We expressed the rat Nav1.3 and Nav1.6 sodium channel α subunit isoforms in Xenopus oocytes either alone or with the rat β1 and β2 auxiliary subunits in various combinations and assessed the sensitivity of the expressed channels to resting and use-dependent modification by the pyrethroid insecticide tefluthrin using the two-electrode voltage clamp technique. Coexpression with the β1 and β2 subunits, either individually or in combination, did not affecting the resting sensitivity of Nav1.6 channels to tefluthrin. Modification by tefluthrin of Nav1.6 channels in the absence of β subunits was not altered by the application of trains of high-frequency depolarizing prepulses. By contrast, coexpression of the Nav1.6 channel with the β1 subunit enhanced the extent of channel modification twofold following repeated depolarization. Coexpression of Nav1.6 with the β2 subunit also slightly enhanced modification following repeated depolarization, but coexpression of Nav1.6 with both β subunits caused enhanced modification following repeated depolarization that was indistinguishable from that found with Nav1.6+β1 channels. In contrast to Nav1.6, the resting modification by tefluthrin of Nav1.3 channels expressed in the absence of β subunits was reduced by repeated depolarization. However, tefluthrin modification of the Nav1.3 α subunit expressed with both β subunits was enhanced 1.7-fold by repeated depolarization, thereby confirming that β subunit modulation of use-dependent effects was not confined to the Nav1.6 isoform. These results show that the actions of pyrethroids on mammalian sodium channels in the Xenopus oocyte expression system are determined in part by the interactions of the sodium channel α subunit with the auxiliary β subunits that are part of the heteromultimeric sodium channel complexes found in neurons and other excitable cells.
voltage-gated sodium channels; Nav1.6 isoform; Nav1.3 isoform; β subunit; voltage clamp; tefluthrin
In rats expression of the Nav1.7 voltage-gated sodium channel isoform is restricted to the peripheral nervous system and is abundant in the sensory neurons of the dorsal root ganglion. We expressed the rat Nav1.7 sodium channel α subunit together with the rat auxiliary β1 and β2 subunits in Xenopus laevis oocytes and assessed the effects of the pyrethroid insecticide tefluthrin on the expressed currents using the two-electrode voltage clamp method. Tefluthrin at 100 µM modified of Nav1.7 channels to prolong inactivation of the peak current during a depolarizing pulse, resulting in a marked "late current" at the end of a 40-ms depolarization, and induced a sodium tail current following repolarization. Tefluthrin modification was enhanced up to two-fold by the application of a train of up to 100 5-ms depolarizing prepulses. These effects of tefluthrin on Nav1.7 channels were qualitatively similar to its effects on rat Nav1.2, Nav1.3 and Nav1.6 channels assayed previously under identical conditions. However, Nav1.7 sodium channels were distinguished by their low sensitivity to modification by tefluthrin, especially compared to Nav1.3 and Nav1.6 channels. It is likely that Nav1.7 channels contribute significantly to the tetrodotoxin-sensitive, pyrethroid-resistant current found in cultured dorsal root ganglion neurons. We aligned the complete amino acid sequences of four pyrethroid-sensitive isoforms (house fly Vssc1; rat Nav1.3, Nav1.6 and Nav1.8) and two pyrethroid-resistant isoforms (rat Nav1.2 and Nav1.7) and found only a single site, located in transmembrane segment 6 of homology domain I, at which the amino acid sequence was conserved among all four sensitive isoform sequences but differed in the two resistant isoform sequences. This position, corresponding to Val410 of the house fly Vssc1 sequence, also aligns with sites of multiple amino acid substitutions identified in the sodium channel sequences of pyrethroid-resistant insect populations. These results implicate this single amino acid polymorphism in transmembrane segment 6 of sodium channel homology domain I as a determinant of the differential pyrethroid sensitivity of rat sodium channel isoforms.
