Signaling across the platelet adhesion receptor glycoprotein Ib-IX induces alpha IIbbeta 3 activation both in platelets and a transfected Chinese hamster ovary cell system

Distinct platelet F-actin patterns and traction forces on von Willebrand factor versus fibrinogen

Upon vascular injury, platelets form a hemostatic plug by binding to the subendothelium and to each other. Platelet-to-matrix binding is initially mediated by von Willebrand factor (VWF) and platelet-to-platelet binding is mediated mainly by fibrinogen and VWF. After binding, the actin cytoskeleton of a platelet drives its contraction, generating traction forces that are important to the cessation of bleeding. Our understanding of the relationship between adhesive environment, F-actin morphology, and traction forces is limited. Here, we examined F-actin morphology of platelets attached to surfaces coated with fibrinogen and VWF. We identified distinct F-actin patterns induced by these protein coatings and found that these patterns were identifiable into three classifications via machine learning: solid, nodular, and hollow. We observed that traction forces for platelets were significantly higher on VWF than on fibrinogen coatings and these forces varied by F-actin pattern. In addition, we analyzed the F-actin orientation in platelets and noted that their filaments were more circumferential when on fibrinogen coatings and having a hollow F-actin pattern, while they were more radial on VWF and having a solid F-actin pattern. Finally, we noted that subcellular localization of traction forces corresponded to protein coating and F-actin pattern: VWF-bound, solid platelets had higher forces at their central region while fibrinogen-bound, hollow platelets had higher forces at their periphery. These distinct F-actin patterns on fibrinogen and VWF and their differences in F-actin orientation, force magnitude, and force localization could have implications in hemostasis, thrombus architecture, and venous versus arterial thrombosis.

The GRKs Reactome: Role in Cell Biology and Pathology

G protein-coupled receptor kinases (GRKs) are protein kinases that function in concert with arrestins in the regulation of a diverse class of G protein-coupled receptors (GPCRs) signaling. Although GRKs and arrestins are key participants in the regulation of GPCR cascades, the complex regulatory mechanisms of GRK expression, its alternation, and their function are not thoroughly understood. Several studies together with the work from our lab in recent years have revealed the critical role of these kinases in various physiological and pathophysiological processes, including cardiovascular biology, inflammation and immunity, neurodegeneration, thrombosis, and hemostasis. A comprehensive understanding of the mechanisms underlying functional interactions with multiple receptor proteins and how these interactions take part in the development of various pathobiological processes may give rise to novel diagnostic and therapeutic strategies. In this review, we summarize the current research linking the role of GRKs to various aspects of cell biology, pathology, and therapeutics, with a particular focus on thrombosis and hemostasis.

Platelets as an indicator of vascular repair in elderly Japanese men

Platelets and circulating CD34-positive cells have been reported to contribute to vascular repair (endothelial repair and developing atherosclerosis). And because hypertension is known to be a strong vascular impairment factors, it should also influence the respective numbers of these factors. To clarify the clinical importance of platelets on vascular repair, we conducted a cross-sectional study of 567 Japanese men aged 60-69 who underwent an annual health check-up between 2013 and 2015. Multiple linear regression analysis of non-hypertensive subjects adjusting for classical cardiovascular risk factors showed that although platelet count did not significantly correlate with carotid intima media thickness (β = −0.05, p = 0.356), it did positively correlate significantly with the natural log of the number of circulating CD34-positive cells (β = 0.26, p < 0.001). In hypertensive subjects, a significant positive correlation was seen between platelets and intima media thickness (β = 0.19, p = 0.008), whereas no significant correlation was seen between platelet count and the natural log of the number of circulating CD34-positive cells (β = 0.11, p = 0.119). Our results indicate that platelet count is an indicator of vascular repair activity (endothelial repair and developing atherosclerosis). Additionally, hypertension might mask the beneficial effects of circulating CD34-positive cells.

Possible mechanism underlying the association between height and vascular remodeling in elderly Japanese men

Height is reported to be inversely associated with cardiovascular disease. And platelets play an important role in vascular remodeling by supporting CD34-positive cells. To clarify the association between height and platelet, we conducted a cross-sectional study of 219 elderly Japanese men. Since hemoglobin concentration is influenced by vascular remodeling activity, an analysis stratified by hemoglobin level was performed. An inverse association was seen between height and platelet count in subjects with a high hemoglobin concentration (≥ 14.5 g/dL), but not in subjects with a low hemoglobin concentration (< 14.5 g/dL). The standardized parameter estimates (β) were β = -0.22, p = 0.019 for subjects with high hemoglobin, and β = -0.01, p = 0.931 for subjects with low hemoglobin. We also found a positive association between platelets and carotid intima media thickness (CIMT) and circulating CD34-positive cells in subjects with high hemoglobin (partial correlation coefficient (r) = 0.21, p = 0.037 and r = 0.40, p =< 0.001), but not in subjects with low hemoglobin (r = 0.04, p = 0.710 and r = 0.06, p = 0.544). In subjects with a high hemoglobin concentration, platelets were inversely associated with height, and positively associated with CIMT and circulating CD34-positive cells. These results indicate that subjects with a short stature activate vascular remodeling to a much greater extent than subjects with a tall stature.

14-3-3ζ regulates the mitochondrial respiratory reserve linked to platelet phosphatidylserine exposure and procoagulant function

The 14-3-3 family of adaptor proteins regulate diverse cellular functions including cell proliferation, metabolism, adhesion and apoptosis. Platelets express numerous 14-3-3 isoforms, including 14-3-3ζ, which has previously been implicated in regulating GPIbα function. Here we show an important role for 14-3-3ζ in regulating arterial thrombosis. Interestingly, this thrombosis defect is not related to alterations in von Willebrand factor (VWF)–GPIb adhesive function or platelet activation, but instead associated with reduced platelet phosphatidylserine (PS) exposure and procoagulant function. Decreased PS exposure in 14-3-3ζ-deficient platelets is associated with more sustained levels of metabolic ATP and increased mitochondrial respiratory reserve, independent of alterations in cytosolic calcium flux. Reduced platelet PS exposure in 14-3-3ζ-deficient mice does not increase bleeding risk, but results in decreased thrombin generation and protection from pulmonary embolism, leading to prolonged survival. Our studies define an important role for 14-3-3ζ in regulating platelet bioenergetics, leading to decreased platelet PS exposure and procoagulant function.

