Azemiopsin from Azemiops feae viper venom, a novel polypeptide ligand of nicotinic acetylcholine receptor

Subtype-Selective Peptide and Protein Neurotoxic Inhibitors of Nicotinic Acetylcholine Receptors Enhance Proliferation of Patient-Derived Glioblastoma Cell Lines

Glioblastoma multiforme (GBM) is the most aggressive type of brain cancer, with a poor prognosis. GBM cells, which develop in the environment of neural tissue, often exploit neurotransmitters and their receptors to promote their own growth and invasion. Nicotinic acetylcholine receptors (nAChRs), which play a crucial role in central nervous system signal transmission, are widely represented in the brain, and GBM cells express several subtypes of nAChRs that are suggested to transmit signals from neurons, promoting tumor invasion and growth. Analysis of published GBM transcriptomes revealed spatial heterogeneity in nAChR subtype expression, and functional nAChRs of α1*, α7, and α9 subtypes are demonstrated in our work on several patient-derived GBM microsphere cultures and on the U87MG GBM cell line using subtype-selective neurotoxins and fluorescent calcium mobilization assay. The U87MG cell line shows reactions to nicotinic agonists similar to those of GBM patient-derived culture. Selective α1*, α7, and α9 nAChR neurotoxins stimulated cell growth in the presence of nicotinic agonists. Several cultivating conditions with varying growth factor content have been proposed and tested. The use of selective neurotoxins confirmed that cell cultures obtained from patients are representative GBM models, but the use of media containing fetal bovine serum can lead to alterations in nAChR expression and functioning.

Biomedical applications of synthetic peptides derived from venom of animal origin: A systematic review

Development of therapeutic agents that have fewer adverse effects and have higher efficacy for diseases, such as cancer, metabolic disorders, neurological diseases, infections, cardiovascular diseases, and respiratory diseases, are required. Recent studies have focused on identifying novel sources for pharmaceutical molecules to develop therapies against these diseases. Among the sources for potentially new therapies, animal venom-derived molecules have generated much interest. Various animal venom-derived proteins and peptides have been isolated, identified, synthesized, and tested to develop drugs. Venom-derived peptides have several biomedical properties, such as proapoptotic, cell migration, and autophagy regulation activities in cancer cell models; induction of vasodilation by nitric oxide and regulation of angiotensin II; modification of insulin response by controlling calcium and potassium channels; regulation of pain receptor activity; modulation of immune cell activity; alteration of motor neuron activity; degradation or inhibition of β-amyloid plaque formation; antibacterial, antifungal, antiviral, and antiprotozoal activities; increase in sperm motility and potentiation of erectile function; reduction of intraocular pressure; anticoagulation, fibrinolytic, and antithrombotic activities; etc. This systematic review compiles these biomedical properties and potential biomedical applications of synthesized animal venom-derived peptides reported in the latest research. In addition, the limitations and areas of opportunity in this research field are discussed so that new studies can be developed based on the data presented.

Nicotinic acetylcholine receptors in cancer: Limitations and prospects

Nicotinic acetylcholine receptors (nAChRs) have long been considered to solely mediate neurotransmission. However, their widespread distribution in the human body suggests a more diverse physiological role. Additionally, the expression of nAChRs is increased in certain cancers, such as lung cancer, and has been associated with cell proliferation, epithelial-to-mesenchymal cell transition, angiogenesis and apoptosis prevention. Several compounds that interact with these receptors have been identified as potential therapeutic agents. They have been tested as drugs for treating nicotine addiction, alcoholism, depression, pain and Alzheimer's disease. This review focuses on nAChR-mediated signalling in cancer, presenting opportunities for the development of innovative nAChR-based anticancer drugs. It displays the differences in expression of each nAChR subunit between normal and cancer cells for selected cancer types, highlighting their possible involvement in specific cases. Antagonists of nAChRs that could complement existing cancer therapies are summarised and critically discussed. We hope that this review will stimulate further research on the role of nAChRs in cancer potentially leading to innovative cancer therapies.

De Novo Assembly of Venom Gland Transcriptome of Tropidolaemus wagleri (Temple Pit Viper, Malaysia) and Insights into the Origin of Its Major Toxin, Waglerin

The venom proteome of Temple Pit Viper (Tropidolaemus wagleri) is unique among pit vipers, characterized by a high abundance of a neurotoxic peptide, waglerin. To further explore the genetic diversity of its toxins, the present study de novo assembled the venom gland transcriptome of T. wagleri from west Malaysia. Among the 15 toxin gene families discovered, gene annotation and expression analysis reveal the dominating trend of bradykinin-potentiating peptide/angiotensin-converting enzyme inhibitor-C-type natriuretic peptide (BPP/ACEI-CNP, 76.19% of all-toxin transcription) in the transcriptome, followed by P-III snake venom metalloproteases (13.91%) and other toxins. The transcript TwBNP01 of BPP/ACEI-CNP represents a large precursor gene (209 amino acid residues) containing the coding region for waglerin (24 residues). TwBNP01 shows substantial sequence variations from the corresponding genes of its sister species, Tropidolaemus subannulatus of northern Philippines, and other viperid species which diversely code for proline-rich small peptides such as bradykinin-potentiating peptides (BPPs). The waglerin/waglerin-like peptides, BPPs and azemiopsin are proline-rich, evolving de novo from multiple highly diverged propeptide regions within the orthologous BPP/ACEI-CNP genes. Neofunctionalization of the peptides results in phylogenetic constraints consistent with a phenotypic dichotomy, where Tropidolaemus spp. and Azemiops feae convergently evolve a neurotoxic trait while vasoactive BPPs evolve only in other species.

