Role of auxiliary beta1-, beta2-, and beta3-subunits and their interaction with Na(v)1.8 voltage-gated sodium channel

A Novel Spider Toxin Inhibits Fast Inactivation of the Nav1.9 Channel by Binding to Domain III and Domain IV Voltage Sensors

Venomous animals have evolved to produce peptide toxins that modulate the activity of voltage-gated sodium (Na_v) channels. These specific modulators are powerful probes for investigating the structural and functional features of Na_v channels. Here, we report the isolation and characterization of δ-theraphotoxin-Gr4b (Gr4b), a novel peptide toxin from the venom of the spider Grammostola rosea. Gr4b contains 37-amino acid residues with six cysteines forming three disulfide bonds. Patch-clamp analysis confirmed that Gr4b markedly slows the fast inactivation of Na_v1.9 and inhibits the currents of Na_v1.4 and Na_v1.7, but does not affect Na_v1.8. It was also found that Gr4b significantly shifts the steady-state activation and inactivation curves of Na_v1.9 to the depolarization direction and increases the window current, which is consistent with the change in the ramp current. Furthermore, analysis of Na_v1.9/Na_v1.8 chimeric channels revealed that Gr4b preferentially binds to the voltage-sensor of domain III (DIII VSD) and has additional interactions with the DIV VSD. The site-directed mutagenesis analysis indicated that N1139 and L1143 in DIII S3-S4 linker participate in toxin binding. In sum, this study reports a novel spider peptide toxin that may slow the fast inactivation of Na_v1.9 by binding to the new neurotoxin receptor site-DIII VSD. Taken together, these findings provide insight into the functional role of the Na_v channel DIII VSD in fast inactivation and activation.

Functional modulation of the human voltage-gated sodium channel NaV1.8 by auxiliary β subunits

The voltage-gated sodium channel Nav1.8 mediates the tetrodotoxin-resistant (TTX-R) Na+ current in nociceptive primary sensory neurons, which has an important role in the transmission of painful stimuli. Here, we describe the functional modulation of the human Nav1.8 α-subunit in Xenopus oocytes by auxiliary β subunits. We found that the β3 subunit down-regulated the maximal Na+ current amplitude and decelerated recovery from inactivation of hNav1.8, whereas the β1 and β2 subunits had no such effects. The specific regulation of Nav1.8 by the β3 subunit constitutes a potential novel regulatory mechanism of the TTX-R Na+ current in primary sensory neurons with potential implications in chronic pain states. In particular, neuropathic pain states are characterized by a down-regulation of Nav1.8 accompanied by increased expression of the β3 subunit. Our results suggest that these two phenomena may be correlated, and that increased levels of the β3 subunit may directly contribute to the down-regulation of Nav1.8. To determine which domain of the β3 subunit is responsible for the specific regulation of hNav1.8, we created chimeras of the β1 and β3 subunits and co-expressed them with the hNav1.8 α-subunit in Xenopus oocytes. The intracellular domain of the β3 subunit was shown to be responsible for the down-regulation of maximal Nav1.8 current amplitudes. In contrast, the extracellular domain mediated the effect of the β3 subunit on hNav1.8 recovery kinetics.

Exercise-Induced Cognitive Improvement Is Associated with Sodium Channel-Mediated Excitability in APP/PS1 Mice

Elevated brain activation, or hyperexcitability, induces cognitive impairment and confers an increased risk of Alzheimer's disease (AD). Blocking the overexcitation of the neural network may be a promising new strategy to prevent, halt, and even reverse this condition. Physical exercise has been shown to be an effective cognitive enhancer that reduces the risk of AD in elderly individuals, but the underlying mechanisms are far from being fully understood. We explored whether long-term treadmill exercise attenuates amyloid precursor protein (APP)/presenilin-1 (PS1) mutation-induced aberrant network activity and thus improves cognition by altering the numbers and/or distribution of voltage-gated sodium channels (Nav) in transgenic mice. APP/PS1 mice aged 2, 3.5, 5, 6.5, 8, and 9 months underwent treadmill exercise with different durations or at different stages of AD. The alterations in memory, electroencephalogram (EEG) recordings, and expression levels and distributions of Nav functional members (Nav1.1α, Nav1.2, Nav1.6, and Navβ2) were evaluated. The results revealed that treadmill exercise with 12- and 24-week durations 1) induced significant improvement in novel object recognition (NOR) memory and Morris water maze (MWM) spatial memory; 2) partially reduced abnormal spike activity; and 3) redressed the disturbed cellular distribution of Nav1.1α, aberrant Navβ2 cleavage augmentation, and Nav1.6 upregulation. Additionally, APP/PS1 mice in the 24-week exercise group showed better performance in the NOR task and a large decrease in Nav1.6 expression, which was close to the wild-type level. This study suggests that exercise improves cognition and neural activity by altering the numbers and distribution of hippocampal Nav in APP/PS1 mice. Long-term treadmill exercise, for about 24 weeks, starting in the preclinical stage, is a promising therapeutic strategy for preventing AD and halting its progress.

Neuroprotection induced by Navβ2‑knockdown in APP/PS1 transgenic neurons is associated with NEP regulation

Voltage‑gated sodium channel β2 (Navβ2), as an unconventional substrate of β‑site amyloid precursor protein cleaving enzyme 1, is involved in regulating the neuronal surface expression of sodium channels. A previous study demonstrated that knockdown of Navβ2 protected neurons and induced spatial cognition improvement by partially reducing pathological amyloidogenic processing of amyloid precursor protein (APP) in aged APP/presenilin 1 (PS1) transgenic mice. The present study aimed to investigate whether Navβ2 knockdown altered APP metabolism via regulation of the Aβ‑degrading enzyme neprilysin (NEP). APPswe/PS1ΔE9 mice (APP/PS1 transgenic mice with a C57BL/6J genetic background) carrying a Navβ2‑knockdown mutation (APP/PS1/Navβ2‑kd) or without Navβ2 knockdown (APP/PS1) were used for cell culture and further analysis. The present results demonstrated that in APP/PS1 mouse‑derived neurons, Navβ2 knockdown partially reversed the reduction in pathological APP cleavage, and the recovery of neurite extension and neuron area. Additionally, Navβ2 knockdown increased NEP activity and levels, and the levels of intracellular domain fragment binding to the NEP promoter. The present findings suggested that knockdown of Navβ2 reversed the APP/PS1 mutation‑induced deficiency in amyloid β degradation by regulating NEP.

Functional characterization of SCN10A variants in several cases of sudden unexplained death

BACKGROUND

Multiple genome-wide association studies (GWAS) and targeted gene sequencing have identified common variants in SCN10A in cases of PR and QRS duration abnormalities, atrial fibrillation and Brugada syndrome. The New York City Office of Chief Medical Examiner has now also identified five SCN10A variants of uncertain significance in six separate cases within a cohort of 330 sudden unexplained death events. The gene product of SCN10A is the Nav1.8 sodium channel. The purpose of this study was to characterize effects of these variants on Nav1.8 channel function to provide better information for the reclassification of these variants.

