Rhodocetin, a novel platelet aggregation inhibitor from the venom of Calloselasma rhodostoma (Malayan pit viper): synergistic and noncovalent interaction between its subunits

High throughput identification of human monoclonal antibodies and heavy-chain-only antibodies to treat snakebite

Snakebite envenoming is a priority Neglected Tropical Disease that causes an estimated 81,000-135,000 fatalities each year. The development of a new generation of safer, affordable, and accessible antivenom therapies is urgently needed. With this goal in mind, rigorous characterisation of the specific toxins in snake venom is key to generating novel therapies for snakebite. Monoclonal antibodies directed against venom toxins are emerging as potentially strong candidates in the development of new snakebite diagnostics and treatment. Venoms comprise many different toxins of which several are responsible for their pathological effects. Due to the large variability of venoms within and between species, formulations of combinations of human antibodies are proposed as the next generation antivenoms. Here a high-throughput screening method employing antibody-based ligand fishing of venom toxins in 384 filter-well plate format has been developed to determine the antibody target/s The approach uses Protein G beads for antibody capture followed by exposure to a full venom or purified toxins to bind their respective ligand toxin(s). This is followed by a washing/centrifugation step to remove non-binding toxins and an in-well tryptic digest. Finally, peptides from each well are analysed by nanoLC-MS/MS and subsequent Mascot database searching to identify the bound toxin/s for each antibody under investigation. The approach was successfully validated to rapidly screen antibodies sourced from hybridomas, derived from venom-immunised mice expressing either regular human antibodies or heavy-chain-only human antibodies (HCAbs).

Snake venom disintegrins update: insights about new findings

Snake venom disintegrins are low molecular weight, non-enzymatic proteins rich in cysteine, present in the venom of snakes from the families Viperidae, Crotalidae, Atractaspididae, Elapidae, and Colubridae. This family of proteins originated in venom through the proteolytic processing of metalloproteinases (SVMPs), which, in turn, evolved from a gene encoding an A Disintegrin And Metalloprotease (ADAM) molecule. Disintegrins have a recognition motif for integrins in their structure, allowing interaction with these transmembrane adhesion receptors and preventing their binding to proteins in the extracellular matrix and other cells. This interaction gives disintegrins their wide range of biological functions, including inhibition of platelet aggregation and antitumor activity. As a result, many studies have been conducted in an attempt to use these natural compounds as a basis for developing therapies for the treatment of various diseases. Furthermore, the FDA has approved Tirofiban and Eptifibatide as antiplatelet compounds, and they are synthesized from the structure of echistatin and barbourin, respectively. In this review, we discuss some of the main functional and structural characteristics of this class of proteins and their potential for therapeutic use.

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.

Basal but divergent: Clinical implications of differential coagulotoxicity in a clade of Asian vipers

Envenomations by Asian pitvipers can induce multiple clinical complications resulting from coagulopathic and neuropathic effects. While intense research has been undertaken for some species, functional coagulopathic effects have been neglected. As these species' venoms affect the blood coagulation cascade we investigated their effects upon the human clotting cascade using venoms of species from the Azemiops, Calloselasma, Deinagkistrodon and Hypnale genera. Calloselasma rhodostoma, Deinagkistrodon acutus, and Hypnale hypnale produced net anticoagulant effects through pseudo-procoagulant clotting of fibrinogen, resulting in weak, unstable, transient fibrin clots. Tropidolaemus wagleri was only weakly pseudo-procoagulant, clotting fibrinogen with only a negligible net anticoagulant effect. Azemiops feae and Tropidolaemus subannulatus did not affect clotting. This is the first study to examine in a phylogenetic context the coagulotoxic effects of related genera of basal Asiatic pit-vipers. The results reveal substantial variation between sister genera, providing crucial information about clinical effects and implications for antivenom cross-reactivity.

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.

