1
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Genge CE, Muralidharan P, Kemp J, Hull CM, Yip M, Simpson K, Hunter DV, Claydon TW. Zebrafish cardiac repolarization does not functionally depend on the expression of the hERG1b-like transcript. Pflugers Arch 2024; 476:87-99. [PMID: 37934265 DOI: 10.1007/s00424-023-02875-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2023] [Revised: 10/25/2023] [Accepted: 10/27/2023] [Indexed: 11/08/2023]
Abstract
Zebrafish provide a translational model of human cardiac function. Their similar cardiac electrophysiology enables screening of human cardiac repolarization disorders, drug arrhythmogenicity, and novel antiarrhythmic therapeutics. However, while zebrafish cardiac repolarization is driven by delayed rectifier potassium channel current (IKr), the relative role of alternate channel transcripts is uncertain. While human ether-a-go-go-related-gene-1a (hERG1a) is the dominant transcript in humans, expression of the functionally distinct alternate transcript, hERG1b, modifies the electrophysiological and pharmacologic IKr phenotype. Studies of zebrafish IKr are frequently translated without consideration for the presence and impact of hERG1b in humans. Here, we performed phylogenetic analyses of all available KCNH genes from Actinopterygii (ray-finned fishes). Our findings confirmed zebrafish cardiac zkcnh6a as the paralog of human hERG1a (hKCNH2a), but also revealed evidence of a hERG1b (hKCNH2b)-like N-terminally truncated gene, zkcnh6b, in zebrafish. zkcnh6b is a teleost-specific variant that resulted from the 3R genome duplication. qRT-PCR showed dominant expression of zkcnh6a in zebrafish atrial and ventricular tissue, with low levels of zkcnh6b. Functional evaluation of zkcnh6b in a heterologous system showed no discernable function under the conditions tested, and no influence on zkcnh6a function during the zebrafish ventricular action potential. Our findings provide the first descriptions of the zkcnh6b gene, and show that, unlike in humans, zebrafish cardiac repolarization does not rely upon co-assembly of zERG1a/zERG1b. Given that hERG1b modifies IKr function and drug binding in humans, our findings highlight the need for consideration when translating hERG variant effects and toxicological screens in zebrafish, which lack a functional hERG1b-equivalent gene.
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Affiliation(s)
- Christine E Genge
- Department of Biomedical Physiology and Kinesiology, Simon Fraser University, 8888 University Drive, Burnaby, V5A 1S6, Burnaby, B.C, Canada
| | - Padmapriya Muralidharan
- Department of Biomedical Physiology and Kinesiology, Simon Fraser University, 8888 University Drive, Burnaby, V5A 1S6, Burnaby, B.C, Canada
| | - Jake Kemp
- Department of Biomedical Physiology and Kinesiology, Simon Fraser University, 8888 University Drive, Burnaby, V5A 1S6, Burnaby, B.C, Canada
| | - Christina M Hull
- Department of Biomedical Physiology and Kinesiology, Simon Fraser University, 8888 University Drive, Burnaby, V5A 1S6, Burnaby, B.C, Canada
| | - Mandy Yip
- Department of Biomedical Physiology and Kinesiology, Simon Fraser University, 8888 University Drive, Burnaby, V5A 1S6, Burnaby, B.C, Canada
| | - Kyle Simpson
- Department of Biomedical Physiology and Kinesiology, Simon Fraser University, 8888 University Drive, Burnaby, V5A 1S6, Burnaby, B.C, Canada
| | - Diana V Hunter
- Department of Biomedical Physiology and Kinesiology, Simon Fraser University, 8888 University Drive, Burnaby, V5A 1S6, Burnaby, B.C, Canada
| | - Thomas W Claydon
- Department of Biomedical Physiology and Kinesiology, Simon Fraser University, 8888 University Drive, Burnaby, V5A 1S6, Burnaby, B.C, Canada.
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2
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Baldwin SN, Jepps TA, Greenwood IA. Cycling matters: Sex hormone regulation of vascular potassium channels. Channels (Austin) 2023; 17:2217637. [PMID: 37243715 PMCID: PMC10228406 DOI: 10.1080/19336950.2023.2217637] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/23/2023] [Revised: 05/07/2023] [Accepted: 05/19/2023] [Indexed: 05/29/2023] Open
Abstract
Sex hormones and the reproductive cycle (estrus in rodents and menstrual in humans) have a known impact on arterial function. In spite of this, sex hormones and the estrus/menstrual cycle are often neglected experimental factors in vascular basic preclinical scientific research. Recent research by our own laboratory indicates that cyclical changes in serum concentrations of sex -hormones across the rat estrus cycle, primary estradiol, have significant consequences for the subcellular trafficking and function of KV. Vascular potassium channels, including KV, are essential components of vascular reactivity. Our study represents a small part of a growing field of literature aimed at determining the role of sex hormones in regulating arterial ion channel function. This review covers key findings describing the current understanding of sex hormone regulation of vascular potassium channels, with a focus on KV channels. Further, we highlight areas of research where the estrus cycle should be considered in future studies to determine the consequences of physiological oscillations in concentrations of sex hormones on vascular potassium channel function.
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Affiliation(s)
- Samuel N Baldwin
- Vascular Biology Group, Department of Biomedical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Thomas A Jepps
- Vascular Biology Group, Department of Biomedical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Iain A Greenwood
- Vascular Biology Research Centre, Institute of Molecular and Clinical Sciences, St George’s University of London, London, UK
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3
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Ukachukwu CU, Jimenez-Vazquez EN, Jain A, Jones DK. hERG1 channel subunit composition mediates proton inhibition of rapid delayed rectifier potassium current (I Kr) in cardiomyocytes derived from hiPSCs. J Biol Chem 2022; 299:102778. [PMID: 36496073 PMCID: PMC9867984 DOI: 10.1016/j.jbc.2022.102778] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/25/2022] [Revised: 11/29/2022] [Accepted: 12/04/2022] [Indexed: 12/13/2022] Open
Abstract
The voltage-gated channel, hERG1, conducts the rapid delayed rectifier potassium current (IKr) and is critical for human cardiac repolarization. Reduced IKr causes long QT syndrome and increases the risk for cardiac arrhythmia and sudden death. At least two subunits form functional hERG1 channels, hERG1a and hERG1b. Changes in hERG1a/1b abundance modulate IKr kinetics, magnitude, and drug sensitivity. Studies from native cardiac tissue suggest that hERG1 subunit abundance is dynamically regulated, but the impact of altered subunit abundance on IKr and its response to external stressors is not well understood. Here, we used a substrate-driven human-induced pluripotent stem cell-derived cardiomyocyte (hiPSC-CM) maturation model to investigate how changes in relative hERG1a/1b subunit abundance impact the response of native IKr to extracellular acidosis, a known component of ischemic heart disease and sudden infant death syndrome. IKr recorded from immatured hiPSC-CMs displays a 2-fold greater inhibition by extracellular acidosis (pH 6.3) compared with matured hiPSC-CMs. Quantitative RT-PCR and immunocytochemistry demonstrated that hERG1a subunit mRNA and protein were upregulated and hERG1b subunit mRNA and protein were downregulated in matured hiPSC-CMs compared with immatured hiPSC-CMs. The shift in subunit abundance in matured hiPSC-CMs was accompanied by increased IKr. Silencing hERG1b's impact on native IKr kinetics by overexpressing a polypeptide identical to the hERG1a N-terminal Per-Arnt-Sim domain reduced the magnitude of IKr proton inhibition in immatured hiPSC-CMs to levels comparable to those observed in matured hiPSC-CMs. These data demonstrate that hERG1 subunit abundance is dynamically regulated and determines IKr proton sensitivity in hiPSC-CMs.
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Affiliation(s)
- Chiamaka U. Ukachukwu
- Department of Pharmacology, University of Michigan Medical School, Ann Arbor, Michigan, USA
| | | | - Abhilasha Jain
- Department of Pharmacology, University of Michigan Medical School, Ann Arbor, Michigan, USA
| | - David K. Jones
- Department of Pharmacology, University of Michigan Medical School, Ann Arbor, Michigan, USA,Department of Internal Medicine, University of Michigan Medical School,For correspondence: David K. Jones
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4
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Sanchez-Conde FG, Jimenez-Vazquez EN, Auerbach DS, Jones DK. The ERG1 K+ Channel and Its Role in Neuronal Health and Disease. Front Mol Neurosci 2022; 15:890368. [PMID: 35600076 PMCID: PMC9113952 DOI: 10.3389/fnmol.2022.890368] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/05/2022] [Accepted: 04/11/2022] [Indexed: 11/30/2022] Open
Abstract
The ERG1 potassium channel, encoded by KCNH2, has long been associated with cardiac electrical excitability. Yet, a growing body of work suggests that ERG1 mediates physiology throughout the human body, including the brain. ERG1 is a regulator of neuronal excitability, ERG1 variants are associated with neuronal diseases (e.g., epilepsy and schizophrenia), and ERG1 serves as a potential therapeutic target for neuronal pathophysiology. This review summarizes the current state-of-the-field regarding the ERG1 channel structure and function, ERG1’s relationship to the mammalian brain and highlights key questions that have yet to be answered.
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Affiliation(s)
| | - Eric N. Jimenez-Vazquez
- Department of Pharmacology, University of Michigan Medical School, Ann Arbor, MI, United States
| | - David S. Auerbach
- Department of Pharmacology, State University of New York Upstate Medical University, Syracuse, NY, United States
- *Correspondence: David S. Auerbach,
| | - David K. Jones
- Department of Pharmacology, University of Michigan Medical School, Ann Arbor, MI, United States
- Department of Internal Medicine, University of Michigan Medical School, Ann Arbor, MI, United States
- David K. Jones,
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5
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Jones DK. Hysteretic hERG channel gating current recorded at physiological temperature. Sci Rep 2022; 12:5950. [PMID: 35396394 PMCID: PMC8993916 DOI: 10.1038/s41598-022-10003-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2021] [Accepted: 03/24/2022] [Indexed: 11/10/2022] Open
Abstract
Cardiac hERG channels comprise at least two subunits, hERG 1a and hERG 1b, and drive cardiac action potential repolarization. hERG 1a subunits contain a cytoplasmic PAS domain that is absent in hERG 1b. The hERG 1a PAS domain regulates voltage sensor domain (VSD) movement, but hERG VSD behavior and its regulation by the hERG 1a PAS domain have not been studied at physiological temperatures. We recorded gating charge from homomeric hERG 1a and heteromeric hERG 1a/1b channels at near physiological temperatures (36 ± 1 °C) using pulse durations comparable in length to the human ventricular action potential. The voltage dependence of deactivation was hyperpolarized relative to activation, reflecting VSD relaxation at positive potentials. These data suggest that relaxation (hysteresis) works to delay pore closure during repolarization. Interestingly, hERG 1a VSD deactivation displayed a double Boltzmann distribution, but hERG 1a/1b deactivation displayed a single Boltzmann. Disabling the hERG 1a PAS domain using a PAS-targeting antibody similarly transformed hERG 1a deactivation from a double to a single Boltzmann, highlighting the contribution of the PAS in regulating VSD movement. These data represent, to our knowledge, the first recordings of hERG gating charge at physiological temperature and demonstrate that VSD relaxation (hysteresis) is present in hERG channels at physiological temperature.
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Affiliation(s)
- David K Jones
- Department of Pharmacology, University of Michigan School of Medicine, Ann Arbor, MI, 48109, USA. .,Department of Internal Medicine, University of Michigan School of Medicine, Ann Arbor, MI, 48109, USA.
