Co-expression of Na(V)β subunits alters the kinetics of inhibition of voltage-gated sodium channels by pore-blocking μ-conotoxins

Sodium Channel β Subunits-An Additional Element in Animal Tetrodotoxin Resistance?

Tetrodotoxin (TTX) is a neurotoxic molecule used by many animals for defense and/or predation, as well as an important biomedical tool. Its ubiquity as a defensive agent has led to repeated independent evolution of tetrodotoxin resistance in animals. TTX binds to voltage-gated sodium channels (VGSC) consisting of α and β subunits. Virtually all studies investigating the mechanisms behind TTX resistance have focused on the α subunit of voltage-gated sodium channels, where tetrodotoxin binds. However, the possibility of β subunits also contributing to tetrodotoxin resistance was never explored, though these subunits act in concert. In this study, we present preliminary evidence suggesting a potential role of β subunits in the evolution of TTX resistance. We gathered mRNA sequences for all β subunit types found in vertebrates across 12 species (three TTX-resistant and nine TTX-sensitive) and tested for signatures of positive selection with a maximum likelihood approach. Our results revealed several sites experiencing positive selection in TTX-resistant taxa, though none were exclusive to those species in subunit β1, which forms a complex with the main physiological target of TTX (VGSC Nav1.4). While experimental data validating these findings would be necessary, this work suggests that deeper investigation into β subunits as potential players in tetrodotoxin resistance may be worthwhile.

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.

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.

Pain-causing stinging nettle toxins target TMEM233 to modulate NaV1.7 function

Voltage-gated sodium (NaV) channels are critical regulators of neuronal excitability and are targeted by many toxins that directly interact with the pore-forming α subunit, typically via extracellular loops of the voltage-sensing domains, or residues forming part of the pore domain. Excelsatoxin A (ExTxA), a pain-causing knottin peptide from the Australian stinging tree Dendrocnide excelsa, is the first reported plant-derived NaV channel modulating peptide toxin. Here we show that TMEM233, a member of the dispanin family of transmembrane proteins expressed in sensory neurons, is essential for pharmacological activity of ExTxA at NaV channels, and that co-expression of TMEM233 modulates the gating properties of NaV1.7. These findings identify TMEM233 as a previously unknown NaV1.7-interacting protein, position TMEM233 and the dispanins as accessory proteins that are indispensable for toxin-mediated effects on NaV channel gating, and provide important insights into the function of NaV channels in sensory neurons.

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.

Inhibition of voltage-gated sodium ion channel by corannulene and computational inversion blockage underlying mechanisms

Corannulene (Cor), a special carbon material, evidenced strong protein binding capacity which regulating lysozyme crystallization and controlling reactive oxygen species (ROS) generation. Ion channel protein play role in regulating ion channel functions to affect physiological functions. However, the interaction between Cor and ion channel protein have not been studied. In this study, PEG/Cor nanoparticles (PEG/Cor Nps) were prepared by mPEG-DSPE. The PEG/Cor Nps localized in cytoplasm and produced cytotoxicity at high concentration. Whole cell patch clamp examined ion channel functions after incubate PEG/Cor Nps with PC-12 cell. we found that PEG/Cor Nps inhibited voltage-gated Na⁺ ion channels in a dose- and time-dependent manner but not act on voltage-gated K⁺ ion channels. The potential mechanisms were revealed by all-atom molecular dynamic (MD) simulations. The results showed that PEG/Cor Nps block the pore of sodium ion channel protein due to dose- and time-dependent accumulation. Besides, the orientation angle (θ) configuration of PEG/Cor Nps will be inverted with the accumulation to generate two blocking mechanisms. Different from other carbon nanomaterials, the blockage mechanism of PEG/Cor Nps provides novel insights into the mechanisms of interaction between carbon nanomaterials and protein.

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.

Recent progress in the structural study of ion channels as insecticide targets

Ion channels, many expressed in insect neural and muscular systems, have drawn huge attention as primary targets of insecticides. With the recent technical breakthroughs in structural biology, especially in cryo-electron microscopy (cryo-EM), many new high-resolution structures of ion channel targets, apo or in complex with insecticides, have been solved, shedding light on the molecular mechanism of action of the insecticides and resistance mutations. These structures also provide accurate templates for structure-based insecticide screening and rational design. This review summarizes the recent progress in the structural studies of 5 ion channel families: the ryanodine receptor (RyR), the nicotinic acetylcholine receptor (nAChR), the voltage-gated sodium channel (VGSC), the transient receptor potential (TRP) channel, and the ligand-gated chloride channel (LGCC). We address the selectivity of the channel-targeting insecticides by examining the conservation of key coordinating residues revealed by the structures. The possible resistance mechanisms are proposed based on the locations of the identified resistance mutations on the 3D structures of the target channels and their impacts on the binding of insecticides. Finally, we discuss how to develop "green" insecticides with a novel mode of action based on these high-resolution structures to overcome the resistance.

