescue

von Willebrand factor (VWF) is a large multimeric adhesive glycoprotein with complex roles in thrombosis and hemostasis. Abnormalities in VWF give rise to a variety of bleeding complications, known as von Willebrand disease (VWD), the most common inherited bleeding disorder in humans. Current treatment of VWD is based on the replacement of the deficient or dysfunctional protein either by endogenous release from endothelial Weibel-Palade bodies or by administration of plasma-derived VWF concentrates. During the last years, several efforts have been made to optimize existing therapies for VWD, but also to devise new approaches, such as inducing endogenous expression with interleukin-11, administering exogenous recombinant VWF, or introducing the protein via gene delivery. Clearly, the efficacy of any strategy will depend on several factors, including, for example, the quantity, activity, and stability of the delivered VWF. The inherent complexity of VWF biosynthesis, which involves extensive posttranslational processing, may be limiting in terms of producing active VWF outside of its native cellular sources. This review summarizes recent progress in the development of different treatment strategies for VWD, including those that are established and those that are at the experimental stage. Potential pitfalls and benefits of each strategy are discussed.

Integrins during evolution: evolutionary trees and model organisms

The integrins form a large family of cell adhesion receptors. All multicellular animals express integrins, indicating that the family evolved relatively early in the history of metazoans, and homologous sequences of the component domains of integrin alpha and beta subunits are seen in prokaryotes. Some integrins, however, seem to be much younger. For example, the alphaI domain containing integrins, including collagen receptors and leukocyte integrins, have been found in chordates only. Here, we will discuss what conclusions can be drawn about integrin function by studying the evolutionary conservation of integrins. We will also look at how studying integrins in organisms such as the fruit fly and mouse has helped our understanding of integrin evolution-function relationships. As an illustration of this, we will summarize the current understanding of integrin involvement in skeletal muscle formation.

Function of von Willebrand factor in haemostasis and thrombosis

The physiological protection against bleeding is secured by platelet adhesion to the site of injury and sealing of the defect. The first step involves the arrest of platelets that have adhered to subendothelial structures, primarily collagen, at the site of injury. Under conditions of low shear rates, platelet adhesion to the damaged vessel wall is mediated by several proteins, including von Willebrand factor (VWF). However, under conditions of high shear, aggregation occurs only in the presence of soluble VWF. In solution, VWF becomes immobilized via its A3 domain on the fibrillar collagen of the vessel wall and acts as an intermediary between collagen and the platelet receptor glycoprotein Ibalpha (GPIbalpha), which is the only platelet receptor that does not require prior activation for bond formation. After GPIbalpha binds to the A1 domain of its main ligand VWF, further activation of the platelet via intracellular signalling occurs, allowing other receptors to engage VWF and collagen and thereby reinforcing permanent adhesion. On this first layer of adherent platelets, soluble VWF binds and uncoils, thereby attracting more platelets. Platelet interaction with immobilized and soluble VWF may also generate platelet-derived microparticles that exhibit pro-coagulant activity. Full growth of a multilayered platelet aggregate comprises binding of the platelet receptor integrin alphaIIbbeta3 to VWF and fibrinogen. In addition, the surface of the activated platelets accelerates the coagulation cascade, which, by its end product fibrin, stabilizes the growing platelet thrombus. This article summarizes the characteristics and role of VWF in the coagulation cascade.

Structural basis of sequence-specific collagen recognition by SPARC

Protein interactions with the collagen triple helix play a critical role in collagen fibril formation, cell adhesion, and signaling. However, structural insight into sequence-specific collagen recognition is limited to an integrin-peptide complex. A GVMGFO motif in fibrillar collagens (O denotes 4-hydroxyproline) binds 3 unrelated proteins: von Willebrand factor (VWF), discoidin domain receptor 2 (DDR2), and the extracellular matrix protein SPARC/osteonectin/BM-40. We report the crystal structure at 3.2 A resolution of human SPARC bound to a triple-helical 33-residue peptide harboring the promiscuous GVMGFO motif. SPARC recognizes the GVMGFO motifs of the middle and trailing collagen chains, burying a total of 720 A(2) of solvent-accessible collagen surface. SPARC binding does not distort the canonical triple helix of the collagen peptide. In contrast, a critical loop in SPARC is substantially remodelled upon collagen binding, creating a deep pocket that accommodates the phenylalanine residue of the trailing collagen chain ("Phe pocket"). This highly restrictive specificity pocket is shared with the collagen-binding integrin I-domains but differs strikingly from the shallow collagen-binding grooves of the platelet receptor glycoprotein VI and microbial adhesins. We speculate that binding of the GVMGFO motif to VWF and DDR2 also results in structural changes and the formation of a Phe pocket.

