Integrins: dynamic scaffolds for adhesion and signaling in platelets

The Small GTPase Rap1b: A Bidirectional Regulator of Platelet Adhesion Receptors

Integrins and other families of cell adhesion receptors are responsible for platelet adhesion and aggregation, which are essential steps for physiological haemostasis, as well as for the development of thrombosis. The modulation of platelet adhesive properties is the result of a complex pattern of inside-out and outside-in signaling pathways, in which the members of the Rap family of small GTPases are bidirectionally involved. This paper focuses on the regulation of the main Rap GTPase expressed in circulating platelets, Rap1b, downstream of adhesion receptors, and summarizes the most recent achievements in the investigation of the function of this protein as regulator of platelet adhesion and thrombus formation.

Key role of integrin α(IIb)β (3) signaling to Syk kinase in tissue factor-induced thrombin generation

The fibrin(ogen) receptor, integrin α_IIbβ₃, has a well-established role in platelet spreading, aggregation and clot retraction. How α_IIbβ₃ contributes to platelet-dependent coagulation is less well resolved. Here, we demonstrate that the potent suppressing effect of clinically used α_IIbβ₃ blockers on tissue factor-induced thrombin generation is linked to diminished platelet Ca²⁺ responses and phosphatidylserine (PS) exposure. The same blockers suppress these responses in platelets stimulated with collagen and thrombin receptor agonists, whereas added fibrinogen potentiates these responses. In platelets spreading on fibrinogen, outside-in α_IIbβ₃ signaling similarly enhances thrombin-induced Ca²⁺ rises and PS exposure. These responses are reduced in α_IIbβ₃-deficient platelets from patients with Glanzmann’s thrombasthenia. Furthermore, the contribution of α_IIbβ₃ to tissue factor-induced platelet Ca²⁺ rises, PS exposure and thrombin generation in plasma are fully dependent on Syk kinase activity. Tyrosine phosphorylation analysis confirms a key role of Syk activation, which is largely but not exclusively dependent on α_IIbβ₃ activation. It is concluded that the majority of tissue factor-induced procoagulant activity of platelets relies on Syk activation and ensuing Ca²⁺ signal generation, and furthermore that a considerable part of Syk activation relies on α_IIbβ₃ signaling. These results hence point to a novel role of Syk in integrin-dependent thrombin generation.

In vivo imaging visualizes discoid platelet aggregations without endothelium disruption and implicates contribution of inflammatory cytokine and integrin signaling

The mechanism by which thrombotic vessel occlusion occurs independently of plaque development or endothelial cell (EC) disruption remains unclear, largely because of an inability to visualize the formation of thrombus, especially at the single-platelet level in real time. Here we demonstrate that rapidly developing thrombi composed of discoid platelets can be induced in the mesenteric capillaries, arterioles, and large-sized arteries of living mice, enabling characterization of the kinetics of thrombosis initiation and the multicellular interrelationships during thrombus development. Platelet aggregation without EC disruption was triggered by reactive oxygen species (ROS) photochemically induced by moderate power laser irradiation. The inflammatory cytokines TNF-α and IL-1 could be key components of the EC response, acting through regulation of VWF mobilization to the cell surface. Thrombus formation was then initiated by the binding of platelet GPIbα to endothelial VWF in our model, and this effect was inhibited by the ROS scavenger N-acetylcysteine. Actin linker talin-dependent activation of alphaIIb-beta3 integrin or Rac1 in platelets was required for late-phase thrombus stability. Our novel imaging technology illustrates the molecular mechanism underlying inflammation-based thrombus formation by discoid platelets on undisrupted ECs and suggests control of ROS could be a useful therapeutic target for the prevention of thrombotic diseases.

A mutation in the β3 cytoplasmic tail causes variant Glanzmann thrombasthenia by abrogating transition of αIIb β3 to an active state

BACKGROUND

The cytoplasmic tails of α(IIb) and β(3) regulate essential α(IIb) β(3) functions. We previously described a variant Glanzmann thrombasthenia mutation in the β(3) cytoplasmic tail, IVS14: -3C>G, which causes a frameshift with an extension of β(3) by 40 residues.

OBJECTIVES

The aim of this study was to characterize the mechanism by which the mutation abrogates transition of α(IIb) β(3) from a resting state to an active state.

METHODS

 We expressed the natural mutation, termed 742ins, and three artificial mutations in baby hamster kidney (BHK) cells along with wild-type (WT) α(IIb) as follows: β(3) -742stop, a truncated mutant to evaluate the effect of deleted residues; β(3) -749stop, a truncated mutant that preserves the NPLY conserved sequence; and β(3) -749ins, in which the aberrant tail begins after the conserved sequence. Flow cytometry was used to determine ligand binding to BHK cells.

RESULTS AND CONCLUSIONS

Surface expression of α(IIb) β(3) of all four mutants was at least 60% of WT expression, but there was almost no binding of soluble fibrinogen following activation with activating antibodies (anti-ligand-induced-binding-site 6 [antiLIBS6] or PT25-2). Activation of the α(IIb) β(3) mutants was only achieved when both PT25-2 and antiLIBS6 were used together or following treatment with dithiothreitol. These data suggest that the ectodomain of the four mutants is tightly locked in a resting conformation but can be forced to become active by strong stimuli. These data and those of others indicate that the middle part of the β(3) tail is important for maintaining α(IIb) β(3) in a resting conformation.

Extracellular matrix proteins in hemostasis and thrombosis

The adhesion and aggregation of platelets during hemostasis and thrombosis represents one of the best-understood examples of cell-matrix adhesion. Platelets are exposed to a wide variety of extracellular matrix (ECM) proteins once blood vessels are damaged and basement membranes and interstitial ECM are exposed. Platelet adhesion to these ECM proteins involves ECM receptors familiar in other contexts, such as integrins. The major platelet-specific integrin, αIIbβ3, is the best-understood ECM receptor and exhibits the most tightly regulated switch between inactive and active states. Once activated, αIIbβ3 binds many different ECM proteins, including fibrinogen, its major ligand. In addition to αIIbβ3, there are other integrins expressed at lower levels on platelets and responsible for adhesion to additional ECM proteins. There are also some important nonintegrin ECM receptors, GPIb-V-IX and GPVI, which are specific to platelets. These receptors play major roles in platelet adhesion and in the activation of the integrins and of other platelet responses, such as cytoskeletal organization and exocytosis of additional ECM ligands and autoactivators of the platelets.

Dehydroepiandrosterone-sulfate inhibits thrombin-induced platelet aggregation

Dehydroepiandrosterone (DHEA) and its sulfated form, DHEA-S, are the most abundant steroids circulating in human blood. DHEA stimulates endothelial cells to release high amounts of nitric oxide in the circulation. Nitric oxide activates guanylyl cyclase in platelets thus decreasing the responsiveness of these cells to physiological agonists. However, the impact of DHEA-S and DHEA on platelet function and their possible role in modulating the response of human platelets to physiological agonists were not yet investigated. Here, DHEA-S, but not DHEA, inhibited in vitro thrombin-dependent platelet aggregation in a dose-dependent manner. DHEA-S exerted this effect by decreasing thrombin-dependent dense granule secretion, and so impairing the positive feed-back loop provided by ADP. Furthermore, DHEA-S inhibited thrombin-dependent activation of Akt, ERK1/2, and p38 MAP kinase. Although both DHEA-S and DHEA directly activated in platelets the inhibitory cGMP/PGK/VASP pathway, these events were not responsible for the inhibitory action of DHEA-S in platelets. In addition DHEA-S acted in synergism with nitric oxide in inhibiting platelet aggregation. In conclusion DHEA-S inhibited platelet activation caused by a mild stimulus without completely hampering platelet functionality and thus DHEA-S may participate in the physiological mechanisms that maintain circulating platelets in a resting state. The role played by DHEA-S could be relevant mainly when the functionality of the vascular endothelium is compromised.

