Purification and characterization of bitiscetin, a novel von Willebrand factor modulator protein from Bitis arietans snake venom

Anti-Metalloproteases: Production and Characterization of Polyclonal IgG Anti-F2 Fraction Antibodies Purified from the Venom of the Snake Bitis arietans

Bitis arietans is a medically important snake found in Sub-Saharan Africa. The envenomation is characterized by local and systemic effects, and the lack of antivenoms aggravates the treatment. This study aimed to identify venom toxins and develop antitoxins. The F2 fraction obtained from Bitis arietans venom (BaV) demonstrated the presence of several proteins in its composition, including metalloproteases. Titration assays carried out together with the immunization of mice demonstrated the development of anti-F2 fraction antibodies by the animals. The determination of the affinity of antibodies against different Bitis venoms was evaluated, revealing that only BaV had peptides recognized by anti-F2 fraction antibodies. In vivo analyses demonstrated the hemorrhagic capacity of the venom and the effectiveness of the antibodies in inhibiting up to 80% of the hemorrhage and 0% of the lethality caused by BaV. Together, the data indicate: (1) the prevalence of proteins that influence hemostasis and envenomation; (2) the effectiveness of antibodies in inhibiting specific activities of BaV; and (3) isolation and characterization of toxins can become crucial steps in the development of new alternative treatments. Thus, the results obtained help in understanding the envenoming mechanism and may be useful for the study of new complementary therapies.

Ancestrally reconstructed Von Willebrand Factor (vWF) reveals evidence for trench warfare coevolution between opossums and pit vipers

Opossums in the tribe Didelphini are resistant to pit viper venoms and are hypothesized to be coevolving with venomous snakes. Specifically, a protein involved in blood clotting (von Willebrand factor [vWF] which is targeted by snake venom C-type lectins [CTLs]) has been found to undergo rapid adaptive evolution in Didelphini. Several unique amino acid changes in vWF could explain their resistance; however, experimental evidence that these changes disrupt binding to venom CTLs was lacking. Furthermore, without explicit testing of ancestral phenotypes to reveal the mode of evolution, the assertion that this system represents an example of coevolution rather than noncoevolutionary adaptation remains unsupported. Using expressed vWF proteins and purified venom CTLs, we quantified binding affinity for vWF proteins from all resistant taxa, their venom-sensitive relatives, and their ancestors. We show that CTL-resistant vWF is present in opossums outside clade Didelphini and likely across a wider swath of opossums (family Didelphidae) than previously thought. Ancestral reconstruction and in vitro testing of vWF phenotypes in a clade of rapidly evolving opossums reveal a pattern consistent with trench warfare coevolution between opossums and their venomous snake prey.

Bitiscetin-3, a Novel C-Type Lectin-like Protein Cloned from the Venom Gland of the Viper Bitis arietans, Induces Platelet Agglutination and Inhibits Binding of Von Willebrand Factor to Collagen

Bitiscetin-1 (aka bitiscetin) and bitiscetin-2 are C-type lectin-like proteins purified from the venom of Bitis arietans (puff adder). They bind to von Willebrand factor (VWF) and—at least bitiscetin-1—induce platelet agglutination via enhancement of VWF binding to platelet glycoprotein Ib (GPIb). Bitiscetin-1 and -2 bind the VWF A1 and A3 domains, respectively. The A3 domain includes the major site of VWF for binding collagen, explaining why bitiscetin-2 blocks VWF-to-collagen binding. In the present study, sequences for a novel bitiscetin protein—bitiscetin-3—were identified in cDNA constructed from the B. arietans venom gland. The deduced amino acid sequences of bitiscetin-3 subunits α and β share 79 and 80% identity with those of bitiscetin-1, respectively. Expression vectors for bitiscetin-3α and -3β were co-transfected to 293T cells, producing the heterodimer protein recombinant bitiscetin-3 (rBit-3). Functionally, purified rBit-3 (1) induced platelet agglutination involving VWF and GPIb, (2) did not compete with bitiscetin-1 for binding to VWF, (3) blocked VWF-to-collagen binding, and (4) lost its platelet agglutination inducing ability in the presence of an anti-VWF monoclonal antibody that blocked VWF-to-collagen binding. These combined results suggest that bitiscetin-3 binds to the A3 domain, as does bitiscetin-2. Except for a small N-terminal fragment of a single subunit—which differs from that of both bitiscetin-3 subunits—the sequences of bitiscetin-2 have never been determined. Therefore, by identifying and analyzing bitiscetin-3, the present study is the first to present the full-length α- and β-subunit sequences and recombinant expression of a bitiscetin-family toxin that blocks the binding of VWF to collagen.

