Regulation of mammalian Shaker-related K+ channels: evidence for non-conducting closed and non-conducting inactivated states

Unmasking subtype-dependent susceptibility to C-type inactivation in mammalian Kv1 channels

Shaker potassium channels have been an essential model for studying inactivation of ion channels and shaped our earliest understanding of N-type vs. C-type mechanisms. In early work describing C-type inactivation, López-Barneo and colleagues systematically characterized numerous mutations of Shaker residue T449, demonstrating that this position was a key determinant of C-type inactivation rate. In most of the closely related mammalian Kv1 channels, however, a persistent enigma has been that residue identity at this position has relatively modest effects on the rate of inactivation in response to long depolarizations. In this study, we report alternative ways to measure or elicit conformational changes in the outer pore associated with C-type inactivation. Using a strategically substituted cysteine in the outer pore, we demonstrate that mutation of Kv1.2 V381 (equivalent to Shaker T449) or W366 (Shaker W434) markedly increases susceptibility to modification by extracellularly applied MTSET. Moreover, due to the cooperative nature of C-type inactivation, Kv1.2 assembly in heteromeric channels markedly inhibits MTSET modification of this substituted cysteine in neighboring subunits. The identity of Kv1.2 residue V381 also markedly influences function in conditions that bias channels toward C-type inactivation, namely when Na+ is substituted for K+ as the permeant ion or when channels are blocked by an N-type inactivation particle (such as Kvβ1.2). Overall, our findings illustrate that in mammalian Kv1 channels, the identity of the T449-equivalent residue can strongly influence function in certain experimental conditions, even while having modest effects on apparent inactivation during long depolarizations. These findings contribute to reconciling differences in experimental outcomes in many Kv1 channels vs. Shaker.

Mutations within the selectivity filter reveal that Kv1 channels have distinct propensities to slow inactivate

Voltage-activated potassium (Kv) channels open in response to membrane depolarization and subsequently inactivate through distinct mechanisms. For the model Shaker Kv channel from Drosophila, fast N-type inactivation is thought to occur by a mechanism involving blockade of the internal pore by the N-terminus, whereas slow C-type inactivation results from conformational changes in the ion selectivity filter in the external pore. Kv channel inactivation plays critical roles in shaping the action potential and regulating firing frequency, and has been implicated in a range of diseases including episodic ataxia and arrhythmias. Although structures of the closely related Shaker and Kv1.2 channels containing mutations that promote slow inactivation both support a mechanism involving dilation of the outer selectivity filter, mutations in the outer pores of these two Kv channels have been reported to have markedly distinct effects on slow inactivation, raising questions about the extent to which slow inactivation is related in both channels. In this study, we characterized the influence of a series of mutations within the external pore of Shaker and Kv1.2 channels and observed many distinct mutant phenotypes. We find that mutations at four positions near the selectivity filter promote inactivation less dramatically in Kv1.2 when compared to Shaker, and they identify one key variable position (T449 in Shaker and V381 in Kv1.2) underlying the different phenotypes in the two channels. Collectively, our results suggest that Kv1.2 is less prone to inactivate compared to Shaker, yet support a common mechanism of inactivation in the two channels.

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.

Rapid constriction of the selectivity filter underlies C-type inactivation in the KcsA potassium channel

C-type inactivation in K⁺ channels is thought to be a result of constriction of the selectivity filter. By using MD simulations, Li et al. show that rapid constriction occurs within 1–2 s when the intracellular activation gate is fully open, but not when the gate is closed or partially open.

C-type inactivation is a time-dependent process observed in many K⁺ channels whereby prolonged activation by an external stimulus leads to a reduction in ionic conduction. While C-type inactivation is thought to be a result of a constriction of the selectivity filter, the local dynamics of the process remain elusive. Here, we use molecular dynamics (MD) simulations of the KcsA channel to elucidate the nature of kinetically delayed activation/inactivation gating coupling. Microsecond-scale MD simulations based on the truncated form of the KcsA channel (C-terminal domain deleted) provide a first glimpse of the onset of C-type inactivation. We observe over multiple trajectories that the selectivity filter consistently undergoes a spontaneous and rapid (within 1–2 µs) transition to a constricted conformation when the intracellular activation gate is fully open, but remains in the conductive conformation when the activation gate is closed or partially open. Multidimensional umbrella sampling potential of mean force calculations and nonequilibrium voltage-driven simulations further confirm these observations. Electrophysiological measurements show that the truncated form of the KcsA channel inactivates faster and greater than full-length KcsA, which is consistent with truncated KcsA opening to a greater degree because of the absence of the C-terminal domain restraint. Together, these results imply that the observed kinetics underlying activation/inactivation gating reflect a rapid conductive-to-constricted transition of the selectivity filter that is allosterically controlled by the slow opening of the intracellular gate.

The incretin hormone glucagon-like peptide 1 increases mitral cell excitability by decreasing conductance of a voltage-dependent potassium channel

Key points

The gut hormone called glucagon‐like peptide 1 (GLP‐1) is a strong moderator of energy homeostasis and communication between the peripheral organs and the brain.

GLP‐1 signalling occurs in the brain; using a newly developed genetic reporter line of mice, we have discovered GLP‐synthesizing cells in the olfactory bulb.

GLP‐1 increases the firing frequency of neurons (mitral cells) that encode olfactory information by decreasing activity of voltage‐dependent K channels (Kv1.3).

Modifying GLP‐1 levels, either therapeutically or following the ingestion of food, could alter the excitability of neurons in the olfactory bulb in a nutrition or energy state‐dependent manner to influence olfactory detection or metabolic sensing.

The results of the present study uncover a new function for an olfactory bulb neuron (deep short axon cells, Cajal cells) that could be capable of modifying mitral cell activity through the release of GLP‐1. This might be of relevance for the action of GLP‐1 mimetics now widely used in the treatment of diabetes.