METHODS AND RESULTS

Patch clamp studies were performed to assess effects of the variants on whole-cell Nav1.8 currents. We also performed RNA-seq analysis and immunofluorescence confocal microcopy to determine Nav1.8 expression in heart. We show that four of the five rare 'variants of unknown significance' (L388M, L867F, P1102S and V1518I) are associated with altered functional phenotypes. The R756W variant behaved similar to wild-type under our experimental conditions. We failed to detect Nav1.8 protein expression in immunofluorescence microscopy in rat heart. Furthermore, RNA-seq analysis failed to detect full-length SCN10A mRNA transcripts in human ventricle or mouse specialized cardiac conduction system, suggesting that the effect of Nav1.8 on cardiac function is likely to be extra-cardiac in origin.

CONCLUSIONS

We have demonstrated that four of five SCN10A variants of uncertain significance, identified in unexplained death, have deleterious effects on channel function. These data extend the genetic testing of SUD cases, but significantly more clinical evidence is needed to satisfy the criteria needed to associate these variants with the onset of SUD.

The NaV1.7 Channel Subtype as an Antinociceptive Target for Spider Toxins in Adult Dorsal Root Ganglia Neurons

Although necessary for human survival, pain may sometimes become pathologic if long-lasting and associated with alterations in its signaling pathway. Opioid painkillers are officially used to treat moderate to severe, and even mild, pain. However, the consequent strong and not so rare complications that occur, including addiction and overdose, combined with pain management costs, remain an important societal and economic concern. In this context, animal venom toxins represent an original source of antinociceptive peptides that mainly target ion channels (such as ASICs as well as TRP, Ca_V, K_V and Na_V channels) involved in pain transmission. The present review aims to highlight the Na_V1.7 channel subtype as an antinociceptive target for spider toxins in adult dorsal root ganglia neurons. It will detail (i) the characteristics of these primary sensory neurons, the first ones in contact with pain stimulus and conveying the nociceptive message, (ii) the electrophysiological properties of the different Na_V channel subtypes expressed in these neurons, with a particular attention on the Na_V1.7 subtype, an antinociceptive target of choice that has been validated by human genetic evidence, and (iii) the features of spider venom toxins, shaped of inhibitory cysteine knot motif, that present high affinity for the Na_V1.7 subtype associated with evidenced analgesic efficacy in animal models.

Loss-of-function of Nav1.8/D1639N linked to human pain can be rescued by lidocaine

Mutations in voltage-gated sodium channels are associated with altered pain perception in humans. Most of these mutations studied to date present with a direct and intuitive link between the altered electrophysiological function of the channel and the phenotype of the patient. In this study, we characterize a variant of Nav1.8, D1639N, which has been previously identified in a patient suffering from the chronic pain syndrome "small fiber neuropathy". Using a heterologous expression system and patch-clamp analysis, we show that Nav1.8/D1639N reduces current density without altering biophysical gating properties of Nav1.8. Therefore, the D1639N variant causes a loss-of-function of the Nav1.8 sodium channel in a patient suffering from chronic pain. Using immunocytochemistry and biochemical approaches, we show that Nav1.8/D1639N impairs trafficking of the channel to the cell membrane. Neither co-expression of β1 or β3 subunit, nor overnight incubation at 27 °C rescued current density of the D1639N variant. On the other hand, overnight incubation with lidocaine fully restored current density of Nav1.8/D1639N most likely by overcoming the trafficking defect, whereas phenytoin failed to do so. Since lidocaine rescues the loss-of-function of Nav1.8/D1639N, it may offer a future therapeutic option for the patient carrying this variant. These results demonstrate that the D1639N variant, identified in a patient suffering from chronic pain, causes loss-of-function of the channel due to impaired cell surface trafficking and that this trafficking defect can be rescued by lidocaine.

Distribution and function of voltage-gated sodium channels in the nervous system

Voltage-gated sodium channels (VGSCs) are the basic ion channels for neuronal excitability, which are crucial for the resting potential and the generation and propagation of action potentials in neurons. To date, at least nine distinct sodium channel isoforms have been detected in the nervous system. Recent studies have identified that voltage-gated sodium channels not only play an essential role in the normal electrophysiological activities of neurons but also have a close relationship with neurological diseases. In this study, the latest research findings regarding the structure, type, distribution, and function of VGSCs in the nervous system and their relationship to neurological diseases, such as epilepsy, neuropathic pain, brain tumors, neural trauma, and multiple sclerosis, are reviewed in detail.

Navβ2 knockdown improves cognition in APP/PS1 mice by partially inhibiting seizures and APP amyloid processing

Voltage-gated sodium channels beta 2 (Navβ2, encoded by SCN2B) is a substrate of β-site amyloid precursor protein cleaving enzyme 1 (BACE1) and regulates cell surface expression of channels in neurons. Previous studies reported enhanced Navβ2 processing by BACE1 in Alzheimer’s disease (AD) model and patients. We investigated whether changes in Navβ2 expression affect neuronal seizure and amyloid precursor protein (APP) processing in an AD mouse model. Our study used eight-month-old APP/presenilin 1 (PS1) mice and transgenic Navβ2 knockdown [by 61% vs. wild type (WT)] APP/PS1 mice (APP/PS1/Navβ2-kd), with age-matched WT and Navβ2 knockdown (Navβ2-kd) mice as controls. We found that Navβ2 knockdown in APP/PS1 mice partially reversed the abnormal Navβ2 cleavage and the changes in intracellular and total Nav1.1α expression. It also restored sodium currents density in hippocampal neurons and neuronal activity, as indicated by EEG tracing; improved Morris water maze performance; and shifted APP amyloidogenic metabolism towards non-amyloidogenic processing. There were no differences in these indicators between WT and Navβ2-kd mice. These results suggest Navβ2 knockdown may be a promising strategy for treating AD.

Voltage-gated sodium channel β subunits: The power outside the pore in brain development and disease

Voltage gated sodium channels (VGSCs) were first identified in terms of their role in the upstroke of the action potential. The underlying proteins were later identified as saxitoxin and scorpion toxin receptors consisting of α and β subunits. We now know that VGSCs are heterotrimeric complexes consisting of a single pore forming α subunit joined by two β subunits; a noncovalently linked β1 or β3 and a covalently linked β2 or β4 subunit. VGSC α subunits contain all the machinery necessary for channel cell surface expression, ion conduction, voltage sensing, gating, and inactivation, in one central, polytopic, transmembrane protein. VGSC β subunits are more than simple accessories to α subunits. In the more than two decades since the original cloning of β1, our knowledge of their roles in physiology and pathophysiology has expanded immensely. VGSC β subunits are multifunctional. They confer unique gating mechanisms, regulate cellular excitability, affect brain development, confer distinct channel pharmacology, and have functions that are independent of the α subunits. The vast array of functions of these proteins stems from their special station in the channelome: being the only known constituents that are cell adhesion and intra/extracellular signaling molecules in addition to being part of channel complexes. This functional trifecta and how it goes awry demonstrates the power outside the pore in ion channel signaling complexes, broadening the term channelopathy beyond defects in ion conduction. This article is part of the Special Issue entitled 'Channelopathies.'