Bioactive Molecules Derived from Snake Venoms with Therapeutic Potential for the Treatment of Thrombo-Cardiovascular Disorders Associated with COVID-19

As expected, several new variants of Severe Acute Respiratory Syndrome-CoronaVirus-2 (SARS-CoV-2) emerged and have been detected around the world throughout this Coronavirus Disease of 2019 (COVID-19) pandemic. Currently, there is no specific developed drug against COVID-19 and the challenge of developing effective antiviral strategies based on natural agents with different mechanisms of action becomes an urgent need and requires identification of genetic differences among variants. Such data is used to improve therapeutics to combat SARS-CoV-2 variants. Nature is known to offer many biotherapeutics from animal venoms, algae and plant that have been historically used in traditional medicine. Among these bioresources, snake venom displays many bioactivities of interest such as antiviral, antiplatelet, antithrombotic, anti-inflammatory, antimicrobial and antitumoral. COVID-19 is a viral respiratory sickness due to SARS-CoV-2 which induces thrombotic disorders due to cytokine storm, platelet hyperactivation and endothelial dysfunction. This review aims to: (1) present an overview on the infection, the developed thrombo-inflammatory responses and mechanisms of induced thrombosis of COVID-19 compared to other similar pathogenesis; (2) underline the role of natural compounds such as anticoagulant, antiplatelet and thrombolytic agents; (3) investigate the management of coagulopathy related to COVID-19 and provide insight on therapeutic such as venom compounds. We also summarize the updated advances on antiviral proteins and peptides derived from snake venoms that could weaken coagulopathy characterizing COVID-19.

Baltetin: a new C-type lectin-like isolated from Bothrops alternatus snake venom which act as a platelet aggregation inhibiting

C-type lectin-like proteins found in snake venom, known as snaclecs, have important effects on hemostasis through targeting membrane receptors, coagulation factors and other hemostatic proteins. Here, we present the isolation and functional characterization of a snaclec isolated from Bothrops alternatus venom, designated as Baltetin. We purified the protein in three chromatographic steps (anion-exchange, affinity and reversed-phase chromatography). Baltetin is a dimeric snaclec that is approximately 15 and 25 kDa under reducing and non-reducing conditions, respectively, as estimated by SDS-PAGE. Matrix-assisted laser desorption and ionization time-of-flight mass spectrometry and Edman degradation sequencing revealed that Baltetin is a heterodimer. The first 40 amino acid residues of the N-terminal region of Baltetin subunits share a high degree of sequence identity with other snaclecs. Baltetin had a specific, dose-dependent inhibitory effect on epinephrine-induced platelet aggregation in human platelet-rich plasma, inhibiting up to 69% of platelet aggregation. Analysis of the infrared spectra suggested that the interaction between Baltetin and platelets can be attributed to the formation of hydrogen bonds between the PO32- groups in the protein and PO2- groups in the platelet membrane. This interaction may lead to membrane lipid peroxidation, which prevents epinephrine from binding to its receptor. The present work suggests that Baltetin, a new C-type lectin-like protein isolated from B. alternatus venom, is the first snaclec to inhibit epinephrine-induced platelet aggregation. This could be of medical interest as a new tool for the development of novel therapeutic agents for the prevention and treatment of thrombotic disorders.

Resistance of South American opossums to vWF-binding venom C-type lectins

Opossums in the clade Didelphini are well known to be resistant to snake venom due to endogenous circulating inhibitors which target metalloproteinases and phospholipases. However, the mechanisms through which these opossums cope with a variety of other damaging venom proteins are unknown. A protein involved in blood clotting (von Willebrand Factor) has been found to have undergone rapid adaptive evolution in venom-resistant opossums. This protein is a known target for a subset of snake venom C-type lectins (CTLs), which bind it and then induce it to bind platelets, causing hemostatic disruption. Several amino acid changes in vWF unique to these opossums could explain their resistance; however, experimental evidence that these changes disrupt venom CTL binding was lacking. We used platelet aggregation assays to quantify resistance to a venom-induced platelet response in two species of venom-resistant opossums (Didelphis virginiana, Didelphis aurita), and one venom-sensitive opossum (Monodelphis domestica). We found that all three species have lost nearly all their aggregation response to the venom CTLs tested. Using washed platelet assays we showed that this loss of aggregation response is not due to inhibitors in the plasma, but rather to the failure of either vWF or platelets (or both) to respond to venom CTLs. These results demonstrate the potential adaptive function of a trait previously shown to be evolving under positive selection. Surprisingly, these findings also expand the list of potentially venom tolerant species to include Monodelphis domestica and suggest that an ecological relationship between opossums and vipers may be a broader driver of adaptive evolution across South American marsupials than previously thought.