The second von Willebrand type A domain of cochlin has high affinity for type I, type II and type IV collagens

Cochlin is colocalized with type II collagen in the extracellular matrix of cochlea and has been suggested to interact with this collagen. Here we show that the second von Willebrand type A domain of cochlin has affinity for type II collagen, as well as type I and type IV collagens whereas the LCCL-domain of cochlin has no affinity for these proteins. The implications of these findings for the mechanism whereby cochlin mutations cause the dominant negative DFNA9-type hearing loss are discussed.

Integrins alpha1beta1 and alpha2beta1 are receptors for the rotavirus enterotoxin

Rotavirus NSP4 is a viral enterotoxin capable of causing diarrhea in neonatal mice. This process is initiated by the binding of extracellular NSP4 to target molecule(s) on the cell surface that triggers a signaling cascade leading to diarrhea. We now report that the integrins alpha1beta1 and alpha2beta1 are receptors for NSP4. NSP4 specifically binds to the alpha1 and alpha2 I domains with apparent K(d) = 1-2.7 muM. Binding is mediated by the I domain metal ion-dependent adhesion site motif, requires Mg(2+) or Mn(2+), is abolished with EDTA, and an NSP4 point mutant, E(120)A, fails to bind alpha2 integrin I domain. NSP4 has two distinct integrin interaction domains. NSP4 amino acids 114-130 are essential for binding to the I domain, and NSP4 peptide 114-135 blocks binding of the natural ligand, collagen I, to integrin alpha2. NSP4 amino acids 131-140 are not associated with the initial binding to the I domain, but elicit signaling that leads to the spreading of attached C2C12-alpha2 cells, mouse myoblast cells stably expressing the human alpha2 integrin. NSP4 colocalizes with integrin alpha2 on the basolateral surface of rotavirus-infected polarized intestinal epithelial (Caco-2) cells as well as surrounding noninfected cells. NSP4 mutants that fail to bind or signal through integrin alpha2 were attenuated in diarrhea induction in neonatal mice. These results indicate that NSP4 interaction with integrin alpha1 and alpha2 is an important component of enterotoxin function and rotavirus pathogenesis, further distinguishing this viral virulence factor from other microbial enterotoxins.

Adhesion molecules in the nervous system: structural insights into function and diversity

The unparalleled complexity of intercellular connections in the nervous system presents requirements for high levels of both specificity and diversity for the proteins that mediate cell adhesion. Here we describe recent advances toward understanding the molecular mechanisms that underlie adhesive binding, specificity, and diversity for several well-characterized families of adhesion molecules in the nervous system. Although many families of adhesion proteins, including cadherins and immunoglobulin superfamily members, are utilized in neural and nonneural contexts, nervous system-specific diversification mechanisms, such as precisely regulated alternative splicing, provide an important means to enable their function in the complex context of the nervous system.

Purified A2 domain of von Willebrand factor binds to the active conformation of von Willebrand factor and blocks the interaction with platelet glycoprotein Ibalpha

BACKGROUND

von Willebrand factor (VWF) does not interact with circulating platelets unless it is induced to expose the binding site for platelet glycoprotein (GP)Ibalpha in the A1 domain by high shear stress, immobilization, and/or a modulator. Previous studies have implied indirectly that the A2 domain may be involved in regulating A1-GPIbalpha binding.

OBJECTIVE AND METHODS

Because the relationship between the A1 and A2 domains has not been defined, we have investigated the effect of the A2 domain on the binding activity of the A1 domain using recombinant A domain polypeptides, multimeric VWF, and monoclonal antibodies (mAb).

