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J. Biol. Chem., Vol. 282, Issue 52, 37864-37874, December 28, 2007

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Triadins Modulate Intracellular Ca²⁺ Homeostasis but Are Not Essential for Excitation-Contraction Coupling in Skeletal Muscle^*

Xiaohua Shen, Clara Franzini-Armstrong, Jose R. Lopez, Larry R. Jones^¶, Yvonne M. Kobayashi^¶1, Ying Wang^||, W. Glenn L. Kerrick^||, Anthony H. Caswell^**, James D. Potter^**, Todd Miller^**, Paul D. Allen, and Claudio F. Perez²

From the Department of Anesthesiology, Brigham and Women's Hospital, Boston, Massachusetts 02115, the Deptartment of Cell and Developmental Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, the ^¶Krannert Institute of Cardiology, Department of Medicine, Indiana University School of Medicine, Indianapolis, Indiana 46202, and the Departments of ^||Physiology and Biophysics and ^**Molecular and Cellular Pharmacology, University of Miami Miller School of Medicine, Miami, Florida 33101

To unmask the role of triadin in skeletal muscle we engineered pan-triadin-null mice by removing the first exon of the triadin gene. This resulted in a total lack of triadin expression in both skeletal and cardiac muscle. Triadin knockout was not embryonic or birth-lethal, and null mice presented no obvious functional phenotype. Western blot analysis of sarcoplasmic reticulum (SR) proteins in skeletal muscle showed that the absence of triadin expression was associated with down-regulation of Junctophilin-1, junctin, and calsequestrin but resulted in no obvious contractile dysfunction. Ca²⁺ imaging studies in null lumbricalis muscles and myotubes showed that the lack of triadin did not prevent skeletal excitation-contraction coupling but reduced the amplitude of their Ca²⁺ transients. Additionally, null myotubes and adult fibers had significantly increased myoplasmic resting free Ca²⁺.[³H]Ryanodine binding studies of skeletal muscle SR vesicles detected no differences in Ca²⁺ activation or Ca²⁺ and Mg²⁺ inhibition between wild-type and triadin-null animals. Subtle ultrastructural changes, evidenced by the appearance of longitudinally oriented triads and the presence of calsequestrin in the sacs of the longitudinal SR, were present in fast but not slow twitch-null muscles. Overall, our data support an indirect role for triadin in regulating myoplasmic Ca²⁺ homeostasis and organizing the molecular complex of the triad but not in regulating skeletal-type excitation-contraction coupling.

Received for publication, July 11, 2007 , and in revised form, September 20, 2007.

^* This work was supported by American Heart Association Grant 0530250N (to C. F. P.) and National Institute of Health Grant PO1AR47605 (to P. D. A.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1-S3.

¹ Present address: Dept. of Molecular Physiology and Biophysics, University of Iowa, Iowa City, IA 52242.

² To whom correspondence should be addressed: Brigham and Women's Hospital, 75 Francis St., Boston, MA 02115. Tel.: 617-525-6486; Fax: 617-732-6927; E-mail: cperez{at}zeus.bwh.harvard.edu.

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