Snake Venoms in Drug Discovery: Valuable Therapeutic Tools for Life Saving

Animal venoms are used as defense mechanisms or to immobilize and digest prey. In fact, venoms are complex mixtures of enzymatic and non-enzymatic components with specific pathophysiological functions. Peptide toxins isolated from animal venoms target mainly ion channels, membrane receptors and components of the hemostatic system with high selectivity and affinity. The present review shows an up-to-date survey on the pharmacology of snake-venom bioactive components and evaluates their therapeutic perspectives against a wide range of pathophysiological conditions. Snake venoms have also been used as medical tools for thousands of years especially in tradition Chinese medicine. Consequently, snake venoms can be considered as mini-drug libraries in which each drug is pharmacologically active. However, less than 0.01% of these toxins have been identified and characterized. For instance, Captopril^® (Enalapril), Integrilin^® (Eptifibatide) and Aggrastat^® (Tirofiban) are drugs based on snake venoms, which have been approved by the FDA. In addition to these approved drugs, many other snake venom components are now involved in preclinical or clinical trials for a variety of therapeutic applications. These examples show that snake venoms can be a valuable source of new principle components in drug discovery.

Kn-Ba: a novel serine protease isolated from Bitis arietans snake venom with fibrinogenolytic and kinin-releasing activities

Background

Bitis arietans is a venomous snake found in sub-Saharan Africa and in parts of Morocco and Saudi Arabia. The envenomation is characterized by local and systemic reactions including pain, blistering, edema and tissue damage, besides hemostatic and cardiovascular disturbances, which can cause death or permanent disabilities in its victims. However, the action mechanisms that provoke these effects remain poorly understood, especially the activities of purified venom components. Therefore, in order to elucidate the molecular mechanisms that make the Bitis arietans venom so potent and harmful to human beings, this study reports the isolation and biochemical characterization of a snake venom serine protease (SVSP).

Methods

Solubilized venom was fractionated by molecular exclusion chromatography and the proteolytic activity was determined using fluorescent substrates. The peaks that showed serine protease activity were determined by blocking the proteolytic activity with site-directed inhibitors. In sequence, the fraction of interest was submitted to another cycle of molecular exclusion chromatography. The purified serine protease was identified by mass spectrometry and characterized biochemically and immunochemically.

Results

A serine protease of 33 kDa with fibrinogen-degrading and kinin-releasing activities was isolated, described, and designated herein as Kn-Ba. The experimental Butantan Institute antivenom produced against Bitis arietans venom inhibited the Kn-Ba activity.

Conclusions

The in vitro activities of Kn-Ba can be correlated with the capacity of the venom to provoke bleeding and clotting disorders as well as hypotension, which are common symptoms presented by envenomed victims. Obtaining satisfactory Kn-Ba inhibition through the experimental antivenom is important, given the WHO’s recommendation of immunotherapy in cases of human accidents with venomous snakes.

Electronic supplementary material

The online version of this article (10.1186/s40409-018-0176-5) contains supplementary material, which is available to authorized users.

The role of platelets in hemostasis and the effects of snake venom toxins on platelet function

The human body has a set of physiological processes, known as hemostasis, which keeps the blood fluid and free of clots in normal vessels; in the case of vascular injury, this process induces the local formation of a hemostatic plug, preventing hemorrhage. The hemostatic system in humans presents complex physiological interactions that involve platelets, plasma proteins, endothelial and subendothelial structures. Disequilibrium in the regulatory mechanisms that control the growth and the size of the thrombus is one of the factors that favors the development of diseases related to vascular disorders such as myocardial infarction and stroke, which are among the leading causes of death in the western world. Interfering with platelet function is a strategy for the treatment of thrombotic diseases. Antiplatelet drugs are used mainly in cases related to arterial thrombosis and interfere in the formation of the platelet plug by different mechanisms. Aspirin (acetylsalicylic acid) is the oldest and most widely used antithrombotic drug. Although highly effective in most cases, aspirin has limitations compared to other drugs used in the treatment of homeostatic disorders. For this reason, research related to molecules that interfere with platelet aggregation are of great relevance. In this regard, snake venoms are known to contain a number of molecules that interfere with hemostasis, including platelet function. The mechanisms by which snake venom components inhibit or activate platelet aggregation are varied and can be used as tools for the diagnosis and the treatment of several hemostatic disorders. The aim of this review is to present the role of platelets in hemostasis and the mechanisms by which snake venom toxins interfere with platelet function.

Mutant botrocetin-2 inhibits von Willebrand factor-induced platelet agglutination

Essentials Botrocetin-2 (Bot2) binds to von Willebrand factor (VWF) and induces platelet agglutination. We identified Bot2 residues that are required for binding to VWF and glycoprotein (GP) Ib. We produced a mutant Bot2 that binds to VWF but inhibits platelet agglutination. Mutant Bot2 could be used as a potential anti-thrombotic reagent to block VWF-GPIb interaction.