Venomics of Calloselasma rhodostoma, the Malayan pit viper: A complex toxin arsenal unraveled

The venom of Malayan pit viper (Calloselasma rhodostoma) is highly toxic but also valuable in drug discovery. However, a comprehensive proteome of the venom that details its toxin composition and abundance is lacking. This study aimed to unravel the venom complexity through a multi-step venomic approach. At least 96 distinct proteins (29 basic, 67 acidic) in 11 families were identified from the venom. The venom consists of mainly snake venom metalloproteinases (SVMP, 41.17% of total venom proteins), within which the P-I (kistomin, 20.4%) and P-II (rhodostoxin, 19.8%) classes predominate. This is followed by C-type lectins (snaclec, 26.3%), snake venom serine protease (SVSP, 14.9%), L-amino acid oxidase (7.0%), phospholipase A2 (4.4%), cysteine-rich secretory protein (2.5%), and five minor toxins (nerve growth factor, neurotrophin, phospholipase B, 5' nucleotidase and phosphodiesterase, totaling 2.6%) not reported in the proteome hitherto. Importantly, all principal hemotoxins unveiled correlate with the syndrome: SVSP ancrod causes venom-induced consumptive coagulopathy, aggravated by thrombocytopenia caused by snaclec rhodocytin, a platelet aggregation inducer, while P-II rhodostoxin mediates hemorrhage, exacerbated by P-I kistomin and snaclec rhodocetin that inhibit platelet plug formation. These toxins exist in multiple isoforms and/or complex subunits, deserving further characterization for the development of an effective, polyspecific regional antivenom.

BIOLOGICAL SIGNIFICANCE

Advents in proteomics and bioinformatics have vigorously propelled the scientific discoveries of toxins from various lineages of venomous snakes. The Malayan pit viper, Calloselasma rhodostoma, is a medically important species in Southeast Asia as its bite can cause envenomation, while the venom is also a source of bioactive compounds for drug discovery. Detailed profiling of the venom, however, is inadequate possibly due to the complex nature of the venom and technical limitation in separating the constituents into details. Integrating a multi-step fractionation method, this study successfully revealed a comprehensive and quantitative profile of the composition of the venom of this medically important venomous snake. The relative abundance of the various venom proteins is determined in a global profile, providing useful information for understanding the pathogenic roles of the different toxins in C. rhodostoma envenomation. Notably, the principal hemotoxins were identified in great details, including the variety of toxin subunits and isoforms. The findings indicate that these toxins are the principal targets for effective antivenom neutralization, and should be addressed in the production of a pan-regional polyspecific antivenom. In addition, minor toxin components not reported previously in the venom were also detected in this study, enriching the current toxin database for the venomous snakes.

Proteomic investigation of Sri Lankan hump-nosed pit viper (Hypnale hypnale) venom

The hump-nosed pit viper, Hypanle hypnale, contributes to snakebite mortality and morbidity in Sri Lanka. Studies showed that the venom is hemotoxic and nephrotoxic, with some biochemical and antigenic properties similar to the venom of Calloselasma rhodostoma (Malayan pit viper). To further characterize the complexity composition of the venom, we investigated the proteome of a pooled venom sample from >10 Sri Lankan H. hypnale with reverse-phase high performance liquid chromatography (rp-HPLC), sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and peptide sequencing (tandem mass-spectrometry and/or N-terminal sequencing). The findings ascertained that two phospholipase A2 subtypes (E6-PLA2, W6-PLA2) dominate the toxin composition by 40.1%, followed by snake venom metalloproteases (36.9%), l-amino acid oxidase (11.9%), C-type lectins (5.5%), serine proteases (3.3%) and others (2.3%). The presence of the major toxins correlates with the venom's major pathogenic effects, indicating these to be the principal target toxins for antivenom neutralization. This study supports the previous finding of PLA2 dominance in the venom but diverges from the view that H. hypnale venom has low expression of large enzymatic toxins. The knowledge of the composition and abundance of toxins is essential to elucidate the pathophysiology of H. hypnale envenomation and to optimize antivenom formulation in the future.

Comparative analysis of the venom proteome of four important Malaysian snake species

BACKGROUND

Naja kaouthia, Ophiophagus hannah, Bungarus fasciatus and Calloselasma rhodostoma are four venomous snakes indigenous to Malaysia. In the present study, their proteomic profile by two-dimensional gel electrophoresis (2-DE) have been separated and compared.

RESULTS

The 2-DE of venoms of the four species snake demonstrated complexity and obvious interspecies differences in proteome profiles. A total of 63 proteins were identified in the four species: C. rhodostoma - 26, N. kaouthia - 16, O. hannah - 15 and B. fasciatus - 6.

