Amidation of beta-amyloid peptide strongly reduced the amyloidogenic activity without alteration of the neurotoxicity

beta-Amyloid accumulates in cerebral deposits in Alzheimer's disease, so to test the correlation between the neurotoxic and fibrillogenic capacity of beta-amyloid, we synthesized a peptide homologous to fragment 25-35 of beta-amyloid (beta 25-35) and amidated at the C-terminus (beta 25-35-NH2). As the amidation strongly reduced the amyloidogenic capacity of beta 25-35, we compared its neurotoxic activity in the amidated (beta 25-35-NH2) and nonamidated forms. The viability of primary cultures from fetal rat hippocampus was reduced in a dose-related manner (10-100 mu M) similarly by beta 25-35 and beta 25-35-NH2 whereas a scrambled peptide, amidated or nonamidated, did not alter the neuronal viability. The neurotoxic activity of beta 25-35-NH2 is mediated by apoptosis as demonstrated by morphological and biochemical investigations, Electron microscopy examination of culture media with beta 25-35 or beta 25-35-NH2 incubated with neuronal cells for 7 days confirmed the high level of fibrillogenic activity of beta 25-35 and the almost total absence of fibrils in the solution with beta 25-35-NH2. Furthermore, staining with thioflavine S was used to identify amyloid fibrils, and only the cultures exposed to beta 25-35 exhibited intense staining associated with neuronal membranes. These data indicate that the neurotoxic activity of the beta-amyloid fragment is independent of the aggregated state of the peptide.