Human Endothelial Cells in Culture and In Vivo Express on Their Surface All Four Components of the Glycoprotein Ib/IX/V Complex

Viral Sepsis in Children

Sepsis in children is typically presumed to be bacterial in origin until proven otherwise, but frequently bacterial cultures ultimately return negative. Although viruses may be important causative agents of culture-negative sepsis worldwide, the incidence, disease burden and mortality of viral-induced sepsis is poorly elucidated. Consideration of viral sepsis is critical as its recognition carries implications on appropriate use of antibacterial agents, infection control measures, and, in some cases, specific, time-sensitive antiviral therapies. This review outlines our current understanding of viral sepsis in children and addresses its epidemiology and pathophysiology, including pathogen-host interaction during active infection. Clinical manifestation, diagnostic testing, and management options unique to viral infections will be outlined.

Selective Tropism of Dengue Virus for Human Glycoprotein Ib

Since the hemorrhage in severe dengue seems to be primarily related to the defect of the platelet, the possibility that dengue virus (DENV) is selectively tropic for one of its surface receptors was investigated. Flow cytometric data of DENV-infected megakaryocytic cell line superficially expressing human glycoprotein Ib (CD42b) and glycoprotein IIb/IIIa (CD41 and CD41a) were analyzed by our custom-written software in MATLAB. In two-dimensional analyses, intracellular DENV was detected in CD42b+, CD41+ and CD41a+ cells. In three-dimensional analyses, the DENV was exclusively detected in CD42b+ cells but not in CD42b- cells regardless of the other expressions. In single-cell virus-protein analyses, the amount of DENV was directly correlated with those of CD42b at the Pearson correlation coefficient of 0.9. Moreover, RT- PCR and apoptosis assays showed that DENV was able to replicate itself and release its new progeny from the infected CD42b+ cells and eventually killed those cells. These results provide evidence for the involvement of CD42b in DENV infection.

Cab4b, the first human platelet antigen carried by glycoprotein IX discovered in a context of severe neonatal thrombocytopenia

Essentials Life-threatening maternofetal thrombocytopenias mostly depend on αIIb β3 antigens. We performed serological, genomic and in vitro studies of two life-threatening thrombocytopenias. Identification of a c.368C>T variation leading to Pro123Leu substitution in GPIX. A rare GPIX variant reported in a genomic database define a new alloantigen.

SUMMARY

Background After three miscarriages, a 39-year-old woman gave birth, with a 1-year interval, to two severely thrombocytopenic neonates (4 ×109 L-1 and 33 ×109 L-1 ) with intracranial hemorrhages. Transfusion of platelet concentrates corrected the thrombocytopenia. The outcome was favorable for the first child, but the second one died 10 days after cesarean delivery (31 weeks of gestation + 6 days). Methods Serologic studies were performed with mAb-specific immobilization of platelet antigens and flow cytometry techniques. Human platelet alloantigen (HPA) genotyping was performed with the BioArray HPA BeadChip and PCR-sequence-specific primer techniques. Genomic DNA was studied by direct sequencing of PCR products. The mutant glycoprotein (GP) was expressed in transiently transfected HEK293 cells. Results In MAIPA assay, the maternal serum faintly reacted with GPIbIX from paternal and child 1 platelets, but not with maternal or panel platelets. No maternofetal incompatibility was found in the 22 known HPA systems, tested except for HPA-1b in child 2. A new alloantigen carried by GPIbIX was suspected. Genomic sequencing revealed a paternal GPIX variation (NM_000174.4:c.368C>T). The father and children were heterozygous and incompatible with the mother, who was NM_000174.4:c.368C homozygous. The maternal serum reacted with the GPIX NP_000165.1:p.Leu123 form coexpressed with GPIb in transfected HEK293 cells. The NM_000174.4:c.368T allele (rs202229101) has a minor allele frequency of 0.0002, and was not detected in 120 French subjects (families with fetal and neonatal alloimmune thrombocytopenia [FNAIT]), suggesting that it is rarely implicated in alloimmunization. Conclusion The NP_000165.1:p.Leu123 allele named Cab4b is the first platelet alloantigen described on GPIX. In the absence of other known maternofetal incompatibility, the child 1 case suggests that anti-Cab4b alloantibodies can induce severe thrombocytopenias.

Platelets and platelet alloantigens: Lessons from human patients and animal models of fetal and neonatal alloimmune thrombocytopenia

Platelets play critical roles in hemostasis and thrombosis. Emerging evidence indicates that they are versatile cells and also involved in many other physiological processes and disease states. Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a life threatening bleeding disorder caused by fetal platelet destruction by maternal alloantibodies developed during pregnancy. Gene polymorphisms cause platelet surface protein incompatibilities between mother and fetus, and ultimately lead to maternal alloimmunization. FNAIT is the most common cause of intracranial hemorrhage in full-term infants and can also lead to intrauterine growth retardation and miscarriage. Proper diagnosis, prevention and treatment of FNAIT is challenging due to insufficient knowledge of the disease and a lack of routine screening as well as its frequent occurrence in first pregnancies. Given the ethical difficulties in performing basic research on human fetuses and neonates, animal models are essential to improve our understanding of the pathogenesis and treatment of FNAIT. The aim of this review is to provide an overview on platelets, hemostasis and thrombocytopenia with a focus on the advancements made in FNAIT by utilizing animal models.

Hepatic sinusoidal endothelium avidly binds platelets in an integrin-dependent manner, leading to platelet and endothelial activation and leukocyte recruitment

Platelets have recently been shown to drive liver injury in murine models of viral hepatitis and promote liver regeneration through the release of serotonin. Despite their emerging role in inflammatory liver disease, little is known about the mechanisms by which platelets bind to the hepatic vasculature. Therefore, we referenced public expression data to determine the profile of potential adhesive receptors expressed by hepatic endothelium. We then used a combination of tissue-binding and flow-based endothelial-binding adhesion assays to show that resting platelets bind to human hepatic sinusoidal endothelial cells and that the magnitude of adhesion is greatly enhanced by thrombin-induced platelet activation. Adhesion was mediated by the integrins Gp1b, αIIbβIII, and αvβ3, as well as immobilized fibrinogen. Platelet binding to hepatic endothelial cells resulted in NF-κB activation and increased chemokine secretion. The functional relevance of platelet binding was confirmed by experiments that showed markedly increased binding of neutrophils and lymphocytes to hepatic endothelial cells under shear conditions replicating those found in the hepatic sinusoid, which was in part dependent on P-selectin expression. Thus the ability of platelets to activate endothelium and promote leukocyte adhesion may reflect an additional mechanism through which they promote liver injury.

Platelet receptors

Well-understood functions for "traditional" platelet receptors are described, but "newer" receptors are equally discussed. Receptors are described biochemically (structure, ligand(s), protein partners, and function) and whenever possible, their clinical importance (mutations, polymorphisms, syndrome) are highlighted.

