Flow Analysis of Von Willebrand Factor Collagen Binding Mutants

Blood ◽  
2011 ◽  
Vol 118 (21) ◽  
pp. 2213-2213
Author(s):  
Thomas A J McKinnon ◽  
Agata Anna Nowak ◽  
Alina Hua ◽  
Carolyn Millar ◽  
Michael Laffan
Keyword(s):  
Shear Stress ◽  
Collagen Type ◽  
Collagen Type I ◽  
Type I ◽  
Flow Conditions ◽  
Collagen Binding ◽  
Binding Function ◽  
Static Conditions ◽  
A1 Domain ◽  
Von Willebrand

Abstract Abstract 2213 Von Willebrand Factor (VWF) binds to exposed sub-endothelial collagen at sites of vessel injury principally via its A3 domain, although some evidence suggests that the A1 domain can compensate for the A3 domain under flow conditions if the A3 domain is absent or non-functional. Recently, several naturally occurring Von Willebrand disease-causing mutations have been indentified in the A3 domain; S1731T, W1745C, S1783, H1786D and most recently M1761K, as well as one mutation in the A1 domain (I1343V) all of which have defective collagen binding. While the collagen binding function of these mutations has been assessed under static conditions it remains to be established if these affect collagen binding under shear stress. In the present study the collagen binding mutants were expressed in HEK293T cells and collagen binding function determined using an in vitro flow assay. All of the mutations were expressed at similar levels to wild type (wt) VWF and demonstrated normal multimeric patterns and binding to GPIbα under static conditions. As expected, collagen binding analysis under static conditions confirmed the collagen binding defect of all the mutants, with reduced or abolished binding to both collagens type I and III for all the mutants except S1731T which demonstrated normal binding to collagen type III and slightly reduced binding to collagen type I. Analysis of platelet capture under flow conditions confirmed that all the mutants were able to capture platelets similarly to wtVWF. Analysis of VWF mediated platelet capture to a collagen surface under flow conditions confirmed the phenotype of the collagen binding mutants. With the exception of S1731T, which demonstrated normal platelet capture on both collagens, none of the mutants were able to bind to collagen type I or III under flow conditions, or mediate platelet capture at high shear stress. The collagen binding function of these mutants under flow was partially restored when co-expressed with wtVWF. Interestingly, in contrast to a previous study, a VWF variant lacking the A3 domain (VWF-ΔA3) failed to bind to collagen under shear stress and was not able to mediate platelet capture to collagen. Together these data confirm that the major collagen binding site in VWF is located in the A3 domain and demonstrate that collagen binding mutations affect VWF mediated platelet capture under shear stress. Disclosures: No relevant conflicts of interest to declare.

scholarly journals Platelet adhesion to cyanogen-bromide fragments of collagen alpha 1(I) under flow conditions

Blood ◽  
1993 ◽  
Vol 82 (10) ◽  
pp. 3029-3033 ◽  
Author(s):  
EU Saelman ◽  
LF Horton ◽  
MJ Barnes ◽  
HR Gralnick ◽  
KM Hese ◽  
...  
Keyword(s):  
Cyanogen Bromide ◽  
Platelet Adhesion ◽  
Collagen Type I ◽  
Type I ◽  
Flow Conditions ◽  
Human Collagen ◽  
Collagen Fragment ◽  
Static Conditions ◽  
Von Willebrand ◽  
Willebrand Factor

