Two independent mouse lines carrying the Nav1.7 I228M gain-of-function variant display dorsal root ganglion neuron hyperexcitability but a minimal pain phenotype

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.

Piezo2 voltage-block regulates mechanical pain sensitivity

PIEZO2 is a trimeric mechanically-gated ion channel expressed by most sensory neurons in the dorsal root ganglia. Mechanosensitive PIEZO2 channels are also genetically required for normal touch sensation in both mice and humans. We previously showed that PIEZO2 channels are also strongly modulated by membrane voltage. Specifically, it is only at very positive voltages that all channels are available for opening by mechanical force. Conversely, most PIEZO2 channels are blocked at normal negative resting membrane potentials. The physiological function of this unusual biophysical property of PIEZO2 channels, however, remained unknown. We characterized the biophysical properties of three PIEZO2 ion channel mutations at an evolutionarily conserved arginine (R2756). Using genome engineering in mice we generated Piezo2R2756H/R2756H and Piezo2R2756K/R2756K knock-in mice to characterize the physiological consequences of altering PIEZO2 voltage sensitivity in vivo. We measured endogenous mechanosensitive currents in sensory neurons isolated from the dorsal root ganglia and characterized mechanoreceptor and nociceptor function using electrophysiology. Mice were also assessed behaviourally and morphologically. Mutations at the conserved Arginine (R2756) dramatically changed the biophysical properties of the channel relieving voltage block and lowering mechanical thresholds for channel activation. Piezo2R2756H/R2756H and Piezo2R2756K/R2756K knock-in mice that were homozygous for gain-of-function mutations were viable and were tested for sensory changes. Surprisingly, mechanosensitive currents in nociceptors, neurons that detect noxious mechanical stimuli, were substantially sensitized in Piezo2 knock-in mice, but mechanosensitive currents in most mechanoreceptors that underlie touch sensation were only mildly affected by the same mutations. Single-unit electrophysiological recordings from sensory neurons innervating the glabrous skin revealed that rapidly-adapting mechanoreceptors that innervate Meissner's corpuscles exhibited slightly decreased mechanical thresholds in Piezo2 knock-in mice. Consistent with measurements of mechanically activated currents in isolated sensory neurons essentially all cutaneous nociceptors, both fast conducting Aδ-mechanonociceptors and unmyelinated C-fibre nociceptors were substantially more sensitive to mechanical stimuli and indeed acquired receptor properties similar to ultrasensitive touch receptors in Piezo2 knock-in mice. Mechanical stimuli also induced enhanced ongoing activity in cutaneous nociceptors in Piezo2 knock-in mice and hyper-sensitive PIEZO2 channels were sufficient alone to drive ongoing activity, even in isolated nociceptive neurons. Consistently, Piezo2 knock-in mice showed substantial behavioural hypersensitivity to noxious mechanical stimuli. Our data indicate that ongoing activity and sensitization of nociceptors, phenomena commonly found in human chronic pain syndromes, can be driven by relieving the voltage-block of PIEZO2 ion channels. Indeed, membrane depolarization caused by multiple noxious stimuli may sensitize nociceptors by relieving voltage-block of PIEZO2 channels.

Age-dependent small fiber neuropathy: Mechanistic insights from animal models

Small fiber neuropathy (SFN) is a common and debilitating disease in which the terminals of small diameter sensory axons degenerate, producing sensory loss, and in many patients neuropathic pain. While a substantial number of cases are attributable to diabetes, almost 50% are idiopathic. An underappreciated aspect of the disease is its late onset in most patients. Animal models of human genetic mutations that produce SFN also display age-dependent phenotypes suggesting that aging is an important contributor to the risk of development of the disease. In this review we define how particular sensory neurons are affected in SFN and discuss how aging may drive the disease. We also evaluate how animal models of SFN can define disease mechanisms that will provide insight into early risk detection and suggest novel therapeutic interventions.

Carbenoid-involved reactions integrated with scaffold-based screening generates a Nav1.7 inhibitor

The discovery of selective Nav1.7 inhibitors is a promising approach for developing anti-nociceptive drugs. In this study, we present a novel oxindole-based readily accessible library (OREAL), which is characterized by readily accessibility, unique chemical space, ideal drug-like properties, and structural diversity. We used a scaffold-based approach to screen the OREAL and discovered compound C4 as a potent Nav1.7 inhibitor. The bioactivity characterization of C4 reveals that it is a selective Nav1.7 inhibitor and effectively reverses Paclitaxel-induced neuropathic pain (PINP) in rodent models. Preliminary toxicology study shows C4 is negative to hERG. The consistent results of molecular docking and molecular simulations further support the reasonability of the in-silico screening and show the insight of the binding mode of C4. Our discovery of C4 paves the way for pushing the Nav1.7-based anti-nociceptive drugs forward to the clinic.

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 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.

