The platelet cytoskeleton regulates the affinity of the integrin alpha(IIb)beta(3) for fibrinogen

Cucurbitacins Elicit Anti-Platelet Activity via Perturbation of the Cytoskeleton and Integrin Function

Cucurbitacins are dietary compounds that have been shown to elicit a range of anti-tumour, anti-inflammatory and anti-atherosclerotic activities. Originally identified as signal transducer and activator of transcription, STAT, inhibitors, a variety of mechanisms of action have since been described, including dysregulation of the actin cytoskeleton and disruption of integrin function. Integrin outside-in signalling and cytoskeletal rearrangements are critical for the propagation of stable thrombus formation and clot retraction following platelet adhesion at the site of vessel damage. The effects of cucurbitacins on platelet function and thrombus formation are unknown. We report for the first time anti-platelet and anti-thrombotic effects of cucurbitacins B, E and I in human platelets. Treatment of platelets with cucurbitacins resulted in attenuation of platelet aggregation, secretion and fibrinogen binding following stimulation by platelet agonists. Cucurbitacins were also found to potently inhibit other integrin- and cytoskeleton-mediated events, including adhesion, spreading and clot retraction. Further investigation of cytoskeletal dynamics found treatment with cucurbitacins altered cofilin phosphorylation, enhanced activation and increased F actin polymerisation and microtubule assembly. Disruption to cytoskeletal dynamics has been previously shown to impair integrin activation, platelet spreading and clot retraction. Anti-platelet properties of cucurbitacins were found to extend to a disruption of stable thrombus formation, with an increase in thrombi instability and de-aggregation under flow. Our research identifies novel, anti-platelet and anti-thrombotic actions of cucurbitacins that appear to be linked to dysregulation of cytoskeletal dynamics and integrin function.

Multiple protein disulfide isomerases support thrombosis

PURPOSE OF REVIEW

The present review provides an overview of recent findings on new members of the protein disulfide isomerase (PDI) family required for thrombosis.

RECENT FINDINGS

Twenty years ago PDI was shown to mediate platelet aggregation, and 10 years ago PDI was shown to support thrombosis in vivo. Subsequently, other members of this endoplasmic reticulum family of enzymes, ERp57 and ERp5, were demonstrated to support thrombosis. A fourth member, ERp72, was recently shown to be required for platelet accumulation and fibrin deposition in vivo. None of these enzymes can individually support these processes. Moreover, aggregation of platelets deficient in a specific PDI is only recovered by the PDI that is missing. This implies that each PDI has a distinct role in activation of the αIIbβ3 fibrinogen receptor and platelet aggregation. Free thiols can be labeled in both subunits of αIIbβ3, suggesting cysteine-based reactions are involved in relaying conformational changes from the cytoplasmic tails to the integrin headpiece of this integrin.

SUMMARY

Multiple members of the PDI family support platelet function, and hemostasis and thrombosis with distinct roles in these processes. The individual cysteine targets of each enzyme and how these enzymes are integrated into a network that supports hemostasis and thrombosis remain to be elucidated.

Profilin 1-mediated cytoskeletal rearrangements regulate integrin function in mouse platelets

Profilin 1–mediated cytoskeletal dynamics regulate platelet β1- and β3-integrin function and turnover. Profilin 1 deficiency in platelets impairs hemostasis and results in a marked protection from arterial thrombosis.

Cucurbitacin I inhibits STAT3, but enhances STAT1 signaling in human cancer cells in vitro through disrupting actin filaments

STAT1 and STAT3 are two important members of the STAT (signal transducers and activators of transcription) protein family and play opposing roles in regulating cancer cell growth. Suppressing STAT3 and/or enhancing STAT1 signaling are considered to be attractive anticancer strategies. Cucurbitacin I (CuI) isolated from the cucurbitacin family was reported to be an inhibitor of STAT3 signaling and a disruptor of actin cytoskeleton. In this study we investigated the function and mechanisms of CuI in regulating STAT signaling in human cancer cells in vitro. CuI (0.1-10 mmol/L) dose-dependently inhibited the phosphorylation of STAT3, and enhanced the phosphorylation of STAT1 in lung adenocarcinoma A549 cells possibly through disrupting actin filaments. We further demonstrated that actin filaments physically associated with JAK2 and STAT3 in A549 cells and regulated their phosphorylation through two signaling complexes, the IL-6 receptor complex and the focal adhesion complex. Actin filaments also interacted with STAT1 in A549 cells and regulated its dephosphorylation. Taken together, this study reveals the molecular mechanisms of CuI in the regulation of STAT signaling and in a possible inhibition of human cancer cell growth. More importantly, this study uncovers a novel role of actin and actin-associated signaling complexes in regulating STAT signaling.

Phosphorylation of vasodilator-stimulated phosphoprotein contributes to myocardial ischemic preconditioning

Ischemic preconditioning (IP) is a well-known strategy to protect organs against cell death following ischemia. The previous work has shown that vasodilator-stimulated phosphoprotein (VASP) is involved in cytoskeletal reorganization and that it holds significant importance for the extent of myocardial ischemia reperfusion injury. Yet, the role of VASP during myocardial IP is, to date, not known. We report here that VASP phosphorylation at serine¹⁵⁷ and serine²³⁹ is induced during hypoxia in vitro and during IP in vivo. The preconditioning-induced VASP phosphorylation inactivates the GP IIb/IIIa integrin receptor on platelets, which results in the reduced formation of organ compromising platelet neutrophil complexes. Experiments in chimeric mice confirmed the importance of VASP phosphorylation during myocardial IP. When studying this in VASP^-/- animals and in an isolated heart model, we were able to confirm the important role of VASP on myocardial IP. In conclusion, we were able to show that IP-induced VASP phosphorylation in platelets is a protective mechanism against the deleterious effects of ischemia.

Functional links between Disabled-2 Ser723 phosphorylation and thrombin signaling in human platelets

Essentials Disabled-2 (Dab2) phosphorylation status in thrombin signaling of human platelet was investigated. Ser723 was the major Dab2 phosphorylation site in human platelets stimulated by thrombin. Dab2 S723 phosphorylation (pS723) caused the dissociation of Dab2-CIN85 protein complex. Dab2-pS723 regulated ADP release and integrin αIIbβ3 activation in thrombin-treated platelets.

SUMMARY

Background Disabled-2 (Dab2) is a platelet protein that is functionally involved in thrombin signaling in mice. It is unknown whether or not Dab2 undergoes phosphorylation during human platelet activation. Objectives To investigate the phosphorylation status of Dab2 and its functional consequences in thrombin-stimulated human platelets. Methods Dab2 was immunoprecipitated from resting and thrombin-stimulated platelet lysates for differential isotopic labeling. After enrichment of the phosphopeptides, the phosphorylation sites were analyzed by mass spectrometry. The corresponding phospho-specific antibody was generated. The protein kinases responsible for and the functional significance of Dab2 phosphorylation were defined by the use of signaling pathway inhibitors/activators, protein kinase assays, and various molecular approaches. Results Dab2 was phosphorylated at Ser227, Ser394, Ser401 and Ser723 in thrombin-stimulated platelets, with Ser723 phosphorylation being the most significantly increased by thrombin. Dab2 was phosphorylated by protein kinase C at Ser723 in a Gαq -dependent manner. ADP released from the stimulated platelets further activated the Gβγ -dependent pathway to sustain Ser723 phosphorylation. The Cbl-interacting protein of 85 kDa (CIN85) bound to Dab2 at a motif adjacent to Ser723 in resting platelets. The consequence of Ser723 phosphorylation was the dissociation of CIN85 from the Dab2-CIN85 complex. These molecular events led to increases in fibrinogen binding and platelet aggregation in thrombin-stimulated platelets by regulating αIIb β3 activation and ADP release. Conclusions Dab2 Ser723 phosphorylation is a key molecular event in thrombin-stimulated inside-out signaling and platelet activation, contributing to a new function of Dab2 in thrombin signaling.

Twinfilin 2a regulates platelet reactivity and turnover in mice

Regulated reorganization of the actin cytoskeleton is a prerequisite for proper platelet production and function. Consequently, defects in proteins controlling actin dynamics have been associated with platelet disorders in humans and mice. Twinfilin 2a (Twf2a) is a small actin-binding protein that inhibits actin filament assembly by sequestering actin monomers and capping filament barbed ends. Moreover, Twf2a binds heterodimeric capping proteins, but the role of this interaction in cytoskeletal dynamics has remained elusive. Even though Twf2a has pronounced effects on actin dynamics in vitro, only little is known about its function in vivo. Here, we report that constitutive Twf2a-deficient mice (Twf2a^-/-) display mild macrothrombocytopenia due to a markedly accelerated platelet clearance in the spleen. Twf2a^-/- platelets showed enhanced integrin activation and α-granule release in response to stimulation of (hem) immunoreceptor tyrosine-based activation motif (ITAM) and G-protein-coupled receptors, increased adhesion and aggregate formation on collagen I under flow, and accelerated clot retraction and spreading on fibrinogen. In vivo, Twf2a deficiency resulted in shortened tail bleeding times and faster occlusive arterial thrombus formation. The hyperreactivity of Twf2a^-/- platelets was attributed to enhanced actin dynamics, characterized by an increased activity of n-cofilin and profilin 1, leading to a thickened cortical cytoskeleton and hence sustained integrin activation by limiting calpain-mediated integrin inactivation. In summary, our results reveal the first in vivo functions of mammalian Twf2a and demonstrate that Twf2a-controlled actin rearrangements dampen platelet activation responses in a n-cofilin- and profilin 1-dependent manner, thereby indirectly regulating platelet reactivity and half-life in mice.

