Modulation of contractile protein gene expression in fetal murine crural muscles: Emergence of muscle diversity

Open Access
Publisher Website
Google Scholar
Abstract
The modulation of contractile protein gene expression in mouse crural muscles (i.e., muscles located in the region between the knee and ankle) during the fetal period (defined as 15 days gestation to birth), resulting in diversity among and within these muscles, has been evaluated with in situ hybridization and correlated with morphogenetic events in the extensor digitorum longus and soleus muscles. During the fetal period extensive secondary myotube formation occurs in the crural muscles, and the myotubes become innervated (Ontell and Kozeka [1984a, b] Am. J. Anat. 171:133–148, 149–161; Ontell et al. [1988a, b] Am. J. Anat. 181:267–278, 181:278–288). At 15 days gestation, hybridization with 35S-labeled antisense cRNA probes demonstrates the accumulation of transcripts forα-cardiac andα-skeletal actin; MLC1A, MLC1F, and MLC3F; and MHCemb, MHCpn, and MHCβ/slow. At 16 days gestation, accumulation of MHCemb transcripts is reduced (as compared with earlier developmental stages); intensity of signal following hybridization with the probe forα-skeletal actin is, for the first time, equal to that for the cardiac isoform; and MLC1V mRNA accumulation is discernible. At this stage, variation in transcript accumulation for some mRNAs among and within crural muscles becomes evident. Two factors may play a role in the selective distribution of these transcripts: (1) the stage of muscle maturation; and (2) the future myofiber type. At 16 days gestation anterior crural muscles (which mature ˜ 2 days before posterior crural muscles; Ontell and Kozeka [1984a, b], ibid., Ontell et al. [1988a, b], ibid.) exhibit a greater accumulation of transcripts forα-skeletal actin and for MLC3F than is found in posterior crural muscles. In muscles that in the neonate are composed, in large part, of slow myofibers, MHCβ/slow and MLC1V mRNAs accumulate in greater amounts, whereas MHCpn transcripts are less abundant in the soleus muscle than in other crural muscles. By 19 days gestation regionalization of transcript accumulation is more pronounced. The soleus muscle, a predominantly slow twitch muscle in the newborn mouse (Wirtz et al. [1983] J. Anat. 137:109–126) exhibits strong signal after hybridization with probes specific for MHCβ/slow and MLC1V. While the level of transcript accumulation for the developmetal isoforms, MHCemb, MLC1A, andα-cardiac actin, is greatly reduced in most crural muscles at 19 days gestation, these transcripts persist in the soleus muscle at levels equal to or exceeding their amount in limb muscles of 13 day gestation mouse embryos. By 19 days gestation both MyoD and myogenin are “down-regulated” (as compared with their expression at earlier developmental stages) in all muscle masses. Alterations in contractile protein gene expression are correlated with changes in the myogenic regulatory factors present in fetal hindlimbs during development.