Structure and function of μ-conotoxins, peptide-based sodium channel blockers with analgesic activity

Identification of sodium channel toxins from marine cone snails of the subgenera Textilia and Afonsoconus

Voltage-gated sodium (NaV) channels are transmembrane proteins that play a critical role in electrical signaling in the nervous system and other excitable tissues. µ-Conotoxins are peptide toxins from the venoms of marine cone snails (genus Conus) that block NaV channels with nanomolar potency. Most species of the subgenera Textilia and Afonsoconus are difficult to acquire; therefore, their venoms have yet to be comprehensively interrogated for µ-conotoxins. The goal of this study was to find new µ-conotoxins from species of the subgenera Textilia and Afonsoconus and investigate their selectivity at human NaV channels. Using RNA-seq of the venom gland of Conus (Textilia) bullatus, we identified 12 µ-conotoxin (or µ-conotoxin-like) sequences. Based on these sequences we designed primers which we used to identify additional µ-conotoxin sequences from DNA extracted from historical specimens of species from Textilia and Afonsoconus. We synthesized six of these µ-conotoxins and tested their activity on human NaV1.1-NaV1.8. Five of the six synthetic peptides were potent blockers of human NaV channels. Of these, two peptides (BuIIIB and BuIIIE) were potent blockers of hNaV1.3. Three of the peptides (BuIIIB, BuIIIE and AdIIIA) had submicromolar activity at hNaV1.7. This study serves as an example of the identification of new peptide toxins from historical DNA and provides new insights into structure-activity relationships of µ-conotoxins with activity at hNaV1.3 and hNaV1.7.

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

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

Pathophysiological Responses to Conotoxin Modulation of Voltage-Gated Ion Currents

Voltage-gated ion channels are plasma membrane proteins that generate electrical signals following a change in the membrane voltage. Since they are involved in several physiological processes, their dysfunction may be responsible for a series of diseases and pain states particularly related to neuronal and muscular systems. It is well established for decades that bioactive peptides isolated from venoms of marine mollusks belonging to the Conus genus, collectively known as conotoxins, can target different types and isoforms of these channels exerting therapeutic effects and pain relief. For this reason, conotoxins are widely used for either therapeutic purposes or studies on ion channel mechanisms of action disclosure. In addition their positive property, however, conotoxins may generate pathological states through similar ion channel modulation. In this narrative review, we provide pieces of evidence on the pathophysiological impacts that different members of conotoxin families exert by targeting the three most important voltage-gated channels, such as sodium, calcium, and potassium, involved in cellular processes.

Structural and functional insights into the inhibition of human voltage-gated sodium channels by μ-conotoxin KIIIA disulfide isomers

μ-Conotoxins are components of cone snail venom, well-known for their analgesic activity through potent inhibition of voltage-gated sodium channel (Na_V) subtypes, including Na_V1.7. These small, disulfide-rich peptides are typically stabilized by three disulfide bonds arranged in a ‘native’ CysI-CysIV, CysII-CysV, CysIII-CysVI pattern of disulfide connectivity. However, μ-conotoxin KIIIA, the smallest and most studied μ-conotoxin with inhibitory activity at Na_V1.7, forms two distinct disulfide bond isomers during thermodynamic oxidative folding, including Isomer 1 (CysI-CysV, CysII-CysIV, CysIII-CysVI) and Isomer 2 (CysI-CysVI, CysII-CysIV, CysIII-CysV), but not the native μ-conotoxin arrangement. To date, there has been no study on the structure and activity of KIIIA comprising the native μ-conotoxin disulfide bond arrangement. Here, we evaluated the synthesis, potency, sodium channel subtype selectivity, and 3D structure of the three isomers of KIIIA. Using a regioselective disulfide bond-forming strategy, we synthetically produced the three μ-conotoxin KIIIA isomers displaying distinct bioactivity and Na_V subtype selectivity across human Na_V channel subtypes 1.2, 1.4, and 1.7. We show that Isomer 1 inhibits Na_V subtypes with a rank order of potency of Na_V1.4 > 1.2 > 1.7 and Isomer 2 in the order of Na_V1.4≈1.2 > 1.7, while the native isomer inhibited Na_V1.4 > 1.7≈1.2. The three KIIIA isomers were further evaluated by NMR solution structure analysis and molecular docking with hNa_V1.2. Our study highlights the importance of investigating alternate disulfide isomers, as disulfide connectivity affects not only the overall structure of the peptides but also the potency and subtype selectivity of μ-conotoxins targeting therapeutically relevant Na_V subtypes.

