Molecular regulation of intestinal calcium absorption

钙（Ca2+）是无数生理过程的核心。因此，其在血浆中保持在较窄的范围内。这是通过肠吸收、骨沉积/再吸收以及过滤和随后的肾脏重吸收之间的动态相互作用实现的。第一步是肠道Ca2+吸收，通过两种途径之一发生。Ca2+通过主动跨细胞（通过上皮细胞）途径或被动细胞旁途径（上皮细胞之间）吸收。当饮食中存在足够量的Ca2+时，被动的细胞旁吸收占主导地位。当膳食中钙离子浓度低时，跨细胞转运是重要的。这是由一系列运输和载体事件介导的（图1）。十二指肠被认为是主动跨细胞钙离子吸收的主要部位，尽管新出现的证据，包括我的研究计划，表明近端大肠的作用。肠道钙吸收的主要调节剂是维生素D，它增加了主动的经肠钙转运。肾脏通过钙敏感受体（CaSR）直接对细胞外Ca2+作出反应。肠也表达CaSR，然而，其在调节Ca2+吸收中的作用尚不清楚。 我在这一领域的研究计划主要集中在调节肠道Ca2+吸收的基本分子机制。我使用小鼠模型，因为它们：i）允许在整个生物体的背景下研究Ca 2+稳态; ii）易于接受药理学和遗传学方法，包括关键参与者的过表达和敲低;和iii）允许在电压和浓度梯度受到严格控制的Ussing室中直接测量穿过肠上皮的Ca2+通量，使得能够独立地询问细胞旁和跨细胞途径。 我的研究计划在肠道钙吸收在未来5年的主要目标是三倍。1)确定近端大肠在Ca2+吸收中的重要性。我们假设盲肠是一个Ca2+抢救的网站，最大限度地减少Ca2+的损失。为此，我们将测量小鼠近端大肠制备物中的Ca2+吸收：i）用钙激素或Ca2+稳态的药理学调节剂处理; ii）喂食改变的含Ca2+饮食;和iii）缺乏跨细胞Ca2+通量所需组分的转基因小鼠。2)使用类似的方法，我们将描绘负责维持生命早期的正钙平衡的分子。3)我们将探讨血清Ca 2+水平对肠道Ca 2+吸收的直接影响以及CaSR在此过程中的作用。为此，我们将使用CaSR的药理学调节剂，并在小鼠肠中敲低和过表达CaSR。这些研究将解决介导Ca2+稳态的基本机制。

Calcium (Ca2+) is central to a myriad of physiological processes. Consequently it is maintained within narrow limits in plasma. This is accomplished by dynamic interplay between intestinal absorption, deposition/resorption from bone and by filtration and subsequent reabsorption by the kidney. The first step is intestinal Ca2+ absorption, which occurs via one of two routes. Ca2+ is absorbed via an active transcellular (through the epithelial cell) pathway, or by a passive paracellular pathway (between epithelial cells). When adequate amounts of Ca2+ are present in the diet, passive paracellular absorption predominates. When, dietary Ca2+ is low, transcellular transport is important. This is mediated by a series of transport and carrier events (Fig 1). The duodenum is thought to be the primary site of active transcellular Ca2+ absorption, although emerging evidence, including from my research program, suggests a role for the proximal large bowel. The primary regulator of intestinal Ca2+ absorption is vitamin D, which increases active transintestinal Ca2+ transport. The kidney responds directly to extracellular Ca2+ via the calcium sensing receptor (CaSR). The intestine also expresses the CaSR, however, its role in regulating Ca2+ absorption is unknown. My research program in this area focuses on basic molecular mechanisms regulating intestinal Ca2+ absorption. I use murine models because they: i) permit the study of Ca2+ homeostasis within the context of the whole organism; ii) are readily amenable to pharmacologic and genetic approaches, including over-expression and knockdown of key players; and iii) permit the direct measurement of Ca2+ flux across intestinal epithelium in Ussing chambers where voltage and concentration gradients are tightly controlled,  enabling the interrogation of paracellular and transcellular pathways independently. The main objectives of my research program in intestinal Ca2+ absorption over the next 5 years are three-fold. 1) To determine the importance of proximal large bowel in Ca2+ absorption. We hypothesize the cecum is a site of Ca2+ salvaging minimizing Ca2+ loss. To this end, we will measure Ca2+ absorption across proximal large bowel preparations of mice: i) treated with calciotropic hormones or pharmacological regulators of Ca2+ homeostasis; ii) fed altered Ca2+-containing diets; and iii) genetically modified mice lacking components required for transcellular Ca2+ flux. 2) Using similar approaches we will delineate the molecules responsible for maintaining a positive Ca2+ balance early in life. 3) We will explore the direct effect of serum Ca2+ levels on intestinal Ca2+ absorption and the role of the CaSR in this process. To do so we will use pharmacological modulators of the CaSR and both knockdown and over-express the CaSR in the intestine of mice. These studies will address fundamental mechanisms mediating Ca2+ homeostasis.