Signal-transducing mechanisms involved in activation of the platelet collagen receptor integrin alpha(2)beta(1)

Dimers of the platelet collagen receptor glycoprotein VI bind specifically to fibrin fibers during clot formation, but not to intact fibrinogen

OBJECTIVE

The platelet collagen receptor glycoprotein VI (GPVI) has an independent role as a receptor for fibrin produced via the coagulation cascade. However, various reports of GPVI binding to immobilized fibrin(ogen) are not consistent. As a collagen receptor, GPVI-dimer is the functional form, but whether GPVI dimers or monomers bind to fibrin remains controversial. To resolve this, we analyzed GPVI binding to nascent fibrin clots, which more closely approximate physiological conditions.

METHODS AND RESULTS

ELISA using biotinyl-fibrinogen immobilized on streptavidin-coated wells indicated that GPVI dimers do not bind intact fibrinogen. Clots were formed by adding thrombin to a mixture of near-plasma level of fibrinogen and recombinant GPVI ectodomain: GPVI dimer (GPVI-Fc2 or Revacept) or monomer (GPVI-His: single chain of Revacept GPVI domain, with His tag). Clot-bound proteins were analyzed by SDS-PAGE/immunoblotting. GPVI-dimer bound to noncrosslinked fibrin clots with classical one-site binding kinetics, with µM-level KD , and to crosslinked clots with higher affinity. Anti-GPVI-dimer (mFab-F) inhibited the binding. However, GPVI-His binding to either type of clot was nonsaturable and nearly linear, indicating very low affinity or nonspecific binding. In clots formed in the presence of platelets, clot-bound platelet-derived proteins were integrin αIIbβ3, present at high levels, and GPVI.

CONCLUSIONS

We conclude that dimeric GPVI is the receptor for fibrin, exhibiting a similar KD to those obtained for its binding to fibrinogen D-fragment and D-dimer, suggesting that fibrin(ogen)'s GPVI-binding site becomes exposed after fibrin formation or cleavage to fragment D. Analysis of platelets bound to fibrin clots indicates that platelet GPVI binds to fibrin fibers comprising the clot.

GPR56/ADGRG1 is a platelet collagen-responsive GPCR and hemostatic sensor of shear force

We identified the known collagen receptor GPR56/ADGRG1 on platelets. GPR56 is an adhesion G protein-coupled receptor that becomes activated following forced dissociation of its N-terminal fragment and C-terminal fragment or seven-transmembrane spanning domain (7TM). Fragment dissociation reveals the cryptic stalk of the 7TM, which acts as a tethered peptide agonist, and for GPR56, this activates platelet G13 signaling. GPR56 pharmacological probes activated platelets to undergo shape change and aggregation, which are critical for the formation of hemostatic plugs. Gpr56^−/− mice exhibit prolonged bleeding, defective platelet plug formation in vessel injury assays, and delayed thrombotic vessel occlusion. Shear-force dependency of platelet adhesion to immobilized collagen was found to be GPR56 dependent.

Circulating platelets roll along exposed collagen at vessel injury sites and respond with filipodia protrusion, shape change, and surface area expansion to facilitate platelet adhesion and plug formation. Various glycoproteins were considered to be both collagen responders and mediators of platelet adhesion, yet the signaling kinetics emanating from these receptors do not fully account for the rapid platelet cytoskeletal changes that occur in blood flow. We found the free N-terminal fragment of the adhesion G protein-coupled receptor (GPCR) GPR56 in human plasma and report that GPR56 is the platelet receptor that transduces signals from collagen and blood flow-induced shear force to activate G protein 13 signaling for platelet shape change. Gpr56^−/− mice have prolonged bleeding, defective platelet plug formation, and delayed thrombotic occlusion. Human and mouse blood perfusion studies demonstrated GPR56 and shear-force dependence of platelet adhesion to immobilized collagen. Our work places GPR56 as an initial collagen responder and shear-force transducer that is essential for platelet shape change during hemostasis.

Interplay between platelets and coagulation

Haemostasis stops bleeding at the site of vascular injury and maintains the integrity of blood vessels through clot formation. This regulated physiological process consists of complex interactions between endothelial cells, platelets, von Willebrand factor and coagulation factors. Haemostasis is initiated by a damaged vessel wall, followed with a rapid adhesion, activation and aggregation of platelets to the exposed subendothelial extracellular matrix. At the same time, coagulation factors aggregate on the procoagulant surface of activated platelets to consolidate the platelet plug by forming a mesh of cross-linked fibrin. Platelets and coagulation mutually influence each other and there are strong indications that, thanks to the interplay between platelets and coagulation, haemostasis is far more effective than the two processes separately. Clinically this is relevant because impaired interaction between platelets and coagulation may result in bleeding complications, while excessive platelet-coagulation interaction induces a high thrombotic risk. In this review, platelets, coagulation factors and the complex interaction between them will be discussed in detail.

Disulfide Bonds as Regulators of Integrin Function in Thrombosis and Hemostasis

SIGNIFICANCE

Disulfide bonds are generally viewed as structure-stabilizing elements in proteins, but some display an alternative functional role as redox switches. Functional disulfide bonds have recently emerged as important regulators of integrin function in thrombosis and hemostasis.

RECENT ADVANCES

Functional disulfide bonds were identified in the β subunit of the major platelet integrin αIIbβ3 and in other integrins involved in thrombus formation that is, αvβ3 and α2β1. Most of these functional bonds are located in the four epidermal growth factor-like domains of the integrins. Redox agents such as glutathione and nitric oxide and enzymatic thiol isomerase activity were shown to regulate the function of these integrins by disulfide bond reduction and thiol/disulfide exchange.

CRITICAL ISSUES

Increasing evidence suggests that thiol isomerases such as protein disulfide isomerase (PDI) and Erp57 directly bind to the β3 subunit of αIIbβ3 and αvβ3 and regulate their function during thrombus formation. αIIbβ3 also exhibits an endogenous thiol isomerase activity. The specific functional disulfide bonds identified in the β3 subunit might be the targets for both exogenous and endogenous thiol isomerase activity.

FUTURE DIRECTIONS

Targeting redox sites of integrins or redox agents and enzymes that regulate their function can provide a useful tool for development of anti-thrombotic therapy. Hence, inhibitors of PDI are currently studied for this purpose.

[Activators, receptors and signal transduction pathways of blood platelets]

Platelet participation in hemostatic plug formation requires transition into an activated state (or, rather, variety of states) upon action of agonists like ADP, thromboxane A , collagen, thrombin, and others. The mechanisms of action for different agonists, their receptors and signaling pathways associated with them, as well as the mechanisms of platelet response inhibition are the subject of the present review. Collagen exposed upon vessel wall damage induced initial platelet attachment and start of thrombus formation, which involves numerous processes such as aggregation, activation of integrins, granule secretion and increase of intracellular Ca2+. Thrombin, ADP, thromboxane A , and ATP activated platelets that were not initially in contact with the wall and induce additional secretion of activating substances. Vascular endothelium and secretory organs also affect platelet activation, producing both positive (adrenaline) an d negative (prostacyclin, nitric oxide) regulators, thereby determining the relation of activation and inhibition signals, which plays a significant role in the formation of platelet aggregate under normal and pathological conditions. The pathways of platelet signaling are still incompletely understood, and their exploration presents an important objective both for basic cell biology and for the development of new drugs, the methods of diagnostics and of treatment of hemostasis disorders.

