Heterologous expression and functional analysis of rat NaV1.8 (SNS) voltage-gated sodium channels in the dorsal root ganglion neuroblastoma cell line ND7–23

Nociceptor sodium channels shape subthreshold phase, upstroke, and shoulder of action potentials

Voltage-gated sodium channels (VGSCs) in the peripheral nervous system shape action potentials (APs) and thereby support the detection of sensory stimuli. Most of the nine mammalian VGSC subtypes are expressed in nociceptors, but predominantly, three are linked to several human pain syndromes: while Nav1.7 is suggested to be a (sub-)threshold channel, Nav1.8 is thought to support the fast AP upstroke. Nav1.9, as it produces large persistent currents, is attributed a role in determining the resting membrane potential. We characterized the gating of Nav1.1-Nav1.3 and Nav1.5-Nav1.9 in manual patch clamp with a focus on the AP subthreshold depolarization phase. Nav1.9 exhibited the most hyperpolarized activation, while its fast inactivation resembled the depolarized inactivation of Nav1.8. For some VGSCs (e.g., Nav1.1 and Nav1.2), a positive correlation between ramp current and window current was detected. Using a modified Hodgkin-Huxley model that accounts for the time needed for inactivation to occur, we used the acquired data to simulate two nociceptive nerve fiber types (an Aδ- and a mechano-insensitive C-nociceptor) containing VGSC conductances according to published human RNAseq data. Our simulations suggest that Nav1.9 is supporting both the AP upstroke and its shoulder. A reduced threshold for AP generation was induced by enhancing Nav1.7 conductivity or shifting its activation to more hyperpolarized potentials, as observed in Nav1.7-related pain disorders. Here, we provide a comprehensive, comparative functional characterization of VGSCs relevant in nociception and describe their gating with Hodgkin-Huxley-like models, which can serve as a tool to study their specific contributions to AP shape and sodium channel-related diseases.

Molecular determinants of resurgent sodium currents mediated by Navβ4 peptide and A-type FHFs

Introduction

Resurgent current (INaR ) generated by voltage-gated sodium channels (VGSCs) plays an essential role in maintaining high-frequency firing of many neurons and contributes to disease pathophysiology such as epilepsy and painful disorders. Targeting INaR may present a highly promising strategy in the treatment of these diseases. Navβ4 and A-type fibroblast growth factor homologous factors (FHFs) have been identified as two classes of important INaR mediators; however, their receptor sites in VGSCs remain unknown, which hinders the development of novel agents to effectively target INaR .

Methods

Navβ4 and FHF4A can mediate INaR generation through the amino acid segment located in their C-terminus and N-terminus, respectively. We mainly employed site-directed mutagenesis, chimera construction and whole-cell patch-clamp recording to explore the receptor sites of Navβ4 peptide and FHF4A in Nav1.7 and Nav1.8.

Results

We show that the receptor of Navβ4-peptide involves four residues, N395, N945, F1737 and Y1744, in Nav1.7 DI-S6, DII-S6, and DIV-S6. We show that A-type FHFs generating INaR depends on the segment located at the very beginning, not at the distal end, of the FHF4 N-terminus domain. We show that the receptor site of A-type FHFs also resides in VGSC inner pore region. We further show that an asparagine at DIIS6, N891 in Nav1.8, is a major determinant of INaR generated by A-type FHFs in VGSCs.

Discussion

Cryo-EM structures reveal that the side chains of the critical residues project into the VGSC channel pore. Our findings provide additional evidence that Navβ4 peptide and A-type FHFs function as open-channel pore blockers and highlight channel inner pore region as a hotspot for development of novel agents targeting INaR .

Automated Patch Clamp for the Detection of Tetrodotoxin in Pufferfish Samples

Tetrodotoxin (TTX) is a marine toxin responsible for many intoxications around the world. Its presence in some pufferfish species and, as recently reported, in shellfish, poses a serious health concern. Although TTX is not routinely monitored, there is a need for fast, sensitive, reliable, and simple methods for its detection and quantification. In this work, we describe the use of an automated patch clamp (APC) system with Neuro-2a cells for the determination of TTX contents in pufferfish samples. The cells showed an IC50 of 6.4 nM for TTX and were not affected by the presence of muscle, skin, liver, and gonad tissues of a Sphoeroides pachygaster specimen (TTX-free) when analysed at 10 mg/mL. The LOD achieved with this technique was 0.05 mg TTX equiv./kg, which is far below the Japanese regulatory limit of 2 mg TTX equiv./kg. The APC system was applied to the analysis of extracts of a Lagocephalus sceleratus specimen, showing TTX contents that followed the trend of gonads > liver > skin > muscle. The APC system, providing an in vitro toxicological approach, offers the advantages of being sensitive, rapid, and reliable for the detection of TTX-like compounds in seafood.

TRPM8 inhibits substance P release from primary sensory neurons via PKA/GSK-3beta to protect colonic epithelium in colitis

Transient receptor potential melastatin 8 (TRPM8) is a cold sensory receptor in primary sensory neurons that regulates various neuronal functions. Substance P (SP) is a pro-inflammatory neuropeptide secreted by the neurons, and it aggravates colitis. However, the regulatory role of TRPM8 in SP release is still unclear. Our study aimed to investigate TRPM8's role in SP release from primary sensory neurons during colitis and clarify the effect of SP on colonic epithelium. We analyzed inflammatory bowel disease patients' data from the Gene Expression Omnibus dataset. Dextran sulfate sodium (DSS, 2.5%)-induced colitis in mice, mouse dorsal root ganglion (DRG) neurons, ND7/23 cell line, and mouse or human colonic organoids were used for this experiment. Our study found that TRPM8, TAC1 and WNT3A expression were significantly correlated with the severity of ulcerative colitis in patients and DSS-induced colitis in mice. The TRPM8 agonist (menthol) and the SP receptor antagonist (Aprepitant) can attenuate colitis in mice, but the effects were not additive. Menthol promoted calcium ion influx in mouse DRG neurons and inhibited the combination and phosphorylation of PKAca from the cAMP signaling pathway and GSK-3β from the Wnt/β-catenin signaling pathway, thereby inhibiting the effect of Wnt3a-driven β-catenin on promoting SP release in ND7/23 cells. Long-term stimulation with SP inhibited proliferation and enhanced apoptosis in both mouse and human colonic organoids. Conclusively, TRPM8 inhibits SP release from primary sensory neurons by inhibiting the interaction between PKAca and GSK-3β, thereby inhibiting the role of SP in promoting colonic epithelial apoptosis and relieving colitis.

In vitro effects of eslicarbazepine (S-licarbazepine) as a potential precision therapy on SCN8A variants causing neuropsychiatric disorders

BACKGROUND AND PURPOSE

Variants in SCN8A, the Na_V 1.6 channel's coding gene, are characterized by a variety of symptoms, including intractable epileptic seizures, psychomotor delay, progressive cognitive decline, autistic features, ataxia or dystonia. Standard anticonvulsant treatment has a limited impact on the course of disease.

EXPERIMENTAL APPROACH

We investigated the therapeutic potential of eslicarbazepine (S-licarbazepine; S-lic), an enhancer of slow inactivation of voltage gated sodium channels, on two variants with biophysical and neuronal gain-of-function (G1475R and M1760I) and one variant with biophysical gain-of-function but neuronal loss-of-function (A1622D) in neuroblastoma cells and in murine primary hippocampal neuron cultures. These three variants cover the broad spectrum of Na_V 1.6-associated disease and are linked to representative phenotypes of mild to moderate epilepsy (G1475R), developmental and epileptic encephalopathy (M1760I) and intellectual disability without epilepsy (A1622D).

KEY RESULTS

Similar to known effects on Na_V 1.6 wildtype channels, S-lic predominantly enhances slow inactivation on all tested variants, irrespective of their particular biophysical mechanisms. Beyond that, S-lic exhibits variant-specific effects including a partial reversal of pathologically slowed fast inactivation dynamics (A1622D and M1760I) and a trend to reduce enhanced persistent Na⁺ current by A1622D variant channels. Furthermore, our data in primary transfected neurons reveal that not only variant-associated hyperexcitability (M1760I and G1475R) but also hypoexcitability (A1622D) can be modulated by S-lic.

CONCLUSIONS AND IMPLICATIONS

S-lic has not only substance-specific effects but also variant-specific effects. Personalized treatment regimens optimized to achieve such variant-specific pharmacological modulation may help to reduce adverse side effects and improve the overall therapeutic outcome of SCN8A-related disease.

