Guillaume D, Bertrand P, Dea D, Davignon J, Poirier J
Douglas Hospital Research Centre, Department of Psychiatry and Neurology, McGill University, Verdun, Quebec, Canada.
J Neurochem. 1996 Jun;66(6):2410-8. doi: 10.1046/j.1471-4159.1996.66062410.x.
Apolipoprotein (apo) E is likely involved in redistributing cholesterol and phospholipids during compensatory synaptogenesis in the injured CNS. Three common isoforms of apoE exist in human (E2, E3, and E4). The apoE4 allele frequency is markedly increased in both late-onset sporadic and familial Alzheimer's disease (AD). ApoE concentration in the brain of AD subjects follows a gradient: ApoE levels decrease as a function of E2 > E3 >> E4. It has been proposed that the poor reinnervation capacity reported in AD may be caused by impairment of the apoE/low-density lipoprotein (LDL) receptor activity. To understand further the role of this particular axis in lipid homeostasis in the CNS, we have characterized binding, internalization, and degradation of human 125I-LDL to primary cultures of rat astrocytes. Specific binding was saturable, with a KD of 1.8 nM and a Bmax of 0.14 pmol/mg of proteins. Excess unlabeled human LDL or very LDL (VLDL) displaced 70% of total binding. Studies at 37 degrees C confirmed that astrocytes bind, internalize, and degrade 125I-LDL by a specific, saturable mechanism. Reconstituted apoE (E2, E3, and E4)-liposomes were labeled with 125I and incubated with primary cultures of rat astrocytes and hippocampal neurons to examine specific binding. Human LDL and VLDL displaced binding and internalization of all apoE isoforms similarly in both astrocytes and neurons. 125I-ApoE2 binding was significantly lower than that of the other 125I-apoE isoforms in both cell types. 125I-ApoE4 binding was similar to that of 125I-apoE3 in both astrocytes and neurons. On the other hand, 125I-apoE3 binding was significantly higher in neurons than in astrocytes. These isoform-specific alterations in apoE-lipoprotein pathway could explain some of the differences reported in the pathophysiology of AD subjects carrying different apoE alleles.
载脂蛋白（apo）E可能在受损中枢神经系统的代偿性突触形成过程中参与胆固醇和磷脂的重新分布。人类存在三种常见的载脂蛋白E异构体（E2、E3和E4）。在晚发性散发性和家族性阿尔茨海默病（AD）中，载脂蛋白E4等位基因频率均显著增加。AD患者大脑中的载脂蛋白E浓度呈梯度变化：载脂蛋白E水平随E2>E3>>E4的顺序降低。有人提出，AD中报道的神经再支配能力差可能是由于载脂蛋白E/低密度脂蛋白（LDL）受体活性受损所致。为了进一步了解这一特定轴在中枢神经系统脂质稳态中的作用，我们对大鼠星形胶质细胞原代培养物中人类125I-LDL的结合、内化和降解进行了表征。特异性结合具有饱和性，解离常数（KD）为1.8 nM，最大结合容量（Bmax）为0.14 pmol/mg蛋白质。过量未标记的人类LDL或极低密度脂蛋白（VLDL）可取代70%的总结合。37℃的研究证实，星形胶质细胞通过一种特异性、可饱和的机制结合、内化和降解125I-LDL。用125I标记重组载脂蛋白E（E2、E3和E4）脂质体，并与大鼠星形胶质细胞和海马神经元原代培养物一起孵育，以检测特异性结合。人类LDL和VLDL在星形胶质细胞和神经元中对所有载脂蛋白E异构体的结合和内化的取代作用相似。在两种细胞类型中，125I-载脂蛋白E2的结合均显著低于其他125I-载脂蛋白E异构体。在星形胶质细胞和神经元中，125I-载脂蛋白E4的结合与125I-载脂蛋白E3相似。另一方面，125I-载脂蛋白E3在神经元中的结合显著高于星形胶质细胞。载脂蛋白E-脂蛋白途径中这些异构体特异性的改变可以解释携带不同载脂蛋白E等位基因的AD患者在病理生理学上报道的一些差异。
