MURF1 deficiency suppresses unloading-induced effects on osteoblasts and osteoclasts to lead to bone loss

Loss of mechanical stress or unloading causes disuse osteoporosis that leads to fractures and deteriorates body function and affects mortality rate in aged population. This bone loss is due to reduction in osteoblastic bone formation and increase in osteoclastic bone resorption. MuRF1 is a muscle RING finger protein which is involved in muscle wasting and its expression is enhanced in the muscle of mice subjected to disuse condition such as hind limb unloading (HU). However, whether MuRF1 is involved in bone loss due to unloading is not known. We therefore examined the effects of MuRF1 deficiency on unloading-induced bone loss. We conducted hind limb unloading of MuRF1 KO mice and wild-type control mice. Unloading induced about 60% reduction in cancellous bone volume (BV/TV) in WT mice. In contrast, MuRF1 deficiency suppressed unloading-induced cancellous bone loss. The cortical bone mass was also reduced by unloading in WT mice. In contrast, MuRF1 deficiency suppressed this reduction in cortical bone mass. To understand whether the effects of MuRF1 deficiency suppress bone loss is on the side of bone formation or bone resorption, histomorphometry was conducted. Unloading reduced bone osteoblastic formation rate (BFR) in WT. In contrast, MuRF1 deficiency suppressed this reduction. Regarding bone resorption, unloading increased osteoclast number in WT. In contrast, MURF1 deficiency suppressed this osteoclast increase. These data indicated that the ring finger protein, MURF1 is involved in disuse-induced bone loss in both of the two major bone remodeling activities, osteoblastic bone formation and osteoclastic bone resorption.