The maternal immune response to fetal platelet GPIbα causes frequent miscarriage in mice that can be prevented by intravenous IgG and anti-FcRn therapies

Fetal and neonatal immune thrombocytopenia (FNIT) is a severe bleeding disorder caused by maternal antibody-mediated destruction of fetal/neonatal platelets. It is the most common cause of severe thrombocytopenia in neonates, but the frequency of FNIT-related miscarriage is unknown, and the mechanism(s) underlying fetal mortality have not been explored. Furthermore, although platelet αIIbβ3 integrin and GPIbα are the major antibody targets in immune thrombocytopenia, the reported incidence of anti-GPIbα-mediated FNIT is rare. Here, we developed mouse models of FNIT mediated by antibodies specific for GPIbα and β3 integrin and compared their pathogenesis. We found, unexpectedly, that miscarriage occurred in the majority of pregnancies in our model of anti-GPIbα-mediated FNIT, which was far more frequent than in anti-β3-mediated FNIT. Dams with anti-GPIbα antibodies exhibited extensive fibrin deposition and apoptosis/necrosis in their placentas, which severely impaired placental function. Furthermore, anti-GPIbα (but not anti-β3) antiserum activated platelets and enhanced fibrin formation in vitro and thrombus formation in vivo. Importantly, treatment with either intravenous IgG or a monoclonal antibody specific for the neonatal Fc receptor efficiently prevented anti-GPIbα-mediated FNIT. Thus, the maternal immune response to fetal GPIbα causes what we believe to be a previously unidentified, nonclassical FNIT (i.e., spontaneous miscarriage but not neonatal bleeding) in mice. These results suggest that a similar pathology may have masked the severity and frequency of human anti-GPIbα-mediated FNIT, but also point to possible therapeutic interventions.

Platelet binding and activity of a factor VIIa variant with enhanced tissue factor independent activity

BACKGROUND AND OBJECTIVES

Platelet binding and activity play important roles in the efficacy of factor VIIa (FVIIa) as a bypassing agent for hemophilia treatment. An analog of FVIIa with increased tissue factor (TF)-independent activity, NN1731, has been produced by introducing three amino acid changes in the protease domain. NN1731 has a conformation similar to TF-bound FVIIa, even in the absence of TF. This results in much greater intrinsic proteolytic activity, but similar activity in the presence of TF.

OBJECTIVES

We hypothesized that these changes would not alter binding to platelets or phospholipid, a characteristic thought to be localized to the Gla domain. The goal of the current work was to compare platelet binding and activity of NN1731 and wild-type FVIIa.

METHODS/RESULTS

FVIIa and NN1731 bound identically to phospholipid vesicles as assessed by both activity assays and electrophoretic quasielastic light scattering techniques. However, NN1731 bound to a greater number of sites on activated platelets than FVIIa, as assessed by flow cytometry. Removal of the Gla domain abolished binding of both FVIIa and NN1731. Inhibition of the active site did not reduce NN1731 binding to the level of FVIIa. When corrected for the amount of protein bound, NN1731 had greater activity than FVIIa on platelet surfaces.

CONCLUSIONS

While the Gla domain is essential for FVIIa binding to platelets, changes in the protease domain in NN1731 enhanced platelet binding as well as proteolytic activity. Features in addition to lipid composition appear to contribute to binding of rFVIIa and, especially, NN1731 to platelets.

Internalization of microparticles by endothelial cells promotes platelet/endothelial cell interaction under flow

BACKGROUND

Microparticles (MPs) released by activated or apoptotic cells increase in number in the blood of subjects with vascular or metabolic diseases and may contribute to thrombotic complications.

OBJECTIVES

In this study, we investigated whether MPs promoted platelet recruitment to endothelial cells in flow conditions, and by which mechanism.

METHODS

Human umbilical vein endothelial cells (HUVECs) grown in microslide perfusion chambers were exposed to MPs prepared in vitro from HUVECs, monocytes or platelets.

RESULTS

Videomicroscopy of DIOC-labelled blood perfused at arterial rate on human umbilical vein ECs demonstrated that, irrespective of their cell origin, MPs promoted the formation of platelet strings at the surface of HUVECs. This platelet/endothelial cell interaction was dependent on von Willebrand factor (VWF) expression at the HUVEC surface and involved Glycoprotein Ib and P-selectin. Interestingly, HUVECs internalized MPs within a few hours through a process involving anionic phospholipids, lactadherin and αvβ3 integrin. This uptake generated the production of reactive oxygen species via the xanthine/xanthine oxidase system (inhibited by allopurinol and the ROCK inhibitor Y-27632) and the NADPH oxidase (inhibited by SOD). Reactive oxygen species appeared essential for VWF expression at the endothelial cell surface and subsequent platelet/endothelial cell interaction under flow. The pathophysiological relevance of this process is underlined by the fact that circulating MPs from Type I diabetic patients induced platelet/endothelial cell interaction under flow, with an intensity correlated with the severity of the vasculopathy.

Platelets as delivery systems for disease treatments

Platelets are small, anucleate, discoid shaped blood cells that play a fundamental role in hemostasis. Platelets contain a large number of biologically active molecules within cytoplasmic granules that are critical to normal platelet function. Because platelets circulate in blood through out the body, release biological molecules and mediators on demand and participate in hemostasis as well as many other pathophysiologic processes, targeting expression of proteins of interest to platelets and utilizing platelets as delivery systems for disease treatment would be a logical approach. This paper reviews the genetic therapy for inherited bleeding disorders utilizing platelets as delivery system, with a particular focus on platelet-derived FVIII for hemophilia A treatment.

In vivo efficacy of platelet-delivered, high specific activity factor VIII variants

Ectopically expressed, human B-domainless (hB) factor 8 (F8) in platelets improves hemostasis in hemophilia A mice in several injury models. However, in both a cuticular bleeding model and a cremaster laser arteriole/venule injury model, there were limitations to platelet-derived (p) hBF8 efficacy, including increased clot embolization. We now address whether variants of F8 with enhanced activity, inactivation resistant F8 (IR8) and canine (c) BF8, would improve clotting efficacy. In both transgenic and lentiviral murine model approaches, pIR8 expressed at comparable levels to phBF8, but pcBF8 expressed at only approximately 30%. Both variants were more effective than hBF8 in cuticular bleeding and FeCl(3) carotid artery models. However, in the cremaster injury model, only pcBF8 was more effective, markedly decreasing clot embolization. Because inhibitors of F8 are stored in platelet granules and IR8 is not protected by binding to von Willebrand factor, we also tested whether pIR8 was effective in the face of inhibitors and found that pIR8 is protected from the inhibitors. In summary, pF8 variants with high specific activity are more effective in controlling bleeding, but this improved efficacy was inconsistent between bleeding models, perhaps reflecting the underlying mechanism(s) for the increased specific activity of the studied F8 variants.

