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

LmNaTx15, a novel scorpion toxin, enhances the activity of Nav channels and induces pain in mice

Polypeptide toxins are major bioactive components found in venomous animals. Many polypeptide toxins can specifically act on targets, such as ion channels and voltage-gated sodium (Nav) channels, in the nervous, muscle, and cardiovascular systems of the recipient to increase defense and predation efficiency. In this study, a novel polypeptide toxin, LmNaTx15, was isolated from the venom of the scorpion Lychas mucronatus, and its activity was analyzed. LmNaTx15 slowed the fast inactivation of Nav1.2, Nav1.3, Nav1.4, Nav1.5, and Nav1.7 and inhibited the peak current of Nav1.5, but it did not affect Nav1.8. In addition, LmNaTx15 altered the voltage-dependent activation and inactivation of these Nav channel subtypes. Furthermore, like site 3 neurotoxins, LmNaTx15 induced pain in mice. These results show a novel scorpion toxin with a modulatory effect on specific Nav channel subtypes and pain induction in mice. Therefore, LmNaTx15 may be a key bioactive component for scorpion defense and predation. Besides, this study provides a basis for analyzing structure-function relationships of the scorpion toxins affecting Nav channel activity.

A scorpion toxin affecting sodium channels shows double cis-trans isomerism

Scorpion α-toxins (α-NaTx) inhibiting the inactivation of voltage-gated sodium channels (Nav ) are a well-studied family of small proteins. We previously showed that the structure of α-NaTx specificity module responsible for selective Nav binding is governed by an interplay between the nest and niche protein motifs. Here, we report the solution structure of the toxin Lqq4 from the venom of the scorpion Leiurus quinquestriatus. Unexpectedly, we find that this toxin presents an ensemble of long-lived structurally distinct states. We unequivocally assign these states to the alternative configurations (cis-trans isomers) of two peptide bonds: V56-P57 and C17-G18; neither of the cis isomers has been described in α-NaTx so far. We argue that the native conformational space of α-NaTx is wider than assumed previously.

Methionine 58 is a key residue in the modulation of BmK scorpion toxin AGP-SYPU2 activity through in silico and in vivo studies

Protein dynamic networks play an important role in the regulation of many protein systems. Some residues that are far away from the interface between proteins and their targets have a critical role in modulating the activity of some scorpion toxins. Here, conservation analysis combined with an in vivo experiment has reveals that Met58 is a key residue of BmK scorpion toxin AGP-SYPU2 in the modulation of analgesic activity. Molecular dynamics simulations clearly reveal the conformational changes that allow the loop between the β2 and β3 sheets to be exposed on the toxin surface to interact with its targets. Our results emphasize specific roles for the residue Met58 in the NC domain and our work gives valuable information for further modification of scorpion toxins to obtain new analgesic peptides with enhanced activity. Communicated by Ramaswamy H. Sarma.

Structure of MeuNaTxα-1 toxin from scorpion venom highlights the importance of the nest motif

Old world scorpions produce an abundance of toxins called α-NaTx, which interfere with the fast inactivation of voltage-gated sodium channels. Their selectivity to channels of mammals or insects depends on a part of toxin named the specificity module. We report here the spatial structure of a major and broadly active toxin MeuNaTxα-1 from the venom of Mesobuthus eupeus. Notably, its specificity module is markedly different from other α-NaTx with known 3D structure. Close inspection shows that its conformation is a result of an interplay between protein motifs such as the nest and niche, which eventually shape α-NaTx structural diversity.

