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

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

CGRP sensory neurons promote tissue healing via neutrophils and macrophages

The immune system has a critical role in orchestrating tissue healing. As a result, regenerative strategies that control immune components have proved effective1,2. This is particularly relevant when immune dysregulation that results from conditions such as diabetes or advanced age impairs tissue healing following injury2,3. Nociceptive sensory neurons have a crucial role as immunoregulators and exert both protective and harmful effects depending on the context4-12. However, how neuro-immune interactions affect tissue repair and regeneration following acute injury is unclear. Here we show that ablation of the NaV1.8 nociceptor impairs skin wound repair and muscle regeneration after acute tissue injury. Nociceptor endings grow into injured skin and muscle tissues and signal to immune cells through the neuropeptide calcitonin gene-related peptide (CGRP) during the healing process. CGRP acts via receptor activity-modifying protein 1 (RAMP1) on neutrophils, monocytes and macrophages to inhibit recruitment, accelerate death, enhance efferocytosis and polarize macrophages towards a pro-repair phenotype. The effects of CGRP on neutrophils and macrophages are mediated via thrombospondin-1 release and its subsequent autocrine and/or paracrine effects. In mice without nociceptors and diabetic mice with peripheral neuropathies, delivery of an engineered version of CGRP accelerated wound healing and promoted muscle regeneration. Harnessing neuro-immune interactions has potential to treat non-healing tissues in which dysregulated neuro-immune interactions impair tissue healing.

Protein arginine methyltransferase 7 modulates neuronal excitability by interacting with NaV1.9

ABSTRACT

Human NaV1.9 (hNaV1.9), encoded by SCN11A, is preferentially expressed in nociceptors, and its mutations have been linked to pain disorders. NaV1.9 could be a promising drug target for pain relief. However, the modulation of NaV1.9 activity has remained elusive. Here, we identified a new candidate NaV1.9-interacting partner, protein arginine methyltransferase 7 (PRMT7). Whole-cell voltage-clamp recordings showed that coelectroporation of human SCN11A and PRMT7 in dorsal root ganglion (DRG) neurons of Scn11a-/- mice increased the hNaV1.9 current density. By contrast, a PRMT7 inhibitor (DS-437) reduced mNaV1.9 currents in Scn11a+/+ mice. Using the reporter molecule CD4, we observed an increased distribution of hLoop1 on the cell surface of PRMT7-overexpressing HKE293T cells. Furthermore, we found that PRMT7 mainly binds to residues 563 to 566 within the first intracellular loop of hNaV1.9 (hLoop1) and methylates hLoop1 at arginine residue 519. Moreover, overexpression of PRMT7 increased the number of action potential fired in DRG neurons of Scn11a+/+ mice but not Scn11a-/- mice. However, DS-437 significantly inhibited the action potential frequency of DRG neurons and relieved pain hypersensitivity in Scn11aA796G/A796G mice. In summary, our observations revealed that PRMT7 modulates neuronal excitability by regulating NaV1.9 currents, which may provide a potential method for pain treatment.

Nociceptive sensory neurons promote CD8 T cell responses to HSV-1 infection

Host protection against cutaneous herpes simplex virus 1 (HSV-1) infection relies on the induction of a robust adaptive immune response. Here, we show that Nav1.8+ sensory neurons, which are involved in pain perception, control the magnitude of CD8 T cell priming and expansion in HSV-1-infected mice. The ablation of Nav1.8-expressing sensory neurons is associated with extensive skin lesions characterized by enhanced inflammatory cytokine and chemokine production. Mechanistically, Nav1.8+ sensory neurons are required for the downregulation of neutrophil infiltration in the skin after viral clearance to limit the severity of tissue damage and restore skin homeostasis, as well as for eliciting robust CD8 T cell priming in skin-draining lymph nodes by controlling dendritic cell responses. Collectively, our data reveal an important role for the sensory nervous system in regulating both innate and adaptive immune responses to viral infection, thereby opening up possibilities for new therapeutic strategies.

Antinociceptive properties of an isoform-selective inhibitor of Nav1.7 derived from saxitoxin in mouse models of pain

ABSTRACT

The voltage-gated sodium channel Nav1.7 is highly expressed in nociceptive afferents and is critically involved in pain signal transmission. Nav1.7 is a genetically validated pain target in humans because loss-of-function mutations cause congenital insensitivity to pain and gain-of-function mutations cause severe pain syndromes. Consequently, pharmacological inhibition has been investigated as an analgesic therapeutic strategy. We describe a small molecule Nav1.7 inhibitor, ST-2530, that is an analog of the naturally occurring sodium channel blocker saxitoxin. When evaluated against human Nav1.7 by patch-clamp electrophysiology using a protocol that favors the resting state, the Kd of ST-2530 was 25 ± 7 nM. ST-2530 exhibited greater than 500-fold selectivity over human voltage-gated sodium channel isoforms Nav1.1-Nav1.6 and Nav1.8. Although ST-2530 had lower affinity against mouse Nav1.7 (Kd = 250 ± 40 nM), potency was sufficient to assess analgesic efficacy in mouse pain models. A 3-mg/kg dose administered subcutaneously was broadly analgesic in acute pain models using noxious thermal, mechanical, and chemical stimuli. ST-2530 also reversed thermal hypersensitivity after a surgical incision on the plantar surface of the hind paw. In the spared nerve injury model of neuropathic pain, ST-2530 transiently reversed mechanical allodynia. These analgesic effects were demonstrated at doses that did not affect locomotion, motor coordination, or olfaction. Collectively, results from this study indicate that pharmacological inhibition of Nav1.7 by a small molecule agent with affinity for the resting state of the channel is sufficient to produce analgesia in a range of preclinical pain models.

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

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

Pathogenic mutations perturb calmodulin regulation of Nav1.8 channel

The voltage-gated sodium channels play a key role in the generation and propagation of the cardiac action potential. Emerging data indicate that the Nav1.8 channel, encoded by the SCN10A gene, is a modulator of cardiac conduction and variation in the gene has been associated with arrhythmias such as atrial fibrillation (AF) and Brugada syndrome (BrS). The voltage gated sodium channels contain a calmodulin (CaM)-binding IQ domain involved in channel slow inactivation, we here investigated the role of CaM regulation of Nav1.8 channel function, and showed that CaM enhanced slow inactivation of the Nav1.8 channel and hyperpolarized steady-state inactivation curve of sodium currents. The effects of CaM on the channel gating were disrupted in the Nav1.8 channel truncated IQ domain. We studied Nav1.8 IQ domain mutations associated with AF and BrS, and found that a BrS-linked mutation (R1863Q) reduced the CaM-induced hyperpolarization shift, AF-linked mutations (R1869C and R1869G) disrupted CaM-induced enhanced inactivation, and effects of CaM on both development and recovery from slow inactivation were attenuated in all pathogenic mutations. Our findings indicate a role of CaM in the regulation of Nav1.8 channel function in cardiac arrhythmias.

