Relative antithrombotic effect of soluble GPVI dimer compared with anti-GPVI antibodies in mice

An EF hand mutation in Stim1 causes premature platelet activation and bleeding in mice

Changes in cytoplasmic Ca2+ levels regulate a variety of fundamental cellular functions in virtually all cells. In nonexcitable cells, a major pathway of Ca2+ entry involves receptor-mediated depletion of intracellular Ca2+ stores followed by the activation of store-operated calcium channels in the plasma membrane. We have established a mouse line expressing an activating EF hand motif mutant of stromal interaction molecule 1 (Stim1), an ER receptor recently identified as the Ca2+ sensor responsible for activation of Ca2+ release-activated (CRAC) channels in T cells, whose function in mammalian physiology is not well understood. Mice expressing mutant Stim1 had macrothrombocytopenia and an associated bleeding disorder. Basal intracellular Ca2+ levels were increased in platelets, which resulted in a preactivation state, a selective unresponsiveness to immunoreceptor tyrosine activation motif-coupled agonists, and increased platelet consumption. In contrast, basal Ca2+ levels, but not receptor-mediated responses, were affected in mutant T cells. These findings identify Stim1 as a central regulator of platelet function and suggest a cell type-specific activation or composition of the CRAC complex.

Blocking of platelets or intrinsic coagulation pathway-driven thrombosis does not prevent cerebral infarctions induced by photothrombosis

BACKGROUND AND PURPOSE

Models of photochemically-induced thrombosis are widely used in cerebrovascular research. Photothrombotic brain infarctions can be induced by systemic application of photosensitizing dyes followed by focal illumination of the cerebral cortex. Although the ensuing activation of platelets is well established, their contribution for thrombosis and tissue damage has not formally been proved.

METHODS

Infarction to the cerebral cortex was induced in mice by Rose Bengal and a cold light source. To assess the functional role of platelets, animals were platelet-depleted by anti-GPIbalpha antibodies or treated with GPIIb/IIIa-blocking F(ab)(2) fragments. The significance of the plasmatic coagulation cascade was determined by using blood coagulation factor XII (FXII)-deficient mice or heparin. Infarct development and infarct volumes were determined by serial MRI and conventional and electron microscopy.

RESULTS

There was no difference in development and final size of photothrombotic infarctions in mice with impaired platelet function. Moreover, deficiency of FXII, which initiates the intrinsic pathway of coagulation and is essential for thrombus formation, or blockade of FXa, the key protease during the waterfall cascade of plasmatic coagulation, by heparin likewise did not affect lesion development.

CONCLUSIONS

Our data demonstrate that platelet activation, factor XII-driven thrombus formation, and plasmatic coagulation pathways downstream of FX are not a prerequisite for ensuing tissue damage in models of photothrombotic vessel injury indicating that other pathomechanisms are involved. We suggest that this widely used model does not depend on platelet- or plasmatic coagulation-derived thrombosis.

Diverging signaling events control the pathway of GPVI down-regulation in vivo

Coronary artery thrombosis is often initiated by platelet activation on collagen-rich subendothelial layers in the disrupted atherosclerotic plaque. The activating platelet collagen receptor glycoprotein VI (GPVI) noncovalently associates with the Fc receptor γ-chain (FcRγ), which signals through its immunoreceptor-tyrosine–based activation motif (ITAM) via the adaptor LAT leading to the activation of phospholipase Cγ2 (PLCγ2). GPVI is a promising antithrombotic target as anti-GPVI antibodies induce the irreversible loss of the receptor from circulating platelets by yet undefined mechanisms in humans and mice and long-term antithrombotic protection in the latter. However, the treatment is associated with transient but severe thrombocytopenia and reduced platelet reactivity to thrombin questioning its clinical usefulness. Here we show that GPVI down-regulation occurs through 2 distinct pathways, namely ectodomain shedding or internalization/intracellular clearing, and that both processes are abrogated in mice carrying a point mutation in the FcRγ-associated ITAM. In mice lacking LAT or PLCγ2, GPVI shedding is abolished, but the receptor is irreversibly down-regulated through internalization/intracellular clearing. This route of GPVI loss is not associated with thrombocytopenia or altered thrombin responses. These results reveal the existence of 2 distinct signaling pathways downstream of the FcRγ-ITAM and show that it is possible to uncouple GPVI down-regulation from undesired side effects with obvious therapeutic implications.

