An Appetite for Destruction: Detecting Prey-Selective Binding of α-Neurotoxins in the Venom of Afro-Asian Elapids

Sugar-coated survival: N-glycosylation as a unique bearded dragon venom resistance trait within Australian agamid lizards

In the ongoing evolutionary arms race between predators and prey, adaptive innovations often trigger a reciprocal response. For instance, the emergence of α-neurotoxins in snake venom has driven prey species targeted by these snakes to evolve sophisticated defense mechanisms. This study zeroes in on the particular motifs within the orthosteric sites of post-synaptic nicotinic acetylcholine receptors (nAChR) that confer resistance to α-neurotoxins, often through structural alterations of nAChR. This research examined Australian agamid lizards, a primary prey group for Australian elapid snakes, which are subject to predatory selection pressures. We previously showed that Pogona vitticeps (Central bearded dragon) was resistant to α-neurotoxic snake venoms through a steric hindrance form resistance evolving within the nAChR orthosteric, specifically through the 187-189NVT motif resulting in the presence of N-glycosylation, with the branching carbohydrate chains impeding the binding by the neurotoxins. This adaptive trait is thought to be a compensatory mechanism for the lizard's limited escape capabilities. Despite the significance of this novel adaptation, the prevalence and evolutionary roots of such venom resistance in Australian agamids have not been thoroughly investigated. To fill this knowledge gap, we undertook a comprehensive sequencing analysis of the nAChR ligand-binding domain across the full taxonomical diversity of Australian agamid species. Our findings reveal that the N-glycosylation resistance mechanism is a trait unique to the Pogona genus and absent in other Australian agamids. This aligns with Pogona's distinctive morphology, which likely increases vulnerability to neurotoxic elapid snakes, thereby increasing selective pressures for resistance. In contrast, biolayer interferometry experiments with death adder (Acanthophis species) venoms did not indicate any resistance-related binding patterns in other agamids, suggesting a lack of similar resistance adaptations, consistent with these lineages either being fast-moving, covered with large defensive spines, or being arboreal. This research not only uncovers a novel α-neurotoxin resistance mechanism in Australian agamids but also highlights the complex dynamics of the predator-prey chemical arms race. It provides a deeper understanding of how evolutionary pressures shape the interactions between venomous snakes and their prey.

Prey specificity of predatory venoms

Venom represents a key adaptation of many venomous predators, allowing them to immobilise prey quickly through chemical rather than physical warfare. Evolutionary arms races between prey and a predator are believed to be the main factor influencing the potency and composition of predatory venoms. Predators with narrowly restricted diets are expected to evolve specifically potent venom towards their focal prey, with lower efficacy on alternative prey. Here, we evaluate hypotheses on the evolution of prey-specific venom, focusing on the effect of restricted diet, prey defences, and prey resistance. Prey specificity as a potential evolutionary dead end is also discussed. We then provide an overview of the current knowledge on venom prey specificity, with emphasis on snakes, cone snails, and spiders. As the current evidence for venom prey specificity is still quite limited, we also overview the best approaches and methods for its investigation and provide a brief summary of potential model groups. Finally, possible applications of prey-specific toxins are discussed.

Red-on-Yellow Queen: Bio-Layer Interferometry Reveals Functional Diversity Within Micrurus Venoms and Toxin Resistance in Prey Species

Snakes in the family Elapidae largely produce venoms rich in three-finger toxins (3FTx) that bind to the α 1 subunit of nicotinic acetylcholine receptors (nAChRs), impeding ion channel activity. These neurotoxins immobilize the prey by disrupting muscle contraction. Coral snakes of the genus Micrurus are specialist predators who produce many 3FTx, making them an interesting system for examining the coevolution of these toxins and their targets in prey animals. We used a bio-layer interferometry technique to measure the binding interaction between 15 Micrurus venoms and 12 taxon-specific mimotopes designed to resemble the orthosteric binding region of the muscular nAChR subunit. We found that Micrurus venoms vary greatly in their potency on this assay and that this variation follows phylogenetic patterns rather than previously reported patterns of venom composition. The long-tailed Micrurus tend to have greater binding to nAChR orthosteric sites than their short-tailed relatives and we conclude this is the likely ancestral state. The repeated loss of this activity may be due to the evolution of 3FTx that bind to other regions of the nAChR. We also observed variations in the potency of the venoms depending on the taxon of the target mimotope. Rather than a pattern of prey-specificity, we found that mimotopes modeled after snake nAChRs are less susceptible to Micrurus venoms and that this resistance is partly due to a characteristic tryptophan → serine mutation within the orthosteric site in all snake mimotopes. This resistance may be part of a Red Queen arms race between coral snakes and their prey.

High-Voltage Toxin'Roll: Electrostatic Charge Repulsion as a Dynamic Venom Resistance Trait in Pythonid Snakes

The evolutionary interplay between predator and prey has significantly shaped the development of snake venom, a critical adaptation for subduing prey. This arms race has spurred the diversification of the components of venom and the corresponding emergence of resistance mechanisms in the prey and predators of venomous snakes. Our study investigates the molecular basis of venom resistance in pythons, focusing on electrostatic charge repulsion as a defense against α-neurotoxins binding to the alpha-1 subunit of the postsynaptic nicotinic acetylcholine receptor. Through phylogenetic and bioactivity analyses of orthosteric site sequences from various python species, we explore the prevalence and evolution of amino acid substitutions that confer resistance by electrostatic repulsion, which initially evolved in response to predatory pressure by Naja (cobra) species (which occurs across Africa and Asia). The small African species Python regius retains the two resistance-conferring lysines (positions 189 and 191) of the ancestral Python genus, conferring resistance to sympatric Naja venoms. This differed from the giant African species Python sebae, which has secondarily lost one of these lysines, potentially due to its rapid growth out of the prey size range of sympatric Naja species. In contrast, the two Asian species Python brongersmai (small) and Python bivittatus (giant) share an identical orthosteric site, which exhibits the highest degree of resistance, attributed to three lysine residues in the orthosteric sites. One of these lysines (at orthosteric position 195) evolved in the last common ancestor of these two species, which may reflect an adaptive response to increased predation pressures from the sympatric α-neurotoxic snake-eating genus Ophiophagus (King Cobras) in Asia. All these terrestrial Python species, however, were less neurotoxin-susceptible than pythons in other genera which have evolved under different predatory pressure as: the Asian species Malayopython reticulatus which is arboreal as neonates and juveniles before rapidly reaching sizes as terrestrial adults too large for sympatric Ophiophagus species to consider as prey; and the terrestrial Australian species Aspidites melanocephalus which occupies a niche, devoid of selection pressure from α-neurotoxic predatory snakes. Our findings underline the importance of positive selection in the evolution of venom resistance and suggest a complex evolutionary history involving both conserved traits and secondary evolution. This study enhances our understanding of the molecular adaptations that enable pythons to survive in environments laden with venomous threats and offers insights into the ongoing co-evolution between venomous snakes and their prey.

