Insulin control of glycogen metabolism in knockout mice lacking the muscle-specific protein phosphatase PP1G/RGL

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 similar to 50% of the wild-type level in heterozygotes. Both the level and activity of C1 protein are also decreased by similar to 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 similar to 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,, is genetically linked to glycogen metabolism, since its loss decreases PPI and basal GS activities and glycogen accumulation. However, PP1G/R-GL is not required for insulin activation of CS in skeletal muscle, and rather another GS-specific phosphatase appears to be involved.