Biophysical properties of Na(v) 1.8/Na(v) 1.2 chimeras and inhibition by µO-conotoxin MrVIB

O1-conotoxin Tx6.7 cloned from the genomic DNA of Conus textile that inhibits calcium currents

Background

Conotoxins exhibit great potential as neuropharmacology tools and therapeutic candidates due to their high affinity and specificity for ion channels, neurotransmitter receptors or transporters. The traditional methods to discover new conotoxins are peptide purification from the crude venom or gene amplification from the venom duct.

Methods

In this study, a novel O1 superfamily conotoxin Tx6.7 was directly cloned from the genomic DNA of Conus textile using primers corresponding to the conserved intronic sequence and 3' UTR elements. The mature peptide of Tx6.7 (DCHERWDWCPASLLGVIYCCEGLICFIAFCI) was synthesized by solid-phase chemical synthesis and confirmed by mass spectrometry.

Results

Patch clamp experiments on rat DRG neurons showed that Tx6.7 inhibited peak calcium currents by 59.29 ± 2.34% and peak potassium currents by 22.33 ± 7.81%. In addition, patch clamp on the ion channel subtypes showed that 10 μM Tx6.7 inhibited 56.61 ± 3.20% of the hCa_V1.2 currents, 24.67 ± 0.91% of the hCa_V2.2 currents and 7.30 ± 3.38% of the hNa_V1.8 currents. Tx6.7 had no significant toxicity to ND7/23 cells and increased the pain threshold from 0.5 to 4 hours in the mouse hot plate assay.

Conclusion

Our results suggested that direct cloning of conotoxin sequences from the genomic DNA of cone snails would be an alternative approach to obtaining novel conotoxins. Tx6.7 could be used as a probe tool for ion channel research or a therapeutic candidate for novel drug development.

A nutraceutical product, extracted from Cannabis sativa, modulates voltage-gated sodium channel function

Background

Purified cannabidiol (CBD), a non-psychoactive phytocannabinoid, has gained regulatory approval to treat intractable childhood epilepsies. Despite this, artisanal and commercial CBD-dominant hemp-based products continue to be used by epilepsy patients. Notably, the CBD doses used in these latter products are much lower than that found to be effective in reducing seizures in clinical trials with purified CBD. This might be because these CBD-dominant hemp products contain other bioactive compounds, including phytocannabinoids and terpenes, which may exert unique effects on epilepsy-relevant drug targets. Voltage-gated sodium (Na_V) channels are vital for initiation of neuronal action potential propagation and genetic mutations in these channels result in epilepsy phenotypes. Recent studies suggest that Na_V channels are inhibited by purified CBD. However, the effect of cannabis-based products on the function of Na_V channels is unknown.

Methods

Using automated-planar patch-clamp technology, we profile a hemp-derived nutraceutical product (NP) against human Na_V1.1–Na_V1.8 expressed in mammalian cells to examine effects on the biophysical properties of channel conductance, steady-state fast inactivation and recovery from fast inactivation.

Results

NP modifies peak current amplitude of the Na_V1.1–Na_V1.7 subtypes and has variable effects on the biophysical properties for all channel subtypes tested. NP potently inhibits Na_V channels revealing half-maximal inhibitory concentration (IC₅₀) values of between 1.6 and 4.2 μg NP/mL. Purified CBD inhibits Na_V1.1, Na_V1.2, Na_V1.6 and Na_V1.7 to reveal IC₅₀ values in the micromolar range. The CBD content of the product equates to IC₅₀ values (93–245 nM), which are at least an order of magnitude lower than purified CBD. Unlike NP, hemp seed oil vehicle alone did not inhibit Na_V channels, suggesting that the inhibitory effects of NP are independent of hemp seed oil.

Conclusions

This CBD-dominant NP potently inhibits Na_V channels. Future study of the individual elements of NP, including phytocannabinoids and terpenes, may reveal a potent individual component or that its components interact to modulate Na_V channels.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42238-022-00136-x.

Voltage-Gated Sodium Channels: A Prominent Target of Marine Toxins

Voltage-gated sodium channels (VGSCs) are considered to be one of the most important ion channels given their remarkable physiological role. VGSCs constitute a family of large transmembrane proteins that allow transmission, generation, and propagation of action potentials. This occurs by conducting Na⁺ ions through the membrane, supporting cell excitability and communication signals in various systems. As a result, a wide range of coordination and physiological functions, from locomotion to cognition, can be accomplished. Drugs that target and alter the molecular mechanism of VGSCs’ function have highly contributed to the discovery and perception of the function and the structure of this channel. Among those drugs are various marine toxins produced by harmful microorganisms or venomous animals. These toxins have played a key role in understanding the mode of action of VGSCs and in mapping their various allosteric binding sites. Furthermore, marine toxins appear to be an emerging source of therapeutic tools that can relieve pain or treat VGSC-related human channelopathies. Several studies documented the effect of marine toxins on VGSCs as well as their pharmaceutical applications, but none of them underlined the principal marine toxins and their effect on VGSCs. Therefore, this review aims to highlight the neurotoxins produced by marine animals such as pufferfish, shellfish, sea anemone, and cone snail that are active on VGSCs and discuss their pharmaceutical values.

