Protein kinase C is activated in platelets subjected to pathological shear stress

Microfluidic assay of antiplatelet agents for inhibition of shear-induced platelet adhesion and activation

We have developed a microfluidic system to perfuse whole blood through a flow channel with an upstream stenotic region and a downstream protein capture region. This flow-based system was used to assay how effectively antiplatelet agents suppress shear-induced platelet adhesion and activation downstream of the stenotic region. Microcontact printing was used to covalently attach one of three platelet binding proteins [fibrinogen, collagen, or von Willebrand factor (vWf)] to the surface of the downstream capture region. Whole blood with an antiplatelet agent was transiently exposed to an upstream high wall shear rate (either 4860 s-1 or 11 560 s-1), and subsequently flowed over the downstream capture region where the platelet adhesion was measured. Several antiplatelet agents (acetylsalicylic acid, tirofiban, eptifibatide, anti-vWf, and anti-GPIbα) were evaluated for their efficacy in attenuating downstream adhesion. Following antibody blocking of vWf or GPIbα, downstream platelet activation was also assessed in perfused blood by flow cytometry using two activation markers (active GPIIb/IIIa and P-selectin). Acetylsalicylic acid demonstrated its inability to diminish shear-induced platelet adhesion to all three binding proteins. GPIIb/IIIa inhibitors (tirofiban and eptifibatide) significantly reduced platelet adhesion to fibrinogen. Antibody blocking of vWf or GPIbα effectively diminished platelet adhesion to all three capture proteins as well as platelet activation in perfused blood, indicating an essential role of vWf-GPIbα interaction in mediating shear-induced platelet aggregation.

Mechanical Regulation Underlies Effects of Exercise on Serotonin-Induced Signaling in the Prefrontal Cortex Neurons

Mechanical forces are known to be involved in various biological processes. However, it remains unclear whether brain functions are mechanically regulated under physiological conditions. Here, we demonstrate that treadmill running and passive head motion (PHM), both of which produce mechanical impact on the head, have similar effects on the hallucinogenic 5-hydroxytryptamine (5-HT) receptor subtype 2A (5-HT_2A) signaling in the prefrontal cortex (PFC) of rodents. PHM generates interstitial fluid movement that is estimated to exert shear stress of a few pascals on cells in the PFC. Fluid shear stress of a relevant magnitude on cultured neuronal cells induces ligand-independent internalization of 5-HT_2A receptor, which is observed in mouse PFC neurons after treadmill running or PHM. Furthermore, inhibition of interstitial fluid movement by introducing polyethylene glycol hydrogel eliminates the effect of PHM on 5-HT_2A receptor signaling in the PFC. Our findings indicate that neuronal cell function can be physiologically regulated by mechanical forces in the brain.

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Mechanical forces regulate brain functions under physiological conditions

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Intracerebral interstitial fluid has mechanical roles in regulating brain functions

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Mechanical impact on the head mediates effects of exercise on the brain

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Fluid shear stress physiologically modulates signaling in nervous cells

Downstream platelet adhesion and activation under highly elevated upstream shear forces

Elevated shear force caused by an anastomotic stenosis is a common complication at the blood vessel-vascular implant interface. Although elevated shear forces were found to cause platelet aggregation around a stenotic region, transient platelet exposure to elevated shear forces and subsequent downstream events occurring under lower shear force were not extensively studied. We hypothesize that effects of elevated shear forces on pre-activation of platelets for downstream adhesion and activation are relevant in understanding the increased thrombotic risk associated with blood-contacting devices. We designed a microfluidic flow system to mimic the hemodynamic environment of vasculature with an upstream anastomotic stenosis with five wall shear strain rates ranging from 1620 s^-1 to 11560 s^-1. Under shear flow conditions, transient exposure of whole blood to elevated shear forces resulted in higher downstream platelet adhesion onto three different immobilized platelet agonists: fibrinogen, collagen, or von Willebrand factor. Platelet expression of four activation markers (P-selectin, GPIIb/IIIa, lysosomal glycoprotein, and phosphatidylserine) significantly increased after transient exposure to higher upstream wall shear strain rates of 2975-11560 s^-1. A significant lysis was observed when platelets were primed by upstream wall shear strain rate of 11560 s^-1. These experimental results could be helpful to understand how altered hemodynamics around an anastomotic stenosis promotes thrombus formation downstream. STATEMENT OF SIGNIFICANCE: Studying the downstream response of platelets following transient exposure to an upstream agonist is important because of significant clinical implications to the implantation of vascular devices. Due to intimal fibrous hyperplasia, vascular biomaterials such as synthetic small-diameter vascular grafts sometimes become stenotic (narrow), leading to transient platelet exposure to elevated shear forces. In this study, a microfluidic flow system was developed to mimic a stenosed vascular graft and to investigate how highly elevated, transient upstream shear forces, typically found in severe stenosis, results in the pre-activation of platelets for downstream adhesion and activation. The findings of the present study have implications for optimizing the design of blood-contacting biomaterials in order to minimize thrombotic risk associated with transiently elevated shear forces. The findings also provide additional insights into the mechanisms of thrombus formation at the post-stenotic regions of vascular implants.

Effects of upstream shear forces on priming of platelets for downstream adhesion and activation

Platelets in flowing blood are sometimes exposed to elevated shear forces caused by anastomotic stenosis at the blood vessel-vascular implant interface. The objective of this study was to determine how effective upstream shear forces are in priming platelets for downstream adhesion and activation. Flow chambers with upstream stenotic regions (shear rates of 400-1000 s^-1) were manufactured by relief molding of polydimethylsiloxane. Downstream from the stenotic regions, microcontact printing was used to covalently immobilize three different proteins (fibrinogen, collagen, or von Willebrand factor) to serve as platelet capture agents. Anticoagulated whole blood was perfused through the flow chambers and platelet adhesion to the downstream capture region was quantified. It was found that transient exposure of platelets to increased shear forces resulted in higher platelet adhesion on all three proteins. The duration of the platelet exposure to elevated shear forces was varied by changing the length of the stenotic regions. The results indicated that, in addition to the magnitude of shear forces, the duration of exposure to these forces was also an important factor in priming platelets. The effect of upstream shear forces on platelet activation was assessed by quantifying P-selectin, integrin α_IIbβ₃, lysosomal glycoprotein, and phosphatidylserine exposure using flow cytometry. The results suggested that increased shear forces were capable of increasing the priming of platelets for downstream activation. This study implicates the anastomotic region(s) of vascular implants as a locus of platelet pre-activation that may lead to thrombus formation downstream.

STATEMENT OF SIGNIFICANCE

A synthetic small-diameter vascular graft can often become stenotic due to intimal fibrous hyperplasia, either generally along the inside of the graft or at the anastomotic regions, leading to an increased shear force on flowing platelets. Our lab is studying how the upstream platelet preactivation (aka "priming") in flowing blood affects their downstream adhesion and activation. This manuscript describes a study in which priming of platelets is achieved by upstream stenotic narrowing in a microfluidic flow chamber. Such experimental design was intended to mimic a vascular implant with stenotic upstream anastomosis and downstream exposed platelet protein agonists. Understanding how the pre-activated platelets respond to imperfect vascular implant surfaces downstream is an important factor in designing better vascular implants.

The relationship between fractional flow reserve, platelet reactivity and platelet leukocyte complexes in stable coronary artery disease

BACKGROUND

The presence of stenoses that significantly impair blood flow and cause myocardial ischemia negatively affects prognosis of patients with stable coronary artery disease. Altered platelet reactivity has been associated with impaired prognosis of stable coronary artery disease. Platelets are activated and form complexes with leukocytes in response to microshear gradients caused by friction forces on the arterial wall or flow separation. We hypothesized that the presence of significantly flow-limiting stenoses is associated with altered platelet reactivity and formation of platelet-leukocyte complexes.

METHODS

One hundred patients with stable angina were studied. Hemodynamic significance of all coronary stenoses was assessed with Fractional Flow Reserve (FFR). Patients were classified FFR-positive (at least one lesion with FFR≤0.75) or FFR-negative (all lesions FFR>0.80). Whole blood samples were stimulated with increasing concentrations of ADP, TRAP, CRP and Iloprost with substimulatory ADP. Expression of P-selectin as platelet activation marker and platelet-leukocyte complexes were measured by flowcytometry. Patients were stratified on clopidogrel use. FFR positive and negative patient groups were compared on platelet reactivity and platelet-leukocyte complexes.

