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Abstract
In a normal human life span, the heart beats about 2 to 3 billion times. Under diseased conditions, a heart may lose its normal rhythm and degenerate suddenly into much faster and irregular rhythms, called arrhythmias, which may lead to sudden death. The transition from a normal rhythm to an arrhythmia is a transition from regular electrical wave conduction to irregular or turbulent wave conduction in the heart, and thus this medical problem is also a problem of physics and mathematics. In the last century, clinical, experimental, and theoretical studies have shown that dynamical theories play fundamental roles in understanding the mechanisms of the genesis of the normal heart rhythm as well as lethal arrhythmias. In this article, we summarize in detail the nonlinear and stochastic dynamics occurring in the heart and their links to normal cardiac functions and arrhythmias, providing a holistic view through integrating dynamics from the molecular (microscopic) scale, to the organelle (mesoscopic) scale, to the cellular, tissue, and organ (macroscopic) scales. We discuss what existing problems and challenges are waiting to be solved and how multi-scale mathematical modeling and nonlinear dynamics may be helpful for solving these problems.
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Affiliation(s)
- Zhilin Qu
- Department of Medicine (Cardiology), David Geffen School of Medicine, University of California, Los Angeles, California 90095, USA
- Correspondence to: Zhilin Qu, PhD, Department of Medicine, Division of Cardiology, David Geffen School of Medicine at UCLA, A2-237 CHS, 650 Charles E. Young Drive South, Los Angeles, CA 90095, Tel: 310-794-6050, Fax: 310-206-9133,
| | - Gang Hu
- Department of Physics, Beijing Normal University, Beijing 100875, China
| | - Alan Garfinkel
- Department of Medicine (Cardiology), David Geffen School of Medicine, University of California, Los Angeles, California 90095, USA
- Department of Integrative Biology and Physiology, University of California, Los Angeles, California 90095, USA
| | - James N. Weiss
- Department of Medicine (Cardiology), David Geffen School of Medicine, University of California, Los Angeles, California 90095, USA
- Department of Physiology, David Geffen School of Medicine, University of California, Los Angeles, California 90095, USA
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Subramanyam P, Colecraft HM. Ion channel engineering: perspectives and strategies. J Mol Biol 2014; 427:190-204. [PMID: 25205552 DOI: 10.1016/j.jmb.2014.09.001] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2014] [Accepted: 09/01/2014] [Indexed: 01/19/2023]
Abstract
Ion channels facilitate the passive movement of ions down an electrochemical gradient and across lipid bilayers in cells. This phenomenon is essential for life and underlies many critical homeostatic processes in cells. Ion channels are diverse and differ with respect to how they open and close (gating) and to their ionic conductance/selectivity (permeation). Fundamental understanding of ion channel structure-function mechanisms, their physiological roles, how their dysfunction leads to disease, their utility as biosensors, and development of novel molecules to modulate their activity are important and active research frontiers. In this review, we focus on ion channel engineering approaches that have been applied to investigate these aspects of ion channel function, with a major emphasis on voltage-gated ion channels.
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Affiliation(s)
- Prakash Subramanyam
- Department of Physiology and Cellular Biophysics, Columbia University, NY, 10032, USA
| | - Henry M Colecraft
- Department of Physiology and Cellular Biophysics, Columbia University, NY, 10032, USA.
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Conservation of Ca2+/calmodulin regulation across Na and Ca2+ channels. Cell 2014; 157:1657-70. [PMID: 24949975 DOI: 10.1016/j.cell.2014.04.035] [Citation(s) in RCA: 80] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2014] [Revised: 03/25/2014] [Accepted: 04/15/2014] [Indexed: 11/20/2022]
Abstract
Voltage-gated Na and Ca2+ channels comprise distinct ion channel superfamilies, yet the carboxy tails of these channels exhibit high homology, hinting at a long-shared and purposeful module. For different Ca2+ channels, carboxyl-tail interactions with calmodulin do elaborate robust and similar forms of Ca2+ regulation. However, Na channels have only shown subtler Ca2+ modulation that differs among reports, challenging attempts at unified understanding. Here, by rapid Ca2+ photorelease onto Na channels, we reset this view of Na channel regulation. For cardiac-muscle channels (NaV1.5), reported effects from which most mechanistic proposals derive, we observe no Ca2+ modulation. Conversely, for skeletal-muscle channels (NaV1.4), we uncover fast Ca2+ regulation eerily similar to that of Ca2+ channels. Channelopathic myotonia mutations halve NaV1.4 Ca2+ regulation, and transplanting the NaV1.4 carboxy tail onto Ca2+ channels recapitulates Ca2+ regulation. Thus, we argue for the persistence and physiological relevance of an ancient Ca2+ regulatory module across Na and Ca2+ channels.
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Vocke K, Dauner K, Hahn A, Ulbrich A, Broecker J, Keller S, Frings S, Möhrlen F. Calmodulin-dependent activation and inactivation of anoctamin calcium-gated chloride channels. ACTA ACUST UNITED AC 2014; 142:381-404. [PMID: 24081981 PMCID: PMC3787769 DOI: 10.1085/jgp.201311015] [Citation(s) in RCA: 58] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
Calcium-dependent chloride channels serve critical functions in diverse biological systems. Driven by cellular calcium signals, the channels codetermine excitatory processes and promote solute transport. The anoctamin (ANO) family of membrane proteins encodes three calcium-activated chloride channels, named ANO 1 (also TMEM16A), ANO 2 (also TMEM16B), and ANO 6 (also TMEM16F). Here we examined how ANO 1 and ANO 2 interact with Ca2+/calmodulin using nonstationary current analysis during channel activation. We identified a putative calmodulin-binding domain in the N-terminal region of the channel proteins that is involved in channel activation. Binding studies with peptides indicated that this domain, a regulatory calmodulin-binding motif (RCBM), provides two distinct modes of interaction with Ca2+/calmodulin, one at submicromolar Ca2+ concentrations and one in the micromolar Ca2+ range. Functional, structural, and pharmacological data support the concept that calmodulin serves as a calcium sensor that is stably associated with the RCBM domain and regulates the activation of ANO 1 and ANO 2 channels. Moreover, the predominant splice variant of ANO 2 in the brain exhibits Ca2+/calmodulin-dependent inactivation, a loss of channel activity within 30 s. This property may curtail ANO 2 activity during persistent Ca2+ signals in neurons. Mutagenesis data indicated that the RCBM domain is also involved in ANO 2 inactivation, and that inactivation is suppressed in the retinal ANO 2 splice variant. These results advance the understanding of Ca2+ regulation in anoctamin Cl− channels and its significance for the physiological function that anoctamin channels subserve in neurons and other cell types.
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Affiliation(s)
- Kerstin Vocke
- Department of Molecular Physiology, Centre for Organismal Studies, Heidelberg University, 69120 Heidelberg, Germany
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Yin G, Hassan F, Haroun AR, Murphy LL, Crotti L, Schwartz PJ, George AL, Satin J. Arrhythmogenic calmodulin mutations disrupt intracellular cardiomyocyte Ca2+ regulation by distinct mechanisms. J Am Heart Assoc 2014; 3:e000996. [PMID: 24958779 PMCID: PMC4309107 DOI: 10.1161/jaha.114.000996] [Citation(s) in RCA: 69] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
Background Calmodulin (CaM) mutations have been identified recently in subjects with congenital long QT syndrome (LQTS) or catecholaminergic polymorphic ventricular tachycardia (CPVT), but the mechanisms responsible for these divergent arrhythmia‐susceptibility syndromes in this context are unknown. We tested the hypothesis that LQTS‐associated CaM mutants disrupt Ca2+ homeostasis in developing cardiomyocytes possibly by affecting either late Na current or Ca2+‐dependent inactivation of L‐type Ca2+ current. Methods and Results We coexpressed CaM mutants with the human cardiac Na channel (NaV1.5) in tsA201 cells, and we used mammalian fetal ventricular cardiomyocytes to investigate LQTS‐ and CPVT‐associated CaM mutations (LQTS‐ and CPVT‐CaM). LQTS‐CaM mutants do not consistently affect L‐type Na current in heterologous cells or native cardiomyocytes, suggesting that the Na channel does not contribute to LQTS pathogenesis in the context of CaM mutations. LQTS‐CaM mutants (D96V, D130G, F142L) impaired Ca2+‐dependent inactivation, whereas the CPVT‐CaM mutant N54I had no effect on Ca2+‐dependent inactivation. LQTS‐CaM mutants led to loss of Ca2+‐transient entrainment with the rank order from greatest to least effect: CaM‐D130G~CaM‐D96V>>CaM‐F142L. This rank order follows measured Ca2+‐CaM affinities for wild‐type and mutant CaM. Acute isoproterenol restored entrainment for CaM‐130G and CaM‐D96V but caused irreversible cytosolic Ca2+ overload for cells expressing a CPVT‐CaM mutant. Conclusions CaM mutations associated with LQTS may not affect L‐type Na+ current but may evoke defective Ca2+‐dependent inactivation of L‐type Ca2+ current.
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Affiliation(s)
- Guo Yin
- Department of Physiology, University of Kentucky College of Medicine, Lexington, KY (G.Y., F.H., A.R.H., J.S.)
| | - Faisal Hassan
- Department of Physiology, University of Kentucky College of Medicine, Lexington, KY (G.Y., F.H., A.R.H., J.S.)
| | - Ayman R Haroun
- Department of Physiology, University of Kentucky College of Medicine, Lexington, KY (G.Y., F.H., A.R.H., J.S.)
| | - Lisa L Murphy
- Department of Pharmacology, Vanderbilt University School of Medicine, Nashville, TN (L.L.M., A.L.G.)
| | - Lia Crotti
- Section of Cardiology, Department of Molecular Medicine, University of Pavia, Pavia, Italy (L.C.) Institute of Human Genetics, Helmholtz Zentrum München, Neuherberg, Germany (L.C.) IRCCS Istituto Auxologico Italiano, Center for Cardiac Arrhythmias of Genetic Origin and Laboratory of Cardiovascular Genetics, Milan, Italy (L.C., P.J.S.)
| | - Peter J Schwartz
- IRCCS Istituto Auxologico Italiano, Center for Cardiac Arrhythmias of Genetic Origin and Laboratory of Cardiovascular Genetics, Milan, Italy (L.C., P.J.S.)
| | - Alfred L George
- Department of Pharmacology, Vanderbilt University School of Medicine, Nashville, TN (L.L.M., A.L.G.) Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN (A.L.G.) Department of Pharmacology, Northwestern University Feinberg School of Medicine, Chicago, IL (A.L.G.)
| | - Jonathan Satin
- Department of Physiology, University of Kentucky College of Medicine, Lexington, KY (G.Y., F.H., A.R.H., J.S.)
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Bers DM, Morotti S. Ca(2+) current facilitation is CaMKII-dependent and has arrhythmogenic consequences. Front Pharmacol 2014; 5:144. [PMID: 24987371 PMCID: PMC4060732 DOI: 10.3389/fphar.2014.00144] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/01/2014] [Accepted: 06/02/2014] [Indexed: 11/13/2022] Open
Abstract
The cardiac voltage gated Ca2+ current (ICa) is critical to the electrophysiological properties, excitation-contraction coupling, mitochondrial energetics, and transcriptional regulation in heart. Thus, it is not surprising that cardiac ICa is regulated by numerous pathways. This review will focus on changes in ICa that occur during the cardiac action potential (AP), with particular attention to Ca2+-dependent inactivation (CDI), Ca2+-dependent facilitation (CDF) and how calmodulin (CaM) and Ca2+-CaM dependent protein kinase (CaMKII) participate in the regulation of Ca2+ current during the cardiac AP. CDI depends on CaM pre-bound to the C-terminal of the L-type Ca2+ channel, such that Ca2+ influx and Ca2+ released from the sarcoplasmic reticulum bind to that CaM and cause CDI. In cardiac myocytes CDI normally pre-dominates over voltage-dependent inactivation. The decrease in ICa via CDI provides direct negative feedback on the overall Ca2+ influx during a single beat, when myocyte Ca2+ loading is high. CDF builds up over several beats, depends on CaMKII-dependent Ca2+ channel phosphorylation, and results in a staircase of increasing ICa peak, with progressively slower inactivation. CDF and CDI co-exist and in combination may fine-tune the ICa waveform during the cardiac AP. CDF may partially compensate for the tendency for Ca2+ channel availability to decrease at higher heart rates because of accumulating inactivation. CDF may also allow some reactivation of ICa during long duration cardiac APs, and contribute to early afterdepolarizations, a form of triggered arrhythmias.
