Two novel sodium channel inhibitors from Heriaeus melloteei spider venom differentially interacting with mammalian channel's isoforms

Pain-related toxins in scorpion and spider venoms: a face to face with ion channels

Pain is a common symptom induced during envenomation by spiders and scorpions. Toxins isolated from their venom have become essential tools for studying the functioning and physiopathological role of ion channels, as they modulate their activity. In particular, toxins that induce pain relief effects can serve as a molecular basis for the development of future analgesics in humans. This review provides a summary of the different scorpion and spider toxins that directly interact with pain-related ion channels, with inhibitory or stimulatory effects. Some of these toxins were shown to affect pain modalities in different animal models providing information on the role played by these channels in the pain process. The close interaction of certain gating-modifier toxins with membrane phospholipids close to ion channels is examined along with molecular approaches to improve selectivity, affinity or bioavailability in vivo for therapeutic purposes.

Spider Knottin Pharmacology at Voltage-Gated Sodium Channels and Their Potential to Modulate Pain Pathways

Voltage-gated sodium channels (Na_Vs) are a key determinant of neuronal signalling. Neurotoxins from diverse taxa that selectively activate or inhibit Na_V channels have helped unravel the role of Na_V channels in diseases, including chronic pain. Spider venoms contain the most diverse array of inhibitor cystine knot (ICK) toxins (knottins). This review provides an overview on how spider knottins modulate Na_V channels and describes the structural features and molecular determinants that influence their affinity and subtype selectivity. Genetic and functional evidence support a major involvement of Na_V subtypes in various chronic pain conditions. The exquisite inhibitory properties of spider knottins over key Na_V subtypes make them the best lead molecules for the development of novel analgesics to treat chronic pain.

Role of human Hv1 channels in sperm capacitation and white blood cell respiratory burst established by a designed peptide inhibitor

Using a de novo peptide inhibitor, Corza6 (C6), we demonstrate that the human voltage-gated proton channel (hHv1) is the main pathway for H⁺ efflux that allows capacitation in sperm and permits sustained reactive oxygen species (ROS) production in white blood cells (WBCs). C6 was identified by a phage-display strategy whereby ∼1 million novel peptides were fabricated on an inhibitor cysteine knot (ICK) scaffold and sorting on purified hHv1 protein. Two C6 peptides bind to each dimeric channel, one on the S3-S4 loop of each voltage sensor domain (VSD). Binding is cooperative with an equilibrium affinity (K _d) of ∼1 nM at -50 mV. As expected for a VSD-directed toxin, C6 inhibits by shifting hHv1 activation to more positive voltages, slowing opening and speeding closure, effects that diminish with membrane depolarization.

The insecticidal spider toxin SFI1 is a knottin peptide that blocks the pore of insect voltage-gated sodium channels via a large β-hairpin loop

Spider venoms contain a plethora of insecticidal peptides that act on neuronal ion channels and receptors. Because of their high specificity, potency and stability, these peptides have attracted much attention as potential environmentally friendly insecticides. Although many insecticidal spider venom peptides have been isolated, the molecular target, mode of action and structure of only a small minority have been explored. Sf1a, a 46-residue peptide isolated from the venom of the tube-web spider Segesteria florentina, is insecticidal to a wide range of insects, but nontoxic to vertebrates. In order to investigate its structure and mode of action, we developed an efficient bacterial expression system for the production of Sf1a. We determined a high-resolution solution structure of Sf1a using multidimensional 3D/4D NMR spectroscopy. This revealed that Sf1a is a knottin peptide with an unusually large β-hairpin loop that accounts for a third of the peptide length. This loop is delimited by a fourth disulfide bond that is not commonly found in knottin peptides. We showed, through mutagenesis, that this large loop is functionally critical for insecticidal activity. Sf1a was further shown to be a selective inhibitor of insect voltage-gated sodium channels, consistent with its 'depressant' paralytic phenotype in insects. However, in contrast to the majority of spider-derived sodium channel toxins that function as gating modifiers via interaction with one or more of the voltage-sensor domains, Sf1a appears to act as a pore blocker.

