Muscle-specific overexpression of wild type and R225Q mutant AMP-activated protein kinase γ3-subunit differentially regulates glycogen accumulation

AMP-activated protein kinase ( AMPK) is a heterotrimeric complex that works as an energy sensor to integrate nutritional and hormonal signals. The naturally occurring R225Q mutation in the gamma 3-subunit in pigs is associated with abnormally high glycogen content in skeletal muscle. Becauses skeletal muscle accounts for most of the body's glucose uptake, and gamma 3 is specifically expressed in skeletal muscle, it is important to understand the underlying mechanism of this mutation in regulating glucose and glycogen metabolism. Using skeletal muscle-specific transgenic mice overexpressing wild type mu 3 (WT mu 3) and R225Q mutant mu 3 (MUT gamma 3), we show that both WT gamma 3 and MUT gamma 3 mice have 1.5- to 2-fold increases in muscle glycogen content. In WT gamma 3 mice, increased glycogen content was associated with elevated total glycogen synthase activity and reduced glycogen phosphorylase activity, whereas alterations in activities of these enzymes could not explain elevated glycogen in MUT gamma 3 mice. Basal, 5-aminoimidazole-AICAR- and phenformin-stimulated AMPK alpha 2 isoform-specific activities were decreased only in MUT gamma 3 mice. Basal rates of 2-DG glucose uptake were decreased in both WT gamma 3 and MUT gamma 3 mice. However, AICAR- and phenformin-stimulated 2-DG glucose uptake were blunted only in MUT gamma 3 mice. In conclusion, expression of either wild type or mutant gamma 3-subunit of AMPK results in increased glycogen concentrations in muscle, but the mechanisms underlying this alteration appear to be different. Furthermore, mutation of the gamma 3-subunit is associated with decreases in AMPK alpha 2 isoform-specific activity and impairment in AICAR- and phenformin-stimulated skeletal muscle glucose uptake.