Convulxin induces platelet activation by a tyrosine-kinase-dependent pathway and stimulates tyrosine phosphorylation of platelet proteins, including PLC gamma 2, independently of integrin alpha IIb beta 3

RUNX1-deficient human megakaryocytes demonstrate thrombopoietic and platelet half-life and functional defects

Heterozygous defects in runt-related transcription factor 1 (RUNX1) are causative of a familial platelet disorder with associated myeloid malignancy (FPDMM). Because RUNX1-deficient animal models do not mimic bleeding disorder or leukemic risk associated with FPDMM, development of a proper model system is critical to understanding the underlying mechanisms of the observed phenotype and to identifying therapeutic interventions. We previously reported an in vitro megakaryopoiesis system comprising human CD34+ hematopoietic stem and progenitor cells that recapitulated the FPDMM quantitative megakaryocyte defect through a decrease in RUNX1 expression via a lentiviral short hairpin RNA strategy. We now show that shRX-megakaryocytes have a marked reduction in agonist responsiveness. We then infused shRX-megakaryocytes into immunocompromised NOD scid gamma (NSG) mice and demonstrated that these megakaryocytes released fewer platelets than megakaryocytes transfected with a nontargeting shRNA, and these platelets had a diminished half-life. The platelets were also poorly responsive to agonists, unable to correct thrombus formation in NSG mice homozygous for a R1326H mutation in von Willebrand Factor (VWFR1326H), which switches the species-binding specificity of the VWF from mouse to human glycoprotein Ibα. A small-molecule inhibitor RepSox, which blocks the transforming growth factor β1 (TGFβ1) pathway and rescued defective megakaryopoiesis in vitro, corrected the thrombopoietic defect, defects in thrombus formation and platelet half-life, and agonist response in NSG/VWFR1326H mice. Thus, this model recapitulates the defects in FPDMM megakaryocytes and platelets, identifies previously unrecognized defects in thrombopoiesis and platelet half-life, and demonstrates for the first time, reversal of RUNX1 deficiency-induced hemostatic defects by a drug.

Antineoplastic properties and pharmacological applications of Crotalus durissus terrificus snake venom

Snake toxins are widely studied owing to their importance in snakebite accidents, a serious public health issue in tropical countries, and their broad therapeutic potential. Isolated fractions from venom produced by snakes of the genus Crotalus sp. present a wide variety of pharmacological uses such as antifungal, antiviral, antibacterial, and antitumor properties, among other therapeutic potentialities. Given the direct effect of this venom on tumor cells, isolation of its compounds is important for the characterization of its anticarcinogenic actions. Crotalus durissus terrificus venom and its toxins have been widely evaluated as potential candidates for the development of new antineoplastic therapies that are efficient against different tumor lines and cellular targets. This review highlights the venom toxins of this species, with a focus on their antineoplastic properties.

Plasmodium falciparum infection induces expression of a mosquito salivary protein (Agaphelin) that targets neutrophil function and inhibits thrombosis without impairing hemostasis

Background

Invasion of mosquito salivary glands (SGs) by Plasmodium falciparum sporozoites is an essential step in the malaria life cycle. How infection modulates gene expression, and affects hematophagy remains unclear.

Principal Findings

Using Affimetrix chip microarray, we found that at least 43 genes are differentially expressed in the glands of Plasmodium falciparum-infected Anopheles gambiae mosquitoes. Among the upregulated genes, one codes for Agaphelin, a 58-amino acid protein containing a single Kazal domain with a Leu in the P1 position. Agaphelin displays high homology to orthologs present in Aedes sp and Culex sp salivary glands, indicating an evolutionarily expanded family. Kinetics and surface plasmon resonance experiments determined that chemically synthesized Agaphelin behaves as a slow and tight inhibitor of neutrophil elastase (K_D∼10 nM), but does not affect other enzymes, nor promotes vasodilation, or exhibit antimicrobial activity. TAXIscan chamber assay revealed that Agaphelin inhibits neutrophil chemotaxis toward fMLP, affecting several parameter associated with cell migration. In addition, Agaphelin reduces paw edema formation and accumulation of tissue myeloperoxidase triggered by injection of carrageenan in mice. Agaphelin also blocks elastase/cathepsin-mediated platelet aggregation, abrogates elastase-mediated cleavage of tissue factor pathway inhibitor, and attenuates neutrophil-induced coagulation. Notably, Agaphelin inhibits neutrophil extracellular traps (NETs) formation and prevents FeCl₃-induced arterial thrombosis, without impairing hemostasis.

