The G1662S NaV1.8 mutation in small fibre neuropathy: impaired inactivation underlying DRG neuron hyperexcitability

Reduced Gray Matter Volume and Cortical Thickness in Patients With Small-Fiber Neuropathy

Small-fiber neuropathy (SFN) is defined by degeneration or dysfunction of peripheral sensory nerve endings. Central correlates have been identified on the level of gray matter volume (GMV) and cortical thickness (CT) changes. However, across SFN etiologies knowledge about a common structural brain signature is still lacking. Therefore, we recruited 26 SFN patients and 25 age- and sex-matched healthy controls to conduct voxel-based- and surface-based morphometry. Across all patients, we found reduced GMV in widespread frontal regions, left caudate, insula and superior parietal lobule. Surface-based morphometry analysis revealed reduced CT in the right precentral gyrus of SFN patients. In a region-based approach, patients had reduced GMV in the left caudate. Since pathogenic gain-of-function variants in voltage-gated sodium channels (Nav) have been associated with SFN pathophysiology, we explored brain morphological patterns in a homogenous subsample of patients carrying rare heterozygous missense variants. Whole brain- and region-based approaches revealed GMV reductions in the bilateral caudate for Nav variant carriers. Further research is needed to analyze the specific role of Nav variants for structural brain alterations. Together, we conclude that SFN patients have specific GMV and CT alterations, potentially forming potential new central biomarkers for this condition. Our results might help to better understand underlying or compensatory mechanisms of chronic pain perception in the future. PERSPECTIVE: This study reveals structural brain changes in small-fiber neuropathy (SFN) patients, particularly in frontal regions, caudate, insula, and parietal lobule. Notably, individuals with SFN and specific Nav variants exhibit bilateral caudate abnormalities. These findings may serve as potential central biomarkers for SFN and provide insights into chronic pain perception mechanisms.

Insights into the voltage-gated sodium channel, NaV1.8, and its role in visceral pain perception

Pain is a major issue in healthcare throughout the world. It remains one of the major clinical issues of our time because it is a common sequela of numerous conditions, has a tremendous impact on individual quality of life, and is one of the top drivers of cost in medicine, due to its influence on healthcare expenditures and lost productivity in those affected by it. Patients and healthcare providers remain desperate to find new, safer and more effective analgesics. Growing evidence indicates that the voltage-gated sodium channel Nav1.8 plays a critical role in transmission of pain-related signals throughout the body. For that reason, this channel appears to have strong potential to help develop novel, more selective, safer, and efficacious analgesics. However, many questions related to the physiology, function, and clinical utility of Nav1.8 remain to be answered. In this article, we discuss the latest studies evaluating the role of Nav1.8 in pain, with a particular focus on visceral pain, as well as the steps taken thus far to evaluate its potential as an analgesic target. We also review the limitations of currently available studies related to this topic, and describe the next scientific steps that have already been undertaken, or that will need to be pursued, to fully unlock the capabilities of this potential therapeutic target.

Heterologous expression of the human wild-type and variant NaV 1.8 (A1073V) in rat sensory neurons

BACKGROUND

Silent inflammatory bowel disease (IBD) is a condition in which individuals with the active disease experience minor to no pain. Voltage-gated Na+ (NaV ) channels expressed in sensory neurons play a major role in pain perception. Previously, we reported that a NaV 1.8 genetic polymorphism (A1073V, rs6795970) was more common in a cohort of silent IBD patients. The expression of this variant (1073V) in rat sympathetic neurons activated at more depolarized potentials when compared to the more common variant (1073A). In this study, we investigated whether expression of either NaV 1.8 variant in rat sensory neurons would exhibit different biophysical characteristics than previously observed in sympathetic neurons.

METHODS

Endogenous NaV 1.8 channels were first silenced in DRG neurons and then either 1073A or 1073V human NaV 1.8 cDNA constructs were transfected. NaV 1.8 currents were recorded with the whole-cell patch-clamp technique.

KEY RESULTS

The results indicate that 1073A and 1073V NaV 1.8 channels exhibited similar activation values. However, the slope factor (k) for activation determined for this same group of neurons decreased by 5 mV, suggesting an increase in voltage sensitivity. Comparison of inactivation parameters indicated that 1073V channels were shifted to more depolarized potentials than 1073A-expressing neurons, imparting a proexcitatory characteristic.

CONCLUSIONS AND INFERENCES

These findings differ from previous observations in other expression models and underscore the challenges with heterologous expression systems. Therefore, the use of human sensory neurons derived from induced pluripotent stem cells may help address these inconsistencies and better determine the effect of the polymorphism present in IBD patients.

Pharmacologic Characterization of LTGO-33, a Selective Small Molecule Inhibitor of the Voltage-Gated Sodium Channel NaV1.8 with a Unique Mechanism of Action

Discovery and development of new molecules directed against validated pain targets is required to advance the treatment of pain disorders. Voltage-gated sodium channels (NaVs) are responsible for action potential initiation and transmission of pain signals. NaV1.8 is specifically expressed in peripheral nociceptors and has been genetically and pharmacologically validated as a human pain target. Selective inhibition of NaV1.8 can ameliorate pain while minimizing effects on other NaV isoforms essential for cardiac, respiratory, and central nervous system physiology. Here we present the pharmacology, interaction site, and mechanism of action of LTGO-33, a novel NaV1.8 small molecule inhibitor. LTGO-33 inhibited NaV1.8 in the nM potency range and exhibited over 600-fold selectivity against human NaV1.1-NaV1.7 and NaV1.9. Unlike prior reported NaV1.8 inhibitors that preferentially interacted with an inactivated state via the pore region, LTGO-33 was state-independent with similar potencies against closed and inactivated channels. LTGO-33 displayed species specificity for primate NaV1.8 over dog and rodent NaV1.8 and inhibited action potential firing in human dorsal root ganglia neurons. Using chimeras combined with mutagenesis, the extracellular cleft of the second voltage-sensing domain was identified as the key site required for channel inhibition. Biophysical mechanism of action studies demonstrated that LTGO-33 inhibition was relieved by membrane depolarization, suggesting the molecule stabilized the deactivated state to prevent channel opening. LTGO-33 equally inhibited wild-type and multiple NaV1.8 variants associated with human pain disorders. These collective results illustrate LTGO-33 inhibition via both a novel interaction site and mechanism of action previously undescribed in NaV1.8 small molecule pharmacologic space. SIGNIFICANCE STATEMENT: NaV1.8 sodium channels primarily expressed in peripheral pain-sensing neurons represent a validated target for the development of novel analgesics. Here we present the selective small molecule NaV1.8 inhibitor LTGO-33 that interdicts a distinct site in a voltage-sensor domain to inhibit channel opening. These results inform the development of new analgesics for pain disorders.

Peripheral temperature dysregulation associated with functionally altered NaV1.8 channels

The voltage-gated sodium channel NaV1.8 is prominently expressed in the soma and axons of small-caliber sensory neurons, and pathogenic variants of the corresponding gene SCN10A are associated with peripheral pain and autonomic dysfunction. While most disease-associated SCN10A variants confer gain-of-function properties to NaV1.8, resulting in hyperexcitability of sensory neurons, a few affect afferent excitability through a loss-of-function mechanism. Using whole-exome sequencing, we here identify a rare heterozygous SCN10A missense variant resulting in alteration p.V1287I in NaV1.8 in a patient with a 15-year history of progressively worsening temperature dysregulation in the distal extremities, particularly in the feet. Further symptoms include increasingly intensifying tingling and numbness in the fingers and increased sweating. To assess the impact of p.V1287I on channel function, we performed voltage-clamp recordings demonstrating that the alteration confers loss- and gain-of-function characteristics to NaV1.8 characterized by a right-shifted voltage dependence of channel activation and inactivation. Current-clamp recordings from transfected mouse dorsal root ganglion neurons further revealed that NaV1.8-V1287I channels broaden the action potentials of sensory neurons and increase their firing rates in response to depolarizing current stimulations, indicating a gain-of-function mechanism of the variant at the cellular level in a heterozygous setting. The data support the hypothesis that the properties of NaV1.8 p.V1287I are causative for the patient's symptoms and that nonpainful peripheral paresthesias should be considered part of the clinical spectrum of NaV1.8-associated disorders.