SUMMARY

Background Botrocetin-2 (Bot2) is a botrocetin-like protein composed of α and β subunits that have been cloned from the snake Bothrops jararaca. Bot2 binds specifically to von Willebrand factor (VWF), and the complex induces glycoprotein (GP) Ib-dependent platelet agglutination. Objectives To exploit Bot2's VWF-binding capacity in order to attempt to create a mutant Bot2 that binds to VWF but inhibits platelet agglutination. Methods and Results Several point mutations were introduced into Bot2 cDNA, and the recombinant protein (recombinant Bot2 [rBot2]) was purified on an anti-botrocetin column. The mutant rBot2 with either Ala at Asp70 in the β subunit (Aspβ70Ala), or Argβ115Ala and Lysβ117Ala, showed reduced platelet agglutination-inducing activity. rBot2 with Aspβ70Ala showed little binding activity towards immobilized VWF on an ELISA plate, whereas rBot2 with Argβ115Ala/Lysβ117Ala showed reduced binding activity towards GPIb (glycocalicin) after forming a complex with VWF. rBot2 point-mutated to oppositely charged Glu at both Argβ115 and Lysβ117 showed normal binding activity towards VWF but no platelet-agglutinating activity. Furthermore, this doubly mutated protein inhibited ristocetin-induced or high shear stress-induced platelet aggregation, and restrained thrombus formation under flow conditions. Conclusions Asp70 in the β subunit of botrocetin is important for VWF binding, and Arg115 and Lys117 in the β subunit are essential for interaction with GPIb. Doubly mutated rBot2, with Argβ115Glu and Lysβ117Glu, repels GPIb and might have potential as an antithrombotic reagent that specifically blocks VWF function. This is the first report on an artificial botrocetin that can inhibit the VWF-GPIb interaction.

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.

Haemostatically active proteins in snake venoms

Snake venom proteins that affect the haemostatic system can cause (a) lowering of blood coagulability, (b) damage to blood vessels, resulting in bleeding, (c) secondary effects of bleeding, e.g. hypovolaemic shock and organ damage, and (d) thrombosis. These proteins may, or may not, be enzymes. We review the data on the most relevant haemostatically active proteinases, phospholipases A₂, L-amino acid oxidases and 5'-nucleotidases from snake venoms. We also survey the non-enzymatic effectors of haemostasis from snake venoms--disintegrins, C-type lectins and three-finger toxins. Medical applications have already been found for some of these snake venom proteins. We describe those that have already been approved as drugs to treat haemostatic disorders or are being used to diagnose such health problems. No clinical applications, however, currently exist for the majority of snake venom proteins acting on haemostasis. We conclude with the most promising potential uses in this respect.

Interaction of the cysteine-rich domain of snake venom metalloproteinases with the A1 domain of von Willebrand factor promotes site-specific proteolysis of von Willebrand factor and inhibition of von Willebrand factor-mediated platelet aggregation

Snake venom metalloproteinases (SVMPs) have recently been shown to interact with proteins containing von Willebrand factor A (VWA) domains, including the extracellular matrix proteins collagen XII, collagen XIV, matrilins 1, 3 and 4, and von Willebrand factor (VWF) via their cysteine-rich domain. We extended those studies using surface plasmon resonance to investigate the interaction of SVMPs with VWF, and demonstrated that jararhagin, a PIII SVMP containing a metalloproteinase domain followed by disintegrin-like and cysteine-rich domains, catrocollastatin C, a disintegrin-like/cysteine-rich protein, and the recombinant cysteine-rich domain of atrolysin A (A/C) all interacted with immobilized VWF in a dose-dependent fashion. Binding of VWF in solution to immobilized A/C was inhibited by ristocetin and preincubation of platelets with A/C abolished ristocetin/VWF-induced platelet aggregation, indicating that the interaction of A/C with VWF is mediated by the VWA1 domain. Jararhagin cleaved VWF at sites adjacent to the VWA1 domain, whereas atrolysin C, a SVMP lacking the cysteine-rich domain, cleaved VWF at dispersed sites. A/C and catrocollastatin C completely inhibited the digestion of VWF by jararhagin, demonstrating that the specific interaction of jararhagin with VWF via the VWA1 domain is necessary for VWF proteolysis. In summary, we localized the binding site of PIII SVMPs in VWF to the A1 domain. This suggests additional mechanisms by which SVMPs may interfere with the adhesion of platelets at the site of envenoming. Thus, specific interaction of cysteine-rich domain-containing SVMPs with VWF may function to promote the hemorrhage caused by SVMP proteolysis of capillary basements and surrounding stromal extracellular matrix.

