Molecular basis of the mammalian potency of the scorpion α‐like toxin, BmK M1

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.

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.

Target-Driven Evolution of Scorpion Toxins

It is long known that peptide neurotoxins derived from a diversity of venomous animals evolve by positive selection following gene duplication, yet a force that drives their adaptive evolution remains a mystery. By using maximum-likelihood models of codon substitution, we analyzed molecular adaptation in scorpion sodium channel toxins from a specific species and found ten positively selected sites, six of which are located at the core-domain of scorpion α-toxins, a region known to interact with two adjacent loops in the voltage-sensor domain (DIV) of sodium channels, as validated by our newly constructed computational model of toxin-channel complex. Despite the lack of positive selection signals in these two loops, they accumulated extensive sequence variations by relaxed purifying selection in prey and predators of scorpions. The evolutionary variability in the toxin-bound regions of sodium channels indicates that accelerated substitutions in the multigene family of scorpion toxins is a consequence of dealing with the target diversity. This work presents an example of atypical co-evolution between animal toxins and their molecular targets, in which toxins suffered from more prominent selective pressure from the channels of their competitors. Our discovery helps explain the evolutionary rationality of gene duplication of toxins in a specific venomous species.

Characterization of a novel BmαTX47 toxin modulating sodium channels: the crucial role of expression vectors in toxin pharmacological activity

Long-chain scorpion toxins with four disulfide bridges exhibit various pharmacological features towards the different voltage-gated sodium channel subtypes. However, the toxin production still remains a huge challenge. Here, we reported the effects of different expression vectors on the pharmacological properties of a novel toxin BmαTX47 from the scorpion Buthus martensii Karsch. The recombinant BmαTX47 was obtained using the expression vector pET-14b and pET-28a, respectively. Pharmacological experiments showed that the recombinant BmαTX47 was a new α-scorpion toxin which could inhibit the fast inactivation of rNa_v1.2, mNa_v1.4 and hNa_v1.5 channels. Importantly, the different expression vectors were found to strongly affect BmαTX47 pharmacological activities while toxins were obtained by the same expression and purification procedures. When 10 µM recombinant BmαTX47 from the pET-28a vector was applied, the values of I_5ms/I_peak for rNa_v1.2, mNa_v1.4 and hNa_v1.5 channels were 44.12% ± 3.17%, 25.40% ± 4.89% and 65.34% ± 3.86%, respectively, which were better than those values of 11.33% ± 1.46%, 15.96% ± 1.87% and 5.24% ± 2.38% for rNa_v1.2, mNa_v1.4 and hNa_v1.5 channels delayed by 10 µM recombinant BmαTX47 from the pET-14b vector. The dose-response experiments further indicated the EC₅₀ values of recombinant BmαTX47 from the pET-28a vector were 7262.9 ± 755.9 nM for rNa_v1.2 channel and 1005.8 ± 118.6 nM for hNav1.5 channel, respectively. Together, these findings highlighted the important role of expression vectors in scorpion toxin pharmacological properties, which would accelerate the understanding of the structure-function relationships of scorpion toxins and promote the potential application of toxins in the near future.

Development and implementation of Baltimore Healthy Eating Zones: a youth-targeted intervention to improve the urban food environment

Poor accessibility to affordable healthy foods is associated with higher rates of obesity and diet-related chronic diseases. We present our process evaluation of a youth-targeted environmental intervention (Baltimore Healthy Eating Zones) that aimed to increase the availability of healthy foods and promote these foods through signage, taste tests and other interactive activities in low-income Baltimore City. Trained peer educators reinforced program messages. Dose, fidelity and reach-as measured by food stocking, posting of print materials, distribution of giveaways and number of interactions with community members-were collected in six recreation centers and 21 nearby corner stores and carryouts. Participating stores stocked promoted foods and promotional print materials with moderate fidelity. Interactive sessions were implemented with high reach and dose among both adults and youth aged 10-14 years, with more than 4000 interactions. Recreation centers appear to be a promising location to interact with low-income youth and reinforce exposure to messages.

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.

The role of glycine residues at the C-terminal peptide segment in antinociceptive activity: a molecular dynamics simulation

Elucidating structural determinants in the functional regions of toxins can provide useful knowledge for designing novel analgesic peptides. Glycine residues at the C-terminal region of the neurotoxin BmK AGP-SYPU2 from the scorpion Buthus martensii Karsch (BmK) have been shown to be crucial to its analgesic activity. However, there has been no research on the structure-function relationship between the C-terminal segment of this toxin and its analgesic activity. To address this issue, we performed three MD simulations: one on the native structure and the other two on mutants of that structure. Results of these calculations suggest that the existence of glycine residues at the C-terminal segment stabilizes the protruding topology of the NC domain, which is considered an important determinant of the analgesic activity of BmK AGP-SYPU2.