RESULTS

Platelet reactivity between FFR-positive patients and FFR-negative patients did not differ. A significantly lower percentage of circulating platelet-neutrophil complexes in FFR-positive patients and a similar non-significant decrease in percentage of circulating platelet-monocyte complexes in FFR-positive patients was observed.

CONCLUSION

The presence of hemodynamically significant coronary stenoses does not alter platelet reactivity but is associated with reduced platelet-neutrophil complexes in peripheral blood of patients with stable coronary artery disease.

High shear flow induces migration of adherent human platelets

Shear forces are generated in all parts of the vascular system and contribute directly and indirectly to vascular disease progression. Endothelial cells are able to adapt to flow conditions, and are known to polarize and migrate in response to shear forces. Platelets exposed to shear stress are activated and release bioactive molecules from their alpha granules. So far, platelets have been considered to be static cells that do not leave the site of tight adhesion. However, we have recently been able to demonstrate the capacity of platelets to migrate in response to stromal derived factor-1 (SDF-1). In this project, we have demonstrated that platelets accumulate in areas with a high concentration of SDF-1 under flow conditions and respond to high shear stress by cellular polarization, cytoskeletal reorganisation, and flow-directed migration. In this context, we have shown increased Wiskott-Aldrich Syndrome protein (WASP) phosphorylation and intracellular redistribution of focal adhesion kinase (FAK) under high-shear stress conditions. The effect of flow-induced platelet migration has not previously been recognized and offers a new role for platelets as mobile cells. Their migratory potential may enable platelets to cover intimal lesions and contribute to vascular repair.

Systems analysis of the role of bone morphogenic protein 4 in endothelial inflammation

Shear stress is an important factor in the onset and progression of atherosclerosis. High and unidirectional laminar stress is seen as protective, while low and oscillatory shear stress is considered pro-inflammatory and pro-atherogenic. The mechanosensitive response of endothelial cells is governed by a complex system of genes, proteins, and signals that operate at distinctly different time scales. We propose a dynamic mathematical model that quantitatively describes this mechanosensing system and permits novel insights into its functioning. The model, the first of its kind, is constructed within the guidelines of Biochemical Systems Theory and accounts for different time scales by means of approximated delays. Parameter values are obtained directly from biochemical observations in an ad hoc fashion. The model reflects most documented observations well and leads to a number of predictions and novel hypotheses. In particular, it demonstrates the crucial role of Bone Morphogenic Protein 4 and p47(phox)-dependent NADPH oxidases in endothelial inflammation.

Influence of vitamin E on the antiplatelet effect of acetylsalicylic acid in human blood

We analysed the in vitro interaction between acetylsalicylic acid and vitamin E on the principal antiplatelet sites of action of acetylsalicylic acid, i.e., platelet aggregation, prostanoid production in platelets and leukocytes, and nitric oxide synthesis. Aggregation was measured in whole blood and in platelet-rich plasma (PRP) with ADP, collagen or arachidonic acid as platelet inducers, and we measured the production of thromboxane B2, prostacyclin and nitric oxide. Vitamin E potentiated the antiplatelet effect of acetylsalicylic acid in both whole blood and PRP. In PRP induced with collagen the IC50 for acetylsalicylic acid alone was 339+/-11.26, and that of acetylsalicylic acid+vitamin E was 0.89+/-0.09 (P<0.05). Vitamin E did not enhance inhibition of platelet thromboxane production by acetylsalicylic acid. Vitamin E spared or even increased prostacyclin levels, and acetylsalicylic acid+vitamin E diminished the inhibition of prostacyclin synthesis by acetylsalicylic acid (IC50 acetylsalicylic acid alone=1.81+/-0.15 microM; IC50 acetylsalicylic acid+vitamin E= 12.92+/-1.10 microM, P<0.05). Vitamin E increased the effect of acetylsalicylic acid on neutrophil nitric oxide production 42-fold (P<0.05). We conclude that vitamin E potentiates the antiplatelet effect of acetylsalicylic acid in vitro, and thus merits further research in ex vivo studies.

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.

Local glycoprotein IIb/IIIa receptor inhibitor delivery from the pump surface attenuates platelet adhesion in continuous flow ventricular assist devices

Shear-induced platelet activation (SIPA) has been identified to induce platelet adhesion and thrombus formation in continuous flow left ventricular assist devices (LVAD). Platelet glycoprotein (GP) IIb/IIIa receptor inhibitors are effective to prevent SIPA. However, systemic GP IIb/IIIa receptor inhibitor application is associated with severe bleeding complications. The aim of the study was to evaluate (i) the feasibility of absorption and elution of the GP IIb/IIIa receptor blocker TAK-029 from the Ti6Al4V surface of the pump; and (ii) the effect of local GP IIb/IIIa receptor blocker delivery regarding platelet adhesion on the surface of a continuous flow VAD model. Saturating concentrations of TAK-029 were adsorbed on the surface of a centrifugal pump. Whole human blood was perfused in circulatory mock loops using untreated (control), albumin-coated, or TAK-029-coated pumps. Peripheral resistance of the circulatory systems were adjusted accordingly to generate 5 L flow per min with impeller rotational speeds of 3500 (high-shear group) and 1500 rpm (low-shear group), respectively. Platelet adhesions on the respective impellers were quantified by ELISA and scanning electron microscopy (SEM). TAK-029 elution and half-life time were determined by ELISA. Compared with control, albumin-coated pumps showed 64 and 20% less platelet adhesions in the high- and low-shear group, respectively. TAK-029 coated pumps reduced platelet adhesion by additional 33 and 65%, respectively, compared with the albumin group. Elution of TAK 029 was initially very rapid and continued slowly. The results show that it is possible to adsorb and elute a small molecular weight GP IIb/IIIa receptor blocker from the pump surface. This drug elution reduced platelet adhesion on the pump significantly. Further studies are necessary to find a suitable drug bonding that will prolong the antiplatelet effect and preclude any bleeding complication caused by this procedure.

Fluid shear stress enhances the sphingosine 1-phosphate responses in cell-cell interactions between platelets and endothelial cells

Fluid shear stress modulates the functional responses of platelets and vascular cells, and plays an important role in the pathogenesis of vascular disorders, including atherosclerosis and restenosis. Since shear stress induces activation of platelets, which abundantly store sphingosine 1-phosphate (Sph-1-P), and upregulates the mRNA expression of S1P(1), the most important Sph-1-P receptor expressed on the endothelial cells, we examined the effects of shear stress on the Sph-1-P-related responses involving these cells. Shear stress was found to induce Sph-1-P release from the platelets in a shear intensity- and time-dependent manner. Inhibitors of protein kinase C suppressed this mechanical force-induced Sph-1-P release, suggesting involvement of this kinase. On the other hand, in vascular endothelial cells, shear stress increased S1P(1) protein expression, as revealed by flow-cytometric analysis, and the responsiveness to Sph-1-P, which was assessed by monitoring the intracellular Ca(2+) concentration. These results indicate that shear stress enhances the Sph-1-P responses in cell-cell interactions between platelets and endothelial cells.

Pathological shear stress directly regulates platelet αIIbβ3signaling

Integrin mechanotransduction is a ubiquitous biological process. Mechanical forces are transduced transmembranously by an integrin's ligand-bound extracellular domain through its β-subunit's cytoplasmic domain connected to the cytoskeleton. This often culminates in the activation of tyrosine kinases directing cell responses. The delicate balance between hemostasis and thrombosis requires exquisitely fine-tuned integrin function, and balance is maintained in vivo despite that the major platelet integrin αIIbβ3is continuously subjected to frictional or shearing forces generated by laminar blood flow. To test the hypothesis that platelet function is regulated by the direct effects of mechanical forces on αIIbβ3, we examined αIIbβ3/cytoskeletal interactions in human platelets exposed to shear stress in a cone-plate viscometer. We observed that α-actinin, myosin heavy chain, and Syk coimmunoprecipitate with αIIbβ3in resting platelets and that 120 dyn/cm2shear stress leads to their disassociation from αIIbβ3. Shear-induced disassociation of α-actinin and myosin heavy chain from the β3tail is unaffected by blocking von Willebrand factor (VWF) binding to glycoprotein (Gp) Ib-IX-V but abolished by blocking VWF binding to αIIbβ3. Syk's disassociation from β3is inhibited when VWF binding to either GpIb-IX-V or αIIbβ3is blocked. Shear stress-induced phosphorylation of SLP-76 and its association with tyrosine-phosphorylated adhesion and degranulation-promoting adapter protein are inhibited by blocking ligand binding to αIIbβ3but not by blocking ligand binding to GpIb-IX-V. Chinese hamster ovary cells expressing αIIbβ3with β3truncated of its cytoskeletal binding domains demonstrate diminished shear-dependent adhesion and cohesion. These results support the hypothesis that shear stress directly modulates αIIbβ3function and suggest that shear-induced αIIbβ3-mediated signaling contributes to the regulation of platelet aggregation by directing the release of constraining cytoskeletal elements from the β3-tail.

