Solution structure of mu-conotoxin PIIIA, a preferential inhibitor of persistent tetrodotoxin-sensitive sodium channels

Voltage-gated sodium channels in cancer and their specific inhibitors

Voltage-gated sodium channels (VGSCs) participate in generating and spreading action potentials in electrically excited cells such as neurons and muscle fibers. Abnormal expression of VGSCs has been observed in various types of tumors, while they are either not expressed or expressed at a low level in the matching normal tissue. Hence, this abnormal expression suggests that VGSCs confer some advantage or viability on tumor cells, making them a valuable indicator for identifying tumor cells. In addition, overexpression of VGSCs increased the ability of cancer cells to metastasize and invade, as well as correlated with the metastatic behavior of different cancers. Therefore, blocking VGSCs presents a new strategy for the treatment of cancers. A portion of this review summarizes the structure and function of VGSCs and also describes the correlation between VGSCs and cancers. Most importantly, we provide an overview of current research on various subtype-selective VGSC inhibitors and updates on ongoing clinical 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.

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.

Discovery of Cyclotides from Australasian Plants

This article is part of a special issue celebrating the contributions of Professor Paul Alewood to peptide science. We begin by providing a summary of collaborative projects between the Alewood and Craik groups at The University of Queensland and highlighting the impacts of some of these studies. In particular, studies on the discovery, synthesis, structures, and bioactivities of disulfide-rich toxins from animal venoms have led to a greater understanding of the biology of ion channels and to applications of these bioactive peptides in drug design. The second part of the article focuses on plant-derived disulfide-rich cyclic peptides, known as cyclotides, and includes an analysis of the geographical distribution of Australasian plant species that contain cyclotides as well as an analysis of the diversity of cyclotide sequences found in Australasian plants. This should provide a useful resource for researchers to access native cyclotides and explore their chemistry and biology.

Role of the disulfide bond on the structure and activity of μ-conotoxin PIIIA in the inhibition of NaV1.4

μ-Conotoxin PIIIA, a peptide toxin isolated from Conus purpurascens, blocks the skeletal muscle voltage-gated sodium channel NaV1.4 with significant potency. PIIIA has three disulfide bonds, which contribute largely to its highly constrained and stable structure. In this study, a combination of experimental studies and computational modeling were performed to assess the effects of deletion of the disulfide bonds on the structure and activity of PIIIA. The final results indicate that the three disulfide bonds of PIIIA are required to produce the effective inhibition of NaV1.4, and the removal of any one of the disulfide bonds significantly reduces its binding affinity owing to secondary structure variation, among which the Cys11-Cys22 is the most important for sustaining the structure and activity of PIIIA.

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.

Synthesis and Structure Determination of µ-Conotoxin PIIIA Isomers with Different Disulfide Connectivities

Peptides with a high number of cysteines are usually influenced regarding the three-dimensional structure by their disulfide connectivity. It is thus highly important to avoid undesired disulfide bond formation during peptide synthesis, because it may result in a completely different peptide structure, and consequently altered bioactivity. However, the correct formation of multiple disulfide bonds in a peptide is difficult to obtain by using standard self-folding methods such as conventional buffer oxidation protocols, because several disulfide connectivities can be formed. This protocol represents an advanced strategy required for the targeted synthesis of multiple disulfide-bridged peptides which cannot be synthesized via buffer oxidation in high quality and quantity. The study demonstrates the application of a distinct protecting group strategy for the synthesis of all possible 3-disulfide-bonded peptide isomers of µ-conotoxin PIIIA in a targeted way. The peptides are prepared by Fmoc-based solid phase peptide synthesis using a protecting group strategy for defined disulfide bond formation. The respective pairs of cysteines are protected with trityl (Trt), acetamidomethyl (Acm), and tert-butyl (tBu) protecting groups to make sure that during every oxidation step only the required cysteines are deprotected and linked. In addition to the targeted synthesis, a combination of several analytical methods is used to clarify the correct folding and generation of the desired peptide structures. The comparison of the different 3-disulfide-bonded isomers indicates the importance of accurate determination and knowledge of the disulfide connectivity for the calculation of the three-dimensional structure and for interpretation of the biological activity of the peptide isomers. The analytical characterization includes the exact disulfide bond elucidation via tandem mass spectrometry (MS/MS) analysis which is performed with partially reduced and alkylated derivatives of the intact peptide isomer produced by an adapted protocol. Furthermore, the peptide structures are determined using 2D nuclear magnetic resonance (NMR) experiments and the knowledge obtained from MS/MS analysis.