Abstract

The olfactory system is intricately linked with the endocrine system where it may serve as a detector of the internal metabolic state or energy homeostasis in addition to its classical function as a sensor of external olfactory information. The recent development of transgenic mGLU‐yellow fluorescent protein mice that express a genetic reporter under the control of the preproglucagon reporter suggested the presence of the gut hormone, glucagon‐like peptide (GLP‐1), in deep short axon cells (Cajal cells) of the olfactory bulb and its neuromodulatory effect on mitral cell (MC) first‐order neurons. A MC target for the peptide was determined using GLP‐1 receptor binding assays, immunocytochemistry for the receptor and injection of fluorescence‐labelled GLP‐1 analogue exendin‐4. Using patch clamp recording of olfactory bulb slices in the whole‐cell configuration, we report that GLP‐1 and its stable analogue exendin‐4 increase the action potential firing frequency of MCs by decreasing the interburst interval rather than modifying the action potential shape, train length or interspike interval. GLP‐1 decreases Kv1.3 channel contribution to outward currents in voltage clamp recordings as determined by pharmacological blockade of Kv1.3 or utilizing mice with Kv1.3 gene‐targeted deletion as a negative control. Because fluctuations in GLP‐1 concentrations monitored by the olfactory bulb can modify the firing frequency of MCs, olfactory coding could change depending upon nutritional or physiological state. As a regulator of neuronal activity, GLP‐1 or its analogue may comprise a new metabolic factor with a potential therapeutic target in the olfactory bulb (i.e. via intranasal delivery) for controlling an imbalance in energy homeostasis.

The gut hormone called glucagon‐like peptide 1 (GLP‐1) is a strong moderator of energy homeostasis and communication between the peripheral organs and the brain.

GLP‐1 signalling occurs in the brain; using a newly developed genetic reporter line of mice, we have discovered GLP‐synthesizing cells in the olfactory bulb.

GLP‐1 increases the firing frequency of neurons (mitral cells) that encode olfactory information by decreasing activity of voltage‐dependent K channels (Kv1.3).

Modifying GLP‐1 levels, either therapeutically or following the ingestion of food, could alter the excitability of neurons in the olfactory bulb in a nutrition or energy state‐dependent manner to influence olfactory detection or metabolic sensing.

The results of the present study uncover a new function for an olfactory bulb neuron (deep short axon cells, Cajal cells) that could be capable of modifying mitral cell activity through the release of GLP‐1. This might be of relevance for the action of GLP‐1 mimetics now widely used in the treatment of diabetes.

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.

Charybdotoxin and margatoxin acting on the human voltage-gated potassium channel hKv1.3 and its H399N mutant: an experimental and computational comparison

The effect of the pore-blocking peptides charybdotoxin and margatoxin, both scorpion toxins, on currents through human voltage-gated hK(v)1.3 wild-type and hK(v)1.3_H399N mutant potassium channels was characterized by the whole-cell patch clamp technique. In the mutant channels, both toxins hardly blocked current through the channels, although they did prevent C-type inactivation by slowing down the current decay during depolarization. Molecular dynamics simulations suggested that the fast current decay in the mutant channel was a consequence of amino acid reorientations behind the selectivity filter and indicated that the rigidity-flexibility in that region played a key role in its interactions with scorpion toxins. A channel with a slightly more flexible selectivity filter region exhibits distinct interactions with scorpion toxins. Our studies suggest that the toxin-channel interactions might partially restore rigidity in the selectivity filter and thereby prevent the structural rearrangements associated with C-type inactivation.

Effect of verapamil on the action of methanethiosulfonate reagents on human voltage-gated K(v)1.3 channels: implications for the C-type inactivated state

BACKGROUND AND PURPOSE

Voltage-gated K(v)1.3 channels appear on T-lymphocytes and are characterized by their typical C-type inactivation. In order to develop drugs stabilizing the C-type inactivated state and thus potentially useful in treatment of autoimmune diseases, it is important to know more about the three-dimensional structure of this inactivated state of the channel.

EXPERIMENTAL APPROACH

The patch-clamp technique was used to study effects of methanethiosulphonate (MTS) compounds on currents through wild-type human K(v)1.3 (hK(v)1.3) and two mutant channels, hK(v)1.3 V417C and hK(v) 1.3 H399T-V417C, in the closed, open and inactivated states.

KEY RESULTS

Extracellular application of 2-aminoethyl methanethiosulphonate (MTSEA) irreversibly reduced currents through hK(v) 1.3 V417C channels in the open and inactivated, but not in the closed state, indicating that a modification was possible. Co-application of verapamil prevented this reduction. Intracellular application of MTSEA and [2-(trimethylammonium)ethyl] methanethiosulphonate (MTSET) also modified the mutant channels, whereas extra- and intracellular application of sodium (2-sulfonatoethyl)methanethiosulphonate (MTSES) and intracellular application of MTSET did not.

CONCLUSIONS AND IMPLICATIONS

Our experiments showed that the binding site for MTS compounds was intracellular in the mutant channels and that the V417C mutant channels were modified in the open and the inactivated states, and this modification was prevented by verapamil. Therefore, the activation gate on the intracellular side of the selectivity filter must be open during inactivation. Furthermore, although the S6 segment is moving further apart during inactivation, this change does not include a movement of the side chain of the amino acid at position 417, away from lining the channel pore.

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.

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.

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).