Structural basis for high-voltage activation and subtype-specific inhibition of human Nav1.8

Pain management represents an unmet healthcare need in many countries. Na_v1.8 represents a potential target for developing nonaddictive analgesics. Here we present the cryogenic electron microscopy (cryo-EM) structures of human Na_v1.8 alone and bound to a selective pore blocker, A-803467. Unlike reported structures of eukaryotic Na_v channels wherein the first voltage-sensing domain (VSD_I) is well-resolved in one stable conformation, different conformations of VSD_I are observed in the cryo-EM maps of Na_v1.8. An extracellular interface between VSD_I and the pore domain was identified to be a determinant for Na_v1.8’s dependence on higher voltage for activation. A-803467 clenches S6_IV within the central cavity. Unexpectedly, the channel selectivity for A-803467 is determined by nonligand coordinating residues through an allosteric mechanism.

The dorsal root ganglia–localized voltage-gated sodium (Na_v) channel Na_v1.8 represents a promising target for developing next-generation analgesics. A prominent characteristic of Na_v1.8 is the requirement of more depolarized membrane potential for activation. Here we present the cryogenic electron microscopy structures of human Na_v1.8 alone and bound to a selective pore blocker, A-803467, at overall resolutions of 2.7 to 3.2 Å. The first voltage-sensing domain (VSD_I) displays three different conformations. Structure-guided mutagenesis identified the extracellular interface between VSD_I and the pore domain (PD) to be a determinant for the high-voltage dependence of activation. A-803467 was clearly resolved in the central cavity of the PD, clenching S6_IV. Our structure-guided functional characterizations show that two nonligand binding residues, Thr397 on S6_I and Gly1406 on S6_III, allosterically modulate the channel’s sensitivity to A-803467. Comparison of available structures of human Na_v channels suggests the extracellular loop region to be a potential site for developing subtype-specific pore-blocking biologics.

A-type FHFs mediate resurgent currents through TTX-resistant voltage-gated sodium channels

Resurgent currents (I_NaR) produced by voltage-gated sodium channels are required for many neurons to maintain high-frequency firing, and contribute to neuronal hyperexcitability and disease pathophysiology. Here we show, for the first time, that I_NaR can be reconstituted in a heterologous system by co-expression of sodium channel α-subunits and A-type fibroblast growth factor homologous factors (FHFs). Specifically, A-type FHFs induces I_NaR from Nav1.8, Nav1.9 tetrodotoxin-resistant neuronal channels and, to a lesser extent, neuronal Nav1.7 and cardiac Nav1.5 channels. Moreover, we identified the N-terminus of FHF as the critical molecule responsible for A-type FHFs-mediated I_NaR. Among the FHFs, FHF4A is the most important isoform for mediating Nav1.8 and Nav1.9 I_NaR. In nociceptive sensory neurons, FHF4A knockdown significantly reduces I_NaR amplitude and the percentage of neurons that generate I_NaR, substantially suppressing excitability. Thus, our work reveals a novel molecular mechanism underlying TTX-resistant I_NaR generation and provides important potential targets for pain treatment.

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

The voltage-gated sodium channel Nav1.8 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 Nav1.8 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 hNav1.8, whereas the β1 and β2 subunits had no such effects. The specific regulation of Nav1.8 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 Nav1.8 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 Nav1.8. To determine which domain of the β3 subunit is responsible for the specific regulation of hNav1.8, we created chimeras of the β1 and β3 subunits and co-expressed them with the hNav1.8 α-subunit in Xenopus oocytes. The intracellular domain of the β3 subunit was shown to be responsible for the down-regulation of maximal Nav1.8 current amplitudes. In contrast, the extracellular domain mediated the effect of the β3 subunit on hNav1.8 recovery kinetics.

Trigeminal neuralgia: An overview from pathophysiology to pharmacological treatments

The trigeminal nerve (V) is the fifth and largest of all cranial nerves, and it is responsible for detecting sensory stimuli that arise from the craniofacial area. The nerve is divided into three branches: ophthalmic (V1), maxillary (V2), and mandibular (V3); their cell bodies are located in the trigeminal ganglia and they make connections with second-order neurons in the trigeminal brainstem sensory nuclear complex. Ascending projections via the trigeminothalamic tract transmit information to the thalamus and other brain regions responsible for interpreting sensory information. One of the most common forms of craniofacial pain is trigeminal neuralgia. Trigeminal neuralgia is characterized by sudden, brief, and excruciating facial pain attacks in one or more of the V branches, leading to a severe reduction in the quality of life of affected patients. Trigeminal neuralgia etiology can be classified into idiopathic, classic, and secondary. Classic trigeminal neuralgia is associated with neurovascular compression in the trigeminal root entry zone, which can lead to demyelination and a dysregulation of voltage-gated sodium channel expression in the membrane. These alterations may be responsible for pain attacks in trigeminal neuralgia patients. The antiepileptic drugs carbamazepine and oxcarbazepine are the first-line pharmacological treatment for trigeminal neuralgia. Their mechanism of action is a modulation of voltage-gated sodium channels, leading to a decrease in neuronal activity. Although carbamazepine and oxcarbazepine are the first-line treatment, other drugs may be useful for pain control in trigeminal neuralgia. Among them, the anticonvulsants gabapentin, pregabalin, lamotrigine and phenytoin, baclofen, and botulinum toxin type A can be coadministered with carbamazepine or oxcarbazepine for a synergistic approach. New pharmacological alternatives are being explored such as the active metabolite of oxcarbazepine, eslicarbazepine, and the new Nav1.7 blocker vixotrigine. The pharmacological profiles of these drugs are addressed in this review.

Modulations of Nav1.8 and Nav1.9 Channels in Monosodium Urate-Induced Gouty Arthritis in Mice

The aim of the present study was to observe the changes of TTX-R, Na_v1.8, and Na_v1.9 Na⁺ currents in MSU-induced gouty arthritis mice, and to explore the possibility of Na_v1.8 and Na_v1.9 channels as potential targets for gout pain treatment. Acute gouty arthritis was induced by monosodium urate (MSU) in mice. Swelling degree was evaluated by measuring the circumference of the ankle joint. Mechanical allodynia was assessed by applying the electronic von Frey. Na⁺ currents were recorded by patch-clamp techniques in acute isolated dorsal root ganglion (DRG) neurons. MSU treatment significantly increased the swelling degree of ankle joint and decreased the mechanical pain threshold. The amplitude of TTX-R Na⁺ current was significantly increased and reached its peak on the 4th day after injection of MSU. For TTX-R Na⁺ channel subunits, Na_v1.8 current density was significantly increased, but Na_v1.9 current density was markedly decreased after MSU treatment. MSU treatment shifted the steady-state activation curves of TTX-R Na⁺ channel, Na_v1.8 and Na_v1.9 channels, and the inactivation curves of TTX-R Na⁺ channel and Na_v1.8 channels to the depolarizing direction, but did not affect the inactivation curve of Na_v1.9 channel. Compared with the normal group, the recovery of Na_v1.8 channel was faster, while that of Na_v1.9 channel was slower. The recovery of TTX-R Na⁺ channel remained unchanged after MSU treatment. Additionally, MSU treatment increased DRG neurons excitability by reducing action potential threshold. Na_v1.8 channel, not Na_v1.9 channel, may be involved in MSU-induced gout pain by increasing nerve excitability.

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.

Immortalized Dorsal Root Ganglion Neuron Cell Lines

Pain is one of the most significant causes of suffering and disability world-wide, and arguably the most burdensome global health challenge. The growing number of patients suffering from chronic pain conditions such as fibromyalgia, complex regional pain syndrome, migraine and irritable bowel syndrome, not only reflect the complexity and heterogeneity of pain types, but also our lack of understanding of the underlying mechanisms. Sensory neurons within the dorsal root ganglia (DRG) have emerged as viable targets for effective chronic pain therapy. However, DRG's contain different classes of primary sensory neurons including pain-associated nociceptive neurons, non-nociceptive temperature sensing, mechanosensory and chemoreceptive neurons, as well as multiple types of immune and endothelial cells. This cell-population heterogeneity makes investigations of individual subgroups of DRG neurons, such as nociceptors, difficult. In attempts to overcome some of these difficulties, a limited number of immortalized DRG-derived cell lines have been generated over the past few decades. In vitro experiments using DRG-derived cell lines have been useful in understanding sensory neuron function. In addition to retaining phenotypic similarities to primary cultured DRG neurons, these cells offer greater suitability for high throughput assays due to ease of culture, maintenance, growth efficiency and cost-effectiveness. For accurate interpretation and translation of results it is critical, however, that phenotypic similarities and differences of DRG-derived cells lines are methodically compared to native neurons. Published reports to date show notable variability in how these DRG-derived cells are maintained and differentiated. Understanding the cellular and molecular differences stemming from different culture methods, is essential to validate past and future experiments, and enable these cells to be used to their full potential. This review describes currently available DRG-derived cell lines, their known sensory and nociceptor specific molecular profiles, and summarize their morphological features related to differentiation and neurite outgrowth.

