Mechanisms of WNT Signaling in Bone

描述(由申请人提供)：WNT信号为开发新型骨合成代谢药物提供了一个很有前途的靶向途径。到目前为止，大多数研究都支持一种模型，即成骨细胞系细胞中的自分泌或旁分泌Wnt信号直接控制成骨细胞的生物学。然而，这一模型受到了最近一项研究的挑战，该研究得出结论，Wnt蛋白的共同受体LRP5并不直接在成骨细胞中发挥作用，而是通过调节肠道中5-羟色胺的产生来发挥作用。因此，这项研究对骨骼中直接的Wnt信号的生理相关性产生了不确定性。不确定的一个主要原因是2.3Col1-Cre基因缺失了成骨细胞中的连环蛋白(Wnt信号的强制性效应因子)，并不影响出生后动物的成骨细胞数量或功能。然而，先前在小鼠胚胎中的研究表明，Wnt/LRP5/？-catenin信号可能在2.3Col1-Cre激活之前的某个阶段发挥作用。由于缺乏适当的遗传工具，直接在出生后生活中测试这一概念是不可行的。我们现在已经开发了一种新的Tet-on系统，它允许在骨祖细胞中进行基因操作，特别是在出生后的小鼠中。因此，我们建议在出生后删除骨祖细胞中的β-catenin，以检验β-catenin在出生后直接调节骨形成的假设(目标1)。该领域进展的第二个关键障碍是缺乏对成骨细胞系细胞中介导Wnt功能的分子机制的了解。由于缺乏一种健壮的小鼠模型，可以在体内操纵和评估Wnt蛋白的急性信号能力，这一研究一直受到阻碍。我们现在已经开发了这样一种模型，其中有效的骨合成代谢Wnt配体可以以受控的方式被激活。因此，在AIMS 2和AIMS 3中，我们将使用这种新的小鼠模型从生化和遗传学两方面研究Wnt7b在体内诱导骨形成的信号转导机制。 公共卫生相关性：需要新的策略来安全地促进骨形成以治疗骨质疏松症。WNT信号已被认为可以刺激骨形成，并为开发新型骨合成药物提供了一条很有前途的靶向途径，但其潜在的分子机制尚不清楚。这项建议旨在了解Wnt蛋白强大的骨刺激功能的机制。本研究的研究结果将为开发新型骨增强药物提供分子框架。

DESCRIPTION (provided by applicant): Wnt signaling provides a promising target pathway for developing novel bone anabolic agents. Most studies to date have supported a model in which autocrine or paracrine Wnt signaling in osteoblast-lineage cells directly controls osteoblast biology. This model however, was challenged by a recent study that concluded that LRP5, a co-receptor for Wnt proteins, does not function directly in osteoblasts, but rather through regulating enteric production of serotonin. This study therefore has cast uncertainty about the physiological relevance of direct Wnt signaling in bone. A major cause for the uncertainty is that genetic deletion of ¿-catenin (an obligatory effector of canonical Wnt signaling) in osteoblasts by 2.3Col1-Cre did not affect osteoblast number or function in postnatal animals. However, previous work in the mouse embryo indicates that Wnt/Lrp5/¿-catenin signaling may function at a stage before 2.3Col1-Cre becomes active. Directly testing this notion in postnatal life has not been feasible because of the lack of proper genetic tools. We have now developed a novel Tet-on system that allows for gene manipulation in osteoprogenitors specifically in postnatal mice. Therefore, we propose to delete ¿-catenin in osteoprogenitors postnatally to test the hypothesis that ¿-catenin directly regulates bone formation in postnatal life (Aim 1). A second critical barrier to progress in the field is the lack of understanding of the molecular mechanisms that mediate Wnt function in osteoblast-lineage cells. Research has been hindered by the lack of a robust mouse model in which a Wnt protein can be manipulated and assessed for its acute signaling ability in vivo. We have now developed such a model wherein a potent bone anabolic Wnt ligand can be activated in a controlled manner. Therefore, in Aims 2 and 3, we will employ this new mouse model to investigate both biochemically and genetically the signal transduction mechanisms through which Wnt7b induces bone formation in vivo. PUBLIC HEALTH RELEVANCE: Novel strategies are required to safely promote bone formation to treat osteoporosis. Wnt signaling has been known to stimulate bone formation and provides a promising target pathway for developing novel bone anabolic agents, but the underlying molecular mechanisms are not well understood. This proposal is designed to understand the mechanism responsible for the potent bone-stimulating function of Wnt proteins. Research results from this study will provide a molecular framework for developing novel bone-enhancing pharmaceutics.