Mutations in SCN9A, encoding a sodium channel alpha subunit, in patients with primary erythermalgia

Sensory neuron sodium channels as pain targets; from cocaine to Journavx (VX-548, suzetrigine)

Voltage-gated sodium channels underpin electrical signaling in sensory neurons. Their activity is an essential element in the vast majority of pain conditions, making them significant drug targets. Sensory neuron sodium channels play roles not only in afferent signaling but also in a range of efferent regulatory mechanisms. Side effects through actions on other cell types and efferent signaling are thus important issues to address during analgesic drug development. As an example, the human genetic evidence for NaV1.7 as an ideal pain target contrasts with the side effects of NaV1.7 antagonists. In this review, we describe the history and progress toward the development of useful analgesic drugs and the renewed focus on NaV1.8 as a key target in pain treatment. NaV1.8 antagonists alone or in combination with other analgesics are likely to provide new opportunities for pain relief for the vast number of people (about 33% of the population) impacted by chronic pain, particularly present in aging populations.

SCN9A should not be considered an epilepsy gene; Refuting a gene-disease association

OBJECTIVE

The SCN9A gene is primarily expressed in nociceptive pathways within the peripheral nervous system, and pathogenic variants are associated with human pain disorders. In recent years, several studies have proposed SCN9A as a monogenic cause of epilepsy. Our objective was to critically appraise the SCN9A-epilepsy gene-disease relationship.

METHODS

We assessed "epilepsy-associated" SCN9A variants from four sources: (1) the literature up to December 2023 (n = 27), (2) epilepsy patients referred for genetic testing at a regional service in Glasgow, UK over a 5-year period (n = 30), (3) the Human Genetics Mutation Database (n = 25), and (4) ClinVar (n = 1546). The latter two are genome-wide variant databases, accepting submissions from genetic laboratories and research groups. We checked whether each SCN9A variant is present in the Genome Aggregation Database (gnomAD) V4 (a reference population database for variant interpretation), and classified its pathogenicity based on the American College of Molecular Genetics and Genomics/Association of Molecular Pathologists guidelines.

RESULTS

Only three SCN9A variants were classified as "likely pathogenic," of which two were identified in healthy individuals in gnomAD. A total of 1540 of the 1546 SCN9A variants in ClinVar labeled as being associated with epilepsy were also reported in association with hereditary sensory and autonomic neuropathy. No further clinical data were provided in 1482 of these submissions.

SIGNIFICANCE

There is no convincing genetic evidence to support SCN9A as a causative epilepsy gene. As such, the inclusion of SCN9A in epilepsy genetic testing panels should be reassessed. Research centers and genetic testing laboratories should be rigorous and consistent in their submissions to variant databases.

Genetic associations of neuropathic pain and sensory profile in a deeply phenotyped neuropathy cohort

ABSTRACT

We aimed to investigate the genetic associations of neuropathic pain in a deeply phenotyped cohort. Participants with neuropathic pain were cases and compared with those exposed to injury or disease but without neuropathic pain as control subjects. Diabetic polyneuropathy was the most common aetiology of neuropathic pain. A standardised quantitative sensory testing protocol was used to categorize participants based on sensory profile. We performed genome-wide association study, and in a subset of participants, we undertook whole-exome sequencing targeting analyses of 45 known pain-related genes. In the genome-wide association study of diabetic neuropathy (N = 1541), a top significant association was found at the KCNT2 locus linked with pain intensity (rs114159097, P = 3.55 × 10 -8 ). Gene-based analysis revealed significant associations between LHX8 and TCF7L2 and neuropathic pain. Polygenic risk score for depression was associated with neuropathic pain in all participants. Polygenic risk score for C-reactive protein showed a positive association, while that for fasting insulin showed a negative association with neuropathic pain, in individuals with diabetic polyneuropathy. Gene burden analysis of candidate pain genes supported significant associations between rare variants in SCN9A and OPRM1 and neuropathic pain. Comparison of individuals with the "irritable" nociceptor profile to those with a "nonirritable" nociceptor profile identified a significantly associated variant (rs72669682, P = 4.39 × 10 -8 ) within the ANK2 gene. Our study on a deeply phenotyped cohort with neuropathic pain has confirmed genetic associations with the known pain-related genes KCNT2 , OPRM1 , and SCN9A and identified novel associations with LHX8 and ANK2 , genes not previously linked to pain and sensory profiles, respectively.

Comparative Efficacy and Tolerability of Treatments for Erythromelalgia: A Systematic Review

Background and Objectives: Erythromelalgia (EM) is an uncommon condition marked by recurring redness, intense burning sensations, and elevated limb warmth. This syndrome can be significantly debilitating, and finding effective treatment options often proves to be quite difficult. The symptoms can severely impact the quality of life of those affected, resulting in considerable disability. This systematic review aims to compare available medical treatments for EM by evaluating their efficacy and safety. Materials and Methods: Following PRISMA guidelines, the search included the PubMed, Medline, and Web of Science databases, using the keywords ("Erythromelalgia" OR "Mitchell's Disease") AND ("Erythromelalgia Treatment" OR "Erythromelalgia Management"). Results: From the 103 papers extracted through the database search, six articles were considered suitable for the systematic review. The included studies investigated various interventions used for a total of 120 patients, including iloprost (n = 8), misoprostol (n = 21), topical amitriptyline-ketamine (n = 36), lidocaine (n = 27), chemical lumbar sympathectomy (CLS, n = 13), and various pharmacological agents (n = 11). The outcomes showed significant improvements in areas like pain reduction, cooling scores, and temperature regulation. Iloprost and misoprostol exhibited notable benefits in cooling scores, sympathetic dysfunction, and EM severity compared to placebos. About 75% of the patients reported pain relief with topical amitriptyline-ketamine, while lidocaine reduced nociceptive feelings in a dose-dependent manner. Conclusions: Comparing interventions demonstrated consistent clinical benefit with varied tolerability. However, adverse events ranged from mild gastrointestinal symptoms to severe complications such as disability and depression, requiring careful monitoring. Given EM's diverse symptoms and comorbidities, treatment efficacy varies among individuals. A personalized approach incorporating genetic testing, multidisciplinary care, and long-term monitoring is essential to optimize outcomes. Continued research is vital to advance understanding of EM's pathophysiology and improve patient care.

Voltage-gated sodium channels in excitable cells as drug targets

Excitable cells - including neurons, muscle cells and cardiac myocytes - are unique in expressing high densities of voltage-gated sodium (NaV) channels. This molecular adaptation enables these cells to produce action potentials, and is essential to their function. With the advent of the molecular revolution, the concept of 'the' sodium channel has been supplanted by understanding that excitable cells in mammals can express any of nine different forms of sodium channels (NaV1.1-NaV1.9). Selective expression in particular types of cells, together with a key role in controlling action potential firing, makes some of these NaV subtypes especially attractive molecular targets for drug development. Although these different channel subtypes display a common overall structure, differences in their amino acid sequences have provided a basis for the development of subtype-specific drugs. This approach has resulted in exciting progress in the development of drugs for epilepsy, cardiac disorders and pain. In this Review, we discuss recent progress in the development of drugs that selectively target each of the sodium channel subtypes.

Pharmacology and Mechanism of Action of Suzetrigine, a Potent and Selective NaV1.8 Pain Signal Inhibitor for the Treatment of Moderate to Severe Pain

INTRODUCTION

There is a high unmet need for safe and effective non-opioid medicines to treat moderate to severe pain without risk of addiction. Voltage-gated sodium channel 1.8 (NaV1.8) is a genetically and pharmacologically validated pain target that is selectively expressed in peripheral pain-sensing neurons and not in the central nervous system (CNS). Suzetrigine (VX-548) is a potent and selective inhibitor of NaV1.8, which has demonstrated clinical efficacy and safety in multiple acute pain studies. Our study was designed to characterize the mechanism of action of suzetrigine and assess both nonclinical and clinical data to test the hypothesis that selective NaV1.8 inhibition translates into clinical efficacy and safety, including lack of addictive potential.

