Protein tyrosine phosphorylation in human platelets induced by interaction between glycoprotein Ib and von Willebrand factor

Jararaca GPIb-binding protein causes thrombocytopenia during Bothrops jararaca envenomation

Inoculation of Bothrops jararaca snake venom (BjV) induces thrombocytopenia in humans and various animal species. Although several BjV toxins acting on hemostasis have been well characterized in vitro, it is not known which one is responsible for inducing thrombocytopenia in vivo. In previous studies, we showed that BjV incubated with metalloproteinase or serine proteinase inhibitors and/or anti-botrocetin antibodies still induced thrombocytopenia in rats and mice. Thus, herein we identified and characterized BjV toxins responsible for inducing thrombocytopenia. Initially, by filtering BjV on ultrafiltration systems, proteins with molecular masses between 30 and 50 kDa were shown to induce thrombocytopenia in mice, but they were not associated with hemorrhagic or coagulating activities. The 50 kDa ultrafiltrate was chromatographed, and two proteins (named fraction D and fraction E) induced thrombocytopenia in mice. However, neither fraction D nor fraction E induced platelet aggregation in platelet-rich plasma or whole blood from humans or mice. By mass spectrometry analysis, fraction E was identified as jararaca glycoprotein Ib (GPIb)-binding protein. Injection of these fractions caused thrombocytopenia in control or Vwf-/- mice, showing that the axis platelet GPIb - von Willebrand factor is not involved in their biological action in vivo. New studies are necessary to understand how these proteins act in vivo.

Extracellular signal-regulated kinase 5 associates with casein kinase II to regulate GPIb-IX-mediated platelet activation via the PTEN/PI3K/Akt pathway

Essentials The mechanisms of extracellular signal-regulated kinase 5 (ERK5) in GPIb-IX signaling are unclear. Function of ERK5 in GPIb-IX was tested using aggregation, western blotting, and mass spectrometry. The protein interacting with ERK5 in human platelets was identified as casein kinase II (CKII). ERK5 associates with CKII to regulate the activation of the PI3K/Akt pathway in GPIb-IX signaling.

SUMMARY

Background The platelet glycoprotein (GP) Ib-IX complex plays essential roles in thrombosis and hemostasis. The mitogen-activated protein kinases (MAPKs) ERK1/2 and p38 have been shown to be important in the GPIb-IX-mediated signaling leading to integrin activation. However, the roles of the MAPK extracellular signal-regulated kinase 5 (ERK5) in GPIb-IX-mediated platelet activation are unknown. Objective To reveal the function and mechanisms of ERK5 in GPIb-IX-mediated platelet activation. Methods The functions of ERK5 in GPIb-IX-mediated human platelet activation were assessed using botrocetin/VWF, ristocetin/VWF, or platelet adhesion to von Willebrand factor (VWF) under shear stress in the presence of a specific inhibitor of ERK5. ERK5-associated proteins were pulled down from Chinese hamster ovary (CHO) cells transfected with HA-tagged-ERK5, identified by mass spectrometry, and confirmed in human platelets. Roles of ERK5-associated proteins in GPIb-IX-mediated platelet activation were clarified using specific inhibitors. Results The phosphorylation levels of ERK5 were significantly enhanced in human platelets stimulated with botrocetin/VWF or ristocetin/VWF. The ERK5 inhibitor XMD8-92 suppressed the second wave of human platelet aggregation induced by botrocetin/VWF or ristocetin/VWF and inhibited human platelet adhesion on immobilized VWF under shear stress. Casein kinase II (CKII) was identified as an ERK5-associated protein in human platelets. The CKII inhibitor TBB, similar to the ERK5 inhibitor XMD8-92, specifically restrained PTEN phosphorylation, therefore suppressing Akt phosphorylation in human platelets treated with botrocetin/VWF. Conclusion ERK5 associates with CKII to play essential roles in GPIb-IX-mediated platelet activation via the PTEN/PI3K/Akt pathway.