voltage-gated sodium channel; Nav1.7 isoform; pyrethroid; tefluthrin; peripheral nervous system; dorsal root ganglion
We expressed rat Nav1.6 sodium channels in combination with the rat β1 and β2 auxiliary subunits in human embryonic kidney (HEK293) cells and evaluated the effects of the pyrethroid insecticides tefluthrin and deltamethrin on expressed sodium currents using the whole-cell patch clamp technique. Both pyrethroids produced concentration-dependent, resting modification of Nav1.6 channels, prolonging the kinetics of channel inactivation and deactivation to produce persistent “late” currents during depolarization and tail currents following repolarization. Both pyrethroids also produced concentration dependent hyperpolarizing shifts in the voltage dependence of channel activation and steady-state inactivation. Maximal shifts in activation, determined from the voltage dependence of the pyrethroid-induced late and tail currents, were ~25 mV for tefluthrin and ~20 mV for deltamethrin. The highest attainable concentrations of these compounds also caused shifts of ~5–10 mV in the voltage dependence of steady-state inactivation. In addition to their effects on the voltage dependence of inactivation, both compounds caused concentration-dependent increases in the fraction of sodium current that was resistant to inactivation following strong depolarizing prepulses. We assessed the use-dependent effects of tefluthrin and deltamethrin on Nav1.6 channels by determining the effect of trains of 1 to 100 5-ms depolarizing prepulses at frequencies of 20 or 66.7 Hz on the extent of channel modification. Repetitive depolarization at either frequency increased modification by deltamethrin by ~2.3-fold but had no effect on modification by tefluthrin. Tefluthrin and deltamethrin were equally potent as modifiers of Nav1.6 channels in HEK293 cells using the conditions producing maximal modification as the basis for comparison. These findings show that the actions of tefluthrin and deltamethrin of Nav1.6 channels in HEK293 cells differ from the effects of these compounds on Nav1.6 channels in Xenopus oocytes and more closely reflect the actions of pyrethroids on channels in their native neuronal environment.
voltage-gated sodium channel; Nav1.6 isoform; pyrethroid; deltamethrin; tefluthrin; HEK293 cells
We expressed rat Nav1.6 sodium channels in combination with the rat β1 and β2 auxiliary subunits in Xenopus laevis oocytes and evaluated the effects of the pyrethroid insecticides S-bioallethrin, deltamethrin and tefluthrin on expressed sodium currents using the two-electrode voltage clamp technique. S-Bioallethrin, a Type I structure, produced transient modification evident in the induction of rapidly-decaying sodium tail currents, weak resting modification (5.7% modification at 100 μM), and no further enhancement of modification upon repetitive activation by high-frequency trains of depolarizing pulses. By contrast deltamethrin, a Type II structure, produced sodium tail currents that were ~9-fold more persistent than those caused by S-bioallethrin, barely detectable resting modification (2.5% modification at 100 μM), and 3.7-fold enhancement of modification upon repetitive activation. Tefluthrin, a Type I structure with high mammalian toxicity, exhibited properties intermediate between S-bioallethrin and deltamethrin: intermediate tail current decay kinetics, much greater resting modification (14.1% at 100 μM), and 2.8-fold enhancement of resting modification upon repetitive activation. Comparison of concentration–effect data showed that repetitive depolarization increased the potency of tefluthrin ~15-fold and that tefluthrin was ~10-fold more potent than deltamethrin as a use-dependent modifier of Nav1.6 sodium channels. Concentration–effect data from parallel experiments with the rat Nav1.2 sodium channel co-expressed with the rat β1 and β2 subunits in oocytes showed that the Nav1.6 isoform was at least 15-fold more sensitive to tefluthrin and deltamethrin than the Nav1.2 isoform. These results implicate sodium channels containing the Nav1.6 isoform as potential targets for the central neurotoxic effects of pyrethroids.
voltage-gated sodium channel; Nav1.6 isoform; pyrethroid; S-bioallethrin; deltamethrin; tefluthrin
The Nav1.6 voltage-gated sodium channel α subunit isoform is the most abundant isoform in the brain and is implicated in the transmission of high frequency action potentials. Purification and immunocytochemical studies imply that Nav1.6 exist predominantly as Nav1.6+β1+β2 heterotrimeric complexes. We assessed the independent and joint effects of the rat β1 and β2 subunits on the gating and kinetic properties of rat Nav1.6 channels by recording whole-cell currents in the two-electrode voltage clamp configuration following transient expression in Xenopus oocytes. The β1 subunit accelerated fast inactivation of sodium currents but had no effect on the voltage dependence of their activation and steady-state inactivation and also prevented the decline of currents following trains of high-frequency depolarizing prepulses. The β2 subunit selectively retarded the fast phase of fast inactivation and shifted the voltage dependence of activation towards depolarization without affecting other gating properties and had no effect on the decline of currents following repeated depolarization. The β1 and β2 subunits expressed together accelerated both kinetic phases of fast inactivation, shifted the voltage dependence of activation towards hyperpolarization, and gave currents with a persistent component typical of those recorded from neurons expressing Nav1.6 sodium channels. These results identify unique effects of the β1 and β2 subunits and demonstrate that joint modulation by both auxiliary subunits gives channel properties that are not predicted by the effects of individual subunits.
