A novel µ-conopeptide, CnIIIC, exerts potent and preferential inhibition of NaV1.2/1.4 channels and blocks neuronal nicotinic acetylcholine receptors

Voltage-Gated Sodium Channel Inhibition by µ-Conotoxins

µ-Conotoxins are small, potent pore-blocker inhibitors of voltage-gated sodium (NaV) channels, which have been identified as pharmacological probes and putative leads for analgesic development. A limiting factor in their therapeutic development has been their promiscuity for different NaV channel subtypes, which can lead to undesirable side-effects. This review will focus on four areas of µ-conotoxin research: (1) mapping the interactions of µ-conotoxins with different NaV channel subtypes, (2) µ-conotoxin structure-activity relationship studies, (3) observed species selectivity of µ-conotoxins and (4) the effects of µ-conotoxin disulfide connectivity on activity. Our aim is to provide a clear overview of the current status of µ-conotoxin research.

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.

Recent advances on marine mollusk-derived natural products: chemistry, chemical ecology and therapeutical potential

Covering: 2011-2021Marine mollusks, which are well known as rich sources of diverse and biologically active natural products, have attracted significant attention from researchers due to their chemical and pharmacological properties. The occurrence of some of these marine mollusk-derived natural products in their preys, predators, and associated microorganisms has also gained interest in chemical ecology research. Based on previous reviews, herein, we present a comprehensive summary of the recent advances of interesting secondary metabolites from marine mollusks, focusing on their structural features, possible chemo-ecological significance, and promising biological activities, covering the literature from 2011 to 2021.

Preparation of Conotoxin-Encapsulated Chitosan Nanoparticles and Evaluation of Their Skin Permeability

μ-Conotoxin CnIIIC (conotoxin, CTX)-loaded chitosan nanoparticles (CTX-NPs) were prepared using the ionic cross-linking method. The CTX-NPs were spherical and well with a polydispersity index of 0.292 ± 0.039, drug loading efficiency of 25.9 ± 1.2%, and encapsulation efficiency of 95.6 ± 1.3%. In vitro release studies showed that the release behavior of CTX-NPs in a pH 5.0 acetate buffer followed zero-order kinetics. In vitro transdermal experiments using Franz diffusion cells mounted with mouse abdominal skin demonstrated that the cumulative intradermal deposition amount of CTX per unit area in 8 h (D₈) and permeability coefficient (P_f) of CTX loaded on CTX-NPs were 2.30- and 7.71-times that of the CTX solution. In vivo transdermal experiments in mice showed that the amount of CTX deposited in the skin after 8 h of CTX saline administration was significantly lower than that of CTX deposited in the skin after administration of CTX-NPs. In vitro fluorescence labeling transdermal studies through Franz diffusion cells mounted with mouse abdominal skin indicated that CTX-NPs aggregated at hair follicles. Skin irritation tests in mice indicated that the irritation due to CTX-NPs was negligible. The cytotoxicity experiment showed that the viability of Balb/c 3T3 cells with CTX-NPs containing 230 μg/mL (0.08 μM) CTX was greater than 75%. CTX-NPs increase intradermal deposition of CTX by accumulating in hair follicles, which has positive implications for transdermal penetration of CTX.

µ-Conotoxins Targeting the Human Voltage-Gated Sodium Channel Subtype NaV1.7

µ-Conotoxins are small, potent, peptide voltage-gated sodium (Na_V) channel inhibitors characterised by a conserved cysteine framework. Despite promising in vivo studies indicating analgesic potential of these compounds, selectivity towards the therapeutically relevant subtype Na_V1.7 has so far been limited. We recently identified a novel µ-conotoxin, SxIIIC, which potently inhibits human Na_V1.7 (hNa_V1.7). SxIIIC has high sequence homology with other µ-conotoxins, including SmIIIA and KIIIA, yet shows different Na_V channel selectivity for mammalian subtypes. Here, we evaluated and compared the inhibitory potency of µ-conotoxins SxIIIC, SmIIIA and KIIIA at hNa_V channels by whole-cell patch-clamp electrophysiology and discovered that these three closely related µ-conotoxins display unique selectivity profiles with significant variations in inhibitory potency at hNa_V1.7. Analysis of other µ-conotoxins at hNa_V1.7 shows that only a limited number are capable of inhibition at this subtype and that differences between the number of residues in loop 3 appear to influence the ability of µ-conotoxins to inhibit hNa_V1.7. Through mutagenesis studies, we confirmed that charged residues in this region also affect the selectivity for hNa_V1.4. Comparison of µ-conotoxin NMR solution structures identified differences that may contribute to the variance in hNa_V1.7 inhibition and validated the role of the loop 1 extension in SxIIIC for improving potency at hNa_V1.7, when compared to KIIIA. This work could assist in designing µ-conotoxin derivatives specific for hNa_V1.7.

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.

