Fibronectin type III domain-containing protein 5 (FNDC5)-like immunoreactivity and mRNA abundance in domestic animal tissues

Irisin is a 112-amino acid peptide hormone that is cleaved from fibronectin type III domain-containing protein 5 (FNDC5), a type I transmembrane protein abundantly found in muscle tissue. Irisin is a putative mediator of the benefits of exercise, neuroprotection, bone growth, and cardiac health. However, few studies have focused on irisin in domestic animals. Further, whether processed irisin is detectable in domestic animal tissues remains uncertain. To address this, we determined FNDC5 mRNA and protein concentration in anatine (duck) and porcine (pig) skeletal muscle, and in equine (horse), swine, and anatine serum samples. RT-PCR analysis identified FNDC5 mRNA in all pig and duck skeletal muscle samples. An approximately 25 kDa band representing FNDC5 was detected in both pig and duck skeletal muscle. Fluorescence immunohistochemistry using a rabbit monoclonal FNDC5/irisin primary antibody and a goat polyclonal anti-rabbit secondary antibody localized FNDC5/irisin-like immunoreactivity in both the glandular and muscular regions of pig stomach. FNDC5/irisin-like immunoreactivity was also identified in horse, pig, and duck serum using a multispecies irisin ELISA. The average values of irisin-like immunoreactivity were 13.7 (duck), 15.4 (horse), and 7.0 (pig) ng/mL in samples tested. Our results support the presence of irisin precursor in several domestic animals. Processed irisin, however, was not detectable. Further studies are required to validate reliable tools to detect and quantify processed irisin in domestic animals.