Early Over-Expressing of microRNA-145 Effectively Precludes the Development of Neuropathic Mechanical Hyperalgesia via Suppressing Nav1.8 in Diabetic Rats

BACKGROUND

Painful diabetic neuropathy (PDN) is a common complication secondary to diabetes mellitus. Nav1.8 is an isoform of voltage-gated sodium channels and its expression regulation is closely related with PDN. MicroRNA-145 (miR-145) is involved in the occurrence and development of neuropathic pain. TargetScan software has revealed that Nav1.8 (SCN10A) is the major target of miR-145. However, its function between miR-145 and Nav1.8 in PDN is unknown.

OBJECTIVES

We aim to explore the regulatory effect of miR-145 on the expression and function of Nav1.8, which plays a pivotal role in precluding the advancement of neuropathic mechanical hyperalgesia in diabetic pain.

STUDY DESIGN

An experimental, animal study.

SETTING

An animal research facility at Nanjing Maternal and Child Health Institute, China.

METHODS

The paw mechanical withdrawal threshold (PMWT) of rats was assessed with the von Frey test. The adverse regulation of Nav1.8 by miR-145 was confirmed by a dual luciferase detection system in HEK293T cells. The mRNA level and expression of Nav1.8 in dorsal root ganglion (DRG) neurons were assessed with real-time polymerase chain reaction (real-time PCR), western blotting and immunofluorescence assays following intrathecal injection of agomiR-145 in vitro and in vivo. Whole-cell patch-clamping was applied to assess alterations in the tetrodotoxin-resistant (TTX-R) sodium current (Nav1.8) in DRGs.

RESULTS

The PMWT was significantly decreased in rats following streptozotocin (STZ) injection on Day 7 and was maintained at a lower level on Day 28; this change was accompanied by changes in the expression of Nav1.8 in DRG neurons, which was increased 3 days after STZ injection and reached a maximal level on Day 14. The early knockdown of Nav1.8 with siRNA or agomiR-145 treatment on Day 8 effectively precluded the deterioration of pain behaviors in STZ-treated rats. The luciferase intensity was significantly decreased in HEK293T cells expressing wild-type SCN10A infected with miR-145 mimic. In addition, Nav1.8 overexpression was significantly repressed via overexpression of miR-145 in cultured DRG neurons, and neuronal hyperexcitability was concomitantly decreased. Furthermore, the intrathecal administration of agomiR-145 elicited a significant decrease in Nav1.8 expression in DRG neurons from STZ-treated rats on Day 14.

LIMITATIONS

The causes of PDN are likely to be multifactorial and inflammatory markers, such as IL-6, IL-2, and TNF-?, are elevated in hyperglycemia and might be the precipitating factors that contribute to miR-145 dysregulation. The curative effect of miR-145 upregulation in reversal of pain behaviors at the stage of well-established PDN wasn't investigated in this study.

CONCLUSION

Early infection with a lentiviral vector overexpressing miR-145 adversely regulated the expression and function of TTX-resistant Nav1.8 and abrogated the development of PDN. Therefore, miR-145 might be a potential therapeutic target for preventing PDN in the near future.

Genetic Determinants of Electrocardiographic P-Wave Duration and Relation to Atrial Fibrillation

BACKGROUND

The P-wave duration (PWD) is an electrocardiographic measurement that represents cardiac conduction in the atria. Shortened or prolonged PWD is associated with atrial fibrillation (AF). We used exome-chip data to examine the associations between common and rare variants with PWD.

METHODS

Fifteen studies comprising 64 440 individuals (56 943 European, 5681 African, 1186 Hispanic, 630 Asian) and ≈230 000 variants were used to examine associations with maximum PWD across the 12-lead ECG. Meta-analyses summarized association results for common variants; gene-based burden and sequence kernel association tests examined low-frequency variant-PWD associations. Additionally, we examined the associations between PWD loci and AF using previous AF genome-wide association studies.

RESULTS

We identified 21 common and low-frequency genetic loci (14 novel) associated with maximum PWD, including several AF loci (TTN, CAND2, SCN10A, PITX2, CAV1, SYNPO2L, SOX5, TBX5, MYH6, RPL3L). The top variants at known sarcomere genes (TTN, MYH6) were associated with longer PWD and increased AF risk. However, top variants at other loci (eg, PITX2 and SCN10A) were associated with longer PWD but lower AF risk.

CONCLUSIONS

Our results highlight multiple novel genetic loci associated with PWD, and underscore the shared mechanisms of atrial conduction and AF. Prolonged PWD may be an endophenotype for several different genetic mechanisms of AF.

Inhibition of NaV1.8 prevents atrial arrhythmogenesis in human and mice

Pharmacologic approaches for the treatment of atrial arrhythmias are limited due to side effects and low efficacy. Thus, the identification of new antiarrhythmic targets is of clinical interest. Recent genome studies suggested an involvement of SCN10A sodium channels (Na_V1.8) in atrial electrophysiology. This study investigated the role and involvement of Na_V1.8 (SCN10A) in arrhythmia generation in the human atria and in mice lacking Na_V1.8. Na_V1.8 mRNA and protein were detected in human atrial myocardium at a significant higher level compared to ventricular myocardium. Expression of Na_V1.8 and Na_V1.5 did not differ between myocardium from patients with atrial fibrillation and sinus rhythm. To determine the electrophysiological role of Na_V1.8, we investigated isolated human atrial cardiomyocytes from patients with sinus rhythm stimulated with isoproterenol. Inhibition of Na_V1.8 by A-803467 or PF-01247324 showed no effects on the human atrial action potential. However, we found that Na_V1.8 significantly contributes to late Na⁺ current and consequently to an increased proarrhythmogenic diastolic sarcoplasmic reticulum Ca²⁺ leak in human atrial cardiomyocytes. Selective pharmacological inhibition of Na_V1.8 potently reduced late Na⁺ current, proarrhythmic diastolic Ca²⁺ release, delayed afterdepolarizations as well as spontaneous action potentials. These findings could be confirmed in murine atrial cardiomyocytes from wild-type mice and also compared to SCN10A^-/- mice (genetic ablation of Na_V1.8). Pharmacological Na_V1.8 inhibition showed no effects in SCN10A^-/- mice. Importantly, in vivo experiments in SCN10A^-/- mice showed that genetic ablation of Na_V1.8 protects against atrial fibrillation induction. This study demonstrates that Na_V1.8 is expressed in the murine and human atria and contributes to late Na⁺ current generation and cellular arrhythmogenesis. Blocking Na_V1.8 selectively counteracts this pathomechanism and protects against atrial arrhythmias. Thus, our translational study reveals a new selective therapeutic target for treating atrial arrhythmias.