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6
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Lubberding AF, Juhl CR, Skovhøj EZ, Kanters JK, Mandrup‐Poulsen T, Torekov SS. Celebrities in the heart, strangers in the pancreatic beta cell: Voltage-gated potassium channels K v 7.1 and K v 11.1 bridge long QT syndrome with hyperinsulinaemia as well as type 2 diabetes. Acta Physiol (Oxf) 2022; 234:e13781. [PMID: 34990074 PMCID: PMC9286829 DOI: 10.1111/apha.13781] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2021] [Revised: 12/20/2021] [Accepted: 01/02/2022] [Indexed: 12/13/2022]
Abstract
Voltage‐gated potassium (Kv) channels play an important role in the repolarization of a variety of excitable tissues, including in the cardiomyocyte and the pancreatic beta cell. Recently, individuals carrying loss‐of‐function (LoF) mutations in KCNQ1, encoding Kv7.1, and KCNH2 (hERG), encoding Kv11.1, were found to exhibit post‐prandial hyperinsulinaemia and episodes of hypoglycaemia. These LoF mutations also cause the cardiac disorder long QT syndrome (LQTS), which can be aggravated by hypoglycaemia. Interestingly, patients with LQTS also have a higher burden of diabetes compared to the background population, an apparent paradox in relation to the hyperinsulinaemic phenotype, and KCNQ1 has been identified as a type 2 diabetes risk gene. This review article summarizes the involvement of delayed rectifier K+ channels in pancreatic beta cell function, with emphasis on Kv7.1 and Kv11.1, using the cardiomyocyte for context. The functional and clinical consequences of LoF mutations and polymorphisms in these channels on blood glucose homeostasis are explored using evidence from pre‐clinical, clinical and genome‐wide association studies, thereby evaluating the link between LQTS, hyperinsulinaemia and type 2 diabetes.
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Affiliation(s)
- Anniek F. Lubberding
- Department of Biomedical Sciences Faculty of Health and Medical Sciences University of Copenhagen Copenhagen Denmark
| | - Christian R. Juhl
- Department of Biomedical Sciences Faculty of Health and Medical Sciences University of Copenhagen Copenhagen Denmark
| | - Emil Z. Skovhøj
- Department of Biomedical Sciences Faculty of Health and Medical Sciences University of Copenhagen Copenhagen Denmark
| | - Jørgen K. Kanters
- Department of Biomedical Sciences Faculty of Health and Medical Sciences University of Copenhagen Copenhagen Denmark
| | - Thomas Mandrup‐Poulsen
- Department of Biomedical Sciences Faculty of Health and Medical Sciences University of Copenhagen Copenhagen Denmark
| | - Signe S. Torekov
- Department of Biomedical Sciences Faculty of Health and Medical Sciences University of Copenhagen Copenhagen Denmark
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7
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Feng L, Zhang J, Lee C, Kim G, Liu F, Petersen AJ, Lim E, Anderson CL, Orland KM, Robertson GA, Eckhardt LL, January CT, Kamp TJ. Long QT Syndrome KCNH2 Variant Induces hERG1a/1b Subunit Imbalance in Patient-Specific Induced Pluripotent Stem Cell-Derived Cardiomyocytes. Circ Arrhythm Electrophysiol 2021; 14:e009343. [PMID: 33729832 PMCID: PMC8058932 DOI: 10.1161/circep.120.009343] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
[Figure: see text].
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Affiliation(s)
- Li Feng
- Cellular and Molecular Arrhythmia Research Program, Department of Medicine, Division of Cardiovascular Medicine, University of Wisconsin-Madison, Madison, WI
- Department of Cardiology, Beijing Anzhen Hospital, Capital Medical University, National Clinical Research Center for Cardiovascular Diseases, Beijing, China
| | - Jianhua Zhang
- Cellular and Molecular Arrhythmia Research Program, Department of Medicine, Division of Cardiovascular Medicine, University of Wisconsin-Madison, Madison, WI
| | - ChangHwan Lee
- Department of Biological Sciences, University at Albany, State University of New York, Albany, NY
| | - Gina Kim
- Cellular and Molecular Arrhythmia Research Program, Department of Medicine, Division of Cardiovascular Medicine, University of Wisconsin-Madison, Madison, WI
| | - Fang Liu
- Department of Neuroscience, Wisconsin Institutes for Medical Research, University of Wisconsin-Madison, Madison, WI
| | | | - Evi Lim
- Cellular and Molecular Arrhythmia Research Program, Department of Medicine, Division of Cardiovascular Medicine, University of Wisconsin-Madison, Madison, WI
| | - Corey L. Anderson
- Cellular and Molecular Arrhythmia Research Program, Department of Medicine, Division of Cardiovascular Medicine, University of Wisconsin-Madison, Madison, WI
| | - Kate M. Orland
- Cellular and Molecular Arrhythmia Research Program, Department of Medicine, Division of Cardiovascular Medicine, University of Wisconsin-Madison, Madison, WI
| | - Gail A. Robertson
- Department of Neuroscience, Wisconsin Institutes for Medical Research, University of Wisconsin-Madison, Madison, WI
| | - Lee L. Eckhardt
- Cellular and Molecular Arrhythmia Research Program, Department of Medicine, Division of Cardiovascular Medicine, University of Wisconsin-Madison, Madison, WI
| | - Craig T. January
- Cellular and Molecular Arrhythmia Research Program, Department of Medicine, Division of Cardiovascular Medicine, University of Wisconsin-Madison, Madison, WI
| | - Timothy J. Kamp
- Cellular and Molecular Arrhythmia Research Program, Department of Medicine, Division of Cardiovascular Medicine, University of Wisconsin-Madison, Madison, WI
- Department of Cell and Regenerative Biology, University of Wisconsin-Madison, Madison, WI
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8
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Identification and Characterization of a Transcribed Distal Enhancer Involved in Cardiac Kcnh2 Regulation. Cell Rep 2020; 28:2704-2714.e5. [PMID: 31484079 DOI: 10.1016/j.celrep.2019.08.007] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/2018] [Revised: 06/05/2019] [Accepted: 07/30/2019] [Indexed: 12/26/2022] Open
Abstract
The human ether-a-go-go-related gene KCNH2 encodes the voltage-gated potassium channel underlying IKr, a current critical for the repolarization phase of the cardiac action potential. Mutations in KCNH2 that cause a reduction of the repolarizing current can result in cardiac arrhythmias associated with long-QT syndrome. Here, we investigate the regulation of KCNH2 and identify multiple active enhancers. A transcribed enhancer ∼85 kbp downstream of Kcnh2 physically contacts the promoters of two Kcnh2 isoforms in a cardiac-specific manner in vivo. Knockdown of its ncRNA transcript results in reduced expression of Kcnh2b and two neighboring mRNAs, Nos3 and Abcb8, in vitro. Genomic deletion of the enhancer, including the ncRNA transcription start site, from the mouse genome causes a modest downregulation of both Kcnh2a and Kcnh2b in the ventricles. These findings establish that the regulation of Kcnh2a and Kcnh2b is governed by a complex regulatory landscape that involves multiple partially redundantly acting enhancers.
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9
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Palme D, Misovic M, Ganser K, Klumpp L, Salih HR, Zips D, Huber SM. hERG K + Channels Promote Survival of Irradiated Leukemia Cells. Front Pharmacol 2020; 11:489. [PMID: 32390841 PMCID: PMC7194033 DOI: 10.3389/fphar.2020.00489] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2019] [Accepted: 03/27/2020] [Indexed: 12/19/2022] Open
Abstract
Many tumor cells express highly elevated activities of voltage-gated K+ channels in the plasma membrane which are indispensable for tumor growth. To test for K+ channel function during DNA damage response, we subjected human chronic myeloid leukemia (CML) cells to sub-lethal doses of ionizing radiation (0-8 Gy, 6 MV photons) and determined K+ channel activity, K+ channel-dependent Ca2+ signaling, cell cycle progression, DNA repair, and clonogenic survival by whole-cell patch clamp recording, fura-2 Ca2+ imaging, Western blotting, flow cytometry, immunofluorescence microscopy, and pre-plating colony formation assay, respectively. As a result, the human erythroid CML cell line K562 and primary human CML cells functionally expressed hERG1. Irradiation stimulated in both cell types an increase in the activity of hERG1 K+ channels which became apparent 1-2 h post-irradiation. This increase in K+ channel activity was paralleled by an accumulation in S phase of cell cycle followed by a G2/M cell cycle arrest as analyzed between 8 and 72 h post-irradiation. Attenuating the K+ channel function by applying the hERG1 channel inhibitor E4031 modulated Ca2+ signaling, impaired inhibition of the mitosis promoting subunit cdc2, overrode cell cycle arrest, and decreased clonogenic survival of the irradiated cells but did not affect repair of DNA double strand breaks suggesting a critical role of the hERG1 K+ channels for the Ca2+ signaling and the cell cycle control during DNA damage response.
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Affiliation(s)
- Daniela Palme
- Department of Radiation Oncology, University Hospital Tübingen, Tübingen, Germany
| | - Milan Misovic
- Department of Radiation Oncology, University Hospital Tübingen, Tübingen, Germany
| | - Katrin Ganser
- Department of Radiation Oncology, University Hospital Tübingen, Tübingen, Germany
| | - Lukas Klumpp
- Department of Radiation Oncology, University Hospital Tübingen, Tübingen, Germany
| | - Helmut R Salih
- Clinical Collaboration Unit Translational Immunology, German Cancer Consortium (DKTK), University Hospital Tübingen, Tübingen, Germany
| | - Daniel Zips
- Department of Radiation Oncology, University Hospital Tübingen, Tübingen, Germany.,German Cancer Consortium (DKTK), Partner Site Tübingen, Tübingen, Germany.,German Cancer Research Center (DKFZ), Heidelberg, Germany
| | - Stephan M Huber
- Department of Radiation Oncology, University Hospital Tübingen, Tübingen, Germany
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10
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Tay YL, Amanah A, Adenan MI, Wahab HA, Tan ML. Mitragynine, an euphoric compound inhibits hERG1a/1b channel current and upregulates the complexation of hERG1a-Hsp90 in HEK293-hERG1a/1b cells. Sci Rep 2019; 9:19757. [PMID: 31874991 PMCID: PMC6930223 DOI: 10.1038/s41598-019-56106-6] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2019] [Accepted: 11/06/2019] [Indexed: 11/24/2022] Open
Abstract
Mitragyna speciosa Korth (M. speciosa) has been widely used as a recreational product, however, there are growing concerns on the abuse potentials and toxicity of the plant. Several poisoning and fatal cases involving kratom and mitragynine have been reported but the underlying causes remain unclear. The human ether-a-go-go-related gene 1 (hERG1) encodes the pore-forming subunit underlying cardiac rapidly delayed rectifier potassium current (IKr). Pharmacological blockade of the IKr can cause acquired long QT syndrome, leading to lethal cardiac arrhythmias. This study aims to elucidate the mechanisms of mitragynine-induced inhibition on hERG1a/1b current. Electrophysiology experiments were carried out using Port-a-Patch system. Quantitative RT-PCR, Western blot analysis, immunofluorescence and co-immunoprecipitation methods were used to determine the effects of mitragynine on hERG1a/1b expression and hERG1-cytosolic chaperones interaction. Mitragynine was found to inhibit the IKr current with an IC50 value of 332.70 nM. It causes a significant reduction of the fully-glycosylated (fg) hERG1a protein expression but upregulates both core-glycosylated (cg) expression and hERG1a-Hsp90 complexes, suggesting possible impaired hERG1a trafficking. In conclusion, mitragynine inhibits hERG1a/1b current through direct channel blockade at lower concentration, but at higher concentration, it upregulates the complexation of hERG1a-Hsp90 which may be inhibitory towards channel trafficking.
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Affiliation(s)
- Yea Lu Tay
- Malaysian Institute of Pharmaceuticals & Nutraceuticals, NIBM, Ministry of Energy, Science, Technology, Environment and Climate Change (MESTECC), Pulau Pinang, 11700, Malaysia
| | - Azimah Amanah
- Malaysian Institute of Pharmaceuticals & Nutraceuticals, NIBM, Ministry of Energy, Science, Technology, Environment and Climate Change (MESTECC), Pulau Pinang, 11700, Malaysia
| | - Mohd Ilham Adenan
- Atta-ur-Rahman Institute for Natural Product Discovery, Universiti Teknologi MARA (UiTM), Selangor Darul Ehsan, 42300, Malaysia
| | - Habibah Abdul Wahab
- School of Pharmaceutical Sciences, Universiti Sains Malaysia, Pulau Pinang, 11700, Malaysia
| | - Mei Lan Tan
- Malaysian Institute of Pharmaceuticals & Nutraceuticals, NIBM, Ministry of Energy, Science, Technology, Environment and Climate Change (MESTECC), Pulau Pinang, 11700, Malaysia. .,School of Pharmaceutical Sciences, Universiti Sains Malaysia, Pulau Pinang, 11700, Malaysia. .,Advanced Medical and Dental Institute, Universiti Sains Malaysia, SAINS@BERTAM, Kepala Batas, Pulau Pinang, 13200, Malaysia.