Genetic Profiling of Sodium Channels in Diabetic Painful and Painless and Idiopathic Painful and Painless Neuropathies

Neuropathic pain is a frequent feature of diabetic peripheral neuropathy (DPN) and small fiber neuropathy (SFN). Resolving the genetic architecture of these painful neuropathies will lead to better disease management strategies, counselling and intervention. Our aims were to profile ten sodium channel genes (SCG) expressed in a nociceptive pathway in painful and painless DPN and painful and painless SFN patients, and to provide a perspective for clinicians who assess patients with painful peripheral neuropathy. Between June 2014 and September 2016, 1125 patients with painful-DPN (n = 237), painless-DPN (n = 309), painful-SFN (n = 547) and painless-SFN (n = 32), recruited in four different centers, were analyzed for SCN3A, SCN7A-SCN11A and SCN1B-SCN4B variants by single molecule Molecular inversion probes-Next Generation Sequence. Patients were grouped based on phenotype and the presence of SCG variants. Screening of SCN3A, SCN7A-SCN11A, and SCN1B-SCN4B revealed 125 different (potential) pathogenic variants in 194 patients (17.2%, n = 194/1125). A potential pathogenic variant was present in 18.1% (n = 142/784) of painful neuropathy patients vs. 15.2% (n = 52/341) of painless neuropathy patients (17.3% (n = 41/237) for painful-DPN patients, 14.9% (n = 46/309) for painless-DPN patients, 18.5% (n = 101/547) for painful-SFN patients, and 18.8% (n = 6/32) for painless-SFN patients). Of the variants detected, 70% were in SCN7A, SCN9A, SCN10A and SCN11A. The frequency of SCN9A and SCN11A variants was the highest in painful-SFN patients, SCN7A variants in painful-DPN patients, and SCN10A variants in painless-DPN patients. Our findings suggest that rare SCG genetic variants may contribute to the development of painful neuropathy. Genetic profiling and SCG variant identification should aid in a better understanding of the genetic variability in patients with painful and painless neuropathy, and may lead to better risk stratification and the development of more targeted and personalized pain treatments.

The retinal pigment epithelium displays electrical excitability and lateral signal spreading

BACKGROUND

The non-neuronal retinal pigment epithelium (RPE) functions in intimate association with retinal photoreceptors, performing a multitude of tasks critical for maintaining retinal homeostasis and collaborating with retinal glial cells to provide metabolic support and ionic buffering. Accordingly, the RPE has recently been shown to display dynamic properties mediated by an array of ion channels usually more characteristic of astrocytes and excitable cells. The recent discovery of canonical voltage-activated Na⁺ channels in the RPE and their importance for phagocytosis of photoreceptor outer segments raises a question about their electrogenic function. Here, we performed a detailed electrophysiological analysis related to the functioning of these channels in human embryonic stem cell (hESC)-derived RPE.

RESULTS

Our studies examining the electrical properties of the hESC-RPE revealed that its membrane mainly displays passive properties in a broad voltage range, with the exception of depolarization-induced spikes caused by voltage-activated Na⁺ current (I_Na). Spike amplitude depended on the availability of I_Na and spike kinetics on the membrane time constant, and the spikes could be largely suppressed by TTX. Membrane resistance fluctuated rapidly and strongly, repeatedly changing over the course of recordings and causing closely correlated fluctuations in resting membrane potential. In a minority of cells, we found delayed secondary I_Na-like inward currents characterized by comparatively small amplitudes and slow kinetics, which produced secondary depolarizing spikes. Up to three consecutive delayed inward current waves were detected. These currents could be rapidly and reversibly augmented by applying L-type Ca²⁺ channel blocker nifedipine to diminish influx of calcium and thus increase gap junctional conductance.

CONCLUSIONS

This work shows, for the first time, that I_Na and I_Na-mediated voltage spikes can spread laterally through gap junctions in the monolayer of cells that are traditionally considered non-excitable. Our findings support a potential role of the RPE that goes beyond giving homeostatic support to the retina.

Expanding the genetic causes of small-fiber neuropathy: SCN genes and beyond

Small-fiber neuropathy (SFN) is a disorder that exclusively affects the small nerve fibers, sparing the large nerve fibers. Thinly myelinated Aδ-fibers and unmyelinated C-fibers are damaged, leading to development of neuropathic pain, thermal dysfunction, sensory symptoms, and autonomic disturbances. Although many SFNs are secondary and due to immunological causes or metabolic disturbances, the etiology is unknown in up to half of the patients. Over the years, this proportion of "idiopathic SFN" has decreased, as familial and genetic causes have been discovered, thus shifting a proportion of once "idiopathic" cases to the genetic category. After the discovery of SCN9A-gene variants in 2012, SCN10A and SCN11A variants have been found to be pathogenic in SFN. With improved accessibility of SFN diagnostic tools and genetic tests, many non-SCN variants and genetically inherited systemic diseases involving the small nerve fibers have also been described, but only scattered throughout the literature. There are 80 SCN variants described as causing SFN, 8 genes causing hereditary sensory autonomic neuropathies (HSAN) described with pure SFN, and at least 7 genes involved in genetically inherited systemic diseases associated with SFN. This systematic review aims to consolidate and provide an updated overview on the genetic variants of SFN to date---SCN genes and beyond. Awareness of these genetic causes of SFN is imperative for providing treatment directions, prognostication, and management of expectations for patients and their health-care providers.

Normalization of Neuroinflammation: A New Strategy for Treatment of Persistent Pain and Memory/Emotional Deficits in Chronic Pain

Chronic pain, which affects around 1/3 of the world population and is often comorbid with memory deficit and mood depression, is a leading source of suffering and disability. Studies in past decades have shown that hyperexcitability of primary sensory neurons resulting from abnormal expression of ion channels and central sensitization mediated pathological synaptic plasticity, such as long-term potentiation in spinal dorsal horn, underlie the persistent pain. The memory/emotional deficits are associated with impaired synaptic connectivity in hippocampus. Dysregulation of numerous endogenous proteins including receptors and intracellular signaling molecules is involved in the pathological processes. However, increasing knowledge contributes little to clinical treatment. Emerging evidence has demonstrated that the neuroinflammation, characterized by overproduction of pro-inflammatory cytokines and glial activation, is reliably detected in humans and animals with chronic pain, and is sufficient to induce persistent pain and memory/emotional deficits. The abnormal expression of ion channels and pathological synaptic plasticity in spinal dorsal horn and in hippocampus are resulting from neuroinflammation. The neuroinflammation is initiated and maintained by the interactions of circulating monocytes, glial cells and neurons. Obviously, unlike infectious diseases and cancer, which are caused by pathogens or malignant cells, chronic pain is resulting from alterations of cells and molecules which have numerous physiological functions. Therefore, normalization (counterbalance) but not simple inhibition of the neuroinflammation is the right strategy for treating neuronal disorders. Currently, no such agent is available in clinic. While experimental studies have demonstrated that intracellular Mg²⁺ deficiency is a common feature of chronic pain in animal models and supplement Mg²⁺ are capable of normalizing the neuroinflammation, activation of upregulated proteins that promote recovery, such as translocator protein (18k Da) or liver X receptors, has a similar effect. In this article, relevant experimental and clinical evidence is reviewed and discussed.

