Extracellular K+ specifically modulates a rat brain K+ channel

Mechanism of external K+ sensitivity of KCNQ1 channels

KCNQ1 voltage-gated K+ channels are involved in a wide variety of fundamental physiological processes and exhibit the unique feature of being markedly inhibited by external K+. Despite the potential role of this regulatory mechanism in distinct physiological and pathological processes, its exact underpinnings are not well understood. In this study, using extensive mutagenesis, molecular dynamics simulations, and single-channel recordings, we delineate the molecular mechanism of KCNQ1 modulation by external K+. First, we demonstrate the involvement of the selectivity filter in the external K+ sensitivity of the channel. Then, we show that external K+ binds to the vacant outermost ion coordination site of the selectivity filter inducing a diminution in the unitary conductance of the channel. The larger reduction in the unitary conductance compared to whole-cell currents suggests an additional modulatory effect of external K+ on the channel. Further, we show that the external K+ sensitivity of the heteromeric KCNQ1/KCNE complexes depends on the type of associated KCNE subunits.

Mechanisms Underlying C-type Inactivation in Kv Channels: Lessons From Structures of Human Kv1.3 and Fly Shaker-IR Channels

Voltage-gated potassium (Kv) channels modulate the function of electrically-excitable and non-excitable cells by using several types of “gates” to regulate ion flow through the channels. An important gating mechanism, C-type inactivation, limits ion flow by transitioning Kv channels into a non-conducting inactivated state. Here, we highlight two recent papers, one on the human Kv1.3 channel and the second on the Drosophila Shaker Kv channel, that combined cryogenic electron microscopy and molecular dynamics simulation to define mechanisms underlying C-type inactivation. In both channels, the transition to the non-conducting inactivated conformation begins with the rupture of an intra-subunit hydrogen bond that fastens the selectivity filter to the pore helix. The freed filter swings outwards and gets tethered to an external residue. As a result, the extracellular end of the selectivity filter dilates and K⁺ permeation through the pore is impaired. Recovery from inactivation may entail a reversal of this process. Such a reversal, at least partially, is induced by the peptide dalazatide. Binding of dalazatide to external residues in Kv1.3 frees the filter to swing inwards. The extracellular end of the selectivity filter narrows allowing K⁺ to move in single file through the pore typical of conventional knock-on conduction. Inter-subunit hydrogen bonds that stabilize the outer pore in the dalazatide-bound structure are equivalent to those in open-conducting conformations of Kv channels. However, the intra-subunit bond that fastens the filter to the pore-helix is absent, suggesting an incomplete reversal of the process. These mechanisms define how Kv channels self-regulate the flow of K⁺ by changing the conformation of the selectivity filter.

Voltage-Gated Potassium Channels Beyond the Action Potential

Bioelectricity goes far beyond electrical signaling in the nervous system, but this was initially not obvious for me. This article describes the journey from studying the biophysics of ion channels in classical electrically excitable tissues to focusing on the pathogenic roles of the Kv10.1 potassium channel in cancers.

Conductance stability and Na+ interaction with Shab K+ channels under low K+ conditions

K+ ions exert a structural effect that brings stability to K+ selective pores. Thus, upon bathing Shab channels in 0 K+ solutions the ion conductance, GK, irreversibly collapses. Related to this, studies with isolated KcsA channels have suggested that there is a transition [K+] around which the pore takes one of two conformations, either the low (non-conducting) or high K+ (conducting) crystal structures. We examined this premise by looking at the K+-dependency of GK stability of Shab channels within the cell membrane environment. We found that: K+ effect on GK stability is highly asymmetrical, and that as internal K+ is replaced by Na+ GK drops in a way that suggests a transition internal [K+]. Additionally, we found that external permeant ions inhibit GK drop with a potency that differs from the global selectivity-sequence of K+ pores; the non-permeant TEA inhibited GK drop in a K+-dependent manner. Upon lowering internal [K+] we observed an influx of Na+ at negative potentials. Na+ influx was halted by physiological external [K+], which also restored GK stability. Hyperpolarized potentials afforded GK stability but, as expected, do not restore GK selectivity. For completeness, Na+ interaction with Shab was also assessed at depolarized potentials by looking at Na block followed by permeation (pore unblock) at positive potentials, in solutions approaching the 0 K+ limit. The stabilizing effect of negative potentials along with the non-parallel variation of Na+ permeability and conductance-stability herein reported, show that pore stability and selectivity, although related, are not strictly coupled.

The activation gate controls steady-state inactivation and recovery from inactivation in Shaker

Despite major advances in the structure determination of ion channels, the sequence of molecular rearrangements at negative membrane potentials in voltage-gated potassium channels of the Shaker family remains unknown. Four major composite gating states are documented during the gating process: closed (C), open (O), open-inactivated (OI), and closed-inactivated (CI). Although many steps in the gating cycle have been clarified experimentally, the development of steady-state inactivation at negative membrane potentials and mandatory gating transitions for recovery from inactivation have not been elucidated. In this study, we exploit the biophysical properties of Shaker-IR mutants T449A/V474C and T449A/V476C to evaluate the status of the activation and inactivation gates during steady-state inactivation and upon locking the channel open with intracellular Cd²⁺. We conclude that at negative membrane potentials, the gating scheme of Shaker channels can be refined in two aspects. First, the most likely pathway for the development of steady-state inactivation is C→O→OI⇌CI. Second, the OI→CI transition is a prerequisite for recovery from inactivation. These findings are in accordance with the widely accepted view that tight coupling is present between the activation and C-type inactivation gates in Shaker and underscore the role of steady-state inactivation and recovery from inactivation as determinants of excitability.

Structural Insights into the Mechanisms and Pharmacology of K2P Potassium Channels

Leak currents, defined as voltage and time independent flows of ions across cell membranes, are central to cellular electrical excitability control. The K_2P (KCNK) potassium channel class comprises an ion channel family that produces potassium leak currents that oppose excitation and stabilize the resting membrane potential in cells in the brain, cardiovascular system, immune system, and sensory organs. Due to their widespread tissue distribution, K_2Ps contribute to many physiological and pathophysiological processes including anesthesia, pain, arrythmias, ischemia, hypertension, migraine, intraocular pressure regulation, and lung injury responses. Structural studies of six homomeric K_2Ps have established the basic architecture of this channel family, revealed key moving parts involved in K_2P function, uncovered the importance of asymmetric pinching and dilation motions in the K_2P selectivity filter (SF) C-type gate, and defined two K_2P structural classes based on the absence or presence of an intracellular gate. Further, a series of structures characterizing K_2P:modulator interactions have revealed a striking polysite pharmacology housed within a relatively modestly sized (~70 kDa) channel. Binding sites for small molecules or lipids that control channel function are found at every layer of the channel structure, starting from its extracellular side through the portion that interacts with the membrane bilayer inner leaflet. This framework provides the basis for understanding how gating cues sensed by different channel parts control function and how small molecules and lipids modulate K_2P activity. Such knowledge should catalyze development of new K_2P modulators to probe function and treat a wide range of disorders.

