Tamoxifen inhibits inward rectifier K+ 2.x family of inward rectifier channels by interfering with phosphatidylinositol 4,5-bisphosphate-channel interactions

Pharmacological inhibition of Kir4.1 evokes rapid-onset antidepressant responses

Major depressive disorder, a prevalent and severe psychiatric condition, necessitates development of new and fast-acting antidepressants. Genetic suppression of astrocytic inwardly rectifying potassium channel 4.1 (Kir4.1) in the lateral habenula ameliorates depression-like phenotypes in mice. However, Kir4.1 remains an elusive drug target for depression. Here, we discovered a series of Kir4.1 inhibitors through high-throughput screening. Lys05, the most potent one thus far, effectively suppressed native Kir4.1 channels while displaying high selectivity against established targets for rapid-onset antidepressants. Cryogenic-electron microscopy structures combined with electrophysiological characterizations revealed Lys05 directly binds in the central cavity of Kir4.1. Notably, a single dose of Lys05 reversed the Kir4.1-driven depression-like phenotype and exerted rapid-onset (as early as 1 hour) antidepressant actions in multiple canonical depression rodent models with efficacy comparable to that of (S)-ketamine. Overall, we provided a proof of concept that Kir4.1 is a promising target for rapid-onset antidepressant effects.

The network of cardiac KIR2.1: its function, cellular regulation, electrical signaling, diseases and new drug avenues

The functioning of the human heart relies on complex electrical and communication systems that coordinate cardiac contractions and sustain rhythmicity. One of the key players contributing to this intricate system is the KIR2.1 potassium ion channel, which is encoded by the KCNJ2 gene. KIR2.1 channels exhibit abundant expression in both ventricular myocytes and Purkinje fibers, exerting an important role in maintaining the balance of intracellular potassium ion levels within the heart. And by stabilizing the resting membrane potential and contributing to action potential repolarization, these channels have an important role in cardiac excitability also. Either gain- or loss-of-function mutations, but also acquired impairments of their function, are implicated in the pathogenesis of diverse types of cardiac arrhythmias. In this review, we aim to elucidate the system functions of KIR2.1 channels related to cellular electrical signaling, communication, and their contributions to cardiovascular disease. Based on this knowledge, we will discuss existing and new pharmacological avenues to modulate their function.

Verapamil inhibits Kir2.3 channels by binding to the pore and interfering with PIP2 binding

The inwardly rectifying potassium current of the cardiomyocyte (IK1) is the main determinant of the resting potential. Ion channels Kir2.1, Kir2.2, and Kir2.3 form tetramers and are the molecular correlate of macroscopic IK1 current. Verapamil is an antiarrhythmic drug used to suppress atrial and ventricular arrhythmias. Its primary mechanism of action is via blocking calcium channels. In addition, it has been demonstrated to block IK1 current and the Kir2.1 subunit. Its effect on other subunits that contribute to IK1 current has not been studied to date. We therefore analyzed the effect of verapamil on the Kir channels 2.1, 2.2, and 2.3 in the Xenopus oocyte expression system. Kir2.1, Kir2.2, and Kir2.3 channels were heterologously expressed in Xenopus oocytes. Respective currents were measured with the voltage clamp technique and the effect of verapamil on the current was measured. At a concentration of 300 µM, verapamil inhibited Kir2.1 channels by 41.36% ± 2.7 of the initial current, Kir2.2 channels by 16.51 ± 3.6%, and Kir2.3 by 69.98 ± 4.2%. As a verapamil effect on kir2.3 was a previously unknown finding, we analyzed this effect further. At wash in with 300 µM verapamil, the maximal effect was seen within 20 min of the infusion. After washing out with control solution, there was only a partial current recovery. The current reduction from verapamil was the same at − 120 mV (73.2 ± 3.7%), − 40 mV (85.5 ± 6.5%), and 0 mV (61.5 ± 10.6%) implying no voltage dependency of the block. Using site directed mutations in putative binding sites, we demonstrated a decrease of effect with pore mutant E291A and absence of verapamil effect for D251A. With mutant I214L, which shows a stronger affinity for PIP2 binding, we observed a normalized current reduction to 61.9 ± 0.06% of the control current, which was significantly less pronounced compared to wild type channels. Verapamil blocks Kir2.1, Kir2.2, and Kir2.3 subunits. In Kir2.3, blockade is dependent on sites E291 and D251 and interferes with activation of the channel via PIP2. Interference with these sites and with PIP2 binding has also been described for other Kir channels blocking drugs. As Kir2.3 is preferentially expressed in atrium, a selective Kir2.3 blocking agent would constitute an interesting antiarrhythmic concept.

Challenges and innovation: Disease modeling using human-induced pluripotent stem cell-derived cardiomyocytes

Disease modeling using human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) has both challenges and promise. While patient-derived iPSC-CMs provide a unique opportunity for disease modeling with isogenic cells, the challenge is that these cells still demonstrate distinct properties which make it functionally less akin to adult cardiomyocytes. In response to this challenge, numerous innovations in differentiation and modification of hiPSC-CMs and culture techniques have been developed. Here, we provide a focused commentary on hiPSC-CMs for use in disease modeling, the progress made in generating electrically and metabolically mature hiPSC-CMs and enabling investigative platforms. The solutions are bringing us closer to the promise of modeling heart disease using human cells in vitro.

Kir Channel Molecular Physiology, Pharmacology, and Therapeutic Implications

For the past two decades several scholarly reviews have appeared on the inwardly rectifying potassium (Kir) channels. We would like to highlight two efforts in particular, which have provided comprehensive reviews of the literature up to 2010 (Hibino et al., Physiol Rev 90(1):291-366, 2010; Stanfield et al., Rev Physiol Biochem Pharmacol 145:47-179, 2002). In the past decade, great insights into the 3-D atomic resolution structures of Kir channels have begun to provide the molecular basis for their functional properties. More recently, computational studies are beginning to close the time domain gap between in silico dynamic and patch-clamp functional studies. The pharmacology of these channels has also been expanding and the dynamic structural studies provide hope that we are heading toward successful structure-based drug design for this family of K⁺ channels. In the present review we focus on placing the physiology and pharmacology of this K⁺ channel family in the context of atomic resolution structures and in providing a glimpse of the promising future of therapeutic opportunities.

