Developmental potential of rat L6 myoblasts in vivo following injection into regenerating muscles.

To examine the relative importance of myoblast lineage and environmental influences on the development of muscle fiber types in vivo, the phenotype of muscle fibers formed from rat L6 myoblasts was examined following their injection into different regenerating adult muscles. Myoblasts were infected with a retroviral vector carrying a LacZ reporter gene and their fate in vivo was examined using a panel of antibodies against various myosin heavy chain (MyHC) isoforms. Since L6 myoblasts express IIX MyHC following differentiation in vitro, we wanted to determine if they would form IIX muscle fibers in vivo and whether innervation would alter this fate. Following injection, L6 cells either fused with each other to form homotypic fibers or fused with host muscle cells to form heterotypic fibers. Initially, homotypic fibers expressed embryonic MyHC-similar to L6 myotubes in vitro. However, by 4 weeks postinjection IIX MyHC had replaced embryonic MyHC as the predominant isoform. Single fiber analysis using an antibody specific for NCAM indicated that this transition was independent of innervation. Analysis of heterotypic fibers resulting from the incorporation of donor L6 myoblasts into host fast IIA and IIB fibers revealed that L6-derived nuclei express embryonic and IIX MyHCs for up to 8 weeks postinjection, often as nuclear domains surrounding L6 nuclei. These results suggest that MyHC expression in muscle fibers derived from L6 myoblasts is regulated, in part, by intrinsic factors that limit the fiber type potential of these cells in vivo.