RESULTS

The A2 domain polypeptide bound specifically to the immobilized A1 domain polypeptide or full-length VWF, with half-maximal binding being obtained at 60 or 168 nm, respectively. This A1-A2 interaction was inhibited by mAb against the A2 or A1 domain and by the A1 domain polypeptide. The A2 domain polypeptide effectively blocked GPIbalpha-mediated platelet adhesion under high flow conditions. The A2 domain polypeptide specifically recognizes the GPIbalpha-binding conformation in the A1 domain, as it only interacted with VWF activated by the modulator ristocetin or immobilized VWF. Furthermore, in contrast to plasma VWF, the ultra-large (UL)VWF multimers or a recombinant VWF-A1A2A3 polypeptide containing a gain-of-function mutation (R1308 L) of type 2B von Willebrand disease bound to the A2 domain polypeptide without the need for ristocetin.

CONCLUSIONS

The recombinant A2 domain polypeptide specifically binds to the active conformation of the A1 domain in VWF and effectively blocks the interaction with platelet GPIbalpha under high-flow conditions.

Complement and the multifaceted functions of VWA and integrin I domains

The recent crystal structure of complement protein component C2a reveals an interface between its VWA and serine protease domains that could not exist in the zymogen C2. The implied change in VWA domain conformation between C2 and C2a differs from that described for other VWA domains, including the I domains in integrins. Here, the remarkable diversity in both conformational regulation and ligand binding among VWA domains that function in complement, hemostasis, cell adhesion, anthrax toxin binding, vesicle transport, DNA break repair, and RNA quality control is reviewed. Finally, implications for metastability of complement convertases are discussed.

Solution structure of human von Willebrand factor studied using small angle neutron scattering

von Willebrand factor (VWF) binding to platelets under high fluid shear is an important step regulating atherothrombosis. We applied light and small angle neutron scattering to study the solution structure of human VWF multimers and protomer. Results suggest that these proteins resemble prolate ellipsoids with radius of gyration (R(g)) of approximately 75 and approximately 30 nm for multimer and protomer, respectively. The ellipsoid dimensions/radii are 175 x 28 nm for multimers and 70 x 9.1 nm for protomers. Substructural repeat domains are evident within multimeric VWF that are indicative of elements of the protomer quarternary structure (16 nm) and individual functional domains (4.5 nm). Amino acids occupy only approximately 2% of the multimer and protomer volume, compared with 98% for serum albumin and 35% for fibrinogen. VWF treatment with guanidine.HCl, which increases VWF susceptibility to proteolysis by ADAMTS-13, causes local structural changes at length scales <10 nm without altering protein R(g). Treatment of multimer but not protomer VWF with random homobifunctional linker BS(3) prior to reduction of intermonomer disulfide linkages and Western blotting reveals a pattern of dimer and trimer units that indicate the presence of stable intermonomer non-covalent interactions within the multimer. Overall, multimeric VWF appears to be a loosely packed ellipsoidal protein with non-covalent interactions between different monomer units stabilizing its solution structure. Local, and not large scale, changes in multimer conformation are sufficient for ADAMTS-13-mediated proteolysis.

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.

Fluorescently labeled collagen binding proteins allow specific visualization of collagen in tissues and live cell culture

Visualization of the formation and orientation of collagen fibers in tissue engineering experiments is crucial for understanding the factors that determine the mechanical properties of tissues. In this study, collagen-specific fluorescent probes were developed using a new approach that takes advantage of the inherent specificity of collagen binding protein domains present in bacterial adhesion proteins (CNA35) and integrins (GST-alpha1I). Both collagen binding domains were obtained as fusion proteins from an Escherichia coli expression system and fluorescently labeled using either amine-reactive succinimide (CNA35) or cysteine-reactive maleimide (GST-alpha1I) dyes. Solid-phase binding assays showed that both protein-based probes are much more specific than dichlorotriazinyl aminofluorescein (DTAF), a fluorescent dye that is currently used to track collagen formation in tissue engineering experiments. The CNA35 probe showed a higher affinity for human collagen type I than did the GST-alpha1I probe (apparent K(d) values of 0.5 and 50 microM, respectively) and showed very little cross-reactivity with noncollagenous extracellular matrix proteins. The CNA35 probe was also superior to both GST-alpha1I and DTAF in visualizing the formation of collagen fibers around live human venous saphena cells. Immunohistological experiments on rat tissue showed colocalization of the CNA35 probe with collagen type I and type III antibodies. The fluorescent probes described here have important advantages over existing methods for visualization of collagen, in particular for monitoring the formation of collagen in live tissue cultures over prolonged time periods.