3'-Methoxydaidzein exerts analgesic activity by inhibiting voltage-gated sodium channels

Isoflavones are widely consumed by people around the world in the form of soy products, dietary supplements and drugs. Many isoflavones or related crude extracts have been reported to exert pain-relief activities, but the mechanism remains unclear. Voltage-gated sodium channels (VGSCs) play important roles in excitability of pain sensing neurons and many of them are important nociceptors. Here, we report that several isoflavones including 3'-methoxydaidzein (3MOD), genistein (GEN) and daidzein (DAI) show abilities to block VGSCs and thus to attenuate chemicals and heat induced acute pain or chronic constriction injury (CCI) induced pain hypersensitivity in mice. Especially, 3MOD shows strong analgesic potential without inducing addiction through inhibiting subtypes Na_V1.7, Na_V1.8 and Na_V1.3 with the IC₅₀ of 181 ± 14, 397 ± 26, and 505 ± 46 nmol·L^-1, respectively, providing a promising compound or parent structure for the treatment of pain pathologies. This study reveals a pain-alleviating mechanism of dietary isoflavones and may provide a convenient avenue to alleviate pain.

Newly Discovered Action of HpTx3 from Venom of Heteropoda venatoria on Nav1.7 and Its Pharmacological Implications in Analgesia

It has been reported that Heteropodatoxin3 (HpTx3), a peptidic neurotoxin purified from the venom of the spider species Heteropoda venatoria, could inhibit K_v4.2 channels. Our present study newly found that HpTx3 also has potent and selective inhibitory action on Na_v1.7, with an IC₅₀ of 135.61 ± 12.98 nM. Without effect on the current–voltage (I-V) relationship of Na_v1.7, HpTx3 made minor alternation in the voltage-dependence of activation and steady-state inactivation of Na_v1.7 (4.15 mV and 7.29 mV, respectively) by interacting with the extracellular S3–S4 loop (S3b–S4 sequence) in domain II and the domain IV of the Na_v channel subtype, showing the characteristics of both pore blocker and gate modifier toxin. During the interaction of HpTx3 with the S3b–S4 sequence of Na_v1.7, the amino acid residue D in the sequence played a key role. When administered intraperitoneally or intramuscularly, HpTx3 displayed potent analgesic activity in a dose-dependent manner in different mouse pain models induced by formalin, acetic acid, complete Freund’s adjuvant, hot plate, or spared nerve injury, demonstrating that acute, inflammatory, and neuropathic pains were all effectively inhibited by the toxin. In most cases HpTx3 at doses of ≥ 1mg/kg could produce the analgesic effect comparable to that of 1 mg/kg morphine. These results suggest that HpTx3 not only can be used as a molecular probe to investigate ion channel function and pain mechanism, but also has potential in the development of the drugs that treat the Na_v1.7 channel-related pain.

A Kinetic Map of the Homomeric Voltage-Gated Potassium Channel (Kv) Family

The voltage-gated potassium (Kv) channels, encoded by 40 genes, repolarize all electrically excitable cells, including plant, cardiac, and neuronal cells. Although these genes were fully sequenced decades ago, a comprehensive kinetic characterization of all Kv channels is still missing, especially near physiological temperature. Here, we present a standardized kinetic map of the 40 homomeric Kv channels systematically characterized at 15, 25, and 35°C. Importantly, the Kv kinetics at 35°C differ significantly from commonly reported kinetics, usually performed at room temperature. We observed voltage-dependent Q₁₀ for all active Kv channels and inherent heterogeneity in kinetics for some of them. Kinetic properties are consistent across different host cell lines and conserved across mouse, rat, and human. All electrophysiology data from all Kv channels are made available through a public website (Channelpedia). This dataset provides a solid foundation for exploring kinetics of heteromeric channels, roles of auxiliary subunits, kinetic modulation, and for building accurate Kv models.

NaV1.6 and NaV1.7 channels are major endogenous voltage-gated sodium channels in ND7/23 cells

ND7/23 cells are gaining traction as a host model to express peripheral sodium channels such as NaV1.8 and NaV1.9 that have been difficult to express in widely utilized heterologous cells, like CHO and HEK293. Use of ND7/23 as a model cell to characterize the properties of sodium channels requires clear understanding of the endogenous ion channels. To define the nature of the background sodium currents in ND7/23 cells, we aimed to comprehensively profile the voltage-gated sodium channel subunits by endpoint and quantitative reverse transcription-PCR and by whole-cell patch clamp electrophysiology. We found that untransfected ND7/23 cells express endogenous peak sodium currents that average -2.12nA (n = 15) and with kinetics typical of fast sodium currents having activation and inactivation completed within few milliseconds. Furthermore, sodium currents were reduced to virtually nil upon exposure to 100nM tetrodotoxin, indicating that ND7/23 cells have essentially null background for tetrodotoxin-resistant (TTX-R) currents. qRT-PCR profiling indicated a major expression of TTX-sensitive (TTX-S) NaV1.6 and NaV1.7 at similar levels and very low expression of TTX-R NaV1.9 transcripts. There was no expression of TTX-R NaV1.8 in ND7/23 cells. There was low expression of NaV1.1, NaV1.2, NaV1.3 and no expression of cardiac or skeletal muscle sodium channels. As for the sodium channel auxiliary subunits, β1 and β3 subunits were expressed, but not the β2 and β4 subunits that covalently associate with the α-subunits. In addition, our results also showed that only the mouse forms of NaV1.6, NaV1.7 and NaV1.9 sodium channels were expressed in ND7/23 cells that was originally generated as a hybridoma of rat embryonic DRG and mouse neuroblastoma cell-line. By molecular profiling of auxiliary β- and principal α-subunits of the voltage gated sodium channel complex, our results define the background sodium channels expressed in ND7/23 cells, and confirm their utility for detailed functional studies of emerging pain channelopathies ascribed to mutations of the TTX-R sodium channels of sensory neurons.

Functional characterization of SCN10A variants in several cases of sudden unexplained death

BACKGROUND

Multiple genome-wide association studies (GWAS) and targeted gene sequencing have identified common variants in SCN10A in cases of PR and QRS duration abnormalities, atrial fibrillation and Brugada syndrome. The New York City Office of Chief Medical Examiner has now also identified five SCN10A variants of uncertain significance in six separate cases within a cohort of 330 sudden unexplained death events. The gene product of SCN10A is the Nav1.8 sodium channel. The purpose of this study was to characterize effects of these variants on Nav1.8 channel function to provide better information for the reclassification of these variants.

METHODS AND RESULTS

Patch clamp studies were performed to assess effects of the variants on whole-cell Nav1.8 currents. We also performed RNA-seq analysis and immunofluorescence confocal microcopy to determine Nav1.8 expression in heart. We show that four of the five rare 'variants of unknown significance' (L388M, L867F, P1102S and V1518I) are associated with altered functional phenotypes. The R756W variant behaved similar to wild-type under our experimental conditions. We failed to detect Nav1.8 protein expression in immunofluorescence microscopy in rat heart. Furthermore, RNA-seq analysis failed to detect full-length SCN10A mRNA transcripts in human ventricle or mouse specialized cardiac conduction system, suggesting that the effect of Nav1.8 on cardiac function is likely to be extra-cardiac in origin.

CONCLUSIONS

We have demonstrated that four of five SCN10A variants of uncertain significance, identified in unexplained death, have deleterious effects on channel function. These data extend the genetic testing of SUD cases, but significantly more clinical evidence is needed to satisfy the criteria needed to associate these variants with the onset of SUD.

