BjalphaIT: a novel scorpion alpha-toxin selective for insects--unique pharmacological tool

The fusion protein of scorpion neurotoxin BjαIT and Galanthus nivalis agglutinin (GNA) enhanced the injection insecticidal activity against silkworms, but only has lethal activity against newly hatched larva when administered orally

Fusing insect derived neurotoxic peptides with Galanthus nivalis agglutinin (GNA) has been shown to enhance the insecticidal activity of the neuropeptides, especially when administered orally. This study produced a recombinant scorpion insect specific neurotoxin BjαIT, GNA, and a fusion protein BjαIT/GNA using Pichia pastoris as an expression host. Recombinant rBjαIT/GNA was found to be easily degraded during expression in yeast which and produced a main protein product with a molecular weight of approximately 14 kDa. Cytotoxicity results showed that rBjαIT, rGNA, and rBjαIT/GNA had no toxicity to mammalian NIH/3T3 cells. Adding rBjαIT or rBjαIT/GNA at a concentration as low as 1 ng/mL to insect cell culture medium inhibited the proliferation of insect Sf9 cells, with rBjαIT exhibiting stronger cytotoxicity, while 20 ng/mL rGNA did not inhibit the proliferation of Sf9 cells. Silkworm larval injection results showed that rBjαIT/GNA was the most toxic of the three proteins, followed by rBjαIT, and rGNA. When rBjαIT/GNA was injected at a concentration of 0.129 nmol/g body weight 46.7% of silkworm died within 48 h. Feeding newly hatched silkworms with rBjαIT/GNA at a leaf surface concentration of 40 µg/cm2 resulted in 76.7% mortality within 24 h. However, rBjαIT/GNA showed almost no oral insecticidal activity against second instar silkworms. The results indicated that rBjαIT/GNA has stronger injection insecticidal activity and feeding insecticidal activity than rBjαIT and rGNA individually, making it more suitable for biological control.

Insecticidal activities of three recombinant venom proteins of the predatory stink bug, Arma custos

BACKGROUND

Widespread resistance of insect pests to insecticides and transgenic crops in the field is a significant challenge for sustainable agriculture, and calls for the development of novel alternative strategies to control insect pests. One potential resource for the discovery of novel insecticidal molecules is natural toxins, particularly those derived from the venoms of insect predators.

RESULTS

In this study, we identified three insecticidal proteinaceous toxins from the venom glands (VGs) of the predatory stink bug, Arma custos (Hemiptera: Asopinae). Transcriptomic analysis of A. custos VGs revealed 151 potentially secreted VG-rich venom proteins. Three VG-rich venom proteins (designated AcVP1 ~ 3) were produced by overexpression in Escherichia coli. Injection of the recombinant proteins into tobacco cutworm (Spodoptera litura) larvae showed that all of the three recombinant proteins caused paralysis, liquefaction and death. Injection of recombinant proteins into rice brown planthopper (Nilaparvata lugens) nymphs showed higher insecticidal activities, among which a trypsin (AcVP2) caused 100% mortality postinjection at 1.27 pmol mg-1 body weight.

CONCLUSION

A natural toolkit for the discovery of insecticidal toxins from predatory insects has been revealed by the present study. © 2024 Society of Chemical Industry.

Biochemical characterization and insecticidal activity of isolated peptides from the venom of the scorpion Centruroides tecomanus

