Positively selected sites of scorpion depressant toxins: possible roles in toxin functional divergence

Behavior and oxidative stress evaluation of scorpion Tityusserrulatus (Lutz & Mello,1922) envenomation with genomic modulation and dopaminergic neutralization by antiscorpionic serum treatment

Tityus serrulatus accident promote vast symptomatology related to toxins of the venom, which leads to a massive release of neurotransmitters, notably dopamine, affecting behavior and neurochemistry. The recommended treatment for envenomation is the antiscorpionic serum (SAEsc) administration. Related to this complexity of the Tityus serrulatus envenomation, this study aimed to assess organism responses to the venom, its impact on behavior, oxidative stress, neurochemistry, and genetic impacts, as well as the efficacy of SAEsc, especially concerning dopamine levels and genetic interactions. Swiss mice were divided into groups and administered different venom concentrations intracerebroventricularly to assess behavioral impacts and brain oxidative stress. Oxidative stress was evaluated through reactive oxygen species (ROS) analysis and antioxidant assays, including dichloro-dihydro-fluorescein diacetate (DCF), thiobarbituric acid reactive substances (TBARS), superoxide dismutase (SOD), and glutathione (GSH) measurements. Swiss mice were divided into four groups to evaluate genomic modulation, micronucleus enhancement, and dopamine levels. Additionally, SAEsc's neutralizing effect on dopamine was also investigated. Results showed that venom doses (100-300 ng/μL) increased lipid peroxidation in the brain, with SAEsc maintaining dopamine balance and neutralizing venom up to 24 h post-envenomation. After 24 h, cellular repair became less efficient, leading to mutagenic damage in both treated and untreated animals. The results highlight the importance of considering genomic and neurotransmitter function modulation in the treatment of Tityus serrulatus envenomation.

Dopaminergic metabolism is affected by intracerebral injection of Tb II-I isolated from Tityus bahiensis scorpion venom

Tb II-I isolated from Tityus bahiensis venom causes epileptic-discharges when injected into the hippocampus of rats. The involvement of neurotransmitters in this activity was investigated. Our results demonstrated that Tb II-I increases the concentrations of dopamine metabolite but does not alter other neurotransmitters. Thus, dopaminergic system seems to be partially responsible for the convulsive process. Specific action on particular neurotransmitter can make this toxin a useful tool to better understand the functioning of the system.

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The dopaminergic system is affected by intracerebral injection of Tb II-I

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The dopaminergic system seems to be partially responsible for the convulsive process.

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The hippocampal level of glutamate and GABA is not affected by Tb II-I

Scorpion Toxins: Positive Selection at a Distal Site Modulates Functional Evolution at a Bioactive Site

The bioactive sites of proteins are those that directly interact with their targets. In many immunity- and predation-related proteins, they frequently experience positive selection for dealing with the changes of their targets from competitors. However, some sites that are far away from the interface between proteins and their targets are also identified to evolve under positive selection. Here, we explore the evolutionary implication of such a site in scorpion α-type toxins affecting sodium (Na⁺) channels (abbreviated as α-ScNaTxs) using a combination of experimental and computational approaches. We found that despite no direct involvement in interaction with Na⁺ channels, mutations at this site by different types of amino acids led to toxicity change on both rats and insects in three α-ScNaTxs, accompanying differential effects on their structures. Molecular dynamics simulations indicated that the mutations changed the conformational dynamics of the positively selected bioactive site-containing functional regions by allosteric communication, suggesting a potential evolutionary correlation between these bioactive sites and the distant nonbioactive site. Our results reveal for the first time the cause of fast evolution at nonbioactive sites of scorpion neurotoxins, which is presumably to adapt to the change of their bioactive sites through coevolution to maintain an active conformation for channel binding. This might aid rational design of scorpion Na⁺ channel toxins with improved phyletic selectivity via modification of a distant nonbioactive site.

