Chimeric study of sodium channels from rat skeletal and cardiac muscle

Voltage-Gated Sodium Channel Inhibition by µ-Conotoxins

µ-Conotoxins are small, potent pore-blocker inhibitors of voltage-gated sodium (NaV) channels, which have been identified as pharmacological probes and putative leads for analgesic development. A limiting factor in their therapeutic development has been their promiscuity for different NaV channel subtypes, which can lead to undesirable side-effects. This review will focus on four areas of µ-conotoxin research: (1) mapping the interactions of µ-conotoxins with different NaV channel subtypes, (2) µ-conotoxin structure-activity relationship studies, (3) observed species selectivity of µ-conotoxins and (4) the effects of µ-conotoxin disulfide connectivity on activity. Our aim is to provide a clear overview of the current status of µ-conotoxin research.

Activation of reverse Na+-Ca2+ exchanger by skeletal Na+ channel isoform increases excitation-contraction coupling efficiency in rabbit cardiomyocytes

Our prior work has shown that Na⁺ current (I_Na) affects sarcoplasmic reticular (SR) Ca²⁺ release by activating early reverse of the Na⁺-Ca²⁺ exchanger (NCX). The resulting Ca²⁺ entry primes the dyadic cleft, which appears to increase Ca²⁺ channel coupling fidelity. It has been shown that the skeletal isoform of the voltage-gated Na⁺ channel (Na_v1.4) is the main tetrodotoxin (TTX)-sensitive Na_v isoform expressed in adult rabbit ventricular cardiomyocytes. Here, I tested the hypothesis that it is also the principal isoform involved in the priming mechanism. Action potentials (APs) were evoked in isolated rabbit ventricular cells loaded with fluo-4, and simultaneously recorded Ca²⁺ transients before and after the application of either relatively low doses of TTX (100 nM), the specific Na_v1.4 inhibitor μ-Conotoxin GIIIB or the specific Na_v1.1 inhibitor ICA 121430. Although APs changes after the application of each drug reflected the relative abundance of each isoform, the effects of TTX and GIIIB on SR Ca²⁺ release (measured as the transient maximum upstroke velocity) were no different. Furthermore, this reduction in SR Ca²⁺ release was comparable with the value that we obtained previously when total I_Na was inactivated with a ramp applied under voltage clamp. Finally, SR Ca²⁺ release was unaltered by the same ramp in the presence of TTX or GIIB. In contrast, application of ICA had no effect of SR Ca²⁺ release. These results suggest that Na_v1.4 is the main Na_v isoform involved in regulating the efficiency of excitation-contraction coupling in rabbit cardiomyocytes by priming the junction via activation of reverse-mode NCX.NEW & NOTEWORTHY A number of studies suggest that the Na⁺-Ca²⁺ exchanger (NCX) activated by Na⁺ currents is involved in the process of excitation-contraction (EC) coupling in cardiac ventricular myocytes. Although insufficient to trigger sarcoplasmic Ca²⁺ release alone, the Ca²⁺ entering through reverse NCX during an action potential can prime the dyadic cleft and increase the Ca²⁺ current coupling fidelity. Using specific Na⁺ inhibitors in this study, we show that in rabbit ventricular cells the skeletal Na⁺ channel isoform (Na_v1.4) is the main isoform responsible for this priming. Our study provides insights into a mechanism that may have an increased relevance where EC coupling is remodeled.

Biophysical characterization of the honeybee DSC1 orthologue reveals a novel voltage-dependent Ca2+ channel subfamily: CaV4

Insect DSC1 channels have sequences that are intermediate between voltage-gated Na⁺ and Ca²⁺ channels but have hitherto been classified as the former. Gosselin-Badaroudine et al. clone and characterize honeybee DSC1, revealing high selectivity for Ca²⁺ and suggesting reclassification of DSC1 homologues as Ca²⁺ channels.

Bilaterian voltage-gated Na⁺ channels (Na_V) evolved from voltage-gated Ca²⁺ channels (Ca_V). The Drosophila melanogaster Na⁺ channel 1 (DSC1), which features a D-E-E-A selectivity filter sequence that is intermediate between Ca_V and Na_V channels, is evidence of this evolution. Phylogenetic analysis has classified DSC1 as a Ca²⁺-permeable Na⁺ channel belonging to the Na_V2 family because of its sequence similarity with Na_V channels. This is despite insect Na_V2 channels (DSC1 and its orthologue in Blatella germanica, BSC1) being more permeable to Ca²⁺ than Na⁺. In this study, we report the cloning and molecular characterization of the honeybee (Apis mellifera) DSC1 orthologue. We reveal several sequence variations caused by alternative splicing, RNA editing, and genomic variations. Using the Xenopus oocyte heterologous expression system and the two-microelectrode voltage-clamp technique, we find that the channel exhibits slow activation and inactivation kinetics, insensitivity to tetrodotoxin, and block by Cd²⁺ and Zn²⁺. These characteristics are reminiscent of Ca_V channels. We also show a strong selectivity for Ca²⁺ and Ba²⁺ ions, marginal permeability to Li⁺, and impermeability to Mg²⁺ and Na⁺ ions. Based on current ion channel nomenclature, the D-E-E-A selectivity filter, and the properties we have uncovered, we propose that DSC1 homologues should be classified as Ca_V4 rather than Na_V2. Indeed, channels that contain the D-E-E-A selectivity sequence are likely to feature the same properties as the honeybee’s channel, namely slow activation and inactivation kinetics and strong selectivity for Ca²⁺ ions.

Sodium channel diversity in the vestibular ganglion: NaV1.5, NaV1.8, and tetrodotoxin-sensitive currents

Firing patterns differ between subpopulations of vestibular primary afferent neurons. The role of sodium (NaV) channels in this diversity has not been investigated because NaV currents in rodent vestibular ganglion neurons (VGNs) were reported to be homogeneous, with the voltage dependence and tetrodotoxin (TTX) sensitivity of most neuronal NaV channels. RT-PCR experiments, however, indicated expression of diverse NaV channel subunits in the vestibular ganglion, motivating a closer look. Whole cell recordings from acutely dissociated postnatal VGNs confirmed that nearly all neurons expressed NaV currents that are TTX-sensitive and have activation midpoints between -30 and -40 mV. In addition, however, many VGNs expressed one of two other NaV currents. Some VGNs had a small current with properties consistent with NaV1.5 channels: low TTX sensitivity, sensitivity to divalent cation block, and a relatively negative voltage range, and some VGNs showed NaV1.5-like immunoreactivity. Other VGNs had a current with the properties of NaV1.8 channels: high TTX resistance, slow time course, and a relatively depolarized voltage range. In two NaV1.8 reporter lines, subsets of VGNs were labeled. VGNs with NaV1.8-like TTX-resistant current also differed from other VGNs in the voltage dependence of their TTX-sensitive currents and in the voltage threshold for spiking and action potential shape. Regulated expression of NaV channels in primary afferent neurons is likely to selectively affect firing properties that contribute to the encoding of vestibular stimuli.

Characterization of the honeybee AmNaV1 channel and tools to assess the toxicity of insecticides

Pollination is important for both agriculture and biodiversity. For a significant number of plants, this process is highly, and sometimes exclusively, dependent on the pollination activity of honeybees. The large numbers of honeybee colony losses reported in recent years have been attributed to colony collapse disorder. Various hypotheses, including pesticide overuse, have been suggested to explain the disorder. Using the Xenopus oocytes expression system and two microelectrode voltage-clamp, we report the functional expression and the molecular, biophysical, and pharmacological characterization of the western honeybee’s sodium channel (Apis Mellifera Na_V1). The Na_V1 channel is the primary target for pyrethroid insecticides in insect pests. We further report that the honeybee’s channel is also sensitive to permethrin and fenvalerate, respectively type I and type II pyrethroid insecticides. Molecular docking of these insecticides revealed a binding site that is similar to sites previously identified in other insects. We describe in vitro and in silico tools that can be used to test chemical compounds. Our findings could be used to assess the risks that current and next generation pesticides pose to honeybee populations.

