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The sodium channels Na v 1.7, Na v 1.8 and Na v 1.9 are critical for pain perception in peripheral nociceptors. Loss of function of Na v 1.7 leads to congenital insensitivity to pain in humans. Here we show that the spider peptide toxin called HpTx1, first identified as an inhibitor of K v 4.2, restores nociception in Na v 1.7 knockout (Na v 1.7-KO) mice by enhancing the excitability of dorsal root ganglion neurons. HpTx1 inhibits Na v 1.7 and activates Na v 1.9 but does not affect Na v 1.8. This toxin produces pain in wild-type (WT) and Na v 1.7-KO mice, and attenuates nociception in Na v 1.9-KO mice, but has no effect in Na v 1.8-KO mice. These data indicate that HpTx1-induced hypersensitivity is mediated by Na v 1.9 activation and offers pharmacological insight into the relationship of the three Na v channels in pain signalling.

Acute pruritus occurs in various disorders. Despite severe repercussions on quality of life treatment options remain limited. Voltage-gated sodium channels (Na V ) are indispensable for transformation and propagation of sensory signals implicating them as drug targets. Here, Na V 1.7, 1.8 and 1.9 were compared for their contribution to itch by analysing Na V -specific knockout mice. Acute pruritus was induced by a comprehensive panel of pruritogens (C48/80, endothelin, 5-HT, chloroquine, histamine, lysophosphatidic acid, trypsin, SLIGRL, β-alanine, BAM8-22), and scratching was assessed using a magnet-based recording technology. We report an unexpected stimulus-dependent diversity in Na V channel-mediated itch signalling. Na V 1.7 -/- showed substantial scratch reduction mainly towards strong pruritogens. Na V 1.8 -/- impaired histamine and 5-HT-induced scratching while Na V 1.9 was involved in itch signalling towards 5-HT, C48/80 and SLIGRL. Furthermore, similar microfluorimetric calcium responses of sensory neurons and expression of itch-related TRP channels suggest no change in sensory transduction but in action potential transformation and conduction. The cumulative sum of scratching over all pruritogens confirmed a leading role of Na V 1.7 and indicated an overall contribution of Na V 1.9. Beside the proposed general role of Na V 1.7 and 1.9 in itch signalling, scrutiny of time courses suggested Na V 1.8 to sustain prolonged itching. Therefore, Na V 1.7 and 1.9 may represent targets in pruritus therapy.

Genetic and functional studies have confirmed an important role for the voltage-gated sodium channel Nav1.9 in human pain disorders. However, low functional expression of Nav1.9 in heterologous systems ( e.g. in human embryonic kidney 293 (HEK293) cells) has hampered studies of its biophysical and pharmacological properties and the development of high-throughput assays for drug development targeting this channel. The mechanistic basis for the low level of Nav1.9 currents in heterologous expression systems is not understood. Here, we implemented a multidisciplinary approach to investigate the mechanisms that govern functional Nav1.9 expression. Recombinant expression of a series of Nav1.9-Nav1.7 C-terminal chimeras in HEK293 cells identified a 49-amino-acid-long motif in the C terminus of the two channels that regulates expression levels of these chimeras. We confirmed the critical role of this motif in the context of a full-length channel chimera, Nav1.9-Ct49aa Nav1.7 , which displayed significantly increased current density in HEK293 cells while largely retaining the characteristic Nav1.9-gating properties. High-resolution live microscopy indicated that the newly identified C-terminal motif dramatically increases the number of channels on the plasma membrane of HEK293 cells. Molecular modeling results suggested that this motif is exposed on the cytoplasmic face of the folded C terminus, where it might interact with other channel partners. These findings reveal that a 49-residue-long motif in Nav1.9 regulates channel trafficking to the plasma membrane.

The voltage dependent sodium channel Nav1.9, is expressed preferentially in peripheral sensory neurons and has been linked to human genetic pain disorders, which makes it target of interest for the development of new pain therapeutics. However, characterization of Nav1.9 pharmacology has been limited due in part to the historical difficulty of functionally expressing recombinant channels. Here we report the successful generation and characterization of human, mouse and rat Nav1.9 stably expressed in human HEK-293 cells. These cells exhibit slowly activating and inactivating inward sodium channel currents that have characteristics of native Nav1.9. Optimal functional expression was achieved by coexpression of Nav1.9 with β1/β2 subunits. While recombinantly expressed Nav1.9 was found to be sensitive to sodium channel inhibitors TC-N 1752 and tetracaine, potency was up to 100-fold less than reported for other Nav channel subtypes despite evidence to support an interaction with the canonical local anesthetic (LA) binding region on Domain 4 S6. Nav1.9 Domain 2 S6 pore domain contains a unique lysine residue (K799) which is predicted to be spatially near the local anesthetic interaction site. Mutation of this residue to the consensus asparagine (K799N) resulted in an increase in potency for tetracaine, but a decrease for TC-N 1752, suggesting that this residue can influence interaction of inhibitors with the Nav1.9 pore. In summary, we have shown that stable functional expression of Nav1.9 in the widely used HEK-293 cells is possible, which opens up opportunities to better understand channel properties and may potentially aid identification of novel Nav1.9 based pharmacotherapies., Competing Interests: The authors have declared that no competing interests exist.