A Russian Doll of Resistance: Nested Gains and Losses of Venom Immunity in Varanid Lizards

The interplay between predator and prey has catalyzed the evolution of venom systems, with predators honing their venoms in response to the evolving resistance of prey. A previous study showed that the African varanid species Varanus exanthematicus has heightened resistance to snake venoms compared to the Australian species V. giganteus, V. komodoensis, and V. mertensi, likely due to increased predation by sympatric venomous snakes on V. exanthematicus. To understand venom resistance among varanid lizards, we analyzed the receptor site targeted by venoms in 27 varanid lizards, including 25 Australian varanids. The results indicate an active evolutionary arms race between Australian varanid lizards and sympatric neurotoxic elapid snakes. Large species preying on venomous snakes exhibit inherited neurotoxin resistance, a trait potentially linked to their predatory habits. Consistent with the 'use it or lose it' aspect of venom resistance, this trait was secondarily reduced in two lineages that had convergently evolved gigantism (V. giganteus and the V. komodoensis/V. varius clade), suggestive of increased predatory success accompanying extreme size and also increased mechanical protection against envenomation due to larger scale osteoderms. Resistance was completely lost in the mangrove monitor V. indicus, consistent with venomous snakes not being common in their arboreal and aquatic niche. Conversely, dwarf varanids demonstrate a secondary loss at the base of the clade, with resistance subsequently re-evolving in the burrowing V. acanthurus/V. storri clade, suggesting an ongoing battle with neurotoxic predators. Intriguingly, within the V. acanthurus/V. storri clade, resistance was lost again in V. kingorum, which is morphologically and ecologically distinct from other members of this clade. Resistance was also re-evolved in V. glebopalma which is terrestrial in contrast to the arboreal/cliff dwelling niches occupied by the other members of its clade (V. glebopalma, V. mitchelli, V. scalaris, V. tristis). This 'Russian doll' pattern of venom resistance underscores the dynamic interaction between dwarf varanids and Australian neurotoxic elapid snakes. Our research, which included testing Acanthophis (death adder) venoms against varanid receptors as models for alpha-neurotoxic interactions, uncovered a fascinating instance of the Red Queen Hypothesis: some death adders have developed more potent toxins specifically targeting resistant varanids, a clear sign of the relentless predator-prey arms race. These results offer new insight into the complex dynamics of venom resistance and highlight the intricate ecological interactions that shape the natural world.

Comparative Analysis of Alpha-1 Orthosteric-Site Binding by a Clade of Central American Pit Vipers (Genera Atropoides, Cerrophidion, Metlapilcoatlus, and Porthidium)

The distribution and relative potency of post-synaptic neurotoxic activity within Crotalinae venoms has been the subject of less investigation in comparison with Elapidae snake venoms. No previous studies have investigated post-synaptic neurotoxic activity within the Atropoides, Metlapilcoatlus, Cerrophidion, and Porthidium clade. Given the specificity of neurotoxins to relevant prey types, we aimed to uncover any activity present within this clade of snakes that may have been overlooked due to lower potency upon humans and thus not appearing as a clinical feature. Using biolayer interferometry, we assessed the relative binding of crude venoms to amphibian, lizard, bird, rodent and human α-1 nAChR orthosteric sites. We report potent alpha-1 orthosteric site binding in venoms from Atropoides picadoi, Metlapilcoatlus occiduus, M. olmec, M. mexicanus, M. nummifer. Lower levels of binding, but still notable, were evident for Cerrophidion godmani, C. tzotzilorum and C. wilsoni venoms. No activity was observed for Porthidium venoms, which is consistent with significant alpha-1 orthosteric site neurotoxicity being a trait that was amplified in the last common ancestor of Atropoides/Cerrophidion/Metlapilcoatlus subsequent to the split by Porthidium. We also observed potent taxon-selective activity, with strong selection for non-mammalian targets (amphibian, lizard, and bird). As these are poorly studied snakes, much of what is known about them is from clinical reports. The lack of affinity towards mammalian targets may explain the knowledge gap in neurotoxic activity within these species, since symptoms would not appear in bite reports. This study reports novel venom activity, which was previously unreported, indicating toxins that bind to post-synaptic receptors may be more widespread in pit vipers than previously considered. While these effects appear to not be clinically significant due to lineage-specific effects, they are of significant evolutionary novelty and of biodiscovery interest. This work sets the stage for future research directions, such as the use of in vitro and in vivo models to determine whether the alpha-1 orthosteric site binding observed within this study confers neurotoxic venom activity.

Resistance Is Not Futile: Widespread Convergent Evolution of Resistance to Alpha-Neurotoxic Snake Venoms in Caecilians (Amphibia: Gymnophiona)

Predatory innovations impose reciprocal selection pressures upon prey. The evolution of snake venom alpha-neurotoxins has triggered the corresponding evolution of resistance in the post-synaptic nicotinic acetylcholine receptors of prey in a complex chemical arms race. All other things being equal, animals like caecilians (an Order of legless amphibians) are quite vulnerable to predation by fossorial elapid snakes and their powerful alpha-neurotoxic venoms; thus, they are under strong selective pressure. Here, we sequenced the nicotinic acetylcholine receptor alpha-1 subunit of 37 caecilian species, representing all currently known families of caecilians from across the Americas, Africa, and Asia, including species endemic to the Seychelles. Three types of resistance were identified: (1) steric hindrance from N-glycosylated asparagines; (2) secondary structural changes due to the replacement of proline by another amino acid; and (3) electrostatic charge repulsion of the positively charged neurotoxins, through the introduction of a positively charged amino acid into the toxin-binding site. We demonstrated that resistance to alpha-neurotoxins convergently evolved at least fifteen times across the caecilian tree (three times in Africa, seven times in the Americas, and five times in Asia). Additionally, as several species were shown to possess multiple resistance modifications acting synergistically, caecilians must have undergone at least 20 separate events involving the origin of toxin resistance. On the other hand, resistance in non-caecilian amphibians was found to be limited to five origins. Together, the mutations underlying resistance in caecilians constitute a robust signature of positive selection which strongly correlates with elapid presence through both space (sympatry with caecilian-eating elapids) and time (Cenozoic radiation of elapids). Our study demonstrates the extent of convergent evolution that can be expected when a single widespread predatory adaptation triggers parallel evolutionary arms races at a global scale.