Abstract The aim of this investigation was to identify domains of collagen type I that can support platelet adhesion under flow conditions. Four cyanogen bromide (CB) fragments composing 87% of the collagen alpha 1(I)-chain were studied under static and flow conditions. Under static conditions, bovine and human collagen fragment alpha 1(I)CB3 induced aggregate formation, whereas alpha 1(I)CB7 and alpha 1(I)CB8 supported adhesion of dendritic and contact platelets. Bovine alpha 1(I)CB6 weakly supported platelet adhesion. At shear rate 300/s, collagen fragment alpha 1(I)CB3 strongly supported platelet adhesion, whereas lower platelet adhesion was observed to alpha 1(I)CB7 and alpha 1(I)CB8. The fragment alpha 1(I)CB6 did not support platelet adhesion under flow conditions. Adhesion to alpha 1(I)CB3 was completely inhibited by a low concentration (0.6 IgG microgram/mL) of anti-GPIa monoclonal antibody (MoAb), whereas this concentration of antibody partially inhibited adhesion to alpha 1(I)CB7 and alpha 1(I)CB8. At higher concentrations (3 micrograms/mL) the anti-glycoprotein Ia (GPIa) antibody completely inhibited adhesion to alpha 1(I)CB8 and further reduced adhesion to alpha 1(I)CB7. Platelet adhesion to alpha 1(I)CB3, alpha 1(I)CB7, and alpha 1(I)CB8 was strongly inhibited by an anti-GPIb MoAb. A MoAb against the GPIb-binding site of von Willebrand factor (vWF) strongly inhibited platelet adhesion to alpha 1(I)CB7 and alpha 1(I)CB8, whereas platelet adhesion to alpha 1(I)CB3 was not inhibited. We conclude that under flow conditions alpha 1(I)CB3, alpha 1(I)CB7, and alpha 1(I)CB8 support GPIa/IIa-dependent platelet adhesion. The GPIb-vWF interaction is important under flow conditions for adhesion to alpha 1(I)CB7 and alpha 1(I)CB8 and probably also to alpha 1(I)CB3.

scholarly journals Platelet adhesion to cyanogen-bromide fragments of collagen alpha 1(I) under flow conditions

Blood ◽  
1993 ◽  
Vol 82 (10) ◽  
pp. 3029-3033 ◽  
Author(s):  
EU Saelman ◽  
LF Horton ◽  
MJ Barnes ◽  
HR Gralnick ◽  
KM Hese ◽  
...  
Keyword(s):  
Cyanogen Bromide ◽  
Platelet Adhesion ◽  
Collagen Type I ◽  
Type I ◽  
Flow Conditions ◽  
Human Collagen ◽  
Collagen Fragment ◽  
Static Conditions ◽  
Von Willebrand ◽  
Willebrand Factor

The aim of this investigation was to identify domains of collagen type I that can support platelet adhesion under flow conditions. Four cyanogen bromide (CB) fragments composing 87% of the collagen alpha 1(I)-chain were studied under static and flow conditions. Under static conditions, bovine and human collagen fragment alpha 1(I)CB3 induced aggregate formation, whereas alpha 1(I)CB7 and alpha 1(I)CB8 supported adhesion of dendritic and contact platelets. Bovine alpha 1(I)CB6 weakly supported platelet adhesion. At shear rate 300/s, collagen fragment alpha 1(I)CB3 strongly supported platelet adhesion, whereas lower platelet adhesion was observed to alpha 1(I)CB7 and alpha 1(I)CB8. The fragment alpha 1(I)CB6 did not support platelet adhesion under flow conditions. Adhesion to alpha 1(I)CB3 was completely inhibited by a low concentration (0.6 IgG microgram/mL) of anti-GPIa monoclonal antibody (MoAb), whereas this concentration of antibody partially inhibited adhesion to alpha 1(I)CB7 and alpha 1(I)CB8. At higher concentrations (3 micrograms/mL) the anti-glycoprotein Ia (GPIa) antibody completely inhibited adhesion to alpha 1(I)CB8 and further reduced adhesion to alpha 1(I)CB7. Platelet adhesion to alpha 1(I)CB3, alpha 1(I)CB7, and alpha 1(I)CB8 was strongly inhibited by an anti-GPIb MoAb. A MoAb against the GPIb-binding site of von Willebrand factor (vWF) strongly inhibited platelet adhesion to alpha 1(I)CB7 and alpha 1(I)CB8, whereas platelet adhesion to alpha 1(I)CB3 was not inhibited. We conclude that under flow conditions alpha 1(I)CB3, alpha 1(I)CB7, and alpha 1(I)CB8 support GPIa/IIa-dependent platelet adhesion. The GPIb-vWF interaction is important under flow conditions for adhesion to alpha 1(I)CB7 and alpha 1(I)CB8 and probably also to alpha 1(I)CB3.

