Department of Biomembrane and Biofunctional Chemistry, Faculty of Advanced Life Science, Hokkaido University, Sapporo 001-0021, Japan.
J Biol Chem. 2012 Mar 30;287(14):10977-89. doi: 10.1074/jbc.M111.324616. Epub 2012 Feb 2.
Amyloid β-peptide (Aβ), the pathogenic agent of Alzheimer disease, is a physiological metabolite whose levels are constantly controlled in normal brain. Recent studies have demonstrated that a fraction of extracellular Aβ is associated with exosomes, small membrane vesicles of endosomal origin, although the fate of Aβ in association with exosome is largely unknown. In this study, we identified novel roles for neuron-derived exosomes acting on extracellular Aβ, i.e. exosomes drive conformational changes in Aβ to form nontoxic amyloid fibrils and promote uptake of Aβ by microglia. The Aβ internalized together with exosomes was further transported to lysosomes and degraded. We also found that blockade of phosphatidylserine on the surface of exosomes by annexin V not only prevented exosome uptake but also suppressed Aβ incorporation into microglia. In addition, we demonstrated that secretion of neuron-derived exosomes was modulated by the activities of sphingolipid-metabolizing enzymes, including neutral sphingomyelinase 2 (nSMase2) and sphingomyelin synthase 2 (SMS2). In transwell experiments, up-regulation of exosome secretion from neuronal cells by treatment with SMS2 siRNA enhanced Aβ uptake into microglial cells and significantly decreased extracellular levels of Aβ. Our findings indicate a novel mechanism responsible for clearance of Aβ through its association with exosomes. The modulation of the vesicle release and/or elimination may alter the risk of AD.
淀粉样 β-肽 (Aβ) 是阿尔茨海默病的致病因子，是一种生理代谢物，在正常大脑中其水平受到持续控制。最近的研究表明，细胞外 Aβ 的一部分与外泌体有关，外泌体是起源于内体的小膜囊泡，尽管 Aβ 与外泌体结合的命运在很大程度上尚不清楚。在这项研究中，我们确定了神经元衍生的外泌体对细胞外 Aβ 的作用的新作用，即外泌体驱动 Aβ 的构象变化形成无毒的淀粉样纤维，并促进小胶质细胞摄取 Aβ。与外泌体一起内化的 Aβ 进一步被转运到溶酶体并被降解。我们还发现，通过 annexin V 阻断外泌体表面的磷脂酰丝氨酸不仅可以防止外泌体摄取，还可以抑制 Aβ 掺入小胶质细胞。此外，我们证明神经元衍生的外泌体的分泌受鞘脂代谢酶的活性调节，包括中性鞘磷脂酶 2 (nSMase2) 和鞘磷脂合酶 2 (SMS2)。在 Transwell 实验中，用 SMS2 siRNA 处理增强神经元细胞中外泌体分泌可增加 Aβ 进入小胶质细胞的摄取，并显著降低细胞外 Aβ 的水平。我们的研究结果表明了一种通过与外泌体结合清除 Aβ 的新机制。囊泡释放和/或消除的调节可能会改变 AD 的风险。
