Substitutions in the domain III voltage-sensing module enhance the sensitivity of an insect sodium channel to a scorpion beta-toxin

A Novel Spider Toxin Inhibits Fast Inactivation of the Nav1.9 Channel by Binding to Domain III and Domain IV Voltage Sensors

Venomous animals have evolved to produce peptide toxins that modulate the activity of voltage-gated sodium (Na_v) channels. These specific modulators are powerful probes for investigating the structural and functional features of Na_v channels. Here, we report the isolation and characterization of δ-theraphotoxin-Gr4b (Gr4b), a novel peptide toxin from the venom of the spider Grammostola rosea. Gr4b contains 37-amino acid residues with six cysteines forming three disulfide bonds. Patch-clamp analysis confirmed that Gr4b markedly slows the fast inactivation of Na_v1.9 and inhibits the currents of Na_v1.4 and Na_v1.7, but does not affect Na_v1.8. It was also found that Gr4b significantly shifts the steady-state activation and inactivation curves of Na_v1.9 to the depolarization direction and increases the window current, which is consistent with the change in the ramp current. Furthermore, analysis of Na_v1.9/Na_v1.8 chimeric channels revealed that Gr4b preferentially binds to the voltage-sensor of domain III (DIII VSD) and has additional interactions with the DIV VSD. The site-directed mutagenesis analysis indicated that N1139 and L1143 in DIII S3-S4 linker participate in toxin binding. In sum, this study reports a novel spider peptide toxin that may slow the fast inactivation of Na_v1.9 by binding to the new neurotoxin receptor site-DIII VSD. Taken together, these findings provide insight into the functional role of the Na_v channel DIII VSD in fast inactivation and activation.

Charge substitutions at the voltage-sensing module of domain III enhance actions of site-3 and site-4 toxins on an insect sodium channel

Scorpion α-toxins bind at the pharmacologically-defined site-3 on the sodium channel and inhibit channel inactivation by preventing the outward movement of the voltage sensor in domain IV (IVS4), whereas scorpion β-toxins bind at site-4 on the sodium channel and enhance channel activation by trapping the voltage sensor of domain II (IIS4) in its outward position. However, limited information is available on the role of the voltage-sensing modules (VSM, comprising S1-S4) of domains I and III in toxin actions. We have previously shown that charge reversing substitutions of the innermost positively-charged residues in IIIS4 (R4E, R5E) increase the activity of an insect-selective site-4 scorpion toxin, Lqh-dprIT3-c, on BgNav1-1a, a cockroach sodium channel. Here we show that substitutions R4E and R5E in IIIS4 also increase the activity of two site-3 toxins, LqhαIT from Leiurusquinquestriatus hebraeus and insect-selective Av3 from Anemonia viridis. Furthermore, charge reversal of either of two conserved negatively-charged residues, D1K and E2K, in IIIS2 also increase the action of the site-3 and site-4 toxins. Homology modeling suggests that S2-D1 and S2-E2 interact with S4-R4 and S4-R5 in the VSM of domain III (III-VSM), respectively, in the activated state of the channel. However, charge swapping between S2-D1 and S4-R4 had no compensatory effects on gating or toxin actions, suggesting that charged residue interactions are complex. Collectively, our results highlight the involvement of III-VSM in the actions of both site 3 and site 4 toxins, suggesting that charge reversing substitutions in III-VSM allosterically facilitate IIS4 or IVS4 voltage sensor trapping by these toxins.