Platelets from mice lacking the aryl hydrocarbon receptor exhibit defective collagen-dependent signaling

BACKGROUND

We previously identified aryl hydrocarbon receptor (AHR) as a novel regulator of megakaryocytic differentiation and polyploidization and reported that AHR-null mice have approximately 15% fewer platelets than do wild-type mice, yet they exhibit a dramatic, unexplained bleeding phenotype.

OBJECTIVES

The current work tests our hypothesis that AHR-null platelets are functionally deficient, contributing to the previously reported (yet unexplained) bleeding phenotype present in AHR-null mice.

METHODS

AHR-null bone marrow was ex vivo differentiated with thrombopoietin with or without AHR ligands or AHR inhibitors and analyzed for degree of megakaryopoiesis and polyploidization. Platelet function of AHR-null mice was assessed with aggregation and spreading assays. Platelet signaling was examined using Western analysis and Rac activity assays.

RESULTS

AHR ligands differentiate murine bone marrow-derived progenitors into polyploid megakaryocytes in the absence of thrombopoietin, and AHR inhibitors block thrombopoietin-induced megakaryocytic differentiation. Despite their responsiveness toward thrombin, AHR-null platelets demonstrate decreased aggregation and spreading in response to collagen compared with wild-type platelets. AHR-null platelets bind fibrinogen after stimulation with thrombin or AYPGKF and aggregate in response to AYPGKF and adenosine diphosphate. Mechanistically, AHR absence led to down-regulation of Vav1 and Vav3, altered phospholipase Cγ2 phosphorylation, decreased Rac1 activation, and reduced platelet activation in response to collagen.

CONCLUSIONS

These results are consistent with a role for AHR in platelet function, especially as it relates to platelet aggregation and spreading in response to collagen. Our work suggests AHR is a critical component of the physiologic response that platelets undergo in response to collagen and may provide novel treatment options for patients with bleeding disorders.

Platelet flow cytometry

Flow cytometry is a powerful and versatile tool which can be used to provide substantial phenotypic data on platelets by yielding quantitative information of their physical and antigenic properties. This includes surface expression of functional receptors, bound ligands, expression of granule components, interaction of platelets with other platelets via aggregation, or interaction with other blood components, such as leukocytes or the plasma coagulation system. Quantitative assessment of these parameters may facilitate the diagnosis of inherited or acquired platelet disorders, assist in the diagnosis of diseases associated with platelet activation, or assist in the monitoring of safety and efficacy of antiplatelet therapy.

Platelet function in health and disease: from molecular mechanisms, redox considerations to novel therapeutic opportunities

Increased oxidative stress appears to be of fundamental importance in the pathogenesis and development of several disease processes. Indeed, it is well known that reactive oxygen species (ROS) exert critical regulatory functions within the vascular wall, and it is, therefore, plausible that platelets represent a relevant target for their action. Platelet activation cascade (including receptor-mediated tethering to the endothelium, rolling, firm adhesion, aggregation, and thrombus formation) is tightly regulated. In addition to already well-defined platelet regulatory factors, ROS may participate in the regulation of platelet activation. It is already established that enhanced ROS release from the vascular wall can indirectly affect platelet activity by scavenging nitric oxide (NO), thereby decreasing the antiplatelet properties of endothelium. On the other hand, recent data suggest that platelets themselves generate ROS, which may evoke pro-thrombotic responses, triggering many biological processes participating in atherosclerosis initiation, progression, and complication. That oxidative stress may alter platelet function is conceivable when considering that antioxidants play a role in the prevention of cardiovascular disease, although the precise mechanism accounting for changes attributable to antioxidants in atherosclerosis remains unknown. It is possible that the effects of antioxidants may be a consequence of their enhancing or promoting the antiplatelet effects of NO derived from both endothelial cells and platelets. This review focuses on current knowledge regarding ROS-dependent regulation of platelet function in health and disease, and summarizes in vitro and in vivo evidence for their physiological and potential therapeutic relevance.

Novel α2β1 integrin inhibitors reveal that integrin binding to collagen under shear stress conditions does not require receptor preactivation

The interaction between α2β1 integrin (GPIa/IIa, VLA-2) and vascular collagen is one of the initiating events in thrombus formation. Here, we describe two structurally similar sulfonamide derivatives, BTT-3033 and BTT-3034, and show that, under static conditions, they have an almost identical effect on α2-expressing CHO cell adhesion to collagen I, but only BTT-3033 blocks platelet attachment under flow (90 dynes/cm(2)). Differential scanning fluorimetry showed that both molecules bind to the α2I domain of the recombinant α2 subunit. To further study integrin binding mechanism(s) of the two sulfonamides, we created an α2 Y285F mutant containing a substitution near the metal ion-dependent adhesion site motif in the α2I domain. The action of BTT-3033, unlike that of BTT-3034, was dependent on Tyr-285. In static conditions BTT-3034, but not BTT-3033, inhibited collagen binding by an α2 variant carrying a conformationally activating E318W mutation. Conversely, in under flow conditions (90 dynes/cm(2)) BTT-3033, but not BTT-3034, inhibited collagen binding by an α2 variant expressing E336A loss-of-function mutation. Thus, the binding sites for BTT-3033 and BTT-3034 are differentially available in distinct integrin conformations. Therefore, these sulfonamides can be used to study the biological role of different functional stages of α2β1. Furthermore, only the inhibitor that recognized the non-activated conformation of α2β1 integrin under shear stress conditions effectively blocked platelet adhesion, suggesting that the initial interaction between integrin and collagen takes place prior to receptor activation.