Determination of the μ-Conotoxin PIIIA Specificity Against Voltage-Gated Sodium Channels from Binding Energy Calculations

Voltage-gated sodium (NaV) channels generate and propagate action potentials in excitable cells, and several NaV subtypes have become important targets for pain management. The μ-conotoxins inhibit subtypes of the NaV with varied specificity but often lack of specificity to interested subtypes. Engineering the selectivity of the μ-conotoxins presents considerable complexity and challenge, as it involves the optimization of their binding affinities to multiple highly conserved NaV subtypes. In this study, a model of NaV1.4 bound with μ-conotoxin PIIIA complex was constructed using homology modeling, docking, molecular dynamic simulations and binding energy calculations. The accuracy of this model was confirmed based on the experimental mutagenesis data. The complex models of PIIIA bound with varied subtypes of NaV1.x (x = 1, 2, 3, 5, 6, 7, 8, or 9) were built using NaV1.4/PIIIA complex as a template, and refined using molecular dynamic simulations. The binding affinities of PIIIA to varied subtypes of NaV1.x (x = 1 to 9) were calculated using the Molecular Mechanics Generalized Born/Surface Area (MMGB/SA) and umbrella sampling, and were compared with the experimental values. The binding affinities calculated using MMGB/SA and umbrella sampling are correlated with the experimental values, with the former and the latter giving correlation coefficient of 0.41 (R²) and 0.68 (R²), respectively. Binding energy decomposition suggests that conserved and nonconserved residues among varied NaV subtypes have a synergistic effect on the selectivity of PIIIA.

Selective Voltage-Gated Sodium Channel Peptide Toxins from Animal Venom: Pharmacological Probes and Analgesic Drug Development

Voltage-gated sodium channels (Navs) play critical roles in action potential generation and propagation. Nav channelopathy as well as pathological sensitization contribute to allodynia and hyperalgesia. Recent evidence has demonstrated the significant roles of Nav subtypes (Nav1.3, 1.7, 1.8, and 1.9) in nociceptive transduction, and therefore these Navs may represent attractive targets for analgesic drug discovery. Animal toxins are structurally diverse peptides that are highly potent yet selective on ion channel subtypes and therefore represent valuable probes to elucidate the structures, gating properties, and cellular functions of ion channels. In this review, we summarize recent advances on peptide toxins from animal venom that selectively target Nav1.3, 1.7, 1.8, and 1.9, along with their potential in analgesic drug discovery.

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.

Analgesic Effects of GpTx-1, PF-04856264 and CNV1014802 in a Mouse Model of NaV1.7-Mediated Pain

Loss-of-function mutations of Na(V)1.7 lead to congenital insensitivity to pain, a rare condition resulting in individuals who are otherwise normal except for the inability to sense pain, making pharmacological inhibition of Na(V)1.7 a promising therapeutic strategy for the treatment of pain. We characterized a novel mouse model of Na(V)1.7-mediated pain based on intraplantar injection of the scorpion toxin OD1, which is suitable for rapid in vivo profiling of Na(V)1.7 inhibitors. Intraplantar injection of OD1 caused spontaneous pain behaviors, which were reversed by co-injection with Na(V)1.7 inhibitors and significantly reduced in Na(V)1.7(-/-) mice. To validate the use of the model for profiling Na(V)1.7 inhibitors, we determined the Na(V) selectivity and tested the efficacy of the reported Na(V)1.7 inhibitors GpTx-1, PF-04856264 and CNV1014802 (raxatrigine). GpTx-1 selectively inhibited Na(V)1.7 and was effective when co-administered with OD1, but lacked efficacy when delivered systemically. PF-04856264 state-dependently and selectively inhibited Na(V)1.7 and significantly reduced OD1-induced spontaneous pain when delivered locally and systemically. CNV1014802 state-dependently, but non-selectively, inhibited Na(V) channels and was only effective in the OD1 model when delivered systemically. Our novel model of Na(V)1.7-mediated pain based on intraplantar injection of OD1 is thus suitable for the rapid in vivo characterization of the analgesic efficacy of Na(V)1.7 inhibitors.

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.

A tyrosine-containing analog of mu-conotoxin GIIIA as ligand in the receptor binding assay for paralytic shellfish poisons

Development of novel analytical tools to detect marine biotoxins has been warranted in view of the apparent global pervasiveness of algal-derived shellfish poisoning, and the limitations of existing methods. Here, we describe the initial phase in the development and evaluation of a tyrosine-containing analog of μ-conotoxin (μ-CTX) GIIIA as an alternative to saxitoxin (STX) in a receptor binding assay (RBA) for paralytic shellfish poisons. The peptide analog was synthesized and characterized for structure and bioactivity. The major product of oxidation elicited paralytic symptoms in mice at a minimum dose of 1.31 mg kg(-1) (i.p.). Mass spectrometry analysis of the bioactive peptide gave a molecular mass of 2637.52 Da that was close to the predicted value. Iodination via chloramine-T produced non-, mono- and di-iodinated peptides (respectively, NIP, MIP and DIP). Competition assays against (3)H-STX revealed higher Ki and EC50 (P < 0.0001, ANOVA) indicating reduced affinity for the receptor, and limited displacement of receptor-bound STX. However, subsequent use of MIP may extend the application of RBA to detect small changes in toxin levels owing to its likely enhanced displacement by STX. This may be useful in analyzing samples with toxicities near the regulatory limit, or in establishing baseline values in high risk environments.