Platelets express negatively charged phosphatidylserine (PS) on their plasma membrane when propagating coagulation within a developing thrombus. Here the authors show that an adaptor protein 14-3-3 regulates mitochondrial function and PS exposure and thus platelet procoagulant activity, promising a new therapy to reduce thrombosis.

Vinculin regulates assembly of talin: β3 integrin complexes

Vinculin is a talin-binding protein that promotes integrin-mediated cell adhesion, but the mechanisms are not understood. Because talin is a direct activator of integrins, we asked whether and how vinculin regulates the formation of integrin: talin complexes. We report that VD1 (aa 1-258) and its talin-binding mutant, VD1A50I, bind directly and equally to several β integrin cytoplasmic tails (βCT). Results from competition assays show that VD1, but not VD1A50I, inhibits the interaction of talin (Tn) and talin rod (TnR), but not talin head (TnH) with β3CT. The inhibition observed could be the result of VD1 binding to one or more of the 11 vinculin binding sites (VBSs) in the TnR domain. Our studies demonstrate that VD1 binding to amino acids 482-911, a VBS rich region, in TnR perturbs the interaction of rod with β3CT. The integrin activation assays done using CHOA5 cells show that activated vinculin enhances αIIbβ3 integrin activation and that the effect is dependent on talin. The TnR domain however shows no integrin activation unlike TnH that shows enhanced integrin activation. The overall results indicate that activated vinculin promotes talin-mediated integrin activation by binding to accessible VBSs in TnR and thus displacing the TnR from the β3 subunit. The study presented, defines a novel direct interaction of VD1 with β3CT and provides an attractive explanation for vinculin's ability to potentiate integrin-mediated cell adhesion through directly binding to both TnR and the integrin cytoplasmic tail.

Tissue-specific neutrophil recruitment into the lung, liver, and kidney

The recruitment of immune cells is crucial for the development of inflammatory processes. The classical recruitment cascade of neutrophils into inflamed tissues is well understood and consists of capturing, rolling, slow rolling, arrest, postadhesion strengthening, crawling, and transmigration. While this commonly agreed paradigm might be applicable to most peripheral tissues, recruitment mechanisms may substantially vary in different organs such as the lung, liver, and kidney. These organs are highly specialized tissues with unique cell populations and structural organization, which enables them to fulfill their individual functions. The published research over the last decade has shed some light on organ-specific mechanisms of neutrophil recruitment and helped to generate a deeper understanding of the specific recruitment mechanisms involved in this process. The aim of this review is to highlight current concepts of tissue-specific differences and similarities of neutrophil recruitment into the lung, liver, and kidney.

Role of 14-3-3ζ in platelet glycoprotein Ibα-von Willebrand factor interaction-induced signaling

The interaction of platelet glycoprotein (GP) Ib-IX with von Willebrand factor (VWF) exposed at the injured vessel wall or atherosclerotic plaque rupture initiates platelet transient adhesion to the injured vessel wall, which triggers intracellular signaling cascades leading to platelet activation and thrombus formation. 14-3-3ζ has been verified to regulate the VWF binding function of GPIb-IX by interacting with the cytoplasmic domains of GPIb-IX. However, the data regarding the role of 14-3-3ζ in GPIb-IX-VWF interaction-induced signaling still remain controversial. In the present study, the data indicate that the S609A mutation replacing Ser(609) of GPIbα with alanine (S609A) significantly prevented the association of 14-3-3ζ with GPIbα before and after the VWF binding to GPIbα. GPIb-IX-VWF interaction-induced activations of Src family kinases and protein kinase C were clearly reduced in S609A mutation. Furthermore, S609A mutation significantly inhibited GPIb-IX-VWF interaction-induced elevation of cytoplasmic Ca(2+) levels in flow cytometry analysis. Taken together, these data indicate that the association of 14-3-3ζ with the cytoplasmic domain of GPIbα plays an important role in GPIb-IX-VWF interaction-induced signaling.

A GPIb-IX-V complex signaling environment

See also David T, Strassel C, Eckly A, Cazenave J-P, Gachet C, Lanza F. The platelet glycoprotein GPIbbeta intracellular domain participates in von Willebrand factor induced-filopodia formation independently of the Ser 166 phosphorylation site. This issue, pp 1077-87.

The role of platelets in acute lung injury (ALI)

Acute lung injury (ALI) is a common syndrome associated with a high mortality rate. Better understanding of the pathophysiology of acute lung injury and progress in supportive care and mechanical ventilation have led to slightly improved clinical outcomes. New evidence shows that the interplay between platelets, leukocytes and endothelial cells is critical in the pathogenesis of ALI. Key molecules involved in this interaction include P-selectin and the eicosanoid thromboxane A2 (TXA2), suggesting potential new targets for pharmacological intervention. In this review, we summarize the aspects of the interactions between platelets, leukocytes, and endothelial cells that are relevant for the pathogenesis of ALI.

A role for glycosphingolipid-enriched microdomains in platelet glycoprotein Ib-mediated platelet activation

BACKGROUND

Glycoprotein (GP) Ib, a platelet von Willebrand factor (VWF) receptor, plays a crucial role in thrombosis and hemostasis. As recent reports have suggested that GPIb partially locates in a particular region, designated as glycosphingolipid-enriched microdomains (GEMs), we hypothesized that GEMs play a central role in GPIb-mediated platelet activation.

METHODS

Platelets were stimulated by VWF/botrocetin to activate platelets through GPIb. GEMs and non-GEMs were isolated by sucrose density gradient ultracentrifugation and the location of signaling molecules characterized. The role of GEMs-mediated signaling in platelet behavior was tested by platelet aggregation and by platelet interaction with immobilized VWF under flow conditions when GEMs were disrupted by methyl-beta-cyclodextrin (MbetaCD).