Homology modeling and molecular dynamics simulation of odonthubuthus doriae (Od1) scorpion toxin in comparison to the BmK M1

All of the α-subgroups share similarity in their sequence and structure but different in the toxicity to various voltage-gated sodium channels (VGSCs). We modeled the first 3D structural model of the Od1 based on BmK M1 using homology modeling. The reliability of model for more investigation and compare to BmK M1 has been examined and confirmed. Then the model structure is further refined by energy minimization and molecular dynamics methods. The purpose of this modeling and simulation is comparison toxicity of two mentioned toxins by investigation structural feature of functional regions including core domain, 5-turn and C-terminal which make NC domain. In the one hand, it is intriguing that Od1 in comparison to BmK M1 shows same solvent accessible surface area (SASA) in 5-turn region but a little more exposed and feasibility (more SASA) in C-terminal region and key functional residues of C-terminal such as positive residues Arg58, lys62 and Arg (His)64. These data suggested that Od1 has similarity with BmK M1 but has more toxicity to sodium channel. In the other hand 5-turn proximity of C-terminal to 5-turn in BmK M1with cis peptide bond is less than Od1 without cis peptide bond which is a confirmation with experimental data about BmK M1.A better understanding of the 3-D structure of Od1and comparison to BmK M1 will be helpful for more investigation of functional characters action of natural toxins with a specialized role for VGSCs.

Recombinant expression, purification, and characterization of scorpion toxin BmαTX14

Long-chain and cysteine-rich scorpion toxins exhibit various pharmacological profiles for different voltage-gated sodium channel subtypes. However, the exploration of toxin structure-function relationships has progressed slowly due to the difficulty of obtaining synthetic or recombinant peptides. We now report that we have established an effective expression and purification approach for the novel scorpion toxin BmαTX14. BmαTX14 was over-expressed as inclusion bodies in Escherichia coli. The insoluble pellet was successfully transformed into active peptide by using a refolding procedure. One-step purification by reverse-phase HPLC was sufficient to generate chromatographically pure peptide. The yield of recombinant toxin reached 4mg from 1L LB medium. The pharmacological data further showed that BmαTX14 selectively inhibited the fast inactivation of mNa(v)1.4 (EC(50)=82.3±15.7nM) rather than that of rNa(v)1.2 (EC(50)>30μM), which indicates that BmαTX14 is a new α-like toxin. This work enables further structural, functional, and pharmacological studies of BmαTX14 and similar toxins.

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.

Elucidation of the molecular basis of selective recognition uncovers the interaction site for the core domain of scorpion alpha-toxins on sodium channels

Neurotoxin receptor site-3 at voltage-gated Na(+) channels is recognized by various peptide toxin inhibitors of channel inactivation. Despite extensive studies of the effects of these toxins, their mode of interaction with the channel remained to be described at the molecular level. To identify channel constituents that interact with the toxins, we exploited the opposing preferences of LqhαIT and Lqh2 scorpion α-toxins for insect and mammalian brain Na(+) channels. Construction of the DIV/S1-S2, DIV/S3-S4, DI/S5-SS1, and DI/SS2-S6 external loops of the rat brain rNa(v)1.2a channel (highly sensitive to Lqh2) in the background of the Drosophila DmNa(v)1 channel (highly sensitive to LqhαIT), and examination of toxin activity on the channel chimera expressed in Xenopus oocytes revealed a substantial decrease in LqhαIT effect, whereas Lqh2 was as effective as at rNa(v)1.2a. Further substitutions of individual loops and specific residues followed by examination of gain or loss in Lqh2 and LqhαIT activities highlighted the importance of DI/S5-S6 (pore module) and the C-terminal region of DIV/S3 (gating module) of rNa(v)1.2a for Lqh2 action and selectivity. In contrast, a single substitution of Glu-1613 to Asp at DIV/S3-S4 converted rNa(v)1.2a to high sensitivity toward LqhαIT. Comparison of depolarization-driven dissociation of Lqh2 and mutant derivatives off their binding site at rNa(v)1.2a mutant channels has suggested that the toxin core domain interacts with the gating module of DIV. These results constitute the first step in better understanding of the way scorpion α-toxins interact with voltage-gated Na(+)-channels at the molecular level.