Absence of Functional Nav1.8 Channels in Non-diseased Atrial and Ventricular Cardiomyocytes

PURPOSE

Several studies have indicated a potential role for SCN10A/NaV1.8 in modulating cardiac electrophysiology and arrhythmia susceptibility. However, by which mechanism SCN10A/NaV1.8 impacts on cardiac electrical function is still a matter of debate. To address this, we here investigated the functional relevance of NaV1.8 in atrial and ventricular cardiomyocytes (CMs), focusing on the contribution of NaV1.8 to the peak and late sodium current (INa) under normal conditions in different species.

METHODS

The effects of the NaV1.8 blocker A-803467 were investigated through patch-clamp analysis in freshly isolated rabbit left ventricular CMs, human left atrial CMs and human-induced pluripotent stem cell-derived CMs (hiPSC-CMs).

RESULTS

A-803467 treatment caused a slight shortening of the action potential duration (APD) in rabbit CMs and hiPSC-CMs, while it had no effect on APD in human atrial cells. Resting membrane potential, action potential (AP) amplitude, and AP upstroke velocity were unaffected by A-803467 application. Similarly, INa density was unchanged after exposure to A-803467 and NaV1.8-based late INa was undetectable in all cell types analysed. Finally, low to absent expression levels of SCN10A were observed in human atrial tissue, rabbit ventricular tissue and hiPSC-CMs.

CONCLUSION

We here demonstrate the absence of functional NaV1.8 channels in non-diseased atrial and ventricular CMs. Hence, the association of SCN10A variants with cardiac electrophysiology observed in, e.g. genome wide association studies, is likely the result of indirect effects on SCN5A expression and/or NaV1.8 activity in cell types other than CMs.

Identification of Leptin Receptor-Expressing Cells in the Nodose Ganglion of Male Mice

Leptin has been proposed to modulate viscerosensory information directly at the level of vagal afferents. In support of this view, broad expression for the leptin receptor (Lepr) has previously been reported in vagal afferents. However, the exact identity and distribution of leptin-sensitive vagal afferents has not been elucidated. Using quantitative PCR, we found that the whole mouse nodose ganglion was predominantly enriched in the short form of Lepr, rather than its long form. Consistent with this observation, the acute administration of leptin did not stimulate JAK-STAT signaling in the nodose ganglion. Using chromogenic in situ hybridization in wild-type mice and several reporter mouse models, we demonstrated that Lepr mRNA was restricted to nonneuronal cells in the epineurium and parenchyma of the nodose ganglion and a subset of vagal afferents, which accounted for only 3% of all neuronal profiles. Double labeling studies further established that Lepr-expressing vagal afferents were Nav1.8-negative fibers that did not supply the peritoneal cavity. Finally, double chromogenic in situ hybridization revealed that many Lepr-expressing neurons coexpressed the angiotensin 1a receptor (At1ar), which is a gene expressed in baroreceptors. Taken together, our data challenge the commonly held view that Lepr is broadly expressed in vagal afferents. Instead, our data suggest that leptin may exert a previously unrecognized role, mainly via its short form, as a direct modulator of a very small group of At1ar-positive vagal fibers.

Nociceptor Translational Profiling Reveals the Ragulator-Rag GTPase Complex as a Critical Generator of Neuropathic Pain

Nociceptors, sensory neurons in the DRG that detect damaging or potentially damaging stimuli, are key drivers of neuropathic pain. Injury to these neurons causes activation of translation regulation signaling, including the mechanistic target of rapamycin complex 1 (mTORC1) and mitogen-activated protein kinase interacting kinase (MNK) eukaryotic initiation factor (eIF) 4E pathways. This is a mechanism driving changes in excitability of nociceptors that is critical for the generation of chronic pain states; however, the mRNAs that are translated to lead to this plasticity have not been elucidated. To address this gap in knowledge, we used translating ribosome affinity purification in male and female mice to comprehensively characterize mRNA translation in Scn10a-positive nociceptors in chemotherapy-induced neuropathic pain (CIPN) caused by paclitaxel treatment. This unbiased method creates a new resource for the field, confirms many findings in the CIPN literature and also find extensive evidence for new target mechanisms that may cause CIPN. We provide evidence that an underlying mechanism of CIPN is sustained mTORC1 activation driven by MNK1-eIF4E signaling. RagA, a GTPase controlling mTORC1 activity, is identified as a novel target of MNK1-eIF4E signaling. This demonstrates a novel translation regulation signaling circuit wherein MNK1-eIF4E activity drives mTORC1 via control of RagA translation. CIPN and RagA translation are strongly attenuated by genetic ablation of eIF4E phosphorylation, MNK1 elimination or treatment with the MNK inhibitor eFT508. We identify a novel translational circuit for the genesis of neuropathic pain caused by chemotherapy with important implications for therapeutics.SIGNIFICANCE STATEMENT Neuropathic pain affects up to 10% of the population, but its underlying mechanisms are incompletely understood, leading to poor treatment outcomes. We used translating ribosome affinity purification technology to create a comprehensive translational profile of DRG nociceptors in naive mice and at the peak of neuropathic pain induced by paclitaxel treatment. We reveal new insight into how mechanistic target of rapamycin complex 1 is activated in neuropathic pain pointing to a key role of MNK1-eIF4E-mediated translation of a complex of mRNAs that control mechanistic target of rapamycin complex 1 signaling at the surface of the lysosome. We validate this finding using genetic and pharmacological techniques. Our work strongly suggests that MNK1-eIF4E signaling drives CIPN and that a drug in human clinical trials, eFT508, may be a new therapeutic for neuropathic pain.

Polymorphisms of the SCN10A Gene in Patients With Sick Sinus Syndrome

PURPOSE

To study association of rs6795970 polymorphism of SCN10A gene with development of idiopathic sick sinus syndrome (ISSS).

MATERIALS AND METHODS

We examined 109 patients with ISSS, 59 their healthy 1-st-, 2-nd-, and 3-rd-degree relatives, and 630 controls. Patients with ISSS were divided into subgroups according to gender and clinical variant of the disease. All patients underwent cardiologic examination and molecular genetic testing of DNA.

RESULTS

We revealed significant preponderance of homozygous genotype with rare allele of the studied gene among patients with ISSS compared with control group. In addition, this genotype significantly prevailed among men with SSSU in comparison with the control group.