Adaptive evolution of insect selective excitatory β-type sodium channel neurotoxins from scorpion venom

Insect selective excitatory β-type sodium channel neurotoxins from scorpion venom (β-NaScTxs) are composed of about 70-76 amino acid residues and share a common scaffold stabilized by four unique disulfide bonds. The phylogenetic analysis of these toxins was hindered by limited sequence data. In our recent study, two new insect selective excitatory β-NaScTxs, LmIT and ImIT, were isolated from Lychas mucronatus and Isometrus maculatus, respectively. With the sequences previously reported, we examined the adaptive molecular evolution of insect selective excitatory β-NaScTxs by estimating the nonsynonymous-to-synonymous rate ratio (ω=d_N/d_S). The results revealed 12 positively selected sites in the genes of insect selective excitatory β-NaScTxs. Moreover, these positively selected sites match well with the sites important for interacting with sodium channels, as demonstrated in previous mutagenesis study. These results reveal that adaptive evolution after gene duplication is one of the most important genetic mechanisms of scorpion neurotoxin diversification.

Profile comparison revealed deviation from structural constraint at the positively selected sites

The amino acid substitutions at a site are affected by mixture of various constraints. It is also known that the amino acid substitutions are accelerated at sites under positive selection. However, the relationship between the substitutions at positively selected sites and the constraints has not been thoroughly examined. The advances in computational biology have enabled us to divide the mixture of the constraints into the structural constraint and the remainings by using the amino acid sequences and the tertiary structures, which is expressed as the deviation of the mixture of constraints from the structural constraint. Here, two types of profiles, or matrices with the size of 20 x (site length), are compared. One of the profiles represents the mixture of constraints, and is generated from a multiple amino acid sequence alignment, whereas the other is designed to represent the structural constraints. We applied the profile comparison method to proteins under positive selection to examine the relationship between the positive selection and constraints. The results suggested that the constraint at a site under positive selection tends to be deviated from the structural constraint at the site.

Target-Driven Positive Selection at Hot Spots of Scorpion Toxins Uncovers Their Potential in Design of Insecticides

Positive selection sites (PSSs), a class of amino acid sites with an excess of nonsynonymous to synonymous substitutions, are indicators of adaptive molecular evolution and have been detected in many protein families involved in a diversity of biological processes by statistical approaches. However, few studies are conducted to evaluate their functional significance and the driving force behind the evolution (i.e., agent of selection). Scorpion α-toxins are a class of multigene family of peptide neurotoxins affecting voltage-gated Na(+ )(Nav) channels, whose members exhibit differential potency and preference for insect and mammalian Nav channels. In this study, we undertook a systematical molecular dissection of nearly all the PSSs newly characterized in the Mesobuthus α-toxin family and a two-residue insertion ((19)AlaPhe(20)) located within a positively selected loop via mutational analysis of α-like MeuNaTxα-5, one member affecting both insect and mammalian Nav channels. This allows to identify hot-spot residues on its functional face involved in interaction with the receptor site of Nav channels, which comprises two PSSs (Ile(40) and Leu(41)) and the small insertion, both located on two spatially separated functional loops. Mutations at these hot-spots resulted in a remarkably decreased anti-mammalian activity in MeuNaTxα-5 with partially impaired or enhanced insecticide activity, suggesting the potential of PSSs in designing promising candidate insecticides from scorpion α-like toxins. Based on an experiment-guided toxin-channel complex model and high evolutionary variability in the receptor site of predators and prey of scorpions, we provide new evidence for target-driven adaptive evolution of scorpion toxins to deal with their targets' diversity.

Effect of maternal exposure to Tityus bahiensis scorpion venom during lactation on the offspring of rats

Scorpion stings are a public health problem in Brazil and lactating women may be affected. We aimed to study the effects of Tityus bahiensis venom in the offspring of rats treated during lactation. Mothers received a subcutaneous injection of saline (1.0ml/kg) or venom (2.5mg/kg) or an intraperitoneal injection of LPS (lipopolysaccharide) (100μg/kg) on postnatal (PN) days 2 (PN2), 10 (PN10) or 16 (PN16). The offspring were evaluated during the childhood and adulthood. Pups showed a delay in physical and reflexological development, and a decrease in motor activity. Adults displayed low anxiety. There was an increase in the number of viable neuronal cells in hippocampal areas CA1 and CA4. The levels of IFN-γ (interferon-gamma) increased in the experimental groups. Several of the parameters analyzed showed important differences between the sexes. Thus, the scorpion venom affects the development in the offspring of mothers envenomed during the lactation.

