A PROCESSING PATHWAY FOR AMYLOID BETA-PROTEIN PRECURSOR GENERATES A SECRETED FORM WITH AN INTACT CARBOXYL-TERMINUS

Extracellular deposition of amyloid beta-protein (A beta) is a key neuropathological feature of Alzheimer's disease (AD), Down's syndrome and hereditary cerebral hemorrhage with amyloidoses-Dutch type (HCWA-D). A beta is part of a larger transmembrane glycoprotein, the amyloid beta-protein precursor (A beta PP). Two processing pathways are known for A beta PP. In the secretory pathway A beta PP is secreted following cleavage within A beta thereby preventing A beta formation. The endosomal/lysosomal pathway generates intracellular carboxyl terminal fragments that contain the A beta region. Recently, A beta has been detected in human cerebrospinal fluid, plasma and in conditioned medium from a variety of cell culture systems. This indicates that A beta is formed by a normal processing pathway that has not yet been understood. To investigate this pathway we have expressed A beta PP751 and two disease related point mutants of A beta PP resulting in glu(693)-to-gln and val(717)-to-ile in the baculovirus expression system. In each case we detected secreted A beta PP molecules that had their carboxyl terminus intact (sA beta PPc). This unique species contains the A beta region intact and could therefore serve as the source of extracellular A beta. The levels of sA beta PPc appear to increase in A beta PP with mutations. In addition sA beta PPc molecules were defected in the conditioned medium of fibroblasts from individuals with a Val(717)-to-Ile mutation and from individuals with Down's syndrome. Further, sA beta PPc molecules were also detected in a human Ewing's sarcoma cell line, 5838. Taken together these results point to a third processing pathway for A beta PP that generates secreted A beta PP with its carboxyl terminus intact (sA beta PPc). sA beta PPc molecules could serve as a substrate for extracellular amyloid formation.