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

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.

Anti-Ovarian Cancer Conotoxins Identified from Conus Venom

Conotoxins constitute a treasury of drug resources and have attracted widespread attention. In order to explore biological candidates from the marine cone snail, we isolated and identified three novel conopeptides named as Vi14b, Vi002, Vi003, three conotoxin variants named as Mr3d.1, Mr3e.1, Tx3a.1, and three known conotoxins (Vi15a, Mr3.8 and TCP) from crude venoms of Conus virgo, Conus marmoreus and Conus texile. Mr3.8 (I-V, II-VI, III-IV) and Tx3a.1 (I-III, II-VI, IV-V) both showed a novel pattern of disulfide connectivity, different from that previously established for the µ- and ψ-conotoxins. Concerning the effect on voltage-gated sodium channels, Mr3e.1, Mr3.8, Tx3a.1, TCP inhibited Na_v1.4 or Na_v1.8 by 21.51~24.32% of currents at semi-activated state (TP2) at 10 μmol/L. Certain anti-ovarian cancer effects on ID-8 cells were exhibited by Tx3a.1, Mr3e.1 and Vi14b with IC₅₀ values of 24.29 µM, 54.97 µM and 111.6 µM, respectively. This work highlights the role of conotoxin libraries in subsequent drug discovery for ovarian cancer treatment.

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.

Characterization in Inhibitory Effectiveness of Carbamazepine in Voltage-Gated Na+ and Erg-Mediated K+ Currents in a Mouse Neural Crest-Derived (Neuro-2a) Cell Line

Carbamazepine (CBZ, Tegretol^®) is an anticonvulsant used in the treatment of epilepsy and neuropathic pain; however, several unwanted effects of this drug have been noticed. Therefore, the regulatory actions of CBZ on ionic currents in electrically excitable cells need to be reappraised, although its efficacy in suppressing voltage-gated Na⁺ current (I_Na) has been disclosed. This study was undertaken to explore the modifications produced by CBZ on ionic currents (e.g., I_Na and erg-mediated K⁺ current [I_K(erg)]) measured from Neuro-2a (N2a) cells. In these cells, we found that this drug differentially suppressed the peak (transient, I_Na(T)) and sustained (late, I_Na(L)) components of I_Na in a concentration-dependent manner with effective IC₅₀ of 56 and 18 μM, respectively. The overall current–voltage relationship of I_Na(T) with or without the addition of CBZ remained unchanged; however, the strength (i.e., ∆area) in the window component of I_Na (I_Na(W)) evoked by the short ascending ramp pulse (V_ramp) was overly lessened in the CBZ presence. Tefluthrin (Tef), a synthetic pyrethroid, known to stimulate I_Na, augmented the strength of the voltage-dependent hysteresis (Hys_(V)) of persistent I_Na (I_Na(P)) in response to the isosceles-triangular V_ramp; moreover, further application of CBZ attenuated Tef-mediated accentuation of I_Na(P)’s Hys_(V). With a two-step voltage protocol, the recovery of I_Na(T) inactivation seen in Neuro-2a cells became progressively slowed by adding CBZ; however, the cumulative inhibition of I_Na(T) evoked by pulse train stimulation was enhanced during exposure to this drug. Neuro-2a-cell exposure to CBZ (100 μM), the magnitude of erg-mediated K⁺ current measured throughout the entire voltage-clamp steps applied was mildly inhibited. The docking results regarding the interaction of CBZ and voltage-gate Na⁺ (Na_V) channel predicted the ability of CBZ to bind to some amino-acid residues in Na_V due to the existence of a hydrogen bond or hydrophobic contact. It is conceivable from the current investigations that the I_Na (I_Na(T), I_Na(L), I_Na(W), and I_Na(P)) residing in Neuro-2a cells are susceptible to being suppressed by CBZ, and that its block on I_Na(L) is larger than that on I_Na(T). Collectively, the magnitude and gating of Na_V channels produced by the CBZ presence might have an impact on its anticonvulsant and analgesic effects occurring in vivo.