The primacy of β1 integrin activation in the metastatic cascade

After neoplastic cells leave the primary tumor and circulate, they may extravasate from the vasculature and colonize tissues to form metastases. β1 integrins play diverse roles in tumorigenesis and tumor progression, including extravasation. In blood cells, activation of β1 integrins can be regulated by “inside-out” signals leading to extravasation from the circulation into tissues. However, a role for inside-out β1 activation in tumor cell metastasis is uncertain. Here we show that β1 integrin activation promotes tumor metastasis and that activated β1 integrin may serve as a biomarker of metastatic human melanoma. To determine whether β1 integrin activation can influence tumor cell metastasis, the β1 integrin subunit in melanoma and breast cancer cell lines was stably knocked down with shRNA and replaced with wild-type or constitutively-active β1. When tumor cells expressing constitutively-active β1 integrins were injected intravenously into chick embryos or mice, they demonstrated increased colonization of the liver when compared to cells expressing wild-type β1 integrins. Rescue expression with mutant β1 integrins revealed that tumor cell extravasation and hepatic colonization required extracellular ligand binding to β1 as well as β1 interaction with talin, an intracellular mediator of integrin activation by the Rap1 GTPase. Furthermore, shRNA-mediated knock down of talin reduced hepatic colonization by tumor cells expressing wild-type β1, but not constitutively-active β1. Overexpression in tumor cells of the tumor suppressor, Rap1GAP, inhibited Rap1 and β1 integrin activation as well as hepatic colonization. Using an antibody that detects activated β1 integrin, we found higher levels of activated β1 integrins in human metastatic melanomas compared to primary melanomas, suggesting that activated β1 integrin may serve as a biomarker of invasive tumor cells. Altogether, these studies establish that inside-out activation of β1 integrins promotes tumor cell extravasation and colonization, suggesting diagnostic and therapeutic approaches for targeting of β1 integrin signaling in neoplasia.

Inhibitors of the interactions between collagen and its receptors on platelets

At sites of vascular injury, collagen-mediated platelet adhesion and activation have long been known as one of the first events in platelet-dependent thrombus formation. Studying patients with bleeding disorders that are caused by defective platelet adhesion to collagen resulted in the identification of several platelet collagen receptors, with glycoprotein VI and integrin α2β1 being the most important ones. Subsequent development of specific collagen receptor knockout mice and various inhibitors of platelet binding to collagen have further proven the role of these receptors in haemostasis and thrombosis. The search for clinically applicable inhibitors for use as antithrombotic drug has led to the identification of inhibitory antibodies, soluble receptor fragments, peptides, collagen-mimetics and proteins from snake venoms or haematophagous animals. In experimental settings, these inhibitors have a good antithrombotic effect, with little prolongation of bleeding times, suggesting a larger therapeutic window than currently available antiplatelet drugs. However, at present, none of the collagen receptor blockers are in clinical development yet.

Blood platelet biochemistry

Defects in platelet function or formation increase the risk for bleeding or thrombosis, which indicates the crucial role for platelets in maintaining haemostasis in normal life. Upon vascular injury, platelets instantly adhere to the exposed extracellular matrix which results in platelet activation and aggregation and the formation a haemostatic plug that stops bleeding. To prevent excessive platelet aggregate formation that eventually would occlude the vessels, this self-amplifying process nevertheless requires a tight control. This review intends to give a comprehensive overview of the currently established main mechanisms in platelet function.

Thrombin-dependent modulation of β1-integrin-mediated signaling up-regulates prolidase and HIF-1α through p-FAK in colorectal cancer cells

Products of prolidase [E.C. 3.4.13.9] activity, proline or hydroxyproline, contribute to up-regulation of hypoxia-inducible factor-1α (HIF-1α). Prolidase activity is regulated by β(1)-integrin signaling. We studied the effects of echistatin (a well-known disintegrin) and thrombin (a serine protease capable of activation of integrin α(2)β(1) receptor) on prolidase activity and expressions of prolidase, α(2)β(1)-integrin receptor, focal adhesion kinase (FAK), MAP-kinases (ERK(1) and ERK(2)), and nuclear HIF-1α in human colon adenocarcinoma (DLD-1) cells. It has been found that treatment of the cells with thrombin contributes to decrease in the expression of prolidase and simultaneously increase in its phosphorylation, resulting in maintenance of the enzyme activity. The phenomenon was accompanied by thrombin-dependent recovery of depressed autophosphorylation of FAK (pY(397)) under the effect of FAK inhibitor (1,2,4,5-benzenetetramine tetrahydrochloride). Although integrin α(2)β(1) receptor expression was not affected by thrombin, the signaling induced by thrombin up-regulated nuclear HIF-1α expression. It was accompanied by increase in the expression of MAP kinases, ERK1 and ERK2. It suggests that integrin-dependent signaling through p-FAK is up-regulated in DLD-1 cells and it may represent potential target for anti-cancer therapy.

Platelets at work in primary hemostasis

When platelet numbers are low or when their function is disabled, the risk of bleeding is high, which on the one hand indicates that in normal life vascular damage is a rather common event and that hence the role of platelets in maintaining a normal hemostasis is a continuously ongoing physiological process. Upon vascular injury, platelets instantly adhere to the exposed extracellular matrix resulting in platelet activation and aggregation to form a hemostatic plug. This self-amplifying mechanism nevertheless requires a tight control to prevent uncontrolled platelet aggregate formation that eventually would occlude the vessel. Therefore endothelial cells produce inhibitory compounds such as prostacyclin and nitric oxide that limit the growth of the platelet thrombus to the damaged area. With this review, we intend to give an integrated survey of the platelet response to vascular injury in normal hemostasis.

Functional Effect of Platelet Membrane Glycoprotein Ia Gene Polymorphism in the Pathogenesis of Unstable Angina Pectoris

The functional effect of platelet membrane glycoprotein Ia ( GPIa) gene 807C/T polymorphism in unstable angina pectoris (UAP) pathogenesis was investigated in Chinese Han individuals. Collagen type I-induced platelet aggregation was measured in 33 healthy subjects. Plasma levels of α-granule membrane protein (GMP-140) were measured in 33 healthy subjects and in 35 patients with recentonset angina at rest within 24 h of hospitalization. Platelet membrane GPIa gene 807C/T polymorphism was determined in all subjects. Lag-time before 30% platelet aggregation was significantly longer in CC genotype than in TC genotype healthy subjects, although there was no significant difference in maximal platelet aggregation between healthy subjects with either genotype. Plasma GMP-140 levels were significantly higher in TC genotype patients compared with CC genotype patients or healthy subjects; a significant difference was also observed between the latter two groups. It was concluded that rapid initiation of collagen-induced platelet aggregation may be associated with the platelet membrane GPIa T807 allele, which may be important in UAP pathogenesis.