Platelet "first responders" in wound response, cancer, and metastasis

Platelets serve as "first responders" during normal wounding and homeostasis. Arising from bone marrow stem cell lineage megakaryocytes, anucleate platelets can influence inflammation and immune regulation. Biophysically, platelets are optimized due to size and discoid morphology to distribute near vessel walls, monitor vascular integrity, and initiate quick responses to vascular lesions. Adhesion receptors linked to a highly reactive filopodia-generating cytoskeleton maximizes their vascular surface contact allowing rapid response capabilities. Functionally, platelets normally initiate rapid clotting, vasoconstriction, inflammation, and wound biology that leads to sterilization, tissue repair, and resolution. Platelets also are among the first to sense, phagocytize, decorate, or react to pathogens in the circulation. These platelet first responder properties are commandeered during chronic inflammation, cancer progression, and metastasis. Leaky or inflammatory reaction blood vessel genesis during carcinogenesis provides opportunities for platelet invasion into tumors. Cancer is thought of as a non-healing or chronic wound that can be actively aided by platelet mitogenic properties to stimulate tumor growth. This growth ultimately outstrips circulatory support leads to angiogenesis and intravasation of tumor cells into the blood stream. Circulating tumor cells reengage additional platelets, which facilitates tumor cell adhesion, arrest and extravasation, and metastasis. This process, along with the hypercoagulable states associated with malignancy, is amplified by IL6 production in tumors that stimulate liver thrombopoietin production and elevates circulating platelet numbers by thrombopoiesis in the bone marrow. These complex interactions and the "first responder" role of platelets during diverse physiologic stresses provide a useful therapeutic target that deserves further exploration.

Platelet Shape Changes and Cytoskeleton Dynamics as Novel Therapeutic Targets for Anti-Thrombotic Drugs

Platelets play an essential role in hemostasis through aggregation and adhesion to vascular injury sites but their unnecessary activation can often lead to thrombotic diseases. Upon exposure to physical or biochemical stimuli, remarkable platelet shape changes precede aggregation or adhesion. Platelets shape changes facilitate the formation and adhesion of platelet aggregates, but are readily reversible in contrast to the irrevocable characteristics of aggregation and adhesion. In this dynamic phenomenon, complex molecular signaling pathways and a host of diverse cytoskeleton proteins are involved. Platelet shape change is easily primed by diverse pro-thrombotic xenobiotics and stimuli, and its inhibition can modulate thrombosis, which can ultimately contribute to the development or prevention of thrombotic diseases. In this review, we discussed the current knowledge on the mechanisms of platelet shape change and also pathological implications and therapeutic opportunities for regulating the related cytoskeleton dynamics.

Wdr1-Dependent Actin Reorganization in Platelet Activation

In resting platelets, the integrin αIIbβ3 is present in a low-affinity “bent” state. During platelet aggregation, intracytoplasmic signals induce conformational changes (inside-out signaling) that result in a “swung-out” conformation competent to bind ligands such as fibrinogen. The cytoskeleton plays an essential role in αIIbβ3 activation. We investigated the role of the actin interacting protein Wdr1 in αIIbβ3 activation. Wdr1-hypomorphic mice had a prolonged bleeding time (> 10 minutes) compared to that of wild-type mice (2.1 ± 0.7 minutes). Their platelets had impaired aggregation to collagen and thrombin. In a FeCl₃ induced carotid artery thrombosis model, vessel occlusion in Wdr1-hypomorphic mice was prolonged significantly compared to wild-type mice (9.0 ± 10.5 minutes versus 5.8 ± 12.6 minutes (p = 0.041). Activation-induced binding of JON/A (a conformation-specific antibody to activated αIIbβ3) was significantly less in Wdr1-hypomorphic platelets at various concentrations of collagen, indicating impaired inside-out activation of αIIbβ3, despite a normal calcium response. Actin turnover, assessed by measuring F-actin and G-actin ratios during collagen- and thrombin-induced platelet aggregation, was highly impaired in Wdr1-hypomorphic platelets. Furthermore, talin failed to redistribute and translocate to the cytoskeleton following activation in Wdr1-hypomorphic platelets. These studies show that Wdr1 is essential for talin-induced activation of αIIbβ3 during platelet activation.

Intracellular Trafficking, Localization, and Mobilization of Platelet-Borne Thiol Isomerases

OBJECTIVE

Thiol isomerases facilitate protein folding in the endoplasmic reticulum, and several of these enzymes, including protein disulfide isomerase and ERp57, are mobilized to the surface of activated platelets, where they influence platelet aggregation, blood coagulation, and thrombus formation. In this study, we examined the synthesis and trafficking of thiol isomerases in megakaryocytes, determined their subcellular localization in platelets, and identified the cellular events responsible for their movement to the platelet surface on activation.

APPROACH AND RESULTS

Immunofluorescence microscopy imaging was used to localize protein disulfide isomerase and ERp57 in murine and human megakaryocytes at various developmental stages. Immunofluorescence microscopy and subcellular fractionation analysis were used to localize these proteins in platelets to a compartment distinct from known secretory vesicles that overlaps with an inner cell-surface membrane region defined by the endoplasmic/sarcoplasmic reticulum proteins calnexin and sarco/endoplasmic reticulum calcium ATPase 3. Immunofluorescence microscopy and flow cytometry were used to monitor thiol isomerase mobilization in activated platelets in the presence and absence of actin polymerization (inhibited by latrunculin) and in the presence or absence of membrane fusion mediated by Munc13-4 (absent in platelets from Unc13d(Jinx) mice).

CONCLUSIONS

Platelet-borne thiol isomerases are trafficked independently of secretory granule contents in megakaryocytes and become concentrated in a subcellular compartment near the inner surface of the platelet outer membrane corresponding to the sarco/endoplasmic reticulum of these cells. Thiol isomerases are mobilized to the surface of activated platelets via a process that requires actin polymerization but not soluble N-ethylmaleimide-sensitive fusion protein attachment receptor/Munc13-4-dependent vesicular-plasma membrane fusion.

Cytoskeletal perturbation leads to platelet dysfunction and thrombocytopenia in variant forms of Glanzmann thrombasthenia

Several patients have been reported to have variant dominant forms of Glanzmann thrombasthenia, associated with macrothrombocytopenia and caused by gain-of-function mutations of ITGB3 or ITGA2B leading to reduced surface expression and constitutive activation of integrin αIIbβ3. The mechanisms leading to a bleeding phenotype of these patients have never been addressed. The aim of this study was to unravel the mechanism by which ITGB3 mutations causing activation of αIIbβ3 lead to platelet dysfunction and macrothrombocytopenia. Using platelets from two patients carrying the β3 del647-686 mutation and Chinese hamster ovary cells expressing different αIIbβ3-activating mutations, we showed that reduced surface expression of αIIbβ3 is due to receptor internalization. Moreover, we demonstrated that permanent triggering of αIIbβ3-mediated outside-in signaling causes an impairment of cytoskeletal reorganization arresting actin turnover at the stage of polymerization. The induction of actin polymerization by jasplakinolide, a natural toxin that promotes actin nucleation and prevents depolymerization of stress fibers, in control platelets produced an impairment of platelet function similar to that of patients with variant forms of dominant Glanzmann thrombasthenia. del647-686β3-transduced murine megakaryocytes generated proplatelets with a reduced number of large tips and asymmetric barbell-proplatelets, suggesting that impaired cytoskeletal rearrangement is the cause of macrothrombocytopenia. These data show that impaired cytoskeletal remodeling caused by a constitutively activated αIIbβ3 is the main effector of platelet dysfunction and macrothrombocytopenia, and thus of bleeding, in variant forms of dominant Glanzmann thrombasthenia.

Platelets and cancer: a casual or causal relationship: revisited

Human platelets arise as subcellular fragments of megakaryocytes in bone marrow. The physiologic demand, presence of disease such as cancer, or drug effects can regulate the production circulating platelets. Platelet biology is essential to hemostasis, vascular integrity, angiogenesis, inflammation, innate immunity, wound healing, and cancer biology. The most critical biological platelet response is serving as "First Responders" during the wounding process. The exposure of extracellular matrix proteins and intracellular components occurs after wounding. Numerous platelet receptors recognize matrix proteins that trigger platelet activation, adhesion, aggregation, and stabilization. Once activated, platelets change shape and degranulate to release growth factors and bioactive lipids into the blood stream. This cyclic process recruits and aggregates platelets along with thrombogenesis. This process facilitates wound closure or can recognize circulating pathologic bodies. Cancer cell entry into the blood stream triggers platelet-mediated recognition and is amplified by cell surface receptors, cellular products, extracellular factors, and immune cells. In some cases, these interactions suppress immune recognition and elimination of cancer cells or promote arrest at the endothelium, or entrapment in the microvasculature, and survival. This supports survival and spread of cancer cells and the establishment of secondary lesions to serve as important targets for prevention and therapy.