METHODS

Preclinical pharmacology and mechanism of action studies were performed in vitro using electrophysiology and radiolabeled binding methods in cells recombinantly expressing human NaV channels, human proteins, and primary human dorsal root ganglion (DRG) sensory neurons. Safety and addictive potential assessments included in vitro secondary pharmacology studies, nonclinical repeat-dose toxicity and dependence studies in rats and/or monkeys, and a systematic analysis of adverse event data generated from 2447 participants from phase 3 acute pain studies of suzetrigine.

RESULTS

Suzetrigine is selective against all other NaV subtypes (≥ 31,000-fold) and 180 other molecular targets. Suzetrigine inhibits NaV1.8 by binding to the protein's second voltage sensing domain (VSD2) to stabilize the closed state of the channel. This novel allosteric mechanism results in tonic inhibition of NaV1.8 and reduces pain signals in primary human DRG sensory neurons. Nonclinical and clinical safety assessments with suzetrigine demonstrate no adverse CNS, cardiovascular or behavioral effects and no evidence of addictive potential or dependence.

CONCLUSIONS

The comprehensive pharmacology assessment presented here indicates that suzetrigine represents the first in a new class of non-opioid analgesics that are selective NaV1.8 pain signal inhibitors acting in the peripheral nervous system to safely treat pain without addictive potential.

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.

Discordance between preclinical and clinical testing of Na V 1.7-selective inhibitors for pain

ABSTRACT

The voltage-gated sodium channel Na V 1.7 plays an important role in pain processing according to genetic data. Those data made Na V 1.7 a popular drug target, especially since its relatively selective expression in nociceptors promised pain relief without the adverse effects associated with broader sodium channel blockade. Despite encouraging preclinical data in rodents, Na V 1.7-selective inhibitors have not yet proven effective in clinical trials. Discrepancies between preclinical and clinical results should raise alarms. We reviewed preclinical and clinical reports on the analgesic efficacy of Na V 1.7-selective inhibitors and found critical differences in several factors. Putting aside species differences, most preclinical studies tested young male rodents with limited genetic variability, inconsistent with the clinical population. Inflammatory pain was the most common preclinical chronic pain model whereas nearly all clinical trials focused on neuropathic pain despite some evidence suggesting Na V 1.7 channels are not essential for neuropathic pain. Preclinical studies almost exclusively measured evoked pain whereas most clinical trials assessed average pain intensity without distinguishing between evoked and spontaneous pain. Nearly all preclinical studies gave a single dose of drug unlike the repeat dosing used clinically, thus precluding preclinical data from demonstrating whether tolerance or other slow processes occur. In summary, preclinical testing of Na V 1.7-selective inhibitors aligned poorly with clinical testing. Beyond issues that have already garnered widespread attention in the pain literature, our results highlight the treatment regimen and choice of pain model as areas for improvement.

Small-Fiber Neuropathy: An Etiology-Oriented Review

BACKGROUND

Small-fiber neuropathy (SFN), affecting Aδ or C nerve fibers, is characterized by alterations of pain and temperature sensation, as well as autonomic dysfunction. Its diagnosis may still remain challenging as methods specifically assessing small nerve fibers are not always readily available, and standard techniques for large-fiber neuropathies, such as electroneuromyography, yield negative results. Still, skin biopsy for epidermal innervation and quantitative sensory testing allow for diagnosis in the presence of a congruent clinical picture.

OBJECTIVES

Many different etiologies may underlie small-fiber neuropathy, of which metabolic (diabetes mellitus/impaired glucose tolerance) and idiopathic remain prevalent. The aim of this narrative review is to provide a general picture of SFN while focusing on the different etiologies described in the literature in order to raise awareness of the variegated set of different causes of SFN and promote adequate diagnostic investigation.

METHODS

The term "Small-Fiber Neuropathy" was searched on the PubMed database with its different recognized etiologies: the abstracts of the articles were reviewed and described in the article if relevant for a total of 40 studies.

RESULTS

Many different disorders have been associated with SFN, even though often in the form of case reports or small case series.

CONCLUSIONS

Idiopathic forms of SFN remain the most prevalent in the literature, but association with different disorders (e.g., infectious, autoimmune) should prompt investigation for SFN in the presence of a congruent clinical picture (e.g., pain with neuropathic features).

Inhibition of TTX-S Na+ currents by a novel blocker QLS-278 for antinociception

Genetic loss-of-function mutations of the NaV1.7 channel, abundantly expressed in peripheral nociceptive neurons, cause congenital insensitivity to pain in humans, indicating that selective inhibition of the channel may lead to potential therapy for pain disorders. In this study, we investigated a novel compound, 5-chloro-N-(cyclopropylsulfonyl)-2-fluoro-4-(2-(8-(furan-2-ylmethyl)-8-azaspiro [4.5] decan-2-yl) ethoxy) benzamide (QLS-278) that inhibits NaV1.7 channels and exhibits antinociceptive activity. Compound QLS-278 exhibits inactivation- and concentration-dependent inhibition of macroscopic currents of NaV1.7 channels stably expressed in HEK293 cells with an IC50 of 1.2 ± 0.2 μM. QLS-278 causes a hyperpolarization shift of the channel inactivation and delays recovery from inactivation, without any noticeable effect on voltage-dependent activation. In mouse dorsal root ganglion neurons, QLS-278 suppresses native tetrodotoxin-sensitive NaV currents and also reduces neuronal firings. Moreover, QLS-278 dose-dependently relieves neuropathic pain induced by spared nerve injury and inflammatory pain induced by formalin without significantly altering spontaneous locomotor activity in mice. Therefore, our identification of the novel compound QLS-278 may hold developmental potential in chronic pain treatment. SIGNIFICANCE STATEMENT: QLS-278, a novel voltage-gated sodium NaV1.7 channel blocker, inhibits native tetrodotoxin-sensitive Na+ current and reduces action potential firings in dorsal root ganglion sensory neurons. QLS-278 also exhibits antinociceptive activity in mouse models of pain, demonstrating the potential for the development of a chronic pain treatment.

Genetic Variants in the SCN9A Gene are Detected in a Minority of Erythromelalgia Patients

Gain-of-function variants in the voltage-gated sodium channel Nav1.7, encoded by the SCN9A gene, have previously been identified in patients with erythromelalgia, a clinical diagnosis defined by intermittent attacks of painful, hot, swollen, and red skin, predominantly involving the hands and feet. Symptoms are induced or aggravated by warming and relieved by cooling. In primary erythromelalgia there is no known underlying disease. This study investigated the frequency of SCN9A variants in a cohort of primary erythromelalgia patients collected at a single centre, and examined the clinical signs and symptoms associated with identified variants. One hundred patients with possible erythromelalgia were collected prospectively and evaluated by clinical examination. Thirty-five patients fulfilling the clinical criteria of primary erythromelalgia were screened for variants in SCN9A. Five were found to carry likely causal variants, including a variant found in 2 related individuals and a variant not previously described in patients with erythromelalgia. The clinical findings differed significantly between the patients. Overall, in this cohort only 4/34 (11.7%) of unrelated patients had erythromelalgia likely caused by gain-of-function variants in SCN9A. Variants in SCN9A are therefore likely to cause or contribute to primary erythromelalgia in only a small proportion of patients.