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11
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Goversen B, Jonsson MK, van den Heuvel NH, Rijken R, Vos MA, van Veen TA, de Boer TP. The influence of hERG1a and hERG1b isoforms on drug safety screening in iPSC-CMs. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2019; 149:86-98. [PMID: 30826123 DOI: 10.1016/j.pbiomolbio.2019.02.003] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/15/2018] [Revised: 01/14/2019] [Accepted: 02/08/2019] [Indexed: 01/03/2023]
Abstract
The human Ether-à-go-go Related Gene (hERG) encodes the pore forming subunit of the channel that conducts the rapid delayed rectifier potassium current IKr. IKr drives repolarization in the heart and when IKr is dysfunctional, cardiac repolarization delays, the QT interval on the electrocardiogram (ECG) prolongs and the risk of developing lethal arrhythmias such as Torsade de Pointes (TdP) increases. TdP risk is incorporated in drug safety screening for cardiotoxicity where hERG is the main target since the IKr channels appear highly sensitive to blockage. hERG block is also included as an important read-out in the Comprehensive in Vitro Proarrhythmia Assay (CiPA) initiative which aims to combine in vitro and in silico experiments on induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) to screen for cardiotoxicity. However, the hERG channel has some unique features to consider for drug safety screening, which we will discuss in this study. The hERG channel consists of different isoforms, hERG1a and hERG1b, which individually influence the kinetics of the channel and the drug response in the human heart and in iPSC-CMs. hERG1b is often underappreciated in iPSC-CM studies, drug screening assays and in silico models, and the fact that its contribution might substantially differ between iPSC-CM and healthy but also diseased human heart, adds to this problem. In this study we show that the activation kinetics in iPSC-CMs resemble hERG1b kinetics using Cs+ as a charge carrier. Not including hERG1b in drug safety testing might underestimate the actual role of hERG1b in repolarization and drug response, and might lead to inappropriate conclusions. We stress to focus more on including hERG1b in drug safety testing concerning IKr.
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Affiliation(s)
- Birgit Goversen
- Department of Medical Physiology, Division Heart & Lungs, University Medical Center Utrecht, the Netherlands
| | - Malin Kb Jonsson
- Department of Medical Physiology, Division Heart & Lungs, University Medical Center Utrecht, the Netherlands; Bioscience Heart Failure, Cardiovascular, Renal and Metabolic Diseases, IMED Biotech Unit, AstraZeneca, Gothenburg, Sweden
| | - Nikki Hl van den Heuvel
- Department of Medical Physiology, Division Heart & Lungs, University Medical Center Utrecht, the Netherlands
| | - Rianne Rijken
- Department of Medical Physiology, Division Heart & Lungs, University Medical Center Utrecht, the Netherlands
| | - Marc A Vos
- Department of Medical Physiology, Division Heart & Lungs, University Medical Center Utrecht, the Netherlands
| | - Toon Ab van Veen
- Department of Medical Physiology, Division Heart & Lungs, University Medical Center Utrecht, the Netherlands
| | - Teun P de Boer
- Department of Medical Physiology, Division Heart & Lungs, University Medical Center Utrecht, the Netherlands.
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12
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Aromolaran AS, Srivastava U, Alí A, Chahine M, Lazaro D, El-Sherif N, Capecchi PL, Laghi-Pasini F, Lazzerini PE, Boutjdir M. Interleukin-6 inhibition of hERG underlies risk for acquired long QT in cardiac and systemic inflammation. PLoS One 2018; 13:e0208321. [PMID: 30521586 PMCID: PMC6283635 DOI: 10.1371/journal.pone.0208321] [Citation(s) in RCA: 95] [Impact Index Per Article: 15.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/09/2018] [Accepted: 11/15/2018] [Indexed: 12/19/2022] Open
Abstract
Increased proinflammatory interleukin-6 (IL-6) levels are associated with acquired long QT-syndrome (LQTS) in patients with systemic inflammation, leading to higher risks for life-threatening polymorphic ventricular tachycardia such as Torsades de Pointes. However, the functional and molecular mechanisms of this association are not known. In most cases of acquired LQTS, the target ion channel is the human ether-á-go-go-related gene (hERG) encoding the rapid component of the delayed rectifier K current, IKr, which plays a critical role in cardiac repolarization. Here, we tested the hypothesis that IL-6 may cause QT prolongation by suppressing IKr. Electrophysiological and biochemical assays were used to assess the impact of IL-6 on the functional expression of IKr in HEK293 cells and adult guinea-pig ventricular myocytes (AGPVM). In HEK293 cells, IL-6 alone or in combination with the soluble IL-6 receptor (IL-6R), produced a significant depression of IKr peak and tail current densities. Block of IL-6R or Janus kinase (JAK) reversed the inhibitory effects of IL-6 on IKr. In AGPVM, IL-6 prolonged action potential duration (APD) which was further prolonged in the presence of IL-6R. Similar to heterologous cells, IL-6 reduced endogenous guinea pig ERG channel mRNA and protein expression. The data are first to demonstrate that IL-6 inhibition of IKr and the resulting prolongation of APD is mediated via IL-6R and JAK pathway activation and forms the basis for the observed clinical QT interval prolongation. These novel findings may guide the development of targeted anti-arrhythmic therapeutic interventions in patients with LQTS and inflammatory disorders.
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Affiliation(s)
- Ademuyiwa S. Aromolaran
- Cardiovascular Research Program, VA New York Harbor Healthcare System, Brooklyn, New York, United States of America
- Department of Cell Biology and Pharmacology, State University of New York Downstate Medical Center, Brooklyn, New York, United States of America
| | - Ujala Srivastava
- Cardiovascular Research Program, VA New York Harbor Healthcare System, Brooklyn, New York, United States of America
- Department of Cell Biology and Pharmacology, State University of New York Downstate Medical Center, Brooklyn, New York, United States of America
| | - Alessandra Alí
- Department of Molecular and Developmental Medicine, University of Siena, Siena, Italy
| | - Mohamed Chahine
- Centre de Recherche, Institut Universitaire en Santé Mentale de Québec, Department of Medicine, Université Laval, Quebec City, Quebec, Canada
| | - Deana Lazaro
- Cardiovascular Research Program, VA New York Harbor Healthcare System, Brooklyn, New York, United States of America
| | - Nabil El-Sherif
- Cardiovascular Research Program, VA New York Harbor Healthcare System, Brooklyn, New York, United States of America
| | - Pier Leopoldo Capecchi
- Department of Medical Sciences, Surgery and Neurosciences, University of Siena, Siena, Italy
| | - Franco Laghi-Pasini
- Department of Medical Sciences, Surgery and Neurosciences, University of Siena, Siena, Italy
| | - Pietro Enea Lazzerini
- Department of Medical Sciences, Surgery and Neurosciences, University of Siena, Siena, Italy
| | - Mohamed Boutjdir
- Cardiovascular Research Program, VA New York Harbor Healthcare System, Brooklyn, New York, United States of America
- Department of Cell Biology and Pharmacology, State University of New York Downstate Medical Center, Brooklyn, New York, United States of America
- Departments of Medicine, State University of New York Downstate Medical Center, Brooklyn, New York, United States of America
- Department of Medicine, New York University School of Medicine, New York, United States of America
- * E-mail:
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13
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Perissinotti LL, De Biase PM, Guo J, Yang PC, Lee MC, Clancy CE, Duff HJ, Noskov SY. Determinants of Isoform-Specific Gating Kinetics of hERG1 Channel: Combined Experimental and Simulation Study. Front Physiol 2018; 9:207. [PMID: 29706893 PMCID: PMC5907531 DOI: 10.3389/fphys.2018.00207] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2017] [Accepted: 02/23/2018] [Indexed: 12/22/2022] Open
Abstract
IKr is the rapidly activating component of the delayed rectifier potassium current, the ion current largely responsible for the repolarization of the cardiac action potential. Inherited forms of long QT syndrome (LQTS) (Lees-Miller et al., 1997) in humans are linked to functional modifications in the Kv11.1 (hERG) ion channel and potentially life threatening arrhythmias. There is little doubt now that hERG-related component of IKr in the heart depends on the tetrameric (homo- or hetero-) channels formed by two alternatively processed isoforms of hERG, termed hERG1a and hERG1b. Isoform composition (hERG1a- vs. the b-isoform) has recently been reported to alter pharmacologic responses to some hERG blockers and was proposed to be an essential factor pre-disposing patients for drug-induced QT prolongation. Very little is known about the gating and pharmacological properties of two isoforms in heart membranes. For example, how gating mechanisms of the hERG1a channels differ from that of hERG1b is still unknown. The mechanisms by which hERG 1a/1b hetero-tetramers contribute to function in the heart, or what role hERG1b might play in disease are all questions to be answered. Structurally, the two isoforms differ only in the N-terminal region located in the cytoplasm: hERG1b is 340 residues shorter than hERG1a and the initial 36 residues of hERG1b are unique to this isoform. In this study, we combined electrophysiological measurements for HEK cells, kinetics and structural modeling to tease out the individual contributions of each isoform to Action Potential formation and then make predictions about the effects of having various mixture ratios of the two isoforms. By coupling electrophysiological data with computational kinetic modeling, two proposed mechanisms of hERG gating in two homo-tetramers were examined. Sets of data from various experimental stimulation protocols (HEK cells) were analyzed simultaneously and fitted to Markov-chain models (M-models). The minimization procedure presented here, allowed assessment of suitability of different Markov model topologies and the corresponding parameters that describe the channel kinetics. The kinetics modeling pointed to key differences in the gating kinetics that were linked to the full channel structure. Interactions between soluble domains and the transmembrane part of the channel appeared to be critical determinants of the gating kinetics. The structures of the full channel in the open and closed states were compared for the first time using the recent Cryo-EM resolved structure for full open hERG channel and an homology model for the closed state, based on the highly homolog EAG1 channel. Key potential interactions which emphasize the importance of electrostatic interactions between N-PAS cap, S4-S5, and C-linker are suggested based on the structural analysis. The derived kinetic parameters were later used in higher order models of cells and tissue to track down the effect of varying the ratios of hERG1a and hERG1b on cardiac action potentials and computed electrocardiograms. Simulations suggest that the recovery from inactivation of hERG1b may contribute to its physiologic role of this isoform in the action potential. Finally, the results presented here contribute to the growing body of evidence that hERG1b significantly affects the generation of the cardiac Ikr and plays an important role in cardiac electrophysiology. We highlight the importance of carefully revisiting the Markov models previously proposed in order to properly account for the relative abundance of the hERG1 a- and b- isoforms.
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Affiliation(s)
- Laura L Perissinotti
- Centre for Molecular Simulations, Department of Biological Sciences, Faculty of Science, University of Calgary, Calgary, AB, Canada
| | - Pablo M De Biase
- Centre for Molecular Simulations, Department of Biological Sciences, Faculty of Science, University of Calgary, Calgary, AB, Canada
| | - Jiqing Guo
- Libin Cardiovascular Institute of Alberta, Faculty of Medicine, University of Calgary, Calgary, AB, Canada
| | - Pei-Chi Yang
- Department of Physiology and Membrane Biology, University of California, Davis, Davis, CA, United States
| | - Miranda C Lee
- Centre for Molecular Simulations, Department of Biological Sciences, Faculty of Science, University of Calgary, Calgary, AB, Canada
| | - Colleen E Clancy
- Department of Physiology and Membrane Biology, University of California, Davis, Davis, CA, United States
| | - Henry J Duff
- Libin Cardiovascular Institute of Alberta, Faculty of Medicine, University of Calgary, Calgary, AB, Canada
| | - Sergei Y Noskov
- Centre for Molecular Simulations, Department of Biological Sciences, Faculty of Science, University of Calgary, Calgary, AB, Canada
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14
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Roy S, Mathew MK. Fluid flow modulates electrical activity in cardiac hERG potassium channels. J Biol Chem 2018; 293:4289-4303. [PMID: 29305421 DOI: 10.1074/jbc.ra117.000432] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2017] [Revised: 12/28/2017] [Indexed: 01/01/2023] Open
Abstract
Fluid movement within the heart generates substantial shear forces, but the effect of this mechanical stress on the electrical activity of the human heart has not been examined. The fast component of the delayed rectifier potassium currents responsible for repolarization of the cardiac action potential, Ikr, is encoded by the human ether-a-go-go related gene (hERG) channel. Here, we exposed hERG1a channel-expressing HEK293T cells to laminar shear stress (LSS) and observed that this mechanical stress increased the whole-cell current by 30-40%. LSS shifted the voltage dependence of steady-state activation of the hERG channel to the hyperpolarizing direction, accelerated the time course of activation and recovery from inactivation, slowed down deactivation, and shifted the steady-state inactivation to the positive direction, all of which favored the hERG open state. In contrast, the time course of inactivation was faster, favoring the closed state. Using specific inhibitors of focal adhesion kinase, a regulator of mechano-transduction via the integrin pathway, we also found that the LSS-induced modulation of the whole-cell current depended on the integrin pathway. The hERG1b channel variant, which lacks the Per-Arnt-Sim (PAS) domain, and long QT syndrome-associated variants having point mutations in the PAS domain were unaffected by LSS, suggesting that the PAS domain in hERG1a channel may be involved in sensing mechanical shear stress. We conclude that a mechano-electric feedback pathway modulates hERG channel activity through the integrin pathway, indicating that mechanical forces in the heart influence its electrical activity.
