Description
Irisin is a recently described exercise-induced hormone secreted by skeletal muscle in mice and humans. Irisin activates beige fat cells (beige cells have a gene expression pattern distinct from either white or brown fat and are preferentially sensitive to the polypeptide hormone irisin). Irisin is cleaved from the type I membrane protein FNDC5 and improves systemic metabolism by increasing energy expenditure. FNDC4 is an ortholog of FNDC5 with 50% identity and 86% similarity compared to Irisin. FNDC4 as well as FNDC5 are extremely well conserved between species. The function of FNDC4 is unknown. The human FNDC4 gene is highly enriched in liver, brain tissue and adipocytes