Block of tetrodotoxin-resistant Na+ channel pore by multivalent cations: gating modification and Na+ flow dependence

Prefrontal cortex pyramidal neurons express functional Nav1.8 tetrodotoxin-resistant sodium currents

It has been repeatedly proved that Nav1.8 tetrodotoxin (TTX)-resistant sodium currents are expressed in peripheral sensory neurons where they play important role in nociception. There are very few publications that show the presence of TTX-resistant sodium currents in central neurons. The aim of this study was to assess if functional Nav1.8 TTX-resistant sodium currents are expressed in prefrontal cortex pyramidal neurons. All recordings were performed in the presence of TTX in the extracellular solution to block TTX-sensitive sodium currents. The TTX-resistant sodium current recorded in this study was mainly carried by the Nav1.8 sodium channel isoform because the Nav1.9 current was inhibited by the -65 mV holding potential that we used throughout the study. Moreover, the sodium current that we recorded was inhibited by treatment with the selective Nav1.8 inhibitor A-803467. Confocal microscopy experiments confirmed the presence of the Nav1.8 α subunit in prefrontal cortex pyramidal neurons. Activation and steady state inactivation properties of TTX-resistant sodium currents were also assessed in this study and they were similar to activation and inactivation properties of TTX-resistant sodium currents expressed in dorsal root ganglia (DRG) neurons. Moreover, this study showed that carbamazepine (60 µM) inhibited the maximal amplitude of the TTX-resistant sodium current. Furthermore, we found that carbamazepine shifts steady state inactivation curve of TTX-resistant sodium currents toward hyperpolarization. This study suggests that the Nav1.8 TTX-resistant sodium channel is expressed not only in DRG neurons, but also in cortical neurons and may be molecular target for antiepileptic drugs such as carbamazepine.

Cationic Modulation of Voltage-Gated Sodium Channel (Nav1.5): Neonatal Versus Adult Splice Variants-2. Divalent (Cd2+) and Trivalent (Gd3+) Ions

Background: A "neonatal" splice-form of the voltage-gated sodium channel Nav1.5 is functionally expressed in human cancers and potentiates metastatic cell behaviors. Splicing causes the replacement of 7 amino acids, including a negatively charged aspartate211 in the "adult" Nav1.5 (aNav1.5) to a positively charged lysine in the "neonatal" (nNav1.5). These changes occur in the region surrounding the DI:S3-S4 extracellular linker. The splice variants respond differently to changes in extracellular H+ and this could be of pathophysiological significance. However, how the two differentially charged splice variants would react to cations of higher valency is not known. Materials and Methods: We used patch-clamp recording to compare the electrophysiological effects of Cd2+ and Gd3+ on "adult" and "neonatal" Nav1.5 expressed stably in EBNA-293 cells. Several parameters were determined for the two channels and statistically compared. Results: Both cations inhibited peak I Na through reducing G max and induced a positive shift in the voltage range of activation. However, unlike Gd3+, Cd2+ had only a weak effect on voltage dependence of activation, and no effect on voltage dependence of inactivation, recovery from inactivation, or the kinetics of activation/inactivation. Conclusions: The electrophysiological effects of Cd2+ and Gd3+ studied were essentially the same for "neonatal" and "adult" Nav1.5, although these splice variants possess differences in their external charges. In contrast, the effects of H+ were shown earlier to be significantly differential. Taken together, these results suggest that limited adjustment of the charged structure of pharmacological agents could enable selective targeting of neonatal Nav1.5 associated with several cancers.

Steady-State Activation and Modulation of the Concatemeric α1β2γ2L GABAA Receptor

The two-state coagonist model has been successfully used to analyze and predict peak current responses of the γ-aminobutyric acid type A (GABA_A) receptor. The goal of the present study was to provide a model-based description of GABA_A receptor activity under steady-state conditions after desensitization has occurred. We describe the derivation and properties of the cyclic three-state resting-active-desensitized (RAD) model. The relationship of the model to receptor behavior was tested using concatemeric α1β2γ2 GABA_A receptors expressed in Xenopus oocytes. The receptors were activated by the orthosteric agonists GABA or β-alanine, the allosteric agonist propofol, or combinations of GABA, propofol, pentobarbital, and the steroid allopregnanolone, and the observed steady-state responses were compared with those predicted by the model. A modified RAD model was employed to analyze and describe the actions on steady-state current of the inhibitory steroid pregnenolone sulfate. The findings indicate that the steady-state activity in the presence of multiple active agents that interact with distinct binding sites follows standard energetic additivity. The derived equations enable prediction of peak and steady-state activity in the presence of orthosteric and allosteric agonists, and the inhibitory steroid pregnenolone sulfate. SIGNIFICANCE STATEMENT: The study describes derivation and properties of a three-state resting-active-desensitized model. The model and associated equations can be used to analyze and predict peak and steady-state activity in the presence of one or more active agents.