Pharmacological determination of the fractional block of Nav channels required to impair neuronal excitability and ex vivo seizures

Voltage-gated sodium channels (Nav) are essential for the initiation and propagation of action potentials in neurons. Of the nine human channel subtypes, Nav1.1, Nav1.2 and Nav1.6 are prominently expressed in the adult central nervous system (CNS). All three of these sodium channel subtypes are sensitive to block by the neurotoxin tetrodotoxin (TTX), with TTX being almost equipotent on all three subtypes. In the present study we have used TTX to determine the fractional block of Nav channels required to impair action potential firing in pyramidal neurons and reduce network seizure-like activity. Using automated patch-clamp electrophysiology, we first determined the IC₅₀s of TTX on mouse Nav1.1, Nav1.2 and Nav1.6 channels expressed in HEK cells, demonstrating this to be consistent with previously published data on human orthologs. We then compared this data to the potency of block of Nav current measured in pyramidal neurons from neocortical brain slices. Interestingly, we found that it requires nearly 10-fold greater concentration of TTX over the IC₅₀ to induce significant block of action potentials using a current-step protocol. In contrast, concentrations near the IC₅₀ resulted in a significant reduction in AP firing and increase in rheobase using a ramp protocol. Surprisingly, a 20% reduction in action potential generation observed with 3 nM TTX resulted in significant block of seizure-like activity in the 0 Mg²⁺ model of epilepsy. Additionally, we found that approximately 50% block in pyramidal cell intrinsic excitability is sufficient to completely block all seizure-like events. Furthermore, we also show that the anticonvulsant drug phenytoin blocked seizure-like events in a manner similar to TTX. These data serve as a critical starting point in understanding how fractional block of Nav channels affect intrinsic neuronal excitability and seizure-like activity. It further suggests that seizures can be controlled without significantly compromising intrinsic neuronal activity and determines the required fold over IC₅₀ for novel and clinically relevant Nav channel blockers to produce efficacy and limit side effects.

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.

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.

Computational design of peptides to target NaV1.7 channel with high potency and selectivity for the treatment of pain

The voltage-gated sodium Na_V1.7 channel plays a key role as a mediator of action potential propagation in C-fiber nociceptors and is an established molecular target for pain therapy. ProTx-II is a potent and moderately selective peptide toxin from tarantula venom that inhibits human Na_V1.7 activation. Here we used available structural and experimental data to guide Rosetta design of potent and selective ProTx-II-based peptide inhibitors of human Na_V1.7 channels. Functional testing of designed peptides using electrophysiology identified the PTx2-3127 and PTx2-3258 peptides with IC₅₀s of 7 nM and 4 nM for hNa_V1.7 and more than 1000-fold selectivity over human Na_V1.1, Na_V1.3, Na_V1.4, Na_V1.5, Na_V1.8, and Na_V1.9 channels. PTx2-3127 inhibits Na_V1.7 currents in mouse and human sensory neurons and shows efficacy in rat models of chronic and thermal pain when administered intrathecally. Rationally designed peptide inhibitors of human Na_V1.7 channels have transformative potential to define a new class of biologics to treat pain.

Evidence that toxin resistance in poison birds and frogs is not rooted in sodium channel mutations and may rely on "toxin sponge" proteins

Many poisonous organisms carry small-molecule toxins that alter voltage-gated sodium channel (NaV) function. Among these, batrachotoxin (BTX) from Pitohui poison birds and Phyllobates poison frogs stands out because of its lethality and unusual effects on NaV function. How these toxin-bearing organisms avoid autointoxication remains poorly understood. In poison frogs, a NaV DIVS6 pore-forming helix N-to-T mutation has been proposed as the BTX resistance mechanism. Here, we show that this variant is absent from Pitohui and poison frog NaVs, incurs a strong cost compromising channel function, and fails to produce BTX-resistant channels in poison frog NaVs. We also show that captivity-raised poison frogs are resistant to two NaV-directed toxins, BTX and saxitoxin (STX), even though they bear NaVs sensitive to both. Moreover, we demonstrate that the amphibian STX "toxin sponge" protein saxiphilin is able to protect and rescue NaVs from block by STX. Taken together, our data contradict the hypothesis that BTX autoresistance is rooted in the DIVS6 N→T mutation, challenge the idea that ion channel mutations are a primary driver of toxin resistance, and suggest the possibility that toxin sequestration mechanisms may be key for protecting poisonous species from the action of small-molecule toxins.

The Voltage-Gated Sodium Channel Beta4 Subunit Maintains Epithelial Phenotype in Mammary Cells

The SCN4B gene, coding for the Na_Vβ4 subunit of voltage-gated sodium channels, was recently found to be expressed in normal epithelial cells and down-regulated in several cancers. However, its function in normal epithelial cells has not been characterized. In this study, we demonstrated that reducing Na_Vβ4 expression in MCF10A non-cancer mammary epithelial cells generated important morphological changes observed both in two-dimensional cultures and in three-dimensional cysts. Most notably, the loss of Na_Vβ4 induced a complete loss of epithelial organisation in cysts and increased proteolytic activity towards the extracellular matrix. Loss of epithelial morphology was associated with an increased degradation of β-catenin, reduced E-cadherin expression and induction of mesenchymal markers N-cadherin, vimentin, and α-SMA expression. Overall, our results suggest that Navβ4 may participate in the maintenance of the epithelial phenotype in mammary cells and that its downregulation might be a determining step in early carcinogenesis.

μ-conotoxin TsIIIA, a peptide inhibitor of human voltage-gated sodium channel hNav1.8

TsIIIA, the first μ-conotoxin from Conus tessulatus, can selectively inhibit rat tetrodotoxin-resistant sodium channels. TsIIIA also shows potent analgesic activity in a mice hotplate analgesic assay, but its effect on human sodium channels remains unknown. In this study, eight human sodium channel subtypes, hNa_v1.1- hNa_v1.8, were expressed in HEK293 or ND7/23 cells and tested on the chemically synthesized TsIIIA. Patch clamp experiments showed that 10 μM TsIIIA had no effects on the tetrodotoxin-sensitive hNa_v1.1, hNa_v1.2, hNa_v1.3, hNa_v1.4, hNa_v1.6 and hNa_v1.7, as well as tetrodotoxin-resistant hNa_v1.5. For tetrodotoxin-resistant hNa_v1.8, concentrations of 1, 5 and 10 μM TsIIIA reduced the hNa_v1.8 currents to 59.26%, 36.21% and 24.93% respectively. Further detailed dose-effect experiments showed that TsIIIA inhibited hNa_v1.8 currents with an IC₅₀ value of 2.11 μM. In addition, 2 μM TsIIIA did not induce a shift in the current-voltage relationship of hNa_v1.8. Taken together, the hNa_v1.8 peptide inhibitor TsIIIA provides a pharmacological probe for sodium channels and a potential therapeutic agent for pain.

