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1.  Characterization of a Novel BmαTX47 Toxin Modulating Sodium Channels: The Crucial Role of Expression Vectors in Toxin Pharmacological Activity 
Toxins  2014;6(3):816-829.
Long-chain scorpion toxins with four disulfide bridges exhibit various pharmacological features towards the different voltage-gated sodium channel subtypes. However, the toxin production still remains a huge challenge. Here, we reported the effects of different expression vectors on the pharmacological properties of a novel toxin BmαTX47 from the scorpion Buthus martensii Karsch. The recombinant BmαTX47 was obtained using the expression vector pET-14b and pET-28a, respectively. Pharmacological experiments showed that the recombinant BmαTX47 was a new α-scorpion toxin which could inhibit the fast inactivation of rNav1.2, mNav1.4 and hNav1.5 channels. Importantly, the different expression vectors were found to strongly affect BmαTX47 pharmacological activities while toxins were obtained by the same expression and purification procedures. When 10 µM recombinant BmαTX47 from the pET-28a vector was applied, the values of I5ms/Ipeak for rNav1.2, mNav1.4 and hNav1.5 channels were 44.12% ± 3.17%, 25.40% ± 4.89% and 65.34% ± 3.86%, respectively, which were better than those values of 11.33% ± 1.46%, 15.96% ± 1.87% and 5.24% ± 2.38% for rNav1.2, mNav1.4 and hNav1.5 channels delayed by 10 µM recombinant BmαTX47 from the pET-14b vector. The dose-response experiments further indicated the EC50 values of recombinant BmαTX47 from the pET-28a vector were 7262.9 ± 755.9 nM for rNav1.2 channel and 1005.8 ± 118.6 nM for hNav1.5 channel, respectively. Together, these findings highlighted the important role of expression vectors in scorpion toxin pharmacological properties, which would accelerate the understanding of the structure-function relationships of scorpion toxins and promote the potential application of toxins in the near future.
PMCID: PMC3968363  PMID: 24577584
Buthus martensii Karsch; BmαTX47; recombinant expression; sodium channels; pET-28a vector; pET-14b vector
2.  Localization of Receptor Site on Insect Sodium Channel for Depressant β-toxin BmK IT2 
PLoS ONE  2011;6(1):e14510.
BmK IT2 is regarded as a receptor site-4 modulator of sodium channels with depressant insect toxicity. It also displays anti-nociceptive and anti-convulsant activities in rat models. In this study, the potency and efficacy of BmK IT2 were for the first time assessed and compared among four sodium channel isoforms expressed in Xenopus oocytes. Combined with molecular approach, the receptor site of BmK IT2 was further localized.
Principal Findings
2 µM BmK IT2 strongly shifted the activation of DmNav1, the sodium channel from Drosophila, to more hyperpolarized potentials; whereas it hardly affected the gating properties of rNav1.2, rNav1.3 and mNav1.6, three mammalian central neuronal sodium channel subtypes. (1) Mutations of Glu896, Leu899, Gly904 in extracellular loop Domain II S3–S4 of DmNav1 abolished the functional action of BmK IT2. (2) BmK IT2-preference for DmNav1 could be conferred by Domain III. Analysis of subsequent DmNav1 mutants highlighted the residues in Domain III pore loop, esp. Ile1529 was critical for recognition and binding of BmK IT2.
In this study, BmK IT2 displayed total insect-selectivity. Two binding regions, comprising domains II and III of DmNav1, play separated but indispensable roles in the interaction with BmK IT2. The insensitivity of Nav1.2, Nav1.3 and Nav1.6 to BmK IT2 suggests other isoforms or mechanism might be involved in the suppressive activity of BmK IT2 in rat pathological models.
PMCID: PMC3021515  PMID: 21264295
3.  Revealing the Function and the Structural Model of Ts4: Insights into the “Non-Toxic” Toxin from Tityus serrulatus Venom 
Toxins  2015;7(7):2534-2550.
The toxin, previously described as a “non-toxic” toxin, was isolated from the scorpion venom of Tityus serrulatus (Ts), responsible for the most severe and the highest number of accidents in Brazil. In this study, the subtype specificity and selectivity of Ts4 was investigated using six mammalian Nav channels (Nav1.2→Nav1.6 and Nav1.8) and two insect Nav channels (DmNav1 and BgNav). The electrophysiological assays showed that Ts4 specifically inhibited the fast inactivation of Nav1.6 channels, the most abundant sodium channel expressed in the adult central nervous system, and can no longer be classified as a “non-toxic peptide”. Based on the results, we could classify the Ts4 as a classical α-toxin. The Ts4 3D-structural model was built based on the solved X-ray Ts1 3D-structure, the major toxin from Ts venom with which it shares high sequence identity (65.57%). The Ts4 model revealed a flattened triangular shape constituted by three-stranded antiparallel β-sheet and one α-helix stabilized by four disulfide bonds. The absence of a Lys in the first amino acid residue of the N-terminal of Ts4 is probably the main responsible for its low toxicity. Other key amino acid residues important to the toxicity of α- and β-toxins are discussed here.
