Scorpion toxins affecting sodium current inactivation bind to distinct homologous receptor sites on rat brain and insect sodium channels

Scorpion α-toxin LqhαIT specifically interacts with a glycan at the pore domain of voltage-gated sodium channels

Voltage-gated sodium (Nav) channels sense membrane potential and drive cellular electrical activity. The deathstalker scorpion α-toxin LqhαIT exerts a strong action potential prolonging effect on Nav channels. To elucidate the mechanism of action of LqhαIT, we determined a 3.9 Å cryoelectron microscopy (cryo-EM) structure of LqhαIT in complex with the Nav channel from Periplaneta americana (NavPas). We found that LqhαIT binds to voltage sensor domain 4 and traps it in an "S4 down" conformation. The functionally essential C-terminal epitope of LqhαIT forms an extensive interface with the glycan scaffold linked to Asn330 of NavPas that augments a small protein-protein interface between NavPas and LqhαIT. A combination of molecular dynamics simulations, structural comparisons, and prior mutagenesis experiments demonstrates the functional importance of this toxin-glycan interaction. These findings establish a structural basis for the specificity achieved by scorpion α-toxins and reveal the conserved glycan as an essential component of the toxin-binding epitope.

Historical Perspective of the Characterization of Conotoxins Targeting Voltage-Gated Sodium Channels

Marine toxins have potent actions on diverse sodium ion channels regulated by transmembrane voltage (voltage-gated ion channels) or by neurotransmitters (nicotinic acetylcholine receptor channels). Studies of these toxins have focused on varied aspects of venom peptides ranging from evolutionary relationships of predator and prey, biological actions on excitable tissues, potential application as pharmacological intervention in disease therapy, and as part of multiple experimental approaches towards an understanding of the atomistic characterization of ion channel structure. This review examines the historical perspective of the study of conotoxin peptides active on sodium channels gated by transmembrane voltage, which has led to recent advances in ion channel research made possible with the exploitation of the diversity of these marine toxins.

Recombinant expression and antigenicity of two peptide families of neurotoxins from Androctonus sp

Background

Scorpion neurotoxins such as those that modify the mammalian voltage-gated sodium ion channels (Nav) are the main responsible for scorpion envenomation. Their neutralization is crucial in the production of antivenoms against scorpion stings.

Methods

In the present study, two in silico designed genes - one that codes for a native neurotoxin from the venom of the Anatolian scorpion Androctonus crassicauda, named Acra 4 - and another non-native toxin - named consensus scorpion toxin (SccTx) obtained from the alignment of the primary structures of the most toxic neurotoxins from the Middle Eastern and North African scorpions - were recombinantly expressed in E. coli Origami.

Results

Following bacterial expression, the two expressed neurotoxins, hereafter named HisrAcra4 and HisrSccTx, were obtained from inclusion bodies. Both recombinant neurotoxins were obtained in multiple Cys-Cys isoforms. After refolding, the active protein fractions were identified with molecular masses of 8,947.6 and 9,989.1 Da for HisrAcra4 and HisrSccTx, respectively, which agreed with their expected theoretical masses. HisrAcra4 and HisrSccTx were used as antigens to immunize two groups of rabbits, to produce either anti-HisrAcra4 or anti-HisrSccTx serum antibodies, which in turn could recognize and neutralize neurotoxins from venoms of scorpion species from the Middle East and North Africa. The antibodies obtained from rabbits neutralized the 3LD₅₀ of Androctonus australis, Leiurus quinquestriatus hebraeus and Buthus occitanus venoms, but they did not neutralize A. crassicauda and A. mauritanicus venoms. In addition, the anti-HisrAcra4 antibodies did not neutralize any of the five scorpion venoms tested. However, an antibody blend of anti-HisrAcra4 and anti-HisrSccTx was able to neutralize A. crassicauda and A. mauritanicus venoms.

Conclusions

Two recombinant Nav neurotoxins, from different peptide families, were used as antigens to generate IgGs for neutralizing scorpion venoms of species from the Middle East and North Africa.

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.

Scorpion venom increases acetylcholine release by prolonging the duration of somatic nerve action potentials

Scorpionism is frequently accompanied by a massive release of catecholamines and acetylcholine from peripheral nerves caused by neurotoxic peptides present in these venoms, which have high specificity and affinity for ion channels. Tityus bahiensis is the second most medically important scorpion species in Brazil but, despite this, its venom remains scarcely studied, especially with regard to its pharmacology on the peripheral (somatic and autonomic) nervous system. Here, we evaluated the activity of T. bahiensis venom on somatic neurotransmission using myographic (chick and mouse neuromuscular preparations), electrophysiological (MEPP, EPP, resting membrane potentials, perineural waveforms, compound action potentials) and calcium imaging (on DRG neurons and muscle fibres) techniques. Our results show that the major toxic effects of T. bahiensis venom on neuromuscular function are presynaptically driven by the increase in evoked and spontaneous neurotransmitter release. Low venom concentrations prolong the axonal action potential, leading to a longer depolarization of the nerve terminals that enhances neurotransmitter release and facilitates nerve-evoked muscle contraction. The venom also stimulates the spontaneous release of neurotransmitters, probably through partial neuronal depolarization that allows calcium influx. Higher venom concentrations block the generation of action potentials and resulting muscle twitches. These effects of the venom were reversed by low concentrations of TTX, indicating voltage-gated sodium channels as the primary target of the venom toxins. These results suggest that the major neuromuscular toxicity of T. bahiensis venom is probably mediated mainly by α- and β-toxins interacting with presynaptic TTX-sensitive ion channels on both axons and nerve terminals.

