P1576Association of colorectal cancer with genetic and epigenetic variation in PEAR1 - a population-based cohort study

Background

Platelet Endothelial Aggregation Receptor 1 (PEAR1) modulates angiogenesis and platelet contact-induced activation, which play a role in the pathogenesis of colorectal cancer.

Purpose

To study the association of colorectal cancer with genetic and epigenetic variation in PEAR1.

Methods

Among 2532 randomly recruited participants enrolled in the family-based Flemish Study on Environment, Genes and Health Outcomes (51.2% women; mean age 44.8 years), we recorded the incidence of colorectal cancer and genotyped SNP rs12566888 located in intron 1 of the PEAR1 gene. In 929 participants, we also measured the methylation at 16 CpG sites in the PEAR1 promoter. In multivariable-adjusted analyses, we contrasted the risk of colorectal cancer in minor-allele (T) carriers vs. major allele (GG) homozygotes. We applied partial least square discriminant analysis (PLS-DA) to identify methylation sites associated with colorectal cancer.

Results

Over a median follow-up of 18.1 years, 49 patients developed colorectal cancer. While accounting for clustering within families and adjusting for sex, age, body mass index, the total-to-HDL cholesterol ratio, serum creatinine, plasma glucose, smoking and drinking, use of antiplatelet and nonsteroidal anti-inflammatory drug, the hazard ratio contrasting minor allele carriers vs. major allele homozygotes was 2.17 (95% confidence interval, 1.18–3.99; P=0.013). Bootstrapped analyses, from which we randomly excluded from two to nine cancer cases, provided confirmatory results. PLS-DA identified two methylation sites in the PEAR1 promoter associated with higher colorectal cancer risk and two with lower risk. In-silico analysis suggested that methylation of the PEAR1 promoter at these four sites affects binding of the transcription factors p53, PAX5, and E2F-1, thereby modulating gene expression.

Potential pathways

Conclusions

Our findings suggest that genetic and epigenetic variation in PEAR1 modulates the risk of colorectal cancer in white Flemish.

Acknowledgement/Funding

The European Union, the European Research Council, the European Research Area Net for Cardiovascular Diseases, and the Research Foundation Flanders.

Fast and Reproducible Vascular Neointima Formation in the Hamster Carotid Artery: Effects of Trapidil and Captopril

Neointima formation was induced in the hamster carotid artery by mechanical intraluminal injury with a catheter covered with roughened dental cement. Neointimal thickening occurred as early as 7 days after denudation and further increased during the next 1 to 2 weeks. Proliferation indices of smooth muscle cells (SMCs) showed the highest proportion of proliferating cells in the media and neointima respectively 1 and 5 days after the vascular injury. Transmission and scanning electron microscopy of damaged carotid artery sections as well as immuno-histochemical stainings of von Willebrand factor (vWF) confirmed that reendothelialization was progressive and already complete on day 14, at which time the neointima formation was almost complete.

In order to pharmacologically characterize this model further, the effects on neointima formation of trapidil (triazolopyrimidine), a platelet-derived growth factor (PDGF) antagonist, and captopril, an angiotensin converting enzyme inhibitor, were investigated. Trapidil administered orally twice daily at total doses of 25, 50 and 100 mg/kg/day, started 3 days prior to infliction of injury and up to 7 or 14 days after the catheterization, significantly reduced neointima formation. Captopril administered orally three times daily at a total dose of 100 mg/kg/day, equally reduced neointima formation, with 100 mg/kg/day trapidil being more effective than 100 mg/kg/day captopril 7 days after injury. When the treatment by either one of these drugs was arrested on day 7, neointima formation resumed quickly.

The hamster appears to be a small, reproducible and fast model for the study of SMC proliferation, requiring only relatively small amounts of experimental drugs. The model furthermore is sensitive to substances known to reduce neointima formation in other animal models.

Platelet rescue by macrophage depletion in obese ADAMTS-13-deficient mice at risk of thrombotic thrombocytopenic purpura

Essentials Obesity is a potential risk factor for development of thrombotic thrombocytopenic purpura (TTP). Obese ADAMTS-13-deficient mice were triggered with von Willebrand factor (VWF). Depletion of hepatic and splenic macrophages protects against thrombocytopenia in this model. VWF enhances phagocytosis of platelets by macrophages, dose-dependently.

SUMMARY

Background Thrombotic thrombocytopenic purpura (TTP) is caused by the absence of ADAMTS-13 activity. Thrombocytopenia is presumably related to the formation of microthrombi rich in von Willebrand factor (VWF) and platelets. Obesity may be a risk factor for TTP; it is associated with abundance of macrophages that may phagocytose platelets. Objectives To evaluate the role of obesity and ADAMTS-13 deficiency in TTP, and to establish whether macrophages contribute to thrombocytopenia. Methods Lean or obese ADAMTS-13-deficient (Adamts-13-/- ) and wild-type (WT) mice were injected with 250 U kg-1 of recombinant human VWF (rVWF), and TTP characteristics were evaluated 24 h later. In separate experiments, macrophages were depleted in the liver and spleen of lean and obese WT or Adamts-13-/- mice by injection of clodronate-liposomes, 48 h before injection of rVWF. Results Obese Adamts-13-/- mice had a lower platelet count than their lean counterparts, suggesting that they might be more susceptible to TTP development. Lean Adamts-13-/- mice triggered with a threshold dose of rVWF did not develop TTP, whereas typical TTP symptoms developed in obese Adamts-13-/- mice, including severe thrombocytopenia and higher lactate dehydrogenase (LDH) levels. Removal of hepatic and splenic macrophages by clodronate injection in obese Adamts-13-/- mice before treatment with rVWF preserved the platelet counts measured 24 h after the trigger. In vitro experiments with cultured macrophages confirmed a VWF dose-dependent increase of platelet phagocytosis. Conclusions Obese Adamts-13-/- mice are more susceptible to the induction of TTP-related thrombocytopenia than lean mice. Phagocytosis of platelets by macrophages contributes to thrombocytopenia after rVWF injection in this model.

Clumping factor A, von Willebrand factor-binding protein and von Willebrand factor anchor Staphylococcus aureus to the vessel wall

Essentials Staphylococcus aureus (S. aureus) binds to endothelium via von Willebrand factor (VWF). Secreted VWF-binding protein (vWbp) mediates S. aureus adhesion to VWF under shear stress. vWbp interacts with VWF and the Sortase A-dependent surface protein Clumping factor A (ClfA). VWF-vWbp-ClfA anchor S. aureus to vascular endothelium under shear stress.