Amino acid sequence and characterization of C-type lectin purified from the snake venom of Crotalus ruber

Galactoside-binding lectin was purified from the snake venom of Crotalus ruber by affinity chromatography on a lactose-agarose column, and the complete amino acid sequence was determined. The C. ruber venom lectin (CRL) showed a single band of 28 kDa by SDS-polyacrylamide electrophoresis under non-reducing conditions, but it showed a single band of 15 kDa under reducing conditions, indicating that CRL is a disulfide-linked homodimer of 15 kDa subunit. CRL specifically recognized beta-galactosides such as thiodigalactoside followed by N-acetylgalactosamine when examined with their inhibitory effects on CRL-induced hemagglutination. A CRL subunit was composed of 135 residues containing nine Cys residues and showed a high similarity to other C-type galactoside-binding lectins from snake venoms. C. atrox lectin (CAL) showed almost the same sequence except for eight amino acid residues. Neither CRL nor CAL induced platelet aggregation by itself or inhibited platelet aggregation mediated by von Willebrand factor or fibrinogen with agonists. CRL showed a similar oligomeric form and the sugar specificity as CAL, but it showed different divalent cation sensitivity such as Mn(2+) and Ni(2+). Homology modeling suggested that the amino acid substitution found in CRL does not affect sugar recognition of the lectin but might alter the conformation and influence the sugar binding pocket induced by the metal-ion binding.

Characterization of bitiscetin-2, a second form of bitiscetin from the venom of Bitis arietans : comparison of its binding site with the collagen-binding site on the von Willebrand factor A3-domain

BACKGROUND

Bitiscetin, a heterodimeric snake venom protein purified from Bitis arietans, binds to the A1 domain of von Willebrand factor (VWF) and induces binding of this domain to platelet glycoprotein (GP) Ib. We previously purified a distinct form of dimeric bitiscetin (herein called bitiscetin-2) that also induces the VWF A1 domain-GPIb interaction, but does not bind to the A1 domain. Instead, it interacts with the collagen-binding A3 domain of VWF.

METHODS

In the current study we identify the amino terminal sequence of the bitiscetin-2 as DEGCLPDDSSRT, showing conclusively that the protein is distinct form the originally described bitiscetin. We further studied the interaction of bitiscetin-2 and VWF using DeltaA3 VWF and a series of 33 VWF point mutants previously prepared to map the collagen-binding site.

RESULTS

Our results confirm that DeltaA3 VWF, even though containing the A1-domain, is unable to interact with bitiscetin-2. Mutation of VWF-A3 residues Ile975, Asp979, Pro981, Ser1020 and His1023 reduces binding by 80% while mutation of residues Val980, Glu1001 and Arg1021 reduces binding by 30-60%. A 2- to 6-fold increase of binding is caused by mutation of residues Val985, Glu987, and Arg1016.

CONCLUSION

Nearly all of these mutations also affect collagen binding showing that the binding sites for bitiscetin-2 and collagen type III in the VWF-A3 domain closely overlap.

Structure-Function Relationships of C-Type Lectin-Related Proteins

The structural and functional studies of the first identified C-type lectin-like protein (CLP), blood coagulation factor IX/factor X-binding protein (IX/X-bp), have been instrumental in defining how new functionally heterodimeric CLPs are generated from monomeric carbohydrate recognition domain in C-type lectins by three-dimensional domain swapping. The crystal structures of gamma-carboxyglutamic acid domains of coagulation factors X and IX have recently been clarified in structural studies of complexes between the gamma-carboxyglutamic acid domain of factors X and X-bp (a venom CLP) and between the gamma-carboxyglutamic acid domain of factors IX and IX-bp (a venom CLP).

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.

Characterization and molecular cloning of dabocetin, a potent antiplatelet C-type lectin-like protein from Daboia russellii siamensis venom

A novel C-type lectin-like protein, dabocetin, was purified from Daboia russellii siamensis venom. On SDS-polyacrylamide gel electrophoresis, it showed a single band with an apparent molecular weight of 28 kDa and two distinct bands with the apparent molecular weights of 15.0 kDa and 14.5 kDa under non-reducing and reducing conditions, respectively. cDNA clones containing the coding sequences for dabocetin alpha and beta subunits were isolated and sequenced. The deduced protein sequences of both subunits were confirmed by N-terminal amino acid sequencing and trypsin-digested peptide mass fingerprinting. Dabocetin did not induce platelet aggregation in platelet-rich plasma. It also had little effect on the platelet aggregation induced by ADP, TMVA or stejnulxin. Whereas, dabocetin inhibited ristocetin-induced platelet agglutination in platelet-rich plasma in a dose-dependent manner with an IC50 value of 0.35 microM. Flow cytometry analysis showed that dabocetin significantly inhibited mAb SZ2 binding to platelet membrane glycoprotein Ib alpha, indicating that platelet membrane glycoprotein Ib is involved in the inhibitory effect of dabocetin on ristocetin-induced platelet agglutination.

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.