The Role of Voltage-Gated Sodium Channels in Pain Signaling

Acute pain signaling has a key protective role and is highly evolutionarily conserved. Chronic pain, however, is maladaptive, occurring as a consequence of injury and disease, and is associated with sensitization of the somatosensory nervous system. Primary sensory neurons are involved in both of these processes, and the recent advances in understanding sensory transduction and human genetics are the focus of this review. Voltage-gated sodium channels (VGSCs) are important determinants of sensory neuron excitability: they are essential for the initial transduction of sensory stimuli, the electrogenesis of the action potential, and neurotransmitter release from sensory neuron terminals. Nav1.1, Nav1.6, Nav1.7, Nav1.8, and Nav1.9 are all expressed by adult sensory neurons. The biophysical characteristics of these channels, as well as their unique expression patterns within subtypes of sensory neurons, define their functional role in pain signaling. Changes in the expression of VGSCs, as well as posttranslational modifications, contribute to the sensitization of sensory neurons in chronic pain states. Furthermore, gene variants in Nav1.7, Nav1.8, and Nav1.9 have now been linked to human Mendelian pain disorders and more recently to common pain disorders such as small-fiber neuropathy. Chronic pain affects one in five of the general population. Given the poor efficacy of current analgesics, the selective expression of particular VGSCs in sensory neurons makes these attractive targets for drug discovery. The increasing availability of gene sequencing, combined with structural modeling and electrophysiological analysis of gene variants, also provides the opportunity to better target existing therapies in a personalized manner.

Differential Inhibition of Neuronal Sodium Channel Subtypes by the General Anesthetic Isoflurane

Volatile anesthetics depress neurotransmitter release in a brain region- and neurotransmitter-selective manner by unclear mechanisms. Voltage-gated sodium channels (Na_vs), which are coupled to synaptic vesicle exocytosis, are inhibited by volatile anesthetics through reduction of peak current and modulation of gating. Subtype-selective effects of anesthetics on Na_v might contribute to observed neurotransmitter-selective anesthetic effects on release. We analyzed anesthetic effects on Na⁺ currents mediated by the principal neuronal Na_v subtypes Na_v1.1, Na_v1.2, and Na_v1.6 heterologously expressed in ND7/23 neuroblastoma cells using whole-cell patch-clamp electrophysiology. Isoflurane at clinically relevant concentrations induced a hyperpolarizing shift in the voltage dependence of steady-state inactivation and slowed recovery from fast inactivation in all three Na_v subtypes, with the voltage of half-maximal steady-state inactivation significantly more positive for Na_v1.1 (-49.7 ± 3.9 mV) than for Na_v1.2 (-57.5 ± 1.2 mV) or Na_v1.6 (-58.0 ± 3.8 mV). Isoflurane significantly inhibited peak Na⁺ current (I _Na) in a voltage-dependent manner: at a physiologically relevant holding potential of -70 mV, isoflurane inhibited peak I _Na of Na_v1.2 (16.5% ± 5.5%) and Na_v1.6 (18.0% ± 7.8%), but not of Na_v1.1 (1.2% ± 0.8%). Since Na_v subtypes are differentially expressed both between neuronal types and within neurons, greater inhibition of Na_v1.2 and Na_v1.6 compared with Na_v1.1 could contribute to neurotransmitter-selective effects of isoflurane on synaptic transmission.

Topical Ambroxol 20% for the Treatment of Classical Trigeminal Neuralgia - A New Option? Initial Clinical Case Observations

BACKGROUND

Trigeminal neuralgia is difficult to treat and shows upregulation of sodium channels. The expectorant ambroxol acts as a strong local anesthetic, about 40 times stronger than lidocaine. It preferentially inhibits the channel subtype Na_v 1.8, expressed especially in nociceptive C-fibers. It seemed reasonable to try ambroxol for the treatment with neuropathic facial pain unresponsive to other standard options.

MATERIAL AND METHODS

Medical records of patients suffering from classical trigeminal neuralgia (n = 5) and successful pain reduction following topical ambroxol 20% cream in addition to standard treatment are reported.

RESULTS

All patients reported pain attacks with pain intensity between 4 and 10 NRS (numeric pain scale). In all cases they could be triggered, 3 patients reported additional spontaneous pain. Attacks were reduced in all 5 patients. Pain reduction achieved following ambroxol 20% cream was 2-8 points (NRS) earliest within 15-30 minutes and lasted for 4-6 hours mostly. This was reproducible in all cases; in one case pain was eliminated after 1 week. No patient reported side effects or skin changes; oral medication was reduced in 2 patients.

CONCLUSION

For the first time, a clinically significant pain relief following topical ambroxol 20% cream in patients with trigeminal neuralgia is reported. In view of the positive side effect profile, topical ambroxol for patients with such a highly impaired quality of life should be investigated further as a matter of urgency.

Inhibition of Voltage-Gated Na+ Channels by Bupivacaine Is Enhanced by the Adjuvants Buprenorphine, Ketamine, and Clonidine

BACKGROUND AND OBJECTIVES

Regional anesthesia includes application of local anesthetics (LAs) into the vicinity of peripheral nerves. Prolongation or improvement of nerve blocks with LAs can be accomplished by coapplication with adjuvants, including buprenorphine, ketamine, and clonidine. While the mechanisms mediating prolonged or improved LA-induced effects by adjuvants are poorly understood, we hypothesized that they are likely to increase LA-induced block of voltage-gated Na channels. In this study, we investigated the inhibitory effects of the LA bupivacaine alone and in combination with the adjuvants on neuronal Na channels.

METHODS

Effects of bupivacaine, buprenorphine, ketamine, and clonidine on endogenous Na channels in ND7/23 neuroblastoma cells were investigated with whole-cell patch clamp.

RESULTS

Bupivacaine, buprenorphine, ketamine, and clonidine are concentration- and state-dependent inhibitors of Na currents in ND7/23 cells. Tonic block of resting channels revealed an order of potency of bupivacaine (half-maximal inhibitory concentration [IC50] 178 ± 8 μM) > buprenorphine (IC50 172 ± 25) > clonidine (IC50 824 ± 55 μM) > ketamine (IC50 1377 ± 92 μM). Bupivacaine and buprenorphine, but not clonidine and ketamine, induced a strong use-dependent block at 10 Hz. Except for clonidine, all substances enhanced fast and slow inactivation. The combination of bupivacaine with one of the adjuvants resulted in a concentration-dependent potentiation bupivacaine-induced block.

CONCLUSIONS

We demonstrate that buprenorphine, ketamine, and clonidine directly inhibit Na channels and that they potentiate the blocking efficacy of bupivacaine on Na channels. These data indicate that block of Na channels may account for the additive effects of adjuvants used for regional anesthesia.

Nonmuscle myosin II isoforms interact with sodium channel alpha subunits

Sodium channels play pivotal roles in health and diseases due to their ability to control cellular excitability. The pore-forming α-subunits (sodium channel alpha subunits) of the voltage-sensitive channels (i.e., Na_v1.1–1.9) and the nonvoltage-dependent channel (i.e., Na_x) share a common structural motif and selectivity for sodium ions. We hypothesized that the actin-based nonmuscle myosin II motor proteins, nonmuscle myosin heavy chain-IIA/myh9, and nonmuscle myosin heavy chain-IIB/myh10 might interact with sodium channel alpha subunits to play an important role in their transport, trafficking, and/or function. Immunochemical and electrophysiological assays were conducted using rodent nervous (brain and dorsal root ganglia) tissues and ND7/23 cells coexpressing Na_v subunits and recombinant myosins. Immunoprecipitation of myh9 and myh10 from rodent brain tissues led to the coimmunoprecipitation of Na_x, Na_v1.2, and Na_v1.3 subunits, but not Na_v1.1 and Na_v1.6 subunits, expressed there. Similarly, immunoprecipitation of myh9 and myh10 from rodent dorsal root ganglia tissues led to the coimmunoprecipitation of Na_v1.7 and Na_v1.8 subunits, but not Na_v1.9 subunits, expressed there. The functional implication of one of these interactions was assessed by coexpressing myh10 along with Na_v1.8 subunits in ND7/23 cells. Myh10 overexpression led to three-fold increase (P < 0.01) in the current density of Na_v1.8 channels expressed in ND7/23 cells. Myh10 coexpression also hyperpolarized voltage-dependent activation and steady-state fast inactivation of Na_v1.8 channels. In addition, coexpression of myh10 reduced (P < 0.01) the offset of fast inactivation and the amplitude of the ramp currents of Na_v1.8 channels. These results indicate that nonmuscle myosin heavy chain-IIs interact with sodium channel alpha subunits subunits in an isoform-dependent manner and influence their functional properties.