Syk Activity Is Dispensable for Platelet GP1b-IX-V Signaling

The binding of von Willebrand factor (VWF) to the platelet membrane glycoprotein 1b-IX (GP1b-IX) leads to activation of platelets. GP1b was shown to signal via the FcRγ-ITAM (Fc Receptor γ-Immunoreceptor tyrosine-based activation motif) pathway, activating spleen tyrosine kinase (Syk) and other tyrosine kinases. However, there have been conflicting reports regarding the role of Syk in GP1b signaling. In this study, we sought to resolve these conflicting reports and clarify the role of Syk in VWF-induced platelet activation. The inhibition of Syk with the selective Syk inhibitors, OXSI-2 and PRT-060318, did not inhibit VWF-induced platelet adhesion, agglutination, aggregation, or secretion. In contrast, platelets stimulated with the Glycoprotein VI (GPVI) agonist, collagen-related peptide (CRP), failed to cause any aggregation or secretion in presence of the Syk inhibitors. Furthermore, GP1b-induced platelet signaling was unaffected in the presence of Syk inhibitors, but GPVI-induced signaling was abolished under similar conditions. Thus, we conclude that Syk kinase activity does not play any functional role downstream of GP1b-mediated platelet activation.

Platelet receptors activated via mulitmerization: glycoprotein VI, GPIb-IX-V, and CLEC-2

While very different in structure, GPVI - the major collagen receptor on platelet membranes, the GPIb-IX-V complex - the receptor for von Willebrand factor, and CLEC-2, a novel platelet activation receptor for podoplanin, share several common features in terms of function and platelet activation signal transduction pathways. All employ Src family kinases (SFK), Syk, and other signaling molecules involving tyrosine phosphorylation, similar to those of immunoreceptors for T and B cells. There appear to be overlapping functional roles for these glycoproteins, and in some cases, they can compensate for each other, suggesting a degree of redundancy. New ligands for these receptors are being identified, which broadens their functional relevancy. This is particularly true for CLEC-2, whose functions beyond hemostasis are being explored. The common mode of signaling, clustering, and localization to glycosphingolipid-enriched microdomains (GEMs) suggest that GEMs are central to signaling function by ligand-dependent association of these receptors, SFK, Syk, phosphotyrosine phosphatases, and other signaling molecules.

The role of Rac1 in glycoprotein Ib-IX-mediated signal transduction and integrin activation

OBJECTIVE

The platelet receptor for von Willebrand factor, the glycoprotein Ib-IX (GPIb-IX) complex, mediates platelet adhesion at sites of vascular injury and transmits signals leading to platelet activation. von Willebrand factor/GPIb-IX interaction sequentially activates the Src family kinase Lyn (SFK), phosphoinositide 3-kinase (PI3K), and Akt, leading to activation of integrin α(IIb)β(3) and integrin-dependent stable platelet adhesion and aggregation. It remains unclear how Lyn activates the PI3K/Akt pathway after ligand binding to GPIb-IX.

METHODS AND RESULTS

Using platelet-specific Rac1(-/-) mice and the Rac1 inhibitor NSC23766, we examined the role of Rac1 in GPIb-IX-dependent platelet activation. Rac1(-/-) mouse platelets and NSC23766-treated human platelets were defective in GPIb-dependent stable adhesion to von Willebrand factor under shear stress, integrin activation, thromboxane A(2) synthesis, and platelet aggregation. Interestingly, GPIb-induced activation of Rac1 and the guanine nucleotide exchange factor for Rac1, Vav, was abolished in both Lyn(-/-) and SFK inhibitor-treated platelets but was unaffected by the PI3K inhibitor LY294002, indicating that Lyn mediates activation of Vav and Rac1 independently of PI3K. Furthermore, GPIb-induced activation of Akt was abolished in Rac1-deficient platelets, suggesting that Rac1 is upstream of the PI3K/Akt pathway.

CONCLUSIONS

A Lyn-Vav-Rac1-PI3K-Akt pathway mediates von Willebrand factor-induced activation of integrin α(IIb)β(3) to promote GPIb-IX-dependent platelet activation.