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.

The Human SCN9AR185H Point Mutation Induces Pain Hypersensitivity and Spontaneous Pain in Mice

The voltage-gated sodium channel Nav1.7 is encoded by SCN9A gene and plays a critical role in pain sensitivity. Several SCN9A gain-of-function (GOF) mutations have been found in patients with small fiber neuropathy (SFN) having chronic pain, including the R185H mutation. However, for most of these variants, their involvement in pain phenotype still needs to be experimentally elucidated. In order to delineate the impact of R185H mutation on pain sensitivity, we have established the Scn9aR185H mutant mouse model using the CRISPR/Cas9 technology. The Scn9aR185H mutant mice show no cellular alteration in the dorsal root ganglia (DRG) containing cell bodies of sensory neurons and no alteration of growth or global health state. Heterozygous and homozygous animals of both sexes were investigated for pain sensitivity. The mutant mice were more sensitive than the wild-type mice in the tail flick and hot plate tests, acetone, and von Frey tests for sensitivity to heat, cold, and touch, respectively, although with sexual dimorphic effects. The newly developed bioinformatic pipeline, Gdaphen is based on general linear model (GLM) and random forest (RF) classifiers as well as a multifactor analysis of mixed data and shows the qualitative and quantitative variables contributing the most to the pain phenotype. Using Gdaphen, tail flick, Hargreaves, hot plate, acetone, cold plate, and von Frey tests, sex and genotype were found to be contributing most to the pain phenotype. Importantly, the mutant animals displayed spontaneous pain as assessed in the conditioned place preference (CPP) assay. Altogether, our results indicate that Scn9aR185H mice show a pain phenotype, suggesting that the SCN9AR185H mutation identified in patients with SFN having chronic pain contributes to their symptoms. Therefore, we provide genetic evidence for the fact that this mutation in Nav1.7 channel plays an important role in nociception and in the pain experienced by patients with SFN who have this mutation. These findings should aid in exploring further pain treatments based on the Nav1.7 channel.

A phase I, randomized, double-blind, placebo-controlled, single- and multiple dose escalation study evaluating the safety, pharmacokinetics and pharmacodynamics of VX-128, a highly selective Nav 1.8 inhibitor, in healthy adults

Selective inhibition of certain voltage‐gated sodium channels (Na_vs), such as Na_v1.8, is of primary interest for pharmacological pain research and widely studied as a pharmacological target due to its contribution to repetitive firing, neuronal excitability, and pain chronification. VX‐128 is a highly potent and selective Na_v1.8 inhibitor that was being developed as a treatment for pain. We evaluated the safety, tolerability, and pharmacokinetics of VX‐128 in healthy subjects in a single‐ and multiple‐ascending dose (MAD) first‐in‐human study. Pharmacodynamics were evaluated in the MAD part using a battery of evoked pain tests. Overall, single doses of VX‐128 up to 300 mg were well‐tolerated, although adverse effect (AE) incidence was higher in subjects receiving VX‐128 (41.7%) compared with placebo (25.0%). After multiple dosing of up to 10 days, skin rash events were observed at all dose levels (up to 100 mg once daily [q.d.]), in five of 26 (19.2%) subjects, including one subject receiving VX‐128 (100 mg q.d.) who had a serious AE of angioedema. A trend in pain tolerance were observed for cold pressor‐ and pressure pain, which was dose‐dependent for the latter. VX‐128 was rapidly absorbed (median time to maximum plasma concentration between 1 and 2 h) with a half‐life of ~80 h at 10 mg q.d., and approximately two‐fold accumulation ratio after 10 and 30 mg q.d. Although VX‐128, when given in a multiple dose fashion, resulted in early study termination due to tolerability issues, effects were observed on multiple pain tests that may support further investigation of Na_v1.8 inhibitors as pain treatments.

The Human SCN10AG1662S Point Mutation Established in Mice Impacts on Mechanical, Heat, and Cool Sensitivity

The voltage-gated sodium channel NAV1.8 is expressed in primary nociceptive neurons and is involved in pain transmission. Mutations in the SCN10A gene (encoding NAV1.8 channel) have been identified in patients with idiopathic painful small fiber neuropathy (SFN) including the SCN10A^G1662S gain-of-function mutation. However, the role of this mutation in pain sensation remains unknown. We have generated the first mouse model for the G1662S mutation by using homologous recombination in embryonic stem cells. The corresponding Scn10a^G1663S mouse line has been analyzed for Scn10a expression, intraepidermal nerve fiber density (IENFD), and nociception using behavioral tests for thermal and mechanical sensitivity. The Scn10a^G1663S mutants had a similar Scn10a expression level in dorsal root ganglia (DRG) to their wild-type littermates and showed normal IENFD in hindpaw skin. Mutant mice were more sensitive to touch than wild types in the von Frey test. In addition, sexual dimorphism was observed for several pain tests, pointing to the relevance of performing the phenotypical assessment in both sexes. Female homozygous mutants tended to be more sensitive to cooling stimuli in the acetone test. For heat sensitivity, male homozygous mutants showed shorter latencies to radiant heat in the Hargreaves test while homozygous females had longer latencies in the tail flick test. In addition, mutant males displayed a shorter reaction latency on the 54°C hot plate. Collectively, Scn10a^G1663S mutant mice show a moderate but consistent increased sensitivity in behavioral tests of nociception. This altered nociception found in Scn10a^G1663S mice demonstrates that the corresponding G1662 mutation of SCN10A found in SFN patients with pain contributes to their pain symptoms.

A Phase 1, Randomized, Double-Blind, Placebo-Controlled, Crossover Study to Evaluate the Pharmacodynamic Effects of VX-150, a Highly Selective NaV1.8 Inhibitor, in Healthy Male Adults

Objective

To evaluate the analgesic potential, safety, tolerability, and pharmacokinetics of VX-150, a pro-drug of a highly selective Na_V1.8 inhibitor, in healthy subjects.

Design

This was a randomized, double-blind, placebo-controlled, crossover study in healthy subjects.

Subjects

Twenty healthy male subjects with an age of 18–55 years, inclusive, were enrolled. Eligibility was based on general fitness, absence of current or previous medical conditions that could compromise subject safety, and a training assessment of pain tolerance across pain tests to exclude highly tolerant individuals whose tolerance could compromise the ability to detect analgesic responses. All dosed subjects completed the study.

Methods

Subjects were randomized 1:1 to one of two sequences receiving a single VX-150 dose and subsequently placebo, or vice versa, with at least 7 days between dosing. A battery of pain tests (pressure, electrical stair, [capsaicin-induced] heat, and cold pressor) was administered before dosing and repetitively up to 10 h after dosing, with blood sampling up to 24 h after dosing. Safety was monitored throughout the study. Data were analyzed with a repeated-measures mixed-effects model.

Results

VX-150 induced analgesia in a variety of evoked pain tests, without affecting subject safety. Significant effects were reported for the cold pressor and heat pain thresholds. Maximum median concentration for the active moiety was 4.30 µg/mL at 4 h after dosing.

Conclusion

Results of this proof-of-mechanism study are supportive of the potential of VX-150, a highly selective Na_V1.8 channel inhibitor, to treat various pain indications.