voltage-gated sodium channels; Nav1.6; β subunits; voltage clamp; kinetics; steady-state properties
The voltage-gated sodium channel is the primary target site of pyrethroids, which constitute a major class of insecticides used worldwide. Pyrethroids prolong the opening of sodium channels by inhibiting deactivation and inactivation. Despite numerous attempts to characterize pyrethroid binding to sodium channels in the past several decades, the molecular determinants of the pyrethroid binding site on the sodium channel remain elusive. Here, we show that an F-to-I substitution at 1519 (F1519I) in segment 6 of domain III (IIIS6) abolished the sensitivity of the cockroach sodium channel expressed in Xenopus laevis oocytes to all eight structurally diverse pyrethroids examined, including permethrin and deltamethrin. In contrast, substitution by tyrosine or tryptophan reduced the channel sensitivity to deltamethrin only by 3- to 10-fold, indicating that an aromatic residue at this position is critical for the interaction of pyrethroids with sodium channels. The F1519I mutation, however, did not alter the action of two other classes of sodium channel toxins, batrachotoxin (a site 2 toxin) and Lqhα-IT (a site 3 toxin). Schild analysis using competitive interaction of pyrethroid-stereospecific isomers demonstrated that the F1519W mutation and a previously known pyrethroid-resistance mutation, L993F in IIS6, reduced the binding affinity of 1S-cis-permethrin, an inactive isomer that shares the same binding site with the active isomer 1R-cis-permethrin. Our results provide the first direct proof that Leu993 and Phe1519 are part of the pyrethroid receptor site on an insect sodium channel.
Voltage-gated sodium (Nav) channels are responsible for initiation and propagation of action potential in the neurons. To explore the mechanisms for chronic heart failure (CHF)-induced baroreflex dysfunction, we measured the expression and current density of Nav channel subunits (Nav1.7, Nav1.8, and Nav1.9) in the aortic baroreceptor neurons and investigated the role of Nav channels on aortic baroreceptor neuron excitability and baroreflex sensitivity in sham and CHF rats. CHF was induced by left coronary artery ligation. The development of CHF (6–8 weeks after the coronary ligation) was confirmed by hemodynamic and morphological characteristics. Immunofluorescent data indicated that Nav1.7 was expressed in A-type (myelinated) and C-type (unmyelinated) nodose neurons but Nav1.8 and Nav1.9 were expressed only in C-type nodose neurons. Real-time RT-PCR and western blot data showed that CHF reduced mRNA and protein expression levels of Nav channels in nodose neurons. In addition, using the whole cell patch-clamp technique, we found that Nav current density and cell excitability of the aortic baroreceptor neurons were lower in CHF rats than that in sham rats. Aortic baroreflex sensitivity was blunted in anesthetized CHF rats, compared with that in sham rats. Furthermore, Nav channel activator (rATX II, 100 nM) significantly enhanced Nav current density and cell excitability of aortic baroreceptor neurons and improved aortic baroreflex sensitivity in CHF rats. These results suggest that reduced expression and activation of the Nav channels is involved in the attenuation of baroreceptor neuron excitability, which subsequently contributes to the impairment of baroreflex in CHF state.
Aortic baroreceptor neuron; Baroreflex; Heart failure; Sodium channel
The voltage-gated sodium channel Nav1.6 plays unique roles in the nervous system, but its functional properties and neuromodulation are not as well established as for NaV1.2 channels. We found no significant differences in voltage-dependent activation or fast inactivation between NaV1.6 and NaV1.2 channels expressed in non-excitable cells. In contrast, the voltage dependence of slow inactivation was more positive for Nav1.6 channels, they conducted substantially larger persistent sodium currents than Nav1.2 channels, and they were much less sensitive to inhibtion by phosphorylation by cAMP-dependent protein kinase and protein kinase C. Resurgent sodium current, a hallmark of Nav1.6 channels in neurons, was not observed for NaV1.6 expressed alone or with the auxiliary β4 subunit. The unique properties of NaV1.6 channels, together with the resurgent currents that they conduct in neurons, make these channels well-suited to provide the driving force for sustained repetitive firing, a crucial property of neurons.