Initial segment Kv2.2 channels mediate a slow delayed rectifier and maintain high frequency action potential firing in medial nucleus of the trapezoid body neurons

The medial nucleus of the trapezoid body (MNTB) is specialized for high frequency firing by expression of Kv3 channels, which minimize action potential (AP) duration, and Kv1 channels, which suppress multiple AP firing, during each calyceal giant EPSC. However, the outward K(+) current in MNTB neurons is dominated by another unidentified delayed rectifier. It has slow kinetics and a peak conductance of approximately 37 nS; it is half-activated at -9.2 +/- 2.1 mV and half-inactivated at -35.9 +/- 1.5 mV. It is blocked by several non-specific potassium channel antagonists including quinine (100 microm) and high concentrations of extracellular tetraethylammonium (TEA; IC(50) = 11.8 mM), but no specific antagonists were found. These characteristics are similar to recombinant Kv2-mediated currents. Quantitative RT-PCR showed that Kv2.2 mRNA was much more prevalent than Kv2.1 in the MNTB. A Kv2.2 antibody showed specific staining and Western blots confirmed that it recognized a protein approximately 110 kDa which was absent in brainstem tissue from a Kv2.2 knockout mouse. Confocal imaging showed that Kv2.2 was highly expressed in axon initial segments of MNTB neurons. In the absence of a specific antagonist, Hodgkin-Huxley modelling of voltage-gated conductances showed that Kv2.2 has a minor role during single APs (due to its slow activation) but assists recovery of voltage-gated sodium channels (Nav) from inactivation by hyperpolarizing interspike potentials during repetitive AP firing. Current-clamp recordings during high frequency firing and characterization of Nav inactivation confirmed this hypothesis. We conclude that Kv2.2-containing channels have a distinctive initial segment location and crucial function in maintaining AP amplitude by regulating the interspike potential during high frequency firing.

Kv4 (A-type) potassium currents in the mouse medial nucleus of the trapezoid body

Principal neurones of the mouse medial nucleus of the trapezoid body (MNTB) possess multiple voltage-gated potassium currents, including a transient outward current (or A-current), which is characterized here. The A-current exhibited rapid voltage-dependent inactivation and was half inactivated at resting membrane potentials. Following a hyperpolarizing pre-pulse to remove inactivation, the peak transient current was 1.07 nA at -17 mV. The pharmacological characteristics of this A-current were consistent with Kv4 subunits in expression studies; the A-current was resistant to block by tetraethylammonium and dendrotoxin-I but sensitive to millimolar concentrations of 4-aminopyridine and 5 microM hanatoxin. Immunohistochemistry confirmed that Kv4.3 sub-units are present in the MNTB. In a single-compartment model of an MNTB neurone, the A-current served to accelerate the decay of the initial action potentials in a stimulus train and suggested that removal of A-current steady-state inactivation could raise firing threshold for non-calyceal synaptic inputs. This A-type current was not observed in the rat.

Novel pharmacological activity of loperamide and CP-339,818 on human HCN channels characterized with an automated electrophysiology assay

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels underlie the pacemaker currents in neurons (I(h)) and cardiac (I(f)) cells. As such, the identification and characterization of novel blockers of HCN channels is important to enable the dissection of their function in vivo. Using a new IonWorks HT electrophysiology assay with human HCN1 and HCN4 expressed stably in cell lines, four HCN channel blockers are characterized. Two blockers known for their activity at opioid/Ca(2+) channels and K(+) channels, loperamide and CP-339,818 (respectively), are described to block HCN1 more potently than HCN4. The known HCN blocker ZD7288 was also found to be more selective for HCN1 over HCN4, while the HCN blocker DK-AH269 was equipotent on HCN4 and HCN1. Partial replacement of the intracellular Cl(-) with gluconate reduced the potency on both channels, but to varying degrees. For both HCN1 and HCN4, ZD7288 was most sensitive in lower Cl(-) solutions, while the potency of loperamide was not affected by the differing solutions. The block of HCN1 for all compounds was voltage-dependent, being relieved at more negative potentials. The voltage-dependent, Cl(-) dependent, HCN1 preferring compounds described here elaborate on the current known pharmacology of HCN channels and may help provide novel tools and chemical starting points for the investigation of HCN channel function in natively expressing systems.

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.

K+-dependent stability and ion conduction of Shab K+ channels: a comparison with Shaker channels

K+ depletion exerts dramatically variable effects on different potassium channels. Here we report that Shab channels are rather stable in the absence of either internal or external K+ alone; however, its stability is greater with K+ outside the cell. In contrast, with 0 K+ (non-added) solutions on both sides of the membrane, the conductance (G(K)) is rapidly and irreversibly lost. G(K) is lost with the channels closed and regardless of the composition of the 0 K+ solutions. In comparison, it is known that the Shaker B G(K) collapses only if the channels are gated in 0 K+, Na+-containing solutions. In order to compare the behavior of Shab to that of Shaker, we show that after extensively gating the channels in 0 K+ N-methyl-D-glucamine solutions, most Shaker channels remain stable, and in a conformation where G(K) collapses as soon as there is Na+ in the solutions. Regarding ion conduction, in contrast to Kv2.1 and Shaker A463C that have a sizable G(Na) in 0 K+, Shab, which shares a 463-cysteine and an identical signature sequence with these channels, does not appreciably conduct Na+, although it presents a significant Cs+ conductance. The observations suggest that there are at least two sites where K+ binds and thus maintains Shab G(K) stable, one internal and the other(s) most likely located outside the selectivity filter.

Regulation by external K+ in a maize inward shaker channel targets transport activity in the high concentration range

An inward Shaker K(+) channel identified in Zea mays (maize), ZmK2.1, displays strong regulation by external K(+) when expressed in Xenopus laevis (African clawed frog) oocytes or COS cells. ZmK2.1 is specifically activated by K(+) with an apparent K(m) close to 15 mM independent of the membrane hyperpolarization level. In the absence of K(+), ZmK2.1 appears to enter a nonconducting state. Thus, whatever the membrane potential, this maize channel cannot mediate K(+) influx in the submillimolar concentration range, unlike its relatives in Arabidopsis thaliana. Its expression is restricted to the shoots, the strongest signal (RT-PCR) being associated with vascular/bundle sheath strands. Based on sequence and gene structure, the closest relatives of ZmK2.1 in Arabidopsis are K(+) Arabidopsis Transporter 1 (KAT1) (expressed in guard cells) and KAT2 (expressed in guard cells and leaf phloem). Patch-clamp analyses of guard cell protoplasts reveal a higher functional diversity of K(+) channels in maize than in Arabidopsis. Channels endowed with regulation by external K(+) similar to that of ZmK2.1 (channel activity regulated by external K(+) with a K(m) close to 15 mM, regulation independent of external Ca(2+)) constitute a major component of the maize guard cell inward K(+) channel population. The presence of such channels in maize might reflect physiological traits of C4 and/or monocotyledonous plants.