The pharmacology of voltage-gated sodium channel activators

Toxins and venom components that target voltage-gated sodium (Na_V) channels have evolved numerous times due to the importance of this class of ion channels in the normal physiological function of peripheral and central neurons as well as cardiac and skeletal muscle. Na_V channel activators in particular have been isolated from the venom of spiders, wasps, snakes, scorpions, cone snails and sea anemone and are also produced by plants, bacteria and algae. These compounds have provided key insight into the molecular structure, function and pathophysiological roles of Na_V channels and are important tools due to their at times exquisite subtype-selectivity. We review the pharmacology of Na_V channel activators with particular emphasis on mammalian isoforms and discuss putative applications for these compounds. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Differential effects of the recombinant toxin PnTx4(5-5) from the spider Phoneutria nigriventer on mammalian and insect sodium channels

The toxin PnTx4(5-5) from the spider Phoneutria nigriventer is extremely toxic/lethal to insects but has no macroscopic behavioral effects observed in mice after intracerebral injection. Nevertheless, it was demonstrated that it inhibits the N-methyl-d-aspartate (NMDA) - subtype of glutamate receptors of cultured rat hippocampal neurons. PnTx4(5-5) has 63% identity to PnTx4(6-1), another insecticidal toxin from P. nigriventer, which can slow down the sodium current inactivation in insect central nervous system, but has no effect on Nav1.2 and Nav1.4 rat sodium channels. Here, we have cloned and heterologous expressed the toxin PnTx4(5-5) in Escherichia coli. The recombinant toxin rPnTx4(5-5) was tested on the sodium channel NavBg from the cockroach Blatella germanica and on mammalian sodium channels Nav1.2-1.6, all expressed in Xenopus leavis oocytes. We showed that the toxin has different affinity and mode of action on insect and mammalian sodium channels. The most remarkable effect was on NavBg, where rPnTx4(5-5) strongly slowed down channel inactivation (EC50 = 212.5 nM), and at 1 μM caused an increase on current peak amplitude of 105.2 ± 3.1%. Interestingly, the toxin also inhibited sodium current on all the mammalian channels tested, with the higher current inhibition on Nav1.3 (38.43 ± 8.04%, IC50 = 1.5 μM). Analysis of activation curves on Nav1.3 and Nav1.5 showed that the toxin shifts channel activation to more depolarized potentials, which can explain the sodium current inhibition. Furthermore, the toxin also slightly slowed down sodium inactivation on Nav1.3 and Nav1.6 channels. As far as we know, this is the first araneomorph toxin described which can shift the sodium channel activation to more depolarized potentials and also slows down channel inactivation.

Exposure to 50 Hz electromagnetic field changes the efficiency of the scorpion alpha toxin

BACKGROUND

Extremely low-frequency (50 Hz) electromagnetic field (ELF-EMF) is produced by electric power transmission lines and electronic devices of everyday use. Some phenomena are proposed as "first effects" of ELF-EMF: the discrete changes in the membrane potential and the increase of the calcium channel activity as well as the intracellular concentration of Ca(2+). Interaction of the scorpion alpha toxin with the sodium channel depends on the orientation of the charges and may be perturbed by changes in the membrane polarization. The toxin induces overexcitability in the nervous system and an increase in the neurotransmitters released with different consequences, mainly the paralysis of muscles. We assumed that the exposure to ELF-EMF 0.7 mT will change the effects of the insect selective scorpion alpha toxin (recombinant LqhαIT from Leiurus quinquestriatus hebraeus) at the level of the cercal nerve function, the synaptic transmission and on the level of entire insect organism. Taking into account the compensatory mechanisms in organisms, we tested in addition ten times higher ELF-EMF on whole insects.

METHODS

Experiments were performed in vivo on cockroaches (Periplaneta americana) and in vitro - on isolated cockroach abdominal nerve cord with cerci. In biotests, the effects of LqhαIT (10(-8) M) were estimated on the basis of the insect ability to turn back from dorsal to ventral side. Three groups were compared: the control one and the two exposed to ELF-EMF - 0.7 and 7 mT. Bioelectrical activity of the cercal nerve and of the connective nerve that leaves the terminal abdominal ganglion was recorded using extracellular electrodes. LqhαIT (5 × 10(-8) M) induced modifications of neuronal activity that were observed in the control cockroach preparations and in the ones exposed to ELF-EMF (0.7 mT). The exposure to ELF-EMF was carried out using coils with a size appropriate to the examined objects.

