von Willebrand factor A1 domain can adequately substitute for A3 domain in recruitment of flowing platelets to collagen

Von Willebrand factor and hematogenous cancer metastasis under flow

Hematogenous metastasis involves cancer cell migration to different locations from the primary tumor through the blood circulation. Von Willebrand factor (VWF) has been shown to play an important role in tumor cell adhesion to and extravasation from the endothelial cell lining of blood vessel walls during cancer metastasis. VWF may contribute to this process by interacting with tumor cells, endothelial cells, and platelets through various cell membrane receptors, such as platelet glycoprotein (GP)Ibα, P-selectin, ανβ3 and αIIbβ3 integrins, and glycocalyx. Blood flow can mechanically extend and activate VWF to bind platelets and associate intermolecularly with other VWF molecules in plasma or on the surface of endothelial cells, cancer cells, or platelets. This suggests a mechanoregulatory role of VWF in mediating the interactions between VWF and these cells to promote cancer cell adhesion to blood vessels. In this review, we will summarize the current knowledge of VWF function and the role of hydrodynamic forces in hematogenous cancer metastasis.

Increased Binding of von Willebrand Factor to Sub-Endothelial Collagen May Facilitate Thrombotic Events Complicating Bothrops lanceolatus Envenomation in Humans

Consumption coagulopathy and hemorrhagic syndrome exacerbated by blood anticoagulability remain the most important causes of lethality associated with Bothrops snake envenomation. Bothrops venom also engages platelet aggregation on the injured endothelium via von Willebrand factor (vWF) interactions. Besides platelet aggregation, some Bothrops venom toxins may induce qualitative thrombopathy, which has been in part related to the inhibition of vWF activation. We tested whether B. lanceolatus venom impaired vWF to collagen(s) binding (vWF:CB) activity. Experiments were performed with B. lanceolatus crude venom, in the presence or absence of Bothrofav, a monospecific B. lanceolatus antivenom. Venom of B. lanceolatus fully inhibited vWF to collagen type I and III binding, suggesting venom interactions with the vWF A3 domain. In contrast, B. lanceolatus venom increased vWF to collagen type VI binding, suggesting the enhancement of vWF binding to collagen at the vWF A1 domain. Hence, B. lanceolatus venom exhibited contrasting in vitro effects in terms of the adhesive properties of vWF to collagen. On the other hand, the antivenom Bothrofav reversed the inhibitory effects of B. lanceolatus venom on vWF collagen binding activity. In light of the respective distribution of collagen type III and collagen type VI in perivascular connective tissue and the sub-endothelium, a putative association between an increase in vWF:CB activity for collagen type VI and the onset of thrombotic events in human B. lanceolatus envenomation might be considered.

Von Willebrand disease type 2M: Correlation between genotype and phenotype

BACKGROUND

An appropriate clinical diagnosis of von Willebrand disease (VWD) can be challenging because of a variable bleeding pattern and laboratory phenotype. Genotyping is a powerful diagnostic tool and may have an essential role in the diagnostic field of VWD.

OBJECTIVES

To unravel the clinical and laboratory heterogeneity of genetically confirmed VWD type 2M patients and to investigate their relationship.

METHODS

Patients with a confirmed VWD type 2M genetic variant in the A1 or A3 domain of von Willebrand factor (VWF) and normal or only slightly aberrant VWF multimers were selected from all subjects genotyped at the Radboud university medical center because of a high suspicion of VWD. Bleeding scores and laboratory results were analyzed.

RESULTS

Fifty patients had a clinically relevant genetic variant in the A1 domain. Median bleeding score was 5. Compared with the nationwide Willebrand in the Netherlands study type 2 cohort, bleeding after surgery or delivery was reported more frequently and mucocutaneous bleedings less frequently. Median VWF activity/VWF antigen (VWF:Act/VWF:Ag) ratio was 0.32, whereas VWF collagen binding activity/VWF antigen (VWF:CB/VWF:Ag) ratio was 0.80. Variants in the A3 domain were only found in two patients with low to normal VWF:Act/VWF:Ag ratios (0.45, 1.03) and low VWF:CB/VWF:Ag ratios (0.45, 0.63).

CONCLUSION

Genetically confirmed VWD type 2M patients have a relatively mild clinical phenotype, except for bleeding after surgery and delivery. Laboratory phenotype is variable and depends on the underlying genetic variant. Addition of genotyping to the current phenotypic characterization may improve diagnosis and classification of VWD.

von Willebrand factor binding to myosin assists in coagulation

von Willebrand factor (VWF) binds to platelets and collagen as a means of facilitating coagulation at sites of injury. Recent evidence has shown that myosin can serve as a surface for thrombin generation and binds to activated factor V and factor X. We studied whether VWF can also bind myosin as a means of bringing factor VIII (FVIII) to sites of clot formation. A myosin-binding assay was developed using skeletal muscle myosin to measure VWF binding, and plasma-derived and recombinant VWF containing molecular disruptions at key VWF sites were tested. Competition assays were performed using anti-VWF antibodies. FVIII binding to myosin was measured using a chromogenic FVIII substrate. Thrombin generation was measured using a fluorogenic substrate with and without myosin. Wild-type recombinant VWF and human plasma VWF from healthy controls bound myosin, whereas plasma lacking VWF exhibited no detectable myosin binding. Binding was multimer dependent and blocked by anti-VWF A1 domain antibodies or A1 domain VWF variants. The specific residues involved in myosin binding were similar, but not identical, to those required for collagen IV binding. FVIII did not bind myosin directly, but FVIII activity was detected when VWF and FVIII were bound to myosin. Myosin enhanced thrombin generation in platelet-poor plasma, although no difference was detected with the addition of myosin to platelet-rich plasma. Myosin may help to facilitate delivery of FVIII to sites of injury and indirectly accelerate thrombin generation by providing a surface for VWF binding in the setting of trauma and myosin exposure.