Venom phenotype conservation suggests integrated specialization in a lizard-eating snake

Biological specialization reduces the size of niche space while increasing efficiency in the use of available resources. Specialization often leads to phenotypic changes via natural selection aligning with niche space constraints. Commonly observed changes are in size, shape, behavior, and traits associated with feeding. One often selected trait for dietary specialization is venom, which, in snakes, often shows variation dependent on diet across and within species. The Neotropical Blunt-headed Treesnake (Imantodes cenchoa) is a highly specialized, rear-fanged, arboreal, lizard hunter that displays a long thin body, enlarged eyes, and a large Duvernoy's gland. However, toxin characterization of I. cenchoa has never been completed. Here, we use RNA-seq and mass spectrometry to assemble, annotate, and analyze the venom gland transcriptomes of four I. cenchoa from across their range. We find a lack of significant venom variation at the sequence and expression levels, suggesting venom conservation across the species. We propose this conservation provides evidence of a specialized venom repertoire, adapted to maximize efficiency of capturing and processing lizards. Importantly, this study provides the most complete venom gland transcriptomes of I. cenchoa and evidence of venom specialization in a rear-fanged snake, giving insight into selective pressures of venom across all snake species.

Venom biotechnology: casting light on nature's deadliest weapons using synthetic biology

Venoms are complex chemical arsenals that have evolved independently many times in the animal kingdom. Venoms have attracted the interest of researchers because they are an important innovation that has contributed greatly to the evolutionary success of many animals, and their medical relevance offers significant potential for drug discovery. During the last decade, venom research has been revolutionized by the application of systems biology, giving rise to a novel field known as venomics. More recently, biotechnology has also made an increasing impact in this field. Its methods provide the means to disentangle and study venom systems across all levels of biological organization and, given their tremendous impact on the life sciences, these pivotal tools greatly facilitate the coherent understanding of venom system organization, development, biochemistry, and therapeutic activity. Even so, we lack a comprehensive overview of major advances achieved by applying biotechnology to venom systems. This review therefore considers the methods, insights, and potential future developments of biotechnological applications in the field of venom research. We follow the levels of biological organization and structure, starting with the methods used to study the genomic blueprint and genetic machinery of venoms, followed gene products and their functional phenotypes. We argue that biotechnology can answer some of the most urgent questions in venom research, particularly when multiple approaches are combined together, and with other venomics technologies.

Composition and toxicity of venom produced by araneophagous white-tailed spiders (Lamponidae: Lampona sp.)

Prey-specialised spiders are adapted to capture specific prey items, including dangerous prey. The venoms of specialists are often prey-specific and less complex than those of generalists, but their venom composition has not been studied in detail. Here, we investigated the venom of the prey-specialised white-tailed spiders (Lamponidae: Lampona), which utilise specialised morphological and behavioural adaptations to capture spider prey. We analysed the venom composition using proteo-transcriptomics and taxon-specific toxicity using venom bioassays. Our analysis identified 208 putative toxin sequences, comprising 103 peptides < 10 kDa and 105 proteins > 10 kDa. Most peptides belonged to one of two families characterised by scaffolds containing eight or ten cysteine residues. Toxin-like proteins showed similarity to galectins, leucine-rich repeat proteins, trypsins and neprilysins. The venom of Lampona was shown to be more potent against the preferred spider prey than against alternative cricket prey. In contrast, the venom of a related generalist was similarly potent against both prey types. These data provide insights into the molecular adaptations of venoms produced by prey-specialised spiders.

Molecular dynamics simulation analysis of alpha-cobra toxin docked with phytochemical compounds

It is of interest to document data on the molecular dynamics simulation analysis of alpha-cobratoxin docked with phytochemical compounds. This can be used as effective drug candidates against the snake and scorpion venom. It should be noted experimental verification is needed to further validate the current data.

The Target Selects the Toxin: Specific Amino Acids in Snake-Prey Nicotinic Acetylcholine Receptors That Are Selectively Bound by King Cobra Venoms

Snake venom is an adaptive ecological trait that has evolved primarily as a form of prey subjugation. Thus, the selection pressure for toxin diversification is exerted by the prey’s physiological targets, with this pressure being particularly acute for specialist feeders, such as the King Cobra species, all of which are snake-prey specialists. However, while extensive research has been undertaken to elucidate key amino acids that guide toxin structure–activity relationships, reciprocal investigations into the specific sites guiding prey-lineage selective effects have been lacking. This has largely been due to the lack of assay systems amenable to systematic amino acid replacements of targeted proteins in the prey’s physiological pathways. To fill this knowledge gap, we used a recently described approach based upon mimotope peptides corresponding to the orthosteric site of nicotinic acetylcholine receptor alpha-1 subunits, a major binding site for snake venom neurotoxins that cause flaccid paralysis. We investigated the venoms of four different types of King Cobra (Cambodian, Javan, Malaysian, and Thai). This approach allowed for the determination of the key amino acid positions in King Cobra snake prey that are selectively bound by the toxins, whereby replacing these amino acids in the snake-prey orthosteric site with those from lizards or rats resulted in a significantly lower level of binding by the venoms, while conversely replacing the lizard or rat amino acids with those from the snake at that position increased the binding. By doing such, we identified three negatively charged amino acids in the snake orthosteric site that are strongly bound by the positively charged neurotoxic three-finger toxins found in King Cobra venom. This study, thus, sheds light on the selection pressures exerted by a specialist prey item for the evolution of lineage-selective toxins.

Differential coagulotoxic and neurotoxic venom activity from species of the arboreal viperid snake genus Bothriechis (palm-pitvipers)

The viperid snake genus Bothriechis consists of eleven species distributed among Central and South America, living across low and high-altitude habitats. Despite Bothriechis envenomations being prominent across the Central and South American region, the functional effects of Bothriechis venoms are poorly understood. Thus, the aim of this study was to investigate the coagulotoxic and neurotoxic activities of Bothriechis venoms to fill this knowledge gap. Coagulotoxic investigations revealed Bothriechis nigroviridis and B. schlegelii to have pseudo-procoagulant venom activity, forming weak clots that rapidly break down, thereby depleting fibrinogen levels and thus contributing to a net anticoagulant state. While one sample of B. lateralis also showed weaker pseudo-procoagulant activity, directly clotting fibrinogen, two samples of B. lateralis venom were anticoagulant through the inhibition of thrombin and factor Xa activity. Differential efficacy of PoliVal-ICP antivenom was also observed, with the pseudo-procoagulant effect of B. nigroviridis venom poorly neutralised, despite this same activity in the venom of B. schlegelii being effectively neutralised. Significant specificity of these fibrinogen cleaving toxins was also observed, with no activity upon model amphibian, avian, lizard or rodent plasma observed. However, upon avian plasma the venom of B. nigroviridis exerted a complete anticoagulant effect, in contrast to the pseudo-procoagulant effect seen on human plasma. Neurotoxic investigations revealed B. schlegelii to be unique among the genus in having potent binding to the orthosteric site of the alpha-1 postsynaptic nicotinic acetylcholine receptor (with B. lateralis having a weaker but still discernible effect). This represents the first identification of postsynaptic nAChR neurotoxic activity for Bothriechis. In conclusion this study identifies notable differential activity within the coagulotoxic and postsynaptic neurotoxic activity of Bothriechis venoms, supporting previous research, and highlights the need for further studies with respect to antivenom efficacy as well as coagulotoxin specificity for Bothriechis venoms.