RESULTS

The exposure to ELF-EMF (0.7 mT) modified the effects of LqhαIT (5 × 10(-8) M) on activity of the cercal nerve and of the connective nerve. We observed a decrease of the toxin effect on the cercal nerve activity, but the toxic effect of LqhαIT on the connective nerve was increased. Biotests showed that toxicity of LqhαIT (10(-8) M) on cockroaches was reduced by the exposure to ELF-EMF (0.7 and 7 mT).

CONCLUSIONS

The exposure to 50 Hz ELF-EMF modified the mode of action of the anti-insect scorpion alpha toxin LqhαIT at cellular level of the cockroach nervous system and in biotests. Toxin appeared as a usefull tool in distinguishing between the primary and the secondary effects of ELF-EMF.

A comparison between the recombinant expression and chemical synthesis of a short cysteine-rich insecticidal spider peptide

Background

The choice between heterologous expression versus chemical synthesis for synthesizing short cysteine-rich insecticidal peptides from arthropods may impact the obtainment of yields and well-folded bioactive molecules for scientific research. Therefore, two recombinant expression systems were compared to that of chemical synthesis for producing Ba1, a cysteine-rich spider neurotoxin.

Methods

The transcription of the insecticidal neurotoxin Ba1 was obtained from a cDNA library of venom glands of the spider Brachypelma albiceps. It was cloned into the pCR®2.1-TOPO® cloning vector and then introduced in two different expression vectors, pQE40 and pET28a⁺. Each vector was transfected into E. coli M15 and BL21 cells, respectively, and expressed under induction with isopropyl thiogalactoside (IPTG). The chemical synthesis of Ba1 was performed in an Applied Biosystems 433A peptide synthesizer.

Results

Both expression systems pQE40 and pET28a⁺ expressed the His-tagged recombinant protein products, HisrDFHRBa1 and HisrBa1, respectively, as inclusion bodies. The recombinant proteins HisrDFHRBa1 and HisrBa1 presented respective molecular masses of 28,289 and 8274.6 Da, and were not biologically active. These results suggested that both HisrDFHRBa1 and HisrBa1 were oxidized after cell extraction, and that their insecticidal activities were affected by their N-terminal pro-peptides and different disulfide bridge arrangements. The respective protein expression yields for HisrDFHRBa1 and HisrBa1 were 100 μg/L and 900 μg/L of culture medium. HisrBa1 was reduced and folded under in vitro conditions. The in vitro folding of HisrBa1 produced several isoforms, one of which, after removing its N-terminal pro-peptide by enzymatic cleavage, presented elevated insecticidal activities compared to the native Ba1. Furthermore, the His-tagged protein HisrDFHRBa1 underwent enzymatic cleavage to obtain recombinant Ba1 (rBa1). As expected, the molecular mass of rBa1 was 4406.4 Da. On the other hand, Ba1 was chemically synthesized (sBa1) with a yield of 11 mg per 0.1 mmol of amino acid assembly.

Conclusions

The two recombinant insecticidal peptides and the one synthesized chemically were as active as the native Ba1; however, toxin yields differed drastically.

Electrophysiological characterization of the first Tityus serrulatus alpha-like toxin, Ts5: Evidence of a pro-inflammatory toxin on macrophages

Tityus serrulatus (Ts) venom is composed of mainly neurotoxins specific for voltage-gated K(+) and Na(+) channels, which are expressed in many cells such as macrophages. Macrophages are the first line of defense invasion and they participate in the inflammatory response of Ts envenoming. However, little is known about the effect of Ts toxins on macrophage activation. This study investigated the effect of Ts5 toxin on different sodium channels as well as its role on the macrophage immunomodulation. The electrophysiological assays showed that Ts5 inhibits the rapid inactivation of the mammalian sodium channels Nav1.2, Nav1.3, Nav1.4, Nav1.5, Nav1.6 and Nav1.7. Interestingly, Ts5 also inhibits the inactivation of the insect Drosophila melanogaster sodium channel (DmNav1), and it is therefore classified as the first Ts α-like toxin. The immunological experiments on macrophages reveal that Ts5 is a pro-inflammatory toxin inducing the cytokine production of tumor necrosis factor (TNF)-α and interleukin (IL)-6. On the basis of recent literature, our study also stresses a possible mechanism responsible for venom-associated molecular patterns (VAMPs) internalization and macrophage activation and moreover we suggest two main pathways of VAMPs signaling: direct and indirect. This work provides useful insights for a better understanding of the involvement of VAMPs in macrophage modulation.

