Rap1B and Rap2B translocation to the cytoskeleton by von Willebrand factor involves FcgammaII receptor-mediated protein tyrosine phosphorylation

Rap2B GTPase: structure, functions, and regulation

Rap2B GTPase, a member of Ras-related protein superfamily, was first discovered from a platelet cDNA library in the early 1990s. Since then, it has been reported to play an important role in regulating cellular processes including cytoskeletal organization, cell growth, and proliferation. It can be stimulated and suppressed by a wide range of external and internal inducers, circulating between GTP-bound active state and GDP-bound inactive state. Increasing focus on Ras signaling pathway reveals critical effects of Rap2B on tumorigenesis. In particular, Rap2B behaves in a p53-dependent manner in regulation of apoptosis and migration. Apart from being an oncogenic activator, Rap2B has been found to participate in many other physiological events via diverse downstream effectors. In this review, we present recent studies on the structure, regulation, and multiple biological functions of Rap2B, shedding light on its potential status in treatment of cancer as well as other diseases.

Structure, functional regulation and signaling properties of Rap2B

The Ras family small guanosine 5'-triphosphate (GTP)-binding protein Rap2B is is a member of the Ras oncogene family and a novel target of p53 that regulates the p53-mediated pro-survival function of cells. The Rap2B protein shares ~90% homology with Rap2A, and its sequence is 70% identical to other members of the Rap family such as RaplA and RaplB. As a result, Rap2B has been theorized to have similar signaling effectors to the GTPase-binding protein Rap, which mediates various biological functions, including the regulation of sterile 20/mitogen-activated proteins. Since its identification in the early 1990s, Rap2B has elicited a considerable interest. Numerous studies indicate that Rap2B exerts specific biological functions, including binding and stimulating phospholipase C-ε and interferon-γ. In addition, downregulation of Rap2B affects the growth of melanoma cells. The present review summarizes the possible effectors and biological functions of Rap2B. Increasing evidence clearly supports the association between Rap2B function and tumor development. Therefore, it is conceivable that anticancer drugs targeting Rap2B may be generated as novel therapies against cancer.

Activation of phosphatidylinositol 3-kinase β by the platelet collagen receptors integrin α2β1 and GPVI: The role of Pyk2 and c-Cbl

Phosphatidylinositol 3-kinaseβ (PI3Kβ) plays a predominant role in integrin outside-in signaling and in platelet activation by GPVI engagement. We have shown that the tyrosine kinase Pyk2 mediates PI3Kβ activation downstream of integrin αIIbβ3, and promotes the phosphorylation of the PI3K-associated adaptor protein c-Cbl. In this study, we compared the functional correlation between Pyk2 and PI3Kβ upon recruitment of the two main platelet collagen receptors, integrin α2β1 and GPVI. PI3Kβ-mediated phosphorylation of Akt was inhibited in Pyk2-deficient platelets adherent to monomeric collagen through integrin α2β1, but occurred normally upon GPVI ligation. Integrin α2β1 engagement led to Pyk2-independent association of c-Cbl with PI3K. However, c-Cbl was not phosphorylated in adherent platelets, and phosphorylation of Akt occurred normally in c-Cbl-deficient platelets, indicating that the c-Cbl is dispensable for Pyk2-mediated PI3Kβ activation. Stimulation of platelets with CRP, a selective GPVI ligand, induced c-Cbl phosphorylation in the absence of Pyk2, but failed to promote its association with PI3K. Pyk2 activation was completely abrogated in PI3KβKD, but not in PI3KγKD platelets, and was strongly inhibited by Src kinases and phospholipase C inhibitors, and by BAPTA-AM. The absence of PI3Kβ activity also hampered GPVI-induced tyrosine-phosphorylation and activation of PLCγ2, preventing intracellular Ca2+ increase and phosphorylation of pleckstrin. Moreover, GPVI-induced intracellular Ca2+ increase and pleckstrin phosphorylation were also strongly inhibited in human platelets treated with the PI3Kβ inhibitor TGX-221. These results outline important differences in the regulation of PI3Kβ by GPVI and integrin α2β1 and suggest that inhibition of Pyk2 may target PI3Kβ activation in a selective context of platelet stimulation.

Interaction of von Willebrand factor with platelets and the vessel wall

The initiation of thrombus formation at sites of vascular injury to secure haemostasis after tissue trauma requires the interaction of surface-exposed von Willebrand factor (VWF) with its primary platelet receptor, the glycoprotein (GP) Ib-IX-V complex. As an insoluble component of the extracellular matrix (ECM) of endothelial cells, VWF can directly initiate platelet adhesion. Circulating plasma VWF en-hances matrix VWF activity by binding to structures that become exposed to flowing blood, notably collagen type I and III in deeper layers of the vessel along with microfibrillar collagen type VI in the subendothelium. Moreover, plasma VWF is required to support platelet-to-platelet adhesion - i. e. aggregation - which promotes thrombus growth and consolidation. For these reasons, understanding how plasma VWF interaction with platelet receptors is regulated, particularly any distinctive features of GPIb binding to soluble as opposed to immobilized VWF, is of paramount importance in vascular biology. This brief review will highlight knowledge acquired and key problems that remain to be solved to elucidate fully the role of VWF in normal haemostasis and pathological thrombosis.

Amyloid β-peptide-dependent activation of human platelets: essential role for Ca2+ and ADP in aggregation and thrombus formation

Alzheimer's disease is associated with the accumulation of Aβ (amyloid β)-peptides in the brain. Besides their cytotoxic effect on neurons, Aβ-peptides are thought to be responsible for the atherothrombotic complications associated with Alzheimer's disease, which are collectively known as cerebrovascular disease. In the present study, we investigated the effect of Aβ-peptides on human platelet signal transduction and function. We discovered that the 25-35 domain of Aβ-peptides induce an increase in platelet intracellular Ca2+ that stimulates α-granule and dense granule secretion and leads to the release of the secondary agonist ADP. Released ADP acts in an autocrine manner as a stimulant for critical signalling pathways leading to the activation of platelets. This includes the activation of the protein kinases Syk, protein kinase C, Akt and mitogen-activated protein kinases. Ca2+-dependent release of ADP is also the main component of the activation of the small GTPase Rap1b and the fibrinogen receptor integrin αIIbβ3, which leads to increased platelet aggregation and increased thrombus formation in human whole blood. Our discoveries complement existing understanding of cerebrovascular dementia and suggest that Aβ-peptides can induce vascular complications of Alzheimer's disease by stimulating platelets in an intracellular Ca2+-dependent manner. Despite a marginal ADP-independent component suggested by low levels of signalling activity in the presence of apyrase or P2Y receptor inhibitors, Ca2+-dependent release of ADP by Aβ-peptides clearly plays a critical role in platelet activation. Targeting ADP signalling may therefore represent an important strategy to manage the cerebrovascular component of Alzheimer's disease.

