Isolation and characterization of the alpha and beta subunits of the platelet-activating glycoprotein from the venom of Crotalus durissus cascavella

Human platelets activation by convulxin is accompanied by tyrosyl-phosphorylation of PLCgamma2 and occurs independently of integrin alphaIIbbeta3

In the present report we show that convulxin (Cvx), a C-type lectin from Crotalus durissus terrificus venom, induces platelet agregation and phospholipase C (PLC) activation by a protein tyrosine kinase (PTK)-dependent pathway. In addition, Cvx stimulates a rapid increase in tyrosine phosphorylation of human platelet proteins with molecular masses of 40, 72/74, 78/80 and 120 kDa, followed by dephosphorylation of some proteins. However, platelet aggregation was accompanied by the phosphorylation of a 105-kDa molecular mass protein. Furthermore, Cvx stimulates a rapid-tyrosyl phosphorylation of a 145-kDa protein that was identified as PLC gamma 2. Protein tyrose phosphatase (PTP) induced by Cvx was not blocked when platelets were stimulated in the presence of indomethacin, apyrase, EDTA or RGDS peptide, but inhibited by staurosporine and genistein. These results indicate that PTP is chronologically proximal to Cvx binding to platelets, and it is independent of platelet aggregation or fibrinogen binding to integrin alphaIIbbeta3. On the other hand, the phosphorylation step, and the phosphorylation of the 105-kDa protein, were both inhibited by RGDS and EDTA, which suggests that the integrin alphaIIbbeta3 beta is involved in these steps. Our results, taken together, show that Cvx induces platelet IIb 3 aggregation in a similar manner as collagen and collagen-related peptides that also trigger platelet aggregation by a PTK-dependent pathway, and stimulate tyrosyl-phosphorylation of PLC gamma 2. However, Cvx is unique among platelet receptor agonists, because under test-tube stirring conditions it induces a PTP profile independently of integrin alphaIIbbeta3.

Effect of multimer size and a natural dimorphism on the binding of convulxin to platelet glycoprotein (GP)VI

BACKGROUND

Convulxin (CVX), a C-type lectin from the venom of Crotalus durissus terrificus, is a potent activator of human platelets, binding predominantly to glycoprotein (GP)VI. Native CVX is an octamer composed of four alphabeta-heterodimers [(alphabeta)(4)]. Two different native sequences have been reported, one bearing lysine (K), the other glutamic acid (E), at beta chain residue 89, but the physiological relevance of this difference is unknown.

OBJECTIVE

We used the Drosophila S2 system to express recombinant CVX (rCVX) heterodimers (alphabeta) and site-directed mutagenesis to evaluate the influence of multimer size and the substitution betaK89E on CVX function.

METHODS

By flow cytometry, native CVX and both recombinant forms bind to human platelets in whole blood. By surface plasmon resonance (BIAcore, Piscataway, NJ, USA), the calculated equilibrium dissociation constants (K(D)) were: rCVX alphabeta89K, 11.3 x 10(-8) m; rCVX alphabeta89E, 9 x 10(-8) m; and native CVX, 2.8 x 10(-8) m.

RESULTS

Thus, the affinities of the two rCVX forms for human, recombinant GPVI are essentially the same, but the relative affinity of native CVX is about 3-fold higher. The minimum concentration of native CVX that induces maximal human platelet aggregation (70 pm) is roughly 400-fold lower than that of either rCVX (29 nm).

CONCLUSIONS

These results are consistent with the hypothesis that the ability of the native CVX octamer to cluster mobile GPVI molecules within the platelet membrane may be the single most important factor that contributes to the efficiency with which CVX is able to induce platelet activation.