The extent of platelet activation under shear depends on platelet count: differential expression of anionic phospholipid and factor Va

It is widely accepted that shear stress activates platelets. However, this may have two linked but separate causes: a direct effect of shear stress on individual platelets, and secondary inter-platelet activation dependent on the release of agonists caused by shear. Gel-filtered platelets were exposed to intermittent low shear at 20,000 and 200,000 platelets/microl and their activation was measured with a prothrombinase-based assay. At the lower count, activation was slow and essentially linear, but at the higher count, it rose exponentially with time, leading to 3-fold more prothrombinase activity. Inclusion of apyrase and/or prostaglandin I(2) slightly reduced activation at high platelet counts, but did not abolish the nonlinear kinetics, and antibodies against von Willebrand factor had no significant effect. The contributions of anionic phospholipid and factor Va to the prothrombinase activity were assessed by measurements in the presence of exogenous factor Va. The results strongly suggest that anionic phospholipid appearance is caused directly by shear exposure, but that factor Va release from the alpha-granules is a secondary event and largely the result of platelet-platelet signaling.

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.

The roles of ADP and TXA in botrocetin/VWF-induced aggregation of washed platelets

BACKGROUND

Binding of von Willebrand factor (VWF) to the platelet membrane glycoprotein (GP) Ib-IX-V complex initiates a cascade of events leading to alphaIIbbeta3 activation and platelet aggregation. The roles of ADP and thromboxane A2 (TXA2) in agglutination-induced GPIbalpha-mediated platelet activation have not been fully described.

METHODS

Botrocetin and human VWF were used to stimulate washed mouse platelets. Platelets deficient in TXA2 receptors, Galphaq, or alphaIIbbeta3, and inhibitors and chelating agents were used to investigate the roles of TXA2, ADP, alphaIIbbeta3 and Ca2+ in botrocetin/VWF-induced signaling.

RESULTS

Our data demonstrate that botrocetin/VWF/GPIbalpha-mediated agglutination results in calcium-independent protein kinase C (PKC) and phospholipase A2 (PLA2) activities required for GPIbalpha-elicited TXA2 production that in turn causes dense granule secretion. Aggregation of washed platelets requires TXA2-induced alphaIIbbeta3 activation and ADP signaling. TXA2 or ADP can activate alphaIIbbeta3, but both are required for alpha-granule secretion and aggregation. Botrocetin/VWF-induced dense granule secretion is Galphaq-dependent. alpha-Granule secretion requires initial ADP signaling through P2Y1 and subsequent signaling through P2Y12. Signaling initiated by agglutination is propagated and amplified in an alphaIIbbeta3-dependent manner.

CONCLUSIONS

In contrast to adhesion or shear stress-induced GPIb-elicited signaling, agglutination-elicited GPIb signaling that activates alphaIIbbeta3 requires TXA2. Agglutination-elicited TXA2 production is independent of Ca2+ influx and mobilization of internal Ca2+ stores. Therefore, our results demonstrate that agglutination-elicited GPIb signaling causes alphaIIbbeta3 activation by a mechanism that is distinct from those used by adhesion, or shear stress-induced GPIb signaling.

ATP augments von Willebrand factor-dependent shear-induced platelet aggregation through Ca2+-calmodulin and myosin light chain kinase activation

Shear stress triggers von Willebrand factor (VWF) binding to platelet glycoprotein Ibalpha and subsequent integrin alpha(IIb)beta(3)-dependent platelet aggregation. Concomitantly, nucleotides are released from plateletdense granules, and ADP is known to contribute to shear-induced platelet aggregation (SIPA). We found that the impaired SIPA of platelets from a Hermansky-Pudlak patient lacking dense granules was restored by exogenous l-beta,gamma-methylene ATP, a stable P2X(1) agonist, as well as by ADP, confirming that in addition to ADP (via P2Y(1) and P2Y(12)), ATP (via P2X(1)) also contributes to SIPA. Likewise, SIPA of apyrase-treated platelets was restored upon P2X(1) activation with l-beta,gamma-methylene ATP, which promoted granule centralization within platelets and stimulated P-selectin expression, which is a marker of alpha-granule release. In addition, during SIPA, platelet degranulation required both extracellular Ca(2+) and VWF-glycoprotein Ibalpha interactions without involving alpha(IIb)beta(3). Neither platelet release nor SIPA was affected by protein kinase C inactivation, even though protein kinase C blockade inhibits platelet responses to collagen and thrombin in stirring conditions. In contrast, inhibiting myosin light chain (MLC) kinase with ML-7 reduced platelet release and SIPA by 30%. Accordingly, the potentiating effect of P2X(1) stimulation on the aggregation of apyrase-treated platelets coincided with intensified phosphorylation of MLC and was abrogated by ML-7. SIPA-induced MLC phosphorylation occurred exclusively through released nucleotides and selective antagonism of P2X(1) with MRS2159-reduced SIPA, ATP release, and potently inhibited MLC phosphorylation. We conclude that the P2X(1) ion channel induces MLC-mediated cytoskeletal rearrangements, thus contributing to SIPA and degranulation during VWF-triggered platelet activation.

von Willebrand factor binding to platelet glycoprotein Ib-IX-V stimulates the assembly of an alpha-actinin-based signaling complex

BACKGROUND

Pathological shear stress induces platelet aggregation that is dependent on von Willebrand factor (VWF) binding to glycoprotein (Gp)Ib-IX-V and phosphatidylinositol 3-kinase activation. We tested the hypothesis that pathological shear stress stimulates phosphatidylinositol 3,4,5-trisphosphate (PIP3) synthesis by directing the assembly of a molecular signaling complex that includes class IA phosphatidylinositol 3-kinase (PI 3-KIA).

METHODS

Platelets were subjected to 120 dynes cm-2 shear stress in a cone-plate viscometer. Resting and sheared platelets were lyzed, immunoprecipitations of PI 3-KIA performed, or lipids extracted for PIP3 measurements. alpha-Actinin was incubated with phosphatidylinositol 4,5-bisphosphate (PIP2), immunoprecipitated, and used as a substrate for in vitro PI 3-KIA activity.

RESULTS

Pathological shear stress induces biphasic PIP3 production. In resting platelets, PI 3-KIA associates with alpha-actinin and PIP2. After exposure to shear stress, alpha-actinin and PIP2 rapidly disassociate from PI 3-KIA. PI 3-KIA then gradually re-associates with PIP2 and alpha-actinin, and this complex becomes linked to GpIb alpha through the cytoskeleton. PIP3 production and the observed changes in the association between alpha-actinin, PIP2, and PI 3-KIA are inhibited when VWF binding to GpIb alpha is blocked. In a cell-free system, alpha-actinin binds PIP2 and when the alpha-actinin-PIP2 complex is added to platelet PI 3-KIA, PIP3 production is stimulated.

CONCLUSIONS

These results suggest that pathological shear-induced VWF binding to GpIb-IX-V stimulates PIP3 production through the assembly of an alpha-actinin-based complex that colocalizes PI 3-KIA with substrate PIP2.

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.