Collagen XXVIII, a novel von Willebrand factor A domain-containing protein with many imperfections in the collagenous domain

Here we describe a novel collagen belonging to the class of von Willebrand factor A (VWA) domain-containing proteins. This novel protein was identified by screening the EST data base and was subsequently recombinantly expressed and characterized as an authentic tissue component. The COL28A1 gene on human chromosome 7p21.3 and on mouse chromosome 6A1 encodes a novel protein that structurally resembles the beaded filament-forming collagens. The collagenous domain contains several very short interruptions arranged in a repeat pattern. As shown for other novel minor collagens, the expression of collagen XXVIII protein in mouse is very restricted. In addition to small amounts in skin and calvaria, the major signals were in dorsal root ganglia and peripheral nerves. By immunoelectron microscopy, collagen XXVIII was detected in the sciatic nerve, at the basement membrane of certain Schwann cells surrounding the nerve fibers. Even though the protein is present in the adult sciatic nerve, collagen XXVIII mRNA was only detected in sciatic nerve of newborn mice, indicating that the protein persists for an extended period after synthesis.

Paratope determination of the antithrombotic antibody 82D6A3 based on the crystal structure of its complex with the von Willebrand factor A3-domain

The antithrombotic monoclonal antibody 82D6A3 is directed against amino acids Arg-963, Pro-981, Asp-1009, Arg-1016, Ser-1020, Met-1022, and His-1023 of the von Willebrand factor A3-domain (Vanhoorelbeke, K., Depraetere, H., Romijn, R. A., Huizinga, E., De Maeyer, M., and Deckmyn, H. (2003) J. Biol. Chem. 278, 37815-37821). By this, it potently inhibits the interaction of von Willebrand factor to collagens, which is a prerequisite for blood platelet adhesion to the injured vessel wall at sites of high shear. To fully understand the mode of action of 82D6A3 at the molecular level, we resolved its crystal structure in complex with the A3-domain and fine mapped its paratope by construction and characterization of 13 mutants. The paratope predominantly consists of two short sequences in the heavy chain CDR1 (Asn-31 and Tyr-32) and CDR3 (Asp-99, Pro-101, Tyr-102 and Tyr-103), forming one patch on the surface of the antibody. Trp-50 of the heavy and His-49 of the light chain, both situated adjacent to the patch, play ancillary roles in antigen binding. The crystal structure furthermore confirms the epitope location, which largely overlaps with the collagen binding site deduced from mutagenesis of the A3-domain (Romijn, R. A., Westein, E., Bouma, B., Schiphorst, M. E., Sixma, J. J., Lenting, P. J., and Huizinga, E. G. (2003) J. Biol. Chem. 278, 15035-15039). We herewith further consolidate the location of the collagen binding site and reveal that the potent action of the antibody is due to direct competition for the same interaction site. This information allows the design of a paratope-mimicking peptide with antithrombotic properties.