Conclusions

Blockade of neutrophil elastase emerges as a novel antihemostatic mechanism in hematophagy; it also supports the notion that neutrophils and the innate immune response are targets for antithrombotic therapy. In addition, Agaphelin is the first antihemostatic whose expression is induced by Plasmodium sp infection. These results suggest that an important interplay takes place in parasite-vector-host interactions.

Malaria is transmitted by Plasmodium falciparum-infected Anopheles gambiae mosquitoes. Salivary gland contributes to the development of the parasite by creating a favorable environment for the infection and facilitating blood feeding and reproduction of the vector. However, the molecular mechanism by which the vector salivary gland modulates parasite/host interactions is not understood. We discovered that infection of the mosquito salivary gland upregulates several genes; among them, one codes for a protease inhibitor named Agaphelin. Notably, Agaphelin was found to exhibit multiple antihemostatic functions by targeting elastase. As a result, it inhibits platelet function which is required for blood to clot, and it prevents cleavage of TFPI, an anticoagulant that has recently been found to play a crucial role in thrombus formation in vivo. Agaphelin also attenuates neutrophils chemotaxis and the release of Neutrophil Extracellular Traps. These results provide evidence that neutrophils serve as a link between coagulation and the innate immune response. Agaphelin also exhibits anti-inflammatory and antithrombotic effects in vivo. Furthermore, Agaphelin did not promote bleeding, suggesting that targeting neutrophil exhibits potential therapeutic value. Altogether, these results highlight that the interplay between parasite, vector and host is a dynamic process that contributes and sustains the interface among Plasmodium, Anopheles and humans.

Salivary antigen-5/CAP family members are Cu2+-dependent antioxidant enzymes that scavenge O₂₋. and inhibit collagen-induced platelet aggregation and neutrophil oxidative burst

The function of the antigen-5/CAP family of proteins found in the salivary gland of bloodsucking animals has remained elusive for decades. Antigen-5 members from the hematophagous insects Dipetalogaster maxima (DMAV) and Triatoma infestans (TIAV) were expressed and discovered to attenuate platelet aggregation, ATP secretion, and thromboxane A2 generation by low doses of collagen (<1 μg/ml) but no other agonists. DMAV did not interact with collagen, glycoprotein VI, or integrin α2β1. This inhibitory profile resembles the effects of antioxidants Cu,Zn-superoxide dismutase (Cu,Zn-SOD) in platelet function. Accordingly, DMAV was found to inhibit cytochrome c reduction by O2[Symbol: see text] generated by the xanthine/xanthine oxidase, implying that it exhibits antioxidant activity. Moreover, our results demonstrate that DMAV blunts the luminescence signal of O2[Symbol: see text] generated by phorbol 12-myristate 13-acetate-stimulated neutrophils. Mechanistically, inductively coupled plasma mass spectrometry and fluorescence spectroscopy revealed that DMAV, like Cu,Zn-SOD, interacts with Cu(2+), which provides redox potential for catalytic removal of O2[Symbol: see text]. Notably, surface plasmon resonance experiments (BIAcore) determined that DMAV binds sulfated glycosaminoglycans (e.g. heparin, KD ~100 nmol/liter), as reported for extracellular SOD. Finally, fractions of the salivary gland of D. maxima with native DMAV contain Cu(2+) and display metal-dependent antioxidant properties. Antigen-5/CAP emerges as novel family of Cu(2+)-dependent antioxidant enzymes that inhibit neutrophil oxidative burst and negatively modulate platelet aggregation by a unique salivary mechanism.

