A kinetic study on the stereospecific inhibition of KCNQ1 and I(Ks) by the chromanol 293B

Physiological Functions, Biophysical Properties, and Regulation of KCNQ1 (KV7.1) Potassium Channels

KCNQ1 (K_V7.1) K⁺ channels are expressed in multiple tissues, including the heart, pancreas, colon, and inner ear. The gene encoding the KCNQ1 protein was discovered by a positional cloning effort to determine the genetic basis of long QT syndrome, an inherited ventricular arrhythmia that can cause sudden death. Mutations in KCNQ1 can also cause other types of arrhythmia (i.e., short QT syndrome, atrial fibrillation) and the gene may also have a role in diabetes and certain cancers. KCNQ1 α-subunits can partner with accessory β-subunits (KCNE1-KCNE5) to form K⁺-selective channels that have divergent biophysical properties. In the heart, KCNQ1 α-subunits coassemble with KCNE1 β-subunits to form channels that conduct I_Ks, a very slowly activating delayed rectifier K⁺ current. K_V7.1 channels are highly regulated by PIP₂, calmodulin, and phosphorylation, and rich pharmacology includes blockers and gating modulators. Recent biophysical studies and a cryo-EM structure of the KCNQ1-calmodulin complex have provided new insights into K_V7.1 channel function, and how interactions between KCNQ1 and KCNE subunits alter the gating properties of heteromultimeric channels.

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.

Do KV 7.1 channels contribute to control of arterial vascular tone?

BACKGROUND AND PURPOSE

K_V 7.1 voltage-gated potassium channels are expressed in vascular smooth muscle cells (VSMC) of diverse arteries, including mesenteric arteries. Based on pharmacological evidence using R-L3 (K_V 7.1 channel opener), HMR1556, chromanol 293B (K_V 7.1 channel blockers), stimulation of these channels has been suggested to evoke profound relaxation in various vascular beds of rats. However, the specificity of these drugs in vivo is uncertain.

EXPERIMENTAL APPROACH

We used Kcnq1^-/- mice and pharmacological tools to determine whether K_V 7.1 channels play a role in the regulation of arterial tone.

KEY RESULTS

R-L3 produced similar concentration-dependent relaxations (EC₅₀  ~ 1.4 μM) of arteries from wild-type (Kcnq1^+/+ ) and Kcnq1^-/- mice, pre-contracted with either phenylephrine or 60 mM KCl. This relaxation was not affected by 10 μM chromanol 293B, 10 μM HMR1556 or 30 μM XE991 (pan-K_V 7 channel blocker). The anti-contractile effects of the perivascular adipose tissue (PVAT) were normal in Kcnq1^-/- arteries. Chromanol 293B and HMR1556 did not affect the anti-contractile effects of (PVAT). Isolated VSMCs from Kcnq1^-/- mice exhibited normal peak K_V currents. The K_V 7.2-5 channel opener retigabine caused similar relaxations in Kcnq1^-/- and wild-type vessels.

CONCLUSION AND IMPLICATIONS

We conclude that K_V 7.1 channels were apparently not involved in the control of arterial tone by α₁ -adrenoceptor agonists and PVAT. In addition, R-L3 is an inappropriate pharmacological tool for studying the function of native vascular K_V 7.1 channels in mice.

Computational modeling reveals key contributions of KCNQ and hERG currents to the malleability of uterine action potentials underpinning labor

The electrical excitability of uterine smooth muscle cells is a key determinant of the contraction of the organ during labor and is manifested by spontaneous, periodic action potentials (APs). Near the end of term, APs vary in shape and size reflecting an ability to change the frequency, duration and amplitude of uterine contractions. A recent mathematical model quantified several ionic features of the electrical excitability in uterine smooth muscle cells. It replicated many of the experimentally recorded uterine AP configurations but its limitations were evident when trying to simulate the long-duration bursting APs characteristic of labor. A computational parameter search suggested that delayed rectifying K(+) currents could be a key model component requiring improvement to produce the longer-lasting bursting APs. Of the delayed rectifying K(+) currents family it is of interest that KCNQ and hERG channels have been reported to be gestationally regulated in the uterus. These currents exhibit features similar to the broadly defined uterine IK1 of the original mathematical model. We thus formulated new quantitative descriptions for several I(KCNQ) and I(hERG). Incorporation of these currents into the uterine cell model enabled simulations of the long-lasting bursting APs. Moreover, we used this modified model to simulate the effects of different contributions of I(KCNQ) and I(hERG) on AP form. Our findings suggest that the alterations in expression of hERG and KCNQ channels can potentially provide a mechanism for fine tuning of AP forms that lends a malleability for changing between plateau-like and long-lasting bursting-type APs as uterine cells prepare for parturition.

