GENERATION AND REGULATION OF BETA-AMYLOID PEPTIDE VARIANTS BY NEURONS

Studies of processing of the Alzheimer beta-amyloid precursor protein (PAPP) have been performed to dale mostly in continuous cell lines and indicate the existence of two principal metabolic pathways: the ''bet a-secretase'' pathway, which generates beta-amyloid (A beta(1-40/42); similar to 4 kDa), and the ''alpha-secretase'' pathway, which generate s a smaller fragment, the ''p3'' peptide (A beta(17-40/42); similar to 3 kDa), To determine whether similar processing events underlie PAPP metabolism in neurons, media were examined following conditioning by p rimary neuronal cultures derived from embryonic day 17 rats. Immunopre cipitates of conditioned media derived from [S-35]methionine pulse-lab eled primary neuronal cultures contained 4- and 3-kDa A beta-related s pecies. Radiosequencing analysis revealed that the 4-kDa band correspo nded to conventional A beta beginning at position A beta(Asp(1)), wher eas both radiosequencing and immunoprecipitation-mass spectrometry ana lyses indicated that the 3-kDa species in these conditioned media bega n with A beta(Glu(11)) at the N terminus, rather than A beta(Leu(17)) as does the conventional p3 peptide. Either activation of protein kina se C or inhibition of protein phosphatase 1/2A increased soluble PAPP, release and decreased generation of both the 4-kDa A beta and the 3-k Da N-truncated A beta. Unlike results obtained with continuously cultu red cells, protein phosphatase 1/2A inhibitors were more potent at red ucing A beta secretion by neurons than were protein kinase C activator s. These data indicate that rodent neurons generate abundant A beta va riant peptides and emphasize the role of protein phosphatases in modul ating neuronal A beta generation.