Neutrophil integrin affinity regulation in adhesion, migration, and bacterial clearance

During an infection, neutrophils are the first immune cells to arrive armed to clear the invading pathogen. In order to do so, neutrophils need to transmigrate from the peripheral blood through the endothelial layer toward the site of inflammation. This process is in most cases dependent on integrins, adhesion molecules present on all immune cells. These molecules are functionally regulated by "inside-out" signaling, where stimulus-induced signaling pathways act on the intracellular integrin tail to regulate the activity of the receptor on the outside. Both a change in conformation (affinity) and clustering (avidity/valency) of the receptors occurs and many factors have been linked to regulation of integrins on neutrophils. Control of integrin conformation and clustering is of pivotal importance for proper cell adhesion, migration, and bacterial clearance. Recently, gelsolin was found to be involved in β 1-integrin affinity regulation and cell adhesion. Here, I summarize the role of neutrophil integrin regulation in the essential steps to reach the site of inflammation and clearance of bacterial pathogens.

Gelsolin expression increases β1 -integrin affinity and L1210 cell adhesion

Integrins are functionally regulated by "inside-out" signaling, in that stimulus-induced signaling pathways act on the intracellular integrin tail to regulate the activity of the receptor on the outside. Both a change in conformation (affinity) and clustering (avidity/valency) of the receptors occurs, but the mechanisms that regulate inside out signaling are not completely understood. Previously, we identified gelsolin in a proteomics screen to identify proteins involved in inside-out control of integrins using the lymphocytic leukemia cell line L1210. Furthermore, we showed that gelsolin was involved in affinity regulation of β1 -integrins in the leukemic cell line U937. Here, we examined how gelsolin regulates β1 -integrin affinity in the leukemia cell line L1210. We show that gelsolin is mainly expressed at the cell membrane and is present near β1 -integrins. The role for actin polymerization in integrin affinity regulation was examined using the actin modulating agent jasplakinolide, which decreased β1 -integrin affinity. Similarly, knock-down of gelsolin in L1210 cells also decreased β1 -integrin affinity and cell adhesion to collagen. These data suggest that increased actin polymerization through gelsolin regulates β1 -integrin affinity and cell adhesion.

Phosphorylation of vasodilator-stimulated phosphoprotein prevents platelet-neutrophil complex formation and dampens myocardial ischemia-reperfusion injury

BACKGROUND

Recent work has suggested that the formation of platelet-neutrophil complexes (PNCs) aggravates the severity of inflammatory tissue injury. Given the importance of vasodilator-stimulated phosphoprotein (VASP) for platelet function, we pursued the role of VASP on the formation of PNCs and its impact on the extent of myocardial ischemia-reperfusion (IR) injury.

METHODS AND RESULTS

In initial in vitro studies we found that neutrophils facilitated the movement of platelets across endothelial monolayers. Phosphorylation of VASP reduced the formation of PNCs and transendothelial movement of PNCs. During myocardial IR injury, VASP(-/-) animals demonstrated reduced intravascular formation of PNCs and reduced presence of PNCs within the ischemic myocardial tissue. This was associated with reduced IR injury. Studies using platelet transfer and bone marrow chimeric animals showed that hematopoietic VASP expression was crucial for the intravascular formation of PNCs the presence of PNCs within ischemic myocardial tissue and the extent of myocardial IR injury. Furthermore, phosphorylation of VASP on Ser153 or Ser235 reduced intravascular PNC formation and presence of PNCs within ischemic myocardial tissue. This finding was associated with reduced myocardial IR injury.

CONCLUSION

Previously unappreciated, the phosphorylation of VASP performs a key function for the formation of PNCs that is crucially important for the extent of myocardial IR injury.

Clot strength: a comparison between cord and adult blood by means of thrombelastometry

OBJECTIVE

To evaluate the clot strength in cord versus adult blood.

METHOD

Thrombelastometry (TEM) was the method of choice as it provides information on the clot strength in terms of the maximum clot firmness (MCF) and on the fibrin polymerization process in terms of the clot formation time and the alpha angle.

RESULTS

The MCFs were significantly lower in cord versus adult platelet rich plasma (PRP, 63.0+/-3.8 vs. 67.0+/-3.9 mm, P=0.006) and in cord versus adult whole blood (WB, 55.3+/-3.8 vs. 59.3+/-3.6 mm, P=0.001) employing the thrombelastometry with extrinsic activator assay. We suggest that the diminished clot strength in cord versus adult blood and plasma samples is attributable to an impaired polymerization of neonatal fibrin: (i) the thrombelastometry with extrinsic activator and inactivated platelets (FIBTEM) assay revealed significantly lower MCFs in cord versus adult PRP (23.0+/-3.1 mm vs. 27.3+/-3.9 mm, P=0.002) and in cord versus adult WB (11.6+/-2.3 mm vs. 15.3+/-3.3 mm, P<0.001); (ii) the alpha angle in the FIBTEM assay was significantly lower in cord versus adult WB (39.0+/-12.8 degrees vs. 55.5+/-12.3 degrees, P=0.02); (iii) the clot formation times in the FIBTEM assay were significantly longer in cord versus adult PRP (248.0+/-143.5 s vs. 81.5+/-39.8 s, P=0.001).

CONCLUSIONS

Neonatal fibrin shows impaired polymerization properties under our experimental conditions resulting in reduced clot strength compared with fibrin of adult origin.

Platelet integrin alpha(IIb)beta(3): activation mechanisms

Integrin alpha(IIb)beta(3) plays a critical role in platelet aggregation, a central response in hemostasis and thrombosis. This function of alpha(IIb)beta(3) depends upon a transition from a resting to an activated state such that it acquires the capacity to bind soluble ligands. Diverse platelet agonists alter the cytoplasmic domain of alpha(IIb)beta(3) and initiate a conformational change that traverses the transmembrane region and ultimately triggers rearrangements in the extracellular domain to permit ligand binding. The membrane-proximal regions of alpha(IIb) and beta(3) cytoplasmic tails, together with the transmembrane segments of the subunits, contact each other to form a complex which restrains the integrin in the resting state. It is unclasping of this complex that induces integrin activation. This clasping/unclasping process is influenced by multiple cytoplasmic tail binding partners. Among them, talin appears to be a critical trigger of alpha(IIb)beta(3) activation, but other binding partners, which function as activators or suppressors, are likely to act as co-regulators of integrin activation.

The Contributions of Integrin Affinity and Integrin-Cytoskeletal Engagement in Endothelial and Smooth Muscle Cell Adhesion to Vitronectin

The serine proteinase inhibitor, plasminogen activator inhibitor type-1 (PAI-1), binds to the adhesion protein vitronectin with high affinity at a site that is located directly adjacent to the vitronectin RGD integrin binding sequence. The binding of PAI-1 to vitronectin sterically blocks integrin access to this site and completely inhibits the binding of purified integrins to vitronectin; however, its inhibition of endothelial and smooth muscle cell adhesion to vitronectin is at most 50-75%. Because PAI-1 binds vitronectin with approximately 10-100-fold higher affinity than purified integrins, we have analyzed the mechanism whereby these cells are able to overcome this obstacle. Our studies exclude proteolytic removal of PAI-1 from vitronectin as the mechanism, and show instead that cell adhesion in the presence of PAI-1 is dependent on integrin-cytoskeleton engagement. Disrupting endothelial or smooth muscle cell actin polymerization and/or focal adhesion assembly reduces cell adhesion to vitronectin in the presence of PAI-1 to levels similar to that observed for the binding of purified integrins to vitronectin. Furthermore, endothelial cell, but not smooth muscle cell adhesion to vitronectin in the presence of PAI-1 requires both polymerized microtubules and actin, further demonstrating the importance of the cytoskeleton for integrin-mediated adhesion. Finally, we show that cell adhesion in the presence of PAI-1 leads to colocalization of PAI-1 with the integrins alphavbeta3 and alphavbeta5 at the cell-matrix interface.

VASP-dependent regulation of actin cytoskeleton rigidity, cell adhesion, and detachment

Enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) proteins are established regulators of actin-based motility, platelet aggregation, and growth cone guidance. However, the molecular mechanisms involved essentially remain elusive. Here we report on a novel mechanism of VASP action, namely the regulation of tensile strength, contractility, and rigidity of the actin cytoskeleton. Compared to wild-type cells fibroblasts derived from VASP-deficient mice have thicker and more stable actin stress fibres. Furthermore focal adhesions are enlarged, myosin light chain phosphorylation is increased, and the rigidity of the filament-supported plasma membrane is elevated about three- to fourfold, as is evident from atomic force microscopy. Moreover, fibronectin-coated beads adhere stronger to the surface of VASP-deficient cells. The resistance of these beads to mechanical displacement by laser tweezers is dramatically increased in an F-actin-dependent mode. Cytoskeletal stabilization coincides with slower cell adhesion and detachment, while overall adhesion is increased. Interestingly, many of these effects observed in VASP (-/-) cells are recapitulated in VASP-overexpressing cells, hinting towards a balanced stoichiometry necessary for appropriate VASP function. Taken together, our results suggest that VASP regulates surface protrusion formation and cell adhesion through modulation of the mechanical properties of the actin cytoskeleton.