Voltage-gated sodium channels are the primary target of pyrethroid insecticides. Numerous point mutations in sodium channel genes have been identified in pyrethroid-resistant insect species, and many have been confirmed to reduce or abolish sensitivity of channels expressed in Xenopus oocytes to pyrethroids. Recently, several novel mutations were reported in sodium channel genes of pyrethroid-resistant Aedes mosquito populations. One of the mutations is a phenylalanine (F) to cysteine (C) change in segment 6 of domain III (IIIS6) of the Aedes mosquito sodium channel. Curiously, a previous study showed that alanine substitution of this F did not alter the action of deltamethrin, a type II pyrethroid, on a cockroach sodium channel. In this study, we changed this F to C in a pyrethroid-sensitive cockroach sodium channel and examined mutant channel sensitivity to permethrin as well as five other type I or type II pyrethroids in Xenopus oocytes. Interestingly, the F to C mutation drastically reduced channel sensitivity to three type I pyrethroids, permethrin, NRDC 157 (a deltamethrin analogue lacking the α-cyano group) and bioresemthrin, but not to three type II pyrethroids, cypermethrin, deltamethrin and cyhalothrin. These results confirm the involvement of the F to C mutation in permethrin resistance, and raise the possibility that rotation of type I and type II pyrethroids might be considered in the control of insect pest populations where this particular mutation is present.
Knockdown resistance; sodium channel; pyrethroid resistance; Type I & type II pyrethroids; Xenopus oocyte expression system
The excitotoxic conopeptide ι-RXIA induces repetitive action potentials in frog motor axons and seizures upon intracranial injection into mice. We recently discovered that ι-RXIA shifts the voltage-dependence of activation of voltage-gated sodium channel NaV1.6 to a more hyperpolarized level. Here, we performed voltage-clamp experiments to examine its activity against rodent NaV1.1 through NaV1.7 co-expressed with the β1 subunit in Xenopus oocytes and NaV1.8 in dissociated mouse DRG neurons. The order of sensitivity to ι-RXIA was NaV1.6 > 1.2 > 1.7, and the remaining subtypes were insensitive. The time course of ι-RXIA-activity on NaV1.6 during exposure to different peptide concentrations were well fit by single-exponential curves that provided kobs. The plot of kobs versus [ι-RXIA] was linear, consistent with a bimolecular reaction with a Kd of ~3 μM, close to the steady-state EC50 of ~2 μM. ι-RXIA has an unusual residue, D-Phe, and the analog with an L-Phe instead, ι-RXIA[L-Phe44], had a two-fold lower affinity and two-fold faster off-rate than ι-RXIA on NaV1.6 and furthermore was inactive on NaV1.2. ι-RXIA induced repetitive action potentials in mouse sciatic nerve with conduction velocities of both A- and C-fibers, consistent with the presence of NaV1.6 at nodes of Ranvier as well as in unmyelinated axons. Sixteen peptides homologous to ι-RXIA have been identified from a single species of Conus, so these peptides represent a rich family of novel sodium channel-targeting ligands.
channel-activation; conopeptide; excitotoxin; iota-conotoxin RXIA; neurotoxin; voltage-gated sodium channel
Voltage-gated sodium channels are the primary target of pyrethroids, an important class of synthetic insecticides. Pyrethroids bind to a distinct receptor site on sodium channels and prolong the open state by inhibiting channel deactivation and inactivation. Recent studies have begun to reveal sodium channel residues important for pyrethroid binding. However, how pyrethroid binding leads to inhibition of sodium channel deactivation and inactivation remains elusive. In this study, we show that a negatively charged aspartic acid residue at position 802 (D802) located in the extracellular end of transmembrane segment 1 of domain II (IIS1) is critical for both the action of pyrethroids and the voltage dependence of channel activation. Charge-reversing or -neutralizing substitutions (K, G, or A) of D802 shifted the voltage dependence of activation in the depolarizing direction and reduced channel sensitivity to deltamethrin, a pyrethroid insecticide. The charge-reversing mutation D802K also accelerated open-state deactivation, which may have counteracted the inhibition of sodium channel deactivation by deltamethrin. In contrast, the D802G substitution slowed open-state deactivation, suggesting an additional mechanism for neutralizing the action of deltamethrin. Importantly, Schild analysis showed that D802 is not involved in pyrethroid binding. Thus, we have identified a sodium channel residue that is critical for regulating the action of pyrethroids on the sodium channel without affecting the receptor site of pyrethroids.
Pyrethroid insecticide; Sodium channel; Activation; Deactivation
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.