The Collagen-binding Leech Products rLAPP and Calin Prevent both von Willebrand Factor and α2β1 (GPIa/IIa)-I-domain Binding to Collagen in a Different Manner

1999 ◽  
Vol 82 (09) ◽  
pp. 1160-1163 ◽  
Author(s):  
H. Deckmyn ◽  
H. Depraetere ◽  
A. Kerekes
Keyword(s):  
Binding Sites ◽  
Collagen Type ◽  
Collagen Type I ◽  
The Other ◽  
Type I ◽  
Flow Conditions ◽  
Human Collagen ◽  
Von Willebrand ◽  
Willebrand Factor ◽  
Natural Inhibitors

SummaryCalin and rLAPP are two natural inhibitors that are able to inhibit the vWF-binding and platelet adhesion to collagen both under static and flow conditions. In this study we demonstrate that both rLAPP and Calin prevent α2I-domain binding to human collagen type I with an IC50 of 5 μg/ml. However, although both vWF and α2I-domain binding to collagen is prevented by rLAPP and Calin, the latter two do not bind to the same collagen site since Calin only partially could compete with rLAPP for binding to collagen. Also vWF and the α2I-domain were unable to compete completely with each other for the binding to collagen. So the following hypothesis can be made: the binding sites of vWF and of the α2I-domain on human collagen type I are different but close to each other since rLAPP could inhibit both interactions, and thus should bind to an overlapping epitope. The Calin preparation on the other hand may still contain two active principles, one interfering with vWF-binding, the other with the α2I-domain-binding to collagen.

IDENTIFICATION OF A SECOND COLLAGEN-BINDING DOMAIN IN HUMAN VON WILLEBRAND FACTOR

1987 ◽  
Author(s):  
F I Pareti ◽  
K Nliya ◽  
P J Kostel ◽  
J M McPherson ◽  
T S Zimmerman ◽  
...  
Keyword(s):  
Collagen Type ◽  
Collagen Type I ◽  
Binding Domain ◽  
Type I ◽  
Collagen Binding ◽  
Tryptic Fragment ◽  
Binding Domains ◽  
Bovine Collagen ◽  
Von Willebrand ◽  
Willebrand Factor

We have recently reported (Journal of Biological Chemistry 261: 15310-15315, 1986) that von Willebrand factor (vWF) possesses a collagen-binding domain localized in a reduced and alkylated tryptic fragment of apparent 52/48 kDa molecular weight extending between residues Val (449) and Lys (728) of the constituent subunit. This proteolytic fragment of vWF also contains a glycoprotein lb-binding domain and a heparin-binding domain. We have now identified a second collagen-binding domain in the Staphylococcus aureus V8 protease-generated fragment I that extends from residue Gly (911) to Glu (1365). The two binding domains exhibit different interaction with collagens of different origin. The reduced and alkylated 52/48 kDa tryptic fragment was a potent inhibitor of vWF binding to equine collagen type I, but had no effect on the binding to bovine collagen type I and III. In contrast, a purified fraction containing the unreduced 52/48 kDa domain inhibited vWF binding to all types of collagen, as did anti-52/48 kDa monoclonal antibodies. Some of these antibodies, however, were more effective in inhibiting binding to equine collagen. On the other hand, fragment I markedly inhibited the binding of vWF to bovine collagen type I and III, but was less effective with equine collagen type I. Direct binding studies using 425j_qabeled fragment I demonstrated that the association constant was 5 to 10 times greater with the bovine collagens than with the equine collagen. The Staphylococcus aureus V8 protease-generated fragment III, which extends from residue Ser (1) to Glu (1365) and contains both collagen-binding domains, was the most potent inhibitor of vWF binding to all types of collagen tested. Thus, vWF has at least two collagen-binding domains. Native conformation appears to be necessary for binding of the 52/48 kDa domain to bovine collagen type I and III, but not to the equine collagen type I tested. The two domains appear to function concurrently in mediating vWF binding to collagen.

