ANOMALOUS RECTIFICATION IN THE SQUID GIANT AXON INJECTED WITH TETRAETHYLAMMONIUM CHLORIDE

Voltage-Gated Ion Channels: Structure, Pharmacology and Photopharmacology

Voltage-gated ion channels are transmembrane proteins responsible for the generation and propagation of action potentials in excitable cells. Over the last decade, advancements have enabled the elucidation of crystal structures of ion channels. This progress in structural understanding, particularly in identifying the binding sites of local anesthetics, opens avenues for the design of novel compounds capable of modulating ion conduction. However, many traditional drugs lack selectivity and come with adverse side effects. The emergence of photopharmacology has provided an orthogonal way of controlling the activity of compounds, enabling the regulation of ion conduction with light. In this review, we explore the central pore region of voltage-gated sodium and potassium channels, providing insights from both structural and pharmacological perspectives. We discuss the different binding modes of synthetic compounds that can physically occlude the pore and, therefore, block ion conduction. Moreover, we examine recent advances in the photopharmacology of voltage-gated ion channels, introducing molecular approaches aimed at controlling their activity by using photosensitive drugs.

The Long Journey from Animal Electricity to the Discovery of Ion Channels and the Modelling of the Human Brain

This retrospective begins with Galvani's experiments on frogs at the end of the 18th century and his discovery of 'animal electricity'. It goes on to illustrate the numerous contributions to the field of physical chemistry in the second half of the 19th century (Nernst's equilibrium potential, based on the work of Wilhelm Ostwald, Max Planck's ion electrodiffusion, Einstein's studies of Brownian motion) which led Bernstein to propose his membrane theory in the early 1900s as an explanation of Galvani's findings and cell excitability. These processes were fully elucidated by Hodgkin and Huxley in 1952 who detailed the ionic basis of resting and action potentials, but without addressing the question of where these ions passed. The emerging question of the existence of ion channels, widely debated over the next two decades, was finally accepted and, a decade later, many of them began to be cloned. This led to the possibility of modelling the activity of individual neurons in the brain and then that of simple circuits. Taking advantage of the remarkable advances in computer science in the new millennium, together with a much deeper understanding of brain architecture, more ambitious scientific goals were dreamed of to understand the brain and how it works. The retrospective concludes by reviewing the main efforts in this direction, namely the construction of a digital brain, an in silico copy of the brain that would run on supercomputers and behave just like a real brain.

A charged existence: A century of transmembrane ion transport in plants

If the past century marked the birth of membrane transport as a focus for research in plants, the past 50 years has seen the field mature from arcane interest to a central pillar of plant physiology. Ion transport across plant membranes accounts for roughly 30% of the metabolic energy consumed by a plant cell, and it underpins virtually every aspect of plant biology, from mineral nutrition, cell expansion, and development to auxin polarity, fertilization, plant pathogen defense, and senescence. The means to quantify ion flux through individual transporters, even single channel proteins, became widely available as voltage clamp methods expanded from giant algal cells to the fungus Neurospora crassa in the 1970s and the cells of angiosperms in the 1980s. Here, I touch briefly on some key aspects of the development of modern electrophysiology with a focus on the guard cells of stomata, now without dispute the premier plant cell model for ion transport and its regulation. Guard cells have proven to be a crucible for many technical and conceptual developments that have since emerged into the mainstream of plant science. Their study continues to provide fundamental insights and carries much importance for the global challenges that face us today.

A Life of Biophysics

Biophysics is a way of approaching biological problems through numbers, physical laws, models, and quantitative logic. In a long scientific career, I have seen the formation and fruition of the ion channel concept through biophysical study. Marvelous discoveries were made as our instruments evolved from vacuum tubes to transistors; computers evolved from the size of an entire building to a few chips inside our instruments; and genome sequencing, gene expression, and atom-level structural biology became accessible to all laboratories. Science is rewarding and exhilarating. Expected final online publication date for the Annual Review of Biophysics, Volume 51 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Tetraoctylammonium, a Long Chain Quaternary Ammonium Blocker, Promotes a Noncollapsed, Resting-Like Inactivated State in KcsA

Alkylammonium salts have been used extensively to study the structure and function of potassium channels. Here, we use the hydrophobic tetraoctylammonium (TOA⁺) to shed light on the structure of the inactivated state of KcsA, a tetrameric prokaryotic potassium channel that serves as a model to its homologous eukaryotic counterparts. By the combined use of a thermal denaturation assay and the analysis of homo-Förster resonance energy transfer in a mutant channel containing a single tryptophan (W67) per subunit, we found that TOA⁺ binds the channel cavity with high affinity, either with the inner gate open or closed. Moreover, TOA⁺ bound at the cavity allosterically shifts the equilibrium of the channel's selectivity filter conformation from conductive to an inactivated-like form. The inactivated TOA⁺-KcsA complex exhibits a loss in the affinity towards permeant K⁺ at pH 7.0, when the channel is in its closed state, but maintains the two sets of K⁺ binding sites and the W67-W67 intersubunit distances characteristic of the selectivity filter in the channel resting state. Thus, the TOA⁺-bound state differs clearly from the collapsed channel state described by X-ray crystallography and claimed to represent the inactivated form of KcsA.