Folic acid-conjugated 4-amino-phenylboronate, a boron-containing compound designed for boron neutron capture therapy, is an unexpected agonist for human neutrophils and platelets

Boron neutron capture therapy (BNCT) is an anticancer treatment based on the accumulation in the tumor cells of (10) B-containing molecules and subsequent irradiation with low-energy neutrons, which bring about the decay of (10) B to very toxic (7) Li(3+) and (4) He(2+) ions. The effectiveness of BNCT is limited by the low delivery and accumulation of the used (10) B-containing compounds. Here, we report the development of folic acid-conjugated 4-amino-phenylboronate as a novel possible compound for the selective delivery of (10) B in BNCT. An extensive analysis about its biocompatibility to mature blood cells and platelet progenitors revealed that the compound markedly supports platelet aggregation, neutrophil oxidative burst, and inhibition of megakaryocyte development, while it does not have any manifest effect on red blood cells.

Fatty acid synthase is a key target in multiple essential tumor functions of prostate cancer: uptake of radiolabeled acetate as a predictor of the targeted therapy outcome

Fatty acid synthase (FASN) expression is elevated in several cancers, and this over-expression is associated with poor prognosis. Inhibitors of FASN, such as orlistat, reportedly show antitumor effects against cancers that over-express FASN, making FASN a promising therapeutic target. However, large variations in FASN expression levels in individual tumors have been observed, and methods to predict FASN-targeted therapy outcome before treatment are required to avoid unnecessary treatment. In addition, how FASN inhibition affects tumor progression remains unclear. Here, we showed the method to predict FASN-targeted therapy outcome using radiolabeled acetate uptake and presented mechanisms of FASN inhibition with human prostate cancer cell lines, to provide the treatment strategy of FASN-targeted therapy. We revealed that tumor uptake of radiolabeled acetate reflected the FASN expression levels and sensitivity to FASN-targeted therapy with orlistat in vitro and in vivo. FASN-targeted therapy was noticeably effective against tumors with high FASN expression, which was indicated by high acetate uptake. To examine mechanisms, we established FASN knockdown prostate cancer cells by transduction of short-hairpin RNA against FASN and investigated the characteristics by analyses on morphology and cell behavior and microarray-based gene expression profiling. FASN inhibition not only suppressed cell proliferation but prevented pseudopodia formation and suppressed cell adhesion, migration, and invasion. FASN inhibition also suppressed genes involved in production of intracellular second messenger arachidonic acid and androgen hormones, both of which promote tumor progression. Collectively, our data demonstrated that uptake of radiolabeled acetate is a useful predictor of FASN-targeted therapy outcome. This suggests that [1-(11)C]acetate positron emission tomography (PET) could be a powerful tool to accomplish personalized FASN-targeted therapy by non-invasive visualization of tumor acetate uptake and selection of responsive tumors. FASN-targeted therapy could be an effective treatment to suppress multiple steps related to tumor progression in prostate cancers selected by [1-(11)C]acetate PET.

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.

GPIb potentiates GPVI-induced responses in human platelets

Platelet adhesion to vascular subendothelial proteins at the site of blood vessel injury is critical for initiating haemostasis. Collagen is a major matrix protein that binds plasma von Willebrand factor (vWF) when the endothelium becomes damaged and therefore in vivo platelets are likely to encounter both of these agonists simultaneously, through glycoprotein VI (GPVI) and alpha2beta1 receptors for collagen and GPIb-V-IX and alphaIIbbeta3 receptors for vWF. We hypothesised a potentiatory role for vWF that would synergise with collagen leading to functional activation and show this to be the case for platelet aggregation, 5-HT secretion and calcium responses. Synergy between these two agonists is likely to involve receptors GPVI and GPIb-V-IX, for collagen and vWF, respectively, since 5-HT secretion in response to collagen is potentiated by vWF in the presence of either EGTA or EDTA, which prevent binding to integrins alphaIIbbeta3 (EGTA) or both alphaIIbbeta3 and alpha2beta1 (EDTA). In addition, vWF is also able to potentiate 5-HT secretion responses to collagen-related peptide, confirming that GPVI is able to support synergy with vWF. These findings are important in that they reveal a novel role for vWF in platelet activation as a potentiator of platelet activation by collagen.

FcgammaRII mediates platelet aggregation caused by disintegrins and GPIIb/IIIa monoclonal antibody, AP2

OBJECTIVE

Disintegrins, snake venom-derived Arg-Gly-Asp (RGD)-containing polypeptides, and GPIIb/IIIa antagonist (AP2) block fibrinogen binding to GPIIb/IIIa of activated platelets, however, the combination of these two agents caused platelet aggregation. We hypothesize that disintegrin initially binds to specific epitope of GPIIb/IIIa, causing conformational change, and the recruitment of FcgammaRII, which can be bound by AP2, and finally triggering platelet aggregation.

MATERIALS AND METHODS

We prepared human platelet suspensions and measured platelet aggregation, Ca2+ mobilization, thromboxane B2 formation, and signal transduction.

RESULTS

Disintegrin (e.g., accutin) and AP2 (a monoclonal antibody [mAb]-raised against GPIIb/IIIa) individually inhibited human platelet aggregation caused by collagen. However, as both accutin and AP2 were sequentially added into platelet suspension, platelet aggregation occurred. Accutin/AP2 caused shape change, cytosolic Ca2+ mobilization, P-selectin expression, and thromboxane A2 formation. Tirofiban, FcgammaRII mAb, or indomethacin completely inhibited platelet aggregation caused by accutin/AP2. Accutin/AP2 also caused tyrosine phosphorylation of signal molecules. Disintegrins enhanced AP2 binding to platelets, and AP2 also promoted disintegrin binding to platelets. FcgammaRII mAb inhibited the enhanced fluorescein isothiocyanate-disintegrin binding to platelet caused by AP2. Immunoprecipitation of the lysates of disintegrin/AP2-treated platelets using FcgammaRII Ab showed complex formation of GPIIb/IIIa and FcgammaRII.

CONCLUSION

FcgammaRII mediates platelet aggregation caused by disintegrin and AP2, triggering a phospholipase C, phospholipase A2, Src-, Syk kinases, and Ca2+-dependent activation process. AP2 triggers platelet aggregation via binding to accessible FcgammaRII and the conformation-altered GPIIb/IIIa caused by disintegrin.

