A Novel Gain-of-Function Nav1.9 Mutation in a Child With Episodic Pain

The influence of Nav1.9 channels on intestinal hyperpathia and dysmotility

The Nav1.9 channel is a voltage-gated sodium channel. It plays a vital role in the generation of pain and the formation of neuronal hyperexcitability after inflammation. It is highly expressed in small diameter neurons of dorsal root ganglions and Dogiel II neurons in enteric nervous system. The small diameter neurons in dorsal root ganglions are the primary sensory neurons of pain conduction. Nav1.9 channels also participate in regulating intestinal motility. Functional enhancements of Nav1.9 channels to a certain extent lead to hyperexcitability of small diameter dorsal root ganglion neurons. The hyperexcitability of the neurons can cause visceral hyperalgesia. Intestinofugal afferent neurons and intrinsic primary afferent neurons in enteric nervous system belong to Dogiel type II neurons. Their excitability can also be regulated by Nav1.9 channels. The hyperexcitability of intestinofugal afferent neurons abnormally activate entero-enteric inhibitory reflexes. The hyperexcitability of intrinsic primary afferent neurons disturb peristaltic waves by abnormally activating peristaltic reflexes. This review discusses the role of Nav1.9 channels in intestinal hyperpathia and dysmotility.

Peripheral temperature dysregulation associated with functionally altered NaV1.8 channels

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

Understanding the physiological role of NaV1.9: Challenges and opportunities for pain modulation

Voltage-activated Na⁺ (Na_V) channels are crucial contributors to rapid electrical signaling in the human body. As such, they are among the most targeted membrane proteins by clinical therapeutics and natural toxins. Several of the nine mammalian Na_V channel subtypes play a documented role in pain or other sensory processes such as itch, touch, and smell. While causal relationships between these subtypes and biological function have been extensively described, the physiological role of Na_V1.9 is less understood. Yet, mutations in Na_V1.9 can cause striking disease phenotypes related to sensory perception such as loss or gain of pain and chronic itch. Here, we explore our current knowledge of the mechanisms by which Na_V1.9 may contribute to pain and elaborate on the challenges associated with establishing links between experimental conditions and human disease. This review also discusses the lack of comprehensive insights into Na_V1.9-specific pharmacology, an unfortunate situation since modulatory compounds may have tremendous potential in the clinic to treat pain or as precision tools to examine the extent of Na_V1.9 participation in sensory perception processes.

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.

Recent advances for using human induced-pluripotent stem cells as pain-in-a-dish models of neuropathic pain

Neuropathic pain is amongst the most common non-communicable disorders and the poor effectiveness of current treatment is an unmet need. Although pain is a universal experience, there are significant inter-individual phenotypic differences. Developing models that can accurately recapitulate the clinical pain features is crucial to better understand underlying pathophysiological mechanisms and find innovative treatments. Current data from heterologous expression systems that investigate properties of specific molecules involved in pain signaling, and from animal models, show limited success with their translation into the development of novel treatments for pain. This is in part because they do not recapitulate the native environment in which a particular molecule functions, and due to species-specific differences in the properties of several key molecules that are involved in pain signaling. The limited availability of post-mortem tissue, in particular dorsal root ganglia (DRG), has hampered research using human cells in pre-clinical studies. Human induced-pluripotent stem cells (iPSCs) have emerged as an exciting alternative platform to study patient-specific diseases. Sensory neurons that are derived from iPSCs (iPSC-SNs) have provided new avenues towards elucidating peripheral pathophysiological mechanisms, the potential for development of personalized treatments, and as a cell-based system for high-throughput screening for discovering novel analgesics. Nevertheless, reprogramming and differentiation protocols to obtain nociceptors have mostly yielded immature homogenous cell populations that do not recapitulate the heterogeneity of native sensory neurons. To close the gap between native human tissue and iPSCs, alternative strategies have been developed. We will review here recent developments in differentiating iPSC-SNs and their use in pre-clinical translational studies. Direct conversion of stem cells into the cells of interest has provided a more cost- and time-saving method to improve reproducibility and diversity of sensory cell types. Furthermore, multi-cellular strategies that mimic in vivo microenvironments for cell maturation, by improving cell contact and communication (co-cultures), reproducing the organ complexity and architecture (three-dimensional organoid), and providing iPSCs with the full spatiotemporal context and nutrients needed for acquiring a mature phenotype (xenotransplantation), have led to functional sensory neuron-like systems. Finally, this review touches on novel prospective strategies, including fluorescent-tracking to select the differentiated neurons of relevance, and dynamic clamp, an electrophysiological method that allows direct manipulation of ionic conductances that are missing in iPSC-SNs.