MmTX1 and MmTX2 from coral snake venom potently modulate GABAA receptor activity

GABAA receptors shape synaptic transmission by modulating Cl(-) conductance across the cell membrane. Remarkably, animal toxins that specifically target GABAA receptors have not been identified. Here, we report the discovery of micrurotoxin1 (MmTX1) and MmTX2, two toxins present in Costa Rican coral snake venom that tightly bind to GABAA receptors at subnanomolar concentrations. Studies with recombinant and synthetic toxin variants on hippocampal neurons and cells expressing common receptor compositions suggest that MmTX1 and MmTX2 allosterically increase GABAA receptor susceptibility to agonist, thereby potentiating receptor opening as well as desensitization, possibly by interacting with the α(+)/β(-) interface. Moreover, hippocampal neuron excitability measurements reveal toxin-induced transitory network inhibition, followed by an increase in spontaneous activity. In concert, toxin injections into mouse brain result in reduced basal activity between intense seizures. Altogether, we characterized two animal toxins that enhance GABAA receptor sensitivity to agonist, thereby establishing a previously unidentified class of tools to study this receptor family.

Clinical comparison of scorpion envenomation by Androctonus mauritanicus and Buthus occitanus in children

The clinical results of scorpion stings by Androctonus mauritanicus (Am) and Buthus occitanus (Bo) (main sources of scorpionism in Morocco) were evaluated in this work. The objective was to compare the clinical manifestations of envenoming from these species by investigating possible correlations among symptoms/signs and laboratory abnormalities of envenomed patients. 41 children (25 males, 18 months - 11 years) were admitted at the Provincial Hospital of El Jadida-Morocco. Their minor (18 children) or severe (23 children) systemic signs such as pallor (48.8%), pulmonary edema (APE) (36.6%), convulsion (26.8%), coma (7.3%) were more frequent in children envenomed by Am than Bo, but angioedema (Quincke's edema) (4.9%) was particularly developed in the latter group. The laboratory blood abnormalities (hyperglycemia, high levels of aspartate aminotransferase (AST), lactate dehydrogenase (LDH), creatinine, bilirubin, leukocytes, neutrophils, monocytes, platelets and low levels of lymphocytes and hemoglobin) were significantly higher (p < 0.05) in patients envenomed by Am than Bo, and in all population in comparison to control group. The correlation among these biological analyzes and clinical status showed that higher levels of LDH and value of leukocytes ≥19 × 10(3)/mm(3) were indices of cardiac dysfunction with APE. Pallor sign was correlated with a state of shock and/or low level of hemoglobin, associated or not to bilirubin increase. Fatalities (7.3%), presenting toxic myocarditis, had lowest count of lymphocytes (≤4.2%) in comparison to survivors. This is the first report on lymphopenia which may be useful for forecast the fatal outcome in scorpion envenomation.

A first exploration of the venom of the Buthus occitanus scorpion found in southern France

Even though Buthus occitanus scorpions are found throughout the Mediterranean region, a lack of distinctive characteristics has hampered their classification into different subspecies. Yet, stings from this particular scorpion family are reported each year to result in pain followed by various toxic symptoms. In order to determine the toxicity origin of the rare French B. occitanus Amoreux scorpion, we collected several specimens and studied their venom composition using a nano ultra high performance liquid chromatography and matrix assisted laser desorption/ionisation time-of-flight mass spectrometry (nano UHPLC/MALDI-TOF-MS) automated workflow combined with an enzyme-linked immunosorbent assay (ELISA) approach. Moreover, we compared this dataset to that obtained from highly lethal Androctonus australis and Androctonus mauretanicus scorpions collected in North Africa. As a result, we found that the B. occitanus Amoreux venom is toxic to mice, an observation that is most likely caused by venom components that inhibit voltage-gated sodium channel inactivation. Moreover, we identified similarities in venom composition between B. occitanus scorpions living in the South of France and other Buthidae collected in Morocco and Algeria. As such, the results of this study should be taken into consideration when treating stings from the B. occitanus species living in the South of France.

Two recombinant α-like scorpion toxins from Mesobuthus eupeus with differential affinity toward insect and mammalian Na(+) channels

α-Scorpion toxins are modulators of voltage-gated Na(+) channels (Navs), which bind to the receptor site 3 to inhibit the fast inactivation of the channels. MeuNaTxα-12 and MeuNaTxα-13 are two new α-scorpion toxin-like peptides identified by cDNA cloning from the scorpion Mesobuthus eupeus with unknown functions. Here, we report their recombinant production, oxidative refolding, structural and functional features. By in vitro renaturation from bacterial inclusion bodies and further purification through reverse phase high-performance liquid chromatography, we obtained high purity recombinant products with a native-like conformation identified by circular dichroism analysis. Two-electrode voltage clamp recordings on five cloned mammalian Nav subtypes (rNav1.1, rNav1.2, rNav1.4, rNav1.5, and mNav1.6) and the insect counterpart DmNav1, all expressed in Xenopus laevis oocytes, showed that these two peptides inhibited rapid inactivation of the sensitive Na(+) channels with significant preference for DmNav1. The half maximal effective concentrations (EC50) of MeuNaTxα-12 and MeuNaTxα-13 for this channel are 19.95 ± 2.99 nM and 65.50 ± 7.28 nM, respectively, showing 45 and 38 folds higher affinities than for rNav1.1, the most sensitive mammalian channel among the five isoforms. Our functional data confirms that these two peptides belong to the α-like scorpion toxin group. A combined analysis of the site 3 sequences and the pharmacological data illuminates the importance of the loop LD4:S5-S6 of the channel in interacting with the toxins whereas affinity variations between MeuNaTxα-12 and MeuNaTxα-13 highlight a key functional role of a cationic side chain at position 28 of MeuNaTxα-12. Successful expression together with structural and functional characterization of these two new α-like scorpion toxins lays basis for further studies of their structure-function relationship.

