Role of EGFR signaling in bone formation and the anabolic actions of PTH

描述（由申请人提供）：间歇性注射甲状旁腺激素（PTH）是治疗骨质疏松症最有效的合成代谢疗法之一，因为它对骨形成有显著的作用。成骨细胞来源于间充质祖细胞，包括间充质干细胞（MSCs）和固定的骨祖细胞。人们提出了多种机制来解释甲状旁腺激素如何发挥其有益作用，但这些作用是否包括骨髓间充质祖细胞活性的刺激，或由其介导，尚不清楚。我们的新数据支持这一机制，证明了小梁骨内存在功能独特的间充质祖细胞群，它们对甲状旁腺激素非常敏感。甲状旁腺激素强烈刺激双调节蛋白的表达，双调节蛋白是一种表皮生长因子受体（EGFR）配体，在成骨细胞和骨细胞中。EGFR在间充质祖细胞中高度表达，其激活刺激这些细胞的增殖、存活和迁移。我们的初步数据表明，在小鼠中阻断EGFR活性会导致骨形成缺陷和骨质减少表型，并伴有间充质祖细胞数量的减少。有趣的是，pth处理的成骨细胞的条件培养基以双调节- egfr依赖的方式化学吸引间充质祖细胞。此外，成骨细胞EGFR活性缺乏的小鼠对PTH注射的合成代谢反应较差。这些和其他数据导致我们的中心假设，即EGFR信号是骨髓间充质祖细胞的重要调节因子，至少部分介导间歇性PTH给药的合成代谢效应。我们将测试

DESCRIPTION (provided by applicant): Intermittent parathyroid hormone (PTH) injection is one of the most effective anabolic treatments for osteoporosis because of its remarkable actions on bone formation. Osteoblasts are derived from mesenchymal progenitors, including mesenchymal stem cells (MSCs) and committed osteoprogenitors. Multiple mechanisms have been proposed to explain how PTH exerts its beneficial effects, but whether these effects include, or are mediated by, a stimulation of bone marrow mesenchymal progenitor activities, is not clear. Our new data support this mechanism by demonstrating the presence of a functionally distinct population of mesenchymal progenitors within the trabecular bone that are very responsive to PTH. PTH strongly stimulates the expression of amphiregulin, an epidermal growth factor receptor (EGFR) ligand, in osteoblasts and osteocytes. EGFR is highly expressed in mesenchymal progenitors and its activation stimulates proliferation, survival, and migration of these cells. Our preliminary data show that blocking EGFR activity in mice leads to defective bone formation and an osteopenic phenotype which are accompanied by a reduction in the number of mesenchymal progenitors. Interestingly, conditioned media from PTH-treated osteoblastic cells chemoattract mesenchymal progenitors in an amphiregulin-EGFR-dependent manner. Moreover, mice with deficient EGFR activity in osteoblast lineage cells have a poor anabolic response to PTH injection. These and other data lead to our central hypothesis that EGFR signaling is an essential regulator of bone marrow mesenchymal progenitors and mediates at least in part the anabolic effects of intermittent PTH administration. We will test our central hypothesis by pursuing the following aims: 1) investigate whether EGFR inactivation in MSCs inhibits bone formation and the anabolic response to PTH by assessing skeletal phenotypes of conditional EGFR knockout mice with a nestin promoter-driven inducible Cre with or without PTH treatment; 2) elucidate whether EGFR plays a critical role in maintaining the bone marrow mesenchymal progenitor population under conditions of normal and PTH-induced bone formation. We will use animal models with deficient EGFR activity in mesenchymal progenitors at different stages of lineage commitment to investigate the relationship between EGFR activity and mesenchymal progenitor populations residing in different regions of the long bone. We will also use cell culture approaches to determine whether a group of transcription factors, Egrs, mediates and regulates EGFR-stimulated proliferation and survival of mesenchymal progenitors; 3) determine whether EGFR signaling activated by PTH injection recruits mesenchymal progenitors toward the bone surface by using an in vivo transplantation approach, in which GFP-labeled mesenchymal progenitors can be visualized. Completion of these three aims will enable us to determine for the first time the role of EGFR in bone formation and the anabolic response of bone to the osteoporosis therapy PTH. Long-term, we seek to determine whether we can target EGFR signaling as a novel anabolic strategy to treat osteoporosis and other bone-related diseases.