Identification of the crucial molecular events during the large-scale myoblast fusion in sheep

Abstract

It is well known that in sheep most myofibers are formed before birth; however, the crucial myogenic stage and the cellular and molecular mechanisms underpinning phenotypic variation of fetal muscle development remain to be ascertained. We used histological, microarray, and quantitative realtime PCR (qPCR) methods to examine the developmental characteristics of fetal muscle at 70, 85, 100, 120, and 135 days of gestation in sheep. We show that day 100 is an important checkpoint for change in muscle transcriptome and histomorphology in fetal sheep and that the period of 85-100 days is the vital developmental stage for large-scale myoblast fusion. Furthermore, we identified the cisregulatory motifs for E2F1 or MEF2A in a list of decreasingly or increasingly expressed genes between 85 and 100 days, respectively. Further analysis demonstrated that the mRNA and phosphorylated protein levels of E2F1 and MEF2A significantly declined with myogenic progression in vivo and in vitro. qRT-PCR analysis indicated that PI3K and FST, as targets of E2F1, may be involved in myoblast differentiation and fusion and that downregulation of MEF2A contributes to transition of myofiber types by differential regulation of the target genes involved at the stage of 85-100 days. We clarify for the first time the timing of myofiber proliferation and development during gestation in sheep, which would be beneficial to meat sheep production. Our findings present a repertoire of gene expression in muscle during large-scale myoblast fusion at transcriptome-wide level, which contributes to elucidate the regulatory network of myogenic differentiation.sheep; muscle; cis-regulatory motif; histology; microarray UNLIKE THE SMALL MAMMALS with two myogenic waves during development, there are at least three generations of myotubes in sheep (63). Myofiber formation in the sheep fetus begins at ϳ32 days of gestation, and the full complement of myofibers is achieved by days 80 -120 of gestation (3,39). Previous studies provide the valuable information on association between gene expression and prenatal skeletal muscle development in sheep at transcriptome-wide level (10,50,62). However, to our best knowledge, we still do not know the specific developmental stage at which large-scale fetal myoblasts undergo differentiation and fusion in sheep, or the ongoing molecular mechanisms underpinning the phenotypic variations in muscle during the fetal period.Myogenesis is a multiple-step process, including cell determination, proliferation, differentiation, and fusion, that is orchestrated by the expression of a cascade of transcriptional regulators such as Pax3/Pax7, members of the MyoD, MEF2, and E2F family, and so on (9, 44, 59). Myoblast fusion is a critical process that contributes to the growth of muscle during development and to the regeneration of myofibers upon injury. Myoblasts fuse with each other as well as with multinucleated myotubes to enlarge the myofiber. At the cellular level, the fusion process is characterized by the alignment of myoblast and myotu...

Signals of Ezh2, Src, and Akt Involve in Myostatin-Pax7 Pathways Regulating the Myogenic Fate Determination during the Sheep Myoblast Proliferation and Differentiation

“…Mutations in Myostatin gene lead to a hypertrophic phenotype in mice, sheep, cattle, dog, and human [ 1 – 6 ], suggesting that this growth factor alters the myogenic signals during the prenatal development. Surely, we recently demonstrate that there are crucial molecular events involved in the large-scale myoblast fusion in fetal sheep [ 7 ]. On the other hand, an important myogenic regulatory factor, the paired box transcription factor Pax7 plays an essential role for early and postnatal myogenesis [ 8 – 11 ].…”