Alpha1G T-type calcium channel selectively regulates P-selectin surface expression in pulmonary capillary endothelium

Regulated P-selectin surface expression provides a rapid measure for endothelial transition to a proinflammatory phenotype. In general, P-selectin surface expression results from Weibel-Palade body (WPb) exocytosis. Yet, it is unclear whether pulmonary capillary endothelium possesses WPbs or regulated P-selectin surface expression and, if so, how inflammatory stimuli initiate exocytosis. We used immunohistochemistry, immunofluorescence labeling, ultrastructural assessment, and an isolated perfused lung model to demonstrate that capillary endothelium lacks WPbs but possesses P-selectin. Thrombin stimulated P-selectin surface expression in both extra-alveolar vessel and alveolar capillary endothelium. Only in capillaries was the thrombin-stimulated P-selectin surface expression considerably mitigated by pharmacologic blockade of the T-type channel or genetic knockout of the T-type channel alpha(1G)-subunit. Depolarization of endothelial plasma membrane via high K(+) perfusion capable of eliciting cytosolic Ca(2+) transients also provoked P-selectin surface expression in alveolar capillaries that was abolished by T-type channel blockade or alpha(1G) knockout. Our findings reveal an intracellular WPb-independent P-selectin pool in pulmonary capillary endothelium, where the regulated P-selectin surface expression is triggered by Ca(2+) transients evoked through activation of the alpha(1G) T-type channel.

Inhibition of platelet GPIb alpha and promotion of melanoma metastasis

Platelet glycoprotein Ibalpha (GPIb alpha) is part of the receptor complex GPIb-V-IX, which has a critical role in hemostasis, especially through interactions with the subendothelial von Willebrand factor. As there is accumulating evidence for a contribution of platelet receptors to hematogenous tumor metastasis, GPIb alpha is an interesting molecule to study in this context. We have investigated the effect of GPIb alpha inhibition by monovalent Fab fragments on experimental pulmonary metastasis in a syngeneic mouse model using C57BL/6 mice and B16F10 melanoma cells. The early fate of green fluorescent protein (GFP)-transfected melanoma cells under GPIb alpha blockade was also assessed, as was the effect of GPIb alpha inhibition on pulmonary metastasis in mice lacking P-selectin. Surprisingly and, to our knowledge previously unreported, GPIb alpha inhibition led to a significant increase in pulmonary metastasis, and assessment of the early fate of circulating GFP-labeled B16F10 showed improved survival and pulmonary arrest of tumor cells shortly after GPIb alpha inhibition, indicating that inhibition of a platelet protein can, in some cases, promote metastasis of a malignant tumor. In contrast, GPIb alpha blockade in P-selectin-deficient mice had no enhancing effect on metastasis, suggesting the involvement of GPIb alpha in the initial, P-selectin-dependent steps of metastasis. These findings suggest that GPIb alpha contributes to the control of tumor metastasis, in addition to its role in hemostasis.

Pathophysiological levels of soluble P-selectin mediate adhesion of leukocytes to the endothelium through Mac-1 activation

Plasma soluble P-selectin (sP-selectin) levels are increased in pathologies associated with atherosclerosis, including peripheral arterial occlusive disease (PAOD). However, the role of sP-selectin in regulating leukocyte-endothelial adhesion is unclear. The aim of this study was to assess the ability of exogenous and endogenous sP-selectin to induce leukocyte responses that promote their adhesion to various forms of endothelium. In flow chamber assays, sP-selectin dose-dependently increased neutrophil adhesion to resting human iliac artery endothelial cells. Similarly, sP-selectin induced neutrophil adhesion to the endothelial surface of murine aortae and human radial venous segments in ex vivo flow chamber experiments. Using intravital microscopy to examine postcapillary venules in the mouse cremaster muscle, in vivo administration of sP-selectin was also found to significantly increase leukocyte rolling and adhesion in unstimulated postcapillary venules. Using a Mac-1-specific antibody and P-selectin knockout mouse, it was demonstrated that this finding was dependent on a contribution of Mac-1 to leukocyte rolling and endothelial P-selectin expression. This was confirmed in an ex vivo perfusion model using viable mouse aorta and human radial vessels. In contrast, with tumor necrosis factor-alpha-activated endothelial cells and intact endothelium, where neutrophil adhesion was already elevated, sP-selectin failed to further increase adhesion. Plasma samples from PAOD patients containing pathologically elevated concentrations of sP-selectin also increased neutrophil adhesion to the endothelium in a sP-selectin-dependent manner, as demonstrated by immunodepletion of sP-selectin. These studies demonstrate that raised plasma sP-selectin may influence the early progression of vascular disease by promoting leukocyte adhesion to the endothelium in PAOD, through Mac-1-mediated rolling and dependent on endothelial P-selectin expression.

The endothelial glycocalyx: composition, functions, and visualization

This review aims at presenting state-of-the-art knowledge on the composition and functions of the endothelial glycocalyx. The endothelial glycocalyx is a network of membrane-bound proteoglycans and glycoproteins, covering the endothelium luminally. Both endothelium- and plasma-derived soluble molecules integrate into this mesh. Over the past decade, insight has been gained into the role of the glycocalyx in vascular physiology and pathology, including mechanotransduction, hemostasis, signaling, and blood cell–vessel wall interactions. The contribution of the glycocalyx to diabetes, ischemia/reperfusion, and atherosclerosis is also reviewed. Experimental data from the micro- and macrocirculation alludes at a vasculoprotective role for the glycocalyx. Assessing this possible role of the endothelial glycocalyx requires reliable visualization of this delicate layer, which is a great challenge. An overview is given of the various ways in which the endothelial glycocalyx has been visualized up to now, including first data from two-photon microscopic imaging.

Hydrolysis of phosphatidylserine-exposing red blood cells by secretory phospholipase A2 generates lysophosphatidic acid and results in vascular dysfunction

Secretory phospholipase A(2) (sPLA(2)) type IIa, elevated in inflammation, breaks down membrane phospholipids and generates arachidonic acid. We hypothesized that sPLA(2) will hydrolyze red blood cells that expose phosphatidylserine (PS) and generate lysophosphatidic acid (LPA) from phosphatidic acid that is elevated in PS-exposing red blood cells. In turn, LPA, a powerful lipid mediator, could affect vascular endothelial cell function. Although normal red blood cells were not affected by sPLA(2), at levels of sPLA(2) observed under inflammatory conditions (100 ng/ml) PS-exposing red blood cells hemolyzed and generated LPA (1.2 nM/10(8) RBC). When endothelial cell monolayers were incubated in vitro with LPA, a loss of confluence was noted. Moreover, a dose-dependent increase in hydraulic conductivity was identified in rat mesenteric venules in vivo with 5 microM LPA, and the combination of PS-exposing red blood cells with PLA(2) caused a similar increase in permeability. In the presence of N-palmitoyl L-serine phosphoric acid, a competitive inhibitor for the endothelial LPA receptor, loss of confluence in vitro and the hydraulic permeability caused by 5 microM LPA in vivo were abolished. The present study demonstrates that increased sPLA(2) activity in inflammation in the presence of cells that have lost their membrane phospholipid asymmetry can lead to LPA-mediated endothelial dysfunction and loss of vascular integrity.