Src family tyrosine kinase Lyn mediates VWF/GPIb-IX-induced platelet activation via the cGMP signaling pathway

The platelet receptor for von Willebrand factor (VWF), glycoprotein (GP) Ib-IX, mediates initial platelet adhesion and transmits signals leading to platelet activation. Src family tyrosine kinases (SFKs) play an important role in VWF-induced GPIb-IX signaling. However, the SFK-dependent downstream signaling pathway is unclear but is thought to involve thromboxane A2 (TXA2) synthesis. Here we show that, although platelets deficient in SFK members, Lyn or Fyn, were defective in the TXA2-dependent second wave of platelet aggregation induced by botrocetin/VWF, only Lyn-knockout platelets were also defective in stable platelet adhesion to VWF under shear stress that is independent of the TXA2 pathway. Lyn-knockout platelets also spread poorly on VWF but spread normally on fibrinogen, indicating an important role for Lyn in VWF-mediated GPIb signaling but not in integrin outside-in signaling. Importantly, Lyn knockout abrogated VWF-induced cGMP elevation. Addition of low concentrations of 8-bromo-cGMP, however, corrected the defective stable adhesion of Lyn-knockout platelets or PP2-treated platelets on VWF. These results demonstrate an important role for Lyn in VWF/GPIb-IX-induced integrin activation mediated via the cGMP signaling pathway independently of TXA2 synthesis and also indicate that Lyn is critically important in GPIb-IX-mediated activation of the cGMP pathway.

The role of Akt in the signaling pathway of the glycoprotein Ib-IX induced platelet activation

The platelet von Willebrand factor (vWF) receptor, glycoprotein Ib-IX (GPIb-IX), mediates platelet adhesion and induces signaling leading to integrin activation. Phosphoinositol 3-kinase (PI3K) is important in GPIb-IX-mediated signaling. PI3K-dependent signaling mechanisms, however, are unclear. We show that GPIb-IX-induced platelet aggregation and stable adhesion under flow were impaired in mouse platelets deficient in PI3K effectors, Akt1 and Akt2, and in human platelets treated with an Akt inhibitor, SH-6. Akt1 and Akt2 play important roles in early GPIb-IX signaling independent of Syk, adenosine diphosphate (ADP), or thromboxane A2 (TXA2), in addition to their recognized roles in ADP- and TXA2-dependent secondary amplification pathways. Knockout of Akt1 or Akt2 diminished platelet spreading on vWF but not on immobilized fibrinogen. Thus, Akt1 and Akt2 are both required only in the GPIb-IX-mediated integrin activation (inside-out signaling). In contrast, PI3K inhibitors abolished platelet spreading on both vWF and fibrinogen, indicating a role for PI3K in integrin outside-in signaling distinct from that in GPIb-IX-mediated inside-out signaling. Furthermore, Akt1- or Akt2-deficiency diminished vWF-induced cGMP elevation, and their inhibitory effects on GPIb-IX-dependent platelet adhesion were reversed by exogenous cGMP. Thus, Akt1 and Akt2 mediate GPIb-IX signaling via the cGMP-dependent signaling pathway.

Von Willebrand factor activates endothelial nitric oxide synthase in blood platelets by a glycoprotein Ib-dependent mechanism

BACKGROUND

The molecular regulation of endothelial nitric oxide synthase (eNOS) in blood platelets and the signalling events induced by platelet-derived NO are poorly defined. In particular, the ability of von Willebrand factor (VWF) to stimulate cyclic guanosine monophosphate (cGMP) formation in platelets has produced conflicting data.

OBJECTIVES

To determine the mechanisms leading to eNOS activation and clarify the downstream signaling pathways activated by platelet-derived NO in response to VWF.

METHODS

We used three independent markers of NO signaling, [3H] l-citrulline production, cGMP accrual and immunoblotting of vasodilator-stimulated phosphoprotein (VASP) to examine the NO signaling cascade in response to VWF.

