Myoblast and myotube nuclei display similar patterns of heterogeneous acetylcholine receptor subunit mRNA expression

Muscle progenitor cells differentiate to myoblasts, and subsequently myotubes, upon expression of muscle specific genes. We and others have previously shown that myotube nuclei, even in the absence of nerve, express AChR alpha subunit RNA at varying levels, with a small subset (about ten percent) of the nuclei expressing at high levels. These findings raised two important questions: 1) is the observed heterogeneity a unique property of the alpha subunits, and 2) when does the heterogeneity begin? In particular, is it induced only at or after the time of fusion, or does it exist at the myoblast stage? We have, therefore, extended our observations to the gamma and delta subunits and we also have examined the distributions of AChR alpha, gamma, and delta subunit RNAs in both myoblasts and myotubes. We used intron and intron-exon probes to detect prespliced transcripts or mature mRNAs in the cells. Because intron-containing transcripts are not transported out of the nuclei, the distributions of these transcripts can indicate their expression patterns among nuclei in the same myotubes. Our results show that both myotubes and myoblasts have distributions of the AChR alpha, gamma, and delta subunit RNAs which differ sharply from that of the U1 RNA or Myo D. Thus, the heterogeneous expression of AChR genes is not only an intrinsic property of muscle cell nuclei (in the sense that it does not require the presence of nerves), but it also exists prior to fusion. Our results suggest that muscle nuclei attain individualized capacities for AChR subunit mRNA production early in their development. Conceptual models consistent with such individuality imply an additional level of regulation beyond the known diffusible transcriptional factors.