Overview: imaging in the study of integrins

Integrins play critical adhesion and signaling roles during development, wound healing, immunity, and cancer. Central to their function is a unique ability to dynamically modulate their adhesiveness and signaling properties through changes in conformation, both homo- and heterotypic protein-protein interactions and cellular distribution. Genetic, biochemical and structural studies have been instrumental in uncovering overall functions, describing ligand and regulatory protein interactions and elucidating the molecular architecture of integrins. However, such approaches alone are inadequate to describe how dynamic integrin behaviors are orchestrated in intact cells. To fill this void, a wide array of distinct light microscopy (largely fluorescence-based) imaging approaches have been developed and employed. Various microscopy technologies, including wide-field, optical sectioning (laser-scanning confocal, spinning-disk confocal, and multiphoton), TIRF and range of novel "Super-Resolution" techniques have been used in combination with diverse imaging modalities (such as IRM, FRET, FRAP, CALI, and fluorescence speckle imaging) to address distinct aspects of integrin function and regulation. This chapter provides an overview of these imaging approaches and how they have advanced our understanding of integrins.

Clot retraction

The study of clot retraction in vitro has been adopted as a simple and reproducible approach to assess platelet function. Plasma clots should retract away from the sides of a glass tube within a few hours allowing the rapid characterization of outside-in signaling through platelet integrin α(IIb)β(3). In this chapter, we describe the role of platelets in fibrin clot retraction and provide a detailed description of the methods used to assess this process.

Platelet signaling

This chapter summarizes current ideas about the intracellular signaling that drives platelet responses to vascular injury. After a brief overview of platelet activation intended to place the signaling pathways into context, the first section considers the early events of platelet activation leading up to integrin activation and platelet aggregation. The focus is on the G protein-mediated events utilized by agonists such as thrombin and ADP, and the tyrosine kinase-based signaling triggered by collagen. The second section considers the events that occur after integrin engagement, some of which are dependent on close physical contact between platelets. A third section addresses the regulatory events that help to avoid unprovoked or excessive platelet activation, after which the final section briefly considers individual variations in platelet reactivity and the role of platelet signaling in the innate immune response and embryonic development.

In vitro generation of megakaryocytes and platelets from human embryonic stem cells and induced pluripotent stem cells

Human embryonic stem cells (hESCs) represent a potential source of blood cells for transfusion therapies and a promising tool for studying the ontogeny of hematopoiesis. Moreover, human-induced pluripotent stem cells (hiPSCs), recently established by defined reprogramming factors expressed in somatic cells, represent a further source for the generation of hematopoietic cells. When undifferentiated hESCs or hiPSCs are cultured on either mesenchymal C3H10T1/2 cells or OP-9 stromal cells, they can be differentiated into a hematopoietic niche that concentrates hematopoietic progenitors, which we named "embryonic stem cell-derived sacs" (ES-sacs). We have optimized the in vitro culture condition for obtaining mature megakaryocytes derived from the hematopoietic progenitors within ES-sacs, which are then able to release platelets. These in vitro-generated platelets display integrin activation capability, indicating normal hemostatic function. This novel protocol thus provides a means of generating platelets from hESCs as well as hiPSCs, for the study of normal human thrombopoiesis and also thrombopoiesis in disease conditions using patient-specific hiPSCs.

Positive charge of chitosan retards blood coagulation on chitosan films

In this study, a series of chitosan films with different protonation degrees were prepared by deacidification with NaOH aqueous or ethanol solutions. The films were then used as a model to investigate the effects of the positive charge of chitosan on blood coagulation. The results showed that the positive charge of chitosan acted as a double-edged sword, in that it promoted erythrocyte adhesion, fibrinogen adsorption, and platelet adhesion and activation, but inhibited activation of the contact system. In contrast to prevailing views, we found that the positive charge of chitosan retarded thrombin generation and blood coagulation on these films. At least two reasons were responsible for this phenomenon. First, the positive charge inhibited the contact activation, and second, the positive charge could not significantly promote the activation of non-adherent platelets in the bulk phase during the early stage of coagulation. The present findings improve our understanding of the events leading to blood coagulation on chitosan films, which will be useful for the future development of novel chitosan-based hemostatic devices.

Acidosis downregulates platelet haemostatic functions and promotes neutrophil proinflammatory responses mediated by platelets

Acidosis is one of the hallmarks of tissue injury such as trauma, infection, inflammation, and tumour growth. Although platelets participate in the pathophysiology of all these processes, the impact of acidosis on platelet biology has not been studied outside of the quality control of laboratory aggregation assays or platelet transfusion optimization. Herein, we evaluate the effect of physiologically relevant changes in extracellular acidosis on the biological function of platelets, placing particular emphasis on haemostatic and secretory functions. Platelet haemostatic responses such as adhesion, spreading, activation of αIIbβ3 integrin, ATP release, aggregation, thromboxane B2 generation, clot retraction and procoagulant activity including phosphatidilserine exposure and microparticle formation, showed a statistically significant inhibition of thrombin-induced changes at pH of 7.0 and 6.5 compared to the physiological pH (7.4). The release of alpha granule content was differentially regulated by acidosis. At low pH, thrombin or collagen-induced secretion of vascular endothelial growth factor and endostatin were dramatically reduced. The release of von Willebrand factor and stromal derived factor-1α followed a similar, albeit less dramatic pattern. In contrast, the induction of CD40L was not changed by low pH, and P-selectin exposure was significantly increased. While the generation of mixed platelet-leukocyte aggregates and the increased chemotaxis of neutrophils mediated by platelets were further augmented under acidic conditions in a P-selectin dependent manner, the increased neutrophil survival was independent of P-selectin expression. In conclusion, our results indicate that extracellular acidosis downregulates most of the haemostatic platelet functions, and promotes those involved in amplifying the neutrophil-mediated inflammatory response.

A point mutation in the EGF-4 domain of β(3) integrin is responsible for the formation of the Sec(a) platelet alloantigen and affects receptor function

Neonatal alloimmune thrombocytopenia (NAIT) is caused by fetomaternal platelet incompatibility with maternal antibodies crossing the placenta and destroying fetal platelets. Antibodies against human platelet antigen-1a (HPA-1a) and HPA-5b are responsible for the majority of NAIT cases. We observed a suspected NAIT in a newborn with a platelet count of 25 G/l and petechial haemorrhages. Serological analysis of maternal serum revealed an immunisation against αIIbβ3 on paternal platelets only, indicating the presence of an antibody against a new rare alloantigen (Sec(a)) residing on αIIbβ3. The location of Sec(a) on αIIbβ3 was confirmed by immunoprecipitation. Nucleotide sequence analysis of paternal β3 revealed a single nucleotide exchange (G(1818)T) in exon 11 of the β3 gene (ITGB3), changing Lys(580) (wild-type) to Asn(580) (Sec(a)). Two additional members of the family Sec were typed Sec(a) positive, but none of 300 blood donors. Chinese hamster ovary cells expressing Asn(580), but not Lys(580) αIIbβ3, bound anti-Sec(a), which was corroborated by immunoprecipitation. Adhesion of transfected cells onto immobilised fibrinogen showed reduced binding of the Asn(580) variant compared to wild-type αIIbβ3. Analysis of transfected cells with anti-LIBS and PAC-1 antibody showed reduced binding when compared to the wild-type. No such effects were observed with Sec(a) positive platelets, which, however, are heterozygous for the Lys(580)Asn mutation. In this study, we describe a NAIT case caused by maternal alloimmunisation against a new antigen on αIIbβ3. Analysis with mutant transfected cells showed that the Lys(580)Asn mutation responsible for the formation of the Sec(a) antigenic determinant affects αIIbβ3 receptor function.

