Platelet physiology and thrombosis

Saxatilin, a Snake Venom Disintegrin, Regulates Platelet Activation Associated with Human Vascular Endothelial Cell Migration and Invasion

BACKGROUND

Platelet activation results in platelet aggregation and the secretion of granules, which contain a variety of constituents including nonprotein molecules, adhesive proteins and hydrolases. The platelet-derived supernatant (PDS), which contains these granules, is known to trigger the activation of endothelium and chemotaxis of monocytes.

METHODS AND RESULTS

PDS derived from collagen-activated platelets stimulated human umbilical vein endothelial cell (HUVEC) migration and invasion, as measured through the use of a Boyden chamber. This collagen-induced PDS also triggered integrin alpha(v)beta(3) upregulation in HUVECs. The inclusion of a neutralizing antibody to platelet-derived growth factor (PDGF)-B abolished HUVEC migration/invasion and integrin alpha(v)beta(3) upregulation, showing that PDGF-AB mediates the proangiogenic effects of collagen-activated PDS. Saxatilin, a snake venom disintegrin known to interrupt platelet aggregation by antagonizing integrin alpha(IIb)beta(3), inhibited the collagen-induced platelet activation and abolished the angiogenic properties of PDS. Saxatilin also inhibited the collagen-induced phosphorylation of Syk, a key mediator of inside-out signaling in platelet activation.

CONCLUSION

Saxatilin inhibits platelet activation, platelet PDGF-AB release as well as subsequent endothelial cell migration and invasion.

Activities of enzymes that hydrolyze adenine nucleotides in platelets from rats experimentally demyelinated with ethidium bromide and treated with interferon-beta

The activities of the enzymes NTPDase (EC 3.6.1.5, apyrase, CD39) and 5'-nucleotidase (EC 3.1.3.5, CD73) were analyzed in platelets from rats submitted to demyelination by ethidium bromide (EB) and treated with interferon beta (IFN-beta). The following groups were studied: I - control (saline), II - (saline and IFN-beta), III - (EB) and IV - (EB and IFN-beta). After 7, 15 and 30 days, the animals (n=7) were sacrificed and the platelets were separated by the method of Lunkes et al. [Lunkes, G., Lunkes D., Morsch, V., Mazzanti, C., Morsch, A., Miron, V., Schetinger, M.R.C., 2004. NTPDase and 5'-nucleotidase in rats alloxan- induced diabetes. Diabetes Research and Clinical Practice 65, 1-6]. NTPDase activity for ATP and ADP substrates was significantly lower in groups II and III after seven days, when compared to control (p<0.001). At fifteen days, ATP hydrolysis was significantly lower in group III and IV and higher in group II (p<0.001), while there was an activation of ADP hydrolysis in group II (p<0.001), when compared with the control. 5'-nucleotidase activity was significantly higher in group IV (p<0.001) after seven days, and lower in the groups III and IV (p<0.001) after fifteen days in relation to the control. No significant differences were observed in NTPDase and 5'-nucleotidase activities after thirty days. In conclusion, our study demonstrated that the hydrolysis of adenine nucleotides is modified in platelets of rats demyelinated and treated with IFN-beta.

Cell Adhesion Molecules for Targeted Drug Delivery

Rapid advancement of the understanding of the structure and function of cell adhesion molecules (i.e., integrins, cadherins) has impacted the design and development of drugs (i.e., peptide, proteins) with the potential to treat cancer and heart and autoimmune diseases. For example, RGD peptides/peptidomimetics have been marketed as anti-thrombic agents and are being investigated for inhibiting tumor angiogenesis. Other cell adhesion peptides derived from ICAM-1 and LFA-1 sequences were found to block T-cell adhesion to vascular endothelial cells and epithelial cells; these peptides are being investigated for treating autoimmune diseases. Recent findings suggest that cell adhesion receptors such as integrins can internalize their peptide ligands into the intracellular space. Thus, many cell adhesion peptides (i.e., RGD peptide) were used to target drugs, particles, and diagnostic agents to a specific cell that has increased expression of cell adhesion receptors. This review is focused on the utilization of cell adhesion peptides and receptors in specific targeted drug delivery, diagnostics, and tissue engineering. In the future, more information on the mechanism of internalization and intracellular trafficking of cell adhesion molecules will be exploited for delivering drug molecules to a specific type of cell or for diagnosis of cancer and heart and autoimmune diseases.

