A single high-affinity binding site for von Willebrand factor in collagen III, identified using synthetic triple-helical peptides

The NC2 domain of type IX collagen determines the chain register of the triple helix

Precise mapping and unraveling the mechanism of interaction or degradation of a certain type of collagen triple helix requires the generation of short and stable collagenous fragments. This is a great challenge especially for hetero-trimeric collagens, where chain composition and register (stagger) are important factors. No system has been reported that can be efficiently used to generate a natural collagenous fragment with exact chain composition and desired chain register. The NC2 domain (only 35-50 residues) of FACIT collagens is a potent trimerization domain. In the case of type IX collagen it provides the efficient selection and hetero-trimerization of three distinct chains. The ability of the NC2 domain to determine the chain register of the triple helix is studied. We generated three possible sequence combinations (α1α1α2, α1α2α1, α2α1α1) of a type I collagen fragment (the binding region for the von Willebrand factor A3 domain) attached to the NC2 domain. In addition, two control combinations were produced that constitute homo-trimers of (α1)(3) or (α2)(3). For the hetero-trimeric constructs, α1α1α2 demonstrated a higher melting temperature than the other two. Binding experiments with the von Willebrand factor A3 domain revealed the homo-trimer of (α1)(3) as the strongest binding construct, whereas the homo-trimer of (α2)(3) showed no binding. For hetero-trimers, α1α1α2 was found to be the strongest binding construct. Differences in thermal stability and binding to the A3 domain unambiguously demonstrate that the NC2 domain of type IX collagen determines not only the chain composition but also the chain register of the adjacent triple helix.

Collagen recognition and transmembrane signalling by discoidin domain receptors

The discoidin domain receptors, DDR1 and DDR2, are two closely related receptor tyrosine kinases that are activated by triple-helical collagen in a slow and sustained manner. The DDRs have important roles in embryo development and their dysregulation is associated with human diseases, such as fibrosis, arthritis and cancer. The extracellular region of DDRs consists of a collagen-binding discoidin (DS) domain and a DS-like domain. The transmembrane region mediates the ligand-independent dimerisation of DDRs and is connected to the tyrosine kinase domain by an unusually long juxtamembrane domain. The major DDR binding site in fibrillar collagens is a GVMGFO motif (O is hydroxyproline), which is recognised by an amphiphilic trench at the top of the DS domain. How collagen binding leads to DDR activation is not understood. GVMGFO-containing triple-helical peptides activate DDRs with the characteristic slow kinetics, suggesting that the supramolecular structure of collagen is not required. Activation can be blocked allosterically by monoclonal antibodies that bind to the DS-like domain. Thus, collagen most likely causes a conformational change within the DDR dimer, which may lead to the formation of larger DDR clusters. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.

Measurement of whole blood thrombus formation using parallel-plate flow chambers - a practical guide

Custom-made and commercial parallel-plate flow chambers are widely used for studies of platelet activation and thrombus formation in whole blood at defined shear rates. When used in a reproducible way, such flow chamber devices give valuable information on the thrombogenic potential of human, mouse, or rat blood. This article aims to provide a practical guide for the use of parallel-plate flow chambers in combination with routine microscopic imaging techniques. The following methodological aspects are addressed: preparation of surface coatings, calculation of blood flow and shear rate, control of pre-analytical variables, protocols for routine performing of flow chamber tests with non-coagulating or coagulating blood, and procedures for real-time and end-point analysis of thrombus formation. Frequently encountered experimental problems and artifacts are discussed, as well as possibilities for using flow chamber devices as a diagnostic tool to test antithrombotic medication.

The properties conferred upon triple-helical collagen-mimetic peptides by the presence of cysteine residues

Recently, the ability of polymeric collagen-like peptides to regulate cell behavior has generated great interest. A triple-helical peptide known as collagen-related peptide (CRP) contains the sequence (Gly-Pro-Hyp)(10). With Gly-Pro-Cys triplets appended to both of its termini, designated CRP(cys), chemical cross-linking using heterobifunctional reagents generates CRP(cys)-XL, a potent, widely used, polymeric agonist for platelet Glycoprotein VI, whereas non-cross-linked, monomeric CRP(cys) antagonizes Glycoprotein VI. Here, we describe how cysteine in these triplets may also undergo random air-induced oxidation, especially upon prolonged storage or repeated freeze-thawing, to form disulphide bonds, resulting in a lesser degree of polymerization than with chemical cross-linking. We investigated the monomeric and polymeric states of these and other cysteine-containing collagen-derived peptides, using gel filtration and dynamic light scattering, allowing the size of a CRP-XL aggregate to be estimated. The effect of cysteine thiols upon peptide adsorption to surfaces and subsequent platelet responses was investigated. This demonstrated that cysteine is required for strong binding to glass coverslips and to plastic plates used in ELISA assays.

