Astrocytes and microglia but not neurons preferentially generate N-terminally truncated Aβ peptides

The neuropathological hallmarks of Alzheimer's disease include extracellular neuritic plaques and neurofibrillary tangles. The neuritic plaques contain beta-amyloid peptides (A beta peptides) as the major proteinaceous constituent and are surrounded by activated microglia and astrocytes as well as dystrophic neurites. N-terminally truncated forms of A beta peptides are highly prevalent in neuritic plaques, including A beta 3-x beginning at Glu eventually modified to pyroglutamate (A beta N3pE-x), A beta 2-x, A beta 4-x, and A beta 5-x. The precise origin of the different N-terminally modified A beta peptides currently remains unknown. To assess the contribution of specific cell types to the formation of different N-terminally truncated A beta peptides, supernatants from serum-free primary cell cultures of chicken neurons, astrocytes, and microglia, as well as human astrocytes, were analyzed by A beta-ELISA and one- and two-dimensional SDS-urea polyacrylamide gel electrophoresis followed by immunoblot analysis. To evaluate the contribution of beta- and gamma-secretase to the generation of N-terminally modified A beta, cultured astrocytes were treated with membrane-anchored tripartite beta-secretase (BACE1) inhibitors and the gamma-secretase inhibitor DAPT. Neurons, astrocytes, and microglia each exhibited cell type-specific patterns of secreted to peptides. Neurons predominantly secreted A beta peptides that begin at Asp1, whereas those released from astrocytes and microglia included high proportions of N-terminally modified A beta peptides, presumably including A beta 2/3-x and 4/5-x. The inhibition of BACE1 reduced the amount of A beta 1-x in cell culture supernatants but not the amount of A beta 2-x.(C) 2015 The Authors. Published by Elsevier Inc.