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Affiliation(s)
- Donald M Bers
- Department of Pharmacology, University of California Davis Davis, CA, USA
| | - Stefano Morotti
- Department of Pharmacology, University of California Davis Davis, CA, USA
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Limpitikul WB, Dick IE, Joshi-Mukherjee R, Overgaard MT, George AL, Yue DT. Calmodulin mutations associated with long QT syndrome prevent inactivation of cardiac L-type Ca(2+) currents and promote proarrhythmic behavior in ventricular myocytes. J Mol Cell Cardiol 2014; 74:115-24. [PMID: 24816216 DOI: 10.1016/j.yjmcc.2014.04.022] [Citation(s) in RCA: 121] [Impact Index Per Article: 12.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/11/2014] [Accepted: 04/28/2014] [Indexed: 01/13/2023]
Abstract
Recent work has identified missense mutations in calmodulin (CaM) that are associated with severe early-onset long-QT syndrome (LQTS), leading to the proposition that altered CaM function may contribute to the molecular etiology of this subset of LQTS. To date, however, no experimental evidence has established these mutations as directly causative of LQTS substrates, nor have the molecular targets of CaM mutants been identified. Here, therefore, we test whether expression of CaM mutants in adult guinea-pig ventricular myocytes (aGPVM) induces action-potential prolongation, and whether affiliated alterations in the Ca(2+) regulation of L-type Ca(2+) channels (LTCC) might contribute to such prolongation. In particular, we first overexpressed CaM mutants in aGPVMs, and observed both increased action potential duration (APD) and heightened Ca(2+) transients. Next, we demonstrated that all LQTS CaM mutants have the potential to strongly suppress Ca(2+)/CaM-dependent inactivation (CDI) of LTCCs, whether channels were heterologously expressed in HEK293 cells, or present in native form within myocytes. This attenuation of CDI is predicted to promote action-potential prolongation and boost Ca(2+) influx. Finally, we demonstrated how a small fraction of LQTS CaM mutants (as in heterozygous patients) would nonetheless suffice to substantially diminish CDI, and derange electrical and Ca(2+) profiles. In all, these results highlight LTCCs as a molecular locus for understanding and treating CaM-related LQTS in this group of patients.
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Affiliation(s)
- Worawan B Limpitikul
- Calcium Signals Laboratory, Department of Biomedical Engineering, The Johns Hopkins University School of Medicine, Baltimore, MD 21205
| | - Ivy E Dick
- Calcium Signals Laboratory, Department of Biomedical Engineering, The Johns Hopkins University School of Medicine, Baltimore, MD 21205
| | - Rosy Joshi-Mukherjee
- Calcium Signals Laboratory, Department of Biomedical Engineering, The Johns Hopkins University School of Medicine, Baltimore, MD 21205
| | - Michael T Overgaard
- Department of Biotechnology, Chemistry and Environmental Engineering, Aalborg University, Denmark
| | - Alfred L George
- Department of Medicine, Vanderbilt University, Nashville, TN 37232, USA; Department of Pharmacology, Vanderbilt University, Nashville, TN 37232, USA
| | - David T Yue
- Calcium Signals Laboratory, Department of Biomedical Engineering, The Johns Hopkins University School of Medicine, Baltimore, MD 21205; Department of Neuroscience, The Johns Hopkins University School of Medicine, Baltimore, MD 21205.
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Simms BA, Souza IA, Zamponi GW. Effect of the Brugada syndrome mutation A39V on calmodulin regulation of Cav1.2 channels. Mol Brain 2014; 7:34. [PMID: 24775099 PMCID: PMC4012176 DOI: 10.1186/1756-6606-7-34] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2014] [Accepted: 04/23/2014] [Indexed: 01/22/2023] Open
Abstract
Background The L-type calcium channel Cav1.2 is important for brain and heart function. The ubiquitous calcium sensing protein calmodulin (CaM) regulates calcium dependent gating of Cav1.2 channels by reducing calcium influx, a process known as calcium-dependent inactivation (CDI). Dissecting the calcium-dependence of CaM in this process has benefited greatly from the use of mutant CaM molecules which are unable to bind calcium to their low affinity (N-lobe) and high affinity (C-lobe) binding sites. Unlike CDI, it is unknown whether CaM can modulate the activation gating of Cav1.2 channels. Results We examined a Cav1.2 point mutant in the N-terminus region of the channel (A39V) that has been previously linked to Brugada syndrome. Using mutant CaM constructs in which the N- and/or C-lobe calcium binding sites were ablated, we were able to show that this Brugada syndrome mutation disrupts N-lobe CDI of the channel. In the course of these experiments, we discovered that all mutant CaM molecules were able to alter the kinetics of channel activation even in the absence of calcium for WT-Cav1.2, but not A39V-Cav1.2 channels. Moreover, CaM mutants differentially shifted the voltage-dependence of activation for WT and A39V-Cav1.2 channels to hyperpolarized potentials. Our data therefore suggest that structural changes in CaM that arise directly from site directed mutagenesis of calcium binding domains alter activation gating of Cav1.2 channels independently of their effects on calcium binding, and that the N-terminus of the channel contributes to this CaM dependent process. Conclusions Our data indicate that caution must be exercised when interpreting the effects of CaM mutants on ion channel gating.
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Affiliation(s)
| | | | - Gerald W Zamponi
- Department of Physiology and Pharmacology, Hotchkiss Brain Institute, University of Calgary, 3330 Hospital Dr, NW, Calgary T2N 4N1, Canada.
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Grégoire-Lacoste F, Jacquemet V, Vinet A. Bifurcations, sustained oscillations and torus bursting involving ionic concentrations dynamics in a canine atrial cell model. Math Biosci 2014; 250:10-25. [PMID: 24530894 DOI: 10.1016/j.mbs.2014.01.010] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/12/2013] [Revised: 01/28/2014] [Accepted: 01/31/2014] [Indexed: 11/28/2022]
Abstract
Atrial fibrillation is a disorganization of the electrical propagation in the atria often initiated by ectopic beats. This spontaneous activity might be associated with the appearance of sustained oscillations in some portion of the tissue. Adrenergic stress and specific gene polymorphisms known to promote atrial fibrillation are notably related to calcium and potassium channel conductances. We performed codimension-one and two bifurcation analysis along these conductances in an ionic canine atrial myocyte model. Two Hopf bifurcations were found, related to two distinct mechanisms: (1) a fast calcium gating-driven oscillator, and (2) a slow concentration-driven oscillator. These two mechanisms interact through a double Hopf bifurcation (HH) in a neighborhood of which a torus (Neimark-Sacker) bifurcation leads to bursting. A complex codimension-two theoretical scenario was identified around HH, through systematic comparison with the attractors found numerically. The concentration oscillator was further decomposed to reveal the minimal oscillating subnetwork, in which the Na(+)/Ca(2+) exchanger plays a prominent role.
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Affiliation(s)
- François Grégoire-Lacoste
- Institut de Génie Biomédical, Department of Physiology, Faculty of Medicine, Université de Montréal, CP 6128, Succ Centre-Ville, Montréal H3C 3J7, Canada
| | - Vincent Jacquemet
- Institut de Génie Biomédical, Department of Physiology, Faculty of Medicine, Université de Montréal, CP 6128, Succ Centre-Ville, Montréal H3C 3J7, Canada; Centre de Recherche, Hôpital du Sacré-Coeur de Montréal, Canada
| | - Alain Vinet
- Institut de Génie Biomédical, Department of Physiology, Faculty of Medicine, Université de Montréal, CP 6128, Succ Centre-Ville, Montréal H3C 3J7, Canada; Centre de Recherche, Hôpital du Sacré-Coeur de Montréal, Canada.
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60
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Allostery in Ca²⁺ channel modulation by calcium-binding proteins. Nat Chem Biol 2014; 10:231-8. [PMID: 24441587 DOI: 10.1038/nchembio.1436] [Citation(s) in RCA: 36] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2013] [Accepted: 11/25/2013] [Indexed: 12/17/2022]
Abstract
Distinguishing between allostery and competition among modulating ligands is challenging for large target molecules. Out of practical necessity, inferences are often drawn from in vitro assays on target fragments, but such inferences may belie actual mechanisms. One key example of such ambiguity concerns calcium-binding proteins (CaBPs) that tune signaling molecules regulated by calmodulin (CaM). As CaBPs resemble CaM, CaBPs are believed to competitively replace CaM on targets. Yet, brain CaM expression far surpasses that of CaBPs, raising questions as to whether CaBPs can exert appreciable biological actions. Here, we devise a live-cell, holomolecule approach that reveals an allosteric mechanism for calcium channels whose CaM-mediated inactivation is eliminated by CaBP4. Our strategy is to covalently link CaM and/or CaBP to holochannels, enabling live-cell fluorescence resonance energy transfer assays to resolve a cyclical allosteric binding scheme for CaM and CaBP4 to channels, thus explaining how trace CaBPs prevail. This approach may apply generally for discerning allostery in live cells.
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Manning JR, Yin G, Kaminski CN, Magyar J, Feng H, Penn J, Sievert G, Thompson K, Jin J, Andres DA, Satin J. Rad GTPase deletion increases L-type calcium channel current leading to increased cardiac contraction. J Am Heart Assoc 2013; 2:e000459. [PMID: 24334906 PMCID: PMC3886777 DOI: 10.1161/jaha.113.000459] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
BACKGROUND The small GTPase Rad is a negative regulator of voltage-dependent L-type calcium channel current (ICaL); however, the effects of Rad ablation on cardiomyocyte function are unknown. The objective of this study is to test the hypothesis that Rad-depletion causes positive inotropic effects without inducing cardiac hypertrophy. METHODS AND RESULTS Ventricular myocytes from adult Rad(-/-) mice were isolated and evaluated by patch-clamp recordings for I(Ca,L) and action potentials, Ca(2+) transients, and sarcomere shortening. Maximum I(CaL) is elevated in Rad(-/-) (maximal conductance 0.35 ± 0.04 picoSiemens/picoFarad (pS/pF) wild-type; 0.61 ± 0.14 pS/pF Rad(-/-)), decay kinetics are faster, and I(Ca,L) activates at lower voltages (activation midpoint -7.2 ± 0.6 wild-type; -11.7 ± 0.9 Rad(-/-)) mimicking effects of β-adrenergic receptor stimulation. Diastolic and twitch calcium are elevated in Rad(-/-) (F340/380: 1.03 diastolic and 0.35 twitch for wild-type; 1.47 diastolic and 0.736 twitch for Rad(-/-)) and sarcomere shortening is enhanced (4.31% wild-type; 14.13% Rad(-/-)) at lower pacing frequencies. Consequentially, frequency-dependence of Ca(2+) transients is less in Rad(-/-), and the frequency dependence of relaxation is also blunted. In isolated working hearts, similar results were obtained; chiefly, +dP/dt was elevated at baseline and developed pressure was relatively nonresponsive to acute β-adrenergic receptor stimulation. In single cells, at subphysiological frequencies, nonstimulated calmodulin-dependent protein kinase-sensitive calcium release is observed. Remarkably, Rad(-/-) hearts did not show hypertrophic growth despite elevated levels of diastolic calcium. CONCLUSIONS This study demonstrates that the depletion of Rad GTPase is equivalent to sympathomimetic β-adrenergic receptor, without stimulating cardiac hypertrophy. Thus, targeting Rad GTPase is a novel potential therapeutic target for Ca(2+)-homeostasis-driven positive inotropic support of the heart.
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Affiliation(s)
- Janet R. Manning
- Department of Physiology, University of Kentucky College of Medicine, Lexington, KY (J.R.M., G.Y., J.M., J.P., G.S., J.S.)
