Morphology and antibody recognition of synthetic beta-amyloid peptides

To elucidate the relationship between amyloid fibril formation in Alzheimer disease (AD) and the primary structure of the beta-amyloid protein (beta-AP), we investigated the ability of peptides sharing sequences with beta-AP to form fibrils in vitro and to recognize anti-beta-amyloid antisera. The peptides, which were synthesized using a FMOC solid phase procedure and purified by HPLC, consisted of residues 6-25 from the putative aqueous domain, residues 22-35, which overlaps the putative aqueous and transmembrane domains, and residues 1-38 and 1-40 representing nearly the full length of beta-AP. Electron microscopy of negative-stained or thin-sectioned preparations revealed that the peptides assembled into fibrils having different morphologies, some of which resembled in situ AD amyloid. Peptide 6-25 fibrils had diameters of 50-80 A and occasionally showed a central groove suggestive of constituent filaments. Cross sections of the fibril showed a penta- or hexameric arrangement of globular subunits with diameters of 25-30 A. Peptide 22-35 fibrils were helical, with a pitch of 1,100 A and a width of 120 A at its greatest and 50-60 A at its narrowest. The fibrils formed by peptides 1-38 and 1-40 were 70-90 A in diameter. When the peptide assemblies were singly oriented by sedimentation or doubly oriented in a magnetic field, their X-ray diffraction patterns all showed reflections typical of a cross-beta pleated sheet conformation. The patterns differed mainly in their small-angle equatorial intensity, which arises from the packing of fibrils having different widths. Antiserum raised to either native amyloid or to synthetic peptide beta-(1-28) was highly reactive in an inhibition-ELISA assay to beta-(6-25) and beta-(1-38), but not to beta-(22-35), and immunostained beta-(1-40) on Western blots. These studies show that the beta-(6-25), beta-(1-38) and beta-(1-40) peptides can assemble into cross-beta fibrils that retain epitopes characteristic of AD amyloid.