CONCLUSIONS

Despite the identifications of major proteins in the four snake species, a large number of protein spots from the 2-DE were unidentified even though the spots displayed high-quality MALDI-TOF-MS spectra. Those identified included phospholipase A2 proteins in all four venoms, long neurotoxins in both cobra species and the C. rhodostoma venom found with the most varied types of peptidases, i.e. metalloproteinase kistomin, halystase and L-amino acid oxidase.

Applications of snake venom components to modulate integrin activities in cell-matrix interactions

Snake venom proteins are broadly investigated in the different areas of life science. Direct interaction of these compounds with cells may involve a variety of mechanisms that result in diverse cellular responses leading to the activation or blocking of physiological functions of the cell. In this review, the snake venom components interacting with integrins will be characterized in context of their effect on cellular response. Currently, two major families of snake venom proteins are considered as integrin-binding molecules. The most attention has been devoted to the disintegrin family, which binds certain types of integrins through specific motifs recognized as a tri-peptide structurally localized on an integrin-binding loop. Other snake venom integrin-binding proteins belong to the C-type lectin family. Snake venom molecules bind to the cellular integrins resulting in a modulation of cell signaling and in consequence, the regulation of cell proliferation, migration and apoptosis. Therefore, snake venom research on the integrin-binding molecules may have significance in biomedicine and basic cell biology.

Identification of inhibitors of α2β1 integrin, members of C-lectin type proteins, in Echis sochureki venom

Snake venom antagonists of α2β1 integrin have been identified as members of a C-lectin type family of proteins (CLP). In the present study, we characterized three new CLPs isolated from Echis sochureki venom, which interact with this integrin. These proteins were purified using a combination of gel filtration, ion exchange chromatography and reverse phase HPLC. Sochicetin-A and sochicetin-B potently inhibited adhesion of cells expressing α2β1 integrin and binding of isolated α2β1 ectodomain to collagen I, as well as bound to recombinant GST-α2A domain in ELISA, whereas activity of sochicetin-C in these assays was approximately two orders of magnitude lower. Structurally, sochicetin-B and sochicetin-C are typical heterodimeric αβ CLPs, whereas sochicetin-A exhibits a trimer of its subunits (αβ)₃ in the quaternary structure. Immobilized sochicetins supported adhesion of glioma cell lines, LN18 and LBC3, whereas in a soluble form they partially inhibited adhesion of these cells to collagen I. Glioma cells spread very poorly on sochicetin-A, showing no cytoskeleton rearrangement typical for adhesion to collagen I or fibronectin. Adhesion on CLP does not involve focal adhesion elements, such as vinculin. Sochicetin-A also inhibited collagen-induced platelet aggregation, similar to other CLPs' action on the blood coagulation system.

Rhinocetin, a venom-derived integrin-specific antagonist inhibits collagen-induced platelet and endothelial cell functions

Snaclecs are small non-enzymatic proteins present in viper venoms reported to modulate hemostasis of victims through effects on platelets, vascular endothelial, and smooth muscle cells. In this study, we have isolated and functionally characterized a snaclec that we named "rhinocetin" from the venom of West African gaboon viper, Bitis gabonica rhinoceros. Rhinocetin was shown to comprise α and β chains with the molecular masses of 13.5 and 13 kDa, respectively. Sequence and immunoblot analysis of rhinocetin confirmed this to be a novel snaclec. Rhinocetin inhibited collagen-stimulated activation of human platelets in a dose-dependent manner but displayed no inhibitory effects on glycoprotein VI (collagen receptor) selective agonist, CRP-XL-, ADP-, or thrombin-induced platelet activation. Rhinocetin antagonized the binding of monoclonal antibodies against the α2 subunit of integrin α2β1 to platelets and coimmunoprecipitation analysis confirmed integrin α2β1 as a target for this venom protein. Rhinocetin inhibited a range of collagen-induced platelet functions such as fibrinogen binding, calcium mobilization, granule secretion, aggregation, and thrombus formation. It also inhibited integrin α2β1-dependent functions of human endothelial cells. Together, our data suggest rhinocetin to be a modulator of integrin α2β1 function and thus may provide valuable insights into the role of this integrin in physiological and pathophysiological scenarios, including hemostasis, thrombosis, and envenomation.