K2P channel C-type gating involves asymmetric selectivity filter order-disorder transitions

K_2P potassium channels regulate cellular excitability using their selectivity filter (C-type) gate. C-type gating mechanisms, best characterized in homotetrameric potassium channels, remain controversial and are attributed to selectivity filter pinching, dilation, or subtle structural changes. The extent to which such mechanisms control C-type gating of innately heterodimeric K_2Ps is unknown. Here, combining K_2P2.1 (TREK-1) x-ray crystallography in different potassium concentrations, potassium anomalous scattering, molecular dynamics, and electrophysiology, we uncover unprecedented, asymmetric, potassium-dependent conformational changes that underlie K_2P C-type gating. These asymmetric order-disorder transitions, enabled by the K_2P heterodimeric architecture, encompass pinching and dilation, disrupt the S1 and S2 ion binding sites, require the uniquely long K_2P SF2-M4 loop and conserved "M3 glutamate network," and are suppressed by the K_2P C-type gate activator ML335. These findings demonstrate that two distinct C-type gating mechanisms can operate in one channel and underscore the SF2-M4 loop as a target for K_2P channel modulator development.

Impact of intracellular hemin on N-type inactivation of voltage-gated K+ channels

N-type inactivation of voltage-gated K⁺ channels is conferred by the N-terminal “ball” domains of select pore-forming α subunits or of auxiliary β subunits, and influences electrical cellular excitability. Here, we show that hemin impairs inactivation of K⁺ channels formed by Kv3.4 α subunits as well as that induced by the subunits Kvβ1.1, Kvβ1.2, and Kvβ3.1 when coexpressed with α subunits of the Kv1 subfamily. In Kvβ1.1, hemin interacts with cysteine and histidine residues in the N terminus (C7 and H10) with high affinity (EC₅₀ 100 nM). Similarly, rapid inactivation of Kv4.2 channels induced by the dipeptidyl peptidase-like protein DPP6a is also sensitive to hemin, and the DPP6a mutation C13S eliminates this dependence. The results suggest a common mechanism for a dynamic regulation of Kv channel inactivation by heme/hemin in N-terminal ball domains of Kv α and auxiliary β subunits. Free intracellular heme therefore has the potential to regulate cellular excitability via modulation of Kv channel inactivation.

A twin histidine motif is the core structure for high-affinity substrate selection in plant ammonium transporters

Ammonium transporters (AMT), methylamine permeases (Mep), and the more distantly related rhesus factors (Rh) are trimeric membrane proteins present in all domains of life. AMT/Mep/Rhs are highly selective membrane proteins required for ammonium uptake or release, and they efficiently exclude the similarly sized K⁺ ion. Previously reported crystal structures have revealed that each transporter subunit contains a unique hydrophobic but occluded central pore, but it is unclear whether the base (NH₃) or NH₃ coupled with an H⁺ are transported. Here, using expression of two plant AMTs (AtAMT1;2 and AMT2) in budding yeast, we found that systematic replacements in the conserved twin-histidine motif, a hallmark of most AMT/Mep/Rh, alter substrate recognition, transport capacities, N isotope selection, and selectivity against K⁺ AMT-specific differences were found for histidine variants. Variants that completely lost ammonium N isotope selection, a feature likely associated with NH₄ ⁺ deprotonation during passage, substantially transported K⁺ in addition to NH₄ ⁺ Of note, the twin-histidine motif was not essential for ammonium transport. However, it conferred key AMT features, such as high substrate affinity and selectivity against alkali cations via an NH₄ ⁺ deprotonation mechanism. Our findings indicate that the twin-His motif is the core structure responsible for substrate deprotonation and isotopic preferences in AMT pores and that decreased deprotonation capacity is associated with reduced selectivity against K⁺ We conclude that optimization for ammonium transport in plant AMT represents a compromise between substrate deprotonation for optimal selectivity and high substrate affinity and transport rates.

A perspective on Na and K channel inactivation

Armstrong and Hollingworth discuss inactivation in the light of modern structural data from K and Na channels.

We are wired with conducting cables called axons that rapidly transmit electrical signals (e.g., “Ouch!”) from, for example, the toe to the spinal cord. Because of the high internal resistance of axons (salt water rather than copper), a signal must be reinforced after traveling a short distance. Reinforcement is accomplished by ion channels, Na channels for detecting the signal and reinforcing it by driving it further positive (to near 50 mV) and K channels for then restoring it to the resting level (near −70 mV). The signal is called an action potential and has a duration of roughly a millisecond. The return of membrane voltage (V_m) to the resting level after an action potential is facilitated by “inactivation” of the Na channels: i.e., an internal particle diffuses into the mouth of any open Na channel and temporarily blocks it. Some types of K channels also show inactivation after being open for a time. N-type inactivation of K channels has a relatively fast time course and involves diffusion of the N-terminal of one of the channel’s four identical subunits into the channel’s inner mouth, if it is open. This mechanism is similar to Na channel inactivation. Both Na and K channels also display slower inactivation processes. C inactivation in K channels involves changes in the channel’s outer mouth, the “selectivity filter,” whose normal function is to prevent Na⁺ ions from entering the K channel. C inactivation deforms the filter so that neither K⁺ nor Na⁺ can pass.

Inactivation of KCNQ1 potassium channels reveals dynamic coupling between voltage sensing and pore opening

In voltage-activated ion channels, voltage sensor (VSD) activation induces pore opening via VSD-pore coupling. Previous studies show that the pore in KCNQ1 channels opens when the VSD activates to both intermediate and fully activated states, resulting in the intermediate open (IO) and activated open (AO) states, respectively. It is also well known that accompanying KCNQ1 channel opening, the ionic current is suppressed by a rapid process called inactivation. Here we show that inactivation of KCNQ1 channels derives from the different mechanisms of the VSD-pore coupling that lead to the IO and AO states, respectively. When the VSD activates from the intermediate state to the activated state, the VSD-pore coupling has less efficacy in opening the pore, producing inactivation. These results indicate that different mechanisms, other than the canonical VSD-pore coupling, are at work in voltage-dependent ion channel activation.

KCNQ1 is a voltage-gated potassium channel that is important in cardiac and epithelial function. Here the authors present a mechanism for KCNQ1 activation and inactivation in which voltage sensor activation promotes pore opening more effectively in the intermediate open state than the fully open state, generating inactivation.

Developmental nicotine exposure alters potassium currents in hypoglossal motoneurons of neonatal rat

We previously showed that nicotine exposure in utero and after birth via breast milk [developmental nicotine exposure (DNE)] is associated with many changes in the structure and function of hypoglossal motoneurons (XIIMNs), including a reduction in the size of the dendritic arbor and an increase in cell excitability. Interestingly, the elevated excitability was associated with a reduction in the expression of glutamate receptors on the cell body. Together, these observations are consistent with a homeostatic compensation aimed at restoring cell excitability. Compensation for increased cell excitability could also occur by changing potassium conductance, which plays a critical role in regulating resting potential, spike threshold, and repetitive spiking behavior. Here we test the hypothesis that the previously observed increase in the excitability of XIIMNs from DNE animals is associated with an increase in whole cell potassium currents. Potassium currents were measured in XIIMNs in brain stem slices derived from DNE and control rat pups ranging in age from 0 to 4 days by whole cell patch-clamp electrophysiology. All currents were measured after blockade of action potential-dependent synaptic transmission with tetrodotoxin. Compared with control cells, XIIMNs from DNE animals showed significantly larger transient and sustained potassium currents, but this was observed only under conditions of increased cell and network excitability, which we evoked by raising extracellular potassium from 3 to 9 mM. These observations suggest that the larger potassium currents in nicotine-exposed neurons are an important homeostatic compensation that prevents "runaway" excitability under stressful conditions, when neurons are receiving elevated excitatory synaptic input.NEW & NOTEWORTHY Developmental nicotine exposure is associated with increased cell excitability, which is often accompanied by compensatory changes aimed at normalizing excitability. Here we show that whole cell potassium currents are also increased in hypoglossal motoneurons from nicotine-exposed neonatal rats under conditions of increased cell and network excitability. This is consistent with a compensatory response aimed at preventing instability under conditions in which excitatory synaptic input is high and is compatible with the concept of homeostatic plasticity.