Choline and NMDG directly reduce outward currents: reduced outward current when these substances replace Na+ is alone not evidence of Na+-activated K+ currents

Choline chloride is often, and N-methyl-d-glucamine (NMDG) sometimes, used to replace sodium chloride in studies of sodium-activated potassium channels. Given the high concentrations used in sodium replacement protocols, it is essential to test that it is not the replacement substances themselves, as opposed to the lack of sodium, that cause any observed effects. We therefore compared, in lobster stomatogastric neurons and leech Retzius cells, the effects of applying salines in which choline chloride replaced sodium chloride, and in which choline hydroxide or sucrose was added to normal saline. We also tested, in stomatogastric neurons, the effect of adding NMDG to normal saline. These protocols allowed us to measure the direct effects (i.e., effects not due to changes in sodium concentration or saline osmolarity or ionic strength) of choline on stomatogastric and leech currents, and of NMDG on stomatogastric currents. Choline directly reduced transient and sustained depolarization-activated outward currents in both species, and NMDG directly reduced transient depolarization-activated outward currents in stomatogastric neurons. Experiments with lower choline concentrations showed that adding as little as 150 mM (stomatogastric) or 5 mM (leech) choline reduced at least some depolarization-activated outward currents. Reductions in outward current with choline chloride or NMDG replacement alone are thus not evidence of sodium-activated potassium currents. NEW & NOTEWORTHY We show that choline or N-methyl-d-glucamine (NMDG) directly (i.e., not due to changes in extracellular sodium) decrease outward currents. Prior work studying sodium-activated potassium channels in which sodium was replaced with choline or NMDG without an addition control may therefore be artifactual.

A thermodynamic description for physiological transmembrane transport

A general formulation for both passive and active transmembrane transport is derived from basic thermodynamical principles. The derivation takes into account the energy required for the motion of molecules across membranes and includes the possibility of modeling asymmetric flow. Transmembrane currents can then be described by the general model in the case of electrogenic flow. As it is desirable in new models, it is possible to derive other well-known expressions for transmembrane currents as particular cases of the general formulation. For instance, the conductance-based formulation for current turns out to be a linear approximation of the general formula for current. Also, under suitable assumptions, other formulas for current based on electrodiffusion, like the constant field approximation by Goldman, can be recovered from the general formulation. The applicability of the general formulations is illustrated first with fits to existing data, and after, with models of transmembrane potential dynamics for pacemaking cardiocytes and neurons. The general formulations presented here provide a common ground for the biophysical study of physiological phenomena that depend on transmembrane transport.

Voltage-sensitive potassium channels expressed after 20-Hydroxyecdysone treatment of a mosquito cell line

The goal of this research was to express receptors and ion channels in hormone-treated insect cell lines. Treatment of Anopheles gambiae Sua1B cells with 20-hydroxyecdysone showed an inhibition of cell growth over a time course of three days, with no change in cellular morphology. The effect of 20-hydroxyecdysone was enhanced in the presence of the potassium channel blocker 4-aminopyridine, but not tetraethylammonium. Concentration-response curves of 4-aminopyridine in the presence of 42 μM (1 mg/ml) 20-hydroxyecdysone showed similar IC₅₀ values (6-10 μM) across 3 day exposures. Whole cell patch clamp confirmed the expression of delayed-rectifier (K_v2) potassium channels in hormone-supplemented Sua1B cells, whereas untreated Sua1B cells showed no evidence of Kv2 expression. The hormone-induced expression of K_v2 channels occurred in as little as 4 h after treatment, but were not observed after 24 h of exposure to 20-hydroxyecdysone, suggesting they played a role in cell death. The expressed channels had current-voltage relationships diagnostic for the K_v2 subtype, and were inhibited with an IC₅₀ = 13 mM of tetraethylammonium. Overall, these parameters were similar to Anopheles gambiae Kv2 potassium channels expressed in HEK-293 cells. The induced presence of ion channels (and possibly receptors) in these cells has potential utility for high throughput screening and basic neuroscience research.

The Cole-Moore Effect: Still Unexplained?