Temperature Dependence of the Biophysical Mechanisms Underlying the Inhibition and Enhancement Effect of Amiodarone on hERG Channels

hERG K⁺ channel is important for controlling the duration of cardiac action potentials. Amiodarone (AMD), a widely prescribed class III antiarrhythmic, could inhibit hERG currents with relatively few tachyarrhythmic adverse events. We use injected Xenopus oocyte with two-electrode voltage clamp techniques to characterize the action of AMD on hERG channels. We found that AMD binds to the resting hERG channel with an apparent dissociation constant of ∼1.4 μM, and inhibits hERG currents at mild and strong depolarization pulses by slowing activation and enhancing inactivation, respectively, at 22°C. The activation kinetics of hERG channel activation are much faster, but inactivation kinetics are slower at 37°C. AMD accordingly has a 15% to 20% weaker and stronger inhibitory effect at mild and strong depolarization (e.g., -60 and +30 mV, 0.3-second pulse), respectively. In the meanwhile, the resurgent hERG tail currents are dose-dependently inhibited by AMD without altering the kinetics of current decay at both 22°C and 37°C, indicating facilitation of recovery from inactivation via the silent route. Most importantly, AMD no longer inhibits but enhances hERG currents at a mild pulse shortly after a prepulse at 37°C, but not so much at 22°C. We conclude that AMD is an effective hERG channel-gating modifier capable of lengthening the plateau phase of cardiac action potential (without increasing the chance of afterdepolarization). AMD, however, should be used with caution in hypothermia or the other scenarios that slow hERG channel activation. SIGNIFICANCE STATEMENT: It is known that amiodarone (AMD) acts on hERG K⁺ channels to treat cardiac arrhythmias with relatively little arrhythmogenicity. We found that AMD enhances hERG channel inactivation but slows activation as well as recovery from inactivation, and thus has a differential inhibition and enhancement effect on hERG currents at different phases of membrane voltage changes, especially at 37°C, but not so much at 22°C. AMD is therefore a relatively ideal agent against tachyarrhythmia at 37°C, but should be more cautiously used at lower temperatures or relevant pathophysiological/pharmacological scenarios associated with slower hERG channel activation because of the increased chances of adverse events.

Zinc as a Neuromodulator in the Central Nervous System with a Focus on the Olfactory Bulb

The olfactory bulb (OB) is central to the sense of smell, as it is the site of the first synaptic relay involved in the processing of odor information. Odor sensations are first transduced by olfactory sensory neurons (OSNs) before being transmitted, by way of the OB, to higher olfactory centers that mediate olfactory discrimination and perception. Zinc is a common trace element, and it is highly concentrated in the synaptic vesicles of subsets of glutamatergic neurons in some brain regions including the hippocampus and OB. In addition, zinc is contained in the synaptic vesicles of some glycinergic and GABAergic neurons. Thus, zinc released from synaptic vesicles is available to modulate synaptic transmission mediated by excitatory (e.g., N-methyl-D aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)) and inhibitory (e.g., gamma-aminobutyric acid (GABA), glycine) amino acid receptors. Furthermore, extracellular zinc can alter the excitability of neurons through effects on a variety of voltage-gated ion channels. Consistent with the notion that zinc acts as a regulator of neuronal activity, we and others have shown zinc modulation (inhibition and/or potentiation) of amino acid receptors and voltage-gated ion channels expressed by OB neurons. This review summarizes the locations and release of vesicular zinc in the central nervous system (CNS), including in the OB. It also summarizes the effects of zinc on various amino acid receptors and ion channels involved in regulating synaptic transmission and neuronal excitability, with a special emphasis on the actions of zinc as a neuromodulator in the OB. An understanding of how neuroactive substances such as zinc modulate receptors and ion channels expressed by OB neurons will increase our understanding of the roles that synaptic circuits in the OB play in odor information processing and transmission.

Sodium Channels and Venom Peptide Pharmacology

Venomous animals including cone snails, spiders, scorpions, anemones, and snakes have evolved a myriad of components in their venoms that target the opening and/or closing of voltage-gated sodium channels to cause devastating effects on the neuromuscular systems of predators and prey. These venom peptides, through design and serendipity, have not only contributed significantly to our understanding of sodium channel pharmacology and structure, but they also represent some of the most phyla- and isoform-selective molecules that are useful as valuable tool compounds and drug leads. Here, we review our understanding of the basic function of mammalian voltage-gated sodium channel isoforms as well as the pharmacology of venom peptides that act at these key transmembrane proteins.

The Biophysical Basis Underlying Gating Changes in the p.V1316A Mutant Nav1.7 Channel and the Molecular Pathogenesis of Inherited Erythromelalgia

The Na_v1.7 channel critically contributes to the excitability of sensory neurons, and gain-of-function mutations of this channel have been shown to cause inherited erythromelalgia (IEM) with neuropathic pain. In this study, we report a case of a severe phenotype of IEM caused by p.V1316A mutation in the Na_v1.7 channel. Mechanistically, we first demonstrate that the Na_vβ4 peptide acts as a gating modifier rather than an open channel blocker competing with the inactivating peptide to give rise to resurgent currents in the Na_v1.7 channel. Moreover, there are two distinct open and two corresponding fast inactivated states in the genesis of resurgent Na⁺ currents. One is responsible for the resurgent route and practically existent only in the presence of Na_vβ4 peptide, whereas the other is responsible for the “silent” route of recovery from inactivation. In this regard, the p.V1316A mutation makes hyperpolarization shift in the activation curve, and depolarization shift in the inactivation curve, vividly uncoupling inactivation from activation. In terms of molecular gating operation, the most important changes caused by the p.V1316A mutation are both acceleration of the transition from the inactivated states to the activated states and deceleration of the reverse transition, resulting in much larger sustained as well as resurgent Na⁺ currents. In summary, the genesis of the resurgent currents in the Na_v1.7 channel is ascribable to the transient existence of a distinct and novel open state promoted by the Na_vβ4 peptide. In addition, S4–5 linker in domain III where V1316 is located seems to play a critical role in activation–inactivation coupling, chiefly via direct modulation of the transitional kinetics between the open and the inactivated states. The sustained and resurgent Na⁺ currents may therefore be correlatively enhanced by specific mutations involving this linker and relevant regions, and thus marked hyperexcitability in corresponding neural tissues as well as IEM symptomatology.