Spider venom-derived peptide induces hyperalgesia in Nav1.7 knockout mice by activating Nav1.9 channels

The sodium channels Na_v1.7, Na_v1.8 and Na_v1.9 are critical for pain perception in peripheral nociceptors. Loss of function of Na_v1.7 leads to congenital insensitivity to pain in humans. Here we show that the spider peptide toxin called HpTx1, first identified as an inhibitor of K_v4.2, restores nociception in Na_v1.7 knockout (Na_v1.7-KO) mice by enhancing the excitability of dorsal root ganglion neurons. HpTx1 inhibits Na_v1.7 and activates Na_v1.9 but does not affect Na_v1.8. This toxin produces pain in wild-type (WT) and Na_v1.7-KO mice, and attenuates nociception in Na_v1.9-KO mice, but has no effect in Na_v1.8-KO mice. These data indicate that HpTx1-induced hypersensitivity is mediated by Na_v1.9 activation and offers pharmacological insight into the relationship of the three Na_v channels in pain signalling.

Loss of function of Na_v1.7 leads to congenital insensitivity to pain in humans. Here the authors found that activation of Na_v1.9 can restore nociception in Na_v1.7 knockout mice, revealed by a venom-derived peptide as a probe.

Venom-Derived Peptide Modulators of Cation-Selective Channels: Friend, Foe or Frenemy

Ion channels play a key role in our body to regulate homeostasis and conduct electrical signals. With the help of advances in structural biology, as well as the discovery of numerous channel modulators derived from animal toxins, we are moving toward a better understanding of the function and mode of action of ion channels. Their ubiquitous tissue distribution and the physiological relevancies of their opening and closing suggest that cation channels are particularly attractive drug targets, and years of research has revealed a variety of natural toxins that bind to these channels and alter their function. In this review, we provide an introductory overview of the major cation ion channels: potassium channels, sodium channels and calcium channels, describe their venom-derived peptide modulators, and how these peptides provide great research and therapeutic value to both basic and translational medical research.

Selective NaV1.1 activation rescues Dravet syndrome mice from seizures and premature death

Spider venom is a rich source of peptides, many targeting ion channels. We assessed a venom peptide, Hm1a, as a potential targeted therapy for Dravet syndrome, the genetic epilepsy linked to a mutation in the gene encoding the sodium channel alpha subunit Na_V1.1. Cell-based assays showed Hm1a was selective for hNa_V1.1 over other sodium and potassium channels. Utilizing a mouse model of Dravet syndrome, Hm1a restored inhibitory neuron function and significantly reduced seizures and mortality in heterozygote mice. Evidence from the structure of Hm1a and modeling suggest Hm1a interacts with Na_V1.1 inactivation domains, providing a structural correlate of the functional mechanisms. This proof-of-concept study provides a promising strategy for future drug development in genetic epilepsy and other neurogenetic disorders.

Dravet syndrome is a catastrophic, pharmacoresistant epileptic encephalopathy. Disease onset occurs in the first year of life, followed by developmental delay with cognitive and behavioral dysfunction and substantially elevated risk of premature death. The majority of affected individuals harbor a loss-of-function mutation in one allele of SCN1A, which encodes the voltage-gated sodium channel Na_V1.1. Brain Na_V1.1 is primarily localized to fast-spiking inhibitory interneurons; thus the mechanism of epileptogenesis in Dravet syndrome is hypothesized to be reduced inhibitory neurotransmission leading to brain hyperexcitability. We show that selective activation of Na_V1.1 by venom peptide Hm1a restores the function of inhibitory interneurons from Dravet syndrome mice without affecting the firing of excitatory neurons. Intracerebroventricular infusion of Hm1a rescues Dravet syndrome mice from seizures and premature death. This precision medicine approach, which specifically targets the molecular deficit in Dravet syndrome, presents an opportunity for treatment of this intractable epilepsy.

Heat-resistant action potentials require TTX-resistant sodium channels NaV1.8 and NaV1.9

Nociceptors prevent damage by being able to detect and transmit noxious stimuli, such as hot temperatures. Touska et al. show that the TTX-resistant Na_V channels, Na_V1.8 and Na_V1.9, are required for heat-resistant nociceptors to encode noxious heat and that the current through Na_V1.9 increases at higher temperatures.

Damage-sensing nociceptors in the skin provide an indispensable protective function thanks to their specialized ability to detect and transmit hot temperatures that would block or inflict irreversible damage in other mammalian neurons. Here we show that the exceptional capacity of skin C-fiber nociceptors to encode noxiously hot temperatures depends on two tetrodotoxin (TTX)-resistant sodium channel α-subunits: Na_V1.8 and Na_V1.9. We demonstrate that Na_V1.9, which is commonly considered an amplifier of subthreshold depolarizations at 20°C, undergoes a large gain of function when temperatures rise to the pain threshold. We also show that this gain of function renders Na_V1.9 capable of generating action potentials with a clear inflection point and positive overshoot. In the skin, heat-resistant nociceptors appear as two distinct types with unique and possibly specialized features: one is blocked by TTX and relies on Na_V1.9, and the second type is insensitive to TTX and composed of both Na_V1.8 and Na_V1.9. Independent of rapidly gated TTX-sensitive Na_V channels that form the action potential at pain threshold, Na_V1.8 is required in all heat-resistant nociceptors to encode temperatures higher than ∼46°C, whereas Na_V1.9 is crucial for shaping the action potential upstroke and keeping the Na_V1.8 voltage threshold within reach.

