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，很少有人会简单地将食物视为一种能源。食物的准备和消费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得我们小时候最喜欢的菜肴的味道、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食物，因为直到最近，饥饿还是一个非常真实的威胁。然而，目前我们发现自己每天24小时都被高糖、高脂肪的食物包围，陷入了深深的麻烦之中。任何一个节食过的人都直观地知道，食欲调节远比在感觉饱和感觉饿之间进行简单的拉锯战复杂得多。多种生理信号调节食欲和新陈代谢。饥饿感是由迷走神经引起的空腹萎缩和饥饿荷尔蒙--胃促生长素的同时分泌引起的。这些信号作用于大脑，刺激饥饿感。食物到达胃后不久，饱腹感就会传递给大脑；胃扩张和胃促生长素水平急剧下降会减弱大脑的饥饿信号，消化后的食物进入小肠会激活大量荷尔蒙分泌细胞，即所谓的肠道内分泌细胞。由肠内分泌细胞分泌的激素调节着来自胆囊和胰腺的消化酶的分泌、食物在肠道中的移动速度、来自胰腺的胰岛素分泌以及大脑中的饱腹感。因此，胃肠激素有许多下游靶点，并作用于迷走神经，迷走神经支配肠道、大脑、胰腺远处的激素分泌细胞，也作用于它们的内分泌邻居。肠道荷尔蒙分泌细胞和它们的靶细胞之间的这种不断的闲聊是我研究的重点。消化的食物是由肠内分泌细胞膜上表达的受体检测到的。这些受体的激活会引发一系列事件，最终导致内分泌细胞分泌激素，这一过程被称为胞吐作用。相反，也应该有一个刹车，一旦释放了足够的荷尔蒙，就会关闭排泄作用。肠道中营养物质的检测已经得到了很好的研究，但目前我们对终止激素释放和发生在肠内分泌细胞和神经末梢共同定位的连接处的通讯的事件序列知之甚少。具体地说，我的研究调查了一种膜转运蛋白ABCC5的作用，它已经被证明可以跨细胞膜运输神经递质。这种蛋白质似乎在关闭胞吐信号方面起着核心作用。当在模型细胞系中通过重组过表达而人为地提高ABCC5蛋白水平时，激素释放被抑制。当ABCC5蛋白水平被敲除时，胞吐作用就会增强。此外，ABCC5底物，一种称为NAAG的神经递质，也能抑制胞吐作用。综上所述，这可能表明ABCC5及其跨肠内分泌细胞膜运输的分子参与了胞吐功能的关闭。我们的研究从两个角度研究了肠内分泌细胞的作用。首先，我们的目标是确定参与调节和终止激素胞吐的信号级联反应的受体和酶。其次，我们采取更广泛的全系统观察，并调查缺乏ABCC5的基因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荷尔蒙，以应对消化的食物到达小肠，将有更强大的胰岛素反应，并将显示出“瘦削的表型”。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.