PRIMARY BINDING SITE OF VON WILLEBRAND FACTOR IN THE SUBENDOTHELIUM WHICH MEDIATES PLATELET ADHESION IS NOT COLLAGEN

1987 ◽  
Author(s):  
Philip G de Groot ◽  
Jan A van Mourik ◽  
Jan J Sixma
Keyword(s):  
Monoclonal Antibody ◽  
Von Willebrand Factor ◽  
Platelet Adhesion ◽  
Collagen Type ◽  
Collagen Type I ◽  
Extracellular Matrices ◽  
Type I ◽  
Type Iii ◽  
Von Willebrand ◽  
Willebrand Factor

We have studies the binding of von Willebrand factor (vWF) to extracellular matrices of endothelial cells and smooth muscle cells and to the vessel wall of human umbilical arteries in relation to its function in supporting platelet adhesion at high shear rates. CLB-RAg 38, a monoclonal antibody directed against vWF inhibits the binding of 125I-vWF extracellular matrices completely. The binding of 125I-vWF to subendothelium is not inhibited, because there are many different binding sites. CLB-RAg 38 inhibits platelet adhesion to extracellular matrices and subendothelium, in sofar as it is dependent on plasma vWF. CLB-RAg 38 has no effect on adhesion depending on vWF already bound to the matrix or subendothelium. CLB-RAg 38 does not inhibit binding of vWF to collagen type I and type III. Another monoclonal antibody against vWF, CLB-RAg 201, completely inhibits binding of vWF to collagen type I and type III. CLB-RAg 201 does not inhibit binding of 125I-vWF ot the extracellular matrices. CLB-RAg 201 partly inhibits platelet adhesion but this inhibition is also present when the adhesion depends on vWF already present in matrix or subendothelium, indicating that CLB-RAg 201 also inhibits the adhesion of platelets directly, this in contrast to CLB-RAg 38. The epitopes for CLB-RAg 201 and 38 were found on different tryptic fragments of vWF. These data indicate that vWF binds to subendothelium and to matrices of cultured cells by mechanism that is different from binding to collagen.

Elevated Threshold Shear Rate for the Dependence of Glycoprotein Ibα-Mediated Platelet Thrombus Formation onto Immobilized von Willebrand Factor in Mouse Blood.

Blood ◽  
2004 ◽  
Vol 104 (11) ◽  
pp. 3662-3662
Author(s):  
Patrizia Marchese ◽  
Taisuke Kanaji ◽  
Denisa D. Wagner ◽  
Jerry Ware ◽  
Zaverio M. Ruggeri
Keyword(s):  
Shear Rate ◽  
Collagen Type ◽  
Thrombus Formation ◽  
Collagen Type I ◽  
Type I ◽  
Shear Rates ◽  
Normal Platelet ◽  
Platelet Thrombus ◽  
Von Willebrand ◽  
Willebrand Factor

Abstract The interaction between platelet glycoprotein (GP) Ibα and von Willebrand Factor (VWF) is essential to initiate platelet deposition at sites of vascular injury and sustain platelet thrombus formation when the shear rate exceeds a threshold value. With human blood, the dependence of normal platelet adhesion and aggregation on VWF-GP Ibα function becomes evident at shear rates above 1,000 s−1. In the last several years, mouse models have been increasingly used to study the mechanisms of thrombus formation in circulating blood, and mice deficient in both VWF and GP Ibα have been generated. These animals offer the opportunity to evaluate whether the pathways of platelet adhesion and aggregation mediated by VWF and GP Ibα are equally important in mouse and human blood as well as to define the threshold shear rate at which the function of these pathways may become essential in the mouse circulation. To address this issue, we used an ex vivo perfusion system using fibrillar collagen type I as the thrombogenic surface and a flow chamber in which the shear rate varied according to a predictable function from the inlet to the outlet in relation to the x,y position in the flow path. Thus, wall shear rates between 5,000 at the inlet and 0 s−1 at the outlet could be evaluated in a single experiment, allowing a precise definition of the threshold at which platelet deposition on the surface could initiate. In these studies we used wild type control animals (WT), mice deficient in VWF (VWF-KO) and mice in which most of the extracellular domain of GP Ibα was replaced by a domain of the human interleukin 4 receptor (GPIb-KO/IL-4R). In the latter case, the ligand binding function of GP Ibα was obliterated, but unlike in GP Ib-KO mice platelet morphology and count were essentially normal. Blood was obtained from the retroorbital vein plexus and contained 100 u/ml heparin as an anticoagulant. Experiments were recorded in real time for the visualization of platelet-surface contacts and confocal videomicroscopy was used for the direct measurement of platelet thrombus volume. With normal mouse blood, platelet formed large thrombi throughout the tested range of shear rates. In contrast, with VWF-KO and GPIb-KO/IL-4R blood, thrombus volume was less than 5% of normal at 5,000 s−1, approximately 50% of normal at 3,000 s−1, but entirely normal at 1,500 s−1. Essentially the same results were observed when the extracellular matrix of mouse fibroblasts, which may better represent the complex thrombogenic properties of the vascular wall, was used as a reactive substrate instead of isolated collagen type I. The different threshold shear rate at which VWF and GP Ibα function are essential for thrombus formation with human and mouse platelets may be explained by the smaller size of the latter, which consequently are subjected to a lower drag at equivalent shear rate levels. Moreover, the similar behavior of VWF-KO and GPIb-KO/IL-4R platelets suggests that, under the conditions of these studies, VWF binding is the predominant GP Ibα function required for normal platelet thrombus formation at high shear rates. The present results should allow a more critical evaluation of the findings derived from mouse models of hemostasis and thrombosis.

