Trafficking of cell-surface amyloid beta-protein precursor. II. Endocytosis, recycling and lysosomal targeting detected by immunolocalization.

Amyloid beta-protein (A beta) is a proteolytic fragment of the amyloid beta-protein precursor (beta PP). Progressive cerebral deposition of A beta is an early and invariant feature of Alzheimer's disease. The cellular trafficking of beta PP is of particular interest because understanding the production of A beta requires a comprehensive elucidation of the metabolic pathways of this protein. In addition, beta PP is a type I integral membrane glycoprotein that belongs to a class of molecules with both full length and secreted products. Recent evidence suggests that beta PP can be processed in an endosomal/lysosomal pathway. In the latter organelles, a number of beta PP carboxy-terminal derivatives are found, but the precise pathway and kinetics of beta PP trafficking from the cell surface remain unclear. To address these questions, we visualized directly the beta PP internalization pathway by following the localization and distribution of beta PP monoclonal antibodies added to intact beta PP-transfected Chinese hamster ovary cells. Using immunofluorescence and immunoelectron microscopy, beta PP was shown to be rapidly internalized via coated pits and vesicles, after which the molecules were transported to endosomes, prelysosomes, and lysosomes. Using a modified immunodetection protocol, we demonstrated the rapid recycling of endocytosed beta PP to the cell surface and its ultimate targeting to lysosomes. Because we recently found that endocytosis of cell surface beta PP is one route for the constitutive production of A beta, the recycling pathway for cell surface beta PP demonstrated here is a probable route for production of the critical A beta fragment.