Functional suppression of Kcnq1 leads to early sodium channel remodelling and cardiac conduction system dysmorphogenesis

AIMS

Ion channel remodelling and ventricular conduction system (VCS) alterations play relevant roles in the generation of cardiac arrhythmias, but the interaction between ion channel remodelling and cardiac conduction system dysfunctions in an arrhythmogenic context remain unexplored.

METHODS AND RESULTS

We have used a transgenic mouse line previously characterized as an animal model of Long QT Syndrome (LQTS) to analyse ion channel remodelling and VCS configuration. Reverse transcriptase-PCR and immunohistochemistry analysis showed early cardiac sodium channel upregulation at embryonic stages prior to the onset of Kv potassium channel remodelling, and cardiac hypertrophy at foetal stages. In line with these findings, patch-clamp assays demonstrated changes in sodium current density and a slowing of recovery from inactivation. Functional analysis by optical mapping revealed an immature ventricular activation pattern as well as an increase in the total left ventricle activation time in foetal transgenic hearts. Morphological analysis of LQTS transgenic mice in a Cx40(GFP/+)background demonstrated VCS dysmorphogenesis during heart development.

CONCLUSIONS

Our data demonstrate early sodium channel remodelling secondary to IKs blockage in a mouse model of LQTS leading to morphological and functional anomalies in the developing VCS and cardiac hypertrophy. These results provide new insights into the mechanisms underlying foetal and neonatal cardiac electrophysiological disorders, which might help understand how molecular, functional, and morphological alterations are linked to clinical pathologies such as cardiac congenital anomalies, arrhythmias, and perinatal sudden death.

Single-channel basis for the slow activation of the repolarizing cardiac potassium current, I(Ks)

Coassembly of potassium voltage-gated channel, KQT-like subfamily, member 1 (KCNQ1) with potassium voltage-gated channel, Isk-related family, member 1 (KCNE1) the delayed rectifier potassium channel I(Ks). Its slow activation is critically important for membrane repolarization and for abbreviating the cardiac action potential, especially during sympathetic activation and at high heart rates. Mutations in either gene can cause long QT syndrome, which can lead to fatal arrhythmias. To understand better the elementary behavior of this slowly activating channel complex, we quantitatively analyzed direct measurements of single-channel I(Ks). Single-channel recordings from transiently transfected mouse ltk(-) cells confirm a channel that has long latency periods to opening (1.67 ± 0.073 s at +60 mV) but that flickers rapidly between multiple open and closed states in non-deactivating bursts at positive membrane potentials. Channel activity is cyclic with periods of high activity followed by quiescence, leading to an overall open probability of only ∼0.15 after 4 s under our recording conditions. The mean single-channel conductance was determined to be 3.2 pS, but unlike any other known wild-type human potassium channel, long-lived subconductance levels coupled to activation are a key feature of both the activation and deactivation time courses of the conducting channel complex. Up to five conducting levels ranging from 0.13 to 0.66 pA could be identified in single-channel recordings at 60 mV. Fast closings and overt subconductance behavior of the wild-type I(Ks) channel required modification of existing Markov models to include these features of channel behavior.

Probucol and the cholesterol synthesis inhibitors simvastatin and triparanol regulate I ks channel function differently

Channels responsible for slowly activating delayed-rectifier potassium current (I(Ks)) are composed of KCNQ1 and KCNE1 subunits, and these channels play a role in the repolarization of cardiac action potentials. Recently, we showed that the antihyperlipidemic drug probucol, which induces QT prolongation, decreases the I(Ks) after 24-h treatment. In the present study, we investigated the effects of three cholesterol-lowering agents (probucol, an enhancer of cholesterol efflux; simvastatin, a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor; and triparanol, a 3β-hydroxysterol-▵24-reductase inhibitor) on cholesterol synthesis, the KCNQ1 current (I KCNQ1), and the I(Ks) to clarify the differences in the modes of action of these agents on the I(Ks). Probucol did not inhibit cholesterol synthesis and had no effect on I KCNQ1, while I(Ks) decreased after 24-h treatment. Simvastatin inhibited cholesterol synthesis and decreased I KCNQ1 and I(Ks). Additionally, the activation kinetics of I(Ks) became faster, compared with that of control I(Ks). Triparanol inhibited cholesterol synthesis but did not reduce I KCNQ1 and I(Ks). However, the activation kinetics of I(Ks) became faster. Our data indicated that the mechanism by which probucol inhibits I(Ks) was not mediated by the inhibition of cholesterol synthesis but depended on an interaction with the KCNQ1/KCNE1 complex. Meanwhile, the reduction in cholesterol induced by simvastatin and triparanol is one of the mechanisms that affects the kinetics of I(ks).