In the first issue, on the first page of the Biophysical Journal in 1960, Cole and Moore provided the first confirmation of the Hodgkin and Huxley formulation of the sodium and potassium conductances that underlie the action potential. In addition, working with the squid giant axon, Cole and Moore noted that strong hyperpolarization preceding a depolarizing voltage-clamp pulse delayed the rise of the potassium conductance: once started, the time course of the rise was always the same but after significant hyperpolarization there was a long lag before the rise began. This phenomenon has come to be known as the Cole-Moore effect. Their article examines and disproves the hypothesis that the lag reflects the time required to refill the membrane with potassium ions after the ions are swept out of the membrane into the axoplasm by hyperpolarization. The work by Cole and Moore indirectly supports the idea of a membrane channel for potassium conductance. However, the mechanism of the Cole-Moore effect remains a mystery even now, buried in the structure of the potassium channel, which was completely unknown at the time.

Structural Insights into GIRK Channel Function

G protein-gated inwardly rectifying potassium (GIRK; Kir3) channels, which are members of the large family of inwardly rectifying potassium channels (Kir1-Kir7), regulate excitability in the heart and brain. GIRK channels are activated following stimulation of G protein-coupled receptors that couple to the G(i/o) (pertussis toxin-sensitive) G proteins. GIRK channels, like all other Kir channels, possess an extrinsic mechanism of inward rectification involving intracellular Mg(2+) and polyamines that occlude the conduction pathway at membrane potentials positive to E(K). In the past 17 years, more than 20 high-resolution atomic structures containing GIRK channel cytoplasmic domains and transmembrane domains have been solved. These structures have provided valuable insights into the structural determinants of many of the properties common to all inward rectifiers, such as permeation and rectification, as well as revealing the structural bases for GIRK channel gating. In this chapter, we describe advances in our understanding of GIRK channel function based on recent high-resolution atomic structures of inwardly rectifying K(+) channels discussed in the context of classical structure-function experiments.

Hydrophobic gating in ion channels

Biological ion channels are nanoscale transmembrane pores. When water and ions are enclosed within the narrow confines of a sub-nanometer hydrophobic pore, they exhibit behavior not evident from macroscopic descriptions. At this nanoscopic level, the unfavorable interaction between the lining of a hydrophobic pore and water may lead to stochastic liquid-vapor transitions. These transient vapor states are "dewetted", i.e. effectively devoid of water molecules within all or part of the pore, thus leading to an energetic barrier to ion conduction. This process, termed "hydrophobic gating", was first observed in molecular dynamics simulations of model nanopores, where the principles underlying hydrophobic gating (i.e., changes in diameter, polarity, or transmembrane voltage) have now been extensively validated. Computational, structural, and functional studies now indicate that biological ion channels may also exploit hydrophobic gating to regulate ion flow within their pores. Here we review the evidence for this process and propose that this unusual behavior of water represents an increasingly important element in understanding the relationship between ion channel structure and function.

Structures of KcsA in complex with symmetrical quaternary ammonium compounds reveal a hydrophobic binding site

Potassium channels allow for the passive movement of potassium ions across the cell membrane and are instrumental in controlling the membrane potential in all cell types. Quaternary ammonium (QA) compounds block potassium channels and have long been used to study the functional and structural properties of these channels. Here we describe the interaction between three symmetrical hydrophobic QAs and the prokaryotic potassium channel KcsA. The structures demonstrate the presence of a hydrophobic pocket between the inner helices of KcsA and provide insight into the binding site and blocking mechanism of hydrophobic QAs. The structures also reveal a structurally hidden pathway between the central cavity and the outside membrane environment reminiscent of the lateral fenestration observed in sodium channels that can be accessed through small conformational changes in the pore wall. We propose that the hydrophobic binding pocket stabilizes the alkyl chains of long-chain QA molecules and may play a key role in hydrophobic drug binding in general.

The voltage-dependent gate in MthK potassium channels is located at the selectivity filter

Understanding how ion channels open and close their pores is crucial for understanding their physiological roles. We used intracellular quaternary ammonium blockers to locate the voltage-dependent gate in MthK potassium channels from Methanobacterium thermoautotrophicum with electrophysiology and X-ray crystallography. Blockers bind in an aqueous cavity between two putative gates, an intracellular gate and the selectivity filter. Thus, these blockers directly probe gate location: an intracellular gate will prevent binding when closed, whereas a selectivity filter gate will always allow binding. A kinetic analysis of tetrabutylammonium block of single MthK channels combined with X-ray crystallographic analysis of the pore with tetrabutylantimony unequivocally determined that the voltage-dependent gate, like the C-type inactivation gate in eukaryotic channels, is located at the selectivity filter. State-dependent binding kinetics suggests that MthK inactivation leads to conformational changes within the cavity and intracellular pore entrance.