RESULTS

GPIb was partially translocated to GEMs upon VWF/botrocetin stimulation. Immunoprecipitation of GPIb in GEMs and non-GEMs revealed that the tyrosine kinases, Src and Lyn, were associated with GPIb only in GEMs after GPIb-stimulation, and not in non-GEMs. Activation of PLCgamma2 was more intense in GEMs than non-GEMs. Disruption of GEMs by MbetaCD strongly inhibited tyrosine phosphorylation of Syk and PLCgamma2. Functional studies revealed that stable adhesion of platelets to a VWF-coated surface under flow was impaired by GEM disruption by MbetaCD.

CONCLUSION

The combined results suggest that GEMs play an important role in GPIb-mediated platelet activation.

Platelet-neutrophil-interactions: Linking hemostasis and inflammation

Platelets are essential for primary hemostasis, but they also play an important pro-inflammatory role. Platelets normally circulate in a quiescent state. Upon activation, platelets can secrete and present various molecules, change their shape as well as the expression pattern of adhesion molecules. These changes are associated with the adhesion of platelets to leukocytes and the vessel wall. The interaction of platelets with neutrophils promotes the recruitment of neutrophils into inflammatory tissue and thus participates in host defense. This interaction of neutrophils with platelets is mainly mediated through P-selectin and beta(2) and beta(3) integrins (CD11b/CD18, CD41/CD61). Platelets can also interact with endothelial cells and monocytes. Adherent platelets promote the 'secondary capture' of neutrophils and other leukocytes. In addition, platelets secrete neutrophil and endothelial activators inducing production of inflammatory cytokines. Thus, platelets are important amplifiers of acute inflammation.

Activation of Individual αIIbβ3 Integrin Molecules by Disruption of Transmembrane Domain Interactions in the Absence of Clustering

We used laser tweezers-based force spectroscopy to measure the binding strength between fibrinogen molecules covalently bound to latex beads and either wild-type alphaIIbbeta3 molecules or alphaIIbbeta3 molecules containing the transmembrane domain mutations beta3 G708N or alphaIIb G972N expressed on Chinese hamster ovary cells. As we demonstrated previously for alphaIIbbeta3 on agonist-stimulated platelets and for purified alphaIIbbeta3 molecules incubated with Mn(2+), two regimes of rupture forces were present when wild-type alphaIIbbeta3 was activated by the monoclonal antibody PT25-2: rupture forces of 20-60 pN with an exponentially decreasing probability of detection and rupture forces in the range of 60-150 pN with a maximum at approximately 70-80 pN. Both rupture force regimes were specific for fibrinogen binding to the activated conformation of alphaIIbbeta3 because they were inhibited by alphaIIbbeta3-specific antagonists. Identical rupture force regimes were present constitutively when cells expressing the alphaIIb and beta3 transmembrane domain mutants were studied, confirming that these mutations induced an active alphaIIbbeta3 conformation. Moreover, there were no significant differences in the yield strength of the low-to-moderate and strong force regimes when alphaIIbbeta3 was activated by PT25-2 or the transmembrane domain mutations, implying that there was no fundamental difference in the way these forms of activated alphaIIbbeta3 interacted with fibrinogen. Thus, the two-step pathway of the interaction of alphaIIbbeta3 with fibrinogen we have identified appears to be a fundamental property of the high-affinity state of alphaIIbbeta3 and is identical regardless of whether this affinity state is achieved by intracellular, extracellular, or membrane-associated events.

Translocation of GPIb and Fc receptor gamma-chain to cytoskeleton in mucetin-activated platelets

Recent studies have implied that GPIb-IX-V as well as functioning as an adhesion receptor may also induce signaling to mediate binding of platelets to damaged vessel wall to prevent bleeding. Reorganization of the cytoskeleton and redistribution of platelet structural proteins and signaling molecules are thought to be important in this early activation process, though the molecular mechanisms remain to be fully defined. In this study, we have used mucetin, a snake venom lectin protein that activates platelets via GPIb, to study the redistribution of GPIb in platelets. In unstimulated platelets, a minor portion of GPIb localized to Triton-insoluble cytoskeleton fractions (TIC). This portion increased considerably after platelet activation by mucetin. We also find increased contents of the FcRgamma chain in TIC. Anti-GPIb antibodies, mocarhagin or cytochalasin D completely inhibited the cytoskeletal translocation. In addition, BAPTA-AM, a cytoplasmic calcium chelator, strongly inhibited this process. On the other hand, inhibitors of alphaIIbbeta3, PLCgamma, PKC, tyrosine kinases, ADP receptor, PI3-kinase or EDTA are effective in preventing GPIb relocation in convulxin- but not in mucetin-activated platelets. We propose that cytoskeletal translocation of GPIb is upstream of alphaIIbbeta3 activation and cross-linking of GPIb is sufficient to induce this event in mucetin-activated platelets.

A critical role for 14-3-3zeta protein in regulating the VWF binding function of platelet glycoprotein Ib-IX and its therapeutic implications

The platelet receptor for von Willebrand factor (VWF), glycoprotein (GP) Ib-IX, mediates platelet adhesion and activation. The cytoplasmic domains of the GPIb alpha and beta subunits contain binding sites for the phosphorylation-dependent signaling molecule, 14-3-3zeta. Here we show that a novel membrane-permeable inhibitor of 14-3-3zeta-GPIbalpha interaction, MPalphaC, potently inhibited VWF binding to platelets and VWF-mediated platelet adhesion under flow conditions. MPalphaC also inhibited VWF-dependent platelet agglutination induced by ristocetin. Furthermore, activation of the VWF binding function of GPIb-IX induced by GPIbbeta dephosphorylation is diminished by mutagenic disruption of the 14-3-3zeta binding site in the C-terminal domain of GPIbalpha, mimicking MPalphaC-induced inhibition, indicating that the inhibitory effect of MPalphaC is likely to be caused by disruption of 14-3-3zeta binding to GPIbalpha. These data suggest a novel 14-3-3zeta-dependent regulatory mechanism that controls the VWF binding function of GPIb-IX, and also suggest a new type of antiplatelet agent that may be potentially useful in preventing or treating thrombosis.

