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Thrombosis |
From INSERM U311, EFS-Alsace, ULP, Strasbourg, France.
Correspondence to François Lanza, INSERM U.311, Etablissement Français du Sang-Alsace, 10 rue Spielmann, BP 36, 67065 Strasbourg Cedex, France. E-mail francois.lanza{at}efs-alsace.fr
| Abstract |
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Methods and Results Complete deletion (GPIbß/) or an intracellular truncation (GPIbß
IC/) reproduced typical and variant forms of Bernard-Soulier, with absent and partial (20%) expression of the complex on the platelet surface. Both strains exhibited thrombocytopenia and enlarged platelets with abnormal microtubular structures but normal granule composition. They exhibited prolonged tail bleeding times, which were less pronounced in GPIbß
IC/. Decreased thrombus formation was observed after blood perfusion over a collagen coated surface at high shear. Resistance to vascular occlusion and an abnormal thrombus composition were observed in a model of FeCl3-induced lesion of carotid arteries. In a model of laser-induced lesion of mesenteric arterioles, thrombosis was strongly reduced in GPIbß/ mice, while a more modest effect was observed in GPIbß
IC/ animals. Finally, the two strains were protected against death in a model of systemic thromboembolism.
Conclusions This study provides in vivo evidence of a decreased thrombotic tendency linked to defective platelet GPIb-V-IX in mouse models of Bernard-Soulier syndrome.
Mouse models of Bernard-Soulier bleeding disorder were generated by targeting the GP GPIbß. These mice displayed, in addition to their defective hemostasis, decreased thrombotic responses in different models of arterial and systemic thrombosis.
Key Words: GPIb-V-IX complex von Willebrand factor knockout thrombosis models hemostasis
| Introduction |
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The extreme rarity of Bernard-Soulier has prevented correct assessment of the incidence of thrombotic events in this disease. One report described the occurrence of unstable angina in a Bernard-Soulier patient, suggesting that the absence of the complex does not fully protect against thrombosis.5 Indications of decreased thrombosis have also come from assays involving perfusion of Bernard-Soulier blood over thrombogenic surfaces such as exteriorized blood vessels or collagen.6,7 Mice deleted of the GPIb
or GPIbß subunit have been reported to reproduce the bleeding and platelet morphological defects found in Bernard-Soulier patients, but there is as yet no information concerning their thrombotic tendency.8,9 Recently, transgenic mice missing the GPIb
extracellular domain, which display a partial Bernard Soulier phenotype, without the additional abnormalities in platelet morphology and count, have been reported with decreased arterial thrombosis.10
To evaluate the degree of protection against thrombosis in Bernard-Soulier syndrome we generated two mouse strains by genetic recombination: a knockout strain lacking receptor expression by inactivating the GPIbß gene (GPIbß/) and a knock-in strain by interrupting the GPIbß intracellular domain (GPIbß
IC/). Thrombosis was studied in vitro in collagen perfusion assays and in several in vivo artery injury and thromboembolism models.
| Materials and Methods |
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IC/ Animals
IC/) was obtained by introducing a stop codon after the transmembrane domain. The targeting vectors were electroporated into ES cells followed by injection into blastocytes and implantation into pseudopregnant females. GPIbß/, GPIbß
IC/, and GPIbß+/+ mouse colonies were established by breeding heterozygotes. For detailed methods please see the supplemental materials (available online at http://atvb.ahajournals.org).
Platelet Preparation, Flow Cytometry, Western Blotting, and Electron Microscopy
Please see the supplemental materials for details
Bleeding Time Assays
The bleeding time was measured by severing a 3-mm segment from the distal end of the tail of 6- to 8-week-old mice. Please see the supplemental materials for details.
In Vitro Model of Thrombosis on Immobilized Collagen in a Flow System
Platelet adhesion under flow was studied as described previously.11 Whole blood anticoagulated with hirudin (100 U/mL) was perfused at 1500 s1 or 3000 s1 through a collagen-coated glass capillary and surface coverage was evaluated by off-line analysis. Some samples were prepared for scanning electron microscopy. Please see the supplemental materials for details.
