Domain 2 of Drosophila para voltage-gated sodium channel confers insect properties to a rat brain channel

Toxins That Affect Voltage-Gated Sodium Channels

Voltage-gated sodium channels (VGSCs) are critical in generation and conduction of electrical signals in multiple excitable tissues. Natural toxins, produced by animal, plant, and microorganisms, target VGSCs through diverse strategies developed over millions of years of evolutions. Studying of the diverse interaction between VGSC and VGSC-targeting toxins has been contributing to the increasing understanding of molecular structure and function, pharmacology, and drug development potential of VGSCs. This chapter aims to summarize some of the current views on the VGSC-toxin interaction based on the established receptor sites of VGSC for natural toxins.

The specificity of Av3 sea anemone toxin for arthropods is determined at linker DI/SS2-S6 in the pore module of target sodium channels

Av3 is a peptide neurotoxin from the sea anemone Anemonia viridis that shows specificity for arthropod voltage-gated sodium channels (Navs). Interestingly, Av3 competes with a scorpion α-toxin on binding to insect Navs and similarly inhibits the inactivation process, and thus has been classified as 'receptor site-3 toxin', although the two peptides are structurally unrelated. This raises questions as to commonalities and differences in the way both toxins interact with Navs. Recently, site-3 was partly resolved for scorpion α-toxins highlighting S1-S2 and S3-S4 external linkers at the DIV voltage-sensor module and the juxtaposed external linkers at the DI pore module. To uncover channel determinants involved in Av3 specificity for arthropods, the toxin was examined on channel chimaeras constructed with the external linkers of the mammalian brain Nav1.2a, which is insensitive to Av3, in the background of the Drosophila DmNav1. This approach highlighted the role of linker DI/SS2-S6, adjacent to the channel pore, in determining Av3 specificity. Point mutagenesis at DI/SS2-S6 accompanied by functional assays highlighted Trp404 and His405 as a putative point of Av3 interaction with DmNav1. His405 conservation in arthropod Navs compared with tyrosine in vertebrate Navs may represent an ancient substitution that explains the contemporary selectivity of Av3. Trp404 and His405 localization near the membrane surface and the hydrophobic bioactive surface of Av3 suggest that the toxin possibly binds at a cleft by DI/S6. A partial overlap in receptor site-3 of both toxins nearby DI/S6 may explain their binding competition capabilities.

Pharmacological kinetics of BmK AS, a sodium channel site 4-specific modulator on Nav1.3

OBJECTIVE

In this study, the pharmacological kinetics of Buthus martensi Karsch (BmK) AS, a specific modulator of voltage-gated sodium channel site 4, was investigated on Na(v)1.3 expressed in Xenopus oocytes.

METHODS

Two-electrode voltage clamp was used to record the whole-cell sodium current.

RESULTS

The peak currents of Na(v)1.3 were depressed by BmK AS over a wide range of concentrations (10, 100, and 500 nmol/L). Most remarkably, BmK AS at 100 nmol/L hyperpolarized the voltage-dependence and increased the voltage-sensitivity of steady-state activation/inactivation. In addition, BmK AS was capable of hyperpolarizing not only the fast inactivation but also the slow inactivation, with a greater preference for the latter. Moreover, BmK AS accelerated the time constant and increased the ratio of recovery in Na(v)1.3 at all concentrations.

CONCLUSION

This study provides direct evidence that BmK AS facilitates steady-state activation and inhibits slow inactivation by stabilizing both the closed and open states of the Na(v)1.3 channel, which might result from an integrative binding to two receptor sites on the voltage-gated sodium channels. These results may shed light on therapeutics against Na(v)1.3-targeted pathology.

The insecticidal potential of venom peptides

Pest insect species are a burden to humans as they destroy crops and serve as vectors for a wide range of diseases including malaria and dengue. Chemical insecticides are currently the dominant approach for combating these pests. However, the de-registration of key classes of chemical insecticides due to their perceived ecological and human health risks in combination with the development of insecticide resistance in many pest insect populations has created an urgent need for improved methods of insect pest control. The venoms of arthropod predators such as spiders and scorpions are a promising source of novel insecticidal peptides that often have different modes of action to extant chemical insecticides. These peptides have been optimized via a prey-predator arms race spanning hundreds of millions of years to target specific types of insect ion channels and receptors. Here we review the current literature on insecticidal venom peptides, with a particular focus on their structural and pharmacological diversity, and discuss their potential for deployment as insecticides.

