Osteocyte Control of Bone Remodeling

摘要 破骨细胞的形成依赖于NF κ B B配体（RANKL）的细胞因子受体激活剂，其作用是 被诱骗受体骨保护素（OPG）抑制。骨细胞是RANKL的重要来源， 最近表明，成骨细胞，而不是骨细胞，是抑制OPG的重要来源， 松质骨的吸收。相反，成骨细胞和骨细胞仅提供OPG的一部分 保护皮质骨因此，提供OPG的细胞的身份，保护皮质的大区域， 骨尚不清楚，但可能涉及成骨祖细胞和血管内皮细胞，两者都是 表达OPG。以前的研究表明，β-连环蛋白的主要功能是促进OPG的表达， 骨细胞我们发现成骨细胞，而不是骨细胞，是OPG的主要来源， 重新评估β-连环蛋白在骨细胞中的作用。以前的β-连环蛋白功能丧失研究已经被证明是 由于缺乏能够区分成骨细胞和骨细胞的Cre驱动菌株， 晚期成骨细胞中β-连环蛋白缺失导致的严重骨丢失。造骨细胞是一种 OPG的一个重要来源也表明， 停止抗RANKL（地舒单抗）治疗可能部分是由于缺乏成骨细胞，以及 因此，OPG在停药后期间。基于这些发现，我们提出了假设 成骨祖细胞或血管内皮细胞是OPG保护皮质骨的重要来源， 骨细胞中的β-连环蛋白通路有助于骨重建，而不受其控制 OPG的表达。我们还提出，缺乏成骨细胞，因此OPG，有助于反弹 地舒单抗停药后的吸收。目的1将确定控制皮层的OPG的细胞来源 通过使用Prx 1-Cre小鼠删除间充质祖细胞中的条件OPG等位基因的骨吸收，和 血管内皮细胞使用Tek-Cre小鼠，并比较对皮质骨的影响，在OPG- 无效小鼠。目的2：利用Sost-Cre基因敲除β-连环蛋白，确定β-连环蛋白在骨细胞中的作用 小鼠，删除骨细胞而不是成骨细胞中的靶基因。目标3将决定是否反弹 狄诺塞单抗停药引起的吸收部分是由于严重缺乏成骨细胞 使用地舒单抗处理的新型人源化RANKL小鼠系生产OPG。这些老鼠将被用于 绘制与停用地舒单抗相关的细胞和分子状况并确定 通过给予抗sclerostin抗体促进成骨细胞形成是否可以恢复OPG水平 并改善回弹性再吸收。

Abstract Osteoclast formation depends on the cytokine receptor activator of NFB ligand (RANKL), whose actions are inhibited by the decoy receptor osteoprotegerin (OPG). Osteocytes are an important source of RANKL and we have recently shown that osteoblasts, but not osteocytes, are an essential source of the OPG that suppresses resorption of cancellous bone. In contrast, osteoblasts and osteocytes provide only a portion of the OPG protecting cortical bone. Thus, the identity of the cells providing the OPG that protects large regions of cortical bone remains unclear, but may involve osteoblast progenitors and vascular endothelial cells, both of which express OPG. Previous studies suggest that a major function of beta-catenin is to promote OPG expression in osteocytes. Our finding that osteoblasts, but not osteocytes, are a major source of OPG necessitates a reevaluation of the role of beta-catenin in osteocytes. Previous beta-catenin loss-of-function studies have been hampered by the lack of Cre driver strains that can distinguish between osteoblasts and osteocytes and by the dramatic bone loss caused by beta-catenin deletion in late-stage osteoblastic cells. That osteoblasts are an important source of OPG also suggests that the rapid increase in resorption and rebound bone loss following discontinuation of anti-RANKL (denosumab) therapy may be due, in part, to the absence of osteoblasts, and thus OPG, during the period following discontinuation. Based on these findings, we propose the hypotheses that osteoblast progenitors or vascular endothelial cells are important sources of the OPG protecting cortical bone and that the beta-catenin pathway in osteocytes contributes to bone remodeling independent of its control of OPG expression. We also propose that the lack of osteoblasts, and thus OPG, contributes to the rebound resorption following discontinuation of denosumab. Aim 1 will identify cellular sources of OPG that control cortical bone resorption by deleting a conditional OPG allele in mesenchymal progenitors using Prx1-Cre mice, and vascular endothelial cells using Tek-Cre mice, and compare the effects on cortical bone to those seen in OPG- null mice. Aim 2 will determine the role of beta-catenin in osteocytes by deletion of beta-catenin using Sost-Cre mice, which delete target genes in osteocytes but not osteoblasts. Aim 3 will determine whether the rebound resorption caused by discontinuation of denosumab results in part from the profound lack of osteoblasts producing OPG using a novel humanized RANKL mouse line treated with denosumab. These mice will be used to map the cellular and molecular conditions associated with discontinuation of denosumab and to determine whether promotion of osteoblast formation via anti-sclerostin antibody administration can restore OPG levels and ameliorate rebound resorption.