FeCl3-Induced Carotid Artery Thrombosis
FeCl3-induced arterial injury was performed according to published procedures.12,13 The right common carotid artery was exposed to 20% FeCl3 for 3 minutes, rinsed with saline, and the blood flow was monitored for 30 minutes. The time to first occlusion and the numbers of arteries respectively patent and occluded at 30 minutes were recorded. Histological samples were prepared by perfusing the left ventricle with 3 mL of 4% paraformaldehyde in PBS, after which the artery was removed, incubated overnight in 4% paraformaldehyde solution, and embedded in paraffin. Samples for light and transmission electron microscopy (TEM) were prepared by fixing the arteries with 2.5% glutaraldehyde. Please see the supplemental materials for details.
Laser-Induced Mesenteric Artery Thrombosis
Laser-induced arterial thrombosis was studied as described by Nonne et al.11 Reproducible superficial lesions inducing reversible parietal thrombi or more extensive almost occlusive thrombi were produced by adjusting the firing time and laser intensity. Please see the supplemental materials for details.
Thromboembolism Model
The model of acute systemic vascular thromboembolism by infusion of a mixture of collagen (0.3 mg/kg) and adrenaline (60 µg/kg) has been described previously.14 Please see the supplemental materials for details.
Statistical Analyses
Non parametric MannWhitney tests, area under curves, and Logrank tests were performed using GraphPad PrisM version 3.00 for Windows (GraphPad Software; ***P<0.0001, ** P<0.01, *P<0.05; ns P>0.05).
| Results |
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IC/; supplemental Figure I). GPIbß/ was obtained by inserting a neo cassette into the second exon, thereby eliminating most of the coding sequence. GPIbß
IC/ was generated by inserting a stop codon into the native sequence three amino acids after the transmembrane domain. After electroporation and karyotype analysis, one KO clone (194 ES) and two KI clones (59 ES and 22 ES) were selected for injection into blastocytes. Animals with germline transmission were obtained and the offspring from crossing of heterozygotes were screened for the +/+, +/, and / genotypes. A Mendelian distribution and no overt developmental or morphological abnormalities were observed for the two mutations.
The lack of expression of GPIbß in platelets from GPIbß/ mice was confirmed by Western blotting using the mAb RAM.1 against the extracellular domain of mouse GPIbß15 (supplemental Figure I). In GPIbß
IC/ cells, a GPIbß reactive band with a lower molecular weight (18 kDa) was revealed by RAM.1, confirming the deletion of this domain.
Hematologic Parameters, Platelet Properties and Bleeding Tendencies in GPIbß/ and GPIbß
IC/ Mice
Analysis of the hematologic parameters of GPIbß/ and GPIbß
IC/ revealed the characteristic decreased platelet counts and enlarged platelets of the Bernard-Soulier syndrome but normal leukocyte and erythrocyte counts (Figure 1A). Heterozygotes from both strains had normal platelet counts and morphology (data not shown).
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Transmission electronic microscopy (TEM) analysis revealed discoid platelets with an enlarged diameter (6±1 for GPIbß/ and 6±2 µm for GPIbß
IC /) compared with control GPIbß+/+ (2±1 µm; Figure 1B). Intracellularly,
- granules were present at a normal density and size but a 2-fold increase in the number of microtubules per platelet was observed as compared with control platelets with however a normal equatorial localization (Figure 1C).
Flow cytometry analysis showed that GPIbß/ platelets lacked GPIbß expression and expressed little GPIb
(3% of GPIbß+/+ cells). By contrast, in the GPIbß
IC/ mice, GPIbß, and GPIb
represented close to 20% of normal levels (Figure 1D). Expression of integrin
IIbß3 in GPIbß/ and GPIbß
IC/ represented 170% as compared with GPIbß+/+ attriubutable to increased platelet size (data not shown).
Platelet counts were reduced by 73% and 71% in GPIbß/ and GPIbß
IC/ mice, respectively, compared with wild-type littermates (Figure 1A). Heterozygotes had normal platelet counts and morphology (data not shown).