Mutations in the Na_v1.7 sodium channel cause idiopathic erythromelalgia. This study shows that the pathogenic resurgent sodium currents arise via modification of gating behavior rather than via competing pore block by the Na_vβ4 peptide.

The gain-of-function mutation (p.V1316A) of the Na_v1.7 channel causes inherited erythromelalgia (IEM), a disease characterized by extremely enhanced activity in relevant neural tissues that results in neuropathic pain. We found that the p.V1316A mutation alters the basic gating properties of the channel, leading to increased sustained currents during membrane depolarization and resurgent currents during repolarization. Neurons expressing these mutant channels are more difficult to maintain in a hyperpolarized state and are thus more excitable. We demonstrate that there is very likely a distinct set of open/inactivated (O/I) states responsible for the genesis of resurgent currents. We show that the p.V1316A mutation chiefly accelerates the I to O transition in this set, but also decelerates the transitions between different sets of O/I states, to cause the channel gating and cellular excitability changes. Contrary to the conventional view, we find that the Na_vβ4 peptide, a key element responsible for sizable resurgent currents, does not seem to act as a pore blocker that competes with the inactivation peptide. Instead, we show that it acts as a gating modifier of the Na_v1.7 channel. Thus, the DIII/S4–5 linker, where V1316 is located, may play a critical role not only in O/I coupling but also in the couplings between different sets of O/I in the Na_v1.7 channel.

Zinc as Allosteric Ion Channel Modulator: Ionotropic Receptors as Metalloproteins

Zinc is an essential metal to life. This transition metal is a structural component of many proteins and is actively involved in the catalytic activity of cell enzymes. In either case, these zinc-containing proteins are metalloproteins. However, the amino acid residues that serve as ligands for metal coordination are not necessarily the same in structural proteins compared to enzymes. While crystals of structural proteins that bind zinc reveal a higher preference for cysteine sulfhydryls rather than histidine imidazole rings, catalytic enzymes reveal the opposite, i.e., a greater preference for the histidines over cysteines for catalysis, plus the influence of carboxylic acids. Based on this paradigm, we reviewed the putative ligands of zinc in ionotropic receptors, where zinc has been described as an allosteric modulator of channel receptors. Although these receptors do not strictly qualify as metalloproteins since they do not normally bind zinc in structural domains, they do transitorily bind zinc at allosteric sites, modifying transiently the receptor channel's ion permeability. The present contribution summarizes current information showing that zinc allosteric modulation of receptor channels occurs by the preferential metal coordination to imidazole rings as well as to the sulfhydryl groups of cysteine in addition to the carboxyl group of acid residues, as with enzymes and catalysis. It is remarkable that most channels, either voltage-sensitive or transmitter-gated receptor channels, are susceptible to zinc modulation either as positive or negative regulators.

Genetic alteration of the metal/redox modulation of Cav3.2 T-type calcium channel reveals its role in neuronal excitability

KEY POINTS

In this study, we describe a new knock-in (KI) mouse model that allows the study of the H191-dependent regulation of T-type Cav3.2 channels. Sensitivity to zinc, nickel and ascorbate of native Cav3.2 channels is significantly impeded in the dorsal root ganglion (DRG) neurons of this KI mouse. Importantly, we describe that this H191-dependent regulation has discrete but significant effects on the excitability properties of D-hair (down-hair) cells, a sub-population of DRG neurons in which Cav3.2 currents prominently regulate excitability. Overall, this study reveals that the native H191-dependent regulation of Cav3.2 channels plays a role in the excitability of Cav3.2-expressing neurons. This animal model will be valuable in addressing the potential in vivo roles of the trace metal and redox modulation of Cav3.2 T-type channels in a wide range of physiological and pathological conditions.

ABSTRACT

Cav3.2 channels are T-type voltage-gated calcium channels that play important roles in controlling neuronal excitability, particularly in dorsal root ganglion (DRG) neurons where they are involved in touch and pain signalling. Cav3.2 channels are modulated by low concentrations of metal ions (nickel, zinc) and redox agents, which involves the histidine 191 (H191) in the channel's extracellular IS3-IS4 loop. It is hypothesized that this metal/redox modulation would contribute to the tuning of the excitability properties of DRG neurons. However, the precise role of this H191-dependent modulation of Cav3.2 channel remains unresolved. Towards this goal, we have generated a knock-in (KI) mouse carrying the mutation H191Q in the Cav3.2 protein. Electrophysiological studies were performed on a subpopulation of DRG neurons, the D-hair cells, which express large Cav3.2 currents. We describe an impaired sensitivity to zinc, nickel and ascorbate of the T-type current in D-hair neurons from KI mice. Analysis of the action potential and low-threshold calcium spike (LTCS) properties revealed that, contrary to that observed in WT D-hair neurons, a low concentration of zinc and nickel is unable to modulate (1) the rheobase threshold current, (2) the afterdepolarization amplitude, (3) the threshold potential necessary to trigger an LTCS or (4) the LTCS amplitude in D-hair neurons from KI mice. Together, our data demonstrate that this H191-dependent metal/redox regulation of Cav3.2 channels can tune neuronal excitability. This study validates the use of this Cav3.2-H191Q mouse model for further investigations of the physiological roles thought to rely on this Cav3.2 modulation.

