Permeation of large tetra-alkylammonium cations through mutant and wild-type voltage-gated sodium channels as revealed by relief of block at high voltage

Anthrax Toxin: Model System for Studying Protein Translocation

Dedicated translocase channels are nanomachines that often, but not always, unfold and translocate proteins through narrow pores across the membrane. Generally, these molecular machines utilize external sources of free energy to drive these reactions, since folded proteins are thermodynamically stable, and once unfolded they contain immense diffusive configurational entropy. To catalyze unfolding and translocate the unfolded state at appreciable timescales, translocase channels often utilize analogous peptide-clamp active sites. Here we describe how anthrax toxin has been used as a biophysical model system to study protein translocation. The tripartite bacterial toxin is composed of an oligomeric translocase channel, protective antigen (PA), and two enzymes, edema factor (EF) and lethal factor (LF), which are translocated by PA into mammalian host cells. Unfolding and translocation are powered by the endosomal proton gradient and are catalyzed by three peptide-clamp sites in the PA channel: the α clamp, the ϕ clamp, and the charge clamp. These clamp sites interact nonspecifically with the chemically complex translocating chain, serve to minimize unfolded state configurational entropy, and work cooperatively to promote translocation. Two models of proton gradient driven translocation have been proposed: (i) an extended-chain Brownian ratchet mechanism and (ii) a proton-driven helix-compression mechanism. These models are not mutually exclusive; instead the extended-chain Brownian ratchet likely operates on β-sheet sequences and the helix-compression mechanism likely operates on α-helical sequences. Finally, we compare and contrast anthrax toxin with other related and unrelated translocase channels.

In Silico Analysis of Tetrodotoxin Binding in Voltage-Gated Sodium Ion Channels from Toxin-Resistant Animal Lineages

Multiple animal species have evolved resistance to the neurotoxin tetrodotoxin (TTX) through changes in voltage-gated sodium ion channels (VGSCs). Amino acid substitutions in TTX-resistant lineages appear to be positionally convergent with changes in homologous residues associated with reductions in TTX block. We used homology modeling coupled with docking simulations to test whether positionally convergent substitutions generate functional convergence at the level of TTX–channel interactions. We found little evidence that amino acids at convergent positions generated similar patterns among TTX-resistant animal lineages across several metrics, including number of polar contacts, polar contact position, and estimates of binding energy. Though binding energy values calculated for TTX docking were reduced for some TTX-resistant channels, not all TTX-resistant channels and not all of our analyses returned reduced binding energy values for TTX-resistant channels. Our results do not support a simple model of toxin resistance where a reduced number of bonds between TTX and the channel protein prevents blocking. Rather models that incorporate flexibility and movement of the protein overall may better describe how homologous substitutions in the channel cause changes in TTX block.

TRESK channel contributes to depolarization-induced shunting inhibition and modulates epileptic seizures

Glutamatergic and GABAergic synaptic transmission controls excitation and inhibition of postsynaptic neurons, whereas activity of ion channels modulates neuronal intrinsic excitability. However, it is unclear how excessive neuronal excitation affects intrinsic inhibition to regain homeostatic stability under physiological or pathophysiological conditions. Here, we report that a seizure-like sustained depolarization can induce short-term inhibition of hippocampal CA3 neurons via a mechanism of membrane shunting. This depolarization-induced shunting inhibition (DShI) mediates a non-synaptic, but neuronal intrinsic, short-term plasticity that is able to suppress action potential generation and postsynaptic responses by activated ionotropic receptors. We demonstrate that the TRESK channel significantly contributes to DShI. Disruption of DShI by genetic knockout of TRESK exacerbates the sensitivity and severity of epileptic seizures of mice, whereas overexpression of TRESK attenuates seizures. In summary, these results uncover a type of homeostatic intrinsic plasticity and its underlying mechanism. TRESK might represent a therapeutic target for antiepileptic drugs.

The hitchhiker's guide to the voltage-gated sodium channel galaxy

Eukaryotic voltage-gated sodium (Na_v) channels contribute to the rising phase of action potentials and served as an early muse for biophysicists laying the foundation for our current understanding of electrical signaling. Given their central role in electrical excitability, it is not surprising that (a) inherited mutations in genes encoding for Na_v channels and their accessory subunits have been linked to excitability disorders in brain, muscle, and heart; and (b) Na_v channels are targeted by various drugs and naturally occurring toxins. Although the overall architecture and behavior of these channels are likely to be similar to the more well-studied voltage-gated potassium channels, eukaryotic Na_v channels lack structural and functional symmetry, a notable difference that has implications for gating and selectivity. Activation of voltage-sensing modules of the first three domains in Na_v channels is sufficient to open the channel pore, whereas movement of the domain IV voltage sensor is correlated with inactivation. Also, structure–function studies of eukaryotic Na_v channels show that a set of amino acids in the selectivity filter, referred to as DEKA locus, is essential for Na⁺ selectivity. Structures of prokaryotic Na_v channels have also shed new light on mechanisms of drug block. These structures exhibit lateral fenestrations that are large enough to allow drugs or lipophilic molecules to gain access into the inner vestibule, suggesting that this might be the passage for drug entry into a closed channel. In this Review, we will synthesize our current understanding of Na_v channel gating mechanisms, ion selectivity and permeation, and modulation by therapeutics and toxins in light of the new structures of the prokaryotic Na_v channels that, for the time being, serve as structural models of their eukaryotic counterparts.

Royal Jelly: An ancient remedy with remarkable antibacterial properties

Royal Jelly (RJ), a honeybee hypopharyngeal gland secretion of young nurse and an exclusive nourishment for bee queen, has been used since ancient times for care and human health and it is still very important in traditional and folkloristic medicine, especially in Asia within the apitherapy. Recently, RJ and its protein and lipid components have been subjected to several investigations on their antimicrobial activity due to extensive traditional uses and for a future application in medicine. Antimicrobial activities of crude Royal Jelly, Royalisin, 10-hydroxy-2-decenoic acid, Jelleines, Major Royal Jelly Proteins against different bacteria have been reported. All these beehive products showed antimicrobial activities that lead their potential employment in several fields as natural additives. RJ and its derived compounds show a highest activity especially against Gram positive bacteria. The purpose of this Review is to summarize the results of antimicrobial studies of Royal Jelly following the timescale of the researches. From the first scientific applications to the isolation of the single components in order to better understand its application in the past years and propose an employment in future studies as a natural antimicrobial agent.