Marine Origin Ligands of Nicotinic Receptors: Low Molecular Compounds, Peptides and Proteins for Fundamental Research and Practical Applications

The purpose of our review is to briefly show what different compounds of marine origin, from low molecular weight ones to peptides and proteins, offer for understanding the structure and mechanism of action of nicotinic acetylcholine receptors (nAChRs) and for finding novel drugs to combat the diseases where nAChRs may be involved. The importance of the mentioned classes of ligands has changed with time; a protein from the marine snake venom was the first excellent tool to characterize the muscle-type nAChRs from the electric ray, while at present, muscle and α7 receptors are labeled with the radioactive or fluorescent derivatives prepared from α-bungarotoxin isolated from the many-banded krait. The most sophisticated instruments to distinguish muscle from neuronal nAChRs, and especially distinct subtypes within the latter, are α-conotoxins. Such information is crucial for fundamental studies on the nAChR revealing the properties of their orthosteric and allosteric binding sites and mechanisms of the channel opening and closure. Similar data are provided by low-molecular weight compounds of marine origin, but here the main purpose is drug design. In our review we tried to show what has been obtained in the last decade when the listed classes of compounds were used in the nAChR research, applying computer modeling, synthetic analogues and receptor mutants, X-ray and electron-microscopy analyses of complexes with the nAChRs, and their models which are acetylcholine-binding proteins and heterologously-expressed ligand-binding domains.

Neurocosmetics in Skincare—The Fascinating World of Skin–Brain Connection: A Review to Explore Ingredients, Commercial Products for Skin Aging, and Cosmetic Regulation

The “modern” cosmetology industry is focusing on research devoted to discovering novel neurocosmetic functional ingredients that could improve the interactions between the skin and the nervous system. Many cosmetic companies have started to formulate neurocosmetic products that exhibit their activity on the cutaneous nervous system by affecting the skin’s neuromediators through different mechanisms of action. This review aims to clarify the definition of neurocosmetics, and to describe the features of some functional ingredients and products available on the market, with a look at the regulatory aspect. The attention is devoted to neurocosmetic ingredients for combating skin stress, explaining the stress pathways, which are also correlated with skin aging. “Neuro-relaxing” anti-aging ingredients derived from plant extracts and neurocosmetic strategies to combat inflammatory responses related to skin stress are presented. Afterwards, the molecular basis of sensitive skin and the suitable neurocosmetic ingredients to improve this problem are discussed. With the aim of presenting the major application of Botox-like ingredients as the first neurocosmetics on the market, skin aging is also introduced, and its theory is presented. To confirm the efficacy of the cosmetic products on the market, the concept of cosmetic claims is discussed.

A novel proline-rich M-superfamily conotoxin that can simultaneously affect sodium, potassium and calcium currents

Background

Conotoxins have become a research hotspot in the neuropharmacology field for their high activity and specificity in targeting ion channels and neurotransmitter receptors. There have been reports of a conotoxin acting on two ion channels, but rare reports of a conotoxin acting on three ion channels.

Methods

Vr3a, a proline-rich M-superfamily conotoxin from a worm-hunting Conus varius, was obtained by solid-phase synthesis and identified by mass spectrometry. The effects of synthesized Vr3a on sodium, potassium and calcium currents were tested on rat DRG cells by patch clamp experiments. The further effects of Vr3a on human Ca_v1.2 and Ca_v2.2 currents were tested on HEK293 cells.

Results

About 10 μM Vr3a has no effects on the peak sodium currents, but can induce a ~10 mV shift in a polarizing direction in the current-voltage relationship. In addition, 10 μM Vr3a can increase 19.61 ± 5.12% of the peak potassium currents and do not induce a shift in the current-voltage relationship. An amount of 10 μM Vr3a can inhibit 31.26% ± 4.53% of the peak calcium currents and do not induce a shift in the current-voltage relationship. The IC₅₀ value of Vr3a on calcium channel currents in rat DRG neurons is 19.28 ± 4.32 μM. Moreover, 10 μM Vr3a can inhibit 15.32% ± 5.41% of the human Ca_v1.2 currents and 12.86% ± 4.93% of the human Ca_v2.2 currents.

Conclusions

Vr3a can simultaneously affect sodium, potassium and calcium currents. This novel triple-target conotoxin Vr3a expands understanding of conotoxin functions.

Advances in venom peptide drug discovery: where are we at and where are we heading?

Introduction: Animal venoms are a complex mixture of bioactive molecules that have evolved over millions of years for prey capture and defense from predators. Venom consists of many different types of molecules, with disulfide-rich peptides being a major component in most venoms. The study of these potent and highly selective molecules has led to the development of venom-derived drugs for diseases such as type 2 diabetes mellitus and chronic pain. As technologies have improved, more bioactive peptides have been discovered from venomous animals. Many of these molecules may have applications as tools for understanding normal and disease physiology, therapeutics, cosmetics or in agriculture.Areas covered: This article reviews venom-derived drugs approved by the FDA and venom-derived peptides currently in development. It discusses the challenges faced by venom-derived peptide drugs during drug development and the future for venom-derived peptides.Expert opinion: New techniques such as toxin driven discovery are expanding the pipeline of venom-derived peptides. There are many venom-derived peptides currently in preclinical and clinical trials that would have remained undiscovered using traditional approaches. A renewed focus on venoms, with advances in technology, will broaden the diversity of venom-derived peptide therapeutics and expand our knowledge of their molecular targets.

