Structural and functional characterization of a novel homodimeric three-finger neurotoxin from the venom of Ophiophagus hannah (king cobra)

Exploring Snake Venoms Beyond the Primary Sequence: from Proteoforms to Protein-Protein Interactions

Snakebite envenomation has been a long-standing global issue that is difficult to treat, largely owing to the flawed nature of current immunoglobulin-based antivenom therapy and the complexity of snake venoms as sophisticated mixtures of bioactive proteins and peptides. Comprehensive characterisation of venom compositions is essential to better understanding snake venom toxicity and inform effective and rationally designed antivenoms. Additionally, a greater understanding of snake venom composition will likely unearth novel biologically active proteins and peptides that have promising therapeutic or biotechnological applications. While a bottom-up proteomic workflow has been the main approach for cataloguing snake venom compositions at the toxin family level, it is unable to capture snake venom heterogeneity in the form of protein isoforms and higher-order protein interactions that are important in driving venom toxicity but remain underexplored. This review aims to highlight the importance of understanding snake venom heterogeneity beyond the primary sequence, in the form of post-translational modifications that give rise to different proteoforms and the myriad of higher-order protein complexes in snake venoms. We focus on current top-down proteomics workflows to identify snake venom proteoforms and further discuss alternative or novel separation, instrumentation, and data processing strategies that may improve proteoform identification. The current higher-order structural characterisation techniques implemented for snake venom proteins are also discussed; we emphasise the need for complementary and higher resolution structural bioanalytical techniques such as mass spectrometry-based approaches, X-ray crystallography and cryogenic electron microscopy, to elucidate poorly characterised tertiary and quaternary protein structures. We envisage that the expansion of the snake venom characterisation "toolbox" with top-down proteomics and high-resolution protein structure determination techniques will be pivotal in advancing structural understanding of snake venoms towards the development of improved therapeutic and biotechnology applications.

Protein Painting Mass Spectrometry in the Discovery of Interaction Sites within the Acetylcholine Binding Protein

Nicotinic acetylcholine receptors (nAChRs) are a family of ligand-gated ion channel receptors that contribute to cognition, memory, and motor control in many organisms. The pharmacological targeting of these receptors, using small molecules or peptides, presents an important strategy for the development of drugs that can treat important human diseases, including neurodegenerative disorders. The Aplysia californica acetylcholine binding protein (Ac-AChBP) is a structural surrogate of the nAChR with high homology to the extracellular ligand binding domain of homopentameric nAChRs. In this study, we optimized protein-painting-based mass spectrometry to identify regions of interaction between the Ac-AChBP and several nAChR ligands. Using molecular dyes that adhere to the surface of a solubilized Ac-AChBP complex, we identified amino acid residues that constitute a contact site within the Ac-AChBP for α-bungarotoxin, choline, nicotine, and amyloid-β 1-42. By integrating innovation in protein painting mass spectrometry with computational structural modeling, we present a new experimental tool for analyzing protein interactions of the nAChR.

Antibodies against a single fraction of Micrurus dumerilii venom neutralize the lethal effect of whole venom

The coralsnake Micrurus dumerilii (Elapidae) is reported to cause envenomings of medical importance. Previous studies characterized the protein composition of its venom, with phospholipase A₂ (PLA₂) proteins the most abundant. However, it is unknown which venom components are responsible for its lethal toxicity. Fractionation of M. dumerilii venom from Colombia was carried out using RP-HPLC and each fraction was screened for lethal effect in mice at a dose of 20 μg by intraperitoneal route. Results showed that only one fraction, F9, was lethal. This fraction displayed PLA₂ activity, induced indirect hemolysis in vitro, as well as edema and myotoxicity in vivo. SDS-PAGE of unreduced F9 evidenced two bands of 8 and 15 kDa, respectively, consistent with the detection of proteins with masses of 13,217.77 Da, 7144.06 Da, and 7665.55 Da. Tryptic digestion of F9 followed by nESI-MS/MS revealed peptide sequences matching proteins of the three-finger toxin (3FTx) and PLA₂ families. Immunization of a rabbit with F9 proteins elicited antibody titers up to 1:10,000 by ELISA. After serum fractionation with caprylic acid, the obtained IgG was able to neutralize the lethal effect of the complete venom of M. dumerilii using a challenge of 2 ×LD₅₀ at the IgG/venom ratio of 50:1 (w/w). In conclusion, present results show that the lethal effect of M. dumerilii venom in mice is mainly driven by one fraction which contains 3FTx and PLA₂ proteins. The antibodies produced against this fraction cross-recognized other PLA₂s and neutralized the lethal effect of whole M. dumerilii venom, pointing out to the potential usefulness of F9 as a relevant antigen for improving current coral snake antivenoms.

The molecular mechanism of snake short-chain α-neurotoxin binding to muscle-type nicotinic acetylcholine receptors

Bites by elapid snakes (e.g. cobras) can result in life-threatening paralysis caused by venom neurotoxins blocking neuromuscular nicotinic acetylcholine receptors. Here, we determine the cryo-EM structure of the muscle-type Torpedo receptor in complex with ScNtx, a recombinant short-chain α-neurotoxin. ScNtx is pinched between loop C on the principal subunit and a unique hairpin in loop F on the complementary subunit, thereby blocking access to the neurotransmitter binding site. ScNtx adopts a binding mode that is tilted toward the complementary subunit, forming a wider network of interactions than those seen in the long-chain α-Bungarotoxin complex. Certain mutations in ScNtx at the toxin-receptor interface eliminate inhibition of neuronal α7 nAChRs, but not of human muscle-type receptors. These observations explain why ScNtx binds more tightly to muscle-type receptors than neuronal receptors. Together, these data offer a framework for understanding subtype-specific actions of short-chain α-neurotoxins and inspire strategies for design of new snake antivenoms.

The structural and functional divergence of a neglected three-finger toxin subfamily in lethal elapids

Bungarus multicinctus is a widely distributed and medically important elapid snake that produces lethal neurotoxic venom. To study and enhance existing antivenom, we explore the complete repertoire of its toxin genes based on de novo chromosome-level assembly and multi-tissue transcriptome data. Comparative genomic analyses suggest that the three-finger toxin family (3FTX) may evolve through the neofunctionalization of flanking LY6E. A long-neglected 3FTX subfamily (i.e., MKA-3FTX) is also investigated. Only one MKA-3FTX gene, which evolves a different protein conformation, is under positive selection and actively transcribed in the venom gland, functioning as a major toxin effector together with MKT-3FTX subfamily homologs. Furthermore, this lethal snake may acquire self-resistance to its β-bungarotoxin via amino acid replacements on fast-evolving KCNA2. This study provides valuable resources for further evolutionary and structure-function studies of snake toxins, which are fundamental for the development of effective antivenoms and drug candidates.