PMCID: PMC4516927  PMID: 26153865
Tityus serrulatus; Nav1.6; envenomation; scorpion toxin; sodium channel
4.  Differential Effects of TipE and a TipE-Homologous Protein on Modulation of Gating Properties of Sodium Channels from Drosophila melanogaster 
PLoS ONE  2013;8(7):e67551.
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.
PMCID: PMC3715519  PMID: 23874427
5.  Isolation and Characterization of CvIV4: A Pain Inducing α- Scorpion Toxin 
PLoS ONE  2011;6(8):e23520.
Among scorpion species, the Buthidae produce the most deadly and painful venoms. However, little is known regarding the venom components that cause pain and their mechanism of action. Using a paw-licking assay (Mus musculus), this study compared the pain-inducing capabilities of venoms from two species of New World scorpion (Centruroides vittatus, C. exilicauda) belonging to the neurotoxin-producing family Buthidae with one species of non-neurotoxin producing scorpion (Vaejovis spinigerus) in the family Vaejovidae. A pain-inducing α-toxin (CvIV4) was isolated from the venom of C. vittatus and tested on five Na+ channel isoforms.
Principal Findings
C. vittatus and C. exilicauda venoms produced significantly more paw licking in Mus than V. spinigerus venom. CvIV4 produced paw licking in Mus equivalent to the effects of whole venom. CvIV4 slowed the fast inactivation of Nav1.7, a Na+ channel expressed in peripheral pain-pathway neurons (nociceptors), but did not affect the Nav1.8-based sodium currents of these neurons. CvIV4 also slowed the fast inactivation of Nav1.2, Nav1.3 and Nav1.4. The effects of CvIV4 are similar to Old World α-toxins that target Nav1.7 (AahII, BmK MI, LqhIII, OD1), however the primary structure of CvIV4 is not similar to these toxins. Mutant Nav1.7 channels (D1586A and E1589Q, DIV S3–S4 linker) reduced but did not abolish the effects of CvIV4.
This study: 1) agrees with anecdotal evidence suggesting that buthid venom is significantly more painful than non-neurotoxic venom; 2) demonstrates that New World buthids inflict painful stings via toxins that modulate Na+ channels expressed in nociceptors; 3) reveals that Old and New World buthids employ similar mechanisms to produce pain. Old and New World α-toxins that target Nav1.7 have diverged in sequence, but the activity of these toxins is similar. Pain-inducing toxins may have evolved in a common ancestor. Alternatively, these toxins may be the product of convergent evolution.
PMCID: PMC3160894  PMID: 21887265
6.  A quantitative and comparative study of the effects of a synthetic ciguatoxin CTX3C on the kinetic properties of voltage-dependent sodium channels 
British Journal of Pharmacology  2004;142(5):879-889.
Ciguatoxins (CTXs) are known to bind to receptor site 5 of the voltage-dependent Na channel, but the toxin's physiological effects are poorly understood. In this study, we investigated the effects of a ciguatoxin congener (CTX3C) on three different Na-channel isoforms, rNav1.2, rNav1.4, and rNav1.5, which were transiently expressed in HEK293 cells.The toxin (1.0 μmol l−1) shifted the activation potential (V1/2 of activation curve) in the negative direction by 4–9 mV and increased the slope factor (k) from 8 mV to between 9 and 12 mV (indicative of decreased steepness of the activation curve), thereby resulting in a hyperpolarizing shift of the threshold potential by 30 mV for all Na channel isoforms.The toxin (1.0 μmol l−1) significantly accelerated the time-to-peak current from 0.62 to 0.52 ms in isoform rNav1.2. Higher doses of the toxin (3–10 μmol l−1) additionally decreased time-to-peak current in rNav1.4 and rNav1.5.A toxin effect on decay of INa at −20 mV was either absent or marginal even at relatively high doses of CTX3C.The toxin (1 μmol l−1) shifted the inactivation potential (V1/2 of inactivation curve) in the negative direction by 15–18 mV in all isoforms.INa maxima of the I–V curve (at −20 mV) were suppressed by application of 1.0 μmol l−1 CTX3C to a similar extent (80–85% of the control) in all the three isoforms. Higher doses of CTX3C up to 10 μmol l−1 further suppressed INa to 61–72% of the control.Recovery from slow inactivation induced by a depolarizing prepulse of intermediate duration (500 ms) was dramatically delayed in the presence of 1.0 μmol l−1 CTX3C, as time constants describing the monoexponential recovery were increased from 38±8 to 588±151 ms (n=5), 53±6 to 338±85 ms (n=4), and 23±3 to 232±117 ms (n=3) in rNav1.2, rNav1.4, and rNav1.5, respectively.CTX3C exerted multimodal effects on sodium channels, with simultaneous stimulatory and inhibitory aspects, probably due to the large molecular size (3 nm in length) and lipophilicity of this membrane-spanning toxin.