The venom of scorpions is a mixture of components that constitute a source of bioactive molecules. The venom of the scorpion Centruroides tecomanus contains peptides toxic to insects, however, to date no toxin responsible for this activity has yet been isolated and fully characterized. This communication describes two new peptides Ct-IT1 and Ct-IT2 purified from this scorpion. Both peptides contain 63 amino acids with molecular weight 6857.85 for Ct-IT1 and 6987.77 Da for Ct-IT2. The soluble venom was separated using chromatographic techniques of molecular size exclusion, cationic exchange, and reverse phase chromatography, allowing the identification of at least 99 components of which in 53 the insecticidal activity was evaluated. The LD₅₀ determined for Ct-IT1 is 3.81 μg/100 mg of cricket weight, but low amounts of peptides (0.8 μg of peptide) already cause paralysis in crickets. The relative abundance of these two peptides in the venom is 2.1% for Ct-IT1 and 1% for Ct-IT2. The molecular masses and N-terminal sequences of both insecticidal toxins were determined by mass spectrometry and Edman degradation. The primary structure of both toxins was compared with other known peptides isolated from other scorpion venoms. The analysis of the sequence alignments revealed the position of a highly conserved amino acid residue, Gly39, exclusively present in anti-insect selective depressant β-toxins (DBTXs), which in Ct-IT1 and Ct-IT2 is at position Gly40. Similarly, a three-dimensional structure of this toxins was obtained by homology modeling and compared to the structure of known insect toxins of scorpions. An important similarity of the cavity formed by the trapping apparatus region of the depressant toxin LqhIT2, isolated from the scorpion Leiurus quinquestriatus hebraeus, was found in the toxins described here. These results indicate that Ct-IT1 and Ct-IT2 toxins have a high potential to be evaluated on pests that affect economically important crops to eventually consider them as a potential biological control method.

Armed stem to stinger: a review of the ecological roles of scorpion weapons

Scorpions possess two systems of weapons: the pincers (chelae) and the stinger (telson). These are placed on anatomically and developmentally well separated parts of the body, that is, the oral appendages and at the end of the body axis. The otherwise conserved body plan of scorpions varies most in the shape and relative dimensions of these two weapon systems, both across species and in some cases between the sexes. We review the literature on the ecological function of these two weapon systems in each of three contexts of usage: (i) predation, (ii) defense and (iii) sexual contests. In the latter context, we will also discuss their usage in mating. We first provide a comparative background for each of these contexts of usage by giving examples of other weapon systems from across the animal kingdom. Then, we discuss the pertinent aspects of the anatomy of the weapon systems, particularly those aspects relevant to their functioning in their ecological roles. The literature on the functioning and ecological role of both the chelae and the telson is discussed in detail, again organized by context of usage. Particular emphasis is given on the differences in morphology or usage between species or higher taxonomic groups, or between genders, as such cases are most insightful to understand the roles of each of the two distinct weapon systems of the scorpions and their evolutionary interactions. We aimed to synthesize the literature while minimizing conjecture, but also to point out gaps in the literature and potential future research opportunities.

Derivative of Scorpion Neurotoxin BeM9 Is Selective for Insect Voltage-Gated Sodium Channels

Abstract

Scorpion α-toxins are small proteins inhibiting the inactivation of voltage-gated sodium channels. They can selectively act on either mammalian (mammal toxins) or insect channels (insect toxins), or affect both types of channels (α-like toxins). Currently no model has been proposed that fully explains the dependence of selectivity upon amino acid sequence, but some patterns have already been established. Thus, most mammal toxins have an aspartic acid residue in position 8, which is involved in the formation of the nest motif, but it is still not clear whether this residue interacts directly with channels. The objective of our study was to obtain a derivative of the α-like toxin BeM9 with the replacement of lysine in position 8 by glutamate (K8E), changing the charge, but excluding the formation of the nest motif. In addition, we replaced the tyrosine in position 17 with glycine (Y17G), which is characteristic of mammal toxins. Surprisingly, the double-mutant derivative BeM9EG lost its activity on mammalian channels, becoming an insect toxin. To explain these changes, we constructed models of BeM9 and BeM9EG complexes with channels, and also performed molecular dynamics of isolated toxins. Analysis of intermolecular contacts in the complexes did not explain the reason for the selectivity change. Nevertheless, the structure of intramolecular contacts and data on molecular mobility indicate an important role of residues K8 and Y17 in stabilizing a certain conformation of BeM9 loops. We assume that the replacement of these residues allosterically affects the efficiency of toxin binding to channels.