Ligand modulation of KCNQ-encoded (KV7) potassium channels in the heart and nervous system

KCNQ-encoded (K_V7) potassium channels are diversely distributed in the human tissues, associated with many physiological processes and pathophysiological conditions. These channels are increasingly used as drug targets for treating diseases. More selective and potent molecules on various types of the K_V7 channels are desirable for appropriate therapies. The recent knowledge of the structure and function of human KCNQ-encoded channels makes it more feasible to achieve these goals. This review discusses the role and mechanism of action of many molecules in modulating the function of the KCNQ-encoded potassium channels in the heart and nervous system. The effects of these compounds on K_V7 channels help to understand their involvement in many diseases, and to search for more selective and potent ligands to be used in the treatment of many disorders such as various types of cardiac arrhythmias, epilepsy, and pain.

Next-generation inward rectifier potassium channel modulators: discovery and molecular pharmacology

Inward rectifying potassium (Kir) channels play important roles in both excitable and nonexcitable cells of various organ systems and could represent valuable new drug targets for cardiovascular, metabolic, immune, and neurological diseases. In nonexcitable epithelial cells of the kidney tubule, for example, Kir1.1 (KCNJ1) and Kir4.1 (KCNJ10) are linked to sodium reabsorption in the thick ascending limb of Henle's loop and distal convoluted tubule, respectively, and have been explored as novel-mechanism diuretic targets for managing hypertension and edema. G protein-coupled Kir channels (Kir3) channels expressed in the central nervous system are critical effectors of numerous signal transduction pathways underlying analgesia, addiction, and respiratory-depressive effects of opioids. The historical dearth of pharmacological tool compounds for exploring the therapeutic potential of Kir channels has led to a molecular target-based approach using high-throughput screen (HTS) of small-molecule libraries and medicinal chemistry to develop "next-generation" Kir channel modulators that are both potent and specific for their targets. In this article, we review recent efforts focused specifically on discovery and improvement of target-selective molecular probes. The reader is introduced to fluorescence-based thallium flux assays that have enabled much of this work and then provided with an overview of progress made toward developing modulators of Kir1.1 (VU590, VU591), Kir2.x (ML133), Kir3.X (ML297, GAT1508, GiGA1, VU059331), Kir4.1 (VU0134992), and Kir7.1 (ML418). We discuss what is known about the small molecules' molecular mechanisms of action, in vitro and in vivo pharmacology, and then close with our view of what critical work remains to be done.

Inhibition of inwardly rectifying Kir2.x channels by the novel anti-cancer agent gambogic acid depends on both pore block and PIP2 interference

The caged xanthone gambogic acid (GA) is a novel anti-cancer agent which exhibits anti-proliferative, anti-inflammatory and cytotoxic effects in many types of cancer tissues. In a recent phase IIa study, GA exhibits a favourable safety profile. However, limited data are available concerning its interaction with cardiac ion channels. Heteromeric assembly of Kir2.x channels underlies the cardiac inwardly rectifying I_K1 current which is responsible for the stabilization of the diastolic resting membrane potential. Inhibition of the cardiac I_K1 current may lead to ventricular arrhythmia due to delayed afterdepolarizations. Compared to Kv2.1, hERG and Kir1.1, a slow, delayed inhibition of Kir2.1 channels by GA in a mammalian cell line was reported before but no data exist in literature concerning action of GA on homomeric Kir2.2 and Kir2.3 and heteromeric Kir2.x channels. Therefore, the aim of this study was to provide comparative data on the effect of GA on homomeric and heteromeric Kir2.x channels. Homomeric and heteromeric Kir2.x channels were heterologously expressed in Xenopus oocytes, and the two-microelectrode voltage-clamp technique was used to record Kir2.x currents. To investigate the mechanism of the channel inhibition by GA, alanine-mutated Kir2.x channels with modifications in the channels pore region or at phosphatidylinositol 4,5-bisphosphate (PIP₂)-binding sites were employed. GA caused a slow inhibition of homomeric and heteromeric Kir2.x channels at low micromolar concentrations (with IC₅₀ Kir2.1/2.2 < Kir2.2 < Kir2.2/2.3 < Kir2.3 < Kir2.1 < Kir2.1/2.3). The effect did not reach saturation within 60 min and was not reversible upon washout for 30 min. The inhibition showed no strong voltage dependence. We provide evidence for a combination of direct channel pore blockade and a PIP₂-dependent mechanism as a molecular basis for the observed effect. We conclude that Kir2.x channel inhibition by GA may be relevant in patients with pre-existing cardiac disorders such as chronic heart failure or certain rhythm disorders and recommend a close cardiac monitoring for those patients when treated with GA.