Profiling and targeting connective tissue remodeling in autoimmunity - A novel paradigm for diagnosing and treating chronic diseases

Connective tissue (ConT) remodeling is an essential process in tissue regeneration, where a balanced replacement of old tissue by new tissue occurs. This balance is disturbed in chronic diseases, often autoimmune diseases, usually resulting in the buld up of fibrosis and a gradual loss of organ function. During progression of liver, lung, skin, heart, joint, skeletal and kidney diseasesboth ConT formation and degradation are elevated, which is tightly linked to immune cell activation and a loss of specific cell types and extracellular matrix (ECM) structures that are required for normal organ function. Here, we address the balance of key general and organ specific components of the ECM during homeostasis and in disease, with a focus on collagens, which are emerging as both structural and signaling molecules harbouring neoepitopes and autoantigens that are released during ConT remodeling. Specific collagen molecular signatures of ConT remodeling are linked to disease activity and stage, and to prognosis across different organs. These signatures accompany and further drive disease progression, and often become detectable before clinical disease manifestation (illness). Recent advances allow to quantify and define the nature of ConT remodeling via blood-based assays that measure the levels of well-defined collagen fragments, reflecting different facets of ConT formation and degradation, and associated immunological processes. These novel serum assays are becoming important tools of precision medicine, to detect various chronic and autoimmune diseases before their clinical manifestation, and to non-invasively monitor the efficacy of a broad range of pharmacological interventions.

von Willebrand factor promotes platelet-induced metastasis of osteosarcoma through activation of the VWF-GPIb axis

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Immunohistochemistry results directly show VWF is increased during tumor progression.

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VWF is expressed as low molecular weight multimer in OS cell line SAOS2.

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VWF promotes platelet-induced metastasis of OS through VWF-GPIb pathway.

von Willebrand factor (VWF) is exclusively expressed in endothelial cells (ECs) and megakaryocytes, which plays a crucial role in the initiation of arterial thrombosis. Recent studies have shown that VWF is also expressed in osteosarcoma (OS) cells and participates in adhesion of cancer cells to platelets, thus promoting metastasis of OS cells. However, it is unclear how OS cell-derived VWF-platelet interaction contributes to the metastasis of OS. We hypothesized that the interaction is mediated by the binding between VWF A1 and GPIbα of platelets, a molecular mechanism similar to that of thrombosis. The increased expression of VWF in SAOS2 cells may contribute to the enhancement of platelet adhesion through the VWF-GPIb pathway, which could promote the migration and invasion capacities of SAOS2 cells in vitro. Antibodies that block the pathway could significantly inhibit the platelet-induced metastasis of OS cells. Our results suggest a theoretical basis for the development of new anti-OS metastasis drugs, and further enrich the mechanism of OS metastasis.

Cleavage by MMP-13 renders VWF unable to bind to collagen but increases its platelet reactivity

BACKGROUND

Atherosclerotic plaque rupture and subsequent thrombosis underpin thrombotic syndromes. Under inflammatory conditions in the unstable plaque, perturbed endothelial cells secrete von Willebrand Factor (VWF) which, via its interaction with GpIbα, enables platelet rolling across and adherence to the damaged endothelium. Following plaque rupture, VWF and platelets are exposed to subendothelial collagen, which supports stable platelet adhesion, activation, and aggregation. Plaque-derived matrix metalloproteinase (MMP)-13 is also released into the surrounding lumen where it may interact with VWF, collagen, and platelets.

OBJECTIVES

We sought to discover whether MMP-13 can cleave VWF and whether this might regulate its interaction with both collagen and platelets.

METHODS

We have used platelet adhesion assays and whole blood flow experiments to assess the effects of VWF cleavage by MMP-13 on platelet adhesion and thrombus formation.

RESULTS

Unlike the shear-dependent cleavage of VWF by a disintegrin and metalloprotease with thrombospondin motif member 13 (ADAMTS13), MMP-13 is able to cleave VWF under static conditions. Following cleavage by MMP-13, immobilized VWF cannot bind to collagen but interacts more strongly with platelets, supporting slower platelet rolling in whole blood under shear. Compared with intact VWF, the interaction of cleaved VWF with platelets results in greater GpIbα upregulation and P-selectin expression, and the thrombi formed on cleaved VWF-collagen co-coatings are larger and more contractile than platelet aggregates on intact VWF-collagen co-coatings or on collagen alone.

CONCLUSIONS

Our data suggest a VWF-mediated role for MMP-13 in the recruitment of platelets to the site of vascular injury and may provide new insights into the association of MMP-13 in atherothrombotic and stroke pathologies.

The CEACAM1-derived peptide QLSN impairs collagen-induced human platelet activation through glycoprotein VI

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) regulates collagen-mediated platelet activation through its cytoplasmic immunoreceptor tyrosine-based inhibition motifs (ITIMs). However, the function of CEACAM1's extracellular cleavage fragments is currently unknown. In the present study, we used mass spectrometry (MS) to identify 9 cleavage fragments shed by matrix metallopeptidase 12 (MMP-12), and then we synthesized peptides with sequences corresponding to the fragments. QLSNGNRTLT (QLSN), a peptide from the A1-domain of CEACAM1, significantly attenuated collagen-induced platelet aggregation. QLSN also attenuated platelet static adhesion to collagen. Additionally, QLSN reduced human platelet secretion and integrin α_IIbβ₃ activation in response to glycoprotein VI (GPVI)-selective agonist, convulxin. Correspondingly, QLSN treatment significantly decreased convulxin-mediated phosphorylation of Src, protein kinase B (Akt), spleen tyrosine kinase (Syk) and phospholipase Cγ2 (PLCγ2) in human platelets. These data indicate that the CEACAM1-derived peptide QLSN inhibits GPVI-mediated human platelet activation. QLSN could potentially be developed as a novel antiplatelet agent.

