Wang Baoli, Jin Hongting, Shu Bing, Mira Ranim R, Chen Di
Key Lab of Hormone and Development (Ministry of Health), Metabolic Diseases Hospital and Tianjin Institute of Endocrinology, Tianjin Medical University, Tianjin 300070, China.
Department of Orthopaedics, Center for Musculoskeletal Research, University of Rochester School of Medicine, Rochester, NY 14642, USA.
Sci Rep. 2015 Sep 2;5:13667. doi: 10.1038/srep13667.
Bone marrow stromal cells/osteoblasts were originally thought to be the major player in regulating osteoclast differentiation through expressing RANKL/OPG cytokines. Recent studies have established that chondrocytes also express RANKL/OPG and support osteoclast formation. Till now, the in vivo function of chondrocyte-produced OPG in osteoclast formation and postnatal bone growth has not been directly investigated. In this study, chondrocyte-specific Opg transgenic mice were generated by using type II collagen promoter. The Col2-Opg transgenic mice showed delayed formation of secondary ossification center and localized increase of bone mass in proximal metaphysis of tibiae. TRAP staining showed that osteoclast numbers were reduced in both secondary ossification center and proximal metaphysis. This finding was further confirmed by in vitro chondrocyte/spleen cell co-culture assay. In contrast, the mineral apposition rates were not changed in Col2-Opg transgenic mice. TUNEL staining revealed more apoptotic hypertrophic chondrocytes in the growth plate of Col2-Opg mice. Flow cytometry analysis showed fewer RANK-expressing cells in the marrow of Col2a1-Opg mice, suggesting the role of OPG in blocking the differentiation of early mesenchymal progenitors into RANK-expressing pre-osteoclasts. Our results demonstrated that OPG expression in chondrocyte increases bone mass in the proximal metaphysis of tibiae through negative regulation of osteoclast formation.
骨髓基质细胞/成骨细胞最初被认为是通过表达RANKL/OPG细胞因子来调节破骨细胞分化的主要细胞。最近的研究表明，软骨细胞也表达RANKL/OPG并支持破骨细胞形成。到目前为止，软骨细胞产生的OPG在破骨细胞形成和出生后骨骼生长中的体内功能尚未得到直接研究。在本研究中，通过使用II型胶原启动子生成了软骨细胞特异性Opg转基因小鼠。Col2-Opg转基因小鼠显示出二级骨化中心形成延迟，胫骨近端干骺端骨量局部增加。TRAP染色显示，二级骨化中心和近端干骺端的破骨细胞数量均减少。体外软骨细胞/脾细胞共培养试验进一步证实了这一发现。相比之下，Col2-Opg转基因小鼠的矿物质沉积率没有变化。TUNEL染色显示Col2-Opg小鼠生长板中凋亡的肥大软骨细胞更多。流式细胞术分析显示Col2a1-Opg小鼠骨髓中表达RANK的细胞较少，提示OPG在阻止早期间充质祖细胞分化为表达RANK的破骨细胞前体细胞中的作用。我们的结果表明，软骨细胞中OPG的表达通过对破骨细胞形成的负调节增加了胫骨近端干骺端的骨量。