Dual Mechanism for Inhibition of Inwardly Rectifying Kir2.x Channels by Quinidine Involving Direct Pore Block and PIP2-interference

Class IA antiarrhythmic drug quinidine was one of the first clinically used compounds to terminate atrial fibrillation and acts as multichannel inhibitor with well-documented inhibitory effects on several cardiac potassium channels. In the mammalian heart, heteromeric assembly of Kir2.1-2.3 channels underlies I_K1 current. Although a low-affinity block of quinidine on Kir2.1 has already been described, a comparative analysis of effects on other Kir2.x channels has not been performed to date. Therefore, we analyzed the effects of quinidine on wild-type and mutant Kir2.x channels in the Xenopus oocyte expression system. Quinidine exerted differential inhibitory effects on Kir2.x channels with the highest affinity toward Kir2.3 subunits. Onset of block was slow and solely reversible in Kir2.2 subunits. Quinidine inhibited Kir2.x currents in a voltage-independent manner. By means of comparative Ala-scanning mutagenesis, we further found that residues E224, F254, D259, and E299 are essential for quinidine block in Kir2.1 subunits. Analogously, quinidine mediated Kir2.3 inhibition by binding corresponding residues E216, D247, D251, and E291. In contrast, Kir2.2 current block merely involved corresponding residue D260. Using channel mutants with altered (phosphatidylinositol 4,5-bisphosphate PIP₂) affinities, we were able to demonstrate that high PIP₂ affinities (i.e., Kir2.3 I214L) correlate with low quinidine sensitivity. Inversely, mutant channels interacting only weakly with PIP₂ (i.e., Kir2.1 K182Q, and L221I) are prone to a higher inhibitory effect. Thus, we conclude that inhibition of Kir2.x channels by quinidine is mediated by joint modes of action involving direct cytoplasmic pore block and an impaired channel stabilization via interference with PIP₂.

Styrax blocks inward and outward current of Kir2.1 channel

Kir2.1 plays key roles in setting rest membrane potential and modulation of cell excitability. Mutations of Kir2.1, such as D172N or E299V, inducing gain-of-function, can cause type3 short QT syndrome (SQT3) due to the enlarged outward currents. So far, there is no clinical drug target to block the currents of Kir2.1. Here, we identified a novel blocker of Kir2.1, styrax, which is a kind of natural compound selected from traditional Chinese medicine. Our data show that styrax can abolish the inward and outward currents of Kir2.1. The IC₅₀ of styrax for WT, D172N and E299V are 0.0113 ± 0.00075, 0.0204 ± 0.0048 and 0.0122 ± 0.0012 (in volume), respectively. The results indicate that styrax can serve as a novel blocker for Kir2.1.

Pharmacological inhibition of IK1 by PA-6 in isolated rat hearts affects ventricular repolarization and refractoriness

The inwardly rectifying potassium current (I_K1) conducted through K_ir2.X channels contribute to repolarization of the cardiac action potential and to stabilization of the resting membrane potential in cardiomyocytes. Our aim was to investigate the effect of the recently discovered I_K1 inhibitor PA‐6 on action potential repolarization and refractoriness in isolated rat hearts. Transiently transfected HEK‐293 cells expressing I_K1 were voltage‐clamped with ramp protocols. Langendorff‐perfused heart experiments were performed on male Sprague–Dawley rats, effective refractory period, Wenckebach cycle length, and ventricular effective refractory period were determined following 200 nmol/L PA‐6 perfusion. 200 nmol/L PA‐6 resulted in a significant time‐latency in drug effect on the I_K1 current expressed in HEK‐293 cells, giving rise to a maximal effect at 20 min. In the Langendorff‐perfused heart experiments, PA‐6 prolonged the ventricular action potential duration at 90% repolarization (from 41.8 ± 6.5 msec to 72.6 ± 21.1 msec, 74% compared to baseline, P < 0.01, n = 6). In parallel, PA‐6 significantly prolonged the ventricular effective refractory period compared to baseline (from 34.8 ± 4.6 msec to 58.1 ± 14.7 msec, 67%, P < 0.01, n = 6). PA‐6 increased the short‐term beat‐to‐beat variability and ventricular fibrillation was observed in two of six hearts. Neither atrial ERP nor duration of atrial fibrillation was altered following PA‐6 application. The results show that pharmacological inhibition of cardiac I_K1 affects ventricular action potential repolarization and refractoriness and increases the risk of ventricular arrhythmia in isolated rat hearts.

Rapid estrogen actions on ion channels: A survey in search for mechanisms

A survey of nearly two hundred reports shows that rapid estrogenic actions can be detected across a range of kinds of estrogens, a range of doses, on a wide range of tissue, cell and ion channel types. Striking is the fact that preparations of estrogenic agents that do not permeate the cell membrane almost always mimic the actions of the estrogenic agents that do permeate the membrane. All kinds of estrogens, ranging from natural ones, through receptor modulators, endocrine disruptors, phytoestrogens, agonists, and antagonists to novel G-1 and STX, have been reported to be effective. For actions on specific types of ion channels, the possibility of opposing actions, in different cases, is the rule, not the exception. With this variety there is no single, specific action mechanism for estrogens per se, although in some cases estrogens can act directly or via some signaling pathways to affect ion channels. We infer that estrogens can bind a large number of substrates/receptors at the membrane surface. As against the variety of subsequent routes of action, this initial step of the estrogen's binding action is the key.

The ICl,swell inhibitor DCPIB blocks Kir channels that possess weak affinity for PIP2

Inwardly rectifying K(+) (Kir) channels are important contributors to the resting membrane potential and regulate cellular excitability. The activity of Kir channels depends critically on the phospholipid PIP2. Several modulators of the activity of Kir channels alter the apparent affinity of the channel to PIP2. Channels with high apparent affinity to PIP2 may not respond to a given modulator, but mutations that decrease such affinity can render the channel susceptible to modulation. Here, we identify a known inhibitor of the swelling-activated Cl(-) current, DCPIB, as an effective inhibitor of a number of Kir channels both in native cardiac cells and in heterologous expression systems. We show that the apparent affinity to PIP2 determines whether DCPIB will serve as an efficient blocker of Kir channels. These effects are consistent with a model in which DCPIB competes with PIP2 for a common binding site.

Cardiac ion channels

Ion channels are critical for all aspects of cardiac function, including rhythmicity and contractility. Consequently, ion channels are key targets for therapeutics aimed at cardiac pathophysiologies such as atrial fibrillation or angina. At the same time, off-target interactions of drugs with cardiac ion channels can be the cause of unwanted side effects. This manuscript aims to review the physiology and pharmacology of key cardiac ion channels. The intent is to highlight recent developments for therapeutic development, as well as elucidate potential mechanisms for drug-induced cardiac side effects, rather than present an in-depth review of each channel subtype.

