Role and Mechanism of Action of Ksr2, an Obesity and Type 2 Diabetes Gene, in Bone Metabolism

摘要 在日益老龄化和肥胖的人群中，2型糖尿病（T2 DM）和骨质疏松性骨折是主要的公共卫生问题 性问题在美国，肥胖影响超过三分之一的成年人，是T2 DM最重要的风险因素。是 现在已经确定肥胖和T2 DM对骨折风险有影响，T2 DM中的骨折与更大的 发病率高于一般人群。尽管通常已知肥胖和T2 DM患者的骨密度会增加， 患者中，导致T2 DM/肥胖受试者骨折风险增加的根本原因仍有待确定。 因此，了解肥胖、T2 DM和骨折的相互作用正成为减少社会风险的迫切需要。 和骨折的个人成本。我们努力识别控制分子和它们的信号通路， T2 DM、骨质疏松症和肥胖之间的联系导致了令人兴奋的发现， 2（Ksr 2），肥胖和T2 DM基因，导致4个月大的股骨远端骨小梁骨量增加>2倍。 此外，小鼠诱导肥胖和T2 DM。相比之下，破坏一个密切相关的Ksr 1基因没有显著的影响， 骨骼或肥胖表型。KSR 1和2被发现是协调Raf组装的支架蛋白， 经典Ras-Raf-MAPKs通路中的MEK和ERK。虽然下丘脑表达的Ksr 2的中心作用是 被牵连在控制进食行为和能量平衡，从而肥胖，我们有新的令人兴奋的 初步数据表明，成骨细胞中表达的KSR 2以细胞自主方式局部作用， 功能协调发展的基于我们的初步数据和已发表的数据，我们提出了一个新的假设，即KSR 2调节成骨细胞 通过调节mTORC 1信号及其下游缺氧信号来促进骨分化和骨形成。在这 竞争性更新RO 1应用程序，我们将测试KSR 2作用的此模型如下：1）为了测试假设， KSR 2以细胞自主方式调节骨小梁形成，我们将产生成骨细胞特异性KSR 2， 条件性基因敲除小鼠，并通过显微CT、组织学、力学测试和基因分析来表征其骨骼表型 表情2)为了检验KSR 2对成骨细胞的作用是通过其对mTORC 1信号传导的调节介导的假设， 我们将通过评估KSR 2/mTORC 1通路相互作用的功能后果， 成骨细胞中的Raptor消除了Ksr 2敲低成骨细胞增加的合成代谢功能。3)为了检验这一假设 KSR 2/mTORC 1对成骨细胞的作用是通过HIF 1 α信号介导的，我们将评估缺氧信号的变化， Ksr 2和/或Raptor条件性破坏的成骨细胞中。我们还将确定是否干扰Hif 1 α表达 在成骨细胞特异性Ksr 2条件性敲除小鼠中，成功完成 我们提出的研究将为KSR 2调控成骨细胞和骨的途径提供重要信息 骨髓脂肪细胞的功能，并可能导致更好地了解肥胖和高骨量的机制 大量表型。

Abstract In an increasingly aging and obese population, type 2 diabetes (T2DM) and osteoporotic fractures are major public health concerns. Affecting more than 1 in 3 adults in the United States, obesity is the most important risk factor for T2DM. It is now well established that obesity and T2DM have effects on fracture risk, and fractures in T2DM are associated with greater morbidity than in the general population. Although bone density is generally known to be increased in obese and T2DM patients, the underpinning causes that contribute to increased fracture risk in T2DM/obese subjects remain to be established. Therefore, understanding the interactions of obesity, T2DM and fracture is becoming a pressing need to reduce the societal and individual costs of fracture. Our efforts to identify control molecules and their signaling pathways that delineate a link between T2DM, osteoporosis and obesity led to the exciting discovery that targeted disruption of kinase suppressor of Ras 2 (Ksr2), an obesity and T2DM gene, led to a >2-fold increase in trabecular bone mass at the distal femur of 4-month-old mice besides inducing obesity and T2DM. By contrast, disruption of a closely related Ksr1 gene had no significant effect on bone or obese phenotypes. KSR1 and 2 were discovered as scaffolding proteins that orchestrate the assembly of Raf, MEK and ERK in the canonical Ras-Raf-MAPKs pathway. While a central role for Ksr2 expressed in hypothalamus has been implicated in the control of feeding behavior and energy balance and, thereby obesity, we have new exciting preliminary data that suggest that KSR2 expressed in osteoblasts acts locally in a cell autonomous fashion to regulate its functions. Based on our preliminary data and published data, we propose a novel hypothesis that KSR2 regulates osteoblast differentiation and bone formation via regulating mTORC1 signaling and its downstream hypoxia signaling. In this competitive renewal RO1 application, we will test this model of KSR2 action as follows: 1) To test the hypothesis that KSR2 acts in a cell autonomous fashion to regulate trabecular bone formation, we will generate osteoblast-specific Ksr2 conditional knockout mice and characterize its skeletal phenotype by micro-CT, histology, mechanical testing and gene expression. 2) To test the hypothesis that KSR2 effects on osteoblasts are mediated via its regulation of mTORC1 signaling, we will determine the functional consequence of KSR2/mTORC1 pathway interactions by evaluating if knockdown of Raptor in osteoblasts abolishes the increased anabolic functions of Ksr2 knockdown osteoblasts. 3) To test the hypothesis that KSR2/mTORC1 effects on osteoblasts are mediated via HIF1α signaling, we will evaluate changes in hypoxia signaling in osteoblasts with conditional disruption of Ksr2 and/or Raptor. We will also determine if disruption of Hif1α expression in osteoblasts rescues the skeletal phenotype in osteoblast-specific Ksr2 conditional knockout mice. Successful completion of our proposed studies will provide important information on the pathway/s by which KSR2 regulates osteoblast and bone marrow adipocyte functions and could lead to improved understanding of the mechanisms linking obesity and a high bone mass phenotype.