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).

Megakaryocyte proliferation and ploidy regulated by the cytoplasmic tail of glycoprotein Ibα

We have investigated the ability of glycoprotein (GP) Ibα, a megakaryocytic gene product, to sequester the signal transduction protein 14-3-3ξ and to influence megakaryocytopoiesis. Using a Gp1ba–/– mouse colony, we compared the rescued phenotypes produced by a wild-type human GP Ibα allele or a similar allele containing a 6-residue cytoplasmic tail truncation that abrogates binding to 14-3-3ξ. The observed phenotypes illustrate an involvement for GP Ibα in thrombopoietin-mediated events of megakaryocyte proliferation, polyploidization, and the expression of apoptotic markers in maturing megakaryocytes. We developed a hypothesis for the involvement of a GP Ibα/14-3-3ξ/PI-3 kinase complex in regulating thrombopoietin-mediated responses. An observed increase in thrombopoietin-mediated Akt phosphorylation in the truncated variant supported the hypothesis and led to the development of a model in which the GP Ibα cytoplasmic tail sequestered signaling proteins during megakaryocytopoiesis and, as such, became a critical regulator in the temporal sequence of events that led to normal megakaryocyte maturation.

Identification of a novel 14-3-3ζ binding site within the cytoplasmic tail of platelet glycoprotein Ibα

The glycoprotein Ib-V-IX (GPIb-V-IX) complex interacts with subendothelial von Willebrand factor (VWF) to ensure recruitment of platelets at sites of vascular injury, a process that culminates in integrin αIIbβ3-dependent stable adhesion and spreading. Interaction of the 14-3-3ζ adaptor protein with the C-terminal 606-610 phosphoserine motif of the GPIbα subunit has been implicated in the control of αIIbβ3 activation and cell spreading. In this study, we have examined potentially novel 14-3-3ζ binding sites by expressing mutant forms of GPIbα in Chinese-hamster-ovary (CHO) cells. Analysis of a series of neighboring 11-12 residue deletions identified a critical role for the 580-LVAGRRPSALS-590 sequence in promoting GPIbα-14-3-3ζ interaction. Development of a phosphospecific antibody demonstrated high levels of phosphorylation of the Ser587 and Ser590 residues in resting platelets (which became dephosphorylated during platelet spreading on VWF), and peptides containing these phosphorylated residues effectively displaced 14-3-3ζ from GPIbα. Analysis of single and double alanine substitutions of Ser587 and Ser590 demonstrated a major role for these residues in promoting GPIbα-14-3-3ζ binding. Moreover, these cell lines exhibited a defect in cell spreading on immobilized VWF. These studies demonstrate the existence of a second major 14-3-3ζ binding site within the cytoplasmic tail of GPIbα that has an important functional role in regulating integrin-dependent cell spreading. (Blood. 2004;104:420-427)

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.

Membrane type-1 matrix metalloproteinase stimulates tumour cell-induced platelet aggregation: role of receptor glycoproteins

1. Matrix metalloproteinase-2 (MMP-2) plays a role in agonist- and tumour cell-induced platelet aggregation (TCIPA). 2. MMP-2 is synthesized as a proenzyme and is activated at the cell surface by membrane type-1 matrix metalloproteinase (MT1-MMP, MMP-14). 3. The significance of tumour cell-associated MT1-MMP for TCIPA was investigated using human breast carcinoma MCF7 cells stably coexpressing the integrin alphavbeta3 with MT1-MMP, cells expressing alphavbeta3 alone and mock-transfected cells. 4. Western blot and zymography confirmed that alphavbeta3/MT1-MMP cells expressed MT1-MMP and efficiently processed proMMP-2 to MMP-2. 5. Aggregometry, phase-contrast and transmission electron microscopy and flow cytometry were used to characterize TCIPA induced by MCF7 cell lines. 6. The aggregating potency of cells was: alphavbeta3/MT1-MMP >alphavbeta3=mock cells, as shown by aggregometry and phase-contrast microscopy. 7. Electron microscopy revealed close, membrane-membrane interactions between activated platelets and alphavbeta3/MT1-MMP cells during TCIPA. 8. Inhibition of MMP-2 with the neutralizing anti-MMP-2 antibody (5 microg ml(-1)) and o-phenanthroline (100 microm) reduced aggregation induced by alphavbeta3/MT1-MMP cells. 9. TCIPA induced by alphavbeta3/MT1-MMP cells was also reduced by inhibiting the generation and actions of ADP with apyrase (250 microg ml(-1)) and 2-methylthio-AMP (2-MeSAMP) (30 microm), but not N(6)-methyl-2'-deoxyadenosine-3',5'-bisphosphate (MRS2179) (30 microm). 10. Flow cytometry demonstrated that TCIPA enhanced expression of glycoprotein (GP) Ib and IIb/IIIa receptors not only on platelets but also on breast cancer cells. 11. Thus, (a) human breast carcinoma cell surface-associated MT1-MMP, via activating proMMP-2, stimulates TCIPA; (b) ADP amplifies the effects of MMPs via stimulation of P2Y(12) receptors and (c) both tumour- and platelet-derived GPIb and GPIIb/IIIa are involved in the aggregatory effects of MT1-MMP.