Zinc and calcium ions cooperatively modulate ADAMTS13 activity

ADAMTS13 is a metalloproteinase that cleaves von Willebrand factor (VWF) multimers. The metal ion dependence of ADAMTS13 activity was examined with multimeric VWF and a fluorescent peptide substrate based on Asp(1596)-Arg(1668) of the VWF A2 domain, FRETS-VWF73. ADAMTS13 activity in citrate-anticoagulated plasma was enhanced approximately 2-fold by zinc ions, approximately 3-fold by calcium ions, and approximately 6-fold by both ions, suggesting cooperative activation. Cleavage of VWF by recombinant ADAMTS13 was activated up to approximately 200-fold by zinc ions (K(D) (app) approximately 0.5 microM), calcium ions (K(D) (app) approximately 4.8 microM), and barium ions (K(D) (app) approximately 1.7 mM). Barium ions stimulated ADAMTS13 activity in citrated plasma but not in citrate-free plasma. Therefore, the stimulation by barium ions of ADAMTS13 in citrated plasma appears to reflect the release of chelated calcium and zinc ions from complexes with citrate. At optimal zinc and calcium concentrations, ADAMTS13 cleaved VWF with a K(m) (app) of 3.7 +/- 1.4 microg/ml (approximately 15 nM for VWF subunits), which is comparable with the plasma VWF concentration of 5-10 microg/ml. ADAMTS13 could cleave approximately 14% of VWF pretreated with guanidine HCl, suggesting that this substrate is heterogeneous in susceptibility to proteolysis. ADAMTS13 cleaved FRETS-VWF73 with a K(m) (app) of 3.2 +/- 1.1 microM, consistent with an approximately 200-fold decrease in affinity compared with VWF. ADAMTS13 cleaved VWF and FRETS-VWF73 with roughly comparable catalytic efficiency of 55 microM(-1) min(-1) and 18 microM(-1) min(-1), respectively. The striking preference of ADAMTS13 for VWF suggests that substrate recognition depends on structural features or exosites on multimeric VWF that are missing from FRETS-VWF73.

Cleavage of ultra-large von Willebrand factor by ADAMTS-13 under flow conditions

von Willebrand factor (VWF) is a critical ligand for platelet adhesion and aggregation. It is synthesized and released as multimers composed of various numbers of monomers. When first released from the storage granules of endothelial cells, VWF multimers are rich in the ultra-large (UL) forms that spontaneously bind the GP Ib-IX complex and aggregate platelets. These prothrombotic ULVWF multimers are rapidly cleaved by the metalloprotease ADAMTS-13 (A Disintegrin and Metalloprotease with ThromboSpondin motif) to smaller and much less active forms. Recently, several methods have been developed to measure ADAMTS-13 activity in vitro and to link its deficiency to thrombotic thrombocytopenic purpura. Correlations between the structure and functions of the metalloprotease have also been extensively studied using recombinant technologies. However, questions remain regarding the proper substrate for the metalloprotease, the time and location of ULVWF proteolysis, and the role of fluid shear stress. In this brief review, we have discussed a potential model for ULVWF proteolysis by ADAMTS-13 in vivo. In this model, ULVWF is anchored to the surface of endothelial cells to form string-like structures under fluid shear stress. Such an elongated conformation facilitates ULVWF cleavage by exposing either the cleavage or binding sites for the metalloprotease. When ADAMTS-13 is deficient, the uncleaved ULVWF accumulates in plasma and on endothelial cells to capture platelets. This leads to platelet aggregation and thromboembolism. Dissecting the process of ULVWF proteolysis is important for not only understanding the pathophysiology of thrombotic microangiopathies, but also developing more effective means to treat these deadly diseases.

New perspectives on von Willebrand factor functions in hemostasis and thrombosis

The adhesive protein, von Willebrand factor (VWF), mediates the initiation and progression of thrombus formation at sites of vascular injury by means of specific interactions with extracellular matrix components and platelet receptors. The essential biologic properties of VWF have been elucidated, with progress particularly in the areas of genetic regulation, biosynthesis, and specific bimolecular interactions. The three-dimensional structure of selected domains has been solved, but our understanding of detailed structure-function relationships is still fragmented, partly because of the complexity and size of the VWF molecule. The biomechanical properties of the interaction between the VWF A1 domain and the platelet receptor glycoprotein (GP) Ibalpha also are better known, but we can still only hypothesize how this adhesive bond can oppose the fluid dynamic effects of rapidly flowing blood to initiate thrombus formation and contribute to platelet activation. Elucidating the details of VWF and GPIbalpha function will lead to a more satisfactory definition of the role of platelets in atherothrombosis, since hemodynamic forces greatly influence responses to vascular injury in stenosed and partially occluded arteries. Progress in this direction is also aided by rapidly expanding novel information on the mechanisms that regulate VWF multimer size in the circulation, a topic of relevance to explain microvascular thrombosis and, perhaps, arterial thrombosis in general. Developments in these areas of research will refine our understanding of the role played by VWF in vascular biology and pathology.