Trafficking of cell-surface amyloid β-protein precursor: I. Secretion, endocytosis and recycling as detected by labeled monoclonal antibody

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Abstract
Amyloid beta-protein, the principal constituent of amyloid fibrils found in senile plaques and blood vessels in Alzheimer's disease, is constitutively produced and released into medium of cultured cells. Amyloid beta-protein is derived by proteolysis of the beta-amyloid precursor protein by unclear mechanisms. Beta-amyloid precursor protein is a transmembrane protein which can be processed to release a large secretory product or processed in the endosomal/lysosomal pathway without secretion. Previous studies have shown that from the cell surface, beta-amyloid precursor protein may be released after cleavage or internalized without cleavage, the latter in a pathway that both produces amyloid beta-protein and also targets some molecules to the lysosomal compartment. Analysis of beta-amyloid precursor protein trafficking is confounded by the concomitant secretion and internalization of molecules from the cell surface. To address this issue, we developed an assay, based on the binding of radioiodinated monoclonal antibody, to measure the release and internalization of cell surface beta-amyloid precursor protein in transfected cells. With this approach, we showed that surface beta-amyloid precursor protein is either rapidly released or internalized, such that the duration at the cell surface is very short. Approximately 30% of cell surface beta-amyloid precursor protein molecules are released. Following internalization, a fraction of molecules are recycled while the majority of molecules are rapidly sorted to the lysosomal compartment for degradation When the C terminus of beta-amyloid precursor protein is deleted, secretion is increased by approximately 2.5-fold as compared to wild-type molecules. There is concomitant decrease in internalization in these mutant molecules as well as prolongation of the resident time on the cell surface. This observation is consistent with recent evidence that signals within the cytoplasmic domain mediate beta-amyloid precursor protein internalization.