A palmitylated peptide derived from the glycoprotein Ib beta cytoplasmic tail inhibits platelet activation

The platelet receptor GPIb/IX/V mediates a crucial role in hemostasis, yet the signaling mechanisms involved are incompletely understood. The complex consists of four polypeptides GPIb alpha, GPIb beta, GPIX and GPV. We identified an amino acid sequence in the cytoplasmic tail of the GPIb beta subunit between residues R151 and A161 that is highly conserved across species and hypothesized that it has functional importance. To target this motif, we synthesized a corresponding cell-permeable palmitylated peptide (Pal-RRLRARARARA) and investigated its effect on platelet function. Pal-RRLRARARARA completely inhibited low dose thrombin- and ristocetin-induced aggregation in washed platelets but only partially inhibited collagen- and U46619-induced aggregation. Thromboxane production in platelets stimulated with thrombin was significantly reduced by Pal-RRLRARARARA compared with collagen. Activation of the integrin alpha IIb beta 3 in response to thrombin was significantly reduced when platelets were preincubated with Pal-RRLRARARARA. The adhesion of washed platelets to von Willebrand factor (VWF) under static conditions was significantly reduced by Pal-RRLRARARARA. Under conditions of high shear, the velocity of platelets rolling on VWF was significantly increased when platelets are preincubated with Pal-RRLRARARARA. This study defines a novel function for the RRLRARARARA motif of GPIb beta in platelet activation.

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.

Calcium-and integrin-binding protein regulates focal adhesion kinase activity during platelet spreading on immobilized fibrinogen

Platelet spreading on the subendothelium in response to vascular injury is fundamental to the regulation of physiologic hemostasis. Previously, we have shown that, when bound to glycoprotein IIb (GPIIb), calcium- and integrin-binding protein (CIB) regulates platelet spreading on immobilized fibrinogen (Fg). In this study, we investigated the signaling events that occur downstream of CIB in the absence of signaling that occurs as a result of granular secretion. Using Chinese hamster ovary (CHO) cells as a model, we demonstrate that CIB induces cell migration. Immunofluorescence analysis of CIB localization indicates that endogenous CIB accumulates in areas of focal adhesions, and its overexpression up-regulates the formation of focal adhesion complexes compared with control cells. Immunoprecipitation analysis indicates that CIB associates with focal adhesion kinase (FAK), a key regulator in focal complex formation, and up-regulates its activity. Overexpression of dominant-negative FAK, FRNK, along with CIB in CHO cells completely inhibits CIB-induced cell migration. Further, confirmation of these data in the platelet system indicates that CIB and FAK associate throughout all stages of platelet spreading but only on Fg binding to GPIIb/IIIa. Taken together, our results suggest that CIB regulates platelet spreading through the regulation of FAK activation.

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.

Redistribution of glycoprotein Ib within platelets in response to protease-activated receptors 1 and 4: roles of cytoskeleton and calcium

Thrombin activates human platelets by hydrolyzing the protease-activated receptors PAR-1 and PAR-4, exposing new N-terminal sequences which act as tethered ligands, and binding to glycoprotein (GP) Ib, whose surface accessibility transiently decreases when platelets are stimulated by the enzyme. In an attempt to better understand this latter process, we used the peptides SFLLRNPNDKYEPF (PAR-1-AP or TRAP) and AYPGKF (PAR-4-AP) to study whether hydrolysis of both PAR receptors leads to GPIb redistribution. Both peptides induced surface clearance of GPIb with a maximum at 2 min and 5 min for PAR-1-AP and PAR-4-AP, respectively, followed by a slow return to the surface with levels normalizing between 30 and 60 min. Translocation was associated with the formation of clusters of GPIb as revealed by fluorescence microscopy. This transient redistribution of GPIb was blocked by cytochalasin D and in large part by the membrane permeable Ca2+ chelator, BAPTA. The inhibitor of phosphatidylinositol 3-kinase and myosin light chain kinase, wortmannin, did not significantly modify internalization of GPIb, although its return to the surface was delayed for PAR-1-AP. PAR receptor-mediated association of GPIb to the insoluble cytoskeleton was blocked by cytochalasin D, while BAPTA alone increased and stabilized the presence of GPIb. Globally, immunoprecipitation experiments and analysis of the cytoskeleton confirmed that GPIb translocation is powered by a contractile mechanism involving Ca2+ mobilization, actin polymerization, and myosin incorporation into the cytoskeleton and that both PAR-1 and PAR-4 can activate this process.

Integrin alpha IIb beta 3-dependent calcium signals regulate platelet-fibrinogen interactions under flow. Involvement of phospholipase C gamma 2

Platelet adhesion to fibrinogen is important for platelet aggregation and thrombus growth. In this study we have examined the mechanisms regulating platelet adhesion on immobilized fibrinogen under static and shear conditions. We demonstrate that integrin alpha IIb beta 3 engagement of immobilized fibrinogen is sufficient to induce an oscillatory calcium response, necessary for lamellipodial formation and platelet spreading. Released ADP increases the proportion of platelets exhibiting a cytosolic calcium response but is not essential for calcium signaling or lamellipodial extension. Pretreating platelets with the Src kinase inhibitor PP2, the inositol 1,4,5-trisphosphate (IP3) receptor antagonist 2-aminoethoxydiphenyl borate (APB-2), or the phospholipase C (PLC) inhibitor U73122 abolished calcium signaling and platelet spreading, suggesting a major role for Src kinase-regulated PLC isoforms in these processes. Analysis of PLC gamma 2-/- mouse platelets revealed a major role for this isoform in regulating cytosolic calcium flux and platelet spreading on fibrinogen. Under flow conditions, platelets derived from PLC gamma 2-/- mice formed less stable adhesive interactions with fibrinogen, particularly in the presence of ADP antagonists. Our studies define an important role for PLC gamma 2 in integrin alpha IIb beta 3-dependent calcium flux, necessary for stable platelet adhesion and spreading on fibrinogen. Furthermore, they establish an important cooperative signaling role for PLC gamma 2 and ADP in regulating platelet adhesion efficiency on fibrinogen.