The good and the bad collagens of fibrosis - Their role in signaling and organ function

Usually the dense extracellular structure in fibrotic tissues is described as extracellular matrix (ECM) or simply as collagen. However, fibrosis is not just fibrosis, which is already exemplified by the variant morphological characteristics of fibrosis due to viral versus cholestatic, autoimmune or toxic liver injury, with reticular, chicken wire and bridging fibrosis. Importantly, the overall composition of the ECM, especially the relative amounts of the many types of collagens, which represent the most abundant ECM molecules and which centrally modulate cellular functions and physiological processes, changes dramatically during fibrosis progression. We hypothesize that there are good and bad collagens in fibrosis and that a change of location alone may change the function from good to bad. Whereas basement membrane collagen type IV anchors epithelial and other cells in a polarized manner, the interstitial fibroblast collagens type I and III do not provide directional information. In addition, feedback loops from biologically active degradation products of some collagens are examples of the importance of having the right collagen at the right place and at the right time controlling cell function, proliferation, matrix production and fate. Examples are the interstitial collagen type VI and basement membrane collagen type XVIII. Their carboxyterminal propeptides serve as an adipose tissue hormone, endotrophin, and as a regulator of angiogenesis, endostatin, respectively. We provide an overview of the 28 known collagen types and propose that the molecular composition of the ECM in fibrosis needs careful attention to assess its impact on organ function and its potential to progress or reverse. Consequently, to adequately assess fibrosis and to design optimal antifibrotic therapies, we need to dissect the molecular entity of fibrosis for the molecular composition and spatial distribution of collagens and the associated ECM.

The Von Willebrand Factor A1-Collagen III Interaction Is Independent of Conformation and Type 2 Von Willebrand Disease Phenotype

The blood von Willebrand factor (VWF) mediates platelet adhesion to injured vessels by sequestering platelets from blood flow and depositing them to collagen and other exposed subendothelial matrix proteins. This process of capturing platelets to facilitate formation of platelet plugs occurs through transient interactions with platelet glycoprotein Ibα via the VWF A1 domain which also binds collagen. Using a conformationally diverse collection of natively folded and mutation-induced misfolded von Willebrand disease (VWD) variants, we test a recently proposed affinity up-regulation hypothesis which states that collagen binding changes the conformation of the A1 domain to a high-affinity GPIbα binding competent state. With surface plasmon resonance (SPR), we present this diversified collection to collagen and quantify the kinetics of association and dissociation to ascertain the conformational selectivity of collagen. With analytical rheology, we quantify real-time platelet pause times and translocation velocities across a Cu²⁺ HisTag-chelated and collagen-bound A1 single domain and A1A2A3 tridomain fragment of VWF under shear stress in an ex vivo shear flow microfluidic chamber. In contrast to expected hypothetical outcomes, collagen has limited conformational selectivity for binding A1. A1-collagen binding is independent of gain- or loss-of-function phenotype and under shear stress, platelet translocation pause times on collagen-bound A1A2A3 are either normal or shorter depending on whether A1 is concertedly bound with the A3 domain to collagen. With respect to A1, collagen has an inhibitory role that provides an explanation for the lack of thrombosis in patients with gain-of-function VWD.

Collagen-mediated hemostasis

Collagens mediate essential hemostasis by maintaining the integrity and stability of the vascular wall. Imbalanced turnover of collagens by uncontrolled formation and/or degradation may result in pathologic conditions such as fibrosis. Thickening of the vessel wall because of accumulation of collagens may lead to arterial occlusion or thrombosis. Thinning of the wall because of collagen degradation or deficiency may lead to rupture of the vessel wall or aneurysm. Preventing excessive hemorrhage or thrombosis relies on collagen-mediated actions. Von Willebrand factor, integrins and glycoprotein VI, as well as clotting factors, can bind collagen to restore normal hemostasis after trauma. This review outlines the essential roles of collagens in mediating hemostasis, with a focus on collagens types I, III, IV, VI, XV, and XVIII.

Mutational Constraints on Local Unfolding Inhibit the Rheological Adaptation of von Willebrand Factor

Unusually large von Willebrand factor (VWF), the first responder to vascular injury in primary hemostasis, is designed to capture platelets under the high shear stress of rheological blood flow. In type 2M von Willebrand disease, two rare mutations (G1324A and G1324S) within the platelet GPIbα binding interface of the VWF A1 domain impair the hemostatic function of VWF. We investigate structural and conformational effects of these mutations on the A1 domain's efficacy to bind collagen and adhere platelets under shear flow. These mutations enhance the thermodynamic stability, reduce the rate of unfolding, and enhance the A1 domain's resistance to limited proteolysis. Collagen binding affinity is not significantly affected indicating that the primary stabilizing effect of these mutations is to diminish the platelet binding efficiency under shear flow. The enhanced stability stems from the steric consequences of adding a side chain (G1324A) and additionally a hydrogen bond (G1324S) to His(1322) across the β2-β3 hairpin in the GPIbα binding interface, which restrains the conformational degrees of freedom and the overall flexibility of the native state. These studies reveal a novel rheological strategy in which the incorporation of a single glycine within the GPIbα binding interface of normal VWF enhances the probability of local unfolding that enables the A1 domain to conformationally adapt to shear flow while maintaining its overall native structure.

Misfolding of vWF to pathologically disordered conformations impacts the severity of von Willebrand disease

The primary hemostatic von Willebrand factor (vWF) functions to sequester platelets from rheological blood flow and mediates their adhesion to damaged subendothelium at sites of vascular injury. We have surveyed the effect of 16 disease-causing mutations identified in patients diagnosed with the bleeding diathesis disorder, von Willebrand disease (vWD), on the structure and rheology of vWF A1 domain adhesiveness to the platelet GPIbα receptor. These mutations have a dynamic phenotypical range of bleeding from lack of platelet adhesion to severe thrombocytopenia. Using new rheological tools in combination with classical thermodynamic, biophysical, and spectroscopic metrics, we establish a high propensity of the A1 domain to misfold to pathological molten globule conformations that differentially alter the strength of platelet adhesion under shear flow. Rheodynamic analysis establishes a quantitative rank order between shear-rate-dependent platelet-translocation pause times that linearly correlate with clinically reported measures of patient platelet counts and the severity of thrombocytopenia. These results suggest that specific secondary structure elements remaining in these pathological conformations of the A1 domain regulate GPIbα binding and the strength of vWF-platelet interactions, which affects the vWD functional phenotype and the severity of thrombocytopenia.