C1q induces a rapid up-regulation of P-selectin and modulates collagen- and collagen-related peptide-triggered activation in human platelets

Blood platelets are emerging as important immunomodulatory cells, but complement interaction with platelets is not well understood. Several platelet structures have been described as complement protein 1q (C1q) binding receptors, such as C1qRp/CD93 and gC1qR. However, there are conflicting results whether these receptors are C1q binding structures, or even at all expressed on the cell surface. Recently, the collagen-binding integrin αIIβI was reported to bind C1q on mast cells, and this receptor is also present on platelets. The aim of this study was to further characterize the effects of C1q on platelets, by quantifying the platelet surface expression of P-selectin (CD62P) and monitoring the formation of platelet-neutrophil aggregates. Using flow cytometry, we found that C1q dose-dependently triggered a rapid but moderate and transient up-regulation of P-selectin already within 5s of C1q exposure. Pre-incubation with an antibody directed against gC1qR significantly inhibited (with 57% compared to control) the up-regulation, whereas an antibody towards the αIIβI-integrin showed no effect. Stimulation with C1q did not change the cytosolic calcium-levels, as measured with the fluorescent ratiometric probe Fura-2, however, a protein kinase C inhibitor (GF109203x) blocked the C1q-induced P-selectin expression. Furthermore, pre-incubation of platelets with C1q diminished both the collagen as well as the collagen-related peptide-induced up-regulation of P-selectin, most evident after 90s of stimulation. This indicates that C1q may regulate platelet activation via the GPVI receptor, which is a novel finding. Moreover, C1q antagonized the collagen-induced formation of platelet-neutrophil aggregates, indicating a reduced interaction between platelet P-selectin and neutrophil P-selectin glycoprotein ligand-1(PSGL-1/CD162). In summary, C1q induces a moderate rapid platelet P-selectin expression, modulates subsequent collagen and collagen-related peptide stimulation of platelets, and inhibits the formation of platelet-neutrophil aggregates. These immuno-regulatory effects of C1q may have a crucial role in innate immunity and inflammation.

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.

Suboptimal activation of protease-activated receptors enhances alpha2beta1 integrin-mediated platelet adhesion to collagen

Thrombin and fibrillar collagen are potent activators of platelets at sites of vascular injury. Both agonists cause platelet shape change, granule secretion, and aggregation to form the primary hemostatic plug. Human platelets express two thrombin receptors, protease-activated receptors 1 and 4 (PAR1 and PAR4) and two collagen receptors, the alpha(2)beta(1) integrin (alpha(2)beta(1)) and the glycoprotein VI (GPVI)/FcRgamma chain complex. Although these receptors and their signaling mechanisms have been intensely studied, it is not known whether and how these receptors cooperate in the hemostatic function of platelets. This study examined cooperation between the thrombin and collagen receptors in platelet adhesion by utilizing a collagen-related peptide (alpha2-CRP) containing the alpha(2)beta(1)-specific binding motif, GFOGER, in conjunction with PAR-activating peptides. We demonstrate that platelet adhesion to alpha2-CRP is substantially enhanced by suboptimal PAR activation (agonist concentrations that do not stimulate platelet aggregation) using the PAR4 agonist peptide and thrombin. The enhanced adhesion induced by suboptimal PAR4 activation was alpha(2)beta(1)-dependent and GPVI/FcRgamma-independent as revealed in experiments with alpha(2)beta(1)- or FcRgamma-deficient mouse platelets. We further show that suboptimal activation of other platelet G(q)-linked G protein-coupled receptors (GPCRs) produces enhanced platelet adhesion to alpha2-CRP. The enhanced alpha(2)beta(1)-mediated platelet adhesion is controlled by phospholipase C (PLC), but is not dependent on granule secretion, activation of alpha(IIb)beta(3) integrin, or on phosphoinositol-3 kinase (PI3K) activity. In conclusion, we demonstrate a platelet priming mechanism initiated by suboptimal activation of PAR4 or other platelet G(q)-linked GPCRs through a PLC-dependent signaling cascade that promotes enhanced alpha(2)beta(1) binding to collagens containing GFOGER sites.

Integrin alpha2beta1 induces phosphorylation-dependent and phosphorylation-independent activation of phospholipase Cgamma2 in platelets: role of Src kinase and Rac GTPase

BACKGROUND

Platelet adhesion promoted by integrin alpha2beta1 induces integrin alpha(IIb)beta3 activation through the phospholipase C (PLC)-dependent stimulation of the small GTPase Rap1b.

OBJECTIVE

To analyze the mechanism of PLC activation downstream of alpha2beta1 that is required for regulation of Rap1b and alpha(IIb)beta3.

METHODS

Human and murine platelets were allowed to adhere to immobilized type I monomeric collagen through alpha2beta1. Tyrosine phosphorylation of PLCgamma2, PLC activation, accumulation of GTP-bound Rap1b and fibrinogen binding were measured and compared.

RESULTS

Integrin alpha2beta1 recruitment induced an evident PLC activation that was concomitant with robust tyrosine phosphorylation of PLCgamma2, and was suppressed in platelets from PLCgamma2-knockout mice. Moreover, PLCgamma2(-/-) platelets were unable to accumulate active Rap1b and to activate alpha(IIb)beta3 upon adhesion through alpha2beta1. Inhibition of Src kinases completely prevented tyrosine phosphorylation of PLCgamma2 in adherent platelets, but did not affect its activation, and both Rap1b and alpha(IIb)beta3 stimulation occurred normally. Importantly, alpha(IIb)beta3-induced phosphorylation and activation of PLCgamma2, as well as accumulation of active Rap1b, were totally suppressed by Src inhibition. Integrin alpha2beta1 recruitment triggered the Src kinase-independent activation of the small GTPase Rac1, and activation of Rac1 was not required for PLCgamma2 phosphorylation. However, when phosphorylation of PLCgamma2 was blocked by the Src kinase inhibitor PP2, prevention of Rac1 activation significantly reduced PLCgamma2 activation, GTP-Rap1b accumulation, and alpha(IIb)beta3 stimulation.

CONCLUSIONS

Src kinases and the Rac GTPases mediate independent pathways for PLCgamma2 activation downstream of alpha2beta1.

Negative regulation of activated α2 integrins during thrombopoiesis

Circulating platelets exhibit rapid signaling and adhesive responses to collagen that facilitate hemostasis at sites of vessel injury. Because platelets are anuclear, their collagen receptors must be expressed by megakaryocytes, platelet precursors that arise in the collagen-rich environment of the bone marrow. Whether and how megakaryocytes regulate collagen adhesion during their development in the bone marrow are unknown. We find that surface expression of activated, but not wild-type, α2 integrins in hematopoietic cells in vivo results in the generation of platelets that lack surface α2 receptors. Culture of hematopoietic progenitor cells ex vivo reveals that surface levels of activated, but not wild-type, α2 integrin receptors are rapidly down-regulated during cell growth on collagen but reach wild-type levels when cells are grown in the absence of collagen. Progenitor cells that express activated α2 integrins are normally distributed in the bone marrow in vivo and exhibit normal migration across a collagen-coated membrane ex vivo. This migration is accompanied by rapid down-regulation of activated surface integrins. These studies identify ligand-dependent removal of activated α2 receptors from the cell surface as a mechanism by which integrin function can be negatively regulated in hematopoietic cells during migration between the adhesive environment of the bone marrow and the nonadhesive environment of the circulating blood.