Mapping the interaction surface of scorpion β-toxins with an insect sodium channel

The interaction of insect-selective scorpion depressant β-toxins (LqhIT2 and Lqh-dprIT3 from Leiurus quinquestriatus hebraeus) with the Blattella germanica sodium channel, BgNav1-1a, was investigated using site-directed mutagenesis, electrophysiological analyses, and structural modeling. Focusing on the pharmacologically defined binding site-4 of scorpion β-toxins at the voltage-sensing domain II (VSD-II), we found that charge neutralization of D802 in VSD-II greatly enhanced the channel sensitivity to Lqh-dprIT3. This was consistent with the high sensitivity of the splice variant BgNav2-1, bearing G802, to Lqh-dprIT3, and low sensitivity of BgNav2-1 mutant, G802D, to the toxin. Further mutational and electrophysiological analyses revealed that the sensitivity of the WT = D802E < D802G < D802A < D802K channel mutants to Lqh-dprIT3 correlated with the depolarizing shifts of activation in toxin-free channels. However, the sensitivity of single mutants involving IIS4 basic residues (K4E = WT << R1E < R2E < R3E) or double mutants (D802K = K4E/D802K = R3E/D802K > R2E/D802K > R1E/D802K > WT) did not correlate with the activation shifts. Using the cryo-EM structure of the Periplaneta americana channel, NavPaS, as a template and the crystal structure of LqhIT2, we constructed structural models of LqhIT2 and Lqh-dprIT3-c in complex with BgNav1-1a. These models along with the mutational analysis suggest that depressant toxins approach the salt-bridge between R1 and D802 at VSD-II to form contacts with linkers IIS1-S2, IIS3-S4, IIIP5-P1 and IIIP2-S6. Elimination of this salt-bridge enables deeper penetration of the toxin into a VSD-II gorge to form new contacts with the channel, leading to increased channel sensitivity to Lqh-dprIT3.

Cytotoxic and lethal effects of recombinant β-BUTX-Lqq1a peptide against Lepidopteran insects and cell lines

Extensive usage of synthetic chemical pesticides have collateral effect in harming the health, environment and development of resistance in insect pests. Scorpion produces variety of molecules that are specific to insects, mammals and to both. Insect specific molecules act as potential candidature as an alternative to synthetic chemical pesticides. We have successfully expressed and purified recombinant Scorpion Leiurus quinquestriatus quinquestriaus β-BUTX-Lqq1a toxin in bacterial system. Cytotoxic activity assay with the help of insect cell line Sf-21 from Spodoptera frugiperda reveals that mean IC₅₀ 1.72-3.0 μg ml ^-1 significantly reduced the cell proliferation when compared with control. Microscopic examination of treated Sf-21 cell lines also showed changes in the cell morphology such as cell membrane blebbing, cell shrinkage and granulated apoptotic bodies. When β-BUTX-Lqq1a was hemocoelly injected with various doses, significant reduction in survival of Helicoverpa armigera (LC₅₀ = 0.13 μg insect^-1) and Spodoptera litura (LC₅₀ = 0.147 μg insect ^-1) were noticeable with immediate paralysis, and reduced feeding when compared with control. Toxicity with purified recombinant β-BUTX-Lqq1a protein towards insect cell line Sf-21 and major agricultural pest was demonstrated by various bioassays. Cytotoxicity and insect bioassay demonstrated the potential use of β-BUTX-Lqq1a protein as an effective insecticide against lepidopteran insects. These results strongly suggest that the development of rational insecticidal molecule against with significant promise.

Revisiting Polybia paulista wasp venom using shotgun proteomics - Insights into the N-linked glycosylated venom proteins