Using antibodies against P2Y and P2X receptors in purinergic signaling research

The broad expression pattern of the G protein-coupled P2Y receptors has demonstrated that these receptors are fundamental determinants in many physiological responses, including neuromodulation, vasodilation, inflammation, and cell migration. P2Y receptors couple either G(q) or G(i) upon activation, thereby activating different signaling pathways. Ionotropic ATP (P2X) receptors bind extracellular nucleotides, a signal which is transduced within the P2X protein complex into a cation channel opening, which usually leads to intracellular calcium concentration elevation. As such, this family of proteins initiates or shapes several cellular processes including synaptic transmission, gene expression, proliferation, migration, and apoptosis. The ever-growing range of applications for antibodies in the last 30 years attests to their major role in medicine and biological research. Antibodies have been used as therapeutic tools in cancer and inflammatory diseases, as diagnostic reagents (flow cytometry, ELISA, and immunohistochemistry, to name a few applications), and in widespread use in biological research, including Western blot, immunoprecipitation, and ELISPOT. In this article, we will showcase several of the advances that scientists around the world have achieved using the line of antibodies developed at Alomone Labs for P2Y and P2X receptors.

Diagnostic importance of platelet parameters in patients with acute coronary syndrome admitted to a tertiary care hospital in southwest region, Saudi Arabia

OBJECTIVE

Identifying risk factors for acute coronary syndrome (ACS) is important for both diagnostic and prognostic purposes. Abnormal platelet parameters, mainly platelet count (PC), mean platelet volume (MPV) and platelet distribution width (PDW) are thought to be among these risk factors. In this study, the associations between PC, MPV and PDW and ACS were investigated in patients admitted to the tertiary care hospital in the south west region of Saudi Arabia.

MATERIALS AND METHODS

A retrospective cohort of 212 patients with the diagnosis of ACS admitted to Aseer Central Hospital during the period extending from February 1, 2008 to October 31, 2008 were included. The control group consisted of 49 matched subjects who were admitted for chest pain investigation and subsequently found to be non-cardiac chest pain after performing relevant investigations. Blood samples were taken at the time of admission for platelet parameters. Statistical analysis was made using SPSS software and P-values were considered significant if <0.05.

RESULTS

A total of 212 patients with acute coronary syndrome (80 patients with MI and 132 patients with UA) and 49 matched controls were studied. The PC was not statistically different among the three groups (283.3 ± 94.8 × 10(9) L(-1) for MI cases, 262 ± 60.8 × 10(9) L(-1) for UA cases and 275.8 ± 58.9 × 10(9) L(-1) for controls). The MPV was significantly larger in MI cases compared to controls (8.99 ± 1.5 fl vs. 8.38 ± 0.51 fl, respectively, P < 0.009), similarly, the MPV was significantly larger in UA cases compared to controls (9.23 ± 1.19 fl vs. 8.38 ± 0.51 fl, respectively, P < 0.001). The PDW was significantly higher in MI cases compared to controls (15.88 ± 1.5 fl vs. 11.96 ± 1.8 fl, respectively, P < 0.001), similarly, the PDW as also significantly larger in UA cases compared to controls (18.1 ± 18 fl vs. 11.96 ± 1.8 fl, respectively, P < 0.019).

CONCLUSION

Platelet parameters mainly MPV and PDW are readily available and relatively simple and inexpensive laboratory tests which we detected to be significantly raised in patients who have suffered an acute coronary syndrome compared with controls.

Reconstruction of integrin activation

Integrins are integral membrane proteins that mediate cell-matrix and cell-cell adhesion. They are important for vascular development and hematopoiesis, immune and inflammatory responses, and hemostasis. Integrins are also signaling receptors that can transmit information bidirectionally across plasma membranes. Research in the past 2 decades has made progress in unraveling the mechanisms of integrin signaling and brings the field to the moment of attempting synthetic reconstruction of the signaling pathways in vitro. Reconstruction of biologic processes provides stringent tests of our understanding of the process, as evidenced by studies of other biologic machines, such as ATP synthase, lactose permease, and G-protein-coupled receptors. Here, we review recent progress in reconstructing integrin signaling and the insights that we have gained through these experiments.

A few immobilized thrombins are sufficient for platelet spreading

Eukaryotic cells respond to signaling molecules with picomolar to nanomolar sensitivities. However, molar concentrations give no suggestion of the sufficient number of molecules per cell and are confusing when referring to physiological situations in which signaling molecules act in an immobilized state. Here, we studied platelet adhesion by thrombin, a key step in normal hemostasis and pathological arterial thrombosis. We generated a biofunctional nanosheet surface to mimic the in vivo solid-state interaction between platelets and thrombin at sites of injured tissues. We observed that <10 molecules readily activate platelets with high specificity, resulting in platelet adhesion and spreading. This number is much lower than expected from previous experiments in solution, in which the sole activation of platelets required a >1000-fold stoichiometric excess of thrombin. We conclude that immobilizing thrombin apposed to the membrane receptor allows platelets to respond with very high sensitivity. Moreover, we propose that irreversible cell activation may require several ligands to avoid activation by single, mislocalized signaling molecules.

An important role for Akt3 in platelet activation and thrombosis

The Akt family of serine/threonine kinases includes Akt1, Akt2, and Akt3 isoforms. Prior studies have reported that Akt1 and Akt2, but not Akt3, are expressed in platelets. Here, we show that Akt3 is expressed in substantial amounts in platelets. Akt3(-/-) mouse platelets selectively exhibit impaired platelet aggregation and secretion in response to low concentrations of thrombin receptor agonists and thromboxane A₂ (TXA₂), but not collagen or VWF. In contrast, platelets from Akt1(-/-) or Akt2(-/-) mice are defective in platelet activation induced by thrombin, TXA₂, and VWF, but only Akt1(-/-) platelets show significant defects in response to collagen, indicating differences among Akt isoforms. Akt3(-/-) platelets exhibit a significant reduction in thrombin-induced phosphorylation of glycogen synthase kinase 3β (GSK-3β) at Ser9, which is known to inhibit GSK-3β function. Thus, Akt3 is important in inhibiting GSK-3β. Accordingly, treatment of Akt3(-/-) platelets with a GSK-3β inhibitor rescued the defect of Akt3(-/-) platelets in thrombin-induced aggregation, suggesting that negatively regulating GSK-3β may be a mechanism by which Akt3 promotes platelet activation. Importantly, Akt3(-/-) mice showed retardation in FeCl₃-induced carotid artery thrombosis in vivo. Thus, Akt3 plays an important and distinct role in platelet activation and in thrombosis.

Hyperthermia inhibits platelet hemostatic functions and selectively regulates the release of alpha-granule proteins

BACKGROUND

Hyperthermia is one of the main disturbances of homeostasis occurring during sepsis or hypermetabolic states such as cancer. Platelets are important mediators of the inflammation that accompanies these processes, but very little is known about the changes in platelet function that occur at different temperatures.

OBJECTIVES

To explore the effect of higher temperatures on platelet physiology.

METHODS

Platelet responses including adhesion, spreading (fluorescence microscopy), α(IIb)β(3) activation (flow cytometry), aggregation (turbidimetry), ATP release (luminescence), thromboxane A(2) generation, alpha-granule protein secretion (ELISA) and protein phosphorylation from different signaling pathways (immunoblotting) were studied.