Structure of membrane-active toxin from crab spider Heriaeus melloteei suggests parallel evolution of sodium channel gating modifiers in Araneomorphae and Mygalomorphae

We present a structural and functional study of a sodium channel activation inhibitor from crab spider venom. Hm-3 is an insecticidal peptide toxin consisting of 35 amino acid residues from the spider Heriaeus melloteei (Thomisidae). We produced Hm-3 recombinantly in Escherichia coli and determined its structure by NMR spectroscopy. Typical for spider toxins, Hm-3 was found to adopt the so-called "inhibitor cystine knot" or "knottin" fold stabilized by three disulfide bonds. Its molecule is amphiphilic with a hydrophobic ridge on the surface enriched in aromatic residues and surrounded by positive charges. Correspondingly, Hm-3 binds to both neutral and negatively charged lipid vesicles. Electrophysiological studies showed that at a concentration of 1 μm Hm-3 effectively inhibited a number of mammalian and insect sodium channels. Importantly, Hm-3 shifted the dependence of channel activation to more positive voltages. Moreover, the inhibition was voltage-dependent, and strong depolarizing prepulses attenuated Hm-3 activity. The toxin is therefore concluded to represent the first sodium channel gating modifier from an araneomorph spider and features a "membrane access" mechanism of action. Its amino acid sequence and position of the hydrophobic cluster are notably different from other known gating modifiers from spider venom, all of which are described from mygalomorph species. We hypothesize parallel evolution of inhibitor cystine knot toxins from Araneomorphae and Mygalomorphae suborders.

Molecular surface of JZTX-V (β-Theraphotoxin-Cj2a) interacting with voltage-gated sodium channel subtype NaV1.4

Voltage-gated sodium channels (VGSCs; Na_V1.1–Na_V1.9) have been proven to be critical in controlling the function of excitable cells, and human genetic evidence shows that aberrant function of these channels causes channelopathies, including epilepsy, arrhythmia, paralytic myotonia, and pain. The effects of peptide toxins, especially those isolated from spider venom, have shed light on the structure–function relationship of these channels. However, most of these toxins have not been analyzed in detail. In particular, the bioactive faces of these toxins have not been determined. Jingzhaotoxin (JZTX)-V (also known as β-theraphotoxin-Cj2a) is a 29-amino acid peptide toxin isolated from the venom of the spider Chilobrachys jingzhao. JZTX-V adopts an inhibitory cysteine knot (ICK) motif and has an inhibitory effect on voltage-gated sodium and potassium channels. Previous experiments have shown that JZTX-V has an inhibitory effect on TTX-S and TTX-R sodium currents on rat DRG cells with IC₅₀ values of 27.6 and 30.2 nM, respectively, and is able to shift the activation and inactivation curves to the depolarizing and the hyperpolarizing direction, respectively. Here, we show that JZTX-V has a much stronger inhibitory effect on Na_V1.4, the isoform of voltage-gated sodium channels predominantly expressed in skeletal muscle cells, with an IC₅₀ value of 5.12 nM, compared with IC₅₀ values of 61.7–2700 nM for other heterologously expressed Na_V1 subtypes. Furthermore, we investigated the bioactive surface of JZTX-V by alanine-scanning the effect of toxin on Na_V1.4 and demonstrate that the bioactive face of JZTX-V is composed of three hydrophobic (W5, M6, and W7) and two cationic (R20 and K22) residues. Our results establish that, consistent with previous assumptions, JZTX-V is a Janus-faced toxin which may be a useful tool for the further investigation of the structure and function of sodium channels.