Triplatin, a platelet aggregation inhibitor from the salivary gland of the triatomine vector of Chagas disease, binds to TXA(2) but does not interact with glycoprotein PVI

Salivary glands from haematophagous animals express a notable diversity of negative modulators of platelet function. Triplatin is an inhibitor of collagen-induced platelet aggregation which has been described as an antagonist of glycoprotein VI (GPVI). Because triplatin displays sequence homology to members of the lipocalin family of proteins, we investigated whether triplatin mechanism of action could be explained by interaction with pro-haemostatic prostaglandins. Our results demonstrate that triplatin inhibits platelet aggregation induced by low doses of collagen, thromboxane A2 (TXA(2)) mimetic (U46619), and arachidonic acid (AA). On the other hand, it does not inhibit platelet aggregation by convulxin, PMA, or low-dose ADP. Isothermal titration calorimetry (ITC) revealed that triplatin binds AA, cTXA(2), TXB(2), U46619 or prostaglandin (PG)H(2) mimetic (U51605). Consistent with its ligand specificity, triplatin induces relaxation of rat aorta contracted with U46619. Triplatin also interacts with PGF(2α) and PGJ(2), but not with leukotrienes, AA or biogenic amines. Surface plasmon resonance experiments failed to demonstrate interaction of triplatin with GPVI; it also did to inhibit platelet adhesion to fibrillar or soluble collagen. Because triplatin displays sequence similarity to apolipoprotein D (ApoD) - a lipocalin associated with high-density lipoprotein, ApoD was tested as a putative TXA(2)-binding molecule. ITC failed to demonstrate binding of ApoD to all prostanoids described above, or to AA. Furthermore, ApoD was devoid of inhibitory properties towards platelets activation by AA, collagen, or U46619. In conclusion, triplatin mechanism of action has been elucidated without ambiguity as a novel TXA(2)- and PGF(2α)- binding protein. It conceivably blocks platelet aggregation and vasoconstriction, thus contributing to successful blood feeding at the vector-host interface.

Essential Role of Protein Kinase Cδ in Platelet Signaling, αIIbβ3 Activation, and Thromboxane A2 Release

The protein kinase C (PKC) family is an essential signaling mediator in platelet activation and aggregation. However, the relative importance of the major platelet PKC isoforms and their downstream effectors in platelet signaling and function remain unclear. Using isolated human platelets, we report that PKCdelta, but not PKCalpha or PKCbeta, is required for collagen-induced phospholipase C-dependent signaling, activation of alpha(IIb)beta(3), and platelet aggregation. Analysis of PKCdelta phosphorylation and translocation to the membrane following activation by both collagen and thrombin indicates that it is positively regulated by alpha(IIb)beta(3) outside-in signaling. Moreover, PKCdelta triggers activation of the mitogen-activated protein kinase-kinase (MEK)/extracellular-signal regulated kinase (ERK) and the p38 MAPK signaling. This leads to the subsequent release of thromboxane A(2), which is essential for collagen-induced but not thrombin-induced platelet activation and aggregation. This study adds new insight to the role of PKCs in platelet function, where PKCdelta signaling, via the MEK/ERK and p38 MAPK pathways, is required for the secretion of thromboxane A(2).