Pyrethroid insecticides alter the normal gating of voltage-gated sodium channels in the nervous system. Three sodium channel mutations (E434K, C764R, L993F) were recently identified in pyrethroid resistant German cockroach populations. In this report, we show that the L993F mutation decreased sodium channel sensitivity to the pyrethroid, deltamethrin, by five-fold in Xenopus oocytes. In contrast, neither E434K nor C764R alone decreased channel sensitivity to deltamethrin. However, E434K or C764R combined with L993F reduced deltamethrin sensitivity by 100-fold. Furthermore, concomitant presence of all three mutations (KRF) reduced channel sensitivity to deltamethrin by 500-fold. None of the mutations significantly affected channel gating. However, sodium current amplitudes from the mutant sodium channel carrying either E434K or C764R alone were much reduced compared to those of the wild-type channel or the channel carrying the double or triple mutations (KF, RF and KRF). These results indicated that evolution of sodium channel insensitivity in the German cockroach is achieved by sequential selection of a primary mutation L993F and two secondary mutations E434K and C764R, and concomitant presence of all three mutations dramatically reduced sodium channel sensitivity to deltamethrin.
Knockdown resistance; Pyrethroids; Insecticide resistance; Sodium channel; Xenopus oocyte expression system
Reversible phosphorylation of ion channels underlies cellular plasticity in mammalian neurons. Voltage-gated sodium or Nav channels underlie action potential initiation and propagation, dendritic excitability, and many other aspects of neuronal excitability. Various protein kinases have been suggested to phosphorylate the primary α subunit of Nav channels, affecting diverse aspects of channel function. Previous studies of Nav α subunit phosphorylation have led to the identification of a small set of phosphorylation sites important in meditating aspects of Nav channel function. Here we use nanoflow liquid chromatography tandem mass spectrometry (nano-LC MS/MS) on Nav α subunits affinity-purified from rat brain with two distinct monoclonal antibodies to identify 15 phosphorylation sites on Nav1.2, 12 of which have not been previously reported. We also found 3 novel phosphorylation sites on Nav1.1. In general, commonly used phosphorylation site prediction algorithms did not accurately predict these novel in vivo phosphorylation sites. Our results demonstrate that specific Nav α subunits isolated from rat brain are highly phosphorylated, and suggest extensive modulation of Nav channel activity in mammalian brain. Identification of phosphorylation sites using monoclonal antibody-based immunopurification and mass spectrometry is an effective approach to define the phosphorylation status of Nav channels and important membrane proteins in mammalian brain.
voltage-gated sodium channels; brain; phosphorylation; tandem mass spectrometry; immunopurification; monoclonal antibody; nanoflow liquid chromatography
Tetrodotoxin (TTX) is the quintessential ligand of voltage-gated sodium channels (NaVs). Like TTX, μ-conotoxin peptides are pore blockers, and both toxins have helped to define the properties of neurotoxin receptor Site 1 of NaVs. Here, we report unexpected results showing that the recently discovered μ-conotoxin KIIIA and TTX can simultaneously bind to Site 1 and act in concert. Results with saturating concentrations of peptide applied to voltage-clamped Xenopus oocytes expressing brain NaV1.2, and single-channel recordings from brain channels in lipid bilayers, show that KIIIA or its analog, KIIIA[K7A], block partially, with a residual current that can be completely blocked by TTX. In addition, the kinetics of block by TTX and peptide are each affected by the prior presence of the other toxin. For example, bound peptide slows subsequent binding of TTX (an antagonistic interaction) and slows TTX dissociation when both toxins are bound (a synergistic effect on block). The overall functional consequence resulting from the combined action of the toxins depends on the quantitative balance between these opposing actions. The results lead us to postulate that in the bi-liganded NaV complex, TTX is bound between the peptide and the selectivity filter. These observations refine our view of Site 1 and open new possibilities in NaV pharmacology.
conotoxin; contratoxin; NaV1.2; oocyte; sodium channel; site 1; syntoxin; tetrodotoxin; voltage clamp
Pyrethroids disrupt nerve function by altering the rapid kinetic transitions between conducting and nonconducting states of voltage-gated sodium channels that underlie the generation of nerve action potentials. Recent studies of pyrethroid action on cloned insect and mammalian sodium channel isoforms expressed in Xenopus laevis oocytes show that in some cases pyrethroid modification is either absolutely dependent on or significantly enhanced by repeated channel activation. These use-dependent effects have been interpreted as evidence of preferential binding of at least some pyrethroids to the open, rather than resting, state of the sodium channel. This paper reviews the evidence for state-dependent modification of insect and mammalian sodium channels expressed in oocytes by pyrethroids and considers the implications of state-dependent effects for understanding the molecular mechanism of pyrethroid action and the development and testing of models of the pyrethroid receptor.