Fifty Years of Animal Toxin Research at the Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry RAS

This review covers briefly the work carried out at our institute (IBCh), in many cases in collaboration with other Russian and foreign laboratories, for the last 50 years. It discusses the discoveries and studies of various animal toxins, including protein and peptide neurotoxins acting on the nicotinic acetylcholine receptors (nAChRs) and on other ion channels. Among the achievements are the determination of the primary structures of the α-bungarotoxin-like three-finger toxins (TFTs), covalently bound dimeric TFTs, glycosylated cytotoxin, inhibitory cystine knot toxins (ICK), modular ICKs, and such giant molecules as latrotoxins and peptide neurotoxins from the snake, as well as from other animal venoms. For a number of toxins, spatial structures were determined, mostly by 1H-NMR spectroscopy. Using this method in combination with molecular modeling, the molecular mechanisms of the interactions of several toxins with lipid membranes were established. In more detail are presented the results of recent years, among which are the discovery of α-bungarotoxin analogs distinguishing the two binding sites in the muscle-type nAChR, long-chain α-neurotoxins interacting with α9α10 nAChRs and with GABA-A receptors, and the strong antiviral effects of dimeric phospholipases A2. A summary of the toxins obtained from arthropod venoms includes only highly cited works describing the molecules' success story, which is associated with IBCh. In marine animals, versatile toxins in terms of structure and molecular targets were discovered, and careful work on α-conotoxins differing in specificity for individual nAChR subtypes gave information about their binding sites.

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.

What Are the Neurotoxins in Hemotoxic Snake Venoms?

Snake venoms as tools for hunting are primarily aimed at the most vital systems of the prey, especially the nervous and circulatory systems. In general, snakes of the Elapidae family produce neurotoxic venoms comprising of toxins targeting the nervous system, while snakes of the Viperidae family and most rear-fanged snakes produce hemotoxic venoms directed mainly on blood coagulation. However, it is not all so clear. Some bites by viperids results in neurotoxic signs and it is now known that hemotoxic venoms do contain neurotoxic components. For example, viperid phospholipases A₂ may manifest pre- or/and postsynaptic activity and be involved in pain and analgesia. There are other neurotoxins belonging to diverse families ranging from large multi-subunit proteins (e.g., C-type lectin-like proteins) to short peptide neurotoxins (e.g., waglerins and azemiopsin), which are found in hemotoxic venoms. Other neurotoxins from hemotoxic venoms include baptides, crotamine, cysteine-rich secretory proteins, Kunitz-type protease inhibitors, sarafotoxins and three-finger toxins. Some of these toxins exhibit postsynaptic activity, while others affect the functioning of voltage-dependent ion channels. This review represents the first attempt to systematize data on the neurotoxins from "non-neurotoxic" snake venom. The structural and functional characteristic of these neurotoxins affecting diverse targets in the nervous system are considered.

Toxins' classification through Raman spectroscopy with principal component analysis

The method based on the combination of Raman spectroscopy and principal component analysis (PCA) was applied to the set of peptide and protein toxins from animal venoms and to synthetic analogues of peptides. The study demonstrated the possibility of toxin classification according to their primary and secondary structures based on Raman spectroscopy. The method described here allows discrimination of snake venom three-finger toxins from predatory marine mollusks α-conotoxins. Moreover, PCA of the Raman spectra of toxins revealed differences within the group of three-finger toxins and also within the group of conotoxins, related to their spatial structure. In particular, on the basis of the developed technique it is possible to distinguish the disulfide isomers of the same peptide toxin. The results obtained have been confirmed by bioinformatic methods. So, we have proposed a method for the rapid analysis of newly discovered venom-derived protein or peptide toxins by establishing their similarity with other already studied toxins by referring to a particular class. Taking into account a low specimen consumption by Raman spectroscopy, the proposed method could represent a first step in the study of toxins from rare and/or endangered venomous animals. The ability to distinguish configuration of disulfide bonds allows to synthesize the correct isomer of the toxin.

De Novo Genome Assembly Highlights the Role of Lineage-Specific Duplications in the Evolution of Venom in Fea’s Viper

Despite the medical significance to humans and important ecological roles filled by vipers, few high-quality genomic resources exist for these snakes outside of a few genera of pitvipers. Here we sequence, assemble, and annotate the genome of Fea’s Viper (Azemiops feae). This taxon is distributed in East Asia and belongs to a monotypic subfamily, sister to the pitvipers. The newly sequenced genome resulted in a 1.56 Gb assembly, a contig N50 of 1.59 Mb, with 97.6% of the genome assembly in contigs >50 Kb, and a BUSCO completeness of 92.4%. We found that A. feae venom is primarily composed of phospholipase A₂ (PLA₂) proteins expressed by genes that likely arose from lineage-specific PLA₂ gene duplications. Additionally, we show that renin, an enzyme associated with blood pressure regulation in mammals and known from the venoms of two viper species including A. feae, is expressed in the venom gland at comparative levels to known toxins and is present in the venom proteome. The cooption of this gene as a toxin may be more widespread in viperids than currently known. To investigate the historical population demographics of A. feae, we performed coalescent-based analyses and determined that the effective population size has remained stable over the last 100 kyr. This suggests Quaternary glacial cycles likely had minimal influence on the demographic history of A. feae. This newly assembled genome will be an important resource for studying the genomic basis of phenotypic evolution and understanding the diversification of venom toxin gene families.

Interaction of α9α10 Nicotinic Receptors With Peptides and Proteins From Animal Venoms

Unlike most neuronal nicotinic acetylcholine receptor (nAChR) subunits, α7, α9, and α10 subunits are able to form functional homo- or heteromeric receptors without any β subunits. While the α7 subtype is widely distributed in the mammalian brain and several peripheral tissues, α9 and α9α10 nAChRs are mainly found in the cochlea and immune cells. α-Conotoxins that specifically block the α9α10 receptor showed anti-nociceptive and anti-hyperalgesic effects in animal models. Hence, this subtype is considered a drug target for analgesics. In contrast to the α9α10-selective α-conotoxins, the three-finger toxin α-bungarotoxin inhibits muscle-type and α7 nAChRs in addition to α9α10 nAChRs. However, the selectivity of α-neurotoxins at the α9α10 subtype was less intensively investigated. Here, we compared the potencies of α-conotoxins and α-neurotoxins at the human α9α10 nAChR by two-electrode voltage clamp analysis upon expression in Xenopus oocytes. In addition, we analyzed effects of several α9α10-selective α-conotoxins on mouse granulocytes from bone marrow to identify possible physiological functions of the α9α10 nAChR subtype in these cells. The α-conotoxin-induced IL-10 release was measured upon LPS-stimulation. We found that α-conotoxins RgIA, PeIA, and Vc1.1 enhance the IL-10 expression in granulocytes which might explain the known anti-inflammatory and associated analgesic activities of α9α10-selective α-conotoxins. Furthermore, we show that two long-chain α-neurotoxins from the cobra Naja melanoleuca venom that were earlier shown to bind to muscle-type and α7 nAChRs, also inhibit the α9α10 subtype at nanomolar concentrations with one of them showing a significantly slower dissociation from this receptor than α-bungarotoxin.