Human β-defensin 4 with non-native disulfide bridges exhibit antimicrobial activity

Human defensins play multiple roles in innate immunity including direct antimicrobial killing and immunomodulatory activity. They have three disulfide bridges which contribute to the stability of three anti-parallel β-strands. The exact role of disulfide bridges and canonical β-structure in the antimicrobial action is not yet fully understood. In this study, we have explored the antimicrobial activity of human β-defensin 4 (HBD4) analogs that differ in the number and connectivity of disulfide bridges. The cysteine framework was similar to the disulfide bridges present in μ-conotoxins, an unrelated class of peptide toxins. All the analogs possessed enhanced antimicrobial potency as compared to native HBD4. Among the analogs, the single disulfide bridged peptide showed maximum potency. However, there were no marked differences in the secondary structure of the analogs. Subtle variations were observed in the localization and membrane interaction of the analogs with bacteria and Candida albicans, suggesting a role for disulfide bridges in modulating their antimicrobial action. All analogs accumulated in the cytosol where they can bind to anionic molecules such as nucleic acids which would affect several cellular processes leading to cell death. Our study strongly suggests that native disulfide bridges or the canonical β-strands in defensins have not evolved for maximal activity but they play important roles in determining their antimicrobial potency.

Systematic study of binding of μ-conotoxins to the sodium channel NaV1.4

Voltage-gated sodium channels (NaV) are fundamental components of the nervous system. Their dysfunction is implicated in a number of neurological disorders, such as chronic pain, making them potential targets for the treatment of such disorders. The prominence of the NaV channels in the nervous system has been exploited by venomous animals for preying purposes, which have developed toxins that can block the NaV channels, thereby disabling their function. Because of their potency, such toxins could provide drug leads for the treatment of neurological disorders associated with NaV channels. However, most toxins lack selectivity for a given target NaV channel, and improving their selectivity profile among the NaV1 isoforms is essential for their development as drug leads. Computational methods will be very useful in the solution of such design problems, provided accurate models of the protein-ligand complex can be constructed. Using docking and molecular dynamics simulations, we have recently constructed a model for the NaV1.4-μ-conotoxin-GIIIA complex and validated it with the ample mutational data available for this complex. Here, we use the validated NaV1.4 model in a systematic study of binding other μ-conotoxins (PIIIA, KIIIA and BuIIIB) to NaV1.4. The binding mode obtained for each complex is shown to be consistent with the available mutation data and binding constants. We compare the binding modes of PIIIA, KIIIA and BuIIIB to that of GIIIA and point out the similarities and differences among them. The detailed information about NaV1.4-μ-conotoxin interactions provided here will be useful in the design of new NaV channel blocking peptides.

Bioactive marine drugs and marine biomaterials for brain diseases

Marine invertebrates produce a plethora of bioactive compounds, which serve as inspiration for marine biotechnology, particularly in drug discovery programs and biomaterials development. This review aims to summarize the potential of drugs derived from marine invertebrates in the field of neuroscience. Therefore, some examples of neuroprotective drugs and neurotoxins will be discussed. Their role in neuroscience research and development of new therapies targeting the central nervous system will be addressed, with particular focus on neuroinflammation and neurodegeneration. In addition, the neuronal growth promoted by marine drugs, as well as the recent advances in neural tissue engineering, will be highlighted.

Analgesic effects of clinically used compounds in novel mouse models of polyneuropathy induced by oxaliplatin and cisplatin

BACKGROUND

Peripheral neuropathy is the major dose-limiting side effect of cisplatin and oxaliplatin, and there are currently no effective treatments available. The aim of this study was to assess the pharmacological mechanisms underlying chemotherapy-induced neuropathy in novel animal models based on intraplantar administration of cisplatin and oxaliplatin and to systematically evaluate the analgesic efficacy of a range of therapeutics.

METHODS

Neuropathy was induced by a single intraplantar injection of cisplatin or oxaliplatin in C57BL/6J mice and assessed by quantification of mechanical and thermal allodynia. The pharmacological basis of cisplatin-induced neuropathy was characterized using a range of selective pharmacological inhibitors. The analgesic effects of phenytoin, amitriptyline, oxcarbazepine, mexiletine, topiramate, retigabine, gabapentin, fentanyl, and Ca(2+/)Mg(2+) were assessed 24 hours after induction of neuropathy.

RESULTS

Intraplantar administration of cisplatin led to the development of mechanical allodynia, mediated through Nav1.6-expressing sensory neurons. Unlike intraplantar injection of oxaliplatin, cold allodynia was not observed with cisplatin, consistent with clinical observations. Surprisingly, only fentanyl was effective at alleviating cisplatin-induced mechanical allodynia despite a lack of efficacy in oxaliplatin-induced cold allodynia. Conversely, lamotrigine, phenytoin, retigabine, and gabapentin were effective at reversing oxaliplatin-induced cold allodynia but had no effect on cisplatin-induced mechanical allodynia. Oxcarbazepine, amitriptyline, mexiletine, and topiramate lacked efficacy in both models of acute chemotherapy-induced neuropathy.

CONCLUSION

This study established a novel animal model of cisplatin-induced mechanical allodynia consistent with the A-fiber neuropathy seen clinically. Systematic assessment of a range of therapeutics identified several candidates that warrant further clinical investigation.

