Evidence that phospholipase C-gamma2 interacts with SLP-76, Syk, Lyn, LAT and the Fc receptor gamma-chain after stimulation of the collagen receptor glycoprotein VI in human platelets

Material matters: exploring the interplay between natural biomaterials and host immune system

Biomaterials are widely used for various medical purposes, for instance, implants, tissue engineering, medical devices, and drug delivery systems. Natural biomaterials can be obtained from proteins, carbohydrates, and cell-specific sources. However, when these biomaterials are introduced into the body, they trigger an immune response which may lead to rejection and failure of the implanted device or tissue. The immune system recognizes natural biomaterials as foreign substances and triggers the activation of several immune cells, for instance, macrophages, dendritic cells, and T cells. These cells release pro-inflammatory cytokines and chemokines, which recruit other immune cells to the implantation site. The activation of the immune system can lead to an inflammatory response, which can be beneficial or detrimental, depending on the type of natural biomaterial and the extent of the immune response. These biomaterials can also influence the immune response by modulating the behavior of immune cells. For example, biomaterials with specific surface properties, such as charge and hydrophobicity, can affect the activation and differentiation of immune cells. Additionally, biomaterials can be engineered to release immunomodulatory factors, such as anti-inflammatory cytokines, to promote a tolerogenic immune response. In conclusion, the interaction between biomaterials and the body's immune system is an intricate procedure with potential consequences for the effectiveness of therapeutics and medical devices. A better understanding of this interplay can help to design biomaterials that promote favorable immune responses and minimize adverse reactions.

The prohibitin-binding compound fluorizoline affects multiple components of the translational machinery and inhibits protein synthesis

Fluorizoline (FLZ) binds to prohibitin-1 and -2 (PHB1/2), which are pleiotropic scaffold proteins known to affect signaling pathways involved in several intracellular processes. However, it is not yet clear how FLZ exerts its effect. Here, we show that exposure of three different human cancer cell lines to FLZ increases the phosphorylation of key translation factors, particularly of initiation factor 2 (eIF2) and elongation factor 2 (eEF2), modifications that inhibit their activities. FLZ also impaired signaling through mTOR complex 1, which also regulates the translational machinery, e.g. through the eIF4E-binding protein 4E-BP1. In line with these findings, FLZ potently inhibited protein synthesis. We noted that the first phase of this inhibition involves very rapid eEF2 phosphorylation, which is catalyzed by a dedicated Ca²⁺-dependent protein kinase, eEF2 kinase (eEF2K). We also demonstrate that FLZ induces a swift and marked rise in intracellular Ca²⁺ levels, likely explaining the effects on eEF2. Disruption of normal Ca²⁺ homeostasis can also induce endoplasmic reticulum stress, and our results suggest that induction of this stress response contributes to the increased phosphorylation of eIF2, likely because of activation of the eIF2-modifying kinase PKR-like endoplasmic reticulum kinase (PERK). We show that FLZ induces cancer cell death and that this effect involves contributions from the phosphorylation of both eEF2 and eIF2. Our findings provide important new insights into the biological effects of FLZ and thus the roles of PHBs, specifically in regulating Ca²⁺ levels, cellular protein synthesis, and cell survival.

Tyrosine Kinase Inhibitor Pazopanib Inhibits Platelet Procoagulant Activity in Renal Cell Carcinoma Patients

Pazopanib is an angiostatic tyrosine kinase inhibitor (TKI) presently used for cancer treatment, particularly in patients with renal cell carcinoma (RCC). This treatment can be accompanied by mild bleeding as an adverse effect. Given the role of protein tyrosine kinases in platelet activation processes, we investigated whether and how pazopanib can affect platelet functions in purified systems and during treatment of advanced RCC patients. In isolated platelets from healthy volunteers, pazopanib dose-dependently reduced collagen-induced integrin activation and secretion, as well as platelet aggregation. Pazopanib addition diminished glycoprotein (GP) VI-dependent tyrosine phosphorylation of multiple platelet proteins, including the tyrosine kinase Syk. Furthermore, pazopanib inhibited GPVI-induced Ca²⁺ elevation, resulting in reduced exposure of the procoagulant phospholipid phosphatidylserine (PS). Upon perfusion of control blood over a collagen surface, pazopanib inhibited thrombus size as well as PS exposure. Blood samples from 10 RCC patients were also analyzed before and after 14 days of pazopanib treatment as monotherapy. This treatment caused an overall lowering in platelet count, with 3 out of 10 patients experiencing mild bleeding. Platelets isolated from pazopanib-treated patients showed a significant lowering of PS exposure upon activation. In addition, platelet procoagulant activity was inhibited in thrombi formed under flow conditions. Control experiments indicated that higher pazopanib concentrations were required to inhibit GPVI-mediated PS exposure in the presence of plasma. Together, these results indicated that pazopanib suppresses GPVI-induced platelet activation responses in a way partly antagonized by the presence of plasma. In treated cancer patients, pazopanib effects were confined to a reduction in GPVI-dependent PS exposure. Together with the reduced platelet count, this may explain the mild bleeding tendency observed in pazopanib-treated patients.

Platelet immunoreceptor tyrosine-based activation motif (ITAM) and hemITAM signaling and vascular integrity in inflammation and development

Platelets are essential for maintaining hemostasis following mechanical injury to the vasculature. Besides this established function, novel roles of platelets are becoming increasingly recognized, which are critical in non-injury settings to maintain vascular barrier integrity. For example, during embryogenesis platelets act to support the proper separation of blood and lymphatic vessels. This role continues beyond birth, where platelets prevent leakage of blood into the lymphatic vessel network. During the course of inflammation, platelets are necessary to prevent local hemorrhage due to neutrophil diapedesis and disruption of endothelial cell-cell junctions. Surprisingly, platelets also work to secure tumor-associated blood vessels, inhibiting excessive vessel permeability and intra-tumor hemorrhaging. Interestingly, many of these novel platelet functions depend on immunoreceptor tyrosine-based activation motif (ITAM) signaling but not on signaling via G protein-coupled receptors, which plays a crucial role in platelet plug formation at sites of mechanical injury. Murine platelets express two ITAM-containing receptors: the Fc receptor γ-chain (FcRγ), which functionally associates with the collagen receptor GPVI, and the C-type lectin-like 2 (CLEC-2) receptor, a hemITAM receptor for the mucin-type glycoprotein podoplanin. Human platelets express an additional ITAM receptor, FcγRIIA. These receptors share common downstream effectors, including Syk, SLP-76 and PLCγ2. Here we will review the recent literature that highlights a critical role for platelet GPVI/FcRγ and CLEC-2 in vascular integrity during development and inflammation in mice and discuss the relevance to human disease.