pyrethroid; cismethrin; cypermethrin; deltamethrin; permethrin; tefluthrin; sodium channel; insect; rat; human
Because of their prominent role in electro-excitability, voltage-gated sodium (NaV) channels have become the foremost important target of animal toxins. These toxins have developed the ability to discriminate between closely related NaV subtypes, making them powerful tools to study NaV channel function and structure. CgNa is a 47-amino acid residue type I toxin isolated from the venom of the Giant Caribbean Sea Anemone Condylactis gigantea. Previous studies showed that this toxin slows the fast inactivation of tetrodotoxin-sensitive NaV currents in rat dorsal root ganglion neurons. To illuminate the underlying NaV subtype-selectivity pattern, we have assayed the effects of CgNa on a broad range of mammalian isoforms (NaV1.2–NaV1.8) expressed in Xenopus oocytes. This study demonstrates that CgNa selectively slows the fast inactivation of rNaV1.3/β1, mNaV1.6/β1 and, to a lesser extent, hNaV1.5/β1, while the other mammalian isoforms remain unaffected. Importantly, CgNa was also examined on the insect sodium channel DmNaV1/tipE, revealing a clear phyla-selectivity in the efficacious actions of the toxin. CgNa strongly inhibits the inactivation of the insect NaV channel, resulting in a dramatic increase in peak current amplitude and complete removal of fast and steady-state inactivation. Together with the previously determined solution structure, the subtype-selective effects revealed in this study make of CgNa an interesting pharmacological probe to investigate the functional role of specific NaV channel subtypes. Moreover, further structural studies could provide important information on the molecular mechanism of NaV channel inactivation.
sea anemone; toxin; inactivation; sodium channel; subtype; selectivity
Pyrethroid insecticides have been used for more than 40 years and account for 25% of the worldwide insecticide market. Although their acute neurotoxicity to adults has been well characterized, information regarding the potential developmental neurotoxicity of this class of compounds is limited. There is a large age dependence to the acute toxicity of pyrethroids in which neonatal rats are at least an order of magnitude more sensitive than adults to two pyrethroids. There is no information on age-dependent toxicity for most pyrethroids. In the present review we examine the scientific data related to potential for age-dependent and developmental neurotoxicity of pyrethroids. As a basis for understanding this neurotoxicity, we discuss the heterogeneity and ontogeny of voltage-sensitive sodium channels, a primary neuronal target of pyrethroids. We also summarize 22 studies of the developmental neurotoxicity of pyrethroids and review the strengths and limitations of these studies. These studies examined numerous end points, with changes in motor activity and muscarinic acetylcholine receptor density the most common. Many of the developmental neurotoxicity studies suffer from inadequate study design, problematic statistical analyses, use of formulated products, and/or inadequate controls. These factors confound interpretation of results. To better understand the potential for developmental exposure to pyrethroids to cause neurotoxicity, additional, well-designed and well-executed developmental neurotoxicity studies are needed. These studies should employ state-of-the-science methods to promote a greater understanding of the mode of action of pyrethroids in the developing nervous system.
age-dependent toxicity; biologically based dose–response model; developmental neurotoxicity; mode of action; physiologically based pharmacokinetic model; pyrethroid; risk assessment; voltage-sensitive sodium channel
The voltage-gated sodium channel (Nav1) plays an important role in initiating and propagating action potentials in neuronal cells. We and others have recently found that the Alzheimer’s disease-related secretases BACE1 and presenilin (PS)/γ-secretase regulate Nav1 function by cleaving auxiliary subunits of the channel complex. We have also shown that elevated BACE1 activity significantly decreases sodium current densities in neuroblastoma cells and acutely dissociated adult hippocampal neurons. For detailed molecular studies of sodium channel regulation, biochemical methods are now complementing classical electrophysiology. To understand how BACE1 regulates sodium current densities in our studies, we setup conditions to analyze surface levels of the pore-forming Nav1 α-subunits. By using a cell surface biotinylation protocol, we found that elevated BACE1 activity significantly decreases surface Nav1 α-subunit levels in both neuroblastoma cells and acutely prepared hippocampal slices. This finding would explain the decreased sodium currents shown by standard electrophysiological methods. The biochemical methods used in our studies would be applicable to analyses of surface expression levels of other ion channels as well as Nav1 in cells and adult hippocampal neurons.