Platelets undergo phosphorylation of Syk at Y525/526 and Y352 in response to pathophysiological shear stress

Atherosclerotic plaques can lead to partial vascular occlusions that produce abnormally high levels of arterial wall shear stress. Such pathophysiological shear stress can promote shear-induced platelet aggregation (SIPA), which has been linked to acute myocardial infarction, unstable angina, and stroke. This study investigated the role of the tyrosine kinase Syk in shear-induced human platelet signaling. The extent of Syk tyrosine phosphorylation induced by pathophysiological levels of shear stress (100 dyn/cm(2)) was significantly greater than that resulting from physiological shear stress (10 dyn/cm(2)). With the use of phospho-Syk specific antibodies, these data are the first to show that key regulatory sites of Syk at tyrosines 525/526 (Y525/526) and tyrosine 352 (Y352) were phosphorylated in response to pathophysiological shear stress. Increased phosphorylation at both sites was attenuated by pharmacological inhibition of Syk using two different Syk inhibitors, piceatannol and 3-(1-methyl-1H-indol-3-yl-methylene)-2-oxo-2,3-dihydro-1H-indole-5-sulfonamide (OXSI-2), and by inhibition of upstream Src-family kinases (SFKs). Shear-induced response at the Syk 525/526 site was ADP dependent but not contingent on glycoprotein (GP) IIb-IIIa ligation or the generation of thromboxane (Tx) A(2). Pretreatment with Syk inhibitors not only reduced SIPA and Syk phosphorylation in isolated platelets, but also diminished, up to 50%, the platelet-mediated thrombus formation when whole blood was perfused over type-III collagen. In summary, this study demonstrated that Syk is a key molecule in both SIPA and thrombus formation under flow. Pharmacological regulation of Syk may prove efficacious in treating occlusive vascular disease.

The low-molecular-weight phosphotyrosine phosphatase is a negative regulator of FcγRIIA-mediated cell activation

Activation of human platelets by cross-linking of the low-affinity receptor for immunoglobulin G (FcγRIIA) is initiated by Src kinase–mediated phosphorylation of the immunoreceptor tyrosine–based activation motif (ITAM) within the receptor, but the identity of the enzyme responsible for its dephosphorylation and inactivation is unknown. Here we report that the 18-kDa low-molecular-weight phosphotyrosine phosphatase (LMW-PTP) is expressed in human platelets and undergoes subcellular redistribution upon FcγRIIA cross-linking. In vitro, LMW-PTP was found to efficiently dephosphorylate activated FcγRIIA and LAT, but not Syk or phospholipase Cγ2. In the megakaryocytic cell line DAMI, antibody-induced phosphorylation of FcγRIIA was rapid and transient. The late dephosphorylation of FcγRIIA was dramatically delayed upon reduction of LMW-PTP expression by siRNA. Strikingly, overexpression of LMW-PTP resulted in the inhibition of antibody-induced phosphorylation of FcγRIIA, and caused a more rapid dephosphorylation. In addition, overexpression of LMW-PTP inhibited activation of Syk downstream of FcγRIIA and reduced intracellular Ca2+ mobilization. These results demonstrate that LMW-PTP is responsible for FcγRIIA dephosphorylation, and is implicated in the down-regulation of cell activation mediated by this ITAM-bearing immunoreceptor.

A role for PLCγ2 in platelet activation by homocysteine

The aim of this study was to examine the homocysteine effect on phospholipase Cgamma2 (PLCgamma2) activation and to investigate the signaling pathway involved. We found that homocysteine stimulated the tyrosine phosphorylation and activation of platelet PLCgamma2. The tyrosine kinases p60src and p72syk appeared to be involved upstream. Reactive oxygen species were increased in homocysteine treated platelets. Likely oxidative stress could prime the non receptor-mediated tyrosine kinase p60src, inducing phosphorylation and activation of p72syk. The antioxidant N-acetyl-L-cysteine prevented the activation of these kinases. The phosphorylation and activation of PLCgamma2 were greatly reduced by the inhibition of p72syk through piceatannol. Moreover indomethacin diminished the homocysteine effect on p60src, p72syk and PLCgamma2, suggesting that thromboxane A(2) could be involved. In addition the treatment of platelets with homocysteine caused intracellular calcium rise and protein kinase C activation. Finally homocysteine induced platelet aggregation, that was partially reduced by indomethacin and by N-acetyl-L-cysteine of 35% or 50% respectively, while the PLCgamma2 specific inhibitor U73122 diminished platelet response to homocysteine of 70%. Altogether the data indicate that PLCgamma2 plays an important role in platelet activation by homocysteine and that the stimulation of this pathway requires signals through oxygen free radicals and thromboxane A(2).

Platelet GPIb-IX-V-dependent signaling

Although the signaling pathways related to GPIb-IX-V have not been fully elucidated, an accumulating body of evidence suggests that phospholipase C (PLC)gamma2 activation, subsequent Ca++ release and oscillations constitute an essential signal transduction pathway related to GPIb-IX-V. Src family kinases are required for PLCgamma2 activation, while FcR gamma-chain/Fc gammaRIIA may be dispensable for PLCgamma2 activation. Although PI-3K serves to potentiate various signaling events culminating in alpha(IIb)beta3 activation, PI-3K activity may be dispensable for Src-PLCgamma2 activation in GPIb-IX-V-mediated signaling. Glycosphingolipid-enriched microdomains (GEMs) appear to provide platforms for the signal transduction pathway related to GIb-IX-V, as the interaction between GPIb-IX-V and Src or PLCgamma2 tyrosine phosphorylation occurs exclusively in GEMs.

New perspectives on von Willebrand factor functions in hemostasis and thrombosis

The adhesive protein, von Willebrand factor (VWF), mediates the initiation and progression of thrombus formation at sites of vascular injury by means of specific interactions with extracellular matrix components and platelet receptors. The essential biologic properties of VWF have been elucidated, with progress particularly in the areas of genetic regulation, biosynthesis, and specific bimolecular interactions. The three-dimensional structure of selected domains has been solved, but our understanding of detailed structure-function relationships is still fragmented, partly because of the complexity and size of the VWF molecule. The biomechanical properties of the interaction between the VWF A1 domain and the platelet receptor glycoprotein (GP) Ibalpha also are better known, but we can still only hypothesize how this adhesive bond can oppose the fluid dynamic effects of rapidly flowing blood to initiate thrombus formation and contribute to platelet activation. Elucidating the details of VWF and GPIbalpha function will lead to a more satisfactory definition of the role of platelets in atherothrombosis, since hemodynamic forces greatly influence responses to vascular injury in stenosed and partially occluded arteries. Progress in this direction is also aided by rapidly expanding novel information on the mechanisms that regulate VWF multimer size in the circulation, a topic of relevance to explain microvascular thrombosis and, perhaps, arterial thrombosis in general. Developments in these areas of research will refine our understanding of the role played by VWF in vascular biology and pathology.