A Chimeric NaV1.8 Channel Expression System Based on HEK293T Cell Line

Among the nine voltage-gated sodium channel (NaV) subtypes, NaV1.8 is an attractive therapeutic target for pain. The heterologous expression of recombinant NaV1.8 currents is of particular importance for its electrophysiological and pharmacological studies. However, NaV1.8 expresses no or low-level functional currents when transiently transfected into non-neuronal cell lines. The present study aims to explore the molecular determinants limiting its functional expression and accordingly establish a functional NaV1.8 expression system. We conducted screening analysis of the NaV1.8 intracellular loops by constructing NaV chimeric channels and confirmed that the NaV1.8 C-terminus was the only limiting factor. Replacing this sequence with that of NaV1.4, NaV1.5, or NaV1.7 constructed functional channels (NaV1.8/1.4L5, NaV1.8/1.5L5, and NaV1.8/1.7L5, respectively), which expressed high-level NaV1.8-like currents in HEK293T cells. The chimeric channel NaV1.8/1.7L5 displayed much faster inactivation of its macroscopic currents than NaV1.8/1.4L5 and NaV1.8/1.5L5, and it was the most similar to wild-type NaV1.8 expressed in ND7/23 cells. Its currents were very stable during repetitive depolarizations, while its repriming kinetic was different from wild-type NaV1.8. Most importantly, NaV1.8/1.7L5 pharmacologically resembled wild-type NaV1.8 as revealed by testing their susceptibility to two NaV1.8 selective antagonists, APETx-2 and MrVIB. NaV chimeras study showed that at least the domain 2 and domain 4 of NaV1.8 were involved in binding with APETx-2. Our study provided new insights into the function of NaV1.8 intracellular loops, as well as a reliable and convenient expression system which could be useful in NaV1.8 studies.

Endoplasmic reticulum (ER) Ca2+-channel activity contributes to ER stress and cone death in cyclic nucleotide-gated channel deficiency

Endoplasmic reticulum (ER) stress and mislocalization of improperly folded proteins have been shown to contribute to photoreceptor death in models of inherited retinal degenerative diseases. In particular, mice with cone cyclic nucleotide-gated (CNG) channel deficiency, a model for achromatopsia, display both early-onset ER stress and opsin mistrafficking. By 2 weeks of age, these mice show elevated signaling from all three arms of the ER-stress pathway, and by 1 month, cone opsin is improperly distributed away from its normal outer segment location to other retinal layers. This work investigated the role of Ca²⁺-release channels in ER stress, protein mislocalization, and cone death in a mouse model of CNG-channel deficiency. We examined whether preservation of luminal Ca²⁺ stores through pharmacological and genetic suppression of ER Ca²⁺ efflux protects cones by attenuating ER stress. We demonstrated that the inhibition of ER Ca²⁺-efflux channels reduced all three arms of ER-stress signaling while improving opsin trafficking to cone outer segments and decreasing cone death by 20-35%. Cone-specific gene deletion of the inositol-1,4,5-trisphosphate receptor type I (IP₃R1) also significantly increased cone density in the CNG-channel-deficient mice, suggesting that IP₃R1 signaling contributes to Ca²⁺ homeostasis and cone survival. Consistent with the important contribution of organellar Ca²⁺ signaling in this achromatopsia mouse model, significant differences in dynamic intraorganellar Ca²⁺ levels were detected in CNG-channel-deficient cones. These results thus identify a novel molecular link between Ca²⁺ homeostasis and cone degeneration, thereby revealing novel therapeutic targets to preserve cones in inherited retinal degenerative diseases.

Characterization of Endogenous Sodium Channels in the ND7-23 Neuroblastoma Cell Line: Implications for Use as a Heterologous Ion Channel Expression System Suitable for Automated Patch Clamp Screening

The rodent neuroblastoma cell line, ND7-23, is used to express voltage-dependent sodium (Nav) and other neuronal ion channels resistant to heterologous expression in Chinese hamster ovary (CHO) or human embryonic kidney (HEK) cells. Their advantage is that they provide endogenous factors and signaling pathways to promote ion channel peptide folding, expression, and function at the cell surface and are also amenable to automated patch clamping. However, ND7-23 cells exhibit endogenous tetrodotoxin (TTX)-sensitive Nav currents, and molecular profiling has revealed the presence of Nav1.2, Nav1.3, Nav1.6, and Nav1.7 transcripts, but no study has determined which subtypes contribute to functional channels at the cell surface. We profiled the repertoire of functional Nav channels endogenously expressed in ND7-23 cells using the QPatch automated patch clamp platform and selective toxins and small molecules. The potency and subtype selectivity of the ligands (Icagen compound 68 from patent US-20060025415-A1-20060202, 4,9 anhydro TTX, and Protoxin-II) were established in human Nav1.3, Nav1.6, and Nav1.7 channel cell lines before application of selective concentrations to ND7-23 cells. Our data confirm previous studies that >97% of macroscopic Nav current in ND7-23 cells is carried by TTX-sensitive channels (300 nM TTX) and that Nav1.7 is the predominant channel contributing to this response (65% of peak inward current), followed by Nav1.6 (∼20%) and negligible Nav1.3 currents (∼2%). In addition, our data are the first to assess the Nav1.6 potency (50% inhibitory concentration [IC₅₀] of 33 nM) and selectivity (50-fold over Nav1.7) of 4,9 anhydro TTX in human Nav channels expressed in mammalian cells, confirming previous studies of rodent Nav channels expressed in oocytes and HEK cells.

Trafficking regulates the subcellular distribution of voltage-gated sodium channels in primary sensory neurons

Voltage-gated sodium channels (Na_vs) comprise at least nine pore-forming α subunits. Of these, Na_v1.6, Na_v1.7, Na_v1.8 and Na_v1.9 are the most frequently studied in primary sensory neurons located in the dorsal root ganglion and are mainly localized to the cytoplasm. A large pool of intracellular Na_vs raises the possibility that changes in Na_v trafficking could alter channel function. The molecular mediators of Na_v trafficking mainly consist of signals within the Na_vs themselves, interacting proteins and extracellular factors. The surface expression of Na_vs is achieved by escape from the endoplasmic reticulum and proteasome degradation, forward trafficking and plasma membrane anchoring, and it is also regulated by channel phosphorylation and ubiquitination in primary sensory neurons. Axonal transport and localization of Na_vs in afferent fibers involves the motor protein KIF5B and scaffold proteins, including contactin and PDZ domain containing 2. Localization of Na_v1.6 to the nodes of Ranvier in myelinated fibers of primary sensory neurons requires node formation and the submembrane cytoskeletal protein complex. These findings inform our understanding of the molecular and cellular mechanisms underlying Na_v trafficking in primary sensory neurons.

Activity-dependent depression of neuronal sodium channels by the general anaesthetic isoflurane

BACKGROUND

The mechanisms by which volatile anaesthetics such as isoflurane alter neuronal function are poorly understood, in particular their presynaptic mechanisms. Presynaptic voltage-gated sodium channels (Na(v)) have been implicated as a target for anaesthetic inhibition of neurotransmitter release. We hypothesize that state-dependent interactions of isoflurane with Na(v) lead to increased inhibition of Na(+) current (I(Na)) during periods of high-frequency neuronal activity.

METHODS

The electrophysiological effects of isoflurane, at concentrations equivalent to those used clinically, were measured on recombinant brain-type Na(v)1.2 expressed in ND7/23 neuroblastoma cells and on endogenous Na(v) in isolated rat neurohypophysial nerve terminals. Rate constants determined from experiments on the recombinant channel were used in a simple model of Na(v) gating.

RESULTS

At resting membrane potentials, isoflurane depressed peak I(Na) and shifted steady-state inactivation in a hyperpolarizing direction. After membrane depolarization, isoflurane accelerated entry (τ(control)=0.36 [0.03] ms compared with τ(isoflurane)=0.33 [0.05] ms, P<0.05) and slowed recovery (τ(control)=6.9 [1.1] ms compared with τ(isoflurane)=9.0 [1.9] ms, P<0.005) from apparent fast inactivation, resulting in enhanced depression of I(Na), during high-frequency stimulation of both recombinant and endogenous nerve terminal Na(v). A simple model of Na(v) gating involving stabilisation of fast inactivation, accounts for this novel form of activity-dependent block.