Familial Episodic Pain Syndromes

Over the past decades, advances in genetic sequencing have opened a new world of discovery of causative genes associated with numerous pain-related syndromes. Familial episodic pain syndromes (FEPS) are one of the distinctive syndromes characterized by early-childhood onset of severe episodic pain mainly affecting the distal extremities and tend to attenuate or diminish with age. According to the phenotypic and genetic properties, FEPS at least includes four subtypes of FEPS1, FEPS2, FEPS3, and FEPS4, which are caused by mutations in the TRPA1, SCN10A, SCN11A, and SCN9A genes, respectively. Functional studies have revealed that all missense mutations in these genes are closely associated with the gain-of-function of cation channels. Because some FEPS patients may show a relative treatability and favorable prognosis, it is worth paying attention to the diagnosis and management of FEPS as early as possible. In this review, we state the common clinical manifestations, pathogenic mechanisms, and potential therapies of the disease, and provide preliminary opinions about future research for FEPS.

Pathological changes of the sural nerve in patients with familial episodic pain syndrome

BACKGROUND

Familial episodic pain syndrome type 3 (FEPS3) is an inherited disorder characterized by the early-childhood onset of severe episodic pain that primarily affects the distal extremities. As skin biopsy has revealed a reduction in intraepidermal nerve fiber density and degeneration of the unmyelinated axons, it remains unclear whether FEPS3 patients have pathological changes in the peripheral nerve.

METHODS

The clinical features of patients with FEPS3 were summarized in a large autosomal dominant family. Sural nerve biopsies were conducted in two patients. Whole exome sequencing (WES) was performed in the index patient. Sanger sequencing was used to analyze family co-segregation.

RESULTS

Fourteen members exhibited typical and uniform clinical phenotypes characterized by length-dependent and age-dependent severe episodic pain affecting the distal extremities, which can be relieved with anti-inflammatory medicine. The WES revealed a heterozygous mutation c.665G > A (p.R222H) in the SCN11A gene, which was co-segregated with the clinical phenotype in this family. A sural biopsy in patient V:1, who was experiencing episodic pain at 16 years old, showed normal structure, while the sural nerve in patient IV:1, whose pain attack had completely diminished at 42 years old, displayed a decrease of the density of unmyelinated axons with the axonal degeneration.

CONCLUSIONS

The clinical phenotype of FEPS3 showed distinctive characteristics that likely arise from dysfunctional nociceptive neurons that lack detectable pathological alterations in the nerve fibers. Nevertheless, long-term dysfunction of the Nav1.9 channel may cause degeneration of the unmyelinated fibers in FEPS3 patient with pain remission.

The Gain-of-Function R222S Variant in Scn11a Contributes to Visceral Hyperalgesia and Intestinal Dysmotility in Scn11a R222S/R222S Mice

Background

The SCN11A gene encodes the α-subunit of the Nav1. 9 channel, which is a regulator of primary sensory neuron excitability. Nav1.9 channels play a key role in somatalgia. Humans with the gain-of-function mutation R222S in SCN11A exhibit familial episodic pain. As already known, R222S knock-in mice carrying a mutation orthologous to the human R222S variant demonstrate somatic hyperalgesia. This study investigated whether Scn11a^R222S/R222S mice developed visceral hyperalgesia and intestinal dysmotility.

Methods

We generated Scn11a^R222S/R222S mice using the CRISPR/Cas9 system. The somatic pain threshold in Scn11a^R222S/R222S mice was assessed by Hargreaves' test and formalin test. The excitability of dorsal root ganglia (DRG) neurons was assessed by whole-cell patch-clamp recording. Visceralgia was tested using the abdominal withdrawal reflex (AWR), acetic acid-induced writhing, and formalin-induced visceral nociception tests. Intestinal motility was detected by a mechanical recording of the intestinal segment and a carbon powder propelling test. The excitability of the enteric nervous system (ENS) could influence gut neurotransmitters. Gut neurotransmitters participate in regulating intestinal motility and secretory function. Therefore, vasoactive intestinal peptide (VIP) and substance P (SP) were measured in intestinal tissues.

