Calsequestrin 1 Is an Active Partner of Stromal Interaction Molecule 2 in Skeletal Muscle

Abstract

Calsequestrin 1 (CASQ1) in skeletal muscle buffers and senses Ca2+ in the sarcoplasmic reticulum (SR). CASQ1 also regulates store-operated Ca2+ entry (SOCE) by binding to stromal interaction molecule 1 (STIM1). Abnormal SOCE and/or abnormal expression or mutations in CASQ1, STIM1, or STIM2 are associated with human skeletal, cardiac, or smooth muscle diseases. However, the functional relevance of CASQ1 along with STIM2 has not been studied in any tissue, including skeletal muscle. First, in the present study, it was found by biochemical approaches that CASQ1 is bound to STIM2 via its 92 N-terminal amino acids (C1 region). Next, to examine the functional relevance of the CASQ1-STIM2 interaction in skeletal muscle, the full-length wild-type CASQ1 or the C1 region was expressed in mouse primary skeletal myotubes, and the myotubes were examined using single-myotube Ca2+ imaging experiments and transmission electron microscopy observations. The CASQ1-STIM2 interaction via the C1 region decreased SOCE, increased intracellular Ca2+ release for skeletal muscle contraction, and changed intracellular Ca2+ distributions (high Ca2+ in the SR and low Ca2+ in the cytosol were observed). Furthermore, the C1 region itself (which lacks Ca2+-buffering ability but has STIM2-binding ability) decreased the expression of Ca2+-related proteins (canonical-type transient receptor potential cation channel type 6 and calmodulin 1) and induced mitochondrial shape abnormalities. Therefore, in skeletal muscle, CASQ1 plays active roles in Ca2+ movement and distribution by interacting with STIM2 as well as Ca2+ sensing and buffering.