Taxon-selective venom variation in adult and neonate Daboia russelii (Russell's Viper), and antivenom efficacy

Major variations in venom composition can occur between juvenile and adult venomous snakes. However, due to logistical constraints, antivenoms are produced using adult venoms in immunising mixtures, possibly resulting in limited neutralisation of juvenile snake venoms. Daboia russelii is one of the leading causes of snakebite death across South Asia. Its venom is potently procoagulant, causing stroke in prey animals but causing in humans consumptive coagulopathy-a net anticoagulant state-and sometimes death resulting from hemorrhage. In this in vitro study, we compared the venom activity of-and antivenom efficacy against-six 2-week-old D. russelii relative to that of their parents. Using a coagulation analyser, we quantified the relative coagulotoxicity of these venoms in human, avian, and amphibian plasma. The overall potency on human plasma was similar across all adult and neonate venoms, and SII (Serum Institute of India) antivenom was equipotent in neutralising these coagulotoxic effects. In addition, all venoms were also similar in their action upon avian plasma. In contrast, the neonate venoms were more potent on amphibian plasma, suggesting amphibians make up a larger proportion of neonate diet than adult diet. A similar venom potency in human and avian plasmas but varying selectivity for amphibian plasma suggests ontogenetic differences in toxin isoforms within the factor X or factor V activating classes, thereby providing a testable hypothesis for future transcriptomics work. By providing insights into the functional venom differences between adult and neonate D. russelii venoms, we hope to inform clinical treatment of patients envenomated by this deadly species and to shed new light on the natural history of these extremely medically important snakes.

Snake Venom Proteomics of Samar Cobra (Naja samarensis) from the Southern Philippines: Short Alpha-Neurotoxins as the Dominant Lethal Component Weakly Cross-Neutralized by the Philippine Cobra Antivenom

The Samar Cobra, Naja samarensis, is endemic to the southern Philippines and is a WHO-listed Category 1 venomous snake species of medical importance. Envenomation caused by N. samarensis results in neurotoxicity, while there is no species-specific antivenom available for its treatment. The composition and neutralization of N. samarensis venom remain largely unknown to date. This study thus aimed to investigate the venom proteome of N. samarensis for a comprehensive profiling of the venom composition, and to examine the immunorecognition as well as neutralization of its toxins by a hetero-specific antivenom. Applying C₁₈ reverse-phase high-performance liquid chromatography (RP-HPLC) and tandem mass spectrometry (LC-MS/MS), three-finger toxins (3FTx) were shown to dominate the venom proteome by 90.48% of total venom proteins. Other proteins in the venom comprised snake venom metalloproteinases, phospholipases A_2, cysteine-rich secretory proteins, venom nerve growth factors, L-amino acid oxidases and vespryn, which were present at much lower abundances. Among all, short-chain alpha-neurotoxins (SαNTX) were the most highly expressed toxin within 3FTx family, constituting 65.87% of the total venom proteins. The SαNTX is the sole neurotoxic component of the venom and has an intravenous median lethal dose (LD₅₀) of 0.18 μg/g in mice. The high abundance and low LD₅₀ support the potent lethal activity of N. samarensis venom. The hetero-specific antivenom, Philippine Cobra Antivenom (PCAV, raised against Naja philippinensis) were immunoreactive toward the venom and its protein fractions, including the principal SαNTX. In efficacy study, PCAV was able to cross-neutralize the lethality of SαNTX albeit the effect was weak with a low potency of 0.20 mg/ml (defined as the amount of toxin completely neutralized per milliliter of the antivenom). With a volume of 5 ml, each vial of PCAV may cross-neutralize approximately 1 mg of the toxin in vivo. The findings support the potential para-specific use of PCAV in treating envenomation caused by N. samarensis while underscoring the need to improve the potency of its neutralization activity, especially against the highly lethal alpha-neurotoxins.

Monkeying around with venom: an increased resistance to α-neurotoxins supports an evolutionary arms race between Afro-Asian primates and sympatric cobras

Background

Snakes and primates have a multi-layered coevolutionary history as predators, prey, and competitors with each other. Previous work has explored the Snake Detection Theory (SDT), which focuses on the role of snakes as predators of primates and argues that snakes have exerted a selection pressure for the origin of primates’ visual systems, a trait that sets primates apart from other mammals. However, primates also attack and kill snakes and so snakes must simultaneously avoid primates. This factor has been recently highlighted in regard to the movement of hominins into new geographic ranges potentially exerting a selection pressure leading to the evolution of spitting in cobras on three independent occasions.

Results

Here, we provide further evidence of coevolution between primates and snakes, whereby through frequent encounters and reciprocal antagonism with large, diurnally active neurotoxic elapid snakes, Afro-Asian primates have evolved an increased resistance to α-neurotoxins, which are toxins that target the nicotinic acetylcholine receptors. In contrast, such resistance is not found in Lemuriformes in Madagascar, where venomous snakes are absent, or in Platyrrhini in the Americas, where encounters with neurotoxic elapids are unlikely since they are relatively small, fossorial, and nocturnal. Within the Afro-Asian primates, the increased resistance toward the neurotoxins was significantly amplified in the last common ancestor of chimpanzees, gorillas, and humans (clade Homininae). Comparative testing of venoms from Afro-Asian and American elapid snakes revealed an increase in α-neurotoxin resistance across Afro-Asian primates, which was likely selected against cobra venoms. Through structure-activity studies using native and mutant mimotopes of the α-1 nAChR receptor orthosteric site (loop C), we identified the specific amino acids responsible for conferring this increased level of resistance in hominine primates to the α-neurotoxins in cobra venom.

Conclusion

We have discovered a pattern of primate susceptibility toward α-neurotoxins that supports the theory of a reciprocal coevolutionary arms-race between venomous snakes and primates.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01195-x.

Promoting co-existence between humans and venomous snakes through increasing the herpetological knowledge base

Snakebite incidence at least partly depends on the biology of the snakes involved. However, studies of snake biology have been largely neglected in favour of anthropic factors, with the exception of taxonomy, which has been recognised for some decades to affect the design of antivenoms. Despite this, within-species venom variation and the unpredictability of the correlation with antivenom cross-reactivity has continued to be problematic. Meanwhile, other aspects of snake biology, including behaviour, spatial ecology and activity patterns, distribution, and population demography, which can contribute to snakebite mitigation and prevention, remain underfunded and understudied. Here, we review the literature relevant to these aspects of snakebite and illustrate how demographic, spatial, and behavioural studies can improve our understanding of why snakebites occur and provide evidence for prevention strategies. We identify the large gaps that remain to be filled and urge that, in the future, data and relevant metadata be shared openly via public data repositories so that studies can be properly replicated and data used in future meta-analyses.

