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

The inhibition of voltage-gated sodium (NaV) 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 NaV 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 NaV1.8 and NaV1.9 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 NaV1.8 current density and action potential half-width, overshoot, and repetitive firing. We additionally quantify the effect varying NaV1.9 current densities have on neuronal membrane potential and rheobase. Furthermore, we examined the simultaneous interplay between NaV1.8 and NaV1.9 on neuronal excitability. Finally, we show that minor biophysical changes in the gating of NaV1.8 can render human iPSC-SNs hyperexcitable, in a first-of-its-kind investigation of a gain-of-function NaV1.8 mutation in a human neuronal background.

The insecticide deltamethrin enhances sodium channel slow inactivation of human Nav1.9, Nav1.8 and Nav1.7.

The insecticide deltamethrin of the pyrethroid class mainly targets voltage-gated sodium channels (Navs). Deltamethrin prolongs the opening of Navs by slowing down fast inactivation and deactivation. Pyrethroids are supposedly safe for humans, however, they have also been linked to the gulf-war syndrome, a neuropathic pain condition that can develop following exposure to certain chemicals. Inherited neuropathic pain conditions have been linked to mutations in the Nav subtypes Nav1.7, Nav1.8, and Nav1.9. Here, we examined the effect of deltamethrin on the human isoforms Nav1.7, Nav1.8, and Nav1.9_C4 (chimera containing the C-terminus of rat Nav1.4) heterologously expressed in HEK293T and ND7/23 cells using whole-cell patch-clamp electrophysiology. For all three Nav subtypes, we observed increased persistent and tail currents that are typical for Nav channels modified by deltamethrin. The most surprising finding was an enhanced slow inactivation induced by deltamethrin in all three Nav subtypes. An enhanced slow inactivation is contrary to the prolonged opening caused by pyrethroids and has not been described for deltamethrin or any other pyrethroid before. Furthermore, we found that the fraction of deltamethrin-modified channels increased use-dependently. However, for Nav1.8, the use-dependent potentiation occurred only when the holding potential was increased to -90 mV, a potential at which the tail currents decay more slowly. This indicates that use-dependent modification is due to an accumulation of tail currents. In summary, our findings support a novel mechanism whereby deltamethrin enhances slow inactivation of voltage-gated sodium channels, which may, depending on the cellular resting membrane potential, reduce neuronal excitability and counteract the well-described pyrethroid effects of prolonging channel opening.

Complementary roles of murine NaV1.7, NaV1.8 and NaV1.9 in acute itch signalling.

Acute pruritus occurs in various disorders. Despite severe repercussions on quality of life treatment options remain limited. Voltage-gated sodium channels (NaV) are indispensable for transformation and propagation of sensory signals implicating them as drug targets. Here, NaV1.7, 1.8 and 1.9 were compared for their contribution to itch by analysing NaV-specific knockout mice. Acute pruritus was induced by a comprehensive panel of pruritogens (C48/80, endothelin, 5-HT, chloroquine, histamine, lysophosphatidic acid, trypsin, SLIGRL, ß-alanine, BAM8-22), and scratching was assessed using a magnet-based recording technology. We report an unexpected stimulus-dependent diversity in NaV channel-mediated itch signalling. NaV1.7-/- showed substantial scratch reduction mainly towards strong pruritogens. NaV1.8-/- impaired histamine and 5-HT-induced scratching while NaV1.9 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 NaV1.7 and indicated an overall contribution of NaV1.9. Beside the proposed general role of NaV1.7 and 1.9 in itch signalling, scrutiny of time courses suggested NaV1.8 to sustain prolonged itching. Therefore, NaV1.7 and 1.9 may represent targets in pruritus therapy.

Membrane cholesterol depletion as a trigger of Nav1.9 channel-mediated inflammatory pain.