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.

Differential Inhibition of Nav1.7 and Neuropathic Pain by Hybridoma-Produced and Recombinant Monoclonal Antibodies that Target Nav1.7 : Differential activities of Nav1.7-targeting monoclonal antibodies

The voltage-gated Na+ channel subtype Nav1.7 is important for pain and itch in rodents and humans. We previously showed that a Nav1.7-targeting monoclonal antibody (SVmab) reduces Na+ currents and pain and itch responses in mice. Here, we investigated whether recombinant SVmab (rSVmab) binds to and blocks Nav1.7 similar to SVmab. ELISA tests revealed that SVmab was capable of binding to Nav1.7-expressing HEK293 cells, mouse DRG neurons, human nerve tissue, and the voltage-sensor domain II of Nav1.7. In contrast, rSVmab showed no or weak binding to Nav1.7 in these tests. Patch-clamp recordings showed that SVmab, but not rSVmab, markedly inhibited Na+ currents in Nav1.7-expressing HEK293 cells. Notably, electrical field stimulation increased the blocking activity of SVmab and rSVmab in Nav1.7-expressing HEK293 cells. SVmab was more effective than rSVmab in inhibiting paclitaxel-induced mechanical allodynia. SVmab also bound to human DRG neurons and inhibited their Na+ currents. Finally, potential reasons for the differential efficacy of SVmab and rSVmab and future directions are discussed.

Voltage-gated sodium channel β subunits: The power outside the pore in brain development and disease

Voltage gated sodium channels (VGSCs) were first identified in terms of their role in the upstroke of the action potential. The underlying proteins were later identified as saxitoxin and scorpion toxin receptors consisting of α and β subunits. We now know that VGSCs are heterotrimeric complexes consisting of a single pore forming α subunit joined by two β subunits; a noncovalently linked β1 or β3 and a covalently linked β2 or β4 subunit. VGSC α subunits contain all the machinery necessary for channel cell surface expression, ion conduction, voltage sensing, gating, and inactivation, in one central, polytopic, transmembrane protein. VGSC β subunits are more than simple accessories to α subunits. In the more than two decades since the original cloning of β1, our knowledge of their roles in physiology and pathophysiology has expanded immensely. VGSC β subunits are multifunctional. They confer unique gating mechanisms, regulate cellular excitability, affect brain development, confer distinct channel pharmacology, and have functions that are independent of the α subunits. The vast array of functions of these proteins stems from their special station in the channelome: being the only known constituents that are cell adhesion and intra/extracellular signaling molecules in addition to being part of channel complexes. This functional trifecta and how it goes awry demonstrates the power outside the pore in ion channel signaling complexes, broadening the term channelopathy beyond defects in ion conduction. This article is part of the Special Issue entitled 'Channelopathies.'

The pharmacology of voltage-gated sodium channel activators

Toxins and venom components that target voltage-gated sodium (Na_V) channels have evolved numerous times due to the importance of this class of ion channels in the normal physiological function of peripheral and central neurons as well as cardiac and skeletal muscle. Na_V channel activators in particular have been isolated from the venom of spiders, wasps, snakes, scorpions, cone snails and sea anemone and are also produced by plants, bacteria and algae. These compounds have provided key insight into the molecular structure, function and pathophysiological roles of Na_V channels and are important tools due to their at times exquisite subtype-selectivity. We review the pharmacology of Na_V channel activators with particular emphasis on mammalian isoforms and discuss putative applications for these compounds. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Sodium Channels and Venom Peptide Pharmacology

Venomous animals including cone snails, spiders, scorpions, anemones, and snakes have evolved a myriad of components in their venoms that target the opening and/or closing of voltage-gated sodium channels to cause devastating effects on the neuromuscular systems of predators and prey. These venom peptides, through design and serendipity, have not only contributed significantly to our understanding of sodium channel pharmacology and structure, but they also represent some of the most phyla- and isoform-selective molecules that are useful as valuable tool compounds and drug leads. Here, we review our understanding of the basic function of mammalian voltage-gated sodium channel isoforms as well as the pharmacology of venom peptides that act at these key transmembrane proteins.

Mining Protein Evolution for Insights into Mechanisms of Voltage-Dependent Sodium Channel Auxiliary Subunits

Voltage-gated sodium channel (VGSC) beta (β) subunits have been called the "overachieving" auxiliary ion channel subunit. Indeed, these subunits regulate the trafficking of the sodium channel complex at the plasma membrane and simultaneously tune the voltage-dependent properties of the pore-forming alpha-subunit. It is now known that VGSC β-subunits are capable of similar modulation of multiple isoforms of related voltage-gated potassium channels, suggesting that their abilities extend into the broader voltage-gated channels. The gene family for these single transmembrane immunoglobulin beta-fold proteins extends well beyond the traditional VGSC β1-β4 subunit designation, with deep roots into the cell adhesion protein family and myelin-related proteins - where inherited mutations result in a myriad of electrical signaling disorders. Yet, very little is known about how VGSC β-subunits support protein trafficking pathways, the basis for their modulation of voltage-dependent gating, and, ultimately, their role in shaping neuronal excitability. An evolutionary approach can be useful in yielding new clues to such functions as it provides an unbiased assessment of protein residues, folds, and functions. An approach is described here which indicates the greater emergence of the modern β-subunits roughly 400 million years ago in the early neurons of Bilateria and bony fish, and the unexpected presence of distant homologues in bacteriophages. Recent structural breakthroughs containing α and β eukaryotic sodium channels containing subunits suggest a novel role for a highly conserved polar contact that occurs within the transmembrane segments. Overall, a mixture of approaches will ultimately advance our understanding of the mechanism for β-subunit interactions with voltage-sensor containing ion channels and membrane proteins.