Evidence for Resistance to Coagulotoxic Effects of Australian Elapid Snake Venoms by Sympatric Prey (Blue Tongue Skinks) but Not by Predators (Monitor Lizards)

Some Australian elapids possess potently procoagulant coagulotoxic venoms which activate the zymogen prothrombin into the functional enzyme thrombin. Although the activity of Australian elapid prothrombin-activators has been heavily investigated with respect to the mammalian, and in particular, human clotting cascades, very few studies have investigated the activity of their venom upon reptile plasmas. This is despite lizards representing both the primary diet of most Australian elapids and also representing natural predators. This study investigated the procoagulant actions of a diverse range of Australian elapid species upon plasma from known prey species within the genera Tiliqua (blue tongue skinks) as well as known predator species within the genera Varanus (monitor lizards). In addition to identifying significant variation in the natural responses of the coagulation cascade between species from the genera Tiliqua and Varanus relative to each other, as well as other vertebrate lineages, notable differences in venom activity were also observed. Within the genus Tiliqua, both T. rugosa and T. scincoides plasma displayed significant resistance to the procoagulant activity of Pseudechis porphyriacus venom, despite being susceptible to all other procoagulant elapid venoms. These results indicate that T. rugosa and T. scincoides have evolved resistance within their plasma to the coagulotoxic venom activity of the sympatric species P. porphyriacus. Other venoms were able to activate Tiliqua prothrombin, which suggests that the lessened activity of P. porphyriacus venom is not due to modifications of the prothrombin and may instead be due to a serum factor that specifically binds to P. porphyriacus toxins, as has been previously seen for squirrels resistant to rattlesnake venom. In contrast, none of the predatory lizards studied (Varanus giganteus, V. mertensi and V. varius) demonstrated resistance to the venom. This suggests that the mechanical protection afforded by thick osteodermic scales, and prey handling behaviour, removes a selection pressure for the evolution of resistance in these large predatory lizards. These results therefore reveal differential interactions between venoms of snakes with sympatric lizards that are on opposite sides of the predator-prey arms race.

The Dragon's Paralysing Spell: Evidence of Sodium and Calcium Ion Channel Binding Neurotoxins in Helodermatid and Varanid Lizard Venoms

Bites from helodermatid lizards can cause pain, paresthesia, paralysis, and tachycardia, as well as other symptoms consistent with neurotoxicity. Furthermore, in vitro studies have shown that Heloderma horridum venom inhibits ion flux and blocks the electrical stimulation of skeletal muscles. Helodermatids have long been considered the only venomous lizards, but a large body of robust evidence has demonstrated venom to be a basal trait of Anguimorpha. This clade includes varanid lizards, whose bites have been reported to cause anticoagulation, pain, and occasionally paralysis and tachycardia. Despite the evolutionary novelty of these lizard venoms, their neuromuscular targets have yet to be identified, even for the iconic helodermatid lizards. Therefore, to fill this knowledge gap, the venoms of three Heloderma species (H. exasperatum, H. horridum and H. suspectum) and two Varanus species (V. salvadorii and V. varius) were investigated using Gallus gallus chick biventer cervicis nerve–muscle preparations and biolayer interferometry assays for binding to mammalian ion channels. Incubation with Heloderma venoms caused the reduction in nerve-mediated muscle twitches post initial response of avian skeletal muscle tissue preparation assays suggesting voltage-gated sodium (Na_V) channel binding. Congruent with the flaccid paralysis inducing blockage of electrical stimulation in the skeletal muscle preparations, the biolayer interferometry tests with Heloderma suspectum venom revealed binding to the S3–S4 loop within voltage-sensing domain IV of the skeletal muscle channel subtype, Na_V1.4. Consistent with tachycardia reported in clinical cases, the venom also bound to voltage-sensing domain IV of the cardiac smooth muscle calcium channel, Ca_V1.2. While Varanus varius venom did not have discernable effects in the avian tissue preparation assay at the concentration tested, in the biointerferometry assay both V. varius and V. salvadorii bound to voltage-sensing domain IV of both Na_V1.4 and Ca_V1.2, similar to H. suspectum venom. The ability of varanid venoms to bind to mammalian ion channels but not to the avian tissue preparation suggests prey-selective actions, as did the differential potency within the Heloderma venoms for avian versus mammalian pathophysiological targets. This study thus presents the detailed characterization of Heloderma venom ion channel neurotoxicity and offers the first evidence of varanid lizard venom neurotoxicity. In addition, the data not only provide information useful to understanding the clinical effects produced by envenomations, but also reveal their utility as physiological probes, and underscore the potential utility of neglected venomous lineages in the drug design and development pipeline.

Venom-Induced Blood Disturbances by Palearctic Viperid Snakes, and Their Relative Neutralization by Antivenoms and Enzyme-Inhibitors

Palearctic vipers are medically significant snakes in the genera Daboia, Macrovipera, Montivipera, and Vipera which occur throughout Europe, Central Asia, Near and Middle East. While the ancestral condition is that of a small-bodied, lowland species, extensive diversification has occurred in body size, and niche specialization. Using 27 venom samples and a panel of in vitro coagulation assays, we evaluated the relative coagulotoxic potency of Palearctic viper venoms and compared their neutralization by three antivenoms (Insoserp Europe, VIPERFAV and ViperaTAb) and two metalloprotease inhibitors (prinomastat and DMPS). We show that variation in morphology parallels variation in the Factor X activating procoagulant toxicity, with the three convergent evolutions of larger body sizes (Daboia genus, Macrovipera genus, and Vipera ammodytes uniquely within the Vipera genus) were each accompanied by a significant increase in procoagulant potency. In contrast, the two convergent evolutions of high altitude specialization (the Montivipera genus and Vipera latastei uniquely within the Vipera genus) were each accompanied by a shift away from procoagulant action, with the Montivipera species being particularly potently anticoagulant. Inoserp Europe and VIPERFAV antivenoms were both effective against a broad range of Vipera species, with Inoserp able to neutralize additional species relative to VIPERFAV, reflective of its more complex antivenom immunization mixture. In contrast, ViperaTAb was extremely potent in neutralizing V. berus but, reflective of this being a monovalent antivenom, it was not effective against other Vipera species. The enzyme inhibitor prinomastat efficiently neutralized the metalloprotease-driven Factor X activation of the procoagulant venoms. In contrast, DMPS (2,3-dimercapto-1-propanesulfonic acid), which as been suggested as another potential treatment option in the absence of antivenom, DMPS failed against all venoms tested. Overall, our results highlight the evolutionary variations within Palearctic vipers and help to inform clinical management of viper envenomation.