Platelet adhesion signalling and the regulation of thrombus formation

Platelets perform a central role in haemostasis and thrombosis. They adhere to subendothelial collagens exposed at sites of blood vessel injury via the glycoprotein (GP) Ib-V-IX receptor complex, GPVI and integrin alpha(2)beta(1). These receptors perform distinct functions in the regulation of cell signalling involving non-receptor tyrosine kinases (e.g. Src, Fyn, Lyn, Syk and Btk), adaptor proteins, phospholipase C and lipid kinases such as phosphoinositide 3-kinase. They are also coupled to an increase in cytosolic calcium levels and protein kinase C activation, leading to the secretion of paracrine/autocrine platelet factors and an increase in integrin receptor affinities. Through the binding of plasma fibrinogen and von Willebrand Factor to integrin alpha(IIb)beta(3), a platelet thrombus is formed. Although increasing evidence indicates that each of the adhesion receptors GPIb-V-IX and GPVI and integrins alpha(2)beta(1) and alpha(IIb)beta(3) contribute to the signalling that regulates this process, the individual roles of each are only beginning to be dissected. By contrast, adhesion receptor signalling through platelet endothelial cell adhesion molecule 1 (PECAM-1) is implicated in the inhibition of platelet function and thrombus formation in the healthy circulation. Recent studies indicate that understanding of platelet adhesion signalling mechanisms might enable the development of new strategies to treat and prevent thrombosis.

Altered cytoskeleton organization in platelets from patients with MYH9-related disease

MYH9-related disease (MYH9-RD) is an autosomal dominant disorder deriving from mutations in the MYH9 gene encoding for the heavy chain of non-muscle myosin IIA, and characterized by thrombocytopenia and giant platelets. Isoform IIA of myosin is the only one expressed in platelets, but the possibility that MYH9 mutations affect the organization of contractile structures in these blood elements has never been investigated. In this work we have analyzed the composition and the agonist-induced reorganization of the platelet cytoskeleton from seven MYH9-RD patients belonging to four different families. We found that an increased amount of myosin was constitutively associated with actin in the cytoskeleton of resting MYH9-RD platelets. Upon platelet stimulation, an impaired increase in the total cytoskeletal proteins was observed. Moreover, selected membrane glycoproteins, tyrosine kinases, and small GTPases failed to interact with the cytoskeleton in agonist-stimulated MYH9-RD platelets. These results demonstrate for the first time that mutations of MYH9 result in an alteration of the composition and agonist-induced reorganization of the platelet cytoskeleton. We suggest that these abnormalities may represent the biochemical basis for the previously reported functional alterations of MYH9-RD platelets, and for the abnormal platelet formation from megakaryocytes, resulting in thrombocytopenia and giant platelets.

Signalling through the platelet glycoprotein Ib-V–IX complex

The glycoprotein Ib-V-IX is one of the major adhesive receptors expressed on the surface of circulating platelets. It is composed of four different polypeptides-GPIbalpha, GPIbbeta, GPIX, and GPV-and represents a multifunctional receptor able to interact with a number of ligands, including the adhesive protein von Willebrand factor, the coagulation factors thrombin, factors XI and XII, and the membrane glycoproteins P-selectin and Mac-1. Interaction of GPIb-V-IX with the subendothelial von Willebrand factor is essential for primary haemostasis, as it initiates platelet adhesion to the subendothelial matrix at the sites of vascular injury even under high flow conditions. Upon interaction with von Willebrand factor, GPIb-V-IX initiates transmembrane signalling events for platelet activation, which eventually result in integrin alpha(IIb)beta(3) stimulation and platelet aggregation. The investigation of the biochemical mechanisms for platelet activation by GPIb-V-IX has attracted increasing attention during the last years. This review will describe and discuss recent findings that have provided new insights into the events underlying GPIb-V-IX transmembrane signalling. In particular, it will summarise basic concepts on the structure of this receptor, extracellular ligands, and intracellular interactors potentially involved in transmembrane signalling. The recently suggested role of membrane Fc receptors in GPIb-V-IX-initiated platelet activation will also be discussed, along with the involvement of lipid metabolising enzymes, tyrosine kinases, and the cytoskeleton in the crosstalk between GPIb-V-IX and integrin alpha(IIb)beta(3).

Autonomous SHIP-dependent FcγR signaling in pre-B cells leads to inhibition of cell migration and induction of cell death

Mature B cells express a single immunoglobulin Fc receptor, FcgammaRIIB, that functions to block downstream signaling by co-aggregated antigen receptors. Co-aggregation of receptors is essential because BCR activated kinases must phosphorylate FcgammaRIIB to recruit SHIP and mediate inhibitory signals. Pre-B cells also express FcgammaRIIB, but since they do not yet express antigen receptor, it is unclear when they are activated physiologically. Here, we demonstrate that aggregation of the FcR on pre-B cells leads to potent inhibitory signaling. Aggregation of the FcR alone leads to downstream effects including the induction of cell death and the blockade of SDF-1 induced migration. The biochemical circuitry that mediates this response is unique because although SHIP is required for this signaling and is phosphorylated upon receptor aggregation, this occurs in the absence of FcgammaRIIB phosphorylation. Results indicate that immune complexes may inhibit B cell production in the bone marrow by antigen non-specific mechanisms.

GPIb-dependent platelet activation is dependent on Src kinases but not MAP kinase or cGMP-dependent kinase

Glycoprotein Ib-IX-V (GPIb-IX-V) mediates platelet tethering to von Willebrand factor (VWF), recruiting platelets into the thrombus, and activates integrin alphaIIbbeta3 through a pathway that is dependent on Src kinases. In addition, recent reports indicate that activation of alphaIIbbeta3 by VWF is dependent on protein kinase G (PKG) and mitogen-activated protein (MAP) kinases. The present study compares the importance of these signaling pathways in the activation of alphaIIbbeta3 by GPIb-IX-V. In contrast to a recent report, VWF did not promote an increase in cyclic guanosine monophosphate (cGMP), while agents that elevate cGMP, such as the nitrous oxide (NO) donor glyco-SNAP-1 (N-(beta-D-glucopyranosyl)-N2-acetyl-S-nitroso-D,L-penicillaminamide) or the type 5 phosphosdiesterase inhibitor, sildenafil, inhibited rather than promoted activation of alphaIIbbeta3 by GPIb-IX-V and blocked aggregate formation on collagen at an intermediate rate of shear (800 s(-1)). Additionally, sildenafil increased blood flow in a rabbit model of thrombus formation in vivo. A novel inhibitor of the MAP kinase pathway, which is active in plasma, PD184161, had no effect on aggregate formation on collagen under flow conditions, whereas a novel inhibitor of Src kinases, which is also active in plasma, PD173952, blocked this response. These results demonstrate a critical role for Src kinases but not MAP kinases in VWF-dependent platelet activation and demonstrate an inhibitory role for cGMP-elevating agents in regulating this process.