In wild-type animals, 82% had bleeding times shorter than 12 minutes, whereas 76% of the GPIbß/ mice bled for more than 20 minutes (Figure 2). GPIbß
IC/ mice had a decreased bleeding tendency than GPIbß/ with 52% of the mice bleeding after 20 minutes. Heterozygotes from the GPIbß/ and GPIbß
IC/ strains had bleeding times similar to those of their respective +/+ genotypes (data not shown).
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Decreased Thrombus Formation During Perfusion of GPIbß/ and GPIbß
IC/ Blood Over a Collagen Surface at High Shear Rates
The thrombotic tendency was first explored in blood perfusion assays over immobilized collagen (Figure 3). In GPIbß+/+, single adherent platelets progressively formed aggregates which were individually larger at 3000 s1 than at 1500 s1 after 2 minutes (Figure 3A and 3D), with a slightly increased total surface coverage at 3000 s1 (Figure 3B and 3E). A major defect was observed in GPIbß/ and GPIbß
IC/ blood at 3000 s1, where platelet adhesion was nearly abolished with respective decreases of 98 and 83% in the total surface coverage and no evidence of aggregate formation at 2 minutes (Figure 3B). Electron microscopy analysis revealed the rare attachment of individual GPIbß/ platelets to the collagen fibers and attachment of a few GPIbß
IC/ platelets in strings along the fibers (Figure 3C). These cells were however almost incapable of attracting circulating platelets to form an aggregate. At 1500 s1 some adhesion and aggregates were observed in GPIbß/ blood which were greatly decreased in size as compared with the wild-type (Figure 3F). At this shear rate, adhesion and aggregate formation occurred in GPIbß
IC/ blood and was approximately 50 to 60% of that in wild-type blood (Figure 3E). Therefore, the absence of the GPIb-IX complex or a decrease in surface expression, following truncation of its intracellular domain, resulted in a decreased tendency to thrombus formation in vitro under high shear conditions.
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Decreased Arterial Thrombosis in GPIbß/ and GPIbß
IC/ Mice
Thrombotic tendency was then evaluated in vivo in two arterial models. The first was a carotid artery model where injury was induced by application of FeCl3 (Figure 4). In wild-type (n=8), stable occlusion occurred in 75% of the arteries, with permanent cessation of flow (Figure 4A). In GPIbß/ (n=8), stable occlusion occurred in only 12.5% of the arteries, whereas 25% exhibited unstable occlusion and 62.5% did not occlude. A decrease in stable occlusion was also observed in GPIbß
IC/ (n=8) animals where 62.5% of the arteries were patent (unstable or no occlusion) at the end of the 30 minutes period (Figure 4A). The time to first occlusion, either stable or transient, was increased in GPIbß/ (1646±84 s) or GPIbß
IC/ (1242±169 s) as compared with their wild-type littermates (997±135 s and 631±68 s) (Figure 4B). Histological analysis revealed that the thrombi formed in GPIbß/ and GPIbß
IC/ animals were less extensive than in the wild-type (Figure 4C) and ultrastructural analysis showed loose thrombi mainly composed of well demarcated platelets displaying less extensive shape change and little sign of degranulation (Figure 4D). This contrasted with the intricate appearance of wild-type aggregates which consisted of intertwined platelets having undergone degranulation.
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In a second model, lesions of mesenteric arterioles were induced with a laser beam so as to obtain desquamation of the endothelial layer or rupture of the underlying layers, respectively.11 In wild-type mice, a reversible parietal thrombus formed after superficial injury, while a larger almost occlusive thrombus was produced after severe injury (Figure 5A and 5B). In GPIbß/, thrombus formation was almost absent on superficial lesions and on more severe lesions reached a size representing only 25% of that in the wild type at 160 s (Figure 5A). In GPIbß
IC/, the maximum peak after superficial injury was decreased by 45% as compared with controls, while initial thrombus growth was delayed after more severe injury but reached at later times a size comparable to that of wild-type (Figure 5B).
