Polarized secretion of β-amyloid precursor protein and amyloid β-peptide in MDCK cells

Abstract

The β-amyloid precursor protein (βAPP) is a widely expressed integral membrane protein that is proteolytically processed to yield several secreted derivatives, including soluble APP (APPs), the 4-kDa amyloid β-peptide (Aβ), and a related 3-kDa peptide (p3). To understand βAPP trafficking and processing, we analyzed the sorting of βAPP in Madin-Darby canine kidney (MDCK) cells, an epithelial cell known to possess physiologically distinct apical and basolateral plasma membranes. Processing of βAPP resulted in highly polarized secretion of APPs. More than 90% of APPs was detected in the basolateral compartment, and less than 10% was found in the apical compartment. This was associated with a preferential localization of βAPP on the basolateral cell surface. Activation of protein kinase C, which is known to enhance the secretion of APPs, did not change the polarity of APPs release but significantly increased the amount secreted. Aβ and p3 peptides were also secreted predominantly basolaterally. In addition, MDCK cells secreted a truncated form of Aβ beginning at Arg-5. These data show that the proteolytic processing products of βAPP undergo polarized secretion. Moreover, the results suggest that the amyloidogenic Aβ peptide is generated following the polarized sorting of βAPP. The polarized basolateral secretion of Aβ in these epithelial cells provides a potential mechanism for the accumulation of Aβ in the abluminal basement membrane of brain micro vessels during Alzheimer disease.