Convergent evolution of sodium ion selectivity in metazoan neuronal signaling

Ion selectivity of metazoan voltage-gated Na⁺ channels is critical for neuronal signaling and has long been attributed to a ring of four conserved amino acids that constitute the ion selectivity filter (SF) at the channel pore. Yet, in addition to channels with a preference for Ca²⁺ ions, the expression and characterization of Na⁺ channel homologs from the sea anemone Nematostella vectensis, a member of the early-branching metazoan phylum Cnidaria, revealed a sodium-selective channel bearing a noncanonical SF. Mutagenesis and physiological assays suggest that pore elements additional to the SF determine the preference for Na⁺ in this channel. Phylogenetic analysis assigns the Nematostella Na⁺-selective channel to a channel group unique to Cnidaria, which diverged >540 million years ago from Ca²⁺-conducting Na⁺ channel homologs. The identification of Cnidarian Na⁺-selective ion channels distinct from the channels of bilaterian animals indicates that selectivity for Na⁺ in neuronal signaling emerged independently in these two animal lineages.

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.

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

BACKGROUND

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

PRINCIPAL FINDINGS

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

CONCLUSIONS/SIGNIFICANCE

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

Molecular determination of selectivity of the site 3 modulator (BmK I) to sodium channels in the CNS: a clue to the importance of Nav1.6 in BmK I-induced neuronal hyperexcitability

BmK I, a site-3-specific modulator of VGSCs (voltage-gated sodium channels) from the Chinese scorpion Buthus martensi Karsch, can induce spontaneous nociception and hyperalgesia and generate epileptiform responses in rats, which is attributed to the modulation of VGSCs in the neural system. However, which VGSC subtype is targeted by BmK I remains to be identified. Using two-electrode voltage-clamp recording, we studied the efficacy and selectivity of BmK I to three neuronal VGSCs co-expressed with the auxiliary β1 subunit in Xenopus oocytes. Results revealed that BmK I induced a large increase in both transient and persistent currents in mNav1.6α/β1 (where m indicates mouse), which correlated with a prominent reduction in the fast component of inactivating current. In comparison, BmK I-increased currents of rNav1.2α/β1 (where r indicates rat) and rNav1.3α/β1 were much smaller. The EC50 values of BmK I for rNav1.2α/β1 (252±60 nM) and mNav1.6α/β1 (214±30 nM) were similar and roughly half of that for rNav1.3α/β1 (565±16 nM). Moreover, BmK I only accelerated the slow inactivation development and delay recovery of mNav1.6α/β1 through binding to the channel in the open state. Residue-swap analysis verified that an acidic residue (e.g. Asp1602 in mNav1.6) within the domain IV S3-S4 extracellular loop of VGSCs was crucial for the selectivity and modulation pattern of BmK I. Our findings thus provide the molecular determinant explaining the divergent and intriguing behaviour of neuronal VGSCs in response to site-3-specific modulators, indicating that these subtypes play different roles in BmK I-induced hyperexcitablity in rat models.

Scorpion venom and the inflammatory response

Scorpion venoms consist of a complex of several toxins that exhibit a wide range of biological properties and actions, as well as chemical compositions, toxicity, and pharmacokinetic and pharmacodynamic characteristics. These venoms are associated with high morbility and mortality, especially among children. Victims of envenoming by a scorpion suffer a variety of pathologies, involving mainly both sympathetic and parasympathetic stimulation as well as central manifestations such as irritability, hyperthermia, vomiting, profuse salivation, tremor, and convulsion. The clinical signs and symptoms observed in humans and experimental animals are related with an excessive systemic host inflammatory response to stings and stings, respectively. Although the pathophysiology of envenomation is complex and not yet fully understood, venom and immune responses are known to trigger the release of inflammatory mediators that are largely mediated by cytokines. In models of severe systemic inflammation produced by injection of high doses of venom or venoms products, the increase in production of proinflammatory cytokines significantly contributes to immunological imbalance, multiple organ dysfunction and death. The cytokines initiate a cascade of events that lead to illness behaviors such as fever, anorexia, and also physiological events in the host such as activation of vasodilatation, hypotension, and increased of vessel permeability.

