Treatment with synthetic glucocorticoid impairs bone metabolism, as revealed by in vivo imaging of osteoblasts and osteoclasts in medaka fish

Glucocorticoids (GCs) play an important role in the stress reaction and function in the development of multiple tissues in our body. When given chronically in supraphysiologic doses, GCs are associated with orthodontic tooth movement, with serious side effects and particularly adverse effects on bone metabolism. However, the effects of steroids on bone cell dynamics are incompletely understood. Therefore, in this present study we examined the participation of osteoblasts and osteoclasts in osterix-DsRed/TRAP-EGFP double transgenic (Tg) medaka treated with synthetic GCs. Chronic continuous administration of prednisolone (PN) significantly reduced the fluorescence signals in the whole body of both osterix-DsRed and TRAP-EGFP medaka at 18 days, and those of the pharyngeal bone and tooth region at 32 days. To examine the capacity of the medaka for fracture healing during chronic administration of PN, we caused a fracture of a part of the bony fin ray at 18 days after the initiation of PN continuous administration. The bone fracture healing was significantly delayed by 32 days, accompanied by decreased signal area of both osterix-DsRed and TRAP-EGFP compared with that of the control. Next, to investigate the effect of acute administration of GC on the fracture healing, we started administration of dexamethasone (DX) immediately after the bone fracture, and this administration lasted during the 11 days of fracture healing. The results showed that the TRAP-EGFP-positive osteoclasts were reduced in area, but not the osterix-DsRed-positive osteoblasts. Lastly, to confirm the function of the glucocorticoid receptor in bone healing, we generated glucocorticoid receptor 2-deficient medaka (gr2(-/-)). The fluorescent signal area of osterix-DsRed and TRAP-EGFP were increased at bone fracture sites in these fish, and DX treatment of them decreased the TRAP-EGFP signal area compared with that for the control fish. Our results indicate that GRs negatively regulated osteoclast recruitment and the accumulation of osteoblasts in bone fracture healing.