Identification of the active region responsible for the anti-thrombotic activity of anopheline anti-platelet protein from a malaria vector mosquito

We previously identified an anti-platelet protein, anopheline anti-platelet protein (AAPP), from the salivary gland of female Anopheles stephensi (a mosquito vector of human malaria). AAPP specifically blocks platelet adhesion to collagen by binding directly to collagen and subsequently causing platelet aggregation. The aim of this study was to identify the active region of AAPP responsible for the anti-thrombotic activity because we hypothesized that AAPP could be used as a candidate anti-platelet drug. Various truncated forms of AAPP were produced using an Escherichia coli expression system. Each protein was examined for binding activities to soluble/fibrillar collagen and anti-thrombotic activity using a plate assay and platelet/whole blood aggregation study, respectively. Among the truncated forms examined, only a protein encoded by exon 3-4 (rAAPPex3-4) effectively bound to soluble/fibrillar collagen in a concentration-dependent and saturable manner. The EC50 values of full-length AAPP and rAAPPex3-4 for soluble collagen binding were 35 nM and 36 nM, respectively. In contrast to soluble collagen, there was a difference in binding affinity to fibrillar collagen between full-length AAPP and rAAPPex3-4, with EC50 values of 31 nM and 51 nM, respectively. rAAPPex3-4 also inhibited aggregation of platelets/whole blood, and the IC50 values of full-length AAPP and rAAPPex3-4 for platelet aggregation were 35 nM and 93 nM, respectively. These results indicated that the essential moiety of AAPP for collagen binding and anti-thrombotic activity was in the region encoded by exon 3-4, which is highly conserved among the counterpart regions of other mosquito species.

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.

Platelet interaction with the vessel wall

Platelets have attracted a growing interest among basic scientists and clinicians, as they have been shown to play an important role in many physiological and pathophysiological conditions. Beyond hemostasis, platelets participate in wound healing, inflammation, infectious diseases, maintenance of the endothelial barrier function, angiogenesis, and tumor metastasis. Over the last 50 years enormous progress has been made in our understanding of the role of platelets in hemostasis. Platelets circulate in blood in a resting state, but they are able to react immediately upon a vessel wall injury by adhering to the exposed collagen, followed by platelet-platelet interaction to form a plug that effectively seals the injured vessel wall to prevent excessive blood loss. Comparable events will take place on a rupturing atherosclerotic plaque, which may result in a platelet-rich thrombus. This chapter will address the molecular basis of platelet adhesion and aggregation, the regulation of platelet function and the interaction of primary and secondary hemostasis.

Transmembrane collagen receptors

Collagen, the most abundant protein in animals, is a key component of extracellular matrices. Not only do collagens provide essential structural support for connective tissues, but they are also intimately involved in controlling a spectrum of cellular functions such as growth, differentiation, and morphogenesis. All collagens possess triple-helical regions through which they interact with a host of other proteins including cell surface receptors. A structurally diverse group of transmembrane receptors mediates the recognition of the collagen triple helix: integrins, discoidin domain receptors, glycoprotein VI, and leukocyte-associated immunoglobulin-like receptor-1. These collagen receptors regulate a wide range of behaviors including cell adhesion and migration, hemostasis, and immune function. Here these collagen receptors are discussed in terms of their molecular basis of collagen recognition, their signaling and developmental functions, and their roles in disease.

Platelet adhesion to collagen

Platelets play a central role in maintaining hemostasis mainly by binding to subendothelial collagen exposed upon vascular injury, thereby initiating thrombus formation. Platelets can bind directly to the exposed collagen through two major receptors i.e. the integrin a2b1 and glycoprotein (GP) VI. However, under high shear conditions the GPIb-V-IX receptor complex and its main ligand von Willebrand Factor are additionally needed for firm platelet adhesion to the vessel wall. In this review, we summarize the current knowledge on the individual roles and structure-function relationships of these main platelet adhesion receptors.

Mapping structural landmarks, ligand binding sites, and missense mutations to the collagen IV heterotrimers predicts major functional domains, novel interactions, and variation in phenotypes in inherited diseases affecting basement membranes

Collagen IV is the major protein found in basement membranes. It comprises three heterotrimers (α1α1α2, α3α4α5, and α5α5α6) that form distinct networks, and are responsible for membrane strength and integrity.We constructed linear maps of the collagen IV heterotrimers ("interactomes") that indicated major structural landmarks, known and predicted ligand-binding sites, and missense mutations, in order to identify functional and disease-associated domains, potential interactions between ligands, and genotype–phenotype relationships. The maps documented more than 30 known ligand-binding sites as well as motifs for integrins, heparin, von Willebrand factor (VWF), decorin, and bone morphogenetic protein (BMP). They predicted functional domains for angiogenesis and haemostasis, and disease domains for autoimmunity, tumor growth and inhibition, infection, and glycation. Cooperative ligand interactions were indicated by binding site proximity, for example, between integrins, matrix metalloproteinases, and heparin. The maps indicated that mutations affecting major ligand-binding sites, for example, for Von Hippel Lindau (VHL) protein in the α1 chain or integrins in the α5 chain, resulted in distinctive phenotypes (Hereditary Angiopathy, Nephropathy, Aneurysms, and muscle Cramps [HANAC] syndrome, and early-onset Alport syndrome, respectively). These maps further our understanding of basement membrane biology and disease, and suggest novel membrane interactions, functions, and therapeutic targets.