Cloning and expression of OdTx12, a β excitatory toxin from Odontobuthus doriae, in Escherichia coli and evaluation of its bioactivity in Locusta migratoria

The venom of Odontobuthus doriae contains several peptide toxins that interfere with the sodium channel function of cell membranes, some of which specifically act on the insect's sodium channel without affecting mammalian cells. In this study sodium channel toxins of Odontobuthus doriae were aligned to other closely related toxins by BLAST and ClustalW servers. Among these toxins, NaTx12 (OdTx12) showed more than 90% similarity to the most known beta excitatory toxin, AaHIT1; furthermore, our modeling studies confirmed high tertiary structure similarity of these proteins. OdTx12 was cloned and expressed in E.coli, using pET26-b and pET28-a expression vectors. Tris-tricine SDS-PAGE and Western blot analysis showed OdTx12 expression by pET28-a, only. After purification, bioactivity of the purified protein was analyzed by injection and oral administration to Locusta migratoria larvae, and toxicity to mammals was tested on mice. Injection of OdTx12 resulted in the killing of larvae with LD50 of 0.4 and 0.2 after 48 and 72 h respectively, but oral administration of OdTx12 had no significant effect on Locusta migratoria, nor did the injection to mice show any signs of toxicity. These results showed that OdTx12, as a novel β excitatory toxin can be considered as a candidate for insect control purposes.

High-level expression and purification of active scorpion long-chain neurotoxin BjαIT from Pichia pastoris

As an insect-selective neurotoxin, scorpion long-chain BjαIT is a promising prospect for insecticidal application; however, the difficulty of obtaining natural BjαIT represents the major obstacle preventing analysis of its insecticidal activity against agricultural insect pests. Here, we screened recombinant Pichia pastoris transformants showing high levels of secretory recombinant (r)BjαIT. Secreted rBjαIT was expressed at levels as high as 340 mg/L following methanol induction in a fed-batch reactor, with ∼21 mg of pure rBjαIT obtained from 200-mL fed-batch culture supernatant by Ni²⁺-nitriloacetic acid affinity chromatography and CM Sepharose ion-exchange chromatography. Injection of purified rBjαIT induced neurotoxicity symptoms in locust (Locusta migratoria) larvae, and the half-lethal dose of rBjαIT for locusts at 24-h post-injection ranged from 11 to 14 μg/g body weight. These results demonstrated that large amounts of active rBjαIT were efficiently prepared from P. pastoris, suggesting this system as efficacious for determining rBjαIT insecticidal activity against other agricultural insect pests.

Recombinant production of the insecticidal scorpion toxin BjαIT in Escherichia coli

Scorpion long-chain insect neurotoxins have important potential application value in agricultural pest control. The difficulty of obtaining natural toxins is the major obstacle preventing analyses of their insecticidal activity against more agricultural insect pests. Here we cloned the insect neurotoxin BjαIT gene into the pET32 expression vector and expressed the resulting thioredoxin (Trx)-BjαIT fusion protein in Escherichia coli. Soluble Trx-BjαIT was expressed at a high level when induced at 18 °C with 0.1 mM isopropyl β-d-1-thiogalactopyranoside, and it was purified by Ni²⁺-nitriloacetic acid affinity chromatography. After cleaving the Trx tag with recombinant enterokinase, the digestion products were purified by CM Sepharose FF ion-exchange chromatography, and 1.5 mg of purified recombinant BjαIT (rBjαIT) was obtained from 100 ml of induced bacterial cells. Injecting rBjαIT induced obvious neurotoxic symptoms and led to death in locust (Locusta migratoria) larvae. Dietary toxicity was not observed in locusts. The results demonstrate that active rBjαIT could be obtained efficiently from an E. coli expression system, which is helpful for determining its insecticidal activity against agricultural insect pests.