Ligand action on sodium, potassium, and calcium channels: role of permeant ions

Ion channels are targets for many naturally occurring toxins and small-molecule drugs. Despite great progress in the X-ray crystallography of ion channels, we still do not have a complete understanding of the atomistic mechanisms of channel modulation by ligands. In particular, the importance of the simultaneous interaction of permeant ions with the ligand and the channel protein has not been the focus of much attention. Considering these interactions often allows one to rationalize the highly diverse experimental data within the framework of relatively simple structural models. This has been illustrated in earlier studies on the action of local anesthetics, sodium channel activators, as well as blockers of potassium and calcium channels. Here, we discuss the available data with a view to understanding the use-, voltage-, and current carrying cation-dependence of the ligand action, paradoxes in structure--activity relationships, and effects of mutations in these ion channels.

Use-dependent block of the voltage-gated Na(+) channel by tetrodotoxin and saxitoxin: effect of pore mutations that change ionic selectivity

Voltage-gated Na(+) channels (NaV channels) are specifically blocked by guanidinium toxins such as tetrodotoxin (TTX) and saxitoxin (STX) with nanomolar to micromolar affinity depending on key amino acid substitutions in the outer vestibule of the channel that vary with NaV gene isoforms. All NaV channels that have been studied exhibit a use-dependent enhancement of TTX/STX affinity when the channel is stimulated with brief repetitive voltage depolarizations from a hyperpolarized starting voltage. Two models have been proposed to explain the mechanism of TTX/STX use dependence: a conformational mechanism and a trapped ion mechanism. In this study, we used selectivity filter mutations (K1237R, K1237A, and K1237H) of the rat muscle NaV1.4 channel that are known to alter ionic selectivity and Ca(2+) permeability to test the trapped ion mechanism, which attributes use-dependent enhancement of toxin affinity to electrostatic repulsion between the bound toxin and Ca(2+) or Na(+) ions trapped inside the channel vestibule in the closed state. Our results indicate that TTX/STX use dependence is not relieved by mutations that enhance Ca(2+) permeability, suggesting that ion-toxin repulsion is not the primary factor that determines use dependence. Evidence now favors the idea that TTX/STX use dependence arises from conformational coupling of the voltage sensor domain or domains with residues in the toxin-binding site that are also involved in slow inactivation.

Extracellular quaternary ammonium blockade of transient receptor potential vanilloid subtype 1 channels expressed in Xenopus laevis oocytes

Transient receptor potential vanilloid subtype 1 (TRPV1) channels are essential nociceptive integrators in primary afferent neurons. These nonselective cation channels are inhibited by local anesthetic compounds through an undefined mechanism. Here, we show that lidocaine inhibits TRPV1 channels expressed in Xenopus laevis oocytes, whereas the neutral local anesthetic, benzocaine, does not, suggesting that a titratable amine is required for high-affinity inhibition. Consistent with this possibility, extracellular tetraethylammonium (TEA) and tetramethylammonium application produces potent, voltage-dependent pore block. Alanine substitutions at Phe649 and Glu648, residues in the putative TRPV1 pore region, significantly abrogated the concentration-dependent TEA inhibition. The results suggest that large cations, shown previously to enter cells through activated transient receptor potential channels, can also act as channel blockers.

Gating transitions in the selectivity filter region of a sodium channel are coupled to the domain IV voltage sensor

Voltage-dependent ion channels are crucial for generation and propagation of electrical activity in biological systems. The primary mechanism for voltage transduction in these proteins involves the movement of a voltage-sensing domain (D), which opens a gate located on the cytoplasmic side. A distinct conformational change in the selectivity filter near the extracellular side has been implicated in slow inactivation gating, which is important for spike frequency adaptation in neural circuits. However, it remains an open question whether gating transitions in the selectivity filter region are also actuated by voltage sensors. Here, we examine conformational coupling between each of the four voltage sensors and the outer pore of a eukaryotic voltage-dependent sodium channel. The voltage sensors of these sodium channels are not structurally symmetric and exhibit functional specialization. To track the conformational rearrangements of individual voltage-sensing domains, we recorded domain-specific gating pore currents. Our data show that, of the four voltage sensors, only the domain IV voltage sensor is coupled to the conformation of the selectivity filter region of the sodium channel. Trapping the outer pore in a particular conformation with a high-affinity toxin or disulphide crossbridge impedes the return of this voltage sensor to its resting conformation. Our findings directly establish that, in addition to the canonical electromechanical coupling between voltage sensor and inner pore gates of a sodium channel, gating transitions in the selectivity filter region are also coupled to the movement of a voltage sensor. Furthermore, our results also imply that the voltage sensor of domain IV is unique in this linkage and in the ability to initiate slow inactivation in sodium channels.