- Department of Molecular and Cellular Biochemistry, University of Kentucky College of Medicine, Lexington, KY (J.R.M., C.N.K., D.A.A.)
| | - Guo Yin
- Department of Physiology, University of Kentucky College of Medicine, Lexington, KY (J.R.M., G.Y., J.M., J.P., G.S., J.S.)
| | - Catherine N. Kaminski
- Department of Molecular and Cellular Biochemistry, University of Kentucky College of Medicine, Lexington, KY (J.R.M., C.N.K., D.A.A.)
| | - Janos Magyar
- Department of Physiology, University of Kentucky College of Medicine, Lexington, KY (J.R.M., G.Y., J.M., J.P., G.S., J.S.)
- Department of Physiology, University of Debrecen, Hungary (J.M.)
| | - Han‐Zhong Feng
- Department of Physiology, Wayne State University School of Medicine, Detroit, MI (H.Z.F., J.)
| | - John Penn
- Department of Physiology, University of Kentucky College of Medicine, Lexington, KY (J.R.M., G.Y., J.M., J.P., G.S., J.S.)
| | - Gail Sievert
- Department of Physiology, University of Kentucky College of Medicine, Lexington, KY (J.R.M., G.Y., J.M., J.P., G.S., J.S.)
| | - Katherine Thompson
- Department of Statistics, University of Kentucky College of Medicine, Lexington, KY (K.T.)
| | - J.‐P. Jin
- Department of Physiology, Wayne State University School of Medicine, Detroit, MI (H.Z.F., J.)
| | - Douglas A. Andres
- Department of Molecular and Cellular Biochemistry, University of Kentucky College of Medicine, Lexington, KY (J.R.M., C.N.K., D.A.A.)
| | - Jonathan Satin
- Department of Physiology, University of Kentucky College of Medicine, Lexington, KY (J.R.M., G.Y., J.M., J.P., G.S., J.S.)
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Schwartz PJ, Ackerman MJ, George AL, Wilde AAM. Impact of genetics on the clinical management of channelopathies. J Am Coll Cardiol 2013; 62:169-180. [PMID: 23684683 DOI: 10.1016/j.jacc.2013.04.044] [Citation(s) in RCA: 220] [Impact Index Per Article: 20.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/25/2013] [Revised: 04/11/2013] [Accepted: 04/24/2013] [Indexed: 12/29/2022]
Abstract
There are few areas in cardiology in which the impact of genetics and genetic testing on clinical management has been as great as in cardiac channelopathies, arrhythmic disorders of genetic origin related to the ionic control of the cardiac action potential. Among the growing number of diseases identified as channelopathies, 3 are sufficiently prevalent to represent significant clinical and societal problems and to warrant adequate understanding by practicing cardiologists: long QT syndrome, catecholaminergic polymorphic ventricular tachycardia, and Brugada syndrome. This review will focus selectively on the impact of genetic discoveries on clinical management of these 3 diseases. For each disorder, we will discuss to what extent genetic knowledge and clinical genetic test results modify the way cardiologists should approach and manage affected patients. We will also address the optimal use of genetic testing, including its potential limitations and the potential medico-legal implications when such testing is not performed. We will highlight how important it is to understand the ways that genotype can affect clinical manifestations, risk stratification, and responses to the therapy. We will also illustrate the close bridge between molecular biology and clinical medicine, and will emphasize that consideration of the genetic basis for these heritable arrhythmia syndromes and the proper use and interpretation of clinical genetic testing should remain the standard of care.
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Affiliation(s)
- Peter J Schwartz
- Department of Molecular Medicine, University of Pavia, Pavia, Italy; Department of Cardiology, Fondazione IRCCS Policlinico San Matteo, Pavia, Italy; Cardiovascular Genetics Laboratory, Hatter Institute for Cardiovascular Research in Africa, Department of Medicine, University of Cape Town, Cape Town, South Africa; Department of Medicine, University of Stellenbosch, Stellenbosch, South Africa; Chair of Sudden Death, Department of Family and Community Medicine, College of Medicine, King Saud University, Riyadh, Saudi Arabia.
| | - Michael J Ackerman
- Department of Medicine, Division of Cardiovascular Diseases, Mayo Clinic, Rochester, Minnesota; Department of Pediatrics, Division of Pediatric Cardiology, Mayo Clinic, Rochester, Minnesota; Department of Molecular Pharmacology & Experimental Therapeutics, Windland Smith Rice Sudden Death Genomics Laboratory, Mayo Clinic, Rochester, Minnesota
| | - Alfred L George
- Division of Genetic Medicine, Department of Medicine, Vanderbilt University, Nashville, Tennessee; Institute for Integrative Genomics, Vanderbilt University, Nashville, Tennessee
| | - Arthur A M Wilde
- Department of Cardiology, Heart Failure Research Centre, Academic Medical Centre, Amsterdam, the Netherlands; Princess Al Jawhara Albrahim Centre of Excellence in Research of Hereditary Disorders, King Abdulaziz University, Jeddah, Saudi Arabia
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Qu Z, Xie LH, Olcese R, Karagueuzian HS, Chen PS, Garfinkel A, Weiss JN. Early afterdepolarizations in cardiac myocytes: beyond reduced repolarization reserve. Cardiovasc Res 2013; 99:6-15. [PMID: 23619423 DOI: 10.1093/cvr/cvt104] [Citation(s) in RCA: 119] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Early afterdepolarizations (EADs) are secondary voltage depolarizations during the repolarizing phase of the action potential, which can cause lethal cardiac arrhythmias. The occurrence of EADs requires a reduction in outward current and/or an increase in inward current, a condition called reduced repolarization reserve. However, this generalized condition is not sufficient for EAD genesis and does not explain the voltage oscillations manifesting as EADs. Here, we summarize recent progress that uses dynamical theory to build on and advance our understanding of EADs beyond the concept of repolarization reserve, towards the goal of developing a holistic and integrative view of EADs and their role in arrhythmogenesis. We first introduce concepts from nonlinear dynamics that are relevant to EADs, namely, Hopf bifurcation leading to oscillations and basin of attraction of an equilibrium or oscillatory state. We then present a theory of phase-2 EADs in nonlinear dynamics, which includes the formation of quasi-equilibrium states at the plateau voltage, their stabilities, and the bifurcations leading to and terminating the oscillations. This theory shows that the L-type calcium channel plays a unique role in causing the nonlinear dynamical behaviours necessary for EADs. We also summarize different mechanisms of phase-3 EADs. Based on the dynamical theory, we discuss the roles of each of the major ionic currents in the genesis of EADs, and potential therapeutic targets.
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Affiliation(s)
- Zhilin Qu
- Department of Medicine , David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA.
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65
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Xie Y, Grandi E, Puglisi JL, Sato D, Bers DM. β-adrenergic stimulation activates early afterdepolarizations transiently via kinetic mismatch of PKA targets. J Mol Cell Cardiol 2013; 58:153-61. [PMID: 23481579 DOI: 10.1016/j.yjmcc.2013.02.009] [Citation(s) in RCA: 52] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/12/2012] [Revised: 01/25/2013] [Accepted: 02/11/2013] [Indexed: 02/04/2023]
Abstract
Sympathetic stimulation regulates cardiac excitation-contraction coupling in hearts but can also trigger ventricular arrhythmias caused by early afterdepolarizations (EADs) in pathological conditions. Isoproterenol (ISO) stimulation can transiently cause EADs which could result from differential kinetics of L-type Ca current (ICaL) vs. delayed rectifier potassium current (IKs) effects, but multiple PKA targets complicate mechanistic analysis. Utilizing a biophysically detailed model integrating Ca and β-adrenergic signaling, we investigate how different phosphorylation kinetics and targets influence β-adrenergic-induced transient EADs. We found that: 1) The faster time course of ICaL vs. IKs increases recapitulates experimentally observed ISO-induced transient EADs (which are due to ICaL reactivation). These EADs disappear at steady state ISO and do not occur during more gradual ISO application. 2) This ICaL vs. IKs kinetic mismatch with ISO can also induce transient EADs due to spontaneous sarcoplasmic reticulum (SR) Ca release and Na/Ca exchange current. The increased ICaL, SR Ca uptake and action potential duration (APD) raise SR Ca to cause spontaneous SR Ca release, but eventual IKs activation and APD shortening abolish these EADs. 3) Phospholemman (PLM) phosphorylation decreases both types of EADs by increasing outward Na/K-ATPase current (INaK) for ICaL-mediated EADs, and reducing intracellular Na and Ca loading for SR Ca-release-mediated EADs. Slowing PLM phosphorylation kinetics abolishes this protective effect. 4) Blocking phospholamban (PLB) phosphorylation has little effect on ICaL-mediated transient EADs, but abolishes SR Ca-release-mediated transient EADs by limiting SR Ca loading. 5) RyR phosphorylation has little effect on either transient EAD type. Our study emphasizes the importance of understanding non-steady state kinetics of several systems in mediating β-adrenergic-induced EADs and arrhythmias.
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Affiliation(s)
- Yuanfang Xie
- Department of Pharmacology, University of California Davis, Davis, CA, USA
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66
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Electrical storm: recent pathophysiological insights and therapeutic consequences. Basic Res Cardiol 2013; 108:336. [DOI: 10.1007/s00395-013-0336-2] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/07/2013] [Revised: 01/29/2013] [Accepted: 02/04/2013] [Indexed: 01/01/2023]
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67
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Shaw RM, Colecraft HM. L-type calcium channel targeting and local signalling in cardiac myocytes. Cardiovasc Res 2013; 98:177-86. [PMID: 23417040 DOI: 10.1093/cvr/cvt021] [Citation(s) in RCA: 90] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
In the heart, Ca(2+) influx via Ca(V)1.2 L-type calcium channels (LTCCs) is a multi-functional signal that triggers muscle contraction, controls action potential duration, and regulates gene expression. The use of LTCC Ca(2+) as a multi-dimensional signalling molecule in the heart is complicated by several aspects of cardiac physiology. Cytosolic Ca(2+) continuously cycles between ~100 nM and ~1 μM with each heartbeat due to Ca(2+) linked signalling from LTCCs to ryanodine receptors. This rapid cycling raises the question as to how cardiac myocytes distinguish the Ca(2+) fluxes originating through L-type channels that are dedicated to contraction from Ca(2+) fluxes originating from other L-type channels that are used for non-contraction-related signalling. In general, disparate Ca(2+) sources in cardiac myocytes such as current through differently localized LTCCs as well as from IP3 receptors can signal selectively to Ca(2+)-dependent effectors in local microdomains that can be impervious to the cytoplasmic Ca(2+) transients that drive contraction. A particular challenge for diversified signalling via cardiac LTCCs is that they are voltage-gated and, therefore, open and presumably flood their microdomains with Ca(2+) with each action potential. Thus spatial localization of Cav1.2 channels to different types of microdomains of the ventricular cardiomyocyte membrane as well as the existence of particular macromolecular complexes in each Cav1.2 microdomain are important to effect different types of Cav1.2 signalling. In this review we examine aspects of Cav1.2 structure, targeting and signalling in two specialized membrane microdomains--transverse tubules and caveolae.