Mechanism of μ-conotoxin PIIIA binding to the voltage-gated Na+ channel NaV1.4

Several subtypes of voltage-gated Na+ (NaV) channels are important targets for pain management. μ-Conotoxins isolated from venoms of cone snails are potent and specific blockers of different NaV channel isoforms. The inhibitory effect of μ-conotoxins on NaV channels has been examined extensively, but the mechanism of toxin specificity has not been understood in detail. Here the known structure of μ-conotoxin PIIIA and a model of the skeletal muscle channel NaV1.4 are used to elucidate elements that contribute to the structural basis of μ-conotoxin binding and specificity. The model of NaV1.4 is constructed based on the crystal structure of the bacterial NaV channel, NaVAb. Six different binding modes, in which the side chain of each of the basic residues carried by the toxin protrudes into the selectivity filter of NaV1.4, are examined in atomic detail using molecular dynamics simulations with explicit solvent. The dissociation constants (Kd) computed for two selected binding modes in which Lys9 or Arg14 from the toxin protrudes into the filter of the channel are within 2 fold; both values in close proximity to those determined from dose response data for the block of NaV currents. To explore the mechanism of PIIIA specificity, a double mutant of NaV1.4 mimicking NaV channels resistant to μ-conotoxins and tetrodotoxin is constructed and the binding of PIIIA to this mutant channel examined. The double mutation causes the affinity of PIIIA to reduce by two orders of magnitude.

A novel α4/7-conotoxin LvIA from Conus lividus that selectively blocks α3β2 vs. α6/α3β2β3 nicotinic acetylcholine receptors

This study was performed to discover and characterize the first potent α3β2-subtype-selective nicotinic acetylcholine receptor (nAChR) ligand. A novel α4/7-conotoxin, α-CTxLvIA, was cloned from Conus lividus. Its pharmacological profile at Xenopus laevis oocyte-expressed rat nAChR subtypes was determined by 2-electrode voltage-clamp electrophysiology, and its 3-dimensional (3D) structure was determined by NMR spectroscopy. α-CTx LvIA is a 16-aa C-terminally-amidated peptide with 2-disulfide bridges. Using rat subunits expressed in Xenopus oocytes, we found the highest affinity of α-CTxLvIA was for α3β2 nAChRs (IC50 8.7 nM), where blockade was reversible within 2 min. IC50 values were >100 nM at α6/α3β2β3, α6/α3β4, and α3β4 nAChRs, and ≥3 μM at all other subtypes tested. α3β2 vs. α6β2 subtype selectivity was confirmed for human-subunit nAChRs with much greater preference (300-fold) for α3β2 over α6β2 nAChRs. This is the first α-CTx reported to show high selectivity for human α3β2 vs. α6β2 nAChRs. α-CTxLvIA adopts two similarly populated conformations water: one (assumed to be bioactive) is highly structured, whereas the other is mostly random coil in nature. Selectivity differences with the similarly potent, but less selective, α3β2 nAChR antagonist α-CTx PeIA probably reside within the three residues, which differ in loop 2, given their otherwise similar 3D structures. α4/7-CTx LvIA is a new, potent, selective α3β2 nAChR antagonist, which will enable detailed studies of α3β2 nAChR structure, function, and physiological roles.

[Conotoxins: from the biodiversity of gastropods to new drugs]

A review describes general trends in research of conotoxins that are peptide toxins isolated from sea gastropods of the Conus genus, since the toxins were discovered in 1970th. There are disclosed a conotoxin classification, their structure diversity and different ways of action to their molecular targets, mainly, ion channels. In the applied aspect of conotoxin research, drug discovery and development is discussed, the drugs being based on conotoxin structure. A first exemplary drug is a ziconotide, which is an analgesic of new generation.

Complex structures between the N-type calcium channel (CaV2.2) and ω-conotoxin GVIA predicted via molecular dynamics

The N-type voltage-gated Ca(2+) channel CaV2.2 is one of the important targets for pain management. ω-Conotoxins isolated from venoms of cone snails, which specifically inhibit CaV2.2, are promising scaffolds for novel analgesics. The inhibitory action of ω-conotoxins on CaV2.2 has been examined experimentally, but the modes of binding of the toxins to this and other related subfamilies of Ca(2+) channels are not understood in detail. Here molecular dynamics simulations are used to construct models of ω-conotoxin GVIA in complex with a homology model of the pore domain of CaV2.2. Three different binding modes in which the side chain of Lys2, Arg17, or Lys24 from the toxin protrudes into the selectivity filter of CaV2.2 are considered. In all the modes, the toxin forms a salt bridge with an aspartate residue of subunit II just above the EEEE ring of the selectivity filter. Using the umbrella sampling technique and potential of mean force calculations, the half-maximal inhibitory concentration (IC50) values are calculated to be 1.5 and 0.7 nM for the modes in which Lys2 and Arg17 occlude the ion conduction pathway, respectively. Both IC50 values compare favorably with the values of 0.04-1.0 nM determined experimentally. The similar IC50 values calculated for the different binding modes demonstrate that GVIA can inhibit CaV2.2 with alternative binding modes. Such a multiple-binding mode mechanism may be common for ω-conotoxins.

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.