A novel gain-of-function sodium channel β2 subunit mutation in idiopathic small fiber neuropathy

Small fiber neuropathy (SFN) is a common condition affecting thinly myelinated Aδ and unmyelinated C fibers, often resulting in excruciating pain and dysautonomia. SFN has been associated with several conditions, but a significant number of cases have no discernible cause. Recent genetic studies have identified potentially pathogenic gain-of-function mutations in several the pore-forming voltage-gated sodium channel α subunits (Na_Vs) in a subset of patients with SFN, but the auxiliary sodium channel β subunits have been less implicated in the development of the disease. β subunits modulate Na_V trafficking and gating, and several mutations have been linked to epilepsy and cardiac dysfunction. Recently, we provided the first evidence for the contribution of a mutation in the β2-subunit to pain in human painful diabetic neuropathy. Here, we provide the first evidence for the involvement of a sodium channel β subunit mutation in the pathogenesis of SFN with no other known causes. We show, through current-clamp analysis, that the newly-identified Y69H variant of the β2 subunit induces neuronal hyperexcitability in dorsal root ganglion neurons, lowering the threshold for action potential firing and allowing for increased repetitive action potential spiking. Underlying the hyperexcitability induced by the β2-Y69H variant, we demonstrate an upregulation in tetrodotoxin-sensitive, but not tetrodotoxin-resistant sodium currents. This provides the first evidence for the involvement of β2 subunits in SFN and strengthens the link between sodium channel β subunits and the development of neuropathic pain in humans.

The Potential Effect of Na v 1.8 in Autism Spectrum Disorder: Evidence From a Congenital Case With Compound Heterozygous SCN10A Mutations

Apart from the most prominent symptoms in Autism spectrum disorder (ASD), namely deficits in social interaction, communication and repetitive behavior, patients often show abnormal sensory reactivity to environmental stimuli. Especially potentially painful stimuli are reported to be experienced in a different way compared to healthy persons. In our present study, we identified an ASD patient carrying compound heterozygous mutations in the voltage-gated sodium channel (VGSC) Na v 1.8, which is preferentially expressed in sensory neurons. We expressed both mutations, p.I1511M and p.R512∗, in a heterologous expression system and investigated their biophysical properties using patch-clamp recordings. The results of these experiments reveal that the p.R512∗ mutation renders the channel non-functional, while the p.I1511M mutation showed only minor effects on the channel's function. Behavioral experiments in a Na v 1.8 loss-of-function mouse model additionally revealed that Na v 1.8 may play a role in autism-like symptomatology. Our results present Na v 1.8 as a protein potentially involved in ASD pathophysiology and may therefore offer new insights into the genetic basis of this disease.

Identification and Characterization of Novel Proteins from Arizona Bark Scorpion Venom That Inhibit Nav1.8, a Voltage-Gated Sodium Channel Regulator of Pain Signaling

The voltage-gated sodium channel Nav1.8 is linked to neuropathic and inflammatory pain, highlighting the potential to serve as a drug target. However, the biophysical mechanisms that regulate Nav1.8 activation and inactivation gating are not completely understood. Progress has been hindered by a lack of biochemical tools for examining Nav1.8 gating mechanisms. Arizona bark scorpion (Centruroides sculpturatus) venom proteins inhibit Nav1.8 and block pain in grasshopper mice (Onychomys torridus). These proteins provide tools for examining Nav1.8 structure–activity relationships. To identify proteins that inhibit Nav1.8 activity, venom samples were fractioned using liquid chromatography (reversed-phase and ion exchange). A recombinant Nav1.8 clone expressed in ND7/23 cells was used to identify subfractions that inhibited Nav1.8 Na⁺ current. Mass-spectrometry-based bottom-up proteomic analyses identified unique peptides from inhibitory subfractions. A search of the peptides against the AZ bark scorpion venom gland transcriptome revealed four novel proteins between 40 and 60% conserved with venom proteins from scorpions in four genera (Centruroides, Parabuthus, Androctonus, and Tityus). Ranging from 63 to 82 amino acids, each primary structure includes eight cysteines and a “CXCE” motif, where X = an aromatic residue (tryptophan, tyrosine, or phenylalanine). Electrophysiology data demonstrated that the inhibitory effects of bioactive subfractions can be removed by hyperpolarizing the channels, suggesting that proteins may function as gating modifiers as opposed to pore blockers.

Growth Differentiation Factor-15 Produces Analgesia by Inhibiting Tetrodotoxin-Resistant Nav1.8 Sodium Channel Activity in Rat Primary Sensory Neurons

Growth differentiation factor 15 (GDF-15) is a member of the transforming growth factor-β superfamily. It is widely distributed in the central and peripheral nervous systems. Whether and how GDF-15 modulates nociceptive signaling remains unclear. Behaviorally, we found that peripheral GDF-15 significantly elevated nociceptive response thresholds to mechanical and thermal stimuli in naïve and arthritic rats. Electrophysiologically, we demonstrated that GDF-15 decreased the excitability of small-diameter dorsal root ganglia (DRG) neurons. Furthermore, GDF-15 concentration-dependently suppressed tetrodotoxin-resistant sodium channel Nav1.8 currents, and shifted the steady-state inactivation curves of Nav1.8 in a hyperpolarizing direction. GDF-15 also reduced window currents and slowed down the recovery rate of Nav1.8 channels, suggesting that GDF-15 accelerated inactivation and slowed recovery of the channel. Immunohistochemistry results showed that activin receptor-like kinase-2 (ALK2) was widely expressed in DRG medium- and small-diameter neurons, and some of them were Nav1.8-positive. Blockade of ALK2 prevented the GDF-15-induced inhibition of Nav1.8 currents and nociceptive behaviors. Inhibition of PKA and ERK, but not PKC, blocked the inhibitory effect of GDF-15 on Nav1.8 currents. These results suggest a functional link between GDF-15 and Nav1.8 in DRG neurons via ALK2 receptors and PKA associated with MEK/ERK, which mediate the peripheral analgesia of GDF-15.

The physiological function of different voltage-gated sodium channels in pain

Evidence from human genetic pain disorders shows that voltage-gated sodium channel α-subtypes Nav1.7, Nav1.8 and Nav1.9 are important in the peripheral signalling of pain. Nav1.7 is of particular interest because individuals with Nav1.7 loss-of-function mutations are congenitally insensitive to acute and chronic pain, and there is considerable hope that phenocopying these effects with a pharmacological antagonist will produce a new class of analgesic drug. However, studies in these rare individuals do not reveal how and where voltage-gated sodium channels contribute to pain signalling, which is of critical importance for drug development. More than a decade of research utilizing rodent genetic models and pharmacological tools to study voltage-gated sodium channels in pain has begun to unravel the role of different subtypes. Here, we review the contribution of individual channel subtypes in three key physiological processes necessary for transmission of sensory information to the CNS: transduction of stimuli at peripheral nerve terminals, axonal transmission of action potentials and neurotransmitter release from central terminals. These data suggest that drugs seeking to recapitulate the analgesic effects of loss of function of Nav1.7 will need to be brain-penetrant - which most of those developed to date are not.

Pain behavior in SCN9A (Nav1.7) and SCN10A (Nav1.8) mutant rodent models

The two voltage gated sodium channels Nav1.7 and Nav1.8 are expressed in the peripheral nervous system and involved in various pain conditions including inflammatory and neuropathic pain. Rodent models bearing deletions or mutations of the corresponding genes, Scn9a and Scn10a, were created in order to understand the role of these channels in the pathophysiological mechanism underlying pain symptoms. This review summarizes the pain behavior profiles reported in Scn9a and Scn10a rodent models. The complete loss-of-function or knockout (KO) of Scn9a or Scn10a and the conditional KO (cKO) of Scn9a in specific cell populations were shown to decrease sensitivity to various pain stimuli. The Possum mutant mice bearing a dominant hypermorphic mutation in Scn10a revealed higher sensitivity to noxious stimuli. Several gain-of-function mutations were identified in patients with painful small fiber neuropathy. Future knowledge obtained from preclinical models bearing these mutations will allow understanding how these mutations affect pain. In addition, the review gives perspectives for creating models that better mimic patients' pain symptoms in view to developing novel analgesic strategies.

