A novel Nav1.7 mutation producing carbamazepine-responsive erythromelalgia

Erythromelalgia caused by the missense mutation p.Arg220Pro in an alternatively spliced exon of SCN9A (NaV1.7)

Erythromelalgia (EM), is a familial pain syndrome characterized by episodic 'burning' pain, warmth, and erythema. EM is caused by monoallelic variants in SCN9A, which encodes the voltage-gated sodium channel (NaV) NaV1.7. Over 25 different SCN9A mutations attributed to EM have been described to date, all identified in the SCN9A transcript utilizing exon 6N. Here we report a novel SCN9A missense variant identified in seven related individuals with stereotypic episodes of bilateral lower limb pain presenting in childhood. The variant, XM_011511617.3:c.659G>C;p.(Arg220Pro), resides in the exon 6A of SCN9A, an exon previously shown to be selectively incorporated by developmentally regulated alternative splicing. The mutation is located in the voltage-sensing S4 segment of domain I, which is important for regulating channel activation. Functional analysis showed the p.Arg220Pro mutation altered voltage-dependent activation and delayed channel inactivation, consistent with a NaV1.7 gain-of-function molecular phenotype. These results demonstrate that alternatively spliced isoforms of SCN9A should be included in all genomic testing of EM.

Clinical Characterization of Pediatric Erythromelalgia: A Single-Center Case Series

Erythromelalgia is a descriptive term for severe burning pain and erythema in the distal extremities relieved by cold and exacerbated by heat. Pediatric case series to date are relatively small. We extracted and analyzed medical record data for 42 pediatric patients to describe clinical characteristics, associated conditions, and responses to treatments. Informed consent was obtained according to an IRB-approved protocol that included gene discovery. Three patients had confirmed Nav1.7 sodium channelopathies, with six additional patients under investigation with novel gene candidates. There was a female predominance (2.5:1), and the median onset age was 12 years (IQR = 3-14). Patients saw a median of three specialists (IQR = 2-3) for a diagnosis. The majority (90%) reported bilateral symptoms. Cooling methods usually provided partial relief, while heat and exercise exacerbated pain. No medication appeared to be consistently effective; commonly prescribed medications included sodium channel blockers (n = 37), topical analgesics (n = 26), gabapentin (n = 22), and aspirin (n = 15). Based on the currently published literature, we believe this cohort is the largest pediatric study of erythromelalgia to date. Many findings are consistent with those of previously published case series. Work is in progress to establish a prospective cohort and multi-center registry.

Structural Conformation and Activity of Spider-Derived Inhibitory Cystine Knot Peptide Pn3a Are Modulated by pH

Numerous spider venom-derived gating modifier toxins exhibit conformational heterogeneity during purification by reversed-phase high-performance liquid chromatography (RP-HPLC). This conformational exchange is especially peculiar for peptides containing an inhibitor cystine knot motif, which confers excellent structural stability under conditions that are not conducive to disulfide shuffling. This phenomenon is often attributed to proline cis/trans isomerization but has also been observed in peptides that do not contain a proline residue. Pn3a is one such peptide forming two chromatographically distinguishable peaks that readily interconvert following the purification of either conformer. The nature of this exchange was previously uncharacterized due to the fast rate of conversion in solution, making isolation of the conformers impossible. In the present study, an N-terminal modification of Pn3a enabled the isolation of the individual conformers, allowing activity assays to be conducted on the individual conformers using electrophysiology. The conformers were analyzed separately by nuclear magnetic resonance spectroscopy (NMR) to study their structural differences. RP-HPLC and NMR were used to study the mechanism of exchange. The later-eluting conformer was the active conformer with a rigid structure that corresponds to the published structure of Pn3a, while NMR analysis revealed the earlier-eluting conformer to be inactive and disordered. The exchange was found to be pH-dependent, arising in acidic solutions, possibly due to reversible disruption and formation of intramolecular salt bridges. This study reveals the nature of non-proline conformational exchange observed in Pn3a and possibly other disulfide-rich peptides, highlighting that the structure and activity of some disulfide-stabilized peptides can be dramatically susceptible to disruption.

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.

Changes in Potency and Subtype Selectivity of Bivalent NaV Toxins are Knot-Specific

Disulfide-rich peptide toxins have long been studied for their ability to inhibit voltage-gated sodium channel subtype Na_V1.7, a validated target for the treatment of pain. In this study, we sought to combine the pore blocking activity of conotoxins with the gating modifier activity of spider toxins to design new bivalent inhibitors of Na_V1.7 with improved potency and selectivity. To do this, we created an array of heterodimeric toxins designed to target human Na_V1.7 by ligating a conotoxin to a spider toxin and assessed the potency and selectivity of the resulting bivalent toxins. A series of spider-derived gating modifier toxins (GpTx-1, ProTx-II, gHwTx-IV, JzTx-V, CcoTx-1, and Pn3a) and two pore-blocker μ-conotoxins, SxIIIC and KIIIA, were used for this study. We employed either enzymatic ligation with sortase A for C- to N-terminal ligation or click chemistry for N- to N-terminal ligation. The bivalent peptide resulting from ligation of ProTx-II and SxIIIC (Pro[LPATG₆]Sx) was shown to be the best combination as native ProTx-II potency at hNa_V1.7 was conserved following ligation. At hNa_V1.4, a synergistic effect between the pore blocker and gating modifier toxin moieties was observed, resulting in altered sodium channel subtype selectivity compared to the parent peptides. Further studies including mutant bivalent peptides and mutant hNa_V1.7 channels suggested that gating modifier toxins have a greater contribution to the potency of the bivalent peptides than pore blockers. This study delineated potential benefits and drawbacks of designing pharmacological hybrid peptides targeting hNa_V1.7.