Extension of Brownian dynamics for studying blockers of ion channels

We present new Brownian dynamics techniques for studying blockers of ion channels. By treating the channel as a fixed body, simulating the blocker molecules using rigid bodies, and using an implicit water force field with explicit ions, we are able to carry out fast simulations that can be used to investigate the dynamics of block and unblock, deduce binding modes, and calculate binding affinities. We test our program using the NavAb bacterial sodium channel, whose structure was recently solved (Payandeh et al. Nature, 2011, 475, 353-358) in conjunction with the μ-conotoxin PIIIA blocker. We derive an ohmic current-voltage relationship for channel permeation, calculate potentials of mean force for blocker unbinding, and deduce multiple binding modes for the blocker. Our results are shown to be compatible with other computational and experimental results. Finally, we discuss future improvements such as the inclusion of flexible side chains. After these improvements are carried out, we anticipate our program will be an extremely useful new tool that could be used to help develop new drugs to treat a range of ion-channel related diseases.

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.

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

BACKGROUND AND PURPOSE

The µ-conopeptide family is defined by its ability to block voltage-gated sodium channels (VGSCs), a property that can be used for the development of myorelaxants and analgesics. We characterized the pharmacology of a new µ-conopeptide (µ-CnIIIC) on a range of preparations and molecular targets to assess its potential as a myorelaxant.

EXPERIMENTAL APPROACH

µ-CnIIIC was sequenced, synthesized and characterized by its direct block of elicited twitch tension in mouse skeletal muscle and action potentials in mouse sciatic and pike olfactory nerves. µ-CnIIIC was also studied on HEK-293 cells expressing various rodent VGSCs and also on voltage-gated potassium channels and nicotinic acetylcholine receptors (nAChRs) to assess cross-interactions. Nuclear magnetic resonance (NMR) experiments were carried out for structural data.

KEY RESULTS

Synthetic µ-CnIIIC decreased twitch tension in mouse hemidiaphragms (IC(50) = 150 nM), and displayed a higher blocking effect in mouse extensor digitorum longus muscles (IC = 46 nM), compared with µ-SIIIA, µ-SmIIIA and µ-PIIIA. µ-CnIIIC blocked Na(V)1.4 (IC(50) = 1.3 nM) and Na(V)1.2 channels in a long-lasting manner. Cardiac Na(V)1.5 and DRG-specific Na(V)1.8 channels were not blocked at 1 µM. µ-CnIIIC also blocked the α3β2 nAChR subtype (IC(50) = 450 nM) and, to a lesser extent, on the α7 and α4β2 subtypes. Structure determination of µ-CnIIIC revealed some similarities to α-conotoxins acting on nAChRs.

CONCLUSION AND IMPLICATIONS

µ-CnIIIC potently blocked VGSCs in skeletal muscle and nerve, and hence is applicable to myorelaxation. Its atypical pharmacological profile suggests some common structural features between VGSCs and nAChR channels.

N- and C-terminal extensions of μ-conotoxins increase potency and selectivity for neuronal sodium channels

μ-Conotoxins are peptide blockers of voltage-gated sodium channels (sodium channels), inhibiting tetrodotoxin-sensitive neuronal (Na(v) 1.2) and skeletal (Na(v) 1.4) subtypes with highest affinity. Structure-activity relationship studies of μ-conotoxins SIIIA, TIIIA, and KIIIA have shown that it is mainly the C-terminal part of the three-loop peptide that is involved in binding to the sodium channel. In this study, we characterize the effect of N- and C-terminal extensions of μ-conotoxins SIIIA, SIIIB, and TIIIA on their potency and selectivity for neuronal versus muscle sodium channels. Interestingly, extending the N- or C-terminal of the peptide by introducing neutral, positive, and/or negatively charged residues, the selectivity of the native peptide can be altered from neuronal to skeletal and the other way around. The results from this study provide further insight into the binding profile of μ-conotoxins at voltage-gated sodium channels, revealing that binding interactions outside the cysteine-stablilized loops can contribute to μ-conotoxin affinity and sodium channel selectivity.

Binding modes of μ-conotoxin to the bacterial sodium channel (NaVAb)

Polypeptide toxins isolated from the venom of cone snails, known as μ-conotoxins, block voltage-gated sodium channels by physically occluding the ion-conducting pathway. Using molecular dynamics, we show that one subtype of μ-conotoxins, PIIIA, effectively blocks the bacterial voltage-gated sodium channel Na(V)Ab, whose crystal structure has recently been elucidated. The spherically shaped toxin, carrying a net charge of +6 e with six basic residues protruding from its surface, is attracted by the negatively charged residues on the vestibular wall and the selectivity filter of the channel. The side chain of each of these six arginine and lysine residues can wedge into the selectivity filter, whereas the side chains of other basic residues form electrostatic complexes with two acidic residues on the channel. We construct the profile of potential of mean force for the unbinding of PIIIA from the channel, and predict that PIIIA blocks the bacterial sodium channel with subnanomolar affinity.

