Osteoprotegerin-Deficient Male Mice as a Model for Severe Alveolar Bone Loss: Comparison With RANKL-Overexpressing Transgenic Male Mice

Periodontitis, an inflammatory disease of periodontal tissues, is characterized by excessive alveolar bone resorption. An increase in the receptor activator of nuclear factor-κB ligand (RANKL) to osteoprotegerin (OPG) ratio is thought to reflect the severity of periodontitis. Here, we examined alveolar bone loss in OPG-deficient (OPG−/−) mice and RANKL-overexpressing transgenic (RANKL-Tg) mice. Alveolar bone loss in OPG−/− mice at 12 weeks was significantly higher than that in RANKL-Tg mice. OPG−/− but not RANKL-Tg mice exhibited severe bone resorption especially in cortical areas of the alveolar bone. An increased number of osteoclasts was observed in the cortical areas in OPG−/− but not in RANKL-Tg mice. Immunohistochemical analyses showed many OPG-positive signals in osteocytes but not osteoblasts. OPG-positive osteocytes in the cortical area of alveolar bones and long bones were abundant in both wild-type and RANKL-Tg mice. This suggests the resorption in cortical bone areas to be prevented by OPG produced locally. To test the usefulness of OPG−/− mice as an animal model for screening drugs to prevent alveolar bone loss, we administered an antimouse RANKL antibody or risedronate, a bisphosphonate, to OPG−/− mice. They suppressed alveolar bone resorption effectively. OPG−/− mice are useful for screening therapeutic agents against alveolar bone loss.