Post-translationally modified conopeptides: Biological activities and pharmacological applications

Conus venoms comprise a large variety of biologically active peptides (conopeptides or conotoxins) that are employed for prey capture and other biological functions. Throughout the course of evolution of the cone snails, they have developed an envenomation scheme that necessitates a potent mixture of peptides, most of which are highly post-translationally modified, that can cause rapid paralysis of their prey. The great diversity of these peptides defines the ecological interactions and evolutionary strategy of cone snails. Such scheme has led to some pharmacological applications for pain, epilepsy, and myocardial infarction, that could be further explored to ultimately find unique peptide-based therapies. This review focuses on ∼ 60 representative post-translationally modified conopeptides that were isolated from Conus venoms. Various conopeptides reveal post-translational modifications of specific amino acids, such as hydroxylation of proline and lysine, gamma-carboxylation of glutamate, formation of N-terminal pyroglutamate, isomerization of l- to d-amino acid, bromination of tryptophan, O-glycosylation of threonine or serine, sulfation of tyrosine, and cysteinylation of cysteine, other than the more common disulfide crosslinking and C-terminal amidation. Many of the post-translationally modified peptides paved the way for the characterization, by alternative analytical methods, of other pharmacologically important peptides that are classified under 27 conopeptide families denoting pharmacological classes.

Prevention of scar hyperplasia in the skin by conotoxin: A prospective review

Scars are often considered to be skin problems that affect beauty. The tension acting on the edge of the wound is the main factor causing the scar hyperplasia. At present, the clinical use of botulinum toxin A (BTX-A) around the wound to cause transient muscle paralysis reduce the muscle movement around the wound and wound tension to prevent scar hyperplasia during wound healing. But the use of BTX-A to prevent scarring requires the use of a syringe. The syringe can cause trauma and pain when it pricks the skin for BTX-A injection. The conotoxin which is secreted by the poison glands on the inside of the venom tube and capsule of the snail provides a simple and effective way to prevent skin scar hyperplasia. We reviewed the classification of conotoxin, the conotoxin's mechanism of preventing scar hyperplasia, and the research direction of conotoxin in the future and provided reference for promoting the application of conotoxin in preventing skin scar hyperplasia.

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.

Curses or Cures: A Review of the Numerous Benefits Versus the Biosecurity Concerns of Conotoxin Research

Conotoxins form a diverse group of peptide toxins found in the venom of predatory marine cone snails. Decades of conotoxin research have provided numerous measurable scientific and societal benefits. These include their use as a drug, diagnostic agent, drug leads, and research tools in neuroscience, pharmacology, biochemistry, structural biology, and molecular evolution. Human envenomations by cone snails are rare but can be fatal. Death by envenomation is likely caused by a small set of toxins that induce muscle paralysis of the diaphragm, resulting in respiratory arrest. The potency of these toxins led to concerns regarding the potential development and use of conotoxins as biological weapons. To address this, various regulatory measures have been introduced that limit the use and access of conotoxins within the research community. Some of these regulations apply to all of the ≈200,000 conotoxins predicted to exist in nature of which less than 0.05% are estimated to have any significant toxicity in humans. In this review we provide an overview of the many benefits of conotoxin research, and contrast these to the perceived biosecurity concerns of conotoxins and research thereof.

αM-Conotoxin MIIIJ Blocks Nicotinic Acetylcholine Receptors at Neuromuscular Junctions of Frog and Fish

We report the discovery and functional characterization of αM-Conotoxin MIIIJ, a peptide from the venom of the fish-hunting cone snail Conus magus. Injections of αM-MIIIJ induced paralysis in goldfish (Carassius auratus) but not mice. Intracellular recording from skeletal muscles of fish (C. auratus) and frog (Xenopus laevis) revealed that αM-MIIIJ inhibited postsynaptic nicotinic acetylcholine receptors (nAChRs) with an IC₅₀ of ~0.1 μM. With comparable potency, αM-MIIIJ reversibly blocked ACh-gated currents (I_ACh) of voltage-clamped X. laevis oocytes exogenously expressing nAChRs cloned from zebrafish (Danio rerio) muscle. αM-MIIIJ also protected against slowly-reversible block of I_ACh by α-bungarotoxin (α-BgTX, a snake neurotoxin) and α-conotoxin EI (α-EI, from Conus ermineus another fish hunter) that competitively block nAChRs at the ACh binding site. Furthermore, assessment by fluorescence microscopy showed that αM-MIIIJ inhibited the binding of fluorescently-tagged α-BgTX at neuromuscular junctions of X. laevis, C. auratus, and D. rerio. (Note, we observed that αM-MIIIJ can block adult mouse and human muscle nAChRs exogenously expressed in X. laevis oocytes, but with IC₅₀s ~100-times higher than those of zebrafish nAChRs.) Taken together, these results indicate that αM-MIIIJ inhibits muscle nAChRs and furthermore apparently does so by interfering with the binding of ACh to its receptor. Comparative alignments with homologous sequences identified in other fish hunters revealed that αM-MIIIJ defines a new class of muscle nAChR inhibitors from cone snails.