Neuronal Nav1.8 Channels as a Novel Therapeutic Target of Acute Atrial Fibrillation Prevention

BACKGROUND

Ganglionated plexus have been developed as additional ablation targets to improve the outcome of atrial fibrillation (AF) besides pulmonary vein isolation. Recent studies implicated an intimate relationship between neuronal sodium channel Nav1.8 (encoded by SCN10A) and AF. The underlying mechanism between Nav1.8 and AF remains unclear. This study aimed to determine the role of Nav1.8 in cardiac electrophysiology in an acute AF model and explore possible therapeutic targets.

METHODS AND RESULTS

Immunohistochemical study was used on canine cardiac ganglionated plexus. Both Nav1.5 and Nav1.8 were expressed in ganglionated plexus with canonical neuronal markers. Sixteen canines were randomly administered either saline or the Nav1.8 blocker A-803467. Electrophysiological study was compared between the 2 groups before and after 6-hour rapid atrial pacing. Compared with the control group, administration of A-803467 decreased the incidence of AF (87.5% versus 25.0%, P<0.05), shortened AF duration, and prolonged AF cycle length. A-803467 also significantly suppressed the decrease in the effective refractory period and the increase in effective refractory period dispersion and cumulative window of vulnerability caused by rapid atrial pacing in all recording sites. Patch clamp study was performed under 100 nmol/L A-803467 in TSA201 cells cotransfected with SCN10A-WT, SCN5A-WT, and SCN3B-WT. INa,P was reduced by 45.34% at -35 mV, and INa,L by 68.57% at -20 mV. Evident fast inactivation, slow recovery, and use-dependent block were also discovered after applying the drug.

CONCLUSIONS

Our study demonstrates that Nav1.8 could exert its effect on electrophysiological characteristics through cardiac ganglionated plexus. It indicates that Nav1.8 is a novel target in understanding cardiac electrophysiology and SCN10A-related arrhythmias.

Post-translational modifications of voltage-gated sodium channels in chronic pain syndromes

In the peripheral sensory nervous system the neuronal expression of voltage-gated sodium channels (Navs) is very important for the transmission of nociceptive information since they give rise to the upstroke of the action potential (AP). Navs are composed of nine different isoforms with distinct biophysical properties. Studying the mutations associated with the increase or absence of pain sensitivity in humans, as well as other expression studies, have highlighted Nav1.7, Nav1.8, and Nav1.9 as being the most important contributors to the control of nociceptive neuronal electrogenesis. Modulating their expression and/or function can impact the shape of the AP and consequently modify nociceptive transmission, a process that is observed in persistent pain conditions. Post-translational modification (PTM) of Navs is a well-known process that modifies their expression and function. In chronic pain syndromes, the release of inflammatory molecules into the direct environment of dorsal root ganglia (DRG) sensory neurons leads to an abnormal activation of enzymes that induce Navs PTM. The addition of small molecules, i.e., peptides, phosphoryl groups, ubiquitin moieties and/or carbohydrates, can modify the function of Navs in two different ways: via direct physical interference with Nav gating, or via the control of Nav trafficking. Both mechanisms have a profound impact on neuronal excitability. In this review we will discuss the role of Protein Kinase A, B, and C, Mitogen Activated Protein Kinases and Ca++/Calmodulin-dependent Kinase II in peripheral chronic pain syndromes. We will also discuss more recent findings that the ubiquitination of Nav1.7 by Nedd4-2 and the effect of methylglyoxal on Nav1.8 are also implicated in the development of experimental neuropathic pain. We will address the potential roles of other PTMs in chronic pain and highlight the need for further investigation of PTMs of Navs in order to develop new pharmacological tools to alleviate pain.

Structure and function of μ-conotoxins, peptide-based sodium channel blockers with analgesic activity

μ-Conotoxins block voltage-gated sodium channels (VGSCs) and compete with tetrodotoxin for binding to the sodium conductance pore. Early efforts identified µ-conotoxins that preferentially blocked the skeletal muscle subtype (NaV1.4). However, the last decade witnessed a significant increase in the number of µ-conotoxins and the range of VGSC subtypes inhibited (NaV1.2, NaV1.3 or NaV1.7). Twenty µ-conotoxin sequences have been identified to date and structure-activity relationship studies of several of these identified key residues responsible for interactions with VGSC subtypes. Efforts to engineer-in subtype specificity are driven by in vivo analgesic and neuromuscular blocking activities. This review summarizes structural and pharmacological studies of µ-conotoxins, which show promise for development of selective blockers of NaV1.2, and perhaps also NaV1.1,1.3 or 1.7.

Sodium Channel Voltage-Gated Beta 2 Plays a Vital Role in Brain Aging Associated with Synaptic Plasticity and Expression of COX5A and FGF-2

The role of sodium channel voltage-gated beta 2 (SCN2B) in brain aging is largely unknown. The present study was therefore designed to determine the role of SCN2B in brain aging by using the senescence-accelerated mice prone 8 (SAMP8), a brain senescence-accelerated animal model, together with the SCN2B transgenic mice. The results showed that SAMP8 exhibited impaired learning and memory functions, assessed by the Morris water maze test, as early as 8 months of age. The messenger RNA (mRNA) and protein expressions of SCN2B were also upregulated in the prefrontal cortex at this age. Treatment with traditional Chinese anti-aging medicine Xueshuangtong (Panax notoginseng saponins, PNS) significantly reversed the SCN2B expressions in the prefrontal cortex, resulting in improved learning and memory. Moreover, SCN2B knockdown transgenic mice were generated and bred to determine the roles of SCN2B in brain senescence. A reduction in the SCN2B level by 60.68% resulted in improvement in the hippocampus-dependent spatial recognition memory and long-term potential (LTP) slope of field excitatory postsynaptic potential (fEPSP), followed by an upregulation of COX5A mRNA levels and downregulation of fibroblast growth factor-2 (FGF-2) mRNA expression. Together, the present findings indicated that SCN2B could play an important role in the aging-related cognitive deterioration, which is associated with the regulations of COX5A and FGF-2. These findings could provide the potential strategy of candidate target to develop antisenescence drugs for the treatment of brain aging.