PMCID: PMC1575065  PMID: 15197105
Ciguatoxin; brevetoxin; Nav1.2; Nav1.4; Nav1.5; slow inactivation
7.  Intron Retention in mRNA Encoding Ancillary Subunit of Insect Voltage-Gated Sodium Channel Modulates Channel Expression, Gating Regulation and Drug Sensitivity 
PLoS ONE  2013;8(8):e67290.
Insect voltage-gated sodium (Nav) channels are formed by a well-known pore-forming α-subunit encoded by para-like gene and ancillary subunits related to TipE from the mutation “temperature-induced-paralysis locus E.” The role of these ancillary subunits in the modulation of biophysical and pharmacological properties of Na+ currents are not enough documented. The unique neuronal ancillary subunit TipE-homologous protein 1 of Drosophila melanogaster (DmTEH1) strongly enhances the expression of insect Nav channels when heterologously expressed in Xenopus oocytes. Here we report the cloning and functional expression of two neuronal DmTEH1-homologs of the cockroach, Periplaneta americana, PaTEH1A and PaTEH1B, encoded by a single bicistronic gene. In PaTEH1B, the second exon encoding the last 11-amino-acid residues of PaTEH1A is shifted to 3′UTR by the retention of a 96-bp intron-containing coding-message, thus generating a new C-terminal end. We investigated the gating and pharmacological properties of the Drosophila Nav channel variant (DmNav1-1) co-expressed with DmTEH1, PaTEH1A, PaTEH1B or a truncated mutant PaTEH1Δ(270-280) in Xenopus oocytes. PaTEH1B caused a 2.2-fold current density decrease, concomitant with an equivalent α-subunit incorporation decrease in the plasma membrane, compared to PaTEH1A and PaTEH1Δ(270-280). PaTEH1B positively shifted the voltage-dependences of activation and slow inactivation of DmNav1-1 channels to more positive potentials compared to PaTEH1A, suggesting that the C-terminal end of both proteins may influence the function of the voltage-sensor and the pore of Nav channel. Interestingly, our findings showed that the sensitivity of DmNav1-1 channels to lidocaine and to the pyrazoline-type insecticide metabolite DCJW depends on associated TEH1-like subunits. In conclusion, our work demonstrates for the first time that density, gating and pharmacological properties of Nav channels expressed in Xenopus oocytes can be modulated by an intron retention process in the transcription of the neuronal TEH1-like ancillary subunits of P. americana.
PMCID: PMC3744522  PMID: 23967047
8.  Molecular basis of differential sensitivity of insect sodium channels to DCJW, a bioactive metabolite of the oxadiazine insecticide indoxacarb 
Neurotoxicology  2005;27(2):237-244.
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.
PMCID: PMC3057067  PMID: 16325912
Insect sodium channel; Insecticide; Indoxacarb; DCJW; Xenopus oocyte
9.  Human Nav1.6 Channels Generate Larger Resurgent Currents than Human Nav1.1 Channels, but the Navβ4 Peptide Does Not Protect Either Isoform from Use-Dependent Reduction 
PLoS ONE  2015;10(7):e0133485.
Voltage-gated sodium channels are responsible for the initiation and propagation of action potentials (APs). Two brain isoforms, Nav1.1 and Nav1.6, have very distinct cellular and subcellular expression. Specifically, Nav1.1 is predominantly expressed in the soma and proximal axon initial segment of fast-spiking GABAergic neurons, while Nav1.6 is found at the distal axon initial segment and nodes of Ranvier of both fast-spiking GABAergic and excitatory neurons. Interestingly, an auxiliary voltage-gated sodium channel subunit, Navβ4, is also enriched in the axon initial segment of fast-spiking GABAergic neurons. The C-terminal tail of Navβ4 is thought to mediate resurgent sodium current, an atypical current that occurs immediately following the action potential and is predicted to enhance excitability. To better understand the contribution of Nav1.1, Nav1.6 and Navβ4 to high frequency firing, we compared the properties of these two channel isoforms in the presence and absence of a peptide corresponding to part of the C-terminal tail of Navβ4. We used whole-cell patch clamp recordings to examine the biophysical properties of these two channel isoforms in HEK293T cells and found several differences between human Nav1.1 and Nav1.6 currents. Nav1.1 channels exhibited slower closed-state inactivation but faster open-state inactivation than Nav1.6 channels. We also observed a greater propensity of Nav1.6 to generate resurgent currents, most likely due to its slower kinetics of open-state inactivation, compared to Nav1.1. These two isoforms also showed differential responses to slow and fast AP waveforms, which were altered by the Navβ4 peptide. Although the Navβ4 peptide substantially increased the rate of recovery from apparent inactivation, Navβ4 peptide did not protect either channel isoform from undergoing use-dependent reduction with 10 Hz step-pulse stimulation or trains of slow or fast AP waveforms. Overall, these two channels have distinct biophysical properties that may differentially contribute to regulating neuronal excitability.
PMCID: PMC4504674  PMID: 26182346
10.  Three Peptide Modulators of the Human Voltage-Gated Sodium Channel 1.7, an Important Analgesic Target, from the Venom of an Australian Tarantula 
Toxins  2015;7(7):2494-2513.
