The Roles of Extracellular Vesicles Released by Mechanically Stimulated Osteocytes in Regulating Osteoblast and Osteoclast Functions

Bone adapts to mechanical loading by changing its shape and mass. Osteocytes, as major mechanosensors, are critical for bone modeling/remodeling in response to mechanical stimuli. Intracellular calcium oscillation is one of the early responses in osteocytes, and this further facilitates bone cell communication through released biochemical signals. Our previous study has found that mechanically induced calcium oscillations in osteocytes enhance the release of extracellular vesicles (EVs), and those released EVs can elevate bone formation activity. However, the mechanism of mechanically stimulated EVs' regulation of bone formation and resorption is still unclear. Here, using in vitro studies, we exposed OCY454 cells, with relatively high sclerostin expression, to steady fluid flow (SFF) and characterized the functions of rapidly released EVs in osteoblast and osteoclast regulation. Our study demonstrates that SFF stimulates intracellular calcium response in OCY454 cells and further induces sclerostin, osteoprotegerin (OPG), receptor activator of NF-κB ligand (RANKL) inside or outside EVs to regulate osteoblast and osteoclast activities. This load-induced protein and EVs release is load-duration dependent. Moreover, stimulated osteocytes rapidly regulate osteoclast maturation through EVs capsulated RANKL. In contrast, other regulating proteins, OPG, and sclerostin, are mainly released directly into the medium without EV capsulation.