14-3-3 zeta mediates integrin-induced activation of Cdc42 and Rac. Platelet glycoprotein Ib-IX regulates integrin-induced signaling by sequestering 14-3-3 zeta

Integrin-induced cytoskeletal reorganizations are initiated by Cdc42 and Rac1 but little is known about mechanisms by which integrins activate these Rho GTPases. 14-3-3 proteins are adaptors implicated in binding and regulating the function and subcellular location of numerous signaling molecules. In platelets, the 14-3-3 zeta isoform interacts with the glycoprotein (GP) Ibalpha subunit of the adhesion receptor GP Ib-IX. In this study, we show that integrin-induced activation of Cdc42, activation of Rac, cytoskeletal reorganizations, and cell spreading were inhibited in Chinese hamster ovary cells expressing full-length GP Ibalpha compared with GP Ibalpha lacking the 14-3-3 zeta binding site. Activation of Rho GTPases and cytoskeletal reorganizations were restored by expression of 14-3-3 zeta. Spreading in cells expressing truncated GP Ibalpha was inhibited by co-expressing a chimeric receptor containing interleukin 2 receptor alpha and GP Ibalpha cytoplasmic domain. These results identify a previously unrecognized function of 14-3-3 zeta, that of mediating integrin-induced signaling. They show that 14-3-3 zeta mediates Cdc42 and Rac activation. They also reveal a novel function of platelet GP Ib-IX, that of regulating integrin-induced cytoskeletal reorganizations by sequestering 14-3-3 zeta. Signaling across integrins initiates changes in cell behavior such as spreading, migration, differentiation, apoptosis, or cell division. Thus, introduction of the 14-3-3 zeta binding domain of GP Ibalpha into target cells might provide a method for regulating integrin-induced pathways in a variety of pathological conditions.

Critical roles for the COOH-terminal NITY and RGT sequences of the integrin beta3 cytoplasmic domain in inside-out and outside-in signaling

Bidirectional signaling of integrin alphaIIbbeta3 requires the beta3 cytoplasmic domain. To determine the sequence in the beta3 cytoplasmic domain that is critical to integrin signaling, cell lines were established that coexpress the platelet receptor for von Willebrand factor (vWF), glycoprotein Ib-IX, integrin alphaIIb, and mutants of beta3 with truncations at sites COOH terminal to T741, Y747, F754, and Y759. Truncation at Y759 did not affect integrin activation, as indicated by vWF-induced fibrinogen binding, but affected cell spreading and stable adhesion. Thus, the COOH-terminal RGT sequence of beta3 is important for outside-in signaling but not inside-out signaling. In contrast, truncation at F754, Y747, or T741 completely abolished integrin activation. A point mutation replacing Y759 with alanine also abolished integrin activation. Thus, the T755NITY759 sequence of beta3, containing an NXXY motif, is critical to inside-out signaling, whereas the intact COOH terminus is important for outside-in signaling. In addition, we found that the calcium-dependent protease calpain preferentially cleaves at Y759 in a population of beta3 during platelet aggregation and adhesion, suggesting that calpain may selectively regulate integrin outside-in signaling.

Platelet-collagen interaction: is GPVI the central receptor?

At sites of vascular injury, platelets come into contact with subendothelial collagen, which triggers their activation and the formation of a hemostatic plug. Besides glycoprotein Ib (GPIb) and alphaIIbbeta3 integrin, which indirectly interact with collagen via von Willebrand factor (VWF), several collagen receptors have been identified on platelets, most notably alpha2beta1 integrin and the immunoglobulin (Ig) superfamily member GPVI. Within the last few years, major advances have been made in understanding platelet-collagen interactions including the molecular cloning of GPVI, the generation of mouse strains lacking individual collagen receptors, and the development of collagen receptor-specific antibodies and synthetic peptides. It is now recognized that platelet adhesion to collagen requires prior activation of integrins through "inside-out" signals generated by GPVI and reinforced by released second-wave mediators adenosine diphosphate (ADP) and thromboxane A2. These developments have led to revision of the original "2-site, 2-step" model, which now places GPVI in a central position in the complex processes of platelet tethering, activation, adhesion, aggregation, degranulation, and procoagulant activity on collagen. This review discusses these recent developments and proposes possible mechanisms for how GPVI acts in concert with other receptors and signaling pathways to initiate hemostasis and arterial thrombosis.

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.

Integrin alphaIIbbeta3 and shear-dependent action of glycoprotein Ibalpha stimulate platelet-dependent thrombin formation in stirred plasma

Under conditions of arterial-wall shear rates, platelets bind to von Willebrand factor (vWf) by way of the glycoprotein Ib (GP Ib) complex and integrin alpha(IIb)beta(3). Both adhesive receptors may also play roles in the development of procoagulant activity of platelets. Here, we investigated the effect of shear stress, as provided by a rotating cylinder, on GP Ib- and integrin alpha(IIb)beta(3)-dependent thrombin generation in coagulating platelet-rich plasma (PRP). We measured thrombin continuously with the use of fluorometry from the cleavage rate of a fluorescent low-affinity substrate. The integrin alpha(IIb)beta(3) antagonist abciximab progressively reduced the peak of thrombin formation up to 43% when rate of stirring and shear stress were increased (estimated shear rates of 105-420 s(-1)). Abciximab did not lower the peak of thrombin formation in stirred PRP from patients with Glanzmann's thrombasthenia lacking alpha(IIb)beta(3) but, surprisingly, shortened the time until onset. In PRP from control subjects, antibodies specifically directed against vWf-binding epitopes on GP Ibalpha reduced thrombin formation, with 25% to 30% at the high but not at the low stirring rate. In combination with the anti-GP Ib antibody, abciximab retained its strong inhibitory effect only at the high stirring rate. We conclude that thrombin formation and coagulation in stirred PRP depend, to a large extent, on platelet adhesion to integrin alpha(IIb)beta(3) and, in a shear-dependent way, on GP Ib.

