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Heterokaryons provide a model system in which to examine how tissue-specific phenotypes arise and are maintained. When muscle cells are fused with nonmuscle cells, muscle gene expression is activated in the nonmuscle cell type. Gene expression was studied either at a single cell level with monoclonal antibodies or in mass cultures at a biochemical and molecular level. In all of the nonmuscle cell types tested, including representatives of different embryonic lineages, phenotypes, and developmental stages, muscle gene expression was induced. Differences among cell types in the kinetics, frequency, and gene dosage requirements for gene expression provide clues to the underlying regulatory mechanisms. These results show that the expression of genes in the nuclei of differentiated cells is remarkably plastic and susceptible to modulation by the cytoplasm. The isolation of the genes encoding the tissue-specific trans-acting regulators responsible for muscle gene activation should now be possible.

We have developed an in vitro system for the study of postnatal human muscle under standardized conditions. The technique utilizes cloning to isolate pure populations ofmuscle cells. By manipulating culture conditions we can maximize either proliferation or differentiation of individual clones or of clones pooled to yield mass cultures of muscle cells. The muscle phenotype is stable; cells can be stored in liquid nitrogen for long-term use without loss of proliferative or differentiative potential. We have determined proliferative capacity of muscle cells from an analysis of clonal growth kinetics; we determined differentiative capacity from morphological evidence (cell fusion, striations, contractions, and the appearance of acetylcholine receptors) and biochemical analysis of muscle protein synthesis (creatine kinase, aactin, tropomyosin, and myosin light chains). Our approach eliminates the variability in cellular composition that has complicated studies of primary muscle to date. We can now study in a controlled fashion the interactions and contributions of different cell types to the development of normal and genetically dystrophic human muscle.

A defect in the proliferative capacity of satellite cells, mononucleated precursors of mature muscle fibers, was found in clonal analyses of cells cultured from Duchenne muscular dystrophy (DMD) patients. The total yield of myoblasts per gram of muscle biopsy was decreased to 5% of normal. Of the DMD myoblast clones obtained, a large proportion -contained a morphological class of flat distended cells that had an increased generation time and ceased to proliferate beyond 100-1,000 cells but could be induced to fuse and form myotubes. The altered muscle phenotype was detected in all cultures from DMD patients but was rarely found among myoblasts of controls. By age 14 yr, it comprised as many as 90% of DMD myoblasts. The remaining DMD myoblast clones, which initially grew well, had severely impaired proliferative capacity upon passage and further cultivation. Eventually all myoblasts from DMD muscle tissue exhibited defective growth potential. In contrast, the fibroblast yield and proliferative capacity from DMD samples did not differ from normal. Based on these findings, we propose a hypothesis for the etiology of DMD: Dividing myoblasts are required for muscle growth and maintenance, and the limited capacity of DMD myoblasts to grow is directly related to the progressive muscle degeneration characteristic of the disease.Duchenne muscular dystrophy (DMD) is a chronic, lethal Xlinked disease that affects 1 in 4,800 males (1). Although DMD is well defined clinically, the cellular and biochemical basis for the disease is unknown. A major problem in the biological study of muscular dystrophy has been the heterogeneous nature of muscle tissue. Muscle is composed of a mixture of cell types present in variable ratios, especially in disease states. When muscle is dissociated and the cells plated in culture, both myoblasts and fibroblasts are obtained and the rate of proliferation of the fibroblasts frequently exceeds that of the myoblasts. DMD muscle cultures have been compared with normal muscle cultures and a number of differences reported, including increased collagen synthesis (2), decreased specific activity of creatine kinase (3), increased ratio of nonmuscle to muscle creatine kinase isozymes (3-5), altered lipid biosynthesis (6), and clustered myotube morphology (5,7,8). However, it is unclear whether these differences are intrinsic to the DMD myoblasts and related to the disease or a reflection of the proportion of fibroblasts present in the mixed cultures analyzed.We have developed methods for obtaining pure populations of human myoblasts that can be studied under standardized conditions (9). Our methods use postnatal muscle as a source.Consequently, the myoblasts obtained in culture are muscle satellite cells, the mononucleated myoblasts of mature muscle fibers, positioned between the basement membrane and the sarcolemma (10). It is these satellite cells that are capable of proliferating in vitro (11, 12) and are probably responsible for postnatal muscle regeneration and fiber growth in vivo. We have...