Results

The R222S mutation induced hyperexcitability of dorsal root ganglion neurons in Scn11a^R222S/R222S mice. Scn11a^R222S/R222S mice exhibited somatic hyperalgesia. In addition, Scn11a^R222S/R222S mice showed lower visceralgia thresholds and slowed intestinal movements when compared with wild-type controls. Moreover, Scn11a^R222S/R222S mice had lower SP and VIP concentrations in intestinal tissues.

Conclusions

These results indicated that Scn11a^R222S/R222S mice showed visceral hyperalgesia and intestinal dysmotility.

The AMIGO1 adhesion protein activates Kv2.1 voltage sensors

Kv2 voltage-gated potassium channels are modulated by amphoterin-induced gene and open reading frame (AMIGO) neuronal adhesion proteins. Here, we identify steps in the conductance activation pathway of Kv2.1 channels that are modulated by AMIGO1 using voltage-clamp recordings and spectroscopy of heterologously expressed Kv2.1 and AMIGO1 in mammalian cell lines. AMIGO1 speeds early voltage-sensor movements and shifts the gating charge-voltage relationship to more negative voltages. The gating charge-voltage relationship indicates that AMIGO1 exerts a larger energetic effect on voltage-sensor movement than is apparent from the midpoint of the conductance-voltage relationship. When voltage sensors are detained at rest by voltage-sensor toxins, AMIGO1 has a greater impact on the conductance-voltage relationship. Fluorescence measurements from voltage-sensor toxins bound to Kv2.1 indicate that with AMIGO1, the voltage sensors enter their earliest resting conformation, yet this conformation is less stable upon voltage stimulation. We conclude that AMIGO1 modulates the Kv2.1 conductance activation pathway by destabilizing the earliest resting state of the voltage sensors.

Familial episodic pain syndrome: a case report and literature review

The purpose of this case report and literature review is to show that familial episodic pain syndrome (FEPS) is a non-inflammatory genetically inherited pain syndrome. A 3-year-old boy presented at our hospital with pain in both his forearms and lower limbs below the knees for more than 3 years. There were no abnormalities in the blood tests, blood smears, liver and kidney function tests, trace elements tests, cellular immunity test, humoral immunity test, autoantibody tests, C-reactive protein (CRP) test, erythrocyte sedimentation rate (ESR) test, and tumor-related and bone marrow cytology examinations. Additionally, the imaging examination results showed no abnormalities. From the patient's medical history, we found that the mother of the child had a family history of a similar disease. To date, only 21 cases of FEPS3 caused by the sodium voltage-gated channel alpha subunit 11A (SCN11A) gene mutation have been reported. Although the age of onset is different, most of them are inherited in families. The results of the genetic examination revealed that the pain mainly came from the genetic inheritance of the maternal family line. The whole exon gene test revealed that the pain was caused by 2 heterozygous mutations of c.674G > T and c.671T > C in the SCN11A gene.

Contributions of NaV1.8 and NaV1.9 to excitability in human induced pluripotent stem-cell derived somatosensory neurons

The inhibition of voltage-gated sodium (Na_V) channels in somatosensory neurons presents a promising novel modality for the treatment of pain. However, the precise contribution of these channels to neuronal excitability, the cellular correlate of pain, is unknown; previous studies using genetic knockout models or pharmacologic block of Na_V channels have identified general roles for distinct sodium channel isoforms, but have never quantified their exact contributions to these processes. To address this deficit, we have utilized dynamic clamp electrophysiology to precisely tune in varying levels of Na_V1.8 and Na_V1.9 currents into induced pluripotent stem cell-derived sensory neurons (iPSC-SNs), allowing us to quantify how graded changes in these currents affect different parameters of neuronal excitability and electrogenesis. We quantify and report direct relationships between Na_V1.8 current density and action potential half-width, overshoot, and repetitive firing. We additionally quantify the effect varying Na_V1.9 current densities have on neuronal membrane potential and rheobase. Furthermore, we examined the simultaneous interplay between Na_V1.8 and Na_V1.9 on neuronal excitability. Finally, we show that minor biophysical changes in the gating of Na_V1.8 can render human iPSC-SNs hyperexcitable, in a first-of-its-kind investigation of a gain-of-function Na_V1.8 mutation in a human neuronal background.