A critical role for the regulation of Syk from agglutination to aggregation in human platelets

Agglucetin, a tetrameric glycoprotein (GP) Ibα agonist from Formosan Agkistrodon acutus venom, has been characterized as an agglutination inducer in human washed platelets (WPs). In platelet-rich plasma (PRP), agglucetin dramatically elicits a biphasic response of agglutination and subsequent aggregation. For clarifying the intracellular signaling events from agglutination to aggregation in human platelets, we examined the essential signaling molecules involved through the detection of protein tyrosine phosphorylation (PTP). In WPs, an anti-GPIbα monoclonal antibody (mAb) AP1, but not a Src kinase inhibitor PP1, completely inhibited agglucetin-induced agglutination. However, PP1 but not AP1 had a potent suppression on platelet aggregation by a GPVI activator convulxin. The PTP analyses showed agglucetin alone can cause a weak pattern involving sequential phosphorylation of Lyn/Fyn, Syk, SLP-76 and phospholipase Cγ2 (PLCγ2). Furthermore, a Syk-selective kinase inhibitor, piceatannol, significantly suppressed the aggregating response in agglucetin-activated PRP. Analyzed by flow cytometry, the binding capacity of fluorophore-conjugated PAC-1, a mAb recognizing activated integrin αIIbβ3, was shown to increase in agglucetin-stimulated platelets. Again, piceatannol but not PP1 had a concentration-dependent suppression on agglucetin-induced αIIbβ3 exposure. Moreover, the formation of signalosome, including Syk, SLP-76, VAV, adhesion and degranulation promoting adapter protein (ADAP) and PLCγ2, are required for platelet aggregation in agglucetin/fibrinogen-activated platelets. In addition, GPIbα-ligation via agglucetin can substantially promote the interactions between αIIbβ3 and fibrinogen. Therefore, the signal pathway of Lyn/Fyn/Syk/SLP-76/ADAP/VAV/PLCγ2/PKC is sufficient to trigger platelet aggregation in agglucetin/fibrinogen-pretreated platelets. Importantly, Syk may function as a major regulator for the response from GPIbα-initiated agglutination to integrin αIIbβ3-dependent aggregation in human platelets.

Signal transducer and activator of transcription 3 (STAT3) regulates collagen-induced platelet aggregation independently of its transcription factor activity

BACKGROUND

Platelet hyperactivity induced by inflammation is a known risk factor for atherosclerosis and thrombosis, but its underlying mechanisms remain poorly understood.

METHODS AND RESULTS

The signal transducer and activator of transcription 3 (STAT3) was activated in collagen-stimulated platelets. Activated STAT3 served as a protein scaffold to facilitate the catalytic interaction between the kinase Syk (spleen tyrosine kinase) and the substrate PLCγ2 to enhance collagen-induced calcium mobilization and platelet activation. The same interaction of STAT3 with Syk and PLCγ2 was detected in HEK293 cells transfected with cDNAs for Syk and PLCγ2 and stimulated with interleukin-6. Pharmacological inhibition of STAT3 blocked ≈50% of collagen- and a collagen-related peptide-induced but not thrombin receptor-activating peptide- or ADP-induced aggregation and ≈80% of thrombus formation of human platelets on a collagen matrix. This in vitro phenotype was reproduced in mice infused with STAT3 inhibitors and mice with platelet-specific STAT3 deficiency. By forming a complex with its soluble receptor, the proinflammatory cytokine interleukin-6 enhanced the collagen-induced STAT3 activation in human platelets.

CONCLUSIONS

These data demonstrate a nontranscriptional activity of STAT3 that facilitates a crosstalk between proinflammatory cytokine and hemostasis/thrombosis signals in platelets. This crosstalk may be responsible for the platelet hyperactivity found in conditions of inflammation.

Phospholipase Cγ2 plays a role in TCR signal transduction and T cell selection

One of the important signaling events following TCR engagement is activation of phospholipase Cγ (PLCγ). PLCγ has two isoforms, PLCγ1 and PLCγ2. It is known that PLCγ1 is important for TCR signaling and TCR-mediated T cell selection and functions, whereas PLCγ2 is critical for BCR signal transduction and BCR-mediated B cell maturation and functions. In this study, we report that PLCγ2 was expressed in primary T cells, and became associated with linker for activated T cells and Src homology 2-domain containing leukocyte protein of 76 kDa and activated upon TCR stimulation. PLCγ1/PLCγ2 double-deficient T cells displayed further block from CD4 and CD8 double-positive to single-positive transition compared with PLCγ1 single-deficient T cells. TCR-mediated proliferation was further impaired in PLCγ1/PLCγ2 double-deficient T cells compared with PLCγ1 single-deficient T cells. TCR-mediated signal transduction, including Ca²⁺ mobilization and Erk activation, was further impaired in PLCγ1/PLCγ2 double-deficient relative to PLCγ1 single-deficient T cells. In addition, in HY TCR transgenic mouse model, thymic positive and negative selections were reduced in PLCγ1 heterozygous- and PLCγ2 homozygous-deficient (PLCγ1⁺/⁻PLCγ2⁻/⁻) relative to wild-type, PLCγ2 single-deficient (PLCγ2⁻/⁻), or PLCγ1 heterozygous-deficient (PLCγ1⁺/⁻) mice. Taken together, these data demonstrate that PLCγ2 participates in TCR signal transduction and plays a role in T cell selection.

Curcumin inhibits GPVI-mediated platelet activation by interfering with the kinase activity of Syk and the subsequent activation of PLCgamma2

Turmeric (Curcuma longa), a herbal remedy and culinary spice, has been used in traditional Indian culture for millennia. An active ingredient found in turmeric is curcumin (diferuloylmethane). In the current study, we investigated the antiplatelet properties of this naturally occurring compound. Curcumin inhibited human platelet aggregation and dense granule secretion induced by GPVI agonist convulxin in a concentration-dependent manner. At 50 microM, it effectively inhibited the maximal extent of aggregation and dense granule secretion to as much as 75%. It also dramatically inhibited the activation-dependent tyrosine phosphorylation of Y753 and Y759 on PLCgamma2, but did not affect the phosphorylation of Y145 residue on the cytosolic adaptor protein SLP-76. Interestingly, curcumin had no significant effect on the phosphorylation of Y525/Y526 present on the activation loop of Syk (spleen tyrosine kinase), but had a significant inhibitory effect on in vitro Syk kinase activity. Moreover, the inhibitory action of curcumin is not due to an inhibition of thromboxane generation because all our studies were performed using aspirin-treated platelets. We conclude that curcumin inhibits platelet activation induced by GPVI agonists through interfering with the kinase activity of Syk and the subsequent activation of PLCgamma2.

LAB/NTAL/Lat2: a force to be reckoned with in all leukocytes?

LAB/NTAL/Lat2 is a transmembrane adaptor protein closely related to LAT. It is expressed in various myeloid and lymphoid cells, many of which also express LAT. Phosphorylation of LAB occurs following engagement of various ITAM- and non-ITAM-linked receptors and can play positive and negative roles following receptor engagement. LAT binds PLCγ directly, resulting in efficient Ca²+ flux and degranulation. However, LAB does not contain a PLCγ-binding motif and only binds PLCγ indirectly, possibly via Grb2, thereby resulting in suboptimal signaling. As LAT can signal more efficiently than LAB, competition between the 2 for space/substrates in the lipid rafts can attenuate signaling. This competition model requires coexpression of LAT; however, LAB is repressive, even in cells lacking substantial LAT expression such as macrophages and mature B cells. The reported interaction between LAB and the ubiquitin E3-ligase c-Cbl suggests 1 possible mechanism for LAT-independent inhibition by LAB, but such a model requires further investigation. Given the wide-reaching expression pattern of LAB, LAB has the ability to modulate signaling in virtually every type of leukocyte. Regardless of its ultimate mode of action, the potent regulatory capability of LAB proves this protein to be a complex adaptor that warrants continued, substantial scrutiny by biochemists and immunologists alike.

Non-genomic effects of PPARgamma ligands: inhibition of GPVI-stimulated platelet activation

BACKGROUND

Peroxisome proliferator-activated receptor-(gamma) (PPAR(gamma)) is expressed in human platelets although in the absence of genomic regulation in these cells, its functions are unclear.

OBJECTIVE

In the present study, we aimed to demonstrate the ability of PPAR(gamma) ligands to modulate collagen-stimulated platelet function and suppress activation of the glycoprotein VI (GPVI) signaling pathway.

