Analyzing the Role of Wnt Signaling in Bone Development

骨质疏松性假性胶质瘤（OPPG）是一种以骨密度极低为特征的人类综合征，由Wnt共受体LRP5缺乏引起。同一受体的点突变通过编码具有不适当信号活性的蛋白质而导致高骨量。高度相关的LRP6蛋白也影响骨量。鉴于将这些发现转化为治疗低骨量的疗法的强烈兴趣，迫切需要了解LRP5和LRP6蛋白如何控制骨生长。LRP5和LRP6都可以转导来自Wnt配体的信号，导致细胞质中（-catenin）的稳定。为了评估携带突变LRP5和LRP6基因的人类和小鼠的表型是否受（-catenin）调控改变的影响，我们和其他人建立了小鼠模型，其中（-catenin）基因在成骨细胞分化的特定阶段被删除。我们使用骨钙素-cre （OC-cre）在分化成骨细胞中删除（-catenin）。OC-cre;(-cateninflox/flox小鼠在一个月龄时死亡，破骨细胞数量升高，骨量严重降低。Dermo1-cre对骨软骨前体（-catenin）的消融阻断成骨细胞分化，促进软骨形成。在这两种情况下，(-catenin消融的表型比Lrp5基因敲除的表型严重得多。对于p-catenin缺失和Lrp5缺失之间表型严重程度差异的一种解释是，Lrp6也可以调节成骨细胞中的p-catenin。在本研究中，我们研究了Lrp6和Lrp5在成骨细胞分化早期和晚期的作用。我们预测Lrp5或Lrp6的消融将在这些阶段产生独特的后果，并且这两个基因的丢失将导致更严重的表型。与公共卫生相关：解决这些问题具有总体意义：弄清楚这些受体如何工作将有助于确定和验证新的靶点，并为如何治疗低骨量开发新的策略，同时为了解Wnt信号通路如何控制骨骼发育提供重要的见解。

DESCRIPTION (provided by applicant): Osteoporosis pseudoglioma (OPPG), a human syndrome characterized by extremely low bone density, is caused by a deficiency in the Wnt co-receptor LRP5. Point mutations in this same receptor cause high bone mass by encoding for proteins with inappropriate signaling activity. The highly related LRP6 protein also influences bone mass. Given the intense interest in translating these findings into therapies to treat low bone mass, there is a critical need to understand how LRP5 and LRP6 protein control bone growth. Both LRP5 and LRP6 can transduce signals from Wnt ligands resulting in stabilization of (-catenin in the cytosol. To assess whether altered regulation of (-catenin underlies the phenotypes in humans an mice carrying mutated LRP5 and LRP6 genes, we and others have created mouse models in which the (-catenin gene was deleted at specific stages of osteoblast differentiation. We used osteocalcin-cre (OC-cre) to delete (-catenin in differentiated osteoblasts. OC-cre;(-cateninflox/flox mice die by one month of age, have elevated numbers of osteoclasts, and display severely low bone mass. Ablation of (-catenin in osteochondral precursors by Dermo1-cre blocks osteoblast differentiation and enhances chondrogenesis. In both cases, the phenotype of (-catenin ablation is much more severe than that of a global Lrp5 knockout. One explanation for difference in phenotypic severity between loss of p-catenin and deletion of Lrp5 is that Lrp6 can also regulate p-catenin in osteoblasts. In this proposal we examine the roles of Lrp6 and Lrp5 in early (commitment) and late stages of osteoblast differentiation. We predict that ablation of either Lrp5 or Lrp6 will have unique consequences at these stages, and that loss of both genes will cause more severe phenotypes. Relevant to Public Health: Addressing these questions has overarching implications: Figuring out how these receptors work will help to identify and validate novel targets and develop novel strategies for how to treat low bone mass as well as provide important insights into how the Wnt signaling pathway controls bone development.