Insulin Control of Glycogen Metabolism in Knockout Mice Lacking the Muscle-Specific Protein Phosphatase PP1G/R GL

Abstract

ABSTRACT The regulatory-targeting subunit (R GL , also called G M ) of the muscle-specific glycogen-associated protein phosphatase PP1G targets the enzyme to glycogen where it modulates the activity of glycogen-metabolizing enzymes. PP1G/R GL has been postulated to play a central role in epinephrine and insulin control of glycogen metabolism via phosphorylation of R GL . To investigate the function of the phosphatase, R GL knockout mice were generated. Animals lacking R GL show no obvious defects. The R GL protein is absent from the skeletal and cardiac muscle of null mutants and present at ∼50% of the wild-type level in heterozygotes. Both the level and activity of C1 protein are also decreased by ∼50% in the R GL -deficient mice. In skeletal muscle, the glycogen synthase (GS) activity ratio in the absence and presence of glucose-6-phosphate is reduced from 0.3 in the wild type to 0.1 in the null mutant R GL mice, whereas the phosphorylase activity ratio in the absence and presence of AMP is increased from 0.4 to 0.7. Glycogen accumulation is decreased by ∼90%. Despite impaired glycogen accumulation in muscle, the animals remain normoglycemic. Glucose tolerance and insulin responsiveness are identical in wild-type and knockout mice, as are basal and insulin-stimulated glucose uptakes in skeletal muscle. Most importantly, insulin activated GS in both wild-type and R GL null mutant mice and stimulated a GS-specific protein phosphatase in both groups. These results demonstrate that R GL is genetically linked to glycogen metabolism, since its loss decreases PP1 and basal GS activities and glycogen accumulation. However, PP1G/R GL is not required for insulin activation of GS in skeletal muscle, and rather another GS-specific phosphatase appears to be involved.