LOCALIZATION OF SECRETORY ALZHEIMER AMYLOID PRECURSOR PROTEIN IN THE BRUSH BORDER MEMBRANE OF HUMAN INTESTINAL EPITHELIAL CELLS

Abstract

101 Dysregulation in the processing of the Alzheimer amyloid precursor protein (APP) is thought to be central to the deposition of the beta-A4 peptide and to the pathogenesis of Alzheimer's disease. APP and Alzheimer amyloid precursor-like proteins (APLP) also mediate cell-matrix interactions and participate in the regulation of cell proliferation. AIMS: To test the hypothesis that surface proteins of the brush border membrane of human intestinal epithelial cells might interact with cell proliferation, the presence and localization of APP in biopsies from human intestines were investigated. METHODS: Normal intestinal biopsies from children undergoing diagnostic procedures were frozen and processed for immunostaining with specific antibodies against APP using confocal laser microscopy. In parallel biopsies were cultured with standard organ culture technique, labelled with 35S-methionin and homogenized. Homogenates and culture media were immunoprecipitated with specific antibodies against APP and separated onto SDS-gel electrophoresis. RESULTS: APP was detected on the apical membrane of mature intestinal enterocytes, less in crypt cells and not in the basolateral compartment of the cell nor in the lamina mucosae. Biosynthesis of APP occured within 30 minutes of organ culture as triple band of a 160 kDa protein which after reaching the apical membrane is secreted into the culture medium in hight amounts. The secreted form of sAPP was shown to be less in molecular weight of approximately 130 kDa suggesting a proteolytic cleavage within the apical membrane due to an unidentified membrane bound peptidase. CONCLUSION: APP has been identified to be localized within the apical membrane of human intestinal epithelial cells. It is secreted into the gut lumen after a specific proteolysis as secretory APP. As it was shown in other epithelial cell like in thyrocytes or in keratinoctes of the skin that sAPP plays a role in cell proliferation. It is speculated that sAPP may play a role in intestinal proliferation or in apoptosis. The physiological role of sAPP in the intestine remains to be elucidated.