Influence of apoE on LRP1 function and Beta-Amyloid Transport Across the BBB

描述（由申请人提供）：阿尔茨海默病（AD）是美国第六大死亡原因，其特征在于神经元缠结的形成和β-淀粉样蛋白（A？）在脑中的沉积。脑中的蓄积是AD发病机制中的关键因素，并且是认知障碍的预测因子。AD中的A？减少策略主要针对A？产生机制，到目前为止，该机制在临床试验中基本上不成功。现在有证据表明，AD大脑中A <$的过度积累不是由于A <$的异常产生，而是A <$从大脑中清除受损的结果。因此，靶向清除相关途径可能证明在减弱AD脑中的A？积累方面最有效。血脑屏障（BBB）转运蛋白主要负责大脑消除A？是低密度脂蛋白受体相关蛋白1（LRP 1）。与LRP 1相互作用的配体之一是载脂蛋白E（apoE），E4等位基因的拥有代表了AD最强的遗传风险因素。然而，载脂蛋白E在A？跨血脑屏障交换中的作用尚不清楚。在细胞培养中的几项研究表明，在apoE存在的情况下，A？内化受到刺激，而其他研究则报道apoE对A？穿过BBB的清除具有破坏性。在我们的初步研究中，我们试图阐明apoE在A？的BBB清除中的作用。我们的研究和其他人的工作表明，当apoE与A <$结合时，A <$穿过BBB的转运显著减弱。然而，我们第一次证明了大脑中未结合的apoE可以以同种型特异性的方式促进A <$BBB的清除（apoE 3>> apoE 4）。在BBB水平，很少有研究调查了apoE同种型对LRP 1功能的影响，特别是LRP 1脱落（即，形成可溶性LRP 1，它是无功能的，不转胞苷A）。在apoE 4转基因小鼠和携带E4等位基因的AD患者中，我们观察到与apoE 3基因型相比，LRP 1在脑中的脱落增加。当结合我们的体内A <$BBB转运数据时，我们的初步研究表明LRP 1脱落和A <$通过BBB的转运之间存在反比关系，这是apoE基因型特异性的。我们假设apoE 4在防止LRP脱落方面比其他apoE亚型效率低，这导致A？穿过BBB的清除率降低。我们的初步研究结果还表明，LRP 1内化的调节导致增强的LRP 1脱落和减少的A？转运通过体外模型的血脑屏障。由于LRP 1内化和转胞吞作用是从大脑中消除A？的重要步骤，我们将研究apoE亚型对LRP 1介导的A？在BBB中的内化和亚细胞运输的影响。总的来说，目前的建议将调查几个潜在的机制，以阐明apoE依赖的差异，我们观察到在LRP 1脱落和A ²运输通过BBB。这些研究将提高我们对AD中apoE和LRP 1之间关系的理解，并可能为这种疾病提供新的治疗模式。 公共卫生相关性：阿尔茨海默病（AD）现在是美国第六大死亡原因-影响数百万人。该提案将研究一种新的途径对AD进展的贡献，目的是确定这种疾病的新治疗策略。

DESCRIPTION (provided by applicant): Alzheimer's disease (AD) is the sixth leading cause of death in the United States and is characterized by the formation of neurofibrillary tangles and the deposition of beta-amyloid (A¿) proteins in the brain. A¿ accumulation in the brain is a key factor in AD pathogenesis and is a predictor of cognitive impairment. A¿ reducing strategies in AD have primarily targeted A¿ production mechanisms which, to this point, have been largely unsuccessful in clinical trials. Evidence now suggests that the excessive accumulation of A¿ in the AD brain is not due to aberrant A¿ production, but the result of impaired A¿ removal from the brain. Thus, targeting clearance-related pathways may prove most effective in attenuating A¿ accumulation in the AD brain. The blood-brain barrier (BBB) transporter primarily responsible for the brain elimination of A¿ is the low density lipoprotein receptor-related protein 1 (LRP1). One of the ligands with which LRP1 interacts is apolipoprotein E (apoE) and possession of the E4 allele represents the strongest genetic risk factor for AD. However, the role of apoE in the exchange of A¿ across the BBB is not clearly defined. Several studies in cell culture have shown a stimulation of A¿ internalization in the presence apoE, while others have reported apoE is disruptive to A¿ clearance across the BBB. In our preliminary studies we sought to clarify the role of apoE in the BBB clearance of A¿. Our studies and the work of others indicate that when apoE is bound to A¿, the transport of A¿ across the BBB is dramatically attenuated. However, for the first time, we demonstrate that unbound apoE in the brain can facilitate A¿ BBB clearance in an isoform-specific manner (apoE3 >> apoE4). At the BBB level, few studies have investigated the influence of apoE isoforms on LRP1 function, in particular LRP1 shedding (i.e., formation soluble LRP1, which is nonfunctional and does not transcytose A¿). In apoE4 transgenic mice and AD patients carrying the E4 allele, we observe elevated LRP1 shedding in the brain compared to apoE3 genotypes. When combined with our in vivo A¿ BBB transit data, our preliminary studies demonstrate an inverse relationship between LRP1 shedding and A¿ transport across the BBB, one that is apoE genotype-specific. We hypothesize that apoE4 is less efficient than other apoE isoforms in preventing LRP shedding, which leads to reduced A¿ clearance across the BBB. Our preliminary findings also show that modulation of LRP1 internalization leads to enhanced LRP1 shedding and reduced A¿ transport across an in vitro model of the BBB. Since LRP1 internalization and transcytosis are essential steps in eliminating A¿ from the brain, we will examine the effect of apoE isoforms on LRP1-mediated internalization and subcellular trafficking of A¿ in the BBB. Overall, the current proposal will investigate severa potential mechanisms to elucidate the apoE- dependant differences we observe in LRP1 shedding and A¿ transit across the BBB. These studies will improve our understanding of the relationship between apoE and LRP1 in AD and may provide new treatment modalities for this disease. PUBLIC HEALTH RELEVANCE: Alzheimer's disease (AD) is now the sixth leading cause of death in the United States -- affecting millions of people. This proposal will investigate the contribution of a novel pathway to the progression of AD with the purpose of identifying new treatment strategies for this disease.