Membrane-proximal α/β Stalk Interactions Differentially Regulate Integrin Activation

The affinity of integrin-ligand interaction is regulated extracellularly by divalent cations and intracellularly by inside-out signaling. We report here that the extracellular, membrane-proximal alpha/beta stalk interactions not only regulate cation-induced integrin activation but also play critical roles in propagating inside-out signaling. Two closely related integrins, alphaIIbbeta3 and alphaVbeta3, share high structural homology and bind to similar ligands in an RGD-dependent manner. Despite these structural and functional similarities, they exhibit distinct responses to Mn(2+). Although alphaVbeta3 showed robust ligand binding in the presence of Mn(2+), alphaIIbbeta3 showed a limited increase but failed to achieve full activation. Swapping alpha stalk regions between alphaIIb and alphaV revealed that the alpha stalk, but not the ligand-binding head region, was responsible for the difference. A series of alphaIIb/alphaV domain-swapping chimeras were constructed to identify the responsible domain. Surprisingly, the minimum component required to render alphaIIbbeta3 susceptible to Mn(2+) activation was the alphaV calf-2 domain, which does not contain any divalent cation-binding sites. The calf-2 domain makes interface with beta epidermal growth factor 4 and beta tail domain in three-dimensional structure. The effect of calf-2 domain swapping was partially reproduced by mutating the specific amino acid residues in the calf-2/epidermal growth factor 4-beta tail domain interface. When this interface was constrained by an artificially introduced disulfide bridge, the Mn(2+)-induced alphaVbeta3-fibrinogen interaction was significantly impaired. Notably, a similar disulfide bridge completely abrogated fibrinogen binding to alphaIIbbeta3 when alphaIIbbeta3 was activated by cytoplasmic tail truncation to mimic inside-out signaling. Thus, disruption/formation of the membrane-proximal alpha/beta stalk interface may act as an on/off switch that triggers integrin-mediated bidirectional signaling.

Normal values for thrombelastography (ROTEM) and selected coagulation parameters in porcine blood

The pig is a suitable animal model for researching blood coagulation and fibrinolysis. The present study therefore aimed to investigate in porcine blood the applicability of commercially available tests of coagulation and thrombelastography (ROTEM) and above all to determine normal values for coagulation parameters (e.g. prothrombin time (PT), activated partial thromboplastin time (aPTT), thrombin-antithrombin complexes (TAT), fibrinogen, antithrombin III (AT III), D-dimers, protein C). Except for the FibTEM and aPTT tests, all commercially available coagulation tests used were fully applicable for porcine blood. Normal values and reference intervals for porcine blood are given. As compared to the human reference intervals for the coagulation parameters investigated, porcine blood was found to be hypercoagulable.

The role of the cytoskeleton in differentially regulating pressure-mediated effects on malignant colonocyte focal adhesion signaling and cell adhesion

Increased extracellular pressure stimulates colon cancer cell adhesion by activating focal adhesion kinase (FAK) and Src. We investigated the role of the cytoskeleton in pressure-induced inside-out FAK and Src phosphorylation and pressure-stimulated adhesion. We perturbed actin polymerization with phalloidin, cytochalasin D and latrunculin B, and microtubule organization with colchicine and paclitaxol. We compared the effects of these agents on pressure-induced SW620 and human primary colon cancer cell adhesion and inside-out FAK/Src activation with outside-in adhesion-dependent FAK/Src activation. Cells pretreated with cytoskeletal inhibitors were subjected to 15 mmHg increased pressure and allowed to adhere to collagen I coated plates or prevented from adhesion to pacificated plates for 30 min. Phalloidin, cytochalasin D, latrunculin B and colchicine pretreatment completely prevented pressure-stimulated and significantly inhibited basal SW620 cell adhesion. Taxol did not inhibit pressure-induced colon cancer cell adhesion, but significantly lowered basal adhesion. Cytochalasin D and colchicine had similar effects in pressure-stimulated primary human malignant colonocytes. Phalloidin, cytochalasin D, latrunculin B and colchicine prevented pressure-induced SW620 FAK phosphorylation but not Src phosphorylation. FAK phosphorylation in response to collagen I adhesion was significantly attenuated but not completely prevented by these inhibitors. Although Src phosphorylation was not increased on adhesion, the cytoskeleton disrupting agents significantly lowered basal Src phosphorylation in adherent cells. These results suggest that both cytoskeleton-dependent FAK activation and cytoskeleton-independent Src activation may be required for extracellular pressure to stimulate colon cancer cell adhesion. Furthermore, the cytoskeleton plays a different role in pressure-activated FAK and Src signaling than in FAK and Src activation in adherent cells. We, therefore, hypothesize that cytoskeletal interactions with focal adhesion signals mediate the effects of extracellular pressure on colon cancer cell adhesion.

Negative regulation of platelet function by a secreted cell repulsive protein, semaphorin 3A

Semaphorin 3A (Sema3A) is a secreted disulfide-bound homodimeric molecule that induces growth cone collapse and repulsion of axon growth in the nervous system. Recently, it has been demonstrated that Sema3A is produced by endothelial cells and inhibits integrin function in an autocrine fashion. In this study, we investigated the effects of Sema3A on platelet function by using 2 distinct human Sema3A chimera proteins. We detected expression of functional Sema3A receptors in platelets and dose-dependent and saturable binding of Sema3A to platelets. Sema3A dose-dependently inhibited activation of integrin alphaIIbbeta3 by all agonists examined including adenosine diphosphate (ADP), thrombin, convulxin, phorbol 12-myristate 13-acetate, and A23187. Sema3A inhibited not only platelet aggregation induced by thrombin or collagen but also platelet adhesion and spreading on immobilized fibrinogen. Moreover, Sema3A impaired alphaIIbbeta3-independent spreading on glass coverslips and aggregation-independent granular secretion. Sema3A inhibited agonist-induced elevation of filamentous action (F-actin) contents, phosphorylation of cofilin, and Rac1 activation. In contrast, Sema3A did not affect the levels of cyclic nucleotides or agonist-induced increase of intracellular Ca2+ concentrations. Thus, the extensive inhibition of platelet function by Sema3A appears to be mediated, at least in part, through impairment of agonist-induced Rac1-dependent actin rearrangement.

alpha3beta1 integrin modulates neuronal migration and placement during early stages of cerebral cortical development

We show that alpha3 integrin mutation disrupts distinct aspects of neuronal migration and placement in the cerebral cortex. The preplate develops normally in alpha3 integrin mutant mice. However, time lapse imaging of migrating neurons in embryonic cortical slices indicates retarded radial and tangential migration of neurons, but not ventricular zone-directed migration. Examination of the actin cytoskeleton of alpha3 integrin mutant cortical cells reveals aberrant actin cytoskeletal dynamics at the leading edges. Deficits are also evident in the ability of developing neurons to probe their cellular environment with filopodial and lamellipodial activity. Calbindin or calretinin positive upper layer neurons as well as the deep layer neurons of alpha3 integrin mutant mice expressing EGFP were misplaced. These results suggest that alpha3beta1 integrin deficiency impairs distinct patterns of neuronal migration and placement through dysregulated actin dynamics and defective ability to search and respond to migration modulating cues in the developing cortex.

Mechanism of CD47-induced α4β1 Integrin Activation and Adhesion in Sickle Reticulocytes

We recently reported that CD47 (integrin-associated protein) on sickle red blood cells (SS RBCs) activates G-protein-dependent signaling, which promotes cell adhesion to immobilized thrombospondin (TSP) under relevant shear stress. These data suggested that signal transduction in SS RBCs may contribute to the vaso-occlusive pathology observed in sickle cell disease. However, the CD47-activated SS RBC adhesion receptor(s) that mediated adhesion to immobilized TSP remained unknown. Here we demonstrate that the alpha4beta1 integrin (VLA-4) is the receptor that mediates CD47-stimulated SS RBC adhesion to immobilized TSP. This adhesion requires both the N-terminal heparin-binding domain and the RGD site of TSP. CD47 signaling induces an "inside-out" activation of alpha4beta1 on SS RBCs as indicated by an RGD-dependent interaction of this integrin with soluble, plasma fibronectin. However, CD47 engagement also induces an alpha4beta1-mediated, RGD-independent adhesion of SS RBCs to immobilized vascular cell adhesion molecule-1 (VCAM-1). CD47 signaling in SS RBCs appears to be independent of large scale changes in cAMP formation but nonetheless promotes alpha4beta1-mediated adhesion via a protein kinase A-dependent, serine phosphorylation of the alpha4 cytoplasmic domain. CD47-activated SS RBC adhesion absolutely requires the Src family tyrosine kinases and is also enhanced by treatment of SS RBCs with low concentrations of cytochalasin D, which may release alpha4beta1 from cytoskeletal restraints. In addition, CD47 co-immunoprecipitates with alpha4beta1 in a sickle reticulocyte-enriched fraction of SS RBCs. These studies therefore identify the alpha4beta1 integrin on SS RBCs as a CD47-activated receptor for TSP, VCAM-1, and plasma fibronectin, revealing novel binding characteristics of this integrin.