hERG K+ Channels: Structure, Function, and Clinical Significance

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

Mechanism of estradiol-induced block of voltage-gated K+ currents in rat medial preoptic neurons

The present study was conducted to characterize possible rapid effects of 17-β-estradiol on voltage-gated K⁺ channels in preoptic neurons and, in particular, to identify the mechanisms by which 17-β-estradiol affects the K⁺ channels. Whole-cell currents from dissociated rat preoptic neurons were studied by perforated-patch recording. 17-β-estradiol rapidly (within seconds) and reversibly reduced the K⁺ currents, showing an EC₅₀ value of 9.7 µM. The effect was slightly voltage dependent, but independent of external Ca²⁺, and not sensitive to an estrogen-receptor blocker. Although 17-α-estradiol also significantly reduced the K⁺ currents, membrane-impermeant forms of estradiol did not reduce the K⁺ currents and other estrogens, testosterone and cholesterol were considerably less effective. The reduction induced by estradiol was overlapping with that of the K_V-2-channel blocker r-stromatoxin-1. The time course of K⁺ current in 17-β-estradiol, with a time-dependent inhibition and a slight dependence on external K⁺, suggested an open-channel block mechanism. The properties of block were predicted from a computational model where 17-β-estradiol binds to open K⁺ channels. It was concluded that 17-β-estradiol rapidly reduces voltage-gated K⁺ currents in a way consistent with an open-channel block mechanism. This suggests a new mechanism for steroid action on ion channels.

Voltage profile along the permeation pathway of an open channel

For ion channels, the transmembrane potential plays a critical role by acting as a driving force for permeant ions. At the microscopic level, the transmembrane potential is thought to decay nonlinearly across the ion permeation pathway because of the irregular three-dimensional shape of the channel's pore. By taking advantage of the current structural and functional understanding of cyclic nucleotide-gated channels, in this study we experimentally explore the transmembrane potential's distribution across the open pore. As a readout for the voltage drop, we engineered cysteine residues along the selectivity filter and scanned the sensitivity of their modification rates by Ag(+) to the transmembrane potential. The experimental data, which indicate that the majority of the electric field drops across the selectivity filter, are in good agreement with continuum electrostatic calculations using a homology model of an open CNG channel. By focusing the transmembrane potential across the selectivity filter, the electromotive driving force is coupled with the movement of permeant ions in the filter, maximizing the efficiency of this process.

Leaky membrane dynamics

A concentration difference of particles across a membrane perforated by pores will induce a diffusive flux. If the diffusing objects are of the same length scale as the pores, diffusion may not be simple; objects can move into the pore in a configuration that requires them to back up in order to continue forward. A configuration that blocks flow through the pore may be statistically preferred, an attracting metastable state of the system. This effect is purely kinetic, and not dependent on potentials, friction, or dissipation. We discuss several geometries which generate this effect, and introduce a heuristic model which captures the qualitative features.

Calcium-activated and voltage-gated potassium channels of the pancreatic islet impart distinct and complementary roles during secretagogue induced electrical responses

Glucose-induced β-cell action potential (AP) repolarization is regulated by potassium efflux through voltage gated (Kv) and calcium activated (K(Ca)) potassium channels. Thus, ablation of the primary Kv channel of the β-cell, Kv2.1, causes increased AP duration. However, Kv2.1(-/-) islet electrical activity still remains sensitive to the potassium channel inhibitor tetraethylammonium. Therefore, we utilized Kv2.1(-/-) islets to characterize Kv and K(Ca) channels and their respective roles in modulating the β-cell AP. The remaining Kv current present in Kv2.1(-/-) β-cells is inhibited with 5 μM CP 339818. Inhibition of the remaining Kv current in Kv2.1(-/-) mouse β-cells increased AP firing frequency by 39.6% but did not significantly enhance glucose stimulated insulin secretion (GSIS). The modest regulation of islet AP frequency by CP 339818 implicates other K(+) channels, possibly K(Ca) channels, in regulating AP repolarization. Blockade of the K(Ca) channel BK with slotoxin increased β-cell AP amplitude by 28.2%, whereas activation of BK channels with isopimaric acid decreased β-cell AP amplitude by 30.6%. Interestingly, the K(Ca) channel SK significantly contributes to Kv2.1(-/-) mouse islet AP repolarization. Inhibition of SK channels decreased AP firing frequency by 66% and increased AP duration by 67% only when Kv2.1 is ablated or inhibited and enhanced GSIS by 2.7-fold. Human islets also express SK3 channels and their β-cell AP frequency is significantly accelerated by 4.8-fold with apamin. These results uncover important repolarizing roles for both Kv and K(Ca) channels and identify distinct roles for SK channel activity in regulating calcium- versus sodium-dependent AP firing.

