The NaV1.7 voltage-gated sodium channel is critical for pain signaling in humans. Gain-of-function mutations are associated with several pain syndromes including inherited erythromelalgia (IEM). Most IEM patients with NaV1.7 mutations are resistant to pharmacotherapy, but carbamazepine (CBZ) normalizes activation of NaV1.7-V400M mutant channels from a family with CBZ-responsive IEM. Here we show that structural modeling and mutant cycle analysis predict pharmacoresponsiveness to CBZ of a NaV1.7 mutant channel that substitutes a residue 159 amino acids distant from V400M in the channel peptide. Structural modeling reveals that this IEM mutation (S241T) is only 2.4-angstrom (Å) apart from V400M in the folded NaV1.7 channel and mutant cycle analysis demonstrates that V400M is energetically coupled to S241T during channel activation. We further show that the atomic proximity and energetic coupling of V400M and S241T are paralleled by pharmacological coupling, as CBZ at therapeutic concentration (30 μM) causes a depolarizing shift of S214T mutant channel activation curve, similar to that previously reported for V400M mutant channel. This pharmacoresponsiveness of S241T to CBZ was further evident at a cellular level, where CBZ normalized the hyperexcitability of dorsal root ganglion (DRG) neurons expressing S241T mutant channel. We suggest that a similar approach might facilitate screening for amino acid variants of a variety of channels that confer enhanced pharmacoresponsiveness on the channel.
Human and animal studies have shown that Nav1.7 sodium channels, which are preferentially expressed within nociceptors and sympathetic neurons, play a major role in inflammatory and neuropathic pain. Inherited erythromelalgia (IEM) has been linked to gain-of-function mutations of Nav1.7. We now report a novel mutation (V400M) in a three-generation Canadian family in which pain is relieved by carbamazepine (CBZ).
We extracted genomic DNA from blood samples of eight members of the family, and the sequence of SCN9A coding exons was compared with the reference Nav1.7 complementary DNA. Wild-type Nav1.7 and V400M cell lines were then analyzed using whole-cell patch-clamp recording for changes in activation, deactivation, steady-state inactivation, and ramp currents.
Whole-cell patch-clamp studies of V400M demonstrate changes in activation, deactivation, steady-state inactivation, and ramp currents that can produce dorsal root ganglia neuron hyperexcitability that underlies pain in these patients. We show that CBZ, at concentrations in the human therapeutic range, normalizes the voltage dependence of activation and inactivation of this inherited erythromelalgia mutation in Nav1.7 but does not affect these parameters in wild-type Nav1.7.
Our results demonstrate a normalizing effect of CBZ on mutant Nav1.7 channels in this kindred with CBZ-responsive inherited erythromelalgia. The selective effect of CBZ on the mutant Nav1.7 channel appears to explain the ameliorative response to treatment in this kindred. Our results suggest that functional expression and pharmacological studies may provide mechanistic insights into hereditary painful disorders.
Traumatic brain injury (TBI) contributes to about 10% of acquired epilepsy. Even though the mechanisms of post-traumatic epileptogenesis are poorly known, a disruption of neuronal networks predisposing to altered neuronal synchrony remains a viable candidate mechanism. We tested a hypothesis that resting state BOLD-fMRI functional connectivity can reveal network abnormalities in brain regions that are connected to the lesioned cortex, and that these changes associate with functional impairment, particularly epileptogenesis. TBI was induced using lateral fluid-percussion injury in seven adult male Sprague-Dawley rats followed by functional imaging at 9.4T 4 months later. As controls we used six sham-operated animals that underwent all surgical operations but were not injured. Electroencephalogram (EEG)-functional magnetic resonance imaging (fMRI) was performed to measure resting functional connectivity. A week after functional imaging, rats were implanted with bipolar skull electrodes. After recovery, rats underwent pentyleneterazol (PTZ) seizure-susceptibility test under EEG. For image analysis, four pairs of regions of interests were analyzed in each hemisphere: ipsilateral and contralateral frontal and parietal cortex, hippocampus, and thalamus. High-pass and low-pass filters were applied to functional imaging data. Group statistics comparing injured and sham-operated rats and correlations over time between each region were calculated. In the end, rats were perfused for histology. None of the rats had epileptiform discharges during functional imaging. PTZ-test, however revealed increased seizure susceptibility in injured rats as compared to controls. Group statistics revealed decreased connectivity between the ipsilateral and contralateral parietal cortex and between the parietal cortex and hippocampus on the side of injury as compared to sham-operated animals. Injured animals also had abnormal negative connectivity between the ipsilateral and contralateral parietal cortex and other regions. Our data provide the first evidence on abnormal functional connectivity after experimental TBI assessed with resting state BOLD-fMRI.
