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

Ito K, Komazaki S, Sasamoto K, Yoshida M, Nishi M, Kitamura K, Takeshima H - J. Cell Biol. (2001)

Bottom Line: Of the three JP subtypes, both type 1 (JP-1) and type 2 (JP-2) are abundantly expressed in skeletal muscle.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.

View Article: PubMed Central - PubMed

Affiliation: Institute of Life Science, Kurume University and CREST, Japan Science and Technology Corporation, Fukuoka 839-0861, Japan.

ABSTRACT
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.

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Quantitative analysis of junctional membrane structures between wild-type and JP-1 knockout skeletal muscles. Skeletal muscles were prepared from tongue, jaw (digastric muscle), diaphragm, and hindlimb (thigh region) of newborn mice immediately after birth. The junctional membrane structures at the A-I junctions were analyzed by electron microscopic observations, and the results of random samples were compared between the genotypes. The data were obtained from at least 3,000 A-I junctions of three neonates and are shown as the mean ± SEM. Statistical differences between the genotypes are indicated by asterisks (t test, *P < 0.05 and **P < 0.01).
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fig5: Quantitative analysis of junctional membrane structures between wild-type and JP-1 knockout skeletal muscles. Skeletal muscles were prepared from tongue, jaw (digastric muscle), diaphragm, and hindlimb (thigh region) of newborn mice immediately after birth. The junctional membrane structures at the A-I junctions were analyzed by electron microscopic observations, and the results of random samples were compared between the genotypes. The data were obtained from at least 3,000 A-I junctions of three neonates and are shown as the mean ± SEM. Statistical differences between the genotypes are indicated by asterisks (t test, *P < 0.05 and **P < 0.01).

Mentions: In mutant muscle cells prepared immediately after birth, both diads and triads were apparently normal in morphology. The T-SR gap size in the junctional membrane structures was 13 ± 1 nm in both wild-type and mutant muscles. However, statistical analysis demonstrated that the number of triad junctions was significantly reduced in all examined muscles from the JP-1 knockout neonates immediately after birth (Fig. 5) . These observations clearly suggest that the loss of JP-1 affects the triad formation in skeletal muscle.


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

Ito K, Komazaki S, Sasamoto K, Yoshida M, Nishi M, Kitamura K, Takeshima H - J. Cell Biol. (2001)

Quantitative analysis of junctional membrane structures between wild-type and JP-1 knockout skeletal muscles. Skeletal muscles were prepared from tongue, jaw (digastric muscle), diaphragm, and hindlimb (thigh region) of newborn mice immediately after birth. The junctional membrane structures at the A-I junctions were analyzed by electron microscopic observations, and the results of random samples were compared between the genotypes. The data were obtained from at least 3,000 A-I junctions of three neonates and are shown as the mean ± SEM. Statistical differences between the genotypes are indicated by asterisks (t test, *P < 0.05 and **P < 0.01).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2196186&req=5

fig5: Quantitative analysis of junctional membrane structures between wild-type and JP-1 knockout skeletal muscles. Skeletal muscles were prepared from tongue, jaw (digastric muscle), diaphragm, and hindlimb (thigh region) of newborn mice immediately after birth. The junctional membrane structures at the A-I junctions were analyzed by electron microscopic observations, and the results of random samples were compared between the genotypes. The data were obtained from at least 3,000 A-I junctions of three neonates and are shown as the mean ± SEM. Statistical differences between the genotypes are indicated by asterisks (t test, *P < 0.05 and **P < 0.01).
Mentions: In mutant muscle cells prepared immediately after birth, both diads and triads were apparently normal in morphology. The T-SR gap size in the junctional membrane structures was 13 ± 1 nm in both wild-type and mutant muscles. However, statistical analysis demonstrated that the number of triad junctions was significantly reduced in all examined muscles from the JP-1 knockout neonates immediately after birth (Fig. 5) . These observations clearly suggest that the loss of JP-1 affects the triad formation in skeletal muscle.

Bottom Line: Of the three JP subtypes, both type 1 (JP-1) and type 2 (JP-2) are abundantly expressed in skeletal muscle.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.

View Article: PubMed Central - PubMed

Affiliation: Institute of Life Science, Kurume University and CREST, Japan Science and Technology Corporation, Fukuoka 839-0861, Japan.

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus