Analysis of Bone Remodeling using Osteopetrotic and Osteosclerotic Mouse Models

1.骨稳态需要在骨吸收破骨细胞和骨形成成骨细胞的活动之间保持微妙的平衡。各种分子协调破骨细胞与成骨细胞的功能，然而，介导破骨细胞-成骨细胞的相互作用，同时在两种细胞类型的信号转导的分子尚未被确定。我们发现破骨细胞表达NFATc 1靶基因ephrinB 2，而成骨细胞表达受体EphB 4，沿着的还有其他ephrin-Eph家族成员。通过功能获得和功能丧失实验，我们证明了通过ephrinB 2进入破骨细胞前体的反向信号通过抑制破骨细胞生成c-Fos-NFATc 1级联反应来抑制破骨细胞分化。此外，通过EphB 4进入成骨细胞的正向信号传导增强了成骨分化，并且成骨细胞中EphB 4的过表达增加了转基因小鼠的骨量。这些数据表明肝配蛋白-Eph双向信号传导连接细胞分化的两种主要分子机制-一种在破骨细胞中，另一种在成骨细胞中-从而维持骨稳态（Zhao等人，2006年）。雌性转基因（Tg）小鼠发生骨质疏松症，并表现出骨基质蛋白表达的改变。我们发现，Thbsl和Thbs 2的表达在Fral Tg成骨细胞中减少。Fral Tg和非成骨细胞Thbsl-/-Thbs 2-/-小鼠共享边缘对边缘咬合。因此，血小板反应蛋白的表达减少可能导致颅面畸形，与骨质疏松无关（Nishiwaki et al.，2006年）。我们还观察到，在骨折后，Fral Tg小鼠中炎性细胞因子的产生减少，成骨细胞分化的启动延迟（手稿在准备中）。

1. Bone homeostasis requires a delicate balance between the activities of bone-resorbing osteoclasts and bone-forming osteoblasts. Various molecules coordinate osteoclast function with that of osteoblasts; however, molecules that mediate osteoclast-osteoblast interactions by simultaneous signal transduction in both cell types have not yet been identified. Here we show that osteoclasts express the NFATc 1 target gene ephrinB2, while osteoblasts express the receptor EphB4, along with other ephrin-Eph family members. Using gain-and loss-of-function experiments, we demonstrate that reverse signaling through ephrinB2 into osteoclast precursors suppresses osteoclast differentiation by inhibiting the osteoclastogenic c-Fos-NFATc 1 cascade. In addition, forward signaling through EphB4 into osteoblasts enhances osteogenic differentiation, and overexpression of EphB4 in osteoblasts increases bone mass in transgenic mice. These data demonstrate that ephrin-Eph bidirectional signaling links two major molecular mechanisms for cell differentiation-one in osteoclasts and the other in osteoblasts-thereby maintaining bone homeostasis (Zhao et al., 2006).2. Fral transgenic (Tg) mice develop osteosclerosis and exhibit altered expression of bone matrix proteins. We found that expression of Thbsl and Thbs2 was reduced in Fral Tg osteoblasts. Fral Tg and non-osteosclerotic Thbsl-/-Thbs2-/-mice share an edge-to-edge bite. Therefore, reduced expression of thrombospondins may contribute to craniofacial dysmorphism independently of osteosclerosis (Nishiwaki et al., 2006). We also observed that inflammatory cytokine production is reduced in Fral Tg mice and initiation of osteoblastic differentiation is delayed after bone fracture (manuscript in preparation).