METHODS

Washed platelets were stimulated with PPAR(gamma) ligands in the presence and absence of PPAR(gamma) antagonist GW9662 and collagen-induced aggregation was measured using optical aggregometry. Calcium levels were measured by spectrofluorimetry in Fura-2AM-loaded platelets and tyrosine phosphorylation levels of receptor-proximal components of the GPVI signaling pathway were measured using immunoblot analysis. The role of PPAR(gamma) agonists in thrombus formation was assessed using an in vitro model of thrombus formation under arterial flow conditions.

RESULTS

PPAR(gamma) ligands inhibited collagen-stimulated platelet aggregation that was accompanied by a reduction in intracellular calcium mobilization and P-selectin exposure. PPAR(gamma) ligands inhibited thrombus formation under arterial flow conditions. The incorporation of GW9662 reversed the inhibitory actions of PPAR(gamma) agonists, implicating PPAR(gamma) in the effects observed. Furthermore, PPAR(gamma) ligands were found to inhibit tyrosine phosphorylation levels of multiple components of the GPVI signaling pathway. PPAR(gamma) was found to associate with Syk and LAT after platelet activation. This association was prevented by PPAR(gamma) agonists, indicating a potential mechanism for PPAR(gamma) function in collagen-stimulated platelet activation.

CONCLUSIONS

PPAR(gamma) agonists inhibit the activation of collagen-stimulation of platelet function through modulation of early GPVI signalling.

The novel Syk inhibitor R406 reveals mechanistic differences in the initiation of GPVI and CLEC-2 signaling in platelets

BACKGROUND

Syk is a key mediator of signaling pathways downstream of several platelet surface receptors including GPVI/FcRgamma collagen receptor, the C-type lectin receptor CLEC-2, and integrin alphaIIbbeta3. A recent study identified the novel small molecule R406 as a selective inhibitor of Syk.

OBJECTIVES

The present study evaluates the role of Syk in human platelets using the novel inhibitor R406.

METHODS

Agonist-induced GPVI and CLEC-2 signaling were assessed using aggregometry, immunoprecipitation and western blotting to determine the effects of R406 on platelet activation.

RESULTS

We demonstrate R406 to be a powerful inhibitor of Syk in human platelets. R406 abrogated shape change and aggregation induced by activation of GPVI and CLEC-2, and reduced platelet spreading on fibrinogen. The inhibitory effect of R406 was associated with inhibition of tyrosine phosphorylation of signaling proteins that lay downstream of Syk for all three receptors, including PLCgamma2. Strikingly, R406 markedly inhibited tyrosine phosphorylation of CLEC-2 and Syk downstream of CLEC-2 activation, whereas phosphorylation of Syk downstream of GPVI and integrin alphaIIbbeta3 was unaffected.

CONCLUSIONS

The inhibitory effect of R406 provides direct evidence of a role for Syk in GPVI, CLEC-2 and integrin alphaIIbbeta3 signaling in human platelets. Further, the results demonstrate a critical role for Syk in mediating tyrosine phosphorylation of CLEC-2, suggesting a novel model in which both Src and Syk kinases regulate tyrosine phosphorylation of the C-type lectin receptor leading to platelet activation.

Phosphorylation regulates tau interactions with Src homology 3 domains of phosphatidylinositol 3-kinase, phospholipase Cgamma1, Grb2, and Src family kinases

The microtubule-associated protein tau can associate with various other proteins in addition to tubulin, including the SH3 domains of Src family tyrosine kinases. Tau is well known to aggregate to form hyperphosphorylated filamentous deposits in several neurodegenerative diseases (tauopathies) including Alzheimer disease. We now report that tau can bind to SH3 domains derived from the p85alpha subunit of phosphatidylinositol 3-kinase, phospholipase Cgamma1, and the N-terminal (but not the C-terminal) SH3 of Grb2 as well as to the kinases Fyn, cSrc, and Fgr. However, the short inserts found in neuron-specific isoforms of Src prevented the binding of tau. The experimentally determined binding of tau peptides is well accounted for when modeled into the peptide binding cleft in the SH3 domain of Fyn. After phosphorylation in vitro or in transfected cells, tau showed reduced binding to SH3 domains; no binding was detected with hyperphosphorylated tau isolated from Alzheimer brain, but SH3 binding was restored by phosphatase treatment. Tau mutants with serines and threonines replaced by glutamate, to mimic phosphorylation, showed reduced SH3 binding. These results strongly suggest that tau has a potential role in cell signaling in addition to its accepted role in cytoskeletal assembly, with regulation by phosphorylation that may be disrupted in the tauopathies including Alzheimer disease.

Platelet integrins and immunoreceptors

Stable platelet adhesion to extracellular matrices and the formation of a hemostatic or pathological thrombus are dependent on integrin alphaIIbbeta3, also known as GPIIb-IIIa. However, maximal platelet responses to vascular injury may involve the participation of other integrins expressed in platelets (alphaVbeta3, alpha2beta1, alpha5beta1, and alpha6beta1). Platelet membrane 'immunoreceptors' contain at least one subunit with an extracellular immunoglobulin superfamily domain and/or an intracellular stimulatory immunoreceptor tyrosine-based activation motif (ITAM) or immunoreceptor tyrosine-based inhibitory motif (ITIM). Platelet ITAM receptors, such as FcgammaRIIA and the GPVI-FcRgamma complex, promote activation of integrins, while ITIM receptors, such as platelet-endothelial cell adhesion molecule-1, may promote their inhibition. This review summarizes the structure and function of platelet integrins and immunoreceptors, the emerging functional relationships between these receptor classes, and the consequences of their interaction for platelet function in hemostasis and thrombosis.

Glycoprotein VI agonists have distinct dependences on the lipid raft environment

BACKGROUND

It has been reported that the association of glycoprotein VI (GPVI) with lipid rafts regulates GPVI signaling in platelets.

OBJECTIVE

Secreted adenosine 5'-diphosphate (ADP) potentiates GPVI-induced platelet aggregation at particular agonist concentrations. We have investigated whether the decrease in GPVI signaling, previously reported in platelets with disrupted rafts, is a result of the loss of agonist potentiation by ADP.

METHODS

We disrupted platelet lipid rafts with methyl-beta-cyclodextrin and measured signaling events downstream of GPVI activation.

RESULTS

Lipid raft disruption decreases aggregation induced by low concentrations of convulxin, but this decrease is almost eliminated in the presence of ADP antagonists. Signaling indicators, such as protein phosphorylation and calcium mobilization, were not affected by raft disruption in collagen or convulxin stimulated platelets. Interestingly, however, raft disruption directly reduced GPVI signaling induced by collagen-related peptide.

CONCLUSIONS

Lipid rafts do not directly contribute to signaling by the physiologic agonist collagen. The effects of disruption of lipid rafts in in vitro assays can be attributed to inhibition of ADP feedback that potentiates GPVI signaling.