Role of protein dynamics in ion selectivity and allosteric coupling in the NaK channel

Flux-dependent inactivation that arises from functional coupling between the inner gate and the selectivity filter is widespread in ion channels. The structural basis of this coupling has only been well characterized in KcsA. Here we present NMR data demonstrating structural and dynamic coupling between the selectivity filter and intracellular constriction point in the bacterial nonselective cation channel, NaK. This transmembrane allosteric communication must be structurally different from KcsA because the NaK selectivity filter does not collapse under low-cation conditions. Comparison of NMR spectra of the nonselective NaK and potassium-selective NaK2K indicates that the number of ion binding sites in the selectivity filter shifts the equilibrium distribution of structural states throughout the channel. This finding was unexpected given the nearly identical crystal structure of NaK and NaK2K outside the immediate vicinity of the selectivity filter. Our results highlight the tight structural and dynamic coupling between the selectivity filter and the channel scaffold, which has significant implications for channel function. NaK offers a distinct model to study the physiologically essential connection between ion conduction and channel gating.

The direct modulatory activity of zinc toward ion channels

The divalent zinc ion is a cation that plays an indispensable role as a structural constituent of numerous proteins, including enzymes and transcription factors. Recently, it has been suggested that zinc also plays a dynamic role in extracellular and intracellular signaling as well. Ion channels are pore-forming proteins that control the flow of specific ions across the membrane, which is important to maintain ion gradients. In this review, we outline the modulatory effect of zinc on the activities of several ion channels through direct binding of zinc into histidine, cysteine, aspartate, and glutamate moieties of channel proteins. The binding of zinc to ion channels results in the activation or inhibition of the channel due to conformational changes. These novel aspects of ion-channel activity modulation by zinc provide new insights into the physiological regulation of ion channels.

Voltage-gated calcium channels: Determinants of channel function and modulation by inorganic cations

Voltage-gated calcium channels (VGCCs) represent a key link between electrical signals and non-electrical processes, such as contraction, secretion and transcription. Evolved to achieve high rates of Ca(2+)-selective flux, they possess an elaborate mechanism for selection of Ca(2+) over foreign ions. It has been convincingly linked to competitive binding in the pore, but the fundamental question of how this is reconcilable with high rates of Ca(2+) transfer remains unanswered. By virtue of their similarity to Ca(2+), polyvalent cations can interfere with the function of VGCCs and have proven instrumental in probing the mechanisms underlying selective permeation. Recent emergence of crystallographic data on a set of Ca(2+)-selective model channels provides a structural framework for permeation in VGCCs, and warrants a reconsideration of their diverse modulation by polyvalent cations, which can be roughly separated into three general mechanisms: (I) long-range interactions with charged regions on the surface, affecting the local potential sensed by the channel or influencing voltage-sensor movement by repulsive forces (electrostatic effects), (II) short-range interactions with sites in the ion-conducting pathway, leading to physical obstruction of the channel (pore block), and in some cases (III) short-range interactions with extracellular binding sites, leading to non-electrostatic modifications of channel gating (allosteric effects). These effects, together with the underlying molecular modifications, provide valuable insights into the function of VGCCs, and have important physiological and pathophysiological implications. Allosteric suppression of some of the pore-forming Cavα1-subunits (Cav2.3, Cav3.2) by Zn(2+) and Cu(2+) may play a major role for the regulation of excitability by endogenous transition metal ions. The fact that these ions can often traverse VGCCs can contribute to the detrimental intracellular accumulation of metal ions following excessive release of endogenous Cu(2+) and Zn(2+) or exposure to non-physiological toxic metal ions.

Zinc inactivates melastatin transient receptor potential 2 channels via the outer pore

Zinc ion (Zn(2+)) is an endogenous allosteric modulator that regulates the activity of a wide variety of ion channels in a reversible and concentration-dependent fashion. Here we used patch clamp recording to study the effects of Zn(2+) on the melastatin transient receptor potential 2 (TRPM2) channel. Zn(2+) inhibited the human (h) TRPM2 channel currents, and the steady-state inhibition was largely not reversed upon washout and concentration-independent in the range of 30-1000 μM, suggesting that Zn(2+) induces channel inactivation. Zn(2+) inactivated the channels fully when they conducted inward currents, but only by half when they passed outward currents, indicating profound influence of the permeant ion on Zn(2+) inactivation. Alanine substitution scanning mutagenesis of 20 Zn(2+)-interacting candidate residues in the outer pore region of the hTRPM2 channel showed that mutation of Lys(952) in the extracellular end of the fifth transmembrane segment and Asp(1002) in the large turret strongly attenuated or abolished Zn(2+) inactivation, and mutation of several other residues dramatically changed the inactivation kinetics. The mouse (m) TRPM2 channels were also inactivated by Zn(2+), but the kinetics were remarkably slower. Reciprocal mutation of His(995) in the hTRPM2 channel and the equivalent Gln(992) in the mTRPM2 channel completely swapped the kinetics, but no such opposing effects resulted from exchanging another pair of species-specific residues, Arg(961)/Ser(958). We conclude from these results that Zn(2+) inactivates the TRPM2 channels and that residues in the outer pore are critical determinants of the inactivation.