Evolutionary diversification of Mesobuthus α-scorpion toxins affecting sodium channels

α-Scorpion toxins constitute a family of peptide modulators that induce a prolongation of the action potential of excitable cells by inhibiting voltage-gated sodium channel inactivation. Although they all adopt a conserved structural scaffold, the potency and phylogentic preference of these toxins largely vary, which render them an intriguing model for studying evolutionary diversification among family members. Here, we report molecular characterization of a new multigene family of α-toxins comprising 13 members (named MeuNaTxα-1 to MeuNaTxα-13) from the scorpion Mesobuthus eupeus. Of them, five native toxins (MeuNaTxα-1 to -5) were purified to homogeneity from the venom and the solution structure of MeuNaTxα-5 was solved by nuclear magnetic resonance. A systematic functional evaluation of MeuNaTxα-1, -2, -4, and -5 was conducted by two-electrode voltage-clamp recordings on seven cloned mammalian voltage-gated sodium channels (Na(v)1.2 to Na(v)1.8) and the insect counterpart DmNa(v)1 expressed in Xenopus oocytes. Results show that all these four peptides slow inactivation of DmNa(v)1 and are inactive on Na(v)1.8 at micromolar concentrations. However, they exhibit differential specificity for the other six channel isoforms (Na(v)1.2 to Na(v)1.7), in which MeuNaTxα-4 shows no activity on these isoforms and thus represents the first Mesobuthus-derived insect-selective α-toxin identified so far with a half maximal effective concentration of 130 ± 2 nm on DmNa(v)1 and a half maximal lethal dose of about 200 pmol g(-1) on the insect Musca domestica; MeuNaTxα-2 only affects Na(v)1.4; MeuNaTxα-1 and MeuNaTxα-5 have a wider range of channel spectrum, the former active on Na(v)1.2, Na(v)1.3, Na(v)1.6, and Na(v)1.7, whereas the latter acting on Na(v)1.3-Na(v)1.7. Remarkably, MeuNaTxα-4 and MeuNaTxα-5 are two nearly identical peptides differing by only one point mutation at site 50 (A50V) but exhibit rather different channel subtype selectivity, highlighting a switch role of this site in altering the target specificity. By the maximum likelihood models of codon substitution, we detected nine positively selected sites (PSSs) that could be involved in functional diversification of Mesobuthus α-toxins. The PSSs include site 50 and other seven sites located in functional surfaces of α-toxins. This work represents the first thorough investigation of evolutionary diversification of α-toxins derived from a specific scorpion lineage from the perspectives of sequence, structure, function, and evolution.

The role of Ser54 in the antinociceptive activity of BmK9, a neurotoxin from the scorpion Buthus martensii Karsch

Residue 54 has been shown to be important for bioactivity in several toxins. However, its role in the antinociceptive activity of toxins has not been evaluated yet. In this study, site-directed mutagenesis and mouse acetic acid writhing test were used to investigate the role of Ser54 in the antinociceptive activity of BmK9 neurotoxin from the Buthus martensii Karsch scorpion. Detailed mutagenesis analysis revealed that substitution of Ser54 by various polar amino acids produced no significant change in the antinociceptive activity, while all substitutions of nonpolar amino acid for Ser54 led to a significant loss of antinociceptive activity. Following the conformational analysis, it was suggested that Ser54 in BmK9 plays a functional role in the antinociceptive activity, the residue exerts its effect by means of a side-chain hydrogen bond.

A naturally-occurring carboxyl-terminally truncated α-scorpion toxin is a blocker of sodium channels

α-Scorpion toxins constitute a multigene family of evolutionarily conserved venom peptides that inhibit sodium channel inactivation and increase its peak current. Here, we describe the characterization of a new α-scorpion toxin gene expressed in the venom gland of Mesobuthus eupeus that encodes a carboxyl-terminally truncated product of 38 residues (named MeuNaTxα(NT)-1). Synthetic MeuNaTxα(NT)-1 was oxidized to form two disulfide bridges in an alkaline environment and the refolded peptide exhibits different structure and function from the classical α-scorpion toxin. MeuNaTxα(NT)-1 blocks sodium channels on rat dorsal root ganglia (DRG) neurons without impact on the inactivation of the channels. This work provides a clue for evolution-guided design of channel blockers for therapeutic aims.