Voltage-Gated Potassium Channels: A Structural Examination of Selectivity and Gating

Voltage-gated potassium channels play a fundamental role in the generation and propagation of the action potential. The discovery of these channels began with predictions made by early pioneers, and has culminated in their extensive functional and structural characterization by electrophysiological, spectroscopic, and crystallographic studies. With the aid of a variety of crystal structures of these channels, a highly detailed picture emerges of how the voltage-sensing domain reports changes in the membrane electric field and couples this to conformational changes in the activation gate. In addition, high-resolution structural and functional studies of K(+) channel pores, such as KcsA and MthK, offer a comprehensive picture on how selectivity is achieved in K(+) channels. Here, we illustrate the remarkable features of voltage-gated potassium channels and explain the mechanisms used by these machines with experimental data.

Complex interactions among residues within pore region determine the K+ dependence of a KAT1-type potassium channel AmKAT1

KAT1-type channels mediate K(+) influx into guard cells that enables stomatal opening. In this study, a KAT1-type channel AmKAT1 was cloned from the xerophyte Ammopiptanthus mongolicus. In contrast to most KAT1-type channels, its activation is strongly dependent on external K(+) concentration, so it can be used as a model to explore the mechanism for the K(+) -dependent gating of KAT1-type channels. Domain swapping between AmKAT1 and KAT1 reveals that the S5-pore-S6 region controls the K(+) dependence of AmKAT1, and residue substitutions show that multiple residues within the S5-Pore linker and Pore are involved in its K(+) -dependent gating. Importantly, complex interactions occur among these residues, and it is these interactions that determine its K(+) dependence. Finally, we analyzed the potential mechanism for the K(+) dependence of AmKAT1, which could originate from the requirement of K(+) occupancy in the selectivity filter to maintain its conductive conformation. These results provide new insights into the molecular basis of the K(+) -dependent gating of KAT1-type channels.

Novel phenotype associated with a mutation in the KCNA1(Kv1.1) gene

Episodic ataxia type 1 (EA1) is an autosomal dominant K(+) channelopathy which manifests with short attacks of cerebellar ataxia and dysarthria, and may also show interictal myokymia. Episodes can be triggered by emotional or physical stress, startle response, sudden postural change or fever. Here we describe a 31-year-old man displaying markedly atypical symptoms, including long-lasting attacks of jerking muscle contractions associated with hyperthermia, severe migraine, and a relatively short-sleep phenotype. A single nucleotide change in KCNA1 (c.555C>G) was identified that changes a highly conserved residue (p.C185W) in the first transmembrane segment of the voltage-gated K(+) channel Kv1.1. The patient is heterozygous and the mutation was inherited from his asymptomatic mother. Next generation sequencing revealed no variations in the CACNA1A, CACNB4, KCNC3, KCNJ10, PRRT2 or SCN8A genes of either the patient or mother, except for a benign variant in SLC1A3. Functional analysis of the p.C185W mutation in KCNA1 demonstrated a deleterious dominant-negative phenotype where the remaining current displayed slower activation kinetics, subtle changes in voltage-dependence and faster recovery from slow inactivation. Structural modeling also predicts the C185W mutation to be functionally deleterious. This description of novel clinical features, associated with a Kv1.1 mutation highlights a possibly unrecognized relationship between K(+) channel dysfunction, hyperthermia and migraine in EA1, and suggests that thorough assessments for these symptoms should be carefully considered for all patients affected by EA1.

Lack of negatively charged residues at the external mouth of Kir2.2 channels enable the voltage-dependent block by external Mg2+

Kir channels display voltage-dependent block by cytosolic cations such as Mg2+ and polyamines that causes inward rectification. In fact, cations can regulate K channel activity from both the extracellular and intracellular sides. Previous studies have provided insight into the up-regulation of Kir channel activity by extracellular K+ concentration. In contrast, extracellular Mg2+ has been found to reduce the amplitude of the single-channel current at milimolar concentrations. However, little is known about the molecular mechanism of Kir channel blockade by external Mg2+ and the relationship between the Mg2+ blockade and activity potentiation by permeant K+ ions. In this study, we applied an interactive approach between theory and experiment. Electrophysiological recordings on Kir2.2 and its mutants were performed by heterologous expression in Xenopus laevis oocytes. Our results confirmed that extracellular Mg2+ could reduce heterologously expressed WT Kir2.2 currents in a voltage dependent manner. The kinetics of inhibition and recovery of Mg2+ exhibit a 3∼4s time constant. Molecular dynamics simulation results revealed a Mg2+ binding site located at the extracellular mouth of Kir2.2 that showed voltage-dependent Mg2+ binding. The mutants, G119D, Q126E and H128D, increased the number of permeant K+ ions and reduced the voltage-dependent blockade of Kir2.2 by extracellular Mg2+.

Cooperative subunit interactions mediate fast C-type inactivation of hERG1 K+ channels

At depolarized membrane potentials, the conductance of some voltage-gated K(+) channels is reduced by C-type inactivation. This gating process is voltage independent in Kv1 and involves a conformational change in the selectivity filter that is mediated by cooperative subunit interactions. C-type inactivation in hERG1 K(+) channels is voltage-dependent, much faster in onset and greatly attenuates currents at positive potentials. Here we investigate the potential role of subunit interactions in C-type inactivation of hERG1 channels. Point mutations in hERG1 known to eliminate (G628C/S631C), inhibit (S620T or S631A) or enhance (T618A or M645C) C-type inactivation were introduced into subunits that were combined with wild-type subunits to form concatenated tetrameric channels with defined subunit composition and stoichiometry. Channels were heterologously expressed in Xenopus oocytes and the two-microelectrode voltage clamp was used to measure the kinetics and steady-state properties of inactivation of whole cell currents. The effect of S631A or T618A mutations on inactivation was a graded function of the number of mutant subunits within a concatenated tetramer as predicted by a sequential model of cooperative subunit interactions, whereas M645C subunits increased the rate of inactivation of concatemers, as predicted for subunits that act independently of one another. For mutations located within the inactivation gate proper (S620T or G628C/S631C), the presence of a single subunit in a concatenated hERG1 tetramer disrupted gating to the same extent as that observed for mutant homotetramers. Together, our findings indicate that the final step of C-type inactivation of hERG1 channels involves a concerted, all-or-none cooperative interaction between all four subunits, and that probing the mechanisms of channel gating with concatenated heterotypic channels should be interpreted with care, as conclusions regarding the nature of subunit interactions may depend on the specific mutation used to probe the gating process.