Dynamic genetic differentiation drives the widespread structural and functional convergent evolution of snake venom proteinaceous toxins

Background

The explosive radiation and diversification of the advanced snakes (superfamily Colubroidea) was associated with changes in all aspects of the shared venom system. Morphological changes included the partitioning of the mixed ancestral glands into two discrete glands devoted for production of venom or mucous respectively, as well as changes in the location, size and structural elements of the venom-delivering teeth. Evidence also exists for homology among venom gland toxins expressed across the advanced snakes. However, despite the evolutionary novelty of snake venoms, in-depth toxin molecular evolutionary history reconstructions have been mostly limited to those types present in only two front-fanged snake families, Elapidae and Viperidae. To have a broader understanding of toxins shared among extant snakes, here we first sequenced the transcriptomes of eight taxonomically diverse rear-fanged species and four key viperid species and analysed major toxin types shared across the advanced snakes.

Results

Transcriptomes were constructed for the following families and species: Colubridae - Helicops leopardinus, Heterodon nasicus, Rhabdophis subminiatus; Homalopsidae – Homalopsis buccata; Lamprophiidae - Malpolon monspessulanus, Psammophis schokari, Psammophis subtaeniatus, Rhamphiophis oxyrhynchus; and Viperidae – Bitis atropos, Pseudocerastes urarachnoides, Tropidolaeumus subannulatus, Vipera transcaucasiana. These sequences were combined with those from available databases of other species in order to facilitate a robust reconstruction of the molecular evolutionary history of the key toxin classes present in the venom of the last common ancestor of the advanced snakes, and thus present across the full diversity of colubroid snake venoms. In addition to differential rates of evolution in toxin classes between the snake lineages, these analyses revealed multiple instances of previously unknown instances of structural and functional convergences. Structural convergences included: the evolution of new cysteines to form heteromeric complexes, such as within kunitz peptides (the beta-bungarotoxin trait evolving on at least two occasions) and within SVMP enzymes (the P-IIId trait evolving on at least three occasions); and the C-terminal tail evolving on two separate occasions within the C-type natriuretic peptides, to create structural and functional analogues of the ANP/BNP tailed condition. Also shown was that the de novo evolution of new post-translationally liberated toxin families within the natriuretic peptide gene propeptide region occurred on at least five occasions, with novel functions ranging from induction of hypotension to post-synaptic neurotoxicity. Functional convergences included the following: multiple occasions of SVMP neofunctionalised in procoagulant venoms into activators of the clotting factors prothrombin and Factor X; multiple instances in procoagulant venoms where kunitz peptides were neofunctionalised into inhibitors of the clot destroying enzyme plasmin, thereby prolonging the half-life of the clots formed by the clotting activating enzymatic toxins; and multiple occasions of kunitz peptides neofunctionalised into neurotoxins acting on presynaptic targets, including twice just within Bungarus venoms.

Conclusions

We found novel convergences in both structural and functional evolution of snake toxins. These results provide a detailed roadmap for future work to elucidate predator–prey evolutionary arms races, ascertain differential clinical pathologies, as well as documenting rich biodiscovery resources for lead compounds in the drug design and discovery pipeline.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01208-9.

Snake Toxins Labeled by Green Fluorescent Protein or Its Synthetic Chromophore are New Probes for Nicotinic acetylcholine Receptors

Fluorescence can be exploited to monitor intermolecular interactions in real time and at a resolution up to a single molecule. It is a method of choice to study ligand-receptor interactions. However, at least one of the interacting molecules should possess good fluorescence characteristics, which can be achieved by the introduction of a fluorescent label. Gene constructs with green fluorescent protein (GFP) are widely used to follow the expression of the respective fusion proteins and monitor their function. Recently, a small synthetic analogue of GFP chromophore (p-HOBDI-BF₂) was successfully used for tagging DNA molecules, so we decided to test its applicability as a potential fluorescent label for proteins and peptides. This was done on α-cobratoxin (α-CbTx), a three-finger protein used as a molecular marker of muscle-type, neuronal α7 and α9/α10 nicotinic acetylcholine receptors (nAChRs), as well as on azemiopsin, a linear peptide neurotoxin selectively inhibiting muscle-type nAChRs. An activated N-hydroxysuccinimide ester of p-HOBDI-BF₂ was prepared and utilized for toxin labeling. For comparison we used a recombinant α-CbTx fused with a full-length GFP prepared by expression of a chimeric gene. The structure of modified toxins was confirmed by mass spectrometry and their activity was characterized by competition with iodinated α-bungarotoxin in radioligand assay with respective receptor preparations, as well as by thermophoresis. With the tested protein and peptide neurotoxins, introduction of the synthetic GFP chromophore induced considerably lower decrease in their affinity for the receptors as compared with full-length GFP attachment. The obtained fluorescent derivatives were used for nAChR visualization in tissue slices and cell cultures.

Structural and bioinformatic analyses of Azemiops venom serine proteases reveal close phylogeographic relationships to pitvipers from eastern China and the New World

The semi-fossil and pit-less Azemiops feae is possibly the most primitive crotalid species. Here, we have cloned and sequenced cDNAs encoding four serine proteases (vSPs) from the venom glands of Chinese A. feae. Full amino-acid sequences of the major vSP (designated as AzKNa) and three minor vSPs (designated as AzKNb, AzKNc and Az-PA) were deduced. Using Protein-BLAST search, the ten most-similar vSPs for each Azemiops vSP have been selected for multiple sequence alignment, and all the homologs are crotalid vSPs. The results suggest that the A. feae vSPs are structurally most like those of eastern-Chinese Gloydius, Viridovipera, Protobothrops and North American pitvipers, and quite different from more-specialized vSPs such as Agkistrodon venom Protein-C activators. The vSPs from Chinese A. feae and those from Vietnamese A. feae show significant sequence variations. AzKNa is acidic and contains six potential N-glycosylation sites and its surface-charge distribution differs greatly from that of AzKNb, as revealed by 3D-modeling. AzKNb and AzKNc do not contain N-glycosylation sites although most of their close homologs contain one or two. Az-PA belongs to the plasminogen-activator subtype with a conserved N²⁰-glycosylation site. The evolution of this subtype of vSPs in Azemiops and related pitvipers has been traced by phylogenetic analysis.