Inhibition of the collapse of the Shaker K+ conductance by specific scorpion toxins

The Shaker B K(+) conductance (G(K)) collapses when the channels are closed (deactivated) in Na(+) solutions that lack K(+) ions. Also, it is known that external TEA (TEA(o)) impedes the collapse of G(K), and that channel block by TEA(o) and scorpion toxins are two mutually exclusive events. Therefore, we tested the ability of scorpion toxins to inhibit the collapse of G(K) in 0 K(+). We have found that these toxins are not uniform regarding the capacity to protect G(K). Those toxins, whose binding to the channels is destabilized by external K(+), are also effective inhibitors of the collapse of G(K). In addition to K(+), other externally added cations also destabilize toxin block, with an effectiveness that does not match the selectivity sequence of K(+) channels. The inhibition of the drop of G(K) follows a saturation relationship with [toxin], which is fitted well by the Michaelis-Menten equation, with an apparent Kd bigger than that of block of the K(+) current. However, another plausible model is also presented and compared with the Michaelis-Menten model. The observations suggest that those toxins that protect G(K) in 0 K(+) do so by interacting either with the most external K(+) binding site of the selectivity filter (suggesting that the K(+) occupancy of only that site of the pore may be enough to preserve G(K)) or with sites capable of binding K(+) located in the outer vestibule of the pore, above the selectivity filter.

pH-dependent modulation of Kv1.3 inactivation: role of His399

The Kv1.3 K(+) channel lacks N-type inactivation, but during prolonged depolarized periods it inactivates via the slow (P/C type) mechanism. It bears a titratable histidine residue in position 399 (equivalent of Shaker 449), a site known to influence the rate of slow inactivation. As opposed to several other voltage-gated K(+) channels, slow inactivation of Kv1.3 is slowed when extracellular pH (pH(o)) is lowered under physiological conditions. Our findings are as follows. First, when His399 was mutated to a lysine, arginine, leucine, valine or tyrosine, extracellular acidification (pH 5.5) accelerated inactivation reminiscent of other Kv channels. Second, inactivation of the wild-type channel was accelerated by low pH(o) when the ionic strength of the external solution was raised. Inactivation of the H399K mutant was also accelerated by high ionic strength at pH 7.35 but not the inactivation of H399L. Third, after the external application of blocking barium ions, recovery of the wild-type current during washout was slower in low pH(o). Fourth, the dissociation rate of Ba(2+) was pH insensitive for both H399K and H399L. Furthermore, Ba(2+) dissociation rates were equal for H399K and the wild type at pH 5.5 and were equal for H399L and the wild type at pH 7.35. These observations support a model in which the electric field of the protonated histidines creates a potential barrier for potassium ions just outside the external mouth of the pore that hinders their exit from the binding site controlling inactivation. In Kv1.3, this effect overrides the generally observed speeding of slow inactivation when pH(o) is reduced.

Metabolic regulation of potassium channels

Potassium (K+) channels exist in all three domains of organisms: eubacteria, archaebacteria, and eukaryotes. In higher animals, these membrane proteins participate in a multitude of critical physiological processes, including food and fluid intake, locomotion, stress response, and cognitive functions. Metabolic regulatory factors such as O2, CO2/pH, redox equivalents, glucose/ATP/ADP, hormones, eicosanoids, cell volume, and electrolytes regulate a diverse group of K+ channels to maintain homeostasis.

Mechanisms of the inhibition of Shaker potassium channels by protons

Potassium channels are regulated by protons in various ways and, in most cases, acidification results in potassium current reduction. To elucidate the mechanisms of proton-channel interactions we investigated N-terminally truncated Shaker potassium channels (Kv1 channels) expressed in Xenopus oocytes, varying pH at the intracellular and the extracellular face of the membrane. Intracellular acidification resulted in rapid and reversible channel block. The block was half-maximal at pH 6.48, thus even physiological excursions of intracellular pH will have an impact on K+ current. The block displayed only very weak voltage dependence and C-type inactivation and activation were not affected. Extracellular acidification (up to pH 4) did not block the channel, indicating that protons are effectively excluded from the selectivity filter. Channel current, however, was reduced greatly due to marked acceleration of C-type inactivation at low pH. In contrast, inactivation was not affected in the T449V mutant channel, in which C-type inactivation is impaired. The pH effect on inactivation of the wild-type channel had an apparent pK of 4.7, suggesting that protonation of extracellular acidic residues in Kv channels makes them subject to pH regulation.

Structure of the BgK-Kv1.1 complex based on distance restraints identified by double mutant cycles. Molecular basis for convergent evolution of Kv1 channel blockers

A structural model of BgK, a sea anemone toxin, complexed with the S5-S6 region of Kv1.1, a voltage-gated potassium channel, was determined by flexible docking under distance restraints identified by a double mutant cycles approach. This structure provides the molecular basis for identifying the major determinants of the BgK-Kv1.1 channel interactions involving the BgK dyad residues Lys(25) and Tyr(26). These interactions are (i) electrostatic interactions between the extremity of Lys(25) side chain and carbonyl oxygen atoms of residues from the channel selectivity filter that may be strengthened by solvent exclusion provided by (ii) hydrophobic interactions involving BgK residues Tyr(26) and Phe(6) and Kv1.1 residue Tyr(379) whose side chain protrudes in the channel vestibule. In other Kv1 channel-BgK complexes, these interactions are likely to be conserved, implicating both conserved and variable residues from the channels. The data suggest that the conservation in sea anemone and scorpion potassium channel blockers of a functional dyad composed of a lysine, and a hydrophobic residue reflects their use of convergent binding solutions based on a crucial interplay between these important conserved interactions.

