Haass C, Hung A Y, Selkoe D J
Department of Neurology, Harvard Medical School, Boston, Massachusetts.
J Neurosci. 1991 Dec;11(12):3783-93. doi: 10.1523/JNEUROSCI.11-12-03783.1991.
Microglial cells and astrocytes are closely associated with nearly all compact deposits of the amyloid beta-protein found in the senile plaques characteristic of Alzheimer's disease and trisomy 21. The biosynthesis and metabolic fate of the beta-amyloid precursor protein (beta APP) in astrocytes has not been characterized, and its identification in microglia has not been described. Here, we report the expression of beta APP by astrocytes and microglia in primary cultures of cerebral cortex from newborn rats. Using metabolic labeling followed by immunoprecipitation, we show that both astrocytes and microglia express substantial amounts of the major isoforms of beta APP. This is confirmed by PCR-mediated amplification of the corresponding mRNAs, showing that all three major transcripts (beta APP695, beta APP751, and beta APP770) are present in relatively equal amounts. Despite rapid turnover of the precursor, astrocytes and microglia show a reduced production of soluble fragments of beta APP compared to cells transfected with beta APP cDNAs. The relative amount of soluble beta APP molecules generated is both cell type and isoform specific. Immunocytochemistry reveals that full-length beta APP is located in internal membranous vesicles, with only very little insertion at the cell surface. The latter data are in agreement with the reduced ability of microglia and astrocytes to cleave the beta APP into soluble derivatives. Our findings indicate that both astrocytes and microglia strongly express all three major forms of beta APP but apparently process these molecules by an alternative pathway that generates very small amounts of soluble beta APP. The immunocytochemical localization and the biochemical data lead to the suggestion that beta APP may not function principally as a cell surface or secreted protein in vivo but may have an important intracellular function.
小胶质细胞和星形胶质细胞与几乎所有在阿尔茨海默病和21三体综合征特征性老年斑中发现的淀粉样β蛋白紧密沉积物密切相关。星形胶质细胞中β淀粉样前体蛋白（βAPP）的生物合成和代谢命运尚未明确，并且在小胶质细胞中的鉴定也未被描述。在此，我们报道新生大鼠大脑皮质原代培养物中星形胶质细胞和小胶质细胞表达βAPP。通过代谢标记随后进行免疫沉淀，我们表明星形胶质细胞和小胶质细胞都表达大量的βAPP主要异构体。通过PCR介导的相应mRNA扩增证实了这一点，表明所有三种主要转录本（βAPP695、βAPP751和βAPP770）以相对相等的量存在。尽管前体周转迅速，但与用βAPP cDNA转染的细胞相比，星形胶质细胞和小胶质细胞显示出βAPP可溶性片段产生减少。产生的可溶性βAPP分子的相对量是细胞类型和异构体特异性的。免疫细胞化学显示全长βAPP位于内部膜泡中，在细胞表面仅有极少的插入。后一数据与小胶质细胞和星形胶质细胞将βAPP切割成可溶性衍生物的能力降低一致。我们的研究结果表明，星形胶质细胞和小胶质细胞都强烈表达βAPP的所有三种主要形式，但显然通过一种替代途径加工这些分子，该途径产生极少量的可溶性βAPP。免疫细胞化学定位和生化数据表明，βAPP在体内可能主要不作为细胞表面或分泌蛋白起作用，而可能具有重要的细胞内功能。