Antithrombotic action of annexin V proved as efficient as direct inhibition of tissue factor or thrombin

The role of phospholipid platelet membrane and tissue factor in thrombin generation and thrombus formation is accepted. In the present study we have explored antithrombotic action of strategies aimed to block exposure of negatively charged phospholipids and we compared effects with those obtained through tissue factor or a direct thrombin inhibition. Type III collagen was exposed to flowing blood (5 min, 300 s(-1)). Effects of inhibition of platelet deposition by annexin A5 (ANXA5), hirudin (HIR) or by an antibody against tissue factor (TF) were evaluated. Prothrombin fragment F1 + 2 (F1 + 2) was monitored. Pre-incubation of whole blood with HIR or ANXA5 resulted in a statistically significant reduction of platelet deposition (12.2 +/- 0.6% in control experiments vs. 8.3 +/- 0.4% and 8.5 +/- 0.5%, respectively, P < 0.05). A similar decrease was found when blood was incubated with an antibody against TF. Furthermore, ANXA5 and HIR inhibited the recruitment of platelets into forming aggregates. The height of platelet aggregates generated was decreased in the presence of HIR or ANXA5, but only incubation with both inhibitors reached levels of statistical significance. The presence of ANXA5 or HIR decreased levels of F1 + 2 suggesting a reduced activation of the coagulation system. In our experimental studies, the inhibitory potential of ANXA5 on platelet-thrombus formation was as effective as that of a direct thrombin inhibitor, as HIR, or an antibody against TF. Negatively charged phospholipids exposed on activated platelets potentiate the formation of platelet aggregates on a collagen surface and further suggest that inhibition of platelet procoagulant activity might be a specific target for antithrombotic drugs.

The synergy site of fibronectin is required for strong interaction with the platelet integrin alphaIIbbeta3

Integrins are a class of cell adhesion molecules that bind to ligands containing the RGD peptide sequence. There is increasing evidence that peptide sites other than the RGD site are required for optimal binding of integrins with their ligands. We have examined the sites on the protein fibronectin that are needed for optimal binding to the platelet integrin alphaIIbbeta3 using a strategy of site directed mutagenesis. Single amino acids near the RGD site or near the synergy site of fibronectin were mutated and the resultant proteins were expressed in a bacterial expression system. The purified protein was coated onto glass cover slips. Platelets, expressing alphaIIbbeta3 were perfused over the surface at physiologically relevant shear rates and the extent of adhesion was quantified. We found that the single amino acid substitution of the aspartic acid in the RGD sequence, D1495A, completely abolished adhesion. Surprisingly, the mutants R1445A and R1448Q that are near the RGD site also abolished adhesion of platelets under flow. Additionally, the synergy site mutants R1371A, R1374Q, or R1379A displayed only minimal adhesion of platelets. These results show that the binding site for alphaIIbbeta3 on fibronectin extends over a considerable distance from the RGD site and that these distant sites are required for optimal attachment of cells in the presence of physiologically relevant shear stress.

Lipoprotein(a) and Thrombocytes: Potential Mechanisms Underlying Cardiovascular Risk

Plasma levels of lipoprotein(a), Lp(a), is an independent risk factor for cardiovascular disease. Lp(a) has many properties in common with low-density lipoprotein (LDL), including a cholesteryl ester-rich lipid core and the presence of one copy of apolipoprotein B-100; both apoB-100 and the lipid core are pro-atherogenic. In addition, Lp(a) contains a unique hydrophilic, carbohydrate-rich protein, apo(a), linked to apoB through a single disulfide bond connecting the C-terminal regions of the two proteins. The similarities between apolipoprotein(a), apo(a), and plasminogen has initiated numerous studies on the possible role of Lp(a) as a prothrombotic agent. Studies to date suggest that Lp(a) has antifibrinolytic and procoagulant properties. In this review, we summarize recent studies focused on the interaction between Lp(a) and platelets. Collectively, results to date illustrate that thrombogenicity associated with Lp(a) could be due to risk associated with the LDL moiety, with the apo(a) moiety, or from the combination of those in Lp(a). Present findings suggest that the various components of Lp(a) may impact to a varying degree on different underlying pathways involved in platelet activation and aggregation. On balance, results indicate an effect by Lp(a) on platelet function and future studies focused on specific Lp(a) components, such as the role of apo(a) and of the LDL-like lipid moiety, are needed.

