Processing of β-amyloid precursor protein in microglia and astrocytes favors an internal localization over constitutive secretion

Abstract

Microglial cells and astrocytes are closely associated with nearly all compact deposits of the amyloid β-protein found in the senile plaques characteristic of Alzheimer's disease and trisomy 21. The biosynthesis and metabolic fate of the β-amyloid precursor protein (βAPP) in astrocytes has not been characterized, and its identification in microglia has not been described. Here, we report the expression of βAPP 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 βAPP. This is confirmed by PCR-mediated amplification of the corresponding mRNAs, showing that all three major transcripts (βAPP695, βAPP751, and βAPP770) are present in relatively equal amounts. Despite rapid turnover of the precursor, astrocytes and microglia show a reduced production of soluble fragments of βAPP compared to cells transfected with βAPP cDNAs. The relative amount of soluble βAPP molecules generated is both cell type and isoform specific. Immunocytochemistry reveals that full-length βAPP 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 βAPP into soluble derivatives. Our findings indicate that both astrocytes and microglia strongly express all three major forms of βAPP but apparently process these molecules by an alternative pathway that generates very small amounts of soluble βAPP. The immunocytochemical localization and the biochemical data lead to the suggestion that βAPP may not function principally as a cell surface or secreted protein in vivo but may have an important intracellular function.