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Muscle fibers consist of multi-nucleated cells that are the basic cellular building block of muscle. In adults, muscle growth and regeneration mainly occur through the activation, proliferation and differentiation of a resident population of muscle stem cells called satellite cells. Once activated, (known as myoblast) these cells differentiate and either fuse to existing myofibers (hypertrophy) or fuse together to form new myofibers (hyperplasia). Contrary to muscle growth in terrestrial livestock animals, post-larval muscle growth in large and rapidly growing fish, such as trout, results from both hypertrophy and hyperplasia. Thus, in fish, satellite cells have the choice to fuse with an existing myofiber or with another satellite cell. The factors that control the fusion with either a myofiber or another myoblast to enhance the growth of muscle are of great interest from an agronomic and therapeutic standpoint. Our project aims to identify new molecular factors involved in the commitment of a satellite cell to fuse with a myoblast or with a myotube. We are particularly interested in the identification of the differences between the genetic programs underlying myoblast-myoblast fusion and myoblast-myotube fusion.

For the newly identified genes, we will study their expression (task 3) during the embryonic development of zebrafish, as well as during the fusion of satellite cellsin vitro. We will determine the function of the identified genesin vivoduring embryonic myogenesis using a morpholino. Finally, we will determine the involvement of the identified genes in the balance between hyperplasia and hypertrophy. We will carry out gain-of-function and loss-of-function experiments for the candidate genes in trout muscle, which exhibits a high level of hyperplasia.