Potassium channel block by a tripartite complex of two cationophilic ligands and a potassium ion

Voltage-gated potassium channels (Kv) are targets for drugs of large chemical diversity. Although hydrophobic cations block Kv channels with Hill coefficients of 1, uncharged electron-rich (cationophilic) molecules often display Hill coefficients of 2. The mechanism of the latter block is unknown. Using a combination of computational and experimental approaches, we mapped the receptor for the immunosuppressant PAP-1 (5-(4-phenoxybutoxy)psoralen), a high-affinity blocker of Kv1.3 channels in lymphocytes. Ligand-docking using Monte Carlo minimizations suggested a model in which two cationophilic PAP-1 molecules coordinate a K(+) ion in the pore with their coumarin moieties, whereas the hydrophobic phenoxyalkoxy side chains extend into the intrasubunit interfaces between helices S5 and S6. We tested the model by generating 58 point mutants involving residues in and around the predicted receptor and then determined their biophysical properties and sensitivity to PAP-1 by whole-cell patch-clamp. The model correctly predicted the key PAP-1-sensing residues in the outer helix, the P-loop, and the inner helix and explained the Hill coefficient of 2 by demonstrating that the Kv1.3 pore can accommodate two or even four PAP-1 molecules. The model further explained the voltage-dependence of block by PAP-1 and its thousand-fold selectivity for Kv1.3 over non-Kv1 channels. The 23- to 125-fold selectivity of PAP-1 for Kv1.3 over other Kv1 channels is probably due to its preferential affinity to the C-type inactivated state, in which cessation of K(+) flux stabilizes the tripartite PAP-1:K(+):PAP-1 complex in the pore. Our study provides a new concept for potassium channel block by cationophilic ligands.

Competitive effects of some cations on active potassium transport in the human red blood cell

The effect of some cations on the active potassium transport system of the human red blood cell has been investigated. At low extracellular potassium concentrations, extracellular sodium competitively inhibits the active potassium influx at all sodium concentrations investigated, and tetraethylammonium behaves in a fashion similar to that of sodium. At low extracellular concentrations of potassium, ammonium at low concentrations at first stimulates the active potassium influx, but at higher concentrations inhibits it. Tetramethylammonium at most slightly stimulates the active potassium influx, and calcium is without effect. The behavior is consistent with a model in which potassium is required at more than one site before transport occurs, and the sites are indistinguishable as far as their behavior toward the ions investigated is concerned. The affinity of the alkali metal cations for the sites appears to be explicable in terms of their physical characteristics.

Going native: voltage-gated potassium channels controlling neuronal excitability

In this review we take a physiological perspective on the role of voltage-gated potassium channels in an identified neuron in the auditory brainstem. The large number of KCN genes for potassium channel subunits and the heterogeneity of the subunit combination into K(+) channels make identification of native conductances especially difficult. We provide a general pharmacological and biophysical profile to help identify the common voltage-gated K(+) channel families in a neuron. Then we consider the physiological role of each of these conductances from the perspective of the principal neuron in the medial nucleus of the trapezoid body (MNTB). The MNTB is an inverting relay, converting excitation generated by sound from one cochlea into inhibition of brainstem nuclei on the opposite side of the brain; this information is crucial for binaural comparisons and sound localization. The important features of MNTB action potential (AP) firing are inferred from its inhibitory projections to four key target nuclei involved in sound localization (which is the foundation of auditory scene analysis in higher brain centres). These are: the medial superior olive (MSO), the lateral superior olive (LSO), the superior paraolivary nucleus (SPN) and the nuclei of the lateral lemniscus (NLL). The Kv families represented in the MNTB each have a distinct role: Kv1 raises AP firing threshold; Kv2 influences AP repolarization and hyperpolarizes the inter-AP membrane potential during high frequency firing; and Kv3 accelerates AP repolarization. These actions are considered in terms of fidelity of transmission, AP duration, firing rates and temporal jitter. An emerging theme is activity-dependent phosphorylation of Kv channel activity and suggests that intracellular signalling has a dynamic role in refining neuronal excitability and homeostasis.

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.

Ion channels: from conductance to structure

In this perspective I tell the story (albeit a clearly abridged version) of how our knowledge of ion conduction through ion channels has evolved from a purely electrical concept to a structural dynamics view of ions interacting with a membrane protein. Our progress in this field has shown steady growth over the years but has also been interspersed with sudden jumps of discovery. These leaps have normally been associated with the introduction of a new technical advance or the development of a new biological preparation; therefore, it is quite certain that we have not seen them all.

