Flavocetin-A and -B, two high molecular mass glycoprotein Ib binding proteins with high affinity purified from Trimeresurus flavoviridis venom, inhibit platelet aggregation at high shear stress

The Clot Thickens: Differential Coagulotoxic and Cardiotoxic Activities of Anguimorpha Lizard Venoms

Despite their evolutionary novelty, lizard venoms are much less studied in comparison to the intense research on snake venoms. While the venoms of helodermatid lizards have long been assumed to be for defensive purposes, there is increasing evidence of toxic activities more useful for predation than defence (such as paralytic neurotoxicity). This study aimed to ascertain the effects of Heloderma, Lanthanotus, and Varanus lizard venoms on the coagulation and cardiovascular systems. Anticoagulant toxicity was demonstrated for the Varanus species studied, with the venoms prolonging clotting times in human and bird plasma due to the destructive cleavage of fibrinogen. In contrast, thromboelastographic analyses on human and bird plasmas in this study demonstrated a procoagulant bioactivity for Heloderma venoms. A previous study on Heloderma venom using factor-depleted plasmas as a proxy model suggested a procoagulant factor was present that activated either Factor XI or Factor XII, but could not ascertain the precise target. Our activation studies using purified zymogens confirmed FXII activation. Comparisons of neonate and adult H. exasperatum, revealed the neonates to be more potent in the ability to activate FXII, being more similar to the venom of the smaller species H. suspectum than the adult H. exasperatum. This suggests potent FXII activation a basal trait in the genus, present in the small bodied last common ancestor. This also indicates an ontogenetic difference in prey preferences in the larger Heloderma species paralleing the change in venom biochemistry. In addition, as birds lack Factor XII, the ability to clot avian plasma suggested an additional procoagulant site of action, which was revealed to be the activation of Factor VII, with H. horridum being the most potent. This study also examined the effects upon the cardiovascular system, including the liberation of kinins from kininogen, which contributes to hypotension induction. This form of toxicity was previously described for Heloderma venoms, and was revealed in this study was to also be a pathophysiological effect of Lanthanotus and Varanus venoms. This suggests that this toxic activity was present in the venom of the last common ancestor of the anguimorph lizards, which is consistent with kallikrein enzymes being a shared toxin trait. This study therefore uncovered novel actions of anguimorph lizard venoms, not only contributing to the evolutionary biology body of knowledge but also revealing novel activities to mine for drug design lead compounds.

Structurally Robust and Functionally Highly Versatile-C-Type Lectin (-Related) Proteins in Snake Venoms

Snake venoms contain an astounding variety of different proteins. Among them are numerous C-type lectin family members, which are grouped into classical Ca²⁺- and sugar-binding lectins and the non-sugar-binding snake venom C-type lectin-related proteins (SV-CLRPs), also called snaclecs. Both groups share the robust C-type lectin domain (CTLD) fold but differ in a long loop, which either contributes to a sugar-binding site or is expanded into a loop-swapping heterodimerization domain between two CLRP subunits. Most C-type lectin (-related) proteins assemble in ordered supramolecular complexes with a high versatility of subunit numbers and geometric arrays. Similarly versatile is their ability to inhibit or block their target molecules as well as to agonistically stimulate or antagonistically blunt a cellular reaction triggered by their target receptor. By utilizing distinct interaction sites differentially, SV-CLRPs target a plethora of molecules, such as distinct coagulation factors and receptors of platelets and endothelial cells that are involved in hemostasis, thrombus formation, inflammation and hematogenous metastasis. Because of their robust structure and their high affinity towards their clinically relevant targets, SV-CLRPs are and will potentially be valuable prototypes to develop new diagnostic and therapeutic tools in medicine, provided that the molecular mechanisms underlying their versatility are disclosed.

Snake venom components in medicine: From the symbolic rod of Asclepius to tangible medical research and application

Both mythologically and logically, snakes have always fascinated man. Snakes have attracted both awe and fear not only because of the elegant movement of their limbless bodies, but also because of the potency of their deadly venoms. Practically, in 2017, the world health organization (WHO) listed snake envenomation as a high priority neglected disease, as snakes inflict up to 2.7 million poisonous bites, around 100.000 casualties, and about three times as many invalidities on man. The venoms of poisonous snakes are a cocktail of potent compounds which specifically and avidly target numerous essential molecules with high efficacy. The individual effects of all venom toxins integrate into lethal dysfunctions of almost any organ system. It is this efficacy and specificity of each venom component, which after analysis of its structure and activity may serve as a potential lead structure for chemical imitation. Such toxin mimetics may help in influencing a specific body function pharmaceutically for the sake of man's health. In this review article, we will give some examples of snake venom components which have spurred the development of novel pharmaceutical compounds. Moreover, we will provide examples where such snake toxin-derived mimetics are in clinical use, trials, or consideration for further pharmaceutical exploitation, especially in the fields of hemostasis, thrombosis, coagulation, and metastasis. Thus, it becomes clear why a snake captured its symbolic place at the Asclepius rod with good reason still nowadays.

Comprehensive Snake Venomics of the Okinawa Habu Pit Viper, Protobothrops flavoviridis, by Complementary Mass Spectrometry-Guided Approaches

The Asian world is home to a multitude of venomous and dangerous snakes, which are used to induce various medical effects in the preparation of traditional snake tinctures and alcoholics, like the Japanese snake wine, named Habushu. The aim of this work was to perform the first quantitative proteomic analysis of the Protobothrops flavoviridis pit viper venom. Accordingly, the venom was analyzed by complimentary bottom-up and top-down mass spectrometry techniques. The mass spectrometry-based snake venomics approach revealed that more than half of the venom is composed of different phospholipases A2 (PLA₂). The combination of this approach and an intact mass profiling led to the identification of the three main Habu PLA₂s. Furthermore, nearly one-third of the total venom consists of snake venom metalloproteinases and disintegrins, and several minor represented toxin families were detected: C-type lectin-like proteins (CTL), cysteine-rich secretory proteins (CRISP), snake venom serine proteases (svSP), l-amino acid oxidases (LAAO), phosphodiesterase (PDE) and 5'-nucleotidase. Finally, the venom of P. flavoviridis contains certain bradykinin-potentiating peptides and related peptides, like the svMP inhibitors, pEKW, pEQW, pEEW and pENW. In preliminary MTT cytotoxicity assays, the highest cancerous-cytotoxicity of crude venom was measured against human neuroblastoma SH-SY5Y cells and shows disintegrin-like effects in some fractions.