VWF Interaction With Type IV Collagen Is Mediated Through Critical VWF A1 Domain Residues

Blood ◽  
2013 ◽  
Vol 122 (21) ◽  
pp. 29-29
Author(s):  
Veronica H. Flood ◽  
Abraham C. Schlauderaff ◽  
Paula M. Jacobi ◽  
Tricia L. Slobodianuk ◽  
Robert R. Montgomery ◽  
...  
Keyword(s):  
Binding Sites ◽  
Type Iv Collagen ◽  
Platelet Glycoprotein ◽  
Collagen Binding ◽  
Common Component ◽  
Type Iv ◽  
Static Conditions ◽  
A1 Domain ◽  
Von Willebrand

Abstract Von Willebrand factor (VWF) plays a key role in coagulation by tethering platelets to injured subendothelium via binding sites for platelet glycoprotein Ib and collagen. The binding sites for types I and III collagen in the VWF A3 domain are well characterized, and defects in this region have been implicated in von Willebrand disease (VWD). Additional collagens present in the vasculature may also be involved in interactions with VWF. A VWF A1 sequence variation, p.R1399H, has been associated with decreased binding to type VI collagen, but the clinical significance of this observation remains unclear. Type IV collagen is a common component of the basement membrane and as such may be an important ligand for VWF. While some VWD testing utilizes types I or III collagen, current clinical testing does not include collagen IV or VI. To characterize the role of the VWF A1 domain in VWF-type IV collagen interactions, we generated several A1 domain variant human and/or murine recombinant VWF (rVWF) constructs including R1399H and several type 2M VWD variants localized to the same region (S1387I, Q1402P, and an 11 amino acid deletion mutant encompassing amino acids 1392-1402). These constructs were then expressed in HEK 293T cells. To further assess the role of the A1 domain, scanning alanine mutagenesis (SAM) of residues 1387 through 1412 was conducted. VWF antigen levels (VWF:Ag), collagen binding with type III (VWF:CB3), IV (VWF:CB4), or VI (VWF:CB6) collagen were determined, and multimer distribution was assessed for all recombinant VWF variants. The role of R1399H in the context of human rVWF was characterized initially. Although VWF:Ag, VWF:CB3, and multimer distribution were normal for R1399H compared to wild-type (WT VWF), VWF:CB4 was undetectable. To examine this effect in a mouse model, the R1399H variant was expressed in the context of murine rVWF and collagen binding was determined. Similar to the human variant, murine R1399H rVWF demonstrated significantly reduced binding to murine type IV collagen, at only 7% of the binding seen with WT murine rVWF. In order to examine the behavior of R1399H under shear conditions, either WT or R1399H murine rVWF DNA was hydrodynamically injected into the tail veins of VWF -/- mice to induce expression of the proteins; blood was drawn from the vena cava 24 hours later and then examined on the VenaFlux flow apparatus. VWF expression levels and multimer distribution were similar for the R1399H- and WT-injected mice. Under static conditions, the murine plasma-derived R1399H demonstrated decreased VWF:CB4, at only 16% of the levels seen with WT VWF. No defect was seen in VWF:CB3. Furthermore, when binding to type IV collagen was assessed under flow conditions by VenaFlux, platelet adhesion was significantly decreased in mice expressing R1399H VWF as compared to mice expressing WT VWF. When examining other A1 domain variants, Q1402P and del1392-1402 demonstrated absent VWF:CB4 while S1387I demonstrated a significant reduction in VWF:CB4 compared to WT VWF. All SAM VWF A1 domain variants demonstrated normal expression, multimerization, and VWF:CB3. However, type IV collagen binding was absent for R1392A, R1395A, R1399A, and K1406A and was reduced to less than 50% of WT VWF for Q1402A, K1405A, and K1407A. These residues map to an outside face of the VWF A1 domain crystal structure, and are likely the critical residues for VWF binding to type IV collagen. Taken together, these data demonstrate that the type IV collagen binding site localizes to a specific region of the VWF A1 domain. Mutations in this region of VWF may be clinically significant due to a defect in the ability of VWF to attract platelets to exposed type IV collagen which may contribute to bleeding symptoms seen in VWD. Disclosures: No relevant conflicts of interest to declare.