End-point targeted molecular dynamics: large-scale conformational changes in potassium channels

Large-scale conformational changes in proteins that happen often on biological time scales may be relatively rare events on the molecular dynamics time scale. We have implemented an approach to targeted molecular dynamics called end-point targeted molecular dynamics that transforms proteins between two specified conformational states through the use of nonharmonic "soft" restraints. A key feature of the method is that the protein is free to discover its own conformational pathway through the plethora of possible intermediate states. The method is applied to the Shaker K(v)1.2 potassium channel in implicit solvent. The rate of cycling between the open and closed states was varied to explore how slow the cycling rate needed to be to ensure that microscopic reversibility along the transition pathways was well approximated. Results specific to the K(+) channel include: 1), a variation in backbone torsion angles of residues near the Pro-Val-Pro motif in the inner helix during both opening and closing; 2), the identification of possible occlusion sites in the closed channel located among Pro-Val-Pro residues and downstream; 3), a difference in the opening and closing pathways of the channel; and 4), evidence of a transient intermediate structural substate. The results also show that likely intermediate conformations during the opening-closing process can be generated in computationally tractable simulation times.

Geometry-induced asymmetric diffusion

Past work has shown that ions can pass through a membrane more readily in one direction than the other. We demonstrate here in a model and an experiment that for a mixture of small and large particles such asymmetric diffusion can arise solely from an asymmetry in the geometry of the pores of the membrane. Our deterministic simulation considers a two-dimensional gas of elastic disks of two sizes diffusing through a membrane, and our laboratory experiment examines the diffusion of glass beads of two sizes through a metal membrane. In both experiment and simulation, the membrane is permeable only to the smaller particles, and the asymmetric pores lead to an asymmetry in the diffusion rates of these particles. The presence of even a small percentage of large particles can clog a membrane, preventing passage of the small particles in one direction while permitting free flow of the small particles in the other direction. The purely geometric kinetic constraints may play a role in common biological contexts such as membrane ion channels.

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.

Life Among the Axons

A blink in history's eye has brought us an understanding of electricity, and with it a revolution in human life. From the frog leg twitch experiments of Galvani and the batteries of Volta, we have progressed to telegraphs, motors, telephones, computers, and the Internet. In the same period, the ubiquitous role of electricity in animal and plant life has become clear. A great milestone in this journey was the elucidation of electrical signaling by Hodgkin & Huxley in 1952. This chapter gives a personal account of a small part of this story, the transformation of the rather abstract electrical conductances of Hodgkin & Huxley into the more tangible gated ion channel.

Crystallographic study of the tetrabutylammonium block to the KcsA K+ channel

K(+) channels play essential roles in regulating membrane excitability of many diverse cell types by selectively conducting K(+) ions through their pores. Many diverse molecules can plug the pore and modulate the K(+) current. Quaternary ammonium (QA) ions are a class of pore blockers that have been used for decades by biophysicists to probe the pore, leading to important insights into the structure-function relation of K(+) channels. However, many key aspects of the QA-blocking mechanisms remain unclear to date, and understanding these questions requires high resolution structural information. Here, we address the question of whether intracellular QA blockade causes conformational changes of the K(+) channel selectivity filter. We have solved the structures of the KcsA K(+) channel in complex with tetrabutylammonium (TBA) and tetrabutylantimony (TBSb) under various ionic conditions. Our results demonstrate that binding of TBA or TBSb causes no significant change in the KcsA structure at high concentrations of permeant ions. We did observe the expected conformational change of the filter at low concentration of K(+), but this change appears to be independent of TBA or TBSb blockade.

Sphingosine-1-phosphate via activation of a G-protein-coupled receptor(s) enhances the excitability of rat sensory neurons

Sphingosine-1-phosphate (S1P) is released by immune cells and is thought to play a key role in chemotaxis and the onset of the inflammatory response. The question remains whether this lipid mediator also contributes to the enhanced sensitivity of nociceptive neurons that is associated with inflammation. Therefore we examined whether S1P alters the excitability of small diameter, capsaicin-sensitive sensory neurons by measuring action potential (AP) firing and two of the membrane currents critical in regulating the properties of the AP. External application of S1P augments the number of APs evoked by a depolarizing current ramp. The enhanced firing is associated with a decrease in the rheobase and an increase in the resistance at firing threshold although neither the firing threshold nor the resting membrane potential are changed. Treatment with S1P enhanced the tetrodotoxin-resistant sodium current and decreased the total outward potassium current (IK). When sensory neurons were internally perfused with GDP-beta-S, a blocker of G protein activation, the S1P-induced increase in APs was completely blocked and suggests the excitatory actions of S1P are mediated through G-protein-coupled receptors called endothelial differentiation gene or S1PR. In contrast, internal perfusion with GDP-beta-S and S1P increased the number of APs evoked by the current ramp. These results and our finding that the mRNAs for S1PRs are expressed in both the intact dorsal root ganglion and cultures of adult sensory neurons supports the notion that S1P acts on S1PRs linked to G proteins. Together these findings demonstrate that S1P can regulate the excitability of small diameter sensory neurons by acting as an external paracrine-type ligand through activation of G-protein-coupled receptors and thus may contribute to the hypersensitivity during inflammation.