CONCLUSION

Genotype AA of the SCN10A gene is associated with a predisposition to the development of ISSS.

[Rs17042171 at chromosome 4q25 is associated with atrial fibrillation in the Chinese Han population from the central plains]

OBJECTIVE

To determine the correlations of single nucleotide polymorphisms (SNPs) with atrial fibrillation (AF) in the Chinese Han population from the central plains.
 Methods: A total of 168 hospitalized patients, including 56 AF and 112 controls, were recruited in this case-control study. The clinical data were obtained from the medical records. All 5 SNPs, rs337711 in KCNN2, rs11264280 near KCNN3, rs17042171 near PITX2, rs6771157 and rs6795970 in SCN10A, were genotyped using amplification refractory mutation system-polymerase chain reaction or direct sequencing. The χ2 test was used to compare categorical variables and preliminarily examine correlations between the genotype frequencies and AF. Subsequently, a logistic regression model was constructed to determine the associations between the SNPs and AF based on the above screened results. Odds ratios (ORs) and 95% confidence interval (CI) were calculated to assess the strength of the correlations. Moreover, we downloaded the genotype data from the HapMap Project for linkage disequilibrium analysis of rs17042171.
 Results: AF patients were likely to be of older age and longer left atrial diameter and had more coronary artery disease and higher hypertension compared with the control group (P<0.05). Among the 5 SNPs, the frequency distribution of genotype AA for rs17042171 was significantly different between the AF and control groups (P<0.05). After adjusting for several covariates, there was still a high risk ratio in patients with the AA genotype compared with the AC+CC genotype (OR: 5.591, 95%CI 2.176 to 14.365, P-B<0.008). Similarly, stratification analysis on the AA genotype demonstrated significant differences between rs17042171 and persistent AF. However, there were not significant correlations between AF and the control groups for the other 4 SNPs (P<0.05).
 Conclusion: Rs17042171, near PITX2 on chromosome 4q25, is associated with AF susceptibility in the Chinese Han population from the central plains, suggesting that this SNP can provide a new strategy for clinical diagnosis in AF patients.

Characteristics of sensory neuronal groups in CGRP-cre-ER reporter mice: Comparison to Nav1.8-cre, TRPV1-cre and TRPV1-GFP mouse lines

Peptidergic sensory neurons play a critical role in nociceptive pathways. To precisely define the function and plasticity of sensory neurons in detail, new tools such as transgenic mouse models are needed. We employed electrophysiology and immunohistochemistry to characterize in detail dorsal root ganglion (DRG) neurons expressing an inducible CGRP^cre-ER (CGRP-cre⁺); and compared them to DRG neurons expressing Nav1.8^cre (Nav1.8-cre⁺), TRPV1^cre (TRPV1-cre⁺) and TRPV1-GFP (V1-GFP⁺). Tamoxifen effectively induced CGRP^cre-ER production in DRG. ≈87% of CGRP^cre-ER-expressing neurons were co-labeled CGRP antibody. Three small and two medium-large-sized (5HT3a⁺/NPY2R^- and NPY2R⁺) neuronal groups with unique electrophysiological profiles were CGRP-cre⁺. Nav1.8-cre⁺ neurons were detected in all CGRP-cre⁺ groups, as well as in 5 additional neuronal groups: MrgprD⁺/TRPA1^-, MrgprD⁺/TRPA1⁺, TRPV1⁺/CGRP^-, vGLUT3⁺ and ≈30% of trkC⁺ neurons. Differences between TRPV1^cre and Nav1.8^cre reporters were that unlike TRPV1-cre⁺, Nav1.8-cre⁺ expression was detected in non-nociceptive vGLUT3⁺ and trkC⁺ populations. Many TRPV1-cre⁺ neurons did not respond to capsaicin. In contrast, V1-GFP⁺ neurons were in 4 groups, each of which was capsaicin-sensitive. Finally, none of the analyzed reporter lines showed cre-recombination in trkB⁺, calbindin⁺, 70% of trkC⁺ or parvalbumin⁺ neurons, which together encompassed ≈20% of Nav1.8-cre^- DRG neurons. The data presented here increases our knowledge of peptidergic sensory neuron characteristics, while showing the efficiency and specificity manipulation of peptidergic neurons by the CGRP^cre-ER reporter. We also demonstrate that manipulation of all C- and A-nociceptors is better achieved with TRPV1-cre reporter. Finally, the described approach for detailed characterization of sensory neuronal groups can be applied to a variety of reporter mice.

Profiling of G protein-coupled receptors in vagal afferents reveals novel gut-to-brain sensing mechanisms

OBJECTIVES

G protein-coupled receptors (GPCRs) act as transmembrane molecular sensors of neurotransmitters, hormones, nutrients, and metabolites. Because unmyelinated vagal afferents richly innervate the gastrointestinal mucosa, gut-derived molecules may directly modulate the activity of vagal afferents through GPCRs. However, the types of GPCRs expressed in vagal afferents are largely unknown. Here, we determined the expression profile of all GPCRs expressed in vagal afferents of the mouse, with a special emphasis on those innervating the gastrointestinal tract.

METHODS

Using a combination of high-throughput quantitative PCR, RNA sequencing, and in situ hybridization, we systematically quantified GPCRs expressed in vagal unmyelinated Nav1.8-expressing afferents.

RESULTS

GPCRs for gut hormones that were the most enriched in Nav1.8-expressing vagal unmyelinated afferents included NTSR1, NPY2R, CCK1R, and to a lesser extent, GLP1R, but not GHSR and GIPR. Interestingly, both GLP1R and NPY2R were coexpressed with CCK1R. In contrast, NTSR1 was coexpressed with GPR65, a marker preferentially enriched in intestinal mucosal afferents. Only few microbiome-derived metabolite sensors such as GPR35 and, to a lesser extent, GPR119 and CaSR were identified in the Nav1.8-expressing vagal afferents. GPCRs involved in lipid sensing and inflammation (e.g. CB1R, CYSLTR2, PTGER4), and neurotransmitters signaling (CHRM4, DRD2, CRHR2) were also highly enriched in Nav1.8-expressing neurons. Finally, we identified 21 orphan GPCRs with unknown functions in vagal afferents.

CONCLUSION

Overall, this study provides a comprehensive description of GPCR-dependent sensing mechanisms in vagal afferents, including novel coexpression patterns, and conceivably coaction of key receptors for gut-derived molecules involved in gut-brain communication.

