Central Role of Gut Hormone GIP in Leptin Resistance and Obesity

 描述(申请人提供)：肥胖症显然是美国最明显的公共健康问题之一。肥胖现在影响了超过30%的美国成年人口，增加了患严重慢性病的风险，并缩短了预期寿命。这突显出迫切需要更好地了解肥胖的病因，并开发更有效的肥胖治疗方法。瘦素是一种关键的脂肪细胞来源的激素，它能有效地抑制食物摄入，减轻体重，增加能量消耗。因此，瘦素曾被认为是治疗肥胖症的“灵丹妙药”。然而，由于瘦素抵抗的发展，瘦素在肥胖者中不起作用。这一观察产生了该领域需要解决的基本问题之一：神经元瘦素抵抗的机制是什么，这是人类肥胖的一个标志。为了解决这个问题，我们一直使用器官型脑片模型作为体外工具来研究下丘脑神经元细胞瘦素抵抗的分子机制，下丘脑神经元是瘦素作用的主要部位。利用这一工具，我们初步发现，cAMP相关途径通过EPAC-Rap1信号有效地诱导瘦素抵抗。EPAC是小G蛋白Rap1的GTP/GDP的交换因子。此外，我们还寻找了一种细胞外上游因子(S)，它可以同时诱导EPAC-RAP1信号的激活和瘦素抵抗。为此，我们一直在进行候选配体方法，基于EPAC-Rap1信号可以被产生cAMP的各种G蛋白偶联受体(GPCRs)激活的事实。我们一直在系统地筛选与cAMP信号偶联的已知GPCRs的配体。在我们的初步筛选中，我们已经确定胃肠激素葡萄糖依赖的促胰岛素多肽(GIP)是一个有希望的候选药物。基于这些先前的观察和我们的初步数据，我们假设肠道来源的GIP作为一个以前未被识别的循环信号，通过在肥胖过程中直接激活EPAC-Rap1信号来驱动神经元瘦素抵抗。在我们的具体目标中，在Aim1中，我们将使用Cre依赖的条件性Rap1基因敲除小鼠来确定瘦素反应神经元表达的Rap1在饮食诱导的瘦素抵抗和肥胖中的生理学相关性。在AIM2中，我们将通过使用GIP受体缺陷的小鼠来确定瘦素反应神经元中的GIP受体是否是饮食诱导的瘦素抵抗和肥胖所必需的。在AIM3中，通过对啮齿动物的药理学和遗传学研究的结合，我们还将探索针对GIP受体和EPAC的潜在干预策略，并将使用相同的方法，确定脑GIP-EPAC-Rap1信号调节瘦素敏感性的分子机制。这些研究的结果潜在地改变了范式，应该为更好地理解中枢性瘦素抵抗提供一个框架。

 DESCRIPTION (provided by applicant): Obesity is clearly one of the most visible public health problems in the US. Obesity now affects more than 30 % of the US adult population, increasing the risks for serious chronic diseases and reducing the life expectancy. This highlights urgent need to better understand the etiology of obesity and develop more effective therapies against obesity. Leptin is a key adipocyte-derived hormone that potently suppresses food intake, reduces body weight, and increases energy expenditure. Thus, leptin was once thought to be a "magic bullet" for the treatment of obesity. However, leptin does not work in obese people because of the development of leptin resistance. This observation creates one of the fundamental questions to be addressed in the field: what are the mechanisms of neuronal leptin resistance, a hallmark of human obesity. To address this question, we have been using an organotypic brain slice model as an in vitro tool to investigate the molecular mechanisms underlying cellular leptin resistance in hypothalamic neurons, a primary site of leptin action. Using this tool, we initially found that the cAMP-related pathway potently induces leptin resistance through Epac-Rap1 signaling. Epac is an exchange factor for GTP/GDP for the small G protein Rap1. Furthermore, we have searched for an extracellular upstream factor(s) that induces both the activation of Epac-Rap1 signaling and leptin resistance. To this end, we have been conducting a candidate-ligand approach based on the fact that Epac-Rap1 signaling can be activated by a variety of G protein-coupled receptors (GPCRs) that produce cAMP. We have been systematically screening the ligands of known GPCRs that couple to cAMP signaling. During our initial screening, we have identified the gut hormone glucose-dependent insulinotropic polypeptide (GIP) as a promising candidate. Based on these previous observations and our preliminary data, we hypothesize that the gut-derived GIP acts as a previously unrecognized circulating signal that drives neuronal leptin resistance via directly activating Epac-Rap1 signaling during obesity. In our specific aims, in Aim1, we will use a Cre- dependent conditional Rap1 knockout mouse to determine the physiological relevance of Rap1 expressed by leptin responsive neurons in diet-induced leptin resistance and obesity. In Aim2, we will determine if GIP receptor in leptin responsive neurons is required for diet-induced leptin resistance and obesity by using GIP receptor deficient mice. In Aim3, through a combination of pharmacological and genetic studies in rodents, we will also explore potential intervention strategies targeting GIP receptor and Epac, and using the same approach, we will determine the molecular mechanisms mediating the effect of brain GIP-Epac-Rap1 signaling on modulating leptin sensitivity. The results of these studies are potentially paradigm-shifting, and should provide a framework for a better understanding of central leptin resistance.