Isolation and Pharmacological Characterization of α-Elapitoxin-Oh3a, a Long-Chain Post-Synaptic Neurotoxin From King Cobra (Ophiophagus hannah) Venom

The King Cobra (Ophiophagus hannah) is the world's largest venomous snake and has a widespread geographical distribution throughout Southeast Asia. Despite proteomic studies indicating the presence of postsynaptic neurotoxins in O. hannah venom, there are few pharmacological investigations of these toxins. We isolated and characterized α-elapitoxin-Oh3a (α-EPTX-Oh3a; 7,938 Da), a long-chain postsynaptic neurotoxin, which constitutes 5% of O. hannah venom. α-EPTX-Oh3a (100-300 nM) caused concentration-dependent inhibition of indirect twitches and inhibited contractile responses of tissues to exogenous acetylcholine and carbachol, in the chick biventer cervicis nerve-muscle preparation. The prior incubation of tissues with Thai Red Cross Society King Cobra antivenom (1 ml/0.8 mg) prevented the in vitro neurotoxic effects of α-EPTX-Oh3a (100 nM). The addition of Thai Red Cross Society King Cobra antivenom (1 ml/0.8 mg), at the t₉₀ time point partially reversed the in vitro neurotoxicity of α-EPTX-Oh3a (100 nM). Repeatedly washing the tissue did not allow significant recovery from the in vitro neurotoxic effects of α-EPTX-Oh3a (100 nM). α-EPTX-Oh3a demonstrated pseudo-irreversible antagonism of concentration-response curves to carbachol, with a pA₂ of 8.99. De novo sequencing of α-EPTX-Oh3a showed a long-chain postsynaptic neurotoxin with 72 amino acids, sharing 100% sequence identity with Long neurotoxin OH-55. In conclusion, the antivenom is useful for reversing the clinically important long-chain α-neurotoxin-mediated neuromuscular paralysis.

Venom-Derived Neurotoxins Targeting Nicotinic Acetylcholine Receptors

Acetylcholine was the first neurotransmitter described. The receptors targeted by acetylcholine are found within organisms spanning different phyla and position themselves as very attractive targets for predation, as well as for defense. Venoms of snakes within the Elapidae family, as well as those of marine snails within the Conus genus, are particularly rich in proteins and peptides that target nicotinic acetylcholine receptors (nAChRs). Such compounds are invaluable tools for research seeking to understand the structure and function of the cholinergic system. Proteins and peptides of venomous origin targeting nAChR demonstrate high affinity and good selectivity. This review aims at providing an overview of the toxins targeting nAChRs found within venoms of different animals, as well as their activities and the structural determinants important for receptor binding.

Three-Finger Toxins from Brazilian Coral Snakes: From Molecular Framework to Insights in Biological Function

Studies on 3FTxs around the world are showing the amazing diversity in these proteins both in structure and function. In Brazil, we have not realized the broad variety of their amino acid sequences and probable diversified structures and targets. In this context, this work aims to conduct an in silico systematic study on available 3FTxs found in Micrurus species from Brazil. We elaborated a specific guideline for this toxin family. First, we grouped them according to their structural homologue predicted by HHPred server and further curated manually. For each group, we selected one sequence and constructed a representative structural model. By looking at conserved features and comparing with the information available in the literature for this toxin family, we managed to point to potential biological functions. In parallel, the phylogenetic relationship was estimated for our database by maximum likelihood analyses and a phylogenetic tree was constructed including the homologous 3FTx previously characterized. Our results highlighted an astonishing diversity inside this family of toxins, showing some groups with expected functional similarities to known 3FTxs, and pointing out others with potential novel roles and perhaps structures. Moreover, this classification guideline may be useful to aid future studies on these abundant toxins.

Unusual quaternary structure of a homodimeric synergistic-type toxin from mamba snake venom defines its molecular evolution

Snake venoms are complex mixtures of enzymes and nonenzymatic proteins that have evolved to immobilize and kill prey animals or deter predators. Among them, three-finger toxins (3FTxs) belong to the largest superfamily of nonenzymatic proteins. They share a common structure of three β-stranded loops extending like fingers from a central core containing all four conserved disulfide bonds. Most 3FTxs are monomers and through subtle changes in their amino acid sequences, they interact with different receptors, ion channels and enzymes to exhibit a wide variety of biological effects. The 3FTxs have further expanded their pharmacological space through covalent or noncovalent dimerization. Synergistic-type toxins (SynTxs) isolated from the deadly mamba venoms, although nontoxic, have been known to enhance the toxicity of other venom proteins. However, the details of three-dimensional structure and molecular mechanism of activity of this unusual class of 3FTxs are unclear. We determined the first three-dimensional structure of a SynTx isolated from Dendroaspis jamesoni jamesoni (Jameson's mamba) venom. The SynTx forms a unique homodimer that is held together by an interchain disulfide bond. The dimeric interface is elaborate and encompasses loops II and III. In addition to the inter-subunit disulfide bond, the hydrogen bonds and hydrophobic interactions between the monomers contribute to the dimer formation. Besides, two sulfate ions that mediate interactions between the monomers. This unique quaternary structure is evolved through noncovalent homodimers such as κ-bungarotoxins. This novel dimerization further enhances the diversity in structure and function of 3FTxs.

An Investigation of Three-Finger Toxin-nAChR Interactions through Rosetta Protein Docking

Three-finger toxins (3FTX) are a group of peptides that affect multiple receptor types. One group of proteins affected by 3FTX are nicotinic acetylcholine receptors (nAChR). Structural information on how neurotoxins interact with nAChR is limited and is confined to a small group of neurotoxins. Therefore, in silico methods are valuable in understanding the interactions between 3FTX and different nAChR subtypes, but there are no established protocols to model 3FTX-nAChR interactions. We followed a homology modeling and protein docking protocol to address this issue and tested its success on three different systems. First, neurotoxin peptides co-crystallized with acetylcholine binding protein (AChBP) were re-docked to assess whether Rosetta protein-protein docking can reproduce the native poses. Second, experimental data on peptide binding to AChBP was used to test whether the docking protocol can qualitatively distinguish AChBP-binders from non-binders. Finally, we docked eight peptides with known α7 and muscle-type nAChR binding properties to test whether the protocol can explain the differential activities of the peptides at the two receptor subtypes. Overall, the docking protocol predicted the qualitative and some specific aspects of 3FTX binding to nAChR with reasonable success and shed light on unknown aspects of 3FTX binding to different receptor subtypes.

Snake three-finger α-neurotoxins and nicotinic acetylcholine receptors: molecules, mechanisms and medicine

Snake venom three-finger α-neurotoxins (α-3FNTx) act on postsynaptic nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction (NMJ) to produce skeletal muscle paralysis. The discovery of the archetypal α-bungarotoxin (α-BgTx), almost six decades ago, exponentially expanded our knowledge of membrane receptors and ion channels. This included the localisation, isolation and characterization of the first receptor (nAChR); and by extension, the pathophysiology and pharmacology of neuromuscular transmission and associated pathologies such as myasthenia gravis, as well as our understanding of the role of α-3FNTxs in snakebite envenomation leading to novel concepts of targeted treatment. Subsequent studies on a variety of animal venoms have yielded a plethora of novel toxins that have revolutionized molecular biomedicine and advanced drug discovery from bench to bedside. This review provides an overview of nAChRs and their subtypes, classification of α-3FNTxs and the challenges of typifying an increasing arsenal of structurally and functionally unique toxins, and the three-finger protein (3FP) fold in the context of the uPAR/Ly6/CD59/snake toxin superfamily. The pharmacology of snake α-3FNTxs including their mechanisms of neuromuscular blockade, variations in reversibility of nAChR interactions, specificity for nAChR subtypes or for distinct ligand-binding interfaces within a subtype and the role of α-3FNTxs in neurotoxic envenomation are also detailed. Lastly, a reconciliation of structure-function relationships between α-3FNTx and nAChRs, derived from historical mutational and biochemical studies and emerging atomic level structures of nAChR models in complex with α-3FNTxs is discussed.