Going beyond nutrition: regulation of potassium homoeostasis as a common denominator of plant adaptive responses to environment

Partially and fully completed plant genome sequencing projects in both lower and higher plants allow drawing a comprehensive picture of the molecular and structural diversities of plant potassium transporter genes and their encoded proteins. While the early focus of the research in this field was aimed on the structure-function studies and understanding of the molecular mechanisms underlying K(+) transport, availability of Arabidopsis thaliana mutant collections in combination with micro-array techniques have significantly advanced our understanding of K(+) channel physiology, providing novel insights into the transcriptional regulation of potassium homeostasis in plants. More recently, posttranslational regulation of potassium transport systems has moved into the center stage of potassium transport research. The current review is focused on the most exciting developments in this field. By summarizing recent work on potassium transporter regulation we show that potassium transport in general, and potassium channels in particular, represent important targets and are mediators of the cellular responses during different developmental stages in a plant's life cycle. We show that regulation of intracellular K(+) homeostasis is essential to mediate plant adaptive responses to a broad range of abiotic and biotic stresses including drought, salinity, and oxidative stress. We further link post-translational regulation of K(+) channels with programmed cell death and show that K(+) plays a critical role in controlling the latter process. Thus, is appears that K(+) is not just the essential nutrient required to support optimal plant growth and yield but is also an important signaling agent mediating a wide range of plant adaptive responses to environment.

Ginseng ginsenoside pharmacology in the nervous system: involvement in the regulation of ion channels and receptors

Ginseng, the root of Panax ginseng C.A. Meyer, is one of the oldest traditional medicines and is thought to be a tonic. It has been claimed that ginseng may improve vitality and health. Recent studies have advanced ginseng pharmacology and shown that ginseng has various pharmacological effects in the nervous system. Ginsenosides, steroid glycosides extracted from ginseng, were one of the first class of biologically active plant glycosides identified. The diverse pharmacological effects of ginsenosides have been investigated through the regulation of various types of ion channels and receptors in neuronal cells and heterologous expression systems. Ginsenoside Rg3 regulates voltage-gated ion channels such as Ca(2+), K(+), and Na(+) channels, and ligand-gated ion channels such as GABAA, 5-HT3, nicotinic acetylcholine, and N-methyl-D-aspartate (NMDA) receptors through interactions with various sites including channel blocker binding sites, toxin-binding sites, channel gating regions, and allosteric channel regulator binding sites when the respective ion channels or receptors are stimulated with depolarization or ligand treatment. Treatment with ginsenoside Rg3 has been found to stabilize excitable cells by blocking influxes of cations such as Ca(2+) and Na(+), or by enhancing Cl(-) influx. The aim of this review is to present recent findings on the pharmacological functions of the ginsenosides through the interactions with ion channels and receptors. This review will detail the pharmacological applications of ginsenosides as neuroprotective drugs that target ion channels and ligand-gated ion channels.

ShakerIR and Kv1.5 mutant channels with enhanced slow inactivation also exhibit K⁺ o-dependent resting inactivation

Previous studies have shown that in N-type inactivation-removed Shaker (ShakerIR) channels, the T449K and T449A mutations result in enhanced slow inactivation. These mutant channels also show a loss of conductance in 0 mM K⁺ o that was attributed to an inactivation process occurring from the closed, resting state and which we refer to as resting inactivation. Similar behavior has also been observed in the Kv1.5 H463G mutant channel. To date, the time courses for the onset of and recovery from resting inactivation have been unknown, but a comparison of the kinetics for resting inactivation induced at -80 mV and slow inactivation evoked at +50 mV may provide information on whether these two processes are mechanistically related. Here, we present an analysis of the time courses for the onset of and recovery from [K⁺]o-dependent resting inactivation and depolarization-induced inactivation of these mutant channels. Despite the enhancement of slow inactivation in the ShakerIR T449K, T449A, and Kv1.5 H463G mutants, the time constant for slow inactivation at +50 mV (τ inact) was relatively insensitive to the increases or decreases of [K(+)]o, confirming that accelerated inactivation from the open state does not underlie the loss of conductance in 0 mM K⁺. For all three mutants, the time constant for resting inactivation (τ RI), induced by exposure to 0 mM K⁺ o solution at -80 mV, was at least an order of magnitude larger than τ inact. On the other hand, the time course of recovery at -80 mV of each mutant from 0 mM K(+) o-induced resting inactivation was the same as that from depolarization-induced slow inactivation. This latter result suggests that the 0 mM K⁺ o-induced resting inactivation of these mutant ShakerIR and Kv1.5 channels is mechanistically related to slow inactivation.

Stabilization of the conductive conformation of a voltage-gated K+ (Kv) channel: the lid mechanism

Voltage-gated K(+) (Kv) channels are molecular switches that sense membrane potential and in response open to allow K(+) ions to diffuse out of the cell. In these proteins, sensor and pore belong to two distinct structural modules. We previously showed that the pore module alone is a robust yet dynamic structural unit in lipid membranes and that it senses potential and gates open to conduct K(+) with unchanged fidelity. The implication is that the voltage sensitivity of K(+) channels is not solely encoded in the sensor. Given that the coupling between sensor and pore remains elusive, we asked whether it is then possible to convert a pore module characterized by brief openings into a conductor with a prolonged lifetime in the open state. The strategy involves selected probes targeted to the filter gate of the channel aiming to modulate the probability of the channel being open assayed by single channel recordings from the sensorless pore module reconstituted in lipid bilayers. Here we show that the premature closing of the pore is bypassed by association of the filter gate with two novel open conformation stabilizers: an antidepressant and a peptide toxin known to act selectively on Kv channels. Such stabilization of the conductive conformation of the channel is faithfully mimicked by the covalent attachment of fluorescein at a cysteine residue selectively introduced near the filter gate. This modulation prolongs the occupancy of permeant ions at the gate. It is this longer embrace between ion and gate that we conjecture underlies the observed stabilization of the conductive conformation. This study provides a new way of thinking about gating.

Regulation of Kv2.1 K(+) conductance by cell surface channel density

The Kv2.1 voltage-gated K(+) channel is found both freely diffusing over the plasma membrane and concentrated in micron-sized clusters localized to the soma, proximal dendrites, and axon initial segment of hippocampal neurons. In transfected HEK cells, Kv2.1 channels within cluster microdomains are nonconducting. Using total internal reflection fluorescence microscopy, the number of GFP-tagged Kv2.1 channels on the HEK cell surface was compared with K(+) channel conductance measured by whole-cell voltage clamp of the same cell. This approach indicated that, as channel density increases, nonclustered channels cease conducting. At the highest density observed, only 4% of all channels were conducting. Mutant Kv2.1 channels that fail to cluster also possessed the nonconducting state with 17% conducting K(+) at higher surface densities. The nonconducting state was specific to Kv2.1 as Kv1.4 was always conducting regardless of the cell-surface expression level. Anti-Kv2.1 immunofluorescence intensity, standardized to Kv2.1 surface density in transfected HEK cells, was used to determine the expression levels of endogenous Kv2.1 in cultured rat hippocampal neurons. Endogenous Kv2.1 levels were compared with the number of conducting channels determined by whole-cell voltage clamp. Only 13 and 27% of the endogenous Kv2.1 was conducting in neurons cultured for 14 and 20 d, respectively. Together, these data indicate that the nonconducting state depends primarily on surface density as opposed to cluster location and that this nonconducting state also exists for native Kv2.1 found in cultured hippocampal neurons. This excess of Kv2.1 protein relative to K(+) conductance further supports a nonconducting role for Kv2.1 in excitable tissues.