The partial proteome of Polybia paulista wasp venom was previously reported elsewhere using a gel-dependent approach and resulted in the identification of a limited number of venom toxins. Here, we reinvestigated the P. paulista venom using a gel-free shotgun proteomic approach; the highly dynamic range of this approach facilitated the detection and identification of 1673 proteins, of which 23 venom proteins presented N-linked glycosylation as a posttranslational modification. Three different molecular forms of PLA1 were identified as allergenic proteins, and two of these forms were modified by N-linked glycosylation. This study reveals an extensive repertoire of hitherto undescribed proteins that were classified into the following six different functional groups: (i) typical venom proteins; (ii) proteins related to the folding/conformation and PTMs of toxins; (iii) proteins that protect toxins from oxidative stress; (iv) proteins involved in chemical communication; (v) housekeeping proteins; and (vi) uncharacterized proteins. It was possible to identify venom toxin-like proteins that are commonly reported in other animal venoms, including arthropods such as spiders and scorpions. Thus, the findings reported here may contribute to improving our understanding of the composition of P. paulista venom, its envenoming mechanism and the pathologies experienced by the victim after the wasp stinging accident. BIOLOGICAL SIGNIFICANCE: The present study significantly expanded the number of proteins identified in P. paulista venom, contributing to improvements in our understanding of the envenoming mechanism produced by sting accidents caused by this wasp. For example, novel wasp venom neurotoxins have been identified, but no studies have assessed the presence of this type of toxin in social wasp venoms. In addition, 23 N-linked glycosylated venom proteins were identified in the P. paulista venom proteome, and some of these proteins might be relevant allergens that are immunoreactive to human IgE.

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.

To4, the first Tityus obscurus β-toxin fully electrophysiologically characterized on human sodium channel isoforms

Many scorpion toxins that act on sodium channels (NaScTxs) have been characterized till date. These toxins may act modulating the inactivation or the activation of sodium channels and are named α- or β-types, respectively. Some venom toxins from Tityus obscurus (Buthidae), a scorpion widely distributed in the Brazilian Amazon, have been partially characterized in previous studies; however, little information about their electrophysiological role on sodium ion channels has been published. In the present study, we describe the purification, identification and electrophysiological characterization of a NaScTx, which was first described as Tc54 and further fully sequenced and renamed To4. This toxin shows a marked β-type effect on different sodium channel subtypes (hNa_v1.1-hNa_v1.7) at low concentrations, and has more pronounced activity on hNa_v1.1, hNa_v1.2 and hNa_v1.4. By comparing To4 primary structure with other Tityus β-toxins which have already been electrophysiologically tested, it is possible to establish some key amino acid residues for the sodium channel activity. Thus, To4 is the first toxin from T. obscurus fully electrophysiologically characterized on different human sodium channel isoforms.

Molecular basis for the toxin insensitivity of scorpion voltage-gated potassium channel MmKv1

Our work is the first to uncover the mechanisms by which scorpions resist their own venoms at the ion channel receptor level and enriched our knowledge of the co-evolution of venomous animals and their venoms for hundreds of million years.

A distinct sodium channel voltage-sensor locus determines insect selectivity of the spider toxin Dc1a

β-Diguetoxin-Dc1a (Dc1a) is a toxin from the desert bush spider Diguetia canities that incapacitates insects at concentrations that are non-toxic to mammals. Dc1a promotes opening of German cockroach voltage-gated sodium (Na_v) channels (BgNa_v1), whereas human Na_v channels are insensitive. Here, by transplanting commonly targeted S3b-S4 paddle motifs within BgNa_v1 voltage sensors into K_v2.1, we find that Dc1a interacts with the domain II voltage sensor. In contrast, Dc1a has little effect on sodium currents mediated by PaNa_v1 channels from the American cockroach even though their domain II paddle motifs are identical. When exploring regions responsible for PaNa_v1 resistance to Dc1a, we identified two residues within the BgNa_v1 domain II S1–S2 loop that when mutated to their PaNa_v1 counterparts drastically reduce toxin susceptibility. Overall, our results reveal a distinct region within insect Na_v channels that helps determine Dc1a sensitivity, aconcept that will be valuable for the design of insect-selective insecticides.