A role for specific collagen motifs during wound healing and inflammatory response of fibroblasts in the teleost fish gilthead seabream

Specific sites and sequences in collagen to which cells can attach, either directly or through protein intermediaries, were identified using Toolkits of 63-amino acid triple-helical peptides and specific shorter GXX'GEX″ motifs, which have different intrinsic affinity for integrins that mediate cell adhesion and migration. We have previously reported that collagen type I (COL-I) was able to prime in vitro the respiratory burst and induce a specific set of immune- and extracellular matrix-related molecules in phagocytes of the teleost fish gilthead seabream (Sparus aurata L.). It was also suggested that COL-I would provide an intermediate signal during the early inflammatory response in gilthead seabream. Since fibroblasts are highly involved in the initiation of wound repair and regeneration processes, in the present study SAF-1 cells (gilthead seabream fibroblasts) were used to identify the binding motifs in collagen by end-point and real-time cell adhesion assays using the collagen peptides and Toolkits. We identified the collagen motifs involved in the early magnesium-dependent adhesion of these cells. Furthermore, we found that peptides containing the GFOGER and GLOGEN motifs (where O is hydroxyproline) present high affinity for SAF-1 adhesion, expressed as both cell number and surface covering, while in cell suspensions, these motifs were also able to induce the expression of the genes encoding the proinflammatory molecules interleukin-1β and cyclooxygenase-2. These data suggest that specific collagen motifs are involved in the regulation of the inflammatory and healing responses of teleost fish.

Collagen binding specificity of the discoidin domain receptors: binding sites on collagens II and III and molecular determinants for collagen IV recognition by DDR1

The discoidin domain receptors, DDR1 and DDR2 are cell surface receptor tyrosine kinases that are activated by triple-helical collagen. While normal DDR signalling regulates fundamental cellular processes, aberrant DDR signalling is associated with several human diseases. We previously identified GVMGFO (O is hydroxyproline) as a major DDR2 binding site in collagens I–III, and located two additional DDR2 binding sites in collagen II. Here we extend these studies to the homologous DDR1 and the identification of DDR binding sites on collagen III. Using sets of overlapping triple-helical peptides, the Collagen II and Collagen III Toolkits, we located several DDR2 binding sites on both collagens. The interaction of DDR1 with Toolkit peptides was more restricted, with DDR1 mainly binding to peptides containing the GVMGFO motif. Triple-helical peptides containing the GVMGFO motif induced DDR1 transmembrane signalling, and DDR1 binding and receptor activation occurred with the same amino acid requirements as previously defined for DDR2. While both DDRs exhibit the same specificity for binding the GVMGFO motif, which is present only in fibrillar collagens, the two receptors display distinct preferences for certain non-fibrillar collagens, with the basement membrane collagen IV being exclusively recognised by DDR1. Based on our recent crystal structure of a DDR2-collagen complex, we designed mutations to identify the molecular determinants for DDR1 binding to collagen IV. By replacing five amino acids in DDR2 with the corresponding DDR1 residues we were able to create a DDR2 construct that could function as a collagen IV receptor.

Efficient inhibition of collagen-induced platelet activation and adhesion by LAIR-2, a soluble Ig-like receptor family member

LAIR-1 (Leukocyte Associated Ig-like Receptor -1) is a collagen receptor that functions as an inhibitory receptor on immune cells. It has a soluble family member, LAIR-2, that also binds collagen and can interfere with LAIR-1/collagen interactions. Collagen is a main initiator for platelet adhesion and aggregation. Here, we explored the potential of soluble LAIR proteins to inhibit thrombus formation in vitro. LAIR-2/Fc but not LAIR-1/Fc inhibited collagen-induced platelet aggregation. In addition, LAIR-2/Fc also interfered with platelet adhesion to collagen at low shear rate (300 s(-1); IC(50) = 18 microg/ml) and high shear rate (1500 s(-1); IC(50) = 30 microg/ml). Additional experiments revealed that LAIR-2/Fc leaves interactions between collagen and alpha2beta1 unaffected, but efficiently prevents binding of collagen to Glycoprotein VI and von Willebrand factor. Thus, LAIR-2/Fc has the capacity to interfere with platelet-collagen interactions mediated by Glycoprotein VI and the VWF/Glycoprotein Ib axis.

Synthetic collagen mimics: self-assembly of homotrimers, heterotrimers and higher order structures

Collagen is a fascinating system of proteins that undergo a multi-step, hierarchical self-assembly which starts from individual peptide chains that assemble into a canonical triple helix. These triple helices then assemble into higher order structures which are often, but not always, fibrous in nature. While collagen is the most abundant protein in the human body, the details of its structure and mechanism of assembly are surprisingly poorly understood. This critical review will focus on small peptide systems, commonly referred to as collagen mimetic peptides (CMPs) which have been used successfully to help unravel some of the mystery of this complex structure. We will discuss homotrimeric CMPs, which are the most commonly researched subject in this field, and the structure of the collagen triple helix in detail and the factors that contribute to its stabilization. We will also cover how CMPs have been used to study breaks in triple helical domains as models for connective tissue diseases and, finally, how they have been used to understand the interactions of collagenous proteins with cell-surface receptors. Additionally, we will focus on heterotrimeric CMPs, a relatively new area of collagen research. Finally, we will deal with CMPs used as models for higher level self-assembly and also as materials that are designed to mimic the function of collagens in the extracellular matrix (178 references).

Neutrophil granulocyte-dependent proteolysis enhances platelet adhesion to the arterial wall under high-shear flow

SUMMARY BACKGROUND

Under high shear stress platelets adhere preferentially to the adventitia layer of the arterial vessel wall in a von Willebrand factor (VWF)-dependent manner.