Class III antiarrhythmic drug dronedarone inhibits cardiac inwardly rectifying Kir2.1 channels through binding at residue E224

Dronedarone is a novel class III antiarrhythmic drug that is widely used in atrial fibrillation. It has been shown in native cardiomyocytes that dronedarone inhibits cardiac inwardly rectifying current IK1 at high concentrations, which may contribute both its antifibrillatory efficacy and its potential proarrhythmic side effects. However, the underlying mechanism has not been studied in further detail to date. In the mammalian heart, heterotetrameric assembly of Kir2.x channels is the molecular basis of IK1 current. Therefore, we studied the effects of dronedarone on wild-type and mutant Kir2.x channels in the Xenopus oocyte expression system. Dronedarone inhibited Kir2.1 currents but had no effect on Kir2.2 or Kir2.3 currents. Onset of block was slow but completely reversible upon washout. Blockade of Kir2.1 channels did not exhibit strong voltage dependence or frequency dependence. In a screening with different Kir2.1 mutants lacking specific binding sites within the cytoplasmic pore region, we found that residue E224 is essential for binding of dronedarone to Kir2.1 channels. In conclusion, direct block of Kir2.1 channel subunits by dronedarone through binding at E224 may underlie its inhibitory effects on cardiac IK1 current.

Electrophysiological effects of tamoxifen: mechanism of protection against reperfusion arrhythmias in isolated rat hearts

Reperfusion arrhythmias are currently attributed to ionic imbalance and oxidative stress. Tamoxifen is a potent antioxidant that also modulates some ionic transport pathways. In this work, we tried to correlate the electrophysiological effects of 1, 2, and 5 µM of tamoxifen with the incidence and severity of arrhythmias appearing on reperfusion after 10 minutes of coronary occlusion in isolated hearts from female rats. All tamoxifen concentrations inhibited the action potential shortening observed in the control hearts during late ischemia (6-10 minutes), whereas 2 and 5 µM also reduced the resting membrane potential depolarization. The incidence of sustained ventricular tachycardia and/or ventricular fibrillation on reperfusion decreased from 10 of 12 (control group) to 5 of 10 (1 µM, P = 0.1718), 4 of 12 (2 µM, P = 0.0361), and 2 of 10 (5 µM, P = 0.0083). The possible role of chloride currents activated by cell swelling in these effects was explored in hearts submitted to a 10-minute hypotonic challenge, where tamoxifen (5 µM) blocked the action potential shortening and the late resting membrane potential depolarization produced by hypotonicity, mimicking its action in late ischemia. Tamoxifen produced a similar increase of the total antioxidant capacity of myocardial samples at all the concentration tested. In conclusion, our data strongly suggest that the antiarrhythmic action of this agent is mediated by its electrophysiological effect derived from modulation of chloride currents activated by cell swelling.

Direct, differential effects of tamoxifen, 4-hydroxytamoxifen, and raloxifene on cardiac myocyte contractility and calcium handling

Tamoxifen (Tam), a selective estrogen receptor modulator, is in wide clinical use for the treatment and prevention of breast cancer. High Tam doses have been used for treatment of gliomas and cancers with multiple drug resistance, but long QT Syndrome is a side effect. Tam is also used experimentally in mice for inducible gene knockout in numerous tissues, including heart; however, the potential direct effects of Tam on cardiac myocyte mechanical function are not known. The goal of this study was to determine the direct, acute effects of Tam, its active metabolite 4-hydroxytamoxifen (4OHT), and related drug raloxifene (Ral) on isolated rat cardiac myocyte mechanical function and calcium handling. Tam decreased contraction amplitude, slowed relaxation, and decreased Ca²⁺ transient amplitude. Effects were primarily observed at 5 and 10 μM Tam, which is relevant for high dose Tam treatment in cancer patients as well as Tam-mediated gene excision in mice. Myocytes treated with 4OHT responded similarly to Tam-treated cells with regard to both contractility and calcium handling, suggesting an estrogen-receptor independent mechanism is responsible for the effects. In contrast, Ral increased contraction and Ca²⁺ transient amplitudes. At 10 μM, all drugs had a time-dependent effect to abolish cellular contraction. In conclusion, Tam, 4OHT, and Ral adversely and differentially alter cardiac myocyte contractility and Ca²⁺ handling. These findings have important implications for understanding the Tam-induced cardiomyopathy in gene excision studies and may be important for understanding effects on cardiac performance in patients undergoing high-dose Tam therapy.

Tamoxifen inhibition of kv7.2/kv7.3 channels

KCNQ genes encode five Kv7 K⁺ channel subunits (Kv7.1–Kv7.5). Four of these (Kv7.2–Kv7.5) are expressed in the nervous system. Kv7.2 and Kv7.3 are the principal molecular components of the slow voltage-gated M-channel, which regulates neuronal excitability. In this study, we demonstrate that tamoxifen, an estrogen receptor antagonist used in the treatment of breast cancer, inhibits Kv7.2/Kv7.3 currents heterologously expressed in human embryonic kidney HEK-293 cells. Current inhibition by tamoxifen was voltage independent but concentration-dependent. The IC₅₀ for current inhibition was 1.68 ± 0.44 µM. The voltage-dependent activation of the channel was not modified. Tamoxifen inhibited Kv7.2 homomeric channels with a higher potency (IC₅₀ = 0.74 ± 0.16 µM). The mutation Kv7.2 R463E increases phosphatidylinositol- 4,5-bisphosphate (PIP₂) - channel interaction and diminished dramatically the inhibitory effect of tamoxifen compared with that for wild type Kv7.2. Conversely, the mutation Kv7.2 R463Q, which decreases PIP₂ -channel interaction, increased tamoxifen potency. Similar results were obtained on the heteromeric Kv7.2 R463Q/Kv7.3 and Kv7.2 R463E/Kv7.3 channels, compared to Kv7.2/Kv7.3 WT. Overexpression of type 2A PI(4)P5-kinase (PIP5K 2A) significantly reduced tamoxifen inhibition of Kv7.2/Kv7.3 and Kv7.2 R463Q channels. Our results suggest that tamoxifen inhibited Kv7.2/Kv7.3 channels by interfering with PIP₂-channel interaction because of its documented interaction with PIP₂ and the similar effect of tamoxifen on various PIP2 sensitive channels.