IMPACT_S: integrated multiprogram platform to analyze and combine tests of selection

Among the major goals of research in evolutionary biology are the identification of genes targeted by natural selection and understanding how various regimes of evolution affect the fitness of an organism. In particular, adaptive evolution enables organisms to adapt to changing ecological factors such as diet, temperature, habitat, predatory pressures and prey abundance. An integrative approach is crucial for the identification of non-synonymous mutations that introduce radical changes in protein biochemistry and thus in turn influence the structure and function of proteins. Performing such analyses manually is often a time-consuming process, due to the large number of statistical files generated from multiple approaches, especially when assessing numerous taxa and/or large datasets. We present IMPACT_S, an easy-to-use Graphical User Interface (GUI) software, which rapidly and effectively integrates, filters and combines results from three widely used programs for assessing the influence of selection: Codeml (PAML package), Datamonkey and TreeSAAP. It enables the identification and tabulation of sites detected by these programs as evolving under the influence of positive, neutral and/or negative selection in protein-coding genes. IMPACT_S further facilitates the automatic mapping of these sites onto the three-dimensional structures of proteins. Other useful tools incorporated in IMPACT_S include Jmol, Archaeopteryx, Gnuplot, PhyML, a built-in Swiss-Model interface and a PDB downloader. The relevance and functionality of IMPACT_S is shown through a case study on the toxicoferan-reptilian Cysteine-rich Secretory Proteins (CRiSPs). IMPACT_S is a platform-independent software released under GPLv3 license, freely available online from http://impact-s.sourceforge.net.

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.

Evolution stings: the origin and diversification of scorpion toxin peptide scaffolds

The episodic nature of natural selection and the accumulation of extreme sequence divergence in venom-encoding genes over long periods of evolutionary time can obscure the signature of positive Darwinian selection. Recognition of the true biocomplexity is further hampered by the limited taxon selection, with easy to obtain or medically important species typically being the subject of intense venom research, relative to the actual taxonomical diversity in nature. This holds true for scorpions, which are one of the most ancient terrestrial venomous animal lineages. The family Buthidae that includes all the medically significant species has been intensely investigated around the globe, while almost completely ignoring the remaining non-buthid families. Australian scorpion lineages, for instance, have been completely neglected, with only a single scorpion species (Urodacus yaschenkoi) having its venom transcriptome sequenced. Hence, the lack of venom composition and toxin sequence information from an entire continent’s worth of scorpions has impeded our understanding of the molecular evolution of scorpion venom. The molecular origin, phylogenetic relationships and evolutionary histories of most scorpion toxin scaffolds remain enigmatic. In this study, we have sequenced venom gland transcriptomes of a wide taxonomical diversity of scorpions from Australia, including buthid and non-buthid representatives. Using state-of-art molecular evolutionary analyses, we show that a majority of CSα/β toxin scaffolds have experienced episodic influence of positive selection, while most non-CSα/β linear toxins evolve under the extreme influence of negative selection. For the first time, we have unraveled the molecular origin of the major scorpion toxin scaffolds, such as scorpion venom single von Willebrand factor C-domain peptides (SV-SVC), inhibitor cystine knot (ICK), disulphide-directed beta-hairpin (DDH), bradykinin potentiating peptides (BPP), linear non-disulphide bridged peptides and antimicrobial peptides (AMP). We have thus demonstrated that even neglected lineages of scorpions are a rich pool of novel biochemical components, which have evolved over millions of years to target specific ion channels in prey animals, and as a result, possess tremendous implications in therapeutics.

