Phosphorylation-dependent Translocation of Glycogen Synthase to a Novel Structure during Glycogen Resynthesis

Clara Prats,Joan A Cadefau,Roser Cussó,Klaus Qvortrup,Jakob N Nielsen,Jørgen F.P Wojtaszewki,D Grahame Hardie,Greg Stewart,Bo F Hansen,Thorkil Ploug

doi:10.1074/jbc.m502713200

Abstract

Glycogen metabolism has been the subject of extensive research, but the mechanisms by which it is regulated are still not fully understood. It is well accepted that the rate-limiting enzymes in glycogenesis and glycogenolysis are glycogen synthase (GS) and glycogen phosphorylase (GPh), respectively. Both enzymes are regulated by reversible phosphorylation and by allosteric effectors. However, evidence in the literature indicates that changes in muscle GS and GPh intracellular distribution may constitute a new regulatory mechanism of glycogen metabolism. Already in the 1960s, it was proposed that glycogen was present in dynamic cellular organelles that were termed glycosomas but no such cellular entities have ever been demonstrated. The aim of this study was to characterize muscle GS and GPh intracellular distribution and to identify possible translocation processes of both enzymes. Using in situ stimulation of rabbit tibialis anterior muscle, we show GS and GPh intracellular redistribution at the beginning of glycogen resynthesis after contraction-induced glycogen depletion. We identify a new "player," a new intracellular compartment involved in skeletal muscle glycogen metabolism. They are spherical structures that were not present in basal muscle, and we present evidence that indicate that they are products of actin cytoskeleton remodeling. Furthermore, for the first time, we show a phosphorylation-dependent intracellular distribution of GS. Here, we present evidence of a new regulatory mechanism of skeletal muscle glycogen metabolism based on glycogen enzyme intracellular compartmentalization.

Highlights

Glycogen metabolism has been the subject of extensive research, but the mechanisms by which it is regulated are still not fully understood
We showed that the lack of glycogen resynthesis was not due to a high glycogen phosphorylase (GPh) activity ratio
We showed a clear intracellular redistribution of both glycogen synthase (GS) and GPh after 3 h of continuous low frequency (CLF) stimulation (Figs. 3 and 4), which coincides with the beginning of muscle glycogen resynthesis

Summary

Introduction

Glycogen metabolism has been the subject of extensive research, but the mechanisms by which it is regulated are still not fully understood. It is well accepted that the ratelimiting enzymes in glycogenesis and glycogenolysis are glycogen synthase (GS) and glycogen phosphorylase (GPh), respectively Both enzymes are regulated by reversible phosphorylation and by allosteric effectors. We present evidence of a new regulatory mechanism of skeletal muscle glycogen metabolism based on glycogen enzyme intracellular compartmentalization. The rate-limiting enzymes in glycogenesis and glycogenolysis are considered to be GS and glycogen phosphorylase (GPh), respectively Compartmentalized Glycogen Metabolism in Skeletal Muscle the allosteric activator glucose 6-phosphate (Glc-6-P) reverses inactivation of GS by phosphorylation [7] and increases the susceptibility of the enzyme to dephosphorylation by GS phosphatases [8]. The ratio of the activities in the presence and absence of the allosteric activator is a useful measure of the phosphorylation state for both enzymes

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Results

Conclusion