MHC Class I–Associated Peptides Identified From Normal Platelets and From Individuals With Idiopathic Thrombocytopenic Purpura

Major histocompatibility complex (MHC) class I molecules bind and display peptide antigens on the cell surface. CD8(+) T lymphocytes recognize peptides in association with class I proteins to initiate a cytotoxic immune response. To understand the specificity of such immune responses and to facilitate the development of therapies for disease, it is important to identify MHC-presented peptides. In this study, platelets, easily obtainable and often associated with immune-mediated disease, were selected to identify MHC class I-associated peptides. MHC-associated peptides presented on platelets of normal individuals and individuals with idiopathic thrombocytopenic purpura (ITP) were characterized. ITP is characterized by the premature immune destruction of platelets. It is associated with the production of antiplatelet autoantibodies, most often targeting platelet membrane GPIIb/IIIa or GPIb/IX. In addition to characterizing five fully and several partially sequenced peptides from platelets, the peptide GPRGA(L/I)S(L/I)(L/I) was identified from four of the five ITP patients. The anchor motif of this peptide correlates with the presence of the HLA-B7 allele. A BLAST search identified this peptide as GPIb (4-12). In conclusion, platelets from normal and ITP individuals can present peptides from general cellular proteins and platelet specific proteins, such as GPIb, to the immune system via MHC class I.

Protease-activated receptor (PAR)-independent growth and pro-inflammatory actions of thrombin on human cultured airway smooth muscle

(1) Thrombin, a mitogen for human cultured airway smooth muscle (HASM), has many actions that have been attributed to activation of protease-activated receptor (PARs). However, the role of PARs in the proliferative action has not been clearly identified. Moreover, thrombin elicits cytokine production in a number of cell types, but these effects have not been characterized in human ASM. (2) Thrombin (0.03-3 U ml(-1))-stimulated increases in the levels of the pro-inflammatory and fibrogenic cytokine, granulocyte-macrophage colony-stimulating factor (GM-CSF) were observed over the same concentration range observed for thrombin-stimulated mitogenesis. (3) Inhibition of thrombin proteolytic activity, with either D-phenylalanyl-L-prolyl-L-arginine chloromethyl ketone (PPACK)- or hirudin-treated thrombin (0.3 U ml(-1)) or in the presence of the thrombin serine protease-selective inhibitor, SDZ 217-766 (0.15 micro M), reduced the thrombin-stimulated GM-CSF levels by 91+/-3, 65+/-12 and 83+/-9% (n=8, P<0.05), respectively. PPACK treatment, hirudin and SDZ 217-766 inhibited thrombin-stimulated increase in cell number by 70+/-8, 63+/-11 and 69+/-8%, respectively. (4) PAR-selective peptides SFLLRN (PAR1; 10 micro M), SLIGKV (PAR2; 10 micro M), GYPGQV (PAR4; 100 micro M) or the combination of SFLLRN and GYPGQV elicited mitogenic responses of only 15% of that to thrombin and surprisingly, had no effect on GM-CSF levels (n=8). Nevertheless, inhibition of thrombin responses by pertussis toxin (50 ng ml(-1)) suggests that the PAR-independent actions also involve a G-protein-coupled receptor. (5) PAR1 receptor expression was evident by immunohistochemistry and these receptors were coupled to increases in intracellular calcium, but not to the phosphorylation of ERK or the increases in cyclin D1 protein levels that are essential for cell proliferation. Cross-desensitization of intracellular calcium increases by thrombin and the PAR1-selective peptide provides evidence that the PAR1 receptor responds to both ligands. (6) The failure of PAR-selective peptides to mimic thrombin responses together with the inhibition of thrombin responses by serine protease inhibitors suggest the involvement of novel proteolytic receptor targets for thrombin-induced mitogenesis and cytokine production.

Identification and characterisation of a platelet GPIb/V/IX-like complex on human breast cancers: implications for the metastatic process

The glycoprotein (GP) Ib /V/IX receptor complex is an important adhesion molecule, originally thought to be unique to the megakaryocytic lineage. Recent evidence now indicates that GPIb /V/IX may be more widely expressed. In this study we report the presence of all subunits of the complex on four breast cancer cell lines, and 51 / 80 primary breast tumours. The surface expression of GPIb /V/IX was confirmed by flow cytometry, and by immunoprecipitation of biotin surface-labelled tumour cells. Western blotting of cell lysates under reducing conditions revealed that tumour cell-GPIb alpha had a relative molecular weight of 95 kDa as compared to 135 kDa on platelets. Despite the discrepant protein size, molecular analyses on the tumour cell-GPIb alpha subunit using RT-PCR and DNA sequencing revealed 100% sequence homology to platelet GPIb alpha. Tumour cell-GPIb /V/IX was capable of binding human von Willebrand factor (vWf), and this binding caused aggregation of tumour cells in suspension. Tumour cells bound to immobilised vWf in the presence of EDTA and demonstrated prominent filapodial extensions indicative of cytoskeletal reorganisation. Furthermore, in a modified Boyden chamber assay, prior exposure to vWf or a GPIb alpha monoclonal antibody, AK2, enhanced cell migration. The presence of a functional GPIb /V/IX-like complex in tumour cells suggests that this complex may participate in the process of haematogenous breast cancer metastasis.