SUMMARY

Objective When establishing endovascular infections, Staphylococcus aureus (S. aureus) overcomes shear forces of flowing blood by binding to von Willebrand factor (VWF). Staphylococcal VWF-binding protein (vWbp) interacts with VWF, but it is unknown how this secreted protein binds to the bacterial cell wall. We hypothesized that vWbp interacts with a staphylococcal surface protein, mediating the adhesion of S. aureus to VWF and vascular endothelium under shear stress. Methods We studied the binding of S. aureus to vWbp, VWF and endothelial cells in a micro-parallel flow chamber using various mutants deficient in Sortase A (SrtA) and SrtA-dependent surface proteins, and Lactococcus lactis expressing single staphylococcal surface proteins. In vivo adhesion of bacteria was evaluated in the murine mesenteric circulation using real-time intravital vascular microscopy. Results vWbp bridges the bacterial cell wall and VWF, allowing shear-resistant binding of S. aureus to inflamed or damaged endothelium. Absence of SrtA and Clumping factor A (ClfA) reduced adhesion of S. aureus to vWbp, VWF and activated endothelial cells. ADAMTS-13 and an anti-VWF A1 domain antibody, when combined, reduced S. aureus adhesion to activated endothelial cells by 90%. Selective overexpression of ClfA in the membrane of Lactococcus lactis enabled these bacteria to bind to VWF and activated endothelial cells but only in the presence of vWbp. Absence of ClfA abolished bacterial adhesion to the activated murine vessel wall. Conclusions vWbp interacts with VWF and with the SrtA-dependent staphylococcal surface protein ClfA. The complex formed by VWF, secreted vWbp and bacterial ClfA anchors S. aureus to vascular endothelium under shear stress.

Effects of GPI-anchored TNAP on the dynamic structure of model membranes

Tissue-nonspecific alkaline phosphatase (TNAP) plays a crucial role during skeletal mineralization, and TNAP deficiency leads to the soft bone disease hypophosphatasia. TNAP is anchored to the external surface of the plasma membranes by means of a GPI (glycosylphosphatidylinositol) anchor. Membrane-anchored and solubilized TNAP displays different kinetic properties against physiological substrates, indicating that membrane anchoring influences the enzyme function. Here, we used Electron Spin Resonance (ESR) measurements along with spin labeled phospholipids to probe the possible dynamic changes prompted by the interaction of GPI-anchored TNAP with model membranes. The goal was to systematically analyze the ESR data in terms of line shape changes and of alterations in parameters such as rotational diffusion rates and order parameters obtained from non-linear least-squares simulations of the ESR spectra of probes incorporated into DPPC liposomes and proteoliposomes. Overall, the presence of TNAP increased the dynamics and decreased the ordering in the three distinct regions probed by the spin labeled lipids DOPTC (headgroup), and 5- and 16-PCSL (acyl chains). The largest change was observed for 16-PCSL, thus suggesting that GPI-anchored TNAP can give rise to long reaching modifications that could influence membrane processes halfway through the bilayer.

Whole blood thrombin generation in Bmal1-deficient mice

The Calibrated Automated Thrombogram (CAT) assay that measures thrombin generation (TG) in platelet-poor and -rich plasma, is increasingly being recognised as a more sensitive tool to determine the overall function of the haemostatic system. We developed a method enabling the measurement of TG in a small aliquot of blood. The objective was to validate this assay in mouse blood and to examine the rate and extent of TG in a mouse model of premature aging. TG was assayed in blood from 20- to 28-week-old brain and muscle ARNT-like protein-1 (Bmal1)-deficient (knockout, KO) mice and wild-type (WT) littermates. Bmal1-KO mice are known to display symptoms of premature aging. TG was initiated by adding calcium, tissue factor and a thrombin specific substrate. After TG, the samples were prepared for scanning electron microscopy (SEM). The intra-assay variations (%) in mouse blood of the endogenous thrombin potential (ETP), peak height, lag time, time-to-peak and velocity index were 10% or less (n=24). We found that Bmal1-KO mice have a significantly (p<0.001) higher ETP (437 ± 7 nM.min; mean ± SD, n=7) when compared with WT mice (ETP=220 ± 45 nM.min; mean ± SD, n=5). The peak heights also differed significantly (p=0.027). By applying SEM we found that Bmal1 deficient mice display a denser fibrin network with smaller pores compared to WT mice. In conclusion, the whole blood TG assay in mice revealed to be reproducible. As a proof-of-principle we have shown that the whole blood TG assay is capable of detecting a prothrombotic phenotype in Bmal1-KO mice.

In vitro sealing of iatrogenic fetal membrane defects by a collagen plug imbued with fibrinogen and plasma

OBJECTIVES

We aimed to demonstrate local thrombin generation by fetal membranes, as well as its ability to generate fibrin from fibrinogen concentrate. Furthermore, we aimed to investigate the efficacy of collagen plugs, soaked with plasma and fibrinogen, to seal iatrogenic fetal membrane defects.

METHODS

Thrombin generation by homogenized fetal membranes was measured by calibrated automated thrombography. To identify the coagulation caused by an iatrogenic membrane defect, we analyzed fibrin formation by optical densitometry, upon various concentrations of fibrinogen. The ability of a collagen plug soaked with fibrinogen and plasma was tested in an ex vivo model for its ability to seal an iatrogenic fetal membrane defect.

RESULTS

Fetal membrane homogenates potently induced thrombin generation in amniotic fluid and diluted plasma. Upon the addition of fibrinogen concentrate, potent fibrin formation was triggered. Measured by densiometry, fibrin formation was optimal at 1250 µg/mL fibrinogen in combination with 4% plasma. A collagen plug soaked with fibrinogen and plasma sealed an iatrogenic membrane defect about 35% better than collagen plugs without these additives (P = 0.037).

CONCLUSIONS

These in vitro experiments suggest that the addition of fibrinogen and plasma may enhance the sealing efficacy of collagen plugs in closing iatrogenic fetal membrane defects.

The hyperfibrinolytic state of mice with combined thrombin-activatable fibrinolysis inhibitor (TAFI) and plasminogen activator inhibitor-1 gene deficiency is critically dependent on TAFI deficiency

BACKGROUND

Mice with single gene deficiency of thrombin-activatable fibrinolysis inhibitor (TAFI) or plasminogen activator inhibitor-1 (PAI-1) have an enhanced fibrinolytic capacity.

OBJECTIVES

To unravel the function and relevance of both antifibrinolytic proteins through the generation and characterization of mice with combined TAFI and PAI-1 gene deficiency.

RESULTS

Mating of TAFI knockout (KO) mice with PAI-1 KO mice resulted in the production of TAFI/PAI-1 double-KO mice that were viable, were fertile, and developed normally. In a tail vein bleeding model, the bleeding time and hemoglobin content of the TAFI/PAI-1 double-KO mice did not deviate significantly from those of the single-KO mice or of the wild-type (WT) counterparts. Interestingly, in ex vivo rotational thromboelastometry measurements with whole blood samples, TAFI KO mice and TAFI/PAI-1 double-KO mice were more sensitive to fibrinolytic activation with tissue-type plasminogen activator than WT or PAI-1 KO mice. This enhanced fibrinolytic capacity was confirmed in vivo in a mouse thromboembolism model, as shown by decreased fibrin deposition in the lungs of TAFI KO mice and TAFI/PAI-1 double-KO mice as compared with WT or PAI-1 KO mice.

CONCLUSIONS

TAFI gene inactivation predominantly contributes to the increased fibrinolytic capacity of TAFI and PAI-1 double-gene-deficient mice, as observed in some basic thrombosis models.

Inhibition of staphylothrombin by dabigatran reduces Staphylococcus aureus virulence

BACKGROUND

Staphylocoagulase and von Willebrand binding protein (VWbp) bind to prothrombin to form the staphylothrombin complex that converts fibrinogen into fibrin.

OBJECTIVES

To study the role of staphylothrombin and its inhibition by dabigatran on Staphylococcus aureus virulence.

METHODS

We studied the effect of staphylothrombin inhibition on bacterial attachment to polystyrene surfaces, leukocyte activation and bactericidal activity for S. aureus ATCC 25923, S. aureus Newman, and staphylocoagulase- and VWbp-negative S. aureus Newman mutants in the presence or absence of prothrombin and fibrinogen. We measured the abscess size after subcutaneous (s.c.) injection of S. aureus ATCC 25923 and S. aureus Newman, as well as an S. aureus Newman mutant strain lacking staphylocoagulase and VWbp, in mice treated with either dabigatran or placebo.

RESULTS

Staphylothrombin-mediated fibrin increased the association of S. aureus to polystyrene surfaces and reduced the bactericidal activity of leukocytes. The absence or inhibition of staphylothrombin decreased the bacterial association, enhanced leukocyte activation and reduced bacterial survival in vitro. Abscess size was smaller in mice treated with dabigatran or infected with a coagulase-negative mutant.

CONCLUSION

Inhibition or the absence of staphylothrombin reduced S. aureus virulence in in vitro and in vivo models.

The effect of air pollution on haemostasis

Ambient environmental air pollutants include gaseous and particulate components. In polluted air, especially particulate matter seems responsible for cardiovascular complications: It consists of a heterogeneous mixture of solid and liquid particles with different diameters ranging from large thoracic to ultrafine particles, with a diameter <100 nm. Ultrafines can penetrate deeply into the lung to deposit in the alveoli. Cardiovascular manifestations result both from short-term and long-term exposure and have been linked to interference with the autonomic nervous system, direct translocation into the systemic circulation, pulmonary inflammation and oxidative stress. Thrombotic complications associated with air pollution comprise arterial and probably venous thrombogenicity. This review describes the existing epidemiological and experimental evidence to explain the rapid induction of myocardial infarction within 1-2 hours after exposure to polluted air and advances several explanations as to why more chronic exposure will lead to enhanced venous thrombogenicity. Mechanisms such as platelet activation, endothelial dysfunction, coagulation factor changes and microvesicle production are discussed.

Short-term exposure to particulate matter induces arterial but not venous thrombosis in healthy mice

BACKGROUND

Epidemiological findings suggest an association between exposure to particulate matter (PM) and venous thrombo-embolism.

OBJECTIVES

  To investigate arterial vs. venous thrombosis, inflammation and coagulation in mice, (sub)acutely exposed to two types of PM.

METHODS

Various doses (25, 100 and 200 μg per animal) of urban particulate matter (UPM) or diesel exhaust particles (DEP) were intratracheally (i.t.) instilled in C57Bl6/n mice and several endpoints measured at 4, 10 and 24 h. Mice were also repeatedly exposed to 100 μg per animal on three consecutive days with endpoints measured 24 h after the last instillation.

RESULTS

Exposure to 200 μg per mouse UPM enhanced arterial thrombosis, but neither UPM nor DEP significantly enhanced venous thrombosis. Both types of PM induced dose-dependent increases in broncho-alveolar lavage fluid (BALF) total cell numbers (mainly neutrophils) and cytokines (IL-6, KC, MCP-1, RANTES, MIP-1α), with peaks at 4 h and overall higher values for UPM than for DEP. Systemic inflammation was limited to increased serum IL-6 levels, 4 h after UPM. Both types of PM induced similar and dose-dependent but modest increases in factor (F)VII, FVIII and fibrinogen. Three repeated instillations did not or only modestly enhance the proinflammatory and procoagulant status.

CONCLUSIONS

Compared with DEP, UPM induced more pronounced pulmonary inflammation, but both particle types triggered similar and mild short-term systemic effects. Hence, acute exposure to PM triggers activation of primary hemostasis in the mouse, but no substantial secondary hemostasis activation, resulting in arterial but not venous thrombogenicity.

Potentiating role of Gas6 and Tyro3, Axl and Mer (TAM) receptors in human and murine platelet activation and thrombus stabilization

BACKGROUND

 Interaction of murine Gas6 with the platelet Gas6 receptors Tyro3, Axl and Mer (TAM) plays an important role in arterial thrombus formation. However, a role for Gas6 in human platelet activation has been questioned.

OBJECTIVE

 To determine the role of Gas6 in human and murine platelet activation and thrombus formation.

METHODS AND RESULTS

 Gas6 levels appeared to be 20-fold higher in human plasma than in platelets, suggesting a predominant role of plasma-derived Gas6. Human Gas6 synergizes with ADP-P2Y(12) by enhancing and prolonging the phosphorylation of Akt. Removal of Gas6 from plasma impaired ADP-induced platelet aggregation. Under flow conditions, absence of human Gas6 provoked gradual platelet disaggregation and integrin α(IIb) β(3) inactivation. Recombinant human Gas6 reversed the effects of Gas6 removal. In mouse blood, deficiency in Gas6 or in one of the TAM receptors led to reduced thrombus formation and increased disaggregation, which was completely antagonized by external ADP. In contrast, collagen-induced platelet responses were unchanged by the absence of Gas6 in both human and mouse systems.