RESULTS

VWF increased NO synthesis and bioavailability, as evidenced by increased [3H] l-citrulline production and cGMP accrual, respectively. VWF-induced eNOS activation was GPIb-IX-dependent and independent of integrin alpha(IIb)beta3. cGMP formation in response to VWF required Ca2+ mobilization, Src family kinases, phosphatidylinositol 3-kinase and phospholipase C, but not protein kinase C. This suggests that a cross-talk between the signaling mechanisms regulates platelet activation and NO synthesis. VWF-induced cGMP accrual was completely blocked by apyrase and indomethacin, demonstrating an essential role for platelet-derived ADP and thromboxane A2 (TxA2). Elevated cGMP levels led to increased VASP phosphorylation at serine239 that was both protein kinase G (PKG)- and protein kinase A (PKA)-dependent.

CONCLUSIONS

We demonstrate that VWF activates eNOS through a specific Ca2+-dependent GPIb receptor-signaling cascade that relies on the generation of platelet-derived ADP and TxA2. Furthermore, we provide the first evidence to suggest that platelet derived-NO/cGMP activates PKA in addition to PKG.

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.

Platelet adhesion signalling and the regulation of thrombus formation

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

Theoretical analysis on ratiometric fluorescent indicators caused biased estimates of intracellular free calcium concentrations

Ratiometric fluorescent calcium indicator dyes have been widely used for the study of the role of Ca2+ in cell physiopathology. Although these ratiometric dyes offer several advantages over others, they suffer some drawbacks which cause serious errors in measurement of intracellular Ca2+ concentration, [Ca2+]i. The present study systematically analyzes theoretical reasons and technical sources of discrepancies occurring in the measurement of the characteristics of the agonists-induced cells [Ca2+]i. In order to avoid the errors and achieve the accurate determination of [Ca2+]i, this study proposes solutions and suggests some critical measures in both theoretical and technical aspects. Therefore, this analysis can be a valuable tool in clarifying proper usages of fluorescent dyes for [Ca2+]i measurements.

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).

Effects of platelet inhibitors on propyl gallate-induced platelet aggregation, protein tyrosine phosphorylation, and platelet factor 3 activation

Propyl gallate (PG) is a platelet agonist characterized by inducing platelet aggregation, protein tyrosine phosphorylation, and platelet factor 3 activity. The mechanisms of platelet activation following PG stimulation were examined by pre-incubating platelets with well-defined platelet inhibitors using platelet aggregation, protein tyrosine phosphorylation, activated plasma clotting time, and annexin V binding by flow cytometry. PG-induced platelet aggregation and tyrosine phosphorylation of multiple proteins were substantially abolished by aspirin, apyrase, and abciximab (c7E3), suggesting that PG is associated with activation of platelet cyclooxygenase 1, adenosine phosphate receptors, and glycoprotein IIb/IIIa, respectively. The phosphorylation of the cytoskeletal enzyme pp60(c-src) increased following PG stimulation, but was blunted by pre-incubation of platelets with aspirin, apyrase, and c7E3, suggesting that tyrosine kinase is important for the signal transduction of platelet aggregation. Propyl gallate also activates platelet factor 3 by decreasing the platelet coagulation time and increasing platelet annexin V binding. Platelet incubation with aspirin, apyrase, and c7E3 did not alter PG-induced platelet coagulation and annexin V binding. The results suggest that platelet factor 3 activation and membrane phosphotidylserine expression were not involved with activation of platelet cyclooxygenase, adenosine phosphate receptors, and glycoprotein IIb/IIIa. PG is unique in its ability to stimulate platelet aggregation and coagulation simultaneously, and platelet inhibitors in this study affect only platelet aggregation but not platelet coagulation.

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

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

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

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

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.

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.

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.