SCN4B acts as a metastasis-suppressor gene preventing hyperactivation of cell migration in breast cancer

The development of metastases largely relies on the capacity of cancer cells to invade extracellular matrices (ECM) using two invasion modes termed ‘mesenchymal' and ‘amoeboid', with possible transitions between these modes. Here we show that the SCN4B gene, encoding for the β4 protein, initially characterized as an auxiliary subunit of voltage-gated sodium channels (Na_V) in excitable tissues, is expressed in normal epithelial cells and that reduced β4 protein levels in breast cancer biopsies correlate with high-grade primary and metastatic tumours. In cancer cells, reducing β4 expression increases RhoA activity, potentiates cell migration and invasiveness, primary tumour growth and metastatic spreading, by promoting the acquisition of an amoeboid–mesenchymal hybrid phenotype. This hyperactivated migration is independent of Na_V and is prevented by overexpression of the intracellular C-terminus of β4. Conversely, SCN4B overexpression reduces cancer cell invasiveness and tumour progression, indicating that SCN4B/β4 represents a metastasis-suppressor gene.

The capacity of cancer cells to migrate is intimately linked to their ability to induce metastasis. Here the authors show that the sodium channel β4 subunit regulates breast cancer cell migration via inhibition of RhoA activation, independently from its function as an auxiliary protein of the sodium channel.

The hitchhiker's guide to the voltage-gated sodium channel galaxy

Eukaryotic voltage-gated sodium (Na_v) channels contribute to the rising phase of action potentials and served as an early muse for biophysicists laying the foundation for our current understanding of electrical signaling. Given their central role in electrical excitability, it is not surprising that (a) inherited mutations in genes encoding for Na_v channels and their accessory subunits have been linked to excitability disorders in brain, muscle, and heart; and (b) Na_v channels are targeted by various drugs and naturally occurring toxins. Although the overall architecture and behavior of these channels are likely to be similar to the more well-studied voltage-gated potassium channels, eukaryotic Na_v channels lack structural and functional symmetry, a notable difference that has implications for gating and selectivity. Activation of voltage-sensing modules of the first three domains in Na_v channels is sufficient to open the channel pore, whereas movement of the domain IV voltage sensor is correlated with inactivation. Also, structure–function studies of eukaryotic Na_v channels show that a set of amino acids in the selectivity filter, referred to as DEKA locus, is essential for Na⁺ selectivity. Structures of prokaryotic Na_v channels have also shed new light on mechanisms of drug block. These structures exhibit lateral fenestrations that are large enough to allow drugs or lipophilic molecules to gain access into the inner vestibule, suggesting that this might be the passage for drug entry into a closed channel. In this Review, we will synthesize our current understanding of Na_v channel gating mechanisms, ion selectivity and permeation, and modulation by therapeutics and toxins in light of the new structures of the prokaryotic Na_v channels that, for the time being, serve as structural models of their eukaryotic counterparts.

Human neuronal changes in brain edema and increased intracranial pressure

Functional and molecular changes associated with pathophysiological conditions are relatively easily detected based on tissue samples collected from patients. Population specific cellular responses to disease might remain undiscovered in samples taken from organs formed by a multitude of cell types. This is particularly apparent in the human cerebral cortex composed of a yet undefined number of neuron types with a potentially different involvement in disease processes. We combined cellular electrophysiology, anatomy and single cell digital PCR in human neurons identified in situ for the first time to assess mRNA expression and corresponding functional changes in response to edema and increased intracranial pressure. In single pyramidal cells, mRNA copy numbers of AQP1, AQP3, HMOX1, KCNN4, SCN3B and SOD2 increased, while CACNA1B, CRH decreased in edema. In addition, single pyramidal cells increased the copy number of AQP1, HTR5A and KCNS1 mRNAs in response to increased intracranial pressure. In contrast to pyramidal cells, AQP1, HMOX1and KCNN4 remained unchanged in single cell digital PCR performed on fast spiking cells in edema. Corroborating single cell digital PCR results, pharmacological and immunohistochemical results also suggested the presence of KCNN4 encoding the α-subunit of KCa3.1 channels in edema on pyramidal cells, but not on interneurons. We measured the frequency of spontaneous EPSPs on pyramidal cells in both pathophysiological conditions and on fast spiking interneurons in edema and found a significant decrease in each case, which was accompanied by an increase in input resistances on both cell types and by a drop in dendritic spine density on pyramidal cells consistent with a loss of excitatory synapses. Our results identify anatomical and/or physiological changes in human pyramidal and fast spiking cells in edema and increased intracranial pressure revealing cell type specific quantitative changes in gene expression. Some of the edema/increased intracranial pressure modulated and single human pyramidal cell verified gene products identified here might be considered as novel pharmacological targets in cell type specific neuroprotection.