Pharmacotherapeutic Options for Managing Neuropathic Pain: A Systematic Review and Meta-Analysis

Despite an increasing number of available therapies, the treatment of neuropathic pain remains a major issue. Systematic reviews and meta-analyses indicate that only a minority of patients with neuropathic pain have an adequate response to pharmacological treatment and that most drugs have dose-limiting side effects. We conducted a systematic review and meta-analysis of randomised controlled trials published in the last five years. We searched for relevant papers within PubMed, EMBASE, the Cochrane Database of Systematic Reviews, and the Clinical Trials database (ClinicalTrials.gov). Two authors independently selected studies for inclusion, data extraction, and bias assessment. We identified 39 randomised controlled trials and included 16 in the meta-analysis. Trial outcomes were generally modest even for first-line drugs such as tricyclic antidepressants, serotonin-noradrenaline reuptake inhibitors, and gabapentinoids. Many drugs acting on new pain targets are currently under development. Clinical data are currently available for sodium channel isoform-specific antagonists, anti-nerve growth factor molecules, and fatty acid amide hydrolase inhibitors.

Hydropathicity-based prediction of pain-causing NaV1.7 variants

Background

Mutation-induced variations in the functional architecture of the NaV1.7 channel protein are causally related to a broad spectrum of human pain disorders. Predicting in silico the phenotype of NaV1.7 variant is of major clinical importance; it can aid in reducing costs of in vitro pathophysiological characterization of NaV1.7 variants, as well as, in the design of drug agents for counteracting pain-disease symptoms.

Results

In this work, we utilize spatial complexity of hydropathic effects toward predicting which NaV1.7 variants cause pain (and which are neutral) based on the location of corresponding mutation sites within the NaV1.7 structure. For that, we analyze topological and scaling hydropathic characteristics of the atomic environment around NaV1.7’s pore and probe their spatial correlation with mutation sites. We show that pain-related mutation sites occupy structural locations in proximity to a hydrophobic patch lining the pore while clustering at a critical hydropathic-interactions distance from the selectivity filter (SF). Taken together, these observations can differentiate pain-related NaV1.7 variants from neutral ones, i.e., NaV1.7 variants not causing pain disease, with 80.5\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\%$$\end{document}% sensitivity and 93.7\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\%$$\end{document}% specificity [area under the receiver operating characteristics curve = 0.872].

Conclusions

Our findings suggest that maintaining hydrophobic NaV1.7 interior intact, as well as, a finely-tuned (dictated by hydropathic interactions) distance from the SF might be necessary molecular conditions for physiological NaV1.7 functioning. The main advantage for using the presented predictive scheme is its negligible computational cost, as well as, hydropathicity-based biophysical rationalization.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12859-021-04119-2.

Trigeminal neuralgia: An overview from pathophysiology to pharmacological treatments

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

Status of peripheral sodium channel blockers for non-addictive pain treatment

The effective and safe treatment of pain is an unmet health-care need. Current medications used for pain management are often only partially effective, carry dose-limiting adverse effects and are potentially addictive, highlighting the need for improved therapeutic agents. Most common pain conditions originate in the periphery, where dorsal root ganglion and trigeminal ganglion neurons feed pain information into the CNS. Voltage-gated sodium (Na_V) channels drive neuronal excitability and three subtypes - Na_V1.7, Na_V1.8 and Na_V1.9 - are preferentially expressed in the peripheral nervous system, suggesting that their inhibition might treat pain while avoiding central and cardiac adverse effects. Genetic and functional studies of human pain disorders have identified Na_V1.7, Na_V1.8 and Na_V1.9 as mediators of pain and validated them as targets for pain treatment. Consequently, multiple Na_V1.7-specific and Na_V1.8-specific blockers have undergone clinical trials, with others in preclinical development, and the targeting of Na_V1.9, although hampered by technical constraints, might also be moving ahead. In this Review, we summarize the clinical and preclinical literature describing compounds that target peripheral Na_V channels and discuss the challenges and future prospects for the field. Although the potential of peripheral Na_V channel inhibition for the treatment of pain has yet to be realized, this remains a promising strategy to achieve non-addictive analgesia for multiple pain conditions.

Computational pipeline to probe NaV1.7 gain-of-function variants in neuropathic painful syndromes

Applications of machine learning and graph theory techniques to neuroscience have witnessed an increased interest in the last decade due to the large data availability and unprecedented technology developments. Their employment to investigate the effect of mutational changes in genes encoding for proteins modulating the membrane of excitable cells, whose biological correlates are assessed at electrophysiological level, could provide useful predictive clues. We apply this concept to the analysis of variants in sodium channel NaV1.7 subunit found in patients with chronic painful syndromes, by the implementation of a dedicated computational pipeline empowering different and complementary techniques including homology modeling, network theory, and machine learning. By testing three templates of different origin and sequence identities, we provide an optimal condition for its use. Our findings reveal the usefulness of our computational pipeline in supporting the selection of candidates for cell electrophysiology assay and with potential clinical applications.

Differential effect of lacosamide on Nav1.7 variants from responsive and non-responsive patients with small fibre neuropathy

Small fibre neuropathy is a common pain disorder, which in many cases fails to respond to treatment with existing medications. Gain-of-function mutations of voltage-gated sodium channel Na_v1.7 underlie dorsal root ganglion neuronal hyperexcitability and pain in a subset of patients with small fibre neuropathy. Recent clinical studies have demonstrated that lacosamide, which blocks sodium channels in a use-dependent manner, attenuates pain in some patients with Na_v1.7 mutations; however, only a subgroup of these patients responded to the drug. Here, we used voltage-clamp recordings to evaluate the effects of lacosamide on five Na_v1.7 variants from patients who were responsive or non-responsive to treatment. We show that, at the clinically achievable concentration of 30 μM, lacosamide acts as a potent sodium channel inhibitor of Na_v1.7 variants carried by responsive patients, via a hyperpolarizing shift of voltage-dependence of both fast and slow inactivation and enhancement of use-dependent inhibition. By contrast, the effects of lacosamide on slow inactivation and use-dependence in Na_v1.7 variants from non-responsive patients were less robust. Importantly, we found that lacosamide selectively enhances fast inactivation only in variants from responders. Taken together, these findings begin to unravel biophysical underpinnings that contribute to responsiveness to lacosamide in patients with small fibre neuropathy carrying select Na_v1.7 variants.