Anti-sera raised in rabbits against crotoxin and phospholipase A2 from Crotalus durissus cascavella venom neutralize the neurotoxicity of the venom and crotoxin

Crotoxin, the principal neurotoxin in venom of the South American rattlesnakes Crotalus durissus terrificus and Crotalus durissus cascavella, contains a basic phospholipase A2 (PLA2) and an acidic protein, crotapotin. In this work, we examined the ability of rabbit anti-sera against crotoxin and its PLA2 subunit to neutralize the neurotoxicity of venom and crotoxin from C. d. cascavella in mouse phrenic nerve-diaphragm and chick biventer cervicis preparations. Immunoblotting showed that the anti-sera recognized C. d. cascavella crotoxin and PLA2. This was confirmed by ELISA, with both anti-sera having end-point dilutions of 3 x 10(-6). Anti-crotoxin serum neutralized the neuromuscular blockade in phrenic nerve-diaphragm muscle preparations at venom or crotoxin:anti-serum ratios of 1:2 and 1:3, respectively. Anti-PLA2 serum also neutralized this neuromuscular activity at a venom or crotoxin:anti-serum ratio of 1:1. In biventer cervicis preparations, the corresponding ratio for anti-crotoxin serum was 1:3 for venom and crotoxin, and 1:1 and 1:2 for anti-PLA2 serum. The neutralizing capacity of the sera in mouse preparations was comparable to that of commercial anti-serum raised against C. d. terrificus venom. These results show that anti-sera against crotoxin and PLA2 from C. d. cascavella venom neutralized the neuromuscular blockade induced by venom and crotoxin in both nerve-muscle preparations, with the anti-serum against crotoxin being slightly less potent than that against crotoxin.

Ophioluxin, a convulxin-like C-type lectin from Ophiophagus hannah (King cobra) is a powerful platelet activator via glycoprotein VI

Ophioluxin, a potent platelet agonist, was purified from the venom of Ophiophagus hannah (King cobra). Under nonreducing conditions it has a mass of 85 kDa, similar to convulxin, and on reduction gives two subunits with masses of 16 and 17 kDa, slightly larger than those of convulxin. The N-terminal sequences of both subunits are very similar to those of convulxin and other C-type lectins. Ophioluxin induces a pattern of tyrosine-phosphorylated proteins in platelets like that caused by convulxin, when using appropriate concentrations based on aggregation response, because it is about 2-4 times more powerful as agonist than the latter. Ophioluxin and convulxin induce [Ca(2+)](i) elevation both in platelets and in Dami megakaryocytic cells, and each of these C-type lectins desensitizes responses to the other. Convulxin agglutinates fixed platelets at 2 microg/ml, whereas ophioluxin does not, even at 80 microg/ml. Ophioluxin resembles convulxin more than echicetin or alboaggregin B because polyclonal anti-ophioluxin antibodies recognize both ophioluxin and convulxin, but not echicetin, and platelets adhere to and spread on ophioluxin- or convulxin-precoated surfaces in the same way that is clearly different from their behavior on an alboaggregin B surface. Immobilized ophioluxin was used to isolate the glycoprotein VI-Fcgamma complex from resting platelets, which also contained Fyn, Lyn, Syk, LAT, and SLP76. Ophioluxin is the first multiheterodimeric, convulxin-like snake C-type lectin, as well as the first platelet agonist, to be described from the Elapidae snake family.

Purification and cloning of a novel C-type lectin-like protein with platelet aggregation activity from Trimeresurus mucrosquamatus venom

TMVA, a novel C-type lectin-like protein that induces platelet aggregation in a dose-dependent manner, was purified from the venom of Trimeresurus mucrosquamatus. It consists of two subunits, alpha (15536 Da) and beta (14873 Da). The mature amino acid sequences of the alpha (135 amino acids) and beta subunits (123 amino acids) were deduced from cloned cDNAs. Both of the sequences show great similarity to C-type lectin-like venom proteins, including a carbohydrate recognition domain. The cysteine residues of TMVA are conserved at positions corresponding to those of flavocetin-A and convulxin, including the additional Cys135 in the alpha subunit and Cys3 in the beta subunit. SDS-PAGE, mass spectrometry analysis and amino acid sequence showed that native TMVA exists as two convertible multimers of (alpha beta)(2) and (alpha beta)(4) with molecular weights of 63680 and 128518 Da, respectively. The (alpha beta)(2) complex is stabilized by an interchain disulfide bridge between the two alpha beta-heterodimers, whereas the stabilization of the (alpha beta)(4) complex seems to involve non-covalent interactions between the (alpha beta)(2) complexes.