Quantitative high-throughput profiling of snake venom gland transcriptomes and proteomes (Ovophis okinavensis and Protobothrops flavoviridis)

Background

Advances in DNA sequencing and proteomics have facilitated quantitative comparisons of snake venom composition. Most studies have employed one approach or the other. Here, both Illumina cDNA sequencing and LC/MS were used to compare the transcriptomes and proteomes of two pit vipers, Protobothrops flavoviridis and Ovophis okinavensis, which differ greatly in their biology.

Results

Sequencing of venom gland cDNA produced 104,830 transcripts. The Protobothrops transcriptome contained transcripts for 103 venom-related proteins, while the Ovophis transcriptome contained 95. In both, transcript abundances spanned six orders of magnitude. Mass spectrometry identified peptides from 100% of transcripts that occurred at higher than contaminant (e.g. human keratin) levels, including a number of proteins never before sequenced from snakes. These transcriptomes reveal fundamentally different envenomation strategies. Adult Protobothrops venom promotes hemorrhage, hypotension, incoagulable blood, and prey digestion, consistent with mammalian predation. Ovophis venom composition is less readily interpreted, owing to insufficient pharmacological data for venom serine and metalloproteases, which comprise more than 97.3% of Ovophis transcripts, but only 38.0% of Protobothrops transcripts. Ovophis venom apparently represents a hybrid strategy optimized for frogs and small mammals.

Conclusions

This study illustrates the power of cDNA sequencing combined with MS profiling. The former quantifies transcript composition, allowing detection of novel proteins, but cannot indicate which proteins are actually secreted, as does MS. We show, for the first time, that transcript and peptide abundances are correlated. This means that MS can be used for quantitative, non-invasive venom profiling, which will be beneficial for studies of endangered species.

The collagen-binding integrin α2β1 is a novel interaction partner of the Trimeresurus flavoviridis venom protein flavocetin-A

Many snake venoms are known for their antithrombotic activity. They contain components that specifically target different platelet-activating receptors such as the collagen-binding integrin α2β1 and the von Willebrand factor receptor GPIb. In a search for an α2β1 integrin-blocking component from the venom of the habu snake (Trimeresurus flavoviridis), we employed two independent purification protocols. First, we used the integrin α2A domain, a major collagen-binding domain, as bait for affinity purification of an α2β1 integrin-binding toxin from the crude venom. Second, in parallel, we used classical protein separation protocols and tested for α2β1 integrin-inhibiting capabilities by ELISA. Using both approaches, we identified flavocetin-A as an inhibitor of α2β1 integrin. Hitherto, flavocetin-A has been reported as a GPIb inhibitor. However, flavocetin-A inhibited collagen-induced platelet aggregation even after GPIb was blocked with other inhibitors. Moreover, flavocetin-A antagonized α2β1 integrin-mediated adhesion and migration of HT1080 human fibrosarcoma cells, which lack any GPIb, on collagen. Protein chemical analyses proved that flavocetin-A binds to α2β1 integrin and its α2A domain with high affinity and in a cooperative manner, which most likely is due to its quaternary structure. Kinetic measurements confirmed the formation of a strong complex between integrin and flavocetin-A, which dissociates very slowly. This study proves that flavocetin-A, which has long been known as a GPIb inhibitor, efficiently targets α2β1 integrin and thus blocks collagen-induced platelet activation. Moreover, our findings suggest that the separation of GPIb- and α2β1 integrin-blocking members within the C-type lectin-related protein family is less strict than previously assumed.

Toxins in thrombosis and haemostasis: potential beyond imagination

Exogenous factors isolated from venoms of snakes and saliva of haematophagous animals that affect thrombosis and haemostasis have contributed significantly to the development of diagnostic agents, research tools and life-saving drugs. Here, I discuss recent advances in the discovery, structural and functional characterisation, and mechanism of action of new procoagulant and anti-haemostatic proteins. In nature, these factors have evolved to target crucial 'bottlenecks' in the coagulation cascade and platelet aggregation. Several simple protein scaffolds are used to target a wide variety of target proteins and receptors exhibiting functional divergence. Different protein scaffolds have also evolved to target identical, physiologically relevant key enzymes or receptors exhibiting functional convergence. At times, exogenous factors bind to the same target protein, but at distinct sites, to differentially attenuate their functions exhibiting mechanistic divergence within the same family of proteins. The structure-function relationships of these factors are subtle and complicated but represent an exciting challenge. These studies provide ample opportunities to design highly specific and precise ligands to achieve desired biological target function. Although only a small number of them have been characterised to date, the molecular and mechanical diversities of these exogenous factors and their contributions to understanding molecular and cellular events in thrombosis and haemostasis as well as developing diagnostic and research tools and therapeutic agents, is outstanding. Based on the current status, I have attempted to identify future potential and prospects in this area of research.

