Endocytosis in skeletal muscle fibers

Defining the organization of endocytic pathway in multinucleated skeletal m yofibers 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 myof ibers. We found that sarcolemmal-coated pits and transferrin receptors were concentrated in the I band areas. Fluid phase markers were taken up into v esicles in the same areas along the whole length of the fibers and were the n delivered into structures around and between the nuclei. These markers al so accumulated beneath the neuromuscular and myotendinous junctions. The re cycling 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 tra nsported into sparsely distributed perinuclear and interfibrillar dots that contacted microtubules. A majority of these dots did not colocalize with i nternalized transferrin, indicating that the recycling and the lysosome-dir ected pathways were distinct. In conclusion, the I band areas were active i n endocytosis along the whole length of the multinucleated myofibers. The s orting endosomes distributed in a cross-striated fashion while the recyclin g and late endosomal compartments showed perinuclear and interfibrillar loc alizations and followed the course of microtubules. (C) 1999 Academic Press .