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(Circulation. 2007;115:300-309.)
© 2007 American Heart Association, Inc.

Arrhythmia/Electrophysiology

Sarcoplasmic Reticulum Calcium Overloading in Junctin Deficiency Enhances Cardiac Contractility but Increases Ventricular Automaticity

Qunying Yuan, MD; Guo-Chang Fan, PhD; Min Dong, BS; Beth Altschafl, PhD; Abhinav Diwan, MD; Xiaoping Ren, MD; Harvey H. Hahn, MD; Wen Zhao, MD, PhD; Jason R. Waggoner, PhD; Larry R. Jones, MD, PhD; W. Keith Jones, PhD; Donald M. Bers, PhD; Gerald W. Dorn, II, MD; Hong-Sheng Wang, PhD; Héctor H. Valdivia, MD, PhD; Guoxiang Chu, MD, PhD; Evangelia G. Kranias, PhD

From the Department of Pharmacology and Cell Biophysics (Q.Y., G.-C.F., M.D., X.R., W.Z., J.R.W., W.K.J., H.-S.W., G.C., E.G.K.) and Department of Medicine (A.D., H.H.H., G.W.D.), University of Cincinnati College of Medicine, Cincinnati, Ohio; Foundation of Biomedical Research of the Academy of Athens (E.G.K.), Athens, Greece; Department of Physiology (B.A., H.H.V.), University of Wisconsin Medical School, Madison; Krannert Institute of Cardiology and the Department of Medicine (L.R.J.), Indiana University School of Medicine, Indianapolis, Ind; and Department of Physiology (D.M.B.), Stritch School of Medicine Loyola University, Maywood, Ill.

Correspondence to Evangelia G. Kranias, PhD, Department of Pharmacology and Cell Biophysics, University of Cincinnati College of Medicine, Cincinnati, OH 45267-0575. E-mail Litsa.Kranias{at}uc.edu

Received July 28, 2006; accepted November 10, 2006.

Background— Abnormal sarcoplasmic reticulum calcium (Ca) cycling is increasingly recognized as an important mechanism for increased ventricular automaticity that leads to lethal ventricular arrhythmias. Previous studies have linked lethal familial arrhythmogenic disorders to mutations in the ryanodine receptor and calsequestrin genes, which interact with junctin and triadin to form a macromolecular Ca-signaling complex. The essential physiological effects of junctin and its potential regulatory roles in sarcoplasmic reticulum Ca cycling and Ca-dependent cardiac functions, such as myocyte contractility and automaticity, are unknown.

Methods and Results— The junctin gene was targeted in embryonic stem cells, and a junctin-deficient mouse was generated. Ablation of junctin was associated with enhanced cardiac function in vivo, and junctin-deficient cardiomyocytes exhibited increased contractile and Ca-cycling parameters. Short-term isoproterenol stimulation elicited arrhythmias, including premature ventricular contractions, atrioventricular heart block, and ventricular tachycardia. Long-term isoproterenol infusion also induced premature ventricular contractions and atrioventricular heart block in junctin-null mice. Further examination of the electrical activity revealed a significant increase in the occurrence of delayed afterdepolarizations. Consistently, 25% of the junctin-null mice died by 3 months of age with structurally normal hearts.

Conclusions— Junctin is an essential regulator of sarcoplasmic reticulum Ca release and contractility in normal hearts. Ablation of junctin is associated with aberrant Ca homeostasis, which leads to fatal arrhythmias. Thus, normal intracellular Ca cycling relies on maintenance of junctin levels and an intricate balance among the components in the sarcoplasmic reticulum quaternary Ca-signaling complex.

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