Voltage-gated sodium (NaV) channels are responsible for propagating action potentials in excitable cells. NaV1.7 plays a crucial role in the human pain signalling pathway and it is an important therapeutic target for treatment of chronic pain. Numerous spider venom peptides have been shown to modulate the activity of NaV channels and these peptides represent a rich source of research tools and therapeutic lead molecules. The aim of this study was to determine the diversity of NaV1.7-active peptides in the venom of an Australian Phlogius sp. tarantula and to characterise their potency and subtype selectivity. We isolated three novel peptides, μ-TRTX-Phlo1a, -Phlo1b and -Phlo2a, that inhibit human NaV1.7 (hNaV1.7). Phlo1a and Phlo1b are 35-residue peptides that differ by one amino acid and belong in NaSpTx family 2. The partial sequence of Phlo2a revealed extensive similarity with ProTx-II from NaSpTx family 3. Phlo1a and Phlo1b inhibit hNaV1.7 with IC50 values of 459 and 360 nM, respectively, with only minor inhibitory activity on rat NaV1.2 and hNaV1.5. Although similarly potent at hNaV1.7 (IC50 333 nM), Phlo2a was less selective, as it also potently inhibited rNaV1.2 and hNaV1.5. All three peptides cause a depolarising shift in the voltage-dependence of hNaV1.7 activation.
PMCID: PMC4516925  PMID: 26134258
Phlogius sp.; spider venom; venom peptide; voltage-gated sodium channel; NaV1.7; two-electrode voltage clamp electrophysiology; ion channel; mass spectrometry
11.  Deconstructing voltage sensor function and pharmacology in sodium channels 
Nature  2008;456(7219):202-208.
Voltage-activated sodium (Nav) channels are crucial for the generation and propagation of nerve impulses, and as such are amongst the most widely targeted ion channels by toxins and drugs. The four voltage sensors in Nav channels have distinct amino acid sequences, raising fundamental questions about their relative contributions to the function and pharmacology of the channel. Here we use four-fold symmetric voltage-activated potassium (Kv) channels as reporters to examine the contributions of individual Nav channel S3b-S4 paddle motifs to the kinetics of voltage sensor activation and to forming toxin receptors. Our results uncover binding sites for toxins from tarantula and scorpion venom on each of the four paddle motifs in Nav channels and reveal how paddle-specific interactions can be used to reshape Nav channel activity. One paddle motif is unique in that it slows voltage sensor activation and toxins selectively targeting this motif impede Nav channel inactivation. This reporter approach and the principles that emerge will be useful in developing new drugs for treating pain and Nav channelopathies.
PMCID: PMC2587061  PMID: 19005548
12.  The Scorpion Toxin Tf2 from Tityus fasciolatus Promotes Nav1.3 Opening 
PLoS ONE  2015;10(6):e0128578.
We identified Tf2, the first β-scorpion toxin from the venom of the Brazilian scorpion Tityus fasciolatus. Tf2 is identical to Tb2-II found in Tityus bahiensis. We found that Tf2 selectively activates human (h)Nav1.3, a neuronal voltage-gated sodium (Nav) subtype implicated in epilepsy and nociception. Tf2 shifts hNav1.3 activation voltage to more negative values, thereby opening the channel at resting membrane potentials. Seven other tested mammalian Nav channels (Nav1.1-1.2; Nav1.4-1.8) expressed in Xenopus oocytes are insensitive upon application of 1 μM Tf2. Therefore, the identification of Tf2 represents a unique addition to the repertoire of animal toxins that can be used to investigate Nav channel function.
PMCID: PMC4470819  PMID: 26083731
13.  Systematic Study of Binding of µ-Conotoxins to the Sodium Channel NaV1.4 
Toxins  2014;6(12):3454-3470.
Voltage-gated sodium channels (NaV) are fundamental components of the nervous system. Their dysfunction is implicated in a number of neurological disorders, such as chronic pain, making them potential targets for the treatment of such disorders. The prominence of the NaV channels in the nervous system has been exploited by venomous animals for preying purposes, which have developed toxins that can block the NaV channels, thereby disabling their function. Because of their potency, such toxins could provide drug leads for the treatment of neurological disorders associated with NaV channels. However, most toxins lack selectivity for a given target NaV channel, and improving their selectivity profile among the NaV1 isoforms is essential for their development as drug leads. Computational methods will be very useful in the solution of such design problems, provided accurate models of the protein-ligand complex can be constructed. Using docking and molecular dynamics simulations, we have recently constructed a model for the NaV1.4-µ-conotoxin-GIIIA complex and validated it with the ample mutational data available for this complex. Here, we use the validated NaV1.4 model in a systematic study of binding other µ-conotoxins (PIIIA, KIIIA and BuIIIB) to NaV1.4. The binding mode obtained for each complex is shown to be consistent with the available mutation data and binding constants. We compare the binding modes of PIIIA, KIIIA and BuIIIB to that of GIIIA and point out the similarities and differences among them. The detailed information about NaV1.4-µ-conotoxin interactions provided here will be useful in the design of new NaV channel blocking peptides.