Controlled shedding of platelet glycoprotein (GP)VI and GPIb-IX-V by ADAM family metalloproteinases

BACKGROUND

Platelet glycoprotein (GP)VI that binds collagen, and GPIb-IX-V that binds von Willebrand factor, initiate thrombus formation.

OBJECTIVES

In this study, we investigated the mechanisms of metalloproteinase-mediated ectodomain shedding that regulate the surface expression of GPVI, GPIbalpha (the major ligand-binding subunit) and GPV (that regulates thrombin-dependent activation via GPIbalpha).

METHODS AND RESULTS

Immunoblotting human platelet lysates using affinity-purified antibodies against cytoplasmic domains of GPVI, GPIbalpha or GPV allowed simultaneous analysis of intact and cleaved receptor, and revealed (i) that a significant fraction of GPIbalpha, but not GPVI, exists in a cleaved state on platelets, even when isolated in the presence of metalloproteinase inhibitor (GM6001) or EDTA; (ii) the same-sized membrane-associated fragments of GPVI or GPIbalpha are generated by phorbol-ester (PMA), the mitochondrial-targeting reagent CCCP, the calmodulin inhibitor W7, or the thiol-modifying reagent, N-ethylmaleimide, that directly activates ADAM10/ADAM17; and (iii) GPV is shed by both metalloproteinase- and thrombin-dependent mechanisms, depending on the concentration of thrombin. Based on the predicted cleavage area defined by these studies, ADAM10, but not ADAM17, cleaved a GPVI-based synthetic peptide within the extracellular membrane-proximal sequence (PAR;Q(243)YY) as analyzed by MALDI-TOF-MS. In contrast, ADAM17, but not ADAM10, cleaved within the GPIbalpha-based peptide (LRG;V(465)LQ). Both ADAM10 and ADAM17 cleaved within a GPV-based peptide (AQP;V(494)TT). Metalloproteinase-mediated shedding of GPIbalpha from GPIb-IX-transfected or GPVI-transfected cells induced by W7 or N-ethylmaleimide was inhibited by mutagenesis of sequences identified from peptide analysis.

CONCLUSIONS

These findings suggest surface levels of GPVI, GPIbalpha and GPV may be controlled by distinct mechanisms involving ADAM10 and/or ADAM17.

In vivo thrombus formation in murine models

Platelets play a central role in hemostasis, but also in atherothrombosis, as they rapidly adhere to tissue and to one another as a response to any vascular injury. This process involves a large number of surface receptors, signaling pathways, and enzymatic cascades as well as their complex interplay. Although in vitro experiments proved successful in both identifying new receptors and pathways and developing potent and selective antithrombotic drugs, in vitro research cannot mimic the myriad hemodynamic and spatiotemporal cellular and molecular interactions that occur during the generation and propagation of thrombi in vivo. Animal models, and, with the availability of genetically modified mouse strains and of modern intravital imaging techniques, mouse models in particular, have opened new ways to identify both individual roles and the interplay of platelet proteins in complex in vivo settings. In vivo models revealed the important role of, eg, Gas6 or blood coagulation factor XII in thrombus formation, and results obtained in in vivo models raised the interesting possibility that (physiologic) hemostasis and (pathologic) thrombosis might represent 2 mechanistically different processes. This review summarizes in vivo findings that contributed significantly to our understanding of hemostatic and thrombotic processes and which may help to guide future research.

Glycoprotein VI oligomerization in cell lines and platelets

BACKGROUND

Glycoprotein VI (GPVI) is a physiologic receptor for collagen expressed at the surface of platelets and megakaryocytes. Constitutive dimerization of GPVI has been proposed as being necessary for the interaction with collagen, although direct evidence of dimerization has not been reported in cell lines or platelets.

OBJECTIVES

To investigate oligomerization of GPVI in transfected cell lines and in platelets under non-stimulated conditions.

METHODS AND RESULTS

By using a combination of molecular and biochemical techniques, we demonstrate that GPVI association occurs at the surface of transfected 293T cells under basal conditions, through an interaction at the extracellular domain of the receptor. Bioluminescence resonance energy transfer was used to confirm oligomerization of GPVI under these conditions. A chemical crosslinker was used to detect constitutive oligomeric forms of GPVI at the surface of platelets, which contain the Fc receptor (FcR) gamma-chain.