Voltage-gated sodium channel isoform-specific effects of pompilidotoxins

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

Fusion and retrotransposition events in the evolution of the sea anemone Anemonia viridis neurotoxin genes

Sea anemones are sessile predators that use a variety of toxins to paralyze prey and foe. Among these toxins, Types I, II and III are short peptides that affect voltage-gated sodium channels. Anemonia viridis is the only sea anemone species that produces both Types I and III neurotoxin. Although the two toxin types are unrelated in sequence and three-dimensional structure, cloning and comparative analysis of their loci revealed a highly similar sequence at the 5' region, which encodes a signal peptide. This similarity was likely generated by gene fusion and could be advantageous in transcript stability and intracellular trafficking and secretion. In addition, these analyses identified the processed pseudogenes of the two gene families in the genome of A. viridis, probably resulting from retrotransposition events. As presence of processed pseudogenes in the genome requires transcription in germ-line cells, we analyzed oocyte-rich ovaries and found that indeed they contain Types I and III transcripts. This result raises questions regarding the role of toxin transcripts in these tissues. Overall, the retrotransposition and gene fusion events suggest that the genes of both Types I and III neurotoxins evolved in a similar fashion and share a partial common ancestry.

Drosomycin, an innate immunity peptide of Drosophila melanogaster, interacts with the fly voltage-gated sodium channel

Several peptide families, including insect antimicrobial peptides, plant protease inhibitors, and ion channel gating modifiers, as well as blockers from scorpions, bear a common CSalphabeta scaffold. The high structural similarity between two peptides containing this scaffold, drosomycin and a truncated scorpion beta-toxin, has prompted us to examine and compare their biological effects. Drosomycin is the most expressed antimicrobial peptide in Drosophila melanogaster immune response. A truncated scorpion beta-toxin is capable of binding and inducing conformational alteration of voltage-gated sodium channels. Here, we show that both peptides (i) exhibit anti-fungal activity at micromolar concentrations; (ii) enhance allosterically at nanomolar concentration the activity of LqhalphaIT, a scorpion alpha toxin that modulates the inactivation of the D. melanogaster voltage-gated sodium channel (DmNa(v)1); and (iii) inhibit the facilitating effect of the polyether brevetoxin-2 on DmNa(v)1 activation. Thus, the short CSalphabeta scaffold of drosomycin and the truncated scorpion toxin can maintain more than one bioactivity, and, in light of this new observation, we suggest that the biological role of peptides bearing this scaffold should be carefully examined. As for drosomycin, we discuss the intriguing possibility that it has additional functions in the fly, as implied by its tight interaction with DmNa(v)1.

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.

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.

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.

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.

Design of a specific activator for skeletal muscle sodium channels uncovers channel architecture

Gating modifiers of voltage-gated sodium channels (Na(v)s) are important tools in neuroscience research and may have therapeutic potential in medicinal disorders. Analysis of the bioactive surface of the scorpion beta-toxin Css4 (from Centruroides suffusus suffusus) toward rat brain (rNa(v)1.2a) and skeletal muscle (rNa(v)1.4) channels using binding studies revealed commonality but also substantial differences, which were used to design a specific activator, Css4(F14A/E15A/E28R), of rNa(v)1.4 expressed in Xenopus oocytes. The therapeutic potential of Css4(F14A/E15A/E28R) was tested using an rNa(v)1.4 mutant carrying the same mutation present in the genetic disorder hypokalemic periodic paralysis. The activator restored the impaired gating properties of the mutant channel expressed in oocytes, thus offering a tentative new means for treatment of neuromuscular disorders with reduced muscle excitability. Mutant double cycle analysis employing toxin residues involved in the construction of Css4(F14A/E15A/E28R) and residues whose equivalents in the rat brain channel rNa(v)1.2a were shown to affect Css4 binding revealed significant coupling energy (>1.3 kcal/mol) between F14A and E592A at Domain-2/voltage sensor segments 1-2 (D2/S1-S2), R27Q and E1251N at D3/SS2-S6, and E28R with both E650A at D2/S3-S4 and E1251N at D3/SS2-S6. These results show that despite the differences in interactions with the rat brain and skeletal muscle Na(v)s, Css4 recognizes a similar region on both channel subtypes. Moreover, our data indicate that the S3-S4 loop of the voltage sensor module in Domain-2 is in very close proximity to the SS2-S6 segment of the pore module of Domain-3 in rNa(v)1.4. This is the first experimental evidence that the inter-domain spatial organization of mammalian Na(v)s resembles that of voltage-gated potassium channels.

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.