Ionic flow enhances low-affinity binding: a revised mechanistic view into Mg2+ block of NMDA receptors

The N-methyl-d-aspartate receptor (NMDAR) channel is one of the major excitatory amino acid receptors in the mammalian brain. Since external Mg(2+) blocks the channel in an apparently voltage-dependent fashion, this ligand-gated channel displays intriguing voltage-dependent control of Na(+) and Ca(2+) permeability and thus plays an important role in synaptic physiology. We found that the essential features of Mg(2+) block could not be solely envisaged by binding of a charged blocker in the membrane electric field. Instead, the blocking effect of Mg(2+) is critically regulated by, and quantitatively correlated with, the relative tendency of outward and inward ionic fluxes. The 'intrinsic' affinity of Mg(2+) to the binding sites, however, is low (in the millimolar range) in the absence of net ionic flow at 0 mV. Besides, extracellular and intracellular Mg(2+) blocks the channel at distinct sites of electrical distances 0.7 and 0.95 from the outside, respectively. The two sites are separated by a high energy barrier for the movement of Mg(2+) (but not Na(+) or the other ions), and functionally speaking, each could accommodate 1.1 and 0.8 coexisting permeating ions, respectively. Mg(2+) block of the ionic flow thus is greatly facilitated by the flux-coupling effect or the ionic flow (the preponderant direction of permeant ion movement) per se, as if the poorly permeable Mg(2+) is 'pushed' against a high energy barrier by the otherwise permeating ions. Extracellular and intracellular Mg(2+) block then is in essence 'use dependent', more strongly inhibiting both Na(+) and Ca(2+) fluxes with stronger tendencies of influx and efflux, respectively. In conclusion, although permeant ions themselves could compete with Mg(2+), the flow or the tendency of movement of the permeant ions may actually enhance rather than interfere with Mg(2+) block, making the unique current-voltage relationship of NMDAR and the molecular basis of many important neurobiological phenomena.

Differential effects of Tityus bahiensis scorpion venom on tetrodotoxin-sensitive and tetrodotoxin-resistant sodium currents

We examined modification of sodium channel gating by Tityus bahiensis scorpion venom (TbScV), and compared effects on native tetrodotoxin-sensitive and tetrodotoxin-resistant sodium currents from rat dorsal root ganglion neurons and cardiac myocytes. In neurons, TbScV dramatically reduced the rate of sodium current inactivation, increased current amplitude, and caused a negative shift in the voltage-dependence of activation and inactivation of tetrodotoxin-sensitive channels. Enhanced activation of modified sodium channels was independent of a depolarizing prepulse. We identified two components of neuronal tetrodotoxin-resistant current with biophysical properties similar to those described for NaV1.8 and NaV1.9. In contrast to its effects on neuronal tetrodotoxin-sensitive current, TbScV caused a small decrease in neuronal tetrodotoxin-resistant sodium current amplitude and the gating modifications described above were absent. A third tetrodotoxin-resistant current, NaV1.5 recorded in rat cardiac ventricular myocytes, was inhibited approximately 50% by TbScV, and the remaining current exhibited markedly slowed activation and inactivation. In conclusion, TbScV has very different effects on different sodium channel isoforms. Among the neuronal types, currents resistant to tetrodotoxin are also resistant to gating modification by TbScV. The cardiac tetrodotoxin-resistant current has complex sensitivity that includes both inhibition of current amplitude and slowing of activation and inactivation.

The external pore loop interacts with S6 and S3-S4 linker in domain 4 to assume an essential role in gating control and anticonvulsant action in the Na(+) channel

Carbamazepine, phenytoin, and lamotrigine are widely prescribed anticonvulsants in neurological clinics. These drugs bind to the same receptor site, probably with the diphenyl motif in their structure, to inhibit the Na⁺ channel. However, the location of the drug receptor remains controversial. In this study, we demonstrate close proximity and potential interaction between an external aromatic residue (W1716 in the external pore loop) and an internal aromatic residue (F1764 in the pore-lining part of the sixth transmembrane segment, S6) of domain 4 (D4), both being closely related to anticonvulsant and/or local anesthetic binding to the Na⁺ channel. Double-mutant cycle analysis reveals significant cooperativity between the two phenyl residues for anticonvulsant binding. Concomitant F1764C mutation evidently decreases the susceptibility of W1716C to external Cd²⁺ and membrane-impermeable methanethiosulfonate reagents. Also, the W1716E/F1764R and G1715E/F1764R double mutations significantly alter the selectivity for Na⁺ over K⁺ and markedly shift the activation curve, respectively. W1716 and F1764 therefore very likely form a link connecting the outer and inner compartments of the Na⁺ channel pore (in addition to the selectivity filter). Anticonvulsants and local anesthetics may well traverse this “S6 recess” without trespassing on the selectivity filter. Furthermore, we found that Y1618K, a point mutation in the S3-4 linker (the extracellular extension of D4S4), significantly alters the consequences of carbamazepine binding to the Na⁺ channel. The effect of Y1618K mutation, however, is abolished by concomitant point mutations in the vicinity of Y1618, but not by those in the internally located inactivation machinery, supporting a direct local rather than a long-range allosteric action. Moreover, Y1618 could interact with D4 pore residues W1716 and L1719 to have a profound effect on both channel gating and anticonvulsant action. We conclude that there are direct interactions among the external S3-4 linker, the external pore loop, and the internal S6 segment in D4, making the external pore loop a pivotal point critically coordinating ion permeation, gating, and anticonvulsant binding in the Na⁺ channel.