Pediatric erythromelalgia from multidisciplinary perspectives: a scoping review

Erythromelalgia is a rare, chronic pain disorder characterized by the triad of intense burning sensation, warmth, and redness, primarily involving the hands and feet, and usually alleviated by cold and worsened by heat. The objective of this scoping review was to: 1) map the existing literature on erythromelalgia in youth, 2) identify knowledge gaps, and 3) inform directions for future research in pediatric erythromelalgia. One hundred and sixty-seven studies reporting 411 cases of childhood-onset erythromelalgia were identified. Variability was found in reporting of clinical symptoms, the clinical presentations and diagnostic criteria used for classification of erythromelagia, the clinical assessments and investigations performed, and the types of interventions and management plans utilised. While factors to aid early recognition and optimize management have been identified, there are also significant gaps for future research to address. Ongoing efforts to develop a multicenter registry of pediatric erythromelalgia cases, with standardized data collection and reporting, will be beneficial to establish consensus recommendations for the diagnosis and management of pediatric erythromelalgia. IMPACT: This scoping review maps the existing literature on pediatric erythromelalgia. Variability was found in reporting of clinical symptoms, the clinical presentations and diagnostic criteria used for classification of erythromelagia, the clinical assessments and investigations performed, and the types of interventions and management plans utilised. The development of an international registry would immensely benefit multidisciplinary experts involved in the care of pediatric erythromelalgia and those with lived experience.

Previously defined variants of uncertain significance may play an important role in epilepsy and interactions between certain variants may become pathogenic

OBJECTIVE

Epilepsy is a chronic neurological disorder related to various etiologies, and the prevalence of active epilepsy is estimated to be between 4 and 10 per 1000 individuals having a significant role in genetic mutations. Next-Generation Sequencing (NGS) panels are utilized for genetic testing, but a substantial proportion of the results remain uncertain and are not considered directly causative of epilepsy. This study aimed to reevaluate pediatric patients diagnosed with epilepsy who underwent genetic investigation using NGS panels, focusing on inconclusive variant findings or multiple variants of uncertain significance (VUSs).

METHODS

A subgroup of pediatric patients aged 0-25 years, diagnosed with epilepsy, who underwent genetic investigation with an NGS epilepsy panel at the Child Neurology Unit, The Edmond and Lily Safra Children's Hospital, Sheba Medical Center, between 2018 and 2022 through Invitae, was reevaluated. Patients with inconclusive variant findings or multiple VUSs in their test results were included. Genetic data were analyzed to identify potentially pathogenic variants and frequent genetic combinations.

RESULTS

Two unrelated potentially pathogenic variants were identified in the SCN9A and QARS1 genes. A frequent genetic combination, RANBP2&RYR3, was also observed among other combinations. The RANBP2 gene consistently co-occurred with RYR3 variants in uncertain results, suggesting potential pathogenicity. Analysis of unaffected parents' data revealed certain combinations inherited from different parents, suggesting specific gene combinations as possible risk factors for the disease.

SIGNIFICANCE

This study highlights the importance of reevaluating genetic data from pediatric epilepsy patients with inconclusive variant findings or multiple VUSs. Identification of potentially pathogenic variants and frequent genetic combinations, such as RANBP2&RYR3, could aid in understanding the genetic basis of epilepsy and identifying potential hotspots.

PLAIN LANGUAGE SUMMARY

We have performed a retrospective analysis on a subpopulation of pediatric patients diagnosed with epilepsy. We found that specific genetic variants were repeatable, indicating their potential pathogenicity to the disease.

Drug discovery targeting Nav1.8: Structural insights and therapeutic potential

Voltage-gated sodium (Nav) channels are crucial in transmitting action potentials in neurons. The tetrodotoxin-resistant subtype Nav1.8 is predominantly expressed in the peripheral nervous system, offering a unique opportunity to design selective inhibitors for pain relief. A number of compounds have been reported to specifically block Nav1.8. Among these, VX-548 is already in regulatory review for the treatment of moderate-to-severe acute pain and holds the promise to be the first non-opioid pain killer over the past twenty years. Recent structural studies using cryogenic electron microscopy (cryo-EM) and structure-based predictive modeling have provided unprecedented insights into the structural pharmacology of Nav1.8. In this review, we summarize the latest developments in Nav1.8-selective inhibitors, focusing on the druggable sites and mechanisms that confer subtype specificity. These structural insights highlight the potential for Nav1.8 inhibitors to deliver non-addictive pain management, thus illuminating the avenue to next-generation analgesic development.

Molecular mechanisms of neuropathic pain

Peripheral neuropathic pain, which occurs after a lesion or disease affecting the peripheral somatosensory nervous system, is a complex and challenging condition to treat. This chapter will cover molecular mechanisms underlying the pathophysiology of peripheral neuropathic pain, focusing on (1) sensitization of nociceptors, (2) neuro-immune crosstalk, and (3) axonal degeneration and regeneration. The chapter will also emphasize the importance of identifying novel therapeutic targets in non-neuronal cells. A comprehensive understanding of how changes at both neuronal and non-neuronal levels contribute to peripheral neuropathic pain may significantly improve pain management and treatment options, expanding to topical application that bypass the side effects associated with systemic administration.

Spider and scorpion knottins targeting voltage-gated sodium ion channels in pain signaling

In sensory neurons that transmit pain signals, whether acute or chronic, voltage-gated sodium channels (VGSCs) are crucial for regulating excitability. NaV1.1, NaV1.3, NaV1.6, NaV1.7, NaV1.8, and NaV1.9 have been demonstrated and defined their functional roles in pain signaling based on their biophysical properties and distinct patterns of expression in each subtype of sensory neurons. Scorpions and spiders are traditional Chinese medicinal materials, belonging to the arachnid class. Most of the studied species of them have evolved venom peptides that exhibit a wide variety of knottins specifically targeting VGSCs with subtype selectivity and conformational specificity. This review provides an overview on the exquisite knottins from scorpion and spider venoms targeting pain-related NaV channels, describing the sequences and the structural features as well as molecular determinants that influence their selectivity on special subtype and at particular conformation, with an aim for the development of novel research tools on NaV channels and analgesics with minimal adverse effects.

ST-2560, a selective inhibitor of the NaV1.7 sodium channel, affects nocifensive and cardiovascular reflexes in non-human primates

BACKGROUND AND PURPOSE

The voltage-gated sodium channel isoform NaV1.7 is a high-interest target for the development of non-opioid analgesics due to its preferential expression in pain-sensing neurons. NaV1.7 is also expressed in autonomic neurons, yet its contribution to involuntary visceral reflexes has received limited attention. The small molecule inhibitor ST-2560 was advanced into pain behaviour and cardiovascular models to understand the pharmacodynamic effects of selective inhibition of NaV1.7.

EXPERIMENTAL APPROACH

Potency of ST-2560 at NaV1.7 and off-target ion channels was evaluated by whole-cell patch-clamp electrophysiology. Effects on nocifensive reflexes were assessed in non-human primate (NHP) behavioural models, employing the chemical capsaicin and mechanical stimuli. Cardiovascular parameters were monitored continuously in freely-moving, telemetered NHPs following administration of vehicle and ST-2560.

KEY RESULTS

ST-2560 is a potent inhibitor (IC50 = 39 nM) of NaV1.7 in primates with ≥1000-fold selectivity over other isoforms of the human NaV1.x family. Following systemic administration, ST-2560 (0.1-0.3 mg·kg-1, s.c.) suppressed noxious mechanical- and chemical-evoked reflexes at free plasma concentrations threefold to fivefold above NaV1.7 IC50. ST-2560 (0.1-1.0 mg·kg-1, s.c.) also produced changes in haemodynamic parameters, most notably a 10- to 20-mmHg reduction in systolic and diastolic arterial blood pressure, at similar exposures.

CONCLUSIONS AND IMPLICATIONS

Acute pharmacological inhibition of NaV1.7 is antinociceptive, but also has the potential to impact the cardiovascular system. Further work is merited to understand the role of NaV1.7 in autonomic ganglia involved in the control of heart rate and blood pressure, and the effect of selective NaV1.7 inhibition on cardiovascular function.