Desmopressin treatment improves platelet function under flow in patients with postoperative bleeding

BACKGROUND

Patients undergoing major cardiothoracic surgery are subjected to dilution, owing to massive fluid infusion and blood component transfusion. These patients may experience bleeding perioperatively, and are frequently treated with the endothelium-activating agent desmopressin.

OBJECTIVES

To investigate the effect of desmopressin administration on von Willebrand factor (VWF)-dependent coagulant and platelet functions under flow conditions.

PATIENTS/METHODS

Blood from 16 patients with postoperative bleeding was obtained before and after desmopressin treatment (0.3 μg kg(-1) body weight), and assessed for coagulant properties and platelet function. Furthermore, VWF antigen levels and multimer composition were determined in both samples.

RESULTS

Desmopressin treatment did not change thrombin generation in plasma or whole blood thromboelasticity. Also coagulation factor levels (other than factor VIII) and coagulation times were unchanged, suggesting that desmopressin treatment did not have a major effect on the coagulant activity. On the other hand, desmopressin treatment raised the already high plasma levels of VWF from a median of 116 IU mL(-1) (interquartile range [IQR] 102-154 IU mL(-1) ) to a median of 160 IU mL(-1) (IQR 126-187 IU mL(-1) ) (P = 0.007), owing to accumulation of the high molecular weight VWF multimers. Furthermore, desmopressin treatment caused an increase in collagen-dependent thrombus formation and platelet phosphatidylserine exposure. Markers of thrombus formation correlated with the plasma levels of VWF. In vitro control experiments confirmed a major contribution of VWF to thrombus formation and procoagulant activity under conditions of blood dilution.

CONCLUSIONS

Desmopressin treatment of patients with bleeding complications after cardiothoracic surgery induces the release of high molecular weight VWF multimers, which enhance platelet activation and thrombus formation under flow conditions.

Crucial role for the VWF A1 domain in binding to type IV collagen

Von Willebrand factor (VWF) contains binding sites for platelets and for vascular collagens to facilitate clot formation at sites of injury. Although previous work has shown that VWF can bind type IV collagen (collagen 4), little characterization of this interaction has been performed. We examined the binding of VWF to collagen 4 in vitro and extended this characterization to a murine model of defective VWF-collagen 4 interactions. The interactions of VWF and collagen 4 were further studied using plasma samples from a large study of both healthy controls and subjects with different types of von Willebrand disease (VWD). Our results show that collagen 4 appears to bind VWF exclusively via the VWF A1 domain, and that specific sequence variations identified through VWF patient samples and through site-directed mutagenesis in the VWF A1 domain can decrease or abrogate this interaction. In addition, VWF-dependent platelet binding to collagen 4 under flow conditions requires an intact VWF A1 domain. We observed that decreased binding to collagen 4 was associated with select VWF A1 domain sequence variations in type 1 and type 2M VWD. This suggests an additional mechanism through which VWF variants may alter hemostasis.

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.

Platelet GpIba binding to von Willebrand Factor under fluid shear:contributions of the D′D3-domain, A1-domain flanking peptide and O-linked glycans

BACKGROUND

Von Willebrand Factor (VWF) A1-domain binding to platelet receptor GpIbα is an important fluid-shear dependent interaction that regulates both soluble VWF binding to platelets, and platelet tethering onto immobilized VWF. We evaluated the roles of different structural elements at the N-terminus of the A1-domain in regulating shear dependent platelet binding. Specifically, the focus was on the VWF D'D3-domain, A1-domain N-terminal flanking peptide (NFP), and O-glycans on this peptide.

METHODS AND RESULTS

Full-length dimeric VWF (ΔPro-VWF), dimeric VWF lacking the D'D3 domain (ΔD'D3-VWF), and ΔD'D3-VWF variants lacking either the NFP (ΔD'D3NFP(─)-VWF) or just O-glycans on this peptide (ΔD'D3OG(─)-VWF) were expressed. Monomeric VWF-A1 and D'D3-A1 were also produced. In ELISA, the apparent dissociation constant (KD) of soluble ΔPro-VWF binding to immobilized GpIbα (KD≈100 nmol/L) was 50- to 100-fold higher than other proteins lacking the D'D3 domain (KD~0.7 to 2.5 nmol/L). Additionally, in surface plasmon resonance studies, the on-rate of D'D3-A1 binding to immobilized GpIbα (kon=1.8±0.4×10(4) (mol/L)(-1)·s(-1); KD=1.7 μmol/L) was reduced compared with the single VWF-A1 domain (kon=5.1±0.4×10(4) (mol/L)(-1)·s(-1); KD=1.2 μmol/L). Thus, VWF-D'D3 primarily controls soluble VWF binding to GpIbα. In contrast, upon VWF immobilization, all molecular features regulated A1-GpIbα binding. Here, in ELISA, the number of apparent A1-domain sites available for binding GpIbα on ΔPro-VWF was ≈50% that of the ΔD'D3-VWF variants. In microfluidics based platelet adhesion measurements on immobilized VWF and thrombus formation assays on collagen, human platelet recruitment varied as ΔPro-VWF<ΔD'D3-VWF<ΔD'D3NFP(─)-VWF<ΔD'D3OG(─)-VWF.