Three-finger proteins from snakes and humans acting on nicotinic receptors: Old and new

The first toxin to give rise to the three-finger protein (TFP) family was α-bungarotoxin (α-Bgt) from Bungarus multicinctus krait venom. α-Bgt was crucial for research on nicotinic acetylcholine receptors (nAChRs), and in this Review article we focus on present data for snake venom TFPs and those of the Ly6/uPAR family from mammalians (membrane-bound Lynx1 and secreted SLURP-1) interacting with nAChRs. Recently isolated from Bungarus candidus venom, αδ-bungarotoxins differ from α-Bgt: they bind more reversibly and distinguish two binding sites in Torpedo californica nAChR. Naja kaouthia α-cobratoxin, classical blocker of nAChRs, was shown to inhibit certain GABA-A receptor subtypes, whereas α-cobratoxin dimer with 2 intermolecular disulfides has a novel type of 3D structure. Non-conventional toxin WTX has additional 5th disulfide not in the central loop, as α-Bgt, but in the N-terminal loop, like all Ly6/uPAR proteins, and inhibits α7 and Torpedo nAChRs. A water-soluble form of Lynx1, ws-Lynx1, was expressed in E. coli, its ¹ H-NMR structure and binding to several nAChRs determined. For SLURP-1, similar information was obtained with its recombinant analogue rSLURP-1. A common feature of ws-Lynx1, rSLURP-1, and WTX is their activity against nAChRs and muscarinic acetylcholine receptors. Synthetic SLURP-1, identical to the natural protein, demonstrated some differences from rSLURP-1 in distinguishing nAChR subtypes. The loop II fragment of the Lynx1 was synthesized having the same µM affinity for the Torpedo nAChR as ws-Lynx1. This review illustrates the productivity of parallel research of nAChR interactions with the two TFP groups.

Snake venom three-finger toxins and their potential in drug development targeting cardiovascular diseases

Cardiovascular diseases such as coronary and peripheral artery diseases, venous thrombosis, stroke, hypertension, and heart failure are enormous burden to health and economy globally. Snake venoms have been the sources of discovery of successful therapeutics targeting cardiovascular diseases. For example, the first-in-class angiotensin-converting enzyme inhibitor captopril was designed largely based on bradykinin-potentiating peptides from Bothrops jararaca venom. In the recent years, sensitive and high throughput approaches drive discovery and cataloging of new snake venom toxins. As one of the largest class of snake venom toxin, there are now>700 sequences of three-finger toxins (3FTxs) available, many of which are yet to be studied. While the function of 3FTxs are normally associated with neurotoxicity, increasingly more 3FTxs have been characterized to have pharmacological effects on cardiovascular systems. Here we focus on this family of snake venom toxins and their potential in developing therapeutics against cardiovascular diseases.

Fulditoxin, representing a new class of dimeric snake toxins, defines novel pharmacology at nicotinic ACh receptors

BACKGROUND AND PURPOSE

Animal toxins have contributed significantly to our understanding of the neurobiology of receptors and ion channels. We studied the venom of the coral snake Micrurus fulvius fulvius and identified and characterized the structure and pharmacology of a new homodimeric neurotoxin, fulditoxin, that exhibited novel pharmacology at nicotinic ACh receptors (nAChRs).

EXPERIMENTAL APPROACH

Fulditoxin was isolated by chromatography, chemically synthesized, its structure determined by X-ray crystallography, and its pharmacological actions on nAChRs characterized by organ bath assays and two-electrode voltage clamp electrophysiology.

KEY RESULTS

Fulditoxin's distinct 1.95-Å quaternary structure revealed two short-chain three-finger α-neurotoxins (α-3FNTxs) non-covalently bound by hydrophobic interactions and an ability to bind metal and form tetrameric complexes, not reported previously for three-finger proteins. Although fulditoxin lacked all conserved amino acids canonically important for inhibiting nAChRs, it produced postsynaptic neuromuscular blockade of chick muscle at nanomolar concentrations, comparable to the prototypical α-bungarotoxin. This neuromuscular blockade was completely reversible, which is unusual for snake α-3FNTxs. Fulditoxin, therefore, interacts with nAChRs by utilizing a different pharmacophore. Unlike short-chain α-3FNTxs that bind only to muscle nAChRs, fulditoxin utilizes dimerization to expand its pharmacological targets to include human neuronal α4β2, α7, and α3β2 nAChRs which it blocked with IC₅₀ values of 1.8, 7, and 12 μM respectively.

CONCLUSIONS AND IMPLICATIONS

Based on its distinct quaternary structure and unusual pharmacology, we named this new class of dimeric Micrurus neurotoxins represented by fulditoxin as Σ-neurotoxins, which offers greater insight into understanding the interactions between nAChRs and peptide antagonists.

Crystal Structures of Human C4.4A Reveal the Unique Association of Ly6/uPAR/α-neurotoxin Domain

Ly6/uPAR/α-neurotoxin domain (LU-domain) is characterized by the presence of 4-5 disulfide bonds and three flexible loops that extend from a core stacked by several conversed disulfide bonds (thus also named three-fingered protein domain). This highly structurally stable protein domain is typically a protein-binder at extracellular space. Most LU proteins contain only single LU-domain as represented by Ly6 proteins in immunology and α-neurotoxins in snake venom. For Ly6 proteins, many are expressed in specific cell lineages and in differentiation stages, and are used as markers. In this study, we report the crystal structures of the two LU-domains of human C4.4A alone and its complex with a Fab fragment of a monoclonal anti-C4.4A antibody. Interestingly, both structures showed that C4.4A forms a very compact globule with two LU-domain packed face to face. This is in contrast to the flexible nature of most LU-domain-containing proteins in mammals. The Fab combining site of C4.4A involves both LU-domains, and appears to be the binding site for AGR2, a reported ligand of C4.4A. This work reports the first structure that contain two LU-domains and provides insights on how LU-domains fold into a compact protein and interacts with ligands.

The Indian cobra reference genome and transcriptome enables comprehensive identification of venom toxins

Snakebite envenoming is a serious and neglected tropical disease that kills ~100,000 people annually. High-quality, genome-enabled comprehensive characterization of toxin genes will facilitate development of effective humanized recombinant antivenom. We report a de novo near-chromosomal genome assembly of Naja naja, the Indian cobra, a highly venomous, medically important snake. Our assembly has a scaffold N50 of 223.35 Mb, with 19 scaffolds containing 95% of the genome. Of the 23,248 predicted protein-coding genes, 12,346 venom-gland-expressed genes constitute the ‘venom-ome’ and this included 139 genes from 33 toxin families. Among the 139 toxin genes were 19 ‘venom-ome-specific toxins’ (VSTs) that showed venom-gland-specific expression, and these probably encode the minimal core venom effector proteins. Synthetic venom reconstituted through recombinant VST expression will aid in the rapid development of safe and effective synthetic antivenom. Additionally, our genome could serve as a reference for snake genomes, support evolutionary studies and enable venom-driven drug discovery.

