RESUMO
The interaction of the beta-amyloid peptide (Abeta) with neuronal membranes could play a key role in the pathogenesis of Alzheimer's disease. Recent studies have focused on the interactions of Abeta oligomers to explain the neuronal toxicity accompanying Alzheimer's disease. In our study, we have investigated the role of lipid interactions with soluble Abeta(28-35) (wild-type) and its mutants A30G and A30I in their aggregation and conformational preferences. CD and Trp fluorescence spectroscopic studies indicated that, immediately on dissolution, these peptides adopted a random coil structure. Upon addition of negatively charged 1,2-dipalmitoyl-syn-glycero-3-phospho-rac-(glycerol) sodium salt (PG) lipid, the wild-type and A30I mutant underwent reorganization into a predominant beta-sheet structure. However, no conformational changes were observed in the A30G mutant on interaction with PG. In contrast, the presence of zwitterionic 1,2-dipalmitoyl-syn-glycero-3-phosphatidylcholine (PC) lipid had no effect on the conformation of these three peptides. These observations were also confirmed with atomic force microscopy and the thioflavin-T assay. In the presence of PG vesicles, both the wild-type and A30I mutant formed fibrillar structures within 2 days of incubation in NaCl/P(i), but not in their absence. Again, no oligomerization was observed with PC vesicles. The Trp studies also revealed that both ends of the three peptides are not buried deep in the vesicle membrane. Furthermore, fluorescence spectroscopy using the environment-sensitive probe 1,6-diphenyl-1,3,5-hexatriene showed an increase in the membrane fluidity upon exposure of the vesicles to the peptides. The latter effect may result from the lipid head group interactions with the peptides. Fluorescence resonance energy transfer experiments revealed that these peptides undergo a random coil-to-sheet conversion in solution on aging and that this process is accelerated by negatively charged lipid vesicles. These results indicate that aggregation depends on hydrophobicity and propensity to form beta-sheets of the amyloid peptide, and thus offer new insights into the mechanism of amyloid neurodegenerative disease.