Fluorinated amino acids: compatibility with native protein structures and effects on protein-protein interactions

Fluorinated analogues of the canonical α-L-amino acids have gained widespread attention as building blocks that may endow peptides and proteins with advantageous biophysical, chemical and biological properties. This critical review covers the literature dealing with investigations of peptides and proteins containing fluorinated analogues of the canonical amino acids published over the course of the past decade including the late nineties. It focuses on side-chain fluorinated amino acids, the carbon backbone of which is identical to their natural analogues. Each class of amino acids--aliphatic, aromatic, charged and polar as well as proline--is presented in a separate section. General effects of fluorine on essential properties such as hydrophobicity, acidity/basicity and conformation of the specific side chains and the impact of these altered properties on stability, folding kinetics and activity of peptides and proteins are discussed (245 references).

Possible roles of exceptionally conserved residues around the selectivity filters of sodium and calcium channels

In the absence of x-ray structures of sodium and calcium channels their homology models are used to rationalize experimental data and design new experiments. A challenge is to model the outer-pore region that folds differently from potassium channels. Here we report a new model of the outer-pore region of the NaV1.4 channel, which suggests roles of highly conserved residues around the selectivity filter. The model takes from our previous study (Tikhonov, D. B., and Zhorov, B. S. (2005) Biophys. J. 88, 184-197) the general disposition of the P-helices, selectivity filter residues, and the outer carboxylates, but proposes new intra- and inter-domain contacts that support structural stability of the outer pore. Glycine residues downstream from the selectivity filter are proposed to participate in knob-into-hole contacts with the P-helices and S6s. These contacts explain the adapted tetrodotoxin resistance of snakes that feed on toxic prey through valine substitution of isoleucine in the P-helix of repeat IV. Polar residues five positions upstream from the selectivity filter residues form H-bonds with the ascending-limb backbones. Exceptionally conserved tryptophans are engaged in inter-repeat H-bonds to form a ring whose π-electrons would facilitate passage of ions from the outer carboxylates to the selectivity filter. The outer-pore model of CaV1.2 derived from the NaV1.4 model is also stabilized by the ring of exceptionally conservative tryptophans and H-bonds between the P-helices and ascending limbs. In this model, the exceptionally conserved aspartate downstream from the selectivity-filter glutamate in repeat II facilitates passage of calcium ions to the selectivity-filter ring through the tryptophan ring. Available experimental data are discussed in view of the models.

Mu-conotoxins as leads in the development of new analgesics

Voltage-gated sodium channels (VGSCs) contain a specific binding site for a family of cone shell toxins known as mu-conotoxins. As some VGSCs are involved in pain perception and mu-conotoxins are able to block these channels, mu-conotoxins show considerable potential as analgesics. Recent studies have advanced our understanding of the three-dimensional structures and structure-function relationships of the mu-conotoxins, including their interaction with VGSCs. Truncated peptide analogues of the native toxins have been created in which secondary structure elements are stabilized by non-native linkers such as lactam bridges. Ultimately, it would be desirable to capture the favourable analgesic properties of the native toxins, in particular their potency and channel sub-type selectivity, in non-peptide mimetics. Such mimetics would constitute lead compounds in the development of new therapeutics for the treatment of pain.

Synergistic and antagonistic interactions between tetrodotoxin and mu-conotoxin in blocking voltage-gated sodium channels

Tetrodotoxin (TTX) is the quintessential ligand of voltage-gated sodium channels (NaVs). Like TTX, mu-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 mu-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.

Neuroecology, chemical defense, and the keystone species concept

Neuroecology unifies principles from diverse disciplines, scaling from biophysical properties of nerve and muscle cells to community-wide impacts of trophic interactions. Here, these principles are used as a common fabric, woven from threads of chemosensory physiology, behavior, and population and community ecology. The "keystone species" concept, for example, is seminal in ecological theory. It defines a species whose impacts on communities are far greater than would be predicted from its relative abundance and biomass. Similarly, neurotoxins could function in keystone roles. They are rare within natural habitats but exert strong effects on species interactions at multiple trophic levels. Effects of two guanidine alkaloids, tetrodotoxin (TTX) and saxitoxin (STX), coalesce neurobiological and ecological perspectives. These molecules compose some of the most potent natural poisons ever described, and they are introduced into communities by one, or only a few, host species. Functioning as voltage-gated sodium channel blockers for nerve and muscle cells, TTX and STX serve in chemical defense. When borrowed by resistant consumer species, however, they are used either in chemical defense against higher order predators or for chemical communication as chemosensory excitants. Cascading effects of the compounds profoundly impact community-wide attributes, including species compositions and rates of material exchange. Thus, a diverse array of physiological traits, expressed differentially across many species, renders TTX and STX fully functional as keystone molecules, with vast ecological consequences at multiple trophic levels.

Calcium block of single sodium channels: role of a pore-lining aromatic residue

Extracellular Ca(2+) ions cause a rapid block of voltage-gated sodium channels, manifest as an apparent reduction of the amplitude of single-channel currents. We examined the influence of residue Tyr-401 in the isoform rNa(V)1.4 on both single-channel conductance and Ca(2+) block. An aromatic residue at this position in the outer mouth of the pore plays a critical role in high-affinity block by the guanidinium toxin tetrodotoxin, primarily due to an electrostatic attraction between the cationic blocker and the system of pi electrons on the aromatic face. We tested whether a similar attraction between small metal cations (Na(+) and Ca(2+)) and this residue would enhance single-channel conductance or pore block, using a series of fluorinated derivatives of phenylalanine at this position. Our results show a monotonic decrease in Ca(2+) block as the aromatic ring is increasingly fluorinated, a result in accord with a cation-pi interaction between Ca(2+) and the aromatic ring. This occurred without a change of single-channel conductance, consistent with a greater electrostatic effect of the pi system on divalent than on monovalent cations. High-level quantum mechanical calculations show that Ca(2+) ions likely do not bind directly to the aromatic ring because of the substantial energetic penalty of dehydrating a Ca(2+) ion. However, the complex of a Ca(2+) ion with its inner hydration shell, Ca(2+)(H(2)O)(6), interacts electrostatically with the aromatic ring in a way that affects the local concentration of Ca(2+) ions in the extracellular vestibule.

A cation-pi interaction discriminates among sodium channels that are either sensitive or resistant to tetrodotoxin block

Voltage-gated sodium channels control the upstroke of the action potential in excitable cells of nerve and muscle tissue, making them ideal targets for exogenous toxins that aim to squelch electrical excitability. One such toxin, tetrodotoxin (TTX), blocks sodium channels with nanomolar affinity only when an aromatic Phe or Tyr residue is present at a specific location in the external vestibule of the ion-conducting pore. To test whether TTX is attracted to Tyr401 of NaV1.4 through a cation-pi interaction, this aromatic residue was replaced with fluorinated derivatives of Phe using in vivo nonsense suppression. Consistent with a cation-pi interaction, increased fluorination of Phe401, which reduces the negative electrostatic potential on the aromatic face, caused a monotonic increase in the inhibitory constant for block. Trifluorination of the aromatic ring decreased TTX affinity by approximately 50-fold, a reduction similar to that caused by replacement with the comparably hydrophobic residue Leu. Furthermore, we show that an energetically equivalent cation-pi interaction underlies both use-dependent and tonic block by TTX. Our results are supported by high level ab initio quantum mechanical calculations applied to a model of TTX binding to benzene. Our analysis suggests that the aromatic side chain faces the permeation pathway where it orients TTX optimally and interacts with permeant ions. These results are the first of their kind to show the incorporation of unnatural amino acids into a voltage-gated sodium channel and demonstrate that a cation-pi interaction is responsible for the obligate nature of an aromatic at this position in TTX-sensitive sodium channels.

Conotoxins down under

In the four decades since toxinologists in Australia and elsewhere started to investigate the active constituents of venomous cone snails, a wealth of information has emerged on the various classes of peptides and proteins that make their venoms such potent bioactive cocktails. This article provides an overview of the current state of knowledge of these venom constituents, several of which are of interest as potential human therapeutics as a consequence of their high potency and exquisite target specificity. With the promise of as many as 50,000 venom components across the entire Conus genus, many more interesting peptides can be anticipated.