Target-Specificity in Scorpions; Comparing Lethality of Scorpion Venoms across Arthropods and Vertebrates

Scorpions use their venom in defensive situations as well as for subduing prey. Since some species of scorpion use their venom more in defensive situations than others, this may have led to selection for differences in effectiveness in defensive situations. Here, we compared the LD₅₀ of the venom of 10 species of scorpions on five different species of target organisms; two insects and three vertebrates. We found little correlation between the target species in the efficacy of the different scorpion venoms. Only the two insects showed a positive correlation, indicating that they responded similarly to the panel of scorpion venoms. We discuss the lack of positive correlation between the vertebrate target species in the light of their evolution and development. When comparing the responses of the target systems to individual scorpion venoms pairwise, we found that closely related scorpion species tend to elicit a similar response pattern across the target species. This was further reflected in a significant phylogenetic signal across the scorpion phylogeny for the LD₅₀ in mice and in zebrafish. We also provide the first mouse LD₅₀ value for Grosphus grandidieri.

A biomechanical view on stinger diversity in scorpions

Scorpions have elongated metasomas that bear a telson, which is used as a stinger for venom injection. There is a remarkable diversity in the use of the stinger among scorpions, comprising defensive behavior, prey subjugation and mating. This diversity could be reflected by the shape of the telson, as different stinging behaviors will result in very different functional demands. Here we explored the diversity of telson shapes in scorpions by providing morphological measurements, such as curvature and tip angle, as well as by testing stingers under load using finite element analysis (FEA). FEA models were loaded with forces scaled to the surface area of the models, to allow comparison of the relative strain energy based on shape alone. Load force angle was rotated to identify the optimal stinging angle based on the lowest strain energy. Aculeus length and mean aculeus height correlated with minimal strain energy. Optimal stinging angle correlated with tip angle, and differed from the tip angle by about 28.4 ± 6.22 °. We found that species that are more venomous have long aculei (stinger barbs) with a larger radius of curvature. FEA models of these longer aculei showed basal stress concentrations, indicating a potential greater risk of basal breakage due to shape alone. Telsons with shorter and thicker aculeus shapes showed stress concentrations at the tip only. Despite these marked differences in shape, we found no difference in the scaled strain energy between groups of species that are more venomous and less venomous groups of species. These results show that scorpion stingers may be biomechanically optimized, and this may indicate different usage of the stinger in different species.

Scorpion toxin peptide action at the ion channel subunit level

This review categorizes functionally validated actions of defined scorpion toxin (SCTX) neuropeptides across ion channel subclasses, highlighting key trends in this rapidly evolving field. Scorpion envenomation is a common event in many tropical and subtropical countries, with neuropharmacological actions, particularly autonomic nervous system modulation, causing significant mortality. The primary active agents within scorpion venoms are a diverse group of small neuropeptides that elicit specific potent actions across a wide range of ion channel classes. The identification and functional characterisation of these SCTX peptides has tremendous potential for development of novel pharmaceuticals that advance knowledge of ion channels and establish lead compounds for treatment of excitable tissue disorders. This review delineates the unique specificities of 320 individual SCTX peptides that collectively act on 41 ion channel subclasses. Thus the SCTX research field has significant translational implications for pathophysiology spanning neurotransmission, neurohumoral signalling, sensori-motor systems and excitation-contraction coupling. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

The unfulfilled promises of scorpion insectotoxins

Since the description and biochemical characterization of the first insect-specific neurotoxins from scorpion venoms, almost all contributions have highlighted their potential application as leads for the development of potent bioinsecticides. Their practical use, however, has been hindered by different factors, some of which are intrinsically related to the toxins and other external determinants. Recent developments in the understanding of the action mechanisms of the scorpion insectotoxins and their bioactive surfaces, coupled with the exploration of novel bioinsecticide delivery systems have renewed the expectations that the scorpion insectotoxins could find their way into commercial applications in agriculture, as part of integrated pest control strategies. Herein, we review the current arsenal of available scorpion neurotoxins with a degree of specificity for insects, the progress made with alternative delivery methods, and the drawbacks that still preclude their practical use.