Analysis of a near-chromosomal genome assembly and transcriptome profiling of the Indian cobra identifies genes expressed in the venom glands. These data should help develop a new antivenom.

Omics Technologies for Profiling Toxin Diversity and Evolution in Snake Venom: Impacts on the Discovery of Therapeutic and Diagnostic Agents

Snake venoms are primarily composed of proteins and peptides, and these toxins have developed high selectivity to their biological targets. This makes venoms interesting for exploration into protein evolution and structure-function relationships. A single venom protein superfamily can exhibit a variety of pharmacological effects; these variations in activity originate from differences in functional sites, domains, posttranslational modifications, and the formations of toxin complexes. In this review, we discuss examples of how the major venom protein superfamilies have diversified, as well as how newer technologies in the omics fields, such as genomics, transcriptomics, and proteomics, can be used to characterize both known and unknown toxins.Because toxins are bioactive molecules with a rich diversity of activities, they can be useful as therapeutic and diagnostic agents, and successful examples of toxin applications in these areas are also reviewed. With the current rapid pace of technology, snake venom research and its applications will only continue to expand.

A Decoy-Receptor Approach Using Nicotinic Acetylcholine Receptor Mimics Reveals Their Potential as Novel Therapeutics Against Neurotoxic Snakebite

Snakebite is a neglected tropical disease that causes 138,000 deaths each year. Neurotoxic snake venoms contain small neurotoxins, including three-finger toxins (3FTxs), which can cause rapid paralysis in snakebite victims by blocking postsynaptic transmission via nicotinic acetylcholine receptors (nAChRs). These toxins are typically weakly immunogenic and thus are often not effectively targeted by current polyclonal antivenom therapies. We investigated whether nAChR mimics, also known as acetylcholine binding proteins (AChBPs), could effectively capture 3FTxs and therefore be developed as a novel class of snake-generic therapeutics for combatting neurotoxic envenoming. First, we identified the binding specificities of 3FTx from various medically important elapid snake venoms to nAChR using two recombinant nAChR mimics: the AChBP from Lymnaea stagnalis and a humanized neuronal α7 version (α7-AChBP). We next characterized these AChBP-bound and unbound fractions using SDS-PAGE and mass spectrometry. Interestingly, both mimics effectively captured long-chain 3FTxs from multiple snake species but largely failed to capture the highly related short-chain 3FTxs, suggesting a high level of binding specificity. We next investigated whether nAChR mimics could be used as snakebite therapeutics. We showed that while α7-AChBP alone did not protect against Naja haje (Egyptian cobra) venom lethality in vivo, it significantly prolonged survival times when coadministered with a nonprotective dose of antivenom. Thus, nAChR mimics are capable of neutralizing specific venom toxins and may be useful adjunct therapeutics for improving the safety and affordability of existing snakebite treatments by reducing therapeutic doses. Our findings justify exploring the future development of AChBPs as potential snakebite treatments.

Neurobiology and therapeutic utility of neurotoxins targeting postsynaptic mechanisms of neuromuscular transmission

The neuromuscular junction (NMJ) is the principal site for the translation of motor neurochemical signals to muscle activity. Therefore, the release and sensing machinery of acetylcholine (ACh) along with muscle contraction are two of the main targets of natural toxins and pathogens, causing paralysis. Given pharmacology and medical advances, the active ingredients of toxins that target postsynaptic mechanisms have become of major interest, showing promise as drug leads. Herein, we review key facets of prevalent toxins modulating the mechanisms of ACh sensing and generation of the postsynaptic response, with muscle contraction. We consider the correlation between their outstanding selectivity and potency plus effects on motor function, and discuss emerging data advocating their usage for the development of therapies alleviating neuromuscular dysfunction.

Evolution and Medical Significance of LU Domain-Containing Proteins

Proteins containing Ly6/uPAR (LU) domains exhibit very diverse biological functions and have broad taxonomic distributions in eukaryotes. In general, they adopt a characteristic three-fingered folding topology with three long loops projecting from a disulfide-rich globular core. The majority of the members of this protein domain family contain only a single LU domain, which can be secreted, glycolipid anchored, or constitute the extracellular ligand binding domain of type-I membrane proteins. Nonetheless, a few proteins contain multiple LU domains, for example, the urokinase receptor uPAR, C4.4A, and Haldisin. In the current review, we will discuss evolutionary aspects of this protein domain family with special emphasis on variations in their consensus disulfide bond patterns. Furthermore, we will present selected cases where missense mutations in LU domain-containing proteins leads to dysfunctional proteins that are causally linked to genesis of human disease.

Crystal structure of the unoccupied murine urokinase-type plasminogen activator receptor (uPAR) reveals a tightly packed DII-DIII unit

The urokinase-type plasminogen activator receptor (uPAR) is a cell surface receptor that is capable of binding to a range of extracellular proteins and triggering a series of proteolytic and signaling events. Previous structural studies of uPAR with its ligands uPA and vitronectin revealed that its three domains (DI, DII, and DIII) form a large hydrophobic cavity to accommodate uPA. In the present study, the structure of unoccupied murine uPAR (muPAR) is determined. The structure of DII and DIII of muPAR is well defined and forms a compact globular unit, while DI could not be traced. Molecular dynamic simulations further confirm the rigid binding interface between DII and DIII. This study shows overall structural flexibility of uPAR in the absence of uPA.

Drysdalin, an antagonist of nicotinic acetylcholine receptors highlights the importance of functional rather than structural conservation of amino acid residues

Snake venom neurotoxins are potent antagonists of nicotinic acetylcholine receptors (nAChRs). Here, we describe a novel member of class 3c long-chain neurotoxin drysdalin from the venom of Drysdalia coronoides. Drysdalin lacks three of the eight conserved classical functional residues critical for nAChRs interaction. Despite such a drastic alteration of the functional site, recombinant drysdalin showed irreversible postsynaptic neurotoxicity with nanomolar potency and selectively antagonizes the rodent muscle (α1)2β1δε, and human α7 and α9α10 nAChRs, but had no significant activity at the human α3β2, α3β4, α4β2, and α4β4 nAChRs. Substitution of Leu34 and Ala37 residues with the conserved Arg had minimal impact on the potency whereas conserved Phe replacement of residue Arg30 substantially reduced or abolished inhibitory activity. In contrast, truncation of the 24-residue long C-terminal tail leads to complete loss in (a) activity at α9α10 nAChR; and (b) irreversibility with reduced potency at the muscle and α7 nAChRs. Overall, the non-conserved Arg30 residue together with the uniquely long C-terminal tail contribute to the inhibitory activity of drysdalin at the nAChRs suggesting, at least for drysdalin, functional rather than sequence conservation plays a critical role in determining the activity of the toxin.

Last decade update for three-finger toxins: Newly emerging structures and biological activities

Three-finger toxins (TFTs) comprise one of largest families of snake venom toxins. While they are principal to and the most toxic components of the venoms of the Elapidae snake family, their presence has also been detected in the venoms of snakes from other families. The first TFT, α-bungarotoxin, was discovered almost 50 years ago and has since been used widely as a specific marker of the α7 and muscle-type nicotinic acetylcholine receptors. To date, the number of TFT amino acid sequences deposited in the UniProt Knowledgebase free-access database is more than 700, and new members are being added constantly. Although structural variations among the TFTs are not numerous, several new structures have been discovered recently; these include the disulfide-bound dimers of TFTs and toxins with nonstandard pairing of disulfide bonds. New types of biological activities have also been demonstrated for the well-known TFTs, and research on this topic has become a hot topic of TFT studies. The classic TFTs α-bungarotoxin and α-cobratoxin, for example, have now been shown to inhibit ionotropic receptors of γ-aminobutyric acid, and some muscarinic toxins have been shown to interact with adrenoceptors. New, unexpected activities have been demonstrated for some TFTs as well, such as toxin interaction with interleukin or insulin receptors and even TFT-activated motility of sperm. This minireview provides a summarization of the data that has emerged in the last decade on the TFTs and their activities.

Snake Venom Peptides: Tools of Biodiscovery

Nature endowed snakes with a lethal secretion known as venom, which has been fine-tuned over millions of years of evolution. Snakes utilize venom to subdue their prey and to survive in their natural habitat. Venom is known to be a very poisonous mixture, consisting of a variety of molecules, such as carbohydrates, nucleosides, amino acids, lipids, proteins and peptides. Proteins and peptides are the major constituents of the dry weight of snake venoms and are of main interest for scientific investigations as well as for various pharmacological applications. Snake venoms contain enzymatic and non-enzymatic proteins and peptides, which are grouped into different families based on their structure and function. Members of a single family display significant similarities in their primary, secondary and tertiary structures, but in many cases have distinct pharmacological functions and different bioactivities. The functional specificity of peptides belonging to the same family can be attributed to subtle variations in their amino acid sequences. Currently, complementary tools and techniques are utilized to isolate and characterize the peptides, and study their potential applications as molecular probes, and possible templates for drug discovery and design investigations.

Transcriptome-facilitated proteomic characterization of rear-fanged snake venoms reveal abundant metalloproteinases with enhanced activity

High-throughput technologies were used to identify venom gland toxin expression and to characterize the venom proteomes of two rear-fanged snakes, Ahaetulla prasina (Asian Green Vine Snake) and Borikenophis portoricensis (Puerto Rican Racer). Sixty-nine complete toxin-coding transcripts from 12 venom protein superfamilies (A. prasina) and 50 complete coding transcripts from 11 venom protein superfamilies (B. portoricensis) were identified in the venom glands. However, only 18% (A. prasina) and 32% (B. portoricensis) of the translated protein isoforms were detected in the proteome of these venoms. Both venom gland transcriptomes and venom proteomes were dominated by P-III metalloproteinases. Three-finger toxins, cysteine-rich secretory proteins, and C-type lectins were present in moderate amounts, but other protein superfamilies showed very low abundances. Venoms contained metalloproteinase activity comparable to viperid snake venom levels, but other common venom enzymes were absent or present at negligible levels. Western blot analysis showed metalloproteinase and cysteine-rich secretory protein epitopes shared with the highly venomous Boomslang (Dispholidus typus). The abundance of metalloproteinases emphasizes the important trophic role of these toxins. Comprehensive, transcriptome-informed definition of proteomes and functional characterization of venom proteins in rear-fanged snake families help to elucidate toxin evolution and provide models for protein structure-function analyses.

Comparative Study of Biological Activities of Venom from Colubrid Snakes Rhabdophis tigrinus (Yamakagashi) and Rhabdophis lateralis

Rhabdophis lateralis, a colubrid snake distributed throughout the continent of Asia, has recently undergone taxonomic revisions. Previously, Rhabdophis lateralis was classified as a subspecies of R. tigrinus (Yamakagashi) until 2012, when several genetic differences were discovered which classified this snake as its own species. To elucidate the toxicity of venom from this poorly studied colubrid, various biological activities were compared between the venom from the two snake species. The components of their venom were compared by the elution profiles of reversed-phase HPLC and SDS-PAGE, and gel filtrated fractions were tested for effects on blood coagulation. Proteolytic activities of these fractions were also assayed by using synthetic substrates, fibrinogen, and matrix proteins. Similar to the R. tigrinus venom, the higher molecular weight fraction of R. lateralis venom contained a prothrombin activator. Both prothrombin time (PT) and activated partial thromboplastin time (APTT) of human plasma were shortened by the addition of R. lateralis and R. tigrinus venom. The thrombin formation was estimated by the uses of SDS-PAGE and chromogenic substrates. These venom fractions also possessed very specific proteinase activity on human fibrinogen, but the substrates for matrix metalloproteinase, such as collagen and laminin, were not hydrolyzed. However, there were some notable differences in reactivity to synthetic substrates for matrix metalloproteinase, and R. tigrinus venom possessed relatively higher activity. Our chemical investigation indicates that the components included in both venoms resemble each other closely. However, the ratio of components and proteolytic activity of some ingredients are slightly different, indicating differences between two closely-related snakes.

Nicotinic acetylcholine receptor inhibitors derived from snake and snail venoms

The nicotinic acetylcholine receptor (nAChR) represents the prototype of ligand-gated ion channels. It is vital for neuromuscular transmission and an important regulator of neurotransmission. A variety of toxic compounds derived from diverse species target this receptor and have been of elemental importance in basic and applied research. They enabled milestone discoveries in pharmacology and biochemistry ranging from the original formulation of the receptor concept, the first isolation and structural analysis of a receptor protein (the nAChR) to the identification, localization, and differentiation of its diverse subtypes and their validation as a target for therapeutic intervention. Among the venom-derived compounds, α-neurotoxins and α-conotoxins provide the largest families and still represent indispensable pharmacological tools. Application of modified α-neurotoxins provided substantial structural and functional details of the nAChR long before high resolution structures were available. α-bungarotoxin represents not only a standard pharmacological tool and label in nAChR research but also for unrelated proteins tagged with a minimal α-bungarotoxin binding motif. A major advantage of α-conotoxins is their smaller size, as well as superior selectivity for diverse nAChR subtypes that allows their development into ligands with optimized pharmacological and chemical properties and potentially novel drugs. In the following, these two groups of nAChR antagonists will be described focusing on their respective roles in the structural and functional characterization of nAChRs and their development into research tools. In addition, we provide a comparative overview of the diverse α-conotoxin selectivities that can serve as a practical guide for both structure activity studies and subtype classification. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Exactin: A specific inhibitor of Factor X activation by extrinsic tenase complex from the venom of Hemachatus haemachatus

Unwanted clots lead to heart attack and stroke that result in a large number of deaths. Currently available anticoagulants have some drawbacks including their non-specific actions. Therefore novel anticoagulants that target specific steps in the coagulation pathway are being sought. Here we describe the identification and characterization of a novel anticoagulant protein from the venom of Hemachatus haemachatus (African Ringhals cobra) that specifically inhibits factor X (FX) activation by the extrinsic tenase complex (ETC) and thus named as exactin. Exactin belongs to the three-finger toxin (3FTx) family, with high sequence identity to neurotoxins and low identity to the well-characterized 3FTx anticoagulants-hemextin and naniproin. It is a mixed-type inhibitor of ETC with the kinetic constants, Ki’ and Ki determined as 30.62 ± 7.73 nM and 153.75 ± 17.96 nM, respectively. Exactin does not bind to the active site of factor VIIa and factor Xa based on its weak inhibition (IC₅₀ ≫ 300 μM) to the amidolytic activities of these proteases. Exactin shows exquisite macromolecular specificity to FX activation as compared to factor IX activation by ETC. Exactin thus displays a distinct mechanism when compared to other anticoagulants targeting ETC, with its selective preference to ETC-FX [ES] complex.

Mapping Proteoforms and Protein Complexes From King Cobra Venom Using Both Denaturing and Native Top-down Proteomics

Characterizing whole proteins by top-down proteomics avoids a step of inference encountered in the dominant bottom-up methodology when peptides are assembled computationally into proteins for identification. The direct interrogation of whole proteins and protein complexes from the venom of Ophiophagus hannah (king cobra) provides a sharply clarified view of toxin sequence variation, transit peptide cleavage sites and post-translational modifications (PTMs) likely critical for venom lethality. A tube-gel format for electrophoresis (called GELFrEE) and solution isoelectric focusing were used for protein fractionation prior to LC-MS/MS analysis resulting in 131 protein identifications (18 more than bottom-up) and a total of 184 proteoforms characterized from 14 protein toxin families. Operating both GELFrEE and mass spectrometry to preserve non-covalent interactions generated detailed information about two of the largest venom glycoprotein complexes: the homodimeric l-amino acid oxidase (∼130 kDa) and the multichain toxin cobra venom factor (∼147 kDa). The l-amino acid oxidase complex exhibited two clusters of multiproteoform complexes corresponding to the presence of 5 or 6 N-glycans moieties, each consistent with a distribution of N-acetyl hexosamines. Employing top-down proteomics in both native and denaturing modes provides unprecedented characterization of venom proteoforms and their complexes. A precise molecular inventory of venom proteins will propel the study of snake toxin variation and the targeted development of new antivenoms or other biotherapeutics.

Ringhalexin from Hemachatus haemachatus: A novel inhibitor of extrinsic tenase complex

Anticoagulant therapy is used for the prevention and treatment of thromboembolic disorders. Blood coagulation is initiated by the interaction of factor VIIa (FVIIa) with membrane-bound tissue factor (TF) to form the extrinsic tenase complex which activates FX to FXa. Thus, it is an important target for the development of novel anticoagulants. Here, we report the isolation and characterization of a novel anticoagulant ringhalexin from the venom of Hemachatus haemachatus (African Ringhals Cobra). Amino acid sequence of the protein indicates that it belongs to the three-finger toxin family and exhibits 94% identity to an uncharacterized Neurotoxin-like protein NTL2 from Naja atra. Ringhalexin inhibited FX activation by extrinsic tenase complex with an IC50 of 123.8 ± 9.54 nM. It is a mixed-type inhibitor with the kinetic constants, Ki and Ki' of 84.25 ± 3.53 nM and 152.5 ± 11.32 nM, respectively. Ringhalexin also exhibits a weak, irreversible neurotoxicity on chick biventer cervicis muscle preparations. Subsequently, the three-dimensional structure of ringhalexin was determined at 2.95 Å resolution. This study for the first time reports the structure of an anticoagulant three-finger toxin. Thus, ringhalexin is a potent inhibitor of the FX activation by extrinsic tenase complex and a weak, irreversible neurotoxin.

Proteomic characterization and comparison of venoms from two elapid snakes (Bungarus multicinctus and Naja atra) from China

Bungarus multicinctus (many-banded krait) and Naja atra (Chinese cobra) are widely distributed and medically important venomous snakes in China; however, their venom proteomic profiles have not been fully compared. Here, we fractionated crude venoms and analyzed them using a combination of proteomic techniques. Three-finger toxins (3-FTx) and phospholipase A2 (PLA2) were most abundant in both species, respectively accounting for 32.6% and 66.4% of total B. multicinctus venom, and 84.3% and 12.2% of total N. atra venom. Venoms from these two species contained one common protein family and six less abundant species-specific protein families. The proteomic profiles of B. multicinctus and N. atra venoms and analysis of toxicological activity in mice suggested that 3-FTx and PLA2 are the major contributors to clinical symptoms caused by envenomation. The venoms differed in enzymatic activity, likely the result of inter-specific variation in the amount of related venom components. Antivenomics assessment revealed that a small number of venom components (3-FTxs and PLA2s in B. multicinctus, and 3-FTxs in N. atra) could not be immunocaptured completely, suggesting that we should pay attention to enhancing the immune response of these components in designing commercial antivenoms for B. multicinctus and N. atra.

BIOLOGICAL SIGNIFICANCE

The proteomic profiles of venoms from two medically important snake species - B. multicinctus and N. atra - have been explored. Quantitative and qualitative differences are evident in both venoms when proteomic profiles and transcriptomic results are compared; this is a reminder that combined approaches are needed to explore the precise composition of snake venom. Two protein families (3-FTx and PLA2) of high abundance in these snake venoms are major players in the biochemical and pharmacological effects of envenomation. Elucidation of the proteomic profiles of these snake venoms is helpful in understanding composition-function relationships and will facilitate the clinical application of antivenoms.

A Distinct Functional Site in Ω-Neurotoxins: Novel Antagonists of Nicotinic Acetylcholine Receptors from Snake Venom

Snake venom α-neurotoxins from the three-finger toxin (3FTx) family are competitive antagonists with nanomolar affinity and high selectivity for nicotinic acetylcholine receptors (nAChR). Here, we report the characterization of a new group of competitive nAChR antagonists: Ω-neurotoxins. Although they belong to the 3FTx family, the characteristic functional residues of α-neurotoxins are not conserved. We evaluated the subtype specificity and structure-function relationships of Oh9-1, an Ω-neurotoxin from Ophiophagus hannah venom. Recombinant Oh9-1 showed reversible postsynaptic neurotoxicity in the micromolar range. Experiments with different nAChR subtypes expressed in Xenopus oocytes indicated Oh9-1 is selective for rat muscle type α1β1εδ (adult) and α1β1γδ (fetal) and rat neuronal α3β2 subtypes. However, Oh9-1 showed low or no affinity for other human and rat neuronal subtypes. Twelve individual alanine-scan mutants encompassing all three loops of Oh9-1 were evaluated for binding to α1β1εδ and α3β2 subtypes. Oh9-1's loop-II residues (M25, F27) were the most critical for interactions and formed the common binding core. Mutations at T23 and F26 caused a significant loss in activity at α1β1εδ receptors but had no effect on the interaction with the α3β2 subtype. Similarly, mutations at loop-II (H7, K22, H30) and -III (K45) of Oh9-1 had a distinctly different impact on its activity with these subtypes. Thus, Oh9-1 interacts with these nAChRs via distinct residues. Unlike α-neurotoxins, the tip of loop-II is not involved. We reveal a novel mode of interaction, where both sides of the β-strand of Oh9-1's loop-II interact with α1β1εδ, but only one side interacts with α3β2. Phylogenetic analysis revealed functional organization of the Ω-neurotoxins independent of α-neurotoxins. Thus, Ω-neurotoxin: Oh9-1 may be a new, structurally distinct class of 3FTxs that, like α-neurotoxins, antagonize nAChRs. However, Oh9-1 binds to the ACh binding pocket via a different set of functional residues.

