R-spondin 2 promotes osteoblastic differentiation of immature human periodontal ligament cells through the Wnt/β-catenin signaling pathway.

Abstract

In this study, we measured the expression of R-spondin 2 (RSPO2) in periodontal ligament (PDL) tissue and cells. Further, we examined the effects of RSPO2 on osteoblastic differentiation of immature human PDL cells (HPDLCs). R-spondin (RSPO) family proteins are secreted glycoproteins that play important roles in embryonic development and tissue homeostasis through activation of the Wnt/β-catenin signaling pathway. RSPO2, a member of the RSPO family, has been reported to enhance osteogenesis in mice. However, little is known regarding the roles of RSPO2 in PDL tissues. Expression of RSPO2 in rat PDL tissue and primary HPDLCs was examined by immunohistochemical and immunofluorescence staining, as well as by semiquantitative RT-PCR. The effects of stretch loading on the expression of RSPO2 and Dickkopf-related protein 1 (DKK1) were assessed by quantitative RT-PCR. Expression of receptors for RSPOs, such as Leucine-rich repeat-containing G-protein-coupled receptors (LGRs) 4, 5, and 6 in immature human PDL cells (cell line 2-14, or 2-14 cells), was investigated by semiquantitative RT-PCR. Mineralized nodule formation in 2-14 cells treated with RSPO2 under osteoblastic inductive condition was examined by Alizarin Red S and von Kossa stainings. Nuclear translocation of β-catenin and expression of active β-catenin in 2-14 cells treated with RSPO2 were assessed by immunofluorescence staining and Western blotting analysis, respectively. In addition, the effect of Dickkopf-related protein 1 (DKK1), an inhibitor of Wnt/β-catenin signaling, was also examined. Rat PDL tissue and HPDLCs expressed RSPO2, and HPDLCs also expressed RSPO2, while little was found in 2-14 cells. Expression of RSPO2 as well as DKK1 in HPDLCs was significantly upregulated by exposure to stretch loading. LGR4 was predominantly expressed in 2-14 cells, which expressed low levels of LGR5 and LGR6. RSPO2 enhanced the Alizarin Red S and von Kossa-positive reactions in 2-14 cells. In addition, DKK1 suppressed nuclear translocation of β-catenin, activation of β-catenin, and increases of Alizarin Red S and von Kossa-positive reactions in 2-14 cells, all of which were induced by RSPO2 treatment. RSPO2, which is expressed in PDL tissue and cells, might play an important role in regulating the osteoblastic differentiation of immature human PDL cells through the Wnt/β-catenin signaling pathway.