A C-type lectin-like protein from Agkistrodon acutus venom binds to both platelet glycoprotein Ib and coagulation factor IX/factor X

Agkisacutacin, a C-type lectin-like protein (CLP) isolated from Agkistrodon acutus venom, had been previously identified as an antagonist of platelet aggregation induced by ristocetin, as well as a certain extent fibrinogenlytic activity. In this study, agkisacutacin was further purified by three-step chromatography, and its biological functions were characterized. Agkisacutacin after further purification retained the effect on ristocetin-induced, von Willebrand factor-dependent platelet aggregation, while it lost the fibrinogenlytic activity. FACS and ELISA assays showed that agkisacutacin belongs to membrane glycoprotein Ib-binding protein (GPIb-bp) for it could block and inhibit the binding of anti-GPIb antibody to GPIb. Especially, agkisacutacin exhibits anti-coagulant activity and the function as IX/X-binding protein was confirmed by PAGE and ELISA. So, agkisacutacin is the first reported CLP that binds to both platelet membrane receptors and coagulation factors.

Structure and function of snake venom toxins interacting with human von Willebrand factor

Hemostatic plug formation is a complex event mediated by platelets, subendothelial matrices and von Willebrand factor (VWF) at the vascular injury. Snake venom proteins have an excellent potency to regulate the interaction between VWF and platelet membrane receptors in vitro. Two protein families, C-type lectin-like proteins and Zn(2+)-metalloproteinases, have been found to affect platelet-VWF interaction. Botrocetin and bitiscetin from viper venom are disulfide-linked heterodimers with C-type lectin-like motif, and modulate VWF to elicit platelet glycoprotein Ib (GPIb)-binding activity via the A1 domain of VWF leading to the platelet agglutination. The crystal structures of botrocetin and bitiscetin together with complex from the VWF A1 domain indicate the following: (1) a central concave domain formed by two subunits of botrocetin or bitiscetin provides the binding site for VWF, (2) these modulators directly bind to the A1 domain of VWF in close proximity to the GPIb binding site, (3) both modulators induce no significant conformational change on the GPIb-binding site of the A1 domain but could provide a supplemental platform fitting for GPIb. These results suggest that the modulating mechanisms of these venoms are different from those performed by either antibiotic ristocetin in vitro or extremely high shear stress in vivo. Other modulator toxins include kaouthiagin and jararhagin, chimeric proteins composed of metalloproteinase, disintegrin-like and Cys-rich domains. These toxins cleave VWF and reduce its platelet agglutinating or collagen-binding activity. Kaouthiagin from cobra venom specifically cleaves between Pro708 and Asp709 in the C-terminal VWF A1 domain resulting in the decrease of the multimer structure of VWF. Recently a plasma proteinase, which specifically cleaves VWF into a smaller multimer, has been elucidated to be a reprolysin-like metalloproteinase with thrombospondin motif family (ADAMTS). This endogenous metalloproteinase (ADAMTS-13) specifically cleaves between Tyr842 and Met843 in the A2 domain of VWF regulating its physiological hemostatic activity. These VWF-binding snake venom proteins are suitable probes for basic research on platelet plug formation mediated by VWF, for subsidiary diagnostic use for von Willebrand disease or platelet disorder, and might be potently applicable to the regulation of VWF in thrombosis and hemostasis. Structural information of these venom proteins together with recombinant technology might strongly promote the construction of a new antihemostatic drug in the near future.

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.

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.

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.

Evolutionary analysis reveals collective properties and specificity in the C-type lectin and lectin-like domain superfamily

Members of the C-type lectin/C-type lectin-like domain (CTL/CTLD) superfamily share a common fold and are involved in a variety of functions, such as generalized defense mechanisms against foreign agents, discrimination between healthy and pathogen-infected cells, and endocytosis and blood coagulation. In this work we used ConSurf, a computer program recently developed in our lab, to perform an evolutionary analysis of this superfamily in order to further identify characteristics of all or part of its members. Given a set of homologous proteins in the form of multiple sequence alignment (MSA) and an inferred phylogenetic tree, ConSurf calculates the conservation score in every alignment position, taking into account the relationships between the sequences and the physicochemical similarity between the amino acids. The scores are then color-coded onto the three-dimensional structure of one of the homologous proteins. We provide here and at http://ashtoret.tau.ac.il/ approximately sharon a detailed analysis of the conservation pattern obtained for the entire superfamily and for two subgroups of proteins: (a) 21 CTLs and (b) 11 heterodimeric CTLD toxins. We show that, in general, proteins of the superfamily have one face that is constructed mostly of conserved residues and another that is not, and we suggest that the former face is involved in binding to other proteins or domains. In the CTLs examined we detected a region of highly conserved residues, corresponding to the known calcium- and carbohydrate-binding site of the family, which is not conserved throughout the entire superfamily, and in the CTLD toxins we found a patch of highly conserved residues, corresponding to the known dimerization region of these proteins. Our analysis also detected patches of conserved residues with yet unknown function(s).