Painful and painless mutations of SCN9A and SCN11A voltage-gated sodium channels

Chronic pain is a global problem affecting up to 20% of the world’s population and has a significant economic, social and personal cost to society. Sensory neurons of the dorsal root ganglia (DRG) detect noxious stimuli and transmit this sensory information to regions of the central nervous system (CNS) where activity is perceived as pain. DRG neurons express multiple voltage-gated sodium channels that underlie their excitability. Research over the last 20 years has provided valuable insights into the critical roles that two channels, Na_V1.7 and Na_V1.9, play in pain signalling in man. Gain of function mutations in Na_V1.7 cause painful conditions while loss of function mutations cause complete insensitivity to pain. Only gain of function mutations have been reported for Na_V1.9. However, while most Na_V1.9 mutations lead to painful conditions, a few are reported to cause insensitivity to pain. The critical roles these channels play in pain along with their low expression in the CNS and heart muscle suggest they are valid targets for novel analgesic drugs.

Manipulation of a spider peptide toxin alters its affinity for lipid bilayers and potency and selectivity for voltage-gated sodium channel subtype 1.7

Huwentoxin-IV (HwTx-IV) is a gating modifier peptide toxin from spiders that has weak affinity for the lipid bilayer. As some gating modifier toxins have affinity for model lipid bilayers, a tripartite relationship among gating modifier toxins, voltage-gated ion channels, and the lipid membrane surrounding the channels has been proposed. We previously designed an HwTx-IV analogue (gHwTx-IV) with reduced negative charge and increased hydrophobic surface profile, which displays increased lipid bilayer affinity and in vitro activity at the voltage-gated sodium channel subtype 1.7 (NaV1.7), a channel targeted in pain management. Here, we show that replacements of the positively-charged residues that contribute to the activity of the peptide can improve gHwTx-IV's potency and selectivity for NaV1.7. Using HwTx-IV, gHwTx-IV, [R26A]gHwTx-IV, [K27A]gHwTx-IV, and [R29A]gHwTx-IV variants, we examined their potency and selectivity at human NaV1.7 and their affinity for the lipid bilayer. [R26A]gHwTx-IV consistently displayed the most improved potency and selectivity for NaV1.7, examined alongside off-target NaVs, compared with HwTx-IV and gHwTx-IV. The lipid affinity of each of the three novel analogues was weaker than that of gHwTx-IV, but stronger than that of HwTx-IV, suggesting a possible relationship between in vitro potency at NaV1.7 and affinity for lipid bilayers. In a murine NaV1.7 engagement model, [R26A]gHwTx-IV exhibited an efficacy comparable with that of native HwTx-IV. In summary, this study reports the development of an HwTx-IV analogue with improved in vitro selectivity for the pain target NaV1.7 and with an in vivo efficacy similar to that of native HwTx-IV.

Translational issues in precision medicine in neuropathic pain

Neuropathic pain remains poorly treated, with most new drugs falling through the translational gap. The traditional model of bench-to-bedside research has relied on identifying new mechanisms/targets in animal models and then developing clinical applications. Several have advocated bridging the translational gap by beginning with clinical observations and back-translating to animal models for further investigation of mechanisms. There is good evidence that phenotyping of patients through quantitative sensory testing can lead to improved treatment selection and hence improved patient outcomes. This practice has been widely adopted in clinical investigations, but its application in preclinical research is not mainstream. In this review, we retrospectively examine our historical rodent data sets with the aim of reconsidering drug effects on sensory neuronal endpoints, their alignment with clinical observations, and how these might guide future clinical studies.

The Genetics of Neuropathic Pain from Model Organisms to Clinical Application

Neuropathic pain (NeuP) arises due to injury of the somatosensory nervous system and is both common and disabling, rendering an urgent need for non-addictive, effective new therapies. Given the high evolutionary conservation of pain, investigative approaches from Drosophila mutagenesis to human Mendelian genetics have aided our understanding of the maladaptive plasticity underlying NeuP. Successes include the identification of ion channel variants causing hyper-excitability and the importance of neuro-immune signaling. Recent developments encompass improved sensory phenotyping in animal models and patients, brain imaging, and electrophysiology-based pain biomarkers, the collection of large well-phenotyped population cohorts, neurons derived from patient stem cells, and high-precision CRISPR generated genetic editing. We will discuss how to harness these resources to understand the pathophysiological drivers of NeuP, define its relationship with comorbidities such as anxiety, depression, and sleep disorders, and explore how to apply these findings to the prediction, diagnosis, and treatment of NeuP in the clinic.

A case of sporadic erythromelalgia presenting with small fibre neuropathy

Erythromelalgia (EM) is a rare disorder of small nerve fibres that leads to painful flushing and burning paresthesisas of the distal extremities and is typically associated with heat or physical activity; relief is found using cooling measures. Its effects are often debilitating in the general population, but this patient had an excellent response to specific treatment options and continues to maintain employment, something many individuals suffering from EM are unable to do. His presentation was also unique in that he had isolated, proximal involvement as his condition progressed whereas typically only the distal extremities are affected. Routine electromyography and nerve conduction studies were normal, whereas nerve biopsy demonstrated findings of small fibre neuropathy. Ultimately, his condition was managed with carbamazepine and his symptoms have almost entirely resolved to date.

