CHANGES IN CHROMATIN STRUCTURE OF THE CONTRACTILE PROTEIN GENES DURING MUSCLE DIFFERENTIATION

Abstract

Myogenesis in the rat myoblast cell line, L(,6)E(,9), involves an irreversible withdrawal from the cell cycle, an increase in the steady state levels of muscle-specific mRNAs and proteins, and fusion to form multinucleate myotubes. I studied several inheritable aspects of contractile protein gene and chromatin structure, which could be involved in the maintenance of the expression of this potential in the differentiated myotubes. By pulse labeling the histone proteins in myoblasts and at various times after myogenic induction, I found a cessation of histone, as well as DNA synthesis, as the cells irreversibly withdrew from the cell cycle.;Using cloned portions of the genes and mRNAs encoding the myosin heavy chains (MHC), myosin light chains, actins, troponins and tropomyosins as probes in blot hybridization studies, I found: (1) an allelic polymorphism for 1 cardiac muscle MHC gene in the laboratory rat population, (2) no evidence for rearrangement or change in copy number of the contractile protein genes during muscle differentiation, (3) preferential DNase I sensitivity for the contractile protein genes in the L(,6)E(,9) myotubes, not in L(,6)E(,9) myoblasts, where they are committed, but unexpressed, (4) a tissue-specific set of methylation patterns for the embryonic MHC gene in L(,6)E(,9) myoblasts and myotubes, compared to sperm or adult rat tissues. This specific methylation pattern precedes the transcriptional activation of the MHC gene during L(,6)E(,9) myogenesis and may be one factor which accounts for the maintenance of the myogenic potential of dividing myoblasts.