Bacillus pasteurii urease shares with plant ureases the ability to induce aggregation of blood platelets

Ureases (EC 3.5.1.5) are highly homologous enzymes found in plants, bacteria and fungi. Canatoxin, an isoform Canavalia ensiformis urease, has several biological properties unrelated to its ureolytic activity, like platelet-aggregating and pro-inflammatory effects. Here, we describe that Bacillus pasteurii urease (BPU) also induces aggregation of rabbit platelets, similar to the canatoxin-induced effect (ED(50) 0.4 and 0.015 mg/mL, respectively). BPU induced-aggregation was blocked in platelets pretreated with dexamethasone and esculetin, a phospholipase A(2) and a lipoxygenase inhibitor, respectively, while platelets treated with indomethacin, a cyclooxygenase inhibitor, showed increased response to BPU. Methoxyverapamil (Ca(2+) channel blocker) and AMP (ADP antagonist) abrogated urease-induced aggregation, whereas the PAF-acether antagonist Web2170 had no effect. We concluded that platelet aggregation induced by BPU is mediated by lipoxygenase-derived eicosanoids and secretion of ADP from the platelets through a calcium-dependent mechanism. Potential relevance of these findings for bacterium-plant interactions and pathogenesis of bacterial infections are discussed.

Identification and characterization of a collagen-induced platelet aggregation inhibitor, triplatin, from salivary glands of the assassin bug, Triatoma infestans

To facilitate feeding, certain hematophagous invertebrates possess inhibitors of collagen-induced platelet aggregation in their saliva. However, their mechanisms of action have not been fully elucidated. Here, we describe two major salivary proteins, triplatin-1 and -2, from the assassin bug, Triatoma infestans, which inhibited platelet aggregation induced by collagen but not by other agents including ADP, arachidonic acid, U46619 and thrombin. Furthermore, these triplatins also inhibited platelet aggregation induced by collagen-related peptide, a specific agonist of the major collagen-signaling receptor glycoprotein (GP)VI. Moreover, triplatin-1 inhibited Fc receptor gamma-chain phosphorylation induced by collagen, which is the first step of GPVI-mediated signaling. These results strongly suggest that triplatins target GPVI and inhibit signal transduction necessary for platelet activation by collagen. This is the first report on the mechanism of action of collagen-induced platelet aggregation inhibitors from hematophagus invertebrates.

Releasing or expression modulating mediator involved in hemostasis by Berythractivase and Jararhagin (SVMPs)

PIII snake venom metalloproteases (SVMPs) are structurally related to ADAMs (a disintegrin and metalloprotease human family of proteins). Berythractivase and Jararhagin are PIII SVMPs with 69% homology with different hemostatic properties. In order to clarify these differences and further characterize the biological effects of these proteins, we compared the effect of both proteases on human umbilical vein endothelial cell (HUVEC) for evaluating the release and modulation of coagulation and fibrinolysis mechanisms as well as the expression of their correlated genes. We found that both proteins increase the von Willebrand factor liberation, but did not modulate gene expression. Berythractivase, differently from Jararhagin increased the expression of tissue factor. Our results showed that both SVMPs (Berythractivase and Jararhagin) activate HUVEC releasing or modulating mediators involved in hemostasis. Meanwhile, we can suggest through the up-regulation of TF gene that the studied SVMP acts in a specific manner, suggesting that Jararhagin has preferentially a local action, while Berythractivase can be assumed as a systemic pro-coagulant protein with activity on the surface of HUVECs.

From normal to pathological hemostasis

PURPOSE

To review the evolution of knowledge on physiological hemostasis and the main abnormalities that may interfere with hemostasis in the perioperative period.

METHODS

Narrative review of the literature, including relevant papers published in English.

