J Physiol 595.7 (2017) pp 2209–2228 2209 WHITE PAPER Potassium channels in the heart: structure, function and regulation Eleonora Grandi1, Michael C. Sanguinetti2, Daniel C. Bartos1,DonaldM.Bers1, Ye Chen-Izu1,3, Nipavan Chiamvimonvat3, Henry M. Colecraft4, Brian P. Delisle5, Jordi Heijman6 , Manuel F. Navedo1, Sergei Noskov7, Catherine Proenza8, Jamie I. Vandenberg9 and Vladimir Yarov-Yarovoy10 1Department of Pharmacology, University of California, Davis, Davis, CA 95616, USA 2Department of Internal Medicine, University of Utah, Nora Eccles Harrison Cardiovascular Research and Training Institute, Salt Lake City, UT 84112, USA 3Department of Internal Medicine, Division of Cardiology, University of California, Davis, CA 95616, USA 4Department of Physiology and Cellular Biophysics, Columbia University, New York, NY 10032, USA 5Department of Physiology, University of Kentucky, Lexington, KY 40536, USA 6Department of Cardiology, Cardiovascular Research Institute Maastricht, Faculty of Health, Medicine, and Life Sciences, Maastricht University, Maastricht, The Netherlands 7Centre for Molecular Simulation, Department of Biological Sciences, University of Calgary, Calgary, AB, T2N 1N4, Canada 8Department of Physiology and Biophysics, University of Colorado - Anschutz Medical Campus, Denver, CO 80045, USA 9Victor Chang Cardiac Research Institute, Darlinghurst, NSW, 2010, Australia 10Department of Physiology and Membrane Biology, University of California, Davis, CA 95616, USA Abstract This paper is the outcome of the fourth UC Davis Systems Approach to Understanding Cardiac Excitation–Contraction Coupling and Arrhythmias Symposium, a biannual event that aims to bring together leading experts in subfields of cardiovascular biomedicine to focus on topics of importance to the field. The theme of the 2016 symposium was ‘K+ Channels and Regulation’. Experts in the field contributed their experimental and mathematical modelling perspectives and discussed emerging questions, controversies and challenges on the topic of cardiac K+ channels. This paper summarizes the topics of formal presentations and informal discussions from the symposium on the structural basis of voltage-gated K+ channel function, as well as the mechanisms involved in regulation of K+ channel gating, expression and membrane localization. Given the critical role for K+ channels in determining the rate of cardiac repolarization, it is hardly surprising that essentially every aspect of K+ channel function is exquisitely regulated in cardiac myocytes. This regulation is complex and highly interrelated to other aspects of myocardial function. K+ channel regulatory mechanisms alter, and are altered by, physiological challenges, The Journal of Physiology pathophysiological conditions, and pharmacological agents. An accompanying paper focuses on the integrative role of K+ channels in cardiac electrophysiology, i.e. how K+ currents shape the cardiac action potential, and how their dysfunction can lead to arrhythmias, and discusses K+ channel-based therapeutics. A fundamental understanding of K+ channel regulatory mechanisms and disease processes is fundamental to reveal new targets for human therapy. (Received 28 May 2016; accepted after revision 18 July 2016) Corresponding author E. Grandi: Department of Pharmacology, Genome and Biomedical Sciences Facility, Room 3502, University of California, Davis, Davis, CA, USA. Email: [email protected] Abbreviations AF, atrial fibrillation; AP, action potential; APD, action potential duration; CaMKII, Ca2+/calmodulin-dependent protein kinase II; COP, coatamer associated protein; cryo-EM, cryo-electron microscopy; ER, endoplasmic reticulum; HF, heart failure; hiPSC-CM, human induced pluripotent stem cell-derived cardiomyocyte; + IPC, ischaemic preconditioning; KV,voltage-gatedK ; LQTS, long QT syndrome; MD, molecular dynamics; PIP2, phosphatidylinositol-4,5-bisphosphate; REFER, rate-equilibrium free energy; RMP, resting membrane potential; SR, sarcoplasmic reticulum; SUR, sulfonylurea receptor; VSD, voltage sensor domain. C 2016 The Authors. The Journal of Physiology C 2016 The Physiological Society DOI: 10.1113/JP272864 2210 E. Grandi and others J Physiol 595.7 Voltage-gated K+ channel structure voltage-gated channels; however, we are still trying to and function work out the precise details of the structural basis of channel function. Perhaps the most prescient prediction In this session Toby Allen, Francisco Bezanilla, Jianmin made by Hodgkin and Huxley, was that ‘changes in Cui, Peter Larsson, Daniel Minor, Sergei Noskov, Jon ionic permeability depend on the movement of some Sack, Matthew Trudeau, Jamie Vandenberg and Vladimir component of the membrane which behaves as though Yarov-Yarovoy participated as speakers, discussion