Purinergic P2Y12 receptor blockade inhibits shear-induced platelet phosphatidylinositol 3-kinase activation

Pathologically elevated shear stress triggers aspirin-insensitive platelet thrombosis. Signaling mechanisms involved in shear-induced platelet thrombosis are not well understood. To investigate these, we examined the hypothesis that functionally important platelet phosphatidylinositol 3-kinase (PI3-K) activity is stimulated by an in vitro shear stress of 120 dynes/cm(2) (shear rate of 6,000 sec(-1)). Phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) production was examined in washed human platelets subjected to pathological shear stress in a cone-plate viscometer. PIP(3) production peaks 30 s after shear begins and is initiated by von Willebrand factor (VWF) binding to the glycoprotein (Gp) Ib-IX-V complex. Inhibiting PI3-K with wortmannin or 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) results in the inhibition of shear-induced platelet aggregation. In resting platelets, class IA PI3-K associates with the tyrosine kinase Syk. Within 30 s of beginning shear, PI3-K-associated Syk becomes tyrosine phosphorylated. Inhibiting Syk activation with piceatannol results in the inhibition of PIP(3) production and aggregation. Selective blockade of the P2Y(12) receptor results in the inhibition of Syk phosphorylation, PIP(3) production, and aggregation. These results indicate that shear-induced VWF binding to platelet GpIb-IX-V stimulates functionally important PI3-K activity. PI3-K activation is signaled by rapid feedback amplification that involves P2Y(12) receptor-mediated activation of Syk.

von Willebrand factor (VWF)-dependent human platelet activation: porcine VWF utilizes different transmembrane signaling pathways than does thrombin to activate platelets, but both require protein phosphatase function

The interaction between von Willebrand factor (VWF) and glycoprotein (GP) Ib results in platelet agglutination and activation of many signaling intermediates. To determine if VWF-dependent platelet activation requires the participation of pivotal transmembrane signaling pathways, we analyzed VWF-dependent platelet activation profiles following inhibition of several transmembrane signaling intermediates. This was accomplished using porcine VWF, which has been shown to interact with human GPIb independently of shear stress or ristocetin. Platelet alpha (CD62) and lysozomal granule release (CD63), microparticle formation, and platelet agglutination/aggregation were evaluated. The ability of signaling inhibitors to prevent VWF-dependent platelet activation was compared to their ability to inhibit thrombin-dependent activation. The results demonstrate that VWF-dependent platelet activation can occur independently of the activities of protein kinase C (PKC), wortmannin-sensitive phosphatidylinositide 3-kinase, and phospholipase C, as well as independently of elevations in the concentration of intracellular calcium. In sharp contrast, these transmembrane signaling intermediates are required for thrombin-dependent platelet activation. In addition, thrombin-dependent but not VWF-dependent platelet activation was associated with elevations in the concentration of intracellular calcium under the conditions used. The family of signaling intermediates which appeared to be pivotal for both thrombin- and VWF-dependent platelet activation were the protein tyrosine phosphatases and the serine/threonine phosphatases. It is concluded that thrombin-dependent platelet activation relies on the activation of several transmembrane signaling pathways, whereas VWF-dependent platelet activation is dependent upon the activity of protein phosphatases. Inhibition of these phosphatases in vivo may provide a novel therapeutic approach for treating VWF-dependent platelet disorders such as thrombotic thrombocytopenic purpura or arterial thrombosis.

Interaction of Bruton's tyrosine kinase and protein kinase Ctheta in platelets. Cross-talk between tyrosine and serine/threonine kinases

The nonreceptor Bruton's tyrosine kinase (Btk) has been previously shown to associate physically and functionally with members of the protein kinase C (PKC) family of serine/threonine kinases in a variety of cell types. Here we show evidence for a novel interaction between Btk and PKCtheta; in platelets activated through the adhesion receptors GP Ib-V-IX and GP VI. Alboaggregin A, a snake venom component capable of activating both receptors in combination, leads to tyrosine phosphorylation of Btk downstream of Src family kinases. Inhibition of Btk by the selective antagonist LFM-A13 causes a reduction in calcium entry, although secretion of 5-hydroxytryptamine is potentiated. Btk is also phosphorylated on threonine residues in a PKC-dependent manner and associates with PKCtheta; upon platelet activation by either alboaggregin A or activation of GP Ib-V-IX alone by von Willebrand factor/ristocetin. PKCtheta; in turn becomes tyrosine-phosphorylated in a manner dependent upon Src family and Btk kinase activity. Inhibition of Btk activity by LFM-A13 leads to enhancement of PKCtheta; activity, whereas nonselective inhibition of PKC activity by bisindolylmaleimide I leads to reduction in Btk activity. We propose a reciprocal feedback interaction between Btk and PKCtheta; in platelets, in which PKCtheta; positively modulates activity of Btk, which in turn feeds back negatively upon PKCtheta;.

Pathological shear stress stimulates the tyrosine phosphorylation of alpha-actinin associated with the glycoprotein Ib-IX complex

Shear-induced platelet responses are triggered by VWF binding to the platelet GpIb-IX complex, and there is evidence that this ligand-receptor coupling stimulates transmembranous signaling through the cytoplasmic tail of glycoprotein (Gp) Ib alpha. To investigate the mechanism by which signaling is effected, new molecular interactions involving GpIb-IX that develop in response to pathological shearing stress were examined in intact human platelets. Exposure to shear, but not alpha-thrombin, results in the co-immunoprecipitation of the actin cross-linking protein alpha-actinin with the GpIb-IX complex. Blockers of VWF binding to GpIb alpha or actin polymerization inhibit the association of alpha-actinin with the GpIb-IX complex, but the association of alpha-actinin with the GpIb-IX complex is not affected by inhibiting VWF binding to platelet integrin alpha IIb beta 3 (GpIIb-IIIa). alpha-Actinin becomes tyrosine phosphorylated in response to pathological shear stress, and phosphorylated alpha-actinin associates with GpIb-IX. In resting platelets, class IA heterodimeric phosphatidylinositol 3-kinase (PI 3-K) and protein kinase N (PKN) associate with nonphosphorylated alpha-actinin. Shear stress causes PI 3-K to disassociate from alpha-actinin, while it stimulates PKN binding to alpha-actinin. These results demonstrate that shear-induced VWF binding to GpIb alpha causes enhanced binding of cytoskeletal alpha-actinin to GpIb-IX and suggest that alpha-actinin, perhaps through tyrosine phosphorylation, serves as an adapter for a signaling complex that could regulate VWF-induced platelet aggregation.

A novel viper venom metalloproteinase, alborhagin, is an agonist at the platelet collagen receptor GPVI

The interaction of platelet membrane glycoprotein VI (GPVI) with collagen can initiate (patho)physiological thrombus formation. The viper venom C-type lectin family proteins convulxin and alboaggregin-A activate platelets by interacting with GPVI. In this study, we isolated from white-lipped tree viper (Trimeresurus albolabris) venom, alborhagin, which is functionally related to convulxin because it activates platelets but is structurally different and related to venom metalloproteinases. Alborhagin-induced platelet aggregation (EC50, <7.5 microg/ml) was inhibitable by an anti-alphaIIbbeta3 antibody, CRC64, and the Src family kinase inhibitor PP1, suggesting that alborhagin activates platelets, leading to alphaIIbbeta3-dependent aggregation. Additional evidence suggested that, like convulxin, alborhagin activated platelets by a mechanism involving GPVI. First, alborhagin- and convulxin-treated platelets showed a similar tyrosine phosphorylation pattern, including a similar level of phospholipase Cgamma2 phosphorylation. Second, alborhagin induced GPVI-dependent responses in GPVI-transfected K562 and Jurkat cells. Third, alborhagin-dependent aggregation of mouse platelets was inhibited by the anti-GPVI monoclonal antibody JAQ1. Alborhagin had minimal effect on convulxin binding to GPVI-expressing cells, indicating that these venom proteins may recognize distinct binding sites. Characterization of alborhagin as a GPVI agonist that is structurally distinct from convulxin demonstrates the versatility of snake venom toxins and provides a novel probe for GPVI-dependent platelet activation.