Conformational dynamics in peptide toxins: Implications for receptor interactions and molecular design

Peptide toxins are potent and often exquisitely selective probes of the structure and function of ion channels and receptors, and are therefore of significant interest to the pharmaceutical and biotech industries as both pharmacological tools and therapeutic leads. The three-dimensional structures of peptide toxins are essential as a basis for understanding their structure-activity relationships and their binding to target receptors, as well as in guiding the design of analogues with modified potency and/or selectivity for key targets. NMR spectroscopy has played a key role in elucidating the structures of peptide toxins and probing their structure-function relationships. In this article, we highlight the additional important contribution of NMR to characterising the dynamics of peptide toxins. We also compare the information available from NMR measurements with that afforded by molecular dynamics simulations. We describe several examples of the importance of dynamics measurements over a range of timescales for understanding the structure-function relationships of peptide toxins and their receptor engagement. Peptide toxins that inhibit the voltage-gated potassium channel K_V1.3 with pM affinities display different degrees of conformational flexibility, even though they contain multiple disulfide bonds, and this flexibility can affect the relative orientation of residues that have been shown to be critical for channel binding. Information on the dynamic properties of peptide toxins is important in the design of analogues or mimetics where receptor-bound structures are not available.

New application of an old drug proparacaine in treating epilepsy via liposomal hydrogel formulation

Proparacaine (PPC) is a previously discovered topical anesthetic for ophthalmic optometry and surgery by blocking the central Nav1.3. In this study, we found that proparacaine hydrochloride (PPC-HCl) exerted an acute robust antiepileptic effect in pilocarpine-induced epilepsy mice. More importantly, chronic treatment with PPC-HCl totally terminated spontaneous recurrent seizure occurrence without significant toxicity. Chronic treatment with PPC-HCl did not cause obvious cytotoxicity, neuropsychiatric adverse effects, hepatotoxicity, cardiotoxicity, and even genotoxicity that evaluated by whole genome-scale transcriptomic analyses. Only when in a high dose (50 mg/kg), the QRS interval measured by electrocardiography was slightly prolonged, which was similar to the impact of levetiracetam. Nevertheless, to overcome this potential issue, we adopt a liposome encapsulation strategy that could alleviate cardiotoxicity and prepared a type of hydrogel containing PPC-HCl for sustained release. Implantation of thermosensitive chitosan-based hydrogel containing liposomal PPC-HCl into the subcutaneous tissue exerted immediate and long-lasting remission from spontaneous recurrent seizure in epileptic mice without affecting QRS interval. Therefore, this new liposomal hydrogel formulation of proparacaine could be developed as a transdermal patch for treating epilepsy, avoiding the severe toxicity after chronic treatment with current antiepileptic drugs in clinic.

Insertions and Deletions Play an Important Role in the Diversity of Conotoxins

Previous studies have indicated that each conotoxin precursor has a hyperconserved signal region, a rather conserved pro region and a hypervariable mature region, and nucleotide mutations are the main driven factor. However, in this study, we made an in-depth analysis on the M-superfamily conotoxin precursors and found that the diversity of the signal, pro and mature regions are more complicated than previous findings. Different conotoxin precursors can have same signal, pro and/or mature regions, especially different conotoxin precursors with same mature region but different signal and pro regions. In addition, insertions and deletions (indels) were detected in conotoxin precursors. Indels are infrequent in the signal region but frequent in the pro and mature regions. In contrast to deletions that dominate in the pro region, insertions dominate in the mature region. The number of amino acids is crucial for the physiological functions of mature conotoxins, therefore indels, especially insertions in the mature region, play an important role in the sequence and function diversity of conotoxins.