CONCLUSIONS

Whereas VWF-D'D3 is the major regulator of soluble VWF binding to platelet GpIbα, both the D'D3-domain and N-terminal peptide regulate platelet translocation and thrombus formation.

Of von Willebrand factor and platelets

Hemostasis and pathological thrombus formation are dynamic processes that require multiple adhesive receptor-ligand interactions, with blood platelets at the heart of such events. Many studies have contributed to shed light on the importance of von Willebrand factor (VWF) interaction with its platelet receptors, glycoprotein (GP) Ib-IX-V and αIIbβ3 integrin, in promoting primary platelet adhesion and aggregation following vessel injury. This review will recapitulate our current knowledge on the subject from the rheological aspect to the spatio-temporal development of thrombus formation. We will also discuss the signaling events generated by VWF/GPIb-IX-V interaction, leading to platelet activation. Additionally, we will review the growing body of evidence gathered from the recent development of pathological mouse models suggesting that VWF binding to GPIb-IX-V is a promising target in arterial and venous pathological thrombosis. Finally, the pathological aspects of VWF and its impact on platelets will be addressed.

Analysis of the role of von Willebrand factor, platelet glycoprotein VI-, and α2β1-mediated collagen binding in thrombus formation

Rare missense mutations in the von Willebrand factor (VWF) A3 domain that disrupt collagen binding have been found in patients with a mild bleeding phenotype. However, the analysis of these aberrant VWF-collagen interactions has been limited. Here, we have developed mouse models of collagen-binding mutants and analyzed the function of the A3 domain using comprehensive in vitro and in vivo approaches. Five loss-of-function (p.S1731T, p.W1745C, p.S1783A, p.H1786D, A3 deletion) and 1 gain-of-function (p.L1757A) variants were generated in the mouse VWF complementary DNA. The results of these various assays were consistent, although the magnitude of the effects were different: the gain-of-function (p.L1757A) variant showed consistent enhanced collagen binding whereas the loss-of-function mutants showed variable degrees of functional deficit. We further analyzed the impact of direct platelet-collagen binding by blocking glycoprotein VI (GPVI) and integrin α2β1 in our ferric chloride murine thrombosis model. The inhibition of GPVI demonstrated a comparable functional defect in thrombosis formation to the VWF(-/-) mice whereas α2β1 inhibition demonstrated a milder bleeding phenotype. Furthermore, a delayed and markedly reduced thrombogenic response was still evident in VWF(-/-), GPVI, and α2β1 blocked animals, suggesting that alternative primary hemostatic mechanisms can partially rescue the bleeding phenotype associated with these defects.

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.

On the versatility of von Willebrand factor

Von Willebrand factor (VWF) is a large multimeric protein, the function of which has been demonstrated to be pivotal to the haemostatic system. Indeed, quantitative and/or qualitative abnormalities of VWF are associated with the bleeding disorder Von Willebrand disease (VWD). Moreover, increased plasma concentrations of VWF have been linked to an increased risk for thrombotic complications. In the previous decades, many studies have contributed to our understanding of how VWF is connected to the haemostatic system, particularly with regard to structure-function relationships. Interactive sites for important ligands of VWF (such as factor VIII, collagen, glycoprotein Ibα, integrin αIIbβ3 and protease ADAMTS13) have been identified, and mutagenesis studies have confirmed the physiological relevance of the interactions between VWF and these ligands. However, we have also become aware that VWF has a more versatile character than previously thought, given its potential role in various non-hemostatic processes, like intimal thickening, tumor cell apoptosis and inflammatory processes. In the presence review, a summary of our knowledge on VWF structure-function relationships is provided in the context of the “classical” haemostatic task of VWF and in perspective of pathological processes beyond haemostasis.

Wound modulation after filtration surgery

Filtration surgery is the standard invasive procedure for the management of intraocular pressure in advanced glaucoma. The key to a successful outcome is to modulate the normal wound healing cascade that leads to closure of the newly created aqueous outflow pathway. Antifibrotic agents such as mitomycin C and 5-fluorouracil have been increasingly used to modulate the wound healing process and increase surgical success. Although these agents have proven efficacy, they also increase the risk of complications. Efforts have centered on the identification of novel agents and techniques that can influence wound modulation without these complications. We detail new agents and methods under investigation to control wound healing after filtration surgery.

Sources of variability in platelet accumulation on type 1 fibrillar collagen in microfluidic flow assays

Microfluidic flow assays (MFA) that measure shear dependent platelet function have potential clinical applications in the diagnosis and treatment of bleeding and thrombotic disorders. As a step towards clinical application, the objective of this study was to measure how phenotypic and genetic factors, as well as experimental conditions, affect the variability of platelet accumulation on type 1 collagen within a MFA. Whole blood was perfused over type 1 fibrillar collagen at wall shear rates of 150, 300, 750 and 1500 s⁻¹ through four independent channels with a height of 50 µm and a width of 500 µm. The accumulation of platelets was characterized by the lag time to 1% platelet surface coverage (Lag_T), the rate of platelet accumulation (V_PLT), and platelet surface coverage (SC). A cohort of normal donors was tested and the results were correlated to plasma von Willebrand factor (VWF) levels, platelet count, hematocrit, sex, and collagen receptors genotypes. VWF levels were the strongest determinant of platelet accumulation. VWF levels were positively correlated to V_PLT and SC at all wall shear rates. A longer Lag_T for platelet accumulation at arterial shear rates compared to venous shear rates was attributed to the time required for plasma proteins to adsorb to collagen. There was no association between platelet accumulation and hematocrit or platelet count. Individuals with the AG genotype of the GP6 gene had lower platelet accumulation than individuals with the AA genotype at 150 s⁻¹ and 300 s⁻¹. Recalcified blood collected into sodium citrate and corn trypsin inhibitor (CTI) resulted in diminished platelet accumulation compared to CTI alone, suggesting that citrate irreversibly diminishes platelet function. This study the largest association study of MFA in healthy donors (n = 104) and will likely set up the basis for the determination of the normal range of platelet responses in this type of assay.