Positive selection-guided mutational analysis revealing two key functional sites of scorpion ERG K(+) channel toxins

Scorpion γ-KTx toxins are important molecular tools for studying physiological and pharmacological functions of human ether-á-go-go related gene (hERG) K(+) channels. To pinpoint functional residues of this class of toxins involved in channel binding, we employed a combined approach that integrates evolutionary information and site-directed mutagenesis. Among three positively selected sites (PSSs) identified here, two (Gln18 and Met35) were found to be associated with the toxin's function because their changes significantly decreased the potency of ErgTx1 (also called CnErg1) on hERG1 channel. On the contrary, no potency alteration was observed at the third PSS (Ala42) when the mutation was introduced, which could be due to its location far from the functional surface of the toxin. Our strategy will accelerate the research of structure-function relationship of scorpion K(+) channel toxins.

Effects of a toxin isolated from Tityus bahiensis scorpion venom on the hippocampus of rats

AIMS

The objective of the present study was to determine the effects of a toxin from T. bahiensis scorpion venom on the hippocampus of rats. This toxin, called Tb V-4, was chosen since it shows remarkable convulsive activity.

MAIN METHODS

Male Wistar rats weighing 250g were used. The toxin (1.0μg/μl) was injected into the hippocampus. The animals were then submitted to electroencephalographic and behavioral examinations or to microdialysis to determine the levels of neurotransmitters. The location of the implanted guide cannulae and electrodes was checked histologically. The number of cells in the CA1, CA3 and CA4 areas of the hippocampus was determined by light microscopy. Changes in the concentration of cytosolic free calcium were evaluated by confocal microscopy.

KEY FINDINGS

The toxin evoked behavioral alterations such as wet dog shakes, myoclonus, yawning and orofacial automatisms. Electroencephalographic recordings exhibited alterations such as isolated or grouped spikes and epileptic-like discharges. Injection of the toxin augmented glutamate concentration in the extracellular fluid in some animals. There was also a decrease in the number of pyramidal cells, mainly in the CA1 and CA4 areas for some rats. In some slices of the hippocampus, an increase in intracellular calcium mobilization was seen.

SIGNIFICANCE

The present results suggest that the Tb V-4 toxin may be responsible for the epileptic and behavioral effects observed with the crude venom. We suggest that the convulsive and degenerative effects induced by the toxin could be due to the enhanced release of excitatory amino acids involved in the most important pathways of the hippocampus.

Molding the business end of neurotoxins by diversifying evolution

A diverse range of organisms utilize neurotoxins that target specific ion channels and modulate their activity. Typically, toxins are clustered into several multigene families, providing an organism with the upper hand in the never-ending predator-prey arms race. Several gene families, including those encoding certain neurotoxins, have been subject to diversifying selection forces, resulting in rapid gene evolution. Here we sought a spatial pattern in the distribution of both diversifying and purifying selection forces common to neurotoxin gene families. Utilizing the mechanistic empirical combination model, we analyzed various toxin families from different phyla affecting various receptors and relying on diverse modes of action. Through this approach, we were able to detect clear correlations between the pharmacological surface of a toxin and rapidly evolving domains, rich in positively selected residues. On the other hand, patches of negatively selected residues were restricted to the nontoxic face of the molecule and most likely help in stabilizing the tertiary structure of the toxin. We thus propose a mutual evolutionary strategy of venomous animals in which adaptive molecular evolution is directed toward the toxin active surface. Furthermore, we propose that the binding domains of unstudied toxins could be readily predicted using evolutionary considerations.

Localization of receptor site on insect sodium channel for depressant β-toxin BmK IT2

BACKGROUND

BmK IT2 is regarded as a receptor site-4 modulator of sodium channels with depressant insect toxicity. It also displays anti-nociceptive and anti-convulsant activities in rat models. In this study, the potency and efficacy of BmK IT2 were for the first time assessed and compared among four sodium channel isoforms expressed in Xenopus oocytes. Combined with molecular approach, the receptor site of BmK IT2 was further localized.

PRINCIPAL FINDINGS

2 µM BmK IT2 strongly shifted the activation of DmNa(v)1, the sodium channel from Drosophila, to more hyperpolarized potentials; whereas it hardly affected the gating properties of rNa(v)1.2, rNa(v)1.3 and mNa(v)1.6, three mammalian central neuronal sodium channel subtypes. (1) Mutations of Glu(896), Leu(899), Gly(904) in extracellular loop Domain II S3-S4 of DmNa(v)1 abolished the functional action of BmK IT2. (2) BmK IT2-preference for DmNa(v)1 could be conferred by Domain III. Analysis of subsequent DmNa(v)1 mutants highlighted the residues in Domain III pore loop, esp. Ile(1529) was critical for recognition and binding of BmK IT2.