The rhodocetin αβ subunit targets GPIb and inhibits von Willebrand factor induced platelet activation

Rhodocetin, a heterotetrameric snake C-type lectin from Calloselasma rhodostoma is a specific antagonist of α2β1 integrin. Its γδ subunit is responsible for binding to α2β1 integrin. In this study we show that the rhodocetin αβ subunit can bind to platelet glycoprotein GPIb. Binding of the rhodocetin αβ subunit does not depend on divalent cations. When added to washed human platelets the rhodocetin αβ subunit effectively inhibits platelet aggregation induced by von Willebrand factor plus ristocetin. In contrast, it does not affect collagen-induced platelet activation. By itself the rhodocetin αβ subunit does not induce any changes when added to washed platelets or platelet-rich plasma. However, rhodocetin αβ, after biotinylation and cross-linkage with avidin induces small platelet agglutination but not aggregation. These agglutinated platelets change their pattern of protein tyrosine phosphorylation slightly as kinase p72SYK but not p125FAK is phosphorylated.

Structure and function of snake venom proteins affecting platelet plug formation

Many snake venom proteins have been isolated that affect platelet plug formation by interacting either with platelet integrins, membrane glycoprotein Ib (GPIb), or plasma von Willebrand factor (VWF). Among them, disintegrins purified from various snake venoms are strong inhibitors of platelet aggregation. Botrocetin and bitiscetin derived from Bothrops jararaca and Bitis arietans venom, respectively, induce VWF-dependent platelet agglutination in vitro. Several GPIb-binding proteins have also been isolated from snake venoms. In this review, we focus on the structure and function of those snake venom proteins that influence platelet plug formation. These proteins are potentially useful as reagents for the sub-diagnosis of platelet disorder or von Willebrand disease, as well as for clinical and basic research of thrombosis and hemostasis.

Protein complexes in snake venom

Snake venom contains mixture of bioactive proteins and polypeptides. Most of these proteins and polypeptides exist as monomers, but some of them form complexes in the venom. These complexes exhibit much higher levels of pharmacological activity compared to individual components and play an important role in pathophysiological effects during envenomation. They are formed through covalent and/or non-covalent interactions. The subunits of the complexes are either identical (homodimers) or dissimilar (heterodimers; in some cases subunits belong to different families of proteins). The formation of complexes, at times, eliminates the non-specific binding and enhances the binding to the target molecule. On several occasions, it also leads to recognition of new targets as protein-protein interaction in complexes exposes the critical amino acid residues buried in the monomers. Here, we describe the structure and function of various protein complexes of snake venoms and their role in snake venom toxicity.

Blood cells as targets of snake toxins

Snake venoms are mixtures of enzymes and peptides which exert toxicological effects by targeting their substrates or receptors upon envenomation. Snake venom proteins widely affect vascular system including circulating blood cells, coagulation factors, and vascular wall components. Many of the toxic proteins have multiple targets. For example, some metalloproteinase domain-containing snake venom protein cleaves not only fibrinogen but also receptors on platelets. Also, it is frequent that toxins from different snake venom protein families are capable of binding to a common target on cells. Most of the cytotoxic effects in the venom are usually results of the activities of metalloproteinase, C-type lectin, disintegrin, cysteine-rich protein, as well as phospholipase A(2). There has been a growing interest in studying the structure and function of these snake venom proteins because many of them have high structural homologies to proteins found in human. Therefore, the understanding of how these toxins interact with their targets may contribute to the discovery of novel physiological processes and the development of therapeutic agents for cardiovascular diseases. In this review, we summarize how snake toxins target blood cells with an emphasis on their effects on platelet function.

A novel 25 kDa protein from the venom of Bitis arietans with similarity to C-type lectins causes fibrinogen-dependent platelet agglutination

Snake venoms affect blood coagulation and platelet functions in various ways. Venom from the Viperidae and Crotalidae family of snakes contains biologically active proteins that possess coagulant and anticoagulant activities, as well as platelet aggregating and inhibitory activities. Many of these proteins belong to the C-type lectin family. C-type lectins from viper venoms can act by prohibiting the interaction between platelet receptors, such as GPIIbIIIa and the GPIb/V/IX complex, and their ligands. We report on the purification of a novel 25 kDa protein, Ba25, from Bitis arietans with a primary structure that possesses similarity to other C-type lectins from viper venom. This protein has a profound effect on the clotting of whole blood, as well as being able to cause agglutination of platelets in platelet rich plasma without degranulation of the cells, but not of washed platelets in the absence of fibrinogen. Ba25 interacts with the platelet via the GPIb/V/IX, as well as the GPIIbIIIa receptor, and causes an increase in binding of fibrinogen to platelets. These results suggest that Ba25 may be a potent mediator of platelet-platelet interactions, and other coagulatory mechanisms.

Snake venom C-type lectins interacting with platelet receptors. Structure–function relationships and effects on haemostasis

Snake venoms contain components that affect the prey either by neurotoxic or haemorrhagic effects. The latter category affect haemostasis either by inhibiting or activating platelets or coagulation factors. They fall into several types based upon structure and mode of action. A major class is the snake C-type lectins or C-type lectin-like family which shows a typical folding like that in classic C-type lectins such as the selectins and mannose-binding proteins. Those in snake venoms are mostly based on a heterodimeric structure with two subunits alpha and beta, which are often oligomerized to form larger molecules. Simple heterodimeric members of this family have been shown to inhibit platelet functions by binding to GPIb but others activate platelets via the same receptor. Some that act via GPIb do so by inducing von Willebrand factor to bind to it. Another series of snake C-type lectins activate platelets by binding to GPVI while yet another series uses the integrin alpha(2)beta(1) to affect platelet function. The structure of more and more of these C-type lectins have now been, and are being, determined, often together with their ligands, casting light on binding sites and mechanisms. In addition, it is relatively easy to model the structure of the C-type lectins if the primary structure is known. These studies have shown that these proteins are quite a complex group, often with more than one platelet receptor as ligand and although superficially some appear to act as inhibitors, in fact most function by inducing thrombocytopenia by various routes. The relationship between structure and function in this group of venom proteins will be discussed.