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Affiliation(s)
- Samrat Roy
- From the National Centre for Biological Sciences, Tata Institute of Fundamental Research, Bengaluru 560065.,the Biocon Bristol Myers Squibb Research Center, Bengaluru 560099, and.,the School of Biotechnology, Kalinga Institute of Industrial Technology (KIIT) University, Bhubaneswar 751024, India
| | - M K Mathew
- From the National Centre for Biological Sciences, Tata Institute of Fundamental Research, Bengaluru 560065,
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15
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Barrese V, Cidad P, Yeung SY, López-López JR, McNeish AJ, Ohya S, Pérez-García MT, Greenwood IA. Proliferative Role of Kv11 Channels in Murine Arteries. Front Physiol 2017; 8:500. [PMID: 28747891 PMCID: PMC5506201 DOI: 10.3389/fphys.2017.00500] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2017] [Accepted: 06/29/2017] [Indexed: 12/22/2022] Open
Abstract
K+ channels encoded by the ether-a-go-go related gene (ERG1 or KCNH2) are important determinants of the cardiac action potential. Expression of both cardiac isoforms (ERG1a and ERG1b) were identified in murine portal vein and distinctive voltage-gated K+ currents were recorded from single myocytes. The aim of the present study was to ascertain the expression and functional impact of ERG channels in murine arteries. Methods: Quantitative RT-PCR was undertaken on RNA extracted from a number of murine arteries. Immunofluorescence was performed on single vascular smooth muscle cells using antibodies against the ERG1 expression product (Kv11.1). Single cell electrophysiology was performed on myocytes from portal vein and several different arteries, complimented by isometric tension recordings. Proliferation assays were undertaken on smooth muscle cells isolated from femoral arteries. Results: ERG1 transcripts were detected in all murine blood vessels, and Kv11.1 immunofluorescence was observed in all smooth muscle cells. However, K+ currents with properties consistent with ERG channels were only recorded in portal vein myocytes. Moreover, ERG channel blockers (E4031 or dofetilide, 1 μM) failed to depolarize carotid arteries or produce contraction. Proliferation of arterial smooth muscle cells was associated with a marked increase in ERG1 expression and ERG blockers suppressed proliferation significantly. Conclusions: These data reveal that arterial blood vessels express ERG channels that appear to be functional silent in contractile smooth muscle but contribute to proliferative response.
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Affiliation(s)
- Vincenzo Barrese
- Vascular Research Centre, Institute of Molecular & Clinical Sciences, St George's University of LondonLondon, United Kingdom
| | - Pilar Cidad
- Departmento de Fisiología, Universidad de ValladolidValladolid, Spain
| | - Shuk Y Yeung
- Vascular Research Centre, Institute of Molecular & Clinical Sciences, St George's University of LondonLondon, United Kingdom
| | | | - Alister J McNeish
- Reading School of Pharmacy, University of ReadingReading, United Kingdom
| | - Susumu Ohya
- Department of Pharmacology, Kyoto Pharmaceutical UniversityKyoto, Japan
| | | | - Iain A Greenwood
- Vascular Research Centre, Institute of Molecular & Clinical Sciences, St George's University of LondonLondon, United Kingdom
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16
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Mauerhöfer M, Bauer CK. Effects of Temperature on Heteromeric Kv11.1a/1b and Kv11.3 Channels. Biophys J 2017; 111:504-523. [PMID: 27508435 DOI: 10.1016/j.bpj.2016.07.002] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2016] [Revised: 06/30/2016] [Accepted: 07/01/2016] [Indexed: 01/17/2023] Open
Abstract
Kv11.1 channels are crucial in cardiac physiology, and there is increasing evidence of physiological roles of different Kv11 channels outside the heart. The HERG (human Kv11.1a) channel has previously been shown to carry substantially more current at elevated temperatures, and we have now comparably investigated the temperature dependence of neuronal Kv11.3 channels and the more ubiquitous heteromeric Kv11.1a/1b channels. Transiently expressed rat Kv11 channels were studied at 21°C, 30°C, and 35°C. At near-physiological temperature, the maximal sustained outward current density was almost three times the mean value obtained at room temperature for Kv11.1a/1b, and increased by ∼150% for Kv11.3. For both channels, reduced inactivation contributed to the current increase at higher temperature. Elevated temperature moved Kv11.1a/1b isochronal activation curves to more negative potentials, but shifted the potential of half-maximal Kv11.3 channel activation to more depolarized values and reduced its voltage sensitivity. Thus, increased temperature stabilized the open state over the closed state of Kv11.1a/1b channels and exerted the opposite effect on Kv11.3 channel activation. Both Kv11 channels exhibited an overall high temperature sensitivity of most gating parameters, with remarkably high Q10 factors of ∼5 for the rate of Kv11.1a/1b activation. The Q10 factors for Kv11.3 gating were more uniform, but still higher for activation than for inactivation kinetics. The results demonstrate that characteristic differences between Kv11.1a/1b and Kv11.3 determined at room temperature do not necessarily apply to physiological conditions. The data provided here can aid in the design of models that will enhance our understanding of the role of Kv11 currents in excitable cells.
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Affiliation(s)
- Maike Mauerhöfer
- Department of Cellular and Integrative Physiology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany
| | - Christiane K Bauer
- Department of Cellular and Integrative Physiology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.
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17
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Sale H, Roy S, Warrier J, Thangathirupathy S, Vadari Y, Gopal SK, Krishnamurthy P, Ramarao M. Modulation of K v 11.1 (hERG) channels by 5-(((1H-indazol-5-yl)oxy)methyl)-N-(4-(trifluoromethoxy)phenyl)pyrimidin-2-amine (ITP-2), a novel small molecule activator. Br J Pharmacol 2017; 174:2484-2500. [PMID: 28500657 DOI: 10.1111/bph.13859] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2016] [Revised: 05/04/2017] [Accepted: 05/05/2017] [Indexed: 01/10/2023] Open
Abstract
BACKGROUND AND PURPOSE Activators of Kv 11.1 (hERG) channels have potential utility in the treatment of acquired and congenital long QT (LQT) syndrome. Here, we describe a new hERG channel activator, 5-(((1H-indazol-5-yl)oxy)methyl)-N-(4-(trifluoromethoxy)phenyl)pyrimidin-2-amine (ITP-2), with a chemical structure distinct from previously reported compounds. EXPERIMENTAL APPROACH Conventional electrophysiological methods were used to assess the effects of ITP-2 on hERG1a and hERG1a/1b channels expressed heterologously in HEK-293 cells. KEY RESULTS ITP-2 selectively increased test pulse currents (EC50 1.0 μM) and decreased tail currents. ITP-2 activated hERG1a homomeric channels primarily by causing large depolarizing shifts in the midpoint of voltage-dependent inactivation and hyperpolarizing shifts in the voltage-dependence of activation. In addition, ITP-2 slowed rates of inactivation and made recovery from inactivation faster. hERG1a/1b heteromeric channels showed reduced sensitivity to ITP-2 and their inactivation properties were differentially modulated. Effects on midpoint of voltage-dependent inactivation and rates of inactivation were less pronounced for hERG1a/1b channels. Effects on voltage-dependent activation and activation kinetics were not different from hERG1a channels. Interestingly, hERG1b channels were inhibited by ITP-2. Inactivation-impairing mutations abolished activation by ITP-2 and led to inhibition of hERG channels. ITP-2 exerted agonistic effect from extracellular side of the membrane and could activate one of the arrhythmia-associated trafficking-deficient LQT2 mutants. CONCLUSIONS AND IMPLICATIONS ITP-2 may serve as another novel lead molecule for designing robust activators of hERG channels. hERG1a/1b gating kinetics were differentially modulated by ITP-2 leading to altered sensitivity. ITP-2 is capable of activating an LQT2 mutant and may be potentially useful in the development of LQT2 therapeutics.
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Affiliation(s)
- Harinath Sale
- Disease Sciences and Technology, Biocon Bristol-Myers Squibb Research and Development Center, Syngene International Limited, Bangalore, India
| | - Samrat Roy
- Disease Sciences and Technology, Biocon Bristol-Myers Squibb Research and Development Center, Syngene International Limited, Bangalore, India
| | - Jayakumar Warrier
- Medicinal Chemistry, Biocon Bristol Myers-Squibb Research and Development Center, Syngene International Limited, Bangalore, India
| | - Srinivasan Thangathirupathy
- Medicinal Chemistry, Biocon Bristol Myers-Squibb Research and Development Center, Syngene International Limited, Bangalore, India
| | - Yoganand Vadari
- Disease Sciences and Technology, Biocon Bristol-Myers Squibb Research and Development Center, Syngene International Limited, Bangalore, India
| | - Shruthi K Gopal
- Disease Sciences and Technology, Biocon Bristol-Myers Squibb Research and Development Center, Syngene International Limited, Bangalore, India
| | - Prasad Krishnamurthy
- Disease Sciences and Technology, Biocon Bristol-Myers Squibb Research and Development Center, Syngene International Limited, Bangalore, India
| | - Manjunath Ramarao
- Disease Sciences and Technology, Biocon Bristol-Myers Squibb Research and Development Center, Syngene International Limited, Bangalore, India
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18
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Bohnen MS, Peng G, Robey SH, Terrenoire C, Iyer V, Sampson KJ, Kass RS. Molecular Pathophysiology of Congenital Long QT Syndrome. Physiol Rev 2017; 97:89-134. [PMID: 27807201 PMCID: PMC5539372 DOI: 10.1152/physrev.00008.2016] [Citation(s) in RCA: 114] [Impact Index Per Article: 16.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
Ion channels represent the molecular entities that give rise to the cardiac action potential, the fundamental cellular electrical event in the heart. The concerted function of these channels leads to normal cyclical excitation and resultant contraction of cardiac muscle. Research into cardiac ion channel regulation and mutations that underlie disease pathogenesis has greatly enhanced our knowledge of the causes and clinical management of cardiac arrhythmia. Here we review the molecular determinants, pathogenesis, and pharmacology of congenital Long QT Syndrome. We examine mechanisms of dysfunction associated with three critical cardiac currents that comprise the majority of congenital Long QT Syndrome cases: 1) IKs, the slow delayed rectifier current; 2) IKr, the rapid delayed rectifier current; and 3) INa, the voltage-dependent sodium current. Less common subtypes of congenital Long QT Syndrome affect other cardiac ionic currents that contribute to the dynamic nature of cardiac electrophysiology. Through the study of mutations that cause congenital Long QT Syndrome, the scientific community has advanced understanding of ion channel structure-function relationships, physiology, and pharmacological response to clinically employed and experimental pharmacological agents. Our understanding of congenital Long QT Syndrome continues to evolve rapidly and with great benefits: genotype-driven clinical management of the disease has improved patient care as precision medicine becomes even more a reality.