The insecticidal potential of scorpion β-toxins

Voltage-gated sodium channels are a major target for toxins and insecticides due to their central role in excitability, but due to the conservation of these channels in Animalia most insecticides do not distinguish between those of insects and mammals, thereby imposing risks to humans and livestock. Evidently, as long as modern agriculture depends heavily on the use of insecticides there is a great need for new substances capable of differentiating between sodium channel subtypes. Such substances exist in venomous animals, but ways for their exploitation have not yet been developed due to problems associated with manufacturing, degradation, and delivery to the target channels. Engineering of plants for expression of anti-insect toxins or use of natural vectors that express toxins near their target site (e.g. baculoviruses) are still problematic and raise public concern. In this problematic reality a rational approach might be to learn from nature how to design highly selective anti-insect compounds preferably in the form of peptidomimetics. This is a complex task that requires the elucidation of the face of interaction between insect-selective toxins and their sodium channel receptor sites. This review delineates current progress in: (i) elucidation of the bioactive surfaces of scorpion beta-toxins, especially the excitatory and depressant groups, which show high preference for insects and bind insect sodium channels with high affinity; (ii) studies of the mode of interaction of scorpion beta-toxins with receptor site-4 on voltage-gated sodium channels; and (iii) clarification of channel elements that constitute receptor site-4. This information may be useful in future attempts to mimic the bioactive surface of the toxins for the design of anti-insect selective peptidomimetics.

Novel paradigms on scorpion toxins that affects the activating mechanism of sodium channels

Scorpion toxins classified as beta-class are reviewed using a new paradigm. Four distinct sub types are recognized: "classical", "Tsgamma-like", "excitatory" and "depressant"beta-scorpion toxins. Recent experimental data have made possible to identify the interacting interfaces of the Na(+) channel-receptor site 4 with some of these toxins. The voltage-sensor trapping mechanism proposed for the action of these toxic peptides is analyzed in the context of what causes a modification of the activating mechanism of Na(+) channels. A cartoon model is presented with the purpose of summarizing the most current knowledge on the field. Finally, the recent advances on the knowledge of the specific interactions of beta-toxins and different sub types of Na(+) channels are also reviewed.

Solution structure and alanine scan of a spider toxin that affects the activation of mammalian voltage-gated sodium channels

Magi 5, from the hexathelid spider Macrothele gigas, is a 29-residue polypeptide containing three disulfide bridges. It binds specifically to receptor site 4 on mammalian voltage-gated sodium channels and competes with scorpion beta-toxins, such as Css IV from Centruroides suffusus suffusus. As a consequence, Magi 5 shifts the activation voltage of the mammalian rNav1.2a channel to more hyperpolarized voltages, whereas the insect channel, DmNav1, is not affected. To gain insight into toxin-channel interactions, Magi 5 and 23 analogues were synthesized. The three-dimensional structure of Magi 5 in aqueous solution was determined, and its voltage-gated sodium channel-binding surfaces were mapped onto this structure using data from electrophysiological measurements on a series of Ala-substituted analogues. The structure clearly resembles the inhibitor cystine knot structural motif, although the triple-stranded beta-sheet typically found in that motif is partially distorted in Magi 5. The interactive surface of Magi 5 toward voltage-gated sodium channels resembles in some respects the Janus-faced atracotoxins, with functionally important charged residues on one face of the toxin and hydrophobic residues on the other. Magi 5 also resembles the scorpion beta-toxin Css IV, which has distinct nonpolar and charged surfaces that are critical for channel binding and has a key Glu involved in voltage sensor trapping. These two distinct classes of toxin, with different amino acid sequences and different structures, may utilize similar groups of residues on their surface to achieve the common end of modifying voltage-gated sodium channel function.

X-ray structure and mutagenesis of the scorpion depressant toxin LqhIT2 reveals key determinants crucial for activity and anti-insect selectivity