Structures and functions of snake venom CLPs (C-type lectin-like proteins) with anticoagulant-, procoagulant-, and platelet-modulating activities

C-type lectin-like proteins (CLPs) have a variety of biological activities, including anticoagulant- and platelet-modulating activities but have no lectin activity. CLPs are made up of heterodimers or oligomers of heterodimers, while C-type lectins from snake venom are composed exclusively of homodimers or homooligomers. In the last decade, numerous CLPs, such as blood coagulation factor IX/X-binding protein and botrocetin, have been isolated from various snake venoms, sequenced, and characterized. In addition, RVV-X (factor X activator) and carinactivase-1 (prothrombin activator) are metalloproteases composed of two C-type lectin-like domains that recognize the Gla domain of factor X and prothrombin, respectively. The basic structures of these CLPs include two homologous subunits: subunit alpha (A chain) of 14-15 kDa and subunit beta (B chain) of 13-14 kDa. CLPs occur in a variety of oligomeric forms, including alphabeta, (alphabeta)(2), and (alphabeta)(4). The basic homologous dimer (alphabeta) of these CLPs is formed by three-dimensional (3D) domain swapping. The CLPs constitute a new protein family and are useful tools for elucidating the mechanisms involved in clotting and platelet activation as well as the structure-function relationships of both blood clotting factors and platelet glycoproteins.

Molecular diversity and accelerated evolution of C-type lectin-like proteins from snake venom

A number of C-type lectin-like proteins that affect thrombosis and hemostasis by inhibiting or activating specific platelet membrane receptors or blood coagulation factors have been isolated from the venom of various snake species and characterized and more than 80 have been sequenced. Recent data on the primary sequences and 3D structures of C-type lectins and C-type lectin-like proteins from snake venoms have enabled us to analyze their molecular evolution. Statistical analysis of their cDNA sequences shows that C-type lectin-like proteins, with some exceptions, have evolved in an accelerated manner to acquire their diverse functions. Phylogenetic analysis shows that the A and B chains of C-type lectin-like proteins are clearly separated from C-type lectins and that the A and B chains are further divided into a group of platelet receptor-binding proteins and a group of coagulation factor-binding proteins. Elucidation of the tertiary structures of several C-type lectin-like proteins led to the discovery of a unique domain-swapping interaction between heterodimeric subunits, which creates a concave surface for ligand binding.

Crystallization and preliminary X-ray crystallographic analysis of agkicetin-C from Deinagkistrodon acutus venom

The crystallization and preliminary crystallographic analysis of agkicetin-C, a well known platelet glycoprotein Ib (GPIb) antagonist from the venom of Deinagkistrodon acutus found in Anhui Province, China is reported. Crystals of agkicetin-C suitable for structure determination were obtained from 1.8 M ammonium sulfate, 40 mM MES pH 6.5 with 2%(v/v) PEG 400. Interestingly, low buffer concentrations of MES seem to be necessary for crystal growth. The crystals of agkicetin-C belong to space group C2, with unit-cell parameters a = 177.5, b = 97.7, c = 106.8 A, beta = 118.5 degrees, and diffract to 2.4 A resolution. Solution of the phase problem by the molecular-replacement method shows that there are four agkicetin-C molecules in the asymmetric unit, with a VM value of 3.4 A3 Da(-1), which corresponds to a high solvent content of approximately 64%. Self-rotation function calculations show a single well defined non-crystallographic twofold axis with features that may represent additional elements of non-crystallographic symmetry.

TMVA, a snake C-type lectin-like protein from Trimeresurus mucrosquamatus venom, activates platelet via GPIb

TMVA is a C-type lectin-like protein with potent platelet activating activity from Trimeresurus mucrosquamatus venom. In the absence of von Willebrand factor (vWF), TMVA dose-dependently induced aggregation of washed platelets. Anti-GP Ib monoclonal antibodies (mAbs), HIP1, specifically inhibited TMVA-induced aggregation in a dose-dependent manner. The aggregation was also inhibited by mAb P2 (an anti-GP IIb mAb). Flow cytometric analysis revealed that FITC-TMVA bound to human formalin-fixed platelets in a saturable manner, and its binding was specifically blocked by HIP1 in a dose-dependent manner. Flow cytometric analysis showed that TMVA did not bind to platelet GPIX, GPIIb, GPIIIa, GPIa, GPIIa and GPIV. Moreover, the platelet aggregation induced by TMVA was partially inhibited when platelet was pretreated with mocarhagin, a snake venom protease that specifically cleaves human GPIb. These results suggest that TMVA is a strong platelet agonist via GPIb and it might have multiple functional binding-sites on GPIb molecule or on other unknown receptor.

Crystal structure of a platelet-agglutinating factor isolated from the venom of Taiwan habu (Trimeresurus mucrosquamatus)

Platelet glycoprotein Ib (GPIb)-binding proteins (GPIb-BPs) from snake venoms are usually C-type lectins, which target specific sites of GPIbalpha and elicit distinct effects on platelets. In the present paper, we report a tetrameric platelet-agglutinating factor (molecular mass 121.1 kDa), termed mucrocetin, purified from the venom of Taiwan habu (Trimeresurus mucrosquamatus ). Mucrocetin is a GPIbalpha agonist with a binding site distinct from that of flavocetin-A (a snake venom GPIbalpha antagonist) on GPIbalpha, in spite of the high sequence identity (94.6%) between the two venom lectins. The crystal structure of mucrocetin was solved and refined to 2.8 A (1 A=0.1 nm) resolution, which shows an interesting crystal packing of six-layer cylinders of doughnut-shaped molecules. The four alphabeta heterodimers are arranged in an unusual square-shaped ring stabilized by four interdimer 'head-to-tail' disulphide bridges. Detailed structural comparison between mucrocetin and flavocetin-A suggests that their disparate platelet effects are probably attributable to different charge distributions on the putative concave binding surface. A unique positively charged patch on the binding surface of mucrocetin, formed by Lys102, Lys108, Lys109 and Arg123 in the alpha-subunit coupled with Lys22, Lys102, Lys116 and Arg117 in the beta-subunit, appears to be the primary determinant of its platelet-agglutinating activity. Conceivably, this interesting venom factor may provide a useful tool to study platelet agglutination by binding to the GPIb-IX-V complex.