Mathematical Modeling of Ion Quantum Tunneling Reveals Novel Properties of Voltage-Gated Channels and Quantum Aspects of Their Pathophysiology in Excitability-Related Disorders

Voltage-gated channels are crucial in action potential initiation and propagation and there are many diseases and disorders related to them. Additionally, the classical mechanics are the main mechanics used to describe the function of the voltage-gated channels and their related abnormalities. However, the quantum mechanics should be considered to unravel new aspects in the voltage-gated channels and resolve the problems and challenges that classical mechanics cannot solve. In the present study, the aim is to mathematically show that quantum mechanics can exhibit a powerful tendency to unveil novel electrical features in voltage-gated channels and be used as a promising tool to solve the problems and challenges in the pathophysiology of excitability-related diseases. The model of quantum tunneling of ions through the intracellular hydrophobic gate is used to evaluate the influence of membrane potential and gating free energy on the tunneling probability, single channel conductance, and quantum membrane conductance. This evaluation is mainly based on graphing the mathematical relationships between these variables. The obtained mathematical graphs showed that ions can achieve significant quantum membrane conductance, which can affect the resting membrane potential and the excitability of cells. In the present work, quantum mechanics reveals original electrical properties associated with voltage-gated channels and introduces new insights and implications into the pathophysiology of excitability- related disorders. In addition, the present work sets a mathematical and theoretical framework that can be utilized to conduct experimental studies in order to explore the quantum aspects of voltage-gated channels and the quantum bioelectrical property of biological membranes.

Mechanical allodynia triggered by cold exposure in mice with the Scn11a p.R222S mutation: a novel model of drug therapy for neuropathic pain related to NaV1.9

Mutations within the SCN11A gene which encodes the voltage-gated sodium channel NaV1.9 mainly expressed in small fiber sensory neurons have been associated with neuropathic disorders; however, suitable medications have not been fully investigated. To develop drug therapies against NaV1.9-related neuropathic pain, we aimed to establish a novel model using mice carrying the Scn11a p.R222S mutation initially identified in patients with familial episodic limb pain that is characterized by paroxysmal pain induced by fatigue or bad weather conditions. We investigated the influence of cold exposure (4 °C, overnight) on the behavioral and biochemical phenotypes of Scn11a p.R222S mutant (R222S) and wild type C57BL/6N (WT) mice. We also tested the effects of acetaminophen (125, 250 mg/kg, perorally, p.o.) and traditional Japanese medicine, goshajinkigan (0.5 or 1.0 g/kg, p.o.), which are analgesic drugs prescribed to patients with neuropathic pain, in this model of cold-induced mechanical allodynia in R222S mice.Cold-exposed R222S mice exhibited enhanced mechanical allodynia and thermal hypersensitivity compared with WT mice. The decrease of the mechanical withdrawal threshold in R222S mice was reversible 24 h after housing at room temperature. There was no significant change in the levels of interleukin-1β, interleukin-6, tumor necrosis factor-α, or interferon-γ in the plasma or spinal cords of WT and R222S mice after cold exposure. Both acetaminophen (250 mg/kg) and goshajinkigan (1.0 g/kg) significantly attenuated mechanical allodynia in R222S mice. The model of cold-induced mechanical allodynia in mice with the Scn11a p.R222S mutation is novel and useful for evaluating analgesic drugs for intractable neuropathies related to NaV1.9.

The development of somatosensory neurons: Insights into pain and itch

Primary nociceptors are a heterogeneous class of peripheral somatosensory neurons, responsible for detecting noxious, pruriceptive, and thermal stimuli. These neurons are further divided into several molecularly defined subtypes that correlate with their functional sensory modalities and morphological features. During development, all nociceptors arise from a common pool of embryonic precursors, and then segregate progressively into their mature specialized phenotypes. In this review, we summarize the intrinsic transcriptional programs and extrinsic trophic factor signaling mechanisms that interact to control nociceptor diversification. We also discuss how recent transcriptome profiling studies have significantly advanced the field of sensory neuron development.

Neuropathic Itch

Neurologic insults as varied as inflammation, stroke, and fibromyalgia elicit neuropathic pain and itch. Noxious sensation results when aberrantly increased afferent signaling reaches percept-forming cortical neurons and can occur due to increased sensory signaling, decreased inhibitory signaling, or a combination of both processes. To treat these symptoms, detailed knowledge of sensory transmission, from innervated end organ to cortex, is required. Molecular, genetic, and behavioral dissection of itch in animals and patients has improved understanding of the receptors, cells, and circuits involved. In this review, we will discuss neuropathic itch with a focus on the itch-specific circuit.

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.