Progressive abnormalities in skeletal muscle and neuromuscular junctions of transgenic mice expressing the Huntington's disease mutation

Huntington's disease (HD) is a neurodegenerative disorder with complex symptoms dominated by progressive motor dysfunction. Skeletal muscle atrophy is common in HD patients. Because the HD mutation is expressed in skeletal muscle as well as brain, we wondered whether the muscle changes arise from primary pathology. We used R6/2 transgenic mice for our studies. Unlike denervation atrophy, skeletal muscle atrophy in R6/2 mice occurs uniformly. Paradoxically however, skeletal muscles show age-dependent denervation-like abnormalities, including supersensitivity to acetylcholine, decreased sensitivity to mu-conotoxin, and anode-break action potentials. Morphological abnormalities of neuromuscular junctions are also present, particularly in older R6/2 mice. Severely affected R6/2 mice show a progressive increase in the number of motor endplates that fail to respond to nerve stimulation. Surprisingly, there was no constitutive sprouting of motor neurons in R6/2 muscles, even in severely atrophic muscles that showed other denervation-like characteristics. In fact, there was an age-dependent loss of regenerative capacity of motor neurons in R6/2 mice. Because muscle fibers appear to be released from the activity-dependent cues that regulate membrane properties and muscle size, and motor axons and nerve terminals become impaired in their capacity to release neurotransmitter and to respond to stimuli that normally evoke sprouting and adaptive reinnervation, we speculate that in these mice there is a progressive dissociation of trophic signalling between motor neurons and skeletal muscle. However, irrespective of the cause, the abnormalities at neuromuscular junctions we report here are likely to contribute to the pathological phenotype in R6/2 mice, particularly in late stages of the disease.

Serum concentration modifies amplitude and kinetics of voltage-gated Na+ current in the Mat-LyLu cell line of rat prostate cancer

Voltage-gated Na+ channel (VGSC) expression has previously been shown to be upregulated in strongly metastatic prostate cancer cells (rat and human) and its activity shown to potentiate a variety of cellular behaviours integral to the metastatic cascade. However, the mechanism(s) responsible for the Na+ channel upregulation is not known. As a step towards evaluating the role of the extracellular biochemical environment in this regard, we have determined the effects of serum concentration on characteristics of Na+ channel expressed in the strongly metastatic Mat-LyLu rat prostate cancer cell line. Whole-cell patch-clamp recording techniques were used to study the effects of serum concentrations, above and below the normal 1%. Both the amplitude and the kinetics of the currents were analysed. The following results were obtained: (1) Adding 1% foetal calf serum to cells starved of serum for 24h increased Na+ current density; however, increasing serum concentration further (to 5%) caused a reduction. (2) Serum-free medium produced Na+ currents with slower kinetics of activation (time to peak) and inactivation (exponential decay). (3) Increased serum concentration (a) shifted steady-state inactivation to more positive potentials without affecting conductance and (b) increased tetrodotoxin sensitivity. It is concluded that serum concentration is an important determinant of the Na+ channel characteristics leading to possible transcriptional and post-translational modifications of channel expression and/or activity. Experiments are now needed to determine which constituents (protein hormones, growth factors, etc.) are responsible for these effects.

Novel interactions identified between micro -Conotoxin and the Na+ channel domain I P-loop: implications for toxin-pore binding geometry

micro -Conotoxins ( micro -CTX) are peptides that inhibit Na(+) flux by blocking the Na(+) channel pore. Toxin residue arginine 13 is critical for both high affinity binding and for complete block of the single channel current, prompting the simple conventional view that residue 13 (R13) leads toxin docking by entering the channel along the pore axis. To date, the strongest interactions identified are between micro -CTX and domain II (DII) or DIII pore residues of the rat skeletal muscle (Na(v)1.4) Na(+) channels, but little data is available for the role of the DI P-loop in micro -CTX binding due to the lack of critical determinants identified in this domain. Despite being an essential determinant of isoform-specific tetrodotoxin sensitivity, the DI-Y401C variant had little effect on micro -CTX block. Here we report that the charge-changing substitution Y401K dramatically reduced the micro -CTX affinity ( approximately 300-fold). Using mutant cycle analysis, we demonstrate that K401 couples strongly to R13 (DeltaDeltaG > 3.0 kcal/mol) but not R1, K11, or R14 (<<1 kcal/mol). Unlike K401, however, a significant coupling was detected between toxin residue 14 and DI-E403K (DeltaDeltaG = 1.4 kcal/mol for the E403K-Q14D pair). This appears to underlie the ability of DI-E403K channels to discriminate between the GIIIA and GIIIB isoforms of micro -CTX (p < 0.05), whereas Y401K, DII-E758Q, and DIII-D1241K do not. We also identify five additional, novel toxin-channel interactions (>0.75 kcal/mol) in DII (E758-K16, D762-R13, D762-K16, E765-R13, E765-K16). Considered together, these new interactions suggest that the R13 side chain and the bulk of the bound toxin micro -CTX molecule may be significantly tilted with respect to pore axis.

Conus venoms: a rich source of novel ion channel-targeted peptides

The cone snails (genus Conus) are venomous marine molluscs that use small, structured peptide toxins (conotoxins) for prey capture, defense, and competitor deterrence. Each of the 500 Conus can express approximately 100 different conotoxins, with little overlap between species. An overwhelming majority of these peptides are probably targeted selectively to a specific ion channel. Because conotoxins discriminate between closely related subtypes of ion channels, they are widely used as pharmacological agents in ion channel research, and several have direct diagnostic and therapeutic potential. Large conotoxin families can comprise hundreds or thousands of different peptides; most families have a corresponding ion channel family target (i.e., omega-conotoxins and Ca channels, alpha-conotoxins and nicotinic receptors). Different conotoxin families may have different ligand binding sites on the same ion channel target (i.e., mu-conotoxins and delta-conotoxins to sites 1 and 6 of Na channels, respectively). The individual peptides in a conotoxin family are typically each selectively targeted to a diverse set of different molecular isoforms within the same ion channel family. This review focuses on the targeting specificity of conotoxins and their differential binding to different states of an ion channel.

Structural basis for tetrodotoxin-resistant sodium channel binding by mu-conotoxin SmIIIA

SmIIIA is a new micro-conotoxin isolated recently from Conus stercusmuscarum. Although it shares several biochemical characteristics with other micro-conotoxins (the arrangement of cysteine residues and a conserved arginine believed to interact with residues near the channel pore), it has several distinctive features, including the absence of hydroxyproline, and is the first specific antagonist of tetrodotoxin-resistant voltage-gated sodium channels to be characterized. It therefore represents a potentially useful tool to investigate the functional roles of these channels. We have determined the three-dimensional structure of SmIIIA in aqueous solution. Consistent with the absence of hydroxyprolines, SmIIIA adopts a single conformation with all peptide bonds in the trans configuration. The spatial orientations of several conserved Arg and Lys side chains, including Arg14 (using a consensus numbering system), which plays a key role in sodium channel binding, are similar to those in other micro-conotoxins but the N-terminal regions differ, reflecting the trans conformation for the peptide bond preceding residue 8 in SmIIIA, as opposed to the cis conformation in micro-conotoxins GIIIA and GIIIB. Comparison of the surfaces of SmIIIA with other micro-conotoxins suggests that the affinity of SmIIIA for TTX-resistant channels is influenced by the Trp15 side chain, which is unique to SmIIIA. Arg17, which replaces Lys in the other micro-conotoxins, may also be important. Consistent with these inferences from the structure, assays of two chimeras of SmIIIA and PIIIA in which their N- and C-terminal halves were recombined, indicated that residues in the C-terminal half of SmIIIA confer affinity for tetrodotoxin-resistant sodium channels in the cell bodies of frog sympathetic neurons. SmIIIA and the chimera possessing the C-terminal half of SmIIIA also inhibit tetrodotoxin-resistant sodium channels in the postganglionic axons of sympathetic neurons, as indicated by their inhibition of C-neuron compound action potentials that persist in the presence of tetrodotoxin.