Purpureotin: a novel di-dimeric C-type lectin-like protein from Trimeresurus purpureomaculatus venom is stabilized by noncovalent interactions

Purpureotin, a novel di-dimeric C-type lectin-like protein (CLP) from Trimeresurus purpureomaculatus, was purified and sequenced. While its native molecular mass was determined to be 63kDa, purpureotin showed a single band of 30kDa on nonreducing SDS-PAGE and two polypeptide chains (16.0 and 14.5kDa) under reducing condition. These results were subsequently confirmed by mass spectrometric analyses. Based on these results, we postulate that purpureotin is a dimer of the alpha,beta-heterodimer which is held together by noncovalent interactions. Molecular modeling studies indicate that a dimer of alpha,beta-heterodimers can be formed where the alpha chains are held together by electrostatic charges and beta chains via hydrophobic interactions. Functionally, purpureotin induced platelet aggregation without any cofactor in a dose-dependent manner. However, the platelet aggregation effect was blocked by echicetin. Therefore, purpureotin is assumed to be a GPIb-binding protein which binds to the same or a closely related GPIb site on platelets as echicetin.

Lebecetin, a potent antiplatelet C-type lectin from Macrovipera lebetina venom

A novel C-type lectin protein (CLP), lebecetin, was purified to homogeneity from the venom of Macrovipera lebetina by gel filtration on a Sephadex G75 column and ion exchange chromatography on Mono S column. Lebecetin is a basic protein with a pHi=9.9 and migrates in SDS-PAGE as a single band or two distinct bands under nonreducing and reducing conditions, respectively. These results are further confirmed by MALDI-TOF mass spectrometry that indicates a molecular mass of 29779 Da for native lebecetin and molecular masses of 15015 and 16296 Da for alpha and beta subunits, respectively. The N-terminal amino acid sequences of lebecetin subunits show a high degree of similarity with those of C-type lectin-like proteins. In addition, functional studies showed that lebecetin has a potent inhibitory effect on platelet aggregation induced by thrombin in a concentration-dependent manner. In contrast, no inhibitory effect is observed when platelets are exposed to thromboxane A2 (TxA2) mimetic (U46619) or arachidonic acid. Moreover, there was no effect either on blood coagulation or A, B and O washed human erythrocytes agglutination. Furthermore, flow cytometric analysis revealed that fluoro-isothiocyanate (FITC)-labelled lebecetin bound to human formalin fixed platelets in a saturable and concentration manner and this binding was specifically prevented by anti-glycoprotein Ib (GPIb) mAb. These observations suggest that lebecetin is a C-type lectin-like protein that selectively binds to platelet GPIb.

Calcium-binding analysis and molecular modeling reveal echis coagulation factor IX/factor X-binding protein has the Ca-binding properties and Ca ion-independent folding of other C-type lectin-like proteins

Many biologically active heterodimeric proteins of snake venom consist of two C-type lectin-like subunits. One of these proteins, habu IX/X-bp, is a Gla domain-binding protein whose subunits both bind to a Ca2+ ion, with a total of two Ca2+-binding sites. The molecular modeling and Ca2+-binding analysis of echis IX/X-bp revealed that it lacks one of two Ca2+-binding sites, though the folding of this subunit is conserved. It is concluded that heterodimeric C-type lectin-like proteins function independent of Ca2+ and have essentially a similar folding to habu IX/X-bp.

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.

A novel dimer of a C-type lectin-like heterodimer from the venom of Calloselasma rhodostoma (Malayan pit viper)

We have isolated a potent platelet aggregation inducer from the crude venom of Calloselasma rhodostoma (Malayan pit viper), termed rhodoaggretin, with a novel oligomeric structure consisting of a dimer of C-type lectin-like heterodimers. On the basis of its native molecular mass of 66 kDa, and a M(r) of 30 kDa for its disulfide-linked alphabeta-heterodimer, we propose that rhodoaggretin exists as a (alphabeta)2 complex in the native state. We postulate that the di-dimer is stabilized by non-covalent interactions as well as by an intersubunit disulfide bridge between the two alphabeta-heterodimers. This conclusion is based on the following observations: (a) sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of the non-reduced rhodoaggretin gave a major 28 and a minor 52 kDa band. (b) Prior treatment of rhodoaggretin with a limited amount of 2-mercaptoethanol (2-ME; 0.1%) resulted in the complete abolishment of the 52 kDa band in SDS-PAGE. (c) Two-dimensional SDS-PAGE in the presence of 3% 2-ME showed that both the 28 and 52 kDa bands gave two bands each with M(r)s of 18 (alpha-subunit) and 15 (beta-subunit) kDa. (d) Mass spectrometric analyses showed that purified rhodoaggretin had a M(r) of 30155.39+/-3.25 Da while its s-pyridylethylated alpha- and beta-subunits had M(r)s of 16535.62+/-2.98 and 15209.89+/-1.61 Da respectively. These molecular weight data suggested the presence of 15 cysteinyl residues in rhodoaggretin as compared to the 14 that are reported for the heterodimeric C-type lectin-like proteins. This extra cysteinyl residue is a candidate for the formation of the intersubunit disulfide bond in the (alphabeta)2 complex. (e) Homology structural modeling studies showed that the extra cysteinyl residue can indeed form a disulfide bond that covalently links the two alphabeta-heterodimers as proposed above.