RESULTS

Preincubation of platelets at temperatures higher than 37 °C (38.5-42 °C) inhibited thrombin-induced hemostasis, including platelet adhesion, aggregation, ATP release and thromboxane A(2) generation. The expression of P-selectin and CD63, as well as vascular endothelial growth factor (VEGF) release, was completely inhibited by hyperthermia, whereas von Willebrand factor (VWF) and endostatin levels remained substantially increased at high temperatures. This suggested that release of proteins from platelet granules is modulated not only by classical platelet agonists but also by microenvironmental factors. The observed gradation of response involved not only antiangiogenesis regulators, but also other cargo proteins. Some signaling pathways were more stable than others. While ERK1/2 and AKT phosphorylation were resistant to changes in temperature, Src, Syk, p38 phosphorylation and IkappaB degradation were decreased in a temperature-dependent fashion.

CONCLUSIONS

Higher temperatures, such as those observed with fever or tissue invasion, inhibit the hemostatic functions of platelets and selectively regulate the release of alpha-granule proteins.

Platelet adhesion and activation mechanisms in arterial thrombosis and ischaemic stroke

Platelet adhesion, activation and aggregation on the exposed subendothelial extracellular matrix (ECM) are essential for haemostasis, but may also lead to occlusion of diseased vessels. Binding of the glycoprotein (GP)Ib-V-IX complex to immobilised von Willebrand factor (VWF) initiates adhesion of flowing platelets to the ECM, and thereby enables the collagen receptor GPVI to interact with its ligand and to mediate platelet activation. This process is reinforced by locally produced thrombin and platelet-derived secondary mediators, such as adenosine diphosphate (ADP) and thromboxane A(2) (TxA(2)). Together, these events promote a shift of β1 and β3 integrins from a low to a high affinity state for their ligands through 'inside-out' signalling allowing firm platelet adhesion and aggregation. Formed platelet aggregates are stabilised by fibrin formation and signalling events between adjacent platelets involving multiple platelet receptors, such as the newly discovered C-type lectin-like receptor 2 (CLEC-2). While occlusive thrombus formation is the principal pathogenic event in myocardial infarction, the situation is more complex in ischaemic stroke where infarct development often progresses despite sustained early reperfusion of previously occluded major intracranial arteries, a process referred to as 'reperfusion injury'. Increasing experimental evidence now suggests that early platelet adhesion and activation events, orchestrate a 'thrombo-inflammatory' cascade in this setting, whereas platelet aggregation and thrombus formation are not required. This review summarises recent developments in understanding the principal platelet adhesion receptor systems with a focus on their involvement in arterial thrombosis and ischaemic stroke models.

The adaptor Lnk (SH2B3): an emerging regulator in vascular cells and a link between immune and inflammatory signaling

A better knowledge of the process by which inflammatory extracellular signals are relayed from the plasma membrane to specific intracellular sites is a key step to understand how inflammation develops and how it is regulated. This review focuses on Lnk (SH2B3) a member, with SH2B1 and SH2B2, of the SH2B family of adaptor proteins that influences a variety of signaling pathways mediated by Janus kinase and receptor tyrosine kinases. SH2B adaptor proteins contain conserved dimerization, pleckstrin homology, and SH2 domains. Initially described as a regulator of hematopoiesis and lymphocyte differentiation, Lnk now emerges as a key regulator in hematopoeitic and non hematopoeitic cells such as endothelial cells (EC) moderating growth factor and cytokine receptor-mediated signaling. In EC, Lnk is a negative regulator of TNF signaling that reduce proinflammatory phenotype and prevent EC from apoptosis. Lnk is a modulator in integrin signaling and actin cytoskeleton organization in both platelets and EC with an impact on cell adhesion, migration and thrombosis. In this review, we discuss some recent insights proposing Lnk as a key regulator of bone marrow-endothelial progenitor cell kinetics, including the ability to cell growth, endothelial commitment, mobilization, and recruitment for vascular regeneration. Finally, novel findings also provided evidences that mutations in Lnk gene are strongly linked to myeloproliferative disorders but also autoimmune and inflammatory syndromes where both immune and vascular cells display a role. Overall, these studies emphasize the importance of the Lnk adaptor molecule not only as prognostic marker but also as potential therapeutic target.

Regulation of integrin activation

Regulation of cell-cell and cell-matrix interaction is essential for the normal physiology of metazoans and is important in many diseases. Integrin adhesion receptors can rapidly increase their affinity (integrin activation) in response to intracellular signaling events in a process termed inside-out signaling. The transmembrane domains of integrins and their interactions with the membrane are important in inside-out signaling. Moreover, integrin activation is tightly regulated by a complex network of signaling pathways. Here, we review recent progress in understanding how the membrane environment can, in cooperation with integrin-binding proteins, regulate integrin activation.

Platelets at work in primary hemostasis

When platelet numbers are low or when their function is disabled, the risk of bleeding is high, which on the one hand indicates that in normal life vascular damage is a rather common event and that hence the role of platelets in maintaining a normal hemostasis is a continuously ongoing physiological process. Upon vascular injury, platelets instantly adhere to the exposed extracellular matrix resulting in platelet activation and aggregation to form a hemostatic plug. This self-amplifying mechanism nevertheless requires a tight control to prevent uncontrolled platelet aggregate formation that eventually would occlude the vessel. Therefore endothelial cells produce inhibitory compounds such as prostacyclin and nitric oxide that limit the growth of the platelet thrombus to the damaged area. With this review, we intend to give an integrated survey of the platelet response to vascular injury in normal hemostasis.

A1/A2 polymorphism of the glycoprotein IIIa gene and diabetic retinopathy in Caucasians with type 2 diabetes

BACKGROUND

A PlA1/A2 polymorphism of glycoprotein IIIa is known to be involved in the pathogenesis of arterial thrombosis, myocardial infarction, stroke and type 2 diabetes, but there is no evidence of association with diabetic retinopathy. The aim of this study was to examine the role of the PlA1/A2 polymorphism of the glycoprotein IIIa gene in the development of diabetic retinopathy in Caucasians with type 2 diabetes.

DESIGN

Cross-sectional case-control study.

PARTICIPANTS

Totally 222 patients with diabetic retinopathy and 120 diabetic subjects without clinical signs of diabetic retinopathy from the Eye Clinic, University Medical Centre Ljubljana were enrolled in the study.

METHODS

Fundus examination and blood biochemical analysis were performed. The polymerase chain reaction and restriction fragment length polymorphism were used.

MAIN OUTCOME MEASURES

The total cholesterol, triglyceride, high-density lipoprotein levels, fasting blood glucose and HbA(1c) were measured, and the genotypes of the PlA1/A2 polymorphism were determined.

RESULTS

Patients with diabetic retinopathy had earlier onset, longer duration of type 2 diabetes and a higher incidence of insulin therapy compared to the diabetic patients without diabetic retinopathy. A significantly lower frequency of the A2A2 genotype of glycoprotein IIIa was found in diabetic patients with retinopathy compared to those without retinopathy (odds ratio = 0.49, 95% confidence interval = 0.28-0.89; P = 0.018).

CONCLUSIONS

The A2A2 genotype of the glycoprotein IIIa polymorphism was associated with lower risk for diabetic retinopathy in Caucasians with type 2 diabetes. Further studies are needed to elucidate its protective role in the development of diabetic retinopathy in Caucasians.

A Label-Free Approach to Identify Inhibitors of α4β7-Mediated Cell Adhesion to MadCAM

Traditionally, cell adhesion assays are performed in a manual workstation format using fluorescence-based readouts. Herein, the authors describe a label-free homogeneous assay to identify inhibitors of α4β7 integrin-mediated cell adhesion to its ligand, the mucosal addressin cell adhesion molecule (MadCAM), using the SRU BIND platform. The biosensor is optically based and comprises a subwavelength polymer grating. The assay was validated using standard compounds and an α4 blocking antibody and correlated very closely with the manual assay format when running a battery of test compounds of varying potencies. Cell adhesion was strictly dependent on the presence of divalent cations where Mg2+ was greater than Ca2+ at promoting cell adhesion. This homogeneous and label-free format exhibited low variability with a calculated Z′ of 0.6. In addition to measuring α4β7-mediated 8866 cell adhesion to MadCAM, the authors also demonstrate that this platform can measure adhesion of Jurkat cells expressing α4β1 to the vascular cell adhesion molecule. Thus, the SRU BIND platform is widely applicable to measuring cell adhesion events mediated by other integrins binding to their receptors in an assay format that is amenable to high-throughput screening.