Redirecting drug repositioning to discover innovative cosmeceuticals

Skin appearance is essential for self-esteem and quality of life; consequently, skin care products represent a huge market. In particular, cosmeceuticals constitute a hybrid category of skin care formulations, at the interphase of cosmetics and pharmaceuticals, rationally designed to target (patho) physiological mechanisms aiming to enhance skin health and appearance. Cosmeceuticals are marketed as anti-ageing, anti-wrinkle, hair regrowth, skin whitening and wound healing agents with special emphasis on scar-free healing. An overview on recent cutting-edge advances concerning the discovery and development of enhanced performance cosmeceuticals by drug repositioning approaches is presented here. In this context, we propose "target repositioning," a new term, to highlight that druggable protein targets implicated in multiple diseases (hubs in the diseasome) can be exploited to accelerate the discovery of molecularly targeted cosmeceuticals that can promote skin health as an added benefit, which is a novel concept not described before. In this direction, emphasis is placed on the role of mouse models, for often untreatable skin diseases, as well as recent breakthroughs on monogenic rare skin syndromes, in promoting compound repositioning to innovative cosmeceuticals.

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.

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.

Novel Bradykinin-Potentiating Peptides and Three-Finger Toxins from Viper Venom: Combined NGS Venom Gland Transcriptomics and Quantitative Venom Proteomics of the Azemiops feae Viper

Feae's viper Azemipos feae belongs to the Azemiopinae subfamily of the Viperidae family. The effects of Viperidae venoms are mostly coagulopathic with limited neurotoxicity manifested by phospholipases A2. From A. feae venom, we have earlier isolated azemiopsin, a novel neurotoxin inhibiting the nicotinic acetylcholine receptor. To characterize other A. feae toxins, we applied label-free quantitative proteomics, which revealed 120 unique proteins, the most abundant being serine proteinases and phospholipases A2. In total, toxins representing 14 families were identified, among which bradykinin-potentiating peptides with unique amino acid sequences possessed biological activity in vivo. The proteomic analysis revealed also basal (commonly known as non-conventional) three-finger toxins belonging to the group of those possessing neurotoxic activity. This is the first indication of the presence of three-finger neurotoxins in viper venom. In parallel, the transcriptomic analysis of venom gland performed by Illumina next-generation sequencing further revealed 206 putative venom transcripts. Together, the study unveiled the venom proteome and venom gland transciptome of A. feae, which in general resemble those of other snakes from the Viperidae family. However, new toxins not found earlier in viper venom and including three-finger toxins and unusual bradykinin-potentiating peptides were discovered.

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.

Snake C-type lectin-like proteins inhibit nicotinic acetylcholine receptors

Venoms of viperid snakes affect mostly hemostasis, while C-type lectin-like proteins (CTLPs), one of the main components of viperid venoms, act as anticoagulants, procoagulants, or agonists/antagonists of platelet activation. However, we have shown earlier that CTLPs from the saw-scaled viper Echis multisquamatus, called emunarecins EM1 and EM2, were able to inhibit nicotinic acetylcholine receptors (nAChRs) in neurons of a pond snail (Lymnaea stagnalis). Here we analysed the structure of the emunarecins by mass spectrometry and report that EM1 and EM2 inhibit fluorescent α-bungarotoxin binding to both muscle-type nAChRs from Torpedo californica and human neuronal α7 nAChRs. EM1 at 23µM and EM2 at 9µM almost completely prevented fluorecsent α-bungarotoxin binding to muscle-type nAChRs. Interaction with human neuronal α7 nAChR was weaker; EM1 at the concentration of 23µM blocked the α-bungarotoxin binding only by about 40% and EM2 at 9µM by about 20%. The efficiency of the EM2 interaction with nAChRs was comparable to that of a non-conventional toxin, WTX, from Naja kaouthia cobra venom. Together with the data obtained earlier, these results show that CTLPs may represent new nAChR ligands.

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.

Arachidonoylcholine and Other Unsaturated Long-Chain Acylcholines Are Endogenous Modulators of the Acetylcholine Signaling System

Cholines acylated with unsaturated fatty acids are a recently discovered family of endogenous lipids. However, the data on the biological activity of acylcholines remain very limited. We hypothesized that acylcholines containing residues of arachidonic (AA-CHOL), oleic (Ol-CHOL), linoleic (Ln-CHOL), and docosahexaenoic (DHA-CHOL) acids act as modulators of the acetylcholine signaling system. In the radioligand binding assay, acylcholines showed inhibition in the micromolar range of both α7 neuronal nAChR overexpressed in GH4C1 cells and muscle type nAChR from Torpedo californica, as well as Lymnaea stagnalis acetylcholine binding protein. Functional response was checked in two cell lines endogenously expressing α7 nAChR. In SH-SY5Y cells, these compounds did not induce Ca2+ rise, but inhibited the acetylcholine-evoked Ca2+ rise with IC50 9 to 12 μM. In the A549 lung cancer cells, where α7 nAChR activation stimulates proliferation, Ol-CHOL, Ln-CHOL, and AA-CHOL dose-dependently decreased cell viability by up to 45%. AA-CHOL inhibited human erythrocyte acetylcholinesterase (AChE) and horse serum butyrylcholinesterase (BChE) by a mixed type mechanism with Ki = 16.7 ± 1.5 μM and αKi = 51.4 ± 4.1 μM for AChE and Ki = 70.5 ± 6.3 μM and αKi = 214 ± 17 μM for BChE, being a weak substrate of the last enzyme only, agrees with molecular docking results. Thus, long-chain unsaturated acylcholines could be viewed as endogenous modulators of the acetylcholine signaling system.

