Myogenic conversion of mammalian fibroblasts induced by differentiating muscle cells

Somite-derived skeletal myoblasts are supposed to be the sole source of muscle fibre nuclei during pre- and postnatal development, but evidence is accumulating for unorthodox contributions to muscle fibre nuclei from other cell types. For example, in tissue culture, fibroblasts can fuse with dysgenic myoblasts and restore correct membrane function. We report here the results of a series of experiments investigating this phenomenon and its possible mechanism, 10T1/2 cells, infected with a replication defective retrovirus encoding the bacterial enzyme β-galactosidase, fused to form β-galactosidase positive, differentiated myotubes when cocultured with differentiating uninfected C2C12 or primary myogenic cells, but this did not occur when they were cocultured with other cells such as 3T3 fibroblasts or PC12 pheochromocytoma cells. Myogenic conversion ranged from 1 to 10% of the 10T1/2 cell population and required close cell interaction between the different cells types: it was not induced by conditioned medium or extracellular matrix deposited by C2C12 cells. Myogenic conversion was also observed in vivo, after injection of similarly infected 10T1/2 cells into regenerating muscle. Conversion was seen also after coculture of uninfected 10T1/2 cells with primary chick myoblasts, thus demonstrating that it was not dependent upon viral infection and that there is no species or class barrier in this phenomenon. Primary fibroblasts, isolated from different organs of transgenic mice carrying a Lac Z marker under the control of a muscle-specific promoter, restricting β-galactosidase expression to striated muscle cells, also underwent myogenic conversion, when cocultured with C2C12 myoblasts. The efficiency of this conversion varied with their embryological origin, being common in cells with a dorsal mesoderm lineage but rare in cells of ventral mesoderm origin. These experiments demonstrate that myogenic conversion is a true embryological feature of mammalian mesodermal cells. Conversion of mononucleated cells was also observed, showing that fusion is not a pre-requisite for myogenic differentiation and may indeed be a consequence of differentiation induced by short-range local signalling. We conclude that a proportion of adult cells of mesodermal origin may conserve a bi- or multi-potential state of determination throughout the life of an animal, enhancing the regenerative capacity of the tissues in which they reside.