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Affiliation(s)
- M S Bohnen
- Department of Pharmacology, Columbia University Medical Center, New York, New York; and The New York Stem Cell Foundation Research Institute, New York, New York
| | - G Peng
- Department of Pharmacology, Columbia University Medical Center, New York, New York; and The New York Stem Cell Foundation Research Institute, New York, New York
| | - S H Robey
- Department of Pharmacology, Columbia University Medical Center, New York, New York; and The New York Stem Cell Foundation Research Institute, New York, New York
| | - C Terrenoire
- Department of Pharmacology, Columbia University Medical Center, New York, New York; and The New York Stem Cell Foundation Research Institute, New York, New York
| | - V Iyer
- Department of Pharmacology, Columbia University Medical Center, New York, New York; and The New York Stem Cell Foundation Research Institute, New York, New York
| | - K J Sampson
- Department of Pharmacology, Columbia University Medical Center, New York, New York; and The New York Stem Cell Foundation Research Institute, New York, New York
| | - R S Kass
- Department of Pharmacology, Columbia University Medical Center, New York, New York; and The New York Stem Cell Foundation Research Institute, New York, New York
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19
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Holzem KM, Gomez JF, Glukhov AV, Madden EJ, Koppel AC, Ewald GA, Trenor B, Efimov IR. Reduced response to IKr blockade and altered hERG1a/1b stoichiometry in human heart failure. J Mol Cell Cardiol 2016; 96:82-92. [PMID: 26093152 PMCID: PMC4683114 DOI: 10.1016/j.yjmcc.2015.06.008] [Citation(s) in RCA: 29] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/22/2014] [Revised: 05/27/2015] [Accepted: 06/01/2015] [Indexed: 01/27/2023]
Abstract
Heart failure (HF) claims 250,000 lives per year in the US, and nearly half of these deaths are sudden and presumably due to ventricular tachyarrhythmias. QT interval and action potential (AP) prolongation are hallmark proarrhythmic changes in the failing myocardium, which potentially result from alterations in repolarizing potassium currents. Thus, we aimed to examine whether decreased expression of the rapid delayed rectifier potassium current, IKr, contributes to repolarization abnormalities in human HF. To map functional IKr expression across the left ventricle (LV), we optically imaged coronary-perfused LV free wall from donor and end-stage failing human hearts. The LV wedge preparation was used to examine transmural AP durations at 80% repolarization (APD80), and treatment with the IKr-blocking drug, E-4031, was utilized to interrogate functional expression. We assessed the percent change in APD80 post-IKr blockade relative to baseline APD80 (∆APD80) and found that ∆APD80s are reduced in failing versus donor hearts in each transmural region, with 0.35-, 0.43-, and 0.41-fold reductions in endo-, mid-, and epicardium, respectively (p=0.008, 0.037, and 0.022). We then assessed hERG1 isoform gene and protein expression levels using qPCR and Western blot. While we did not observe differences in hERG1a or hERG1b gene expression between donor and failing hearts, we found a shift in the hERG1a:hERG1b isoform stoichiometry at the protein level. Computer simulations were then conducted to assess IKr block under E-4031 influence in failing and nonfailing conditions. Our results confirmed the experimental observations and E-4031-induced relative APD80 prolongation was greater in normal conditions than in failing conditions, provided that the cellular model of HF included a significant downregulation of IKr. In human HF, the response to IKr blockade is reduced, suggesting decreased functional IKr expression. This attenuated functional response is associated with altered hERG1a:hERG1b protein stoichiometry in the failing human LV, and failing cardiomyoctye simulations support the experimental findings. Thus, of IKr protein and functional expression may be important determinants of repolarization remodeling in the failing human LV.
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Affiliation(s)
- Katherine M Holzem
- Department of Biomedical Engineering, Washington University in St. Louis, MO 63130, USA
| | - Juan F Gomez
- Polytechnic University of Valencia, Valencia, Spain
| | - Alexey V Glukhov
- Department of Biomedical Engineering, Washington University in St. Louis, MO 63130, USA
| | - Eli J Madden
- Department of Biomedical Engineering, Washington University in St. Louis, MO 63130, USA
| | - Aaron C Koppel
- Department of Biomedical Engineering, Washington University in St. Louis, MO 63130, USA
| | - Gregory A Ewald
- Department of Biomedical Engineering, Washington University in St. Louis, MO 63130, USA
| | | | - Igor R Efimov
- Department of Biomedical Engineering, Washington University in St. Louis, MO 63130, USA; Moscow Institute of Physics and Technology, Dolgoprudny, Moscow Region, Russia.
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20
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Foo B, Williamson B, Young JC, Lukacs G, Shrier A. hERG quality control and the long QT syndrome. J Physiol 2016; 594:2469-81. [PMID: 26718903 DOI: 10.1113/jp270531] [Citation(s) in RCA: 37] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2015] [Accepted: 12/07/2015] [Indexed: 11/08/2022] Open
Abstract
Long-QT syndrome type-2 (LQT2) is characterized by reduced functional expression of the human ether-à-go-go related (hERG) gene product, resulting in impaired cardiac repolarization and predisposition to fatal arrhythmia. Previous studies have implicated abnormal trafficking of misfolded hERG as the primary mechanism of LQT2, with misfolding being caused by mutations in the hERG gene (inherited) or drug treatment (acquired). More generally, environmental and metabolic stresses present a constant challenge to the folding of proteins, including hERG, and must be countered by robust protein quality control (QC) systems. Disposal of partially unfolded yet functional plasma membrane (PM) proteins by protein QC contributes to the loss-of-function phenotype in various conformational diseases including cystic fibrosis (CF) and long-QT syndrome type-2 (LQT2). The prevalent view has been that the loss of PM expression of hERG is attributed to biosynthetic block by endoplasmic reticulum (ER) QC pathways. However, there is a growing appreciation for protein QC pathways acting at post-ER cellular compartments, which may contribute to conformational disease pathogenesis. This article will provide a background on the structure and cellular trafficking of hERG as well as inherited and acquired LQT2. We will review previous work on hERG ER QC and introduce the more novel view that there is a significant peripheral QC at the PM and peripheral cellular compartments. Particular attention is drawn to the unique role of the peripheral QC system in acquired LQT2. Understanding the QC process and players may provide targets for therapeutic intervention in dealing with LQT2.
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Affiliation(s)
- Brian Foo
- Department of Physiology, McGill University, Montréal, Québec, Canada, H3G 1Y6
| | - Brittany Williamson
- Department of Biochemistry, McGill University, Montréal, Québec, Canada, H3G 1Y6
| | - Jason C Young
- Department of Biochemistry, McGill University, Montréal, Québec, Canada, H3G 1Y6
| | - Gergely Lukacs
- Department of Physiology, McGill University, Montréal, Québec, Canada, H3G 1Y6
| | - Alvin Shrier
- Department of Physiology, McGill University, Montréal, Québec, Canada, H3G 1Y6
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21
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Puckerin A, Aromolaran KA, Chang DD, Zukin RS, Colecraft HM, Boutjdir M, Aromolaran AS. hERG 1a LQT2 C-terminus truncation mutants display hERG 1b-dependent dominant negative mechanisms. Heart Rhythm 2016; 13:1121-1130. [PMID: 26775140 DOI: 10.1016/j.hrthm.2016.01.012] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/19/2015] [Indexed: 12/01/2022]
Abstract
BACKGROUND The human ether-à-go-go-related gene (hERG 1a) potassium channel is critical for cardiac repolarization. hERG 1b, another variant subunit, co-assembles with hERG 1a, modulates channel biophysical properties and plays an important role in repolarization. Mutations of hERG 1a lead to type 2 long QT syndrome (LQT2), and increased risk for fatal arrhythmias. The functional consequences of these mutations in the presence of hERG 1b are not known. OBJECTIVE To investigate whether hERG 1a mutants exert dominant negative gating and trafficking defects when co-expressed with hERG 1b. METHODS Electrophysiology, co-immunoprecipitation, and fluorescence resonance energy transfer (FRET) experiments in HEK293 cells and guinea pig cardiomyocytes were used to assess the mutants on gating and trafficking. Mutations of 1a-G965X and 1a-R1014X, relevant to gating and trafficking were introduced in the C-terminus region. RESULTS The hERG 1a mutants when expressed alone did not result in decreased current amplitude. Compared to wild-type hERG 1a currents, 1a-G965X currents were significantly larger, whereas those produced by the 1a-R1014X mutant were similar in magnitude. Only when co-expressed with wild-type hERG 1a and 1b did a mutant phenotype emerge, with a marked reduction in surface expression, current amplitude, and a corresponding positive shift in the V1/2 of the activation curve. Co-immunoprecipitation and FRET assays confirmed association of mutant and wild-type subunits. CONCLUSION Heterologously expressed hERG 1a C-terminus truncation mutants, exert a dominant negative gating and trafficking effect only when co-expressed with hERG 1b. These findings may have potentially profound implications for LQT2 therapy.
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Affiliation(s)
- Akil Puckerin
- Department of Physiology & Cellular Biophysics, Columbia University, New York, New York
| | - Kelly A Aromolaran
- Dominick P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, New York, New York
| | - Donald D Chang
- Department of Physiology & Cellular Biophysics, Columbia University, New York, New York
| | - R Suzanne Zukin
- Dominick P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, New York, New York
| | - Henry M Colecraft
- Department of Physiology & Cellular Biophysics, Columbia University, New York, New York
| | - Mohamed Boutjdir
- Cardiovascular Research Program, VA New York Harbor Healthcare System, Brooklyn, New York; Departments of Medicine, Cell Biology and Pharmacology, State University of New York Downstate Medical Center, Brooklyn, New York,; Department of Medicine, New York University School of Medicine, New York, New York
| | - Ademuyiwa S Aromolaran
- Department of Physiology & Cellular Biophysics, Columbia University, New York, New York.
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Abstract
Optimal cardiac function depends on proper timing of excitation and contraction in various regions of the heart, as well as on appropriate heart rate. This is accomplished via specialized electrical properties of various components of the system, including the sinoatrial node, atria, atrioventricular node, His-Purkinje system, and ventricles. Here we review the major regionally determined electrical properties of these cardiac regions and present the available data regarding the molecular and ionic bases of regional cardiac function and dysfunction. Understanding these differences is of fundamental importance for the investigation of arrhythmia mechanisms and pharmacotherapy.
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Affiliation(s)
- Daniel C Bartos
- Department of Pharmacology, University of California Davis, Davis, California, USA
| | - Eleonora Grandi
- Department of Pharmacology, University of California Davis, Davis, California, USA
| | - Crystal M Ripplinger
- Department of Pharmacology, University of California Davis, Davis, California, USA
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Tong WC, Tribe RM, Smith R, Taggart MJ. Computational modeling reveals key contributions of KCNQ and hERG currents to the malleability of uterine action potentials underpinning labor. PLoS One 2014; 9:e114034. [PMID: 25474527 PMCID: PMC4256391 DOI: 10.1371/journal.pone.0114034] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2014] [Accepted: 11/03/2014] [Indexed: 11/19/2022] Open
Abstract
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.
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Affiliation(s)
- Wing-Chiu Tong
- Institute of Cellular Medicine, Newcastle University, Newcastle upon Tyne, United Kingdom
| | - Rachel M. Tribe
- Division of Women's Health, King's College London and King's Health Partners, London, United Kingdom
| | - Roger Smith
- Hunter Medical Research Institute, University of Newcastle, New Lambton, New South Wales, Australia
| | - Michael J. Taggart
- Institute of Cellular Medicine, Newcastle University, Newcastle upon Tyne, United Kingdom
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Morais-Cabral JH, Robertson GA. The enigmatic cytoplasmic regions of KCNH channels. J Mol Biol 2014; 427:67-76. [PMID: 25158096 DOI: 10.1016/j.jmb.2014.08.008] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2014] [Revised: 08/14/2014] [Accepted: 08/15/2014] [Indexed: 01/09/2023]
Abstract
KCNH channels are expressed across a vast phylogenetic and evolutionary spectrum. In humans, they function in a wide range of tissues and serve as biomarkers and targets for diseases such as cancer and cardiac arrhythmias. These channels share a general architecture with other voltage-gated ion channels but are distinguished by the presence of an N-terminal PAS (Per-Arnt-Sim) domain and a C-terminal domain with homology to cyclic nucleotide binding domains (referred to as the CNBh domain). Cytosolic regions outside these domains show little conservation between KCNH families but are strongly conserved across species within a family, likely reflecting variability that confers specificity to individual channel types. PAS and CNBh domains participate in channel gating, but at least twice in evolutionary history, the PAS domain has been lost and it is omitted by alternate transcription to create a distinct channel subunit in one family. In this focused review, we present current knowledge of the structure and function of these cytosolic regions, discuss their evolution as modular domains and provide our perspective on the important questions moving forward.