Deleterious protein-altering mutations in the SCN10A voltage-gated sodium channel gene are associated with prolonged QT

BACKGROUND

Long QT syndrome (LQT) is a pro-arrhythmogenic condition with life-threatening complications. Fifteen genes have been associated with congenital LQT, however, the genetic causes remain unknown in more than 20% of cases.

MATERIALS AND METHODS

Eighteen patients with history of palpitations, pre-syncope, syncope and prolonged QT were referred to the Yale Cardiovascular Genetics Program. All subjects underwent whole-exome sequencing (WES) followed by confirmatory Sanger sequencing. Mutation burden analysis was carried out using WES data from 16 subjects with no identifiable cause of LQT.

RESULTS

Deleterious and novel SCN10A mutations were identified in 3 of the 16 patients (19%) with idiopathic LQT. These included 2 frameshifts and 1 missense variants (p.G810fs, p.R1259Q, and p.P1877fs). Further analysis identified 2 damaging SCN10A mutations with allele frequencies of approximately 0.2% (p.R14L and p.R1268Q) in 2 independent cases. None of the SCN10A mutation carriers had mutations in known arrhythmia genes. Damaging SCN10A mutations (p.R209H and p.R485C) were also identified in the 2 subjects on QT prolonging medications.

CONCLUSION

Our findings implicate SCN10A in LQT. The presence of frameshift mutations suggests loss-of-function as the underlying disease mechanism. The common association with atrial fibrillation suggests a unique mechanism of disease for this LQT gene.

[Polymorphisms of the SCN10A Gene in Patients With Sick Sinus Syndrome]

PURPOSE

To study association of rs6795970 polymorphism of SCN10A gene with development of idiopathic sick sinus syndrome (ISSS).

MATERIALS AND METHODS

We examined 109 patients with ISSS, 59 their healthy 1‑st-, 2‑nd-, and 3‑rd-degree relatives, and 630 controls. Patients with ISSS were divided into subgroups according to gender and clinical variant of the disease. All patients underwent cardiologic examination and molecular genetic testing of DNA.

RESULTS

We revealed significant preponderance of homozygous genotype with rare allele of the studied gene among patients with ISSS compared with control group. In addition, this genotype significantly prevailed among men with SSSU in comparison with the control group.

CONCLUSION

Genotype AA of the SCN10A gene is associated with a predisposition to the development of ISSS.

Fifteen Genetic Loci Associated With the Electrocardiographic P Wave

BACKGROUND

The P wave on an ECG is a measure of atrial electric function, and its characteristics may serve as predictors for atrial arrhythmias. Increased mean P-wave duration and P-wave terminal force traditionally have been used as markers for left atrial enlargement, and both have been associated with increased risk of atrial fibrillation. Here, we explore the genetic basis of P-wave morphology through meta-analysis of genome-wide association study results for P-wave duration and P-wave terminal force from 12 cohort studies.

METHODS AND RESULTS

We included 44 456 individuals, of which 6778 (16%) were of African ancestry. Genotyping, imputation, and genome-wide association study were performed at each study site. Summary-level results were meta-analyzed centrally using inverse-variance weighting. In meta-analyses of P-wave duration, we identified 6 significant (P<5×10^-8) novel loci and replicated a prior association with SCN10A. We identified 3 loci at SCN5A, TBX5, and CAV1/CAV2 that were jointly associated with the PR interval, PR segment, and P-wave duration. We identified 6 novel loci in meta-analysis of P-wave terminal force. Four of the identified genetic loci were significantly associated with gene expression in 329 left atrial samples. Finally, we observed that some of the loci associated with the P wave were linked to overall atrial conduction, whereas others identified distinct phases of atrial conduction.

CONCLUSIONS

We have identified 6 novel genetic loci associated with P-wave duration and 6 novel loci associated with P-wave terminal force. Future studies of these loci may aid in identifying new targets for drugs that may modify atrial conduction or treat atrial arrhythmias.

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

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

Carboplatin/taxane-induced gastrointestinal toxicity: a pharmacogenomics study on the SCOTROC1 trial

Carboplatin/taxane combination is first-line therapy for ovarian cancer. However, patients can encounter treatment delays, impaired quality of life, even death because of chemotherapy-induced gastrointestinal (GI) toxicity. A candidate gene study was conducted to assess potential association of genetic variants with GI toxicity in 808 patients who received carboplatin/taxane in the Scottish Randomized Trial in Ovarian Cancer 1 (SCOTROC1). Patients were randomized into discovery and validation cohorts consisting of 404 patients each. Clinical covariates and genetic variants associated with grade III/IV GI toxicity in discovery cohort were evaluated in replication cohort. Chemotherapy-induced GI toxicity was significantly associated with seven single-nucleotide polymorphisms in the ATP7B, GSR, VEGFA and SCN10A genes. Patients with risk genotypes were at 1.53 to 18.01 higher odds to develop carboplatin/taxane-induced GI toxicity (P<0.01). Variants in the VEGF gene were marginally associated with survival time. Our data provide potential targets for modulation/inhibition of GI toxicity in ovarian cancer patients.

Progression of motor axon dysfunction and ectopic Nav1.8 expression in a mouse model of Charcot-Marie-Tooth disease 1B

Mice heterozygously deficient for the myelin protein P0 gene (P0+/-) develop a slowly progressing neuropathy modeling demyelinating Charcot-Marie-Tooth disease (CMT1B). The aim of the study was to investigate the long-term progression of motor dysfunction in P0+/- mice at 3, 7, 12 and 20months. By comparison with WT littermates, P0+/- showed a decreasing motor performance with age. This was associated with a progressive reduction in amplitude and increase in latency of the plantar compound muscle action potential (CMAP) evoked by stimulation of the tibial nerve at ankle. This progressive functional impairment was in contrast to the mild demyelinating neuropathy of the tibial nerve revealed by histology. "Threshold-tracking" studies showed impaired motor axon excitability in P0+/- from 3months. With time, there was a progressive reduction in threshold deviations during both depolarizing and hyperpolarizing threshold electrotonus associated with increasing resting I/V slope and increasing strength-duration time constant. These depolarizing features in excitability in P0+/- as well as the reduced CMAP amplitude were absent in P0+/- NaV1.8 knockouts, and could be acutely reversed by selective pharmacologic block of NaV1.8 in P0+/-. Mathematical modeling indicated an association of altered passive cable properties with a depolarizing shift in resting membrane potential and increase in the persistent Na(+) current in P0+/-. Our data suggest that ectopic NaV1.8 expression precipitates depolarizing conduction failure in CMT1B, and that motor axon dysfunction in demyelinating neuropathy is pharmacologically reversible.