Effects of DK-002, a synthesized (6aS,cis)-9,10-Dimethoxy-7,11b-dihydro-indeno[2,1-c]chromene-3,6a-diol, on platelet activity

In the present study, the mechanism of antiplatelet activity of DK-002, a synthesized (6aS,cis)-9,10-Dimethoxy-7,11b-dihydro-indeno[2,1-c]chromene-3,6a-diol, was investigated. DK-002 inhibited the thrombin, collagen, and ADP-induced rat platelet aggregation in a concentration-dependent manner, with IC50 values of 120, 27, and 47 microM, respectively. DK-002 also inhibited thrombin-induced dense granule secretion, thromboxane A2 synthesis, and [Ca2+]i elevation in platelets. DK-002 did not show any significant effect on ADP-induced inhibition of cyclic AMP elevation by prostaglandin E1, but DK-002 was confirmed to inhibit ADP-induced [Ca2+]i elevation and shape change. DK-002 inhibited 4-bromo-A23187-induced [Ca2+]i elevation in the presence of creatine phosphate/creatine phosphokinase (CP/CPK, a ADP scavenging system) and indomethacin (a specific inhibitor of cyclooxygenase). DK-002 also inhibited Ca2+ mobilization in thrombin- or 4-bromo-A23187-stimulated platelets through its inhibitory effects on both Ca2+ release from intracellular stores and Ca2+ influx, in the presence of CP/CPK and indomethacin. Taken together, the present study shows that DK-002 has inhibitory effects on stimulation of platelets, and suggests that its antiplatelet activity might be related to the inhibitory mechanism on Ca2+ mobilization in stimulated platelets.

Importance of Platelet Phospholipase Cγ2 Signaling in Arterial Thrombosis as a Function of Lesion Severity

Objective— Platelet activation occurs in response to adhesion receptors for von Willebrand factor (GPIb-V-IX) and collagen (GPVI and α 2 β 1 integrin) acting upstream of phospholipase C (PLC) γ2. However, PLCβ transduces signals from Gαq protein-coupled receptors for soluble agonists (P2y 1 , TxA 2 /TP, and thrombin/PAR). A Gi-dependent pathway amplifies most of these responses.

Methods and Results— To evaluate the role of adhesion receptors signaling in arterial thrombosis, PLCγ2 knockout mice were studied in blood perfusion assays over fibrillar collagen and in a laser-induced mesenteric artery model of thrombosis. In vitro, PLCγ2-deficient platelets formed a single layer incapable of generating a thrombus on collagen, whereas Gαq-deficient platelets formed reduced size aggregates compared with wild-type cells. In the in vivo model, PLCγ2 −/− mice displayed defective thrombus formation in superficial lesions but productive thrombosis after a more severe laser injury. In contrast, resistance to thrombosis was observed in Gαq −/− mice in both levels of injury.

Conclusions— These results demonstrate that signaling through PLCγ2 plays an important role in arterial thrombosis, but that its contribution depends on the severity of the vascular lesion.

GPVI and alpha2beta1 play independent critical roles during platelet adhesion and aggregate formation to collagen under flow

The roles of the 2 major platelet-collagen receptors, glycoprotein VI (GPVI) and integrin alpha2beta1, have been intensely investigated using a variety of methods over the past decade. In the present study, we have used pharmacologic and genetic approaches to study human and mouse platelet adhesion to collagen under flow conditions. Our studies demonstrate that both GPVI and integrin alpha2beta1 play significant roles for platelet adhesion to collagen under flow and that the loss of both receptors completely ablates this response. Intracellular signaling mediated by the cytoplasmic adaptor Src homology 2 domain-containing leukocyte protein of 76 kDa (SLP-76) but not by the transmembrane adaptor linker for activation of T cells (LAT) is critical for platelet adhesion to collagen under flow. In addition, reduced GPVI receptor density results in severe defects in platelet adhesion to collagen under flow. Defective adhesion to collagen under flow is associated with prolonged tail-bleeding times in mice lacking one or both collagen receptors. These studies establish platelet-collagen responses under physiologic flow as the consequence of a close partnership between 2 structurally distinct receptors and suggest that both receptors play significant hemostatic roles in vivo.

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.

PRAM-1 is required for optimal integrin-dependent neutrophil function

PML-retinoic acid receptor alpha (RARalpha) regulated adaptor molecule 1 (PRAM-1) is an intracellular adaptor molecule that is upregulated during the induced granulocytic differentiation of promyelocytic leukemic cells and during normal human myelopoiesis. This report describes the generation of PRAM-1-deficient mice and an analysis of the function of this adaptor in neutrophil differentiation and mature neutrophil function. We demonstrate here that neutrophil differentiation is not impaired in PRAM-1-deficient mice and that PRAM-1-deficient neutrophils function normally following engagement of Fcgamma receptors. In contrast, mature PRAM-1-null neutrophils exhibit significant defects in adhesion-dependent reactive oxygen intermediate production and degranulation. Surprisingly, other integrin-dependent responses, such as cell spreading and activation of several signaling pathways, are normal. Together, these findings demonstrate the uncoupling of key integrin-dependent responses in the absence of PRAM-1 and show this adaptor to be critical for select integrin functions in neutrophils.

Ingestion of quercetin inhibits platelet aggregation and essential components of the collagen-stimulated platelet activation pathway in humans

BACKGROUND

Quercetin, a flavonoid present in the human diet, which is found in high levels in onions, apples, tea and wine, has been shown previously to inhibit platelet aggregation and signaling in vitro. Consequently, it has been proposed that quercetin may contribute to the protective effects against cardiovascular disease of a diet rich in fruit and vegetables.

OBJECTIVES

A pilot human dietary intervention study was designed to investigate the relationship between the ingestion of dietary quercetin and platelet function.

METHODS

Human subjects ingested either 150 mg or 300 mg quercetin-4'-O-beta-D-glucoside supplement to determine the systemic availability of quercetin. Platelets were isolated from subjects to analyse collagen-stimulated cell signaling and aggregation.

RESULTS

Plasma quercetin concentrations peaked at 4.66 microm (+/- 0.77) and 9.72 microm (+/- 1.38) 30 min after ingestion of 150-mg and 300-mg doses of quercetin-4'-O-beta-D-glucoside, respectively, demonstrating that quercetin was bioavailable, with plasma concentrations attained in the range known to affect platelet function in vitro. Platelet aggregation was inhibited 30 and 120 min after ingestion of both doses of quercetin-4'-O-beta-D-glucoside. Correspondingly, collagen-stimulated tyrosine phosphorylation of total platelet proteins was inhibited. This was accompanied by reduced tyrosine phosphorylation of the tyrosine kinase Syk and phospholipase Cgamma2, components of the platelet glycoprotein VI collagen receptor signaling pathway.

CONCLUSIONS

This study provides new evidence of the relatively high systemic availability of quercetin in the form of quercetin-4'-O-beta-D-glucoside by supplementation, and implicates quercetin as a dietary inhibitor of platelet cell signaling and thrombus formation.

Binding of Cytoplasmic Proteins to the CD19 Intracellular Domain Is High Affinity, Competitive, and Multimeric

CD19 is required for the development of B1 and marginal zone B cells, for Ab responses, and for B cell memory. CD19 immunoprecipitates contain a complex of cytoplasmic proteins, including Lyn, Vav, phospholipase Cgamma2 (PLCgamma2), Grb2, and the p85 subunit of phosphatidylinositol 3-kinase. Which of these bind directly to CD19 and the strengths of the interactions are unknown. These issues are important in understanding the signaling functions of CD19, which are crucial for normal B cell physiology. Using purified, recombinant proteins, we now show that each of these signaling proteins contains at least one Src homology 2 (SH2) domain that interacts directly with the phosphorylated CD19 cytoplasmic domain. The affinities of binding of the SH2 domains of Vav, p85, and Grb2 to CD19 are each in the nanomolar range by surface plasmon resonance (Biacore) analysis. Binding of Lyn and PLCgamma2 do not fit 1:1 modeling. However, analyses of binding data (Lyn) and competition experiments (PLCgamma2) suggest that these bind with comparable affinity. Competition experiments demonstrate that SH2 domains whose binding is dependent on the same CD19 tyrosine(s) compete for binding, but these SH2 domains do not impede binding of different SH2 domains to other CD19 tyrosines. We conclude that binding to the CD19 cytoplasmic domain is multimeric, high affinity, and competitive. The high affinity of the interactions also suggests that tyrosines that were nonessential in vivo are nevertheless functional. A preliminary structural model suggests that CD19 forms a signaling complex containing multiple cytoplasmic proteins in close proximity to each other and to the plasma membrane.