Monoclonal antibodies reveal the alteration of the rhodocetin structure upon α2β1 integrin binding

The α2β1 antagonist rhodocetin from Calloselasma rhodostoma is a heterotetrameric CLRP (C-type lectin-related protein) consisting of four distinct chains, α, β, γ and δ. Via their characteristic domain-swapping loops, the individual chains form two subunits, αβ and γδ. To distinguish the four chains which share similar molecular masses and high sequence homologies, we generated 11 mAbs (monoclonal antibodies) with different epitope specificities. Four groups of distinct mAbs were generated: the first targeted the rhodocetin β chain, the second group bound to the αβ subunit mostly in a conformation-dependent manner, the third group recognized the γδ subunit only when separated from the αβ subunit, whereas a fourth group interacted with the γδ subunit both in the heterotetrameric molecule and complexed with the integrin α2 A-domain. Using the specific mAbs, we have shown that the rhodocetin heterotetramer dissociates into the αβ and γδ subunit upon binding to the integrin α2 A-domain at both the molecular and cellular levels. After dissociation, the γδ subunit firmly interacts with the α2β1 integrin, thereby blocking it, whereas the rhodocetin αβ subunit is released from the complex. The small molecular interface between the αβ and γδ subunits within rhodocetin is mostly mediated by charged residues, which causes the two dissociated subunits to have hydrophilic surfaces.

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.

Identification of a novel family of snake venom proteins Veficolins from Cerberus rynchops using a venom gland transcriptomics and proteomics approach

Cerberus rynchops (dog-faced water snake) belongs to Homalopsidae of Colubroidea (rear-fanged snakes). So far, venom compositions of snakes of the Homalopsidae family are not known. To determine the venom composition of C. rynchops, we have used both transcriptomics and proteomics approaches. The venom gland transcriptome revealed 104 ESTs and the presence of three known snake protein families, namely, metalloprotease, CRISP, and C-type lectin. In addition, we identified two proteins that showed sequence homology to ficolin, a mammalian protein with collagen-like and fibrinogen-like domains. We named them as ryncolin 1 and ryncolin 2 (rynchops ficolin) and this new family of snake venom proteins as veficolins (venom ficolins). On the basis of its structural similarity to ficolin, we speculate that ryncolins may induce platelet aggregation and/or initiate complement activation. To determine the proteome, the whole C. rynchops venom was trypsinized and fractionated by reverse phase HPLC followed by MALDI-MS/MS analysis of the tryptic peptides. Analysis of the tandem mass spectrometric data indicated the presence of all protein families compared to the translated cDNA library. Overall, our combined approach of transcriptomics and proteomics revealed that C. rynchops venom is among the least complex snake venom characterized to date despite the presence of a new family of snake venom proteins.

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.

Biophysical characterization of anticoagulant hemextin AB complex from the venom of snake Hemachatus haemachatus

Hemextin AB complex from the venom of Hemachatus haemachatus is the first known natural anticoagulant that specifically inhibits the enzymatic activity of blood coagulation factor VIIa in the absence of factor Xa. It is also the only known heterotetrameric complex of two three-finger toxins. Individually only hemextin A has mild anticoagulant activity, whereas hemextin B is inactive. However, hemextin B synergistically enhances the anticoagulant activity of hemextin A and their complex exhibits potent anticoagulant activity. In this study we characterized the nature of molecular interactions leading to the complex formation. Circular dichroism studies indicate the stabilization of beta-sheet in the complex. Hemextin AB complex has an increased apparent molecular diameter in both gas and liquid phase techniques. The complex formation is enthalpically favorable and entropically unfavorable with a negative change in the heat capacity. Thus, the anticoagulant complex shows less structural flexibility than individual subunits. Both electrostatic and hydrophobic interactions are important for the complexation; the former driving the process and the latter helping in the stabilization of the tetramer. The tetramer dissociates into dimers and monomers with the increase in the ionic strength of the solution and also with increase in the glycerol concentration in the buffer. The two dimers formed under each of these conditions display distinct differences in their apparent molecular diameters and anticoagulant properties. Based on these results, we have proposed a model for this unique anticoagulant complex.

Crystallization and preliminary X-ray diffraction analysis of hemextin A: a unique anticoagulant protein from Hemachatus haemachatus venom

Hemextin A was isolated and purified from African Ringhals cobra (Hemachatus haemachatus). It is a three-finger toxin that specifically inhibits blood coagulation factor VIIa and clot formation and that also interacts with hemextin B to form a unique anticoagulant complex. Hemextin A was crystallized by the hanging-drop vapour-diffusion method by equilibration against 0.2 M ammonium acetate, 0.1 M sodium acetate trihydrate pH 4.6 and 30% PEG 4000 as the precipitating agent. The crystals belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 49.27, b = 49.51, c = 57.87 A and two molecules in the asymmetric unit. They diffracted to 1.5 A resolution at beamline X25 at BNL.

