Insulin-stimulated endosomal trafficking of GLUT4 - a change to the model?

Recent advances in spatiotemporally resolved imaging and single-molecule labeling technologies have provided new mechanistic insight into the very early phases of insulin-responsive GLUT4 (also known as SLC2A4) trafficking. Live-cell assays combining quantum dot tracking and fluorescence-based fusion reporters have uncovered a previously overlooked, insulin-induced initial and fundamental step - that static GLUT4 vesicles undergo heterotypic fusion with transferrin receptor-positive endosomes. This insulin-induced fusion functions as a molecular gateway - termed 'fusion-guided GLUT4 entry' (FGG4E) - that enables GLUT4 molecules, originally sequestered in static vesicles, to circulate within a dynamic endosomal network when insulin is present, escaping the trafficking itinerary that leads to static retention. Through this pathway, insulin-stimulated GLUT4 is efficiently delivered to the plasma membrane and continues to recycle dynamically between the plasma membrane and endosomal compartments. After insulin withdrawal, GLUT4 molecules are retrieved from the endosomal system and returned to the static pool. In this Opinion article, we propose that this revised model highlights a key regulatory role for heterotypic vesicle fusion as a gateway linking basal retention with dynamic mobilization and recycling, and redefines GLUT4 trafficking beyond the classical view of vesicle mobilization.