Verotoxin-1-induced up-regulation of adhesive molecules renders microvascular endothelial cells thrombogenic at high shear stress

Verotoxin-1 (VT-1)-producing Escherichia coli is the causative agent of postdiarrheal hemolytic uremic syndrome (D+HUS) of children, which leads to renal and other organ microvascular thrombosis. Why thrombi form only on arterioles and capillaries is not known. This study investigated whether VT-1 directly affected endothelial antithrombogenic properties promoting platelet deposition and thrombus formation on human microvascular endothelial cell line (HMEC-1) under high shear stress. Human umbilical vein endothelial cells (HUVECs) were used for comparison as a large-vessel endothelium. HMEC-1 and HUVECs were pre-exposed for 24 hours to increasing concentrations of VT-1 (2-50 pM) and then perfused at 60 dynes/cm(2) with heparinized human blood prelabeled with mepacrine. Results showed that VT-1 significantly increased platelet adhesion and thrombus formation on HMEC-1 in comparison with unstimulated control cells. An increase in thrombus formation was also observed on HUVECs exposed to VT-1, but to a remarkably lower extent. The greater sensitivity of HMEC-1 to the toxin in comparison with HUVECs was at least in part due to a higher expression of VT-1 receptor (20-fold more) as documented by FACS analysis. The HMEC-1 line had a comparable susceptibility to the thrombogenic effect of VT-1 as primary human microvascular cells of the same dermal origin (HDMECs). The adhesive molecules involved in VT-induced thrombus formation were also studied. Blocking the binding of von Willebrand factor to platelet glycoprotein Ib by aurintricarboxylic acid (ATA) or inhibition of platelet alpha(IIb)beta(3)-integrin by chimeric 7E3 Fab resulted in a significant reduction of VT-1-induced thrombus formation, suggesting the involvement of von Willebrand factor-platelet interaction at high shear stress in this phenomenon. Functional blockade of endothelial beta(3)-integrin subunit, vitronectin receptor, P-selectin, and PECAM-1 with specific antibodies was associated with a significant decrease of the endothelial area covered by thrombi. Confocal microscopy studies revealed that VT-1 increased the expression of vitronectin receptor and P-selectin and redistributed PECAM-1 away from the cell-cell border of HMEC-1, as well as of HDMECs, thus indicating that the above endothelial adhesion molecules are directly involved and possibly determine the effect of VT-1 on enhancing platelet adhesion and thrombus formation in microvascular endothelium. These results might help to explain why thrombi in HUS localize in microvessels rather than in larger ones and provide insights on the molecular events involved in the process of microvascular thrombosis associated with D+HUS.

Sickle erythrocytes adhere to fibronectin-thrombospondin-integrin complexes exposed by thrombin-induced endothelial cell contraction

Vascular damage appears to be associated with sickle erythrocyte (SS RBC) adherence to the endothelium. Thrombin, which has been found in abnormal levels in many sickle patients, causes endothelial cell (EC) retraction and increased SS RBC adherence, and SS RBC adhere in the gaps opened between the EC. Our objective was to elucidate the mechanism of adherence to activated EC monolayers and to determine whether the matrix proteins thrombospondin (TSP) and fibronectin (FN) are mediators of this adherence. Thrombin activation elicited the same 2.5-fold increase in adherence whether 10 or 35% of the matrix was exposed, and the majority of the RBC adhered at the edges of the EC regardless of the extent of matrix exposed. Using static adherence assays we investigated whether TSP, FN, or the integrins alpha(v)beta(3) and alpha(5)beta(1) mediated adherence. Blocking antibodies to any of these four had no effect on adherence to untreated monolayers. However, all the increased adherence elicited by thrombin was abrogated by each one, whereas control antibodies had no effect. Immunofluorescent microscopy demonstrated that both integrins were present on the luminal surface of confluent EC. Neither TSP nor FN was exposed in confluent cultures but they both became available as receptors after EC retraction. These data suggest that SS RBC adhere to a complex of matrix TSP and FN maintained in an adhesive conformation by interactions with both integrins.

GATA and NF-Y participate in transcriptional regulation of FcgammaRIIA in megakaryocytic cells

Human FcgammaRIIA, expressed on platelets, neutrophils, and macrophages, plays a major role in platelet activation and immune clearance. Clinical observations indicate that regulation of expression of this receptor is an important factor influencing the course of immune thrombocytopenia. We used both transient transfection with FcgammaRIIA promoter constructs and electrophoretic mobility shift assays (EMSA) to study the regulation of FcgammaRIIA transcription. In HEL (erythromegakaryocytic) cells, the 200 bp immediately 5' of the ATG start codon accounted for the majority of the activity of a 3.6-kb promoter fragment. Putative GATA (-161) and NF-Y (-119) sites are present. EMSA analyses demonstrate specific binding of both GATA-1 and GATA-2 to labeled oligonucleotides containing the putative GATA site with HEL but not U937 (myelomonocytic) nuclear extracts. Antibodies to NF-Y supershift the specific -119 NF-Y complex with HEL, U937, Jurkat (T-lymphocytic), and HeLa (nonhematopoietic) nuclear extracts. Comparison of the activity of GATA and NF-Y mutant constructs in HEL and U937 demonstrates that while either GATA or NF-Y mutation results in a large decrease in the promoter activity (2.2- and 2.3-fold, respectively) in HEL cells, neither mutation is effective in reducing activity in U937 cells. This is the first example of a promoter active in the megakaryocyte lineage in which NF-Y cooperates additively with GATA factors to regulate transcription. Identification of other factors that must be operational for FcgammaRIIA transcription in myelomonocytic cells which lack GATA factors will bolster our ongoing efforts to dissect the function of these Fc receptors in megakaryocytic and myelomonocytic cells in vivo.

Platelets adhere to and translocate on von Willebrand factor presented by endothelium in stimulated veins

With the use of intravital microscopy, a new type of platelet–endothelial interaction in mouse mesenteric venules at low shear (80-100 seconds−1) is described. Stimulation of these vessels with calcium ionophore A23187, a known secretagogue of Weibel-Palade bodies, induced immediate platelet adhesion (within 15 seconds) and translocation without the formation of aggregates. This stop-and-go process reached a maximum in approximately 1 minute, when approximately 25 000 platelets adhered/mm2·s, and then adhesion progressively decreased. This adhesion process was dependent on von Willebrand factor (vWF) and independent of P-selectin. Immunohistologic analysis showed that the venules were not denuded withA23187 treatment, suggesting that platelets adhered to vWF secreted on the luminal face of the endothelial cells. Histamine treatment induced a similar adhesion phenomenon. Platelet adhesion was not abolished in β3-deficient mice or when the platelets were treated with inhibitory antibodies to PECAM-1 or PSGL-1, indicating that these molecules are not required for platelet–endothelium interaction at low shear. The adhesion was mediated by platelet glycoprotein Ibα (GPIbα) because the adhesion of murine platelets expressing exclusively the human GPIbα could be prevented by a pretreatment with mocarhagin, a snake venom protease that cleaves human GPIbα. The results indicate that vWF released from Weibel-Palade bodies can dramatically increase the concentration of platelets along the vessel wall through an interaction with GPIbα. It is proposed that this process may rapidly recruit platelets to sites of injury or inflammation in veins.