Molecular basis of isoform-specific micro-conotoxin block of cardiac, skeletal muscle, and brain Na+ channels

mu-Conotoxins (mu-CTXs) block skeletal muscle Na(+) channels with an affinity 1-2 orders of magnitude higher than cardiac and brain Na(+) channels. Although a number of conserved pore residues are recognized as critical determinants of mu-CTX block, the molecular basis of isoform-specific toxin sensitivity remains unresolved. Sequence comparison of the domain II (DII) S5-S6 loops of rat skeletal muscle (mu1, Na(v)1.4), human heart (hh1, Na(v)1.5), and rat brain (rb1, Na(v)1.1) Na(+) channels reveals substantial divergence in their N-terminal S5-P linkers even though the P-S6 and C-terminal P segments are almost identical. We used Na(v)1.4 as the backbone and systematically converted these DII S5-P isoform variants to the corresponding residues in Na(v)1.1 and Na(v)1.5. The Na(v)1.4-->Na(v)1.5 variant substitutions V724R, C725S, A728S, D730S, and C731S (Na(v)1.4 numbering) reduced block of Na(v)1.4 by 4-, 86-, 12-, 185-, and 55-fold respectively, rendering the skeletal muscle isoform more "cardiac-like." Conversely, an Na(v)1.5--> Na(v)1.4 chimeric construct in which the Na(v)1.4 DII S5-P linker replaces the analogous segment in Na(v)1.5 showed enhanced mu-CTX block. However, these variant determinants are conserved between Na(v)1.1 and Na(v)1.4 and thus cannot explain their different sensitivities to mu-CTX. Comparison of their sequences reveals two variants at Na(v)1.4 positions 729 and 732: Ser and Asn in Na(v)1.4 compared with Thr and Lys in Na(v)1.1, respectively. The double mutation S729T/N732K rendered Na(v)1.4 more "brain-like" (30-fold downward arrow in block), and the converse mutation T925S/K928N in Na(v)1.1 reproduced the high affinity blocking phenotype of Na(v)1.4. We conclude that the DII S5-P linker, although lying outside the conventional ion-conducting pore, plays a prominent role in mu-CTX binding, thus shaping isoform-specific toxin sensitivity.

Na(v)1.5 underlies the 'third TTX-R sodium current' in rat small DRG neurons

In addition to slow-inactivating and persistent TTX-R Na(+) currents produced by Na(v)1.8 and Na(v)1.9 Na(+) channels, respectively, a third TTX-R Na(+) current with fast activation and inactivation can be recorded in 80% of small neurons of dorsal root ganglia (DRG) from E15 rats, but in only 3% of adult small DRG neurons. The half-time for activation, the time constant for inactivation, and the midpoints of activation and inactivation of the third TTX-R Na(+) currents are significantly different from those of Na(v)1.8 and Na(v)1.9 Na(+) currents. The estimated TTX K(i) (2.11+/-0.34 microM) of the third TTX-R Na(+) current is significantly lower than those of Na(v)1.8 and Na(v)1.9 Na(+) currents. The Cd(2+) sensitivity of third TTX-R Na(+) current is closer to cardiac Na(+) currents. A concentration of 1 mM Cd(2+) is required to completely block this current, which is significantly lower than the 5 mM required to block Na(v)1.8 and Na(v)1.9 currents. The third TTX-R Na(+) channel is not co-expressed with Na(v)1.8 and Na(v)1.9 Na(+) channels in DRG neurons of E18 rats, at a time when all three currents show comparable densities. The physiological and pharmacological profiles of the third TTX-R Na(+) current are similar to those of the cardiac Na(+) channel Na(v)1.5 and RT-PCR and restriction enzyme polymorphism analysis, show a parallel pattern of expression of Na(v)1.5 in DRG during development. Taken together, these results demonstrate that Na(v)1.5 is expressed in a developmentally regulated manner in DRG neurons and suggest that Na(v)1.5 Na(+) channel produces the third TTX-R current.

Critical molecular determinants of voltage-gated sodium channel sensitivity to mu-conotoxins GIIIA/B

GIIIA/B mu-conotoxins block the rat skeletal muscle sodium channel (rNa(v)1.4) with high affinity by binding to specific residues in the pore. However, human Na(v)1.4 (hNa(v)1.4) channels, which are resistant to block by GIIIA/B, have these same pore residues. We used chimera constructs, site-directed mutagenesis, and electrophysiological techniques to investigate which residues determine GIIIA/B selectivity. Exchange of serine 729 in the D2/S5-S6 linker of rat Na(v)1.4 with leucine (S729L), the corresponding residue in hNa(v)1.4, reduces the sensitivity of rNa(v)1.4 by approximately 20-fold and largely accounts for the differential sensitivity of rNa(v)1.4 and hNa(v)1.4 to both GIIIA and GIIIB. To determine whether D2/S5-S6 linker residues might contribute to the resistance of neuronal channels to GIIIA/B, we exchanged residues in this linker that differed between rNa(v)1.4 and neuronal channels. Substitution of aspargine 732 with lysine (N732K), the corresponding residue in rNa(v)1.1a and rNa(v)1.7, reduced the GIIIB sensitivity of rNa(v)1.4 by approximately 20-fold. The N732K substitution, however, only reduced GIIIA sensitivity of rNa(v)1.4 by approximately 4-fold, demonstrating that GIIIA and GIIIB have distinct interactions with the D2/S5-S6 linker. Our data indicate that naturally occurring variants in the extra-pore region of the D2/S5-S6 linker contribute to the isoform-specific sensitivity of sodium channels to GIIIA/B. Because S729 and N732 are not part of the high-affinity binding site for mu-conotoxins, these extra-pore residues probably influence the accessibility of the toxin to the binding site within the pore and/or the stability of the toxin-channel complex. Our results should aid the development of toxins that block specific neuronal sodium channel isoforms.

Isoform-specific effects of sialic acid on voltage-dependent Na+ channel gating: functional sialic acids are localized to the S5-S6 loop of domain I

The isoform specific role of sialic acid in human voltage-gated sodium channel gating was investigated through expression and chimeric analysis of two human isoforms, Na(v1.4) (hSkM1), and Na(v1.5) (hH1) in Chinese hamster ovary (CHO) cell lines. Immunoblot analyses indicate that both hSkM1 and hH1 are glycosylated and that hSkM1 is more glycosylated than hH1. Four sets of voltage-dependent parameters, the voltage of half-activation (V(a)), the voltage of half-inactivation (V(i)), the time constants for fast inactivation (tau(h)), and the time constants for recovery from inactivation (tau(rec)), were measured for hSkM1 and hH1 expressed in two CHO cell lines, Pro5 and Lec2, to determine the effect of changing sialylation on channel gating under conditions of full (Pro5) or reduced (Lec2) sialylation. For all parameters measured, hSkM1 gating showed a consistent 11-15 mV depolarizing shift under conditions of reduced sialylation, while hH1 showed no significant change in any gating parameter. Shifts in channel V(a) with changing external [Ca2+] indicated that sialylation of hSkM1, but not hH1, directly contributes to a negative surface potential. Functional analysis of two chimeras, hSkM1P1 and hH1P1, indicated that the responsible sialic acids are localized to the hSkM1 S5-S6 loop of domain I. When hSkM1 IS5-S6 was replaced by the analogous hH1 loop (hSkM1P1), changing sialylation had no significant effect on any voltage-dependent parameter. Conversely, when hSkM1 IS5-S6 was added to hH1 (hH1P1), all four parameters shifted by 6-7 mV in the depolarized direction under conditions of reduced sialylation. In summary, the gating of two human sodium channel isoforms show very different dependencies on sialic acid, with hSkM1 gating uniformly altered by sialic acid levels through an apparent electrostatic mechanism, while hH1 gating is unaffected by changing sialylation. Sialic acid-dependent gating can be removed or created by replacing or inserting hSkM1 IS5-S6, respectively, indicating that the functionally relevant sialic acid residues are localized to the first domain of the channel.