Development of improved fluorescent voltage indicators is a key challenge in neuroscience, but progress has been hampered by the low throughput of patch-clamp characterization. We introduce a line of non-fluorescent HEK cells that stably express NaV 1.3 and KIR 2.1 and generate spontaneous electrical action potentials. These cells enable rapid, electrode-free screening of speed and sensitivity of voltage sensitive dyes or fluorescent proteins on a standard fluorescence microscope. We screened a small library of mutants of archaerhodopsin 3 (Arch) in spiking HEK cells and identified two mutants with greater voltage-sensitivity than found in previously published Arch voltage indicators.
Voltage-gated sodium channels Nav1.8 and Nav1.9 are expressed preferentially in small diameter sensory neurons, and are thought to play a role in the generation of ectopic activity in neuronal cell bodies and/or their axons following peripheral nerve injury. The expression of Nav1.8 and Nav1.9 has been quantified in human lingual nerves that have been previously injured inadvertently during lower third molar removal, and any correlation between the expression of these ion channels and the presence or absence of dysaesthesia investigated.
Immunohistochemical processing and quantitative image analysis revealed that Nav1.8 and Nav1.9 were expressed in human lingual nerve neuromas from patients with or without symptoms of dysaesthesia. The level of Nav1.8 expression was significantly higher in patients reporting pain compared with no pain, and a significant positive correlation was observed between levels of Nav1.8 expression and VAS scores for the symptom of tingling. No significant differences were recorded in the level of expression of Nav1.9 between patients with or without pain.
These results demonstrate that Nav1.8 and Nav1.9 are present in human lingual nerve neuromas, with significant correlations between the level of expression of Nav1.8 and symptoms of pain. These data provide further evidence that changes in expression of Nav1.8 are important in the development and/or maintenance of nerve injury-induced pain, and suggest that Nav1.8 may be a potential therapeutic target.
Lingual nerve; Nerve injury; Trigeminal; Neuropathic pain; Dysaesthesia; Tingling; Nav1.8; Nav1.9
Central nervous system plasticity is essential for normal function, but can also reinforce abnormal network behavior, leading to epilepsy and other disorders. The role of altered ion channel expression in abnormal plasticity has not been thoroughly investigated. Nav1.6 is the most abundantly expressed sodium channel in the nervous system. Because of its distribution in the cell body and axon initial segment, Nav1.6 is crucial for action potential generation. The goal of the present study was to investigate the possible role of changes in Nav1.6 expression in abnormal, activity-dependent plasticity of hippocampal circuits.
We studied kindling, a form of abnormal activity-dependent facilitation. We investigated: 1. sodium channel protein expression by immunocytochemistry and sodium channel mRNA by in situ hybridization, 2. sodium current by patch clamp recordings, and 3. rate of kindling by analysis of seizure behavior. The initiation, development, and expression of kindling in wild type mice were compared to Nav1.6 +/− medtg mice, which have reduced expression of Nav1.6.
We found that kindling was associated with increased expression of Nav1.6 protein and mRNA, which occurred selectively in hippocampal CA3 neurons. Hippocampal CA3 neurons also showed increased persistent sodium current in kindled animals compared to sham-kindled controls. Conversely, Nav1.6 +/− medtg mice resisted the initiation and development of kindling.
These findings suggest an important mechanism for enhanced excitability, in which Nav1.6 may participate in a self-reinforcing cycle of activity-dependent facilitation in the hippocampus. This mechanism could contribute to both normal hippocampal function, and to epilepsy and other common nervous system disorders.
epilepsy; kindling; hippocampus; persistent sodium current; LTP
NaV1.7 is preferentially expressed, at relatively high levels, in peripheral neurons, and is often referred to as a “peripheral” sodium channel, and NaV1.7-specific blockers are under study as potential pain therapeutics which might be expected to have minimal CNS side effects. However, occasional reports of patients with NaV1.7 gain-of-function mutations and apparent hypothalamic dysfunction have appeared. The two sodium channels previously studied within the rat hypothalamic supraoptic nucleus, NaV1.2 and NaV1.6, display up-regulated expression in response to osmotic stress.
Here we show that NaV1.7 is present within vasopressin-producing neurons and oxytocin-producing neurons within the rat hypothalamus, and demonstrate that the level of Nav1.7 immunoreactivity is increased in these cells in response to osmotic stress.
NaV1.7 is present within neurosecretory neurons of rat supraoptic nucleus, where the level of immunoreactivity is dynamic, increasing in response to osmotic stress. Whether NaV1.7 levels are up-regulated within the human hypothalamus in response to environmental factors or stress, and whether NaV1.7 plays a functional role in human hypothalamus, is not yet known. Until these questions are resolved, the present findings suggest the need for careful assessment of hypothalamic function in patients with NaV1.7 mutations, especially when subjected to stress, and for monitoring of hypothalamic function as NaV1.7 blocking agents are studied.
