MECHANISMS OF INHIBITING BONE FORMATION THROUGH ANTAGONISM

描述（由申请人提供）：一个重要的研究领域是鉴定和表征促进骨形成的关键分子和途径，从头开始。目前最有希望用于骨质疏松症全身治疗的合成代谢药物是甲状旁腺激素（PTH），最近有研究表明，骨形成的负调节因子硬化蛋白（sost）是甲状旁腺激素（PTH）的直接靶点，提示sost调节可能在甲状旁腺激素依赖性骨形成中发挥重要作用。这在生物医学上具有重要意义，因为Sost在人类中丢失会导致两种严重的骨发育不良：硬化症（MIM 269500）和Van Buchem （VB）病（MIM 239100），这两种疾病都以骨质增生为特征，因此将Sost定位为治疗骨质流失的理想新靶点。我们最近的研究表明，在转基因小鼠中过度表达人Sost会导致骨质减少，而表达一种模仿VB等位基因的转基因小鼠则不会受到影响。通过跨物种序列比较以及体外和体内增强子试验的结合，我们已经确定了一个高度保守的元件，ECR5，它在体外和体内都具有成骨细胞/骨细胞增强子的功能。此外，我们已经在体外证明ECR5对PTH有反应，PTH介导的sost抑制需要ECR5远端增强子，而独立于近端sost启动子。由于调节调节骨形成的蛋白质表达具有重要的生物医学意义，并且由于sost在调节成骨细胞活性和骨形成方面的作用得到了充分的证明，因此我们研究的首要目标是了解涉及骨形成负调节或“骨拮抗剂”的激素调节和信号转导途径。特别是，我们感兴趣的是阐明在成骨、骨模式和骨代谢过程中sost表达被调节的转录机制。我们假设这种拮抗剂受到几种已知在骨稳态过程中至关重要的信号通路的严格转录控制，如PTH和BMP信号通路，并且根据肌细胞增强因子2 （Mef2）家族成员的需要优先上调或下调，通过微调其转录水平来调节其效应功能。

DESCRIPTION (provided by applicant): An important area of research is the identification and characterization of key molecules and pathways that promote bone formation, de novo. Currently the most promising anabolic agent used for the systemic treatment of osteoporosis is parathyroid hormone (PTH) and recently it has been shown that sclerostin (sost), a negative regulator of bone formation, is a direct target of PTH, suggesting that sost regulation may play an important role in PTH-dependent bone formation. This is of great biomedical importance since loss of Sost in humans causes two severe bone dysplasias: sclerosteosis (MIM 269500) and Van Buchem (VB) disease (MIM 239100), both characterized by hyperostosis and thus positioning sost as an ideal new target for the treatment of bone loss. We have recently shown that over-expressing human Sost in transgenic mice results in osteopenia, while mice expressing a transgene that mimics the VB allele, are unaffected. Through a combination of cross-species sequence comparisons followed by in vitro and in vivo enhancer assays, we have identified a highly-conserved element, ECR5 that functions as an osteoblast/osteocyte enhancer in vitro and in vivo. Further, we have shown in vitro that ECR5 is PTH-responsive, and that PTH- mediated suppression of sost requires the ECR5 distal enhancer, independently of the proximal sost promoter. Since modulating expression of proteins that regulate bone formation is of great biomedical importance and because of the well-documented role of sost in regulating osteoblast activity and bone formation, the overarching goal of our research is to understand the hormonal regulation and signal transduction pathways involving negative regulators of bone formation or `bone- antagonists'. In particular, we are interested in elucidating the transcriptional mechanisms by which sost expression is modulated during osteogenesis, bone patterning, and bone metabolism. We hypothesize that this antagonist is under tight transcriptional control from several signaling pathways known to be critical during bone homeostasis, such as the PTH- and BMP- signaling, and is preferentially up-regulated or down-regulated as needed by members of the Myocyte Enhancer Factor 2 (Mef2) family to mediate its effector function via fine-tuning of its transcript levels.