Channel cytoplasmic loops alter voltage-dependent sodium channel activation in an isoform-specific manner

1. The isoform-specific functional role of cytoplasmic structures of two voltage-gated sodium channel isoforms, the human cardiac channel (hH1) and the adult human skeletal muscle channel (hSkM1) was investigated through functional comparison of chimeras. 2. The voltage of half-activation (V(a)) for hH1 was shifted by > 20 mV in the hyperpolarised direction following internal papain treatment ('papain sensitive'), while V(a) for hSkM1 was unaffected ('papain insensitive'). 3. The hH1 region(s) responsible for this papain sensitivity was localised by testing a series of hH1/hSkM1 chimeras in which combinations of the large hH1 cytoplasmic loops joining the four transmembrane domains replaced analogous hSkM1 loops. Various chimeras were used to determine the smallest subset of loops that converted fully the papain-insensitive hSkM1 into a papain-sensitive channel. Then three converse chimeras were tested in which hSkM1 loops replaced hH1 loops to determine the smallest subset of loops necessary and sufficient to convert the papain-sensitive hH1 into a papain-insensitive channel. 4. Functional studies of this inclusive set of chimeras indicate that the first two cytoplasmic loops of the cardiac sodium channel that join domain I to II (loop A), and domain II to III (loop B), are both necessary, and together are sufficient to produce a papain-induced hyperpolarising shift in the voltage at which channels activate. When both loops are present (wild-type hH1 and the chimera hSkM1AB), V(a) for the channel shifts in the hyperpolarised direction by > 20 mV with papain treatment. When the analogous hSkM1 loops are present (wild-type hSkM1 and the chimera hH1AB), V(a) for the channel is not sensitive to treatment with papain. For channels that contain only one of the two hH1 loops, the effect of papain on V(a) is intermediary. 5. Experiments performed in the absence of papain showed that the activation voltages of the double loop chimeras, hSkM1AB and hH1AB, were shifted significantly from V(a) for hSkM1 and V(a) for hH1, respectively, indicating that these loops directly alter channel activation voltage. The resulting shifts in V(a) were in opposing directions, suggesting that cytoplasmic control of activation voltage is isoform specific. V(a) for hSkM1AB was about 20 mV more depolarised than V(a) for hSkM1, and V(a) for hH1AB was about 9 mV more negative than V(a) for hH1. 6. These data are the first to indicate isoform-specific cytoplasmic regions of the voltage-gated sodium channel that directly and differently alter the voltage of channel activation.

Resurgence of sodium channel research

A variety of isoforms of mammalian voltage-gated sodium channels have been described. Ten genes encoding sodium channel alpha subunits have been identified, and nine of those isoforms have been functionally expressed in exogenous systems. The alpha subunit is associated with accessory beta subunits in some tissues, and three genes encoding different beta subunits have been identified. The alpha subunit isoforms have distinct patterns of development and localization in the nervous system, skeletal and cardiac muscle. In addition, many of the isoforms demonstrate subtle differences in their functional properties. However, there are no clear subfamilies of the channels, unlike the situation with potassium and calcium channels. The subtle differences in the functional properties of the sodium channel isoforms result in unique conductances in specific cell types, which have important physiological effects for the organism. Small alterations in the electrophysiological properties of the channel resulting from mutations in specific isoforms cause human diseases such as periodic paralysis, long QT syndrome, and epilepsy.

Clockwise domain arrangement of the sodium channel revealed by (mu)-conotoxin (GIIIA) docking orientation

mu-Conotoxins (mu-CTXs) specifically inhibit Na(+) flux by occluding the pore of voltage-gated Na(+) channels. Although the three-dimensional structures of mu-CTXs are well defined, the molecular configuration of the channel receptor is much less certain; even the fundamental question of whether the four homologous Na(+) channel domains are arranged in a clockwise or counter-clockwise configuration remains unanswered. Residues Asp(762) and Glu(765) from domain II and Asp(1241) from domain III of rat skeletal muscle Na(+) channels are known to be critical for mu-CTX binding. We probed toxin-channel interactions by determining the potency of block of wild-type, D762K, E765K, and D1241C channels by wild-type and point-mutated mu-CTXs (R1A, Q14D, K11A, K16A, and R19A). Individual interaction energies for different toxin-channel pairs were quantified from the half-blocking concentrations using mutant cycle analysis. We find that Asp(762) and Glu(765) interact strongly with Gln(14) and Arg(19) but not Arg(1) and that Asp(1241) is tightly coupled to Lys(16) but not Arg(1) or Lys(11). These newly identified toxin-channel interactions within adjacent domains, interpreted in light of the known asymmetric toxin structure, fix the orientation of the toxin with respect to the channel and reveal that the four internal domains of Na(+) channels are arranged in a clockwise configuration as viewed from the extracellular surface.

Structural and gating changes of the sodium channel induced by mutation of a residue in the upper third of IVS6, creating an external access path for local anesthetics

Membrane-impermeant quaternary amine local anesthetics QX314 and QX222 can access their binding site on the cytoplasmic side of the selectivity filter from the outside in native cardiac Na(+) channels. Mutation of domain IV S6 Ile-1760 of rat brain IIA Na(+) channel or the equivalent (Ile-1575) in the adult rat skeletal muscle isoform (mu 1) creates an artificial access path for QX. We examined the characteristics of mutation of mu 1-I1575 and the resulting QX path. In addition to allowing external QX222 access, I1575A accelerated decay of Na(+) current and shifted steady-state availability by -27 mV. I1575A had negligible effects on inorganic or organic cation selectivity and block by tetrodotoxin (TTX), saxitoxin (STX), or mu-conotoxin (mu-CTX). It exposed a site within the protein that binds membrane-permeant methanethiosulfonate ethylammonium (MTSEA), but not membrane-impermeant methanethiosulfonate ethyltrimethylammonium (MTSET) and methanethiosulfonate ethylsulfonate (MTSES). MTSEA binding abolished the QX path created by this mutation, without effects on toxin binding. The mu-CTX derivative R13N, which partially occluded the pore, had no effect on QX access. I1575A exposed two Cys residues because a disulfide bond was formed under oxidative conditions, but the exposed Cys residues are not those in domain IV S6, adjacent to Ile-1575. The Cys mutant I1575C was insensitive to external Cd(2+) and MTS compounds (MTSEA, MTSET, MTSES), and substitution of Ile with a negatively charged residue (I1575E) did not affect toxin binding. Ile-1575 seems to be buried in the protein, and its mutation disrupts the protein structure to create the QX path without disturbing the outer vestibule and its selectivity function.

Involvement of Na+ channels in pain pathways

Voltage-dependent Na+ channels in sensory nerves contribute to the control of membrane excitability and underlie action potential generation. Na+ channel subtypes exhibit a neurone-specific and developmentally regulated pattern of expression, and changes in both channel expression and function are caused by disease. Recent evidence implicates specific roles for Na+ channel subtypes Na(v)1.3 and Na(v)1.8 in pain states that are associated with nerve injury and inflammation, respectively. Insight into the role of Na(v)1.8 in pain pathways has been gained by the generation of a null mutant. Although drugs discriminate poorly between subtypes, the molecular diversity of channels and subtype-specific modulation might provide opportunities to target pain pathways selectively.

mu-conotoxin GIIIA interactions with the voltage-gated Na(+) channel predict a clockwise arrangement of the domains

Voltage-gated Na(+) channels underlie the electrical activity of most excitable cells, and these channels are the targets of many antiarrhythmic, anticonvulsant, and local anesthetic drugs. The channel pore is formed by a single polypeptide chain, containing four different, but homologous domains that are thought to arrange themselves circumferentially to form the ion permeation pathway. Although several structural models have been proposed, there has been no agreement concerning whether the four domains are arranged in a clockwise or a counterclockwise pattern around the pore, which is a fundamental question about the tertiary structure of the channel. We have probed the local architecture of the rat adult skeletal muscle Na(+) channel (mu1) outer vestibule and selectivity filter using mu-conotoxin GIIIA (mu-CTX), a neurotoxin of known structure that binds in this region. Interactions between the pore-forming loops from three different domains and four toxin residues were distinguished by mutant cycle analysis. Three of these residues, Gln-14, Hydroxyproline-17 (Hyp-17), and Lys-16 are arranged approximately at right angles to each other in a plane above the critical Arg-13 that binds directly in the ion permeation pathway. Interaction points were identified between Hyp-17 and channel residue Met-1240 of domain III and between Lys-16 and Glu-403 of domain I and Asp-1532 of domain IV. These interactions were estimated to contribute -1.0+/-0.1, -0.9+/-0.3, and -1.4+/-0.1 kcal/mol of coupling energy to the native toxin-channel complex, respectively. mu-CTX residues Gln-14 and Arg-1, both on the same side of the toxin molecule, interacted with Thr-759 of domain II. Three analytical approaches to the pattern of interactions predict that the channel domains most probably are arranged in a clockwise configuration around the pore as viewed from the extracellular surface.