Small-molecule inhibitors of integrin alpha2beta1 that prevent pathological thrombus formation via an allosteric mechanism

There is a grave need for safer antiplatelet therapeutics to prevent heart attack and stroke. Agents targeting the interaction of platelets with the diseased vessel wall could impact vascular disease with minimal effects on normal hemostasis. We targeted integrin alpha(2)beta(1), a collagen receptor, because its overexpression is associated with pathological clot formation whereas its absence does not cause severe bleeding. Structure-activity studies led to highly potent and selective small-molecule inhibitors. Responses of integrin alpha(2)beta(1) mutants to these compounds are consistent with a computational model of their mode of inhibition and shed light on the activation mechanism of I-domain-containing integrins. A potent compound was proven efficacious in an animal model of arterial thrombosis, which demonstrates in vivo efficacy for inhibition of this platelet receptor. These results suggest that targeting integrin alpha(2)beta(1) could be a potentially safe, effective approach to long-term therapy for cardiovascular disease.

Nitric oxide specifically inhibits integrin-mediated platelet adhesion and spreading on collagen

BACKGROUND

Nitric oxide (NO) inhibits platelet adhesion to collagen, although the precise molecular mechanisms underlying this process are unclear.

OBJECTIVES

Collagen-mediated adhesion is a multifaceted event requiring multiple receptors and platelet-derived soluble agonists. We investigated the influence of NO on these processes.

RESULTS

S-nitrosoglutathione (GSNO) induced a concentration-dependent inhibition of platelet adhesion to immobilized collagen. Maximal adhesion to collagen required platelet-derived ADP and TxA(2). GSNO-mediated inhibition was lost in the presence of apyrase and indomethacin, suggesting that NO reduced the availability of, or signaling by, ADP and TxA(2). Exogenous ADP, but not the TxA(2) analogue U46619, reversed the inhibitory actions of GSNO on adhesion. Under adhesive conditions NO inhibited dense granule secretion but did not influence TxA(2) generation. These data indicated that NO may block signaling by TxA(2) required for dense granule secretion, thereby reducing the availability of ADP. Indeed, we found TxA(2)-mediated activation of PKC was required to drive dense granule secretion, a pathway that was inhibited by NO. Because our data demonstrated that NO only inhibited the activation-dependent component of adhesion, we investigated the effects of NO on individual collagen receptors. GSNO inhibited platelet adhesion and spreading on alpha(2)beta(1) specific peptide ligand GFOGER. In contrast, GSNO did not inhibit GPVI-mediated adhesion to collagen, or adhesion to the GPVI specific ligand, collagen related peptide (CRP).

CONCLUSIONS

NO targets activation-dependent adhesion mediated by alpha(2)beta(1), possibly by reducing bioavailability of platelet-derived ADP, but has no effect on activation-independent adhesion mediated by GPVI. Thus, NO regulates platelet spreading and stable adhesion to collagen.

Evaluation of the physiological significance of botrocetin/ von Willebrand factor in vitro signaling

BACKGROUND

A signaling pathway is difficult, if not impossible, to elucidate in platelets using only in vivo studies. Likewise, the physiological significance of signaling information obtained exclusively from in vitro observations is unknown. Therefore, both in vitro and in vivo experiments are required to establish the physiological significance of a signaling pathway.

OBJECTIVE

To evaluate the physiological significance of signaling data obtained from botrocetin (bt)/von Willebrand factor (VWF)-stimulated washed platelets.

METHOD

Stable thrombus formation in response to FeCl(3)-induced injury of the mouse carotid artery was used to evaluate the physiological significance of signaling data obtained from bt/VWF-stimulated washed platelets.

RESULTS

Syk, PLCgamma2, Galphaq and P2Y12, but not LAT, were found either to be required for or to affect stable thrombus formation. Prior in vitro studies had demonstrated that LAT is not required for bt/VWF-induced platelet aggregation in the presence of exogenous fibrinogen. These data provide the first demonstration of the in vivo role for these signaling molecules in GPIb-dependent/initiated signal transduction and are consistent with the signaling pathway deduced from in vitro studies of bt/VWF-stimulated washed platelets using metabolic inhibitors and knockout mice.

CONCLUSION

The broad agreement between the in vitro and the in vivo results establish that bt/VWF stimulation of washed platelets can provide physiologically significant glycoprotein Ib-dependent/initiated signaling data.

Dynamic Tyrosine Kinase-Regulated Signaling and Actin Polymerisation Mediate Aggregate Stability Under Shear

Objective— Aggregate formation on collagen at arteriolar rates of shear is mediated by coordinated signaling between tyrosine kinase–linked and G protein–coupled receptors. We have investigated the role of these receptors and the actin cytoskeleton in maintaining aggregate stability under shear.

Methods and Results— Platelet aggregates are rapidly formed when blood is flowed over collagen at 1000 s −1 and remain stable over 20 minutes. A novel fibrin-independent mechanism of retraction against the direction of flow occurs at the aggregate front and recruits platelets into the main aggregate. Stable aggregates are not observed in the presence of cytochalasin D, which blocks de novo actin polymerization. When exposed to the Src family kinase inhibitor, PD0173952, preformed aggregates spread in the direction of flow and rounded platelets appear within the aggregate body and are lost in the direction of flow. A similar set of observations is observed in the presence of latrunculin A, which disrupts preexisting actin filaments, but not in the combined presence of inhibitors of ADP and thromboxane A 2 formation.

Conclusions— Maintenance of stable aggregates at high shear is a dynamic process mediated by Src kinases and actin polymerization. These signals maintain aggregates in a compact structure and prevent continuous streaming of platelets.

Future innovations in anti-platelet therapies

Platelets have long been recognized to be of central importance in haemostasis, but their participation in pathological conditions such as thrombosis, atherosclerosis and inflammation is now also well established. The platelet has therefore become a key target in therapies to combat cardiovascular disease. Anti-platelet therapies are used widely, but current approaches lack efficacy in a proportion of patients, and are associated with side effects including problem bleeding. In the last decade, substantial progress has been made in understanding the regulation of platelet function, including the characterization of new ligands, platelet-specific receptors and cell signalling pathways. It is anticipated this progress will impact positively on the future innovations towards more effective and safer anti-platelet agents. In this review, the mechanisms of platelet regulation and current anti-platelet therapies are introduced, and strong, and some more speculative, potential candidate target molecules for future anti-platelet drug development are discussed.

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.

Platelet integrins and immunoreceptors

Stable platelet adhesion to extracellular matrices and the formation of a hemostatic or pathological thrombus are dependent on integrin alphaIIbbeta3, also known as GPIIb-IIIa. However, maximal platelet responses to vascular injury may involve the participation of other integrins expressed in platelets (alphaVbeta3, alpha2beta1, alpha5beta1, and alpha6beta1). Platelet membrane 'immunoreceptors' contain at least one subunit with an extracellular immunoglobulin superfamily domain and/or an intracellular stimulatory immunoreceptor tyrosine-based activation motif (ITAM) or immunoreceptor tyrosine-based inhibitory motif (ITIM). Platelet ITAM receptors, such as FcgammaRIIA and the GPVI-FcRgamma complex, promote activation of integrins, while ITIM receptors, such as platelet-endothelial cell adhesion molecule-1, may promote their inhibition. This review summarizes the structure and function of platelet integrins and immunoreceptors, the emerging functional relationships between these receptor classes, and the consequences of their interaction for platelet function in hemostasis and thrombosis.

