Characterization of fibronectin type III domain-containing protein 5 (FNDC5) gene in chickens: Cloning, tissue expression, and regulation of its expression in the muscle by fasting and cold exposure.

Irisin, a novel myokine encoded by fibronectin type III domain-containing protein 5 gene (FNDC5), is reported to stimulate brown fat-like development of white fat tissue and thermogenesis in mammals recently. However, information about the structure, tissue expression, and roles of FNDC5/irisin remains unknown in non-mammalian vertebrates including birds. In this study, we first cloned the FNDC5 (cFNDC5) cDNA from chickens. cFNDC5 is predicted to encode a 220-amino acid precursor containing the putative 'irisin peptide' of 112 amino acids, which shows high amino acid sequence identity with irisin of humans (97%), mice (97%), anole lizards (93%) and zebrafish (~80%). Using quantitative real-time PCR, we further examined cFNDC5 mRNA expression in chicken tissues. The results showed that in adult chickens, cFNDC5 is abundantly expressed in the muscle, heart, pituitary, ovary and various brain regions, and moderately expressed in adipose tissue, kidneys, lung, testes and small intestine. Moreover, cFNDC5 is also abundantly expressed in the muscle, brain, hypothalamus and pituitary of developing embryos and post-hatching chicks. Interestingly, we noted that cFNDC5 expression in the muscle of 3-week-old chicks could be induced by fasting and cold exposure, while its expression decreases during differentiation of pre-adipocytes cultured in vitro. Collectively, our data suggest that FNDC5/irisin is more than a 'myokine' and may be related to the development/functions of many tissues (e.g. muscle, brain, fat), as well as metabolic status of chickens.