PECAM-1 negatively regulates GPIb/V/IX signaling in murine platelets

Platelet adhesion at sites of vascular injury is mediated, in part, by interaction of the platelet plasma membrane glycoprotein (GP) Ib/V/IX complex with von Willebrand Factor (VWF) presented on collagen-exposed surfaces. Recent studies indicate that GPIb/V/IX may be functionally coupled with the Fc receptor gamma (FcR gamma)-chain, which, by virtue of its cytoplasmic immunoreceptor tyrosine-based activation motif, sends activation signals into the cell. Platelet endothelial cell adhesion molecule-1 (PECAM-1) is an inhibitory receptor that has previously been shown to negatively regulate platelet responses to collagen, which transduces activation signals via the GPVI/FcR gamma-chain complex. To determine whether PECAM-1 might similarly regulate signals emanating from GPIb/FcR gamma, we compared activation and aggregation responses to VWF of PECAM-1-positive and PECAM-1-deficient murine platelets. PECAM-1 and the FcR gamma-chain became rapidly tyrosine phosphorylated in platelets following botrocetin-induced VWF binding, but FcR gamma-chain tyrosine phosphorylation was delayed in PECAM-1-positive, versus PECAM-1-deficient, platelets. PECAM-1-deficient platelets were hyperaggregable to VWF, exhibited enhanced spreading and, under conditions of arterial flow, formed markedly larger thrombi on immobilized VWF than did wild-type platelets. Taken together, these data support the notion that engagement of the GPIb complex, in addition to sending activation signals, also initiates a negative feedback loop involving PECAM-1 that controls the rate and extent of platelet activation.

Signaling role for phospholipase C gamma 2 in platelet glycoprotein Ib alpha calcium flux and cytoskeletal reorganization. Involvement of a pathway distinct from FcR gamma chain and Fc gamma RIIA

Interaction of the platelet GPIb-V-IX complex with surface immobilized von Willebrand factor (vWf) is required for the capture of circulating platelets and their ensuing activation. In previous work, it was found that GPIb/vWf-mediated platelet adhesion triggers Ca2+ release from intracellular stores, leading to cytoskeletal reorganization and filopodia extension. Despite the potential functional importance of GPIb-induced cytoskeletal changes, the signaling mechanisms regulating this process have remained ill-defined. The studies presented here demonstrate an important role for phospholipase C (PLC)-dependent phosphoinositide turnover for GPIb-dependent cytoskeletal remodeling. This is supported by the findings that the vWf-GPIb interaction induced a small increase in inositol 1,4,5-triphosphate (IP3) and that treating platelets with the IP3 receptor antagonist APB-2 or the PLC inhibitor U73122 blocked cytosolic Ca2+ flux and platelet shape change. Normal shape change was observed in G alpha q-/- mouse platelets, excluding a role for PLC beta isoforms in this process. However, decreased shape change and Ca2+ mobilization were observed in mice lacking PLC gamma 2, demonstrating that this isotype played an important, albeit incomplete, role in GPIb signaling. The signaling pathways utilized by GPIb involved one or more members of the Src kinase family as platelet shape change and Ca2+ flux were inhibited by the Src kinase inhibitors PP1 and PP2. Strikingly, shape change and Ca2+ release occurred independently of immunoreceptor tyrosine-based activation motif (ITAM)-containing receptors, because these platelet responses were normal in human platelets treated with the anti-Fc gamma RIIA blocking monoclonal antibody IV.3 and in mouse platelets deficient in the FcR gamma chain. Taken together, these studies define an important role for PLC gamma 2 in GPIb signaling linked to platelet shape change. Moreover, they demonstrate that GPIb-dependent calcium flux and cytoskeletal reorganization involves a signaling pathway distinct from that utilized by ITAM-containing receptors.

Signaling events underlying thrombus formation

Recent in vivo studies have highlighted the dynamic and complex nature of platelet thrombus growth and the requirement for multiple adhesive receptor-ligand interactions in this process. In particular, the importance of von Willebrand factor (VWF) in promoting both primary adhesion and aggregation under high shear conditions is now well established. In general, the efficiency with which platelets adhere and aggregate at sites of vessel wall injury is dependent on the synergistic action of various adhesive and soluble agonist receptors, with the contribution of each of the individual receptors dependent on the prevailing blood flow conditions. In this review, we will discuss the major platelet adhesive interactions regulating platelet thrombus formation under high shear, with specific focus on the VWF (GPIb and integrin alphaIIbbeta3) and collagen receptors (GPVI and integrin alpha2beta1). We will also discuss the signaling mechanisms utilized by these receptors to induce platelet activation with specific emphasis on the role of cytosolic calcium flux in regulating platelet adhesion dynamics. The role of soluble agonists in promoting thrombus growth will be highlighted and a model to explain the synergistic requirement for adhesive and soluble stimuli for efficient platelet aggregation will be discussed.

Helicobacter pylori binds von Willebrand factor and interacts with GPIb to induce platelet aggregation

BACKGROUND & AIMS

Clinical studies have suggested an association between cardiovascular disease and infection with Helicobacter pylori. We examined the effect of H. pylori on platelets and the mechanism of the interaction.

METHODS

Three of 5 strains of H. pylori induced platelet aggregation with a lag time of 5 +/- 2 minutes that was independent of the toxigenic genes cagA and vacA. Aggregation was inhibited completely by aspirin and a glycoprotein (GP) IIb/IIIa antagonist. Aggregation also was inhibited by monoclonal antibodies that prevented the von Willebrand factor (vWF) interaction with GPIb. vWF-coated H. pylori bound to cells transfected with GPIbalpha but not to mock transfected cells and this was inhibited by an antibody to GPIb.

RESULTS

The interaction with platelets appeared to be mediated by vWF because platelet aggregation was blocked by an antibody to vWF. Moreover, a strain of H. pylori that induced platelet aggregation bound vWF to a greater extent than a nonaggregating strain. Aggregation also required IgG and could be inhibited by an antibody to the platelet IgG receptor (FcgammaRIIA).

CONCLUSIONS

Some strains of H. pylori induce platelet activation mediated by H. pylori-bound vWF interacting with GPIb, and supported by IgG. These platelet-H. pylori interactions may contribute to the pathogenesis of H. pylori-associated peptic ulcer disease and to the association between H. pylori infection and cardiovascular disease, whereas local platelet effects may contribute to the pathogenesis of H. pylori-associated peptic ulcer disease.

Activation of pp125FAK by type 2B recombinant von Willebrand factor binding to platelet GPIb at a high shear rate occurs independently of alpha IIb beta 3 engagement

Shear-induced platelet aggregation (SIPA) involves the sequential interaction of von Willebrand factor (VWF) with both glycoprotein Ib (GPIb) and alphaIIbbeta3 receptors. Type 2B recombinant VWF (2B-rVWF), characterized by an increased affinity for GPIb, induces strong SIPA at a high shear rate (4000 s-1). Despite the increased affinity of 2B-rVWF for GPIb, patients with type 2B von Willebrand disease have a paradoxical bleeding disorder, which is not well understood. The purpose of this study was to determine if SIPA induced by 2B-rVWF was associated with alphaIIbbeta3-dependent platelet activation. To this end, we have addressed the influence of 2B-rVWF (Val553Met substitution) on SIPA-dependent variations of tyrosine protein phosphorylation (P-Tyr) and the effect of alphaIIbbeta3 blockers. At a high shear rate, 2B-rVWF induced a strong SIPA, as shown by a 92.7% +/- 0.4% disappearance of single platelets (DSP) after 4.5 minutes. In these conditions, increased P-Tyr of proteins migrating at positions 64 kd, 72 kd, and 125 kd were observed. The band at 125 kd was identified as pp125FAK using anti-phospho-FAK antibody. This effect, which required a high level of SIPA (> 70% DSP), was observed at 4000 s-1 but not at 200 s-1. Monoclonal antibodies (MoAbs) 6D1 (anti-GPIb) and 328 (anti-VWF A1 domain), completely abolished SIPA and p125FAK phosphorylation mediated by 2B-rVWF. In contrast, neither RGDS peptide nor MoAb 7E3, both known to block alphaIIbbeta3 engagement, had any effect on SIPA and pp125FAK. The size of aggregates formed at a high shear rate in the presence of 2B-rVWF was decreased by genistein, demonstrating the biologic relevance of pp125FAK. These findings provide a unique mechanism whereby the enhanced interaction of 2B-rVWF with GPIb, without engagement of alphaIIbbeta3, is sufficient to induce SIPA but does not lead to stable thrombus formation.