Important role of raft aggregation in the signaling events of cold-induced platelet activation

When human platelets are chilled below 20 degrees C, they undergo cold-induced activation. We have previously shown that cold activation correlates with the main phospholipid phase transition (10-20 degrees C) and induces the formation of large raft aggregates. In addition, we found that the glycoprotein CD36 is selectively enriched within detergent-resistant membranes (DRMs) of cold-activated platelets and is extremely sensitive to treatment with methyl-beta-cyclodextrin (MbetaCD). Here, we further studied the partitioning of downstream signaling molecules within the DRMs. We found that the phospholipase Cgamma2 (PLCgamma2) and the protein tyrosine kinase Syk do not partition exclusively within the DRMs, but their distribution is perturbed by cholesterol extraction. In addition, PLCgamma2 activity increases in cold-activated cells compared to resting platelets and is entirely inhibited after treatment with MbetaCD. The Src-family protein tyrosine kinases Src and Lyn preferentially partition within the DRMs and are profoundly affected by removal of cholesterol. These kinases are non-redundant in cold-activation. CD36, active Lyn, along with inactive Src and PLCgamma2 co-localize in small raft complexes in resting platelets. Cold-activation induces raft aggregation, resulting in changes in the activity of these proteins. These data suggest a crucial role of raft aggregation in the early events of cold-induced platelet activation.

T cell aggregation induced through CD43: intracellular signals and inhibition by the immunomodulatory drug leflunomide

The CD43 coreceptor molecule has been shown to participate in lymphocyte adhesion and activation. Leukocyte homotypic aggregation results from a cascade of intracellular signals delivered to the cells upon engagement of different cell-surface molecules with their natural ligands. This phenomenon requires an active metabolism, reorganization of the cytoskeleton, and relocalization of cell-surface molecules. The aim of this study was to identify some of the key members of the signaling cascade leading to T lymphocyte homotypic aggregation following CD43 engagement. CD43-mediated homotypic aggregation of T lymphocytes required the participation of Src kinases, phospholipase C-gamma2, protein kinase C, phosphatidylinositol-3 kinase, as well as extracellular-regulated kinase 1/2 and p38. Data shown here suggest that these signaling molecules play a central role in regulating actin cytoskeleton remodeling after CD43 ligation. We also evaluated the ability of immunomodulatory drugs such as leflunomide to block the CD43-mediated homotypic aggregation. Leflunomide blocked the recruitment of targets of the Src family kinases as well as actin polymerization, diminishing the ability of T lymphocytes to aggregate in response to CD43-specific signals, suggesting that this drug might control the migration and recruitment of lymphoid cells to inflamed tissues.

The tyrosine phosphatase 1B regulates linker for activation of T-cell phosphorylation and platelet aggregation upon FcgammaRIIa cross-linking

Human platelets express the receptor for immunoglobulin G, FcgammaRIIa, that triggers cell aggregation upon interaction with immune complexes. Here, we report that the rapid tyrosine phosphorylation of the Linker for Activation of T-cell (LAT) in human platelets stimulated by FcgammaRIIa cross-linking was followed by its complete dephosphorylation in an alphaIIb/beta3 integrin-dependent manner. Concomitant to LAT dephosphorylation, the protein tyrosine phosphatase 1B (PTP1B) was activated through a mechanism involving its proteolysis by calpains downstream of integrins. Both PTP1B and LAT were associated with the actin cytoskeleton complex formed during platelet aggregation. Moreover, phospho-LAT appeared as a good substrate of activated PTP1B in vitro and these two proteins interacted upon platelet activation by FcgammaRIIa cross-linking. The permeant substrate-trapping PTP1B (TAT-PTP1B D181A) partly inhibited LAT dephosphorylation in human platelets, strongly suggesting that this tyrosine phosphatase was involved in this regulatory pathway. Using a pharmacological inhibitor, we provide evidence that PTP1B activation and LAT dephosphorylation processes were required for irreversible platelet aggregation. Altogether, our results demonstrate that PTP1B plays an important role in the integrin-mediated dephosphorylation of LAT in human platelets and is involved in the control of irreversible aggregation upon FcgammaRIIa stimulation.

A critical role for phospholipase Cgamma2 in alphaIIbbeta3-mediated platelet spreading

The interaction of fibrinogen with the integrin alphaIIbbeta3 plays a crucial role in platelet adhesion and platelet activation leading to the generation of intracellular signals that nucleate the reorganization of the cytoskeleton. Presently, we have only a limited understanding of the signaling cascades and effector proteins through which changes in the cytoskeletal architecture are mediated. The present study identifies phospholipase Cgamma2 (PLCgamma2) as an important target of the Src-dependent signaling cascade regulated by alphaIIbbeta3. Real time phasecontrast microscopy is used to show that formation of filopodia and lamellapodia in murine platelets on a fibrinogen surface is dramatically inhibited in the absence of PLCgamma2. Significantly, the formation of these structures is mediated by Ca2+ elevation and activation of protein kinase C, both directly regulated by PLC activity. With the involvement of Syk, SLP-76, and Btk, alphaIIbbeta3-induced PLCgamma2 activation partly overlaps with the pathway used by the collagen receptor glycoprotein VI. Important differences, however, exist between the two signaling cascades in that activation of PLCgamma2 by alphaIIbbeta3 is unaltered in murine platelets, which lack the FcR gamma-chain or the adaptor LAT, but is abolished in the presence of cytochalasin D. Therefore, PLCgamma2 plays not only a crucial role in activation of alphaIIbbeta3 by collagen receptors but also in alphaIIbbeta3-mediated responses.

COAT platelets

PURPOSE OF REVIEW

COAT platelets are a recently described subpopulation of cells resulting from simultaneous activation with collagen and thrombin. The complete process by which COAT platelets are produced is still not clear, although significant recent progress has been made.

RECENT FINDINGS

COAT platelets retain several procoagulant proteins on their surface by a previously unrecognized mechanism involving transglutaminase mediated conjugation of serotonin to released alpha-granule proteins. Fibrinogen and thrombospondin have been found to bind serotonin-conjugated proteins and thereby provide the requisite link for stabilization of serotonin-derivatized, procoagulant proteins on COAT platelets.

SUMMARY

Multivalent interactions, resulting from traditional receptor interactions and binding of conjugated serotonin by fibrinogen and thrombospondin, result in exceptionally strong retention of procoagulant alpha-granule proteins on the surface of COAT platelets. The physiologic significance of this new subclass of platelets remains to be determined.

A PLC gamma 2-independent platelet collagen aggregation requiring functional association of GPVI and integrin alpha2beta1

The role of the phospholipase C (PLC)gamma 2 isotype in platelet activation was evaluated by studying PLC gamma 2 -/- mice. These mice have a prolonged bleeding time but their platelets respond normally to non-collagenous agonists. PLC gamma 2-null platelets show residual aggregation response to collagen fibres (6% versus 74% for wild-type) with minimal granule secretion and no shape change. A delayed shape change is observed at later aggregation times. Specific activation by glycoprotein (GP)VI agonists (convulxin, collagen-related peptide and GPVI crosslinking) is, however, abolished. Antibodies against integrin alpha(2)beta(1) and GPVI each inhibit the residual collagen response, implying a role of alpha(2)beta(1) in platelet activation and a functional association with GPVI. These responses are also prevented by blocking integrin alpha(IIb)beta(3) and phosphoinositide 3-kinase, whereas aspirin treatment and ADP receptor blockade only inhibit shape change. These results provide evidence for a PLC gamma 2-independent collagen activation pathway requiring cooperation between GPVI and alpha(2)beta(1) leading to alpha(IIb)beta(3)-dependent aggregation and shape change by released ADP and thromboxane A(2).