Small cyclic sodium channel inhibitors

Voltage-gated sodium (Na_V) channels play crucial roles in a range of (patho)physiological processes. Much interest has arisen within the pharmaceutical industry to pursue these channels as analgesic targets following overwhelming evidence that Na_V channel subtypes Na_V1.7-Na_V1.9 are involved in nociception. More recently, Na_V1.1, Na_V1.3 and Na_V1.6 have also been identified to be involved in pain pathways. Venom-derived disulfide-rich peptide toxins, isolated from spiders and cone snails, have been used extensively as probes to investigate these channels and have attracted much interest as drug leads. However, few peptide-based leads have made it as drugs due to unfavourable physiochemical attributes including poor in vivo pharmacokinetics and limited oral bioavailability. The present work aims to bridge the gap in the development pipeline between drug leads and drug candidates by downsizing these larger venom-derived Na_V inhibitors into smaller, more "drug-like" molecules. Here, we use molecular engineering of small cyclic peptides to aid in the determination of what drives subtype selectivity and molecular interactions of these downsized inhibitors across Na_V subtypes. We designed a series of small, stable and novel Na_V probes displaying Na_V subtype selectivity and potency in vitro coupled with potent in vivo analgesic activity, involving yet to be elucidated analgesic pathways in addition to Na_V subtype modulation.

The first Kunitz-type proteins from a viperid venom that potentiate neuromuscular transmission

Kunitz-type proteins that interfere with neuronal transmission have been thus far exclusively detected in venoms of elapid snakes. Here, we report for the first time that such proteins are also present in the venom of a viperid snake. From the venom of the nose-horned viper (Vipera ammodytes ammodytes; Vaa), we isolated Kunitz-type chymotrypsin inhibitors (VaaChi) and demonstrated that these molecules also significantly increase the amplitudes of an indirectly evoked simple muscle contraction of the mouse hemidiaphragm, the end-plate potential and the miniature end-plate potential. By facilitating neuromuscular transmission, these proteins resemble structurally homologous dendrotoxins from mamba (Dendroaspis spp.) venoms, which are blockers of voltage-dependent K⁺ channels at the presynaptic site of the neuromuscular junction. What is the mechanism behind facilitation of neuromuscular transmission by VaaChi has not been established yet, however, blocking of K⁺ channels does not seem to be the most probable option.

Neurobiological activity of conotoxins via sodium channel modulation

Conotoxins (CnTX) are bioactive peptides produced by marine molluscs belonging to Conus genus. The biochemical structure of these venomous peptides is characterized by a low number of amino acids linked with disulfide bonds formed by a high degree of post-translational modifications and glycosylation steps which increase the diversity and rate of evolution of these molecules. CnTX different isoforms are known to target ion channels and, in particular, voltage-gated sodium (Na⁺) channels (Na_v channels). These are transmembrane proteins fundamental in excitable cells for generating the depolarization of plasma membrane potential known as action potential which propagates electrical signals in muscles and nerves for physiological functions. Disorders in Na_v channel activity have been shown to induce neurological pathologies and pain states. Here, we describe the current knowledge of CnTX isoform modulation of the Na_v channel activity, the mechanism of action and the potential therapeutic use of these toxins in counteracting neurological dysfunctions.