Mutations in the A3 domain of von Willebrand factor inducing combined qualitative and quantitative defects in the protein

Two unrelated families were recruited in the French Reference Center for von Willebrand Disease with moderate bleeding symptoms associated with low von Willebrand factor (VWF) antigen levels, decreased collagen binding assay, and no or partial response to desmopressin. Genetic analysis showed the presence of heterozygous mutations in the A3 domain away from the collagen-binding surface: 1 never reported previously (p.L1696R) and another (p.P1824H) described in a Spanish family. The mutations were reproduced by site-directed mutagenesis and mutant VWF was expressed in different expression systems, COS-7 cells, baby hamster kidney cells, and in VWF-deficient mice through hydrodynamic injection. p.L1696R and p.P1824H were associated with very low expression levels both in vitro and in vivo, with intracellular retention for p.P1824H. Both homozygous mutants displayed decreased binding to collagen types I and III but also decreased binding to platelet glycoproteins Ib and IIbIIIa. Co-transfections with wild-type VWF partially corrected these defects, except that collagen binding remained abnormal. The in vivo thrombosis response was severely reduced for both heterozygous mutants. In conclusion, we report 2 VWF A3 domain mutations that induce a combined qualitative and quantitative defect.

von Willebrand factor: more than a regulator of hemostasis and thrombosis

von Willebrand factor (vWF) was first identified as an adhesive glycoprotein involved in hemostasis by Zimmermann in 1971. Since then, vWF has been shown to play a vital role in platelet adhesion, platelet binding to collagen and factor VIII protection. Recent studies have implicated vWF as a regulator of angiogenesis, smooth muscle cell proliferation, tumor cell metastasis and crosstalk in the immune system. In this review, we will discuss the aspects of vWF structure that facilitate its biological effects and speculate on its newly discovered and hypothesized roles in the pathogenesis of several diseases.

Prevention of ocular scarring post glaucoma filtration surgery using the inflammatory cell and platelet binding modulator saratin in a rabbit model

Clinical Relevance

Late complications can occur with use of current antimetabolites to prevent scarring following glaucoma filtration surgery (GFS). Safer, more targeted, anti-fibrosis agents are sought.

Objectives

The protein saratin has been shown to exhibit anti-fibrotic and anti-thrombotic properties in response to injury, but had not been used for glaucoma surgery. The goal of this study was to compare the efficacy of saratin with that of the widely accepted mitomycin-C (MMC) in prolonging bleb survival following GFS in the rabbit model. Two saratin delivery routes were compared; a single intraoperative topical application versus a combination of intraoperative topical application with two additional postoperative injections.

Methods

Twenty-four New Zealand White rabbits underwent GFS and received either intraoperative topical saratin, intraoperative topical saratin plus two injections on post-operative days 4 and 8, balanced saline solution (BSS), or MMC. The bleb tissues and their elevation durations were compared based on clinical and histological findings.

Results

Rabbits receiving topical+injections of saratin had a mean bleb survival of 33.6±8.5 days, significantly higher than the negative BSS controls, which averaged 17.4±6.0 days (p = 0.018). No improvement over BSS was seen for rabbits receiving topical saratin only (15.5±4.8 days, p = 0.749). Rabbits receiving saratin did not develop bleb avascularity and thinning associated with MMC treatment and there were no apparent clinical signs of toxicity.

Conclusions

Treatment with a single intraoperative topical application plus two additional postoperative injections significantly prolonged bleb elevation comparable to MMC, but without toxicity; however, topical application alone was ineffective.

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

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

Characterization of collagen thin films for von Willebrand factor binding and platelet adhesion

Von Willebrand factor (VWF) binding and platelet adhesion to subendothelial collagens are initial events in thrombus formation at sites of vascular injury. These events are often studied in vitro using flow assays designed to mimic vascular hemodynamics. Flow assays commonly employ collagen-functionalized substrates, but a lack of standardized methods of surface ligation limits their widespread use as a clinical diagnostic. Here, we report the use of collagen thin films (CTF) in flow assays. Thin films were grown on hydrophobic substrates from type I collagen solutions of increasing concentration (10, 100, and 1000 μg/mL). We found that the corresponding increase in fiber surface area determined the amount of VWF binding and platelet adhesion. The association rate constant (k(a)) of plasma VWF binding at a wall shear stress of 45 dyn/cm(2) was 0.3 × 10(5), 1.8 × 10(5), and 1.6 × 10(5) M(-1) s(-1) for CTF grown from 10, 100, and 1000 μg/mL solutions, respectively. We observed a 5-fold increase in VWF binding capacity with each 10-fold increase in collagen solution concentration. The association rates of Ser1731Thr and His1786Asp VWF mutants with collagen binding deficiencies were 9% and 22%, respectively, of wild-type rates. Using microfluidic devices for blood flow assays, we observed that CTF supported platelet adhesion at a wall shear rate of 1000 s(-1). CTF grown from 10 and 100 μg/mL solutions had variable levels of platelet surface coverage between multiple normal donors. However, CTF substrates grown from 1000 μg/mL solutions had reproducible surface coverage levels (74 ± 17%) between normal donors, and there was significantly diminished surface coverage from two type 1 von Willebrand disease patients (8.0% and 24%). These results demonstrate that collagen thin films are homogeneous and reproducible substrates that can measure dysfunctions in VWF binding and platelet adhesion under flow in a clinical microfluidic assay format.