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

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

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

The persistence of increased excitability and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain. This aberrant activity involves increased activity and/or expression of voltage-gated Na+ and Ca2+ channels and hyperpolarization activated cyclic nucleotide gated (HCN) channels as well as decreased function of K+ channels. Because they display limited central side effects, peripherally restricted Na+ and Ca2+ channel blockers and K+ channel activators offer potential therapeutic approaches to pain management. This review outlines the current status and future therapeutic promise of peripherally acting channel modulators. Selective blockers of Nav1.3, Nav1.7, Nav1.8, Cav3.2, and HCN2 and activators of Kv7.2 abrogate signs of neuropathic pain in animal models. Unfortunately, their performance in the clinic has been disappointing; some substances fail to meet therapeutic end points whereas others produce dose-limiting side effects. Despite this, peripheral voltage-gated cation channels retain their promise as therapeutic targets. The way forward may include (i) further structural refinement of K+ channel activators such as retigabine and ASP0819 to improve selectivity and limit toxicity; use or modification of Na+ channel blockers such as vixotrigine, PF-05089771, A803467, PF-01247324, VX-150 or arachnid toxins such as Tap1a; the use of Ca2+ channel blockers such as TTA-P2, TTA-A2, Z 944, ACT709478, and CNCB-2; (ii) improving methods for assessing "pain" as opposed to nociception in rodent models; (iii) recognizing sex differences in pain etiology; (iv) tailoring of therapeutic approaches to meet the symptoms and etiology of pain in individual patients via quantitative sensory testing and other personalized medicine approaches; (v) targeting genetic and biochemical mechanisms controlling channel expression using anti-NGF antibodies such as tanezumab or re-purposed drugs such as vorinostat, a histone methyltransferase inhibitor used in the management of T-cell lymphoma, or cercosporamide a MNK 1/2 inhibitor used in treatment of rheumatoid arthritis; (vi) combination therapy using drugs that are selective for different channel types or regulatory processes; (vii) directing preclinical validation work toward the use of human or human-derived tissue samples; and (viii) application of molecular biological approaches such as clustered regularly interspaced short palindromic repeats (CRISPR) technology.

Developing nociceptor-selective treatments for acute and chronic pain

Despite substantial efforts dedicated to the development of new, nonaddictive analgesics, success in treating pain has been limited. Clinically available analgesic agents generally lack efficacy and may have undesirable side effects. Traditional target-based drug discovery efforts that generate compounds with selectivity for single targets have a high rate of attrition because of their poor clinical efficacy. Here, we examine the challenges associated with the current analgesic drug discovery model and review evidence in favor of stem cell–derived neuronal-based screening approaches for the identification of analgesic targets and compounds for treating diverse forms of acute and chronic pain.

Dissecting the precise nature of itch-evoked scratching

Itch is a discrete and irritating sensation tightly coupled to a drive to scratch. Acute scratching developed evolutionarily as an adaptive defense against skin irritants, pathogens, or parasites. In contrast, the itch-scratch cycle in chronic itch is harmful, inducing escalating itch and skin damage. Clinically and preclinically, scratching incidence is currently evaluated as a unidimensional motor parameter and believed to reflect itch severity. We propose that scratching, when appreciated as a complex, multidimensional motor behavior, will yield greater insight into the nature of itch and the organization of neural circuits driving repetitive motor patterns. We outline the limitations of standard measurements of scratching in rodent models and present new approaches to observe and quantify itch-evoked scratching. We argue that accurate quantitative measurements of scratching are critical for dissecting the molecular, cellular, and circuit mechanisms underlying itch and for preclinical development of therapeutic interventions for acute and chronic itch disorders.

Calmodulin Interactions with Voltage-Gated Sodium Channels

Calmodulin (CaM) is a small protein that acts as a ubiquitous signal transducer and regulates neuronal plasticity, muscle contraction, and immune response. It interacts with ion channels and plays regulatory roles in cellular electrophysiology. CaM modulates the voltage-gated sodium channel gating process, alters sodium current density, and regulates sodium channel protein trafficking and expression. Many mutations in the CaM-binding IQ domain give rise to diseases including epilepsy, autism, and arrhythmias by interfering with CaM interaction with the channel. In the present review, we discuss CaM interactions with the voltage-gated sodium channel and modulators involved in CaM regulation, as well as summarize CaM-binding IQ domain mutations associated with human diseases in the voltage-gated sodium channel family.

Human inducible pluripotent stem cells: Realization of initial promise in drug discovery

Over the course of the last decade, the biopharmaceutical industry has slowly adopted human inducible pluripotent stem cell (hiPSC) technology to enable the development of humanized model systems to test new therapeutic molecules and drug modalities. The adoption of hiPSC-based models by the industry has increased appreciably in the past 3-5 years. This increase has paralleled the explosion in availability of high-quality human genetic data to mine for new drug targets and the emergence of human-specific therapeutic modalities.