CONTRACTILE PROTEIN GENE-EXPRESSION IN PRIMARY MYOTUBES OF EMBRYONIC MOUSE HINDLIMB MUSCLES

The time course of contractile protein [actin, myosin heavy chain (MHC ) and myosin light chain (MLC)] gene expression in the hindlimb muscle s 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, second ary myotubes are not yet present in crural muscles (M. Ontell and K. K ozeka (1984) Am. J. Anat. 171, 133-148; M. Ontell, D. Bourke and D. Hu ghes (1988) Am. J. Anat. 181, 267-278); therefore, all transcripts for contractile proteins found in these muscles must be produced in prima ry myotubes. In situ hybridization, with S-35-labeled antisense cRNAs, demonstrates the versatility of primary myotubes in that transcripts for (1) alpha-cardiac and alpha-skeletal actin, (2) MHC(embryonic), MH C(perinatal) and MHC(beta/slow), and (3) MLC1A, MLC1F and MLC3F are de tectable at 14 days gestation. While the general patterns of early act ivation of the cardiac genes and early activation of the genes for the developmental isoforms are preserved in both myotomal and limb muscle s (D. Sassoon, 1. 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 11 1, 1465-1476 for myotomal muscle), there are a number of differences in contractile protein gene expression. For exam ple, in the myotome, when myosin light chain genes are initially trans cribed, hybridization signal with probe for MLC1A mRNA is greater than that with probe for MLC1F transcripts, whereas the relative intensity of signal with these same probes is reversed in the hindlimb. The ord er in which myosin heavy chain genes are activated is also different, with MHC(embryonic) and MHC(perinatal) preceding the appearance of MHC (beta/slow) transcripts in limb muscles, while MHC(embryonic) and MHC( beta/slow) appear simultaneously in the myotomes prior to MHC(perinata l.) In the myotome, an intense hybridization signal for alpha-cardiac and a weak signal for alpha-skeletal actin transcripts are detectable prior to myosin mRNAs, whereas in the limb alpha-cardiac actin transcr ipts accumulate with myosin transcripts before alpha-skeletal actin mR NA is detectable. These differences indicate that there is no single c oordinate pattern of expression of contractile protein genes during in itial formation of the muscles of the mouse. The fact that different m yogenic factors are present initially in myotomes than are present ini tially in the hindlimb (cf. M. E. Buckingham (1992) Trends Genet. 8, 1 44-149) suggests that there may be subtle differences in the capacity of these factors to activate different muscle genes. These results are discussed in the context of muscle cell lineages.