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Affiliation(s)
- Robin M Shaw
- Cardiovascular Research Institute and Department of Medicine, University of California, San Francisco, CA 94143, USA
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68
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Crotti L, Johnson CN, Graf E, De Ferrari GM, Cuneo BF, Ovadia M, Papagiannis J, Feldkamp MD, Rathi SG, Kunic JD, Pedrazzini M, Wieland T, Lichtner P, Beckmann BM, Clark T, Shaffer C, Benson DW, Kääb S, Meitinger T, Strom TM, Chazin WJ, Schwartz PJ, George AL. Calmodulin mutations associated with recurrent cardiac arrest in infants. Circulation 2013; 127:1009-17. [PMID: 23388215 DOI: 10.1161/circulationaha.112.001216] [Citation(s) in RCA: 271] [Impact Index Per Article: 24.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
BACKGROUND Life-threatening disorders of heart rhythm may arise during infancy and can result in the sudden and tragic death of a child. We performed exome sequencing on 2 unrelated infants presenting with recurrent cardiac arrest to discover a genetic cause. METHODS AND RESULTS We ascertained 2 unrelated infants (probands) with recurrent cardiac arrest and dramatically prolonged QTc interval who were both born to healthy parents. The 2 parent-child trios were investigated with the use of exome sequencing to search for de novo genetic variants. We then performed follow-up candidate gene screening on an independent cohort of 82 subjects with congenital long-QT syndrome without an identified genetic cause. Biochemical studies were performed to determine the functional consequences of mutations discovered in 2 genes encoding calmodulin. We discovered 3 heterozygous de novo mutations in either CALM1 or CALM2, 2 of the 3 human genes encoding calmodulin, in the 2 probands and in 2 additional subjects with recurrent cardiac arrest. All mutation carriers were infants who exhibited life-threatening ventricular arrhythmias combined variably with epilepsy and delayed neurodevelopment. Mutations altered residues in or adjacent to critical calcium binding loops in the calmodulin carboxyl-terminal domain. Recombinant mutant calmodulins exhibited several-fold reductions in calcium binding affinity. CONCLUSIONS Human calmodulin mutations disrupt calcium ion binding to the protein and are associated with a life-threatening condition in early infancy. Defects in calmodulin function will disrupt important calcium signaling events in heart, affecting membrane ion channels, a plausible molecular mechanism for potentially deadly disturbances in heart rhythm during infancy.
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Affiliation(s)
- Lia Crotti
- Section of Cardiology, Department of Molecular Medicine, University of Pavia, Pavia, Italy
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69
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Localized calcineurin confers Ca2+-dependent inactivation on neuronal L-type Ca2+ channels. J Neurosci 2013; 32:15328-37. [PMID: 23115171 DOI: 10.1523/jneurosci.2302-12.2012] [Citation(s) in RCA: 50] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Excitation-driven entry of Ca(2+) through L-type voltage-gated Ca(2+) channels controls gene expression in neurons and a variety of fundamental activities in other kinds of excitable cells. The probability of opening of Ca(V)1.2 L-type channels is subject to pronounced enhancement by cAMP-dependent protein kinase (PKA), which is scaffolded to Ca(V)1.2 channels by A-kinase anchoring proteins (AKAPs). Ca(V)1.2 channels also undergo negative autoregulation via Ca(2+)-dependent inactivation (CDI), which strongly limits Ca(2+) entry. An abundance of evidence indicates that CDI relies upon binding of Ca(2+)/calmodulin (CaM) to an isoleucine-glutamine motif in the carboxy tail of Ca(V)1.2 L-type channels, a molecular mechanism seemingly unrelated to phosphorylation-mediated channel enhancement. But our work reveals, in cultured hippocampal neurons and a heterologous expression system, that the Ca(2+)/CaM-activated phosphatase calcineurin (CaN) is scaffolded to Ca(V)1.2 channels by the neuronal anchoring protein AKAP79/150, and that overexpression of an AKAP79/150 mutant incapable of binding CaN (ΔPIX; CaN-binding PXIXIT motif deleted) impedes CDI. Interventions that suppress CaN activity-mutation in its catalytic site, antagonism with cyclosporine A or FK506, or intracellular perfusion with a peptide mimicking the sequence of the phosphatase's autoinhibitory domain-interfere with normal CDI. In cultured hippocampal neurons from a ΔPIX knock-in mouse, CDI is absent. Results of experiments with the adenylyl cyclase stimulator forskolin and with the PKA inhibitor PKI suggest that Ca(2+)/CaM-activated CaN promotes CDI by reversing channel enhancement effectuated by kinases such as PKA. Hence, our investigation of AKAP79/150-anchored CaN reconciles the CaM-based model of CDI with an earlier, seemingly contradictory model based on dephosphorylation signaling.
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70
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Rokita AG, Anderson ME. New therapeutic targets in cardiology: arrhythmias and Ca2+/calmodulin-dependent kinase II (CaMKII). Circulation 2013; 126:2125-39. [PMID: 23091085 DOI: 10.1161/circulationaha.112.124990] [Citation(s) in RCA: 90] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Affiliation(s)
- Adam G Rokita
- Department of Internal Medicine, University of Iowa Carver College of Medicine, Iowa City, IA 52242, USA
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71
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Ben Johny M, Yang PS, Bazzazi H, Yue DT. Dynamic switching of calmodulin interactions underlies Ca2+ regulation of CaV1.3 channels. Nat Commun 2013; 4:1717. [PMID: 23591884 PMCID: PMC3856249 DOI: 10.1038/ncomms2727] [Citation(s) in RCA: 93] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2012] [Accepted: 03/08/2013] [Indexed: 11/30/2022] Open
Abstract
Calmodulin regulation of CaV channels is a prominent Ca(2+) feedback mechanism orchestrating vital adjustments of Ca(2+) entry. The long-held structural correlation of this regulation has been Ca(2+)-bound calmodulin, complexed alone with an IQ domain on the channel carboxy terminus. Here, however, systematic alanine mutagenesis of the entire carboxyl tail of an L-type CaV1.3 channel casts doubt on this paradigm. To identify the actual molecular states underlying channel regulation, we develop a structure-function approach relating the strength of regulation to the affinity of underlying calmodulin/channel interactions, by a Langmuir relation (individually transformed Langmuir analysis). Accordingly, we uncover frank exchange of Ca(2+)-calmodulin to interfaces beyond the IQ domain, initiating substantial rearrangements of the calmodulin/channel complex. The N-lobe of Ca(2+)-calmodulin binds an N-terminal spatial Ca(2+) transforming element module on the channel amino terminus, whereas the C-lobe binds an EF-hand region upstream of the IQ domain. This system of structural plasticity furnishes a next-generation blueprint for CaV channel modulation.
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Affiliation(s)
- Manu Ben Johny
- Calcium Signals Laboratory, Departments of Biomedical Engineering and Neuroscience, The Johns Hopkins University School of Medicine, Ross Building, Room 713, 720 Rutland Avenue, Baltimore, MD 21205, voice: (410) 955-0078, fax: (410) 614-8269,
| | - Philemon S. Yang
- Calcium Signals Laboratory, Departments of Biomedical Engineering and Neuroscience, The Johns Hopkins University School of Medicine, Ross Building, Room 713, 720 Rutland Avenue, Baltimore, MD 21205, voice: (410) 955-0078, fax: (410) 614-8269,
| | - Hojjat Bazzazi
- Calcium Signals Laboratory, Departments of Biomedical Engineering and Neuroscience, The Johns Hopkins University School of Medicine, Ross Building, Room 713, 720 Rutland Avenue, Baltimore, MD 21205, voice: (410) 955-0078, fax: (410) 614-8269,
| | - David T. Yue
- Calcium Signals Laboratory, Departments of Biomedical Engineering and Neuroscience, The Johns Hopkins University School of Medicine, Ross Building, Room 713, 720 Rutland Avenue, Baltimore, MD 21205, voice: (410) 955-0078, fax: (410) 614-8269,
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72
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Stockner T, Koschak A. What can naturally occurring mutations tell us about Ca(v)1.x channel function? BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2012; 1828:1598-607. [PMID: 23219801 PMCID: PMC3787742 DOI: 10.1016/j.bbamem.2012.11.026] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 11/02/2012] [Revised: 11/16/2012] [Accepted: 11/17/2012] [Indexed: 11/18/2022]
Abstract
Voltage-gated Ca2 + channels allow for Ca2 +-dependent intracellular signaling by directly mediating Ca2 + ion influx, by physical coupling to intracellular Ca2 + release channels or functional coupling to other ion channels such as Ca2 + activated potassium channels. L-type Ca2 + channels that comprise the family of Cav1 channels are expressed in many electrically excitable tissues and are characterized by their unique sensitivity to dihydropyridines. In this issue, we summarize genetic defects in L-type Ca2 + channels and analyze their role in human diseases (Ca2 + channelopathies); e.g. mutations in Cav1.2 α1 cause Timothy and Brugada syndrome, mutations in Cav1.3 α1 are linked to sinoatrial node dysfunction and deafness while mutations in Cav1.4 α1 are associated with X-linked retinal disorders such as an incomplete form of congenital stationary night blindness. Herein, we also put the mutations underlying the channel's dysfunction into the structural context of the pore-forming α1 subunit. This analysis highlights the importance of combining functional data with structural analysis to gain a deeper understanding for the disease pathophysiology as well as for physiological channel function. This article is part of a Special Issue entitled: Calcium channels.
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Affiliation(s)
- Thomas Stockner
- Medical University Vienna, Center for Physiology and Pharmacology, Department of Pharmacology, Währingerstrasse 13A, 1090 Vienna, Austria
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73
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Qu Z, Chung D. Mechanisms and determinants of ultralong action potential duration and slow rate-dependence in cardiac myocytes. PLoS One 2012; 7:e43587. [PMID: 22952713 PMCID: PMC3428352 DOI: 10.1371/journal.pone.0043587] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/25/2012] [Accepted: 07/23/2012] [Indexed: 11/28/2022] Open
Abstract
In normal cardiac myocytes, the action potential duration (APD) is several hundred milliseconds. However, experimental studies showed that under certain conditions, APD could be excessively long (or ultralong), up to several seconds. Unlike the normal APD, the ultralong APD increases sensitively with pacing cycle length even when the pacing rate is very slow, exhibiting a sensitive slow rate-dependence. In addition, these long action potentials may or may not exhibit early afterdepolarizations (EADs). Although these phenomena are well known, the underlying mechanisms and ionic determinants remain incompletely understood. In this study, computer simulations were performed with a simplified action potential model. Modifications to the L-type calcium current (ICa,L) kinetics and the activation time constant of the delayed rectifier K current were used to investigate their effects on APD. We show that: 1) the ultralong APD and its sensitive slow rate-dependence are determined by the steady-state window and pedestal ICa,L currents and the activation speed and the recovery of the delayed rectifier K current; 2) whether an ultralong action potential exhibits EADs or not depends on the kinetics of ICa,L; 3) increasing inward currents elevates the plateau voltage, which in general prolongs APD, however, this can also shorten APD when the APD is already ultralong under certain conditions; and 4) APD alternans occurs at slow pacing rates due to the sensitive slow rate-dependence and the ionic determinants are different from the ones causing APD alternans at fast heart rates.
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Affiliation(s)
- Zhilin Qu
- Department of Medicine (Cardiology), David Geffen School of Medicine, University of California Los Angeles, Los Angeles, California, United States of America.
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74
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Swaminathan PD, Purohit A, Hund TJ, Anderson ME. Calmodulin-dependent protein kinase II: linking heart failure and arrhythmias. Circ Res 2012; 110:1661-77. [PMID: 22679140 DOI: 10.1161/circresaha.111.243956] [Citation(s) in RCA: 215] [Impact Index Per Article: 17.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Understanding relationships between heart failure and arrhythmias, important causes of suffering and sudden death, remains an unmet goal for biomedical researchers and physicians. Evidence assembled over the past decade supports a view that activation of the multifunctional Ca(2+) and calmodulin-dependent protein kinase II (CaMKII) favors myocardial dysfunction and cell membrane electrical instability. CaMKII activation follows increases in intracellular Ca(2+) or oxidation, upstream signals with the capacity to transition CaMKII into a Ca(2+) and calmodulin-independent constitutively active enzyme. Constitutively active CaMKII appears poised to participate in disease pathways by catalyzing the phosphorylation of classes of protein targets important for excitation-contraction coupling and cell survival, including ion channels and Ca(2+) homeostatic proteins, and transcription factors that drive hypertrophic and inflammatory gene expression. This rich diversity of downstream targets helps to explain the potential for CaMKII to simultaneously affect mechanical and electrical properties of heart muscle cells. Proof-of-concept studies from a growing number of investigators show that CaMKII inhibition is beneficial for improving myocardial performance and for reducing arrhythmias. We review the molecular physiology of CaMKII and discuss CaMKII actions at key cellular targets and results of animal models of myocardial hypertrophy, dysfunction, and arrhythmias that suggest CaMKII inhibition may benefit myocardial function while reducing arrhythmias.