Methylglyoxal mediates streptozotocin-induced diabetic neuropathic pain via activation of the peripheral TRPA1 and Nav1.8 channels

OBJECTIVE

Methylglyoxal is known to be associated with the development of nephropathy, retinopathy, and other complications in diabetes. The present study tested the hypothesis that endogenously increased levels of methylglyoxal in diabetes are causally associated with the induction of neuropathic pain.

MATERIALS AND METHODS

Streptozotocin- and methylglyoxal-induced pain models were established in rats, and the anti-nociceptive effects of the methylglyoxal scavenging agents, selective transient receptor potential channel ankyrin 1 (TRPA1) antagonist, and Nav1.8 antagonist were tested.

RESULTS

Systemic injection of streptozotocin in rats induced a prolonged increase in plasma methylglyoxal by approximately 60%, which was correlated with the progressive development of mechanical allodynia and thermal hyperalgesia. Local subcutaneous injection of methylglyoxal into the hindpaw produced dose-dependent and biphasic flinching nociceptive responses, which resembled formaldehyde (formalin)-induced nociception. The local methylglyoxal nociception was significantly blocked by co-injection into the hindpaw of the selective transient receptor potential channel ankyrin 1 (TRPA1) antagonist, A967079, and the Nav1.8 antagonist, A803467. Co-incubation with the methylglyoxal scavengers, aminoguanidine, d-arginine, and metformin, reduced the level of free methylglyoxal by more than 90%, and injection of their incubation solutions into the hindpaw produced negligible (3-17%) nociception. Like the clinically effective anti-diabetic neuropathic pain drug gabapentin, systemic injection of aminoguanidine, d-arginine, and metformin at doses that effectively inhibit paw-injected methylglyoxal-induced nociception significantly blocked streptozotocin-induced mechanical allodynia.

CONCLUSION

Endogenously increased methylglyoxal may mediate diabetic neuropathic pain via activation of both TRPA1 and Nav1.8 expressed on primary afferent sensory neurons, and injection of methylglyoxal into the hindpaw may serve as a simple and robust model for testing the anti-diabetic pain drugs.

Psychiatric Risk Gene Transcription Factor 4 Regulates Intrinsic Excitability of Prefrontal Neurons via Repression of SCN10a and KCNQ1

Transcription Factor 4 (TCF4) is a clinically pleiotropic gene associated with schizophrenia and Pitt-Hopkins syndrome (PTHS). To gain insight about the neurobiology of TCF4, we created an in vivo model of PTHS by suppressing Tcf4 expression in rat prefrontal neurons immediately prior to neurogenesis. This cell-autonomous genetic insult attenuated neuronal spiking by increasing the afterhyperpolarization. At the molecular level, using a novel technique called iTRAP that combined in utero electroporation and translating ribosome affinity purification, we identified increased translation of two ion channel genes, Kcnq1 and Scn10a. These ion channel candidates were validated by pharmacological rescue and molecular phenocopy. Remarkably, similar excitability deficits were observed in prefrontal neurons from a Tcf4(+/tr) mouse model of PTHS. Thus, we identify TCF4 as a regulator of neuronal intrinsic excitability in part by repression of Kcnq1 and Scn10a and suggest that this molecular function may underlie pathophysiology associated with neuropsychiatric disorders.

Functional up-regulation of Nav1.8 sodium channel on dorsal root ganglia neurons contributes to the induction of scorpion sting pain

BmK I, purified from the venom of scorpion Buthus martensi Karsch (BmK), is a receptor site-3-specific modulator of voltage-gated sodium channels (VGSCs) and can induce pain-related behaviors in rats. The tetrodotoxin-resistant (TTX-R) sodium channel Nav1.8 contributes to most of the sodium current underlying the action potential upstroke in dorsal root ganglia (DRG) neurons and may serve as a critical ion channel targeted by BmK I. Herein, using electrophysiological, molecular, and behavioral approaches, we investigated whether the aberrant expression of Nav1.8 in DRG contributes to generation of pain induced by BmK I. The expression of Nav1.8 was found to be significantly increased at both mRNA and protein levels following intraplantar injection of BmK I in rats. In addition, the current density of TTX-R Nav1.8 sodium channel is significantly increased and the gating kinetics of Nav1.8 is also altered in DRG neurons from BmK I-treated rats. Furthermore, spontaneous pain and mechanical allodynia, but not thermal hyperalgesia induced by BmK I, are significantly alleviated through either blockade of the Nav1.8 sodium channel by its selective blocker A-803467 or knockdown of the Nav1.8 expression in DRG by antisense oligodeoxynucleotide (AS-ODN) targeting Nav1.8 in rats. Finally, BmK I was shown to induce enhanced pain behaviors in complete freund's adjuvant (CFA)-inflamed rats, which was partly due to the over-expression of Nav1.8 in DRG. Our results suggest that functional up-regulation of Nav1.8 channel on DRG neurons contributes to the development of BmK I-induced pain in rats.

IL-1β Is Involved with the Generation of Pain in Experimental Autoimmune Encephalomyelitis

Pain is one of the main symptoms of multiple sclerosis, a demyelinating disease of the central nervous system that affects millions of people worldwide. The experimental autoimmune encephalomyelitis (EAE) is considered an experimental model of multiple sclerosis, and besides motor weakness, hypernociception is one of the clinical signs of animals with EAE. In this study, we investigated the influence of some cytokines in the generation of the hypernociceptive response in a mouse model of EAE using MOG_35-55. We measured some cytokines in the dorsal root ganglia (DRG), an important anatomical structure involved in pain. We found increased levels of the cytokines TNF-α, IL-1β, and Kc in DRGs of animals with EAE. We used the antibody IL-1ra to antagonize the effects of IL-1β, and animals presented a decrease in the hypernociceptive response. Thus, our results suggest that hypernociception in this experimental model of EAE may be a consequence of the increase in some cytokines in DRGs, especially IL-1β.

Role of common and rare variants in SCN10A: results from the Brugada syndrome QRS locus gene discovery collaborative study

AIMS

Brugada syndrome (BrS) remains genetically heterogeneous and is associated with slowed cardiac conduction. We aimed to identify genetic variation in BrS cases at loci associated with QRS duration.

