Endocytosis of alpha 2-macroglobulin is developmentally regulated during myogenesis

A monoclonal antibody, H143, reacts with an intracellular antigen present and accumulated in E63 rat myoblasts. H143 is directed against a species-specific determinant on purified equine serum alpha 2-macroglobulin. Immunofluorescence analyses of differentiating myoblasts grown in horse serum demonstrate that the capacity to take up alpha 2-macroglobulin is stage-specific: the rapid uptake of alpha 2-macroglobulin characteristic of myoblasts ceases prior to their fusion to form multinucleate fibers (myotubes). Neither rat fibroblasts nor a developmentally defective mutant of E63 exhibit this change in alpha 2-macroglobulin uptake. The temperature and calcium requirements for the uptake of H143 antigen, and its accumulation as effected by lysosomotropic amines, indicate that alpha 2-macroglobulin is taken up by myoblasts via a developmentally regulated endocytic process. Electron microscopy using equine alpha 2-macroglobulin labeled with colloidal gold supports this finding.