Integration of an insecticidal scorpion toxin (BjαIT) gene into Metarhizium acridum enhances fungal virulence towards Locusta migratoria manilensis

BACKGROUND

Entomopathogenic fungi have been developed as biopesticides, but poor efficacy has blocked their application. One approach to improving virulence is by genetic manipulation. BjαIT from the venom of Buthotus judaicus is an insect-selective neurotoxin. To clarify the insecticidal potency of BjαIT as a virulence candidate in microbial biocontrol agents, the entomopathogenic fungus Metarhizium acridum was genetically modified with BjαIT, and its resulting activity against locusts (Locusta migratoria manilensis) was assessed.

RESULT

In comparison with the wild-type strain, the engineered isolate BjαIT-102 grew significantly quicker in locust haemolymph. Correspondingly, the median lethal dose (LC50 ) for BjαIT-102 was 18.2-fold lower, and the median lethal times (LT50 ) for BjαIT-102 were reduced by 28.1 and 30.4%, respectively, after topical inoculation and injection. BjαIT-102 formed conidia on dead locusts, although the conidial yield was reduced 1.58-fold. Moreover, there were no significant differences in germination and appressorium formation between the BjαIT-102 and wild-type strains.

CONCLUSION

Expression of BjαIT in M. acridum significantly increased virulence against locusts by shortening the in vivo infection period without affecting conidium formation on the carcasses. This study demonstrated that engineering entomopathogenic fungi to incorporate BjαIT offers great potential for increasing their virulence.

Expression of scorpion toxin LqhIT2 increases the virulence of Metarhizium acridum towards Locusta migratoria manilensis

LqhIT2 is an insect-specific neurotoxin from the venom of scorpion. In this study, the LqhIT2 gene was introduced into the entomopathogenic fungus, Metarhizium acridum. The virulence of the genetically modified strain MaLqhIT2 was then evaluated against locusts (Locusta migratoria manilensis). Compared with the wild-type strain, the median lethal cell density (LC50) for MaLqhIT2 was a 22.6-fold lower, and the median times to death (LT50) for MaLqhIT2 were reduced by 30.3 and 29.6 %, respectively, after topical inoculation and injection. MaLqhIT2 also grew significantly faster in the hemolymph than wild-type strain. There were no significant differences in germination, appressorium formation and sporulation in locust carcasses between the MaLqhIT2 and wild-type strain. These results indicate that LqhIT2 increased the virulence of M. acridum towards locusts by shortening the in vivo infection period, without affecting cuticle penetration or conidia formation in the carcasses. LqhIT2 thus shows considerable potential for increasing fungal virulence against locusts.

Potency of insect-specific scorpion toxins on mosquito control using Bacillus thuringiensis Cry4Aa

Two insect-specific scorpion toxins, BjαIT and AahIT were produced as alkali-soluble protein inclusions in Escherichia coli. The inclusion bodies themselves exhibited no toxicity against Culex pipiens larvae. However, coadministration with Cry4Aa toxin enhanced the mosquitocidal activity by 2-3 fold. Insect-specific scorpion toxins can be good supplements for Cry toxin-based bioinsecticides.

Modular organization of α-toxins from scorpion venom mirrors domain structure of their targets, sodium channels

To gain success in the evolutionary "arms race," venomous animals such as scorpions produce diverse neurotoxins selected to hit targets in the nervous system of prey. Scorpion α-toxins affect insect and/or mammalian voltage-gated sodium channels (Na(v)s) and thereby modify the excitability of muscle and nerve cells. Although more than 100 α-toxins are known and a number of them have been studied into detail, the molecular mechanism of their interaction with Na(v)s is still poorly understood. Here, we employ extensive molecular dynamics simulations and spatial mapping of hydrophobic/hydrophilic properties distributed over the molecular surface of α-toxins. It is revealed that despite the small size and relatively rigid structure, these toxins possess modular organization from structural, functional, and evolutionary perspectives. The more conserved and rigid "core module" is supplemented with the "specificity module" (SM) that is comparatively flexible and variable and determines the taxon (mammal versus insect) specificity of α-toxin activity. We further show that SMs in mammal toxins are more flexible and hydrophilic than in insect toxins. Concomitant sequence-based analysis of the extracellular loops of Na(v)s suggests that α-toxins recognize the channels using both modules. We propose that the core module binds to the voltage-sensing domain IV, whereas the more versatile SM interacts with the pore domain in repeat I of Na(v)s. These findings corroborate and expand the hypothesis on different functional epitopes of toxins that has been reported previously. In effect, we propose that the modular structure in toxins evolved to match the domain architecture of Na(v)s.