Novel sequences encoding venom C-type lectins are conserved in phylogenetically and geographically distinct Echis and Bitis viper species

Envenoming by Echis saw scaled vipers and Bitis arietans puff adders is the leading cause of death and morbidity in Africa due to snake bite. Despite their medical importance, the composition and constituent functionality of venoms from these vipers remains poorly understood. Here, we report the cloning of cDNA sequences encoding seven clusters or isoforms of the haemostasis-disruptive C-type lectin (CTL) proteins from the venom glands of Echis ocellatus, E. pyramidum leakeyi, E. carinatus sochureki and B. arietans. All these CTL sequences encoded the cysteine scaffold that defines the carbohydrate-recognition domain of mammalian CTLs. All but one of the Echis and Bitis CTL sequences showed greater sequence similarity to the beta than alpha CTL subunits in venoms of related Asian and American vipers. Four of the new CTL clusters showed marked inter-cluster sequence conservation across all four viper species which were significantly different from that of previously published viper CTLs. The other three Echis and Bitis CTL clusters showed varying degrees of sequence similarity to published viper venom CTLs. Because viper venom CTLs exhibit a high degree of sequence similarity and yet exert profoundly different effects on the mammalian haemostatic system, no attempt was made to assign functionality to the new Echis and Bitis CTLs on the basis of sequence alone. The extraordinary level of inter-specific and inter-generic sequence conservation exhibited by the Echis and Bitis CTLs leads us to speculate that antibodies to representative molecules should neutralise the biological function of this important group of venom toxins in vipers that are distributed throughout Africa, the Middle East and the Indian subcontinent.

Crystal structure of von Willebrand factor A1 domain complexed with snake venom, bitiscetin: insight into glycoprotein Ibalpha binding mechanism induced by snake venom proteins

Bitiscetin, a platelet adhesion inducer isolated from venom of the snake Bitis arietans, activates the binding of the von Willebrand factor (VWF) A1 domain to glycoprotein Ib (GPIb) in vitro. This activation requires the formation of a bitiscetin-VWF A1 complex, suggesting an allosteric mechanism of action. Here, we report the crystal structure of bitiscetin-VWF A1 domain complex solved at 2.85 A. In the complex structure, helix alpha5 of VWF A1 domain lies on a concave depression on bitiscetin, and binding sites are located at both ends of the depression. The binding sites correspond well with those proposed previously based on alanine-scanning mutagenesis (Matsui, T., Hamako, J., Matsushita, T., Nakayama, T., Fujimura, Y., and Titani, K. (2002) Biochemistry 41, 7939-7946). Against our expectations, the structure of the VWF A1 domain bound to bitiscetin does not differ significantly from the structure of the free A1 domain. These results are similar to the case of botrocetin, another snake-derived inducer of platelet aggregation, although the binding modes of botrocetin and bitiscetin are different. The modeled structure of the ternary bitiscetin-VWF A1-GPIb complex suggests that an electropositive surface of bitiscetin may interact with a favorably positioned anionic region of GPIb. These results suggest that snake venom proteins induce VWF A1-GPIbalpha binding by interacting with both proteins, and not by causing conformational changes in VWF A1.

A new mutation, S1285F, within the A1 loop of von Willebrand factor induces a conformational change in A1 loop with abnormal binding to platelet GPIb and botrocetin causing type 2M von Willebrand disease

We report the identification of a new mutation in exon 28 of the von Willebrand factor (VWF) gene in two related patients with type 2M von Willebrand disease (VWD). The molecular abnormality changes the Ser 1285 to Phe within the A1 loop of VWF. The S1285F mutation was reproduced by site-directed mutagenesis on the full-length VWF cDNA. The mutated recombinant VWF (rVWF), F1285rVWF, and the hybrid, S/F1285rVWF, were expressed in COS-7 cells. F1285rVWF exhibited a slight decrease of high-molecular-weight multimers and markedly reduced ristocetin- or botrocetin-induced binding of VWF to platelets in association with a decreased binding to botrocetin. The hybrid S/F1285rVWF showed a normal multimeric profile and bound to platelets in a similar way to the patients' plasma VWF, in the presence of ristocetin or botrocetin. Thus, the new S1285F mutation within the A1 loop was responsible for the type 2M VWD observed in these patients, and was involved in the binding of VWF to botrocetin and to platelet glycoprotein Ib (GPIb). Three anti-VWF monoclonal antibodies, with conformational epitopes within the A1 loop but distinct GPIb binding inhibitory properties, showed a different interaction with F1285rVWF. These results indicate that the S1285F substitution alters the folding of the A1 loop and prevents the correct exposure of the VWF binding sites to botrocetin and GPIb.

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.

