IDENTIFICATION OF FULL-LENGTH BETA-AMYLOID PRECURSOR PROTEIN IN HUMAN NEURONAL AND NONNEURONAL CELL-CULTURE SUPERNATANT - A POSSIBLE EXTRACELLULAR SOURCE FOR THE GENERATION OF A-BETA

beta protein (A beta) which is deposited in the amyloid plaques and vasculature in brains of Alzheimer's Disease (AD) patients is a 39 to 43 amino acid peptide proteolytically derived from the amyloid precursor protein (A beta PP). Three major isoforms are expressed in the brain: A beta PP751 and A beta PP770, which contain a Kunitz-like plateaus inhibitor domain (KPI), and A beta PP695. To date it is still unknown which A beta PP isoforms are the precursors of A beta, which proteolytic pathways are involved in its production, and if the processing occurs intracellularly and/or extracellularly. We now report the identification, by Western blot analysis, of an A beta-containing A beta PP protein which co-migrates with full length recombinant A beta PP751 in the culture supernatant of two human neuroblastoma cell lines and in one human kidney cell line. This protein is recognized with six different antibodies towards A beta PP targeting intracellular, extracellular, and the A beta region of A beta PP. The immunodetection of this A beta precursor is shown to be specific by absorption. The presence of full length A beta PP in culture supernatant strongly suggests that some processing of A beta PP may occur extracellularly. The recent identification of two soluble and/or secreted proteases from AD and monkey brain both capable of processing recombinant A beta PP in vitro(1,2) suggests that A beta production may occur extracellularly in vivo by an undescribed mechanism.