A fast Na+/Ca2+-based action potential in a marine diatom

BACKGROUND

Electrical impulses in animals play essential roles in co-ordinating an array of physiological functions including movement, secretion, environmental sensing and development. Underpinning many of these electrical signals is a fast Na+-based action potential that has been fully characterised only in cells associated with the neuromuscular systems of multicellular animals. Such rapid action potentials are thought to have evolved with the first metazoans, with cnidarians being the earliest representatives. The present study demonstrates that a unicellular protist, the marine diatom Odontella sinensis, can also generate a fast Na+/Ca2+ based action potential that has remarkably similar biophysical and pharmacological properties to invertebrates and vertebrate cardiac and skeletal muscle cells.

METHODOLOGY/PRINCIPAL FINDINGS

The kinetic, ionic and pharmacological properties of the rapid diatom action potential were examined using single electrode current and voltage clamp techniques. Overall, the characteristics of the fast diatom currents most closely resemble those of vertebrate and invertebrate muscle Na+/Ca2+ currents.

CONCLUSIONS/SIGNIFICANCE

This is the first demonstration of voltage-activated Na+ channels and the capacity to generate fast Na+-based action potentials in a unicellular photosynthetic organism. The biophysical and pharmacological characteristics together with the presence of a voltage activated Na+/Ca2+ channel homologue in the recently sequenced genome of the diatom Thalassiosira pseudonana, provides direct evidence supporting the hypothesis that this rapid signalling mechanism arose in ancestral unicellular eukaryotes and has been retained in at least two phylogenetically distant lineages of eukaryotes; opisthokonts and the stramenopiles. The functional role of the fast animal-like action potential in diatoms remains to be elucidated but is likely involved in rapid environmental sensing of these widespread and successful marine protists.

Low concentrations of tetrodotoxin interact with tetrodotoxin-resistant voltage-gated sodium channels

BACKGROUND AND PURPOSE

Tetrodotoxin (TTX) is used to distinguish between two classes of voltage-gated sodium channel (VGSC)--TTX sensitive (TTXS) and TTX resistant (TTXR). The resistance of TTXR VGSCs is thought to result from a low binding affinity of TTX, although at high TTX concentrations channel block does occur. Here, we show that, at concentrations below those which produce block, TTX can bind to TTXR VGSCs.

EXPERIMENTAL APPROACH

Whole-cell voltage clamp recordings were made from dissociated rat dorsal root ganglion neurones that expressed both TTXS and TTXR sodium currents. Voltage-gated calcium currents were blocked by 10 microM extracellular lanthanum chloride. TTXS, but not TTXR, current was suppressed by using a holding potential of -50 mV, and the effect of TTX on the isolated TTXR current was explored.

KEY RESULTS

Extracellular application of 0.5 microM TTX produced a 40% increase in TTXR current amplitude, a negative shift in the voltage-dependence of current activation (approximately -8 mV) and inactivation (approximately -10 mV) and increased rates of current activation and inactivation. The effect of TTX on current amplitude was dose-dependent (EC50 = 364 nM). Removal of lanthanum prevented the effect of TTX on TTXR current amplitude, whereas reducing extracellular calcium did not.

CONCLUSIONS AND IMPLICATIONS

The findings are consistent with an interpretation that TTX relieves a tonic block of the TTXR VGSC by lanthanum. We conclude that TTX binds to the TTXR VGSC at low concentrations, without blocking it. This appears to be the first demonstration of a clear distinction between binding affinity and blocking potency of a channel-blocking agent.

Ca2+-dependent inactivation of CaV1.2 channels prevents Gd3+ block: does Ca2+ block the pore of inactivated channels?

Lanthanide gadolinium (Gd(3+)) blocks Ca(V)1.2 channels at the selectivity filter. Here we investigated whether Gd(3+) block interferes with Ca(2+)-dependent inactivation, which requires Ca(2+) entry through the same site. Using brief pulses to 200 mV that relieve Gd(3+) block but not inactivation, we monitored how the proportions of open and open-blocked channels change during inactivation. We found that blocked channels inactivate much less. This is expected for Gd(3+) block of the Ca(2+) influx that enhances inactivation. However, we also found that the extent of Gd(3+) block did not change when inactivation was reduced by abolition of Ca(2+)/calmodulin interaction, showing that Gd(3+) does not block the inactivated channel. Thus, Gd(3+) block and inactivation are mutually exclusive, suggesting action at a common site. These observations suggest that inactivation causes a change at the selectivity filter that either hides the Gd(3+) site or reduces its affinity, or that Ca(2+) occupies the binding site at the selectivity filter in inactivated channels. The latter possibility is supported by previous findings that the EEQE mutation of the selectivity EEEE locus is void of Ca(2+)-dependent inactivation (Zong Z.Q., J.Y. Zhou, and T. Tanabe. 1994. Biochem. Biophys. Res. Commun. 201:1117-11123), and that Ca(2+)-inactivated channels conduct Na(+) when Ca(2+) is removed from the extracellular medium (Babich O., D. Isaev, and R. Shirokov. 2005. J. Physiol. 565:709-717). Based on these results, we propose that inactivation increases affinity of the selectivity filter for Ca(2+) so that Ca(2+) ion blocks the pore. A minimal model, in which the inactivation "gate" is an increase in affinity of the selectivity filter for permeating ions, successfully simulates the characteristic U-shaped voltage dependence of inactivation in Ca(2+).