Characterization of tetraethylammonium uptake across the basolateral membrane of theDrosophilaMalpighian (renal) tubule

Basolateral transport of the prototypical type I organic cation tetraethylammonium (TEA) by the Malpighian tubules of Drosophila melanogaster was studied using measurements of basolateral membrane potential (Vbl) and uptake of [14C]-labeled TEA. TEA uptake was metabolically dependent and saturable (maximal rate of mediated TEA uptake by all potential transport processes, reflecting the total transport capacity of the membrane, 0.87 pmol·tubule−1·min−1; concentration of TEA at 0.5 of the maximal rate of TEA uptake value, 24 μM). TEA uptake in Malpighian tubules was inhibited by a number of type I (e.g., cimetidine, quinine, and TEA) and type II (e.g., verapamil) organic cations and was dependent on Vbl. TEA uptake was reduced in response to conditions that depolarized Vbl(high-K+saline, Na+-free saline, NaCN) and increased in conditions that hyperpolarized Vbl(low-K+saline). Addition of TEA to the saline bathing Malpighian tubules rapidly depolarized the Vbl, indicating that TEA uptake was electrogenic. Blockade of K+channels with Ba2+did not block effects of TEA on Vblor TEA uptake indicating that TEA uptake does not occur through K+channels. This is the first study to provide physiological evidence for an electrogenic carrier-mediated basolateral organic cation transport mechanism in insect Malpighian tubules. Our results also suggest that the mechanism of basolateral TEA uptake by Malpighian tubules is distinct from that found in vertebrate renal tubules.

Mechanism of the voltage sensitivity of IRK1 inward-rectifier K+ channel block by the polyamine spermine

IRK1 (Kir2.1) inward-rectifier K+ channels exhibit exceedingly steep rectification, which reflects strong voltage dependence of channel block by intracellular cations such as the polyamine spermine. On the basis of studies of IRK1 block by various amine blockers, it was proposed that the observed voltage dependence (valence approximately 5) of IRK1 block by spermine results primarily from K+ ions, not spermine itself, traversing the transmembrane electrical field that drops mostly across the narrow ion selectivity filter, as spermine and K+ ions displace one another during channel block and unblock. If indeed spermine itself only rarely penetrates deep into the ion selectivity filter, then a long blocker with head groups much wider than the selectivity filter should exhibit comparably strong voltage dependence. We confirm here that channel block by two molecules of comparable length, decane-bis-trimethylammonium (bis-QA(C10)) and spermine, exhibit practically identical overall voltage dependence even though the head groups of the former are much wider ( approximately 6 A) than the ion selectivity filter ( approximately 3 A). For both blockers, the overall equilibrium dissociation constant differs from the ratio of apparent rate constants of channel unblock and block. Also, although steady-state IRK1 block by both cations is strongly voltage dependent, their apparent channel-blocking rate constant exhibits minimal voltage dependence, which suggests that the pore becomes blocked as soon as the blocker encounters the innermost K+ ion. These findings strongly suggest the existence of at least two (potentially identifiable) sequentially related blocked states with increasing numbers of K+ ions displaced. Consequently, the steady-state voltage dependence of IRK1 block by spermine or bis-QA(C10) should increase with membrane depolarization, a prediction indeed observed. Further kinetic analysis identifies two blocked states, and shows that most of the observed steady-state voltage dependence is associated with the transition between blocked states, consistent with the view that the mutual displacement of blocker and K+ ions must occur mainly as the blocker travels along the long inner pore.

Fifty Years of Progress in Ion Channel Research

Fifty years ago, ion channels were but a reasonable hypothesis. I outline some major steps in transforming this idea from a plausible description of the biological assemblies responsible for controlling passive ion transport across membranes to established fact. Important electrophysiological, biochemical, molecular biological, structural, and theoretical tools are discussed in the context of the transition from studying whole cell preparations, containing many channels, to investigating single channel behavior. Six channel families are exemplified: the model peptide, gramicidin, the acetylcholine receptor, and the sodium, potassium, calcium, and chloride channels. Some questions of current interest are posed.