A versatile residue numbering scheme for Nav and Cav channels

Voltage-gated sodium (Nav) and calcium (Cav) channels are responsible for the initiation of electrical signals. They have long been targeted for the treatment of various diseases. The mounting number of cryoelectron microscopy (cryo-EM) structures for diverse subtypes of Nav and Cav channels from multiple organisms necessitates a generic residue numbering system to establish the structure-function relationship and to aid rational drug design or optimization. Here we suggest a structure-based residue numbering scheme, centering around the most conserved residues on each of the functional segments. We elaborate the generic numbers through illustrative examples, focusing on representative drug-binding sites of eukaryotic Nav and Cav channels. We also extend the numbering scheme to compare common disease mutations among different Nav subtypes. Application of the generic residue numbering scheme affords immediate insights into hotspots for pathogenic mutations and critical loci for drug binding and will facilitate drug discovery targeting Nav and Cav channels.

Shining a Light on Venom-Peptide Receptors: Venom Peptides as Targeted Agents for In Vivo Molecular Imaging

Molecular imaging has revolutionised the field of biomedical research by providing a non-invasive means to visualise and understand biochemical processes within living organisms. Optical fluorescent imaging in particular allows researchers to gain valuable insights into the dynamic behaviour of a target of interest in real time. Ion channels play a fundamental role in cellular signalling, and they are implicated in diverse pathological conditions, making them an attractive target in the field of molecular imaging. Many venom peptides exhibit exquisite selectivity and potency towards ion channels, rendering them ideal agents for molecular imaging applications. In this review, we illustrate the use of fluorescently-labelled venom peptides for disease diagnostics and intraoperative imaging of brain tumours and peripheral nerves. Finally, we address challenges for the development and clinical translation of venom peptides as nerve-targeted imaging agents.

Role of NaV1.7 in postganglionic sympathetic nerve function in human and guinea-pig arteries

NaV1.7 plays a crucial role in inducing and conducting action potentials in pain-transducing sensory nociceptor fibres, suggesting that NaV1.7 blockers could be effective non-opioid analgesics. While SCN9A is expressed in both sensory and autonomic neurons, its functional role in the autonomic system remains less established. Our single neuron rt-PCR analysis revealed that 82% of sympathetic neurons isolated from guinea-pig stellate ganglia expressed NaV1.7 mRNA, with NaV1.3 being the only other tetrodotoxin-sensitive channel expressed in approximately 50% of neurons. We investigated the role of NaV1.7 in conducting action potentials in postganglionic sympathetic nerves and in the sympathetic adrenergic contractions of blood vessels using selective NaV1.7 inhibitors. Two highly selective NaV1.7 blockers, GNE8493 and PF 05089771, significantly inhibited postganglionic compound action potentials by approximately 70% (P < 0.01), with residual activity being blocked by the NaV1.3 inhibitor, ICA 121431. Electrical field stimulation (EFS) induced rapid contractions in guinea-pig isolated aorta, pulmonary arteries, and human isolated pulmonary arteries via stimulation of intrinsic nerves, which were inhibited by prazosin or the NaV1 blocker tetrodotoxin. Our results demonstrated that blocking NaV1.7 with GNE8493, PF 05089771, or ST2262 abolished or strongly inhibited sympathetic adrenergic responses in guinea-pigs and human vascular smooth muscle. These findings support the hypothesis that pharmacologically inhibiting NaV1.7 could potentially reduce sympathetic and parasympathetic function in specific vascular beds and airways. KEY POINTS: 82% of sympathetic neurons isolated from the stellate ganglion predominantly express NaV1.7 mRNA. NaV1.7 blockers inhibit action potential conduction in postganglionic sympathetic nerves. NaV1.7 blockade substantially inhibits sympathetic nerve-mediated adrenergic contractions in human and guinea-pig blood vessels. Pharmacologically blocking NaV1.7 profoundly affects sympathetic and parasympathetic responses in addition to sensory fibres, prompting exploration into the broader physiological consequences of NaV1.7 mutations on autonomic nerve activity.

Similar excitability through different sodium channels and implications for the analgesic efficacy of selective drugs

Nociceptive sensory neurons convey pain-related signals to the CNS using action potentials. Loss-of-function mutations in the voltage-gated sodium channel NaV1.7 cause insensitivity to pain (presumably by reducing nociceptor excitability) but clinical trials seeking to treat pain by inhibiting NaV1.7 pharmacologically have struggled. This may reflect the variable contribution of NaV1.7 to nociceptor excitability. Contrary to claims that NaV1.7 is necessary for nociceptors to initiate action potentials, we show that nociceptors can achieve similar excitability using different combinations of NaV1.3, NaV1.7, and NaV1.8. Selectively blocking one of those NaV subtypes reduces nociceptor excitability only if the other subtypes are weakly expressed. For example, excitability relies on NaV1.8 in acutely dissociated nociceptors but responsibility shifts to NaV1.7 and NaV1.3 by the fourth day in culture. A similar shift in NaV dependence occurs in vivo after inflammation, impacting ability of the NaV1.7-selective inhibitor PF-05089771 to reduce pain in behavioral tests. Flexible use of different NaV subtypes exemplifies degeneracy - achieving similar function using different components - and compromises reliable modulation of nociceptor excitability by subtype-selective inhibitors. Identifying the dominant NaV subtype to predict drug efficacy is not trivial. Degeneracy at the cellular level must be considered when choosing drug targets at the molecular level.

Analgesic potential of voltage gated sodium channel modulators for the management of pain

Neuronal electrochemical signals involve the flux of sodium ions through voltage-gated sodium channels (NaV) located in the neurolemma. Of the nine sodium channel subtypes, NaV-1.7, 1.8, and 1.9 are predominantly located on nociceptors, making them prime targets to control pain. This review highlights some of the latest discoveries targeting NaV channel activity, including: (1) charged local anaesthetic derivatives; (2) NaV channel toxins and associated small peptide blockers; (3) regulation of NaV channel accessory proteins; and (4) genetic manipulation of NaV channel function. While the translation of preclinical findings to a viable treatment in humans has remained a challenge, a greater understanding of NaV channel physiology could lead to the development of a new stream of therapies aimed at alleviating chronic pain.

Nav1.7 Modulator Bearing a 3-Hydroxyindole Backbone Holds the Potential to Reverse Neuropathic Pain

Chronic pain is a growing global health problem affecting at least 10% of the world's population. However, current chronic pain treatments are inadequate. Voltage-gated sodium channels (Navs) play a pivotal role in regulating neuronal excitability and pain signal transmission and thus are main targets for nonopioid painkiller development, especially those preferentially expressed in dorsal root ganglial (DRG) neurons, such as Nav1.6, Nav1.7, and Nav1.8. In this study, we screened in virtual hits from dihydrobenzofuran and 3-hydroxyoxindole hybrid molecules against Navs via a veratridine (VTD)-based calcium imaging method. The results showed that one of the molecules, 3g, could inhibit VTD-induced neuronal activity significantly. Voltage clamp recordings demonstrated that 3g inhibited the total Na+ currents of DRG neurons in a concentration-dependent manner. Biophysical analysis revealed that 3g slowed the activation, meanwhile enhancing the inactivation of the Navs. Additionally, 3g use-dependently blocked Na+ currents. By combining with selective Nav inhibitors and a heterozygous expression system, we demonstrated that 3g preferentially inhibited the TTX-S Na+ currents, specifically the Nav1.7 current, other than the TTX-R Na+ currents. Molecular docking experiments implicated that 3g binds to a known allosteric site at the voltage-sensing domain IV(VSDIV) of Nav1.7. Finally, intrathecal injection of 3g significantly relieved mechanical pain behavior in the spared nerve injury (SNI) rat model, suggesting that 3g is a promising candidate for treating chronic pain.