Site-directed mutagenesis of the toxin from the Chinese scorpion Buthus martensii Karsch (BmKAS): insight into sites related to analgesic activity

This study utilized the E. coli expression system to investigate the role of amino acid residues in toxin from the Chinese scorpion--Buthus martensii Karsch (BmKAS). To evaluate the extent to which residues of the toxin core contribute to its analgesic activity, ten mutants of BmKAS were obtained by PCR. Using site-directed mutagenesis, all of these residues were substituted with different amino acids. This study represents a thorough mapping and elucidation of the epitopes that form the molecular basis of the toxin's analgesic activity. Our results showed large mutant-dependent differences that emphasize the important roles of the studied residues.

Molecular requirements for recognition of brain voltage-gated sodium channels by scorpion alpha-toxins

The scorpion alpha-toxin Lqh2 (from Leiurus quinquestriatus hebraeus) is active at various mammalian voltage-gated sodium channels (Na(v)s) and is inactive at insect Na(v)s. To resolve the molecular basis of this preference we used the following strategy: 1) Lqh2 was expressed in recombinant form and key residues important for activity at the rat brain channel rNa(v)1.2a were identified by mutagenesis. These residues form a bipartite functional surface made of a conserved "core domain" (residues of the loops connecting the secondary structure elements of the molecule core), and a variable "NC domain" (five-residue turn and the C-tail) as was reported for other scorpion alpha-toxins. 2) The functional role of the two domains was validated by their stepwise construction on the similar scaffold of the anti-insect toxin LqhalphaIT. Analysis of the activity of the intermediate constructs highlighted the critical role of Phe(15) of the core domain in toxin potency at rNa(v)1.2a, and has suggested that the shape of the NC-domain is important for toxin efficacy. 3) Based on these findings and by comparison with other scorpion alpha-toxins we were able to eliminate the activity of Lqh2 at rNa(v)1.4 (skeletal muscle), hNa(v)1.5 (cardiac), and rNa(v)1.6 channels, with no hindrance of its activity at Na(v)1.1-1.3. These results suggest that by employing a similar approach the design of further target-selective sodium channel modifiers is imminent.

The unique pharmacology of the scorpion α-like toxin Lqh3 is associated with its flexible C-tail

The affinity of scorpion alpha-toxins for various voltage-gated sodium channels (Na(v)s) differs considerably despite similar structures and activities. It has been proposed that key bioactive residues of the five-residue-turn (residues 8-12) and the C-tail form the NC domain, whose topology is dictated by a cis or trans peptide-bond conformation between residues 9 and 10, which correlates with the potency on insect or mammalian Na(v)s. We examined this hypothesis using Lqh3, an alpha-like toxin from Leiurus quinquestriatus hebraeus that is highly active in insects and mammalian brain. Lqh3 exhibits slower association kinetics to Na(v)s compared with other alpha-toxins and its binding to insect Na(v)s is pH-dependent. Mutagenesis of Lqh3 revealed a bi-partite bioactive surface, composed of the Core and NC domains, as found in other alpha-toxins. Yet, substitutions at the five-residue turn and stabilization of the 9-10 bond in the cis conformation did not affect the activity. However, substitution of hydrogen-bond donors/acceptors at the NC domain reduced the pH-dependency of toxin binding, while retaining its high potency at Drosophila Na(v)s expressed in Xenopus oocytes. Based on these results and the conformational flexibility and rearrangement of intramolecular hydrogen-bonds at the NC domain, evident from the known solution structure, we suggest that acidic pH or specific mutations at the NC domain favor toxin conformations with high affinity for the receptor by stabilizing the bound toxin-receptor complex. Moreover, the C-tail flexibility may account for the slower association rates and suggests a novel mechanism of dynamic conformer selection during toxin binding, enabling alpha-like toxins to affect a broad range of Na(v)s.

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'.

Potent Modulation of the Voltage-Gated Sodium Channel Nav1.7 by OD1, a Toxin from the Scorpion Odonthobuthus doriae

Voltage-gated sodium channels are essential for the propagation of action potentials in nociceptive neurons. Nav1.7 is found in peripheral sensory and sympathetic neurons and involved in short-term and inflammatory pain. Nav1.8 and Nav1.3 are major players in nociception and neuropathic pain, respectively. In our effort to identify isoform-specific and high-affinity ligands for these channels, we investigated the effects of OD1, a scorpion toxin isolated from the venom of the scorpion Odonthobuthus doriae. Nav1.3, Nav1.7, and Nav1.8 channels were coexpressed with beta1-subunits in Xenopus laevis oocytes. Na+ currents were recorded with the two-electrode voltage-clamp technique. OD1 modulates Nav1.7 at low nanomolar concentrations: 1) fast inactivation is dramatically impaired, with an EC50 value of 4.5 nM; 2) OD1 substantially increases the peak current at all voltages; and 3) OD1 induces a substantial persistent current. Nav1.8 was not affected by concentrations up to 2 microM, whereas Nav1.3 was sensitive only to concentrations higher than 100 nM. OD1 impairs the inactivation process of Nav1.3 with an EC50 value of 1127 nM. Finally, the effects of OD1 were compared with a classic alpha-toxin, AahII from Androctonus australis Hector and a classic alpha-like toxin, BmK M1 from Buthus martensii Karsch. At a concentration of 50 nM, both toxins affected Nav1.7. Nav1.3 was sensitive to AahII but not to BmK M1, whereas Nav1.8 was affected by neither toxin. In conclusion, the present study shows that the scorpion toxin OD1 is a potent modulator of Nav1.7, with a unique selectivity pattern.

