δL- and γ-Sarcoglycan Localization in the Sarcoplasmic Reticulum of Skeletal Muscle

Abstract

Sarcoglycans are transmembrane proteins that are members of the dystrophin complex. Sarcoglycans cluster together to form a complex, which is localized in the cell membrane of skeletal, cardiac, and smooth muscle fibers. However, it is still unclear whether or not sarcoglycans are restricted to the sarcolemma. To address this issue, we examined α-, β-, δ-, and γ-sarcoglycan expression in femoral skeletal muscle from control and dystrophin-deficient mice and rats using confocal microscopy and immunoelectron microscopy. Confocal microscopy of the tissues in cross-section showed that all sarcoglycans were detected under the sarcolemma in rats and control mice. δ- and γ-sarcoglycan labeling demonstrated striations in the longitudinal section, suggesting that the proteins were expressed in the sarcoplasmic reticulum (SR) or transverse tubules (T-tubules). Moreover, such striations of both sarcoglycans were recognized in the dystrophin-deficient mouse skeletal muscle. Double labeling with phalloidin or α-actinin and δ- or γ-sarcoglycan showed different labeling patterns, indicating that δ-sarcoglycan localization was distinct from that of γ-sarcoglycan. Immunoelectron microscopy clarified that δ-sarcoglycan was localized in the terminal cisternae of the SR, while γ-sarcoglycan was found in the terminal cisternae and longitudinal SR over I-bands but not over A-bands. These data demonstrate that δ- and γ-sarcoglycans are components of the SR in skeletal muscle, suggesting that both sarcoglycans function independent of the dystrophin complex in the SR.