leaders, it had a large charge or dipole moment’ (Hodgkin & or panelists. + Huxley, 1952). Twenty years passed before the small Voltage-gated K (KV) channels are gated pores in transient current associated with the movement of this the membranes of excitable cells that open and close in charge, the gating current, was first recorded (Armstrong response to changes in transmembrane voltage to allow + & Bezanilla, 1973). Measuring gating currents remains the selective passage of K ions across the cell membrane to this day at the centre of ongoing efforts to under- (Bezanilla, 2000). The existence of KV channels was first stand how voltage sensors work (see e.g. Lacroix et al. inferred over 60 years ago through a combination of 2014). It was another decade before cloning of the first functional and theoretical studies in the squid giant axon members of the voltage-gated ion channel family led to (Hodgkin & Huxley, 1952). Many of the principles under- the identification of the S4 transmembrane domain, with lying how these channels sense change in membrane its multiple arginine residues, as the principal component voltage and then transduce that into the opening and of the voltage-sensing domain in these channels (Noda closing of the voltage-dependent gates are now well et al. 1984; Papazian et al. 1987). In the last 20 years, established, but there is still a way to go before we will the development of methods to attach chemical probes understand voltage sensing and voltage sensor to pore ranging from H+ (Starace & Bezanilla, 2001) to fluoro- coupling at the same atomic level resolution that we under- phores (Mannuzzu et al. 1996) to electron paramagnetic stand the basis of ion selectivity (Zhou et al. 2001; Medovoy resonance probes (Cuello et al. 2004) has allowed indirect et al. 2016). monitoring of the motion of the voltage-sensor domain. Our current understanding of the molecular basis + These spectroscopic methods have complemented gating of cardiac K channel function, including selective current measurements in our quest to understand the ion permeation, voltage sensing and gating, is derived extraordinary molecular gymnastics that underlie the from a combination of experimental observations and function of these channels. Recent studies utilizing these modelling. Heterologous expression of cloned channels techniques have revealed the gating mechanisms for has greatly facilitated quantitative biophysical and several cardiac KV channels, including KV11.1 (ERG1) structural studies of K+ channels. Static structures + (Es-Salah-Lamoureux et al. 2010), KV1.5 (Vaid et al. 2009), of prokaryotic K channels (e.g. KcsA, MthK, KVAP, + and IKs channels formed by co-assembly of KV7.1 with KirBac1.1, Kir3.1/KirBac1.3 chimera) and eukaryotic K KCNE1 (minK) accessory subunits (Barro-Soria et al. channels (e.g. Kir2.2, KV1.2) determined by X-ray 2014; Zaydman et al. 2014). crystallography and more recently by cryo-electron micro- In most KV channels that have been investigated, the scopy (cryo-EM) for Slo2.2 (Hite et al. 2015) have provided voltage dependence of the voltage sensor domain (VSD) crucial information for constructing homology models + movement occurs over a more negative voltage range than of cardiac K channels whose structures have yet to that required for channel activation. Channel opening only be solved. These structures have also enabled molecular occurs after all the VSDs have been activated, and involves dynamics (MD) simulations that provide insights into the a final highly cooperative state transition. In a process mechanisms of ion selectivity and permeation, channel called electromechanical coupling, S4–S5 linkers act as gating and ligand binding at an atomic level of detail. mechanical levers that couple outward movement of the Structures have not yet been determined for most of S4 segments to opening of the pore domain (Long et al. the cardiac K+ channels. However, structures of highly + 2005b; Jensen et al. 2012). Recent studies have revealed related mammalian K channels (i.e. KV1.2, Kir3.2 and that gating of KV7.1 and KV11.1 channels differs from this Slo2.2) as well as the cytoplasmic domains of Kir2.1, Kir3.1 canonical model. Quite unexpectedly, channel gating is and KV11.1 are available and provide useful templates to relatively normal when KV11.1 subunits are split at the construct homology models. S4/S5 linker into separate VSD and pore domains and Questions, controversies, challenges: heterologously expressed in oocytes (Lorinczi et al. 2015). r Split KV1orKV7.1 channels are non-functional. Thus, a What do we know about the mechanisms of voltage rigid covalent linkage between the S4 and S5 segments is sensing? not required for normal gating of all KV channels. Given In a seminal series of papers published