The snake venom toxin alboaggregin-A activates glycoprotein VI

The glycoprotein (GP)-Ib-IX-V receptor complex has recently been reported to signal through a pathway similar to that used by the collagen receptor GPVI, with a critical role described for the Fc receptor gamma-chain. The evidence for this was based in part on studies with the GPIbalpha-selective snake venom toxin, alboaggregin-A. In the present study, it is reported that alboaggregin-A has activity at the collagen receptor GPVI in addition to GPIbalpha, and evidence is provided that this contributes to protein tyrosine phosphorylation, shape change, and GPIIb-IIIa-dependent aggregation. This may explain why responses to alboaggregin-A are distinct from those to von Willebrand factor-ristocetin. (Blood. 2001;97:3989-3991)

Dynamic aspects of platelet adhesion under flow

1. Cell-cell and cell-matrix adhesive interactions are critical for a wide range of physiological processes, including embryogenesis, inflammation, immunity and haemostasis. 2. The ability of circulating blood cells, such as platelets and leucocytes, to adhere to sites of vascular injury is complicated by the presence of blood flow, which imposes hydrodynamic forces on adhesion contacts. 3. To overcome this problem, platelets and leucocytes have evolved specific adhesion receptors with unique biomechanical properties that enable these cells to adhere to the vessel wall under flow conditions. 4. Platelet adhesion in the normal circulation appears to be a multiple-step process involving an initial reversible interaction between the platelet adhesion receptor glycoprotein Ib-IX-V and the vascular adhesion protein von Willebrand factor. Once tethered to the vessel wall, platelets form irreversible adhesion contacts through the binding of one or more platelet integrins to specific subendothelial matrix proteins. 5. There is now a wealth of evidence demonstrating that these receptors not only mediate platelet adhesion, but also transduce signals leading to platelet activation. 6. In the present review, we will briefly discuss the current understanding of the specific roles of individual platelet receptors in supporting the haemostatic function of platelets and discuss mechanisms by which these receptors induce platelet activation.

Porcine von Willebrand factor and thrombin induce the activation of c-Jun amino-terminal kinase (JNK/SAPK) whereas only thrombin induces activation of extracellular signal-related kinase 2 (ERK2) in human platelets

The interaction of platelets with subendothelial von Willebrand factor (VWF), especially under high shear stress, is considered to be the first activation step which primes platelets for subsequent haemostatic events. The signalling cascade which results from the interaction of VWF and its receptor GPIbIX has only been partially defined. Mitogen-activated protein kinases (MAPKs) are a family of downstream transmembrane signalling serine-threonine kinases and have been demonstrated to be present and functional in platelets; these include the extracellular signal-related kinases (ERKs), c-Jun amino-terminal kinases (JNKs) and p38 MAPK. Previously, we showed that p38 MAPK was not required in VWF-induced human platelet activation. It is not known whether VWF-dependent platelet activation involves the activation of the JNK and ERK family of signalling molecules. This report demonstrates that porcine von Willebrand factor (pVWF) induced a sustained and stable JNK activation measurable by 1 min after activation. Thrombin also induced JNK activation assessed at 1 min after activation. In contrast to thrombin, pVWF did not induce ERK2 activation at any time point tested. To ensure that ERK activation was unnecessary for pVWF-dependent platelet activation, we functionally inhibited ERK-dependent signalling with PD98059, a potent and selective inhibitor of the MAP kinase kinase (MEK-1), which is the upstream kinase of ERK1 and ERK2. Although PD98059 inhibited ERK2 activation in platelets, it had no effect on pVWF- or thrombin-induced platelet alpha or lysozomal granule release, modulation of membrane glycoprotein CD41, microparticle formation, platelet shape change or platelet agglutination. It is concluded that pVWF and thrombin induced JNK activation, but whereas thrombin induced ERK2 activation VWF did not; functional ERK2 activity was also not required for pVWF- or thrombin-dependent platelet activation.

Association between platelet glycoprotein Ibalpha genotype and ischemic cerebrovascular disease

BACKGROUND AND PURPOSE

Platelets play pivotal roles in the development of ischemic cerebrovascular disease (CVD). The platelet glycoprotein (GP) Ib/IX/V complex is a receptor for von Willebrand factor, which plays a major role in the initial phase of platelet activation under high shear stress conditions. This study was designed to investigate the association between a genetic variation of this receptor and the prevalence of CVD.

METHODS

Two hundred patients with ischemic CVD, as confirmed by brain CT and/or MRI, and 317 age- and sex-matched control subjects without clinical evidence of CVD or cardiovascular disease were analyzed for their genotype frequencies of the (145)Thr/Met dimorphism of the alpha-chain of GPIb (GPIbalpha).

RESULTS

Genotypes with (145)Met (T/M and M/M) were more frequently found in the CVD patients (26.5%) than in control subjects (14.2%, P=0.0005). The genotype effect was more obvious in those <60 years of age or without acquired cardiovascular risk factors. The odds ratio for nonsmoking women <60 years of age was 10. 6 (95% confidence intervals, 2.2 to 51.7). Although the number of patients studied was small (n=24), transient ischemic attack showed the highest odds ratio (4.3, P=0.0004), followed by lacunar infarction (OR=2.2, P=0.0024) and atherothrombotic infarction (OR=1. 5, P=0.3143). Logistic regression analysis revealed that the presence of Met-allele was independently associated with CVD.

CONCLUSIONS

Our study suggests that the platelet GPIbalpha genotype is a genetic risk factor for ischemic CVD.

Apheresis-induced platelet activation:comparison of three types of cell separators

BACKGROUND

Platelet-harvesting technology differs in various cell separators. Alteration in shear stress and biocompatibility of surfaces may give variable platelet activation and thereby affect the quality of the component.

STUDY DESIGN AND METHODS

Four groups (n = 10) of single-needle apheresis procedures using three cell separators, were compared: 1) Spectra LRS, 90-minute harvesting time; 2) MCS+, 90-minute harvest; 3) Amicus, 90-minute; and 4) Amicus, 45-minute. Whole-blood samples were collected from the donors as were samples from the final components at intervals during the first 4 hours after cessation of the apheresis. Platelet activation status and platelet activation capacity after agonist stimulation were assessed by flow cytometry.

RESULTS

No activated platelets were found in preapheresis and postapheresis samples from the donors. The platelets in the components from the Amicus (90-min) were significantly more activated than those in the other groups of components: that is, there was increased size of platelet aggregates, increased fraction of microparticles, increased degranulation, increased fibrinogen receptor activation, and decreased von Willebrand factor receptor expression. Moreover, the response of these platelets to agonist stimulation was reduced for all activation variables.

CONCLUSIONS

After 90 minutes' processing time, platelets obtained with the Amicus cell separator were significantly more activated than platelets harvested with the Spectra and the MCS+.

Cytoplasmic domains of GpIbα and GpIbβ regulate 14-3-3ζ binding to GpIb/IX/V

Shear stress causes the platelet glycoprotein (Gp) Ib/IX/V to bind to von Willebrand factor, resulting in platelet adhesion. GpIb/IX/V also functions to stimulate transmembranous signaling, leading to platelet activation and the expression of a ligand-receptive GpIIb-IIIa complex. The highly conserved cytoplasmic domain of GpIb binds directly to a dimeric 14-3-3 adapter protein ζ isoform. To explore structural determinants of GpIb/IX/V binding to 14-3-3ζ, the authors examined 14-3-3ζ interactions with GpIb and GpIbβ in heterologous cells and platelets. Truncations of GpIb at amino acid 542 or 594, or deletions of residues 542 through 590, inhibited binding of 14-3-3ζ. Deletion of GpIb from Trp570 to Ser590 eliminated 14-3-3ζ binding, and deletion of the sequence from Arg542-Trp570 enhanced binding of 14-3-3ζ to GpIb. All GpIb mutations that eliminated GpIb binding to the GST-14-3-3ζ fusion protein also eliminated GpIbβ binding to the fusion protein. Forskolin treatment of Chinese hamster ovary cells expressing wild-type GpIb/β/IX resulted in the phosphorylation of GpIbβ associated with enhanced binding of GpIbβ to GST-14-3-3ζ fusion protein and increased 14-3-3ζ coimmunoprecipitated with GpIb. When intact human platelets aggregated in response to 90 dynes/cm2 shear stress, 14-3-3ζ disassociated from GpIb. Prostacyclin treatment of platelets inhibited shear stress-induced aggregation and the release of 14-3-3ζ from GpIb. These data demonstrate that amino acid residues in the cytoskeletal interaction domains of GpIb regulate 14-3-3ζ binding to GpIb/β/IX, and suggest that protein kinase A-dependent phosphorylation of GpIbβ enhances 14-3-3ζ binding to the GpIb/IX/V complex in human platelets.