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Affiliation(s)
- Paari Dominic Swaminathan
- Division of Cardiovascular Medicine, Department of Internal Medicine, Carver College of Medicine, University of Iowa, Iowa City, IA 52242, USA
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75
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Dynamics of early afterdepolarization-mediated triggered activity in cardiac monolayers. Biophys J 2012; 102:2706-14. [PMID: 22735520 DOI: 10.1016/j.bpj.2012.05.011] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2011] [Revised: 04/22/2012] [Accepted: 05/08/2012] [Indexed: 11/20/2022] Open
Abstract
Early afterdepolarizations (EADs) are voltage oscillations that occur during the repolarizing phase of the cardiac action potential and cause cardiac arrhythmias in a variety of clinical settings. EADs occur in the setting of reduced repolarization reserve and increased inward-over-outward currents, which intuitively explains the repolarization delay but does not mechanistically explain the time-dependent voltage oscillations that are characteristic of EADs. In a recent theoretical study, we identified a dual Hopf-homoclinic bifurcation as a dynamical mechanism that causes voltage oscillations during EADs, depending on the amplitude and kinetics of the L-type Ca(2+) channel (LTCC) current relative to the repolarizing K(+) currents. Here we demonstrate this mechanism experimentally. We show that cardiac monolayers exposed to the LTCC agonists BayK8644 and isoproterenol produce EAD bursts that are suppressed by the LTCC blocker nitrendipine but not by the Na(+) current blocker tetrodoxin, depletion of intracellular Ca(2+) stores with thapsigargin and caffeine, or buffering of intracellular Ca(2+) with BAPTA-AM. These EAD bursts exhibited a key dynamical signature of the dual Hopf-homoclinic bifurcation mechanism, namely, a gradual slowing in the frequency of oscillations before burst termination. A detailed cardiac action potential model reproduced the experimental observations, and identified intracellular Na(+) accumulation as the likely mechanism for terminating EAD bursts. Our findings in cardiac monolayers provide direct support for the Hopf-homoclinic bifurcation mechanism of EAD-mediated triggered activity, and raise the possibility that this mechanism may also contribute to EAD formation in clinical settings such as long QT syndromes, heart failure, and increased sympathetic output.
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76
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Morotti S, Grandi E, Summa A, Ginsburg KS, Bers DM. Theoretical study of L-type Ca(2+) current inactivation kinetics during action potential repolarization and early afterdepolarizations. J Physiol 2012; 590:4465-81. [PMID: 22586219 DOI: 10.1113/jphysiol.2012.231886] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
Sarcoplasmic reticulum (SR) Ca(2+) release mediates excitation–contraction coupling (ECC) in cardiac myocytes. It is triggered upon membrane depolarization by entry of Ca(2+) via L-type Ca(2+) channels (LTCCs), which undergo both voltage- and Ca(2+)-dependent inactivation (VDI and CDI, respectively). We developed improved models of L-type Ca(2+) current and SR Ca(2+) release within the framework of the Shannon-Bers rabbit ventricular action potential (AP) model. The formulation of SR Ca(2+) release was modified to reproduce high ECC gain at negative membrane voltages. An existing LTCC model was extended to reflect more faithfully contributions of CDI and VDI to total inactivation. Ba(2+) current inactivation included an ion-dependent component (albeit small compared with CDI), in addition to pure VDI. Under physiological conditions (during an AP) LTCC inactivates predominantly via CDI, which is controlled mostly by SR Ca(2+) release during the initial AP phase, but by Ca(2+) through LTCCs for the remaining part. Simulations of decreased CDI or K(+) channel block predicted the occurrence of early and delayed after depolarizations. Our model accurately describes ECC and allows dissection of the relative contributions of different Ca(2+) sources to total CDI, and the relative roles of CDI and VDI, during normal and abnormal repolarization.
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Affiliation(s)
- Stefano Morotti
- Department of Pharmacology, University of California, Davis, CA 95616-8636, USA
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77
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Abstract
Due to its high external and low internal concentration the Ca(2+) ion is used ubiquitously as an intracellular signaling molecule, and a great many Ca(2+)-sensing proteins have evolved to receive and propagate Ca(2+) signals. Among them are ion channel proteins, whose Ca(2+) sensitivity allows internal Ca(2+) to influence the electrical activity of cell membranes and to feedback-inhibit further Ca(2+) entry into the cytoplasm. In this review I will describe what is understood about the Ca(2+) sensing mechanisms of the three best studied classes of Ca(2+)-sensitive ion channels: Large-conductance Ca(2+)-activated K(+) channels, small-conductance Ca(2+)-activated K(+) channels, and voltage- gated Ca(2+) channels. Great strides in mechanistic understanding have be made for each of these channel types in just the past few years.
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Affiliation(s)
- Daniel H Cox
- Department of Neuroscience, Tufts University School of Medicine, Boston MA, 02420, USA.
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78
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Christel C, Lee A. Ca2+-dependent modulation of voltage-gated Ca2+ channels. Biochim Biophys Acta Gen Subj 2011; 1820:1243-52. [PMID: 22223119 DOI: 10.1016/j.bbagen.2011.12.012] [Citation(s) in RCA: 69] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/05/2011] [Revised: 12/15/2011] [Accepted: 12/16/2011] [Indexed: 01/06/2023]
Abstract
BACKGROUND Voltage-gated (Cav) Ca2+ channels are multi-subunit complexes that play diverse roles in a wide variety of tissues. A fundamental mechanism controlling Cav channel function involves the Ca2+ ions that permeate the channel pore. Ca2+ influx through Cav channels mediates feedback regulation to the channel that is both negative (Ca2+-dependent inactivation, CDI) and positive (Ca2+-dependent facilitation, CDF). SCOPE OF REVIEW This review highlights general mechanisms of CDI and CDF with an emphasis on how these processes have been studied electrophysiologically in native and heterologous expression systems. MAJOR CONCLUSIONS Electrophysiological analyses have led to detailed insights into the mechanisms and prevalence of CDI and CDF as Cav channel regulatory mechanisms. All Cav channel family members undergo some form of Ca2+-dependent feedback that relies on CaM or a related Ca2+ binding protein. Tremendous progress has been made in characterizing the role of CaM in CDI and CDF. Yet, what contributes to the heterogeneity of CDI/CDF in various cell-types and how Ca2+-dependent regulation of Cav channels controls Ca2+ signaling remain largely unexplored. GENERAL SIGNIFICANCE Ca2+ influx through Cav channels regulates diverse physiological events including excitation-contraction coupling in muscle, neurotransmitter and hormone release, and Ca2+-dependent gene transcription. Therefore, the mechanisms that regulate channels, such as CDI and CDF, can have a large impact on the signaling potential of excitable cells in various physiological contexts. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.
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Affiliation(s)
- Carl Christel
- Department of Molecular Physiology and Biophysics, University of Iowa, Iowa City, IA 52242, USA
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79
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Different subcellular populations of L-type Ca2+ channels exhibit unique regulation and functional roles in cardiomyocytes. J Mol Cell Cardiol 2011; 52:376-87. [PMID: 21888911 DOI: 10.1016/j.yjmcc.2011.08.014] [Citation(s) in RCA: 61] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/14/2011] [Revised: 07/11/2011] [Accepted: 08/17/2011] [Indexed: 11/23/2022]
Abstract
Influx of Ca(2+) through L-type Ca(2+) channels (LTCCs) contributes to numerous cellular processes in cardiomyocytes including excitation-contraction (EC) coupling, membrane excitability, and transcriptional regulation. Distinct subpopulations of LTCCs have been identified in cardiac myocytes, including those at dyadic junctions and within different plasma membrane microdomains such as lipid rafts and caveolae. These subpopulations of LTCCs exhibit regionally distinct functional properties and regulation, affording precise spatiotemporal modulation of L-type Ca(2+) current (I(Ca,L)). Different subcellular LTCC populations demonstrate variable rates of Ca(2+)-dependent inactivation and sometimes coupled gating of neighboring channels, which can lead to focal, persistent I(Ca,L). In addition, the assembly of spatially defined macromolecular signaling complexes permits compartmentalized regulation of I(Ca,L) by a variety of neurohormonal pathways. For example, β-adrenergic receptor subtypes signal to different LTCC subpopulations, with β(2)-adrenergic activation leading to enhanced I(Ca,L) through caveolar LTCCs and β(1)-adrenergic stimulation modulating LTCCs outside of caveolae. Disruptions in the normal subcellular targeting of LTCCs and associated signaling proteins may contribute to the pathophysiology of a variety of cardiac diseases including heart failure and certain arrhythmias. Further identifying the characteristic functional properties and array of regulatory molecules associated with specific LTCC subpopulations will provide a mechanistic framework to understand how LTCCs contribute to diverse cellular processes in normal and diseased myocardium. This article is part of a Special Issue entitled "Local Signaling in Myocytes".
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Saucerman JJ, Bers DM. Calmodulin binding proteins provide domains of local Ca2+ signaling in cardiac myocytes. J Mol Cell Cardiol 2011; 52:312-6. [PMID: 21708171 DOI: 10.1016/j.yjmcc.2011.06.005] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/07/2011] [Revised: 05/24/2011] [Accepted: 06/02/2011] [Indexed: 11/25/2022]
Abstract
Calmodulin (CaM) acts as a common Ca(2+) sensor for many signaling pathways, transducing local Ca(2+) signals into specific cellular outcomes. Many of CaM's signaling functions can be explained by its unique biochemical properties, including high and low affinity Ca(2+)-binding sites with slow and fast kinetics, respectively. CaM is expected to have a limited spatial range of action, emphasizing its role in local Ca(2+) signaling. Interactions with target proteins further fine-tune CaM signal transduction. Here, we focus on only three specific cellular targets for CaM signaling in cardiac myocytes: the L-type Ca(2+) channel, the ryanodine receptor, and the IP(3) receptor. We elaborate a working hypothesis that each channel is regulated by two distinct functional populations of CaM: dedicated CaM and promiscuous CaM. Dedicated CaM is typically tethered to each channel and directly regulates channel activity. In addition, a local pool of promiscuous CaM appears poised to sense local Ca(2+) signals and trigger downstream pathways such as Ca(2+)/CaM dependent-protein kinase II and calcineurin. Understanding how promiscuous CaM coordinates multiple distinct signaling pathways remains a challenge, but is aided by the use of mathematical modeling and a new generation of fluorescent biosensors. This article is part of a special issue entitled "Local Signaling in Myocytes."
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Affiliation(s)
- Jeffrey J Saucerman
- Department of Biomedical Engineering, Robert M. Berne Cardiovascular Research Center, University of Virginia, Charlottesville, VA 22908, USA.
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81
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Fernández-Velasco M, Ruiz-Hurtado G, Rueda A, Neco P, Mercado-Morales M, Delgado C, Napolitano C, Priori SG, Richard S, María Gómez A, Benitah JP. RyRCa2+ leak limits cardiac Ca2+ window current overcoming the tonic effect of calmodulinin mice. PLoS One 2011; 6:e20863. [PMID: 21673970 PMCID: PMC3108979 DOI: 10.1371/journal.pone.0020863] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2010] [Accepted: 05/13/2011] [Indexed: 11/19/2022] Open
Abstract
Ca2+ mediates the functional coupling between L-type Ca2+ channel (LTCC) and sarcoplasmic reticulum (SR) Ca2+ release channel (ryanodine receptor, RyR), participating in key pathophysiological processes. This crosstalk manifests as the orthograde Ca2+-induced Ca2+-release (CICR) mechanism triggered by Ca2+ influx, but also as the retrograde Ca2+-dependent inactivation (CDI) of LTCC, which depends on both Ca2+ permeating through the LTCC itself and on SR Ca2+ release through the RyR. This latter effect has been suggested to rely on local rather than global Ca2+ signaling, which might parallel the nanodomain control of CDI carried out through calmodulin (CaM). Analyzing the CICR in catecholaminergic polymorphic ventricular tachycardia (CPVT) mice as a model of RyR-generated Ca2+ leak, we evidence here that increased occurrence of the discrete local SR Ca2+ releases through the RyRs (Ca2+ sparks) causea depolarizing shift in activation and a hyperpolarizing shift inisochronic inactivation of cardiac LTCC current resulting in the reduction of window current. Both increasing fast [Ca2+]i buffer capacity or depleting SR Ca2+ store blunted these changes, which could be reproduced in WT cells by RyRCa2+ leak induced with Ryanodol and CaM inhibition.Our results unveiled a new paradigm for CaM-dependent effect on LTCC gating and further the nanodomain Ca2+ control of LTCC, emphasizing the importance of spatio-temporal relationships between Ca2+ signals and CaM function.
