Distribution of SIBLING Proteins in Condylar Cartilage of Rat Mandible

Objectives: SIBLING family includes bone sialoprotein (BSP), osteopontin (OPN), dentin sialophosphoprotein (DSPP) and dentin matrix protein 1 (DMP1); these acidic phosphoproteins, principally found in mineralized tissues, are believed to play key biological roles in the formation of bone. The primary objective of this study was to assess the presence and distribution of the SIBLING proteins in the condylar cartilage of rat mandible and to compare the results obtained from the condylar cartilage with data from the growth plate of long bones. Methods: 1) Temporomandibular joints from rats aged 2, 5, and 8 weeks were evaluated by immunohistochemistry using antibodies against individual SIBLING proteins, BSP, OPN, and fragments of DSPP and DMP1. 2) Condylar cartilage was mechanically dissected from the mandible of 10-week-old rats under dissecting microscope; the SIBLING proteins in the cartilage were extracted by Gdm-HCl/EDTA and the extract was separated by ion-exchange chromatography; individual SIBLING proteins from the chromatographic fractions were assessed by Western immunoblotting. Results: 1) Immunohistochemical studies revealed that while the first two layers of the mandibular condylar cartilage (articular and prechondroblastic layers) showed no immunoreactivity to any of the SIBLING proteins, the other two layers (chondroblastic and hypertrophic layers) contained all of these proteins in all age groups studied. 2) Western immunoblotting analysis confirmed the presence of OPN and fragments of DSPP and DMP1 in the Gdm-HCl/EDTA extract of mandibular condylar cartilage. Conclusions: The four SIBLING proteins, BSP, OPN, DSPP and DMP1 are expressed in the mandibular condylar cartilage, following a general pattern observed in the growth plate of long bone. The interplay among these SIBLING proteins in the cartilage may play an important role in controlling the development of this tissue and in modulating the conversion from cartilage to bone during endochondral osteogenesis. Supported by NIH grant DE005092 (CQ) and Baylor Oral Health Foundation.