Fps/Fes and Fer non-receptor protein-tyrosine kinases regulate collagen- and ADP-induced platelet aggregation

Fps/Fes and Fer proto-oncoproteins are structurally related non-receptor protein-tyrosine kinases implicated in signaling downstream from cytokines, growth factors and immune receptors. We show that Fps/Fes and Fer are expressed in human and mouse platelets, and are activated following stimulation with collagen and collagen-related peptide (CRP), suggesting a role in GPVI receptor signaling. Fer was also activated following stimulation with thrombin and a protease-activated receptor4 (PAR4)-activating peptide, suggesting a role in signaling downstream from the G protein-coupled PAR4. There were no detectable perturbations in CRP-induced activation of Syk, PLCgamma2, cortactin, Erk, Jnk, Akt or p38 in platelets from mice lacking Fps/Fes, Fer, or both kinases. Platelets lacking Fps/Fes, from a targeted fps/fes null strain of mice, showed increased rates and amplitudes of collagen-induced aggregation, relative to wild-type platelets. P-Selectin expression was also elevated on the surface of Fps/Fes-null platelets in response to CRP. Fer-deficient platelets, from mice targeted with a kinase-inactivating mutation, disaggregated more rapidly than wild-type platelets in response to ADP. This report provides the first evidence that Fps/Fes and Fer are expressed in platelets and become activated downstream from the GPVI collagen receptor, and that Fer is activated downstream from a G-protein coupled receptor. Furthermore, using targeted mouse models we show that deficiency in Fps/Fes or Fer resulted in disregulated platelet aggregation and disaggregation, demonstrating a role for these kinases in regulating platelet functions.

Lipid rafts are critical membrane domains in blood platelet activation processes

Among the various hematopoi;etic cells, platelets are critical for maintaining the integrity of the vascular system. They must be rapidly activated by sequential and coordinated mechanisms in order to efficiently prevent haemorrhage upon vascular injury. Several signal transduction pathways lead to platelet activation in vitro and in vivo, among them, several are initiated via receptors or co-receptors containing immuno-receptor tyrosine-based activation motifs (ITAM) which trigger downstream signalling like the immune receptors in lymphocytes. However, in contrast to immune cells for which the role of lipid rafts in signalling has largely been described, the involvement of laterally segregated membrane microdomains in platelet activation has been investigated only recently. The results obtained until now strongly suggest that early steps of platelet activation via the collagen receptor GpVI or via FcgammaRIIa occur preferentially in these microdomains where specific proteins efficiently organize key downstream signalling pathways. In addition, lipid rafts also contribute to platelet activation via heterotrimeric G-protein-coupled receptors. They are sites where the phosphoinositide (PI) metabolism is highly active, leading to a local generation of lipid second messengers such as phosphatidylinositol 3,4,5-trisphosphate. Here, evidence is accumulating that cholesterol-enriched membrane microdomains are part of a general process that contributes to the efficiency and the coordination of platelet activation mechanisms. Here we will discuss the biochemical and functional characterizations of human platelet rafts and their potential impact in platelet physiopathology.

Differential role of glycolipid-enriched membrane domains in glycoprotein VI- and integrin-mediated phospholipase Cgamma2 regulation in platelets

The platelet collagen receptor glycoprotein VI (GPVI) and the fibrinogen receptor integrin alphaIIbbeta3 trigger intracellular signalling cascades involving the tyrosine kinase Syk, the adapter SLP-76 and phospholipase Cgamma2 (PLCgamma2). Similar pathways are activated downstream of immune receptors in lymphocytes, where they have been localized in part to glycolipid-enriched membrane domains (GEMs). Here we provide several lines of evidence that GPVI-mediated tyrosine phosphorylation of PLCgamma2 in platelets is dependent on GEM-organized signalling and utilizes the GEM resident adapter protein LAT (linker for activation of T cells). In sharp contrast, although fibrinogen binding to platelets stimulates alphaIIbbeta3-dependent activation of Syk and tyrosine phosphorylation of SLP-76 and PLCgamma2, it does not utilize GEMs to promote these responses or to support platelet aggregation. These results establish that GPVI and alphaIIbbeta3 trigger distinct patterns of receptor signalling in platelets, leading to tyrosine phosphorylation of PLCgamma2, and they highlight the role of GEMs in compartmentalizing signalling reactions involved in haemostasis.

Association of Fyn and Lyn with the proline-rich domain of glycoprotein VI regulates intracellular signaling

The glycoprotein VI (GPVI)-Fc receptor (FcR) gamma-chain complex, a key activatory receptor for collagen on platelet surface membranes, is constitutively associated with the Src family kinases Fyn and Lyn. Molecular cloning of GPVI has revealed the presence of a proline-rich domain in the sequence of GPVI cytoplasmic tail which has the consensus for interaction with the Src homology 3 (SH3) domains of Fyn and Lyn. A series of in vitro experiments demonstrated the ability of the SH3 domains of both Src kinases to bind the proline-rich domain of GPVI. Furthermore, depletion of the proline-rich domain in GPVI (Pro(-)-GPVI) prevented binding of Fyn and Lyn and markedly reduced phosphorylation of FcR gamma-chain in transiently transfected COS-7 cells, but did not affect the association of the gamma-chain with GPVI. Jurkat cells stably transfected with wild type GPVI show robust increases in tyrosine phosphorylation and intracellular Ca2+ in response to the snake venom convulxin that targets GPVI. Importantly, convulxin is not able to activate cells transfected with Pro(-)-GPVI, even though the association with the immunoreceptor tyrosine-based activation motif-containing chains is maintained. These findings demonstrate that the proline-rich domain of GPVI mediates the association with Fyn/Lyn via their SH3 domain and that this interaction initiates activation signals through GPVI.

The roles of LAT in platelet signaling induced by collagen, TxA2, or ADP

The work presented here demonstrates that platelets from mice lacking LAT (linker for the activation of T cells) show reversible aggregation in response to concentrations of collagen that cause TxA2/ADP-dependent irreversible aggregation of control platelets. The aggregation defect of the LAT-deficient platelets was shown to be the result of almost no TxA2 production and significantly diminished ADP secretion. In contrast, the LAT deficiency does not affect aggregation induced by high concentrations of collagen because that aggregation is not dependent on TxA2 and/or ADP. Even though ADP and TxA2 provide amplification signals for platelet activation in response to low concentrations of collagen, LAT-deficient platelets hyperaggregate to low levels of U46619, a TxA2 analog, or ADP. Though the mechanism(s) of costimulatory signals by collagen, ADP, and TxA2 remains unidentified, it is clear that LAT plays a positive role in collagen-induced, TxA2/ADP-dependent aggregation, and a negative role in TxA2 or ADP-induced platelet aggregation.