PMCID: PMC4280544  PMID: 25529306
sodium channels; conotoxins; homology modeling; docking; molecular dynamics; potential of mean force; binding free energy
14.  Mechanism of μ-Conotoxin PIIIA Binding to the Voltage-Gated Na+ Channel NaV1.4 
PLoS ONE  2014;9(3):e93267.
Several subtypes of voltage-gated Na+ (NaV) channels are important targets for pain management. μ-Conotoxins isolated from venoms of cone snails are potent and specific blockers of different NaV channel isoforms. The inhibitory effect of μ-conotoxins on NaV channels has been examined extensively, but the mechanism of toxin specificity has not been understood in detail. Here the known structure of μ-conotoxin PIIIA and a model of the skeletal muscle channel NaV1.4 are used to elucidate elements that contribute to the structural basis of μ-conotoxin binding and specificity. The model of NaV1.4 is constructed based on the crystal structure of the bacterial NaV channel, NaVAb. Six different binding modes, in which the side chain of each of the basic residues carried by the toxin protrudes into the selectivity filter of NaV1.4, are examined in atomic detail using molecular dynamics simulations with explicit solvent. The dissociation constants (Kd) computed for two selected binding modes in which Lys9 or Arg14 from the toxin protrudes into the filter of the channel are within 2 fold; both values in close proximity to those determined from dose response data for the block of NaV currents. To explore the mechanism of PIIIA specificity, a double mutant of NaV1.4 mimicking NaV channels resistant to μ-conotoxins and tetrodotoxin is constructed and the binding of PIIIA to this mutant channel examined. The double mutation causes the affinity of PIIIA to reduce by two orders of magnitude.
PMCID: PMC3968119  PMID: 24676211
15.  Single-cell analysis of sodium channel expression in dorsal root ganglion neurons 
Sensory neurons of the dorsal root ganglia (DRG) express multiple voltage-gated sodium (Na) channels that substantially differ in gating kinetics and pharmacology. Small-diameter (<25 µm) neurons isolated from the rat DRG express a combination of fast tetrodotoxin-sensitive (TTX-S) and slow TTX-resistant (TTX-R) Na currents while large-diameter neurons (>30 µm) predominately express fast TTX-S Na current. Na channel expression was further investigated using single-cell RT-PCR to measure the transcripts present in individually harvested DRG neurons. Consistent with cellular electrophysiology, the small neurons expressed transcripts encoding for both TTX-S (Nav1.1, Nav1.2, Nav1.6, Nav1.7) and TTX-R (Nav1.8, Nav1.9) Na channels. Nav1.7, Nav1.8 and Nav1.9 were the predominant Na channels expressed in the small neurons. The large neurons highly expressed TTX-S isoforms (Nav1.1, Nav1.6, Nav1.7) while TTX-R channels were present at comparatively low levels. A unique subpopulation of the large neurons was identified that expressed TTX-R Na current and high levels of Nav1.8 transcript. DRG neurons also displayed substantial differences in the expression of neurofilaments (NF200, peripherin) and Necl-1, a neuronal adhesion molecule involved in myelination. The preferential expression of NF200 and Necl-1 suggests that large-diameter neurons give rise to thick myelinated axons. Small-diameter neurons expressed peripherin, but reduced levels of NF200 and Necl-1, a pattern more consistent with thin unmyelinated axons. Single-cell analysis of Na channel transcripts indicates that TTX-S and TTX-R Na channels are differentially expressed in large myelinated (Nav1.1, Nav1.6, Nav1.7) and small unmyelinated (Nav1.7, Nav1.8, Nav1.9) sensory neurons.
PMCID: PMC3005531  PMID: 20816971
Sodium channel; dorsal root ganglia; single-cell RT-PCR; Necl-1; NF200; peripherin
16.  Actions of Tefluthrin on Rat Nav1.7 Voltage-Gated Sodium Channels Expressed in Xenopus Oocytes 
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.
PMCID: PMC3181098  PMID: 21966053
voltage-gated sodium channel; Nav1.7 isoform; pyrethroid; tefluthrin; peripheral nervous system; dorsal root ganglion
17.  Molecular Surface of JZTX-V (β-Theraphotoxin-Cj2a) Interacting with Voltage-Gated Sodium Channel Subtype NaV1.4 
Toxins  2014;6(7):2177-2193.
