维生素A缺乏影响APP代谢对阿尔兹海默病患病风险及机制研究

Alzheimer's Disease (AD) is the most common neurodegenerative disorder leading to dementia. AD neuropathology including senile plaques （SP） and neurofibrillary tangles after middle age. The underlying mechanism leading to AD pathogenesis is not well defined and there is no effective disease-modified treatment for AD. VA study provides enormous potentials for defining underlying mechanism of AD pathogenesis, identifying early diagnostic markers and finding novel drug targets for preventing and treating AD. Our studies in elders have shown that vitamin A (VA) deficiency could enhance the risk with AD, and retinoic acid receptors （RARs） could play important role in AD pathogenesis (such as in APP metabolic pathway). BACE1 and α-secretase are the two key metabolic enzymes in APP metabolic pathway. Our studies in 44-week MVAD rats have shown that APP protein heightened obviously in MVAD rats' hippocampus. However, it's well known that BACE1 expression hardly any in rats' hippocampus. So, APP protein heightened in MVAD 44-week rats' hippocampus suggested that MVAD could inhibit one of the APP metabolic pathwayes, which one is mediated by α-secretase. Our another study in 6-month APP/PS1 transgenic mice with MVAD have shown that SP heightened obviously in their hippocampus. It suggested that MVAD could activate another APP metabolic pathwayes, which one is mediated by BACE1. Bioinformatics researches suggested that there are some close genetic locuses between RARs and AD virulence genes. Our working hypothesis is that RARs plays an important role in AD pathogenesis in APP metabolic pathway. However, there is no report about mechanism of abnormal APP metabolic pathway and high Alzheimer's disease risk caused by vitamin A deficiency. For these reasons, the aims of this proposal are to study the relationships and mechanism of vitamin A deficiency, RARs, BACE1, α-secretase, and SP.

阿尔兹海默症（AD）是最常见的神经退行性疾病，其发病机制及防治方法尚不清楚。本课题组前一项NSFC面上项目研究发现老年人群维生素A（VA）缺乏提高AD患病风险，其机制可能与视黄酸核受体（RARs）对淀粉样前蛋白（APP）代谢通路影响有关。BACE1及α-分泌酶分别介导APP代谢的两条通路。课题组前期动物研究发现微量表达BACE1的44周MVAD大鼠海马APP蛋白异常增高，提示MVAD抑制α-分泌酶代谢途径；6月MVAD APP/PS1双转基因小鼠海马SP异常增高，提示MVAD激活BACE1代谢途径。国外生物信息学研究发现, RAR基因编码与AD发病的多个基因位点密切相关。但是，有关VA缺乏对APP代谢途径影响的机制研究却未见报道。因此，本课题拟通过不同VA水平APP/PS1双转基因小鼠模型，深入探讨VA缺乏、RARs与APP代谢关键酶及AD主要病理改变老年斑（SP）的关系及分子作用机制。

大脑中淀粉样β蛋白（Aβ）沉积形成老年斑（SP）是阿尔茨海默病（AD）的最主要病理特征。目前认为淀粉样前体蛋白（APP）代谢通路是AD的经典发病通路。APP首先由β-分泌酶切割产生C99，后者再经γ-分泌酶切割产生Aβ，脑内Aβ主要是40和42。在正常情况下，绝大多数APP由α-分泌酶在Aβ结构内进行切割，从而阻止了Aβ的产生。在发展中国家，边缘性维生素A缺乏（MVAD）在孕妇和儿童中广泛存在，已成为常见的公共卫生营养问题。越来越多的研究提示，维生素A（VA）可能参与AD的病理过程。然而，VA在AD发生发展过程中的作用尚未完全明确。我们前期对重庆市15家敬老院的老年人群研究结果显示，维生素A不足可能增加AD的患病风险，而视黄酸受体（RARs）在APP代谢通路中可能起到重要的作用。为了进一步明确VA在AD病理过程中的作用，我们在APP/PS1双转基因AD经典的模型小鼠中建立了从孕期开始的MVAD小鼠模型并开展相关研究。我们的研究结果显示，孕期开始的MVAD小鼠脑中的Aβ40和Aβ42水平显著增加；此外，APP和β-分泌酶(BACE1) 蛋白表达显著增加，α-分泌酶的重要组成部分ADAM10蛋白表达显著减少，而γ-分泌酶的重要组成部分PS1蛋白表达没有明显改变。此外，免疫组化以及体视学方法检测显示，孕期开始MVAD显著增加AD小鼠脑中老年斑的数量和体积。水迷宫行为学测试也显示，MVAD小鼠的空间学习和记忆能力明显低于对照组。以上研究结果表明，MVAD可通过增加APP和BACE1切割APP，以及减少ADAM10介导的α-分泌酶切割APP，而增加Aβ的产生和老年斑的沉积，从而加重认知功能的损害。研究结果提示维生素A缺乏是AD的高危因素，研究为孕期开始的MVAD在AD发病机制中的作用提供了重要的理论依据。我们认为，在孕妇和儿童中，VA应当成为预防远期AD患病的一级预防内容的重要靶点。