Gating transitions in the selectivity filter region of a sodium channel are coupled to the domain IV voltage sensor

Voltage-dependent ion channels are crucial for generation and propagation of electrical activity in biological systems. The primary mechanism for voltage transduction in these proteins involves the movement of a voltage-sensing domain (D), which opens a gate located on the cytoplasmic side. A distinct conformational change in the selectivity filter near the extracellular side has been implicated in slow inactivation gating, which is important for spike frequency adaptation in neural circuits. However, it remains an open question whether gating transitions in the selectivity filter region are also actuated by voltage sensors. Here, we examine conformational coupling between each of the four voltage sensors and the outer pore of a eukaryotic voltage-dependent sodium channel. The voltage sensors of these sodium channels are not structurally symmetric and exhibit functional specialization. To track the conformational rearrangements of individual voltage-sensing domains, we recorded domain-specific gating pore currents. Our data show that, of the four voltage sensors, only the domain IV voltage sensor is coupled to the conformation of the selectivity filter region of the sodium channel. Trapping the outer pore in a particular conformation with a high-affinity toxin or disulphide crossbridge impedes the return of this voltage sensor to its resting conformation. Our findings directly establish that, in addition to the canonical electromechanical coupling between voltage sensor and inner pore gates of a sodium channel, gating transitions in the selectivity filter region are also coupled to the movement of a voltage sensor. Furthermore, our results also imply that the voltage sensor of domain IV is unique in this linkage and in the ability to initiate slow inactivation in sodium channels.

Sensitivity of cloned muscle, heart and neuronal voltage-gated sodium channels to block by polyamines: a possible basis for modulation of excitability in vivo

Spermidine and spermine, are endogenous polyamines (PAs) that regulate cell growth and modulate the activity of numerous ion channel proteins. In particular, intracellular PAs are potent blockers of many different cation channels and are responsible for strong suppression of outward K (+) current, a phenomenon known as inward rectification characteristic of a major class of KIR K (+) channels. We previously described block of heterologously expressed voltage-gated Na (+) channels (NaV) of rat muscle by intracellular PAs and PAs have recently been found to modulate excitability of brain neocortical neurons by blocking neuronal NaV channels. In this study, we compared the sensitivity of four different cloned mammalian NaV isoforms to PAs to investigate whether PA block is a common feature of NaV channel pharmacology. We find that outward Na (+) current of muscle (NaV 1.4), heart (NaV 1.5), and neuronal (NaV 1.2, NaV 1.7) NaV isoforms is blocked by PAs, suggesting that PA metabolism may be linked to modulation of action potential firing in numerous excitable tissues. Interestingly, the cardiac NaV 1.5 channel is more sensitive to PA block than other isoforms. Our results also indicate that rapid binding of PAs to blocking sites in the NaV 1.4 channel is restricted to access from the cytoplasmic side of the channel, but plasma membrane transport pathways for PA uptake may contribute to long-term NaV channel modulation. PAs may also play a role in drug interactions since spermine attenuates the use-dependent effect of the lidocaine, a typical local anesthetic and anti-arrhythmic drug.

Antillatoxin is a sodium channel activator that displays unique efficacy in heterologously expressed rNav1.2, rNav1.4 and rNav1.5 α subunits

BACKGROUND

Antillatoxin (ATX) is a structurally unique lipopeptide produced by the marine cyanobacterium Lyngbya majuscula. ATX activates voltage-gated sodium channel α-subunits at an undefined recognition site and stimulates sodium influx in neurons. However, the pharmacological properties and selectivity of ATX on the sodium channel α-subunits were not fully characterized.

RESULTS

In this study, we characterized the pharmacological properties and selectivity of ATX in cells heterologously expressing rNa(v)1.2, rNa(v)1.4 or rNa(v)1.5 α-subunits by using the Na(+) selective fluorescent dye, sodium-binding benzofuran isophthalate. ATX produced sodium influx in cells expressing each sodium channel α-subunit, whereas two other sodium channel activators, veratridine and brevetoxin-2, were without effect. The ATX potency at rNa(v)1.2, rNa(v)1.4 and rNa(v)1.5 did not differ significantly. Similarly, there were no significant differences in the efficacy for ATX-induced sodium influx between rNa(v)1.2, rNa(v)1.4 and rNa(v)1.5 α-subunits. ATX also produced robust Ca²(+) influx relative to other sodium channel activators in the calcium-permeable DEAA mutant of rNa(v)1.4 α-subunit. Finally, we demonstrated that the 8-demethyl-8,9-dihydro-antillatoxin analog was less efficacious and less potent in stimulating sodium influx.

CONCLUSIONS

ATX displayed a unique efficacy with respect to stimulation of sodium influx in cells expressing rNa(v)1.2, rNa(v)1.4 and rNa(v)1.5 α-subunits. The efficacy of ATX was distinctive inasmuch as it was not shared by activators of neurotoxin sites 2 and 5 on VGSC α-subunits. Given the unique pharmacological properties of ATX interaction with sodium channel α-subunits, decoding the molecular determinants and mechanism of action of antillatoxin may provide further insight into sodium channel gating mechanisms.

A molecular switch between the outer and the inner vestibules of the voltage-gated Na+ channel

Voltage-gated ion channels are transmembrane proteins that undergo complex conformational changes during their gating transitions. Both functional and structural data from K⁺ channels suggest that extracellular and intracellular parts of the pore communicate with each other via a trajectory of interacting amino acids. No crystal structures are available for voltage-gated Na⁺ channels, but functional data suggest a similar intramolecular communication involving the inner and outer vestibules. However, the mechanism of such communication is unknown. Here, we report that amino acid Ile-1575 in the middle of transmembrane segment 6 of domain IV (DIV-S6) in the adult rat skeletal muscle isoform of the voltage-gated sodium channel (rNa_V1.4) may act as molecular switch allowing for interaction between outer and inner vestibules. Cysteine scanning mutagenesis of the internal part of DIV-S6 revealed that only mutations at site 1575 rescued the channel from a unique kinetic state (“ultra-slow inactivation,” I_US) produced by the mutation K1237E in the selectivity filter. A similar effect was seen with I1575A. Previously, we reported that conformational changes of both the internal and the external vestibule are involved in the generation of I_US. The fact that mutations at site 1575 modulate I_US produced by K1237E strongly suggests an interaction between these sites. Our data confirm a previously published molecular model in which Ile-1575 of DIV-S6 is in close proximity to Lys-1237 of the selectivity filter. Furthermore, these functional data define the position of the selectivity filter relative to the adjacent DIV-S6 segment within the ionic permeation pathway.