Conus coronatus and Conus frigidus Venom: A New Source of Conopeptides with Analgesic Activity

BACKGROUND

Cone snails are a natural source of complex peptides with analgesic properties called conotoxins. These peptides are secreted in a complex venomic mixture and are predominantly smaller than 5 kDa. The present study aimed to document the analgesic activity of two species of Conus coronatus (C. coronatus) and Conus frigidus (C. frigidus) venom collected off the Iranian coast in a mouse behavioral test.

METHODS

Conotoxin containing fractions was extracted from the venom ducts and initially purified by column chromatography. The analgesic effect of the fractions was determined on formalin pain model and hot-plate test.

RESULTS

The results led to the identification of four fractions with analgesic activity in C. coronatus and two in C. frigidus. Only one fraction was able to reduce the flinching and licking in both acute pain and chronic pain phases of the formalin test. Moreover, the activity of this fraction remained 30 minutes on the hot-plate test. Purification of the fractions was carried out by RP-HPLC. LC-ESI-MS analysis of the fractions showed that the conotoxins of the analgesic fraction had molecular weights not previously reported.

CONCLUSION

The findings give insight into the venom of two previously under-investigated Conus species and reveal the therapeutic potential of the containing conopeptides.

A Distinct Pool of Nav1.5 Channels at the Lateral Membrane of Murine Ventricular Cardiomyocytes

Background: In cardiac ventricular muscle cells, the presence of voltage-gated sodium channels Na_v1.5 at the lateral membrane depends in part on the interaction between the dystrophin–syntrophin complex and the Na_v1.5 C-terminal PDZ-domain-binding sequence Ser-Ile-Val (SIV motif). α1-Syntrophin, a PDZ-domain adaptor protein, mediates the interaction between Na_v1.5 and dystrophin at the lateral membrane of cardiac cells. Using the cell-attached patch-clamp approach on cardiomyocytes expressing Na_v1.5 in which the SIV motif is deleted (ΔSIV), sodium current (I_Na) recordings from the lateral membrane revealed a SIV-motif-independent I_Na. Since immunostaining has suggested that Na_v1.5 is expressed in transverse (T-) tubules, this remaining I_Na might be carried by channels in the T-tubules. Of note, a recent study using heterologous expression systems showed that α1-syntrophin also interacts with the Na_v1.5 N-terminus, which may explain the SIV-motif independent I_Na at the lateral membrane of cardiomyocytes.

Aim: To address the role of α1-syntrophin in regulating the I_Na at the lateral membrane of cardiac cells.

Methods and Results: Patch-clamp experiments in cell-attached configuration were performed on the lateral membranes of wild-type, α1-syntrophin knockdown, and ΔSIV ventricular mouse cardiomyocytes. Compared to wild-type, a reduction of the lateral I_Na was observed in myocytes from α1-syntrophin knockdown hearts. Similar to ΔSIV myocytes, a remaining I_Na was still recorded. In addition, cell-attached I_Na recordings from lateral membrane did not differ significantly between non-detubulated and detubulated ΔSIV cardiomyocytes. Lastly, we obtained evidence suggesting that cell-attached patch-clamp experiments on the lateral membrane cannot record currents carried by channels in T-tubules such as calcium channels.

Conclusion: Altogether, these results suggest the presence of a sub-pool of sodium channels at the lateral membrane of cardiomyocytes that is independent of α1-syntrophin and the PDZ-binding motif of Na_v1.5, located in membrane domains outside of T-tubules. The question of a T-tubular pool of Na_v1.5 channels, however, remains open.

Structural Diversity of Ultralong CDRH3s in Seven Bovine Antibody Heavy Chains

Antigen recognition by mammalian antibodies represents the most diverse setting for protein-protein interactions, because antibody variable regions contain exceptionally diverse variable gene repertoires of DNA sequences containing combinatorial, non-templated junctional mutational diversity. Some animals use additional strategies to achieve structural complexity in the antibody combining site, and one of the most interesting of these is the formation of ultralong heavy chain complementarity determining region 3 loops in cattle. Repertoire sequencing studies of bovine antibody heavy chain variable sequences revealed that bovine antibodies can contain heavy chain complementarity determining region 3 (CDRH3) loops with 60 or more amino acids, with complex structures stabilized by multiple disulfide bonds. It is clear that bovine antibodies can achieve long, peculiarly structured CDR3s, but the range of diversity and complexity of those structures is poorly understood. We determined the atomic resolution structure of seven ultralong bovine CDRH3 loops. The studies, combined with five previous structures, reveal a large diversity of cysteine pairing variations, and highly diverse globular domains.

