Glycogen metabolism and its regulation

描述(申请人提供)：糖原是葡萄糖的一种分支储存聚合物，在许多细胞类型中作为能量储备。就绝对值而言，肝脏和骨骼肌拥有哺乳动物中最大的糖原沉淀物，对全身葡萄糖代谢至关重要。这项建议的总体目标是提高对糖原合成的机制和控制的了解，它与全身葡萄糖代谢的相关性，以及它在代谢性疾病如糖尿病和某些糖原储存性疾病中的损害。一个重要的调节部位是糖原合成酶，这是一种关键的糖原生物合成酶，它受几种激素控制，包括胰岛素和肾上腺素以及运动。目的(1)运动过程中糖原合成酶的控制。运动对糖原合成酶活性的影响复杂且知之甚少。小鼠的力竭运动导致肌肉糖原合成酶的稳定失活，其机制不能用已知的通过磷酸化对该酶进行共价控制来解释。这一目标将试图确定这一潜在的新控制机制的性质。目的(Ii)新的潜在的糖原合成酶活性调节剂。在上一次资助期间，出现了两个新的潜在的糖原合成酶调节因子。首先，发现了一种双特异性磷酸酶，BEDP，当在培养细胞中共表达时，它可以激活糖原合成酶。这种激活的机制和负责任的BEDP底物(S)的身份将被寻找。其次，一种蛋白激酶，PAS激酶，其酵母同源蛋白使酵母糖原合成酶磷酸化，也被发现是哺乳动物酶的有效失活剂。其目的将试图确定该激酶是否在体内发挥作用。目的(Iii)小鼠葡萄糖稳态模型中的糖原。在葡萄糖耐量试验中，肌肉糖原合成酶基因(GYS1)被破坏的小鼠的葡萄糖耐量有所改善，血清胰岛素的升高比野生型小鼠更持久。这些动物的胰岛功能是否发生了变化将是目标之一。它还将通过其他小鼠模型解决肝脏和肌肉糖原储存对全身葡萄糖代谢的相对重要性，包括缺乏肝糖原的小鼠(也是0型糖原储存疾病的模型)或组织特异性去除GYS1基因的小鼠。目的(3)糖原分支、拉福林和拉福拉病。拉福林是一种含有糖原结合域的双特异性蛋白磷酸酶。编码拉福林的EPM2A基因突变会导致拉福拉病，拉福拉病是一种癫痫，在这种情况下，分支不良的糖原沉积拉福拉小体，会在神经元和其他组织中积聚。目的是了解这种糖原相关的磷酸酶缺陷是如何导致分支不良的糖原积累的。目的(4)UDP-葡萄糖焦磷酸酶(UGPPase)。最近发现的这种酶可以将糖原合成酶的底物UDP-葡萄糖水解为葡萄糖-1-P和UMP。因此，它的作用可能会深刻地影响糖原代谢。其目的是确定UGPPase是否影响体内的UDP-葡萄糖和糖原水平。

DESCRIPTION (provided by applicant): Glycogen is a branched storage polymer of glucose that serves as an energy reserve in many cell types. In absolute terms, liver and skeletal muscle house the largest glycogen deposits in mammals and are critical to whole body glucose metabolism. The overall goal of this proposal is to improve understanding of the mechanism and control of glycogen synthesis, its relevance to whole body glucose metabolism and its impairment in metabolic diseases like diabetes and certain glycogen storage diseases. An important site of regulation is glycogen synthase, a key glycogen biosynthetic enzyme, which is controlled by several hormones, including insulin and epinephrine as well as by exercise. Aim (i) Control of glycogen synthase during exercise. Exercise has complex and poorly understood effects on the activity of glycogen synthase. Exhaustive exercise of mice led to a stable inactivation of muscle glycogen synthase by a mechanism not explained by known covalent control of the enzyme by phosphorylation. This aim would attempt to determine the nature of this potentially novel control mechanism. Aim (ii) Novel potential regulators of glycogen synthase activity. During the last funding period, two novel potential regulators of glycogen synthase emerged. First, a dual specificity phosphatase, BEDP, was discovered that activates glycogen synthase when co-expressed in cultured cells. The mechanism for this activation and the identity of responsible BEDP substrate(s) will be sought. Second, a protein kinase, PAS kinase, whose yeast orthologs phosphorylate yeast glycogen synthase, was found to be also a potent inactivator of the mammalian enzyme. The aim will attempt to establish whether the kinase has a role in vivo. Aim (iii) Glycogen in mouse models of glucose homeostasis. Mice with the muscle glycogen synthase gene (GYS1) disrupted have improved glucose tolerance and elevated serum insulin is more sustained than in wild type mice during a glucose tolerance test. Whether there is altered islet function in these animals will be one focus of the aim. It will also address the relative importance of the liver and muscle glycogen stores for whole body glucose metabolism through other mouse models, including mice lacking liver glycogen (also a model for glycogen storage disease type 0) or with tissue-specific ablation of the GYS1 gene. Aim (iii) Glycogen branching, laforin and Lafora disease. Laforin is a dual specificity protein phosphatase that contains a glycogen binding domain. Mutations in the EPM2A gene, encoding laforin, cause Lafora disease, a form of epilepsy in which poorly branched glycogen deposits, Lafora bodies, accumulate in neurons and other tissues. The goal is to understand how defects in this glycogen associated phosphatase cause the accumulation of poorly branched glycogen. Aim (iv) UDP-glucose pyrophosphatase (UGPPase). This recently discovered enzyme hydrolyzes the substrate for glycogen synthase, UDP-glucose, to glucose-1-P and UMP. Its action could therefore profoundly affect glycogen metabolism. The goal of this aim is to establish whether UGPPase influences UDP-glucose and glycogen levels in vivo.