[Erythromelalgia: skin redness and pain]

Erythromelalgia is a rare disease that is associated with hemato-oncological diseases or after taking certain drugs and toxins, but it can also occur as an independent clinical picture, for example, due to mutations in the sodium channel Na_V1.7. Clinically, there is a characteristic triad of attack-like burning pain and skin redness in the area of the distal extremities, which can be alleviated by excessive cooling. The attacks are triggered by heat, exertion, and stress. The diagnosis is primarily made clinically and can be confirmed by genetic testing if a sodium channel Na_V1.7 mutation is present. Important differential diagnoses are complex regional pain syndrome, the non-freezing cold injury, and small fiber neuropathies. Therapy is multidisciplinary and has to be planned individually and include physical therapy and psychotherapy as well as drug therapy as integral components.

Pharmacogenomics in chronic pain therapy: from disease to treatment and challenges for clinical practice

Pharmacogenomics (PGx) has emerged as an encouraging tool in chronic pain therapy. Genetic variations associated with drug effectiveness or adverse reactions (amitriptyline/nortriptyline/codeine/oxycodone/tramadol-CYP2D6, amitriptyline-CYP2C19, carbamazepine-HLA-A, carbamazepine/oxcarbazepine-HLA-B) can be used to guide chronic pain management. Despite this evidence, many obstacles still need to be overcome for the effective clinical implementation of PGx. To translate the pharmacogenetic testing into actionable clinical decisions, the Clinical Pharmacogenetics Implementation Consortium has been developing guidelines for several drug–gene pairs. This review will show the applicability of PGx in chronic pain from disease to treatment; report the drug–gene pairs with strongest evidences in the clinic; and the challenges for the clinical implementation of PGx.

Sodium Channels in Human Pain Disorders: Genetics and Pharmacogenomics

Acute pain is adaptive, but chronic pain is a global challenge. Many chronic pain syndromes are peripheral in origin and reflect hyperactivity of peripheral pain-signaling neurons. Current treatments are ineffective or only partially effective and in some cases can be addictive, underscoring the need for better therapies. Molecular genetic studies have now linked multiple human pain disorders to voltage-gated sodium channels, including disorders characterized by insensitivity or reduced sensitivity to pain and others characterized by exaggerated pain in response to normally innocuous stimuli. Here, we review recent developments that have enhanced our understanding of pathophysiological mechanisms in human pain and advances in targeting sodium channels in peripheral neurons for the treatment of pain using novel and existing sodium channel blockers.

Pointer-kindreds and pain: big lessons from small families

Small families carrying rare mutations, which I call "pointer-kindreds," can teach us important lessons. Here, I provide some examples from the field of 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.

Pediatric Erythromelalgia and SCN9A Mutations: Systematic Review and Single-Center Case Series

OBJECTIVES

To evaluate the clinical features of erythromelalgia in childhood associated with gain-of-function SCN9A mutations that increase activity of the Na_v1.7 voltage-gated sodium channel, we conducted a systematic review of pediatric presentations of erythromelalgia related to SCN9A mutations, and compared pediatric clinical presentations of symptomatic erythromelalgia, with or without SCN9A mutations.

STUDY DESIGN

PubMed, Embase, and PsycINFO Databases were searched for reports of inherited erythromelalgia in childhood. Clinical features, management, and genotype were extracted. Case notes of pediatric patients with erythromelalgia from the Great Ormond Street Hospital Pain Service were reviewed for clinical features, patient-reported outcomes, and treatments. Children aged over 10 years were recruited for quantitative sensory testing.

RESULTS

Twenty-eight publications described erythromelalgia associated with 15 different SCN9A gene variants in 25 children. Pain was severe and often refractory to multiple treatments, including nonspecific sodium channel blockers. Skin damage or other complications of cold immersion for symptomatic relief were common (60%). SCN9A mutations resulting in greater hyperpolarizing shifts in Na_v1.7 sodium channels correlated with symptom onset at younger ages (P = .016). Variability in reporting, and potential publication bias toward severe cases, limit any estimations of overall prevalence. In our case series, symptoms were similar but comorbidities were more common in children with SCN9A mutations. Quantitative sensory testing revealed marked dynamic warm allodynia.

CONCLUSIONS

Inherited erythromelalgia in children is associated with difficult-to-manage pain and significant morbidity. Standardized reporting of outcome and management in larger series will strengthen identification of genotype-phenotype relationships. More effective long-term therapies are a significant unmet clinical need.

Neuropathy following spinal nerve injury shares features with the irritable nociceptor phenotype: A back-translational study of oxcarbazepine

BACKGROUND

The term 'irritable nociceptor' was coined to describe neuropathic patients characterized by evoked hypersensitivity and preservation of primary afferent fibres. Oxcarbazepine is largely ineffectual in an overall patient population, but has clear efficacy in a subgroup with the irritable nociceptor profile. We examine whether neuropathy in rats induced by spinal nerve injury shares overlapping pharmacological sensitivity with the irritable nociceptor phenotype using drugs that target sodium channels.

METHODS

In vivo electrophysiology was performed in anaesthetized spinal nerve ligated (SNL) and sham-operated rats to record from wide dynamic range (WDR) neurones in the ventral posterolateral thalamus (VPL) and dorsal horn.

RESULTS

In neuropathic rats, spontaneous activity in the VPL was substantially attenuated by spinal lidocaine, an effect that was absent in sham rats. The former measure was in part dependent on ongoing peripheral activity as intraplantar lidocaine also reduced aberrant spontaneous thalamic firing. Systemic oxcarbazepine had no effect on wind-up of dorsal horn neurones in sham and SNL rats. However, in SNL rats, oxcarbazepine markedly inhibited punctate mechanical-, dynamic brush- and cold-evoked neuronal responses in the VPL and dorsal horn, with minimal effects on heat-evoked responses. In addition, oxcarbazepine inhibited spontaneous activity in the VPL. Intraplantar injection of the active metabolite licarbazepine replicated the effects of systemic oxcarbazepine, supporting a peripheral locus of action.