Voltage-gated potassium channel (K(v) 1) autoantibodies in patients with chagasic gut dysmotility and distribution of K(v) 1 channels in human enteric neuromusculature (autoantibodies in GI dysmotility)

BACKGROUND

Autoantibodies directed against specific neuronal antigens are found in a significant number of patients with gastrointestinal neuromuscular diseases (GINMDs) secondary to neoplasia. This study examined the presence of antineuronal antibodies in idiopathic GINMD and GINMD secondary to South American Trypanosomiasis. The GI distribution of voltage-gated potassium channels (VGKCs) was also investigated.

METHODS

Seventy-three patients were included in the study with diagnoses of primary achalasia, enteric dysmotility, chronic intestinal pseudo-obstruction, esophageal or colonic dysmotility secondary to Chagas' disease. Sera were screened for specific antibodies to glutamic acid decarboxylase, voltage-gated calcium channels (VGCCs; P/Q subtype), nicotinic acetylcholine receptors (nAChRs; α3 subtype), and voltage-gated potassium channels (VGKCs, K(V) 1 subtype) using validated immunoprecipitation assays. The distribution of six VGKC subunits (K(V) 1.1-1.6), including those known to be antigenic targets of anti-VGKC antibodies was immunohistochemically investigated in all main human GI tract regions.

KEY RESULTS

Three patients (14%) with chagasic GI dysmotility were found to have positive anti-VGKC antibody titers. No antibodies were detected in patients with idiopathic GINMD. The VGKCs were found in enteric neurons at every level of the gut in unique yet overlapping distributions. The VGKC expression in GI smooth muscle was found to be limited to the esophagus.

CONCLUSIONS & INFERENCES

A small proportion of patients with GI dysfunction secondary to Chagas' disease have antibodies against VGKCs. The presence of these channels in the human enteric nervous system may have pathological relevance to the growing number of GINMDs with which anti-VGKC antibodies have been associated.

Voltage sensor inactivation in potassium channels

In voltage-gated potassium (Kv) channels membrane depolarization causes movement of a voltage sensor domain. This conformational change of the protein is transmitted to the pore domain and eventually leads to pore opening. However, the voltage sensor domain may interact with two distinct gates in the pore domain: the activation gate (A-gate), involving the cytoplasmic S6 bundle crossing, and the pore gate (P-gate), located externally in the selectivity filter. How the voltage sensor moves and how tightly it interacts with these two gates on its way to adopt a relaxed conformation when the membrane is depolarized may critically determine the mode of Kv channel inactivation. In certain Kv channels, voltage sensor movement leads to a tight interaction with the P-gate, which may cause conformational changes that render the selectivity filter non-conductive (“P/C-type inactivation”). Other Kv channels may preferably undergo inactivation from pre-open closed-states during voltage sensor movement, because the voltage sensor temporarily uncouples from the A-gate. For this behavior, known as “preferential” closed-state inactivation, we introduce the term “A/C-type inactivation”. Mechanistically, P/C- and A/C-type inactivation represent two forms of “voltage sensor inactivation.”

Structural correlates of selectivity and inactivation in potassium channels

Potassium channels are involved in a tremendously diverse range of physiological applications requiring distinctly different functional properties. Not surprisingly, the amino acid sequences for these proteins are diverse as well, except for the region that has been ordained the "selectivity filter". The goal of this review is to examine our current understanding of the role of the selectivity filter and regions adjacent to it in specifying selectivity as well as its role in gating/inactivation and possible mechanisms by which these processes are coupled. Our working hypothesis is that an amino acid network behind the filter modulates selectivity in channels with the same signature sequence while at the same time affecting channel inactivation properties. This article is part of a Special Issue entitled: Membrane protein structure and function.

TWIK-1 two-pore domain potassium channels change ion selectivity and conduct inward leak sodium currents in hypokalemia

Background potassium (K+) channels, which are normally selectively permeable to K+, maintain the cardiac resting membrane potential at around -80 mV. In subphysiological extracellular K+ concentrations ([K+]o), which occur in pathological hypokalemia, the resting membrane potential of human cardiomyocytes can depolarize to around -50 mV, whereas rat and mouse cardiomyocytes become hyperpolarized, consistent with the Nernst equation for K+. This paradoxical depolarization of cardiomyocytes in subphysiological [K+]o, which may contribute to cardiac arrhythmias, is thought to involve an inward leak sodium (Na+) current. Here, we show that human cardiac TWIK-1 (also known as K2P1) two-pore domain K+ channels change ion selectivity, becoming permeable to external Na+, and conduct inward leak Na+ currents in subphysiological [K+]o. A specific threonine residue (Thr118) within the pore selectivity sequence TxGYG was required for this altered ion selectivity. Mouse cardiomyocyte-derived HL-1 cells exhibited paradoxical depolarization with ectopic expression of TWIK-1 channels, whereas TWIK-1 knockdown in human spherical primary cardiac myocytes eliminated paradoxical depolarization. These findings indicate that ion selectivity of TWIK-1 K+ channels changes during pathological hypokalemia, elucidate a molecular basis for inward leak Na+ currents that could trigger or contribute to cardiac paradoxical depolarization in lowered [K+]o, and identify a mechanism for regulating cardiac excitability.

Extracellular K+ elevates outward currents through Kir2.1 channels by increasing single-channel conductance

Outward currents through inward rectifier K+ channels (Kir) play a pivotal role in determining resting membrane potential and in controlling excitability in many cell types. Thus, the regulation of outward Kir current (IK1) is important for appropriate physiological functions. It is known that outward IK1 increases with increasing extracellular K+ concentration ([K+]o), but the underlying mechanism is not fully understood. A "K+-activation of K+-channel" hypothesis and a "blocking-particle" model have been proposed to explain the [K+]o-dependence of outward IK1. Yet, these mechanisms have not been examined at the single-channel level. In the present study, we explored the mechanisms that determine the amplitudes of outward IK1 at constant driving forces [membrane potential (Vm) minus reversal potential (EK)]. We found that increases in [K+]o elevated the single-channel current to the same extent as macroscopic IK1 but did not affect the channel open probability at a constant driving force. In addition, spermine-binding kinetics remained unchanged when [K+]o ranged from 1 to 150 mM at a constant driving force. We suggest the regulation of K+ permeation by [K+]o as a new mechanism for the [K+]o-dependence of outward IK1.

