Localization of SOST/sclerostin in cementocytes in vivo and in mineralizing periodontal ligament cells in vitro

Background and Objective: Cementum and bone are rather similar hard tissues, and osteocytes and cementocytes, together with their canalicular network, share many morphological and cell biological characteristics. However, there is no clear evidence that cementocytes have a function in tissue homeostasis of cementum comparable to that of osteocytes in bone. Recent studies have established an important role for the secreted glycoprotein sclerostin, the product of the SOST gene, as an osteocyte-derived signal to control bone remodelling. In this study, we investigated the expression of sclerostin in cementocytes in vivo as well as the expression of SOST and sclerostin in periodontal ligament cell cultures following induction of mineralization. Material and Method: Immunolocalization of sclerostin was performed in decalcified histological sections of mouse and human teeth and alveolar bone. Additionally, periodontal ligament cells from human donors were cultured in osteogenic conditions, namely in the presence of dexamethasone, ascorbic acid and beta-glycerophosphate, for up to 3 wk. The induction of calcified nodules was visualized by von Kossa stain. SOST mRNA was detected by real-time PCR, and the presence of sclerostin was verified using immunohistochemistry and western blots. Results: Expression of sclerostin was demonstrated in osteocytes of mouse and human alveolar bone. Distinct immunolocalization in the cementocytes was shown. In periodontal ligament cultures, following mineralization treatment, increasing levels of SOST mRNA as well as of sclerostin protein could be verified. Conclusion: The identification of SOST/sclerostin in cementocytes and mineralizing periodontal ligament cells adds to our understanding of the biology of the periodontium, but the functional meaning of these findings can only be unravelled after additional in vitro and in vivo studies.