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.

Convulxin binding to platelet receptor GPVI: competition with collagen related peptides

Convulxin (CVX), a potent platelet aggregating protein from the venom of the snake Crotalus durissus terrificus, is known to bind to the platelet collagen receptor, glycoprotein VI (GPVI). CVX binding to human platelets was investigated by flow cytometry, using fluorescein labeled convulxin (FITC-CVX). Scatchard analysis indicated high and low affinity binding sites with Kd values of 0.6 and 4 nM and Bmax values of 1200 and 2000 binding sites per platelet. FITC-CVX binding was inhibited by collagen related peptides (CRPs) comprising a repeated GPO sequence, namely GCO(GPO)(10)GCOGNH(2) and GKO(GPO)(10)GKOGNH(2), which also bind to receptor GPVI. These peptides (monomeric or cross-linked forms) gave a high affinity inhibition of 10-20% for concentrations between 10 ng/ml and 5 microg/ml, followed by a second phase of inhibition at concentrations greater than 5 microg/ml. It was shown also that the inhibition of FITC-CVX binding by CRPs was independent on the time of preincubation of platelets with CRPs, and the same percentage of inhibition was seen with various concentrations of convulxin. Confocal microscopy of the distribution of FITC-CVX binding sites on platelets showed an homogeneous distribution of FITC-CVX bound to GPVI, although some limited clustering may exist.

Molecular cloning and sequence analysis of aggretin, a collagen-like platelet aggregation inducer

A cDNA library derived from the Malayan-pit-viper (Calloselasma rhodostoma) venom gland was constructed in the phagemid vector. Using the information of the N-terminal amino acid sequences of two subunits of aggretin, synthetic mixed-base oligonucleotides were employed as a screening probe for colony hybridization. Separate cDNA clones encoding for the alpha and beta chains of aggretin were isolated and sequenced. The results revealed that mature alpha and beta chains contain 136 and 123 amino acid residues, respectively. Aggretin subunits show high degrees of identity with respective subunits (50-60% for alpha, 49-58% for beta) of C-type lectin-like snake venoms. The identity to rattlesnake lectin is relatively lower (i.e., 39 and 30%). All cysteine residues in each chain of aggretin are well conserved and located at the positions corresponding to those of C-type lectins. Thus, three intracatenary disulfide bridges and an interchain disulfide bond between Cys83(alpha) and Cys75(beta) may be allocated. This is the first report regarding the entire sequence of venom GPIa/IIa agonist. According to the alignment of amino acid sequences, hypervariable regions among these C-type lectin-like proteins were revealed. These hypervariable regions are proposed to be the counterparts directly interacting with different receptors or different domains of a receptor on the surface of platelet.

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.

Cloning of subunits of convulxin, a collagen-like platelet-aggregating protein from Crotalus durissus terrificus venom

Convulxin (CVX) is a potent platelet-aggregating glycoprotein from the venom of the snake Crotalus durissus terrificus. It consists of two subunits, alpha and beta, joined by disulphide bridges in a hexameric structure. A cDNA library from venom gland was constructed in the vector pT3T7. The cloned cDNAs encoding the two chains of CVX were sequenced. Both are preceded by an identical 23-amino acid peptide signal sequence and encode sequences of 135 amino acids for the alpha chain and 125 amino acids for the beta chain. These polypeptides include a carbohydrate-recognition domain (CRD) in which some of the specific amino acids required for binding Ca2+ and galactose or mannose are absent. The presence of such a domain means that CVX can be included in the family of C-type lectins along with other snake venom proteins, although it is not a true lectin. Assuming that the localization of intracatenary disulphide bridges of each CVX chain is similar to that of the CRD and that an intercatenary bridge between the alpha and beta chains is similar to that of the C-type lectin botrocetin, we postulate the existence of an additional intercatenary bridge, which explains the tridimeric structure (alphabeta)3 of CVX.