β1- and β3- voltage-gated sodium channel subunits modulate cell surface expression and glycosylation of Nav1.7 in HEK293 cells

Voltage-gated sodium channels (Navs) are glycoproteins composed of a pore-forming α-subunit and associated β-subunits that regulate Nav α-subunit plasma membrane density and biophysical properties. Glycosylation of the Nav α-subunit also directly affects Navs gating. β-subunits and glycosylation thus comodulate Nav α-subunit gating. We hypothesized that β-subunits could directly influence α-subunit glycosylation. Whole-cell patch clamp of HEK293 cells revealed that both β1- and β3-subunits coexpression shifted V ½ of steady-state activation and inactivation and increased Nav1.7-mediated I Na density. Biotinylation of cell surface proteins, combined with the use of deglycosydases, confirmed that Nav1.7 α-subunits exist in multiple glycosylated states. The α-subunit intracellular fraction was found in a core-glycosylated state, migrating at ~250 kDa. At the plasma membrane, in addition to the core-glycosylated form, a fully glycosylated form of Nav1.7 (~280 kDa) was observed. This higher band shifted to an intermediate band (~260 kDa) when β1-subunits were coexpressed, suggesting that the β1-subunit promotes an alternative glycosylated form of Nav1.7. Furthermore, the β1-subunit increased the expression of this alternative glycosylated form and the β3-subunit increased the expression of the core-glycosylated form of Nav1.7. This study describes a novel role for β1- and β3-subunits in the modulation of Nav1.7 α-subunit glycosylation and cell surface expression.

[ICU acquired neuromyopathy]

ICU acquired neuromyopathy (IANM) is the most frequent neurological pathology observed in ICU. Nerve and muscle defects are merged with neuromuscular junction abnormalities. Its physiopathology is complex. The aim is probably the redistribution of nutriments and metabolism towards defense against sepsis. The main risk factors are sepsis, its severity and its duration of evolution. IANM is usually diagnosed in view of difficulties in weaning from mechanical ventilation, but electrophysiology may allow an earlier diagnosis. There is no curative therapy, but early treatment of sepsis, glycemic control as well as early physiotherapy may decrease its incidence. The outcomes of IANM are an increase in morbi-mortality and possibly long-lasting neuromuscular abnormalities as far as tetraplegia.

Differential effects of TipE and a TipE-homologous protein on modulation of gating properties of sodium channels from Drosophila melanogaster

β subunits of mammalian sodium channels play important roles in modulating the expression and gating of mammalian sodium channels. However, there are no orthologs of β subunits in insects. Instead, an unrelated protein, TipE in Drosophila melanogaster and its orthologs in other insects, is thought to be a sodium channel auxiliary subunit. In addition, there are four TipE-homologous genes (TEH1-4) in D. melanogaster and three to four orthologs in other insect species. TipE and TEH1-3 have been shown to enhance the peak current of various insect sodium channels expressed in Xenopus oocytes. However, limited information is available on how these proteins modulate the gating of sodium channels, particularly sodium channel variants generated by alternative splicing and RNA editing. In this study, we compared the effects of TEH1 and TipE on the function of three Drosophila sodium channel splice variants, DmNav9-1, DmNav22, and DmNav26, in Xenopus oocytes. Both TipE and TEH1 enhanced the amplitude of sodium current and accelerated current decay of all three sodium channels tested. Strikingly, TEH1 caused hyperpolarizing shifts in the voltage-dependence of activation, fast inactivation and slow inactivation of all three variants. In contrast, TipE did not alter these gating properties except for a hyperpolarizing shift in the voltage-dependence of fast inactivation of DmNav26. Further analysis of the gating kinetics of DmNav9-1 revealed that TEH1 accelerated the entry of sodium channels into the fast inactivated state and slowed the recovery from both fast- and slow-inactivated states, thereby, enhancing both fast and slow inactivation. These results highlight the differential effects of TipE and TEH1 on the gating of insect sodium channels and suggest that TEH1 may play a broader role than TipE in regulating sodium channel function and neuronal excitability in vivo.

Tetrodotoxin-resistant voltage-dependent sodium channels in identified muscle afferent neurons

Muscle afferents are critical regulators of motor function (Group I and II) and cardiovascular responses to exercise (Group III and IV). However, little is known regarding the expressed voltage-dependent ion channels. We identified muscle afferent neurons in dorsal root ganglia (DRGs), using retrograde labeling to examine voltage-dependent sodium (Na(V)) channels. In patch-clamp recordings, we found that the dominant Na(V) current in the majority of identified neurons was insensitive to tetrodotoxin (TTX-R), with Na(V) current in only a few (14%) neurons showing substantial (>50%) TTX sensitivity (TTX-S). The TTX-R current was sensitive to a Na(V)1.8 channel blocker, A803467. Immunocytochemistry demonstrated labeling of muscle afferent neurons by a Na(V)1.8 antibody, which further supported expression of these channels. A portion of the TTX-R Na(V) current appeared to be noninactivating during our 25-ms voltage steps, which suggested activity of Na(V)1.9 channels. The majority of the noninactivating current was insensitive to A803467 but sensitive to extracellular sodium. Immunocytochemistry showed labeling of muscle afferent neurons by a Na(V)1.9 channel antibody, which supports expression of these channels. Further examination of the muscle afferent neurons showed that functional TTX-S channels were expressed, but were largely inactivated at physiological membrane potentials. Immunocytochemistry showed expression of the TTX-S channels Na(V)1.6 and Na(V)1.7 but not Na(V)1.1. Na(V)1.8 and Na(V)1.9 appear to be the dominant functional sodium channels in small- to medium-diameter muscle afferent neurons. The expression of these channels is consistent with the identification of these neurons as Group III and IV, which mediate the exercise pressor reflex.

Differential expression of sodium channel β subunits in dorsal root ganglion sensory neurons

The small-diameter (<25 μm) and large-diameter (>30 μm) sensory neurons of the dorsal root ganglion (DRG) express distinct combinations of tetrodotoxin sensitive and tetrodotoxin-resistant Na(+) channels that underlie the unique electrical properties of these neurons. In vivo, these Na(+) channels are formed as complexes of pore-forming α and auxiliary β subunits. The goal of this study was to investigate the expression of β subunits in DRG sensory neurons. Quantitative single-cell RT-PCR revealed that β subunit mRNA is differentially expressed in small (β(2) and β(3)) and large (β(1) and β(2)) DRG neurons. This raises the possibility that β subunit availability and Na(+) channel composition and functional regulation may differ in these subpopulations of sensory neurons. To further explore these possibilities, we quantitatively compared the mRNA expression of the β subunit with that of Na(v)1.7, a TTX-sensitive Na(+) channel widely expressed in both small and large DRG neurons. Na(v)1.7 and β subunit mRNAs were significantly correlated in small (β(2) and β(3)) and large (β(1) and β(2)) DRG neurons, indicating that these subunits are coexpressed in the same populations. Co-immunoprecipitation and immunocytochemistry indicated that Na(v)1.7 formed stable complexes with the β(1)-β(3) subunits in vivo and that Na(v)1.7 and β(3) co-localized within the plasma membranes of small DRG neurons. Heterologous expression studies showed that β(3) induced a hyperpolarizing shift in Na(v)1.7 activation, whereas β(1) produced a depolarizing shift in inactivation and faster recovery. The data indicate that β(3) and β(1) subunits are preferentially expressed in small and large DRG neurons, respectively, and that these auxiliary subunits differentially regulate the gating properties of Na(v)1.7 channels.