Propafenone blocks human cardiac Kir2.x channels by decreasing the negative electrostatic charge in the cytoplasmic pore

Human cardiac inward rectifier current (IK1) is generated by Kir2.x channels. Inhibition of IK1 could offer a useful antiarrhythmic strategy against fibrillatory arrhythmias. Therefore, elucidation of Kir2.x channels pharmacology, which still remains elusive, is mandatory. We characterized the electrophysiological and molecular basis of the inhibition produced by the antiarrhythmic propafenone of the current generated by Kir2.x channels (IKir2.x) and the IK1 recorded in human atrial myocytes. Wild type and mutated human Kir2.x channels were transiently transfected in CHO and HEK-293 cells. Macroscopic and single-channel currents were recorded using the patch-clamp technique. At concentrations >1μM propafenone inhibited IKir2.x the order of potency being Kir2.3∼IK1>Kir2.2>Kir2.1 channels. Blockade was irrespective of the extracellular K(+) concentration whereas markedly increased when the intracellular K(+) concentration was decreased. Propafenone decreased inward rectification since at potentials positive to the K(+) equilibrium potential propafenone-induced block decreased in a voltage-dependent manner. Importantly, propafenone favored the occurrence of subconductance levels in Kir2.x channels and decreased phosphatidylinositol 4,5-bisphosphate (PIP2)-channel affinity. Blind docking and site-directed mutagenesis experiments demonstrated that propafenone bound Kir2.x channels at the cytoplasmic domain, close to, but not in the pore itself, the binding site involving two conserved Arg residues (residues 228 and 260 in Kir2.1). Our results suggested that propafenone incorporated into the cytoplasmic domain of the channel in such a way that it decreased the net negative charge sensed by K(+) ions and polyamines which, in turn, promotes the appearance of subconductance levels and the decrease of PIP2 affinity of the channels.

KCNJ2 mutation in short QT syndrome 3 results in atrial fibrillation and ventricular proarrhythmia

We describe a mutation (E299V) in KCNJ2, the gene that encodes the strong inward rectifier K(+) channel protein (Kir2.1), in an 11-y-old boy. The unique short QT syndrome type-3 phenotype is associated with an extremely abbreviated QT interval (200 ms) on ECG and paroxysmal atrial fibrillation. Genetic screening identified an A896T substitution in a highly conserved region of KCNJ2 that resulted in a de novo mutation E299V. Whole-cell patch-clamp experiments showed that E299V presents an abnormally large outward IK1 at potentials above -55 mV (P < 0.001 versus wild type) due to a lack of inward rectification. Coexpression of wild-type and mutant channels to mimic the heterozygous condition still resulted in a large outward current. Coimmunoprecipitation and kinetic analysis showed that E299V and wild-type isoforms may heteromerize and that their interaction impairs function. The homomeric assembly of E299V mutant proteins actually results in gain of function. Computer simulations of ventricular excitation and propagation using both the homozygous and heterozygous conditions at three different levels of integration (single cell, 2D, and 3D) accurately reproduced the electrocardiographic phenotype of the proband, including an exceedingly short QT interval with merging of the QRS and the T wave, absence of ST segment, and peaked T waves. Numerical experiments predict that, in addition to the short QT interval, absence of inward rectification in the E299V mutation should result in atrial fibrillation. In addition, as predicted by simulations using a geometrically accurate three-dimensional ventricular model that included the His-Purkinje network, a slight reduction in ventricular excitability via 20% reduction of the sodium current should increase vulnerability to life-threatening ventricular tachyarrhythmia.

Multiple cationic amphiphiles induce a Niemann-Pick C phenotype and inhibit Ebola virus entry and infection

Ebola virus (EBOV) is an enveloped RNA virus that causes hemorrhagic fever in humans and non-human primates. Infection requires internalization from the cell surface and trafficking to a late endocytic compartment, where viral fusion occurs, providing a conduit for the viral genome to enter the cytoplasm and initiate replication. In a concurrent study, we identified clomiphene as a potent inhibitor of EBOV entry. Here, we screened eleven inhibitors that target the same biosynthetic pathway as clomiphene. From this screen we identified six compounds, including U18666A, that block EBOV infection (IC₅₀ 1.6 to 8.0 µM) at a late stage of entry. Intriguingly, all six are cationic amphiphiles that share additional chemical features. U18666A induces phenotypes, including cholesterol accumulation in endosomes, associated with defects in Niemann–Pick C1 protein (NPC1), a late endosomal and lysosomal protein required for EBOV entry. We tested and found that all six EBOV entry inhibitors from our screen induced cholesterol accumulation. We further showed that higher concentrations of cationic amphiphiles are required to inhibit EBOV entry into cells that overexpress NPC1 than parental cells, supporting the contention that they inhibit EBOV entry in an NPC1-dependent manner. A previously reported inhibitor, compound 3.47, inhibits EBOV entry by blocking binding of the EBOV glycoprotein to NPC1. None of the cationic amphiphiles tested had this effect. Hence, multiple cationic amphiphiles (including several FDA approved agents) inhibit EBOV entry in an NPC1-dependent fashion, but by a mechanism distinct from that of compound 3.47. Our findings suggest that there are minimally two ways of perturbing NPC1-dependent pathways that can block EBOV entry, increasing the attractiveness of NPC1 as an anti-filoviral therapeutic target.