Predicting functional effects of missense variants in voltage-gated sodium and calcium channels

Malfunctions of voltage-gated sodium and calcium channels (encoded by SCNxA and CACNA1x family genes, respectively) have been associated with severe neurologic, psychiatric, cardiac, and other diseases. Altered channel activity is frequently grouped into gain or loss of ion channel function (GOF or LOF, respectively) that often corresponds not only to clinical disease manifestations but also to differences in drug response. Experimental studies of channel function are therefore important, but laborious and usually focus only on a few variants at a time. On the basis of known gene-disease mechanisms of 19 different diseases, we inferred LOF (n = 518) and GOF (n = 309) likely pathogenic variants from the disease phenotypes of variant carriers. By training a machine learning model on sequence- and structure-based features, we predicted LOF or GOF effects [area under the receiver operating characteristics curve (ROC) = 0.85] of likely pathogenic missense variants. Our LOF versus GOF prediction corresponded to molecular LOF versus GOF effects for 87 functionally tested variants in SCN1/2/8A and CACNA1I (ROC = 0.73) and was validated in exome-wide data from 21,703 cases and 128,957 controls. We showed respective regional clustering of inferred LOF and GOF nucleotide variants across the alignment of the entire gene family, suggesting shared pathomechanisms in the SCNxA/CACNA1x family genes.

Social defeat stress-induced hyperalgesia is mediated by nav 1.8+ nociceptive fibers

Recently the voltage-gated sodium (Nav) channels began to be studied as possible targets for analgesic drugs. In addition, specific Nav 1.8 blockers are currently being used to treat some types of chronic pain pathologies such as neuropathies and fibromyalgia. Nav 1.8⁺ fibers convey nociceptive information to brain structures belonging to the limbic system, which is involved in the pathophysiology of major depressive disorders. From this, using a model of chronic social defeat stress (SDS) and intrathecal injections of Nav 1.8 antisense, this study investigated the possible involvement of Nav 1.8⁺ nociceptive fibers in SDS- induced hyperalgesia in C57/BL mice. Our results showed that SDS induced a depressive-like behavior of social avoidance and increased the sensitivity to mechanical (electronic von Frey test) and chemical (capsaicin test) nociceptive stimuli. We also showed that intrathecal injection of Nav 1.8 antisense reversed the SDS-induced hyperalgesia as demonstrated by both, mechanical and chemical nociceptive tests. We confirmed the antisense efficacy and specificity in a separate no-defeated cohort through real-time PCR, which showed a significant reduction of Nav 1.8 mRNA and no reduction of Nav 1.7 and Nav 1.9 in the L4, L5 and L6 dorsal root ganglia (DRG). The present study advances the understanding of SDS-induced hyperalgesia, which seems to be dependent on Nav 1.8⁺ nociceptive fibers.

A gain-of-function sodium channel β2-subunit mutation in painful diabetic neuropathy

Diabetes mellitus is a global challenge with many diverse health sequelae, of which diabetic peripheral neuropathy is one of the most common. A substantial number of patients with diabetic peripheral neuropathy develop chronic pain, but the genetic and epigenetic factors that predispose diabetic peripheral neuropathy patients to develop neuropathic pain are poorly understood. Recent targeted genetic studies have identified mutations in α-subunits of voltage-gated sodium channels (Na_vs) in patients with painful diabetic peripheral neuropathy. Mutations in proteins that regulate trafficking or functional properties of Na_vs could expand the spectrum of patients with Na_v-related peripheral neuropathies. The auxiliary sodium channel β-subunits (β1–4) have been reported to increase current density, alter inactivation kinetics, and modulate subcellular localization of Na_v. Mutations in β-subunits have been associated with several diseases, including epilepsy, cancer, and diseases of the cardiac conducting system. However, mutations in β-subunits have never been shown previously to contribute to neuropathic pain. We report here a patient with painful diabetic peripheral neuropathy and negative genetic screening for mutations in SCN9A, SCN10A, and SCN11A—genes encoding sodium channel α-subunit that have been previously linked to the development of neuropathic pain. Genetic analysis revealed an aspartic acid to asparagine mutation, D109N, in the β2-subunit. Functional analysis using current-clamp revealed that the β2-D109N rendered dorsal root ganglion neurons hyperexcitable, especially in response to repetitive stimulation. Underlying the hyperexcitability induced by the β2-subunit mutation, as evidenced by voltage-clamp analysis, we found a depolarizing shift in the voltage dependence of Na_v1.7 fast inactivation and reduced use-dependent inhibition of the Na_v1.7 channel.

Voltage gated sodium channels as therapeutic targets for chronic pain

Being maladaptive and frequently unresponsive to pharmacotherapy, chronic pain presents a major unmet clinical need. While an intact central nervous system is required for conscious pain perception, nociceptor hyperexcitability induced by nerve injury in the peripheral nervous system (PNS) is sufficient and necessary to initiate and maintain neuropathic pain. The genesis and propagation of action potentials is dependent on voltage-gated sodium channels, in particular, Nav1.7, Nav1.8 and Nav1.9. However, nerve injury triggers changes in their distribution, expression and/or biophysical properties, leading to aberrant excitability. Most existing treatment for pain relief acts through non-selective, state-dependent sodium channel blockage and have narrow therapeutic windows. Natural toxins and developing subtype-specific and molecular-specific sodium channel blockers show promise for treatment of neuropathic pain with minimal side effects. New approaches to analgesia include combination therapy and gene therapy. Here, we review how individual sodium channel subtypes contribute to pain, and the attempts made to develop more effective analgesics for the treatment of chronic pain.

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.

Yield of peripheral sodium channels gene screening in pure small fibre neuropathy

BACKGROUND

Neuropathic pain is common in peripheral neuropathy. Recent genetic studies have linked pathogenic voltage-gated sodium channel (VGSC) variants to human pain disorders. Our aims are to determine the frequency of SCN9A, SCN10A and SCN11A variants in patients with pure small fibre neuropathy (SFN), analyse their clinical features and provide a rationale for genetic screening.

METHODS

Between September 2009 and January 2017, 1139 patients diagnosed with pure SFN at our reference centre were screened for SCN9A, SCN10A and SCN11A variants. Pathogenicity of variants was classified according to established guidelines of the Association for Clinical Genetic Science and frequencies were determined. Patients with SFN were grouped according to the VGSC variants detected, and clinical features were compared.

RESULTS

Among 1139 patients with SFN, 132 (11.6%) patients harboured 73 different (potentially) pathogenic VGSC variants, of which 50 were novel and 22 were found in ≥ 1 patient. The frequency of (potentially) pathogenic variants was 5.1% (n=58/1139) for SCN9A, 3.7% (n=42/1139) for SCN10A and 2.9% (n=33/1139) for SCN11A. Only erythromelalgia-like symptoms and warmth-induced pain were significantly more common in patients harbouring VGSC variants.

CONCLUSION

(Potentially) pathogenic VGSC variants are present in 11.6% of patients with pure SFN. Therefore, genetic screening of SCN9A, SCN10A and SCN11A should be considered in patients with pure SFN, independently of clinical features or underlying conditions.

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.