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.

Neurotoxins and their binding areas on voltage-gated sodium channels

Voltage-gated sodium channels (VGSCs) are large transmembrane proteins that conduct sodium ions across the membrane and by doing so they generate signals of communication between many kinds of tissues. They are responsible for the generation and propagation of action potentials in excitable cells, in close collaboration with other channels like potassium channels. Therefore, genetic defects in sodium channel genes can cause a wide variety of diseases, generally called “channelopathies.” The first insights into the mechanism of action potentials and the involvement of sodium channels originated from Hodgkin and Huxley for which they were awarded the Nobel Prize in 1963. These concepts still form the basis for understanding the function of VGSCs. When VGSCs sense a sufficient change in membrane potential, they are activated and consequently generate a massive influx of sodium ions. Immediately after, channels will start to inactivate and currents decrease. In the inactivated state, channels stay refractory for new stimuli and they must return to the closed state before being susceptible to a new depolarization. On the other hand, studies with neurotoxins like tetrodotoxin (TTX) and saxitoxin (STX) also contributed largely to our today’s understanding of the structure and function of ion channels and of VGSCs specifically. Moreover, neurotoxins acting on ion channels turned out to be valuable lead compounds in the development of new drugs for the enormous range of diseases in which ion channels are involved. A recent example of a synthetic neurotoxin that made it to the market is ziconotide (Prialt^®, Elan). The original peptide, ω-MVIIA, is derived from the cone snail Conus magus and now FDA/EMA-approved for the management of severe chronic pain by blocking the N-type voltage-gated calcium channels in pain fibers. This review focuses on the current status of research on neurotoxins acting on VGSC, their contribution to further unravel the structure and function of VGSC and their potential as novel lead compounds in drug development.

The sea anemone Bunodosoma caissarum toxin BcIII modulates the sodium current kinetics of rat dorsal root ganglia neurons and is displaced in a voltage-dependent manner

Sea anemone toxins bind to site 3 of the sodium channels, which is partially formed by the extracellular linker connecting S3 and S4 segments of domain IV, slowing down the inactivation process. In this work we have characterized the actions of BcIII, a sea anemone polypeptide toxin isolated from Bunodosoma caissarum, on neuronal sodium currents using the patch clamp technique. Neurons of the dorsal root ganglia of Wistar rats (P5-9) in primary culture were used for this study (n=65). The main effects of BcIII were a concentration-dependent increase in the sodium current inactivation time course (IC(50)=2.8 microM) as well as an increase in the current peak amplitude. BcIII did not modify the voltage at which 50% of the channels are activated or inactivated, nor the reversal potential of sodium current. BcIII shows a voltage-dependent action. A progressive acceleration of sodium current fast inactivation with longer conditioning pulses was observed, which was steeper as more depolarizing were the prepulses. The same was observed for other two anemone toxins (CgNa, from Condylactis gigantea and ATX-II, from Anemonia viridis). These results suggest that the binding affinity of sea anemone toxins may be reduced in a voltage-dependent manner, as has been described for alpha-scorpion toxins.

Sea anemone toxins affecting voltage-gated sodium channels--molecular and evolutionary features

The venom of sea anemones is rich in low molecular weight proteinaceous neurotoxins that vary greatly in structure, site of action, and phyletic (insect, crustacean or vertebrate) preference. This toxic versatility likely contributes to the ability of these sessile animals to inhabit marine environments co-habited by a variety of mobile predators. Among these toxins, those that show prominent activity at voltage-gated sodium channels and are critical in predation and defense, have been extensively studied for more than three decades. These studies initially focused on the discovery of new toxins, determination of their covalent and folded structures, understanding of their mechanisms of action on different sodium channels, and identification of the primary sites of interaction of the toxins with their channel receptors. The channel binding site for Type I and the structurally unrelated Type III sea anemone toxins was identified as neurotoxin receptor site 3, a site previously shown to be targeted by scorpion alpha-toxins. The bioactive surfaces of toxin representatives from these two sea anemone types have been characterized by mutagenesis. These analyses pointed to heterogeneity of receptor site 3 at various sodium channels. A turning point in evolutionary studies of sea anemone toxins was the recent release of the genome sequence of Nematostella vectensis, which enabled analysis of the genomic organization of the corresponding genes. This analysis demonstrated that Type I toxins in Nematostella and other species are encoded by gene families and suggested that these genes developed by concerted evolution. The current review provides a brief historical description of the discovery and characterization of sea anemone toxins that affect voltage-gated sodium channels and delineates recent advances in the study of their structure-activity relationship and evolution.