Regulatory Role of Voltage-Gated Na Channel β Subunits in Sensory Neurons

Voltage-gated sodium Na(+) channels are membrane-bound proteins incorporating aqueous conduction pores that are highly selective for sodium Na(+) ions. The opening of these channels results in the rapid influx of Na(+) ions that depolarize the cell and drive the rapid upstroke of nerve and muscle action potentials. While the concept of a Na(+)-selective ion channel had been formulated in the 1940s, it was not until the 1980s that the biochemical properties of the 260-kDa and 36-kDa auxiliary β subunits (β(1), β(2)) were first described. Subsequent cloning and heterologous expression studies revealed that the α subunit forms the core of the channel and is responsible for both voltage-dependent gating and ionic selectivity. To date, 10 isoforms of the Na(+) channel α subunit have been identified that vary in their primary structures, tissue distribution, biophysical properties, and sensitivity to neurotoxins. Four β subunits (β(1)-β(4)) and two splice variants (β(1A), β(1B)) have been identified that modulate the subcellular distribution, cell surface expression, and functional properties of the α subunits. The purpose of this review is to provide a broad overview of β subunit expression and function in peripheral sensory neurons and examine their contributions to neuropathic pain.

Navβ subunits modulate the inhibition of Nav1.8 by the analgesic gating modifier μO-conotoxin MrVIB

Voltage-gated sodium channels (VGSCs) consist of a pore-forming α-subunit and regulatory β-subunits. Several families of neuroactive peptides of Conus snails target VGSCs, including μO-conotoxins and μ-conotoxins. Unlike μ-conotoxins and the guanidinium alkaloid saxitoxin (STX), which are pore blockers, μO-conotoxins MrVIA and MrVIB inhibit VGSCs by modifying channel gating. μO-MrVIA/B can block Na(v)1.8 (a tetrodotoxin-resistant isoform of VGSCs) and have analgesic properties. The effect of Na(v)β-subunit coexpression on susceptibility to block by μO-MrVIA/B and STX has, until now, not been reported. Here, we show that β1-, β2-, β3-, and β4-subunits, when individually coexpressed with Na(v)1.8 in Xenopus laevis oocytes, increased the k(on) of the block produced by μO-MrVIB (by 3-, 32-, 2-, and 7-fold, respectively) and modestly decreased the apparent k(off). Strong depolarizing prepulses markedly accelerated MrVIB washout with rates dependent on β-subunit coexpression. Thus, coexpression of β-subunits with Na(v)1.8 can strongly influence the affinity of the conopeptide for the channel. This observation is of particular interest because β-subunit expression can be dynamic, e.g., β2-expression is up-regulated after nerve injury (J Neurosci, 25:10970-10980, 2005); therefore, the effectiveness of a μO-conotoxin as a channel blocker could be enhanced by the conditions that may call for its use therapeutically. In contrast to MrVIB's action, the STX-induced block of Na(v)1.8 was only marginally, if at all, affected by coexpression of any of the β-subunits. Our results raise the possibility that μO-conotoxins and perhaps other gating modifiers may provide a means to functionally assess the β-subunit composition of VGSC complexes in neurons.

Regulation of Nav1.6 and Nav1.8 peripheral nerve Na+ channels by auxiliary β-subunits

Voltage-gated Na(+) (Na(v)) channels are composed of a pore-forming α-subunit and one or more auxiliary β-subunits. The present study investigated the regulation by the β-subunit of two Na(+) channels (Na(v)1.6 and Na(v)1.8) expressed in dorsal root ganglion (DRG) neurons. Single cell RT-PCR was used to show that Na(v)1.8, Na(v)1.6, and β(1)-β(3) subunits were widely expressed in individually harvested small-diameter DRG neurons. Coexpression experiments were used to assess the regulation of Na(v)1.6 and Na(v)1.8 by β-subunits. The β(1)-subunit induced a 2.3-fold increase in Na(+) current density and hyperpolarizing shifts in the activation (-4 mV) and steady-state inactivation (-4.7 mV) of heterologously expressed Na(v)1.8 channels. The β(4)-subunit caused more pronounced shifts in activation (-16.7 mV) and inactivation (-9.3 mV) but did not alter the current density of cells expressing Na(v)1.8 channels. The β(3)-subunit did not alter Na(v)1.8 gating but significantly reduced the current density by 31%. This contrasted with Na(v)1.6, where the β-subunits were relatively weak regulators of channel function. One notable exception was the β(4)-subunit, which induced a hyperpolarizing shift in activation (-7.6 mV) but no change in the inactivation or current density of Na(v)1.6. The β-subunits differentially regulated the expression and gating of Na(v)1.8 and Na(v)1.6. To further investigate the underlying regulatory mechanism, β-subunit chimeras containing portions of the strongly regulating β(1)-subunit and the weakly regulating β(2)-subunit were generated. Chimeras retaining the COOH-terminal domain of the β(1)-subunit produced hyperpolarizing shifts in gating and increased the current density of Na(v)1.8, similar to that observed for wild-type β(1)-subunits. The intracellular COOH-terminal domain of the β(1)-subunit appeared to play an essential role in the regulation of Na(v)1.8 expression and gating.

Na+ channel Scn1b gene regulates dorsal root ganglion nociceptor excitability in vivo

Nociceptive dorsal root ganglion (DRG) neurons express tetrodotoxin-sensitive (TTX-S) and -resistant (TTX-R) Na(+) current (I(Na)) mediated by voltage-gated Na(+) channels (VGSCs). In nociceptive DRG neurons, VGSC β2 subunits, encoded by Scn2b, selectively regulate TTX-S α subunit mRNA and protein expression, ultimately resulting in changes in pain sensitivity. We hypothesized that VGSCs in nociceptive DRG neurons may also be regulated by β1 subunits, encoded by Scn1b. Scn1b null mice are models of Dravet Syndrome, a severe pediatric encephalopathy. Many physiological effects of Scn1b deletion on CNS neurons have been described. In contrast, little is known about the role of Scn1b in peripheral neurons in vivo. Here we demonstrate that Scn1b null DRG neurons exhibit a depolarizing shift in the voltage dependence of TTX-S I(Na) inactivation, reduced persistent TTX-R I(Na), a prolonged rate of recovery of TTX-R I(Na) from inactivation, and reduced cell surface expression of Na(v)1.9 compared with their WT littermates. Investigation of action potential firing shows that Scn1b null DRG neurons are hyperexcitable compared with WT. Consistent with this, transient outward K(+) current (I(to)) is significantly reduced in null DRG neurons. We conclude that Scn1b regulates the electrical excitability of nociceptive DRG neurons in vivo by modulating both I(Na) and I(K).

