BMP-2 Regulation of Bone Homeostasis

描述（由申请人提供）：骨形态发生蛋白（BMP）基于其在成年动物中诱导从头骨形成的能力而被鉴定为有效的骨形成剂，并且BMP活性的这种独特特征导致BMP用作骨修复中的治疗剂。它也产生了强烈的兴趣，在确定内源性骨形成蛋白在骨骼中发挥的作用。在胚胎发育过程中去除单个成骨BMP（BMP 2、4、5、6、7）表明，任何单个BMP的损失都可以由存在的其他BMP补偿。已去除成骨BMP组合的小鼠显示出多种骨骼表型，这与单个BMP在骨骼发育期间协同工作以提供适当水平的BMP活性的想法一致。BMP活性也是维持成人正常骨功能所必需的，增加骨中BMP拮抗剂的水平会导致骨质减少、骨脆性和自发性骨折。我们最近的数据显示，肢体骨骼中BMP 2的缺失导致出生后小鼠普遍的自发性骨折（Tsuji et al.，2006）将BMP 2鉴定为成人骨骼稳态的主要参与者。在这个提议中，我们假设BMP 2是骨骼稳态所必需的，因为它在出生后的肢体骨骼中具有独特的空间和时间表达模式，因此不能被其他BMP所补偿。我们在三个具体目标中解决这个假设。在SA 1中，我们创建了一种小鼠，其中BMP 2基因被视觉报告基因取代，提供了一种精确的方法来跟踪BMP 2表达。在SA 2中，我们研究了BMP 2在维持骨髓基质细胞微环境中的作用，该微环境是支持成人骨骼中成骨细胞分化所必需的。如果需要BMP 2来维持允许成骨细胞分化的骨髓基质微环境，则BMP 2的损失将具有更大的后果。在SA 3中，我们表征了Wnt介导的成骨细胞分化中对BMP 2的需求。如果骨中最佳Wnt信号传导需要BMP 2，则BMP 2的损失将具有更大的后果。实验计划的完成将使我们能够确定BMP 2在骨稳态中的作用。公共卫生相关性：骨重建发生在整个成年生活中，需要成骨细胞（骨形成细胞）和破骨细胞（骨清除细胞）协同工作以维持骨量。当新骨形成不能跟上骨吸收的步伐时，就会导致骨质减少，从而导致骨骼衰弱，骨折风险增加，以及骨质疏松症，这是一种随着年龄增长而患病率增加的失能性疾病。由于成人骨中BMP活性的变化与骨质疏松症和随年龄增长发生的骨折愈合能力降低有关（Badley，1995; Enomoto-Iwamoto等人，1998; Mundy等人，1999; Rountree等人，2004; Styrkarsdottir等人，2003），并且我们的数据确定BMP 2是成人中正常BMP活性的关键，我们相信发展对BMP 2调节骨稳态的方式的更好理解与改善美国老龄人口的健康高度相关。

DESCRIPTION (provided by applicant): Bone morphogenetic proteins (BMPs) were identified as potent bone forming agents based on their ability to induce de novo bone formation in adult animals and this unique feature of BMP activity has led to use of BMPs as therapeutic agents in bone repair. It has also generated intense interest in defining the role endogenous BMPs play in the skeleton. Removal of individual osteogenic BMPs (BMPs 2, 4, 5, 6, 7) during embryonic development shows that loss of any individual BMP can be compensated for by the other BMPs present. Mice in which combinations of osteogenic BMPs have been removed display a variety of skeletal phenotypes, consistent with idea that individual BMPs work in concert during skeletal development to provide the appropriate levels of BMP activity. BMP activity is also required to maintain normal bone function in adults, and increasing the levels of BMP antagonists in bone leads to osteopenia, bone fragility and spontaneous fracture. Our recent data showing that loss of BMP2 from the limb skeleton leads to pervasive spontaneous fractures in the postnatal mice (Tsuji et al., 2006) identifies BMP2 as a primary participant in skeletal homeostasis in adults. In this proposal, we hypothesize that BMP2 is required for skeletal homeostasis because it has a unique spatial and temporal expression pattern in the postnatal limb skeleton and so cannot be compensated for by other BMPs. We address this hypothesis in three specific aims. In SA 1 we create a mouse in which the BMP2 gene is replaced by a visual reporter, providing a precise way to follow BMP2 expression. In SA 2 we investigate the role of BMP2 in maintaining the bone marrow stromal cell microenvironment that is required to support osteoblast differentiation in the adult skeleton. Loss of BMP2 would be of greater consequence if BMP2 were required for maintenance of the bone marrow stromal microenvironment that allows for osteoblast differentiation. In SA 3 we characterize the requirement for BMP2 in Wnt-mediated osteoblast differentiation. Loss of BMP2 would be of greater consequence if BMP2 were required for optimal Wnt signaling in bone. Completion of the experimental plan will allow us to define the role of BMP2 in bone homeostasis. PUBLIC HEALTH RELEVANCE: Bone remodeling occurs throughout adult life, and requires that osteoblasts (bone forming cells) and osteoclasts (bone removing cells) work in concert to maintain bone mass. Osteopenia results when new bone formation cannot keep pace with bone resorption, leading to a weakened skeleton, increased risk of fracture, and osteoporosis, an incapacitating disease of increased prevalence with aging. As changes in BMP activity in adult bone have been associated with osteoporosis and in the reduced ability to heal fractures that occur with aging (Badley, 1995; Enomoto-Iwamoto et al., 1998; Mundy et al., 1999; Rountree et al., 2004; Styrkarsdottir et al., 2003), and our data identify BMP2 as key to normal BMP activity in adults, we believe developing a greater understanding of the way BMP2 regulates bone homeostasis is highly relevant to improving the health of the aging US population.