Increased Resurgent Sodium Currents in Nav1.8 Contribute to Nociceptive Sensory Neuron Hyperexcitability Associated with Peripheral Neuropathies

Neuropathic pain is a significant public health challenge, yet the underlying mechanisms remain poorly understood. Painful small fiber neuropathy (SFN) may be caused by gain-of-function mutations in Nav1.8, a sodium channel subtype predominantly expressed in peripheral nociceptive neurons. However, it is not clear how Nav1.8 disease mutations induce sensory neuron hyperexcitability. Here we studied two mutations in Nav1.8 associated with hypersensitive sensory neurons: G1662S reported in painful SFN; and T790A, which underlies increased pain behaviors in the Possum transgenic mouse strain. We show that, in male DRG neurons, these mutations, which impair inactivation, significantly increase TTX-resistant resurgent sodium currents mediated by Nav1.8. The G1662S mutation doubled resurgent currents, and the T790A mutation increased them fourfold. These unusual currents are typically evoked during the repolarization phase of action potentials. We show that the T790A mutation greatly enhances DRG neuron excitability by reducing current threshold and increasing firing frequency. Interestingly, the mutation endows DRG neurons with multiple early afterdepolarizations and leads to substantial prolongation of action potential duration. In DRG neurons, siRNA knockdown of sodium channel β4 subunits fails to significantly alter T790A current density but reduces TTX-resistant resurgent currents by 56%. Furthermore, DRG neurons expressing T790A channels exhibited reduced excitability with fewer early afterdepolarizations and narrower action potentials after β4 knockdown. Together, our data demonstrate that open-channel block of TTX-resistant currents, enhanced by gain-of-function mutations in Nav1.8, can make major contributions to the hyperexcitability of nociceptive neurons, likely leading to altered sensory phenotypes including neuropathic pain in SFN.SIGNIFICANCE STATEMENT This work demonstrates that two disease mutations in the voltage-gated sodium channel Na_v1.8 that induce nociceptor hyperexcitability increase resurgent currents. Nav1.8 is crucial for pain sensations. Because resurgent currents are evoked during action potential repolarization, they can be crucial regulators of action potential activity. Our data indicate that increased Nav1.8 resurgent currents in DRG neurons greatly prolong action potential duration and enhance repetitive firing. We propose that Nav1.8 open-channel block is a major factor in Nav1.8-associated pain mechanisms and that targeting the molecular mechanism underlying these unique resurgent currents represents a novel therapeutic target for the treatment of aberrant pain sensations.

Neurophysiological Effects of Dorsal Root Ganglion Stimulation (DRGS) in Pain Processing at the Cortical Level

OBJECTIVES

Dorsal root ganglion stimulation (DRGS) has been used successfully against localized neuropathic pain. Nevertheless, the effects of DRGS on pain processing, particularly at the cortical level, remain largely unknown. In this study, we investigated whether positive responses to DRGS treatment would alter patients' laser-evoked potentials (LEP).

METHODS

We prospectively enrolled 12 adult patients with unilateral localized neuropathic pain in the lower limbs or inguinal region and followed them up for six months. LEPs were assessed at baseline, after one month of DRGS, and after six months of DRGS. Clinical assessment included the Numerical Rating Scale (NRS), Brief Pain Inventory (BPI), SF-36, and Beck Depression Inventory (BDI). For each patient, LEP amplitudes and latencies of the N2 and P2 components on the deafferented side were measured and compared to those of the healthy side and correlated with pain intensity, as measured with the NRS.

RESULTS

At the one- and six-month follow-ups, N2-P2 amplitudes were significantly greater and NRS scores were significantly lower compared with baseline (all p's < 0.01). There was a negative correlation between LEP amplitudes and NRS scores (r_s = -0.31, p < 0.10).

CONCLUSIONS

DRGS is able to restore LEPs to normal values in patients with localized neuropathic pain, and LEP alterations are correlated with clinical response in terms of pain intensity.

The Novel Activity of Carbamazepine as an Activation Modulator Extends from NaV1.7 Mutations to the NaV1.8-S242T Mutant Channel from a Patient with Painful Diabetic Neuropathy

Neuropathic pain in patients carrying sodium channel gain-of-function mutations is generally refractory to pharmacotherapy. However, we have shown that pretreatment of cells with clinically achievable concentration of carbamazepine (CBZ; 30 μM) depolarizes the voltage dependence of activation in some Na_V1.7 mutations such as S241T, a novel CBZ mode of action of this drug. CBZ reduces the excitability of dorsal root ganglion (DRG) neurons expressing Na_V1.7-S241T mutant channels, and individuals carrying the S241T mutation respond to treatment with CBZ. Whether the novel activation-modulating activity of CBZ is specific to Na_V1.7, and whether this pharmacogenomic approach can be extended to other sodium channel subtypes, are not known. We report here the novel Na_V1.8-S242T mutation, which corresponds to the Na_V1.7-S241T mutation, in a patient with neuropathic pain and diabetic peripheral neuropathy. Voltage-clamp recordings demonstrated hyperpolarized and accelerated activation of Na_V1.8-S242T. Current-clamp recordings showed that Na_V1.8-S242T channels render DRG neurons hyperexcitable. Structural modeling shows that despite a substantial difference in the primary amino acid sequence of Na_V1.7 and Na_V1.8, the S242 (Na_V1.8) and S241 (Na_V1.7) residues have similar position and orientation in the domain I S4-S5 linker of the channel. Pretreatment with a clinically achievable concentration of CBZ corrected the voltage dependence of activation of Na_V1.8-S242T channels and reduced DRG neuron excitability as predicted from our pharmacogenomic model. These findings extend the novel activation modulation mode of action of CBZ to a second sodium channel subtype, Na_V1.8.

Loss-of-function of Nav1.8/D1639N linked to human pain can be rescued by lidocaine

Mutations in voltage-gated sodium channels are associated with altered pain perception in humans. Most of these mutations studied to date present with a direct and intuitive link between the altered electrophysiological function of the channel and the phenotype of the patient. In this study, we characterize a variant of Nav1.8, D1639N, which has been previously identified in a patient suffering from the chronic pain syndrome "small fiber neuropathy". Using a heterologous expression system and patch-clamp analysis, we show that Nav1.8/D1639N reduces current density without altering biophysical gating properties of Nav1.8. Therefore, the D1639N variant causes a loss-of-function of the Nav1.8 sodium channel in a patient suffering from chronic pain. Using immunocytochemistry and biochemical approaches, we show that Nav1.8/D1639N impairs trafficking of the channel to the cell membrane. Neither co-expression of β1 or β3 subunit, nor overnight incubation at 27 °C rescued current density of the D1639N variant. On the other hand, overnight incubation with lidocaine fully restored current density of Nav1.8/D1639N most likely by overcoming the trafficking defect, whereas phenytoin failed to do so. Since lidocaine rescues the loss-of-function of Nav1.8/D1639N, it may offer a future therapeutic option for the patient carrying this variant. These results demonstrate that the D1639N variant, identified in a patient suffering from chronic pain, causes loss-of-function of the channel due to impaired cell surface trafficking and that this trafficking defect can be rescued by lidocaine.

Loss-of-function mutations of SCN10A encoding NaV1.8 α subunit of voltage-gated sodium channel in patients with human kidney stone disease

Human kidney stone disease (KSD) causes significant morbidity and public health burden worldwide. The etiology of KSD is heterogeneous, ranging from monogenic defects to complex interaction between genetic and environmental factors. However, the genetic defects causing KSD in the majority of affected families are still unknown. Here, we report the discovery of mutations of SCN10A, encoding Na_V1.8 α subunit of voltage-gated sodium channel, in families with KSD. The region on chromosome 3 where SCN10A locates was initially identified in a large family with KSD by genome-wide linkage analysis and exome sequencing. Two mutations (p.N909K and p.K1809R) in the same allele of SCN10A co-segregated with KSD in the affected family. Additional mutation (p.V1149M) of SCN10A was identified in another affected family, strongly supporting the causal role of SCN10A for KSD. The amino acids at these three positions, N909, K1809, and V1149, are highly conserved in vertebrate evolution, indicating their structural and functional significances. Na_V1.8 α subunit mRNA and protein were found to express in human kidney tissues. The mutant proteins expressed in cultured cells were unstable and causing reduced current density as analyzed by whole-cell patch-clamp technique. Thus, loss-of-function mutations of SCN10A were associated with KSD in the families studied.