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

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

Physiology and Pathophysiology of Sodium Channel Inactivation

Voltage-gated sodium channels are present in different tissues within the human body, predominantly nerve, muscle, and heart. The sodium channel is composed of four similar domains, each containing six transmembrane segments. Each domain can be functionally organized into a voltage-sensing region and a pore region. The sodium channel may exist in resting, activated, fast inactivated, or slow inactivated states. Upon depolarization, when the channel opens, the fast inactivation gate is in its open state. Within the time frame of milliseconds, this gate closes and blocks the channel pore from conducting any more sodium ions. Repetitive or continuous stimulations of sodium channels result in a rate-dependent decrease of sodium current. This process may continue until the channel fully shuts down. This collapse is known as slow inactivation. This chapter reviews what is known to date regarding, sodium channel inactivation with a focus on various mutations within each NaV subtype and with clinical implications.

Voltage-Gated Na+ Channels: Not Just for Conduction

Voltage-gated sodium channels (VGSCs), composed of a pore-forming α subunit and up to two associated β subunits, are critical for the initiation of the action potential (AP) in excitable tissues. Building on the monumental discovery and description of sodium current in 1952, intrepid researchers described the voltage-dependent gating mechanism, selectivity of the channel, and general structure of the VGSC channel. Recently, crystal structures of bacterial VGSC α subunits have confirmed many of these studies and provided new insights into VGSC function. VGSC β subunits, first cloned in 1992, modulate sodium current but also have nonconducting roles as cell-adhesion molecules and function in neurite outgrowth and neuronal pathfinding. Mutations in VGSC α and β genes are associated with diseases caused by dysfunction of excitable tissues such as epilepsy. Because of the multigenic and drug-resistant nature of some of these diseases, induced pluripotent stem cells and other novel approaches are being used to screen for new drugs and further understand how mutations in VGSC genes contribute to pathophysiology.

Binary architecture of the Nav1.2-β2 signaling complex

To investigate the mechanisms by which β-subunits influence Nav channel function, we solved the crystal structure of the β2 extracellular domain at 1.35Å. We combined these data with known bacterial Nav channel structural insights and novel functional studies to determine the interactions of specific residues in β2 with Nav1.2. We identified a flexible loop formed by (72)Cys and (75)Cys, a unique feature among the four β-subunit isoforms. Moreover, we found that (55)Cys helps to determine the influence of β2 on Nav1.2 toxin susceptibility. Further mutagenesis combined with the use of spider toxins reveals that (55)Cys forms a disulfide bond with (910)Cys in the Nav1.2 domain II pore loop, thereby suggesting a 1:1 stoichiometry. Our results also provide clues as to which disulfide bonds are formed between adjacent Nav1.2 (912/918)Cys residues. The concepts emerging from this work will help to form a model reflecting the β-subunit location in a Nav channel complex.

A new look at sodium channel β subunits

Voltage-gated sodium (Nav) channels are intrinsic plasma membrane proteins that initiate the action potential in electrically excitable cells. They are a major focus of research in neurobiology, structural biology, membrane biology and pharmacology. Mutations in Nav channels are implicated in a wide variety of inherited pathologies, including cardiac conduction diseases, myotonic conditions, epilepsy and chronic pain syndromes. Drugs active against Nav channels are used as local anaesthetics, anti-arrhythmics, analgesics and anti-convulsants. The Nav channels are composed of a pore-forming α subunit and associated β subunits. The β subunits are members of the immunoglobulin (Ig) domain family of cell-adhesion molecules. They modulate multiple aspects of Nav channel behaviour and play critical roles in controlling neuronal excitability. The recently published atomic resolution structures of the human β3 and β4 subunit Ig domains open a new chapter in the study of these molecules. In particular, the discovery that β3 subunits form trimers suggests that Nav channel oligomerization may contribute to the functional properties of some β subunits.

Voltage-gated sodium channels and cancer: is excitability their primary role?

Voltage-gated sodium channels (Na_V) are molecular characteristics of excitable cells. Their activation, triggered by membrane depolarization, generates transient sodium currents that initiate action potentials in neurons and muscle cells. Sodium currents were discovered by Hodgkin and Huxley using the voltage clamp technique and reported in their landmark series of papers in 1952. It was only in the 1980's that sodium channel proteins from excitable membranes were molecularly characterized by Catterall and his collaborators. Non-excitable cells can also express Na_V channels in physiological conditions as well as in pathological conditions. These Na_V channels can sustain biological roles that are not related to the generation of action potentials. Interestingly, it is likely that the abnormal expression of Na_V in pathological tissues can reflect the re-expression of a fetal phenotype. This is especially true in epithelial cancer cells for which these channels have been identified and sodium currents recorded, while it was not the case for cells from the cognate normal tissues. In cancers, the functional activity of Na_V appeared to be involved in regulating the proliferative, migrative, and invasive properties of cells. This review is aimed at addressing the non-excitable roles of Na_V channels with a specific emphasis in the regulation of cancer cell biology.

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.

Voltage-gated sodium channels

Voltage-gated sodium channels play a key role in the transmission of sensory information about the status of organs in the periphery. Sensory fibers contain a heterogeneous yet specific distribution of voltage-gated sodium channel isoforms. Major efforts by industry and academic groups are underway to develop medicines that interrupt inappropriate signaling for a number of clinical indications by taking advantage of this specific distribution of channel isoforms. This review highlights recent advances in the study of human channelopathies, animal toxins and channel structure that may facilitate the development of selective voltage-gated sodium channel blockers.

