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Published 3 September 2001. doi:10.1083/jcb.200105040
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© The Rockefeller University Press, 0021-9525/2001/9/1059 $5.00
The Journal of Cell Biology, Volume 154, Number 5, September 3, 2001 1059-1068

Article

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

Koichi Ito1, Shinji Komazaki2, Kazushige Sasamoto3, Morikatsu Yoshida1, Miyuki Nishi1, Kenji Kitamura4 and Hiroshi Takeshima1

1 Institute of Life Science, Kurume University and CREST, Japan Science and Technology Corporation, Fukuoka 839-0861, Japan
2 Department of Anatomy, Saitama Medical School, Moroyama, Saitama 350-0495, Japan
3 Section of Oral Neuroscience, Graduate School of Dental Sciences, Kyushu University, Fukuoka 812-8582, Japan
4 Department of Pharmacology, Fukuoka Dental School, Fukuoka 814-0193, Japan

Address correspondence to Hiroshi Takeshima, Institute of Life Science, Kurume University, 2432-3 Aikawa, Kurume, Fukuoka 839-0861, Japan. Tel.: 81-942-37-6317. Fax: 81-942-31-3320. E-mail: takeshim{at}lsi.kurume-u.ac.jp

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.

Key Words: dihydropyridine receptor; excitation–contraction coupling; junctophilin; ryanodine receptor; triad junction


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