CONCLUSIONS/SIGNIFICANCE

In this study, BmK IT2 displayed total insect-selectivity. Two binding regions, comprising domains II and III of DmNa(v)1, play separated but indispensable roles in the interaction with BmK IT2. The insensitivity of Na(v)1.2, Na(v)1.3 and Na(v)1.6 to BmK IT2 suggests other isoforms or mechanism might be involved in the suppressive activity of BmK IT2 in rat pathological models.

Study of the binding residues between ANEPII and insect sodium channel receptor

The present study aimed at determining the functional characteristics of anti-neuroexcitation peptide II (ANEPII). The depressant insect toxin ANEPII from the Chinese scorpion Buthus martensii Karsch had an effect on insect sodium channels. Previous studies showed that scorpion depressant toxins induce insect flaccid paralysis upon binding to receptor site-4, so we tried to predict the functional residues involved using computational techniques. In this study, three-dimensional structure modeling of ANEPII and site-4 of the insect sodium channel were carried out by homology modeling, and these models were used as the starting point for nanosecond-duration molecular dynamics simulations. Docking studies of ANEPII in the sodium channel homology model were conducted, and likely ANEPII binding loci were investigated. Based on these analyses, the residues Tyr34, Trp36, Gly39, Leu40, Trp53, Asn58, Gly61 and Gly62 were predicted to interact with sodium channel receptor and to act as functional residues.

Drosotoxin, a selective inhibitor of tetrodotoxin-resistant sodium channels

The design of animal toxins with high target selectivity has long been a goal in protein engineering. Based on evolutionary relationship between the Drosophila antifungal defensin (drosomycin) and scorpion depressant Na(+) channel toxins, we exploited a strategy to create a novel chimeric molecule (named drosotoxin) with high selectivity for channel subtypes, which was achieved by using drosomycin to substitute the structural core of BmKITc, a depressant toxin acting on both insect and mammalian Na(+) channels. Recombinant drosotoxin selectively inhibited tetrodotoxin-resistant (TTX-R) Na(+) channels in rat dorsal root ganglion (DRG) neurons with a 50% inhibitory concentration (IC(50)) of 2.6+/-0.5muM. This chimeric peptide showed no activity on K(+), Ca(2+) and TTX-sensitive (TTX-S) Na(+) channels in rat DRG neurons and Drosophila para/tipE channels at micromolar concentrations. Drosotoxin represents the first chimeric toxin and example of a non-toxic core scaffold with high selectivity on mammalian TTX-R Na(+) channels.

Assembling an arsenal, the scorpion way

Background

For survival, scorpions depend on a wide array of short neurotoxic polypeptides. The venoms of scorpions from the most studied group, the Buthida, are a rich source of small, 23–78 amino acid-long peptides, well packed by either three or four disulfide bridges that affect ion channel function in excitable and non-excitable cells.

Results

In this work, by constructing a toxin transcripts data set from the venom gland of the scorpion Buthus occitanus israelis, we were able to follow the evolutionary path leading to mature toxin diversification and suggest a mechanism for leader peptide hyper-conservation. Toxins from each family were more closely related to one another than to toxins from other species, implying that fixation of duplicated genes followed speciation, suggesting early gene conversion events. Upon fixation, the mature toxin-coding domain was subjected to diversifying selection resulting in a significantly higher substitution rate that can be explained solely by diversifying selection. In contrast to the mature peptide, the leader peptide sequence was hyper-conserved and characterized by an atypical sub-neutral synonymous substitution rate. We interpret this as resulting from purifying selection acting on both the peptide and, as reported here for the first time, the DNA sequence, to create a toxin family-specific codon bias.

Conclusion

We thus propose that scorpion toxin genes were shaped by selective forces acting at three levels, namely (1) diversifying the mature toxin, (2) conserving the leader peptide amino acid sequence and intriguingly, (3) conserving the leader DNA sequences.