Non-Equilibrium Gating in Cardiac Na
            <sup>+</sup>
            Channels

Clancy, Colleen E.; Tateyama, Michihiro; Liu, Huajun; Wehrens, Xander H.T.; Kass, Robert S.

doi:10.1161/01.cir.0000069273.51375.bd

Cited by 132 publications

(82 citation statements)

References 30 publications

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“…Importantly, it is not known how Ca 2+ or Ca 2+ /CaM interactions with the C-terminal EF-hand and IQ motifs are relayed to channel inactivation, but we (9) and others (10) Table S4. in Ca 2+ regulation, a possibility suggested previously (28). The 53-amino acid DIII-IV linker is considered the inactivation gate of the sodium channel because experimentally introduced or inherited mutations in this highly conserved region have profound effects on the fast inactivation of all sodium channel isoforms (12,14,16,31,(36)(37)(38). Here we show through a highresolution crystal structure, ITC analysis, and electrophysiology that the Ca 2+ /CaM C lobe interacts with the DIII-IV linker.…”

Section: Discussionmentioning

confidence: 61%

Crystallographic basis for calcium regulation of sodium channels

Sarhan

Tung²,

Petegem³

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

126

140

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Voltage-gated sodium channels underlie the rapid regenerative upstroke of action potentials and are modulated by cytoplasmic calcium ions through a poorly understood mechanism. We describe the 1.35 Å crystal structure of Ca 2+ -bound calmodulin (Ca 2+ /CaM) in complex with the inactivation gate (DIII-IV linker) of the cardiac sodium channel (Na V 1.5). The complex harbors the positions of five disease mutations involved with long Q-T type 3 and Brugada syndromes. In conjunction with isothermal titration calorimetry, we identify unique inactivation-gate mutations that enhance or diminish Ca 2+ /CaM binding, which, in turn, sensitize or abolish Ca 2+ regulation of full-length channels in electrophysiological experiments. Additional biochemical experiments support a model whereby a single Ca 2+ /CaM bridges the C-terminal IQ motif to the DIII-IV linker via individual N and C lobes, respectively. The data suggest that Ca 2+ /CaM destabilizes binding of the inactivation gate to its receptor, thus biasing inactivation toward more depolarized potentials.crystallography | patch-clamp electrophysiology | structural biology | cardiac arrhythmia V oltage-gated sodium channels (Na V s) support excitability in the cardiovascular and nervous systems, where they contribute to the rhythm and rate of action potentials. These large (∼220-kDa) transmembrane protein complexes are expressed at a high density in excitable cells, where they conduct large macroscopic inward sodium currents. These channels are exquisitely sensitive to subtle changes in the transmembrane potential, and modest alterations in channel gating can fine-tune or disorder electrical signaling at the organ and systemic level. The α-subunit of the channel contains cytoplasmic amino and carboxyl termini and is composed of four homologous transmembrane domains (DI-DIV) that are connected by intracellular linkers. Each domain contains voltage-sensing (S1-S4) and pore-forming (S5 and S6) domains that form the selectivity filter and putative activation gates. A crystal structure of a bacterial Na V was recently described (1) showing a similar overall fold compared with potassium channels. However, this bacterial variant is homotetrameric, and seems to lack a conserved fast-inactivation mechanism. As such, it has no homology with several relevant domains in mammalian Na V channels, and no crystal structure of any eukaryotic Na V region has yet been reported.Calcium ions (Ca 2+ ) are universal second messengers, and in the heart they form the electrochemical link between plasma membrane depolarization and myocyte contraction. Consequently, their cytoplasmic levels oscillate between nanomolar and micromolar levels with each excitation-contraction cycle (2). Sodium channel steady-state inactivation, a process that controls channel availability at a given transmembrane potential, is modulated through interactions with Ca 2+ and calmodulin (CaM) (3-10). The mechanistic details of Ca 2+ modulation of sodium channel inactivation are sparse, but the C-terminal region of the cha...

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Section: Discussionmentioning

confidence: 61%

Crystallographic basis for calcium regulation of sodium channels

Sarhan

Tung²,

Petegem³

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

126

140

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“…The late sodium current was measured as the mean current in the window between 200 and 220 ms from the start of the depolarisation, and the current recorded following addition of 100 μM TTX was used as the zero current level. Furthermore, in atrial myocytes, a simplified action potential pulse of a 100-ms step to +20 mV followed by a 100-ms ramp back to -80 mV and finally 100 ms at -80 mV was used (modified from (Clancy et al, 2003)). …”

Section: Currents In Isolated Cardiomyocytes (Papers III V)mentioning

confidence: 99%

Functional effects of the late sodium current inhibition by AZD7009 and lidocaine in rabbit isolated atrial and ventricular tissue and Purkinje fibre

