Do increases in enzyme activities during muscle differentiation reflect expression of new genes?

Abstract

TRANSITIONS from embryonic isoenzymic forms to mature adult forms are known to occur during in vitro myogenesis for creatine phosphokinase¹ (CPK) and other muscle enzymes². More recently, similar transitions have been shown to occur for myosin^3,4, and actin^5,6. Both CPK activity^7,8 and amino acid incorporation into myosin^9–11 increase during the early stages of differentiation in vitro when cells fuse to form multinucleate myotubes, and it is tempting to assume that these increases require the production of adult form mRNA after activation of a “battery” of genes for myogenesis². Yaffe and Dym¹¹ have proposed a model of myogenesis in which mRNAs for characteristic muscle proteins are synthesised before cell fusion and stored in a “masked” form until activated by the fusion process, thus giving rise to simultaneous increases in muscle-specific protein synthesis and cell fusion. We describe here experiments which show large increases in CPK activity at the time of early cell fusion which cannot result from activation of “masked” muscle-specific mRNAs and which do not require adult form mRNAs at all. Using a new technique which allows measurement of low levels of CPK isoenzymes, we have shown that the early increase in CPK activity during the first 2 d of culture is of the embryonic, or brain, form, and only later does the adult muscle form make a major contribution to the total increase in enzyme activity.