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.

Role of glycoprotein Ia gene polymorphisms in determining platelet function in myocardial infarction patients undergoing percutaneous coronary intervention on dual antiplatelet treatment

Response variability to antiplatelet treatment has been described and the widespread use of acetylsalicylic acid (ASA) and clopidogrel requires clarification of the residual platelet reactivity (RPR). Various glycoprotein Ia (GpIa) polymorphisms have been investigated, but their influence on platelet reactivity in myocardial infarction (MI) patients undergoing percutaneous coronary intervention (PCI) on dual antiplatelet treatment is not still elucidated. Aim of this study was to evaluate the effect of C807T, G873A and T837C polymorphisms of GpIa on modulating platelet function in MI patients on dual antiplatelet treatment undergoing PCI. We measured platelet function by both a point-of-care assay (PFA100) and platelet-rich-plasma aggregation in 289 MI patients undergoing PCI and receiving dual antiplatelet treatment. Our data show that C807T/G873A polymorphisms, but not T837C, are associated with higher platelet reactivity. Carriers of the 807T/873A allele had significantly higher platelet aggregation values after arachidonic acid (AA) and collagen stimuli and, even if they did not reach the statistical significance, after 2 and 10 microM ADP stimuli; 807T/873A allele carriers had also significantly shorter closure times on PFA100/epinephrine membranes. At the multiple analyses, C807T/G873A polymorphisms resulted an independent risk factor for RPR defined by both AA induced platelet aggregation (OR=3.0, 95%CI 1.17-7.89, p=0.022) or by PFA100/epinephrine (OR=4.1, 95%CI 1.53-10.89, p=0.005). In conclusion, this study shows the 807T/873A allele of the GpIa gene is an independent risk factor for the RPR on dual antiplatelet treatment, and extends, in a larger acute coronary syndrome population, the observation that the 807T/873A allele is associated with higher platelet reactivity.

Novel Platelet and Vascular Roles for Immunoreceptor Signaling

The immunoreceptor signaling pathway has classically been defined by its role in mediating intracellular signals downstream of immune receptors on circulating cells, but recent studies have revealed new and unexpected roles for this pathway in vascular biology. In platelets the immunoreceptor signaling pathway is coupled to 2 structurally distinct platelet collagen receptors, glycoprotein VI and integrin α2β1, and is required for the activation of platelets after exposure to vessel wall collagen during plaque rupture. During vascular development immunoreceptor signaling is required for proper formation of the lymphatic system, a role that has revealed the contribution of hematopoietic endothelial progenitors to that process. In conjunction with the identification of new biological roles in vascular cell types, new molecular mechanisms of activating this signaling pathway have been discovered, including activation by integrins and immunoreceptor tyrosine activation motifs (ITAMs) on receptors that do not function as part of the immune response. Here we discuss some of these recent findings and their implications for vascular biology and the treatment of human vascular diseases.

Characterization of a patient with glycoprotein (GP) VI deficiency possessing neither anti-GPVI autoantibody nor genetic aberration

BACKGROUND

There have been only seven reported cases of glycoprotein (GP) VI deficiency. However, the pathogenesis of this disorder has not been well-elucidated.

OBJECTIVES

We characterized a novel patient with GPVI deficiency and used these platelets to investigate the role of GPVI in normal hemostasis.

PATIENT

A 31-year-old female with immune thrombocytopenic purpura who had been suffering from mild bleeding diathesis even after recovery from thrombocytopenia.

RESULTS AND CONCLUSION

The patient's platelets did not aggregate in response to either convulxin or collagen-related peptide. Immunoblotting revealed complete absence of the GPVI molecule, whereas a significantly reduced but substantial amount of Fc receptor (FcR) gamma-chain was expressed. Platelet stimulation with convulxin did not induce tyrosine-phosphorylation of FcR gamma-chain, indicating a defect in GPVI-mediated signaling. Concerning the underlying pathogenesis, we found normal level of GPVI-mRNA expression, no aberration of the sequence of the entire coding region of GPVI, and presence of degraded GPVI in her plasma. However, no anti-GPVI autoantibody was detected either by the binding assay to GPVI-Fc2 fusion protein or by immunoblotting/immunoprecipitation using the patient's immunoglobulin. We thus consider that either a short-time exposure to anti-GPVI autoantibody or a continuous exposure to low titers of the autoantibody has resulted in persistent GPVI deficiency. Under high shear flow, the patient's platelets could not form large aggregates, although initial platelet attachment was obviously observed. These results suggest that GPVI deficiency in this patient resulted in defective platelet thrombi development, manifesting as bleeding diathesis. Furthermore, our observations indicate that coordination of GPVI with integrin alpha2beta1 is essential for physiological platelet thrombus formation.

Activation of αIIbβ3 is a sufficient but also an imperative prerequisite for activation of α2β1 on platelets

Platelet integrins α2β1 and αIIbβ3 play critical roles in platelet adhesion and thrombus formation after vascular injury. On resting platelets, both integrins are in a low-affinity state. However, agonist stimulation results in conformational changes that enable ligand binding that can be detected with conformation dependent monoclonal antibodies (mAbs). By using such conformation-dependent mAbs, we could demonstrate that activation of integrin αIIbβ3 is not only sufficient, but also a prerequisite for α2β1 activation. Compared with platelets in plasma, stimulation of washed platelets resulted in only a minor activation of α2β1, as detected with the activation-sensitive mAb IAC-1. Addition of fibrinogen to stimulated washed platelets greatly potentiated activation of this integrin. Also, treatment of αIIbβ3 with the ligand-mimetic peptide RGDS, resulting in outside-in signaling, led to a powerful α2β1 activation, even in the absence of overall platelet activation, involving tyrosine kinase activity but no protein kinase C activation. The absolute necessity of αIIbβ3 for proper α2β1 activation on platelets was demonstrated by using the αIIbβ3 antagonist aggrastat, which was able to completely abolish α2β1 activation, both under static and flow conditions. In addition, analogous experiments with Glanzmann platelets lacking αIIbβ3 confirmed the indispensability of αIIbβ3 for α2β1 activation.

In vivo beta1 integrin function requires phosphorylation-independent regulation by cytoplasmic tyrosines

Integrins are heterodimeric adhesion receptors associated with bidirectional signaling. In vitro studies support a role for the binding of evolutionarily conserved tyrosine motifs (NPxY) in the beta integrin cytoplasmic tail to phosphotyrosine-binding (PTB) domain-containing proteins, an interaction proposed to be dynamically regulated by tyrosine phosphorylation. Here we show that replacement of both beta1 integrin cytoplasmic tyrosines with alanines, resulting in the loss of all PTB domain interaction, causes complete loss of beta1 integrin function in vivo. In contrast, replacement of beta1 integrin cytoplasmic tyrosines with phenylalanines, a mutation that prevents tyrosine phosphorylation, conserves in vivo integrin function. These results have important implications for the molecular mechanism and regulation of integrin function.