RgIA4 Accelerates Recovery from Paclitaxel-Induced Neuropathic Pain in Rats

Chemotherapeutic drugs are widely utilized in the treatment of human cancers. Painful chemotherapy-induced neuropathy is a common, debilitating, and dose-limiting side effect for which there is currently no effective treatment. Previous studies have demonstrated the potential utility of peptides from the marine snail from the genus Conus for the treatment of neuropathic pain. α-Conotoxin RgIA and a potent analog, RgIA4, have previously been shown to prevent the development of neuropathy resulting from the administration of oxaliplatin, a platinum-based antineoplastic drug. Here, we have examined its efficacy against paclitaxel, a chemotherapeutic drug that works by a mechanism of action distinct from that of oxaliplatin. Paclitaxel was administered at 2 mg/kg (intraperitoneally (IP)) every other day for a total of 8 mg/kg. Sprague Dawley rats that were co-administered RgIA4 at 80 µg/kg (subcutaneously (SC)) once daily, five times per week, for three weeks showed significant recovery from mechanical allodynia by day 31. Notably, the therapeutic effects reached significance 12 days after the last administration of RgIA4, which is suggestive of a rescue mechanism. These findings support the effects of RgIA4 in multiple chemotherapeutic models and the investigation of α9α10 nicotinic acetylcholine receptors (nAChRs) as a non-opioid target in the treatment of chronic pain.

Conotoxins: Chemistry and Biology

The venom of the marine predatory cone snails (genus Conus) has evolved for prey capture and defense, providing the basis for survival and rapid diversification of the now estimated 750+ species. A typical Conus venom contains hundreds to thousands of bioactive peptides known as conotoxins. These mostly disulfide-rich and well-structured peptides act on a wide range of targets such as ion channels, G protein-coupled receptors, transporters, and enzymes. Conotoxins are of interest to neuroscientists as well as drug developers due to their exquisite potency and selectivity, not just against prey but also mammalian targets, thereby providing a rich source of molecular probes and therapeutic leads. The rise of integrated venomics has accelerated conotoxin discovery with now well over 10,000 conotoxin sequences published. However, their structural and pharmacological characterization lags considerably behind. In this review, we highlight the diversity of new conotoxins uncovered since 2014, their three-dimensional structures and folds, novel chemical approaches to their syntheses, and their value as pharmacological tools to unravel complex biology. Additionally, we discuss challenges and future directions for the field.

Closer to Nature Through Dynamic Culture Systems

Mechanics in the human body are required for normal cell function at a molecular level. It is now clear that mechanical stimulations play significant roles in cell growth, differentiation, and migration in normal and diseased cells. Recent studies have led to the discovery that normal and cancer cells have different mechanosensing properties. Here, we discuss the application and the physiological and pathological meaning of mechanical stimulations. To reveal the optimal conditions for mimicking an in vivo microenvironment, we must, therefore, discern the mechanotransduction occurring in cells.

Structural and Functional Analyses of Cone Snail Toxins

Cone snails are marine gastropod mollusks with one of the most powerful venoms in nature. The toxins, named conotoxins, must act quickly on the cone snails´ prey due to the fact that snails are extremely slow, reducing their hunting capability. Therefore, the characteristics of conotoxins have become the object of investigation, and as a result medicines have been developed or are in the trialing process. Conotoxins interact with transmembrane proteins, showing specificity and potency. They target ion channels and ionotropic receptors with greater regularity, and when interaction occurs, there is immediate physiological decompensation. In this review we aimed to evaluate the structural features of conotoxins and the relationship with their target types.

Molecular basis for pore blockade of human Na+ channel Nav1.2 by the μ-conotoxin KIIIA

The voltage-gated sodium channel Na_v1.2 is responsible for the initiation and propagation of action potentials in the central nervous system. We report the cryo-electron microscopy structure of human Na_v1.2 bound to a peptidic pore blocker, the μ-conotoxin KIIIA, in the presence of an auxiliary subunit, β2, to an overall resolution of 3.0 angstroms. The immunoglobulin domain of β2 interacts with the shoulder of the pore domain through a disulfide bond. The 16-residue KIIIA interacts with the extracellular segments in repeats I to III, placing Lys⁷ at the entrance to the selectivity filter. Many interacting residues are specific to Na_v1.2, revealing a molecular basis for KIIIA specificity. The structure establishes a framework for the rational design of subtype-specific blockers for Na_v channels.

