Graphical abstract

Aggregation of amyloid-β (Aβ) by self-assembly into oligomers or amyloids is a central event in Alzheimer's disease. Coordination of transition-metal ions, mainly copper and zinc, to Aβ occurs in vivo and modulates the aggregation process. A survey of the impact of Cu(II) and Zn(II) on the aggregation of Aβ reveals some general trends: (i) Zn(II) and Cu(II) at high micromolar concentrations and/or in a large superstoichiometric ratio compared to Aβ have a tendency to promote amorphous aggregations (precipitation) over the ordered formation of fibrillar amyloids by self-assembly; (ii) metal ions affect the kinetics of Aβ aggregations, with the most significant impact on the nucleation phase; (iii) the impact is metal-specific; (iv) Cu(II) and Zn(II) affect the concentrations and/or the types of aggregation intermediates formed; (v) the binding of metal ions changes both the structure and the charge of Aβ. The decrease in the overall charge at physiological pH increases the overall driving force for aggregation but may favor more precipitation over fibrillation, whereas the induced structural changes seem more relevant for the amyloid formation.

The two hallmarks of Alzheimer's disease (AD) are the presence of neurofibrillary tangles (NFT) made of aggregates of the hyperphosphorylated tau protein and of amyloid plaques composed of amyloid-β (Aβ) peptides, primarily Aβ1-40 and Aβ1-42. Targeting the production, aggregation, and toxicity of Aβ with small molecule drugs or antibodies is an active area of AD research due to the general acceptance of the amyloid cascade hypothesis, but thus far all drugs targeting Aβ have failed. From a review of the recent literature and our own experience based on in vitro, in silico, and in vivo studies, we present some reasons to explain this repetitive failure.

Formation of fibrillar structures of proteins that deposit into aggregates has been suggested to play a key role in various neurodegenerative diseases. However mechanisms and dynamics of fibrillization remains to be elucidated. We have previously established that lithostathine, a protein overexpressed in the pre-clinical stages of Alzheimer's disease and present in the pathognomonic lesions associated with this disease, form fibrillar aggregates after its N-terminal truncation. In this paper we visualized, using high-speed atomic force microscopy (HS-AFM), growth and assembly of lithostathine protofibrils under physiological conditions with a time resolution of one image/s. Real-time imaging highlighted a very high velocity of elongation. Formation of fibrils via protofibril lateral association and stacking was also monitored revealing a zipper-like mechanism of association. We also demonstrate that, like other amyloid ß peptides, two lithostathine protofibrils can associate to form helical fibrils. Another striking finding is the propensity of the end of a growing protofibril or fibril to associate with the edge of a second fibril, forming false branching point. Taken together this study provides new clues about fibrillization mechanism of amyloid proteins.

No abstract

Bacteriorhodopsin (BR) is a robust light-driven proton pump embedded in the purple membrane of the extremophilic archae Halobacterium salinarium. Its photoactivity remains in the dry state, making BR of significant interest for nanotechnological use. Here, in a novel configuration, BR was depleted from most of its endogenous lipids and covalently and asymmetrically anchored onto a gold electrode through a strategically located and highly responsive cysteine mutation; BR has no indigenous cysteines. Chemisorption on gold was characterized by surface plasmon resonance, reductive striping voltammetry, ellipsometry, and atomic force microscopy (AFM). For the first time, the conductance of isolated protein trimers, intimately probed by conducting AFM, was reproducibly and reversibly switched under wavelength-specific conditions (mean resistance of 39 ± 12 MΩ under illumination, 137 ± 18 MΩ in the dark), demonstrating a surface stability that is relevant to potential nanodevice applications.

Inhibition of the aggregation of the monomeric peptide β-amyloid (Aβ) into oligomers is a widely studied therapeutic approach in Alzheimer's disease (AD). Many small molecules have been reported to work in this way, including 1,4-naphthoquinon-2-yl-L-tryptophan (NQ-Trp). NQ-Trp has been reported to inhibit aggregation, to rescue cells from Aβ toxicity, and showed complete phenotypic recovery in an in vivo AD model. In this work we investigated its molecular mechanism by using a combined approach of experimental and theoretical studies, and obtained converging results. NQ-Trp is a relatively weak inhibitor and the fluorescence data obtained by employing the fluorophore widely used to monitor aggregation into fibrils can be misinterpreted due to the inner filter effect. Simulations and NMR experiments showed that NQ-Trp has no specific "binding site"-type interaction with mono- and dimeric Aβ, which could explain its low inhibitory efficiency. This suggests that the reported anti-AD activity of NQ-Trp-type molecules in in vivo models has to involve another mechanism. This study has revealed the potential pitfalls in the development of aggregation inhibitors for amyloidogenic peptides, which are of general interest for all the molecules studied in the context of inhibiting the formation of toxic aggregates.

Bacteriorhodopsin, BR, is a natural, photoresponsive, biomolecule that has potential application in data storage, imaging and sensing. Being membrane‐bound, however, it is coupled with metallic electronic surfaces only with some difficulty. We report herein a facile method to generate uniformly orientated, anchored and active monolayers of BR on metallic electrodes. In the present study, the cytoplasmic side of the BR is equipped with an engineered cysteine to achieve largely lipid‐free, orientation‐specific, highly stable, covalent immobilization on gold surfaces. By using non‐invasive Kelvin probe force microscopy, it is possible to measure the light‐induced proton accumulation at the extracellular protein surface at truly molecular scales. The intimate probe–BR interaction possible on lipid removal facilitates the detection of photoinduced surface potential switching substantially larger ((20.4±7.5) mV) with functional single delipidated mutant BR trimers than for the wild‐type protein. The proton pumping detected is also notably highly unidirectional with the orientated protein.