Analyzing the mechanism of Rap1 activation in platelets: Rap1 activation is related to the release reaction mediated through the collagen receptor GPVI

The abundant Rap1 in platelets becomes activated when these cells are stimulated by various agonists, but its function has remained unknown. In view of this, we developed an assay to quantitatively measure activated Rap1 and used it to determine relationships between Rap1 activation and several platelet functions: integrin alpha2beta1 activation, tyrosine phosphorylation, and the release reaction. We looked at how these processes are affected by the protein kinase C inhibitor BIMI, tyrosine kinase inhibitor PP2, PI 3-kinase inhibitor wortmannin, and ADP scavenger apyrase. In CRP (collagen related peptide)-activated platelets, all the inhibitors severely inhibited Rap1 activation, but had little effect on integrin alpha2beta1 activation, indicating that the integrin activation mechanism is different from the Rap1 activation mechanism, at least in GPVI-dependent activation. With p85alpha-null mouse platelets, we demonstrated that Rap1 activation involves PI 3-kinase p85alpha-dependent tyrosine phosphorylation. All the inhibitors similarly decreased Rap1 activation and the serotonin release reaction, and the inhibition of Rap1 activation was not due to the lack of released ADP. Our results indicate that platelet Rap1 activation is closely related to the release reaction and not to integrin alpha2beta1 activation in GPVI-mediated platelet activation.

The small GTPase Rap1b regulates the cross talk between platelet integrin alpha2beta1 and integrin alphaIIbbeta3

The involvement of the small GTPase Rap1b in platelet integrin alpha2beta1-dependent outside-in signaling was investigated. Platelet adhesion to 4 different specific ligands for integrin alpha2beta1, monomeric collagen, decorin, and collagen-derived peptides CB8(II) and CB11(II), induced a robust and rapid activation of Rap1b. This process did not require secreted ADP or thromboxane A2 production but was critically regulated by phospholipase C (PLC)-derived second messengers. Both Ca2+ and protein kinase C were found to organize independent but additive pathways for Rap1b activation downstream of integrin-alpha2beta1, which were completely blocked by inhibition of PLC with U73122. Moreover, integrin alpha2beta1 engagement failed to trigger Rap1b activation in murine platelets lacking CalDAG-GEFI, a guanine nucleotide exchange factor regulated by Ca2+ and diacylglycerol, despite normal phosphorylation and activation of PLCgamma2. In addition, CalDAG-GEFI-deficient platelets showed defective integrin alpha2beta1-dependent adhesion and spreading. We found that outside-in signaling through integrin alpha2beta1 triggered inside-out activation of integrin alphaIIbbeta3 and promoted fibrinogen binding. Similarly to Rap1b stimulation, this process occurred downstream of PLC activation and was dramatically impaired in murine platelets lacking the Rap1 exchange factor CalDAG-GEFI. These results demonstrate that Rap1b is an important element in integrin-dependent outside-in signaling during platelet adhesion and regulates the cross talk between adhesive receptors.

Use of synthetic peptides to locate novel integrin alpha2beta1-binding motifs in human collagen III

A set of 57 synthetic peptides encompassing the entire triplehelical domain of human collagen III was used to locate binding sites for the collagen-binding integrin alpha(2)beta(1). The capacity of the peptides to support Mg(2+)-dependent binding of several integrin preparations was examined. Wild-type integrins (recombinant alpha(2) I-domain, alpha(2)beta(1) purified from platelet membranes, and recombinant soluble alpha(2)beta(1) expressed as an alpha(2)-Fos/beta(1)-Jun heterodimer) bound well to only three peptides, two containing GXX'GER motifs (GROGER and GMOGER, where O is hydroxyproline) and one containing two adjacent GXX'GEN motifs (GLKGEN and GLOGEN). Two mutant alpha(2) I-domains were tested: the inactive T221A mutant, which recognized no peptides, and the constitutively active E318W mutant, which bound a larger subset of peptides. Adhesion of activated human platelets to GER-containing peptides was greater than that of resting platelets, and HT1080 cells bound well to more of the peptides compared with platelets. Binding of cells and recombinant proteins was abolished by anti-alpha(2) monoclonal antibody 6F1 and by chelation of Mg(2+). We describe two novel high affinity integrin-binding motifs in human collagen III (GROGER and GLOGEN) and a third motif (GLKGEN) that displays intermediate activity. Each motif was verified using shorter synthetic peptides.

Differential effects of echistatin and thrombin on collagen production and prolidase activity in human dermal fibroblasts and their possible implication in beta1-integrin-mediated signaling

Prolidase [E.C. 3.4.13.9] is a cytosolic imidodipeptidase that plays an important role in collagen biosynthesis. The enzyme contributes to the recovery of proline from protein degradation products (mainly collagen) for collagen resynthesis. Prolidase activity and collagen biosynthesis are supposed to be regulated by beta(1)-integrins, which initiate a signaling pathway in which several kinases and intracellular proteins are involved, including focal adhesion kinase pp125(FAK) (FAK), Src, Shc, growth factor receptor bound protein 2 (Grb-2), son of sevenless protein (SOS), Ras, Raf and mitogen-activated protein kinases (MAPK), extracellular-signal regulated kinase 1 (ERK(1)) and kinase 2 (ERK(2)). We studied the effects of echistatin, a well-known disintegrin and thrombin, a serine protease capable of activation of platelet integrin alpha(2)beta(1) receptor on collagen production, prolidase activity, expression of prolidase, beta(1)-integrin receptor, FAK, SOS-protein and phosphorylated MAP-kinases (ERK(1) and ERK(2)) in confluent human dermal fibroblasts. It has been found that treatment of the cells with 100nM echistatin contributes to inhibition of collagen production, as well as prolidase activity and expression compared to control cells. These phenomena were accompanied by a decrease in the expression of FAK, SOS-protein and phosphorylated MAP-kinases, ERK(1) and ERK(2). An opposite phenomenon was observed in fibroblasts treated with 0.1IU thrombin. In this case, a significant increase in collagen production and prolidase activity, accompanied by a distinct raise in the expression of prolidase, FAK and phosphorylated MAP-kinases and a slight increase in expression of SOS compared to controls were found. The results suggest that regulation of prolidase activity and collagen biosynthesis in human dermal fibroblasts may involve beta(1)-integrin-dependent signaling.