OBJECTIVE

The present study was undertaken in an attempt to characterize the structural background of the relative thromboresistance of the media and the impact of neutrophil leukocyte-derived proteases (matrix metalloproteinases, neutrophil elastase) on platelet adhesion in this layer of the arteries.

METHODS AND RESULTS

Platelet adhesion to cross-sections of the human iliac artery was monitored by indirect immunofluorescent detection of GpIIb/IIIa antigen. Exposure of the vessel wall to activated neutrophils or neutrophil-derived proteases increased platelet adhesion to the media about tenfold over the control level at 3350 s(-1) surface shear rate. In parallel with this enhanced thrombogenicity morphological changes in the media were evidenced by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The fine proteoglycan meshwork seen with Cupromeronic Blue enhancement of the SEM images was removed by the proteolytic treatment and the typical collagen fiber structure was exposed on the AFM images of the media.

CONCLUSION

Through their proteases activated neutrophils degrade proteoglycans, unmask VWF binding sites and thus abolish the thromboresistance of the media in human arteries.

First analysis of a bacterial collagen-binding protein with collagen Toolkits: promiscuous binding of YadA to collagens may explain how YadA interferes with host processes

The Yersinia adhesin YadA mediates the adhesion of the human enteropathogen Yersinia enterocolitica to collagens and other components of the extracellular matrix. Though YadA has been proposed to bind to a specific site in collagens, the exact binding determinants for YadA in native collagen have not previously been elucidated. We investigated the binding of YadA to collagen Toolkits, which are libraries of triple-helical peptides spanning the sequences of type II and III human collagens. YadA bound to many of them, in particular to peptides rich in hydroxyproline but with few charged residues. We were able to block the binding of YadA to collagen type IV with the triple-helical peptide (Pro-Hyp-Gly)(10), suggesting that the same site in YadA binds to triple-helical regions in network-forming collagens as well. We showed that a single Gly-Pro-Hyp triplet in a triple-helical peptide was sufficient to support YadA binding, but more than six triplets were required to form a tight YadA binding site. This is significantly longer than the case for eukaryotic collagen-binding proteins. YadA-expressing bacteria bound promiscuously to Toolkit peptides. Promiscuous binding could be advantageous for pathogenicity in Y. enterocolitica and, indeed, for other pathogenic bacteria. Many of the tightly binding peptides are also targets for eukaryotic collagen-binding proteins, and YadA was able to inhibit the interaction between selected Toolkit peptides and platelets. This leads to the intriguing possibility that YadA may interfere in vivo with host processes mediated by endogenous collagen-binding proteins.

Synergism between platelet collagen receptors defined using receptor-specific collagen-mimetic peptide substrata in flowing blood

Exposed subendothelial collagen acts as a substrate for platelet adhesion and thrombus formation after vascular injury. Synthetic collagen-derived triple-helical peptides, designated collagen-related peptide (CRP), GFOGER, and VWF-III, can specifically engage the platelet collagen receptors, glycoprotein VI and integrin alpha(2)beta(1), and plasma von Willebrand factor (VWF), respectively. Hitherto, the role of these 3 collagen-binding axes has been studied indirectly. Use of these uniform peptide substrates, rather than collagen fibers, provides independent control of each axis. Here, we use confocal imaging and novel image analysis techniques to investigate the effects of receptor-ligand engagement on platelet binding and activation during thrombus formation under flow conditions. At low shear (100s(-1) and 300s(-1)), both GFOGER and CRP are required for thrombus formation. At 1000s(-1), a combination of either CRP or GFOGER with VWF-III induces comparable thrombus formation, and VWF-III increases thrombus deposition at all shear rates, being indispensable at 3000s(-1). A combination of CRP and VWF-III is sufficient to support extensive platelet deposition at 3000s(-1), with slight additional effect of GFOGER. Measurement of thrombus height after specific receptor blockade or use of altered proportions of peptides indicates a signaling rather than adhesive role for glycoprotein VI, and primarily adhesive roles for both alpha(2)beta(1) and the VWF axis.

Crystallographic insight into collagen recognition by discoidin domain receptor 2

The discoidin domain receptors, DDR1 and DDR2, are widely expressed receptor tyrosine kinases that are activated by triple-helical collagen. They control important aspects of cell behavior and are dysregulated in several human diseases. The major DDR2-binding site in collagens I–III is a GVMGFO motif (O is hydroxyproline) that also binds the matricellular protein SPARC. We have determined the crystal structure of the discoidin domain of human DDR2 bound to a triple-helical collagen peptide. The GVMGFO motifs of two collagen chains are recognized by an amphiphilic pocket delimited by a functionally critical tryptophan residue and a buried salt bridge. Collagen binding results in structural changes of DDR2 surface loops that may be linked to the process of receptor activation. A comparison of the GVMGFO-binding sites of DDR2 and SPARC reveals a striking case of convergent evolution in collagen recognition.

Synthesis and biological applications of collagen-model triple-helical peptides

Triple-helical peptides (THPs) have been utilized as collagen models since the 1960s. The original focus for THP-based research was to unravel the structural determinants of collagen. In the last two decades, virtually all aspects of collagen structural biochemistry have been explored with THP models. More specifically, secondary amino acid analogs have been incorporated into THPs to more fully understand the forces that stabilize triple-helical structure. Heterotrimeric THPs have been utilized to better appreciate the contributions of chain sequence diversity on collagen function. The role of collagen as a cell signaling protein has been dissected using THPs that represent ligands for specific receptors. The mechanisms of collagenolysis have been investigated using THP substrates and inhibitors. Finally, THPs have been developed for biomaterial applications. These aspects of THP-based research are overviewed herein.