Pharmacological dissection and distribution of NaN/Nav1.9, T-type Ca2+ currents, and mechanically activated cation currents in different populations of DRG neurons

Low voltage-activated (LVA) T-type Ca(2+) (I(Ca)T) and NaN/Nav1.9 currents regulate DRG neurons by setting the threshold for the action potential. Although alterations in these channels have been implicated in a variety of pathological pain states, their roles in processing sensory information remain poorly understood. Here, we carried out a detailed characterization of LVA currents in DRG neurons by using a method for better separation of NaN/Nav1.9 and I(Ca)T currents. NaN/Nav1.9 was inhibited by inorganic I(Ca) blockers as follows (IC(50), microM): La(3+) (46) > Cd(2+) (233) > Ni(2+) (892) and by mibefradil, a non-dihydropyridine I(Ca)T antagonist. Amiloride, however, a preferential Cav3.2 channel blocker, had no effects on NaN/Nav1.9 current. Using these discriminative tools, we showed that NaN/Nav1.9, Cav3.2, and amiloride- and Ni(2+)-resistant I(Ca)T (AR-I(Ca)T) contribute differentially to LVA currents in distinct sensory cell populations. NaN/Nav1.9 carried LVA currents into type-I (CI) and type-II (CII) small nociceptors and medium-Adelta-like nociceptive cells but not in low-threshold mechanoreceptors, including putative Down-hair (D-hair) and Aalpha/beta cells. Cav3.2 predominated in CII-nociceptors and in putative D-hair cells. AR-I(Ca)T was restricted to CII-nociceptors, putative D-hair cells, and Aalpha/beta-like cells. These cell types distinguished by their current-signature displayed different types of mechanosensitive channels. CI- and CII-nociceptors displayed amiloride-sensitive high-threshold mechanical currents with slow or no adaptation, respectively. Putative D-hair and Aalpha/beta-like cells had low-threshold mechanical currents, which were distinguished by their adapting kinetics and sensitivity to amiloride. Thus, subspecialized DRG cells express specific combinations of LVA and mechanosensitive channels, which are likely to play a key role in shaping responses of DRG neurons transmitting different sensory modalities.

Subunit-specific modulation of T-type calcium channels by zinc

Zinc (Zn2+) functions as a signalling molecule in the nervous system and modulates many ionic channels. In this study, we have explored the effects of Zn2+ on recombinant T-type calcium channels (CaV3.1, CaV3.2 and CaV3.3). Using tsA-201 cells, we demonstrate that CaV3.2 current (IC50, 0.8 microm) is significantly more sensitive to Zn2+ than are CaV3.1 and CaV3.3 currents (IC50, 80 microm and approximately 160 microm, respectively). This inhibition of CaV3 currents is associated with a shift to more negative membrane potentials of both steady-state inactivation for CaV3.1, CaV3.2 and CaV3.3 and steady-state activation for CaV3.1 and CaV3.3 currents. We also document changes in kinetics, especially a significant slowing of the inactivation kinetics for CaV3.1 and CaV3.3, but not for CaV3.2 currents. Notably, deactivation kinetics are significantly slowed for CaV3.3 current (approximately 100-fold), but not for CaV3.1 and CaV3.2 currents. Consequently, application of Zn2+ results in a significant increase in CaV3.3 current in action potential clamp experiments, while CaV3.1 and CaV3.2 currents are significantly reduced. In neuroblastoma NG 108-15 cells, the duration of CaV3.3-mediated action potentials is increased upon Zn2+ application, indicating further that Zn2+ behaves as a CaV3.3 channel opener. These results demonstrate that Zn2+ exhibits differential modulatory effects on T-type calcium channels, which may partly explain the complex features of Zn2+ modulation of the neuronal excitability in normal and disease states.

Sodium channel inactivation in heart: a novel role of the carboxy-terminal domain

Electrical activity in the heart depends critically on the interactions of multiple ion channels to coordinate the timing of excitation and contraction of the ventricles. Voltage-gated sodium channels underlie the rapid spread of impulses through the atria and ventricles, but the importance of sodium (Na(+)) channels to the control of the ventricular action potential has only most recently become apparent through the investigation of the relationship between mutation-induced clinical phenotypes and the altered function of mutant Na(+) channels linked to inherited arrhythmias. Investigation into the structural basis of disease-associated mutations of the cardiac Na(+) channel has led to the discovery of novel role of the Na(+) channel carboxy-terminal (CT) domain in controlling channel inactivation. Intramolecular interactions between the carboxy-terminal domain and an intracellular peptide loop that forms the inactivation gate are required to minimize channel reopening during prolonged depolarization. Disruption of this interaction leads to persistent sodium channel current, action potential prolongation, and elevated risk of cardiac arrhythmia.

