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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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Morphological abnormalities of membrane system in JP-1 knockout skeletal muscle. Longitudinal ultrathin sections of hindlimb (A–D) and jaw (E–H) muscles were analyzed using electron microscopy. Muscle preparations were derived from wild-type (A and E) and JP-1 knockout (C and G) neonates immediately after birth, and wild-type (B and F) and JP-1 knockout (D and H) neonates ∼15 h after birth. In JP-1 knockout muscles, morphology of the SR turned worsened after birth, although mitochondria and myofibril retained normal structures. Bar, 1.0 μm.
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fig4: Morphological abnormalities of membrane system in JP-1 knockout skeletal muscle. Longitudinal ultrathin sections of hindlimb (A–D) and jaw (E–H) muscles were analyzed using electron microscopy. Muscle preparations were derived from wild-type (A and E) and JP-1 knockout (C and G) neonates immediately after birth, and wild-type (B and F) and JP-1 knockout (D and H) neonates ∼15 h after birth. In JP-1 knockout muscles, morphology of the SR turned worsened after birth, although mitochondria and myofibril retained normal structures. Bar, 1.0 μm.

Mentions: Because JP-1 is predominantly expressed in skeletal muscle, we surveyed morphological defects in skeletal muscles from the JP-1 knockout neonates. Cell density, diameter, and the shape of mutant muscle fibers seemed to be normal, and photomicroscopic examination detected no obvious abnormalities. However, the electron microscopic examination detected ultrastructural abnormalities in the membrane systems of the mutant muscles (Fig. 4) . In muscles from newborn mice, formation of the junctional complexes between the T-tubule and the SR are not yet completed, and both diads and triads partially occupy A-I junctions. In contrast to flattened T-tubular structures in mature triads, the T-tubules pinched from both sides by the SR are often elliptical in shape in immature muscles from neonates. No obvious ultrastructural abnormalities in the membrane system were observed in mutant muscles from JP-1 knockout neonates immediately after birth. However, mutant muscles from the knockout neonates that were near death (15–20 h after birth) showed the following abnormal SR features: the terminal regions of the SR were frequently swollen, longitudinal SR regions were partially vacuolated, and the orientation of SR networks was irregular. Therefore, junctional membrane complexes of both diads and triads were structurally abnormal in mutant muscles. These morphological abnormalities were detected in all muscle types examined in neonates 15–20 h after birth, and jaw muscle cells showed the most severe defects. On the other hand, mitochondria and myofibrils retained normal structures in JP-1 knockout muscles.


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)

Morphological abnormalities of membrane system in JP-1 knockout skeletal muscle. Longitudinal ultrathin sections of hindlimb (A–D) and jaw (E–H) muscles were analyzed using electron microscopy. Muscle preparations were derived from wild-type (A and E) and JP-1 knockout (C and G) neonates immediately after birth, and wild-type (B and F) and JP-1 knockout (D and H) neonates ∼15 h after birth. In JP-1 knockout muscles, morphology of the SR turned worsened after birth, although mitochondria and myofibril retained normal structures. Bar, 1.0 μm.
© Copyright Policy
Related In: Results  -  Collection

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

fig4: Morphological abnormalities of membrane system in JP-1 knockout skeletal muscle. Longitudinal ultrathin sections of hindlimb (A–D) and jaw (E–H) muscles were analyzed using electron microscopy. Muscle preparations were derived from wild-type (A and E) and JP-1 knockout (C and G) neonates immediately after birth, and wild-type (B and F) and JP-1 knockout (D and H) neonates ∼15 h after birth. In JP-1 knockout muscles, morphology of the SR turned worsened after birth, although mitochondria and myofibril retained normal structures. Bar, 1.0 μm.
Mentions: Because JP-1 is predominantly expressed in skeletal muscle, we surveyed morphological defects in skeletal muscles from the JP-1 knockout neonates. Cell density, diameter, and the shape of mutant muscle fibers seemed to be normal, and photomicroscopic examination detected no obvious abnormalities. However, the electron microscopic examination detected ultrastructural abnormalities in the membrane systems of the mutant muscles (Fig. 4) . In muscles from newborn mice, formation of the junctional complexes between the T-tubule and the SR are not yet completed, and both diads and triads partially occupy A-I junctions. In contrast to flattened T-tubular structures in mature triads, the T-tubules pinched from both sides by the SR are often elliptical in shape in immature muscles from neonates. No obvious ultrastructural abnormalities in the membrane system were observed in mutant muscles from JP-1 knockout neonates immediately after birth. However, mutant muscles from the knockout neonates that were near death (15–20 h after birth) showed the following abnormal SR features: the terminal regions of the SR were frequently swollen, longitudinal SR regions were partially vacuolated, and the orientation of SR networks was irregular. Therefore, junctional membrane complexes of both diads and triads were structurally abnormal in mutant muscles. These morphological abnormalities were detected in all muscle types examined in neonates 15–20 h after birth, and jaw muscle cells showed the most severe defects. On the other hand, mitochondria and myofibrils retained normal structures in JP-1 knockout muscles.

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