Getting stoned: Characterisation of the coagulotoxic and neurotoxic effects of reef stonefish (Synanceia verrucosa) venom

The reef stonefish (Synanceia verrucosa) is a venomous fish which causes excruciatingly painful envenomations. While some research on the pathophysiology and functions of the venom have been conducted, there are still some gaps in the understanding of the venom effects due to the extreme lability of fish venom toxins and the lack of available testing platforms. Here we set out to assess new functions of the venom whilst also attempting to address some unclear pathophysiological effects from previous literature. Utilising a biolayer interferometry assay, our results highlight that the venom binds to the orthosteric site of the α-1 nicotinic acetylcholine receptor as well as the domain IV of voltage-gated Ca²⁺ (Ca_V1.2) channel mimotopes. Both these results add some clarity to the previously ambiguous literature. We further assessed the coagulotoxic effects of the venom using thromboelastography and Stago STA-R Max coagulation analyser assays. We reveal that the venom produced anticoagulant activity and significantly delayed time until clot formation of recalcified human plasma which is likely through the degradation of phospholipids. There was a difference between fresh and lyophilised venom activity toward the nicotinic acetylcholine receptor mimotopes and coagulation assays, whilst no difference was observed in the activity toward the domain IV of Ca_V1.2 mimotopes. This research adds further insights into the neglected area of fish venom whilst also highlighting the extreme labile nature of fish venom toxins.

Not Goanna Get Me: Mutations in the Savannah Monitor Lizard (Varanus exanthematicus) Nicotinic Acetylcholine Receptor Confer Reduced Susceptibility to Sympatric Cobra Venoms

Antagonistic coevolutionary relationships provide intense selection pressure which drive changes in the genotype. Predator-prey interactions have caused some venomous snakes and their predators/prey to evolve α-neurotoxin resistance through changes at the orthosteric site of nicotinic acetylcholine receptors. The presence of negatively charged amino acids at orthosteric site positions 191 and 195 is the ancestral state. These negatively charged amino acids have exerted a selection pressure for snake venom α-neurotoxins to evolve with strong positive charges on their molecular surface, with the opposite-charge attraction facilitating the binding by the neurotoxins. We aimed to test the effects of a series of mutations whereby one or both negatively charged amino acids are replaced by uncharged residues to ascertain if this was a novel form of reduced venom susceptibility in the varanid species. Using a biolayer interferometry assay, we tested the relative binding of α-neurotoxin-rich snake venoms against the orthosteric sites of V. giganteus (Perentie) and V. komodoensis (Komodo dragon), which both possess the negatively charged aspartic acid at position 191; V. mertensi (Merten's water monitor), which also has aspartic acid at position 195; and Varanus exanthematicus (savannah monitor), which lacks negatively charged amino acids at both positions 191 and 195. The orthosteric sites of these species are otherwise identical. In order to complete the structure-function relationship examination, we also tested a mutant version with the negatively charged aspartic acid at both positions 191 and 195. It was demonstrated that the presence of a negatively charged amino acid at either position 191 or 195 is crucial for the successful binding of snake venom α-neurotoxins, with V. giganteus, V. komodoensis and V. mertensi all strongly bound. The mutant version containing a negatively charged amino acid at both positions was bound equipotently to the native forms of V. giganteus, V. komodoensis and V. mertensi. Thus, the presence of a negatively charged amino acid at both positions does not increase binding affinity. In contrast, Varanus exanthematicus, lacking a negatively charged amino acid at either position, displayed dramatically less sensitivity to neurotoxins compared with the other species. V. exanthematicus is distinguished from the other species examined in this study by being a small, terrestrial, slow-moving species living sympatrically with a high density of large cobra species that have neurotoxin-rich venoms. Thus, this vulnerable prey item seems to have evolved a novel form of reduced susceptibility to snake venom neurotoxins under a strong selection pressures from these neurotoxic predators. These results therefore contribute to the body of knowledge of predator/prey chemical arm races while providing novel insights into the structure-activity relationships of the orthosteric site of the nicotinic acetylcholine receptor alpha-subunit.

Utilising venom activity to infer dietary composition of the Kenyan horned viper (Bitis worthingtoni)

Bitis are well known for being some of the most commonly encountered and medically important snake species in all of Africa. While the majority of species possess potently anticoagulant venom, only B. worthingtoni is known to possess procoagulant venom. Although known to be the basal species within the genus, B. worthingtoni is an almost completely unstudied species with even basic dietary information lacking. This study investigated various aspects of the unique procoagulant effects of B. worthingtoni venom. Coagulation assays determined the primary procoagulant effect to be driven by Factor X activating snake venom metalloprotease toxins. In addition to acting upon the mammalian blood clotting cascade, B. worthingtoni venom was also shown to clot amphibian plasma. As previous studies have shown differences in clotting factors between amphibian and mammalian plasmas, individual enzymes in snake venoms acting on plasma clotting factors can be taxon-selective. As venoms evolve under purifying selection pressures, this suggests that the procoagulant snake venom metalloprotease toxins present in B. worthingtoni have likely been retained from a recent common ancestor shared with the related amphibian-feeding Proatheris superciliaris, and that both amphibians and mammals represent a substantial proportion of B. worthingtoni current diet. Thus, taxon-specific actions of venoms may have utility in inferring dietary composition for rare or difficult to study species. An important caveat is that to validate this hypothesis field studies investigating the dietary ecology of B. worthingtoni must be conducted, as well as further investigations of its venom composition to reconstruct the molecular evolutionary history of the toxins present.

Electrostatic resistance to alpha-neurotoxins conferred by charge reversal mutations in nicotinic acetylcholine receptors

The evolution of venom resistance through coevolutionary chemical arms races has arisen multiple times throughout animalia. Prior documentation of resistance to snake venom α-neurotoxins consists of the N-glycosylation motif or the hypothesized introduction of arginine at positions 187 at the α-1 nicotinic acetylcholine receptor orthosteric site. However, no further studies have investigated the possibility of other potential forms of resistance. Using a biolayer interferometry assay, we first confirm that the previously hypothesized resistance conferred by arginine at position 187 in the honey badger does reduce binding to α-neurotoxins, which has never been functionally tested. We further discovered a novel form of α-neurotoxin resistance conferred by charge reversal mutations, whereby a negatively charged amino acid is replaced by the positively charged amino acid lysine. As venom α-neurotoxins have evolved strong positive charges on their surface to facilitate binding to the negatively charged α-1 orthosteric site, these mutations result in a positive charge/positive charge interaction electrostatically repelling the α-neurotoxins. Such a novel mechanism for resistance has gone completely undiscovered, yet this form of resistance has convergently evolved at least 10 times within snakes. These coevolutionary innovations seem to have arisen through convergent phenotypes to ultimately evolve a similar biophysical mechanism of resistance across snakes.