Novel structural determinants of mu-conotoxin (GIIIB) block in rat skeletal muscle (mu1) Na+ channels

mu-Conotoxin (mu-CTX) specifically occludes the pore of voltage-dependent Na(+) channels. In the rat skeletal muscle Na(+) channel (mu1), we examined the contribution of charged residues between the P loops and S6 in all four domains to mu-CTX block. Conversion of the negatively charged domain II (DII) residues Asp-762 and Glu-765 to cysteine increased the IC(50) for mu-CTX block by approximately 100-fold (wild-type = 22.3 +/- 7.0 nm; D762C = 2558 +/- 250 nm; E765C = 2020 +/- 379 nm). Restoration or reversal of charge by external modification of the cysteine-substituted channels with methanethiosulfonate reagents (methanethiosulfonate ethylsulfonate (MTSES) and methanethiosulfonate ethylammonium (MTSEA)) did not affect mu-CTX block (D762C: IC(50, MTSEA+) = 2165.1 +/- 250 nm; IC(50, MTSES-) = 2753.5 +/- 456.9 nm; E765C: IC(50, MTSEA+) = 2200.1 +/- 550.3 nm; IC(50, MTSES-) = 3248.1 +/- 2011.9 nm) compared with their unmodified counterparts. In contrast, the charge-conserving mutations D762E (IC(50) = 21.9 +/- 4.3 nm) and E765D (IC(50) = 22.0 +/- 7.0 nm) preserved wild-type blocking behavior, whereas the charge reversal mutants D762K (IC(50) = 4139.9 +/- 687.9 nm) and E765K (IC(50) = 4202.7 +/- 1088.0 nm) destabilized mu-CTX block even further, suggesting a prominent electrostatic component of the interactions between these DII residues and mu-CTX. Kinetic analysis of mu-CTX block reveals that the changes in toxin sensitivity are largely due to accelerated toxin dissociation (k(off)) rates with little changes in association (k(on)) rates. We conclude that the acidic residues at positions 762 and 765 are key determinants of mu-CTX block, primarily by virtue of their negative charge. The inability of the bulky MTSES or MTSEA side chain to modify mu-CTX sensitivity places steric constraints on the sites of toxin interaction.

A critical residue for isoform difference in tetrodotoxin affinity is a molecular determinant of the external access path for local anesthetics in the cardiac sodium channel

Membrane-impermeant quaternary derivatives of lidocaine (QX222 and QX314) block cardiac Na(+) channels when applied from either side of the membrane, but they block neuronal and skeletal muscle channels poorly from the outside. To find the molecular determinants of the cardiac external QX access path, mutations of adult rat skeletal muscle (micro1) and rat heart (rH1) Na(+) channels were studied by two-electrode voltage clamp in Xenopus oocytes. Mutating the micro1 domain I P-loop Y401, which is the critical residue for isoform differences in tetrodotoxin block, to the heart sequence (Y401C) allowed outside QX222 block, but its mutation to brain type (Y401F) showed little block. mu1-Y401C accelerated recovery from block by internal QX222. Block by external QX222 in mu1-Y401C was diminished by chemical modification with methanethiosulfonate ethylammonium (MTSEA) to the outer vestibule or by a double mutant (mu1-Y401C/F1579A), which altered the putative local anesthetic binding site. The reverse mutation in heart rH1-C374Y reduced outside QX314 block and slowed dissociation of internal QX222. Mutation of mu1-C1572 in IVS6 to Thr, the cardiac isoform residue (C1572T), allowed external QX222 block, and accelerated recovery from internal QX222 block, as reported. Blocking efficacy of outside QX222 in mu1-Y401C was more than that in mu1-C1572T, and the double mutant (mu1-Y401C/C1572T) accelerated internal QX recovery more than mu1-Y401C or mu1-C1572T alone. We conclude that the isoform-specific residue (Tyr/Phe/Cys) in the P-loop of domain I plays an important role in drug access as well as in tetrodotoxin binding. Isoform-specific residues in the IP-loop and IVS6 determine outside drug access to an internal binding site.

Binding properties of (3)H-PbTx-3 and (3)H-saxitoxin to brain membranes and to skeletal muscle membranes of puffer fish Fugu pardalis and the primary structure of a voltage-gated Na(+) channel alpha-subunit (fMNa1) from skeletal muscle of F. pardalis

The dissociation constants for (3)H-saxitoxin to brain membranes and to skeletal muscle membranes of puffer fish Fugu pardalis have been estimated to be 190- and 460-fold, respectively, larger than those to corresponding membranes of rat, by a rapid filtration assay, while these values for (3)H-PbTx-3 have been estimated to be one-third and one-half of those to rat, respectively. We have obtained a cDNA, encoding an entire voltage-gated Na(+) channel alpha-subunit (fMNa1, 1880 residues) from skeletal muscle of F. pardalis by composition of the fragments obtained from cDNA library and RT-PCR products. In fMNa1 protein, the residues for ion-selective filter and voltage sensor and the charged residues in SS2 regions of domains I-IV were conserved, but the aromatic amino acid (Phe/Tyr), commonly located in the SS2 region of domain I of tetrodotoxin-sensitive Na(+) channels, was replaced by Asn. With this particular criterion, we propose that the fMNa1 protein is a tetrodotoxin-resistant Na(+) channel.

Effects of channel cytoplasmic regions on the activation mechanisms of cardiac versus skeletal muscle Na(+) channels

Functional comparison of skeletal muscle (rSkM1) and cardiac (hH1) voltage-gated sodium channel isoforms expressed in Chinese hamster ovary cells showed rSkM1 half-activation (V(a)) and inactivation (V(i)) voltages 7 and 10 mV more depolarized than hH1 V(a) and V(i), respectively. Internal papain perfusion removed fast inactivation from each isoform and caused a 20-mV hyperpolarizing shift in hH1 V(a), with an insignificant change in rSkM1 V(a). Activation voltage of the inactivation-deficient hH1 mutant, hH1Q3, was nearly identical to wild-type hH1 V(a), both before and after papain treatment, with hH1Q3 V(a) also shifted by nearly 20 mV after internal papain perfusion. These data indicate that while papain removes both hH1 and rSkM1 inactivation, it has a second effect only on hH1 that causes a shift in activation voltage. Internal treatment with an antibody directed against the III-IV linker essentially mimicked papain treatment by removing some inactivation from each isoform and causing a 12-mV shift in hH1 V(a), while rSkM1 V(a) remained constant. This suggests that some channel segment within, near, or interacting with the III-IV linker is involved in establishing hH1 activation voltage. Together the data show that rSkM1 and hH1 activation mechanisms are different and are the first to suggest a role for a cytoplasmic structure in the voltage-dependent activation of cardiac sodium channels.

Functional roles of the extracellular segments of the sodium channel alpha subunit in voltage-dependent gating and modulation by beta1 subunits

Voltage-gated sodium channels consist of a pore-forming alpha subunit associated with beta1 subunits and, for brain sodium channels, beta2 subunits. Although much is known about the structure and function of the alpha subunit, there is little information on the functional role of the 16 extracellular loops. To search for potential functional activities of these extracellular segments, chimeras were studied in which an individual extracellular loop of the rat heart (rH1) alpha subunit was substituted for the corresponding segment of the rat brain type IIA (rIIA) alpha subunit. In comparison with rH1, wild-type rIIA alpha subunits are characterized by more positive voltage-dependent activation and inactivation, a more prominent slow gating mode, and a more substantial shift to the fast gating mode upon coexpression of beta1 subunits in Xenopus oocytes. When alpha subunits were expressed alone, chimeras with substitutions from rH1 in five extracellular loops (IIS5-SS1, IISS2-S6, IIIS1-S2, IIISS2-S6, and IVS3-S4) had negatively shifted activation, and chimeras with substitutions in three of these (IISS2-S6, IIIS1-S2, and IVS3-S4) also had negatively shifted steady-state inactivation. rIIA alpha subunit chimeras with substitutions from rH1 in five extracellular loops (IS5-SS1, ISS2-S6, IISS2-S6, IIIS1-S2, and IVS3-S4) favored the fast gating mode. Like wild-type rIIA alpha subunits, all of the chimeric rIIA alpha subunits except chimera IVSS2-S6 were shifted almost entirely to the fast gating mode when coexpressed with beta1 subunits. In contrast, substitution of extracellular loop IVSS2-S6 substantially reduced the effectiveness of beta1 subunits in shifting rIIA alpha subunits to the fast gating mode. Our results show that multiple extracellular loops influence voltage-dependent activation and inactivation and gating mode of sodium channels, whereas segment IVSS2-S6 plays a dominant role in modulation of gating by beta1 subunits. Evidently, several extracellular loops are important determinants of sodium channel gating and modulation.