Platelet glycoprotein VI: its structure and function

Glycoprotein (GP) VI is a platelet membrane protein with a molecular weight of 62 kDa that was identified as a physiological collagen receptor from studies of patients deficient in this protein. GPVI-deficient platelets lacked specifically collagen-induced aggregation and the ability to form thrombi on a collagen surface under flow conditions, suggesting that GPVI makes an indispensable contribution to collagen-induced platelet activation. On the platelet surface, GPVI is present as a complex with the Fc receptor (FcR) gamma-chain, probably composed of two GPVI molecules and one FcR gamma-chain dimer. GPVI must form such a dimeric complex to exhibit high affinity binding to collagen. The GPVI-induced activation mechanism is initiated by tyrosine phosphorylation of the immunoreceptor tyrosine-based activation motif (ITAM) of the FcR gamma-chain, and then this signal is transduced to many related proteins, mainly by tyrosine phosphorylation. GPVI is widely recognized as a requisite factor for the formation of platelet aggregates on a collagen surface under blood flow. However, individuals with GPVI-deficient or null platelets do not exhibit any strong bleeding tendency. Analyzing this apparent dichotomy should provide us with a more precise understanding of the mechanism of thrombus formation.

Platelet adhesion and signaling induced by the octapeptide primary binding sequence (KOGEOGPK) from type III collagen

Platelet adhesion to vascular collagens is an essential step in the initiation of hemostasis and thrombosis. Several platelet receptors interact with type I and type III collagens, including GP Ia/IIa and GP VI. We recently described a new platelet receptor (TIIICBP) specific for a type III collagen-related primary binding sequence, the KOGEOGPK octapeptide. Here, we characterize platelet adhesion to the immobilized octapeptide and demonstrate that this adhesion 1) is Ca2+ and Mg2+ independent, suggesting a noninvolvement of GP Ia/IIa; 2) is not inhibited by an antibody against GP VI; and 3) triggers platelet protein tyrosine phosphorylation. Whereas TXA2 has minimal effects, released ADP via only P2Y12 potentiates platelet adhesion to the octapeptide. Octapeptide-induced platelet adhesion triggers platelet signaling through tyrosine phosphorylation of the 68 kDa subunit of TIIICBP, Syk, PLCgamma2, and FAK. Tyrosine phosphorylation of the FcR gamma-chain and LAT is also observed but to a lesser extent than with type III collagen, suggesting the requirement of GP VI for full tyrosine phosphorylation of FcR gamma-chain and LAT. The present study provides evidence for a critical role for the type III collagen-related KOGEOGPK octapeptide in mediating platelet adhesion and signaling, and consequently in platelet-collagen interactions.-

Purification and characterization of a phospholipase A2 isoenzyme isolated from Lachesis muta snake venom

A new phospholipase A2 (PLA2) isoenzyme was isolated from Lachesis muta crude venom, and was named LM-PLA2-II. This enzyme was purified by gel filtration on a Sephacryl S-200 HR column followed by reverse-phase chromatography on a C2/C18 column. LM-PLA2-II consists of a single polypeptide chain with an apparent molecular mass of 18 kDa and an isoelectric point at pH 5.4. The amino terminal sequence of the enzyme revealed a high degree of homology with other PLA2s from several sources. LM-PLA2-II has a high indirect hemolytic activity and a potent inhibitory effect on platelet aggregation induced by ADP and collagen. It also produces a significant paw edema reaction in rats. The edematous response in rats was abolished by pretreatment with either indomethacin or dexamethasone, suggesting the involvement of cyclo-oxygenase. Pretreatment of LM-PLA2-II with p-bromophenacyl bromide abolished all of these actions, clearly indicating that the biological activities, including the edematogenic effect, are dependent entirely on its enzymatic activity.

Isolation and characterization of a convulxin-like protein from Crotalus durissus collilineatus venom

A convulxin (Cvx)-like protein was isolated from Crotalus durissus collilineatus venom by a combination of molecular exclusion and reversed-phase HPLC chromatographies. The molecular mass of the Cvx-like protein in the absence and presence of DTT was 78 kDa and 12-13 kDa, respectively. The Cvx-like protein consisted of two nonidentical polypeptide chains (alpha and beta). The N-terminal amino-acid sequences of the alpha and beta subunits were GLHCPSDWYAYDGHCYKIFNEEMNWED and GFCCPSHWSSYSRYCYKFFSQEMNWEDAEK, respectively, with both subunits having a high content of Glu, Ser, Cys, and Asp. The Cvx-like protein showed high homology with other venom C-type lectins, but had low hemagglutinating activity on intact and trypsinized erythrocytes. The Cvx-like protein stimulated insulin receptor phosphorylation and potentiated insulin secretion from isolated islets in the presence of sub- (2.8 mM) or supra-physiological (16.7 mM) glucose concentrations. These results suggest that the increase in insulin secretion induced by Cvx-like protein may be mediated by a protein tyrosine kinase-dependent pathway and may involve other membrane receptors, such as GP VI or Scr family proteins.