Voltage-gated sodium channel isoform-specific effects of pompilidotoxins

Pompilidotoxins (PMTXs, alpha and beta) are small peptides consisting of 13 amino acids purified from the venom of the solitary wasps Anoplius samariensis (alpha-PMTX) and Batozonellus maculifrons (beta-PMTX). They are known to facilitate synaptic transmission in the lobster neuromuscular junction, and to slow sodium channel inactivation. By using beta-PMTX, alpha-PMTX and four synthetic analogs with amino acid changes, we conducted a thorough study of the effects of PMTXs on sodium current inactivation in seven mammalian voltage-gated sodium channel (VGSC) isoforms and one insect VGSC (DmNa(v)1). By evaluating three components of which the inactivating current is composed (fast, slow and steady-state components), we could distinguish three distinct groups of PMTX effects. The first group concerned the insect and Na(v)1.6 channels, which showed a large increase in the steady-state current component without any increase in the slow component. Moreover, the dose-dependent increase in this steady-state component was correlated with the dose-dependent decrease in the fast component. A second group of effects concerned the Na(v)1.1, Na(v)1.2, Na(v)1.3 and Na(v)1.7 isoforms, which responded with a large increase in the slow component, and showed only a small steady-state component. As with the first group of effects, the slow component was dose-dependent and correlated with the decrease in the fast component. Finally, a third group of effects concerned Na(v)1.4 and Na(v)1.5, which did not show any change in the slow or steady-state component. These data shed light on the complex and intriguing behavior of VGSCs in response to PMTXs, helping us to better understand the molecular determinants explaining isoform-specific effects.

Molecular analysis of the sea anemone toxin Av3 reveals selectivity to insects and demonstrates the heterogeneity of receptor site-3 on voltage-gated Na+ channels

Av3 is a short peptide toxin from the sea anemone Anemonia viridis shown to be active on crustaceans and inactive on mammals. It inhibits inactivation of Na(v)s (voltage-gated Na+ channels) like the structurally dissimilar scorpion alpha-toxins and type I sea anemone toxins that bind to receptor site-3. To examine the potency and mode of interaction of Av3 with insect Na(v)s, we established a system for its expression, mutagenized it throughout, and analysed it in toxicity, binding and electrophysiological assays. The recombinant Av3 was found to be highly toxic to blowfly larvae (ED50=2.65+/-0.46 pmol/100 mg), to compete well with the site-3 toxin LqhalphaIT (from the scorpion Leiurus quinquestriatus) on binding to cockroach neuronal membranes (K(i)=21.4+/-7.1 nM), and to inhibit the inactivation of Drosophila melanogaster channel, DmNa(v)1, but not that of mammalian Na(v)s expressed in Xenopus oocytes. Moreover, like other site-3 toxins, the activity of Av3 was synergically enhanced by ligands of receptor site-4 (e.g. scorpion beta-toxins). The bioactive surface of Av3 was found to consist mainly of aromatic residues and did not resemble any of the bioactive surfaces of other site-3 toxins. These analyses have portrayed a toxin that might interact with receptor site-3 in a different fashion compared with other ligands of this site. This assumption was corroborated by a D1701R mutation in DmNa(v)1, which has been shown to abolish the activity of all other site-3 ligands, except Av3. All in all, the present study provides further evidence for the heterogeneity of receptor site-3, and raises Av3 as a unique model for design of selective anti-insect compounds.