Erythromelalgia. Part I: Pathogenesis, clinical features, evaluation, and complications

Erythromelalgia is a rare pain disorder that is underrecognized and difficult-to-treat. It is characterized by episodes of extremity erythema and pain that can be disabling; it may be genetic, related to an underlying systemic disease, or idiopathic. Considering the prominent cutaneous features characteristic of the condition, dermatologists can play an important role in early recognition and limitation of morbidity. The first article in this 2-part continuing medical education series reviews the epidemiology, pathogenesis, clinical manifestations, evaluation, and complications.

Identification and In-Silico study of non-synonymous functional SNPs in the human SCN9A gene

Single nucleotide polymorphisms are the most common form of DNA alterations at the level of a single nucleotide in the genomic sequence. Genome-wide association studies (GWAS) were carried to identify potential risk genes or genomic regions by screening for SNPs associated with disease. Recent studies have shown that SCN9A comprises the NaV1.7 subunit, Na+ channels have a gene encoding of 1988 amino acids arranged into 4 domains, all with 6 transmembrane regions, and are mainly found in dorsal root ganglion (DRG) neurons and sympathetic ganglion neurons. Multiple forms of acute hypersensitivity conditions, such as primary erythermalgia, congenital analgesia, and paroxysmal pain syndrome have been linked to polymorphisms in the SCN9A gene. Under this study, we utilized a variety of computational tools to explore out nsSNPs that are potentially damaging to heath by modifying the structure or activity of the SCN9A protein. Over 14 potentially damaging and disease-causing nsSNPs (E1889D, L1802P, F1782V, D1778N, C1370Y, V1311M, Y1248H, F1237L, M936V, I929T, V877E, D743Y, C710W, D623H) were identified by a variety of algorithms, including SNPnexus, SNAP-2, PANTHER, PhD-SNP, SNP & GO, I-Mutant, and ConSurf. Homology modeling, structure validation, and protein-ligand interactions also were performed to confirm 5 notable substitutions (L1802P, F1782V, D1778N, V1311M, and M936V). Such nsSNPs may become the center of further studies into a variety of disorders brought by SCN9A dysfunction. Using in-silico strategies for assessing SCN9A genetic variations will aid in organizing large-scale investigations and developing targeted therapeutics for disorders linked to these variations.

IUPHAR review: Navigating the role of preclinical models in pain research

Chronic pain is a complex and challenging medical condition that affects millions of people worldwide. Understanding the underlying mechanisms of chronic pain is a key goal of preclinical pain research so that more effective treatment strategies can be developed. In this review, we explore nociception, pain, and the multifaceted factors that lead to chronic pain by focusing on preclinical models. We provide a detailed look into inflammatory and neuropathic pain models and discuss the most used animal models for studying the mechanisms behind these conditions. Additionally, we emphasize the vital role of these preclinical models in developing new pain-relief drugs, focusing on biologics and the therapeutic potential of NMDA and cannabinoid receptor antagonists. We also discuss the challenges of TRPV1 modulation for pain treatment, the clinical failures of neurokinin (NK)- 1 receptor antagonists, and the partial success story of Ziconotide to provide valuable lessons for preclinical pain models. Finally, we highlight the overall success and limitations of current treatments for chronic pain while providing critical insights into the development of more effective therapies to alleviate the burden of chronic pain.

Differential expression of slow and fast-repriming tetrodotoxin-sensitive sodium currents in dorsal root ganglion neurons

Sodium channel Nav1.7 triggers the generation of nociceptive action potentials and is important in sending pain signals under physiological and pathological conditions. However, studying endogenous Nav1.7 currents has been confounded by co-expression of multiple sodium channel isoforms in dorsal root ganglion (DRG) neurons. In the current study, slow-repriming (SR) and fast-repriming (FR) tetrodotoxin-sensitive (TTX-S) currents were dissected electrophysiologically in small DRG neurons of both rats and mice. Three subgroups of small DRG neurons were identified based on the expression pattern of SR and FR TTX-S currents. A majority of rat neurons only expressed SR TTX-S currents, while a majority of mouse neurons expressed additional FR TTX-S currents. ProTx-II inhibited SR TTX-S currents with variable efficacy among DRG neurons. The expression of both types of TTX-S currents was higher in Isolectin B4-negative (IB4-) compared to Isolectin B4-positive (IB4+) neurons. Paclitaxel selectively increased SR TTX-S currents in IB4- neurons. In simulation experiments, the Nav1.7-expressing small DRG neuron displayed lower rheobase and higher frequency of action potentials upon threshold current injections compared to Nav1.6. The results suggested a successful dissection of endogenous Nav1.7 currents through electrophysiological manipulation that may provide a useful way to study the functional expression and pharmacology of endogenous Nav1.7 channels in DRG neurons.

The C-Terminal of NaV1.7 Is Ubiquitinated by NEDD4L

NaV1.7, the neuronal voltage-gated sodium channel isoform, plays an important role in the human body's ability to feel pain. Mutations within NaV1.7 have been linked to pain-related syndromes, such as insensitivity to pain. To date, the regulation and internalization mechanisms of the NaV1.7 channel are not well known at a biochemical level. In this study, we perform biochemical and biophysical analyses that establish that the HECT-type E3 ligase, NEDD4L, ubiquitinates the cytoplasmic C-terminal (CT) region of NaV1.7. Through in vitro ubiquitination and mass spectrometry experiments, we identify, for the first time, the lysine residues of NaV1.7 within the CT region that get ubiquitinated. Furthermore, binding studies with an NEDD4L E3 ligase modulator (ubiquitin variant) highlight the dynamic partnership between NEDD4L and NaV1.7. These investigations provide a framework for understanding how NEDD4L-dependent regulation of the channel can influence the NaV1.7 function.

Genetic, electrophysiological, and pathological studies on patients with SCN9A-related pain disorders

BACKGROUND AND AIMS

Voltage-gated sodium channel Nav1.7, encoded by the SCN9A gene, has been linked to diverse painful peripheral neuropathies, represented by the inherited erythromelalgia (EM) and paroxysmal extreme pain disorder (PEPD). The aim of this study was to determine the genetic etiology of patients experiencing neuropathic pain, and shed light on the underlying pathogenesis.

METHODS

We enrolled eight patients presenting with early-onset painful peripheral neuropathies, consisting of six cases exhibiting EM/EM-like disorders and two cases clinically diagnosed with PEPD. We conducted a gene-panel sequencing targeting 18 genes associated with hereditary sensory and/or autonomic neuropathy. We introduced novel SCN9A mutation (F1624S) into a GFP-2A-Nav1.7rNS plasmid, and the constructs were then transiently transfected into HEK293 cells. We characterized both wild-type and F1624S Nav1.7 channels using an automated high-throughput patch-clamp system.

RESULTS

From two patients displaying EM-like/EM phenotypes, we identified two SCN9A mutations, I136V and P1308L. Among two patients diagnosed with PEPD, we found two additional mutations in SCN9A, F1624S (novel) and A1632E. Patch-clamp analysis of Nav1.7-F1624S revealed depolarizing shifts in both steady-state fast inactivation (17.4 mV, p < .001) and slow inactivation (5.5 mV, p < .001), but no effect on channel activation was observed.

INTERPRETATION

Clinical features observed in our patients broaden the phenotypic spectrum of SCN9A-related pain disorders, and the electrophysiological analysis enriches the understanding of genotype-phenotype association caused by Nav1.7 gain-of-function mutations.