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

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

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

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

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.

p38 MAPK is activated but not necessary in porcine von Willebrand factor-dependent platelet activation

We have investigated the role of p38 mitogen-activated protein kinase (MAPK) in von Willebrand factor (VWF)-dependent platelet activation. The interaction of platelets with subendothelial VWF, especially under high shear stress, is considered to be the first activation step which primes platelets for subsequent haemostatic events. As a model of VWF-dependent platelet activation, porcine VWF was employed. Porcine VWF induced p38 MAPK activation by 1 min post-addition; assessed by phosphorylation of a recombinant p38 MAPK fusion protein substrate termed glutathione S-transferase-MAPK activated protein kinase-2. To determine if p38 MAPK was necessary for porcine VWF-induced platelet activation, we functionally inhibited p38 MAPK activity with SB203580 before exposure of the platelets to porcine VWF. Inhibition of p38 MAPK had no effect on VWF-induced platelet alpha or lysozomal granule release, expression of activated GPIIb IIIa, modulation of membrane glycoprotein CD41, expression of phosphatidylserine as assessed by annexin V binding, microparticle formation, or platelet agglutination. It was concluded that SB203580-inhibitable p38 MAPK activity induced by porcine VWF is not necessary for platelet activation.

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.

Marked Temperature Dependence of the Platelet Calcium Signal Induced by Human von Willebrand Factor

Interaction of von Willebrand factor (vWF) with the platelet is essential to hemostasis when vascular injury occurs. This interaction elevates the intracellular free calcium concentration ([Ca2+]i) and promotes platelet activation. The present study investigated the temperature dependence of vWF-induced [Ca2+]i signaling in human platelets. The influence of temperature can provide invaluable insight into the underlying mechanism. Platelet [Ca2+]i was monitored with Fura-PE3. Ristocetin-mediated binding of vWF induced a transient platelet [Ca2+]i increase at 37°C, but no response at lower temperatures (20°C to 25°C). This temperature dependence could not be attributed to a reduction in vWF binding, as ristocetin-mediated platelet aggregation and agglutination were essentially unaffected by temperature. Most other platelet agonists (U-46619, -thrombin, and adenosine 5′-diphosphate [ADP]) induced a [Ca2+]isignal whose amplitude did not diminish at lower temperatures. The [Ca2+]i signal in response to arachidonic acid, however, showed similar temperature dependence to that seen with vWF. Assessment of thromboxane A2 production showed a strong temperature dependence for metabolism of arachidonic acid by the cyclo-oxygenase pathway. vWF induced thromboxane A2production in the platelet. Aspirin treatment abolished the vWF-induced [Ca2+]i signal. These observations suggest that release of arachidonic acid and its conversion to thromboxane A2 play a central role in vWF-mediated [Ca2+]i signaling in the platelet at physiological temperatures.

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

Stimulation of human platelets with von Willebrand factor (vWF) induced the translocation of the small GTPases Rap1B and Rap2B to the cytoskeleton. This effect was specifically prevented by an anti-glycoprotein Ib monoclonal antibody or by the omission of stirring, but was not affected by the peptide RGDS, which antagonizes binding of adhesive proteins to platelet integrins. Association of Rap2B with the cytoskeleton was very rapid, while translocation of Rap1B occurred in a later phase of platelet activation and was totally inhibited by cytochalasin D. vWF also induced the rapid tyrosine phosphorylation of several proteins that was prevented by the tyrosine kinases inhibitor genistein and by cAMP-increasing agents. Under these conditions, also the association of Rap1B and Rap2B with the cytoskeleton was prevented. Translocation of Rap proteins to the cytoskeleton induced by vWF, but not by thrombin, was inhibited by a monoclonal antibody against the FcgammaII receptor. The same antibody inhibited vWF-induced tyrosine phosphorylation of selected substrates with molecular masses of about 75, 95, and 150 kDa. Three of these substrates were identified as the tyrosine kinase pp72(syk), the phospholipase Cgamma2, and the inositol 5-phosphatase SHIP. Our results indicate that translocation of Rap1B and Rap2B to the cytoskeleton is regulated by tyrosine kinases and suggest a novel role for the FcgammaII receptor in the mechanism of platelet activation by vWF.