Orientation of μ-conotoxin PIIIA in a sodium channel vestibule, based on voltage dependence of its binding

Mutant cycle analysis has been used in previous studies to constrain possible docking orientations for various toxins. As an independent test of the bound orientation of μ-conotoxin PIIIA, a selectively targeted sodium channel pore blocker, we determined the contributions to binding voltage dependence of specific residues on the surface of the toxin. A change in the "apparent valence" (zδ) of the block, which is associated with a change of a specific toxin charge, reflects a change in the charge movement within the transmembrane electric field as the toxin binds. Toxin derivatives with charge-conserving mutations (R12K, R14K, and K17R) showed zδ values similar to those of wild type (0.61 ± 0.01, mean ± S.E.M.). Charge-changing mutations produced a range of responses. Neutralizing substitutions for Arg14 and Lys17 showed the largest reductions in zδ values, to 0.18 ± 0.06 and 0.20 ± 0.06, respectively, whereas unit charge-changing substitutions for Arg12, Ser13, and Arg20 gave intermediate values (0.24 ± 0.07, 0.33 ± 0.04, and 0.32 ± 0.05), which suggests that each of these residues contributes to the dependence of binding on the transmembrane voltage. Two mutations, R2A and G6K, yielded no significant change in zδ. These observations suggest that the toxin binds with Arg2 and Gly6 facing the extracellular solution, and Arg14 and Lys17 positioned most deeply in the pore. In this study, we used molecular dynamics to simulate toxin docking and performed Poisson-Boltzmann calculations to estimate the changes in local electrostatic potential when individual charges were substituted on the toxin's surface. Consideration of two limiting possibilities suggests that most of the charge movement associated with toxin binding reflects sodium redistribution within the narrow part of the pore.

Multiple, distributed interactions of μ-conotoxin PIIIA associated with broad targeting among voltage-gated sodium channels

The first μ-conotoxin studied, μCTX GIIIA, preferentially blocked voltage-gated skeletal muscle sodium channels, Na(v)1.4, while μCTX PIIIA was the first to show significant blocking action against neuronal voltage-gated sodium channels. PIIIA shares >60% sequence identity with the well-studied GIIIA, and both toxins preferentially block the skeletal muscle sodium channel isoform. Two important features of blocking by wild-type GIIIA are the toxin's high binding affinity and the completeness of block of a single channel by a bound toxin molecule. With GIIIA, neutral replacement of the critical residue, Arg-13, allows a residual single-channel current (~30% of the unblocked, unitary amplitude) when the mutant toxin is bound to the channel and reduces the binding affinity of the toxin for Na(v)1.4 (~100-fold) [Becker, S., et al. (1992) Biochemistry 31, 8229-8238]. The homologous residue in PIIIA, Arg-14, is also essential for completeness of block but less important in the toxin's binding affinity (~55% residual current and ~11-fold decrease in affinity when substituted with alanine or glutamine). The weakened dominance of this key arginine in PIIIA is also seen in the fact that there is not just one (R13 in GIIIA) but three basic residues (R12, R14, and K17) for which individual neutral replacement enables a substantial residual current through the bound channel. We suggest that, despite a high degree of sequence conservation between GIIIA and PIIIA, the weaker dependence of PIIIA's action on its key arginine and the presence of a nonconserved histidine near the C-terminus may contribute to the greater promiscuity of its interactions with different sodium channel isoforms.

Stereochemistry of 4-hydroxyproline affects the conformation of conopeptides

The cis/trans isomerization of 4-hydroxyproline is shown to remarkably affect the conformation of conopeptides with or without disulfide bonds.

Mu-conotoxins as leads in the development of new analgesics

Voltage-gated sodium channels (VGSCs) contain a specific binding site for a family of cone shell toxins known as mu-conotoxins. As some VGSCs are involved in pain perception and mu-conotoxins are able to block these channels, mu-conotoxins show considerable potential as analgesics. Recent studies have advanced our understanding of the three-dimensional structures and structure-function relationships of the mu-conotoxins, including their interaction with VGSCs. Truncated peptide analogues of the native toxins have been created in which secondary structure elements are stabilized by non-native linkers such as lactam bridges. Ultimately, it would be desirable to capture the favourable analgesic properties of the native toxins, in particular their potency and channel sub-type selectivity, in non-peptide mimetics. Such mimetics would constitute lead compounds in the development of new therapeutics for the treatment of pain.

Biochemical characterization of kappaM-RIIIJ, a Kv1.2 channel blocker: evaluation of cardioprotective effects of kappaM-conotoxins

Conus snail (Conus) venoms are a valuable source of pharmacologically active compounds; some of the peptide toxin families from the snail venoms are known to interact with potassium channels. We report the purification, synthesis, and characterization of kappaM-conotoxin RIIIJ from the venom of a fish-hunting species, Conus radiatus. This conopeptide, like a previously characterized peptide in the same family, kappaM-RIIIK, inhibits the homotetrameric human Kv1.2 channels. When tested in Xenopus oocytes, kappaM-RIIIJ has an order of magnitude higher affinity (IC(50) = 33 nm) to Kv1.2 than kappaM-RIIIK (IC(50) = 352 nm). Chimeras of RIIIK and RIIIJ tested on the human Kv1.2 channels revealed that Lys-9 from kappaM-RIIIJ is a determinant of its higher potency against hKv1.2. However, when compared in a model of ischemia/reperfusion, kappaM-RIIIK (100 mug/kg of body weight), administered just before reperfusion, significantly reduces the infarct size in rat hearts in vivo without influencing hemodynamics, providing a potential compound for cardioprotective therapeutics. In contrast, kappaM-RIIIJ does not exert any detectable cardioprotective effect. kappaM-RIIIJ shows more potency for Kv1.2-Kv1.5 and Kv1.2-Kv1.6 heterodimers than kappaM-RIIIK, whereas the affinity of kappaM-RIIIK to Kv1.2-Kv1.7 heterodimeric channels is higher (IC(50) = 680 nm) than that of kappaM-RIIIJ (IC(50) = 3.15 mum). Thus, the cardioprotection seems to correlate to antagonism to heteromultimeric channels, involving the Kv1.2 alpha-subunit rather than antagonism to Kv1.2 homotetramers. Furthermore, kappaM-RIIIK and kappaM-RIIIJ provide a valuable set of probes for understanding the underlying mechanism of cardioprotection.