Two heteromeric Kv1 potassium channels differentially regulate action potential firing

Low-threshold voltage-gated potassium currents (I(LT)) activating close to resting membrane potentials play an important role in shaping action potential (AP) firing patterns. In the medial nucleus of the trapezoid body (MNTB), I(LT) ensures generation of single APs during each EPSP, so that the timing and pattern of AP firing is preserved on transmission across this relay synapse (calyx of Held). This temporal information is critical for computation of sound location using interaural timing and level differences. I(LT) currents are generated by dendrotoxin-I-sensitive, Shaker-related K+ channels; our immunohistochemistry confirms that MNTB neurons express Kv1.1, Kv1.2, and Kv1.6 subunits. We used subunit-specific toxins to separate I(LT) into two components, each contributing approximately one-half of the total low-threshold current: (1) I(LTS), a tityustoxin-Kalpha-sensitive current (TsTX) (known to block Kv1.2 containing channels), and (2) I(LTR), an TsTX-resistant current. Both components were sensitive to the Kv1.1-specific toxin dendrotoxin-K and were insensitive to tetraethylammonium (1 mm). This pharmacological profile excludes homomeric Kv1.1 or Kv1.2 channels and is consistent with I(LTS) channels being Kv1.1/Kv1.2 heteromers, whereas I(LTR) channels are probably Kv1.1/Kv1.6 heteromers. Although they have similar kinetic properties, I(LTS) is critical for generating the phenotypic single AP response of MNTB neurons. Immunohistochemistry confirms that Kv1.1 and Kv1.2 (I(LTS) channels), but not Kv1.6, are concentrated in the first 20 microm of MNTB axons. Our results show that heteromeric channels containing Kv1.2 subunits govern AP firing and suggest that their localization at the initial segment of MNTB axons can explain their dominance of AP firing behavior.

Outer pore residues control the H(+) and K(+) sensitivity of the Arabidopsis potassium channel AKT3

The Arabidopsis phloem channel AKT3 is the founder of a subfamily of shaker-like plant potassium channels characterized by weak rectification, Ca(2+) block, proton inhibition, and, as shown in this study, K(+) sensitivity. In contrast to inward-rectifying, acid-activated K(+) channels of the KAT1 family, extracellular acidification decreases AKT3 currents at the macroscopic and single-channel levels. Here, we show that two distinct sites within the outer mouth of the K(+)-conducting pore provide the molecular basis for the pH sensitivity of this phloem channel. After generation of mutant channels and functional expression in Xenopus oocytes, we identified the His residue His-228, which is proximal to the K(+) selectivity filter (GYGD) and the distal Ser residue Ser-271, to be involved in proton susceptibility. Mutations of these sites, H228D and S271E, drastically reduced the H(+) and K(+) sensitivity of AKT3. Although in K(+)-free bath solutions outward K(+) currents were abolished completely in wild-type AKT3, S271E as well as the AKT3-HDSE double mutant still mediated K(+) efflux. We conclude that the pH- and K(+)-dependent properties of the AKT3 channel involve residues in the outer mouth of the pore. Both properties, H(+) and K(+) sensitivity, allow the fine-tuning of the phloem channel and thus seem to represent important elements in the control of membrane potential and sugar loading.

Molecular determinants of the inhibition of human Kv1.5 potassium currents by external protons and Zn(2+)

Using human Kv1.5 channels expressed in HEK293 cells we assessed the ability of H+o to mimic the previously reported action of Zn(2+) to inhibit macroscopic hKv1.5 currents, and using site-directed mutagenesis, we addressed the mechanistic basis for the inhibitory effects of H(+)(o) and Zn(2+). As with Zn(2+), H(+)(o) caused a concentration-dependent, K(+)(o)-sensitive and reversible reduction of the maximum conductance (g(max)). With zero, 5 and 140 mM K(+)(o) the pK(H) for this decrease of g(max) was 6.8, 6.2 and 6.0, respectively. The concentration dependence of the block relief caused by increasing [K(+)](o) was well fitted by a non-competitive interaction between H(+)(o) and K(+)(o), for which the K(D) for the K(+) binding site was 0.5-1.0 mM. Additionally, gating current analysis in the non-conducting mutant hKv1.5 W472F showed that changing from pH 7.4 to pH 5.4 did not affect Q(max) and that charge immobilization, presumed to be due to C-type inactivation, was preserved at pH 5.4. Inhibition of hKv1.5 currents by H+o or Zn(2+) was substantially reduced by a mutation either in the channel turret (H463Q) or near the pore mouth (R487V). In light of the requirement for R487, the homologue of Shaker T449, as well as the block-relieving action of K(+)(o), we propose that H(+) or Zn(2+) binding to histidine residues in the pore turret stabilizes a channel conformation that is most likely an inactivated state.

A Shaker homologue encodes an A-type current in Xenopus laevis

In Xenopus laevis, several distinct K(+)-channels (xKv1.1, xKv1.2, xKv2,1, xKv2.2, xKv3.1) have been cloned, sequenced, and electrophysiologically characterized. K(+)-channels significantly shape neuronal excitability by setting the membrane potential, and latency and duration of action potentials. We identified a further Shaker homologue, xKv1.4, in X. laevis. The open reading frame encodes a K(+)-channel that shares 72% of its 698 amino acids with the human Shaker homologue, hKv1.4. Northern blot analysis revealed xKv1.4 in the brain, muscle, and spleen but not in the ovary, intestine, heart, liver, kidney, lung, and skin. Whole-cell patch clamp recording from rat basophilic leukaemia (RBL) cells transfected with xKv1.4 revealed a voltage-gated, outward rectifying, transient A-type, K(+) selective current. xKv1.4 was strongly dependent on extracellular K(+). Exposure of cells to K(+) free bath solution almost completely abolished the current, whereas in the presence of high K(+), inactivation in response to a maintained depolarizing step and the frequency-dependent cumulative inactivation decreased. Ion channels encoded by xKv1.4 are sensitive to 4-aminopyridine and quinidine but insensitive to tetraethylammonium and the peptide toxins, charybdotoxin, margatoxin, and dendrotoxin. In conclusion, our results indicate that the biophysical and pharmacological signature of xKv1.4 closely resemble those of the A-current described in Xenopus embryonic neurons and is similar to the human Shaker homologue, hKv1.4.