A Taxon-Specific and High-Throughput Method for Measuring Ligand Binding to Nicotinic Acetylcholine Receptors

The binding of compounds to nicotinic acetylcholine receptors is of great interest in biomedical research. However, progress in this area is hampered by the lack of a high-throughput, cost-effective, and taxonomically flexible platform. Current methods are low-throughput, consume large quantities of sample, or are taxonomically limited in which targets can be tested. We describe a novel assay which utilizes a label-free bio-layer interferometry technology, in combination with adapted mimotope peptides, in order to measure ligand binding to the orthosteric site of nicotinic acetylcholine receptor alpha-subunits of diverse organisms. We validated the method by testing the evolutionary patterns of a generalist feeding species (Acanthophis antarcticus), a fish specialist species (Aipysurus laevis), and a snake specialist species (Ophiophagus hannah) for comparative binding to the orthosteric site of fish, amphibian, lizard, snake, bird, marsupial, and rodent alpha-1 nicotinic acetylcholine receptors. Binding patterns corresponded with diet, with the Acanthophis antarcticus not showing bias towards any particular lineage, while Aipysurus laevis showed selectivity for fish, and Ophiophagus hannah a selectivity for snake. To validate the biodiscovery potential of this method, we screened Acanthophis antarcticus and Tropidolaemus wagleri venom for binding to human alpha-1, alpha-2, alpha-3, alpha-4, alpha-5, alpha-6, alpha-7, alpha-9, and alpha-10. While A. antarcticus was broadly potent, T. wagleri showed very strong but selective binding, specifically to the alpha-1 target which would be evolutionarily selected for, as well as the alpha-5 target which is of major interest for drug design and development. Thus, we have shown that our novel method is broadly applicable for studies including evolutionary patterns of venom diversification, predicting potential neurotoxic effects in human envenomed patients, and searches for novel ligands of interest for laboratory tools and in drug design and development.

Encapsulation of Neurotoxins, Blockers of Nicotinic Acetylcholine Receptors, in Nanomaterials Based on Sulfated Polysaccharides

Three-finger snake neurotoxins are selective antagonists of some nicotinic acetylcholine receptor subtypes and are widely used to study these receptors. The peptide neurotoxin azemiopsin, recently isolated from the venom of Azemipos feae, is a selective blocker of muscle-type nicotinic acetylcholine receptor. In order to reduce their toxicity and increase resistance under physiological conditions, we have encapsulated these toxins into nanomaterials. The study of nanomaterials after interaction with neurotoxins by the methods of transmission electron microscopy and dynamic light scattering revealed an increase in the size of nanoparticles, which indicates the inclusion of neurotoxins in nanomaterials.

Oligoarginine Peptides, a New Family of Nicotinic Acetylcholine Receptor Inhibitors

Many peptide ligands of nicotinic acetylcholine receptors (nAChRs) contain a large number of positively charged amino acid residues, a striking example being conotoxins RgIA and GeXIVA from marine mollusk venom, with an arginine content of >30%. To determine whether peptides built exclusively from arginine residues will interact with different nAChR subtypes or with their structural homologs such as the acetylcholine-binding protein and ligand-binding domain of the nAChR α9 subunit, we synthesized a series of R3, R6, R8, and R16 oligoarginines and investigated their activity by competition with radioiodinated α-bungarotoxin, two-electrode voltage-clamp electrophysiology, and calcium imaging. R6 and longer peptides inhibited muscle-type nAChRs, α7 nAChRs, and α3β2 nAChRs in the micromolar range. The most efficient inhibition of ion currents was detected for muscle nAChR by R16 (IC₅₀ = 157 nM) and for the α9α10 subtype by R8 and R16 (IC₅₀ = 44 and 120 nM, respectively). Since the R8 affinity for other tested nAChRs was 100-fold lower, R8 appears to be a selective antagonist of α9α10 nAChR. For R8, the electrophysiological and competition experiments indicated the existence of two distinct binding sites on α9α10 nAChR. Since modified oligoarginines and other cationic molecules are widely used as cell-penetrating peptides, we studied several cationic polymers and demonstrated their nAChR inhibitory activity. SIGNIFICANT STATEMENT: By using radioligand analysis, electrophysiology, and calcium imaging, we found that oligoarginine peptides are a new group of inhibitors for muscle nicotinic acetylcholine receptors (nAChRs) and some neuronal nAChRs, the most active being those with 16 and 8 Arg residues. Such compounds and other cationic polymers are cell-penetrating tools for drug delivery, and we also demonstrated the inhibition of nAChRs for several of the latter. Possible positive and negative consequences of such an action should be taken into account.

Scorpion toxins interact with nicotinic acetylcholine receptors

Neurotoxins are among the main components of scorpion and snake venoms. Scorpion neurotoxins affect voltage-gated ion channels, while most snake neurotoxins target ligand-gated ion channels, mainly nicotinic acetylcholine receptors (nAChRs). We report that scorpion venoms inhibit α-bungarotoxin binding to both muscle-type nAChR from Torpedo californica and neuronal human α7 nAChR. Toxins inhibiting nAChRs were identified as OSK-1 (α-KTx family) from Orthochirus scrobiculosus and HelaTx1 (κ-KTx family) from Heterometrus laoticus, both being blockers of voltage-gated potassium channels. With an IC₅₀ of 1.6 μm, OSK1 inhibits acetylcholine-induced current through mouse muscle-type nAChR heterologously expressed in Xenopus oocytes. Other well-characterized scorpion toxins from these families also bind to Torpedo nAChR with micromolar affinities. Our results indicate that scorpion neurotoxins present target promiscuity.

Toxins for decoding interface selectivity in nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels that play crucial roles in neurotransmission and regulate complex processes in brain functions, including anxiety, learning and memory, food intake, drug addiction, cognition and nociception. To perform these and other functions, a diverse array of nAChR subtypes are generated by homomeric or heteromeric assembly of 17 homologous nAChR subunits. Agonists, acetylcholine and nicotine, bind to the interface formed between two α subunits and between α and non-α subunits to activate the nAChR and allow cation influx. The diversity of subunit interfaces determines the channel properties, the responses to different agonists/antagonists, desensitization and downstream signaling and thus, define specialized properties and functions. Over the last several decades, snake venom neurotoxins have contributed to the purification, localization and characterization of molecular details of various nAChRs. Utkin et al. have described the purification and characterization of αδ-bungarotoxins, a novel class of neurotoxins in a recent paper published in the Biochemical Journal [Biochem. J. (2019) 476, 1285-1302]. These toxins from Bungarus candidus venom preferably bind to α-δ site with two orders of magnitude higher affinity compared with α-γ or α-ε sites. The subtle changes in the structure of αδ-bungarotoxins led to variation in interface selectivity. Such new classes of antagonists will offer us great opportunity to delineate the pharmacophores and design new highly selective antagonists. Thus, their findings provide new impetus to re-evaluate molecular details of pharmacological properties of α-neurotoxins with careful consideration towards subtype-, interface- and species-selectivity.