Crystal structure and collagen-binding site of immune inhibitory receptor LAIR-1: unexpected implications for collagen binding by platelet receptor GPVI

Leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1), one of the most widely spread immune receptors, attenuates immune cell activation when bound to specific sites in collagen. The collagen-binding domain of LAIR-1 is homologous to that of glycoprotein VI (GPVI), a collagen receptor crucial for platelet activation. Because LAIR-1 and GPVI also display overlapping collagen-binding specificities, a common structural basis for collagen recognition would appear likely. Therefore, it is crucial to gain insight into the molecular interaction of both receptors with their ligand to prevent unwanted cross-reactions during therapeutic intervention. We determined the crystal structure of LAIR-1 and mapped its collagen-binding site by nuclear magnetic resonance (NMR) titrations and mutagenesis. Our data identify R59, E61, and W109 as key residues for collagen interaction. These residues are strictly conserved in LAIR-1 and GPVI alike; however, they are located outside the previously proposed GPVI collagen-binding site. Our data provide evidence for an unanticipated mechanism of collagen recognition common to LAIR-1 and GPVI. This fundamental insight will contribute to the exploration of specific means of intervention in collagen-induced signaling in immunity and hemostasis.

Aegyptin displays high-affinity for the von Willebrand factor binding site (RGQOGVMGF) in collagen and inhibits carotid thrombus formation in vivo

Aegyptin is a 30 kDa mosquito salivary gland protein that binds to collagen and inhibits platelet aggregation. We have studied the biophysical properties of aegyptin and its mechanism of action. Light-scattering plot showed that aegyptin has an elongated monomeric form, which explains the apparent molecular mass of 110 kDa estimated by gel-filtration chromatography. Surface plasmon resonance identified the sequence RGQOGVMGF (where O is hydroxyproline) that mediates collagen interaction with von Willebrand factor (vWF) as a high-affinity binding site for aegyptin, with a K(D) of approximately 5 nM. Additionally, aegyptin interacts with the linear peptide RGQPGVMGF and heat-denatured collagen, indicating that the triple helix and hydroxyproline are not a prerequisite for binding. However, aegyptin does not interact with scrambled RGQPGVMGF peptide. Aegyptin also recognizes the peptides (GPO)(10) and GFOGER with low affinity (microM range), which respectively represent glycoprotein VI and integrin alpha2beta1 binding sites in collagen. Truncated forms of aegyptin were engineered, and the C-terminus fragment was shown to interact with collagen and to attenuate platelet aggregation. In addition, aegyptin prevents laser-induced carotid thrombus formation in the presence of Rose Bengal in vivo, without significant bleeding in rats. In conclusion, aegyptin interacts with distinct binding sites in collagen, and is useful tool to inhibit platelet-collagen interaction in vitro and in vivo.

A collagen-mimetic triple helical supramolecule that evokes integrin-dependent cell responses

Collagen is an abundantly distributed extracellular matrix protein in mammalian bodies that maintains structural integrity of the organs and tissues. Besides its function as a structural protein, collagen has various biological functions which regulate cell adhesion, migration and differentiation. In order to develop totally synthetic collagen-surrogates, we recently reported a basic concept for preparing collagen-like triple helical supramolecules based on the self-assembly of staggered trimeric peptides with self-complementary shapes. In this paper, we add one of the specific cellular functions of the native collagen to the collagen-mimetic supramolecule. We synthesized a self-assembling peptide unit containing the integrin-binding sequence Gly-Phe-Hyp-Gly-Glu-Arg. The supramolecule carrying the sequence exhibited significant binding activity to human dermal fibroblasts. The supramolecular structure was found to be essential for function in in vitro cell culture. Cell adhesion was shown to be comparable to that of native collagen, and was further demonstrated to be mediated solely by integrin alpha 2 beta 1. Well-grown focal contacts and stress fibers were observed in cells spread on the supramolecular collagen-mimetic. The results demonstrate the potential of peptide-based artificial collagen as a biomaterial for regulating specific cellular function and fate.

COLdb, a database linking genetic data to molecular function in fibrillar collagens

Fibrillar collagens are ubiquitous proteins essential for the structural integrity of bones, skin, blood vessels, and other tissues. Mutations in collagen genes result in disorders including osteogenesis imperfecta, chondrodysplasias, and Ehlers-Danlos syndromes, but the molecular basis for the heterogeneity of clinical phenotypes is not well understood. A more complete understanding of the relationship between sequence and phenotype requires synthesis of multiple facets of collagen structure and function. To facilitate such an analysis, we developed COLdb, a freely available database integrating collagen biological and physicochemical properties with known variants. A Web-based, interactive, graphical user interface displays the data as annotations on the collagen protein sequences. Collagen gene-level data are provided as custom tracks for display in the UCSC genome browser. COLdb currently includes 35,582 data points spanning collagen types I, II, and III, and, importantly, users can add their own data to the display. The database is the first comprehensive integration of disparate functional information on the three major fibrillar collagens, and the first electronic collection of mutations in the COL2A1 gene.