Integrins: dynamic scaffolds for adhesion and signaling in platelets

The major platelet integrin, alphaIIbbeta3, is required for platelet interactions with proteins in plasma and the extracellular matrices (ECMs) that are essential for platelet adhesion and aggregation during hemo stasis and arterial thrombosis. Lig and binding to alphaIIbbeta3 is controlled by inside-out signals that modulate receptor conformation and clustering. In turn, ligand binding triggers outside-in signals through alphaIIbbeta3 that, when disrupted, can cause a bleeding diathesis. In the past 5 years there has been an explosion of knowledge about the structure and function ofalphaIIbbeta3 and the related integrin, alphaVbeta3. These developments are discussed here, and current models of bidirectional alphaIIbbeta3 signaling are presented as frameworks for future investigations. An understanding that alphaIIbbeta3 functions as a dynamic molecular scaffold for extracellular and intracellular proteins has translated into diagnostic and therapeutic insights relevant to hematology and cardiovascular medicine, and further advances can be anticipated.

The major outer sheath protein of Treponema denticola inhibits the binding step of collagen phagocytosis in fibroblasts

Bacterial infections contribfute to the chronicity of connective tissue lesions in part by perturbing extracellular matrix remodelling processes. We examined a novel mechanism by which the major outer sheath protein (Msp) of the spirochaete Treponema denticola disrupts matrix remodelling mediated by intracellular digestion of collagen. The initial collagen-binding step of phagocytosis was examined in human gingival fibroblasts and Rat-2 fibroblasts. Cells were pretreated with Msp or vehicle, and binding of collagen-coated beads was measured by flow cytometry. Exposure to Msp induced a dose- and time-dependent decrease in cells that bound collagen beads; the inhibition of binding was reversed by absorption with anti-Msp antibodies. Msp-treated fibroblasts remained viable but underwent actin reorganization, including the assembly of a dense meshwork of subcortical actin filaments. Shear force assays showed that Msp abrogated collagen-binding interactions in the minimal affinity range required for stable adhesion. Fluorescence microscopy and immunoblotting showed equivalent amounts of beta1 integrin associated with collagen beads bound to Msp- and vehicle-treated cells. Photobleaching experiments found a similar percentage mobile fraction of beta1 integrins recovered in bleached areas of the plasma membrane. In contrast, Msp-induced inhibition of collagen binding was reversed by beta1 integrin affinity-activating antibodies and by latrunculin B, which prevented subcortical actin assembly. We conclude that native Msp of T. denticola inhibits the binding step of collagen phagocytosis in fibroblasts by inducing subcortical actin filament assembly and restricting affinity modulation of beta1 integrins. We suggest that, like Msp, bacterial toxins that target the cytoskeleton may also perturb the signalling networks required for cellular engagement of matrix ligands.

Caspase-12: a developmental link between G-protein-coupled receptors and integrin alphaIIbbeta3 activation

Fibrinogen binding by integrin alphaIIbbeta3 is promoted by platelet agonists that increase the affinity and avidity of alphaIIbbeta3 for fibrinogen through a process called "inside-out" signaling. Having previously demonstrated that inside-out activation of alphaIIbbeta3 is defective in murine megakaryocytes that lack the transcription factor NF-E2, we screened for NF-E2-regulated genes that affect alphaIIbbeta3 activation. Caspase-12 is the most down-regulated gene we identified in NF-E2(-/-) megakaryocytes. Therefore, the role of this protein in alphaIIbbeta3 activation was determined using platelets from caspase-12(-/-) mice. Despite wild-type levels of alphaIIbbeta3, caspase-12(-/-) platelets exhibit reduced fibrinogen binding to alphaIIbbeta3 following stimulation by adenosine diphosphate (ADP) or protease-activated receptor 4 (PAR4) receptor-activating peptide. The defect in alphaIIbbeta3 activation is associated with decreased cytosolic free calcium and inositol triphosphate levels, and with reduced aggregation, despite wild-type phospholipase Cbeta expression levels. In contrast, agonist-induced surface expression of P-selectin, suppression of cAMP levels following ADP stimulation, and spreading on immobilized fibrinogen are unimpaired. Moreover, although caspase-12 is highly expressed in mature megakaryocytes, it is undetectable in platelets. Taken together, these studies establish that caspase-12 expression in murine megakaryocytes is regulated, directly or indirectly, by NF-E2, and suggest that caspase-12 participates in the development of fully functional signaling pathways linking some G-protein-coupled receptors to alphaIIbbeta3 activation.

Colon cancer cell adhesion in response to Src kinase activation and actin-cytoskeleton by non-laminar shear stress

Malignant cells shed from tumors during surgical resection or spontaneous metastasis experience physical forces such as shear stress and turbulence within the peritoneal cavity during irrigation, laparoscopic air insufflation, or surgical manipulation, and within the venous or lymphatic system. Since physical forces can activate intracellular signals that modulate the biology of various cell types in vitro, we hypothesized that shear stress and turbulence might increase colon cancer cell adhesion to extracellular matrix, potentiating metastatic implantation. Primary human malignant colon cancer cells isolated from resected tumors and SW620 were subjected to shear stress and turbulence by stirring cells in suspension at 600 rpm for 10 min. Shear stress for 10 min increased subsequent SW620 colon cancer cell adhesion by 40.0 +/- 3.0% (n = 3; P < 0.001) and primary cancer cells by 41.0 +/- 3.0% to collagen I when compared to control cells. In vitro kinase assay (1.5 +/- 0.13 fold) and Western analysis (1.34 +/- 0.04 fold) demonstrated a significant increase in Src kinase activity in cells exposed shear stress. Src kinase inhibitors PP1 (0.1 microM), PP2 (20 microM), and actin-cytoskeleton stabilizer phalloidin (10 microM) prevented the shear stress stimulated cell adhesion to collagen I. Furthermore, PP2 inhibited basal (50.0 +/- 2.8%) and prevented shear stress induced src activation but phalloidin pretreatment did not. These results raise the possibility that shear stress and turbulence may stimulate the adhesion of malignant cells shed from colon cancers by a mechanism that requires both actin-cytoskeletal reorganization an independent physical force activation of Src kinase. Blocking this pathway might reduce tumor metastasis during surgical resection.

Activation of integrin alpha IIb beta 3 in the glycoprotein Ib-high population of a megakaryocytic cell line, CMK, by inside-out signaling

Affinity/avidity state of integrin alpha IIb beta 3 is regulated by intracellular inside-out signaling. Although several megakaryocytic cell lines have been established, soluble ligand binding to alpha IIb beta 3 expressed in these cells by cellular agonists has not been demonstrated. We have re-examined agonist-induced alpha IIb beta 3 activation on megakaryocytic cell lines with a marker of the late stage of megakaryocytic differentiation, glycoprotein Ib (GPIb). Activation of alpha IIb beta 3 was assessed by PAC1 and soluble fibrinogen binding to the cells. We found that alpha IIb beta 3 expressed in CMK cells with high GPIb expression was activated by a phorbor ester, phorbol myristate acetate (PMA). Although the population of the GPIbhigh cells was <0.5% of the total cells, incubation with a nucleoside analog, ribavirin, efficiently increased the PMA-reactive GPIbhigh cells. Not only PMA but also a calcium ionophore, A23187, induced alpha IIb beta 3 activation, and PMA and A23187 had an additive effect on alpha IIb beta 3 activation. Ligand binding to the activated alpha IIb beta 3 in the GPIbhigh CMK cells is totally abolished by an alpha IIb beta 3-specific antagonist, and inhibited by wortmannin, cytochalasin-D and prostaglandin E1, and the effects of these inhibitors on alpha IIb beta 3 activation in the GPIbhigh CMK cells were compatible with those in platelets. We have also demonstrated that the ribavirin-treated CMK cells express PKC-alpha, -beta, -delta and -theta, and suggested that PKC-alpha and/or -beta appear to be responsible for PMA-induced activation of alpha IIb beta 3 in CMK cells.