CONCLUSIONS

 The ADP-P2Y(12) and Gas6-TAM activation pathways synergize to achieve persistent α(IIb) β(3) activation and platelet aggregation. We postulate a model of thrombus stabilization in which plasma Gas6, by signaling via the TAM receptors, extends and enhances the platelet-stabilizing effect of autocrine ADP, particularly when secretion becomes limited.

Explanations for coagulation activation after air travel

SUMMARY BACKGROUND

It is unknown whether venous thrombosis after long haul air travel is exclusively attributable to immobilization.

OBJECTIVES

We determined whether the following mechanisms were involved: hypoxia, stress, inflammation or viral infection.

PATIENTS/METHODS

In a case crossover setting in 71 healthy volunteers who were exposed to an 8-h flight, 8-h movie marathon and 8 h of regular activities, we compared markers for several hypothetical pathways: plasminogen activator inhibitor-1 (PAI-1), stress, plasma factor (F)VIII coagulant activity (FVIIIc), soluble P-selectine (sP-selectine), interleukin-8 (IL-8) and neutrophil elastase. We reported earlier an activated clotting system, as evidenced by thrombin generation, in 17% of volunteers after the flight.

RESULTS

PAI-1 increased by 4.2 ng mL(-1) (CI95:-49.5 to 6.5) in volunteers with an activated clotting system whereas it decreased in those without (-20.0 ng mL(-1), CI95:-33.2 to -14.0). FVIIIc levels rose more in individuals with clotting activation (18.0%, CI95:-1.0 to 33.0) than in those without (2.0%, CI95:-2.0 to 5.0). The increases in FVIIIc were not associated with stress, which appeared unrelated to clotting activation. sP-selectin increased in those with clotting activation (3.5 microg L(-1), CI95: -3.0 to 10.0), but decreased in those without (-0.5 microg L(-1), CI95: -2.0 to 2.0). Changes in levels of neutrophil elastase or IL-8 were not different between the subjects with and without clotting activation.

CONCLUSIONS

Our results do not support the hypotheses that stress, infection or air pollution are involved in the development of a prothrombotic state in air travellers. After long haul air travel, this state is more pronounced in patients with risk factors and may be caused by hypoxia, triggering systemic inflammation and platelet activation, leading to coagulation induction and degranulation of platelets.

In vitro evaluation of the ability of platelet-rich plasma to seal an iatrogenic fetal membrane defect

OBJECTIVES

The purpose of the study is to evaluate the ability of platelet-rich plasma (PRP) to seal an iatrogenic fetal membrane defect.

METHODS

First, we evaluated the stability of a PRP plug in an amniotic fluid environment. Further, we evaluated the sealing capability of PRP plugs in an in vitro model that mimics a fetoscopic membrane defect. Finally, we examined its influence on membrane repair and cell proliferation in monolayer cell cultures and amnion-chorion tissue explants.

RESULTS

PRP plugs persisted in an amniotic fluid for a median time of 7 weeks. PRP plugs also provided waterproof sealing of a fetoscopic membrane defect. Finally, PRP stimulated cell proliferation in a monolayer cell culture and provided a good matrix for cell proliferation and migration in amnion-chorion tissue explants.

CONCLUSION

Our in vitro experiments suggest that PRP plugs may provide a long-lasting, waterproof sealing of fetal membrane defects and stimulate fetal membrane repair.

The evolving role of thrombospondin-1 in hemostasis and vascular biology

Thrombospondin-1 (TSP1) is a multi-domain, multi-functional glycoprotein synthesized by many cells. Matricellular TSP1 modulates cell adhesion and proliferation. TSP1 is involved in angiogenesis, inflammation, wound healing and cancer. As a major platelet protein, for a long time it was postulated to control hemostasis via platelet aggregate stabilization. However, these in vitro findings have been questioned in the absence of corroborating clinical data and of obvious hemostatic defects in TSP1 gene-deficient mice.Yet, the past few years have provided indices to implicate TSP1 in hemostasis. In clinical studies, a correlation exists between a welldefined TSP1 polymorphism and a significant risk of myocardial infarction. At the same time, recent in vivo animal model data imply TSP1 in the multimer size control of von Willebrand factor, in smooth muscle cell regulation and in vascular perfusion. These findings shed new light on the role of TSP1 in hemostasis and prothrombotic vascular pathologies. (Part of a Multi-author Review).

Major coagulation disturbances during fractionated plasma separation and adsorption

Fractionated Plasma Separation and Adsorption (FPSA) is a novel nonbiologic detoxification system for the removal of protein-bound solutes. FPSA is used to bridge patients during fulminant liver failure, either to functional recovery or to liver transplantation. Besides liver failure associated protein bound solutes, several important uremic retention solutes share important protein binding. We observed repeated occlusive thrombosis of the arterio-venous conduit during FPSA in hemodialysis (HD) patients, resulting in acute loss of function. A major reduction of several coagulation factors was demonstrated, exceeding 50% for factor II, factor X and protein C. Broad disturbances of the coagulation system were confirmed in FPSA treated liver failure patients. An ex vivo recirculation model demonstrated nonspecific adsorption of coagulation factors protein S and protein C on the anion exchange cartridge. Direct contact between fractionated plasma and the Prometh02 anion exchanger causes significant adsorption of procoagulant and anti-coagulant factors, associated with clinically relevant adverse events.

Enhanced peripheral thrombogenicity after lung inflammation is mediated by platelet-leukocyte activation: role of P-selectin

BACKGROUND

Inhaled ultrafine particles trigger peripheral thrombotic complications.

METHODS

We have analyzed the systemic prothrombotic risk following lung inflammation induced by pulmonary carbon nanotubes (CNTs).

RESULTS

Intratracheal instillation in Swiss mice of 200 and 400 microg of multiwall ground CNTs triggered substantial lung neutrophil, but not macrophage influx, 24 h later. The detection of circulating platelet-leukocyte conjugates exclusively 6 h after CNT instillation pointed to early but transient activation of circulating platelets. At 24 h, elevated plasma procoagulant microvesicular tissue factor activity was found in CNT-exposed but not in saline-exposed mice. However, at 24 h, both the tail and jugular vein bleeding times were prolonged in CNT-exposed but not in saline-exposed mice, arguing against strong CNT-induced platelet activation at this point. Nevertheless, at 24 h, enhanced peripheral thrombogenicity was detected in CNT-exposed but not in saline-exposed mice, via quantitative photochemically induced carotid artery thrombosis measurements. P-selectin neutralization abrogated platelet-leukocyte conjugate formation and microvesicular tissue factor generation, and abolished the CNT-induced thrombogenicity amplification. In contrast, the weak vascular injury-triggered thrombus formation in saline-treated mice was not affected by P-selectin neutralization at 24 h.