Functional modulation of the human voltage-gated sodium channel NaV1.8 by auxiliary β subunits

The voltage-gated sodium channel Na_v1.8 mediates the tetrodotoxin-resistant (TTX-R) Na⁺ current in nociceptive primary sensory neurons, which has an important role in the transmission of painful stimuli. Here, we describe the functional modulation of the human Na_v1.8 α-subunit in Xenopus oocytes by auxiliary β subunits. We found that the β3 subunit down-regulated the maximal Na⁺ current amplitude and decelerated recovery from inactivation of hNa_v1.8, whereas the β1 and β2 subunits had no such effects. The specific regulation of Na_v1.8 by the β3 subunit constitutes a potential novel regulatory mechanism of the TTX-R Na⁺ current in primary sensory neurons with potential implications in chronic pain states. In particular, neuropathic pain states are characterized by a down-regulation of Na_v1.8 accompanied by increased expression of the β3 subunit. Our results suggest that these two phenomena may be correlated, and that increased levels of the β3 subunit may directly contribute to the down-regulation of Na_v1.8. To determine which domain of the β3 subunit is responsible for the specific regulation of hNa_v1.8, we created chimeras of the β1 and β3 subunits and co-expressed them with the hNa_v1.8 α-subunit in Xenopus oocytes. The intracellular domain of the β3 subunit was shown to be responsible for the down-regulation of maximal Na_v1.8 current amplitudes. In contrast, the extracellular domain mediated the effect of the β3 subunit on hNa_v1.8 recovery kinetics.

Structural and dynamical effects of targeted mutations on μO-Conotoxin MfVIA: Molecular simulation studies

Conotoxins are a group of cysteine-rich, neurotoxic peptides isolated from the venom of marine cone snails. MfVIA is a member of the μO-conotoxin family, and acts as an inhibitor of subtype 1.8 voltage-gated sodium ion channels (Na_V1.8). The unique selectivity of MfVIA as an inhibitor of Na_V1.8 makes it an ideal peptide for elucidation of the physiological functions of this voltage-gated ion channel. Previous experimental studies of point mutations of MfVIA showed that the double mutant [E5K,E8K] exhibited greater activity at Na_V1.8 relative to the wild-type toxin. The present study employs molecular dynamics (MD) simulations to examine the effects of various mutations at these key residues (E5 and E8) on the structure and dynamics of MfVIA. Five double mutants were studied, in which the positions 5 and 8 residues were mutated to amino acids with a range of different physicochemical properties, namely [E5A,E8A], [E5D,E8D], [E5F,E8F], [E5K,E8K], and [E5R,E8R]. Except for [E5D,E8D], all of the mutants tend to show decreased contacts at the N-terminus owing to the loss of the R1-E5 salt bridge relative to that of the wild-type, which subsequently lead to greater exposure and flexibility of the N-terminus for most of the mutant peptides studied, potentially rendering them more able to interact with other species, including Na_V1.8. Molecular docking studies of the peptides to Na_V1.8 via different binding mechanisms suggest that the [E5R, E8R] mutant may be especially worthy of further investigation owing to its predicted binding mode, which differs markedly from those of the other peptides in this study.

The Role of Toxins in the Pursuit for Novel Analgesics

Chronic pain is a major medical issue which reduces the quality of life of millions and inflicts a significant burden on health authorities worldwide. Currently, management of chronic pain includes first-line pharmacological therapies that are inadequately effective, as in just a portion of patients pain relief is obtained. Furthermore, most analgesics in use produce severe or intolerable adverse effects that impose dose restrictions and reduce compliance. As the majority of analgesic agents act on the central nervous system (CNS), it is possible that blocking pain at its source by targeting nociceptors would prove more efficient with minimal CNS-related side effects. The development of such analgesics requires the identification of appropriate molecular targets and thorough understanding of their structural and functional features. To this end, plant and animal toxins can be employed as they affect ion channels with high potency and selectivity. Moreover, elucidation of the toxin-bound ion channel structure could generate pharmacophores for rational drug design while favorable safety and analgesic profiles could highlight toxins as leads or even as valuable therapeutic compounds themselves. Here, we discuss the use of plant and animal toxins in the characterization of peripherally expressed ion channels which are implicated in pain.

Development of a μO-Conotoxin Analogue with Improved Lipid Membrane Interactions and Potency for the Analgesic Sodium Channel NaV1.8

The μO-conotoxins MrVIA, MrVIB, and MfVIA inhibit the voltage-gated sodium channel NaV1.8, a well described target for the treatment of pain; however, little is known about the residues or structural elements that define this activity. In this study, we determined the three-dimensional structure of MfVIA, examined its membrane binding properties, performed alanine-scanning mutagenesis, and identified residues important for its activity at human NaV1.8. A second round of mutations resulted in (E5K,E8K)MfVIA, a double mutant with greater positive surface charge and greater affinity for lipid membranes compared with MfVIA. This analogue had increased potency at NaV1.8 and was analgesic in the mouse formalin assay.