LmNaTx15, a novel scorpion toxin, enhances the activity of Nav channels and induces pain in mice

Polypeptide toxins are major bioactive components found in venomous animals. Many polypeptide toxins can specifically act on targets, such as ion channels and voltage-gated sodium (Nav) channels, in the nervous, muscle, and cardiovascular systems of the recipient to increase defense and predation efficiency. In this study, a novel polypeptide toxin, LmNaTx15, was isolated from the venom of the scorpion Lychas mucronatus, and its activity was analyzed. LmNaTx15 slowed the fast inactivation of Nav1.2, Nav1.3, Nav1.4, Nav1.5, and Nav1.7 and inhibited the peak current of Nav1.5, but it did not affect Nav1.8. In addition, LmNaTx15 altered the voltage-dependent activation and inactivation of these Nav channel subtypes. Furthermore, like site 3 neurotoxins, LmNaTx15 induced pain in mice. These results show a novel scorpion toxin with a modulatory effect on specific Nav channel subtypes and pain induction in mice. Therefore, LmNaTx15 may be a key bioactive component for scorpion defense and predation. Besides, this study provides a basis for analyzing structure-function relationships of the scorpion toxins affecting Nav channel activity.

Exploring the Spectrum of RHOBTB2 Variants Associated with Developmental Encephalopathy 64: A Case Series and Literature Review

Background

Rho-related BTB domain-containing protein 2 (RHOBTB2) is a protein that interacts with cullin-3, a crucial E3 ubiquitin ligase for mitotic cell division. RHOBTB2 has been linked to early infantile epileptic encephalopathy, autosomal dominant type 64 (OMIM618004), in 34 reported patients.

Methods

We present a case series of seven patients with RHOBTB2-related disorders (RHOBTB2-RD), including a description of a novel heterozygous variant. We also reviewed previously published cases of RHOBTB2-RD.

Results

The seven patients had ages ranging from 2 years and 8 months to 26 years, and all had experienced seizures before the age of one (onset, 4-12 months, median, 4 months), including various types of seizures. All patients in this cohort also had a movement disorder (onset, 0.3-14 years, median, 1.5 years). Six of seven had a baseline movement disorder, and one of seven only had paroxysmal dystonia. Stereotypies were noted in four of six, choreodystonia in three of six, and ataxia in one case with multiple movement phenotypes at baseline. Paroxysmal movement disorders were observed in six of seven patients for whom carbamazepine or oxcarbazepine treatment was effective in controlling acute or paroxysmal movement disorders. Four patients had acute encephalopathic episodes at ages 4 (one patient) and 6 (three patients), which improved following treatment with methylprednisolone. Magnetic resonance imaging scans revealed transient fluid-attenuated inversion recovery abnormalities during these episodes, as well as myelination delay, thin corpus callosum, and brain atrophy. One patient had a novel RHOBTB2 variant (c.359G>A/p.Gly120Glu).

Conclusion

RHOBTB2-RD is characterized by developmental delay or intellectual disability, early-onset seizures, baseline movement disorders, acute or paroxysmal motor phenomena, acquired microcephaly, and episodes of acute encephalopathy. Early onsets of focal dystonia, acute encephalopathic episodes, episodes of tongue protrusion, or peripheral vasomotor disturbances are important diagnostic clues. Treatment with carbamazepine or oxcarbazepine was found to be effective in controlling acute or paroxysmal movement disorders. Our study highlights the clinical features and treatment response of RHOBTB2-RD.

Neanderthal introgression in SCN9A impacts mechanical pain sensitivity

The Nav1.7 voltage-gated sodium channel plays a key role in nociception. Three functional variants in the SCN9A gene (encoding M932L, V991L, and D1908G in Nav1.7), have recently been identified as stemming from Neanderthal introgression and to associate with pain symptomatology in UK BioBank data. In 1000 genomes data, these variants are absent in Europeans but common in Latin Americans. Analysing high-density genotype data from 7594 Latin Americans, we characterized Neanderthal introgression in SCN9A. We find that tracts of introgression occur on a Native American genomic background, have an average length of ~123 kb and overlap the M932L, V991L, and D1908G coding positions. Furthermore, we measured experimentally six pain thresholds in 1623 healthy Colombians. We found that Neanderthal ancestry in SCN9A is significantly associated with a lower mechanical pain threshold after sensitization with mustard oil and evidence of additivity of effects across Nav1.7 variants. Our findings support the reported association of Neanderthal Nav1.7 variants with clinical pain, define a specific sensory modality affected by archaic introgression in SCN9A and are consistent with independent effects of the Neanderthal variants on Nav1.7 function.

Licochalcone Mediates the Pain Relief by Targeting the Voltage-Gated Sodium Channel

Licorice is a traditional Chinese medicine and recorded to have pain relief effects in national pharmacopoeia, but the mechanisms behind these effects have not been fully explored. Among the hundreds of compounds in licorice, licochalcone A (LCA) and licochalcone B (LCB) are two important components belonging to the chalcone family. In this study, we compared the analgesic effects of these two licochalcones and the molecular mechanisms. LCA and LCB were applied in cultured dorsal root ganglion (DRG) neurons, and the voltage-gated sodium (NaV) currents and action potentials were recorded. The electrophysiological experiments showed that LCA can inhibit NaV currents and dampen excitabilities of DRG neurons, whereas LCB did not show inhibition effect on NaV currents. Because the NaV1.7 channel can modulate Subthreshold membrane potential oscillations in DRG neuron, which can palliate neuropathic pain, HEK293T cells were transfected with NaV1.7 channel and recorded with whole-cell patch clamp. LCA can also inhibit NaV1.7 channels exogenously expressed in HEK293T cells. We further explored the analgesic effects of LCA and LCB on formalin-induced pain animal models. The animal behavior tests revealed that LCA can inhibit the pain responses during phase 1 and phase 2 of formalin test, and LCB can inhibit the pain responses during phase 2. The differences of the effects on NaV currents between LCA and LCB provide us with the basis for developing NaV channel inhibitors, and the novel findings of analgesic effects indicate that licochalcones can be developed into effective analgesic medicines. SIGNIFICANCE STATEMENT: This study found that licochalcone A (LCA) can inhibit voltage-gated sodium (NaV) currents, dampen excitabilities of dorsal root ganglion neurons, and inhibit the NaV1.7 channels exogenously expressed in HEK293T cells. Animal behavior tests showed that LCA can inhibit the pain responses during phase 1 and phase 2 of formalin test, whereas licochalcone B can inhibit the pain responses during phase 2. These findings indicate that licochalcones could be the leading compounds for developing NaV channel inhibitors and effective analgesic medicines.

Naphthylisoquinoline alkaloids, a new structural template inhibitor of Nav1.7 sodium channel

Voltage-gated sodium channel 1.7 (Nav1.7) remains one of the most promising drug targets for pain relief. In the current study, we conducted a high-throughput screening of natural products in our in-house compound library to discover novel Nav1.7 inhibitors, then characterized their pharmacological properties. We identified 25 naphthylisoquinoline alkaloids (NIQs) from Ancistrocladus tectorius to be a novel type of Nav1.7 channel inhibitors. Their stereostructures including the linkage modes of the naphthalene group at the isoquinoline core were revealed by a comprehensive analysis of HRESIMS, 1D, and 2D NMR spectra as well as ECD spectra and single-crystal X-ray diffraction analysis with Cu Kα radiation. All the NIQs showed inhibitory activities against the Nav1.7 channel stably expressed in HEK293 cells, and the naphthalene ring in the C-7 position displayed a more important role in the inhibitory activity than that in the C-5 site. Among the NIQs tested, compound 2 was the most potent with an IC50 of 0.73 ± 0.03 µM. We demonstrated that compound 2 (3 µM) caused dramatical shift of steady-state slow inactivation toward the hyperpolarizing direction (V1/2 values were changed from -39.54 ± 2.77 mV to -65.53 ± 4.39 mV, which might contribute to the inhibition of compound 2 against the Nav1.7 channel. In acutely isolated dorsal root ganglion (DRG) neurons, compound 2 (10 μM) dramatically suppressed native sodium currents and action potential firing. In the formalin-induced mouse inflammatory pain model, local intraplantar administration of compound 2 (2, 20, 200 nmol) dose-dependently attenuated the nociceptive behaviors. In summary, NIQs represent a new type of Nav1.7 channel inhibitors and may act as structural templates for the following analgesic drug development.

