The Role of Irisin in Initiating Resorption During the Skeletal Response to Exercise

This project investigates the role of the myokine irisin in directly regulating osteoclasts and promoting bone resorption, reframing irisin’s contribution to muscle–bone crosstalk and skeletal remodeling. Prior work had emphasized irisin’s actions on osteoblasts and osteocytes; Estell et al. addressed the unresolved question of whether irisin also acts directly on osteoclast progenitors to influence differentiation and resorptive function. Using primary murine bone marrow progenitors from C57BL/6J mice and complementary cell line models, the team treated cultures with physiologic concentrations of recombinant irisin (2–20 ng/mL, with experimental focus on 2–10 ng/mL and 10 ng/mL doses). Continuous exposure to irisin increased osteoclast differentiation as judged by TRAP-positive multinucleated cells, and short exposures (first 4 or 24 hours) produced measurable effects compared with untreated controls. T

he stimulatory effect was observed across primary murine genders and in RAW 264.7 macrophage-derived osteoclast cultures. Key mechanistic interrogation revealed that the effect of irisin on osteoclastogenesis depended on an integrin receptor; expression of integrin subunits αV (ITGAV) and β5 (ITGB5) was detected in primary osteoclast cultures, and a neutralizing antibody to integrin αVβ5 blocked the increase in osteoclast formation induced by irisin, implicating this receptor complex in irisin signaling to osteoclast progenitors. Beyond differentiation, the study demonstrated that irisin augmented osteoclastic resorptive activity. Multiple in situ assays showed increased bone resorption with irisin treatment: dentin cultures revealed larger resorption pits and greater resorption area per culture, and Corning OsteoAssay resorbable calcium phosphate substrate assays confirmed enhanced resorption with irisin. To explore transcriptional responses, RNA sequencing of treated cultures identified differential gene expression induced by irisin, including upregulation of markers associated with osteoclast differentiation and resorption as well as increased expression of osteoblast-stimulating 'clastokines,' signaling molecules secreted by osteoclasts that can influence osteoblast activity.

Complementary in vivo genetic approaches further supported a net resorptive effect: forced expression of the irisin precursor Fndc5 in transgenic C57BL/6J mice yielded lower bone mass at three examined ages and produced bone marrow progenitors that exhibited greater in vitro osteoclastogenesis than wild-type controls. Together, these in vitro, in situ, transcriptional, and transgenic in vivo results support a model in which irisin acts directly on osteoclast progenitors via an integrin αVβ5-mediated mechanism to increase differentiation and resorptive function. The findings position irisin not only as a promoter of bone remodeling but also as a potential counter-regulatory hormone that can heighten bone resorption, with implications for understanding exercise-related skeletal adaptations and the balance between bone formation and resorption. This body of work provides mechanistic insight into how muscle-derived signals can directly modulate osteoclast biology and overall bone homeostasis.