Hypothalamus; Nav1.7; Salt-loading; Supraoptic nucleus
Neurons are highly polarized cells with functionally distinct axonal and somatodendritic compartments. Voltage-gated sodium channels Nav1.2 and Nav1.6 are highly enriched at axon initial segments (AIS) and nodes of Ranvier, where they are necessary for generation and propagation of action potentials. Previous studies using reporter proteins in unmyelinated cultured neurons suggest that an ankyrinG-binding motif within intracellular loop 2 (L2) of sodium channels is sufficient for targeting these channels to the AIS, but mechanisms of channel targeting to nodes remain poorly understood. Using a CD4-Nav1.2/L2 reporter protein in rat dorsal root ganglion neuron-Schwann cell myelinating co-cultures, we show that the ankyrinG-binding motif is sufficient for protein targeting to nodes of Ranvier. However, reporter proteins cannot capture the complexity of full-length channels. To determine how native, full-length sodium channels are clustered in axons, and to show the feasibility of studying these channels in vivo, we constructed fluorescently-tagged and functional mouse Nav1.6 channels for in vivo analysis using in utero brain electroporation. We show here that wild-type tagged-Nav1.6 channels are efficiently clustered at nodes and AIS in vivo. Furthermore, we show that mutation of a single invariant glutamic acid residue (E1100) within the ankyrinG-binding motif blocked Nav1.6 targeting in neurons both in vitro and in vivo. Additionally, we show that caseine kinase phosphorylation sites within this motif, while not essential for targeting, can modulate clustering at the AIS. Thus, the ankyrinG- binding motif is both necessary and sufficient for the clustering of sodium channels at nodes of Ranvier and the AIS.
Ion Channel; Axon Initial Segment; Nodes of Ranvier; cytoskeleton; in utero electroporation
Sodium channel Nav1.7 has emerged as a target of considerable interest in pain research, since loss-of-function mutations in SCN9A, the gene that encodes Nav1.7, are associated with a syndrome of congenital insensitivity to pain, gain-of-function mutations are linked to the debiliting chronic pain conditions erythromelalgia and paroxysmal extreme pain disorder, and upregulated expression of Nav1.7 accompanies pain in diabetes and inflammation. Since Nav1.7 has been implicated as playing a critical role in pain pathways, we examined by immunocytochemical methods the expression and distribution of Nav1.7 in rat dorsal root ganglia neurons, from peripheral terminals in the skin to central terminals in the spinal cord dorsal horn.
Nav1.7 is robustly expressed within the somata of peptidergic and non-peptidergic DRG neurons, and along the peripherally- and centrally-directed C-fibers of these cells. Nav1.7 is also expressed at nodes of Ranvier in a subpopulation of Aδ-fibers within sciatic nerve and dorsal root. The peripheral terminals of DRG neurons within skin, intraepidermal nerve fibers (IENF), exhibit robust Nav1.7 immunolabeling. The central projections of DRG neurons in the superficial lamina of spinal cord dorsal horn also display Nav1.7 immunoreactivity which extends to presynaptic terminals.
The expression of Nav1.7 in DRG neurons extends from peripheral terminals in the skin to preterminal central branches and terminals in the dorsal horn. These data support a major contribution for Nav1.7 in pain pathways, including action potential electrogenesis, conduction along axonal trunks and depolarization/invasion of presynaptic axons. The findings presented here may be important for pharmaceutical development, where target engagement in the right compartment is essential.
Dorsal root ganglia; Dorsal horn; Intraepidermal nerve fiber; Pain pathway; Sodium channel; Spinal cord
Transplantation of human mesenchymal stem cells has been shown to reduce infarct size and improve functional outcome in animal models of stroke. Here, we report a study designed to assess feasibility and safety of transplantation of autologous human mesenchymal stem cells expanded in autologous human serum in stroke patients. We report an unblinded study on 12 patients with ischaemic grey matter, white matter and mixed lesions, in contrast to a prior study on autologous mesenchymal stem cells expanded in foetal calf serum that focused on grey matter lesions. Cells cultured in human serum expanded more rapidly than in foetal calf serum, reducing cell preparation time and risk of transmissible disorders such as bovine spongiform encephalomyelitis. Autologous mesenchymal stem cells were delivered intravenously 36–133 days post-stroke. All patients had magnetic resonance angiography to identify vascular lesions, and magnetic resonance imaging prior to cell infusion and at intervals up to 1 year after. Magnetic resonance perfusion-imaging and 3D-tractography were carried out in some patients. Neurological status was scored using the National Institutes of Health Stroke Scale and modified Rankin scores. We did not observe any central nervous system tumours, abnormal cell growths or neurological deterioration, and there was no evidence for venous thromboembolism, systemic malignancy or systemic infection in any of the patients following stem cell infusion. The median daily rate of National Institutes of Health Stroke Scale change was 0.36 during the first week post-infusion, compared with a median daily rate of change of 0.04 from the first day of testing to immediately before infusion. Daily rates of change in National Institutes of Health Stroke Scale scores during longer post-infusion intervals that more closely matched the interval between initial scoring and cell infusion also showed an increase following cell infusion. Mean lesion volume as assessed by magnetic resonance imaging was reduced by >20% at 1 week post-cell infusion. While we would emphasize that the current study was unblinded, did not assess overall function or relative functional importance of different types of deficits, and does not exclude placebo effects or a contribution of recovery as a result of the natural history of stroke, our observations provide evidence supporting the feasibility and safety of delivery of a relatively large dose of autologous mesenchymal human stem cells, cultured in autologous human serum, into human subjects with stroke and support the need for additional blinded, placebo-controlled studies on autologous mesenchymal human stem cell infusion in stroke.