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Decreased Systemic Thromboembolism in GPIbß/ and GPIbß
IC/ Mice
The GPIbß/ and GPIbß
IC/ strains were finally evaluated in a model of platelet-dependent intravascular thrombosis induced by intravenous injection of a mixture of collagen (0.3 mg/kg) and adrenaline (60 µg/kg). In wild-type mice, collagen/adrenaline injection led to widespread pulmonary vascular thrombosis with 90% mortality
6 minutes after injection (Figure 6). GPIbß/ and GPIbß
IC/ mice had a lower death rate, with 40% and 60% of the mice being still alive 30 minutes after collagen-adrenaline injection.
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| Discussion |
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expression, and an increased bleeding tendency. A difference was noted at the ultrastructural level with normal density and size of platelet granules. This contrasted with the report of
-granules of increased size9 which was attributed to upregulation of SEPT5 expression which gene is located 5' to the GPIbß gene and modulates exocytosis. This discrepancy is difficult to explain but does not appear to be attributable to the targeting strategy which is very similar in the two studies with insertion of a Neo cassette at the same 5'-restriction site, and only differed in the length of the 3' untranslated region. In the present study, a novel finding was the observation of a doubling in the number of microtubule rings in the knock-out strain which retained an equatorial location and the capacity to maintain a discoid shape despite the enlarged platelet size.
The GPIbß
IC/ strain, which was originally developed to explore signaling functions of the receptor, happened to exhibit decreased level of GPIb-IX on the platelet surface. This suggests that the GPIbß intracellular domain is required for efficient expression of the complex. In fact, the GPIbß
IC/ resembled the knockout for the ultrastructural defects and decreased number of circulating cells, and was classified as a variant form of Bernard-Soulier. Reduced bleeding complications were however observed compared with the knockout implying that an 80% decrease in GPIb, even with the added intracellular mutation of GPIbß, is sufficient to support minimal hemostasis.
The GPIbß/ and GPIbß
IC/ models allowed us to assess the thrombotic tendency associated with Bernard-Soulier. Defective thrombus formation was documented in collagen flow assays at high shear rates known to depend on GPIb/VWF interaction.1,2 These defects are in line with those observed in earlier studies of Bernard-Soulier patients after blood perfusion over collagen or vascular matrices.6,7 More recently a similar defect in collagen flow studies has been reported in transgenic mice engineered on a GPIb
knock out and missing the GPIb
extracellular domain.10 Compared with this strain, the GPIbß/ and GPIbß
IC/ have the added defect of enlarged platelets which could result in increased drag-forces exerted on the platelets. Nevertheless these two strains behaved very similarly to the GPIb
transgenic mice in this flow assay.
Other receptors involved in platelet-collagen interaction have been evaluated in similar ex vivo perfusion systems, such as integrin
2ß1 and GPVI, which are the major receptors contributing to platelet adhesion and activation after direct interaction with collagen. GPVI-FcR
deficiency induced by immunodepletion or after genetic ablation have established a critical role for GPVI in such perfusion assays over collagen1618 whereas
2ß1 deficiency resulted in normal thrombus formation.19 GPVI-deficient platelets tethered normally, but failed to spread and extend aggregates, in contrast with the deficient initial tethering in GPIb-deficient mice. In the experiments presented here, platelet tethering and aggregation nevertheless occurred in GPIb-deficient mice under conditions of intermediate shear (1500 s1), which in human blood have been described as being GPIb-dependent. Platelet capture and activation by
2ß1 and GPVI can probably take place at these shears in mouse blood10
In vivo results in a FeCl3 carotid thrombosis model were consistent with the flow experiments demonstrating a lower incidence of vessel occlusion in both GPIbß/ and GPIbß
IC/ mice. Histology and ultrastructural analyses of the carotids showed that thrombus formation was not completely prevented but that thrombi consisted of less extensive loose aggregates. This suggested a defective propagation of activation through the platelet layers, resulting in decreased thrombus growth. Transgenic mice lacking the GPIb
extracellular domain evaluated in a similar carotid model appeared to be similarly protected against vessel occlusion.10 No information was presented on the presence and histology of the thrombi. Thrombosis was also severely impaired after laser-induced arterial injuries, especially in the GPIbß/ strain. A more modest protection of GPIbß
IC/ contrasted with the carotid FeCl3 model where both strains were similarly protected. Such variable responses in the same mouse strain depending on the nature of the vessel injury have now been observed in several mouse strains. For example, FcR
/GPVI deficiency has been reported to induce full or only minor protection against arterial thrombosis in different studies17,20 depending on the type of injury and exposure to collagen and thrombin.10,17,21 The degree of collagen exposure and thrombin activation have been reported to vary depending on the concentration of FeCl3 applied to arteries21 and the sensitivity to thrombin blockade can change with the extent of laser injury in mesenteric arteries.11,17 In spite of these differences, which await standardization between laboratories, this study and the recent work by Konstantinides et al establish that the GPIb-VWF interaction is, contrary to the GPVI-collagen axis, critical to arterial thrombosis independently of the nature of the lesion.