Platelets adhere to and translocate on von Willebrand factor presented by endothelium in stimulated veins

With the use of intravital microscopy, a new type of platelet–endothelial interaction in mouse mesenteric venules at low shear (80-100 seconds−1) is described. Stimulation of these vessels with calcium ionophore A23187, a known secretagogue of Weibel-Palade bodies, induced immediate platelet adhesion (within 15 seconds) and translocation without the formation of aggregates. This stop-and-go process reached a maximum in approximately 1 minute, when approximately 25 000 platelets adhered/mm2·s, and then adhesion progressively decreased. This adhesion process was dependent on von Willebrand factor (vWF) and independent of P-selectin. Immunohistologic analysis showed that the venules were not denuded withA23187 treatment, suggesting that platelets adhered to vWF secreted on the luminal face of the endothelial cells. Histamine treatment induced a similar adhesion phenomenon. Platelet adhesion was not abolished in β3-deficient mice or when the platelets were treated with inhibitory antibodies to PECAM-1 or PSGL-1, indicating that these molecules are not required for platelet–endothelium interaction at low shear. The adhesion was mediated by platelet glycoprotein Ibα (GPIbα) because the adhesion of murine platelets expressing exclusively the human GPIbα could be prevented by a pretreatment with mocarhagin, a snake venom protease that cleaves human GPIbα. The results indicate that vWF released from Weibel-Palade bodies can dramatically increase the concentration of platelets along the vessel wall through an interaction with GPIbα. It is proposed that this process may rapidly recruit platelets to sites of injury or inflammation in veins.

Bernard-Soulier syndrome: common ancestry in two African American families with the GP Ib alpha Leu129Pro mutation

Bernard-Soulier syndrome (BSs) is a rare bleeding disorder characterized by circulating giant platelets, thrombocytopenia, and a prolonged bleeding time. BSs usually has an autosomal recessive inheritance pattern, with a preponderance of Caucasian and Japanese ancestry when the ethnic background has been reported. Underlying this disorder of platelet function is a defect in the platelet glycoprotein (GP) Ib-IX-V complex, composed of four polypeptides, GP Ib alpha, GP Ib beta, GP IX, and GP V. Molecular characterization of individuals with BSs has identified mutations in the GP Ib alpha, GP Ib beta, and GP IX genes responsible for the expressed phenotype. In this study, we report a family of African-American descent, with autosomal recessive BSs showing a point mutation in codon 129 of the GP Ib alpha gene. This mutation, CTC:wild-type to CCC:mutant, is similar to that of another African American family where the resulting leucine to proline substitution in the 5(th) leucine-rich repeat of GP Ib alpha is responsible for the observed BSs phenotype. Comparison of the intragenic polymorphisms of GP Ib alpha, as well as microsatellite markers in a 17.5 cM region of chromosome 17p12 that contains the GP Ib alpha gene, suggests that, although socially unrelated, the Leu129Pro mutation in these two families has a common founder.

Agkistin, a snake venom-derived glycoprotein Ib antagonist, disrupts von Willebrand factor-endothelial cell interaction and inhibits angiogenesis

Glycoprotein (GP) Ib, an adhesion receptor expressed on both platelets and endothelial cells, mediates the binding of von Willebrand factor (vWF). Platelet GPIb plays an important role in platelet adhesion and activation, whereas the interaction of vWF and endothelial GPIb is not fully understood. We report here that agkistin, a snake venom protein, selectively blocks the interaction of vWF with human endothelial GPIb and inhibits angiogenesis in vivo. Agkistin specifically blocked human umbilical vein endothelial cell (HUVEC) adhesion to immobilized vWF in a concentration-dependent manner. Fluorescein isothiocyanate (FITC)-conjugated agkistin bound to HUVECs in a saturable manner. AP1, a monoclonal antibody (mAb) raised against GPIb, specifically inhibited the binding of FITC-conjugated agkistin to HUVECs in a dose-dependent manner, but other anti-integrin mAbs raised against alpha(v)beta(3), alpha(2)beta(1), and alpha(5)beta(1) did not affect this binding reaction. However, neither agkistin (2 microgram/ml) nor AP1 (40 microgram/ml) apparently reduced HUVEC viability. Both agkistin and AP1 exhibited a profound anti-angiogenic effect in vivo when assayed by using the 10-day-old embryo chick chorioallantoic membrane model. These results suggest endothelial GPIb plays a role in spontaneous angiogenesis in vivo, and the anti-angiogenic effect of agkistin may be because of disruption of the interaction of endogenous vWF with endothelial GPIb.

Mast cells and dendritic cells in basal cell carcinoma stroma

BACKGROUND

The interaction of tumors with the surrounding stroma has become an important topic in tumor biology. Basal cell carcinoma (BCC) stroma has been characterized as hypervascular and rich in mast cells. The presence of dermal dendrocytes thought to have both antigen presenting and wound healing functions has recently been reported in BCC stroma. GP1b-alpha is a newly described vascular adhesion molecule with potential significance in tumor biology.

OBJECTIVE

To further characterize the cellular phenotype of BCC stroma.

METHODS

Eleven BCCs (8 nodular, 2 sclerosing, 1 adenoid-cystic) were examined using immunohistochemical techniques for the presence of antigens specific to vascular endothelium, mast cells, and dermal dendrocytes.

RESULTS

The stroma of all BCCs demonstrated increased vascularity, increased numbers of mast cells, and increased numbers of dermal dendrocytes expressing both CD34 and GP1b-alpha adjacent to tumor nests. No differences in antigen expression were observed between histologic subtypes of BCC.

CONCLUSION

The close proximity of stromal mast cells and dermal dendrocytes surrounding BCC nests suggests a biologically significant interaction. The pattern observed is similar to that observed in healing wounds.

Adherence of phosphatidylserine-exposing erythrocytes to endothelial matrix thrombospondin

Phospholipid asymmetry is well maintained in erythrocyte (RBC) membranes with phosphatidylserine (PS) exclusively present in the inner leaflet. The appearance of PS on the surface of the cell can have major physiologic consequences, including increased cell-cell interactions. Because increased adherence of PS-exposing RBCs to endothelial cells (ECs) may be pathologically important in hemoglobinopathies such as sickle cell disease and thalassemia, we studied the role of PS exposure in calcium ionophore-treated normal RBC adherence to human umbilical vein endothelial cell (HUVEC) monolayers. When HUVEC monolayers were incubated with these PS-exposing RBCs, the ECs retracted and the RBCs adhered primarily in the gaps opened between the ECs. A linear correlation was found between the number of PS-exposing RBCs in the population and the number of adhering RBCs to the monolayer. Pretreatment of RBCs with annexin V significantly decreased adherence by shielding PS on the RBCs. Similarly, PS-containing lipid vesicles decreased RBC binding by competing for the PS binding sites in the monolayer. PS-exposing RBCs and PS-containing lipid vesicles adhered to immobilized thrombospondin (TSP) and matrix TSP, respectively, and adherence of PS-exposing RBCs to EC monolayers was reduced by antibodies to TSP and to its EC receptor, vβ3. Together, these results indicate a role for PS and matrix TSP in the adherence of PS-exposing RBCs to EC monolayers, and suggest an important contribution of PS-exposing RBCs in pathologies with reported vascular damage, such as sickle cell anemia.