Binding of sodium channel inhibitors to hyperpolarized and depolarized conformations of the channel

Sodium channels are inhibited by a chemically diverse group of compounds. In the last decade entirely new structural classes with superior properties have been discovered, and novel therapeutic uses of sodium channel inhibitors (SCIs) have been suggested. Many promising novel drug candidates have been described and characterized. Published structure-activity relationship studies, pharmacophore models, and mutagenesis studies seem to lag behind, dealing with only a limited group of inhibitor compounds. The abundance of novel compounds requires an organized comparison of drug potencies. The affinity of sodium channel inhibitors can vary typically ten- to thousand-fold depending on the voltage protocol; therefore comparison of electrophysiology data is difficult. In this study we describe a method for standardization of these data with the help of a simple model of state-dependence. We derived hyperpolarized (resting) and depolarized (generally termed "inactivated") state affinities for the studied drugs, which made the measurements comparable. We show a rank order of SCIs based on resting and inactivated affinity values. In an attempt to define basic chemical requirements for sodium channel inhibitor activity we investigated the dependence of both resting and inactivated state affinities on individual chemical descriptors. Lipophilicity (most often expressed by the logP value) is the single most important determinant of SCI potency. We investigated the independent impact of several other calculated chemical properties by standardizing drug potencies for logP values. By combining these two approaches: standardization of affinity values, and standardization of potencies, we concluded that while resting affinity is mostly determined by lipophilicity, inactivated state affinity is determined by a more complex interaction of chemical properties, including hydrogen bond acceptors, aromatic rings, and molecular weight.

Intrinsic versus extrinsic voltage sensitivity of blocker interaction with an ion channel pore

Many physiological and synthetic agents act by occluding the ion conduction pore of ion channels. A hallmark of charged blockers is that their apparent affinity for the pore usually varies with membrane voltage. Two models have been proposed to explain this voltage sensitivity. One model assumes that the charged blocker itself directly senses the transmembrane electric field, i.e., that blocker binding is intrinsically voltage dependent. In the alternative model, the blocker does not directly interact with the electric field; instead, blocker binding acquires voltage dependence solely through the concurrent movement of permeant ions across the field. This latter model may better explain voltage dependence of channel block by large organic compounds that are too bulky to fit into the narrow (usually ion-selective) part of the pore where the electric field is steep. To date, no systematic investigation has been performed to distinguish between these voltage-dependent mechanisms of channel block. The most fundamental characteristic of the extrinsic mechanism, i.e., that block can be rendered voltage independent, remains to be established and formally analyzed for the case of organic blockers. Here, we observe that the voltage dependence of block of a cyclic nucleotide-gated channel by a series of intracellular quaternary ammonium blockers, which are too bulky to traverse the narrow ion selectivity filter, gradually vanishes with extreme depolarization, a predicted feature of the extrinsic voltage dependence model. In contrast, the voltage dependence of block by an amine blocker, which has a smaller "diameter" and can therefore penetrate into the selectivity filter, follows a Boltzmann function, a predicted feature of the intrinsic voltage dependence model. Additionally, a blocker generates (at least) two blocked states, which, if related serially, may preclude meaningful application of a commonly used approach for investigating channel gating, namely, inferring the properties of the activation gate from the kinetics of channel block.

Trafficking of systemic fluorescent gentamicin into the cochlea and hair cells

Aminoglycosides enter inner ear hair cells across their apical membranes via endocytosis, or through the mechanoelectrical transduction channels in vitro, suggesting that these drugs enter cochlear hair cells from endolymph to exert their cytotoxic effect. We used zebrafish to determine if fluorescently tagged gentamicin (GTTR) also enters hair cells via apically located calcium-sensitive cation channels and the cytotoxicity of GTTR to hair cells. We then examined the serum kinetics of GTTR following systemic injection in mice and which murine cochlear sites preferentially loaded with systemically administered GTTR over time by confocal microscopy. GTTR is taken up by, and is toxic to, wild-type zebrafish neuromast hair cells. Neuromast hair cell uptake of GTTR is attenuated by high concentrations of extracellular calcium or unconjugated gentamicin and is blocked in mariner mutant zebrafish, suggestive of entry via the apical mechanotransduction channel. In murine cochleae, GTTR is preferentially taken up by the stria vascularis compared to the spiral ligament, peaking 3 h after intra-peritoneal injection, following GTTR kinetics in serum. Strial marginal cells display greater intensity of GTTR fluorescence compared to intermediate and basal cells. Immunofluorescent detection of gentamicin in the cochlea also revealed widespread cellular labeling throughout the cochlea, with preferential labeling of marginal cells. Only GTTR fluorescence displayed increasing cytoplasmic intensity with increasing concentration, unlike the cytoplasmic intensity of fluorescence from immunolabeled gentamicin. These data suggest that systemically administered aminoglycosides are trafficked from strial capillaries into marginal cells and clear into endolymph. If so, this will facilitate electrophoretically driven aminoglycoside entry into hair cells from endolymph. Trans-strial trafficking of aminoglycosides from strial capillaries to marginal cells will be dependent on as-yet-unidentified mechanisms that convey these drugs across the intra-strial electrical barrier and into marginal cells.

Amphiphilic blockers punch through a mutant CLC-0 pore

Intracellularly applied amphiphilic molecules, such as p-chlorophenoxy acetate (CPA) and octanoate, block various pore-open mutants of CLC-0. The voltage-dependent block of a particular pore-open mutant, E166G, was found to be multiphasic. In symmetrical 140 mM Cl(-), the apparent affinity of the blocker in this mutant increased with a negative membrane potential but, paradoxically, decreased when the negative membrane potential was greater than -80 mV, a phenomenon similar to the blocker "punch-through" shown in many blocker studies of cation channels. To provide further evidence of the punch-through of CPA and octanoate, we studied the dissociation rate of the blocker from the pore by measuring the time constant of relief from the block under various voltage and ionic conditions. Consistent with the voltage dependence of the effect on the steady-state current, the rate of CPA dissociation from the E166G pore reached a minimum at -80 mV in symmetrical 140 mM Cl(-), and the direction of current recovery suggested that the bound CPA in the pore can dissociate into both intracellular and extracellular solutions. Moreover, the CPA dissociation depends upon the Cl(-) reversal potential with a minimal dissociation rate at a voltage 80 mV more negative than the Cl(-) reversal potential. That the shift of the CPA-dissociation rate follows the Cl(-) gradient across the membrane argues that these blockers can indeed punch through the channel pore. Furthermore, a minimal CPA-dissociation rate at a voltage 80 mV more negative than the Cl(-) reversal potential suggests that the outward blocker movement through the CLC-0 pore is more difficult than the inward movement.