Voltage-dependent open-channel block of G protein-gated inward-rectifying K(+) (GIRK) current in rat atrial myocytes by tamoxifen

Tamoxifen (Tmx) is a nonsteroidal selective estrogen receptor antagonist and is frequently used in the treatment and prevention of breast cancer. The compound and its metabolites have been reported to inhibit functions of different classes of membrane proteins, including various ion channels. For members of the inward-rectifying K(+) (Kir) channel family, interference of Tmx with binding of phosphatidylinositol 4,5-bisphosphate (PIP(2)) has been suggested as the mechanism underlying such inhibition. We have studied the inhibition of G protein-activated K(+) (GIRK) current by Tmx in isolated myocytes from hearts of adult rats using whole-cell voltage clamp and experimental conditions for measuring K(+) currents as inward currents (E (K) -50 mV; holding potential -90 mV). Extracellular Tmx reversibly inhibited GIRK current activated by acetylcholine (I (K(ACh))) with an EC(50) of 7.4 × 10(-7) M. This inhibition was composed of two components, a basal reduction in peak current and a block that required opening of channels by ACh. The open-channel block was partially relieved by depolarizing voltage steps in a voltage- and time-dependent fashion. A voltage-dependent open-channel block was not observed when I (K(ACh)) was measured as outward current (E (K) -90 mV; holding potential -40 mV). Intracellular application of Tmx via the patch clamp pipette at a concentration (7 × 10(-6) M) that caused a rapid inhibition of I (K(ACh)) upon extracellular application did not affect the current. Intracellular application of the H(2)O-soluble PIP(2) analog diC(8)-PIP(2) reduced the voltage-independent component of inhibition but had no effect on voltage-dependent open-channel block. The effects of 4-hydroxy-Tmx, a major active metabolite, tested at 2 × 10(-6) M, had effects on I (K(ACh)) analogous to those of Tmx. Inhibition of constitutive inward-rectifying K(+) current (I (K1)) in ventricular myocytes, carried by Kir2 complexes, by Tmx was devoid of a voltage-dependent component. This study suggests the voltage-dependent open-channel block of GIRK inward current as a novel mechanism of Tmx action.

Molecular mechanisms of chloroquine inhibition of heterologously expressed Kir6.2/SUR2A channels

Chloroquine and related compounds can inhibit inwardly rectifying potassium channels by multiple potential mechanisms, including pore block and allosteric effects on channel gating. Motivated by reports that chloroquine inhibition of cardiac ATP-sensitive inward rectifier K(+) current (I(KATP)) is antifibrillatory in rabbit ventricle, we investigated the mechanism of chloroquine inhibition of ATP-sensitive potassium (K(ATP)) channels (Kir6.2/SUR2A) expressed in human embryonic kidney 293 cells, using inside-out patch-clamp recordings. We found that chloroquine inhibits the Kir6.2/SUR2A channel by interacting with at least two different sites and by two mechanisms of action. A fast-onset effect is observed at depolarized membrane voltages and enhanced by the N160D mutation in the central cavity, probably reflecting direct channel block resulting from the drug entering the channel pore from the cytoplasmic side. Conversely, a slow-onset, voltage-independent inhibition of I(KATP) is regulated by chloroquine interaction with a different site and probably involves disruption of interactions between Kir6.2/SUR2A and phosphatidylinositol 4,5-bisphosphate. Our findings reveal multiple mechanisms of K(ATP) channel inhibition by chloroquine, highlighting the numerous convergent regulatory mechanisms of these ligand-dependent ion channels.

Discovery, characterization, and structure-activity relationships of an inhibitor of inward rectifier potassium (Kir) channels with preference for Kir2.3, Kir3.x, and Kir7.1

The inward rectifier family of potassium (Kir) channels is comprised of at least 16 family members exhibiting broad and often overlapping cellular, tissue, or organ distributions. The discovery of disease-causing mutations in humans and experiments on knockout mice has underscored the importance of Kir channels in physiology and in some cases raised questions about their potential as drug targets. However, the paucity of potent and selective small-molecule modulators targeting specific family members has with few exceptions mired efforts to understand their physiology and assess their therapeutic potential. A growing body of evidence suggests that G protein-coupled inward rectifier K (GIRK) channels of the Kir3.X subfamily may represent novel targets for the treatment of atrial fibrillation. In an effort to expand the molecular pharmacology of GIRK, we performed a thallium (Tl⁺) flux-based high-throughput screen of a Kir1.1 inhibitor library for modulators of GIRK. One compound, termed VU573, exhibited 10-fold selectivity for GIRK over Kir1.1 (IC₅₀ = 1.9 and 19 μM, respectively) and was therefore selected for further study. In electrophysiological experiments performed on Xenopus laevis oocytes and mammalian cells, VU573 inhibited Kir3.1/3.2 (neuronal GIRK) and Kir3.1/3.4 (cardiac GIRK) channels with equal potency and preferentially inhibited GIRK, Kir2.3, and Kir7.1 over Kir1.1 and Kir2.1.Tl⁺ flux assays were established for Kir2.3 and the M125R pore mutant of Kir7.1 to support medicinal chemistry efforts to develop more potent and selective analogs for these channels. The structure–activity relationships of VU573 revealed few analogs with improved potency, however two compounds retained most of their activity toward GIRK and Kir2.3 and lost activity toward Kir7.1. We anticipate that the VU573 series will be useful for exploring the physiology and structure–function relationships of these Kir channels.