Novel long-chain neurotoxins from Bungarus candidus distinguish the two binding sites in muscle-type nicotinic acetylcholine receptors

αδ-Bungarotoxins, a novel group of long-chain α-neurotoxins, manifest different affinity to two agonist/competitive antagonist binding sites of muscle-type nicotinic acetylcholine receptors (nAChRs), being more active at the interface of α-δ subunits. Three isoforms (αδ-BgTx-1-3) were identified in Malayan Krait (Bungarus candidus) from Thailand by genomic DNA analysis; two of them (αδ-BgTx-1 and 2) were isolated from its venom. The toxins comprise 73 amino acid residues and 5 disulfide bridges, being homologous to α-bungarotoxin (α-BgTx), a classical blocker of muscle-type and neuronal α7, α8, and α9α10 nAChRs. The toxicity of αδ-BgTx-1 (LD₅₀ = 0.17-0.28 µg/g mouse, i.p. injection) is essentially as high as that of α-BgTx. In the chick biventer cervicis nerve-muscle preparation, αδ-BgTx-1 completely abolished acetylcholine response, but in contrast with the block by α-BgTx, acetylcholine response was fully reversible by washing. αδ-BgTxs, similar to α-BgTx, bind with high affinity to α7 and muscle-type nAChRs. However, the major difference of αδ-BgTxs from α-BgTx and other naturally occurring α-neurotoxins is that αδ-BgTxs discriminate the two binding sites in the Torpedo californica and mouse muscle nAChRs showing up to two orders of magnitude higher affinity for the α-δ site as compared with α-ε or α-γ binding site interfaces. Molecular modeling and analysis of the literature provided possible explanations for these differences in binding mode; one of the probable reasons being the lower content of positively charged residues in αδ-BgTxs. Thus, αδ-BgTxs are new tools for studies on nAChRs.

Improving therapeutic potential of antibacterial spider venom peptides: coarse-grain molecular dynamics guided approach

Aim: Spider venom is a rich source of antibacterial peptides, whose hemolytic activity is often excessive. Methodology: How to get rid of it? Using latarcins from Lachesana tarabaevi and oxyopinin Oxt 4a from Oxyopes takobius spider venoms we performed coarse-grained molecular dynamics simulations of these peptides in the presence of lipid bilayers, mimicking erythrocyte membranes. This identified hemolytically active fragments within Oxt 4a and latarcins. Then, we synthesized five 20-residue peptides, containing different parts of the Oxt 4a and latarcin-1 sequence, carrying mutations within the identified regions. Conclusion: The antibacterial and hemolytic tests suggested that the three of the synthesized peptides demonstrated substantial decrease in hemolytic activity, retaining, or even exceeding antibacterial potential of the parent peptides.

The First Recombinant Viper Three-Finger Toxins: Inhibition of Muscle and Neuronal Nicotinic Acetylcholine Receptors

Genes encoding two three-finger toxins TFT-AF and TFT-VN, nucleotide sequences of which were earlier determined by cloning cDNA from venom glands of vipers Azemiops feae and Vipera nikolskii, respectively, were expressed for the first time in E. coli cells. The biological activity of these toxins was studied by electrophysiological techniques, calcium imaging, and radioligand analysis. It was shown for the first time that viper three-finger toxins are antagonists of nicotinic acetylcholine receptors of neuronal and muscle type.

Azemiopsin, a Selective Peptide Antagonist of Muscle Nicotinic Acetylcholine Receptor: Preclinical Evaluation as a Local Muscle Relaxant

Azemiopsin (Az), a linear peptide from the Azemiops feae viper venom, contains no disulfide bonds, is a high-affinity and selective inhibitor of nicotinic acetylcholine receptor (nAChR) of muscle type and may be considered as potentially applicable nondepolarizing muscle relaxant. In this study, we investigated its preclinical profile in regard to in vitro and in vivo efficacy, acute and chronic toxicity, pharmacokinetics, allergenic capacity, immunotoxicity and mutagenic potency. The peptide effectively inhibited (IC50 ~ 19 nM) calcium response of muscle nAChR evoked by 30 μM (EC100) acetylcholine but was less potent (IC50 ~ 3 μM) at α7 nAChR activated by 10 μM (EC50) acetylcholine and had a low affinity to α4β2 and α3-containing nAChR, as well as to GABAA or 5HT₃ receptors. Its muscle relaxant effect was demonstrated at intramuscular injection to mice at doses of 30-300 µg/kg, 30 µg/kg being the initial effective dose and 90 µg/kg-the average effective dose. The maximal muscle relaxant effect of Az was achieved in 10 min after the administration and elimination half-life of Az in mice was calculated as 20-40 min. The longest period of Az action observed at a dose of 300 µg/kg was 55 min. The highest acute toxicity (LD50 510 μg/kg) was observed at intravenous injection of Az, at intramuscular or intraperitoneal administration it was less toxic. The peptide showed practically no immunotoxic, allergenic or mutagenic capacity. Overall, the results demonstrate that Az has good drug-like properties for the application as local muscle relaxant and in its parameters, is not inferior to the relaxants currently used. However, some Az modification might be effective to extend its narrow therapeutic window, a typical characteristic and a weak point of all nondepolarizing myorelaxants.