Where cone snails and spiders meet: design of small cyclic sodium-channel inhibitors

A 13 aa residue voltage-gated sodium (Na_V) channel inhibitor peptide, Pn, containing 2 disulfide bridges was designed by using a chimeric approach. This approach was based on a common pharmacophore deduced from sequence and secondary structural homology of 2 Na_V inhibitors: Conus kinoshitai toxin IIIA, a 14 residue cone snail peptide with 3 disulfide bonds, and Phoneutria nigriventer toxin 1, a 78 residue spider toxin with 7 disulfide bonds. As with the parent peptides, this novel Na_V channel inhibitor was active on Na_V1.2. Through the generation of 3 series of peptide mutants, we investigated the role of key residues and cyclization and their influence on Na_V inhibition and subtype selectivity. Cyclic PnCS1, a 10 residue peptide cyclized via a disulfide bond, exhibited increased inhibitory activity toward therapeutically relevant Na_V channel subtypes, including Na_V1.7 and Na_V1.9, while displaying remarkable serum stability. These peptides represent the first and the smallest cyclic peptide Na_V modulators to date and are promising templates for the development of toxin-based therapeutic agents.-Peigneur, S., Cheneval, O., Maiti, M., Leipold, E., Heinemann, S. H., Lescrinier, E., Herdewijn, P., De Lima, M. E., Craik, D. J., Schroeder, C. I., Tytgat, J. Where cone snails and spiders meet: design of small cyclic sodium-channel inhibitors.

NMR Structure of μ-Conotoxin GIIIC: Leucine 18 Induces Local Repacking of the N-Terminus Resulting in Reduced NaV Channel Potency

μ-Conotoxins are potent and highly specific peptide blockers of voltage-gated sodium channels. In this study, the solution structure of μ-conotoxin GIIIC was determined using 2D NMR spectroscopy and simulated annealing calculations. Despite high sequence similarity, GIIIC adopts a three-dimensional structure that differs from the previously observed conformation of μ-conotoxins GIIIA and GIIIB due to the presence of a bulky, non-polar leucine residue at position 18. The side chain of L18 is oriented towards the core of the molecule and consequently the N-terminus is re-modeled and located closer to L18. The functional characterization of GIIIC defines it as a canonical μ-conotoxin that displays substantial selectivity towards skeletal muscle sodium channels (Na_V), albeit with ~2.5-fold lower potency than GIIIA. GIIIC exhibited a lower potency of inhibition of Na_V1.4 channels, but the same Na_V selectivity profile when compared to GIIIA. These observations suggest that single amino acid differences that significantly affect the structure of the peptide do in fact alter its functional properties. Our work highlights the importance of structural factors, beyond the disulfide pattern and electrostatic interactions, in the understanding of the functional properties of bioactive peptides. The latter thus needs to be considered when designing analogues for further applications.

Purification and Characterization of JZTx-14, a Potent Antagonist of Mammalian and Prokaryotic Voltage-Gated Sodium Channels

Exploring the interaction of ligands with voltage-gated sodium channels (Na_Vs) has advanced our understanding of their pharmacology. Herein, we report the purification and characterization of a novel non-selective mammalian and bacterial Na_Vs toxin, JZTx-14, from the venom of the spider Chilobrachys jingzhao. This toxin potently inhibited the peak currents of mammalian Na_V1.2–1.8 channels and the bacterial NaChBac channel with low IC₅₀ values (<1 µM), and it mainly inhibited the fast inactivation of the Na_V1.9 channel. Analysis of Na_V1.5/Na_V1.9 chimeric channel showed that the Na_V1.5 domain II S3–4 loop is involved in toxin association. Kinetics data obtained from studying toxin–Na_V1.2 channel interaction showed that JZTx-14 was a gating modifier that possibly trapped the channel in resting state; however, it differed from site 4 toxin HNTx-III by irreversibly blocking Na_V currents and showing state-independent binding with the channel. JZTx-14 might stably bind to a conserved toxin pocket deep within the Na_V1.2–1.8 domain II voltage sensor regardless of channel conformation change, and its effect on Na_Vs requires the toxin to trap the S3–4 loop in its resting state. For the NaChBac channel, JZTx-14 positively shifted its conductance-voltage (G–V) and steady-state inactivation relationships. An alanine scan analysis of the NaChBac S3–4 loop revealed that the 108th phenylalanine (F108) was the key residue determining the JZTx-14–NaChBac interaction. In summary, this study provided JZTx-14 with potent but promiscuous inhibitory activity on both the ancestor bacterial Na_Vs and the highly evolved descendant mammalian Na_Vs, and it is a useful probe to understand the pharmacology of Na_Vs.