Scorpion depressant beta-toxins show high preference for insect voltage-gated sodium channels (Na(v)s) and modulate their activation. Although their pharmacological and physiological effects were described, their three-dimensional structure and bioactive surface have never been determined. We utilized an efficient system for expression of the depressant toxin LqhIT2 (from Leiurus quinquestriatushebraeus), mutagenized its entire exterior, and determined its X-ray structure at 1.2 A resolution. The toxin molecule is composed of a conserved cysteine-stabilized alpha/beta-core (core-globule), and perpendicular to it an entity constituted from the N and C-terminal regions (NC-globule). The surface topology and overall hydrophobicity of the groove between the core and NC-globules (N-groove) is important for toxin activity and plays a role in selectivity to insect Na(v)s. The N-groove is flanked by Glu24 and Tyr28, which belong to the "pharmacophore" of scorpion beta-toxins, and by the side-chains of Trp53 and Asn58 that are important for receptor site recognition. Substitution of Ala13 by Trp in the N-groove uncoupled activity from binding, suggesting that this region of the molecule is also involved in "voltage-sensor trapping", the mode of action that typifies scorpion beta-toxins. The involvement of the N-groove in recognition of the receptor site, which seems to require a defined topology, as well as in sensor trapping, which involves interaction with a moving channel region, is puzzling. On the basis of the mutagenesis studies we hypothesize that following binding to the receptor site, the toxin undergoes a conformational change at the N-groove region that facilitates the trapping of the voltage-sensor in its activated position.

Allosteric interactions between scorpion toxin receptor sites on voltage‐gated Na channels imply a novel role for weakly active components in arthropod venom

Scorpion beta and alpha-toxins modify the activation and inactivation of voltage-gated sodium channels. Although the two types of toxin bind at two distinct receptor sites on the same sodium channel, they exhibit synergic effects when coinjected into insects. To clarify the basis of this synergism we examined the mutual effects of alpha and beta toxin representatives in radio-ligand binding assays. We found positive allosteric interactions between receptor site-4 of the excitatory Bj-xtrIT and the depressant LqhIT2 beta toxins and receptor site-3 of the alpha toxin LqhalphaIT, on locust neuronal membranes. Unexpectedly, a nontoxic mutant Bj-xtrIT-E15R, which binds with high affinity to receptor site-4, was able to enhance LqhalphaIT binding and toxicity similarly to the unmodified Bj-xtrIT. This result indicates that mere binding of a nontoxic ligand to receptor site-4 ("silent binding") induces a conformational change that does not alter channel gating, but influences toxin binding at receptor site-3 leading to enhanced toxicity. This finding suggests a new functional role for weakly toxic polypeptides in that they enhance the effect of other active neurotoxins in the arthropod venom. Such silent binding may have also valuable implications in attempts to improve drug efficacy by combining potent drugs with nonactive allosteric enhancers.

Comparative pharmacology and cloning of two novel arachnid sodium channels: Exploring the adaptive insensitivity of scorpion to its toxins

Scorpion toxins have been found lacking effect on Na(+) current of its own sodium channel, whereas the molecular mechanism remains mystery. In this study, the binding affinity of pharmacologically distinct scorpion toxins was found much weaker to scorpion (Buthus martensii) nerve synaptosomes than to spider (Ornithoctonus huwena) ones. The sodium channel cDNA from these two species were further cloned. The deduced proteins contain 1871 and 1987 amino acids respectively. Several key amino acid substitutions, i.e., A1610V, I1611L and S1617K, are found in IVS3-S4 constituting receptor site-3, and for receptor site-4, two residues (Leu-Pro) are inserted near IIS4 of scorpion sodium channel.

Subtype specificity of scorpion beta-toxin Tz1 interaction with voltage-gated sodium channels is determined by the pore loop of domain 3

Voltage-gated sodium (Nav) channels are modulated by a variety of specific neurotoxins. Scorpion beta-toxins affect the voltage-dependence of channel gating: In their presence, Nav channels activate at subthreshold membrane voltages. Previous mutagenesis studies have revealed that the beta-toxin Css4 interacts with the extracellular linker between segments 3 and 4 in domain 2 of Nav channels with the effect to trap this voltage sensor in an open position (Neuron 21: 919-931, 1998 ). The voltage sensor of domain 2 was thus identified to constitute a major part of neurotoxin receptor site 4. In this work, we studied the effects of the beta-toxin Tz1 from the Venezuelan scorpion Tityus zulianus on various mammalian Nav channel types expressed in HEK 293 cells. Although skeletal muscle channels (Nav1.4) were strongly affected by Tz1, the neuronal channels Nav1.6 and Nav1.2 were less sensitive, and the cardiac Nav1.5 and the peripheral nerve channel Nav1.7 were essentially insensitive. Analysis of channel chimeras in which whole domains of Nav1.2 were inserted into a Nav1.4 background revealed that the Nav1.2 phenotype was not conferred to Nav1.4 by domain 2 but by domain 3. The interaction epitope could be narrowed down to residues Glu1251, Lys1252, and His1257 located in the C-terminal pore loop in domain 3. The receptor site for beta-toxin interaction with Nav channels thus spans domains 2 and 3, where the pore loop in domain 3 specifies the pharmacological properties of individual neuronal Nav channel types.