Hemextin AB Complex – A Snake Venom Anticoagulant Protein Complex That Inhibits Factor VIIa Activity

Snake venom is a veritable gold mine of bioactive molecules, capable of binding to a wide variety of pharmacological targets, including the blood coagulation cascade. Here, we report the isolation and characterization of two synergistically acting anticoagulant three-finger proteins, hemextin A and hemextin B, from the venom of Hemachatus haemachatus (African Ringhals cobra). Hemextin A but not hemextin B exhibits mild anticoagulant activity. However, hemextin B interacts with hemextin A and forms a complex (hemextin AB complex), and synergistically enhances its anticoagulant potency. Prothrombin time assay showed that these two proteins form a 1:1 complex. Using a 'a dissection approach', we found that hemextins A and AB complex prolong clotting by inhibiting extrinsic tenase activity. Further studies showed that hemextin AB complex potently inhibits the proteolytic activity of factor VIIa (FVIIa) and its complexes. Kinetic studies showed that hemextin AB complex is a non-competitive inhibitor of FVIIa-soluble tissue factor proteolytic activity with a K(i) of 25 nM. Hemextin AB complex is the first reported natural inhibitor of FVIIa that does not require either tissue factor or factor Xa scaffold to mediate its inhibitory activity. Molecular interactions of hemextin AB complex with FVIIa/tissue factor-FVIIa may provide a new paradigm in the search for anticoagulants inhibiting the initiation of blood coagulation.

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.

Hemextin AB Complex, a Unique Anticoagulant Protein Complex from Hemachatus haemachatus (African Ringhals Cobra) Venom That Inhibits Clot Initiation and Factor VIIa Activity

During injury or trauma, blood coagulation is initiated by the interaction of factor VIIa (FVIIa) in the blood with freshly exposed tissue factor (TF) to form the TF.FVIIa complex. However, unwanted clot formation can lead to death and debilitation due to vascular occlusion, and hence, anticoagulants are important for the treatment of thromboembolic disorders. Here, we report the isolation and characterization of two synergistically acting anticoagulant proteins, hemextins A and B, from the venom of Hemachatus haemachatus (African Ringhals cobra). N-terminal sequences and CD spectra of the native proteins indicate that these proteins belong to the three-finger toxin family. Hemextin A (but not hemextin B) exhibits mild anticoagulant activity. However, hemextin B forms a complex (hemextin AB complex) with hemextin A and synergistically enhances its anticoagulant potency. Prothrombin time assay showed that these two proteins form a 1:1 complex. Complex formation was supported by size-exclusion chromatography. Using a "dissection approach," we determined that hemextin A and the hemextin AB complex prolong clotting by inhibiting TF.FVIIa activity. The site of anticoagulant effects was supported by their inhibitory effect on the reconstituted TF.FVIIa complex. Furthermore, we demonstrated their specificity of inhibition by studying their effects on 12 serine proteases; the hemextin AB complex potently inhibited the amidolytic activity of FVIIa in the presence and absence of soluble TF. Kinetic studies showed that the hemextin AB complex is a noncompetitive inhibitor of soluble TF.FVIIa amidolytic activity, with a Ki of 50 nm. Isothermal titration calorimetric studies showed that the hemextin AB complex binds directly to FVIIa with a binding constant of 1.62 x 10(5) m(-1). The hemextin AB complex is the first reported natural inhibitor of FVIIa that does not require a scaffold to mediate its inhibitory activity. Molecular interactions of the hemextin AB complex with FVIIa/TF.FVIIa will provide a new paradigm in the search for anticoagulants that inhibit the initiation of blood coagulation.

