Liu Dawen, Wise Gary E
Department of Comparative Biomedical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803, USA.
Bone. 2007 Aug;41(2):266-72. doi: 10.1016/j.bone.2007.04.181. Epub 2007 Apr 24.
The dental follicle, a loose connective tissue sac that surrounds the unerupted tooth, appears to regulate the osteoclastogenesis needed for eruption; i.e., bone resorption to form an eruption pathway. Thus, DNA microarray studies were conducted to determine which chemokines and their receptors were expressed chronologically in the dental follicle, chemokines that might attract osteoclast precursors. In the rat first mandibular molar, a major burst of osteoclastogenesis occurs at day 3 with a minor burst at day 10. The results of the microarray confirmed our previous studies showing the gene expression of molecules such as CSF-1 and MCP-1 in the dental follicle cells. Other new genes also were detected, including secreted frizzled-related protein-1 (SFRP-1), which was found to be downregulated at days 3 and 9. Using rat bone marrow cultures to conduct in vitro osteoclastogenic assays, it was demonstrated that SFRP-1 inhibited osteoclast formation in a concentration-dependent fashion. However, with increasing concentrations of SFRP-1, the number of TRAP-positive mononuclear cells increased suggesting that SFRP-1 inhibits osteoclast formation by inhibiting the fusion of mononuclear cells (osteoclast precursors). Co-culturing bone marrow mononuclear cells and dental follicle cells demonstrated that the dental follicle cells were secreting a product(s) that inhibited osteoclastogenesis, as measured by counting of TRAP-positive osteoclasts. Adding an antibody either to SFRP-1 or OPG partially restored osteoclastogenesis. Adding both anti-SFRP-1 and anti-OPG fully negated the inhibitory effect of the follicle cells upon osteoclastogenesis. These results strongly suggest that SFRP-1 and OPG, both secreted by the dental follicle cells, use different pathways to exert their inhibitory effect on osteoclastogenesis. Based on these in vitro studies of osteoclastogenesis, it is likely that the downregulation of SFRP-1 gene expression in the dental follicle at days 3 and 9 is a contributory factor in allowing the major and minor bursts of osteoclastogenesis to occur. Thus, inhibition of SFRP-1 gene expression in combination with inhibition of OPG gene expression likely are critical events in enabling alveolar bone resorption to occur such that teeth will erupt.
牙囊是围绕未萌出牙齿的疏松结缔组织囊，似乎能调节牙齿萌出所需的破骨细胞生成，即通过骨吸收形成萌出通道。因此，开展了DNA微阵列研究，以确定哪些趋化因子及其受体在牙囊中按时间顺序表达，这些趋化因子可能吸引破骨细胞前体。在大鼠第一下颌磨牙中，破骨细胞生成的主要高峰出现在第3天，次要高峰出现在第10天。微阵列结果证实了我们之前的研究，显示牙囊细胞中存在CSF-1和MCP-1等分子的基因表达。还检测到了其他新基因，包括分泌型卷曲相关蛋白-1（SFRP-1），发现其在第3天和第9天表达下调。利用大鼠骨髓培养进行体外破骨细胞生成试验，结果表明SFRP-1以浓度依赖方式抑制破骨细胞形成。然而，随着SFRP-1浓度增加，抗酒石酸酸性磷酸酶（TRAP）阳性单核细胞数量增加，这表明SFRP-1通过抑制单核细胞（破骨细胞前体）融合来抑制破骨细胞形成。将骨髓单核细胞与牙囊细胞共培养表明，通过计数TRAP阳性破骨细胞发现，牙囊细胞分泌一种抑制破骨细胞生成的产物。添加针对SFRP-1或骨保护素（OPG）的抗体可部分恢复破骨细胞生成。同时添加抗SFRP-1和抗OPG可完全消除牙囊细胞对破骨细胞生成的抑制作用。这些结果强烈表明，牙囊细胞分泌的SFRP-1和OPG通过不同途径对破骨细胞生成发挥抑制作用。基于这些破骨细胞生成的体外研究，牙囊中SFRP-1基因表达在第3天和第9天的下调可能是导致破骨细胞生成主要和次要高峰出现的一个促成因素。因此，抑制SFRP-1基因表达并结合抑制OPG基因表达可能是使牙槽骨吸收发生从而使牙齿萌出的关键事件。