The M-superfamily of conotoxins: a review

The focus of this review is the M-superfamily of Conus venom peptides. Disulfide rich peptides belonging to the M-superfamily have three loop regions and the cysteine arrangement: CC-C-C-CC, where the dashes represent loops one, two, and three, respectively. Characterization of M-superfamily peptides has demonstrated that diversity in cystine connectivity occurs between different branches of peptides even though the cysteine pattern remains consistent. This superfamily is subdivided into five branches, M-1 through M-5, based on the number of residues in the third loop region, between the fourth and fifth cysteine residues. M-superfamily peptides appear to be ubiquitous in Conus venom. They are largely unexplained in indigenous biological function, and they represent an active area of research within the scientific community.

Structurally minimized mu-conotoxin analogues as sodium channel blockers: implications for designing conopeptide-based therapeutics

Disulfide bridges that stabilize the native conformation of conotoxins pose a challenge in the synthesis of smaller conotoxin analogues. Herein we describe the synthesis of a minimized analogue of the analgesic mu-conotoxin KIIIA that blocks two sodium channel subtypes, the neuronal Na(V)1.2 and skeletal muscle Na(V)1.4. Three disulfide-deficient analogues of KIIIA were initially synthesized in which the native disulfide bridge formed between either C1-C9, C2-C15, or C4-C16 was removed. Deletion of the first bridge only slightly affected the peptide's bioactivity. To further minimize this analogue, the N-terminal residue was removed and two nonessential serine residues were replaced by a single 5-amino-3-oxapentanoic acid residue. The resulting "polytide" analogue retained the ability to block sodium channels and to produce analgesia. Until now, the peptidomimetic approach applied to conotoxins has progressed only modestly at best; thus, the disulfide-deficient analogues containing backbone spacers provide an alternative advance toward the development of conopeptide-based therapeutics.

Conotoxins: molecular and therapeutic targets

Marine molluscs known as cone snails produce beautiful shells and a complex array of over 50,000 venom peptides evolved for prey capture and defence. Many of these peptides selectively modulate ion channels and transporters, making them a valuable source of new ligands for studying the role these targets play in normal and disease physiology. A number of conopeptides reduce pain in animal models, and several are now in pre-clinical and clinical development for the treatment of severe pain often associated with diseases such as cancer. Less than 1% of cone snail venom peptides are pharmacologically characterised.

NMR-based mapping of disulfide bridges in cysteine-rich peptides: application to the mu-conotoxin SxIIIA

Disulfide-rich peptides represent a megadiverse group of natural products with very promising therapeutic potential. To accelerate their functional characterization, high-throughput chemical synthesis and folding methods are required, including efficient mapping of multiple disulfide bridges. Here, we describe a novel approach for such mapping and apply it to a three-disulfide-bridged conotoxin, mu-SxIIIA (from the venom of Conus striolatus), whose discovery is also reported here for the first time. Mu-SxIIIA was chemically synthesized with three cysteine residues labeled 100% with (15)N/(13)C, while the remaining three cysteine residues were incorporated using a mixture of 70%/30% unlabeled/labeled Fmoc-protected residues. After oxidative folding, the major product was analyzed by NMR spectroscopy. Sequence-specific resonance assignments for the isotope-enriched Cys residues were determined with 2D versions of standard triple-resonance ((1)H, (13)C, (15)N) NMR experiments and 2D [(13)C, (1)H] HSQC. Disulfide patterns were directly determined with cross-disulfide NOEs confirming that the oxidation product had the disulfide connectivities characteristic of mu-conotoxins. Mu-SxIIIA was found to be a potent blocker of the sodium channel subtype Na(V)1.4 (IC50 = 7 nM). These results suggest that differential incorporation of isotope-labeled cysteine residues is an efficient strategy to map disulfides and should facilitate the discovery and structure-function studies of many bioactive peptides.

Structure, dynamics, and selectivity of the sodium channel blocker mu-conotoxin SIIIA