Localization of the adhesion receptor glycoprotein Ib-IX-V complex to lipid rafts is required for platelet adhesion and activation

The platelet glycoprotein (GP) Ib-IX-V complex mediates the attachment of platelets to the blood vessel wall by binding von Willebrand factor (VWF), an interaction that also transmits signals for platelet activation and aggregation. Because the complex is extensively palmitoylated, a modification known to target proteins to lipid rafts, we investigated the role of raft localization in GP Ib-IX-V functions. In unstimulated platelets, a minor portion of the complex localized to Triton-insoluble raft fractions; this portion increased three to sixfold with platelet activation by VWF. Raft-associated GP Ib-IX-V was selectively palmitoylated, with GP Ib-IX-V-associated palmitate increasing in the raft fraction on VWF-mediated activation. The raft fraction was also the site of association between GP Ib-IX-V and the Fc receptor FcgammaRIIA. The importance of this association was demonstrated by the ability of the FcgammaRIIA antibody IV.3 to inhibit shear-induced platelet aggregation. Disruption of rafts by depleting membrane cholesterol impaired several GP Ib-IX-V-dependent platelet fractions: aggregation to VWF under static conditions and under shear stress, tyrosine phosphorylation, and adhesion to a VWF surface. Partial restoration of membrane cholesterol content partially restored shear-induced platelet aggregation and tyrosine phosphorylation. Thus, localization of the GP Ib-IX-V complex within rafts is crucial for both platelet adhesion and postadhesion signaling.

The small proteoglycan decorin supports adhesion and activation of human platelets

Decorin is a small leucine-rich proteoglycan able to interact with several molecules of the subendothelial matrix, such as collagen and fibronectin. In this work, we investigated the ability of purified decorin to support adhesion of human platelets. We found that gel-filtered platelets were actually able to interact with immobilized decorin. Platelet adhesion to decorin was time dependent, required the presence of Mg2+ ions, and was totally mediated by the protein core of the proteoglycan. Platelet stimulation with either adenosine diphosphate (ADP) or a thrombin receptor–activating peptide significantly increased interaction of these cells with the proteoglycan. Upon adhesion to immobilized decorin a number of platelet proteins were found to become tyrosine-phosphorylated. By immunoprecipitation experiments with specific antibodies, the tyrosine phosphorylation of the tyrosine kinase Syk and the phospholipase Cγ2 (PLCγ2) isozyme was demonstrated in decorin-adherent platelets. Interaction of platelets with decorin was selectively prevented by 2 different antibodies against membrane integrin α2β1, but not by a number of antibodies against other membrane receptors. In addition, integrin α2β1, purified from platelet membranes, was able to specifically interact with immobilized decorin. Finally, purified decorin bound to Sepharose beads could precipitate integrin α2β1 from a platelet membrane glycoprotein preparation. Therefore, these results demonstrate that human platelets can bind to immobilized decorin through integrin α2β1, and that this interaction results in the tyrosine phosphorylation of intracellular proteins.

Dietary phytoestrogens and their synthetic structural analogues as calcium channel blockers in human platelets

Phytoestrogens have been shown to inhibit platelet activation by blocking platelet calcium channels. This study examined the effect of several synthetic derivatives of trans-resveratrol, genistein, and daidzein on platelet free intracellular calcium ([Ca2+]i) elevation in thrombin-activated platelets and the possible mechanisms of this inhibitory effect. Studies were conducted on fresh human platelets from healthy volunteers. The fluorescent dye fura-2 was used to monitor [Ca2+]i in platelets. At 10 microM-resveratrol, triacetyl-trans-resveratrol, and trimethoxy-trans-resveratrol produced, respectively, 57 +/- 4%, 40 +/- 4%, and 21 +/- 1% inhibition; genistein, acetylgenistein, and dihydrogenistein produced 51 +/- 10%, 26 +/- 7%, and 16 +/- 2% inhibition, respectively; daidzein and diacetyldaidzein produced 56 +/- 5% and 45 +/- 10% inhibition of thrombin-induced [Ca2+]i elevation. The inhibitory effect was immediate and appeared to directly affect the calcium influx channels. Phytoestrogen action on [Ca2+]i did not cause alteration in nitric oxide signaling. Tyrosine phosphorylation was not involved in the inhibition of [Ca2+]i elevation by phytoestrogens, because the percent inhibition produced by the tyrosine kinase inhibitor genistein and its inactive analogue daidzein on thrombin-induced and thapsigargin-induced [Ca2+]i elevation was not significantly different for either compound at any concentration tested. Structure-activity relationship studies on this limited set of compounds reveal the requirements for the stilbene pharmacophore for the calcium-blocking activity.

A role for glycoprotein Ib in Streptococcus sanguis-induced platelet aggregation

Numerous studies have implicated bacteria in cardiovascular disease, but there is a paucity of information on the mechanism involved. In this study we show how the common oral bacterium Streptococcus sanguis can directly interact with platelets, resulting in activation and aggregate formation. Platelet aggregation was dependent on glycoprotein IIb/IIIa (GPIIb/IIIa) and thromboxane. Platelets could also directly bind to S sanguis, but this interaction was not inhibited by GPIIb/IIIa antagonists. Antibodies to GPIb could inhibit both platelet aggregation and platelet adhesion to bacteria. This suggested a direct interaction between GPIb and S sanguis; however, this interaction did not require von Willebrand factor, the normal ligand for GPIb. By use of a range of monoclonal antibodies to GPIb and the enzyme mocharagin, which cleaves GPIb at amino acid 282, the interaction was localized to a region within the N-terminal 1-225 portion of GPIbalpha. Furthermore S sanguis failed to induce aggregation of platelets from a patient with Bernard-Soulier disease, the organism bound to Chinese hamster ovary cells transfected with the GPIbalpha gene but did not bind to mock-transfected cells and biotin-labeled S sanguis cells bound to purified GPIb in ligand blots. It is suggested that the interaction between S sanguis and GPIb is important in the pathogenesis of infective endocarditis and may also play a contributory role in some cases of myocardial infarction.