Na(+) interaction with the pore of Shaker B K(+) channels: zero and low K(+) conditions

The Shaker B K(+) conductance (G(K)) collapses (in a reversible manner) if the membrane is depolarized and then repolarized in, 0 K(+), Na(+)-containing solutions (Gómez-Lagunas, F. 1997. J. Physiol. 499:3-15; Gómez-Lagunas, F. 1999. Biophys. J. 77:2988-2998). In this work, the role of Na(+) ions in the collapse of G(K) in 0-K(+) solutions, and in the behavior of the channels in low K(+) was studied. The main findings are as follows. First, in 0-K(+) solutions, the presence of Na(+) ions is an important factor that speeds the collapse of G(K). Second, external Na(+) fosters the drop of G(K) by binding to a site with a K(d) = 3.3 mM. External K(+) competes, in a mutually exclusive manner, with Na(o)(+) for binding to this site, with an estimated K(d) = 80 microM. Third, NMG and choline are relatively inert regarding the stability of G(K); fourth, with [K(o)(+)] = 0, the energy required to relieve Na(i)(+) block of Shaker (French, R.J., and J.B. Wells. 1977. J. Gen. Physiol. 70:707-724; Starkus, J.G., L. Kuschel, M. Rayner, and S. Heinemann. 2000. J. Gen. Physiol. 110:539-550) decreases with the molar fraction of Na(i)(+) (X(Na,i)), in an extent not accounted for by the change in Delta(mu)(Na). Finally, when X(Na,i) = 1, G(K) collapses by the binding of Na(i)(+) to two sites, with apparent K(d)s of 2 and 14.3 mM.

Regulation of a mammalian Shaker-related potassium channel, hKv1.5, by extracellular potassium and pH

Using the whole-cell recording mode of the patch-clamp technique we studied the effects of removal of extracellular potassium, [K(+)](o), on a mammalian Shaker-related K(+) channel, hKv1.5. In the absence of [K(+)](o), current through hKv1.5 was similar to currents obtained in the presence of 4.5 mM [K(+)](o). This observation was not expected as earlier results had suggested that either positively charged residues or the presence of a nitrogen-containing residue at the external TEA(+) binding site (R487 in hKv1.5) caused current loss upon removal of [K(+)](o). However, the current loss in hKv1.5 was observed when the extracellular pH, pH(o), was reduced from 7.4 to 6.0, a behavior similar to that observed previously for current through mKv1.3 with a histidine at the equivalent position (H404). These observations suggested that the charge at R487 in hKv1.5 channels was influenced by other amino acids in the vicinity. Replacement of a histidine at position 463 in hKv1.5 by glycine confirmed this hypothesis making this H463G mutant channel sensitive to removal of [K(+)](o) even at pH(o) 7.4. We conclude that the protonation of H463 at pH 7.4 might induce a pK(a) shift of R487 that influences the effective charge at this position leading to a not fully protonated arginine. Furthermore, we assume that the charge at position 487 in hKv1.5 can directly or indirectly disturb the occupation of a K(+) binding site within the channel pore possibly by electrostatic interaction. This in turn might interfere with the concerted transition of K(+) ions resulting in a loss of K(+) conduction.

Collapse of conductance is prevented by a glutamate residue conserved in voltage-dependent K(+) channels

Voltage-dependent K(+) channel gating is influenced by the permeating ions. Extracellular K(+) determines the occupation of sites in the channels where the cation interferes with the motion of the gates. When external [K(+)] decreases, some K(+) channels open too briefly to allow the conduction of measurable current. Given that extracellular K(+) is normally low, we have studied if negatively charged amino acids in the extracellular loops of Shaker K(+) channels contribute to increase the local [K(+)]. Surprisingly, neutralization of the charge of most acidic residues has minor effects on gating. However, a glutamate residue (E418) located at the external end of the membrane spanning segment S5 is absolutely required for keeping channels active at the normal external [K(+)]. E418 is conserved in all families of voltage-dependent K(+) channels. Although the channel mutant E418Q has kinetic properties resembling those produced by removal of K(+) from the pore, it seems that E418 is not simply concentrating cations near the channel mouth, but has a direct and critical role in gating. Our data suggest that E418 contributes to stabilize the S4 voltage sensor in the depolarized position, thus permitting maintenance of the channel open conformation.

Regulation of transient Na+ conductance by intra- and extracellular K+ in the human delayed rectifier K+ channel Kv1.5

1. Significant Na+ conductance has been described in only a few native and cloned K+ channels, but has been used to characterize inactivation and K+ binding within the permeation pathway, and to refine models of K+ flux through multi-ion pores. Here we use Na+ permeation of the delayed rectifier K+ channel Kv1.5 to study extra- and intracellular K+ (K+o and K+i, respectively) regulation of conductance and inactivation, using whole-cell recording from human embryonic kidney (HEK)-293 cells. 2. Kv1.5 Na+ currents in the absence of K+o and K+i were confirmed by: (i) resistance of outward Na+ currents to dialysis by K+-free solutions; (ii) tail current reversal potential changes with Na+o with a slope of 55.8 mV per decade; (iii) block by 4-aminopyridine (50 % at 50 microM), and resistance to Cl- channel inhibition. 3. Na+ currents were transient followed by a small sustained current. An envelope test confirmed that activated Kv1.5 channels conducted Na+, and that rapid current decay reflected C-type inactivation. Sustained currents ( approximately 13 % of peak) represented Na+ flux through inactivated Kv1.5 channels. 4. K+o could modulate the maximum available Na+ conductance in the stable cell line while channels were closed. Before the first pulse of a train, increasing K+o concentration increased the subsequent Na+ conductance from approximately 15 (0 mM K+o) to 30 nS (5 mM K+o), with a Kd of 23 microM. Repeated low rate depolarizations in Na+i/Na+o solutions induced a use-dependent loss of Kv1.5 channel Na+ conductance, distinct from that caused by C-type inactivation. K+o binding that sensed little of the electric field could prevent this secondary loss of available Kv1.5 channels with a Kd of 230 microM. These two effects on conductance were both voltage independent, and had no effect on channel inactivation rate. 5. K+o concentrations >= 0.3 mM slowed the inactivation rate in a strongly voltage-dependent manner. This suggested it could compete for binding at a K+ site or sites deeper in the pore, as well as restoring the Na+ conductance. K+i was able to modulate the inactivation rate but was unable to affect conductance. 6. Mutation of arginine 487 in the outer pore region of the channel to valine (R487V) greatly reduced C-type inactivation in Na+ solutions, caused loss of channel use dependence, and prevented any conductance increase upon the addition of 0.1 mM K+o. Our results confirm the existence of a high affinity binding site at the selectivity filter that regulates inactivation, and also reveals the presence of at least one additional high affinity outer mouth site that predominantly regulates conductance of resting channels, and protects channels activated by depolarization when they conduct Na+.