Different effects of abciximab and cytochalasin D on clot strength in thrombelastography

Maximum amplitude (MA) in thrombelastography (TEG) consists of a plasmatic and a platelet component. To assess the magnitude of the plasmatic component, pharmacological approaches have been proposed to eliminate the platelet component. We evaluated the individual and combined effects of abciximab and cytochalasin D on the MA of TEG. Whole blood, platelet-rich plasma (PRP) and homologous platelet-poor plasma (PPP) from 20 healthy volunteers were spiked with abciximab or cytochalasin D or a combination of both and TEGs performed. Abciximab and cytochalasin D decreased MA in all samples. MA of whole blood (18.6 +/- 3.1 mm) and PRP (33.7 +/- 3.5 mm) spiked with abciximab or cytochalasin D alone (15.0 +/- 2.9 mm and 25.0 +/- 4.0 mm) were significantly higher when compared with abciximab and cytochalasin D combined (10.4 +/- 3.0 and 20.2 +/- 3.5 mm). While MA of PRP and homologous PPP were significantly (P < 0.001) different after individual administration of abciximab and cytochalasin D, combination of both abolished this difference (20.2 +/- 3.5 mm and 20.4 +/- 3.7 mm, P = 0.372). In whole blood of critically ill patients or patients undergoing major surgery there was also a significant difference of MA between abciximab alone and in combination with cytochalasin D (16.5 +/- 11.3 mm and 11.3 +/- 7.7 mm, P < 0.001). This indicates that in contrast to individual administration of abciximab or cytochalasin D, a combination of both compounds eliminates the platelet-specific effect on MA of TEG tracings.

Effects of thrombin on interactions between beta3-integrins and extracellular matrix in platelets and vascular cells

The beta3-integrin family consists of alphaIIbbeta3 (also known as glycoprotein IIb/IIIa) and alpha(v)beta3. alphaIIbbeta3 is found on platelets and megakaryocytes and has an essential role in hemostasis. alpha(v)beta3 has a broader distribution, and it functions in angiogenesis, neointimal formation after vascular injury, and leukocyte trafficking. There are important interactions between thrombin and beta3-integrins relative to both "inside-out" (integrin activation) and "outside-in" (modification of cellular events by ligand binding to integrins) signaling. Thrombin, by binding to G protein-coupled, protease-activated receptors, is a potent activator of alphaIIbbeta3. Conversely, outside-in signaling through alphaIIbbeta3 amplifies events initiated by thrombin and is necessary for full platelet spreading, platelet aggregation, granule secretion, and the formation of a stable platelet thrombus. In smooth muscle cells, alpha(v)beta3-integrins influence various responses to thrombin, including proliferation, c-Jun NH2-terminal kinase-1 activation, and focal adhesion formation. Other interactions between beta3-integrins and thrombin include beta3-integrin promotion of the generation of thrombin by localizing prothrombin to cellular surfaces and/or enhancing the formation of procoagulant microparticles and the requirement of beta3-integrin function for platelet-dependent clot retraction. In summary, there is increasing evidence that interactions between beta3-integrins and thrombin play important roles in the regulation of hemostatic and vascular functions.

Disruption of the beta3 663-687 disulfide bridge confers constitutive activity to beta3 integrins

The platelet fibrinogen receptor, integrin alphaIIbbeta3, is a noncovalent heterodimer of glycoproteins IIb and IIIa. This work was aimed at elucidating the role played by the carboxy-terminal extracellular, trans-membrane, and cytoplasmic regions of the glycoprotein beta3 in the formation of functional complexes with alpha subunits. Progressive carboxy-terminal deletions of beta3 revealed that surface exposure of alphaIIbbeta3 or alphavbeta3 could not occur in the absence of the transmembrane domain of beta3. In contrast, internal deletions 616 to 690 of the carboxy-terminal regions of the beta3 ectodomain led to surface exposure of constitu tive active receptors in CHO cells, as indicated by the enhanced rate of cell adhesion to immobilized ligands and spontaneous binding to soluble fibrinogen or activation-dependent antibody PAC-1. The functional analysis of cysteine mutations within the 616 to 690 region of beta3 or chimeric beta3-beta7 subunits revealed that disruption of the C663-C687 disulfide bridge endows constitutive activity to the alphaIIbbeta3 receptor. It is concluded that the carboxy-terminal tail of the beta3 ectodomain, so-called beta tail domain (betaTD), is not essential for cell surface expression of beta3 receptors. However, a basal, nonactivated, low ligand-affinity state of the beta3 integrins demands a normal conformation of this domain.

Recombinant protein 1/secretoglobin 1A1 participates in the actin polymerization of human platelets

BACKGROUND

Human protein 1 which was originally isolated from pathological urine is identified with clara cell 10 kDa protein and uteroglobin. It has immunomodulatory and anti-inflammatory effects in many vertebrates. Although there have been thorough studies on the structure, molecular biology and biochemical characteristics, the precise mechanism of its biological activities is unknown. The purpose of this research is to clarify the biochemical mechanism of protein 1 through its effect on the platelet aggregation process.

METHODS

Changes in calcium mobilization, actin filament concentrations and functions of integrin alphaIIbbeta3 resulting from platelet stimulation were measured in the presence or absence of recombinant protein 1 (rP1).

RESULTS

Recombinant protein 1 inhibited U46619- and thapsigargin-induced platelet aggregation by preventing store mediated calcium entry (SMCE). The binding of rP1 to resting platelets induced an increase in the actin filament that accompanied the structural changes of alphaIIbbeta3. When rP1-pretreated platelets were activated by thrombin or thapsigargin, the alterations in the actin filament and alphaIIbbeta3 resulting from the stimulation decreased.

CONCLUSION

These results suggest that rP1 inhibits platelet aggregation by participation in the actin polymerization through which SMCE is mediated.

Cytoplasmic regions of the beta3 subunit of integrin alphaIIbbeta3 involved in platelet adhesion on fibrinogen under flow conditions

Platelet adhesion to surface-bound fibrinogen depends on integrin alphaIIbbeta3. In the present study, we investigated the role of the regions 749EATSTFT756N and 755TNITYRG762T of the beta3 cytoplasmic tail in the regulation of platelet adhesion under flow conditions, by introducing peptide mimetics in platelets. Introduction of peptide EATSTFTN (E-N) increased surface coverage by 35%, an effect caused by 25% more adhesion. In contrast, peptide TNITYRGT (T-T) decreased surface coverage by 16%, as a result of 25% less adhesion. An S-->P substitution in the E-N peptide, thereby mimicking a mutation in Glanzmann's thrombasthenia, abolished the effect of E-N. A suboptimal concentration of cytochalasin D is known to enhance ligand binding to alphaIIbbeta3 in platelet suspensions. Under flow, cytochalasin D (1 micro mol L-1) induced 50% more platelet adhesion, with a strong reduction in platelet spreading. Both peptides opposed the increase in adhesion by cytochalasin D and partly (E-N) and completely (T-T) restored platelet spreading. Thus, the 749EATSTFT756N and 755TNITYRG762T regions of beta3 contribute to the regulation of alphaIIbbeta3 anchorage to the cytoskeleton and platelet spreading to an adhesive surface.

Detection of integrin alpha IIbbeta 3 clustering in living cells

In platelets, bidirectional signaling across integrin alpha(IIb)beta(3) regulates fibrinogen binding, cytoskeletal reorganization, cell aggregation, and spreading. Because these responses may be influenced by the clustering of alpha(IIb)beta(3) heterodimers into larger oligomers, we established two independent methods to detect integrin clustering and evaluate factors that regulate this process. In the first, weakly complementing beta-galactosidase mutants were fused to the C terminus of individual alpha(IIb) subunits, and the chimeras were stably expressed with beta(3) in Chinese hamster ovary cells. Clustering of alpha(IIb)beta(3) should bring the mutants into proximity and reconstitute beta-galactosidase activity. In the second method, alpha(IIb) was fused to either a green fluorescent protein (GFP) or Renilla luciferase and transiently expressed with beta(3). Here, integrin clustering should stimulate bioluminescence resonance energy transfer between a cell-permeable luciferase substrate and GFP. These methods successfully detected integrin clustering induced by anti-alpha(IIb)beta(3) antibodies. Significantly, they also detected clustering upon soluble fibrinogen binding to alpha(IIb)beta(3). In contrast, no clustering was observed following direct activation of alpha(IIb)beta(3) by MnCl(2) or an anti-alpha(IIb)beta(3)-activating antibody Fab in the absence of fibrinogen. Intracellular events also influenced alpha(IIb)beta(3) clustering. For example, a cell-permeable, bivalent FK506-binding protein (FKBP) ligand stimulated clustering when added to cells expressing an alpha(IIb)(FKBP)(2) chimera complexed with beta(3). Furthermore, alpha(IIb)beta(3) clustering occurred in the presence of latrunculin A or cytochalasin D, inhibitors of actin polymerization. These effects were enhanced by fibrinogen, suggesting that actin-regulated clustering modulates alpha(IIb)beta(3) interaction with ligands. These studies in living cells establish that alpha(IIb)beta(3) clustering is modulated by fibrinogen and actin dynamics. More broadly, they should facilitate investigations of the mechanisms and consequences of integrin clustering.