Ristocetin-mediated interaction of human von Willebrand factor with platelet glycoprotein lb evokes a transient calcium signal: observations with Fura-PE3

High shear stress in narrowed arteries causes von Willebrand factor (vWf) to bind to its platelet receptor, glycoprotein Ib (GpIb). This binding is reported to promote an increase in intracellular free calcium concentration ((Ca2+)i), which may be responsible for platelet activation. The present study examined the platelet (Ca2+)i signal that arises when ristocetin mediates vWf-GpIb binding. Platelet (Ca2+)i was monitored with Fura-PE3 (Vorndran C, Minta A, Poenie M. Biophys J 1995;69:2112-24), a new ratiometric calcium indicator. Fura-PE3 has calcium-binding characteristics (Kd = 146 nmol/L) and fluorescent properties similar to those of Fura-2. However, its zwitterionic nature ensured much slower extrusion from the platelet (0.2% per minute) than that for Fura-2. This eliminated one of the technical problems that seriously distorted previous measurements of vWf-induced changes in platelets (Ca2+)i. Design of a novel stirring arrangement avoided the other major problem, which is the tendency of platelet aggregates to settle to the bottom of the cuvette, beneath the detection zone of the spectrofluorometer. With Fura-PE3 and the new stirrer used in the present study, vWf-induced changes in (Ca2+)i could be measured reliably in aggregating platelets. Ristocetin-mediated vWf-GpIb binding induced a transient increase in platelet (Ca2+)i. This increase occurred after a significant lag phase; platelet (Ca2+)i rose gradually, followed by a decline to almost the resting level. Binding of vWf to platelet Gplb was responsible for the (Ca2+)i signal. A similar signal was found in the absence of extracellular calcium. These characteristics differ substantially from those described in previous reports, in which the vWf-induced rise in (Ca2+)i was attributed to calcium influx through channels in the plasma membrane. Data from those earlier studies, however, were severely distorted by indicator extrusion and loss of platelet aggregates. The present findings are a more accurate representation of the vWf-induced platelet (Ca2+]i signal.

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.

Molecular mechanisms of platelet adhesion and activation

When a blood vessel is injured, control of bleeding starts with the rapid adhesion of circulating platelets to the site of damage. Within seconds, the adhered platelets are activated, secrete the contents of storage organelles, spread out over the damaged area and recruit more platelets to the developing thrombus. However, if this same process occurs in a diseased, sclerotic or occluded vessel, the resulting platelet thrombus may break away and block the coronary artery, causing a heart attack, or restrict blood supply to the brain, causing a stroke. The glycoprotein (GP) Ib-IX-V complex, a member of the leucine-rich protein family, is a constitutive platelet membrane receptor for von Willebrand Factor (vWF), a multimeric adhesive glycoprotein found in the matrix underlying the endothelial cell lining of the blood vessel wall and in the plasma. Binding of vWF to the GP. Ib-IX-V complex regulates adhesion of platelets to the subendothelium at high shear flow, and initiates signal transduction leading to platelet activation. The GP Ib-IX-V complex also constitutes a binding site for alpha-thrombin, an interaction that facilitates thrombin-dependent platelet activation. This review will focus on recent detailed analysis of the GP Ib-IX-V complex and vWF that has identified discrete amino acid sequences that mediate their interaction. An anionic/sulfated tyrosine sequence of the GP Ib alpha-chain that is critical for binding of the GP Ib-IX-V complex to both vWF and alpha-thrombin is analogous to sulfated anionic amino acid sequences mediating interactions of other adhesive proteins, including P-selectin binding to PSGL-1 and Factor VIII binding to vWF.

Integrin-mediated calcium signaling and regulation of cell adhesion by intracellular calcium

Integrins are ubiquitous trans-membrane adhesion molecules that mediate the interaction of cells with the extracellular matrix (ECM). Integrins link cells to the ECM by interacting with the cell cytoskeleton. In cases such as leukocyte binding, integrins mediate cell-cell interactions and cell-ECM interactions. Recent research indicates that integrins also function as signal transduction receptors, triggering a number of intracellular signaling pathways that regulate cell behavior and development. A number of integrins are known to stimulate changes in intracellular calcium levels, resulting in integrin activation. Although changes in intracellular calcium regulate a vast number of cellular functions, this review will discuss the stimulation of calcium signaling by integrins and the role of intracellular calcium in the regulation of integrin-mediated adhesion.