On the mechanism of TBA block of the TRPV1 channel

The transient receptor potential vanilloid 1 (TRPV1) channel is a nonselective cation channel activated by capsaicin and responsible for thermosensation. To date, little is known about the gating characteristics of these channels. Here we used tetrabutylammonium (TBA) to determine whether this molecule behaves as an ion conduction blocker in TRPV1 channels and to gain insight into the nature of the activation gate of this protein. TBA belongs to a family of classic potassium channel blockers that have been widely used as tools for determining the localization of the activation gate and the properties of the pore of several ion channels. We found TBA to be a voltage-dependent pore blocker and that the properties of block are consistent with an open-state blocker, with the TBA molecule binding to multiple open states, each with different blocker affinities. Kinetics of channel closure and burst-length analysis in the presence of blocker are consistent with a state-dependent blocking mechanism, with TBA interfering with closing of an activation gate. This activation gate may be located cytoplasmically with respect to the binding site of TBA ions, similar to what has been observed in potassium channels. We propose an allosteric model for TRPV1 activation and block by TBA, which explains our experimental data.

Organic cation permeation through the channel formed by polycystin-2

Polycystin-2 (PC2), a member of the transient receptor potential family of ion channels (TRPP2), forms a calcium-permeable cation channel. Mutations in PC2 lead to polycystic kidney disease. From the primary sequence and by analogy with other channels in this family, PC2 is modeled to have six transmembrane domains. However, most of the structural features of PC2, such as how large the channel is and how many subunits make up the pore of the channel, are unknown. In this study, we estimated the pore size of PC2 from the permeation properties of the channel. Organic cations of increasing size were used as current carriers through the PC2 channel after PC2 was incorporated into lipid bilayers. We found that dimethylamine, triethylamine, tetraethylammonium, tetrabutylammonium, tetrapropylammonium, and tetrapentylammonium were permeable through the PC2 channel. The slope conductance of the PC2 channel decreased as the ionic diameter of the organic cation increased. For each organic cation tested, the currents were inhibited by gadolinium and anti-PC2 antibody. Using the dimensions of the largest permeant cation, the minimum pore diameter of the PC2 channel was estimated to be at least 11 A. The large pore size suggests that the primary state of this channel found in vivo is closed to avoid rundown of cation gradients across the plasma membrane and excessive calcium leak from endoplasmic reticulum stores.

Block of inactivation-deficient cardiac Na(+) channels by acetyl-KIFMK-amide

The Na(+) channel alpha-subunit contains an IFM motif that is critical for the fast inactivation process. In this study, we sought to determine whether an IFM-containing peptide, acetyl-KIFMK-amide, blocks open cardiac Na(+) channels via the inner cavity. Intracellular acetyl-KIFMK-amide at 2mM elicited a rapid time-dependent block (tau=0.24 ms) of inactivation-deficient human heart Na(+) channels (hNav1.5-L409C/A410W) at +50 mV. In addition, a peptide-induced tail current appeared conspicuously upon repolarization, suggesting that the activation gate cannot close until acetyl-KIFMK-amide is cleared from the open pore. Repetitive pulses (+50 mV for 20 ms at 1Hz) produced a substantial use-dependent block of both peak and tail currents by approximately 65%. A F1760K mutation (hNav1.5-L409C/A410W/F1760K) abolished the use-dependent block by acetyl-KIFMK-amide and hindered the time-dependent block. Competition experiments showed that acetyl-KIFMK-amide antagonized bupivacaine binding. These results are consistent with a model that two acetyl-KIFMK-amide receptors exist in proximity within the Na(+) channel inner cavity.

Interactions of local anesthetics with voltage-gated Na+ channels

Voltage-gated Na+ channels are dynamic transmembrane proteins responsible for the rising phase of the action potential in excitable membranes. Local anesthetics (LAs) and structurally related antiarrhythmic and anticonvulsant compounds target specific sites in voltage-gated Na+ channels to block Na+ currents, thus reducing excitability in neuronal, cardiac, or central nervous tissue. A high-affinity LA block is produced by binding to open and inactivated states of Na+ channels rather than to resting states and suggests a binding site that converts from a low- to a high-affinity conformation during gating. Recent findings using site-directed mutagenesis suggest that multiple S6 segments together form an LA binding site within the Na+ channel. While the selectivity filter may form the more extracellular-located part of this binding site, the role of the fast inactivation gate in LA binding has not yet been resolved. The receptor of the neurotoxin batrachotoxin (BTX) is adjacent to or even overlaps with the LA binding site. The close proximity of the LA and BTX binding sites to residues critical for inactivation, together with gating transitions through S6 segments, might explain the strong impact of LAs and BTX on inactivation of voltage-gated Na+ channels and might help elucidate the mechanisms underlying voltage- and frequency-dependent LA block.

Revisiting voltage-dependent relief of block in ion channels: a mechanism independent of punchthrough

Voltage dependence of block was investigated in a simple model for permeation in a multiion pore. Internal blocker could bind to three states of the open channel that differed in the locations and number of permeant ion bound; blocker dissociation occurs exclusively to the internal solution, and the blocker does not itself enter the electric field. By changing the relative stability of blocker binding to these three states, block displayed voltage dependence with relief of block at high potentials. Similar patterns of block could also be generated in more detailed models of ion permeation. These results illustrate that the observation of relief of block at high potentials is not a sufficient criterion for establishing that a blocker is permeant in a channel that has a complex permeation cycle.