The KCNE Tango - How KCNE1 Interacts with Kv7.1

The classical tango is a dance characterized by a 2/4 or 4/4 rhythm in which the partners dance in a coordinated way, allowing dynamic contact. There is a surprising similarity between the tango and how KCNE β-subunits "dance" to the fast rhythm of the cell with their partners from the Kv channel family. The five KCNE β-subunits interact with several members of the Kv channels, thereby modifying channel gating via the interaction of their single transmembrane-spanning segment, the extracellular amino terminus, and/or the intracellular carboxy terminus with the Kv α-subunit. Best studied is the molecular basis of interactions between KCNE1 and Kv7.1, which, together, supposedly form the native cardiac I(Ks) channel. Here we review the current knowledge about functional and molecular interactions of KCNE1 with Kv7.1 and try to summarize and interpret the tango of the KCNEs.

Modification of K+ channel-drug interactions by ancillary subunits

Reconciling ion channel alpha-subunit expression with native ionic currents and their pharmacological sensitivity in target organs has proved difficult. In native tissue, many K(+) channel alpha-subunits co-assemble with ancillary subunits, which can profoundly affect physiological parameters including gating kinetics and pharmacological interactions. In this review, we examine the link between voltage-gated potassium ion channel pharmacology and the biophysics of ancillary subunits. We propose that ancillary subunits can modify the interaction between pore blockers and ion channels by three distinct mechanisms: changes in (1) binding site accessibility; (2) orientation of pore-lining residues; (3) the ability of the channel to undergo post-binding conformational changes. Each of these subunit-induced changes has implications for gating, drug affinity and use dependence of their respective channel complexes. A single subunit may modulate its associated alpha-subunit by more than one of these mechanisms. Voltage-gated potassium channels are the site of action of many therapeutic drugs. In addition, potassium channels interact with drugs whose primary target is another channel, e.g. the calcium channel blocker nifedipine, the sodium channel blocker quinidine, etc. Even when K(+) channel block is the intended mode of action, block of related channels in non-target organs, e.g. the heart, can result in major and potentially lethal side-effects. Understanding factors that determine specificity, use dependence and other properties of K(+) channel drug binding are therefore of vital clinical importance. Ancillary subunits play a key role in determining these properties in native tissue, and so understanding channel-subunit interactions is vital to understanding clinical pharmacology.

Chromanol 293B binding in KCNQ1 (Kv7.1) channels involves electrostatic interactions with a potassium ion in the selectivity filter

The chromanol 293B (293B, trans-6-cyano-4-(N-ethylsulfonyl-N-methylamino)-3-hydroxy-2,2-dimethyl-chroman) is a lead compound of potential class III antiarrhythmics that inhibit cardiac I(Ks) potassium channels. These channels are formed by the coassembly of KCNQ1 (Kv7.1, KvLQT1) and KCNE1 subunits. Although homomeric KCNQ1 channels are the principal molecular targets, entry of KCNE1 to the channel complex enhances the chromanol block. Because closely related neuronal KCNQ2 potassium channels are insensitive to the drug, we used KCNQ1/KCNQ2 chimeras to identify the binding site of the inhibitor. We localized the putative drug receptor to the H5 selectivity filter and the S6 transmembrane segment. Single residues affecting 293B inhibition were subsequently identified through systematic exchange of amino acids that were either different in KCNQ1 and KCNQ2 or predicted by a docking model of 293B in the open and closed conformation of KCNQ1. Mutant channel proteins T312S, I337V, and F340Y displayed dramatically lowered sensitivity to chromanol block. The predicted drug binding receptor lies in the inner pore vestibule containing the lower part of the selectivity filter, and the S6 transmembrane domain also reported to be important for binding of benzodiazepines. We propose that the block of the ion permeation pathway involves hydrophobic interactions with the S6 transmembrane residues Ile337 and Phe340, and stabilization of chromanol 293B binding through electrostatic interactions of its oxygen atoms with the most internal potassium ion within the selectivity filter.