Mechanisms of closed-state inactivation in voltage-gated ion channels

Inactivation of voltage-gated ion channels is an intrinsic auto-regulatory process necessary to govern the occurrence and shape of action potentials and establish firing patterns in excitable tissues. Inactivation may occur from the open state (open-state inactivation, OSI) at strongly depolarized membrane potentials, or from pre-open closed states (closed-state inactivation, CSI) at hyperpolarized and modestly depolarized membrane potentials. Voltage-gated Na(+), K(+), Ca(2+) and non-selective cationic channels utilize both OSI and CSI. Whereas there are detailed mechanistic descriptions of OSI, much less is known about the molecular basis of CSI. Here, we review evidence for CSI in voltage-gated cationic channels (VGCCs) and recent findings that shed light on the molecular mechanisms of CSI in voltage-gated K(+) (Kv) channels. Particularly, complementary observations suggest that the S4 voltage sensor, the S4S5 linker and the main S6 activation gate are instrumental in the installment of CSI in Kv4 channels. According to this hypothesis, the voltage sensor may adopt a distinct conformation to drive CSI and, depending on the stability of the interactions between the voltage sensor and the pore domain, a closed-inactivated state results from rearrangements in the selectivity filter or failure of the activation gate to open. Kv4 channel CSI may efficiently exploit the dynamics of the subthreshold membrane potential to regulate spiking properties in excitable tissues.

Kinetic analysis of the effects of H+ or Ni2+ on Kv1.5 current shows that both ions enhance slow inactivation and induce resting inactivation

External H+ and Ni2+ ions inhibit Kv1.5 channels by increasing current decay during a depolarizing pulse and reducing the maximal conductance. Although the former may be attributed to an enhancement of slow inactivation occurring from the open state, the latter cannot. Instead, we propose that the loss of conductance is due to the induction, by H+ or Ni2+, of a resting inactivation process. To assess whether the two inactivation processes are mechanistically related, we examined the time courses for the onset of and recovery from H+- or Ni2+-enhanced slow inactivation and resting inactivation. Compared to the time course of H+- or Ni2+-enhanced slow inactivation at +50 mV, the onset of resting inactivation induced at 80 mV with either ion involves a relatively slower process. Recovery from slow inactivation under control conditions was bi-exponential, indicative of at least two inactivated states. Recovery following H+- or Ni2+-enhanced slow inactivation or resting inactivation had time constants similar to those for recovery from control slow inactivation, although H+ and Ni2+ biased inactivation towards states from which recovery was fast and slow, respectively. The shared time constants suggest that the H+- and Ni2+-enhanced slow inactivated and induced resting inactivated states are similar to those visited during control slow inactivation at pH 7.4. We conclude that in Kv1.5 H+ and Ni2+ differentially enhance a slow inactivation process that involves at least two inactivated states and that resting inactivation is probably a close variant of slow inactivation.

Quinidine interaction with Shab K+ channels: pore block and irreversible collapse of the K+ conductance

Quinidine is a commonly used antiarrhythmic agent and a tool to study ion channels. Here it is reported that quinidine equilibrates within seconds across the Sf9 plasma membrane, blocking the open pore of Shab channels from the intracellular side of the membrane in a voltage-dependent manner with 1:1 stoichiometry. On binding to the channels, quinidine interacts with pore K(+) ions in a mutually destabilizing manner. As a result, when the channels are blocked by quinidine with the cell bathed in an external medium lacking K(+), the Shab conductance G(K) collapses irreversibly, despite the presence of a physiological [K(+)] in the intracellular solution. The quinidine-promoted collapse of Shab G(K) resembles the collapse of Shaker G(K) observed with 0 K(+) solutions on both sides of the membrane: thus the extent of G(K) drop depends on the number of activating pulses applied in the presence of quinidine, but is independent of the pulse duration. Taken together the observations indicate that, as in Shaker, the quinidine-promoted collapse of Shab G(K) occurs during deactivation of the channels, at the end of each activating pulse, with a probability of 0.1 per pulse at 80 mV. It appears that when Shab channels are open, the pore conformation able to conduct is stable in the absence of K(+), but on deactivation this conformation collapses irreversibly.

The extracellular K+ concentration dependence of outward currents through Kir2.1 channels is regulated by extracellular Na+ and Ca2+

It has been known for more than three decades that outward Kir currents (I(K1)) increase with increasing extracellular K(+) concentration ([K(+)](o)). Although this increase in I(K1) can have significant impacts under pathophysiological cardiac conditions, where [K(+)](o) can be as high as 18 mm and thus predispose the heart to re-entrant ventricular arrhythmias, the underlying mechanism has remained unclear. Here, we show that the steep [K(+)](o) dependence of Kir2.1-mediated outward I(K1) was due to [K(+)](o)-dependent inhibition of outward I(K1) by extracellular Na(+) and Ca(2+). This could be accounted for by Na(+)/Ca(2+) inhibition of I(K1) through screening of local negative surface charges. Consistent with this, extracellular Na(+) and Ca(2+) reduced the outward single-channel current and did not increase open-state noise or decrease the mean open time. In addition, neutralizing negative surface charges with a carboxylate esterifying agent inhibited outward I(K1) in a similar [K(+)](o)-dependent manner as Na(+)/Ca(2+). Site-directed mutagenesis studies identified Asp(114) and Glu(153) as the source of surface charges. Reducing K(+) activation and surface electrostatic effects in an R148Y mutant mimicked the action of extracellular Na(+) and Ca(2+), suggesting that in addition to exerting a surface electrostatic effect, Na(+) and Ca(2+) might inhibit outward I(K1) by inhibiting K(+) activation. This study identified interactions of K(+) with Na(+) and Ca(2+) that are important for the [K(+)](o) dependence of Kir2.1-mediated outward I(K1).

Endocytic control of ion channel density as a target for cardiovascular disease

Ion channels encoded by the human ether-a-go-go-related gene (HERG) give rise to the rapidly activating delayed rectifier K+ current (IKr), the perturbation of which causes ventricular arrhythmias associated with inherited and acquired long QT syndrome. Electrolyte imbalances, such as reduced serum K+ levels (hypokalemia), also trigger these potentially fatal arrhythmias. In this issue of the JCI, Guo et al. report that physiological levels of serum K+ are required to maintain normal HERG surface density in HEK 293 cells and IKr in rabbit cardiomyocytes. They found that hypokalemia evoked HERG channel ubiquitination, enhanced internalization via endocytosis, and ultimately degradation at the lysosome, thus identifying unbridled turnover as a mechanism of hypokalemia-induced arrhythmia. But too little channel turnover can also cause disease, as suggested by Kruse et al. in a study also in this issue. The authors identified mutations in TRPM4--a nonselective cation channel--in a large family with progressive familial heart block type I and showed that these mutations prevented channel internalization (see the related articles beginning on pages 2745 and 2737, respectively).

Distinct roles of the last transmembrane domain in controlling Arabidopsis K+ channel activity

The family of voltage-gated potassium channels in plants presumably evolved from a common ancestor and includes both inward-rectifying (K(in)) channels that allow plant cells to accumulate K(+) and outward-rectifying (K(out)) channels that mediate K(+) efflux. Despite their close structural similarities, the activity of K(in) channels is largely independent of K(+) and depends only on the transmembrane voltage, whereas that of K(out) channels responds to the membrane voltage and the prevailing extracellular K(+) concentration. Gating of potassium channels is achieved by structural rearrangements within the last transmembrane domain (S6). Here we investigated the functional equivalence of the S6 helices of the K(in) channel KAT1 and the K(out) channel SKOR by domain-swapping and site-directed mutagenesis. Channel mutants and chimeras were analyzed after expression in Xenopus oocytes. We identified two discrete regions that influence gating differently in both channels, demonstrating a lack of functional complementarity between KAT1 and SKOR. Our findings are supported by molecular models of KAT1 and SKOR in the open and closed states. The role of the S6 segment in gating evolved differently during specialization of the two channel subclasses, posing an obstacle for the transfer of the K(+)-sensor from K(out) to K(in) channels.