Antiplatelet activity of Phellinus baummii methanol extract is mediated by cyclic AMP elevation and inhibition of collagen-activated integrin-α(IIb) β₃ and MAP kinase

Phellinus baumii is a mushroom that has been used as folk medicine against various diseases and is reported to have antidiabetic, anticancer, antioxidant, antiinflammatory and antihypertensive activities. However, information on the effects of P. baumii extract in platelet function is limited. Therefore, the aim of this study was to examine the impact of a P. baumii methanol extract (PBME) on platelet activation and to investigate the mechanism behind its antiplatelet activity. PBME effects on agonist-induced platelet aggregation, granule secretion, [Ca²⁺](i) mobilization, α(IIb) β₃ activation, cyclic AMP release and mitogen-activated protein kinase (MAPK) phosphorylations were studied using rat platelets. PBME dose-dependently inhibited collagen, thrombin and ADP-induced platelet aggregation with an IC₅₀ of 51.0 ± 2.4, 54.0 ± 2.1 and 53.0 ± 4.3 μg/mL, respectively. Likewise, thrombin-induced [Ca²⁺](i) and collagen-activated ATP secretions were suppressed in PBME treated platelets. Aggregation and ATP secretion were also markedly attenuated by PBME alone or in combination with PP2 (Src inhibitor) and U-73122 (PLC inhibitor) in collagen-stimulated platelets. Besides, PBME treatment elevated basal cyclic AMP levels and inhibited collagen-induced integrin-α(IIb) β₃ activation. Moreover, PBME attenuated extracellular-signal-regulated protein kinase 2 (ERK2) and c-Jun N-terminal kinase 1 (JNK1) phosphorylations. Further PD98059 (ERK inhibitor) and SP60025 (JNK inhibitor) reduced collagen-induced platelet aggregation and ATP secretion. In conclusion, the observed PBME antiplatelet activity may be mediated by activation of cyclic AMP and inhibition of ERK2 and JNK1 phosphorylations. Finally, these data suggest that PBME may have therapeutic potential for the treatment of cardiovascular diseases that involve aberrant platelet function.

G(12/13) signaling pathways substitute for integrin αIIbβ3-signaling for thromboxane generation in platelets

BACKGROUND

We have previously shown that ADP-induced TXA(2) generation requires signaling from αIIbβ3 integrin in platelets. Here we observed that, unlike ADP, protease-activated receptor (PAR)-mediated TXA(2) generation occurs independently of αIIbβ3. PAR agonists, but not ADP, activate G(12/13) signaling pathways. Hence, we evaluated the role of these pathways in TXA(2) generation.

PRINCIPAL FINDINGS

Inhibition of ADP-induced thromboxane generation by fibrinogen receptor antagonist SC57101 was rescued by co-stimulation of G(12/13) pathways with YFLLRNP. This observation suggested an existence of a common signaling effector downstream of integrins and G(12/13) pathways. Hence, we evaluated role of three potential tyrosine kinases; c-Src, Syk and FAK (Focal Adhesion Kinase) that are known to be activated by integrins. c-Src and Syk kinase did not play a role in ADP-induced functional responses in platelets. Selective activation of G(12/13) pathways resulted in the activation of FAK, in the absence of integrin signaling. Interestingly, αIIbβ3-mediated FAK activation occurred in a Src family kinase (SFK)-independent manner whereas G(12/13) pathway caused FAK activation in a SFK and RhoA-dependent manner. A FAK selective inhibitor TAE-226, blocked TXA(2) generation. However, in comparison to WT mice, Pf4-Cre/Fak-Floxed mice did not show any difference in platelet TXA(2) generation.

CONCLUSIONS

Therefore, we conclude that differential activation of FAK occurs downstream of Integrins and G(12/13) pathways. However, the common effector molecule, possibly a tyrosine kinase downstream of integrins and G(12/13) pathways contributing to TXA(2) generation in platelets remains elusive.

Human platelets express and are activated by galectin-8

Gals (galectins) are proteins with glycan affinity that are emerging as mediators of atherosclerosis. Despite the similarities in structure and sequence, different Gals exert distinct effects on their target cells. We have shown that Gal-1 triggers platelet activation, suggesting a role for Gals in thrombus formation. Since Gal-8 is expressed upon endothelial activation and also contributes to inflammation, to understand further the role of these lectins in haemostasis, we evaluated the effect of Gal-8 on human platelets. Gal-8 bound specific glycans in the platelet membrane and triggered spreading, calcium mobilization and fibrinogen binding. It also promoted aggregation, thromboxane generation, P-selectin expression and granule secretion. GP (glycoprotein) αIIb and Ib-V were identified as putative Gal-8 counter-receptors by MS. Studies performed using platelets from Glanzmann's thromboasthenia and Bernard-Soulier syndrome patients confirmed that GPIb is essential for transducing Gal-8 signalling. Accordingly, Src, PLC2γ (phospholipase C2γ), ERK (extracellular-signal-regulated kinase) and PI3K (phosphoinositide 3-kinase)/Akt downstream molecules were involved in the Gal-8 signalling pathway. Gal-8 fragments containing either the N- or C-terminal carbohydrate-recognition domains showed that activation is exerted through the N-terminus. Western blotting and cytometry showed that platelets not only contain Gal-8, but also expose Gal-8 after thrombin activation. These findings reveal Gal-8 as a potent platelet activator, supporting a role for this lectin in thrombosis and inflammation.

The integrin LFA-1 signals through ZAP-70 to regulate expression of high-affinity LFA-1 on T lymphocytes

The integrin lymphocyte function-associated antigen 1 (LFA-1) controls many functions of T lymphocytes and is particularly essential during lymphocyte migration from blood into tissues. LFA-1 is considered to initiate "outside-in" signaling when bound to ligand intercellular adhesion molecule 1 (ICAM-1), but little is known about the proteins involved or where in the cell such LFA-1-mediated signaling might be operating. Here we show that LFA-1 is constitutively associated with the protein tyrosine kinases Lck and zeta chain-associated protein of 70 kDa (ZAP-70). When LFA-1 binds ICAM-1, both kinases become phosphorylated and the consequence of kinase activation is the conversion of intermediate- to high-affinity LFA-1 and an increase in close contact with ICAM-1. In the polarized T lymphocyte, phospho-ZAP-70 is concentrated within a region of high-affinity LFA-1 that includes talin and encompasses the lamella/lamellipodial interface as well as further back in the cell. Deficiency of ZAP-70 through inhibition or knockdown in T lymphocytes decreases the speed of migration on ICAM-1, as well as reducing firm adhesion under shear-flow conditions. Through its control of high-affinity LFA-1, the LFA-1/Lck/ZAP-70 complex is in position to initiate the rapid adhesion strengthening and migration necessary for T-lymphocyte responses when stimulated vasculature is encountered at sites of infection or injury.

Overview: structural biology of integrins

Integrins are cell adhesion molecules that play important roles in many biological processes including hemostasis, immune responses, development, and cancer. Their adhesiveness is dynamically regulated through a process termed inside-out signaling. In addition, ligand binding transduces outside-in signals from the extracellular domain to the cytoplasm. Advances in the past several years have shed light on structural basis for integrin regulation and signaling, especially how the large-scale reorientations of the ectodomain are related to the inter-domain and intra-domain shape shifting that changes ligand-binding affinity. Experiments have also shown how the conformational changes of the ectodomain are linked to changes in the α- and β-subunit transmembrane and cytoplasmic domains.

