BRAIN LIPID METABOLISM, DENDRITES AND SYNAPSES IN AGING AND ALZHEIMER'S DISEASE

项目摘要/摘要 胆固醇等脑脂类在神经细胞膜动态平衡和突触中起关键作用 功能。然而，支配它们的生物发生和运输到神经元的机制很差。 明白了。载脂蛋白E(ApoE)是脑内主要的脂质转运体。人类载脂蛋白E三种亚型中的 (E_2、E_3和E_4)，apoE_4是晚发性AD的主要危险等位基因。脑载脂蛋白E/脂蛋白颗粒， 主要由星形胶质细胞产生，通过载脂蛋白E受体将胆固醇和其他脂质传递给神经元， 属于低密度脂蛋白受体(LDLR)家族。要最终理解为什么apoE4是一种风险 阿尔茨海默病的发病因素中，有必要研究载脂蛋白E亚型在脑内脂类转运和代谢中的差异功能。 突触的功能，以及载脂蛋白E受体在这些过程中扮演的具体角色。我们已经证明了 脑内载脂蛋白E的代谢是由LDLR和LDLR相关蛋白1(LRP1)共同介导的。然而，神经元 LRP1的缺失，而不是Ldlr的缺失，会损害小鼠的胆固醇代谢。这表明LRP1是 神经元中主要的胆固醇转运受体。条件性LRP1前脑基因敲除(LRP1-KO)小鼠 降低脑胆固醇、硫脂和脑苷脂；减少树突棘密度和分支； 突触减少；突触功能减弱。LRP1-KO小鼠存在记忆缺陷和运动障碍 与树突棘/突触完整性和突触功能受损相一致的疾病。有趣的是， 在人类AD大脑和apoE4靶向替换(TR)小鼠中，LRP1水平显著降低。 ApoE4-tr，而不是apoE3-tr小鼠，也表现出脂代谢和突触功能受损，apoE4 与apoE3相比，其稳定性较差。基于这些观察，我们假设apoE4不如 APOE3在脑脂质运输和支持树突棘/突触完整性中的作用，特别是在 这些apoE4缺陷可以通过恢复表达和 载脂蛋白E受体LRP1的功能。我们提出了三个目标来检验我们的假设。在目标1中，我们将剖析 载脂蛋白E亚型转运脂质和调节神经元功能的分子和细胞机制 星形胶质细胞分泌载脂蛋白E/脂蛋白颗粒和神经胶质细胞的LRP1和LDLR依赖通路 共培养体系。在目标2中，我们将定义载脂蛋白E4对脑脂代谢的年龄依赖性影响和 并检查老化的大脑是否对apoE4的劣质功能更敏感。在……里面 目的3，我们将确定在小鼠脑中过表达的LRP1是否足以挽救脂质和突触 用LRP1转基因小鼠培育apoE3-tr和apoE4-tr小鼠对apoE4-tr小鼠的损伤。 总之，我们提出的研究应该会产生关于载脂蛋白E亚型如何不同的关键知识。 通过载脂蛋白E受体调节脑脂代谢和突触功能，以及为什么载脂蛋白E4是一个强大的风险 AD的因素。我们的研究也可能将载脂蛋白E和载脂蛋白E受体定义为AD治疗的关键靶点。

PROJECT SUMMARY/ABSTRACT Brain lipids such as cholesterol play critical roles in neuronal membrane homeostasis and synapse functions. However, the mechanisms that govern their biogenesis and transport to neurons are poorly understood. Apolipoprotein E (apoE) is a major lipid transporter in the brain. Of the three human apoE isoforms (E2, E3 and E4), apoE4 is the predominant risk allele for late-onset AD. Brain apoE/lipoprotein particles, produced primarily by astrocytes, deliver cholesterol and other lipids to neurons via apoE receptors, which belong to the low-density lipoprotein receptor (LDLR) family. To ultimately understand why apoE4 is a risk factor for AD, it is essential to study the differential functions of apoE isoforms in brain lipid transport and synapse functions, and what specific roles apoE receptors play in these processes. We have demonstrated that brain apoE metabolism is mediated by both LDLR and LDLR-related protein 1 (LRP1). However, neuronal deletion of Lrp1, but not Ldlr, impairs cholesterol metabolism in mice. This suggests that LRP1 is the predominant cholesterol transport receptor in neurons. Conditional Lrp1 forebrain knockout (LRP1-KO) mice have decreased brain cholesterol, sulfatide and cerebroside; reduced dendritic spine density and branching; fewer synapses; and diminished synaptic functions. LRP1-KO mice have memory deficits and movement disorders consistent with compromised dendritic spine/synaptic integrity and synaptic functions. Interestingly, LRP1 levels are significantly reduced in human AD brains and in the apoE4-targeted replacement (TR) mice. ApoE4-TR, but not apoE3-TR mice, also exhibit impaired lipid metabolism and synaptic functions, and apoE4 is less stable compared to apoE3. Based on these observations, we hypothesize that apoE4 is inferior to apoE3 in transporting brain lipid and in supporting dendritic spine/synaptic integrity, particularly in aging brains, and that these apoE4 defects can be partially rescued by restoring the expression and function of apoE receptor LRP1. We propose three aims to test our hypothesis. In Aim 1, we will dissect the molecular and cellular mechanisms by which apoE isoforms transport lipids and regulate neuronal functions via LRP1- and LDLR-dependent pathways using astrocytes-secreted apoE/lipoprotein particles and glia-neuron co-culture system. In Aim 2, we will define age-dependent effects of apoE4 on brain lipid metabolism and synaptic functions and examine whether aging brains are more sensitive to the inferior functions of apoE4. In Aim 3, we will determine if overexpressed LRP1 in mouse brains is sufficient to rescue lipid and synaptic impairments in apoE4-TR mice by breeding apoE3-TR and apoE4-TR mice with LRP1 transgenic mice. Together, our proposed studies should generate critical knowledge on how apoE isoforms differentially regulate brain lipid metabolism and synaptic functions via apoE receptors, and why apoE4 is a strong risk factor for AD. Our studies may also define apoE and apoE receptors as critical targets for AD therapy.