PRINCIPAL FINDINGS

Physiological hemostasis controls blood fluidity and rapidly induces hemostatic plug formation in order to stop or limit bleeding. The three distinct phases of the hemostatic process, primary hemostasis, coagulation and fibrinolysis are closely linked to each other and precisely regulated in order to efficiently close vessel wounds, promote vascular healing and maintain vessel patency. Primary hemostasis is the result of complex interactions between the vascular wall, platelets and adhesive proteins. Initiation of the coagulation pathway in vivo is secondary to the exposure of tissue factor (TF) and the formation of TF/VIIa complex which can activate both FIX and FX. This initiation phase is followed by a propagation phase with amplification of thrombin generation. Several control mechanisms exist for localizing fibrin formation to the site of injury including tissue factor pathway inhibitor, protein C system, antithrombin, and glycosaminoglycans on the vessel wall. Fibrinolysis is also a highly regulated system that controls fibrin dissolution. Both constitutive and acquired hemostasic defects exist. The consequences of these abnormalities are highly variable according to the type of defect, and to the genetic and environmental background.

CONCLUSION

Hemostasis is one of the most complex physiological self-defence systems, not only involved in control of blood fluidity but also interfering in major physiopathological processes. The evolution of our knowledge of the physiology of hemostasis has numerous implications for therapy.

von Willebrand factor mediates platelet spreading through glycoprotein Ib and alpha(IIb)beta3 in the presence of botrocetin and ristocetin, respectively

BACKGROUND

von Willebrand factor (VWF) plays a critical role in the process of hemostasis by mediating flow-dependent adhesion and spreading of platelets on exposed extracellular matrix proteins following vascular injury. To accomplish this, VWF binds to two distinct platelet receptors: glycoprotein (GP)Ib-IX-V and integrin alpha(IIb)beta3.

OBJECTIVE

To evaluate the ability of GPIb and alpha(IIb)beta3 to mediate platelet adhesion and lamellipodia formation on immobilized VWF in the presence of the biochemical modulators, ristocetin and botrocetin.

RESULTS

In the presence of botrocetin and inhibitors of adenosine diphosphate (ADP) and thromboxane A2 (TxA2), VWF is able to support formation of lamellipodia through a GPIb-dependent mechanism that is independent of alpha(IIb)beta3 and PI3-kinase. Lamellipodia formation under these conditions is incomplete. In marked contrast, in the presence of ristocetin, VWF stimulates formation of fully spread lamellipodia through a pathway that is dependent upon alpha(IIb)beta3 and PI3-kinase. Furthermore, alpha(IIb)beta3 also supports platelet spreading on VWF alone, but only in the absence of inhibitors of ADP and TxA2. The localization of filamentous actin and the Arp2/3 complex in platelets on VWF in the presence of botrocetin and ristocetin are distinct, yielding disparate lamellipodium kinetic signatures. Interestingly, botrocetin significantly enhances platelet adhesion to VWF under flow in whole blood in an alpha(IIb)beta3-independent manner, while ristocetin augments washed platelet adhesion and spreading to VWF under flow in an alpha(IIb)beta3-dependent manner.

CONCLUSIONS

These observations demonstrate that VWF is able to induce lamellipodia formation through distinct receptors, and has important consequences for investigation of the role of VWF-GPIb interactions in the context of platelet regulation.

Impact of fibronectin assembly on platelet thrombus formation in response to type I collagen and von Willebrand factor

Plasma fibronectin enhances platelet thrombus formation on surfaces coated with collagen. We investigated the role of fibronectin assembly in this process. Platelets adherent to fibrillar type I collagen, but not platelets adherent to von Willebrand factor (VWF), supported assembly of plasma fibronectin under static conditions. At a shear rate of 1250 s(-1), platelets adherent to collagen assembled coperfused plasma fibronectin and formed larger thrombi in a fibronectin-concentration-dependent manner, with a maximum effect at 250 mug/mL. Enhanced thrombus formation on collagen was blocked by a peptide that binds to the N-terminal region of fibronectin and inhibits fibronectin assembly. Cross-linking of fibronectin to collagen prior to exposure to platelets had no effect on thrombus formation. Collagen-induced platelet thrombus formation at a shear rate of 5000 s(-1) required coperfusion with VWF and did not result in assembly of coperfused fibronectin. VWF-mediated increase in platelet thrombi on collagen was not enhanced and indeed was somewhat attenuated by coperfused fibronectin at a shear rate of 5000 s(-1). These results indicate that, at moderately high but not very high shear rates, fibronectin assembly in platelet aggregates that form in response to collagen enhances thrombus formation and serves as an alternative to VWF-mediated enhancement.

