Secretion of amyloid precursor protein via signal transduction pathway

阿尔茨海默病(AD)的特点是进行性神经变性。AD的特征之一是形成老年斑，其中主要含有β蛋白，这是一种由大膜结合的淀粉样前体蛋白(APP)加工而成的小肽，由39-43个氨基酸残基组成。在AD大脑中，APP通过异常处理转换为β蛋白，而几乎所有的APP通常都是在属于β蛋白分子的氨基酸16处的前体被裂解后分泌出来的。这种断裂也导致残存的C-末端非淀粉样蛋白10 kDa片段保留在膜内。为了研究APP的细胞内分选和加工的机制，我们通过定点突变的方法删除了APP的两个潜在的N-连接糖基化位点，即G蛋白结合位点、内小体滞留信号和溶酶体靶向信号。将野生型APPS和突变型APPS在COS-1细胞中表达，观察到蛋白产物的表达和N末端大片段的分泌。与野生型相比，突变型APP的初始分泌较慢，而PKC转染组的APP分泌量明显高于未转染组。这些结果表明，PKC通过一条信号通路调节APP的分泌。

Alzheimer's disease (AD) is characterized by progressive neurodegeneration. One of the features of AD is the formation of senile plaques, which contain mainly beta protein, small peptides comprising 39-43 amino acid residues, processed from a large membrane-bound amyloid precursor protein (APP). In AD brain, APP is converted to beta protein by abnormal processing, while almost all APP is normally secreted following cleavage of the precursor at amino acid 16 which falls within the beta protein molecule. This scission also leads to the retention of the residual C-terminal non-amyloidogenic 10 kDa fragment within the membrane. The regulation of beta protein formation by aberrant processing or of a secreted form by constitutive processing from the intact APP molecule remains unclear.In order to study the mechanism of intracellular sorting and processing of the APP,we deleted two potential N-linked glycosylation sites, G-protein binding site, endosome-retension signal and lysosome-targeting signal of APP by site-directed mutagenesis. Wild-type and mutant APPs were expressed in COS-1 cells by cDNA transfection and the expression of the protein products and secretion of N-terminal large fragment was observed. The initial secretion of the mutant APP appeared to be slow compared with wild type.The amount of APP secreted from PKC-transfected cells was higher than those from untransfected. These results suggest that PKC regulates the secretion of APP through a signaling pathway.