Receptor activator of nuclear factor-κB ligand and sclerostin expression in osteocytes of alveolar bone in rats with ligature-induced periodontitis

Background: Osteocytes are increasingly recognized as significant sources of osteoclast differentiation factor, receptor activator of nuclear factor-κB ligand (RANKL), and osteoblast differentiation inhibitory factor, sclerostin. In this study, RANKL and sclerostin expression of osteocytes is investigated in rats with ligature-induced periodontitis. Methods: Rats were divided into control and periodontitis groups, and periodontitis was induced by ligature on the mandibular first molars. At 1, 3, 10, and 20 days after ligature, histologic analyses of alveolar bone (AB) and osteoid areas in the molar furcation were performed. The numbers of osteoclasts and RANKL- and sclerostin-positive osteocytes were estimated by tartrate-resistant acid phosphatase staining and immunohistochemistry, respectively. Results: The AB area gradually decreased at day 10 after ligature and increased at day 20. The number of osteoclasts markedly increased at day 3 and then decreased. Conversely, osteoid formation was suppressed up to day 3 and then showed a remarkable increase above control level at day 20. The number of RANKL-positive osteocytes increased at days 1 and 3 and then decreased. Sclerostin-positive osteocytes markedly increased at days 3 and 10 but decreased below control level at day 20. Conclusions: These results show that AB loss is accompanied by enhanced osteoclast formation and suppressed osteoid formation. Osteocytes express RANKL when osteoclast formation increases, and they express sclerostin when osteoid formation is suppressed. Conversely, osteocytic sclerostin expression decreases when osteoid formation increases. These findings suggest that osteocytes may be important in AB loss via RANKL and sclerostin expression in periodontitis.