Expression and Mutagenesis of the Sea Anemone Toxin Av2 Reveals Key Amino Acid Residues Important for Activity on Voltage-Gated Sodium Channels

Type I sea anemone toxins are highly potent modulators of voltage-gated Na-channels (Na(v)s) and compete with the structurally dissimilar scorpion alpha-toxins on binding to receptor site-3. Although these features provide two structurally different probes for studying receptor site-3 and channel fast inactivation, the bioactive surface of sea anemone toxins has not been fully resolved. We established an efficient expression system for Av2 (known as ATX II), a highly insecticidal sea anemone toxin from Anemonia viridis (previously named A. sulcata), and mutagenized it throughout. Each toxin mutant was analyzed in toxicity and binding assays as well as by circular dichroism spectroscopy to discern the effects derived from structural perturbation from those related to bioactivity. Six residues were found to constitute the anti-insect bioactive surface of Av2 (Val-2, Leu-5, Asn-16, Leu-18, and Ile-41). Further analysis of nine Av2 mutants on the human heart channel Na(v)1.5 expressed in Xenopus oocytes indicated that the bioactive surfaces toward insects and mammals practically coincide but differ from the bioactive surface of a structurally similar sea anemone toxin, Anthopleurin B, from Anthopleura xanthogrammica. Hence, our results not only demonstrate clear differences in the bioactive surfaces of Av2 and scorpion alpha-toxins but also indicate that despite the general conservation in structure and importance of the Arg-14 loop and its flanking residues Gly-10 and Gly-20 for function, the surface of interaction between different sea anemone toxins and Na(v)s varies.

Direct evidence that receptor site-4 of sodium channel gating modifiers is not dipped in the phospholipid bilayer of neuronal membranes

In a recent note to Nature, R. MacKinnon has raised the possibility that potassium channel gating modifiers are able to partition in the phospholipid bilayer of neuronal membranes and that by increasing their partial concentration adjacent to their receptor, they affect channel function with apparent high affinity (Lee and MacKinnon (2004) Nature 430, 232-235). This suggestion was adopted by Smith et al. (Smith, J. J., Alphy, S., Seibert, A. L., and Blumenthal, K. M. (2005) J. Biol. Chem. 280, 11127-11133), who analyzed the partitioning of sodium channel modifiers in liposomes. They found that certain modifiers were able to partition in these artificial membranes, and on this basis, they have extrapolated that scorpion beta-toxins interact with their channel receptor in a similar mechanism as that proposed by MacKinnon. Since this hypothesis has actually raised a new conception, we examined it in binding assays using a number of pharmacologically distinct scorpion beta-toxins and insect and mammalian neuronal membrane preparations, as well as by analyzing the rate by which the toxin effect on gating of Drosophila DmNa(v)1 and rat brain rNa(v)1.2a develops. We show that in general, scorpion beta-toxins do not partition in neuronal membranes and that in the case in which a depressant beta-toxin partitions in insect neuronal membranes, this partitioning is unrelated to its interaction with the receptor site and the effect on the gating properties of the sodium channel. These results negate the hypothesis that the high affinity of beta-toxins for sodium channels is gained by their ability to partition in the phospholipid bilayer and clearly indicate that the receptor site for scorpion beta-toxins is accessible to the extracellular solvent.

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.

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

Long-chain neurotoxins derived from the venom of the Buthidae scorpions, which affect voltage-gated sodium channels (VGSCs) can be subdivided according to their toxicity to insects into insect-selective excitatory and depressant toxins (beta-toxins) and the alpha-like toxins which affect both mammals and insects. In the present study by the aid of reverse-phase HPLC column chromatography, RT-PCR, cloning and various toxicity assays, a new insect selective toxin designated as BjalphaIT was isolated from the venom of the Judean Black Scorpion (Buthotus judaicus), and its full primary sequence was determined: MNYLVVICFALLLMTVVESGRDAYIADNLNCAYTCGSNSYCNTECTKNGAVSGYCQWLGKYGNACWCINLPDKVPIRIPGACR (leader sequence is underlined). Despite its lack of toxicity to mammals and potent toxicity to insects, BjalphaIT reveals an amino acid sequence and an inferred spatial arrangement that is characteristic of the well-known scorpion alpha-toxins highly toxic to mammals. BjalphaITs sharp distinction between insects and mammals was also revealed by its effect on sodium conductance of two cloned neuronal VGSCs heterloguously expressed in Xenopus laevis oocytes and assayed with the two-electrode voltage-clamp technique. BjalphaIT completely inhibits the inactivation process of the insect para/tipE VGSC at a concentration of 100 nM, in contrast to the rat brain Na(v)1.2/beta1 which is resistant to the toxin. The above categorical distinction between mammal and insect VGSCs exhibited by BjalphaIT enables its employment in the clarification of the molecular basis of the animal group specificity of scorpion venom derived neurotoxic polypeptides and voltage-gated sodium channels.

