Amyloid beta-protein (A beta), the 40-43-amino acid polypeptide that is the principal constituent of senile plaques found in Alzheimer's disease, is constitutively produced and released into medium of cultured cells by an unclear mechanism. In this study, we report that one route of A beta generation involves the internalization of cell surface amyloid precursor protein (beta PP) via the coated pit-mediated endocytic pathway. Radiolabeled A beta can be recovered in medium following selective cell surface radioiodination, indicating that cell surface beta PP is a direct precursor to A beta. In addition, deletion of the cytoplasmic domains of beta PP or depletion of potassium in medium, both of which resulted in reduced beta PP internalization, significantly diminished A beta release. Moreover, pulse-chase experiments after surface radioiodination showed that the kinetics of beta PP secretion and A beta release was different, with the latter occurring at a significantly slower rate. We therefore hypothesize that the internalization of cell surface beta PP via coated pit-mediated endocytosis is one pathway leading to A beta generation and release into medium.