Echicetin, a GPIb-binding snake C-type lectin from Echis carinatus, also contains a binding site for IgMkappa responsible for platelet agglutination in plasma and inducing signal transduction

Echicetin, a heterodimeric snake C-type lectin from Echis carinatus, is known to bind specifically to platelet glycoprotein (GP)Ib. We now show that, in addition, it agglutinates platelets in plasma and induces platelet signal transduction. The agglutination is caused by binding to a specific protein in plasma. The protein was isolated from plasma and shown to cause platelet agglutination when added to washed platelets in the presence of echicetin. It was identified as immunoglobulin Mkappa (IgMkappa) by peptide sequencing and dot blotting with specific heavy and light chain anti-immunoglobulin reagents. Platelet agglutination by clustering echicetin with IgMkappa induced P-selectin expression and activation of GPIIb/IIIa as well as tyrosine phosphorylation of several signal transduction molecules, including p53/56(LYN), p64, p72(SYK), p70 to p90, and p120. However, neither ethylenediaminetetraacetic acid nor specific inhibition of GPIIb/IIIa affected platelet agglutination or activation by echicetin. Platelet agglutination and induction of signal transduction could also be produced by cross-linking biotinylated echicetin with avidin. These data indicate that clustering of GPIb alone is sufficient to activate platelets. In vivo, echicetin probably activates platelets rather than inhibits platelet activation, as previously proposed, accounting for the observed induction of thrombocytopenia.

Alboaggregin A activates platelets by a mechanism involving glycoprotein VI as well as glycoprotein Ib

The snake venom C-type lectin alboaggregin A (or 50-kd alboaggregin) from Trimeresurus albolabris was previously shown to be a platelet glycoprotein (GP) Ib agonist. However, investigations of the signal transduction induced in platelets showed patterns of tyrosine phosphorylation that were different from those of other GPIb agonists and suggested the presence of an additional receptor. In this study, the binding of biotinylated alboaggregin A to platelet lysates, as well as affinity chromatography evaluations of platelet lysates on an alboaggregin A-coated column, indicated that this other receptor is GPVI. Additional experiments with reagents that inhibit either GPIb or GPVI specifically supported this finding. These experiments also showed that both GPIb and GPVI have a role in the combined signaling and that the overall direction this takes can be influenced by inhibitors of one or the other receptor pathway.

Pharmacological characterization and antithrombotic effect of agkistin, a platelet glycoprotein Ib antagonist

1. Agkistin, purified from the snake venom of Formosan Agkistrodon acutus, belongs to the family of C-type lectin GPIb binding proteins. It is a heterodimeric molecule, consisting of alpha- (16.5 kDa) and beta- (15.5 kDa) subunits with a molecular mass of 32,512 Daltons examined by SDS - PAGE and mass spectrometry. 2. In vitro, agkistin concentration-dependently inhibited ristocetin-induced human platelet agglutination and aggregation in the presence of vWF. It also inhibited TXA2 formation and prolonged the latent period in triggering aggregation by a low concentration of thrombin (0.03 u x ml(-1)). 3. 125I-agkistin specifically bound to unactivated human platelets in a saturable manner with a KD value of 223+/-10.6 nM. This binding reaction was rapid and reversible. Monoclonal antibodies, AP1 and 6D1 raised against platelet GPIb, almost completely blocked 125I-agkistin binding to platelets. However, monoclonal antibody 7E3 raised against GPIIb/IIIa complex, trigramin, a GPIIb/IIIa antagonist, ADP and EDTA did not affect 125I-agkistin binding reaction. 4. Agkistin (250 microg x kg(-1)) significantly prolonged the bleeding time and induced transient thrombocytopenia of mice when given intravenously. Furthermore, it markedly inhibited platelet plug formation in irradiated mesenteric venules of fluorescein-treated mice in vivo. 5. In conclusion, agkistin inhibits ristocetin induced platelet aggregation mainly through its specific binding to platelet GPIb, thereby blocking the interaction between GPIb and vWF. In addition, agkistin exhibits antithrombotic activity in vivo.

Molecular cloning of glycoprotein Ib-binding protein, flavocetin-A, which inhibits platelet aggregation

Flavocetin-A is a strong platelet aggregation inhibitor isolated from the venom of Trimeresurus flavoviridis. It binds specifically to platelet glycoprotein Ib with high affinity and inhibits von Willebrand factor-dependent platelet aggregation. The apparent molecular weight of flavocetin-A is 149 kDa. It consists of two subunits, alpha (17 kDa) and beta (14 kDa). The amino acid sequences of the alpha and beta subunits were determined from cloned cDNAs. Deduced amino acid sequences showed signal peptide-sequences of 23 amino acids for both alpha and beta subunits, mature peptide sequences of 135 amino acids for the alpha subunit, and 125 amino acids for the beta subunit. The amino acid sequences of alpha and beta subunits show high degrees of homology to those of C-type lectin-like venom proteins such as habu coagulation factors IX/X-binding protein (IX/X-bp), botrocetin, and alboaggregin-B. The cysteinyl residues of flavocetin-A, IX/X-bp, and botrocetin are conserved, except that flavocetin-A contains Cys 135 in the alpha subunit and Cys 3 in the beta subunit. We assumed that the arrangements of disulfide bridges in flavocetin-A are similar to those of IX/X-bp and botrocetin, and the additional Cys 135 of the alpha subunit and Cys 3 of the beta subunit are involved in novel disulfide bridges. These findings suggested that the additional disulfide bridges formed with Cys 135 of the alpha subunit and Cys 3 of the beta subunit cause polymerization of C-type lectin-like heterodimers.