In Vitro Tests for Assessing the Neutralizing Ability of Snake Antivenoms: Toward the 3Rs Principles

There is an urgent need to strengthen the implementation of the 3Rs principle (Replacement, Reduction and Refinement) in the use of experimental animals in toxinological research and in the assessment of the neutralizing efficacy of snake antivenoms. This is a challenging task owing to the inherent complexity of snake venoms. The state of the art on this topic is hereby reviewed, with emphasis on the studies in which a correlation has been observed between in vivo toxicity tests and in vitro surrogate assays, particularly in the study of lethal activity of venoms and its neutralization. Correlations have been described with some venoms-antivenoms when using: (a) enzyme immunoassays, (b) hemagglutination, (c) enzyme assays (proteinase, phospholipase A₂), (d) in vitro coagulant effect on plasma, (e) cell culture assays for cytotoxicity, (f) functional assays for assessing neurotoxicity in vitro, (g) use of hens’ eggs, and (h) antivenomics. Additionally, the routine introduction of analgesia in these assays and the design of more ‘humane’ protocols for the lethality test are being pursued. It is expected that the next years will witness a growing awareness of the relevance of the 3Rs principles in antivenom testing, and that new in vitro alternatives and more ‘humane’ experimental designs will emerge in this field.

Widespread and Differential Neurotoxicity in Venoms from the Bitis Genus of Viperid Snakes

Research into the neurotoxic activity of venoms from species within the snake family Viperidae is relatively neglected compared with snakes in the Elapidae family. Previous studies into venoms from the Bitis genus of vipers have identified the presence of presynaptic phospholipase A₂ neurotoxins in B. atropos and B. caudalis, as well as a postsynaptic phospholipase A₂ in B. arietans. Yet, no studies have investigated how widespread neurotoxicity is across the Bitis genus or if they exhibit prey selectivity of their neurotoxins. Utilising a biolayer interferometry assay, we were able to assess the binding of crude venom from 14 species of Bitis to the neuromuscular α-1 nAChR orthosteric site across a wide range of vertebrate taxa mimotopes. Postsynaptic binding was seen for venoms from B. arietans, B. armata, B. atropos, B. caudalis, B. cornuta, B. peringueyi and B. rubida. To further explore the types of neurotoxins present, venoms from the representatives B. armata, B. caudalis, B. cornuta and B. rubida were additionally tested in the chick biventer cervicis nerve muscle preparation, which showed presynaptic and postsynaptic activity for B. caudalis and only presynaptic neurotoxicity for B. cornuta and B. rubida, with myotoxicity also evident for some species. These results, combined with the biolayer interferometry results, indicate complex neurotoxicity exerted by Bitis species, which varies dramatically by lineage tested upon. Our data also further support the importance of sampling across geographical localities, as significant intraspecific variation of postsynaptic neurotoxicity was reported across the different localities.

Assessing the Binding of Venoms from Aquatic Elapids to the Nicotinic Acetylcholine Receptor Orthosteric Site of Different Prey Models

The evolution of an aquatic lifestyle from land dwelling venomous elapids is a radical ecological modification, bringing about many evolutionary changes from morphology to diet. Diet is an important ecological facet which can play a key role in regulating functional traits such as venom composition and prey-specific targeting of venom. In addition to predating upon novel prey (e.g., fish, fish eggs and invertebrates), the venoms of aquatic elapids also face the challenge of increased prey-escape potential in the aquatic environment. Thus, despite the independent radiation into an aquatic niche on four separate occasions, the venoms of aquatic elapids are evolving under convergent selection pressures. Utilising a biolayer interferometry binding assay, this study set out to elucidate whether crude venoms from representative aquatic elapids were target-specific to the orthosteric site of postsynaptic nicotinic acetylcholine receptor mimotopes of fish compared to other terrestrial prey types. Representatives of the four aquatic lineages were: aquatic coral snakes representative was Micrurus surinamensis;, sea kraits representative was Laticauda colubrina; sea snakes representatives were two Aipysurus spp. and eight Hydrophis spp; and water cobras representative was Naja annulata. No prey-specific differences in crude venom binding were observed from any species tested, except for Aipysurus laevis, which showed slight evidence of prey-potency differences. For Hydrophis caerulescens, H. peronii, H. schistosus and M. surinamensis, there was a lack of binding to the orthosteric site of any target lineage. Subsequent testing on the in vitro chick-biventer cervicis muscle preparation suggested that, while the venoms of these species bound postsynaptically, they bound to allosteric sites rather than orthosteric. Allosteric binding is potentially a weaker but faster-acting form of neurotoxicity and we hypothesise that the switch to allosteric binding is likely due to selection pressures related to prey-escape potential. This research has potentially opened up the possibility of a new functional class of toxins which have never been assessed previously while shedding light on the selection pressures shaping venom evolution.

Widespread Evolution of Molecular Resistance to Snake Venom α-Neurotoxins in Vertebrates

Venomous snakes are important subjects of study in evolution, ecology, and biomedicine. Many venomous snakes have alpha-neurotoxins (α-neurotoxins) in their venom. These toxins bind the alpha-1 nicotinic acetylcholine receptor (nAChR) at the neuromuscular junction, causing paralysis and asphyxia. Several venomous snakes and their predators have evolved resistance to α-neurotoxins. The resistance is conferred by steric hindrance from N-glycosylated asparagines at amino acids 187 or 189, by an arginine at position 187 that has been hypothesized to either electrostatically repulse positively charged neurotoxins or sterically interfere with α-neurotoxin binding, or proline replacements at positions 194 or 197 of the nAChR ligand-binding domain to inhibit α-neurotoxin binding through structural changes in the receptor. Here, we analyzed this domain in 148 vertebrate species, and assessed its amino acid sequences for resistance-associated mutations. Of these sequences, 89 were sequenced de novo. We find widespread convergent evolution of the N-glycosylation form of resistance in several taxa including venomous snakes and their lizard prey, but not in the snake-eating birds studied. We also document new lineages with the arginine form of inhibition. Using an in vivo assay in four species, we provide further evidence that N-glycosylation mutations reduce the toxicity of cobra venom. The nAChR is of crucial importance for normal neuromuscular function and is highly conserved throughout the vertebrates as a result. Our research shows that the evolution of α-neurotoxins in snakes may well have prompted arms races and mutations to this ancient receptor across a wide range of sympatric vertebrates. These findings underscore the inter-connectedness of the biosphere and the ripple effects that one adaption can have across global ecosystems.

An Examination of the Neutralization of In Vitro Toxicity of Chinese Cobra (Naja atra) Venom by Different Antivenoms