Peptides of arachnid venoms with insecticidal activity targeting sodium channels

Arachnids have a venom apparatus and secrete a complex chemical mixture of low molecular mass organic molecules, enzymes and polypeptide neurotoxins designed to paralyze or kill their prey. Most of these toxins are specific for membrane voltage-gated sodium channels, although some may also target calcium or potassium channels and other membrane receptors. Scorpions and spiders have provided the greatest number of the neurotoxins studied so far, for which, a good number of primary and 3D structures have been obtained. Structural features, comprising a folding that determines a similar spatial distribution of charged and hydrophobic side chains of specific amino acids, are strikingly common among the toxins from spider and scorpion venoms. Such similarities are, in turn, the key feature to target and bind these proteins to ionic channels. The search for new insecticidal compounds, as well as the study of their modes of action, constitutes a current approach to rationally design novel insecticides. This goal tends to be more relevant if the resistance to the conventional chemical products is considered. A promising alternative seems to be the biotechnological approach using toxin-expressing recombinant baculovirus. Spider and scorpion toxins having insecticidal activity are reviewed here considering their structures, toxicities and action mechanisms in sodium channels of excitable membranes.

The differential preference of scorpion α-toxins for insect or mammalian sodium channels: Implications for improved insect control

Receptor site-3 on voltage-gated sodium channels is targeted by a variety of structurally distinct toxins from scorpions, sea anemones, and spiders whose typical action is the inhibition of sodium current inactivation. This site interacts allosterically with other topologically distinct receptors that bind alkaloids, lipophilic polyether toxins, pyrethroids, and site-4 scorpion toxins. These features suggest that design of insecticides with specificity for site-3 might be rewarding due to the positive cooperativity with other toxins or insecticidal agents. Yet, despite the central role of scorpion alpha-toxins in envenomation and their vast use in the study of channel functions, molecular details on site-3 are scarce. Scorpion alpha-toxins vary greatly in preference for sodium channels of insects and mammals, and some of them are highly active on insects. This implies that despite its commonality, receptor site-3 varies on insect vs. mammalian channels, and that elucidation of these differences could potentially be exploited for manipulation of toxin preference. This review provides current perspectives on (i) the classification of scorpion alpha-toxins, (ii) their mode of interaction with sodium channels and pharmacological divergence, (iii) molecular details on their bioactive surfaces and differences associated with preference for channel subtypes, as well as (iv) a summary of the present knowledge about elements involved in constituting receptor site-3. These details, combined with the variations in allosteric interactions between site-3 and the other receptor sites on insect and mammalian sodium channels, may be useful in new strategies of insect control and future design of anti-insect selective ligands.

Voltage-gated sodium channel modulation by scorpion alpha-toxins

Voltage-gated Na(+) channels are integral membrane proteins that function as a gateway for a selective permeation of sodium ions across biological membranes. In this way, they are crucial players for the generation of action potentials in excitable cells. Voltage-gated Na(+) channels are encoded by at least nine genes in mammals. The different isoforms have remarkably similar functional properties, but small changes in function and pharmacology are biologically well-defined, as underscored by mutations that cause several diseases and by modulation of a myriad of compounds, respectively. This review will stress on the modulation of voltage-gated Na(+) channels by scorpion alpha-toxins. Nature has designed these two classes of molecules as if they were predestined to each other: an inevitable 'encounter' between a voltage-gated Na(+) channel isoform and an alpha-toxin from scorpion venom indeed results in a dramatically changed Na(+) current phenotype with clear-cut consequences on electrical excitability and sometimes life or death. This fascinating aspect justifies an overview on scorpion venoms, their alpha-toxins and the Na(+) channel targets they are built for, as well as on the molecular determinants that govern the selectivity and affinity of this 'inseparable duo'.

