Maine Medical Center, Medical Education, Maine Medical Center Research Institute
Osteoclasts, Central Nervous System Stimulants, Hematopoiesis, Transcription Factors
Irisin is the cleaved product of the protein FNDC5 expressed in skeletal muscle, and is increased by exercise. Recent work suggests this myokine is a modulator of muscle-bone crosstalk. Irisin has been shown to increase bone formation (Colaiannia, PNAS 2015), prevent disuse-associated bone loss (Colaiannia, Sci Rep 2017), and directly stimulate osteoblasts (Colaiannia, Int J Endo 2014). These results demonstrate an anabolic function, but the full mechanisms of action on each key cell type in bone, and the relative impacts of irisin on bone formation, resorption, and coupled remodeling have yet to be elucidated. Converse to an anabolic role, genetic loss of FNDC5 protects against ovariectomy-induced bone loss by blocking bone resorption and osteocytic osteolysis. Additionally irisin acts directly on the osteocyte via the ?V?5 receptor to increase production of sclerostin and RANKL, key regulators of osteoclast differentiation (Kim, Cell 2018). The present work continues this investigation with the hypothesis that irisin acts directly on the osteoclast itself as well, stimulating differentiation and modulating downstream resorption and signaling functions.
To characterize the effects of physiologic elevated concentrations of irisin on osteoclast differentiation in vitro, we cultured primary BMSCs from C57BL/6J (B6) female mice with mCSF and RANKL, with or without 10 ng/mL irisin. Continuous irisin treatment for 7 days led to significantly increased osteoclast numbers and heightened expression of differentiation markers (c-Fos, c-Src, fam10, Itgb3, NfkB, NFATC, RANK, Rela). Irisin-treated osteoclasts demonstrated larger, more nucleated cells, with increased expression of the fusion markers Atp6vod2 and DCstamp. Treatment with neutralizing antibodies against integrin ?V?5 abolished the increase in osteoclast number with irisin, indicating this integrin pair as the functional receptor for osteoclasts, as it has been demonstrated in the osteocyte. Irisin treatment also increased total resorption for osteoclasts on Corning Osteo Assay Surface plates, and heightened expression of bone resorption markers (TRAP5b, ADAMTS5, CLCN7, cathepsinK, MMP2). Irisin-treated osteoclasts also demonstrated increased expression of osteoblast-stimulating markers (CTHRC1, TGFB, WNT10a), indicating secretion of clastokines that may stimulate osteoblast function and promote coupled remodeling.
The present work demonstrates that in addition to the osteoblast and osteocyte, irisin acts directly on the osteoclast via the integrin receptor ?V?5 to stimulate differentiation and resorption. These results support the inhibition of bone resorption observed with the genetic deletion of FNDC5. The stimulation of clastokine production further suggests that irisin may dynamically regulate coupled remodeling, as opposed to independently stimulating formation or resorption. However, the net effect of irisin on bone structure appears to vary depending on experimental conditions. Elucidating the cellular mechanisms of irisin and their interaction with the dose and timing of treatment could pave the way for development of this myokine as an anabolic therapy for bone diseases.
Estell, Eben; Le, Phuong; Vegting, Yosta; Kim, Hyeonwoo; Baron, Roland; Spiegelman, Bruce; and Rosen, Clifford, "Irisin Stimulates Osteoclast Differentiation and Function" (2020). Costas T. Lambrew Research Retreat 2020. 35.