Structural and functional characterization of the mouse von Willebrand factor receptor GPIb-IX with novel monoclonal antibodies

Five novel monoclonal antibodies (mAbs; p0p 1-5) were used to characterize the structural and functional properties and the in vivo expression of the murine GPIb-IX complex (von Willebrand factor receptor). The molecular weights of the subunits are similar to the human homologs: GPIb (150 kd), GPIbβ (25 kd), and GPIX (25 kd). Activation of platelets with thrombin or PMA predominantly induced shedding of glycocalicin (GC; 130 kd) but only low levels of receptor internalization. The GC concentration in normal mouse plasma was found to be at least 10 times higher than that described for human plasma (approximately 25 μg/mL versus 1-2 μg/mL). Two additional cleavage sites for unidentified platelet-derived proteases were found on GPIb, as demonstrated by the generation of 3 N-terminal fragments during in vitro incubation of washed platelets (GC, 60 kd, 45 kd). Occupancy of GPIb with p0p mAbs or F(ab)2-fragments resulted in aggregate formation in vitro and rapid irreversible thrombocytopenia in vivo, irrespective of the exact binding epitopes of the individual antibodies. GPIb-IX was not detectable immunohistochemically on endothelial cells in the major organs under normal or inflammatory conditions. The authors conclude that the mouse system might become an interesting model for studies on GPIb-IX function and regulation.

Glycoprotein V-Deficient Platelets Have Undiminished Thrombin Responsiveness and Do Not Exhibit a Bernard-Soulier Phenotype

Adhesion of platelets to extracellular matrix via von Willebrand factor (vWF) and activation of platelets by thrombin are critical steps in hemostasis. Glycoprotein (GP) V is a component of the GPIb-V-IX complex, the platelet receptor for vWF. GPV is also cleaved by thrombin. Deficiency of GPIb or GPIX results in Bernard-Soulier syndrome (BSS), a bleeding disorder in which platelets are giant and have multiple functional defects. Whether GPV-deficiency might also cause BSS is unknown as are the roles of GPV in platelet-vWF interaction and thrombin signaling. We report that GPV-deficient mice developed normally, had no evidence of spontaneous bleeding, and had tail bleeding times that were not prolonged compared with wild-type mice. GPV-deficient platelets were normal in size and structure as assessed by flow cytometry and electron microscopy. GPV-deficient and wild-type platelets were indistinguishable in botrocetin-mediated platelet agglutination and in their ability to adhere to mouse vWF A1 domain. Platelet aggregation and ATP secretion in response to low and high concentrations of thrombin were not decreased in GPV-deficient platelets compared with wild-type. Our results show that (1) GPV is not necessary for GPIb expression and function in platelets and that GPV deficiency is not likely to be a cause of human BSS and (2) GPV is not necessary for robust thrombin signaling. Whether redundancy accounts for the lack of phenotype of GPV-deficiency or whether GPV serves subtle or as yet unprobed functions in platelets or other cells remains to be determined.

Glycoprotein Ibalpha can mediate endothelial cell migration on von Willebrand factor-containing substrata

We have previously shown that, in addition to the vitronectin receptor (VNR, alpha(v)beta(3)), the GP Ib complex can participate in endothelial cell (EC) attachment to von Willebrand Factor (vWF) (D. A. Beacham, M. S. Cruz, and R. I. Handin, 1995, Thromb. Haemostas. 73, 309-317; D. A. Beacham, L.-P. Tran, and S. S. Shapiro, 1997, Blood 89, 4071-4077). In this study we have investigated the functional roles of these vWF receptors in the migration of untreated and TNFalpha-treated EC on vWF, a mixture of vWF and type I collagen, and on vitronectin (VN). In agreement with previous studies (D. I. Leavesley, M. A. Schwartz, M. Rosenfeld, and D. A. Cheresh, 1993, J. Cell Biol. 121, 163-170), the migration of untreated and TNFalpha-treated EC on VN was dependent entirely on the VNR. Migration of untreated EC on vWF was inhibited 10-15% by recombinant vWF-A1, the GP Ibalpha-binding domain on vWF which abrogates the platelet GP Ibalpha-vWF interaction. In contrast, migration of TNFalpha-treated EC on vWF was inhibited 50-60% by vWF-A1 or the anti-GP Ibalpha mAb AS-7 but only 20% by the anti-VNR mAb LM609. On a mixed vWF-collagen substratum, vWF-A1 inhibited untreated EC migration by 45%, and TNFalpha-treated EC migration by 75%. The possible role of EC proliferation was eliminated, since hydroxyurea completely inhibited EC proliferation without reducing migration significantly. The anti-GP Ibalpha mAb Ib1 inhibited EC migration by 50%, but reduced proliferation by only 15%. Taken together, our data demonstrate that EC migration on vWF-containing substrata involves the GP Ib complex as well as the VNR and raises the possibility that the VNR and GP Ib act cooperatively in supporting EC migration.

Von Willebrand factor receptor GPIb alpha is expressed by human factor XIIIa-positive dermal dendrocytes and is upregulated by mast cell degranulation

GPIb alpha, a glycoprotein component of the GPIb-IX-V complex, serves as a platelet membrane receptor that mediates adhesion to von Willebrand factor normally present in the vascular subendothelium. Recent data have demonstrated that GPIb alpha is not restricted to platelets, but is also expressed by endothelium in vitro. In this study, we describe the expression and distribution of GPIb alpha in normal adult and neonatal human skin. GPIb alpha is present, as detected by immunohistochemistry, on endothelial cells and on highly dendritic cells localized within the perivascular space, dermal-epidermal junction, and reticular dermis. By dual-labeling immunofluorescence and confocal microscopy, GPIb alpha-positive cells within the dermal interstitium are demonstrated to represent factor XIIIa-positive dermal dendrocytes. In organ cultures of neonatal human foreskin, mast cell degranulation induced by either substance P or compound 48/80 resulted in transiently increased GPIb alpha expression by dermal dendrocytes. Because the GPIb-IX-V complex plays a part in regulating hemostasis and may be important for cellular interactions with extracellular matrix molecules, these data provide additional insight into the potential function of FXIIIa-positive dermal dendrocytes in skin remodeling and repair.