Voltage clamp fluorimetry reveals a novel outer pore instability in a mammalian voltage-gated potassium channel

Voltage-gated potassium (Kv) channel gating involves complex structural rearrangements that regulate the ability of channels to conduct K(+) ions. Fluorescence-based approaches provide a powerful technique to directly report structural dynamics underlying these gating processes in Shaker Kv channels. Here, we apply voltage clamp fluorimetry, for the first time, to study voltage sensor motions in mammalian Kv1.5 channels. Despite the homology between Kv1.5 and the Shaker channel, attaching TMRM or PyMPO fluorescent probes to substituted cysteine residues in the S3-S4 linker of Kv1.5 (M394C-V401C) revealed unique and unusual fluorescence signals. Whereas the fluorescence during voltage sensor movement in Shaker channels was monoexponential and occurred with a similar time course to ionic current activation, the fluorescence report of Kv1.5 voltage sensor motions was transient with a prominent rapidly dequenching component that, with TMRM at A397C (equivalent to Shaker A359C), represented 36 +/- 3% of the total signal and occurred with a tau of 3.4 +/- 0.6 ms at +60 mV (n = 4). Using a number of approaches, including 4-AP drug block and the ILT triple mutation, which dissociate channel opening from voltage sensor movement, we demonstrate that the unique dequenching component of fluorescence is associated with channel opening. By regulating the outer pore structure using raised (99 mM) external K(+) to stabilize the conducting configuration of the selectivity filter, or the mutations W472F (equivalent to Shaker W434F) and H463G to stabilize the nonconducting (P-type inactivated) configuration of the selectivity filter, we show that the dequenching of fluorescence reflects rapid structural events at the selectivity filter gate rather than the intracellular pore gate.

The Arabidopsis vacuolar malate channel is a member of the ALMT family

In plants, malate is a central metabolite and fulfills a large number of functions. Vacuolar malate may reach very high concentrations and fluctuate rapidly, whereas cytosolic malate is kept at a constant level allowing optimal metabolism. Recently, a vacuolar malate transporter (Arabidopsis thaliana tonoplast dicarboxylate transporter, AttDT) was identified that did not correspond to the well-characterized vacuolar malate channel. We therefore hypothesized that a member of the aluminum-activated malate transporter (ALMT) gene family could code for a vacuolar malate channel. Using GFP fusion constructs, we could show that AtALMT9 (A. thaliana ALMT9) is targeted to the vacuole. Promoter-GUS fusion constructs demonstrated that this gene is expressed in all organs, but is cell-type specific as GUS activity in leaves was detected nearly exclusively in mesophyll cells. Patch-clamp analysis of an Atalmt9 T-DNA insertion mutant exhibited strongly reduced vacuolar malate channel activity. In order to functionally characterize AtALMT9 as a malate channel, we heterologously expressed this gene in tobacco and in oocytes. Overexpression of AtALMT9-GFP in Nicotiana benthamiana leaves strongly enhanced the malate current densities across the mesophyll tonoplasts. Functional expression of AtALMT9 in Xenopus oocytes induced anion currents, which were clearly distinguishable from endogenous oocyte currents. Our results demonstrate that AtALMT9 is a vacuolar malate channel. Deletion mutants for AtALMT9 exhibit only slightly reduced malate content in mesophyll protoplasts and no visible phenotype, indicating that AttDT and the residual malate channel activity are sufficient to sustain the transport activity necessary to regulate the cytosolic malate homeostasis.

Speeding the recovery from ultraslow inactivation of voltage-gated Na+ channels by metal ion binding to the selectivity filter: a foot-on-the-door?

Slow inactivated states in voltage-gated ion channels can be modulated by binding molecules both to the outside and to the inside of the pore. Thus, external K(+) inhibits C-type inactivation in Shaker K(+) channels by a "foot-in-the-door" mechanism. Here, we explore the modulation of a very long-lived inactivated state, ultraslow inactivation (I(US)), by ligand binding to the outer vestibule in voltage-gated Na(+) channels. Blocking the outer vestibule by a mutant mu-conotoxin GIIIA substantially accelerated recovery from I(US). A similar effect was observed if Cd(2+) was bound to a cysteine engineered to the selectivity filter (K1237C). In K1237C channels, exposed to 30 microM Cd(2+), the time constant of recovery from I(US) was decreased from 145.0 +/- 10.2 s to 32.5 +/- 3.3 s (P < 0.001). Recovery from I(US) was only accelerated if Cd(2+) was added to the bath solution during recovery (V = -120 mV) from I(US), but not when the channels were selectively exposed to Cd(2+) during the development of I(US) (-20 mV). These data could be explained by a kinetic model in which Cd(2+) binds with high affinity to a slow inactivated state (I(S)), which is transiently occupied during recovery from I(US). A total of 50 microM Cd(2+) produced an approximately 8 mV hyperpolarizing shift of the steady-state inactivation curve of I(S), supporting this kinetic model. Binding of lidocaine to the internal vestibule significantly reduced the number of channels entering I(US), suggesting that I(US) is associated with a conformational change of the internal vestibule of the channel. We propose a molecular model in which slow inactivation (I(S)) occurs by a closure of the outer vestibule, whereas I(US) arises from a constriction of the internal vestibule produced by a widening of the selectivity filter region. Binding of Cd(2+) to C1237 promotes the closure of the selectivity filter region, thereby hastening recovery from I(US). Thus, Cd(2+) ions may act like a foot-on-the-door, kicking the I(S) gate to close.

SCAM analysis reveals a discrete region of the pore turret that modulates slow inactivation in Kv1.5

In Kv1.5, protonation of histidine 463 in the S5-P linker (turret) increases the rate of depolarization-induced inactivation and decreases the peak current amplitude. In this study, we examined how amino acid substitutions that altered the physico-chemical properties of the side chain at position 463 affected slow inactivation and then used the substituted cysteine accessibility method (SCAM) to probe the turret region (E456-P468) to determine whether residue 463 was unique in its ability to modulate the macroscopic current. Substitutions at position 463 of small, neutral (H463G and H463A) or large, charged (H463R, H463K, and H463E) side groups accelerated inactivation and induced a dependency of the current amplitude on the external potassium concentration. When cysteine substitutions were made in the distal turret (T462C-P468C), modification with either the positively charged [2-(trimethylammonium)ethyl] methanethiosulfonate bromide (MTSET) or negatively charged sodium (2-sulfonatoethyl) methanethiosulfonate reagent irreversibly inhibited current. This inhibition could be antagonized either by the R487V mutation (homologous to T449V in Shaker) or by raising the external potassium concentration, suggesting that current inhibition by MTS reagents resulted from an enhancement of inactivation. These results imply that protonation of residue 463 does not modulate inactivation solely by an electrostatic interaction with residues near the pore mouth, as proposed by others, and that residue 463 is part of a group of residues within the Kv1.5 turret that can modulate P/C-type inactivation.