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

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

von Willebrand factor self-association on platelet GpIbalpha under hydrodynamic shear: effect on shear-induced platelet activation

The function of the mechanosensitive, multimeric blood protein von Willebrand factor (VWF) is dependent on its size. We tested the hypothesis that VWF may self-associate on the platelet glycoprotein Ibα (GpIbα) receptor under hydrodynamic shear. Consistent with this proposition, whereas Alexa-488-conjugated VWF (VWF-488) bound platelets at modest levels, addition of unlabeled VWF enhanced the extent of VWF-488 binding. Recombinant VWF lacking the A1-domain was conjugated with Alexa-488 to produce ΔA1-488. Although ΔA1-488 alone did not bind platelets under shear, this protein bound GpIbα on addition of either purified plasma VWF or recombinant full-length VWF. The extent of self-association increased with applied shear stress more than ∼ 60 to 70 dyne/cm(2). ΔA1-488 bound platelets in the milieu of plasma. On application of fluid shear to whole blood, half of the activated platelets had ΔA1-488 bound, suggesting that VWF self-association may be necessary for cell activation. Shearing platelets with 6-μm beads bearing either immobilized VWF or anti-GpIbα mAb resulted in cell activation at shear stress down to 2 to 5 dyne/cm(2). Taken together, the data suggest that fluid shear in circulation can increase the effective size of VWF bound to platelet GpIbα via protein self-association. This can trigger mechanotransduction and cell activation by enhancing the drag force applied on the cell-surface receptor.

Unfolding the A2 domain of von Willebrand factor with the optical trap

Von Willebrand factor (VWF) is a multimeric plasma glycoprotein involved in both hemostasis and thrombosis. VWF conformational changes, especially unfolding of the A2 domain, may be required for efficient enzymatic cleavage in vivo. It has been shown that a single A2 domain unfolds at most probable unfolding forces of 7-14 pN at force loading rates of 0.35-350 pN/s and A2 unfolding facilitates A2 cleavage in vitro. However, it remains unknown how much force is required to unfold the A2 domain in the context of a VWF multimer where A2 may be stabilized by other domains like A1 and A3. With the optical trap, we stretched VWF multimers and a poly-protein (A1A2A3)3 that contains three repeats of the triplet A1A2A3 domains at constant speeds of 2000 nm/s and 400 nm/s, respectively, which yielded corresponding average force loading rates of 90 and 22 pN/s. We found that VWF multimers became stiffer when they were stretched and extended by force. After force increased to a certain level, sudden extensional jumps that signify domain unfolding were often observed. Histograms of the unfolding force and the unfolded contour length showed two or three peaks that were integral multiples of approximately 21 pN and approximately 63 nm, respectively. Stretching of (A1A2A3)3 yielded comparable distributions of unfolding force and unfolded contour length, showing that unfolding of the A2 domain accounts for the behavior of VWF multimers under tension. These results show that the A2 domain can be indeed unfolded in the presence of A1, A3, and other domains. Compared with the value in the literature, the larger most probable unfolding force measured in this study suggests that the A2 domain is mechanically stabilized by A1 or A3 although variations in experimental setups and conditions may complicate this interpretation.

Identification of a VWF peptide antagonist that blocks platelet adhesion under high shear conditions by selectively inhibiting the VWF-collagen interaction

BACKGROUND

Because the collagen-VWF-GPIb/IX/V axis plays an important role in thrombus formation, it represents a promising target for development of new antithrombotic agents.

OBJECTIVES

We used phage display to identify potential small peptides that interfere with the VWF-collagen binding and might serve as lead products for the development of possible oral antithrombotic compounds.

METHODS

A random linear heptamer peptide library was used to select VWF-binding peptides.

RESULTS

We identified a phage clone, displaying the YDPWTPS sequence, further referred to as L7-phage, that bound to VWF in a specific and a dose-dependent manner. This L7-phage specifically inhibited the VWF-collagen interaction under both static and flow conditions. Epitope mapping using deletion mutants of VWF revealed that the L7-phage does not bind to the known collagen-binding A3 domain within VWF, but to the more carboxyterminal situated C domain. This inhibition was not due to steric hindrance of the A3 domain-collagen interaction by the L7-phage. Indeed, a tetrabranched multi-antigen peptide (MAP) presenting four copies of the peptide, but not the scrambled MAP, also inhibited VWF-collagen interaction under conditions of high shear stress at a concentration of 148 nmol L(-1).

CONCLUSIONS

Based on these results, we conclude that we have identified the first peptide antagonist that binds to the VWF C domain and by this specifically inhibits the VWF binding to collagen, suppressing platelet adhesion and aggregation under high shear conditions. As a consequence, this peptide and its future derivates are potentially interesting antithrombotic agents.

Characterization of W1745C and S1783A: 2 novel mutations causing defective collagen binding in the A3 domain of von Willebrand factor

Investigation of 3 families with bleeding symptoms demonstrated a defect in the collagen-binding activity of von Willebrand factor (VWF) in association with a normal VWF multimeric pattern. Genetic analysis showed affected persons to be heterozygous for mutations in the A3 domain of VWF: S1731T, W1745C, and S1783A. One person showed compound heterozygosity for W1745C and R760H. W1745C and S1783A have not been reported previously. The mutations were reproduced by site-directed mutagenesis and mutant VWF expressed in HEK293T cells. Collagen-binding activity measured by immunosorbent assay varied according to collagen type: W1745C and S1783A were associated with a pronounced binding defect to both type I and type III collagen, whereas the principal abnormality in S1731T patients was a reduction in binding to type I collagen only. The multimer pattern and distribution of mutant proteins were indistinguishable from wild-type recombinant VWF, confirming that the defect in collagen binding resulted from the loss of affinity at the binding site and not impairment of high-molecular-weight multimer formation. Our findings demonstrate that mutations causing an abnormality in the binding of VWF to collagen may contribute to clinically significant bleeding symptoms. We propose that isolated collagen-binding defects are classified as a distinct subtype of von Willebrand disease.

