Osteocyte-derived exosomes inhibit osteoblast activity and induce osteoclast formation

Osteocytes, which are the most abundant cell type in bone, regulate osteoblasts and osteoclasts via both cell–cell interactions and paracrine signaling, and osteocyte-derived exosomes might contribute to this paracrine action. In this study, we investigated the effects of osteocyte-derived exosomes on regulating osteoblasts and osteoclasts and studied the potential mechanism. Materials and Methods: Osteocyte-derived exosomes were extracted and identified. PKH67-labeled exosomes were incubated with MC3T3-E1 cells and RAW264.7 cells, and fluorescence confocal microscopy was used to analyze the uptake of exosomes. ALP stain- ing and TRAP staining were used to analyze osteoblast activity and osteoclast formation. The level of miR-214-3p in exosomes was analyzed by qPCR and the incorporation of FAM-labeled miR-214-3p from exosomes into MC3T3-E1 cells was evaluated. The expressions of ephrinA2 and RANKL in exosomes were studied. Results: Our results demonstrated that osteocyte-derived exosomes might recognize osteoblasts through the ephrinA2 protein; thus, miR-214-3p in exosomes was transferred into osteoblasts to inhibit osteoblast activity. Meanwhile, we found that osteocyte-derived exosomes could be transferred into osteoclasts to induce osteoclast formation by releasing RANKL. Conclusion: These findings suggest that osteocyte-derived exosomes play an important role in the regulation of osteoblast and osteoclast activity, which might occur via miR-214-3p and RANKL.