Voltage-gated sodium channels (VGSCs; NaV1.1–NaV1.9) have been proven to be critical in controlling the function of excitable cells, and human genetic evidence shows that aberrant function of these channels causes channelopathies, including epilepsy, arrhythmia, paralytic myotonia, and pain. The effects of peptide toxins, especially those isolated from spider venom, have shed light on the structure–function relationship of these channels. However, most of these toxins have not been analyzed in detail. In particular, the bioactive faces of these toxins have not been determined. Jingzhaotoxin (JZTX)-V (also known as β-theraphotoxin-Cj2a) is a 29-amino acid peptide toxin isolated from the venom of the spider Chilobrachys jingzhao. JZTX-V adopts an inhibitory cysteine knot (ICK) motif and has an inhibitory effect on voltage-gated sodium and potassium channels. Previous experiments have shown that JZTX-V has an inhibitory effect on TTX-S and TTX-R sodium currents on rat DRG cells with IC50 values of 27.6 and 30.2 nM, respectively, and is able to shift the activation and inactivation curves to the depolarizing and the hyperpolarizing direction, respectively. Here, we show that JZTX-V has a much stronger inhibitory effect on NaV1.4, the isoform of voltage-gated sodium channels predominantly expressed in skeletal muscle cells, with an IC50 value of 5.12 nM, compared with IC50 values of 61.7–2700 nM for other heterologously expressed NaV1 subtypes. Furthermore, we investigated the bioactive surface of JZTX-V by alanine-scanning the effect of toxin on NaV1.4 and demonstrate that the bioactive face of JZTX-V is composed of three hydrophobic (W5, M6, and W7) and two cationic (R20 and K22) residues. Our results establish that, consistent with previous assumptions, JZTX-V is a Janus-faced toxin which may be a useful tool for the further investigation of the structure and function of sodium channels.
PMCID: PMC4113750  PMID: 25055801
spider toxin; voltage gated sodium channels; JZTX-V; NaV1.4
18.  Lidocaine reduces the transition to slow inactivation in Nav1.7 voltage-gated sodium channels 
British Journal of Pharmacology  2011;164(2b):719-730.
The primary use of local anaesthetics is to prevent or relieve pain by reversibly preventing action potential propagation through the inhibition of voltage-gated sodium channels. The tetrodotoxin-sensitive voltage-gated sodium channel subtype Nav1.7, abundantly expressed in pain-sensing neurons, plays a crucial role in perception and transmission of painful stimuli and in inherited chronic pain syndromes. Understanding the interaction of lidocaine with Nav1.7 channels could provide valuable insight into the drug's action in alleviating pain in distinct patient populations. The aim of this study was to determine how lidocaine interacts with multiple inactivated conformations of Nav1.7 channels.
We investigated the interactions of lidocaine with wild-type Nav1.7 channels and a paroxysmal extreme pain disorder mutation (I1461T) that destabilizes fast inactivation. Whole cell patch clamp recordings were used to examine the activity of channels expressed in human embryonic kidney 293 cells.
Depolarizing pulses that increased slow inactivation of Nav1.7 channels also reduced lidocaine inhibition. Lidocaine enhanced recovery of Nav1.7 channels from prolonged depolarizing pulses by decreasing slow inactivation. A paroxysmal extreme pain disorder mutation that destabilizes fast inactivation of Nav1.7 channels decreased lidocaine inhibition.
Lidocaine decreased the transition of Nav1.7 channels to the slow inactivated state. The fast inactivation gate (domain III–IV linker) is important for potentiating the interaction of lidocaine with the Nav1.7 channel.
PMCID: PMC3188891  PMID: 21232038
Nav1.7; voltage-gated sodium channel; neuropathic pain; inactivation; channelopathies; local anaesthetics
19.  A tarantula-venom peptide that antagonises the TRPA1 nociceptor ion channel by binding to the S1-S4 gating domain 
Current biology : CB  2014;24(5):473-483.
The venoms of predators such as spiders, scorpions, cone snails, sea anemones, and snakes, have been an excellent source of pharmacological diversity for drug discovery and as pharmacological tools for elucidating the structure, function, and physiological properties of ion channels. Here we describe the first known peptide antagonist of the nociceptor ion channel transient receptor potential ankyrin 1 (TRPA1).
We constructed a recombinant cDNA library encoding ∼100 diverse GPI-anchored peptide toxins (t-toxins) derived from spider venoms and screened this library by co-expression in Xenopus oocytes with TRPA1. This screen resulted in identification of protoxin-I (ProTx-I), a 35-residue peptide from the venom of the Peruvian green-velvet tarantula, Thrixopelma pruriens, as the first known high-affinity peptide TRPA1 antagonist. Interestingly, ProTx-I was previously identified as an antagonist of voltage-gated sodium (NaV) channels. To identify the surfaces of ProTx-I by which it binds to these distinct ion channel types, we constructed a t-toxin library of ProTx-I alanine-scanning mutants and screened this library against NaV1.2 and TRPA1. This revealed distinct partially overlapping surfaces of ProTx-I by which it binds to these two ion channels, and whose specific chemical features explain its higher affinity for NaV1.2 than for TRPA1. Importantly, this mutagenesis yielded two novel ProTx-I variants that are only active against either TRPA1or NaV1.2, but not both. By testing its activity against chimeric channels, we identified the extracellular loops of the TRPA1 S1-S4 gating domain as the ProTx-I binding site.
These studies establish screening of t-toxin libraries of native and mutated toxins, which we term “toxineering”, as a generally applicable method for isolation of novel ion channel modifiers and for design of ion channel modifiers with altered target selectivity. They also suggest that ProTx-I will be a valuable pharmacological reagent for addressing the biophysical mechanisms of TRPA1 gating, the physiology and pathophysiology of TRPA1 function in nociceptors, and for potential clinical application in the context of pain and inflammation.