Snails In Silico: A Review of Computational Studies on the Conopeptides

Marine cone snails are carnivorous gastropods that use peptide toxins called conopeptides both as a defense mechanism and as a means to immobilize and kill their prey. These peptide toxins exhibit a large chemical diversity that enables exquisite specificity and potency for target receptor proteins. This diversity arises in terms of variations both in amino acid sequence and length, and in posttranslational modifications, particularly the formation of multiple disulfide linkages. Most of the functionally characterized conopeptides target ion channels of animal nervous systems, which has led to research on their therapeutic applications. Many facets of the underlying molecular mechanisms responsible for the specificity and virulence of conopeptides, however, remain poorly understood. In this review, we will explore the chemical diversity of conopeptides from a computational perspective. First, we discuss current approaches used for classifying conopeptides. Next, we review different computational strategies that have been applied to understanding and predicting their structure and function, from machine learning techniques for predictive classification to docking studies and molecular dynamics simulations for molecular-level understanding. We then review recent novel computational approaches for rapid high-throughput screening and chemical design of conopeptides for particular applications. We close with an assessment of the state of the field, emphasizing important questions for future lines of inquiry.

Discovery Methodology of Novel Conotoxins from Conus Species

Cone snail venoms provide an ideal resource for neuropharmacological tools and drug candidates discovery, which have become a research hotspot in neuroscience and new drug development. More than 1,000,000 natural peptides are produced by cone snails, but less than 0.1% of the estimated conotoxins has been characterized to date. Hence, the discovery of novel conotoxins from the huge conotoxin resources with high-throughput and sensitive methods becomes a crucial key for the conotoxin-based drug development. In this review, we introduce the discovery methodology of new conotoxins from various Conus species. It focuses on obtaining full N- to C-terminal sequences, regardless of disulfide bond connectivity through crude venom purification, conotoxin precusor gene cloning, venom duct transcriptomics, venom proteomics and multi-omic methods. The protocols, advantages, disadvantages, and developments of different approaches during the last decade are summarized and the promising prospects are discussed as well.

Neuronal Nicotinic Acetylcholine Receptor Modulators from Cone Snails

Marine cone snails are a large family of gastropods that have evolved highly potent venoms for predation and defense. The cone snail venom has exceptional molecular diversity in neuropharmacologically active compounds, targeting a range of receptors, ion channels, and transporters. These conotoxins have helped to dissect the structure and function of many of these therapeutically significant targets in the central and peripheral nervous systems, as well as unravelling the complex cellular mechanisms modulated by these receptors and ion channels. This review provides an overview of α-conotoxins targeting neuronal nicotinic acetylcholine receptors. The structure and activity of both classical and non-classical α-conotoxins are discussed, along with their contributions towards understanding nicotinic acetylcholine receptor (nAChR) structure and function.

Peptide therapeutics from venom: Current status and potential

Peptides are recognized as being highly selective, potent and relatively safe as potential therapeutics. Peptides isolated from the venom of different animals satisfy most of these criteria with the possible exception of safety, but when isolated as single compounds and used at appropriate concentrations, venom-derived peptides can become useful drugs. Although the number of venom-derived peptides that have successfully progressed to the clinic is currently limited, the prospects for venom-derived peptides look very optimistic. As proteomic and transcriptomic approaches continue to identify new sequences, the potential of venom-derived peptides to find applications as therapeutics, cosmetics and insecticides grows accordingly.

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.

Marine Pharmacology in 2012-2013: Marine Compounds with Antibacterial, Antidiabetic, Antifungal, Anti-Inflammatory, Antiprotozoal, Antituberculosis, and Antiviral Activities; Affecting the Immune and Nervous Systems, and Other Miscellaneous Mechanisms of Action

The peer-reviewed marine pharmacology literature from 2012 to 2013 was systematically reviewed, consistent with the 1998–2011 reviews of this series. Marine pharmacology research from 2012 to 2013, conducted by scientists from 42 countries in addition to the United States, reported findings on the preclinical pharmacology of 257 marine compounds. The preclinical pharmacology of compounds isolated from marine organisms revealed antibacterial, antifungal, antiprotozoal, antituberculosis, antiviral and anthelmitic pharmacological activities for 113 marine natural products. In addition, 75 marine compounds were reported to have antidiabetic and anti-inflammatory activities and affect the immune and nervous system. Finally, 69 marine compounds were shown to display miscellaneous mechanisms of action which could contribute to novel pharmacological classes. Thus, in 2012–2013, the preclinical marine natural product pharmacology pipeline provided novel pharmacology and lead compounds to the clinical marine pharmaceutical pipeline, and contributed significantly to potentially novel therapeutic approaches to several global disease categories.