The insecticidal neurotoxin Aps III is an atypical knottin peptide that potently blocks insect voltage-gated sodium channels

One of the most potent insecticidal venom peptides described to date is Aps III from the venom of the trapdoor spider Apomastus schlingeri. Aps III is highly neurotoxic to lepidopteran crop pests, making it a promising candidate for bioinsecticide development. However, its disulfide-connectivity, three-dimensional structure, and mode of action have not been determined. Here we show that recombinant Aps III (rAps III) is an atypical knottin peptide; three of the disulfide bridges form a classical inhibitor cystine knot motif while the fourth disulfide acts as a molecular staple that restricts the flexibility of an unusually large β hairpin loop that often houses the pharmacophore in this class of toxins. We demonstrate that the irreversible paralysis induced in insects by rAps III results from a potent block of insect voltage-gated sodium channels. Channel block by rAps III is voltage-independent insofar as it occurs without significant alteration in the voltage-dependence of channel activation or steady-state inactivation. Thus, rAps III appears to be a pore blocker that plugs the outer vestibule of insect voltage-gated sodium channels. This mechanism of action contrasts strikingly with virtually all other sodium channel modulators isolated from spider venoms that act as gating modifiers by interacting with one or more of the four voltage-sensing domains of the channel.

Advances in targeting voltage-gated sodium channels with small molecules

Blockade of voltage-gated sodium channels (VGSCs) has been used successfully in the clinic to enable control of pathological firing patterns that occur in conditions as diverse as chronic pain, epilepsy, and arrhythmias. Herein we review the state of the art in marketed sodium channel inhibitors, including a brief compendium of their binding sites and of the cellular and molecular biology of sodium channels. Despite the preferential action of this drug class toward over-excited cells, which significantly limits potential undesired side effects on other cells, the need to develop a second generation of sodium channel inhibitors to overcome their critical clinical shortcomings is apparent. Current approaches in drug discovery to deliver novel and truly innovative sodium channel inhibitors is next presented by surveying the most recent medicinal chemistry breakthroughs in the field of small molecules and developments in automated patch-clamp platforms. Various strategies aimed at identifying small molecules that target either particular isoforms of sodium channels involved in specific diseases or anomalous sodium channel currents, irrespective of the isoform by which they have been generated, are critically discussed and revised.

Molecular diversity of spider venom

Spider venom, a factor that has played a decisive role in the evolution of one of the most successful groups of living organisms, is reviewed. Unique molecular diversity of venom components including substances of variable structure (from simple low molecular weight compounds to large multidomain proteins) with different functions is considered. Special attention is given to the structure, properties, and biosynthesis of toxins of polypeptide nature.

Unique bell-shaped voltage-dependent modulation of Na+ channel gating by novel insect-selective toxins from the spider Agelena orientalis

Spider venoms provide a highly valuable source of peptide toxins that act on a wide diversity of membrane-bound receptors and ion channels. In this work, we report isolation, biochemical analysis, and pharmacological characterization of a novel family of spider peptide toxins, designated beta/delta-agatoxins. These toxins consist of 36-38 amino acid residues and originate from the venom of the agelenid funnel-web spider Agelena orientalis. The presented toxins show considerable amino acid sequence similarity to other known toxins such as mu-agatoxins, curtatoxins, and delta-palutoxins-IT from the related spiders Agelenopsis aperta, Hololena curta, and Paracoelotes luctuosus. beta/delta-Agatoxins modulate the insect Na(V) channel (DmNa(V)1/tipE) in a unique manner, with both the activation and inactivation processes being affected. The voltage dependence of activation is shifted toward more hyperpolarized potentials (analogous to site 4 toxins) and a non-inactivating persistent Na(+) current is induced (site 3-like action). Interestingly, both effects take place in a voltage-dependent manner, producing a bell-shaped curve between -80 and 0 mV, and they are absent in mammalian Na(V) channels. To the best of our knowledge, this is the first detailed report of peptide toxins with such a peculiar pharmacological behavior, clearly indicating that traditional classification of toxins according to their binding sites may not be as exclusive as previously assumed.