From zebrafish (Danio rerio) to mouse, the function of Nfix in skeletal muscle development is partially conserved during the evolution

In amniotes, skeletal muscle is formed by different classes of myogenic precursors (embryonic, fetal myoblasts and satellite cells) that give rise to subsequent generation of muscle fibers in distinct developmental stages. Genes that are differentially expressed in murine embryonic and fetal myoblasts have been identified and among all, we have been demonstrated that the transcription factor Nfix regulates fetal-specific transcription in developing skeletal muscle. Zebrafish muscles, though similar to mouse, develop at a faster rate to enable fish to escape from predators and find food. In fact, only 24 hours post fertilization (24 hpf) a functional myotome has been formed with primary slow and fast fibers. Following this primary muscle wave, at 48 hpf, secondary slow fibers differentiate. Until now, primary and secondary myogenic waves in zebrafish have not been correlated with their murine counterparts. We use a loss-of-function approach to specifically abrogate nfixa function in vivo and, by combining techniques as In Situ Hybridization, Immunohistochemistry, TEM, qRT-PCR and Western Blot, we demonstrate that lack of nfixa impairs larvae motility. At variance with mouse, this phenotype is due to a strong reduction of the sarcoplasmic reticulum. However, much as in mouse, nfixa strongly regulates slow twitch fiber differentiation during the second myogenic wave. Despite differences in body plan organization between fish and mouse, we show here the first evidence of a conserved role for Nfix in regulating a more mature and organized fiber during skeletal muscle development. Moreover, we highlight a new role for zebrafish nfixa in sarcoplasmic reticulum formation.