Effects of BmKNJX11, a bioactive polypeptide purified from Buthus martensi Karsch, on sodium channels in rat dorsal root ganglion neurons

A long-chain polypeptide BmKNJX11 was purified from the venom of Asian scorpion Buthus martensi Karsch (BmK) by a combination of gel filtration, ion-exchange chromatography, and reverse-phase high-performance liquid chromatography. The molecular mass was found to be 7036.85 Da by electrospray ionization mass spectrometry. The first 15 N-terminal amino acid sequence of BmKNJX11 was determined to be GRDAY IADSE NCTYT by Edman degradation. With whole cell recording, BmKNJX11 inhibited tetrodotoxin-sensitive voltage-gated sodium channels (TTX-S VGSC) in freshly isolated rat dorsal root ganglion (DRG) neurons in a concentration- and voltage-dependent manner. At a concentration of 40 mug/ml BmKNJX11 lowered the activation threshold and produced negative shifting of TTX-S sodium current (I(Na)) activation curve. In addition, BmKNJX11 induced shifting of the steady-state inactivation curve to the left, delayed the recovery of TTX-S I(Na) from inactivation, and also reduced the fraction of available sodium channels. These results suggested that BmKNJX11 might exert effects on VGSC by binding to a specific site. Considering that TTX-S VGSC expressed in DRG neurons play a critical role in nociceptive transmission, the interaction of BmKNJX11 with TTX-S VGSC might lead to a change in excitability of nociceptive afferent fibers, which may be involved in the observed peripheral pain expression.

Miniaturization of scorpion beta-toxins uncovers a putative ancestral surface of interaction with voltage-gated sodium channels

The bioactive surface of scorpion beta-toxins that interact with receptor site-4 at voltage-gated sodium channels is constituted of residues of the conserved betaalphabetabeta core and the C-tail. In an attempt to evaluate the extent by which residues of the toxin core contribute to bioactivity, the anti-insect and anti-mammalian beta-toxins Bj-xtrIT and Css4 were truncated at their N and C termini, resulting in miniature peptides composed essentially of the core secondary structure motives. The truncated beta-toxins (DeltaDeltaBj-xtrIT and DeltaDeltaCss4) were non-toxic and did not compete with the parental toxins on binding at receptor site-4. Surprisingly, DeltaDeltaBj-xtrIT and DeltaDeltaCss4 were capable of modulating in an allosteric manner the binding and effects of site-3 scorpion alpha-toxins in a way reminiscent of that of brevetoxins, which bind at receptor site-5. While reducing the binding and effect of the scorpion alpha-toxin Lqh2 at mammalian sodium channels, they enhanced the binding and effect of LqhalphaIT at insect sodium channels. Co-application of DeltaDeltaBj-xtrIT or DeltaDeltaCss4 with brevetoxin abolished the brevetoxin effect, although they did not compete in binding. These results denote a novel surface at DeltaDeltaBj-xtrIT and DeltaDeltaCss4 capable of interaction with sodium channels at a site other than sites 3, 4, or 5, which prior to the truncation was masked by the bioactive surface that interacts with receptor site-4. The disclosure of this hidden surface at both beta-toxins may be viewed as an exercise in "reverse evolution," providing a clue as to their evolution from a smaller ancestor of similar scaffold.

Immunological characterization of a non-toxic peptide conferring protection against the toxic fraction (AahG50) of the Androctonus australis hector venom

KAaH1 and KAaH2 are non-toxic peptides, isolated from the venom of the Androctonus australis hector (Aah) scorpion. In a previous study, we showed these peptides to be the most abundant (approximately 10% each) in the toxic fraction (AahG50) of the Aah venom. KAaH1 and KAaH2 showed high sequence identities (approximately 60%) with birtoxin-like peptides, which likewise are the major peptidic components of Parabuthus transvaalicus scorpion venom. Here, we report the immunological characterization of KAaH1 and KAaH2. These peptides were found to be specifically recognized by polyclonal antibodies raised against AahII, the most toxic peptide of Aah venom, and represents the second antigenic group, including toxins from different scorpion species in the world. Moreover, KAaH1 partially inhibits AahII binding to its specific antibody, suggesting some common epitopes between these two peptides. The identification of possible key antigenic residues in KAaH1 was deduced from comparison of its 3-D model with the experimental structure of AahII. Two clusters of putative antigenically important residues were found at the exposed surface; one could be constituted of V3 and D53, the other of D10, T15 and Y16. Polyclonal antibodies raised against KAaH1 in mice were found to cross-react with both AahII and AahG50, and neutralizing 5LD(50)/ml of the toxic fraction. Mice vaccinated with KAaH1 were protected against a challenge of 2LD(50) of AahG50 fraction. All these data suggest that KAaH1 has clear advantages over the use of the whole or part of the venom. KAaH1 is not toxic and could produce sera-neutralizing scorpion toxins, not only from Aah venom, but also toxins of other venoms from Buthus, Leiurus, or Parabuthus scorpion species presenting antigenically related toxins.