The use of snake venom toxins as tools to study platelet receptors for collagen and von Willebrand factor

A large proportion of the biologically active proteins and peptides present within snake venoms interact with components of the haemostatic system to promote or inhibit the normal sequence of events that lead to clot formation. The venom proteins achieve their effects through interaction with various components of the coagulation cascade, endothelial matrix and platelets. Within the latter group, a number of venom proteins target the interaction of platelets with the major adhesive proteins, von Willebrand factor and collagen. The venom proteins bind either the adhesive protein itself or their receptors on the platelet surface, notably GP-Ib-IX-V and GPVI. This review discusses the substantial contribution that venom proteins have made to our understanding of the role of these two adhesive proteins and their receptors (excluding GPIIb-IIIa) in platelet regulation.

Primary structure and functional characterization of bilitoxin-1, a novel dimeric P-II snake venom metalloproteinase from Agkistrodon bilineatus venom

The amino acid sequence of the hemorrhagic toxin, bilitoxin-1, isolated from the venom of Agkistrodon bilineatus was determined by the Edman sequencing procedure of peptides derived from digests utilizing cyanogen bromide, clostripain, lysyl endopeptidase, and Staphylococcus aureus V8 protease. A molecular mass of 80,000 Da was observed in the nonreduced state and 48,000 Da was observed in the reduced state, as demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Each subunit consists of 291 amino acid residues and has a calculated molecular mass of 32,276 Da. The toxin contains fucose, galactosamine, glucosamine, galactose, mannose, and N-acetylneuraminic acid and three N-linked glycosylation consensus sites. Hydrazinolysis and ESI mass spectrometry revealed that asparagine was the carboxyl-terminal amino acid. The disintegrin-like domain of bilitoxin-1 lacks the RGD cell-binding sequence, which is substituted by the MGD sequence. Under certain conditions, the disintegrin domain is autoproteolytically processed from the native protein. Studies with the bilitoxin disintegrin demonstrated that it lacks platelet aggregation inhibitory activity, probably reflecting the substitution of RGD by MGD. The hemorrhagic activity of the asialobilitoxin-1 was only 25% of bilitoxin-1, while proteolytic activity was unaffected. The three-dimensional structure of this toxin was modeled and was shown to likely possess a structure similar to that of adamalysin II (Gomis-Rüth et al., EMBO J. 12, 151-157 (1993)) and the disintegrin kistrin (Adler et al., Biochemistry 32, 282-289 (1993)). In summary, here we report the first primary structure of a dimeric, P-II snake venom metalloproteinase and the biological role of bilitoxin-1 glycosylation and the disintegrin domain.

Purification, cloning, expression, and mechanism of action of a novel platelet aggregation inhibitor from the salivary gland of the blood-sucking bug, Rhodnius prolixus