CONCLUSIONS

The present results directly demonstrate GPVI-FcR gamma-chain oligomerization at the surface of the platelet, and thereby add to the growing evidence that oligomerization of GPVI may be a prerequisite for binding of the receptor to collagen, and therefore for proper functioning of platelets upon vascular damage.

The Fab Fragment of a Novel Anti-GPVI Monoclonal Antibody, OM4, Reduces In Vivo Thrombosis Without Bleeding Risk in Rats

Background— Inhibition of GPVI has been proposed as a useful antithrombotic strategy; however, in vivo proof-of-concept animal studies targeting GPVI are lacking. We evaluated a novel anti-human GPVI monoclonal antibody OM4 Fab in rats.

Methods and Results— OM4 Fab specifically inhibited collagen-induced aggregation of rat platelets in vitro with an IC 50 of 20 to 30 μg/mL but not ADP and AA-induced platelet aggregation. After intravenous administration of OM4 Fab, a rapid inhibition of ex vivo platelet aggregation was observed with a gradual recovery within 60 to 90 minutes which corresponded to the decline in OM4 Fab plasma concentration and time-dependent decrease in platelet-bound OM4 Fab. In contrast to previous reports in mice, intravenous OM4 Fab did not deplete platelet GPVI. Injection of OM4 IgG caused acute thrombocytopenia. In a modified Folts model of cyclic flow reduction in rat carotid artery, the number of complete occlusions was significantly reduced by intravenous administration of OM4 Fab (20 mg/kg) before or after mechanical injury to the vessel, without prolongation of bleeding time.

Conclusion— Fab fragment of the monoclonal antibody OM4 effectively inhibits collagen induced platelet aggregation in vitro and ex vivo, and in vivo thrombosis in rats without prolonging bleeding time. Antibodies against GPVI may have therapeutic potential, inhibiting thrombosis without prolonging bleeding time.

Structural basis for platelet collagen responses by the immune-type receptor glycoprotein VI

Activation of circulating platelets by exposed vessel wall collagen is a primary step in the pathogenesis of heart attack and stroke, and drugs to block platelet activation have successfully reduced cardiovascular morbidity and mortality. In humans and mice, collagen activation of platelets is mediated by glycoprotein VI (GPVI), a receptor that is homologous to immune receptors but bears little sequence similarity to known matrix protein adhesion receptors. Here we present the crystal structure of the collagen-binding domain of human GPVI and characterize its interaction with a collagen-related peptide. Like related immune receptors, GPVI contains 2 immunoglobulin-like domains arranged in a perpendicular orientation. Significantly, GPVI forms a back-to-back dimer in the crystal, an arrangement that could explain data previously obtained from cell-surface GPVI inhibition studies. Docking algorithms identify 2 parallel grooves on the GPVI dimer surface as collagen-binding sites, and the orientation and spacing of these grooves precisely match the dimensions of an intact collagen fiber. These findings provide a structural basis for the ability of an immunetype receptor to generate signaling responses to collagen and for the development of GPVI inhibitors as new therapies for human cardiovascular disease.

Reduced thrombus stability in mice lacking the α2A-adrenergic receptor

Platelet activation plays a central role in hemostasis and thrombosis. Many platelet agonists function through G-protein–coupled receptors. Epinephrine activates the α2A-adrenergic receptor (α2A) that couples to Gz in platelets. Although α2A was originally cloned from platelets, its role in thrombosis and hemostasis is still unclear. Through analysis of α2A-deficient mice, variable tail bleeding times were observed. In vitro, epinephrine potentiated activation/aggregation responses of wild-type but not α2A-deficient platelets as determined by flow cytometry and aggregometry, whereas perfusion studies showed no differences in platelet adhesion and thrombus formation on collagen. To test the in vivo relevance of α2A deficiency, mice were subjected to 3 different thrombosis models. As expected, α2A-deficient mice were largely protected from lethal pulmonary thromboembolism induced by the infusion of collagen/epinephrine. In a model of FeCl3-induced injury in mesenteric arterioles, α2A–/– mice displayed a 2-fold increase in embolus formation, suggesting thrombus instability. In a third model, the aorta was mechanically injured, and blood flow was measured with an ultrasonic flow probe. In wild-type mice, all vessels occluded irreversibly, whereas in 24% of α2A-deficient mice, the initially formed thrombi embolized and blood flow was reestablished. These results demonstrate that α2A plays a significant role in thrombus stabilization.

