Myomixer is expressed during embryonic and post-larval hyperplasia, muscle regeneration and fusion of myoblats in rainbow trout (Oncorhynchus mykiss)

Abstract

1.AbstractIn contrast to mice or zebrafish, trout exhibits post-larval muscle growth through hypertrophy and formation of new myofibers (hyperplasia). The muscle fibers are formed by the fusion of mononucleated cells (myoblasts) regulated by several muscle-specific proteins such as myomaker or myomixer. In this work, we identified a unique gene encoding a myomixer protein of 77 amino acids in the trout genome. Sequence analysis and phylogenetic tree, showed moderate conservation of the overall protein sequence across teleost fish (61% of aa identity between trout and zebrafish myomixer sequences). Nevertheless, the functionally essential motif, AxLyCxL is perfectly conserved in all studied sequences of vertebrates. Using in situ hybridization, we observed that myomixer was highly expressed in the embryonic myotome, particularly in the hyperplasic area. Moreover, myomixer remained readily expressed in white muscle of juvenile (1 and 20 g) although its expression decreased in mature fish. We also showed that myomixer is up-regulated during muscle regeneration and in vitro myoblasts fusion. Together, these data indicate that myomixer expression is consistently associated with the formation of new myofibers during somitogenesis, post-larval growth and muscle regeneration in trout.

Myomaker and Myomixer Characterization in Gilthead Sea Bream Under Different Myogenesis Conditions.

“…Additionally, the mice myomixer gene had another transcript form, less conserved, which had 3 exons and yielded a protein of 108 aa 9,12 , but until today, a single myomixer transcript has been described in the few sh species where this gene has been studied. Indeed, the only described gilthead sea bream myomixer transcript (ENSSAUT00010028952.1) that was found in the gilthead sea bream genome deposited in Ensembl contained only one exon that encoded a 75 aa protein, similarly to the rainbow trout myomixer (77 aa), although in this salmonid species, the gene is structured in two exons 13 .…”

“…The Myomixer protein sequence showed weak cross-species conservation, with mammals and sh sharing only 36% identity 13,15 . Among sh, the Myomixer gilthead sea bream showed an homology of 70.67% and 69.33% with the zebra sh and rainbow trout Myomixer protein sequences, respectively (unpublished data).…”

“…For instance, in rainbow trout (Oncorhynchus mykiss) Myomaker encodes for 434 aa and in zebra sh (Danio rerio) for 221 aa 7,8 . Although most of the reported experiments were done in mammals, in all vertebrates studied, including sh, myomaker expression is fundamental in both embryogenesis and regeneration of adult skeletal muscle [7][8][9][10][11][12][13] . The expression pattern of myomaker is similar to myod and myogenin because its regulation is in fact carried out through these MRFs 14 .…”

“…In mammals, myomixer encodes for an 84 aa protein 15 . In sh, Myomixer is a 75 aa protein in zebra sh 15 and a 77 aa peptide in rainbow trout 13 . myomixer is expressed during embryogenesis, muscle regeneration and myoblast differentiation in mice and trout 9,13,15 .…”

“…In sh, Myomixer is a 75 aa protein in zebra sh 15 and a 77 aa peptide in rainbow trout 13 . myomixer is expressed during embryogenesis, muscle regeneration and myoblast differentiation in mice and trout 9,13,15 . Similarly to myomaker, MyoD and Myogenin regulate the transcription of myomixer by binding to the three E-boxes of its promoter during myogenesis 12 .…”