Protein surface topography as a tool to enhance the selective activity of a potassium channel blocker

Tk-hefu is an artificial peptide designed based on the α-hairpinin scaffold, which selectively blocks voltage-gated potassium channels K_v1.3. Here we present its spatial structure resolved by NMR spectroscopy and analyze its interaction with channels using computer modeling. We apply protein surface topography to suggest mutations and increase Tk-hefu affinity to the K_v1.3 channel isoform. We redesign the functional surface of Tk-hefu to better match the respective surface of the channel pore vestibule. The resulting peptide Tk-hefu-2 retains K_v1.3 selectivity and displays ∼15 times greater activity compared with Tk-hefu. We verify the mode of Tk-hefu-2 binding to the channel outer vestibule experimentally by site-directed mutagenesis. We argue that scaffold engineering aided by protein surface topography represents a reliable tool for design and optimization of specific ion channel ligands.

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

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

Expression of recombinant α-toxin BmKM9 from scorpion Buthus martensii Karsch and its functional characterization on sodium channels

Scorpion toxins are invaluable pharmacological tools for studying ion channels and potential drugs for channelopathies. The long-chain toxins from scorpion venom with four disulfide bridges exhibit their unusual bioactivity or biotoxicity by acting on the sodium channels. However, the functional properties of most toxins are still unclear due to their tiny amounts in crude venom and their challenging production by chemical and gene engineering techniques. Here, we expressed one of the long-chain α-toxins, BmKM9, found in the venom of the scorpion Buthus martensii Karsch and characterized its pharmacological properties on sodium channels. Unlike previous toxin production, the recombinant BmKM9 (rBmKM9) possessed no additional amino acid residues such as the His-tag and thrombin cleavage site. The refolded toxin could inhibit the inactivation of rNa_v1.4, hNa_v1.5 and hNa_v1.7 sodium channels. Dose-response experiments were further conducted on these channels. The calculated EC₅₀ values were 131.7±6.6nM for rNa_v1.4, 454.2±50.1nM for hNa_v1.5 and 30.9±10.3μM for hNa_v1.7. The channel activation experiments indicated that the rBmKM9 toxin could shift the activation curves of rNa_v1.4 and hNa_v1.5 channels toward a more negative direction and present the typical features of a β-toxin. However, instead of the same activation property on sodium channels, the rBmKM9 toxin could result in different inactivation shift capabilities on rNa_v1.4 and hNa_v1.5 channels. The V_1/2 values of the steady-state inactivation were altered to be more positive for rNa_v1.4 and more negative for hNa_v1.5. Moreover, the recovery of the hNa_v1.5 channel from inactivation was more significantly delayed than that of the rNa_v1.4 channel by exposure to rBmKM9. Together, these findings highlighted that the rBmKM9 toxin presents the pharmacological properties of both α- and β-toxins, which would increase the challenge to the classical classification of scorpion toxins. Furthermore, the expression method and functional information on sodium channels would promote the potential application of toxins and contribute to further channel structural and functional studies.

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

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

Target-Driven Evolution of Scorpion Toxins

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

α-Dendrotoxin inhibits the ASIC current in dorsal root ganglion neurons from rat

Dendrotoxins are a group of peptide toxins purified from the venom of several mamba snakes. α-Dendrotoxin (α-DTx, from the Eastern green mamba Dendroaspis angusticeps) is a well-known blocker of voltage-gated K(+) channels and specifically of K(v)1.1, K(v)1.2 and K(v)1.6. In this work we show that α-DTx inhibited the ASIC currents in DRG neurons (IC50=0.8 μM) when continuously perfused during 25 s (including a 5 s pulse to pH 6.1), but not when co-applied with the pH drop. Additionally, we show that α-DTx abolished a transient component of the outward current that, in some experiments, appeared immediately after the end of the acid pulse. Our data indicate that α-DTx inhibits ASICs in the high nM range while some Kv are inhibited in the low nM range. The α-DTx selectivity and its potential interaction with ASICs should be taken in consideration when DTx is used in the high nM range.

