Increased Ca²⁺ storage capacity of the skeletal muscle sarcoplasmic reticulum of transgenic mice over‐expressing membrane bound calcium binding protein junctate

Abstract

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 2þ 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 2þ storage capacity which was paralleled by an increased 4-chloro-mcresol and caffeine-induced Ca 2þ release, whereas it did not affect SR Ca 2þ -dependent ATPase activity and SR Ca 2þ loading rates. In addition, junctate over-expression did not affect the expression levels of SR Ca 2þ binding proteins such as calsequestrin, calreticulin and sarcalumenin. These findings suggest that junctate over-expression is associated with an increase in the SR Ca 2þ storage capacity and releasable Ca 2þ content and support a physiological role for junctate in intracellular Ca 2þ homeostasis.

Calsequestrin content and SERCA determine normal and maximal Ca2+ storage levels in sarcoplasmic reticulum of fast‐ and slow‐twitch fibres of rat

“…When the SR is near maximal capacity, Ca 2+ binding to the high density of SERCA1 proteins present in EDL fibres could account for a further ∼0.15 m m Ca 2+ (Baylor et al 1983), and maximal binding to CSQ2 could account for an additional ∼0.27 m m in SOL (I) fibres. Thus, these figures imply maximal Ca 2+ content values of ∼4.1 and 1.5 m m in EDL and SOL (I) fibres, respectively, without taking into account possible additional Ca 2+ binding to other less abundant proteins, such as sarcalumenin (Leberer et al 1990), histidine‐rich Ca 2+ ‐binding protein (Sacchetto et al 2001), junctate (Divet et al 2007) and the CSQ‐like proteins (Cala et al 1990; Culligan et al 2002). Thus, it appears that Ca 2+ binding to CSQ is likely to account for a large proportion of the maximal SR Ca 2+ content values of ∼3.7 and 1.1 m m measured in the rat EDL and SOL (I) fibres, respectively, in this laboratory (Fryer & Stephenson, 1996).…”

Triadin/Junctin Double Null Mouse Reveals a Differential Role for Triadin and Junctin in Anchoring CASQ to the jSR and Regulating Ca2+ Homeostasis

“…The observations in the double null phenotype suggest that although Jct cannot compensate for the lack of Tdn it does contribute to restrict the deleterious effects of the Tdn-null phenotype, supporting a role for Jct in regulating Ca 2+ homeostasis in skeletal muscle. However, because of the targeting strategy used to knockout junctin may also prevent expression of aspartyl- β -hydroxylase (Asph), humbug and junctate [56], [57], [58], [59]. The use of an antibody against homologous region of the N-termini of the three proteins reveled that expression of junctate in Jct-null mouse has not been altered in cardiac muscles however, the N-termini antibody failed to detected expression of Asph/humbug.…”

Reorganized stores and impaired calcium handling in skeletal muscle of mice lacking calsequestrin‐1

“…in mitochondria‐rich slow‐twitch fibres (Gillis, 1997). Mitochondrial biogenesis in fast‐twitch fibres has previously been shown to be stimulated by the absence of the cyoplasmic calcium buffering protein parvalbumin (Racay et al 2006) and in junctate‐overexpressing mice (Divet et al 2007). Conceivably, prolonged presence of Ca 2+ in the cytosol, even if only slight, induces mitochondrial proliferation as suggested by Rohas et al (2007).…”