The Chinese Cobra (Naja atra) is an elapid snake of major medical importance in southern China. We describe the in vitro neurotoxic, myotoxic, and cytotoxic effects of N. atra venom, as well as examining the efficacy of three Chinese monovalent antivenoms (N. atra antivenom, Gloydius brevicaudus antivenom and Deinagkistrodon acutus antivenom) and an Australian polyvalent snake antivenom. In the chick biventer cervicis nerve-muscle preparation, N. atra venom (1–10 µg/mL) abolished indirect twitches in a concentration-dependent manner, as well as abolishing contractile responses to exogenous acetylcholine chloride (ACh) and carbamylcholine chloride (CCh), indicative of post-synaptic neurotoxicity. Contractile responses to potassium chloride (KCl) were also significantly inhibited by venom indicating myotoxicity. The prior addition of Chinese N. atra antivenom (0.75 U/mL) or Australian polyvalent snake antivenom (3 U/mL), markedly attenuated the neurotoxic actions of venom (3 µg/mL) and prevented the inhibition of contractile responses to ACh, CCh, and KCl. The addition of Chinese antivenom (0.75 U/mL) or Australian polyvalent antivenom (3 U/mL) at the t₉₀ time point after the addition of venom (3 µg/mL), partially reversed the inhibition of twitches and significantly reversed the venom-induced inhibition of responses to ACh and CCh, but had no significant effect on the response to KCl. Venom (30 µg/mL) also abolished direct twitches in the chick biventer cervicis nerve-muscle preparation and caused a significant increase in baseline tension, further indicative of myotoxicity. N. atra antivenom (4 U/mL) prevented the myotoxic effects of venom (30 µg/mL). However, G. brevicaudus antivenom (24 U/mL), D. acutus antivenom (8 U/mL) and Australian polyvalent snake antivenom (33 U/mL) were unable to prevent venom (30 µg/mL) induced myotoxicity. In the L6 rat skeletal muscle myoblast cell line, N. atra venom caused concentration-dependent inhibition of cell viability, with a half maximal inhibitory concentration (IC₅₀) of 2.8 ± 0.48 μg/mL. N. atra antivenom significantly attenuated the cytotoxic effect of the venom, whereas Australian polyvalent snake antivenom was less effective but still attenuated the cytotoxic effects at lower venom concentrations. Neither G. brevicaudus antivenom or D. acutus antivenom were able to prevent the cytotoxicity. This study indicates that Chinese N. atra monovalent antivenom is efficacious against the neurotoxic, myotoxic and cytotoxic effects of N. atra venom but the clinical effectiveness of the antivenom is likely to be diminished, even if given early after envenoming. The use of Chinese viper antivenoms (i.e., G. brevicaudus and D. acutus antivenoms) in cases of envenoming by the Chinese cobra is not supported by the results of the current study.

An in vitro α-neurotoxin-nAChR binding assay correlates with lethality and in vivo neutralization of a large number of elapid neurotoxic snake venoms from four continents

The aim of this study was to develop an in vitro assay for use in place of in vivo assays of snake venom lethality and antivenom neutralizing potency. A novel in vitro assay has been developed based on the binding of post-synaptically acting α-neurotoxins to nicotinic acetylcholine receptor (nAChR), and the ability of antivenoms to prevent this binding. The assay gave high correlation in previous studies with the in vivo murine lethality tests (Median Lethal Dose, LD50), and the neutralization of lethality assays (Median Effective Dose, ED50) by antisera against Naja kaouthia, Naja naja and Bungarus candidus venoms. Here we show that, for the neurotoxic venoms of 20 elapid snake species from eight genera and four continents, the in vitro median inhibitory concentrations (IC50s) for α-neurotoxin binding to purified nAChR correlated well with the in vivo LD50s of the venoms (R2 = 0.8526, p < 0.001). Furthermore, using this assay, the in vitro ED50s of a horse pan-specific antiserum against these venoms correlated significantly with the corresponding in vivo murine ED50s, with R2 = 0.6896 (p < 0.01). In the case of four elapid venoms devoid or having a very low concentration of α-neurotoxins, no inhibition of nAChR binding was observed. Within the philosophy of 3Rs (Replacement, Reduction and Refinement) in animal testing, the in vitro α-neurotoxin-nAChR binding assay can effectively substitute the mouse lethality test for toxicity and antivenom potency evaluation for neurotoxic venoms in which α-neurotoxins predominate. This will greatly reduce the number of mice used in toxicological research and antivenom production laboratories. The simpler, faster, cheaper and less variable in vitro assay should also expedite the development of pan-specific antivenoms against various medically important snakes in many parts of the world.

A symmetry or asymmetry: Functional and compositional comparison of venom from the left and right glands of the Indochinese spitting cobra (Naja siamensis)

Contralaterally positioned maxillary (upper jaw) venom glands in snakes are mechanically independent, being able to discharge venom from either gland separately. This has led some studies to test venom function and composition of each contralaterally positioned venom gland to investigate any differences. However, the data on the subject to-date derives from limited sample sizes, appearing somewhat contradictory, and thus still remains inconclusive. Here, we tested samples obtained from the left and right venom glands of four N. siamensis specimens for their relative binding to the orthosteric site of amphibian, lizard, snake, bird, and rodent alpha-1 nicotinic acetylcholine receptors. We also show the relative proteomic patterns displayed by reversed phase liquid chromatography – mass spectrometry. Our results indicate that three of the venom gland sets showed no difference in both functional binding and composition, whilst one venom gland set showed a slight difference in functional binding (but not in specificity patterns between prey types) or venom composition. We hypothesise that these differences in functional binding may be due to one gland having previously ejected venom at some time prior to venom extraction, whilst its contralateral counterpart did not. This might cause the differential rate of toxin replenishment to be unequal between glands, thus instigating the difference in potency, likely due to uneven toxin proportions between glands at the time of venom extraction. These results demonstrate that the separate venom producing glands in snakes remain under the same genetic control elements and produce identical venom components.

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No differences in venom profile by contralateral venom glands of Naja siamensis.

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No differences in alpha-1 binding potency by contralateral N. siamensis venom glands.

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A single outlier in the data showed a small difference in relative potency.

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Potency is likely affected by differential replenishment rates.

Evolutionary Interpretations of Nicotinic Acetylcholine Receptor Targeting Venom Effects by a Clade of Asian Viperidae Snakes

Ecological variability among closely related species provides an opportunity for evolutionary comparative studies. Therefore, to investigate the origin and evolution of neurotoxicity in Asian viperid snakes, we tested the venoms of Azemiops feae, Calloselasma rhodostoma, Deinagkistrodon acutus, Tropidolaeums subannulatus, and T. wagleri for their relative specificity and potency upon the amphibian, lizard, bird, rodent, and human α-1 (neuromuscular) nicotinic acetylcholine receptors. We utilised a biolayer interferometry assay to test the binding affinity of these pit viper venoms to orthosteric mimotopes of nicotinic acetylcholine receptors binding region from a diversity of potential prey types. The Tropidolaemus venoms were much more potent than the other species tested, which is consistent with the greater prey escape potential in arboreal niches. Intriguingly, the venom of C. rhodostoma showed neurotoxic binding to the α-1 mimotopes, a feature not known previously for this species. The lack of prior knowledge of neurotoxicity in this species is consistent with our results due to the bias in rodent studies and human bite reports, whilst this venom had a greater binding affinity toward amphibian and diapsid α-1 targets. The other large terrestrial species, D. acutus, did not display any meaningful levels of neurotoxicity. These results demonstrate that whilst small peptide neurotoxins are a basal trait of these snakes, it has been independently amplified on two separate occasions, once in Azemiops and again in Tropidolaemus, and with Calloselasma representing a third possible amplification of this trait. These results also point to broader sources of novel neuroactive peptides with the potential for use as lead compounds in drug design and discovery.