Spontaneous aggregation and cytotoxicity of the beta-amyloid Abeta1-40: a kinetic model.

The time dependency of the spontaneous aggregation of the fibrillogenic beta-amyloid peptide, Abeta1-40, was measured by turbidity, circular dichroism, HPLC, and fluorescence polarization. The results by all methods were comparable and they were most consistent with a kinetic model where the peptide first slowly forms an activated monomeric derivative (AM), which is the only species able to initiate, by tetramerization, the formation of linear aggregates. The anti-Abeta antibody 6E10, raised against residues 1-17, at concentrations of 200-300 nM delayed significantly the aggregation of 50 microM amyloid peptide. The anti-Abeta antibody 4G8, raised against residues 17-24, was much less active in that respect, while the antibody A162, raised against the C-terminal residues 39-43 of the full-length Abeta was totally inactive at those concentrations. Concomitant with the aggregation experiments, we also measured the time dependency of the Abeta1-40-induced toxicity toward SH-EPI cells and hippocampal neurons, evaluated by SYTOX Green fluorescence, lactate dehydrogenase release, and activation of caspases. The extent of cell damage measured by all methods reached a maximum at the same time and this maximum coincided with that of the concentration of AM. According to the kinetic scheme, the latter is the only transient peptide species whose concentration passes through a maximum. Thus, it appears that the toxic species of Abeta1-40 is most likely the same transient activated monomer that is responsible for the nucleation of fibril formation. These conclusions should provide a structural basis for understanding the toxicity of Abeta1-40 in vitro and possibly in vivo.

SEL-10 interacts with presenilin 1, facilitates its ubiquitination, and alters A-beta peptide production.

Mutations in the human presenilin genes (PS1 or PS2) have been linked to autosomal dominant, early onset Alzheimer's disease (AD). Presenilins, probably as an essential part of gamma-secretase, modulate gamma-cleavage of the amyloid protein precursor (APP) to the amyloid beta-peptide (Abeta). Mutations in sel-12, a Caenorhabditis elegans presenilin homologue, cause a defect in egg laying that can be suppressed by loss of function mutations in a second gene, SEL-10. SEL-10 protein is a homologue of yeast Cdc4, a member of the SCF (Skp1-Cdc53/CUL1-F-box protein) E2-E3 ubiquitin ligase family. In this study, we show that human SEL-10 interacts with PS1 and enhances PS1 ubiquitination, thus altering cellular levels of unprocessed PS1 and its N- and C-terminal fragments. Co-transfection of sel-10 and APP cDNAs in HEK293 cells leads to an alteration in the metabolism of APP and to an increase in the production of amyloid beta-peptide, the principal component of amyloid plaque in Alzheimer's disease.