High sensitivity analysis of amyloid-beta peptide composition in amyloid deposits from human and PS2APP mouse brain

Cortical amyloid-beta (A beta) deposition is considered essential in Alzheimer's disease (AD) and is also detectable in nondemented individuals with pathologic aging (PA). The present work presents a detailed analysis of the A beta composition in various plaque types from human AD and PA cases, compared with plaque A beta isolated from PS2APP mice. To determine minute amounts of A/3 from 30 to 50 laser-dissected amyloid deposits, we used a highly sensitive mass spectrometry procedure after restriction protease lysyl endopeptidase (Lys-C) digestion. This approach allowed the analysis of the amino-terminus and, including a novel ionization modifier, for the first time the carboxy-terminus of A beta at a detection limit of similar to 200 fmol. In addition, full length A beta 40/42 and pyroglutamate 3-42 were analyzed using a highly sensitive urea-based Western blot procedure. Generally, A/3 fragments were less accessible in human deposits, indicative of more posttranslational modifications. Thioflavine S positive cored plaques in AD were found to contain predominantly A/3 42, whereas thioflavine S positive compact plaques and vascular amyloid consist mostly of A beta 40. Diffuse plaques from AD and PA, as well as from PS2APP mice are composed predominantly of A beta 1-42. Despite biochemical similarities in human and PS2APP mice, immuno-electron microscopy revealed an extensive extracellular matrix associated with A/3 fibrils in AD, specifically in diffuse plaques. Amino-terminal truncations of A beta, especially pyroglutamate 3-40/42, are more frequently found in human plaques. In cored plaques we measured an increase of N-terminal truncations of similar to 20% between Braak stages IV to VI. In contrast, diffuse plaques of AD and PA cases, show consistently only low levels of amino-terminal truncations. Our data support the concept that diffuse plaques represent initial A/3 deposits but indicate a structural difference for A/3 depositions in human AD compared with PS2APP mice already at the stage of diffuse plaque formation. (c) 2006 IBRO. Published by Elsevier Ltd. All rights reserved.