scholarly journals Activation-independent platelet adhesion and aggregation under elevated shear stress

Blood ◽  
2006 ◽  
Vol 108 (6) ◽  
pp. 1903-1910 ◽  
Author(s):  
Zaverio M. Ruggeri ◽  
Jennifer N. Orje ◽  
Rolf Habermann ◽  
Augusto B. Federici ◽  
Armin J. Reininger
Keyword(s):  
Shear Stress ◽  
Platelet Aggregation ◽  
Shear Rate ◽  
Arterial Occlusion ◽  
Threshold Level ◽  
Flow Conditions ◽  
Membrane Bound ◽  
A1 Domain ◽  
Von Willebrand ◽  
Willebrand Factor

AbstractPlatelet aggregation, which contributes to bleeding arrest and also to thrombovascular disorders, is thought to initiate after signaling-induced activation. We found that this paradigm does not apply under blood flow conditions comparable to those existing in stenotic coronary arteries. Platelets interacting with immobilized von Willebrand factor (VWF) aggregate independently of activation when soluble VWF is present and the shear rate exceeds 10 000 s–1 (shear stress = 400 dyn/cm2). Above this threshold, active A1 domains become exposed in soluble VWF multimers and can bind to glycoprotein Ibα, promoting additional platelet recruitment. Aggregates thus formed are unstable until the shear rate approaches 20 000 s–1 (shear stress = 800 dyn/cm.2). Above this threshold, adherent platelets at the interface of surface-immobilized and membrane-bound VWF are stretched into elongated structures and become the core of aggregates that can persist on the surface for minutes. When isolated dimeric A1 domain is present instead of native VWF multimers, activation-independent platelet aggregation occurs without requiring shear stress above a threshold level, but aggregates never become firmly attached to the surface and progressively disaggregate as shear rate exceeds 6000 s–1. Platelet and VWF modulation by hydrodynamic force is a mechanism for activation-independent aggregation that may support thrombotic arterial occlusion.

O-linked glycosylation of von Willebrand factor modulates the interaction with platelet receptor glycoprotein Ib under static and shear stress conditions

Blood ◽  
2012 ◽  
Vol 120 (1) ◽  
pp. 214-222 ◽  
Author(s):  
Agata A. Nowak ◽  
Kevin Canis ◽  
Anne Riddell ◽  
Michael A. Laffan ◽  
Thomas A. J. McKinnon
Keyword(s):  
Shear Stress ◽  
Binding Sites ◽  
Stress Conditions ◽  
Collagen Binding ◽  
Platelet Receptor ◽  
A1 Domain ◽  
Von Willebrand ◽  
Willebrand Factor ◽  
Double Cluster