CONCLUSIONS

The mild CNT-induced lung inflammation translates via rapid but mild and transient activation of platelets into P-selectin-mediated systemic inflammation. Leukocyte activation leads to tissue factor release, in turn eliciting inflammation-induced procoagulant activity and an associated prothrombotic risk.

Haemostasis

When the continuity of the vascular endothelium is disrupted, platelets and fibrin seal off the defect. Haemostatic processes are classified as primary (mainly involving platelets) and secondary (mainly related to fibrin formation or blood coagulation). When the blood clot is no longer required for haemostasis, the fibrinolytic system will dissolve it. The pivotal ligand for initial platelet recruitment to injured vessel wall components is von Willebrand factor (vWF), a multimeric protein present in the subendothelium and in plasma, where it is conformationally activated by shear forces. Adhering activated platelets recruit additional platelets, which are in turn activated and form a platelet aggregate. Coagulation is initiated by a reaction, activating factors IX and X. Once critical amounts of factor Xa are generated, thrombin generation is initiated and soluble fibrinogen is converted into insoluble fibrin. Excessive thrombin generation is prevented via inhibition by antithrombin and also via downregulation of its further generation by activation of the protein C pathway. Activation of the fibrinolytic system results from conversion of the proenzyme plasminogen into the active serine proteinase plasmin by tissue-type or urokinase-type plasminogen activators. Plasmin digests the fibrin component of a blood clot. Inhibition of the fibrinolytic system occurs at the level of the plasminogen activator (by plasminogen activator inhibitors) or at the level of plasmin (by alpha2-antiplasmin). Together, these physiological processes act to maintain normal functioning blood vessels and a non-thrombotic state.

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

BACKGROUND

Binding of von Willebrand factor (VWF) to platelet GPIbalpha and to collagen is attributed to VWF A1 and A3 domains, respectively.

OBJECTIVES

Using VWF, VWF lacking A1 (DeltaA1-VWF) or A3 (DeltaA3-VWF) and VWF with defective A3 (H1786A-VWF), in combination with recombinant A1 (residues 1262-1492) or A3 (residues 1671-1878), fused to glutathione-S-transferase (GST-A1 and GST-A3), we have re-investigated the role of A1 in platelet recruitment to surfaces of collagen.

METHODS AND RESULTS

In flow, measurable binding of DeltaA3-VWF occurred to horse tendon, but also to human type III collagen. GST-A1 and GST-A3 both competed for binding of DeltaA1-VWF and DeltaA3-VWF to horse tendon collagen fibrils in static conditions and to human collagen III during plasmon surface resonance studies, substantiating overlapping binding sites on both collagens for A1 and A3. Heparin did not affect A3-mediated binding of VWF and DeltaA1-VWF, but inhibited binding to horse tendon collagen of GST-A1 and DeltaA3-VWF. Furthermore, A1-mediated binding to type III collagen of DeltaA3-VWF binding was strongly salt-sensitive. During perfusions at wall shear rate 2500 s(-1) of calcein-labeled platelets in reconstituted blood, DeltaA3-VWF and H1786A-VWF triggered platelet binding to horse tendon collagen comparably and as potently as VWF, and to human type III collagen, only fivefold less potently, DeltaA1-VWF being inactive. Additional flow-controlled interaction studies with DeltaA3-VWF, H1786A-VWF, the collagen-VWF antagonist saratin, heparin and the VWF neutralizing antibody 82D6A3 confirmed that H1786A-VWF binds to collagen exclusively via A1.

CONCLUSION

Hence, in shear forces the VWF A1 domain can assume the role of A3 to trigger substantial platelet recruitment to human collagen fibres.

The pivotal role of the endothelium in haemostasis and thrombosis

The endothelium plays a pivotal role in the regulation of the haemostatic balance. The function of endothelial cells exceeds far beyond providing a non-thrombogenic inner layer of the vascular wall which maintains the blood fluidity. In physiological circumstances endothelial cells carefully prevent thrombosis by different anticoagulant and antiplatelet mechanisms. Endothelial cells are involved in all major haemostatic pathways upon vascular injury and limit clot formation to the areas where haemostasis is needed to restore vascular integrity. Failure of this complex balance between pro- and anticoagulant systems because of genetic or acquired disturbances may result in bleeding or thrombosis. Endothelial heterogeneity assures adequate homeostasis in the different organs and parts of the vascular tree. The local environment induces heterogeneous endothelial cell phenotypes determined by local needs. This heterogeneity also explains the diverse pathological responses upon disturbed vascular integrity. Localised manifestation of thrombosis in spite of systemic procoagulant disturbances depends on vascular bed-specific properties. Endothelial dysfunction not only precedes atherogenesis but may also predispose to arterial thrombosis. The potential role of the endothelium in venous thrombosis with and without overt vessel wall injury is discussed. The vast majority of endothelial cells are located in the microvessels. Therefore, it is no surprise that endothelial cells play a key role in microcirculatory diseases such as thrombotic microangiopathies and diffuse intravascular coagulation. Microcirculatory endothelial cell activation is an important feature in all thrombotic microangiopathies. In diffuse intravascular coagulation, the endothelium is the interface between inflammation and inappropriate activation of the coagulation system.

The platelet ATP and ADP receptors

Adenine nucleotides, ADP and ATP, are coreleased from dense granules during platelet activation, as well as from endothelial cells and damaged red blood cells following vascular injury. Through autocrine and paracrine mechanisms, these extracellular signaling molecules interact with the platelet P2 receptors to amplify ongoing platelet activation. Two receptors for ADP, the G(q)-protein-coupled P2Y1 and G(i)-protein-coupled P2Y12 and one receptor for ATP, the P2X1 ion channel, have been identified on platelets. Due to distinct pharmacological properties and differential regulation, the P2Y and P2X receptors essentially operate on different scales of time and distance and trigger selective intracellular signaling cascades. Recent advances in the understanding of the P2Y receptor physiology have reinforced the concept of these receptors as useful targets for antithrombotic therapy. The function of P2X1 in platelet activation only recently started to be unraveled. This review focuses on recent findings on the physiology of these platelet ADP and ATP receptors, their distinct downstream intracellular signaling pathways as well as on the available agonists, antagonists and inhibitors that allow their pharmacological discrimination.