CONCLUSIONS

We provide evidence that ongoing activity in primary afferent fibres drives spontaneous thalamic firing after spinal nerve injury and that oxcarbazepine through a peripheral mechanism exhibits modality-selective inhibitory effects on sensory neuronal processing.

SIGNIFICANCE

The inhibitory effects of lidocaine and oxcarbazepine in this rat model of neuropathy resemble the clinical observations in the irritable nociceptor patient subgroup and support a mechanism-based rationale for bench-to-bedside translation when screening novel drugs.

Bulleyaconitine A attenuates hyperexcitability of dorsal root ganglion neurons induced by spared nerve injury: The role of preferably blocking Nav1.7 and Nav1.3 channels

Background

Oral administration of Bulleyaconitine A, an extracted diterpenoid alkaloid from Aconitum bulleyanum plants, is effective for treating chronic pain in rats and in human patients, but the underlying mechanisms are poorly understood.

Results

As the hyperexcitability of dorsal root ganglion neurons resulting from the upregulation of voltage-gated sodium (Nav) channels has been proved critical for development of chronic pain, we tested the effects of Bulleyaconitine A on Nav channels in rat spared nerve injury model of neuropathic pain. We found that Bulleyaconitine A at 5 nM increased the threshold of action potentials and reduced the firing rate of dorsal root ganglion neurons in spared nerve injury rats but not in sham rats. Bulleyaconitine A preferably blocked tetrodotoxin-sensitive Nav channels over tetrodotoxin-resistant ones in dorsal root ganglion neurons of spared nerve injury rats. Bulleyaconitine A was more potent for blocking Nav1.3 and Nav1.7 than Nav1.8 in cell lines. The half maximal inhibitory concentration (IC₅₀) values for resting Nav1.3, Nav1.7, and Nav1.8 were 995.6 ± 139.1 nM, 125.7 ± 18.6 nM, and 151.2 ± 15.4 μM, respectively, which were much higher than those for inactivated Nav1.3 (20.3 ± 3.4 pM), Nav1.7 (132.9 ± 25.5 pM), and Nav1.8 (18.0 ± 2.5 μM). The most profound use-dependent blocking effect of Bulleyaconitine A was observed on Nav1.7, less on Nav1.3, and least on Nav1.8 at IC₅₀ concentrations. Bulleyaconitine A facilitated the inactivation of Nav channels in each subtype.

Conclusions

Preferably blocking tetrodotoxin-sensitive Nav1.7 and Nav1.3 in dorsal root ganglion neurons may contribute to Bulleyaconitine A’s antineuropathic pain effect.

Inhibition of the Ubc9 E2 SUMO-conjugating enzyme-CRMP2 interaction decreases NaV1.7 currents and reverses experimental neuropathic pain

We previously reported that destruction of the small ubiquitin-like modifier (SUMO) modification site in the axonal collapsin response mediator protein 2 (CRMP2) was sufficient to selectively decrease trafficking of the voltage-gated sodium channel NaV1.7 and reverse neuropathic pain. Here, we further interrogate the biophysical nature of the interaction between CRMP2 and the SUMOylation machinery, and test the hypothesis that a rationally designed CRMP2 SUMOylation motif (CSM) peptide can interrupt E2 SUMO-conjugating enzyme Ubc9-dependent modification of CRMP2 leading to a similar suppression of NaV1.7 currents. Microscale thermophoresis and amplified luminescent proximity homogeneous alpha assay revealed a low micromolar binding affinity between CRMP2 and Ubc9. A heptamer peptide harboring CRMP2's SUMO motif, also bound with similar affinity to Ubc9, disrupted the CRMP2-Ubc9 interaction in a concentration-dependent manner. Importantly, incubation of a tat-conjugated cell-penetrating peptide (t-CSM) decreased sodium currents, predominantly NaV1.7, in a model neuronal cell line. Dialysis of t-CSM peptide reduced CRMP2 SUMOylation and blocked surface trafficking of NaV1.7 in rat sensory neurons. Fluorescence dye-based imaging in rat sensory neurons demonstrated inhibition of sodium influx in the presence of t-CSM peptide; by contrast, calcium influx was unaffected. Finally, t-CSM effectively reversed persistent mechanical and thermal hypersensitivity induced by a spinal nerve injury, a model of neuropathic pain. Structural modeling has now identified a pocket-harboring CRMP2's SUMOylation motif that, when targeted through computational screening of ligands/molecules, is expected to identify small molecules that will biochemically and functionally target CRMP2's SUMOylation to reduce NaV1.7 currents and reverse neuropathic pain.

Pain management in children with erythromelalgia: case report

Erythromelalgia is a neuropathic pain syndrome due to an autosomal dominant gene, characterized by erythema, increased skin temperature and burning pain in hands and feet, whose treatment is often unsatisfactory. In this paper, we report a case of a 9 years old female patient whose first episode of burning pain, erythema and edema of the hands, without triggering factors, had instant relief after immersion in cold water. She presented with systemic arterial hypertension and had seizures. The patient was treated with gabapentin (150 mg.8 h⁻¹) and amitriptyline (12.5 mg) orally, intravenous lidocaine infusion (120 mg), without relieving pain complaints. Due to the lack of response to the proposed treatment, it was decided to gradually reduce these medications and to introduce carbamazepine (200 mg) orally and, after 4 days of treatment, there was complete relief of the manifestations.

Current pain management strategies for patients with erythromelalgia: a critical review

Erythromelalgia (EM) is a rare disorder characterized by erythematous, warm, painful extremities, which is often precipitated by cold conditions. The pathophysiology of EM is incompletely understood. Recent investigations have identified sodium channelopathy as a genetic cause for this pain condition, classified as primary inherited EM. Other subtypes are idiopathic EM and secondary EM. The management of pain in EM is challenging as no single therapy has been found to be effective. There is varying response to pharmacotherapy and significant variability within this clinical population, resulting in a stepwise trial and error approach. Consequently, EM is often associated with poorer health-related quality of life with higher morbidity. There is currently no consensus or guidelines on management of pain in EM. This is a review of the literature on management of pain using pharmacologic, procedural intervention and nonpharmacologic treatment in children and adults with EM.