Multifunctional snake C-type lectins affecting platelets

Snake venoms contain a wide range of components, many of which affect haemostasis by activation or inhibition of platelets or coagulation factors. They can be classified into groups based on structure and mode of action. One group is the snake C-type lectins, so called because of the typical folding which closely resembles that found in classical C-type lectins, such as selectins and mannose-binding proteins. Unlike the classic C-type lectins, those from snakes are generally heterodimeric with two subunits, alpha and beta. Some are multimeric heterodimers. The subunits have homologous sequences and are generally linked by a disulphide bond as well as by swapping loops. One of the first C-type lectins with a defined function was echicetin which was demonstrated to bind to platelet GPIb and block several functions of this receptor. Since then, many proteins with similar structure have been reported to act on platelet receptors or coagulation factors and several have been crystallized. These proteins were thought to be specific for a single platelet receptor or coagulation factor, i.e. they had only one receptor per heterodimer. Recent studies show that most of these C-type lectins have binding sites for more than one ligand and have complex mechanisms of action.

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.

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.

Conformational Changes in the A3 Domain of von Willebrand Factor Modulate the Interaction of the A1 Domain With Platelet Glycoprotein Ib

Bitiscetin has recently been shown to induce von Willebrand factor (vWF)-dependent aggregation of fixed platelets (Hamako J, et al,Biochem Biophys Res Commun 226:273, 1996). We have purified bitiscetin from Bitis arietans venom and investigated the mechanism whereby it promotes a form of vWF that is reactive with platelets. In the presence of bitiscetin, vWF binds to platelets in a dose-dependent and saturable manner. The binding of vWF to platelets involves glycoprotein (GP) Ib because it was totally blocked by monoclonal antibody (MoAb) 6D1 directed towards the vWF-binding site of GPIb. The binding also involves the GPIb-binding site of vWF located on the A1 domain because it was inhibited by MoAb to vWF whose epitopes are within this domain and that block binding of vWF to platelets induced by ristocetin or botrocetin. However, in contrast to ristocetin or botrocetin, the binding site of bitiscetin does not reside within the A1 domain but within the A3 domain of vWF. Thus, among a series of vWF fragments, 125I-bitiscetin only binds to those that overlap the A3 domain, ie, SpIII (amino acid [aa] 1-1365), SpI (aa 911-1365), and rvWF-A3 domain (aa 920-1111). It does not bind to SpII corresponding to the C-terminal part of vWF subunit (aa 1366-2050) nor to the 39/34/kD dispase species (aa 480-718) or T116 (aa 449-728) overlapping the A1 domain. In addition, bitiscetin that does not bind to DeltaA3-rvWF (deleted between aa 910-1113) has no binding site ouside the A3 domain. The localization of the binding site of bitiscetin within the A3 domain was further supported by showing that MoAb to vWF, which are specific for this domain and block the interaction between vWF and collagen, are potent inhibitors of the binding of bitiscetin to vWF and consequently of the bitiscetin-induced binding of vWF to platelets. Thus, our data support the hypothesis that an interaction between the A1 and A3 domains exists that may play a role in the function of vWF by regulating the ability of the A1 domain to bind to platelet GPIb.

A novel high molecular weight fibrinogenase from the venom of Bitis arietans

A fibrinogenase (Ba100) with an apparent molecular mass of 100 kDa under non-reducing conditions and a pI of 5.4 was purified from the venom of the African puff adder (Bitis arietans) by fibrinogen affinity chromatography. Under reducing conditions the protease dissociates into subunits of 21 kDa and 16 kDa. N-Terminal amino acid sequencing showed these two chains to have 66.7% homology and homology to C-type lectins. The fibrinogenase activity of Ba100 cleaves the Aalpha and Bbeta chain of fibrinogen rendering the molecule unable to polymerise into fibrin clots. Ba100 inhibited platelet aggregation in platelet rich plasma, and clot formation in whole blood, in a concentration dependent manner.

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.

Rhodocytin, a functional novel platelet agonist belonging to the heterodimeric C-type lectin family, induces platelet aggregation independently of glycoprotein Ib

We isolated and characterized a functionally novel platelet agonist, designated as rhodocytin, from the Calloselasma rhodostoma venom. Rhodocytin was a disulfide-linked heterodimer consisting of 18- and 15-kDa subunits. The respective N-terminal amino acid sequences of both subunits were homologous to each other and to those of the carbohydrate-recognition domains (CRD) of C-type lectins. Rhodocytin alone induced platelet aggregation. Platelet agonists and antagonists constructed with CRD-like subunits from snake venoms bind to glycoprotein Ib directly or indirectly. However, rhodocytin induced platelet aggregation not by binding to glycoprotein Ib, because rhodocytin-induced platelet aggregation was not influenced by echicetin, a glycoprotein Ib-binding protein, that completely inhibits platelet agglutination by bovine von Willebrand factor. These findings indicate that rhodocytin is a novel protein structurally related to heterodimers of CRD-like subunits, but functionally distinct from venom proteins that induce platelet aggregation via glycoprotein Ib.

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.