Role of mitochondrial calcium transport in the regulation of insulin secretion

胰岛素分泌对血糖水平的正常调节至关重要，在所有形式的糖尿病中都会出现缺陷，这是一种影响西化社会约5%人口的疾病。胰岛β细胞是人体循环中胰岛素的唯一来源，在健康的人中，它会通过糖的代谢增强来应对血糖水平的升高。这导致了一系列的生化事件，最终导致钙离子流入细胞，细胞表面含有胰岛素的颗粒融合。线粒体在β细胞葡萄糖识别中起着特别重要的作用，使这些细胞几乎完全将糖分解为二氧化碳和水。这种高效的“氧化”代谢，随着葡萄糖浓度的升高而急剧增加，帮助细胞合成大量的三磷酸腺苷。由此产生的ATP与其前体ADP比率的变化导致质膜上的ATP敏感K*(KATP)通道关闭。然后，电压敏感的钙通道允许钙离子内流，触发胰岛素的释放(“胞吐”)。细胞内钙离子的增加也促使ATP的消耗增加，以支持离子泵出细胞、分泌颗粒运动等过程。过去几十年的研究结果表明，为了满足这一需求，线粒体中含有一组三种酶，它们参与了“柠檬酸循环”中葡萄糖衍生碳的分解，这些酶受到钙离子的强烈调控。钙敏感脱氢酶，以及ATP合成酶的一个调节亚单位，因此提供了允许线粒体ATP输出与细胞其余部分的ATP需求相匹配的潜力。尽管上述模型提供了一个有吸引力的假说，并存在大量相关证据，但由于缺乏干预分子或药物工具来阻止(或增强)线粒体钙摄取，上述模型已被证明难以进行正式检验。在过去的12个月里，随着我们的合作者(Rosario Rizzuto及同事)和其他人发现了线粒体钙单转运体MCU的身份，这种情况发生了变化。其他成分，包括钙结合亚基MICU1和钠-钙交换蛋白NCLX，也在2010年被发现。为了测试这些成分的参与，以及线粒体钙摄取在触发或维持胰岛素分泌中的作用，我们建立了一个联合系统，用于记录原代胰岛β细胞亚细胞结构域(特别是线粒体和胞浆)内的电活动和钙浓度。进一步结合这项技术和使用慢病毒载体传递的“短发夹RNA”(ShRNAs)，我们建议首先单独探讨这些成分对线粒体运输、新陈代谢和葡萄糖(和其他刺激)调节的胰岛素分泌的影响。接下来，我们将使用一种新的小鼠模型，在该模型中，shRNAs可以在体内传递，并对胰腺β细胞具有高选择性，以确定干扰β细胞线粒体钙摄取对胰岛素分泌和全身葡萄糖稳态的影响。我们的发现将通过在小鼠身上使用更传统的基因“重组”方法实现这些细胞中一个或多个转运蛋白基因的更完整的“敲除”而得到证实。根据进展，我们还将探索在T2糖尿病的各种啮齿动物模型以及健康供者和T2糖尿病患者的人类胰岛中线粒体钙转运体基因表达的变化。因此，所获得的见解将提供对β细胞生物学的一个基本方面的更深层次的理解，并可能提供可以转化为糖尿病新疗法的发现。

Insulin secretion is essential for the normal regulation of blood glucose levels and becomes defective in all forms of diabetes mellitus, a disease affecting ~5 % of the population of westernised societies. Pancreatic beta cells are the body's sole source of circulating insulin and, in healthy individuals, respond to elevated glucose levels with the enhanced metabolism of the sugar. This leads to a cascade of biochemical events culminating in the influx into the cell of Ca2+ and the fusion of insulin-containing granules at the cell surface.Mitochondria play a particularly important role in beta cell glucose recognition and enable these cells to breakdown the sugar almost completely to CO2 and H2O. This efficient "oxidative" metabolism, which increases steeply as glucose concentrations rise, helps the cells to synthesise large quantities of ATP. The resulting shift in the ratio of ATP to its precursor ADP leads to the closure of ATP-sensitive K* (KATP) channels on the plasma membrane. Voltage-sensitive Ca2+ channels then permit Ca2+ influx to trigger the release ("exocytosis") of insulin. Increases in intracellular Ca2+ also prompt the enhanced consumption of ATP to fuel processes including ion pumping out of the cell, secretory granule movement, and so on. Findings over the past few decades have indicated that, in order to meet this demand, mitochondria contain a group of three enzymes involved in the breakdown of glucose-derived carbon in the "citrate cycle", which are strongly regulated by Ca2+ ions. The Ca2+ sensitive dehydrogenases, as well as a regulatory subunit of the ATP synthase, thus offer the potential to permit mitochondrial ATP output to match ATP demand by the rest of the cell.Whilst providing an attractive hypothesis, for which substantial correlative evidence exists, the above model has proved difficult to test formally due to the absence of molecular or pharmacological tools with which to intervene to block (or enhance) mitochondrial Ca2+ uptake. This situation has changed in the last ~12 months with the discovery by our collaborators (Rosario Rizzuto and colleagues), and others, of the identity of the mitochondrial Ca2+ uniporter, as MCU. Other components, including a Ca2+ binding subunit MICU1, and a Na+-Ca2+ exchange protein, NCLX, were also identified in 2010. In order to test both the involvement of these components, and the role of mitochondrial Ca2+ uptake in triggering or sustaining insulin secretion, we have established a combined system for recording both the electrical activity, and Ca2+ concentrations within defined subcellular domains - notably the mitochondria and the cytosol - of primary pancreatic islet beta cells. Further combining this technique with the use of "short hairpin RNAs" (shRNAs), delivered using lentiviral vectors, we propose here firstly to explore the effects on mitochondrial transport, metabolism, and on glucose- (and other stimulus-) regulated insulin secretion of depleting beta cells of each of these components individually. We shall next use a new mouse model in which shRNAs can be delivered in vivo and with high selectivity to the pancreatic beta cell, to determine the impact of interfering with mitochondrial Ca2+ uptake in the beta cell on insulin secretion and hence whole body glucose homeostasis. Our findings will be substantiated by achieving a more complete "knockout" of one or more of the transporter genes in these cells using more conventional genetic "recombination" approaches in mice. Subject to progress, we shall also explore the changes in the expression of the mitochondrial Ca2+ transporter genes in the context of various rodent models of T2 diabetes, and in human islets from healthy donors and T2 diabetes patients. The insights gained will thus provide a deeper understanding of a fundamental aspect of beta cell biology, and may provide findings which can be translated into new treatments for diabetes.