ERK2 activation in arteriolar and venular murine thrombosis: platelet receptor GPIb vs. P2X

The functional significance of extracellular signal-regulated kinase 2 (ERK2) activation was investigated during shear induced human platelet aggregation (SIPA) in vitro and during shear controlled thrombosis in vivo in intestinal arterioles and venules of wild type (WT) and transgenic (TG) mice with platelet-specific overexpression of human P2X(1) (TG). In SIPA, ERK2 was rapidly phosphorylated during GPIb stimulation, its activation contributing to SIPA for 50%, independently of P2X(1) regulation. Thrombotic occlusion of injured arterioles occurred considerably faster in TG (4.3 +/- 2.3 min) than in WT (38 +/- 8 min) arterioles, but occlusion times in TG (19 +/- 12) and WT (48 +/- 4.5 min) venules differed less. Both the alphabeta-meATP triggered desensitization of platelet P2X(1), as well as P2X(1) antagonism by NF279 or NF449 prolonged mean occlusion to about 75 min in WT and 65 min in TG arterioles, but venular occlusion times were less affected. Preventing ERK2 activation by U0126 prolonged occlusion times in TG (41 +/- 10 min) and WT (51 +/- 17) arterioles more than in TG (46 +/- 5 min) and WT (56 +/- 6 min) venules, uncovering a role for ERK2 in shear controlled thrombosis. Antagonism of GPIb by a recombinant murine von Willebrand factor (VWF)-A1 fragment prolonged occlusion times to comparable values, ranging from 55 to 58 min, both in TG and WT arterioles and venules. Further inhibition strategies, combining VWF-A1, U0126 and NF449 in WT and TG mice and resulting in occlusion in various time windows, identified that inhibition by VWF-A1 largely abrogated the ERK2 contribution to thrombosis. In conclusion, P2X(1) and ERK2 both participate in shear stress controlled thrombosis, but ERK2 activation is initiated predominantly via GPIb-VWF interactions.

Ambient air pollution and acute myocardial infarction

This review summarizes the nature of ambient air pollutants, which are either gaseous or particulate of various sizes, the latter determining their penetration into the body, the smallest even translocating from the lung into the systemic circulation. It presents the epidemiological evidence linking air pollution to overall mortality, cardiovascular mortality and myocardial infarction, making the distinction between acute and chronic exposure to the pollutants. It reviews mechanistic investigations that have evaluated the links among exposure to pollutants, thrombosis, pulmonary inflammation, arterial vasoconstriction and heart rate variability. It concludes by attempting to integrate current epidemiological and mechanistic observations into a pathophysiological framework that links ambient air pollution to acute myocardial infarction and cardiovascular mortality.

alphaIIbbeta3 antagonism vs. antiadhesive treatment to prevent platelet interactions with vascular subendothelium

Platelets adhering to blood vessels promote coagulation and inflammation, and release growth factors that trigger smooth muscle cell activation. We have therefore studied the pharmacological modification of platelet deposition quantitatively by comparing adhesion of flowing platelets to various subendothelial ligands in the absence or presence of an antialpha(IIb)beta(3) antagonist with the effects of antiadhesive treatment consisting of von Willebrand factor (VWF) and fibronectin neutralization or of the combined inhibition of platelet adhesion and aggregation. In vitro, perfusion of anticoagulated human blood over calf skin collagen reiterated that alpha(IIb)beta(3) antagonism prevents platelet aggregation, but not adhesion per se: single platelets strongly bound to collagen at wall shear rates of both 1300 and 2700 s(-1), largely VWF-independent. When perfused over a human umbilical vein endothelial cell-derived extracellular matrix, single alpha(IIb)beta(3)-antagonized platelets primarily adhered to matrix-bound VWF when perfused at 2700 s(-1), but at 1300 s(-1) they also adhered significantly to fibronectin. During perfusion of anticoagulated rabbit blood over de-endothelialized rabbit aorta at a wall shear rate of 1100 s(-1), alpha(IIb)beta(3) antagonism even increased the absolute numbers of adhering platelets and VWF neutralization redirected alpha(IIb)beta(3)-antagonized platelets towards other vascular ligands. Finally, in vivo, following photochemically induced blood vessel injury in mice, alpha(IIb)beta(3) antagonism inhibited platelet-rich thrombus formation, but platelet adhesion was only significantly inhibited when associated with fibronectin neutralization. In conclusion, antiadhesive platelet treatment more potently interferes with platelet deposition on injured blood vessels than alpha(IIb)beta(3) antagonism, but abrogating platelet adhesion can only be achieved by carefully selected antiplatelet drug combinations.

Air pollution and thrombosis: an experimental approach

Air pollution is associated with cardiovascular mortality. Inhaled ultrafine particles translocate into the blood. Amine-polystyrene ultrafine particles significantly enhance experimental thrombus formation in a damaged hamster vessel and shorten the closure time in the Platelet Function Analyser. Diesel exhaust particles are thrombogenic within one hour of intratracheal instillation and shorten the closure time ex vivo. These experimental observations provide a plausible biological explanation for the epidemiologically established link between air pollution and acute myocardial infarction.

Passage of inhaled particles into the blood circulation in humans

BACKGROUND

Pollution by particulates has been consistently associated with increased cardiovascular morbidity and mortality. However, the mechanisms responsible for these effects are not well-elucidated.

METHODS AND RESULTS

To assess to what extent and how rapidly inhaled pollutant particles pass into the systemic circulation, we measured, in 5 healthy volunteers, the distribution of radioactivity after the inhalation of "Technegas," an aerosol consisting mainly of ultrafine (99m)Technetium-labeled carbon particles (<100 nm). Radioactivity was detected in blood already at 1 minute, reached a maximum between 10 and 20 minutes, and remained at this level up to 60 minutes. Thin layer chromatography of blood showed that in addition to a species corresponding to oxidized (99m)Tc, ie, pertechnetate, there was also a species corresponding to particle-bound (99m)Tc. Gamma camera images showed substantial radioactivity over the liver and other areas of the body.

CONCLUSIONS

We conclude that inhaled (99m)Tc-labeled ultrafine carbon particles pass rapidly into the systemic circulation, and this process could account for the well-established, but poorly understood, extrapulmonary effects of air pollution.

Anti-vWf antibodies induce GPIbalpha and FcgammaRII mediated platelet aggregation only at low shear forces

BACKGROUND/METHODS

Anti-von Willebrand factor (vWf) antibody mediated platelet activation was studied using 2 monoclonal anti-vWf antibodies promoting the binding of vWf to GPIbalpha: 1C1E7 (IgG2a) reacting with the vWf N-terminus and 75H4B12 (IgM), characterized in this paper and studied in association with 1C1E7.