The high molecular mass, glycoprotein Ib-binding protein flavocetin-A induces only small platelet aggregates in vitro

The direct effects of snake venom glycoprotein (GP) Ib-binding proteins on platelet receptors during the formation of platelet aggregates were determined by a particle counting method using light scattering. Flavocetin-A induces small platelet aggregates, but not medium or large ones. However, neither jararaca GPIb-BP nor tokaracetin induce platelet aggregation. The flavocetin-A dose-response curve for formation of small aggregates is bell-shaped, with maximal effect at 1 to 2 microg/mL. The formation of small aggregates was not observed when fixed human platelets were used. Jararaca GPIb-BP, the anti-GPIb monoclonal antibody GUR83-35, prostaglandin I2, and ethylene diamine-N,N-dimethylformamide all inhibited flavocetin-A-induced small aggregate formation, but acetylsalicylic acid did not. Furthermore, anti-GPIIb/IIIa monoclonal antibodies, Abciximab, and YM337 significantly but partially inhibited aggregate formation, but the anti-von Willebrand factor monoclonal antibody NMC-4 had no effect. The formation of small aggregates required extracellular calcium, but flavocetin-A did not elevate cytosolic calcium. These results suggest that flavocetin-A binds to intact platelets, initiating platelet responses and inducing platelet aggregate formation by cross-linking platelets. Consequently, flavocetin-A may be a useful tool to study the mechanism of GPIb-mediated platelet activation and the structure-function relationships of GPIb.

Thrombin receptor activating peptide (SFLLRN) potentiates shear-induced platelet microvesiculation

Shear-induced activation of platelets plays a major role in vascular thrombosis. Shear stress tends to increase both platelet aggregation and procoagulant activity. One mechanism for increased procoagulant activity is promotion of the transbilayer movement of anionic phospholipids from the inner to the outer leaflet of the platelet membrane bilayer. This is accompanied by vesiculation of the platelet membrane, resulting in the formation of procoagulant membrane particles called microvesicles. In this study we have examined the effect of various platelet agonists on shear-induced platelet microvesiculation and the development of platelet procoagulant activity. Normal citrated whole blood was subjected to laminar shear rate up to 12,500 sec(-1) (shear stress approximately 375 dyne/cm2) in a cone-and-plate viscometer, and the formation of platelet microvesicles was measured by flow cytometry under different conditions. Elevated levels of shear stress induced significant microvesiculation. We investigated the effects of adenosine diphosphate, epinephrine, thromboxane A2 analog, collagen, and thrombin receptor activation peptide (SFLLRN) on shear-induced platelet microvesiculation. The thrombin peptide significantly increased shear-induced microvesicle formation. In contrast, under similar conditions, the other agonists had no significant effect on shear-induced microvesiculation. These studies suggest that thrombin formed in the vicinity of primary hemostatic plugs in areas of elevated shear stress may have a major role in the propagation of thrombi by potentiating shear-induced platelet microvesiculation.

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.

Analysis of the roles of 14-3-3 in the platelet glycoprotein Ib-IX-mediated activation of integrin alpha(IIb)beta(3) using a reconstituted mammalian cell expression model

We have reconstituted the platelet glycoprotein (GP) Ib-IX-mediated activation of the integrin alpha(IIb)beta(3) in a recombinant DNA expression model, and show that 14-3-3 is important in GPIb-IX signaling. CHO cells expressing alpha(IIb)beta(3) adhere poorly to vWF. Cells expressing GPIb-IX adhere to vWF in the presence of botrocetin but spread poorly. Cells coexpressing integrin alpha(IIb)beta(3) and GPIb-IX adhere and spread on vWF, which is inhibited by RGDS peptides and antibodies against alpha(IIb)beta(3). vWF binding to GPIb-IX also activates soluble fibrinogen binding to alpha(IIb)beta(3) indicating that GPIb-IX mediates a cellular signal leading to alpha(IIb)beta(3) activation. Deletion of the 14-3-3-binding site in GPIbalpha inhibited GPIb-IX-mediated fibrinogen binding to alpha(IIb)beta(3) and cell spreading on vWF. Thus, 14-3-3 binding to GPIb-IX is important in GPIb-IX signaling. Expression of a dominant negative 14-3-3 mutant inhibited cell spreading on vWF, suggesting an important role for 14-3-3. Deleting both the 14-3-3 and filamin-binding sites of GPIbalpha induced an endogenous integrin-dependent cell spreading on vWF without requiring alpha(IIb)beta(3), but inhibited vWF-induced fibrinogen binding to alpha(IIb)beta(3). Thus, while different activation mechanisms may be responsible for vWF interaction with different integrins, GPIb-IX-mediated activation of alpha(IIb)beta(3) requires 14-3-3 interaction with GPIbalpha.

Glycoprotein Ib-V-IX, a Receptor for von Willebrand Factor, Couples Physically and Functionally to the Fc Receptor γ-Chain, Fyn, and Lyn to Activate Human Platelets

The adhesion molecule von Willebrand factor (vWF) activates platelets upon binding 2 surface receptors, glycoprotein (GP) Ib-V-IX and integrin IIbβ3. We have used 2 approaches to selectively activate GP Ib using either the snake venom lectin alboaggregin-A or mutant recombinant forms of vWF (▵A1-vWF and RGGS-vWF) with selective binding properties to its 2 receptors. We show that activation of GP Ib induces platelet aggregation, secretion of 5-hydroxy tryptamine (5-HT), and an increase in cytosolic calcium. Syk becomes tyrosine phosphorylated and activated downstream of GP Ib, and associates with several tyrosine-phosphorylated proteins including the Fc receptor γ-chain through interaction with Syk SH2 domains. GP Ib physically associates with the γ-chain in GST-Syk-SH2 precipitates from platelets stimulated through GP Ib, and 2 Src family kinases, Lyn and Fyn, also associate with this signaling complex. In addition, GP Ib stimulation couples to tyrosine phosphorylation of phospholipase Cγ2. The Src family-specific inhibitor PP1 dose-dependently inhibits phosphorylation of Syk, its association with tyrosine-phosphorylated γ-chain, phosphorylation of PLCγ2, platelet aggregation, and 5-HT release. The results indicate that, upon activation, GP Ib is physically associated with FcR γ-chain and members of the Src family kinases, leading to phosphorylation of the γ-chain, recruitment, and activation of Syk. Phosphorylation of PLCγ2 also lies downstream of Src kinase activation and may critically couple early signaling events to functional platelet responses.

Glycoprotein Ib-V-IX, a Receptor for von Willebrand Factor, Couples Physically and Functionally to the Fc Receptor γ-Chain, Fyn, and Lyn to Activate Human Platelets

The adhesion molecule von Willebrand factor (vWF) activates platelets upon binding 2 surface receptors, glycoprotein (GP) Ib-V-IX and integrin IIbβ3. We have used 2 approaches to selectively activate GP Ib using either the snake venom lectin alboaggregin-A or mutant recombinant forms of vWF (▵A1-vWF and RGGS-vWF) with selective binding properties to its 2 receptors. We show that activation of GP Ib induces platelet aggregation, secretion of 5-hydroxy tryptamine (5-HT), and an increase in cytosolic calcium. Syk becomes tyrosine phosphorylated and activated downstream of GP Ib, and associates with several tyrosine-phosphorylated proteins including the Fc receptor γ-chain through interaction with Syk SH2 domains. GP Ib physically associates with the γ-chain in GST-Syk-SH2 precipitates from platelets stimulated through GP Ib, and 2 Src family kinases, Lyn and Fyn, also associate with this signaling complex. In addition, GP Ib stimulation couples to tyrosine phosphorylation of phospholipase Cγ2. The Src family-specific inhibitor PP1 dose-dependently inhibits phosphorylation of Syk, its association with tyrosine-phosphorylated γ-chain, phosphorylation of PLCγ2, platelet aggregation, and 5-HT release. The results indicate that, upon activation, GP Ib is physically associated with FcR γ-chain and members of the Src family kinases, leading to phosphorylation of the γ-chain, recruitment, and activation of Syk. Phosphorylation of PLCγ2 also lies downstream of Src kinase activation and may critically couple early signaling events to functional platelet responses.