Concurrent signaling from Galphaq- and Galphai-coupled pathways is essential for agonist-induced alphavbeta3 activation on human platelets

The integrin alphavbeta3 mediates platelet adhesion to the matrix protein osteopontin and likely is the predominant integrin mediating platelet adhesion to the matrix protein vitronectin. To address the mechanism that regulates alphavbeta3 activity in platelets, we measured the effect of the P2Y1 antagonist adenosine 3'-phosphate-5'-phosphate (A3P5P) and the P2Y12 antagonist AR-C66096 on ADP-stimulated platelet adhesion to osteopontin and vitronectin. Each antagonist completely inhibited platelet adhesion, implying that concurrent stimulation of P2Y1 and P2Y12 was required to activate alphavbeta3. The reducing agent dithiothreitol and Mn2+ also induced platelet adhesion to osteopontin, but did so without stimulating platelet activation. Thus, these data suggest that ADP stimulation regulates alphavbeta3 activity by perturbing the conformation of its extracellular domain. The actin polymerization inhibitors cytochalasin D and latrunculin A also induced platelet adhesion to osteopontin and vitronectin. Thus, alphavbeta3 activity in resting platelets appears to be constrained by the platelet cytoskeleton. Moreover, the effect of these agents was inhibited by A3P5P and AR-C66096 at micromolar and subnanomolar concentrations, respectively, suggesting that subthreshold platelet stimulation by ADP was required. Our data suggest that signals from both Galphaq- and Galphai-coupled receptors converge to release cytoskeletal constraints on alphavbeta3. We propose that the release of cytoskeletal constraints and a concurrent increase in affinity for ligands is responsible for alphavbeta3-mediated platelet adhesion.

Megakaryocytes derived from embryonic stem cells implicate CalDAG-GEFI in integrin signaling

Fibrinogen binding to integrin alphaIIbbeta3 mediates platelet aggregation and requires agonist-induced "inside-out" signals that increase alphaIIbbeta3 affinity. Agonist regulation of alphaIIbbeta3 also takes place in megakaryocytes, the bone marrow cells from which platelets are derived. To facilitate mechanistic studies of inside-out signaling, we describe here the generation of megakaryocytes in quantity from murine embryonic stem (ES) cells. Coculture of ES cells for 8-12 days with OP9 stromal cells in the presence of thrombopoietin, IL-6, and IL-11 resulted in the development of large, polyploid megakaryocytes that produced proplatelets. These cells expressed alphaIIbbeta3 and platelet glycoprotein Ibalpha but were devoid of hematopoietic stem cell, erythrocyte, and leukocyte markers. Mature megakaryocytes, but not megakaryocyte progenitors, specifically bound fibrinogen by way of alphaIIbbeta3 in response to platelet agonists. Retrovirus-mediated expression of the reporter gene, green fluorescent protein, in ES cell-derived megakaryocytes did not affect viability or alphaIIbbeta3 function. On the other hand, retroviral expression of CalDAG-GEFI, a Rap1 exchange factor identified by megakaryocyte gene profiling as a candidate integrin regulator, enhanced agonist-induced activation of Rap1b and fibrinogen binding to alphaIIbbeta3 (P < 0.01). These results establish that ES cells are a ready source of mature megakaryocytes for integrin studies and other biological applications, and they implicate CalDAG-GEFI in inside-out signaling to alphaIIbbeta3.

A Ser752-->Pro substitution in the cytoplasmic domain of beta3 in a Glanzmann thrombasthenia variant fails to prevent interactions between the alphaIIbbeta3 integrin and the platelet granule pool of fibrinogen

A Glanzmann thrombasthenia variant with a beta3 Ser752-->Pro cytoplasmic domain substitution has platelets that fail to aggregate or bind soluble fibrinogen (Fg) after activation. Despite this, Fg is normally present in the alpha-granules. We have used immunoelectron microscopy to examine the reactivity of Fg with the different pools of alphaIIbbeta3 in the patient's platelets. Immunogold labelling was performed on cryosections using an anti-ligand-induced binding site (LIBS) monoclonal antibody (mAb), which binds to alphaIIbbeta3 only when Fg is bound, or a mixture of two anti-receptor-induced binding site (RIBS) mAbs that specifically recognize receptor-bound Fg. Labelling of the alpha-granule membrane and channels of the surface-connected canalicular system in unstimulated platelets confirmed that the mutated alphaIIbbeta3 retains the capacity to transport Fg. When the patient's platelets were stimulated with ADP in the presence of Fg, as expected there was a much-decreased activation of surface-exposed alphaIIbbeta3. However, thrombin-induced activation was associated with both secretion and a rapid increase in the labelling of internal membrane systems by anti-RIBS and anti-LIBS mAbs, with mobilization of the internal Fg pool. Yet labelling on the surface of the patient's platelets was transient. Our studies implied that alphaIIbbeta3 in platelets may bind fibrinogen in different activation states and that this patient specifically lacked high-affinity binding.

Ancient ubiquitous protein 1 binds to the conserved membrane-proximal sequence of the cytoplasmic tail of the integrin alpha subunits that plays a crucial role in the inside-out signaling of alpha IIbbeta 3

Modification of the cytoplasmic tails of the integrin alpha(IIb)beta(3) plays an important role in the signal transduction in platelets. We searched for proteins that bind to the alpha(IIb) cytoplasmic tail using the yeast two-hybrid assay with a cDNA library of the megakaryocyte-derived cell line and identified a protein, ancient ubiquitous protein 1 (Aup1), that is ubiquitously expressed in human cells. Observation of UT7/TPO cells expressing a red fluorescent protein-tagged Aup1 indicated its localization in the cytoplasm. Immunoprecipitation of UT7/TPO cells by an antibody for Aup1 revealed that approximately 40% of alpha(IIb) is complexed with Aup1. Binding study with an alpha(IIb) cytoplasmic tail peptide and glutathione S-transferase-Aup1 fusion protein revealed a low affinity (K(d) = 90 microm). Subsequent yeast two-hybrid assay indicated binding of Aup1 to cytoplasmic tails of other integrin alpha subunits. Binding study with the purified Aup1 and various glutathione S-transferase-alpha(IIb) cytoplasmic tail peptides revealed specific binding of Aup1 to the membrane-proximal sequence (KVGFFKR) that is conserved among the integrin alpha subunits and plays a crucial role in the alpha(IIb)beta(3) inside-out signaling. As Aup1 possesses domains related to signal transduction, these results suggest involvement of Aup1 in the integrin signaling.

Relationships between Rap1b, affinity modulation of integrin alpha IIbbeta 3, and the actin cytoskeleton

The affinity of integrin alpha(IIb)beta(3) for fibrinogen is controlled by inside-out signals that are triggered by agonists like thrombin. Agonist treatment of platelets also activates Rap1b, a small GTPase known to promote integrin-dependent adhesion of other cells. Therefore, we investigated the role of Rap1b in alpha(IIb)beta(3) function by viral transduction of GFP-Rap1 chimeras into murine megakaryocytes, which exhibit inside-out signaling similar to platelets. Expression of constitutively active GFP-Rap1b (V12) had no effect on unstimulated megakaryocytes, but it greatly augmented fibrinogen binding to alpha(IIb)beta(3) induced by a PAR4 thrombin receptor agonist (p < 0.01). The Rap1b effect was cell-autonomous and was prevented by pre-treating cells with cytochalasin D or latrunculin A to inhibit actin polymerization. Rap1b-dependent fibrinogen binding to megakaryocytes was blocked by POW-2, a novel monovalent antibody Fab fragment specific for high affinity murine alpha(IIb)beta(3). In contrast to GFP-Rap1b (V12), expression of GFP-Rap1GAP, which deactivates endogenous Rap1, inhibited agonist-induced fibrinogen binding (p < 0.01), as did dominant-negative GFP-Rap1b (N17) (p < 0.05). None of these treatments affected surface expression of alpha(IIb)beta(3). These studies establish that Rap1b can augment agonist-induced ligand binding to alpha(IIb)beta(3) through effects on integrin affinity, possibly by modulating alpha(IIb)beta(3) interactions with the actin cytoskeleton.

RhoA sustains integrin alpha IIbbeta 3 adhesion contacts under high shear

The small GTPase RhoA modulates the adhesive nature of many cell types; however, despite high levels of expression in platelets, there is currently limited evidence for an important role for this small GTPase in regulating platelet adhesion processes. In this study, we have examined the role of RhoA in regulating the adhesive function of the major platelet integrin, alpha(IIb)beta(3). Our studies demonstrate that activation of RhoA occurs as a general feature of platelet activation in response to soluble agonists (thrombin, ADP, collagen), immobilized matrices (von Willebrand factor (vWf), fibrinogen) and high shear stress. Blocking the ligand binding function of integrin alpha(IIb)beta(3), by pretreating platelets with c7E3 Fab, demonstrated the existence of integrin alpha(IIb)beta(3)-dependent and -independent mechanisms regulating RhoA activation. Inhibition of RhoA (C3 exoenzyme) or its downstream effector Rho kinase had no effect on integrin alpha(IIb)beta(3) activation induced by soluble agonists or adhesive substrates, however, both inhibitors reduced shear-dependent platelet adhesion on immobilized vWf and shear-induced platelet aggregation in suspension. Detailed analysis of the sequential adhesive steps required for stable platelet adhesion on a vWf matrix under shear conditions revealed that RhoA did not regulate platelet tethering to vWf or the initial formation of integrin alpha(IIb)beta(3) adhesion contacts but played a major role in sustaining stable platelet-matrix interactions. These studies define a critical role for RhoA in regulating the stability of integrin alpha(IIb)beta(3) adhesion contacts under conditions of high shear stress.