Modeling P-loops domain of sodium channel: homology with potassium channels and interaction with ligands

A large body of experimental data on Na+ channels is available, but the interpretation of these data in structural terms is difficult in the absence of a high-resolution structure. Essentially different electrophysiological and pharmacological properties of Na+ and K+ channels and poor identity of their sequences obstruct homology modeling of Na+ channels. In this work, we built the P-loops model of the Na+ channel, in which the pore helices are arranged exactly as in the MthK bacterial K+ channel. The conformation of the selectivity-filter region, which includes residues in positions -2 through +4 from the DEKA locus, was shaped around rigid molecules of saxitoxin and tetrodotoxin that are known to form multiple contacts with this region. Intensive Monte Carlo minimization that started from the MthK-like conformation produced practically identical saxitoxin- and tetrodotoxin-based models. The latter was tested to explain a wide range of experimental data that were not used at the model building stage. The docking of tetrodotoxin analogs unambiguously predicted their optimal orientation and the interaction energy that correlates with the experimental activity. The docking of mu-conotoxin produced a binding model consistent with experimentally known toxin-channel contacts. Monte Carlo-minimized energy profiles of tetramethylammonium pulled through the selectivity-filter region explain the paradoxical experimental data that this organic cation permeates via the DEAA but not the AAAA mutant of the DEKA locus. The model is also consistent with earlier proposed concepts on the Na+ channel selectivity as well as Ca2+ selectivity of the EEEE mutant of the DEKA locus. Thus, the model integrates available experimental data on the Na+ channel P-loops domain, and suggests that it is more similar to K+ channels than was believed before.

Unexpected Mexiletine Responses of a Mutant Cardiac Na+ Channel Implicate the Selectivity Filter as a Structural Determinant of Antiarrhythmic Drug Access

Gating properties of Na(+) channels are the critical determinants for the state-dependent block by class I antiarrhythmic drugs; however, recent site-directed mutagenesis studies have shown that the Na(+) channel selectivity filter region controls drug access to and dissociation from the binding site. To validate these observations, we have exploited a naturally occurring cardiac Na(+) channel mutation, S1710L, located next to the putative selectivity filter residue of domain 4, and evaluated the pharmacological properties to mexiletine using whole-cell, patch-clamp recordings. Consistent with the large negative shift of steady-state inactivation and the enhanced slow inactivation, the S1710L channel showed greater mexiletine tonic block than wild-type (WT) channel. In contradiction, S1710L showed attenuated use-dependent block by mexiletine and accelerated recovery from block, suggesting that the drug escape though the external access path is facilitated. Extracellularly applied QX-314, a membrane-impermeant derivative of lidocaine, elicited significantly enhanced tonic block in S1710L similar to mexiletine. However, recovery from internally applied QX-314 was accelerated by 4.4-fold in S1710L compared with WT. These results suggest that the drug access to and dissociation from the binding site through the hydrophilic path are substantially altered. Moreover, K(+) permeability was 1.9-fold increased in S1710L, verifying that the mutated residue is located in the ion-conducting pore. We propose that the Na(+) channel selectivity filter region is a structural determinant for the antiarrhythmic drug sensitivity in addition to gating properties of the indigenous Na(+) channels that govern the state-dependent drug block.

Potassium, sodium, calcium and glutamate-gated channels: pore architecture and ligand action

In the last decade, the idea of common organization of certain ion channel families exhibiting diverse physiological and pharmacological properties has received strong experimental support. Transmembrane topologies and patterns of the pore-facing residues are conserved in P-loop channels that include high-selective cation channels and certain ligand-gated channels. X-ray structures of bacterial K+ channels, KcsA, MthK and KvAP, help to understand structure-function relationships of other P-loop channels. Data on binding sites and mechanisms of action of ligands of K+, Na+, Ca2+ and glutamate gated ion channels are considered in view of their possible structural similarity to the bacterial K+ channels. Emphasized are structural determinants of ligand-receptor interactions within the channels and mechanisms of state-dependent action of the ligands.

Uptake of gentamicin by bullfrog saccular hair cells in vitro

Vertebrate sensory hair cells in the inner ear are pharmacologically sensitive to aminoglycoside antibiotics. Although the ototoxicity of aminoglycosides is well known, the route of drug uptake by hair cells and mechanisms of cytotoxicity remain poorly understood. Previously published studies have documented the intracellular distribution of gentamicin using immunocytochemical, electron microscopic, and autoradiographic methods. In this article, we compare the subcellular distribution of fluorescently conjugated gentamicin (gentamicin-Texas Red, GTTR) with immunolabeled gentamicin using confocal or electron microscopy. Gentamicin (detected by postfixation immunocytochemistry) and GTTR were rapidly taken up by hair cells throughout the bullfrog saccular explant in vitro and preferentially in peripheral hair cells. Immunolabeled gentamicin and GTTR were observed at the apical membranes of hair cells, particularly in their hair bundles. GTTR was also identified within a variety of subcellular compartments within hair cells, including lysosomes, mitochondria, Golgi bodies, endoplasmic reticulum, and nuclei, and in similar structures by immunoelectron microscopy. The distributions of GTTR and immunolabeled gentamicin are largely identical and corroborate a variety of published immunocytochemical and autoradiography studies. Thus, GTTR is a valid fluorescent probe with which to investigate the pharmacokinetics and mechanisms of gentamicin accumulation.