Congenital insensitivity to pain (CIP) comprises the rare heritable disorders without peripheral neuropathy that feature inability to feel pain. Fracturing and joint destruction are common complications, but lack detailed studies of mineral and skeletal homeostasis and bone histology. In 2013, discovery of a heterozygous gain-of-function mutation in SCN11A encoding voltage-gated sodium channel 1.9 (Nav1.9) established a distinctive CIP in three unrelated patients who suffered multiple painless fractures, self-inflicted mutilation, chronic diarrhea, and hyperhidrosis. Here, we studied a mother and two children with CIP by physical examination, biochemical testing, radiological imaging including DXA, iliac crest histology, and mutation analysis. She suffered fractures primarily of her lower extremities beginning at age two years, and had Charcot deformity of both ankles and joint hypermobility. Nerve conduction velocity together with electromyography were normal. Her children had recurrent major fractures beginning in early childhood, joint hypermobility, and chronic diarrhea. She had an excoriated external nare, and both children had hypertrophic scars from scratching. Skin collagen studies were normal. Radiographs revealed fractures and deformities. However, lumbar spine and total hip BMD Z-scores, biochemical parameters of mineral and skeletal homeostasis, and iliac crest histology of the mother (after in vivo tetracycline labeling) were normal. Genomic DNA from the children revealed a unique heterozygous missense mutation in exon 23 (c.3904C>T, p.Leu1302Phe) of SCN11A that is absent in SNP databases and alters an evolutionarily conserved amino acid. This autosomal dominant CIP reflects the second gain-of-function mutation of SCN11A. Perhaps joint hypermobility is an unreported feature. How mutation of Nav1.9 causes fracturing remains unexplained. Lack of injury awareness is typically offered as the reason, and was supported by our unremarkable biochemical, radiological, and histological findings indicating no skeletal pathobiology. However, low-trauma fracturing in these patients suggests an uncharacterized defect in bone quality., (Copyright © 2016 Elsevier Inc. All rights reserved.)

SCN11A encodes the voltage-gated sodium channel NaV1.9, which deviates most strongly from the other eight NaV channels expressed in mammals. It is characterized by resistance to the prototypic NaV channel blocker tetrodotoxin and exhibits slow activation and inactivation gating. Its expression in dorsal root ganglia neurons suggests a role in motor or pain signaling functions as also recently demonstrated by the occurrence of various mutations in human SCN11A leading to altered pain sensation syndromes. The systematic investigation of human NaV1.9, however, is severely hampered because of very poor heterologous expression in host cells. Using patch-clamp and two-electrode voltage-clamp methods, we show that this limitation is caused by the C-terminal structure of NaV1.9. A chimera of NaV1.9 harboring the C terminus of NaV1.4 yields functional expression not only in neuronal cells but also in non-excitable cells, such as HEK 293T or Xenopus oocytes. The major functional difference of the chimeric channel with respect to NaV1.9 is an accelerated activation and inactivation. Since the entire transmembrane domain is preserved, it is suited for studying pharmacological properties of the channel and the functional impact of disease-causing mutations. Moreover, we demonstrate how mutation S360Y makes NaV1.9 channels sensitive to tetrodotoxin and saxitoxin and that the unusual slow open-state inactivation of NaV1.9 is also mediated by the IFM (isoleucine-phenylalanine-methionine) inactivation motif located in the linker connecting domains III and IV.

Painful small fiber neuropathy is a challenging medical condition with no effective treatment. Non-genetic causes can be identified in one half of the subjects. Gain-of-function variants of sodium channels Nav1.7 and Nav1.8 have recently been associated with painful small fiber neuropathy. More recently, mutations of sodium channel Nav1.9 have been linked to human pain disorders, with two gain-of-function mutations found in patients with painful small fiber neuropathy. Here we report a novel Nav1.9 mutation, a glycine 699 substitution by arginine (G699R) in the domain II S4-S5 linker, identified in a patient with painful small fiber neuropathy. In this study, we assayed the mutant channels by voltage-clamp in superior cervical ganglion neurons, which do not produce endogenous Nav1.8 or Nav1.9 currents, and provide a novel platform where Nav1.9 is expressed at relatively high levels. Voltage-clamp analysis showed that the mutation hyperpolarizes (-10.1 mV) channel activation, depolarizes (+6.3 mV) steady-state fast inactivation, slows deactivation, and enhances ramp responses compared with wild-type Nav1.9 channels. Current-clamp analysis showed that the G699R mutant channels render dorsal root ganglion neurons hyperexcitable, via depolarized resting membrane potential, reduced current threshold and increased evoked firing. These observations show that the domain II S4-S5 linker plays an important role in the gating of Nav1.9 and demonstrates that a mutation in this linker is linked to a common pain disorder.