Increased expression of the beta3 subunit of voltage-gated Na+ channels in the spinal cord of the SOD1G93A mouse

Amyotrophic lateral sclerosis (ALS) is an adult-onset disease characterized by the progressive degeneration of motoneurons (MNs). Altered electrical properties have been described in familial and sporadic ALS patients. Cortical and spinal neurons cultured from the mutant Cu,Zn superoxide dismutase 1 (SOD1G93A) mouse, a murine model of ALS, exhibit a marked increase in the persistent Na+ currents. Here, we investigated the effects of the SOD1G93A mutation on the expression of the voltage-gated Na+ channel alpha subunit SCN8A (Nav1.6) and the beta subunits SCN1B (beta1), SCN2B (beta2), and SCN3B (beta3) in MNs of the spinal cord in presymptomatic (P75) and symptomatic (P120) mice. We observed a significant increase, within lamina IX, of the beta3 transcript and protein expression. On the other hand, the beta1 transcript was significantly decreased, in the same area, at the symptomatic stage, while the beta2 transcript levels were unaltered. The SCN8A transcript was significantly decreased at P120 in the whole spinal cord. These data suggest that the SOD1G93A mutation alters voltage-gated Na+ channel subunit expression. Moreover, the increased expression of the beta3 subunit support the hypothesis that altered persistent Na+ currents contribute to the hyperexcitability observed in the ALS-affected MNs.

Loss-of-function mutation of the SCN3B-encoded sodium channel {beta}3 subunit associated with a case of idiopathic ventricular fibrillation

AIMS

Loss-of-function mutations in the SCN5A-encoded sodium channel SCN5A or Nav1.5 have been identified in idiopathic ventricular fibrillation (IVF) in the absence of Brugada syndrome phenotype. Nav1.5 is regulated by four sodium channel auxiliary beta subunits. Here, we report a case with IVF and a novel mutation in the SCN3B-encoded sodium channel beta subunit Navbeta3 that causes a loss of function of Nav1.5 channels in vitro.

METHODS AND RESULTS

Comprehensive open reading frame mutational analysis of KCNQ1, KCNH2, SCN5A, KCNE1, KCNE2, GPD1L, four sodium channel beta subunit genes (SCN1-4B), and targeted scan of RYR2 was performed. A novel missense mutation, Navbeta3-V54G, was identified in a 20-year-old male following witnessed collapse and defibrillation from VF. The ECG exhibited epsilon waves, and imaging studies demonstrated a structurally normal heart. The mutated residue was highly conserved across species, localized to the Navbeta3 extracellular domain, and absent in 800 reference alleles. We found that HEK-293 cells had endogenous Navbeta3, but COS cells did not. Co-expression of Nav1.5 with Navbeta3-V54G (with or without co-expression of the Navbeta1 subunit) in both HEK-293 cells and COS cells revealed a significant decrease in peak sodium current and a positive shift of inactivation compared with WT. Co-immunoprecipitation experiments showed association of Navbeta3 with Nav1.5, and immunocytochemistry demonstrated a dramatic decrease in trafficking to the plasma membrane when co-expressed with mutant Navbeta3-V54G.

CONCLUSION

This study provides molecular and cellular evidence implicating mutations in Navbeta3 as a cause of IVF.

Excitability constraints on voltage-gated sodium channels

We study how functional constraints bound and shape evolution through an analysis of mammalian voltage-gated sodium channels. The primary function of sodium channels is to allow the propagation of action potentials. Since Hodgkin and Huxley, mathematical models have suggested that sodium channel properties need to be tightly constrained for an action potential to propagate. There are nine mammalian genes encoding voltage-gated sodium channels, many of which are more than ≈90% identical by sequence. This sequence similarity presumably corresponds to similarity of function, consistent with the idea that these properties must be tightly constrained. However, the multiplicity of genes encoding sodium channels raises the question: why are there so many? We demonstrate that the simplest theoretical constraints bounding sodium channel diversity—the requirements of membrane excitability and the uniqueness of the resting potential—act directly on constraining sodium channel properties. We compare the predicted constraints with functional data on mammalian sodium channel properties collected from the literature, including 172 different sets of measurements from 40 publications, wild-type and mutant, under a variety of conditions. The data from all channel types, including mutants, obeys the excitability constraint; on the other hand, channels expressed in muscle tend to obey the constraint of a unique resting potential, while channels expressed in neuronal tissue do not. The excitability properties alone distinguish the nine sodium channels into four different groups that are consistent with phylogenetic analysis. Our calculations suggest interpretations for the functional differences between these groups.

There are few quantitative examples for how functional constraints bound and shape evolution. Sodium channels are a central player in the propagation of action potentials. Action potentials fire above a critical voltage threshold. Below the voltage threshold the membrane potential recovers to a resting value, which is assumed to be unique. Here we ask whether the properties of mammalian voltage-gated sodium channels are determined by the simplest possible constraints. We demonstrate that the requirements, (1) a voltage threshold and (2) a unique resting potential, severely constrain sodium channel properties. These constraints contain no free parameters, depending only on the concentrations of potassium inside and outside the cell. We test these predictions on functional data from the nine mammalian genes encoding voltage-gated sodium channels. All measurements obey the excitability constraint, whereas channels expressed in the nervous system systematically violate the constraint for a unique resting potential. These properties alone distinguish the nine sodium channels into four groups consistent with phylogenetic analysis. Our calculations suggest that different channel types have evolved to perform different tasks.

β-Subunits of voltage-gated sodium channels in human prostate cancer: quantitative in vitro and in vivo analyses of mRNA expression

We previously identified high levels of Na(v)1.7 voltage-gated sodium channel alpha-subunit (VGSCalpha) mRNA and protein in human prostate cancer cells and tissues. Here, we investigated auxillary beta-subunit (VGSCbetas) expression. In vitro, the combined expression of all four VGSCbetas was significantly (approximately 4.5-fold) higher in strongly compared to weakly metastatic cells. This was mainly due to increased beta1-expression, which was under androgenic control. In vivo, beta1-beta4 mRNAs were detectable and their expression in CaP vs non-CaP tissues generally reflected the in vitro levels in relation to metastatic potential. The possible role(s) of VGSCbetas (VGSCalpha-associated and VGSCalpha-independent) in prostate cancer are discussed.

Sodium channel auxiliary subunits

Voltage-gated ion channels are well known for their functional roles in excitable tissues. Excitable tissues rely on voltage-gated ion channels and their auxiliary subunits to achieve concerted electrical activity in living cells. Auxiliary subunits are also known to provide functional diversity towards the transport and biogenesis properties of the principal subunits. Recent interests in pharmacological properties of these auxiliary subunits have prompted significant amounts of efforts in understanding their physiological roles. Some auxiliary subunits can potentially serve as drug targets for novel analgesics. Three families of sodium channel auxiliary subunits are described here: beta1 and beta3, beta2 and beta4, and temperature-induced paralytic E (TipE). While sodium channel beta-subunits are encoded in many animal genomes, TipE has only been found exclusively in insects. In this review, we present phylogenetic analyses, discuss potential evolutionary origins and functional data available for each of these subunits. For each family, we also correlate the functional specificity with the history of evolution for the individual auxiliary subunits.