Essential role for phosphoinositide 3-kinase in shear-dependent signaling between platelet glycoprotein Ib/V/IX and integrin alpha(IIb)beta(3)

Platelet adhesion and aggregation at sites of vascular injury are critically dependent on the interaction between von Willebrand factor (VWF) and 2 major platelet adhesion receptors, glycoprotein (GP) Ib/V/IX and integrin alpha(IIb)beta(3). GP Ib/V/IX binding to VWF mediates platelet tethering and translocation, whereas activation of integrin alpha(IIb)beta(3) promotes cell arrest. To date, the signaling pathways used by the VWF-GP Ib/V/IX interaction to promote activation of integrin alpha(IIb)beta(3), particularly under shear, have remained poorly defined. In this study, the potential involvement of type 1 phosphoinositide (PI) 3-kinases in this process was investigated. Results show that platelet adhesion and spreading on immobilized VWF results in a specific increase in the PI 3-kinase lipid product, PtdIns(3,4)P(2). Under static conditions, inhibiting PI 3-kinase with LY294002 or wortmannin did not prevent platelet adhesion, integrin alpha(IIb)beta(3) activation, or platelet spreading although it significantly delayed the onset of these events. In contrast, PI 3-kinase inhibition under shear dramatically reduced both platelet adhesion and spreading. Real-time analysis of intracellular calcium demonstrated that under static conditions inhibiting PI 3-kinase delayed the onset of intracellular fluxes in adherent platelets, but did not affect the final magnitude of the calcium response. However, under shear, inhibiting PI 3-kinase dramatically reduced intracellular calcium mobilization and integrin alpha(IIb)beta(3) activation, resulting in impaired thrombus growth. The studies demonstrate a shear-dependent role for PI 3-kinase in promoting platelet adhesion on immobilized VWF. Under static conditions, platelets appear to mobilize intracellular calcium through both PI 3-kinase-dependent and -independent mechanisms, whereas under shear PI 3-kinase is indispensable for VWF-induced calcium release.

Platelet activation by von Willebrand factor requires coordinated signaling through thromboxane A2 and Fc gamma IIA receptor

Interaction of von Willebrand Factor with glycoprotein Ib-IX-V induces platelet activation through a still poorly defined mechanism. Previous studies have suggested a possible role for the low affinity receptor for immunoglobulin, Fc gamma RIIA, in GPIb-IX-V signaling. Here we show that binding of vWF to platelets induces the tyrosine phosphorylation of Fc gamma RIIA by a Src kinase. Treatment of platelets with the anti-Fc gamma RIIA monoclonal antibody IV.3 specifically inhibits vWF-induced but not thrombin-induced pleckstrin phosphorylation and serotonin secretion. Moreover, vWF fails to induce pleckstrin phosphorylation in mouse platelets, lacking Fc gamma RIIA, and serotonin secretion is impaired. Pleckstrin phosphorylation and serotonin secretion in human platelets stimulated with vWF are blocked by the cyclooxygenase inhibitor acetylsalicylic acid. However, release of arachidonic acid and synthesis of TxA(2) induced by vWF are not affected by the anti-Fc gamma RIIA monoclonal antibody IV.3. Similarly, vWF-induced tyrosine phosphorylation of Fc gamma RIIA, as well as of Syk and PLC gamma 2, occurs normally in aspirinized platelets. Inhibition of the tyrosine kinase Syk by piceatannol does not affect vWF-induced tyrosine phosphorylation of Fc gamma RIIA but prevents phosphorylation of PLC gamma 2. Pleckstrin phosphorylation and platelet secretion induced by vWF, but not by thrombin, are also inhibited by piceatannol. Pleckstrin phosphorylation is also sensitive to the phosphatidylinositol 3-kinase inhibitor wortmannin. These results indicate that PLC gamma 2 plays a central role in platelet activation by vWF and that the stimulation of this enzyme requires coordinated signals through endogenous TxA(2) and Fc gamma RIIA.

Role of Fc receptor gamma-chain in platelet glycoprotein Ib-mediated signaling

Interaction between von Willebrand factor (vWF) and glycoprotein Ib (GPIb) stimulates tyrosine kinases and subsequent tyrosine phosphorylation events in human platelets. This study found that the combination of vWF and botrocetin, by interacting with GPIb, induced tyrosine phosphorylation of Fc receptor gamma-chain (FcR gamma-chain), Syk, linker for activation of T cells (LAT), and phospholipase C gamma2 (PLCgamma2). Pretreatment of platelets with 10 microM PP1 completely inhibited these tyrosine phosphorylation events. On GPIb stimulation, Src and Lyn formed a complex with FcR gamma-chain and Syk, suggesting that Src and Lyn are involved in FcR gamma-chain tyrosine phosphorylation and downstream signals. In spite of the PLCgamma2 tyrosine phosphorylation, however, there was no intracellular calcium release and inositol 1,4,5-trisphosphate production. In Brij 35 lysates, FcR gamma-chain was found to constitutively associate with GPIb. The number of GPIb expressed on FcR gamma-chain-deficient platelets was comparable to that of the wild-type, as assessed by flow cytometry. However, tyrosine phosphorylation of Syk, LAT, and PLCgamma2 in response to vWF plus botrocetin was significantly suppressed, suggesting that FcR gamma-chain mediates activation signals related to GPIb. Compared with the aggregation response of wild-type platelets, that of FcR gamma-chain-deficient platelets in response to vWF plus botrocetin was impaired, implying that FcR gamma-chain is required for the full activation of platelets mediated by GPIb. (Blood. 2001;97:3836-3845)

Heparin modulates integrin function in human platelets

PURPOSE

Heparin binds to human platelets and can cause activation and aggregation, although the mechanisms are unknown. To determine how heparin alters platelet function, we identified platelet-binding sites for heparin and measured heparin's influence on the function of platelet integrin alpha(IIb)beta(3) (glycoprotein IIb/IIIa).

METHODS

Photoaffinity cross-linking and affinity chromatography experiments were performed to identify platelet membrane proteins that bind heparin. Heparin's effect on fibrinogen binding to platelets was measured with a radioligand-binding assay. The translocation to the cytoskeleton of Rap2, a guanosine triphosphate-binding protein, was measured from platelets aggregating in response to heparin and other agonists.

RESULTS

Cross-linking and affinity chromatographic experiments positively identified the integrin alpha(IIb)beta(3) as a heparin-binding site. Heparin aggregation was calcium dependent. Low concentrations of unfractionated porcine mucosal heparin (2-5 U/mL) significantly increased fibrinogen I 125 binding to activated platelets, whereas higher doses did not. Heparin-mediated platelet aggregation was completely blocked by GRGDS peptide (5 mmol/L), a competitive inhibitor of fibrinogen binding, and was blocked by EDTA (2 mmol/L), which dissociates the functional integrin complex. Aggregation was associated with Rap2 translocation to the cytoskeleton, a sign of outside-in signaling.