Mutation-induced blocker permeability and multiion block of the CFTR chloride channel pore

Chloride permeation through the cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel is blocked by a broad range of anions that bind tightly within the pore. Here we show that the divalent anion Pt(NO₂)₄²⁻ acts as an impermeant voltage-dependent blocker of the CFTR pore when added to the intracellular face of excised membrane patches. Block was of modest affinity (apparent K_d 556 μM), kinetically fast, and weakened by extracellular Cl⁻ ions. A mutation in the pore region that alters anion selectivity, F337A, but not another mutation at the same site that has no effect on selectivity (F337Y), had a complex effect on channel block by intracellular Pt(NO₂)₄²⁻ ions. Relative to wild-type, block of F337A-CFTR was weakened at depolarized voltages but strengthened at hyperpolarized voltages. Current in the presence of Pt(NO₂)₄²⁻ increased at very negative voltages in F337A but not wild-type or F337Y, apparently due to relief of block by permeation of Pt(NO₂)₄²⁻ ions to the extracellular solution. This “punchthrough” was prevented by extracellular Cl⁻ ions, reminiscent of a “lock-in” effect. Relief of block in F337A by Pt(NO₂)₄²⁻ permeation was only observed for blocker concentrations above 300 μM; as a result, block at very negative voltages showed an anomalous concentration dependence, with an increase in blocker concentration causing a significant weakening of block and an increase in Cl⁻ current. We interpret this effect as reflecting concentration-dependent permeability of Pt(NO₂)₄²⁻ in F337A, an apparent manifestation of an anomalous mole fraction effect. We suggest that the F337A mutation allows intracellular Pt(NO₂)₄²⁻ to enter deeply into the CFTR pore where it interacts with multiple binding sites, and that simultaneous binding of multiple Pt(NO₂)₄²⁻ ions within the pore promotes their permeation to the extracellular solution.

Lighting up the senses: FM1-43 loading of sensory cells through nonselective ion channels

We describe a novel mechanism for vital fluorescent dye entry into sensory cells and neurons: permeation through ion channels. In addition to the slow conventional uptake of styryl dyes by endocytosis, small styryl dyes such as FM1-43 rapidly and specifically label hair cells in the inner ear by entering through open mechanotransduction channels. This labeling can be blocked by pharmacological or mechanical closing of the channels. This phenomenon is not limited to hair cell transduction channels, because human embryonic kidney 293T cells expressing the vanilloid receptor (TRPV1) or a purinergic receptor (P2X2) rapidly take up FM1-43 when those receptor channels are opened and not when they are pharmacologically blocked. This channel permeation mechanism can also be used to label many sensory cell types in vivo. A single subcutaneous injection of FM1-43 (3 mg/kg body weight) in mice brightly labels hair cells, Merkel cells, muscle spindles, taste buds, enteric neurons, and primary sensory neurons within the cranial and dorsal root ganglia, persisting for several weeks. The pattern of labeling is specific; nonsensory cells and neurons remain unlabeled. The labeling of the sensory neurons requires dye entry through the sensory terminal, consistent with permeation through the sensory channels. This suggests that organic cationic dyes are able to pass through a number of different sensory channels. The bright and specific labeling with styryl dyes provides a novel way to study sensory cells and neurons in vivo and in vitro, and it offers new opportunities for visually assaying sensory channel function.

Coupled movement of permeant and blocking ions in the CFTR chloride channel pore

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel pore is blocked in a voltage-dependent manner by a broad range of anionic substances added to the cytoplasmic side of the membrane. Here we investigate the origin of the voltage dependence of block by intracellular Au(CN)2-, a highly permeant lyotropic anion which also acts as a high-affinity blocker of Cl- permeation. Not only the affinity, but also the voltage dependence of block by intracellular Au(CN)2- ions is strongly dependent on extracellular Cl- concentration; following replacement of most extracellular Cl- by glucose or by impermeant anions, block by Au(CN)2- shows greatly weakened voltage dependence. This suggests that coupled movement of Au(CN)2- and Cl- ions within the pore contributes to the voltage dependence of block. This explanation requires that interactions between different anions take place within the pore, implying simultaneous binding of multiple anions to intrapore sites. Other anions are able to substitute for extracellular Cl- and interact with intracellular Au(CN)2- ions. Analysis of the effects of different extracellular anions on the apparent affinity and voltage dependence of block by intracellular Au(CN)2- ions suggests that extracellular anions do not need to permeate through the channel in order to destabilize Au(CN)2- binding within the pore, implying that this destabilizing effect results from binding to an externally accessible site in the permeation pathway. We propose that multiple anions can bind simultaneously within the CFTR channel pore, and that repulsive interactions between bound anions speeds anion exit from the pore.

Lighting up the senses: FM1-43 loading of sensory cells through nonselective ion channels

We describe a novel mechanism for vital fluorescent dye entry into sensory cells and neurons: permeation through ion channels. In addition to the slow conventional uptake of styryl dyes by endocytosis, small styryl dyes such as FM1-43 rapidly and specifically label hair cells in the inner ear by entering through open mechanotransduction channels. This labeling can be blocked by pharmacological or mechanical closing of the channels. This phenomenon is not limited to hair cell transduction channels, because human embryonic kidney 293T cells expressing the vanilloid receptor (TRPV1) or a purinergic receptor (P2X2) rapidly take up FM1-43 when those receptor channels are opened and not when they are pharmacologically blocked. This channel permeation mechanism can also be used to label many sensory cell types in vivo. A single subcutaneous injection of FM1-43 (3 mg/kg body weight) in mice brightly labels hair cells, Merkel cells, muscle spindles, taste buds, enteric neurons, and primary sensory neurons within the cranial and dorsal root ganglia, persisting for several weeks. The pattern of labeling is specific; nonsensory cells and neurons remain unlabeled. The labeling of the sensory neurons requires dye entry through the sensory terminal, consistent with permeation through the sensory channels. This suggests that organic cationic dyes are able to pass through a number of different sensory channels. The bright and specific labeling with styryl dyes provides a novel way to study sensory cells and neurons in vivo and in vitro, and it offers new opportunities for visually assaying sensory channel function.