Predicting Structural Details of the Sodium Channel Pore Basing on Animal Toxin Studies

Eukaryotic voltage-gated sodium channels play key roles in physiology and are targets for many toxins and medically important drugs. Physiology, pharmacology, and general architecture of the channels has long been the subject of intensive research in academia and industry. In particular, animal toxins such as tetrodotoxin, saxitoxin, and conotoxins have been used as molecular probes of the channel structure. More recently, X-ray structures of potassium and prokaryotic sodium channels allowed elaborating models of the toxin-channel complexes that integrated data from biophysical, electrophysiological, and mutational studies. Atomic level cryo-EM structures of eukaryotic sodium channels, which became available in 2017, show that the selectivity filter structure and other important features of the pore domain have been correctly predicted. This validates further employments of toxins and other small molecules as sensitive probes of fine structural details of ion channels.

µ-Conotoxins Modulating Sodium Currents in Pain Perception and Transmission: A Therapeutic Potential

The Conus genus includes around 500 species of marine mollusks with a peculiar production of venomous peptides known as conotoxins (CTX). Each species is able to produce up to 200 different biological active peptides. Common structure of CTX is the low number of amino acids stabilized by disulfide bridges and post-translational modifications that give rise to different isoforms. µ and µO-CTX are two isoforms that specifically target voltage-gated sodium channels. These, by inducing the entrance of sodium ions in the cell, modulate the neuronal excitability by depolarizing plasma membrane and propagating the action potential. Hyperexcitability and mutations of sodium channels are responsible for perception and transmission of inflammatory and neuropathic pain states. In this review, we describe the current knowledge of µ-CTX interacting with the different sodium channels subtypes, the mechanism of action and their potential therapeutic use as analgesic compounds in the clinical management of pain conditions.

Cloning, expression and functional characterization of a D-superfamily conotoxin Lt28.1 with previously undescribed cysteine pattern

As a class of peptides with 10 cysteine residues (-C-CC-C-CC-C-C-C-C-), D-superfamily conotoxins (D-conotoxins) can specifically act on nicotinic acetylcholine receptors (nAChRs). According to the conserved signal peptides of D-conotoxins, seven D-conotoxin precursor sequences with a previously undescribed Cys arrangement (-C-C-C-CC-C-C-C-C-C-) were identified by PCR-RACE methodology in the present study. The alignment of sequences revealed that signal peptide regions were same as D-VxXXA from Conus vexillum, and their mature peptides were almost different from the D-conotoxins. Analyses of the evolutionary tree demonstrated that they had low homology to those reported conotoxins with 10 cysteine residues (less than 35%) and lied in a separate branch in the evolutionary tree. Furthermore, a previously undescribed D-superfamily conotoxin Lt28.1 was further expressed in Pichia pastoris and then functionally characterized. The results showed that the recombinant Lt28.1 targeted α9α10 nAChRs but not other nAChRs subtypes. These findings defined a new branch of D-superfamily and expanded our knowledge of targets and potential application of D-conotoxins.