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Affiliation(s)
- João H Morais-Cabral
- Instituto de Biologia Molecular e Celular, Universidade do Porto, 4150-180 Porto, Portugal.
| | - Gail A Robertson
- Department of Neuroscience, University of Wisconsin School of Medicine and Public Health, Madison, WI 53705, USA
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Thomson SJ, Hansen A, Sanguinetti MC. Concerted all-or-none subunit interactions mediate slow deactivation of human ether-à-go-go-related gene K+ channels. J Biol Chem 2014; 289:23428-36. [PMID: 25008322 DOI: 10.1074/jbc.m114.582437] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
During the repolarization phase of a cardiac action potential, hERG1 K(+) channels rapidly recover from an inactivated state then slowly deactivate to a closed state. The resulting resurgence of outward current terminates the plateau phase and is thus a key regulator of action potential duration of cardiomyocytes. The intracellular N-terminal domain of the hERG1 subunit is required for slow deactivation of the channel as its removal accelerates deactivation 10-fold. Here we investigate the stoichiometry of hERG1 channel deactivation by characterizing the kinetic properties of concatenated tetramers containing a variable number of wild-type and mutant subunits. Three mutations known to accelerate deactivation were investigated, including R56Q and R4A/R5A in the N terminus and F656I in the S6 transmembrane segment. In all cases, a single mutant subunit induced the same rapid deactivation of a concatenated channel as that observed for homotetrameric mutant channels. We conclude that slow deactivation gating of hERG1 channels involves a concerted, fully cooperative interaction between all four wild-type channel subunits.
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Affiliation(s)
- Steven J Thomson
- From the Nora Eccles Harrison Cardiovascular Research and Training Institute and
| | - Angela Hansen
- From the Nora Eccles Harrison Cardiovascular Research and Training Institute and
| | - Michael C Sanguinetti
- From the Nora Eccles Harrison Cardiovascular Research and Training Institute and Department of Internal Medicine and Division of Cardiovascular Medicine, University of Utah, Salt Lake City, Utah 84112
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Schmitt N, Grunnet M, Olesen SP. Cardiac potassium channel subtypes: new roles in repolarization and arrhythmia. Physiol Rev 2014; 94:609-53. [PMID: 24692356 DOI: 10.1152/physrev.00022.2013] [Citation(s) in RCA: 160] [Impact Index Per Article: 16.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
About 10 distinct potassium channels in the heart are involved in shaping the action potential. Some of the K+ channels are primarily responsible for early repolarization, whereas others drive late repolarization and still others are open throughout the cardiac cycle. Three main K+ channels drive the late repolarization of the ventricle with some redundancy, and in atria this repolarization reserve is supplemented by the fairly atrial-specific KV1.5, Kir3, KCa, and K2P channels. The role of the latter two subtypes in atria is currently being clarified, and several findings indicate that they could constitute targets for new pharmacological treatment of atrial fibrillation. The interplay between the different K+ channel subtypes in both atria and ventricle is dynamic, and a significant up- and downregulation occurs in disease states such as atrial fibrillation or heart failure. The underlying posttranscriptional and posttranslational remodeling of the individual K+ channels changes their activity and significance relative to each other, and they must be viewed together to understand their role in keeping a stable heart rhythm, also under menacing conditions like attacks of reentry arrhythmia.
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Erg potassium currents of neonatal mouse Purkinje cells exhibit fast gating kinetics and are inhibited by mGluR1 activation. J Neurosci 2013; 33:16729-40. [PMID: 24133274 DOI: 10.1523/jneurosci.5523-12.2013] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023] Open
Abstract
We investigated the subthreshold properties of an erg (ether-à-go-go-related gene) K(+) current in Purkinje cells of neonatal mice. Action potentials recorded from Purkinje cells in cerebellar slices exhibited a decreased threshold potential and increased frequency of spontaneous and repetitive activity following application of the specific erg channel blocker E-4031. Accommodation was absent before and after drug application. The erg current of these Purkinje cells activated at membrane potentials near -60 mV and exhibited fast gating kinetics. The functional importance of fast gating subthreshold erg channels in Purkinje cells was corroborated by comparing the results of action potential clamp experiments with erg1a, erg1b, erg2, and erg3 currents heterologously expressed in HEK cells. Computer simulations based on a NEURON model of Purkinje cells only reproduced the effects of the native erg current when an erg channel conductance like that of erg3 was included. Experiments with subunit-sensitive toxins (BeKm-1, APETx1) indicated that erg channels in Purkinje cells are presumably mediated by heteromeric erg1/erg3 or modified erg1 channels. Following mGluR1 activation, the native erg current was reduced by ∼70%, brought about by reduction of the maximal erg current and a shift of the activation curve to more positive potentials. The Purkinje cell erg current contributed to the sustained current component of the biphasic mGluR1 response. Activation of mGluR1 by the agonist 3,4-dihydroxyphenylglycol increased Purkinje cell excitability, similar to that induced by E-4031. The results indicated that erg currents can be modulated and may contribute to the mGluR1-induced plasticity changes in Purkinje cells.
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28
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Crotti L, Tester DJ, White WM, Bartos DC, Insolia R, Besana A, Kunic JD, Will ML, Velasco EJ, Bair JJ, Ghidoni A, Cetin I, Van Dyke DL, Wick MJ, Brost B, Delisle BP, Facchinetti F, George AL, Schwartz PJ, Ackerman MJ. Long QT syndrome-associated mutations in intrauterine fetal death. JAMA 2013; 309:1473-82. [PMID: 23571586 PMCID: PMC3852902 DOI: 10.1001/jama.2013.3219] [Citation(s) in RCA: 110] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/14/2022]
Abstract
IMPORTANCE Intrauterine fetal death or stillbirth occurs in approximately 1 out of every 160 pregnancies and accounts for 50% of all perinatal deaths. Postmortem evaluation fails to elucidate an underlying cause in many cases. Long QT syndrome (LQTS) may contribute to this problem. OBJECTIVE To determine the spectrum and prevalence of mutations in the 3 most common LQTS susceptible genes (KCNQ1, KCNH2, and SCN5A) for a cohort of unexplained cases. DESIGN, SETTING, AND PATIENTS In this case series, retrospective postmortem genetic testing was conducted on a convenience sample of 91 unexplained intrauterine fetal deaths (mean [SD] estimated gestational age at fetal death, 26.3 [8.7] weeks) that were collected from 2006-2012 by the Mayo Clinic, Rochester, Minnesota, or the Fondazione IRCCS Policlinico San Matteo, Pavia, Italy. More than 1300 ostensibly healthy individuals served as controls. In addition, publicly available exome databases were assessed for the general population frequency of identified genetic variants. MAIN OUTCOMES AND MEASURES Comprehensive mutational analyses of KCNQ1 (KV7.1, LQTS type 1), KCNH2 (HERG/KV11.1, LQTS type 2), and SCN5A (NaV1.5, LQTS type 3) were performed using denaturing high-performance liquid chromatography and direct DNA sequencing on genomic DNA extracted from decedent tissue. Functional analyses of novel mutations were performed using heterologous expression and patch-clamp recording. RESULTS The 3 putative LQTS susceptibility missense mutations (KCNQ1, p.A283T; KCNQ1, p.R397W; and KCNH2 [1b], p.R25W), with a heterozygous frequency of less than 0.05% in more than 10 000 publicly available exomes and absent in more than 1000 ethnically similar control patients, were discovered in 3 intrauterine fetal deaths (3.3% [95% CI, 0.68%-9.3%]). Both KV7.1-A283T (16-week male) and KV7.1-R397W (16-week female) mutations were associated with marked KV7.1 loss-of-function consistent with in utero LQTS type 1, whereas the HERG1b-R25W mutation (33.2-week male) exhibited a loss of function consistent with in utero LQTS type 2. In addition, 5 intrauterine fetal deaths hosted SCN5A rare nonsynonymous genetic variants (p.T220I, p.R1193Q, involving 2 cases, and p.P2006A, involving 2 cases) that conferred in vitro electrophysiological characteristics consistent with potentially proarrhythmic phenotypes. CONCLUSIONS AND RELEVANCE In this molecular genetic evaluation of 91 cases of intrauterine fetal death, missense mutations associated with LQTS susceptibility were discovered in 3 cases (3.3%) and overall, genetic variants leading to dysfunctional LQTS-associated ion channels in vitro were discovered in 8 cases (8.8%). These preliminary findings may provide insights into mechanisms of some cases of stillbirth.
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Affiliation(s)
- Lia Crotti
- Department of Molecular Medicine, University of Pavia, and Molecular Cardiology Laboratory, Fondazione IRRCCS Policlinico S Matteo, Pavia, Italy
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29
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Cardiac ion channels and mechanisms for protection against atrial fibrillation. Rev Physiol Biochem Pharmacol 2013; 162:1-58. [PMID: 21987061 DOI: 10.1007/112_2011_3] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/10/2023]
Abstract
Atrial fibrillation (AF) is recognised as the most common sustained cardiac arrhythmia in clinical practice. Ongoing drug development is aiming at obtaining atrial specific effects in order to prevent pro-arrhythmic, devastating ventricular effects. In principle, this is possible due to a different ion channel composition in the atria and ventricles. The present text will review the aetiology of arrhythmias with focus on AF and include a description of cardiac ion channels. Channels that constitute potentially atria-selective targets will be described in details. Specific focus is addressed to the recent discovery that Ca(2+)-activated small conductance K(+) channels (SK channels) are important for the repolarisation of atrial action potentials. Finally, an overview of current pharmacological treatment of AF is included.
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Early LQT2 nonsense mutation generates N-terminally truncated hERG channels with altered gating properties by the reinitiation of translation. J Mol Cell Cardiol 2012; 53:725-33. [PMID: 22964610 DOI: 10.1016/j.yjmcc.2012.08.021] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/23/2012] [Revised: 08/18/2012] [Accepted: 08/24/2012] [Indexed: 11/23/2022]
Abstract
Mutations in the human ether-a-go-go-related gene (hERG) result in long QT syndrome type 2 (LQT2). The hERG gene encodes a K(+) channel that contributes to the repolarization of the cardiac action potential. We have previously shown that hERG mRNA transcripts that contain premature termination codon mutations are rapidly degraded by nonsense-mediated mRNA decay (NMD). In this study, we identified a LQT2 nonsense mutation, Q81X, which escapes degradation by the reinitiation of translation and generates N-terminally truncated channels. RNA analysis of hERG minigenes revealed equivalent levels of wild-type and Q81X mRNA while the mRNA expressed from minigenes containing the LQT2 frameshift mutation, P141fs+2X, was significantly reduced by NMD. Western blot analysis revealed that Q81X minigenes expressed truncated channels. Q81X channels exhibited decreased tail current levels and increased deactivation kinetics compared to wild-type channels. These results are consistent with the disruption of the N-terminus, which is known to regulate hERG deactivation. Site-specific mutagenesis studies showed that translation of the Q81X transcript is reinitiated at Met124 following premature termination. Q81X co-assembled with hERG to form heteromeric channels that exhibited increased deactivation rates compared to wild-type channels. Mutant channels also generated less outward current and transferred less charge at late phases of repolarization during ventricular action potential clamp. These results provide new mechanistic insight into the prolongation of the QT interval in LQT2 patients. Our findings indicate that the reinitiation of translation may be an important pathogenic mechanism in patients with nonsense and frameshift LQT2 mutations near the 5' end of the hERG gene.
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31
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Isoform-specific dominant-negative effects associated with hERG1 G628S mutation in long QT syndrome. PLoS One 2012; 7:e42552. [PMID: 22876326 PMCID: PMC3411645 DOI: 10.1371/journal.pone.0042552] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/06/2012] [Accepted: 07/09/2012] [Indexed: 11/25/2022] Open
Abstract
Background Mutations in the human ether-a-go-go-related gene 1 (hERG1) cause type 2 long QT syndrome (LQT2). The hERG1 gene encodes a K+ channel with properties similar to the rapidly activating delayed rectifying K+ current in the heart. Several hERG1 isoforms with unique structural and functional properties have been identified. To date, the pathogenic mechanisms of LQT2 mutations have been predominantly described in the context of the hERG1a isoform. In the present study, we investigated the functional consequences of the LQT2 mutation G628S in the hERG1b and hERG1aUSO isoforms. Methods A double-stable, mammalian expression system was developed to characterize isoform-specific dominant-negative effects of G628S-containing channels when co-expressed at equivalent levels with wild-type hERG1a. Western blot and co-immunoprecipitation studies were performed to study the trafficking and co-assembly of wild-type and mutant hERG1 isoforms. Patch-clamp electrophysiology was performed to characterize hERG1 channel function and the isoform-specific dominant-negative effects associated with the G628S mutation. Conclusions The non-functional hERG1a-G628S and hERG1b-G628S channels co-assembled with wild-type hERG1a and dominantly suppressed hERG1 current. In contrast, G628S-induced dominant-negative effects were absent in the context of the hERG1aUSO isoform. hERG1aUSO-G628S channels did not appreciably associate with hERG1a and did not significantly suppress hERG1 current when co-expressed at equivalent ratios or at ratios that approximate those found in cardiac tissue. These results suggest that the dominant-negative effects of LQT2 mutations may primarily occur in the context of the hERG1a and hERG1b isoforms.