Voltage-gated sodium channel; BACE1; Presenilin; γ-Secretase; Cell surface biotinylation; Neuroblastoma; Hippocampal neurons; Adult hippocampal slices
Sodium channel-blocker insecticides (SCBIs), such as indoxacarb and metaflumizone, are a new class of insecticides with a mechanism of action different from those of other insecticides that target sodium channels. SCBIs block sodium channels in a manner similar to local anesthetics (LA) such as lidocaine. Several residues, particularly F1579 and Y1586, in the sixth transmembrane segment (S6) of domain IV (IV) of rat Nav1.4 sodium channels are required for the action of LAs and SCBIs and may form part of overlapping receptor sites. However, the binding site for SCBIs in insect sodium channels remains undefined. We used site-directed mutagenesis, the Xenopus laevis oocyte expression system, and the two-electrode voltage clamp technique to study the effects on SCBI activity of mutating F1817 and Y1824 (analogous to those residues identified in mammalian sodium channels) to alanine in the voltage-sensitive sodium channel of the German cockroach, Blattella germanica. The mutant channels showed no effect or a marked increase in channel sensitivity to both DCJW (the active metabolite of indoxacarb) and metaflumizone. Thus, it appeared that although the F1817 residue plays a role in the action of SCBIs and both residues are involved in LA activity in mammalian sodium channels, neither F1817 nor Y1824 are integral determinants of SCBI binding on insect sodium channels. Our results suggest that the receptor site of SCBIs on insect sodium channels may be significantly different from that on mammalian sodium channels.
Indoxacarb (DPX-JW062) was recently developed as a new oxadiazine insecticide with high insecticidal activity and low mammalian toxicity. Previous studies showed that indoxacarb and its bioactive metabolite, N-decarbomethoxyllated JW062 (DCJW), block insect sodium channels in nerve preparations and isolated neurons. However, the molecular mechanism of indoxacarb/DCJW action on insect sodium channels is not well understood. In this study, we identified two cockroach sodium channel variants, BgNav1-1 and BgNav1-4, which differ in voltage dependence of fast and slow inactivation, and channel sensitivity to DCJW. The voltage dependence of fast inactivation and slow inactivation of BgNav1-4 were shifted in the hyperpolarizing direction compared with those of BgNav1-1 channels. At the holding potential of −90 mV, 20 μM of DCJW reduced the peak current of BgNav1-4 by about 40%, but had no effect on BgNav1-1. However, at the holding potential of −60 mV, DCJW also reduced the peak currents of BgNav1-1 by about 50%. Furthermore, DCJW delayed the recovery from slow inactivation of both variants. Substitution of E1689 in segment 4 of domain four (IVS4) of BgNav1-4 with a K, which is present in BgNav1-1, was sufficient to shift the voltage dependence of fast and slow inactivation of BgNav1-4 channels to the more depolarizing membrane potential close to that of BgNav1-1 channels. The E1689K change also eliminated the DCJW inhibition of BgNav1-4 at the hyperpolarizing holding potentials. These results show that the E1689K change is responsible for the difference in channel gating and sensitivity to DCJW between BgNav1-4 and BgNav1-1. Our results support the notion that DCJW preferably acts on the inactivated state of the sodium channel and demonstrate that K1689E is a major molecular determinant of the voltage-dependent inactivation and state-dependent action of DCJW.
Insect sodium channel; Insecticide; Indoxacarb; DCJW; Xenopus oocyte
The expression of voltage-gated sodium channels is regulated at multiple levels, and in this study we addressed the potential for alternative splicing of the Nav1.2, Nav1.3, Nav1.6 and Nav1.7 mRNAs. We isolated novel mRNA isoforms of Nav1.2 and Nav1.3 from adult mouse and rat dorsal root ganglia (DRG), Nav1.3 and Nav1.7 from adult mouse brain, and Nav1.7 from neonatal rat brain. These alternatively spliced isoforms introduce an additional exon (Nav1.2 exon 17A and topologically equivalent Nav1.7 exon 16A) or exon pair (Nav1.3 exons 17A and 17B) that contain an in-frame stop codon and result in predicted two-domain, truncated proteins. The mouse and rat orthologous exon sequences are highly conserved (94-100% identities), as are the paralogous Nav1.2 and Nav1.3 exons (93% identity in mouse) to which the Nav1.7 exon has only 60% identity. Previously, Nav1.3 mRNA has been shown to be upregulated in rat DRG following peripheral nerve injury, unlike the downregulation of all other sodium channel transcripts. Here we show that the expression of Nav1.3 mRNA containing exons 17A and 17B is unchanged in mouse following peripheral nerve injury (axotomy), whereas total Nav1.3 mRNA expression is upregulated by 33% (P=0.003), suggesting differential regulation of the alternatively spliced transcripts. The alternatively spliced rodent exon sequences are highly conserved in both the human and chicken genomes, with 77-89% and 72-76% identities to mouse, respectively. The widespread conservation of these sequences strongly suggests an additional level of regulation in the expression of these channels, that is also tissue-specific.