Clinical Challenges in Primary Erythromelalgia: a Real-Life Experience from a Single Center and a Diagnostic-Therapeutic Flow-Chart Proposal

INTRODUCTION

Primary erythromelalgia (EM) is a rare clinical syndrome characterized by recurrent erythema, burning pain and warmth of the extremities. The symptoms greatly compromise the patients' quality of life leading to severe disability. SCN9A mutations can be the cause of the disease. Dermatologists are often the specialists these patients turn to for assistance.

OBJECTIVES

To describe the demographic and clinical characteristics of patients with primary EM, to assess the presence and mutation types in the SCN9A gene, to evaluate the effectiveness of several therapeutic approaches, and to propose a diagnostic algorithm with therapeutic implications.

METHODS

A monocentric retrospective study using the database of patients with a discharge diagnosis of primary EM of our Center. Demographic, clinical, instrumental and laboratory data of patients were reviewed.

RESULTS

Eleven female patients (age range 16 to 57) were selected. All patients were affected in both the lower and upper extremities. Follow-up ranged from 2 to 9 years. Four patients had four different heterozygous variants of the SCN9A gene. Two patients, although genetically negative, had a suggestive family history. A variety of medications were tried in all our patients to alleviate symptoms, but their efficacy was variable, partial and/or transitory. The most effective therapies were antihistamines, venlafaxine, and mexiletine.

CONCLUSIONS

The diagnosis and treatment of EM remain challenging. Patients with this condition display a wide spectrum of clinical manifestations and severity, as well as a paucity of resources and structures to support them. Mutations in the SCN9A gene are not always detected.

Structural mapping of Nav1.7 antagonists

Voltage-gated sodium (Na_v) channels are targeted by a number of widely used and investigational drugs for the treatment of epilepsy, arrhythmia, pain, and other disorders. Despite recent advances in structural elucidation of Na_v channels, the binding mode of most Na_v-targeting drugs remains unknown. Here we report high-resolution cryo-EM structures of human Na_v1.7 treated with drugs and lead compounds with representative chemical backbones at resolutions of 2.6-3.2 Å. A binding site beneath the intracellular gate (site BIG) accommodates carbamazepine, bupivacaine, and lacosamide. Unexpectedly, a second molecule of lacosamide plugs into the selectivity filter from the central cavity. Fenestrations are popular sites for various state-dependent drugs. We show that vinpocetine, a synthetic derivative of a vinca alkaloid, and hardwickiic acid, a natural product with antinociceptive effect, bind to the III-IV fenestration, while vixotrigine, an analgesic candidate, penetrates the IV-I fenestration of the pore domain. Our results permit building a 3D structural map for known drug-binding sites on Na_v channels summarized from the present and previous structures.

Enhanced sodium channel inactivation by temperature and FHF2 deficiency blocks heat nociception

ABSTRACT

Transient voltage-gated sodium currents are essential for the initiation and conduction of action potentials in neurons and cardiomyocytes. The amplitude and duration of sodium currents are tuned by intracellular fibroblast growth factor homologous factors (FHFs/iFGFs) that associate with the cytoplasmic tails of voltage-gated sodium channels (Na v s), and genetic ablation of Fhf genes disturbs neurological and cardiac functions. Among reported phenotypes, Fhf2null mice undergo lethal hyperthermia-induced cardiac conduction block attributable to the combined effects of FHF2 deficiency and elevated temperature on the cardiac sodium channel (Na v 1.5) inactivation rate. Fhf2null mice also display a lack of heat nociception, while retaining other somatosensory capabilities. Here, we use electrophysiological and computational methods to show that the heat nociception deficit can be explained by the combined effects of elevated temperature and FHF2 deficiency on the fast inactivation gating of Na v 1.7 and tetrodotoxin-resistant sodium channels expressed in dorsal root ganglion C fibers. Hence, neurological and cardiac heat-associated deficits in Fhf2null mice derive from shared impacts of FHF deficiency and temperature towards Na v inactivation gating kinetics in distinct tissues.

Case report: Spinal cord stimulation in the treatment of pediatric erythromelalgia

Introduction

In children, erythromelalgia is a rare chronic pain syndrome characterized by erythema, severe burning pain, and itching of affected feet. Unfortunately, there is no definitive therapy available currently.

Case report

Here, we report a case of primary erythromelalgia and the treatment response in a 10-year-old boy, whose genetic findings for mutations in the SCN9A gene were positive and skin biopsy results were diagnosed as small fiber neuropathy, while he has suffered from excruciating burning pain, itching, erythema, and recurrent infections over the past 3 years. He did not respond well to conventional treatment, and the only way to receive minimal relief was to immerse his feet in ice water. After a successful trial of spinal cord stimulation (SCS), the implantable pulse generator (IPG) was successfully implanted without complications, and it proved partial response to therapy.

Conclusion

There is no specific, efficient treatment for pediatric erythromelalgia currently, but this case demonstrates neuromodulation serves as part of the multimodal regimen to treat pediatric erythromelalgia.

NAN-190, a 5-HT1A antagonist, alleviates inflammatory pain by targeting Nav1.7 sodium channels

AIMS

In the present study, NAN-190 [1-(2-methoxyphenyl)-4-[4-(2-phthalimido) butyl] piperazine] was identified as a Nav1.7 blocker. In the meantime, the compound could alleviate the Complete Freund's Adjuvant (CFA)-induced inflammatory pain. To understand the molecular mechanisms of NAN-190 on pain, the effect of NAN-190 on Nav1.7 sodium channels was studied.

MAIN METHODS

Inflammatory pain was induced by injection of CFA solution into the plantar side of the left hindpaw. Thermal hyperalgesia and mechanical allodynia were measured. Whole-cell patch clamp methods were used to record sodium channels and other pain-related targets in the cultured recombinant cells and dorsal root ganglion neurons.

KEY FINDINGS

Nan-190 was identified as an inhibitor of Nav1.7 sodium channels and animal experiments showed that NAN-190 significantly alleviated CFA-induced inflammatory pain. Mechanism studies demonstrated that NAN-190 was a state-dependent Nav1.7 blocker with IC₅₀ value on the inactivated state ten-fold more potent than that on the rest state. NAN-190 leftward-shifted the fast and slow inactivation curves about 9.07 mV and 38.56 mV, respectively, but had no effects on channel activation. The compound also slowed the recovery from fast and slow inactivation and showed use-dependent properties. Further, the site-directed mutagenesis experiments demonstrated that NAN-190 mainly worked on the open state of Nav1.7 channels by interacting with sites similar as local anesthetics. In DRG neurons, NAN-190 mainly blocks TTX-sensitive currents but is less sensitive to TTX-R sodium currents.

SIGNIFICANCE

Taken together, our results indicated that NAN-190 alleviated pain behaviors by blocking sodium channels by interacting with the open state.

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.