mu-SIIIA, a novel mu-conotoxin from Conus striatus, appeared to be a selective blocker of tetrodotoxin-resistant sodium channels in frog preparations. It also exhibited potent analgesic activity in mice, although its selectivity profile against mammalian sodium channels remains unknown. We have determined the structure of mu-SIIIA in aqueous solution and characterized its backbone dynamics by NMR and its functional properties electrophysiologically. Consistent with the absence of hydroxyprolines, mu-SIIIA adopts a single conformation with all peptide bonds in the trans conformation. The C-terminal region contains a well-defined helix encompassing residues 11-16, while residues 3-5 in the N-terminal region form a helix-like turn resembling 3 10-helix. The Trp12 and His16 side chains are close together, as in the related conotoxin mu-SmIIIA, but Asn2 is more distant. Dynamics measurements show that the N-terminus and Ser9 have larger-magnitude motions on the subnanosecond time scale, while the C-terminus is more rigid. Cys4, Trp12, and Cys13 undergo significant conformational exchange on microsecond to millisecond time scales. mu-SIIIA is a potent, nearly irreversible blocker of Na V1.2 but also blocks Na V1.4 and Na V1.6 with submicromolar potency. The selectivity profile of mu-SIIIA, including poor activity against the cardiac sodium channel, Na V1.5, is similar to that of the closely related mu-KIIIA, suggesting that the C-terminal regions of both are critical for blocking neuronal Na V1.2. The structural and functional characterization described in this paper of an analgesic mu-conotoxin that targets neuronal subtypes of mammalian sodium channels provides a basis for the design of novel analogues with an improved selectivity profile.

Chemical modification of conotoxins to improve stability and activity

Conotoxins are small disulfide-rich peptides from the venom of cone snails. Along with other conopeptides, they target a wide range of membrane receptors, ion channels, and transporters, and because of their high potency and selectivity for defined subtypes of these receptors, they have attracted a great deal of attention recently as leads in drug development. However, like most peptides, conopeptides potentially suffer from the disadvantages of poor absorption, poor stability, or short biological half-lives. Recently, various chemical approaches, including residue substitutions, backbone cyclization, and disulfide-bridge modification, have been reported to increase the stability of conopeptides. These manufactured interventions add to the array of post-translational modifications that occur naturally in conopeptides. They enhance the versatility of these peptides as tools in neuroscience and as drug leads.

Isolation and structure-activity of mu-conotoxin TIIIA, a potent inhibitor of tetrodotoxin-sensitive voltage-gated sodium channels

Mu-conotoxins are three-loop peptides produced by cone snails to inhibit voltage-gated sodium channels during prey capture. Using polymerase chain reaction techniques, we identified a gene sequence from the venom duct of Conus tulipa encoding a new mu-conotoxin-TIIIA (TIIIA). A 125I-TIIIA binding assay was established to isolate native TIIIA from the crude venom of Conus striatus. The isolated peptide had three post-translational modifications, including two hydroxyproline residues and C-terminal amidation, and <35% homology to other mu-conotoxins. TIIIA potently displaced [3H]saxitoxin and 125I-TIIIA from rat brain (Nav1.2) and skeletal muscle (Nav1.4) membranes. Alanine and glutamine scans of TIIIA revealed several residues, including Arg14, that were critical for high-affinity binding to tetrodotoxin (TTX)-sensitive Na+ channels. We were surprised to find that [E15A]TIIIA had a 10-fold higher affinity than TIIIA for TTX-sensitive sodium channels (IC50, 15 vs. 148 pM at rat brain membrane). TIIIA was selective for Nav1.2 and -1.4 over Nav1.3, -1.5, -1.7, and -1.8 expressed in Xenopus laevis oocytes and had no effect on rat dorsal root ganglion neuron Na+ current. 1H NMR studies revealed that TIIIA adopted a single conformation in solution that was similar to the major conformation described previously for mu-conotoxin PIIIA. TIIIA and analogs provide new biochemical probes as well as insights into the structure-activity of mu-conotoxins.

Sequence diversity of O-superfamily conopetides from Conus marmoreus native to Hainan

The full-length cDNAs of six new O-superfamily conotoxins (CTX) were cloned and sequenced from Conus marmoreus native to Hainan in China South Sea using RT-PCR and 3'-RACE. Six novel conotoxin precursors encoded by these cDNAs consist of three typical regions of signal, pro-peptide and mature peptide. All the six toxin regions share a common O-superfamily cysteine pattern (C-C-CC-C-C, with three disulfide bridges). The predicted precursors are composed of 73-88 amino acids, and the predicted mature peptides consist of 26-34 amino acids. Phylogenetic analysis of new conotoxins from C. marmoreus from the present study and published homologue T-superfamily sequences from other Conus species was performed systematically. Patterns of sequence divergence for three regions of signal, pro-region and mature peptides, as well as Cys codon usage define the major O-superfamily branches and suggest how these separate branches arose. Percent identities of the amino acid sequences of the signal region exhibited high conservation, whereas the sequences of the mature peptides ranged from almost identical to highly divergent between inter- and intra-species. Notably, the diversity of the pro-region was also high with intermediate divergence between that observed in signal and toxin regions. Amino acid sequences and their mode of action (target) of previously identified conotoxins from molluscivorous C. marmoreus for the known conotoxins classes are discussed in detail. The data presented are new and should pave the way for chemical synthesis of these unique conotoxins for to allow determination of the molecular targets of these peptides, and also to provide clues for a better understanding of the phylogeny of these peptides.

Conotoxins down under

In the four decades since toxinologists in Australia and elsewhere started to investigate the active constituents of venomous cone snails, a wealth of information has emerged on the various classes of peptides and proteins that make their venoms such potent bioactive cocktails. This article provides an overview of the current state of knowledge of these venom constituents, several of which are of interest as potential human therapeutics as a consequence of their high potency and exquisite target specificity. With the promise of as many as 50,000 venom components across the entire Conus genus, many more interesting peptides can be anticipated.