Mammalian Skeletal Muscle Voltage-Gated Sodium Channels Are Affected by Scorpion Depressant “Insect-Selective” Toxins when Preconditioned

Among scorpion beta- and alpha-toxins that modify the activation and inactivation of voltage-gated sodium channels (Na(v)s), depressant beta-toxins have traditionally been classified as anti-insect selective on the basis of toxicity assays and lack of binding and effect on mammalian Na(v)s. Here we show that the depressant beta-toxins LqhIT2 and Lqh-dprIT3 from Leiurus quinquestriatus hebraeus (Lqh) bind with nanomolar affinity to receptor site 4 on rat skeletal muscle Na(v)s, but their effect on the gating properties can be viewed only after channel preconditioning, such as that rendered by a long depolarizing prepulse. This observation explains the lack of toxicity when depressant toxins are injected in mice. However, when the muscle channel rNa(v)1.4, expressed in Xenopus laevis oocytes, was modulated by the site 3 alpha-toxin LqhalphaIT, LqhIT2 was capable of inducing a negative shift in the voltage-dependence of activation after a short prepulse, as was shown for other beta-toxins. These unprecedented results suggest that depressant toxins may have a toxic impact on mammals in the context of the complete scorpion venom. To assess whether LqhIT2 and Lqh-dprIT3 interact with the insect and rat muscle channels in a similar manner, we examined the role of Glu24, a conserved "hot spot" at the bioactive surface of beta-toxins. Whereas substitutions E24A/N abolished the activity of both LqhIT2 and Lqh-dprIT3 at insect Na(v)s, they increased the affinity of the toxins for rat skeletal muscle channels. This result implies that depressant toxins interact differently with the two channel types and that substitution of Glu24 is essential for converting toxin selectivity.

Development of a bacterial cloning vector for expression of scorpion toxins for biotechnological studies

Scorpion venoms contain toxic peptides that recognize K(+) channels of excitable and non-excitable cells. These toxins comprise three structurally distinct groups designated alpha-KTx, beta-KTx, and gamma-KTx. It is highly desirable to develop systems for the expression of these toxins for further physiological and structural studies. In this work, an expression vector (pTEV3) was constructed by inserting protein D (major capsid of phage lambda) and TEV protease recognition site into plasmid pET21d DNA sequences. Three alpha-KTx toxins (OsK2, PbTx1, and BmKK3) were cloned into vector pTEV3 and expressed as soluble fusion proteins. The fractions containing the purified fusion proteins (protein D-toxin) were treated with TEV protease to remove protein D. The resulting toxins were analyzed by MALDI-TOF Mass Spectrometry. The results showed that the vector is appropriate for the expression of the target toxins in soluble form and that ion exchange purification of these toxins by flow-through recovery is possible. Analysis by MALDI-TOF Mass Spectrometry of Osk2 demonstrated that this toxin was expressed in its native form, as suggested by the values expected for the presence of two disulfide bridges.

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.

Sea anemone venom as a source of insecticidal peptides acting on voltage-gated Na+ channels

Sea anemones produce a myriad of toxic peptides and proteins of which a large group acts on voltage-gated Na+ channels. However, in comparison to other organisms, their venoms and toxins are poorly studied. Most of the known voltage-gated Na+ channel toxins isolated from sea anemone venoms act on neurotoxin receptor site 3 and inhibit the inactivation of these channels. Furthermore, it seems that most of these toxins have a distinct preference for crustaceans. Given the close evolutionary relationship between crustaceans and insects, it is not surprising that sea anemone toxins also profoundly affect insect voltage-gated Na+ channels, which constitutes the scope of this review. For this reason, these peptides can be considered as insecticidal lead compounds in the development of insecticides.

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.

Insect-selective spider toxins targeting voltage-gated sodium channels

The voltage-gated sodium (Na(v)) channel is a target for a number of drugs, insecticides and neurotoxins. These bind to at least seven identified neurotoxin binding sites and either block conductance or modulate Na(v) channel gating. A number of peptide neurotoxins from the venoms of araneomorph and mygalomorph spiders have been isolated and characterized and determined to interact with several of these sites. These all conform to an 'inhibitor cystine-knot' motif with structural, but not sequence homology, to a variety of other spider and marine snail toxins. Of these, spider toxins several show phyla-specificity and are being considered as lead compounds for the development of biopesticides. Hainantoxin-I appears to target site-1 to block Na(v) channel conductance. Magi 2 and Tx4(6-1) slow Na(v) channel inactivation via an interaction with site-3. The delta-palutoxins, and most likely mu-agatoxins and curtatoxins, target site-4. However, their action is complex with the mu-agatoxins causing a hyperpolarizing shift in the voltage-dependence of activation, an action analogous to scorpion beta-toxins, but with both delta-palutoxins and mu-agatoxins slowing Na(v) channel inactivation, a site-3-like action. In addition, several other spider neurotoxins, such as delta-atracotoxins, are known to target both insect and vertebrate Na(v) channels most likely as a result of the conserved structures within domains of voltage-gated ion channels across phyla. These toxins may provide tools to establish the molecular determinants of invertebrate selectivity. These studies are being greatly assisted by the determination of the pharmacophore of these toxins, but without precise identification of their binding site and mode of action their potential in the above areas remains underdeveloped.