Rhodnius prolixus aggregation inhibitor 1 (RPAI-1), a 19-kDa protein isolated from the salivary gland of R. prolixus, was purified by strong cation exchange and reverse-phase high performance liquid chromatographies. Based on 49 amino-terminal amino acid sequences of RPAI-1, primers were produced to generate probes to screen an R. prolixus salivary gland cDNA library. A phage containing the full-length clone of RPAI-1 codes for a mature protein of 155 amino acids. RPAI-1 shows sequence homology to triabin and pallidipin, lipocalins from Triatoma pallidipennis. The cDNA sequence was cloned in Pet17B Escherichia coli expression vector, producing an active peptide. RPAI-1 inhibits human platelet-rich plasma aggregation triggered by low concentrations of ADP, collagen, arachidonic acid, thromboxane A(2) mimetics (U46619), and very low doses of thrombin and convulxin. Here we show that ADP is the target of RPAI-1 since (i) RPAI-1 inhibits ADP-dependent large aggregation formation and secretion triggered by U46619, without affecting Ca(2+) increase and shape change; (ii) ADP restored the inhibition of U46619-induced platelet aggregation by RPAI-1, (iii) PGE(1)-induced increase of cAMP (which is antagonized by U46619 in an ADP-dependent manner) was restored by RPAI-1, (iv) RPAI-1 inhibits low concentrations of ADP-mediated responses of indomethacin-treated platelets, and (v) RPAI-1 binds to ADP, as assessed by large zone chromatography. RPAI-1 affects neither integrin alpha(2)beta(1)- nor glycoprotein VI-mediated platelet responses. We conclude that RPAI-1 is the first lipocalin described that inhibits platelet aggregation by a novel mechanism, binding to ADP.

Role of the recombinant non-integrin platelet collagen receptor P65 on platelet activation induced by convulxin

Convulxin (Cvx) isolated from Crotalus durissus terrificus venom selectively binds with a high affinity to platelets and induces platelet aggregation by a mechanism that resembles that induced by collagen. Taking advantage that P65 has been recently cloned and expressed as a recombinant soluble protein (rec-P65), we examined the role of this non-integrin collagen receptor in platelet activation induced by Cvx. Rec-P65 blocked platelet adhesion to collagen-coated surfaces and inhibited platelet aggregation and ATP secretion induced by type I collagen. On the other hand, rec-P65 did not inhibit platelet aggregation and ATP secretion induced by Cvx, and it did not affect platelet adhesion to Cvx. In addition, ligand-blotting indicated that the Cvx binding to the collagen receptor GPVI was preserved in the presence of rec-P65. These observations indicate that P65 does not play a significant role in platelet activation by Cvx; in contrast, platelet response to collagen involves multiple receptors.

Distinct contributions of glycoprotein VI and alpha(2)beta(1) integrin to the induction of platelet protein tyrosine phosphorylation and aggregation

Platelet activation by collagen depends principally on two receptors, alpha(2)beta(1) integrin (GPIa-IIa) and GPVI. During this activation, the nonreceptor protein tyrosine kinase pp72(syk) is rapidly phosphorylated, but the precise contribution of alpha(2)beta(1) integrin and GPVI to signaling for this phosphorylation is not clear. We have recently found that proteolysis of platelet alpha(2)beta(1) integrin by the snake venom metalloproteinase, jararhagin, results in inhibition of collagen-induced platelet aggregation and pp72(syk) phosphorylation. In order to verify whether the treatment of platelets with jararhagin had any effect on GPVI signaling, in this study we stimulated platelets treated with either jararhagin or anti-alpha(2)beta(1) antibody with two GPVI agonists, an antibody to GPVI and convulxin. Platelet shape change and phosphorylation of pp72(syk) by both GPVI agonists was preserved, as was the structure and function of GPVI shown by (125)I-labeled convulxin binding to immunoprecipitated GPVI from jararhagin-treated platelets. In contrast, defective platelet aggregation in response to GPVI agonists occurred in both jararhagin-treated and alpha(2)beta(1)-blocked platelets. This apparent cosignaling role of alpha(2)beta(1) integrin for platelet aggregation suggests the possibility of a topographical association of this integrin with GPVI. We found that both platelet alpha(2)beta(1) integrin and GPVI coimmunoprecipitated with alpha(IIb)beta(3) integrin. Since platelet aggregation requires activation of alpha(IIb)beta(3) integrin, defective aggregation in the absence of alpha(2)beta(1) suggests that this receptor may provide a signaling link between GPVI and alpha(IIb)beta(3). Our study therefore demonstrates that platelet signaling leading to pp72(syk) phosphorylation initiated with GPVI engagement by either convulxin or GPVI antibody does not depend on alpha(2)beta(1) integrin. However, alpha(IIb)beta(3) integrin may, in this model, require functional alpha(2)beta(1) integrin for its activation.