Glycoprotein VI/Fc receptor gamma chain-independent tyrosine phosphorylation and activation of murine platelets by collagen

We have investigated the ability of collagen to induce signalling and functional responses in suspensions of murine platelets deficient in the FcRgamma (Fc receptor gamma) chain, which lack the collagen receptor GPVI (glycoprotein VI). In the absence of the FcRgamma chain, collagen induced a unique pattern of tyrosine phosphorylation which was potentiated by the thromboxane analogue U46619. Immunoprecipitation studies indicated that neither collagen alone nor the combination of collagen plus U46619 induced phosphorylation of the GPVI-regulated proteins Syk and SLP-76 (Src homology 2-containing leucocyte protein of 76 kDa). A low level of tyrosine phosphorylation of phospholipase Cgamma2 was observed, which was increased in the presence of U46619, although the degree of phosphorylation remained well below that observed in wild-type platelets (approximately 10%). By contrast, collagen-induced phosphorylation of the adapter ADAP (adhesion- and degranulation-promoting adapter protein) was substantially potentiated by U46619 to levels equivalent to those observed in wild-type platelets. Collagen plus U46619 also induced significant phosphorylation of FAK (focal adhesion kinase). The functional significance of collagen-induced non-GPVI signals was highlighted by the ability of U46619 and collagen to induce the secretion of ATP in FcRgamma chain-deficient platelets, even though neither agonist was effective alone. Protein tyrosine phosphorylation and the release of ATP were abolished by the anti-(alpha2 integrin) antibodies Ha1/29 and HMalpha2, but not by blockade of alphaIIbbeta3. These results illustrate a novel mechanism of platelet activation by collagen which is independent of the GPVI-FcRgamma chain complex, and is facilitated by binding of collagen to integrin alpha2beta1.

Platelet glycoprotein VI: its structure and function

Glycoprotein (GP) VI is a platelet membrane protein with a molecular weight of 62 kDa that was identified as a physiological collagen receptor from studies of patients deficient in this protein. GPVI-deficient platelets lacked specifically collagen-induced aggregation and the ability to form thrombi on a collagen surface under flow conditions, suggesting that GPVI makes an indispensable contribution to collagen-induced platelet activation. On the platelet surface, GPVI is present as a complex with the Fc receptor (FcR) gamma-chain, probably composed of two GPVI molecules and one FcR gamma-chain dimer. GPVI must form such a dimeric complex to exhibit high affinity binding to collagen. The GPVI-induced activation mechanism is initiated by tyrosine phosphorylation of the immunoreceptor tyrosine-based activation motif (ITAM) of the FcR gamma-chain, and then this signal is transduced to many related proteins, mainly by tyrosine phosphorylation. GPVI is widely recognized as a requisite factor for the formation of platelet aggregates on a collagen surface under blood flow. However, individuals with GPVI-deficient or null platelets do not exhibit any strong bleeding tendency. Analyzing this apparent dichotomy should provide us with a more precise understanding of the mechanism of thrombus formation.

Platelet adhesion signalling and the regulation of thrombus formation

Platelets perform a central role in haemostasis and thrombosis. They adhere to subendothelial collagens exposed at sites of blood vessel injury via the glycoprotein (GP) Ib-V-IX receptor complex, GPVI and integrin alpha(2)beta(1). These receptors perform distinct functions in the regulation of cell signalling involving non-receptor tyrosine kinases (e.g. Src, Fyn, Lyn, Syk and Btk), adaptor proteins, phospholipase C and lipid kinases such as phosphoinositide 3-kinase. They are also coupled to an increase in cytosolic calcium levels and protein kinase C activation, leading to the secretion of paracrine/autocrine platelet factors and an increase in integrin receptor affinities. Through the binding of plasma fibrinogen and von Willebrand Factor to integrin alpha(IIb)beta(3), a platelet thrombus is formed. Although increasing evidence indicates that each of the adhesion receptors GPIb-V-IX and GPVI and integrins alpha(2)beta(1) and alpha(IIb)beta(3) contribute to the signalling that regulates this process, the individual roles of each are only beginning to be dissected. By contrast, adhesion receptor signalling through platelet endothelial cell adhesion molecule 1 (PECAM-1) is implicated in the inhibition of platelet function and thrombus formation in the healthy circulation. Recent studies indicate that understanding of platelet adhesion signalling mechanisms might enable the development of new strategies to treat and prevent thrombosis.

Adhesion of human and mouse platelets to collagen under shear: a unifying model

There is presently confusion as to the roles of alpha2beta1 and GPVI in supporting platelet adhesion and aggregate formation on collagen at intermediate/high shear. Recent studies have reported essential, partial, or dispensable roles for either receptor in supporting these events, and the possibility that there may be fundamental differences between their roles in human and mouse platelets has been proposed. Further, the recent recognition that Src family tyrosine kinases contribute to signaling by alpha2beta1 and other adhesive receptors, in addition to GPVI, has added to this debate. The present study compares the roles of alpha2beta1, GPVI, and Src-dependent kinases in supporting adhesion and aggregation in human and mouse platelets in whole blood using blocking antibodies, mutant mice, and a novel inhibitor of Src kinases, PD0173952, which is effective in plasma. The results demonstrate that the fundamental processes of adhesion and aggregate formation are conserved in mice and human platelets and that two mechanisms of stable adhesion and activation on collagen exist. These can be distinguished by the contributions of GPVI and alpha2beta1, with GPVI-mediated platelet activation either preceding or following integrin-mediated adhesion. The relative contribution of each pathway depends on environmental conditions and may also reflect platelet heterogeneity. These observations form the basis of a unifying two-state model of platelet adhesion and aggregate formation on collagen that is conserved between human and mouse platelets.

The platelet glycoprotein Ib-von Willebrand factor interaction activates the collagen receptor alpha2beta1 to bind collagen: activation-dependent conformational change of the alpha2-I domain

Integrin alpha2beta1 (glycoprotein [GP] Ia/IIa) is a major platelet receptor for collagen, containing its collagen-binding site within the alpha2 I domain. alpha2beta1 changes conformation upon platelet activation, increasing its affinity for collagen. We observed that 2 antibodies known to bind within the alpha2I domain, 12F1 and 6F1, bound preferentially to adenosine diphosphate (ADP)-activated platelets. Interestingly, when whole blood was perfused over a surface coated with either 12F1 or 6F1, only 6F1 supported the adhesion of unstimulated platelets. To test whether the interaction of GP Ib with von Willebrand factor (VWF) directly activates alpha2beta1, we used 12F1 as a probe of integrin activation. We perfused blood over a surface coated with a mixture of VWF-A1 domain (a GP Ib ligand) and 12F1 or VWF-A1 and mouse immunoglobulin G (IgG). Platelets rolled and did not attach stably on the A1/IgG surface, but they firmly bound and covered the A1/12F1 surface. We corroborated that 12F1 binds an active conformation of the I domain by showing that it binds with higher affinity to a gain-of-function mutant than to either wild-type I domain or a loss-of-function mutant. These results strongly suggest that the interaction of platelet GP Ib with VWF mediates the activation of alpha2beta1, increasing its affinity for collagen.