Ancillary subunits and stimulation frequency determine the potency of chromanol 293B block of the KCNQ1 potassium channel

KCNQ1 (Kv7.1 or KvLQT1) encodes the alpha-subunit of a voltage-gated potassium channel found in tissues including heart, brain, epithelia and smooth muscle. Tissue-specific characteristics of KCNQ1 current are diverse, due to modification by ancillary subunits. In heart, KCNQ1 associates with KCNE1 (MinK), producing a slowly activating voltage-dependent channel. In epithelia, KCNQ1 co-assembles with KCNE3 (Mirp2) producing a constitutively open channel. Chromanol 293B is a selective KCNQ1 blocker. We studied drug binding and frequency dependence of 293B on KCNQ1 and ancillary subunits expressed in Xenopus oocytes. Ancillary subunits altered 293B potency up to 100-fold (IC(50) for KCNQ1 = 65.4 +/- 1.7 microm; KCNQ1/KCNE1 = 15.1 +/- 3.3 microm; KCNQ1/KCNE3 = 0.54 +/- 0.18 microm). Block of KCNQ1 and KCNQ1/KCNE3 was time independent, but 293B altered KCNQ1/KCNE1 activation. We therefore studied frequency-dependent block of KCNQ1/KCNE1. Repetitive rapid stimulation increased KCNQ1/KCNE1 current biphasically, and 293B abolished the slow component. KCNQ1/KCNE3[V72T] activates slowly with a KCNQ1/KCNE1-like phenotype, but retains the high affinity binding of KCNQ1/KCNE3, demonstrating that subunit-mediated changes in gating can be dissociated from subunit-mediated changes in affinity. This study demonstrates the KCNQ1 pharmacology is significantly altered by ancillary subunits. The response of KCNQ1 to specific blockers will therefore be critically dependent on the electrical stimulation pattern of the target organ. Furthermore, the dissociation between gating and overall affinity suggests that mutations in ancillary subunits can potentially strongly alter drug sensitivity without obvious functional changes in gating behaviour, giving rise to unexpected side-effects such as a predisposition to acquired long QT syndrome.

Atrial fibrillation in KCNE1-null mice

Although atrial fibrillation is the most common serious cardiac arrhythmia, the fundamental molecular pathways remain undefined. Mutations in KCNQ1, one component of a sympathetically activated cardiac potassium channel complex, cause familial atrial fibrillation, although the mechanisms in vivo are unknown. We show here that mice with deletion of the KCNQ1 protein partner KCNE1 have spontaneous episodes of atrial fibrillation despite normal atrial size and structure. Isoproterenol abolishes these abnormalities, but vagomimetic interventions have no effect. Whereas loss of KCNE1 function prolongs ventricular action potentials in humans, KCNE1-/- mice displayed unexpectedly shortened atrial action potentials, and multiple potential mechanisms were identified: (1) K+ currents (total and those sensitive to the KCNQ1 blocker chromanol 293B) were significantly increased in atrial cells from KCNE1-/- mice compared with controls, and (2) when CHO cells expressing KCNQ1 and KCNE1 were pulsed very rapidly (at rates comparable to the normal mouse heart and to human atrial fibrillation), the sigmoidicity of IKs activation prevented current accumulation, whereas cells expressing KCNQ1 alone displayed marked current accumulation at these very rapid rates. Thus, KCNE1 deletion in mice unexpectedly leads to increased outward current in atrial myocytes, shortens atrial action potentials, and enhances susceptibility to atrial fibrillation.

Both β-adrenergic receptor stimulation and cardiac tissue type have important roles in elucidating the functional effects of IKs channel blockers in vitro

INTRODUCTION

Conflicting results associated with the use of I(Ks) blockers on the action potential duration (APD) have raised a question as to whether the variable results arise from the use of different cardiac tissues, beta-adrenergic stimulation, or by the "selectivity" of the chosen I(Ks) blockers.

METHODS

We used the highly selective I(Ks) blocker (-)-[3R, 4S] chromanol 293B [(-) chromanol] to mimic drug-induced long QT1 in isolated rabbit Purkinje fibers, papillary muscles, and ventricular trabeculae using the conventional microelectrode technique.

RESULTS

I(Ks) block with (-) chromanol at 1 x 10(-5) M did not significantly change the APD at different stimulation rates in all three cardiac tissues. Isoproterenol (Iso:1 x 10(-7) M) shortened APD(90), and (-) chromanol (1 x 10(-5) M) largely prevented this shortening in isolated papillary muscles at 1 Hz [-3% with Iso combined (-) chromanol group versus -16% with iso group; p<0.05] and also at 2 Hz (+7% versus -25% with Iso group; p<0.05), but did not significantly prevent this shortening in isolated Purkinje fibers. In isolated trabeculae, (-) chromanol combined with Iso significantly prolonged the APD(90) by 15% at 1 Hz (versus -10% with Iso group; p<0.05) and by 5% at 2 Hz (versus -11% with Iso group; p<0.05).

