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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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Related in: MedlinePlus

Force–frequency relationship in skeletal muscles from wild-type and JP-1 knockout neonates. Isometric tension of muscle preparations at several frequencies was determined in modified Krebs-Ringer solution, and typical recordings from wild-type and mutant muscles are shown in A. Force–frequency relationships in wild-type and mutant muscles are shown in B. Each value from at least n =13 from seven mice was normalized to the maximum force and represents 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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fig6: Force–frequency relationship in skeletal muscles from wild-type and JP-1 knockout neonates. Isometric tension of muscle preparations at several frequencies was determined in modified Krebs-Ringer solution, and typical recordings from wild-type and mutant muscles are shown in A. Force–frequency relationships in wild-type and mutant muscles are shown in B. Each value from at least n =13 from seven mice was normalized to the maximum force and represents the mean ± SEM. Statistical differences between the genotypes are indicated by asterisks (t test, *P < 0.05 and ** P < 0.01).

Mentions: To survey functional abnormalities of the JP-1 knockout muscle, we examined the contraction profiles of a hindlimb muscle bundle (musculus vastus lateralis) from the mutant neonates. Fig. 6 shows significant differences in the force–frequency relationship between JP-1 knockout and wild-type muscles in normal bathing solution. Electrical stimuli at low frequencies developed less contractile forces in the mutant muscle compared with wild-type muscle. Twitch tension developed in the mutant muscle was reduced to about half the value of that in control muscle, although there is no significant difference in the maximum force between the genotypes (Table I). Thus, the force–frequency curve of the mutant muscle was shifted downward, indicating that the loss of JP-1 reduced the efficiency of E–C coupling. The low efficiency in the mutant muscle is likely due to the deficiency of triad junctions where functional coupling occurs between DHPR and RyR.


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)

Force–frequency relationship in skeletal muscles from wild-type and JP-1 knockout neonates. Isometric tension of muscle preparations at several frequencies was determined in modified Krebs-Ringer solution, and typical recordings from wild-type and mutant muscles are shown in A. Force–frequency relationships in wild-type and mutant muscles are shown in B. Each value from at least n =13 from seven mice was normalized to the maximum force and represents 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

fig6: Force–frequency relationship in skeletal muscles from wild-type and JP-1 knockout neonates. Isometric tension of muscle preparations at several frequencies was determined in modified Krebs-Ringer solution, and typical recordings from wild-type and mutant muscles are shown in A. Force–frequency relationships in wild-type and mutant muscles are shown in B. Each value from at least n =13 from seven mice was normalized to the maximum force and represents the mean ± SEM. Statistical differences between the genotypes are indicated by asterisks (t test, *P < 0.05 and ** P < 0.01).
Mentions: To survey functional abnormalities of the JP-1 knockout muscle, we examined the contraction profiles of a hindlimb muscle bundle (musculus vastus lateralis) from the mutant neonates. Fig. 6 shows significant differences in the force–frequency relationship between JP-1 knockout and wild-type muscles in normal bathing solution. Electrical stimuli at low frequencies developed less contractile forces in the mutant muscle compared with wild-type muscle. Twitch tension developed in the mutant muscle was reduced to about half the value of that in control muscle, although there is no significant difference in the maximum force between the genotypes (Table I). Thus, the force–frequency curve of the mutant muscle was shifted downward, indicating that the loss of JP-1 reduced the efficiency of E–C coupling. The low efficiency in the mutant muscle is likely due to the deficiency of triad junctions where functional coupling occurs between DHPR and RyR.

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