Rapid intracellular TEA block of the KcsA potassium channel

Intracellular tetraethylammonium (TEA) inhibition was studied at the single-channel level in the KcsA potassium channel reconstituted in planar lipid bilayers. TEA acts as a fast blocker (resulting in decreased current amplitude) with an affinity in the 75 mM range even at high bandwidth. Studies over a wide voltage range reveal that TEA block has a complex voltage-dependence that also depends on the ionic conditions. These observations are examined in the context of permeation models to extend our understanding of the coupling between permeant ions and TEA blockade.

Interaction mechanisms between polyamines and IRK1 inward rectifier K+ channels

Rectification of macroscopic current through inward-rectifier K⁺ (Kir) channels reflects strong voltage dependence of channel block by intracellular cations such as polyamines. The voltage dependence results primarily from the movement of K⁺ ions across the transmembrane electric field, which accompanies the binding–unbinding of a blocker. Residues D172, E224, and E299 in IRK1 are critical for high-affinity binding of blockers. D172 appears to be located somewhat internal to the narrow K⁺ selectivity filter, whereas E224 and E299 form a ring at a more intracellular site. Using a series of alkyl-bis-amines of varying length as calibration, we investigated how the acidic residues in IRK1 interact with amine groups in the natural polyamines (putrescine, spermidine, and spermine) that cause rectification in cells. To block the pore, the leading amine of bis-amines of increasing length penetrates ever deeper into the pore toward D172, while the trailing amine in every bis-amine binds near a more intracellular site and interacts with E224 and E299. The leading amine in nonamethylene-bis-amine (bis-C9) makes the closest approach to D172, displacing the maximal number of K⁺ ions and exhibiting the strongest voltage dependence. Cells do not synthesize bis-amines longer than putrescine (bis-C4) but generate the polyamines spermidine and spermine by attaching an amino-propyl group to one or both ends of putrescine. Voltage dependence of channel block by the tetra-amine spermine is comparable to that of block by the bis-amines bis-C9 (shorter) or bis-C12 (equally long), but spermine binds to IRK1 with much higher affinity than either bis-amine does. Thus, counterintuitively, the multiple amines in spermine primarily confer the high affinity but not the strong voltage dependence of channel block. Tetravalent spermine achieves a stronger interaction with the pore by effectively behaving like a pair of tethered divalent cations, two amine groups in its leading half interacting primarily with D172, whereas the other two in the trailing half interact primarily with E224 and E299. Thus, nature has optimized not only the blocker but also, in a complementary manner, the channel for producing rapid, high-affinity, and strongly voltage-dependent channel block, giving rise to exceedingly sharp rectification.

Unique inner pore properties of BK channels revealed by quaternary ammonium block

Potassium channels have a very wide distribution of single-channel conductance, with BK type Ca²⁺-activated K⁺ channels having by far the largest. Even though crystallographic views of K⁺ channel pores have become available, the structural basis underlying BK channels' large conductance has not been completely understood. In this study we use intracellularly applied quaternary ammonium compounds to probe the pore of BK channels. We show that molecules as large as decyltriethylammonium (C₁₀) and tetrabutylammonium (TBA) have much faster block and unblock rates in BK channels when compared with any other tested K⁺ channel types. Additionally, our results suggest that at repolarization large QA molecules may be trapped inside blocked BK channels without slowing the overall process of deactivation. Based on these findings we propose that BK channels may differ from other K⁺ channels in its geometrical design at the inner mouth, with an enlarged cavity and inner pore providing less spatially restricted access to the cytoplasmic solution. These features could potentially contribute to the large conductance of BK channels.

Mechanism of Rectification in Inward-Rectifier K+ Channels

Inward rectifiers are a class of K+ channels that can conduct much larger inward currents at membrane voltages negative to the K+ equilibrium potential than outward currents at voltages positive to it, even when K+ concentrations on both sides of the membrane are made equal. This conduction property, called inward rectification, enables inward rectifiers to perform many important physiological tasks. Rectification is not an inherent property of the channel protein itself, but reflects strong voltage dependence of channel block by intracellular cations such as Mg2+ and polyamines. This voltage dependence results primarily from the movement of K+ ions across the transmembrane electric field along the pore, which is energetically coupled to the blocker binding and unbinding. This mutual displacement mechanism between several K+ ions and a blocker explains the signature feature of inward rectifier K+ channels, namely, that at a given concentration of intracellular K+, their macroscopic conductance depends on the difference between membrane voltage and the K+ equilibrium potential rather than on membrane voltage itself.