Cholesterol is a major lipid component of the mammalian plasma membrane. While much is known about its metabolism, its transport, and its role in atherosclerotic vascular disease, less is known about its role in neuronal pathophysiology. This study reveals an unexpected function of cholesterol in controlling pain transmission. We show that inflammation lowers cholesterol content in skin tissue and sensory DRG culture. Pharmacological depletion of cellular cholesterol entails sensitization of nociceptive neurons and promotes mechanical and thermal hyperalgesia through the activation of voltage-gated Nav1.9 channels. Inflammatory mediators enhance the production of reactive oxygen species and induce partitioning of Nav1.9 channels from cholesterol-rich lipid rafts to cholesterol-poor non-raft regions of the membrane. Low-cholesterol environment enhances voltage-dependent activation of Nav1.9 channels leading to enhanced neuronal excitability, whereas cholesterol replenishment reversed these effects. Consistently, we show that transcutaneous delivery of cholesterol alleviates hypersensitivity in animal models of acute and chronic inflammatory pain. In conclusion, our data establish that membrane cholesterol is a modulator of pain transmission and shed a new light on the relationship between cholesterol homeostasis, inflammation, and pain.

Translational Model Systems for Complex Sodium Channel Pathophysiology in Pain.

Chronic pain patients are often left with insufficient treatment as the pathophysiology especially of neuropathic pain remains enigmatic. Recently, genetic variations in the genes of the voltage-gated sodium channels (Navs) were linked to inherited neuropathic pain syndromes, opening a research pathway to foster our understanding of the pathophysiology of neuropathic pain. More than 10 years ago, the rare, inherited pain syndrome erythromelalgia was linked to mutations in the subtype Nav1.7, and since then a plethora of mutations and genetic variations in this and other Nav genes were identified. Often the biophysical changes induced by the genetic alteration offer a straightforward explanation for the clinical symptoms, but mutations in some channels, especially Nav1.9, paint a more complex picture. Although efforts were undertaken to significantly advance our knowledge, translation from heterologous or animal model systems to humans remains a challenge. Here we present recent advances in translation using stem cell-derived human sensory neurons and their potential application for identification of better, effective, and more precise treatment for the individual pain patient.

Voltage-Gated Ion Channels in the PNS: Novel Therapies for Neuropathic Pain?

Neuropathic pain arises from injury to the nervous system. Conditions associated with neuropathic pain are diverse, and lesions and/or pathological changes in the central nervous system (CNS) or peripheral nervous system (PNS) can frequently, but not always, be identified. It is difficult to treat, with patients often on multiple, different classes of medications, all with appreciable adverse side effect profiles. Consequently, there is a pressing need for the development of new medications. The development of such therapeutics is predicated on a clear understanding of the relevant molecular and cellular processes that contribute to the development, and maintenance, of the neuropathic pain state. One proposed mechanism thought to contribute to the ontogeny of neuropathic pain is altered expression, trafficking, and functioning of ion channels expressed by primary sensory neurons. Here, we will focus on three voltage-gated ion channel families, CaV, HCN, and NaV, first reviewing the preclinical data and then the human data where it exists.

Stromal Cell-Derived Factor 1 Increases Tetrodotoxin-Resistant Sodium Currents Nav1.8 and Nav1.9 in Rat Dorsal Root Ganglion Neurons via Different Mechanisms.

Stromal cell-derived factor 1 (SDF-1)/chemokine CXC motif ligand 12 (CXCL12), a chemokine that is upregulated in dorsal root ganglion (DRG) during chronic pain models, has recently been found to play a central role in pain hypersensitivity. The purpose of present study is to investigate the functional impact of SDF-1 and its receptor, chemokine CXC motif receptor 4 (CXCR4), on two TTXR sodium channels in rat DRG using electrophysiological techniques. Preincubation with SDF-1 caused a concentration-dependent increase of Nav1.8 and Nav1.9 currents amplitudes in acutely isolated small diameter DRG neurons in short-term culture. As to Nav1.9, changes in current density and kinetic properties of Nav1.9 current evoked by SDF-1(50 ng/ml) was eliminated by CXCR4 antagonist AMD3100 and phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002. The increase in Nav1.9 current was also blocked by pertussis toxin (PTX) but not cholera toxin (CTX), showing involvement of Gi/o but not Gs subunits. As to Nav1.8, inhibitors (AMD3100, PTX, CTX, LY294002) used in present study didn't inhibit the increased amplitude of Nav1.8 current and shifted activation curve of Nav1.8 in a hyperpolarizing direction in the presence of SDF-1 (50 ng/ml). In conclusion, our data demonstrated that SDF-1 may excite primary nociceptive sensory neurons by acting on the biophysical properties of Nav1.8 and Nav1.9 currents but via different mechanisms.