Sensitive Detection of α-Conotoxin GI in Human Plasma Using a Solid-Phase Extraction Column and LC-MS/MS

α-conotoxin GI, a short peptide toxin in the venom of Conus geographus, is composed of 13 amino acids and two disulfide bonds. It is the most toxic component of Conus geographus venom with estimated lethal doses of 0.029–0.038 mg/kg for humans. There is currently no reported analytical method for this toxin. In the present study, a sensitive detection method was developed to quantify GI in human plasma using a solid-phase extraction (SPE) column (polystyrene–divinyl benzene copolymer) combined with liquid chromatography/electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) in the multiple reaction monitoring (MRM) mode. The plasma samples were treated with a protein precipitating solvent (methanol: acetonitrile = 50:50, v/v). GI in the solvent was efficiently extracted with an SPE column and was further separated by a Grace Alltima HP C₁₈ (50 × 2.1 mm, 5 μm) column at a flow rate of 0.4 mL/min. Water (with 2% methanol) acetonitrile (with 0.1% acetic acid) was selected as the mobile phase combination used in a linear gradient system. α-Conotoxin GI was analyzed by an API 4000 triple quadrupole mass spectrometer. In the method validation, the linear calibration curve in the range of 2.0 to 300.0 ng/mL had correlation coefficients (r) above 0.996. The recovery was 57.6–66.8% for GI and the internal standard. The lower limit of quantification (LLOQ) was 2 ng/mL. The intra- and inter-batch precisions were below 6.31% and 8.61%, respectively, and the accuracies were all within acceptance. GI was stable in a bench-top autosampler through long-term storage and freeze/thaw cycles. Therefore, this method is specific, sensitive and reliable for quantitative analysis of α-conotoxin GI in human plasma.

A Transcriptomic Survey of Ion Channel-Based Conotoxins in the Chinese Tubular Cone Snail (Conus betulinus)

Conotoxins in the venom of cone snails (Conus spp.) are a mixture of active peptides that work as blockers, agonists, antagonists, or inactivators of various ion channels. Recently we reported a high-throughput method to identify 215 conotoxin transcripts from the Chinese tubular cone snail, C. betulinus. Here, based on the previous datasets of four transcriptomes from three venom ducts and one venom bulb, we explored ion channel-based conotoxins and predicted their related ion channel receptors. Homologous analysis was also performed for the most abundant ion channel protein, voltage-gated potassium (Kv; with Kv1.1 as the representative), and the most studied ion channel receptor, nicotinic acetylcholine receptor (nAChR; with α2-nAChR as the representative), in different animals. Our transcriptomic survey demonstrated that ion channel-based conotoxins and related ion channel proteins/receptors transcribe differentially between the venom duct and the venom bulb. In addition, we observed that putative κ-conotoxins were the most common conotoxins with the highest transcription levels in the examined C. betulinus. Furthermore, Kv1.1 and α2-nAChR were conserved in their functional domains of deduced protein sequences, suggesting similar effects of conotoxins via the ion channels in various species, including human beings. In a word, our present work suggests a high-throughput way to develop conotoxins as potential drugs for treatment of ion channel-associated human diseases.

Centipede venoms as a source of drug leads

INTRODUCTION

Centipedes are one of the oldest and most successful lineages of venomous terrestrial predators. Despite their use for centuries in traditional medicine, centipede venoms remain poorly studied. However, recent work indicates that centipede venoms are highly complex chemical arsenals that are rich in disulfide-constrained peptides that have novel pharmacology and three-dimensional structure. Areas covered: This review summarizes what is currently known about centipede venom proteins, with a focus on disulfide-rich peptides that have novel or unexpected pharmacology that might be useful from a therapeutic perspective. The authors also highlight the remarkable diversity of constrained three-dimensional peptide scaffolds present in these venoms that might be useful for bioengineering of drug leads. Expert opinion: Like most arthropod predators, centipede venoms are rich in peptides that target neuronal ion channels and receptors, but it is also becoming increasingly apparent that many of these peptides have novel or unexpected pharmacological properties with potential applications in drug discovery and development.

Venom Peptides From Cone Snails: Pharmacological Probes for Voltage-Gated Sodium Channels

The venoms of cone snails provide a rich source of neuroactive peptides (conotoxins). Several venom peptide families have been identified that are either agonists (ι- and δ-conotoxins) or antagonists (μ- and μO-conotoxins) of voltage-gated sodium channels (VGSCs). Members of these conotoxin classes have been integral in identifying and characterizing specific neurotoxin binding sites on the channel. Furthermore, given the specificity of some of these peptides for one sodium channel subtype over another, conotoxins have also proven useful in exploring differences between VGSC subtypes. This chapter summarizes the current knowledge of the structure and function based on the results of conotoxin interactions with VGSCs and correlates the peptides with the phylogeny of the Conus species from which they were derived.

