Relationships between protein and nucleic acid synthesis during muscle differentiation in vitro

The authors studied DNA, RNA, and protein synthesis in differentiating skeletal muscle cell cultures from chicken embryos and newborn rats. The study was performed at different phases of myogenesis, namely, during myoblast division, fusion, and final differentiation of multinucleated muscle fibers. Both types of cultures showed a very active DNA synthesis during cell division, followed by a sharp decrease at the moment of myoblast fusion, and by a complete disappearance of DNA synthesis during fiber development. A parallel reduction of macromolecular RNA synthesis (mainly ribosomal RNA) could be observed. Conversely, protein synthesis increased after myoblast fusion and during fiber development. Experiments of pulse and chase with tritiated and cold uridine showed that the decrease of uridine incorporation is due to a real decrease of RNA synthesis and not to a different permeability of different cell types to the precursor. Moreover it was found that muscle cells have a typical kinetic of intracellular pool saturation, yet this is a property that myoblasts and fibers have in common. The high level of protein synthesis occurring after cell fusion is relatively independent of the RNA de novo synthesis. In differentiated cultures proteins can be synthesized up to 24 hr after the complete inhibition of RNA synthesis which follows actinomycin D treatment. These results could be explained by proposing the existence, in the muscle fiber, of stable messenger RNA molecules. The authors found that even myoblasts and fibroblasts at the earliest stages can sustain protein synthesis, after actinomycin D treatment, for an appreciable length of time. This phenomenon could have two alternative explanations: either all the proteins of any cell belonging to the muscle line are synthesized copying stable messenger RNA molecules, or, if those long life RNA molecules are specific for the contractile proteins, it should be concluded that these highly specialized proteins must be already present at very early stages of muscle cell development. The relationships between macromolecular synthesis and the steps of myogenesis, with a particular emphasis on RNA and protein synthesis relationships, were analyzed.