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

Conotoxins Targeting Voltage-Gated Sodium Ion Channels

Voltage-gated sodium (NaV) channels are intimately involved in the generation and transmission of action potentials, and dysfunction of these channels may contribute to nervous system diseases, such as epilepsy, neuropathic pain, psychosis, autism, and cardiac arrhythmia. Many venom peptides selectively act on NaV channels. These include conotoxins, which are neurotoxins secreted by cone snails for prey capture or self-defense but which are also valuable pharmacological tools for the identification and/or treatment of human diseases. Typically, conotoxins contain two or three disulfide bonds, and these internal crossbraces contribute to conotoxins having compact, well defined structures and high stability. Of the conotoxins containing three disulfide bonds, some selectively target mammalian NaV channels and can block, stimulate, or modulate these channels. Such conotoxins have great potential to serve as pharmacological tools for studying the functions and characteristics of NaV channels or as drug leads for neurologic diseases related to NaV channels. Accordingly, discovering or designing conotoxins targeting NaV channels with high potency and selectivity is important. The amino acid sequences, disulfide bond connectivity, and three-dimensional structures are key factors that affect the biological activity of conotoxins, and targeted synthetic modifications of conotoxins can greatly improve their activity and selectivity. This review examines NaV channel-targeted conotoxins, focusing on their structures, activities, and designed modifications, with a view toward expanding their applications. SIGNIFICANCE STATEMENT: NaV channels are crucial in various neurologic diseases. Some conotoxins selectively target NaV channels, causing either blockade or activation, thus enabling their use as pharmacological tools for studying the channels' characteristics and functions. Conotoxins also have promising potential to be developed as drug leads. The disulfide bonds in these peptides are important for stabilizing their structures, thus leading to enhanced specificity and potency. Together, conotoxins targeting NaV channels have both immediate research value and promising future application prospects.

Synthesis of Asp-based lactam cyclic peptides using an amide-bonded diaminodiacid to prevent aspartimide formation

Asp-based lactam cyclic peptides are considered promising drug candidates. However, using Fmoc solid-phase peptide synthesis (Fmoc-SPPS) for these peptides also causes aspartimide formation, resulting in low yields or even failure to obtain the target peptides. Here, we developed a diaminodiacid containing an amide bond as a β-carboxyl-protecting group for Asp to avoid aspartimide formation. The practicality of this diaminodiacid has been illustrated by the synthesis of lactam cyclic peptide cyclo[Lys9,Asp13] KIIIA7-14 and 1Y.

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.

Conotoxin Prediction: New Features to Increase Prediction Accuracy

Conotoxins are toxic, disulfide-bond-rich peptides from cone snail venom that target a wide range of receptors and ion channels with multiple pathophysiological effects. Conotoxins have extraordinary potential for medical therapeutics that include cancer, microbial infections, epilepsy, autoimmune diseases, neurological conditions, and cardiovascular disorders. Despite the potential for these compounds in novel therapeutic treatment development, the process of identifying and characterizing the toxicities of conotoxins is difficult, costly, and time-consuming. This challenge requires a series of diverse, complex, and labor-intensive biological, toxicological, and analytical techniques for effective characterization. While recent attempts, using machine learning based solely on primary amino acid sequences to predict biological toxins (e.g., conotoxins and animal venoms), have improved toxin identification, these methods are limited due to peptide conformational flexibility and the high frequency of cysteines present in toxin sequences. This results in an enumerable set of disulfide-bridged foldamers with different conformations of the same primary amino acid sequence that affect function and toxicity levels. Consequently, a given peptide may be toxic when its cysteine residues form a particular disulfide-bond pattern, while alternative bonding patterns (isoforms) or its reduced form (free cysteines with no disulfide bridges) may have little or no toxicological effects. Similarly, the same disulfide-bond pattern may be possible for other peptide sequences and result in different conformations that all exhibit varying toxicities to the same receptor or to different receptors. We present here new features, when combined with primary sequence features to train machine learning algorithms to predict conotoxins, that significantly increase prediction accuracy.

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.