Regulation of phospholipase C-gamma 2 via phosphatidylinositol 3-kinase in macrophages

Phospholipase C-gamma (PLC-gamma) isoforms are thought to be activated by both tyrosine phosphorylation and phosphatidylinositol 3,4,5 trisphosphate (PtdIns 3,4,5 P(3)), the product of phosphatidylinositol 3-kinase (PtdIns 3-kinase). In this study, we show that stimulation of mouse macrophages with either zymosan beads or bacteria (Prevotella intermedia) induced tyrosine phosphorylation of PLC-gamma 2. Zymosan stimulation also induced translocation to membrane and cytoskeleton fractions, which was inhibited by the PtdIns 3-kinase inhibitors wortmannin and LY 294002. However, the tyrosine phosphorylation of PLC-gamma 2 induced by zymosan was not affected by the inhibitors wortmannin and LY 294002. In contrast to zymosan and bacteria, PLC-gamma 2 was not phosphorylated by stimulation with lipopolysaccharide (LPS), phorbol ester or calcium ionophore. Moreover, the PLC-gamma 1 isoform was not detected in mouse macrophages. These data indicate that PtdIns 3-kinase is critical for the translocation but not for the tyrosine phosphorylation of PLC-gamma 2 in mouse macrophages and that the latter may be insufficient for enzyme activation.

Differential requirement for SLP-76 domains in T cell development and function

The hematopoietic cell-specific adaptor protein, SLP-76, is critical for T cell development and mature T cell receptor (TCR) signaling; however, the structural requirements of SLP-76 for mediating thymopoiesis and mature T cell function remain largely unknown. In this study, transgenic mice were generated to examine the requirements for specific domains of SLP-76 in thymocytes and peripheral T cells in vivo. Examination of mice expressing various mutants of SLP-76 on the null background demonstrates a differential requirement for specific domains of SLP-76 in thymocytes and T cells and provides new insight into the molecular mechanisms underlying SLP-76 function.

Regulation of phospholipase C gamma isoforms in haematopoietic cells: why one, not the other?

Phospholipase C gamma (PLCgamma) isoforms are critical for the generation of calcium signals in haematopoietic systems in response to the stimulation of immune receptors. PLCgamma is unique amongst phospholipases in that it is tightly regulated by the action of a number of tyrosine kinases. It is itself directly phosphorylated on a number of tyrosines and contains several domains through which it can interact with other signalling proteins and lipid products such as phosphatidylinositol 3,4,5-trisphosphate. Through this network of interactions, PLCgamma is activated and recruited to its substrate, phosphatidylinositol 4,5-bisphosphate, at the membrane. Both isoforms of PLCgamma, PLCgamma1 and PLCgamma2, are present in haematopoietic cells. The signalling cascade involved in the regulation of these two isoforms varies between cells, though the systems are similar for both PLCgamma1 and PLCgamma2. We will compare these cascades for both PLCgamma1 and PLCgamma2 and discuss possible reasons as to why one form of PLCgamma and not the other is required for signalling in specific haematopoietic cells, including T lymphocytes, B lymphocytes, platelets, and mast cells.

High resolution mapping of mast cell membranes reveals primary and secondary domains of Fc(epsilon)RI and LAT

In mast cells, cross-linking the high-affinity IgE receptor (Fc(epsilon)RI) initiates the Lyn-mediated phosphorylation of receptor ITAMs, forming phospho-ITAM binding sites for Syk. Previous immunogold labeling of membrane sheets showed that resting Fc(epsilon)RI colocalize loosely with Lyn, whereas cross-linked Fc(epsilon)RI redistribute into specialized domains (osmiophilic patches) that exclude Lyn, accumulate Syk, and are often bordered by coated pits. Here, the distribution of Fc(epsilon)RI beta is mapped relative to linker for activation of T cells (LAT), Grb2-binding protein 2 (Gab2), two PLCgamma isoforms, and the p85 subunit of phosphatidylinositol 3-kinase (PI3-kinase), all implicated in the remodeling of membrane inositol phospholipids. Before activation, PLCgamma1 and Gab2 are not strongly membrane associated, LAT occurs in small membrane clusters separate from receptor, and PLCgamma2, that coprecipitates with LAT, occurs in clusters and along cytoskeletal cables. After activation, PLCgamma2, Gab2, and a portion of p85 colocalize with Fc(epsilon)RI beta in osmiophilic patches. LAT clusters enlarge within 30 s of receptor activation, forming elongated complexes that can intersect osmiophilic patches without mixing. PLCgamma1 and another portion of p85 associate preferentially with activated LAT. Supporting multiple distributions of PI3-kinase, Fc(epsilon)RI cross-linking increases PI3-kinase activity in anti-LAT, anti-Fc(epsilon)RIbeta, and anti-Gab2 immune complexes. We propose that activated mast cells propagate signals from primary domains organized around Fc(epsilon)RIbeta and from secondary domains, including one organized around LAT.

Phosphatidylinositol 3-kinase-dependent translocation of phospholipase Cgamma2 in mouse megakaryocytes is independent of Bruton tyrosine kinase translocation

Activation of the collagen receptor glycoprotein VI (GPVI) by a collagen-related peptide (CRP) induces stimulation of platelets and megakaryocytes through the phosphatidylinositol (PI) 3-kinase-dependent pathway leading to activation of Bruton tyrosine kinase (Btk) and phospholipase Cgamma2 (PLCgamma2). Here, we present evidence that both proteins undergo PI 3-kinase-dependent translocation to the plasma membrane on CRP stimulation that is markedly inhibited by wortmannin and LY294002. Translocation of PLCgamma2 but not Btk is also seen in megakaryocytes from X-linked immunodeficiency mice, which have a mutation that reduces the affinity of the pleckstrin homology (PH) domain of Btk for PI 3,4,5-trisphosphate (PI 3,4,5-P3). Activation of PC12 cells by epidermal growth factor (EGF) results in increased PI 3-kinase activity and high PI 3,4,5-P3 levels that trigger translocation of the green fluorescent protein (GFP)-labeled PH of Btk, but not the GFP-labeled PH and tandem Src homology 2 (SH2) domains of PLCgamma2. In contrast to the results with CRP, the G protein-coupled receptor agonist thrombin stimulates PI 3-kinase-independent translocation of Btk but not PLCgamma2. In conclusion, these results demonstrate that in mouse megakaryocytes, CRP leads to PI 3-kinase-dependent translocation of PLCgamma2 and Btk that are independent of one another, whereas thrombin only induces translocation of Btk through a pathway that is independent of PI 3-kinase activity.

Evidence for two distinct epitopes within collagen for activation of murine platelets

It has recently been shown that the monoclonal antibody JAQ1 to murine glycoprotein VI (GPVI) can cause aggregation of mouse platelets upon antibody cross-linking and that collagen-induced platelet aggregation can be inhibited by preincubation of platelets with JAQ1 in the absence of cross-linking (Nieswandt, B., Bergmeier, W., Schulte, V., Rackebrandt, K., Gessner, J. E., and Zirngibl, H. (2000) J. Biol. Chem. 275, 23998-24002). In the present study, we have shown that cross-linking of GPVI by JAQ1 results in tyrosine phosphorylation of the same profile of proteins as that induced by collagen, including the Fc receptor (FcR) gamma-chain, Syk, LAT, SLP-76, and phospholipase C gamma 2. In contrast, platelet aggregation and tyrosine phosphorylation of these proteins were inhibited when mouse platelets were preincubated with JAQ1 in the absence of cross-linking and were subsequently stimulated with a collagen-related peptide (CRP) that is specific for GPVI and low concentrations of collagen. However, at higher concentrations of collagen, but not CRP, aggregation of platelets and tyrosine phosphorylation of the above proteins (except for the adapter LAT) is re-established despite the presence of JAQ1. These observations suggest that a second activatory binding site, which is distinct from the CRP binding site on GPVI on mouse platelets, is occupied in the presence of high concentrations of collagen. Although this could be a second site on GPVI that is activated by a novel motif within the collagen molecule, the absence of LAT phosphorylation in response to collagen in the presence of JAQ1 suggests that this is more likely to be caused by activation of a second receptor that is also coupled to the FcR gamma-chain. The possibility that this response is mediated by a receptor that is not coupled to FcR gamma-chain is excluded on the grounds that aggregation is absent in platelets from FcR gamma-chain-deficient mice.