Influence on platelet aggregation of i.v. parecoxib and acetaminophen in healthy volunteers

BACKGROUND

Acetaminophen (paracetamol) alone or in combination with other analgesics is widely used for postoperative analgesia. While acetaminophen and non-steroidal anti-inflammatory drugs inhibit platelet function, the cyclooxygenase-2 (COX-2) selectively inhibiting coxibs show no interference with platelet function. The authors studied the effect of a combination of i.v. parecoxib and acetaminophen on platelet function in healthy volunteers.

METHODS

Eighteen healthy, male volunteers (22-33 yr) received i.v. acetaminophen 1 g, parecoxib 40 mg+acetaminophen 1 g or placebo in a double-blind, crossover study. Platelet function was assessed by photometric aggregometry and by measuring the release of thromboxane B(2). Plasma acetaminophen concentrations were measured by high-performance liquid chromatography.

RESULTS

Platelet aggregation (median area under the curve) triggered with arachidonic acid 500 microM was 24.6, 3.9 and 4.2x10(3) area units (P=0.02, all groups) after placebo, acetaminophen and parecoxib+acetaminophen, respectively. Inhibition of platelet aggregation showed no difference between acetaminophen alone and the combination (P=0.82). Aggregation triggered with arachidonic acid 750 or 1000 microM, adenosine diphosphate (ADP) 1.5 or 3 microM, or epinephrine 5 microM showed no differences between the groups. Release of thromboxane B(2) in response to ADP was inhibited similarly by both acetaminophen and the combination. Plasma acetaminophen concentrations were similar after acetaminophen and the combination.

CONCLUSIONS

Acetaminophen and parecoxib showed no interaction in inhibiting platelet function. In combination they cause a mild degree of COX-1 inhibition corresponding to that of acetaminophen alone.

Wiskott-Aldrich syndrome protein is involved in alphaIIb beta3-mediated cell adhesion

The Wiskott-Aldrich syndrome (WAS) is an X-chromosome-linked immunodeficiency disorder. The most common symptom seen in WAS patients is bleeding. One of the main causes of bleeding is defective platelet aggregation. The causative gene of WAS encodes WAS protein (WASP). Here, we show that WASP binds to the calcium- and integrin-binding protein (CIB) in platelets. CIB was originally identified as a protein binding to the alphaIIb cytoplasmic tail of platelet integrin alphaIIb beta3, which has a primary role in platelet aggregation. We also show that the WASP-CIB complex is important in alphaIIb beta3-mediated cell adhesion, and that in patients mutant forms of WASP are expressed at reduced levels or show lower affinities for CIB than wild-type WASP. Our results indicate that impaired complex formation between mutant WASPs and CIB reduces alphaIIb beta3-mediated cell adhesion and causes defective platelet aggregation, resulting in bleeding.

Clopidogrel resistance: fact and fiction

The antiplatelet agent clopidogrel in combination with aspirin has been shown to reduce thrombotic events in patients with acute coronary syndromes and/or who are undergoing percutaneous coronary intervention. However, a large interindividual response variability to clopidogrel has been described. The reported rates of inadequate clopidogrel response vary considerably depending on the definition and methodologies used to measure the inhibition of platelet function. Recently, several (small) studies have demonstrated the clinical relevance of an inadequate response to clopidogrel. Moreover, several factors have been associated with a high interindividual variability in response to clopidogrel. These are: dosing, impaired intestinal absorption, cytochrome P450 3A4 and 3A5 activity, drug–drug interactions, polymorphisms of the receptors involved in the process of arterial thrombosis and hemostasis, and the method of measurement of platelet function. Future research for the evaluation of clopidogrel resistance should be based on the assessment of selective P2Y12 receptor inhibition (e.g., the vasodilator-stimulated phosphoprotein-assay or the measurement of stabilization of platelet aggregates) with quick and simple tests. Only then can we reveal the true prevalence and impact of clopidogrel resistance.