Voltage-Gated Sodium Channels: Structure, Function, Pharmacology, and Clinical Indications

The tremendous therapeutic potential of voltage-gated sodium channels (Na(v)s) has been the subject of many studies in the past and is of intense interest today. Na(v)1.7 channels in particular have received much attention recently because of strong genetic validation of their involvement in nociception. Here we summarize the current status of research in the Na(v) field and present the most relevant recent developments with respect to the molecular structure, general physiology, and pharmacology of distinct Na(v) channel subtypes. We discuss Na(v) channel ligands such as small molecules, toxins isolated from animal venoms, and the recently identified Na(v)1.7-selective antibody. Furthermore, we review eight characterized ligand binding sites on the Na(v) channel α subunit. Finally, we examine possible therapeutic applications of Na(v) ligands and provide an update on current clinical studies.

Insights into the origins of fish hunting in venomous cone snails from studies of Conus tessulatus

Prey shifts in carnivorous predators are events that can initiate the accelerated generation of new biodiversity. However, it is seldom possible to reconstruct how the change in prey preference occurred. Here we describe an evolutionary "smoking gun" that illuminates the transition from worm hunting to fish hunting among marine cone snails, resulting in the adaptive radiation of fish-hunting lineages comprising ∼100 piscivorous Conus species. This smoking gun is δ-conotoxin TsVIA, a peptide from the venom of Conus tessulatus that delays inactivation of vertebrate voltage-gated sodium channels. C. tessulatus is a species in a worm-hunting clade, which is phylogenetically closely related to the fish-hunting cone snail specialists. The discovery of a δ-conotoxin that potently acts on vertebrate sodium channels in the venom of a worm-hunting cone snail suggests that a closely related ancestral toxin enabled the transition from worm hunting to fish hunting, as δ-conotoxins are highly conserved among fish hunters and critical to their mechanism of prey capture; this peptide, δ-conotoxin TsVIA, has striking sequence similarity to these δ-conotoxins from piscivorous cone snail venoms. Calcium-imaging studies on dissociated dorsal root ganglion (DRG) neurons revealed the peptide's putative molecular target (voltage-gated sodium channels) and mechanism of action (inhibition of channel inactivation). The results were confirmed by electrophysiology. This work demonstrates how elucidating the specific interactions between toxins and receptors from phylogenetically well-defined lineages can uncover molecular mechanisms that underlie significant evolutionary transitions.

Α- and β-subunit composition of voltage-gated sodium channels investigated with μ-conotoxins and the recently discovered μO§-conotoxin GVIIJ

We investigated the identities of the isoforms of the α (NaV1)- and β (NaVβ)-subunits of voltage-gated sodium channels, including those responsible for action potentials in rodent sciatic nerves. To examine α-subunits, we used seven μ-conotoxins, which target site 1 of the channel. With the use of exogenously expressed channels, we show that two of the μ-conotoxins, μ-BuIIIB and μ-SxIIIA, are 50-fold more potent in blocking NaV1.6 from mouse than that from rat. Furthermore, we observed that μ-BuIIIB and μ-SxIIIA are potent blockers of large, myelinated A-fiber compound action potentials (A-CAPs) [but not small, unmyelinated C-fiber CAPs (C-CAPs)] in the sciatic nerve of the mouse (unlike A-CAPs of the rat, previously shown to be insensitive to these toxins). To investigate β-subunits, we used two synthetic derivatives of the recently discovered μO§-conotoxin GVIIJ that define site 8 of the channel, as previously characterized with cloned rat NaV1- and NaVβ-subunits expressed in Xenopus laevis oocytes, where it was shown that μO§-GVIIJ is a potent inhibitor of several NaV1-isoforms and that coexpression of NaVβ2 or -β4 (but not NaVβ1 or -β3) totally protects against block by μO§-GVIIJ. We report here the effects of μO§-GVIIJ on 1) sodium currents of mouse NaV1.6 coexpressed with various combinations of NaVβ-subunits in oocytes; 2) A- and C-CAPs of mouse and rat sciatic nerves; and 3) sodium currents of small and large neurons dissociated from rat dorsal root ganglia. Our overall results lead us to conclude that action potentials in A-fibers of the rodent sciatic nerve are mediated primarily by NaV1.6 associated with NaVβ2 or NaVβ4.

Pharmacological fractionation of tetrodotoxin-sensitive sodium currents in rat dorsal root ganglion neurons by μ-conotoxins

BACKGROUND AND PURPOSE

Adult rat dorsal root ganglion (DRG) neurons normally express transcripts for five isoforms of the α-subunit of voltage-gated sodium channels: NaV 1.1, 1.6, 1.7, 1.8 and 1.9. Tetrodotoxin (TTX) readily blocks all but NaV 1.8 and 1.9, and pharmacological agents that discriminate among the TTX-sensitive NaV 1-isoforms are scarce. Recently, we used the activity profile of a panel of μ-conotoxins in blocking cloned rodent NaV 1-isoforms expressed in Xenopus laevis oocytes to conclude that action potentials of A- and C-fibres in rat sciatic nerve were, respectively, mediated primarily by NaV 1.6 and NaV 1.7.

EXPERIMENTAL APPROACH

We used three μ-conotoxins, μ-TIIIA, μ-PIIIA and μ-SmIIIA, applied individually and in combinations, to pharmacologically differentiate the TTX-sensitive INa of voltage-clamped neurons acutely dissociated from adult rat DRG. We examined only small and large neurons whose respective INa were >50% and >80% TTX-sensitive.

KEY RESULTS

In both small and large neurons, the ability of the toxins to block TTX-sensitive INa was μ-TIIIA < μ-PIIIA < μ-SmIIIA, with the latter blocking ≳90%. Comparison of the toxin-susceptibility profiles of the neuronal INa with recently acquired profiles of rat NaV 1-isoforms, co-expressed with various NaV β-subunits in X. laevis oocytes, were consistent: NaV 1.1, 1.6 and 1.7 could account for all of the TTX-sensitive INa , with NaV 1.1 < NaV 1.6 < NaV 1.7 for small neurons and NaV 1.7 < NaV 1.1 < NaV 1.6 for large neurons.