Roles of SLP-76, phosphoinositide 3-kinase, and gelsolin in the platelet shape changes initiated by the collagen receptor GPVI/FcRγ-chain complex

How platelet shape change initiated by a collagen-related peptide (CRP) specific for the GPVI/FcRγ-chain complex (GPVI/FcRγ-chain) is coupled to SLP-76, phosphoinositide (PI) 3-kinase, and gelsolin is reported. As shown by video microscopy, platelets rapidly round and grow dynamic filopodial projections that rotate around the periphery of the cell after they contact a CRP-coated surface. Lamellae subsequently spread between the projections. All the actin-driven shape changes require SLP-76 expression. SLP-76 is essential for the Ca++mobilization induced by CRP, whereas PI 3-kinase only modulates it. The extension of lamellae requires net actin assembly and an exposure of actin filament barbed ends downstream of PI 3-kinase. Gelsolin expression is also required for the extension of lamellae, but not for the formation of filopodia. Altogether, the data describe the role of SLP-76 in the platelet activation initiated by GPVI/FcRγ-chain and the roles of PI 3-kinase and gelsolin in lamellae spreading.

Roles of SLP-76, phosphoinositide 3-kinase, and gelsolin in the platelet shape changes initiated by the collagen receptor GPVI/FcRγ-chain complex

How platelet shape change initiated by a collagen-related peptide (CRP) specific for the GPVI/FcRγ-chain complex (GPVI/FcRγ-chain) is coupled to SLP-76, phosphoinositide (PI) 3-kinase, and gelsolin is reported. As shown by video microscopy, platelets rapidly round and grow dynamic filopodial projections that rotate around the periphery of the cell after they contact a CRP-coated surface. Lamellae subsequently spread between the projections. All the actin-driven shape changes require SLP-76 expression. SLP-76 is essential for the Ca++mobilization induced by CRP, whereas PI 3-kinase only modulates it. The extension of lamellae requires net actin assembly and an exposure of actin filament barbed ends downstream of PI 3-kinase. Gelsolin expression is also required for the extension of lamellae, but not for the formation of filopodia. Altogether, the data describe the role of SLP-76 in the platelet activation initiated by GPVI/FcRγ-chain and the roles of PI 3-kinase and gelsolin in lamellae spreading.

Interaction of linker for activation of T cells with multiple adapter proteins in platelets activated by the glycoprotein VI-selective ligand, convulxin

The snake venom toxin convulxin activates platelets through the collagen receptor glycoprotein VI (GPVI)/Fc receptor gamma-chain (FcR gamma-chain) complex leading to tyrosine phosphorylation and activation of the tyrosine Syk and phospholipase Cgamma2 (PLCgamma2). In the present study, we demonstrate that convulxin is a considerably more powerful agonist than collagen or the GPVI-selective collagen-related peptide (CRP). Confirmation that the response to convulxin is mediated solely via Syk was provided by studies on Syk-deficient platelets. The increase in phosphorylation of the FcR gamma-chain is associated with marked increases in tyrosine phosphorylation of downstream proteins including Syk, linker for activation of T cells (LAT), SLP-76, and PLCgamma2. The transmembrane adapter LAT coprecipitates with SLP-76 and PLCgamma2, as well as with a number of other adapter proteins, some of which have not been previously described in platelets, including Cbl, Grb2, Gads, and SKAP-HOM. Gads is constitutively associated with SLP-76 and is probably the protein bridging its association with LAT. There was no detectable association between Grb2 and SLP-76 in control or stimulated cells, suggesting that the interaction of LAT with Grb2 is present in a separate complex to that of LAT-Gads-SLP-76. These results show that the trimeric convulxin stimulates a much greater phosphorylation of the FcR gamma-chain and subsequent downstream responses relative to CRP and collagen, presumably because of its ability to cause a greater degree of cross-linking of GPVI. The adapter LAT appears to play a critical role in recruiting a number of other adapter proteins to the surface membrane in response to activation of GPVI, presumably at sites of glycolipid-enriched microdomains, enabling an organized signaling cascade that leads to platelet activation.

Phosphatidylinositol 3,4,5-trisphosphate regulates Ca(2+) entry via btk in platelets and megakaryocytes without increasing phospholipase C activity

The role of phosphatidylinositol 3,4,5-trisphosphate (PI3,4,5P(3)) and Btk in signalling by the collagen receptor glycoprotein VI was investigated. PI3,4,5P(3) was increased in platelets from mice deficient in the SH2 domain-containing inositol 5-phosphatase (SHIP), in response to collagen related peptide (CRP). Tyrosine phosphorylation and activation of phospholipase Cgamma2 (PLCgamma2) were unaltered in SHIP(-/-) platelets, whereas Btk was heavily tyrosine phosphorylated under basal conditions and maximally phosphorylated by low concentrations of CRP. There was an increase in basal Ca(2+), maximal expression of P-selectin, and potentiation of Ca(2+) and aminophospholipid exposure to CRP in SHIP(-/-) platelets in the presence of Ca(2+) (1 mM). Microinjection of PI3,4, 5P(3) into megakaryocytes caused a 3-fold increase in Ca(2+) in response to CRP, which was absent in X-linked immunodeficiency (Xid) mice, which have a mutation in the PH domain of Btk. There was a corresponding partial reduction in the sustained level of intracellular Ca(2+) in response to CRP in Xid mice but no change in PLC activity. These results demonstrate a novel pathway of Ca(2+) entry that involves PI3,4,5P(3) and Btk, and which is independent of increased PLC activity.

Regulation and Function of WASp in Platelets by the Collagen Receptor, Glycoprotein VI

Wiskott Aldrich syndrome (WAS) is an X-linked recessive disorder associated with abnormalities in platelets and lymphocytes giving rise to thrombocytopenia and immunodeficiency. WAS is caused by a mutation in the gene encoding the cytoskeletal protein (WASp). Despite its importance, the role of WASp in platelet function is not established. WASp was recently shown to undergo tyrosine phosphorylation in platelets after activation by collagen, suggesting that it may play a selective role in activation by the adhesion molecule. In the present study, we show that WASp is heavily tyrosine phosphorylated by a collagen-related peptide (CRP) that binds to the collagen receptor glycoprotein (GP) VI, but not to the integrin 2β1. Tyrosine phosphorylation of WASp was blocked by Src family kinase inhibitors and reduced by treatment with wortmannin and in patients with X-linked agammaglobulinemia (XLA), a condition caused by a lack of functional expression of Btk. This indicates that Src kinases, phosphatidylinositol 3-kinase (PI 3-kinase), and Btk all contribute to the regulation of tyrosine phosphorylation of WASp. The functional importance of WASp was investigated in 2 WAS brothers who show no detectable expression of WASp. Platelet aggregation and secretion from dense granules induced by CRP and thrombin was slightly enhanced in the WAS platelets relative to controls. Furthermore, there was no apparent difference in morphology in WAS platelets after stimulation by these agonists. These observations suggest that WASp does not play a critical role in intracellular signaling downstream of tyrosine kinase-linked and G protein-coupled receptors in platelets.