A Disulfide Stabilized β-Sandwich Defines the Structure of a New Cysteine Framework M-Superfamily Conotoxin

The structure of a new cysteine framework (-C-CC-C-C-C-) "M"-superfamily conotoxin, Mo3964, shows it to have a β-sandwich structure that is stabilized by inter-sheet cross disulfide bonds. Mo3964 decreases outward K(+) currents in rat dorsal root ganglion neurons and increases the reversal potential of the NaV1.2 channels. The structure of Mo3964 (PDB ID: 2MW7 ) is constructed from the disulfide connectivity pattern, i.e., 1-3, 2-5, and 4-6, that is hitherto undescribed for the "M"-superfamily conotoxins. The tertiary structural fold has not been described for any of the known conus peptides. NOE (549), dihedral angle (84), and hydrogen bond (28) restraints, obtained by measurement of (h3)JNC' scalar couplings, were used as input for structure calculation. The ensemble of structures showed a backbone root mean square deviation of 0.68 ± 0.18 Å, with 87% and 13% of the backbone dihedral (ϕ, ψ) angles lying in the most favored and additional allowed regions of the Ramachandran map. The conotoxin Mo3964 represents a new bioactive peptide fold that is stabilized by disulfide bonds and adds to the existing repertoire of scaffolds that can be used to design stable bioactive peptide molecules.

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

μ-Conotoxins block voltage-gated sodium channels (VGSCs) and compete with tetrodotoxin for binding to the sodium conductance pore. Early efforts identified µ-conotoxins that preferentially blocked the skeletal muscle subtype (NaV1.4). However, the last decade witnessed a significant increase in the number of µ-conotoxins and the range of VGSC subtypes inhibited (NaV1.2, NaV1.3 or NaV1.7). Twenty µ-conotoxin sequences have been identified to date and structure-activity relationship studies of several of these identified key residues responsible for interactions with VGSC subtypes. Efforts to engineer-in subtype specificity are driven by in vivo analgesic and neuromuscular blocking activities. This review summarizes structural and pharmacological studies of µ-conotoxins, which show promise for development of selective blockers of NaV1.2, and perhaps also NaV1.1,1.3 or 1.7.

The voltage-gated sodium channel: a major target of marine neurotoxins

Voltage-gated sodium channels (Nav) are key components for nerve excitability. They initiate and propagate the action potential in excitable cells, throughout the central and peripheral nervous system, thus enabling a variety of physiological functions to be achieved. The rising phase of the action potential is driven by the opening of Nav channels which activate rapidly and carry Na(+) ions in the intracellular medium, and ends with the Na(+) current inactivation. The biophysical properties of these channels have been elucidated, through the use of pharmacological agents that disrupt the molecular mechanism of the channel functioning. Among them, marine toxins produced by venomous animals or microorganisms have been crucial to map the different allosteric binding sites of the channels, understand their mode of action and represent an emerging source of therapeutic agents to alleviate or cure Na(+) channels-linked human diseases. In this article, we review recent discoveries on the molecular and biophysical properties of the Na(+) channel as a target for marine neurotoxins, and present the ongoing developments of pharmacological agents as therapeutic tools.

Marine natural products

This review covers the literature published in 2012 for marine natural products, with 1035 citations (673 for the period January to December 2012) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1241 for 2012), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Mammalian neuronal sodium channel blocker μ-conotoxin BuIIIB has a structured N-terminus that influences potency

Among the μ-conotoxins that block vertebrate voltage-gated sodium channels (VGSCs), some have been shown to be potent analgesics following systemic administration in mice. We have determined the solution structure of a new representative of this family, μ-BuIIIB, and established its disulfide connectivities by direct mass spectrometric collision induced dissociation fragmentation of the peptide with disulfides intact. The major oxidative folding product adopts a 1-4/2-5/3-6 pattern with the following disulfide bridges: Cys5-Cys17, Cys6-Cys23, and Cys13-Cys24. The solution structure reveals that the unique N-terminal extension in μ-BuIIIB, which is also present in μ-BuIIIA and μ-BuIIIC but absent in other μ-conotoxins, forms part of a short α-helix encompassing Glu3 to Asn8. This helix is packed against the rest of the toxin and stabilized by the Cys5-Cys17 and Cys6-Cys23 disulfide bonds. As such, the side chain of Val1 is located close to the aromatic rings of Trp16 and His20, which are located on the canonical helix that displays several residues found to be essential for VGSC blockade in related μ-conotoxins. Mutations of residues 2 and 3 in the N-terminal extension enhanced the potency of μ-BuIIIB for NaV1.3. One analogue, [d-Ala2]BuIIIB, showed a 40-fold increase, making it the most potent peptide blocker of this channel characterized to date and thus a useful new tool with which to characterize this channel. On the basis of previous results for related μ-conotoxins, the dramatic effects of mutations at the N-terminus were unanticipated and suggest that further gains in potency might be achieved by additional modifications of this region.

