Insulin-like Growth Factors and Muscle Differentiation

描述（由申请人提供）：骨骼肌等复杂组织的分化、成熟和维持需要激素和生长因子激活的信号通路与肌源性转录因子控制的内在调节程序之间的持续合作。胰岛素样生长因子（IGFs）在胎儿正常肌肉发育中起关键作用，在成人损伤后协调肌肉再生中起重要作用，并且在衰老和疾病期间保持肌肉质量至关重要。该应用的重点将放在IGF系统在肌肉发育和修复上，并代表了理解IGF、其受体和结合蛋白在细胞和整个生物体中整合的作用机制的长期努力的一部分。基于目前的观察，我们假设在肌肉细胞中存在一个强大的自分泌信号网络，这对分化至关重要。激活该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基因转录的次要步骤。分泌的IGF-II结合并激活IGF-I受体，导致pi3 -激酶- Akt信号通路的持续刺激。Akt活性，主要是Akt1，在驱动早期分化中发挥核心作用，部分原因是通过稳定肌肉启动子上的活性转录复合物来增强肌肉基因表达。随着分化的进行，随着更多Akt2的产生，信号放大发生，两种Akt2随后参与肌细胞成熟和肌细胞融合，形成多核肌管。我提出以下三个具体目标来验证这种igf介导的信号网络假说：探讨Akt1在肌肉分化中的作用机制。拟议的研究将验证Akt1是成肌细胞分化的专性介质的观点，其主要功能是通过刺激关键的共激活因子和抑制关键的共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在成肌细胞成熟和肌管形成的后期事件中起控制作用的假设。3. 阐明IGF-II基因在肌肉分化过程中的转录调控机制。拟议的实验将验证一个假设，即分化依赖的IGF-II基因转录激活的关键控制区位于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（IGFs）在胎儿正常肌肉发育中起关键作用，在成人损伤后协调肌肉再生中起重要作用，并且在衰老和疾病期间保持肌肉质量至关重要。该应用的重点将放在IGF系统在肌肉发育和修复上，并代表了理解IGF、其受体和结合蛋白在细胞和整个生物体中整合的作用机制的长期努力的一部分。

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.