Clovamide-Type Phenylpropenoic Acid Amides,N-Coumaroyldopamine andN-Caffeoyldopamine, Inhibit Platelet-Leukocyte Interactions via Suppressing P-Selectin Expression

N-Coumaroyldopamine and N-caffeoyldopamine are clovamide-type phenylpropenoic acid amides found in Theobroma cacao. In this article, N-coumaroyldopamine and N-caffeoyldopamine were investigated to determine their effects on P-selectin expression and platelet-leukocyte interactions in vitro and in vivo models. At the concentration of 0.05 microM, they were able to inhibit P-selectin expression on the platelets by 33 (P < 0.011) and 30% (P < 0.012), respectively. The inhibition was partially blocked by beta(2)-adrenoceptor antagonists, suggesting that beta(2) receptors are probably engaged in the inhibition. N-Caffeoyldopamine and N-coumaroyldopamine could also suppress platelet-leukocyte interactions in blood samples by 36 (P < 0.013) and 32% (P < 0.011), respectively, at the same concentration (0.05 microM). In an animal study, mice administrated orally with N-caffeoyldopamine (50 and 100 microg/35 g of body weight) also showed great reduction in the P-selectin expression and platelet-leukocyte interactions by 31 to 45% (P < 0.011) and 34 to 43% (P < 0.014), respectively. These data suggest that the clovamide-type phenylpropenoic acid amides are able to suppress platelet-leukocyte interactions via inhibiting P-selectin expression.

Thrombus formation in vivo

To examine thrombus formation in a living mouse, new technologies involving intravital videomicroscopy have been applied to the analysis of vascular windows to directly visualize arterioles and venules. After vessel wall injury in the microcirculation, thrombus development can be imaged in real time. These systems have been used to explore the role of platelets, blood coagulation proteins, endothelium, and the vessel wall during thrombus formation. The study of biochemistry and cell biology in a living animal offers new understanding of physiology and pathology in complex biologic systems.

Platelet aggregation-induced by caco-2 cells: regulation by matrix metalloproteinase-2 and adenosine diphosphate

Formation of tumor cell-platelet aggregates facilitates hematogenous metastases. However, molecular mechanisms implicated in tumor cell-induced platelet aggregation (TCIPA) in colon cancer are unclear. To investigate mechanisms of TCIPA induced by colon adenocarcinoma cells in vitro, human Caco-2 cells were used to study their interactions with platelets using aggregometry, zymography, phase-contrast microscopy, and flow cytometry. Caco-2-induced platelet aggregation in a concentration-dependent manner. This aggregation resulted in the release of matrix metalloproteinase (MMP)-2, as measured by zymography. In addition, flow cytometry showed a significant up-regulation of activated GpIIb/IIIa, total GpIIb/IIIa, GpIb, and P-selectin receptors on platelets. Inhibition of MMP-2 by phenantroline and degradation of ADP by APT102, respectively, resulted in inhibition of TCIPA. Furthermore, both phenantroline and APT102 significantly down-regulated the surface abundance of platelet receptors. Caco-2 cells aggregate platelets, at least in part, via releasing MMP-2 and ADP. Modulation of MMP-2 and ADP actions could have therapeutic value in colonic cancer.

Pathogenesis of Thrombosis in Essential Thrombocythemia and Polycythemia Vera: The Role of Neutrophils

Thrombotic complications are frequently observed in patients with polycythemia vera (PV) and essential thrombocythemia (ET). Abnormalities of red blood cells and platelets arising from the clonal rearrangement of hematopoietic cells have been considered, although causal relationships between any of these specific abnormalities and thrombosis have not been clearly established. The involvement of neutrophils and macrophages, which participate in thrombosis and hemostasis, has been insufficiently explored in PV and ET. Persistent activation of circulating neutrophils was recently demonstrated in ET and PV patients, in parallel with an increase in plasma concentrations of endothelial damage-derived and prothrombotic substances. Other studies have explored whether the augmentation of adhesion of neutrophils may affect neutrophil/platelet interaction since a significant increase in circulating neutrophil/platelet aggregates is found in ET and PV. This review summarizes the current knowledge of the pathogenesis of thrombosis in PV and ET, with emphasis on the role of neutrophils in hemostasis and their possible involvement in the mechanisms of the acquired thrombophilia of these patients. Available data suggest that these hemostatic markers deserve to be included in prospective clinical studies aimed at identifying their predictive role in the vascular complications of patients with ET and PV.