Zinc and copper: pharmacological probes and endogenous modulators of neuronal excitability

As well as being key structural components of many proteins, increasing evidence suggests that zinc and copper ions function as signaling molecules in the nervous system and are released from the synaptic terminals of certain neurons. In this review, we consider the actions of these two ions on proteins that regulate neuronal excitability. In addition to the established actions of zinc, and to a lesser degree copper, on excitatory and inhibitory ligand-gated ion channels, we show that both ions have a number of actions on selected members of the voltage-gated-like ion channel superfamily. For example, zinc is a much more effective blocker of one subtype of tetrodotoxin (TTX)-insensitive sodium (Na+) channel (NaV1.5) than other Na+ channels, whereas a certain T-type calcium (Ca2+) channel subunit (CaV3.2) is particularly sensitive to zinc. For potassium (K+) channels, zinc can have profound effects on the gating of certain KV channels whereas zinc and copper have distinct actions on closely related members of the 2 pore domain potassium channel (K2P) channel family. In addition to direct actions on these proteins, zinc is able to permeate a number of membrane proteins such as (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptors, Ca2+ channels and some transient receptor potential (trp) channels. There are a number of important physiological and pathophysiological consequences of these many actions of zinc and copper on membrane proteins, in terms of regulation of neuronal excitability and neurotoxicity. Furthermore, the concentration of free zinc and copper either in the synaptic cleft or neuronal cytoplasm may contribute to the etiology of certain disease states such as Alzheimer's disease (AD) and epilepsy.

Sodium Channel Inactivation: Molecular Determinants and Modulation

Voltage-gated sodium channels open (activate) when the membrane is depolarized and close on repolarization (deactivate) but also on continuing depolarization by a process termed inactivation, which leaves the channel refractory, i.e., unable to open again for a period of time. In the “classical” fast inactivation, this time is of the millisecond range, but it can last much longer (up to seconds) in a different slow type of inactivation. These two types of inactivation have different mechanisms located in different parts of the channel molecule: the fast inactivation at the cytoplasmic pore opening which can be closed by a hinged lid, the slow inactivation in other parts involving conformational changes of the pore. Fast inactivation is highly vulnerable and affected by many chemical agents, toxins, and proteolytic enzymes but also by the presence of β-subunits of the channel molecule. Systematic studies of these modulating factors and of the effects of point mutations (experimental and in hereditary diseases) in the channel molecule have yielded a fairly consistent picture of the molecular background of fast inactivation, which for the slow inactivation is still lacking.

Y3+ block demonstrates an intracellular activation gate for the alpha1G T-type Ca2+ channel

Classical electrophysiology and contemporary crystallography suggest that the activation gate of voltage-dependent channels is on the intracellular side, but a more extracellular “pore gate” has also been proposed. We have used the voltage dependence of block by extracellular Y³⁺ as a tool to locate the activation gate of the α1G (Ca_V3.1) T-type calcium channel. Y³⁺ block exhibited no clear voltage dependence from −40 to +40 mV (50% block at 25 nM), but block was relieved rapidly by stronger depolarization. Reblock of the open channel, reflected in accelerated tail currents, was fast and concentration dependent. Closed channels were also blocked by Y³⁺ at a concentration-dependent rate, only eightfold slower than open-channel block. When extracellular Ca²⁺ was replaced with Ba²⁺, the rate of open block by Y³⁺ was unaffected, but closed block was threefold faster than in Ca²⁺, suggesting the slower closed-block rate reflects ion–ion interactions in the pore rather than an extracellularly located gate. Since an extracellular blocker can rapidly enter the closed pore, the primary activation gate must be on the intracellular side of the selectivity filter.

An inactivation stabilizer of the Na+ channel acts as an opportunistic pore blocker modulated by external Na+

The Na⁺ channel is the primary target of anticonvulsants carbamazepine, phenytoin, and lamotrigine. These drugs modify Na⁺ channel gating as they have much higher binding affinity to the inactivated state than to the resting state of the channel. It has been proposed that these drugs bind to the Na⁺ channel pore with a common diphenyl structural motif. Diclofenac is a widely prescribed anti-inflammatory agent that has a similar diphenyl motif in its structure. In this study, we found that diclofenac modifies Na⁺ channel gating in a way similar to the foregoing anticonvulsants. The dissociation constants of diclofenac binding to the resting, activated, and inactivated Na⁺ channels are ∼880 μM, ∼88 μM, and ∼7 μM, respectively. The changing affinity well depicts the gradual shaping of a use-dependent receptor along the gating process. Most interestingly, diclofenac does not show the pore-blocking effect of carbamazepine on the Na⁺ channel when the external solution contains 150 mM Na⁺, but is turned into an effective Na⁺ channel pore blocker if the extracellular solution contains no Na⁺. In contrast, internal Na⁺ has only negligible effect on the functional consequences of diclofenac binding. Diclofenac thus acts as an “opportunistic” pore blocker modulated by external but not internal Na⁺, indicating that the diclofenac binding site is located at the junction of a widened part and an acutely narrowed part of the ion conduction pathway, and faces the extracellular rather than the intracellular solution. The diclofenac binding site thus is most likely located at the external pore mouth, and undergoes delicate conformational changes modulated by external Na⁺ along the gating process of the Na⁺ channel.