Signaling during platelet adhesion and activation

Upon vascular injury, platelets are activated by adhesion to adhesive proteins, such as von Willebrand factor and collagen, or by soluble platelet agonists, such as ADP, thrombin, and thromboxane A(2). These adhesive proteins and soluble agonists induce signal transduction via their respective receptors. The various receptor-specific platelet activation signaling pathways converge into common signaling events that stimulate platelet shape change and granule secretion and ultimately induce the "inside-out" signaling process leading to activation of the ligand-binding function of integrin α(IIb)β(3). Ligand binding to integrin α(IIb)β(3) mediates platelet adhesion and aggregation and triggers "outside-in" signaling, resulting in platelet spreading, additional granule secretion, stabilization of platelet adhesion and aggregation, and clot retraction. It has become increasingly evident that agonist-induced platelet activation signals also cross talk with integrin outside-in signals to regulate platelet responses. Platelet activation involves a series of rapid positive feedback loops that greatly amplify initial activation signals and enable robust platelet recruitment and thrombus stabilization. Recent studies have provided novel insight into the molecular mechanisms of these processes.

Von willebrand factor increases endothelial cell adhesiveness for human mesenchymal stem cells by activating p38 mitogen-activated protein kinase

Introduction

Delivered systemically or natively circulating mesenchymal stem cells accumulate in injured tissues. During homing mesenchymal stem cells adhere to endothelial cells and infiltrate underlying tissue. Previously we have shown that adhesiveness of endothelial cells for mesenchymal stem cells correlates with the inhibition of mitochondrial function of endothelial cells and secretion of von Willebrand factor. We hypothesized that von Willebrand factor is an auto/paracrine regulator of endothelial cell adhesiveness and studied the effect of von Willebrand factor on adhesion of mesenchymal stem cells to endothelial cells.

Methods

We used Affymetrix DNA microarrays, human protein phospho-MAPK array, Western blot, cell-based ELISA and flow cytometry analysis to study the activation of endothelial cells by von Willebrand factor. Cell adhesion assay and protein kinase inhibitors were used to evaluate the role of mitogen-activated protein kinases in the regulation of endothelial cell adhesiveness for mesenchymal stem cell.

Results

Treatment of endothelial cells with von Willebrand factor stimulated the mesenchymal stem cell adhesion in a time- and concentration-dependent manner. Mesenchymal stem cells did not adhere to immobilized von Willebrand factor and did not express receptors for von Willebrand factor suggesting that the stimulation of the mesenchymal stem cell adhesion is a result of endothelial cell activation with von Willebrand factor. Treatment of endothelial cells with von Willebrand factor activated ERK-1,2 and p38 MAPK without an effect on gene or cell surface expression of E-selectin, P-selectin, VCAM1 and ICAM1. Inhibition of p38 MAPK, but not ERK-1,2, in endothelial cells completely abrogated the stimulation of the mesenchymal stem cell adhesion by von Willebrand factor.

Conclusions

Von Willebrand factor is an auto/paracrine regulator of endothelial cells. Activation of p38 MAPK in endothelial cells by von Willebrand factor is responsible for the regulation of endothelial cell adhesiveness for mesenchymal stem cells.

Tests of integrin transmembrane domain homo-oligomerization during integrin ligand binding and signaling

Integrin transmembrane (TM) and/or cytoplasmic domains play a critical role in integrin bidirectional signaling. Although it has been shown that TM and/or cytoplasmic α and β domains associate in the resting state and separation of these domains is required for both inside-out and outside-in signaling, the role of TM homomeric association remains elusive. Formation of TM homo-oligomers was observed in micelles and bacterial membranes previously, and it has been proposed that homomeric association is important for integrin activation and clustering. This study addresses whether integrin TM domains form homo-oligomers in mammalian cell membranes using cysteine scanning mutagenesis. Our results show that TM homomeric interaction does not occur before or after soluble ligand binding or during inside-out activation. In addition, even though the cysteine mutants and the heterodimeric disulfide-bounded mutant could form clusters after adhering to immobilized ligand, the integrin TM domains do not form homo-oligomers, suggesting that integrin TM homomeric association is not critical for integrin clustering or outside-in signaling. Therefore, integrin TM homo-oligomerization is not required for integrin activation, ligand binding, or signaling.

Pathogen sensing, subsequent signalling, and signalosome in human platelets

Beyond haemostasis, platelets exert a potent role in innate immunity and particularly in its inflammatory arm. The extent of this action remains however debatable, despite clear - and old - evidence of a link between platelets and infection. Platelets can sense infectious pathogens by pathogen recognition receptors and they can even discriminate between various types of infectious signatures. In reply, they can shape their capacity to respond by activating a signalosome and by producing different profiles of pro-inflammatory cytokines and related products. The links between pathogen sensing, signalosome activation and protein production, and their finely tuned regulation are still under investigation since platelets lack a nucleus and thus, canonical molecular biology and genomics apparati.

Analysis of morphology of platelet aggregates formed on collagen under laminar blood flow

In a focal injury model, platelets adhere and activate under flow on a collagen-coated surface, creating a field of individual platelet aggregates. These aggregates exhibit distinct structural characteristics that are linked to the local flow conditions. By combining image analysis techniques and epifluorescence microscopy, we developed a robust strategy for quantifying the characteristic instantaneous width and length of a growing platelet deposit. We have confirmed the technique using model images consisting of ellipsoid objects and quantified the shear rate-dependent nature of aggregate morphology. Venous wall shear rate conditions (100 s(-1)) generated small, circular platelet deposits, whereas elevated arterial shear rates (500 and 1000 s(-1)) generated platelet masses elongated twofold in the direction of flow. At 2000 s(-1), an important regime for von Willebrand Factor (vWF)-mediated recruitment, we observed sporadic platelet capture events on collagen that led to rapidly growing deposits. Furthermore, inter-donor differences were investigated with respect to aggregate growth rate. After perfusion at elevated shear rates (1000 s(-1)) for 5 min, we identified a twofold increase in aggregate size (81.5 ± 24.6 μm; p < 0.1) and a threefold increase in growth rate parallel to the flow (0.40 ± 0.09 μm/s; p < 0.01) for an individual donor. Suspecting a role for vWF, we found that this donor had a twofold increase in soluble vWF relative to the other donors and pooled plasma. Microfluidic devices in combination with automated morphology analysis offer new tools for characterizing clot development under flow.

A potential role for α-actinin in inside-out αIIbβ3 signaling

Many different biochemical signaling pathways regulate integrin activation through the integrin cytoplasmic tail. Here, we describe a new role for α-actinin in inside-out integrin activation. In resting human platelets, α-actinin was associated with αIIbβ3, whereas inside-out signaling (αIIbβ3 activation signals) from protease-activated receptors (PARs) dephosphorylated and dissociated α-actinin from αIIbβ3. We evaluated the time-dependent changes of the αIIbβ3 activation state by measuring PAC-1 binding velocity. The initial velocity analysis clearly showed that PAR1-activating peptide stimulation induced only transient αIIbβ3 activation, whereas PAR4-activating peptide induced long-lasting αIIbβ3 activation. When αIIbβ3 activation signaling dwindled, α-actinin became rephosphorylated and reassociated with αIIbβ3. Compared with control platelets, the dissociation of α-actinin from αIIbβ3 was only transient in PAR4-stimulated P2Y(12)-deficient platelets in which the sustained αIIbβ3 activation was markedly impaired. Overexpression of wild-type α-actinin, but not the mutant Y12F α-actinin, increased its binding to αIIbβ3 and inhibited PAR1-induced initial αIIbβ3 activation in the human megakaryoblastic cell line, CMK. In contrast, knockdown of α-actinin augmented PAR-induced αIIbβ3 activation in CMK. These observations suggest that α-actinin might play a potential role in setting integrins to a default low-affinity ligand-binding state in resting platelets and regulating αIIbβ3 activation by inside-out signaling.