SK2 encodes the apamin-sensitive Ca(2+)-activated K(+) channels in the human leukemic T cell line, Jurkat

T cells express two different types of voltage-independent Ca(2+)-activated K(+) channels with small (SK) and intermediate (IK) conductance that serve important roles in the activation of T lymphocytes. In contrast to the IK channels from T lymphocytes which are upregulated upon mitogen stimulation, SK channels of Jurkat T cells, a human leukemic T cell line, are constitutively expressed even in the absence of mitogenic stimulation. We have used patch-clamp recordings from transfected or injected mammalian cells to show that the cloned SK2 channel demonstrates the biophysical and pharmacological properties of the majority of K(Ca) channels in Jurkat T cells. The cloned and native channels are voltage-independent, Ca(2+)-activated, apamin-sensitive, show an equivalent voltage-dependent Ba(2+) block and possess a similar ion selectivity. In addition, we used the polymerase chain reaction to demonstrate the presence of SK2 mRNA in Jurkat T cells, whereas SK3 transcripts encoding the other cloned apamin-sensitive SK channel were not detected. These data suggest that the voltage-independent apamin-sensitive K(Ca) channel in Jurkat T cells represents the recently cloned SK2 channel.

Barium inhibition of the collapse of the Shaker K(+) conductance in zero K(+)

In the absence of K(+) on both sides of the membrane, delivery of standard activating pulses collapses the Shaker B K(+) conductance. Prolonged depolarizations restore the ability to conduct K(+). It has been proposed that the collapse of the conductance results from the dwelling of the channels in a stable closed (noninactivated) state (, J. Physiol. (Lond.). 499:3-15). Here it is shown that 1) Ba(2+) impedes the collapse of the K(+) conductance, protecting it from both sides of the membrane; 2) external Ba(2+) protection (K(d) = 63 microM at -80 mV) decreases slightly as the holding potential (HP) is made more negative; 3) external Ba(2+) cannot restore the previously collapsed conductance; on the other hand, 4) internal Ba(2+) (and K(+)) protection markedly decreases with hyperpolarized HPs (-80 to -120 mV), and it is not dependent on the pulse potential (0 to +60 mV). Ba(2+) is an effective K(+) substitute, inhibiting the passage of the channels into the stable nonconducting (noninactivated) mode of gating.

Localization of the K+ lock-In and the Ba2+ binding sites in a voltage-gated calcium-modulated channel. Implications for survival of K+ permeability

Using Ba2+ as a probe, we performed a detailed characterization of an external K+ binding site located in the pore of a large conductance Ca2+-activated K+ (BKCa) channel from skeletal muscle incorporated into planar lipid bilayers. Internal Ba2+ blocks BKCa channels and decreasing external K+ using a K+ chelator, (+)-18-Crown-6-tetracarboxylic acid, dramatically reduces the duration of the Ba2+-blocked events. Average Ba2+ dwell time changes from 10 s at 10 mM external K+ to 100 ms in the limit of very low [K+]. Using a model where external K+ binds to a site hindering the exit of Ba2+ toward the external side (Neyton, J., and C. Miller. 1988. J. Gen. Physiol. 92:549-568), we calculated a dissociation constant of 2.7 mircoM for K) at this lock-in site. We also found that BK(Ca) channels enter into a long-lasting nonconductive state when the external [K+] is reduced below 4 microM using the crown ether. Channel activity can be recovered by adding K+, Rb+, Cs+, or NH4+ to the external solution. These results suggest that the BK(Ca) channel stability in solutions of very low [K+] is due to K+ binding to a site having a very high affinity. Occupancy of this site by K+ avoids the channel conductance collapse and the exit of Ba2+ toward the external side. External tetraethylammonium also reduced the Ba2+ off rate and impeded the channel from entering into the long-lasting nonconductive state. This effect requires the presence of external K+. It is explained in terms of a model in which the conduction pore contains Ba2+, K+, and tetraethylammonium simultaneously, with the K+ binding site located internal to the tetraethylammonium site. Altogether, these results and the known potassium channel structure (Doyle, D.A., J.M. Cabral, R.A. Pfuetzner, A. Kuo, J.M. Gulbis, S.L. Cohen, B.T. Chait, and R. MacKinnon. 1998. Science. 280:69-77) imply that the lock-in site and the Ba2+ sites are the external and internal ion sites of the selectivity filter, respectively.

Expression in mammalian cells and electrophysiological characterization of two mutant Kv1.1 channels causing episodic ataxia type 1 (EA-1)

Episodic ataxia type 1 (EA-1) is a rare neurological disorder and was the first ionic channel disease to be associated with defects in a potassium channel. Until now 10 different point mutations in the KCNA1-gene have been reported to cause this disorder. We have investigated the functional consequences of two mutations leading to amino acid substitutions in the first and sixth transmembrane segments of a Kv1.1 channel subunit, by means of the patch-clamp technique; we injected cRNA coding for, respectively, F184C and V408A mutant Kv1.1 channels into mammalian cells and compared the resulting currents with those in the wild-type. The expression levels of F184C and V408A mutant channels relative to that of the wild-type was 38 and 68%, respectively. Since the single-channel conductance of the F184C mutant was similar to that of the wild-type (12 pS) without an apparent change in the maximum open probability, we conclude that the lower expression level in the F184C mutant channels is due to a reduced number of functional channels on the cell surface. F184C activated slower, and at more depolarized potentials, and deactivated faster compared with the wild-type. V408A channels deactivated and inactivated faster compared with the wild-type. Studies with different extracellular cations and tetraethylammonium gave no indication that the pore structure was changed in the mutant channels. Acetazolamide, that is helpful in some patients suffering from EA-1, was without effect on Kv1.1 wild-type or mutant channels. This study confirms and extends earlier studies on the functional consequences of Kv1.1 mutations associated with EA-1, in an attempt to understand the pathophysiology of the disease.