CONCLUSIONS

Heparin binds to the alpha(IIb)beta(3) integrin in vitro and ex vivo, and heparin increases fibrinogen binding to the integrin. Heparin-mediated aggregation requires an intact integrin and ligand and leads to Rap2 translocation to the cytoskeleton-an outside-in signal of ligand engagement. Heparin may directly modulate platelet integrin function, most likely through direct binding and modulation of integrin function.

Combinatorial effect of T-cell receptor ligation and CD45 isoform expression on the signaling contribution of the small GTPases Ras and Rap1

By using ligands with various affinities for the T-cell receptor (TCR) and by altering the contribution of the CD45 tyrosine phosphatase, the effects of the potency of TCR-induced signals on the function of small GTPases Ras and Rap1 were studied. T cells expressing low-molecular-weight CD45 isoforms (e.g., CD45RO) exhibited the strongest activation of the Ras-dependent Elk-1 transcription factor and the highest sensitivity to the inhibitory action of dominant negative mutant Ras compared to T cells expressing high-molecular-weight CD45 isoforms (ABC). Moreover, stimulation of CD45RO(+), but not CD45ABC(+), T cells with a high-affinity TCR ligand induced suboptimal Elk-1 activation compared with the stimulation induced by an intermediate-affinity TCR-ligand interaction. This observation suggested that the Ras-dependent signaling pathway is safeguarded in CD45RO(+) expressors by a negative regulatory mechanism(s) which prohibits maximal activation of the Ras-dependent signaling events following high-avidity TCR-ligand engagement. Interestingly, the biochemical activity of another small GTPase, the Ras-like protein Rap1, which has been implicated in the functional suppression of Ras signaling, was inversely correlated with the extent of Elk-1 activation induced by different-affinity TCR ligands. Consistently, overexpression of putative Rap dominant negative mutant RapN17 or the physiologic inhibitor of Rap1, the Rap GTPase-activating protein RapGAP, augmented the Elk-1 response in CD45RO(+) T cells. This is in contrast to the suppressive effect of RapN17 and RapGAP on CD45ABC(+) T cells, underscoring the possibility that Rap1 can act as either a repressor or a potentiator of Ras effector signals, depending on CD45 isoform expression. These observations suggest that cells expressing distinct isoforms of CD45 employ different signal transduction schemes to optimize Ras-mediated signal transduction in activated T lymphocytes.

Cytoskeletal regulation of the platelet glycoprotein Ib/V/IX–von Willebrand factor interaction

Shear-induced binding of von Willebrand factor (vWf) to the platelet glycoprotein (GP) Ib/V/IX complex plays a key role in initiating platelet adhesion and aggregation at sites of vascular injury. This study demonstrated that pretreating human platelets with inhibitors of actin polymerization, cytochalasin D or latrunculin B, dramatically enhances platelet aggregation induced by vWf. The effects of these inhibitors were specific to the vWf-GPIbα interaction because they enhanced vWf-induced aggregation of Glanzmann thrombasthenic platelets and Chinese hamster ovary (CHO) cells transfected with GPIb/V/IX. Moreover, cytochalasin D enhanced the extent of platelet aggregation induced by high shear stress (5000 s−1) and also lowered the shear threshold required to induce aggregation from 3000 s−1 to as low as 500 s−1. Studies of CHO cells expressing GPIbα cytoplasmic tail truncation mutants that failed to bind actin-binding protein-280 (deletion of residues 569-610 or 535-568) demonstrated that the linkage between GPIb and actin-binding protein-280 was not required for vWf-induced actin polymerization, but was critical for the enhancing effects of cytochalasin D on vWf-induced cell aggregation. Taken together, these studies suggest a fundamentally important role for the cytoskeleton in regulating the adhesive function of GPIb/V/IX.

Cytoskeletal regulation of the platelet glycoprotein Ib/V/IX–von Willebrand factor interaction

Shear-induced binding of von Willebrand factor (vWf) to the platelet glycoprotein (GP) Ib/V/IX complex plays a key role in initiating platelet adhesion and aggregation at sites of vascular injury. This study demonstrated that pretreating human platelets with inhibitors of actin polymerization, cytochalasin D or latrunculin B, dramatically enhances platelet aggregation induced by vWf. The effects of these inhibitors were specific to the vWf-GPIbα interaction because they enhanced vWf-induced aggregation of Glanzmann thrombasthenic platelets and Chinese hamster ovary (CHO) cells transfected with GPIb/V/IX. Moreover, cytochalasin D enhanced the extent of platelet aggregation induced by high shear stress (5000 s−1) and also lowered the shear threshold required to induce aggregation from 3000 s−1 to as low as 500 s−1. Studies of CHO cells expressing GPIbα cytoplasmic tail truncation mutants that failed to bind actin-binding protein-280 (deletion of residues 569-610 or 535-568) demonstrated that the linkage between GPIb and actin-binding protein-280 was not required for vWf-induced actin polymerization, but was critical for the enhancing effects of cytochalasin D on vWf-induced cell aggregation. Taken together, these studies suggest a fundamentally important role for the cytoskeleton in regulating the adhesive function of GPIb/V/IX.

The von Willebrand factor-glycoprotein Ib/V/IX interaction induces actin polymerization and cytoskeletal reorganization in rolling platelets and glycoprotein Ib/V/IX-transfected cells

Platelet adhesion to sites of vascular injury is initiated by the binding of the platelet glycoprotein (GP) Ib-V-IX complex to matrix-bound von Willebrand factor (vWf). This receptor-ligand interaction is characterized by a rapid on-off rate that enables efficient platelet tethering and rolling under conditions of rapid blood flow. We demonstrate here that platelets adhering to immobilized vWf under flow conditions undergo rapid morphological conversion from flat discs to spiny spheres during surface translocation. Studies of Glanzmann thrombasthenic platelets (lacking integrin alpha(IIb)beta(3)) and Chinese hamster ovary (CHO) cells transfected with GPIb/IX (CHO-Ib/IX) confirmed that vWf binding to GPIb/IX was sufficient to induce actin polymerization and cytoskeletal reorganization independent of integrin alpha(IIb)beta(3). vWf-induced cytoskeletal reorganization occurred independently of several well characterized signaling processes linked to platelet activation, including calcium influx, prostaglandin metabolism, protein tyrosine phosphorylation, activation of protein kinase C or phosphatidylinositol 3-kinase but was critically dependent on the mobilization of intracellular calcium. Studies of Oregon Green 488 1, 2-bis(o-amino-5-fluorophenoxy)ethane-N,N,N',N-tetraacetic acid tetraacetoxymethyl ester-loaded platelets and CHO-Ib/IX cells demonstrated that these cells mobilize intracellular calcium in a shear-dependent manner during surface translocation on vWf. Taken together, these studies suggest that the vWf-GPIb interaction stimulates actin polymerization and cytoskeletal reorganization in rolling platelets via a shear-sensitive signaling pathway linked to intracellular calcium mobilization.