Chemical lumbar sympathectomy in the treatment of recalcitrant erythromelalgia

OBJECTIVE

Erythromelalgia is highly disabling and treatment is often very challenging. There have been solitary case reports that it might benefit from sympathectomy. This study sought to evaluate the short-term and long-term efficacy of chemical lumbar sympathectomy (CLS) for treatment of recalcitrant erythromelalgia and try to identify a CLS-responsive subset.

METHODS

Patients with recalcitrant erythromelalgia were recruited from a tertiary hospital over a 10-year period. L3 to L4 CLS was performed using 5% phenol. The pain intensity score (visual analog scale [VAS] 0-10) was assessed before CLS and at 1 day, 1 week, 3 months, 6 months, 1 year, and 2 years after CLS. A VAS decrease of 90%-100% is defined as complete response, 60%-89% as major partial response. Relapse was defined by a return of a VAS score of 5 or higher. SCN9A gene mutations were screened.

RESULTS

Thirteen patients were enrolled, with a median age of 15 years. The mean follow-up was 6.2 ± 3.8 years. SCN9A gene mutation was identified in five patients having family histories. The VAS was 8.2 ± 2.0 at baseline; it decreased to 4.9 ± 2.7 at 1 day and 1.9 ± 3.0 at 1 week after CLS. Nine patients (69.2%) achieved complete response at 1 week after CLS, including three patients with SCN9A gene mutation. Among the three complete response patients having the gene mutation, two reverted to major partial response and one relapsed at 2 years after CLS. Among the six complete response patients without mutation, five maintained complete response and one relapsed. Among the four patients who did not achieve complete response, one patient died at 3.5 months and one patient had an amputation performed at 4 months after CLS.

CONCLUSIONS

CLS provides a valid option for the treatment of recalcitrant erythromelalgia. It takes about 1 week to achieve full efficacy. Relapse may occur, especially in patients with an SCN9A gene mutation.

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.

Reverse pharmacogenomics: carbamazepine normalizes activation and attenuates thermal hyperexcitability of sensory neurons due to Nav 1.7 mutation I234T

BACKGROUND AND PURPOSE

Pharmacotherapy for pain currently involves trial and error. A previous study on inherited erythromelalgia (a genetic model of neuropathic pain due to mutations in the sodium channel, Nav 1.7) used genomics, structural modelling and biophysical and pharmacological analyses to guide pharmacotherapy and showed that carbamazepine normalizes voltage dependence of activation of the Nav 1.7-S241T mutant channel, reducing pain in patients carrying this mutation. However, whether this approach is applicable to other Nav channel mutants is still unknown.

EXPERIMENTAL APPROACH

We used structural modelling, patch clamp and multi-electrode array (MEA) recording to assess the effects of carbamazepine on Nav 1.7-I234T mutant channels and on the firing of dorsal root ganglion (DRG) sensory neurons expressing these mutant channels.

KEY RESULTS

In a reverse engineering approach, structural modelling showed that the I234T mutation is located in atomic proximity to the carbamazepine-responsive S241T mutation and that activation of Nav 1.7-I234T mutant channels, from patients who are known to respond to carbamazepine, is partly normalized with a clinically relevant concentration (30 μM) of carbamazepine. There was significantly higher firing in intact sensory neurons expressing Nav 1.7-I234T channels, compared with neurons expressing the normal channels (Nav 1.7-WT). Pre-incubation with 30 μM carbamazepine also significantly reduced the firing of intact DRG sensory neurons expressing Nav 1.7-I234T channels. Although the expected use-dependent inhibition of Nav 1.7-WT channels by carbamazepine was confirmed, carbamazepine did not enhance use-dependent inhibition of Nav 1.7-I234T mutant channels.

CONCLUSION AND IMPLICATIONS

These results support the utility of a pharmacogenomic approach to treatment of pain in patients carrying sodium channel variants.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Co-expression of β Subunits with the Voltage-Gated Sodium Channel NaV1.7: the Importance of Subunit Association and Phosphorylation and Their Effects on Channel Pharmacology and Biophysics

The voltage-gated sodium ion channel Na_V1.7 is crucial in pain signaling. We examined how auxiliary β2 and β3 subunits and the phosphorylation state of the channel influence its biophysical properties and pharmacology. The human Na_V1.7α subunit was co-expressed with either β2 or β3 subunits in HEK-293 cells. The β2 subunits and the Na_V1.7α, however, were barely associated as evidenced by immunoprecipitation. Therefore, the β2 subunits did not change the biophysical properties of the channel. In contrast, β3 subunit was clearly associated with Na_V1.7α. This subunit had a significant degree of glycosylation, and only the fully glycosylated β3 subunit was associated with the Na_V1.7α. Electrophysiological characterisation revealed that the β3 subunit had small but consistent effects: a right-hand shift of the steady-state inactivation and faster recovery from inactivation. Furthermore, the β3 subunit reduced the susceptibility of Na_V1.7α to several sodium channel blockers. In addition, we assessed the functional effect of Na_V1.7α phosphorylation. Inhibition of kinase activity increased channel inactivation, while the blocking phosphatases produced the opposite effect. In conclusion, co-expression of β subunits with Na_V1.7α, to better mimic the native channel properties, may be ineffective in cases when subunits are not associated, as shown in our experiments with β2. The β3 subunit significantly influences the function of Na_V1.7α and, together with the phosphorylation of the channel, regulates its biophysical and pharmacological properties. These are important findings to take into account when considering the role of Na_V1.7 channel in pain signaling.

