Heterotrimeric Gai2 protein function in insulin-secreting beta-cells and insulin-sensitive adipocytes

2型糖尿病(T2 DM)是一种普遍存在的代谢性疾病，在全球范围内影响着3亿多人。无法控制的T2 DM的显著特征是高血糖，这是由β细胞功能障碍和胰岛素抵抗引起的。在大多数患有T2 DM的人中，会发现一组特有的症状，称为代谢综合征：胰岛素抵抗、高脂血症、高血压和肥胖症。生活方式干预，如节食和/或锻炼，效果往往有限。因此，需要新的药物干预措施来减轻体重，改善胰岛素敏感性和高脂血症，以防止T2 DM的进展。G蛋白偶联受体是常见的药理作用靶点。当与配体结合时，GPCRs激活异三聚体G蛋白，这些G蛋白通过调节细胞内的第二信使生成系统而引起细胞反应。抑制性G蛋白(GI)组成一个亚家族，包括在胰岛和白色脂肪组织(WAT)中主要表达的GI2亚型。然而，胰岛素分泌的β细胞和Wat中依赖于GI2的通路的机制和生物学意义仍然不清楚。在第一个资助期内，我们确定β细胞-GI2是L精氨酸和L鸟氨酸诱导的胰岛素分泌的刺激因子。令人惊讶的是，尽管GI2是一种对PTX敏感的G蛋白，因此被认为抑制胰岛素的分泌，但β细胞中的GI2缺失导致体内葡萄糖耐量下降和胰岛素分泌受损，T2 DM患者也存在这种情况。有趣的是，我们对全球GI2缺陷小鼠的研究显示，在对照组(CD)和45%高脂饮食(HFD)的小鼠中，这些小鼠表现出瘦削的表型。Cd和HFD喂养的GI2缺陷小鼠都比它们的仔鼠明显瘦，积累的体脂质量显著减少。首先，对脂肪细胞特异性GI2缺陷小鼠的研究表明，服用HFD的小鼠体重也显着减轻。因此，我们推测，GI2不仅与正常的胰岛素分泌有关，而且对脂肪细胞的分化也很重要，并在饮食诱导的肥胖中发挥作用。因此，一方面，我们资助计划的一个主要目的是通过分析缺乏GI2的胰岛来阐明β细胞中的GI2分子信号通路。另一方面，我们想要阐明GI2在脂肪细胞中的功能。因此，我们打算研究全局的和脂肪细胞特异性的GI2缺陷小鼠。我们将研究脂肪细胞在体外的分化、成熟和组织，以及对CD和HFD小鼠的体重增加、血糖稳态、身体消耗、脂肪质量分布和能量消耗的影响。综上所述，这项研究将为T2 DM相关的信号通路提供新的见解，并将有助于开发新的T2 DM药物干预措施。

Type 2 diabetes mellitus (T2DM) is a prevalent metabolic disorder, which affects more than 300 million people globally. The prominent feature of uncontrolled T2DM is hyperglycemia, which results from beta-cell dysfunction and insulin resistance. In most people suffering T2DM a characteristic set of symptoms is found, which is known as metabolic syndrome: insulin resistance, hyperlipidemia, hypertension and obesity. Lifestyle interventions such as diet and/or exercise have often limited effects. Hence, novel pharmacological interventions are needed to reduce body weight, improve insulin sensitivity and hyperlipidemia in order to prevent the progression of T2DM. G protein coupled receptors (GPCR) are quite frequent pharmacological targets. Upon ligand binding GPCRs activate heterotrimeric G proteins which in turn are eliciting cellular responses through the regulation of intracellular second messenger-generating systems. The class of inhibitory G proteins (Gi) forms one subfamily including the predominantly expressed Gi2 isoform which is found in pancreatic islets and white adipose tissue (WAT). However, the mechanisms and biological implication of Gi2-dependent pathways in insulin-secreting beta-cells and WAT remain cryptic. Within the first funding period we identified beta-cell-Gi2 as a stimulator of L arginine- and L ornithine-induced insulin secretion. Surprisingly, although Gi2 is a PTx-sensitive G protein and therefore thought to inhibit insulin secretion, Gi2 deletion in beta-cells resulted in a decreased glucose tolerance and impaired insulin secretion in vivo, a situation also found in patients suffering from T2DM. Interestingly, our studies in global Gi2-deficient mice demonstrated a lean phenotype of these mice on control (CD) and on a 45% high-fat diet (HFD). Both, CD and HFD-fed Gi2-deficient mice were significantly leaner and accumulated significantly less body fat mass than their littermate controls. First studies on adipocyte-specific Gi2-deficient mice showed also significantly reduced body weight on HFD. Therefore, we hypothesize that Gi2 is not only relevant for proper insulin secretion but also important for adipocyte differentiation and plays a role in diet-induced obesity. Consequently, on the one hand a major aim of our grant proposal is to elucidate the molecular Gi2 signalling pathway in beta-cells by analyzing beta-cell-specific Gi2-deficient islets. On the other hand we want to clarify the function of Gi2 in adipocytes. Therefore, we intend to examine global and adipocytes-specific Gi2-deficient mice. We will study adipocyte differentiation, maturation and organization in vitro and weight gain, glucose homeostasis, body consumption, fat mass distribution and energy expenditure in mice on CD and HFD. Taken together, this study will provide new insights into T2DM-relevant signalling pathways and will help to develop new pharmacological interventions for T2DM.