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.

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.

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.

Discovery, Pharmacological Characterisation and NMR Structure of the Novel µ-Conotoxin SxIIIC, a Potent and Irreversible NaV Channel Inhibitor

Voltage-gated sodium (Na_V) channel subtypes, including Na_V1.7, are promising targets for the treatment of neurological diseases, such as chronic pain. Cone snail-derived µ-conotoxins are small, potent Na_V channel inhibitors which represent potential drug leads. Of the 22 µ-conotoxins characterised so far, only a small number, including KIIIA and CnIIIC, have shown inhibition against human Na_V1.7. We have recently identified a novel µ-conotoxin, SxIIIC, from Conus striolatus. Here we present the isolation of native peptide, chemical synthesis, characterisation of human Na_V channel activity by whole-cell patch-clamp electrophysiology and analysis of the NMR solution structure. SxIIIC displays a unique Na_V channel selectivity profile (1.4 > 1.3 > 1.1 ≈ 1.6 ≈ 1.7 > 1.2 >> 1.5 ≈ 1.8) when compared to other µ-conotoxins and represents one of the most potent human Na_V1.7 putative pore blockers (IC₅₀ 152.2 ± 21.8 nM) to date. NMR analysis reveals the structure of SxIIIC includes the characteristic α-helix seen in other µ-conotoxins. Future investigations into structure-activity relationships of SxIIIC are expected to provide insights into residues important for Na_V channel pore blocker selectivity and subsequently important for chronic pain drug development.

Antibodies and venom peptides: new modalities for ion channels

Ion channels play fundamental roles in both excitable and non-excitable tissues and therefore constitute attractive drug targets for myriad neurological, cardiovascular and metabolic diseases as well as for cancer and immunomodulation. However, achieving selectivity for specific ion channel subtypes with small-molecule drugs has been challenging, and there currently is a growing trend to target ion channels with biologics. One approach is to improve the pharmacokinetics of existing or novel venom-derived peptides. In parallel, after initial studies with polyclonal antibodies demonstrated the technical feasibility of inhibiting channel function with antibodies, multiple preclinical programmes are now using the full spectrum of available technologies to generate conventional monoclonal and engineered antibodies or nanobodies against extracellular loops of ion channels. After a summary of the current state of ion channel drug discovery, this Review discusses recent developments using the purinergic receptor channel P2X purinoceptor 7 (P2X7), the voltage-gated potassium channel KV1.3 and the voltage-gated sodium channel NaV1.7 as examples of targeting ion channels with biologics.

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.

Animal toxins for channelopathy treatment

Ion channels are transmembrane proteins that allow passive flow of ions inside and/or outside of cells or cell organelles. Except mutations lead to nonfunctional protein production or abolished receptor entrance on the membrane surface an altered channel may have two principal conditions that can be corrected. The channel may conduct fewer ions through (loss-of-function mutations) or too many ions (gain-of-function mutations) compared to a normal channel. Toxins from animal venoms are specialised molecules that are generally oriented toward interactions with ion channels. This is a result of long coevolution between predators and their prey. On the molecular level, toxins activate or inhibit ion channels, so they are ideal molecules for restoring conductance in mutated channels. Another aspect of this long coevolution is that a broad variety of toxins have been fine tuned to recognize the channels of different species, keeping many amino acids substitution among sequences. Many peptide ligands with high selectivity to specific receptor subtypes have been isolated from animal venoms, some of which are absolutely non-toxic to humans and mammalians. It is expected that molecules that are selective to each known receptor can be found in animal venoms, but the pool of toxins currently does not override all receptors described as being involved in channelopathies. Modern investigating methods have enhanced the search process for selective ligands. One prominent method is a site-directed mutagenesis of existing toxins to change the selectivity or/and affinity to the selected receptor, which has shown positive results. This article is part of the Special Issue entitled 'Channelopathies.'

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.

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.

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.