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.

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

1Convulxin (Cvx) is a well-characterized platelet aggregating glycoprotein isolated from Crotalus durissus terrificus and C. d. cascavella venoms. In the present report we show that Cvx induces tyrosine phosphorylation of human platelet proteins, including phospholipase C-gamma 2 (PLC gamma 2), and also stimulates [3H]arachidonic acid ([3H]AA) mobilization, pleckstrin phosphorylation, and an increase in the cytosolic Ca2+ concentration ([Ca2+]in) due to both Ca2+ entry and internal Ca2+ mobilization. Staurosporine, a potent protein kinase inhibitor, and genistein, a specific inhibitor of protein tyrosine kinases (PTK), were used to evaluate the role of protein tyrosine phosphorylation (PTP) in the signal transduction evoked by Cvx. Staurosporine and genistein inhibited in a dose-dependent manner platelet aggregation induced by Cvx. Both inhibitors significantly blocked to near basal levels breakdown of phosphatidylinositol 4,5-bisphosphate from [myo-2-3H]inositol-labeled platelets and the production of [3H]AA metabolites from [3H]AA-labeled platelets after challenge with Cvx. Cvx provokes an increase in [Ca2+]in in Fura-2-loaded platelets that was abolished by concentrations of staurosporine which also inhibited Cvx-induced platelet aggregation. In addition, Cvx stimulates a rapid increase in tyrosine phosphorylation of human platelets proteins with molecular masses of 40, 72/74, 78/80, 105, 120, and 145 kDa, followed by dephosphorylation. Furthermore, Cvx stimulates a rapid tyrosyl phosphorylation of a 145-kDa molecular mass protein that was identified as PLC gamma 2. PTP induced by Cvx was not inhibited when platelets were stimulated in the presence of indomethacin, apyrase, EDTA, or RGDS peptide. These results indicate that PTP is chronologically proximal to Cvx binding to platelets, and is independent of aggregation or fibrinogen binding to the integrin alpha IIb beta 3. On the other hand, the dephosphorylation step is inhibited by RGDS peptide or EDTA, suggesting that integrin alpha IIb beta 3 is envolved in this step. The profile obtained with Cvx resembles that obtained in platelets adherent to an immobilized ligand, such as immobilized collagen, in which PTP is independent on integrin alpha IIb beta 3. Thus, we suggest that Cvx is an example of a protein with adhesion molecule-like properties; i.e., it is an adhesin. In conclusion, our results show that Cvx induces multiple signaling pathways in platelets via a PTK-dependent pathway involving PLC gamma 2 tyrosyl phosphorylation, with the subsequent platelet responses. Cvx is unique among platelet soluble agonists because under test tube stirring conditions it induces a PTP profile independently of integrin alpha IIb beta 3.

Convulxin, a potent platelet-aggregating protein from Crotalus durissus terrificus venom, specifically binds to platelets

Convulxin, a very potent aggregating protein from rattlesnake venom, was purified by a new procedure and its heterodimeric structure alpha 3 beta 3 was confirmed. The polypeptide N-terminal sequences of convulxin subunits were determined by Edman degradation. They are very similar and appear homologous to botrocetin from Bothrops jararaca venom and to rattlesnake lectin from Crotalus atrox venom, both being classified among the C-type lectin family. The binding of 125I-labelled convulxin to blood platelets has also been analysed under equilibrium conditions. These studies indicated that convulxin binds to platelets with a high affinity (Kd = 30 pM) on a small number of binding sites (1000 binding sites per cell). The high-affinity binding of convulxin appears specific to platelets, since it is not observed on other cell types such as neutrophils and erythrocytes. Also, the high-affinity binding of convulxin to membranes platelet is not inhibited by alpha-thrombin, fibrinogen, collagen, laminin binding inhibitor, RGDS peptide, adenosine diphosphate, platelet-activating factor-acether, serotonin or epinephrine. This, together with the recent observation that platelet activation by convulxin is partially mediated by phospholipase C and involves other mechanisms as well, indicates that convulxin may interact with a specific platelet acceptor (receptor) protein which has yet to be characterized.

