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Basolateral Secretion of Amyloid Precursor Protein in Madin-Darby Canine Kidney Cells Is Disturbed by Alterations of Intracellular pH and by Introducing a Mutation Associated with Familial Alzheimer's Disease (*)

The analysis of potential sorting signals in amyloid precursor protein (APP) by site-directed mutagenesis and the disturbance of metabolic pathways by drugs is used here to define the parameters that determine polarized secretion of APP in Madin-Darby canine kidney cells. Endogenously produced APP751/770 and APP695 produced from transfected constructs are secreted almost exclusively into the basolateral compartment. The sorting mechanism is highly dependent on intracellular pH as demonstrated by its sensitivity to primary amines and inhibitors of the acidifying vacuolar proton ATPase. The role of potential basolateral sorting signals in the cytoplasmic, transmembrane, and βA4 amyloid region of APP was investigated. Neither deletion of the endocytosis and putative basolateral sorting signal GY.NPTY nor complete deletion of the cytoplasmic domain causes apical secretion of soluble APP. Further deletion of the transmembrane domain and of the βA4 amyloid region confirmed that the major basolateral sorting determinant resides in the extracellular domain of APP. Increased β-secretase cleavage of APP after introduction of the “swedish” double mutation causes apical missorting of about 20% of β-secretase-cleaved APP. The data underline the complexity of processing and sorting APP in polarized cells and suggest a possible problem of protein sorting in Alzheimer's Disease.