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Jonsson MKB, van der Heyden MAG, van Veen TAB. Deciphering hERG channels: molecular basis of the rapid component of the delayed rectifier potassium current. J Mol Cell Cardiol 2012; 53:369-74. [PMID: 22742967 DOI: 10.1016/j.yjmcc.2012.06.011] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/06/2012] [Revised: 06/07/2012] [Accepted: 06/19/2012] [Indexed: 12/23/2022]
Abstract
The rapid component of the delayed rectifier potassium current (I(Kr)), encoded by the ether-a-go-go-related gene (ERG1, officially denominated as KCNH2), is a major contributor to repolarization in the mammalian heart. Acute (e.g. drug-induced) and chronic (e.g. inherited genetic disorder) disruptions of this current can lead to prolongation of the action potential and potentiate occurrence of lethal arrhythmias. Many cardiac and non-cardiac drugs show high affinity for the I(Kr) channel and it is therefore extensively studied during safety pharmacology. The unique biophysical and pharmacological properties of the I(Kr) channel are largely recapitulated by expressing the human variant (hERG1a) in overexpressing systems. hERG1a channels are tetramers consisting of four 1159 amino acid long proteins and have electrophysiological properties similar, but not identical, to native I(Kr). In the search for an explanation to the discrepancies between I(Kr) and hERG1a channels, two alternative hERG1 proteins have been found. Alternative transcription of hERG1 leads to a protein with a 56 amino acid shorter N-terminus, known as hERG1b. hERG1b can form channels alone or coassemble with hERG1a. Alternative splicing leads to an alternate C-terminus and a protein known as hERGuso. hERGuso and hERG1b regulate hERG1a channel trafficking, functional expression and channel kinetics. Expression of hERGuso leads to a reduced number of channels at the plasma membrane and thereby reduces current density. On the contrary, co-assembly with hERG1b alters channel kinetics resulting in more available channels and a larger current. These findings have implication for understanding mechanisms of disease, acute and chronic drug effects, and potential gender differences.
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Affiliation(s)
- Malin K B Jonsson
- Department of Medical Physiology, Division Heart and Lungs, University Medical Center Utrecht, Utrecht, The Netherlands.
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The voltage-gated channel accessory protein KCNE2: multiple ion channel partners, multiple ways to long QT syndrome. Expert Rev Mol Med 2011; 13:e38. [DOI: 10.1017/s1462399411002092] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023]
Abstract
The single-pass transmembrane protein KCNE2 or MIRP1 was once thought to be the missing accessory protein that combined with hERG to fully recapitulate the cardiac repolarising current IKr. As a result of this role, it was an easy next step to associate mutations in KCNE2 to long QT syndrome, in which there is delayed repolarisation of the heart. Since that time however, KCNE2 has been shown to modify the behaviour of several other channels and currents, and its role in the heart and in the aetiology of long QT syndrome has become less clear. In this article, we review the known interactions of the KCNE2 protein and the resulting functional effects, and the effects of mutations in KCNE2 and their clinical role.
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Abi-Gerges N, Holkham H, Jones EMC, Pollard CE, Valentin JP, Robertson GA. hERG subunit composition determines differential drug sensitivity. Br J Pharmacol 2011; 164:419-32. [PMID: 21449979 PMCID: PMC3188906 DOI: 10.1111/j.1476-5381.2011.01378.x] [Citation(s) in RCA: 40] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2010] [Revised: 03/04/2011] [Accepted: 03/07/2011] [Indexed: 11/28/2022] Open
Abstract
BACKGROUND AND PURPOSE The majority of human ether-a-go-go-related gene (hERG) screens aiming to minimize the risk of drug-induced long QT syndrome have been conducted using heterologous systems expressing the hERG 1a subunit, although both hERG 1a and 1b subunits contribute to the K+ channels producing the repolarizing current I(Kr) . We tested a range of compounds selected for their diversity to determine whether hERG 1a and 1a/1b channels exhibit different sensitivities that may influence safety margins or contribute to a stratified risk analysis. EXPERIMENTAL APPROACH We used the IonWorks™ plate-based electrophysiology device to compare sensitivity of hERG 1a and 1a/1b channels stably expressed in HEK293 cells to 50 compounds previously shown to target hERG channels. Potency was determined as IC₅₀ values (µM) obtained from non-cumulative, eight-point concentration-effect curves of normalized data, fitted to the Hill equation. To minimize possible sources of variability, compound potency was assessed using test plates arranged in alternating columns of cells expressing hERG 1a and 1a/1b. KEY RESULTS Although the potency of most compounds was similar for the two targets, some surprising differences were observed. Fluoxetine (Prozac) was more potent at blocking hERG 1a/1b than 1a channels, yielding a corresponding reduction in the safety margin. In contrast, E-4031 was a more potent blocker of hERG 1a compared with 1a/1b channels, as previously reported, as was dofetilide, another high-affinity blocker. CONCLUSIONS AND IMPLICATIONS The current assays may underestimate the risk of some drugs to cause torsades de pointes arrhythmia, and overestimate the risk of others.
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Affiliation(s)
- N Abi-Gerges
- Safety Pharmacology, Safety Assessment UK, AstraZeneca R&D Alderley Park, Macclesfield, UK.
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35
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Affiliation(s)
- T Jespersen
- Department of Biomedical Sciences 16.5, Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark
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36
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Du C, El Harchi A, McPate M, Orchard C, Hancox J. Enhanced inhibitory effect of acidosis on hERG potassium channels that incorporate the hERG1b isoform. Biochem Biophys Res Commun 2011; 405:222-7. [DOI: 10.1016/j.bbrc.2011.01.014] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/31/2010] [Accepted: 01/04/2011] [Indexed: 10/18/2022]
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Larsen AP, Bentzen BH, Grunnet M. Differential effects of Kv11.1 activators on Kv11.1a, Kv11.1b and Kv11.1a/Kv11.1b channels. Br J Pharmacol 2011; 161:614-28. [PMID: 20880400 DOI: 10.1111/j.1476-5381.2010.00897.x] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
BACKGROUND AND PURPOSE K(v)11.1 channels are involved in regulating cellular excitability in various tissues including brain, heart and smooth muscle. In these tissues, at least two isoforms, K(v)11.1a and K(v)11.1b, with different kinetics, are expressed. K(v)11.1 activators are potential therapeutic agents, but their effects have only been tested on the K(v)11.1a isoform. In this study, the effects of two different K(v)11.1 activators, NS1643 and RPR260243, were characterized on K(v)11.1a and K(v)11.1b channels. EXPERIMENTAL APPROACH K(v)11.1a and K(v)11.1b channels were expressed in Xenopus laevis oocytes, and currents were measured using two-electrode voltage clamp. I/V curves and channel kinetics were measured before and after application of 30 µM NS1643 or 10 µM RPR260243. KEY RESULTS NS1643 increased steady-state currents through Kv11.1b several fold more than through K(v)11.1a channels, without affecting EC(50) values. NS1643 increased activation rates and decreased rates of inactivation, recovery from inactivation and deactivation for both channels. Except for activation, where effect of NS1643 was comparable, relative changes were greater for Kv11.1b than for K(v)11.1a. RPR260243 increased steady-state currents only through Kv11.1a channels, but slowed the process of deactivation for both channels primarily by decreasing time constant of slow deactivation. This effect was greater on K(v)11.1b than on K(v)11.1a. Effects of both compounds on heteromeric K(v)11.1a/K(v)11.1b channels were similar to those on K(v)11.1a. CONCLUSIONS AND IMPLICATIONS Both NS1643 and RPR260243 displayed differential effects on K(v)11.1a and K(v)11.1b channels, the effects being relatively more pronounced on K(v)11.1b channels. This affirms the importance of testing the effect of K(v)11.1 activators on different channel isoforms.
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Affiliation(s)
- A P Larsen
- The Danish National Research Foundation Centre for Cardiac Arrhythmia, Department of Biomedical Sciences, University of Copenhagen, Blegdamsvej 3, Copenhagen N, Denmark
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Grunnet M, Abbruzzese J, Sachse FB, Sanguinetti MC. Molecular determinants of human ether-à-go-go-related gene 1 (hERG1) K+ channel activation by NS1643. Mol Pharmacol 2010; 79:1-9. [PMID: 20876384 DOI: 10.1124/mol.110.067728] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
Human ether-à-go-go-related gene 1 (hERG1) channels conduct the rapid delayed rectifier K+ current, I(Kr), an important determinant of action potential repolarization in mammals, including humans. Reduced I(Kr) function caused by mutations in KCNH2 or drug block of hERG1 channels prolongs the QT interval of the electrocardiogram and increases the risk of ventricular fibrillation and sudden cardiac death. Several activators of hERG1 channels have been discovered in recent years. These compounds shorten the duration of cardiac action potentials and have been proposed as a new therapeutic approach for the treatment of acquired or congenital long QT syndrome. We defined previously the mechanism of action of 1,3-bis-(2-hydroxy-5-trifluoromethyl-phenyl)-urea (NS1643), a compound that increases hERG1 currents by shifting the voltage-dependence of inactivation to more positive potentials. Here, we use scanning mutagenesis of hERG1 and functional characterization of 56 mutant channels heterologously expressed in Xenopus laevis oocytes to define the molecular determinants of the binding site for NS1643. Most point mutations did not alter response to the drug; however, 10 mutant channels had reduced sensitivity, and F619A and I567A exhibited enhanced activation by the drug. Some of these residues form a cluster and, together with molecular modeling, suggest that NS1643 binds to a pocket near the extracellular ends of the S5/S6 segments of two adjacent hERG1 channel subunits. This putative binding site differs from the sites described previously for two other hERG1 activators, (3R,4R)-4-[3-(6-methoxy-quinolin-4-yl)-3-oxo-propyl]-1-[3-(2,3,5-trifluoro-phenyl)-prop-2-ynyl]-piperidine-3-carboxylic acid (RPR260243) and 2-(4-[2-(3,4-dichloro-phenyl)-ethyl]-phenylamino)-benzoic acid (PD-118057).
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Affiliation(s)
- Morten Grunnet
- Department of Drug Discovery Portfolio Management, NeuroSearch A/S, Pederstrupvej 93, 2750 Ballerup, Denmark.
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Role of ERG1 isoforms in modulation of ERG1 channel trafficking and function. Pflugers Arch 2010; 460:803-12. [DOI: 10.1007/s00424-010-0855-8] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2010] [Revised: 06/13/2010] [Accepted: 06/14/2010] [Indexed: 01/31/2023]
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Kolbe K, Schönherr R, Gessner G, Sahoo N, Hoshi T, Heinemann SH. Cysteine 723 in the C-linker segment confers oxidative inhibition of hERG1 potassium channels. J Physiol 2010; 588:2999-3009. [PMID: 20547678 DOI: 10.1113/jphysiol.2010.192468] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
Excess reactive oxygen species (ROS) play a crucial role under pathophysiological conditions, such as ischaemia/reperfusion and diabetes, potentially contributing to cardiac arrhythmia. hERG1 (KCNH2) potassium channels terminate the cardiac action potential and malfunction can lead to long-QT syndrome and fatal arrhythmia. To investigate the molecular mechanisms of hERG1 channel alteration by ROS, hERG1 and mutants thereof were expressed in HEK293 cells and studied with the whole-cell patch-clamp method. Even mild ROS stress induced by hyperglycaemia markedly decreased channel current. Intracellular H2O2 or cysteine-specific modifiers also strongly inhibited channel activity and accelerated deactivation kinetics. Mutagenesis revealed that cysteine 723 (C723), a conserved residue in a structural element linking the C-terminal domain to the channel's gate, is critical for oxidative functional modification. Moreover, kinetics of channel closure strongly influences ROS-induced modification, where rapid channel deactivation diminishes ROS sensitivity. Because of its fast deactivation kinetics, the N-terminally truncated splice variant hERG1b possesses greater resistance to oxidative modification.