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Affiliation(s)
- María Fernández-Velasco
- Inserm, U637, Université Montpellier-1, Université Montpellier-2, Montpellier, France
- Instituto de Investigaciones Biomédicas Alberto Sols CSIC-UAM, Madrid, Spain
| | - Gema Ruiz-Hurtado
- Inserm, U637, Université Montpellier-1, Université Montpellier-2, Montpellier, France
- Inserm, U769, IFR141, Faculté de Pharmacie, Université Paris-Sud 11, Chatenay-Malabry, France
| | - Angélica Rueda
- Inserm, U637, Université Montpellier-1, Université Montpellier-2, Montpellier, France
- Department of Biochemistry, CINVESTAV, Mexico City, Mexico
| | - Patricia Neco
- Inserm, U637, Université Montpellier-1, Université Montpellier-2, Montpellier, France
- Inserm, U769, IFR141, Faculté de Pharmacie, Université Paris-Sud 11, Chatenay-Malabry, France
| | | | - Carmen Delgado
- Departamento de Farmacología, Facultad de Medicina, Universidad Complutense, CyB, CSIC, Madrid, Spain
| | - Carlo Napolitano
- Molecular Cardiology, Fondazione Salvatore Maugeri, Pavia, Italy
- Cardiovascular Genetics, Leon Charney Division of Cardiology, Langone Medical Center, New York University School of Medicine, New York, United States of America
| | - Silvia G. Priori
- Molecular Cardiology, Fondazione Salvatore Maugeri, Pavia, Italy
- Cardiovascular Genetics, Leon Charney Division of Cardiology, Langone Medical Center, New York University School of Medicine, New York, United States of America
- Department of Cardiology, University of Pavia, Italy
| | - Sylvain Richard
- Inserm, U637, Université Montpellier-1, Université Montpellier-2, Montpellier, France
- Inserm, U1046, Université Montpellier-1, Université Montpellier-2, Montpellier, France
| | - Ana María Gómez
- Inserm, U637, Université Montpellier-1, Université Montpellier-2, Montpellier, France
- Inserm, U769, IFR141, Faculté de Pharmacie, Université Paris-Sud 11, Chatenay-Malabry, France
| | - Jean-Pierre Benitah
- Inserm, U637, Université Montpellier-1, Université Montpellier-2, Montpellier, France
- Inserm, U769, IFR141, Faculté de Pharmacie, Université Paris-Sud 11, Chatenay-Malabry, France
- * E-mail:
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82
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Findeisen F, Minor DL. Structural basis for the differential effects of CaBP1 and calmodulin on Ca(V)1.2 calcium-dependent inactivation. Structure 2011; 18:1617-31. [PMID: 21134641 DOI: 10.1016/j.str.2010.09.012] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2010] [Revised: 09/08/2010] [Accepted: 09/17/2010] [Indexed: 01/01/2023]
Abstract
Calcium-binding protein 1 (CaBP1), a calmodulin (CaM) homolog, endows certain voltage-gated calcium channels (Ca(V)s) with unusual properties. CaBP1 inhibits Ca(V)1.2 calcium-dependent inactivation (CDI) and introduces calcium-dependent facilitation (CDF). Here, we show that the ability of CaBP1 to inhibit Ca(V)1.2 CDI and induce CDF arises from interaction between the CaBP1 N-lobe and interlobe linker residue Glu94. Unlike CaM, where functional EF hands are essential for channel modulation, CDI inhibition does not require functional CaBP1 EF hands. Furthermore, CaBP1-mediated CDF has different molecular requirements than CaM-mediated CDF. Overall, the data show that CaBP1 comprises two structural modules having separate functions: similar to CaM, the CaBP1 C-lobe serves as a high-affinity anchor that binds the Ca(V)1.2 IQ domain at a site that overlaps with the Ca²+/CaM C-lobe site, whereas the N-lobe/linker module houses the elements required for channel modulation. Discovery of this division provides the framework for understanding how CaBP1 regulates Ca(V)s.
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Affiliation(s)
- Felix Findeisen
- Cardiovascular Research Institute, University of California, San Francisco, CA 94158-2330, USA
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83
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Abstract
Excitation-contraction coupling in the cardiac myocyte is mediated by a number of highly integrated mechanisms of intracellular Ca²(+) transport. The complexity and integrative nature of heart cell electrophysiology and Ca²(+) cycling has led to an evolution of computational models that have played a crucial role in shaping our understanding of heart function. An important emerging theme in systems biology is that the detailed nature of local signaling events, such as those that occur in the cardiac dyad, have important consequences at higher biological scales. Multiscale modeling techniques have revealed many mechanistic links between microscale events, such as Ca²(+) binding to a channel protein, and macroscale phenomena, such as excitation-contraction coupling gain. Here, we review experimentally based multiscale computational models of excitation-contraction coupling and the insights that have been gained through their application.
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Affiliation(s)
- Joseph L Greenstein
- Center for Cardiovascular Bioinformatics and Modeling, Whitaker Biomedical Engineering Institute, The Johns Hopkins University, Baltimore, MD, USA
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84
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Winslow RL, Cortassa S, O'Rourke B, Hashambhoy YL, Rice JJ, Greenstein JL. Integrative modeling of the cardiac ventricular myocyte. WILEY INTERDISCIPLINARY REVIEWS-SYSTEMS BIOLOGY AND MEDICINE 2010; 3:392-413. [PMID: 20865780 DOI: 10.1002/wsbm.122] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Cardiac electrophysiology is a discipline with a rich 50-year history of experimental research coupled with integrative modeling which has enabled us to achieve a quantitative understanding of the relationships between molecular function and the integrated behavior of the cardiac myocyte in health and disease. In this paper, we review the development of integrative computational models of the cardiac myocyte. We begin with a historical overview of key cardiac cell models that helped shape the field. We then narrow our focus to models of the cardiac ventricular myocyte and describe these models in the context of their subcellular functional systems including dynamic models of voltage-gated ion channels, mitochondrial energy production, ATP-dependent and electrogenic membrane transporters, intracellular Ca dynamics, mechanical contraction, and regulatory signal transduction pathways. We describe key advances and limitations of the models as well as point to new directions for future modeling research. WIREs Syst Biol Med 2011 3 392-413 DOI: 10.1002/wsbm.122
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Affiliation(s)
- Raimond L Winslow
- Institute of Computational Medicine and Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD, USA.
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85
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Striessnig J, Bolz HJ, Koschak A. Channelopathies in Cav1.1, Cav1.3, and Cav1.4 voltage-gated L-type Ca2+ channels. Pflugers Arch 2010; 460:361-74. [PMID: 20213496 PMCID: PMC2883925 DOI: 10.1007/s00424-010-0800-x] [Citation(s) in RCA: 106] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2010] [Revised: 02/03/2010] [Accepted: 02/05/2010] [Indexed: 12/24/2022]
Abstract
Voltage-gated Ca2+ channels couple membrane depolarization to Ca2+-dependent intracellular signaling events. This is achieved by mediating Ca2+ ion influx or by direct conformational coupling to intracellular Ca2+ release channels. The family of Cav1 channels, also termed L-type Ca2+ channels (LTCCs), is uniquely sensitive to organic Ca2+ channel blockers and expressed in many electrically excitable tissues. In this review, we summarize the role of LTCCs for human diseases caused by genetic Ca2+ channel defects (channelopathies). LTCC dysfunction can result from structural aberrations within their pore-forming alpha1 subunits causing hypokalemic periodic paralysis and malignant hyperthermia sensitivity (Cav1.1 alpha1), incomplete congenital stationary night blindness (CSNB2; Cav1.4 alpha1), and Timothy syndrome (Cav1.2 alpha1; reviewed separately in this issue). Cav1.3 alpha1 mutations have not been reported yet in humans, but channel loss of function would likely affect sinoatrial node function and hearing. Studies in mice revealed that LTCCs indirectly also contribute to neurological symptoms in Ca2+ channelopathies affecting non-LTCCs, such as Cav2.1 alpha1 in tottering mice. Ca2+ channelopathies provide exciting disease-related molecular detail that led to important novel insight not only into disease pathophysiology but also to mechanisms of channel function.
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Affiliation(s)
- Jörg Striessnig
- Pharmacology and Toxicology, Institute of Pharmacy, and Center for Molecular Biosciences, University of Innsbruck, Peter-Mayr-Strasse 1, 6020, Innsbruck, Austria.
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86
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Pang C, Crump SM, Jin L, Correll RN, Finlin BS, Satin J, Andres DA. Rem GTPase interacts with the proximal CaV1.2 C-terminus and modulates calcium-dependent channel inactivation. Channels (Austin) 2010; 4:192-202. [PMID: 20458179 DOI: 10.4161/chan.4.3.11867] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
The Rem, Rem2, Rad, and Gem/Kir (RGK) GTPases, comprise a subfamily of small Ras-related GTP-binding proteins, and have been shown to potently inhibit high voltage-activated Ca(2+) channel current following overexpression. Although the molecular mechanisms underlying RGK-mediated Ca(2+) channel regulation remains controversial, recent studies suggest that RGK proteins inhibit Ca(2+) channel currents at the plasma membrane in part by interactions with accessory channel β subunits. In this paper, we extend our understanding of the molecular determinants required for RGK-mediated channel regulation by demonstrating a direct interaction between Rem and the proximal C-terminus of Ca(V)1.2 (PCT), including the CB/IQ domain known to contribute to Ca(2+)/calmodulin (CaM)-mediated channel regulation. The Rem2 and Rad GTPases display similar patterns of PCT binding, suggesting that the Ca(V)1.2 C-terminus represents a common binding partner for all RGK proteins. In vitro Rem:PCT binding is disrupted by Ca(2+)/CaM, and this effect is not due to Ca(2+)/CaM binding to the Rem C-terminus. In addition, co-overexpression of CaM partially relieves Rem-mediated L-type Ca(2+) channel inhibition and slows the kinetics of Ca(2+)-dependent channel inactivation. Taken together, these results suggest that the association of Rem with the PCT represents a crucial molecular determinant in RGK-mediated Ca(2+) channel regulation and that the physiological function of the RGK GTPases must be re-evaluated. Rather than serving as endogenous inhibitors of Ca(2+) channel activity, these studies indicate that RGK proteins may play a more nuanced role, regulating Ca(2+) currents via modulation of Ca(2+)/CaM-mediated channel inactivation kinetics.
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Affiliation(s)
- Chunyan Pang
- Department of Molecular and Cellular Biochemistry and Physiology, University of Kentucky College of Medicine, Lexington, USA
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87
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CaV1.2 beta-subunit coordinates CaMKII-triggered cardiomyocyte death and afterdepolarizations. Proc Natl Acad Sci U S A 2010; 107:4996-5000. [PMID: 20194790 DOI: 10.1073/pnas.0913760107] [Citation(s) in RCA: 101] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Excessive activation of calmodulin kinase II (CaMKII) causes arrhythmias and heart failure, but the cellular mechanisms for CaMKII-targeted proteins causing disordered cell membrane excitability and myocardial dysfunction remain uncertain. Failing human cardiomyocytes exhibit increased CaMKII and voltage-gated Ca(2+) channel (Ca(V)1.2) activity, and enhanced expression of a specific Ca(V)1.2 beta-subunit protein isoform (beta(2a)). We recently identified Ca(V)1.2 beta(2a) residues critical for CaMKII phosphorylation (Thr 498) and binding (Leu 493), suggesting the hypothesis that these amino acids are crucial for cardiomyopathic consequences of CaMKII signaling. Here we show WT beta(2a) expression causes cellular Ca(2+) overload, arrhythmia-triggering cell membrane potential oscillations called early afterdepolarizations (EADs), and premature death in paced adult rabbit ventricular myocytes. Prevention of intracellular Ca(2+) release by ryanodine or global cellular CaMKII inhibition reduced EADs and improved cell survival to control levels in WT beta(2a)-expressing ventricular myocytes. In contrast, expression of beta(2a) T498A or L493A mutants mimicked the protective effects of ryanodine or global cellular CaMKII inhibition by reducing Ca(2+) entry through Ca(V)1.2 and inhibiting EADs. Furthermore, Ca(V)1.2 currents recorded from cells overexpressing CaMKII phosphorylation- or binding-incompetent beta(2a) subunits were incapable of entering a CaMKII-dependent high-activity gating mode (mode 2), indicating that beta(2a) Thr 498 and Leu 493 are required for Ca(V)1.2 activation by CaMKII in native cells. These data show that CaMKII binding and phosphorylation sites on beta(2a) are concise but pivotal components of a molecular and biophysical and mechanism for EADs and impaired survival in adult cardiomyocytes.