CONCLUSIONS

Isoflurane stabilises the fast-inactivated state of neuronal Na(v) leading to greater depression of I(Na) during high-frequency stimulation, consistent with enhanced inhibition of fast firing neurones.

Exon 11 skipping of SCN10A coding for voltage-gated sodium channels in dorsal root ganglia

The voltage-gated sodium channel Na(V)1.8 (encoded by SCN10A) is predominantly expressed in dorsal root ganglia(DRG) and plays a critical role in pain perception. We analyzed SCN10A transcripts isolated from human DRGs using deep sequencing and found a novel splice variant lacking exon 11, which codes for 98 amino acids of the domain I/II linker. Quantitative PCR analysis revealed an abundance of this variant of up to 5–10% in human, while no such variants were detected in mouse or rat. Since no obvious functional differences between channels with and without the exon-11 sequence were detected, it is suggested that SCN10A exon 11 skipping in humans is a tolerated event.

Pharmacological kinetics of BmK AS, a sodium channel site 4-specific modulator on Nav1.3

OBJECTIVE

In this study, the pharmacological kinetics of Buthus martensi Karsch (BmK) AS, a specific modulator of voltage-gated sodium channel site 4, was investigated on Na(v)1.3 expressed in Xenopus oocytes.

METHODS

Two-electrode voltage clamp was used to record the whole-cell sodium current.

RESULTS

The peak currents of Na(v)1.3 were depressed by BmK AS over a wide range of concentrations (10, 100, and 500 nmol/L). Most remarkably, BmK AS at 100 nmol/L hyperpolarized the voltage-dependence and increased the voltage-sensitivity of steady-state activation/inactivation. In addition, BmK AS was capable of hyperpolarizing not only the fast inactivation but also the slow inactivation, with a greater preference for the latter. Moreover, BmK AS accelerated the time constant and increased the ratio of recovery in Na(v)1.3 at all concentrations.

CONCLUSION

This study provides direct evidence that BmK AS facilitates steady-state activation and inhibits slow inactivation by stabilizing both the closed and open states of the Na(v)1.3 channel, which might result from an integrative binding to two receptor sites on the voltage-gated sodium channels. These results may shed light on therapeutics against Na(v)1.3-targeted pathology.

Physiological and genetic analysis of multiple sodium channel variants in a model of genetic absence epilepsy

In excitatory neurons, SCN2A (NaV1.2) and SCN8A (NaV1.6) sodium channels are enriched at the axon initial segment. NaV1.6 is implicated in several mouse models of absence epilepsy, including a missense mutation identified in a chemical mutagenesis screen (Scn8a(V929F)). Here, we confirmed the prior suggestion that Scn8a(V929F) exhibits a striking genetic background-dependent difference in phenotypic severity, observing that spike-wave discharge (SWD) incidence and severity are significantly diminished when Scn8a(V929F) is fully placed onto the C57BL/6J strain compared with C3H. Examination of sequence differences in NaV subunits between these two inbred strains suggested NaV1.2(V752F) as a potential source of this modifier effect. Recognising that the spatial co-localisation of the NaV channels at the axon initial segment (AIS) provides a plausible mechanism for functional interaction, we tested this idea by undertaking biophysical characterisation of the variant NaV channels and by computer modelling. NaV1.2(V752F) functional analysis revealed an overall gain-of-function and for NaV1.6(V929F) revealed an overall loss-of-function. A biophysically realistic computer model was used to test the idea that interaction between these variant channels at the AIS contributes to the strain background effect. Surprisingly this modelling showed that neuronal excitability is dominated by the properties of NaV1.2(V752F) due to "functional silencing" of NaV1.6(V929F) suggesting that these variants do not directly interact. Consequent genetic mapping of the major strain modifier to Chr 7, and not Chr 2 where Scn2a maps, supported this biophysical prediction. While a NaV1.6(V929F) loss of function clearly underlies absence seizures in this mouse model, the strain background effect is apparently not due to an otherwise tempting Scn2a variant, highlighting the value of combining physiology and genetics to inform and direct each other when interrogating genetic complex traits such as absence epilepsy.

Effect of amitriptyline on tetrodotoxin-resistant Nav1.9 currents in nociceptive trigeminal neurons

Background

Amitriptyline (AMI) is tricyclic antidepressant that has been widely used to manage various chronic pains such as migraines. Its efficacy is attributed to its blockade of voltage-gated sodium channels (VGSCs). However, the effects of AMI on the tetrodotoxin-resistant (TTX-r) sodium channel Na_v1.9 currents have been unclear to present.

Results

Using a whole-cell patch clamp technique, this study showed that AMI efficiently inhibited Na_v1.9 currents in a concentration-dependent manner and had an IC₅₀ of 15.16 μM in acute isolated trigeminal ganglion (TG) neurons of the rats. 10 μM AMI significantly shifted the steady-state inactivation of Na_v1.9 channels in the hyperpolarizing direction without affecting voltage-dependent activation. Surprisingly, neither 10 nor 50 μM AMI caused a use-dependent blockade of Na_v1.9 currents elicited by 60 pulses at 1 Hz.

Conclusion

These data suggest that AMI is a state-selective blocker of Na_v1.9 channels in rat nociceptive trigeminal neurons, which likely contributes to the efficacy of AMI in treating various pains, including migraines.

Loureirin B: An Effective Component in Dragon's Blood Modulating Sodium Currents in TG Neurons

To test, analyze and express the relationship between the pharmacological effect of Traditional Chinese Medicine (TCM) dragon's blood and that of its component loureirin B, specify an operational definition for effective component from raw drug of TCM. Using the whole-cell patch-clamp technique, the effects of dragon's blood and its component loureirin B on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium currents in trigeminal ganglion (TG) neurons were observed. The results show that both dragon's blood and loureirin B suppressed two types of peak sodium currents in a dose-dependent way. 0.1% dragon's blood and 0.2mmol/L loureirin B affected the activation and inactivation of sodium channels. The results further prove the analgetic mechanism of dragon's blood interfering with the nociceptive transmission. According to the above definition, loureirin B is the effective component in dragon's blood modulating sodium currents in TG neurons.

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.

The distribution of low-threshold TTX-resistant Na⁺ currents in rat trigeminal ganglion cells

The distribution of low-threshold tetrodotoxin-resistant (TTX-r) Na(+) current and its co-expression with high-threshold TTX-r Na(+) current were studied in randomly selected acutely dissociated rat trigeminal ganglion (non-identified TG cells) and TG cells serving the temporomandibular joint (TMJ-TG cells). Conditions previously shown to enhance Na(V)1.9 channel-mediated currents (holding potential (HP) -80 mV, 130-mM fluoride internally) were employed to amplify the low-threshold Na(+) current. Under these conditions, detectable low-threshold Na(+) current was exhibited by 16 out of 21 non-identified TG cells (average, 1810 ± 358 pA), and by nine of 14 TMJ-TG cells (average, 959 ± 525 pA). The low-threshold Na(+) current began to activate around -55 mV and was inactivated by holding TG cells at -60 mV and delivering 40-ms test potentials (TPs) to 0 mV. The inactivation was long lasting, recovering only 8 ± 3% over a 5-min period after the HP was returned to -80 mV. Following low-threshold Na(+) current inactivation, high-threshold TTX-r Na(+) current, evoked from HP -60 mV, was observed. High-threshold Na(+) current amplitude averaged 16,592 ± 3913 pA for TPs to 0 mV, was first detectable at an average TP of -34 ± 1.3 mV, and was ½ activated at -7.1 ± 2.3 mV. In TG cells expressing prominent low-threshold Na(+) currents, changing the external solution to one containing 0 mM Na(+) reduced the amount of current required to hold the cells at -80 mV through -50 mV, the peak effect being observed at HP -60 mV. TG cells recorded from with a more physiological pipette solution containing chloride instead of fluoride exhibited small low-threshold Na(+) currents, which were greatly increased upon superfusion of the TG cells with the adenylyl cyclase (AC) activator forskolin. These data suggest two hypotheses: (1) low- and high-threshold Na(V)1.9 and Na(V)1.8 channels, respectively, are frequently co-expressed in TG neurons serving the TMJ and other structures, and (2), Na(V)1.9 channel-mediated currents are small under physiological conditions, but may be enhanced by inflammatory mediators that increase AC activity, and may mediate an inward leak that depolarizes TG neurons, increasing their excitability.