Veratridine block of rat skeletal muscle Nav1.4 sodium channels in the inner vestibule

Veratridine (VTD) is an alkaloid toxin found in Liliaceae plants. VTD causes persistent opening of the voltage-gated Na+ channel and reduces its single-channel conductance by 75 %. The mechanisms for these different VTD actions are unknown. Recent reports indicate that the VTD receptor aligns closely with the local anaesthetic (LA) receptor, which resides at D1S6, D3S6 and D4S6 of the Na+ channel alpha-subunit. To study this alignment, we created a mutant with cysteine substitutions at three S6 residues (rNav1.4-N434C/L1280C/F1579C). Under voltage-clamp conditions, amitriptyline and bupivacaine remained as potent blockers of this mutant channel when expressed in human embryonic kidney cells, whereas VTD completely failed to cause persistent opening. Unexpectedly, VTD at 100 microM progressively blocked mutant currents by 90.4 +/- 1.6 % (n = 5), as assayed at 0.1 Hz for 15 min. This VTD block was reversed little during wash-off: approximately 70 % of mutant currents did not return in 30 min. An increase in channel opening either by repetitive pulses at 1 Hz or by the inhibition of the fast inactivation hastened the VTD block. Co-application of amitriptyline or bupivacaine, which targeted the LA receptor, prevented this VTD block. Our data suggest that (a) the VTD receptor and the LA receptor overlap extensively, (b) receptor-bound VTD lies in the inner vestibule, and (c) VTD blocks this mutant channel as a bona fide Na+ channel blocker. We propose that VTD likewise blocks the wild-type open Na+ channel, albeit partially, to decrease the unitary conductance and to stabilize the open conformation for persistent opening.

Cardiac-specific external paths for lidocaine, defined by isoform-specific residues, accelerate recovery from use-dependent block

Local anesthetic antiarrhythmic drugs block voltage-gated Na(+) channels from the cytoplasmic side. In addition, cardiac Na(+) channels can be also blocked by the membrane-impermeant local anesthetic QX via external paths not present in skeletal muscle or brain channels. Introduction of cardiac isoform-specific residues into wild-type skeletal muscle or brain channels creates access paths for external QX block. These paths should affect the characteristics of use-dependent block by influencing drug on- and off-rates. We investigated the effects of these external paths on drug kinetics of lidocaine, a lipophilic drug of clinical relevance, by studying use-dependent block using a two-electrode voltage clamp in Xenopus oocytes. Recovery from use-dependent block was slowed when cardiac isoform-specific residues important for external QX access were mutated to skeletal muscle or brain isoform-specific residues. As the fraction of charged lidocaine was decreased by raising external pH, differences in recovery kinetics diminished, indicating that these mutations mostly influenced block by charged lidocaine molecules. Data were fit into a model in which bound drug distributes into charged and neutral forms based on its pK(a) and external pH with separate dissociation paths and recovery-time constants. These isoform-specific mutations altered the recovery-time constants for the charged molecules with smaller effects on those for the neutral molecules. We conclude that the external egress paths created by isoform-specific residues influence the drug kinetics of lidocaine, and these residues define cardiac-specific external paths for local anesthetic drugs.

Hierarchical approach to predicting permeation in ion channels

A hierarchical computational strategy combining molecular modeling, electrostatics calculations, molecular dynamics, and Brownian dynamics simulations is developed and implemented to compute electrophysiologically measurable properties of the KcsA potassium channel. Models for a series of channels with different pore sizes are developed from the known x-ray structure, using insights into the gating conformational changes as suggested by a variety of published experiments. Information on the pH dependence of the channel gating is incorporated into the calculation of potential profiles for K(+) ions inside the channel, which are then combined with K(+) ion mobilities inside the channel, as computed by molecular dynamics simulations, to provide inputs into Brownian dynamics simulations for computing ion fluxes. The open model structure has a conductance of approximately 110 pS under symmetric 250 mM K(+) conditions, in reasonable agreement with experiments for the largest conducting substate. The dimensions of this channel are consistent with electrophysiologically determined size dependence of quaternary ammonium ion blocking from the intracellular end of this channel as well as with direct structural evidence that tetrabutylammonium ions can enter into the interior cavity of the channel. Realistic values of Ussing flux ratio exponents, distribution of ions within the channel, and shapes of the current-voltage and current-concentration curves are obtained. The Brownian dynamics calculations suggest passage of ions through the selectivity filter proceeds by a "knock-off" mechanism involving three ions, as has been previously inferred from functional and structural studies of barium ion blocking. These results suggest that the present calculations capture the essential nature of K(+) ion permeation in the KcsA channel and provide a proof-of-concept for the integrated microscopic/mesoscopic multitiered approach for predicting ion channel function from structure, which can be applied to other channel structures.

Cytoplasmic polyamines as permeant blockers and modulators of the voltage-gated sodium channel

We report that voltage-gated Na+ channels (Na(V)) from rat muscle (mu1) expressed in HEK293 cells exhibit anomalous rectification of whole-cell outward current under conditions of symmetrical Na+. This behavior gradually fades with time after membrane break-in, as if a diffusible blocking substance in the cytoplasm is slowly diluted by the pipette solution. The degree of such block and rectification is markedly altered by various mutations of the conserved Lys(III) residue in Domain III of the Na(V) channel selectivity filter (DEKA locus), a principal determinant of inorganic ion selectivity and organic cation permeation. Using whole-cell and macropatch recording techniques, we show that two ubiquitous polyamines, spermine and spermidine, are potent voltage-dependent cytoplasmic blockers of mu1 Na(V) current that exhibit relief of block at high positive voltage, a phenomenon that is also enhanced by certain mutations of the Lys(III) residue. In addition, we find that polyamines alter the apparent rate of macroscopic inactivation and exhibit a use-dependent blocking phenomenon reminiscent of the action of local anesthetics. In the presence of a physiological Na+/K+ gradient, spermine also inhibits inward Na(V) current and shifts the voltage dependence of activation and inactivation. Similarities between the endogenous blocking phenomenon observed in whole cells and polyamine block characterized in excised patches suggest that polyamines or related metabolites may function as endogenous modulators of Na(V) channel activity.