stroke; cerebrovascular disease; stem cell; mesenchymal stem cells; bone marrow-derived stem cells; cell transplantation
A novel SCN1A mutation was discovered in a patient who clinically fulfilled the criteria for malignant migrating partial seizures of infancy. The full-term female patient had seizure onset at two months, with progression of hemiclonic, apneic, and generalized tonic-clonic seizures leading to recurrent status epilepticus and fatality at nine months of age. The ictal EEG showed migratory seizure foci and evolved until ictal and interictal EEGs became indistinguishable. We further characterize this novel SCN1A mutation in our patient.
Genomic DNA was isolated from blood and submitted for commercial testing. The missense mutation was confirmed in brain DNA obtained at autopsy. Genomic DNA from patient brain was analyzed by comparative genome hybridization and the coding exons of SCN9A were amplified. Quantitation studies of the mutant transcript were performed.
The heterozygous missense mutation c.C5006A was identified by sequencing genomic DNA from blood and was confirmed in brain DNA. The resulting amino acid substitution p.A1669E alters an evolutionarily conserved residue in an intracellular linker of domain 4 of the channel protein. The mutant transcript is found to be expressed at levels comparable to the wildtype allele in brain RNA. No variation in copy number was detected in the chromosome region 2q24 containing SCN1A or elsewhere in the genome. No mutations were detected in the linked sodium channel gene SCN9A, which has been reported to act as a modifier of SCN1A mutations.
This report expands the spectrum of SCN1A epileptic channelopathies to include malignant migrating partial seizures of infancy.
Mechanical hyperalgesia is a common and potentially disabling complication of many inflammatory and neuropathic conditions. Activation of the enzyme PKCε in primary afferent nociceptors is a major mechanism that underlies mechanical hyperalgesia, but the PKCε substrates involved downstream are not known. Here, we report that in a proteomic screen we identified the NaV1.8 sodium channel, which is selectively expressed in nociceptors, as a PKCε substrate. PKCε-mediated phosphorylation increased NaV1.8 currents, lowered the threshold voltage for activation, and produced a depolarizing shift in inactivation in wild-type — but not in PKCε-null — sensory neurons. PKCε phosphorylated NaV1.8 at S1452, and alanine substitution at this site blocked PKCε modulation of channel properties. Moreover, a specific PKCε activator peptide, ψεRACK, produced mechanical hyperalgesia in wild-type mice but not in Scn10a–/– mice, which lack NaV1.8 channels. These studies demonstrate that NaV1.8 is an important, direct substrate of PKCε that mediates PKCε-dependent mechanical hyperalgesia.
Sodium channel NaV1.7 is preferentially expressed within dorsal root ganglia (DRG), trigeminal ganglia and sympathetic ganglion neurons and their fine-diamter axons, where it acts as a threshold channel, amplifying stimuli such as generator potentials in nociceptors. Gain-of-function mutations and variants (single amino acid substitutions) of NaV1.7 have been linked to three pain syndromes: Inherited Erythromelalgia (IEM), Paroxysmal Extreme Pain Disorder (PEPD), and Small Fiber Neuropathy (SFN). IEM is characterized clinically by burning pain and redness that is usually focused on the distal extremities, precipitated by mild warmth and relieved by cooling, and is caused by mutations that hyperpolarize activation, slow deactivation, and enhance the channel ramp response. PEPD is characterized by perirectal, periocular or perimandibular pain, often triggered by defecation or lower body stimulation, and is caused by mutations that severely impair fast-inactivation. SFN presents a clinical picture dominated by neuropathic pain and autonomic symptoms; gain-of-function variants have been reported to be present in approximately 30% of patients with biopsy-confirmed idiopathic SFN, and functional testing has shown altered fast-inactivation, slow-inactivation or resurgent current. In this paper we describe three patients who house the NaV1.7/I228M variant.
We have used clinical assessment of patients, quantitative sensory testing and skin biopsy to study these patients, including two siblings in one family, in whom genomic screening demonstrated the I228M NaV1.7 variant. Electrophysiology (voltage-clamp and current-clamp) was used to test functional effects of the variant channel.
We report three different clinical presentations of the I228M NaV1.7 variant: presentation with severe facial pain, presentation with distal (feet, hands) pain, and presentation with scalp discomfort in three patients housing this NaV1.7 variant, two of which are from a single family. We also demonstrate that the NaV1.7/I228M variant impairs slow-inactivation, and produces hyperexcitability in both trigeminal ganglion and DRG neurons.