General characterization of Tityus fasciolatus scorpion venom. Molecular identification of toxins and localization of linear B-cell epitopes

This communication describes the general characteristics of the venom from the Brazilian scorpion Tityus fasciolatus, which is an endemic species found in the central Brazil (States of Goiás and Minas Gerais), being responsible for sting accidents in this area. The soluble venom obtained from this scorpion is toxic to mice being the LD50 is 2.984 mg/kg (subcutaneally). SDS-PAGE of the soluble venom resulted in 10 fractions ranged in size from 6 to 10-80 kDa. Sheep were employed for anti-T. fasciolatus venom serum production. Western blotting analysis showed that most of these venom proteins are immunogenic. T. fasciolatus anti-venom revealed consistent cross-reactivity with venom antigens from Tityus serrulatus. Using known primers for T. serrulatus toxins, we have identified three toxins sequences from T. fasciolatus venom. Linear epitopes of these toxins were localized and fifty-five overlapping pentadecapeptides covering complete amino acid sequence of the three toxins were synthesized in cellulose membrane (spot-synthesis technique). The epitopes were located on the 3D structures and some important residues for structure/function were identified.

Wheat germ in vitro translation to produce one of the most toxic sodium channel specific toxins

Envenoming following scorpion sting is a common emergency in many parts of the world. During scorpion envenoming, highly toxic small polypeptides of the venom diffuse rapidly within the victim causing serious medical problems. The exploration of toxin structure-function relationship would benefit from the generation of soluble recombinant scorpion toxins in Escherichia coli. We developed an in vitro wheat germ translation system for the expression of the highly toxic Aah (Androctonus australis hector)II protein that requires the proper formation of four disulphide bonds. Soluble, recombinant N-terminal GST (glutathione S-transferase)-tagged AahII toxin is obtained in this in vitro translation system. After proteolytic removal of the GST-tag, purified rAahII (recombinant AahII) toxin, which contains two extra amino acids at its N terminal relative to the native AahII, is highly toxic after i.c.v. (intracerebroventricular) injection in Swiss mice. An LD₅₀ (median lethal dose)-value of 10 ng (or 1.33 pmol), close to that of the native toxin (LD₅₀ of 3 ng) indicates that the wheat germ in vitro translation system produces properly folded and biological active rAahII. In addition, NbAahII10 (Androctonus australis hector nanobody 10), a camel single domain antibody fragment, raised against the native AahII toxin, recognizes its cognate conformational epitope on the recombinant toxin and neutralizes the toxicity of purified rAahII upon injection in mice.

A wheat germ embryo derived cell-free translation system expresses a biologically active, highly toxic scorpion venom protein that is fully neutralized by a camel single domain antibody fragment raised against the native scorpion toxin.

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

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

The voltage sensor module in sodium channels

The mechanism by which voltage-gated ion channels respond to changes in membrane polarization during action potential signaling in excitable cells has been the subject of research attention since the original description of voltage-dependent sodium and potassium flux in the squid giant axon. The cloning of ion channel genes and the identification of point mutations associated with channelopathy diseases in muscle and brain has facilitated an electrophysiological approach to the study of ion channels. Experimental approaches to the study of voltage gating have incorporated the use of thiosulfonate reagents to test accessibility, fluorescent probes, and toxins to define domain-specific roles of voltage-sensing S4 segments. Crystallography, structural and homology modeling, and molecular dynamics simulations have added computational approaches to study the relationship of channel structure to function. These approaches have tested models of voltage sensor translocation in response to membrane depolarization and incorporate the role of negative countercharges in the S1 to S3 segments to define our present understanding of the mechanism by which the voltage sensor module dictates gating particle permissiveness in excitable cells.

