Molecular cloning of the Atlantic salmon activin receptor IIB cDNA – Localization of the receptor and myostatin in vivo and in vitro in muscle cells

In mammals, the activin receptor type IIB (ActRIIB) binds with high affinity several members of the transforming growth factor-beta (TGF-β) superfamily, including the negative muscle regulator myostatin (MSTN). In this study, an actRIIB cDNA of 1443 bp was isolated by reverse transcription (RT)–PCR from the liver of Atlantic salmon (Salmo salar) encoding almost the complete receptor. The deduced salmon ActRIIB of 481 amino acids (aa) contained the conserved catalytic domain of serine/threonine protein kinases, and showed the highest sequence identity (83–87%) to the zebrafish, chicken and goldfish ActRIIB. Salmon actRIIB mRNA was identified by RT–PCR in all the examined tissues of juvenile fish that was confirmed by in situ hybridization. In comparison, the salmon MSTN signal was less widespread, and co-expression of the receptor and this putative ligand was only demonstrated in skeletal muscle. Consistently, both ActRIIB and MSTN were immunocytologically identified in salmon myoblasts and differentiated myotubes in culture.