RESULTS

75H4B12 binds to an N-terminal epitope in vWf, different from that reacting with 1C1E7. When com-bined, 1C1E7 and 75H4B12 promoted vWf binding to isolated GPIb under static conditions, even in the absence of ristocetin or botrocetin, and induced platelet aggregation synergistically in the presence of zero to subthreshold ristocetin concentrations. Specific inhibitors of GPIbalpha-vWf interactions prevented vWf binding to GPIbalpha in ELISA and during platelet aggregation. In addition, the 1C1E7 dependent platelet aggregation involved Fc receptor mediated platelet activation, a phenomenon even more pronounced when 1C1E7 and 75H4B12 were combined. A 75H4B12 binding phage expressing a peptide homologous with vWf sequence 88-95 neutralized the antibody induced platelet activation. However, at arterial shear rates, both 1C1E7 and 75H4B12 potently prolonged cartridge closure times in the PFA-100, compatible with inhibition of platelets by vWf, unfolded by the combined action of shear stress and antibodies.

CONCLUSIONS

We conclude that antibodies directed against different epitopes in the N-terminus of vWf modify the folded vWf structure synergistically and enhance A1 domain mediated vWf binding to platelet GPIb at low shear forces. In addition, once platelet-bound, IgG antibodies potently activate platelets via the FcgammaII receptor. Thus, such antibodies may promote immune mediated thrombosis at low shear rates, typical of the venous circulation. In contrast, at arterial shear rates, anti-vWf antibodies may rather compromise platelet function following enhanced binding of the unfolded vWf multimers to platelets, shielding platelets from interacting with subendothelial and soluble ligands.

The ATP-gated P2X1 ion channel acts as a positive regulator of platelet responses to collagen

ATP is a potent agonist of the P2X1 ion channel, mediating a rapid, quickly desensitized influx of Ca2+. In hirudinized PRP, containing apyrase, the two stable selective P2X1 agonists, alpha,beta-methylene ATP, and L-beta,gamma-methylene ATP induced extracellular Ca2+-dependent fast and reversible platelet shape change, leading to desensitization of the P2X1 ion channel. Preincubation with HPLC-purified ADP potently antagonized the subsequent alpha,beta-methylene ATP- and L-beta,gamma-methylene ATP-evoked platelet shape change. Accordingly, upon heterologous expression of P2X1 in Xenopus oocytes. HPLC-purified ADP acted as an antagonist of the ATP-induced current, but was inactive itself. Since ATP and ADP are co-released from dense granules during platelet activation, we investigated whether the P2X1 ion channel is involved in the response of platelets to collagen. We found that platelet shape change and aggregation induced by low concentrations of collagen were strongly inhibited after selective desensitization of P2X1 with its agonists or by pretreating the platelets with a low concentration of ADP (0.5 microM), that antagonizes the P2X1 channel without desensitizing the P2Y1 receptor. Our data suggest that, during collagen-initiated platelet activation, the early secretion of ATP results in the activation of the P2X1 ion channel, which plays a role as a positive regulator of further platelet responses.

Passage of intratracheally instilled ultrafine particles from the lung into the systemic circulation in hamster

The mechanisms of particulate pollution-related cardiovascular morbidity and mortality are not well understood. We studied the passage of radioactively labeled ultrafine particles after their intratracheal instillation. Hamsters received a single intratracheal instillation of 100 microg albumin nanocolloid particles (nominal diameter < or = 80 nm) labeled with 100 microCi technetium-99m and were killed after 5, 15, 30, and 60 min. In blood, radioactivity, expressed as percentage of total body radioactivity per gram blood, amounted to 2.88 +/- 0.80%, 1.30 +/- 0.17%, 1.52 +/- 0.46%, and 0.21 +/- 0.06% at 5, 15, 30, and 60 min, respectively. Thin-layer chromatography showed only one peak of radioactivity corresponding to unaltered (99m)Tc-albumin nanocolloid. In the liver, radioactivity, expressed as percentage of total radioactivity per organ, amounted to 0.10 +/- 0.07%, 0.23 +/- 0.06%, 1.24 +/- 0.27%, and 0.06 +/- 0.02% at 5, 15, 30, and 60 min, respectively. Lower values were observed in the heart, spleen, kidneys, and brain. Dose dependence was assessed at 30 min following instillation of 10 microg and 1 microg (99m)Tc-albumin per animal (n = 3 at each dose), and values of the same relative magnitudes as after instillation of 100 microg were obtained. We conclude that a significant fraction of (99m)Tc-albumin, taken as a model of ultrafine particles, rapidly diffuses from the lungs into the systemic circulation.

Genetic variation of the extra-large stimulatory G protein alpha-subunit leads to Gs hyperfunction in platelets and is a risk factor for bleeding

Alternatively spliced GNAS1 and XL-GNAS1, encoding respectively the stimulatory G-protein alpha-subunit (Gsalpha) and the extra-large stimulatory G-protein alpha-subunit (XLsalpha), are located on the imprinted chromosomal region 20q13.12-13. We presently report a functional polymorphism in the imprinted XL-GNAS1 gene. In three patients, a 36 bp insertion and two basepair substitutions flanking this insertion were found in the paternally inherited XL-GNAS1 exon 1. They clinically manifest an enhanced trauma-related bleeding tendency and a variable degree of mental retardation. A platelet aggregation inhibition test to evaluate Gs function was developed. Their platelets display Gs hyperfunction and an enhanced cAMP generation upon stimulation of Gs-coupled receptors. The prevalence of the XLsalpha insertion in a normal control group was 2.2%. Normal controls, inheriting the insertion maternally, had a normal platelet Gs activity, whereas controls inheriting the insertion paternally had increased inducible platelet Gs activity, defining the insertion as a functional polymorphism. This paternally inherited XLsalpha insertion represents a new genetic cause of an inherited bleeding tendency, although to a variable degree.