Scorpion venom components that affect ion-channels function

The number and types of venom components that affect ion-channel function are reviewed. These are the most important venom components responsible for human intoxication, deserving medical attention, often requiring the use of specific anti-venoms. Special emphasis is given to peptides that recognize Na(+)-, K(+)- and Ca(++)-channels of excitable cells. Knowledge generated by direct isolation of peptides from venom and components deduced from cloned genes, whose amino acid sequences are deposited into databanks are nowadays in the order of 1.5 thousands, out of an estimate biodiversity closed to 300,000. Here the diversity of components is briefly reviewed with mention to specific references. Structural characteristic are discussed with examples taken from published work. The principal mechanisms of action of the three different types of peptides are also reviewed. Na(+)-channel specific venom components usually are modifier of the open and closing kinetic mechanisms of the ion-channels, whereas peptides affecting K(+)-channels are normally pore blocking agents. The Ryanodine Ca(++)-channel specific peptides are known for causing sub-conducting stages of the channels conductance and some were shown to be able to internalize penetrating inside the muscle cells.

Direct evidence that scorpion α-toxins (site-3) modulate sodium channel inactivation by hindrance of voltage-sensor movements

The position of the voltage-sensing transmembrane segment, S4, in voltage-gated ion channels as a function of voltage remains incompletely elucidated. Site-3 toxins bind primarily to the extracellular loops connecting transmembrane helical segments S1-S2 and S3-S4 in Domain 4 (D4) and S5-S6 in Domain 1 (D1) and slow fast-inactivation of voltage-gated sodium channels. As S4 of the human skeletal muscle voltage-gated sodium channel, hNav1.4, moves in response to depolarization from the resting to the inactivated state, two D4S4 reporters (R2C and R3C, Arg1451Cys and Arg1454Cys, respectively) move from internal to external positions as deduced by reactivity to internally or externally applied sulfhydryl group reagents, methane thiosulfonates (MTS). The changes in reporter reactivity, when cycling rapidly between hyperpolarized and depolarized voltages, enabled determination of the positions of the D4 voltage-sensor and of its rate of movement. Scorpion α-toxin binding impedes D4S4 segment movement during inactivation since the modification rates of R3C in hNav1.4 with methanethiosulfonate (CH3SO2SCH2CH2R, where R = -N(CH3)3 (+) trimethylammonium, MTSET) and benzophenone-4-carboxamidocysteine methanethiosulfonate (BPMTS) were slowed ~10-fold in toxin-modified channels. Based upon the different size, hydrophobicity and charge of the two reagents it is unlikely that the change in reactivity is due to direct or indirect blockage of access of this site to reagent in the presence of toxin (Tx), but rather is the result of inability of this segment to move outward to the normal extent and at the normal rate in the toxin-modified channel. Measurements of availability of R3C to internally applied reagent show decreased access (slower rates of thiol reaction) providing further evidence for encumbered D4S4 movement in the presence of toxins consistent with the assignment of at least part of the toxin binding site to the region of D4S4 region of the voltage-sensor module.

Structure and function of hainantoxin-III, a selective antagonist of neuronal tetrodotoxin-sensitive voltage-gated sodium channels isolated from the Chinese bird spider Ornithoctonus hainana

In the present study, we investigated the structure and function of hainantoxin-III (HNTX-III), a 33-residue polypeptide from the venom of the spider Ornithoctonus hainana. It is a selective antagonist of neuronal tetrodotoxin-sensitive voltage-gated sodium channels. HNTX-III suppressed Nav1.7 current amplitude without significantly altering the activation, inactivation, and repriming kinetics. Short extreme depolarizations partially activated the toxin-bound channel, indicating voltage-dependent inhibition of HNTX-III. HNTX-III increased the deactivation of the Nav1.7 current after extreme depolarizations. The HNTX-III·Nav1.7 complex was gradually dissociated upon prolonged strong depolarizations in a voltage-dependent manner, and the unbound toxin rebound to Nav1.7 after a long repolarization. Moreover, analysis of chimeric channels showed that the DIIS3-S4 linker was critical for HNTX-III binding to Nav1.7. These data are consistent with HNTX-III interacting with Nav1.7 site 4 and trapping the domain II voltage sensor in the closed state. The solution structure of HNTX-III was determined by two-dimensional NMR and shown to possess an inhibitor cystine knot motif. Structural analysis indicated that certain basic, hydrophobic, and aromatic residues mainly localized in the C terminus may constitute an amphiphilic surface potentially involved in HNTX-III binding to Nav1.7. Taken together, our results show that HNTX-III is distinct from β-scorpion toxins and other β-spider toxins in its mechanism of action and binding specificity and affinity. The present findings contribute to our understanding of the mechanism of toxin-sodium channel interaction and provide a useful tool for the investigation of the structure and function of sodium channel isoforms and for the development of analgesics.