DRG; brain; alternative splicing; Scn2a; Scn3a; Scn9a
Human voltage-activated sodium (Nav) channels are adept at rapidly transmitting electrical signals across long distances in various excitable tissues. As such, they are amongst the most widely targeted ion channels by drugs and animal toxins. Of the nine isoforms, Nav1.8 and Nav1.9 are preferentially expressed in DRG neurons where they are thought to play an important role in pain signaling. Although the functional properties of Nav1.8 have been relatively well characterized, difficulties with expressing Nav1.9 in established heterologous systems limit our understanding of the gating properties and toxin pharmacology of this particular isoform. This review summarizes our current knowledge of the role of Nav1.8 and Nav1.9 in pain perception and elaborates on the approaches used to identify molecules capable of influencing their function.
Nav1.8; Nav1.9; pain; animal toxins; voltage sensor; voltage-activated sodium channel
Voltage-gated sodium channels (Nav) mediate neuronal action potentials. Tetrodotoxin inhibits all Nav isoforms, but Nav1.8 and Nav1.9 are relatively tetrodotoxin-resistant (TTX-r) compared to other isoforms. Nav1.8 is highly expressed in dorsal root ganglion neurons and is functionally linked to nociception, but the sensitivity of TTX-r isoforms to inhaled anesthetics is unclear.
The sensitivities of heterologously expressed rat TTX-r Nav1.8 and endogenous tetrodotoxin-sensitive (TTX-s) Nav to the prototypic inhaled anesthetic isoflurane were tested in mammalian ND7/23 cells using patch-clamp electrophysiology.
From a holding potential of −70 mV, isoflurane (0.53±0.06 mM, ~1.8 MAC at 24°C) reduced normalized peak Na+ current (INa) of Nav1.8 to 0.55±0.03 and of endogenous TTX-s Nav to 0.56±0.06. Isoflurane minimally inhibited INa from a holding potential of −140 mV. Isoflurane did not affect voltage-dependence of activation, but significantly shifted voltage-dependence of steady-state inactivation by −6 mV for Nav1.8 and by −7 mV for TTX-s Nav. IC50 values for inhibition of peak INa were 0.67±0.06 mM for Nav1.8 and 0.66±0.09 mM for TTX-s Nav; significant inhibition occurred at clinically relevant concentrations as low as 0.58 MAC. Isoflurane produced use-dependent block of Nav1.8; at a stimulation frequency of 10 Hz, 0.56±0.08 mM isoflurane reduced INa to 0.64±0.01 vs. 0.78±0.01 for control.
Isoflurane inhibited the tetrodotoxin-resistant isoform Nav1.8 with potency comparable to that for endogenous tetrodotoxin-sensitive Nav isoforms, indicating that sensitivity to inhaled anesthetics is conserved across diverse Nav family members. Block of Nav1.8 in dorsal root ganglion neurons could contribute to the effects of inhaled anesthetics on peripheral nociceptive mechanisms.
In a previous study, we showed that two alternative exons (G1 and G2 encoding IIIS3-S4) were involved in the differential sensitivity of two cockroach sodium channel splice variants, BgNav1-1 and BgNav2-1 (previously called KD1 and KD2), to deltamethrin, a pyrethroid insecticide (Tan, et al., 2002b. Alternative splicing of an insect sodium channel gene generates pharmacologically distinct sodium channels. J. Neurosci. 22, 5300–5309.). Here, we report the identification of an amino acid residue in exon G2 that contributes to the low deltamethrin sensitivity of BgNav2-1. Replacement of A1356 in BgNav2-1 with the corresponding V1356 in BgNav1-1 enhanced the sensitivity of the BgNav2-1 channel to deltamethrin by six-fold. Conversely, substitution of V1356 with A1356 in BgNav1-1 produced a recombinant BgNav1-1 channel that was 5-fold more resistant to deltamethrin. These results demonstrate that A1356 contributes to the low sensitivity of BgNav2-1 to deltamethrin. A1356V substitution also shifted the voltage-dependence of activation by 10 mV in the hyperpolarizing direction. Possible mechanisms by which this amino acid change affects the action of pyrethroids on the sodium channel are discussed.
Sodium channel; Pyrethroids; Alternative splicing