Deduction of functional peptide motifs in scorpion toxins

Scorpion toxins are important physiological probes for characterizing ion channels. Molecular databases have limited functional annotation of scorpion toxins. Their function can be inferred by searching for conserved motifs in sequence signature databases that are derived statistically but are not necessarily biologically relevant. Mutation studies provide biological information on residues and positions important for structure-function relationship but are not normally used for extraction of binding motifs. 3D structure analyses also aid in the extraction of peptide motifs in which non-contiguous residues are clustered spatially. Here we present new, functionally relevant peptide motifs for ion channels, derived from the analyses of scorpion toxin native and mutant peptides.

Structure-activity relationship of an alpha-toxin Bs-Tx28 from scorpion (Buthus sindicus) venom suggests a new alpha-toxin subfamily

Scorpion venoms are among the most widely known source of peptidyl neurotoxins used for callipering different ion channels, e.g., for Na(+), K(+), Ca(+) or Cl(-). An alpha-toxin (Bs-Tx28) has been purified from the venom of scorpion Buthus sindicus, a common yellow scorpion of Sindh, Pakistan. The primary structure of Bs-Tx28 was established using a combination of MALDI-TOF-MS, LC-ESI-MS, and automated Edman degradation analysis. Bs-Tx28 consists of 65 amino acid residues (7274.3+/-2Da), including eight cysteine residues, and shows very high sequence identity (82-94%) with other long-chain alpha-neurotoxins, active against receptor site-3 of mammalian (e.g., Lqq-IV and Lqh-IV from scorpions Leiurus sp.) and insect (e.g., BJalpha-IT and Od-1 from Buthotus judaicus and Odonthobuthus doriae, respectively) voltage-gated Na(+) channels. Multiple sequence alignment and phylogenetic analysis of Bs-Tx28 with other known alpha- and alpha-like toxins suggests the presence of a new and separate subfamily of scorpion alpha-toxins. Bs-Tx28 which is weakly active in both, mammals and insects (LD(50) 0.088 and 14.3microg/g, respectively), shows strong induction of the rat afferent nerve discharge in a dose-dependent fashion (EC(50)=0.01microg/mL) which was completely abolished in the presence of tetrodotoxin suggesting the binding of Bs-Tx28 to the TTX-sensitive Na(+)-channel. Three-dimensional structural features of Bs-Tx28, established by homology modeling, were compared with other known classical alpha-mammal (AaH-II), alpha-insect (Lqh-alphaIT), and alpha-like (BmK-M4) toxins and revealed subtle variations in the Nt-, Core-, and RT-CT-domains (functional domains) which constitute a "necklace-like" structure differing significantly in all alpha-toxin subfamilies. On the other hand, a high level of conservation has been observed in the conserved hydrophobic surface with the only substitution of W43 (Y43/42) and an additional hydrophobic character at position F40 (L40/A/V/G39), as compared to the other mentioned alpha-toxins. Despite major differences within the primary structure and activities of Bs-Tx28, it shares a common structural and functional motif (e.g., transRT-farCT) within the RT-CT domain which is characteristic of scorpion alpha-mammal toxins.

OD1, the first toxin isolated from the venom of the scorpionOdonthobuthus doriaeactive on voltage-gated Na+channels

In this study, we isolated and pharmacologically characterized the first alpha-like toxin from the venom of the scarcely studied Iranian scorpion Odonthobuthus doriae. The toxin was termed OD1 and its primary sequence was determined: GVRDAYIADDKNCVYTCASNGYCNTECTKNGAESGYCQWIGRYGNACWCIKLPDEVPIRIPGKCR. Using the two-electrode voltage clamp technique, the pharmacological effects of OD1 were studied on three cloned voltage-gated Na+ channels expressed in Xenopus laevis oocytes (Na(v)1.2/beta1, Na(v)1.5/beta1, para/tipE). The inactivation process of the insect channel, para/tipE, was severely hampered by 200 nM of OD1 (EC50 = 80+/-14 nM) while Na(v)1.2/beta1 still was not affected at concentrations up to 5 microM. Na(v)1.5/beta1 was influenced at micromolar concentrations.