BMP-2 Regulation of Bone Homeostasis

描述(申请人提供)：骨形态发生蛋白(BMPs)被认为是有效的骨形成剂，基于其诱导成年动物新骨形成的能力，BMP活性的这一独特特征导致BMPs被用作骨修复的治疗剂。这也引起了人们对定义内源性BMP在骨骼中所扮演的角色的浓厚兴趣。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的丢失都可以被存在的其他BMP补偿。在去除成骨BMP组合的小鼠中，显示出各种骨骼表型，这与单个BMP在骨骼发育过程中协同工作以提供适当水平的BMP活动的想法一致。BMP活性也是维持成人正常骨功能所必需的，骨中BMP拮抗剂水平的增加会导致骨量减少、骨脆性和自发性骨折。我们最近的数据显示，肢体骨骼中BMP2的丢失会导致出生后小鼠普遍的自发性骨折(Tsuji等人，2006年)，证实BMP2是成人骨骼动态平衡的主要参与者。在这个方案中，我们假设BMP2是骨骼动态平衡所必需的，因为它在出生后的肢体骨骼中具有独特的时空表达模式，因此不能被其他BMP所补偿。我们在三个具体目标中阐述了这一假设。在SA 1中，我们创造了一只小鼠，其中BMP2基因被视觉报告程序取代，提供了一种跟踪BMP2表达的精确方法。在SA 2中，我们研究了BMP2在维持成人骨骼中支持成骨细胞分化所需的骨髓基质细胞微环境中的作用。如果需要BMP2来维持允许成骨细胞分化的骨髓基质微环境，BMP2的丢失将会造成更严重的后果。在SA 3中，我们描述了在Wnt介导的成骨细胞分化中对BMP2的需求。如果骨中最佳的Wnt信号需要BMP2，那么BMP2的丢失将会造成更严重的后果。实验计划的完成将使我们能够确定BMP2在骨稳态中的作用。公共卫生相关性：成骨细胞(骨形成细胞)和破骨细胞(骨去除细胞)协同工作以维持骨量，骨重建发生在成人的整个生命过程中。当新骨形成跟不上骨吸收的步伐，导致骨骼虚弱，骨折风险增加，以及骨质疏松症，这是一种随着年龄增长而增加的丧失能力的疾病，骨量减少就会导致骨量减少。由于成人骨骼中BMP活性的变化与骨质疏松和修复随年龄增长而发生的骨折的能力降低有关(Badley，1995；Enomoto-Iwamoto等人，1998；Mundy等人，1999；Rountree等人，2004；Styrkarsdottir等人，2003)，我们的数据确定BMP2是成人正常BMP活动的关键，我们相信，更好地了解BMP2调节骨骼动态平衡的方式对于改善美国老龄人口的健康非常重要。

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.