Overview of scorpion toxins specific for Na+ channels and related peptides: biodiversity, structure-function relationships and evolution

Scorpion venoms contain a large number of bioactive components. Several of the long-chain peptides were shown to be responsible for neurotoxic effects, due to their ability to recognize Na(+) channels and to cause impairment of channel functions. Here, we revisited the basic paradigms in the study of these peptides in the light of recent data concerning their structure-function relationships, their functional divergence and extant biodiversity. The reviewed topics include: the criteria for classification of long-chain peptides according to their function, and a revision of the state-of-the-art knowledge concerning the surface areas of contact of these peptides with known Na(+) channels. Additionally, we compiled a comprehensive list encompassing 191 different amino acid sequences from long-chain peptides purified from scorpion venoms. With this dataset, a phylogenetic tree was constructed and discussed taking into consideration their documented functional divergence. A critical view on problems associated with the study of these scorpion peptides is presented, drawing special attention to the points that need revision and to the subjects under intensive research at this moment, regarding scorpion toxins specific for Na(+) channels and the other related long-chain peptides recently described.

Structural basis for the voltage-gated Na+ channel selectivity of the scorpion alpha-like toxin BmK M1

Scorpion alpha-like toxins are proteins that act on mammalian and insect voltage-gated Na+ channels. Therefore, these toxins constitute an excellent target for examining the foundations that underlie their target specificity. With this motive we dissected the role of six critical amino acids located in the five-residue reverse turn (RT) and C-tail (CT) of the scorpion alpha-like toxin BmK M1. These residues were individually substituted resulting in 11 mutants and were subjected to a bioassay on mice, an electrophysiological characterization on three cloned voltage-gated Na+ channels (Nav1.2, Nav1.5 and para), a CD analysis and X-ray crystallography. The results reveal two molecular sites, a couplet of residues (8-9) in the RT and a hydrophobic surface consisting of residues 57 and 59-61 in the CT, where the substitution with specific residues can redirect the alpha-like characteristics of BmK M1 to either total insect or much higher mammal specificity. Crystal structures reveal that the pharmacological ramification of these mutants is accompanied by the reshaping of the 3D structure surrounding position 8. Furthermore, our results also reveal that residues 57 and 59-61, located at the CT, enclose the critical residue 58 in order to form a hydrophobic "gasket". Mutants of BmK M1 that interrupt this hydrophobic surface significantly gain insect selectivity.

The modulation effects of BmK I, an alpha-like scorpion neurotoxin, on voltage-gated Na(+) currents in rat dorsal root ganglion neurons

The present study investigated the effects of BmK I, a Na(+) channel receptor site 3 modulator purified from the Buthus martensi Karsch (BmK) venom, on the voltage-gated sodium currents in dorsal root ganglion (DRG) neurons. Whole-cell patch-clamping was used to record the tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) components of voltage-gated Na(+) currents in small DRG neurons. It was found that the inhibitory effect of BmK I on open-state inactivation of TTX-S Na(+) currents was stronger than that of TTX-R Na(+) currents. In addition, BmK I exhibited a selective enhancing effect on voltage-dependent activation of TTX-S currents, and an opposite effect on time-dependent activation of TTX-S and TTX-R Na(+) currents. The results suggested that the inhibitory effect of BmK I on open-state inactivation might contribute to the increase of peak TTX-S and TTX-R currents, and the enhancing effect of BmK I on time-dependent activation might also contribute to the increase of peak TTX-S currents. It was further suggested that a combined effect of BmK I including inhibiting the inactivation of TTX-S and TTX-R channels, accelerating activation and decreasing the activation threshold of TTX-S channels, might produce a hyperexcitability of small DRG neurons, and thus contribute to the BmK I-induced hyperalgesia.

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.