PMCID: PMC3949122  PMID: 24530065
20.  Comparison of Gating Properties and Use-Dependent Block of Nav1.5 and Nav1.7 Channels by Anti-Arrhythmics Mexiletine and Lidocaine 
PLoS ONE  2015;10(6):e0128653.
Mexiletine and lidocaine are widely used class IB anti-arrhythmic drugs that are considered to act by blocking voltage-gated open sodium currents for treatment of ventricular arrhythmias and relief of pain. To gain mechanistic insights into action of anti-arrhythmics, we characterized biophysical properties of Nav1.5 and Nav1.7 channels stably expressed in HEK293 cells and compared their use-dependent block in response to mexiletine and lidocaine using whole-cell patch clamp recordings. While the voltage-dependent activation of Nav1.5 or Nav1.7 was not affected by mexiletine and lidocaine, the steady-state fast and slow inactivation of Nav1.5 and Nav1.7 were significantly shifted to hyperpolarized direction by either mexiletine or lidocaine in dose-dependent manner. Both mexiletine and lidocaine enhanced the slow component of closed-state inactivation, with mexiletine exerting stronger inhibition on either Nav1.5 or Nav1.7. The recovery from inactivation of Nav1.5 or Nav1.7 was significantly prolonged by mexiletine compared to lidocaine. Furthermore, mexiletine displayed a pronounced and prominent use-dependent inhibition of Nav1.5 than lidocaine, but not Nav1.7 channels. Taken together, our findings demonstrate differential responses to blockade by mexiletine and lidocaine that preferentially affect the gating of Nav1.5, as compared to Nav1.7; and mexiletine exhibits stronger use-dependent block of Nav1.5. The differential gating properties of Nav1.5 and Nav1.7 in response to mexiletine and lidocaine may help explain the drug effectiveness and advance in new designs of safe and specific sodium channel blockers for treatment of cardiac arrhythmia or pain.
PMCID: PMC4465899  PMID: 26068619
21.  Mechanism and molecular basis for the sodium channel subtype specificity of µ-conopeptide CnIIIC 
British Journal of Pharmacology  2012;167(3):576-586.
Voltage-gated sodium channels (NaV channels) are key players in the generation and propagation of action potentials, and selective blockade of these channels is a promising strategy for clinically useful suppression of electrical activity. The conotoxin µ-CnIIIC from the cone snail Conus consors exhibits myorelaxing activity in rodents through specific blockade of skeletal muscle (NaV1.4) NaV channels.
We investigated the activity of µ-CnIIIC on human NaV channels and characterized its inhibitory mechanism, as well as the molecular basis, for its channel specificity.
Similar to rat paralogs, human NaV1.4 and NaV1.2 were potently blocked by µ-CnIIIC, the sensitivity of NaV1.7 was intermediate, and NaV1.5 and NaV1.8 were insensitive. Half-channel chimeras revealed that determinants for the insensitivity of NaV1.8 must reside in both the first and second halves of the channel, while those for NaV1.5 are restricted to domains I and II. Furthermore, domain I pore loop affected the total block and therefore harbours the major determinants for the subtype specificity. Domain II pore loop only affected the kinetics of toxin binding and dissociation. Blockade by µ-CnIIIC of NaV1.4 was virtually irreversible but left a residual current of about 5%, reflecting a ‘leaky’ block; therefore, Na+ ions still passed through µ-CnIIIC-occupied NaV1.4 to some extent. TTX was excluded from this binding site but was trapped inside the pore by µ-CnIIIC.
Of clinical significance, µ-CnIIIC is a potent and persistent blocker of human skeletal muscle NaV1.4 that does not affect activity of cardiac NaV1.5.
PMCID: PMC3449262  PMID: 22537004
sodium channel; muscle relaxant; analgesia; conotoxin; patch-clamp
22.  Calcium-Mediated Dual-Mode Regulation of Cardiac Sodium Channel Gating 
Circulation research  2009;104(7):870-878.
Intracellular Ca2+ ([Ca2+]i) can trigger dual-mode regulation of the voltage gated cardiac sodium channel (NaV1.5). The channel components of the Ca2+ regulatory system are the calmodulin (CaM)-binding IQ motif and the Ca2+ sensing EF hand–like (EFL) motif in the carboxyl terminus of the channel. Mutations in either motif have been associated with arrhythmogenic changes in expressed NaV1.5 currents. Increases in [Ca2+]i shift the steady-state inactivation of NaV1.5 in the depolarizing direction and slow entry into inactivated states. Mutation of the EFL (NaV1.54X) shifts inactivation in the hyperpolarizing direction compared with the wild-type channel and eliminates the Ca2+ sensitivity of inactivation gating. Modulation of the steady-state availability of NaV1.5 by [Ca2+]i is more pronounced after the truncation of the carboxyl terminus proximal to the IQ motif (NaV1.5Δ1885), which retains the EFL. Mutating the EFL (NaV1.54X) unmasks CaM-mediated regulation of the kinetics and voltage dependence of inactivation. This latent CaM modulation of inactivation is eliminated by mutation of the IQ motif (NaV1.54X-IQ/AA). The LQT3 EFL mutant channel NaV1.5D1790G exhibits Ca2+ insensitivity and unmasking of CaM regulation of inactivation gating. The enhanced effect of CaM on NaV1.54X gating is associated with significantly greater fluorescence resonance energy transfer between enhanced cyan fluorescent protein–CaM and NaV1.54X channels than is observed with wild-type NaV1.5. Unlike other isoforms of the Na channel, the IQ-CaM interaction in the carboxyl terminus of NaV1.5 is latent under physiological conditions but may become manifest in the presence of disease causing mutations in the CT of NaV1.5 (particularly in the EFL), contributing to the production of potentially lethal ventricular arrhythmias.