Tuning in C-nociceptors to reveal mechanisms in chronic neuropathic pain

OBJECTIVE

Develop and validate a low-intensity sinusoidal electrical stimulation paradigm to preferentially activate C-fibers in human skin.

METHODS

Sinusoidal transcutaneous stimulation (4Hz) was assessed psychophysically in healthy volunteers (n = 14) and neuropathic pain patients (n = 9). Pursuing laser Doppler imaging and single nociceptor recordings in vivo in humans (microneurography) and pigs confirmed the activation of "silent" C-nociceptors. Synchronized C-fiber compound action potentials were evoked in isolated human nerve fascicles in vitro. Live cell imaging of L4 dorsal root ganglia in anesthetized mice verified the recruitment of small-diameter neurons during transcutaneous 4-Hz stimulation of the hindpaw (0.4mA).

RESULTS

Transcutaneous sinusoidal current (0.05-0.4mA, 4Hz) activated "polymodal" C-fibers (50% at ∼0.03mA) and "silent" nociceptors (50% at ∼0.04mA), intensities substantially lower than that required with transcutaneous 1-ms rectangular pulses ("polymodal" ∼3mA, "silent" ∼50mA). The stimulation induced delayed burning (nonpulsating) pain and a pronounced axon-reflex erythema, both indicative of C-nociceptor activation. Pain ratings to repetitive stimulation (1 minute, 4Hz) adapted in healthy volunteers by Numeric Rating Scale (NRS) -3 and nonpainful skin sites of neuropathic pain patients by NRS -0.5, whereas pain even increased in painful neuropathic skin by approximately NRS +2.

INTERPRETATION

Sinusoidal electrical stimulation at 4Hz enables preferential activation of C-nociceptors in pig and human skin that accommodates during ongoing (1-minute) stimulation. Absence of such accommodation in neuropathic pain patients suggest axonal hyperexcitability that could be predictive of alterations in peripheral nociceptor encoding and offer a potential therapeutic entry point for topical analgesic treatment. Ann Neurol 2018;83:945-957.

Novel Sodium Channel Inhibitor From Leeches

Considering blood-sucking habits of leeches from surviving strategy of view, it can be hypothesized that leech saliva has analgesia or anesthesia functions for leeches to stay undetected by the host. However, no specific substance with analgesic function has been reported from leech saliva although clinical applications strongly indicated that leech therapy produces a strong and long lasting pain-reducing effect. Herein, a novel family of small peptides (HSTXs) including 11 members which show low similarity with known peptides was identified from salivary glands of the leech Haemadipsa sylvestris. A typical HSTX is composed of 22-25 amino acid residues including four half-cysteines, forming two intra-molecular disulfide bridges, and an amidated C-terminus. HSTX-I exerts significant analgesic function by specifically inhibiting voltage-gated sodium (Na_V) channels (Na_V1.8 and Na_V1.9) which contribute to action potential electrogenesis in neurons and potential targets to develop analgesics. This study reveals that sodium channel inhibitors are analgesic substances in the leech. HSTXs are excellent candidates or templates for development of analgesics.

A novel gain-of-function Nav1.7 mutation in a carbamazepine-responsive patient with adult-onset painful peripheral neuropathy

Voltage-gated sodium channel Nav1.7 is a threshold channel in peripheral dorsal root ganglion (DRG), trigeminal ganglion, and sympathetic ganglion neurons. Gain-of-function mutations in Nav1.7 have been shown to increase excitability in DRG neurons and have been linked to rare Mendelian and more common pain disorders. Discovery of Nav1.7 variants in patients with pain disorders may expand the spectrum of painful peripheral neuropathies associated with a well-defined molecular target, thereby providing a basis for more targeted approaches for treatment. We screened the genome of a patient with adult-onset painful peripheral neuropathy characterized by severe burning pain and report here the new Nav1.7-V810M variant. Voltage-clamp recordings were used to assess the effects of the mutation on biophysical properties of Nav1.7 and the response of the mutant channel to treatment with carbamazepine (CBZ), and multi-electrode array (MEA) recordings were used to assess the effects of the mutation on the excitability of neonatal rat pup DRG neurons. The V810M variant increases current density, shifts activation in a hyperpolarizing direction, and slows kinetics of deactivation, all gain-of-function attributes. We also show that DRG neurons that express the V810M variant become hyperexcitable. The patient responded to treatment with CBZ. Although CBZ did not depolarize activation of the mutant channel, it enhanced use-dependent inhibition. Our results demonstrate the presence of a novel gain-of-function variant of Nav1.7 in a patient with adult-onset painful peripheral neuropathy and the responsiveness of that patient to treatment with CBZ, which is likely due to the classical mechanism of use-dependent inhibition.

NaV channel variants in patients with painful and nonpainful peripheral neuropathy

Objective:

To examine the incidence of nonsynonymous missense variants in SCN9A (Na_V1.7), SCN10A (Na_V1.8), and SCN11A (Na_V1.9) in patients with painful and nonpainful peripheral neuropathy.

Methods:

Next-generation sequencing was performed on 457 patient DNA samples provided by the Peripheral Neuropathy Research Registry (PNRR). The patient diagnosis was as follows: 278 idiopathic peripheral neuropathy (67% painful and 33% nonpainful) and 179 diabetic distal polyneuropathy (77% painful and 23% nonpainful).

Results:

We identified 36 (SCN9A), 31 (SCN10A), and 15 (SCN11A) nonsynonymous missense variants, with 47.7% of patients carrying a low-frequency (minor allele frequency <5%) missense variant in at least 1 gene. The incidence of previously reported gain-of-function missense variants was low (≤3%), and these were detected in patients with and without pain. There were no significant differences in missense variant allele frequencies of any gene, or SCN9A haplotype frequencies, between PNRR patients with painful or nonpainful peripheral neuropathy. PNRR patient SCN9A and SCN11A missense variant allele frequencies were not significantly different from the Exome Variant Server, European American (EVS-EA) reference population. For SCN10A, there was a significant increase in the alternate allele frequency of the common variant p.V1073A and low-frequency variant pS509P in PNRR patients compared with EVS-EA and the 1000 Genomes European reference populations.

Conclusions:

These results suggest that identification of a genetically defined subpopulation for testing of Na_V1.7 inhibitors in patients with peripheral neuropathy is unlikely and that additional factors, beyond expression of previously reported disease “mutations,” are more important for the development of painful neuropathy than previously discussed.

Ambroxol for the treatment of fibromyalgia: science or fiction?

Fibromyalgia appears to present in subgroups with regard to biological pain induction, with primarily inflammatory, neuropathic/neurodegenerative, sympathetic, oxidative, nitrosative, or muscular factors and/or central sensitization. Recent research has also discussed glial activation or interrupted dopaminergic neurotransmission, as well as increased skin mast cells and mitochondrial dysfunction. Therapy is difficult, and the treatment options used so far mostly just have the potential to address only one of these aspects. As ambroxol addresses all of them in a single substance and furthermore also reduces visceral hypersensitivity, in fibromyalgia existing as irritable bowel syndrome or chronic bladder pain, it should be systematically investigated for this purpose. Encouraged by first clinical observations of two working groups using topical or oral ambroxol for fibromyalgia treatments, the present paper outlines the scientific argument for this approach by looking at each of the aforementioned aspects of this complex disease and summarizes putative modes of action of ambroxol. Nevertheless, at this point the evidence basis for ambroxol is not strong enough for clinical recommendation.