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88
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Interplay of voltage and Ca-dependent inactivation of L-type Ca current. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2010; 103:44-50. [PMID: 20184915 DOI: 10.1016/j.pbiomolbio.2010.02.001] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/13/2010] [Accepted: 02/16/2010] [Indexed: 11/22/2022]
Abstract
Inactivation of L-type Ca channels (LTCC) is regulated by both Ca and voltage-dependent processes (CDI and VDI). To differentiate VDI and CDI, several experimental and theoretical studies have considered the inactivation of Ba current through LTCC (I(Ba)) as a measure of VDI. However, there is evidence that Ba can weakly mimic Ca, such that I(Ba) inactivation is still a mixture of CDI and VDI. To avoid this complication, some have used the monovalent cation current through LTCC (I(NS)), which can be measured when divalent cation concentrations are very low. Notably, I(NS) inactivation rate does not depend on current amplitude, and hence may reflect purely VDI. However, based on analysis of existent and new data, and modeling, we find that I(NS) can inactivate more rapidly and completely than I(Ba), especially at physiological temperature. Thus VDI that occurs during I(Ba) (or I(Ca)) must differ intrinsically from VDI during I(NS). To account for this, we have extended a previously published LTCC mathematical model of VDI and CDI into an excitation-contraction coupling model, and assessed whether and how experimental I(Ba) inactivation results (traditionally used in VDI experiments and models) could be recapitulated by modifying CDI to account for Ba-dependent inactivation. Thus, the view of a slow and incomplete I(NS) inactivation should be revised, and I(NS) inactivation is a poor measure of VDI during I(Ca) or I(Ba). This complicates VDI analysis experimentally, but raises intriguing new questions about how the molecular mechanisms of VDI differ for divalent and monovalent currents through LTCCs.
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89
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Liu X, Yang PS, Yang W, Yue DT. Enzyme-inhibitor-like tuning of Ca(2+) channel connectivity with calmodulin. Nature 2010; 463:968-72. [PMID: 20139964 PMCID: PMC3553577 DOI: 10.1038/nature08766] [Citation(s) in RCA: 80] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/16/2009] [Accepted: 12/04/2009] [Indexed: 11/23/2022]
Abstract
Ca2+ channels and calmodulin are two prominent signaling hubs1 that synergistically impact functions as diverse as cardiac excitability2, synaptic plasticity3, and gene transcription4. It is thereby fitting that these hubs are in some sense coordinated, as the opening of CaV1-2 Ca2+ channels are regulated by a single calmodulin (CaM) constitutively complexed with channels5. The Ca2+-free form of CaM (apoCaM) is already preassociated with the IQ domain on the channel carboxy terminus, and subsequent Ca2+ binding to this ‘resident’ CaM drives conformational changes that then trigger regulation of channel opening6. Another potential avenue for channel-CaM coordination could arise from the absence of Ca2+ regulation in channels lacking a preassociated CaM6,7. Natural fluctuations in CaM levels might then influence the fraction of regulatable channels, and thereby the overall strength of Ca2+ feedback. However, the prevailing view has been that the ultra-strong affinity of channels for apoCaM ensures their saturation with CaM8, yielding a significant form of concentration independence between Ca2+ channels and CaM. Here, we reveal significant exceptions to this autonomy, by combining electrophysiology to characterize channel regulation, with optical FRET sensor determination of free apoCaM concentration in live cells9. This approach translates quantitative CaM biochemistry from the traditional test-tube context, into the realm of functioning holochannels within intact cells. From this perspective, we find that long splice forms of CaV1.3 and CaV1.4 channels include a distal carboxy tail10-12 that resembles an enzyme competitive inhibitor, which retunes channel affinity for apoCaM so that natural CaM variations affect the strength of Ca2+ feedback modulation. Given the ubiquity of these channels13,14, the connection between ambient CaM levels and Ca2+ entry via channels is broadly significant for Ca2+ homeostasis. Strategies like ours promise key advances for the in situ analysis of signaling molecules resistant to in vitro reconstitution, such as Ca2+ channels.
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Affiliation(s)
- Xiaodong Liu
- Calcium Signals Laboratory, Department of Biomedical Engineering, The Johns Hopkins University School of Medicine, Ross Building, Room 713, 720 Rutland Avenue, Baltimore, Maryland 21205, USA
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90
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Matthes J, Herzig S. Less is more, or enough is enough? Ca(2+)-dependent inactivation revisited. J Physiol 2010; 588:15-6. [PMID: 19948660 DOI: 10.1113/jphysiol.2009.184846] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Affiliation(s)
- Jan Matthes
- Department of Pharmacology, University of Cologne
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91
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Both N- and C-lobes of calmodulin are required for Ca2+-dependent regulations of CaV1.2 Ca2+ channels. Biochem Biophys Res Commun 2010; 391:1170-6. [DOI: 10.1016/j.bbrc.2009.11.171] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2009] [Accepted: 11/28/2009] [Indexed: 11/24/2022]
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92
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Benitah JP, Alvarez JL, Gómez AM. L-type Ca(2+) current in ventricular cardiomyocytes. J Mol Cell Cardiol 2009; 48:26-36. [PMID: 19660468 DOI: 10.1016/j.yjmcc.2009.07.026] [Citation(s) in RCA: 122] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/15/2009] [Revised: 07/09/2009] [Accepted: 07/27/2009] [Indexed: 12/24/2022]
Abstract
L-type Ca(2+) channels are mediators of Ca(2+) influx and the regulatory events accompanying it and are pivotal in the function and dysfunction of ventricular cardiac myocytes. L-type Ca(2+) channels are located in sarcolemma, including the T-tubules facing the sarcoplasmic reticulum junction, and are activated by membrane depolarization, but intracellular Ca(2+)-dependent inactivation limits Ca(2+) influx during action potential. I(CaL) is important in heart function because it triggers excitation-contraction coupling, modulates action potential shape and is involved in cardiac arrhythmia. L-type Ca(2+) channels are multi-subunit complexes that interact with several molecules involved in their regulations, notably by beta-adrenergic signaling. The present review highlights some of the recent findings on L-type Ca(2+) channel function, regulation, and alteration in acquired pathologies such as cardiac hypertrophy, heart failure and diabetic cardiomyopathy, as well as in inherited arrhythmic cardiac diseases such as Timothy and Brugada syndromes.
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93
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Balasubramanian B, Imredy JP, Kim D, Penniman J, Lagrutta A, Salata JJ. Optimization of Ca(v)1.2 screening with an automated planar patch clamp platform. J Pharmacol Toxicol Methods 2009; 59:62-72. [PMID: 19367686 DOI: 10.1016/j.vascn.2009.02.002] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
Abstract
INTRODUCTION Ca(v)1.2 channels play an important role in shaping the cardiac action potential. Screening pharmaceutical compounds for Ca(v)1.2 block is very important in developing drugs without cardiac liability. Ca(v)1.2 screening has been traditionally done using fluorescence assays, but these assays have some limitations. Patch clamping is considered the gold standard for ion channel studies, but is very labor intensive. The purpose of this study was to develop a robust medium throughput Ca(v)1.2 screening assay in PatchXpress 7000A by optimizing cell isolation conditions, recording solutions and experimental parameters. Under the conditions established, structurally different standard Ca(v)1.2 antagonists and an agonist were tested. METHODS HEK-293 cells stably transfected with hCa(v)1.2 L-type Ca channel were used. For experiments, cells were isolated using 0.05% Trypsin. Currents were recorded in the presence of 30 mM extracellular Ba2+ and low magnesium intracellular recording solution to minimize rundown. Ca(v)1.2 currents were elicited from a holding potential of -60 mV at 0.05 Hz to increase pharmacological sensitivity and minimize rundown. Test compounds were applied at increasing concentrations for 5 min followed by a brief washout. RESULTS Averaged peak Ca(v)1.2 current amplitudes were increased from 10 pA/pF to 15 pA/pF by shortening cell incubation and trypsin exposure time from 2.5 min at 37 degrees C to 1 min at room temperature and adding 0.2 mM cAMP to the intracellular solution. Rundown was minimized from 2%/min to 0.5%/min by reducing the intracellular free Mg2+ from 2.7 mM to 0.2 mM and adding 100 nM Ca2+. Under the established conditions, we tested 8 structurally different antagonists and an agonist. The IC(50) values obtained ranked well against published values and results obtained using traditional clamp experiments performed in parallel using the expressed cell line and native myocytes. DISCUSSION This assay can be used as a reliable pharmacological screening tool for Ca(v)1.2 block to assess compounds for cardiac liability during lead optimization.
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Affiliation(s)
- Bharathi Balasubramanian
- Safety and Exploratory Pharmacology, Safety Assessment, Merck Research Laboratories, West Point, PA 19486, USA.
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94
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Tampo A, Hogan CS, Sedlic F, Bosnjak ZJ, Kwok WM. Accelerated inactivation of cardiac L-type calcium channels triggered by anaesthetic-induced preconditioning. Br J Pharmacol 2009; 156:432-43. [PMID: 19154423 DOI: 10.1111/j.1476-5381.2008.00026.x] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
Abstract
BACKGROUND AND PURPOSE Cardioprotection against ischaemia by anaesthetic-induced preconditioning (APC) is well established. However, the mechanism underlying Ca(2+) overload attenuation by APC is unknown. The effects of APC by isoflurane on the cardiac L-type Ca channel were investigated. EXPERIMENTAL APPROACH In a model of in vivo APC, Wistar rats were exposed to isoflurane (1.4%), delivered via a vaporizer in an enclosure, prior to thoracotomy. The Dahl S rats were similarly preconditioned to determine strain-dependent effects. Whole-cell patch clamp using cardiac ventricular myocytes was used to determine the L-type Ca(2+) current (I(Ca,L)) characteristics and calmodulin (CaM) levels were determined by Western blot analysis. Cytosolic Ca(2+) levels were monitored using fluo-4-AM. Action potential (AP) simulations examined the effects of APC. KEY RESULTS In Wistar rats, APC significantly accelerated I(Ca,L) inactivation kinetics. This was abolished when external Ca(2+) was replaced with Ba(2+), suggesting that Ca(2+)-dependent inactivation of I(Ca,L) was modulated by APC. Expression levels of CaM, a determinant of I(Ca,L) inactivation, were not affected. Attenuation of cytosolic Ca(2+) accumulation following oxidative stress was observed in the APC group. Simulations showed that the accelerated inactivation of I(Ca,L) resulted in a shortening of the AP duration. The Dahl S rat strain was resistant to APC and changes in I(Ca,L) inactivation were not observed in cardiomyocytes prepared from these rats. CONCLUSIONS AND IMPLICATIONS APC triggered persistent changes in the inactivation of cardiac L-type Ca channels. This can potentially lead to a reduction in Ca(2+) influx and attenuation of Ca(2+) overload during ischaemia/reperfusion.
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Affiliation(s)
- A Tampo
- Department of Anesthesiology, Medical College of Wisconsin, Milwaukee, 53226, USA
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95
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Yarotskyy V, Gao G, Peterson BZ, Elmslie KS. The Timothy syndrome mutation of cardiac CaV1.2 (L-type) channels: multiple altered gating mechanisms and pharmacological restoration of inactivation. J Physiol 2008; 587:551-65. [PMID: 19074970 DOI: 10.1113/jphysiol.2008.161737] [Citation(s) in RCA: 59] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Timothy syndrome (TS) is a multiorgan dysfunction caused by a Gly to Arg substitution at position 406 (G406R) of the human CaV1.2 (L-type) channel. The TS phenotype includes severe arrhythmias that are thought to be triggered by impaired open-state voltage-dependent inactivation (OSvdI). The effect of the TS mutation on other L-channel gating mechanisms has yet to be investigated. We compared kinetic properties of exogenously expressed (HEK293 cells) rabbit cardiac L-channels with (G436R; corresponding to position 406 in human clone) and without (wild-type) the TS mutation. Our results surprisingly show that the TS mutation did not affect close-state voltage-dependent inactivation, which suggests different gating mechanisms underlie these two types of voltage-dependent inactivation. The TS mutation also significantly slowed activation at voltages less than 10 mV, and significantly slowed deactivation across all test voltages. Deactivation was slowed in the double mutant G436R/S439A, which suggests that phosphorylation of S439 was not involved. The L-channel agonist Bay K8644 increased the magnitude of both step and tail currents, but surprisingly failed to slow deactivation of TS channels. Our mathematical model showed that slowed deactivation plus impaired OSvdI combine to synergistically increase cardiac action potential duration that is a likely cause of arrhythmias in TS patients. Roscovitine, a tri-substituted purine that enhances L-channel OSvdI, restored TS-impaired OSvdI. Thus, inactivation-enhancing drugs are likely to improve cardiac arrhythmias and other pathologies afflicting TS patients.
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Affiliation(s)
- Viktor Yarotskyy
- Department of Anesthesiology, Penn State College of Medicine, 500 University Drive, Hershey, PA 17033, USA
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96
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Grandi E, Pasqualini FS, Pes C, Corsi C, Zaza A, Severi S. Theoretical investigation of action potential duration dependence on extracellular Ca2+ in human cardiomyocytes. J Mol Cell Cardiol 2008; 46:332-42. [PMID: 19121322 DOI: 10.1016/j.yjmcc.2008.12.002] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/30/2008] [Revised: 11/10/2008] [Accepted: 12/03/2008] [Indexed: 12/20/2022]
Abstract
Reduction in [Ca2+]o prolongs the AP in ventricular cardiomyocytes and the QTc interval in patients. Although this phenomenon is relevant to arrhythmogenesis in the clinical setting, its mechanisms are counterintuitive and incompletely understood. To evaluate in silico the mechanisms of APD modulation by [Ca2+]o in human cardiomyocytes. We implemented the Ten Tusscher-Noble-Noble-Panfilov model of the human ventricular myocyte and modified the formulations of the rapidly and slowly activating delayed rectifier K+ currents (IKr and IKs) and L-type Ca2+ current (ICaL) to incorporate their known sensitivity to intra- or extracellular Ca2+. Simulations were run with the original and modified models at variable [Ca2+]o in the clinically relevant 1 to 3 mM range. The original model responds with APD shortening to decrease in [Ca2+]o, i.e. opposite to the experimental observations. Incorporation of Ca2+ dependency of K+ currents cannot reproduce the inverse relation between APD and [Ca2+]o. Only when ICaL inactivation process was modified, by enhancing its dependency on Ca2+, simulations predict APD prolongation at lower [Ca2+]o. Although Ca2+-dependent ICaL inactivation is the primary mechanism, secondary changes in electrogenic Ca2+ transport (by Na+/Ca2+ exchanger and plasmalemmal Ca2+-ATPase) contribute to the reversal of APD dependency on [Ca2+]o. This theoretical investigation points to Ca2+-dependent inactivation of ICaL as a mechanism primarily responsible for the dependency of APD on [Ca2+]o. The modifications implemented here make the model more suitable to analyze repolarization mechanisms when Ca2+ levels are altered.
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Affiliation(s)
- Eleonora Grandi
- Biomedical Engineering Laboratory-D.E.I.S., University of Bologna, Cesena, Italy
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97
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Kawazu T, Murakami S, Adachi-Akahane S, Findlay I, Ait-Haddou R, Kurachi Y, Nomura T. Microstructure-based Monte Carlo simulation of Ca2+ dynamics evoking cardiac calcium channel inactivation. J Physiol Sci 2008; 58:471-80. [PMID: 18928642 DOI: 10.2170/physiolsci.rp013208] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/25/2008] [Accepted: 10/10/2008] [Indexed: 11/05/2022]
Abstract
Ca(2+) dynamics underlying cardiac excitation-contraction coupling are essential for heart functions. In this study, we constructed microstructure-based models of Ca(2+) dynamics to simulate Ca(2+) influx through individual L-type Calcium channels (LCCs), an effective Ca(2+) diffusion within the cytoplasmic space and in the dyadic space, and the experimentally observed calcium-dependent inactivation (CDI) of the LCCs induced by local and global Ca(2+) sensing. The models consisted of LCCs with distal and proximal Ca(2+) (Calmodulin-Ca(2+) complex) binding sites. In one model, the intra-cellular space was organelle-free cytoplasmic space, and the other was with a dyadic space including sarcoplasmic reticulum membrane. The Ca(2+) dynamics and CDI of the LCCs in the model with and without the dyadic space were then simulated using the Monte Carlo method. We first showed that an appropriate set of parameter values of the models with effectively extra-slow Ca(2+) diffusion enabled the models to reproduce major features of the CDI process induced by the local and global sensing of Ca(2+) near LCCs as measured with single and two spatially separated LCCs by Imredy and Yue (Neuron. 1992;9:197-207). The effective slow Ca(2+) diffusion might be due to association and dissociation of Ca(2+) and Calmodulin (CaM). We then examined how the local and global CDIs were affected by the presence of the dyadic space. The results suggested that in microstructure modeling of Ca(2+) dynamics in cardiac myocytes, the effective Ca(2+) diffusion under CaM-Ca(2+) interaction, the nanodomain structure of LCCs for detailed CDI, and the geometry of subcellular space for modeling dyadic space should be considered.
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Affiliation(s)
- Toshihiro Kawazu
- Division of Bioengineering, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan
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98
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Ohrtman J, Ritter B, Polster A, Beam KG, Papadopoulos S. Sequence differences in the IQ motifs of CaV1.1 and CaV1.2 strongly impact calmodulin binding and calcium-dependent inactivation. J Biol Chem 2008; 283:29301-11. [PMID: 18718913 DOI: 10.1074/jbc.m805152200] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The proximal C terminus of the cardiac L-type calcium channel (Ca(V)1.2) contains structural elements important for the binding of calmodulin (CaM) and calcium-dependent inactivation, and exhibits extensive sequence conservation with the corresponding region of the skeletal L-type channel (Ca(V)1.1). However, there are several Ca(V)1.1 residues that are both identical in six species and are non-conservatively changed from the corresponding Ca(V)1.2 residues, including three of the "IQ motif." To investigate the functional significance of these residue differences, we used native gel electrophoresis and expression in intact myotubes to compare the binding of CaM to extended regions (up to 300 residues) of the C termini of Ca(V)1.1 and Ca(V)1.2. We found that in the presence of Ca(2+) (either millimolar or that in resting myotubes), CaM bound strongly to C termini of Ca(V)1.2 but not of Ca(V)1.1. Furthermore, replacement of two residues (Tyr(1657) and Lys(1662)) within the IQ motif of a C-terminal Ca(V)1.2 construct with the divergent residues of Ca(V)1.1 (His(1532) and Met(1537)) led to a weakening of CaM binding (native gels), whereas the reciprocal substitution in Ca(V)1.1 caused a gain of CaM binding. In full-length Ca(V)1.2, substitution of these same two divergent residues with those of Ca(V)1.1 (Y1657H, K1662M) eliminated calcium-dependent inactivation of the heterologously expressed channel. Thus, our results reveal that a conserved difference between the IQ motifs of Ca(V)1.2 and Ca(V)1.1 has a profound effect on both CaM binding and calcium-dependent inactivation.
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Affiliation(s)
- Joshua Ohrtman
- Department of Physiology and Biophysics, University of Colorado Health Sciences Center, Aurora, Colorado 80045, USA
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99
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Contribution of L-type Ca2+ channels to early afterdepolarizations induced by I Kr and I Ks channel suppression in guinea pig ventricular myocytes. J Membr Biol 2008; 222:151-66. [PMID: 18566732 DOI: 10.1007/s00232-008-9113-9] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2008] [Accepted: 05/16/2008] [Indexed: 10/21/2022]
Abstract
Early afterdepolarizations (EADs) induced by suppression of cardiac delayed rectifier I (Kr) and/or I (Ks) channels cause fatal ventricular tachyarrhythmias. In guinea pig ventricular myocytes, partial block of one of the channels with complete block of the other reproducibly induced EADs. Complete block of both I (Kr) and I (Ks) channels depolarized the take-off potential and reduced the amplitude of EADs, which in some cases were not clearly separated from the preceding action potentials. A selective L-type Ca(2+) (I (Ca,L)) channel blocker, nifedipine, effectively suppressed EADs at submicromolar concentrations. As examined with the action potential-clamp method, I (Ca,L) channels mediated inward currents with a spike and dome shape during action potentials. I (Ca,L) currents decayed mainly due to inactivation in phase 2 and deactivation in phase 3 repolarization. When EADs were induced by complete block of I (Kr) channels with partial block of I (Ks) channels, repolarization of the action potential prior to EAD take-off failed to increase I (K1) currents and thus failed to completely deactivate I (Ca,L) channels, which reactivated and mediated inward currents during EADs. When both I (Kr) and I (Ks) channels were completely blocked, I (Ca,L) channels were not deactivated and mediated sustained inward currents until the end of EADs. Under this condition, the recovery and reactivation of I (Ca,L) channels were absent before EADs. Therefore, an essential mechanism underlying EADs caused by suppression of the delayed rectifiers is the failure to completely deactivate I (Ca,L) channels.
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100
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Sodi R, Dubuis E, Shenkin A, Hart G. B-type natriuretic peptide (BNP) attenuates the L-type calcium current and regulates ventricular myocyte function. ACTA ACUST UNITED AC 2008; 151:95-105. [PMID: 18616964 DOI: 10.1016/j.regpep.2008.06.006] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/12/2008] [Revised: 05/22/2008] [Accepted: 06/15/2008] [Indexed: 10/21/2022]
Abstract
A fundamental question in physiology is how hormones regulate the functioning of a cell or organ. It was therefore the aim of this study to investigate the effect(s) of BNP-32 on calcium handling by ventricular myocytes obtained from the rat left ventricle. We specifically tested the hypothesis that BNP-32 decreased the L-type calcium current (I(Ca,L)). Perforated patch clamp technique was used to record I(Ca,L) and action potential (AP) in voltage and current clamp mode, respectively. Myocyte shortening was measured using a photodiode array edge-detection system and intracellular calcium transients were measured by fluorescence photometry. Western blotting was used to determine the relative change in the expression of proteins. At the concentrations tested, BNP-32 significantly decreased cell shortening in a dose-dependent manner; increased the phase II slope of the AP by 53.0%; increased the APD(50) by 16.9%; reduced the I(Ca,L) amplitude with a 22.9% decrease in the peak amplitude and reduced Ca(2+)-dependent inactivation; increased the V(1/2) activation of the L-type calcium channel by 51.1% and decreased V(1/2) inactivation by 31.8%; and, intracellular calcium transient amplitude was significantly decreased by 32.0%, whereas the time to peak amplitude and T(1/2) were both significantly increased by 38.7% and 89.4% respectively. Sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a) protein expression was reduced by BNP-32. These data suggest that BNP-32 regulates ventricular myocyte function by attenuating I(Ca,L), altering the AP and reducing SERCA2a activity and/or expression. This study suggests a novel constitutive mechanism for the autocrine action of BNP on the L-type calcium channel in ventricular myocytes.
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Affiliation(s)
- R Sodi
- Department of Clinical Biochemistry & Metabolic Medicine, Royal Liverpool & Broadgreen University Hospital, Prescot street, Liverpool L7 8XP, United Kingdom.
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