PMCID: PMC2860428  PMID: 19265034
voltage-gated sodium channel; EF hand motif; IQ motif; calmodulin; FRET
23.  Acidosis Differentially Modulates Inactivation in NaV1.2, NaV1.4, and NaV1.5 Channels 
NaV channels play a crucial role in neuronal and muscle excitability. Using whole-cell recordings we studied effects of low extracellular pH on the biophysical properties of NaV1.2, NaV1.4, and NaV1.5, expressed in cultured mammalian cells. Low pH produced different effects on different channel subtypes. Whereas NaV1.4 exhibited very low sensitivity to acidosis, primarily limited to partial block of macroscopic currents, the effects of low pH on gating in NaV1.2 and NaV1.5 were profound. In NaV1.2 low pH reduced apparent valence of steady-state fast inactivation, shifted the τ(V) to depolarizing potentials and decreased channels availability during onset to slow and use-dependent inactivation (UDI). In contrast, low pH delayed open-state inactivation in NaV1.5, right-shifted the voltage-dependence of window current, and increased channel availability during onset to slow and UDI. These results suggest that protons affect channel availability in an isoform-specific manner. A computer model incorporating these results demonstrates their effects on membrane excitability.
PMCID: PMC3372088  PMID: 22701426
gating; activation; fast inactivation; slow inactivation; patch-clamp; sodium channels
24.  Scorpion toxin BmK I directly activates Nav1.8 in primary sensory neurons to induce neuronal hyperexcitability in rats 
Protein & Cell  2015;6(6):443-452.
Voltage-gated sodium channels (VGSCs) in primary sensory neurons play a key role in transmitting pain signals to the central nervous system. BmK I, a site-3 sodium channel-specific toxin from scorpion Buthus martensi Karsch, induces pain behaviors in rats. However, the subtypes of VGSCs targeted by BmK I were not entirely clear. We therefore investigated the effects of BmK I on the current amplitude, gating and kinetic properties of Nav1.8, which is associated with neuronal hyperexcitability in DRG neurons. It was found that BmK I dose-dependently increased Nav1.8 current in small-sized (<25 μm) acutely dissociated DRG neurons, which correlated with its inhibition on both fast and slow inactivation. Moreover, voltage-dependent activation and steady-state inactivation curves of Nav1.8 were shifted in a hyperpolarized direction. Thus, BmK I reduced the threshold of neuronal excitability and increased action potential firing in DRG neurons. In conclusion, our data clearly demonstrated that BmK I modulated Nav1.8 remarkably, suggesting BmK I as a valuable probe for studying Nav1.8. And Nav1.8 is an important target related to BmK I-evoked pain.
PMCID: PMC4444811  PMID: 25903152
voltage-gated sodium channel; Nav1.8; primary sensory neurons; BmK I
25.  Scorpion toxin BmK I directly activates Nav1.8 in primary sensory neurons to induce neuronal hyperexcitability in rats 
Protein & Cell  2015;6(6):443-452.
Voltage-gated sodium channels (VGSCs) in primary sensory neurons play a key role in transmitting pain signals to the central nervous system. BmK I, a site-3 sodium channel-specific toxin from scorpion Buthus martensi Karsch, induces pain behaviors in rats. However, the subtypes of VGSCs targeted by BmK I were not entirely clear. We therefore investigated the effects of BmK I on the current amplitude, gating and kinetic properties of Nav1.8, which is associated with neuronal hyperexcitability in DRG neurons. It was found that BmK I dose-dependently increased Nav1.8 current in small-sized (<25 μm) acutely dissociated DRG neurons, which correlated with its inhibition on both fast and slow inactivation. Moreover, voltage-dependent activation and steady-state inactivation curves of Nav1.8 were shifted in a hyperpolarized direction. Thus, BmK I reduced the threshold of neuronal excitability and increased action potential firing in DRG neurons. In conclusion, our data clearly demonstrated that BmK I modulated Nav1.8 remarkably, suggesting BmK I as a valuable probe for studying Nav1.8. And Nav1.8 is an important target related to BmK I-evoked pain.
PMCID: PMC4444811  PMID: 25903152
voltage-gated sodium channel; Nav1.8; primary sensory neurons; BmK I

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