Molecular Regulation of Intestinal Calcium Absorption

背景：钙（Ca 2+）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/再吸收以及肾过滤和随后的重吸收之间的动态相互作用发生的。第一步是肠道Ca 2+吸收，这是NSERC资助的研究项目的重点，通过两种途径之一发生。Ca 2+可以通过主动跨细胞（通过细胞）途径或被动细胞旁（上皮细胞之间）途径吸收。当饮食中有足够的钙时，被动的细胞旁吸收占主导地位。然而，当饮食中的钙离子是低的，跨细胞运输是重要的。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Ca 2+吸收的主要部位，尽管新出现的证据，包括我的项目，表明近端大肠的重要作用。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。肾脏直接响应细胞外Ca 2+，我们已经证明了一个类似的直接Ca 2+感应机制，在肠道，但该途径在整体Ca 2+稳态的作用尚不清楚。模型系统：我在这一领域的研究计划主要集中在调节小鼠模型肠道钙吸收的基本分子机制，因为它们：i）允许在整个生物体的背景下研究钙稳态; ii）易于接受药理学和遗传学方法，包括过表达和关键参与者的敲除;和iii）允许在电压和浓度梯度可以被严格控制的Ussing室中直接测量穿过肠上皮的Ca 2+通量，使得能够独立地询问细胞旁和跨细胞途径。方法与途径：我在未来5年内对肠道钙吸收的研究计划的主要目标有三个方面。1)阐明近端大肠对Ca 2+吸收的调节作用。我们假设这是一个重要的网站的Ca 2+抢救最小化Ca 2+损失或当血浆Ca 2+是高的，有助于显着的细胞旁分泌。为此，我们将测量近端大肠制备物中的Ca 2+吸收，并检查野生型和转基因动物的尿液、粪便和血浆Ca 2+水平。2)我们将探讨血清Ca 2+水平对肠道细胞旁Ca 2+吸收的直接影响以及CaSR在这一过程中的作用。为此，我们将使用CaSR的药理学调节剂并敲除肠中的CaSR。3)使用类似的方法，我们将描绘分子的紧密连接，有助于积极的Ca 2+平衡和适当矿化骨在生命早期。影响：这些研究将描绘维持哺乳动物Ca 2+稳态的基本机制。

BACKGROUND: Calcium (Ca2+) is central to a myriad of physiological processes. Consequently it is maintained within narrow limits in plasma. This occurs via dynamic interplay between intestinal absorption, deposition/resorption from bone and by renal filtration and subsequent reabsorption. The first step is intestinal Ca2+ absorption, the focus of this NSERC funded research program, which occurs via one of two routes. Ca2+ can be absorbed by an active transcellular (through the cell) pathway, or by a passive paracellular (between epithelial cells) pathway. When there is sufficient Ca2+ in the diet, passive paracellular absorption predominates. However, 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 proposed to be the primary site of active transcellular Ca2+ absorption, although emerging evidence, including from my  program, suggests a significant 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+ and we have demonstrated a similar direct Ca2+ sensing mechanism in the intestine, but the role of this pathway in overall Ca2+ homeostasis is not clear. MODEL SYSTEMS: My research program in this area focuses on basic molecular mechanisms regulating intestinal Ca2+ absorption in mouse 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 direct measurement of Ca2+ flux across intestinal epithelium in Ussing chambers where voltage and concentration gradients can be tightly controlled, enabling the interrogation of paracellular and transcellular pathways independently. OBJECTIVES & APPROACH: The main objectives of my research program in intestinal Ca2+ absorption over the next 5 years are three-fold. 1) To delineate the regulatory role of the proximal large bowel in Ca2+ absorption. We hypothesize this is an important site of Ca2+ salvaging minimizing Ca2+ loss or when plasma Ca2+ is high, contributing significant paracellular secretion. To this end, we will measure Ca2+ absorption across proximal large bowel preparations and examine urine, fecal and plasma Ca2+ levels in wild-type and genetically modified animals. 2) We will explore the direct effect of serum Ca2+ levels on paracellular intestinal Ca2+ absorption and the role of the CaSR in this process. To do so we will use pharmacological modulators of the CaSR and knockout the CaSR in the intestine. 3) Using similar approaches we will delineate molecules in the tight junction contributing to a positive Ca2+ balance and properly mineralized bone early in life. IMPACT: These studies will delineate fundamental mechanisms maintaining mammalian Ca2+ homeostasis.