Myonectin in grass carp (Ctenopharyngodon idella): Molecular cloning and effect of nutrition state, insulin and glucagon on myonectin expression

As an endocrine organ, skeletal muscle secretes multiple myokines. Myonectin, a myokine, participates in lipid and glucose metabolism. To explore the functions of myonectin in teleosts, the myonectin gene was cloned from the brain tissue of grass carp. The open reading frame (ORF) of isolated myonectin was 813 bp, encoding 270 amino acids. Grass carp myonectin has four predicted domains: signal peptide, N-terminal domain (NTD), collagen protein domain, and C1q/TNF-like domain. In addition, four predicted glycosylation sites and four conserved cysteine residues were located in grass carp myonectin. Myonectin mRNA was detected in multiple tissues of grass carp and was highly expressed in the brain region, heart and kidney. In the oral glucose tolerance test (OGTT), myonectin mRNA levels were significantly induced in the kidney, hypothalamus, muscle and brain after treatment with glucose. In the fast and refeeding experiment, the serum glucose level was decreased in the fast group. However, serum glucose levels were increased in the refeeding group. The myonectin mRNA levels in the kidney, heart, brain and liver were significantly reduced in the fast groups. Moreover, the myonectin expression was markedly increased in the refeeding groups. In grass carp primary hepatocytes, myonectin mRNA levels were induced by treatment with glucose and oleic acid for 12 or 24 h. Myonectin expression was dramatically promoted by treatment with insulin. Moreover, myonectin expression was markedly decreased with glucagon treatment in primary hepatocytes. The results of this study will provide a basis for exploring the roles of grass carp myonectin in the future.