Glycosylation of conotoxins

Conotoxins are small peptides present in the venom of cone snails. The snail uses this venom to paralyze and capture prey. The constituent conopeptides display a high level of chemical diversity and are of particular interest for scientists as tools employed in neurological studies and for drug development, because they target with exquisite specificity membrane receptors, transporters, and various ion channels in the nervous system. However, these peptides are known to contain a high frequency and variability of post-translational modifications-including sometimes O-glycosylation-which are of importance for biological activity. The potential application of specific conotoxins as neuropharmalogical agents and chemical probes requires a full characterization of the relevant peptides, including the structure of the carbohydrate part. In this review, the currently existing knowledge of O-glycosylation of conotoxins is described.

A novel inhibitor of α9α10 nicotinic acetylcholine receptors from Conus vexillum delineates a new conotoxin superfamily

Conotoxins (CTxs) selectively target a range of ion channels and receptors, making them widely used tools for probing nervous system function. Conotoxins have been previously grouped into superfamilies according to signal sequence and into families based on their cysteine framework and biological target. Here we describe the cloning and characterization of a new conotoxin, from Conus vexillum, named αB-conotoxin VxXXIVA. The peptide does not belong to any previously described conotoxin superfamily and its arrangement of Cys residues is unique among conopeptides. Moreover, in contrast to previously characterized conopeptide toxins, which are expressed initially as prepropeptide precursors with a signal sequence, a ‘‘pro’’ region, and the toxin-encoding region, the precursor sequence of αB-VxXXIVA lacks a ‘‘pro’’ region. The predicted 40-residue mature peptide, which contains four Cys, was synthesized in each of the three possible disulfide arrangements. Investigation of the mechanism of action of αB-VxXXIVA revealed that the peptide is a nicotinic acetylcholine receptor (nAChR) antagonist with greatest potency against the α9α10 subtype. ¹H nuclear magnetic resonance (NMR) spectra indicated that all three αB-VxXXIVA isomers were poorly structured in aqueous solution. This was consistent with circular dichroism (CD) results which showed that the peptides were unstructured in buffer, but adopted partially helical conformations in aqueous trifluoroethanol (TFE) solution. The α9α10 nAChR is an important target for the development of analgesics and cancer chemotherapeutics, and αB-VxXXIVA represents a novel ligand with which to probe the structure and function of this protein.

Mechanism and molecular basis for the sodium channel subtype specificity of µ-conopeptide CnIIIC

BACKGROUND AND PURPOSE

Voltage-gated sodium channels (Na(V) channels) are key players in the generation and propagation of action potentials, and selective blockade of these channels is a promising strategy for clinically useful suppression of electrical activity. The conotoxin µ-CnIIIC from the cone snail Conus consors exhibits myorelaxing activity in rodents through specific blockade of skeletal muscle (Na(V) 1.4) Na(V) channels.

EXPERIMENTAL APPROACH

We investigated the activity of µ-CnIIIC on human Na(V) channels and characterized its inhibitory mechanism, as well as the molecular basis, for its channel specificity.

KEY RESULTS

Similar to rat paralogs, human Na(V) 1.4 and Na(V) 1.2 were potently blocked by µ-CnIIIC, the sensitivity of Na(V) 1.7 was intermediate, and Na(V) 1.5 and Na(V) 1.8 were insensitive. Half-channel chimeras revealed that determinants for the insensitivity of Na(V) 1.8 must reside in both the first and second halves of the channel, while those for Na(V) 1.5 are restricted to domains I and II. Furthermore, domain I pore loop affected the total block and therefore harbours the major determinants for the subtype specificity. Domain II pore loop only affected the kinetics of toxin binding and dissociation. Blockade by µ-CnIIIC of Na(V) 1.4 was virtually irreversible but left a residual current of about 5%, reflecting a 'leaky' block; therefore, Na(+) ions still passed through µ-CnIIIC-occupied Na(V) 1.4 to some extent. TTX was excluded from this binding site but was trapped inside the pore by µ-CnIIIC.

CONCLUSION AND IMPLICATIONS

Of clinical significance, µ-CnIIIC is a potent and persistent blocker of human skeletal muscle Na(V) 1.4 that does not affect activity of cardiac Na(V) 1.5.

Distinct disulfide isomers of μ-conotoxins KIIIA and KIIIB block voltage-gated sodium channels