Purification and Characterization of Nk-3FTx: A Three Finger Toxin from the Venom of North East Indian Monocled Cobra

Snake venom three finger toxins (3FTxs) are a non-enzymatic family of venom proteins abundantly found in elapids. We have purified a 7579.5 ± 0.591 Da 3FTx named as Nk-3FTx from the venom of Naja kaouthia of North East India origin. The primary structure was determined by a combination of N-terminal sequencing and electrospray ionization liquid chromatography-mass spectrometry/mass spectrometry. Biochemical and biological characterization reveal that it is nontoxic to human cell lines and exhibit mild anticoagulant activity when tested on citrated human plasma. Nk-3FTx was found to affect the compound action potential (CAP) and nerve conduction velocity of isolated toad sciatic nerve. This is the first report of a non-conventional 3FTx from Naja kaouthia venom that reduces CAP for its neurotoxic effect. Further studies can be carried out to understand the mechanism of action and to explore its potential therapeutic application.

Ethnopharmacological survey of medicinal plants used by traditional healers and indigenous people in chittagong hill tracts, bangladesh, for the treatment of snakebite

Snakebites are common in tropical countries like Bangladesh where most snakebite victims dwell in rural areas. Among the management options after snakebite in Bangladesh, snake charmers (Ozha in Bengali language) are the first contact following a snakebite for more than 80% of the victims and they are treated mostly with the help of some medicinal plants. Our aim of the study is to compile plants used for the treatment of snakebite occurrence in Bangladesh. The field survey was carried out in a period of almost 3 years. Fieldwork was undertaken in Chittagong Hill Tracts, Bangladesh, including Chittagong, Rangamati, Bandarban, and Khagrachari. Open-ended and semistructured questionnaire was used to interview a total of 110 people including traditional healers and local people. A total of 116 plant species of 48 families were listed. Leaves were the most cited plant part used against snake venom. Most of the reported species were herb in nature and paste mostly used externally is the mode of preparation. The survey represents the preliminary information of certain medicinal plants having neutralizing effects against snake venoms, though further phytochemical investigation, validation, and clinical trials should be conducted before using these plants as an alternative to popular antivenom.

Three-finger snake neurotoxins and Ly6 proteins targeting nicotinic acetylcholine receptors: pharmacological tools and endogenous modulators

Snake venom neurotoxins and lymphocyte antigen 6 (Ly6) proteins, most of the latter being membrane tethered by a glycosylphosphatidylinositol (GPI) anchor, have a variety of biological activities, but their three-finger (3F) folding combines them in one Ly6/neurotoxin family. Subsets of two groups, represented by α-neurotoxins and Lynx1, respectively, interact with nicotinic acetylcholine receptors (nAChR) and, hence, are of therapeutic interest for the treatment of neurodegenerative diseases, pain, and cancer. Information on the mechanisms of action and 3D structure of the binding sites, which is required for drug design, is available from the 3D structure of α-neurotoxin complexes with nAChR models. Here, I compare the structural and functional features of α-neurotoxins versus Lynx1 and its homologs to get a clearer picture of Lynx1-nAChR interactions that is necessary for fundamental science and practical applications.

Comprehensive analysis of the venom gland transcriptome of the spider Dolomedes fimbriatus

A comprehensive transcriptome analysis of an expressed sequence tag (EST) database of the spider Dolomedes fimbriatus venom glands using single-residue distribution analysis (SRDA) identified 7,169 unique sequences. Mature chains of 163 different toxin-like polypeptides were predicted on the basis of well-established methodology. The number of protein precursors of these polypeptides was appreciably numerous than the number of mature polypeptides. A total of 451 different polypeptide precursors, translated from 795 unique nucleotide sequences, were deduced. A homology search divided the 163 mature polypeptide sequences into 16 superfamilies and 19 singletons. The number of mature toxins in a superfamily ranged from 2 to 49, whereas the diversity of the original nucleotide sequences was greater (2-261 variants). We observed a predominance of inhibitor cysteine knot toxin-like polypeptides among the cysteine-containing structures in the analyzed transcriptome bank. Uncommon spatial folds were also found.

Isolation and structural and pharmacological characterization of α-elapitoxin-Dpp2d, an amidated three finger toxin from black mamba venom

We isolated a novel, atypical long-chain three-finger toxin (TFT), α-elapitoxin-Dpp2d (α-EPTX-Dpp2d), from black mamba (Dendroaspis polylepis polylepis) venom. Proteolytic digestion with trypsin and V8 protease, together with MS/MS de novo sequencing, indicated that the mature toxin has an amidated C-terminal arginine, a posttranslational modification rarely observed for snake TFTs. α-EPTX-Dpp2d was found to potently inhibit α7 neuronal nicotinic acetylcholine receptors (nAChR; IC₅₀, 58 ± 24 nM) and muscle-type nAChR (IC₅₀, 114 ± 37 nM) but did not affect α3β2 and α3β4 nAChR isoforms at 1 μM concentrations. Competitive radioligand binding assays demonstrated that α-EPTX-Dpp2d competes with epibatidine binding to the Lymnea stagnalis acetylcholine-binding protein (Ls-AChBP; IC₅₀, 4.9 ± 2.3 nM). The activity profile and binding data are reminiscent of classical long-chain TFTs with a free carboxyl termini, suggesting that amidation does not significantly affect toxin selectivity. The crystal structure of α-EPTX-Dpp2d was determined at 1.7 Å resolution and displayed a dimeric toxin assembly with each monomer positioned in an antiparallel orientation. The dimeric structure is stabilized by extensive intermolecular hydrogen bonds and electrostatic interactions, which raised the possibility that the toxin may exist as a noncovalent homodimer in solution. However, chemical cross-linking and size-exclusion chromatography coupled with multiangle laser light scattering (MALLS) data indicated that the toxin is predominantly monomeric under physiological conditions. Because of its high potency and selectivity, we expect this toxin to be a valuable pharmacological tool for studying the structure and function of nAChRs.

Functional proteomic approach to discover geographic variations of king cobra venoms from Southeast Asia and China

This study deciphers the geographic variations of king cobra (Ophiophagus hannah) venom using functional proteomics. Pooled samples of king cobra venom (abbreviated as Ohv) were obtained from Indonesia, Malaysia, Thailand, and two provinces of China, namely Guangxi and Hainan. Using two animal models to test and compare the lethal effects, we found that the Chinese Ohvs were more fatal to mice, while the Southeast Asian Ohvs were more fatal to lizards (Eutropis multifasciata). Various phospholipases A2 (PLA2s), three-finger toxins (3FTxs) and Kunitz-type inhibitors were purified from these Ohvs and compared. Besides the two Chinese Ohv PLA2s with known sequences, eight novel PLA2s were identified from the five Ohv samples and their antiplatelet activities were compared. While two 3FTxs (namely oh-55 and oh-27) were common in all the Ohvs, different sets of 3FTx markers were present in the Chinese and Southeast Asian Ohvs. All the Ohvs contain the Kunitz inhibitor, OH-TCI, while only the Chinese Ohvs contain the inhibitor variant, Oh11-1. Relative to the Chinese Ohvs which contained more phospholipases, the Southeast Asian Ohvs had higher metalloproteinase, acetylcholine esterase, and alkaline phosphatase activities.