Development of a high-throughput screening system for the compounds that inhibit collagen-protein interactions

Collagen-binding proteins (CBPs) play important roles in various physiological events. Some CBPs are regarded as targets for drug development; for example, platelet glycoprotein VI (GPVI) and heat shock protein 47 (HSP47) are promising targets for the development of novel antiplatelet and antifibrotic drugs, respectively. However, no systematic screening method to search compounds that inhibit collagen-CBP interactions have been developed, and only a few CBP inhibitors have been reported to date. In this study, a facile turbidimetric multiwell plate assay was developed to evaluate inhibitors of CBPs. The assay is based on the finding that CBPs retard spontaneous collagen fibril formation in vitro and that fibril formation is restored in the presence of compounds that interfere with the collagen-CBP interactions. Using the same platform, the assay was performed in various combinations of fibril-forming collagen types and CBPs. This homogeneous assay is simple, convenient, and suitable as an automated high-throughput screening system.

Structural insights into the interactions between platelet receptors and fibrillar collagen

Collagen peptides have been used to identify binding sites for several important collagen receptors, including integrin alpha(2)beta(1), glycoprotein VI, and von Willebrand factor. In parallel, the structures of these collagen receptors have been reported, and their interactions with collagen peptides have been studied. Recently, the three-dimensional structure of the intact type I collagen fiber from rat tail tendon has been resolved by fiber diffraction. It is now possible to map the binding sites of platelet collagen receptors onto the intact collagen fiber in three dimensions. This minireview will discuss these recent findings and their implications for platelet activation by collagen.

Platelet adhesion under flow

Platelet-adhesive mechanisms play a well-defined role in hemostasis and thrombosis, but evidence continues to emerge for a relevant contribution to other pathophysiological processes, including inflammation, immune-mediated responses to microbial and viral pathogens, and cancer metastasis. Hemostasis and thrombosis are related aspects of the response to vascular injury, but the former protects from bleeding after trauma, while the latter is a disease mechanism. In either situation, adhesive interactions mediated by specific membrane receptors support the initial attachment of single platelets to cellular and extracellular matrix constituents of the vessel wall and tissues. In the subsequent steps of thrombus growth and stabilization, adhesive interactions mediate platelet-to-platelet cohesion (i.e., aggregation) and anchoring to the fibrin clot. A key functional aspect of platelets is their ability to circulate in a quiescent state surveying the integrity of the inner vascular surface, coupled to a prompt reaction wherever alterations are detected. In many respects, therefore, platelet adhesion to vascular wall structures, to one another, or to other blood cells are facets of the same fundamental biological process. The adaptation of platelet-adhesive functions to the effects of blood flow is the main focus of this review.

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.

Candidate cell and matrix interaction domains on the collagen fibril, the predominant protein of vertebrates

Type I collagen, the predominant protein of vertebrates, polymerizes with type III and V collagens and non-collagenous molecules into large cable-like fibrils, yet how the fibril interacts with cells and other binding partners remains poorly understood. To help reveal insights into the collagen structure-function relationship, a data base was assembled including hundreds of type I collagen ligand binding sites and mutations on a two-dimensional model of the fibril. Visual examination of the distribution of functional sites, and statistical analysis of mutation distributions on the fibril suggest it is organized into two domains. The "cell interaction domain" is proposed to regulate dynamic aspects of collagen biology, including integrin-mediated cell interactions and fibril remodeling. The "matrix interaction domain" may assume a structural role, mediating collagen cross-linking, proteoglycan interactions, and tissue mineralization. Molecular modeling was used to superimpose the positions of functional sites and mutations from the two-dimensional fibril map onto a three-dimensional x-ray diffraction structure of the collagen microfibril in situ, indicating the existence of domains in the native fibril. Sequence searches revealed that major fibril domain elements are conserved in type I collagens through evolution and in the type II/XI collagen fibril predominant in cartilage. Moreover, the fibril domain model provides potential insights into the genotype-phenotype relationship for several classes of human connective tissue diseases, mechanisms of integrin clustering by fibrils, the polarity of fibril assembly, heterotypic fibril function, and connective tissue pathology in diabetes and aging.

Mapping of SPARC/BM-40/osteonectin-binding sites on fibrillar collagens

The 33-kDa matrix protein SPARC (BM-40, osteonectin) binds several collagen types with moderate affinity. The collagen-binding site resides in helix alphaA of the extracellular calcium-binding domain of SPARC and is partially masked by helix alphaC. Previously, we found that the removal of helix alphaC caused a 10-fold increase in the affinity of SPARC for collagen, and we identified amino acids crucial for binding by site-directed mutagenesis. In this study, we used rotary shadowing, CNBr peptides, and synthetic peptides to map binding sites of SPARC onto collagens I, II, and III. Rotary shadowing and electron microscopy of SPARC-collagen complexes identified a major binding site approximately 180 nm from the C terminus of collagen. SPARC binding was also detected with lower frequency near the matrix metalloproteinase cleavage site. These data fit well with our analysis of SPARC binding to CNBr peptides, denaturation of which abolished binding, indicating triple-helical conformation of collagen to be essential. SPARC binding was substantially decreased in two of seven alpha2(I) mutant procollagen I samples and after N-acetylation of Lys/Hyl side chains in wild-type collagen. Synthetic peptides of collagen III were used to locate the binding sites, and we found SPARC binding activity in a synthetic triple-helical peptide containing the sequence GPOGPSGPRGQOGVMGFOGPKGNDGAO (where O indicates 4-hydroxyproline), with affinity for SPARC comparable with that of procollagen III. This sequence is conserved among alpha chains of collagens I, II, III, and V. In vitro collagen fibrillogenesis was delayed in the presence of SPARC, suggesting that SPARC might modulate collagen fibril assembly in vivo.