Kv1.5 is a major component underlying the A-type potassium current in retinal arteriolar smooth muscle

Little is known about the molecular characteristics of the voltage-activated K⁺ (K_v) channels that underlie the A-type K⁺ current in vascular smooth muscle cells of the systemic circulation. We investigated the molecular identity of the A-type K⁺ current in retinal arteriolar myocytes using patch-clamp techniques, RT-PCR, immunohistochemistry, and neutralizing antibody studies. The A-type K⁺ current was resistant to the actions of specific inhibitors for K_v3 and K_v4 channels but was blocked by the K_v1 antagonist correolide. No effects were observed with pharmacological agents against K_v1.1/2/3/6 and 7 channels, but the current was partially blocked by riluzole, a K_v1.4 and K_v1.5 inhibitor. The current was not altered by the removal of extracellular K⁺ but was abolished by flecainide, indicative of K_v1.5 rather than K_v1.4 channels. Transcripts encoding K_v1.5 and not K_v1.4 were identified in freshly isolated retinal arterioles. Immunofluorescence labeling confirmed a lack of K_v1.4 expression and revealed K_v1.5 to be localized to the plasma membrane of the arteriolar smooth muscle cells. Anti-K_v1.5 antibody applied intracellularly inhibited the A-type K⁺ current, whereas anti-K_v1.4 antibody had no effect. Co-expression of K_v1.5 with K_vβ1 or K_vβ3 accessory subunits is known to transform K_v1.5 currents from delayed rectifers into A-type currents. K_vβ1 mRNA expression was detected in retinal arterioles, but K_vβ3 was not observed. K_vβ1 immunofluorescence was detected on the plasma membrane of retinal arteriolar myocytes. The findings of this study suggest that K_v1.5, most likely co-assembled with K_vβ1 subunits, comprises a major component underlying the A-type K⁺ current in retinal arteriolar smooth muscle cells.

Ion channels and ion transporters of the transverse tubular system of skeletal muscle

This review focuses on the electrical properties of the transverse (T) tubular membrane of skeletal muscle, with reference to the contribution of the T-tubular system (TTS) to the surface action potential, the radial spread of excitation and its role in excitation-contraction coupling. Particularly, the most important ion channels and ion transporters that enable proper depolarization and repolarization of the T-tubular membrane are described. Since propagation of excitation along the TTS into the depth of the fibers is a delicate balance between excitatory and inhibitory currents, the composition of channels and transporters is specific to the TTS and different from the surface membrane. The TTS normally enables the radial spread of excitation and the signal transfer to the sarcoplasmic reticulum to release calcium that activates the contractile apparatus. However, due to its structure, even slight shifts of ions may alter its volume, Nernstian potentials, ion permeabilities, and consequently T-tubular membrane potential and excitability.

A trapped intracellular cation modulates K+ channel recovery from slow inactivation

Upon depolarization, many voltage-gated potassium channels undergo a time-dependent decrease in conductance known as inactivation. Both entry of channels into an inactivated state and recovery from this state govern cellular excitability. In this study, we show that recovery from slow inactivation is regulated by intracellular permeant cations. When inactivated channels are hyperpolarized, closure of the activation gate traps a cation between the activation and inactivation gates. The identity of the trapped cation determines the rate of recovery, and the ability of cations to promote recovery follows the rank order K+ > NH4+ > Rb+ > Cs+ >> Na+, TMA. The striking similarity between this rank order and that for single channel conductance suggests that these two processes share a common feature. We propose that the rate of recovery from slow inactivation is determined by the ability of entrapped cations to move into a binding site in the channel's selectivity filter, and refilling of this site is required for recovery.

Properties of shaker-type potassium channels in higher plants

Potassium (K(+)), the most abundant cation in biological organisms, plays a crucial role in the survival and development of plant cells, modulation of basic mechanisms such as enzyme activity, electrical membrane potentials, plant turgor and cellular homeostasis. Due to the absence of a Na(+)/K(+) exchanger, which widely exists in animal cells, K(+) channels and some type of K(+) transporters function as K(+) uptake systems in plants. Plant voltage-dependent K(+) channels, which display striking topological and functional similarities with the voltage-dependent six-transmembrane segment animal Shaker-type K(+) channels, have been found to play an important role in the plasma membrane of a variety of tissues and organs in higher plants. Outward-rectifying, inward-rectifying and weakly-rectifying K(+) channels have been identified and play a crucial role in K(+) homeostasis in plant cells. To adapt to the environmental conditions, plants must take advantage of the large variety of Shaker-type K(+) channels naturally present in the plant kingdom. This review summarizes the extensive data on the structure, function, membrane topogenesis, heteromerization, expression, localization, physiological roles and modulation of Shaker-type K(+) channels from various plant species. The accumulated results also help in understanding the similarities and differences in the properties of Shaker-type K(+) channels in plants in comparison to those of Shaker channels in animals and bacteria.

Intracellular K+ sensing of SKOR, a Shaker-type K+ channel from Arabidopsis

Most K+ channels in plants are structurally classified into the Shaker family named after the shaker K+ channel in Drosophila. Plant K+ channels function in many physiological processes including osmotic regulation and K+ nutrition. An outwardly rectifying K+ channel, SKOR, mediates the delivery of K+ from stelar cells to the xylem in the roots, a critical step in the long-distance distribution of K+ from roots to the upper parts of the plant. Here we report that SKOR channel activity is strictly dependent on intracellular K+ concentrations. Activation by K+ did not affect the kinetics of voltage dependence in SKOR, indicating that a voltage-independent gating mechanism underlies the K+ sensing process. Further analysis showed that the C-terminal non-transmembrane region of the SKOR protein was required for this sensing process. The intracellular K+ sensing mechanism couples SKOR activity to K+ nutrition status in the 'source cells', thereby establishing a supply-based unloading system for the regulation of K+ distribution.

External K+ modulates the activity of the Arabidopsis potassium channel SKOR via an unusual mechanism

Plant outward-rectifying K+ channels mediate K+ efflux from guard cells during stomatal closure and from root cells into the xylem for root-shoot allocation of potassium (K). Intriguingly, the gating of these channels depends on the extracellular K+ concentration, although the ions carrying the current are derived from inside the cell. This K+ dependence confers a sensitivity to the extracellular K+ concentration ([K+]) that ensures that the channels mediate K+ efflux only, regardless of the [K+] prevailing outside. We investigated the mechanism of K+-dependent gating of the K+ channel SKOR of Arabidopsis by site-directed mutagenesis. Mutations affecting the intrinsic K+ dependence of gating were found to cluster in the pore and within the sixth transmembrane helix (S6), identifying an 'S6 gating domain' deep within the membrane. Mapping the SKOR sequence to the crystal structure of the voltage-dependent K+ channel KvAP from Aeropyrum pernix suggested interaction between the S6 gating domain and the base of the pore helix, a prediction supported by mutations at this site. These results offer a unique insight into the molecular basis for a physiologically important K+-sensory process in plants.