The effect of deep pore mutations on the action of phenylalkylamines on the Kv1.3 potassium channel

We investigated the action of the phenylalkylamines verapamil and N-methyl-verapamil on the Kv1.3 potassium channel using the whole-cell configuration of the patch-clamp technique. Our goal was to identify their binding as a prerequisite for using the phenylalkylamines as small, well-defined molecular probes, not only to expand the structural findings made with peptide toxins or by crystallization, but also to use them as lead compounds for the generation of more potent and therefore more specific K+ channel modulators. Competition experiments with charybdotoxin, known to interact with external residues of Kv1.3, showed no interaction with verapamil. The internal application of quarternary N-methyl-verapamil in combination with verapamil suggested competition for the same internal binding site. Verapamil affinity was decreased 6 fold by a mutation (M395V) in a region of the internal pore which forms part of the internal tetraethylammonium (TEA+) binding site, although mutations at neighbouring residues (T396 and T397) were without effect. Modification of C-type inactivation by mutations in the internal pore suggest that this region participates in the inactivation process. The action of phenylalkylamines and local anaesthetics on L-type Ca2+ channels and Na channels, respectively, and verapamil on Kv1.3 indicate very similar blocking mechanisms. This might allow the use of these compounds as molecular probes to map the internal vestibule of all three channel types.

UK-78,282, a novel piperidine compound that potently blocks the Kv1.3 voltage-gated potassium channel and inhibits human T cell activation

1. UK-78,282, a novel piperidine blocker of the T lymphocyte voltage-gated K+ channel, Kv1.3, was discovered by screening a large compound file using a high-throughput 86Rb efflux assay. This compound blocks Kv1.3 with a IC50 of approximately 200 nM and 1:1 stoichiometry. A closely related compound, CP-190,325, containing a benzyl moiety in place of the benzhydryl in UK-78,282, is significantly less potent. 2 Three lines of evidence indicate that UK-78,282 inhibits Kv1.3 in a use-dependent manner by preferentially blocking and binding to the C-type inactivated state of the channel. Increasing the fraction of inactivated channels by holding the membrane potential at - 50 mV enhances the channel's sensitivity to UK-78,282. Decreasing the number of inactivated channels by exposure to approximately 160 mM external K+ decreases the sensitivity to UK-78,282. Mutations that alter the rate of C-type inactivation also change the channel's sensitivity to UK-78,282 and there is a direct correlation between tau(h) and IC50 values. 3. Competition experiments suggest that UK-78,282 binds to residues at the inner surface of the channel overlapping the site of action of verapamil. Internal tetraethylammonium and external charybdotoxin do not compete UK-78,282's action on the channel. 4. UK-78,282 displays marked selectivity for Kv1.3 over several other closely related K+ channels, the only exception being the rapidly inactivating voltage-gated K+ channel, Kv1.4. 5. UK-78,282 effectively suppresses human T-lymphocyte activation.

Gating current studies reveal both intra- and extracellular cation modulation of K+ channel deactivation

1. The presence of permeant ions can modulate the rate of gating charge return in wild-type human heart K+ (hKv1.5) channels. Here we employ gating current measurements in a non-conducting mutant, W472F, of the hKv1.5 channel to investigate how different cations can modulate charge return and whether the actions can be specifically localized at the internal as well as the external mouth of the channel pore. 2. Intracellular cations were effective at accelerating charge return in the sequence Cs+ > Rb+ > K+ > Na+ > NMG+. Extracellular cations accelerated charge return with the selectivity sequence Cs+ > Rb+ > Na+ = NMG+. 3. Intracellular and extracellular cation actions were of relatively low affinity. The Kd for preventing slowing of the time constant of the off-gating current decay (tau off) was 20.2 mM for intracellular Cs+ (Cs+i) and 358 mM for extracellular Cs+ (Cs+o). 4. Both intracellular and extracellular cations can regulate the rate of charge return during deactivation of hKv1.5, but intracellular cations are more effective. We suggest that ion crystal radius is an important determinant of this action, with larger ions preventing slowing more effectively. Important parallels exist with cation-dependent modulation of slow inactivation of ionic currents in this channel. However, further experiments are required to understand the exact relationship between acceleration of charge return and the slowing of inactivation of ionic currents by cations.

Loss of shaker K channel conductance in 0 K+ solutions: role of the voltage sensor

In potassium-free solutions some types of K channels enter a long-lasting nonconducting or "defunct" state. It is known that Shaker K channels must open in K+-free solutions to become defunct. Gating current studies presented here indicate an abnormal conformation in the defunct state that restricts S4 movement and alters its kinetics. Thus an abnormality initiated in the P region spreads to the gating apparatus. We find that channels most readily become defunct on repolarization to an intermediate voltage, thus prolonging occupancy of one of the several intermediate closed states. The state dependence of becoming defunct was further dissected by using the gating mutant L382A. Simply closing this channel at 0 mV (reversing the last activation step) does not make the mutant channel defunct. Instead, it is necessary to move further left (more fully closed) in the activation sequence. This was confirmed with ShIR experiments showing that channels become defunct only if there is inward gating charge movement. Rapid transit through the intermediate states, achieved at very negative voltage, is relatively ineffective at making channels defunct. Several mutations that removed C-type inactivation also made the channels resistant to becoming defunct. Our results show that normal gating current cannot be stably recorded in the absence of K+.