Endocytosis in skeletal muscle fibers

Defining the organization of endocytic pathway in multinucleated skeletal myofibers is crucial to understand the routing of membrane proteins, such as receptors and glucose transporters, through this system. Here we analyzed the organization of the endocytic trafficking pathways in isolated rat myofibers. We found that sarcolemmal-coated pits and transferrin receptors were concentrated in the I band areas. Fluid phase markers were taken up into vesicles in the same areas along the whole length of the fibers and were then delivered into structures around and between the nuclei. These markers also accumulated beneath the neuromuscular and myotendinous junctions. The recycling compartment, labeled with transferrin, appeared as perinuclear and interfibrillar dots that partially colocalized with the GLUT4 compartment. Low-density lipoprotein, a marker of the lysosome-directed pathway, was transported into sparsely distributed perinuclear and interfibrillar dots that contacted microtubules. A majority of these dots did not colocalize with internalized transferrin, indicating that the recycling and the lysosome-directed pathways were distinct. In conclusion, the I band areas were active in endocytosis along the whole length of the multinucleated myofibers. The sorting endosomes distributed in a cross-striated fashion while the recycling and late endosomal compartments showed perinuclear and interfibrillar localizations and followed the course of microtubules.