Vesicle Translocation and the Metabolic Syndrome

葡萄糖稳态的调节是一个复杂的过程，在2型糖尿病等疾病状态下被破坏。胰岛素是调节葡萄糖稳态的主要激素。胰岛素通过引起含有GLUT4葡萄糖转运体的细胞膜的运动来刺激肌肉和脂肪的葡萄糖摄取，这些转运体融合并将GLUT4插入细胞表面。胰岛素的这种作用在营养过剩、缺乏运动和遗传易感性的情况下受到损害，导致胰岛素抵抗并促进糖尿病的发展。因此，为了了解代谢性疾病的发病机制，有必要了解GLUT4在细胞膜内运输的分子机制，以及这种运输是如何被胰岛素调节并在胰岛素抵抗中中断的。该项目之前的工作确定了TUG蛋白是肌肉和脂肪细胞中GLUT4运输和葡萄糖摄取的主要调节因子。这些数据支持了一种模型，即在未受刺激的细胞中，TUG介导了GLUT4的细胞内保留，以及选定的其他蛋白质在特定的膜泡中。然后胰岛素通过触发TUG内源性蛋白水解裂解来动员这些囊泡。裂解协调了葡萄糖摄取和其他生理效应，这些生理效应是由与GLUT4共同调节的蛋白质的作用引起的，并可能对能量消耗产生影响。在胰岛素抵抗个体中，这种膜运输机制的改变可能导致代谢综合征的多个方面。然而，目前尚不清楚TUG和GLUT4在细胞中的定位，这种定位在胰岛素抵抗中如何受到影响，以及减弱的TUG切割是否会导致体内胰岛素抵抗。为了解决这些问题，将采取三个目标。目的1将描述TUG如何能够在未受刺激的细胞中捕获含有glut4的囊泡，以及它如何将这些囊泡维持在胰岛素反应状态。目的2将研究TUG将这些囊泡锚定到细胞内结构的位置和机制，以及这在胰岛素抵抗中可能受到的影响。目的3将研究肌肉中TUG蛋白水解对整体胰岛素作用和葡萄糖稳态的重要性。我们预计，这些研究将有助于提高对调节葡萄糖代谢和能量消耗的分子机制的理解，对糖尿病和代谢综合征的预测、预防和治疗具有重要意义。

The regulation of glucose homeostasis is a complex process, which is disrupted in disease states such as type 2 diabetes. Insulin is the primary hormone that regulates glucose homeostasis. Insulin stimulates glucose uptake in muscle and fat by causing the movement of intracellular membranes containing GLUT4 glucose transporters, which fuse and insert GLUT4 at the cell surface. This effect of insulin is impaired in the setting of overnutrition, inactivity, and genetic predisposition, resulting in insulin resistance and contributing to the development of diabetes. Therefore, to understand the pathogenesis of metabolic disease, it is necessary to understand the molecular mechanisms that specify the trafficking of GLUT4 among intracellular membranes, and by which this trafficking is modulated by insulin and disrupted in insulin resistance. Previous work by this project identified the TUG protein as a major regulator of GLUT4 trafficking and glucose uptake in muscle and fat cells. The data support a model in which TUG mediates the intracellular retention of GLUT4, together with selected other proteins, in specific membrane vesicles within unstimulated cells. Insulin then mobilizes these vesicles by triggering TUG endoproteolytic cleavage. Cleavage coordinates glucose uptake with other physiologic effects, resulting from the action of proteins that are co-regulated with GLUT4 by this mechanism, as well as with possible effects on energy expenditure. In insulin resistant individuals, alterations in this membrane trafficking mechanism may contribute to multiple aspects of the metabolic syndrome. Yet, it remains unknown where TUG and GLUT4 are localized in cells, how this localization is affected in insulin resistance, and whether attenuated TUG cleavage can cause insulin resistance in vivo. To address these questions, three Aims will be undertaken. Aim 1 will characterize how TUG is able to trap the GLUT4-containing vesicles in unstimulated cells, and how it maintains these vesicles in an insulin-responsive configuration. Aim 2 will study the location and mechanism by which TUG anchors these vesicles to intracellular structures, and how this may be affected in insulin resistance. . Aim 3 will study the importance of TUG proteolysis in muscle for overall insulin action and glucose homeostasis. We anticipate that, together, these studies will result in an improved understanding of molecular mechanisms regulating glucose metabolism and energy expenditure, with implications for the prediction, prevention, and treatment of diabetes and the metabolic syndrome.