Characterization of chicken nicotinamide phosphoribosyltransferase (Nampt): Evidence as a postnatal myogenic regulator

The protein nicotinamide phosphoribosyltransferase (Nampt) has been identified as an enzyme, an inflammatory cytokine, and an adipokine. Recently, Nampt has been implicated in numerous metabolic disorders, such as obesity and type 2 diabetes mellitus. Although Nampt is highly investigated in mammals, no reports are currently available on Nampt expression in the chicken, which is naturally hyperglycemic and undergoes rapid weight gain. The objective of the first study was to clone Nampt mRNA and characterize Nampt gene and protein expressions in the primary metabolic tissues, as well as to evaluate changes in Nampt expression with age in the broiler chicken. Using RT-PCR, Nampt cDNA was identified in multiple tissues in the chicken, with the deduced amino acid sequence chicken Nampt being 92-93% homologous to mammalian Nampt. Skeletal muscle was found to contain 5-fold and 3-fold greater quantities of Nampt mRNA and protein than the abdominal fat pad, respectively. Skeletal muscle Nampt mRNA and protein quantities, as well as plasma Nampt levels determined by enzyme immunoassay, were significantly higher in 8 week-old compared to 4 week-old chickens, suggesting a role for Nampt in myogenesis. A chicken satellite cell line was used to explore this role. The objective of the second study was to characterize Nampt gene and protein expression in myoblasts and myocytes and determine if Nampt influences the expression of muscle-specific transcription factors within these cells. Both myoblasts and myocytes contained Nampt mRNA and protein, although expression did not differ between the two cell types. Forced over-expression of Nampt using a eukaryotic Nampt expression plasmid significantly decreased myf5 expression in myoblasts. Additionally, exogenous Nampt treatment significantly decreased myoD and mrf4 expression in myocytes. Nampt did not alter glucose uptake in either cell type. The objective of the third study was to determine if interleukin-6 (IL-6) regulated myogenic cell Nampt expression. Myogenic cell Nampt mRNA expression was reduced by IL-6, whereas long-term Nampt treatment decreased IL-6 mRNA expression in myocytes. Collectively, the studies presented herein have identified skeletal muscle as the main source for Nampt in the chicken and have suggested this protein to be a regulator of satellite-cell mediated postnatal myogenesis.