Hepatic and skeletal muscle glycogen metabolism in rats with short-term cholestasis

Previous studies have shown that skeletal muscle glycogen and mitochondria are distributed in distinct subcellular localizations, but the role and regulation of these subcellular localizations are unclear. In the present study, we used transmission electron microscopy to investigate the effect of disuse and aging on human skeletal muscle glycogen and mitochondria content in subsarcolemmal (SS), intermyofibrillar (IMF), and intramyofibrillar (intra) localizations. Five young (∼23 yr) and five old (∼66 yr) recreationally active men had their quadriceps muscle immobilized for 2 wk by whole leg casting. Biopsies were obtained from m. vastus lateralis before and after the immobilization period. Immobilization induced a decrement of intra glycogen content by 54% ( P < 0.001) in both age groups and in two ultrastructurally distinct fiber types, whereas the content of IMF and SS glycogen remained unchanged. A localization-dependent decrease ( P = 0.03) in mitochondria content following immobilization was found in both age groups, where SS mitochondria decreased by 33% ( P = 0.02), superficial IMF mitochondria decreased by 20% ( P = 0.05), and central IMF mitochondria remained unchanged. In conclusion, our findings demonstrate a localization-dependent adaptation to immobilization in glycogen and mitochondria content of skeletal muscles of both young and old individuals. Specifically, this suggests that short-term disuse preferentially affects glycogen particles located inside the myofibrils and that mitochondria volume plasticity can be dependent on the distance to the fiber border.

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Determinants of insulin-stimulated skeletal muscle glycogen metabolism in man

European Journal of Clinical Investigation ◽

10.1111/j.1365-2362.1995.tb01988.x ◽

1995 ◽

Vol 25 (9) ◽

pp. 693-698 ◽

Cited By ~ 7

Author(s):

C. SCHALIN-JÄNTTI ◽

E. LAURILA ◽

M. LÖFMAN ◽

L. C. GROOP

Keyword(s):

Skeletal Muscle ◽

Muscle Glycogen ◽

Glycogen Metabolism ◽

Skeletal Muscle Glycogen

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Mutations of the serine phosphorylated in the protein phosphatase-1-binding motif in the skeletal muscle glycogen-targeting subunit

Biochemical Journal ◽

10.1042/bj3460077 ◽

2000 ◽

Vol 346 (1) ◽

pp. 77-82 ◽

Cited By ~ 9

Author(s):

Jun LIU ◽

Jun WU ◽

Carey OLIVER ◽

Shirish SHENOLIKAR ◽

David L. BRAUTIGAN

Keyword(s):

Skeletal Muscle ◽

Muscle Glycogen ◽

Protein Phosphatase ◽

Glycogen Metabolism ◽

Binding Activity ◽

Hormonal Control ◽

Protein Phosphatase 1 ◽

Binding Motif ◽

Unique Contribution ◽

Skeletal Muscle Glycogen

Cellular functions of protein phosphatase-1 (PP1) are determined by regulatory subunits that contain the consensus PP1-binding motif, RVXF. This motif was first identified as the site of phosphorylation by cAMP-dependent protein kinase (PKA) in a skeletal muscle glycogen-targeting subunit (GM). We reported previously that a recombinant fusion protein of glutathione S-transferase (GST) and the N-terminal domain of GM [GST-GM-(1-240)] bound PP1 in a pull down assay, and phosphorylation by PKA prevented PP1 binding. Here we report that substitution of either Ala or Val for Ser-67 in the RVS67F motif in GST-GM-(1-240) essentially eliminated PP1 binding. This was unexpected because other glycogen-targeting subunits have a Val residue at the position corresponding to Ser-67. In contrast, a mutation of Ser-67 to Thr (S67T) in GST-GM(1-240) gave a protein that bound PP1 the same as wild type and was unaffected by PKA phosphorylation. Full length GM tagged with the epitope sequence DYKDDDDK (FLAG) expressed in COS7 cells bound PP1 that was recovered by co-immunoprecipitation, but this association was prevented by treatment of the cells with forskolin. By comparison, PP1 binding with FLAG-GM(S67T) was not disrupted by forskolin treatment. Neither FLAG-GM(S67A) nor FLAG-GM(S67V) formed stable complexes with PP1 in COS7 cells. These results emphasise the unique contribution of Ser-67 in PP1 binding to GM. The constitutive PP1-binding activity shown by GM(S67T) opens the way for studying the role of GM multisite phosphorylation in hormonal control of glycogen metabolism.

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