Increased Ca 2+ storage capacity of the skeletal muscle sarcoplasmic reticulum of transgenic mice over‐expressing membrane bound calcium binding protein junctate

Junctate is an integral sarco(endo)plasmic reticulum protein expressed in many tissues including heart and skeletal muscle. Because of its localization and biochemical characteristics, junctate is deemed to participate in the regulation of the intracellular Ca 2+ concentration. However, its physiological function in muscle cells has not been investigated yet. In this study we examined the effects of junctate over‐expression by generating a transgenic mouse model which over‐expresses junctate in skeletal muscle. Our results demonstrate that junctate over‐expression induced a significant increase in SR Ca 2+ storage capacity which was paralleled by an increased 4‐chloro‐ m ‐cresol and caffeine‐induced Ca 2+ release, whereas it did not affect SR Ca 2+ ‐dependent ATPase activity and SR Ca 2+ loading rates. In addition, junctate over‐expression did not affect the expression levels of SR Ca 2+ binding proteins such as calsequestrin, calreticulin and sarcalumenin. These findings suggest that junctate over‐expression is associated with an increase in the SR Ca 2+ storage capacity and releasable Ca 2+ content and support a physiological role for junctate in intracellular Ca 2+ homeostasis. J. Cell. Physiol. 213: 464–474, 2007. © 2007 Wiley‐Liss, Inc.