High ability of apolipoprotein E4 to stabilize amyloid-ß peptide oligomers, the pathological entities responsible for Alzheimer's disease.

Nowadays, the emerging role of amyloid-ß peptide (Aß) oligomers in Alzheimer's disease (AD) is widely accepted, putting aside the old idea that fibrils are the primary entities responsible for the onset of the disease. Besides, carrying the E4 isoform of apolipoprotein E (apoE) represents the highest risk of developing AD. Nevertheless, the involvement of apoE4 in AD remains confusing. The goal of this study was to bring new insights into the role of apoE4 in Aß aggregation. We used infrared spectroscopy, thioflavin T fluorescence, and Western blots to evaluate the influence of apoE isoforms on Aß aggregation in vitro. Comparing Aß controls with Aß incubated either with the apoE3 or apoE4 isoform, we report a 30% reduction of the Aß fibrillar content, whereas the oligomeric content is 2 times higher on incubation with the pathological isoform apoE4. ApoE4 would bind and block Aß in its oligomeric conformation, inhibiting further formation of less toxic fibrillar forms of Aß. While previous studies mostly correlated E4 with fibrils, our report underlines a link between apoE4 and Aß oligomers and therefore reconciles apoE4 with the new amyloid cascade hypothesis. Our observations suggest that apoE4 strongly stabilizes Aß oligomers, the pathological species responsible for Alzheimer's disease.

Transformation of amyloid ß(1-40) oligomers into fibrils is characterized by a major change in secondary structure.

Alzheimer's disease (AD) is a neurodegenerative disorder occurring in the elderly. It is widely accepted that the amyloid beta peptide (Aß) aggregation and especially the oligomeric states rather than fibrils are involved in AD onset. We used infrared spectroscopy to provide structural information on the entire aggregation pathway of Aß(1-40), starting from monomeric Aß to the end of the process, fibrils. Our structural study suggests that conversion of oligomers into fibrils results from a transition from antiparallel to parallel ß-sheet. These structural changes are described in terms of H-bonding rupture/formation, ß-strands reorientation and ß-sheet elongation. As antiparallel ß-sheet structure is also observed for other amyloidogenic proteins forming oligomers, reorganization of the ß-sheet implicating a reorientation of ß-strands could be a generic mechanism determining the kinetics of protein misfolding. Elucidation of the process driving aggregation, including structural transitions, could be essential in a search for therapies inhibiting aggregation or disrupting aggregates.

Antiparallel beta-sheet: a signature structure of the oligomeric amyloid beta-peptide.

AD (Alzheimer's disease) is linked to Abeta (amyloid beta-peptide) misfolding. Studies demonstrate that the level of soluble Abeta oligomeric forms correlates better with the progression of the disease than the level of fibrillar forms. Conformation-dependent antibodies have been developed to detect either Abeta oligomers or fibrils, suggesting that structural differences between these forms of Abeta exist. Using conditions which yield well-defined Abeta-(1-42) oligomers or fibrils, we studied the secondary structure of these species by ATR (attenuated total reflection)-FTIR (Fourier-transform infrared) spectroscopy. Whereas fibrillar Abeta was organized in a parallel beta-sheet conformation, oligomeric Abeta displayed distinct spectral features, which were attributed to an antiparallel beta-sheet structure. We also noted striking similarities between Abeta oligomers spectra and those of bacterial outer membrane porins. We discuss our results in terms of a possible organization of the antiparallel beta-sheets in Abeta oligomers, which may be related to reported effects of these highly toxic species in the amyloid pathogenesis associated with AD.

Multidrug resistance protein 1 is not associated to detergent-resistant membranes.

Multidrug resistance protein 1 (MRP1) is a member of the ATP-binding cassette superfamily. Using the energy provided by ATP hydrolysis, it transports a broad spectrum of substrates across the plasma membrane, including hormones, leukotriene C(4), bile salts, and anti-cancer drugs. Recent works have suggested that P-glycoprotein is associated to cholesterol and sphingolipid-rich membrane microdomains and that cholesterol upregulates its ATPase and drug transport activities. Confocal microscopy experiments and Triton X-100 extraction of detergent-resistant membranes provide evidence that MRP1 is not located in raft-like structures and that its activity is downregulated by cholesterol. The data are discussed in terms of cholesterol-protein interaction and topology.