Structure-based assessment of disease-related mutations in human voltage-gated sodium channels

Voltage-gated sodium (Na_v) channels are essential for the rapid upstroke of action potentials and the propagation of electrical signals in nerves and muscles. Defects of Na_v channels are associated with a variety of channelopathies. More than 1000 disease-related mutations have been identified in Na_v channels, with Na_v1.1 and Na_v1.5 each harboring more than 400 mutations. Na_v channels represent major targets for a wide array of neurotoxins and drugs. Atomic structures of Na_v channels are required to understand their function and disease mechanisms. The recently determined atomic structure of the rabbit voltage-gated calcium (Ca_v) channel Ca_v1.1 provides a template for homology-based structural modeling of the evolutionarily related Na_v channels. In this Resource article, we summarized all the reported disease-related mutations in human Na_v channels, generated a homologous model of human Na_v1.7, and structurally mapped disease-associated mutations. Before the determination of structures of human Na_v channels, the analysis presented here serves as the base framework for mechanistic investigation of Na_v channelopathies and for potential structure-based drug discovery.

Sodium Channels and Venom Peptide Pharmacology

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

A painful neuropathy-associated Nav1.7 mutant leads to time-dependent degeneration of small-diameter axons associated with intracellular Ca2+ dysregulation and decrease in ATP levels

Small fiber neuropathy is a painful sensory nervous system disorder characterized by damage to unmyelinated C- and thinly myelinated Aδ- nerve fibers, clinically manifested by burning pain in the distal extremities and dysautonomia. The clinical onset in adulthood suggests a time-dependent process. The mechanisms that underlie nerve fiber injury in small fiber neuropathy are incompletely understood, although roles for energetic stress have been suggested. In the present study, we report time-dependent degeneration of neurites from dorsal root ganglia neurons in culture expressing small fiber neuropathy-associated G856D mutant Nav1.7 channels and demonstrate a time-dependent increase in intracellular calcium levels [Ca2+]i and reactive oxygen species, together with a decrease in ATP levels. Together with a previous clinical report of burning pain in the feet and hands associated with reduced levels of Na+/K+-ATPase in humans with high altitude sickness, the present results link energetic stress and reactive oxygen species production with the development of a painful neuropathy that preferentially affects small-diameter axons.

SCN10A Mutation in a Patient with Erythromelalgia Enhances C-Fiber Activity Dependent Slowing

Gain-of-function mutations in the tetrodotoxin (TTX) sensitive voltage-gated sodium channel (Nav) Nav1.7 have been identified as a key mechanism underlying chronic pain in inherited erythromelalgia. Mutations in TTX resistant channels, such as Nav1.8 or Nav1.9, were recently connected with inherited chronic pain syndromes. Here, we investigated the effects of the p.M650K mutation in Nav1.8 in a 53 year old patient with erythromelalgia by microneurography and patch-clamp techniques. Recordings of the patient’s peripheral nerve fibers showed increased activity dependent slowing (ADS) in CMi and less spontaneous firing compared to a control group of erythromelalgia patients without Nav mutations. To evaluate the impact of the p.M650K mutation on neuronal firing and channel gating, we performed current and voltage-clamp recordings on transfected sensory neurons (DRGs) and neuroblastoma cells. The p.M650K mutation shifted steady-state fast inactivation of Nav1.8 to more hyperpolarized potentials and did not significantly alter any other tested gating behaviors. The AP half-width was significantly broader and the stimulated action potential firing rate was reduced for M650K transfected DRGs compared to WT. We discuss the potential link between enhanced steady state fast inactivation, broader action potential width and the potential physiological consequences.

A SCN10A SNP biases human pain sensitivity

Background:

Nav1.8 sodium channels, encoded by SCN10A, are preferentially expressed in nociceptive neurons and play an important role in human pain. Although rare gain-of-function variants in SCN10A have been identified in individuals with painful peripheral neuropathies, whether more common variants in SCN10A can have an effect at the channel level and at the dorsal root ganglion, neuronal level leading to a pain disorder or an altered normal pain threshold has not been determined.

Results:

Candidate single nucleotide polymorphism association approach together with experimental pain testing in human subjects was used to explore possible common SCN10A missense variants that might affect human pain sensitivity. We demonstrated an association between rs6795970 (G > A; p.Ala1073Val) and higher thresholds for mechanical pain in a discovery cohort (496 subjects) and confirmed it in a larger replication cohort (1005 female subjects). Functional assessments showed that although the minor allele shifts channel activation by −4.3 mV, a proexcitatory attribute, it accelerates inactivation, an antiexcitatory attribute, with the net effect being reduced repetitive firing of dorsal root ganglion neurons, consistent with lower mechanical pain sensitivity.

Conclusions:

At the association and mechanistic levels, the SCN10A single nucleotide polymorphism rs6795970 biases human pain sensitivity.

Advanced Genetic Testing Comes to the Pain Clinic to Make a Diagnosis of Paroxysmal Extreme Pain Disorder

Objective. To describe the use of an advanced genetic testing technique, whole exome sequencing, to diagnose a patient and their family with a SCN9A channelopathy. Setting. Academic tertiary care center. Design. Case report. Case Report. A 61-year-old female with a history of acute facial pain, chronic pain, fibromyalgia, and constipation was found to have a gain of function SCN9A mutation by whole exome sequencing. This mutation resulted in an SCN9A channelopathy that is most consistent with a diagnosis of paroxysmal extreme pain disorder. In addition to the patient being diagnosed, four siblings have a clinical diagnosis of SCN9A channelopathy as they have consistent symptoms and a sister with a known mutation. For treatment, gabapentin was ineffective and carbamazepine was not tolerated. Nontraditional therapies improved symptoms and constipation resolved with pelvic floor retraining with biofeedback. Conclusion. Patients with a personal and family history of chronic pain may benefit from a referral to Medical Genetics. Pelvic floor retraining with biofeedback should be considered for patients with a SCN9A channelopathy and constipation.

Physiology and Pathophysiology of Sodium Channel Inactivation

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

Infantile Pain Episodes Associated with Novel Nav1.9 Mutations in Familial Episodic Pain Syndrome in Japanese Families

Painful peripheral neuropathy has been correlated with various voltage-gated sodium channel mutations in sensory neurons. Recently Nav1.9, a voltage-gated sodium channel subtype, has been established as a genetic influence for certain peripheral pain syndromes. In this study, we performed a genetic study in six unrelated multigenerational Japanese families with episodic pain syndrome. Affected participants (n = 23) were characterized by infantile recurrent pain episodes with spontaneous mitigation around adolescence. This unique phenotype was inherited in an autosomal-dominant mode. Linkage analysis was performed for two families with 12 affected and nine unaffected members, and a single locus was identified on 3p22 (LOD score 4.32). Exome analysis (n = 14) was performed for affected and unaffected members in these two families and an additional family. Two missense variants were identified: R222H and R222S in SCN11A. Next, we generated a knock-in mouse model harboring one of the mutations (R222S). Behavioral tests (Hargreaves test and cold plate test) using R222S and wild-type C57BL/6 (WT) mice, young (8-9 weeks old; n = 10-12 for each group) and mature (36-38 weeks old; n = 5-6 for each group), showed that R222S mice were significantly (p < 0.05) more hypersensitive to hot and cold stimuli than WT mice. Electrophysiological studies using dorsal root ganglion neurons from 8-9-week-old mice showed no significant difference in resting membrane potential, but input impedance and firing frequency of evoked action potentials were significantly increased in R222S mice compared with WT mice. However, there was no significant difference among Nav1.9 (WT, R222S, and R222H)-overexpressing ND7/23 cell lines. These results suggest that our novel mutation is a gain-of-function mutation that causes infantile familial episodic pain. The mouse model developed here will be useful for drug screening for familial episodic pain syndrome associated with SCN11A mutations.