Inactivation properties of sodium channel Nav1.8 maintain action potential amplitude in small DRG neurons in the context of depolarization

Background

Small neurons of the dorsal root ganglion (DRG) express five of the nine known voltage-gated sodium channels. Each channel has unique biophysical characteristics which determine how it contributes to the generation of action potentials (AP). To better understand how AP amplitude is maintained in nociceptive DRG neurons and their centrally projecting axons, which are subjected to depolarization within the dorsal horn, we investigated the dependence of AP amplitude on membrane potential, and how that dependence is altered by the presence or absence of sodium channel Na_v1.8.

Results

In small neurons cultured from wild type (WT) adult mouse DRG, AP amplitude decreases as the membrane potential is depolarized from -90 mV to -30 mV. The decrease in amplitude is best fit by two Boltzmann equations, having V_1/2 values of -73 and -37 mV. These values are similar to the V_1/2 values for steady-state fast inactivation of tetrodotoxin-sensitive (TTX-s) sodium channels, and the tetrodotoxin-resistant (TTX-r) Na_v1.8 sodium channel, respectively. Addition of TTX eliminates the more hyperpolarized V_1/2 component and leads to increasing AP amplitude for holding potentials of -90 to -60 mV. This increase is substantially reduced by the addition of potassium channel blockers. In neurons from Na_v1.8(-/-) mice, the voltage-dependent decrease in AP amplitude is characterized by a single Boltzmann equation with a V_1/2 value of -55 mV, suggesting a shift in the steady-state fast inactivation properties of TTX-s sodium channels. Transfection of Na_v1.8(-/-) DRG neurons with DNA encoding Na_v1.8 results in a membrane potential-dependent decrease in AP amplitude that recapitulates WT properties.

Conclusion

We conclude that the presence of Na_v1.8 allows AP amplitude to be maintained in DRG neurons and their centrally projecting axons even when depolarized within the dorsal horn.

The intracellular domain of the beta 2 subunit modulates the gating of cardiac Na v 1.5 channels

We have previously shown that the transmembrane segment plus either the extracellular or intracellular domain of the beta1 subunit are required to modify cardiac Na(v)1.5 channels. In this study, we coexpressed the intracellular domain of the beta2 subunit in a beta1/beta2 chimera with Na(v)1.5 channels in Xenopus oocytes and obtained an atypical recovery behavior of Na(v)1.5 channels not reported before for other Na(+) channels: Recovery times of up to 20 ms at -120 mV produced a similar fast recovery as observed for Na(v)1.5/beta1 channels, but the current amplitude decreased again at longer recovery times and reached a steady-state level after 1-2 s with current amplitudes of only 43 +/- 2% of the value at 20 ms. Current reduction was accompanied by slowed inactivation and by a shift of steady-state activation toward depolarized potentials by 9 mV. All effects were reversible and they were not seen when deleting the beta2 intracellular domain. These results describe the first functional effects of a beta2 subunit region on Na(v)1.5 channels and suggest a novel closed state in cardiac Na(+) channels accessible at hyperpolarized potentials.

Voltage-sensitive ion channels and cancer

Plasma membrane voltage-sensitive ion channels classically have been associated with a variety of inherited diseases or "channelopathies" that range in the severity of symptoms from mild to lethal. Ion channels are found throughout the body and are responsible for facilitated diffusion of ions down the electrochemical gradient across cells membranes in various tissues. Voltage-sensitive ion channels open in response to changes in the membrane potential and are primarily found in excitable cells and tissues. Potassium, calcium, and sodium channels play critical roles in the development of major diseases, such as hyperkalemia, epilepsy, congenital myotonia and several cardiac arrythmias. Recently, cancer studies have begun to define the role of voltage-sensitive ion channels in the progression of cancer to a more malignant phenotype. In cancer, the increased expression or increased kinetics of voltage-sensitive ion channels is associated with an increasing malignant potential as evinced by their role in cell proliferation, migration and survival; as such, these channels are becoming the targets of significant drug development efforts to block or reduce voltage-sensitive ion channel activity in order to prevent or combat malignant disease.

Multiple sodium channels and their roles in electrogenesis within dorsal root ganglion neurons

Dorsal root ganglion neurons express an array of sodium channel isoforms allowing precise control of excitability. An increasing body of literature indicates that regulation of firing behaviour in these cells is linked to their patterns of expression of specific sodium channel isoforms, which have been discovered to possess distinct biophysical characteristics. The pattern of expression of sodium channels differs in different subclasses of DRG neurons and is not fixed but, on the contrary, changes in response to a variety of disease insults. Moreover, modulation of channels by their environment has been found to play an important role in the response of these neurons to stimuli. In this review we illustrate how excitability can be finely tuned to provide contrasting firing templates in different subclasses of DRG neurons by selective deployment of various sodium channel isoforms, by plasticity of expression of these proteins, and by interactions of these sodium channel isoforms with each other and with other modulatory molecules.

Isoform-specific Effects of the β2 Subunit on Voltage-gated Sodium Channel Gating

Voltage-gated sodium channels (Nav) are complex glycoproteins comprised of an alpha subunit and often one to several beta subunits. We have shown that sialic acid residues linked to Nav alpha and beta1 subunits alter channel gating. To determine whether beta2-linked sialic acids similarly impact Nav gating, we co-expressed beta2 with Nav1.5 or Nav1.2 in Pro5 (complete sialylation) and in Lec2 (essentially no sialylation) cells. Beta2 sialic acids caused a significant hyperpolarizing shift in Nav1.5 voltage-dependent gating, thus describing for the first time an effect of beta2 on Nav1.5 gating. In contrast, beta2 caused a sialic acid-independent depolarizing shift in Nav1.2 gating. A deglycosylated mutant, beta(2-DeltaN), had no effect on Nav1.5 gating, indicating further the impact of beta2 N-linked sialic acids on Nav1.5 gating. Conversely, beta(2-DeltaN) modulated Nav1.2 gating virtually identically to beta2, confirming that beta2 N-linked sugars have no impact on Nav1.2 gating. Thus, beta2 modulates Nav gating through multiple mechanisms possibly determined by the associated alpha subunit. Beta1 and beta2 were expressed together with Nav1.5 or Nav1.2 in Pro5 and Lec2 cells. Together beta1 and beta2 produced a significantly larger sialic acid-dependent hyperpolarizing shift in Nav1.5 gating. Under fully sialylating conditions, the Nav1.2.beta1.beta2 complex behaved like Nav1.2 alone. When sialylation was reduced, only the sialic acid-independent depolarizing effects of beta2 on Nav1.2 gating were apparent. Thus, the varied effects of beta1 and beta2 on Nav1.5 and Nav1.2 gating are apparently synergistic and highlight the complex manner, through subunit- and sugar-dependent mechanisms, by which Nav activity is modulated.

