Differential expression of myogenic determination genes in muscle cells: Possible autoactivation by the Myf gene products

The development of muscle cells involves the action of myogenic determination factors. In this report, we show that human skeletal muscle tissue contains, besides the previously described Myf-5, two additional factors Myf-3 and Myf-4 which represent the human homologues of the rodent proteins MyoD1 aned myogenin. The genes encoding Myf-3, Myf-4 and Myf-5 are located on human chromosomes 11, 1, and 12 respectively. Constitutive expression of a single factor is sufficient to convert mouse C3H 10T1/2 fibroblasts to phenotypically normal muscle cells. The myogenic conversion of 10T1/2 fibroblasts results in the activation of the endogenous MyoD1 and Myf-4 (myogenin) genes. This observation suggests that the expression of Myf proteins leads to positive autoregulation of the members of the Myf gene family. Individual myogenic colonies derived from MCA Cl15 cells (10T1/2 fibroblast transformed by methylcholanthrene) express various levels of endogenous MyoD1 mRNA ranging from nearly zero to high levels. The Myf-5 gene was generally not activated in 10T1/2 derived myogenic cell lines but was expressed in some MCA myoblasts. In primary human muscle cells Myf-3 and Myf-4 mRNA but very little Myf-5 mRNA is expressed. In mouse C2 and P2 muscle cell lines MyoD1 is abundantly synthesized together with myogenin. In contrast, the rat muscle lines L8 and L6 and the mouse BC3H1 cells express primarily myogenin and low levels of Myf-5 but no MyoD1. Myf-4 (myogenin) mRNA is present in all muscle cell lines at the onset of differentiation. From these differential patterns of Myf expression we conclude that, although in principle each Myf factor is capable of activating its own and related Myf genes, only a subset of myogenic factors is actually expressed in most muscle cells and this is sufficient to generate and maintain the differentiated phenotype.