The physiological role of ABCC5 (MRP5) in the regulation of hormone secretion from gut endocrine cells.

作为现代人，我们与食物有着复杂的关系，很少有人会把食物简单地视为一种能量来源。食物的准备和食用在我们的日常生活中扮演着重要的角色，我们中的许多人都能非常详细地记住我们童年时最喜欢的食物的味道、气味和质地。此外，我们的大脑已经进化到奖励我们吃高热量的食物，因为直到最近，饥饿是一个非常现实的威胁。然而，目前我们发现自己每天24小时都被含糖、高脂肪的食物包围着，而且深陷困境。任何有过节食经历的人都凭直觉知道，食欲调节远比饱腹感和饥饿感之间的跷跷板要复杂得多。许多生理信号调节着食欲和新陈代谢。饥饿感是由迷走神经引起的萎缩、空虚的胃，同时分泌“饥饿激素”——胃饥饿素。这些信号作用于大脑，刺激饥饿感。食物进入胃后不久，饱腹感就会传递给大脑；胃膨胀和胃促生长素水平的急剧下降使大脑接收到的饥饿信号减弱，消化后的食物到达小肠时，大量激素分泌细胞（即肠内分泌细胞）参与其中。肠内分泌细胞分泌的激素调节胆囊和胰腺消化酶的分泌、食物通过肠道的运动速度、胰腺胰岛素的分泌和大脑的饱腹感。因此，肠道激素有许多下游目标，并作用于迷走神经，迷走神经支配肠道、大脑、胰腺的远处激素分泌细胞，以及它们的肠内分泌邻居。肠道激素分泌细胞和它们的目标细胞之间的这种持续的闲聊是我研究的重点。消化的食物是通过肠内分泌细胞膜上表达的受体来检测的。这些受体的激活会引发一系列事件，最终导致内内分泌细胞分泌激素，这一过程被称为胞吐。相反，也应该有一个刹车，一旦足够的激素被释放，就会停止胞吐。肠道中营养物质的检测已经得到了很好的研究，但我们目前对终止激素释放的事件序列以及在肠内分泌细胞和神经末梢共定位的连接处发生的通信知之甚少。具体来说，我的研究调查了一种膜转运蛋白ABCC5的作用，它已被证明可以跨细胞膜运输神经递质。这种蛋白似乎在关闭胞吐信号中起着核心作用。当ABCC5蛋白水平在模型细胞系中通过重组过表达人为提高时，激素释放受到抑制。当ABCC5蛋白水平被敲除时，胞吐作用增强。此外，ABCC5底物，一种叫做NAAG的神经递质，也能抑制胞吐。综上所述，这表明ABCC5及其通过肠内分泌细胞膜运输的分子参与了胞吐作用的关闭。我们的研究从两个角度探讨了肠内分泌细胞的作用。首先，我们的目标是确定参与信号级联反应的受体和酶，这些信号级联反应调节和终止激素胞吐。其次，我们采取更广泛的全系统观察并研究缺乏ABCC5的敲除小鼠的代谢影响。如果我们的假设成立，缺乏ABCC5的小鼠将分泌更多的激素，以响应消化的食物到达小肠，将有更强的胰岛素反应，并将显示“瘦表型”。我们的研究将帮助我们了解肠道激素对全身的影响，并将帮助来自几个不同领域的科学家在以前孤立研究的系统之间建立联系。

As modern humans, we have a complicated relationship with food and few among us would view food simply as an energy source. The preparation and consumption of food plays a central role in our daily lives and many of us can remember in great detail the tastes, smells and textures of our favourite childhood dishes. Furthermore, our brains have evolved to reward us for eating calorie-dense food because until very recently, starvation was a very real threat. However, currently we find ourselves surrounded by sugary, fatty foods 24/7, and in deep trouble.Anyone who has ever been on a diet knows intuitively that appetite regulation is far more complex than a simple see-saw between feeling full and feeling hungry. A multitude of physiological signals regulate appetite and metabolism. Feelings of hunger is initiated by a shrunken, empty stomach through the vagus nerve, and the simultaneous secretion of the 'hunger hormone', ghrelin. These signals act on the brain to stimulate feelings of hunger. Feelings of satiety are signalled to the brain shortly after food arrives in the stomach; a distended stomach and a sharp drop in ghrelin levels mute hunger signals to the brain and the arrival of digested food in the small intestine engages a large array of hormone secreting cells, known as enteroendocrine cells. Hormones secreted from enteroendocrine cells regulate the secretion of digestive enzymes from the gall bladder and pancreas, the rate of movement of food through the gut, the secretion of insulin from the pancreas and feelings of satiety in the brain. Gut hormones therefore have many down-stream targets and act on the vagus nerve which innervate the gut, the brain, the distant hormone-secreting cells of the pancreas, and also on their enteroendocrine neighbours. This constant chit-chat between the hormone secreting cells of the gut and their targets is the focus of my research. Digested food is detected by receptors expressed on the membranes of enteroedocrine cells. Activation of these receptors trigger a cascade of events that culminates in the secretion of hormones from endetroendocrine cells, a process called exocytosis. Conversely, there should also be a brake in place, to turn exocytosis off once adequate hormone had been released. The detection of nutrients in the gut have been well studied, but we currently know little about the sequence of events which terminate hormone release and the communication taking place at the junctions where enteroendocrine cells and nerve endings colocalize. Specifically, my research investigates the role of a membrane transporter, ABCC5, which has been shown to transport neurotransmitters across cell membranes. This protein appears to play a central role in turning exocytosis signals off. When ABCC5 protein levels is artificially boosted by recombinant overexpression in a model cell line, hormone release is inhibited. When ABCC5 protein levels are knocked out, exocytosis is enhanced. In addition, an ABCC5 substrate, a neurotransmitter called NAAG, also inhibits exocytosis. Taken together, this would suggest that ABCC5 and molecules it transports across the enteroendocrine cell membrane is involved in switching exocytosis off.Our research investigates the role of enteroendocrine cells from two angles. First, we aim to identify the receptors and enzymes involved in the signalling cascades which regulate and terminate hormone exocytosis. Secondly, we take a wider whole-systems look and investigate the metabolic implications for knock-out mice, which lack ABCC5. If our hypothesis holds true, mice lacking ABCC5 will secrete more hormone in response to the arrival of digested food in the small intestine, will have a more robust insulin response and will display a 'lean phenotype'. Our research will help us understand the whole body impact of gut hormones and will help scientist from several different areas to make connections between systems previously studied in isolation.