A Spider Toxin That Induces a Typical Effect of Scorpion α-Toxins but Competes with β-Toxins on Binding to Insect Sodium Channels

Delta-palutoxins from the spider Paracoelotes luctuosus (Araneae: Amaurobiidae) are 36-37 residue long peptides that show preference for insect sodium channels (NaChs) and modulate their function. Although they slow NaCh inactivation in a fashion similar to that of receptor site 3 modifiers, such as scorpion alpha-toxins, they actually bind with high affinity to the topologically distinct receptor site 4 of scorpion beta-toxins. To resolve this riddle, we scanned by Ala mutagenesis the surface of delta-PaluIT2, a delta-palutoxin variant with the highest affinity for insect NaChs, and compared it to the bioactive surface of a scorpion beta-toxin. We found three regions on the surface of delta-PaluIT2 important for activity: the first consists of Tyr-22 and Tyr-30 (aromatic), Ser-24 and Met-28 (polar), and Arg-8, Arg-26, Arg-32, and Arg-34 (basic) residues; the second is made of Trp-12; and the third is made of Asp-19, whose substitution by Ala uncoupled the binding from toxicity to lepidopteran larvae. Although spider delta-palutoxins and scorpion beta-toxins have developed from different ancestors, they show some commonality in their bioactive surfaces, which may explain their ability to compete for an identical receptor (site 4) on voltage-gated NaChs. Yet, their different mode of channel modulation provides a novel perspective about the structural relatedness of receptor sites 3 and 4, which until now have been considered topologically distinct.

Conversion of a scorpion toxin agonist into an antagonist highlights an acidic residue involved in voltage sensor trapping during activation of neuronal Na + channels

Gating modifiers constitute a large group of polypeptide toxins that interact with the voltage-sensing module of ion channels. Among them, scorpion beta-toxins induce a negative shift in the voltage dependence of sodium channel activation. To explain their effect, a "voltage sensor trapping" model has been proposed in which the voltage sensor of domain-II (DIIS4) is trapped in an outward, activated position by a prebound beta-toxin upon membrane depolarization. Whereas toxin effect on channel activation was enhanced upon neutralization of the two outermost arginines in DIIS4, toxin residues involved in sensor trapping have not been identified. Using the scorpion excitatory beta-toxin, Bj-xtrIT, we found two conserved acidic residues, Glu15 and Glu30, mandatory for toxin action. Whereas mutagenesis of Glu30 affected both toxicity and binding affinity, substitutions E15A/F abolished activity but had minor effects on binding. Complete uncoupling of activity from binding was obtained with mutant E15R, acting as an efficient antagonist of Bj-xtrIT. On the basis of the voltage sensor trapping model and our results, we propose that Glu15 interacts with the emerging gating charges of DIIS4 upon membrane depolarization. Conserved acidic residues found in a variety of gating modifiers from scorpions and spiders may interact similarly with the voltage sensor.

Dissection of the Functional Surface of an Anti-insect Excitatory Toxin Illuminates a Putative “Hot Spot” Common to All Scorpion β-Toxins Affecting Na+ Channels