Anticoagulant Activity of Low-Molecular Weight Compounds from Heterometrus laoticus Scorpion Venom

Scorpion venoms are complex polypeptide mixtures, the ion channel blockers and antimicrobial peptides being the best studied components. The coagulopathic properties of scorpion venoms are poorly studied and the data about substances exhibiting these properties are very limited. During research on the Heterometrus laoticus scorpion venom, we have isolated low-molecular compounds with anticoagulant activity. Determination of their structure has shown that one of them is adenosine, and two others are dipeptides LeuTrp and IleTrp. The anticoagulant properties of adenosine, an inhibitor of platelet aggregation, are well known, but its presence in scorpion venom is shown for the first time. The dipeptides did not influence the coagulation time in standard plasma coagulation tests. However, similarly to adenosine, both peptides strongly prolonged the bleeding time from mouse tail and in vitro clot formation in whole blood. The dipeptides inhibited the secondary phase in platelet aggregation induced by ADP, and IleTrp decreased an initial rate of platelet aggregation induced by collagen. This suggests that their anticoagulant effects may be realized through the deterioration of platelet function. The ability of short peptides from venom to slow down blood coagulation and their presence in scorpion venom are established for the first time. Further studies are needed to elucidate the precise molecular mechanism of dipeptide anticoagulant activity.

Detection of human neuronal α7 nicotinic acetylcholine receptors by conjugates of snake α-neurotoxin with quantum dots

Fluorescent derivatives are widely used to study the structure and functions of proteins. Quantum dots (QDs), fluorescent semiconductor nanocrystals, have a high quantum yield and are much more resistant to bleaching compared to organic dyes. Conjugates of α-neurotoxins with QDs were used for visualization of human α7 acetylcholine receptors heterologously expressed in GH4C1 pituitary adenoma cells. Specific staining of cells by the conjugated toxins was observed.

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.'

Venomics of Tropidolaemus wagleri, the sexually dimorphic temple pit viper: Unveiling a deeply conserved atypical toxin arsenal

Tropidolaemus wagleri (temple pit viper) is a medically important snake in Southeast Asia. It displays distinct sexual dimorphism and prey specificity, however its venomics and inter-sex venom variation have not been thoroughly investigated. Applying reverse-phase HPLC, we demonstrated that the venom profiles were not significantly affected by sex and geographical locality (Peninsular Malaya, insular Penang, insular Sumatra) of the snakes. Essentially, venoms of both sexes share comparable intravenous median lethal dose (LD₅₀) (0.56-0.63 μg/g) and cause neurotoxic envenomation in mice. LCMS/MS identified six waglerin forms as the predominant lethal principles, comprising 38.2% of total venom proteins. Fourteen other toxin-protein families identified include phospholipase A₂, serine proteinase, snaclec and metalloproteinase. In mice, HPLC fractions containing these proteins showed insignificant contribution to the overall venom lethality. Besides, the unique elution pattern of approximately 34.5% of non-lethal, low molecular mass proteins (3-5 kDa) on HPLC could be potential biomarker for this primitive crotalid species. Together, the study unveiled the venom proteome of T. wagleri that is atypical among many pit vipers as it comprises abundant neurotoxic peptides (waglerins) but little hemotoxic proteinases. The findings also revealed that the venom is relatively well conserved intraspecifically despite the drastic morphological differences between sexes.

Viper Venom Botox: The Molecular Origin and Evolution of the Waglerin Peptides Used in Anti-Wrinkle Skin Cream

The molecular origin of waglerin peptides has remained enigmatic despite their industrial application in skin cream products to paralyse facial muscles and thus reduce the incidence of wrinkles. Here we show that these neurotoxic peptides are the result of de novo evolution within the prepro region of the C-type natriuretic peptide gene in Tropidolaemus venoms, at a site distinct from the domain encoding for the natriuretic peptide. It is the same region that yielded the azemiopsin peptides from Azemiops feae, indicative of a close relationship of this toxin gene between these two genera. The precursor region for the molecular evolution is a biodiversity hotspot that has yielded other novel bioactive peptides with novel activities. We detail the diversity of components in this and other species in order to explore what characteristics enable it to be such a biodiscovery treasure trove. The unusual function of Tropidolaemus venoms may have been selected for due to evolutionary pressures brought about by a high likelihood of prey escape.

Peptides from puff adder Bitis arietans venom, novel inhibitors of nicotinic acetylcholine receptors

Phospholipase A₂ (named bitanarin) possessing capability to block nicotinic acetylcholine receptors (nAChRs) was isolated earlier (Vulfius et al., 2011) from puff adder Bitis arietans venom. Further studies indicated that low molecular weight fractions of puff adder venom inhibit nAChRs as well. In this paper, we report on isolation from this venom and characterization of three novel peptides called baptides 1, 2 and 3 that reversibly block nAChRs. To isolate the peptides, the venom of B. arietans was fractionated by gel-filtration and reversed phase chromatography. The amino acid sequences of peptides were established by de novo sequencing using MALDI mass spectrometry. Baptide 1 comprised 7, baptides 2 and 3-10 amino acid residues, the latter being acetylated at the N-terminus. This is the first indication for the presence of such post-translational modification in snake venom proteins. None of the peptides contain cysteine residues. For biological activity studies the peptides were prepared by solid phase peptide synthesis. Baptide 3 and 2 blocked acetylcholine-elicited currents in isolated Lymnaea stagnalis neurons with IC₅₀ of about 50 μM and 250 μM, respectively. In addition baptide 2 blocked acetylcholine-induced currents in muscle nAChR heterologously expressed in Xenopus oocytes with IC₅₀ of about 3 μM. The peptides did not compete with radioactive α-bungarotoxin for binding to Torpedo and α7 nAChRs at concentration up to 200 μM that suggests non-competitive mode of inhibition. Calcium imaging studies on α7 and muscle nAChRs heterologously expressed in mouse neuroblastoma Neuro2a cells showed that on α7 receptor baptide 2 inhibited acetylcholine-induced increasing intracellular calcium concentration with IC₅₀ of 20.6 ± 3.93 μM. On both α7 and muscle nAChRs the suppression of maximal response to acetylcholine by about 50% was observed at baptide 2 concentration of 25 μM, the value being close to IC₅₀ on α7 nAChR. These data are in accord with non-competitive inhibition as follows from α-bungarotoxin binding experiments. The described peptides are the shortest peptides without disulfide bridges isolated from animal venom and capable to inhibit nAChR by non-competitive way.

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.

Nonconventional three-finger toxin BMLCL from krait Bungarus multicinctus venom with high affinity interacts with nicotinic acetylcholine receptors

Nonconventional three-finger toxin BMLCL was isolated from B. multicinctus venom, and its interaction with different subtypes of nicotinic acetylcholine receptor (nAChR) was studied. It was found that BMLCL is able to interact with high efficiency with both α7 and muscle type nAChRs.

