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有希望的靶标途径。迄今为止，大多数研究都支持一种模型，即成骨细胞谱系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蛋白的共受体LRP5并不直接在成骨细胞中起作用，而是通过调节肠内血清素的产生。因此，这项研究对骨中直接Wnt信号的生理相关性提出了不确定性。不确定性的一个主要原因是，2.3Col1-Cre在成骨细胞中基因缺失¿-catenin（典型Wnt信号的强制性效应物）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信号传导可能在2.3Col1-Cre变得活跃之前的一个阶段起作用。由于缺乏适当的遗传工具，在出生后的生活中直接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，允许对骨祖细胞进行基因操作，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¿-catenin，以验证¿-catenin在出生后直接调节骨形成的假设（Aim 1）。该领域进展的第二个关键障碍是缺乏对在成骨细胞谱系中介导Wnt功能的分子机制的理解。由于缺乏一种强大的小鼠模型，可以操纵Wnt蛋白并评估其在体内的急性信号传导能力，研究一直受到阻碍。我们现在已经开发了这样一个模型，其中一个有效的骨合成代谢Wnt配体可以以一种受控的方式被激活。因此，在Aims 2和Aims 3中，我们将采用这种新的小鼠模型来研究Wnt7b诱导体内骨形成的生化和遗传信号转导机制。

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.