Snake venomics: Comparative analysis of the venom proteomes of the Tunisian snakesCerastes cerastes, Cerastes vipera andMacrovipera lebetina

The protein composition of the crude venoms of the three most important vipers of Tunisia was analyzed by RP-HPLC, N-terminal sequence analysis, MALDI-TOF mass determination, and in-gel tryptic digestion followed by PMF and CID-MS/MS of selected peptide ions in a quadrupole-linear IT instrument. Our results show that the venom proteomes of Cerastes cerastes, Cerastes vipera, and Macrovipera lebetina are composed of proteins belonging to a few protein families. However, each venom showed distinct degree of protein composition complexity. The three venoms shared a number of protein classes though the relative occurrence of these toxins was different in each snake species. On the other hand, the venoms of the Cerastes species and Macrovipera lebetina each contained unique components. The comparative proteomic analysis of Tunisian snake venoms provides a comprehensible catalogue of secreted proteins, which may contribute to a deeper understanding of the biological effects of the venoms, and may also serve as a starting point for studying structure-function correlations of individual toxins.

cDNA cloning and functional expression of jerdostatin, a novel RTS-disintegrin from Trimeresurus jerdonii and a specific antagonist of the alpha1beta1 integrin

Jerdostatin represents a novel RTS-containing short disintegrin cloned by reverse transcriptase-PCR from the venom gland mRNA of the Chinese Jerdons pit viper Trimeresurus jerdonii. The jerdostatins precursor cDNA contained a 333-bp open reading frame encoding a signal peptide, a pre-peptide, and a 43-amino acid disintegrin domain, whose amino acid sequence displayed 80% identity with that of the KTS-disintegrins obtustatin and viperistatin. The jerdostatin cDNA structure represents the first complete open reading frame of a short disintegrin and points to the emergence of jerdostatin from a short-coding gene. The different residues between jerdostatin and obtustatin/viperistatin are segregated within the integrin-recognition loop and the C-terminal tail. Native jerdostatin (r-jerdostatin-R21) and a R21K mutant (r-jerdostatin-K21) were produced in Escherichia coli. In each case, two conformers were isolated. One-dimensional (1)H NMR showed that conformers 1 and 2 of r-jerdostatin-R21 represent, respectively, well folded and unfolded proteins. The two conformers of the wild-type and the R21K mutant inhibited the adhesion of alpha(1)-K562 cells to collagen IV with IC(50) values of 180 and 703 nm, respectively. The IC(50) values of conformers 2 of r-jerdostatin-R21 and r-jerdostatin-K21 were, respectively, 5.95 and 12.5 microm. Neither r-jerdostatin-R21 nor r-jerdostatin-K21 showed inhibitory activity toward other integrins, including alpha(IIb)beta(3), alpha(v)beta(3), alpha(2)beta(1), alpha(5)beta(1), alpha(4)beta(1), alpha(6)beta(1), and alpha(9)beta(1) up to a concentration of 24 mum. Although the RTS motif appears to be more potent than KTS inhibiting the alpha(1)beta(1) integrin, r-jerdostatin-R21 is less active than the KTS-disintegrins, strongly suggesting that substitutions outside the integrin-binding motif and/or C-terminal proteolytic processing are responsible for the decreased inhibitory activity.

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.

Structural determinants of the selectivity of KTS-disintegrins for the alpha1beta1 integrin

KTS-disintegrins are a subfamily of short monomeric disintegrins that are potent and selective inhibitors of alpha1beta1 integrin. The amino acid sequence of the new KTS-disintegrin, viperistatin, differs from previously characterized obtustatin in three residues at position 24 (within the integrin binding loop), 38 (hydrophobic core) and 40 (C-terminal region). Noteworthy, viperistatin is about 25-fold more potent than obtustatin inhibiting the binding of this integrin to collagen IV. Synthetic peptides representing the full-length of integrin-binding loops of these disintegrins showed that the Leu24/Arg substitution appears to be partly responsible for the increased inhibitory activity of viperistatin over obtustatin.

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.

Structure of rhodocetin reveals noncovalently bound heterodimer interface

Rhodocetin is a unique heterodimer consisting of alpha- and beta-subunits of 133 and 129 residues, respectively. The molecule, purified from the crude venom of the Malayan pit viper, Calloselasma rhodostoma, functions as an inhibitor of collagen-induced aggregation. Rhodocetin has been shown to have activity only when present as a dimer. The dimer is formed without an intersubunit disulfide bridge, unlike all the other Ca(2+)-dependent lectin-like proteins. We report here the 1.9 A resolution structure of rhodocetin, which reveals the compensatory interactions that occur in the absence of the disulfide bridge to preserve activity.

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.