SCN9A rs6746030 Polymorphism and Pain Perception in Combat Athletes and Non-Athletes

One of the genes associated with pain perception is SCN9A, which encodes an α-subunit of the voltage gated sodium channel, NaV1.7, a crucial player in peripheral pain sensation. It has been suggested that a common missense polymorphism within SCN9A (rs6746030; G>A; R1150W) may affect nociception in the general population, but its effects of pain perception in athletes remain unknown. Therefore, the aim of the study was to investigate the association between a polymorphism within SCN9A (rs6746030) and pain perception (pain threshold and pain tolerance) in the group of combat athletes (n = 214) and students (n = 92) who did not participate in sports at a professional level. Genotyping was carried out using TaqMan Real-Time PCR method. No significant differences were found between the SCN9A genotype distributions with respect to the pain threshold. However, the probability of having a high pain threshold was higher in the combat sports group than in the control group. The probability of having a decreased pain tolerance was higher in the carriers of the GA and AA genotype than in the homozygotes of the GG genotype. Moreover, the possibility of having a high pain threshold was higher in the combat athlete group than in the control group. The results of our study suggest that the SCN9A rs6746030 polymorphism may affect pain perception. However, the additional effect of the experimental group may suggest that pain tolerance is significantly modulated by other factors, such as the systematic exposure of the athletes’ bodies to short-term high-intensity stimuli during training sessions.

Nav1.7 P610T mutation in two siblings with persistent ocular pain after corneal axon transection: impaired slow inactivation and hyperexcitable trigeminal neurons

Despite extensive study, the mechanisms underlying pain after axonal injury remain incompletely understood. Pain after corneal refractive surgery provides a model, in humans, of the effect of injury to trigeminal afferent nerves. Axons of trigeminal ganglion neurons that innervate the cornea are transected by laser-assisted in situ keratomileusis (LASIK). Although most patients do not experience postoperative pain, a small subgroup develop persistent ocular pain. We previously carried out genomic analysis and determined that some patients with persistent pain after axotomy of corneal axons during refractive surgery carry mutations in genes that encode the electrogenisome of trigeminal ganglion neurons, the ensemble of ion channels and receptors that regulate excitability within these cells, including SCN9A, which encodes sodium channel Nav1.7, a threshold channel abundantly expressed in sensory neurons that has been implicated in a number of pain-related disorders. Here, we describe the biophysical and electrophysiological profiling of the P610T Nav1.7 mutation found in two male siblings with persistent ocular pain after refractive surgery. Our results indicate that this mutation impairs the slow inactivation of Nav1.7. As expected from this proexcitatory change in channel function, we also demonstrate that this mutation produces increased spontaneous activity in trigeminal ganglion neurons. These findings suggest that this gain-of-function mutation in Nav1.7 may contribute to pain after injury to the axons of trigeminal ganglion neurons.NEW & NOTEWORTHY Mechanisms underlying pain after axonal injury remain elusive. A small subgroup of patients experience pain after corneal refractive surgery, providing a human pain model after well-defined injury to axons. Here we analyze a mutation (P610T) in Nav1.7, a threshold sodium channel expressed in nociceptors, found in two siblings with persistent ocular pain after refractive surgery. We show that it impairs channel slow inactivation, thereby triggering inappropriate repetitive activity in trigeminal ganglion axons that signal eye pain.

A severe case of primary erythromelalgia presenting as small fiber neuropathy with a novel SCN9A mutation

Primary erythromelalgia (PEM) is a rare condition characterized by severe burning pain, erythema, and increased temperature in the extremeties. Mutations in the Nav1.7 sodium channel encoded by the SCN9A are responsible for PEM. The pathophysiology of PEM is unclear, but the involvement of neurogenic and vasogenic mechanisms has been suggested. Here we report a case of severe PEM in a 9-year-old child with a novel SCN9A mutation and examine the distribution of nerve fibers and expression of neuropeptides in the affected skin. Gene mutation analysis revealed a novel mutation p.L951I (c.2851C>A) in the heterozygous form of the SCN9A. An immunofluorescence study showed that intraepidermal nerve fibers were decreased in the affected leg, suggesting small fiber neuropathy. There was no increase in the expression of substance P (SP) or calcitonin gene-related peptide (CGRP) in the lesional skin tissue. These findings suggest SP and CGRP do not play a major role in the pathophysiology of primary erythromelalgia.

Investigating genotype-phenotype relationship of extreme neuropathic pain disorders in a UK national cohort

The aims of our study were to use whole genome sequencing in a cross-sectional cohort of patients to identify new variants in genes implicated in neuropathic pain, to determine the prevalence of known pathogenic variants and to understand the relationship between pathogenic variants and clinical presentation. Patients with extreme neuropathic pain phenotypes (both sensory loss and gain) were recruited from secondary care clinics in the UK and underwent whole genome sequencing as part of the National Institute for Health and Care Research Bioresource Rare Diseases project. A multidisciplinary team assessed the pathogenicity of rare variants in genes previously known to cause neuropathic pain disorders and exploratory analysis of research candidate genes was completed. Association testing for genes carrying rare variants was completed using the gene-wise approach of the combined burden and variance-component test SKAT-O. Patch clamp analysis was performed on transfected HEK293T cells for research candidate variants of genes encoding ion channels. The results include the following: (i) Medically actionable variants were found in 12% of study participants (205 recruited), including known pathogenic variants: SCN9A(ENST00000409672.1): c.2544T>C, p.Ile848Thr that causes inherited erythromelalgia, and SPTLC1(ENST00000262554.2):c.340T>G, p.Cys133Tr variant that causes hereditary sensory neuropathy type-1. (ii) Clinically relevant variants were most common in voltage-gated sodium channels (Nav). (iii) SCN9A(ENST00000409672.1):c.554G>A, pArg185His variant was more common in non-freezing cold injury participants than controls and causes a gain of function of NaV1.7 after cooling (the environmental trigger for non-freezing cold injury). (iv) Rare variant association testing showed a significant difference in distribution for genes NGF, KIF1A, SCN8A, TRPM8, KIF1A, TRPA1 and the regulatory regions of genes SCN11A, FLVCR1, KIF1A and SCN9A between European participants with neuropathic pain and controls. (v) The TRPA1(ENST00000262209.4):c.515C>T, p.Ala172Val variant identified in participants with episodic somatic pain disorder demonstrated gain-of-channel function to agonist stimulation. Whole genome sequencing identified clinically relevant variants in over 10% of participants with extreme neuropathic pain phenotypes. The majority of these variants were found in ion channels. Combining genetic analysis with functional validation can lead to a better understanding as to how rare variants in ion channels lead to sensory neuron hyper-excitability, and how cold, as an environmental trigger, interacts with the gain-of-function NaV1.7 p.Arg185His variant. Our findings highlight the role of ion channel variants in the pathogenesis of extreme neuropathic pain disorders, likely mediated through changes in sensory neuron excitability and interaction with environmental triggers.

NaV1.7 mRNA and protein expression in putative projection neurons of the human spinal dorsal horn

NaV1.7, a membrane-bound voltage-gated sodium channel, is preferentially expressed along primary sensory neurons, including their peripheral & central nerve endings, axons, and soma within the dorsal root ganglia and plays an integral role in amplifying membrane depolarization and pain neurotransmission. Loss- and gain-of-function mutations in the gene encoding NaV1.7, SCN9A, are associated with a complete loss of pain sensation or exacerbated pain in humans, respectively. As an enticing pain target supported by human genetic validation, many compounds have been developed to inhibit NaV1.7 but have disappointed in clinical trials. The underlying reasons are still unclear, but recent reports suggest that inhibiting NaV1.7 in central terminals of nociceptor afferents is critical for achieving pain relief by pharmacological inhibition of NaV1.7. We report for the first time that NaV1.7 mRNA is expressed in putative projection neurons (NK1R+) in the human spinal dorsal horn, predominantly in lamina 1 and 2, as well as in deep dorsal horn neurons and motor neurons in the ventral horn. NaV1.7 protein was found in the central axons of sensory neurons terminating in lamina 1-2, but also was detected in the axon initial segment of resident spinal dorsal horn neurons and in axons entering the anterior commissure. Given that projection neurons are critical for conveying nociceptive information from the dorsal horn to the brain, these data support that dorsal horn NaV1.7 expression may play an unappreciated role in pain phenotypes observed in humans with genetic SCN9A mutations, and in achieving analgesic efficacy in clinical trials.