Snake venom modulators of platelet adhesion receptors and their ligands

In thrombosis, platelet aggregation is initiated by a specific membrane glycoprotein (GP) Ib-IX-V complex binding to its adhesive ligand, von Willebrand factor, in the matrix of ruptured atherosclerotic plaques or in plasma exposed to high hydrodynamic shear stress. This process closely resembles normal haemostasis at high shear, where GP Ib-IX-V-dependent platelet adhesion to von Willebrand factor in the injured blood vessel wall initiates platelet activation and integrin alphaIIb beta3 (GP IIb-IIIa)-dependent platelet aggregation. At low shear, other receptors such as those that bind collagen, the integrin alpha2beta1 (GP Ia-IIa) or GP VI, mediate platelet adhesion. Recently, snake venom proteins have been identified that selectively modulate platelet function, either promoting or inhibiting platelet aggregation by targeting GP Ib-IX-V, alpha2beta1, GP VI, alphaIIb beta3, or their respective ligands. Interestingly, these venom proteins typically belong to one of two major protein families, the C-type lectin family or the metalloproteinase-disintegrins. This review focuses on recent insights into structure-activity relationships of snake venom proteins that regulate platelet function, and the ways in which these novel probes have contributed in unexpected ways to our understanding of the molecular mechanisms underlying thrombosis.

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

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

Primary structure and biological activity of snake venom lectin (APL) from Agkistrodon p. piscivorus (Eastern cottonmouth)

A lectin (APL) was purified from the venom of Agkistrodon piscivorus piscivorus (Eastern cottonmouth moccasin). APL is a disulfide-linked, homodimeric protein consisting of identical monomers of molecular weight 16,200. Native rabbit and human erythrocytes were agglutinated by APL and the activity was found to be calcium-dependent. Galactose, lactose, rhamnose and EGTA strongly inhibited the hemagglutination activity of APL. The complete amino acid sequence determined by Edman sequencing of the S-pyridylethylated derivative and its peptides derived from enzymatic digestion indicate the structure of APL to be highly homologous with lectins and the platelet glycoprotein Ib (GPIb)-binding proteins isolated from other snake venoms. These results suggest that APL belongs to the C-type beta-galactoside binding lectin family which possess structural similarities with the C-terminal carbohydrate-recognition domain (CRD) of animal membrane lectins.

Identification and Characterization of Endothelial Glycoprotein Ib Using Viper Venom Proteins Modulating Cell Adhesion

The expression and function of a glycoprotein Ib (GPIb) complex on human umbilical vein endothelial cells (HUVECs) is still a matter of controversy. We characterized HUVEC GPIb using viper venom proteins: alboaggregins A and B, echicetin, botrocetin, and echistatin. Echicetin is an antagonist, and alboaggregins act as agonists of the platelet GPIb complex. Botrocetin is a venom protein that alters von Willebrand factor (vWF) conformation and increases its binding affinity for the GPIb complex. Echistatin is a disintegrin that blocks vβ3. Echistatin, but not echicetin, inhibited the adhesion to vWF of Chinese hamster ovary (CHO) cells transfected with vβ3. We found the following: (1) Binding of monoclonal antibodies against GPIb to HUVECs was moderately increased after stimulation with cytokines and phorbol ester. Echicetin demonstrated an inhibitory effect. (2) Both echicetin and echistatin, an vβ3 antagonist, inhibited the adhesion of HUVECs to immobilized vWF in a dose-dependent manner. The inhibitory effect was additive when both proteins were used together. (3) Botrocetin potentiated the adhesion of HUVECs to vWF, and this effect was completely abolished by echicetin, but not by echistatin. (4) CHO cells expressing GPIbβ/IX adhered to vWF (in the presence of botrocetin) and to alboaggregins; GPIb was required for this reaction. Echicetin, but not echistatin, inhibited the adhesion of cells transfected with GPIbβ/IX to immobilized vWF. (5) HUVECs adhered strongly to immobilized vWF and alboaggregins with extensive spreading, which was inhibited by LJ1b1, a monoclonal antibody against GPIb. The purified vβ3 receptor did not interact with the alboaggregins, thereby excluding the contribution of vβ3 in inducing HUVEC spreading on alboaggregins. In conclusion, our data confirm the presence of a functional GPIb complex expressed on HUVECs in low density. This complex may mediate HUVEC adhesion and spreading on immobilized vWF and alboaggregins.

Identification and Characterization of Endothelial Glycoprotein Ib Using Viper Venom Proteins Modulating Cell Adhesion

The expression and function of a glycoprotein Ib (GPIb) complex on human umbilical vein endothelial cells (HUVECs) is still a matter of controversy. We characterized HUVEC GPIb using viper venom proteins: alboaggregins A and B, echicetin, botrocetin, and echistatin. Echicetin is an antagonist, and alboaggregins act as agonists of the platelet GPIb complex. Botrocetin is a venom protein that alters von Willebrand factor (vWF) conformation and increases its binding affinity for the GPIb complex. Echistatin is a disintegrin that blocks vβ3. Echistatin, but not echicetin, inhibited the adhesion to vWF of Chinese hamster ovary (CHO) cells transfected with vβ3. We found the following: (1) Binding of monoclonal antibodies against GPIb to HUVECs was moderately increased after stimulation with cytokines and phorbol ester. Echicetin demonstrated an inhibitory effect. (2) Both echicetin and echistatin, an vβ3 antagonist, inhibited the adhesion of HUVECs to immobilized vWF in a dose-dependent manner. The inhibitory effect was additive when both proteins were used together. (3) Botrocetin potentiated the adhesion of HUVECs to vWF, and this effect was completely abolished by echicetin, but not by echistatin. (4) CHO cells expressing GPIbβ/IX adhered to vWF (in the presence of botrocetin) and to alboaggregins; GPIb was required for this reaction. Echicetin, but not echistatin, inhibited the adhesion of cells transfected with GPIbβ/IX to immobilized vWF. (5) HUVECs adhered strongly to immobilized vWF and alboaggregins with extensive spreading, which was inhibited by LJ1b1, a monoclonal antibody against GPIb. The purified vβ3 receptor did not interact with the alboaggregins, thereby excluding the contribution of vβ3 in inducing HUVEC spreading on alboaggregins. In conclusion, our data confirm the presence of a functional GPIb complex expressed on HUVECs in low density. This complex may mediate HUVEC adhesion and spreading on immobilized vWF and alboaggregins.