Voltage-gated ion channels and hereditary disease

By the introduction of technological advancement in methods of structural analysis, electronics, and recombinant DNA techniques, research in physiology has become molecular. Additionally, focus of interest has been moving away from classical physiology to become increasingly centered on mechanisms of disease. A wonderful example for this development, as evident by this review, is the field of ion channel research which would not be nearly as advanced had it not been for human diseases to clarify. It is for this reason that structure-function relationships and ion channel electrophysiology cannot be separated from the genetic and clinical description of ion channelopathies. Unique among reviews of this topic is that all known human hereditary diseases of voltage-gated ion channels are described covering various fields of medicine such as neurology (nocturnal frontal lobe epilepsy, benign neonatal convulsions, episodic ataxia, hemiplegic migraine, deafness, stationary night blindness), nephrology (X-linked recessive nephrolithiasis, Bartter), myology (hypokalemic and hyperkalemic periodic paralysis, myotonia congenita, paramyotonia, malignant hyperthermia), cardiology (LQT syndrome), and interesting parallels in mechanisms of disease emphasized. Likewise, all types of voltage-gated ion channels for cations (sodium, calcium, and potassium channels) and anions (chloride channels) are described together with all knowledge about pharmacology, structure, expression, isoforms, and encoding genes.

Diversity of mammalian voltage-gated sodium channels

A variety of different isoforms of mammalian voltage-gated sodium channels have been identified. These channels can be classified into three different types. Eight type 1 isoforms have been identified in the CNS, PNS, skeletal muscle, and heart. All of these channels have been expressed in exogenous systems, and all of the genes have been mapped. Three type 2 isoforms have been identified in heart, uterus, and muscle. These channels diverge from the type 1 channels in critical regions, and have not been functionally expressed, so their significance is unknown. A single isoform identified in the PNS may represent a third class of channels, in that it diverges from both type 1 and 2 channels. The type 3 channel has not been functionally expressed.

Differences in saxitoxin and tetrodotoxin binding revealed by mutagenesis of the Na+ channel outer vestibule

The marine guanidinium toxins, saxitoxin (STX) and tetrodotoxin (TTX), have played crucial roles in the study of voltage-gated Na+ channels. Because they have similar actions, sizes, and functional groups, they have been thought to associate with the channel in the same manner, and early mutational studies supported this idea. Recent experiments by. Biophys. J. 67:2305-2315) have suggested that the toxins bind differently to the isoform-specific domain I Phe/Tyr/Cys location. In the adult skeletal muscle Na+ channel isoform (microliter), we compared the effects on both TTX and STX affinities of mutations in eight positions known to influence toxin binding. The results permitted the assignment of energies contributed by each amino acid to the binding reaction. For neutralizing mutations of Asp400, Glu755, and Lys1237, all thought to be part of the selectivity filter of the channel, the loss of binding energy was identical for the two toxins. However, the loss of binding energy was quite different for vestibule residues considered to be more superficial. Specifically, STX affinity was reduced much more by neutralizations of Glu758 and Asp1532. On the other hand, mutation of Tyr401 to Cys reduced TTX binding energy twice as much as it reduced STX binding energy. Kinetic analysis suggested that all outer vestibule residues tested interacted with both toxins early in the binding reaction (consistent with larger changes in the binding than unbinding rates) before the transition state and formation of the final bound complex. We propose a revised model of TTX and STX binding in the Na+ channel outer vestibule in which the toxins have similar interactions at the selectivity filter, TTX has a stronger interaction with Tyr401, and STX interacts more strongly with the more extracellular residues.

Extrapore residues of the S5-S6 loop of domain 2 of the voltage-gated skeletal muscle sodium channel (rSkM1) contribute to the mu-conotoxin GIIIA binding site

The tetradomain voltage-gated sodium channels from rat skeletal muscle (rSkM1) and from human heart (hH1) possess different sensitivities to the 22-amino-acid peptide toxin, mu-conotoxin GIIIA (mu-CTX). rSkM1 is sensitive (IC50 = 51.4 nM) whereas hH1 is relatively resistant (IC50 = 5700 nM) to the action of the toxin, a difference in sensitivity of >100-fold. The affinity of the mu-CTX for a chimera formed from domain 1 (D1), D2, and D3 from rSkM1and D4 from hH1 (SSSH; S indicates origin of domain is skeletal muscle and H indicates origin of domain is heart) was paradoxically increased approximately fourfold relative to that of rSkM1. The source of D3 is unimportant regarding the difference in the relative affinity of rSkM1 and hH1 for mu-CTX. Binding of mu-CTX to HSSS was substantially decreased (IC50 = 1145 nM). Another chimera with a major portion of D2 deriving form hH1 showed no detectable binding of mu-CTX (IC50 > 10 microM). These data indicate that D1 and, especially, D2 play crucial roles in forming the mu-CTX receptor. Charge-neutralizing mutations in D1 and D2 (Asp384, Asp762, and Glu765) had no effect on toxin binding. However, mutations at a neutral and an anionic site (residues 728 and 730) in S5-S6/D2 of rSkM1, which are not in the putative pore region, were found to decrease significantly the mu-CTX affinity with little effect on tetrodotoxin binding (</=1.3-fold increase in affinity). Furthermore, substitution at Asp730 with cysteine and exposure to Cd2+ or methanethiosulfonate reagents had no significant effect on sodium currents, consistent with this residue not contributing to the pore.

Role of the S4 in cooperativity of voltage-dependent potassium channel activation

Charged residues in the S4 transmembrane segment of voltage-gated cation channels play a key role in opening channels in response to changes in voltage across the cell membrane. However, the molecular mechanism of channel activation is not well understood. To learn more about the role of the S4 in channel gating, we constructed chimeras in which S4 segments from several divergent potassium channels, Shab, Shal, Shaw, and Kv3.2, were inserted into a Shaker potassium channel background. These S4 donor channels have distinctly different voltage-dependent gating properties and S4 amino acid sequences. None of the S4 chimeras have the gating behavior of their respective S4 donor channels. The conductance-voltage relations of all S4 chimeras are shifted to more positive voltages and the slopes are decreased. There is no consistent correlation between the nominal charge content of the S4 and the slope of the conductance-voltage relation, suggesting that the mutations introduced by the S4 chimeras may alter cooperative interactions in the gating process. We compared the gating behavior of the Shaw S4 chimera with its parent channels, Shaker and Shaw, in detail. The Shaw S4 substitution alters activation gating profoundly without introducing obvious changes in other channel functions. Analysis of the voltage-dependent gating kinetics suggests that the dominant effect of the Shaw S4 substitution is to alter a single cooperative transition late in the activation pathway, making it rate limiting. This interpretation is supported further by studies of channels assembled from tandem heterodimer constructs with both Shaker and Shaw S4 subunits. Activation gating in the heterodimer channels can be predicted from the properties of the homotetrameric channels only if it is assumed that the mutations alter a cooperative transition in the activation pathway rather than independent transitions.

TTX resistivity of Na+ channel in newt retinal neuron

We examined voltage-dependent, TTX-resistant Na+ channels of newt retina (nRNaCh) electrophysiologically. IC50-TTX value of nRNaCh is more than 20 microM. We determined partial cDNA sequences of nRNaCh restricted to TTX binding sites (SS2 regions of all four repeats). While the amino acid sequences of SS2 regions of repeats II, III and IV of nRNaCh are identical to those of TTX-sensitive Na+ channels, only one amino acid in SS2 of repeat I of nRNaCh is different. nRNaCh have nonaromatic amino acid (Ala) in this site instead of the aromatic amino acid in a case of TTX-sensitive Na+ channels. Many studies suggested that the differences of TTX sensitivity of Na+ channels are decided by whether amino acid in this site is aromatic or not. Therefore nRNaCh acquire their TTX resistivity with the same mechanism as TTX-resistant cardiac Na+ channels do.