Persson

Andersson

Duker

et al. 2007

European Journal of Pharmacology

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Atrial fibrillation (AF) is the most common tachyarrhythmia in the adult population and is a major cause of morbidity and mortality. AF can be terminated and sinus rhythm restored by prolonging the action potential duration (APD) and the refractory period. Unfortunately, antiarrhythmic agents that prolong the APD and increase the refractory period via selective inhibition of the rapid delayed rectifier potassium current (IK r ), i.e. class III antiarrhythmic drugs, are associated with an increased risk of the ventricular tachycardia Torsades de Pointes. AZD7009 is an antiarrhythmic agent with predominant actions on atrial electrophysiology that shows high antiarrhythmic efficacy and low proarrhythmic potential in animals and man. The aim of the current studies was to characterize the effect of AZD7009 on cardiac ion currents and APD in order to provide a mechanistic explanation for its predominant atrial effects and low proarrhythmic potential.The human cardiac ion channels hERG (IK r ), Kv1.5 (IK ur ), Kv4.3/KChIP2.2 (I to ), KvLQT1/minK (IK s ), Kir3.1/Kir3.4 (IK ACh ) and Nav1.5 (INa) were expressed in mammalian cells. Whole-cell currents were inhibited by AZD7009 with the following IC 50 values: hERG 0.6 μM, Nav1.5 8 μM, Kv4.3/KChIP2.2 24 μM, Kv1.5 27 μM, Kir3.1/Kir3.4 166 μM and KvLQT1/minK 193 μM. Whole-cell sodium and calcium currents were recorded in isolated rabbit atrial and ventricular myocytes using amphotericin B perforated patch. The late sodium current in rabbit atrial and ventricular myocytes was inhibited by AZD7009 in a concentration dependent way, with approximately 50% inhibition at 10 μM AZD7009. The L-type Ca 2+ current (ICa L ) in rabbit ventricular myocytes was inhibited with an IC 50 of 90 μM. Transmembrane action potentials were recorded in tissue pieces from rabbit atrium, ventricle and Purkinje fibre in control, during exposure to the selective IK r blocker E-4031 and to E-4031 in combination with AZD7009. In Purkinje fibres, but not in ventricular tissue, AZD7009 attenuated the E-4031-induced APD prolongation. In contrast, in atrial cells, AZD7009 further prolonged the APD. In addition, AZD7009 was able to suppress early afterdepolarisations (EADs) induced by E-4031 in Purkinje fibre preparations.In conclusion, AZD7009 delays repolarisation and increases refractoriness in atrial tissue through synergistic inhibition of IK r , I to , IK ur and INa, a mixed ion channel blockade that may underlie its high antiarrhythmic efficacy. Inhibition of the late sodium current, counteracting excessive APD prolongation and EADs in susceptible cells (midmyocardial and Purkinje cells), may explain the low proarrhythmic potential of AZD7009.

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“…These channels experience a transient failure of inactivation. A different kinetic mechanism has been suggested by computer simulations to underlie LQT3 associated with the SCN5A I1768V mutation (Clancy et al 2003). For this mutation, I Na mutant channels recover from inactivation at a faster rate than WT and reopen during the repolarization phase of the AP, when the membrane voltage is decreasing (non-equilibrium conditions).…”

mentioning

confidence: 99%

Computational biology in the study of cardiac ion channels and cell electrophysiology

Rudy

Silva

2006

Quart. Rev. Biophys.

241

181

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The cardiac cell is a complex biological system where various processes interact to generate electrical excitation (the action potential, AP) and contraction. During AP generation, membrane ion channels interact nonlinearly with dynamically changing ionic concentrations and varying transmembrane voltage, and are subject to regulatory processes. In recent years, a large body of knowledge has accumulated on the molecular structure of cardiac ion channels, their function, and their modification by genetic mutations that are associated with cardiac arrhythmias and sudden death. However, ion channels are typically studied in isolation (in expression systems or isolated membrane patches), away from the physiological environment of the cell where they interact to generate the AP. A major challenge remains the integration of ion-channel properties into the functioning, complex and highly interactive cell system, with the objective to relate molecular-level processes and their modification by disease to whole-cell function and clinical phenotype. In this article we describe how computational biology can be used to achieve such integration. We explain how mathematical (Markov) models of ion-channel kinetics are incorporated into integrated models of cardiac cells to compute the AP. We provide examples of mathematical (computer) simulations of physiological and pathological phenomena, including AP adaptation to changes in heart rate, genetic mutations in SCN5A and HERG genes that are associated with fatal cardiac arrhythmias, and effects of the CaMKII regulatory pathway and β-adrenergic cascade on the cell electrophysiological function. Prologue'Things should be made as simple as possible, but not any simpler.' Albert EinsteinCardiac muscle can generate propagating electrical impulses (action potentials), a property that classifies it as an excitable tissue similar to skeletal muscle and nerve. At the single-cell level, the electrical action potential (AP) triggers mechanical contraction by inducing a transient rise of intracellular calcium which, in turn, carries the contraction message to the contractile proteins of the cell. This process that couples electrical excitation to mechanical function is termed excitation-contraction coupling. APs are generated by individual cells and are conducted from cell to cell through intercellular gap junctions, forming waves of excitation that activate and synchronize the blood pumping action of the heart. Similar to nerve and skeletal muscle, AP initiation and conduction in cardiac ventricular tissue rely mostly on a single membrane process, namely the flow of sodium ions through sodium-specific ion channels. However, unlike the short-duration APs of skeletal muscle and nerve, the cardiac ventricular AP is characterized by long plateau and repolarization phases that prevent premature arrhythmogenic excitation and provide control of mechanical contraction. In NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript contrast to the 'single-current mechanism' of...

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Non-Equilibrium Gating in Cardiac Na ⁺ Channels

Cited by 132 publications

References 30 publications

Crystallographic basis for calcium regulation of sodium channels

Crystallographic basis for calcium regulation of sodium channels

Functional effects of the late sodium current inhibition by AZD7009 and lidocaine in rabbit isolated atrial and ventricular tissue and Purkinje fibre

Computational biology in the study of cardiac ion channels and cell electrophysiology

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Non-Equilibrium Gating in Cardiac Na + Channels

Cited by 132 publications

References 30 publications

Crystallographic basis for calcium regulation of sodium channels

Crystallographic basis for calcium regulation of sodium channels

Functional effects of the late sodium current inhibition by AZD7009 and lidocaine in rabbit isolated atrial and ventricular tissue and Purkinje fibre

Computational biology in the study of cardiac ion channels and cell electrophysiology

Contact Info

Product

Resources

About

Non-Equilibrium Gating in Cardiac Na ⁺ Channels