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

Platelet activation by collagen is mediated by the sequential tyrosine phosphorylation of the Fc receptor gamma-chain (FcR gamma-chain), which is part of the collagen receptor glycoprotein VI, the tyrosine kinase Syk and phospholipase C-gamma2 (PLC-gamma2). In this study tyrosine-phosphorylated proteins that associate with PLC-gamma2 after stimulation by a collagen-related peptide (CRP) were characterized using glutathione S-transferase fusion proteins of PLC-gamma2 Src homology (SH) domains and by immunoprecipitation of endogenous PLC-gamma2. The majority of the tyrosine-phosphorylated proteins that associate with PLC-gamma2 bind to its C-terminal SH2 domain. These were found to include PLC-gamma2, Syk, SH2-domain-containing leucocyte protein of 76 kDa (SLP-76), Lyn, linker for activation of T cells (LAT) and the FcR gamma-chain. Direct association was detected between PLC-gamma2 and SLP-76, and between PLC-gamma2 and LAT upon CRP stimulation of platelets by far-Western blotting. FcR gamma-chain and Lyn were found to co-immunoprecipitate with PLC-gamma2 as well as with unidentified 110-kDa and 75-kDa phosphoproteins. The absence of an in vivo association between Syk and PLC-gamma2 in platelets is in contrast with that for PLC-gamma1 and Syk in B cells. The in vivo function of PLC-gamma2 SH2 domains was examined through measurement of Ca2+ increases in mouse megakaryocytes that had been microinjected with recombinant proteins. This revealed that the C-terminal SH2 domain is involved in the regulation of PLC-gamma2. These data indicate that the C-terminal SH2 domain of PLC-gamma2 is important for PLC-gamma2 regulation through possible interactions with SLP-76, Syk, Lyn, LAT and the FcR gamma-chain.

Phosphatidylinositol 3'-kinase and tyrosine-phosphatase activation positively modulate Convulxin-induced platelet activation. Comparison with collagen

In this report we have studied the role of phosphatidylinositol 3'-kinase (PI3-K) and tyrosine phosphatase activation on platelet activation by Convulxin (Cvx). Wortmannin, a specific PI3-K inhibitor, and phenylarsine oxide (PAO), a sulfhydryl reagent that inhibits tyrosine phosphatase (PTPase), block Cvx-induced platelet aggregation, granule secretion, inositol phosphate production, and increase in [Ca2+]i. However, PAO does not inhibit Cvx-induced tyrosine phosphorylation of platelet proteins, including Syk and PLCgamma2, but blocked collagen-induced platelet aggregation as well as tyrosine phosphorylation of PLCgamma2. In contrast, Cvx-induced PLCgamma2 tyrosyl phosphorylation was partially inhibited by wortmannin. We conclude that (i) although Cvx and collagen activate platelets by a similar mechanism, different regulatory processes are specific to each agonist; (ii) mechanisms other than tyrosine phosphorylation regulate PLCgamma2 activity; and (iii) besides protein tyrosine kinases, PI3-K (and PTPase) positively modulate platelet activation by both Cvx and collagen, and this enzyme is required for effective transmission of GPVI-Fc receptor gamma chain signal to result in full activation and tyrosine phosphorylation of PLCgamma2 in Cvx-stimulated platelets.