Selective inhibition of the K(ir)2 family of inward rectifier potassium channels by a small molecule probe: the discovery, SAR, and pharmacological characterization of ML133

The K(ir) inward rectifying potassium channels have a broad tissue distribution and are implicated in a variety of functional roles. At least seven classes (K(ir)1-K(ir)7) of structurally related inward rectifier potassium channels are known, and there are no selective small molecule tools to study their function. In an effort to develop selective K(ir)2.1 inhibitors, we performed a high-throughput screen (HTS) of more than 300,000 small molecules within the MLPCN for modulators of K(ir)2.1 function. Here we report one potent K(ir)2.1 inhibitor, ML133, which inhibits K(ir)2.1 with an IC(50) of 1.8 μM at pH 7.4 and 290 nM at pH 8.5 but exhibits little selectivity against other members of Kir2.x family channels. However, ML133 has no effect on K(ir)1.1 (IC(50) > 300 μM) and displays weak activity for K(ir)4.1 (76 μM) and K(ir)7.1 (33 μM), making ML133 the most selective small molecule inhibitor of the K(ir) family reported to date. Because of the high homology within the K(ir)2 family-the channels share a common design of a pore region flanked by two transmembrane domains-identification of site(s) critical for isoform specificity would be an important basis for future development of more specific and potent K(ir) inhibitors. Using chimeric channels between K(ir)2.1 and K(ir)1.1 and site-directed mutagenesis, we have identified D172 and I176 within M2 segment of K(ir)2.1 as molecular determinants critical for the potency of ML133 mediated inhibition. Double mutation of the corresponding residues of K(ir)1.1 to those of K(ir)2.1 (N171D and C175I) transplants ML133 inhibition to K(ir)1.1. Together, the combination of a potent, K(ir)2 family selective inhibitor and identification of molecular determinants for the specificity provides both a tool and a model system to enable further mechanistic studies of modulation of K(ir)2 inward rectifier potassium channels.

The antimalarial drug mefloquine inhibits cardiac inward rectifier K+ channels: evidence for interference in PIP2-channel interaction

The antimalarial drug mefloquine was found to inhibit the KATP channel by an unknown mechanism. Because mefloquine is a Cationic amphiphilic drug and is known to insert into lipid bilayers, we postulate that mefloquine interferes with the interaction between PIP2 and Kir channels resulting in channel inhibition. We studied the inhibitory effects of mefloquine on Kir2.1, Kir2.3, Kir2.3(I213L), and Kir6.2/SUR2A channels expressed in HEK-293 cells, and on IK1 and IKATP from feline cardiac myocytes. The order of mefloquine inhibition was Kir6.2/SUR2A ≈ Kir2.3 (IC50 ≈ 2 μM) > Kir2.1 (IC50 > 30 μM). Similar results were obtained in cardiac myocytes. The Kir2.3(I213L) mutant, which enhances the strength of interaction with PIP2 (compared to WT), was significantly less sensitive (IC50 = 9 μM). In inside-out patches, continuous application of PIP2 strikingly prevented the mefloquine inhibition. Our results support the idea that mefloquine interferes with PIP2-Kir channels interactions.

Small-molecule modulators of inward rectifier K+ channels: recent advances and future possibilities

Inward rectifier potassium (Kir) channels have been postulated as therapeutic targets for several common disorders including hypertension, cardiac arrhythmias and pain. With few exceptions, however, the small-molecule pharmacology of this family is limited to nonselective cardiovascular and neurologic drugs with off-target activity toward inward rectifiers. Consequently, the actual therapeutic potential and 'drugability' of most Kir channels has not yet been determined experimentally. The purpose of this review is to provide a comprehensive summary of publicly disclosed Kir channel small-molecule modulators and highlight recent targeted drug-discovery efforts toward Kir1.1 and Kir2.1. The review concludes with a brief speculation on how the field of Kir channel pharmacology will develop over the coming years and a discussion of the increasingly important role academic laboratories will play in this progress.

Carvedilol inhibits Kir2.3 channels by interference with PIP₂-channel interaction

Carvedilol, a β- and α-adrenoceptor blocker, is used to treat congestive heart failure, mild to moderate hypertension, and myocardial infarction. It has been proposed to block K(ATP) channels by binding to the bundle crossing region at a domain including cysteine at position 166, and thereby plugging the pore region. However, carvedilol was reported not to affect Kir2.1 channels, which lack 166 Cys. Here, we demonstrate that carvedilol inhibits Kir2.3 carried current by an alternative mechanism. Carvedilol inhibited Kir2.3 channels with at least 100 fold higher potency (IC(50)=0.49 μM) compared to that for Kir2.1 (IC(50)>50 μM). Kir2.3 channel inhibition was concentration-dependent and voltage-independent. Increasing Kir2.3 channel affinity for PIP(2), by a I213L point mutation, decreased the inhibitory effect of carvedilol more than twentyfold (IC(50)=11.1 μM). In the presence of exogenous PIP(2), Kir2.3 channel inhibition by carvedilol was strongly reduced (80 vs. 2% current inhibition). These results suggest that carvedilol, as other cationic amphiphilic drugs, inhibits Kir2.3 channels by interfering with the PIP(2)-channel interaction.

Emerging concepts in the pharmacogenomics of arrhythmias: ion channel trafficking

Continuous, rhythmic beating of the heart requires exquisite control of expression, localization and function of cardiac ion channels - the foundations of the cardiac myocyte action potential. Disruption of any of these processes can alter the shape of the action potential, predisposing to cardiac arrhythmias. These arrhythmias can manifest in a variety of ways depending on both the channels involved and the type of disruption (i.e., gain or loss of function). As much as 1% of the population of developed countries is affected by cardiac arrhythmia each year, and a detailed understanding of the mechanism of each arrhythmia is crucial to developing and prescribing the proper therapies. Many of the antiarrhythmic drugs currently on the market were developed before the underlying cause of the arrhythmia was known, and as a result lack specificity, causing side effects. The majority of the available drugs target the conductance of cardiac ion channels, either by blocking or enhancing current through the channel. In recent years, however, it has become apparent that specific targeting of ion channel conductance may not be the most effective means for treatment. Here we review increasing evidence that suggests defects in ion channel trafficking play an important role in the etiology of arrhythmias, and small molecule approaches to correct trafficking defects will likely play an important role in the future of arrhythmia treatment.