P2Y Receptors Sensitize Mouse and Human Colonic Nociceptors.

Activation of visceral nociceptors by inflammatory mediators contributes to visceral hypersensitivity and abdominal pain associated with many gastrointestinal disorders. Purine and pyrimidine nucleotides (e.g., ATP and UTP) are strongly implicated in this process following their release from epithelial cells during mechanical stimulation of the gut, and from immune cells during inflammation. Actions of ATP are mediated through both ionotropic P2X receptors and metabotropic P2Y receptors. P2X receptor activation causes excitation of visceral afferents; however, the impact of P2Y receptor activation on visceral afferents innervating the gut is unclear. Here we investigate the effects of stimulating P2Y receptors in isolated mouse colonic sensory neurons, and visceral nociceptor fibers in mouse and human nerve-gut preparations. Additionally, we investigate the role of Nav1.9 in mediating murine responses. The application of UTP (P2Y2 and P2Y4 agonist) sensitized colonic sensory neurons by increasing action potential firing to current injection and depolarizing the membrane potential. The application of ADP (P2Y1, P2Y12, and P2Y13 agonist) also increased action potential firing, an effect blocked by the selective P2Y1 receptor antagonist MRS2500. UTP or ADP stimulated afferents, including mouse and human visceral nociceptors, in nerve-gut preparations. P2Y1 and P2Y2 transcripts were detected in 80% and 56% of retrogradely labeled colonic neurons, respectively. Nav1.9 transcripts colocalized in 86% of P2Y1-positive and 100% of P2Y2-positive colonic neurons, consistent with reduced afferent fiber responses to UTP and ADP in Na(v)1.9(-/-) mice. These data demonstrate that P2Y receptor activation stimulates mouse and human visceral nociceptors, highlighting P2Y-dependent mechanisms in the generation of visceral pain during gastrointestinal disease.

The voltage-gated sodium channel NaV 1.9 in visceral pain.

BACKGROUND: Visceral pain is a common symptom for patients with gastrointestinal (GI) disease. It is unpleasant, debilitating, and represents a large unmet medical need for effective clinical treatments. Recent studies have identified NaV 1.9 as an important regulator of afferent sensitivity in visceral pain pathways to mechanical and inflammatory stimuli, suggesting that NaV 1.9 could represent an important therapeutic target for the treatment of visceral pain. This potential has been highlighted by the identification of patients who have an insensitivity to pain or painful neuropathies associated with mutations in SCN11A, the gene encoding voltage-gated sodium channel subtype 1.9 (NaV 1.9). PURPOSE: Here, we address the role of NaV 1.9 in visceral pain and what known human NaV 1.9 mutants can tell us about NaV 1.9 function in gut physiology and pathophysiology.

Sodium channels and pain.

Human and mouse genetic studies have led to significant advances in our understanding of the role of voltage-gated sodium channels in pain pathways. In this chapter, we focus on Nav1.7, Nav1.8, Nav1.9 and Nav1.3 and describe the insights gained from the detailed analyses of global and conditional transgenic Nav knockout mice in terms of pain behaviour. The spectrum of human disorders caused by mutations in these channels is also outlined, concluding with a summary of recent progress in the development of selective Nav1.7 inhibitors for the treatment of pain.

The Domain II S4-S5 Linker in Nav1.9: A Missense Mutation Enhances Activation, Impairs Fast Inactivation, and Produces Human Painful Neuropathy.

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.

Activation of tetrodotoxin-resistant sodium channel NaV1.9 in rat primary sensory neurons contributes to melittin-induced pain behavior.