Enhancing inactivation rather than reducing activation of Nav1.7 channels by a clinically effective analgesic CNV1014802

The Nav1.7 channel represents a promising target for pain relief. In the recent decades, a number of Nav1.7 channel inhibitors have been developed. According to the effects on channel kinetics, these inhibitors could be divided into two major classes: reducing activation or enhancing inactivation. To date, however, only several inhibitors have moved forward into phase 2 clinical trials and most of them display a less than ideal analgesic efficacy, thus intensifying the controversy regarding if an ideal candidate should preferentially affect the activation or inactivation state. In the present study, we investigated the action mechanisms of a recently clinically confirmed inhibitor CNV1014802 using both electrophysiology and site-directed mutagenesis. We found that CNV1014802 inhibited Nav1.7 channels through stabilizing a nonconductive inactivated state. When the cells expressing Nav1.7 channels were hold at 70 mV or 120 mV, the half maximal inhibitory concentration (IC50) values (with 95% confidence limits) were 1.77 (1.20-2.33) and 71.66 (46.85-96.48) μmol/L, respectively. This drug caused dramatic hyperpolarizing shift of channel inactivation but did not affect activation. Moreover, CNV1014802 accelerated the onset of inactivation and delayed the recovery from inactivation. Notably, application of CNV1014802 (30 μmol/L) could rescue the Nav1.7 mutations expressed in CHO cells that cause paroxysmal extreme pain disorder (PEPD), thereby restoring the impaired inactivation to those of the wild-type channel. Our study demonstrates that CNV1014802 enhances the inactivation but does not reduce the activation of Nav1.7 channels, suggesting that identifying inhibitors that preferentially affect inactivation is a promising approach for developing drugs targeting Nav1.7.

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.

Between fire and ice: refractory hypothermia and warmth-induced pain in inherited erythromelalgia

Inherited erythromelalgia (IEM) is a well-described pain disorder caused by mutations of sodium channel Na_v1.7, a peripheral channel expressed within dorsal root ganglion and the sympathetic ganglion neurons. Clinically, IEM is characterised by paroxysmal attacks of severe pain, usually in the distal extremities, triggered by warmth or exercise. Pain is not adequately treated by existing pharmacological agents. Individuals with IEM classically cool their limbs for relief, in some cases resulting in tissue injury. We describe a patient from a family with IEM due to the L858F mutation of Na_v1.7 who presented with refractory hypothermia due to overcooling. This presentation of refractory hypothermia necessitating warming strategies, complicated by severe warmth-induced pain, posed a substantial therapeutic challenge. We report our experience in overcoming hypothermia lasting 3 weeks in a child with IEM, discuss possible pathophysiological mechanisms underlying this unusual complication and suggest potential therapeutic interventions.

Network topology of NaV1.7 mutations in sodium channel-related painful disorders

BACKGROUND

Gain-of-function mutations in SCN9A gene that encodes the voltage-gated sodium channel NaV1.7 have been associated with a wide spectrum of painful syndromes in humans including inherited erythromelalgia, paroxysmal extreme pain disorder and small fibre neuropathy. These mutations change the biophysical properties of NaV1.7 channels leading to hyperexcitability of dorsal root ganglion nociceptors and pain symptoms. There is a need for better understanding of how gain-of-function mutations alter the atomic structure of Nav1.7.

RESULTS

We used homology modeling to build an atomic model of NaV1.7 and a network-based theoretical approach, which can predict interatomic interactions and connectivity arrangements, to investigate how pain-related NaV1.7 mutations may alter specific interatomic bonds and cause connectivity rearrangement, compared to benign variants and polymorphisms. For each amino acid substitution, we calculated the topological parameters betweenness centrality (B ct ), degree (D), clustering coefficient (CC ct ), closeness (C ct ), and eccentricity (E ct ), and calculated their variation (Δ value  = mutant value -WT value ). Pathogenic NaV1.7 mutations showed significantly higher variation of |ΔB ct | compared to benign variants and polymorphisms. Using the cut-off value ±0.26 calculated by receiver operating curve analysis, we found that ΔB ct correctly differentiated pathogenic NaV1.7 mutations from variants not causing biophysical abnormalities (nABN) and homologous SNPs (hSNPs) with 76% sensitivity and 83% specificity.

CONCLUSIONS

Our in-silico analyses predict that pain-related pathogenic NaV1.7 mutations may affect the network topological properties of the protein and suggest |ΔB ct | value as a potential in-silico marker.

Nonlinear effects of hyperpolarizing shifts in activation of mutant Nav1.7 channels on resting membrane potential

The Na_v1.7 sodium channel is preferentially expressed within dorsal root ganglion (DRG) and sympathetic ganglion neurons. Gain-of-function mutations that cause the painful disorder inherited erythromelalgia (IEM) shift channel activation in a hyperpolarizing direction. When expressed within DRG neurons, these mutations produce a depolarization of resting membrane potential (RMP). The biophysical basis for the depolarized RMP has to date not been established. To explore the effect on RMP of the shift in activation associated with a prototypical IEM mutation (L858H), we used dynamic-clamp models that represent graded shifts that fractionate the effect of the mutation on activation voltage dependence. Dynamic-clamp recording from DRG neurons using a before-and-after protocol for each cell made it possible, even in the presence of cell-to-cell variation in starting RMP, to assess the effects of these graded mutant models. Our results demonstrate a nonlinear, progressively larger effect on RMP as the shift in activation voltage dependence becomes more hyperpolarized. The observed differences in RMP were predicted by the "late" current of each mutant model. Since the depolarization of RMP imposed by IEM mutant channels is known, in itself, to produce hyperexcitability of DRG neurons, the development of pharmacological agents that normalize or partially normalize activation voltage dependence of IEM mutant channels merits further study.NEW & NOTEWORTHY Inherited erythromelalgia (IEM), the first human pain disorder linked to a sodium channel, is widely regarded as a genetic model of neuropathic pain. IEM is produced by Na_v1.7 mutations that hyperpolarize activation. These mutations produce a depolarization of resting membrane potential (RMP) in dorsal root ganglion neurons. Using dynamic clamp to explore the effect on RMP of the shift in activation, we demonstrate a nonlinear effect on RMP as the shift in activation voltage dependence becomes more hyperpolarized.

