Junctophilin (JP) mediates the close contact between cell surface and intracellular membranes in muscle cells ensuring efficient excitation-contraction coupling. Here we demonstrate that disruption of triad junction structure formed by the transverse tubular (TT) invagination of plasma membrane and terminal cisternae of sarcoplasmic reticulum (SR) by reduction of JP expression leads to defective Ca2+ homeostasis in muscle cells. Using adenovirus with small hairpin interference RNA (shRNA) against both JP1 and JP2 genes, we could achieve acute suppression of JPs in skeletal muscle fibers. The shRNA-treated muscles exhibit deformed triad junctions and reduced store-operated Ca2+ entry (SOCE), which is likely due to uncoupled retrograde signaling from SR to TT. Knockdown of JP also causes a reduction in SR Ca2+ storage and altered caffeine-induced Ca2+ release, suggesting an orthograde regulation of the TT membrane on the SR Ca2+ release machinery. Our data demonstrate that JPs play an important role in controlling overall intracellular Ca2+ homeostasis in muscle cells. We speculate that altered expression of JPs may underlie some of the phenotypic changes associated with certain muscle diseases and aging.