PKCε phosphorylation of the sodium channel NaV1.8 increases channel function and produces mechanical hyperalgesia in mice

Mechanical hyperalgesia is a common and potentially disabling complication of many inflammatory and neuropathic conditions. Activation of the enzyme PKCε in primary afferent nociceptors is a major mechanism that underlies mechanical hyperalgesia, but the PKCε substrates involved downstream are not known. Here, we report that in a proteomic screen we identified the NaV1.8 sodium channel, which is selectively expressed in nociceptors, as a PKCε substrate. PKCε-mediated phosphorylation increased NaV1.8 currents, lowered the threshold voltage for activation, and produced a depolarizing shift in inactivation in wild-type - but not in PKCε-null - sensory neurons. PKCε phosphorylated NaV1.8 at S1452, and alanine substitution at this site blocked PKCε modulation of channel properties. Moreover, a specific PKCε activator peptide, ψεRACK, produced mechanical hyperalgesia in wild-type mice but not in Scn10a-/- mice, which lack NaV1.8 channels. These studies demonstrate that NaV1.8 is an important, direct substrate of PKCε that mediates PKCε-dependent mechanical hyperalgesia.

Recent developments regarding voltage-gated sodium channel blockers for the treatment of inherited and acquired neuropathic pain syndromes

Chronic and neuropathic pain constitute significant health problems affecting millions of individuals each year. Pain sensations typically originate in sensory neurons of the peripheral nervous system which relay information to the central nervous system (CNS). Pathological pain sensations can arise as result of changes in excitability of these peripheral sensory neurons. Voltage-gated sodium channels are key determinants regulating action potential generation and propagation; thus, changes in sodium channel function can have profound effects on neuronal excitability and pain signaling. At present, most of the clinically available sodium channel blockers used to treat pain are non-selective across sodium channel isoforms and can contribute to cardio-toxicity, motor impairments, and CNS side effects. Numerous strides have been made over the last decade in an effort to develop more selective and efficacious sodium channel blockers to treat pain. The purpose of this review is to highlight some of the more recent developments put forth by research universities and pharmaceutical companies alike in the pursuit of developing more targeted sodium channel therapies for the treatment of a variety of neuropathic pain conditions.

Regulation of Nav1.6 and Nav1.8 peripheral nerve Na+ channels by auxiliary β-subunits

Voltage-gated Na(+) (Na(v)) channels are composed of a pore-forming α-subunit and one or more auxiliary β-subunits. The present study investigated the regulation by the β-subunit of two Na(+) channels (Na(v)1.6 and Na(v)1.8) expressed in dorsal root ganglion (DRG) neurons. Single cell RT-PCR was used to show that Na(v)1.8, Na(v)1.6, and β(1)-β(3) subunits were widely expressed in individually harvested small-diameter DRG neurons. Coexpression experiments were used to assess the regulation of Na(v)1.6 and Na(v)1.8 by β-subunits. The β(1)-subunit induced a 2.3-fold increase in Na(+) current density and hyperpolarizing shifts in the activation (-4 mV) and steady-state inactivation (-4.7 mV) of heterologously expressed Na(v)1.8 channels. The β(4)-subunit caused more pronounced shifts in activation (-16.7 mV) and inactivation (-9.3 mV) but did not alter the current density of cells expressing Na(v)1.8 channels. The β(3)-subunit did not alter Na(v)1.8 gating but significantly reduced the current density by 31%. This contrasted with Na(v)1.6, where the β-subunits were relatively weak regulators of channel function. One notable exception was the β(4)-subunit, which induced a hyperpolarizing shift in activation (-7.6 mV) but no change in the inactivation or current density of Na(v)1.6. The β-subunits differentially regulated the expression and gating of Na(v)1.8 and Na(v)1.6. To further investigate the underlying regulatory mechanism, β-subunit chimeras containing portions of the strongly regulating β(1)-subunit and the weakly regulating β(2)-subunit were generated. Chimeras retaining the COOH-terminal domain of the β(1)-subunit produced hyperpolarizing shifts in gating and increased the current density of Na(v)1.8, similar to that observed for wild-type β(1)-subunits. The intracellular COOH-terminal domain of the β(1)-subunit appeared to play an essential role in the regulation of Na(v)1.8 expression and gating.

Up-regulation of low-threshold tetrodotoxin-resistant Na+ current via activation of a cyclic AMP/protein kinase A pathway in nociceptor-like rat dorsal root ganglion cells

The effects of forskolin on low-threshold tetrodotoxin-resistant (TTX-r) Na(+) currents was studied in small diameter (average ≈ 25 μm) dorsal root ganglion (DRG) cells. All DRG cells included in the study were categorized as type-2 or non-type-2 based on the expression of a low-threshold A-current. In all type-2 and some non-type-2 DRG cells held at -80 mV, the adenylyl cyclase (AC) activator forskolin (10 μM) up-regulated TTX-r Na(+) currents evoked with steps to -55 mV through -35 mV (low-threshold current). Up-regulation of low-threshold current by forskolin was mimicked by the protein kinase A (PKA) agonist Sp-cAMPs and the inflammatory mediator serotonin, and blocked by the PKA antagonist Rp-cAMPs. Forskolin-induced up-regulation of low-threshold current evoked from a holding potential of -60 mV was blocked by 40 ms steps to 0 mV, which presumably induced a long lasting inactivation of the low-threshold channels. Reducing to 3 ms the duration of steps to 0 mV, significantly increased the number of DRG cells where low-threshold current was up-regulated by forskolin, presumably by reducing the long-lasting inactivation of the low-threshold channels. In the same cells, high-threshold current, evoked by 40 ms or 3 ms steps to 0 mV, was consistently up-regulated by forskolin. The selective Na(V)1.8 channel blocker A-803467 markedly blocked high-threshold current but not low-threshold current. The different voltage protocols observed to activate and inactivate the low- and high-threshold currents, and the observation that A-803467 blocked high- but not low-threshold current suggests that the two currents were mediated by different channels, possibly Na(V)1.8 and Na(V)1.9, respectively. Inflammatory mediators may simultaneously up-regulate Na(V)1.8 and Na(V)1.9 channels in the same nociceptor via a AC/PKA signaling pathway, increasing nociceptor signaling strength, and lowering nociceptor threshold, respectively.

Single-cell analysis of sodium channel expression in dorsal root ganglion neurons

Sensory neurons of the dorsal root ganglia (DRG) express multiple voltage-gated sodium (Na) channels that substantially differ in gating kinetics and pharmacology. Small-diameter (<25 μm) neurons isolated from the rat DRG express a combination of fast tetrodotoxin-sensitive (TTX-S) and slow TTX-resistant (TTX-R) Na currents while large-diameter neurons (>30 μm) predominately express fast TTX-S Na current. Na channel expression was further investigated using single-cell RT-PCR to measure the transcripts present in individually harvested DRG neurons. Consistent with cellular electrophysiology, the small neurons expressed transcripts encoding for both TTX-S (Nav1.1, Nav1.2, Nav1.6, and Nav1.7) and TTX-R (Nav1.8 and Nav1.9) Na channels. Nav1.7, Nav1.8 and Nav1.9 were the predominant Na channels expressed in the small neurons. The large neurons highly expressed TTX-S isoforms (Nav1.1, Nav1.6, and Nav1.7) while TTX-R channels were present at comparatively low levels. A unique subpopulation of the large neurons was identified that expressed TTX-R Na current and high levels of Nav1.8 transcript. DRG neurons also displayed substantial differences in the expression of neurofilaments (NF200, peripherin) and Necl-1, a neuronal adhesion molecule involved in myelination. The preferential expression of NF200 and Necl-1 suggests that large-diameter neurons give rise to thick myelinated axons. Small-diameter neurons expressed peripherin, but reduced levels of NF200 and Necl-1, a pattern more consistent with thin unmyelinated axons. Single-cell analysis of Na channel transcripts indicates that TTX-S and TTX-R Na channels are differentially expressed in large myelinated (Nav1.1, Nav1.6, and Nav1.7) and small unmyelinated (Nav1.7, Nav1.8, and Nav1.9) sensory neurons.