Abnormal laser-induced arterial thrombosis in the GPIbß-deficient strain presented some similarities with results in vWF-deficient mice in a FeCl3 mesenteric artery injury model.22 In the absence of this GPIb ligand a defect was also observed at the early stages of adhesion and in thrombus growth. Interestingly, thrombosis was still observed in vWF-deficient mice23 similar to the present detection of parietal thrombi in the carotid artery model. This analogous response further supports the hypothesis that mechanisms in addition to GPIb-vWFdependent responses can support arterial thrombus formation.
Less expectedly, GPIbß-deficient mice were also protected against systemic thromboembolism, after intra venous collagen-adrenaline injection. These results suggest that this model, which is widely thought to reflect platelet responses to soluble agonists, could also depend on GPIb/VWF-dependent responses. The mechanisms of platelet activation in this model are not entirely understood, but vWF-dependent activation could theoretically occur in the lung microcirculation where high shear conditions are encountered.23
GPIbß/ and GPIbß
IC/ present abnormalities of platelet morphology and count in addition to vWF-binding deficiencies. These additional defects could also contribute to decrease thrombus formation. Enlarged platelets are potentially more susceptible to stress and would have harder time to adhere. However this parameter is probably not sufficient to explain the whole defect as different thrombotic tendencies were observed in GPIbß/ and GPIbß
IC/, despite a similar platelet size increase. Low platelet counts have also been linked to decreased thrombus formation in perfusion models.24 To evaluate the influence of thrombocytopenia in our models, GPIbß
IC/ mice were injected with TPO to increase platelet numbers. Despite doubling of the platelet count, comparable defects in thrombus formation were observed in collagen flow assays (data not shown). Similarly, doubling of the platelet count in splenectomized GPIbß
IC/ mice did not correct the defect in superficial laser-induced lesions (data not shown).
In conclusion, this study has provided in vivo demonstration of a decreased thrombotic tendency in mouse models of the Bernard-Soulier bleeding disorder. This antithrombotic protection linked to a GPIb defect is in line with in vitro and in vivo thrombosis studies using GPIb/VWF blocking agents2527 and recent results in GPIb
transgenic mice.10 The finding is still difficult to extrapolate to man, especially in the face of a report of an acute coronary syndrome in a BSS patient.5 However, the GPIbß/ model should permit further evaluation of the role of this receptor in thrombosis under atherosclerotic conditions, particularly after crossing with APOE- or LDLR-deficient mice.
| Acknowledgments |
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Sources of Funding
Catherine Strassel was supported by a grant 2004.14 from Etablissement Français du Sang and Tovo David by a grant from Association de Recherche et de Développement en Médecine et en Santé Publique.
Disclosures
None.
| Footnotes |
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| References |
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and VI deficiency in a mouse model of arterial thrombosis. J Thromb Haemost. 2006; 4: 20142022.[CrossRef][Medline]
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