Production and characterization of saratin, an inhibitor of von Willebrand factor-dependent platelet adhesion to collagen

Platelets tether to collagen in both a von Willebrand factor (vWF)-dependent and a vWF-independent manner. We have recently characterized a recombinant protein, saratin, isolated from the saliva of the leech Hirudo medicinalis, expressed it in Hansenula polymorpha, and studied its effect on direct and indirect platelet-collagen interactions. Saratin dose dependently inhibited the binding of purified human vWF to human type I and III collagens (IC(50)= 0.23 +/- 0.004 and 0.81 +/- 0.04 microg mL(-1), respectively) and to calf skin collagen (IC(50)= 0.44 +/- 0.008 microg mL(-1)). Furthermore, saratin showed a similar inhibitory potency against the binding of human, rodent, and porcine plasma vWF to these collagens. In a flow chamber under conditions of elevated shear (2700 s(-1)), saratin dose dependently and potently inhibited platelet aggregate formation on a collagen-coated surface (IC(50)= 0.96 +/- 0.25 microg mL(-1)), but at reduced shear (1300 s(-1)) a rightward shift in the dose-response curve was noted (IC(50)= 5.2 +/- 1.4 microg mL(-1)). Surface plasmon resonance analysis revealed both high and low affinity binding sites for saratin on human collagen type III (K(d) 5 x 10(-8) M and 2 x 10(-6) M, respectively). Although low concentrations of saratin, which inhibited platelet adhesion under increased shear (i.e., saturation of high-affinity binding sites), had no effect on vWF-independent collagen-induced platelet aggregation, high concentrations (i.e., saturation of low-affinity binding sites) were found to inhibit platelet aggregation. These data demonstrate that saratin is a potent inhibitor of vWF-dependent platelet adhesion to collagen and hence may have therapeutic potential as an antithrombotic agent.

Congenital disorders of glycosylation type Ia and IIa are associated with different primary haemostatic complications

Congenital disorders of glycosylation (CDG) type I are mostly due to a deficient phosphomannomutase activity, called CDG Ia. CDG IIa (mutations in the MGAT2 gene) results from a deficient activity of the Golgi enzyme N-acetylglucosaminyltransferase II. CDG Ia patients predominantly have a thrombotic tendency, whereas our CDG IIa patient has an increased bleeding tendency, despite similar coagulation factor abnormalities in both types. We have investigated whether abnormally glycosylated platelet membrane glycoproteins are involved in the haemostatic complications of both CDG groups. In flow cytometry, the binding of Ricinus communis lectin (reactive with beta-galactose primarily) to control platelets increased after neuraminidase treatment: this increase was smaller (p < 0.01) in CDG Ia patients (3.1 +/- 0.08 times) than in control platelets (8.5 +/- 1.8 times) and did not occur in the CDG IIa patient. Platelet-rich plasma from CDG Ia patients, but not a CDG IIa patient. aggregated spontaneously and gel-filtered platelets from CDG Ia patients agglutinated at very low concentrations of ristocetin, independently of von Willebrand factor (vWF). Accordingly, in stirred whole blood, the rate of single platelet disappearance of CDG Ia patients was twice that of control platelets. In contrast, perfusion of whole anticoagulated blood of the CDG IIa patient over collagen yielded markedly decreased platelet adherence to collagen at shear rates involving glycoprotein (GP) Ib-vWF interactions. Thus, abnormal glycosylation of platelet glycoproteins in CDG Ia enhances nonspecific platelet interactions, in agreement with a thrombotic tendency. The reduced GP Ib-mediated platelet reactivity with vessel wall components in the CDG IIa patient under flow conditions provides a basis for his bleeding tendency.

Identification of a target site in plasminogen activator inhibitor-1 that allows neutralization of its inhibitor properties concomitant with an allosteric up-regulation of its antiadhesive properties

The serpin plasminogen activator inhibitor-1 (PAI-1) has a dual function: 1) it plays an important role as a direct inhibitor of the plasminogen activation system, and 2) its interaction with the adhesive glycoprotein vitronectin suggests a role in tissue remodeling and metastasis, independent from its proteinase inhibitory properties. Unique to this serpin is the close association between its conformational and functional properties. Indeed, PAI-1 can occur in an active and a latent conformation, but both functions are exclusively present in the active conformation. We report here the epitope localization and functional effects of a monoclonal antibody (MA-124K1) that inhibits rat PAI-1 activity and simultaneously increases the binding of inactive PAI-1 to vitronectin (the affinity constant of PAI-1 for vitronectin is 2 x 10(7) m(-1) in the absence of MA-124K1 and 160 x 10(7) m(-1) in the presence of MA-124K1). To the best of our knowledge, this is the first monoclonal antibody dissociating the proteinase inhibitory properties from the vitronectin binding properties in PAI-1. Mutation of Glu(212) and/or Glu(220) in rat PAI-1 to Ala results in a strongly reduced affinity or absence of binding to MA-124K1. The three-dimensional structure of PAI-1 reveals that these residues constitute a conformational epitope close to the reactive-site loop and compatible with the effect of MA-124K1 on the inhibitory properties of PAI-1. However, the vitronectin binding site is localized at the opposite site of the molecule, indicating that the effect of MA-124K1 involves an allosteric modulation of the vitronectin binding site. Cell culture experiments revealed a significant reduction of cell attachment and migration in the presence of MA-124K1, providing evidence for the functional relevance of this antibody-mediated up-regulation of the vitronectin binding properties of PAI-1. In conclusion, a novel mechanism for interference with PAI-1 functions has been identified and is of importance in the modulation of cell migration and related events (e.g. tumor metastasis).

Platelet characteristics in patients with X-linked macrothrombocytopenia because of a novel GATA1 mutation

A new mutation is described in the X-linked gene GATA1, resulting in macrothrombocytopenia and mild dyserythropoietic features but no marked anemia in a 4-generation family. The molecular basis for the observed phenotype is a substitution of glycine for aspartate in the strictly conserved codon 218 (D218G) of the amino-terminal zinc finger loop of the transcription factor GATA1. Zinc finger interaction studies demonstrated that this mutation results in a weak loss of affinity of GATA1 for its essential cofactor FOG1, whereas direct D218G-GATA1 binding to DNA was normal. The phenotypic effects of this mutation in the patients' platelets have been studied. Semiquantitative RNA analysis, normalized for beta-actin messenger RNA, showed extremely low transcription of the GATA1 target genes GPIbbeta and GPIX but also a significantly lower expression of the nondirectly GATA1-regulated Gsalpha gene, suggestive of incomplete megakaryocyte maturation. In contrast, GPIIIa expression was close to normal in agreement with its early appearance during megakaryocyte differentiation. Flow cytometric analysis of patient platelets confirmed the existence of a platelet population with abnormal size distribution and reduced GPIb complex levels but with normal GPIIIa expression. It also showed the presence of very immature platelets lacking almost all membrane glycoproteins studied (GPIbalpha, GPIbbeta, GPIIIa, GPIX, and GPV). Patients' platelets showed weak ristocetin-induced agglutination, compatible with the disturbed GPIb complex. Accordingly, electron microscopy of the patients' platelets revealed giant platelets with cytoplasmic clusters consisting of smooth endoplasmic reticulum and abnormal membrane complexes. In conclusion, GATA1 mutations can lead to isolated X-linked macrothrombocytopenia without anemia.