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.

Chemical engineering and structural and pharmacological characterization of the α-scorpion toxin OD1

Scorpion α-toxins are invaluable pharmacological tools for studying voltage-gated sodium channels, but few structure-function studies have been undertaken due to their challenging synthesis. To address this deficiency, we report a chemical engineering strategy based upon native chemical ligation. The chemical synthesis of α-toxin OD1 was achieved by chemical ligation of three unprotected peptide segments. A high resolution X-ray structure (1.8 Å) of synthetic OD1 showed the typical βαββ α-toxin fold and revealed important conformational differences in the pharmacophore region when compared with other α-toxin structures. Pharmacological analysis of synthetic OD1 revealed potent α-toxin activity (inhibition of fast inactivation) at Nav1.7, as well as Nav1.4 and Nav1.6. In addition, OD1 also produced potent β-toxin activity at Nav1.4 and Nav1.6 (shift of channel activation in the hyperpolarizing direction), indicating that OD1 might interact at more than one site with Nav1.4 and Nav1.6. Investigation of nine OD1 mutants revealed that three residues in the reverse turn contributed significantly to selectivity, with the triple OD1 mutant (D9K, D10P, K11H) being 40-fold more selective for Nav1.7 over Nav1.6, while OD1 K11V was 5-fold more selective for Nav1.6 than Nav1.7. This switch in selectivity highlights the importance of the reverse turn for engineering α-toxins with altered selectivity at Nav subtypes.

Computational methods of studying the binding of toxins from venomous animals to biological ion channels: theory and applications

The discovery of new drugs that selectively block or modulate ion channels has great potential to provide new treatments for a host of conditions. One promising avenue revolves around modifying or mimicking certain naturally occurring ion channel modulator toxins. This strategy appears to offer the prospect of designing drugs that are both potent and specific. The use of computational modeling is crucial to this endeavor, as it has the potential to provide lower cost alternatives for exploring the effects of new compounds on ion channels. In addition, computational modeling can provide structural information and theoretical understanding that is not easily derivable from experimental results. In this review, we look at the theory and computational methods that are applicable to the study of ion channel modulators. The first section provides an introduction to various theoretical concepts, including force-fields and the statistical mechanics of binding. We then look at various computational techniques available to the researcher, including molecular dynamics, brownian dynamics, and molecular docking systems. The latter section of the review explores applications of these techniques, concentrating on pore blocker and gating modifier toxins of potassium and sodium channels. After first discussing the structural features of these channels, and their modes of block, we provide an in-depth review of past computational work that has been carried out. Finally, we discuss prospects for future developments in the field.

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.

A positively charged surface patch is important for hainantoxin-IV binding to voltage-gated sodium channels