Role of the intracellular domains of GPIb in controlling the adhesive properties of the platelet GPIb/V/IX complex

Glycoprotein (GP) Ib/V/IX complex-dependent platelet adhesion to von Willebrand factor (VWF) is supported by the 45-kd N-terminal extracellular domain of the GPIb alpha subunit. Recent results with an adhesion blocking antibody (RAM.1) against GPIb beta, which is disulfide linked to GPIb alpha, have suggested a novel function of this subunit in regulating VWF-mediated platelet adhesion, possibly involving its intracellular face. A putative cooperation between the GPIb alpha and GPIb beta cytoplasmic domains was investigated by measuring the adhesion under flow to immobilized VWF of K562 and Chinese hamster ovary (CHO) cells transfected with GPIb/(V)/IX containing mutations in this region. Adhesion of cells carrying a glycine substitution of the GPIb beta Ser166 phosphorylation site was 50% lower than normal and became insensitive to inhibition by RAM.1. In contrast, forskolin or PGE(1) treatment increased both the phosphorylation of GPIb beta and adhesion of control cells, both effects being reversed by RAM.1, but had no influence on cells expressing the Ser166Gly mutation. A role of the GPIb alpha intracellular domain was also apparent as the VWF-dependent adhesion of cells containing deletions of the entire (Delta 518-610) or portions (Delta 535-568, Delta 569-610) of the GPIb alpha cytoplasmic tail was insensitive to RAM.1 inhibition. Cells carrying progressive 11 amino acid deletions spanning the GPIb alpha 535-590 region were equally unresponsive to RAM.1, with the exception of those containing GPIb alpha Delta 569-579, which behaved like control cells. These findings support a role of the GPIb beta intracellular domain in controlling the adhesive properties of the GPIb/V/IX complex through phosphorylation of GPIb beta Ser166 and point to the existence of cross-talk between the GPIb beta and GPIb alpha intracellular domains.

Interaction between von Willebrand factor and glycoprotein Ib activates Src kinase in human platelets: role of phosphoinositide 3-kinase

The binding of von Willebrand factor (VWF) to glycoprotein (GP) Ib-IX-V stimulates transmembrane signaling events that lead to platelet adhesion and aggregation. Recent studies have implied that activation of Src family kinases is involved in GPIb-mediated platelet activation, although the related signal transduction pathway remains poorly defined. This study presents evidence for an important role of Src and GPIb association. In platelet lysates containing Complete, a broad-spectrum protease inhibitor mixture, Src and Lyn dynamically associated with GPIb on VWF-botrocetin stimulation. Cytochalasin D, which inhibits translocation of Src kinases to the cytoskeleton, further increased Src and GPIb association. Similar results were obtained with botrocetin and monomeric A1 domain, instead of intact VWF, with induction of both Src activation and association between GPIb and Src. These findings suggest that ligand binding of GPIb, without receptor clustering, is sufficient to activate Src. Immunoprecipitation studies demonstrated that Src, phosphoinositide 3- kinase (PI 3-kinase), and GPIb form a complex in GPIb-stimulated platelets. When the p85 subunit of PI 3-kinase was immunodepleted, association of Src with GPIb was abrogated. However, wortmannin, a specific PI 3-kinase inhibitor, failed to block complex formation between Src and GPIb. The Src-SH3 domain as a glutathione S-transferase (GST)-fusion protein coprecipitated the p85 subunit of PI 3-kinase and GPIb. These findings taken together suggest that the p85 subunit of PI 3-kinase mediates GPIb-related activation signals and activates Src independently of the enzymatic activity of PI 3- kinase.

Plasminogen activator inhibitor-1 detaches cells from extracellular matrices by inactivating integrins

The binding of urokinase plaminogen activator (uPA) to its cell surface receptor (uPAR; CD87) promotes cell adhesion by increasing the affinity of the receptor for both vitronectin (VN) and integrins. We provide evidence that plasminogen activator inhibitor (PAI)-1 can detach cells by disrupting uPAR-VN and integrin-VN interactions and that it does so by binding to the uPA present in uPA-uPAR-integrin complexes on the cell surface. The detached cells cannot reattach to VN unless their surface integrins are first activated by treatment with MnCl2. Immunoprecipitation and subcellular fractionation experiments reveal that PAI-1 treatment triggers deactivation and disengagement of uPA-uPAR-integrin complexes and their endocytic clearance by the low density lipoprotein receptor-related protein. Transfection experiments demonstrate that efficient cell detachment by PAI-1 requires an excess of matrix-engaged uPA-uPAR-integrin complexes over free engaged integrins and that changes in this ratio alter the efficacy of PAI-1. Together, these results suggest a VN-independent, uPA-uPAR-dependent mechanism by which PAI-1 induces cell detachment. This pathway may represent a general mechanism, since PAI-1 also can detach cells from fibronectin and type-1 collagen. This novel "deadhesive" activity of PAI-1 toward a variety of cells growing on different extracellular matrices may begin to explain why high PAI-1 levels often are associated with a poor prognosis in human metastatic disease.

von Willebrand factor stimulates thrombin-induced exposure of procoagulant phospholipids on the surface of fibrin-adherent platelets

Studies from our laboratory have demonstrated that von Willebrand factor (VWF) stimulates thrombin generation in platelet-rich plasma. The precise role of VWF and fibrin in this reaction, however, remained to be clarified. In the present study we utilized thrombin-free planar fibrin layers and washed platelets to examine the relationship between platelet-fibrin interaction and exposure of coagulation-stimulating phosphatidylserine (PS) under conditions of low and high shear stress. Our study confirms that platelet adhesion to fibrin at a shear rate of 1000 s(-1) requires fibrin-bound VWF. The cytosolic calcium concentration ([Ca(2+)]i) of stationary platelets was not elevated and PS exposing platelets were virtually absent (2 +/- 2%). However, thrombin activation resulted in a marked increase in the number of PS exposing platelets (up to 85 +/- 14%) along with a transient elevation in [Ca(2+)]i from 0.05 micro mol L(-1) up to 1.1 +/- 0.2 micro mol L(-1). Platelet adhesion to fibrin at a shear rate of 50 s(-1) is mediated by thrombin but not by fibrin-bound VWF. The [Ca(2+)]i of these thrombin-activated platelets was elevated (0.2 +/- 0.1 micro mol L(-1)), but only a minority of the platelets (11 +/- 8%) exposed PS. The essential role of VWF in this thrombin-induced procoagulant response became apparent from low shear rate perfusion studies over fibrin that was incubated with VWF and botrocetin. After treatment with thrombin, the majority of the adherent platelets (57 +/- 23%) exposed PS and had peak values of [Ca(2+)]i of about 0.6 micro mol L(-1). Taken together, these results demonstrate that thrombin-induced exposure of PS and high calcium response on fibrin-adherent platelets depends on shear- or botrocetin-induced VWF-platelet interaction.

