A review on irisin, a new protagonist that mediates muscle-adipose-bone-neuron connectivity.

Physical activity improves the quality of life and decreases the risk of several diseases (i.e. stroke, hypertension, myocardial infarction, obesity, and malignancies). Skeletal muscles are considered as an endocrine organ that produces myokines characterized by a paracrine or endocrine activity. Irisin is a circulating hormone-like myokine and is secreted as a product of fibronectin type III domain-containing protein 5 from skeletal muscle in response to exercise. This molecule regulates the energy metabolism and acts in adipose tissue, bones, and nervous system. As both animal and clinical studies confirmed the action of irisin in muscle and adipocytes, this protein is considered as adipomyokine. In adipose tissue, irisin stimulates the process of browning of beige precursor fat cells, which are present in white fat cells, and promotes energy expenditure. It affects bone metabolism by increasing osteoblast differentiation and reducing osteoclast maturation. In the nervous system, irisin influences hippocampal neurogenesis and neural differentiation of embryonic stem cells in mice and is considered as a messenger between exercise and brain function. However, the existence of this protein and its role in humans is a matter of debate. This study presents irisin as a new champion of the molecule, which could be considered as the messenger in the muscle-fat-bone-brain axis.