Synthesis and insecticidal activity of cysteine-free conopeptides from Conus betulinus

The cone snail Conus betulinus is a vermivorous species that is widely distributed in the South China Sea. Its crude venom contains various peptides used to prey on marine worms. In previous studies, a systematic analysis of the peptide toxin sequences from C. betulinus was carried out using a multiomics technique. In this study, 10 cysteine-free peptides that may possess insecticidal activity were selected from a previously constructed conopeptide library of C. betulinus using the CPY-Fe conopeptide as a template. These conopeptides were prepared by solid-phase peptide synthesis (SPPS), then characterized by the reverse-phase high performance liquid chromatography (HPLC) and mass spectrometry. Insect cytotoxicity and injection experiments revealed that these cysteine-free peptides exerted favorable insecticidal effects, and two of them (Bt010 and Bt016) exhibited high insecticidal efficacy with LD50 of 9.07 nM and 10.93 nM, respectively. In addition, the 3D structures of these peptides were predicted by homology modeling, and a phylogenetic tree was constructed based on the nucleotide data of conopeptides to analyze the relationships among structures, functions, and evolution. A preliminary mechanism for the insecticidal activity of the cysteine-free conopeptides was predicted by molecular docking. To the best of our knowledge, this is the first study to report the insecticidal activity of cysteine-free conopeptides derived from Conus betulinus, signaling that they could potentially be developed into bioinsecticides with desirable properties such as easy preparation, low cost, and high potency.

Synthesis of the Most Potent Isomer of μ-Conotoxin KIIIA Using Different Strategies

In the chemical synthesis of conotoxins with multiple disulfide bonds, the oxidative folding process can result in diverse disulfide bond connectivities, which presents a challenge for determining the natural disulfide bond connectivities and leads to significant structural differences in the synthesized toxins. Here, we focus on KIIIA, a μ-conotoxin that has high potency in inhibiting Nav1.2 and Nav1.4. The non-natural connectivity pattern (C1-C9, C2-C15, C4-C16) of KIIIA exhibits the highest activity. In this study, we report an optimized Fmoc solid-phase synthesis of KIIIA using various strategies. Our results indicate that free random oxidation is the simplest method for peptides containing triple disulfide bonds, resulting in high yields and a simplified process. Alternatively, the semi-selective strategy utilizing Trt/Acm groups can also produce the ideal isomer, albeit with a lower yield. Furthermore, we performed distributed oxidation using three different protecting groups, optimizing their positions and cleavage order. Our results showed that prioritizing the cleavage of the Mob group over Acm may result in disulfide bond scrambling and the formation of new isomers. We also tested the activity of synthesized isomers on Nav1.4. These findings provide valuable guidance for the synthesis of multi-disulfide-bonded peptides in future studies.

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.

Design, synthesis, and mechanism of action of novel μ-conotoxin KIIIA analogues for inhibition of the voltage-gated sodium channel Nav1.7

μ-Conotoxin KIIIA, a selective blocker of sodium channels, has strong inhibitory activity against several Na_v isoforms, including Na_v1.7, and has potent analgesic effects, but it contains three pairs of disulfide bonds, making structural modification difficult and synthesis complex. To circumvent these difficulties, we designed and synthesized three KIIIA analogues with one disulfide bond deleted. The most active analogue, KIIIA-1, was further analyzed, and its binding pattern to hNa_v1.7 was determined by molecular dynamics simulations. Guided by the molecular dynamics computational model, we designed and tested 32 second-generation and 6 third-generation analogues of KIIIA-1 on hNa_v1.7 expressed in HEK293 cells. Several analogues showed significantly improved inhibitory activity on hNa_v1.7, and the most potent peptide, 37, was approximately 4-fold more potent than the KIIIA Isomer I and 8-fold more potent than the wildtype (WT) KIIIA in inhibiting hNa_v1.7 current. Intraperitoneally injected 37 exhibited potent in vivo analgesic activity in a formalin-induced inflammatory pain model, with activity reaching ∼350-fold of the positive control drug morphine. Overall, peptide 37 has a simplified disulfide-bond framework and exhibits potent in vivo analgesic effects and has promising potential for development as a pain therapy in the future.

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.