Glycoprotein VI-dependent and -independent pathways of thrombus formation in vivo

The role of the collagen receptor glycoprotein VI (GPVI) in arteriolar thrombus formation was studied in FcRgamma-null mice (FcRgamma(-/-)) lacking platelet surface GPVI. Thrombi were induced with severe or mild FeCl(3) injury. Collagen exposure was significantly delayed and diminished in mild compared with severe FeCl(3) injury. Times to initial thrombus formation and vessel occlusion were delayed in FcRgamma(-/-) compared with wild-type mice after severe injury. Platelet accumulation in wild-type mice was decreased after mild compared with severe injury. However, there was little difference between platelet accumulation after severe or mild injury in FcRgamma(-/-). These data indicate a significant role for GPVI in FeCl(3)-induced thrombus formation. Pretreatment of wild-type mice with lepirudin further impaired mild FeCl(3)-induced thrombus formation, demonstrating a role for thrombin. Laser-induced thrombus formation in wild-type and FcRgamma(-/-) was comparable. Collagen exposure to circulating blood was undetectable after laser injury. Normalized for thrombus size, thrombus-associated tissue factor was 5-fold higher in laser-induced thrombi than in severe FeCl(3)-induced thrombi. Thus, platelet activation by thrombin appears to be more important after laser injury than platelet activation by GPVI-collagen. It may thus be important when considering targets for antithrombotic therapy to use multiple animal models with diverse pathways to thrombus formation.

Thrombin overcomes the thrombosis defect associated with platelet GPVI/FcRgamma deficiency

Fibrillar collagens are among the most potent activators of platelets and play an important role in the initiation of thrombosis. The glycoprotein VI (GPVI)/FcRgamma-chain complex is a central collagen receptor and inhibitors of GPVI produce a major defect in arterial thrombogenesis. In this study we have examined arterial thrombus formation in mice lacking the GPVI/FcRgamma-chain complex (FcRgamma(-/-)). Using 3 distinct arterial thrombosis models involving deep vascular injury, we demonstrate that deficiency of GPVI/FcRgamma is not associated with a major defect in arterial thrombus formation. In contrast, with milder vascular injury deficiency of GPVI/FcRgamma was associated with a 30% reduction in thrombus growth. Analysis of FcRgamma(-/-) platelets in vitro, using thrombin-dependent and -independent thrombosis models, demonstrated a major role for thrombin in overcoming the thrombosis defect associated with GPVI/FcRgamma deficiency. Inhibition of thrombin in vivo produced a much greater defect in thrombus formation in mice lacking GPVI/FcRgamma compared with normal controls. Similarly, thrombin inhibition produced a marked prolongation in bleeding time in FcRgamma(-/-) mice relative to wild-type mice. Our studies define an important role for thrombin in overcoming the hemostatic and thrombotic defect associated with GPVI/FcRgamma deficiency. Moreover, they raise the interesting possibility that the full antithrombotic potential of GPVI receptor antagonists may only be realized through the concurrent administration of anticoagulant agents.

The platelet receptor for type III collagen (TIIICBP) is present in platelet membrane lipid microdomains (rafts)

Platelet interactions with collagen are orchestrated by the presence or the migration of platelet receptor(s) for collagen into lipid rafts, which are specialized lipid microdomains from the platelet plasma membrane enriched in signalling proteins. Electron microscopy shows that in resting platelets, TIIICBP, a receptor specific for type III collagen, is present on the platelet membrane and associated with the open canalicular system, and redistributes to the platelet membrane upon platelet activation. After platelet lysis by 1% Triton X-100 and the separation of lipid rafts on a discontinuous sucrose gradient, TIIICBP is recovered in lipid raft-containing fractions and Triton X-100 insoluble fractions enriched in cytoskeleton proteins. Platelet aggregation, induced by type III collagen, was inhibited after disruption of the lipid rafts by cholesterol depletion, whereas platelet adhesion under static conditions did not require lipid raft integrity. These results indicate that TIIICBP, a platelet receptor involved in platelet interaction with type III collagen, is localized within platelet lipid rafts where it could interact with other platelet receptors for collagen (GP VI and alpha2beta1 integrin) for efficient platelet activation.