Molecular biology of insect sodium channels and pyrethroid resistance

Voltage-gated sodium channels are essential for the initiation and propagation of the action potential in neurons and other excitable cells. Because of their critical roles in electrical signaling, sodium channels are targets of a variety of naturally occurring and synthetic neurotoxins, including several classes of insecticides. This review is intended to provide an update on the molecular biology of insect sodium channels and the molecular mechanism of pyrethroid resistance. Although mammalian and insect sodium channels share fundamental topological and functional properties, most insect species carry only one sodium channel gene, compared to multiple sodium channel genes found in each mammalian species. Recent studies showed that two posttranscriptional mechanisms, alternative splicing and RNA editing, are involved in generating functional diversity of sodium channels in insects. More than 50 sodium channel mutations have been identified to be responsible for or associated with knockdown resistance (kdr) to pyrethroids in various arthropod pests and disease vectors. Elucidation of molecular mechanism of kdr led to the identification of dual receptor sites of pyrethroids on insect sodium channels. Many of the kdr mutations appear to be located within or close to the two receptor sites. The accumulating knowledge of insect sodium channels and their interactions with insecticides provides a foundation for understanding the neurophysiology of sodium channels in vivo and the development of new and safer insecticides for effective control of arthropod pests and human disease vectors.

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.

Voltage-Gated Sodium Channels as Insecticide Targets

Voltage-gated sodium channels are critical for the generation and propagation of action potentials. They are the primary target of several classes of insecticides, including DDT, pyrethroids and sodium channel blocker insecticides (SCBIs). DDT and pyrethroids preferably bind to open sodium channels and stabilize the open state, causing prolonged currents. In contrast, SCBIs block sodium channels by binding to the inactivated state. Many sodium channel mutations are associated with knockdown resistance (kdr) to DDT and pyrethroids in diverse arthropod pests. Functional characterization of kdr mutations together with computational modelling predicts dual pyrethroid receptor sites on sodium channels. In contrast, the molecular determinants of the SCBI receptor site remain largely unknown. In this review, we summarize current knowledge about the molecular mechanisms of action of pyrethroids and SCBIs, and highlight the differences in the molecular interaction of these insecticides with insect versus mammalian sodium channels.

Bactridine's effects on DUM cricket neurons under voltage clamp conditions

We describe the effects of six bactridines (150 nM) on cricket dorsal unpaired median (DUM) neurons. The addition of bactridine 2 to DUM neurons induced a large current component with a reversal potential more negative than -30 mV, most evident at the end of the pulses. This current was completely suppressed when 1 μM amiloride was applied before adding the bactridines. Since the amiloride sensitive current is able to distort the aim of our study, i.e. the effect of bactridines on sodium channels, all experiments were done in the presence of 1 μM amiloride. Most bactridines induced voltage shifts of V(1/2) of the Boltzmann inactivation voltage dependency curves in the hyperpolarizing direction. Bactridines 1, 4 and 6 reduced Na current peak by 65, 80 and 24% of the control, respectively. The sodium conductance blockage by bactridines was voltage independent at potentials >20 mV. Bactridines effect on cricket DUM neurons does not correspond to neither α- nor β-toxins. Most bactridines shifted the inactivation curves in the hyperpolarizing direction without any effects on the activation m(∞)-like curves. Also bactridines differ from other NaScpTx in that they increased an amiloride-sensitive conductance in DUM neurons. Our result suggest that the α/β classification of sodium scorpion toxins is not all encompassing. The present work shows that bactridines target more than one site: insect voltage dependent Na channels and an amiloride-sensitive ionic pathway which is under study.

Conserved functional surface of antimammalian scorpion β-toxins

Scorpion β-toxins bind to the voltage-sensing domain of voltage-gated sodium (NaV) channels and trap the voltage-sensing domain in the activated state. Two structurally similar β-toxins from scorpions, Css4 and Cn2, selectively target different subtypes of mammalian NaV channels. While the receptor site on the channels is known, the functional surface of the toxins remains to be understood. Here, we predict the binding modes of Css4 and Cn2 to the voltage-sensing domains of NaV1.2 and NaV1.6, respectively, with a molecular docking method and molecular dynamics simulations. The dissociation constants for the predicted toxin-channel complexes derived with umbrella sampling simulations are in accord with experiment. Our calculations suggest that the functional surface of Cn2 and Css4 is primarily formed by the loop between positions 8 and 18, centered on the two charged residues Lys13 and Glu15.

Neurotoxins and their binding areas on voltage-gated sodium channels

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