Evolutionary role of the skeletal muscle in vertebrates: NfIx role during muscle development in mammals and zebrafish (Danio rerio)

The skeletal muscle tissue in amniotes is formed by different classes of myogenic precursors that act in different steps: embryonic myoblasts, fetal myoblasts and satellite cells. The molecular control of the switch from one phase of muscle development to the further ones is still less known. Only recently, genes that are differentially expressed in murine embryonic and fetal myoblasts have been identified; in particular, the transcription factor Nfix is present only in fetal myoblasts while it is absent from embryonic myoblasts (Biressi et al., 2007). Functional analyses of the NfIx gene evidenced that it is able both to activate the transcription of specific fetal genes (e.g. Mck, beta-enolase) and to inhibit specific embryonic genes (e.g. slow Myosin heavy Chain). The teleostean fish zebrafish (Danio rerio) presents unique features to study NfIx functions and its role during muscle development. In fact, in zebrafish a clear distinction between embryonic and fetal myogenesis is absent in this lower vertebrate. In zebrafish, muscle precursors appeare only 12 hours after birth and it is possible to carefully analyze myogenesis and NfIx homolog's expression territories. Functionally, zebrafish is an ideal model for quick loss- and gain-of-function analyses, by means of microinjection techniques.