BIOLOGICAL SIGNIFICANCE

Remarkable variations in five king cobra geographic samples reveal fast evolution and dynamic translational regulation of the venom which probably adapted to different prey ecology as testified by the lethal tests on mice and lizards. Our results predict possible variations of the king cobra envenoming to human and the importance of using local antivenin for snakebite treatment.

Neurological effects of venomous bites and stings: snakes, spiders, and scorpions

Snake and spider bites, as well as scorpion sting envenoming, are neglected diseases affecting millions of people all over the world. Neurological complications vary according to the offending animal, and are often directly related to toxic effects of the venom, affecting the central nervous system, the neuromuscular transmission, the cardiovascular system, or the coagulation cascade. Snake bite envenoming may result in stroke or muscle paralysis. Metalloproteinases and other substances (common in vipers and colubrids) have anticoagulant or procoagulant activity, and may induce ischemic or hemorrhagic strokes. The venom of elapids is rich in neurotoxins affecting the neuromuscular transmission at either presynaptic or postsynaptic levels. The clinical picture of scorpion sting envenoming is dominated by muscle weakness associated with arterial hypertension, cardiac arrythmias, myocarditis, or pulmonary edema. These manifestations occur as the result of release of catecholamines into the bloodstream or due to direct cardiac toxicity of the venom. Cerebrovascular complications have been reported after the sting of the Indian red scorpion. Intracranial hemorrhages occur in the setting of acute increases in arterial blood pressure related to sympathetic overstimulation, and cerebral infarctions are related to either cerebral hypoperfusion, consumption coagulopathy, vasculitis, or cardiogenic brain embolism. Three main syndromes result from spider bite envenoming: latrodectism, loxoscelism, and funnel-web spider envenoming. Latrodectism is related to neurotoxins present in the venom of widow spiders. Most cases present with headache, lethargy, irritability, myalgia, tremor, fasciculation, or ataxia. Loxoscelism is caused by envenoming by spiders of the family Sicariidae. It may present with a stroke due to a severe coagulopathy. The venom of funnel-web spiders also has neurotoxins that stimulate neurotransmitter release, resulting in sensory disturbances and muscle paralysis. Proper management of the envenomed patient, including prompt transport to the hospital, correction of the hemostatic disorder, ventilatory support, and administration of antivenom, significantly reduce the risk of neurological complications which, in turn, reduce the mortality and improve the functional outcome of survivors.

Identification and structural characterization of a new three-finger toxin hemachatoxin from Hemachatus haemachatus venom

Snake venoms are rich sources of biologically active proteins and polypeptides. Three-finger toxins are non-enzymatic proteins present in elapid (cobras, kraits, mambas and sea snakes) and colubrid venoms. These proteins contain four conserved disulfide bonds in the core to maintain the three-finger folds. Although all three-finger toxins have similar fold, their biological activities are different. A new three-finger toxin (hemachatoxin) was isolated from Hemachatus haemachatus (Ringhals cobra) venom. Its amino acid sequence was elucidated, and crystal structure was determined at 2.43 Å resolution. The overall fold is similar to other three-finger toxins. The structure and sequence analysis revealed that the fold is maintained by four highly conserved disulfide bonds. It exhibited highest similarity to particularly P-type cardiotoxins that are known to associate and perturb the membrane surface with their lipid binding sites. Also, the increased B value of hemachotoxin loop II suggests that loop II is flexible and may remain flexible until its interaction with membrane phospholipids. Based on the analysis, we predict hemachatoxin to be cardiotoxic/cytotoxic and our future experiments will be directed to characterize the activity of hemachatoxin.

Engineered mammalian vector to express EGFP-tagged proteins as biomarkers

Due to its specialized post-translational machinery, mammalian cells represent an interesting and not fully explored system to express snake toxins. Therefore, in this work, we built up a new mammalian expression vector that enhances the feasibility to use mammalian cells to express proteins as biomarkers. Among the modifications, an Igκ signal peptide and a 6xHis tag were inserted into this vector in order to drive the protein to the supernatant and simplify its purification, respectively. In addition, to facilitate selection of high producing clones and also tag proteins which may function as a biomarker, the sequence of enhanced green fluorescent protein (EGFP) was added. The efficiency of the resulting vector (pToxEGFP) was tested by cloning and expressing the viper venom disintegrin echistatin (Ech) that due to its affinity to integrin αvβ3 was tested as a molecular marker. Expression of EGFP-Ech was achieved in CHO-DXB11 cells resulting in a yield of 22 mg/L. The binding activity of this chimera protein was successfully achieved on human umbilical vein endothelial cells which highly express αvβ3. The results indicate that pToxEGFP may constitute an efficient and versatile expression vector to express tagged proteins with potential biomarker activity.

Neurological complications of venomous snake bites: a review

Snake bite envenoming is a neglected tropical disease affecting millions of people living in the developing world. According to the offending snake species, the clinical picture may be dominated by swelling and soft tissue necrosis in the bitten limb, or by systemic or neurological manifestations. Serious neurological complications, including stroke and muscle paralysis, are related to the toxic effects of the venom, which contains a complex mixture of toxins affecting the coagulation cascade, the neuromuscular transmission, or both. Metalloproteinases, serine proteases, and C-type lentins (common in viper and colubrid venoms) have anticoagulant or procoagulant activity and may be either agonists or antagonists of platelet aggregation; as a result, ischemic or hemorrhagic strokes may occur. In contrast, the venom of elapids is rich in phospholipase A(2) and three-finger proteins, which are potent neurotoxins affecting the neuromuscular transmission at either presynaptic or post-synaptic levels. Presynaptic-acting neurotoxins (called β-neurotoxins) inhibit the release of acetylcholine, while post-synaptic-acting neurotoxins (called α-neurotoxins) cause a reversible blockage of acetylcholine receptors. Proper management of the envenomed patient, including prompt transport to the hospital, correction of the hemostatic disorder, ventilatory support, and administration of antivenom, significantly reduces the risk of neurological complications which, in turn, reduce the mortality and improve the functional outcome of survivors.

Receptor-targeting mechanisms of pain-causing toxins: How ow?

Venoms often target vital processes to cause paralysis or death, but many types of venom also elicit notoriously intense pain. While these pain-producing effects can result as a byproduct of generalized tissue trauma, there are now multiple examples of venom-derived toxins that target somatosensory nerve terminals in order to activate nociceptive (pain-sensing) neural pathways. Intriguingly, investigation of the venom components that are responsible for evoking pain has revealed novel roles and/or configurations of well-studied toxin motifs. This review serves to highlight pain-producing toxins that target the capsaicin receptor, TRPV1, or members of the acid-sensing ion channel family, and to discuss the utility of venom-derived multivalent and multimeric complexes.