The Glycoprotein Ib/IX Complex Regulates Cell Proliferation

The glycoprotein (Gp) Ib/IX complex contains three transmembranous leucine-rich repeat polypeptides (GpIb, GpIbβ, and GpIX) that form the platelet von Willebrand factor (vWF) receptor. GpIb/IX functions to effect platelet adhesion, activation, and aggregation under conditions of high shear stress. GpIb/IX is expressed late in the ontogeny of megakaryocytes, the precursor cell that releases platelets when it reaches its terminal stage of differentiation. Because signal pathways can be reused at different stages of development by integration with different effector pathways and because cellular adhesion through other receptor families often modulates cell growth, the hypothesis that GpIb/IX regulates cell growth was investigated. The surface expression of recombinant GpIb decreases the proliferation of transduced CHO cells. GpIb causes growth arrest in the G1 phase of the cell cycle associated with the induction of the cyclin-dependent kinase inhibitor p21. G1 arrest induced by recombinant GpIb in heterologous cells requires signaling through the 14-3-3ζ binding domain of GpIb and is partially dependent on its engagement by the extracellular ligand vWF. Growth arrest induced by the expression of recombinant GpIb/IX is followed by apoptosis of the transduced cells. The endogenous expression of GpIb in human hematopoietic cells is associated with decreased proliferation. These results suggest that the expression of the GpIb/IX complex regulates megakaryocyte growth.

The Glycoprotein Ib/IX Complex Regulates Cell Proliferation

The glycoprotein (Gp) Ib/IX complex contains three transmembranous leucine-rich repeat polypeptides (GpIb, GpIbβ, and GpIX) that form the platelet von Willebrand factor (vWF) receptor. GpIb/IX functions to effect platelet adhesion, activation, and aggregation under conditions of high shear stress. GpIb/IX is expressed late in the ontogeny of megakaryocytes, the precursor cell that releases platelets when it reaches its terminal stage of differentiation. Because signal pathways can be reused at different stages of development by integration with different effector pathways and because cellular adhesion through other receptor families often modulates cell growth, the hypothesis that GpIb/IX regulates cell growth was investigated. The surface expression of recombinant GpIb decreases the proliferation of transduced CHO cells. GpIb causes growth arrest in the G1 phase of the cell cycle associated with the induction of the cyclin-dependent kinase inhibitor p21. G1 arrest induced by recombinant GpIb in heterologous cells requires signaling through the 14-3-3ζ binding domain of GpIb and is partially dependent on its engagement by the extracellular ligand vWF. Growth arrest induced by the expression of recombinant GpIb/IX is followed by apoptosis of the transduced cells. The endogenous expression of GpIb in human hematopoietic cells is associated with decreased proliferation. These results suggest that the expression of the GpIb/IX complex regulates megakaryocyte growth.

Vitamin E and vascular homeostasis: implications for atherosclerosis

Considerable epidemiologic data suggest that dietary consumption of vitamin E reduces the incidence of cardiovascular disease. The precise mechanisms are not clear, but emerging data indicate that vitamin E has numerous activities that may, in part, explain its effect on vascular disease. In particular, vitamin E enhances the bioactivity of nitric oxide, inhibits smooth muscle proliferation, and limits platelet aggregation. One common mechanism to account for these effects of vitamin E is the inhibition of protein kinase C stimulation. In the setting of atherosclerosis, inhibition of protein kinase C by vitamin E would be expected to maintain normal vascular homeostasis and thus reduce the clinical incidence of cardiovascular disease.

A novel ligand-binding site in the zeta-form 14-3-3 protein recognizing the platelet glycoprotein Ibalpha and distinct from the c-Raf-binding site

We reported previously that the zeta-form 14-3-3 protein (14-3-3zeta) binds to a platelet adhesion receptor, glycoprotein (GP) Ib-IX, and this binding is dependent on the SGHSL sequence at the C terminus of GPIbalpha. In this study, we have identified a binding site in the helix I region of 14-3-3zeta (residues 202-231) required for binding to GPIb-IX complex and to the cytoplasmic domain of GPIbalpha. We also show that phosphorylation-dependent binding of c-Raf to 14-3-3zeta requires helix G (residues 163-187) but not helix I. Thus, the GPIbalpha-binding site is distinct from the binding sites for RSXpSXP motif-dependent ligands. Furthermore, we show that wild type 14-3-3zeta has a higher affinity for GPIb-IX complex than recombinant GPIbalpha cytoplasmic domain. Deletion of helices A and B (residues 1-32) disrupts 14-3-3zeta dimerization and decreases its affinity for GPIb-IX. Disruption of 14-3-3zeta dimerization, however, does not reduce 14-3-3zeta binding to recombinant GPIbalpha cytoplasmic domain. This suggests a dual site recognition mechanism in which a 14-3-3zeta dimer interacts with both GPIbalpha and GPIbbeta (known to contain a phosphorylation-dependent binding site), resulting in high affinity binding.

Binding of purified 14-3-3 zeta signaling protein to discrete amino acid sequences within the cytoplasmic domain of the platelet membrane glycoprotein Ib-IX-V complex

The glycoprotein (GP) Ib-IX-V complex constitutively expressed on the platelet plasma membrane mediates initial adhesion of circulating platelets to vessel wall matrix at high shear, and shear-induced platelet aggregation. In both cases, this involves binding of GP Ib-IX-V to the adhesive glycoprotein, von Willebrand Factor (vWF). vWF binding to GP Ib-IX-V rapidly induces platelet activation, leading to cytoskeletal rearrangement, shape change, and secretion that enables alphaIIbbeta3 integrin (GP IIb-IIIa)-dependent platelet aggregation. All these events are critical in (patho)physiological thrombus formation. The recent discovery that the signaling protein, 14-3-3 zeta, copurifies with the GP Ib-IX complex (minus GP V) [Du, X., Harris, S. J., Tetaz, T. J., Ginsberg, M. H., & Berndt, M. C. (1994) J. Biol. Chem. 269, 18287-18290] indicated a potential mechanism for vWF-dependent signaling. The aim of the present study was to identify discrete amino acid sequences that bind 14-3-3 zeta within the cytoplasmic domain of the receptor. As an initial screening assay, overlapping synthetic peptides based on the cytoplasmic domains of GP Ibalpha (100 residues), GP Ibbeta (34 residues), GP IX (5 residues), and GP V (16 residues) were immobilized and assessed for the ability to bind purified 14-3-3 zeta. The C-terminal sequence GHSL of GP Ibalpha was identified as one 14-3-3 zeta interactive sequence, consistent with previous results [Du, X., Fox, J. E., & Pei, S. (1996) J. Biol. Chem. 271, 7362-7367]. Binding of 125I-labeled 14-3-3 zeta to GHSL-containing peptides was inhibitable by unlabeled 14-3-3 zeta and by anti-14-3-3 zeta IgG. Ala-walking through the GHSL sequence suggested all residues were necessary for optimal binding. In addition, 14-3-3 zeta bound with lower affinity to a peptide based on the central region of the GP Ibalpha cytoplasmic domain (Arg-557-Gly-575), whereas peptide sequences within the cytoplasmic domains of GP Ibbeta (Arg-160-Arg-175) and GP V (Lys-529-Gly-544) bound 14-3-3 zeta with comparable affinity to the GHSL-containing peptide. Soluble GHSL-containing peptides, GP Ibbeta- and GP V-based peptides semidissociated 14-3-3 zeta from GP Ib-IX-V or GP Ib-IX in platelet extracts as analyzed by immunoprecipitation, suggesting these sequences, at least partially, mediate the GP Ib-IX-V-14-3-3 zeta interaction in cells. Further, phosphorylation of the GP Ibbeta peptide at a site corresponding to a protein kinase A phosphorylation site (Ser-166) enhanced the affinity of 14-3-3 zeta binding by approximately 8-fold, suggesting phosphorylation as a potential mechanism for regulating 14-3-3 zeta association with the GP Ib-IX-V complex.

Inhibition of shear stress-induced platelet aggregation by cilostazol, a specific inhibitor of cGMP-inhibited phosphodiesterase, in vitro and ex vivo

Cilostazol(6-[4-(1-cyclohexyl-1H-tetrazol-5-yl)-butoxy]-3,4- dihydro-2(1H)-quinolinone) selectively inhibits cGMP-inhibited phosphodiesterase (PDE3) and is a potent inhibitor of platelet aggregation induced by various agonists. Effect of cilostazol on shear stress-induced human platelet aggregation (SIPA) was examined in vitro and ex vivo. Cilostazol inhibited SIPA dose-dependently in vitro. The IC50 value of cilostazol for inhibition of SIPA was 15 +/- 2.6 microM (m +/- SE, n=5), which was very similar to that (12.5 +/- 2.1 microM) for inhibition of ADP-induced platelet aggregation. Cilostazol potentiates the inhibition of SIPA by PGE1 and enhances its ability to increase cAMP concentrations. A single oral adminstration of 100 mg cilostazol to healthy volunteers produced a significant inhibition of SIPA. This study demonstrates that cilostazol is an effective inhibitor of SIPA, which may be important for the prevention and the treatment of arterial occlusive diseases.