Identification of interacting hot spots in the beta3 integrin stalk using comprehensive interface design

Protein-protein interfaces are usually large and complementary surfaces, but specific side chains, representing energetic "hot spots," often contribute disproportionately to binding free energy. We used a computational method, comprehensive interface design, to identify hot spots in the interface between the stalk regions of the β3 and the complementary αIIb and αv integrin subunits. Using the Rosetta alanine-scanning and design algorithms to predict destabilizing, stabilizing, and neutral mutations in the β3 region extending from residues Lys(532) through Gly(690), we predicted eight alanine mutations that would destabilize the αIIbβ3 interface as well as nine predicted to destabilize the αvβ3 interface, by at least 0.3 kcal/mol. The mutations were widely and unevenly distributed, with four between residues 552 and 563 and five between 590 and 610, but none between 565 and 589, and 611 and 655. Further, mutations destabilizing the αvβ3 and αIIbβ3 interfaces were not identical. The predictions were then tested by introducing selected mutations into the full-length integrins expressed in Chinese hamster ovary cells. Five mutations predicted to destabilize αIIb and β3 caused fibrinogen binding to αIIbβ3, whereas three of four predicted to be neutral or stabilizing did not. Conversely, a mutation predicted to destabilize αvβ3, but not αIIbβ3 (D552A), caused osteopontin binding to αvβ3, but not fibrinogen binding to αIIbβ3. These results indicate that stability of the distal stalk interface is involved in constraining integrins in stable, inactive conformations. Further, they demonstrate the ability of comprehensive interface design to identify functionally significant integrin mutations.

Effects of down-regulation of integrin-beta1 expression on migration and hepatic metastasis of human colon carcinoma

Organ-specific tumor cell adhesion to extracellular matrix (ECM) components and cell migration into host organs often involve integrin-mediated cellular processes. Direct integrin-mediated cell adhesion to ECM components in the space of Disse appears to be required for the successful liver metastatic formation of colon cancer. In the present study, human colon cancer HT-29 cells were transfected by liposome with integrin-beta1 antisense oligodeoxynucleotide (ASODN). The integrin-beta1 gene expression in HT-29 cells was significantly down-regulated. The migration of HT-29 cells was assayed using transwell cell culture chambers in vitro. The number of migrating HT-29 cells in experimental group was far less than that in control group (P<0.05). The models of hepatic metastasis in nude mice were established by the intrasplenic injection of transfected HT-29 cells. Thirty days later, the nude mice were killed and the average number of hepatic metastases (4.00+/-0.93 per mouse), average volume (10.10+/-6.50 mm3 per mouse), average weight (0.0440+/-0.0008 g per mouse) in experimental group were remarkably reduced as compared with those in control group (P<0.05). Integrin-beta1 expression in the hepatic metastasis was studied by immunohistochemistry (SP). Positive cell percentage of hepatic metastases in experimental group was markedly decreased as compared with that in control group (P<0.05). It was concluded that integrin-beta1 may take part in hepatic metastasis, and down-regulation of integrin-beta1 expression may play a key role in decreasing migration and hepatic metastasis of human colon carcinoma cells (HT-29).

Mechanisms underlying the inhibitory effects of lipopolysaccharide on human platelet adhesion

Alterations in platelet aggregation in septic conditions are well established. However, little is known about the effects of lipopolysaccharide (LPS) on platelet adhesion. We have therefore investigated the effects of LPS in human platelet adhesion, using an in vitro model of platelet adhesion to fibrinogen-coated wells. Microtiter plates were coated with human fibrinogen, after which washed platelets (6 x 10(8) platelets/ml) were allowed to adhere. Adherent platelets were quantified through measurement of acid phosphatase activity. Calcium mobilization in Fura2-AM-loaded platelets was monitored with a spectrofluorimeter. Platelet flow cytometry in thrombin-stimulated platelets was performed using monoclonal mouse anti-platelet GPIIb/IIIa antibody (PAC-1). Prior incubation of washed platelets with LPS (0.01-300 microg/ml) for 5 to 60 min concentration- and time-dependently inhibited non-activated platelet adhesion. In thrombin-activated (50 mU/ml) platelets, LPS inhibited the adhesion to a significantly lesser extent than non-activated platelets. Cyclohexamide, superoxide dismutase polyethylene glycol (PEG-SOD) or catalase polyethylene glycol did not affect the LPS responses. No alterations in cyclic GMP levels were seen after platelet incubation with LPS, except with the highest concentration employed (300 microg/ml) where an increase of 36% (P < 0.05) was observed. Thrombin increased by 7.5-fold the internal Ca(2+) platelet levels, an effect markedly inhibited by LPS. Thrombin induced concentration-dependent platelet GPIIb/IIIa activation, but LPS failed to affect the activation state of this membrane glycoprotein. In conclusion, LPS inhibits human platelet adhesion to fibrinogen by mechanisms involving blockade of external Ca(2+), independently of cGMP generation and activation of GPIIb/IIIa complex.

Cross-talk between serine/threonine protein phosphatase 2A and protein tyrosine phosphatase 1B regulates Src activation and adhesion of integrin αIIbβ3 to fibrinogen

Integrin α(IIb)β(3) signaling mediated by kinases and phosphatases participate in hemostasis and thrombosis, in part, by supporting stable platelet adhesion. Our previous studies indicate that the genetic manipulation of PP2Acα (α isoform of the catalytic subunit of protein phosphatase 2A) negatively regulate the adhesion of human embryonal kidney 293 cells expressing α(IIb)β(3) to fibrinogen. Here, we demonstrated that small interference RNA (siRNA) mediated knockdown of PP2Acα in 293 α(IIb)β(3) cells led to the dephosphorylation of Src Tyr-529, phosphorylation of Src Tyr-418 and an increased Src kinase activity. Conversely, overexpression of PP2Acα decreased the basal Src activity. Pharmacological inhibition of PP2Ac in human platelets or PP2Acα knockdown in primary murine megakaryocytes resulted in Src activation. PP2Acα-depleted 293 α(IIb)β(3) cells did not alter the serine (Ser) phosphorylation of Src but enhanced the Ser-50 phosphorylation of protein tyrosine phosphatase 1B (PTP-1B) with a concomitant increase in the PTP-1B activity. Src activation in the PP2Acα-depleted 293 α(IIb)β(3) cells was abolished by siRNA mediated knockdown of PTP-1B. Pharmacological inhibition of Src or knockdown of Src, PTP-1B blocked the enhanced activation of extracellular signal-regulated kinase (ERK1/2) and the increased adhesiveness of PP2Acα-depleted 293 α(IIb)β(3) cells to fibrinogen, respectively. Thus, inactivation of PP2Acα promotes hyperphosphorylation of PTP-1B Ser-50, elevates PTP-1B activity, which dephosphorylates Src Tyr-529 to activate Src and its downstream ERK1/2 signaling pathways that regulate α(IIb)β(3) adhesion. Moreover, these studies extend the notion that a cross-talk between Ser/Thr and Tyr phosphatases can fine-tune α(IIb)β(3) outside-in signaling.