Tetrodotoxin-resistant (TTX-R) sodium channels NaV1.8 and NaV1.9 in dorsal root ganglion (DRG) neurons play important roles in pathological pain. We recently reported that melittin, the major toxin of whole bee venom, induced action potential firings in DRG neurons even in the presence of a high concentration (500 nM) of TTX, indicating the contribution of TTX-R sodium channels. This hypothesis is fully investigated in the present study. After subcutaneous injection of melittin, NaV1.8 and NaV1.9 significantly upregulate mRNA and protein expressions, and related sodium currents also increase. Double immunohistochemical results show that NaV1.8-positive neurons are mainly medium- and small-sized, whereas NaV1.9-positive ones are only small-sized. Antisense oligodeoxynucleotides (AS ODNs) targeting NaV1.8 and NaV1.9 are used to evaluate functional significance of the increased expressions of TTX-R sodium channels. Behavioral tests demonstrate that AS ODN targeting NaV1.9, but not NaV1.8, reverses melittin-induced heat hypersensitivity. Neither NaV1.8 AS ODN nor NaV1.9 AS ODN affects melittin-induced mechanical hypersensitivity. These results provide previously unknown evidence that upregulation of NaV1.9, but not NaV1.8, in small-sized DRG neurons contributes to melittin-induced heat hypersensitivity. Furthermore, melittin-induced biological effect indicates a potential strategy to study properties of TTX-R sodium channels.

Animal toxins can alter the function of Nav1.8 and Nav1.9.

Human voltage-activated sodium (Nav) channels are adept at rapidly transmitting electrical signals across long distances in various excitable tissues. As such, they are amongst the most widely targeted ion channels by drugs and animal toxins. Of the nine isoforms, Nav1.8 and Nav1.9 are preferentially expressed in DRG neurons where they are thought to play an important role in pain signaling. Although the functional properties of Nav1.8 have been relatively well characterized, difficulties with expressing Nav1.9 in established heterologous systems limit our understanding of the gating properties and toxin pharmacology of this particular isoform. This review summarizes our current knowledge of the role of Nav1.8 and Nav1.9 in pain perception and elaborates on the approaches used to identify molecules capable of influencing their function.

The distribution of low-threshold TTX-resistant Na⁺ currents in rat trigeminal ganglion cells.

The distribution of low-threshold tetrodotoxin-resistant (TTX-r) Na(+) current and its co-expression with high-threshold TTX-r Na(+) current were studied in randomly selected acutely dissociated rat trigeminal ganglion (non-identified TG cells) and TG cells serving the temporomandibular joint (TMJ-TG cells). Conditions previously shown to enhance Na(V)1.9 channel-mediated currents (holding potential (HP) -80 mV, 130-mM fluoride internally) were employed to amplify the low-threshold Na(+) current. Under these conditions, detectable low-threshold Na(+) current was exhibited by 16 out of 21 non-identified TG cells (average, 1810 ± 358 pA), and by nine of 14 TMJ-TG cells (average, 959 ± 525 pA). The low-threshold Na(+) current began to activate around -55 mV and was inactivated by holding TG cells at -60 mV and delivering 40-ms test potentials (TPs) to 0 mV. The inactivation was long lasting, recovering only 8 ± 3% over a 5-min period after the HP was returned to -80 mV. Following low-threshold Na(+) current inactivation, high-threshold TTX-r Na(+) current, evoked from HP -60 mV, was observed. High-threshold Na(+) current amplitude averaged 16,592 ± 3913 pA for TPs to 0 mV, was first detectable at an average TP of -34 ± 1.3 mV, and was ½ activated at -7.1 ± 2.3 mV. In TG cells expressing prominent low-threshold Na(+) currents, changing the external solution to one containing 0 mM Na(+) reduced the amount of current required to hold the cells at -80 mV through -50 mV, the peak effect being observed at HP -60 mV. TG cells recorded from with a more physiological pipette solution containing chloride instead of fluoride exhibited small low-threshold Na(+) currents, which were greatly increased upon superfusion of the TG cells with the adenylyl cyclase (AC) activator forskolin. These data suggest two hypotheses: (1) low- and high-threshold Na(V)1.9 and Na(V)1.8 channels, respectively, are frequently co-expressed in TG neurons serving the TMJ and other structures, and (2), Na(V)1.9 channel-mediated currents are small under physiological conditions, but may be enhanced by inflammatory mediators that increase AC activity, and may mediate an inward leak that depolarizes TG neurons, increasing their excitability.