Natural compounds interacting with nicotinic acetylcholine receptors: from low-molecular weight ones to peptides and proteins

Nicotinic acetylcholine receptors (nAChRs) fulfill a variety of functions making identification and analysis of nAChR subtypes a challenging task. Traditional instruments for nAChR research are d-tubocurarine, snake venom protein α-bungarotoxin (α-Bgt), and α-conotoxins, neurotoxic peptides from Conus snails. Various new compounds of different structural classes also interacting with nAChRs have been recently identified. Among the low-molecular weight compounds are alkaloids pibocin, varacin and makaluvamines C and G. 6-Bromohypaphorine from the mollusk Hermissenda crassicornis does not bind to Torpedo nAChR but behaves as an agonist on human α7 nAChR. To get more selective α-conotoxins, computer modeling of their complexes with acetylcholine-binding proteins and distinct nAChRs was used. Several novel three-finger neurotoxins targeting nAChRs were described and α-Bgt inhibition of GABA-A receptors was discovered. Information on the mechanisms of nAChR interactions with the three-finger proteins of the Ly6 family was found. Snake venom phospholipases A₂ were recently found to inhibit different nAChR subtypes. Blocking of nAChRs in Lymnaea stagnalis neurons was shown for venom C-type lectin-like proteins, appearing to be the largest molecules capable to interact with the receptor. A huge nAChR molecule sensible to conformational rearrangements accommodates diverse binding sites recognizable by structurally very different compounds.

Venom down under: dynamic evolution of Australian elapid snake toxins

Despite the unparalleled diversity of venomous snakes in Australia, research has concentrated on a handful of medically significant species and even of these very few toxins have been fully sequenced. In this study, venom gland transcriptomes were sequenced from eleven species of small Australian elapid snakes, from eleven genera, spanning a broad phylogenetic range. The particularly large number of sequences obtained for three-finger toxin (3FTx) peptides allowed for robust reconstructions of their dynamic molecular evolutionary histories. We demonstrated that each species preferentially favoured different types of α-neurotoxic 3FTx, probably as a result of differing feeding ecologies. The three forms of α-neurotoxin [Type I (also known as (aka): short-chain), Type II (aka: long-chain) and Type III] not only adopted differential rates of evolution, but have also conserved a diversity of residues, presumably to potentiate prey-specific toxicity. Despite these differences, the different α-neurotoxin types were shown to accumulate mutations in similar regions of the protein, largely in the loops and structurally unimportant regions, highlighting the significant role of focal mutagenesis. We theorize that this phenomenon not only affects toxin potency or specificity, but also generates necessary variation for preventing/delaying prey animals from acquiring venom-resistance. This study also recovered the first full-length sequences for multimeric phospholipase A₂ (PLA₂) ‘taipoxin/paradoxin’ subunits from non-Oxyuranus species, confirming the early recruitment of this extremely potent neurotoxin complex to the venom arsenal of Australian elapid snakes. We also recovered the first natriuretic peptides from an elapid that lack the derived C-terminal tail and resemble the plesiotypic form (ancestral character state) found in viper venoms. This provides supporting evidence for a single early recruitment of natriuretic peptides into snake venoms. Novel forms of kunitz and waprin peptides were recovered, including dual domain kunitz-kunitz precursors and the first kunitz-waprin hybrid precursors from elapid snakes. The novel sequences recovered in this study reveal that the huge diversity of unstudied venomous Australian snakes are of considerable interest not only for the investigation of venom and whole organism evolution but also represent an untapped bioresource in the search for novel compounds for use in drug design and development.

Neurotoxicity in snakebite--the limits of our knowledge

Snakebite is classified by the WHO as a neglected tropical disease. Envenoming is a significant public health problem in tropical and subtropical regions. Neurotoxicity is a key feature of some envenomings, and there are many unanswered questions regarding this manifestation. Acute neuromuscular weakness with respiratory involvement is the most clinically important neurotoxic effect. Data is limited on the many other acute neurotoxic manifestations, and especially delayed neurotoxicity. Symptom evolution and recovery, patterns of weakness, respiratory involvement, and response to antivenom and acetyl cholinesterase inhibitors are variable, and seem to depend on the snake species, type of neurotoxicity, and geographical variations. Recent data have challenged the traditional concepts of neurotoxicity in snake envenoming, and highlight the rich diversity of snake neurotoxins. A uniform system of classification of the pattern of neuromuscular weakness and models for predicting type of toxicity and development of respiratory weakness are still lacking, and would greatly aid clinical decision making and future research. This review attempts to update the reader on the current state of knowledge regarding this important issue.

Differential evolution and neofunctionalization of snake venom metalloprotease domains

Snake venom metalloproteases (SVMP) are composed of five domains: signal peptide, propeptide, metalloprotease, disintegrin, and cysteine-rich. Secreted toxins are typically combinatorial variations of the latter three domains. The SVMP-encoding genes of Psammophis mossambicus venom are unique in containing only the signal and propeptide domains. We show that the Psammophis SVMP propeptide evolves rapidly and is subject to a high degree of positive selection. Unlike Psammophis, some species of Echis express both the typical multidomain and the unusual monodomain (propeptide only) SVMP, with the result that a lower level of variation is exerted upon the latter. We showed that most mutations in the multidomain Echis SVMP occurred in the protease domain responsible for proteolytic and hemorrhagic activities. The cysteine-rich and disintegrin-like domains, which are putatively responsible for making the P-III SVMPs more potent than the P-I and P-II forms, accumulate the remaining variation. Thus, the binding sites on the molecule's surface are evolving rapidly whereas the core remains relatively conserved. Bioassays conducted on two post-translationally cleaved novel proline-rich peptides from the P. mossambicus propeptide domain showed them to have been neofunctionalized for specific inhibition of mammalian a7 neuronal nicotinic acetylcholine receptors. We show that the proline rich postsynaptic specific neurotoxic peptides from Azemiops feae are the result of convergent evolution within the precursor region of the C-type natriuretic peptide instead of the SVMP. The results of this study reinforce the value of studying obscure venoms for biodiscovery of novel investigational ligands.