Our results demonstrate intra- and interfamily phenotypic diversity in pain syndromes produced by a gain-of-function variant of NaV1.7.
Current treatments for epilepsy may control seizures, but have no known effects on the underlying disease. We sought to determine whether early treatment in a model of genetic epilepsy would reduce the severity of the epilepsy phenotype in adulthood.
We used Wistar albino Glaxo rats of Rijswijk (WAG/Rij) rats, an established model of human absence epilepsy. Oral ethosuximide was given from age p21 to 5 months, covering the usual period in which seizures develop in this model (age ~3 months). Two experiments were performed: (1) cortical expression of ion channels Nav1.1, Nav1.6, and HCN1 (previously shown to be dysregulated in WAG/Rij) measured by immunocytochemistry in adult treated rats; and (2) electroencephalogram (EEG) recordings to measure seizure severity at serial time points after stopping the treatment.
Early treatment with ethosuximide blocked changes in the expression of ion channels Nav1.1, Nav1.6, and HCN1 normally associated with epilepsy in this model. In addition, the treatment led to a persistent suppression of seizures, even after therapy was discontinued. Thus, animals treated with ethosuximide from age p21 to 5 months still had a marked suppression of seizures at age 8 months.
These findings suggest that early treatment during development may provide a new strategy for preventing epilepsy in susceptible individuals. If confirmed with other drugs and epilepsy paradigms, the availability of a model in which epileptogenesis can be controlled has important implications both for future basic studies, and human therapeutic trials.
Epileptogenesis; Activity-dependent; Prevention; Idiopathic generalized epilepsy; Sodium channels; HCN1
Voltage-gated sodium channel Nav1.7 is preferentially expressed in dorsal root ganglion (DRG) and sympathetic neurons within the peripheral nervous system. Homozygous or compound heterozygous loss-of-function mutations in SCN9A, the gene which encodes Nav1.7, cause congenital insensitivity to pain (CIP) accompanied by anosmia. Global knock-out of Nav1.7 in mice is neonatal lethal reportedly from starvation, suggesting anosmia. These findings led us to hypothesize that Nav1.7 is the main sodium channel in the peripheral olfactory sensory neurons (OSN, also known as olfactory receptor neurons).
We used multiplex PCR-restriction enzyme polymorphism, in situ hybridization and immunohistochemistry to determine the identity of sodium channels in rodent OSNs.
We show here that Nav1.7 is the predominant sodium channel transcript, with low abundance of other sodium channel transcripts, in olfactory epithelium from rat and mouse. Our in situ hybridization data show that Nav1.7 transcripts are present in rat OSNs. Immunostaining of Nav1.7 and Nav1.6 channels in rat shows a complementary accumulation pattern with Nav1.7 in peripheral presynaptic OSN axons, and Nav1.6 primarily in postsynaptic cells and their dendrites in the glomeruli of the olfactory bulb within the central nervous system.
Our data show that Nav1.7 is the dominant sodium channel in rat and mouse OSN, and may explain anosmia in Nav1.7 null mouse and patients with Nav1.7-related CIP.
Nociception requires transduction and impulse electrogenesis in nerve fibers which innervate the body surface, including the skin. However, the molecular substrates for transduction and action potential initiation in nociceptors are incompletely understood. In this study, we examined the expression and distribution of Na+/Ca2+ exchanger (NCX) and voltage-gated sodium channel isoforms in intra-epidermal free nerve terminals.
Small diameter DRG neurons exhibited robust NCX2, but not NCX1 or NCX3 immunolabeling, and virtually all PGP 9.5-positive intra-epidermal free nerve terminals displayed NCX2 immunoreactivity. Sodium channel NaV1.1 was not detectable in free nerve endings. In contrast, the majority of nerve terminals displayed detectable levels of expression of NaV1.6, NaV1.7, NaV1.8 and NaV1.9. Sodium channel immunoreactivity in the free nerve endings extended from the dermal boundary to the terminal tip. A similar pattern of NCX and sodium channel immunolabeling was observed in DRG neurons in vitro.
NCX2, as well as NaV1.6, NaV1.7, NaV1.8 and NaV1.9, are present in most intra-epidermal free nerve endings. The presence of NCX2, together with multiple sodium channel isoforms, in free nerve endings may have important functional implications.
The Intracellular Fibroblast Growth Factor (iFGF) subfamily includes four members (FGFs 11–14) of the structurally related FGF superfamily. Previous studies showed that the iFGFs interact directly with the pore-forming (α) subunits of voltage-gated sodium (Nav) channels and regulate the functional properties of sodium channel currents. Sequence heterogeneity among the iFGFs is thought to confer specificity to this regulation. Here, we demonstrate that the two N-terminal alternatively spliced FGF14 variants, FGF14-1a and FGF14-1b, differentially regulate currents produced by Nav1.2-and Nav1.6 channels. FGF14-1b, but not FGF14-1a, attenuates both Nav1.2 and Nav1.6 current densities. In contrast, co-expression of an FGF14 mutant, lacking the N-terminus, increased Nav1.6 current densities. In neurons, both FGF14-1a and FGF14-1b localized at the axonal initial segment, and deletion of the N-terminus abolished this localization. Thus, the FGF14 N-terminus is required for targeting and functional regulation of Nav channels, suggesting an important function for FGF14 alternative splicing in regulating neuronal excitability.