The alpha2beta1 integrin inhibitor rhodocetin binds to the A-domain of the integrin alpha2 subunit proximal to the collagen-binding site

Rhodocetin is a snake venom protein that binds to alpha2beta1 integrin, inhibiting its interaction with its endogenous ligand collagen. We have determined the mechanism by which rhodocetin inhibits the function of alpha2beta1. The interaction of alpha2beta1 with collagen and rhodocetin differed: Ca(2+) ions and slightly acidic pH values increased the binding of alpha2beta1 integrin to rhodocetin in contrast with their attenuating effect on collagen binding, suggesting that rhodocetin preferentially binds to a less active conformation of alpha2beta1 integrin. The alpha2A-domain [von Willebrand factor domain A homology domain (A-domain) of the integrin alpha2 subunit] is the major site for collagen binding to alpha2beta1. Recombinant alpha2A-domain bound rhodocetin, demonstrating that the A-domain is also the rhodocetin-binding domain. Although the interaction of alpha2beta1 with rhodocetin is affected by altering divalent cations, the interaction of the A-domain was divalent-cation-independent. The rhodocetin-binding site on the alpha2A-domain was mapped first by identifying an anti-alpha2 antibody that blocked rhodocetin binding and then mapping the epitope of the antibody using human-mouse alpha2A-domain chimaeras; and secondly, by binding studies with alpha2A-domain, which bear point mutations in the vicinity of the mapped epitope. In this way, the rhodocetin-binding site was identified as the alpha3-alpha4 loop plus adjacent alpha-helices. This region is known to form part of the collagen-binding site, thus attaining a mainly competitive mode of inhibition by rhodocetin.

Rhodocetin antagonizes stromal tumor invasion in vitro and other alpha2beta1 integrin-mediated cell functions

The pleiotropic effects of Calloselasma rhodostoma venom is caused by various toxins, among them kistrin and ancrod, which block platelet activation triggered by RGD-dependent integrins and the blood clotting cascade, respectively. Here, we demonstrate that rhodocetin, another component of this venom, acts as alpha2beta1 integrin inhibiting disintegrin and antagonizes important cellular responses to type I collagen. Cell adhesion, migration, and collagen lattice contraction in vitro were specifically inhibited by rhodocetin, whereas expression of collagen-degrading matrix metalloproteases was differently modulated. Moreover, cell invasion of HT1080 fibrosarcoma cells into a type I collagen matrix, but not into a fibrin gel or a basement membrane-extracted matrigel was efficiently blocked by rhodocetin. Unlike its natural ligand collagen, rhodocetin failed to cluster alpha2beta1 integrin, despite similar binding affinities. Hence, in the absence of focal adhesions cells do not attach firmly to rhodocetin and do not respond with any of alpha2beta1-triggered cell reactions, except for MMP-1 production. Therefore, this disintegrin may be a valuable tool to specifically target stromal tumor invasion and to manipulate other alpha2beta1 integrin-mediated functions, such as excessive scar contraction and fibrosis. Rhodocetin might be therapeutically useful because of its lack of interference with RGD-dependent integrins, low molecular mass, high solubility, and biochemical stability.

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.

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.

Purification and characterization of a variant of rhodocetin from Calloselasma rhodostoma (Malayan pit viper) venom

Rhodocetin is a novel C-type lectin-related protein (CLP) purified from the venom of Calloselasma rhodostoma. Thus far, it is the only reported CLP whose alpha and beta subunits are not linked by an interdisulfide bond. We report here the isolation of a variant of rhodocetin from a different source of venom. This variant of rhodocetin exhibited a different elution profile in reverse-phase HPLC as compared to the rhodocetin reported in our original publication [Wang et al., (1999), Biochemistry 38, 7584-7593]. Specifically, the alpha subunit of the variant was eluted at a considerably lower percentage of acetonitrile, which suggested a less hydrophobic polypeptide chain as compared to the original rhodocetin. Using a combination of microcharacterization techniques such as peptide mapping, mass spectrometry, and amino acid sequencing, we identified an amino acid substitution, 163K, in the polypeptide chain that could account for the difference in elution behavior of the alpha subunit. In addition, we also found a conserved E88D substitution in the beta chain which was not apparent during reverse-phase HPLC. However, neither of these substitutions resulted in the alteration of the functional properties of the rhodocetin variant.

alpha 2beta 1 integrin is not recognized by rhodocytin but is the specific, high affinity target of rhodocetin, an RGD-independent disintegrin and potent inhibitor of cell adhesion to collagen