The AMPK Activator A769662 Blocks Voltage-Gated Sodium Channels: Discovery of a Novel Pharmacophore with Potential Utility for Analgesic Development

Voltage-gated sodium channels (VGSC) regulate neuronal excitability by governing action potential (AP) generation and propagation. Recent studies have revealed that AMP-activated protein kinase (AMPK) activators decrease sensory neuron excitability, potentially by preventing sodium (Na⁺) channel phosphorylation by kinases such as ERK or via modulation of translation regulation pathways. The direct positive allosteric modulator A769662 displays substantially greater efficacy than other AMPK activators in decreasing sensory neuron excitability suggesting additional mechanisms of action. Here, we show that A769662 acutely inhibits AP firing stimulated by ramp current injection in rat trigeminal ganglion (TG) neurons. PT1, a structurally dissimilar AMPK activator that reduces nerve growth factor (NGF) -induced hyperexcitability, has no influence on AP firing in TG neurons upon acute application. In voltage-clamp recordings, application of A769662 reduces VGSC current amplitudes. These findings, based on acute A769662 application, suggest a direct channel blocking effect. Indeed, A769662 dose-dependently blocks VGSC in rat TG neurons and in Na_v1.7-transfected cells with an IC₅₀ of ~ 10 μM. A769662 neither displayed use-dependent inhibition nor interacted with the local anesthetic (LA) binding site. Popliteal fossa administration of A769662 decreased noxious thermal responses with a peak effect at 5 mins demonstrating an analgesic effect. These data indicate that in addition to AMPK activation, A769662 acts as a direct blocker/modulator of VGSCs, a potential mechanism enhancing the analgesic property of this compound.

Voltage-gated sodium channels and pain-related disorders

Voltage-gated sodium channels (VGSCs) are heteromeric transmembrane protein complexes. Nine homologous members, SCN1A–11A, make up the VGSC gene family. Sodium channel isoforms display a wide range of kinetic properties endowing different neuronal types with distinctly varied firing properties. Among the VGSCs isoforms, Nav1.7, Nav1.8 and Nav1.9 are preferentially expressed in the peripheral nervous system. These isoforms are known to be crucial in the conduction of nociceptive stimuli with mutations in these channels thought to be the underlying cause of a variety of heritable pain disorders. This review provides an overview of the current literature concerning the role of VGSCs in the generation of pain and heritable pain disorders.

The Biophysical Basis Underlying Gating Changes in the p.V1316A Mutant Nav1.7 Channel and the Molecular Pathogenesis of Inherited Erythromelalgia

The Na_v1.7 channel critically contributes to the excitability of sensory neurons, and gain-of-function mutations of this channel have been shown to cause inherited erythromelalgia (IEM) with neuropathic pain. In this study, we report a case of a severe phenotype of IEM caused by p.V1316A mutation in the Na_v1.7 channel. Mechanistically, we first demonstrate that the Na_vβ4 peptide acts as a gating modifier rather than an open channel blocker competing with the inactivating peptide to give rise to resurgent currents in the Na_v1.7 channel. Moreover, there are two distinct open and two corresponding fast inactivated states in the genesis of resurgent Na⁺ currents. One is responsible for the resurgent route and practically existent only in the presence of Na_vβ4 peptide, whereas the other is responsible for the “silent” route of recovery from inactivation. In this regard, the p.V1316A mutation makes hyperpolarization shift in the activation curve, and depolarization shift in the inactivation curve, vividly uncoupling inactivation from activation. In terms of molecular gating operation, the most important changes caused by the p.V1316A mutation are both acceleration of the transition from the inactivated states to the activated states and deceleration of the reverse transition, resulting in much larger sustained as well as resurgent Na⁺ currents. In summary, the genesis of the resurgent currents in the Na_v1.7 channel is ascribable to the transient existence of a distinct and novel open state promoted by the Na_vβ4 peptide. In addition, S4–5 linker in domain III where V1316 is located seems to play a critical role in activation–inactivation coupling, chiefly via direct modulation of the transitional kinetics between the open and the inactivated states. The sustained and resurgent Na⁺ currents may therefore be correlatively enhanced by specific mutations involving this linker and relevant regions, and thus marked hyperexcitability in corresponding neural tissues as well as IEM symptomatology.

Mutations in the Na_v1.7 sodium channel cause idiopathic erythromelalgia. This study shows that the pathogenic resurgent sodium currents arise via modification of gating behavior rather than via competing pore block by the Na_vβ4 peptide.

The gain-of-function mutation (p.V1316A) of the Na_v1.7 channel causes inherited erythromelalgia (IEM), a disease characterized by extremely enhanced activity in relevant neural tissues that results in neuropathic pain. We found that the p.V1316A mutation alters the basic gating properties of the channel, leading to increased sustained currents during membrane depolarization and resurgent currents during repolarization. Neurons expressing these mutant channels are more difficult to maintain in a hyperpolarized state and are thus more excitable. We demonstrate that there is very likely a distinct set of open/inactivated (O/I) states responsible for the genesis of resurgent currents. We show that the p.V1316A mutation chiefly accelerates the I to O transition in this set, but also decelerates the transitions between different sets of O/I states, to cause the channel gating and cellular excitability changes. Contrary to the conventional view, we find that the Na_vβ4 peptide, a key element responsible for sizable resurgent currents, does not seem to act as a pore blocker that competes with the inactivation peptide. Instead, we show that it acts as a gating modifier of the Na_v1.7 channel. Thus, the DIII/S4–5 linker, where V1316 is located, may play a critical role not only in O/I coupling but also in the couplings between different sets of O/I in the Na_v1.7 channel.