The role of platelets in peripheral vascular disease

Platelets play a major role in acute ischaemic syndromes and peripheral vascular disease. They are involved in the development and progression of atherosclerosis, native vessel and graft thrombosis. They have a central role in the development of restenosis and reocclusion after peripheral percutaneous transluminal angioplasty. Antiplatelet therapy has been shown to be beneficial in patients undergoing peripheral vascular surgery or radiological intervention. Yet current routine therapy, namely aspirin and dipyridamole are limited in their mode of action and efficacy. Recent developments in the understanding of platelet function has led to the development of new more potent drugs such as clopidogrel. Combination of drugs and more specific investigation of individual platelet function may well result in improved bypass and angioplasty patency rates. The results of proposed large randomised controlled trials on the role and safety of aspirin and clopidogrel are awaited with interest. Given the importance of platelets in peripheral vascular disease highlighted in this review, achieving an optimal safe anti-platelet effect for each patient with peripheral vascular disease should be the target of future research.

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.

Sequential cytoplasmic calcium signals in a 2-stage platelet activation process induced by the glycoprotein Ibalpha mechanoreceptor

We found that the interaction of platelets with immobilized von Willebrand factor (VWF) under flow induces distinct elevations of cytosolic Ca(++) concentration ([Ca(++)](i)) that are associated with sequential stages of integrin alpha(IIb)beta(3) activation. Fluid-dynamic conditions that are compatible with the existence of tensile stress on the bonds between glycoprotein Ibalpha (GPIbalpha) and the VWF A1 domain led to Ca(++) release from intracellular stores (type alpha/beta peaks), which preceded stationary platelet adhesion. Raised levels of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate, as well as membrane-permeable calcium chelators, inhibited these [Ca(++)](i) oscillations and prevented stable adhesion without affecting the dynamic characteristics of the typical platelet translocation on VWF mediated by GPIbalpha. Once adhesion was established through the integrin alpha(IIb)beta(3), new [Ca(++)](i) oscillations (type gamma) of greater amplitude and duration, and involving a transmembrane ion flux, developed in association with the recruitment of additional platelets into aggregates. Degradation of released adenosine diphosphate (ADP) to AMP or inhibition of phosphatidylinositol 3-kinase (PI3-K) prevented this response without affecting stationary adhesion and blocked aggregation. These findings indicate that an initial signal induced by stressed GPIbalpha-VWF bonds leads to alpha(IIb)beta(3) activation sufficient to support localized platelet adhesion. Then, additional signals from ADP receptors and possibly ligand-occupied alpha(IIb)beta(3), with the contribution of a pathway involving PI3-K, amplify platelet activation to the level required for aggregation. Our conclusions modify those proposed by others regarding the mechanisms that regulate signaling between GPIbalpha and alpha(IIb)beta(3) and lead to platelet adhesion and aggregation on immobilized VWF.

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.

Agonist-induced aggregation of Chinese hamster ovary cells coexpressing the human receptors for fibrinogen (integrin alphaIIbbeta3) and the platelet-activating factor: dissociation between adhesion and aggregation

This work reports the establishment of a Chinese hamster ovary (CHO) cell line stably coexpressing the human alphaIIbbeta3 integrin and the platelet-activating factor receptor (PAFR). These cells aggregate in response to PAF in a Ca(++), alphaIIbbeta3, and soluble fibrinogen (Fg)-dependent manner that is prevented by PAF antagonists or alphaIIbbeta3 blockade. The aggregating response is accompanied by enhanced binding of fibrinogen and the activation-dependent IgM PAC1. This model has permitted us to identify, for the first time, intracellular signals distinctly associated with either alphaIIbbeta3-mediated adhesion or aggregation. Nonreceptor activation of protein kinase C (PKC) by phorbol ester produced cellular adhesion and spreading onto immobilized Fg, but it was not a sufficient signal to provoke cellular aggregation. Moreover, inhibition of PKC impeded the PAF stimulation of cellular adhesion, whereas the aggregation was not prevented. The PAF-induced cellular aggregation was distinctly associated with signaling events arising from the liganded Fg receptor and the agonist-induced stimulation of a calcium/calmodulin-dependent signaling pathway. Sustained tyrosine phosphorylation of both mitogen-activated protein kinase (MAPK) and an approximately 100-kd protein was associated with the PAF-induced aggregation, whereas phosphorylation of focal adhesion kinase (FAK) was preferably associated with cellular adherence and spreading onto immobilized Fg.

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.

Actin dynamics in platelets

The human blood platelet circulates in the blood as a non-adherent disk. Upon receiving signals of blood vessel damage, the platelet reorganizes its actin cytoskeleton which transforms it into a spiky dynamic adherent glue. This transformation involves a temporal sequence of four morphologically distinct steps which is reproducible in vitro. The actin dynamics underlying these shape changes depend on a large number of actin-binding proteins. Maintenance of the discoid shape requires actin-binding proteins that inhibit these reorganizations, whereas transformation involves other proteins, some to disassemble old filaments and others to polymerize new ones. F-Actin-affinity chromatography identified a large set of actin-binding proteins including VASP, Arp2 and 2E4/kaptin. Recent discoveries show that VASP inhibits filament disassembly and Arp2/3 is required to polymerize new filaments. Morphological analysis of the distribution of these actin-binding proteins in spread platelets together with biochemical measurements of their interactions with actin lead to a model of interactions with actin that mediate shape change.

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.