Hainantoxin-IV (HNTX-IV), isolated from the venom of the spider Ornithoctonus hainana, is a specific antagonist of tetrodotoxin-sensitive (TTX-S) voltage-gated sodium channels in rat dorsal root ganglion (DRG) cells. It adopts an inhibitor cystine knot motif, and structural analysis revealed a positively charged patch consisting of Arg26, Lys27, His28, Arg29 and Lys32 distributed on its molecular surface. Our previous study demonstrated that Lys27 and Arg29 but not Arg26 were critical residues for HNTX-IV binding to TTX-S sodium channels. In the present study, we examined the roles of His28 and Lys32 in the interaction of HNTX-IV with its target. Two mutants, HNTX-IV-H28D and HNTX-IV-K32A, were generated by solid-phase chemical synthesis and purified by reverse-phase HPLC after refolding and oxidation, yielding two compounds of high purity with monoisotopic masses of 3962.66 and 3927.70 Da, respectively, as determined by MALDI-TOF mass spectrometry. This indicated the presence of six cysteine residues forming three disulfide bonds. Moreover, circular dichroism spectroscopy analysis demonstrated that the substitution of His28 or Lys32 did not affect the overall structure of HNTX-IV. The inhibitory activity of HNTX-IV-H28D and HNTX-IV-K32A against TTX-S sodium channels in rat DRG cells was analyzed by whole-cell patch-clamp technique. The IC(50) values for the mutants were 0.57 and 5.80 μM (17-fold and 170-fold lower than the activity of the native toxin), indicating that His28 and Lys32 may be important for the inhibitory activity of HNTX-IV. Taken together, our results suggest that the positively charged patch might be the binding site for the interaction of HNTX-IV with TTX-S sodium channels. These findings might contribute to the elucidation of the structure and function relationship of HNTX-IV.

Solution structure of kurtoxin: a gating modifier selective for Cav3 voltage-gated Ca(2+) channels

Kurtoxin is a 63-amino acid polypeptide isolated from the venom of the South African scorpion Parabuthus transvaalicus. It is the first and only peptide ligand known to interact with Cav3 (T-type) voltage-gated Ca(2+) channels with high affinity and to modify the voltage-dependent gating of these channels. Here we describe the nuclear magnetic resonance (NMR) solution structure of kurtoxin determined using two- and three-dimensional NMR spectroscopy with dynamical simulated annealing calculations. The molecular structure of the toxin was highly similar to those of scorpion α-toxins and contained an α-helix, three β-strands, and several turns stabilized by four disulfide bonds. This so-called "cysteine-stabilized α-helix and β-sheet (CSαβ)" motif is found in a number of functionally varied small proteins. A detailed comparison of the backbone structure of kurtoxin with those of the scorpion α-toxins revealed that three regions [first long loop (Asp(8)-Ile(15)), β-hairpin loop (Gly(39)-Leu(42)), and C-terminal segment (Arg(57)-Ala(63))] in kurtoxin significantly differ from the corresponding regions in scorpion α-toxins, suggesting that these regions may be important for interacting with Cav3 (T-type) Ca(2+) channels. In addition, the surface profile of kurtoxin shows a larger and more focused electropositive patch along with a larger hydrophobic surface compared to those seen on scorpion α-toxins. These distinct surface properties of kurtoxin could explain its binding to Cav3 (T-type) voltage-gated Ca(2+) channels.

Recombinant expression, purification, and characterization of scorpion toxin BmαTX14

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

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

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

Evolutionary diversification of Mesobuthus α-scorpion toxins affecting sodium channels

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

Mapping the receptor site for alpha-scorpion toxins on a Na+ channel voltage sensor

The α-scorpions toxins bind to the resting state of Na(+) channels and inhibit fast inactivation by interaction with a receptor site formed by domains I and IV. Mutants T1560A, F1610A, and E1613A in domain IV had lower affinities for Leiurus quinquestriatus hebraeus toxin II (LqhII), and mutant E1613R had ~73-fold lower affinity. Toxin dissociation was accelerated by depolarization and increased by these mutations, whereas association rates at negative membrane potentials were not changed. These results indicate that Thr1560 in the S1-S2 loop, Phe1610 in the S3 segment, and Glu1613 in the S3-S4 loop in domain IV participate in toxin binding. T393A in the SS2-S6 loop in domain I also had lower affinity for LqhII, indicating that this extracellular loop may form a secondary component of the receptor site. Analysis with the Rosetta-Membrane algorithm resulted in a model of LqhII binding to the voltage sensor in a resting state, in which amino acid residues in an extracellular cleft formed by the S1-S2 and S3-S4 loops in domain IV interact with two faces of the wedge-shaped LqhII molecule. The conserved gating charges in the S4 segment are in an inward position and form ion pairs with negatively charged amino acid residues in the S2 and S3 segments of the voltage sensor. This model defines the structure of the resting state of a voltage sensor of Na(+) channels and reveals its mode of interaction with a gating modifier toxin.