Pore residues critical for mu-CTX binding to rat skeletal muscle Na+ channels revealed by cysteine mutagenesis

We have studied mu-conotoxin (mu-CTX) block of rat skeletal muscle sodium channel (rSkM1) currents in which single amino acids within the pore (P-loop) were substituted with cysteine. Among 17 cysteine mutants expressed in Xenopus oocytes, 7 showed significant alterations in sensitivity to mu-CTX compared to wild-type rSkM1 channel (IC50 = 17.5 +/- 2.8 nM). E758C and D1241C were less sensitive to mu-CTX block (IC50 = 220 +/- 39 nM and 112 +/- 24 nM, respectively), whereas the tryptophan mutants W402C, W1239C, and W1531C showed enhanced mu-CTX sensitivity (IC50 = 1.9 +/- 0.1, 4.9 +/- 0.9, and 5.5 +/- 0.4 nM, respectively). D400C and Y401C also showed statistically significant yet modest (approximately twofold) changes in sensitivity to mu-CTX block compared to WT (p < 0.05). Application of the negatively charged, sulfhydryl-reactive compound methanethiosulfonate-ethylsulfonate (MTSES) enhanced the toxin sensitivity of D1241C (IC50 = 46.3 +/- 12 nM) while having little effect on E758C mutant channels (IC50 = 199.8 +/- 21.8 nM). On the other hand, the positively charged methanethiosulfonate-ethylammonium (MTSEA) completely abolished the mu-CTX sensitivity of E758C (IC50 > 1 microM) and increased the IC50 of D1241C by about threefold. Applications of MTSEA, MTSES, and the neutral MTSBN (benzyl methanethiosulfonate) to the tryptophan-to-cysteine mutants partially or fully restored the wild-type mu-CTX sensitivity, suggesting that the bulkiness of the tryptophan's indole group is a determinant of toxin binding. In support of this suggestion, the blocking IC50 of W1531A (7.5 +/- 1.3 nM) was similar to W1531C, whereas W1531Y showed reduced toxin sensitivity (14.6 +/- 3.5 nM) similar to that of the wild-type channel. Our results demonstrate that charge at positions 758 and 1241 are important for mu-CTX toxin binding and further suggest that the tryptophan residues within the pore in domains I, III, and IV negatively influence toxin-channel interaction.

A single serine residue confers tetrodotoxin insensitivity on the rat sensory-neuron-specific sodium channel SNS

Sensory neurons express a sodium channel (SNS) that is highly resistant to block by tetrodotoxin (IC50 = 60 microM). SNS is 65% homologous to the cardiac sodium channel, in which a single hydrophilic residue in the SS2 segment is critical for tetrodotoxin resistance. By site-directed mutagenesis, we have substituted phenylalanine for serine at the equivalent position in SNS: this mutated (S356F) SNS channel is functionally similar to wild-type SNS when expressed in Xenopus oocytes, but is potently blocked by tetrodotoxin and saxitoxin with IC50s of 2.8 nM and 8.2 nM, respectively. These data provide clues to the rational design of selective blockers of SNS with potential as analgesic drugs.

Solution structure of the sodium channel antagonist conotoxin GS: a new molecular caliper for probing sodium channel geometry

BACKGROUND

The venoms of Conus snails contain small, disulfide-rich inhibitors of voltage-dependent sodium channels. Conotoxin GS is a 34-residue polypeptide isolated from Conus geographus that interacts with the extracellular entrance of skeletal muscle sodium channels to prevent sodium ion conduction. Although conotoxin GS binds competitively with mu conotoxin GIIIA to the sodium channel surface, the two toxin types have little sequence identity with one another, and conotoxin GS has a four-loop structural framework rather than the characteristic three-loop mu-conotoxin framework. The structural study of conotoxin GS will form the basis for establishing a structure-activity relationship and understanding its interaction with the pore region of sodium channels.

RESULTS

The three-dimensional structure of conotoxin GS was determined using two-dimensional NMR spectroscopy. The protein exhibits a compact fold incorporating a beta hairpin and several turns. An unusual feature of conotoxin GS is the exceptionally high proportion (100%) of cis-imide bond geometry for the three proline or hydroxyproline residues. The structure of conotoxin GS bears little resemblance to the three-loop mu conotoxins, consistent with the low sequence identity between the two toxin types and their different structural framework. However, the tertiary structure and cystine-knot motif formed by the three disulfide bonds is similar to that present in several other polypeptide ion channel inhibitors.

CONCLUSIONS

This is the first three-dimensional structure of a 'four-loop' sodium channel inhibitor, and it represents a valuable new structural probe for the pore region of voltage-dependent sodium channels. The distribution of amino acid sidechains in the structure creates several polar and charged patches, and comparison with the mu conotoxins provides a basis for determining the binding surface of the conotoxin GS polypeptide.

Modulation of the human cardiac sodium channel alpha-subunit by cAMP-dependent protein kinase and the responsible sequence domain

1. In order to investigate the modulation of human hH1 sodium channel alpha-subunits by cAMP-dependent protein kinase (PKA), the channel was expressed in oocytes of Xenopus laevis. 2. Cytosolic injection of cAMP, as well as of SP-cyclic 3',5'-hydrogen phosphorothioate adenosine triethylammonium salt (SP-cAMPS, the S-diastereoisomeric configuration of the compound with respect to the phosphorus atom), resulted in a marked and significant increase in peak sodium current (INa,p). Cytosolic injections of RP-cyclic 3',5'-hydrogen phosphorothioate adenosine triethylammonium salt (RP-cAMPS; a compound inhibitory to PKA) had no effect on peak current. 3. Kinetic parameters of steady-state activation, inactivation and recovery from inactivation were unchanged following stimulation of PKA activity, but a 42 +/- 5% (mean +/- S.E.M.) increase in maximal sodium conductance (delta gmax) could account for the observed increase in INa,p. 4. A set of chimerical sodium channels made from portions of the human cardiac hH1 alpha-subunit and the rat skeletal muscle SkM1 alpha-subunit (which is not affected by PKA stimulation) was generated. These were used to localize the structural determinant in the hH1 sequence responsible for PKA modulation of hH1. From our data we conclude that the effects of PKA on hH1 are conferred by the large cytosolic loop interconnecting transmembrane domains I and II, which is not conserved among sodium channel subtypes.

T-type Ca2+ channels and alpha1E expression in spermatogenic cells, and their possible relevance to the sperm acrosome reaction

There is pharmacological evidence that Ca2+ channels play an essential role in triggering the mammalian sperm acrosome reaction, an exocytotic process required for sperm to fertilize the egg. Spermatozoa are small terminally differentiated cells that are difficult to study by conventional electrophysiological techniques. To identify the members of the voltage-dependent Ca2+ channel family possibly present in sperm, we have looked for the expression of the alpha 1A, alpha 1B, alpha 1C, alpha 1D and alpha 1E genes in mouse testis and in purified spermatogenic cell populations with RT-PCR. Our results indicate that all 5 genes are expressed in mouse testis, and in contrast only alpha 1E, and to a minor extent alpha 1A, are expressed in spermatogenic cells. In agreement with these findings, only T-type Ca2+ channels sensitive to the dihydropyridine nifedipine were observed in patch-clamp recordings of pachytene spermatocytes. These results suggest that low-threshold Ca2+ channels are the dihydropyridine-sensitive channels involved in the sperm acrosome reaction.

Structure and function of a novel voltage-gated, tetrodotoxin-resistant sodium channel specific to sensory neurons

Small neurons of the dorsal root ganglia (DRG) are known to play an important role in nociceptive mechanisms. These neurons express two types of sodium current, which differ in their inactivation kinetics and sensitivity to tetrodotoxin. Here, we report the cloning of the alpha-subunit of a novel, voltage-gated sodium channel (PN3) from rat DRG. Functional expression in Xenopus oocytes showed that PN3 is a voltage-gated sodium channel with a depolarized activation potential, slow inactivation kinetics, and resistance to high concentrations of tetrodotoxin. In situ hybridization to rat DRG indicated that PN3 is expressed primarily in small sensory neurons of the peripheral nervous system.

Structural and functional comparison of toxins from the venom of the scorpions Centruroides infamatus infamatus, Centruroides limpidus limpidus and Centruroides noxius

Two novel toxins containing 66 amino acid residues each were isolated from the venom of the scorpions Centruroides infamatus infamatus and Centruroides limpidus limpidus, respectively. Their full amino acid sequences were determined. Comparison of primary structures showed that they share 97% similarity among themselves and 83% to that of toxin 2 from Centruroides noxius. The three toxins studied compete with each other for the same binding sites on membranes prepared from rat brain synaptosomes, suggesting that they are all beta-scorpion toxins. Toxin action was assayed into the microI-2 rat skeletal muscle Na+ channel heterologously expressed into Xenopus oocytes. All three toxins block this Na+ channel in a similar fashion, without affecting inactivation, and showed IC50 values in the micromolar concentration range.