Snake venoms and the hemostatic system

Snake venoms are complex mixtures containing many different biologically active proteins and peptides. A number of these proteins interact with components of the human hemostatic system. This review is focused on those venom constituents which affect the blood coagulation pathway, endothelial cells, and platelets. Only highly purified and well characterized snake venom proteins will be discussed in this review. Hemostatically active components are distributed widely in the venom of many different snake species, particularly from pit viper, viper and elapid venoms. The venom components can be grouped into a number of different categories depending on their hemostatic action. The following groups are discussed in this review: (i) enzymes that clot fibrinogen; (ii) enzymes that degrade fibrin(ogen); (iii) plasminogen activators; (iv) prothrombin activators; (v) factor V activators; (vi) factor X activators; (vii) anticoagulant activities including inhibitors of prothrombinase complex formation, inhibitors of thrombin, phospholipases, and protein C activators; (viii) enzymes with hemorrhagic activity; (ix) enzymes that degrade plasma serine proteinase inhibitors; (x) platelet aggregation inducers including direct acting enzymes, direct acting non-enzymatic components, and agents that require a cofactor; (xi) platelet aggregation inhibitors including: alpha-fibrinogenases, 5'-nucleotidases, phospholipases, and disintegrins. Although many snake venoms contain a number of hemostatically active components, it is safe to say that no single venom contains all the hemostatically active components described here. Several venom enzymes have been used clinically as anticoagulants and other venom components are being used in pre-clinical research to examine their possible therapeutic potential. The disintegrins are an interesting group of peptides that contain a cell adhesion recognition motif, Arg-Gly-Asp (RGD), in the carboxy-terminal half of their amino acid sequence. These agents act as fibrinogen receptor (integrin GPIIb/IIIa) antagonists. Since this integrin is believed to serve as the final common pathway leading to the formation of platelet-platelet bridges and platelet aggregation, blockage of this integrin leads to inhibition of platelet aggregation regardless of the stimulating agent. Clinical trials suggest that platelet GPIIb/IIIa blockade is an effective therapy for the thrombotic events and restenosis frequently accompanying cardiovascular and cerebrovascular disease. Therefore, because of their clinical poten tial, a large number of disintegrins have been isolated and characterized.

Measurement of platelet aggregation peptide inhibitors by ultrasonic interferometry

Several peptide inhibitors of thrombin- or collagen-induced platelet aggregation and of the interaction between glycoprotein Ib and von Willebrand factor were studied by a new method--ultrasonic interferometry (Echo Cell). Inhibition of aggregate formation in a concentration-dependent manner was observed. The sensitivity of the method was 3 to 40 times higher than that of classical turbidimetry.

Are C-type lectin-related proteins derived by proteolysis of metalloproteinase/disintegrin precursor proteins?

Metalloproteinases and disintegrins, non-enzymatic inhibitors of platelet aggregation, are derived by proteolysis from common precursors. A closer examination of the cDNA and amino acid sequences of these precursors indicated that the putative signal peptide may be an internal hydrophobic segment and that the sequences are incomplete at the 5'-region. The studies indicated that C-type lectin-related proteins are also derived from the amino terminal region of these precursors. Based on these findings, a five-domain structure is proposed for the precursors.

Synergistic effect of aurintricarboxylic acid and triflavin in a photochemically induced thrombosis model in rats

We report here the synergistic antithrombotic effect of aurintricarboxylic acid in combination with a snake venom-derived disintegrin, triflavin, in a photochemically induced thrombosis model in rats. The time to initiation of thrombus was prolonged by i.v. bolus injection of aurintricarboxylic acid at 10 mg/kg. In contrast, time to occlusion was dose-dependently prolonged by both agents, this prolongation being significant with aurintricarboxylic acid at 10 mg/kg i.v. and with triflavin at more than 3 mg/kg i.v. Interestingly, the combination of aurintricarboxylic acid at 3 mg/kg i.v. and triflavin at 1 mg/kg i.v. prolonged not only the initiation of thrombus, but also the time to occlusion.

Complete amino acid sequence and identification of the platelet glycoprotein Ib-binding site of jararaca GPIb-BP, a snake venom protein isolated from Bothrops jararaca

Jararaca GPIb-BP, a snake venom protein composed of alpha and beta subunits purified from Bothrops jararaca, binds to platelet glycoprotein (GP)Ib and functions as a receptor blocker for von Willebrand factor binding to GPIb (Fujimura, Y., Ikeda, Y., Miura, S., Yoshida, E., Shima, H., Nishida, S., Suzuki, M., Titani, K., Taniuchi, Y., and Kawasaki, T. (1995) Thromb. Haemostasis 74, 743-750). We present here the entire 142- and 123-residue amino acid sequence of the respective alpha and beta subunits and also demonstrate that the platelet GPIb-binding site resides on the beta and not on the alpha subunit based on an enzyme-linked immunosorbent assay using biotin-labeled jararaca GPIb-BP and competing ligands. Sequences of the alpha and beta subunits were determined by analysis of the intact S-pyridylethylated proteins and their peptides generated by digestion with Achromobacter protease I, Staphyloccocus aureus V8 protease, pepsin, endoproteinase Asp-N, or L-1-tosylamino-2-phenylethyl chloromethyl ketone-trypsin. A 38-39% identity of amino acid sequence between the alpha and beta subunits of jararaca GPIb-BP was observed, as well as a high degree of sequence identities (38-64%) with the respective subunits of botrocetin (Usami, Y., Fujimura, Y., Suzuki, M., Ozeki, Y., Nishio, K., Fukui, H., and Titani, K (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 928-932) and the beta-chain of echicetin (Peng, M., Holt, J. C., and Niewiarowski, S. (1994) Biochem. Biophys. Res. Commun. 205, 68-72).