Tyrosine phosphorylation of SLP-76 is downstream of Syk following stimulation of the collagen receptor in platelets

Collagen-related peptide (CRP), a collagen homologue, induces platelet activation through a tyrosine kinase-dependent pathway, leading to sequential tyrosine phosphorylation of Fc receptor (FcR) gamma-chain, Syk, and phospholipase C-gamma2. Here we report that CRP and the platelet low affinity immune receptor FcgammaRIIA stimulate tyrosine phosphorylation of the T cell adapter SLP-76, whereas the G protein-coupled receptor agonist thrombin induces only minor tyrosine phosphorylation. This suggests that SLP-76 has a specific role downstream of receptors that signal via an immunoreceptor tyrosine-based activation motif. Immunoprecipitation studies demonstrate association of SLP-76 with SLAP-130, Vav, Fyn, Lyn, and the FcR gamma-chain in CRP-stimulated platelets. Several of these proteins, including SLP-76, undergo tyrosine phosphorylation in in vitro kinase assays performed on SLP-76 immunoprecipitates. Tyrosine phosphorylation of all of these proteins in the in vitro kinase assay was abrogated by the Src family kinase inhibitor PP1, suggesting that it is mediated by either Fyn or Lyn. The physiological significance of this is uncertain, however, since tyrosine phosphorylation of SLP-76 in vivo is not altered in either Fyn- or Lyn-deficient platelets. CRP stimulation of Syk-deficient platelets demonstrated that in vivo tyrosine phosphorylation of SLP-76 is downstream of Syk. The absence of Syk in the SLP-76 immunoprecipitates raises the possibility that another protein is responsible for bringing SLP-76 to Syk. Candidates for this include those proteins that co-immunoprecipitate with SLP-76, including the FcR gamma-chain. Tyrosine phosphorylation of PLC-gamma2 and Ca2+ mobilization is markedly attenuated in SLP-76-deficient platelets following CRP stimulation, suggesting that the adapter plays a critical role in the regulation of the phospholipase. The increase in tyrosine phosphorylation of SLAP-130 in response to CRP is also inhibited in SLP-76-deficient platelets, placing it downstream of SLP-76. This work identifies SLP-76 as an important adapter molecule that is regulated by Syk and lies upstream of SLAP-130 and PLC-gamma2 in CRP-stimulated platelets.

cAMP does not inhibit convulxin-induced tyrosyl-phosphorylation of human platelet proteins, including PLCgamma2, but completely blocks the integrin alphaIIb beta3-dependent dephosphorylation step: comparisons with RGDS peptide, cytochalasin D, and phenylarsine oxide

Convulxin (Cvx) isolated from Crotalus durissus terrificus venom, induces platelet aggregation, phospholipase C (PLC) activation, and tyrosyl-phosphorylation (PTP) of multiple proteins, including PLCgamma2 by a mechanism independent of integrin alphaIIb beta3. However, PTP induced by Cvx is followed by a dephosphorylation step in a platelet aggregation-dependent manner. Here we show that increasing intraplatelet content of cAMP with forskolin is associated with the inhibition of Cvx-induced platelet aggregation, ATP secretion, and inositol-phosphates production. However, the early onset of Cvx-induced PTP is not sensitive to cAMP (including PLCgamma2), and it also occurs in the presence of integrin alphaIIb beta3-antagonist (RGDS peptide, RGDS) or inhibitors of actin polymerization (cytochalasin D, CD) and tyrosine-phosphatases (phenylarsine oxide, PAO). However, forskolin, RGDS, and CD prevented the dephosphorylation step together with inhibition of platelet aggregation, whereas in the presence of phenylarsine oxide (PAO) the dephosphorylation step was replaced by an increase in the number and intensity of tyrosyl-phosphorylated proteins. Our data provide evidence to conclude that (i) cAMP inhibits platelet aggregation at a downstream site to PLCgamma2 tyrosyl-phosphorylation; (ii) Cvx-induced PTP is independent on integrin alphaIIb beta3 engagement, actin polymerization, and tyrosine-phosphatases activation; (iii) integrin alphaIIb beta3 mediates the dephosphorylation step in a platelet aggregation-dependent manner; and (iv) Cvx and collagen stimulate platelets by a similar signal transduction pathway.