Novel sodium channel antagonists in the treatment of neuropathic pain

INTRODUCTION

Effective and safe drugs for the treatment of neuropathic pain are still an unmet clinical need. Neuropathic pain, caused by a lesion or disease that affects the somatosensory system, is a debilitating and hampering condition that has a great economic cost and, above all, a tremendous impact on the quality of life. Sodium channels are one of the major players in generating and propagating action potentials. They represent an appealing target for researchers involved in the development of new and safer drugs useful in the treatment of neuropathic pain. The actual goal for researchers is to target sodium channels selectively to stop the abnormal signaling that characterizes neuropathic pain while leaving normal somatosensory functions intact.

AREAS COVERED

This review covers the most recent publications regarding sodium channel blockers and their development as new treatments for neuropathic pain. The main areas discussed are the natural sources of new blockers, such as venom extracts and the recent efforts from many pharmaceutical companies in the field.

EXPERT OPINION

There have been serious efforts by both the pharmaceutical industry and academia to develop new and safer therapeutic options for neuropathic pain. A number of different strategies have been undertaken; the main efforts directed towards the identification of selective blockers starting from both natural products or screening chemical libraries. At this time, researchers have identified and characterized selective compounds against NaV1.7 or NaV1.8 voltage-gated sodium channels but only time will tell if they reach the market.

Bioactive Mimetics of Conotoxins and other Venom Peptides

Ziconotide (Prialt^®), a synthetic version of the peptide ω-conotoxin MVIIA found in the venom of a fish-hunting marine cone snail Conus magnus, is one of very few drugs effective in the treatment of intractable chronic pain. However, its intrathecal mode of delivery and narrow therapeutic window cause complications for patients. This review will summarize progress in the development of small molecule, non-peptidic mimics of Conotoxins and a small number of other venom peptides. This will include a description of how some of the initially designed mimics have been modified to improve their drug-like properties.

Therapeutic conotoxins: a US patent literature survey

INTRODUCTION

Conotoxins are a large family of bioactive peptides derived from cone snail venom. They target specific classes of ion channels and other membrane proteins and may have therapeutic value, primarily in the management of pain.

AREAS COVERED

The authors surveyed the US patent literature covering conotoxins, and their potential therapeutic applications. They describe the various subclasses of conotoxins that are the subject of current patent applications and their therapeutic indications. Limitations that may preclude broader application of these molecules are discussed and strategies for overcoming these limitations are presented.

EXPERT OPINION

Despite more than 25 years of intense global conotoxin research, only one molecule has successfully reached the market. Several other conotoxin-derived candidates failed in clinical trials, indicating that 'from the bench into the clinic' translation has been more difficult than originally anticipated. Nevertheless, we are optimistic that the potent activities of these molecules and the potential for improving their biopharmaceutical properties may lead to next-generation drug candidates with favorable pharmacological properties.

Conotoxin gene superfamilies

Conotoxins are the peptidic components of the venoms of marine cone snails (genus Conus). They are remarkably diverse in terms of structure and function. Unique potency and selectivity profiles for a range of neuronal targets have made several conotoxins valuable as research tools, drug leads and even therapeutics, and has resulted in a concerted and increasing drive to identify and characterise new conotoxins. Conotoxins are translated from mRNA as peptide precursors, and cDNA sequencing is now the primary method for identification of new conotoxin sequences. As a result, gene superfamily, a classification based on precursor signal peptide identity, has become the most convenient method of conotoxin classification. Here we review each of the described conotoxin gene superfamilies, with a focus on the structural and functional diversity present in each. This review is intended to serve as a practical guide to conotoxin superfamilies and to facilitate interpretation of the increasing number of conotoxin precursor sequences being identified by targeted-cDNA sequencing and more recently high-throughput transcriptome sequencing.