Haass C, Koo E H, Mellon A, Hung A Y, Selkoe D J
Department of Neurology, Harvard Medical School, Brigham and Women's Hospital, Boston, Massachusetts 02115.
Nature. 1992 Jun 11;357(6378):500-3. doi: 10.1038/357500a0.
Progressive cerebral deposition of the amyloid beta-peptide is an early and invariant feature of Alzheimer's disease. The beta-peptide is released by proteolytic cleavages from the beta-amyloid precursor protein (beta APP), a membrane-spanning glycoprotein expressed in most mammalian cells. Normal secretion of beta APP involves a cleavage in the beta-peptide region, releasing the soluble extramembranous portion and retaining a 10K C-terminal fragment in the membrane. Because this secretory pathway precludes beta-amyloid formation, we searched for an alternative proteolytic processing pathway that can generate beta-peptide-bearing fragments from full-length beta APP. Incubation of living human endothelial cells with a beta APP antibody revealed reinternalization of mature beta APP from the cell surface and its targeting to endosomes/lysosomes. After cell-surface biotinylation, full-length biotinylated beta APP was recovered inside the cells. Purification of lysosomes directly demonstrated the presence of mature beta APP and an extensive array of beta-peptide-containing proteolytic products. Our results define a second processing pathway for beta APP and suggest that it may be responsible for generating amyloid-bearing fragments in Alzheimer's disease.
β-淀粉样肽在大脑中的进行性沉积是阿尔茨海默病早期且恒定的特征。β-肽是通过β-淀粉样前体蛋白（βAPP）的蛋白水解切割释放出来的，βAPP是一种在大多数哺乳动物细胞中表达的跨膜糖蛋白。βAPP的正常分泌涉及β-肽区域的切割，释放出可溶性膜外部分，并在膜中保留一个10K的C末端片段。由于这种分泌途径可防止β-淀粉样蛋白的形成，我们寻找了一种替代的蛋白水解加工途径，该途径可以从全长βAPP产生含β-肽的片段。用人βAPP抗体孵育活的人内皮细胞，发现成熟的βAPP从细胞表面重新内化并靶向内体/溶酶体。细胞表面生物素化后，全长生物素化的βAPP在细胞内被回收。溶酶体的纯化直接证明了成熟βAPP和大量含β-肽的蛋白水解产物的存在。我们的结果定义了βAPP的第二种加工途径，并表明它可能是阿尔茨海默病中产生含淀粉样蛋白片段的原因。
