The myokine Irisin
Irisin was originally recognized as a hormone-like myokine secreted as a product of fibronectin type III domain containing 5 (FNDC5) from skeletal muscle in response to exercise both in mice and humans. Irisin targets white adipocytes to induce browning response and subsequent non-shivering thermogenesis. Since the skeletal muscle tissue is an important source of thermogenesis per se, it was unconvincing that Irisin produced during exercise may serve only as regulator of thermogenesis. On the other hand, physical exercise physiologically stimulates the skeleton reinforcement, thus pushing us to explore a possible involvement of Irisin on bone metabolism.
Irisin and bone
We recently showed that Irisin acts directly on osteoblasts regulating bone formation. In our first work, we demonstrated that medium enriched of Irisin, synthesized by cultured skeletal muscle cells, was able to enhance the differentiation of bone marrow stromal cells into mature osteoblasts in vitro. More recently, we showed that the myokine Irisin improves cortical bone mass and geometry in vivo, supporting the idea that Irisin recapitulates some of the most important benefits of physical exercise on the skeleton and plays protective role on bone health. Healthy young male mice, treated with a micro-dose of recombinant Irisin (r-Irisin), showed increased cortical bone mineral density (cBMD), periosteal circumference and polar moment of inertia. The enlarged bone perimeter and cross-sectional area, thus distributing bone mass further from the center of bone, indicated improved resistance of tibia from Irisin-treated mice to bending forces. Furthermore, the increase of polar moment of inertia, an index of long bone resistance to torsion, provided evidence that Irisin-treatment might maximize bone to become more structurally efficient for torsion, in order to induce an optimal stress transfer and physical performance. Likewise, three-point bending tests of tibiae showed that bending strength and energy to fracture increased in Irisin-treated mice. Dynamic histomorphometry of the tibial cortical bone using timed injections of xylelol orange and calcein showed a significant increase in bone formation parameters, including bone formation rate (BFR) and mineral apposition rate (MAR).
The action of Irisin on bone is mainly due to an increase in osteoblast activity. It is likely exerted through the rapid up-regulation of the Activating transcription factor 4 (Atf4) via MAP kinase ERK phosphorylation, leading to a dramatic increase in pro-osteoblastic genes, notably Collagen1 (Coll I), Alkaline Phosphatase (ALP) and Osteopontin (OPN), and a decrease of Sclerostin (Sost), one of the major inhibitor of the bone anabolic Wnt pathway.
Inducible brown adipose tissue is anabolic for the skeleton
External stimuli, such as cold exposure or physical activity, through Irisin action as recently reported, stimulate inducible brown adipose tissue (iBAT) expansion, whose cells show morphological characteristics similar to classical brown adipose tissue (cBAT), such as the presence of multilocular lipid droplets and numerous mitochondria. Considering the tight relationship between skeletal metabolism and energy homeostasis, research is giving great importance to the role of BAT in bone metabolism. Transgenic mice overexpressing FoxC2 in adipose tissues are a well-established model for BAT induction. These mice displayed high bone mass due to increased bone formation, triggered by wingless related MMTV integration site 10b (WNT10b) and insulin-like growth factor binding protein 2 (IGFBP2), two pivotal bone anabolic molecules released from BAT.
Since previous work showed that Irisin activates the browning response in white adipose tissue, when injected in mice at a dose of 3500 microg/kg/week, we choose a lower dose of r-Irisin, ruling out an indirect action of Irisin on bone mass via brown adipose tissue induction (iBAT). Hence, we demonstrated that a lower dose of r-Irisin (100 microg/kg/week) was effective on bone but not sufficient to induce the browning response (3). However, in the present project we aim to explore the synergistic effect of both direct and indirect Irisin actions by injecting mice with higher dose of Irisin. Addressing this topic could be relevant considering the well-known inverse relationships among BAT in aging and muscles activity and bone integrity.