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Affiliation(s)
- Katrin Kolbe
- Centre for Molecular Biomedicine, Department of Biophysics, Friedrich Schiller University of Jena, Hans-Knöll-Str. 2, D-07745 Jena, Germany
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Abstract
Since the first discovery of Kvbeta-subunits more than 15 years ago, many more ancillary Kv channel subunits were characterized, for example, KChIPs, KCNEs, and BKbeta-subunits. The ancillary subunits are often integral parts of native Kv channels, which, therefore, are mostly multiprotein complexes composed of voltage-sensing and pore-forming Kvalpha-subunits and of ancillary or beta-subunits. Apparently, Kv channels need the ancillary subunits to fulfill their many different cell physiological roles. This is reflected by the large structural diversity observed with ancillary subunit structures. They range from proteins with transmembrane segments and extracellular domains to purely cytoplasmic proteins. Ancillary subunits modulate Kv channel gating but can also have a great impact on channel assembly, on channel trafficking to and from the cellular surface, and on targeting Kv channels to different cellular compartments. The importance of the role of accessory subunits is further emphasized by the number of mutations that are associated in both humans and animals with diseases like hypertension, epilepsy, arrhythmogenesis, periodic paralysis, and hypothyroidism. Interestingly, several ancillary subunits have in vitro enzymatic activity; for example, Kvbeta-subunits are oxidoreductases, or modulate enzymatic activity, i.e., KChIP3 modulates presenilin activity. Thus different modes of beta-subunit association and of functional impact on Kv channels can be delineated, making it difficult to extract common principles underlying Kvalpha- and beta-subunit interactions. We critically review present knowledge on the physiological role of ancillary Kv channel subunits and their effects on Kv channel properties.
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Affiliation(s)
- Olaf Pongs
- Institut für Neurale Signalverarbeitung, Zentrum für Molekulare Neurobiologie Hamburg, Universitätsklinikum Hamburg-Eppendorf, Universität Hamburg, Hamburg, Germany.
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Mewe M, Mauerhöfer M, Wulfsen I, Szlachta K, Zhou XB, Schwarz JR, Bauer CK. Modulation of cardiac ERG1 K(+) channels by cGMP signaling. J Mol Cell Cardiol 2010; 49:48-57. [PMID: 20188738 DOI: 10.1016/j.yjmcc.2010.02.015] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/16/2009] [Revised: 02/16/2010] [Accepted: 02/17/2010] [Indexed: 12/21/2022]
Abstract
Different K(+) currents have been implicated in the myocardial action potential repolarization including the I(Kr). ERG1 alpha subunits, identified as the molecular correlate of I(Kr), have been shown to form heteromultimeric channels in the heart and their activity is modulated by a complex interplay of signal transduction events. Using electrophysiological techniques, we examined the effects of the cGMP-analogue 8-Br-cGMP on rat and guinea-pig papillary action potential duration (APD), on the biophysical properties of heterologously expressed homo- and heteromeric ERG1 channels, and on cardiac I(Kr). 8-Br-cGMP prolonged APD by about 25% after pharmacological inhibition of L-type Ca(2+) currents and I(Ks). The prolongation was completely abolished by prior application of the hERG channel blocker E-4031 or the protein kinase G (PKG) inhibitor Rp-8-Br-cGMPS. Expression analysis revealed the presence of both ERG1a and -1b subunits in rat papillary muscle. Both 8-Br-cGMP and ANP inhibited heterologously expressed ERG1b and even stronger ERG1a/1b channels, whereas ERG1a channels remained unaffected. The inhibitory 8-Br-cGMP effects were PKG-dependent and involved a profound ERG current reduction, which was also observed with cardiac AP clamp recordings. Measurements of I(Kr) from isolated mouse cardiomyocytes using Cs(+) as charge carrier exhibited faster deactivation kinetics in atrial than in ventricular myocytes consistent with a higher relative expression of ERG1b transcripts in atria than in ventricles. 8-Br-cGMP significantly reduced I(Kr) in atrial, but not in ventricular myocytes. These findings provide first evidence that through heteromeric assembly ERG1 channels become a critical target of cGMP-PKG signaling linking cGMP accumulation to cardiac I(Kr) modulation.
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Affiliation(s)
- Marco Mewe
- Institute of Pharmacology for Pharmacists, University Medical Center, Hamburg-Eppendorf (UKE), Martinistr. 52, D-20246 Hamburg, Germany.
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Differential expression of hERG1 channel isoforms reproduces properties of native I(Kr) and modulates cardiac action potential characteristics. PLoS One 2010; 5:e9021. [PMID: 20126398 PMCID: PMC2814852 DOI: 10.1371/journal.pone.0009021] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2009] [Accepted: 01/14/2010] [Indexed: 11/19/2022] Open
Abstract
BACKGROUND The repolarizing cardiac rapid delayed rectifier current, I(Kr), is composed of ERG1 channels. It has been suggested that two isoforms of the ERG1 protein, ERG1a and ERG1b, both contribute to I(Kr). Marked heterogeneity in the kinetic properties of native I(Kr) has been described. We hypothesized that the heterogeneity of native I(Kr) can be reproduced by differential expression of ERG1a and ERG1b isoforms. Furthermore, the functional consequences of differential expression of ERG1 isoforms were explored as a potential mechanism underlying native heterogeneity of action potential duration (APD) and restitution. METHODOLOGY/PRINCIPAL FINDINGS The results show that the heterogeneity of native I(Kr) can be reproduced in heterologous expression systems by differential expression of ERG1a and ERG1b isoforms. Characterization of the macroscopic kinetics of ERG1 currents demonstrated that these were dependent on the relative abundance of ERG1a and ERG1b. Furthermore, we used a computational model of the ventricular cardiomyocyte to show that both APD and the slope of the restitution curve may be modulated by varying the relative abundance of ERG1a and ERG1b. As the relative abundance of ERG1b was increased, APD was gradually shortened and the slope of the restitution curve was decreased. CONCLUSIONS/SIGNIFICANCE Our results show that differential expression of ERG1 isoforms may explain regional heterogeneity of I(Kr) kinetics. The data demonstrate that subunit dependent changes in channel kinetics are important for the functional properties of ERG1 currents and hence I(Kr). Importantly, our results suggest that regional differences in the relative abundance of ERG1 isoforms may represent a potential mechanism underlying the heterogeneity of both APD and APD restitution observed in mammalian hearts.
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Grunnet M. Repolarization of the cardiac action potential. Does an increase in repolarization capacity constitute a new anti-arrhythmic principle? Acta Physiol (Oxf) 2010; 198 Suppl 676:1-48. [PMID: 20132149 DOI: 10.1111/j.1748-1716.2009.02072.x] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
The cardiac action potential can be divided into five distinct phases designated phases 0-4. The exact shape of the action potential comes about primarily as an orchestrated function of ion channels. The present review will give an overview of ion channels involved in generating the cardiac action potential with special emphasis on potassium channels involved in phase 3 repolarization. In humans, these channels are primarily K(v)11.1 (hERG1), K(v)7.1 (KCNQ1) and K(ir)2.1 (KCNJ2) being the responsible alpha-subunits for conducting I(Kr), I(Ks) and I(K1). An account will be given about molecular components, biophysical properties, regulation, interaction with other proteins and involvement in diseases. Both loss and gain of function of these currents are associated with different arrhythmogenic diseases. The second part of this review will therefore elucidate arrhythmias and subsequently focus on newly developed chemical entities having the ability to increase the activity of I(Kr), I(Ks) and I(K1). An evaluation will be given addressing the possibility that this novel class of compounds have the ability to constitute a new anti-arrhythmic principle. Experimental evidence from in vitro, ex vivo and in vivo settings will be included. Furthermore, conceptual differences between the short QT syndrome and I(Kr) activation will be accounted for.
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Affiliation(s)
- M Grunnet
- NeuroSearch A/S, Ballerup, and Danish National Research Foundation Centre for Cardiac Arrhythmia, University of Copenhagen, Denmark.
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McPate MJ, Zhang H, Cordeiro JM, Dempsey CE, Witchel HJ, Hancox JC. hERG1a/1b heteromeric currents exhibit amplified attenuation of inactivation in variant 1 short QT syndrome. Biochem Biophys Res Commun 2009; 386:111-7. [PMID: 19501051 PMCID: PMC2708341 DOI: 10.1016/j.bbrc.2009.05.134] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/26/2009] [Accepted: 05/30/2009] [Indexed: 11/04/2022]
Abstract
Potassium channels encoded by hERG (human ether-à-go-go-related gene) underlie the cardiac rapid delayed rectifier K+ current (IKr) and hERG mutations underpin clinically important repolarization disorders. Virtually all electrophysiological investigations of hERG mutations have studied exclusively the hERG1a isoform; however, recent evidence indicates that native IKr channels may be comprised of hERG1a together with the hERG1b variant, which has a shorter N-terminus. Here, for the first time, electrophysiological effects were studied of a gain-of-function hERG mutation (N588K; responsible for the ‘SQT1’ variant of the short QT syndrome) on current (IhERG1a/1b) carried by co-expressed hERG1a/1b channels. There were no significant effects of N588K on IhERG1a/1b activation or deactivation, but N588K IhERG1a/1b showed little inactivation up to highly positive voltages (⩽+80 mV), a more marked effect than seen for hERG1a expressed alone. IhERG1a/1b under action potential voltage-clamp, and the effects on this of the N588K mutation, also showed differences from those previously reported for hERG1a. The amplified attenuation of IhERG inactivation for the N588K mutation reported here indicates that the study of co-expressed hERG1a/1b channels should be considered when investigating clinically relevant hERG channel mutations, even if these reside outside of the N-terminus region.
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Affiliation(s)
- M J McPate
- Department of Physiology and Pharmacology, Bristol Heart Institute, School of Medical Sciences, The University of Bristol, University Walk, Bristol BS8 1TD, UK
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Functional properties of human neuronal Kv11 channels. Pflugers Arch 2009; 458:689-700. [DOI: 10.1007/s00424-009-0651-5] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/17/2008] [Accepted: 02/12/2009] [Indexed: 11/25/2022]
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hERG1 channel activators: A new anti-arrhythmic principle. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2008; 98:347-62. [DOI: 10.1016/j.pbiomolbio.2009.01.002] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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Sale H, Wang J, O'Hara TJ, Tester DJ, Phartiyal P, He JQ, Rudy Y, Ackerman MJ, Robertson GA. Physiological properties of hERG 1a/1b heteromeric currents and a hERG 1b-specific mutation associated with Long-QT syndrome. Circ Res 2008; 103:e81-95. [PMID: 18776039 DOI: 10.1161/circresaha.108.185249] [Citation(s) in RCA: 106] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Cardiac I Kr is a critical repolarizing current in the heart and a target for inherited and acquired long-QT syndrome (LQTS). Biochemical and functional studies have demonstrated that I Kr channels are heteromers composed of both hERG 1a and 1b subunits, yet our current understanding of I Kr functional properties derives primarily from studies of homooligomers of the original hERG 1a isolate. Here, we examine currents produced by hERG 1a and 1a/1b channels expressed in HEK-293 cells at near-physiological temperatures. We find that heteromeric hERG 1a/1b currents are much larger than hERG 1a currents and conduct 80% more charge during an action potential. This surprising difference corresponds to a 2-fold increase in the apparent rates of activation and recovery from inactivation, thus reducing rectification and facilitating current rebound during repolarization. Kinetic modeling shows these gating differences account quantitatively for the differences in current amplitude between the 2 channel types. Drug sensitivity was also different. Compared to homomeric 1a channels, heteromeric 1a/1b channels were inhibited by E-4031 with a slower time course and a corresponding 4-fold shift in the IC50. The importance of hERG 1b in vivo is supported by the identification of a 1b-specific A8V missense mutation in 1/269 unrelated genotype-negative LQTS patients that was absent in 400 control alleles. Mutant 1bA8V expressed alone or with hERG 1a in HEK-293 cells dramatically reduced 1b protein levels. Thus, mutations specifically disrupting hERG 1b function are expected to reduce cardiac I Kr and enhance drug sensitivity, and represent a potential mechanism underlying inherited or acquired LQTS.
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Affiliation(s)
- Harinath Sale
- Department of Physiology, University of Wisconsin, Madison, WI, USA
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