Identification of a major GpVI-binding locus in human type III collagen

We have analyzed the adhesion of human and murine platelets, and of recombinant human and murine GpVI ectodomains, to synthetic triple-helical collagen-like peptides. These included 57 peptides derived from the sequence of human type III collagen and 9 peptides derived from the cyanogen bromide fragment of bovine type III collagen, alpha1(III)CB4. We have identified several peptides that interact with GpVI, in particular a peptide designated III-30 with the sequence GAOGLRGGAGPOGPEGGKGAAGPOGPO. Both human and murine platelets bound to peptide III-30 in a GpVI-dependent manner. III-30 also supported binding of recombinant GpVI ectodomains. Cross-linked III-30 induced aggregation of human and murine platelets, although with a lower potency than collagen-related peptide. Modifications of the peptide sequence indicated that the hydroxyproline residues play a significant role in supporting its GpVI reactivity. However, many peptides containing OGP/GPO motifs did not support adhesion to GpVI. These data indicate that the ability of a triple-helical peptide to bind GpVI is not solely determined by the presence or spatial arrangement of these OGP/GPO motifs within the peptides.

Cell–collagen interactions: the use of peptide Toolkits to investigate collagen–receptor interactions

Fibrillar collagens provide the most fundamental platform in the vertebrate organism for the attachment of cells and matrix molecules. We have identified specific sites in collagens to which cells can attach, either directly or through protein intermediaries. Using Toolkits of triple-helical peptides, each peptide comprising 27 residues of collagen primary sequence and overlapping with its neighbours by nine amino acids, we have mapped the binding of receptors and other proteins on to collagens II or III. Integrin α2β1 binds to several GXX′GER motifs within the collagens, the affinities of which differ sufficiently to control cell adhesion and migration independently of the cellular regulation of the integrin. The platelet receptor, Gp (glycoprotein) VI binds well to GPO (where O is hydroxyproline)-containing model peptides, but to very few Toolkit peptides, suggesting that sequence in addition to GPO triplets is important in defining GpVI binding. The Toolkits have been applied to the plasma protein vWF (von Willebrand factor), which binds to only a single sequence, identified by truncation and amino acid substitution within Toolkit peptides, as GXRGQOGVMGFO in collagens II and III. Intriguingly, the receptor tyrosine kinase, DDR2 (discoidin domain receptor 2) recognizes three sites in collagen II, including its vWF-binding site, although the amino acids that support the interaction differ slightly within this motif. Furthermore, the secreted protein BM-40 (basement membrane protein 40) also binds well to this same region. Thus the availability of extracellular collagen-binding proteins may be important in regulating and facilitating direct collagen–receptor interaction.

Characterization of high affinity binding motifs for the discoidin domain receptor DDR2 in collagen

The discoidin domain receptors, DDR1 and DDR2, are receptor tyrosine kinases that are activated by native triple-helical collagen. Here we have located three specific DDR2 binding sites by screening the entire triple-helical domain of collagen II, using the Collagen II Toolkit, a set of overlapping triple-helical peptides. The peptide sequence that bound DDR2 with highest affinity interestingly contained the sequence for the high affinity binding site for von Willebrand factor in collagen III. Focusing on this sequence, we used a set of truncated and alanine-substituted peptides to characterize the sequence GVMGFO (O is hydroxyproline) as the minimal collagen sequence required for DDR2 binding. Based on a recent NMR analysis of the DDR2 collagen binding domain, we generated a model of the DDR2-collagen interaction that explains why a triple-helical conformation is required for binding. Triple-helical peptides comprising the DDR2 binding motif not only inhibited DDR2 binding to collagen II but also activated DDR2 transmembrane signaling. Thus, DDR2 activation may be effected by single triple-helices rather than fibrillar collagen.

Aegyptin, a novel mosquito salivary gland protein, specifically binds to collagen and prevents its interaction with platelet glycoprotein VI, integrin alpha2beta1, and von Willebrand factor

Blood-sucking arthropods have evolved a number of inhibitors of platelet aggregation and blood coagulation. In this study we have molecularly and functionally characterized aegyptin, a member of the family of 30-kDa salivary allergens from Aedes aegypti, whose function remained elusive thus far. Aegyptin displays a unique sequence characterized by glycine, glutamic acid, and aspartic acid repeats and was shown to specifically block collagen-induced human platelet aggregation and granule secretion. Plasmon resonance experiments demonstrate that aegyptin binds to collagen types I-V (K(d) approximately 1 nm) but does not interact with vitronectin, fibronectin, laminin, fibrinogen, and von Willebrand factor (vWf). In addition, aegyptin attenuates platelet adhesion to soluble or fibrillar collagen. Furthermore, aegyptin inhibits vWf interaction with collagen type III under static conditions and completely blocks platelet adhesion to collagen under flow conditions at high shear rates. Notably, aegyptin prevents collagen but not convulxin binding to recombinant glycoprotein VI. These findings suggest that aegyptin recognizes specific binding sites for glycoprotein VI, integrin alpha2beta1, and vWf, thereby preventing collagen interaction with its three major ligands. Aegyptin is a novel tool to study collagen-platelet interaction and a prototype for development of molecules with antithrombotic properties.