Mechanisms of integrin-mediated calcium signaling in MDCK cells: regulation of adhesion by IP3- and store-independent calcium influx

Peptides containing Arg-Gly-Asp (RGD) immobilized on beads bind to integrins and trigger biphasic, transient increases in intracellular free Ca2+ ([Ca2+]i) in Madin-Darby canine kidney epithelial cells. The [Ca2+]i increase participates in feedback regulation of integrin-mediated adhesion in these cells. We examined influx pathways and inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ store release as possible sources of the [Ca2+]i rise. The RGD-induced [Ca2+]i response requires external Ca2+ (threshold approximately 150 microM), and its magnitude is proportional to extracellular calcium. RGD-induced transients were attenuated by Ca2+ channel inhibitors (Ni2+ and carboxy-amidotriazole) or by plasma membrane depolarization, indicating that Ca2+ influx contributes to the response. Loading cells with heparin reduced the size of RGD-induced [Ca2+]i transients, indicating that IP3-mediated release of Ca2+ from stores may also contribute to the RGD response. Depletion of Ca2+ stores with thapsigargin activated Ni(2+)-sensitive Ca2+ influx that might also be expected to occur after IP3-mediated depletion of stored Ca2-. However, RGD elicited a Ni(2+)-sensitive Ca2+ influx even after pretreatment with thapsigargin, indicating that Ca2+ influx is controlled by a mechanism independent of IP3-mediated store depletion. We conclude that RGD-induced [Ca2+]i transients in Madin-Darby canine kidney cells result primarily from the combination of two distinct mechanisms: 1) IP3-mediated release of intracellular stores, and 2) activation of a Ca2+ influx pathway regulated independently of IP3 and Ca2+ store release. Because Ni2+ and carboxy-amidotriazole inhibited adhesion, whereas store depletion with thapsigargin had little effect, we suggest that the Ca2+ influx mechanism is most important for feedback regulation of integrin-mediated adhesion by increased [Ca2+]i.

Fluorescent indicators give biased estimates of intracellular free calcium change in aggregating platelets: implication for studies with human von Willebrand factor

The ratiometric fluorescent indicators Fura-2 and Indo-1 are considered optimal probes for monitoring intracellular free calcium concentration ([Ca2+]i). Unique problems arise, however, in studying [Ca2+]i changes induced in platelets by von Willebrand factor (vWF). Binding of native multimeric vWF causes extensive platelet aggregation, and is reported to evoke a gradual [Ca2+]i increase. the present investigation examined the reliability of platelet [Ca2+]i measurements in these circumstances. Ristocetin-mediated binding of vWF to human platelets promoted a slow rise in Fura-2 fluorescence ratio. Fura-2 extrusion contributed substantially to this rise, unless blocked by probenecid. Despite this precaution, the platelets were invariably contaminated slightly with extracellular indicator. As aggregation progressively reduced the number of platelets in the spectrofluorometer beam, through settling of the larger aggregates, such extracellular Fura-2 contributed proportionately more to the observed fluorescence. This extraneous signal accounted completely for the fluorescence ratio increase, and apparent [Ca2+]i rise, in response to native multimeric vWF. The same problem arose with Indo-1, whereas the single wavelength indicator Fluo-3 showed the opposite pattern of apparent [Ca2+]i changes. Thus, none of these indicators provides reliable data on [Ca2+]i signals in aggregating platelets. Use of a dimeric form of vWF eliminated the problem of platelet aggregates settling out of suspension, but also virtually abolished the [Ca2+]i increase. These observations may explain some of the inconsistencies among previous investigations of vWF-induced calcium signaling. Moreover, similar problems may arise in studies with other adhesive proteins.