Expression analysis of irisin during different development stages of skeletal muscle in mice.

As a newly discovered muscle factor secreted by skeletal muscle cells, irisin is a polypeptide fragment formed from hydrolysis of fibronectin type Ⅲ domain-containing protein 5 (FNDC5). Irisin can promote beigeing of white adipose tissue (WAT) and regulate glucose and lipid metabolisms. However, the functions of irisin in skeletal muscle development remain largely unknown. In order to characterize the expression of irisin, this study investigated the expression of irisin precursor FNDC5 in myoblasts and skeletal muscles during different developmental stages of SPF mice. The Western blot, quantitative real-time PCR (qRT-PCR), and immunofluorescence assay results showed that FNDC5 was expressed in all the developmental stages of myoblasts and gastrocnemius, but its expression differed at different stages. FNDC5 protein exhibited the highest expression in gastrocnemius of sexually mature mice, followed by elderly mice and adolescent mice, and it displayed the lowest expression in pups. Additionally, FNDC5 protein was mainly expressed in cytoplasm, and it had the highest expression in primary myoblasts, followed by the myotubes with the lowest expression in C2C12 myogenic cells. Overall, FNDC5 was mainly expressed in cytoplasm and extracellular matrix with different expression levels at different developmental stages of skeletal muscle cells and tissues in mice. This study will provide new strategies for promoting skeletal muscle development and treating muscle- and metabolism-related disease by using irisin. • High expression of FNDC5 in skeletal muscle of sexually mature mice. • FNDC5 is differentially expressed at different stages of myoblast differentiation. • FNDC5 is localized in the cytoplasm and extracellular matrix of skeletal muscle cells and not in the nucleus. [ABSTRACT FROM AUTHOR]