Identification and Characterization of Endothelial Glycoprotein Ib Using Viper Venom Proteins Modulating Cell Adhesion

The expression and function of a glycoprotein Ib (GPIb) complex on human umbilical vein endothelial cells (HUVECs) is still a matter of controversy. We characterized HUVEC GPIb using viper venom proteins: alboaggregins A and B, echicetin, botrocetin, and echistatin. Echicetin is an antagonist, and alboaggregins act as agonists of the platelet GPIb complex. Botrocetin is a venom protein that alters von Willebrand factor (vWF) conformation and increases its binding affinity for the GPIb complex. Echistatin is a disintegrin that blocks vβ3. Echistatin, but not echicetin, inhibited the adhesion to vWF of Chinese hamster ovary (CHO) cells transfected with vβ3. We found the following: (1) Binding of monoclonal antibodies against GPIb to HUVECs was moderately increased after stimulation with cytokines and phorbol ester. Echicetin demonstrated an inhibitory effect. (2) Both echicetin and echistatin, an vβ3 antagonist, inhibited the adhesion of HUVECs to immobilized vWF in a dose-dependent manner. The inhibitory effect was additive when both proteins were used together. (3) Botrocetin potentiated the adhesion of HUVECs to vWF, and this effect was completely abolished by echicetin, but not by echistatin. (4) CHO cells expressing GPIbβ/IX adhered to vWF (in the presence of botrocetin) and to alboaggregins; GPIb was required for this reaction. Echicetin, but not echistatin, inhibited the adhesion of cells transfected with GPIbβ/IX to immobilized vWF. (5) HUVECs adhered strongly to immobilized vWF and alboaggregins with extensive spreading, which was inhibited by LJ1b1, a monoclonal antibody against GPIb. The purified vβ3 receptor did not interact with the alboaggregins, thereby excluding the contribution of vβ3 in inducing HUVEC spreading on alboaggregins. In conclusion, our data confirm the presence of a functional GPIb complex expressed on HUVECs in low density. This complex may mediate HUVEC adhesion and spreading on immobilized vWF and alboaggregins.

In Vivo Expression of Murine Platelet Glycoprotein Ibα

We have performed a systematic in vivo evaluation of gene expression for the glycoprotein (GP) Ibα subunit of the murine platelet adhesion receptor, GP Ib-IX-V. This study is warranted by in vitro observations of human GP Ibα expression in cells of nonhematopoietic lineage and reports of regulation of the GP Ibα gene by cytokines. However, an in vivo role for a GP Ib-IX-V receptor has not been established beyond that described for normal megakaryocyte/platelet physiology and hemostasis. Our Northern analysis of mouse organs showed high levels of GP Ibα mRNA in bone marrow with a similar expression pattern recapitulated in mice containing a luciferase transgene under the control of the murine GP Ibα promoter. Consistently high levels of luciferase activity were observed in the two hematopoietic organs of mice, bone marrow (1,400 relative light units/μg of protein [RLUs]) and spleen (500 RLUs). Reproducible, but low-levels of luciferase activity were observed in heart, aorta, and lung (30 to 60 RLUs). Among circulating blood cells, the luciferase activity was exclusively localized in platelets. No increase in GP Ibα mRNA or luciferase activity was observed after treatment of mice with lipopolysaccharides (LPS) or tumor necrosis factor-α (TNF-α). We conclude the murine GP Ibα promoter supports a high level of gene expression in megakaryocytes and can express heterologous proteins allowing an in vivo manipulation of platelet-specific proteins in the unique environment of a blood platelet.

Human beta-filamin is a new protein that interacts with the cytoplasmic tail of glycoprotein Ibalpha

We have cloned and sequenced a 9.4-kilobase cDNA specifying a new 280-kDa protein interacting with the cytoplasmic tail of glycoprotein (Gp) Ibalpha and showing considerable homology to actin-binding protein 280 (ABP-280) and chicken retinal filamin. We term this protein human beta-filamin. The gene for beta-filamin localizes to chromosome 3p14.3-p21.1. beta-Filamin mRNA expression was observed in many tissues and in cultured human umbilical vein endothelial cells (HUVECs); only minimal expression was detected in platelets and the megakaryocytic cell line CHRF-288. Like ABP-280, beta-filamin contains an NH2-terminal actin-binding domain, a backbone of 24 tandem repeats, and two "hinge" regions. A polyclonal antibody to the unique beta-filamin first hinge sequence identifies a strong 280-kDa band in HUVECs but only a weak band in platelets, and stains normal human endothelial cells in culture and in situ. We have confirmed the interaction of beta-filamin and GpIbalpha in platelet and HUVEC lysates. In addition, using two-hybrid analysis with deletion mutants, we have localized the binding domain for GpIbalpha in beta-filamin to residues 1862-2148, an area homologous to the GpIbalpha binding domain in ABP-280. beta-Filamin is a new member of the filamin family that may have significance for GpIbalpha function in endothelial cells and platelets.

Adhesion of activated platelets to endothelial cells: evidence for a GPIIbIIIa-dependent bridging mechanism and novel roles for endothelial intercellular adhesion molecule 1 (ICAM-1), alphavbeta3 integrin, and GPIbalpha

Although it has been reported that activated platelets can adhere to intact endothelium, the receptors involved have not been fully characterized. Also, it is not clear whether activated platelets bind primarily to matrix proteins at sites of endothelial cell denudation or directly to endothelial cells. Thus, this study was designed to further clarify the mechanisms of activated platelet adhesion to endothelium. Unstimulated human umbilical vein endothelial cell (HUVEC) monolayers were incubated with washed, stained, and thrombin-activated human platelets. To exclude matrix involvement, HUVEC were harvested mechanically and platelet binding was measured by flow cytometry. Before the adhesion assay, platelets or HUVEC were treated with different receptor antagonists. Whereas blockade of platelet beta1 integrins, GPIbalpha, GPIV, P-selectin, and platelet-endothelial cell adhesion molecule (PECAM)-1 did not reduce platelet adhesion to HUVEC, blockade of platelet GPIIbIIIa by antibodies or Arg-Gly-Asp (RGD) peptides markedly decreased adhesion. Moreover, when platelets were treated with blocking antibodies to GPIIbIIIa-binding adhesive proteins, including fibrinogen and fibronectin, and von Willebrand factor (vWF), platelet binding was also reduced markedly. Addition of fibrinogen, fibronectin, or vWF further increased platelet adhesion, indicating that both endogenous platelet-exposed and exogenous adhesive proteins can participate in the binding process. Evaluation of the HUVEC receptors revealed predominant involvement of intercellular adhesion molecule (ICAM)-1 and alphavbeta3 integrin. Blockade of these two receptors by antibodies decreased platelet binding significantly. Also, there was evidence that a component of platelet adhesion was mediated by endothelial GPIbalpha. Blockade of beta1 integrins, E-selectin, P-selectin, PECAM-1, vascular cell adhesion molecule (VCAM)-1 and different matrix proteins on HUVEC did not affect platelet adhesion. In conclusion, we show that activated platelet binding to HUVEC monolayers is mediated by a GPIIbIIIa-dependent bridging mechanism involving platelet-bound adhesive proteins and the endothelial cell receptors ICAM-1, alphavbeta3 integrin, and, to a lesser extent, GPIbalpha.