Platelet activation and signal transduction by convulxin, a C-type lectin from Crotalus durissus terrificus (tropical rattlesnake) venom via the p62/GPVI collagen receptor

Convulxin, a powerful platelet activator, was isolated from Crotalus durissus terrificus venom, and 20 amino acid N-terminal sequences of both subunits were determined. These indicated that convulxin belongs to the heterodimeric C-type lectin family. Neither antibodies against GPIb nor echicetin had any effect on convulxin-induced platelet aggregation showing that, in contrast to other venom C-type lectins acting on platelets, GPIb is not involved in convulxin-induced platelet activation. In addition, partially reduced/denatured convulxin only affects collagen-induced platelet aggregation. The mechanism of convulxin-induced platelet activation was examined by platelet aggregation, detection of time-dependent tyrosine phosphorylation of platelet proteins, and binding studies with 125I-convulxin. Convulxin induces signal transduction in part like collagen, involving the time-dependent tyrosine phosphorylation of Fc receptor gamma chain, phospholipase Cgamma2, p72(SYK), c-Cbl, and p36-38. However, unlike collagen, pp125(FAK) and some other bands are not tyrosine-phosphorylated. Convulxin binds to a glycosylated 62-kDa membrane component in platelet lysate and to p62/GPVI immunoprecipitated by human anti-p62/GPVI antibodies. Convulxin subunits inhibit both aggregation and tyrosine phosphorylation in response to collagen. Piceatannol, a tyrosine kinase inhibitor with some specificity for p72(SYK), showed differential effects on collagen and convulxin-stimulated signaling. These results suggest that convulxin uses the p62/GPVI but not the alpha2beta1 part of the collagen signaling pathways to activate platelets. Occupation and clustering of p62/GPVI may activate Src family kinases phosphorylating Fc receptor gamma chain and, by a mechanism previously described in T- and B-cells, activate p72(SYK) that is critical for downstream activation of platelets.

Convulxin-induced platelet aggregation is accompanied by a powerful activation of the phospholipase C pathway

Platelet aggregation and stimulation of phosphoinositide-specific phospholipase C (PLC) by thrombin and by convulxin (Cvx), a non-enzymic snake venom glycoprotein, were compared. Cvx-stimulated production of inositol phosphates by washed platelets was independent of the cyclo-oxygenase pathway, formation of platelet-activating factor and ADP release, but prostacyclin (prostaglandin I2), a stimulator of cyclic AMP formation, suppressed its effects on platelet and PLC activation. Kinetic analysis showed that inositol 1,4,5-trisphosphate formation reached its maximal value 15 s after platelet stimulation with Cvx and persisted for at least 5 min. Neomycin sulphate (10 mM), which complexes phosphatidylinositol 4-phosphate and phosphatidyl-inositol 4,5-bisphosphate, decreased the production of inositol phosphates, partially prevented platelet aggregation induced by a high concentration of Cvx (10 nM) and abolished both platelet aggregation and inositol phosphate formation induced by thrombin (2 units/ml) and by a stable prostaglandin H2 analogue, U46619 (1 microM). In contrast with neomycin sulphate, Na2SO4 had no significant effect against all agonists tested. It is concluded that platelet activation by Cvx is partially mediated by PLC and involves other mechanisms as well.

Effects of snake venom proteins on blood platelets

Snake venoms are complex mixtures which contain pharmacologically active polypeptides and proteins. Several snake venom constituents interfere in platelet aggregation, an important cellular process in thrombosis and hemostasis. These components range in size from small molecular weight polypeptides to high molecular weight proteins. Some of the proteins are enzymes, such as phospholipase A2, proteinases, nucleotidases, or L-amino acid oxidase, while others do not exhibit enzymatic activity. These components may initiate and/or inhibit platelet aggregation. Some venom factors induce platelet agglutination. This review deals with the physical characteristics of these venom factors, the mechanisms of their platelet effects, structure-function relationships, and their physiological significance.