A conserved MRF4 promoter drives transgenic expression in Xenopus embryonic somites and adult muscle

Olive flounder (Paralichthys olivaceus) myogenic regulatory factor 4 and its muscle-specific promoter activity

Myogenic regulatory factor 4 (MRF4) is a basic helix-loop-helix (bHLH) transcription factor that plays crucial roles in myoblast differentiation and maturation. Here, we report the isolation of the olive flounder (Paralichthys olivaceus) mrf4 gene and the spatiotemporal analysis of its expression patterns. Sequence analysis indicated that flounder mrf4 shared a similar structure with other vertebrate MRF4, including the conserved bHLH domain. Flounder mrf4 contains 3 exons and 2 introns. Sequence alignment and phylogenetic analysis showed that it was highly homologous with Salmo salar, Danio rerio, Takifugu rubripes, and Tetraodon nigroviridis mrf4. Flounder mrf4 was first expressed in the medial region of somites that give rise to slow muscles, and later spread to the lateral region of somites that give rise to fast muscles. Mrf4 transcript levels decreased significantly in mature somites in the trunk region, and expression could only be detected in the caudal somites, consistent with the timing of somite maturation. Transient expression analysis showed that the 506 bp flounder mrf4 promoter was sufficient to direct muscle-specific GFP expression in zebrafish embryos.

Making muscle: Morphogenetic movements and molecular mechanisms of myogenesis in Xenopus laevis

Xenopus laevis offers unprecedented access to the intricacies of muscle development. The large, robust embryos make it ideal for manipulations at both the tissue and molecular level. In particular, this model system can be used to fate map early muscle progenitors, visualize cell behaviors associated with somitogenesis, and examine the role of signaling pathways that underlie induction, specification, and differentiation of muscle. Several characteristics that are unique to X. laevis include myogenic waves with distinct gene expression profiles and the late formation of dermomyotome and sclerotome. Furthermore, myogenesis in the metamorphosing frog is biphasic, facilitating regeneration studies. In this review, we describe the morphogenetic movements that shape the somites and discuss signaling and transcriptional regulation during muscle development and regeneration. With recent advances in gene editing tools, X. laevis remains a premier model organism for dissecting the complex mechanisms underlying the specification, cell behaviors, and formation of the musculature system.