Effects of Tityus serrulatus scorpion venom and its toxin TsTX-V on neurotransmitter uptake in vitro

Scorpion neurotoxins targeting the Na(v) channel can be classified into two classes: alpha- and beta-neurotoxins and are reported as highly active in mammalian brain. In this work, we evaluate the effects of Tityus serrulatus venom (Ts venom) and its alpha-neurotoxin TsTX-V on gamma-aminobutyric acid (GABA), dopamine (DA) and glutamate (Glu) uptake in isolated rat brain synaptosomes. TsTX-V was isolated from Ts venom by ion exchange chromatography followed by reverse-phase (C18) high-performance liquid chromatography. Neither Ts venom nor TsTX-V was able to affect (3)H-Glu uptake. On the other hand, Ts venom (0.13 microg/mg) significantly inhibited both (3)H-GABA and (3)H-DA uptake ( approximately 50%). TsTX-V showed IC(50) values of 9.37 microM and 22.2 microM for the inhibition of (3)H-GABA and (3)H-DA uptake, respectively. These effects were abolished by pre-treatment with tetrodotoxin (TTX, 1 microM), indicating the involvement of voltage-gated Na(+) channels in this process. In the absence of Ca(2+), and at low Ts venom concentrations, the reduction of (3)H-GABA uptake was not as marked as in the presence of Ca(2+). TsTX-V did not reduce (3)H-GABA uptake in COS-7 cells expressing the GABA transporters GAT-1 and GAT-3, suggesting that this toxin indirectly reduces the transport. The reduced (3)H-GABA uptake by synaptosomes might be due to rapid cell depolarization as revealed by confocal microscopy of C6 glioma cells. Thus, TsTX-V causes a reduction of (3)H-GABA and (3)H-DA uptake in a Ca(2+)-dependent manner, not directly affecting GABA transporters, but, in consequence of depolarization, involving voltage-gated Na(+) channels.

Chinese-scorpion (Buthus martensi Karsch) toxin BmK alphaIV, a novel modulator of sodium channels: from genomic organization to functional analysis

In the present study, BmK alphaIV, a novel modulator of sodium channels, was cloned from venomous glands of the Chinese scorpion (Buthus martensi Karsch) and expressed successfully in Escherichia coli. The BmK alphaIV gene is composed of two exons separated by a 503 bp intron. The mature polypeptide contains 66 amino acids. BmK alphaIV has potent toxicity in mice and cockroaches. Surface-plasmon-resonance analysis found that BmK alphaIV could bind to both rat cerebrocortical synaptosomes and cockroach neuronal membranes, and shared similar binding sites on sodium channels with classical AaH II (alpha-mammal neurotoxin from the scorpion Androctonus australis Hector), BmK AS (beta-like neurotoxin), BmK IT2 (the depressant insect-selective neurotoxin) and BmK abT (transitional neurotoxin), but not with BmK I (alpha-like neurotoxin). Two-electrode voltage clamp recordings on rNav1.2 channels expressed in Xenopus laevis oocytes revealed that BmK alphaIV increased the peak amplitude and prolonged the inactivation phase of Na+ currents. The structural and pharmacological properties compared with those of other scorpion alpha-toxins suggests that BmK alphaIV represents a novel subgroup or functional hybrid of alpha-toxins and might be an evolutionary intermediate neurotoxin for alpha-toxins.

Differential effects of five 'classical' scorpion beta-toxins on rNav1.2a and DmNav1 provide clues on species-selectivity

In general, scorpion beta-toxins have been well examined. However, few in-depth studies have been devoted to species selectivity and affinity comparisons on the different voltage-activated Na(+) channels since they have become available as cloned channels that can be studied in heterologous expression systems. As a result, their classification is largely historical and dates from early in vivo experiments on mice and cockroach and fly larvae. In this study, we aimed to provide an updated overview of selectivity and affinity of scorpion beta-toxins towards voltage-activated Na(+) channels of vertebrates or invertebrates. As pharmacological tools, we used the classic beta-toxins AaHIT, Css II, Css IV, Css VI and Ts VII and tested them on the neuronal vertebrate voltage-activated Na(+) channel, rNa(v)1.2a. For comparison, its invertebrate counterpart, DmNav1, was also tested. Both these channels were expressed in Xenopus laevis oocytes and the currents measured with the two-electrode voltage-clamp technique. We supplemented this data with several binding displacement studies on rat brain synaptosomes. The results lead us to propose a general classification and a novel nomenclature of scorpion beta-toxins based on pharmacological activity.

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'.