A direct role of sodium channels in pain has recently been confirmed by establishing a monogenic link between SCN9A, the gene which encodes sodium channel Nav1.7, and pain disorders in humans, with gain-of-function mutations causing severe pain syndromes, and loss-of-function mutations causing congenital indifference to pain. Expression of sodium channel Nav1.8 in DRG neurons has also been shown to be essential for the manifestation of mutant Nav1.7-induced neuronal hyperexcitability. These findings have confirmed key roles of Nav1.7 and Nav1.8 in pain and identify these channels as novel targets for pain therapeutic development. Ranolazine preferentially blocks cardiac late sodium currents at concentrations that do not significantly reduce peak sodium current. Ranolazine also blocks wild-type Nav1.7 and Nav1.8 channels in a use-dependent manner. However, ranolazine's effects on gain-of-function mutations of Nav1.7 and on DRG neuron excitability have not been investigated. We used voltage- and current-clamp recordings to evaluate the hypothesis that ranolazine may be effective in regulating Nav1.7-induced DRG neuron hyperexcitability.
We show that ranolazine produces comparable block of peak and ramp currents of wild-type Nav1.7 and mutant Nav1.7 channels linked to Inherited Erythromelalgia and Paroxysmal Extreme Pain Disorder. We also show that ranolazine, at a clinically-relevant concentration, blocks high-frequency firing of DRG neurons expressing wild-type but not mutant channels.
Our data suggest that ranalozine can attenuate hyperexcitability of DRG neurons over-expressing wild-type Nav1.7 channels, as occurs in acquired neuropathic and inflammatory pain, and thus merits further study as an alternative to existing non-selective sodium channel blockers.
Two groups of gain-of-function mutations in sodium channel NaV1.7, which are expressed in dorsal root ganglion (DRG) neurons, produce two clinically-distinct pain syndromes - inherited erythromelalgia (IEM) and paroxysmal extreme pain disorder (PEPD). IEM is characterized by intermittent burning pain and skin redness in the feet or hands, triggered by warmth or mild exercise, while PEPD is characterized by episodes of rectal, ocular and mandibular pain accompanied with skin flushing, triggered by bowel movement and perianal stimulation. Most of the IEM mutations are located within channel domains I and II, while most of the PEPD mutations are located within domains III and IV. The structural dichotomy parallels the biophysical effects of the two types of mutations, with IEM mutations shifting voltage-dependence of NaV1.7 activation in a hyperpolarized direction, and PEPD mutations shifting fast-inactivation of NaV1.7 in a depolarized direction. While four IEM and four PEPD mutations are located within cytoplasmic linkers joining segments 4 and 5 (S4-S5 linkers) in the different domains (IEM: domains I and II; PEPD: domains III and IV), no S4-S5 linker has been reported to house both IEM and PEPD mutations thus far.
We have identified a new IEM mutation P1308L within the C-terminus of the DIII/S4-S5 linker of NaV1.7, ten amino acids from a known PEPD mutation V1298F which is located within the N-terminus of this linker. We used voltage-clamp to compare the biophysical properties of the two mutant channels and current-clamp to study their effects on DRG neuron excitability. We confirm that P1308L and V1298F behave as prototypical IEM and PEPD mutations, respectively. We also show that DRG neurons expressing either P1308L or V1298F become hyperexcitable, compared to DRG neurons expressing wild-type channels.
Our results provide evidence for differential roles of the DIII/S4-S5 linker N- and C-termini in channel inactivation and activation, and demonstrate the cellular basis for pain in patients carrying these mutations.
Sensory neurons in aging mammals undergo changes in anatomy, physiology and gene expression that correlate with reduced sensory perception. In this study we compared young and aged mice to identify proteins that might contribute to this loss of sensation. We first show using behavioral testing that thermal sensitivity in aged male and female mice is reduced. Expression of sodium channel (Nav1.8 and Nav1.9) and transient receptor potential vanilloid (TRPV) channels in DRG and peripheral nerves of young and old male mice was then examined. Immunoblotting and RT-PCR assays showed reduced Nav1.8 levels in aged mice. No change was measured in TRPV1 mRNA levels in DRG though TRPV1 protein appeared reduced in the DRG and peripheral nerves. The GFRα3 receptor, which binds the growth factor artemin and is expressed by TRPV1-positive neurons, was also decreased in the DRG of aged animals. These findings indicate that loss of thermal sensitivity in aging animals may result from a decreased level of TRPV1 and Nav1.8 and decreased trophic support that inhibits efficient transport of channel proteins to peripheral afferents.
