G Protein Signaling in Osteoblasts

描述(由申请人提供)： 成骨细胞谱系中的细胞在一生中对控制骨形成和骨吸收至关重要。深入了解成骨细胞系细胞内的信号传递过程与骨形成和骨吸收的关系，对于深入了解骨骼生理学和开辟治疗骨质疏松症的新途径至关重要。在这方面，G蛋白信号通路至关重要，因为FDA批准的唯一治疗骨质疏松症的合成代谢药物是间歇性甲状旁腺激素，它通过成骨细胞中的G蛋白偶联受体(GPCR)激活两种G蛋白-Gs和GQ。以前的研究主要集中在Gs信号通路的作用上，而对其他成骨细胞G蛋白通路如GI和GQ的功能知之甚少。我们必须加深对成骨细胞中特定的G蛋白信号如何与骨形成和骨吸收有关的理解，并确定它们的病理生理学意义。正常骨形成和转换所需的骨骼环境的复杂性使得使用活体模型来解决这些关键问题是必要的。我们的实验室已经建立了一种利用转基因小鼠表达被称为RASSLs(仅由合成配体激活的受体)的转基因小鼠来剖析特定信号通路的作用的方法。到目前为止，我们的结果表明，成熟成骨细胞中Gs信号通路的激活导致骨小梁形成大量增加，骨髓元素显着减少，皮质受到侵蚀。相比之下，成熟成骨细胞中GI信号通路的激活会导致骨小梁减少，这是由于骨形成速度降低所致。最近，我们开发了一种利用百日咳毒素催化亚单位的靶向表达在体内阻断成骨细胞内源性GI信号的方法。在雌性小鼠中，阻断成骨细胞GI信号导致骨生长过程中骨小梁形成增加，并完全防止与年龄相关的骨小梁丢失。这些令人兴奋的发现表明，成骨细胞中的内源性GI信号负性调节骨形成，并表明这一途径是年龄相关性骨丢失的重要因素。在目前的提案中，我们将扩大这些观察，并探讨它们的病理生理学相关性。具体地说，我们将：1)确定成骨细胞GI信号在年龄相关性骨丢失中的病理生理学重要性，并评估成骨细胞GI信号是否限制了甲状旁腺素作为合成代谢剂的有效性；2)确定成骨细胞GI信号负性调节骨形成的机制；以及3)确定GQ信号在成熟OBS中调节骨骼稳态的作用。从这些研究中获得的知识将为控制成骨细胞骨形成提供新的见解，这些将对设计和开发改进的合成代谢疗法治疗骨质疏松症有价值。

DESCRIPTION (provided by applicant): Cells in the osteoblast lineage are crucial for controlling bone formation and bone resorption throughout life. An in-depth understanding of how signaling processes within osteoblast-lineage cells are linked to bone formation and bone resorption is essential to provide new insights into skeletal physiology and to open up new avenues for osteoporosis therapy. In this regard, the G protein signaling pathway is crucial in that the only FDA-approved anabolic treatment for osteoporosis is intermittent parathyroid hormone which activates two G proteins- Gs and Gq- through a G protein coupled receptor (GPCR) in osteoblasts. Previous studies have focused largely on the role of Gs signaling, and little is known about the function of other osteoblast G protein pathways such as Gi and Gq. It is vital that we increase our understanding of how specific G protein signals in osteoblasts are linked to bone formation and bone resorption, and to define their pathophysiological significance. The complexity of the skeletal environment required for normal bone formation and turnover make it necessary to use in vivo models to address these critical issues. Our laboratory has established the utility of an approach to dissect the role of specific signaling pathways using transgenic mice expressing engineered GPCRs termed RASSLs (Receptors Activated Solely by Synthetic Ligands). Our results so far demonstrate that activation of the Gs signaling pathway in mature osteoblasts results in a massive increase in the formation of trabecular bone with a marked reduction in marrow elements and erosion of the cortex. By contract, activation of the Gi signaling pathway in mature osteoblasts results in trabecular osteopenia due to decreased rates of bone formation. Recently, we have developed a method to block endogenous Gi signaling in osteoblasts in vivo using the targeted expression of the catalytic subunit of pertussis toxin. In female mice, blockade of osteoblast Gi signaling results in increased trabecular bone formation during bone growth, and completely prevents age-related trabecular bone loss. These exciting findings indicate that endogenous Gi signaling in osteoblasts negatively regulates bone formation, and suggest that this pathway is an important contributor to age-related bone loss. In the present proposal, we will extend these observations and probe their pathophysiological relevance. Specifically, we will: 1) determine the pathophysiological importance of osteoblast Gi signaling in age-related bone loss, and assess whether osteoblast Gi signaling limits the effectiveness of PTH as an anabolic agent; 2) identify the mechanisms whereby osteoblast Gi signaling negatively regulates bone formation; and 3) determine the role of Gq signaling in mature OBs in regulating skeletal homeostasis. The knowledge gained from these studies will provide new insights into the control of osteoblastic bone formation, and these will be valuable in the design and development of improved anabolic therapies for osteoporosis.