Roles of Smurf1 in Cbfa1 Degradation and Bone Formation

描述(申请人提供)：骨质疏松症是一种以骨量减少和骨微结构恶化为特征的代谢性骨病。了解骨形成的控制机制对于了解骨质疏松症的发病机制和确定治疗这种衰弱疾病的新疗法的分子靶点至关重要。一些证据表明，BMP信号在体内的骨形成中起着关键作用。BMP刺激成骨细胞分化的一个主要机制是通过其下游信号分子Smad1和5激活骨特异性转录因子Cbfa1。最近，我们首次发现成骨细胞蛋白酶体的抑制剂在体内和体外都能刺激骨形成。我们还首次发现E3泛素连接酶SMurf1以泛素蛋白酶体依赖的方式介导Cbfa1的降解。SMurf1介导Cbfa1降解的详细分子机制以及SMurf1在体内骨形成中的调节作用仍不清楚。 在拟议的研究中，我们将确定S-1介导Cbfa1降解的机制以及S-1在体内骨形成中的作用。潜在的假设是，SMurf1是Smad1和Cbfa1功能的重要调节分子，在体内骨形成的调节中发挥关键作用。在特定的目标1中，我们将确定SMurf1介导的Cbfa1降解是否依赖于BMP信号的激活、Smad1的磷酸化和Smad1/Cbfa1蛋白复合体的形成。我们还将确定PTH是否诱导Cbfa1降解，以及SMurf1对PTH诱导的Cbfa1降解的影响。在特定的目的2中，我们将检测野生型和突变型SMurf1过表达对完整和去卵巢动物成骨的影响，以及SMurf1转基因小鼠的成骨细胞功能和蛋白水平。我们还将确定骨生长调节因子对S-1转基因小鼠骨形成的影响。为了研究SMurf1在特定发育阶段(如出生后和成年)骨形成中的特定作用，我们还将产生组织特异性转基因小鼠，在其中转基因表达是可诱导的。我们的工作假设是：(1)SMurf1介导的Cbfa1降解依赖于BMP信号的激活；(2)体内成骨细胞SMurf1活性的调节将导致骨形成的异常，这是E3泛素连接酶家族的其他成员无法弥补的；以及(3)SMurf1在出生后和成年时都调节骨形成。建议的研究将有助于更好地理解SMurf1介导Cbfa1降解的分子机制，以及SMurf1在体内骨形成中的生理作用。这项研究还将有助于确定治疗骨质疏松症和其他骨丢失相关疾病的药物开发的新分子靶点。

DESCRIPTION (provided by applicant): Osteoporosis is a metabolic bone disease characterized by low bone mass and deterioration of bone microarchitecture. An understanding of the mechanisms by which bone formation is controlled is critical to understanding the pathogenesis of osteoporosis and defining molecular targets for development of new therapies for the treatment of this debilitating disease. Several lines of evidence show that BMP signaling plays a critical role in bone formation in vivo. A major mechanism by which BMPs stimulate osteoblast differentiation is by activation of the bone-specific transcription factor Cbfa1 through its downstream signaling molecules, Smad1 and 5. Recently we have discovered for the first time that inhibitors of the osteoblast proteasome stimulate bone formation in vivo and in vitro. We also identified for the first time that the E3 ubiquitin ligase Smurf1 mediates Cbfa1 degradation in an ubiquitin-proteasome-dependent manner. The detailed molecular mechanisms by which Smurf1 mediates Cbfa1 degradation and the regulatory role of Smurf1 in bone formation in vivo remain undefined. In the proposed studies, we will determine the mechanisms by which Smurf1 mediates Cbfa1 degradation and the role of Smurf1 in bone formation in vivo. The underlying hypothesis is that Smurf1 is an important regulatory molecule for Smad1 and Cbfa1 function and plays a critical role in regulation of bone formation in vivo. In Specific Aim 1, we will determine whether Smurf1-mediated Cbfa1 degradation is dependent on activation of BMP signaling, phosphorylation of Smad1 and formation of a Smad1/Cbfa1 protein complex. We will also determine whether PTH induces Cbfa1 degradation and the effects of Smurf1 on PTH-induced Cbfa1 degradation. In Specific Aim 2, we will determine the effects of over-expression of wild-type and mutant Smurf1 on bone formation in intact and ovariectomized animals and osteoblast function and protein levels of Smad1 and Cbfa1 in Smurf1 transgenic mice. We will also determine the effects of bone growth regulatory factors on bone formation in Smurf1 transgenic mice. To investigate the specific role of Smurf1 in bone formation during specific developmental stages, such as postnatal and adult life, we will also generate tissue-specific transgenic mice in which transgene expression is inducible. Our working hypotheses are that (1) Smurf1-mediated Cbfa1 degradation is dependent on activation of BMP signaling; (2) Modulation of osteoblast Smurf1 activity in vivo will result in abnormalities in bone formation that cannot be compensated for by other members of the E3 ubiquitin ligase family; and (3) Smurf1 regulates bone formation in both postnatal and adult life. The proposed studies will lead to a better understanding of the molecular mechanisms by which Smurf1 mediates Cbfa1 degradation and the physiological role of Smurf1 in bone formation in vivo. This study will also help identify a new molecular target for drug development for the treatment of osteoporosis and other bone-loss associated diseases.