Regulation and Function of the Muscle Glycogen-Targeting Subunit of Protein Phosphatase 1 (GM) in Human Muscle Cells Depends on the COOH-Terminal Region and Glycogen Content

Abstract

G M , the muscle-specific glycogen-targeting subunit of protein phosphatase 1 (PP1) targeted to the sarcoplasmic reticulum, was proposed to regulate recovery of glycogen in exercised muscle, whereas mutation truncation of its COOH-terminal domain is known to be associated with type 2 diabetes. Here, we demonstrate differential effects of G M overexpression in human muscle cells according to glycogen concentration. Adenovirus-mediated delivery of G M slightly activated glycogen synthase (GS) and inactivated glycogen phosphorylase (GP) in glycogen-replete cells, causing an overaccumulation of glycogen and impairment of glycogenolysis after glucose deprivation. Differently, in glycogendepleted cells, G M strongly increased GS activation with no further enhancement of early glycogen resynthesis and without affecting GP. Effects of G M on GS and GP were abrogated by treatment with dibutyryl cyclic AMP. Expression of a COOH-terminal deleted-mutant (G M ⌬C), lacking the membrane binding sequence to sarcoplasmic reticulum, failed to activate GS in glycogen-depleted cells, while behaving similar to native G M in glycogen-replete cells. This is explained by loss of stability of the G M ⌬C protein following glycogen-depletion. In summary, G M promotes glycogen storage and inversely regulates GS and GP activities, while, specifically, synthase phosphatase activity of G M -PP1 is inhibited by glycogen. The conditional loss of function of the COOH-terminal deleted G M construct may help to explain the reported association of truncation mutation of G M with insulin resistance in human subjects. Diabetes

The α2–5′AMP-Activated Protein Kinase Is a Site 2 Glycogen Synthase Kinase in Skeletal Muscle and Is Responsive to Glucose Loading

“…Finally, in mouse, G M (muscle glycogen-targeting subunit of protein phosphate 1) is important for maintaining glycogen storage and GS activity, likely because of the action of the associated phosphatase, protein phosphatase 1 (39,40). G M also seems to play an important role in the increase in GS activity after glycogen depletion in human muscle cells (41). Thus, G M -directed phosphatase activity may be an important regulator of GS phosphorylation.…”

Glycogen content and contraction regulate glycogen synthase phosphorylation and affinity for UDP-glucose in rat skeletal muscles

“…In line with this interpretation, high glycogen content has been reported to decrease protein phosphatase-1 (PP-1) activity (39) and overexpression of PP-1 glycogen binding regulatory subunit G M /R GL or PTG in cultured human primary muscle cells increases glycogen content (23), whereas deletion of G M /R GL decreases glycogen content in mouse muscles (2,38). Genetic approaches are required to determine the phosphorylation sites of GS, which regulate glycogen accumulation.…”

Role of PP1 in the regulation of Ca cycling in cardiac physiology and pathophysiology

“…Interestingly, a study demonstrated that the transmembrane domain of PLN interacts directly with the C-terminus of this subunit (76), raising the possibility that G M /R GL may also target PP1c to PLN, providing yet another layer of complexity and regulation. Interestingly, a truncation mutation in the region that binds to the SR in human G M /R GL has been shown to result in aberrant PP1 regulation, impaired glycogen synthase activity and diminished glycogen content in human carriers (77, 78), human muscle cells (79), and in a knock-in mouse model expressing this mutant (78). However, the effects of this mutant were not reported in the heart.…”