Subproteomics analysis of Ca2+-binding proteins demonstrates decreased calsequestrin expression in dystrophic mouse skeletal muscle

Abstract

Duchenne muscular dystrophy represents one of the most common hereditary diseases. Abnormal ion handling is believed to render dystrophin-deficient muscle fibres more susceptible to necrosis. Although a reduced Ca2+ buffering capacity has been shown to exist in the dystrophic sarcoplasmic reticulum, surprisingly no changes in the abundance of the main luminal Ca 2+ reservoir protein calsequestrin have been observed in microsomal preparations. To address this unexpected finding and eliminate potential technical artefacts of subcellular fractionation protocols, we employed a comparative subproteomics approach with total mouse skeletal muscle extracts. Immunoblotting, mass spectrometry and labelling of the entire muscle protein complement with the cationic carbocyanine dye 'Stains-All' was performed in order to evaluate the fate of major Ca2+-binding proteins in dystrophin-deficient skeletal muscle fibres. In contrast to a relatively comparable expression pattern of the main protein population in normal vs. dystrophic fibres, our analysis showed that the expression of key Ca 2+-binding proteins of the luminal sarcoplasmic reticulum is drastically reduced. This included the main terminal cisternae constituent, calsequestrin, and the previously implicated Ca2+-shuttle element, sarcalumenin. In contrast, the 'Stains-All'-positive protein spot, representing the cytosolic Ca2+-binding component, calmodulin, was not changed in dystrophin-deficient fibres. The reduced 2D 'Stains-All' pattern of luminal Ca2+-binding proteins in mdx preparations supports the calcium hypothesis of muscular dystrophy. The previously described impaired Ca 2+ buffering capacity of the dystrophic sarcoplasmic reticulum is probably caused by a reduction in luminal Ca2+-binding proteins, including calsequestrin.