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.
Keywords: Actins; Adenosine Monophosphate; Allosteric Site; Amino Acid Sequence; Animals; Centrifugation; Cytoplasm; Cytoskeleton; Female; Glycogen; Glycogen Phosphorylase; Glycogen Synthase; Image Processing, Computer-Assisted; Immunohistochemistry; Microscopy, Electron, Transmission; Microscopy, Fluorescence; Molecular Sequence Data; Muscle, Skeletal; Muscles; Peptides; Phosphorylation; Protein Conformation; Protein Structure, Tertiary; Protein Transport; Rabbits; Sarcoplasmic Reticulum; Subcellular Fractions; Tibia; Time Factors