Contractile protein gene expression in primary myotubes of embryonic mouse hindlimb muscles

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Abstract
The time course of contractile protein [actin, myosin heavy chain (MHC) and myosin light chain (MLC)] gene expression in the hindlimb muscles of the embryonic mouse (<15 days gestation) has been correlated with the expression of genes for the myogenic regulatory factors, myogenin and MyoD, and with morphogenetic events. At 14 days gestation, secondary myotubes are not yet present in crural muscles (M. Ontell and K. Kozeka (1984) Am. J. Anat. 171, 133-148; M. Ontell, D. Bourke and D. Hughes (1988)Am. J. Anat. 181, 267-278); there-fore, all transcripts for contractile proteins found in these muscles must be produced in primary myotubes. In situ hybridization, with 35S-labeled antisense cRNAs, demonstrates the versatility of primary myotubes in that transcripts for (1) α-cardiac and α-skeletal actin, (2) MHCembryonic, MHCperinatal and MHC /slow, and (3) MLC1A, MLC1F and MLC3F are detectable at 14 days gestation. While the general patterns of early activation of the cardiac genes and early activation of the genes for the developmental isoforms are preserved in both myotomal and limb muscles (D. Sassoon, I. Garner and M. Buckingham (1988)Development 104, 155-164 and G. E. Lyons, M. Ontell, R. Cox, D. Sassoon and M. Buckingham (1990)J. Cell Biol. 111, 1465-1476 for myotomal muscle), there are a number of differences in contractile protein gene expression. For example, in the myotome, when myosin light chain genes are initially transcribed, hybridization signal with probe for MLC1A mRNA is greater than that with probe for MLC1F tran-scripts, whereas the relative intensity of signal with these same probes is reversed in the hindlimb. The order in which myosin heavy chain genes are activated is also different, with MHCembryonic and MHCperinatal preced-ing the appearance of MHC β/slow transcripts in limb muscles, while MHCembryonic and MHC /slow appear simultaneously in the myotomes prior to MHCperinatal. In the myotome, an intense hybridization signal for α-cardiac and a weak signal for α-skeletal actin tran-scripts are detectable prior to myosin mRNAs, whereas in the limb α-cardiac actin transcripts accumulate with myosin transcripts before α-skeletal actin mRNA is detectable. These differences indicate that there is no single coordinate pattern of expression of contractile protein genes during initial formation of the muscles of the mouse. The fact that different myogenic factors are present initially in myotomes than are present initially in the hindlimb (cf. M. E. Buckingham (1992)Trends Genet. 8, 144-149) suggests that there may be subtle dif-ferences in the capacity of these factors to activate dif-ferent muscle genes. These results are discussed in the context of muscle cell lineages.