In the preparation of synthetic conotoxins containing multiple disulfide bonds, oxidative folding can produce numerous permutations of disulfide bond connectivities. Establishing the native disulfide connectivities thus presents a significant challenge when the venom-derived peptide is not available, as is increasingly the case when conotoxins are identified from cDNA sequences. Here, we investigate the disulfide connectivity of μ-conotoxin KIIIA, which was predicted originally to have a [C1-C9,C2-C15,C4-C16] disulfide pattern based on homology with closely related μ-conotoxins. The two major isomers of synthetic μ-KIIIA formed during oxidative folding were purified and their disulfide connectivities mapped by direct mass spectrometric collision-induced dissociation fragmentation of the disulfide-bonded polypeptides. Our results show that the major oxidative folding product adopts a [C1-C15,C2-C9,C4-C16] disulfide connectivity, while the minor product adopts a [C1-C16,C2-C9,C4-C15] connectivity. Both of these peptides were potent blockers of Na(V)1.2 (K(d) values of 5 and 230 nM, respectively). The solution structure for μ-KIIIA based on nuclear magnetic resonance data was recalculated with the [C1-C15,C2-C9,C4-C16] disulfide pattern; its structure was very similar to the μ-KIIIA structure calculated with the incorrect [C1-C9,C2-C15,C4-C16] disulfide pattern, with an α-helix spanning residues 7-12. In addition, the major folding isomers of μ-KIIIB, an N-terminally extended isoform of μ-KIIIA identified from its cDNA sequence, were isolated. These folding products had the same disulfide connectivities as μ-KIIIA, and both blocked Na(V)1.2 (K(d) values of 470 and 26 nM, respectively). Our results establish that the preferred disulfide pattern of synthetic μ-KIIIA and μ-KIIIB folded in vitro is 1-5/2-4/3-6 but that other disulfide isomers are also potent sodium channel blockers. These findings raise questions about the disulfide pattern(s) of μ-KIIIA in the venom of Conus kinoshitai; indeed, the presence of multiple disulfide isomers in the venom could provide a means of further expanding the snail's repertoire of active peptides.

Conotoxins targeting neuronal voltage-gated sodium channel subtypes: potential analgesics?

Voltage-gated sodium channels (VGSC) are the primary mediators of electrical signal amplification and propagation in excitable cells. VGSC subtypes are diverse, with different biophysical and pharmacological properties, and varied tissue distribution. Altered VGSC expression and/or increased VGSC activity in sensory neurons is characteristic of inflammatory and neuropathic pain states. Therefore, VGSC modulators could be used in prospective analgesic compounds. VGSCs have specific binding sites for four conotoxin families: μ-, μO-, δ- and ί-conotoxins. Various studies have identified that the binding site of these peptide toxins is restricted to well-defined areas or domains. To date, only the μ- and μO-family exhibit analgesic properties in animal pain models. This review will focus on conotoxins from the μ- and μO-families that act on neuronal VGSCs. Examples of how these conotoxins target various pharmacologically important neuronal ion channels, as well as potential problems with the development of drugs from conotoxins, will be discussed.

Design of bioactive peptides from naturally occurring μ-conotoxin structures

To date, cone snail toxins ("conotoxins") are of great interest in the pursuit of novel subtype-selective modulators of voltage-gated sodium channels (Na(v)s). Na(v)s participate in a wide range of electrophysiological processes. Consequently, their malfunctioning has been associated with numerous diseases. The development of subtype-selective modulators of Na(v)s remains highly important in the treatment of such disorders. In current research, a series of novel, synthetic, and bioactive compounds were designed based on two naturally occurring μ-conotoxins that target Na(v)s. The initial designed peptide contains solely 13 amino acids and was therefore named "Mini peptide." It was derived from the μ-conotoxins KIIIA and BuIIIC. Based on this Mini peptide, 10 analogues were subsequently developed, comprising 12-16 amino acids with two disulfide bridges. Following appropriate folding and mass verification, blocking effects on Na(v)s were investigated. The most promising compound established an IC(50) of 34.1 ± 0.01 nM (R2-Midi on Na(v)1.2). An NMR structure of one of our most promising compounds was determined. Surprisingly, this structure does not reveal an α-helix. We prove that it is possible to design small peptides based on known pharmacophores of μ-conotoxins without losing their potency and selectivity. These data can provide crucial material for further development of conotoxin-based therapeutics.

Molecular phylogeny, classification and evolution of conopeptides

Conopeptides are toxins expressed in the venom duct of cone snails (Conoidea, Conus). These are mostly well-structured peptides and mini-proteins with high potency and selectivity for a broad range of cellular targets. In view of these properties, they are widely used as pharmacological tools and many are candidates for innovative drugs. The conopeptides are primarily classified into superfamilies according to their peptide signal sequence, a classification that is thought to reflect the evolution of the multigenic system. However, this hypothesis has never been thoroughly tested. Here we present a phylogenetic analysis of 1,364 conopeptide signal sequences extracted from GenBank. The results validate the current conopeptide superfamily classification, but also reveal several important new features. The so-called "cysteine-poor" conopeptides are revealed to be closely related to "cysteine-rich" conopeptides; with some of them sharing very similar signal sequences, suggesting that a distinction based on cysteine content and configuration is not phylogenetically relevant and does not reflect the evolutionary history of conopeptides. A given cysteine pattern or pharmacological activity can be found across different superfamilies. Furthermore, a few conopeptides from GenBank do not cluster in any of the known superfamilies, and could represent yet-undefined superfamilies. A clear phylogenetically based classification should help to disentangle the diversity of conopeptides, and could also serve as a rationale to understand the evolution of the toxins in the numerous other species of conoideans and venomous animals at large.