METHODS AND RESULTS

A multi-centre study sequenced seven candidate genes (SCN10A, HAND1, PLN, CASQ2, TKT, TBX3, and TBX5) in 156 Caucasian SCN5A mutation-negative BrS patients (80% male; mean age 48) with symptoms (64%) and/or a family history of sudden death (47%) or BrS (18%). Forty-nine variants were identified: 18 were rare (MAF <1%) and non-synonymous; and 11/18 (61.1%), mostly in SCN10A, were predicted as pathogenic using multiple bioinformatics tools. Allele frequencies were compared with the Exome Sequencing and UK10K Projects. SKAT methods tested rare variation in SCN10A finding no statistically significant difference between cases and controls. Co-segregation analysis was possible for four of seven probands carrying a novel pathogenic variant. Only one pedigree (I671V/G1299A in SCN10A) showed co-segregation. The SCN10A SNP V1073 was, however, associated strongly with BrS [66.9 vs. 40.1% (UK10K) OR (95% CI) = 3.02 (2.35-3.87), P = 8.07 × 10-19]. Voltage-clamp experiments for NaV1.8 were performed for SCN10A common variants V1073, A1073, and rare variants of interest: A200V and I671V. V1073, A200V and I671V, demonstrated significant reductions in peak INa compared with ancestral allele A1073 (rs6795970).

CONCLUSION

Rare variants in the screened QRS-associated genes (including SCN10A) are not responsible for a significant proportion of SCN5A mutation negative BrS. The common SNP SCN10A V1073 was strongly associated with BrS and demonstrated loss of NaV1.8 function, as did rare variants in isolated patients.

Oral administration of PF-01247324, a subtype-selective Nav1.8 blocker, reverses cerebellar deficits in a mouse model of multiple sclerosis

Cerebellar symptoms significantly diminish quality of life in patients with multiple sclerosis (MS). We previously showed that sodium channel Nav1.8, although normally restricted to peripheral somatosensory neurons, is upregulated in the cerebellum in MS, and that Nav1.8 expression is linked to ataxia and MS-like symptoms in mice. Furthermore, intracerebroventricular administration of the Nav1.8 blocker A-803467 temporarily reversed electrophysiological and behavioral manifestations of disease in a mouse MS model; unfortunately A-803467 is not orally bioavailable, diminishing the potential for translation to human patients. In the present study, we assessed the effect of per os (p.o.) dosing of a new orally bioavailable Nav1.8-selective blocker, PF-01247324, in transgenic mice expressing Nav1.8 in Purkinje neurons, and in wildtype mice in the experimental autoimmune encephalomyelitis (EAE) model. PF-01247324 was administered by oral gavage at 1000 mg/kg; control groups received an equal volume of vehicle. Behavioral assays of motor coordination, grip strength, and ataxia were performed. We observed significant improvements in motor coordination and cerebellar-like symptoms in mice that received PF-01247324 compared to control littermates that received vehicle. These preclinical proof-of-concept data suggest that PF-01247324, its derivatives, or other Nav1.8-selective blockers merit further study for providing symptomatic therapy for cerebellar dysfunction in MS and related disorders.

A common genetic variant within SCN10A modulates cardiac SCN5A expression

Variants in SCN10A, which encodes a voltage-gated sodium channel, are associated with alterations of cardiac conduction parameters and the cardiac rhythm disorder Brugada syndrome; however, it is unclear how SCN10A variants promote dysfunctional cardiac conduction. Here we showed by high-resolution 4C-seq analysis of the Scn10a-Scn5a locus in murine heart tissue that a cardiac enhancer located in Scn10a, encompassing SCN10A functional variant rs6801957, interacts with the promoter of Scn5a, a sodium channel-encoding gene that is critical for cardiac conduction. We observed that SCN5A transcript levels were several orders of magnitude higher than SCN10A transcript levels in both adult human and mouse heart tissue. Analysis of BAC transgenic mouse strains harboring an engineered deletion of the enhancer within Scn10a revealed that the enhancer was essential for Scn5a expression in cardiac tissue. Furthermore, the common SCN10A variant rs6801957 modulated Scn5a expression in the heart. In humans, the SCN10A variant rs6801957, which correlated with slowed conduction, was associated with reduced SCN5A expression. These observations establish a genomic mechanism for how a common genetic variation at SCN10A influences cardiac physiology and predisposes to arrhythmia.

Nav-igating through a complex landscape: SCN10A and cardiac conduction

Genome-wide association studies (GWAS) have implicated SCN10A, which encodes a nociceptor-associated voltage-gated sodium channel subunit, as a modulator of cardiac conduction; however, this role has traditionally been ascribed to SCN5A, which is highly expressed in cardiac muscle. SCN10A is believed to affect cardiac conduction either directly through cardiomyocytes or indirectly via intracardiac neurons. In this issue of the JCI, van den Boogaard and colleagues introduce a third possibility: that the SCN10A locus acts as an enhancer of SCN5A gene expression. The authors demonstrate that SCN10A expression is negligible within human and murine hearts, and that a T-box enhancer within the SCN10A locus drives SCN5A expression within cardiomyocytes. This work reasserts SCN5A as the key determinant of cardiac conduction and highlights the importance of deciphering the functionality of coding versus noncoding regions when interpreting GWAS data.

Chronic itch development in sensory neurons requires BRAF signaling pathways

Chronic itch, or pruritus, is associated with a wide range of skin abnormalities. The mechanisms responsible for chronic itch induction and persistence remain unclear. We developed a mouse model in which a constitutively active form of the serine/threonine kinase BRAF was expressed in neurons gated by the sodium channel Nav1.8 (BRAF(Nav1.8) mice). We found that constitutive BRAF pathway activation in BRAF(Nav1.8) mice results in ectopic and enhanced expression of a cohort of itch-sensing genes, including gastrin-releasing peptide (GRP) and MAS-related GPCR member A3 (MRGPRA3), in nociceptors expressing transient receptor potential vanilloid 1 (TRPV1). BRAF(Nav1.8) mice showed de novo neuronal responsiveness to pruritogens, enhanced pruriceptor excitability, and heightened evoked and spontaneous scratching behavior. GRP receptor expression was increased in the spinal cord, indicating augmented coding capacity for itch subsequent to amplified pruriceptive inputs. Enhanced GRP expression and sustained ERK phosphorylation were observed in sensory neurons of mice with allergic contact dermatitis– or dry skin–elicited itch; however, spinal ERK activation was not required for maintaining central sensitization of itch. Inhibition of either BRAF or GRP signaling attenuated itch sensation in chronic itch mouse models. These data uncover RAF/MEK/ERK signaling as a key regulator that confers a subset of nociceptors with pruriceptive properties to initiate and maintain long-lasting itch sensation.