Sodium channel beta2 subunits regulate tetrodotoxin-sensitive sodium channels in small dorsal root ganglion neurons and modulate the response to pain

Voltage-gated sodium channel (Na(v)1) beta2 subunits modulate channel gating, assembly, and cell-surface expression in CNS neurons in vitro and in vivo. beta2 expression increases in sensory neurons after nerve injury, and development of mechanical allodynia in the spared nerve injury model is attenuated in beta2-null mice. Thus, we hypothesized that beta2 modulates electrical excitability in dorsal root ganglion (DRG) neurons in vivo. We compared sodium currents (I(Na)) in small DRG neurons from beta2+/+ and beta2-/- mice to determine the effects of beta2 on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) Na(v)1 in vivo. Small-fast DRG neurons acutely isolated from beta2-/- mice showed significant decreases in TTX-S I(Na) compared with beta2+/+ neurons. This decrease included a 51% reduction in maximal sodium conductance with no detectable changes in the voltage dependence of activation or inactivation. TTX-S, but not TTX-R, I(Na) activation and inactivation kinetics in these cells were slower in beta2(-/-) mice compared with controls. The selective regulation of TTX-S I(Na) was supported by reductions in transcript and protein levels of TTX-S Na(v)1s, particularly Na(v)1.7. Low-threshold mechanical sensitivity was preserved in beta2-/- mice, but they were more sensitive to noxious thermal stimuli than wild type whereas their response during the late phase of the formalin test was attenuated. Our results suggest that beta2 modulates TTX-S Na(v)1 mRNA and protein expression resulting in increased TTX-S I(Na) and increases the rates of TTX-S Na(v)1 activation and inactivation in small-fast DRG neurons in vivo. TTX-R I(Na) were not significantly modulated by beta2.

Dissecting the role of sodium currents in visceral sensory neurons in a model of chronic hyperexcitability using Nav1.8 and Nav1.9 null mice

Tetrodotoxin-resistant (TTX-R) sodium currents have been proposed to underlie sensory neuronal hyperexcitability in acute inflammatory models, but their role in chronic models is unknown. Since no pharmacological tools to separate TTX-R currents are available, this study employs Na(v)1.8 and Na(v)1.9 null mice to evaluate these currents roles in a chronic hyperexcitability model after the resolution of an inflammatory insult. Transient jejunitis was induced by infection with Nippostrongylus brasiliensis (Nb) in Na(v)1.9 and Na(v)1.8 null, wild-type and naïve mice. Retrogradely labelled dorsal root ganglia (DRG) neurons were harvested on day 20-24 post-infection for patch clamp recording. Rheobase and action potential (AP) parameters were recorded as measures of excitability, and Na(v)1.9 and Na(v)1.8 currents were recorded. DRG neuronal excitability was significantly increased in post-infected mice compared to sham animals, despite the absence of ongoing inflammation (sham = 1.9 +/- 0.3, infected = 3.6 +/- 0.7 APs at 2x rheobase, P = 0.02). Hyperexcitability was associated with a significantly increased amplitude of TTX-R currents. Hyperexcitability was maintained in Na(v)1.9(-/-) mice, but hyperexcitability was absent and APs were blunted in Na(v)1.8(-/-) mice. This study identifies a critical role for Na(v)1.8 in chronic post-infectious visceral hyperexcitability, with no contribution from Na(v)1.9. Nb infection-induced hyperexcitability is not observed in Na(v)1.8(-/-) mice, but is still present in Na(v)1.9(-/-) mice. It is not clear whether hyperexcitability is due to a change in the function of Na(v)1.8 channels or a change in the number of Na(v)1.8 channels.

Multiple types of Na+ currents mediate action potential electrogenesis in small neurons of mouse dorsal root ganglia

Small (<25 microm in diameter) neurons of the dorsal root ganglion (DRG) express multiple voltage-gated Na(+) channel subtypes, two of which being resistant to tetrodotoxin (TTX). Each subtype mediates Na(+) current with distinct kinetic property. However, it is not known how each type of Na(+) channel contributes to the generation of action potentials in small DRG neurons. Therefore, we investigated the correlation between Na(+) currents in voltage-clamp recordings and corresponding action potentials in current-clamp recordings, using wild-type (WT) and Na(V)1.8 knock-out (KO) mice, to clarify the action potential electrogenesis in small DRG neurons. We classified Na(+) currents in small DRG neurons into three categories on the basis of TTX sensitivity and kinetic properties, i.e., TTX-sensitive (TTX-S)/fast Na(+) current, TTX-resistant (TTX-R)/slow Na(+) current, and TTX-R/persistent Na(+) current. Our concurrent voltage- and current-clamp recordings from the same neuron revealed that the action potentials in WT small DRG neurons were mainly dependent on TTX-R/slow Na(+) current mediated by Na(V)1.8. It was surprising that a large portion of TTX-S/fast Na(+) current was switched off in WT small DRG neurons due to a hyperpolarizing shift of the steady-state inactivation (h (infinity)), whereas in KO small DRG neurons which are devoid of TTX-R/slow Na(+) current, the action potentials were generated by TTX-S/fast Na(+) current possibly through a compensatory shift of h (infinity) in the positive direction. We also confirmed that TTX-R/persistent Na(+) current mediated by Na(V)1.9 actually regulates subthreshold excitability in small DRG neurons. In addition, we demon strated that TTX-R/persistent Na(+) current can carry an action potential when the amplitude of this current was abnormally increased. Thus, our results indicate that the action potentials in small DRG neurons are generated and regulated with a combination of multiple mechanisms that may give rise to unique functional properties of small DRG neurons.

Upregulation of the voltage-gated sodium channel beta2 subunit in neuropathic pain models: characterization of expression in injured and non-injured primary sensory neurons

The development of abnormal primary sensory neuron excitability and neuropathic pain symptoms after peripheral nerve injury is associated with altered expression of voltage-gated sodium channels (VGSCs) and a modification of sodium currents. To investigate whether the beta2 subunit of VGSCs participates in the generation of neuropathic pain, we used the spared nerve injury (SNI) model in rats to examine beta2 subunit expression in selectively injured (tibial and common peroneal nerves) and uninjured (sural nerve) afferents. Three days after SNI, immunohistochemistry and Western blot analysis reveal an increase in the beta2 subunit in both the cell body and peripheral axons of injured neurons. The increase persists for >4 weeks, although beta2 subunit mRNA measured by real-time reverse transcription-PCR and in situ hybridization remains unchanged. Although injured neurons show the most marked upregulation,beta2 subunit expression is also increased in neighboring non-injured neurons and a similar pattern of changes appears in the spinal nerve ligation model of neuropathic pain. That increased beta2 subunit expression in sensory neurons after nerve injury is functionally significant, as demonstrated by our finding that the development of mechanical allodynia-like behavior in the SNI model is attenuated in beta2 subunit null mutant mice. Through its role in regulating the density of mature VGSC complexes in the plasma membrane and modulating channel gating, the beta2 subunit may play a key role in the development of ectopic activity in injured and non-injured sensory afferents and, thereby, neuropathic pain.