Sensory neuron; Vanilloid receptor; TRPV1; Sodium channel; Behavior; Thermal sensitivity
Multidrug resistance protein 4 (MRP4; ABCC4) is a member of the MRP/ATP-binding cassette family serving as a transmembrane transporter involved in energy-dependent efflux of anticancer/antiviral nucleotide agents and of physiological substrates, including cyclic nucleotides and prostaglandins (PGs). Phenotypic consequences of mrp4 deficiency were investigated using mrp4-knockout mice and derived immortalized mouse embryonic fibroblast (MEF) cells. Mrp4 deficiency caused decreased extracellular and increased intracellular levels of cAMP in MEF cells under normal and forskolin-stimulated conditions. Mrp4 deficiency and RNA interference-mediated mrp4 knockdown led to a pronounced reduction in extracellular PGE2 but with no accumulation of intracellular PGE2 in MEF cells. This result was consistent with attenuated cAMP-dependent protein kinase activity and reduced cyclooxygenase-2 (Cox-2) expression in mrp4-deficient MEF cells, suggesting that PG synthesis is restrained along with a lack of PG transport caused by mrp4 deficiency. Mice lacking mrp4 exhibited no outward phenotypes but had a decrease in plasma PGE metabolites and an increase in inflammatory pain threshold compared with wild-type mice. Collectively, these findings imply that mrp4 mediates the efflux of PGE2 and concomitantly modulates cAMP mediated signaling for balanced PG synthesis in MEF cells. Abrogation of mrp4 affects the regulation of peripheral PG levels and consequently alters inflammatory nociceptive responses in vivo.
The ENU-induced neurological mutant ataxia3 was mapped to distal mouse chromosome 15. Sequencing of the positional candidate gene Scn8a encoding the sodium channel Nav1.6 identified a T>C transition in exon 1 resulting in the amino acid substitution p.S21P near the N-terminus of the channel. The cytoplasmic N-terminal region is evolutionarily conserved but its function has not been well characterized. ataxia3 homozygotes exhibit a severe disorder that includes ataxia, tremor, and juvenile lethality. Unlike Scn8a null mice, they retain partial hind limb function. The mutant transcript is stable but protein abundance is reduced and the mutant channel is not detected in its usual site of concentration at nodes of Ranvier. In whole cell patch-clamp studies of transfected ND7/23 cells which were maintained at 37°C, the mutant channel did not produce sodium current, and function was not restored by co-expression of β1 and β2 subunits. However, when tranfected cells were maintained at 30°C, the mutant channel generated voltage-dependent inward sodium currents with an average peak current density comparable to wildtype, demonstrating recovery of channel activity. Immunohistochemistry of primary cerebellar granule cells from ataxia3 mice demonstrated that the mutant protein is retained in the cis-Golgi. This trafficking defect can account for the low level of Nav1.6-S21P at nodes of Ranvier in vivo and at the surface of transfected cells. The data demonstrate that the cytoplasmic N-terminal domain of the sodium channel is required for anterograde transport from the Golgi complex to the plasma membrane.
SCN8A; Nav1.6; sodium channel; channelopathy; trafficking; mutant
Whole cell patch-clamp recordings were obtained from dissociated mouse lumbar dorsal root ganglion (DRG) neurons. Recordings were made from control neurons and neurons axotomized by transection of the corresponding spinal nerve 1–2 days prior to dissociation. Medium to large muscle and cutaneous afferent neurons were identified by retrograde transport of True Blue or Fluoro-Gold injected into the corresponding peripheral tissue. Action potentials were classified as non-inflected spikes (A0) and inflected spikes (Ainf). High-frequency, low-amplitude subthreshold membrane potential oscillations were observed in 8% of control A0 neurons, but their incidence increased to 31% in the nerve injury group. Fifty percent of axotomized muscle afferent A0 cells displayed oscillations, while 26% of axotomized cutaneous afferents exhibited oscillations. Lower-frequency oscillations were also observed in a small fraction (4%) of Ainf neurons on strong depolarization. Their numbers were increased after the nerve injury, but the difference was not statistically significant. The oscillations often triggered burst firing in distinct patterns of action potential activity. These results indicate that injury-induced membrane oscillations of DRG neurons, previously observed in whole DRG of rats, are present in dissociated DRG neurons of the adult mouse. Moreover, these observations indicate that both muscle and cutaneous afferents in the Aβ size range give rise to injury-induced membrane oscillations, with muscle afferents being more prone to develop oscillations.