Redox regulation of the coagulation cascade

Enhanced coagulation and thrombosis are linked to a variety of cardiovascular and metabolic diseases, as well as to cancer. Many of these diseases are also associated with enhanced levels of reactive oxygen species (ROS). Indeed, ROS have been made responsible for promoting many of these diseases. They have been shown not only to be cytotoxic, but also to serve as signaling molecules in a variety of cells. Recently, evidence accumulated that ROS and the redox state are also important in the control of blood coagulation and thrombosis.

Two-dimensional Blue Native/sodium dodecyl sulfate gel electrophoresis for analysis of multimeric proteins in platelets

Two-dimensional (2-D) Blue Native/SDS gel electrophoresis combines a first-dimensional separation of monomeric and multimeric proteins in their native state with a second denaturing dimension. These high-resolution 2-D gels aim at identifying multiprotein complexes with respect to their subunit composition. We applied this method for the first time to analyze two human platelet subproteomes: the cytosolic and the microsomal membrane protein fraction. Solubilization of platelet membrane proteins was achieved with the nondenaturing detergent n-dodecyl-beta-D-maltoside. To validate native solubilization conditions, we demonstrated the correct assembly of the Na,K-ATPase, a functional multimeric transmembrane protein, when expressed in Xenopus oocytes. We identified 63 platelet proteins after in-gel tryptic digestion of 58 selected protein spots and liquid chromatography-coupled tandem mass spectrometry. Nine proteins were detected for the first time in platelets by a proteomic approach. We also show that this technology efficiently resolves several known membrane and cytosolic multiprotein complexes. Blue Native/SDS gel electrophoresis is thus a valuable procedure to analyze specific platelet subproteomes, like the membrane(-bound) protein fraction, by mass spectrometry and immunoblotting and could be relevant for the study of protein-protein interactions generated following platelet activation.

Receptors, rafts, and microvesicles in thrombosis and inflammation

Hemostasis at sites of blood vessel injury and its pathologic counterpart, thrombosis, involve a complex interplay between several blood elements: soluble proteins of the blood coagulation system, blood cells (most prominently platelets) and cell fragments, and elements of the vessel wall (endothelial cells and, at sites of injury, the exposed matrix and deeper cellular components). In this review, we focus on ways in which specialized membrane microdomains known as lipid rafts are involved in various phases of hemostasis and thrombosis.

A prothrombin activator (Lopap) modulating inflammation, coagulation and cell survival mechanisms

A severe hemorrhagic syndrome produced by contact with Lonomia obliqua caterpillars has become epidemic in southern Brazil. A significant thrombin production with intense consumption of fibrinogen and high D-dimer production indicates a consumption coagulopathy and secondary fibrinolysis in patients. Lopap is a single-chain 69kDa serine protease isolated from the crude extract of L. obliqua bristles. Experiments in mice showed that the purified protein, similar to the crude extract, causes uncoagulable blood by fibrinogen depletion. In order to characterize the effects of Lopap on cells involved with hemostatic system, we performed experiments using human umbilical vein endothelial cells (HUVECs). Our results show that Lopap exerts a direct effect on endothelial cells by increasing the liberation of molecules involved in the regulation of vascular tone, inhibiting platelet activation and chemotaxis, apart from inducing the expression of cell adhesion molecules which participate in inflammatory responses. The release or new synthesis of mediators involved in coagulation as von Willebrand factor and tissue factor, or in fibrinolysis as tissue plasminogen activator, was not affected by Lopap. Also our results demonstrated that Lopap acts on cell survival of HUVECs, regulating the expression of molecules as NO and avoiding cell death.