Characterization of Pax3 and Pax7 genes and their expression patterns during different development and growth stages of Japanese pufferfish Takifugu rubripes

Pax3 and Pax7 are the regulators and markers of muscle progenitors and satellite cells that contribute to the embryonic development and postembryonic growth of skeletal muscle in vertebrates, as well as to its repair and regeneration. However, information regarding them in vertebrate genome model, torafugu Takifugu rubripes, has remained unknown. Therefore, as an initial step, here we characterized Pax3 and Pax7 from torafugu and investigated their expression patterns during different developmental stages by RT-PCR. In silico analysis with the Fugu genome database (ver. 4.0) yielded two distinct genes each for Pax3 (Pax3a and Pax3b) and Pax7 (Pax7a and Pax7b). The 75th amino acid, glutamine (Gln75), from the N-terminus was replaced by proline in the paired box domain (PD) of Pax3a. One single cDNA clone encoding Pax3a had deletion of Gln75 in PD, suggesting the presence of alternatively spliced variants (Q+/Q-). This was further supported by identification of two adjacent alternative 3' splice acceptor sites which produce Pax3b Q+ (aagCAGGGA) and Q- (aagcagGGA) variants. Interestingly, torafugu Pax7a, but not Pax7b, had an insert encoding five amino acid residues (SGEAS) in a C-terminal region of PD in two out of three cDNA clones. Genomic analysis showed two alternate splice donor sites at exon 4 of Pax7a. In synteny analysis, torafugu Pax3a showed syntenic relationship with the corresponding regions in other teleosts only, whereas Pax3b and Pax7b showed high syntenic relationship with the corresponding regions of both mammals and other teleosts. RT-PCR revealed that expression of Pax3a and Pax3b transcripts was restricted to embryonic stages only, whereas those of Pax7a and Pax7b was continued to be expressed in larvae and importantly those of Pax7a were found in adult skeletal muscles. Therefore, Pax3 appears to be most important for primary myogenesis and Pax7 for secondary myogenesis and growth by hyperplasia in fish. In this regard, the transcripts of torafugu Pax3 and Pax7 genes might be used for further investigation as a marker for identification of muscle precursor cells during different phases of growth, and this ambiguity is the next target of our research.