Fluid shear induces conformation change in human blood protein von Willebrand factor in solution

Many of the physiological functions of von Willebrand Factor (VWF), including its binding interaction with blood platelets, are regulated by the magnitude of applied fluid/hydrodynamic stress. We applied two complementary strategies to study the effect of fluid forces on the solution structure of VWF. First, small-angle neutron scattering was used to measure protein conformation changes in response to laminar shear rates (G) up to 3000/s. Here, purified VWF was sheared in a quartz Couette cell and protein conformation was measured in real time over length scales from 2-140 nm. Second, changes in VWF structure up to 9600/s were quantified by measuring the binding of a fluorescent probe 1,1'-bis(anilino)-4-,4'-bis(naphthalene)-8,8'-disulfonate (bis-ANS) to hydrophobic pockets exposed in the sheared protein. Small angle neutron scattering studies, coupled with quantitative modeling, showed that VWF undergoes structural changes at G < 3000/s. These changes were most prominent at length scales <10 nm (scattering vector (q) range >0.6/nm). A mathematical model attributes these changes to the rearrangement of domain level features within the globular section of the protein. Studies with bis-ANS demonstrated marked increase in bis-ANS binding at G > 2300/s. Together, the data suggest that local rearrangements at the domain level may precede changes at larger-length scales that accompany exposure of protein hydrophobic pockets. Changes in VWF conformation reported here likely regulate protein function in response to fluid shear.

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.

Two novel monoclonal antibodies to VWFA3 inhibit VWF-collagen and VWF-platelet interactions

BACKGROUND

The interaction of collagen-von Willebrand factor (VWF)-GPIb is essential for platelet adhesion, especially under high shear conditions. VWF, which acts as a bridge between platelets and exposed subendothelium, interacts with collagen through its A3 domain, which is a new target for the antithrombotic agent.

OBJECTIVE

To develop functional blockers that specifically inhibit VWF-dependent adhesion of platelets to collagen under high shear stress.

METHODS

To develop murine antihuman VWF A3 monoclonal antibodies (mAbs) by standard hybridoma technology, and characterize their abilities to block interactions between VWF A3 and collagen as well as platelet function.

RESULTS

Thirty anti-VWF-A3 mAbs were obtained. Among them, two mAbs, designated as SZ-123 and SZ-125, were found to inhibit VWF-collagen type III interaction. SZ-123 and SZ-125 inhibited the binding of purified human VWF (1.5 or 3 mug mL(-1)) to human placenta collagen type III (IC(50) = 0.07 +/- 0.02 and 0.15 +/- 0.03 mug mL(-1), respectively) or to calf skin collagen type III (IC(50) = 0.48 +/- 0.06 and 0.51 +/- 0.07 mug mL(-1), respectively) coated on plates. Under flow shear condition (1000 s(-1)), SZ-123 and SZ-125 inhibited platelet adhesion on human placenta collagen- or calf skin collagen-coated surfaces. Both mAbs also inhibited platelet aggregation induced by ristocetin, botrocetin or bovine plasma.

CONCLUSIONS

SZ-123 and SZ-125 inhibited VWF-collagen and VWF-platelet interactions.

Adhesion mechanisms in platelet function

Platelet adhesion is an essential function in response to vascular injury and is generally viewed as the first step during which single platelets bind through specific membrane receptors to cellular and extracellular matrix constituents of the vessel wall and tissues. This response initiates thrombus formation that arrests hemorrhage and permits wound healing. Pathological conditions that cause vascular alterations and blood flow disturbances may turn this beneficial process into a disease mechanism that results in arterial occlusion, most frequently in atherosclerotic vessels of the heart and brain. Besides their relevant role in hemostasis and thrombosis, platelet adhesive properties are central to a variety of pathophysiological processes that extend from inflammation to immune-mediated host defense and pathogenic mechanisms as well as cancer metastasis. All of these activities depend on the ability of platelets to circulate in blood as sentinels of vascular integrity, adhere where alterations are detected, and signal the abnormality to other platelets and blood cells. In this respect, therefore, platelet adhesion to vascular wall structures, to one another (aggregation), or to other blood cells, represent different aspects of the same fundamental biological process. Detailed studies by many investigators over the past several years have been aimed to dissect the complexity of these functions, and the results obtained now permit an attempt to integrate all the available information into a picture that highlights the balanced diversity and synergy of distinct platelet adhesive interactions.

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.

The leech product saratin is a potent inhibitor of platelet integrin α2β1 and von Willebrand factor binding to collagen

Subendothelial collagen plays an important role, via both direct and indirect mechanisms, in the initiation of thrombus formation at sites of vascular injury. Collagen binds plasma von Willebrand factor, which mediates platelet recruitment to collagen under high shear. Subsequently, the direct binding of the platelet receptors glycoprotein VI and alpha2beta1 to collagen is critical for platelet activation and stable adhesion. Leeches, have evolved a number of inhibitors directed towards platelet-collagen interactions so as to prevent hemostasis in the host during hematophagy. In this article, we describe the molecular mechanisms underlying the ability of the leech product saratin to inhibit platelet binding to collagen. In the presence of inhibitors of ADP and thromboxane A2, both saratin and 6F1, a blocking alpha2beta1 mAb, abrogated platelet adhesion to fibrillar and soluble collagen. Additionally, saratin eliminated alpha2beta1-dependent platelet adhesion to soluble collagen in the presence of an Src kinase inhibitor. Moreover, saratin prevented platelet-rich plasma adhesion to fibrillar collagen, a process dependent upon both alpha2beta1 and von Willebrand factor binding to collagen. Furthermore, saratin specifically inhibited the binding of the alpha2 integrin subunit I domain to collagen, and prevented platelet adhesion to collagen under flow to the same extent as observed in the presence of a combination of mAbs to glycoprotein Ib and alpha2beta1. These results demonstrate that saratin interferes with integrin alpha2beta1 binding to collagen in addition to inhibiting von Willebrand factor-collagen binding, presumably by binding to an overlapping epitope on collagen. This has significant implications for the use of saratin as a tool to inhibit platelet-collagen interactions.