Golgi complex reorganization during muscle differentiation: Visualization in living cells and mechanism

During skeletal muscle differentiation, the Golgi complex (GC) undergoes a dramatic reorganization. We have now visualized the differentiation and fus ion of living myoblasts of the mouse muscle cell line C2, permanently expre ssing a mannosidase-green fluorescent protein (GFP) construct. These experi ments reveal that the reorganization of the GC is progressive (1-2 h) and i s completed before the cells start fusing. Fluorescence recovery after phot obleaching (FRAP), immunofluorescence, and immunogold electron microscopy d emonstrate that the GC is fragmented into elements localized near the endop lasmic reticulum (ER) exit sites. FRAP analysis and the ER relocation of en dogenous GC proteins by phospholipase A2 inhibitors demonstrate that Golgi- ER cycling of resident GC proteins takes place in both myoblasts and myotub es. All results support a model in which the GC reorganization in muscle re flects changes in the Golgi-ER cycling. The mechanism is similar to that le ading to the dispersal of the GC caused, in all mammalian cells, by microtu bule-disrupting drugs. We propose that the trigger for the dispersal result s, in muscle, from combined changes in microtubule nucleation and ER exit s ite localization, which place the ER exit sites near microtubule minus ends . Thus, changes in GC organization that initially appear specific to muscle cells, iri fact use pathways common to all mammalian cells.