Common molecular determinants of tarantula huwentoxin-IV inhibition of Na+ channel voltage sensors in domains II and IV

The voltage sensors of domains II and IV of sodium channels are important determinants of activation and inactivation, respectively. Animal toxins that alter electrophysiological excitability of muscles and neurons often modify sodium channel activation by selectively interacting with domain II and inactivation by selectively interacting with domain IV. This suggests that there may be substantial differences between the toxin-binding sites in these two important domains. Here we explore the ability of the tarantula huwentoxin-IV (HWTX-IV) to inhibit the activity of the domain II and IV voltage sensors. HWTX-IV is specific for domain II, and we identify five residues in the S1-S2 (Glu-753) and S3-S4 (Glu-811, Leu-814, Asp-816, and Glu-818) regions of domain II that are crucial for inhibition of activation by HWTX-IV. These data indicate that a single residue in the S3-S4 linker (Glu-818 in hNav1.7) is crucial for allowing HWTX-IV to interact with the other key residues and trap the voltage sensor in the closed configuration. Mutagenesis analysis indicates that the five corresponding residues in domain IV are all critical for endowing HWTX-IV with the ability to inhibit fast inactivation. Our data suggest that the toxin-binding motif in domain II is conserved in domain IV. Increasing our understanding of the molecular determinants of toxin interactions with voltage-gated sodium channels may permit development of enhanced isoform-specific voltage-gating modifiers.

Addition of positive charges at the C-terminal peptide region of CssII, a mammalian scorpion peptide toxin, improves its affinity for sodium channels Nav1.6

CssII is a β-scorpion peptide that modifies preferentially sodium currents of the voltage-dependent Na(+) channel (Nav) sub-type 1.6. Previously, we have found that the C-terminal amidation of CssII increases its affinity for Nav, which opens at more negative potentials in the presence of CssII. Although C-terminal amidation in vitro conditions is possible, five CssII peptide toxin variants with C-terminal residues modified were heterologously expressed (rN66S, rN66H, rN66R, r[T64R/N66S] and r[T64R/N66R], in which r stands for recombinant, the capital letters to the amino acid residues and the numbers indicate the position of the given residue into the primary sequence of the toxin) and correctly folded. A secondary structure prediction of CssII agrees with the experimental secondary structure obtained by circular dichroism; so all bacterial expressed neurotoxin variants maintained the typical α/β secondary structure motif of most Na(+) channel scorpion toxins. The electrophysiological properties of all recombinant variants were examined, and it was found that substitutions of threonine (T) and asparagine (N) at the C-terminal region for arginine (R) (r[T64R/N66R]) increase their affinity for Nav1.6. Although, the molecular interactions involved in this mechanism are still not clearly determined, there is experimental evidence supporting the suspicion that incorporation of basic charged amino acid residues at the C-terminal tail of a group of α-scorpion toxin was favored by natural selection.

Finding new cures for neurological disorders: a possible fringe benefit of biodefense research?

The National Institutes of Health (NIH) supports research about and the development of better therapies for treating exposure to toxic chemicals that could be used in a terrorist attack or released during an industrial accident. A review of recent research published by NIH investigators working in this field indicates that scientific advances in this area also have implications for reducing the burden of other neurological diseases and disorders. Some key examples discussed include studies on the development of therapeutic drugs to treat seizures and the neuropathology caused by chemical nerve agents, which may help find better cures for epilepsy, stroke, and neurodegenerative diseases.