Role of FoxOs in Skeletal Homeostasis- Resubmission

 描述(申请人提供)：Wnt/β-连环蛋白信号对动物和人类的骨形成和骨量的维持是不可或缺的。受损的Wnt信号在获得性骨质疏松症的发展中起着致病作用，而不仅仅是遗传性骨质疏松。同样，NAD+、Sirt1活性和线粒体ATP产量的下降也与几种与年龄有关的疾病有关。FOXO转录因子可抑制系定向成骨细胞前体细胞中的Wnt信号，从而抑制其增殖和基质合成成骨细胞的供应。这是由于FOXO与β-连环蛋白的结合以及β-连环蛋白从TcF介导的转录中隔离的结果。Sirt1对FoxOS的脱乙酰化减弱了FoxOS与β-连环蛋白的结合。因此，骨祖细胞中的Sirt1增加了Wnt信号和骨形成。此外，Wnt信号通过依赖Sirt1的机制增加了ATP的产生，而Foxos则减少了ATP的产生。FOXO还作用于破骨细胞前体细胞，以抑制破骨细胞的生成。这种作用是Foxos与DNA直接结合和过氧化氢酶基因转录上调的结果。SIRT1刺激破骨细胞前体细胞中FoxO介导的转录，从而抑制破骨细胞的形成。上述观察结果形成了假设的基础，即Sirt1通过去乙酰化相应的祖细胞中的FoxO来增加成骨细胞的生成，减少破骨细胞的生成。随着年龄的增长，Sirt1活性降低，使骨形成和骨吸收之间的平衡向后者倾斜，从而导致退化性骨质疏松症的发病。Sirt1的激活可以改善这些影响，因此， 为骨质疏松症的治疗提供了合理的治疗目标。Sirt1对成骨细胞形成的有益作用通过增加ATP的产生而被放大。为了验证这一假设，我们将研究FoxO去乙酰化在Sirt1对骨形成和吸收的影响中的作用。在小鼠中，成骨细胞或破骨细胞前体中的内源性FoxO1、3和4被FoxO1乙酰化突变蛋白取代；或者Sirt1在FoxOS存在或不存在的情况下过表达(目标1)。此外，我们将使用Sirt1在成骨细胞或破骨细胞前体细胞中过表达的小鼠来检验Sirt1的刺激是否能对抗衰老对骨骼的不利影响(目标2)。最后，我们将进行体外研究，以确定线粒体ATP的产生对成骨细胞形成的贡献，以及Foxos、Sirt1和Wnt信号调节线粒体功能的机制。这项工作应该会促进人们对衰老如何降低骨量的了解。此外，它应该提出新的治疗方法来优化骨质疏松症的治疗。

 DESCRIPTION (provided by applicant): Wnt/β-catenin signaling is indispensable for bone formation and the maintenance of bone mass in animals and humans. Compromised Wnt signaling plays a pathogenetic role in the development of the acquired forms of osteoporosis - not just the hereditary forms. Likewise, a decline in NAD+, Sirt1 activity and mitochondria ATP production have been implicated in several age related diseases. FoxO transcription factors attenuate Wnt signaling in lineage-committed osteoblast progenitors and, thereby, restrain their proliferation and the supply of matrix synthesizing osteoblasts. This results from the binding of FoxOs to β-catenin and the sequestration of β-catenin away from TCF-mediated transcription. Deacetylation of FoxOs by Sirt1 attenuates the binding of FoxOs to β-catenin. Thus, Sirt1 in osteoprogenitor cells increases Wnt signaling and bone formation. In addition, Wnt signaling increases ATP production via a Sirt1-dependent mechanism while FoxOs decrease ATP production. FoxOs also act in osteoclast progenitors to restrain osteoclastogenesis. This effect results from direct binding of FoxOs to DNA and upregulation of catalase gene transcription. Sirt1 stimulates FoxO-mediated transcription in osteoclast progenitors, thereby inhibiting osteoclastogenesis. The above observations form the foundation of the hypothesis that Sirt1 increases osteoblastogenesis and decreases osteoclastogenesis by deacetylating FoxOs in the respective progenitors. The age-dependent decrease in Sirt1 activity contributes to the pathogenesis of involutional osteoporosis, by tilting the balance between bone formation and resorption, in favor of the latter. Activation of Sirt1 can ameliorate these effects and may, thus, represent a rational therapeutic target for the management of osteoporosis. The beneficial effects of Sirt1 on osteoblastogenesis are amplified by increased ATP production. To test this hypothesis we will investigate the role of FoxO deacetylation in the effects of Sirt1 on bone formation and resorption using mice in which the endogenous FoxO1, 3 and 4 in osteoblast or osteoclast progenitors are replaced with FoxO1 acetylation mutant proteins; or Sirt1 is overexpressed in the presence or absence of FoxOs (Aim 1). In addition, we will examine whether stimulation of Sirt1 antagonizes the adverse effects of aging on bone using mice in which Sirt1 is overexpressed in osteoblast or osteoclast progenitors (Aim 2). Finally, we will perform in vitro studies to determine the contribution of mitochondria ATP production to osteoblastogenesis and the mechanisms via which FoxOs, Sirt1 and Wnt signaling modulate mitochondria function. This work should advance knowledge of how aging decreases bone mass. Furthermore, it should suggest novel therapies to optimize the treatment of osteoporosis.