We have recombinantly expressed a soluble form of human alpha(2)beta(1) integrin that lacks the membrane-anchoring transmembrane domains as well as the cytoplasmic tails of both integrin subunits. This soluble alpha(2)beta(1) integrin binds to its collagen ligands the same way as the wild-type alpha(2)beta(1) integrin. Furthermore, like the wild-type form, it can be activated by manganese ions and an integrin-activating antibody. However, it does not bind to rhodocytin, a postulated agonist of alpha(2)beta(1) integrin from the snake venom of Calloselasma rhodostoma, which elicits platelet aggregation. Taking advantage of the recombinantly expressed, soluble alpha(2)beta(1) integrin, an inhibition assay was established in which samples can be tested for their capability to inhibit binding of soluble alpha(2)beta(1) integrin to immobilized collagen. Thus, by scrutinizing the C. rhodostoma snake venom in this protein-protein interaction assay, we found a component of the snake venom that inhibits the interaction of soluble alpha(2)beta(1) integrin to type I collagen efficiently. N-terminal sequences identified this inhibitor as rhodocetin, a recently published antagonist of collagen-induced platelet aggregation. We could demonstrate that its inhibitory effect bases on its strong and specific binding to alpha(2)beta(1) integrin, proving that rhodocetin is a disintegrin. Standing apart from the growing group of RGD-dependent snake venom disintegrins, rhodocetin interacts with alpha(2)beta(1) integrin in an RGD-independent manner. Furthermore, its native conformation, which is stabilized by disulfide bridges, is indispensibly required for its inhibitory activity. Rhodocetin does not contain any major collagenous structure despite its high affinity to alpha(2)beta(1) integrin, which binds to collagenous molecules much more avidly than to noncollagenous ligands, such as laminin. Blocking alpha(2)beta(1) integrin as the major collagen receptor on platelets, rhodocetin is responsible for hampering collagen-induced, alpha(2)beta(1) integrin-mediated platelet activation, leading to hemorrhages and bleeding disorders of the snakebite victim. Moreover, having a widespread tissue distribution, alpha(2)beta(1) integrin also mediates cell adhesion, spreading, and migration. We showed that rhodocetin is able to inhibit alpha(2)beta(1) integrin-mediated adhesion of fibrosarcoma cells to type I collagen completely.

Rhodocytin induces platelet aggregation by interacting with glycoprotein Ia/IIa (GPIa/IIa, Integrin alpha 2beta 1). Involvement of GPIa/IIa-associated src and protein tyrosine phosphorylation

Although glycoprotein Ia/IIa (GPIa/IIa, integrin alpha(2)beta(1)) has established its role as a collagen receptor, it remains unclear whether GPIa/IIa mediates activation signals. In this study, we show that rhodocytin, purified from the Calloselasma rhodostoma venom, induces platelet aggregation, which can be blocked by anti-GPIa monoclonal antibodies. Studies with rhodocytin-coupled beads and liposomes loaded with recombinant GPIa/IIa demonstrated that rhodocytin directly binds to GPIa/IIa independently of divalent cations. In vitro kinase assays and Western blotting of GPIa immunoprecipitates revealed that Src and Lyn constitutively associate with GPIa/IIa and that Src activity increases transiently after rhodocytin stimulation. Src specifically associates with p130 Crk-associated substrate (Cas) in a manner dependent upon Cas phosphorylation, suggesting that Src is responsible for Cas tyrosine phosphorylation. While all these phenomena occur early after rhodocytin stimulation in a cAMP-resistant manner, tyrosine phosphorylation of Syk and phospholipase Cgamma2, intracellular Ca(2+) mobilization, and platelet aggregation occur later in a cAMP-sensitive manner. Cytochalasin D, which interferes with actin polymerization and blocks receptor clustering, inhibits all the rhodocytin-mediated signals we examined in this study. We suggest that rhodocytin, by clustering GPIa/IIa, activates GPIa/IIa-associated Src, which then mediates downstream activation signals.

Survey of the 1999 surface plasmon resonance biosensor literature

The application of surface plasmon resonance biosensors in life sciences and pharmaceutical research continues to increase. This review provides a comprehensive list of the commercial 1999 SPR biosensor literature and highlights emerging applications that are of general interest to users of the technology. Given the variability in the quality of published biosensor data, we present some general guidelines to help increase confidence in the results reported from biosensor analyses.