Deficiency of triad junction and contraction in mutant skeletal muscle lacking junctophilin type 1

In skeletal muscle excitation–contraction (E–C) coupling, the depolarization signal is converted from the intracellular Ca²⁺ store into Ca²⁺ release by functional coupling between the cell surface voltage sensor and the Ca²⁺ release channel on the sarcoplasmic reticulum (SR). The signal conversion occurs in the junctional membrane complex known as the triad junction, where the invaginated plasma membrane called the transverse-tubule (T-tubule) is pinched from both sides by SR membranes. Previous studies have suggested that junctophilins (JPs) contribute to the formation of the junctional membrane complexes by spanning the intracellular store membrane and interacting with the plasma membrane (PM) in excitable cells. Of the three JP subtypes, both type 1 (JP-1) and type 2 (JP-2) are abundantly expressed in skeletal muscle. To examine the physiological role of JP-1 in skeletal muscle, we generated mutant mice lacking JP-1. The JP-1 knockout mice showed no milk suckling and died shortly after birth. Ultrastructural analysis demonstrated that triad junctions were reduced in number, and that the SR was often structurally abnormal in the skeletal muscles of the mutant mice. The mutant muscle developed less contractile force (evoked by low-frequency electrical stimuli) and showed abnormal sensitivities to extracellular Ca²⁺. Our results indicate that JP-1 contributes to the construction of triad junctions and that it is essential for the efficiency of signal conversion during E–C coupling in skeletal muscle.