SUMMARY AND CONCLUSIONS
Voltage-dependent Na+ conductances were studied in small (18–25 μm diam) adult rat dorsal root ganglion (DRG) neurons with the use of the whole cell patch-clamp technique. Na+ currents were also recorded from larger (44–50 μm diam) neurons and compared with those of the small neurons.The predominant Na+ conductance in the small neurons was selective over tetramethylammonium by at least 10-fold and was resistant to 1 μM external tetrodotoxin (TTX). Na+ conductances in many larger DRG neurons were kinetically faster and, in contrast, were blocked by 1 μM TTX.The Na+ conductance in the small neurons was kinetically slow. Activation half-times were voltage dependent and ranged from 2 ms at −20 mV to 0.7 ms at +50 mV. Approximately 50% of the activation half-time was comprised of an initial delay. Inactivation half-times were voltage dependent and ranged from 11 ms at −20 mV to 2 ms at +50 mV.Peak slow Na+ conductances were near maximal with conditioning potentials negative to −120 mV and were significantly reduced or eliminated with conditioning potentials positive to −40 mV. The slow Na+ conductance increased gradually with test potentials extending from −40 to +40 mV. In some cells the conductance could be saturated at +10 mV. Peak conductance/voltage relationships, although stable in a given neuron, revealed marked variability among neurons, spanning >20- and 50-mV domains for steady-state activation and inactivation (current availability), respectively.Kinetics remained stable within a given neuron over the course of an experiment. However, considerable kinetic variation was exhibited from neuron to neuron, such that the half-times of activation and of inactivation spanned an order of magnitude. In all small neurons studied there appeared to be a singular kinetic component of the current, based on sensitivity to the conditioning potential, voltage dependence of activation, and inactivation half-time.Unique closing properties were exhibited by Na+ channels of the small neurons. Hyperpolarization following a depolarization-induced fully inactivated state resulted in tail currents that appeared to be the consequence of reactivation of the slow Na+ conductance. Tail currents recorded at various times during a fixed level of depolarization revealed that the underlying channels accumulated into a volatile inactivated state over the course of the preceding depolarization.Larger neurons had a different repertoire of Na+ conductances, with either only a TTX-sensitive, kinetically fast type, or a combination of fast TTX-sensitive and slow currents. In larger neurons the kinetically separable fast current had a greater sensitivity to the conditioning potential, i.e., a left-shifted steady-state inactivation curve.The different properties of the slow Na+ conductance in different neurons is likely to reflect heterogeneity of the structure of the underlying channel molecule. Although consistent with what others have found in equivalent preparations, this heterogeneity is far broader in scope than what has so far been described. We suggest that biosynthetic constraints within a given small neuron maintain ion channel uniformity.
SUMMARY AND CONCLUSIONS
Intraaxonal recordings were obtained in vitro from the sural nerve (SN), the muscle branch of the anterior tibial nerve (ATN), or the deefferented ATN (dATN) in 5- to 7-wk-old rats. Whole-nerve sucrose gap recordings were obtained from the SN and the ATN. This allowed study of cutaneous (SN), mixed motor and muscle afferent (ATN), and isolated muscle afferent (dATN) axons.Application of the potassium channel blocking agent 4-aminopyridine (4-AP) to ATN or dATN resulted in a slight prolongation of the action potential. In contrast, a distinct delayed depolarization followed the axonal action potential in cutaneous afferents (SN) exposed to 4-AP. The delayed depolarization could be induced by a single whole-nerve stimulus or by injection of constant-current depolarizing pulses into individual axons. The delayed depolarization often gave rise to bursts of action potentials and was followed by a prominent afterhyperpolarization (AHP).In paired-pulse experiments on single SN axons, the recovery time (half-amplitude of the action potential) was 3.06 ± 1.82 (SE) ms (n = 12). After exposure to 4-AP the recovery time of the delayed depolarization was considerably longer (half-recovery time: 99.0 ± 28.3 ms; n = 15) than that of the action potential (18.8 ± 9.1 ms; n = 16).Application of tetraethylammonium (TEA) to cutaneous or muscle afferents alone had little effect on single action potential waveform. However, TEA reduced the amplitude of the AHP elicited by a single stimulus in cutaneous afferent axons after exposure to 4-AP and resulted in repetitive spike discharge.The delayed depolarization and spike burst activity induced by 4-AP in SN was present in Ca2+ -free solutions containing 1 mM ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid and was not blocked by Cd2+ (1.0 mM).We obtained whole-cell patch-clamp recordings to study Na+ currents from either randomly selected dorsal root ganglion neurons or cutaneous afferent neurons identified by retrograde labeling with Fluoro-Gold. The majority of the randomly selected neurons had a singular kinetically fast Na+ current. In contrast, no identified cutaneous afferent neurons had a singular fast Na+ current. Rather, they had a combination of kinetically separable fast and slow currents or a singular relatively slow Na+ current.These results demonstrate a difference in the sensitivity of myelinated cutaneous and muscle afferent axons to blockade of a 4-AP-sensitive K+ channel. Cutaneous afferent axons give rise to a prominent depolarizing potential after the action potential, which is not present in the muscle afferent or motor axons. We propose that cutaneous afferent axons have kinetically slow Na+ channels not present in muscle afferent and efferent fibers, whose activation underlies the delayed depolarization and multiple spike discharge. The results indicate a difference in the Na+ channel organization of myelinated cutaneous versus muscle afferent axons and their cell bodies.