Platelet receptor recognition and cross-talk in collagen-induced activation of platelets

Comprehensive mapping of protein-binding sites within human collagen III has allowed the recognition motifs for integrin alpha(2)beta(1) and VWF A3 domain to be identified. Glycoprotein VI-binding sites are understood, although less well defined. This information, together with recent developments in understanding collagen fiber architecture, and crystal structures of the receptor collagen-binding domains, allows a coherent model for the interaction of collagen with the platelet surface to be developed. This complements our understanding of the orchestration of receptor presentation by membrane microdomains, such that the polyvalent collagen surface may stabilize signaling complexes within the heterogeneous receptor composition of the lipid raft. The ensuing interactions lead to the convergence of signals from each of the adhesive receptors, mediated by FcR gamma-chain and/or FcgammaRIIa, leading to concerted and co-operative platelet activation. Each receptor has a shear-dependent role, VWF/GpIb essential at high shear, and alpha(2)beta(1) at low and intermediate shear, whilst GpVI provides core signals that contribute to enhanced integrin affinity, tighter binding to collagen and consequent platelet activation.

A mechanism to safeguard platelet adhesion under high shear flow: von Willebrand factor-glycoprotein Ib and integrin alphabeta-collagen interactions make complementary, collagen-type-specific contributions to adhesion

BACKGROUND

Blood vessels contain different types of collagen, with types I and III being the major components of vascular collagen. Platelet adhesion under high shear stress has been suggested to depend on the binding of von Willebrand factor (VWF) to collagen.

OBJECTIVE

We analyzed the collagen type specificity for the interaction with VWF and high shear stress platelet adhesion.

METHODS

VWF binding to different types of immobilized collagen and effects of antibodies against glycoprotein Ib (gpIb) and integrin alpha(2)beta(1) on platelet adhesion to type I and III collagens under high shear were analyzed.

RESULTS

VWF showed high-affinity, selective binding to human and bovine type III collagens, but weak or no affinity for types I, II, IV and V under static conditions. Anti-integrin alpha(2)beta(1) markedly inhibited adhesion to type I collagen, but did not affect that to type III collagen. Anti-gpIb antibody significantly inhibited adhesion to type III collagen. Adding both antibodies abrogated the adhesion to either type I or III collagen.

CONCLUSIONS

Both the gpIb-VWF interaction and the integrin alpha(2)beta(1)-collagen interaction contribute to platelet adhesion to collagen under high shear stress, and integrin alpha(II)beta(1) makes a greater contribution to adhesion to type I collagen because less VWF is bound to it.

Self-assembled heterotrimeric collagen triple helices directed through electrostatic interactions

Collagen, a fibrous protein, is an essential structural component of all connective tissues such as cartilage, bones, ligaments, and skin. Type I collagen, the most abundant form, is a heterotrimer assembled from two identical alpha1 chains and one alpha2 chain. However, most synthetic systems have addressed homotrimeric triple helices. In this paper we examine the stability of several heterotrimeric collagen-like triple helices with an emphasis on electrostatic interactions between peptides. We synthesize seven 30 amino acid peptides with net charges ranging from -10 to +10. These peptides were mixed, and their ability to form heterotrimers was assessed. We successfully show the assembly of five different AAB heterotrimers and one ABC heterotrimer. The results from this study indicate that intermolecular electrostatic interactions can be utilized to direct heterotrimer formation. Furthermore, amino acids with poor stability in collagen triple helices can be "rescued" in heterotrimers containing amino acids with known high triple helical stability. This mechanism allows collagen triple helices to have greater chemical diversity than would otherwise be allowed.

Platelet adhesion receptors and their ligands in mouse models of thrombosis

Platelet adhesion and aggregation at sites of vascular injury are two key events in hemostasis and thrombosis. Because of exciting advances in genetic engineering, the mouse has become an important and frequently used model to unravel the molecular mechanisms underlying the multistep process leading to the formation of a stable platelet plug. In gene-targeted mice, the crucial importance of platelet adhesion receptors such as glycoprotein Ib alpha or the alphaIIb beta3 integrin has been confirmed and further clarified. Their absence leads to highly impaired thrombus formation, independent of the model used to induce vascular injury. In contrast, the relative contribution of other receptors, such as glycoprotein VI, or of various platelet ligands may be regulated by the severity of injury, the type of vessel injured, and the signaling pathways that are generated. Murine models have also helped improve understanding of the second wave of events that leads to stabilization of the platelet aggregate. Despite the current limitations due to lack of standardization and the virtual absence of thrombosis models in diseased vessels, there is no doubt that the mouse will play a key role in the discovery and characterization of the next generation of antithrombotic agents. This review focuses on key findings about the molecular mechanisms supporting hemostasis and thrombosis that have been obtained with genetically engineered mouse models deficient in various platelet adhesion receptors and ligands. Combination of these models with sophisticated methods allowing direct visualization of platelet-vessel wall interactions after injury greatly contributed to recent advances in the field.