The mammalian K(IR)2.x inward rectifier ion channel family: expression pattern and pathophysiology

Inward rectifier currents based on K(IR)2.x subunits are regarded as essential components for establishing a stable and negative resting membrane potential in many excitable cell types. Pharmacological inhibition, null mutation in mice and dominant positive and negative mutations in patients reveal some of the important functions of these channels in their native tissues. Here we review the complex mammalian expression pattern of K(IR)2.x subunits and relate these to the outcomes of functional inhibition of the resultant channels. Correlations between expression and function in muscle and bone tissue are observed, while we recognize a discrepancy between neuronal expression and function.

Specific residues of the cytoplasmic domains of cardiac inward rectifier potassium channels are effective antifibrillatory targets

Atrial and ventricular tachyarrhythmias can be perpetuated by up-regulation of inward rectifier potassium channels. Thus, it may be beneficial to block inward rectifier channels under conditions in which their function becomes arrhythmogenic (e.g., inherited gain-of-function mutation channelopathies, ischemia, and chronic and vagally mediated atrial fibrillation). We hypothesize that the antimalarial quinoline chloroquine exerts potent antiarrhythmic effects by interacting with the cytoplasmic domains of Kir2.1 (I(K1)), Kir3.1 (I(KACh)), or Kir6.2 (I(KATP)) and reducing inward rectifier potassium currents. In isolated hearts of three different mammalian species, intracoronary chloroquine perfusion reduced fibrillatory frequency (atrial or ventricular), and effectively terminated the arrhythmia with resumption of sinus rhythm. In patch-clamp experiments chloroquine blocked I(K1), I(KACh), and I(KATP). Comparative molecular modeling and ligand docking of chloroquine in the intracellular domains of Kir2.1, Kir3.1, and Kir6.2 suggested that chloroquine blocks or reduces potassium flow by interacting with negatively charged amino acids facing the ion permeation vestibule of the channel in question. These results open a novel path toward discovering antiarrhythmic pharmacophores that target specific residues of the cytoplasmic domain of inward rectifier potassium channels.

Thiopental inhibits function of different inward rectifying potassium channel isoforms by a similar mechanism

Thiopental is a well-known intravenous barbiturate anesthetic with important cardiac side effects. The actions of thiopental on the transmembrane ionic currents that determine the resting potential and action potential duration in cardiomyocytes have been studied widely. We aimed at elucidating the characteristics and mechanism of inhibition by thiopental on members of the subfamily of inward rectifying Kir2.x (Kir2.1, 2.2 and 2.3), Kir1.1 and Kir6.2/SUR2A channels. These inward rectifier potassium channels were transfected in HEK-293 cells and macroscopic currents were recorded in the whole-cell and inside-out configurations of the patch-clamp technique. Thiopental inhibited Kir2.1, Kir2.2, Kir2.3, Kir1.1 and Kir6.2/SUR2A currents with similar potency; in whole-cell experiments 30 microM thiopental decreased Kir2.1, Kir2.2, Kir2.3 and Kir1.1 currents to 55+/-6, 39+/-8, 42+/-5 and 49+/-5% at -120 mV, respectively. Point mutations on Kir2.3 (I213L) or Kir2.1 (L222I) did not modify the potency of block. Thiopental inhibited all Kir channels in a concentration-dependent and voltage-independent manner. Also, the time course of thiopental inhibition was slow (T(1/2) approximately 4 min) and independent of external or internal drug application. However, in the presence of PIP(2), inhibition by thiopental on Kir2.1 was significantly decreased. Thiopental at clinically relevant concentrations significantly inhibited all Kir channels evaluated in this work. The reduction of thiopental effects during PIP(2) treatment suggests that thiopental inhibition on Kir2.1 channels is related to channel-PIP(2) interaction.

The anti-protozoal drug pentamidine blocks KIR2.x-mediated inward rectifier current by entering the cytoplasmic pore region of the channel

BACKGROUND AND PURPOSE

Pentamidine is a drug used in treatment of protozoal infections. Pentamidine treatment may cause sudden cardiac death by provoking cardiac arrhythmias associated with QTc prolongation and U-wave alterations. This proarrhythmic effect was linked to inhibition of hERG trafficking, but not to acute block of ion channels contributing to the action potential. Because the U-wave has been linked to the cardiac inward rectifier current (I(K1)), we examined the action and mechanism of pentamidine-mediated I(K1) block.

EXPERIMENTAL APPROACH

Patch clamp measurements of I(K1) were made on cultured adult canine ventricular cardiomyocytes, K(IR)2.1-HEK293 cells and K(IR)2.x inside-out patches. Pentamidine binding to cytoplasmic amino acid residues of K(IR)2.1 channels was studied by molecular modelling.

KEY RESULTS

Pentamidine application (24 h) decreased I(K1) in cultured canine cardiomyocytes and K(IR)2.1-HEK293 cells under whole cell clamp conditions. Pentamidine inhibited I(K1) in K(IR)2.1-HEK293 cells 10 min after application. When applied to the cytoplasmic side under inside-out patch clamp conditions, pentamidine block of I(K1) was acute (IC(50)= 0.17 microM). Molecular modelling predicted pentamidine-channel interactions in the cytoplasmic pore region of K(IR)2.1 at amino acids E224, D259 and E299. Mutation of these conserved residues to alanine reduced pentamidine block of I(K1). Block was independent of the presence of spermine. K(IR)2.2, and K(IR)2.3 based I(K1) was also sensitive to pentamidine blockade.

CONCLUSIONS AND IMPLICATIONS

Pentamidine inhibits cardiac I(K1) by interacting with three negatively charged amino acids in the cytoplasmic pore region. Our findings may provide new insights for development of specific I(K1) blocking compounds.