Pain without nociceptors? Nav1.7-independent pain mechanisms

Nav1.7, a peripheral neuron voltage-gated sodium channel, is essential for pain and olfaction in mice and humans. We examined the role of Nav1.7 as well as Nav1.3, Nav1.8, and Nav1.9 in different mouse models of chronic pain. Constriction-injury-dependent neuropathic pain is abolished when Nav1.7 is deleted in sensory neurons, unlike nerve-transection-related pain, which requires the deletion of Nav1.7 in sensory and sympathetic neurons for pain relief. Sympathetic sprouting that develops in parallel with nerve-transection pain depends on the presence of Nav1.7 in sympathetic neurons. Mechanical and cold allodynia required distinct sets of neurons and different repertoires of sodium channels depending on the nerve injury model. Surprisingly, pain induced by the chemotherapeutic agent oxaliplatin and cancer-induced bone pain do not require the presence of Nav1.7 sodium channels or Nav1.8-positive nociceptors. Thus, similar pain phenotypes arise through distinct cellular and molecular mechanisms. Therefore, rational analgesic drug therapy requires patient stratification in terms of mechanisms and not just phenotype.

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Phenotypically identical pain models have different underlying molecular mechanisms

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Nav1.7 expression is required for sympathetic sprouting after neuronal damage

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Oxaliplatin and cancer-induced bone pain are both Nav1.7-independent

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Deleting Nav1.7 in adult mice reverses nerve damage-induced neuropathic pain

Mu opioid receptors on primary afferent nav1.8 neurons contribute to opiate-induced analgesia: insight from conditional knockout mice

Opiates are powerful drugs to treat severe pain, and act via mu opioid receptors distributed throughout the nervous system. Their clinical use is hampered by centrally-mediated adverse effects, including nausea or respiratory depression. Here we used a genetic approach to investigate the potential of peripheral mu opioid receptors as targets for pain treatment. We generated conditional knockout (cKO) mice in which mu opioid receptors are deleted specifically in primary afferent Nav1.8-positive neurons. Mutant animals were compared to controls for acute nociception, inflammatory pain, opiate-induced analgesia and constipation. There was a 76% decrease of mu receptor-positive neurons and a 60% reduction of mu-receptor mRNA in dorsal root ganglia of cKO mice. Mutant mice showed normal responses to heat, mechanical, visceral and chemical stimuli, as well as unchanged morphine antinociception and tolerance to antinociception in models of acute pain. Inflammatory pain developed similarly in cKO and controls mice after Complete Freund's Adjuvant. In the inflammation model, however, opiate-induced (morphine, fentanyl and loperamide) analgesia was reduced in mutant mice as compared to controls, and abolished at low doses. Morphine-induced constipation remained intact in cKO mice. We therefore genetically demonstrate for the first time that mu opioid receptors partly mediate opiate analgesia at the level of Nav1.8-positive sensory neurons. In our study, this mechanism operates under conditions of inflammatory pain, but not nociception. Previous pharmacology suggests that peripheral opiates may be clinically useful, and our data further demonstrate that Nav1.8 neuron-associated mu opioid receptors are feasible targets to alleviate some forms of persistent pain.

Common variants at SCN5A-SCN10A and HEY2 are associated with Brugada syndrome, a rare disease with high risk of sudden cardiac death

Brugada syndrome is a rare cardiac arrhythmia disorder, causally related to SCN5A mutations in around 20% of cases. Through a genome-wide association study of 312 individuals with Brugada syndrome and 1,115 controls, we detected 2 significant association signals at the SCN10A locus (rs10428132) and near the HEY2 gene (rs9388451). Independent replication confirmed both signals (meta-analyses: rs10428132, P = 1.0 × 10(-68); rs9388451, P = 5.1 × 10(-17)) and identified one additional signal in SCN5A (at 3p21; rs11708996, P = 1.0 × 10(-14)). The cumulative effect of the three loci on disease susceptibility was unexpectedly large (Ptrend = 6.1 × 10(-81)). The association signals at SCN5A-SCN10A demonstrate that genetic polymorphisms modulating cardiac conduction can also influence susceptibility to cardiac arrhythmia. The implication of association with HEY2, supported by new evidence that Hey2 regulates cardiac electrical activity, shows that Brugada syndrome may originate from altered transcriptional programming during cardiac development. Altogether, our findings indicate that common genetic variation can have a strong impact on the predisposition to rare diseases.

Varicella-zoster viruses associated with post-herpetic neuralgia induce sodium current density increases in the ND7-23 Nav-1.8 neuroblastoma cell line

Post-herpetic neuralgia (PHN) is the most significant complication of herpes zoster caused by reactivation of latent Varicella-Zoster virus (VZV). We undertook a heterologous infection in vitro study to determine whether PHN-associated VZV isolates induce changes in sodium ion channel currents known to be associated with neuropathic pain. Twenty VZV isolates were studied blind from 11 PHN and 9 non-PHN subjects. Viruses were propagated in the MeWo cell line from which cell-free virus was harvested and applied to the ND7/23-Nav1.8 rat DRG x mouse neuroblastoma hybrid cell line which showed constitutive expression of the exogenous Nav 1.8, and endogenous expression of Nav 1.6 and Nav 1.7 genes all encoding sodium ion channels the dysregulation of which is associated with a range of neuropathic pain syndromes. After 72 hrs all three classes of VZV gene transcripts were detected in the absence of infectious virus. Single cell sodium ion channel recording was performed after 72 hr by voltage-clamping. PHN-associated VZV significantly increased sodium current amplitude in the cell line when compared with non-PHN VZV, wild-type (Dumas) or vaccine VZV strains ((POka, Merck and GSK). These sodium current increases were unaffected by acyclovir pre-treatment but were abolished by exposure to Tetrodotoxin (TTX) which blocks the TTX-sensitive fast Nav 1.6 and Nav 1.7 channels but not the TTX-resistant slow Nav 1.8 channel. PHN-associated VZV sodium current increases were therefore mediated in part by the Nav 1.6 and Nav 1.7 sodium ion channels. An additional observation was a modest increase in message levels of both Nav1.6 and Nav1.7 mRNA but not Nav 1.8 in PHN virally infected cells.

Gain-of-function Nav1.8 mutations in painful neuropathy

Painful peripheral neuropathy often occurs without apparent underlying cause. Gain-of-function variants of sodium channel Na(v)1.7 have recently been found in ∼30% of cases of idiopathic painful small-fiber neuropathy. Here, we describe mutations in Na(v)1.8, another sodium channel that is specifically expressed in dorsal root ganglion (DRG) neurons and peripheral nerve axons, in patients with painful neuropathy. Seven Na(v)1.8 mutations were identified in 9 subjects within a series of 104 patients with painful predominantly small-fiber neuropathy. Three mutations met criteria for potential pathogenicity based on predictive algorithms and were assessed by voltage and current clamp. Functional profiling showed that two of these three Na(v)1.8 mutations enhance the channel's response to depolarization and produce hyperexcitability in DRG neurons. These observations suggest that mutations of Na(v)1.8 contribute to painful peripheral neuropathy.