Lateral clustering of platelet GP Ib-IX complexes leads to up-regulation of the adhesive function of integrin alpha IIbbeta 3

Binding of von Willebrand factor (VWF) to GP Ib-IX mediates initial platelet adhesion and increases the subsequent adhesive function of alpha(IIb)beta(3). Because these responses are promoted most effectively by large VWF multimers, we hypothesized that receptor clustering modulates GP Ib-IX function. To test this, GP IX was fused at its cytoplasmic tail to tandem repeats of FKBP, and GP Ib-IX(FKBP)(2) and alpha(IIb)beta(3) were expressed in Chinese hamster ovary cells. Under flow conditions at wall shear rates of up to 2000 s(-1), GP Ib-IX(FKBP)(2) mediated cell tethering to immobilized VWF, just as in platelets. Conditional oligomerization of GP Ib-IX(FKBP)(2) by AP20187, a cell-permeable FKBP dimerizer, caused a decrease in cell translocation velocities on VWF (p < 0.001). Moreover, clustering of GP Ib-IX(FKBP)(2) by AP20187 led to an increase in alpha(IIb)beta(3) function, manifested under static conditions by increased cell adhesion to fibrinogen (p < 0.01) and under flow by increased stable cell adhesion to VWF (p < 0.04). Clustering of GP Ib-IX(FKBP)(2) also stimulated rapid tyrosine phosphorylation of ectopically expressed Syk, a putative downstream effector of GP Ib-IX in platelets. These studies establish that GP Ib-IX oligomerization, per se, affects the interaction of this receptor with VWF and its ability to influence the adhesive function of alpha(IIb)beta(3). By extrapolation, GP Ib-IX clustering in platelets may promote thrombus formation.

Sensing the environment: a historical perspective on integrin signal transduction

Cell adhesion mediated by integrin receptors has a critical function in organizing cells in tissues and in guiding haematopoietic cells to their sites of action. However, integrin adhesion receptors have broader functions in regulating cell behaviour through their ability to transduce bi-directional signals into and out of the cell and to engage in reciprocal interactions with other cellular receptors. This historical perspective traces the key findings that have led to our current understanding of these important functions of integrins.

Localization of an integrin binding site to the C terminus of talin

Talin, consisting of a 47-kDa N-terminal head domain (residues 1-433) and a 190-kDa C-terminal rod domain (residues 434-2541), links integrins to the actin cytoskeleton. We previously reported that the binding stoichiometry of integrin alpha(IIb)beta(3):talin is approximately 2:1. More recently, an integrin binding site has been localized to the talin head domain. In the present study, we identified another integrin binding site at the C-terminal region of the talin rod domain. In a solid phase binding assay, RGD affinity-purified alpha(IIb)beta(3) bound in a dose-dependent manner to microtiter wells coated with the isolated 190-kDa proteolytic fragment of the talin rod domain. Additionally, alpha(IIb)beta(3) also bound to the talin rod domain captured by 8d4, an anti-talin monoclonal antibody. Polyclonal antibodies raised against a recombinant protein fragment corresponding to the entire talin rod domain (anti-talin-R) inhibited alpha(IIb)beta(3) binding to intact talin by approximately 50% but completely blocked alpha(IIb)beta(3) binding to the talin rod domain. To localize the integrin binding site, we examined alpha(IIb)beta(3) binding to recombinant polypeptide fragments corresponding to partial sequences of the talin rod domain. Whereas alpha(IIb)beta(3) bound effectively to talin-(1075-2541) and talin-(1984-2541), it failed to bind to talin-(434-1076) and talin-(434-1975). Furthermore, the binding of alpha(IIb)beta(3) to talin-(1984-2541) was inhibited by anti-talin-R. These results indicate that an integrin binding site is located within residues 1984-2541 of the talin rod domain. Thus, talin contains two integrin binding sites, one in the homologous FERM (band four-point-one, ezrin, radixin, moesin) domain and another near its C terminus. Because talin exists as an anti-parallel homodimer in focal adhesions, the two integrin binding sites in the adjacent talin molecules would be in close proximity with each other.

Taming platelets with cyclic nucleotides

Cardiovascular diseases are often accompanied and aggravated by pathologic platelet activation. Tight regulation of platelet function is an essential prerequisite for intact vessel physiology or effective cardiovascular therapy. Physiological platelet antagonists as well as various pharmacological vasodilators inhibit platelet function by activating adenylyl and guanylyl cyclases and increasing intracellular cyclic AMP (cAMP) and cyclic GMP (cGMP) levels, respectively. Elevation of platelet cyclic nucleotides interferes with basically all known platelet activatory signaling pathways, and effectively blocks complex intracellular signaling networks, cytoskeletal rearrangements, fibrinogen receptor activation, degranulation, and expression of pro-inflammatory signaling molecules. The major target molecules of cyclic nucleotides in platelets are cyclic nucleotide-dependent protein kinases that mediate their effects through phosphorylation of specific substrates. They directly affect receptor/G-protein activation and interfere with a variety of signal transduction pathways, including the phospholipase C, protein kinase C, and mitogen-activated protein kinase pathways. Regulation of these pathways blocks several steps of cytosolic Ca(2+) elevation and controls a multitude of cytoskeleton-associated proteins that are directly involved in organization of the platelet cytoskeleton. Due to their multiple sites of action and strong inhibitory potencies, cyclic nucleotides and their regulatory pathways are of particular interest for developing new approaches for the treatment of thrombotic and cardiovascular disorders.

Oligomerization of the integrin alphaIIbbeta3: roles of the transmembrane and cytoplasmic domains

Integrins are a family of alpha/beta heterodimeric membrane proteins, which mediate cell-cell and cell-matrix interactions. The molecular mechanisms by which integrins are activated and cluster are currently poorly understood. One hypothesis posits that the cytoplasmic tails of the alpha and beta subunits interact strongly with one another in a 1:1 interaction, and that this interaction is modulated in the course of the activation of alphaIIbbeta3 [Hughes, P. E., et al. (1996) J. Biol. Chem. 271, 6571-6574]. To examine the structural basis for this interaction, protein fragments encompassing the transmembrane helix plus cytoplasmic tails of the alpha and beta subunits of alphaIIbbeta3 were expressed and studied in phospholipid micelles at physiological salt concentrations. Analyses of these fragments by analytical ultracentrifugation, NMR, circular dichroism, and electrophoresis indicated that they had very little or no tendency to interact with one another. Instead, they formed homomeric interactions, with the alpha- and beta-fragments forming dimers and trimers, respectively. Thus, these regions of the protein structure may contribute to the clustering of integrins that accompanies cellular adhesion.

Activation of alpha(v)beta3-vitronectin binding is a multistage process in which increases in bond strength are dependent on Y747 and Y759 in the cytoplasmic domain of beta3

Integrin receptors serve as mechanical links between the cell and its structural environment. Using alpha(v)beta3 integrin expressed in K562 cells as a model system, the process by which the mechanical connection between alpha(v)beta3 and vitronectin develops was analyzed by measuring the resistance of these bonds to mechanical separation. Three distinct stages of activation, as defined by increases in the alpha(v)beta3-vitronectin binding strength, were defined by mutational, biochemical, and biomechanical analyses. Activation to the low binding strength stage 1 occurs through interaction with the vitronectin ligand and leads to the phosphorylation of Y747 in the beta3 subunit. Stage 2 is characterized by a 4-fold increase in binding strength and is dependent on stage1 and the phosphorylation of Y747. Stage 3 is characterized by a further 2.5-fold increase in binding strength and is dependent on stage 2 events and the availability of Y759 for interaction with cellular proteins. The Y747F mutant blocked the transition from stage 1 to stage 2, and the Y759F blocked the transition from stage 2 to stage 3. The data suggest a model for tension-induced activation of alpha(v)beta3 integrin.

Distinct ligand-binding modes for integrin alpha(v)beta(3)-mediated adhesion to fibronectin versus vitronectin

Cell surface integrins can adopt distinct conformations in response to ligand binding and intracellular signals. Several integrins including alpha(v)beta(3) can bind to multiple ligands. The binding of alpha(v)beta(3) to fibronectin and vitronectin was used as a model to determine whether the same or distinct forms of the receptor were utilized in strong binding to the two different ligands. A spinning-disc device was used to measure the relative strength of the alpha(v)beta(3)-ligand bonds. The initial binding reaction for both ligands occurred in the absence of metabolic energy and resulted in a strong adhesion to fibronectin but a weak adhesion to vitronectin. Increases in the strength of the alpha(v)beta(3)-vitronectin bond required phosphorylation of the beta(3) cytoplasmic domain, intracellular signals, and the binding of cytoskeletal proteins to cytoplasmic domains of beta(3) controlled by Tyr-747 and Tyr-759. In contrast, alpha(v)beta(3)-mediated adhesion to fibronectin was unaffected by phorbol 12-myristate 13-acetate, mutations of Tyr-747 and Tyr-759 to phenylalanine, or availability of metabolic energy. This suggests that strong adhesion to fibronectin used the initial binding conformation, whereas strong binding to vitronectin required signaling-induced changes in the conformation of alpha(v)beta(3).