DISCUSSION

Our study shows that only during beta-adrenoceptor stimulation, pharmacological inhibition of the I(Ks) current plays an important role in the APD recorded from isolated ventricular trabeculae and papillary muscles, but not from Purkinje fibers. These results indicate that the APD prolonging effects of I(Ks)channel blockers during beta-adrenergic receptor stimulation can only be detected from isolated rabbit papillary muscles and ventricular trabeculae, but not Purkinje fibers.

Dualistic actions of cromakalim and new potent 2H-1,4-benzoxazine derivatives on the native skeletal muscle K ATP channel

1 New 2H-1,4-benzoxazine derivatives were synthesized and tested for their agonist properties on the ATP-sensitive K(+) channels (K(ATP)) of native rat skeletal muscle fibres by using the patch-clamp technique. The novel modifications involved the introduction at position 2 of the benzoxazine ring of alkyl substituents such as methyl (-CH(3)), ethyl (-C(2)H(5)) or propyl (-C(3)H(7)) groups, while maintaining pharmacophore groups critical for conferring agonist properties. 2 The effects of these molecules were compared with those of cromakalim in the presence or absence of internal ATP (10(-4) M). In the presence of internal ATP, all the compounds increased the macropatch K(ATP) currents. The order of potency of the molecules as agonists was -C(3)H(7) (DE(50)=1.63 x 10(-8) M) >-C(2)H(5) (DE(50)=1.11 x 10(-7) M)>-CH(3) (DE(50)=2.81 x 10(-7) M)>cromak-slim (DE(50)= 1.42 x 10(-5) M). Bell-shaped dose-response curves were observed for these compounds and cromakalim indicating a downturn in response when a certain dose was exceeded. 3 In contrast, in the absence of internal ATP, all molecules including cromakalim inhibited the K(ATP) currents. The order of increasing potency as antagonists was cromakalim (IC(50)=1.15 x 10(-8) M)> or =-CH(3) (IC(50)=2.6 x 10(-8) M)>-C(2)H(5) (IC(50)=4.4 x 10(-8) M)>-C(3)H(7) (IC(50)=1.68 x 10(-7) M) derivatives. 4 These results suggest that the newly synthesized molecules and cromakalim act on muscle K(ATP) channel by binding on two receptor sites that have opposite actions. Alternatively, a more simple explanation is to consider the existence of a single site for potassium channel openers regulated by ATP which favours the transduction of the channel opening. The alkyl chains at position 2 of the 2H-1,4-benzoxazine nucleus is pivotal in determining the potency of benzoxazine derivatives as agonists or antagonists.

Blocking action of chromanol 293B on the slow component of delayed rectifier K(+) current in guinea-pig sino-atrial node cells

1. In guinea-pig sino-atrial (SA) node cells the delayed rectifier K(+) current (I(K)) is composed of rapidly and slowly activating components of I(K) (I(Kr) and I(Ks), respectively). The present study was undertaken to characterize the blocking action of the chromanol derivative 293B on I(Ks) in guinea-pig SA node cells using whole-cell patch-clamp technique. 2. Bath application of 293B blocked I(Ks), elicited by 4-s depolarizing voltage pulses from a holding potential of -50 mV, under conditions in which the L-type Ca(2+) current (I(Ca,L)) and I(Kr) were inhibited; the effect was concentration-dependent with an IC(50) of 5.3 microM, when evaluated by the decrease in the amplitude of I(Ks) tail current following 4-s depolarizing voltage steps to +50 mV. 3. The 293B block of I(Ks) progressed with time during depolarizing voltage steps with a more rapid block at higher concentrations. 4. The block of I(Ks) by 293B was fully reversed within a few minutes after washing off the drug, even when a maximal effect (a nearly full block) was achieved at high drug concentration (50 microM). 5. Bath application of 293B at 50 microM greatly and reversibly reduced the amplitude of I(Ks) which is maximally stimulated by beta-adrenergic agonist isoprenaline (1 microM), while the degree of 293B block of the isoprenaline-stimulated I(Ks) was slightly but significantly smaller than that of non-stimulated I(Ks) (94.0+/-0.98% block, n=6 vs 99.4+/-0.45% block, n=6; P<0.01). 6. We conclude that, in guinea-pig SA node cells (i) 293B is a potent and fully reversible blocker of I(Ks) in control and during beta-adrenergic stimulation and (ii) block with 293B occurs in a time-dependent manner during depolarizing voltage steps.