Insulin-like Growth Factors and Muscle Differentiation

描述(申请人提供)：复杂组织的分化、成熟和维持，如骨骼肌，需要激素和生长因子激活的信号通路与生肌转录因子控制的内在调控程序之间的持续合作。胰岛素样生长因子(IGF)在胎儿肌肉的正常发育中起着关键作用，对协调成人损伤后的肌肉再生非常重要，对于在衰老和疾病期间保持肌肉质量也是至关重要的。这一应用的重点将放在肌肉发育和修复中的IGF系统上，并代表着一项长期努力的一部分，目的是了解IGFS、其受体和结合蛋白的作用机制在细胞内和整个有机体中的整合。根据目前的观察，我们假设在肌肉细胞中存在一个强大的自分泌信号网络，这对分化是至关重要的。激活这个网络的第一个关键步骤是通过分化成肌细胞产生和分泌IGF-II，这是诱导IGF-II基因转录的第二步。分泌的IGF-II与IGF-I受体结合并激活，导致PI3-K-Akt信号通路的持续刺激。AKT活性，主要是Akt1，在推动早期分化中发挥核心作用，部分是通过稳定肌肉启动子上的活性转录复合体来增强肌肉基因的表达。随着分化的进行，随着Akt2的产生，信号被放大，然后两个Akt都参与了肌肉细胞的成熟和肌细胞的融合，形成了多核肌管。我提出以下三个具体目标来验证IGF介导的信号网络假说：1.确定Akt1在肌肉分化中的作用机制。拟议中的研究将验证Akt1是成肌细胞分化的专性调节因子，其主要功能是通过刺激关键的辅助激活因子和抑制关键的辅助抑制因子来增强肌源性转录因子对肌肉基因启动子的作用。2.明确Akt2在肌肉分化中的作用。拟议的实验将检验Akt2在肌母细胞成熟和肌管形成中控制后期事件的假设。3.阐明肌肉分化过程中IGF-II基因转录调控机制。拟议的实验将检验这一假设，即依赖分化激活IGF-II基因转录的关键控制区位于H19基因的下游。主要目标将是确定这种染色体反应元件的作用机制。公共卫生相关性：胰岛素样生长因子(IGF)在胎儿正常肌肉发育中发挥关键作用，对于协调成人损伤后的肌肉再生非常重要，对于在衰老和疾病期间保持肌肉质量至关重要。这一应用的重点将放在肌肉发育和修复中的IGF系统上，并代表着一项长期努力的一部分，目的是了解IGFS、其受体和结合蛋白的作用机制在细胞内和整个有机体中的整合。

DESCRIPTION (provided by applicant): The differentiation, maturation, and maintenance of a complex tissue such as skeletal muscle requires ongoing cooperation between signaling pathways activated by hormones and growth factors, and intrinsic regulatory programs controlled by myogenic transcription factors. Insulin-like growth factors (IGFs) play key roles in normal muscle development in the fetus, are important for coordinating muscle regeneration following injury in the adult, and are critical for maintaining muscle mass during aging and in disease. The focus of this application will be on the IGF system in muscle development and repair, and represents part of a long-term effort to understand the mechanisms by which actions of the IGFs, their receptors, and binding proteins are integrated within the cell and in the whole organism. Based on current observations, we have postulated the existence of a robust autocrine signaling network in muscle cells that is critical for differentiation. The first key step in activating this network is production and secretion of IGF-II by differentiating myoblasts, which is secondary to induction of IGF-II gene transcription. Secreted IGF-II binds to and activates the IGF-I receptor, which leads to sustained stimulation of the PI3-kinase - Akt signaling pathway. Akt activity, principally Akt1, then plays a central role in driving early differentiation, in part by enhancing muscle gene expression through stabilization of active transcriptional complexes on muscle promoters. As differentiation proceeds, signal amplification occurs as more Akt2 is produced, and both Akts then participate in muscle cell maturation and in myocyte fusion to form multinucleated myotubes. I propose the following three Specific Aims to test this IGF-mediated signaling network hypothesis: 1. To determine the mechanisms of action of Akt1 in muscle differentiation. Proposed studies will test the idea that Akt1 is an obligate mediator of myoblast differentiation, and that its major functions are to enhance the actions of myogenic transcription factors on muscle gene promoters by stimulating key co-activators and inhibiting critical co-repressors. 2. To define the actions of Akt2 in muscle differentiation. Proposed experiments will examine the hypothesis that Akt2 acts to control late events in myoblast maturation and myotube formation. 3. To elucidate mechanisms of regulation of IGF-II gene transcription during muscle differentiation. Proposed experiments will test the hypothesis that a critical control region for differentiation-dependent activation of IGF-II gene transcription resides downstream of the H19 gene. Major goals will be to define the mechanisms of action of this chromosomal response element. PUBLIC HEALTH RELEVANCE: Insulin-like growth factors (IGFs) play key roles in normal muscle development in the fetus, are important for coordinating muscle regeneration following injury in the adult, and are critical for maintaining muscle mass during aging and in disease. The focus of this application will be on the IGF system in muscle development and repair, and represents part of a long-term effort to understand the mechanisms by which actions of the IGFs, their receptors, and binding proteins are integrated within the cell and in the whole organism.