Platelet JNK1 is involved in secretion and thrombus formation

The role of c-Jun NH2-terminal kinase 1 (JNK1) in hemostasis and thrombosis remains unclear. We show here, with JNK1-deficient (JNK1−/−) mice, that JNK1 plays an important role in platelet biology and thrombus formation. In tail-bleeding assays, JNK1−/− mice exhibited longer bleeding times than wild-type mice (396 ± 39 seconds vs 245 ± 32 seconds). We also carried out in vitro whole-blood perfusion assays on a collagen matrix under arterial shear conditions. Thrombus formation was significantly reduced for JNK1−/− platelets (51%). In an in vivo model of thrombosis induced by photochemical injury to cecum vessels, occlusion times were 4.3 times longer in JNK1−/− arterioles than in wild-type arterioles. Moreover, in vitro studies carried out in platelet aggregation conditions demonstrated that, at low doses of agonists, platelet secretion was impaired in JNK1−/− platelets, leading to altered integrin αIIbβ3 activation and reduced platelet aggregation, via a mechanism involving protein kinase C. JNK1 thus appears to be essential for platelet secretion in vitro, consistent with its role in thrombus growth in vivo. Finally, we showed that ERK2 and another isoform of JNK affect platelet aggregation through 2 pathways, one dependent and another independent of JNK1.

Critical role of Src-Syk-PLC{gamma}2 signaling in megakaryocyte migration and thrombopoiesis

Migration of megakaryocytes (MKs) from the proliferative osteoblastic niche to the capillary-rich vascular niche is essential for proplatelet formation and platelet release. In this study, we explore the role of surface glycoprotein receptors and signaling proteins in regulating MK migration and platelet recovery after immune-induced thrombocytopenia. We show that spreading and migration of mouse primary bone marrow-derived MKs on a fibronectin matrix are abolished by the Src family kinases inhibitor PP1, the Syk kinase inhibitor R406 and the integrin alphaIIbbeta3 antagonist lotrafiban. We also demonstrate that these responses are inhibited in primary phospholipase C gamma2 (PLCgamma2)-deficient MKs. Conversely, MK spreading and migration were unaltered in the absence of the collagen receptor, the glycoprotein VI-FcRgamma-chain complex. We previously reported a correlation between a defect in MK migration and platelet recovery in the absence of platelet endothelial cell adhesion molecule-1 and the tyrosine phosphatase CD148. This correlation also holds for mice deficient in PLCgamma2. This study identifies a model in which integrin signaling via Src family kinases and Syk kinase to PLCgamma2 is required for MK spreading, migration, and platelet formation.

The future of antiplatelet therapy in cardiovascular disease

Mechanisms of platelet inhibition are reviewed with emphasis on the pharmacokinetic and pharmacodynamic determinants of clinical efficacy and safety of antiplatelet drugs. Current developments in antiplatelet therapy are discussed in relation to both primary and secondary prevention of atherothrombotic complications. Interindividual variability in response to antiplatelet agents and new drug targets are outlined within the context of optimizing the balance between the cardiovascular benefits and bleeding risks of antiplatelet therapy. Recent advances in the pharmacogenetics of thienopyridines open the realistic prospect of a personalized choice of the most appropriate antiplatelet agent and tailored dose adjustment for an individual patient.

Effects of limiting extension at the alphaIIb genu on ligand binding to integrin alphaIIbbeta3

Structural data of integrin alphaIIbbeta3 have been interpreted as supporting a model in which: 1) the receptor exists primarily in a "bent," low affinity conformation on unactivated platelets and 2) activation induces an extended, high affinity conformation prior to, or following, ligand binding. Previous studies found that "clasping" the alphaIIb head domain to the beta3 tail decreased fibrinogen binding. To study the role of alphaIIb extension about the genu, we introduced a disulfide "clamp" between the alphaIIb thigh and calf-1 domains. Clamped alphaIIbbeta3 had markedly reduced ability to bind the large soluble ligands fibrinogen and PAC-1 when activated with monoclonal antibody (mAb) PT25-2 but not when activated by Mn(2+) or by coexpressing the clamped alphaIIb with a beta3 subunit containing the activating mutation N339S. The clamp had little effect on the binding of the snake venom kistrin (M(r) 7,500) or alphaIIbbeta3-mediated adhesion to immobilized fibrinogen, but it did diminish the enhanced binding of mAb AP5 in the presence of kistrin. Collectively, our studies support a role for alphaIIb extension about the genu in the binding of ligands of 340,000 and 900,000 M(r) with mAb-induced activation but indicate that it is not an absolute requirement. Our data are consistent with alphaIIb extension resulting in increased access to the ligand-binding site and/or facilitating the conformational change(s) in beta3 that affect the intrinsic affinity of the binding pocket for ligand.

A unique interaction between alphaIIb and beta3 in the head region is essential for outside-in signaling-related functions of alphaIIbbeta3 integrin

The main interface of the 2 subunits of platelet integrin alphaIIbbeta3 comprises the beta-propeller domain of alphaIIb and the betaA domain of beta3. In the center of the beta-propeller, several aromatic residues interact by cation-pi and hydrophobic bonds with Arg261 of betaA. In this study, we substituted alphaIIb-Trp110 or beta3-Arg261 by residues abundant in other alpha or beta subunits at corresponding locations and expressed them in baby hamster kidney cells along with normal beta3 or alphaIIb, respectively. These mutant cells displayed normal surface expression and fibrinogen binding but grossly impaired outside-in signaling-related functions: adhesion to immobilized fibrinogen, cell spreading, focal adhesion kinase phosphorylation, clot retraction, and reduced alphaIIbbeta3 stability in EDTA (ethylenediaminetetraacetic acid). Expression of mutants with substitutions of Arg261 in beta3 by alanine or lysine with normal alphav yielded normal surface expression of alphavbeta3 and soluble fibrinogen binding as well as normal outside-in signaling-related functions, contrasting findings for alphaIIbbeta3. Structural analysis of alphaIIbbeta3 and alphavbeta3 revealed that alphavbeta3 has several strong interactions between alphav and beta3 subunits that are missing in alphaIIbbeta3. Together, these findings indicate that the interaction between Trp110 of alphaIIb and Arg261 of beta3 is critical for alphaIIbbeta3 integrity and outside-in signaling-related functions.

Platelet adhesion and intracellular calcium levels in antigen-challenged rats

There is considerable evidence that platelet activation occurs in allergic airways diseases. In this study we aimed to investigate platelet adhesion to immobilized fibrinogen and intracellular calcium levels in a rat model of allergic inflammation. Male Wistar rats were challenged with ovalbumin (OVA). At 30 min to 24h after OVA-challenge, assays of platelet adhesion to immobilized fibrinogen and intracellular calcium levels using fura 2-AM loaded platelets were performed. The serum levels of IgE were approximately 5-fold greater in OVA-sensitized rats. A marked eosinophil influx in bronchoalveolar lavage (BAL) fluid of OVA-challenged rats at 24h after OVA-challenge was also seen. OVA-challenge resulted in a marked thrombocytopenia, as observed within 12h after OVA-challenge. The agonists ADP (0.5-50 microM) and thrombin (30-100 mU/ml) concentration-dependently increased platelet adhesion to immobilized fibrinogen. At an early time after OVA-challenge (30 min), platelets exhibited greater platelet adhesion compared with the non-sensitized group, whereas at a late time (24h) they exhibited lower platelet adhesion to both agonists. Moreover, at 30 min after OVA-challenge, intracellular calcium levels to ADP (20 microM) and thrombin (100 mU/ml)-activated platelets were greater compared with non-challenged rats. As opposed, at 24h after OVA challenge, a lower intracellular calcium level to ADP- and thrombin-activated platelets was observed. In conclusion, OVA-challenge in rats promotes a biphasic response in platelet adhesion consisting of an increased adhesion and intracellular calcium levels at an early phase (30 min), which progress to a reduction in adhesion and intracellular calcium levels at a late time (24h) after antigen challenge.