Scorpion toxin BmK I directly activates Nav1.8 in primary sensory neurons to induce neuronal hyperexcitability in rats

Voltage-gated sodium channels (VGSCs) in primary sensory neurons play a key role in transmitting pain signals to the central nervous system. BmK I, a site-3 sodium channel-specific toxin from scorpion Buthus martensi Karsch, induces pain behaviors in rats. However, the subtypes of VGSCs targeted by BmK I were not entirely clear. We therefore investigated the effects of BmK I on the current amplitude, gating and kinetic properties of Na_v1.8, which is associated with neuronal hyperexcitability in DRG neurons. It was found that BmK I dose-dependently increased Na_v1.8 current in small-sized (<25 μm) acutely dissociated DRG neurons, which correlated with its inhibition on both fast and slow inactivation. Moreover, voltage-dependent activation and steady-state inactivation curves of Na_v1.8 were shifted in a hyperpolarized direction. Thus, BmK I reduced the threshold of neuronal excitability and increased action potential firing in DRG neurons. In conclusion, our data clearly demonstrated that BmK I modulated Na_v1.8 remarkably, suggesting BmK I as a valuable probe for studying Na_v1.8. And Nav1.8 is an important target related to BmK I-evoked pain.

Chemical engineering and structural and pharmacological characterization of the α-scorpion toxin OD1

Scorpion α-toxins are invaluable pharmacological tools for studying voltage-gated sodium channels, but few structure-function studies have been undertaken due to their challenging synthesis. To address this deficiency, we report a chemical engineering strategy based upon native chemical ligation. The chemical synthesis of α-toxin OD1 was achieved by chemical ligation of three unprotected peptide segments. A high resolution X-ray structure (1.8 Å) of synthetic OD1 showed the typical βαββ α-toxin fold and revealed important conformational differences in the pharmacophore region when compared with other α-toxin structures. Pharmacological analysis of synthetic OD1 revealed potent α-toxin activity (inhibition of fast inactivation) at Nav1.7, as well as Nav1.4 and Nav1.6. In addition, OD1 also produced potent β-toxin activity at Nav1.4 and Nav1.6 (shift of channel activation in the hyperpolarizing direction), indicating that OD1 might interact at more than one site with Nav1.4 and Nav1.6. Investigation of nine OD1 mutants revealed that three residues in the reverse turn contributed significantly to selectivity, with the triple OD1 mutant (D9K, D10P, K11H) being 40-fold more selective for Nav1.7 over Nav1.6, while OD1 K11V was 5-fold more selective for Nav1.6 than Nav1.7. This switch in selectivity highlights the importance of the reverse turn for engineering α-toxins with altered selectivity at Nav subtypes.

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.

Animal toxins can alter the function of Nav1.8 and Nav1.9

Human voltage-activated sodium (Nav) channels are adept at rapidly transmitting electrical signals across long distances in various excitable tissues. As such, they are amongst the most widely targeted ion channels by drugs and animal toxins. Of the nine isoforms, Nav1.8 and Nav1.9 are preferentially expressed in DRG neurons where they are thought to play an important role in pain signaling. Although the functional properties of Nav1.8 have been relatively well characterized, difficulties with expressing Nav1.9 in established heterologous systems limit our understanding of the gating properties and toxin pharmacology of this particular isoform. This review summarizes our current knowledge of the role of Nav1.8 and Nav1.9 in pain perception and elaborates on the approaches used to identify molecules capable of influencing their function.

Isolation, characterization and total regioselective synthesis of the novel μO-conotoxin MfVIA from Conus magnificus that targets voltage-gated sodium channels

The μO-conotoxins are notable for their unique selectivity for Na(v)1.8 over other sodium channel isoforms, making them attractive drug leads for the treatment of neuropathic pain. We describe the discovery of a novel μO-conotoxin, MfVIA, from the venom of Conus magnificus using high-throughput screening approaches. MfVIA was found to be a hydrophobic 32-residue peptide (amino acid sequence RDCQEKWEYCIVPILGFVYCCPGLICGPFVCV) with highest sequence homology to μO-conotoxin MrVIB. To overcome the synthetic challenges posed by μO-conotoxins due to their hydrophobic nature and difficult folding, we developed a novel regioselective approach for the synthesis of μO-conotoxins. Performing selective oxidative deprotections of the cysteine side-chain protecting groups of the fully protected peptide allowed manipulations in organic solvents with no chromatography required between steps. Using this approach, we obtained correctly folded MfVIA with increased synthetic yields. Biological activity of MfVIA was assessed using membrane potential-sensitive dyes and electrophysiological recording techniques. MfVIA preferentially inhibits Na(v)1.8 (IC₅₀ 95.9±74.3 nM) and Na(v)1.4 (IC₅₀ 81±16 nM), with significantly lower affinity for other Na(v) subtypes (IC₅₀ 431-6203 nM; Na(v)1.5>1.6∼1.7∼1.3∼1.1∼1.2). This improved approach to μO-conotoxin synthesis will facilitate the optimization of μO-conotoxins as novel analgesic molecules to improve pain management.