Scorpion beta-toxins affect the activation of voltage-sensitive sodium channels (NaChs). Although these toxins have been instrumental in the study of channel gating and architecture, little is known about their active sites. By using an efficient system for the production of recombinant toxins, we analyzed by point mutagenesis the entire surface of the beta-toxin, Bj-xtrIT, an anti-insect selective excitatory toxin from the scorpion Buthotus judaicus. Each toxin mutant was purified and analyzed using toxicity and binding assays, as well as by circular dichroism spectroscopy to discern the differences among mutations that caused structural changes and those that specifically affected bioactivity. This analysis highlighted a functional discontinuous surface of 1405 A(2), which was composed of a number of non-polar and three charged amino acids clustered around the main alpha-helical motif and the C-tail. Among the charged residues, Glu(30) is a center of a putative "hot spot" in the toxin-receptor binding-interface and is shielded from bulk solvent by a hydrophobic "gasket" (Tyr(26) and Val(34)). Comparison of the Bj-xtrIT structure with that of other beta-toxins that are active on mammals suggests that the hot spot and an adjacent non-polar region are spatially conserved. These results highlight for the first time structural elements that constitute a putative "pharmacophore" involved in the interaction of beta-toxins with receptor site-4 on NaChs. Furthermore, the unique structure of the C-terminal region most likely determines the specificity of excitatory toxins for insect NaChs.

The molecular biology of knockdown resistance to pyrethroid insecticides

The term "knockdown resistance" is used to describe cases of resistance to diphenylethane (e.g. DDT) and pyrethroid insecticides in insects and other arthropods that result from reduced sensitivity of the nervous system. Knockdown resistance, first identified and characterized in the house fly (Musca domestica) in the 1950's, remains a threat to the continued usefulness of pyrethroids in the control of many pest species. Research since 1990 has provided a wealth of new information on the molecular basis of knockdown resistance. This paper reviews these recent developments with emphasis on the results of genetic linkage analyses, the identification of gene mutations associated with knockdown resistance, and the functional characterization of resistance-associated mutations. Results of these studies identify voltage-sensitive sodium channel genes orthologous to the para gene of Drosophila melanogaster as the site of multiple knockdown resistance mutations and define the molecular mechanisms by which these mutations cause pyrethroid resistance. These results also provide new insight into the mechanisms by which pyrethroids modify the function of voltage-sensitive sodium channels.

An 'Old World' scorpion beta-toxin that recognizes both insect and mammalian sodium channels

Scorpion toxins that affect sodium channel (NaCh) gating in excitable cells are divided into alpha- and beta-classes. Whereas alpha-toxins have been found in scorpions throughout the world, anti-mammalian beta-toxins have been assigned, thus far, to 'New World' scorpions while anti-insect selective beta-toxins (depressant and excitatory) have been described only in the 'Old World'. This distribution suggested that diversification of beta-toxins into distinct pharmacological groups occurred after the separation of the continents, 150 million years ago. We have characterized a unique toxin, Lqhbeta1, from the 'Old World' scorpion, Leiurus quinquestriatus hebraeus, that resembles in sequence and activity both 'New World'beta-toxins as well as 'Old World' depressant toxins. Lqhbeta1 competes, with apparent high affinity, with anti-insect and anti-mammalian beta-toxins for binding to cockroach and rat brain synaptosomes, respectively. Surprisingly, Lqhbeta1 also competes with an anti-mammalian alpha-toxin on binding to rat brain NaChs. Analysis of Lqhbeta1 effects on rat brain and Drosophila Para NaChs expressed in Xenopus oocytes revealed a shift in the voltage-dependence of activation to more negative membrane potentials and a reduction in sodium peak currents in a manner typifying beta-toxin activity. Moreover, Lqhbeta1 resembles beta-toxins by having a weak effect on cardiac NaChs and a marked effect on rat brain and skeletal muscle NaChs. These multifaceted features suggest that Lqhbeta1 may represent an ancestral beta-toxin group in 'Old World' scorpions that gave rise, after the separation of the continents, to depressant toxins in 'Old World' scorpions and to various beta-toxin subgroups in 'New World' scorpions.

Allosteric interactions among pyrethroid, brevetoxin, and scorpion toxin receptors on insect sodium channels raise an alternative approach for insect control

Intensive pyrethroid use in insect control has led to resistance buildup among various pests. One alternative to battle this problem envisions the combined use of synergistically acting insecticidal compounds. Pyrethroids, scorpion alpha- and beta-toxins, and brevetoxins bind to distinct receptor sites on voltage-gated sodium channels (NaChs) and modify their function. The binding affinity of scorpion alpha-toxins to locust, but not rat-brain NaChs, is allosterically increased by pyrethroids and by brevetoxin-1. Brevetoxin-1 also increases the binding of an excitatory beta-toxin to insect NaChs. These results reveal differences between insect and mammalian NaChs and may be exploited in new strategies of insect control.