CONCLUSIONS AND IMPLICATIONS

Combinations of μ-conotoxins can be used to determine the probable NaV 1-isoforms underlying the INa in DRG neurons. Preliminary experiments with sympathetic neurons suggest that this approach is extendable to other neurons.

Interactions of disulfide-deficient selenocysteine analogs of μ-conotoxin BuIIIB with the α-subunit of the voltage-gated sodium channel subtype 1.3

Inhibitors of the α-subunit of the voltage-gated sodium channel subtype 1.3 (NaV 1.3) are of interest as pharmacological tools for the study of neuropathic pain associated with spinal cord injury and have potential therapeutic applications. The recently described μ-conotoxin BuIIIB (μ-BuIIIB) from Conus bullatus was shown to block NaV 1.3 with submicromolar potency (Kd = 0.2 μm), making it one of the most potent peptidic inhibitors of this subtype described to date. However, oxidative folding of μ-BuIIIB results in numerous folding isoforms, making it difficult to obtain sufficient quantities of the active form of the peptide for detailed structure-activity studies. In the present study, we report the synthesis and characterization of μ-BuIIIB analogs incorporating a disulfide-deficient, diselenide-containing scaffold designed to simplify synthesis and facilitate structure-activity studies directed at identifying amino acid residues involved in NaV 1.3 blockade. Our results indicate that, similar to other μ-conotoxins, the C-terminal residues (Trp16, Arg18 and His20) are most crucial for NaV 1 blockade. At the N-terminus, replacement of Glu3 by Ala resulted in an analog with an increased potency for NaV 1.3 (Kd = 0.07 μm), implicating this position as a potential site for modification for increased potency and/or selectivity. Further examination of this position showed that increased negative charge, through γ-carboxyglutamate replacement, decreased potency (Kd = 0.33 μm), whereas replacement with positively-charged 2,4-diamonobutyric acid increased potency (Kd = 0.036 μm). These results provide a foundation for the design and synthesis of μ-BuIIIB-based analogs with increased potency against NaV 1.3.

A disulfide tether stabilizes the block of sodium channels by the conotoxin μO§-GVIIJ

A cone snail venom peptide, μO§-conotoxin GVIIJ from Conus geographus, has a unique posttranslational modification, S-cysteinylated cysteine, which makes possible formation of a covalent tether of peptide to its target Na channels at a distinct ligand-binding site. μO§-conotoxin GVIIJ is a 35-aa peptide, with 7 cysteine residues; six of the cysteines form 3 disulfide cross-links, and one (Cys24) is S-cysteinylated. Due to limited availability of native GVIIJ, we primarily used a synthetic analog whose Cys24 was S-glutathionylated (abbreviated GVIIJSSG). The peptide-channel complex is stabilized by a disulfide tether between Cys24 of the peptide and Cys910 of rat (r) NaV1.2. A mutant channel of rNaV1.2 lacking a cysteine near the pore loop of domain II (C910L), was >10(3)-fold less sensitive to GVIIJSSG than was wild-type rNaV1.2. In contrast, although rNaV1.5 was >10(4)-fold less sensitive to GVIIJSSG than NaV1.2, an rNaV1.5 mutant with a cysteine in the homologous location, rNaV1.5[L869C], was >10(3)-fold more sensitive than wild-type rNaV1.5. The susceptibility of rNaV1.2 to GVIIJSSG was significantly altered by treating the channels with thiol-oxidizing or disulfide-reducing agents. Furthermore, coexpression of rNaVβ2 or rNaVβ4, but not that of rNaVβ1 or rNaVβ3, protected rNaV1.1 to -1.7 (excluding NaV1.5) against block by GVIIJSSG. Thus, GVIIJ-related peptides may serve as probes for both the redox state of extracellular cysteines and for assessing which NaVβ- and NaVα-subunits are present in native neurons.

Animal toxins influence voltage-gated sodium channel function

Voltage-gated sodium (Nav) channels are essential contributors to neuronal excitability, making them the most commonly targeted ion channel family by toxins found in animal venoms. These molecules can be used to probe the functional aspects of Nav channels on a molecular level and to explore their physiological role in normal and diseased tissues. This chapter summarizes our existing knowledge of the mechanisms by which animal toxins influence Nav channels as well as their potential application in designing therapeutic drugs.

Crystallographic insights into sodium-channel modulation by the β4 subunit

Voltage-gated sodium (Nav) channels are embedded in a multicomponent membrane signaling complex that plays a crucial role in cellular excitability. Although the mechanism remains unclear, β-subunits modify Nav channel function and cause debilitating disorders when mutated. While investigating whether β-subunits also influence ligand interactions, we found that β4 dramatically alters toxin binding to Nav1.2. To explore these observations further, we solved the crystal structure of the extracellular β4 domain and identified (58)Cys as an exposed residue that, when mutated, eliminates the influence of β4 on toxin pharmacology. Moreover, our results suggest the presence of a docking site that is maintained by a cysteine bridge buried within the hydrophobic core of β4. Disrupting this bridge by introducing a β1 mutation implicated in epilepsy repositions the (58)Cys-containing loop and disrupts β4 modulation of Nav1.2. Overall, the principles emerging from this work (i) help explain tissue-dependent variations in Nav channel pharmacology; (ii) enable the mechanistic interpretation of β-subunit-related disorders; and (iii) provide insights in designing molecules capable of correcting aberrant β-subunit behavior.