Glycoprotein Ib-von Willebrand Factor Interactions Activate Tyrosine Kinases in Human Platelets

von Willebrand factor (vWF ) in the presence of botrocetin induces p72syk activation, assessed as its autophosphorylated level and in vitro kinase assays, the transient association of p72syk with p60c-src, and the translocation of p60c-src and p54/58lyn to cytoskeletal fractions. Jararaca glycoprotein Ib-binding protein (GPIb-BP), which specifically binds to GPIb, abolished these phenomena, suggesting that they are mediated by the vWF-GPIb interaction. These tyrosine kinase-related events were not inhibited by GRGDS peptide (plus EGTA), indicating that GPIIb/IIIa is not involved in the observed responses. Shc, an adaptor protein, was also tyrosine phosphorylated by the botrocetin-vWF activation. When GPIb was immunoprecipitated with nonfunctional monoclonal antibodies (MoAbs) directed against GPIb, a kinase activity was found to associate with GPIb upon botrocetin-vWF activation. On the other hand, anti-GPIb MoAbs that inhibit the vWF-GPIb interaction did not coprecipitate a kinase activity. Because the recovery of GPIb did not differ significantly, it is suggested that the excessive presence of inhibitory anti-GPIb MoAb dissociated a kinase activity from GPIb. Phosphoamino acid analysis showed that the kinase activity was that of a tyrosine kinase. The identity of the tyrosine kinase and the mode of interaction with the cytoplasmic region of GPIb await to be determined. Our findings suggest that the tyrosine kinase associated with GPIb serves at a most proximal step in the signal transduction pathway involved in the vWF-GPIb-induced platelet activation, which leads to other tyrosine kinase-related intracellular signals.

Glycoprotein Ib-von Willebrand Factor Interactions Activate Tyrosine Kinases in Human Platelets

von Willebrand factor (vWF ) in the presence of botrocetin induces p72syk activation, assessed as its autophosphorylated level and in vitro kinase assays, the transient association of p72syk with p60c-src, and the translocation of p60c-src and p54/58lyn to cytoskeletal fractions. Jararaca glycoprotein Ib-binding protein (GPIb-BP), which specifically binds to GPIb, abolished these phenomena, suggesting that they are mediated by the vWF-GPIb interaction. These tyrosine kinase-related events were not inhibited by GRGDS peptide (plus EGTA), indicating that GPIIb/IIIa is not involved in the observed responses. Shc, an adaptor protein, was also tyrosine phosphorylated by the botrocetin-vWF activation. When GPIb was immunoprecipitated with nonfunctional monoclonal antibodies (MoAbs) directed against GPIb, a kinase activity was found to associate with GPIb upon botrocetin-vWF activation. On the other hand, anti-GPIb MoAbs that inhibit the vWF-GPIb interaction did not coprecipitate a kinase activity. Because the recovery of GPIb did not differ significantly, it is suggested that the excessive presence of inhibitory anti-GPIb MoAb dissociated a kinase activity from GPIb. Phosphoamino acid analysis showed that the kinase activity was that of a tyrosine kinase. The identity of the tyrosine kinase and the mode of interaction with the cytoplasmic region of GPIb await to be determined. Our findings suggest that the tyrosine kinase associated with GPIb serves at a most proximal step in the signal transduction pathway involved in the vWF-GPIb-induced platelet activation, which leads to other tyrosine kinase-related intracellular signals.

Coronary artery disease and polymorphisms in a receptor mediating shear stress-dependent platelet activation

BACKGROUND

Platelets play pivotal roles in coronary thrombosis, and antiplatelet therapies are widely used for coronary artery disease (CAD). However, the effects of genetic variation in platelets on CAD are poorly understood. We have assessed the association between CAD and polymorphisms in a platelet receptor for von Willebrand factor, the glycoprotein (GP) Ib/IX complex, which mediates shear stress-dependent platelet activation.

METHODS AND RESULTS

Genotypes of the alpha-chain of the receptor (GP Ib alpha, 145Thr/Met) were determined in 91 patients with myocardial infarction (MI) or angina pectoris whose lesions were confirmed by coronary angiography as well as in 105 individuals from the general population with no history of angina or other heart diseases and normal resting ECGs. There was no homozygote for Met/Met in either the control or patient groups. The prevalence of the Thr/Met genotype (T/M) in all patients was not significantly different from that in the control group. However, the frequency of T/M was significantly higher in patients aged < or = 60 years (31.8%) than in control subjects aged < or = 60 years (16.0%; P<.05, odds ratio=2.5). An association was also demonstrated between CAD and the other polymorphism of GP Ib alpha, a variable number of tandem repeats of a 13-amino acid sequence, which is known to be linked to the 145Thr/Met polymorphism. There was an association between the frequency of the T/M genotype and the angiographic severity of CAD: 11.1% for Gensini score < 40 versus 50.0% for Gensini score > or = 40 (P=.0015). There was no difference in the distribution of GP Ib alpha genotypes between patients with MI and those with angina pectoris.

CONCLUSIONS

This study suggests that the presence of the Met allele in GP Ib alpha is a risk factor for the prevalence and severity of CAD in individuals aged < or = 60 years. The results need to be confirmed in a large-scale study of incident case subjects and matching control subjects.

Platelet activation in flowing blood passing growing arterial thrombi

We investigated the combined effect of wall shear rate and immobilized collagen on platelet activation in flowing nonanticoagulated human blood. By combining an ex vivo model of thrombogenesis with flow cytometry, we showed that activated platelets can be detected in the bloodstream passing growing thrombi at a wall shear rate characteristic of moderately stenosed arteries (2600 s-1). The activation of the circulating platelets was clearly correlated with thrombus growth. Different antibodies against platelet activation-dependent surface markers had distinct sensitivity to the thrombotic process. alpha-Granule release detected by surface expression of CD62P seemed to be the most sensitive marker, as judged by both mean fluorescence intensity and fraction of platelets activated. The conformational change in glycoprotein IIb-IIIa, as detected by PAC-1, also seemed to be a sensitive marker and preceded binding of fibrinogen to activated glycoprotein IIb-IIIa, as detected by anti-fibrinogen. Large thrombi also elicited lysosome exocytosis, detected by surface expression of CD63. Finally, we observed a small decrease of glycoprotein Ib-IX expression, as detected by anti-CD42a. Thus, our study provides further information on the dynamics of platelet activation in relation to thrombus growth at arterial shear conditions in flowing nonanticoagulated human blood.

Calpain activation in shear-induced platelet aggregation

Fluid shear stress has been known to activate platelet reaction such as aggregation, but the exact mechanism of shear-induced platelet aggregation (SIPA) has not been fully understood. Calpain, an intracellular calcium-activated cysteine protease, is abundant in platelets and is considered to be activated and involved in the proteolytic processes during platelet activation. A possible activation of calpain in SIPA was investigated, employing a newly developed aggregometer and specific monoclonal antibodies to detect activation of calpain. When a shear stress gradient varying between 6 and 108 dyn/cm2 was applied to platelets, activation of mu-calpain was observed only in high-shear-stressed platelets, resulting in the proteolysis of talin. At 1 min after the onset of constant high shear stress of 108 dyn/cm2, mu-calpain activation and proteolysis of talin were detected and increased in a time-dependent manner. Constant shear stress more than 50 dyn/cm2, applied for 5 min, caused mu-calpain activation and proteolysis of talin, which were increased in a shear-force-dependent manner. Calpeptin, a calpain-specific peptide antagonist, caused the complete inhibition of both mu-calpain activation and proteolysis of talin, while SIPA profiles with calpeptin showed almost no change compared to those without calpeptin. These results suggest the possibility of calpain involvement in late phases of shear-induced platelet activation such as cytoskeletal reorganization.