Tooth-alveolar Bone Complex Defects In BMP9/GDF2 Knockout Mice

Background: Tooth development is regulated by sequential and reciprocal epithelium-mesenchymal interactions and their related molecular signaling pathways,such as bone morphogenetic proteins(BMPs).Among the 14 types of BMPs which have been isolated,BMP9(as known as,GDF2)is one of the most potent BMPs to induce osteogenic differentiation of mesenchymal stem cells(MSCs).However,the role of BMP9 in tooth development was not clear yet.Objective: The purpose of this study was to examine potential roles of BMP9 signaling in tooth development and its possible mechanism.Methods: In the first part of this study,we detected the expression pattern of BMP9 in tooth germ during postnatal tooth development by immunohistochemical staining.In the second part,gross morphological examination,micro-CT,3-D reconstruction and histologic analysis were used to detect the role of BMP9 in tooth development by the BMP9 knockout mouse model.At last,SCAP cells were infected with recombinant adenovirus silencing or overexpressing BMP9,and osteo/odontoblastic differentiation of SCAP cells were detected by ALP activity or qPCR analysis.Results: We observed the BMP9 signal in different stages of tooth germ development and found that BMP9 was widely expressed in odontoblasts,ameloblasts,dental pulp cells and osteoblasts in alveolar bones.In the second part,gross morphological examination revealed that the tooth cusps of BMP9 knockout mice were significantly abraded with shorter roots.Micro-CT,3-D reconstruction and histologic analysis indicated the first molars of the BMP9 knockout mice exhibited a reduced thickness dentin,enlarged pulp canals and shortened roots,resembling the phenotypes of the common hereditary dental disease dentinogenesis imperfecta.Furthermore,the alveolar bone of the BMP9 knockout mutants was found shorter and had a decreased mineral density and trabecular thickness and bone volume fraction compared with that of the wild-type control.Mechanistically,we demonstrated that the osteoblastic differentiation makers,ALP,and odontoblastic differentiation makers,dentin sialophosphoprotein(Dspp)and dentin matrix protein 1(Dmp1),were reduced in dental stem cells by silencing BMP9,while their expression were induced when BMP9 was overexpressed in vitro.Conclusion: Our results strongly suggest that BMP9 may play an important role in regulating dentinogenesis and tooth-alveolar bone complex development.Further research is recommended into the mechanism of BMP9 regulating tooth development.