Frequency-dependent interaction of ultrashort E-fields with nociceptor membranes and proteins

We examined the influence of ultrashort pulses (USP) on sensory neurons. Single and high frequency bursts of 12 ns E-fields were presented to rat skin nociceptors that expressed distinct combinations of voltage-sensitive proteins. A single E-field pulse produced action potentials in all nociceptor subtypes at a critical threshold (E(c) ) of 403 V/cm. When configured into high frequency bursts, USP charge integrated to reduce the action potential threshold in a frequency and burst duration-dependent manner with E(c) as low as 16 V/cm (4000 Hz, 25 ms burst). There was no evidence of electroporation at field intensities near the E(c) for nociceptor activation. USP bursts activated a late, persistent Ca(++) flux that was identified as a dantrolene-sensitive Ca(++) -induced Ca(++) release (CICR). Influx of Ca(++) into the cell was required for the CICR and resulted in a reduction of the single pulse E(c) by about 50%.

U-shaped dose-dependent effects of BmK AS, a unique scorpion polypeptide toxin, on voltage-gated sodium channels

BACKGROUND AND PURPOSE

Buthus martensi Karsch (BmK) AS is a scorpion polypeptide toxin, said to target the voltage-gated sodium channels (VGSCs). However, the mechanism of action of BmK AS on the VGSCs has yet to be defined.

EXPERIMENTAL APPROACH

We examined the electrophysiological effects of BmK AS in a wide dose range on the rat brain-type VGSC alpha-subunit, rNav1.2a, heterologously expressed in Xenopus oocytes and on the VGSCs endogenously expressed in the dorsal root ganglion neuroblastoma ND7-23 cell line.

KEY RESULTS

In the oocytes, BmK AS depolarized the voltage dependence of activation and inactivation of rNav1.2a at 0.1 and 500 nM whereas these parameters were hyperpolarized at 1 nM. In ND7-23 cells, BmK AS hyperpolarized the voltage dependence of activation and inactivation at 0.1, 1 and 100 nM but not 10 nM. BmK AS also hyperpolarized the voltage dependence of recovery from inactivation at 0.1 and 100 nM and slowed the recovery kinetics at all concentrations, but the effects of 1 and 10 nM were relatively smaller than those at 0.1 and 100 nM. Moreover, the inactivation of VGSCs was potentiated by 10 nM BmK AS in both systems, whereas it was inhibited by 0.1 or 100 nM BmK AS in the oocytes or ND7-23 cells respectively.

CONCLUSIONS AND IMPLICATIONS

BmK AS modulated the VGSCs in a unique U-shaped dose-dependent manner, which could be due to the opposing effects of binding to two distinct receptor sites on the VGSCs.

Sodium channelopathies and pain

Chronic pain often represents a severe, debilitating condition. Up to 10% of the worldwide population are affected, and many patients are poorly responsive to current treatment strategies. Nociceptors detect noxious conditions to produce the sensation of pain, and this signal is conveyed to the CNS by means of action potentials. The fast upstroke of action potentials is mediated by voltage-gated sodium channels, of which nine pore-forming alpha-subunits (Nav1.1-1.9) have been identified. Heterogeneous functional properties and distinct expression patterns denote specialized functions of each subunit. The Nav1.7 and Nav1.8 subunits have emerged as key molecules involved in peripheral pain processing and in the development of an increased pain sensitivity associated with inflammation and tissue injury. Several mutations in the SCN9A gene encoding for Nav1.7 have been identified as important cellular substrates for different heritable pain syndromes. This review aims to cover recent progress on our understanding of how biophysical properties of mutant Nav1.7 translate into an aberrant electrogenesis of nociceptors. We also recapitulate the role of Nav1.8 for peripheral pain processing and of additional sodium channelopathies which have been linked to disorders with pain as a significant component.

Block of sensory neuronal Na+ channels by the secreolytic ambroxol is associated with an interaction with local anesthetic binding sites

Voltage-gated Na(+) channels (Na(v)) regulate the excitability of sensory neurons and are potential targets for novel analgesics. The secreolytic ambroxol reduces pain-related behavior in rodents and alleviates pain in humans. With properties resembling those of local anesthetics, ambroxol has been reported to block Na(+) currents in sensory neurons with a preference for tetrodotoxin-resistant (TTXr) Na(+) currents encoded by Na(v)1.8. However, the molecular determinants for ambroxol-induced block of Na(+) channels and a preferential block of Na(v)1.8 opposed to tetrodotoxin-sensitive (TTXs) Na(v) alpha-subunits have not been studied in detail. By means of whole-cell voltage clamp recordings, we studied the effects of ambroxol and local anesthetics on the recombinant TTXr subunit Na(v)1.8, on TTXs Na(v) alpha-subunits and on mutants of Na(v)1.4 that are insensitive to local anesthetics. Tonic and use-dependent block by ambroxol was strongly alleviated in local anesthetic-insensitive Na(v)1.4 mutants. Use-dependent block, but not tonic block was significantly stronger on Na(v)1.8 than on TTXs channels. The TTXs subunit Na(v)1.3 displayed the least degree of use-dependent block by ambroxol. The local anesthetics mepivacaine and S(-)-bupivacaine also blocked Na(v)1.8 and TTXs channels differentially. While mepivacaine displayed a preferential use-dependent block of Na(v)1.8, S(-)-bupivacaine displayed a preference for TTXs Na(+) channels. Our data show that ambroxol acts as a typical local anesthetic on Na(+) channels interacting with specific residues in the S6 segments. This property probably meditates the analgesic effect of ambroxol. Ambroxol preferentially blocks Na(v)1.8, however shares this property with established local anesthetics like mepivacaine.

Aquaporin-1 tunes pain perception by interaction with Na(v)1.8 Na+ channels in dorsal root ganglion neurons

Aquaporin-1 (AQP1) water channels are expressed in the plasma membrane of dorsal root ganglion (DRG) neurons. We found reduced osmotic water permeability in freshly isolated DRG neurons from AQP1(-/-) versus AQP1(+/+) mice. Behavioral studies showed greatly reduced thermal inflammatory pain perception in AQP1(-/-) mice evoked by bradykinin, prostaglandin E(2), and capsaicin as well as reduced cold pain perception. Patch clamp of freshly isolated DRG neurons showed reduced action potential firing in response to current injections. Single action potentials after pulse current injections showed reduced maximum inward current, suggesting impaired Na(v)1.8 Na(+) function. Whole-cell Na(v)1.8 Na(+) currents in Na(v)1.8-expressing ND7-23 cells showed slowed frequency-dependent inactivation after AQP1 transfection. Immunoprecipitation studies showed AQP1- Na(v)1.8 Na(+) interaction, which was verified in live cells by single-particle tracking of quantum dot-labeled AQP1. Our results implicate the involvement of AQP1 in DRG neurons for the perception of inflammatory thermal pain and cold pain, whose molecular basis is accounted for, in part, by reduced Na(v)1.8-dependent membrane Na(+) current. AQP1 is, thus, a novel target for pain management.

Prostaglandin E2 promotes Na1.8 trafficking via its intracellular RRR motif through the protein kinase A pathway

Voltage-gated sodium channels (Na(v)) are essential for the initiation and propagation of action potentials in neurons. Na(v)1.8 activity is regulated by prostaglandin E(2) (PGE(2)). There is, however, no direct evidence showing the regulated trafficking of Na(v)1.8, and the molecular and cellular mechanism of PGE(2)-induced sodium channel trafficking is not clear. Here, we report that PGE(2) regulates the trafficking of Na(v)1.8 through the protein kinase A (PKA) signaling pathway, and an RRR motif in the first intracellular loop of Na(v)1.8 mediates this effect. In rat dorsal root ganglion (DRG) neurons, prolonged PGE(2) treatment enhanced Na(v)1.8 currents by increasing the channel density on the cell surface. Activation of PKA by forskolin had the same effect on DRG neurons and human embryonic kidney 293T cells expressing Na(v)1.8. Inhibition of PKA completely blocked the PGE(2)-promoted effect on Na(v)1.8. Mutation of five PKA phosphorylation sites or the RRR motif in the first intracellular loop of Na(v)1.8 abolished the PKA-promoted Na(v)1.8 surface expression. Furthermore, a membrane-tethered peptide containing the intracellular RRR motif disrupted the PGE(2)-induced promotion of the Na(v)1.8 current in DRG neurons. Our data indicate that PGE(2) promotes the surface expression of Na(v)1.8 via an intracellular RRR motif, and provide a novel mechanism for functional modulation of Na(v)1.8 by hyperalgesic agents.