AbstractWe have examined the effect of the O-linked glycan (OLG) structures of VWF on its interaction with the platelet receptor glycoprotein Ibα. The 10 OLGs were mutated individually and as clusters (Clus) on either and both sides of the A1 domain: Clus1 (N-terminal side), Clus2 (C-terminal side), and double cluster (DC), in both full-length-VWF and in a VWF construct spanning D′ to A3 domains. Mutations did not alter VWF secretion by HEK293T cells, multimeric structure, or static collagen binding. The T1255A, Clus1, and DC variants caused increased ristocetin-mediated GPIbα binding to VWF. Platelet translocation rate on OLG mutants was increased because of reduced numbers of GPIbα binding sites but without effect on bond lifetime. In contrast, OLG mutants mediated increased platelet capture on collagen under high shear stress that was associated with increased adhesion of these variants to the collagen under flow. These findings suggest that removal of OLGs increases the flexibility of the hinge linker region between the D3 and A1 domain, facilitating VWF unfolding by shear stress, thereby enhancing its ability to bind collagen and capture platelets. These data demonstrate an important functional role of VWF OLGs under shear stress conditions.

The Development of Small Molecule Inhibitors of Collagen Binding to the Integrin α2β1 as Antithrombotic Drugs.

Blood ◽  
2005 ◽  
Vol 106 (11) ◽  
pp. 3677-3677
Author(s):  
Sungwook Choi ◽  
Seth E. Snyder ◽  
David T. Moore ◽  
Gaston Vilaire ◽  
Joel S. Bennett ◽  
...  
Keyword(s):  
Platelet Aggregation ◽  
Small Molecule ◽  
Platelet Adhesion ◽  
Collagen Type ◽  
Small Molecule Inhibitors ◽  
Collagen Type I ◽  
Mouse Tumor ◽  
Type I ◽  
Immunoglobulin Gene ◽  
Collagen Binding

Abstract Platelets tether to collagen in the subendothelial matrix that is exposed by vascular damage. Collagen is a particularly important matrix component in this context, not only because it is a substrate for platelet adhesion, but because it is an agonist for platelet aggregation and secretion as well. There are two platelet collagen receptors, the immunoglobulin gene superfamily member GPVI and the integrin α2Β1. Both are involved in adhesion to exposed collagen and generate downstream activating signals. α2Β1 is widely expressed and has been implicated in hemostasis and thrombosis, as well as cancer metastasis, wound healing, and angiogenesis. In mice, α2Β1 deficiency results in decreased ex vivo platelet aggregation, but normal bleeding times. In mouse tumor models, α2Β1 blockade reduces both metastasis and angiogenesis. Humans lacking α2Β1 have a mild bleeding diathesis. Given this background, α2Β1 appears to be an appropriate target for the development of small-molecule inhibitors to serve as relatively safe anti-platelet and anti-tumor agents, either acting alone or in synergy with other anti-platelet or anti-tumor agents. We have developed two classes of small-molecule α2Β1 inhibitors. The first class is targeted against the collagen binding site located on the α2 I-domain and was designed using molecular modeling to superimpose a dipeptide scaffold onto the published crystal structure of the I-domain bound to a collagen-mimetic peptide (GFOGER). These molecules block recombinant human I-domain binding to immobilized collagen type I with IC50s as low as 10 μM. Although the molecules inhibit platelet adhesion to collagen only at higher concentrations, they readily inhibit melanoma cell adhesion to collagen mimetics. It is also noteworthy that the molecules induce platelet protein phosphorylation and potentiate platelet aggregation induced by other platelet agonists, both of which can be prevented by pre-incubating platelets with monoclonal antibodies directed against the α2 I-domain, but not against GPVI. The second class of molecules was derived from proline-substituted 2,3-diaminopropionic acids and is directed against the Β1 I-like domain, an allosteric site that regulates ligand binding. These molecules are potent inhibitors of platelet adhesion to immobilized soluble collagen type I with IC50s of 10–50 nM and inhibit the adhesion of melanoma cells to collagen mimetics with IC50s of 250–350 nM. These molecules do not inhibit platelet aggregation, nor do they inhibit I-domain binding to immobilized collagen type I, behavior consistent with binding to the Β1 I-like domain. In a murine model of ferric-chloride induced carotid thrombosis, the molecules synergize with aspirin to prevent arterial thrombosis. In summary, we have developed two classes of small molecule inhibitors that impair the interaction of collagen with the integrin α2Β1. Although both classes of inhibitors bind to α2Β1, their effects on its function are substantially different, indicating that there are multiple potential strategies for inhibiting integrin function pharmacologically. Further development of these inhibitors may lead to agents that will be clinically useful in the treatment of thrombosis and cancer.

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